Development of Cold Rolled High Strength Steel Sheet with Excellent Deep Drawability

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.