A novel microstructural design and heat treatment technique based on gradient control of carbon partitioning between austenite and martensite for high strength steels

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Based on gradient control of carbon partitioning between martensite and austenite during heat treatment of steels, a stepping-quenching-partitioning (S-Q-P) process is developed for high strength steels. The S-Q-P process involves several quenching processes at progressively lower temperatures between martensite-start (Ms) and martensite-finish (Mf) temperatures, each followed by a partitioning treatment at either the initial quenching temperature or above that temperature. A novel microstructure is designed based on the S-Q-P process. Sizes and distributions of retained austenite and high-carbon martensite surrounded by martensite laths can be manipulated by the partitioning duration and temperature, and quenching temperature of the S-Q-P process. Alloying element Si is employed in the S-Q-P steel to suppress formation of carbides and create suitable amount of retained austenite. A steel of 0.39C-1.22Mn-1.12Si-0.23Cr (wt.%) treated by the S-Q-P process is endowed with some special microstructural characteristics: some strips of retained austenite located at edges of martensite blocks with high density of dislocations and between martensite laths, some small blocks of twinned martensites distributed among bundles of the low-carbon martensite lath. The mechanical properties of the medium carbon steel after the S-Q-P process can reach ultimate tensile strength (Rm) of 1240 MPa, total elongation (EI) of 25%, and product of strength and ductility (PSD) of 31.2 GPa% that are much more improved than those after the conventional quenching-tempering (Q-T) and currently prevailing quenching-partitioning (Q-P) treatments. The PSD of the tested steel after the S-Q-P process increases by 67% and 32% as compared with those after the Q-T and Q-P processes, respectively.