Development of 780 MPa grade high-strength, hot-rolled steel sheet 2000-05-0050

Increase of load capacity for trucks by weight reduction of the body has been required to cut down transportation costs. The weight reduction of truck body is feasible in terms of the application of high-strength steel sheet to body. Especially, the application to truck frame is important because it is heavy. The required properties of steel sheet for truck frame use are press formability and fatigue strength. Press formability can be optimized based on the previous investigation results. On the other hand, investigations on fatigue strength are relatively few, and the value of fatigue strength is not sufficient. Accordingly, the fatigue strength of high-strength steel sheet for truck frame use was investigated in this study.

As for fatigue strength, the fatigue strength of steel sheet with scale is significant because the steel sheet is painted without pickling to remove the surface scale. It was clarified that the fatigue strength of the steel sheet with scale increased with the decrease in the roughness of the steel sheet surface. On this basis, the surface quality of the steel sheet was improved. The fatigue strength of sheared edges is also important since truck frame has punched holes. The fatigue strength of the sheared edge deteriorates with the increase in the fractured plane roughness of the sheared edge. In case of the microstructure consisting of pearlite and coarse carbides, these phases increase the fractured plane roughness of the sheared edge through the connection of the micro-voids arising around then. Therefore, the reduction of these phases is significant for improving the fatigue strength.

Based on these findings, three types (dual phase type, precipitation hardening type and bainite single phase type) of the 780 MPa grade high- strength, hot-rolled steel sheet with high fatigue strength as well as good formability have been developed.