Development and Application of High-Strength Steel for Auto Outer Panel in Baosteel

The characteristics of China's automotive steel sheets and high strength steels (including bake-hardening, high strength IF and isotropic steel) developed recently by Baosteel for automotive outer panels are briefly reviewed. Among these steels, bake-hardening (BH) steel is often used to make outer panel parts, such as hoods, decklids, and front and rear doors. In this paper, we present results from an investigation of the effect of baking process (temperature and time) on the bake hardening index value of BH220 steel, and through evaluation of tensile specimens cut from a production hood and simulated part. The results show that BH value is more than 30 MPa when the baking time is more than 2 minutes (at a temperature of 170°C) or the baking temperature is more than 90°C (for baking time of 20 minutes). Measurements on an actual hood and simulated part showed that baking has no effect to enhance the yield strength of the panel, but it can enhance the dent resistance. One needs to consider other factors when using the empirical equation (DR∝K×t^a×YS) to evaluate dent resistance of automotive panels.