Development of 780 MPa Grade High Strength Hot Rolled Steel Sheet

The fatigue strength of the high strength steel sheet for truck frame use was investigated. The fatigue strength of the steel sheet with scale increases with the decrease in the roughness of the steel sheet surface. The fatigue strength of the sheared edge increases with the decrease in the roughness of the fractured surface. In case of the microstructure consisting of pearlite and coarse carbides, the fatigue strength of the sheared edge decreases. Based on these findings, two types of the 780MPa grade high strength hot rolled steel sheet with high fatigue strength as well as good formability have been developed.