High-strength steel material having outstanding ultra-low-temperature toughness and a production method therefor

Abstract

The present invention provides steel containing manganese and nickel that is used as a structural material for a cryogenic storage container for liquefied natural gas (LNG) or the like, and a manufacturing method thereof; and more particularly, to steel having good cryogenic temperature toughness and also high strength by adding low-cost Mn instead of relatively expensive Ni at an optimized ratio, refining a microstructure through controlled rolling and cooling, and precipitating retained austenite through tempering, and a manufacturing method of the steel. To achieve the object, the technical feature of the present invention is a method of manufacturing high-strength steel with cryogenic temperature toughness. In the method, a steel slab is heated to a temperature within a range of 1,000 to 1,250° C., wherein the steel slab includes, by weight: 0.01-0.06% of carbon (C), 2.0-8.0% of manganese (Mn), 0.01-6.0% of nickel (Ni), 0.02-0.6% of molybdenum (Mo), 0.03-0.5% of silicon (Si), 0.003-0.05% of aluminum (Al), 0.0015-0.01% of nitrogen (N), 0.02% or less of phosphorous (P), 0.01% or less of sulfur (S), with a remainder of iron (Fe) and other unavoidable impurities. Then, the heated slab is finish-rolled at a temperature of 950° C. or less at a rolling reduction rate of 40% or more. The rolled steel is cooled to a temperature of 400° C. or less at a cooling rate of 2° C./s or more. Thereafter, the steel is tempered for 0.5-4 hours to a temperature within a range of 550 to 650° C. after the cooling.