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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.

Two types of TS 450 to 600 MPa grade hot rolled high strength steels have been developed whose strength increases by a post-deformation heat treatment. One is a nitrogen added steel, produced by a low temperature coiling method. This steel shows an increase in tensile strength by about 60 to 90 MPa after a conventional baking treatment. The other is a Cu-bearing steel, produced also by a low temperature coiling method. This steel shows much larger increase in tensile strength by about 200 MPa due to the precipitation hardening of copper, but it requires an additional heat treatment at a relatively high temperature such as 600° C.

In order to help the gauge reduction of wheels and chassis parts of automobiles, a form-able and weldable hot rolled steel of 550 MPa grade, named FDP-55, has been developed. FDP-55 is an 0.14% C, 0.1% Si, 1.1% Mn and Nb free A1-killed steel obtained by controlled-cooling to a low coiling temperature on a runout table, and it is featured by ferrite-degenerated pearlite microstructure. Results of co-operative works with automotive makers showed that FDP-55 was successful in the application to wheels and chassis parts attaining the large weight reduction. This paper reports the metallurgical features and characteristics of the steel.