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 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.

Effect of Hot-stamping on the Properties Change of Compound Layer in Coated High-strength Steel Plate

“…Regardless of the heating time, all three samples have martensitic structure, and as time increases, the martensitic lath size slightly increases. [18][19][20] It was found that the hot stamped samples consist of a compound layer, a diffusion layer, and a base metal; and as the heating temperature and heating time increase, the thicknesses of the compound layer and the diffusion layer increase.…”

“…In general, when a steel sheet coated with a plating solution containing Al and Si is heat-treated at a high temperature of 850 °C or higher, compounds such as FeAl, Fe3Al, and Fe2Al5 are obtained. [18][19][20] However, the hot stamping of this study obtained compounds of Al5.4Fe structure and Fe2Si structure.…”