| 摘要 |
PROBLEM TO BE SOLVED: To provide a high tensile steel excellent in hot strength and good in weldability and toughness, and to provide its production method. SOLUTION: The high tensile steel is composed of, by mass, 0.05-0.15% C, below 0.6% Si, 0.8-1.6% Mn, below 0.02% P, below 0.01% S, 0.7-1.2% Mo, below 0.06% Al, below 0.006% N, a weld-crack sensitive composition PCM of below 0.2% and the balance Fe with inevitable impurities. In this steel, a micro structure with an area ratio of 80% or more is after than polygonal or pseudopolygonal ferrite, and the average circular equivalent diameter of old-typeγparticle is 150μm or below. This steel contains Cu, Cr, Nb, V, B, Ti, Mg, Ca and REM, according to necessary. Arranging an accumulation of pressure drop at 1000 deg.C or below, to 30% or above, cooling is carried out from 700 deg.C or above after rolling, and hardening and tempering is carried out according to required characteristics.
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