Strength improvement over 2 GPa and austenite grain ultra-refinement in a low carbon martensite steel achieved by ultra-rapid heating and quenching

Abstract

This study focuses on enhancing the strength of low-carbon martensitic steel through the refinement of the prior austenite grain size. After subjecting the steel to heavy cold deformation, ultra-rapid heating in a salt bath was employed. The steel was then quenched in water. The results showed that the ultra-rapid heating for 5 s achieved an impressive tensile strength exceeding 2 GPa, accompanied by a higher hardness of over 500 kgf/mm2. By introducing more nucleation sites through heavy cold rolling, it was possible to achieve an ultrafine-grain structure in the steel, with the prior austenite grain size ranging from 1∼3 μm. In some regions, even smaller grain sizes of around 1 μm or less were observed. The boundaries between martensite packets or blocks disappeared, replaced by two or three lath variations. The study also investigated the effect of high-temperature holding time on the austenite phase. It was found that increasing the holding time resulted in the growth of the austenite grains and a subsequent decrease in tensile strength. Considering carbide dissolution, the optimal temperature holding time was approximately 5 s at 900 °C. A shorter holding time allowed more carbides to remain, which weakened the strengthening effect of the carbon solid solution. The relationship between strength and grain size was analyzed by fitting experimental data. The Hall-Petch slopes were calculated to be about 862 MPa mm1/2 and 1022 MPa mm1/2 for the prior austenite grain size and the effective grain size, respectively. Moreover, excluding the influence of martensite packet or block boundaries, the strength contributions from the carbon solid solution and dislocation strengthening were determined to be approximately 583 MPa and 268 MPa, respectively.