Metal Production- Continuous Casting

Question 1

What forms can continuous castings take?

Answer 1

Blooms, slabs, billets

Question 2

Other main type of casting and when it’s used

Answer 2

95% of cast steel continuously cast and main other is ingot cast. Ingot cast used for certain grades of steel which can’t be continuously cast

Question 3

Sequence of events for continuous casting

Answer 3

Molten steel arrives from secondary steelmaking facilities. This is poured (teemed) inti a refractory lined tundish from the ladle usually under an Ar shroud to prevent contact with air. Last minute refining can be performed in tundish. Steel travels into water cooled Cu mould via a submerged entry nozzle (SEN). As dummy bar withdrawn, thin shell of solid steel forms at mould and travels down with dummy bar. Water sprays further cool shell and when thick enough is bent to horizontal and cut off into required lengths.

Question 4

Useful features of the tundish

Answer 4

Acts as a reservoir for the steel and maintains a uniform flow of liquid steel into the mould. Possible to perform last minute refining in tundish to ensure correct composition and remove inclusions

Question 5

What does the continuous process allow for?

Answer 5

Ladle changes and multiple tundishes

Question 6

What is added to the top of the mould?

Answer 6

Mould powder (flux). Plays an important role in the continuous casting process

Question 7

What is used to control flow in the mould?

Answer 7

Electromagnetic methods

Question 8

What is the tundish designed to do?

Answer 8

Promote inclusion floatation by maximising residence time. Prevent thermal and chemical losses from the melt

Question 9

Problem of tundish clogging

Answer 9

Persists to a certain extent in all casting operations. Reduces productivity by reducing throughput and causing nozzle or tundish exchanges. Reduces internal quality by increasing the inclusion content in the strand. Reduces surface quality by changing the level of the meniscus. Complex phenomenon not very well understood

Question 10

Describe the arrangement of things for the transfer from the tundish to the mould

Answer 10

Liquid steel flows out of bottom of tundish into submerged entry nozzle (SEN). This has slide gate part way down to control flow. Liquid steel flows out of SEN into liquid steel pool in the mould. At top of mould is layer of solid mould powder then liquid flux with boundary to liquid steel as a meniscus. Want the meniscus as flat as possible. At copper mould is the solidifying steel shell which gets thicker as you go down.

Question 11

Describe the geometry and arrangement of the mould

Answer 11

Mad if copper, about 600mm in length, tapered, water cooled. May be straight or mor usually have a curvature of between 4 and 15mm in radius.

Question 12

Why are moulds curved?

Answer 12

Curved moulds tend it allow greater inclusion removal to the mould flux.

Question 13

Lubrication of mould

Answer 13

Mould is lubricated using either oil or more commonly mould flux. Reciprocated or oscillates to prevent the steel from sticking to the copper

Question 14

What does the mould act as?

Answer 14

The primary source for heat extraction and allows a solid shell to grow and support the liquid core

Question 15

Why is controlling the flow of steel important in any type of casting?

Answer 15

If not controlled properly we see turbulent flow which causes defects that cannot be corrected. These include:
Entrapment of air and Ar bubbles,
Entrainment of mould flux and solid inclusions,
Meniscus variations

Question 16

What are mould fluxes usually based around?

Answer 16

SiO2, CaO, Al2O3 and Carbon with smaller amounts of other compounds

Question 17

Where are mould fluxes added and what are they expected to do?

Answer 17

Added onto the steel emerging from SEN either manually or automatically. Expected to:
Protect the steel meniscus from oxidation,
Provide thermal insulation to prevent solidification of the steel surface,
Absorb inclusions into the molten slag pool,
Lubricate the strand and provide uniform heat transfer between the strand and mould

Question 18

Describe where the layers are when mould flux is added

Answer 18

SEN is pointed down but has steel flowing out of its sides and circulates back up. Above the liquid steel is liquid flux then unmelted mould flux. Liquid flux also forms a vertical layer next to the mould between in and the widening (as you go down) steel shell. An uninterrupted layer of liquid flux means there will be a good surface quality.

Question 19

Powder consumption

Answer 19

Powder consumption per unit area of mould (Qs) is a critical parameter. The powder consumption provides a measure of the amount of lubrication being supplied to the strand. Variations from the norm can point to the formation of defects or the occurrence of breakouts

Question 20

What are the most vital functions of the mould flux, what are they for and what are they determine by?

Answer 20

Control lubrication: to control surface quality, determined by the characteristics of the liquid slag layer.
Control heat transfer: to minimise defect formation, governed by the solid slag layer

Question 21

How does the temperature profile vary from the mould through to the steel shell?

Answer 21

Lowest at outer edge of mould. Shallow increase through the mould. Steep short increase through interface between mould and solid flux. Medium increase through solid flux. Slightly steeper increase through melted flux. Shallow increase through steel shell

Question 22

The two types of longitudinal cracks

Answer 22

Gross cracks: up to 400mm long associated with casting problems such as poor mould level control and poor powder choice.
Subsurface cracks: found when casting certain grades of steel and are difficult to detect, prevalent in peritectic (medium C) steels with carbon contents of 0.6-0.18%.

Question 23

Problem of thermal shrinkages and the remedy

Answer 23

Thermal shrinkages of the γ and δ phases result in stresses being produced and will lead to an uneven shell.
Remedy is to reduce the horizontal heat transfer to produce a thin uniform shell by having a thick solid slag layer

Question 24

What are sticker breakouts and what causes them?

Answer 24

Rupturing of the steel shell so liquid steel can pour out and the casting process must be stopped.
Involve a lack of lubrication. Thought to be connected in some way to the blockage of the mould/strand gap by agglomeration of ZrO2.

Question 25

Which steels are prone to breakouts?

Answer 25

High carbon steels

Question 26

Monitoring and remedies for breakouts

Answer 26

Mould thermal monitoring can predict breakouts to a certain extent. A spike in temperature indicates them.
Another remedy is to use a mould powder that will lead to a thin solid slag layer (opposite to longitudinal cracking which wants thick layer).

Question 27

What graph is used for mould powder selection?

Answer 27

Break temperature vs viscosity at a certain temperature graph.
Break temperature is where viscosity suddenly drops. Crack-sensitive grades have a concave increasing curve higher than that for sticker-sensitive grades

Question 28

Two ways of measuring viscosity

Answer 28

Rotating bob: torque on the bob (Mo) while rotating in the molten liquid (in Pt container) is measured.
Inclined plane test: pour the molten liquid down an inclined piece of refractory material and measure the length when it is solidified.

Question 29

What do we want to avoid using in mould fluxes and what has to be considered when avoiding this?

Answer 29

Want fluorine free mould fluxes (may be as fluoride) so that HF can’t form. Whatever replaces the F must not adversely affect the performance of the mould powder. Often use B2O5.

Question 30

Defects to do with meniscus and oscillating mould

Answer 30

A wave-like meniscus near mould can circulate and cut off bits of mould flux so that they become entrained in the middle of the metal product.
The mould is always oscillating which can cause oscillation marks. These can be quite deep with wrong mould powder and then act as cracks

Question 31

Other defects and remedies to them

Answer 31

Slag and gas entrapment caused by turbulent flow in the mould. Also erodes SEN. Solve by manipulating mould powder properties.
Pinholes cause by capture of Ar bubble by the newly solidified surface. Solve by reducing vertical heat losses by increasing the powder layer thickness.
Pencil pip defects or blowholes.
Slivers- entrapment of alumina in low C or ULC steels

Question 32

Problems with turbulent metal flow and solutions

Answer 32

Slag entrapment, SEN erosion (of refractories) and use of non-optimum mould powder to counteract it.
Electromagnetic methods have been used to control the flow and also give a slightly cleaner steel. EM braking and acceleration.
There are also nozzles that produce swirling flow

Question 33

What are some problems that occur during solidification?

Answer 33

Segregation and unwanted precipitation