VŠB - Technical University of Ostrava Faculty of...

VŠB - Technical University of Ostrava

Faculty of Metallurgy and Materials Engineering

Steel Casting Foundry

(study supports)

doc. Ing. Libor Čamek, Ph.D.

Ostrava 2016

http://www.vsb.cz/en

http://www.fmmi.vsb.cz/en/


2

1. Basic parameters of electric arc furnaces and electric induction furnaces.

In steel foundries, the predominant melting units today are electric arc furnace (EAF) and

electric induction furnaces (EIF).

1.1 Melting units in steel foundries. [1,3]

Electric arc furnaces

The diagram of a three-phase electric arc furnace is shown in Fig. 1.1 The furnace is fed

directly from the high-voltage cable 1, via the main switch 2, impedance coil 3 and

transformer 4.

The function of connecting the impedance coil is only in the stage of melting, so that its

inductive resistance reduced voltage fluctuations on the arcs and in the network. From the

transformer, electrical energy is conducted through copper flanges to the outer wall of

transformer vault, and further through the ropes 10 to the arms of electrode holders 6. The

electrodes 7 are held by the holder 8. On the arm of the electrode holder, the current is

further conducted through copper flanges 6. The electrode passes through the furnace

cover via the cooled electrode ring 9.

Fig. 1.1 Example of an electric arc furnace

The installed power of the furnace transformer in foundries usually ranges from 300 to

600 kVA/t.

To melt a ton of burden, the calculated theoretical consumption is 380 kWh/t. The actual

consumption for melting and heating the bath to 1600 °C is higher by about 80 kWh/t.

Foundries operate normally EAFs with melt weight amounting to 4-20 tons. Heavy

castings are produced in foundries at metallurgical steel mills, which supply the liquid

metal.


3

EAFs have alkaline type of lining. The advantage of alkaline furnaces usually lined with

magnesite and chromium-magnesite is the ability to process metal burden with non-

guaranteed phosphorus and sulphur content.

Temperatures in the arc exceed 3000 °C. In the electric arc, dissociation of nitrogen and

hydrogen, which dissolve in the bath, occurs.

Electric induction furnaces

The metallurgical part of electrical medium frequency furnace is shown in Fig. 2.1 In

steel foundries, exclusively the electric induction crucible furnaces are used.

Furnaces typically operate with medium frequency (250-600 Hz). Induction furnaces are

powered by low-voltage network through the furnace transformer. From the furnace

transformer, frequency converter is usually energized by the voltage of up to 6000 V. The

current is initially directed to semiconductor diodes and smoothed by an impedance coil.

The required frequency is produced by power thyristors. Thyristors are controlled and

frequency can be continuously varied. Inductor is powered by medium-frequency current.

Fig. 1.2 Diagram of an electric induction furnace

The construction of the furnace is shielded from the inductor by transformer sheet bundles

which lead the electromagnetic field and reduce losses. The source of heat in induction

furnaces are induced currents. Medium frequency induction furnaces operate with an

input ranging from 500 to 1000 kW/t. In precision casting foundries, furnaces with melt

weighing from 40 to 250 kg are installed most frequently. In other steel foundries, their

capacity ranges from 0.5 to 25 t.

In medium frequency furnaces, exclusively rammed lining are used, since any joints in

the masonry in these furnaces could cause penetration of metal through the crucible


4

lining. The material for the rammed lining is acid ramming mass based on crushed

quartzite (SiO2) or basic ramming mass usually based on MgO spinel – Al2O3 (20 % of

Al2O3), or Al2O3 – MgO (30 % of MgO).

From the metallurgical viewpoint, induction furnaces serve as a unit for burden remelting.

Except for carburizing, alloying and deoxidation, chemical composition of the steel

during melting is intentionally not altered. In the induction furnace, spontaneous self-

carburization of the burden does not occur, therefore they are suitable for the production

of steels with low carbon content.

The induction furnace is an operational melting unit suitable for intermittent operation.

Melting time may be less than one hour, depending on the type of the furnace. Then, if the

time of casting is 30 minutes, two induction furnace can continuously supply the

moulding line with molten metal.

In many foundries, induction furnaces are the only alternative for the production of

stainless steels with a low carbon content.

2. The fundamentals of thermodynamics in steelmaking processes. Solutions of

molten metals, non-metals and gases in iron. [1,2]

2.1 The fundamentals of thermodynamics in steelmaking processes.

All metallurgical reactions are accompanied by consumption or energy release. Chemical

thermodynamics deals with interrelationships between different forms of energy and the

relationship between energy changes and material properties.

The course of each metallurgical process is affected both by the driving force of the

process and internal and external resistance of the reacting system against the course of

this process.

Thermodynamic analysis enables to set the driving force, not the size of resistance against

the analysed response.

Thermodynamic calculations can therefore be used to determine how the monitored

reaction would proceed in the case of no resistance, but it is not possible to determine the

speed of the reaction.

Explanation of the basic concepts

The system or the set is a collection of objects that are subject to thermodynamic

considerations.


5

From the viewpoint of energy transfer and matter transfer, we distinguish the following

systems:

A closed system does not receive matter from its surroundings, and it does not

pass it to its surroundings, either (however, it can exchange energy with it).

An open system exchanges both matter and energy with its surroundings.

In terms of matter properties, we further distinguish the following systems:

A homogeneous system is one whose properties are the same in all parts of the

system or change only continuously (this can be e.g. the case of water or air).

A heterogeneous system is composed of several homogenous sections (phases)

which are separated by phase boundary surfaces (their properties are

changing).

Thermodynamics is interested in states in which the system is found, and in the equilibria

that are established in these systems.

Thermodynamic properties describe the properties of the system. These thermodynamic

properties are divided into extensive and intensive properties.

Extensive properties are dependent on the quantity of the substance in the system and

exert an additive behaviour (their value is equal to the sum of the individual parts of

which the system consists, e.g. matter, volume, material composition, energy).

Intensive properties are independent of the size of the system and the amount of mass in

the system (e.g. pressure, temperature, concentration, density, and all the measurement

values related to the amount of substance or weight).

Thermodynamic process refers to any change in the properties of the system associated

with a change of at least one thermodynamic state variable. In nature, spontaneous

unidirectional processes take place, and they take place with a reduction in the energy

system. Gradually, they come into balance. However, perfect thermodynamic equilibrium

is possible only in an isolated system.

Thermodynamic state variables (p, v, T, C) are independent variables that describe the

system using appropriately chosen, usually directly measurable physical quantities

(temperature, pressure, volume and thermal capacity).

On the basis of state variables, we can then calculate other variables (state functions)

characterizing the system.

Thermodynamic state functions (H, U, S, F, G) are dependent on the thermodynamic state

conditions. It implies that their variation depends only on the initial and final state of the


6

system and does not depend on the mode of switching the system from the initial to the

final state. State functions then mathematically shows a complete differential and the

difference of the initial and final state is independent of the integration path.

State functions are difficult to measure, but they can be expressed as a function of

measurable state variables: pressure, temperature, volume, thermal capacity.

State functions used in thermodynamics include e.g. the internal energy U, enthalpy H,

entropy S, Gibbs energy G and Helmholtz energy F, the chemical potential.

Heat and work do not exhibit thermodynamic state functions, since the transition from the

initial state of the system to the final state is dependent on the initial and final state

conditions, but also on the mode of system switching, i.e. the path of integration, and

therefore thermodynamic functions are not status functions.


7

2.2 Solutions of molten metals, non-metals and gases.

Oxygen in molten iron

The maximum solubility of oxygen in pure iron at a temperature of 1600 °C is 0.25 wt. %

(2500 ppm). With increasing oxygen concentration value approximately above 0.08 %,

negative deviation from Henry’s law occurs due to increasing binding forces between

oxygen and iron, and therefore it is necessary to take into account its effective

concentration – activities.

Currently, oxygen is fed into steel mostly as gas in the oxidation period of melt. Melt

oxidation takes place on each type of melting unit in order to reduce the concentration of

undesirable elements. Thereafter, the reduction of oxygen content before or during the

reduction period of melt is called deoxidation.

Most authors report the solubility of oxygen in iron in the form of O4+

cation, or for

thermodynamic calculations in atomic form, and it can be described by the equation:

OO221

(2.1)

The equilibrium constant K for oxygen of equation (2.1) is expressed by the equation:

21

2O

O

p

K

(2.2)

The oxygen content in the steel, depending on the oxygen partial pressure can be derived

from the equation (2.2) in dependence on the oxygen partial pressure pO .

OO pK (2.3)

In normal steelmaking practice, its concentration usually does not exceed 300 to 400 ppm

in plain steels. The oxygen content in molten steel also controls the other elements present

in the steel as silicon, manganese and carbon. If deoxidizing steel with aluminium in the

ladle is correctly executed, oxygen activity in plain steels is about 5-10 ppm.

Nitrogen in molten iron

In the melt of pure iron, nitrogen is present as an atom, or possibly as N2+

ion. Assuming

atomic nitrogen dissolution in pure iron, the nitrogen transition from the gaseous into the

molten iron phase can be described by the equation:

NN221

(2.4)

Dissolution of nitrogen in pure iron precisely corresponds to Sieverts’ law, which can be,

with respect to Henry’s law, described by the equation:


8

2N

.N% pKN (2.5)

where is partial pressure of nitrogen in the atmosphere [Pa] and KN is the equilibrium

constant of nitrogen dissolution. The equilibrium content of nitrogen in molten iron

corresponds to a given temperature and the partial pressure of nitrogen in the atmosphere.

The solubility of nitrogen in pure iron can be calculated, for example at the temperature of

1600 °C and Pa is about 0.044 % of Nitrogen.

In a similar manner as in iron melt, the nitrogen content equilibrium in the individual

modifications of iron can also be identified in the solid state. For their calculation, it is

necessary to know the temperature dependence of the solubility of nitrogen for the given

modifications of iron. The simple idea of the solubility of nitrogen is shown in Fig. 2.1,

which represents the equilibrium concentration of nitrogen depending on the temperature

of the pure iron. The diagram indicates the concentration of nitrogen, which are in

equilibrium with the atmosphere of pure nitrogen at the pressure Pa.

For the endothermic nature of nitrogen dissolution, with increasing temperature, the

solubility of nitrogen increases as well. Conversely, reducing the solubility of nitrogen in

iron with increasing temperature is related to the formation of nitrides Fe4N, or possibly

Fe2N. The formation of these nitrides is exothermic in nature, which exceeds the

endothermic nature of nitrogen dissolution, which leads to an increase in solubility of

nitrogen in the modification of the iron with a decrease in temperature.

Fig. 2.1 The solubility of nitrogen in pure iron depending on the temperature at

2Np

3251012N

p

3251012N

p

Pa3251012N

p


9

Legend: Obsah dusíku – nitrogen content, dusík – nitrogen, tavenina – melt, teplota –

temperature

In multicomponent systems, especially in high-alloy steels, solubility of nitrogen is also

affected by action of the force with other components. For nitrogen in the case of alloyed

steels, it is necessary, instead of the concentrations in equation (2.5), to use Henry’s

activity and thus Sieverts’ law in the form:

2N

N

N .N% pf

K

(2.6)

In the relation (2.23), represents the activity coefficient of nitrogen in steel, which can

be determined on the basis of the tabulated interaction coefficients.

Steels produced in an electric arc furnace contain about 80 to 120 ppm of nitrogen. Steels

produced in induction furnaces usually contain less nitrogen than steels produced in the

arc furnace, which is mainly due to the transfer of nitrogen to the bath in the area of an

electric arc.

Hydrogen in molten iron

At present, it is assumed that hydrogen is present in iron melts as an atom, or possibly as a

proton H+. Considering atomic dissolving of hydrogen in pure iron, it can be described in

an analogous manner as in the case of nitrogen (2.7). In practical calculations, it is

assumed that hydrogen forms with iron a strongly diluted solution, which is governed by

Henry’s Law. Dissolution of hydrogen in pure iron relatively precisely corresponds to

Sieverts’ law, which can be, with respect to Henry’s law, described by the equation (2.7):

(2.7)

2H

.H% pKH (2.8)

where 2H

p is hydrogen partial pressure in the atmosphere and KH is the equilibrium

constant of hydrogen dissolution.

Since dissolution of hydrogen in iron has an endothermic character, its solubility

increases with increasing temperature. Also for unalloyed steels, Henry’s law (2.2) can be

used to describe the dissolution of hydrogen in steel. The equilibrium hydrogen content in

molten iron corresponds to a given temperature and the partial pressure of hydrogen in the

atmosphere. In a similar manner as in iron melt, the equilibrium hydrogen content can

Nf

HH221


10

also be determined in various modifications of iron in the solid state. For their calculation,

it is necessary to know the temperature dependence of the solubility of hydrogen in the

particular modifications of iron. Fig. 2.2 gives us a simple idea of hydrogen solubility; it

represents the equilibrium hydrogen concentrations depending on the temperature for pure

iron.

hydrogen content

Fig. 2.2 The solubility of hydrogen in pure iron depending on the temperature at

Pa3251012H=p

Water vapour present in the atmosphere decomposes on the metal surface into hydrogen

and oxygen, which dissolve in the steel:

(2.9)

(2.10)

The equilibrium constant equation shows that hydrogen activity in iron under an

atmosphere containing water vapour will increase with increasing partial pressure of

water vapour and with a decrease in oxygen activity. In multicomponent systems,

especially in high-alloy steels, hydrogen solubility is also affected by the action of the

force with other components. For hydrogen in the case of alloyed steel, it is necessary,

instead of the concentrations in equation (2.8), to use Henry’s activity and thus Sieverts’

law in the form:

(2.11)

OH2OH2

OH

O

2

HOH

2

2 p

aaK

[ ]2H

H

H .% pf

KH =


11

In the relation (2.11), represents the activity coefficient of hydrogen in steel, which

can be determined on the basis of the tabulated interaction coefficients [1].

Fig. 2.3 shows the final hydrogen content in steel which is the sum of hydrogen content in

metal and the hydrogen content, which passes into metal from the ladle as well as from

the casting mould.

In the production of steel in electric furnaces, the final hydrogen contents usually range

between 4 and 6 ppm. The final hydrogen contents in steel produced in an acid induction

furnace range between 3.5 and 5 ppm.

Fig. 2.3 Depiction of the sum of metallurgical hydrogen and hydrogen from the mould to

the probability of bubbles forming in the casting

Carbon in molten iron

Carbon, which is the main element in all steels, significantly alters the properties of iron,

namely from very small concentrations. Carbon in steel is an important element also in

terms of metallurgy. In the production of steel in an electric arc furnace, it is

recommended to carburize the burden. The most commonly used carburizing agent for

steel production is coke. For accurate melting or setting the carbon content in steel, crude

iron and carbon-rich ferroalloys (ferromanganese, ferrochromium) are used. High carbon

contents are suitable for the oxidation by gaseous oxygen. If iron ore is used for the

oxidation, it is better to choose somewhat lower carbon contents after melting. The carbon

content after melting should be at least 0.30 % higher than the desired carbon content at

the end of the oxidation, but not less than 0.50 %. For higher levels of carbon or with

alloy steels, it is therefore necessary to use the carbon activity instead of its concentration

in thermodynamic calculations.

Hf


12

Sulphur in molten iron

At higher concentrations, with more massive and heavy castings, sulphur may cause

cracking and it reduces the strength of steel at temperatures below the solidus. Just below

the solidus, there is a temperature zone in which the steel strength value is negligible. In

this zone, low tensions arising in the casting during cooling already lead to cracks, and

sulphur extends this zone even more.

The sulphur source in the production of steel in electric furnaces is the burden. Elements

with high affinity to sulphur form sulphides with sulphur at temperatures of 1600 °C. The

elements with greatest affinity to sulphur are calcium, magnesium and rare earth metals.

Calcium and magnesium are in the gas phase at the temperature of 1600 °C, and their

solubility in steel is small. For desulphurisation of steel under alkaline slag, reducing

solubility of sulphur due to the formation of sulphides, calcium is used. The reaction

between sulphur and the element with a high affinity to sulphur have a great influence on

morphology of sulphides that arise during solidification. Sulphur has a strong tendency to

segregation and in microscale, it segregates to the boundaries of the dendrites. In the

interdendritic areas, melt is enriched with sulphur to the concentrations at which sulphides

of elements originate with a lower affinity to sulphur, e.g. MnS.

In cast steels, for which higher strength values are required even at negative temperatures,

the sulphur content is reduced below 0.015 %. For high-strength cast steel, sulphur

contents below 0.010 % are required. To reduce the segregation of sulphur in heavy

castings, the sulphur content is reduced below 0.003 % and below.

3. Standards and a range of cast steels. Structural steels for castings. Special

stainless steels for castings. Special wear-resistant cast steels. [1,2]

3.1 Standards and a range of cast steel.

Currently, the use of standards in the Czech Republic result from commercial

negotiations.

The conditions of supply of castings, marking, production method, testing, or other

aspects are defined by the technical delivery conditions.

Currently there are valid technical delivery conditions ČSN (Czech National Standard)

EN 1559-1, which specifies the delivery of castings in general.


13

Deliveries of steel castings are particularized in the follow-up standard ČSN EN 1559-2

Additional requirements for steel castings.

This area of standards is supplemented with Technical delivery conditions for steel

castings for pressure vessels from Section 1 to Section 4.

Technical conditions for individual steel grades prescribe in particular chemical

composition, the weldability conditions, heat treatment, and the desired mechanical

properties for different test temperatures.

Marking steels is governed by standards

ČSN EN 10027-1 Steel grades systems Section 1: The system of abbreviated grades –

Basic symbols

ČSN EN 10027-2 Steel grades systems Section 2: The numerical coding System.

Further classification of the range of steels into castings:

Steels for general use ČSN EN 10293 – Steels for castings for general use (structural

steel).

Stainless steels ČSN EN 10283 – Steels for castings of stainless steel.

Heat resistant steels ČSN EN 10295 – Steels for castings of heat-resistant steels.

3.2 Structural steels for castings.

Steels are intended for general use (ČSN EN 10293).

For alloy steels with the Mn content lower than 1.20 %, a maximum content of P and S is

specified.

For the grades GE XXX, the requirement on the chemical composition is prescribed only

for maximum sulphur and phosphorus contents. Other chemical composition is

determined by the foundry operations so as to achieve the required mechanical values. For

the grades GS XXX, only maximum levels of carbon, silicon and sulphur and also basic

mechanical values are prescribed.


14

Unalloyed steel with the Mn content of 1.60 to 1.80 %. The quality of these grades are

widespread not only in the Czech foundries.

The individual grades of this group of steels differ particularly in the carbon content and

in their chemical composition.

Heat treatment and mechanical properties of unalloyed structural steels for castings

containing 1.60 to 1.80 % of Mn can be found in the recommended literature [1].

Low alloy steels can be divided into Mo alloy steels, Cr + Mo alloy steels and steels

containing Ni. In the group of Cr + Mo low alloy steels, the chromium content ranges

from 0.80 to 2.50 %, and the Mo content from 0.15 to 1.20 %. The major effect on the

properties of this group of steels having carbon content and the stability of carbides at

higher temperatures at the molybdenum and chromium-molybdenum steels additive

increases from 0.05 to 0.15% V.

Heat treatment and mechanical properties of low-alloy Cr-Mo- (V) steels for castings can

be found in the recommended literature [1]. The last group of cast steels for general use

are high-martensitic steels with the chromium content above 12 % and with the carbon

content up to 0.06 %. These steels are also classified as high-alloy steels with martensitic

structure.


15

3.3 Stainless steels.

Corrosion resistant steels are materials capable of surface passivation in the presence of

chromium. The lowest concentration of chromium in the matrix which ensures the

passivation is 11.5 %. Because in technical alloys, part of the chromium content may also

be bound to carbides, it is necessary to increase the concentration of chromium to

maintain the same corrosion resistance of steel.

The corrosion resistance of steel depends on the contents of other elements, in particular,

C, Ni, Mo, Mn, or possibly N and Cu.

Casting of stainless steels

Stainless cast steel have a low carbon content, excluding the two grades of martensitic

steels, with other grades, the carbon content is less than 0.070 %.

The steels are divided according to the standard into martensitic, austenitic, fully

austenitic and austenitic-ferritic structure. Austenitic steels contain 8-12 % of nickel and

fully austenitic steels have a high content of nickel in the range of about 24 to 30.5 %.

Besides chromium and nickel, an important alloying element is molybdenum. Fully

austenitic grades contain up to 7 % of Mo.

For alloying of some austenitic, fully austenitic and austenitic-ferritic grades, nitrogen is

also used at concentrations of up to 0.25 %. Up to 4 % of copper is used for alloying some

stainless steels grades.

To summarize the effects austenite-forming and ferrite-forming elements, a concept of so-

called equivalent of nickel Niekv.(3.1) and equivalent of chromium Crekv.(3.2) has been

introduced. Their introducing allows to express the influence of chemical composition on

the structure of stainless steels. The values of the individual equivalents can be

determined using the relations:

%N.30%C.30%Mn.0,5%NiNiekv. (3.1)

%Nb.0,5%Si.1,5%Mo%CrCrekv. (3.2)

Martensitic steels

According to the chemical composition, stainless steels with martensitic structure can be

divided into chrome and chrome-nickel steels. Martensitic stainless steels contain 11.5 to

17.0 % of chromium, and the carbon content depends on the content of nickel. For grades


16

with less carbon (for improving weldability), austenite-forming effect of carbon must be

compensated by the increased content of nickel.

To determine the corrosion resistance of the martensitic steels, they should contain more

than 11.5 % of chromium. To enhance the weldability of martensitic steels, it is required

to reduce the carbon content which is compensated by the addition of nickel. Reducing

the carbon content can be compensated according to the equation (3.1) by increasing the

nickel content. Currently, ratios of 4-6% for nickel content are used, and the carbon

content has decreased below 0.06 %.

Martensitic steels are corrosion resistant in the hardened condition. Heat treatment and the

values of mechanical properties of martensitic stainless steels can be found in the

recommended literature.

Due to the high strength and good weldability, martensitic steels are used in the

construction of hydro turbines, compressors and components working in sea water, the

food industry and in medical technology; they represent the least expensive option of

stainless steels.

Ferritic steels

For steels with a low carbon content (up to about 0.08 % of C), this steel is already ferritic

at the chromium content above 17 %. Commonly used ferrite steels contain 17 to 30 % of

Cr and 0.1 to 0.20 % of C. Purely ferritic stainless steels also include special steels with a

reduced carbon content below 0.01 %, known as super-ferrites. Ferritic steels have good

corrosion and fire resistance, but lower notch strength and high notch sensitivity.

During solidification of these steels, chromium ferrite is eliminated from melt, which is

not further transformed with the decreasing temperature. The solubility of carbon in

chromium ferrite is less than 0.01 %, therefore, practically all carbon present in steel is

eliminated in the form of carbides.

Therefore, ferritic steels with higher carbon content are fragile, and they are used as

refractory steels. Steels containing around 17 % of Cr and with the carbon content below

0.08 % are used as stainless steels.

These steels are applied in particular in the energy industry as parts of heat exchangers, air

preheaters, recuperators and boiler components. High-alloy ferritic chromium steels are

rarely used for castings.

Austenitic steels


17

With a sufficient amount austenite-forming elements, especially Ni (Mn, C, N) the

martensite start temperature is decreased below the room temperature even at a high Cr

content in stainless steels. Then the steels have an austenitic structure. Currently, the Ni

content of 9-12 % is used, while the content of Cr is from 18 to 20 %. The effort to

replace deficient nickel led to austenitic steels, in which a part of nickel content is

replaced by nitrogen, which increases the strength characteristics and nitrogen alloyed

steels have excellent corrosion resistance. Compared to wrought steels, however, the

nitrogen content in austenitic steels is limited to 0.20 %, namely with grades with carbon

content up to 0.03 %. At high contents of nitrogen in castings, there is a risk of

endogenous bubbles. Austenitic steels exhibit the greatest resistance to corrosion

compared with martensite and ferritic steels. They are not suitable for environments

containing sulphur oxides.

Fully austenitic steels

The chemical composition of steels ensures that castings have a fully austenitic structure.

These steels are characterized by a high content of alloying elements and a low carbon

content. With the exception of one grade, the sum of the alloying elements in the steels of

this group may exceed 50 % to 60 %. Nitrogen is widely used as an alloying element in

these steels, partly replacing nickel. Some steels are also relatively high alloyed with

copper. Fully austenitic steels have similar strength properties as austenitic steels.

However, ductility and strength of these steels is higher [1].

Austenitic-ferritic steels

These steels, also called duplex steels, contain in the structure approximately the same

proportion of ferrite and austenite. Of all the groups of stainless steels for castings, they

contain the highest chromium content of 21-27 %. Corrosion resistance of duplex steels is

further increased by alloying with molybdenum and nitrogen. Austenitic-ferritic structure

at the stated chromium content is achieved by nickel alloying at concentrations of from

5.50 to 8.50 %. Except of one grade of steel, austenitic-ferritic steels contain up to 0.03 %

of carbon at maximum.

The advantage of austenitic-ferritic steel is resistance to brittle fracture, because the

ferritic region forms a barrier against propagation of cracks generated in the austenitic

phase.


18

Refractory steels ČSN EN 10295

Steels for casting of this quality are made to a limited extent and only in specialized

foundries. Standard EN 10295 classifies refractory steels according to the structure into

ferritic, ferritic-austenitic and austenitic. The standard also states alloys based on nickel

and cobalt. With regard to the higher carbon content, the production of refractory steels in

foundries is usually easily manageable.

3.4 Special steels for wear resistant castings.

High-alloy austenitic steels are not normalized in the standards ČSN EN; in operations,

old ČSN or DIN standards are used. The principal alloying element in austenitic

manganese steels is manganese, represented by 12 % or more, while the content of carbon

is above 1.00 %, to improve mechanical properties, 0.70 to 1.20 % of Cr is also added.

4. Construction and the thermal mode of an electric arc furnace (EAF). Linings of

electric arc furnaces. The development of EAF technology. [1,2]

In steel foundries, the most common melting aggregate are electric arc furnaces.

Currently, it is estimated that in steel foundries in the Czech Republic, there are about 50

EAF capable of operating with a burden weight of 4-18 t.

4.1 Construction and the thermal mode of an electric arc furnace

The burden is melted in an electric arc furnace (EAF) by an electric arc which burns

between the three graphite electrodes and the burden. The temperature of arches reaches

3000-4000 °C. The diagram of the metallurgical part of an arc furnace is shown in Fig.

1.1. The shell of the furnace vessel is welded from a steel plate and placed on a cradle,

which allows tilting of the furnace. The furnace vessel has a charging hole and a tap hole.

The charging hole is covered with working door operated mechanically in the case of

older furnaces or hydraulically or electro-hydraulically in the case of modern furnaces.

The working door is most often used for slagging off, charging fluxes, charging

ferroalloys, and for other technological operations. The tap hole is located on the opposite

side of the furnace, opposite the door. Behind the tap hole, a tapping trough is welded. A

tilting device allows tilting of the furnace vessel in both directions. The furnace vessel is

covered by a lid, which is controlled mechanically or hydraulically so that it is possible to

deploy the furnace after opening of the lid by charging basket.


19

Fig. 4.1 A diagram of lining in an alkaline arc furnace

4.2 Lining of electric arc furnaces

Lining of an electric arc furnace can be made in different variants of shapes and quality of

refractories. For example, the hearth on magnesite blocks is tampered with grained

magnesite. Vaults (covers) of electric arc furnaces in the foundry are now generally of

magnesite and chromium or from monolithic alumina cast refractory concrete.

For smaller types of arc furnaces (5 -15 t), the lifetime of the cover is about 150-200

melting processes. The lifetime of the cover depends partly on the manufactured range of

materials, and also on controlling the power mode throughout the melt and the slag

regime.

Fig. 4.15 shows a diagram of lining in an arc furnace. Position no. 1 – Tampered layer of

magnesite mass, position no. 2 – magnesite-chromium, or possibly monolithic alumina

cover of the furnace, position no. 3 – insulating lining, position no. 4 and no. 5 –

magnesite and magnesite-chromium working lining, position no. 6 – steel furnace shell.

The reaction between the lining and the slag

Refractoriness of magnesite is reduced by oxides of iron and silicon which are commonly

found in the charge. The greatest wear of the lining occurs in places that are in contact

with the slag layer covering the molten metal, so called slag line. The slag saturated with

MgO is unresponsive to lining. The reaction between the oxides in the slag and the lining

can be affected by the composition of the slag so that the slag concentration contained

preferably more than 10 % of MgO. However, a high content increased the viscosity of

the slag, thereby bringing deterioration of the required metallurgical processes.

The reaction between the lining and the molten metal

The reaction between the lining and the molten metal can be schematically described by

equation (4.1) and (4.2)


20

(4.1)

(4.2)

According to equation (4.1), the reaction of magnesite with deoxidizing element dissolved

after deoxidation in steel, generally designated as Me is given. Magnesite lining does not

contains in particular after the oxidation phase pure MgO, but also other oxides that can

react according to equation (4.1) with a residual concentration of deoxidation agent.

These are oxides of the elements with lower activity towards oxygen than MgO, such as

FeO, or SiO2. The reaction then results in oxides which are present in steel as oxide

inclusions. The reaction (4.2) may occur after deoxidation of steel in a vacuum when the

oxygen activity in steel is lower than what corresponds to dissociation pressures of the

oxides contained in the lining.

4.3 The development of EAF technology

Electric arc furnaces are used both for the production of cast steel, and especially for the

production of moulded steels. Gradual intensification of production technology in electric

arc furnaces required substantial investment funds, particularly in the area of secondary

metallurgy, which is one of the most important measures leading to increased productivity

of arc furnaces. In this case, the arc furnace only serves to melt the burden. During

melting and heating of steel, decarbonisation and dephosphorization takes place. The

melting process finishes by slag-free tapping of the metal into the ladle. Other

metallurgical processes (desulphurisation, fine alloying, etc.) are carried out already at

one of the elements of the secondary metallurgy. Considering the productivity of Czech

foundries, in the Czech conditions, there is not a realistic chance of return of investment

in the entire secondary metallurgy. The purpose of potential realization of secondary

metallurgical equipment in foundries is not to enhance the productivity of the furnaces,

but to improve the quality of steel produced, which could not be achieved in any other

way.

5. The production of unalloyed cast steel in alumina EAF. Progress of individual

technological and metallurgical phases. The practice of melting. [1,2]

5.1 Technology of the production of unalloyed steels

If we denote the time of the melt process τ, then we can divide it into the time of repair of

the lining and charging (τ1), melting time (τ2), oxidation time (τ3) and deoxidation

(reduction) time (τ4): τ = τ1 + τ2 + τ3 + τ4. From the time of melting and burden weight

MgOMeMeMgO xyx xy

OMgMgO


21

(Q), we cane estimate a certain daily productivity of the furnace (G), on the assumption

that the operation is continuous:

(5.1)

5.1.1 Repair of the furnaces and charging τ1

The period of repair and charging begins at the end of tapping of the previous melt and

finished by turning on the oven. After the necessary repairs to the furnace lining, the

burden is prepared. The feedstock can be steel scrap, recovered material, foundry crude

iron and in manufacturing alloy steels also some alloying additives (FeMo, FeW, Ni, Cu).

Normally, ore and lime are added to the burden to enable decarburization and

dephosphorization already during melting. The foamed slag, which in this case is formed

during melting, also increases heat transfer efficiency from the arcs to the burden.

5.1.2 Melting burden τ2

The melting period begins by turning on the furnace and finished by sending the first test

for chemical analysis.

Part of the electrical energy may be replaced by burning gas available today in the

oxygen-fuel burners and exothermic reactions while blowing oxygen into the bath in the

furnace. The energy regimen of the furnace is based on the choice of optimum arc length

and the furnace input power.

During melting, solid burden decreases in the furnace, thus exposing the arcs with

consequent lowering efficiency of heat transfer to the bath. For this reason, it is

appropriate to create foamed slag in the furnace, in which the arches are hidden. Foaming

slag is achieved by addition of ore, lime and carburizing agents to the burden or the empty

furnace. Melting time depends mainly on the furnace transformer input power.

Manufacturers of small arc furnaces (up to about 15 t) state the actual consumption of

electric energy for melting steel about 450 kWh/ton. Simplifying estimate of melting time

can be made based on the installed capacity of the furnace transformer (P) according to

the relation:

(5.2)

where “Q” is the burden weight in tonnes, “k” is the utilization coefficient of the furnace

transformer indicating what parts of the installed furnace performance is used on average

during melting. Product P.k.cos then determines the average power input supplied to the

dentQ

G /244321

cos

4502

kP

Q


22

furnace during melting. Under these conditions, after substituting into formula (5.2), the

resulting melting time for burden of 7 t and the average installed power of the furnace

transformer 3 MVA is approximately 1.5 hours.

5.1.3 Oxidative period τ3

The oxidative period begins by the first test sampling for the chemical analysis and

finished with slagging and putting in oxidation additives.

The most important activity of the production phase is decarburization and

dephosphorization of the bath, reducing the amount of hydrogen or nitrogen in steel, and

heating the bath to the tapping temperature.

For oxidation, iron ore or oxygen gas are used in steel arc furnaces. After the addition of

ore and lime to the burden, mainly the oxidation of Si, Mn, P, and C takes place during

melting. The order in which the individual elements will be oxidized depends on their

affinity for oxygen, and their activity.

The slag composition also has a significant impact both on the composition of the

resulting oxides and also on the equilibrium activities. If the concentration of elements

with high oxygen activity drop to trace amounts, the activities of elements with lower

affinity for oxygen will be decisive for determining the oxygen activity.

Decarburization reaction

When using ore to carbon oxidation, the following reactions take place:

I. Dissolution of ore in slag ,

II. The transition of oxygen from slag into steel ,

III. Decarburization reaction at the slag – metal interface ,

IV. Forming bubble nuclei under the interfacial slag – metal interface. The partial

pressure of carbon monoxide increases with a depth of the bath. In terms of the

partial pressure of CO, the most favourable conditions for the formation of carbon

monoxide bubbles on the surface of the bath of molten steel. Boiling may start at

the surface layers of steel under the surface of slag,

V. The diffusion of carbon and oxygen atoms on the surface of CO bubbles,

VI. The growth of bubbles due to the course of merging of carbon and oxygen on the

surface of bubbles and their flowing out of steel – carbon boil,

FeO3FeOFe 32

OFeFeO

COOC


23

At low activity of oxygen in steel after melting, carbon boil does not occur. During the

entire period of oxidizing, if carbon boil occurs, the oxygen activity is controlled by

carbon.

The importance of carbon boil

- degassing steel, i.e. reducing the hydrogen and nitrogen content in steel

- thermal and chemical homogenization of steel

- controlling the activity of oxygen during oxidation

Dephosphorization of steel

The customer may require the phosphorus content below 0.015 % or even lower.

In accordance with the molecular theory of slag, the dephosphorization reaction can be

described by the equation:

(5.3)

The equilibrium constant of the reaction can be expressed as:

(5.4)

The concentration of oxides in slag are expressed as molar (atomic) fractions N, the

phosphorus content in steel is expressed in weight percent. From the above equations it

follows that the dephosphorization occurs at the slag – metal interface. The oxidative

period finishes with the oxidation test sampling and slagging.

5.1.4 The period of completion (deoxidation, refining, reducing period) 4

The period of completion starts by throwing deoxidizing agents into the furnace after

slagging and lasts until the end of tapping.

The main tasks of the period of completion:

Deoxidation of steel in the furnace, i.e. reducing the oxygen activity in steel and slag

to a value that is suitable for desulphurization, fine alloying and final deoxidation of

steel in the ladle.

Steel desulphurization.

Fine alloying of steel.

Adjusting the tapping temperature.

Maintaining the hydrogen and nitrogen content below the required concentration.

[ ] ( ) ( ) ( ) [ ]Fe5+O4CaO.P=CaO4+FeO5+P2 52

4

CaO

5

FeO

2

5O2P.4CaO

P NN

NK

06,15

06740log

TK


24

Steel deoxidation

The applied ways of technology can be divided into precipitation (deep), extraction

(diffusion) deoxidation, deoxidation under reduced pressure (in vacuum) and deoxidation

by synthetic slags.

Precipitation deoxidation

Precipitation deoxidation is a process during which the oxygen activity is reduced by

adding elements with high affinity for oxygen due to their reaction with oxygen dissolved

in steel. The product of precipitating deoxidation are oxides are in solid, liquid or gaseous

state, which are thermodynamically stable at temperatures of metallurgical processes. A

part of the solid and liquid products of precipitation deoxidation remains in steel as

inclusions, some of them float out and subsequently pass into slag. In the production of

cast steel, mostly aluminium and silicon are used as a strong deoxidizing element for

precipitation deoxidation. In the residual content of aluminium is sufficient, the product of

deoxidation in cast steel is aluminium oxide:

(5.5)

Al and Si deoxidation may be combined with the use of other elements such as Ti, Ca,

rare earth elements, ferroalloys, etc.

Extraction (diffusion) deoxidation

In all the described processes, the product of deoxidation are inclusions, which reduce the

purity of steel. The principle of extraction deoxidation of steel is deoxidation of slag and

reducing the oxygen content in steel and its transfer into slag. The activity of iron oxide in

slag can be usually reduced by the addition of coke or ferrosilicon. Coke is the cheapest

deoxidizing slag additive. Deoxidation of slag can be described by the equations:

(5.6)

(5.7)

When using coke for deoxidation of slag, it is necessary to consider the possibility of steel

carburization, the use of ferrosilicon in turn can increase the silicon content in steel.

Deoxidation products remain in slag, therefore inclusions do not form in steel.

Deoxidation of steel in vacuum

In the case of vacuum processing of steel, it is beneficial to use the reaction between

carbon and oxygen in the conditions of reduced pressure. For the equilibrium between

carbon and oxygen, the equilibrium constant can be expressed by the equation:

32OAlO3Al2 TG 25,3910362071–

( ) Fe+CO=C+FeO g

2Fe+SiO=Si+FeO2 2


25

(5.8)

Reducing the partial pressure of carbon monoxide leads to a reduction in the equilibrium

oxygen activity in steel. In deep vacuum, carbon at higher temperatures is a very strong

deoxidization agent, and it can reduce even the most stable oxides, such as MgO or CaO.

The product of deoxidation of steel is gaseous carbon monoxide practically insoluble in

melt, which leaves the bath in the form of bubbles.

Deoxidation of steel by synthetic slags

Steel refining by synthetic slags leads to deoxidation of steel, provided that the activity of

FeO in slag is lower than the oxygen activity in steel, which is a condition of diffuse

deoxidation.

Most used are slags based on Al2O3 – CaO – SiO2, which, in accordance with the ternary

diagrams, form compounds with a low melting point according to the chemical

composition, mostly in the range of 1350-1450 °C. Consequently, slag with high fluidity

is formed, which makes prerequisites for good desulphurization of steel from a kinetic

viewpoint.

Desulphurization of steel

In the oxidation period, the sulphur content can be reduced by 20-30 %. A prerequisite for

desulphurisation in the oxidation period is strongly alkaline slag. The lowest contents of

sulphur are obtained in the electric arc furnace after deoxidation of steel.

Desulphurization reaction can be described by the equation of molecular theory of slags:

(5.15)

According to the molecular theory of slags, desulphurization takes place at the slag –

metal interface. For desulphurization, it is necessary to work with alkaline well

deoxidized slag. The FeO content in slag should preferably be below 1 %. The condition

for the course of desulphurization is a low activity of sulphur in slag. However, slag must

remain reduction and alkaline all the time.

Fine alloying of steel

The chemical composition of cast steel is prescribed by standards and it is contained in

the purchase agreement. For each alloying element in a specific interval of concentrations

and for sulphur and phosphorus, usually their maximum contents. In the case of unalloyed

OC

COOC,

aa

pK

FeOCaS=FeSCaO

SCaO

FeOCaS

aa

aaKS


26

steel, fine alloying means additive of carbon, manganese and silicon, or possibly any of

the micro-alloying elements, for example niobium, vanadium, titanium, etc.

Temperature measurement

For measuring the temperature of steel, thermocouples of the type Pt-Pt-PtRh10 or

PtRh13 are used most often. Measuring the temperature of liquid steel today is mainly

based on the thermionic phenomenon called Seebeck effect. The hot junction of the

thermocouple is formed by welding platinum wire with platinum wire containing 10 or 13

% of Rh.

6. The technology of the production of alloy cast steel in alkaline EAF. The

function of the main thermodynamic conditions of metallurgical processes.

[1,2,3]

6.1 Classification of cast steel

For most types of wrought steel, there is also a variant of cast steel.

Properties of cast steels, however, are largely influenced by the conditions of

solidification, i.e. especially the cooling rate and chemical heterogeneity generated during

solidification and cooling of steel. They are different from wrought steels despite the

same chemical composition if a melting sample. Cast steels generally have lower values

of mechanical properties (strength, ductility, etc.), which is connected not only with

segregation of some elements and coarser grains, but also with the appearance of micro

shrinkages and sags generated during solidification. The advantage of cast steels is that

they enables virtually unlimited combination of alloying elements and their amounts with

respect to the following processing of castings, and also achieving the final shape of the

product (casting) by casting. The classification of cast steels is usually based on chemical

composition, or according to the content of alloying elements. Standard ČSN EN 10020

divides steel into unalloyed steels, low alloy steels, and high alloy steels.

6.2 Production of low alloy steel in alkaline arc furnace

The basic task of preparing burden for steel production is economical processing of return

alloyed waste. The thing is that during oxidation, oxides of some elements, such as

chromium and vanadium pass into slag, during slagging, the above mentioned elements


27

are lost. In the oxidation of steel by ore, depending on the activity of the individual

elements, silicon is oxidized first, then manganese and chromium. In the presence of more

than 0.50 % of Cr in the bath, the possibilities of dephosphorization are therefore limited.

6.3 Production of high alloy stainless steels in alkaline electric arc furnaces

The influence of carbon on the properties of stainless steels

Stainless steels are characterized by a low carbon content. Except for chromium

martensitic steels, the carbon content lower than 0.07 % is increasingly required. Carbon

forms carbides with chromium, which reduces the content of chromium in solid solution,

it has a higher diffusion speed than chromium, and therefore the formation of carbides

may be associated with heterogeneity of the solid solution. The practical result of the

exclusion of chromium carbides at the grain boundaries is intergranular corrosion.

Intergranular corrosion occurs especially after the welding in the heat affected zone.

Austenitic steels tend to succumb to intercrystalline corrosion, depending on the carbon

content of steel and the temperature.

To prevent it, the carbon content is reduced to a concentration that is equal to or less than

the solubility of carbon in the matrix at temperatures at which carbides are still separated.

The sufficiently low carbon content in austenitic steels, which eliminates extensive

intergranular corrosion, is considered the carbon content below about 0.03 %, and also

alloying of steel with elements, whose affinity for carbon is higher than that of chromium,

i.e. the stabilization of the melt, usually Nb, Ta and Ti.

The main directions of development of stainless steels

Desired development of stainless steels may be characterized by a requirement to increase

resistance to corrosive environments containing chlorides decreasing the content of

carbon and sulphur by alloying with nitrogen and increasing the purity of steels.

The requirements for modern stainless steels:

- High concentrations of elements increasing in resistance to pitting,

which is indicated by the value of PRE (Pitting Resistance Equivalent),

with a value equivalent PRE 40


28

- The carbon content below 0.03 % (or 0.02 %) in the austenitic and

dual phase steels, sulphur content below 0.003 %.

- Nitrogen alloying up to 0.025 % for cast steels. It is used to substitute

nickel in austenitic steels where corrosion resistance is also increased

by nitrogen.

- High purity of steel achieved by secondary metallurgy processes.

- Production of three-phase – duplex steels, e.g. austenitic-ferritic steels.

Austenitic steels, which satisfy the above mentioned requirements on PRE and the

contents of C and S are also called super-austenitic. Nickel is partly replaced with

nitrogen, which also suppresses the adverse effects caused segregations.

In some literature, the evaluation of the resistance of steel against pitting by the value of

P. I (Pitting Index) is used, in which it is essentially the same criterion as the equivalent of

PRE (determination of the index value P. I is based on the same relationship as the

equivalent of PRE).

Austenitic-ferritic steels with 50 % of ferrite are called duplex steels.

The requirements for refractory steels

The properties of refractory steels must be high resistance to oxidation, corrosion and

long-term stability properties in the hot gases. Oxidation resistance at high temperatures is

achieved by alloying steels with Cr, Si, Al, Ni, and their composition is similar to that of

stainless steels. According to the structure, they are divided into ferritic, ferritic-austenitic,

and austenitic. Heat-resistant steels generally have a higher carbon content than stainless

steels, usually in tenths of a percent.

6.3.1 Fundamentals of the stainless steel production

The main metallurgical tasks in the production of stainless steels:

- The production of steel with a low carbon content. From the viewpoint of achieving low

carbon contents, it is necessary to oxidize the melt to as low carbon content as possible

with chromium losses that are economically feasible, and in a later stage of melting, to

minimize carburizing steel by graphite electrodes.


29

The smaller the ratio between the weight of melt mass and the weight of graphite

electrodes and also the longer the period from the end of the oxidation to tapping, the

greater carburizing the bath.

6.3.2 The function of the main thermodynamic conditions of metallurgical processes

Thermodynamic conditions of oxidation of carbon in melts rich in chromium

In the production of stainless chromium steels in EAF, blowing oxygen gas is used for

oxidation. In the production from return waste of high alloyed steels, chromium oxidizes

first, and then, at sufficiently high temperatures of the bath, carbon oxidation occurs as

well. Oxidation of chromium produces oxides whose composition depends on the

concentration chromium in steel. When the chromium content in the melt is up to about 9

%, oxide Cr2O3 results from oxidation; at higher concentrations, oxide Cr3O4 is formed.

For the case of thermodynamic equilibrium and the content of chromium in the melt up to

9 wt. %, oxidation of carbon and chromium can be described by the equations (6.1) and

(6.2) with the tabulated values of free enthalpy. Summation results in the relationship

(6.3), which expresses the reduction of chromium oxide with carbon to form carbon

monoxide and chromium dissolved in the melt. Similar to expressing the chemical

reaction equation by summation, we also express the standard free enthalpy by the

equation.

(6.1)

(6.2)

(6.3)

The equilibrium constant of the reaction can be expressed as:

(6.4)

During oxidation of melts with high chromium content, slags are saturated with

chromium oxides, therefore their activity can be regarded as equal to one.

The chromium content in the burden is governed in particular by the requirement that the

quantity of ferrochromium added to fine alloying after oxidation was sufficient to cool the

bath. In the case of steel having the desired composition of about 18 % of Cr, the most

32OCrO3Cr2 TG .92,356899821–1

C3O3CO3 ).98,4097219(32 TG

C3OCrCO3Cr2 32 TGGG .86,479983761–3 213

3

CO

2

Cr

3

C3O2

Cr

.

.lnln

pa

aaK


30

frequently chosen burden composition results in the chromium content in the melt of

approximately 13 %. During oxidation, the chromium content decreases by from 2 to 3 %.

In the subsequent reduction, about 1 % of Cr passed from the slag back into the bath.

This consideration of the technological progress is based on the assumption of production

of melts rich in chromium only in the EAF, without the possibility of processing in

secondary metallurgy aggregates.

Influence of the temperature on thermodynamic equilibrium

The preparation of burden is usually determined so that after melting the carbon content is

about 0.5 %. With the increasing carbon content in the burden, the temperature at which

carbon boil occurs decreases. After the start of blowing, oxygen first reacts with silicon,

then chromium, and subsequently with carbon. Starting of the carbon boil as soon as

possible after the beginning of blowing is important, because before the beginning of

carbon boil, chromium is oxidized, which results in a loss of chromium and furthermore,

it prolongs melting and increases oxygen consumption.

For the chosen conditions of the beginning of oxygen blowing, i.e. 13 % of Cr and 0.50 %

of C, the equilibrium temperature can be calculated, which may be a guide for

determining the beginning of oxygen blowing. Below this temperature, there are no

thermodynamic conditions for the reaction of oxygen and carbon dissolved in the melt.

After achieving this temperature, oxidation of the components with a higher affinity for

oxygen (Si) occurs, which is followed by chromium oxidation. Only when the heat

released during the exothermic reaction between oxygen and the elements in the melt (Si

and Cr) raises the temperature of the bath above the equilibrium temperature, the carbon

reaction occurs. Practical experience confirms that carbon boil with the carbon contents of

0.50 % begins at the temperature ranging from 1600 to 1630 °C.

Influence of the pressure on thermodynamic equilibrium

According to equation (6.4), the value of the equilibrium constant depends on the activity

of carbon monoxide or on the partial pressure of carbon monoxide in the gaseous mixture,

which is in equilibrium with the melt. In practice, the effect of the carbon monoxide

partial pressure in bubbles emerging from the bath on the carbon equilibrium in steel with

oxygen is used to achieve low carbon contents in steel. The carbon monoxide partial

pressure pCO in bubbles depends both on the concentration of carbon monoxide % VCO in

bubbles and the total pressure on the surface of steel p. At low pressure and high


31

temperatures, it can be assumed that the gas mixture which is in equilibrium with the

melt, acts as a mixture of ideal gases. In this case, the activity of the carbon monoxide in

the mixture can be expressed for a constant temperature by the equation:

pV

pn

np

i

100

%COCO

CO

(6.5)

The partial pressure of carbon monoxide in the gaseous mixture can be reduced either by

decreasing its concentration in the gaseous mixture (% VCO), or decreasing the total

pressure of the mixture (P) above the bath surface.

To reduce the concentration of carbon monoxide in the gaseous phase, which is in contact

with steel, the technology of blowing a mixture of inert gas and oxygen into the bath is

used in practice. Reduction of the total pressure (P) above the surface of steel is achieved

in vacuum metallurgy equipment. For steel foundries, secondary metallurgical equipment

using reduced pressure is an investment with difficult access.

7. Intensification of steel production in EAF. Contemporary trends in steel

production in EAF.

Intensification of the steel production allows to reduce in particular the processing costs

involved in the costs of material in the casting. Intensification of steel production in arc

furnaces in foundries makes sense when all normal precautions to reduce costs are

exhausted:

Burden is quickly loaded to the furnace.

Avoiding any downtime during the time between switching on the furnace after

loading until the tapping.

In terms of maintenance, special attention is paid to the adjustment of regulation

and control of other elements of the furnace.

Foundry operates only the smallest number of furnaces required to provide the

liquid metal production.

Operation of furnaces is motivated to maintain a reduction of selected technical -

economic indicators.

7.1 The development of equipment and technology used in electric arc furnaces

A suitable return on large investments may be in implementing the intensification

measures in metallurgical steel mills at EAF furnaces in furnace equipment and

technology, in their productions in a hundreds of thousands to over one million tons.


32

Electric arc furnaces of new construction and equipment are now metallurgical aggregate

designed for the mass production of steel. Intensification of EAF was achieved, in

particular, by improving the performance of the furnace transformer, increasing the

supply of energy in the form of heat of exothermic reactions and heat from the oxy-fuel

burners. Another measure is the use of heat from the exhaust gases from the furnace. Melt

time is only determined by the time at which sufficient energy is introduced into the

furnace to melt the burden and achieve the desired heating of the bath.

7.2 Contemporary trends in steel production in EAF

New design elements of EAF, which have been developed in recent years, aimed to:

improve the quality of steel produced

reduce production costs - especially for small EAFs

optimize the use of energy - to reduce the consumption of primary electricity and

increase flexibility in the choice of energy

reduce noise and pollution in the production of electrical steel

The basic principles, which are currently used in these newly developed techniques, can

be summarized in the following measures:

heat recovery from outgoing exhaust gases (furnace gas) for preheating the burden

(steel waste),

use of carbon and oxygen, the oxidation of carbon as an additional energy source in

EAF,

use of CO combustion in the furnace or preheater steel waste chamber,

partial replacement of electricity with the energy from the oxy-fuel burners.

The introduction of modern methods of secondary metallurgy fundamentally affected the

changes in the design and manufacturing technology of EAFs. Arc furnaces operating in

tandem with a ladle furnace are only used as a melting unit, in which decarburization and

dephosphorization is made. When tapping is started, oxidizing slag remains in the

furnace. This creates favourable conditions for deep desulphurization and deoxidation of

steel in ladle. Melt time is limited only by the speed of heating the metal to the tapping

temperature.


33

Design changes made recently in modern arc furnaces completely alter the profile of the

whole arc furnace. Fig. 7.1 is an example of diagram representation of the EAF

contemporary design.

Fig. 7.1 Schematic representation of the EAF contemporary design, on the left furnace

with bottom tapping, on the right furnace with burners, equipment for foamed slag

and bottom tuyeres

Legend: koks - coke

8. The production of steel in acid EAFs. Advantages of acidic EAFs versus

alkaline EAFs. [1,2]

8.1 The production of steel in acid electric arc furnaces

Literature suggests that acid arc furnaces are used in American and Russian steel

foundries, while in Europe, acidic arc furnaces are not very common. In the Czech

Republic, acid EAFs are operated.

Lining of acid arc furnaces

Acidic linings based on silicon dioxide are thermodynamically less stable than alkaline

linings based on MgO, they react with components with a higher affinity for oxygen than

that of silicon (Al, Ti, and at higher temperatures also with carbon). In the case of

reducing the oxygen activity below the value corresponding to equilibrium with silicon

under the given conditions, it may lead to the reduction of the silicon dioxide from the

lining, which is accompanied by increase in the oxygen and silicon concentration in the

molten steel.

Therefore, in acidic furnaces, the oxygen activity cannot be reduced to the values

achievable in alkaline furnaces, which has a negative effect on the values of mechanical

properties. Elements dissolved in steel that form basic oxides also react with the lining.

These elements include in particular manganese.


34

8.2 The advantages of acid electric arc furnaces

Steel production in acid EAFs is characterized by up to 20 % lower processing costs

resulting from shorter time of melt, which enables an easier technological process of

melting and thus the possible savings:

in electricity consumption, which is also connected with lower thermal conductivity

of acidic linings and electricity costs can be reduced by up to 20 %,

in the consumption of graphite electrodes by up to 25 %,

in labour costs,

in cost of refractory material,

in consumption of non-metallic additives,

lower costs of transporting and depositing slag, the production technology in acidic

EAFs works with smaller quantities of slag.

In the production of steel in acid arc furnaces, we achieve a lower hydrogen content,

therefore the volume of casting defects caused by bubbles and pinholes is generally lower.

The principal disadvantage especially in today’s possibilities in the field of external metal

waste of acid EAFs is a techno-economic unreality of dephosphorization and

desulphurization on this unit. The production range of steels for acid arc furnaces is

limited to unalloyed and low alloy steel grades.

9. Construction and thermal regime of work of medium frequency electric

induction furnace (EIF). Ramming EIF refractories. The practice of melting.

9.1 Production of cast steel in electric induction furnaces

For the production of steel castings of lower and medium weight, which are produced on

automatic moulding lines, an essential requirement is the continuous supply of molten

metal. For the production of melt, electric induction furnaces are irreplaceable steel-

making units. The time of melts on a modern induction furnace may be shorter than one

hour.

From the viewpoint of the production assortment, a limitation in foundries is the

production of alloy steels, which cannot be economically produced in EAFs. This is the

case of stainless steels with a carbon content below 0.050 %.

From a structural point of view, electric induction furnaces can be classified as follows:

Channel furnaces


35

Crucible furnaces – mains frequency

medium frequency

high frequency

Channel type electric induction furnaces are used in foundries of iron and non-nonferrous

metals as maintenance furnaces. They are powered by the mains frequency. The principle

of heating metal prevents to use in these furnaces for melting in steelmaking, and their

specific electrical power is also limited. Medium frequency electric induction crucible

furnaces (EIFK) are built mostly from a few dozen kilograms up to melt weight of 25

tons. Furnaces weighing 40-250 kg are common in precision casting foundries. High

frequency electrical induction crucible furnaces are used as laboratory furnaces operating

with the charge of a few grams to several hundred grams. They are used for melting most

technical metals. Medium frequency crucible furnaces are now a common melting unit in

steel foundries, and other types of induction furnaces are not built any more.

9.2 Medium frequency electric induction crucible furnaces

Technological advantages of EIFs compared to EAFs:

They enable to the supply melts of lower weight, usually 1-6 tons, to the casting

bed at intervals of 40 to 120 minutes.

They almost continuously provide the moulding shop with liquid metal.

The induction stirring of the melt causes thermal and chemical homogeneity of the

melt.

Made steels generally contain less hydrogen and nitrogen.

During the melting process, no carburization of metal occurs. EIF allows to

produce steel with the lowest carbon content to below 0.03 wt. %.

Operational commissioning of EIFs

Economic benefits EIFs compared to EAFs:

Lower energy consumption for the production of liquid metal at the same

productivity.

Low melting loss of iron and alloying elements and their increased use from

recycled material and external alloyed waste.

Lower consumption of ferroalloys achieved by alloying to the lower limit of the

permitted range of alloying elements.


36

Lower consumption of non-metallic additives.

At the same productivity of aggregates, approximately half the weight of the cast,

which decreases lower investment costs of auxiliary and service facilities.

Lower costs of solving production and working conditions.

Lower consumption of refractory material.

Lower cost of depositing waste.

The environmental advantages of EIFs compared to the EAFs:

Less noise.

Lower emissions.

Less solid waste associated with the operation of the furnace.

9.2.1 The design and equipment of medium frequency electric induction furnaces

Requirements for increasing the productivity of EIFs necessitated the need for

mechanized loading of furnaces. Vibrating troughs are used most frequently for loading.

Requirements for high performance of furnaces also led to the mechanization of making

lining. When using hydraulic displacement of worn lining of the crucible and the use of

vibrators in ramming a new lining, it is possible to prepare a new lining of the furnace

during steel production about 2-4 hours after tapping the last melt of the preceding melt

campaign.

Alternating current of medium frequency is conducted to the crucible by copper strips.

Between the strips and the furnace inductor, current is conducted through cooled copper

cables. The inductor (coil) is formed by a copper pipe (or another profile), to which

cooling water is supplied.

A diagram of the furnace is shown in Fig. 2.1 Electrical power supplied to the inductor

becomes an alternating electromagnetic field inducing eddy currents in burden, which

heat and melt it. Outside the inductor, the magnetic field is routed through packets of

transformer sheets, which also shields the furnace structure.

9.3 Ramming EIF refractories

Furnace crucibles were rammed from acid, alkaline or neutral ramming masses. Lining

must not have gaps or cracks. The inductor consists of a water cooled copper coil,

electrically insulated. Between the individual windings of the coil the voltage drop occurs.


37

The electrical interconnection of the adjacent turns of the coil with metal or condensed

moisture leads to inter-coil short circuit. Burning the coil through and the subsequent

penetration of water into the lining can cause a leak beneath metal furnace, or even an

explosion in the crucible. After each melting process, the crucible lining must be checked

and in case of detection of cracks, cavities or extreme wear, the crucible is put out of

operation.

Acidic ramming mass of crucible electric induction furnaces

In steel foundries, acidic lining of induction furnace crucibles are most frequently used.

On acidic linings, it is possible to produce virtually all types of conventional stainless

steels. Similar to the steel produced in acid EAFs, unalloyed and low alloy steels

produced on an acidic lining are characterized by lower impact strength. Similarly, for

high alloy steels, in steelmaking on acidic lining, the same strength is not achieved as in

the case of the production in alkaline arc furnaces. Despite these shortcomings, steel

melted in the induction furnace with acidic lining has been most frequently used in the

Czech Republic so far.

Alkaline and high alumina ramming masses of crucible electric induction furnaces

Alkaline ramming materials are mostly produced based on magnesium oxide. Adding

alumina into magnesia ramming masses decreases the lining melting point, but increases

the resistance of lining to cracking. Magnesia lining contain about 10 to 25 % of

corundum. High alumina linings contain 20 to 50 % of MgO. The melting point of

corundum linings is lower than that of magnesia linings, but they have a greater crack

resistance.

9.4 Melt management practice

Melting time on electric induction furnace can be divided into the melting time and

finishing:

(9.1)

Where 1 – melting time, 2 – time of completion.

1 depends on the installed power of the converter Pnom. [kW], then the useful power of the

converter

(9.2)

21

nom.už. .PkP


38

where k the coefficient of the converter power utilization. The coefficient k value at the

beginning of melting is about 0.7 to 0.8, in the final stage of melting, its value is nearly 1.

In modern induction furnaces, the convertor utilization almost unitary throughout melting

burden.

To calculate the melting time, the following relation can be used:

(9.3)

Where i is the energy required to melt one ton of burden [kWh.t-1

], Q is the burden weight

[t].

The energy required to melt the burden represents the heat content of metal upon melting,

i.e. the change in enthalpy of metal ΔH in the interval of the starting temperature and the

liquidus temperature, including the total losses of the furnace throughout the melting

process.

The melting phase τ1 begins by turning on the furnace and ends by melting burden (after

the first test sampling).

The phase of completion 2 then includes the times required for test sampling, sending it

to the lab, waiting for the results of chemical analysis, for calculating the weight of added

ferroalloys and deoxidizing additives, their weighting and adding to the crucible, to adjust

the temperature, tapping, or possibly repairing the lining, up to turning on the oven for the

following melt.

Substituting equation (9.3) into equation (9.1), we can, by its modification, get the relation

for the daily productivity of the furnace in the form:

(9.4)

už.

1

.

P

Qi

kPQi

PkQG

...

..24

2nom.

nom.


39

10. The technology of the production of unalloyed and alloyed steels in EIF [1,2]

10.1 The production of unalloyed and low alloy steels on acidic lining

The manufacturing processes for steel production in EIFs cannot be managed, primarily

from the perspective of the economy, in a similar way as in arc furnaces because in EIFs,

it is not possible to create appropriate conditions for certain metallurgical reactions taking

place under sufficiently liquid, warm and active slag of suitable composition. EIF slag is

cooler than metal, and has the function of a cover (it is not active). To achieve the desired

chemical composition of steel, it is particularly necessary to prepare an appropriate

composition of the burden. The burden is constructed from recycled material, external

purchased steel scrap and alloying.

In acid EIFs, slags consist mainly of oxides of silicon and iron. Slags with a high iron

oxide content are aggressive with respect to lining. Silicon in the bath should be kept at a

content of around 0.3 % and above in order to reduce oxides formed. At low

temperatures, silicon dissolved in steel determines the oxygen activity in steel and in slag.

The temperature increase increases deoxidation ability of carbon and when reaching the

critical temperature depending on the chemical composition of steel, carbon boil occurs.

10.2 The production of high alloy steels on acidic lining

High alloy steels produced in induction furnaces are most frequently corrosion-resistant

and refractory steels. Especially for stainless steels, the carbon content generally less than

0.07 % is required. On induction furnaces, it is possible to produce stainless steels with a

carbon content lower than 0.03 %. Between the melt with a high chromium content and

acidic lining, a reaction occurs and the following relation applies

(10.1)

Fig. 10.1 graphically shows thermodynamic equilibrium between chromium and silicon at

the temperature of 1500, 1600 and 1700 °C, provided that the slag is saturated with

chromium trioxide and the lining consists of pure silicon dioxide. Thermodynamically,

the reaction between chromium dissolved in steel and acidic lining at Henry’s activity of

chromium equalling to 18 (corresponding to approximately 18 wt. % of Cr) and the

temperature of 1500 °C can take place, if Henry activity of silicon in steel is less than 0.5

(approximately 0.3 wt. % of Si). Practical experience confirms these theoretical

assumptions. During melting and maintain lower temperatures of metal, these steels

generally contain about 0.40 % of Si at tapping temperatures of up to 1600 °C and

Si3OCr2Cr4SiO3 322 TG 45,32960142


40

reactions between acidic chromium and lining do not occur. At higher tapping

temperatures of these steels of 1620 to 1670 °C, it is no longer necessary to count with

increasing silicon content in the steel.

chromium activity

silicon activity

Fig. 10.1 Thermodynamic equilibrium between Cr and Si in an acidic crucible at the

temperature of 1500, 1600 and 1700 °C under slag saturated with Cr2O3

10.3 Production of steel with alkaline and high aluminate lining

Alkaline linings contain more than 50 % of MgO. High aluminate (corundum - neutral)

lining, containing more than 50 % of Al2O3 and a remainder of MgO. Linings of this type

are formed by oxides having higher thermodynamic stability than SiO2. Steel produced in

these types of linings has lower oxygen activity and higher strength after deoxidation in

the ladle. The reaction between the lining and the molten metal is therefore negligible.

Economic benefits of the production of high alloyed steels alloyed with chromium are

connected with achieving lower melting losses of chromium.

11. Deoxidation of steel in the ladle and casting. Final deoxidation of steel in the

ladle and its effect on steel properties. [1,2,3]

11.1 Deoxidation of steel in the ladle and casting

During casting, properties of castings are affected more than during the previous

manufacturing operations in the melting unit. One of the major influences on the

properties of castings is that of a final deoxidation of steel in the pan with aluminium, or

possibly modification of steel with calcium or rare earth metals. Casting quality is also

influenced by the method of preparation of the foundry ladle, its drying, heating, possible

maintenance of the lining and removal of residual slag. The casting speed is to be

controlled by the diameter of the nozzle. Reoxidation of metal during the casting stream


41

during mould filling can affect the formation of casting defects such as, in particular, sand

inclusions, bubbles and pinholes.

Foundry ladles used for transport and casting of steel

Transport of liquid metal and casting is ensured by means of the foundry ladle (FL). In

steel foundries, pans with bottom outlet and the plug cap are usually used. Steel is cast

from FL nozzle, which closes by the stopper rod, and it has a graphite head in the lower

part. Fig. 11.1 shows a view of a stopper mechanism with the cap and FL lining.

Economically undemanding equipment is a major advantage of the plug stopper. Due to

its thermal vulnerability, the plug system cannot be used for secondary metallurgy

equipment, with metal heating or during vacuum degassing. For these devices, slide

fasteners are used. Fig. 11.2 shows a diagram of the slide closure and tuyere blocks

located in the bottom of the FL. These devices represent an expensive investment and

their use in steel foundries has not been usual so far.

Fig. 11.1 View of a plug stopper of the

foundry ladle

Fig. 11.2 A diagram of a slide closure and a

tuyere block

For lining FLs, acidic refractories with SiO2 content, which is usually higher than 85 %,

and a residual content of Al2O3 are often used in foundries. Refractory material is mostly

supplied in the form of blocks or ramming masses.

For lining alkaline FLs, magnesite and high aluminous mass of a similar composition as

for lining induction furnaces.

11.2 Final deoxidation of steel in the ladle and its effect on steel properties

Metallurgical quality of steel affects the notch strength test and the transition temperature.

These material characteristics are influenced by variations in chemical composition,

purity of steel, the grain size and heat treatment. The achieved values of notch strength of

optimally heat treated steel is mainly affected by the morphology, size and distribution of


42

inclusions. For the final deoxidation of steel in the LPs, aluminium is used. The result of

the deoxidation reaction are aluminium oxides. If the residual aluminium is higher than

0.030 %, which is necessary for most of unalloyed and low alloy steels to prevent

pinholes in castings, stable oxide Al2O3 is formed as a product of the reaction between

aluminium and oxygen.

Therefore, at the temperature of liquid steel, it will exist as a solid phase (melting point

2030 °C), which will, according to Stokes’ Law, float from steel at a certain velocity,

depending on the composition and size of the inclusions and the dynamic viscosity of the

melt

212 rη

ρρg

9

2v

(11.1)

v – floating velocity of inclusions [cm.s-1

], r - particle radius [cm], 1, 2 – specific weight

of inclusions, molten steel [g.cm-3

], - viscosity of molten steel [P], a g – gravity

acceleration (981 cm.s-2

). Non-metallic inclusions have considerable influence on the

properties of steel. Morphology of inclusions depends on their chemical composition,

which is determined by the conditions of deoxidation.

According to the origin, inclusions can be divided into exogenous, whose origin is related

to the formation of erosion and corrosion actions throughout the production process and

casting.

Endogenous inclusions are mainly the product of deoxidizing, desulphurizing reactions,

but also reoxidation processes taking place throughout the course of steel production,

including crystallization of cast steel.

In terms of chemical composition, inclusions are mainly oxidic, sulphidic, which

predominate, then, in lower quantity, oxi-sulphidic inclusions, nitrides, carbides, and a

limited amount of silicates and aluminates.

It is very important to classify inclusions according to the shape, as square, spheroidal and

dendritic type.

From the viewpoint of the temperature of formation of inclusions, they are divided into

primary ones, which arise within the range of steelmaking temperatures, secondary ones,

formed just above the liquidus temperature, and inclusions that arise between the liquidus

and solidus temperatures are tertiary. Precipitation inclusions are formed below the

solidus temperature.


43

Sulphide inclusions, which normally arise during steel solidification, are directly linked

to the oxygen activity in steel and hence the oxygen content in the molten steel. Their

morphology is determined by the type of deoxidizing agent. For precipitating deoxidation,

pure aluminium is usually used, so its residual content determines the chemical

composition of inclusions and the shape, and influences the mechanical properties of

steel. Influence of the residual aluminium content dissolved in steel on the morphology of

sulphide inclusions is shown in schematic Fig. 3.11.

Type I, type II, type III, type Ib, type IV

Fig. 11.3 Influence of the residual content of aluminium dissolved in steel on the

morphology of sulphide inclusions

Oxide inclusions are formed in each period of steel making and casting, while the largest

amount is produced during deoxidation. If the strongest deoxidizing agent is Al, their

morphology and the decomposition products resulting from deoxidation may be

represented according to the diagram in 11.4.

clusters of

Al2O3

Fig. 11.4 Schematic representation of the effect of chemical composition of steel,

depending on the method of deoxidation on the morphology of inclusions and the

resulting deoxidation products


44

If the aluminium content of steel is less than 0.01 %, inclusions are eliminated in the

liquid state, they are globular, and during solidification they disintegrate into oxides of the

nMn.mSiO2.pAl2O3 type. If the aluminium content in steel is greater than 0.01 %, small

crystals of Al2O3 of dendritic character are formed. In the case of excessive content of

aluminium, crystals of Al2O3 form clusters.

Reoxidation of steel

The term reoxidation of the steel is used for oxidation of melt during tapping metal from

the furnace and the subsequent period when steel remains in the ladle, casting, until

solidification of steel. In steel oxidation, Czech terminology uses the term secondary and

tertiary oxidation according to their chronology and temperature. Secondary oxidation of

steel means processes which are associated with increasing concentration and activity of

oxygen in steel after primary deoxidation in the furnace aggregate. Secondary oxidation

takes place during tapping of steel, when the stream of molten steel is in contact with 20

to 30 times larger volume of air, and during the time when the rest of metal remains in the

ladle. Tertiary oxidation is oxidation of steel between the liquidus and solidus

temperature.

12. The possibilities of secondary metallurgy in steel foundries. Using various

methods and principles of individual processes in secondary metallurgy. [1,3]

The term secondary metallurgy (SM) includes a considerable number of options and types

of technological processes that take place outside the melting unit, which is usually an

electric arc furnace or electrical induction furnace in steel foundries. In this case, the

melting phase, or possibly the oxidation phase takes place in EAFs or EIFs. Another

phase of the reduction and completion occurs in some secondary metallurgy equipment.

The objective is the ability to increase the productivity of the melting unit and create

better conditions for example for deep deoxidation, modification of steel, or its

desulphurization.

12.1 Methods of secondary metallurgy

There are many different technological elements of secondary metallurgy. Some are

already overcome by other modern developments in this field. The basic classification of

the most frequently applied secondary metallurgy methods taking place outside the

melting unit, whose processes are realized:


45

Processes taking place at atmospheric pressure

AP (Argon Pouring)

IP (Injection Process)

SL (Scandinavian Lancers)

LF (Ladle Furnace)

AOD (Argon Oxygen Decarburisation)

Processes taking place in a vacuum

VD (Vacuum Degassing)

VOD (Vacuum Oxygen Decarburisation)

VAD (Vacuum Arc Degassing)

ASEA-SKF

RH (Rührstahl Heraeus)

Processes without reheating metal

Process IP, AP, SL, VD

Procedures with reheating metal

Process LF, VAD, AOD, VOD, ASEA-SKF

The melt in the ladle or the converter is heated either by exothermic chemical reactions,

or electrical heating.

In chemical heating, reaction heat of aluminium, silicon and carbon oxidation is most

frequently used.

When steel is alloyed into the ladle or in the case of higher heat losses, metal is generally

heated by electricity, with an electric arc most frequently used as a heat source, which is

same principle as the three-phase electric arc furnace.

The above mentioned classification is now rather formal and it is based on the history of

the individual processes. Currently, various methods are combined in the existing

facilities, and they are further developed.

Metallurgical possibilities of some secondary metallurgy methods

Applying secondary metallurgy in steel foundries is associated with the desired range of

castings and also the foundry producibility. For certain foundries, only basic elements of

secondary metallurgy may be suitable. In contrast, for some foundries, technically and


46

financially more demanding components of secondary metallurgy may be more

preferable. SM processes currently include a large number of individual processes and

their various combinations, which make it possible to achieve very low levels of

undesirable elements, such as sulphur, hydrogen, nitrogen, etc., or which guarantee

economic processing and the production of high alloy steels. Using the elements of SM is

crucial especially in steel mills and metallurgical plants that process large volumes of

material. In steel foundries, usually only technically and economically less demanding

elements of SM are introduced.

12.2 Applying the individual methods and principles of the individual secondary

metallurgy processes

The aim of substantially all of the elements of SM is to convert all, or some of the

operations taking place in the reduction period outside the melting unit, which is usually

EAF in steel foundries. This can be achieved by a significant increase in its productivity

and a simultaneous improvement of some metallurgical parameters compared to the usual

process of steel production in EAFs.

Secondary metallurgy methods operated at atmospheric pressure

Secondary metallurgy methods at atmospheric pressure are carried out either in the ladle

or the converter. The most common methods carried out in the ladle include refining

metal with inert gas, or a combination with blowing dust ingredients or injection using

filled profile, ladle and converter AOD and CLU processes.

Refining metal with inert gases

Blowing inert gas into the melt, referred to as AP (Argon Pourging) is the simplest SM

process. The inert gas used is argon and in some cases, also nitrogen is blown. The said

gases are under the circumstances considered to be inert. Blowing inert gas primarily

results in thermal and chemical homogeneity of metal in the ladle. Inert gas is blown

through a ceramic block.

Argon in the form of bubbles represents atmosphere with the zero partial pressure of

hydrogen and nitrogen in the melt. Gas, such as hydrogen in the bath and in the gaseous

atmosphere (argon bubble) tries to get to a steady state by the process in which hydrogen

diffuses from steel into the argon bubbles where it associates to molecules and along with


47

the argon bubble, it is floated out from the melt. This process is terminated when the

hydrogen partial pressure in the gaseous atmosphere will correspond to a given

concentration of hydrogen in the melt, i.e. at the moment of equilibrium between the gas

mixture and the melt, provided that the Ar bubble with hydrogen remains in the melt for a

sufficient time. The floating velocity of the bubbles depends on their size. The conditions

are less favourable for reducing the nitrogen content in the melt than that of hydrogen due

to impaired diffusion of nitrogen in the melt.

Injecting dust additives with a nozzle

SM components using injection of dust additives include, e.g. the SL method

(Scandinavian Lancers), TN process (Thussen Niederrhein), or IP process (Injection

Process). A nozzle is usually used to inject ground ferro-alloys containing calcium,

silikocalcium, pulverized lime, and possibly also carburizing agents, or ordinary

ferroalloys into steel in the ladle.

The device is primarily intended for desulphurization, which uses mainly injecting

pulverized lime. In this SM process, the content of gases, especially hydrogen increases.

Injecting using a filled profile

A filled profile is a thin-walled tube from a sheet steel with low carbon, typically with a

diameter of 6-20 mm filled with dust, for example, SiCa, FeCaAl, Al, C, milled

ferroalloys, etc. The diameter of the filled profile is chosen according to the weight of the

melt, and also according to the type of the feeding device of the filled profile to the

foundry ladle. For ladle with liquid metal weighing 4-8 tons, a profile diameter of about 8

mm is used. The profile is guided over driving pulley of the feeder and the steel guide

tube into the steel ladle.

The most common use of filled profiles is to modify inclusions to type Ib by SiCa,

wherein the calcium utilization is not greater than 50 %, and also for the injection of

aluminium.

The ladle intended for injection of filled profiles is adjusted to homogenization blowing of

inert gas. Injecting the filled profile that contains calcium is preceded by deoxidation of

steel with aluminium. In the production of carbon steel, oxygen activity of up to 2 ppm

can be achieved in the foundry ladle.


48

Profiles filled with other ferroalloys or alloying elements are used to correct the chemical

composition of steel and for alloying elements having high affinity for oxygen, possibly

also to correct the carbon content.

Summarising the above mentioned SM processes implies that during this secondary

metallurgy steel processing in the foundry ladle under atmospheric pressure without

heating, steel temperature rapidly decreases. Even at high tapping temperatures, the

described procedures of treatment outside the furnace usually cannot be performed longer

than for 5-7 minutes.

13. Defects of steel castings. The causes of the various types of defects and ways of

eliminating them. [1,2]

13.1 Casting defects

Defects are often the result of imperfect and poorly controlled technologies. Checking

during certain production phases is difficult also because there are no affordable and

sufficiently accurate methods for monitoring of all parameters of the applied technologies.

A casting defect means any variation in appearance, shape, size, weight, structure and

properties detected by laboratory or other tests from the agreed technical specifications or

standards associated with the manufacture the type of casting. Casting defects can thus be

apparent and hidden.

Under the current convention, the same deviation from the agreed quality of the casting

between the manufacturer and the customer may be still admissible defect, or

inadmissible, repairable or removable defect.

The applicable ČSN (Czech national standard) distinguishes seven categories of defects in

iron alloy castings. Defects of iron alloy castings, whose division into seven classes of

defects and defect categories, along with the classification, explaining the main causes

and suggestions for their prevention in the foundry production, are also described by Elbel

[5].

13.2 The causes of the various types of defects and ways of eliminating them

The frequent defects in foundries for steel castings are classified as follows: surface

defects, impressions, macroscopic inclusions and defects in macrostructure,

microstructure defects, and defects in chemical composition including properties of

castings.


49

Surface defects

Defects of this type can be considered apparent defects, which can be detected during the

inspection after blasting and cleaning the surface, or possibly using the magnifying glass.

In most cases, defects are removable. In some cases, this requires negotiations between

the purchaser and the manufacturer of the casting, including the determination of repair

method. This class includes the majority of defect types. Fig. 13.1 shows a surface defect

– a flash.

Flash

Fig. 1.13 A fin in oil passage in the cast steel piece of a crankshaft

Disrupted continuity

Defects of the disrupted continuity type are divided into four types of defects: fissures,

cracks, disrupted continuity due to mechanical damage and disrupted continuity related

to unconnected metal. The cause of these defects is a number of parameters related to the

manufacturing technology of casting design, foundry mould design, physical and material

parameters of the produced steel in the solid and liquid phase, and also to the conditions

of melting, casting and solidification. In this class there are most kinds of defects.

Impressions

Defects of this type belong mainly to the apparent, but also among hidden defects. The

impressions in a casting may be either open (apparent), or sealed under the surface of the

casting (hidden). Open impressions can be detected relatively easily. For the identification

of sealed impressions, it is necessary to use special methods. Most often an ultrasound

test, radiography using X-ray radiation or gamma radiation.

Fig. 13.2 shows an impression – a sealed endogenous gas holes. This defect was created

from melt in an electric arc furnace base with an increased concentration of hydrogen and

nitrogen.


50

Fig. 13.2 Shapes of sealed endogenous gas holes

Macroscopic inclusions and defects in macrostructure

Defects of this type include those that are hardest to identify and also most difficult to

remove. The group of slug inclusion defects involves two types of defects: exogenous

slug inclusion and secondary slug inclusion, and the group of defects called non-metallic

inclusions simultaneously comprises six most frequent defects, such as sand inclusions,

floated sand, coating falling off, oxidic covers, carbon covers and black spots. The group

of defects called macrosegregation and exudations includes such types of defects as

gravitational segregation, macrosegregation – physical segregation, “A” segregations

and “V” segregations [5].

Microstructural defects

These defects usually involve limiting deviations of parameters of properties of casts from

the agreed technical specifications and standards, and is not the case of standard casting

defects. These are defects of the type microscopic impressions, which are further divided

into sags, microbubbles, and micro cracks.

However, for their identification, is necessary to adjust the casting surface at the site of

the defect and to use reasonable enlargement – a magnifying glass, or possibly

stereoscopic or metallographic microscope.

Another group includes defects called inclusions, wrong grain size, wrong content of

structural components, hard spot, turbidity, reversed turbidity, surface decarbonisation

and other deviations from the microstructure.

Defects chemical composition and properties of castings


51

This class contains four groups of defects, also usually involving limiting deviations of

the parameters of casting properties from the agreed technical specifications and

standards, and is not the case of standard casting defects.

14. Some possibilities of cost analysis for the steel production management. [1,4]

An important measure for the production management is the cost analysis for the

production of liquid metal. Recently, methods were developed for cost management

aiming at cost optimization, i.e. finding the optimum between costs and the quality of the

manufactured product.

Benchmarking is a continuously organized process of comparing products, services and

processes from the point of view of their quality, efficiency and performance of major

competitors. Internal benchmarking is focused on comparisons within the same

organization. For example, the object of the comparison is the relative productivity per

worker in physical or financial units. Another method for optimizing costs is controlling.

Generally, controlling means a process, whose aim is to monitor the efficiency of the

production.

The technical and economic analysis method is similar to the above mentioned methods.

The technical and economic analysis is based on monitoring the individual cost items in

physical and financial terms in the calculation unit in relation to the technology used. The

technical and economic analysis method focuses on measuring all cost items related to the

production of liquid metal. The above mentioned principle is formulated in the method of

the incomplete costs.

References

[1] Šenberger, J., Stránský, K., Bůžek, Z., Záděra, A., Kafka, V., Metalurgie,

VUTIUM, VUT Brno 2008, 310 s, ISBN 978-80-214-3632-9.

[2] Levíček, P., Stránský, K., Metalurgické vady ocelových odlitků, SNTL, Praha 1984,

269 s.

[3] Fruehan, R. et al. The Making, Shaping and Treating of Steel, Pittsburgh 1998, p.

767, ISBN 0-930767-02-0.

[4] Heyne, P. Ekonomický styl myšlení. VŠE – Praha 1991. ISBN 80-7079-781-9.

[5] Elbel, T., Havlíček, F., Jelínek, P., Levíček, P., Rous, J., Stránský, K.:

Vady odlitků ze slitin železa (klasifikace, příčiny a prevence). MATECS, Brno

1992, 339 s.

VŠB - Technical University of Ostrava Faculty of...

Documents

Transcript of VŠB - Technical University of Ostrava Faculty of...