Cryogenic high-tensile thick steel sheet and method for producing same

Abstract

The present invention addresses the problem of providing a cryogenic steel sheet having a Ni content of less than 9% and also having both of toughness at the same level or a higher level as or than that of a 9%-Ni steel sheet and excellent cold workability. A cryogenic high-tensile thick steel sheet having a specified ingredient component, wherein the micro-structure at a position of one-fourth the thickness of the steel sheet comprises (1) a matrix composed of tempered martensite or a matric composed of tempered martensite and bainite and (2) retained austenite dispersed in the matrix, the content ratio of the retained austenite at a position of one-fourth the thickness of the steel sheet is more than 11% by volume and equal to or less than 20% by volume, and the content ratio of the retained austenite at a position of one-fourth the thickness of the steel sheet becomes more than 11% by volume and equal to or less than 20% by volume after the steel sheet is subjected to such a subzerotreatment that the steel sheet is retained for 15 minutes in liquid nitrogen at -196 DEG C.

Description

technical field [0001] The present invention relates to a high-tensile high-tensile steel plate for ultra-low temperature, particularly a high-tensile high-tensile steel plate for ultra-low temperature that is excellent in ultra-low temperature toughness and cold workability, and can be suitably used for applications such as tanks for storing liquefied natural gas (LNG). In addition, the present invention relates to a method of manufacturing the above-mentioned high-tensile-strength steel plate for extremely low temperature use. Background technique [0002] Tanks for LNG storage often require a high degree of safety. Therefore, steel sheets for low temperature used in tank bodies are required to have excellent toughness at the temperature at which LNG becomes a liquid (approximately -162°C). In addition, severe processing such as forming into a cylindrical pipe is performed in the manufacture of the tank, and thus cold bending workability is also required for the steel she...

AI Technical Summary

Problems solved by technology

[0003] However, Ni is an expensive alloy element, and therefore, from the viewpoint of cost reduction, development of a low-temperature steel sheet with a Ni content of less than 9% and toughness equivalent to or higher than that of a 9% Ni steel sheet is strongly desired.

[0004] Generally, when the Ni content of the low-temperature steel sheet is reduced, the austenite becomes unstable in the low-temperature region, so the low-temperature toughness decreases, and it is difficult to ensure the high level of safety required for LNG storage tanks

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