METHOD OF MANUFACTURING THICK STEEL PRODUCT OF HIGH STRENGTHAND HIGH TOUGHNESS HAVING EXCELLENT WELDABILITY AND MINIMAL VARIATION OF STRUCTURE AND PHYSICAL PROPERTIES

摘要

A method of manufacturing a thick steel product of high strength and high toughness having excellent weldability with minimal variation of material properties, comprises heating a steel raw material to the temperature of Ac3 to 1350.degree.C, hot rolling and then cooling at the cooling rate of 10 .degree.C/sec. or less. The steel raw material has the following composition: C: 0.001 - 0.025 wt%; Mn: 1.0 - 3.0 wt%; Ti: 0.005 - -0.20 wt%; Nb: 0.005 - 0.20 wt%; B : 0.0003 - 0.0050 wt%; and Al: 0.01 - 0.100 wt% balance substantially Fe and incidental impurities. The composition has a transformation start temperature (Bs) of 670.degree.C or less. Since the steel product obtained by the method has no variation in physical properties regardless of variation in cooling rate, it is possible to supply steel products of high strength and high toughness which have uniform microstructure and properties along their thickness direction and are excellent in weldability.