STEEL MATERIAL EXCELLENT IN FATIGUE CRACK PROPAGATION PROPERTY, MANUFACTURING METHOD THEREFOR, AND DETERMINATION METHOD OF STEEL MATERIAL EXCELLENT IN FATIGUE CRACK PROPAGATION PROPERTY

摘要

PROBLEM TO BE SOLVED: To provide a high strength steel material excellent in fatigue crack propagation property.;SOLUTION: There is provided a high strength steel material having a structure containing a composition containing, by mass%, C: 0.02 to 0.4%, Si: 0.01 to 1.0%, Mn: 0.5 to 3.0%, P: 0.05% or less, S: 0.05% or less, Sol.Al: 0.10% or less, and the balance: Fe with inevitable impurities, and a bainite phase and/or a martensite phase having the total area percentage of over 50% as main phase and balance other phase (including 0%), and having excellent low temperature toughness with yield strength: 325 MPa or more and vTrs of -20°C or less. The fatigue crack propagation property of steel material is improved by setting a ratio between crack tip plastic region dimension during propagating fatigue crack γ_p^*(={(K_Imax)²×10⁶/(2πσ_Y²)}×(1-2ν)²(μm)) and an effect crystal unit MU_eff, γ_P^*/MU_eff at 10 or less. Also quality of the fatigue crack propagation property of a composite structure steel material is determined by γ_P^*/MU_eff.;COPYRIGHT: (C)2015,JPO&INPIT