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Fatigue properties in various types of high strength steel sheets for automotive use and their spot and arc welded joints were investigated. In steel sheets and strain-aged sheets, fatigue limit stresses and notch sensitivities depend on not only the tensile strength but also the yield strength. Two methods for improving fatigue strength of spot welded high strength steel joints were found. One is to generate moderate expulsion during welding with a current just above the upper lobe curve and another is to conduct a controlled tempering treatment during the welding schedule. By these methods, fatigue strengths at 107 cycles of spot welded high strength steel joints increase by about twice as compared with those by the conventional welding method.

A new hot rolled high strength sheet steel has been developed by utilizing controlled cooling technique after hot rolling to a 0.05wt%C-1.5wt%Mn-0.08wt%P steel. Phosphorus enhances ferrite transformation at higher temperature, which results in a fine dispersion of austenite phase during the cooling step after hot rolling. At lower temperature, P retards austenite decomposition and enhances martensite transformation when the strip is coiled at 250°C or below. This new high strength steel exhibits low yield-to-tensile strength ratio of 60-70%, while in conventional high strength steels, for example, a 0.15wt%C-1.5wt%Mn steel hot rolled and coiled at 400°C or below after rapid cooling, the yield ratio is about 75%. Besides it exhibits higher ductility, good stretch-flangeability, good fatigue property, good weldability and extremely high bake-hardenability compared to the C-Mn steel. Embrittleness due to segregation of P to grain boundaries was not observed in sheets and welded joints.

Spot weldabilities and fatigue properties of hot-rolled high strength sheet steels for wheel use, with tensile strength of 45 to 60 kgf/mm2 were investigated. The specimens used were phosphorus-niobium bearing steel (P-Nb steel), phosphorus-manganese bearing steel (P-Mn steel) and conventional carbon-manganese bearing steel (C-Mn steel). The P-Nb steel and the P-Mn steel have been developed by the authors for wheel rim use and wheel disc use, respectively. The P-Nb steel showed higher static strengths in welded joints compared with other steels, which is important for steels used for rims. Fatigue strengths of spot welded joints were compared using fracture mechanics. It is confirmed that application of fracture mechanics is a powerful method to assess fatigue strength of various types of spot welded joints. High fatigue strength after strain-aging is required for steels used for wheel disc use.

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.