Austenitic stainless steel and its manufacturing method

Abstract

The present invention discloses an austenitic stainless steel having high elongation and low yield strength to achieve high formability, and a method for producing the same. [Solution] The austenitic stainless steel according to the present invention is characterized by comprising, 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, with the remainder being Fe and unavoidable impurities, and satisfying the following formulas (1) and (2): Formula (1): 80+45×C-10×Si+2×Mn-Ni-Cu-70×N > 55 Equation (2): 60 + 300 × C + 70 × Si - 20 × Mn + 7 × Cr + Ni + 3 × Cu + 530 × N < 300 (Here, C, Si, Mn, Cr, Ni, Cu, and N refer to the content (wt%) of each element.)

Description

[Technical Field]

[0001] The present invention relates to an austenitic stainless steel and a manufacturing method thereof, and more particularly to an austenitic stainless steel having high elongation and low yield strength for forming complex flow channel shapes in separator plates for polymer electrolyte fuel cells, and a manufacturing method thereof. [Background technology]

[0002] A polymer electrolyte fuel cell is a fuel cell that uses a polymer membrane as the electrolyte. When hydrogen is supplied, it separates into hydrogen ions and electrons. The hydrogen ions pass through the electrolyte membrane to the opposite electrode, while the electrons move along the conductor rather than the membrane, generating an electric current. The advantages of a polymer electrolyte fuel cell are its simple structure and manufacturing method, as well as its high weight and space efficiency. These advantages make it suitable for use as a power source for transportation and on-site power generation. A polymer electrolyte fuel cell is made up of unit cells, each of which is connected to a membrane electrode assembly that separates electrons from hydrogen or combines hydrogen ions with oxygen ions to produce water. The unit cells are connected to both sides of the membrane electrode assembly, which is connected to a gas diffusion layer that supplies and discharges hydrogen or air (oxygen) and water, and a separator plate that supplies hydrogen and oxygen to the gas diffusion layer and discharges the generated water. A plurality of these unit cells are connected in series to form a fuel cell stack.

[0003] Separator plates require high formability because they require finely bent flow channels to supply hydrogen and oxygen to the electrodes and efficiently discharge water, a reaction product. Graphite, which was previously used for separator plates, has recently been replaced by stainless steel due to its high forming costs and low impact toughness. When forming separator plate flow channels, high elongation is required to form areas with high deformation, such as fine bends, and low yield strength is required to minimize springback after forming. To achieve this goal, stainless steel materials, particularly austenitic stainless steels with high formability, are being utilized, and there is a demand for austenitic stainless steels with excellent formability, excellent elongation, and low yield strength that can be used for polymer electrolyte fuel cell separator plates. Summary of the Invention [Problem to be solved by the invention]

[0004] The object of the present invention is to provide an austenitic stainless steel having high corrosion resistance in a polymer electrolyte environment, high elongation and low strength that allows for the formation of complex flow channel shapes, by optimizing the basic alloying elements of austenitic stainless steel: C, Si, Mn, Cr, Ni, Cu, and N, and a method for producing the same. [Means for solving the problem]

[0005] The highly elongated, low-strength austenitic stainless steel of the present invention is characterized by comprising, 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, with the remainder being Fe and unavoidable impurities, and satisfying the following formulas (1) and (2): Formula (1): 80+45×C-10×Si+2×Mn-Ni-Cu-70×N > 55 Equation (2): 60 + 300 × C + 70 × Si - 20 × Mn + 7 × Cr + Ni + 3 × Cu + 530 × N < 300 (Here, C, Si, Mn, Cr, Ni, Cu, and N refer to the content (wt%) of each element.)

[0006] Furthermore, the austenitic stainless steel of the present invention preferably has a yield strength of 300 MPa or less and an elongation of 48% or more.

[0007] The method for producing an austenitic stainless steel of the present invention includes the steps of producing a slab containing, by weight, 0.01% to 0.08% C, 0.1% to 1.0% Si, 0.1% to 1.5% Mn, 20.0% to 25.0% Cr, 12.0% to 18.0% Ni, 0.1% to 1.0% Cu, 0.01% to 0.1% N, 0.035% or less P, 0.01% or less S, with the remainder being Fe and unavoidable impurities, and satisfying the following formulas (1) and (2): hot rolling the slab at 1100 to 1300 ° C; and annealing the hot-rolled steel sheet at 1000 to 1200°C for 100 to 300 seconds. Formula (1): 80+45×C-10×Si+2×Mn-Ni-Cu-70×N > 55 Equation (2): 60 + 300 × C + 70 × Si - 20 × Mn + 7 × Cr + Ni + 3 × Cu + 530 × N < 300 (Here, C, Si, Mn, Cr, Ni, Cu, and N refer to the content (wt%) of each element.)

[0008] In the manufacturing method of the present invention, it is preferable that the steel sheet has a yield strength of 300 MPa or less and an elongation of 48% or more after hot rolling and annealing. [Effects of the Invention]

[0009] According to the present invention, a highly elongated and low strength austenitic stainless steel can be provided. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 2 is a diagram showing the range of formula (1) and the elongation percentage after hot rolling and annealing heat treatment for Examples and Comparative Examples. [Figure 2] FIG. 2 is a diagram showing the range of formula (2) and the yield strength after hot rolling and annealing heat treatment for Examples and Comparative Examples. DETAILED DESCRIPTION OF THE INVENTION

[0011] The following describes preferred embodiments of the present invention. However, the embodiments of the present invention can be modified into various other forms, and the technical concept of the present invention is not limited to the embodiments described below. Furthermore, the embodiments of the present invention are provided to more completely explain the present invention to those with average knowledge in the art. The terms used in this application are merely used to describe specific examples. Thus, for example, singular expressions include plural expressions unless the context clearly dictates otherwise. Note that the terms "comprise" or "include" used in this application are used to clearly indicate the presence of features, steps, functions, components, or combinations thereof described in the specification, and are not intended to preclude the presence of other features, steps, functions, components, or combinations thereof.

[0012] On the other hand, unless otherwise defined, all terms used herein should be considered to have the same meaning as commonly understood by a person of ordinary skill in the art to which the present invention belongs. Therefore, unless clearly defined herein, specific terms should not be interpreted as having an overly ideal or formal meaning. For example, singular expressions in this specification include plural expressions unless there is a clear exception in the context. Furthermore, in this specification, the terms "about," "substantially," and the like are used to mean from or near a numerical value when manufacturing and material tolerances inherent in the stated meaning are given, and are used to prevent unscrupulous infringers from unfairly taking advantage of disclosures in which precise or absolute numerical values ​​are stated to aid in the understanding of the present invention.

[0013] The austenitic stainless steel according to the present invention is characterized by containing, by weight percent, 0.01% to 0.08% C, 0.1% to 1.0% Si, 0.1% to 1.5% Mn, 20.0% to 25.0% Cr, 12.0% to 18.0% Ni, 0.1% to 1.0% Cu, 0.01% to 0.1% N, 0.035% or less P, 0.01% or less S, with the remainder being Fe and unavoidable impurities, and satisfying the following formulas (1) and (2): Formula (1): 80+45×C-10×Si+2×Mn-Ni-Cu-70×N > 55 Equation (2): 60 + 300 × C + 70 × Si - 20 × Mn + 7 × Cr + Ni + 3 × Cu + 530 × N < 300 (Here, C, Si, Mn, Cr, Ni, Cu, and N refer to the content (wt%) of each element.)

[0014] The reasons for limiting the range of the content of each of the component elements are as follows.

[0015] The carbon (C) content is 0.01% to 0.08%. Carbon (C) increases the stability of the austenite phase and is an element that is inevitably added during manufacturing, so it is preferable to add 0.01% or more. However, if the content is excessive, Cr carbides are formed between welds, reducing corrosion resistance, so the upper limit is limited to 0.08%. The carbon content is preferably 0.01% to 0.03%.

[0016] The silicon (Si) content is 0.1% to 1.0%. Silicon is an element added to stainless steel for deoxidation in molten steel, and it is preferable to add 0.1% or more. However, excessive silicon content reduces the elongation of the material, degrades the quality of the material after surface pickling, and increases the number of inclusions, which can cause edge cracks during production and lead to poor quality of the material. In consideration of this, the upper limit of the silicon content is preferably set to 1.0%. Preferably, the silicon content is 0.5% to 0.9%.

[0017] The manganese (Mn) content is 0.1% to 1.5%. Mn stabilizes the austenite phase and promotes mechanical twinning during deformation, improving the elongation of the material, so it is recommended to add 0.1% or more. However, excessive addition of Mn generates inclusions, reducing the corrosion resistance of the material, so the addition amount is limited to 1.5% or less. The manganese content is preferably 0.8% to 1.3%.

[0018] The chromium (Cr) content is 20.0% to 25.0%. Cr is an essential element added to stainless steel to improve corrosion resistance, and must be added in an amount of 20.0% or more, especially for use in fuel cell separators. However, because Cr is a strong ferrite stabilizing element, excessive addition reduces the stability of the austenite phase, so the upper limit is limited to 25.0%. Preferably, the chromium content is 20.0% to 23.0%.

[0019] The nickel (Ni) content is 12.0% to 18.0%. Nickel (Ni) is a strong austenite phase stabilizing element, and in the present invention, it must be added in an amount of 12.0% or more. However, Ni is an expensive element, and increasing the amount of Ni added leads to an increase in raw material costs, so the upper limit is limited to 18.0% or less. Preferably, the nickel content is 12.0% to 17.2%.

[0020] The copper (Cu) content is 0.1% to 1.0%. Like Mn and Ni, Cu stabilizes the austenite phase and is an essential element in the stainless steel manufacturing process that uses scrap, and can be added in an amount of 0.1% or more. However, excessive Cu may generate a low-temperature liquid phase, which may cause edge defects during hot rolling, so the amount added is limited to 1.0% or less. Preferably, the Cu content is 0.2% to 0.7%.

[0021] The N content is 0.01% to 0.1%. Nitrogen (N) is a powerful austenite phase stabilizing element, but when added, it increases the yield strength of the material and reduces the elongation, resulting in a decrease in the formability of the material. Therefore, in the present invention, taking into account the manufacturing process using stainless steel scrap, the amount of N added is limited to 0.01% or more and 0.1% or less. Preferably, the nitrogen content is 0.02% to 0.06%.

[0022] The P content is 0.035% or less. Phosphorus (P) is an unavoidable impurity contained in steel, and is an element that is the main cause of intergranular corrosion and impaired hot workability, so it is preferable 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.

[0023] The S content is 0.01% or less. Sulfur (S) is an impurity that is inevitably contained in steel and is an element that segregates at grain boundaries and is the main cause of impairing hot workability, so it is preferable to control the S content as low as possible. In the present invention, the upper limit of the S content is controlled to 0.01% or less.

[0024] The remaining component of the present invention is iron (Fe). However, in a normal manufacturing process, unintentional impurities may inevitably be mixed in from raw materials or the surrounding environment, and this cannot be excluded. Since these impurities are known to anyone skilled in normal manufacturing processes, not all of the contents of these impurities will be specifically mentioned in this specification.

[0025] To develop a highly elongated, low-strength austenitic stainless steel, an equation is required to represent the elongation and yield strength of the alloying elements in the austenite phase. For this purpose, equation (1) representing the elongation and equation (2) representing the strength are used. To achieve an elongation of 48% or more and a yield strength of 300 MPa or less after hot rolling and annealing, equations (1) and (2) must be satisfied. Next, a method for producing an austenitic stainless steel according to another embodiment of the present invention will be described.

[0026] The present invention relates to an austenitic stainless steel that satisfies an elongation of 48% or more and a yield strength of 300 MPa or less after hot rolling and annealing heat treatment, and a method for producing the same, and the stainless steel is composed of, 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 remainder being Fe and unavoidable impurities, A step of producing a slab that satisfies the following formulas (1) and (2): hot rolling the slab at 1100 to 1300 ° C; and annealing the hot-rolled steel sheet at 1000 to 1200°C for 100 to 300 seconds. Formula (1): 80+45×C-10×Si+2×Mn-Ni-Cu-70×N > 55 Equation (2): 60 + 300 × C + 70 × Si - 20 × Mn + 7 × Cr + Ni + 3 × Cu + 530 × N < 300 (Here, C, Si, Mn, Cr, Ni, Cu, and N refer to the content (wt%) of each element.)

[0027] A stainless steel containing the above composition can be produced as a cast piece by continuous casting or ingot casting, and then subjected to a series of hot rolling and hot rolling annealing to form a final product. In the past, to develop a highly elongated, low-strength steel sheet, it was necessary to anneal the hot-rolled steel sheet at high temperatures for a long period of time. However, in the present invention, it is possible to achieve high elongation and low strength properties by simply eliminating the steps of producing a slab, hot-rolling the slab, and annealing the hot-rolled steel sheet under normal conditions.

[0028] Slabs can be hot rolled at the usual rolling temperature of 1,100 to 1,300°C, and hot rolled steel sheets can be hot rolled and annealed at temperatures of 1,000 to 1,200°C. Hot rolled annealing can be carried out for 100 to 300 seconds. Slabs can also be hot rolled to a thickness of 2.5 to 5.0 mm. By controlling the alloying components in this way, it is possible to provide a highly elongated, low-strength austenitic stainless steel even when hot rolling and annealing are carried out under normal process conditions and normal conditions.

[0029] The austenitic stainless steel according to the present invention can be applied to fields requiring high formability, such as fuel cell separators. The present invention will be described in more detail below with reference to examples. [Example]

[0030] For the alloy composition ranges of the examples shown in Table 1 below, slabs were produced by melting ingots, heated at 1,250°C for 2 hours, hot rolled to a thickness of 3.3 mm, and then annealed at 1,100°C for 180 seconds. The alloy composition (wt%) and the values ​​of formula (1) for each experimental steel type are shown in Table 1 below.

[0031] [Table 1]

[0032] The elongation and yield strength of the annealed materials manufactured according to the above composition were measured, and the results are shown in Table 2. The yield strength and elongation were measured by using a tensile tester manufactured by Zwick Roell, using a JIS 13B tensile test piece at a tension rate of 20 mm per minute at room temperature.

[0033] [Table 2]

[0034] Referring to Table 2, Examples 1 to 5, which satisfied the alloy composition proposed by the present invention and formulas (1) and (2), achieved an elongation of 48% or more and a yield strength of 300 MPa or less. Figure 1 is a graph showing formula (1) and elongation for Examples and Comparative Examples, and Figure 2 is a graph showing formula (2) and yield strength. According to Table 2 and Figure 1, Examples that satisfied formulas (1) and (2) for the component ranges achieved an elongation of 48% or more and a yield strength of 300 MPa or less, while Comparative Examples 2 to 4 had elongations of less than 48%, and Comparative Examples 1 to 5 exceeded 300 MPa.

[0035] Although exemplary embodiments of the present invention have been described above, the present invention is not limited thereto, and it will be understood by those skilled in the art that various changes and modifications can be made without departing from the concept and scope of the claims set forth below.

Claims

1. An austenitic stainless steel characterized by comprising, in weight percent, 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, with the remainder being Fe and unavoidable impurities, and satisfying the following formulas (1) and (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 (Here, C, Si, Mn, Cr, Ni, Cu, and N mean the content (wt%) of each element.)

2. 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 step of producing a slab consisting of, in weight percent, 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 remainder being Fe and unavoidable impurities, and satisfying the following formulas (1) and (2): hot rolling the slab at 1100 to 1300°C; and annealing the hot-rolled steel sheet at 1000°C to 1200°C for 100 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 (Here, C, Si, Mn, Cr, Ni, Cu, and N mean the content (wt%) of each element.)

4. 4. The method for producing austenitic stainless steel according to claim 3, wherein the steel has a yield strength of 300 MPa or less and an elongation of 48% or more after hot rolling and annealing.

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