| 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(={(K)×10/(2&pgr;&sgr;)}×(1-2&ngr;)(μm)) and an effect crystal unit MU, γ/MUat 10 or less. Also quality of the fatigue crack propagation property of a composite structure steel material is determined by γ/MU. |
