Improvement of fatigue strength of high strength hot rolled steel sheet for truck frame use

Weight reduction of a truck body brings about increase of load capacity and diminution of fuel consumption. 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 in this study. The fatigue strength of steel sheet with scale is significant for truck frame use. It increases with the decrease in the surface roughness of steel sheet. The fatigue strength of sheared edge is also important. It increases with the decrease in the roughness of sheared edge. In case of the microstructure including pearlite and coarse carbides, these phases increase the roughness of sheared edge through the connection of micro-voids arising around them. Accordingly, the reduction of these phases is significant for improving the fatigue strength of sheared edge. Based on these findings, three types of the 780 MPa grade high strength hot rolled steel sheet were manufactured. From this result, it was suggested that the effect of the roughness reduction on improving the fatigue strength was stable.