Effects of Stress on Martensite Transformation During Continuous Cooling and Mechanical Response of a Medium-Carbon High-Strength Steel

The effects of stress on martensite transformation at different continuous cooling rates and the mechanical response of a medium-carbon high-strength steel were investigated by the metallographic method, dilatometry, and tensile tests. The results show that the microstructure consisted of martensite and retained austenite (RA) regardless of whether stress was applied. The martensite start temperature increased by stress due to additional mechanical driving force. The amount of martensite increased, while the amount of RA decreased at the same cooling rate by applying stress. In addition, the martensite laths were refined and variant selection of martensite orientation was observed by applying stress. Moreover, the tensile strength increased from about 1470 to 2170 MPa by applying stress because of more martensite and the fraction of low-angle grain boundaries. The strength improvement with the increase of cooling rate under stress was larger than that in the specimens without stress.