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 ap plication to truck frame is important because it is heavy. The required pro perties of steel sheet for truck frame use are press formability and fatigu e strength. However, investigations on fatigue strength are relatively few, and the value of fatigue strength is not sufficient. Therefore, the fatigu e strength of high strength steel sheet for truck frame use was investigate d in this study. The fatigue strength of steel sheet with scale is significant for truck fra me use. It increases with the decrease in the surface roughness of steel sh eet. The fatigue strength of sheared edge is also important. It increases w ith the decrease in the roughness of sheared edge. In case of the microstru cture including pearlite and coarse carbides, these phases increase the rou ghness of sheared edge through the connection of micro-voids arising around them. Accordingly, the reduction of these phases is significant for improv ing the fatigue strength of sheared edge. Based on these findings, three ty pes of the 780 MPa grade high strength hot rolled steel sheet were manufact ured. From this result, it was suggested that the effect of the roughness r eduction on improving the fatigue strength was stable.