Development of processing map coupling grain size for the isothermal compression of 300M steel

The expressions for the efficiency of power dissipation and the instability variable coupling grain size in an elevated temperature deformation process are derived using the present physically-based visco-plastic constitutive equations and adopting the dynamic materials model. The approach to construct a processing map is also presented on the basis of the variations of efficiency of power dissipation and instability variable with deformation temperature, strain, strain rate and grain size. The isothermal compression of 300M steel was conducted on a Gleeble-3500 simulator at deformation temperatures ranging from 850°C to 1200°C, strain rates ranging from 0.1s^-1 to 25.0s^-1 and height reductions ranging from 30% to 70%. The microstructure and austenite grain size are obtained using an OLYMPUS PMG3 microscope. According to the experimental results, the processing maps in the isothermal compression of 300M steel are predicted at different strains, in which the peak efficiency of power dissipation increases and the instable region decreases with the increase in strain. The optimal processing parameters of 300M steel at a strain of 0.7 are at the deformation temperatures ranging from 900°C to 1140°C and strain rates ranging from 0.1s^-1 to 1.91s^-1. The comparison between the present processing maps' coupling grain size and the traditional based on Prasad's theory shows that the present processing maps can more accurately describe the elevated temperature deformation behavior, which are validated through the micrographs in the isothermal compression of 300M steel.