| 摘要 |
PROBLEM TO BE SOLVED: To provide a steel having a superfine structure and combining high strength with high toughness, and its production. SOLUTION: In this production of the steel with superfine structure, a hypo- eutectoid steel containing >=0.02 wt.% C and having <=40μm average grain size is heated up to the ferrite-austenite two-phase temp. region where the fraction of austenitic structure becorries 20 to <80%, hot-rolled, and cooled and the above treatment is performed one or more times and then cooling is applied, and this steel has a structure of <=2μm average grain size. In this steel, the principal structure is composed of a two phase structure of bainite.martensite and worked ferrite. Furthermore, >0.5 wt.% Ni is added to the above. Tempering is carried out at a temp. not higher than the Ac1 transformation point. By this treatment, the steel with superfine structure which cannot possibly be obtained by the conventional methods can be obtained.
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