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Three types of hot-rolled high strength steel sheets having tensile strengths of 600 to 800MPa for wheel rim use were developed. For a fundamental study the effects of chemical compositions on the formability in stretch-flanging and bending on the flash butt welded joints were investigated. In precipitation-hardened steels, the main controlling factors are identified as a newly proposed carbon equivalent value, Mn/Si ratio and S content in steel. P-bearing steels and intercritically heat-treated dual-phase steel hoops are able to be used for rim production. Examples of commercial production of rims are presented.

Metallurgical factors affecting the press formability of the newly developed high strength sheet steels have been discussed. Dual phase steels, CHLY(cold-rolled sheet steel) and HTP-D(as-hot-rolled sheet steel), produced by control of cooling conditions after intercritical annealing or hot-rolling exhibit very low yield to tensile strength ratio and high n-value. Rephosphorized extra-low carbon steel, CHRX, produced by continuous annealing with rapid cooling is characterized by its extremely high r-value and low yield strength. CHLY, HTP-D and CHRX have large bake hardenability. Precipitation hardened steel, HTP-F(hot-rolled sheel steel), produced by adjusting carbon equivalent and sulfur content is suitable for manufacturing wheel rims owing to its good formability after flash butt welding.