HIGH-STRENGTH THICK STEEL PRODUCTS EXCELLENT IN TOUGHNESS AND WELDABILITY, HIGH-STRENGTH ULTRA-THICK H SHAPE STEEL AND PROCESSES FOR MANUFACTURING BOTH

摘要

A high-strength thick steel product excellent in toughness and weldability, which is reduced in the contents of C and N and contains Si, Mn, Nb, Ti, B and O in proper amounts while satisfying the relationship: C - N/7.74 = 0.004 (wherein C and Nb are their respective contents) and which has a density of Ti-containing oxide particles whose diameters are 0.05 to 10µm of 30 to 300 particles/mm2 and a density of Ti-containing oxide particles whose diameters are above 10µm of 10 particles/mm2 or below. The steel product can be manufactured by subjecting a slab formed through preliminary deoxidization for the adjustment of dissolved oxygen content to 0.005 to 0.015mass%, the addition of Ti, vacuum degassing treatment for 30 minutes or longer, smelting and continuous casting to heating to 1100 to 1350°C, hot rolling into a product having a thickness of 40 to 150mm and then cooling.