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The effect of a concentration of stress at a welded joint on the fatigue limit was investigated from the point of view of the effective design of automotive parts made of high strength hot rolled steel sheets. The stress concentration at the welded joint was evaluated using FEM analysis and the effect of stress concentration on endurance limit was investigated by fatigue tests using specimens of practical welded joints and specimens with the simulated configuration of welded joints. It is pointed out that another effect besides stress concentration and residual stress should be considered to evaluate the endurance limits for high tensile strength materials of over 590 N/mm2.

The Effects of microstructures on stretch flangeability and stretchability in hot rolled high strength steel was investigated. Stretch flangeability of multi-phase steels was inferior to that of single phase steels at a given tensile strength, although stretchability of the former was superior to that of the latter. In multi-phase steel, the main factor affecting stretch flangeability is the hardness ratio of ferrite to hard phase, on the other hand, that which most affects stretchability is the volume fraction of ferrite. According to these results, several types of high strength hot rolled steel sheets with excellent formability were developed.

High-strength cold-rolled steel sheets having tensile strength of from 350 to 1,500 MPa with a wide variety of features have been developed by using the continuous annealing process consisting of water-quenching system. Newly developed high-strength steels include dual-phase steels with high bake-hardenability, dual-phase steels with low yield ratio and excellent ductility, dual-phase steels with high deep drawability, ultra high-strength steels and extra deep drawing high-strength steels. This paper describes the metallurgical characteristics of the products of the continuous annealing process consisting of water-quenching system, processing conditions and mechanical properties of these high-strength steels, and their application to automobiles.