Short-wave infrared heating of 22MnB5 steel with adjusted surface emissivity for press hardening of tailored-strength components

The increasing requirements for lightweight and safety improvement of modern car bodies have fostered the use of hot-stamped press-hardened ultra-high-strength steel (UHSS) components. Recently, tailored heating has been proven as an effective technology to achieve the tailored properties of UHSS components produced by press hardening. In this work, the thermal absorption of 1.6 mm-thick uncoated sheets of 22MnB5 press hardening steel (PHS) was modified to control the heating rate and further achieve different temperature zones during short-wave infrared heating. The absorption was modified by (i) surface oxidation pre-treatment, (ii) graphite coating with high-reflectivity, and (iii) graphite coating with physical shielding. The surface oxidation pre-treatment was identified to play an important role for the absorptivity of 22MnB5 steel. The absorptivity increased with increasing pre-oxidation temperature due to the changes in both surface roughness and the color of the oxide layer. After heating to 900 °C, the temperature difference between the graphite coating and the physical shielding or the high-reflectivity coating was 258 °C or 167 °C, respectively. After press hardening, the specimens revealed a fully martensitic microstructure in the high-absorption zones and a ferritic microstructure with dispersed carbides in the low-absorption zones. The tailored infrared heating also reduced the width of the transition zone between heated and unheated zones to less than 20 mm. This study demonstrates the high effectiveness of this approach for producing press-hardened components with tailored strength.