Abstract: In order to study the difference of shape defects of NM400 steel at different finishing rolling temperatures, the expansion test under compressive stress load was carried out by means of Gleeble-3500 thermal simulation testing machine. The effect of finishing rolling temperature on the phase transformation kinetics and phase transformation plasticity of the NM400 steel was studied, and the ABAQUS finite element model was established by using the modified phase transformation kinetics and phase transformation plasticity parameters. The residual stress of the NM400 steel was tested by crack flexibility method, and the accuracy of the finite element model was verified by comparing the measured and simulated results. At the same time, EBSD was used to characterize and analyze the microstructure of specimens with different finishing rolling temperatures. The results show that the formation of residual stress in the continuous cooling process is divided into three stages, including the dominant stage of temperature stress, the dominant stage of surface phase transformation and the dominant stage of core phase transformation. When the final rolling temperature is 860 ℃, the phase transformation rate is the largest, resulting in the surface phase transformation volume is 17 % larger than that at 820 ℃, so that the tensile stress level at the beginning of phase transformation becomes larger, and the residual stress after rolling is also the largest, resulting in plate shape problems during production and processing. The finishing rolling temperature has little effect on the microstructure and transformation plasticity parameter K after rolling, which is not the decisive factor causing the difference of residual stress after rolling.

Key words: martensite, phase transformation kinetics, phase transformation plasticity, phase transformation rate, residual stress simulation