Effect of Tool Coatings and Tool Steels on Formability of Advanced High Strength Steels

To improve the formability of advanced high strength steels, the interaction between steel sheet, tool material and tool coating was investigated. Square cup drawing experiments were conducted to determine the range of binder forces for forming good cups without wrinkling or splitting. Binder Span of Control (BSC) tests were conducted for DP590, TRIP590, DP780, DP780 EG and DP980 using three uncoated tool steels and two coatings on a standard tool steel substrate. The experimental results indicate that the binder span for forming good cups is sensitive to the choice of tool material and tool coating and the effect of lubricant on formability also varies with tooling material and coating. The obtained binder spans were compared and the best coating plus tool steel combinations for steels of different grades were identified. In addition, roughness of the tooling surface was measured before and after stamping. The tooling behavior during stamping and the roughness change after stamping depend not only on steel grades and coating but also tooling material and coating. The ranking of surface roughness of different tooling is consistent with their overall performance on binder span. The understanding on the effects of tool material and coating will help to provide solutions to formability issues in the application of advanced high strength steels.