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A wider usage of hot rolled high strength steels (HRHSSs) are intensively studied for more complicated underbody structural parts of cars now. With an emphasis placed on the improvement of sheared-edge flangeability, we have developed new types of HRHSSs ranging from 440N/mm2 to 690N/mm2 in ultimate tensile strength (UTS). The microstructure and chemistry of these steels were controlled: The hard second phase in the polygonal ferrite matrix was controlled both by adjusting the chemical composition, mainly, silicon-content, and by applying thermomechanical treatment to a hot strip mill. Compared to conventional high strength steels such as a high-carbon, low-silicon steel and a niobium-bearing steel, various formability-performances of the developed steels were found to be good.

Recently, many kinds of coated steel sheets are developed and used for automobile panels. Some of them show higher coefficient of friction and poorer formability than a cold rolled steel sheet. New polymer lubricating films are developed and proved to improve frictional characteristics of coated steel sheets in this study. The developed polymer lubricating films are applied to Electro Galvanized steel sheet and Hot Dip Galvannealed steel sheet, and coefficient of friction and formability of them are measured by using model dies. Different coefficients of friction are obtained by a change in a combination of components in a polymer lubricating film. As a result, the most suitable coefficient of friction is able to chose in forming conditions by the developed polymer lubricating films.