| 摘要 |
PURPOSE:To obtain the high tensile steel which is less in the softening of a heateffected-zone and excellent in the weldability by blending C, Si, Mn, Al, N, Nb and B of the specified quantity to steel and making the carbon equivalent a specified range. CONSTITUTION:The composition of steel is made to 0.07-0.23 C by wt%, 0.05-0.35 Si, 0.2-1.3 Mn, 0.01-0.08 Al, <0.004 N, 0.004-0.04 Nb and 0.0005-0.002 B and the carbon equivalent Ceq shown in a formula is 0.22-0.34 and the balance is Fe and the inevitable impurities. Billet is produced by casting continuously this steel and the rolling of billet is started from >=900 deg.C temp. or when it is cooled at <=900 deg.C, the rolling is started after reheating it >=1,050 deg.C. The rolling is performed so that at least >=20% cumulative rolling reduction is secured at 800-900 deg.C temp. range and the billet is acceleratedly cooled in >=3 deg.C/sec average cooling velocity after finishing the rolling at >=800 deg.C.
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