High-strength steel plate and process for production thereof, and high-strength steel pipe

Abstract

The invention provides a high-strength steel plate which exhibits excellent cutting-crack resistance, Charpy absorption energy and DWTT characteristics, a low yield ratio and a tensile strength of 900 MPa or above; a process for the production thereof; and high-strength steel pipes made by using the same. The high-strength steel plate is one which contains by mass C: 0.03 to 0.12%, Si: 0.01 to 0.5%, Mn: 1.5 to 3%, Al: 0.01 to 0.08%, Nb: 0.01 to 0.08%, Ti: 0.005 to 0.025%, N: 0.001 to 0.01%, and at least one of Cu: 0.01 to 2%, Ni: 0.01 to 3%, Cr: 0.01 to 1%, Mo: 0.01 to 1%, and V: 0.01 to 0.1% and satisfies the following relationship (1) as to the contents of Ca, O and S, whose microstructure is composed of ferrite and a hard second phase at an area fraction of ferrite of 10 to 50% with the cementite contained in the hard second phase having a mean particle diameter of 0.5[mu]m or below, and in which the content of Nb contained as carbide in the steel is at most 10% based on the whole Nb content of the steel. 1 < (1 - 130 [O]) [Ca] / (1.25 [S]) < 3 ... (1).

Description

technical field [0001] The invention relates to a thick steel plate for high-strength pipeline steel pipes used for transporting natural gas or crude oil and a manufacturing method thereof. In particular, it is excellent in crack resistance against cutting when it is cut in shear processing, high toughness, especially DWTT (Drop Weight Tear Test, drop weight tear test) characteristics are excellent, and the yield ratio (yield strength divided by tensile strength) The value of strength) is less than 0.85 and the tensile strength is more than 900MPa. The steel plate for high-strength pipeline steel pipe with low yield ratio, its manufacturing method, and the high-strength steel pipe manufactured using the steel plate. Background technique [0002] In recent years, pipeline steel pipes used to transport natural gas or crude oil have been strengthened year by year in order to increase the transmission efficiency through high pressure, or to improve the efficiency of on-site weld...

AI Technical Summary

Problems solved by technology

High deformability is required for a steel plate with a tensile strength of less than 900 MPa, but a yield ratio of 0.85 or less cannot be obtained

[0006] On the other hand, as in Patent Document 2, in the case of heat treatment after accelerated cooling, since the hydrogen in the steel is sufficiently diffused, shear cracking can be suppressed, but cementite is precipitated and coarsened in the microstructure during the heat treatment. , thereby reducing the toughness, especially the DWTT (Drop Weight Tear Test) characteristic for evaluating the brittle crack propagation stop characteristic is deteriorated

In addition, since the purpose of Patent Document 2 is to have high deformability, a yield ratio of 0.85 or less cannot be obtained.

[0007] In addition, the technology described in Patent Document 3, as described in this document, responds to the requirement of high deformation ability to prevent cracks even if the pipeline steel pipe is greatly deformed due to a large earthquake or ground fluctuation in a permafrost zone. Basically, the purpose is to reduce the yield ratio (YR) which divides the yield strength by the tensile strength. However, in this technology, the base material of the steel pipe has a second phase, so that the Charpy absorbed energy decreases, and it is difficult to say that it is caused by an external accident. The crack propagation stop characteristic of the ductile failure (brittle failure test) is carried out by applying static or dynamic load on the test piece or test object that has been subjected to the formation of notches or the processing that can replace it.

In this test, brittle cracks are generated due to impact loads, and the propagation stop characteristics of brittle fractures at various temperatures are found to be excellent, and the first phase is a ferrite structure, so a tensile strength of 900 MPa or more cannot be obtained