Hot stamping is the most efficient manufacturing process to produce high strength automotive parts with reduced springback. Recently, 30MnB5 steel has been developed for hot stamping steel which can provide above 1700 MPa. However, resistance spot weldability of this steel is insufficient due to the need for a large amount of carbon addition. In this study, the effect of the hot stamping heat treatment temperature on resistance spot weldability of Al-10% Si coated 30MnB5 was evaluated to improve the resistance spot weldability of 30MnB5. In terms of nugget diameter, weldable current range, tensile shear strength and cross tension strength, the specimen heat treated at 850 o C achieved better performance than the specimen heat treated at 950 o C. There was a coating microstructure difference between the specimens heat treated at 850 o C and 950 o C whereas both specimens had similar microstructure and mechanical properties of their substrates. The Fe-rich layer of the specimen heat treated at 950 o C was twice as thick as the specimen heat treated at 850 o C because the diffusion rate was much higher at 950 o C. The thick Fe-rich layer deteriorated the resistance spot weldability due to high electrical resistivity during resistance spot welding.

The hot stamping process is beneficial for fabricating high strength automotive parts without spring back. To suppress high temperature oxidation and decarburization, it is necessary to coat the hot stamping steel. In the present work, the performance of galvannealed coated (GA) hot stamped steel was evaluated. During cyclic corrosion tests, the steel substrate began to corrode after the heat treated galvannealed coating fully corroded, because a Zn alloyed coating provides cathodic protection. In order to be applied to the automotive body-in-white, GA coated hot stamped steel must overcome drawbacks such as liquid metal embrittlement (LME) due to the low melting temperature of the Zn, and low or limited weldability due to the thick surface ZnO layer. The results of this study showed that an increase in the α-Fe(Zn) phase fraction of the coating was an effective way to prevent LME. In addition, the resistance spot weldability of GA hot stamped steel can be improved by removing the thick Zn oxide layer by sand blasting.

Hot stamping technology has been steadily developed because it provides both excellent formability and high strength. With the development of TWB hot stamping technology, it is now possible to freely apply the required strength and thickness in the right place. In this study, the microstructure and collision performance of TWB hot stamped parts were evaluated according to their combined strength. Through dilatometry analysis, the hot stamping heat treatment temperatures of 22MnB5 steel and 30MnB5 steel were set at 950 oC and 870 oC. The 5MnB8 steel was composed of Bainite + Martensite when heat-treated at 950 oC and provided a strength of 980 MPa grade. When heat-treated at 870 oC, it was composed of Ferrite + Bainite + Martensite and provided a strength of 780 MPa grade. Simulated rear impact testing showed the 30MnB5- 5MnB8 TWB combination had the best performance, because the 5MnB8 part of the 780 MPa grade absorbed enough energy and the 30MnB5 part of the 1.8 GPa grade fully served as an anti-intrusion.