| 摘要 |
PROBLEM TO BE SOLVED: To easily and simply produce a steel material which has excellent strength, hardness and toughness, and is hard to be cracked on heating. SOLUTION: Raw material steel is coated with a boron compound or is surrounded thereby (a first stage S1). Concretely, a coating film consisting of h-BN or the like is formed on the surface of the raw material steel, or the raw material steel is surrounded by powder of B4 C or the like. In this state, the raw material steel is nitrided by a nitriding gas while heating the same (a second stage S2). In this way, B using the boron compound as a source and N using the nitriding gas as a source respectively diffuse into the raw material steel to obtain a steel material containing B and N. The greater parts of B and N enter into solid solution into the metallic structure composing the steel material, and are present in the steel material in a state of Fe (B, N) based solid solution or Fe (C, B, N) based solid solution. Further, the conditions of the heating and nitriding are set so that the contents of B and N are respectively controlled to, by weight, 7 to 30 ppm, and 10 to 70 ppm.
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