| 摘要 |
PROBLEM TO BE SOLVED: To obtain a high tensile strength steel plate having good HAZ toughness even in welding in the large amt. of welding heat input by allowing it to have a specified chemical componential compsn. and also specifying the content of TiN. SOLUTION: This steel plate is the one having chemical components of, by weight, 0.03 to 0.2%, C, <=0.4% Si, 0.5 to 2% Mn, <=0.015% P, <=0.006 S, <=0.01% Al, 0.007 to 0.025% Ti, 0.0001 to 0.003% Mg <=0.004% O and 0.002 to 0.006% N, contg. one or more kinds among Ca, REM and Zr respectively by 0.0003 to 0.003%, in which the effective TiN content calculated by the formulae I to IV in accordance with the compsns. of O, Mg, Ca, REM and AL is >=0.007%, and the balance iron, and, moreover, as for the average compsns. of oxides of 0.5 to 10μ, Ti<=20%, Mg>=5%, REM+Ca+Zr>=5% and Mg+REM +Ca+Zr>=20% are controlled. In this way, the high tensile strength steel plate of a 500 to 600 Pa class having good HAZ toughness even in large heat input welding in which the amt. of welding heat input exceeds 20 kJ/mm can be obtd.
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