Austenitic stainless steel and manufacturing method therefor

Abstract

Disclosed herein is an austenitic stainless steel having high elongation and low yield strength for realizing high formability. The austenitic stainless steel according to the present disclosure includes, in percent by weight (wt %), 0.01% to 0.08% of carbon (C), 0.1% to 1.0% of silicon (Si), 0.1% to 1.5% of manganese (Mn), 20.0% to 25.0% of chromium (Cr), 12.0% to 18.0% of nickel (Ni), 0.1% to 1.0% of copper (Cu), 0.01% to 0.1% nitrogen (N), 0.035% or less of phosphorus (P), 0.01% or less of sulfur (S), the remainder of iron (Fe) and inevitable impurities, and satisfies Formula (1) and Formula (2) below,(wherein C, Si, Mn, Cr, Ni, Cu, and N represent the content (wt %) of the respective elements).

Description

TECHNICAL FIELDThe present disclosure relates to an austenitic stainless steel having high elongation and low yield strength for realizing high formability, manufacturing method therefor, and more particularly, to an austenitic stainless steel having high elongation and low yield strength for forming a complex flow path shape of a bipolar plate of a polymer electrolyte fuel cell, and manufacturing method therefor.BACKGROUND ART

[0002] A polymer electrolyte fuel cell is a fuel cell that uses a membrane made of a polymer as an electrolyte. When hydrogen is supplied, hydrogen is separated into hydrogen ions and electrons, the hydrogen ions move to the opposite electrode through the electrolyte membrane, and the electrons move along a conductor rather than the membrane, causing current to flow. A polymer electrolyte fuel cell typically has a simple structure and manufacturing method, and high weight and space efficiency. Accordingly, a polymer electrolyte fuel cell may be used as a power source for transportation, on-site power generation, and the like.

[0003] A polymer electrolyte fuel cell consists of a unit cell that integrates a membrane electrode assembly with attached gas diffusion layers and bipolar (separator) plates. In the membrane electrode assembly, water is produced either by separating electrons from hydrogen or by combining hydrogen ions with oxygen ions. On both sides of the MEA, gas diffusion layers supply and remove hydrogen or air (oxygen) along with water. In addition, bipolar plates serve to supply hydrogen and oxygen to the gas diffusion layers as well as to discharge the generated water. A plurality of such unit cells are connected in series to form a fuel cell stack.

[0004] The bipolar plate requires high formability because fine, corrugated flow path processing is necessary to supply hydrogen and oxygen to the electrodes and efficiently discharge the reactant water. Graphite material, traditionally used for bipolar plates, has recently been replaced by stainless steel material due to high forming costs and low impact toughness. In forming the bipolar plate flow paths, high elongation is required to form regions of high deformation, such as fine bends, and low yield strength is required to minimize springback after forming.

[0005] To achieve the above, the present disclosure provides an austenitic stainless steel with excellent elongation and low yield strength for polymer electrolyte fuel cell bipolar plates, by using stainless steel material, particularly austenitic stainless steel having high formability.DISCLOSURETechnical Problem

[0006] The disclosure provides an austenitic stainless steel that has high elongation, low strength, and high corrosion resistance in a polymer electrolyte environment and enables a complex flow path shape by optimizing the basic alloying elements of austenitic stainless steel, C, Si, Mn, Cr, Ni, Cu, and N, and a method for manufacturing the same.Technical Solution

[0007] According to an embodiment of the present disclosure, an austenitic stainless steel having high elongation and low strength includes, in percent by weight (wt %), 0.01% to 0.08% of carbon (C), 0.1% to 1.0% of silicon (Si), 0.1% to 1.5% of manganese (Mn), 20.0% to 25.0% of chromium (Cr), 12.0% to 18.0% of nickel (Ni), 0.1% to 1.0% of copper (Cu), 0.01% to 0.1% nitrogen (N), 0.035% or less of phosphorus (P), 0.01% or less of sulfur (S), the remainder of iron (Fe) and inevitable impurities, and satisfies Formula (1) and Formula (2) below.80+45×C-10×Si+2×Mn-Ni-Cu-70×N>55Formula⁢ (1)60+300×C+70×Si-20×Mn+7×Cr+Ni+3×Cu+530×N<300Formula⁢ (2)(wherein C, Si, Mn, Cr, Ni, Cu, and N represent the content (wt %) of the respective elements).

[0009] In addition, the austenitic stainless steel according to an embodiment of the present disclosure may have a yield strength of 300 MPa or less and an elongation of 48% or more.

[0010] According to an embodiment of the present disclosure, a method for manufacturing an austenitic stainless steel includes: preparing a slab including, in percent by weight (wt %), 0.01% to 0.08% of carbon (C), 0.1% to 1.0% of silicon (Si), 0.1% to 1.5% of manganese (Mn), 20.0% to 25.0% of chromium (Cr), 12.0% to 18.0% of nickel (Ni), 0.1% to 1.0% of copper (Cu), 0.01% to 0.1% nitrogen (N), 0.035% or less of phosphorus (P), 0.01% or less of sulfur (S), the remainder of iron (Fe) and inevitable impurities, and satisfying Formula (1) and Formula (2) below:

[0011] hot rolling the slab at 1100 to 1300° C.; and

[0012] annealing the hot-rolled steel sheet at 1000 to 1200° C. for 100 to 300 seconds,80+45×C-10×Si+2×Mn-Ni-Cu-70×N>55Formula⁢ (1)60+300×C+70×Si-20×Mn+7×Cr+Ni+3×Cu+530×N<300Formula⁢ (2)(wherein C, Si, Mn, Cr, Ni, Cu, and N represent the content (wt %) of the respective elements).

[0014] In the method for manufacturing an austenitic stainless steel according to an embodiment of the present disclosure, the austenitic stainless steel may have a yield strength of 300 MPa or less and an elongation of 48% or more after hot rolling and annealing.Advantageous Effects

[0015] According to the present disclosure, an austenitic stainless steel having high elongation and low strength may be provided.DESCRIPTION OF DRAWINGS

[0016] FIG. 1 is a diagram showing a range of Formula (1) and an elongation after hot rolling and annealing heat treatment for the Examples and the Comparative Examples.

[0017] FIG. 2 is a diagram showing a range of Formula (2) and a yield strength after hot rolling and annealing heat treatment for the Examples and the Comparative Examples.MODE FOR INVENTION

[0018] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. The embodiments of the present disclosure may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art.

[0019] The terms used herein are merely used to describe particular embodiments. An expression used in the singular encompasses the expression of the plural, unless otherwise indicated. Throughout the specification, the terms such as “including” or “having” are intended to indicate the existence of features, operations, functions, components, or combinations thereof disclosed in the specification, and are not intended to preclude the possibility that one or more other features, operations, functions, components, or combinations thereof may exist or may be added.

[0020] Meanwhile, unless otherwise defined, all terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. Thus, these terms should not be interpreted in an idealized or overly formal sense unless expressly so defined herein. As used herein, the singular forms are intended to include the plural forms as well, unless the context clearly indicates otherwise.

[0021] The terms “about”, “substantially”, etc. used throughout the specification means that when a natural manufacturing and a substance allowable error are suggested, such an allowable error corresponds the value or is similar to the value, and such values are intended for the sake of clear understanding of the present disclosure or to prevent an unconscious infringer from illegally using the disclosure of the present disclosure.

[0022] According to the present disclosure, an austenitic stainless steel includes, in percent by weight (wt %), 0.01% to 0.08% of carbon (C), 0.1% to 1.0% of silicon (Si), 0.1% to 1.5% of manganese (Mn), 20.0% to 25.0% of chromium (Cr), 12.0% to 18.0% of nickel (Ni), 0.1% to 1.0% of copper (Cu), 0.01% to 0.1% nitrogen (N), 0.035% or less of phosphorus (P), 0.01% or less of sulfur (S), the remainder of iron (Fe) and inevitable impurities, and satisfies Formula (1) and Formula (2) below.80+45×C-10×Si+2×Mn-Ni-Cu-70×N>55Formula⁢ (1)60+300×C+70×Si-20×Mn+7×Cr+Ni+3×Cu+530×N<300Formula⁢ (2)

[0023] (Here, C, Si, Mn, Cr, Ni, Cu, and N represent the content (wt %) of the respective elements).

[0024] The reasons for numerical limitations on the contents of each of the elements are as follows.

[0025] The content of C is 0.01% to 0.08%.

[0026] Carbon (C) increases the stability of an austenite phase and is an element inevitably added during manufacturing, and thus C may be added in an amount of 0.01% or more. However, an excessive C content may form Cr carbides during welding, deteriorating corrosion resistance. Accordingly, an upper limit of the C content is limited to 0.08%. In addition, the content of C may be 0.01% to 0.03%.

[0027] The content of Si is 0.1% to 1.0%.

[0028] Silicon (Si) is an element added to deoxidize a molten steel in stainless steel and may be added in an amount of 0.1% or more. However, Si decreases the elongation of the material, and an excessive Si content may deteriorate the quality of the material after surface pickling and cause edge cracks due to increased inclusions during manufacturing, impairing material quality. Considering the above, an upper limit of the Si content may be limited to 1.0%. In addition, the content of Si may be 0.5% to 0.9%.

[0029] The content of Mn is 0.1% to 1.5%.

[0030] Manganese (Mn) may stabilize an austenite phase and improve the elongation of the material by developing mechanical twins during deformation, and thus Mn may be added in an amount of 0.1% or more. However, an excessive Mn content may form inclusions and reduce the corrosion resistance of the material. Accordingly, an upper limit of the Mn content is limited to 1.5% or less. In addition, the content of Mn may be 0.8% to 1.3%.

[0031] The content of Cr is 20.0% to 25.0%.

[0032] Chromium (Cr) is an essential element to be added to stainless steel to improve corrosion resistance. In particular, Cr requires to be added in an amount of 20.0% or more for application of a fuel cell bipolar plate. However, because Cr is a strong ferrite stabilizing element, an excessive Cr content may reduce the stability of the austenite phase. Accordingly, an upper limit of the Cr content is limited to 25.0%. In addition, the content of Cr may be 20.0% to 23.0%.

[0033] The content of Ni is 12.0% to 18.0%.

[0034] Nickel (Ni) is a strong austenite phase stabilizing element, and in the present disclosure, Ni is to be added in an amount of 12.0% or more. However, because Ni is a high-priced element, adding a large amount of Ni may increase manufacturing costs. Accordingly, an upper limit of the Ni content is limited to 18.0% or less. In addition, the content of Ni may be 12.0% to 17.2%.

[0035] The content of Cu is 0.1% to 1.0%.

[0036] Copper (Cu), like Mn and Ni, stabilizes the austenite phase, is an element essentially added in the stainless steel manufacturing process using scrap, and Cu may be added in an amount of 0.1% or more. However, an excessive Cu content may create a low-temperature liquid phase, causing edge defects during hot rolling. Accordingly, an upper limit of the Cu content is limited to 1.0% or less. In addition, the content of Cu may be 0.2% to 0.7%.

[0037] The content of N is 0.01% to 0.1%.

[0038] Nitrogen (N) is a strong austenite phase stabilizing element, but when added, N increases the yield strength and decreases the elongation, causing a deterioration in the formability of the material. Accordingly, in the present disclosure, considering the manufacturing process of stainless steel using scrap, the N content is limited to 0.01% or more and 0.1% or less. In addition, the content of N may be 0.02% to 0.06%.

[0039] The content of P is 0.035% or less.

[0040] Phosphorus (P) is an inevitable impurity contained in steel, and is an element that is a major cause of grain boundary corrosion or impairment of hot workability, and thus it is desirable to control its content as low as possible. Accordingly, an upper limit of the P content is controlled to 0.035% or less.

[0041] The content of S is 0.01% or less.

[0042] Sulfur (S) is an inevitable impurity contained in steel, is segregated in grain boundaries and impairs hot workability. Accordingly, it is desirable to control its content as low as possible. In the present disclosure, an upper limit of the S content is limited to 0.01% or less.

[0043] The remaining component of the present disclosure is iron (Fe). However, since unintended impurities may inevitably be introduced from raw materials or the surrounding environment during a typical manufacturing process, this may not be excluded. Since such impurities may be well known to those skilled in the art during a typical manufacturing process, details thereof are not described in this specification.

[0044] To develop a high elongation, low strength austenitic stainless steel, formulas representing elongation and yield strength according to the alloying components of the austenite phase are required. To this end, Formula (1) representing elongation and Formula (2) representing strength were used. To satisfy an elongation of 48% or more and a yield strength of 300 MPa or less after hot rolling and annealing heat treatment, Formula (1) and Formula (2) are to be satisfied.

[0045] Next, a method for manufacturing an austenitic stainless steel according to another aspect of the present disclosure is described.

[0046] The present disclosure relates to an austenitic stainless steel satisfying 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 manufacturing the same. The method includes: preparing a slab including, in percent by weight (wt %), 0.01% to 0.08% of carbon (C), 0.1% to 1.0% of silicon (Si), 0.1% to 1.5% of manganese (Mn), 20.0% to 25.0% of chromium (Cr), 12.0% to 18.0% of nickel (Ni), 0.1% to 1.0% of copper (Cu), 0.01% to 0.1% nitrogen (N), 0.035% or less of phosphorus (P), 0.01% or less of sulfur (S), the remainder of iron (Fe) and inevitable impurities, and satisfying the Formula (1) and Formula (2) below;

[0047] hot rolling the slab at 1100 to 1300° C.; and

[0048] annealing the hot-rolled steel sheet at 1000 to 1200° C. for 100 to 300 seconds,80+45×C−10×Si+2×Mn−Ni−Cu−70×N>55  Formula (1):60+300×C+70×Si−20×Mn+7×Cr+Ni+3×Cu+530×N<300  Formula (2):(Here, C, Si, Mn, Cr, Ni, Cu, and N represent the content (wt %) of the respective elements).

[0050] The stainless steel including the above composition may be manufactured into a slab by continuous casting or ingot casting, and a final product may be formed by performing a series of hot rolling, and hot annealing.

[0051] Conventionally, to develop high elongation, low strength materials, it was necessary to anneal a hot-rolled material at high temperatures for a long time. However, according to the present disclosure, high elongation and low strength characteristics may be acquired merely by preparing a slab; hot rolling the slab; and annealing the hot-rolled steel sheet under normal conditions.

[0052] The slab may be hot-rolled at a normal rolling temperature of 1100 to 1300° C., and the hot-rolled steel sheet may be hot-annealed even at a temperature of 1000 to 1200° C. In this case, the hot annealing may be performed for 100 to 300 seconds. In addition, the slab may be hot-rolled to a thickness of 2.5~5.0 mm.

[0053] As such, by controlling the alloying components, a high elongation, low strength austenitic stainless steel may be provided even when hot annealing is performed under normal manufacturing conditions.

[0054] The austenitic stainless steel according to the present disclosure may be applied to fields requiring high formability, such as fuel cell bipolar plates.

[0055] Hereinafter, the present disclosure is described in more detail through exemplary embodiments.EMBODIMENTS

[0056] With respect to the alloy component ranges of the Examples shown in Table 1 below, a slab was prepared by Ingot melting, heated at 1250° C. for 2 hours, hot-rolled to a thickness of 3.3 mm, and after hot rolling, annealed at 1100° C. for 180 seconds.

[0057] The alloy composition (wt %) for each experimental steel classification and a value of Formula (1) are shown in Table 1 below.TABLE 1SteelComponent (%)ClassificationCSiMnCrNiCuN(1)(2)Example 10.010.71.320.414.60.20.0259.9254.6Example 20.030.91.321.012.10.70.0658.0299.0Example 30.030.51.023.014.50.50.0262.0271.6Example 40.030.90.820.012.00.70.0458.5291.3Example 50.030.51.323.017.20.20.0558.1283.3Comparative0.020.51.020.510.00.00.1756.0324.6Example 1Comparative0.040.51.325.719.60.00.0953.5328.2Example 2Comparative10.070.90.223.015.40.70.0356.5331.4Example 3Comparative0.040.70.121.713.41.00.1054.1337.0Example 4Comparative0.040.60.520.813.50.30.1056.4312.7Example 5

[0058] The results of measuring the elongation and the yield strength of the annealed material manufactured with the above composition are shown in Table 2 below. The yield strength and the elongation were measured by performing a test at room temperature using a JIS13B tensile test specimen at a tensile speed of 20 mm per minute, through a tensile testing machine from Zwick Roell.TABLE 2Yield ElongationstrengthSteel Classification(%)(MPa)Example 150.8253.1Example 249.0298.5Example 353.2270.9Example 449.1289.3Example 548.8281.0Comparative Example 148.5323.4Comparative Example 244.1327.0Comparative Example 346.2330.0Comparative Example 445.1335.7Comparative Example 548.3312.4

[0059] Referring to Table 2, Examples 1 to 5, which satisfied the alloy composition and Formula (1) and Formula (2) provided by the present disclosure, satisfied an elongation of 48% or more and a yield strength of 300 MPa or less. FIG. 1 is a graph showing Formula (1) and the elongation for the Examples and the Comparative Examples, and FIG. 2 is a graph showing Formula (2) and the yield strength. According to Table 2 and FIG. 1, the Examples satisfying the Formula (1) and Formula (2) for the component ranges satisfied the elongation of 48% or more and the yield strength of 300 MPa or less, whereas Comparative Examples 2 to 4 had an elongation of less than 48%, and Comparative Examples 1 to 5 exceeded 300 MPa.

[0060] While exemplary embodiments of the present disclosure have been described above with reference to the accompanying drawings, it will be understood by those of ordinary skill in the art that various modifications and changes can be made without departing from the spirit and scope of the present disclosure as defined by the appended claims.

Claims

1. An austenitic stainless steel comprising, in percent by weight (wt %), 0.01% to 0.08% of carbon (C), 0.1% to 1.0% of silicon (Si), 0.1% to 1.5% of manganese (Mn), 20.0% to 25.0% of chromium (Cr), 12.0% to 18.0% of nickel (Ni), 0.1% to 1.0% of copper (Cu), 0.01% to 0.1% nitrogen (N), 0.035% or less of phosphorus (P), 0.01% or less of sulfur (S), the remainder of iron (Fe) and inevitable impurities, and satisfying Formula (1) and Formula (2) below.80+45×C-10×Si+2×Mn-Ni-Cu-70×N>55Formula⁢ (1)60+300×C+70×Si-20×Mn+7×Cr+Ni+3×Cu+530×N<300Formula⁢ (2)(wherein C, Si, Mn, Cr, Ni, Cu, and N represent the content (wt %) of the respective elements).

2. The austenitic stainless steel of claim 1, having a yield strength of 300 MPa or less and an elongation of 48% or more.

3. A method for manufacturing an austenitic stainless steel, the method comprising:preparing a slab comprising, in percent by weight (wt %), 0.01% to 0.08% of carbon (C), 0.1% to 1.0% of silicon (Si), 0.1% to 1.5% of manganese (Mn), 20.0% to 25.0% of chromium (Cr), 12.0% to 18.0% of nickel (Ni), 0.1% to 1.0% of copper (Cu), 0.01% to 0.1% nitrogen (N), 0.035% or less of phosphorus (P), 0.01% or less of sulfur (S), the remainder of iron (Fe) and inevitable impurities, and satisfying Formula (1) and Formula (2) below;hot rolling the slab at 1100 to 1300° C.; andannealing the hot-rolled steel sheet at 1000 to 1200° C. for 100 to 300 seconds,80+45×C-10×Si+2×Mn-Ni-Cu-70×N>55Formula⁢ (1)60+300×C+70×Si-20×Mn+7×Cr+Ni+3×Cu+530×N<300Formula⁢ (2)(wherein C, Si, Mn, Cr, Ni, Cu, and N represent the content (wt %) of the respective elements).

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

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