Effect of superheat on quality of central equiaxed grain zone of continuously cast bearing steel billet based on two-dimensional segregation ratio

Abstract

The quality of central equiaxed grain zone (CEGZ) of GCr15 bearing steel billets was investigated at different superheats (20, 25 and 35 °C) by experimental observations and a finite element model in order to optimize superheat in continuous casting process. Several GCr15 billets were collected from the continuous casting shop, and the same CEGZ was chosen for comparison of internal quality of GCr15 billets. Considering the limitation of segregation index at some points, two-dimensional segregation ratio in CEGZ was introduced. Firstly, the segregation ratio and the area of center large dark points in CEGZ obtain the minimum at 25 °C superheat, which indicates that the quality of CEGZ at 25 °C superheat is improved compared with those at 20 and 35 °C superheats for corresponding continuously cast billets. The highest superheat and the lowest superheat are not beneficial for improving the central zone quality in the billets. Secondly, the quality of CEGZ of GCr15 billets increases with a decrease in the secondary dendrite arm spacing of CEGZ. Finally, according to the established finite element model, it is deduced that the secondary dendrite arm spacing of CEGZ is closely related to its later solidification time at solid fraction of 0.5–1.0, and the former will be decreased when decreasing the latter.