Patent Description:
An austenitic high-manganese steel material has high toughness because austenite is stable even in room temperature and cryogenic environment by adjusting contents of manganese (Mn) and carbon (C), which are elements increasing stability of austenite, so that it has particularly suitable properties as a material for cryogenic structures such as tanks for LNG storage, tanks for LNG transport, and the like.

However, high-manganese steel has high deformation resistance at high temperatures, and particularly, in the case of thin materials, it is difficult to secure a uniform shape in a longitudinal direction according to a rolling pass, a reduction ratio, and the like. If a shape of the hot-rolled material is poor, a cooling safety is lowered, and there is a possibility of causing equipment damage in a process such as transportation. In addition, when the shape of the hot-rolled material in the longitudinal direction is poor, a subsequent operation such as a shape correction operation, or the like, must be undertaken, which is not preferable in terms of economy and productivity. Further, since there is a technical limitation in securing a uniform shape even through an additional shape correction operation after cooling, or the like, a high-manganese steel material having excellent shape uniformity and a manufacturing method thereof are required without requiring an additional operation such as shape fixing.

<CIT> discloses an economic ultra-low temperature high manganese austenitic steel, having excellent low-temperature toughness.

According to the present invention a cryogenic austenitic high-manganese steel material having an excellent shape and a method of manufacturing the same is provided.

various features of the present invention and advantages and effects thereof will be understood in more detail with reference to the specific embodiments as below.

According to the present invention, it is possible to provide an austenitic high-manganese steel material having excellent cryogenic toughness and an excellent shape, and a method of manufacturing the same.

<FIG> is a view to help in understanding a crest and a trough formed in a steel material in the present disclosure, and <FIG> is a view is an image captured of a steel material according to an example of the present disclosure.

The present invention relates to a cryogenic austenitic high-manganese steel material having an excellent shape and a method of manufacturing the same, and hereinafter, preferable 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. These embodiments are provided to further describe the present invention to a person skilled in the art to which the present invention pertains.

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

The cryogenic austenitic high-manganese steel material having an excellent shape according to an aspect of the present disclosure includes by weight %, <NUM> to <NUM>% of C, <NUM> to <NUM>% of Mn, <NUM> to <NUM> % of si, <NUM> to <NUM>% of Cu <NUM>% or less of P, <NUM>% or less of S, <NUM>
to <NUM>% of Al, <NUM> to <NUM>% of Cr, <NUM> to <NUM>% of B, and a remainder of Fe and other unavoidable impurities.

Carbon (C) is effective in stabilizing austenite and securing strength by solid solution strengthening. Accordingly, in the present disclosure, a lower limit of the carbon (C) content is limited to <NUM>% to secure low-temperature toughness and strength. That is, when the carbon (C) content is less than <NUM>%, austenite stability may be insufficient such that stable austenite may not be obtained at cryogenic temperature, and processing organic transformation into ε-martensite and α'-martensite may easily occur by external stress such that toughness and strength of the steel material may be reduced. On the other hand, when the carbon (C) content exceeds a certain range, toughness of the steel material may be rapidly deteriorated due to precipitation of carbides, and strength of the steel material may increase excessively such that workability of the steel material may significantly degrade. Thus, an upper limit of the carbon (C) content is limited to <NUM>%. Therefore, the carbon (C) content in the present disclosure may be <NUM> to <NUM>%. A preferable carbon (C) content is <NUM> to <NUM>%, and a more preferable carbon (C) content may be <NUM> to <NUM>%.

Manganese (Mn) is an element effectively contributing to austenite stabilization, and thus, in the present disclosure, a lower limit of the manganese (Mn) content is limited to <NUM>% to achieve such an effect. In other words, since <NUM>% or more of manganese (Mn) is included in the present disclosure, stability of austenite may effectively increase, such that the formation of ferrite, ε-martensite, and α'-martensite may be inhibited, thereby effectively securing low-temperature toughness of the steel material. On the other hand, when the manganese (Mn) content exceeds a certain level, an effect of increasing stability of austenite may be saturated, but manufacturing costs may greatly increase, and internal oxidation may excessively occur during hot-rolling, such that surface quality may be deteriorated. Thus, in the present disclosure, an upper limit of the manganese (Mn) content is limited to <NUM>%. Accordingly, the manganese (Mn) content in the present disclosure is <NUM> to <NUM>%, and a more preferable manganese (Mn) content may be <NUM> to <NUM>%.

Silicon (Si)is a deoxidizing agent as aluminum (Al) and is inevitably added in a small amount. However, when silicon (Si) is excessively added, an oxide may be formed on a grain boundary such that high-temperature ductility may be reduced, and cracks may be created such that surface quality may be deteriorated. Thus, in the present disclosure, an upper limit of the silicon (Si) content is limited to <NUM>%. Since excessive costs may be required to reduce the silicon (Si) content in steel, a lower limit of the silicon (Si) content is limited to <NUM>% in the present disclosure. Therefore, the silicon (Si) content in the present disclosure is <NUM> to <NUM>%.

Copper (Cu) is an element stabilizing austenite together with manganese (Mn) and carbon (C), and effectively contributes to improving low-temperature toughness. Also, copper (Cu) has an extremely low solubility in carbides and is slowly diffused in austenite, such that copper (Cu) may be concentrated on an interfacial surface between austenite and carbide and may surround a nuclei of fine carbide, thereby effectively inhibiting formation and growth of carbides caused by additional diffusion of carbon (C). Thus, in the present disclosure, copper (Cu) is essentially added to secure low-temperature toughness, and a lower limit of the copper (Cu) content is <NUM>%. On the other hand, when the copper (Cu) content exceeds <NUM>%, hot workability of the steel material may be deteriorated, and in the present disclosure, an upper limit of the copper (Cu) content is limited to <NUM>%. A more preferable upper limit of the copper (Cu) content may be <NUM>%.

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 an element that deteriorates weldability. Accordingly, in the present disclosure, an upper limit of the phosphorus (P) content is limited to <NUM>% to prevent deterioration of castability and weldability.

Sulfur (S) is not only an impurity element that is unavoidably introduced, but is also an element that causes a hot brittleness defect by forming inclusions. Accordingly, in the present disclosure, an upper limit of the sulfur (S) content is limited to <NUM>% to inhibit hot brittleness.

Aluminum (Al) is a representative element added as a deoxidizer. However, aluminum (Al) may form precipitates by reacting with carbon (C) and nitrogen (N), and hot workability may be deteriorated by the precipitates. Thus, in the present disclosure, an upper limit of the aluminum (Al) content is limited to <NUM>%. The aluminum (Al) content is <NUM> to <NUM>%.

Chromium (Cr) may stabilize austenite in a range of an appropriate amount such that chromium (Cr) may contribute to improving impact toughness at low temperature, and may be solid-solute in austenite and may increase strength of the steel material. Also, chromium may improve corrosion resistance of the steel material. Therefore, in the present disclosure, <NUM>% or more of chromium (Cr) is added to obtain the effect as above. However, chromium (Cr) may be a carbide-forming element and may form carbides on an austenite grain boundary, such that low-temperature impact toughness may be reduced. Thus, an upper limit of the chromium (Cr) content is limited to <NUM>% in consideration of content relationship between carbon (C) and other elements added together. Accordingly, the chromium (Cr) content in the present disclosure is <NUM> to <NUM>%, and a more preferable chromium (Cr) content may be <NUM> to <NUM>%.

Boron (B) is a grain boundary strengthening element which may strengthen an austenite grain boundary, and by even adding boron (B) in a small amount, an austenite grain boundary may be strengthened such that high-temperature cracking sensitivity may be effectively reduced. To achieve the effect as above, in the present disclosure, a lower limit of the boron (B) content is limited to <NUM>%. On the other hand, when the boron (B) content exceeds a certain range, segregation may occur on an austenite grain boundary such that high-temperature cracking sensitivity of the steel material may increase, and surface quality of the steel material may be degraded. Thus, in the present disclosure, an upper limit of the boron (B) content is limited to <NUM>%. The boron (B) content of the present disclosure is <NUM> to <NUM>%, and a more preferable boron (B) content may be <NUM> to <NUM>%.

The cryogenic austenitic high-manganese steel having an excellent shape of the present disclosure has a remainder of Fe and other unavoidable impurities in addition to the above components. However, in a general manufacturing process, inevitable impurities may be inevitably added from raw materials or an ambient environment, and thus, impurities may not be excluded. A person skilled in the art of a general manufacturing process may be aware of the impurities, and thus, the descriptions of the impurities may not be provided in the present disclosure.

The cryogenic austenitic high-manganese steel material having an excellent shape according to an aspect of the present disclosure may include <NUM> area% or more of austenite as a microstructure, thereby effectively securing cryogenic toughness of the steel material. An average grain size of austenite may be <NUM>-<NUM>. An average grain size of austenite implementable in the manufacturing process may be <NUM> or more, and when the average grain size increases significantly, strength of the steel material may be reduced. Thus, the grain size of austenite may be limited to <NUM> or less.

The cryogenic austenitic high-manganese steel material having an excellent shape according to an aspect of the present disclosure may include a carbide and/or ε-martensite as a possible structure other than austenite. When a fraction of carbide and/or ε-martensite exceeds a certain level, toughness and ductility of the steel material may be rapidly deteriorated. Thus, in the present disclosure, the fraction of carbide and/or ε-martensite may be limited to <NUM> area% or less.

The cryogenic austenitic high-manganese steel material having an excellent shape according to the present invention has a yield strength of <NUM> MPa or more, a tensile strength of <NUM> MPa or more, and an elongation of <NUM>% or more. In addition, the cryogenic austenitic high-manganese steel material having an excellent shape according to the present invention has a Charpy impact toughness of -<NUM> of 30J or more (based on a thickness of <NUM>), and thus can have excellent cryogenic properties.

Since the cryogenic austenitic high-manganese steel material having an excellent shape according to an aspect of the present invention has a maximum height difference within <NUM> between the a crest and a trough formed in the steel material in a region within <NUM> of the rolling direction even without performing a separate correction operation after the steel material is manufactured, excellent shape uniformity may be secured.

<FIG> is a view to help in understanding a crest and a trough formed in a steel material in the present disclosure, and <FIG> is an image captured of a steel material according to an example of the present disclosure.

Hereinafter, a manufacturing method in the present disclosure will be described in more detail.

A method of manufacturing a cryogenic austenitic high-manganese steel material having an excellent shape according to an aspect of the present disclosure includes: primarily heating a slab including, by weight%, <NUM> to <NUM>% of C, <NUM> to <NUM>% of Mn, <NUM> to <NUM>% of Si, <NUM> to <NUM>% of Cu, <NUM>% or less of P, <NUM>% or less of S, <NUM> to <NUM>% of Al, <NUM> to <NUM>% of Cr, <NUM> to <NUM>% of B, and a remainder of Fe and unavoidable impurities, to a temperature range of <NUM> to <NUM>; primarily hot-rolling the heated slab at a finishing rolling temperature of <NUM> to <NUM> at a total rolling reduction ratio of <NUM> to <NUM>% to provide an intermediate material;
cutting the intermediate material into a length of <NUM> to <NUM>; secondarily heating the intermediate material to a temperature range of <NUM> to <NUM>; secondarily hot-rolling the secondarily-heated intermediate material at a finishing rolling temperature of (Tnr-<NUM>) to Tnr°C to provide a hot- rolled material; and cooling the hot-rolled material to a temperature range of <NUM> or less at a cooling rate of <NUM> to <NUM>/s, wherein, during the secondary hot-rolling, a total rolling reduction amount of the intermediate material in the temperature range of (Tnr-<NUM>) to Tnr°C may be controlled to <NUM> to <NUM>%.

Since the composition of the slab provided in the manufacturing method in the present disclosure corresponds to the steel composition of the austenitic high-manganese steel material described above, the description of the steel composition of the slab is replaced with the description of the steel composition of the austenitic high-manganese steel material described above.

The slab provided in the above-described steel composition is primarily heated in a temperature range of <NUM> to <NUM>. When a primary heating temperature is lower than a certain range, there may be a problem in which an excessive rolling load may be applied during primary hot-rolling, or an alloy component may not be sufficiently solid solute. Therefore, in the present disclosure, a lower limit of the primary heating temperature range is limited to <NUM>. On the other hand, when the primary heating temperature exceeds a certain range, grains may grow excessively such that strength of the steel material may be deteriorated, or the steel material may be heated by exceeding a solidus temperature of the steel material such that hot-rolling properties of the steel material may be deteriorated. Thus, an upper limit of the primary heating temperature range of slab is limited to <NUM>.

A primary hot-rolling process may include a rough-rolling process and a finishing rolling process, and the primarily-heated slab may be size-rolled during the first hot-rolling and may be provided as an intermediate material. A total reduction ratio of the primary hot-rolling is <NUM> to <NUM>%, and the finishing rolling of the primary hot-rolling is performed in a temperature range of <NUM> tc <NUM>. When the finishing hot-rolling temperature of the primary hot-rolling is less than a certain range, an excessive rolling load due to an increase in rolling load may be a problem, and when the finishing hot-rolling temperature of the primary hot-rolling exceeds a certain range, grains may grow coarse and the target strength cannot be obtained.

In order to load an intermediate material into a heating furnace, the intermediate material is cut to a length of <NUM> to <NUM>. When the length of the intermediate material is less than <NUM>, tracking in the heating furnace is difficult, and when the length of the intermediate material exceeds <NUM>, there may be a risk of bending in a longitudinal direction.

The intermediate material is secondarily heated in a temperature range of <NUM> to <NUM>. When a secondary heating temperature is lower than a certain range, there may be a problem in which an excessive rolling load may occur during the secondary hot-rolling, or a problem in that the alloy component is not sufficiently dissolved may occur. Thus, in the present disclosure, a lower limit of the secondary heating temperature range is limited to <NUM>. On the other hand, when the secondary heating temperature exceeds a certain range, grains may grow excessively such that strength of the steel material may be deteriorated, or the steel material may be heated by exceeding a solidus temperature of the steel material such that hot-rolling properties of the steel material may be deteriorated. Thus, in the present disclosure, an upper limit of the secondary heating temperature range of the intermediate material is limited to <NUM>.

A secondary hot-rolling process may include a rough-rolling process and a finishing-rolling process, and the secondarily-reheated intermediate material is provided as an intermediate material by secondary hot-rolling. In this case, the finishing rolling is performed in a temperature range of (Tnr-<NUM>) to Tnr°C. Here, Tnr can be derived by Equation <NUM> below. <MAT> (where, C, Mn, Cu, Cr, and Si are weight percentages of each component).

When a finishing rolling temperature of the secondary hot rolling is less than (Tnr-<NUM>)°C, strength increases rapidly and the impact toughness tends to be deteriorated. When the finishing rolling temperature of the secondary hot rolling exceeds Tnr°C, grains may grow excessively such that strength of the steel material may be deteriorated. Thus, in the present disclosure, the finishing rolling temperature of the secondary hot rolling is limited to a range of (Tnr-<NUM>) to Tnr°C.

In addition, in the present disclosure, a total rolling reduction amount of the intermediate material in the temperature range of (Tnr-<NUM>) to Tnr°C during the secondary hot rolling is controlled to <NUM> to <NUM>%. When the total rolling reduction amount of the intermediate material in the temperature range of (Tnr-<NUM>) to Tnr°C is less than <NUM>%, the desired shape correction effect cannot be achieved, and when the total rolling reduction amount of the intermediate material in the temperature range of (Tnr-<NUM>) to Tnr°C exceeds <NUM>%, there is a concern about a decrease in impact toughness due to excessive reduction.

The secondarily hot-rolled material is cooled to a cooling stop temperature of <NUM> or less at a cooling rate of <NUM> to <NUM>/s. When the cooling rate is less than a certain range, a decrease in ductility of the steel material and deterioration of abrasion resistance may become problems due to carbides precipitated on a grain boundary during cooling, and thus, in the present disclosure, the cooling rate the hot-rolled material is limited to <NUM>/s or more. A lower limit of the preferred cooling rate may be <NUM>/s, and a cooling method may be accelerated cooling. The higher the cooling rate is, the more advantageous the effect of inhibiting carbide precipitation may be, but in consideration of a situation in which it may be difficult to implement a cooling rate exceeding <NUM>/s in general cooling in terms of characteristics of facility, an upper limit of the cooling rate is limited to <NUM>/s in the present disclosure.

Also, even when a hot-rolled material is cooled by applying a cooling rate of <NUM>/s or more, when the cooling is stopped at a high temperature, it may be highly likely that carbides may be created and grown, and thus, in the present disclosure, the cooling stop temperature is limited to <NUM> or less.

The austenitic high-manganese steel material manufactured as above may include <NUM> area% or more of austenite. The austenitic high-manganese steel material has yield strength of <NUM> MPa or more, tensile strength of <NUM> MPa or more, elongation of <NUM>% or more, and Charpy impact toughness of <NUM> J or more (based on a thickness of <NUM>) at -<NUM>.

In addition, the austenitic high-manganese steel material manufactured as described above has a maximum height difference of within <NUM> or less between a crest and a trough formed in the steel material in an area within <NUM> in the longitudinal direction of the steel material, so that excellent shape uniformity can be ensured.

Hereinafter, the present invention will be described in more detail through examples. However, it is necessary to note that the following examples are only intended to illustrate the present invention in more detail and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by matters described in the claims.

A slab having an alloy composition of Table <NUM> below and a thickness of <NUM> was manufactured. Each slab was primarily heated in a temperature range of <NUM> and then primarily hot-rolled at a finishing rolling temperature of <NUM> with a total rolling reduction ratio of <NUM> to <NUM>% to prepare an intermediate material. Each intermediate material was subjected to secondary heating and secondary hot-rolling under the conditions of Table <NUM> to prepare a hot-rolled material specimen, and yield strength, tensile strength, elongation, Charpy impact toughness, and shape uniformity for each specimen were measured and shown in Table <NUM> below. In this case, shape uniformity was described by measuring a maximum height difference between a crest and a trough formed in an area within <NUM> in a rolling direction of a specimen. Here, tensile properties were tested at room temperature according to ASTM A370, and the impact toughness was also measured at -<NUM> by being processed into a <NUM>-thick impact specimen according to the conditions of the same standard.

As shown in Tables <NUM> and <NUM>, the alloy composition and manufacturing process of the disclosure secures the desired physical properties and shape uniformity of the present disclosure in the case of a satisfactory invention example, but does not satisfy the alloy composition or manufacturing process of the present invention in the case of a comparative example.

In the case of the Comparative example, it can be seen that the present invention does not secure the desired physical properties and shape uniformity.

Claim 1:
A cryogenic austenitic high-manganese steel material having an excellent shape, comprising, by weight%, <NUM> to <NUM> % of C, <NUM> to <NUM> % of Mn, <NUM> to <NUM> % of Si, <NUM> to <NUM> % of Cu, <NUM> % or less of P, <NUM> % or less of S, <NUM> to <NUM> % of Al, <NUM> to <NUM> % of Cr, and <NUM> to <NUM> % of B, with a remainder of Fe and other unavoidable impurities,
wherein Charpy impact toughness at -<NUM> measured according to ASTM A370 standard is at least <NUM> J, based on a thickness of <NUM>, a yield strength of <NUM> MPa or more, a tensile strength of <NUM> MPa or more, and an elongation of <NUM>% or more, wherein yield strength, tensile strength and elongating are measured according to ASTM A370 standard, and
a maximum height difference between a crest and a trough formed within an area of <NUM> in a rolling direction is at most <NUM>.