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Deep drawing quality high strength (400 MPa grade) cold-rolled steel sheet for automotive use has been developed. This steel sheet is characterized by its chemical composition of extra low carbon, high phosphorus and small niobium together with aluminum. It exhibits very high r̄-value and elongation in tensile test and also an excellent deep draw-ability in small cup test and actual press test of automotive panel parts. The resistance to cold-work embrittlement are improved by controlling the compositions and the cooling rate after final annealing. Spot weldability and the weld performances are very good like as ordinary mild steel sheets. These properties are compared with those of DDQ grade mild steel sheet and previously developed high strength dual phase or rephosphorized steel sheets.

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.