Effect of manganese content on sulphide shape control by zirconium in steel

Abstract

For a zirconium content of 0·05–0·06%, the degree of sulphide shape control decreased with increase in manganese content from 0·65 to 1·4%. Increasing the zirconium content to 0·08–0·09% improved sulphide shape control for manganese contents above 1·2%, but reduced sulphide shape control for manganese contents below]· 2%. The higher zirconium content resulted in the formation of a carbosulphide, Zr₄C₂S₂, which had a lamellar as-cast structure. During rolling the lamellae were realigned along the rolling direction so that the shape of the lamellar carbosulphides in the as-cast structure determined the degree of sulphide shape control which could be obtained. Since the degree of sulphide shape control which could be achieved by zirconium additions was dependent on the manganese content, reducing the manganese content of the steel was effective in improving the transverse properties. By reducing the manganese content from 1·4 to 0·65%, a 20% increase in total elongation, an 85% increase in transverse shelf energy, and a reduction in minimum bend radius from 2 to 0·5 times the plate thickness was obtained in steel heat treated to 100 ksi (690 N/mm²) yield strength. Chromium was found to be an effective substitute for manganese, to restore hardenability.