| 摘要 |
PROBLEM TO BE SOLVED: To produce a hot rolled high tensile strength steel plate excellent in strength, ductility, toughness and fatigue characteristics as well in a steel in which the addition of Cr, Ni and Mo causing a problem in the production cost is evaded and a trace amt. of Nb is added. SOLUTION: This steel plate has a compsn. contg., by weight, 0.01 to 0.20% C, <=1.00% Si, <=2.00% Mn, <=0.10% Al, <=0.0070% N, 0.0050 to 0.15% Nb, and the balance substantial Fe with inevitable impurities. The steel plate has a mixed structure in which the area ratio of fine ferrite in which the average grain size of ferrite is regulated to 2 to 3μm is regulated >=70%, the area ratio of the structure contg. bainite and martensite is regulated to <=20%, and the balance area is composed of ferrite having <=10μm average grain size.
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