Investigation of tool damage in shear cutting of ultra-high strength sheet steels

Proportions of high- and ultra-high strength steels are constantly increasing in modern car bodies. In the press shop there are uncertainties concerning the application of steels with strength higher than 1000MPa particularly concerning shear cutting processes. To support the steel user regarding a tool material and coating and the wear behavior, long-term shear cutting test had been carried out. The effects of various shear cutting conditions on tool damage were investigated by continuous shear cutting test with an amount of cutting cuts up to 100,000 times. Tested sheet materials were 780, 1'180 and 1'470MPa grade steels. The changed cutting conditions were sheet steel strength, tool material, cutting clearance, and rake angle. Measurements by laser and confocal microscopes were carried out in order to reveal tool damage mechanisms. The features of tool damage were classified as wear, chipping, and plastic deformation. The amount of plastic deformation rises with the increase of sheet steel strength. Not only plastic deformation but also chipping in the tested tool was observed in the cutting test of 1180MPa sheet steel by standard tool steel (DIN 1.2379) after 40'000 cuts. Large plastic deformation in the tested tool was observed in a cutting test of 1470MPa sheet steel by high-speed steel (DIN 1.3343) after 100'000 cuts although no chipping was detected. Under a rake angle of 5 degrees, serious chipping and tool wear were observed especially near the final sheet-tool contact position. Regarding clearance, the amount of plastic tool deformation increased with the decrease of the clearance. This effect of the clearance can be explained by vertical force and by sheet rotation during cutting.