Secondary steel making: Overview

On completion of primary steel making (either in the Basic Oxygen Furnace or Electric Arc Furnace), the molten steel is 'tapped' into a ladle and transported to the casting facility. While in the ladle, the steel may be subjected to a number of different treatments, such as composition adjustments, stirring, degassing and reheating. Collectively this stage in the overall steel making process is called secondary steel making

Secondary steel making module - outline

*Where secondary steel making fits in the overall production process. *What secondary steel making is all about - description of bits of plant and what they do. *Simulation exercise to convert a ladle of steel from BOS/EAF at a given composition and temperature to one of three specified compositions at a temperature and time ready for casting. *Detailed exercises on 1. deoxidation 2. desulfurization 3. clean steels 4. dehydrogenation 5. decarburization 6. alloy addition 7. reheating

Sequence of stages in the production processBelow are some of the individual stages involved in steel making Place them in the correct sequence starting top left.

Secondary steel making simulationIn this simulation you will play the role of plant metallurgist in charge of secondary steel making operations. You will take charge of a 250 tonne ladle of molten steel from the Basic Oxygen Furnace (BOF), and attempt to deliver it to the appropriate caster at the specified time, composition, temperature and inclusion content (cleanness). You should also aim to complete the simulation at minimum processing cost.

In this simulation you will play the role of plant metallurgist in charge of secondary steel making operations. You will take charge of a 250 tonne ladle of molten steel from the Basic Oxygen Furnace (BOF), and attempt to deliver it to the appropriate caster at the specified time, composition, temperature and inclusion content (cleanness). You should also aim to complete the simulation at minimum processing cost.

 

There are four different steel grades from which to choose, each requiring a different processing route. As a relatively undemanding grade, a general purpose construction steel for beams and columns is recommended for novice users. A TiNb ultra-low carbon (ULC) steel for automotive strip requires different processing in order to remove carbon. A line pipe steel for oil and gas distribution is a much more demanding grade as it requires very low levels of sulfur and hydrogen. Finally, a heat-treatable medium-carbon CrMo engineering steel provides an example of a more highly alloyed grade.

To achieve your objectives, you will need to make decisions based on alloy and slag additions, stirring, vacuum degassing and reheating. The sequencing, timing and quantity of additions and applications of stirring, degassing and/or reheating practices will be critical to the successful completion of the simulation.

At the end of the simulation, the user is given feedback on whether the composition, temperature, inclusion level and delivery time were within the specified values, together with the total operating cost.

Secondary steel making operationsSecondary steel making involves some of the following options. The selection of which are available in any particular steel making shop depends on the types of steel being made and the availability of space within the factory.

*Stirring *Lance 1. Bottom porous plug 2. Electromagnetic Stirring (EMS) *Ladle furnace *Ladle injection 1. Powder 2. Wire *Degassing 1. Tank Degasser 2. Stream Degasser 3. RH Degasser 4. DH Degasser *CAS-OB *AOD  

Steel grades used in simulation

This page provides some information about the steel grades used in the secondary steel making simulation.

Universal beams for building construction

MicrostructureFerrite + ~15% Pearlite, mean grain size 6.6µm.

X200

Typical Composition %C 0.14 %Si 0.26 %Mn1.22 %P0.019max %S0.018max %Al0.022 %Nb 0.033

Mechanical Properties YS 429 MPa UTS 537 MPa %El 25

TiNb ultra-low carbon (ULC) steel for automotive strip

Microstructure

Typical Composition %C 0.003 %Si 0.21%Mn 0.75 %P 0.01max%S 0.012max %Nb 0.01 %Ti 0.01

Mechanical Properties YS 180-280 MPa UTS 310 -375 MPa n value 0.16 - 0.2 %El 34 - 40

Line pipe Steel

Pipes carrying gas or oil require good weldability and a high toughness to withstand cracks. As higher capacity pipes have led to the development of steels with higher yield strength. Deep (170m) water usage requires heavy-wall pipes to withstand buckling, while low temperature conditions (e.g. Alaska, Russia) require special toughness. The transport of sour gas and oil demands steel with resistance to hydrogen-induced cracking and corrosion.

Typical Composition %C %Si %Mn %P %S 0.07 0.18 1.05 0.012max 0.003max

Mechanical Properties X65 YS 448 MPa TS 530 MPa %El 23.5

Engineering Steel

A medium carbon, heat treatable steel for applications requiring high strength in combination with toughness, such as components for engines, drives, equipment, transmissions, and tools, etc.

Typical Composition %C 0.415 %Si 0.4%Mn 0.75%P0.035max %S0.035max %Al0.0225 %Ni0.3 %Cu0.35 %Cr1.05 %Mo 0.225

Mechanical Properties YS 1250 MPa UTS 1450 MPa %El 2

Deoxidation: Overview

Deoxidation is one of the most important processes in secondary steel makingLearning Outcomes for this SectionAfter completing this module, you should be able to:

*Explain why the control of oxygen is important *Describe the chemical reactions, thermodynamics and kinetics for the removal of oxygen from liquid steel *Explain the interactions between liquid steel, slag, refractories and the atmosphere that underpin the control of the oxygen content of steel *Apply this understanding to the control of oxygen content in a simulation of the secondary steel making process Identify the origins of oxygen in steel

Why deoxidize?

The oxygen content of liquid steel in the ladle at the start of the secondary steel making process is 400-1000 ppm (0.04 - 0.1%).The solubility of oxygen in liquid steel is 0.16% but in solid steel it is only 0.003%.Therefore, steps have to be taken to reduce the oxygen content (deoxidize) of the steel before it solidifies in order to prevent blowhole formation during casting and a porous product being created or large quantities of FeO being precipitated.

Sources of oxygen

Oxygen enters the liquid steel in a number of different ways.Which of these sources of oxygen do you think is the dominant one?

Reducing oxygen content

The addition of a strong oxide forming element is the most commonly used method of reducing the oxygen content of liquid steel.Use the Ellingham diagram to help you decide which of the following may be suitable (cheapest) choices for this.

Deoxidation reactions

Aluminum, silicon and manganese are the most common deoxidisers used in steel making The chemical reactions associated with their use are:

Silicon and manganese are often used in conjunction with one another.

N.B.

1. Square brackets [] denote component present in molten steel; 2. Parentheses () denote component present in slag; 3. Thus [Al] represents aluminum in molten steel but (Al2O3) refers to aluminum oxide in slag; 4. Temperature (T) is on the Kelvin scale. The reference state for the solute metal activities in all the thermodynamic values listed is the 1% solution, that is the values of activity and mass % are about equal for low-alloyed steels (see Table I below).

Effectiveness of deoxidizers

Equilibrium concentrations of dissolved oxygen under different concentrations of manganese, silicon and aluminum

Steel cleanness

There are a variety of sources of inclusions:

* indigenous (small): 1. deoxidation product and MnS * exogenous (large) 1. reoxidation (reaction with air or slag) 2. entrainment of slag 3. eroded refractories

Inclusions are formed by chemical reactions (deoxidation, reoxidation and precipitation) or by physical conditions (turbulance or wear).

Effect of inclusions on downstream processing and properties and performance

Most inclusions have a detrimental effect on properties. Solid oxides (alumina or certain calcium aluminates) can cause nozzle blockage during continuous casting and disrupt the process and have to be burnt out. Some inclusions can cause cracking and defects, slivers and delamination in rolled products and also fracture during hot/cold forming and wire drawing.

Deoxidation: Summary

Having completed this section, you should be now able to:

1.Explain why the control of oxygen is important 2. Describe the chemical reactions, thermodynamics and kinetics for the removal of oxygen from liquid steel 3. Explain the interactions between liquid steel, slag, refractories and the atmosphere that underpin the control of the oxygen content of steel 4. Apply this understanding to the control of oxygen content in a simulation of the secondary steel making process 5. Identify the origins of oxygen in steel

Reference :Web site http://ilsap.matter.org.uk/