Low-temperature steel material having excellent toughness in welding portion thereof and manufacturing method therefor

Abstract

Provided according to a preferable aspect of the present invention are a low-temperature steel material having excellent toughness in a welding portion thereof and a manufacturing method therefor, thelow-temperature steel material comprising, by weight %, 0.02-0.06% of C, 6.0-7.5% of Ni, 0.4-1.0% of Mn, 0.02-0.15% of Si, 0.02-0.3% of Mo, 0.02-0.3% of Cr, 50 ppm or less of P, 10 ppm or less of S,0.005-0.015 % of Ti, 60 ppm or less of N, with a Ti / N weight % ratio of 2.5-4, and the balance of iron (Fe) and other inevitable impurities; and having: an effective grain size of 50 micrometers or less, with a boundary angle found to be 15 degrees or greater as measured by EBSD in an area of a fusion line (FL)-FL+1 mm in a weld heat-affected zone of a weld portion welded at a heat input of 5-50 kJ / cm; and an impact toughness of 70 J or higher at -196 DEG C as measured in an area of fusion line (FL)-FL+1 mm.

Description

Technical field [0001] The present invention relates to a low-temperature steel material with excellent weld zone toughness and a manufacturing method thereof, and more specifically, to a low-temperature steel material with nickel-containing weld zone excellent toughness and a manufacturing method thereof. Background technique [0002] Recently, with the strengthening of environmental restrictions around the world caused by global warming, etc., attention to environmentally friendly fuels is increasing day by day. As a typical environmentally friendly fuel, LNG (Liquefied Natura l Gas: Liquefied Natural Gas) has reduced costs and increased efficiency through the development of related technologies. As a result, the world's LNG consumption continues to increase. In 1980, 6 countries only consumed 23 million tons of LNG. , And its scale is increasing at a rate that roughly doubles every 10 years. With the expansion and growth of this LNG market, LNG producing countries are renovat...

AI Technical Summary

Problems solved by technology

[0006] However, in order to ensure hardenability instead of reducing the amount of Ni added by 20%, it is necessary to add other alloying elements, so the reduction in alloy cost is not large. In addition, some steel companies have introduced the DQLT process instead of the QLT process. Micronization and extremely low temperature rolling are used in rolling before heat treatment, so there is still the problem of a significant drop in rolling productivity

[0007] In addition, in low-temperature Ni steel, the part that most needs to ensure toughness is the welded part. As far as the welded part is concerned, the microstructure of the original base metal changes due to the high-temperature heat input, so it is difficult to ensure the impact toughness.

[0008] As far as the heat affected zone is concerned, in the case of the SCHAZ (Sub-Critical Heat Affected Zone: Subcritical Heat Affected Zone) heated below the Ac3 temperature, only a part of the structure reverses the phase transformation, so due to the additional structure refinement and regression It is easy to ensure toughness due to the fire effect, but in the case of CGHAZ (Coarse Grain Heat Affected Zone: coarse-grained zone in the heat-affected zone), while heating to a high temperature, the original low-temperature rolling and heat treatment have made the base metal finer The microstructure is all coarsened, so it is difficult to ensure the impact toughness. Compared with the original 9% Ni steel, the low-Ni steel with 20% Ni has a problem that the impact toughness of the heat-affected zone is very low due to the decrease in Ni.

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