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

摘要

A steel for automobile undercarriage component that excels in fatigue performance, not needing much cost in heat treatment and also excels in molding processability; and a process for manufacturing an automobile undercarriage component with the use of the steel. There is provided a steel doped with Nb in combination with Mo, wherein after bend forming such that the bend radius (R) at plate external surface becomes 2 to 5 times the thickness of the plate, the difference between the Vickers hardness at the center of the plate thickness and the maximum value of Vickers hardness within 0.5 mm from the surface ranges from 50 to 150 points. As the surface hardness is high with the hardness of the center part lowered, excellent fatigue performance and molding processability can be attained. Further, when annealing is carried out under conditions such that temper parameter (lambda) defined by lambda=T(20+log(t)) [wherein T: absolute temperature, t: time (h), and temperature upper limit: 660°C] ranges from 14,000 to 19,000, a relaxation of internal stress can be achieved to thereby attain a further enhancement of fatigue performance.