Ferritic Stainless Steel

Abstract

The present invention provides a ferritic stainless steel having excellent oxidation resistance and workability, which contains, by mass%, predetermined amounts of Cr, Al, S, C, N, O, and Nb, with the balance being Fe and unavoidable impurities, and the content ratios of Al and Cr are expressed by the following formulas (1) and (2): 2Al+Cr≦41 (1) Al+Cr≧23 (2) The ferritic stainless steel has a chemical composition that satisfies the relationship:

Description

[Technical field]

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

[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 for a combustion member. Furthermore, for example, Patent Document 2 discloses an Fe-Cr-Al based 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 a high-temperature member. Furthermore, for example, Patent Document 3 discloses an Fe-Cr-Al alloy as an alloy used for a catalyst carrier for purifying exhaust gas from an internal combustion engine of an automobile or the like, which contains predetermined amounts of Cr, Al, Y, Zr, and Hf, as well as rare earth elements excluding C, N, S, Si, Mn, Ti, Nb, Ta, V, Ce, and Y as unavoidable impurities, with the balance being Fe, and in which the contents of Zr, Hf, Ti, Nb, Ta, V, Al, and Y satisfy a predetermined relationship. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Unexamined Patent Publication No. 48-3927 [Patent Document 2] Japanese Patent Application Publication No. 5-98401 [Patent Document 3] JP 2002-105606 A Summary of the Invention [Problem to be solved by the invention]

[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 desired shape.

[0006] Therefore, an object of the present invention is to provide a ferritic stainless steel having excellent oxidation resistance and workability. [Means for solving the problem]

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

[0008] [1] In mass percent, 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 The remainder is Fe and unavoidable impurities, The content ratios of Al and Cr are expressed by the following formulas (1) and (2): 2Al+Cr≦41 (1) Al+Cr≧23 (2) A ferritic stainless steel having a chemical composition that satisfies the relationship:

[0009] [2] The composition further contains at least one of Zr and Hf, The content ratio of Zr and Hf is expressed by the following formula (3): 0.01≦Zr+Hf≦1.0 (3) The ferritic stainless steel according to [1], which satisfies the relationship:

[0010] [3] The composition further comprises, in mass%, Ti: 0.05% or more and 1.0% or less The ferritic stainless steel according to [1] or [2], containing

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

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

[0013] [6] The ferritic stainless steel according to any one of [1] to [5], which has a reduction in area of ​​40% or more in a tensile test at room temperature. Effect of the Invention

[0014] According to the present invention, a ferritic stainless steel having excellent oxidation resistance and workability can be provided. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[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 indicated 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. Moreover, 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. Moreover, 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 the above formula (1). If 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. In addition, Al and Cr in the formula (1) indicate the content (mass %) of each element.

[0020] Al+Cr≧23 (2) The ferritic stainless steel of the present invention satisfies the relationship of the above formula (2). If 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. In addition, Al and Cr in the formula (2) indicate 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. It is more preferable that Cr / Al is 3.0 or less. In addition, Al and Cr in the formula (5) indicate 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, and oxidation resistance is significantly reduced. 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 the workability decrease significantly, but Cr carbides will precipitate at the grain boundaries, decreasing the 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 content of N exceeds 0.2%, the workability is significantly reduced. Therefore, the content of N is set to 0.2% or less. The lower limit of the content of N 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, oxides such as Al2O3 are formed, 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 is deteriorated. 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. Moreover, 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. Of the component composition of the present invention, the components other than those described above (the balance) 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 can also improve the peeling resistance of passive films such as oxide films that can be formed on the surface of ferritic stainless steel, so it can be added as necessary. The Zr content is preferably 0.01% or more. On the other hand, if the Zr content is too high, the oxidation resistance may decrease, so 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 can also improve 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 necessary. The Hf content is preferably 0.01% or more. On the other hand, if the Hf content is too high, the oxidation resistance may decrease, so 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. In addition, Zr and Hf in the formula (3) indicate 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 can also improve the peeling resistance of an oxide film that can form on the surface of ferritic stainless steel, so it can be added as necessary. 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, the oxidation resistance may decrease, so 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, but if the REM content exceeds 0.01%, abnormal oxidation occurs at high temperatures, and oxidation resistance is significantly reduced. 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, and the REM content here is the total content of these elements.

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

[0035] (Properties) The ferritic stainless steel of the present invention preferably has a reduction in area of ​​40% or more in a tensile test at room temperature (20°C). Ferritic stainless steels having a reduction in area of ​​40% or more have excellent processability and can be easily processed into wire or strip shapes, and are suitable for use in heating elements having these shapes. The reduction in 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 in area of ​​ferritic stainless steel means the maximum change in the cross-sectional area of ​​a 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 ​​round bar before 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 may be, for example, a wire shape, a strip shape, a tube shape, etc. The ferritic stainless steel of the present invention has excellent processability, so that it is easy to process 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] (Application) Applications of the ferritic stainless steel of the present invention are not particularly limited, but include heating elements, high-temperature structures, catalyst carriers for automobiles, etc. The ferritic stainless steel of the present invention has excellent oxidation resistance, so that 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 produced by a conventionally known method, such as a method of producing an alloy by subjecting metal powder produced by a casting method or an atomization method to hot isostatic pressing (HIP) or hot extrusion. EXAMPLES

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

[0040] <Production of ferritic stainless steel> Ferritic stainless steel was produced by casting, adjusting the materials so as 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] <Ablation value of ferritic stainless steel> The reduction in area of ​​the ferritic stainless steel was measured by carrying out a tensile test at room temperature (20°C). Specifically, a tensile test was first performed at room temperature (20°C) using a φ6mm 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 in 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 in area was determined as the maximum change in the cross-sectional area relative to the cross-sectional area of ​​the round bar before the tensile test, expressed as a percentage (maximum change in the cross-sectional area / cross-sectional area of ​​the round bar before the tensile test x 100[%]).

[0042] <Processability evaluation> The obtained ferritic stainless steel was subjected to wire drawing to obtain a wire with a diameter of 0.5 mm, and the workability of the ferritic stainless steel was evaluated. Those that could be drawn were rated "Good," and those that could not be drawn were rated "No good," and the results are shown in Table 1.

[0043] <Oxidation resistance test> Plate pieces measuring 10mm x 20mm x 5mm (length x width x thickness) were cut from the obtained ferritic stainless steel and heated to a specified temperature in an electric furnace (in air). The surface of the plate pieces was observed every 500 hours, and the time (h) at which 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] [Table 1]

[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, in Comparative Example 1, the "Al+Cr" value of the ferritic stainless steel is less than 23, and therefore the oxidation resistance is poor. Moreover, in Comparative Examples 2 and 3, the Al content of the ferritic stainless steel is less than 7%, and therefore the oxidation resistance is poor. Moreover, in Comparative Example 4, the REM content of the ferritic stainless steel exceeds 0.01%, and therefore the oxidation resistance is poor. Moreover, in Comparative Example 5, the ferritic stainless steel has a S content of more than 0.0015%, and therefore has poor oxidation resistance. Moreover, in Comparative Example 6, the ferritic stainless steel has a "Zr+Hf" value of more than 1.0%, and therefore has poor oxidation resistance. Moreover, in Comparative Example 7, the ferritic stainless steel has a Ti content of more than 1.0%, and therefore has poor oxidation resistance. Moreover, in Comparative Examples 8 and 9, the "2Al+Cr" value of the ferritic stainless steel exceeds 41, and therefore the workability is poor. Moreover, in Comparative Example 10, the ferritic stainless steel does not contain the specified amount of Nb, and therefore the workability is poor. [Industrial Applicability]

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

Claims

1. In 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 and the balance being Fe and unavoidable impurities, The content ratios of Al and Cr are expressed by the following formulas (1), (2), and (5): 2Al+Cr≦41...(1) Al+Cr≧23...(2) Cr / Al≦3.2 (5) A ferritic stainless steel having a chemical composition that satisfies the relationship:

2. In 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 and the balance being Fe and unavoidable impurities, The content ratios of Al and Cr are expressed by the following formulas (1) and (2): 2Al+Cr≦41...(1) Al+Cr≧23...(2) The component composition satisfies the relationship The composition further contains at least one of Zr and Hf, The content ratio of Zr and Hf is expressed by the following formula (3): 0.01≦Zr+Hf≦1.0 (3) Fulfilling the relationship, Ferritic stainless steel in wire, strip, tube or plate form.

3. The component composition further comprises, in mass%, Ti: 0.05% or more and 1.0% or less Contains 2. The ferritic stainless steel of claim 1, wherein Al+Cr≧26.

4. The component composition further comprises, in mass %, Ti: 0.05% or more and 1.0% or less Contains 3. The ferritic stainless steel of claim 2, wherein Al+Cr≧26.

5. In 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 and the balance being Fe and unavoidable impurities, The content ratios of Al and Cr are expressed by the following formulas (1) and (2): 2Al+Cr≦41...(1) Al+Cr≧23...(2) The component composition satisfies the relationship The component composition further comprises, in mass%, REM: 0.01% or less Contains The content ratios of Al, Cr, S, and REM are expressed by the following formula (4): -1.0≦(S-REM / 5)×(2Al+Cr) ２ ≦1.3 ・・・(4) Ferritic stainless steel satisfies the above relationship.

6. The composition further contains at least one of Zr and Hf, The content ratio of Zr and Hf is expressed by the following formula (3): 0.01≦Zr+Hf≦1.0 (3) The ferritic stainless steel according to claim 5, which satisfies the relationship:

7. The content ratio of Al and Cr is expressed by the following formula (5): Cr / Al≦3.2 (5) The ferritic stainless steel according to claim 2, which satisfies the relationship:

8. 8. The ferritic stainless steel according to claim 1, wherein the reduction of area in a tensile test at room temperature is 40% or more.

9. The component composition further comprises, in mass%, REM: 0.01% or less Contains The content ratios of Al, Cr, S, and REM are expressed by the following formula (4): -1.0≦(S-REM / 5)×(2Al+Cr) ２ ≦1.3 ・・・(4) The ferritic stainless steel according to claim 3 or 4, which satisfies the relationship: