Patent ID: 8828159

Claim:
A method of production of an automobile chassis part superior in fatigue characteristics, the method comprising producing a steel pipe of a hot-rolled plate of a steel material having a composition of, by mass %, C: 0.05 to 0.23%, Si: 0.05 to 1.0%, Mn: 0.3 to 2.0%, P: 0.03% or less, S: 0.01% or less, Nb: 0.02 to 0.1%, Mo: 0.2 to 0.5%, Sol. Al: 0.01 to 0.05%, N: 0.006% or less, and a balance of Fe, and press forming the steel pipe to give a cross-sectional bending radius of the plate outer surface of 2 to 5 times a plate thickness, then annealing the steel pipe 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 indicates the time (h), and the temperature rise is 660° C.), such that (Nb, Mo)C precipitates are produced, and a Vicker's hardness at a center of the plate thickness is lower than a maximum value of a Vicker's hardness within 0.5 mm of the plate surface by 50 to 150 points.