Ferritic stainless steel

Abstract

It is an aim of the present disclosure to provide a ferritic stainless steel with excellent oxidation resistance and workability. The ferritic stainless steel of the present disclosure includes a chemical composition containing, in mass %, predetermined amounts of Cr, Al, S, C, N, O, and Nb, with the balance being Fe and inevitable impurities, and content ratios of Al and Cr satisfy expressions (1) and (2): 2Al+Cr≤41 (1), and Al+Cr≥23 (2).

Description

TECHNICAL FIELD

[0001] The present disclosure relates to ferritic stainless steel.BACKGROUND

[0002] Ferritic stainless steel is a type of Cr-based stainless steel in which the main chemical components are Fe and Cr. Recently, Fe—Cr—Al 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. Fe—Cr—Al alloys are used in various fields.

[0003] For example, Patent Literature (PTL) 1 discloses a corrosion-resistant alloy that contains a predetermined amount of Cr and Al, with the balance being Fe, as an alloy for use in combustion components.

[0004] As another example, PTL 2 discloses an Fe—Cr—Al powder alloy containing a predetermined amount of Cr, Al, oxygen, and nitrogen, with the balance being inevitable impurities and Fe, as an alloy for use in materials suitable as heater materials and high-temperature components.

[0005] As yet another example, PTL 3 discloses an Fe—Cr—Al alloy containing a predetermined amount of Cr, Al, Y, Zr, and Hf, and rare earth elements other than C, N, S, Si, Mn, Ti, Nb, Ta, V, Ce, and Y as inevitable impurities, the balance being Fe, and the content of Zr, Hf, Ti, Nb, Ta, V, Al, and Y satisfying a predetermined relationship, as an alloy for use in catalyst carriers for purifying exhaust gas from internal combustion engines of automobiles and the like.CITATION LISTPatent LiteraturePTL 1: JP S48-3927 A

[0007] PTL 2: JP H5-98401 A

[0008] PTL 3: JP 2002-105606 ASUMMARYTechnical Problem

[0009] However, conventional ferritic stainless steels such as Fe—Cr—Al alloys have room for further improvement with respect to oxidation resistance at high temperatures and workability into a predetermined shape.

[0010] It is therefore an aim of the present disclosure to provide a ferritic stainless steel with excellent oxidation resistance and workability.Solution to Problem

[0011] An outline of the present disclosure is as follows.

[0012] [1] A ferritic stainless steel comprising:

[0013] a chemical composition containing, in mass %,

[0014] Cr: 14% or more and 27% or less,

[0015] Al: 7% or more and 13.5% or less,

[0016] S: 0.0015% or less,

[0017] C: 0.05% or less,

[0018] N: 0.2% or less,

[0019] O: 0.15% or less, and

[0020] Nb: 0.1% or more and 1.0% or less,

[0021] with the balance being Fe and inevitable impurities, wherein

[0022] content ratios of Al and Cr satisfy expressions (1) and (2):2⁢Al+Cr≤41(1)Al+Cr≥23.(2)

[0023] [2] The ferritic stainless steel according to [1], wherein

[0024] the chemical composition further contains at least one of Zr and Hf, and

[0025] content ratios of Zr and Hf satisfy expression (3):0.01≤Zr+Hf≤1..(3)

[0026] [3] The ferritic stainless steel according to [1] or [2], wherein

[0027] the chemical composition further contains, in mass %,

[0028] Ti: 0.05% or more and 1.0% or less.

[0029] [4] The ferritic stainless steel according to any one of [1] to [3], wherein

[0030] the chemical composition further contains, in mass %,

[0031] REM: 0.01% or less, and

[0032] content ratios of Al, Cr, S, and REM satisfy expression (4):-1.0≤(S-REM / 5)×(2⁢Al+Cr)2≤1.3.(4)

[0033] [5] The ferritic stainless steel according to any one of [1] to [4], wherein

[0034] content ratios of Al and Cr satisfy expression (5):Cr / Al≤3.2.(5)

[0035] [6] The ferritic stainless steel according to any one of [1] to [5], wherein a reduction of area in a tensile test at room temperature is 40% or more.Advantageous Effect

[0036] According to the present disclosure, a ferritic stainless steel with excellent oxidation resistance and workability can be provided.DETAILED DESCRIPTION

[0037] The present disclosure is described below in detail.(Chemical Composition)

[0038] First, the chemical composition of the ferritic stainless steel of the present disclosure will be described. The units of the content ratios of elements in the chemical composition are “mass %” and are hereafter simply indicated by “%” unless otherwise specified.Cr: 14% or More and 27% or Less

[0039] Cr is an effective element for 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 content ratio of Cr is set to be 14% or more and 27% or less. The content ratio of Cr is preferably 16% or more and more preferably 18% or more. The content ratio of Cr is preferably 27% or less and more preferably 25% or less.Al: 7% or More and 13.5% or Less

[0040] Al is an effective element for 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 content ratio of Al is set to be 7% or more and 13.5% or less. The content ratio of Al is preferably 8% or more. The content ratio of Al is preferably 13% or less, more preferably 11% or less, and even more preferably 10% or less.2⁢A⁢ l+Cr≤41(1)

[0041] The ferritic stainless steel of the present disclosure satisfies the relationship in expression (1).

[0042] If 2Al+Cr (the sum of twice the value of the content ratio of Al and the content ratio of Cr) is 41 or less, the ferritic stainless steel can exhibit excellent workability. 2Al+Cr is preferably 40 or less.

[0043] Al and Cr in expression (1) indicate the content ratio (mass %) of each element.Al+Cr≥23(2)

[0044] The ferritic stainless steel of the present disclosure satisfies the relationship in expression (2). If Al+Cr (the sum of the content ratio of Al and the content ratio of Cr) 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.

[0045] Al and Cr in expression (2) indicate the content ratio (mass %) of each element.Cr / Al≤3.2(5)

[0046] The ferritic stainless steel of the present disclosure preferably satisfies the relationship in expression (5). If Cr / Al (the ratio of the content ratio of Cr to the content ratio of Al) is 3.2 or less, the workability of the ferritic stainless steel can be improved. Cr / Al is more preferably 3.0 or less.

[0047] Al and Cr in expression (5) indicate the content ratio (mass %) of each element.S: 0.0015% or Less

[0048] S is an element inevitably contained in steel. A content ratio of S exceeding 0.0015% causes abnormal oxidation at high temperatures, resulting in a significant decrease in oxidation resistance. Therefore, the content ratio of S is set to be 0.0015% or less. The content ratio of S is preferably 0.001% or less. While no lower limit is placed on the content ratio of S, the content ratio of S may, for example, be 0.0001% or more, or 0.0005% or more.C: 0.05% or Less

[0049] If the content ratio of C exceeds 0.05%, not only does the workability decrease markedly, but also Cr carbides precipitate on the grain boundaries, resulting in a decrease in oxidation resistance at high temperatures. Therefore, the content ratio of C is set to be 0.05% or less. The content ratio of C is preferably 0.04% or less.

[0050] On the other hand, from the perspective of workability and strength, the content ratio of C is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.03% or more.N: 0.2% or Less

[0051] If the content ratio of N exceeds 0.2%, workability is significantly reduced. Therefore, the content ratio of N is set to be 0.2% or less. While no lower limit is placed on the content ratio of N, the content ratio of N may, for example, be 0.0005% or more, or 0.001% or more.O: 0.15% or Less

[0052] When O is present, oxides such as Al2O3 are formed and toughness is reduced. Therefore, the content ratio of O is set to be 0.1% or less. The content ratio of O is preferably 0.1% or less and more preferably 0.08% or less. While no lower limit is placed on the content ratio of O, the content ratio of O may, for example, be 0.001% or more, or 0.01% or more.Nb: 0.1% or More and 1.0% or Less

[0053] If the content ratio of Nb is less than 0.1%, workability is significantly reduced. On the other hand, if the content ratio of Nb exceeds 1.0%, the material hardens and workability deteriorates. Therefore, the content ratio of Nb is set to be 0.1% or more and 1.0% or less. The content ratio of Nb is preferably 0.15% or more. The content ratio of Nb is preferably 0.75% or less and more preferably 0.5% or less.

[0054] The basic composition (essential composition) of the presently disclosed ferritic stainless steel has been explained. In the chemical composition according to the present disclosure, components other than those listed above (the balance) are Fe and inevitable impurities.

[0055] The ferritic stainless steel of the present disclosure may further optionally contain Zr, Hf, Ti, and REM at the following ratios. The ferritic stainless steel of the present disclosure may contain one of these components, or may contain two or more.Zr: 0.01% or More and 1.0% or Less

[0056] Zr is an element that can be effective in increasing oxidation resistance and can also be added as needed to improve the peeling resistance of passive films such as oxide films that may form on the surface of ferritic stainless steels. The content ratio of Zr is preferably 0.01% or more.

[0057] On the other hand, too much Zr can decrease oxidation resistance. Therefore, the content ratio of Zr is preferably 1.0% or less, more preferably 0.8% or less, and even more preferably 0.6% or less.Hf: 0.01% or More and 1.0% or Less

[0058] Hf is an element that can be effective in increasing oxidation resistance and can also be added as needed to improve the peeling resistance of passive films such as oxide films that may form on the surface of ferritic stainless steels. The content ratio of Hf is preferably 0.01% or more.

[0059] On the other hand, too much Hf can decrease oxidation resistance. Therefore, the content ratio of Hf is preferably 1.0% or less, more preferably 0.8% or less, and even more preferably 0.6% or less.0.01≤Zr+Hf≤1.(3)

[0060] The ferritic stainless steel of the present disclosure preferably satisfies expression (3). If Zr+Hf (the sum of the content ratio of Zr and the content ratio of Hr) 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, more preferably 0.6% or less.

[0061] Zr and Hf in expression (3) indicate the content ratio (mass %) of each element.Ti: 0.05% or More and 1.0% or Less

[0062] Ti is an element that can be effective in increasing oxidation resistance and can also be added as needed to improve the peeling resistance of passive films such as oxide films that may form on the surface of ferritic stainless steels. The content ratio of Ti is preferably 0.05% or more, more preferably 0.2% or more, and even more preferably 0.3% or more.

[0063] On the other hand, too much Ti can decrease oxidation resistance. Therefore, the content ratio of Ti is preferably 1.0% or less, more preferably 0.8% or less, and even more preferably 0.6% or less.REM: 0.01% or Less

[0064] Rare earth metals (REM) are elements that can be effective in increasing oxidation resistance. A content ratio of REM exceeding 0.01%, however, causes abnormal oxidation at high temperatures, resulting in a significant decrease in oxidation resistance. Therefore, the content ratio of REM is preferably 0.01% or less, more preferably 0.008% or less, and even more preferably 0.006% or less. While no lower limit is placed on the content ratio of REM, the content ratio of REM may, for example, be 0.0005% or more, or 0.001% or more.

[0065] In the present specification, REM is a generic term for Sc, Y, and the 15 elements from La (lanthanum) with atomic number 57 to Lu (lutetium) with atomic number 71, and the content ratio of REM herein is the sum of the content ratios of these elements.-1.0≤(S-REM / 5)×(2⁢Al+Cr)2≤1.3(4)

[0066] The ferritic stainless steel of the present disclosure preferably satisfies expression (4). If (S-REM / 5)×(2Al+Cr) 2 is-1.0 or more and 1.3 or less, REM effectively suppresses S segregation 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 expression (4) is preferably-0.5 or more and more preferably-0.2 or more. The value of expression (4) is also preferably 1.0 or less, more preferably 0.5 or less, and even more preferably 0.2 or less.

[0067] Al, Cr, S, and REM in expression (4) indicate the content ratio (mass %) of each element.(Properties)

[0068] The reduction of area in a tensile test at room temperature (20° C.) is preferably 40% or more for the ferritic stainless steel of the present disclosure. Ferritic stainless steel with a reduction of area of 40% or more has particularly excellent workability, is therefore easy to process into wire or strip shapes, and is suitable for use in heat-generating bodies 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 still more preferably 58% or more.

[0069] In the present disclosure, the reduction in area of the ferritic stainless steel is, in a tensile test of ferritic stainless steel in accordance with JIS Z2241, the maximum change in cross-sectional area of a round bar in the tensile test 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 [%]). The reduction in area can be measured according to the method described in the Examples.

[0070] The shape of the ferritic stainless steel of the present disclosure is not particularly limited, but includes, for example, wire, strip, and tube shapes. By virtue of having excellent workability, the ferritic stainless steel of the present disclosure can, for example, easily be processed into a wire shape. The outer diameter (ϕ) of the wire shape is not particularly limited but may, for example, be 0.1 mm or more to 15.0 mm.(Intended Use)

[0071] The applications of the ferritic stainless steel of the present disclosure are not limited, but examples include heat-generating bodies, high-temperature structural elements, and catalyst carriers for automobiles. By virtue of having excellent oxidation resistance, the ferritic stainless steel of the present disclosure is suitable for use in heat-generating bodies.(Manufacturing Method)

[0072] The ferritic stainless steel of the present disclosure is not limited in terms of manufacturing and can be manufactured by conventionally known methods, such as the manufacturing of alloys by hot isostatic pressing (HIP) or hot extrusion of metal powders manufactured by casting or atomization.EXAMPLES

[0073] The present disclosure is described below through examples, but the present disclosure is not limited to these examples.<Manufacturing of Ferritic Stainless Steel>

[0074] Ferritic stainless steel was manufactured by casting while adjusting the materials to achieve the chemical compositions listed in Table 1 (the balance being Fe and inevitable impurities). The calculation results of the relational expressions for the content ratios of each component are also listed in Table 1.<Reduction in Area of Ferritic Stainless Steel>

[0075] The reduction in area of the ferritic stainless steel was measured by performing tensile tests at room temperature (20° C.).

[0076] Specifically, first, in accordance with JIS Z2241, a tensile test was conducted at room temperature (20° C.) using a ϕ6 mm round bar, and the round bar was fractured. The maximum change in cross-sectional area of the round bar in the tensile test ((cross-sectional area of round bar before tensile test)−(minimum cross-sectional area of round bar at time of fracture)) was calculated, and the maximum change in cross-sectional area 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 [%]), was taken as the reduction in area.<Workability Evaluation>

[0077] The obtained ferritic stainless steels were drawn to yield a ϕ0.5 mm wire, and the workability of the ferritic stainless steel was evaluated. The results are listed in Table 1 as “Good” for those that could be drawn and “No good” for those that could not be drawn.<Oxidation Resistance Test>

[0078] A 10 mm×20 mm×5 mm (length×width×thickness) plate was cut from the obtained ferritic stainless steel and heated to a predetermined temperature in an electric furnace (in atmospheric air). The surface of the plate was observed every 500 hours, and the time (h) at which abnormal oxidation occurred was taken as the test result. The upper limit of the test time was set at 30000 hours for 1150° C., 20000 hours for 1200° C., and 10000 hours for 1300° C. The results are listed in Table 1.TABLE 1Relational expressions for content ratiosAlloy chemical composition (mass %)2Al +CrAlNbZrHfTiYLaSCNOCrExamples11880.2—————0.0010.020.0020.023422480.2—————0.0010.020.0030.0240316110.2—————0.0010.020.0020.033842480.30.20.2———0.0010.030.0030.054052480.4——0.5——0.0010.040.0020.024062480.20.20.20.5——0.0010.020.0030.044072480.40.20.20.50.005—0.0010.040.0020.024082480.20.20.20.5—0.0050.0010.020.0030.06409257.50.30.20.20.50.005—0.0010.030.10.0440Compar-11470.2—————0.0010.030.0030.0228ative22360.2—————0.0010.020.0030.0335Examples32360.20.20.20.5——0.0010.030.0050.063542670.20.20.20.50.05—0.00150.040.0050.024052180.20.20.20.50.01—0.0020.020.0030.043762180.20.70.70.5——0.0010.040.0020.033772180.20.20.21.5——0.0010.020.0030.043782870.2—————0.0010.030.0030.0242918120.2—————0.0010.020.0020.024210218——————0.0010.030.0040.0337Relational expressions for content ratiosReductionAl +Zr +REM(S − REM / 5) ×in areaWorkabilityOxidation resistance test results [h]CrCr / AlHf(Y + La)(2Al + Cr)2[%]evaluation1150° C.1200° C.1300° C.Examples1262.300—75Good>300001450055002323.000—72Good>300001500060003271.500—69Good>300001500060004323.00.40—71Good>300001850070005323.000—68Good>300001800070006323.00.40—70Good>30000>20000>100007323.00.40.0050.080Good>30000>20000>100008323.00.40.0050.078Good>30000>20000>10000932.53.30.40.0050.062Good>30000>20000>10000Compar-1212.000—70Good>3000065002500ative2293.800—68Good>3000070003500Examples3293.80.40—68Good>300001200030004333.70.40.05−13.668Good>300001250010005292.60.40.010.068Good>30000750030006292.61.40—70Good>30000850035007292.60.40—70Good>30000800030008354.000—25No good———9301.500—9No good———10292.600—0No good———

[0079] As is clear from Table 1, the ferritic stainless steels of the Examples have excellent oxidation resistance and workability.

[0080] On the other hand, Comparative Example 1 has poor oxidation resistance because the value of “Al+Cr” for the ferritic stainless steel is less than 23.

[0081] Comparative Examples 2 and 3 have poor oxidation resistance because the content ratio of Al in the ferritic stainless steels is less than 7%.

[0082] Comparative Example 4 has poor oxidation resistance because the content ratio of REM in the ferritic stainless steel exceeds 0.01%.

[0083] Comparative Example 5 has poor oxidation resistance because the content ratio of S in the ferritic stainless steel exceeds 0.0015%.

[0084] Comparative Example 6 has poor oxidation resistance because the value of “Zr+Hf” for the ferritic stainless steel exceeds 1.0%.

[0085] Comparative Example 7 has poor oxidation resistance because the content ratio of Ti in the ferritic stainless steel exceeds 1.0%.

[0086] Comparative Examples 8 and 9 have poor workability because the value of “2Al+Cr” for the ferritic stainless steels exceeds 41.

[0087] Comparative Example 10 has poor workability because the ferritic stainless steel does not contain the predetermined ratio of Nb.INDUSTRIAL APPLICABILITY

[0088] According to the present disclosure, a ferritic stainless steel with excellent oxidation resistance and workability can be provided.

Claims

1-7. (canceled)8. A ferritic stainless steel comprising:a chemical composition containing, 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, andNb: 0.1% or more and 1.0% or less,with the balance being Fe and inevitable impurities, whereincontent ratios of Al and Cr satisfy expressions (1), (2) and (5):2⁢A⁢ l+Cr≤41(1)A⁢l+Cr≥23(2)Cr / A⁢ l≤3.2.(5)9. A ferritic stainless steel comprising:a chemical composition containing, 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, andNb: 0.1% or more and 1.0% or less,with the balance being Fe and inevitable impurities, whereincontent ratios of Al and Cr satisfy expressions (1) and (2):2⁢Al+Cr≤41(1)A⁢l+Cr≥23,(2)the chemical composition further contains at least one of Zr and Hf, content ratios of Zr and Hf satisfy expression (3):0.01≤Zr+H⁢f ≤1.,(3)anda shape of the ferritic stainless steel is wire shape or strip shape.

10. The ferritic stainless steel according to claim 8, whereinthe chemical composition further contains, in mass %,Ti: 0.05% or more and 1.0% or less, andAl+Cr is 26 or more.

11. The ferritic stainless steel according to claim 9, whereinthe chemical composition further contains, in mass %,Ti: 0.05% or more and 1.0% or less, andAl+Cr is 26 or more.

12. The ferritic stainless steel according to claim 9, whereincontent ratios of Al and Cr satisfy expression (5):Cr / Al≤3.2.(5)13. The ferritic stainless steel according to claim 8, wherein a reduction of area in a tensile test at room temperature is 40% or more.

14. The ferritic stainless steel according to claim 10,the chemical composition further contains, in mass %,REM: 0.01% or less, andcontent ratios of Al, Cr, S, and REM satisfy expression (4):-1.≤(S-REM / 5)×(2⁢Al+Cr)2≤1.3.(4)15. A ferritic stainless steel comprising:a chemical composition containing, 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,Nb: 0.1% or more and 1.0% or less, andTi: 0.05% or more and 1.0% or less,with the balance being Fe and inevitable impurities, wherein content ratios of Al and Cr satisfy expressions (1) and (2):2⁢Al+Cr≤41(1)A⁢l+Cr≥26,(2)the chemical composition further contains at least one of Zr and Hf, content ratios of Zr and Hf satisfy expression (3):0.01≤Zr+Hf≤1.,(3)the chemical composition further contains, in mass %,REM: 0.01% or less,content ratios of Al, Cr, S, and REM satisfy expression (4):-1.≤(S-REM / 5)×(2⁢Al+Cr)2≤1.3,(4)anda shape of the ferritic stainless steel is wire shape, strip shape, tube shape or plate shape.

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