eat227-lecture 2.3 - continuous casting

7/22/2019 EAT227-Lecture 2.3 - Continuous Casting

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Lecture 2.3

Continuous Casting


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Continuous casting (right, red arrows) is a method of working steel that conveys steel from

its molten state to blooms, ingots, or slabs. The white-hot metal is poured into open-ended

molds and continues on through rollers cooled by water. A series of guide rollers further

shapes the steel into the desired form. However, hot rolling (left, blue arrows) is still the

primary means of milling steel. This process begins with pre-shaped steel slabs, which are

reheated in a soaking pit. The steel passes through a series of mills: the blooming mill, the

roughing mill, and the finishing mill, which make it progressively thinner. Finally, the steel is

wound into coils and transported elsewhere for further processing.


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Continuous casting

Continuous casting accounts for about 95% of the world cast steel

These castings take the form of blooms, slabs, and billets

Replaced ingot casting which is still used in some steel plants or forcertain grades of steel

Concept is over 150 years old but continuous casting becamewidespread in Europe especially in the 1970s

Perfectly suited to the mini mill concept with electric steel makingfacilities and a continuous caster.


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Casting shapes and sizes


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Continuous casting schematic


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Continuous casting process


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Continuouscasting- sequence of events

Molten steel arrives from the secondary steelmaking facilities

Poured (teemed) into a refractory lined tundish from the ladleusually under an argon shroud to prevent contact with air

Acts as a reservoir for the steel, and maintains a uniform flow of

liquid steel into the mould Possible perform last minute refining in the tundish to ensure

correct composition, and remove inclusions

Steel travels into the mould via a submerged entry nozzles (SEN).

Continuous process, allowing for ladle changes and multiple

tundishes


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Mould is made of copper and is water cooled As the dummy bar is withdrawn, a thin shell of solid steel forms at

the mould, and travels down the mold with the dummy bar Water sprays further cool the shell, and when it is thick enough it is

bent to the horizontal and cut off into required lengths

Mould powder (flux) is added to the top of the mould and plays animportant role in the continuous casting process Electromagnetic methods also used to control in the mould

Continuouscasting- sequence of events


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Tundish metallurgy

The tundish is now seen as a vital part of the steelmaking process though this is not always the case

Particularly important in terms of clean steel practice

Tundish is designed to: Promote inclusion floatation by maximising residence

time

Ensure inclusion removal by the slag

Prevent thermal and chemical losses from the melt


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Tundish clogging is a problem persists to a certain extentin all casting operations

Reduce productivity by reducing throughout and causingnozzle or tundish exchanges

Reduces internal quality by increasing the inclusioncontent in the strand

Reduces surface quality by changing the level of themeniscus

Complex phenomenon, not very well understood

Tundish metallurgy


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Transfer from tundish to mould

Steel flows from the ladle to the tundish and then to the mould bygravity

Between the tundish and mould, the driving force is related to theamount of liquid metal in the tundish

Control of the flow rate is via stopper rods or slide gates Control of flow is important in order to produce a steady stream of

metal flowing into the mold

Turbulent flow must be avoided, as it leads to non uniformsolidification, and defects of various kinds


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Transfer from tundish to mould


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The submerged Entry Nozzle (SEN)

Most SENs fabricated from alumina-carbon refractorymaterial with zirconia or zirconia-carbon inserts (Z band)

The Z band is situated so that it provides wearresistance at the slag metal interface

Some casting operations don not use SENs, openpouring into mould SEN design can be complex or kept simple Design variables include bore size, port angle, wall

thickness, and number of ports


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The mould

The mould is made of copper, is about 600 mm inlength, is tapered, and is water cooled

It may be straight, or more usually have a curvature ofbetween 4 and 15 m in radius

Curved moulds tend to allow greater inclusion removal tothe mould flux

The mould is lubricated using either oil, or morecommonly mould flux, and reciprocates or oscillates toprevent the steel from sticking to the copper

It acts as the primary source for heat extraction, andallows a solid shell to grow, and support the liquid core.


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Flow in the mould

Problems include: Entrapment of air and argon bubbles

Entrapment of mould flux and solid inclusions

Meniscus variations

For these reasons, mould flow can be altered using anumber of techniques

Flow in the mould is important for a number of reasonsIf it is not controlled properly, then it causes defects thatcan not be corrected.


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Mould powders (fluxes)

They are added onto the steel emerging from

the SEN and are expected to:

Protect the steel meniscus from oxidation

Provide thermal insulation to prevent solidification of thesteel surface

Absorb inclusions into the molten slag pool

To lubricate the strand and provide uniform heat transferbetween strand and mould

Mould fluxes are usually based around SiO2 CaO, Al2O3,

and carbon with smaller amounts of other components


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It is related to the casting conditions, and properties of

the flux, specifically:

Viscosity and break temperature of mould powder

Casting speed,Vc

Mould geometry Oscillation characteristics

Powder consumption

Mould fluxes added either manually or automatically.

Powder consumption per unit area of mould Qsis a criticalparameter.


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The powder consumption provides a measure of theamount of lubrication being supplied to the strand

Variation from the normal can point to the formation ofdefects or the occurance of the breakouts

It is of course important that the powder melting rateclosely matches the consumption rate

Carbon can either be added or removed from the moldpowder in order to compensate of this.

Powder consumption


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Lubrication and heat transfer

Lubrication is determined by the characteristics of the liquidslag layer, but heat transfer is governed by the solid slag layer.

This layer is formed when the molten flux solidifies on contactwith the mould

Near the strand a crystalline layer is formed, which reduces

the heat transfer Another factor affecting the heat transfer is the thickness of

the slag layer which is dependent on the break temperatureof the slag, the higher the T br, the thicker the layer

With time, the solid layer contracts and an air gap is formed,

which also contributes to the heat transfer behaviour.


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Heat transfer

Heat transfer in the mould also affect defects formation.The horizontal heat transfer between strand and mould needs to becontrolled to prevent longitudinal cracking.The vertical heat transfer affects the depth of the oscillation mark,pinhole formation, and molten pool depth.

Horizontal heat transfer affected by:

Casting speed

Electromagnetic flow control

Steel grade

Mould level fluctuations


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Defects-cracks

Longitudinal cracks can be classified into 2 groups:

Gross cracks up to 400 mm long associated with castingproblem such as poor mould level control

Subsurface crack found when casting certain grades ofsteel which are difficult to detect

These subsurface cracks are prevalent in peritectic (mediumcarbon) steels with carbon content of 0.06-0.18%.

The thermal shrinkages of the gamma and delta phases result instresses being produced and will lead to an uneven shell.Remedy is to reduce the horizontal heat transfer to produce athin uniform shell,by having a thick solid slag layer.


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Defects- break out

Sticker breakouts involve a lack of lubrication

Thought to be connected in some way to the blockage of the

mold/strand gap by agglomeration of ZrO2

High carbon steels are prone to breakoutsTechniques such as mould thermal monitoring can predict

breakouts to a certain extent.

Another remedy is to use a mould powder that will lead to a thinsolid slag layer (opposite to longitudinal cracking).


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defects

there are other defects such as:Slag and gas entrapment which is caused by turbulent flow in the mould

Remedy is to manipulate the mould powder properties

Pinholes which are caused by the capture of argon bubbles by the newlysolidified surface

Remedy is to reduce the vertical heat losses by increasing the powder layerthickness

Pencil pipe defects or blowholes

Silvers- entrapment of alumina in low or ULC steels.


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Method of flow control

We have seen that turbulent metal flow causes problemssuch as entrapment, and SEN erosion

It can also lead to the use of a non-optimum mould powderin order to counteract this

Recently electromagnetic methods have been used tocontrol the flow

Electromagnetic braking and acceleration


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Summary

continuous casting seem relatively simple

Mould powders inexpensive, but are asked to perform a number ofoperations that are vital to producing good quality steel castings

Powder consumption is key process variable

Control of horizontal heat transfer is vital in minimizing defectformations.

eat227-lecture 2.3 - continuous casting

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