Austenitic stainless steel with improved hydrogen embrittlement resistance and low-temperature impact toughness, and method for manufacturing same

Abstract

The present invention provides an austenitic stainless steel with improved hydrogen embrittlement resistance and low-temperature impact toughness, and a method for manufacturing same. The austenitic stainless steel according to the present invention comprises, by wt%, more than 0% and less than or equal to 0.05% of carbon (C), more than 0% and less than or equal to 1.0% of silicon (Si), more than 0% and less than or equal to 5.0% of manganese (Mn), 16.0-25.0% of chromium (Cr), more than 0% and less than or equal to 2.5% of molybdenum (Mo), 7.0-14.0% of nickel (Ni), more than 0% and less than or equal to 1.0% of copper (Cu), 0.05-0.28% of nitrogen (N), and the balance of Fe and other inevitable impurities, wherein value α satisfies 30.0 or more, the average area fraction of delta ferrite in the region excluding the 1 / 4t region in the thickness direction from the surface is 3% or less, and the density of delta ferrite having a major-axis length of 50 μm or more is 0.04 / cm2 or less. α = Ni+0.65Cr+0.98Mo+1.05Mn+0.35Si+12.6C+33.6N ≥ 30.0 (where Ni, Cr, Mo, Mn, Si, C and N represent the amounts of each element).

Description

Austenitic stainless steel with excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness and its manufacturing method

[0001] The present invention relates to an austenitic stainless steel having excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness and a method for manufacturing the same.

[0002] Austenitic stainless steels maintain relatively excellent toughness when exposed to low temperatures, making them ideal for low-temperature applications. These advantages make them suitable for use in cryogenic environments, such as LNG, liquefied ammonium, liquefied nitrogen, and liquefied CO2. Furthermore, austenitic stainless steels boast superior corrosion resistance, formability, and elongation, allowing them to be shaped and used in a variety of environments to meet diverse customer needs. This makes them ideal for a variety of components, pipes, tanks, equipment, and structural materials. Furthermore, their aesthetic appeal, due to their inherent properties, allows for the production of aesthetically pleasing parts without the need for additional processing.

[0003] The present invention reflects the needs of the market as described above and provides a method for manufacturing an austenitic stainless steel having excellent hydrogen embrittlement resistance and ultra-low temperature toughness that can be applied to parts, equipment, and tanks for the storage, transport, and use of liquefied hydrogen, LNG, liquefied ammonium, liquid nitrogen, liquefied CO2, etc.

[0004] One aspect of the present invention is to provide an austenitic stainless steel having excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness and a method for manufacturing the same.

[0005] According to one embodiment of the present invention, an austenitic stainless steel having excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness comprises, in wt%, carbon (C): more than 0% to 0.05% or less, silicon (Si): more than 0% to 1.0% or less, manganese (Mn): more than 0% to 5.0% or less, chromium (Cr): 16.0% to 25.0%, molybdenum (Mo): more than 0% to 2.5% or less, nickel (Ni): 7.0% to 14.0%, copper (Cu): more than 0% to 1.0% or less, nitrogen (N): 0.05% to 0.28%, the remainder being Fe and other unavoidable impurities, and satisfying the following α value of 30.0 or more, a tensile strength of 780 MPa or less, and an RRA (Relative Reduction of Area, area reduction rate in a hydrogen atmosphere / area reduction rate in an air atmosphere) of It is 0.88 or higher.

[0006] α= Ni+0.65Cr+0.98Mo+1.05Mn+0.35Si+12.6C+33.6N ≥ 30.0

[0007] (Here, Ni, Cr, Mo, Mn, Si, C, N represent the content of each element)

[0008] In addition, the austenitic stainless steel having excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness according to one embodiment of the present invention may have an average delta ferrite area fraction of 3% or less in a region excluding a 1 / 4t region in the thickness direction from the surface.

[0009] In addition, an austenitic stainless steel having excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness according to one embodiment of the present invention has a density of delta ferrite having a major axis length of 50 μm or more of 0.04 / cm 2 It could be as follows:

[0010] In addition, the austenitic stainless steel having excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness according to one embodiment of the present invention may further include phosphorus (P): 0.035% or less and sulfur (S): 0.01% or less.

[0011] In addition, the austenitic stainless steel having excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness according to one embodiment of the present invention may have an α value of 31.0 or more.

[0012] In addition, the austenitic stainless steel having excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness according to one embodiment of the present invention may have an RRA (Relative Reduction of Area, area reduction ratio in hydrogen atmosphere / area reduction ratio in air atmosphere) of 0.90 or more.

[0013] In addition, an austenitic stainless steel having excellent hydrogen embrittlement resistance and cryogenic impact toughness according to one embodiment of the present invention may have a cryogenic impact toughness of 50 J or more at -196°C.

[0014] In addition, an austenitic stainless steel having excellent hydrogen embrittlement resistance and cryogenic impact toughness according to one embodiment of the present invention may have a cryogenic impact toughness of 100 J or more at -196°C.

[0015] According to another embodiment of the present invention, a method for manufacturing an austenitic stainless steel having excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness comprises the steps of: providing a slab containing, in wt%, carbon (C): more than 0% to 0.05% or less, silicon (Si): more than 0% to 1.0% or less, manganese (Mn): more than 0% to 5.0% or less, chromium (Cr): 16.0% to 25.0%, molybdenum (Mo): more than 0% to 2.5% or less, nickel (Ni): 7.0% to 14.0%, copper (Cu): more than 0% to 1.0% or less, nitrogen (N): 0.05% to 0.28%, the remainder being Fe and other unavoidable impurities, and satisfying the following α value of 30.0 or more; heating the slab and then extracting it; hot rolling and finish rolling the extracted slab; A step of cooling the above-mentioned rolled steel and a step of hot-rolling and annealing the cooled steel are included, and the austenitic stainless steel having excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness has a tensile strength of 780 MPa or less and an RRA (Relative Reduction of Area, area reduction rate in a hydrogen atmosphere / area reduction rate in an air atmosphere) of 0.88 or more.

[0016] α = Ni+0.65Cr+0.98Mo+1.05Mn+0.35Si+12.6C+33.6N ≥ 30.0

[0017] (Here, Ni, Cr, Mo, Mn, Si, C, N, Cr, Cu represent the content of each element)

[0018] In addition, a method for manufacturing an austenitic stainless steel having excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness according to another embodiment of the present invention may include the step of extracting after heating, heating the prepared slab at a temperature of β+20°C or lower for 90 to 300 minutes based on the precipitation temperature β value, and then extracting.

[0019] β = 1759+536C-26Si-3Mn+41.3Ni-51.9Cr+40.5Cu-57.3Mo+786.7N

[0020] (Here, Ni, Cr, Mo, Mn, Si, C, N, Cr, Cu represent the content of each element)

[0021] In addition, a method for manufacturing an austenitic stainless steel having excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness according to another embodiment of the present invention may include hot rolling and finish rolling the extracted slab at a temperature of β-70°C or higher with a reduction ratio of 50% or higher.

[0022] In addition, a method for manufacturing an austenitic stainless steel having excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness according to another embodiment of the present invention may include the cooling step including cooling the rolled steel material to 600°C at a cooling rate of 50°C / s or less.

[0023] In addition, a method for manufacturing an austenitic stainless steel having excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness according to another embodiment of the present invention may include the step of hot rolling annealing the cooled steel material at 1,050°C or higher for 1 to 60 minutes.

[0024] In addition, in a method for manufacturing an austenitic stainless steel having excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness according to another embodiment of the present invention, in the hot rolling and finish rolling steps, the average delta ferrite area fraction in a region excluding a 1 / 4t region in the thickness direction from the surface may be 3% or less.

[0025] In addition, according to another embodiment of the present invention, a method for manufacturing an austenitic stainless steel having excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness is provided in which, in the hot rolling and finish rolling steps, the density of delta ferrite having a major axis length of 50 μm or more is 0.04 / cm. 2 It could be as follows:

[0026] In addition, a method for manufacturing an austenitic stainless steel having excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness according to another embodiment of the present invention may further include phosphorus (P) of 0.035% or less and sulfur (S) of 0.01% or less.

[0027] In addition, a method for manufacturing an austenitic stainless steel having excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness according to another embodiment of the present invention can satisfy the above α value of 31.0 or more.

[0028] In addition, a method for manufacturing an austenitic stainless steel having excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness according to another embodiment of the present invention may have an RRA (Relative Reduction of Area, area reduction ratio in hydrogen atmosphere / area reduction ratio in air atmosphere) of 0.90 or more.

[0029] In addition, a method for manufacturing an austenitic stainless steel having excellent hydrogen embrittlement resistance and cryogenic impact toughness according to another embodiment of the present invention may have a cryogenic impact toughness of 50 J or more at -196°C.

[0030] In addition, a method for manufacturing an austenitic stainless steel having excellent hydrogen embrittlement resistance and cryogenic impact toughness according to another embodiment of the present invention may have a cryogenic impact toughness of 100 J or more at -196°C.

[0031] According to the present invention, an austenitic stainless steel having excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness and a method for manufacturing the same can be provided.

[0032] Figure 1 is a photograph of the microstructure observed using an optical microscope in an area excluding the 1 / 4t area in the thickness direction from the surface of Example 1.

[0033] Figure 2 is a photograph of the microstructure observed using an optical microscope in an area excluding the 1 / 4t area in the thickness direction from the surface of Comparative Example 6.

[0034] Hereinafter, the present invention will be described in detail with reference to embodiments. The following embodiments are presented to fully convey the spirit of the present invention to those skilled in the art. The present invention is not limited to the embodiments presented herein and may be embodied in other forms. To clarify the present invention, the drawings may omit portions irrelevant to the description, and the sizes of components may be slightly exaggerated to facilitate understanding.

[0035] Throughout the specification, whenever a part is said to "include" a component, this does not mean that it excludes other components, but rather that it may include other components, unless otherwise stated.

[0036] Singular expressions include plural expressions unless the context clearly indicates otherwise.

[0037] First, an austenitic stainless steel according to one embodiment of the present invention will be described.

[0038] An austenitic stainless steel according to one embodiment of the present invention contains, in wt%, carbon (C): more than 0% to 0.05% or less, silicon (Si): more than 0% to 1.0% or less, manganese (Mn): more than 0% to 5.0% or less, chromium (Cr): 16.0% to 25.0%, molybdenum (Mo): more than 0% to 2.5% or less, nickel (Ni): 7.0% to 14.0%, copper (Cu): more than 0% to 1.0% or less, nitrogen (N): 0.05% to 0.28%, the remainder being Fe and other unavoidable impurities.

[0039] Hereinafter, the reasons for numerical limitations on the alloy component content in the embodiments of the present invention will be described. Hereinafter, unless otherwise specified, the unit is weight percent.

[0040] Carbon (C): More than 0% and less than or equal to 0.05%

[0041] Carbon is an element that is effective in stabilizing the austenite phase and can be added to secure the yield strength of austenitic stainless steel. However, if the content is excessive, it can induce grain boundary precipitation of chromium carbides, which can have a negative effect on ductility, toughness, and corrosion resistance. Therefore, the upper limit is limited to 0.05%, and preferably, it can be included in a range of 0.015% to 0.045%.

[0042] Silicon (Si): greater than 0% and less than or equal to 1.0%

[0043] Silicon can be added to improve the strength of materials while acting as a deoxidizer during the steelmaking process, and can be added as an effective element for improving the stacking fault energy of materials. However, silicon is an element effective in stabilizing the ferrite phase, and if added excessively, it can promote the formation of delta (δ) ferrite in the cast slab, which not only reduces manufacturability but also adversely affect the ductility and cryogenic impact properties of the material. Therefore, the upper limit is limited to 1.0%, and preferably, it can be included in an amount of 0.3% to 0.9%.

[0044] Manganese (Mn): 0% or more and 5.0% or less

[0045] Manganese is an austenite phase stabilizing element that can partially replace nickel (Ni) in the present invention, and can be added to improve austenite stability. However, if the content is excessive, it may form excessive S-type inclusions (MnS), which may reduce the ductility, toughness, and corrosion resistance of the austenitic stainless steel. In addition, it may generate manganese fumes during the steelmaking process, which may pose a manufacturing risk. In addition, excessive addition may cause grain boundary embrittlement, which may lead to a chain deterioration of the material. In addition, manganese deteriorates the ultra-low temperature impact toughness of the material when it exceeds a certain range, so its upper limit is limited to 5.0%, and preferably, it may be included in an amount of 0.4% to 4.7%.

[0046] Chromium (Cr): 16.0% to 25.0%

[0047] Chromium is a ferrite stabilizing element, but it is effective in suppressing the formation of martensite phase, and it can be added in an amount of 16.0% or more as a basic element that secures the corrosion resistance required for stainless steel. However, if the content is excessive, the manufacturing cost increases, and a large amount of delta (δ) ferrite is formed in the slab, which causes a decrease in hot workability and a negative effect on material properties. Therefore, the upper limit is limited to 25.0%, and preferably 16.0% to 22.0%, and more preferably 16.1% to 21.5%. Within the above range, it can be more advantageous in controlling hydrogen embrittlement resistance and ultra-low temperature impact toughness to the target range by acting in combination with the components of the entire present invention.

[0048] Molybdenum (Mo): 0% or more and 2.5% or less

[0049] Considering the frequent use of the present invention in an atmosphere such as seawater, molybdenum can be added to improve Cl-base corrosion resistance. However, since the element is very expensive, excessive content reduces cost competitiveness and, as a strong ferrite stabilizing element, forms a large amount of delta (δ) ferrite within the slab, which reduces hot workability and adversely affects material properties. Therefore, the upper limit is limited to 2.5%, and preferably, it can be included in an amount of 0.6% to 2.3%.

[0050] Nickel (Ni): 7.0% to 14.0%

[0051] Nickel is a powerful austenite-stabilizing element essential for ensuring good material properties. It also contributes to the removal of internal delta-ferrite in steel and improves manufacturability, and its lower limit can be set at 7.0%. However, nickel is an expensive element, and adding large amounts increases raw material costs. Therefore, considering both cost and efficiency of the steel, the upper limit is limited to 14.0%, and preferably, it can be included between 8.8% and 13.8%.

[0052] Copper (Cu): 0% or more and 1.0% or less

[0053] Copper is an austenite-stabilizing element and is added in place of nickel (Ni) in the present invention. Cu can be added to improve corrosion resistance in reducing environments. However, excessive Cu content can lead to reduced corrosion resistance, strength, and material properties, as well as reduced productivity. Therefore, considering the efficiency and material properties of the steel, the upper limit is limited to 1.0%, and preferably, it can be included at 0.2% to 0.8%.

[0054] Nitrogen (N): 0.05% to 0.28%

[0055] Nitrogen is a strong austenite stabilizing element and is an effective element for improving the yield strength of austenitic stainless steel. It can be added in amounts of 0.05%, for example, 0.10% or more. However, if the content is excessive, it can frequently cause wavy slip to change into planar slip due to a decrease in stacking fault energy at extremely low temperatures, or cause a decrease in impact toughness due to short-range ordering. In addition, it can cause problems such as generating pin holes, making manufacturing difficult. Therefore, the upper limit is limited to 0.28%, and preferably, it can be included in an amount of 0.12% to 0.26%.

[0056] In addition, the austenitic stainless steel according to one embodiment of the present invention may further include at least one of P: 0.035% or less and S: 0.01% or less.

[0057] The P content is less than 0.035%.

[0058] Phosphorus (P) is an impurity that is inevitably contained in steel and is a major cause of intergranular corrosion and impaired hot workability. Therefore, it is desirable to control its content as low as possible. In the present invention, the upper limit of the P content is controlled to 0.035% or less.

[0059] The content of S is less than 0.01%.

[0060] Sulfur (S) is an impurity that is inevitably contained in steel. It is an element that segregates at grain boundaries and is the main cause of deteriorating hot workability. Therefore, it is desirable to control its content as low as possible. In the present invention, the upper limit of the S content is controlled to 0.01% or less.

[0061] The remaining component of the present invention is iron (Fe). However, during the typical manufacturing process, unintended impurities from raw materials or the surrounding environment may inevitably be mixed in, and thus cannot be excluded. Since these impurities are readily apparent to anyone skilled in the art of typical manufacturing, their full details are not specifically discussed in this specification.

[0062] The austenitic stainless steel according to one embodiment of the present invention must satisfy the above alloy composition and at the same time, the following formula α must be 30.0 or more. The formula α is an essential formula for the material to be achieved in the present invention, and is a parameter that numerically quantifies the control of the deformation behavior of the material according to the addition of the alloying component. When the formula α is secured as "30.0" or more, sufficient stability of the austenite phase can be secured, and accordingly, the tensile strength is 780 MPa or less, and martensite phase transformation does not occur, so that excellent hydrogen embrittlement resistance can be secured with RRA (Relative Reduction of Area, area reduction rate in hydrogen atmosphere / area reduction rate in air atmosphere) satisfying 0.88 or more.

[0063] α= Ni+0.65Cr+0.98Mo+1.05Mn+0.35Si+12.6C+33.6N ≥ 30.0

[0064] (Here, Ni, Cr, Mo, Mn, Si, C, N represent the content of each element)

[0065] In addition, the austenitic stainless steel having excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness according to one embodiment of the present invention may have an average delta ferrite area fraction of 3% or less in a region excluding a 1 / 4t region in the thickness direction from the surface.

[0066] In addition, an austenitic stainless steel having excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness according to one embodiment of the present invention has a density of delta ferrite having a major axis length of 50 μm or more of 0.04 / cm 2 It may be below. By controlling the average delta ferrite area fraction and the density of delta ferrite within the corresponding range, the ultra-low temperature toughness of -196℃ can be satisfied at 100J or more, and when the delta ferrite area fraction exceeds 3% or the delta ferrite structure with a major axis length of 50μm or more is 0.04 / cm 2 There is a problem that the ultra-low temperature toughness deteriorates when it exceeds .

[0067] The austenitic stainless steel according to one embodiment of the present invention can realize properties applicable to parts, equipment, and tanks for the purpose of storage, transport, and use of liquefied hydrogen, LNG, liquefied ammonium, liquid nitrogen, liquefied CO2, etc. by satisfying the above configuration.

[0068] In addition, the austenitic stainless steel having excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness according to one embodiment of the present invention may have an α value of 30.0 or more. Specifically, it may be 30.0 to 46.4, more specifically, it may be 30.0 to 45.0, and even more specifically, it may be 31.0 to 40.0.

[0069] In addition, the austenitic stainless steel having excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness according to one embodiment of the present invention may have an RRA (Relative Reduction of Area, area reduction ratio in hydrogen atmosphere / area reduction ratio in air atmosphere) of 0.88 or more. Specifically, it may be 0.88 to 1.2, and more specifically, it may be 0.9 to 1.

[0070] If the tensile strength exceeds 780 MPa, it may be impossible to process or the product may be defective when manufacturing large-scale products in extremely low-temperature environments, such as -196°C or -253°C. In particular, at ultra-low temperatures (below -256℃), a higher strength can cause a rapid decline in impact toughness, and in addition, when manufacturing large-scale products such as liquid hydrogen storage tanks, if the strength exceeds a certain standard, it is often impossible to manufacture the product due to insufficient equipment capacity of the manufacturing equipment. However, within a certain range, a higher strength can reduce the thickness of the material used, thereby increasing economic efficiency. Therefore, in the present invention, the tensile strength may 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. In addition, the yield strength may be 200 MPa to 500 MPa, specifically 205 MPa to 450 MPa, and more specifically 310 MPa to 400 MPa. Within the above range, the aforementioned effects can be further improved.

[0071] In addition, the austenitic stainless steel having excellent hydrogen embrittlement resistance and cryogenic impact toughness according to one embodiment of the present invention may have a cryogenic impact toughness of -196°C of 50 J or more, and specifically, 100 J or more.

[0072] Hereinafter, a method for manufacturing austenitic stainless steel according to one embodiment of the present invention will be described.

[0073] A method for manufacturing an austenitic stainless steel according to one embodiment of the present invention comprises the steps of: preparing a slab containing, in wt%, carbon (C): more than 0% and 0.05% or less, silicon (Si): more than 0% and 1.0% or less, manganese (Mn): more than 0% and 5.0% or less, chromium (Cr): 16.0% to 25.0%, molybdenum (Mo): more than 0% and 2.5% or less, nickel (Ni): more than 7.0% and 14.0%, copper (Cu): more than 0% and 1.0% or less, nitrogen (N): 0.05% to 0.28%, the remainder being Fe and other unavoidable impurities, and satisfying the following α value of 30.0 or more;

[0074] A step of extracting after heating the above slab;

[0075] A step of hot rolling and finish rolling the above extracted slab;

[0076] A step of cooling the rolled steel; and

[0077] A step of hot-rolling and annealing the above-mentioned cooled steel material;

[0078] The tensile strength is 780 MPa or less, and the RRA (Relative Reduction of Area, cross-sectional reduction rate in a hydrogen atmosphere / cross-sectional reduction rate in an air atmosphere) is 0.88 or more.

[0079] α = Ni+0.65Cr+0.98Mo+1.05Mn+0.35Si+12.6C+33.6N ≥ 30.0

[0080] (Here, Ni, Cr, Mo, Mn, Si, C, N, Cr, Cu represent the content of each element)

[0081] In addition, a method for manufacturing an austenitic stainless steel having excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness according to another embodiment of the present invention may include the step of extracting after heating, heating the prepared slab at a temperature of β+20°C or lower for 90 to 300 minutes based on the precipitation temperature β value, and then extracting.

[0082] β = 1759+536C-26Si-3Mn+41.3Ni-51.9Cr+40.5Cu-57.3Mo+786.7N

[0083] (Here, Ni, Cr, Mo, Mn, Si, C, N, Cr, Cu represent the content of each element)

[0084] The above composition and formula α are as described above, and the above β is a value that serves as a reference for the process of controlling the fraction and shape of delta ferrite to secure ultra-low temperature impact toughness in the present invention.

[0085] In the present invention, it is necessary to sufficiently heat a slab satisfying the following α value of 30.0 or more at a temperature of β+20℃ or lower for 90 to 300 minutes prior to hot rolling based on the following precipitation temperature β value. If heating is performed prior to hot rolling at a temperature exceeding β+20℃, an excessive level of delta ferrite generated during heating causes an excessive level of delta to remain inside the steel after hot rolling and annealing, thereby deteriorating the ultra-low temperature impact toughness.

[0086] In addition, in a method for manufacturing an austenitic stainless steel having excellent hydrogen embrittlement resistance and cryogenic impact toughness according to another embodiment of the present invention, the hot rolling and finish rolling steps may include hot rolling and finish rolling the extracted slab at a temperature of β-70°C or higher with a reduction ratio of 50% or higher. If a reduction ratio of 50% or higher is not achieved at a temperature of β-70°C or higher, delta ferrite takes on a form that is relatively elongated in the rolling direction, making it impossible to secure cryogenic impact toughness even if the absolute delta ferrite area fraction is low.

[0087] In addition, a method for manufacturing an austenitic stainless steel having excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness according to another embodiment of the present invention may include hot rolling and finish rolling the extracted slab at a temperature of β-70°C or higher with a reduction ratio of 50% or higher.

[0088] In addition, a method for manufacturing an austenitic stainless steel having excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness according to another embodiment of the present invention may include the cooling step including cooling the rolled steel material to 600°C at a cooling rate of 50°C / s or less.

[0089] In addition, a method for manufacturing an austenitic stainless steel having excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness according to another embodiment of the present invention may include the step of hot rolling annealing the cooled steel material at 1,050°C or higher for 1 to 60 minutes.

[0090] In addition, in a method for manufacturing an austenitic stainless steel having excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness according to another embodiment of the present invention, in the hot rolling and finish rolling steps, the average delta ferrite area fraction in a region excluding a 1 / 4t region in the thickness direction from the surface may be 3% or less.

[0091] In addition, according to another embodiment of the present invention, a method for manufacturing an austenitic stainless steel having excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness is provided in which, in the hot rolling and finish rolling steps, the density of delta ferrite having a major axis length of 50 μm or more is 0.04 / cm. 2 It could be as follows:

[0092] Conventionally, when manufacturing austenitic stainless steel, the delta ferrite phase that develops within the material has a problem in that its toughness in cryogenic environments is very poor, which negates the overall advantages of the product. To address this problem, the present invention can control the fraction and shape of delta ferrite by adjusting the components and main manufacturing method as described above. Through this, the present invention can improve the problems of existing products and inventions, thereby providing an austenitic stainless steel and a manufacturing method thereof that can secure high cryogenic toughness.

[0093] Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are intended only to illustrate and further illustrate the present invention and are not intended to limit the scope of the invention. This is because the scope of the invention is determined by the matters set forth in the patent claims and matters reasonably inferred therefrom.

[0094] (Example)

[0095] After preparing a slab having the alloy composition described in Table 1 below, an austenitic stainless steel was manufactured 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 overall reduction ratio was 85%, the cooling rate after hot rolling was 30°C / s or less, and cooling to 600°C was performed, and post-cooling annealing was performed at 1100°C for 20 minutes.

[0096] Table 1 shows the main components of comparative and exemplary austenitic stainless steels. Table 2 also shows α and β derived from Table 1, as well as the furnace temperature and the reduction ratio in the temperature range of β-70°C or higher.

[0097] Ingredients (% by weight) CSiMnNiCrCuMoNComparative Example 10.0 20.4 1.6 8.1 18.10.4 0.10.04Comparative Example 20.0 20.3 1.2 10.2 16.5 0.32 10.02Comparative Example 30.0 30.6 3 7 16 0.41 5 0.19Example 10.0 20.4 1.8 8 5 20.10.5 0.8 0.18Example 20.0 25 0.43 8 7 12 0.70 5 0.70 21Example 30.0 20.4 1.9 9 32 0.50 80 60 18Example 40.0 15 0.60 4 13 8 2 1.5 0.22 30 26Example 50.020.41.88.120.50.50.50.25Example 60.020.33.79.319.50.80.60.16Example 70.0250.42.511.220.80.70.70.17Example 80.0450.94.78.816.10.220.12Comparative Example 40.0400.86.56.516.50.40.10.08Example 90.0300.44.511.524.50.40.60.05Example 100.020.41.99.320.50.80.60.18Example 110.0150.60.413.821.50.22.30.26Example 120.020.33.79.319.50.80.60.16Example 130.0250.42.511.220.80.70.70.17Example 140.0450.94.78.816.10.220.12

[0098] αβ(℃)Heating temperature(℃)β-70℃ or higher reduction ratio Comparative example 123.41191.6120055% Comparative example 225.31230.8124055% Comparative example 329.01288.9124055% Example 130.71177.8122055% Example 232.71114.8124055% Example 331.61213.4122048% Example 439.61285.2122040% Example 532.61212.8124044% Example 632.21246.8128055% Example 734.21259.5124045% Example 831.11261.4124040% Comparative Example 427.61225.6120055% Example 934.9975.7120080% Example 1031.61213.4122065% Example 1139.61285.2126052% Example 1232.21246.8124061% Example 1334.21259.5124059% Example 1431.11261.4126055%

[0099] Test method: For the tensile test specimens according to the ASTM E8 / E8M standard, a tensile test was performed at room temperature in the crosshead range of 10 mm / min to 20 mm / min, and the obtained yield strength YS (MPa) and tensile strength TS (MPa) values ​​were measured. Table 4 below shows the delta ferrite area fraction, delta ferrite shape satisfaction, and secured hydrogen embrittlement resistance (RRA) and -196℃ cryogenic toughness values ​​for the comparative examples and examples according to Tables 1 and 2, and the results of evaluating each property according to the criteria below are shown in Table 3 below. In addition, FIGS. 1 and 2 are scanning electron microscope (SEM) photographs of the microstructures of Example 1 and Comparative Example 6.

[0100] The delta ferrite area fraction was calculated by using a ferrite scope to create a profile in the thickness direction, and using the delta ferrite average excluding the area from the surface to a thickness of 1 / 4t. In addition, the shape of the delta ferrite was analyzed in the thickness direction using optical images at 200x and 500x magnifications, and if it exceeded the standard, it was judged as failing. The cross-sectional area reduction ratio was calculated by performing the SSRT test in accordance with ASTM G142-98 and ASTM G129, and then using a scanning electron microscope (SEM). The impact toughness was expressed by measuring the Charpy impact toughness at a temperature of -196℃ using the ASTM E23 type A specimen specification.

[0101] [evaluation]

[0102] If the α value is less than 30.0, it is evaluated as X, and if it is 30.0 or more, it is evaluated as ○.

[0103] Additionally, if the measured tensile strength value was 780 MPa or less, it was evaluated as ○, and if it exceeded 780 MPa, it was evaluated as X.

[0104] If the internal hydrogen embrittlement is 0.88 or higher, it is evaluated as ○, and if it is less than 0.88, it is evaluated as X.

[0105] In addition, when the impact toughness at -196℃ was 100J or more, it was evaluated as ◎, when it was 65J to 100J, it was evaluated as ○, and when it was 45J to 65J, it was evaluated as .

[0106] α Satisfaction Evaluation Tensile strength evaluation Hydrogen embrittlement resistance evaluation -196℃ Impact toughness Comparative Example 1 X○X◎ Comparative Example 2 X○X◎ Comparative Example 3 X○X◎ Example 130.7 ○○○ Example 232.7 ○○○ Example 331.6 ○○○ Example 439.6 ○○○ Example 532.6 ○○○ Example 632.2 ○○○ Example 734.2 ○○○ Example 831.1 ○○○ Comparative Example 4 X○X◎ Example 934.9 ○○△ Example 1031.6 ○○◎ Example 1139.6 ○○◎ Example 1232.2 ○○◎ Example 1334.2 ○○◎ Example 1431.1 ○○◎

[0107] Delta ferrite area fraction*(%) Delta ferrite shape satisfaction** Hydrogen embrittlement resistance (RRA) -196℃ Impact toughness (J) Comparative Example 12.9 Excellent 0.6 2 1 0 5.8 Comparative Example 22.5 Excellent 0.7 5 1 2 0.3 Comparative Example 32.6 Excellent 0.8 5 1 0 2.4 Exemplary Example 13.7 Excellent 0.8 8 7 2.8 Exemplary Example 25.2 Excellent 0.9 6 5.5 Exemplary Example 32.9 Good 0.9 5 8 1.5 Exemplary Example 41.9 Good 0.9 7 9 7.4 Exemplary Example 54.8 Good 0.9 4 5 6.4 Exemplary Example 63.5 Excellent 0.9 5 8 8.9 Exemplary Example 72.7 Good 0.9 6 9 0.5 Exemplary Example 81.5 Good 0.9 4 9 1.5 Comparative Example 42.9 Good 0.8 5 1 0 4.5 Exemplary Example 9 8.8 Excellent 0.9 1 4 6.9 Exemplary Example 102.2 Excellent 0.94 115.5 Example 111.7 Excellent 0.97 125.2 Example 122 Excellent 0.95 123.1 Example 131.4 Excellent 0.95 140.4 Example 140.9 Excellent 0.94 150.7

[0108] Examples 1 to 14 all satisfied the process conditions based on the α value of 30.0 or more and the β value, and thus satisfied the hydrogen embrittlement resistance of 0.88 or more and the impact toughness at -196°C of 100 J or more, showing excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness.

[0109] On the other hand, Comparative Examples 1 to 3 had poor austenite phase stability α of less than 30.0, resulting in martensite phase transformation and poor hydrogen embrittlement resistance with RRA of less than 0.88.

[0110] Although exemplary embodiments of the present invention have been described above, the present invention is not limited thereto, and those skilled in the art will understand that various changes and modifications are possible within the scope and spirit of the claims set forth below.

Claims

1. In weight%, carbon (C): more than 0% but not more than 0.05%, silicon (Si): more than 0% but not more than 1.0%, manganese (Mn): more than 0% but not more than 5.0%, chromium (Cr): 16.0% to 25.0%, molybdenum (Mo): more than 0% but not more than 2.5%, nickel (Ni): more than 7.0% but not more than 14.0%, copper (Cu): more than 0% but not more than 1.0%, nitrogen (N): 0.05% to 0.28%, the remainder including iron and other unavoidable impurities, The following α value satisfies 30.0 or more, Austenitic stainless steel with excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness, with a tensile strength of 780 MPa or less and an RRA (Relative Reduction of Area, reduction of area in hydrogen atmosphere / reduction of area in air atmosphere) of 0.88 or more. α= Ni+0.65Cr+0.98Mo+1.05Mn+0.35Si+12.6C+33.6N ≥ 30.0 (Here, Ni, Cr, Mo, Mn, Si, C, N represent the content of each element) 2. In claim 1, An austenitic stainless steel having excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness, with an average delta ferrite area fraction of 3% or less in the area excluding the 1 / 4t area in the thickness direction from the surface.

3. In claim 1, The density of delta ferrite with a major axis length of 50 μm or more is 0.04 / cm 2 Austenitic stainless steel with excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness.

4. In claim 1, Austenitic stainless steel with excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness, further containing phosphorus (P) of 0.035% or less and sulfur (S) of 0.01% or less.

5. In claim 1, An austenitic stainless steel having excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness, satisfying the above α value of 31.0 or higher.

6. In claim 1, Austenitic stainless steel with excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness, with an RRA (Relative Reduction of Area, reduction of area in hydrogen atmosphere / reduction of area in air atmosphere) of 0.90 or higher.

7. In claim 1, -Austenitic stainless steel with excellent hydrogen embrittlement resistance and cryogenic impact toughness, with a cryogenic impact toughness of 50J or higher at -196℃.

8. In claim 1, -Austenitic stainless steel with excellent hydrogen embrittlement resistance and cryogenic impact toughness, with a cryogenic impact toughness of 100J or higher at -196℃.

9. A step for preparing a slab containing, by weight%, carbon (C): more than 0% and not more than 0.05%, silicon (Si): more than 0% and not more than 1.0%, manganese (Mn): more than 0% and not more than 5.0%, chromium (Cr): 16.0% to 25.0%, molybdenum (Mo): more than 0% and not more than 2.5%, nickel (Ni): more than 7.0% and not more than 14.0%, copper (Cu): more than 0% and not more than 1.0%, nitrogen (N): 0.05% to 0.28%, the remainder being iron and other unavoidable impurities, and satisfying the following α value of 30.0 or more; A step of extracting after heating the above slab; A step of hot rolling and finish rolling the above extracted slab; A step of cooling the above rolled steel; and A step of hot-rolling and annealing the cooled steel material is included; A method for manufacturing an austenitic stainless steel having excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness, having a tensile strength of 780 MPa or less and an RRA (Relative Reduction of Area, the reduction of area in a hydrogen atmosphere / the reduction of area in an air atmosphere) of 0.88 or more. α = Ni+0.65Cr+0.98Mo+1.05Mn+0.35Si+12.6C+33.6N ≥ 30.0 (Here, Ni, Cr, Mo, Mn, Si, C, N, Cr, Cu represent the content of each element) 10. In claim 9, A method for manufacturing an austenitic stainless steel having excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness, wherein the step of extracting after heating comprises heating the prepared slab at a temperature of β+20℃ or lower for 90 to 300 minutes based on the 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 (Here, Ni, Cr, Mo, Mn, Si, C, N, Cr, Cu represent the content of each element) 11. In claim 9, A method for manufacturing an austenitic stainless steel having excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness, wherein the hot rolling and finish rolling steps include hot rolling and finish rolling the extracted slab at a temperature of β-70°C or higher with a reduction ratio of 50% or higher.

12. In claim 9, A method for manufacturing an austenitic stainless steel having excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness, wherein the cooling step comprises cooling the rolled steel to 600°C at a cooling rate of 50°C / s or less.

13. In claim 9, A method for manufacturing an austenitic stainless steel having excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness, wherein the hot-rolling annealing step comprises hot-rolling the cooled steel material at 1,050°C or higher for 1 to 60 minutes.

14. In claim 9, A method for manufacturing an austenitic stainless steel having excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness, wherein the average delta ferrite area fraction in the area excluding the 1 / 4t area in the thickness direction from the surface is 3% or less in the hot rolling and finish rolling steps described above.

15. In claim 9, In the above hot rolling and finish rolling steps, the density of delta ferrite having a major axis length of 50 μm or more is 0.04 / cm. 2 A method for manufacturing an austenitic stainless steel having excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness.

16. In claim 9, A method for manufacturing an austenitic stainless steel having excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness, wherein the austenitic stainless steel further contains phosphorus (P) of 0.035% or less and sulfur (S) of 0.01% or less.

17. In claim 9, A method for manufacturing an austenitic stainless steel having excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness, wherein the above α value satisfies 31.0 or more.

18. In claim 9, A method for manufacturing an austenitic stainless steel having excellent hydrogen embrittlement resistance and ultra-low temperature impact toughness, having an RRA (Relative Reduction of Area, reduction of area in a hydrogen atmosphere / reduction of area in an air atmosphere) of 0.90 or more.

19. In claim 9, - A method for manufacturing an austenitic stainless steel having an extremely low-temperature impact toughness of 50J or higher at -196℃ and excellent hydrogen embrittlement resistance and extremely low-temperature impact toughness.

20. In claim 9, - A method for manufacturing an austenitic stainless steel having an extremely low-temperature impact toughness of 100J or more at -196℃ and excellent hydrogen embrittlement resistance and extremely low-temperature impact toughness.

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