Metal Production- Secondary Steelmaking Flashcards
Why is secondary steelmaking done?
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.
Other names for secondary steelmaking
Ladle refining
Ladle metallurgy
Secondary refining
What are the 5 things secondary steelmaking is designed to do?
Deoxidise
Decarburise
Desulfurise
Dephosphorise
Remove hydrogen and nitrogen
What do stirring processes do?
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
What do vacuum processes do?
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
What are injection processes and what do they do?
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.
What is ladle stirring and what does it do?
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.
What are CAB and CAS processes?
Developments in stirring practice that give even cleaner steel and compositional control.
Compositional adjustment by bubbling/sealed bubbling
Why is deoxidising necessary?
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
What are the most common deoxidisers and which is the most effective?
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
What can oxygen content get down to using Al, Si and Mn?
For Al killed steels can get down to 2-3ppm.
For Si-Mn can get down to 30-60ppm
What does rate of deoxidation depend on?
The stirring time and the slag area fraction
How does using Al as an oxidation agent affect choice of refractories?
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.
How does using Al as an oxidising agent affect fatigue?
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
What is steel cleanness about?
The number of inclusions in the steel. Fewer inclusions means cleaner
Sources of inclusions
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).
Problem of solid oxide inclusions
Like alumina or certain calcium aluminates. Can cause nozzle blockage during continuous casting and disrupt the process and so have to be burnt out
Problems of som other inclusions
Some can cause cracking and defects, slivers and delamination in rolled products and also fracture during hot/cold forming and wire drawing
How does the amount of inclusions evolve from before Ca injection through to tundish?
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
Fracture energy vs sulfur content graph
Like an exponential decrease curve in fracture energy for increasing sulfur content. Higher curve in longitudinal direction than transverse
Other effect of increasing S content
Increases DBTT
How is sulfur normally present in steel?
As MnS inclusions (tends to form on GBs). Mn is needed to prevent the formation of FeS which is detrimental to hot workability
What determines the size, shape and distribution of MnS inclusions.
S content, O content and solidification rate amongst others.
Problems of MnS inclusions and S in general
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.
