Austenitic stainless steel and method for producing same

Abstract

Disclosed are an austenitic stainless steel having high elongation and low yield strength for achieving high formability, and a method for manufacturing the same. The austenitic stainless steel according to the present invention contains, in wt%, 0.01% to 0.08% of C, 0.1% to 1.0% of Si, 0.1% to 1.5% of Mn, 20.0% to 25.0% of Cr, 12.0% to 18.0% of Ni, 0.1% to 1.0% of Cu, 0.01% to 0.1% of N, 0.035% or less of P, 0.01% or less of S, and the balance Fe and unavoidable impurities, and satisfies the following formulae (1) and (2). The formula (1) is 80 + 45 * C-10 * Si + 2 * Mn-Ni-Cu-70 * Ngt; 55, formula (2): 60 + 300 * C + 70 * Si-20 * Mn + 7 * Cr + Ni + 3 * Cu + 530 * Nlt; 300 (wherein C, Si, Mn, Cr, Ni, Cu, and N represent the content (wt%) of each element)

Description

Technical Field

[0001] The present invention relates to an austenitic stainless steel having a high elongation rate and a low yield strength for achieving high formability, and a manufacturing method thereof. More specifically, the present invention relates to an austenitic stainless steel having a high elongation rate and a low yield strength for forming a complex flow path shape of a polymer electrolyte fuel cell separator, and a manufacturing method thereof. Background Art

[0002] A polymer electrolyte fuel cell is a fuel cell that uses a membrane formed of a polymer as an electrolyte. When hydrogen is supplied, hydrogen is separated into hydrogen ions and electrons. The hydrogen ions move through the electrolyte membrane to the other electrode, and the electrons move along a wire rather than the membrane, thereby generating an electric current. Representative advantages of the polymer electrolyte fuel cell are simple structure and manufacturing method, high weight and high space efficiency. Therefore, using these advantages, it can be usefully used as a power source for transportation, local installation type power generation, etc.

[0003] A polymer electrolyte fuel cell is composed of unit cells, which are combined with a gas diffusion layer that supplies hydrogen or air (oxygen) and water to both sides of a membrane electrode assembly and discharges them, and a separator that supplies hydrogen and oxygen to the gas diffusion layer or discharges generated water. The membrane electrode assembly is used to separate electrons from hydrogen or to combine hydrogen ions and oxygen ions to generate water. A plurality of such unit cells are connected in series to form a fuel cell stack (stack).

[0004] The separator requires micro-scale curved flow path processing to supply hydrogen and oxygen to the electrodes and effectively discharge water as a reactant, and requires high formability. Conventionally, a graphite material used for the separator has been gradually replaced by a stainless steel material in recent years due to high forming costs and low impact toughness. When forming the separator flow path, high elongation is required to form highly deformed regions such as micro-scale bends, and low yield strength is required to minimize springback after forming.

[0005] To achieve this purpose, an austenitic stainless steel material, particularly an austenitic stainless steel with high formability, is used to provide an austenitic stainless steel for a polymer electrolyte fuel cell separator having high formability, excellent elongation, and low yield strength. Summary of the Invention

[0006] (I) Technical Problem to be Solved

[0007] An object of the present invention is to provide an austenitic stainless steel having high ductility and low strength and a method for manufacturing the same. Among them, by optimizing C, Si, Mn, Cr, Ni, Cu, and N, which are the basic alloying elements of the austenitic stainless steel, high corrosion resistance can be achieved in a polymer electrolyte environment, and a complex flow channel shape can be formed.

[0008] (II) Technical Solution

[0009] The austenitic stainless steel with high ductility and low strength according to an embodiment of the present invention contains, by weight%: C: 0.01% to 0.08%, Si: 0.1% to 1.0%, Mn: 0.1% to 1.5%, Cr: 20.0% to 25.0%, Ni: 12.0% to 18.0%, Cu: 0.1% to 1.0%, N: 0.01% to 0.1%, P: 0.035% or less, S: 0.01% or less, the balance being Fe and inevitable impurities, and satisfying the following formula (1) and formula (2).

[0010] Formula (1): 80 + 45×C - 10×Si + 2×Mn - Ni - Cu - 70×N > 55

[0011] Formula (2): 60 + 300×C + 70×Si - 20×Mn + 7×Cr + Ni + 3×Cu + 530×N < 300

[0012] (wherein, C, Si, Mn, Cr, Ni, Cu, and N represent the contents (weight%) of the respective elements)

[0013] In addition, the yield strength of the austenitic stainless steel according to an embodiment of the present invention can be 300 MPa or less, and the elongation can be 48% or more.

[0014] The method for manufacturing an austenitic stainless steel according to an embodiment of the present invention includes the following steps: manufacturing a slab, which contains, by weight%: C: 0.01% to 0.08%, Si: 0.1% to 1.0%, Mn: 0.1% to 1.5%, Cr: 20.0% to 25.0%, Ni: 12.0% to 18.0%, Cu: 0.1% to 1.0%, N: 0.01% to 0.1%, P: 0.035% or less, S: 0.01% or less, the balance being Fe and inevitable impurities, and satisfying the following formula (1) and formula (2); hot rolling the slab at 1100 - 1300 °C; annealing the hot - rolled steel plate at 1000 - 1200 °C for 100 - 300 seconds.

[0015] Formula (1): 80 + 45×C - 10×Si + 2×Mn - Ni - Cu - 70×N > 55

[0016] Formula (2): 60 + 300×C + 70×Si - 20×Mn + 7×Cr + Ni + 3×Cu + 530×N < 300

[0017] (wherein, C, Si, Mn, Cr, Ni, Cu, and N represent the contents (wt%) of the respective elements)

[0018] In the method for manufacturing austenitic stainless steel according to an embodiment of the present invention, the yield strength after hot rolling and annealing can be 300 MPa or less, and the elongation can be 48% or more.

[0019] (III) Advantageous Effects

[0020] According to the present invention, an austenitic stainless steel having high ductility and low strength can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 A graph showing the ranges of formula (1) in the examples and comparative examples and the elongation after hot rolling and annealing heat treatment.

[0022] Figure 2 A graph showing the ranges of formula (2) in the examples and comparative examples and the yield strength after hot rolling and annealing heat treatment. DETAILED DESCRIPTION

[0023] Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention can be modified into various other forms, and the technical idea of the present invention is not limited to the embodiments described below. In addition, the embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art.

[0024] The terms used in this application are only used to explain specific examples. Therefore, unless it is clearly stated in the context that it must be singular, a singular expression includes a plural expression. In addition, it should be noted that the terms "comprising", "including", or "having" used in this application are used to clearly specify the existence of the features, steps, functions, components, or combinations thereof described in the specification, rather than to preclude the existence of other features, steps, functions, components, or combinations thereof in advance.

[0025] In addition, unless otherwise defined, all terms used in this specification should be regarded as having the same meaning as commonly understood by those skilled in the art to which the present invention belongs. Therefore, in this specification, unless otherwise clearly defined, specific terms should not be interpreted in an overly ideal or formal sense. In this specification, unless otherwise clearly stated in the context, a singular expression includes a plural expression.

[0026] In addition, with respect to "about", "substantially", etc. in this specification, when allowing for manufacturing and material tolerances inherent in the recited meanings, the values are used as such or with meanings close thereto, and are used to prevent unscrupulous infringers from illegally using the recited exact or absolute numerical values to aid in understanding the disclosure of the present invention.

[0027] The austenitic stainless steel according to the present invention contains, by weight %: C: 0.01% to 0.08%, Si: 0.1% to 1.0%, Mn: 0.1% to 1.5%, Cr: 20.0% to 25.0%, Ni: 12.0% to 18.0%, Cu: 0.1% to 1.0%, N: 0.01% to 0.1%, P: 0.035% or less, S: 0.01% or less, the balance being Fe and unavoidable impurities, and satisfies the following formulas (1) and (2).

[0028] Formula (1): 80 + 45×C - 10×Si + 2×Mn - Ni - Cu - 70×N > 55

[0029] Formula (2): 60 + 300×C + 70×Si - 20×Mn + 7×Cr + Ni + 3×Cu + 530×N < 300

[0030] (Wherein, C, Si, Mn, Cr, Ni, Cu, N represent the contents (weight %) of the respective elements)

[0031] The reasons for limiting the numerical ranges of the contents of the respective component elements are as follows.

[0032] The content of carbon (C) is 0.01% to 0.08%.

[0033] Carbon (C) can improve the stability of the austenite phase and is an element inevitably added during the manufacturing process. Therefore, carbon (C) of 0.01% or more can be added. However, when the content of carbon (C) is too high, Cr carbides are formed during the welding process, thereby reducing the corrosion resistance. Therefore, the upper limit of the carbon (C) content is limited to 0.08%. In addition, the content of carbon can be 0.01% to 0.03%.

[0034] The content of silicon (Si) is 0.1% to 1.0%.

[0035] Si is an element added to deoxidize the molten steel of the stainless steel, and Si of 0.1% or more can be added. However, the elongation of the material will decrease. When the content of Si is too high, the quality of the material after surface pickling decreases, and due to an increase in inclusions, edge cracks may occur during the manufacturing process, which may lead to a deterioration in the quality of the material. Considering this, the upper limit of the Si content can be limited to 1.0%. In addition, the content of silicon can be 0.5% to 0.9%.

[0036] The content of manganese (Mn) is from 0.1% to 1.5%.

[0037] Mn can stabilize the austenite phase and cause mechanical twins to develop during deformation, thereby increasing the elongation of the material. Therefore, Mn above 0.1% can be added. However, when the addition amount is too high, inclusions are generated and the corrosion resistance of the material decreases. Therefore, the addition amount of Mn is limited to 1.5% or less. In addition, the content of manganese can be from 0.8% to 1.3%.

[0038] The content of chromium (Cr) is from 20.0% to 25.0%.

[0039] Cr is an element that must be added to stainless steel to improve corrosion resistance. In particular, in order to be applied to fuel cell separators, more than 20.0% must be added. However, Cr is a strong ferrite stabilizer. When the addition amount of Cr is too high, the stability of the austenite phase decreases. Therefore, the upper limit of Cr is limited to 25.0%. In addition, the content of chromium can be from 20.0% to 23.0%.

[0040] The content of nickel (Ni) is from 12.0% to 18.0%.

[0041] Nickel (Ni) is a strong austenite phase stabilizer. In the present invention, more than 12.0% must be added. However, Ni is a high-cost element. As the addition amount increases, the raw material cost increases. The upper limit of the content of nickel (Ni) is limited to 18.0% or less. In addition, the content of nickel can be from 12.0% to 17.2%.

[0042] The content of copper (Cu) is from 0.1% to 1.0%.

[0043] Like Mn and Ni, Cu is an element that stabilizes the austenite phase and must be added in the manufacturing process of stainless steel using waste materials. More than 0.1% of Cu can be added. However, when the content of Cu is too high, a low-temperature liquid phase is generated, and edge defects may be caused during hot rolling. Therefore, the addition amount of Cu is limited to 1.0% or less. In addition, the content of copper can be from 0.2% to 0.7%.

[0044] The content of N is from 0.01% to 0.1%.

[0045] Nitrogen (N) is a strong austenite phase stabilizer, but when added, it increases the yield strength of the material and decreases the elongation, thus reducing the formability of the material. Therefore, in the present invention, considering the manufacturing process of using waste materials of stainless steel, the addition amount of nitrogen is limited to more than 0.01% and less than 0.1%. In addition, the content of nitrogen can be from 0.02% to 0.06%.

[0046] The content of P is 0.035% or less.

[0047] Phosphorus (P) is an impurity inevitably contained in steel and is an element that is the main cause of intergranular corrosion or hindrance to hot workability. Therefore, it is preferable to control the content of phosphorus at as low a level as possible. In the present invention, the upper limit of the content of said P is controlled to be 0.035% or less.

[0048] The content of S is 0.01% or less.

[0049] Sulfur (S) is an impurity inevitably contained in steel and is an element that segregates at grain boundaries and hinders hot workability. Therefore, it is preferable to control the content of sulfur at as low a level as possible. In the present invention, the upper limit of the content of said S can be controlled to be 0.01% or less.

[0050] The remaining component of the present invention is iron (Fe). However, in a conventional manufacturing process, undesired impurities may inevitably be mixed in from raw materials or the surrounding environment, and thus these impurities cannot be excluded. These impurities are well known to those skilled in the conventional manufacturing process, and thus all of their details are not particularly described in this specification.

[0051] In order to develop austenitic stainless steel with high ductility and low strength, formulas for representing elongation and yield strength are required according to the alloy composition of the austenite phase. For this purpose, Equation (1) is used to represent elongation and Equation (2) is used to represent strength. In order to satisfy an elongation of 48% or more and a yield strength of 300 MPa or less after hot rolling and annealing heat treatment, Equations (1) and (2) must be satisfied.

[0052] Next, a method for manufacturing austenitic stainless steel according to another aspect of the present invention will be described.

[0053] The present invention relates to an austenitic stainless steel and a method for manufacturing the same, wherein the elongation after hot rolling and annealing heat treatment of the austenitic stainless steel satisfies 48% or more and the yield strength satisfies 300 MPa or less, and the manufacturing method includes the following steps: manufacturing a slab which, by weight%, contains: C: 0.01% to 0.08%, Si: 0.1% to 1.0%, Mn: 0.1% to 1.5%, Cr: 20.0% to 25.0%, Ni: 12.0% to 18.0%, Cu: 0.1% to 1.0%, N: 0.01% to 0.1%, P: 0.035% or less, S: 0.01% or less, the balance being Fe and inevitable impurities, and satisfying the following Equations (1) and (2); hot rolling the slab at 1100 - 1300 °C; annealing the steel sheet subjected to the hot rolling at 1000 - 1200 °C for 100 - 300 seconds.

[0054] Formula (1): 80 + 45×C - 10×Si + 2×Mn - Ni - Cu - 70×N > 55

[0055] Formula (2): 60 + 300×C + 70×Si - 20×Mn + 7×Cr + Ni + 3×Cu + 530×N < 300

[0056] (wherein, C, Si, Mn, Cr, Ni, Cu, and N represent the contents (wt%) of the respective elements)

[0057] The stainless steel containing the above composition can be made into a slab by continuous casting or ingot casting, and a series of hot rolling and hot rolling annealing can be carried out to form the final product.

[0058] In the past, in order to develop materials with high ductility and low strength, it was necessary to perform long-term annealing heat treatment on the hot-rolled materials at high temperatures. However, in the case of the present invention, high ductility and low strength characteristics can be achieved only by manufacturing a slab, hot-rolling the slab, and annealing the hot-rolled steel sheet under normal conditions.

[0059] The slab can be hot-rolled at a temperature of 1100 - 1300°C, which is the normal rolling temperature, and the hot-rolled steel sheet can also be hot-rolled annealed at a temperature of 1000 - 1200°C. At this time, the hot-rolled annealing can be carried out for 100 - 300 seconds. In addition, it can be hot-rolled to a thickness of 2.5 - 5.0 mm for the slab.

[0060] As described above, when controlling the alloy composition, even under normal process conditions for hot-rolling annealing, austenitic stainless steel with high ductility and low strength can be provided.

[0061] The austenitic stainless steel according to the present invention can be applied to fields such as fuel cell separators that require high formability.

[0062] Hereinafter, the present invention will be described in more detail by way of examples.

[0063] [Examples]

[0064] For the alloy example composition ranges shown in Table 1 below, a slab was manufactured by melting an ingot, hot-rolled to a thickness of 3.3 mm after heating at 1250°C for 2 hours, and annealed at 1100°C for 180 seconds after hot-rolling.

[0065] Table 1 below shows the alloy compositions (wt%) of each experimental steel type and the value of Formula (1).

[0066] [Table 1]

[0067]

[0068] The results of measuring the elongation and yield strength of the annealed material manufactured with the above composition are shown in Table 2 below. The yield strength and elongation were measured by testing JIS 13B tensile test pieces at room temperature using a tensile testing machine manufactured by ZwickRoell at a tensile speed of 20 mm per minute.

[0069] [Table 2]

[0070]

[0071] Referring to Table 2, the elongation of Examples 1 to 5 that satisfy the alloy composition and Formulas (1) and (2) proposed in the present invention is 48% or more, and the yield strength is 300 MPa or less. Figure 1 A graph showing Formula (1) and elongation for Examples and Comparative Examples, Figure 2 is a graph showing Formula (2) and yield strength. According to the said Table 2 and Figure 1 , for the composition range, the elongation of the Examples that satisfy Formulas (1) and (2) is 48% or more, and the yield strength is 300 MPa or less. On the other hand, the elongation of Comparative Examples 2 to 4 is less than 48%, and those of Comparative Examples 1 to 5 exceed 300 MPa.

[0072] As described above, exemplary embodiments of the present invention have been described, but the present invention is not limited thereto, and those skilled in the art will understand that various changes and modifications can be made without departing from the concept and scope of the claims.

Claims

1. An austenitic stainless steel, by weight %, the austenitic stainless steel comprises: C: 0.01% to 0.08%, Si: 0.1% to 1.0%, Mn: 0.1% to 1.5%, Cr: 20.0% to 25.0%, Ni: 12.0% to 18.0%, Cu: 0.1% to 1.0%, N: 0.01% to 0.1%, P: below 0.035%, S: below 0.01%, the balance being Fe and inevitable impurities, and satisfying the following formula (1) and formula (2), Formula (1): 80 + 45×C - 10×Si + 2×Mn - Ni - Cu - 70×N > 55 Formula (2): 60 + 300×C + 70×Si - 20×Mn + 7×Cr + Ni + 3×Cu + 530×N < 300 Among them, C, Si, Mn, Cr, Ni, Cu, N represent the contents of the respective elements, in units of weight %.

2. The austenitic stainless steel according to claim 1, wherein, The yield strength is 300 MPa or less, and the elongation is 48% or more.

3. A method for manufacturing an austenitic stainless steel, comprising the following steps: Manufacturing a slab, by weight %, the slab comprises: C: 0.01% to 0.08%, Si: 0.1% to 1.0%, Mn: 0.1% to 1.5%, Cr: 20.0% to 25.0%, Ni: 12.0% to 18.0%, Cu: 0.1% to 1.0%, N: 0.01% to 0.1%, P: below 0.035%, S: below 0.01%, the balance being Fe and inevitable impurities, and satisfying the following formula (1) and formula (2); Hot-rolling the slab at 1100 - 1300 °C; Annealing the steel sheet subjected to the hot-rolling at 1000 °C to 1200 °C for 100 seconds to 300 seconds, Formula (1): 80 + 45×C - 10×Si + 2×Mn - Ni - Cu - 70×N > 55 Formula (2): 60 + 300×C + 70×Si - 20×Mn + 7×Cr + Ni + 3×Cu + 530×N < 300 wherein, C, Si, Mn, Cr, Ni, Cu, N represent the contents of the respective elements, in units of weight %.

4. The manufacturing method of the austenitic stainless steel according to claim 3, wherein, The yield strength after hot-rolling and annealing is 300 MPa or less, and the elongation is 48% or more.

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