Composition Optimization and Experimental Characterization of a Novel Steel Based on CALPHAD

A new steel with high Cr and low W, Mo contents for forged cold work roll was designed based on the composition system of traditional high speed steel (HSS) roll. The Fe-C isopleths of the steel and the mass fraction of equilibrium phases versus temperature were calculated by Thermo–Calc, and the effects of different alloying elements (W, Mo, Cr, V) on austenite, ferrite and carbides (MC, M6C, M7C3, M23C6) were also established to optimize the composition and structure. The designed and optimized specimens were both quenched at 1100℃and then tempered twice at 560℃. The hardness and wear resistance of the samples were measured. The microstructures of quenched tempered and forged specimens were studied. The results show that ferrite crystallization, peritectic reaction, austenite crystallization and the precipitation of MC, M6C, M7C3, M23C6occur during equilibrium solidification process. The alloying elements W, Mo mainly affect the precipitation of M6C, while Cr affects the precipitated region and mass fraction of M7C3. High V content widens the high temperature region of the peritectic reaction and results in a large amount of MC precipitates. The optimized composition (wt%) for cold work roll steel is 1.30-1.35%C, 9-10%Cr, 2.5-3.0%Mo, 0.5-1.0%W, 2.5-3.0%V, 0.5-0.6%Mn, 0.5-0.6%Si. The hardness of the steel after quenching and tempering is 60.8HRC and weight loss after 120min is 6.2mg. This meets the requirement of hardness and wear resistance requirements for cold work roll. The ledeburite in the optimized steel disappears after forging and the carbide network break into a large amount of tiny blocky ones dispersed in the matrix without cracks, which shows a good forgeability of the steel and rationality of the optimized composition