Frictional behaviors of a mild steel and a TRIP780 steel under a wide range of contact stress and sliding speed

Abstract

The application of advanced high-strength steels (AHSS) generally makes it necessary to use higher tool-sheet contact pressures compared with those used for forming low-strength steel, and it leads to significant changes in frictional behavior, which in turn change the final product characteristics. In order to understand frictional behaviors between steel sheets and tool materials under high contact stresses present in real stamping conditions, a novel friction tester was conceived, fabricated, and used. This tester can generate high normal loads, as high as 625 MPa, whereas traditional friction testers were limited to 10 MPa or less. A mild steel and a TRIP780 steel were paired with Cr-coated D2 tool steel, and friction behaviors were observed under various conditions, including the use of two lubricants, wide ranges of sliding speeds, and normal contact stresses. The coefficient of friction (COF) decreased at a low contact pressure as the sliding velocity increased. The contact pressure had a significant effect, albeit too complex to be explained by simple models. It was also evident that lubricant effects must be studied coupled with the contact pressure and sliding speed. In a nonlubricated condition at normal stresses roughly half of the steel's yield strength, the friction event caused plastic deformation that reached up to 0.2 mm from the surface. In this deformed region, the amount of retained austenite in the TRIP steel decreased substantially, and significant residual compressive stress, reaching 350 MPa, also developed in the ferrite phase (plus a minor amount of martensite, which is undistinguishable from ferrite by the X-ray diffraction method used herein). The magnitude of change of friction constant due to changes in contact conditions was enough to significantly affect springback of automotive body panels.