JFE Steel Corporation announced that its 1.5 GPa grade, 1470 Mega Pascal, high tensile strength cold rolled steel sheets are now being utilized in vehicle body structural parts, the world’s first such adoption in a cold press forming application. This constitutes the highest strength of vehicle body structural parts obtained through cold press forming. The steel sheets are being utilized in several applications by automakers, and JFE Steel has begun supplying these sheets to auto parts suppliers.
In order to protect vehicle occupants in the event of a collision and improve fuel economy through weight reductions, efforts are on going to increase the strength of vehicle body structural parts. JFE Steel’s 1.5 GPa grade high-tensile strength cold-rolled steel sheets are already being utilized in parts with simple shapes, such as bumpers and door impact beams. However, the adoption of high tensile strength cold-rolled steel sheets for vehicle body structural parts with complex shapes has been limited to the 1310MPa grade until now, because increasing the strength of sheets can result in decreased cold press formability and delayed fracture resistance. To overcome with these issues, the adoption of 1.5 GPa grade high-tensile strength steel sheets through a hot press forming process2 is becoming more widespread. However, this requires the steel to be heated to high temperature and then maintained at constant temperatures for cooling, which decreases the number of parts that can be pressed per unit of time. Improvements have therefore been desired from a number of perspectives, including manufacturing costs.
With the new 1.5 GPa grade steel sheets, the high cooling capacity of the proprietary water quenching method based continuous annealing process line located at JFE’s West Japan Works in Fukuyama District was used to reduce the addition of alloy elements and minimize non uniformity of the steel sheet microstructure. As a result, particularly high yield strength and delayed fracture resistance were simultaneously realized even with the 1.5 GPa grade high tensile strength steel sheets while maintaining cold press formability equivalent to that of 1310 Mega Pascal grade sheets. This enabled the utilization of the 1.5 GPa grade high tensile strength steel sheets in vehicle body structural parts through a low cost cold press forming process.