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.