Prediction of Final Solidification Position in Continuous Casting Bearing Steel Billets by Slice Moving Method Combined with Kobayashi Approximation and Considering MnS and Fe₃P Precipitation

Abstract

Control of MnS and Fe₃P precipitate are of vital importance for the quality of the bearing steels. The precipitation behavior is not only related to shortening the bearing steel’s fatigue life, but also to another serious engineering problem i.e. changing the billet final solidification position. In order to distinguish the different precipitate behavior on the influencing of the final solidification position, a slice moving method combined with Kobayashi approximation and the MnS and Fe₃P precipitation is developed. The continuous casting billet of seven-component bearing steel, i.e. Fe–C–Cr–Mn–Si–P–S system, is considered as the raw material. Upon the present chemical composition of 0.004 to 0.007 mass% S and 0.011 to 0.012 mass%P in 100Cr6 (DIN-Norm) and RAD1 (GB-Norm) alloy, the MnS but not Fe₃P precipitate covers the billet cross section. The onset of Fe₃P precipitation is at 0.019 mass% P in 100Cr6 alloy and 0.021 mass% P in RAD1 alloy. The distribution of the maximum amount of MnS and Fe₃P precipitate is similar, i.e. concentrating at the billet center. The increase of P composition, besides accelerating the precipitation of MnS and Fe₃P, elongates the liquid core length. In contrast, the increase of S composition and the precipitation of MnS greatly shortens the liquid core length. Thus it is vital to control composition of S, P solute to a low level in bearing steels in order to stabilize the final solidification position.