Thick steel plate having excellent toughness and reduced softening in weld heat-affected zone

Abstract

A thick steel plate which has superior HAZ(heat-affected zone) toughness(specifically, toughness of a bonding part) and suppressed HAZ softening(specifically, deterioration of hardness at the position that is distanced from the bonding part) at the same time is provided. A thick steel plate having superior toughness and reduced softening in a weld heat-affected zone comprises, by mass percent, 0.030 to 0.080% of C, 1.0% or less(excluding 0%) of Si, 0.8 to 2.0% of Mn, 0.01 to 0.10% of Al, not less than 0.015% to less than 0.030% of Ti, greater than 0.0055% to not more than 0.0100% of N, not less than 0.0015% to less than 0.0035% of B, and 0.015% or less(including 0%) of Nb with the balance of Fe and inevitable impurities, wherein the thick steel plate has 1.5 or more of a flatness ratio(major axis / minor axis) of spherical austenite grains, and satisfies the mathematical expression 1, 2.0<=[Ti] / [N[<=4.0, and the mathematical expression 2, 0<P value<23.0 and P value=2000x[B]+300x([Ti]-3.42x[N])+1000x[Nb], wherein [ ] means the content(mass%) of each element.

Description

technical field [0001] The present invention relates to a thick steel plate suitable for welded structures such as ships and marine structures, and particularly relates to a thick steel plate having excellent toughness in a heat-affected zone (HAZ) after welding and suppressed softening. Background technique [0002] In recent years, thick steel plates used in welded structures such as ships and marine structures have to be welded under stricter conditions than before as the size of welded structures increases. For example, for thick steel plates with a plate thickness of about 60mm or more, Perform ultra-large heat input welding with heat input exceeding 60kJ / mm. Under the above-mentioned heat input conditions, the welding wire (bond) part will be subjected to a thermal cycle in which the thick steel plate is heated to 1400°C and held for 50 seconds, and then cooled from 780°C to 500°C for 500 seconds. . [0003] Then, if such super heat input welding is carried out, sinc...

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Problems solved by technology

However, according to this method, even if the softening of the HAZ is suppressed, there is still a problem that the toughness of the HAZ is lowered because a coarse bainite structure is formed at the welding line portion of the HAZ.

[0015] The aforementioned methods of Patent Documents 1 to 4 are all proposed only from the viewpoint of improving HAZ toughness, and the suppression of HAZ softening is not considered at all.

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