STEEL FOR WELDED STRUCTURE EXCELLENT IN TOUGHNESS AT LOW TEMPERATURE, FRACTURE TOUGHNESS, AND FATIGUE CHARACTERISTIC, AND ITS PRODUCTION

摘要

PROBLEM TO BE SOLVED: To provide a steel product which is remarkably improved in toughness at a low temp. and fracture toughness (generation characteristic and arresting property of brittle fracture and crack) of the steel by modifying a steel structure in the surface layer and the rear surface layer part of a steel plate into superfine grains and also is improved in fatigue characteristic (fatigue strength and corrosion fatigue strength) by increasing the texture density of the steel. ;SOLUTION: This steel has a composition consisting of, by weight, 0.04-0.2% C, 0.01-1.0% Si, 0.3-2.0% Mn, ≤0.025% P, ≤0.025% S, and the balance iron with inevitable impurities. Further, the structure of this steel is constituted of a structure where, in the region of ≥5% of the thickness of the steel plate in the surface layer and the rear surface layer part of the steel plate, ≤0.5 μm cementite phase is present in the grain boundaries and/or subgrain boundaries and which has 5% fraction of pearlite and is composed essentially of ferrite or bainite of ≤2.57 μm average grain size.;COPYRIGHT: (C)1999,JPO