Effect of initial rolling temperature on the hydrogen-induced delayed fracture of Nb alloyed hot stamping steel

The effect of initial rolling temperature on the hydrogen-induced delayed fracture resistance of Nb alloyed hot stamping steel was studied by the constant load test and the electrochemical hydrogen permeation method. When the initial rolling temperature drops from 1000℃ to 950℃,the hydrogen diffusion coefficient and the corrosion resistance of the steel decrease,but the hydrogen-induced delayed fracture resistance increases. TEM test shows that when the initial rolling temperature is 1000℃,the MX precipitated phase has an average size of 30 nm,and the Cr₂C₃ precipitated phase can be observed with an average size of 100 nm; when the initial rolling temperature is 950℃,the MX precipitated phase has an average size of 5 nm. The main reason leading to the improvement of delayed fracture resistance is the hydrogen trapping effect of the MX precipitated phase. The reason for the difference of precipitated phases is that a higher rolling start temperature will prompt the coarsening of the MX precipitated phase,which will suppress the precipitation of the Cr₂C₃ phase.