Effects of Cooling Conditions and γ Grain Size on Each Behavior of Transformations, 3-Dimensional Thermal Deformation and Stress Generation during Immersion Cooling of Steel Bloom

Abstract

To optimize reverse transformation treatment, which is effective in preventing hot rolling cracking in the CC-HCR (Continuous Casting - Hot Charge Rolling) process, three-dimensional metallo-thermo-mechanical analyses were performed. The metallo-thermo-mechanics and a CCT (Continuous Cooling Transformation) diagram estimation method using JMatPro were used to obtain the required cooling time for reverse transformation in immersion cooling with water jet and to elucidate the effects of nonuniform cooling and γ grain size on thermal deformation and stress generation behaviors during cooling and the mechanisms of these events.

These analyses clarified the following. When diffusion-controlled transformation occurs during cooling, the required cooling time increases as Dγ (Diameter of γ grain) increases, and becomes substantially constant when only martensitic transformation occurs. In addition, the difference of Dγ causes a difference in the type of transformation that occurs during cooling and the temperature range where transformation occurs during cooling, and these differences produce differences in the amount of transformation expansion, which greatly affects the bloom deformation behavior during cooling. Furthermore, there is a difference in the level of the maximum generated stress during cooling depending on whether the transformation that occurs near the bloom surface layer during cooling is diffusion-controlled transformation or martensitic transformation. In addition, this difference in transformation behavior also causes a difference in the mechanism of maximum stress generation.