Steel for low-temperature service having excellent surface processing quality

Abstract

Steel for low-temperature service having a high degree of surface processing quality comprises: manganese (Mn): 15 wt % to 35 wt %, carbon (C) satisfying conditions of: 23.6C+Mn≧28 and 33.5C−Mn≦23, copper (Cu): 5 wt % or less (excluding 0 wt %), nitrogen (N): 1 wt % or less (excluding 0 wt %), chromium (Cr) satisfying a condition of: 28.5C+4.4Cr≦57, nickel (Ni): 5 wt % or less, molybdenum (Mo): 5 wt % or less, silicon (Si): 4 wt % or less, aluminum (Al): 5 wt % or less, and a balance of iron (Fe) and inevitable impurities. Stacking fault energy (SFE) of the steel is 24 mJ / m2 or greater. The SFE is calculated by a formula: SFE (mJ / m2)=1.6Ni−1.3Mn+0.06Mn2−1.7Cr+0.01Cr2+15Mo−5.6Si+1.6Cu+5.5Al−60(C+1.2N)1 / 2+26.3(C+1.2N)(Cr+Mn+Mo)1 / 2+0.6[Ni(Cr+Mn)]1 / 2.

Description

TECHNICAL FIELD[0001]The present disclosure relates to steel for low-temperature service having a high degree of surface processing quality, and more particularly, to steel having a high degree of surface quality after being processed and usable for low-temperature service or structures such as liquefied gas storage tanks or transportation facilities in a wide temperature range from a low temperature to room temperature.BACKGROUND ART[0002]Steels for manufacturing containers such as liquefied natural gas (LNG) containers or liquid nitrogen containers, marine structures, or structures for use in the Polar Regions are required to have sufficient toughness and strength at very low temperatures. To this end, steels for low-temperature service are used. Steels for low-temperature service are required to have low thermal expansion and thermal conductivity in addition to having a high degree of low-temperature toughness and strength. In addition, magnetic properties of such steels are fact...

AI Technical Summary

Benefits of technology

The patent text states that by adjusting the composition of steel and the ranges of alloying element contents, the stacking fault energy (SFE) of steel is increased, which improves surface processing quality regardless of abnormal grain formation. The technical effect is the improvement of steel quality through a simple composition adjustment.

Problems solved by technology

However, aluminum alloys are expensive, and since aluminum alloys have a low degree of strength, it is required to increase design thicknesses of structures to be formed of aluminum alloys.

In addition, aluminum alloys have a low degree of weldability.

Thus, aluminum alloys are used in limited applications.

Cr—Ni-based stainless steel and 9% Ni steel may incur high manufacturing costs because of the use of expensive nickel (Ni) and the necessity of additional heat treatment processes.

In addition, welding materials for Cr—Ni-based stainless steel and 9% Ni steel are also required to have a large amount of expensive nickel (Ni).

Thus, the application of Cr—Ni-based stainless steel and 9% Ni steel is limited.

However, according to the techniques disclosed in Patent Documents 1 and 2, expensive nickel (Ni) is still used, and a heat treatment process and a tempering process are repeated in many steps to guarantee cryogenic toughness.

That is, the techniques are not advantageous in terms of costs and the complexity of processes.

However, these techniques may increase costs and significantly burden heat treatment equipment because a heat treatment process is repeated many times. Therefore, another technique for guaranteeing cryogenic toughness has been developed.

However, this affects the recrystallization behavior of austenite, and thus partial recrystallization and non-uniform growth of grains are caused in a general rolling temperature range.

As a result, some austenite grains grow excessively, and thus severe unevenness is observed in the size of austenite grains in the microstructure.

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