|JANSTO Steven||CBMM-Reference Metals Company|
Over 200 million tons of Nb-bearing steels were continuously cast and hot rolled globally in 2012. These Nb-bearing plate, bar and sheet products are manufactured throughout the world. Numerous publications discuss the traditional ductility trough for carbon steels with and without microalloy additions of Nb, V and/or Ti. However, the steelmaking and process metallurgy parameters under actual mill conditions are rarely correlated to the hot ductility behavior. The hot ductility troughs associated with simple carbon-manganese steels can also result in surface and internal quality issues if certain steelmaking and casting parameters are not followed. Although higher carbon equivalent steels generally exhibit inherently lower hot ductility behavior, as measured by percent reduction in area at elevated temperature, these steels still exhibit sufficient ductility to satisfactorily meet the unbending stress and strain gradients existing in the straightening section of most casters. The relationship between the steelmaking and caster operation and the resultant slab quality is related through the hot ductility behavior. This global Nb-bearing continuous casting steel research study concludes that the incidence of slab cracking during casting is related more to the steelmaking and caster process parameters. These parameters include the superheat variation, transfer ladle temperature stratification, mould flux incompatibility, casting speed fluctuation, elemental residual chemistry level and excessive secondary cooling. This paper defines these operational root causes supported by physical metallurgy hot ductility data from industrial samples.