Low-yield ratio and high-strength steel having excellent stress corrosion cracking resistance and low temperature toughness

Abstract

An aspect of the present invention relates to a low-yield ratio and high-strength steel having excellent stress corrosion cracking resistance and low temperature toughness, the steel comprising, by weight, 0.02-0.10% of carbon (C), 0.5-2.0% of manganese (Mn), 0.05-0.5% of silicon (Si), 0.05-1.0% of nickel (Ni), 0.005-0.1% of titanium (Ti), 0.005-0.5% of aluminum (Al), 0.005% or less of niobium (Nb), 0.015% or less of phosphorus (P), 0.015% or less of sulfur (S), and the balanced amount of Fe and inevitable impurities, the microstructure of which comprises, by area, 60% or more of acicular ferrite and a balanced amount of one or more phases of bainite, polygonal ferrite and martensite-austenite constituent (MA).

Description

technical field [0001] The invention relates to a high-strength steel with low yield ratio and excellent stress corrosion cracking resistance and low-temperature toughness. Background technique [0002] The steel used for liquefied gas storage tanks varies according to the type of liquefied gas, but the liquefaction temperature of the gas is generally low temperature at normal pressure (LPG is -52°C), so not only the base metal is required to have excellent low temperature toughness, but also welding Some also have excellent low temperature toughness. [0003] In addition, it is known that liquefied ammonia (LAG) can cause stress corrosion cracking (SCC, Stress Corrosion Cracking) of steel, so IGC CODE (International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk) stipulates that oxygen partial pressure, temperature, etc. The operating conditions are limited, while the Ni content of the steel is limited to less than 5%, and the actual yiel...

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Problems solved by technology

[0008] However, in general, if a large amount of Ni is added, many austenite phases with an easily deformable FCC lattice structure will be generated due to the narrow atomic distance. When repeated stress and corrosion environments act on this easily deformable FCC lattice When the structure is prone to corrosion, resulting in cracking

Therefore, the above-mentioned invention has the following problems: due to the high content of high-priced Ni, the economic efficiency is reduced, and the resistance to stress corrosion cracking (SCC) is reduced.

However, although this technology can achieve low temperature toughness and low yield ratio, it is difficult to achieve low temperature toughness and low yield ratio at the same time.

[0010] In addition, as a method of improving the strength of another characteristic steel required for steel, there are, for example, precipitation strengthening, solid solution strengthening, and martensitic (Martensite) strengthening, etc., but these methods increase the strength, but cause deterioration of toughness and elongation.

[0011] In addition, when various manufacturing conditions are used to refine the grains to increase the strength, not only high strength can be obtained, but also the deterioration of toughness can be prevented due to the decrease of the impact toughness transition temperature, but the grain refinement leads to an increase in the yield strength, thereby surpassing ammonia The upper limit of yield strength for stress corrosion cracking (SCC) is 440MPa, making it difficult to ensure a low yield ratio

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