STEEL MATERIAL HAVING EXCELLENT WEAR RESISTANCE AND MANUFACTURING METHOD
The purpose of the present invention is to provide a steel material and a manufacturing method for the same, wherein the steel material has excellent strength, elongation, and impact toughness as well as excellent inside quality and wear resistance. According to the present invention, provided are a steel material having excellent wear resistance and a manufacturing method for the same, wherein. the steel material contains, in weight, 0.55-1.4% carbon (C), 12-23% manganese (Mn), 5% or less (excluding 0%) chromium (Cr), 5% or less (excluding 0%) copper (Cu), 0.5% or less (excluding 0%) Al, 1.0% or less (excluding 0%) Si, 0.02% or less (including 0%) S, 0.04% or less (including 0%) phosphor (P), and the balance Fe and unavoidable impurities, and has a microstructure comprising, in area, 10% or less (including 0%) carbide and the balance austenite.
The present disclosure relates to an austenitic steel material, used for steels in the fields of mining, transportation, storage, and the like, in the oil and gas industries, as steels for industrial machinery, structural materials and slurry pipes, and as sour-resistant steel and the like, and a method of manufacturing the same, and more particularly, to an austenitic steel material having excellent internal quality and wear resistance, and a method of manufacturing the same.
BACKGROUND ARTAustenitic steels are used for a variety of applications due to their excellent work hardenability, low-temperature toughness, and non-magnetic properties. In detail, as carbon steel composed of ferrite or martensite as a main structure, which has been mainly used, has limitations in its properties, the austenitic steel application has recently been increasing as a substitute for overcoming the disadvantages.
In particular, according to the growth of the mining industry, oil and gas industries, wear of steel used in mining, transportation, refining and storage processes has emerged as a major problem. Furthermore, as the development of oil sands as fossil fuels to replace petroleum has recently started, wear of steel by slurry containing oil, rock, gravel, sand, and the like, is pointed out as an important cause of increasing production costs. Accordingly, demand for the development and application of steel materials having excellent wear resistance is greatly increasing.
In the existing parts industry for the mining and machinery industry, Hadfield steel having excellent wear resistance have been mainly used. To increase the wear resistance of steel materials, efforts to generate the austenite structure by including a high content of carbon and a large amount of manganese to increase wear resistance have been made steadily. However, in the case of Hadfield steel, a high carbon content sharply degrades the properties of steel, especially ductility, by forming network-type carbide at high temperature along the austenite grain boundary.
To suppress the precipitation of carbides in the form of a network, a method of manufacturing a high-manganese steel has been proposed by performing a solution heat treatment at a high temperature or quenching to room temperature after hot working. However, it may be difficult to suppress the precipitation of carbides in the form of this network when the change in manufacturing conditions is not easy, such as when the thickness of the steel material is thick or when welding is essential, and thus, it causes a problem that the mechanical properties of the steel material deteriorate rapidly.
In addition, ingots or steel slabs of high-manganese steel inevitably cause segregation by impurity elements such as P, S and the like in addition to alloying elements such as manganese and carbon during solidification. Eventually, coarse carbide is formed along the deep segregation zone in the final product, which eventually causes non-uniformity of the microstructure and deterioration of properties.
In addition, it may result in generating a central portion crack due to heat or stress generated during processing.
To improve wear resistance, it is essential to increase the carbon content, and increasing the manganese content to prevent deterioration of mechanical properties due to carbide precipitation may be a general method, but this leads to an increase in the alloy amount and manufacturing cost.
To solve this, studies on the addition of elements effective for suppressing carbide formation, compared to manganese, are also required. In addition, research on brittleness problems due to segregation, which is common in high-alloy products, is continuously required.
Prior Art Document(Patent Document 1) Korean Patent Application Publication No. 2016-0077558
DISCLOSURE Technical ProblemAn aspect of the present disclosure is to provide a steel material having excellent internal quality and wear resistance as well as excellent strength, elongation and impact toughness.
Another aspect of the present disclosure is to provide a method of manufacturing a steel material having excellent internal quality and wear resistance as well as excellent strength, elongation and impact toughness.
Technical SolutionAccording to an aspect of the present disclosure, a steel material having excellent wear resistance, includes, in weight percent, 0.55 to 1.4% of carbon (C), 12 to 23% of manganese (Mn), 5% or less (excluding 0%) of chromium (Cr) , 5% or less (excluding 0%) of copper (Cu), 0.5% or less (excluding 0%) of Al, 1.0% or less (excluding 0%) of Si, 0.02% or less (including 0%) of S, 0.04% or less (including 0%) of phosphorus (P), and a balance of Fe and unavoidable impurities, wherein the steel material includes, in area o, 10% or less (including 0%) of carbide and balance austenite, as a microstructure.
The steel material may have a component segregation index (S) of 3.0 or less, represented by relational expression 1.
Component segregation index (S)=(C component in central portion of rolled material/C component in molten steel)/1.25 +(Mn component in central portion of rolled material/Mn component in molten steel)/1.15+(P component in central portion of rolled material/P component in molten steel)/3.0, [Relational Expression 1]
where a component in the central portion indicates a component in a range of 50 μm or less in upper and lower portions of a part in which a highest component is measured in microstructure analysis at a position equal to half of a thickness of the rolled material.
The steel material may have a yield strength of 350 MPa or more, a uniform elongation of 20% or more, and an impact toughness of 40 J or more.
According to another aspect of the present disclosure, a method of manufacturing a steel material having excellent wear resistance, includes:
preparing a molten steel containing, in weight percent, 0.55 to 1.4% of carbon (C), 12 to 23% of manganese (Mn), 5% or less (excluding 0%) of chromium (Cr) , 5% or less (excluding 0%) of copper (Cu), 0.5% or less (excluding 0%) of Al, 1.0% or less (excluding 0%) of Si, 0.02% or less (including 0%) of S, 0.04% or less (including 0%) of phosphorus (P), and a balance of Fe and unavoidable impurities;
continuous casting operating of obtaining a slab by continuously casting the molten steel under conditions of a molten steel temperature (TC) satisfying the following relational expression 2 and a casting speed (V) satisfying the following relational expression 3,
K≤TC≤K+60 [Relational Expression 2]
where in relational expression 2, a K value represents a value determined by the following relational expression 4,
V (m/min)≥0.025[TC−K] [Relational Expression 3]
where in relational expression 3, a K value represents a value determined by the following relational expression 4,
K (° C.)=1536−(69[C]+4.2[Mn]+39[P]) [Relational Expression 4]
where [C], [Mn] and [P] each indicate a content (weight %) of an element;
reheating the slab at a reheating temperature (TR) or lower obtained by the following relational expression 5,
TR=1453−165[C]−4.5[Mn]−414[P] [Relational Expression 5]
where TR indicates a reheating temperature (° C.), and [C] and [Mn] each indicate a content (weight %) of an element;
hot rolling the slab reheated in the reheating to a finish rolling temperature of 850 to 1050° C. to obtain a hot rolled steel; and
cooling the hot rolled steel to 600° C. or less at 5° C./sec or more.
Advantageous EffectsAccording to an exemplary embodiment of the present disclosure, a steel material may have excellent wear resistance, and may thus be applied to fields requiring wear resistance, across the mining, transportation, storage or industrial machinery fields in the oil and gas industries in which a relatively large amount of wear occurs. In detail, since internal defects that may occur during the production process, may be significantly reduced, the steel material may be expandably applied to fields requiring relatively high internal quality.
The present inventors have studied steels having superior strength and wear resistance, as compared to existing steels used in technical fields in which wear resistance is required, and have recognized that, in the case of high manganese steels, excellent strength and elongation, unique to austenitic steels, may be secured, and furthermore, excellent wear resistance may be secured as the hardness of the material may be increased due to work hardening of the material itself in an abrasive environment when improving a work hardening rate, thereby completing the present disclosure.
An exemplary embodiment of the present disclosure provides an austenitic steel material having excellent strength as well as superior strength and elongation characteristics unique to austenite-based steel materials, as the hardness of the material was increased due to work hardening of the material itself in an abrasive environment.
Furthermore, in an exemplary embodiment of the present disclosure, casting conditions and reheating conditions may be relatively optimized to provide an improved austenitic wear-resistant steel material having improved internal quality (central portion quality) and a method of manufacturing the same, by controlling the embrittlement of the core due to impurities such as P or the like, and large amounts of carbon and manganese, which are problems with existing austenitic wear-resistant steels.
Hereinafter, a steel material having excellent wear resistance according to an exemplary embodiment of the present disclosure will be described.
A steel material having excellent wear resistance according to an exemplary embodiment of the present disclosure includes, in weight percent, 0.55 to 1.4% of carbon (C), 12 to 23% of manganese (Mn), 5% or less (excluding 0%) of chromium (Cr), 5% or less (excluding 0%) of copper (Cu), 0.5% or less (excluding 0%) of Al, 1.0% or less (excluding 0%) of Si, 0.02% or less (including 0%) of S, 0.04% or less (including 0%) of phosphorus (P) , and a balance of Fe and unavoidable impurities. The steel material includes 10 area % or less (including 0%) of carbide and balance austenite, as a microstructure.
Hereinafter, components and component ranges will be described.
C: 0.55 to 1.4% by weight (hereinafter, also referred to as “%”)
Carbon (C) is an austenite stabilizing element, which not only serves to improve the uniform elongation, but also is a significantly advantageous element for improving strength and increasing a work hardening rate. If the carbon content is less than 0.55%, it maybe difficult to form stable austenite at room temperature, and there is a problem that it may be difficult to secure sufficient strength and work hardening rate. On the other hand, if the content exceeds 1.4%, a large amount of carbide is precipitated to reduce the uniform elongation, and thus, it may be difficult to secure excellent elongation, causing wear resistance deterioration and premature fracture.
Therefore, the content of C may be preferably limited to 0.55 to 1.4%, and in detail, limited to 0.8 to 1.3%.
Mn: 12 to 23%
Manganese (Mn) is a significantly important element that plays a role in stabilizing austenite and improves uniform elongation. To obtain austenite as a main structure in an exemplary embodiment of the present disclosure, it may be preferable that Mn is included in 12% or more.
If the Mn content is lower than 12%, the austenite stability may decrease, and thus, a martensite structure may be formed. Therefore, if the austenite structure is not sufficiently secured, it maybe difficult to secure a sufficient uniform elongation. On the other hand, if the Mn content exceeds 23%, not only does the manufacturing cost increase, but also there are problems such as corrosion resistance deterioration due to manganese addition, difficulty in a manufacturing process, and the like.
Therefore, the Mn content may be preferably limited to 12 to 23%, and in detail, 15 to 21%.
Cr: 5% or less (excluding 0%)
Chromium (Cr) stabilizes austenite up to a range of an appropriate addition amount, thereby improving impact toughness at low temperatures, and is solidified in austenite to increase the strength of steel. In addition, chromium is also an element that improves the corrosion resistance of steel materials. However, if the content of Cr exceeds 5%, it may not be preferable because excessively formed carbides at the austenite grain boundary may significantly reduce toughness of steel. Also, in some cases, the content maybe limited to 3.5% or less.
Cu: 5% or less (excluding 0%) Copper (Cu) has a significantly low solid solubility in carbide and has slow diffusion in austenite, to be concentrated at an austenite and nucleated carbide interface, thereby hindering diffusion of carbon, such that the growth of carbide effectively slows. Therefore, eventually, there is an effect of suppressing generation of carbide. However, if the content of Cu exceeds 5%, there is a problem of deteriorating hot workability of the steel, and thus, it may be preferable to limit the upper limit of the content to 5%.
Al: 0.5% or less (excluding 0%), Si: 1.0% or less (excluding 0%)
Aluminum (Al) and silicon (Si) are components added as a deoxidizer during the steelmaking process, and the upper limit of the aluminum (Al) content is limited to 0.5%, and the upper limit of the silicon (Si) content may be preferably limited to 1.0%.
S: 0.02% or less (including 0%)
S is an impurity and may be preferably suppressed as much as possible, and the upper limit thereof may be preferably managed to be 0.02%.
P: 0.04% or less (including 0%)
In general, P is well known as an element that causes hot brittleness by segregation at the grain boundary. In detail, high alloy steels containing a large amount of C and Mn, such as in the steel according to an exemplary embodiment of the present disclosure, may cause serious brittleness for slabs and products in a case in which P segregation is added. Moreover, if P exceeds a certain content, the segregation degree rises rapidly, and thus, it may be preferable to limit the content to 0.04% or less.
In addition, the balance of Fe and unavoidable impurities are included. However, in the normal manufacturing process, impurities not intended, from the raw material or the surrounding environment, maybe inevitably mixed, and therefore may not be excluded. These impurities are known to anyone skilled in the art and thus, are not specifically mentioned in this specification. In addition, addition of effective ingredients, in addition to the above composition, is not excluded.
A steel material having excellent wear resistance according to an exemplary embodiment of the present disclosure includes, in area o, 10% or less (including 0%) of carbide and residual austenite, as a microstructure.
If the fraction of the carbide exceeds 10% by area, rapid impact toughness deterioration may be caused. The austenite improves ductility and toughness.
The steel material may preferably have a component segregation index (S) of 3.0 or less.
Component segregation index (S)=(C component in central portion of rolled material/C component in molten steel)/1.25+(Mn component in central portion of rolled material/Mn component in molten steel)/1.15+(P component in central portion of rolled material/P component in molten steel)/3.0, [Relational Expression 1]
(where a component in the central portion indicates a component in a range of 50 μm or less in upper and lower portions of a part in which a highest component is measured in microstructure analysis at a position equal to half of a thickness of the rolled material).
If the component segregation index (S) represented by relational expression 1 exceeds 3.0, the probability of occurrence of cracks along the segregation zone at a position of ½t (t: a steel thickness) during processing, for example, during cutting, may increase rapidly.
The steel material may have a yield strength of 350 MPa or more, a uniform elongation of 20% or more, and an impact toughness of 40 J or more.
Hereinafter, a method of manufacturing a steel material having excellent wear resistance according to another exemplary embodiment of the present disclosure will be described in detail.
A method of manufacturing a steel material having excellent wear resistance according to another exemplary embodiment of the present disclosure, includes:
preparing a molten steel containing, in weight percent, 0.55 to 1.4% of carbon (C), 12 to 23% of manganese (Mn), 5% or less (excluding 0%) of chromium (Cr) , 5% or less (excluding 0%) of copper (Cu), 0.5% or less (excluding 0%) of Al, 1.0% or less (excluding 0%) of Si, 0.02% or less (including 0%) of S, 0.04% or less (including 0%) of phosphorus (P), and a balance of Fe and unavoidable impurities;
continuous casting operating of obtaining a slab by continuously casting the molten steel under conditions of a molten steel temperature (TC) satisfying the following relational expression 2 and a casting speed (V) satisfying the following relational expression 3,
K≤TC≤K+60 [Relational Expression 2]
where in relational expression 2, a K value represents a value determined by the following relational expression 4,
V (m/min)≥0.025[TC−K] [Relational Expression 3]
where in relational expression 3, a K value represents a value determined by the following relational expression 4,
K (° C.)=1536−(69[C]+4.2[Mn]+39[P]) [Relational Expression 4]
where [C] , [Mn] and [P] each indicate a content (weight%) of an element;
reheating the slab at a reheating temperature (TR) or lower obtained by the following relational expression 5,
TR=1453−165[]−4.5[Mn]−414[P] [Relational Expression 5]
where TR indicates a reheating temperature (° C.), and [C] and [Mn] each indicate a content (weight %) of an element;
hot rolling the slab reheated in the reheating to a finish rolling temperature of 850 to 1050° C. to obtain a hot rolled steel; and
cooling the hot rolled steel to 600° C. or less at 5° C./sec or more.
Continuous Casting
A steel slab is obtained by continuously casting the molten steel formed as described above under the conditions of a molten steel temperature (TC) satisfying the following relational expression 2 and of a casting speed (V) satisfying the following relational expression 3.
K≤TC≤K+60 [Relational Expression 2]
(In relational expression 2, a K value represents a value determined by the following relational expression 4.)
V (m/min)≥0.025[TC−K] [Relational Expression 3]
(In relational expression 3, a K value represents a value determined by the following relational expression 4.)
K (° C.)=1536−(69[C]+4.2[Mn]+39[P]) [Relational Expression 4]
(where [C], [Mn] and [P] each indicate a content (weight %) of an element.)
In an exemplary embodiment of the present disclosure, to suppress excessive segregation in the slab structure, which may easily occur in high-carbon high-manganese wear-resistant steel, the casting conditions depending on the component changes, as in relational expressions 2 to 4, are derived. Therefore, internal quality (core quality) defects frequently occurring in the final steel may be suppressed.
If the slab is not manufactured under the above casting conditions, an excessive segregation zone may be formed in the slab, resulting in slab brittleness, and the excessive segregation zone may remain even after reheating and rolling, leading to quality defects.
Slab Reheating
The slab obtained by continuous casting as above is reheated.
It maybe preferable that the slab reheating is performed at the reheating temperature (TR) or lower obtained by the following relational expression 5.
TR=1453−165[C]−4.5[Mn]−414[P] [Relational Expression 5]
[TR indicates a reheating temperature (° C.), and [C] and [Mn] each indicate the content (weight%) of the corresponding element]
In an exemplary embodiment of the present disclosure, to suppress the embrittlement of the central portion due to partial melting of a segregation zone during reheating, which may easily occur in high-carbon high-manganese wear-resistant steel, the conditions for limiting the reheating temperature depending on the component change as in relational expression 5 above is derived. Therefore, internal quality (core quality) defects frequently occurring in the final steel may be suppressed.
If the slab reheating temperature exceeds the TR temperature, partial melting may occur in the segregation zone in the slab, and the resulting embrittlement of the core affects a product, causing a component segregation index of the rolled material to exceed 3.0 to cause defects in the core.
Obtaining a Hot Rolled Steel
Hot rolled steel is obtained by hot rolling the reheated slab as described above to a finish rolling temperature of 850 to 1050° C.
If the finish rolling temperature is less than 850° C., carbides may precipitate so that uniform elongation may decrease, and microstructures may become pancakes, resulting in uneven elongation due to anisotropy of the structure. If the finish rolling temperature exceeds 1050° C., grain growth may be active, which may easily cause coarsening of the grain, resulting in a decrease in strength.
Cooling Hot Rolled Steel
The hot-rolled steel is cooled to 600° C. or less at 5° C./sec or more.
If the cooling rate is less than 5° C./sec, or if the cooling stop temperature exceeds 600° C., carbides may be precipitated, resulting in a problem that the elongation decreases. The rapid cooling process helps ensure high solid-solubility of C and N elements in the matrix. Therefore, the cooling may be preferably carried out to 600° C. or less at 5° C./sec or more. The cooling rate may be, in detail, 10° C./sec or more, and in more detail, 15° C./sec or more.
The upper limit of the cooling rate is not particularly limited, and may be limited in consideration of the cooling capability of the equipment. The hot rolled steel may also be cooled to room temperature.
In a method of manufacturing a steel material having excellent wear resistance according to another exemplary embodiment of the present disclosure, for example, a steel material having a yield strength of 350 MPa or more, a uniform elongation of 20% or more, and an impact toughness of 40 J or more may be manufactured.
MODE FOR INVENTIONHereinafter, an exemplary embodiment of the present disclosure will be described in more detail through examples. However, it should be noted that the embodiments described below are only intended to exemplify the present disclosure and are not intended to limit the scope of the present disclosure. This is because the scope of the present disclosure is determined by the items described in the claims and items able to be reasonably inferred therefrom.
EXAMPLESlabs were prepared by continuously casting molten steel satisfying the components and component ranges illustrated in Table 1 under the conditions in Table 2, and then, hot-rolled steels were prepared by reheating, hot rolling and cooling the slabs under the conditions in Table 3.
The microstructure, component segregation index, cut-crack incidence rate (%), wear resistance (g), yield strength (MPa), and uniform elongation (%) of the hot-rolled steel prepared as described above were measured, and the results are illustrated in Table 4 below. In this case, the wear resistance is evaluated by measuring the reduced weight after contacting the specimen to a rotating roll while spraying a predetermined amount of sand with a sand abrasion test according to the ASTM 65 test method.
In addition, the −29° C. impact toughness (impact energy (J)) for the hot-rolled steel was measured, and the results are illustrated in Table 4 below. On the other hand, for comparative steel 4, to observe the occurrence of defects in a central portion of a thickness of the steel sheet, an image was observed, and the result is illustrated in
In Table 2, the casting speed V is V (m/min)=0.025 [TC−K].
1) Component segregation index (S)=(C component in central portion of rolled material/C component in molten steel)/1.25+(Mn component in central portion of rolled material/Mn component in molten steel)/1.15+(P component in central portion of rolled material/P component in molten steel)/3.0
*Component in the central portion: refers to a component in a range of 50 μm or less in upper and lower portions of a part in which a highest component is measured in microstructure analysis at a position equal to half of a thickness of the rolled material.
2) Cut-crack incidence rate: (length of crack in central portion/total cutting length)×100
As illustrated in Tables 1 to 4, in the case of inventive steels 1 to 5 that satisfy all of the steel composition and manufacturing conditions of the present disclosure, it can be seen that not only excellent wear resistance, yield strength, impact toughness and uniform elongation, but also the low cutting crack rate may be exhibited.
On the other hand, in the case of the comparative steels 1 to 9 that do not satisfy the condition of at least one of the steel composition and manufacturing conditions of the present disclosure, it can be seen that at least one property of wear resistance, yield strength, impact toughness and uniform elongation is insufficient or the cutting crack rate is high.
In the case of the comparative steel 4 having a central-portion component segregation index of more than 3.0, it was found that the cracking incidence rate was high, and as illustrated in
Claims
1. A steel material having excellent wear resistance, comprising:
- in weight percent, 0.55 to 1.4% of carbon (C), 12 to 23% of manganese (Mn), 5% or less (excluding 0%) of chromium (Cr), 5% or less (excluding 0%) of copper (Cu), 0.5% or less (excluding 0%) of Al, 1.0% or less (excluding 0%) of Si, 0.02% or less (including 0%) of S, 0.04% or less (including 0%) of phosphorus (P), and a balance of Fe and unavoidable impurities,
- wherein the steel material includes, in area %, 10% or less (including 0%) of carbide and balance austenite, as a microstructure.
2. The steel material having excellent wear resistance of claim 1, wherein the steel material has a component segregation index of 3.0 or less, represented by relational expression 1,
- Component segregation index (S)=(C component in central portion of rolled material/C component in molten steel)/1.25+(Mn component in central portion of rolled material/Mn component in molten steel)/1.15+(P component in central portion of rolled material/P component in molten steel)/3.0, [Relational Expression 1]
- where a component in the central portion indicates a component in a range of 50 μm or less in upper and lower portions of a part in which a highest component is measured in microstructure analysis at a position equal to half of a thickness of the rolled material.
3. The steel material having excellent wear resistance of claim 1, wherein the steel material has a yield strength of 350 MPa or more, a uniform elongation of 20% or more, and an impact toughness of 40 J or more.
4. A method of manufacturing a steel material having excellent wear resistance, comprising:
- preparing a molten steel containing, in weight percent, 0.55 to 1.4% of carbon (C), 12 to 23% of manganese (Mn), 5% or less (excluding 0%) of chromium (Cr), 5% or less (excluding 0%) of copper (Cu), 0.5% or less (excluding 0%) of Al, 1.0% or less (excluding 0%) of Si, 0.02% or less (including 0%) of S, 0.04% or less (including 0%) of phosphorus (P), and a balance of Fe and unavoidable impurities;
- continuous casting operating of obtaining a slab by continuously casting the molten steel under conditions of a molten steel temperature (TC) satisfying the following relational expression 2 and a casting speed (V) satisfying the following relational expression 3, K≤TC≤K+60 [Relational Expression 2]
- where in relational expression 2, a K value represents a value determined by the following relational expression 4, V (m/min)≥0.025[TC−K] [Relational Expression 3]
- where in relational expression 3, a K value represents a value determined by the following relational expression 4, K (° C.)=1536−(69[C]+4.2[Mn]+39[P]) [Relational Expression 4]
- where [C], [Mn] and [P] each indicate a content (weight %) of an element;
- reheating the slab at a reheating temperature (TR) or lower obtained by the following relational expression 5, TR=1453−165[C]−4.5[Mn]−414[P] [Relational Expression 5]
- where TR indicates a reheating temperature (° C.), and [C] and [Mn] each indicate a content (weight %) of an element;
- hot rolling the slab reheated in the reheating to a finish rolling temperature of 850 to 1050° C. to obtain a hot rolled steel; and
- cooling the hot rolled steel to 600° C. or less at 5° C./sec or more.
Type: Application
Filed: Dec 20, 2018
Publication Date: Jun 3, 2021
Inventors: Yong-Jin KIM (Gwangyang-si, Jeollanam-do), Young-Deok JUNG (Gwangyang-si, Jeollanam-do), Myeong-Hun KANG (Gwangyang-si, Jeollanam-do), Yeo-Sun YUN (Gwangyang-si, Jeollanam-do), Soo-Gil PARK (Gwangyang-si, Jeollanam-do)
Application Number: 16/954,672
