The present disclosure relates to an austenitic stainless steel having excellent resistance to hydrogen embrittlement and cryogenic impact toughness, and a method for manufacturing the same.

The ability of austenitic stainless steel to maintain relatively excellent toughness upon exposure to low temperatures makes it a suitable material for low-temperature steel materials. Its advantageous properties make it suitable for use in cryogenic environments, including LNG, liquid-ammonium, liquid nitrogen, and liquid CO₂. Furthermore, the excellent corrosion resistance, formability, and elongation of austenitic stainless steel allow its use without problems in various shapes and environments tailored to customer needs, making it highly applicable to various parts, pipes, tanks, equipment, and structural materials. A particular advantage also arises from the material's characteristics, providing an aesthetically superior appearance without additional processing.

Reflecting such market needs, the present disclosure provides a method for manufacturing an austenitic stainless steel having excellent resistance to hydrogen embrittlement and cryogenic impact toughness, the steel being applicable for parts, equipment, and tanks for the storage, transport, and usage of liquefied hydrogen, LNG, liquid-ammonium, liquid nitrogen, liquefied CO2, etc.

An aspect of the present disclosure provides an austenitic stainless steel having excellent resistance to hydrogen embrittlement and cryogenic impact toughness, and a method for manufacturing the same.

An austenitic stainless steel having excellent resistance to hydrogen embrittlement and cryogenic impact toughness, comprising, in weight percent (wt%): C: more than 0 to 0.05 wt% or less; Si: greater than 0 to 1.0 wt% or less; Mn: greater than 0 to 5.0 wt% or less; Cr: 16.0 to 25.0 wt%; Mo: greater than 0 to 2.5 wt% or less; Ni: 7.0 to 14.0 wt%; Cu: greater than 0 to 1.0 wt% or less; N: 0.05 to 0.28 wt%; and the balance of Fe and other inevitable impurities, wherein a following α value satisfies 30.0 or more, wherein a tensile strength is 780 MPa or less, and wherein an RRA (Relative Reduction of Area, a ratio of a reduction of area in a hydrogen atmosphere to a reduction of area in an air atmosphere) is 0.88 or more.$$\mathrm{\alpha }=\mathrm{Ni}+0.65\mathrm{Cr}+0.98\mathrm{Mo}+1.05\mathrm{Mn}+0.35\mathrm{Si}+12.6\mathrm{C}+33.6\mathrm{N}\ge 30.0$$
(wherein Ni, Cr, Mo, Mn, Si, C, and N represent the content (wt%) of each element).

Additionally, the austenitic stainless steel having excellent resistance to hydrogen embrittlement and cryogenic impact toughness, according to an example of the present disclosure, has an average delta ferrite area fraction of 3% or less in a region excluding a 1/4t region from a surface in a thickness direction.

Further, an austenitic stainless steel having excellent resistance to hydrogen embrittlement and cryogenic impact toughness according to an example of the present disclosure has a density of delta ferrites having a major axis length of 50 µm or more of 0.04 pieces/cm² or less.

Additionally, an austenitic stainless steel having excellent resistance to hydrogen embrittlement and cryogenic impact toughness, according to an example of the present disclosure, may further comprise, in weight percent (wt%): P: 0.035% or less; and S: 0.01% or less.

Further, the austenitic stainless steel, in an example of the present disclosure, having excellent resistance to hydrogen embrittlement and cryogenic impact toughness, satisfies the α value of 31.0 or more.

Furthermore, the austenitic stainless steel according to an example of the present disclosure, having excellent resistance to hydrogen embrittlement and cryogenic impact toughness, has an RRA (Relative Reduction of Area, a ratio of a reduction of area in a hydrogen atmosphere to a reduction of area in an air atmosphere) of 0.90 or more.

Further, in an example of the present disclosure, an austenitic stainless steel having excellent resistance to hydrogen embrittlement and cryogenic impact toughness has a cryogenic impact toughness at -196°C of 50 J or more.

Also, according to an example of the present disclosure, an austenitic stainless steel having excellent resistance to hydrogen embrittlement and cryogenic impact toughness has a cryogenic impact toughness at -196°C of 100 J or more.

A method for manufacturing an austenitic stainless steel having excellent resistance to hydrogen embrittlement and cryogenic impact toughness, in accordance with another example of the present disclosure, the method comprising the steps of: preparing a slab comprising, in weight percent (wt%): C: more than 0 to 0.05 wt% or less; Si: greater than 0 to 1.0 wt% or less; Mn: greater than 0 to 5.0 wt% or less; Cr: 16.0 to 25.0 wt%; Mo: greater than 0 to 2.5 wt% or less; Ni: 7.0 to 14.0 wt%; Cu: greater than 0 to 1.0 wt% or less; N: 0.05 to 0.28 wt%; and the balance of Fe and other inevitable impurities, wherein the slab satisfies a following α value of 30.0 or more, the α value represented by: heating and then extracting the slab; hot rolling and finish rolling the extracted slab; cooling the rolled steel; and hot-annealing the cooled steel; wherein the manufactured austenitic stainless steel has a tensile strength of 780 MPa or less, and an RRA (Relative Reduction of Area, a ratio of a reduction of area in a hydrogen atmosphere to a reduction of area in an air atmosphere) is 0.88 or more.$$\mathrm{\alpha }=\mathrm{Ni}+0.65\mathrm{Cr}+0.98\mathrm{Mo}+1.05\mathrm{Mn}+0.35\mathrm{Si}+12.6\mathrm{C}+33.6\mathrm{N}\ge 30.0$$
(wherein Ni, Cr, Mo, Mn, Si, C, and N represent the content (wt%) of each element).

In another exemplary embodiment of the present disclosure, in the method for manufacturing an austenitic stainless steel having excellent resistance to hydrogen embrittlement and cryogenic impact toughness, the heating and extracting step may include heating the prepared slab at a temperature of β+20°C or less for 90 to 300 minutes, based on a following precipitation temperature β value, and then extracting the heated slab β = 1759 + 536C - 26Si - 3Mn + 41.3Ni - 51.9Cr + 40.5Cu - 57.3Mo + 786.7N
(wherein Ni, Cr, Mo, Mn, Si, C, N, and Cu represent the content (wt%) of each element).

In addition, in a method for manufacturing an austenitic stainless steel having excellent resistance to hydrogen embrittlement and cryogenic impact toughness according to another embodiment of the present disclosure, the hot rolling and finish rolling may include hot rolling and finish rolling the extracted slab at a temperature of β-70°C or higher and at a reduction ratio of 50% or more.

In another exemplary embodiment of the present disclosure, wherein the cooling step (d) comprises cooling the rolled steel to 600°C at a cooling rate of 50°C/s or less.

In another exemplary embodiment of the present disclosure, wherein the hot-annealing step (e) comprises hot-annealing the cooled steel at 1,050°C or more for 1 to 60 minutes.

In another exemplary embodiment of the present disclosure, wherein, in the hot rolling and finish rolling step (c), an average delta ferrite area fraction in a region excluding a 1/4t region from the surface in the thickness direction is 3% or less, thereby providing an austenitic stainless steel having excellent resistance to hydrogen embrittlement and cryogenic impact toughness.

In another exemplary embodiment of the present disclosure, wherein the austenitic stainless steel manufactured by the method exhibits excellent resistance to hydrogen embrittlement and cryogenic impact toughness, and wherein, in the hot rolling and finish rolling steps (c), a density of delta ferrites having a major axis length of 50 µm or more is 0.04 pieces/cm² or less.

Another example of the present disclosure's method for manufacturing an austenitic stainless steel having excellent resistance to hydrogen embrittlement and excellent cryogenic impact toughness, wherein the austenitic stainless steel further comprises, in weight percent: P: 0.035% or less; and S: 0.01% or less.

Furthermore, a method for manufacturing an austenitic stainless steel having excellent resistance to hydrogen embrittlement and excellent cryogenic impact toughness, according to another example of the present disclosure, satisfies the α value of 31.0 or more.

Further, a manufacturing method for an austenitic stainless steel having excellent resistance to hydrogen embrittlement and excellent cryogenic impact toughness, according to another example embodiment of the present disclosure, has an RRA (Relative Reduction of Area, a ratio of a reduction of area in a hydrogen atmosphere to a reduction of area in an air atmosphere) of 0.90 or more.

Furthermore, another example of the method for manufacturing an austenitic stainless steel having excellent resistance to hydrogen embrittlement and cryogenic impact toughness according to the present disclosure provides an austenitic stainless steel wherein the cryogenic impact toughness at -196°C is 50 J or more.

Furthermore, a manufacturing method for an austenitic stainless steel having excellent resistance to hydrogen embrittlement and cryogenic impact toughness, according to another example of the present disclosure, produces an austenitic stainless steel exhibiting a cryogenic impact toughness at -196°C of 100 J or more.

According to the present disclosure, an austenitic stainless steel having excellent resistance to hydrogen embrittlement and excellent cryogenic impact toughness, and a method for manufacturing the same can be provided.

• FIG. 1 is a photograph of a microstructure observed with an optical microscope in a region excluding a 1/4t region from a surface in a thickness direction of example 1.
• FIG. 2 is an image observed with an optical microscope of a microstructure in a region excluding a 1/4t region from a surface in a thickness direction of Comparative Example 6.

The present disclosure is described in detail with reference to examples. The following examples are provided for sufficiently conveying the spirit of the present disclosure to a person of ordinary skill in the art to which the present disclosure pertains. The present disclosure is not limited to the examples presented herein and may be embodied in other forms. The drawings, for clarifying the present disclosure, omit illustration of parts unrelated to the description, and may represent the size of components somewhat exaggeratedly for aiding understanding.

Throughout the disclosure, a statement that a part "comprises" a component, in the absence of an explicit contrary statement, does not exclude other components but means the potential inclusion of further components.

The singular expression includes the plural expression, absent a clear contextual exception.

First, a description of the austenitic stainless steel according to an example of the present disclosure is provided.

An austenitic stainless steel according to an example of the present disclosure comprises, in weight percent (wt%): C: greater than 0 to 0.05 wt% or less; Si: greater than 0 to 1.0 wt% or less; Mn: greater than 0 to 5.0 wt% or less; Cr: 16.0 to 25.0 wt%; Mo: greater than 0 to 2.5 wt% or less; Ni: 7.0 to 14.0 wt%; Cu: greater than 0 to 1.0 wt% or less; N: 0.05 to 0.28 wt%; and the balance of Fe and other inevitable impurities.

Hereinafter, a description of the reasons for the numerical limitations of the alloy component content in the examples of the present disclosure is provided. Herein, unless otherwise specified, the unit is wt%.

Carbon is an effective element for austenite phase stabilization and can be added for securing the yield strength of the austenitic stainless steel. However, the carbon content being excessive induces grain boundary precipitation of Cr carbides, thereby adversely affecting ductility, toughness, and corrosion resistance. Therefore, the upper limit of the carbon content is restricted to 0.05%. Preferably, the carbon content can be included from 0.015% to 0.045%.

Silicon is addable for improving the strength of the material, simultaneously acting as a deoxidizer during the steelmaking process. The addition of silicon is also an effective element for improving the stacking fault energy of the material. However, excessive addition of silicon, an effective element for stabilizing the ferrite phase, promotes delta (δ) ferrite formation in a cast slab. The promotion of delta (δ) ferrite formation degrades manufacturability and may adversely affect the ductility and cryogenic impact properties of the material. Therefore, the upper limit for silicon is restricted to 1.0 wt%, and preferably, the silicon content is 0.3 wt% to 0.9 wt%.

Manganese is an austenite phase stabilizing element capable of partially replacing Ni in the present disclosure, for enhancing austenite stability. However, an excessive content thereof forms an excessive amount of S-based inclusions (MnS), thereby degrading the ductility, toughness, and corrosion resistance of the austenitic stainless steel. Generation of Mn fumes in a steelmaking process accompanies manufacturing risks. Furthermore, excessive addition causes grain boundary embrittlement, accompanying a chained degradation in material properties. Moreover, manganese content exceeding a specific range hinders the cryogenic impact toughness of the material. Therefore, the upper limit is restricted to 5.0 wt%. Preferably, manganese is included in an amount of 0.4 wt% to 4.7 wt%.

Chromium is a ferrite stabilizing element, effective in suppressing martensite phase formation, and a basic element for securing the corrosion resistance required for stainless steel. It can be added in an amount of 16.0 wt% or more. An excessive content thereof increases manufacturing costs and forms a large amount of delta (δ) ferrite within the slab, thereby causing a deterioration in hot workability and an adverse effect on material properties. Accordingly, the upper limit is restricted to 25.0 wt%. Preferably, it can be included from 16.0 wt% to 22.0 wt%. More preferably, it can be included from 16.1 wt% to 21.5 wt%. Within the aforementioned range, a complex interaction with the entire components of the present disclosure can achieve a more advantageous effect in controlling hydrogen embrittlement resistance and cryogenic impact toughness to a desired range.

Molybdenum is added to improve chloride corrosion resistance, considering the use of the present disclosure in environments such as seawater, which involves frequent use. However, the element's very high price leads to a decrease in cost competitiveness with an excessive content. Furthermore, its function as a strong ferrite stabilizing element forms a large amount of delta (δ) ferrites within a slab, causing a decrease in hot formability and adverse effects on material properties. These considerations restrict its upper limit to 2.5 wt%. A preferred content is 0.6 wt% to 2.3 wt%.

Nickel, as a strong austenitic phase stabilizing element, is essential for securing good material properties. Also, as a positive element for removing internal delta ferrite in the steel and improving manufacturability, the lower limit of nickel can be limited to 7.0 wt%. However, a large addition of Ni, as an expensive element, causes an increase in raw material costs. Therefore, a consideration of both the cost and efficiency of the steel restricts the upper limit of nickel to 14.0 wt%, and nickel can preferably be included from 8.8 wt% to 13.8 wt%.

Copper is an austenitic phase stabilizing element, and is an element added in place of nickel (Ni) in the present disclosure. The addition of copper enhances corrosion resistance in reducing environments. However, an excessive content thereof results in degradation of corrosion resistance, strength, and material properties, and a reduction in productivity. Considering the efficiency and material properties of the steel, an upper limit thereof is restricted to 1.0 wt%, and is preferably included in an amount of 0.2 wt% to 0.8 wt%.

Nitrogen is a strong austenitic stabilizing element. Nitrogen's effectiveness for improving the yield strength of austenitic stainless steel allows for an addition of 0.05 wt% or more, an example being 0.10 wt% or more. An excessive content of the element, however, presents issues. A reduction in stacking fault energy at cryogenic temperatures causes frequent changes from wavy slip to planar slip or a degradation of impact toughness due to short range ordering. Furthermore, the generation of pin holes, among other issues, makes manufacturability difficult. The upper limit is therefore restricted to 0.28 wt%. A preferable inclusion range is 0.12 wt% to 0.26 wt%.

Furthermore, the austenitic stainless steel according to an example of the present disclosure further comprises, in weight percent (wt%): one or more of P: 0.035% or less and S: 0.01% or less.

Phosphorus (P), an inevitable impurity contained in steel, is a main cause of intergranular corrosion or inhibited hot workability. Therefore, controlling its content as low as possible is desirable. In the present disclosure, the upper limit of the said P content is controlled to 0.035% or less.

Sulfur (S) is an inevitable impurity contained in steel. The segregation of said S at grain boundaries is a major cause of hindering hot workability. Therefore, controlling the content of said S as low as possible is desirable. The present disclosure controls the upper limit of the S content to 0.01% or less.

The balance of components of the present disclosure is iron (Fe). However, the inevitable incorporation of unintended impurities from raw materials or the surrounding environment occurs during a typical manufacturing process; thus, the inclusion of such impurities cannot be ruled out. Knowledge of these impurities is common to those skilled in the art of typical manufacturing processes; therefore, specific mention of their entire content in the present disclosure is omitted.

An austenitic stainless steel according to an example of the present disclosure simultaneously satisfies the alloy composition, with a following α value satisfying 30.0 or more. The α value is an essential formula for the material desired to be achieved in the present disclosure, parameterizing a quantified value for controlling the deformation behavior of the material according to the addition of alloy components. Securing the α value of 30.0 or more ensures sufficient stabilization of the austenitic phase, thereby resulting in a tensile strength of 780 MPa or less while preventing martensite phase transformation, and obtaining excellent resistance to hydrogen embrittlement, wherein RRA (Relative Reduction of Area, a ratio of a reduction of area in a hydrogen atmosphere to a reduction of area in an air atmosphere) satisfies 0.88 or more.$$\mathrm{\alpha }=\mathrm{Ni}+0.65\mathrm{Cr}+0.98\mathrm{Mo}+1.05\mathrm{Mn}+0.35\mathrm{Si}+12.6\mathrm{C}+33.6\mathrm{N}\ge 30.0$$
(wherein Ni, Cr, Mo, Mn, Si, C, and N represent the content (wt%) of each element).

Further, an austenitic stainless steel having excellent resistance to hydrogen embrittlement and cryogenic impact toughness, according to an example of the present disclosure, has an average delta ferrite area fraction of 3% or less in a region excluding a 1/4t region from a surface in a thickness direction.

In addition, the austenitic stainless steel having excellent resistance to hydrogen embrittlement and cryogenic impact toughness according to an example of the present disclosure may have a density of delta ferrites having a major axis length of 50 µm or more of 0.04 pieces/cm² or less. Control of the average delta ferrite area fraction and the density of the delta ferrites to the corresponding ranges enables satisfying a cryogenic impact toughness at -196°C of 100 J or more. A delta ferrite area fraction exceeding 3%, or a density of delta ferrite structures having a major axis length of 50 µm or more exceeding 0.04 pieces/cm², results in a problem of degraded cryogenic impact toughness.

An austenitic stainless steel according to an example of the present disclosure, wherein its satisfaction of the above composition enables the achievement of properties applicable for parts, equipment, and tanks for storage, transfer, and use of liquefied hydrogen, LNG, liquefied ammonium, liquid nitrogen, liquefied CO2, and the like.

Additionally, an austenitic stainless steel having excellent resistance to hydrogen embrittlement and cryogenic impact toughness according to an example of the present disclosure has the α value of 30.0 or more. The α value specifically may be 30.0 to 46.4, more specifically 30.0 to 45.0, and even more specifically 31.0 to 40.0.

Furthermore, in an example of the present disclosure, an austenitic stainless steel having excellent resistance to hydrogen embrittlement and cryogenic impact toughness may have an RRA (Relative Reduction of Area, a ratio of a reduction of area in a hydrogen atmosphere to a reduction of area in an air atmosphere) of 0.88 or more. Specifically, the RRA may be 0.88 to 1.2, and more specifically, the RRA may be 0.9 to 1.

A tensile strength greater than 780 MPa results in processing impossibility or product defects during the manufacture of large-scale products in cryogenic environments, such as at -196°C and -253°C. In particular, at ultra-cryogenic temperatures (less than or equal to -256°C), a higher strength can result in a rapid decrease in impact toughness. Furthermore, in manufacturing large-scale products such as liquefied hydrogen storage tanks, strength exceeding a certain standard often renders product manufacturing impossible due to insufficient equipment capacity of the manufacturing facility. However, within a certain range, increasing strength allows for reducing the material's thickness, thereby increasing economic efficiency. Accordingly, in the present disclosure, the tensile strength can be 780 MPa or less, specifically 500 MPa to 780 MPa, more specifically 515 MPa to 780 MPa, and even more specifically, preferably 635 MPa to 750 MPa. Furthermore, the yield strength can be 200 MPa to 500 MPa, specifically 205 MPa to 450 MPa, and more specifically 310 MPa to 400 MPa. Within the aforementioned range, the aforementioned effects can be further enhanced.

Further, an austenitic stainless steel having excellent resistance to hydrogen embrittlement and cryogenic impact toughness, according to an example of the present disclosure, has a cryogenic impact toughness at -196°C of 50 J or more, and specifically, 100 J or more.

Hereinafter, a method for manufacturing an austenitic stainless steel according to an example of the present disclosure is described.

A method for manufacturing an austenitic stainless steel according to an example of the present disclosure, the method comprising the steps of: preparing a slab comprising, in weight percent (wt%): C: more than 0 to 0.05 wt% or less; Si: greater than 0 to 1.0 wt% or less; Mn: greater than 0 to 5.0 wt% or less; Cr: 16.0 to 25.0 wt%; Mo: greater than 0 to 2.5 wt% or less; Ni: 7.0 to 14.0 wt%; Cu: greater than 0 to 1.0 wt% or less; N: 0.05 to 0.28 wt%; the balance of Fe and other inevitable impurities, wherein the slab satisfies a following α value of 30.0 or more;

• heating and then extracting the slab;
• hot rolling and finish rolling the extracted slab;
• cooling the rolled steel; and
• comprising the step of hot-annealing the cooled steel;
• wherein a tensile strength is 780 MPa or less, and wherein an RRA (Relative Reduction of Area, a ratio of a reduction of area in a hydrogen atmosphere to a reduction of area in an air atmosphere) is 0.88 or more.
• wherein a following α value satisfies 30.0 or more: α = Ni + 0.65Cr + 0.98Mo + 1.05Mn + 0.35Si + 12.6C + 33.6N ≥ 30.0
• (wherein Ni, Cr, Mo, Mn, Si, C, and N represent the content (wt%) of each element).

Additionally, in another example of the present disclosure, the heating and extracting step for manufacturing an austenitic stainless steel having excellent resistance to hydrogen embrittlement and cryogenic impact toughness can comprise heating the prepared slab for 90 to 300 minutes at a temperature of β+20°C or less, based on the following precipitation temperature β value, and then extracting the heated slab. β = 1759 + 536C - 26Si - 3Mn + 41.3Ni - 51.9Cr + 40.5Cu - 57.3Mo + 786.7N
(wherein Ni, Cr, Mo, Mn, Si, C, N, and Cu represent the content (wt%) of each element)

The composition and Formula α are as set forth above, and said β is a reference value for a process in the present disclosure, the process controlling the fraction and shape of delta ferrite for ensuring cryogenic impact toughness.

The present disclosure requires sufficient heating, before hot rolling, of a slab satisfying a following α value of 30.0 or more. Said heating occurs for 90 to 300 minutes at a temperature of β+20°C or less, based on a following precipitation temperature β value. Heating before hot rolling at a temperature greater than β+20°C causes an excessive level of delta ferrite to remain inside the steel after hot rolling and annealing. This remaining delta ferrite is due to an influence of excessive delta ferrites generated during heating, thereby degrading cryogenic impact toughness.

Further, in another example of the method for manufacturing an austenitic stainless steel having excellent resistance to hydrogen embrittlement and excellent cryogenic impact toughness of the present disclosure, the hot rolling and finish rolling step comprises hot rolling and finish rolling the extracted slab at a temperature of β-70°C or more with a rolling reduction of 50% or more. Non-achievement of a rolling reduction of 50% or more at a temperature of β-70°C or more causes delta ferrites to become relatively elongated in the rolling direction, making securing cryogenic impact toughness impossible even with a low absolute delta ferrite area fraction.

Further, according to another example of the present disclosure, a method for manufacturing an austenitic stainless steel having excellent resistance to hydrogen embrittlement and cryogenic impact toughness comprises the hot rolling and finish rolling step comprising hot rolling and finish rolling the extracted slab at a temperature of β-70°C or more with a rolling reduction of 50% or more.

In one embodiment, the cooling step (d) comprises cooling the rolled steel to 600°C at a cooling rate of 50°C/s or less.

Furthermore, a method for manufacturing an austenitic stainless steel having excellent resistance to hydrogen embrittlement and cryogenic impact toughness, according to another example of the present disclosure, comprises the hot-annealing step (e) comprising hot-annealing the cooled steel at 1,050°C or more for 1 to 60 minutes.

In one embodiment, in the hot rolling and finish rolling step, an average delta ferrite area fraction in a region excluding a 1/4t region from a surface in a thickness direction is 3% or less, thereby manufacturing an austenitic stainless steel having excellent resistance to hydrogen embrittlement and cryogenic impact toughness.

Furthermore, a method for manufacturing an austenitic stainless steel having excellent resistance to hydrogen embrittlement and cryogenic impact toughness, in another example of the present disclosure, is characterized in that, in the said hot rolling and finish rolling step, the density of delta ferrites having a major axis length of 50 µm or more is 0.04 pieces/cm² or less.

Conventional manufacturing of austenitic stainless steel presents a problem wherein delta ferrite phases generated within the material exhibit very weak toughness in cryogenic environments, thereby offsetting the overall advantages of the product. The present disclosure, for solving such problems, enables control of the delta ferrite fraction and shape through adjustment of the composition and main manufacturing method as described above. Through this, the present disclosure provides an austenitic stainless steel and a manufacturing method thereof, enabling the securing of high cryogenic impact toughness by improving the problems of existing products.

Hereinafter, the present disclosure is explained in more detail through examples. However, it should be noted that the following examples are merely for illustrating and explaining the present disclosure in more detail, and do not limit the scope of the present disclosure. The scope of the present disclosure is determined by the matters described in the claims and those reasonably inferable therefrom.

An austenitic stainless steel was manufactured from a slab having the alloy composition described in Table 1 below, under the manufacturing conditions described in Table 2 below. Except for the main manufacturing conditions described in Table 2, the heating time before hot rolling was 200 minutes, the total rolling reduction was 85%, the cooling rate after hot rolling was 30°C/s or less to 600°C, and annealing after cooling proceeded at 1100°C for 20 minutes.

Table 1 shows the major components of the comparative examples and examples of austenitic stainless steel. Also, Table 2 shows the α and β values derived from Table 1, the heating furnace temperature, and the rolling reduction in the temperature range of β-70°C or more. Category Chemical Compositions of Austenitic Stainless Steels (wt%) C Si Mn Ni Cr Cu Mo N Comparative Example 1 0.02 0.4 1.6 8.1 18.1 0.4 0.1 0.04 Comparative Example 2 0.02 0.3 1.2 10.2 16.5 0.3 2.1 0.02 Comparative Example 3 0.03 0.6 3 7 16 0.4 1.5 0.19 Comparative Example 4 0.02 0.4 1.8 8.5 20.1 0.5 0.8 0.18 Comparative Example 5 0.025 0.4 3.8 7.1 20.7 0.5 0.7 0.21 Comparative Example 6 0.02 0.4 1.9 9.3 20.5 0.8 0.6 0.18 Comparative Example 7 0.015 0.6 0.4 13.8 21.5 0.2 2.3 0.26 Comparative Example 8 0.02 0.4 1.8 8.1 20.5 0.5 0.5 0.25 Comparative Example 9 0.02 0.3 3.7 9.3 19.5 0.8 0.6 0.16 Comparative Example 10 0.025 0.4 2.5 11.2 20.8 0.7 0.7 0.17 Comparative Example 11 0.045 0.9 4.7 8.8 16.1 0.2 2 0.12 Comparative Example 12 0.040 0.8 6.5 6.5 16.5 0.4 0.1 0.08 Comparative Example 13 0.030 0.4 4.5 11.5 24.5 0.4 0.6 0.05 Invention Example 1 0.02 0.4 1.9 9.3 20.5 0.8 0.6 0.18 Invention Example 2 0.015 0.6 0.4 13.8 21.5 0.2 2.3 0.26 Invention Example 3 0.02 0.3 3.7 9.3 19.5 0.8 0.6 0.16 Invention Example 4 0.025 0.4 2.5 11.2 20.8 0.7 0.7 0.17 Invention Example 5 0.045 0.9 4.7 8.8 16.1 0.2 2 0.12 Category α Value β Value (°C) Heating Furnace Temperature (°C) Rolling Reduction at β-70°C or Higher(%) Comparative Example 1 23.4 1191.6 1200 55% Comparative Example 2 25.3 1230.8 1240 55% Comparative Example 3 29.0 1288.9 1240 55% Comparative Example 4 30.7 1177.8 1220 55% Comparative Example 5 32.7 1114.8 1240 55% Comparative Example 6 31.6 1213.4 1220 48% Comparative Example 7 39.6 1285.2 1220 40% Comparative Example 8 32.6 1212.8 1240 44% Comparative Example 9 32.2 1246.8 1280 55% Comparative Example 10 34.2 1259.5 1240 45% Comparative Example 11 31.1 1261.4 1240 40% Comparative Example 12 27.6 1225.6 1200 55% Comparative Example 13 34.9 975.7 1200 80% Invention Example 1 31.6 1213.4 1220 65% Invention Example 2 39.6 1285.2 1260 52% Invention Example 3 32.2 1246.8 1240 61% Invention Example 4 34.2 1259.5 1240 59% Invention Example 5 31.1 1261.4 1260 55%

Test method: Yield strength YS (MPa) and tensile strength TS (MPa) values were measured after performing tensile tests at room temperature on ASTM E8/E8M standard tensile test specimens with a crosshead speed in the range of 10 mm/min to 20 mm/min. Table 4 below shows the delta ferrite area fraction, the delta ferrite morphology satisfaction, and the secured resistance to hydrogen embrittlement (RRA) and -196°C cryogenic impact toughness values for the comparative examples and examples according to Tables 1 and 2 above. The results of evaluating each physical property according to the criteria below are shown in Table 3 below. Additionally, FIGS. 1 and 2 are micrographs showing the microstructure of example 1 and comparative example 6 observed by scanning electron microscope (SEM).

The delta ferrite area fraction measurement utilized the average delta ferrite in a region excluding a 1/4t region from a surface in the thickness direction, after creating a profile using a ferrite scope device. Additionally, an analysis of the shape of the delta ferrites was performed by using images at 200x and 500x optical magnifications for each thickness direction, wherein a rejection decision was made for cases exceeding a standard. The reduction of area was calculated through a scanning electron microscope (SEM) after conducting an SSRT test in accordance with ASTM G142-98 and ASTM G129. The cryogenic impact toughness was measured and presented as Charpy impact toughness at a temperature of -196°C using ASTM E23 type A specimen specifications.

The α value less than 30.0 was evaluated as X, and the α value satisfying 30.0 or more was evaluated as O.

Additionally, a measured tensile strength value of 780 MPa or less received an 'O' evaluation, and a value greater than 780 MPa received an 'X' evaluation.

An RRA of 0.88 or more was evaluated as 'O', and an RRA less than 0.88 was evaluated as 'X'.

Further, the cryogenic impact toughness at -196°C is evaluated as ⊚ when 100 J or more, as O when 65 J to 100 J, and as '△' when 45 J to 65 J. Category α Value Satisfaction Tensile Strength Evaluation Hydrogen Embrittlement Resistance -196°C Impact Tough-ness Comparative Example 1 X ○ X ⊚ Comparative Example 2 X ○ X ⊚ Comparative Example 3 X ○ X ⊚ Comparative Example 4 30.7 ○ ○ ○ Comparative Example 5 32.7 ○ ○ ○ Comparative Example 6 31.6 ○ ○ ○ Comparative Example 7 39.6 ○ ○ ○ Comparative Example 8 32.6 ○ ○ ○ Comparative Example 9 32.2 ○ ○ ○ Comparative Example 10 34.2 ○ ○ ○ Comparative Example 11 31.1 ○ ○ ○ Comparative Example 12 X ○ X ⊚ Comparative Example 13 34.9 ○ ○ Δ Invention Example 1 31.6 ○ ○ ⊚ Invention Example 2 39.6 ○ ○ ⊚ Invention Example 3 32.2 ○ ○ ⊚ Invention Example 4 34.2 ○ ○ ⊚ Invention Example 5 31.1 ○ ○ ⊚ Category Delta Ferrite Area Fraction* (%) Delta Ferrite Morphology Satisfaction** Resistance to Hydrogen Embrittlement (RRA) -196°C Impact Toughness (J) Comparative Example 1 2.9 Excellent 0.62 105.8 Comparative Example 2 2.5 Excellent 0.75 120.3 Comparative Example 3 2.6 Excellent 0.85 102.4 Comparative Example 4 3.7 Excellent 0.88 72.8 Comparative Example 5 5.2 Excellent 0.96 65.5 Comparative Example 6 2.9 Good 0.95 81.5 Comparative Example 7 1.9 Good 0.97 97.4 Comparative Example 8 4.8 Good 0.94 56.4 Comparative Example 9 3.5 Excellent 0.95 88.9 Comparative Example 10 2.7 Good 0.96 90.5 Comparative Example 11 1.5 Good 0.94 91.5 Comparative Example 12 2.9 Good 0.85 104.5 Comparative Example 13 8.8 Excellent 0.91 46.9 Invention Example 1 2.2 Excellent 0.94 115.5 Invention Example 2 1.7 Excellent 0.97 125.2 Invention Example 3 2 Excellent 0.95 123.1 Invention Example 4 1.4 Excellent 0.95 140.4 Invention Example 5 0.9 Excellent 0.94 150.7

For each of Examples 1 to 5, satisfaction of an α value of 30.0 or more and process conditions based on the β value resulted in an RRA of 0.88 or more and a cryogenic impact toughness of 100 J or more at -196°C, thereby demonstrating excellent resistance to hydrogen embrittlement and cryogenic impact toughness.

In contrast, Comparative Examples 1 to Comparative Example 3 exhibited inferior resistance to hydrogen embrittlement, with an RRA less than 0.88. This inferiority resulted from the austenite phase stabilization degree α being less than 30.0, causing martensite phase transformation.

In the foregoing, although example modes of the present disclosure have been described, the present disclosure is not limited thereto. A person of ordinary skill in the pertinent technical field understands the possibility of various changes and modifications without departing from the concept and scope of the claims described in the following.