| 摘要 |
PROBLEM TO BE SOLVED: To produce a high tensile strength steel material with low yield ratio for construction use, excellent in fracture resistance, by reducing the amounts of impurities, such as. P, S, and O, and solid-solution N in the steel and forming the structure in a specific region of the surface layer part of the steel material into superfine-grained structure. SOLUTION: A steel stock, having a composition which contains, by weight, 0.01-0.15% C, 0.01-1.0% Si, 0.1-2.0% Mn, 0.003-0.1% Al, and 0.001-0.006% N and satisfies N(%)-Al(%)/3.0<=0 and in which the contents of P, S, and O as impurities are regulated to <=0.01%, <=0.01%, and <0.006%, respectively, is roughed at a temp. in the austenitic region. After heat treatments of cooling and recuperation are applied to the part in the region of specific thickness from the surface layer of the resultant steel bloom, finish rolling is performed. Successively, the surface layer region is recuperated and further subjected to cooling treatment under specific conditions. By this procedure, the average crystalline grain size in the central part of the steel material is regulated to <=30μm and the structure of 10-60% of the steel material is formed into martensitic structure 7 and further, the structure in the region of 10-33% thickness from the surface layer is formed into superfine, grained structure of <=3μm average ferrite grain size. By this method, the high tensile strength steel material with low yield ratio, increased in safety by cyclic plastic strain, can be obtained.
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