Quantitative study of modification of sulphide inclusions by calcium and its effect on the impact toughness of a resulfurised alloy steel

In this paper, the mechanisms of the shape control of sulphide inclusions by calcium in a resulfurised alloy steel and the quantitative effect of the shape factor, the size and the quantity of sulphide inclusions on the transverse impact toughness are investigated. There are three different types of sulphide inclusions in the calcium-treated resulfurised alloy steel, 58CrNiMnMoVSCa, namely elongated MnS, oval (Mn,Ca)S with and without any calcium aluminate nucleus. The proportion of the former is reduced while that of the two tatter raised as the ratio of Ca/S in steel increases. The mechanisms of the shape control of sulphide inclusions by calcium are mainly due to its solid solution hardening effect. Besides, the calcium aluminate, existing as the nucleus of the oval (Mn,Ca)S, restricts the hot deformation of the sulphide inclusions and as a result, enhances further the shape control. Based on the solid solution effect of calcium on sulphide inclusions, the quantitative relationship between the shape factor lambda of sulphide inclusions and the ratio of Ca/S in steel at a hot forging reduction of 85 is developed, Finally, the quantitative relationship between the transverse impact toughness and the parameters of sulphide inclusions, namely the shape factor (lambda), the size (L) and the quantity (N) is also developed.