Secondary steelmaking

A careful balance between the amounts of hot metal and scrap charged into the converter is maintained as a means of controlling the temperature and to ensure that steel of the required specification is produced. After a sample has been taken to check that the chemical content of the steel is correct, the vessel is again tilted to allow the molten steel to flow out. This is known as tapping. The steel is tapped into a ladle, in which secondary steelmaking frequently takes place. During tapping small quantities of other metals and fluxes are often added to... 

Increasingly today, steels after they have been tapped (poured) from the furnace undergo a further stage of processing called secondary steelmaking before the steel is cast. This applies to both the basic oxygen process route and to the electric arc furnace route. 

Direct-bonded magnesia-chrome ore materials exhibit a high strength at high temperatures, superior resistance to slags and volume stability up to 1800 C. They also show a much lower tendency to expansion and subsequent spalling as a result of interaction with iron oxides (cf. the follow ing section on chromite materials). Direct--bonded ware finds application in open-hearth furnaces, in electric arc furnaces, in secondary steelmaking processes, in rotary cement kilns, etc. 

Steels with the lowest sulfur specifications (of less than 0.001 %, or 10 mg/kg) are generally produced by a combination of a pig iron desulfurisation stage, combined with efficient desulfurisation in the BOS and secondary steelmaking processes. 

Secondary steelmaking is used to bring the steels produced by the BOF and the EAF into specification. In many works it acts as a physical and chemical buffer between the primary steelmaking processes and the continuous caster. It has been the main growth area in steelmaking since the early 1980s as a result of ... 

Particularly as a result of (b), the roles of the primary steelmaking processes have changed, with the BOF being used mainly for the removal of carbon from pig iron, and the EAF being used mainly to melt steel scrap. This places increased emphasis on refining the hot metal either before the primary steelmaking operation (i.e., treatment of pig iron — see section 27.3), or after it (i.e., secondary steelmaking). 

The processes used in secondary steelmaking are many and varied [27.2, 27.8, 27.22]. For the purposes of this section, they can be divided into five types of operations [27.5] ... 

Slags play an important role in secondary steelmaking and, as the objectives of the several processes vary, so do the compositions of the slags used [27.5,27.19]. 

Addition rates of quicklime vary widely. The AOD process uses about 90 kg/t steel, whereas other processes use as little as Ikg/t. The average total use in secondary steelmaking is in the range 5 to 15 kg/t [27.5]. 

Calcined dolomite is not widely used in secondary steelmaking, but is often a component of synthetic slags, which may contain up to 10 % of MgO [27.12]. 

Alumina Clogging. Nozzles for secondary steelmaking ladles can become blocked by build-up of dense products of the reaction between the nozzle refractories and the steel and slag, in particular by attack on the alumina phase of the refractory. 

Secondary Steelmaking. The treatment of steel in the ladle (e.g. by vacuum degassing) to improve cleaness, composition control and steelplant productivity. See Appendix B.l. 

Vacuum Degassing. A secondary STEELMAKING PROCESS in which the bath of molten steel in the ladle, or the stream of molten steel from it, is exposed to a vacuum to accelerate the removal of hydrogen from the steel. Vacuum Drying. Reducing the air pressure over ware being dried speeds up moisture removal. 

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