QUANTITATIVE STUDY OF MODIFICATION OF SULFIDE 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 inclusi ons by calcium in a resulfurised alloy steel and the quantitative effe ct of the shape factor, the size and the quantity of sulphide inclusio ns on the transverse impact toughness are investigated. There are thre e different types of sulphide inclusions in the calcium-treated resulf urised alloy steel, 58CrNiMnMoVSCa, namely elongated MnS, oval (Mn,Ca) S with and without any calcium aluminate nucleus. The proportion of th e former is reduced while that of the two tatter raised as the ratio o f Ca/S in steel increases. The mechanisms of the shape control of sulp hide inclusions by calcium are mainly due to its solid solution harden ing effect. Besides, the calcium aluminate, existing as the nucleus of the oval (Mn,Ca)S, restricts the hot deformation of the sulphide incl usions and as a result, enhances further the shape control. Based on t he solid solution effect of calcium on sulphide inclusions, the quanti tative relationship between the shape factor lambda of sulphide inclus ions and the ratio of Ca/S in steel at a hot forging reduction of 85 i s developed, Finally, the quantitative relationship between the transv erse impact toughness and the parameters of sulphide inclusions, namel y the shape factor (lambda), the size (L) and the quantity (N) is also developed.