KR102255827B1 - Low-temperature austenitic high manganese steel having excellent surface quality and manufacturing method for the same - Google Patents

Abstract

Cryogenic austenitic high manganese steel having excellent surface quality according to an aspect of the present invention, by weight, C: 0.4 to 0.5%, Mn: 23 to 26%, Si: 0.03 to 0.5%, Cr: 3 to 5%, Al: 0.05% or less, S: 0.05% or less, P: 0.5% or less, B: 0.005% or less, the balance contains Fe and inevitable impurities, and contains 95 area% or more of austenite as a microstructure, optical When observing a cross section using a microscope, the number of surface flaws formed to a depth of 10 μm or more from the surface is a unit area among the surface flaws observed in the area from the surface to the point t/8 (where t means product thickness (mm)). It may be 0.0001 or less per (mm ^{2 ).}

Description

Low-temperature austenitic high manganese steel having excellent surface quality and manufacturing method for the same}

The present invention relates to a cryogenic austenitic high manganese steel material suitable for storage and transport of liquefied petroleum gas and liquefied natural gas, storage tanks, ship membranes and transport pipes, and a method of manufacturing the same, in detail The present invention relates to a cryogenic austenitic high manganese steel material for effectively securing surface quality by suppressing the formation of surface flaws, and a method of manufacturing the same.

The austenitic high manganese (Mn) steel adjusts the content of manganese (Mn) and carbon (C), which are elements that enhance the phase stability of austenite, so that the austenite phase is stable at room temperature or cryogenic temperature and has high toughness. It can be used as a material for storage and transport of liquefied petroleum gas, liquefied natural gas, etc. that require characteristics, such as fuel tanks, storage tanks, ship membranes, and transport pipes.

However, high manganese (Mn) steel contains a large amount of manganese (Mn), which has a strong tendency to oxidize, so some of the grain boundary oxidation formed when the slab is reheated is removed as scale, but some of it grows into cracks during hot rolling and leaves the surface of the product. It can remain as a surface flaw. Therefore, when manufacturing high manganese (Mn) steel, a grinding process on the product surface is essentially involved, which is not preferable in terms of economy and productivity.

Republic of Korea Patent Publication No. 10-2015-0075275 (published on July 3, 2015)

According to one aspect of the present invention, an austenitic high manganese steel material for cryogenic use and a method of manufacturing the same can be provided which effectively secures surface quality by suppressing the formation of surface flaws.

The subject of the present invention is not limited to the above description. Those skilled in the art will have no difficulty in understanding the additional subject of the present invention from the general contents of the present specification.

Cryogenic austenitic high manganese steel having excellent surface quality according to an aspect of the present invention is, by weight, C: 0.4 to 0.5%, Mn: 23 to 26%, Si: 0.03 to 0.5%, Cr: 3 to 5%, Al: 0.05% or less, S: 0.05% or less, P: 0.5% or less, B: 0.005% or less, the balance contains Fe and inevitable impurities, and contains 95 area% or more of austenite as a microstructure, optical When observing a cross section using a microscope, the number of surface flaws formed to a depth of 10 μm or more from the surface is a unit area among the surface flaws observed in the area from the surface to the point t/8 (where t means product thickness (mm)). It may be 0.0001 or less per (mm ^{2 ).}

The steel material may further contain less than or equal to 0.7% by weight of Cu.

The steel has a yield strength of 400 MPa or more, and a Charpy impact toughness of -196° C. may be 41J or more.

Cryogenic austenitic high manganese steel having excellent surface quality according to an aspect of the present invention is, by weight, C: 0.4 to 0.5%, Mn: 23 to 26%, Si: 0.03 to 0.5%, Cr: 3 to 5%, Al: 0.05% or less, S: 0.05% or less, P: 0.5% or less, B: 0.005% or less, the balance Fe and the slab containing inevitable impurities are reheated in a temperature range of 1000 to 1300 °C, and the reheating The slab is roughly rolled to provide a rough rolled bar, and the rough rolled bar is finish-rolled in a temperature range of 750 to 1000°C to provide a hot rolled material, but the reheating temperature (T _SR ) of the slab and It can be manufactured by controlling the rolling reduction (R _{RM) of the rough rolling.}

[Relationship 1]

R _RM /T _SR > 0.15

_{(R RM} in relation 1 And T _SR means rough rolling reduction (mm) and slab reheating temperature (℃), respectively)

The slab may further contain 0.7% by weight or less of Cu.

The finish-rolled hot-rolled material may be accelerated cooling to 600°C or less at a cooling rate of 10°C/s or more.

The means for solving the above problems are not all of the features of the present invention, and various features of the present invention and advantages and effects thereof will be understood in more detail with reference to the specific embodiments below.

According to an aspect of the present invention, it is possible to provide an austenitic high-manganese steel material having excellent surface quality while having properties particularly suitable for cryogenic use.

In addition, according to an aspect of the present invention, it is possible to provide a method of manufacturing an austenitic high manganese steel material that can effectively secure productivity and economy by securing excellent surface quality without following subsequent processes such as grinding.

1 is a photograph of the surface of specimen 1.
2 is a photograph of the surface of the specimen 3.
3 is a photograph of specimen 1 being cut in the thickness direction and then observed with an optical microscope.

The present invention relates to a cryogenic austenitic high manganese steel and a method for manufacturing the same. Hereinafter, preferred embodiments of the present invention will be described. Embodiments of the present invention may be modified in various forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. Embodiments of the present invention are provided in order to further detail the present invention to those of ordinary skill in the art to which the present invention pertains.

Hereinafter, the steel composition of the present invention will be described in more detail. Hereinafter, unless otherwise indicated,% indicating the content of each element is based on weight.

Cryogenic austenitic high manganese steel having excellent surface quality according to an aspect of the present invention is, by weight, C: 0.4 to 0.5%, Mn: 23 to 26%, Si: 0.03 to 0.5%, Cr: 3 to 5%, Al: 0.05% or less, S: 0.05% or less, P: 0.5% or less, B: 0.005% or less, balance Fe and inevitable impurities may be included. In addition, the austenitic high manganese steel material for cryogenic use having excellent surface quality according to an aspect of the present invention may further contain 0.7% by weight or less of Cu.

Carbon (C): 0.4~0.5%

Carbon (C) is an element effective in stabilizing austenite in steel and securing strength by solid solution strengthening. Accordingly, the present invention may limit the lower limit of the carbon (C) content to 0.4% in order to secure low-temperature toughness and strength. A preferred lower limit of the carbon (C) content may be 0.41%, and a more preferred lower limit of the carbon (C) content may be 0.43%. That is, when the carbon (C) content is less than 0.4%, the yield strength may be lowered, austenite stability may be lowered to form ferrite or martensite, and low temperature toughness may be lowered. On the other hand, when the carbon (C) content exceeds a certain range, excessive carbide may be formed during cooling after rolling, and the present invention may limit the upper limit of the carbon (C) content to 0.5%. The upper limit of the preferable carbon (C) content may be 0.49%, and the upper limit of the more preferable carbon (C) content may be 0.47%.

Manganese (Mn): 23-26%

Manganese (Mn) is an important element that plays a role in stabilizing austenite. Accordingly, the present invention may limit the lower limit of the manganese (Mn) content to 23% in order to achieve such an effect. That is, since the present invention contains more than 23% manganese (Mn), it is possible to effectively increase austenite stability, thereby inhibiting the formation of ferrite, ε-martensite, and α'-martensite to effectively secure low-temperature toughness. I can. A more preferable lower limit of the manganese (Mn) content may be 23.1%. On the other hand, when the manganese (Mn) content is higher than a certain level, the austenite stability increase effect is saturated, while the excessive manufacturing cost is greatly increased, and the surface quality may be deteriorated due to excessive internal oxidation during hot rolling. The upper limit of the manganese (Mn) content can be limited to 26%. A more preferable upper limit of the manganese (Mn) content may be 25.5%.

Silicon (Si): 0.03~0.5%

Silicon (Si), like aluminum (Al), is an element that is indispensably added in trace amounts as a deoxidizing agent. However, when silicon (Si) is excessively added, oxides are formed at grain boundaries to reduce high-temperature ductility, and there is a concern that the surface quality may be degraded by causing cracks, etc. The present invention is the upper limit of the silicon (Si) content. Can be limited to 0.5%. A more preferable upper limit of the silicon (Si) content may be 0.45%. On the other hand, in order to reduce the Si content in steel, an excessive cost is required, and the present invention may limit the lower limit of the silicon (Si) content to 0.03%. A more preferable lower limit of the silicon (Si) content may be 0.04%.

Chrome (Cr): 3~5%

Chromium (Cr) is an element that contributes to increase the strength through solid solution strengthening in austenite. In addition, since chromium (Cr) has excellent corrosion resistance, it is an element that effectively contributes to preventing deterioration of surface quality due to high-temperature oxidation. Therefore, the present invention may limit the lower limit of the chromium (Cr) content to 3% to achieve such an effect. A preferred lower limit of the chromium (Cr) content may be 3.1%, and a more preferred lower limit of the chromium (Cr) content may be 3.3%. On the other hand, when the chromium (Cr) content is higher than a certain level, the cryogenic toughness decreases due to the generation of carbides, and the present invention may limit the upper limit of the chromium (Cr) content to 5%. The upper limit of the preferred chromium (Cr) content may be 4.5%, and the upper limit of the more preferred chromium (Cr) content may be 4.0%.

Sulfur (S): 0.05% or less

Sulfur (S) is not only an impurity element that is unavoidably introduced, but is also an element that causes hot brittle defects due to the formation of inclusions. Accordingly, the present invention can actively suppress the upper limit of the sulfur (S) content, and the upper limit of the preferred sulfur (S) content may be 0.05%.

Phosphorus (P): 0.5% or less

Phosphorus (P) is not only an impurity element that is unavoidably introduced, but is also an element that easily segregates and causes cracking during casting or deteriorates weldability. Accordingly, the present invention can actively suppress the upper limit of the phosphorus (P) content, and the upper limit of the preferred phosphorus (P) content may be 0.5%.

Boron (B): 0.005% or less

Boron (B) is an element that contributes to the improvement of surface quality by suppressing grain boundary fracture through strengthening of grain boundaries, but is also an element that reduces toughness and weldability by formation of coarse precipitates when excessively added. Accordingly, the present invention may contain 0.0005% or more boron (B) to achieve the effect of improving the surface quality, but the upper limit of the boron (B) content may be limited to 0.005% in order to prevent a decrease in weldability.

Copper (Cu): 0.7% or less

Copper (Cu) is an austenite stabilizing element, an element that stabilizes austenite along with manganese (Mn) and carbon (C), and is an element that contributes to improving low-temperature toughness. In addition, copper (Cu) is an element that has a very low solubility in carbides and slows diffusion in austenite, so it is concentrated at the interface between austenite and carbide and surrounds the nuclei of fine carbides, thereby further diffusion of carbon (C). It is an element that effectively suppresses the formation and growth of carbides. Therefore, the present invention may additionally add a certain amount of copper (Cu) to achieve such an effect. The lower limit of the copper (Cu) content may be 0.3%, the preferred lower limit of the copper (Cu) content may be 0.35%, and a more preferable lower limit of the copper (Cu) content may be 0.4%. However, when the copper (Cu) content is higher than a certain level, the surface quality is deteriorated due to hot shortness, and the present invention may limit the upper limit of the copper (Cu) content to 0.7%. The upper limit of the preferred copper (Cu) content may be 0.65%, and the upper limit of the more preferred copper (Cu) content may be 0.6%.

The austenitic high manganese steel for cryogenic use having excellent surface quality according to an aspect of the present invention may contain the balance Fe and other unavoidable impurities in addition to the above components. However, in a typical manufacturing process, since unintended impurities may inevitably be mixed from the raw material or the surrounding environment, this cannot be completely excluded. Since these impurities are known to anyone of ordinary skill in the art, all the contents are not specifically mentioned in the present specification. In addition, addition of effective ingredients other than the above composition is not excluded.

Cryogenic austenitic high manganese steel having excellent surface quality according to an aspect of the present invention contains 95 area% or more of austenite as a microstructure, and when observing a cross section using an optical microscope, t/8 from the surface (where t Means product thickness (mm)) of the surface defects observed in the area up to the point, the number of surface defects formed to a depth of 10 μm or more from the surface may be 0.0001 or less per ^{unit area (mm 2 ).} Here, the observation area means an arbitrary rectangular area formed on the cross-section of the steel material, and one surface of the observation area may be located adjacent to the surface of the steel material. That is, the height of the observation area is t/8 (t: product thickness, mm), and the surface defect number density can be calculated by using the number of surface defects having a depth of at least a certain level among defects formed in the observation area.

That is, the cryogenic austenitic high manganese steel material having excellent surface quality according to an aspect of the present invention actively suppresses the formation of surface flaws on the product surface through strict process condition control as described below, effectively reducing the surface quality. By securing it, it is possible to omit subsequent processes such as the grinding process, and accordingly, economical efficiency and productivity of the product can be effectively secured.

In addition, the austenitic high manganese steel material for cryogenic use having excellent surface quality according to an aspect of the present invention has a yield strength of 400 MPa or more and Charpy impact toughness of 41 J or more at -196°C, so liquefied petroleum gas, which requires cryogenic properties, is liquefied. Austenitic high-manganese steels that are particularly suitable as materials for storage and transport of natural gas, such as fuel tanks, storage tanks, membranes for ships, and pipes for transport can be provided.

Hereinafter, the manufacturing method of the present invention will be described in more detail.

Cryogenic austenitic high manganese steel having excellent surface quality according to an aspect of the present invention reheats a slab provided with the above-described composition in a temperature range of 1000 to 1300°C, and rough rolls the reheated slab by rough rolling A bar is provided and a hot rolled material is provided by finishing rolling in a temperature range of 750 to 1000° C., but the reheating temperature (T _SR ,° C.) _{of the slab and the rolling reduction of the rough rolling (R RM} , mm) can be produced by controlling.

[Relationship 1]

R _RM /T _SR > 0.15

Reheating the slab

Since the steel composition of the slab corresponds to the steel composition of the austenitic high-manganese steel material described above, it is replaced by a description of the steel composition of the austenitic high-manganese steel material described above for the steel composition of the slab.

The slab provided with the above-described steel composition can be uniformly heated in a temperature range of 1000 to 1300°C. The thickness of the slab provided in the slab reheating step may be about 250mm, but the scope of the present invention is not necessarily limited thereto.

In order to prevent excessive rolling load in subsequent hot rolling, the lower limit of the slab reheating temperature may be limited to 1000°C. In addition, the higher the heating temperature, the greater the ease of hot rolling, but steel with a high manganese (Mn) content may severely generate grain boundary oxidation when heated at high temperatures, and the present invention limits the upper limit of the slab reheating temperature to 1300°C. I can.

Hot rolled

After the slab reheating process, a hot rolling process may be involved in which the reheated slab is roughly rolled with a rough rolled bar, and the rough rolled bar is finished rolled in a temperature range of 750 to 1000°C to provide a hot rolled material. The finish rolling temperature of hot rolling also increases the higher the temperature, the lower the deformation resistance, thus ensuring the ease of rolling, but the higher the finish rolling temperature, the lower the surface quality due to grain boundary oxidation, so the finish rolling temperature of the present invention is 750~1000℃. May be limited.

Since the austenitic high manganese steel of the present invention contains a large amount of manganese (Mn) having strong oxidizing properties, grain boundary oxidation inevitably occurs even when the temperature of the heating furnace is limited. Even if some of the grain boundary oxidation formed during reheating of the slab is removed as scale, the remaining part grows into cracks during hot rolling to form surface flaws on the surface of the product, thereby deteriorating the surface quality of the product.

Through in-depth research, the inventors of the present invention came to the conclusion that it is effective to refine the structure by allowing recrystallization to occur as quickly as possible after heating the slab in order to minimize the growth of the grain boundary oxidation remaining on the surface of the slab to cracks during hot rolling. I did. However, to promote recrystallization, increasing the strain rate is the most effective, and the increase in strain rate is a factor that can be reached through an increase in the rolling reduction of rough rolling, but when the rolling reduction is excessively increased, grain boundary oxidation minimizes the growth of cracks. Separately, equipment damage due to excessive rolling load may be a problem.

Accordingly, the inventor of the present invention derives the following relational equation 1 for controlling the rolling load of hot rolling to be below a critical value while actively suppressing the formation of surface flaws of the product through repeated experiments.

[Relationship 1]

R _RM /T _SR > 0.15

_{(R RM} in relation 1 And T _SR means rough rolling reduction (mm) and slab reheating temperature (℃), respectively)

That is, the present invention controls the rough rolling reduction amount with respect to the heating furnace temperature in a certain range as shown in the above relational equation 1, so when the heating furnace temperature is high, the grain boundary oxidation during hot rolling is increased by relatively increasing the rough rolling reduction amount. When the heating furnace temperature is low, the rolling load applied to the rolling mill during hot rolling can be reduced by relatively reducing the rolling amount of rough rolling. Optimal slab heating conditions and hot rolling conditions Can provide.

Accelerated cooling

After the hot rolling process, the finish-rolled hot-rolled material can be accelerated cooling to 600°C or less at a cooling rate of 10°C/s or more. Since the austenitic high-manganese steel material of the present invention contains 3 to 5% of chromium (Cr) and C, the cooling rate of the hot-rolled material is controlled to 10°C/s or more to effectively prevent the decrease in low-temperature toughness due to carbide precipitation. I can. In addition, in the usual accelerated cooling, a cooling rate exceeding 100°C/s is difficult to implement due to the characteristics of the facility, and the present invention may limit the upper limit of the cooling rate to 100°C/s.

In addition, even if the hot-rolled material is cooled by applying a cooling rate of 10°C/s or more, if the cooling is stopped at a high temperature, the possibility of generating and growing carbides is high, so the present invention can limit the cooling stop temperature to 600°C or less. have.

The austenitic high manganese steel manufactured as described above contains 95 area% or more of austenite as a microstructure, but when observing the cross section using an optical microscope, t/8 from the surface (where t means the product thickness (mm)) The number of surface flaws formed with a depth of 10㎛ or more from the surface for the cross-sectional area to the point ^{may be 0.0001 or less per unit area (mm 2} ), and has a yield strength of 400 MPa or more and Charpy impact toughness of 41J or more at -196°C. can do.

Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the examples to be described later are for illustrative and more specific purposes of the present invention, and are not intended to limit the scope of the present invention.

(Example 1)

A slab having a thickness of 250 mm was manufactured using a steel material having the composition shown in Table 1 below, and a specimen was prepared by manufacturing through the process conditions shown in Table 2 below. Each specimen was prepared by finishing rolling in a temperature range of 750 to 1000°C, and accelerated cooling to 600°C or less at a cooling rate of 10°C/s or more. Impact absorption energy, yield strength, and surface flaw formation were evaluated for each specimen, and the results are shown in Table 2 below. Impact absorption energy was evaluated at -196°C using a plate specimen having a notch of 2 mm in accordance with ASTM E23, a standard test method. The tensile test was evaluated by a one-way tensile tester by processing a plate-shaped specimen conforming to the standard test method ASTM E8/E8M. The depth and number of surface flaws are determined by cutting the specimen in the thickness direction, preparing the specimen according to ASTM E112, and using an optical microscope to determine the depth of the largest surface flaw in the observation area and the number of surface flaws with a depth of 10 μm or more per unit area in the observation area. Was evaluated by measuring.

division Mn Cr C Cu B Si P S.Al S One 23.2 3.5 0.44 0.50 0.0012 0.041 0.027 0.036 0.0014 2 24.6 3.4 0.46 0.52 0.0028 0.311 0.014 0.039 0.0013 3 25.2 3.4 0.45 0.49 0.0026 0.318 0.017 0.043 0.0015 4 24.8 3.4 0.45 0.48 0.0029 0.300 0.017 0.033 0.0013 5 24.8 3.4 0.45 0.48 0.0029 0.300 0.017 0.033 0.0014 6 24.8 3.4 0.45 0.48 0.0029 0.300 0.017 0.033 0.0015 7 24.8 3.4 0.45 0.48 0.0029 0.300 0.017 0.033 0.0013 8 24.0 3.4 0.44 0.43 0.0030 0.270 0.013 0.023 0.0014 9 24.0 3.4 0.44 0.43 0.0030 0.270 0.013 0.023 0.0015

division Reheat
Temperature
(℃) Rough rolling
Reduction
(mm) Relation 1
(R _RM /T _SR ) Shock absorption
energy
(J, @-196℃) surrender
burglar
(MPa) maximum
Surface blemishes
depth
(㎛) Surface blemishes
Count
(Pcs/mm ² ) Remark One 1300 132 0.102 123 458 30 0.03 Comparative example 2 1174 130 0.111 90 464 28 0.02 Comparative example 3 1120 180 0.161 96 465 0 0 Invention example 4 1150 105 0.091 86 486 32 0.03 Comparative example 5 1162 135 0.116 84 514 30 0.03 Comparative example 6 1155 175 0.152 84 514 0 0 Invention example 7 1199 203 0.170 84 495 0 0 Invention example 8 1198 207 0.173 71 529 0 0 Invention example 9 1130 207 0.184 70 531 0 0 Invention example

In the case of specimens 3, 6 to 9 that satisfy the relational equation 1, the surface quality is excellent because no surface defect occurs, whereas in the case of the specimens 1, 2, 4, and 5 that do not satisfy the relational equation 1, surface defects occur and the surface quality It can be seen that this is inferior and must be accompanied by subsequent processes such as grinding in order to secure surface quality.

1 is a photograph of the surface of specimen 1, and FIG. 2 is a photograph of the surface of specimen 3. As a result of visual observation, it can be seen that a large amount of fine surface flaws were formed in Specimen 1, whereas no surface flaws were formed in Specimen 3, so that excellent surface quality was secured. In addition, FIG. 3 is a photograph of specimen 1 being cut in the thickness direction and the cross section is observed with an optical microscope, and it can be seen that surface flaws are formed on the surface side of specimen 1 in a direction inclined with respect to the thickness direction of the specimen.

Although the present invention has been described in detail through examples above, other types of examples are also possible. Therefore, the technical spirit and scope of the claims set forth below are not limited to the embodiments.

Claims (6)

delete delete delete In% by weight, C: 0.4 to 0.5%, Mn: 23 to 26%, Si: 0.03 to 0.5%, Cr: 3 to 5%, Al: 0.05% or less, S: 0.05% or less, P: 0.5% or less, B: Reheating the slab containing 0.005% or less, the balance Fe and inevitable impurities in the temperature range of 1000 ~ 1300 ℃,
Rough rolling the reheated slab to provide a rough rolled bar,
Provide a hot rolled material by finishing rolling the rough rolled bar in a temperature range of 750 ~ 1000 ℃
_{Controlling the reheating temperature (T SR} ) of the slab and the rolling reduction (R _RM ) of the rough rolling to satisfy the following relational equation 1, a method for producing a cryogenic austenitic high manganese steel with excellent surface quality.
[Relationship 1]
R _RM /T _SR > 0.15
_{(R RM} and T _SR in relational equation 1 mean rough rolling rolling reduction (mm) and slab reheating temperature (℃), respectively) The method of claim 4,
The slab further comprises 0.7% by weight or less of Cu, a method for producing a cryogenic austenitic high manganese steel having excellent surface quality. The method of claim 4,
A method of producing a cryogenic austenitic high manganese steel material having excellent surface quality by accelerating cooling the finish-rolled hot rolled material to 600°C or less at a cooling rate of 10°C/s or more.

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