关键词: 直接淬火, 离线再加热淬火, 低温回火, 马氏体板条, 析出相, 有效晶粒尺寸

Abstract: A Ti-microalloyed medium carbon steel was designed, and after controlled rolling, the tested steel was conducted by different heat treatment processes, that was online direct quenching+tempering at low temperature (DQ+LT) and off-line reheat quenching+tempering at low temperature (RQ+LT). By using mechanical properties testing and microstructure observation, effect of quenching mode (DQ and RQ) on microstructure and precipitates of the Ti microalloyed medium carbon steel was studied. The results show that the quenching mode has a significant effect on microstructure and mechanical properties of the tested steel. The grain morphology quenched by DQ and RQ processes is flat and equiaxed, respectively. The flattened martensite lath is slender and directional, and the matrix dislocation density is high. The effective grain size of equiaxed grains is finer, the martensite lath in the grain is uniform and non-directional, and there are a lot of fine carbides and (Ti, Mo)C precipitates in the matrix. Due to the microstructure difference, the tested steel treated by DQ process has some shortcomings, such as uneven transverse and longitudinal performance and poor impact property at low temperature, etc. While the tested steel treated by RQ process has uniform transverse and longitudinal performance and the good match of strength and toughness.

Key words: direct quenching, off-line reheat quenching, low-temperature tempering, martensite lath, precipitates, effective grain size