Ferritic stainless steel

Abstract

The purpose of the present invention is to provide a ferritic stainless steel that is excellent in terms of oxidation resistance and workability. The present invention provides a ferritic stainless steel which has a component composition that contains, in mass%, specific amounts of Cr, Al, S, C, N, O, and Nb, with the balance being made up of Fe and unavoidable impurities, wherein the content ratio of Al and Cr satisfies the relationships of formulae (1) and (2). (1): 2Al + Cr ≤ 41, (2): Al + Cr ≥ 23

Description

Ferritic stainless steel

[0001] The present invention relates to a ferritic stainless steel.

[0002] Ferritic stainless steel is a type of Cr-based stainless steel whose main chemical components are Fe and Cr. In recent years, Fe-Cr-Al-based alloys, in which Al is added to Cr-based stainless steel, have been proposed to enable use in relatively high temperature ranges and to improve workability, and these Fe-Cr-Al-based alloys are used in a variety of fields.

[0003] For example, Patent Document 1 discloses a corrosion-resistant alloy containing predetermined amounts of Cr and Al, with the balance being Fe, as an alloy used in combustion components. Patent Document 2, for example, discloses an Fe—Cr—Al powder alloy containing predetermined amounts of Cr, Al, oxygen, and nitrogen, with the balance being unavoidable impurities and Fe, as an alloy suitable for use as a heater material or high-temperature component. Furthermore, Patent Document 3, for example, discloses an Fe—Cr—Al alloy used in a catalyst carrier for purifying exhaust gases from internal combustion engines of automobiles, etc., containing predetermined amounts of Cr, Al, Y, Zr, and Hf, as well as unavoidable impurities including rare earth elements excluding C, N, S, Si, Mn, Ti, Nb, Ta, V, Ce, and Y, with the balance being Fe, and the contents of Zr, Hf, Ti, Nb, Ta, V, Al, and Y satisfying a predetermined relationship.

[0004] JP-A-48-3927, JP-A-5-98401, JP-A-2002-105606

[0005] However, conventional ferritic stainless steels such as Fe-Cr-Al alloys have room for further improvement in terms of oxidation resistance at high temperatures and workability into a predetermined shape.

[0006] Therefore, an object of the present invention is to provide a ferritic stainless steel that is excellent in oxidation resistance and workability.

[0007] The gist and configuration of the present invention are as follows.

[0008] [1] A ferritic stainless steel containing, by mass%, Cr: 14% or more and 27% or less, Al: 7% or more and 13.5% or less, S: 0.0015% or less, C: 0.05% or less, N: 0.2% or less, O: 0.15% or less, and Nb: 0.1% or more and 1.0% or less, with the balance consisting of Fe and unavoidable impurities, wherein the content ratios of Al and Cr satisfy the relationships of the following formulas (1) and (2): 2Al + Cr ≦ 41 ... (1) Al + Cr ≧ 23 ... (2).

[0009] [2] The ferritic stainless steel according to [1], wherein the component composition further contains at least one of Zr and Hf, and the content ratios of Zr and Hf satisfy the relationship of the following formula (3): 0.01≦Zr+Hf≦1.0 (3).

[0010] [3] The ferritic stainless steel according to [1] or [2], wherein the component composition further contains, in mass%, Ti: 0.05% or more and 1.0% or less.

[0011] [4] The composition further contains, in mass %, REM: 0.01% or less, and the content ratio of Al, Cr, S, and REM satisfies the following formula (4): -1.0≦(S−REM / 5)×(2Al+Cr). ２ The ferritic stainless steel according to any one of [1] to [3], which satisfies the relationship: ≦1.3 (4).

[0012] [5] The ferritic stainless steel according to any one of [1] to [4], wherein the content ratio of Al and Cr satisfies the relationship of the following formula (5): Cr / Al≦3.2 (5).

[0013] [6] A ferritic stainless steel according to any one of [1] to [5], having a reduction in area in a tensile test at room temperature of 40% or more.

[0014] According to the present invention, a ferritic stainless steel having excellent oxidation resistance and workability can be provided.

[0015] The present invention will be specifically described below.

[0016] (Composition) First, the composition of the ferritic stainless steel of the present invention will be described. Note that the unit of the content ratio of elements in the composition is "mass %", and hereinafter, unless otherwise specified, it will be simply expressed as "%".

[0017] Cr: 14% or more and 27% or less Cr is an element effective in ensuring oxidation resistance. If the Cr content is less than 14%, sufficient oxidation resistance cannot be obtained at high temperatures. On the other hand, if the Cr content exceeds 27%, workability is significantly reduced. Therefore, the Cr content is set to 14% or more and 27% or less. The Cr content is preferably 16% or more, and more preferably 18% or more. Furthermore, the Cr content is preferably 27% or less, and more preferably 25% or less.

[0018] Al: 7% or more and 13.5% or less Al is an element effective in ensuring oxidation resistance. If the Al content is less than 7%, sufficient oxidation resistance cannot be obtained at high temperatures. On the other hand, if the Al content exceeds 13.5%, workability is significantly reduced. Therefore, the Al content is set to 7% or more and 13.5% or less. The Al content is preferably 8% or more. Furthermore, the Al content is preferably 13% or less, more preferably 11% or less, and even more preferably 10% or less.

[0019] 2Al+Cr≦41 (1) The ferritic stainless steel of the present invention satisfies the relationship of formula (1) above. When 2Al+Cr (the sum of twice the Al content and the Cr content) is 41 or less, the ferritic stainless steel can exhibit excellent workability. 2Al+Cr is preferably 40 or less. Note that Al and Cr in formula (1) represent the content (mass%) of each element.

[0020] Al + Cr ≥ 23 (2) The ferritic stainless steel of the present invention satisfies the relationship of formula (2) above. When Al + Cr (the sum of the Al content and the Cr content) is 23 or more, the ferritic stainless steel can exhibit excellent oxidation resistance. Al + Cr is preferably 26 or more, more preferably 27 or more, and even more preferably 30 or more. Note that Al and Cr in formula (2) represent the content (mass%) of each element.

[0021] Cr / Al≦3.2 (5) The ferritic stainless steel of the present invention preferably satisfies the relationship of formula (5) above. If Cr / Al (the ratio of the Cr content to the Al content) is 3.2 or less, the workability of the ferritic stainless steel can be improved. Cr / Al is more preferably 3.0 or less. Note that Al and Cr in formula (5) represent the content (mass%) of each element.

[0022] S: 0.0015% or less S is an element that is inevitably contained in steel. If the S content exceeds 0.0015%, abnormal oxidation occurs at high temperatures, significantly reducing oxidation resistance. Therefore, the S content is set to 0.0015% or less. The S content is preferably 0.001% or less. The lower limit of the S content is not particularly limited, but may be, for example, 0.0001% or more or 0.0005% or more.

[0023] C: 0.05% or less If the C content exceeds 0.05%, not only will workability be significantly reduced, but Cr carbides will precipitate at grain boundaries, reducing oxidation resistance at high temperatures. Therefore, the C content is set to 0.05% or less. The C content is preferably 0.04% or less. On the other hand, from the viewpoints of workability and strength, the C content is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.03% or more.

[0024] N: 0.2% or less If the N content exceeds 0.2%, workability is significantly reduced. Therefore, the N content is set to 0.2% or less. The lower limit of the N content is not particularly limited, but may be, for example, 0.0005% or more, or 0.001% or more.

[0025] O: 0.15% or less When O is present, Al ２ O ３ The O content forms oxides such as Cr, Ni, and O, which reduces toughness. Therefore, the O content is set to 0.1% or less. The O content is preferably 0.1% or less, and more preferably 0.08% or less. The lower limit of the O content is not particularly limited, but may be, for example, 0.001% or more, or 0.01% or more.

[0026] Nb: 0.1% or more and 1.0% or less If the Nb content is less than 0.1%, the workability is significantly reduced. On the other hand, if the Nb content exceeds 1.0%, the material hardens and the workability deteriorates. Therefore, the Nb content is set to 0.1% or more and 1.0% or less. The Nb content is preferably 0.15% or more. Furthermore, the Nb content is preferably 0.75% or less, and more preferably 0.5% or less.

[0027] The basic components (essential components) of the ferritic stainless steel of the present invention have been described above. The remaining components of the composition of the present invention are Fe and unavoidable impurities.

[0028] The ferritic stainless steel of the present invention may further contain Zr, Hf, Ti, and REM in the following proportions: The ferritic stainless steel of the present invention may contain one or more of these components.

[0029] Zr: 0.01% or more and 1.0% or less Zr is an element that can be effective in improving oxidation resistance and also improves the peeling resistance of passive films such as oxide films that can form on the surface of ferritic stainless steel, so it can be added as needed. The Zr content is preferably 0.01% or more. On the other hand, if the Zr content is too high, oxidation resistance may decrease. Therefore, the Zr content is preferably 1.0% or less, more preferably 0.8% or less, and even more preferably 0.6% or less.

[0030] Hf: 0.01% or more and 1.0% or less Hf is an element that can be effective in improving oxidation resistance and also improves the peeling resistance of passivation films such as oxide films that can form on the surface of ferritic stainless steel, so it can be added as needed. The Hf content is preferably 0.01% or more. On the other hand, if the Hf content is too high, oxidation resistance may decrease. Therefore, the Hf content is preferably 1.0% or less, more preferably 0.8% or less, and even more preferably 0.6% or less.

[0031] 0.01≦Zr+Hf≦1.0 (3) The ferritic stainless steel of the present invention preferably satisfies the above formula (3). When Zr+Hf (the sum of the Zr content and the Hf content) is 0.01 or more and 1.0 or less, the oxidation resistance of the ferritic stainless steel can be improved. Zr+Hf is preferably 0.8% or less, and more preferably 0.6% or less. Note that Zr and Hf in formula (3) represent the content (mass%) of each element.

[0032] Ti: 0.05% or more and 1.0% or less Ti is an element that can be effective in improving oxidation resistance and also improves the peeling resistance of oxide films that can form on the surface of ferritic stainless steel, so it can be added as needed. The Ti content is preferably 0.05% or more, more preferably 0.2% or more, and even more preferably 0.3% or more. On the other hand, if the Ti content is too high, oxidation resistance may decrease. Therefore, the Ti content is preferably 1.0% or less, more preferably 0.8% or less, and even more preferably 0.6% or less.

[0033] REM: 0.01% or less. REM (rare earth metal) is an element that can be effective in improving oxidation resistance. However, if the REM content exceeds 0.01%, abnormal oxidation occurs at high temperatures, significantly reducing oxidation resistance. Therefore, the REM content is preferably 0.01% or less, more preferably 0.008% or less, and even more preferably 0.006% or less. The lower limit of the REM content is not particularly limited, but may be, for example, 0.0005% or more or 0.001% or more. In this specification, REM is a collective term for Sc, Y, and 15 elements ranging from La (lanthanum) with atomic number 57 to Lu (lutetium) with atomic number 71. The REM content here refers to the total content of these elements.

[0034] -1.0≦(S-REM / 5)×(2Al+Cr) ２ The ferritic stainless steel of the present invention preferably satisfies the above formula (4): (S-REM / 5)×(2Al+Cr)≦1.3 (4) ２ When the value of formula (4) is -1.0 or more and 1.3 or less, REM effectively suppresses the segregation of S in the ferritic stainless steel, resulting in an improved reduction of area of ​​the ferritic stainless steel, as described below, and improved workability. The value of formula (4) is preferably -0.5 or more, and more preferably -0.2 or more. The value of formula (4) is preferably 1.0 or less, more preferably 0.5 or less, and even more preferably 0.2 or less. In formula (4), Al, Cr, S, and REM represent the content (mass%) of each element.

[0035] (Properties) The ferritic stainless steel of the present invention preferably has a reduction of area of ​​40% or more in a tensile test at room temperature (20°C). Ferritic stainless steels with a reduction of area of ​​40% or more are particularly excellent in processability, and are easily processed into wire or strip shapes, making them suitable for use in heating elements having these shapes. The reduction of area of ​​the ferritic stainless steel is more preferably 50% or more, even more preferably 55% or more, and even more preferably 58% or more. In the present invention, the reduction of area of ​​the ferritic stainless steel refers to the maximum change in the cross-sectional area of ​​the round bar in a tensile test in accordance with JIS Z2241 relative to the cross-sectional area of ​​the round bar before the tensile test, expressed as a percentage (maximum change in cross-sectional area / cross-sectional area of ​​the round bar before the tensile test × 100 [%]), and can be measured according to the method described in the Examples.

[0036] The shape of the ferritic stainless steel of the present invention is not particularly limited, but examples thereof include wire, strip, and tube shapes. The ferritic stainless steel of the present invention has excellent processability, so it can be easily processed into, for example, a wire shape. The outer diameter (φ) of the wire shape is not particularly limited, but may be, for example, 0.1 mm or more and 15.0 mm or less.

[0037] (Applications) Applications of the ferritic stainless steel of the present invention are not particularly limited, but include heating elements, high-temperature structures, automotive catalyst carriers, etc. The ferritic stainless steel of the present invention has excellent oxidation resistance, so it can be suitably used for heating elements.

[0038] (Manufacturing Method) The ferritic stainless steel of the present invention is not particularly limited and can be manufactured by a conventionally known method, such as a method of manufacturing an alloy by hot isostatic pressing (HIP) or hot extrusion of metal powder manufactured by a casting method or an atomizing method.

[0039] The present invention will be described below with reference to examples, but the present invention is not limited to these examples.

[0040] <Production of Ferritic Stainless Steel> Ferritic stainless steel was produced by casting, adjusting materials to have the composition shown in Table 1 (the balance being Fe and unavoidable impurities). Table 1 also shows the calculation results of the relational expression of the content ratio of each component.

[0041] <Reduction of Area of ​​Ferritic Stainless Steel> Reduction of area of ​​ferritic stainless steel was measured by conducting a tensile test at room temperature (20°C). Specifically, a tensile test was first conducted at room temperature (20°C) using a φ6 mm round bar in accordance with JIS Z2241, and the round bar was broken. The maximum change in the cross-sectional area of ​​the round bar during the tensile test ("cross-sectional area of ​​the round bar before the tensile test" - "minimum cross-sectional area of ​​the round bar at break") was calculated, and the reduction of area was determined as the percentage of the maximum change in cross-sectional area relative to the cross-sectional area of ​​the round bar before the tensile test (maximum change in cross-sectional area / cross-sectional area of ​​the round bar before the tensile test × 100 [%]).

[0042] <Evaluation of Workability> The obtained ferritic stainless steel was subjected to wire drawing to form a wire of 0.5 mm in diameter, and the workability of the ferritic stainless steel was evaluated. The results are shown in Table 1, with those that could be drawn rated as "Good" and those that could not be drawn rated as "Not good."

[0043] <Oxidation Resistance Test> Plates measuring 10 mm x 20 mm x 5 mm (length x width x thickness) were cut from the obtained ferritic stainless steel and heated to a predetermined temperature in an electric furnace (in air). The surface of the plate was observed every 500 hours, and the time (h) until abnormal oxidation occurred was recorded as the test result. The upper limit of the test time was 30,000 hours at 1150°C, 20,000 hours at 1200°C, and 10,000 hours at 1300°C. The results are shown in Table 1.

[0044]

[0045] As is clear from Table 1, the ferritic stainless steels of the examples are excellent in oxidation resistance and workability.

[0046] On the other hand, Comparative Example 1 has poor oxidation resistance because the "Al + Cr" value of the ferritic stainless steel is less than 23. Comparative Examples 2 and 3 have poor oxidation resistance because the Al content of the ferritic stainless steel is less than 7%. Comparative Example 4 has poor oxidation resistance because the REM content of the ferritic stainless steel is greater than 0.01%. Comparative Example 5 has poor oxidation resistance because the S content of the ferritic stainless steel is greater than 0.0015%. Comparative Example 6 has poor oxidation resistance because the "Zr + Hf" value of the ferritic stainless steel is greater than 1.0%. Comparative Example 7 has poor oxidation resistance because the Ti content of the ferritic stainless steel is greater than 1.0%. Comparative Examples 8 and 9 have poor workability because the "2Al + Cr" value of the ferritic stainless steel is greater than 41. Comparative Example 10 has poor workability because the ferritic stainless steel does not contain a specified amount of Nb.

[0047] According to the present invention, a ferritic stainless steel having excellent oxidation resistance and workability can be provided.

Claims

1. A ferritic stainless steel containing, by mass%, Cr: 14% to 27%, Al: 7% to 13.5%, S: 0.0015% or less, C: 0.05% or less, N: 0.2% or less, O: 0.15% or less, and Nb: 0.1% to 1.0%, with the remainder consisting of Fe and unavoidable impurities, and having a component composition in which the content ratios of Al and Cr satisfy the relationships of the following formulas (1) and (2): 2Al + Cr ≦ 41 ... (1) Al + Cr ≧ 23 ... (2).

2. The ferritic stainless steel according to claim 1, wherein the composition further contains at least one of Zr and Hf, and the content ratio of Zr and Hf satisfies the relationship of the following formula (3): 0.01≦Zr+Hf≦1.0 (3).

3. A ferritic stainless steel according to claim 1 or 2, wherein the composition further contains, in mass%, Ti: 0.05% or more and 1.0% or less.

4. The composition further contains, in mass%, REM: 0.01% or less, and the content ratio of Al, Cr, S, and REM satisfies the following formula (4): -1.0≦(S−REM / 5)×(2Al+Cr). ２ The ferritic stainless steel according to claim 1 or 2, which satisfies the relationship: ≦1.3 (4).

5. A ferritic stainless steel according to claim 1 or 2, wherein the content ratio of Al and Cr satisfies the following formula (5): Cr / Al≦3.2 (5).

6. A ferritic stainless steel according to claim 1 or 2, which has a reduction in area of ​​40% or more in a tensile test at room temperature.

7. The composition further contains, in mass %, REM: 0.01% or less, and the content ratio of Al, Cr, S, and REM satisfies the following formula (4): -1.0≦(S−REM / 5)×(2Al+Cr). ２ The ferritic stainless steel according to claim 3, which satisfies the relationship: ≦1.3 (4).

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