Abstract
Increase of load capacity for trucks by weight reduction of the body has been required. The weight reduction 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. However, investigations on fatigue strength are relatively few, and the value of fatigue strength is not sufficient. Therefore, the fatigue strength of 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 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 through the connection of the micro-voids arising around them. Accordingly, the reduction of these phases is significant for improving the fatigue strength. Based on these findings, three types of the 780MPa grade high strength hot rolled steel sheet for truck frame use have been developed.