Metal Production- Secondary Steelmaking

Question 1

Why is secondary steelmaking done?

Answer 1

Removal of unwanted H, S, O, N, C.
Better inclusion removal.
Better control of temperature and analysis procedures.
Results in more superior reproducible and uniform steel properties.

Question 2

Other names for secondary steelmaking

Answer 2

Ladle refining
Ladle metallurgy
Secondary refining

Question 3

What are the 5 things secondary steelmaking is designed to do?

Answer 3

Deoxidise
Decarburise
Desulfurise
Dephosphorise
Remove hydrogen and nitrogen

Question 4

What do stirring processes do?

Answer 4

Usually argon. Promotes homogenisation of the steel and some decarburisation and degassing due to low partial pressures of CO, H2 and N2 in the argon gas. Enhances alloy dissolution, deoxidation and slag/metal reactions

Question 5

What do vacuum processes do?

Answer 5

Facilitate degassing. Enhance the kinetics of decarburisation and deoxidation reactions. Alloy additions can be made at this point to alter the composition. Argon stirring usually an integral part of any vacuum process

Question 6

What are injection processes and what do they do?

Answer 6

Injection of inert gases or powder such as CaSi, CaAl, etc. Various methods of delivery. Sometimes a synthetic slag is used with injection treatments.
Deoxidation, desulfurisation and inclusion modification can all take place almost simultaneously. Argon purging can aid homogenisation and oxide floatation.

Question 7

What is ladle stirring and what does it do?

Answer 7

Deep injection of gas through a porous refractory plug. Causes agitation if the steel. Possible to use EM induction to cause stirring. Stirring makes steel more uniform in T and composition. Makes analysis of these parameters much more reliable. Stirring also causes any non-metallic inclusions to move either to the ladle wall or to the surface slag layer. Results in cleaner steel with reduction in number of harmful inclusions.

Question 8

What are CAB and CAS processes?

Answer 8

Developments in stirring practice that give even cleaner steel and compositional control.
Compositional adjustment by bubbling/sealed bubbling

Question 9

Why is deoxidising necessary?

Answer 9

O content at start of secondary steelmaking between 400 and 1000ppm. Solubility in liquid steel is 0.16% but only 0.03% in solid steel. Need to deoxidise before solidification to:
Prevent blowhole formation during casting,
Prevent porosity in final product,
Prevent large quantities of FeO being precipitated

Question 10

What are the most common deoxidisers and which is the most effective?

Answer 10

Al, Mn, Si.
Al more effective than Si which is more effective than Mn. More effective means lower equilibrium concentration of that deoxidiser to achieve a certain target for dissolved oxygen

Question 11

What can oxygen content get down to using Al, Si and Mn?

Answer 11

For Al killed steels can get down to 2-3ppm.
For Si-Mn can get down to 30-60ppm

Question 12

What does rate of deoxidation depend on?

Answer 12

The stirring time and the slag area fraction

Question 13

How does using Al as an oxidation agent affect choice of refractories?

Answer 13

Al more powerful oxidising agent than Si or Mn so will reduce silica or MnO in refractories used in the ladle or tundish. Silica rich refractories should therefore not be used.

Question 14

How does using Al as an oxidising agent affect fatigue?

Answer 14

Any alumina or calcium aluminates (both inclusions) from the oxidation deoxidation treatments will rescue the fatigue lifetime. Some applications such as bearing steels and tyre wire require good resistance to fatigue so alternative deoxidisers like Mn and/or Si must be used

Question 15

What is steel cleanness about?

Answer 15

The number of inclusions in the steel. Fewer inclusions means cleaner

Question 16

Sources of inclusions

Answer 16

Indigenous (small): deoxidation products and MnS.
Exogenous (large): reoxidation (reaction with air or slag), entrain,ent of slag-eroded refractories.
Inclusions are formed by chemical reactions (deoxidation, reoxidation and precipitation) or by physical conditions (turbulence or wear).

Question 17

Problem of solid oxide inclusions

Answer 17

Like alumina or certain calcium aluminates. Can cause nozzle blockage during continuous casting and disrupt the process and so have to be burnt out

Question 18

Problems of som other inclusions

Answer 18

Some can cause cracking and defects, slivers and delamination in rolled products and also fracture during hot/cold forming and wire drawing

Question 19

How does the amount of inclusions evolve from before Ca injection through to tundish?

Answer 19

Number of inclusions decreases slightly from pre-Ca injection to post Ca injection. Then increases a lot in post degassing then decreases by bit less for tundish

Question 20

Fracture energy vs sulfur content graph

Answer 20

Like an exponential decrease curve in fracture energy for increasing sulfur content. Higher curve in longitudinal direction than transverse

Question 21

Other effect of increasing S content

Answer 21

Increases DBTT

Question 22

How is sulfur normally present in steel?

Answer 22

As MnS inclusions (tends to form on GBs). Mn is needed to prevent the formation of FeS which is detrimental to hot workability

Question 23

What determines the size, shape and distribution of MnS inclusions.

Answer 23

S content, O content and solidification rate amongst others.

Question 24

Problems of MnS inclusions and S in general

Answer 24

The inclusions are more plastic than steel (deform with the steel) and can act as crack initiation sites in the finished product. Sulfur also has adverse effects on toughness, ductility, weldability and corrosion resistance.

Question 25

Benefit of sulfur

Answer 25

Improved machinability

Question 26

Where is sulfur removed and where can’t it be removed?

Answer 26

Not possible in BOS and difficult in EAF. Removed either in torpedo ladle prior to primary steelmaking or in ladle during secondary steelmaking

Question 27

What can be used for desulfurisation and compare their effectiveness?

Answer 27

Can use Mg gas or Ca gas at 1600C.
With mag can get down to 37ppm S.
With Ca can get down to 10^-3ppm S.
Basic reaction for removing S is
[S]+MO=MS+[O]
The relative stabilities of oxides and sulfides being important

Question 28

Calcium injection for desulfurisation

Answer 28

Relatively cheap and widely used. Deep injection of Ca in the form of powdered Ca-Si or Ca-Si wire. Either blow powder through lance or feed in wire (steel tubing with Ca-Si in middle). Usually 1 or 2kg per tonne steel. Boil point of Ca 1491C which is lower than Tm of steel so is vapour

Question 29

Key feature of calcium injection processes

Answer 29

Necessary to ensure that the Ca compounds are injected as deep into the ladle as possible so that the ferroststic pressure stops vaporisation

Question 30

Reactions for desulfurisation using calcium

Answer 30

3CaO+2Al+3S->3CaS+Al2O3
Al already there as a deoxidiser
CaS less harmful than MnS
Products dissolve in liquid slag

Question 31

Example level of desulfurisation using Ca

Answer 31

Using CaO and if steel contains 500pom Al we can reduce the S levels down to less than 50ppm

Question 32

Benefits of calcium injection

Answer 32

Dissolved oxygen content glass to below 2ppm. S content reduced to below 50ppm. MnS and Al2O3 inclusions are modified to CaO.Al2O3(CaS) inclusions which are:
Liquid at casting temperatures and so nozzle clogging not a problem,
Non-deformable during hot working (not drawn out during rolling/forging) and so do not contribute to a deterioration in mechanical properties.