SH-CCT diagram and microstructure and properties of heat-affected-zone of 1 400 MPa ultra high strength steel

The effects of different cooling rates on the microstructure transformation and properties of the coarse grained heat affected zone (HAZ) of the 1 400 MPa ultra-high-strength steel were studied by using a Formastor-F Ⅱ automatic phase changer, combined with metallographic microscope (OM) and Vickers hardness test. According to the simulation results, the welding continuous cooling transformation curve (SH-CCT) is drawn, and the formula method is used to determine the optimal welding parameters combined with the welding process test. The results show that the critical transformation cooling rate of martensite in 1 400 MPa ultra-high strength steel is about 5 ℃/s. When the cooling rate is greater than 5 ℃/s, the microstructure of the coarse grained zone in the heat-affected zone is single lath martensite, and the hardness value is 487 ~ 509 HV5. When the cooling rate is less than 5 ℃/s, medium and high temperature phase transformation products appear in the structure of the coarse-grained region, which are lath bainite, granular bainite and ferrite, and the hardness value drops to 487 ~ 260 HV5. At the same time, M-A component also appeared in the tissue, and its content increased first and then decreased with the decrease of cooling rate. and the shape also changed from dispersed granular to intermittent long strip or block distribution. When welding 1 400 MPa ultra-high-strength steel with a thickness of 8 mm, the welding heat input is controlled within 20 kJ/cm, the microstructure of the coarse-grained zone is lath martensite, the hardness value is maintained at about 500 HV5, and no softening occurs in the coarse-grained region, which meets engineering requirements.