| 摘要 |
PROBLEM TO BE SOLVED: To provide a steel material having high strength with tensile strength of 900 MPa or more and excellent in extensibility and impact property.SOLUTION: A steel material contains a chemical composition containing, by mass%, C:0.050 to 0.40%, Si:0.50 to 3.0%, Mn:3.0 to 8.0%, P:0.05% or less, S:0.01% or less, sol.Al:0.001 to 3.0%, and N:0.01% or less, and a steel structure containing 10 to 40 area% of austenite and having an average C concentration of austenite of 0.30 to 0.60 mass%. The steel material is manufactured by a heat treatment on a steel material having a martensitic single phase structure with an average particle size of old austenite of 20 μm or less including keeping the steel material in a temperature range of 670°C to 780°C and Acfor 5 to 120 seconds and cooling from the temperature range to 150°C with an average cooling rate of 5 to 500°C/sec. |
