Effect of Hot Stamping Heat Treatment Temperature on Resistance Spot Weldability of Al-10% Si Coated 30MnB5 Steel

Abstract

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 oC achieved better performance than the specimen heat treated at 950 oC. There was a coating microstructure difference between the specimens heat treated at 850 oC and 950 oC whereas both specimens had similar microstructure and mechanical properties of their substrates. The Fe-rich layer of the specimen heat treated at 950 oC was twice as thick as the specimen heat treated at 850 oC because the diffusion rate was much higher at 950 oC. The thick Fe-rich layer deteriorated the resistance spot weldability due to high electrical resistivity during resistance spot welding. Key words: hot stamping, Al-10%Si coating, 30MnB5, resistance spot welding, dynamic resistance