| 摘要 |
PROBLEM TO BE SOLVED: To provide a heat treatment method of steel materials capable of obtaining a duplex structure of martensite and bainite that can attain both strength and toughness without using a salt used for a conventional austempering treatment, especially a duplex structure excellent in strength while the toughness of bainite structure is the same level as that of a conventional austempering treatment, and having an extremely simple process.SOLUTION: A heat treatment method includes: a step of heating a steel material higher than an austenitizing temperature and austenitizing the steel material having a chemical composition containing, by mass%, C of 0.25 or more and 1.0 or less, Si of 1.0 or less, Mn of 0.3 or more and 2.0 or less, P of 0.03 or less, S of 0.01 or less, sol. Al of 0.3 or less, and N of 0.01 or less, and the balance being Fe; a step of, by using a heat treating oil having a cooling rate faster than an upper critical cooling rate of the steel material, charging the austenitized steel material into the heat treating oil maintained at an oil temperature of a range from a temperature Ms(°C) shown by following formula (1) to the oil temperature of (Ms-80) (°C), and rapidly cooling the steel material to the oil temperature to harden the steel material; and a step of maintaining the rapidly cooled steel material at the oil temperature for a predetermined period, wherein the formula (1) is expressed by: Ms(°C)=550-361×[C]-39×[Mn]+30×[Al], in the formula, [C], [Mn], and [Al] respectively show C, Mn, and sol. Al contents (mass%) in the steel material. |
