| 摘要 |
PROBLEM TO BE SOLVED: To provide a high strength steel material excellent in fatigue crack propagation property and having excellent balance of properties such as toughness, ductility and weldability and a manufacturing method therefor.SOLUTION: There is provided a high strength steel sheet excellent in strength, ductility, toughness, weldability as well as fatigue crack propagation property, having a structure with a predetermined aggregate structure containing over 80% of bainite phase and the balance of one or more kinds of less than 20% (including 0%) of ferrite, pearlite and martensite by conducting heating at 900 to 1300°C, hot rolling at (Arpoint+200°C) to Arpoint with a cumulative draft of ≥50% and then accelerate cooling from a temperature range of (Arpoint-80°C) or more to 600°C or less at an average cooling speed≥5°C/s on a steel raw material having a composition containing, by mass%, C:0.02 to 0.25%, Si:0.01 to 0.50%, Mn:0.5 to 2.0% and Al:0.10% or less and satisfying Ceq≤0.45% and Pcm≤0.28%. |
