Experiments and analysis of relations among heat, stress and transformation of boron steel for hot forming

Tensile and quenching experiments of boron steel at high temperature are implemented. In the experiment a plate specimen of boron steel was austenitized at 950°C for five minutes, then tensile forming and quenching were applied simultaneously and the force, displacement, expansion and temperature in the experimental process were measured. The experimental results show that martensite starting temperature, martensite transformation rate and transformation plasticity coefficient almost remain unchanged when the stress is lower than certain level. When the stress increases in certain range, the above physical quantities increase linearly with the raise of the stress. The above physical quantities tend to be constant values when the stress is just above certain level. When the stress increases 400 MPa, martensite starting temperature rises to 565°C and is increased by 180°C. Based on the analysis of the above physical quantities' variation and the specimen's microstructure, thermal-mechanical-transformation coupled constitutive models are developed.