High tension steel plate, welded steel pipe and method for production thereof

Abstract

A high tension steel plate, which has a content of carbon equivalent Pcm represented by the following formula (1) of 0.180 to 0.220 % and a surface hardness of 285 or less in terms of Vickers, and has a structure wherein the percentage of a martensite austenite constituent in a surface layer portion is 10 % or less, the percentage of a mixed structure of ferrite and bainite in a portion inside the surface layer portion is 90 % or more and the percentage of bainite in the mixed structure is 10 % or more, a lath of bainite has a thickness of 1 [mu]m or less and a length of 20 [mu]m or less, and the segregation level, which is the ratio of an Mn concentration in the central segregated portion to an Mn concentration in a part having a depth from the surface of 1 / 4 of the thickness of the plate, is 1.3 or less. Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B (1) wherein each symbol in the formula (1) represents the mass % of each element. The high tension steel plate has a yield strength of 551 MPa or greater and a tensile strength of 620MPa or greater, and is excellent in toughness, high-speed ductile fracture characteristics and weldability.

Description

technical field [0001] The present invention relates to a high-tensile steel plate, a welded steel pipe, and a manufacturing method thereof. More specifically, it relates to a high-tensile steel plate, a welded steel pipe, and a manufacturing method thereof used for pipeline pipes for transporting natural gas and crude oil, various pressure vessels, etc. . Background technique [0002] In pipeline pipes used for long-distance transportation of natural gas, crude oil, etc., improvement in transportation efficiency is required. In order to improve the transport efficiency, it is necessary to increase the operating pressure of the line pipe, but it is also necessary to increase the strength of the line pipe in response to the increase in the operating pressure. [0003] If the wall thickness of the line pipe is increased, the strength of the line pipe increases, but the efficiency of welding work on site decreases as the wall thickness increases. In addition, since the weight...

AI Technical Summary

Problems solved by technology

However, since the pipeline pipe disclosed in this document does not specify a manufacturing method, it is considered that segregation and coarsening of crystal grains may occur

In this case, the high-speed ductile fracture stop characteristic is reduced

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