Effect of Stretch-forming on Hydrogen Diffusion Behavior in High-strength Steel Sheet

The hydrogen diffusion behavior in a tempered martensitic steel sheet with 1-GPa grade tensile strength was investigated utilizing a newly developed hydrogen visualization technique using an Ir complex, whose color changes from yellow to orange due to its reaction with hydrogen. The hydrogen permeation through the steel sheet could be monitored from the color change of the Ir complex. Furthermore, the breakthrough time of hydrogen through the specimen could be qualitatively evaluated from the changes in the brightness of the Ir complex. This hydrogen visualization technique was also applied to a steel sheet stretch-formed using a hemisphere punch to simulate press-forming of automotive structural parts. The hydrogen breakthrough time around the top of the specimen was long and decreased with increasing the distance from the top. According to the plastic strain distribution of the specimen calculated by the finite element method, the hydrogen breakthrough time increased with increasing plastic strain. It was considered that the introduction of plastic strain decreased the hydrogen diffusion coefficient due to the introduction of dislocations acting as hydrogen trap sites, resulting in the increase of hydrogen breakthrough time.