Patent ID: 8778261
Filing Date: 2014-07-15
Classification: B60G,C21D,C22C

Abstract:
1. A hot-rolled steel material for automobile chassis parts superior in fatigue characteristics, comprising 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 unavoidable impurities, wherein a plate of the hot-rolled steel material is press formed to give a cross-sectional bending radius of the plate outer surface of 2 to 5 times the plate thickness such that a Vicker's hardness at a center of a 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, and wherein the press-formed plate of the hot-rolled steel material is then annealed under conditions giving a tempering parameter λ, defined by λ=T(20+log(t)), of 14000 to 19000 such that (Nb, Mo)C precipitates are produced, where T is absolute temperature, t is time (h), and an increase in temperature during annealing is 660° C.