Effect of heating speed on SHCCT curve and microstructure of high strength steel

[Objective] The research aimed to study the transformation law of microstructure and properties of heat affected zone of high strength steel under different thermal cycles, so as to provide theoretical basis for the practical engineering application of the same steel grade in the future. [Methods] Taking 30MnCrNiMo high strength steel as the research object, because the steel will undergo several phase transformation processes during continuous heating and cooling, in this process, the lattice structure of the steel will change, resulting in volume change, so there will be a turning point on the normal expansion curve. Based on this principle, the SHCCT curves of low heating speed of 150 ℃/s and high heating speed of 1100 ℃/s were established by using Gleeble-3500 thermal simulation test machine, the law of microstructure transformation is compared and analyzed. [Results] The results show that the SHCCT curves at low heating speed and high heating speed contain four regions, that is M, M+B, B, B+P+F, and the hardness values corresponding to the four regions decrease from 500 HV1 to 200 HV1. The low heating speed SHCCT curve can produce F and P transitions at a cooling speed of 3.0 ℃/s, while the high heating speed SHCCT curve needs to produce F and P transitions at 0.5 ℃/s. Compared with the SHCCT curve at low heating speed, the SHCCT curve at high heating speed has higher A_c1、A_c3 and M_s points, and the temperature difference between A_c1 and A_c3 increases, and the F and P transitions require a longer incubation period. [Conclusion] In practical engineering applications of the same steel grades, in order to obtain lath martensite structure in the welding process of high strength steel to achieve good performance, the cooling speed should be controlled above 7.5 ℃/s in laser-MAG hybrid welding, and the cooling speed should be controlled above 10.0 ℃/s in single MAG welding.