During the indirect hot stamping process of boron steel, the pre-deformed component undergoes air cooling, one-side-contact cooling and both-side-contact cooling phases successively. The effects of pre-deformation and cooling rate on the phase transformation should be understood before conducting indirect hot stamping experiments of vehicle components. Uniaxial tensile tests of boron steel at RT were carried out to obtain specimens with different pre-strain levels. Then they were heated to 900°C according to the indirect hot stamping process and quenching tests were performed on them at different cooling rates. Metallographic observations were performed on the quenched specimens and their hardness was measured. The results show that the pre-strain at RT has little influence on the phase transformation of boron steel. This is due to the dislocation structure introduced by deformation at RT recovered during the heating process and it is good for the indirect hot stamping. Upper B-pillar parts were first cold pre-formed, and then were heated and hot stamped. The microstructure and hardness results at different locations on the indirect hot stamped components are demonstrated qualified.

To study the effects of temperature and thickness on forming limits of boron steel 22MnB5, Nakajima tests have been performed for the steels with thickness of 1.0mm and 1.4mm at 600°C and 700°C, respectively. The forming limit curve (FLC) of the steel at 700°C is higher than that at 600°C, and the FLC of the steel with 1.4mm is higher than that of the steel with 1.0mm. With increasing the forming temperature, the strain rate sensitive exponent m increases, and it results in a longer Swift’s diffuse instability phase and greater limit strains. The effect of thickness on yield path is different from the case at room temperature, due to the half angle of pointed Vertex, θ0, which increases with increasing of the thickness, and then the limit strains increase.

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