Laser confocal observation on the formation of martensite and test on the mechanical properties of steels treated by Q-P-T process

The advanced high strength steels(AHSS) for auto manufacturing have been developed to the 3rd generation. The typical one is steel treated by Q-P-T process, which consists of the prior martensite, fresh martensite and retained austenite. In this paper, the non-uniform distributive formation of martensite and untransformed austenite during the one-step quenching treatment was observed by laser scanning confocal microscope. For the Q-P-T process, the non-uniform distributions of untransformed austenite, prior martensite and segmented austenitic grains are the main reasons to form the multi-scale martenite. Moreover, the fresh martensite possesses three highlights, such as microstructure of finer laths, same orientation in a blocky area considered as a “packet” and higher mechanical properties. The research result of this paper reveals the microstructure evolution of Q-P-T treated steel, and provides the accurate theoretical basis for improving the Q-P-T process and the mechanical properties.