Frictional Behaviors of a TRIP780 Steel and a Mild Steel under a wide range of Contact Stress and at various Sliding Velocities

Abstract

The use of advanced high strength steels (AHSS) requires significant modification of forming process parameters compared with those used to stamp low-strength steels: such as the use of higher tool-sheet contact pressure, more stretching than deep drawing, measures to control spring-back, etc. Such modifications, especially the higher contact pressure, are expected to change frictional behavior significantly, which in turn change the final product characteristics. In order to understand frictional behaviors between steel sheets and tool materials under high contact stresses found in real stamping operations for AHSS, a novel friction tester was fabricated and used. This tester is free from certain deficiencies associated with previous testers

the occurrence of plowing effect is completely eliminated, and high normal loads, as high as 625 MPa, can be used. A mild steel and a TRIP steel were paired with Cr-coated D2 (SKD11) tool steel, and friction behaviors were observed under various conditions, including the use of lubricants, different sliding velocities and different normal contact stresses. The coefficient of friction (COF) decreased as the sliding velocity increased at low load. The contact pressure had a significant effect, albeit too complex to be explained by simple models. The level of change of COF due to changes in experimental conditions was enough to affect spring-back of automotive body panels significantly. Also, it was apparent that high attention must be given to lubricant effects with respect to the contact pressure. The friction event at normal stresses roughly half of the steel’s yield strength caused plastic deformation that reached up to 0.2mm from the surface. In this deformed region, the amount of retained austenite in the TRIP steel decreased substantially and significant compressive stress, reaching 350 MPa, also developed in the ferrite/martensite phase. Frictional behaviors under various contact stresses with lubrications could be explained by the concept of asperity flattening, localized pick-up of wear debris and lubricant buffering.