EP4671399A1 - FERRITIC STAINLESS STEEL SHEET AND EXHAUST COMPONENT - Google Patents

Abstract

A ferritic stainless steel sheet, which has a chemical composition including, in mass%: C: 0.001 to 0.030%, Si: 0.01 to 1.00%, Mn: 0.01 to 0.90%, P: 0.010 to 0.100%, S: 0.0001 to 0.0100%, Cr: 16.0 to 20.0%, Cu: 1.00 to 1.50%, Mo: 0.02 to 0.50%, Ti: 0.050 to 0.300%, Nb: 0.050 to 0.200%, Al: 0.003 to 0.500%, Ni: 0.01 to 0.30%, B: 0.0001 to 0.0050%, V: 0.010 to 0.500%, N: 0.001 to 0.020%, arbitrary elements, and the balance: Fe and impurities, satisfies [830≤250Cu+2216Nb+3083V≤2000] and [900≤1273Nb+1343Mo+7257A1≤3800], and has a grain size number of 6 to 9.

Description

TECHNICAL FIELD
• The disclosure relates to a ferritic stainless steel sheet and an exhaust part.
BACKGROUND ART
• Attributed to its resistance to thermal expansion and low cost, the ferritic stainless steel is used for exhaust parts of automobiles and the like that are repeatedly heated and cooled. Meanwhile, the ferritic stainless steel is inferior in high-temperature resistance compared to an austenitic stainless steel. Accordingly, a ferritic stainless steel containing Nb has been developed. Nb, when it is contained, may cause an increase in recrystallization temperature or cause the degradation of toughness during hot rolling, leading to the degradation of producibility. Accordingly, a steel containing Cu that improves the high-temperature strength similarly to Nb has been developed.
• Meanwhile, the ferritic stainless steel has various high-temperature resistance that are lower than the austenitic stainless steel. Accordingly, ferritic stainless steels that contain Cu and elements such as Al, Nb, Mo, and Ti beyond a certain amount in a combined manner have been developed to improve the high-temperature resistance.
LIST OF PRIOR ART DOCUMENTS PATENT DOCUMENT
• Patent Document 1: WO2015/174079
• Patent Document 2: JP2008-240143A
• Patent Document 3: JP2010-248620A
SUMMARY OF INVENTION TECHNICAL PROBLEM
• Various expensive elements increase production costs when they are contained beyond a certain amount, as described in the abovementioned documents. In some cases, formability may degrade. Partly because exhaust parts have highly complex shapes, the formability is an important one of the characteristics. Accordingly, alloy saving is desirable. On the other hand, while it is desirable that the ferritic stainless steel contains reduced contents of the abovementioned elements to achieve alloy saving, the problem is that it is difficult to improve both the thermal fatigue resistance and the formability, which are high-temperature resistance, while achieving the alloy saving.
• Keeping the above in mind, an objective of the disclosure is to provide a ferritic stainless steel sheet in which both the thermal fatigue resistance and the formability, which are high-temperature resistance, are improved while achieving the alloy saving.
SOLUTION TO PROBLEM
• The gist of the disclosure, which has been made to solve the above problem, is a ferritic stainless steel sheet and an exhaust part as described below.
1. (1) A ferritic stainless steel sheet, which has a chemical composition consisting of, in mass%:
   • C: 0.001 to 0.030%,
   • Si: 0.01 to 1.00%,
   • Mn: 0.01 to 0.90%,
   • P: 0.010 to 0.100%,
   • S: 0.0001 to 0.0100%,
   • Cr: 16.0 to 20.0%,
   • Cu: 1.00 to 1.50%,
   • Mo: 0.02 to 0.50%,
   • Ti: 0.050 to 0.300%,
   • Nb: 0.050 to 0.200%,
   • Al: 0.003 to 0.500%,
   • Ni: 0.01 to 0.30%,
   • B: 0.0001 to 0.0050%,
   • V: 0.010 to 0.500%,
   • N: 0.001 to 0.020%,
   • Mg: 0 to 0.0010%,
   • Ca: 0 to 0.0050%,
   • W: 0 to 3.00%,
   • Co: 0 to 0.30%,
   • Zr: 0 to 0.10%,
   • REM: 0 to 0.10%,
   • Sn: 0 to 0.500%,
   • Sb: 0 to 0.50%,
   • Ga: 0 to 0.30%,
   • Ta: 0 to 1.00%,
   • Hf: 0 to 1.00%,
   • Bi: 0 to 0.02%, and
   • the balance: Fe and impurities,
   • satisfies following Formulas (i) and (ii), and
   • has a grain size number of 6 to 9: $$830\le 250\mathrm{Cu}+2216\mathrm{Nb}+3083\mathrm{V}\le 2000$$ $$900\le 1273\mathrm{Nb}+1343\mathrm{Mo}+7257\mathrm{Al}\le 3800$$
   • where each element symbol in the formulas represents a content (in mass%) of each element contained in the ferritic stainless steel sheet and is zero when the element is not contained.
2. (2) The ferritic stainless steel sheet according to the above (1), wherein the grain size number is 6 to 8.
3. (3) The ferritic stainless steel sheet according to the above (1), wherein the chemical composition contains one or more elements, in mass%, selected from:
   • Mg: 0.0001 to 0.0010%,
   • Ca: 0.0001 to 0.0050%,
   • W: 0.02 to 3.00%,
   • Co: 0.001 to 0.30%,
   • Zr: 0.01 to 0.10%,
   • REM: 0.005 to 0.10%,
   • Sn: 0.005 to 0.500%,
   • Sb: 0.005 to 0.50%,
   • Ga: 0.0002 to 0.30%,
   • Ta: 0.001 to 1.00%,
   • Hf: 0.001 to 1.00%, and
   • Bi: 0.001 to 0.02%.
4. (4) The ferritic stainless steel sheet according to the above (1), wherein an area fraction of Ti precipitates is 5% or less.
5. (5) The ferritic stainless steel sheet according to the above (2), wherein an area fraction of Ti precipitates is 5% or less.
6. (6) The ferritic stainless steel sheet according to the above (1), wherein following Formula (iii) is satisfied: $$\mathrm{KAM}/\mathrm{HAGB}\le 0.05$$ where KAM in the formula represents a KAM value, and HAGB represents a ratio of a grain boundary length with a misorientation of 15° or more to a total grain boundary length.
7. (7) The ferritic stainless steel sheet according to the above (2), wherein following Formula (iii) is satisfied: $$\mathrm{KAM}/\mathrm{HAGB}\le 0.05$$ where KAM in the formula represents a KAM value, and HAGB represents a ratio of a grain boundary length with a misorientation of 15° or more to a total grain boundary length.
8. (8) The ferritic stainless steel sheet according to the above (4), wherein following Formula (iii) is satisfied: $$\mathrm{KAM}/\mathrm{HAGB}\le 0.05$$ where KAM in the formula represents a KAM value, and HAGB represents a ratio of a grain boundary length with a misorientation of 15° or more to a total grain boundary length.
9. (9) An exhaust part, wherein the ferritic stainless steel sheet according to any one of the above (1) to (8) is used.
ADVANTAGEOUS EFFECTS OF INVENTION
• According to the disclosure, it is possible to obtain a ferritic stainless steel sheet in which both the thermal fatigue resistance and the formability, which are high-temperature resistance, are improved while achieving the alloy saving.
DESCRIPTION OF EMBODIMENTS
• The inventors have conducted various studies and obtained the following findings.
1. (a) It is effective for the steel to contain Cu, Nb, V, Mo, Al, and Ti in appropriate ranges while the contents are reduced. As a result, it is possible to inhibit the precipitation of Ti during production, whereas precipitation strengthening and solid-solution strengthening of Ti become evident in a usage environment, so that a ferritic stainless steel sheet that is excellent in the thermal fatigue resistance can be obtained.
2. (b) In other words, the thermal fatigue life is increased by optimizing the contents of Cu, Nb, V, Mo, and Al, which are elements that affect the precipitation strengthening and the solid-solution strengthening. Furthermore, inhibiting the precipitation of Ti compounds during production allows Ti compounds, which contribute to the improvement of the thermal fatigue resistance, to precipitate during the thermal fatigue, leading to more effective improvement of the thermal fatigue resistance.
3. (c) For a high thermal fatigue resistance to become evident, it is necessary to obtain a polycrystalline microstructure that has a grain size number of 6 to 9 and is fully recrystallized. The reason is that in a case in which any uncrystallized microstructure, which is still incompletely recrystallized, is formed, it is likely that the thermal fatigue resistance may degrade.
• An embodiment of the disclosure has been made based on the findings described above. The requirements for the ferritic stainless steel sheet of the embodiment will now be described in detail.
1. Chemical Composition
• The reason for limitation for each element is described below. Note that in the description below, "%" for the content refers to "mass%".
C: 0.001 to 0.030%
• C (carbon) generates carbide and causes the degradation of the corrosion resistance and the high-temperature strength. Accordingly, the reduced contents are preferable, and the content of C is 0.030% or less. The content of C is preferably 0.015% or less, and more preferably 0.010% or less. On the other hand, excessively reducing C leads to an increase in refining costs, and therefore, the content of C is 0.001% or more.
Si: 0.01 to 1.00%
• Si (silicon), which is used as a deoxidizing element, improves the high-temperature resistance and the oxidation resistance. Accordingly, the content of Si is 0.01% or more. To inhibit breakaway oxidation and scale spallation at 950°C and further improve the oxidation resistance, the content of Si is preferably 0.05% or more, preferably 0.10% or more, more preferably 0.13% or more, and further preferably 0.15% or more. However, when Si is contained at more than 1.00%, the steel sheet becomes hardened, leading to the degradation of the formability for parts and the producibility. Accordingly, the content of Si is 1.00% or less. The content of Si is preferably 0.80% or less, more preferably 0.50% or less, and further preferably 0.25% or less.
Mn: 0.01 to 0.90%
• Mn (manganese) is an element that is used as a deoxidizing element and improves the high-temperature strength. Furthermore, Mn oxide is formed on the outer layer during long time use, contributing to the improvement of scale adhesiveness. Accordingly, the content of Mn is 0.01% or more. To further increase the scale adhesiveness, the content of Mn is preferably 0.05% or more, more preferably 0.10% or more, and further preferably 0.11% or more. However, when Mn is contained at more than 0.90%, the uniform elongation at a room temperature is degraded, and MnS is formed to cause the degradation of the corrosion resistance and the oxidation resistance. Accordingly, the content of Mn is 0.90% or less. In consideration of high-temperature ductility, inhibition of breakaway oxidation, and inhibition of oxidation weight gain, the content of Mn is preferably 0.80% or less, more preferably 0.50% or less, and further preferably 0.30% or less.
P: 0.010 to 0.100%
• P (phosphorus) is an element that promotes the hot formability and solidification cracking during production. Furthermore, P precipitates as FeTiP in a high-temperature environment, so as to contribute to the improvement of the thermal fatigue resistance by precipitation strengthening. Accordingly, the content of P is 0.100% or less. The content of P is preferably 0.060% or less, and more preferably 0.050% or less. While it is preferable that the content of P is lowered as much as possible, excessively reducing P increases refining costs. Accordingly, the content of P is 0.010% or more. Furthermore, from the viewpoint of reducing refining costs, the content of P is preferably 0.020% or more, and more preferably 0.030% or more.
S: 0.0001 to 0.0100%
• S (sulfur) is an element that causes the degradation of the hot formability and the corrosion resistance. When coarse sulfide (MnS) is formed, the non-metallic inclusion content of the steel sheet significantly degrades. Furthermore, the oxidation resistance also degrades. Accordingly, the content of S is 0.0100% or less. The content of S is more preferably 0.0050% or less. However, excessively reducing S increases refining costs. Accordingly, the content of S is 0.0001% or more. From the viewpoint of refining costs, the content of S is preferably 0.0005% or more.
Cr: 16.0 to 20.0%
• Cr (chromium) is an element that improves the corrosion resistance and the oxidation resistance. From the viewpoint of inhibiting breakaway oxidation, salt corrosion, and condensate corrosion in an environment of exhaust parts, the content of Cr is 16.0% or more. The content of Cr is preferably 16.5% or more, and more preferably 17.0% or more. However, excessively contained Cr makes the material hardened and leads to an increase in production costs. Accordingly, the content of Cr is 20.0% or less. Furthermore, from the viewpoint of formability, producibility, and production costs, the content of Cr is preferably 19.0% or less, and more preferably 18.0% or less.
Cu: 1.00 to 1.50%
• Cu (copper) is an element that improves the high-temperature strength and the thermal fatigue resistance by precipitation strengthening. The improvement of the characteristics is attributed to precipitation strengthening effects due to the precipitation of bcc-Cu and ε-Cu, and becomes evident when Cu is contained at 1.00% or more. Accordingly, the content of Cu is 1.00% or more. The content of Cu is preferably 1.05% or more, and more preferably 1.15% or more. However, excessively contained Cu leads to hardening and causes the degradation of the uniform elongation and the elongation after fracture. Furthermore, the room-temperature proof stress also increases, leading to the degradation of the producibility for parts, such as press formability. Furthermore, an austenite phase is formed in a high-temperature range, leading to the degradation of the scale adhesiveness and causing breakaway oxidation on the surface. Accordingly, the content of Cu is 1.50% or less. The content of Cu is preferably 1.40% or less, and more preferably 1.30% or less.
Mo: 0.02 to 0.50%
• In addition to improving the corrosion resistance and the oxidation resistance, Mo (molybdenum) is an element that improves the high-temperature strength and the thermal fatigue resistance by solid-solution strengthening. Accordingly, the content of Mo is 0.02% or more. The content of Mo is preferably 0.11 % or more, and more preferably 0.15% or more. However, Mo is expensive and causes the degradation of the uniform elongation at a room temperature. Accordingly, the content of Mo is 0.50% or less. Furthermore, from the viewpoint of non-metallic inclusion content, producibility, and costs, the content of Mo is preferably 0.40% or less, and more preferably 0.30% or less.
Ti: 0.050 to 0.300%
• Ti (titanium) is an element that, combined with C, N, and S, improves the corrosion resistance and the intergranular corrosion resistance. Furthermore, in combined addition with Nb, the high-temperature strength and the high-temperature ductility are improved, so that the high-temperature fatigue resistance and the thermal fatigue resistance are improved. Furthermore, Ti precipitates as FeTiP in a high-temperature environment, so as to contribute to the improvement of the thermal fatigue resistance by precipitation strengthening. Accordingly, the content of Ti is 0.050% or more. From the viewpoint of salt corrosion resistance, condensate corrosion resistance, high-temperature strength, and intergranular corrosion resistance of welded zones, the content of Ti is preferably 0.080% or more, and more preferably 0.100% or more.
• However, excessively contained Ti makes it likely to cause coarse Ti carbonitride, which may be a starting point of a fatigue fracture and the like, or likely to cause nozzle blocking in a casting phase, causing a significant degradation of the producibility. Accordingly, the content of Ti is 0.300% or less. In addition, from the viewpoint of the production costs, the content of Ti is preferably 0.250% or less, and more preferably 0.190% or less.
Nb: 0.050 to 0.200%
• Nb (niobium) is an element that improves the high-temperature strength and the thermal fatigue resistance by solid-solution strengthening. Nb finely precipitates as a Laves phase (mainly, Fe₂Nb) at approximately 750°C, so that the high-temperature strength and the thermal fatigue resistance are improved. In addition, combined with C and N as in Ti, Nb improves the corrosion resistance such as salt corrosion resistance and condensate corrosion resistance, and the intergranular corrosion resistance of welded zones. Accordingly, the content of Nb is 0.050% or more. The content of Nb is preferably 0.080% or more, and more preferably 0.100% or more.
• However, excessively contained Nb leads to a significant degradation of the hot formability and an increase in the recrystallization temperature. As a result, the producibility degrades. Production costs also increase. Accordingly, the content of Nb is 0.200% or less. The content of Nb is preferably 0.180% or less, and more preferably 0.160% or less.
Al: 0.003 to 0.500%
• Al (aluminum) acts as a deoxidizing element and improves the non-metallic inclusion content. Furthermore, the high-temperature strength and the thermal fatigue resistance are improved by solid-solution strengthening. Accordingly, the content of Al is 0.003% or more. The content of Al is preferably 0.010% or more, and more preferably 0.020% or more. However, excessively contained Al leads to the degradation of pickling suitability, generating more surface roughness. Furthermore, the number of inclusions increases, which leaves starting points of fatigue cracks, leading to the degradation of the fatigue strength. The hot formability also degrades. Accordingly, the content of Al is 0.500% or less. From the viewpoint of refining costs and surface conditions, the content of Al is preferably 0.300% or less. Furthermore, from the viewpoint of hot formability, the content of Al is preferably 0.150% or less, and more preferably 0.090% or less.
Ni: 0.01 to 0.30%
• In addition to improving the toughness of the ferritic stainless steel sheet, Ni (nickel) is an element that prevents brittle cracking during die forming. Accordingly, the content of Ni is 0.01% or more. The content of Ni is preferably 0.05% or more, more preferably 0.07% or more, and further preferably 0.10% or more. However, excessively contained Ni leads to the precipitation of an austenite phase, so that breakaway oxidation is induced, and the thermal fatigue resistance degrades. Accordingly, the content of Ni is 0.30% or less. From the viewpoint of producibility, inhibition of breakaway oxidation, thermal fatigue resistance, salt corrosion resistance, and condensate corrosion resistance, the content of Ni is preferably 0.20% or less, and more preferably 0.15% or less.
B: 0.0001 to 0.0050%
• In addition to improving the hot formability, B (boron) is an element that prevents work hardening and secondary-processing cracking at a room temperature. An effect of improving the ductility is also produced. Accordingly, the content of B is 0.0001% or more. From the viewpoint of ductility, the content of B is preferably 0.0002% or more, and more preferably 0.0003% or more. However, excessively contained B leads to the formation of boron carbide, and the non-metallic inclusion content and the intergranular corrosion resistance of the steel sheet degrade. Accordingly, the content of B is 0.0050% or less. From the viewpoint of refining costs or the like, the content of B is preferably 0.0030% or less, more preferably 0.0020% or less, and further preferably 0.0010% or less.
V: 0.010 to 0.500%
• V (vanadium) is an element that improves the corrosion resistance. V also forms carbide and nitride, improving the high-temperature strength and the thermal fatigue resistance. Accordingly, the content of V is 0.010% or more. To increase the thermal fatigue resistance, the content of V is preferably 0.020% or more, and further preferably 0.025% or more. However, excessively contained V leads to an increase in production costs and also the lowering of breakaway oxidation limit temperature. Accordingly, the content of V is 0.500% or less. From the viewpoint of producibility, the content of V is preferably 0.300% or less, more preferably 0.150% or less, more preferably 0.100% or less, and further preferably 0.050% or less.
N: 0.001 to 0.020%
• As in C, N (nitrogen) causes the degradation of the corrosion resistance and the high-temperature strength. Accordingly, the content of N is 0.020% or less. The content of N is preferably 0.015% or less, and more preferably 0.013% or less. While it is preferable that the content of N is lowered as much as possible, excessive reduction leads to an increase in refining costs. Accordingly, the content of N is 0.001% or more. The content of N is preferably 0.003% or more, more preferably 0.004% or more, and further preferably 0.005% or more.
• In addition to the above-described elements, one or more elements selected from Mg, Ca, W, Co, Zr, REM, Sn, Sb, Ga, Ta, Hf, and Bi may further be contained to the extent indicated below. The reason for limitation for each element will be described.
Mg: 0 to 0.0010%
• Mg (magnesium) acts as a deoxidizing element and produces an effect of improving the non-metallic inclusion content and improving the corrosion resistance. Accordingly, Mg may be contained as necessary. However, excessively contained Mg leads to the degradation of the weldability and the corrosion resistance. Accordingly, the content of Mg is 0.0010% or less. From the viewpoint of weldability, the content of Mg is preferably 0.0008% or less, and more preferably 0.0005% or less. On the other hand, to obtain the above-described effects, the content of Mg is preferably 0.0001% or more. From the viewpoint of corrosion resistance, in particular, condensate corrosion resistance, the content of Mg is more preferably 0.00015% or more.
Ca: 0 to 0.0050%
• Ca (calcium) is active in desulfurization and produces an effect of improving the non-metallic inclusion content and improving the corrosion resistance. Accordingly, Ca may be contained as necessary. However, when Ca is contained at more than 0.0050%, water-soluble inclusions CaS are formed. As a result, the non-metallic inclusion content (surface conditions) and the corrosion resistance of the steel sheet significantly degrades. Accordingly, the content of Ca is 0.0050% or less. From the viewpoint of producibility and surface conditions, the content of Ca is preferably 0.0030% or less, and more preferably 0.0020% or less. On the other hand, to obtain the above-described effects, the content of Ca is preferably 0.0001% or more, is preferably 0.0003% or more, more preferably 0.0005% or more, and further preferably 0.0007% or more.
W: 0 to 3.00%
• As in Mo, W (tungsten) produces an effect of improving the high-temperature strength by solid-solution strengthening. An effect of improving the corrosion resistance is also produced. Accordingly, W may be contained as necessary. However, when contained at more than 3.00%, W leads to hardening of the material, the degradation of the toughness during production, and an increase in production costs. Accordingly, the content of W is 3.00% or less. Furthermore, from the viewpoint of producibility and refining costs, the content of W is preferably 2.00% or less, more preferably 1.50% or less, and further preferably 1.00% or less. On the other hand, to obtain the above-described effects, the content of W is preferably 0.02% or more, and more preferably 0.10% or more.
Co: 0 to 0.30%
• Co (cobalt) produces an effect of improving the corrosion resistance and the high-temperature strength. Accordingly, Co may be contained as necessary. However, when contained at more than 0.30%, Co leads to hardening of the steel sheet, the degradation of the toughness during production, and an increase in production costs. Accordingly, the content of Co is 0.30% or less. In consideration of producibility and refining costs, the content of Co is preferably 0.20% or less, and more preferably 0.10% or less. On the other hand, to obtain the above-described effects, the content of Co is preferably 0.001% or more. From the viewpoint of increasing the corrosion resistance, in particular, salt corrosion resistance and condensate corrosion resistance, and improving the high-temperature strength, the content of Co is preferably 0.005% or more, and more preferably 0.010% or more.
Zr: 0 to 0.10%
• As in Ti and Nb, Zr (zirconium) is combined with C or N to form carbide or nitride, improving the high-temperature strength, the oxidation resistance, and the intergranular corrosion resistance of welded zones. Accordingly, Zr may be contained as necessary. However, when contained at more than 0.10%, Zr leads to a significant degradation of the producibility and an increase in production costs. Accordingly, the content of Zr is 0.10% or less. From the viewpoint of producibility and refining costs, the content of Zr is preferably 0.08% or less. On the other hand, to obtain the above-described effects, the content of Zr is preferably 0.01% or more, and more preferably 0.03% or more.
REM: 0 to 0.10%
• REM (rare earth metal) produces an effect of improving the oxidation resistance. Accordingly, REM may be contained as necessary. However, excessively contained REM causes nozzle clogging due to the formation of oxide containing REM in the nozzle through which molten steel is caused to flow during casting, or causes the degradation of the corrosion resistance due to the formation of sulfide containing REM. Accordingly, the content of REM is 0.10% or less. The content of REM is more preferably 0.07% or less, and further preferably 0.05% or less. On the other hand, to obtain the above-described effects, the content of REM is preferably 0.005% or more, and more preferably 0.010% or more.
• REM refers to a total of 17 elements of Sc, Y, and lanthanoid, and the content of REM refers to a total content of the elements. Industrially, REM is often contained as misch metal.
Sn: 0 to 0.500%
• Sn (tin) produces an effect of improving the corrosion resistance and the high-temperature strength. Accordingly, Sn may be contained as necessary. However, when contained at more than 0.500%, Sn may cause slab cracking during production. Accordingly, the content of Sn is 0.500% or less. Furthermore, from the viewpoint of producibility and refining costs, the content of Sn is preferably 0.300% or less. On the other hand, to obtain the above-described effects, the content of Sn is preferably 0.005% or more, and more preferably 0.030% or more.
Sb: 0 to 0.50%
• Sb (antimony) segregates at a grain boundary and produces an effect of improving the high-temperature resistance. Accordingly, Sb may be contained as necessary. However, when contained at more than 0.50%, Sb segregates and causes cracking during welding. Accordingly, the content of Sb is 0.50% or less. Furthermore, in consideration of toughness and production costs, the content of Sb is preferably 0.30% or less. On the other hand, to obtain the above-described effects, the content of Sb is preferably 0.005% or more. To further increase the high-temperature resistance, the content of Sb is more preferably 0.030% or more.
Ga: 0 to 0.30%
• Ga (gallium) produces an effect of improving the corrosion resistance and inhibiting hydrogen embrittlement. Accordingly, Ga may be contained as necessary. However, when contained at more than 0.30%, Ga leads to the formation of coarse sulfide, degrading the formability for parts. Accordingly, the content of Ga is 0.30% or less. From the viewpoint of producibility and production costs, the content of Ga is preferably 0.20% or less, and more preferably 0.10% or less. On the other hand, to obtain the above-described effects, the content of Ga is preferably 0.0002% or more, and more preferably 0.0020% or more.
Ta: 0 to 1.00% Hf: 0 to 1.00% Bi: 0 to 0.02%
• Ta (tantalum) produces an effect of improving the high-temperature strength. Accordingly, Ta may be contained as necessary. However, excessively contained Ta leads to an increase in production costs. Accordingly, the content of Ta is 1.00% or less. The content of Ta is preferably 0.50% or less, and more preferably 0.20% or less. On the other hand, to obtain the above-described effects, the content of Ta is preferably 0.001% or more. Furthermore, to further increase the strength, the content of Ta is more preferably 0.010% or more.
• For the same reason, Hf (hafnium) may also be contained as necessary. However, excessively contained Hf leads to an increase in production costs. Accordingly, the content of Hf is 1.00% or less. The content of Hf is preferably 0.50% or less, and preferably 0.20% or less. Furthermore, for the same reason, the content of Hf is preferably 0.001% or more, and preferably 0.010% or more.
• For the same reason, Bi (bismuth) may also be contained as necessary. However, excessively contained Bi leads to an increase in production costs. Accordingly, the content of Bi is 0.02% or less. The content of Bi is preferably 0.015%, and more preferably 0.01% or less. On the other hand, to obtain the same effects as described above, the content of Bi is preferably 0.001% or more, and more preferably 0.005% or more.
• In the chemical composition of the embodiment, the balance is Fe and impurities. Here, "impunities" refer to components that are introduced due to various factors in raw materials such as ore and scrap and production processes when the ferritic stainless steel sheet is industrially produced and that are acceptable to the extent that they do not adversely affect the embodiment.
Formula (i)
• For the ferritic stainless steel sheet of the embodiment, it is necessary that the contents of Cu, Nb, and V, which affect precipitation strengthening, satisfy Formula (i) below. Cu finely precipitates as bcc-Cu or ε-Cu, Nb as Fe₂Nb (Laves phase), and V as carbonitride to improve the thermal fatigue resistance. $$830\le 250\mathrm{Cu}+2216\mathrm{Nb}+3083\mathrm{V}\le 2000$$ where each element symbol in the formula represents a content (in mass%) of each element contained in the ferritic stainless steel sheet and is zero when the element is not contained.
• When the middle value of Formula (i), 250Cu+2216Nb+3083V, is less than 830, the thermal fatigue resistance cannot be improved. Accordingly, the middle value of Formula (i) is 830 or more. The middle value of Formula (i) is preferably 850 or more, more preferably 870 or more, and further preferably 900 or more. On the other hand, when the middle value of Formula (i) is more than 2000, hardening occurs and the formability for parts may degrade. Accordingly, the middle value of Formula (i) is 2000 or less. The middle value of Formula (i) is preferably 1900 or less, more preferably 1800 or less, and further preferably 1500 or less.
Formula (ii)
• For the ferritic stainless steel sheet of the embodiment, it is necessary that the contents of Nb, Mo, and Al, which affect solid-solution strengthening, satisfy Formula (ii) below. $$900\le 1273\mathrm{Nb}+1343\mathrm{Mo}+7257\mathrm{Al}\le 3800$$ where each element symbol in the formula represents a content (in mass%) of each element contained in the ferritic stainless steel sheet and is zero when the element is not contained.
• When the middle value of Formula (ii), 1273Nb+1343Mo+7257Al, is less than 900, it is difficult to improve the thermal fatigue resistance. Accordingly, the middle value of Formula (ii) is 900 or more. The middle value of Formula (ii) is preferably 1000 or more, more preferably 1100 or more, and further preferably 1300 or more. On the other hand, when the middle value of Formula (ii) is more than 3800, hardening occurs and the formability for parts may degrade. Accordingly, the middle value of Formula (ii) is 3800 or less. The middle value of Formula (ii) is more preferably 3500 or less, further preferably 3000 or less, and further preferably 2700 or less.
• The thermal fatigue is a phenomenon of fatigue fracture occurring in an environment in which heating and cooling are repeated continuously from a low temperature on the order of 200°C to a high temperature of about 900°C. Accordingly, material strengthening at various temperatures is required. In the ferritic stainless steel sheet of the embodiment, studies have been made in a case in which thermal cycle tests are conducted at the lowest temperature of 200°C and the highest temperature of 850°C and a cycle during which fatigue cracks penetrate is considered as the thermal fatigue life. As a result, Formulas (i) and (ii) described above have been experimentally derived. It is necessary to satisfy both Formulas (i) and (ii) to secure the thermal fatigue resistance acceptable for a member for an automobile exhaust part, and if one or both of the formulas are not satisfied, the thermal fatigue resistance will degrade.
2. Grain Size Number
• The grain size number of the ferritic stainless steel sheet of the embodiment is in the range of 6 to 9. When the grain size number is less than 6, the grain becomes too coarse, which leads to the degradation of the material strength or strain-induced microstructural changes caused by heating and cooling during the thermal fatigue. This may cause the degradation of the thermal fatigue resistance. Accordingly, the grain size number is 6 or more. When the grain size number is 6 or more, not only is the strength improved, but strain-induced microstructural changes caused by heating and cooling during the thermal fatigue are also inhibited. As a result, it is possible to secure a sufficient thermal fatigue resistance. On the other hand, when the grain size number is more than 9, the grain becomes too fine, and what is worse, the formability degrades. Furthermore, recrystallization may still be incomplete. Accordingly, the grain size number is 9 or less. The grain size number is preferably 9.0 or less, preferably 8.9 or less, and preferably 8 or less. The grain size number is preferably in the range of 6 to 8.
• The grain size number of the ferritic stainless steel sheet may be measured according to the procedure described below. A steel sheet is embedded in thermosetting resin with a section that is parallel to the rolling direction and the sheet-thickness direction (L-section) being as an observation surface, and is subjected to observation. An area in the range of 1/4 part of the sheet thickness from the center of the sheet thickness is defined as a measurement area, observations are made at five visual fields with 100 to 200× magnification, and a cutting method defined in JIS G 0551: 2020 Annex JB is used to determine the grain size number from total observed visual fields.
3. Area Fraction of Ti Precipitates
• For the ferritic stainless steel sheet of the embodiment, it is preferable to control the area fraction of Ti precipitates to further increase the thermal fatigue resistance. Specifically, the area fraction of Ti precipitates is preferably 5% or less. The reason is that it keeps a small amount of Ti that is present as precipitates in the steel sheet at a room temperature, so that Ti is allowed to finely precipitate in a usage environment to generate FeTiP that contributes to precipitation strengthening. Accordingly, to secure the amount of Ti that precipitates in a usage environment, which reaches a high temperature, the area fraction of Ti precipitates at a room temperature is preferably 5% or less. The area fraction of Ti precipitates is preferably 2% or less, more preferably 1.5% or less, and further preferably 1% or less. From the viewpoint of improvement of the thermal fatigue resistance, the area fraction of Ti precipitates is preferably as low as possible, and the lower limit is preferably 0%. However, it is difficult to achieve 0% of the area fraction of Ti precipitates in the actual production, and therefore, the area fraction of Ti precipitates is preferably 0.1% or more, more preferably 0.2% or more, and further preferably 0.3% or more.
• The area fraction of Ti precipitates may be measured according to the procedure described below. A steel sheet is embedded in thermosetting resin with a section that is parallel to the rolling direction and the sheet-thickness direction (L-section) being as an observation surface, and subjected to mirror polishing by mechanical polishing, followed by observation and analysis by using a scanning electron microscope (hereinafter referred to as "SEM") equipped with an energy dispersive X-ray spectroscopy (hereinafter referred to as "EDS") to calculate the area fraction of precipitates. An area in the range of 1/4 part of the sheet thickness from the center of the sheet thickness is defined as a measurement area, observations are made at ten visual fields with 1000× magnification, and an average value of the areas of precipitates that contain Ti per visual field, which are determined through image analysis, is divided by the visual field area of one visual field to determine area fractions of the respective visual fields. An average value of the area fractions is defined as the area fraction of Ti precipitates. Here, precipitates that contain Ti are defined as those that contain Ti of 1.0 wt% or more and are 3 µm or more in the equivalent circle diameter.
4. Formula (iii)
• It is preferable for the ferritic stainless steel sheet of the embodiment to satisfy the following Formula (iii) to further improve the thermal fatigue resistance: $$\mathrm{KAM}/\mathrm{HAGB}\le 0.05$$ where KAM in the formula represents a KAM value, and HAGB represents a ratio of a grain boundary length with a misorientation of 15° or more to a total grain boundary length.
• Formula (iii) described above represents that there is less strain in the grain of a ferritic stainless steel sheet that has a polycrystalline microstructure. That is, it indicates that the steel sheet is properly produced and fully recrystallized microstructure in which less strain remains in the grain is obtained. In the case of poorly recrystallized crystalline microstructure in which strain remains in the grain, it is considered that microstructural changes occur during thermal fatigue while being heated and the thermal fatigue resistance degrades.
• When KAM/HAGB, which is the left side value of Formula (iii), is 0.05 or less, then it is further facilitated to improve the thermal fatigue resistance. Accordingly, the left side value of Formula (iii) is preferably 0.05 or less, more preferably 0.03 or less, further preferably 0.01 or less, and further preferably 0.005 or less. Note that the lower limit of the left side value of Formula (iii), which is not particularly limited, is most preferably zero because the smaller the left side value of Formula (iii) is, the more preferable.
• KAM and HAGB may be measured according to the procedure described below. With a section that is parallel to the rolling direction and the sheet-thickness direction (L-section) being as an observation surface, chemical mechanical polishing is performed by using mechanical polishing, colloidal silica suspension, and the like to remove the surface strain. A mirrored observation surface is obtained by performing the polishing, followed by observation and measurement by using a field emission scanning electron microscope (made by JEOL Ltd.: hereinafter referred to as "FE-SEM") equipped with an EBSD (also referred to as "Electron Back-Scattering Diffraction pattern") measuring instrument. Then, an OIM (made by TSL: also referred to as "Orientation Imaging Microscopy") is used for analysis to determine KAM and HAGB. Here, "OIM Analysis" software may be used.
• A 19 mm² area in the range of 1/4 part of the sheet thickness from the center of the sheet thickness is defined as a measurement area, and measurements are taken at ten visual fields with a measurement magnification of 1000× and a measurement distance of 0.5 µm. The KAM value refers generally to an average value of crystal orientation differences between measurement points of interest and the surrounding measurement points, and in the application, to a ratio of measurement points that create 1 to 2 degrees in the average value of the crystal orientation differences described above relative to all measurement points. HAGB is the ratio of presence of high-angle grain boundaries, and is determined by dividing grain boundary lengths that create 15 degrees or more in the misorientation by a total grain boundary length, that is, grain boundary lengths that create 2 degrees or more in the misorientation.
5. Application
• The ferritic stainless steel sheet of the embodiment can suitably be used as exhaust parts of automobiles and a member for use in the exhaust parts. In particular, the steel sheet is suitably used for parts for use in an environment in which the parts are exposed to exhaust gas on the order of 600 to 900°C. Specifically, the steel sheet can suitably be used as a member for use in an exhaust manifold, a center pipe, a front pipe, catalytic converter parts, a muffler, exhaust gas cleaning parts, and a housing and internal parts of a turbocharger. The reason is that members for use in the abovementioned parts are prone to thermal fatigues in a usage environment in which heating and cooling are repeated. Note that in the applications described above, a good condensate corrosion resistance is desirable. Here, the condensate corrosion resistance refers to a resistance against corrosion caused by moisture of condensed constituents of exhaust gas.
6. Production Method
• The ferritic stainless steel sheet of the embodiment can be stably produced according to a production method described below, for example. The ferritic stainless steel sheet of the embodiment is preferably produced through the processes from steelmaking to hot rolling to annealing·pickling to cold rolling to annealing·pickling or from steelmaking to hot rolling to pickling to cold rolling to annealing·pickling. Each process will now be described in detail.
6-1. Steelmaking Process
• A preferable method is to melt steel that has a chemical composition described above in an electric furnace or a converter, followed by secondary refining. The molten steel is turned into a slab according to a known casting method (such as continuous casting). It is effective to provide a stabilization period before continuous casting to avoid excessive introduction of inclusions during continuous casting. Such a stabilization period may be provided for one minute or more before continuous casting.
6-2. Hot Rolling Process
• The slab obtained in the steelmaking process described above is subjected to hot rolling by continuous rolling and turned into a hot-rolled sheet. Excluding cooling conditions described below, conditions for hot rolling are not particularly limited. The thickness of the slab and rolling reduction in the hot rolling may be selected as appropriate, whereas heating temperature for the slab, for example, is preferably in the range of 1030 to 1200°C.
• After the hot rolling, the hot-rolled sheet is coiled and cooled. It is preferable to inhibit the precipitation of Ti compounds such as V carbonitride, Ti (C, N), or FeTiP during coiling and cooling after the coiling, and therefore, it is preferable to increase cooling rate. This is to keep the area fraction of Ti precipitates at 5% or less.
• To this end, the cooling rate after hot rolling is preferably 40°C/s or more in the range of hot rolling temperature to 450°C. In consideration of producibility, the cooling rate after hot rolling is more preferably 70°C/s or more. After hot rolling, annealing may be performed as necessary to promote recrystallization. When performing the annealing, it is performed in the range of 10 to 200 seconds at 900 to 1200°C. The steel sheet after hot rolling (or after annealing when the annealing is performed) is generally subjected to pickling treatment. Note that the cooling after hot rolling may be sufficient when a room temperature is reached.
6-3. Cold Rolling and Annealing Process
• Subsequently, cold rolling is performed until a predetermined steel thickness is reached. The rolling reduction of the cold rolling may be selected as appropriate. Furthermore, the roll diameter of the cold rolling mill may be selected as appropriate. After the cold rolling, the cold-rolled sheet may be subjected to annealing and pickling treatment. In the ferritic stainless steel sheet of the embodiment, the amount of Nb is made smaller with the aim of lowering the recrystallization temperature. Accordingly, the annealing temperature is 800 to 980°C to obtain recrystallized microstructures. Furthermore, the annealing time is preferably in the range of 1 to 240 seconds.
• Here, in the production of the ferritic stainless steel sheet of the embodiment, it is preferable to control annealing conditions to achieve recrystallization such that strain does not remain in the grain to secure a high thermal fatigue resistance, that is, satisfy Formula (iii) to obtain a relatively fine microstructure with the grain size number of 6 to 9. Specifically, it is preferable to keep the time during which the temperature of the steel sheet is 880°C or more for 15 seconds or more. Note that in a case in which the cooling rate after hot rolling described above is small and the time during which the temperature is 880°C or more is less than 15 seconds, recrystallization and grain growth is less likely to advance, which may lead to an excessively large grain size number.
• The annealing of the cold-rolled sheet may be performed between passes of the cold rolling and may be either batch annealing or continuous annealing. Heating and cooling in annealing after cold rolling causes the precipitation of precipitates that affect the high-temperature strength, such as a Laves phase that contains Cu particles and Nb, and FeTiP that contains Ti. These precipitates can effectively improve the high-temperature strength and the thermal fatigue resistance by inhibiting the amount of precipitation in the steel sheet as small as possible and allowing them to precipitate in a usage environment.
• In the final annealing of the ferritic stainless steel sheet of the embodiment, the heating rate from 350°C to the annealing temperature is preferably 2.0°C/s or more. The reason is that when the heating rate in the temperature range is 2.0°C/s or more, strain is reduced, and it is likely that Formula (iii) is satisfied. The heating rate in the temperature range is more preferably 10.0°C/s or more. Note that the upper limit of the heating rate in the temperature range is, but not particularly limited to, 150.0°C/s, for example.
• Accordingly, with respect to cooling after final annealing after cold rolling, the cooling rate in the temperature range from 800°C to 350°C, at which the precipitates are likely to precipitate, is preferably 5°C/s or more. When the cooling rate in the temperature range from 800 to 350°C is less than 5°C/s, a large number of precipitates is caused to precipitate, and the heat resistance and the formability may tend to degrade.
• After annealing and cooling, it is preferable to perform pickling treatment. In the pickling treatment, scale formed on the surface of steel due to annealing can be removed. The way of pickling may be any chemical descaling methods such as sulfuric acid, nitric hydrofluoric acid, and nitric acid electrolysis, and an immersion in molten alkali salt may be performed as pre-processing. Conditions for temperature and time of the immersion in molten alkali salt may vary as appropriate depending on desired characteristics of the steel sheet. The steel sheet subjected to cold rolling, annealing, and pickling may be subjected to temper rolling and polishing process.
• Hereinunder, the ferritic stainless steel sheet according to the disclosure will specifically be described with reference to examples, whereas the embodiment is not limited to the examples.
EXAMPLE
• The steel that has a chemical composition shown in Table 1 was cast into a flat 17 kg mold, followed by the processes described below to obtain steel sheets. Specifically, hot-rolled sheets, which had been hot rolled down to a thickness of 5 mm at 1050°C and cooled from 1050 to 450°C at a cooling rate of 5.3 to 75°C/s, were subjected to pickling. Thereafter, cold rolling is performed, which is followed by heating to 920°C, which is an annealing temperature, at a heating rate of 0.8 to 20.3°C/s, annealing at 920°C for 120 seconds, cooling from 800 to 350°C at a cooling rate of 3.1 to 11.2°C/s, and pickling to obtain a steel sheet that has a thickness of 2.0 mm (hereinafter also referred to as "product sheet"). Note that the time taken to 880°C or more in the annealing after the cold rolling was between 5 and 180 seconds as shown in Table 2.
[Table 2]
• Table 2

  No.	Steel	Production method
  		Cooling rate after hot rolling (°C/s)	Heating rate in annealing after cold rolling (°C/s)	Holding time at 880°C or more in the annealing after the cold rolling (s)	Cooling rate in annealing after cold rolling (800-350°C) (°C/s)
  1	A1	70	8.2	120	8.1
  2	A2	65	5.3	150	8.5
  3	A3	60	5.2	150	5.1
  4	A4	55	20.3	130	5.5
  5	A5	50	2.3	130	8.5
  6	A6	45	18.1	50	5.1
  7	A7	40	15.4	80	5.5
  8	A8	75	3.7	180	11.2
  9	A1	5.3	2.8	120	8.5
  10	A1	5.3	3.2	120	5.1
  11	A1	75	2.1	5	7.3
  12	A8	50	2.2	17	9.2
  13	A8	50	3.1	20	9.3
  14	A8	50	10.1	20	9.1
  15	A8	50	1.2	20	9.8
  16	A8	42	3.1	150	5.2
  17	B1	60	3.3	80	8.5
  18	B2	55	3.2	120	5.5
  19	B3	50	2.8	120	5.1
  20	B4	50	2.3	150	8.1
  21	B5	45	2.3	120	8.5
  22	B6	40	3.8	150	8.1
  23	B7	40	5.2	80	5.5
  24	B8	40	10.3	120	7.6
  25	A1	5.3	0.8	5	3.1

• Underline indicates that the value fell out of the requirements of this embodiment.
• Double underline indicates that the value fell out of the preferable or more preferable production conditions of thie embodiment.
• On the resultant product sheet, the grain size number, the area fraction of Ti precipitates, and KAM and HAGB were measured. Furthermore, a thermal fatigue test, a condensate corrosion resistance test, and a tensile test were performed to evaluate the respective values of characteristics.
(Grain Size Number)
• The grain size number was measured according to the procedure described below. A steel sheet was embedded in thermosetting resin with a section in parallel to the rolling direction and the sheet-thickness direction (L-section) being as an observation surface, and was subjected to observation. An area in the range of 1/4 part of the sheet thickness from the center of the sheet thickness was defined as a measurement area, observations were made at five visual fields at 100 to 200× magnification, and a cutting method defined in JIS G 0551: 2020 Annex JB was used to determine the grain size number from total observed visual fields.
(Area Fraction of Ti Precipitates)
• The area fraction of Ti precipitates was measured according to the procedure described below. A steel sheet was embedded in thermosetting resin with a section in parallel to the rolling direction and the sheet-thickness direction (L-section) being as an observation surface, and subjected to mirror polishing by mechanical polishing followed by observation and analysis by using a SEM equipped with an EDS to make calculations. An area in the range of 1/4 part of the sheet thickness from the center of the sheet thickness was defined as a measurement area, observations were made at ten visual fields with 1000× magnification, and an average value of the areas of precipitates that contain Ti per visual field determined through image analysis was divided by the visual field area of one visual field to determine area fractions of the respective visual fields. An average value of the area fractions was defined as the area fraction of Ti precipitates. Here, precipitates that contain Ti were defined as those that contain Ti of 1.0 wt% or more and were 3 µm or more in the equivalent circle diameter.
(KAM and HAGB)
• KAM and HAGB were measured according to the procedure described below. With a section in parallel to the rolling direction and the sheet-thickness direction (L-section) being as an observation surface, chemical mechanical polishing was performed by using mechanical polishing, colloidal silica suspension, and the like to remove the surface strain. A mirrored observation surface was obtained by performing the polishing, followed by observation and measurement by using an FE-SEM equipped with an EBSD measuring instrument. OIM was used for analysis for determination. Here, "OIM Analysis" software was used.
• A 19 mm² area in the range of 1/4 part of the sheet thickness from the center of the sheet thickness was defined as a measurement area, and measurements were taken at ten visual fields with a measurement magnification of 1000× and a measurement distance of 0.5 µm. The KAM value refers to a ratio of measurement points that create 1 to 2 degrees in the average value of the crystal orientation differences described above relative to all measurement points, and HAGB is the ratio of presence of high-angle grain boundary, and was determined by dividing grain boundary lengths that create 15 degrees or more in the misorientation by grain boundary lengths that create 2 degrees or more in the misorientation.
(Thermal Fatigue Test)
• The thermal fatigue test was conducted according to the procedure described below. By using pipe-shaped specimens, each of which is 38.1 mm in the diameter and 1.5 mm in the test-portion sheet thickness, tests were performed by controlling an electro-hydraulic servo fatigue testing machine and a high-frequency induction heating device to continuously repeat heating and cooling such that the highest temperature of 850°C, the lowest temperature of 200°C, the holding time of 120 seconds at the highest temperature, and the restriction ratio of 30% were achieved. The number of cycles at which a generated crack penetrated the pipe was defined as the thermal fatigue life. Those of the specimens that showed 1300 cycles or more in the thermal fatigue life were denoted as A, those that showed 1100 cycles or more and less than 1300 cycles as B, those that showed 1000 cycles or more and less than 1100 cycles as C, and those that showed less than 1000 cycles as D.
(Tensile Test)
• The tensile test was conducted according to the procedure described below. The JIS13B test coupons were produced from the product sheet such that the longitudinal direction was parallel to the rolling direction, and tensile tests were performed. The total elongation (elongation after fracture) at a normal temperature was measured. Those of the specimens that showed 30% or more in the total elongation at the room temperature were determined as being good in formability and denoted as B. On the other hand, those that showed less than 30% in the total elongation were determined as being poor in formability and denoted as D.
(Condensate Corrosion Test)
• The condensate corrosion test was conducted according to the procedure described below. By using specimens cut from the product sheet, each of which had a size of 100 mm (parallel to the longitudinal direction) × 25 mm (parallel to the transverse direction) and was subjected to 600-grit wet-polish finishing over the entire surface, preheat treatment was performed in the atmosphere at 400°C for 8 hours, followed by condensate corrosion tests in compliant with JASO M611-92 A method (temperature: 80°C, time: 600 hours). The specimen after the condensate corrosion test was subjected to descaling, and thereafter, depth of focus measurement was used to determine the maximum corrosion depth of the specimen. The maximum corrosion depth was evaluated as an n=2 average value of the maximum corrosion depths for each specimen. To secure a condensate corrosion resistance acceptable for exhaust parts, those of the specimens that showed 800 µm or less in the maximum corrosion depth were denoted as B, and those that were deeper than 800 µm as D. Note that as a result of comprehensive evaluation on characteristics of exhaust parts in view of various characteristics, those of the specimens that showed good performance as exhaust parts were denoted as B, those that were inferior to those of B but relatively good were denoted as C, and those that were poor were denoted as D in the table. Hereinunder, the results are collectively shown in Table 3.
[Table 3]
• Table 3

  No.	Steel	Microstructure of product sheet			high-temperature resistance	Formability	Condensate corrosion resistance	Comprehensive evaluation
  		Grain size number	Area fraction of Ti precipitates (%)	Left side value of Formula (iii) (KAM)/(HAGB)	200-850°C thermal fatigue resistance	Room-temperature ductility	Condensate corrosion test	Automobile exhaust part characteristics
  							Maximum corrosion depth
  1	A1	7.8	0.7	0.002	A	B	B	B	Inventive example
  2	A2	7.4	0.6	0.003	A	B	B	B
  3	A3	6.8	0.5	0.003	A	B	B	B
  4	A4	7.2	0.5	0.002	A	B	B	B
  5	A5	7.3	0.5	0.005	A	B	B	B
  6	A6	6.7	0.6	0.002	A	B	B	B
  7	A7	6.6	0.6	0.002	A	B	B	B
  8	A8	7.7	0.4	0.004	A	B	B	B
  9	A1	6.3	7.4	0.004	C	B	B	C
  10	A1	6.3	12.2	0.003	C	B	B	C
  11	A1	7.8	0.6	0.120	C	B	B	C
  12	A8	8.9	0.4	0.011	A	B	B	B
  13	A8	8.5	0.4	0.010	A	B	B	B
  14	A8	8.3	0.4	0.006	A	B	B	B
  15	A8	7.1	1.7	0.072	C	B	B	C
  16	A8	6.5	2.6	0.004	B	B	B	B
  17	B1	7.8	0.4	0.005	D	B	B	D	Comparative example
  18	B2	6.3	0.5	0.002	D	B	D	D
  19	B3	6.4	0.5	0.002	D	B	B	D
  20	B4	6.2	0.5	0.002	D	B	D	D
  21	B5	6.3	0.6	0.002	D	B	B	D
  22	B6	6.3	0.5	0.002	D	B	D	D
  23	B7	7.2	0.6	0.004	D	B	B	D
  24	B8	7.6	0.5	0.003	B	D	B	D
  25	Al	9.2	7.4	0.220	D	D	D	D

• Underline indicates that the value fell out of the requirements of this embodiment.
• Double underline indicates that the value fell out of the preferable requirements of thie embodiment.
• The inventive examples were good in both the thermal fatigue resistance and the formability because the chemical composition satisfied the requirements of the embodiment and production conditions were within a preferable range. The maximum corrosion depth after the condensate corrosion test was also good. Among the inventive examples, No. 9 and No. 1 0 showed a degraded thermal fatigue resistance because the cooling rate after the hot rolling was slow, and therefore, the area fraction of Ti precipitates was high and the amount of Ti contributing to precipitation strengthening was small. As a result, they were slightly inferior for automobile exhaust part characteristics. In addition, No. 11 did not satisfy Formula (iii) because the holding time at 880°C or more was short during the annealing, and therefore, the thermal fatigue resistance degraded to some extent. As a result, it was slightly inferior for automobile exhaust part characteristics. No. 15 did not satisfy Formula (iii) because the heating rate was slow during the annealing, and therefore, the thermal fatigue resistance degraded to some extent.
• On the other hand, No. 17 showed a degraded thermal fatigue resistance because the content of Nb was low. No. 18 showed a degraded thermal fatigue resistance because the content of Cr was low and it was considered that the amount of precipitation of Cu contributing to precipitation strengthening was low. The condensate corrosion resistance also degraded. No. 19 showed a degraded thermal fatigue resistance because the content of Cu was low. No. 20 showed a degraded thermal fatigue resistance because the content of Mo was low. The condensate corrosion resistance also degraded. No. 21 showed a degraded thermal fatigue resistance because the content of Al was low.
• No. 22 showed a degraded thermal fatigue resistance because the content of V was low. The condensate corrosion resistance also degraded. No. 23 showed a degraded thermal fatigue resistance because it did not satisfy Formulas (i) and (ii) although the individual content of an element was within the range of the present invention. No. 24 showed degraded formability because it did not satisfy Formulas (i) and (ii) although the individual content of an element was within the range of the embodiment. No. 25 had steel constituents of A1, which was in No. 1. In No. 25, the cooling rate after the hot rolling was slow, the heating rate during annealing after the cold rolling was also low, and the holding time at 880°C or more was short. In addition, the cooling rate from 800 to 350°C was also low. Consequently, the area fraction of Ti precipitates was high, so that the amount of Ti contributing to the improvement of the thermal fatigue resistance as Ti precipitates was small, and moreover, strain remained in the grain due to its poorly recrystallized microstructure, leading to the degradation of the thermal fatigue resistance and formability. The condensate corrosion resistance also degraded.
(Appendix)
1. (1) A ferritic stainless steel sheet, which has a chemical composition consisting of, in mass%:
   • C: 0.001 to 0.030%,
   • Si: 0.01 to 1.00%,
   • Mn: 0.01 to 0.90%,
   • P: 0.010 to 0.100%,
   • S: 0.0001 to 0.0100%,
   • Cr: 16.0 to 20.0%,
   • Cu: 1.00 to 1.50%,
   • Mo: 0.02 to 0.50%,
   • Ti: 0.050 to 0.300%,
   • Nb: 0.050 to 0.200%,
   • Al: 0.003 to 0.500%,
   • Ni: 0.01 to 0.30%,
   • B: 0.0001 to 0.0050%,
   • V: 0.010 to 0.500%,
   • N: 0.001 to 0.020%,
   • Mg: 0 to 0.0010%,
   • Ca: 0 to 0.0050%,
   • W: 0 to 3.00%,
   • Co: 0 to 0.30%,
   • Zr: 0 to 0.10%,
   • REM: 0 to 0.10%,
   • Sn: 0 to 0.500%,
   • Sb: 0 to 0.50%,
   • Ga: 0 to 0.30%,
   • Ta: 0 to 1.00%,
   • Hf: 0 to 1.00%,
   • Bi: 0 to 0.02%, and
   • the balance: Fe and impurities,
   • satisfies following Formulas (i) and (ii), and
   • has a grain size number of 6 to 9: $$830\le 250\mathrm{Cu}+2216\mathrm{Nb}+3083\mathrm{V}\le 2000$$ $$900\le 1273\mathrm{Nb}+1343\mathrm{Mo}+7257\mathrm{Al}\le 3800$$
   • where each element symbol in the formulas represents a content (in mass%) of each element contained in the ferritic stainless steel sheet and is zero when the element is not contained.
2. (2) The ferritic stainless steel sheet according to the above (1), wherein the grain size number is 6 to 8.
3. (3) The ferritic stainless steel sheet according to the above (1) or (2), wherein the chemical composition contains one or more elements, in mass%, selected from:
   • Mg: 0.0001 to 0.0010%,
   • Ca: 0.0001 to 0.0050%,
   • W: 0.02 to 3.00%,
   • Co: 0.001 to 0.30%,
   • Zr: 0.01 to 0.10%,
   • REM: 0.005 to 0.10%,
   • Sn: 0.005 to 0.500%,
   • Sb: 0.005 to 0.50%,
   • Ga: 0.0002 to 0.30%,
   • Ta: 0.001 to 1.00%,
   • Hf: 0.001 to 1.00%, and
   • Bi: 0.001 to 0.02%.
4. (4) The ferritic stainless steel sheet according to any one of the above (1) to (3), wherein an area fraction of Ti precipitates is 5% or less.
5. (5) The ferritic stainless steel sheet according to any one of the above (1) to (4), wherein following Formula (iii) is satisfied: $$\mathrm{KAM}/\mathrm{HAGB}\le 0.05$$ where KAM in the formula represents a KAM value, and HAGB represents a ratio of a grain boundary length with a misorientation of 15° or more to a total grain boundary length.
6. (6) An exhaust part, wherein the ferritic stainless steel sheet according to any one of the above (1) to (5) is used.
INDUSTRIAL APPLICABILITY
• According to the disclosure, it is possible to provide a ferritic stainless steel sheet that is excellent in the thermal fatigue resistance from 200°C to 850°C, while achieving the alloy saving. The ferritic stainless steel sheet makes it possible to provide parts that can address exhaust gas environmental regulations, weight reduction, resource saving, and improvement of fuel efficiency of automobiles. Furthermore, without being limited to exhaust parts for automobiles and two-wheelers, the steel can be applied to parts of various boilers, fuel cell systems, heat exchangers, and the like for use in a high-temperature environment and also in an environment in which the parts are subjected to condensate corrosion or salt corrosion due to dew droplets.

Claims (9)

1. A ferritic stainless steel sheet, which has a chemical composition consisting of, in mass%:

   C: 0.001 to 0.030%,

   Si: 0.01 to 1.00%,

   Mn: 0.01 to 0.90%,

   P: 0.010 to 0.100%,

   S: 0.0001 to 0.0100%,

   Cr: 16.0 to 20.0%,

   Cu: 1.00 to 1.50%,

   Mo: 0.02 to 0.50%,

   Ti: 0.050 to 0.300%,

   Nb: 0.050 to 0.200%,

   Al: 0.003 to 0.500%,

   Ni: 0.01 to 0.30%,

   B: 0.0001 to 0.0050%,

   V: 0.010 to 0.500%,

   N: 0.001 to 0.020%,

   Mg: 0 to 0.0010%,

   Ca: 0 to 0.0050%,

   W: 0 to 3.00%,

   Co: 0 to 0.30%,

   Zr: 0 to 0.10%,

   REM: 0 to 0.10%,

   Sn: 0 to 0.500%,

   Sb: 0 to 0.50%,

   Ga: 0 to 0.30%,

   Ta: 0 to 1.00%,

   Hf: 0 to 1.00%,

   Bi: 0 to 0.02%, and

   the balance: Fe and impurities,

   satisfies following Formulas (i) and (ii), and

   has a grain size number of 6 to 9: $$830\le 250\mathrm{Cu}+2216\mathrm{Nb}+3083\mathrm{V}\le 2000$$ $$900\le 1273\mathrm{Nb}+1343\mathrm{Mo}+7257\mathrm{Al}\le 3800$$

   where each element symbol in the formulas represents a content (in mass%) of each element contained in the ferritic stainless steel sheet and is zero when the element is not contained.
2. The ferritic stainless steel sheet according to claim 1, wherein the grain size number is 6 to 8.
3. The ferritic stainless steel sheet according to claim 1, wherein the chemical composition contains one or more elements, in mass%, selected from:

   Mg: 0.0001 to 0.0010%,

   Ca: 0.0001 to 0.0050%,

   W: 0.02 to 3.00%,

   Co: 0.001 to 0.30%,

   Zr: 0.01 to 0.10%,

   REM: 0.005 to 0.10%,

   Sn: 0.005 to 0.500%,

   Sb: 0.005 to 0.50%,

   Ga: 0.0002 to 0.30%,

   Ta: 0.001 to 1.00%,

   Hf: 0.001 to 1.00%, and

   Bi: 0.001 to 0.02%.
4. The ferritic stainless steel sheet according to claim 1, wherein an area fraction of Ti precipitates is 5% or less.
5. The ferritic stainless steel sheet according to claim 2, wherein an area fraction of Ti precipitates is 5% or less.
6. The ferritic stainless steel sheet according to claim 1, wherein following Formula (iii) is satisfied: $$\mathrm{KAM}/\mathrm{HAGB}\le 0.05$$ where KAM in the formula represents a KAM value, and HAGB represents a ratio of a grain boundary length with a misorientation of 15° or more to a total grain boundary length.
7. The ferritic stainless steel sheet according to claim 2, wherein following Formula (iii) is satisfied: $$\mathrm{KAM}/\mathrm{HAGB}\le 0.05$$ where KAM in the formula represents a KAM value, and HAGB represents a ratio of a grain boundary length with a misorientation of 15° or more to a total grain boundary length.
8. The ferritic stainless steel sheet according to claim 4, wherein following Formula (iii) is satisfied: $$\mathrm{KAM}/\mathrm{HAGB}\le 0.05$$ where KAM in the formula represents a KAM value, and HAGB represents a ratio of a grain boundary length with a misorientation of 15° or more to a total grain boundary length.
9. An exhaust part, wherein the ferritic stainless steel sheet according to any one of claims 1 to 8 is used.