STEEL FOR AUTOMOBILE UNDERCARRIAGE COMPONENT EXCELLING IN FATIGUE PERFORMANCE AND PROCESS FOR MANUFACTURING AUTOMOBILE UNDERCARRIAGE COMPONENT USING THE STEEL

摘要

The present invention provides a steel material for automobile chassis parts which has high fatigue characteristics, does not require much cost for heat treatment, and further is superior in shapeability and a method of production of automobile chassis parts using this steel material, that is, one being a steel material to which Nb and Mo have been compositely added and having a difference 50 to 150 points between a Vicker's hardness of the center of plate thickness and a maximum value of Vicker's hardness within 0.5 mm from the surface after bending by a bending R of the plate outer surface of 2 to 5 times the plate thickness. The surface is high in hardness and the center part is low in hardness, so the fatigue characteristics and shapeability are superior. Note that if annealing under conditions giving a tempering parameter λ defined by λ=T(20+log(t)) of 14000 to 19000 (where T is the absolute temperature, t is the time (h), and the temperature rise is 660° C.), it is possible to relieve the internal stress and further improve the fatigue characteristics.