Effect of Pre-Strain and Baking Temperature on Bake-Hardening Behaviour of BH220 Steel

Light weighting has been one of major driver in automotive industry for few decades. Today when automobile industry is in the transition from internal combustion engine to electric vehicles it becomes even more dominant driver. Many high strength or advanced high strength steels are used in different parts of automotive body for down-gauging and light weighting. BH 220 steel is used in automotive skin panels for its bake hardening property. BH220 provides excellent combination of formability during stamping process and dent resistance in skin panel parts post painting and baking cycle. This material uses CED oven temperature for baking and provide bake hardening effect (BH effect/BH Index) in parts due to increase in yield strength by 35-70 MPa. Current national and international standards specify requirement on BH Index at 170°C for 20 minutes with 2% pre-strain. In order to optimize paint shop CED oven baking temperature, study carried out to know baking temperature effect on BH Index at different pre-strain. In this study effect of baking temperature & %pre-strain on BH Index is studied. For Bake hardening study, tensile samples are prepared, pre-strained to 0, 0.5,1 and 2 % and subjected to bake hardening at different temperature in 130-170°C range for 20 minutes. Tensile testing is performed to evaluate BH Index for these samples. Based on study empirical relation is generated, that can provide BH Index on different baking temperature. The result shows increase in BH Index with same pre-strain however rate of increment is less post 150°C baking temperature. Additionally, BH Index reduces with increase in pre-strain with same baking temperature.