CN109266959B

CN109266959B - Low-temperature steel having excellent surface-finish quality

Info

Publication number: CN109266959B
Application number: CN201811116893.0A
Authority: CN
Inventors: 李淳基; 徐仁植; 李学哲; 朴仁圭
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2013-12-25
Filing date: 2014-12-19
Publication date: 2022-05-24
Anticipated expiration: 2034-12-19
Also published as: CN105849302A; KR101543916B1; KR20150075315A; JP2017507249A; WO2015099363A1; JP6615773B2; CN109266959A; EP3088555A4; US20160319407A1; EP3088555A1; CN105849302B; WO2015099363A8

Abstract

The present invention relates to a low-temperature steel that can be used at a wide range of temperatures from low temperatures to room temperature in liquefied gas storage tanks, transportation facilities, and the like, and provides a low-temperature steel that has excellent surface finish quality even after a processing step such as drawing, and a method for producing the same.

Description

Low-temperature steel having excellent surface-finish quality

The present application claims priority of korean patent application No. 10-2013-0163369, filed 12/25/2013; this application is a divisional application of chinese patent application No. 201480070844.1 filed on 12/19/2014.

Technical Field

The present invention relates to a steel for low temperature use having excellent surface quality, and more particularly, to a steel for low temperature use which can be used at a wide range of temperatures from low temperature to room temperature in liquefied gas storage tanks, transportation facilities, and the like, and which has excellent surface quality after processing.

Background

Steel materials used for storage containers for liquefied natural gas, liquid nitrogen, and the like, marine structures, and polar region structures should be low-temperature steels capable of maintaining sufficient toughness and strength even at ultra-low temperatures. Such a steel for low temperature is required to have not only excellent low temperature toughness and strength but also a small thermal expansion rate and thermal conductivity, and is a steel in which magnetic characteristics are also considered.

As a material usable at a low temperature in a liquefied gas environment, conventionally, a Cr — Ni stainless steel such as AISI304, a 9% Ni steel, a 5000 series aluminum alloy, or the like has been used. However, the aluminum alloy is expensive in material cost, increases the design thickness of the structure due to low strength, and is poor in welding workability, so that its use is limited. On the other hand, Cr — Ni stainless steel, 9% Ni steel, and the like contain expensive nickel and require additional heat treatment, which not only increases the manufacturing cost, but also has a problem in that the welding material contains a large amount of expensive nickel, which is widely used.

In order to solve these problems, as a technique for reducing the content of expensive nickel by adding manganese, chromium, or the like, patent document 1 (korean laid-open patent No. 1998-0058369) and patent document 2 (international patent publication No. WO 2007-080646) can be cited, for example. Patent document 1 is a technique for improving the cryogenic temperature toughness by reducing the nickel content to 1.5 to 4%, and adding 16 to 22% of manganese and 2 to 5.5% of chromium to secure the austenite structure, and patent document 2 is a technique for reducing the nickel content to about 5.5%, and adding 2.0% or less of manganese and 1.5% or less of chromium to each, and repeatedly performing heat treatment and tempering to refine ferrite grains and secure the cryogenic temperature toughness. However, the above-mentioned patent documents 1 and 2 still contain expensive nickel, and are disadvantageous in terms of cost and simplification of the process because a plurality of steps of repeated heat treatment and tempering are performed to secure the extremely low temperature toughness.

Another technique relating to structural steels used in liquefied gases is so-called nickel-free (Ni-free) high manganese steel, which completely excludes nickel. The high manganese steel is classified into ferrite type and austenite type according to the amount of manganese added. For example, patent document 3 (U.S. patent No. 4257808) discloses a technique of adding 5% of manganese in place of 9% of nickel, repeatedly heat-treating the mixture 4 times in a two-phase region temperature range where austenite and ferrite coexist to refine crystal grains, and then tempering the refined grains to improve the cryogenic temperature toughness. Further, patent document 4 (korean laid-open patent No. 1997-0043149) is a technique of adding 13% of manganese, and repeatedly heat-treating it 4 times in a two-phase region temperature zone of austenite and ferrite to refine grains, and then tempering to improve the cryogenic temperature toughness. In patent documents 3 and 4, ferrite is a main structure, and ferrite grains are refined by repeating heat treatment and tempering 4 or more times in order to obtain extremely low temperature toughness. However, this technique has a problem that the number of heat treatments increases, which increases the cost, and the load on the heat treatment equipment increases. Therefore, a technique for obtaining ultra-low temperature toughness has been developed in which austenite (or a mixed structure of austenite and e-martensite) is used as a main structure, instead of ferrite.

In the case of a steel for low temperature use having austenite as a main structure, although austenite can be stabilized by adding a large amount of carbon and manganese, this affects the recrystallization behavior of austenite, and only a specific small number of austenite grains are excessively grown in a conventional rolling temperature range due to partial recrystallization and non-uniform grain growth, resulting in serious unevenness in the size of austenite grains in a fine structure.

Disclosure of Invention

Technical problem to be solved

The present invention provides a low-temperature steel having excellent surface-finished quality even after drawing and bending.

Means for solving the problems

The present invention is achieved by a low-temperature steel having excellent surface finish quality, the low-temperature steel including 15 to 35 wt% of manganese (Mn), carbon (C) satisfying the condition ranges of 23.6C + Mn ≥ 28 and 33.5C-Mn ≤ 23, more than 0 to 5 wt% or less of copper (Cu), more than 0 to 1 wt% or less of nitrogen (N), chromium (Cr) satisfying the condition ranges of 28.5C +4.4Cr ≤ 57, 5 wt% or less of nickel (Ni), 4 wt% or less of silicon (Si), and 5 wt% or less of aluminum (Al), with the balance being iron (Fe) and other unavoidable impurities.

Advantageous effects

The present invention can provide a steel material having excellent surface finish quality even when abnormally coarse grains are formed inside the steel material by adjusting the composition and composition range of the steel material to increase Stacking Fault Energy (Stacking Fault Energy).

Drawings

FIG. 1 is a photograph showing a microstructure of a conventional steel material in which austenite grains are coarsened to form abnormally coarse grains.

FIG. 2 is a photograph taken after drawing the conventional steel material of FIG. 1, showing surface unevenness of the steel material.

FIG. 3 is a photograph showing a microstructure of a steel material according to an embodiment of the present invention in which austenite grains are coarsened to form abnormally coarse grains.

FIG. 4 is a photograph taken after drawing the steel material of FIG. 3 according to one embodiment of the present invention, showing a uniform surface.

Fig. 5 is a graph showing the ranges of carbon and manganese controlled in the present invention.

Best mode for carrying out the invention

The present invention relates to a low-temperature steel having excellent surface finishing quality even after a drawing or bending process, and a method for manufacturing the same, and is not concerned with the formation of abnormally coarse crystal grains in a steel material.

Generally, the deformation behavior of an austenite structure having a high carbon and manganese content is achieved by slippage and twinning, unlike general carbon steel, and although the initial deformation is achieved mainly by slippage as uniform deformation, twinning as non-uniform deformation occurs thereafter. The main variables of the stress required to produce twins are the stacking fault energy and the grain size as a function of the added elements, in particular, the larger the grain size, the less the stress required in twinning, so that twins are easily produced even under slight deformation. When a small number of coarse crystal grains are present in the fine structure, twin deformation occurs in the coarse crystal grains at the initial stage of deformation, resulting in uneven deformation, and therefore, the surface properties of the material deteriorate, and thickness unevenness of the final structure is induced. In particular, when a structure is required to achieve pressure resistance by ensuring a uniform steel thickness as in a low-temperature pressure vessel, a significant problem arises in structural design and use. Therefore, in the case of a steel material in which a fine structure is austenitized by adding carbon and manganese, the surface finish quality can be improved by solving surface unevenness caused by early twin deformation of coarse crystal grains.

Therefore, a steel material containing a large amount of carbon and manganese causes partial recrystallization of an austenite structure and grain growth in a normal rolling temperature range, and abnormally coarse austenite is generated. Generally, the critical stress required for the formation of the twin crystal is higher than that in the case of the slip, but when the crystal grain is large due to the above-mentioned reasons, the stress required for the formation of the twin crystal is reduced, and the twin crystal deformation is generated at the initial stage of the deformation, and therefore, the surface quality is deteriorated due to the discontinuous deformation. In the present invention, even when an abnormally large austenite grain is generated, the generation of a deformation twin can be suppressed by increasing the critical stress required for the twin deformation.

The steel for low temperature use of the present invention having excellent surface finish quality will be described in detail below.

The steel for low temperature use which has excellent surface processing quality comprises 15 to 35 wt% of manganese (Mn), carbon (C) satisfying the condition ranges of 23.6C + Mn ≥ 28 and 33.5C-Mn ≤ 23, more than 0 wt% to 5 wt% or less of copper (Cu), more than 0 wt% to 1 wt% or less of nitrogen (N), chromium (Cr) satisfying the condition ranges of 28.5C +4.4Cr ≤ 57, 5 wt% or less of nickel (Ni), 4 wt% or less of silicon (Si), 5 wt% or less of aluminum (Al), and the balance of iron (Fe) and other unavoidable impurities.

Further, the Stacking Fault Energy (SFE) of the low-temperature steel of the present invention calculated from the following relational expression 1 should be 24mJ/m²The method comprises the following steps:

[ relational expression 1]

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，

[ wherein Mn, C, Cr, Si, Al, Ni, Mo and N in each formula represent the weight% of each component ].

A steel material having a high manganese content is likely to generate local dislocations (dislocations) due to a low stacking fault energy as compared to a general carbon steel, and the deformation behavior of the steel material is changed due to such high density of local dislocations. Thus, the deformation behaviour of the steel can be varied by controlling the stacking fault energy, which, as a function of the alloying elements, increases or decreases the energy value to a different extent depending on the respective element. The formula 1 is a relational expression showing a change in stacking fault energy according to the content of the added alloying element, and is a relational expression calculated by a calculation value of a conventional theory and various experiments of the present inventors.

Fig. 3 shows a photograph of a microstructure of a steel material according to an embodiment of the present invention satisfying the above-described composition ranges and the conditions of formula 1, and fig. 1 shows a photograph of a microstructure of a conventional steel material. It can be confirmed from both fig. 1 and 3 that the microstructure is formed with abnormally coarse crystal grains.

Fig. 2 is a photograph of the surface of a steel material taken after drawing the steel material having the microstructure of fig. 1, which is a conventional steel material, and it can be confirmed that unevenness has occurred. However, if fig. 4, which is an image of the surface of a steel material obtained after drawing the steel material having the microstructure of fig. 3 according to an embodiment of the present invention, is confirmed, it can be confirmed that the microstructure is not uneven even if abnormally coarse crystal grains are formed, unlike fig. 2.

[ relational expression 2]

As shown in fig. 2, the surface after processing is still uniform according to an embodiment of the present invention. When the steel material is deformed by an external force from the outside, slip is caused by movement of dislocation, and in the case of an austenitic steel material having a high carbon and manganese content, twin deformation is further accompanied by low stacking fault energy, so that deformation caused by slip mainly occurs at the initial stage of deformation, but then twin deformation occurs at the same time when the critical stress required for the twin generation is exceeded. In general, the slip deformation by dislocation is uniform deformation, while the deformation by twin is nonuniform deformation, and in particular, when twin deformation occurs due to coarse crystal grains localized in a part of the steel material, the deformation is accompanied by nonuniformity of the microstructure, which is not preferable in use of the steel material.

Generally, the critical stress required for the generation of the twin crystal is higher than that in the case of the slip, but it is confirmed from the formula 2 that if the size of the crystal grain becomes coarse, the stress for the generation of the twin crystal is reduced, and therefore, the stress required for the formation of the twin crystal is reduced, and the twin crystal is locally generated in the coarse crystal grain at the initial stage of the deformation, thereby causing the degradation of the surface quality due to the discontinuous deformation.

However, as is clear from formula 2, the twin generation stress can be increased by increasing the stacking fault energy, and the excellent surface quality can be obtained even after processing regardless of the formation of coarse crystal grains because the size of the crystal grains is not affected.

The occurrence of twins can be suppressed by maintaining the stacking fault energy calculated by the above formula 1 at a predetermined level or more, and the composition of the steel material having the stacking fault energy at a predetermined level or more can provide a steel for low temperature use having excellent surface quality.

The reasons for limiting the respective compositions of the steel material will be explained below.

Manganese (Mn): 15 to 35% by weight

Manganese is an element that functions to stabilize austenite in the present invention. In the present invention, in order to stabilize the austenite phase at a very low temperature, Mn is preferably contained in an amount of 15 wt% or more. That is, when the content of manganese is less than 15% by weight, if the content of carbon is small, metastable-phase e-martensite is formed, and strain-induced transformation at very low temperature is likely to cause transformation into a-martensite, so that toughness cannot be secured, and in order to prevent this, when attempting to stabilize austenite by increasing the content of carbon, physical properties are rapidly deteriorated by precipitation of carbides, which is not preferable. Therefore, the content of manganese is preferably 15% by weight or more. On the other hand, when the content of manganese exceeds 35 wt%, the corrosion rate of the steel material is lowered, and there is a problem that the economical efficiency is lowered due to the increase of the content. Therefore, the manganese content is preferably limited to 15 to 35% by weight.

Carbon (C): satisfies the conditions that 23.6C + Mn is more than or equal to 28 and 33.5C-Mn is less than or equal to 23

Carbon is an element stabilizing austenite and increasing strength, and in particular, acts to reduce M_sAnd M_dThe action of said M_sAnd M_dIs a transformation point at which austenite is transformed into epsilon martensite or alpha-martensite due to a cooling process or working. Therefore, if the added carbon is insufficient, stable austenite cannot be obtained at ultra-low temperatures due to insufficient stability of austenite, and if the carbon content is too large, toughness is rapidly deteriorated due to precipitation of carbides and workability is deteriorated due to excessive increase of strength, while toughness is also decreased due to easy strain-induced transformation to e-martensite or a-martensite due to external stress.

Particularly in the present invention, the carbon content is determined to be preferable by paying attention to the relationship between elements added together with carbon and others, and for these, the relationship between carbon and manganese in forming carbide, which the present inventors found, is shown in fig. 5. As is clear from the drawing, although carbide is formed by carbon, the formation of carbide is not influenced by carbon alone, but by a recombination action with manganese. The appropriate carbon content is shown in the figure. In the figure, in order to prevent the formation of carbide, it is preferable to control the value of 23.6C + Mn (C, M is the content of each component expressed in wt%) to 28 or more on the premise that the other components satisfy the range specified in the present invention. Which represents the slanted left-hand boundary of the parallelogram region of the figure. When 23.6C + Mn is less than 28, the austenite stability is reduced, and strain-induced transformation is induced by impact at a very low temperature, thereby decreasing impact toughness. When the carbon content is too high, that is, 33.5C-Mn is more than 23, excessive addition of carbon precipitates carbide to lower the low-temperature impact toughness. Therefore, in the present invention, it is preferable to add carbon (C) satisfying the conditions of 23.6C + Mn.gtoreq.28 and 33.5C-Mn.ltoreq.23. As can be seen from fig. 5, the lowest limit of the C content is 0 wt% within the range satisfying the above numerical expression.

Copper (Cu): less than 5 wt% (except 0 wt%)

Copper has very low solid solubility in carbides and slow diffusion in austenite, and thus concentrates on the interface with carbides nucleated by austenite, thereby effectively slowing down the growth of carbides by inhibiting the diffusion of carbon, and finally, has the effect of suppressing the formation of carbides. In the base material, although carbide precipitation can be suppressed by accelerated cooling during the production process, the cooling rate is not easily controlled in the welding heat affected zone, and therefore, copper is added as an element very effective for carbide suppression in the present invention. In addition, copper has an effect of improving the very low temperature toughness by stabilizing austenite. However, when the content of Cu exceeds 5 wt%, there is a problem that hot workability of the steel material is lowered, and therefore, it is preferable to limit the upper limit of Cu to 5 wt%. The content of copper for obtaining the above-described carbide-inhibiting effect is preferably 0.5 wt% or more.

Nitrogen (N): 1% by weight or less (except 0% by weight)

Nitrogen is an element that stabilizes austenite together with carbon to improve toughness, and is particularly, as with carbon, an element that is very advantageous in improving strength by solid solution strengthening. In particular, formula 1 shows that the element is an element that promotes slip by effectively increasing stacking fault energy. However, when more than 1% of nitrogen is added, the stress required for the twin generation will exceed the stress value corresponding to the amount of working of conventional steel materials, and thus it is unnecessary, and since coarse nitrides are formed, there is a problem that the surface quality and physical properties of the steel material deteriorate, and therefore, the upper limit is preferably limited to 1% by weight.

In addition to the above elements, the austenitic steel material of the present invention may contain Cr, Ni, Si, and Al.

Chromium (Cr): 28.5C +4.4Cr is less than or equal to 57

Chromium stabilizes austenite in a suitable amount to improve impact toughness at low temperatures, and is dissolved in austenite to increase the strength of steel. Chromium is also an element that improves the corrosion resistance of steel. However, chromium is a carbide element, and particularly, an element which forms carbide at austenite grain boundaries to reduce low-temperature impact. Therefore, the content of chromium added in the present invention is preferably determined by paying attention to the relationship between carbon and other elements added together, and in order to prevent carbide formation, the value of 28.5C +4.4Cr (C, Cr is the content of each component expressed in weight% units) is preferably controlled to 57 or less on the premise that the other components satisfy the range defined in the present invention. When the value of 28.5C +4.4Cr exceeds 57, it is difficult to effectively suppress the formation of carbide in austenite grain boundaries due to excessive chromium and carbon contents, and therefore, there is a problem that impact toughness at low temperature is lowered. Therefore, chromium satisfying 28.5C +4.4 Cr.ltoreq.57 is preferably added in the present invention.

Nickel (Ni): 5% by weight or less

Nickel is an effective austenite stabilizing element and one that acts to reduce M_sAnd M_dThereby improving the toughness of the steel, said M_sAnd M_dIs a transformation point at which austenite is transformed into e-martensite or a-martensite due to a cooling process or working. In particular, as can be seen from formula 1, nickel is an element that promotes slip by effectively increasing stacking fault energy. However, when nickel is added in an amount exceeding 5 wt%, the stress required for twinning exceeds the stress corresponding to the amount of working of conventional steel materials, and is unnecessary and expensive, and therefore there is a problem of lowering the economical efficiency, and therefore, it is preferable to limit the upper limit to 5 wt%.

Silicon (Si): 4% by weight or less

Silicon is an element that improves castability of molten steel, and particularly an element that, when added to an austenitic steel material, is dissolved in the steel material in a solid state, effectively increasing strength. However, when more than 4% of silicon is added, the stacking fault energy is reduced to promote the generation of twins and the toughness is lowered by increasing the strength, so that the upper limit is preferably limited to 4% by weight.

Aluminum (Al): 5% by weight or less

Aluminum stabilizes austenite in a proper addition amount range and reduces M_sAnd M_dThereby improving the toughness of the steel, said M_sAnd M_dIs a transformation point at which austenite is transformed into e-martensite or a-martensite due to a cooling process or working. In addition, the element is an element which is dissolved in a solid solution in the steel material to increase the strength, and particularly, the element affects the mobility of carbon in the steel material to effectively suppress the formation of carbide to increase the toughness. In particular, formula 1 shows that the element promotes slip by effectively increasing stacking fault energy. However, when more than 5% by weight of aluminum is added, the stress required for twinning may exceed that ofThe stress value corresponding to the amount of working of a conventional steel material is unnecessary, and the castability and surface quality of the steel are deteriorated by the formation of oxides and nitrides, so that the upper limit is preferably limited to 5% by weight.

The austenitic steel material of the present invention may further contain Mo.

Molybdenum (Mo): 5% by weight or less

Molybdenum stabilizes austenite in a proper addition amount range and plays a role in reducing M_sAnd M_dThereby improving the toughness of the steel, said M_sAnd M_dIs a transformation point at which austenite is transformed into e-martensite or a-martensite due to a cooling process or working. In addition, the element is an element which is solid-dissolved in the steel material to increase the strength, and particularly, the element acts to improve the stability of grain boundaries by segregating into austenite grain boundaries, and to suppress grain boundary precipitation of carbonitrides by reducing the energy. In particular, formula 1 shows that the element is an element that promotes slip by effectively increasing stacking fault energy. However, when molybdenum is added in excess of 5 wt%, the stress required for the twin generation will exceed the stress value corresponding to the working amount of conventional steels, and thus is unnecessary, and does not contribute much to the grain boundary stability. Further, since molybdenum is an expensive element, it is economically disadvantageous and causes a reduction in toughness due to an increase in strength, and therefore, the upper limit is preferably limited to 5% by weight.

The balance of the invention is iron (Fe) and other unavoidable impurities. However, in a normal steel manufacturing process, unwanted impurities are inevitably mixed from raw materials or the surrounding environment, and therefore, the mixing of these impurities is inevitable. These impurities are well known to those skilled in the art of conventional iron and steel manufacturing processes, and thus, not all of them are specifically described in the present specification.

The low-temperature steel preferably contains 95% or more of austenite structure in terms of area fraction. Typical soft structure austenite showing ductility deterioration even at low temperature is preferably contained in an area fraction of 95% or more as a fine structure necessary for securing low temperature toughness, and when less than 95%, sufficient low temperature toughness cannot be secured, that is, it is difficult to sufficiently secure impact toughness of 41J or more at-196 ℃.

The content of carbides present in the austenite grain boundaries is preferably 5% or less by area fraction. In the present invention, a typical structure that may exist in addition to austenite is carbide, and this carbide precipitates on austenite grain boundaries to cause grain boundary fracture and deteriorate low-temperature toughness and ductility, and therefore, the upper limit is preferably limited to 5%.

The twin generation stress of the low-temperature steel is preferably a tensile stress corresponding to 5% tensile strain of the low-temperature steel or more. Wherein the twin generation stress represents a value calculated according to formula 2, and the tensile strain represents that 5% of the tensile strain occurs upon uniaxial stretching in the stretching experiment. In general, when a sheet material used for manufacturing a low-temperature structure such as a low-temperature container is molded, the amount of deformation to be provided is mostly within a level of 5% in terms of tensile strain, and therefore, the twin generation stress for suppressing the uneven deformation is preferably limited to a tensile stress corresponding to 5% or more of the amount of deformation in uniaxial stretching.

Next, a method for producing a steel for low temperature use excellent in surface finish quality according to the present invention will be described in detail.

The invention comprises the following steps: a step of preparing a steel ingot having the composition of the steel of the present invention described above and having a Stacking Fault Energy (SFE) of 24mJ/m calculated from the above relational expression 1²The above; heating the steel ingot at 1050-1250 ℃; and a hot rolling step of finish rolling the heated steel slab at a temperature of 700 to 950 ℃.

In order to manufacture a steel for low temperature according to the present invention, first, a steel having the above alloy composition and having a Stacking Fault Energy (SFE) of 24mJ/m calculated from the relational expression 1 is prepared²The steel ingot above.

Then, the steel ingot is heated, and the heating temperature is preferably 1050-1250 ℃. It is used for solid solution and homogenization of cast structure, segregation and secondary phases generated in the solid solution and homogenized steel ingot manufacturing step, and when the heating temperature is less than 1050 ℃, deformation resistance during hot rolling increases due to insufficient homogenization or too low temperature of the heating furnace, and when it exceeds 1250 ℃, segregation bands in the cast structure cause partial melting and deterioration of surface quality. Therefore, the reheating temperature range of the steel ingot is preferably 1050-1250 ℃.

The hot rolling is preferably performed at a finish rolling temperature of 700 to 950 ℃, and when the finish rolling temperature is less than 700 ℃, carbides are precipitated on austenite grain boundaries to reduce elongation and low-temperature toughness, and anisotropy of mechanical properties occurs due to anisotropy of a microstructure. When the finish rolling temperature exceeds 950 ℃, austenite grains are coarsened and the strength and the elongation are lowered, which is not preferable, and therefore, the finish rolling temperature range is preferably 700 to 950 ℃.

Detailed Description

Hereinafter, the present invention will be described more specifically by examples. It should be noted that the following embodiments are only for illustrating the present invention and thus the present invention is more embodied, and not for limiting the scope of the present invention.

Steel ingots satisfying the components shown in table 1 below were produced under the production conditions shown in table 2 below, and then the stacking fault energy, microstructure, yield strength, and area fraction of carbide were measured and shown, and physical property values such as elongation and charpy impact toughness were measured and shown in table 3 below. The surface unevenness in table 3 below was evaluated by visually observing the surface of the steel material.

TABLE 1

TABLE 2

As shown in table 2 above, invention examples 1 to 8 satisfying the composition ranges of the present invention and the above formula 1 obtained stable austenite in which the area fraction of austenite in the microstructure was controlled to 95% or more and the carbide was controlled to less than 5%, and thus showed that excellent toughness could be obtained at extremely low temperatures.

TABLE 3

In addition, it can be confirmed from table 3 that inventive examples 1 to 8 are greatly improved in impact toughness as compared with comparative examples 1 to 3. This is because stable austenite is obtained by adding carbon and other elements in an appropriate amount in a relatively low manganese content range, and particularly, when the carbon content is high, the formation of carbides is suppressed by adding copper, and thus, the stability of austenite is also improved.

In particular, in inventive steels 1 to 8, the stacking fault energy was made to satisfy 24mJ/m according to formula 1²Thus, an excellent steel material free from surface unevenness can be obtained. On the other hand, it is known that the stacking fault energies of comparative examples 1 to 3 exceed the range of the above formula 1, and thus surface unevenness occurs even though excellent ultra-low temperature toughness is obtained.

It was confirmed that the contents of carbon and manganese in comparative examples 4 and 6 do not fall within the composition ranges of the present invention, and the expected austenite area fraction cannot be obtained, and therefore, the cryogenic toughness is lowered, and it was also confirmed that the stacking fault energy does not fall within the range of formula 1 in the present invention, and surface unevenness occurs.

It is understood that comparative examples 5 and 7 do not satisfy the composition ranges controlled in the present invention, and thus have poor impact toughness, and particularly have poor impact toughness due to excessive addition of carbon to generate an excessive area fraction of carbide on austenite grain boundaries.

Comparative example 8 does not satisfy the composition range of the present invention, and therefore, even if stacking fault energy exceeds 24mJ/m²Surface unevenness is also generated. In particular, the finish rolling temperature was lower than the controlled temperature, and the elongation and impact toughness were found to be inferior due to the anisotropy of physical properties and the increase in strength.

While the illustrative embodiments of the invention have been shown and described, various modifications and other embodiments may be devised by those skilled in the art. Such variations and other embodiments are contemplated and are within the scope of the claims, and thus do not exceed the spirit and scope of the invention.

Claims

1. A steel for low temperature use having excellent surface finish quality, characterized in that the steel for low temperature use contains 18.2 to 35 wt% of manganese (Mn), carbon (C) satisfying the condition ranges of 23.6C + Mn ≥ 28 and 33.5C-Mn ≤ 23, more than 0 to 5 wt% or less of copper (Cu), more than 0 to 1 wt% or less of nitrogen (N), chromium (Cr) satisfying the condition ranges of 28.5C +4.4Cr ≤ 57, more than 0 to 1.5 wt% or less of aluminum (Al), and

more than 0 to 5 wt% of nickel (Ni) and more than 0 to 4 wt% of silicon (Si), and the balance of iron (Fe) and other unavoidable impurities,

wherein the low-temperature steel contains an austenite structure in an area fraction of 95% or more,

the carbides present in the austenite grain boundaries are 5% or less by area fraction,

the twin generation stress of the low-temperature steel is not less than a tensile stress corresponding to a 5% tensile strain of the low-temperature steel.

2. The steel for low temperature use having excellent surface finish quality according to claim 1, characterized by further comprising 5% by weight or less of molybdenum (Mo).

3. The steel for low temperature use having excellent surface finish quality as claimed in claim 1, wherein the Stacking Fault Energy (SFE) of the steel for low temperature use calculated according to the following relational expression 1 is 24mJ/m²The method comprises the following steps:

relation 1:

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，

wherein Mn, C, Cr, Si, Al, Ni, Mo and N in each formula represent the content by weight% of each component.