Topics in Continuous Casting Science and Technology

摘要

Nonmetallic inclusions, segregation and cracking in cast slabs, blooms and billets have been major three problems in continuous casting, which reduce the yield and casting speed, that is, productivity of the casters. For the removal of nonmetallic inclusions electromagnetic stirring and braking have been applied to control the liquid flow in the mold and in a recent Japanese national project, it was demonstrated that by the application of electromagnetic pinch force to the casting in the continuous casting mold mild cooling are realized and the formation of the oscillation mark is prevented. As the result, subsurface trapping is eliminated of nonmetallic inclusion at the hook and surface treatment of slabs becomes unnecessary to obtain the same revel of defect appearance in the hot coils. The chemical compositions of nonmetallic inclusions were controlled to remove their harmful effect in the cases of steel tire cord, stainless steel wire, etc., but now are for their utilization, such as for precipitation cite of intra granular ferrite in thick plates to obtain finer ferrite grains. Centerline segregation resulted from the movement of residual liquid in inter dendritic region enriched in solutes, which is induced by the solidification and thermal shrinkage or bulging, and its entrapment at the center of slabs near the crater end. Soft reduction of slab are applied to compensate the solidification and thermal shrinkage and finely spaced rolls are adopted as mechanical counter measures and the reduction of solute microsegregation are examined by the increase of solid/liquid partition coefficient of solute as a metallurgical measure. Fine equiaxed grains are tried to obtain by the use of electromagnetic stirring and low super heat in poured steel in order to reduce the centerline segregation size. However, recently it was reported that microsegregation is utilized for obtaining finer ferrite grain: By the use of microsegregation of ferrite stabilizing solute delta ferrite phase is stabilized even at lower temperature and used as pinning of gamma grain. Various mechanical deformations are induced in continuously cast slabs stemming from bulging, bending and unbending, thermal and transformation induced strain, etc. On the other hand steel shows the brittle temperature ranges depending on the chemical compositions and thermal history of casting. In order to prevent from the cracking of casting either the amount of deformation is to be reduced or the brittleness is to be improved by regulating the chemical composition or temperature history of casting. The deformation analysis of unbending shows that shear deformation changes the unbending strain distribution and the usage of this nature is important. Resonance like unsteady slab bulging with oscillation of liquid metal level is also must be minimized. It was reported that first quenching of slab surface from austenite to ferrite temperature range in secondary cooling induced the intragranular ferrite formation even in the slow cooling in the second time after reheating to the austenite temperature range, which prevents from the appearance of ductility dip at austenite-ferrite transformation range.