The growing need for more fuel-efficient and eco-friend vehicles to reduce energy consumption and CO_2 emission is a great challenge for the automotive industry. Al, especially, is one of the most powerful competitors to conventional steels for automotive exposed panel because of its several characteristic properties such as high strength stiffness to weight ratio, and good formability. In general, bake-hardenable steel sheets with 340MPa tensile strength have been applied, so far, to automotive exposed panels such as doors, hoods, etc. However, new kinds of steels with better mechanical properties and thin thickness for automotive exposed panels should be developed to satisfy the "Light-Weight Design of Car Body". We have successfully been developed a new bake-hardenable steel sheets with 390MPa having a dual phase microstructure (ferrite and martensite). This newly developed steels have not only superior mechanical properties such as tensile strength, BH value, non-aging phenomena, and work-hardenability. Steels for exposed panel applications should fulfill several required properties, for example, high dent-resistance (good BH), non-aging at room temperature, superior surface quality for exposed application, and adaptable press-formability. Conventional BH steels with ferritic single microstructure can easily have good surface quality and press-formability, but it is difficult to obtain high BH value and superior non-aging property at the same time. It is, on the other hand, very well-known that DP steels could have higher BH and non-aging properties simultaneously because of high mobile dislocation density caused by the martensitic transformation. However, it is hard to get equivalent surface quality and press-formability compared with conventional BH steels since DP steels should have higher amount of hardenability agent, especially Mn. Therefore, it is very important for DP steels having good combinations of mechanical properties and press-formability to be applied exposed panel sheets. We have been able to increase the elongation of newly developed 390MPa grade steels compared to the existing ones through microstructural control (optimization of ferrite grain size and fraction, and uniform distribution of martensitic islands, etc.) and chemical composition adjustment (ultralow carbon and Mn/Cr addition concept). As a result, the press formability applicable to automotive exterior panels could be secured. In addition, optimization of various process conditions is important in order to ensure the quality of the Zn-coated surface that can be applied to the exposed panels with high alloying contents. If the appropriate SRT and FDT conditions are set in the hot rolling process, grain boundary oxidation and by internal oxidation caused by high amount of Si and Mn can be suppressed to obtain desired Zn-coated surface quality.
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