Mould Fluxess780556004.online.de/wp-content/uploads/CastingFluxes.pdf · Mold fluxes are synthetic slags (form of glass) that are used in the continuous casting process utilizing

Mould Fluxes

What is Mould Flux?

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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

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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

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• 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

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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).


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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


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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

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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

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Liquid Flux

SolidifyingShell

FluxRim

1

2

3

4

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1. Thermal Insulation

2. Prevents Reoxidation

3. Absorbs Inclusions

4. Lubrication5. Uniform Heat

Transfer

Unmelted Flux

Molten SteelSolid Flux

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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


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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


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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


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Note Shell Physically

Entrapping Gas Bubble

Note Physical Entrapment of Slag in Steel


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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


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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


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Liquid Lubrication In Gap Between Mould and Solidifying Shell

Mould Wall Solid Flux Liquid Slag Steel Shell

FluxVelocity

Ferrostatic Pressure

Vc = Casting Velocity


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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


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Key Properties of Mould Flux

• VISCOSITY

• Thermal Breakpoint (Tbr)

• Inclusion Absorption

• Consumption Rate


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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


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Basicity/Vratio and Thermal Breakpoint Temperature

General Rule of Thumb:As basicity/Vratio increases, thermal breakpoints increases


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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


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Mould Fluxess780556004.online.de/wp-content/uploads/CastingFluxes.pdf · Mold fluxes are synthetic slags (form of glass) that are used in the continuous casting process utilizing