JP2023005308A - Ferritic stainless steel sheet and method for producing the same - Google Patents

Abstract

To provide a ferritic stainless steel sheet that achieves excellent oxidation resistance and thermal fatigue resistance and, at the same time, achieves excellent thick deep drawability.SOLUTION: A ferritic stainless steel sheet has a predetermined composition, particularly the composition containing proper amounts of Nb, Mo, Al and Co concurrently in addition to Cr, in which the area ratio of {111}-oriented grains is 20% or more.SELECTED DRAWING: None

Description

TECHNICAL FIELD The present invention relates to a ferritic stainless steel sheet and a method for producing the same.

Exhaust system members such as exhaust manifolds, exhaust pipes, converter cases and mufflers of automobiles (hereinafter also referred to as exhaust system members of automobiles) are required to have excellent thermal fatigue resistance in addition to excellent oxidation resistance. Here, thermal fatigue refers to a low-cycle fatigue phenomenon caused by thermal strain occurring in the material itself. That is, automobile exhaust system members are repeatedly heated and cooled as the engine is started and stopped. Exhaust system members of automobiles are constrained with surrounding parts. Therefore, in exhaust system members for automobiles, thermal expansion and contraction are limited during the repeated heating and cooling described above. As a result, thermal strain occurs in exhaust system members of automobiles, and a low-cycle fatigue phenomenon caused by this thermal strain occurs.

Cr-containing steel such as Type 429 (14 mass% Cr-0.9 mass mass% Si-0.4 mass% Nb system) to which Nb and Si are added in combination is often used as a material for such automobile exhaust system members. It is

In addition, SUS444 (19 mass% Cr-0.4 mass% Nb-2 mass% Mo) specified in JIS G4305, Cr-containing steel with improved high-temperature yield strength by combined addition of Nb and Mo, and Nb and Mo A ferritic stainless steel (hereinafter also referred to as SUS444 or the like) to which W is added in combination has also been developed.

However, the exhaust gas temperature exceeds 900° C. and there is a tendency to further increase in order to comply with the recent tightening of exhaust gas regulations of automobiles and to improve fuel efficiency. Therefore, there are cases where sufficient thermal fatigue resistance is not obtained even with the above-described SUS444 and the like, and the development of a material that provides better thermal fatigue resistance than the above-described SUS444 and the like is desired.

As such a material, for example, in Patent Document 1,
"In mass%, C: 0.020% or less, Si: more than 0.1 to 3.0%, Mn: 2.0% or less, P: 0.050% or less, S: 0.010% or less, Al : 0.05-6.0%, N: 0.020% or less, Cr: 12-30%, Cu: 0.4-4.0%, Nb: 0.02-1.0%, Ti: 0 .01 to 1.0%, Mo: 0.1 to 6.0%, Co: 0.01 to 3.0%, Ni: 0.02 to 1.0%, and satisfies Si + Al ≥ 0.50 ferritic stainless steel, characterized in that it contains as much as possible and the balance consists of Fe and unavoidable impurities.
is disclosed.

In Patent Document 2,
"In mass%, C: 0.020% or less, Si: 3.0% or less, Mn: 0.2% or less, P: 0.040% or less, S: 0.030% or less, Al: 0.01% ~3.0%, Cr: 10.0 to 30.0%, Ni: 0.01 to 0.9%, N: 0.020% or less, Nb: over 0.2% to 1.0%, Co : 0.3 to 10%, B: 0.0001 to 0.0050%, and the balance being Fe and inevitable impurities, a ferritic stainless steel having excellent thermal fatigue properties.
is disclosed.

In Patent Document 3,
"In mass%, C: 0.020% or less, Si: 0.05 to 2.00%, Mn: 0.05 to 2.00%, P: 0.050% or less, S: 0.010% or less , Al: 1.0 to 4.0%, N: 0.020% or less, Cr: 12.0 to 25.0%, Nb: 0.40 to 0.80%, Ti: less than 0.01%, A ferrite system containing Mo: 1.0 to 4.0%, Co: 0.01 to 1.00%, Ni: 0.01 to 1.00%, and the balance being Fe and unavoidable impurities. stainless steel."
is disclosed.

In Patent Document 4,
"In mass%, C: 0.020% or less, Si: 3.0% or less, Mn: 1.0% or less, P: 0.040% or less, S: 0.030% or less, Cr: 10.0% % or more and less than 16.0%, N: 0.020% or less, Al: 1.4 to 4.0%, Ti: more than 0.15% to 0.5%, Ni: 0.05 to 0.5% A ferritic stainless steel containing and the balance being Fe and unavoidable impurities, and satisfying the following formula (1).
Al%/Cr%≧0.14 (1)
In addition, Al % and Cr % in the formula represent the content (% by mass) of Al and Cr, respectively. "
is disclosed.

In Patent Document 5,
"In mass%, C: less than 0.010%, N: 0.020% or less, Si: more than 0.1% to 2.0% or less, Mn: 2.0% or less, Cr: 12.0 to 25.0%, Cu: more than 0.9% to 2.0%, Ti: 0.05 to 0.3%, Nb: 0.001 to 0.1%, Al: 1% or less, B: 0. 0003 to 0.003% or less, Cu/(Ti+Nb) is 5 or more, and the balance is Fe and unavoidable impurities, and has excellent heat resistance and workability.
is disclosed.

In Patent Document 6,
"In mass%, C: 0.020% or less, Si: more than 0.1% and 3.0% or less, Mn: 0.05 to 2.0%, P: 0.050% or less, S: 0.010 % or less, Al: 0.3-6.0%, N: 0.020% or less, Cr: 12-30%, Nb: more than 0.3% and 1.0% or less, Ti: 0.01-0. 5%, Mo: 0.3 to 6.0%, Co: 0.01 to 3.0%, Ni: 0.02 to 1.0%, and the following formulas (1) to (3 ) with the balance being Fe and unavoidable impurities.
Si+Al>1.0% (1)
Al—Mn>0% (2)
Nb-Ti>0% (3)
(Si, Al, Mn, Nb and Ti in formulas (1) to (3) indicate the content (% by mass) of each element.)”
is disclosed.

JP 2015-96648 A JP 2014-214321 A WO2020/080104 WO2014/050016 JP 2011-202257 A Patent No. 6123964

By the way, automobile exhaust system members are sometimes processed into complicated shapes. Therefore, in addition to excellent oxidation resistance and thermal fatigue resistance, materials for automobile exhaust system members are also required to have excellent workability at room temperature, particularly excellent deep drawability.

In addition, from the viewpoint of ensuring durability as the exhaust gas temperature rises, materials with thicker plates than conventional (plate thickness: about 1.2 mm to 2.0 mm) are used as materials for automobile exhaust system parts. Practically, there are many cases where a material having a plate thickness of about 2.0 to 2.5 mm is used. In other words, materials for automotive exhaust system members are required to be excellent in deep drawability (hereinafter also referred to as thick-wall deep drawability) even when the sheet thickness is thicker than in the past.

However, it cannot be said that none of the materials disclosed in Patent Documents 1 to 6 simultaneously satisfies excellent oxidation resistance, excellent thermal fatigue resistance, and excellent thick-wall deep drawability. In particular, no consideration is given to thick-wall deep drawability. Therefore, the current situation is that the development of a material that satisfies these properties at the same time is desired.

The present invention was developed in view of the above-mentioned current situation, and it is an object of the present invention to provide a ferritic stainless steel sheet that achieves excellent thick-wall deep drawability as well as excellent oxidation resistance and thermal fatigue resistance. do.
Another object of the present invention is to provide a method for producing the above ferritic stainless steel sheet.

In addition, "excellent oxidation resistance" means having both excellent continuous oxidation resistance and excellent repeated oxidation resistance.
Here, "excellent continuous oxidation resistance" means that when the test piece is held at 1100 ° C. in an air atmosphere for 200 hours, the test piece does not have abnormal oxidation such that the oxidation weight gain is ≥ 50 g / m ² and , which means that there is no delamination of scale.
Further, the weight gain due to oxidation is calculated by the following formula.
[Oxidation weight gain (g/m ² )] = [weight increase of test piece before and after holding (g)] ÷ [surface area of test piece (m ² )]
In addition, "excellent repeated oxidation resistance" means that when the cycle composed of the following (a) to (d) is repeated 400 times in an air atmosphere, the test piece has an oxidation weight gain of ≥ 50 g / m ² This means that there is no abnormal oxidation and no peeling of scale.
(a) Hold at 200°C for 1 minute (b) Heat up from 200°C to 1100°C (average heating rate 30°C/min)
(c) Hold at 1100°C for 20 minutes (d) Decrease temperature from 1100°C to 200°C (average temperature decrease rate 90°C/min)
The oxidation weight gain is calculated by the following formula.
[Oxidation weight gain (g/m ² )] = [mass increase of test piece before and after cycle repetition (g)] ÷ [surface area of test piece (m ² )]

The term "excellent thermal fatigue resistance" means that the steel exhibits properties superior to those of conventional steel SUS444. More specifically, "excellent thermal fatigue resistance" means that the test piece has a thermal fatigue life of 800 cycles or more when the cycles configured by the following (a) to (d) are repeated. means
(a) Hold at 200°C for 1 minute (b) Heat up from 200°C to 950°C (average heating rate: 7°C/sec)
(C) Hold at 950°C for 2 minutes (D) Decrease temperature from 950°C to 200°C (average temperature decrease rate: 7°C/sec)

“Excellent thick-wall deep drawability” means that the sheet thickness is 2.0 mm or more, the r value is 1.2 or more (preferably 1.4 or more), and the critical drawing ratio is 2.06 or more. means that Note that the r value is determined by a room temperature tensile test. Also, the critical drawing ratio is determined by a deep drawing test (LDR test).

The inventors have made extensive studies in order to achieve the above object.
as a result,
(1) to properly control the component composition, in particular, to contain Nb, Mo, Al and Co in appropriate amounts in addition to Cr, and
(2) The area ratio of {111} oriented grains is 20% or more,
As a result, the inventors have found that excellent thick-wall deep drawability can be realized at the same time as excellent oxidation resistance and thermal fatigue resistance.

That is, by containing 0.30 to 1.00% by mass of Nb and 1.0 to 6.0% by mass of Mo, the high-temperature strength is increased in a wide temperature range and the thermal fatigue property is improved. Both oxidation resistance and creep resistance affect thermal fatigue properties. The content of Al is effective for improving creep resistance in a high temperature range. In particular, by setting the Al content in the range of 1.00 to 6.00% by mass, creep resistance is improved and thermal fatigue characteristics are improved. also greatly improved. In addition, precipitation of the second phase (σ phase) is also suppressed. Furthermore, by containing an appropriate amount of Co, an increase in the coefficient of thermal expansion is suppressed.

In order to improve the deep drawability, it is effective to increase the draft of cold rolling in the manufacturing process, specifically to 60% or more. However, if the thickness of the material to be rolled having the above composition is increased in order to perform cold rolling with a large rolling reduction, the risk of breakage during production increases and the load in cold rolling increases.

Based on this point, when the inventors further studied,
・In the manufacturing process of ferritic stainless steel sheets containing 0.30% or more of Nb, instead of hot-rolled sheet annealing (annealing temperature: 900 ° C. or higher), which is generally performed for the purpose of recrystallization, as a pre-process of cold rolling. Therefore, it is important to perform heat treatment under the conditions of heat treatment temperature: 700 to 850 ° C. and heat treatment time: 60 minutes or more,
・If this heat treatment precipitates an intermetallic work called Laves phase, which has a composition of Fe ₂ Nb, excellent oxidation resistance and thermal fatigue resistance can be secured simultaneously without increasing the reduction ratio of cold rolling. while achieving excellent thick-wall deep drawability,
I found out.

Also, the inventors
In steel sheets manufactured through the above steps, the proportion of {111} oriented grains is increased. It is possible to realize excellent thick-wall deep drawability while ensuring thermal fatigue resistance at the same time.
I also found out.
The present invention has been completed based on the above findings and further studies.

That is, the gist and configuration of the present invention are as follows.
[1] in % by mass,
C: 0.020% or less,
Si: 0.05 to 1.50%,
Mn: 0.05-1.30%,
P: 0.050% or less,
S: 0.010% or less,
Al: 1.00 to 6.00%,
N: 0.020% or less,
Cr: 12.0 to 25.0%,
Nb: 0.30 to 1.00%,
Mo: 1.0 to 6.0%,
Co: 0.01-3.00%, and Ni: 0.01-1.00%
and has a component composition in which the balance is Fe and unavoidable impurities,
A ferritic stainless steel sheet having an area ratio of {111} oriented grains of 20% or more.

[2] The component composition further contains, in % by mass,
Ti: 0.50% or less,
Zr: 0.50% or less,
V: 0.50% or less,
The ferritic stainless steel sheet according to [1] above, containing one or more selected from W: 5.00% or less and Cu: 3.00% or less.

[3] The component composition further contains, in % by mass,
B: 0.0050% or less,
Sn: 0.50% or less,
Sb: 0.50% or less,
The ferritic stainless steel sheet according to [1] or [2] above, containing one or more selected from Ca: 0.0050% or less and Mg: 0.0050% or less.

[4] The ferrite according to any one of [1] to [3], which has a plate thickness of 2.0 mm or more, an r value of 1.2 or more, and a critical drawing ratio of 2.06 or more. system stainless steel plate.

[5] A step of preparing a hot-rolled steel sheet having the composition according to any one of [1] to [3];
A step of subjecting the hot-rolled steel sheet to a heat treatment under conditions of a heat treatment temperature of 700 to 850° C. and a heat treatment time of 60 minutes or more;
a step of cold-rolling the hot-rolled steel sheet under conditions of a total rolling reduction of 40% or more and less than 60% to form a cold-rolled steel sheet;
A method for manufacturing a ferritic stainless steel sheet.

According to the present invention, it is possible to obtain a ferritic stainless steel sheet that achieves excellent thick-wall deep drawability as well as excellent oxidation resistance and thermal fatigue resistance.
Therefore, the ferritic stainless steel sheet of the present invention can be suitably used for exhaust system members of automobiles having various complicated shapes even when the temperature of the exhaust gas is high, and the industrial utility value of the steel sheet is extremely high.

It is a schematic diagram of the test piece used for a thermal fatigue test. It is a figure explaining temperature conditions (temperature control) and restraint conditions (strain control) in a thermal fatigue test.

The present invention will be described based on the following embodiments.
First, the chemical composition of the ferritic stainless steel sheet according to one embodiment of the present invention will be described. Incidentally, although the units in all component compositions are "% by mass", they are indicated simply as "%" unless otherwise specified.

C: 0.020% or less C is an effective element for increasing the strength of steel. However, when the C content exceeds 0.020%, the toughness and formability deteriorate. Therefore, the C content should be 0.020% or less. From the viewpoint of ensuring moldability, the C content is preferably 0.010% or less. The C content is more preferably 0.008% or less. Also, the C content is preferably 0.001% or more from the viewpoint of ensuring strength as an exhaust system member for automobiles. The C content is more preferably 0.003% or more.

Si: 0.05-1.50%
Si is an important element necessary for improving oxidation resistance. The Si content is set to 0.05% or more in order to ensure oxidation resistance in high-temperature exhaust gas. The Si content is preferably 0.10% or more. On the other hand, when the Si content exceeds 1.50%, the workability at room temperature deteriorates. Therefore, the Si content should be 1.50% or less. The Si content is preferably 1.00% or less, more preferably 0.50% or less.

Mn: 0.05-1.30%
Mn has the effect of increasing the peeling resistance of oxide scale. In order to obtain such effects, the Mn content is set to 0.05% or more. The Mn content is preferably 0.10% or more, more preferably 0.20% or more. On the other hand, when the Mn content exceeds 1.30%, the γ phase tends to form at high temperatures, resulting in a decrease in heat resistance. Therefore, the Mn content is set to 1.30% or less. The Mn content is preferably 1.00% or less, more preferably 0.50% or less.

P: 0.050% or less P is a harmful element that lowers the toughness of steel. Therefore, it is desirable to reduce P as much as possible. Therefore, the P content should be 0.050% or less. The P content is preferably 0.030% or less. In addition, the lower limit of the P content is not particularly limited. However, since excessive dephosphorization causes an increase in cost, the P content is preferably 0.005% or more.

S: 0.010% or less S lowers elongation and r-value, and adversely affects formability. S is also a harmful element that reduces corrosion resistance, which is a basic characteristic of stainless steel. Therefore, it is desirable to reduce S as much as possible. Therefore, the S content should be 0.010% or less. The S content is preferably 0.005% or less. In addition, the lower limit of the S content is not particularly limited. However, since excessive desulfurization causes an increase in cost, the S content is preferably 0.0005% or more.

Al: 1.00-6.00%
Al is an element necessary for suppressing high-temperature deformation (creep) and improving thermal fatigue resistance. In general, the higher the service temperature, the more likely the thermal fatigue resistance tends to decrease due to high-temperature deformation. Therefore, Al is an important element in the trend toward higher exhaust gas temperatures. Al also has the effect of improving the oxidation resistance of steel. Furthermore, when Mo is included in the component composition, Al also has the effect of suppressing precipitation of a second phase (σ phase) containing Mo when subjected to thermal fatigue cycles. When such a second phase precipitates, the amount of solid-solution Mo decreases, and the solid-solution strengthening effect of Mo cannot be obtained. In addition, the second phase may coarsen in a short time and become a starting point of crack generation. In order to obtain such effects, the Al content is set to 1.00% or more. The Al content is preferably above 1.50%, more preferably above 2.00%. On the other hand, Al also has the drawback of increasing the coefficient of thermal expansion. In the chemical composition of the ferritic stainless steel sheet according to one embodiment of the present invention, Co is contained to lower the coefficient of thermal expansion and compensate for the above drawbacks. However, if the Al content exceeds 6.00%, even if Co is contained, the increase in the coefficient of thermal expansion cannot be suppressed, and the thermal fatigue resistance deteriorates. In addition, the steel hardens and workability decreases. Therefore, the Al content is set to 6.00% or less. The Al content is preferably 5.00% or less, more preferably 4.00% or less.

N: 0.020% or less N is an element that lowers the toughness and formability of steel. In particular, when the N content exceeds 0.020%, toughness and formability decrease significantly. Therefore, the N content is set to 0.020% or less. Also, from the viewpoint of ensuring toughness and formability, N is preferably reduced as much as possible, and the N content is preferably less than 0.010%.

Cr: 12.0-25.0%
Cr is an element necessary to ensure corrosion resistance, which is a basic characteristic of stainless steel. Cr is also an effective element for improving oxidation resistance. Here, if the Cr content is less than 12.0%, sufficient oxidation resistance cannot be obtained. Therefore, the Cr content is set to 12.0% or more. The Cr content is preferably 14.0% or more, more preferably 18.0% or more. On the other hand, Cr is also an element that solid-solution strengthens steel at room temperature, making it hard and ductile. In particular, when the Cr content exceeds 25.0%, the above point becomes remarkable. Therefore, the Cr content is set to 25.0% or less. The Cr content is preferably 22.0% or less, more preferably 20.0% or less.

Nb: 0.30-1.00%
Nb is an element that forms carbonitride with C and N and fixes C and N. Also, Nb is an element that has the effect of enhancing corrosion resistance, formability, and intergranular corrosion resistance of welded portions. Furthermore, Nb is an element that increases the high-temperature strength and improves the thermal fatigue properties. In addition, as will be described later, Nb is also an element that disperses and precipitates a Laves phase (Fe ₂ Nb), which is an intermetallic compound, in the hot-rolled steel sheet during heat treatment during production. As a result, the ratio of {111} oriented grains in the final product is increased, and the thick wall deep drawability is improved. Such an effect can be obtained by setting the Nb content to 0.30% or more. On the other hand, when the Nb content is less than 0.30%, the strength at high temperatures is insufficient, and excellent thermal fatigue resistance cannot be obtained. In addition, the effect of improving the thick-wall deep drawability due to the precipitation of the Laves phase cannot be sufficiently obtained. Therefore, the Nb content is set to 0.30% or more. The Nb content is preferably 0.40% or more, more preferably 0.43% or more. However, if the Nb content exceeds 1.00%, the Laves phase precipitates coarsely, which causes embrittlement of the steel. Therefore, the Nb content is set to 1.00% or less. The Nb content is preferably 0.80% or less, more preferably 0.60% or less.

Mo: 1.0-6.0%
Mo is an element that dissolves in steel to improve the high-temperature strength of the steel, thereby improving the thermal fatigue resistance. Such an effect is obtained by setting the Mo content to 1.0% or more. On the other hand, when the Mo content is less than 1.0%, the high-temperature strength becomes insufficient, and excellent thermal fatigue resistance cannot be obtained. Therefore, Mo content shall be 1.0% or more. Mo content is preferably 1.5% or more, more preferably 1.8% or more, and still more preferably 2.0% or more. On the other hand, if Mo is contained excessively, the steel becomes hard and workability is lowered. In addition, coarse intermetallic compounds such as the σ phase are likely to be formed. Therefore, the thermal fatigue resistance is rather deteriorated. Therefore, Mo content shall be 6.0% or less. The Mo content is preferably 5.0% or less, more preferably 4.0% or less, still more preferably 3.0% or less.

Co: 0.01-3.00%
Co is an element effective in improving the toughness of steel. Co is also an element that suppresses an increase in thermal expansion coefficient caused by containing a large amount of Al and improves thermal fatigue resistance. In order to obtain such effects, the Co content is set to 0.01% or more. On the other hand, if Co is contained excessively, the toughness of the steel rather decreases. Therefore, the Co content is set to 3.00% or less. The Co content is preferably less than 0.30%, more preferably less than 0.05%.

Ni: 0.01-1.00%
Ni is an element that improves the toughness of steel. In order to obtain such effects, the Ni content is set to 0.01% or more. The Ni content is preferably 0.05% or more, more preferably 0.10% or more. However, Ni is a strong γ-phase forming element. Therefore, Ni forms a γ-phase at high temperatures and lowers the oxidation resistance. Therefore, the Ni content is set to 1.00% or less. The Ni content is preferably less than 0.80%, more preferably less than 0.50%.

The basic chemical composition of the ferritic stainless steel sheet according to one embodiment of the present invention has been described above. In the chemical composition of the ferritic stainless steel sheet according to one embodiment of the present invention, at least one of the following optional additive elements is added. can be included alone or in combination.
Ti: 0.50% or less Zr: 0.50% or less V: 0.50% or less W: 5.00% or less Cu: 3.00% or less

Ti: 0.50% or less Ti, like Nb, is an element that forms carbonitrides with C and N to fix C and N. In addition, Ti is an element that has the effect of enhancing corrosion resistance, formability, and intergranular corrosion resistance of welded portions. In particular, by including Ti, Ti preferentially bonds with C and N over Nb. Therefore, it becomes possible to ensure the amount of dissolved Nb in the steel that is effective for high-temperature strength, and the thermal fatigue resistance is further improved. Ti also effectively contributes to the improvement of oxidation resistance. In order to obtain such effects, the Ti content is preferably 0.01% or more. The Ti content is preferably 0.05% or more, more preferably 0.10% or more. On the other hand, when the Ti content exceeds 0.50%, the effect of improving oxidation resistance is saturated. In addition, the toughness is lowered, and, for example, repeated bending-unbending in the heat treatment line during production causes fracture, which adversely affects manufacturability. Therefore, when Ti is contained, the Ti content shall be 0.50% or less. The Ti content is preferably 0.40% or less, more preferably 0.30% or less.

Zr: 0.50% or less Zr is an element that improves oxidation resistance. In order to obtain such effects, the Zr content is preferably 0.01% or more. However, when the Zr content exceeds 0.50%, Zr intermetallic compounds precipitate and embrittle the steel. Therefore, when Zr is contained, the Zr content should be 0.50% or less.

V: 0.50% or less V is an element effective in improving the workability of steel. In addition, V is an element effective in improving oxidation resistance. In order to obtain such effects, the V content is preferably 0.01% or more. The V content is preferably 0.03% or more, more preferably 0.05% or more. However, when the V content exceeds 0.50%, coarse precipitation of V (C, N) is caused and the toughness is lowered. In addition, the surface properties also deteriorate. Therefore, when V is contained, the V content should be 0.50% or less. The V content is preferably 0.30% or less, more preferably 0.20% or less.

W: 5.00% or less W, like Mo, is an element that improves high-temperature strength by solid-solution strengthening. In order to obtain such effects, the W content is preferably 0.01% or more. The W content is more preferably 0.30% or more, still more preferably 1.00% or more. On the other hand, an excessive W content hardens the steel. In addition, since strong scale is formed in the annealing process during production, descaling during pickling becomes difficult. Therefore, when W is contained, the W content is set to 5.00% or less. The W content is preferably 4.00% or less, more preferably 3.00% or less.

Cu: 3.00% or less Cu is an element that has the effect of improving the corrosion resistance of steel. Cu is also an element that has the effect of improving the high-temperature strength in the vicinity of 600°C. In order to obtain such effects, the Cu content is preferably 0.01% or more. The Cu content is more preferably 0.30% or more, still more preferably 1.00% or more. On the other hand, when the Cu content exceeds 3.00%, the oxide scale tends to peel off, and the repeated oxidation resistance deteriorates. Therefore, when Cu is contained, the Cu content is set to 3.00% or less. The Cu content is preferably 2.00% or less, more preferably 1.50% or less.

In addition, in the chemical composition of the ferritic stainless steel sheet according to one embodiment of the present invention, one or more of the following optional additive elements can be contained singly or in combination.
B: 0.0050% or less Sn: 0.50% or less Sb: 0.50% or less Ca: 0.0050% or less Mg: 0.0050% or less

B: 0.0050% or less B is an element effective in improving the workability of steel, particularly resistance to secondary work embrittlement. In order to obtain such effects, the B content is preferably 0.0002% or more. The B content is more preferably 0.0005% or more, still more preferably 0.0008% or more. On the other hand, when B is contained excessively, BN is generated and workability is lowered. Therefore, when B is contained, the B content is set to 0.0050% or less. The B content is preferably 0.0030% or less, more preferably 0.0020% or less.

Sb: 0.50% or less Sb is an element that has the effect of improving the toughness of steel. In particular, in the chemical composition of the ferritic stainless steel sheet according to one embodiment of the present invention, the toughness of the steel tends to decrease due to the inclusion of various alloying elements, and cracks may occur during processing into parts. . Therefore, it is effective to contain Sb as needed. In order to obtain such effects, the Sb content is preferably 0.01% or more. The Sb content is preferably 0.03% or more. On the other hand, when Sb is contained excessively, the toughness is lowered. Therefore, when Sb is contained, the Sb content is set to 0.50% or less. The Sb content is preferably 0.30% or less, more preferably 0.10% or less.

Sn: 0.50% or less Sn is an element that has the effect of improving the corrosion resistance and high-temperature strength of steel. In order to obtain such effects, the Sn content is preferably 0.01% or more. The Sn content is preferably 0.03% or more. On the other hand, when Sn is excessively contained, the workability of the steel deteriorates. Therefore, when Sn is contained, the Sn content is set to 0.50% or less. The Sn content is preferably 0.30% or less, more preferably 0.20% or less.

Ca: 0.0050% or less Ca is an element effective in preventing clogging of nozzles due to precipitation of Ti-based inclusions that tend to occur during continuous casting. In order to obtain such effects, the Ca content is preferably 0.0002% or more. Ca content is more preferably 0.0005% or more. On the other hand, in order to obtain good surface properties without generating surface defects, the Ca content is preferably 0.0050% or less. Therefore, when Ca is contained, the Ca content is made 0.0050% or less. The Ca content is preferably 0.0030% or less, more preferably 0.0020% or less.

Mg: 0.0050% or less Mg is an element that improves the equiaxed grain ratio of the slab and improves workability and toughness. Moreover, in a steel having a chemical composition containing Nb and Ti, Mg also has the effect of suppressing coarsening of carbonitrides of Nb and Ti. Coarse Ti carbo-nitrides may cause brittle cracking and lower toughness. In addition, when the Nb carbonitrides are coarsened, the amount of dissolved Nb in the steel decreases, which may lead to deterioration in thermal fatigue resistance. In order to obtain such effects, the Mg content is preferably 0.0002% or more. The Mg content is more preferably 0.0004% or more. On the other hand, if the Mg content exceeds 0.0050%, the surface properties of the steel deteriorate. Therefore, when Mg is contained, the Mg content shall be 0.0050% or less. The Mg content is preferably 0.0030% or less, more preferably 0.0020% or less.

The balance other than the above elements is Fe and unavoidable impurities. If the content of the optional additive element is less than the preferred lower limit, the optional additive element is included as an unavoidable impurity.

Next, the steel structure of the ferritic stainless steel sheet according to one embodiment of the present invention will be explained.
The steel structure of the ferritic stainless steel sheet according to one embodiment of the present invention has a ferrite phase with an area ratio of 97% or more. The ferrite phase may have an area ratio of 100%, that is, a single ferrite phase.
The area ratio of the residual structure other than the ferrite phase is 3% or less, and examples of such a residual structure include the martensite phase.

Here, the area ratio of the ferrite phase is determined as follows.
A test piece for cross-sectional observation is prepared from a stainless steel plate as a test material, and etched with a picric acid saturated hydrochloric acid solution. Then, 10 fields of view of the test piece are observed with an optical microscope at a magnification of 100 times, and the martensite phase and ferrite phase are distinguished from the structural shape. Next, the area ratio of the ferrite phase is obtained by obtaining the area ratio of the ferrite phase by image processing and calculating the average value thereof.
Also, the area ratio of the residual structure is obtained by subtracting the area ratio of the ferrite phase from 100%.

In addition, in the ferritic stainless steel sheet according to one embodiment of the present invention, it is essential that the area ratio of the {111} oriented grains is 20% or more in addition to the substantially ferrite single-phase structure as described above. is.

{111} oriented grain area ratio: 20% or more Among them, it is effective to increase the proportion of {111} oriented grains. Therefore, the area ratio of {111} oriented grains is set to 20% or more. The area ratio of {111} oriented grains is preferably 30% or more.

Here, the area ratio of {111} oriented grains is measured as follows.
That is, a test piece is taken from a stainless steel plate as a test material so that a cross section parallel to the rolling direction serves as an observation surface. Then, for the test piece, the crystal orientation of the observed crystal grains is measured by the EBSD (electron beam backscatter diffraction) method using a scanning electron microscope (SEM), and the crystal grains having {111} orientation ({ 111} oriented grains). Then, the area ratio of the {111} oriented grains is obtained by calculating the total area occupied by the {111} oriented grains and dividing this value by the area of the observation region. A {111} oriented grain is a grain in which the <111> direction of the grain is within 15° of the direction perpendicular to the rolled surface.

In addition, the ferritic stainless steel sheet according to one embodiment of the present invention includes tubular ones obtained by forming a sheet.

The thickness of the ferritic stainless steel sheet according to one embodiment of the present invention is not particularly limited, but is preferably 2.0 mm or more, more preferably 2.5 mm or more. Further, the thickness of the ferritic stainless steel sheet according to one embodiment of the present invention is more preferably 3.0 mm or less.

The properties (oxidation resistance, thermal fatigue resistance, and thick-wall deep drawability) of the ferritic stainless steel sheet according to one embodiment of the present invention are as described above.

Next, a method for manufacturing a ferritic stainless steel sheet according to one embodiment of the present invention will be described.

A method for manufacturing a ferritic stainless steel sheet according to one embodiment of the present invention comprises:
A step of preparing a hot-rolled steel sheet having the above chemical composition;
A step of subjecting the hot-rolled steel sheet to a heat treatment under conditions of a heat treatment temperature of 700 to 850° C. and a heat treatment time of 60 minutes or more;
a step of cold-rolling the hot-rolled steel sheet under conditions of a total rolling reduction of 40% or more and less than 60% to form a cold-rolled steel sheet;
It is said to provide.
A method for manufacturing a ferritic stainless steel sheet according to one embodiment of the present invention is a method for manufacturing the ferritic stainless steel sheet according to one embodiment of the present invention.
In addition, unless otherwise specified, the temperature related to the manufacturing method is based on the surface temperature of the slab or hot-rolled steel sheet.

[Preparation process]
First, a hot-rolled steel sheet having the above chemical composition is prepared. The method of preparing the hot-rolled steel sheet is not particularly limited.
For example, in a known melting furnace such as a converter or an electric furnace, the molten steel having the above composition is melted through optional secondary refining, and made into a slab by a continuous casting method or an ingot making-blooming method. Examples of secondary refining include ladle refining and vacuum refining, and the VOD method is particularly preferred. Continuous casting is preferred in terms of productivity and quality. Then, the slab is heated to 1050 to 1250° C. and subjected to hot rolling to prepare a hot-rolled steel sheet having the above chemical composition. The plate thickness of the hot-rolled steel plate to be prepared is preferably 3.5 mm or more and less than 5.0 mm from the viewpoint of manufacturability.
Incidentally, the hot rolling conditions are not particularly limited, and may be carried out according to conventional methods.

[Heat treatment process]
Then, the hot-rolled steel sheet prepared as described above is subjected to heat treatment under the conditions of heat treatment temperature: 700 to 850° C. and heat treatment time: 60 minutes or longer.

Heat treatment temperature: 700-850°C
In this heat treatment step, it is important to precipitate a Laves phase (Fe ₂ Nb) in the hot-rolled steel sheet. By precipitating the Laves phase in the hot-rolled steel sheet, the proportion of {111} oriented grains can be increased without coarsening the crystal grains during annealing after cold rolling, which is a post-process. As a result, it is possible to realize excellent thick-wall deep drawability as well as excellent oxidation resistance and thermal fatigue resistance. Here, when the heat treatment temperature is lower than 700° C., a sufficient amount of Laves phase is not precipitated in the hot rolled steel sheet. Also, when the heat treatment temperature exceeds 850° C., a sufficient amount of Laves phase is not precipitated in the hot-rolled steel sheet. Therefore, the heat treatment temperature is set in the range of 700 to 850.degree. The heat treatment temperature is preferably 750° C. or higher. Moreover, the heat treatment temperature is preferably 800° C. or less. Since stainless steel having a composition containing Nb has a high recrystallization temperature, hot-rolled sheet annealing is generally performed in a temperature range of 900° C. or higher. However, when the hot-rolled sheet is annealed in a temperature range of 900° C. or higher, the Laves phase is not sufficiently precipitated. Therefore, the proportion of {111} oriented grains cannot be increased, and excellent thick-wall deep drawability cannot be obtained.

Heat treatment time: 60 minutes or more As described above, it is important in this heat treatment step to precipitate the Laves phase (Fe ₂ Nb) in the hot-rolled steel sheet. From the viewpoint of precipitating a sufficient amount of Laves phase (Fe ₂ Nb) in the hot-rolled steel sheet, the heat treatment time is set to 60 minutes or longer. The heat treatment time is preferably 120 minutes or longer, more preferably 180 minutes or longer. The upper limit of the heat treatment time is not particularly limited, and for example, the heat treatment time is preferably 600 minutes or less.

Note that the heat treatment temperature is the highest temperature reached in the heat treatment process. Further, the heat treatment time is the holding (residence) time in the temperature range from the heat treatment temperature of −10° C. to the heat treatment temperature, and the temperature during holding may not always be constant.

Then, optionally, the hot-rolled steel sheet is subjected to pickling or the like for descaling. Optionally, the hot rolled steel sheet may also be shot blasted to remove scale prior to pickling.

[Cold rolling process]
Then, the hot-rolled steel sheet is cold-rolled under the conditions of a total rolling reduction of 40% or more and less than 60% to obtain a cold-rolled steel sheet.

Total rolling reduction in cold rolling: 40% or more and less than 60% As described above, generally, a large rolling reduction in cold rolling is preferable for improving deep drawability. However, in the manufacturing process of the ferritic stainless steel sheet according to one embodiment of the present invention, the Laves phase is precipitated in the hot-rolled steel sheet in the heat treatment process. Therefore, even if the total reduction in cold rolling is less than 60%, or even 55% or less, it is possible to achieve excellent deep drawability, and furthermore, excellent thick wall deep drawability. Therefore, even when the total reduction ratio of cold rolling cannot be increased due to manufacturing restrictions (for example, when the thickness of the hot-rolled steel sheet cannot be increased, or when a large thickness is required as the final product), Excellent thick-wall deep drawability can be obtained. Therefore, it is extremely advantageous from the viewpoint of manufacturability. Therefore, the total rolling reduction of cold rolling is set to less than 60%. The total rolling reduction of cold rolling is preferably 55% or less. On the other hand, as described above, in order to improve the deep drawability, it is usually preferable to increase the rolling reduction in cold rolling. Therefore, the total draft of cold rolling is set to 40% or more. The total rolling reduction of cold rolling is preferably 50% or more.

For example, when the final product is a steel plate (cold-rolled steel plate) with a thickness of 2.0 mm, the hot-rolled steel plate must have a thickness of 5.0 mm or more to achieve a total cold rolling reduction of 60% or more. Something is required. When the final product is a steel plate (cold-rolled steel plate) with a thickness of 2.5 mm, the thickness of the hot-rolled steel plate must be 6.3 mm or more in order to achieve a total cold rolling reduction of 60% or more. Something is required. However, according to the method for manufacturing a ferritic stainless steel sheet according to one embodiment of the present invention, it is possible to achieve excellent thick-wall deep drawability without making the total reduction ratio of cold rolling 60% or more. . Therefore, by reducing the total reduction in cold rolling, a final product with a desired thickness can be obtained without excessively increasing the thickness of the hot-rolled steel sheet. For example, when the total reduction in cold rolling is 40%, if the thickness of the hot-rolled steel sheet is 3.4 mm, a steel sheet (cold-rolled steel sheet) with a thickness of 2.0 mm can be obtained. . Further, if the hot-rolled steel sheet has a thickness of 4.2 mm, a steel sheet (cold-rolled steel sheet) having a thickness of 2.5 mm can be obtained. In the case of the same composition, a hot-rolled steel sheet with a thickness of 4.2 mm has higher toughness than a hot-rolled steel sheet with a thickness of 6.3 mm, and the risk of breakage during production is small, so that it is excellent in manufacturability.

Conditions in the cold rolling process other than the above are not particularly limited, and conventional methods may be followed. For example, in the cold rolling process, cold rolling is performed once or twice or more with intermediate annealing. From the viewpoint of productivity and required quality, it is preferable to apply cold rolling twice or more.

After the above cold rolling, the cold rolled steel sheet may optionally be subjected to final annealing and pickling. The conditions for final annealing and pickling are not particularly limited, and conventional methods may be used.
For example, the annealing temperature for finish annealing (hereinafter also referred to as finish annealing temperature) is preferably 900 to 1150°C. If the finish annealing temperature is less than 900°C, sufficient recrystallization may not be achieved, and excellent workability (r value, deep drawability) may not be obtained. On the other hand, if the finish annealing temperature exceeds 1150°C, the recrystallized structure coarsens and the toughness decreases. As a result, cracks are likely to occur during working, and excellent workability (r value, deep drawability) may not be obtained. The finish annealing temperature is more preferably 950°C or higher. Also, the finish annealing temperature is more preferably 1100° C. or lower. In addition, the Laves phase precipitated in the heat treatment step redissolves in the steel when heated to 900° C. or higher in this continuous annealing (finish annealing). Therefore, the Laves phase does not affect the thermal fatigue resistance and toughness of the final product. Annealing time for finish annealing (hereinafter also referred to as finish annealing time) is not particularly limited, but the finish annealing time is preferably 1 minute or more in order to obtain a homogeneous recrystallized structure. On the other hand, from the viewpoint of productivity, the finish annealing time is preferably 10 minutes or less. Note that the finish annealing temperature is the highest temperature reached in the finish annealing process. Further, the finish annealing time is the holding (residence) time in the temperature range from the finishing annealing temperature of −10° C. to the finishing annealing temperature, and the holding temperature may not always be constant. Further, finish annealing may be performed by continuous annealing.

The cold-rolled steel sheets thus obtained are then subjected to processing such as cutting, bending, bulging, drawing, etc., according to their respective uses, and formed into exhaust pipes for automobiles and motorcycles, catalyst outer cylinder materials, and the like. be. The method of welding these members is not particularly limited, and ordinary arc welding such as MIG (Metal Inert Gas), MAG (Metal Active Gas), TIG (Tungsten Inert Gas), spot welding, and seam welding can be used. High-frequency resistance welding such as electric resistance welding, high-frequency induction welding, and the like can be applied.

Manufacturing conditions other than those described above are not particularly limited, and may be in accordance with conventional methods.

A steel having the composition shown in Table 1 (the balance being Fe and unavoidable impurities) was melted in a vacuum melting furnace and cast into a 50 kg steel ingot. A 50 kg steel ingot was then forged and divided into two parts. Then, one of the steel ingots divided into two was heated to 1170° C. and hot-rolled to prepare a hot-rolled steel sheet. In addition, No. 9 and no. The thickness of the hot-rolled steel sheets other than No. 26 is 4.0 mm. The thickness of the hot-rolled steel sheet No. 9 is 4.5 mm. The thickness of the hot-rolled steel sheet No. 26 was 3.4 mm.

Then, the hot-rolled steel sheets were heat-treated under the conditions shown in Table 2. Then, the hot-rolled steel sheet was polished to remove scales. Then, the hot-rolled steel sheet was subjected to cold rolling to obtain a cold-rolled steel sheet. Here, No. 9 and no. All steel sheets other than No. 26 were cold-rolled steel sheets having a thickness of 2.0 mm with a total rolling reduction of 50%. No. In No. 9, a cold-rolled steel sheet having a thickness of 2.5 mm was used with a total rolling reduction of 44%. No. In No. 26, a cold-rolled steel sheet having a thickness of 2.0 mm was used with a total rolling reduction of 41%.

Then, the cold-rolled steel sheet was subjected to finish annealing under conditions of a finish annealing temperature of 900 to 1100° C. and a finish annealing time of 1 minute. Then, the cold-rolled steel sheet was polished to remove scales to obtain a steel sheet as a final product. The steel structures of the obtained steel sheets were all composed of a ferrite phase with an area ratio of 97% or more.

The area ratio of {111} oriented grains of the thus obtained steel sheets was measured by the method described above. Table 2 shows the results. In Table 2, "◎" in the column of the area ratio of {111} oriented grains indicates that "the area ratio of {111} oriented grains is 30% or more", and "◯" indicates that "{111} oriented grains is 20% or more and less than 30%", and "x" means that "the area ratio of {111} oriented grains is less than 20%".

In addition, the obtained steel sheets were evaluated for (1) oxidation resistance, (2) thermal fatigue resistance, and (3) thick-wall deep drawability according to the following test methods. The results are also shown in Table 2.

(1) Oxidation resistance <Continuous oxidation resistance evaluation test>
A test piece of 30 mm×20 mm was cut out from the steel plate obtained as described above, and a hole of 4 mmφ was made in the upper part of the test piece. Then, the surface and end faces of the test piece were polished with #320 emery paper and degreased. Then, the test piece was suspended in a furnace in an air atmosphere heated to 1100° C. and held in that state for 200 hours. After holding, the weight gain by oxidation (g/m ² ) of the test piece was calculated according to the following formula.
[Oxidation weight gain (g/m ² )] = [weight increase of test piece before and after holding (g)] ÷ [surface area of test piece (m ² )]
Here, [mass increase (g) of test piece before and after holding] is calculated by subtracting the mass of the test piece before holding from the mass of the test piece after holding. In addition, the weight gain by oxidation (the weight of the test piece after holding) shall not include the weight of the peeled scale.
The test was conducted twice, and the presence or absence of scale peeling was visually confirmed for both test pieces. Moreover, the presence or absence of abnormal oxidation was determined by the test piece with the greater oxidation weight gain. As for the presence or absence of abnormal oxidation, it was judged that there was no abnormal oxidation when the weight gain by oxidation was <50 g/m ² , and that there was abnormal oxidation when the weight gain by oxidation was ≧50 g/m ² . Then, the continuous oxidation resistance was evaluated according to the following criteria.
○ (Pass): No abnormal oxidation, no scale peeling △ (Fail): No abnormal oxidation, scale peeling × (Fail): Abnormal oxidation

<Repeated oxidation resistance evaluation test>
A test piece is prepared in the same manner as the continuous oxidation resistance evaluation test. Then, the test piece was suspended in a furnace in an air atmosphere, and a cycle consisting of (a) to (d) below was repeated 400 times.
(a) Hold at 200°C for 1 minute (b) Heat up from 200°C to 1100°C (average heating rate 30°C/min)
(c) Hold at 1100°C for 20 minutes (d) Decrease temperature from 1100°C to 200°C (average temperature decrease rate 90°C/min)
After repeating the cycle composed of the above (a) to (d) 400 times, the presence or absence of scale peeling was confirmed and the presence or absence of abnormal oxidation was determined in the same manner as the continuous oxidation resistance evaluation test. rice field. Then, the repetitive oxidation resistance was evaluated according to the following criteria.
○ (Pass): No abnormal oxidation, no scale peeling △ (Fail): No abnormal oxidation, scale peeling × (Fail): Abnormal oxidation

(2) Thermal fatigue characteristics <thermal fatigue test>
The remaining steel ingot of the 50 kg steel ingot divided into two in the above was heated to 1170°C. Then, the steel ingot was hot rolled into a sheet bar having a thickness of 35 mm and a width of 150 mm. Then, the seat bar was forged into a square bar of 30 mm square. Then, the square bar was subjected to heat treatment under the conditions shown in Table 2 (heat treatment temperature and heat treatment time). The square bar was then annealed at a temperature of 1000-1150°C. The annealing temperature was the temperature at which recrystallization was completed after confirming the steel structure of each steel. The square bars were then machined into test pieces having the shape and dimensions shown in FIG. Then, using the test piece, the thermal expansion coefficient was measured and a thermal fatigue test was performed.

In the thermal fatigue test, as shown in FIG. 2, the test piece is restrained at a restraint rate of 0.5, and the cycle composed of the following (a) to (d) is repeated (temperature control). rice field.
(a) Hold at 200°C for 1 minute (60s) (b) Heat up from 200°C to 950°C (average heating rate: 7°C/sec)
(C) Hold at 950°C for 2 minutes (120s) (D) Decrease temperature from 950°C to 200°C (average temperature decrease rate: 7°C/sec)
Note that the above constraint rate can be expressed as constraint rate η=a/(a+b) as shown in the distortion control of FIG. Here, a is (amount of free thermal expansion strain−amount of controlled strain)/2, and b is a amount of controlled strain/2. The free thermal expansion strain amount is the strain amount when the temperature is raised without applying any mechanical stress, and the controlled strain amount indicates the absolute value of the strain amount generated during the test. A substantial amount of restraint strain caused in the material by restraint is (amount of free thermal expansion strain−amount of control strain).

The thermal fatigue life was evaluated according to the following criteria, with the number of cycles at which the stress value decreased to 75% of the stress value in the initial cycle (fifth cycle when the test was stable). The stress value of each cycle is calculated by dividing the load detected at 200° C. by the cross-sectional area of the soaking parallel portion (see FIG. 1) of the test piece.
◎ (passed, excellent): thermal fatigue life is 1200 cycles or more ○ (passed): thermal fatigue life is 800 cycles or more and less than 1200 cycles × (failed): thermal fatigue life is less than 800 cycles

(3) Thick wall deep drawability <room temperature tensile test (measurement of r value)>
From the steel sheets obtained as described above, JIS No. 13B tensile test pieces having longitudinal directions of 0°, 45°, and 90° with respect to the rolling direction were produced by machining. The gauge length (L ₀ ) was set to 50 mm. Next, using these test pieces, a tensile test was performed at room temperature at a tensile speed of 10 mm/min and a strain of up to 10%. Then, the plate width W and the gauge length L of the test piece after the test were measured. Then, the r values (r _0° , r _45° and r _90° ) when the longitudinal direction was 0°, 45° and 90° with respect to the rolling direction were calculated from the following equations.
r-value = -ln(W/W ₀ )/{ln(L/L ₀ )+ln(W/W ₀ )}
Then, from the r values at 0°, 45°, and 90° (r _0° , r _45° , r _90° ), the average value of the r values was obtained by the following equation.
Average r value = (r _0° + 2r _45° + r _90° )/4
Then, the obtained average value of the r values was evaluated according to the following criteria.
◎ (accepted, excellent): the average r-value is 1.4 or more ○ (passed): the average r-value is 1.2 or more and less than 1.4 x (failed): the average r-value is 1.4. less than 2

<Deep drawing test (LDR test)>
A disc having a diameter of 68 mm was punched out from the steel plate obtained as described above to serve as a test piece. Next, the test piece was subjected to deep drawing (drawing ratio=68 mm÷33 mm≈2.06) using a punch having a diameter of 33 mm (shoulder R: 5 mm). The wrinkle pressing force was 7 tons, and the punch speed was 55 mm/min. Further, deep drawing was performed with a polyethylene sheet attached to the surface of the test piece on the punch side. Such a test was performed three times, and the limit drawing ratio was 2.06 or more for those that were drawn through without cracking in all three times, and the limit drawing ratio was 2.06 or more. A ratio of less than 2.06 was determined as x (failed).

As shown in Table 2, all of the inventive examples exhibited excellent oxidation resistance and thermal fatigue resistance, as well as excellent thick-wall deep drawability.

On the other hand, in the comparative examples, at least one of oxidation resistance, thermal fatigue resistance, and thick-wall deep drawability was not sufficient.
That is, Steel No. In No. 27, Cr was less than the appropriate range, and the oxidation resistance (continuous oxidation resistance and repeated oxidation resistance) and thermal fatigue resistance were insufficient.
Steel no. In No. 28, the area ratio of Nb and {111} oriented grains was less than the appropriate range, and the thermal fatigue resistance and thick-wall deep drawability were insufficient.
Steel no. In No. 29, Mo was less than the appropriate range, and the thermal fatigue resistance was insufficient.
Steel no. In No. 30, the Co content was less than the appropriate range, and the thermal fatigue resistance was insufficient.
Steel no. In No. 31, Al was less than the appropriate range, and the oxidation resistance (continuous oxidation resistance and repeated oxidation resistance) and thermal fatigue resistance were insufficient.
Steel no. In Nos. 32 to 34, since the heat treatment conditions were out of the appropriate range, the area ratio of {111} oriented grains was less than the appropriate range, and the thick-wall deep drawability was insufficient.

A ferritic stainless steel sheet according to an embodiment of the present invention is suitable for exhaust system members of automobiles and the like. In addition, the ferritic stainless steel sheet according to one embodiment of the present invention can also be suitably used as an exhaust system member of a thermal power generation system or a solid oxide type fuel cell member (separator, interconnector, reformer, etc.). can be done.

Claims (5)

in % by mass,
C: 0.020% or less,
Si: 0.05 to 1.50%,
Mn: 0.05-1.30%,
P: 0.050% or less,
S: 0.010% or less,
Al: 1.00 to 6.00%,
N: 0.020% or less,
Cr: 12.0 to 25.0%,
Nb: 0.30 to 1.00%,
Mo: 1.0 to 6.0%,
Co: 0.01-3.00%, and Ni: 0.01-1.00%
and has a component composition in which the balance is Fe and unavoidable impurities,
A ferritic stainless steel sheet having an area ratio of {111} oriented grains of 20% or more. The component composition further, in mass %,
Ti: 0.50% or less,
Zr: 0.50% or less,
V: 0.50% or less,
The ferritic stainless steel sheet according to claim 1, containing one or more selected from W: 5.00% or less and Cu: 3.00% or less. The component composition further, in mass %,
B: 0.0050% or less,
Sn: 0.50% or less,
Sb: 0.50% or less,
The ferritic stainless steel sheet according to claim 1 or 2, containing one or more selected from Ca: 0.0050% or less and Mg: 0.0050% or less. The ferritic stainless steel sheet according to any one of claims 1 to 3, having a thickness of 2.0 mm or more, an r value of 1.2 or more, and a critical drawing ratio of 2.06 or more. A step of preparing a hot-rolled steel sheet having the chemical composition according to any one of claims 1 to 3;
A step of subjecting the hot-rolled steel sheet to a heat treatment under conditions of a heat treatment temperature of 700 to 850° C. and a heat treatment time of 60 minutes or more;
a step of cold-rolling the hot-rolled steel sheet under conditions of a total rolling reduction of 40% or more and less than 60% to form a cold-rolled steel sheet;
A method for manufacturing a ferritic stainless steel sheet.

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