Heavy plate excellent in haz toughness and mother plate toughness

Abstract

The invention provides a thick steel plate. The thick steel plate has a great HAZ toughness in the low heat energy input welding and super high heat energy input welding; and the parent material also has a great toughness. The thick steel plate contains C: 0.030-0.10%, Si: below 0.8-2.0%, P: below 0.03%, S: below 0.01%, Al: 0.01-0.10%, Nb: 0.005-0.035%, Ti: 0.015-0.03%, B: 0.0010-0.0035% and N: 0.0050-0.01%; the grain diameter of the old austenite crystal is below 120 um; the island-shaped martensite (MA) is below 3%; in addition, the invention satisfies the following formulas (1) (2): [Ti] / [N] is more than or equal to 1.5 and less than or equal to 4 (1); X value is more than or equal to 40 and less than or equal to 160 (2); the X value=500[C]+32[Si]+8[Mn]-9[Nb]+14[Cu]+17[Ni]-5[Cr]-25[Mo]-34[V].

Description

technical field The present invention relates to thick steel plates suitable for welded structures such as ships and marine structures. Preferably, it is a thick steel plate that has excellent toughness of the heat-affected zone (Heat Affected Zone: HAZ) after welding and excellent toughness of the base material even when welding is performed with a wide range of heat input from low heat input welding to ultra-high heat input welding. Background technique In recent years, for example, container ships and the like have been increased in size, and thick steel plates having a plate thickness of 60 mm or more have been used. In order to efficiently weld such a thick steel plate, it is required to have excellent HAZ toughness not only in low heat input welding but also in super high heat input welding with a heat input of 50KJ / mm or more. However, when performing ultra-high energy heat input welding, since the HAZ is heated to a high-temperature austenite region and then slowly...

AI Technical Summary

Problems solved by technology

However, in Patent Document 3, the precipitation amount of TiN and BN is not sufficient, or is not fine, or due to insufficient Nb, the hardenability is lowered, and the ferrite becomes coarse, so there is room for further improvement of the HAZ toughness.

In addition, the toughness of the base metal is not considered

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