Thick steel sheet having excellent welding heat-affected part toughness

Abstract

A thick steel sheet according to the present invention has a specified chemical composition and contains oxides, wherein constituent elements for the oxides excluding oxygen fulfil the formulae: 2% < Ti < 40%, 5% < Al < 30%, 5% < Ca < 40%, 5% < REM < 50%, 2% < Zr < 30% and 1.0 ≤ REM / Zr in mass%, and wherein, among the particles of the oxides, the number of particles each having an equivalent circle diameter of less than 2 μm is 300 particles / mm2 or more and the number of particles each having an equivalent circle diameter of 2 μm or more is 100 particles / mm2 or less, and wherein, among particles of a Ti nitride, the number of particles each having an equivalent circle diameter of 1 μm or more is 7 particles / mm2 or less and the number of particles each having an equivalent circle diameter of 20 nm or more is 1.0 × 106 particles / mm2 or more, and wherein the relational formula: |da-df| / da ≤ 0.35 is fulfilled.

Description

technical field [0001] The present invention relates to thick steel plates suitable for welded structures such as bridges, high-rise buildings, ships, and transmission line pipes. More specifically, it relates to heat-affected zones (hereinafter also referred to as HAZ) after large heat input Thick steel plate with excellent toughness. Background technique [0002] In recent years, with the increase in the size of welded structures such as bridges, high-rise buildings, ships, and pipelines, thick steel plates with a plate thickness of 50 mm or more are often used in such welded structures, so the thickness of the plate thickness of 50 mm or more Welding of steel plates is inevitable. There is also a thicker actual situation, and large heat input welding for the purpose of improving welding construction efficiency is required. [0003] However, the HAZ during high heat input welding is kept in the high-temperature austenite (γ) region for a long time due to heating, and the...

AI Technical Summary

Problems solved by technology

Through these technologies, thick steel plates with excellent toughness in welded heat-affected zones can be realized, but there are still problems that should be improved in terms of manufacturing.

[0012] In the technique described in Patent Document 11, in order to realize a prescribed oxide form, the amount of Ca addition is controlled based on the amount of dissolved oxygen before Ca addition, but at the same time it is necessary to limit the time from Ti addition to Ca addition to within 3 to 20 minutes, May increase operator's burden

On the other hand, in the technique described in Patent Document 12, it is necessary to hold for 40 minutes to 90 minutes from the time of Ca addition to pouring, and there is a point of improvement in productivity.

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