Influence of Soft Reduction on the Fluid Flow, Porosity and Center Segregation in CC High Carbon- and Stainless Steel Blooms

Abstract

Mechanical soft reduction, MSR, is today an established technique for reducing center segregation and center porosity in the continuous casting of blooms. The bloom cross section is reduced by pinch rollers to compensate for the downward liquid flow in the mushy zone due to solidification shrinkage. In the literature, results are reported on how much MSR quantitatively affects the center segregation, but not on how it influences the liquid flow velocities.

An analytical mathematical model has been developed to calculate the liquid flow velocity along the strand taking into account the effect of height reduction at each pinch roller and the solidification shrinkage. The model considers how much of the total area reduction contributes to an area reduction of the liquid in the center and thus how it affects the flow velocities. The model is easy to apply and brings deeper understanding of the effect of MSR. Calculated results from different scenarios show that the area reduction should increase closer to the crater end. MSR trials on a high carbon steel grade show that the center segregation decreases in proportion to the amount of reduction and clarify the importance of applying a pinch roller just before the crater end. Elongations of the strand above ~1% gave inner cracks in this case. MSR of only 2% height reduction on a stainless steel bloom, type AISI 316, gave a substantial closure of visual center porosity on inspected longitudinal samples.