Thick steel board

Abstract

The invention provides a thick steel board exhibiting low tensile ratio and high toughness, which contains following elements: 0.03-0.15 weight percent of C, less than 1.0 weight percent of Si (0 weight percent not included), 1.0-2.0 weight percent of Mn, less than 0.015 weight percent of P (0 weight percent not included), less than 0.010 weight percent of S (0 weight percent not included); 0.005-0.060 weight percent of Al, 0.008-0.030 weight percent of Ti; 0.0020-0.010 weight percent of N, and less than 0.010 weight percent of O (0 weight percent not included). The microscopic constitution at the t / 4 position of the steel board comprises mixed constitution of ferrite and bainite, wherein t represents thickness of the steel board. Island martensite is dispersed in the bainite, the average particle diameter of the ferrite is 10-50 mu m, and the island martesite in the bainite is 1-20 area percent of the total area.

Description

technical field [0001] The invention relates to a thick steel plate suitable for construction, marine structures, line pipes, shipbuilding, civil engineering, construction machinery and the like, especially relates to a thick steel plate with low yield ratio and high toughness. Background technique [0002] In recent years, among various steel materials for welded structures, in addition to high strength and high toughness, it is also required to reduce the yield strength ratio YR (YR = YS / TS). In general, it is known that the yield ratio of the steel material can be reduced by making the metal structure of the steel material into a hard phase in which bainite, martensite, etc. are moderately dispersed in a soft phase such as ferrite. [0003] As a production method for obtaining the above-mentioned structure in which the hard phase is moderately dispersed in the soft phase, for example, in Patent Document 1, there is known a heat treatment method in which quenching (Q) and...

AI Technical Summary

Problems solved by technology

[0009] However, among the technologies proposed so far, among the processing of circular steel pipes with a plate thickness of 40 mm or more that require high processing degrees (D / t<10, D: steel pipe diameter mm, t: plate thickness), etc., It is difficult to make extremely low yield ratio (YR<70%) and high toughness (vTrs<-20°C) coexist

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