Newly Developed Hot Rolled High Strength Steel Sheets and Their Formability

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.