Uncoated advanced high strength steels (AHSS) are being used for non-exposed automotive applications where high strength is required. Due to higher contact pressures and friction forces between the tooling and sheet steel, increased die wear, die build-up and sheet steel surface galling have been observed. Although various countermeasures have been adopted in production to address these issues, the stamping die tryout process has not been revised accordingly. Proper heat treatment of uncoated tryout dies is required to prevent wear in stamping uncoated, cold-rolled AHSS. Whether die build-up and sheet steel surface galling occur depends on the steel surface condition. The intent of this study is to investigate the effects of steel surface topography on die build-up, sheet steel surface galling and friction in forming uncoated cold-rolled AHSS. DP590 and DP980 with different thickness and different finish rolled textures were tested with the bending under tension (BUT) tester to study these effects. A reciprocal cyclic bend test system (CBTS) of modifying bending under tension test was used to investigate wear and build-up between different surface texture steels in a production condition. Results indicate that the finish roll surface texture is found to be the root cause for the die build-up and steel surface galling that occurs during forming with uncoated dies. The ground-roll surface is the worst in terms of galling and build-up while the Electro-Discharge Textured (EDT) surface is the best. The surface parameters Skewness (Rsk) and Kurtosis (Rku) are found to be able to assess steel galling and die build-up behaviors for uncoated cold rolled AHSS using the uncoated D2 die. A surface with Rsk larger than −0.5 and Rku less than 3 can prevent galling and die build-up in the die tryout process, which is recommended to finish roll the uncoated cold rolled AHSS.
Transcription of Roll Surface Texture onto Deforming Material in Temper Rolling