Development of ultrahigh strength steel through electroslag remelting with inoculation

Abstract

In an earlier work, an ultrahigh strength steel, designated as 0·3C–CrMoV ESR (0·3C–1·0Mo–0·3V), which is a modified version of AFNOR 15CDV6 steel, has been developed by increasing carbon content from 0·15 to 0·28% and by processing the steel through electroslag refining (ESR) with inoculation by either niobium or zirconium during ESR. Depending on the quench rate, the ultimate tensile strength (UTS), 0·2% proof stress (PS), percentage of elongation and Charpy U notch impact energy values of the steel in the as cast condition after heat treatment varied at ranges of 1550–1560 MPa, 1500–1520 MPa, 8–12·5% and 370–790 kJ m⁻² respectively. The microstructure of the 0·3C–CrMoV steel is reported to consist of martensite and bainite. In the present investigation, the UTS and 0·2%PS have been further increasd to 1740–1760 and 1570–1610 MPa respectively by increasing the chromium content of the 0·3C–CrMoV steel to 4% and by subjecting the steel to ESR with inoculation by titanium. Interestingly, the percentage of elongation also increased to 14% simultaneously with UTS and PS without any drop in the Charpy U notch impact energy. The improvement in the mechanical properties of the steel with increasing chromium content may be attributed to an increase in the volume fraction of the martensite in the mixed martensite and bainite microstructure.