Mould Fluxess780556004.online.de/wp-content/uploads/CastingFluxes.pdf · Mold fluxes are synthetic...
Transcript of Mould Fluxess780556004.online.de/wp-content/uploads/CastingFluxes.pdf · Mold fluxes are synthetic...
-
Mould Fluxes
-
What is Mould Flux?
2
Mold fluxes are synthetic slags (form of glass) that are used in the continuous casting process utilizing submerged entry nozzles (SEN’s). These synthetic slags exist as complex mixtures of raw minerals, ceramic based oxides, pre-reacted components, and carbon. Available in many particle sizes, shapes, and types, mold flux is made up of silica (SiO2), lime (CaO), sodium oxide (Na2O), fluorspar (CaF2), and carbon (C). Other components of this slag system include alumina (Al2O3), magnesium oxide (MgO), other alkaline oxides (Li2O, K2O) and some metallic oxides (iron, manganese, titanium) to achieve specific physical properties.
-
Mould Flux Components
3
The components that make up mould fluxes are common minerals and raw materials that can be seen in everyday life.
As an example, a typical synthetic mould flux can contain raw materials such as cement, powdered glass, fluorspar, sodium carbonate, wollastonite, and carbon (added as coke and/or carbon black).
In fact, many of the raw materials used in mould flux are also used throughout the steelmaking process.
-
Mould Flux Composition
4
• SiO2 17% - 56%• CaO 22% - 45%• Al2O3 0% - 13%• Na2O 2% - 25%• K2O 0% - 10%• MgO 0% - 12%• B2O3 0% - 19%• Li2O 0% - 5%• F 0% - 15%• C* 0% - 25%
Network Forming OxidesNetwork Modifying OxidesAmphoteric Oxide (includes Cr2O3, TiO2, Fe2O3)
Mould flux is commercially available in various forms, shapes, and particle sizes. It may exist as
● an intimate mixture of raw minerals and carbon (synthetic powder)
● a pre-melted blend of glass and carbon (fritted/vitreous flux)
● an agglomeration of raw minerals, pre-reacted components and carbon (granulated flux) and
● a blend of raw minerals, exothermic components and carbon (exothermic flux).
Although mould flux can come in many forms, its basic chemical composition is shown here.
*C added to control melting rate and provide thermal insulation
-
Principles of Mould Flux Technology
5
Mould Fluxes are Silicate Chains
Mould slags are silica based ionic melts.
The silica tetrahedron ion (SiO4) serves as the basic building block of the slag structure.
Silica is a network forming oxide (acid oxide).Oxides such as CaO are network modifying oxides (basic oxide).
Oxides that can exhibit the properties of both groups dependent upon the inherent slag structure are called amphoteric oxides (Al2O3).
-
Principles of Mould Flux Technology
6
1. Granulated fluxes are produced via spray drying, extrusion, or high speed pan granulation. These materials represent the majority of fluxes marketed today. At Imerys, spray dried products represent ~70% of volume shipped.
2. Synthetic fluxes are physical blends of various raw materials.
3. Semi-vitreous fluxes are physical blends that contain a significant percentage of smelted (frit) and/or pre-reacted raw materials.
4. Fully fritted (vitreous) fluxes are blends of pulverized frit and free carbons. Use of vitreous mould flux was predominant in the 70’s and 80’s in North America.
5. Fly-ash based fluxes have fly-ash as the major constituent. These products, primarily used in Europe, were predominant in the 70’s and 80’s.
6. Exothermic starting and running fluxes contain exothermic reagents such as the combination of calcium-silicide and FeO.
Six Types of Mould Fluxes
-
Principles of Mould Flux Technology
7
The raw material constituents used in the production of mould flux are key components in determining its physicochemical properties.
These raw materials will ultimately determine the properties of the ionic melt (liquid slag), the viscosity of the slag, its thermal breakpoint, the crystallization temperature, surface tension and fusion behavior.
In addition, these same raw materials will also determine the behavior of the unmelted mould flux in terms of thermal insulation and dry state flowability.
Raw Material Selection and Process Comparison
The angle of repose is the maximum angle of a stable slope determined by friction, cohesion and the shapes of the particles.
http://en.wikipedia.org/wiki/Image:Angleofrepose.png
-
Production of Mould Fluxes
8
Similar to most industrial operations, the production of mould flux includes the following steps:
● Incoming raw materials are tested for consistent chemistry, particle size, moisture and density.
● The verified raw materials are stored in bulk silos.● Raw materials are weighed-up by a computer driven weigh car.● If the mould flux to be produced is a powder product, it would be mechanically
blended. If the mould flux to be produced is a granular product then it would be sent to the slurry tank and then to the spray dryer.
● While the fluxes are mixed and/or spray dried, in process monitoring including chemical analysis, particle size distribution and moisture are continuously verified.
● Packaged per the customer’s specifications.● Quality Control: Quality verification and documentation.
-
In-Mould Functionality of Mould Flux
Date9
Liquid Flux
SolidifyingShell
FluxRim
1
2
3
4
5
1. Thermal Insulation
2. Prevents Reoxidation
3. Absorbs Inclusions
4. Lubrication5. Uniform Heat
Transfer
Unmelted Flux
Molten SteelSolid Flux
9
-
In-Mould Functionality of Mould Flux
10
Liquid Slag Layer1250 - 1460 C
Molten Steel1500 - 1575 C
Unmelted Flux200 - 600 C
Intermediate/Glass-Carbon Matrix650 - 1000 C
Once steady-state casting conditions are established, a tri-layer mould flux profile is developed.
The first layer consists of a molten, vitreous pool of liquid slag.The second layer is an intermediate or glass-carbon-matrix.The third layer consists of unreacted, unmelted mould flux.
-
Slag Rim
Mould Wall Submerged Entry Nozzle
Strand Shell
Liquid Steel
unmelted layer
intermediate layer
liquid layer
In-Mould Functionality of Mould Flux
11
-
In-Mould Functionality of Mould Flux
12
Mould Flux Layer: Thermal Insulation of MeniscusThrough proper engineering of the mould flux composition, the thermal insulation of the molten steel meniscus can be optimized.
The thermal insulating property can be controlled through the mould flux’s bulk density, particle size, and carbon type(s).
Radiant heat is absorbed and reflected/deflected by the unmelted mould flux layer to minimize heat loss to the environment.
Liquid Slag
MoltenSteel
Radiant Heat
Thermal Insulating Barrier
Hotter Meniscus
-
In-Mould Functionality of Mould Flux
13
It is important to optimize the thermal insulating properties of mould flux to prevent the premature solidification of molten steel surface in mold. This can be achieved by applying low bulk density mould flux possessing the proper blend of free carbons.
Perhaps the most important consideration when providing thermal insulation to the molten steel meniscus is to promote lower shell growth along meniscus radius. This allows products of de/reoxidation and/or gas bubbles to enter into the liquid slag pool and not become entrapped by solidifying meniscus hook.
In addition, the proper thermal insulation of meniscus region helps to maintain the slag channel (between mould wall and shell) to facilitate flow of liquid slag and control consumption rate.
GasBubbles
De/reoxidationProducts
Liquid Slag
MoltenSteel
Thermal Insulating Barrier
Heat
Shell forms at lower point along meniscus radius
Slag Channel
-
Principles of Mould Flux Technology
14
Note Shell Physically
Entrapping Gas Bubble
Note Physical Entrapment of Slag in Steel
-
In-Mould Functionality of Mould Flux
15
Protecting the Molten Steel from Reaction with Atmospheric Gases
The tri-layer mould flux system (unmelted flux, glass-carbon matrix, and liquid slag) serves as protective barrier to the components in the molten metal.
This protective barrier prevents the reaction of atmospheric gases with constituents of the of the liquid steel.
ON
H
Unmelted FluxIntermediate/Glass Carbon Matrix
Al
Mn
Fe
Ti
Liquid Slag
-
In-Mould Functionality of Mould Flux
16
Reactivity with Products of De/reoxidation
Mould fluxes are engineered materials that are designed to react with products of deoxidation and reoxidation as well as with complex non-metallics (CNI’s).
The incorporation of these products of de/reoxidation and CNI’s into the liquid slag structure improves the quality of the cast section while facilitating its production.
Al2O3 TiO2TiN
Al3+ Fe3+ Ti4+
Liquid Slag
Mn
Molten Steel
-
In-Mould Functionality of Mould Flux
17
Liquid Lubrication In Gap Between Mould and Solidifying Shell
Mould Wall Solid Flux Liquid Slag Steel Shell
FluxVelocity
Ferrostatic Pressure
Vc = Casting Velocity
-
In-Mould Functionality of Mould Flux
18
Mould flux viscosity and solidification temperature are major determining factors in controlling heat flux/heat transfer rate in mould.Heat flux or heat transfer is controlled in eight steps:
1) Steel shell to liquid slag interface2) Through liquid slag3) Liquid slag to solid flux film4) Through solid flux film5) Solid flux film through air gap 6) Air gap to mold wall7) Through coating and/or mould coppers8) To mould cooling water
Thermal Heat Transfer in the Mould
Solid Flux Film
SolidifyingShell
Air Gap
Heat Flux
Liquid Slag
Mould Copper
-
Principles of Mould Flux Technology
19
Key Properties of Mould Flux
• VISCOSITY
• Thermal Breakpoint (Tbr)
• Inclusion Absorption
• Consumption Rate
-
Principles of Mould Flux Technology
20
Key Properties of Mould Flux – Viscosity
Viscosities of Common Lubricants
• Machine Oil 1.14 poise• 20W Motor Oil 1.21 poise• 40W Motor Oil 2.89 poise• Typical Mould Flux 0.50 - 4.00 poise
Note: Viscosity of H2O is 1 cps (centipoise)
Example of the viscosity of milk and water. Liquids with higher viscosities will not make such a splash when poured at the same velocity.
http://en.wikipedia.org/wiki/Image:Drop_0.jpg
-
Principles of Mould Flux Technology
21
Basicity/Vratio and Thermal Breakpoint Temperature
General Rule of Thumb:As basicity/Vratio increases, thermal breakpoints increases
-
Principles of Mould Flux Technology
22
Increase inComponent Melting Point Viscosity
Thermal Breakpoint (Tbr)
CaO/SiO2 Increase Decrease Increase
SiO2 Decrease Increase Decrease
B2O3 Decrease Decrease Decrease
CaO Increase Decrease Increase
MgO Decrease Decrease Decrease
BaO Decrease Decrease Decrease
Al2O3 Increase Increase Increase
Fe2O3 Decrease Decrease Decrease
TiO2 Increase Little Effect Increase
MnO Decrease Decrease Decrease
Na2O Decrease Decrease Decrease
Li2O Decrease Decrease Decrease
K2O Decrease Decrease Decrease
F Decrease Decrease Decrease
General Relationships Between Melting Point, Viscosity and Thermal Breakpoint
-
Principles of Mould Flux Technology
23