JPWO2020240959A1 - Ferritic stainless steel sheet and its manufacturing method - Google Patents

Abstract

To provide a ferritic stainless steel sheet having a Cr content of less than 15.0% by mass, excellent productivity and corrosion resistance, and 0.2% proof stress equivalent to AISI439, and a method for producing the same. By mass%, C: 0.004 to 0.020%, Si: 0.05 to 0.90%, Mn: 0.05 to 0.60%, P: 0.050% or less, S: 0.030 % Or less, Al: 0.001 to 0.100%, Cr: 13.0% or more and less than 15.0%, Ti: 0.15 to 0.35%, Nb: 0.030 to 0.090%, V : 0.010 to 0.200% and N: 0.004 to 0.020%, the balance is composed of Fe and unavoidable impurities, and the average cross-sectional area of the crystal grains is 200 to 400 μm2. A ferrite-based stainless steel plate having a structure and having a 0.2% resistance in the L, D, and C directions of 230 to 300 MPa.

Description

The present invention relates to a ferritic stainless steel sheet and a method for producing the same, and more particularly to a ferritic stainless steel sheet having excellent corrosion resistance and productivity and having a 0.2% proof stress equivalent to AISI439.

Since stainless steel contains Cr in the steel, a dense and chemically stable passivation film is formed on the steel surface, and the stainless steel is excellent in corrosion resistance. Among stainless steels, ferritic stainless steels are relatively inexpensive because they do not contain many expensive elements, have a small thermal expansion coefficient, and have magnetism, as compared to austenitic stainless steels. Due to its characteristics, it is applied to various applications such as cooking utensils and automobile exhaust system members.

One of the typical ferritic stainless steels is AISI439 (18 mass% Cr-0.3 mass% Ti steel). AISI439 has excellent corrosion resistance, and since Ti is contained in the steel, the occurrence of sensitization is suppressed and the welded portion is excellent in corrosion resistance. Further, AISI439 is a ferrite-based stainless steel having a relatively low recrystallization temperature, and is not an annealing line dedicated to stainless steel having a high maximum annealing temperature in the cold-rolled plate annealing process, which is one of the manufacturing processes. The maximum annealing temperature is as low as about 900 ° C. The annealing line for both ordinary steel and stainless steel can soften the steel, and the productivity is high, so it is relatively inexpensive. Therefore, AISI439 is applied to a wide range of applications including automobile exhaust system members.

On the other hand, in recent years, in the above-mentioned automobile exhaust system members and the like, improvements in the structure of the members to which the steel plate is applied have progressed, and there have been cases where the members for which AISI439 has been conventionally used are not required to have the same corrosion resistance as AISI439. .. In these cases, SUH409L (11% by mass Cr-0.2% by mass Ti steel) was considered as an alternative to AISI439.

SUH409L, like AISI439, has a relatively low recrystallization temperature and thus has high productivity. Further, it is cheaper than AISI439 because the content of Cr that causes an increase in raw material cost and manufacturing cost is low. However, in many cases, AISI439 could not be replaced by SUH409L, and AISI439 has continued to be used.

The reason why SUH409L cannot replace the material of the member in which AISI439 is used is mainly due to the following two points. First, SUH409L has a lower content of Cr, which is an element for improving corrosion resistance, than AISI439, and has lower corrosion resistance than AISI439. In some cases, the steel sheet is not required to have the same corrosion resistance as AISI439 due to the optimization of the member structure, but the SUH409L may have insufficient corrosion resistance.

Next, SUH409L has a lower content of Cr, which is both a corrosion resistance improving element and a solid solution strengthening element, than AISI439, and has a low 0.2% proof stress. The difference in 0.2% proof stress of the steel sheet causes a change in the amount of so-called springback, in which the steel sheet slightly returns to its original shape after being bent or otherwise processed. Such a difference in the amount of springback becomes a problem in the processing of steel sheets.

For example, in bending, the bending angle at the time of processing is set to be larger than the target bending angle. As a result, the sum of the bending angle at the time of machining and the angle returned by the springback becomes exactly the target bending angle, and a desired machining shape can be obtained.

Here, when SUH409L is machined by a conventional machining method optimized for AISI439, the amount of springback becomes small and a desired machining shape cannot be obtained. Since the amount of springback is estimated experimentally and empirically, it takes a lot of time and cost to newly consider the processing method in order to change the conventional processing method to one suitable for SUH409L. In addition, it may be necessary to prepare a new mold for processing. Therefore, SUH409L often does not replace AISI439.

That is, there has been a demand for a ferritic stainless steel sheet that is cheaper than AISI439, has better corrosion resistance than SUH409L, and has 0.2% proof stress equivalent to AISI439. Therefore, the present inventors can use an annealing line for both ordinary steel and stainless steel in the cold-rolled sheet annealing step as in SUH409L and AISI439, and in order to reduce the cost, the Cr content is 15.0 mass. On the premise that the content is less than%, it was decided to study a ferritic stainless steel sheet having improved corrosion resistance with respect to SUH409L and further having 0.2% proof stress equivalent to AISI439.

Techniques for increasing the 0.2% proof stress of ferritic stainless steel are disclosed in, for example, Patent Documents 1 and 2.

Patent Document 1 describes C: 0.015% by mass or less, Si: 0.5% by mass or less, Cr: more than 25.0 to 35.0% by mass, N: 0.020% by mass or less, Ti: 0. Ferritic stainless steel with excellent impact resistance and perforation, containing 50% by mass or less, the balance having Fe composition excluding unavoidable impurities, and a minimum value of 0.2% proof stress in three directions of 320 N / mm ^{2 or more.} Is disclosed.

Patent Document 2 describes C: 0.15% by mass or less, Si: 1.0% by mass or less, Mn: 1.0% by mass or less, S: 0.005% by mass or less, Cr: 10 to 20% by mass, Ni: 0.5% by mass or less, Al: 0.001 to 0.05% by mass, Fe: Substantially the composition of the balance, size: 10 μm or less, Al ₂ O ₃ system and / or Al ₂ O ₃ · MgO A processed hardened material of a stainless steel plate having a processed ferrite structure in which system inclusions are dispersed with a cleanliness of 0.06 or less is disclosed.

Japanese Unexamined Patent Publication No. 2007-9263 International Publication No. 2005/014873

In the technique disclosed in Patent Document 1, in order to refine the crystal grains for the purpose of improving the 0.2% proof stress of steel, Cr that causes an increase in raw material cost and manufacturing cost exceeds 25.0% by mass. It is necessary to contain it, and it has been desired to reduce the Cr content.

In the technique disclosed in Patent Document 2, the softened steel is rolled for the purpose of improving the 0.2% proof stress of the steel. The present inventors have manufactured and evaluated a ferritic stainless steel sheet in a laboratory using the component composition and manufacturing method disclosed in Patent Document 2, and have found that the desired 0.2% proof stress can be stably obtained. It was difficult.

The present invention has been developed in view of the above problems, and is a ferritic stainless steel having a Cr content of less than 15.0% by mass, excellent productivity and corrosion resistance, and 0.2% proof stress equivalent to AISI439. It is an object of the present invention to provide a stainless steel sheet and a method for producing the same.

Here, in the present invention, "excellent in productivity" means that the cold-rolled sheet is annealed at 900 ° C. × 20 s (20 s at 900 ° C.) in the evaluation of the hardness change of the cold-rolled sheet due to annealing described below. It means that the hardness of the cold-rolled annealed sheet decreases until the formula (1) is satisfied. If the formula (1) is satisfied, the cold-rolled plate can be annealed at 900 ° C. × 20 s, and the cold-rolled plate can be annealed on the annealing line for both ordinary steel and stainless steel.

For the evaluation of the change in hardness of the cold-rolled plate due to annealing, the cold-rolled plate (cold-rolled plate not tempered) was evaluated for the cold-rolled plate obtained by cold-rolling the hot-rolled rolled plate at a reduction rate of 67%. The hardness a of the cold-rolled sheet) and the hardness b of the cold-rolled annealed plate that has been annealed for 20 s at 900 ° C. This is carried out by comparing the hardness c of the cold-rolled rolled-rolled plate that has been rolled-rolled. For the evaluation, three test pieces having a length of 15 mm and a width of 20 mm were cut out from the cold rolled plate obtained by cold rolling, and the Vickers hardness (HV) of the cross section of one of the test pieces was used as a test force 9. Measure under the conditions of 8.N and holding time of 15 seconds, and use the above hardness a. The remaining two test pieces were annealed with a cold rolled plate at 900 ° C. for 20 s and 1050 ° C. for 20 s, and then cut into a size of 15 mm in length and 10 mm in width. The hardness (HV) is measured under the above-mentioned conditions, and is defined as the above-mentioned hardnesses b and c, respectively. By applying cold-rolled sheet annealing, the hardness of the steel sheet changes (softens) from a to c, but 90% or more of the hardness decrease due to the softening is achieved by annealing at 900 ° C for 20 s. Those that are satisfied, that is, those that satisfy the following formula (1) are evaluated as "excellent in productivity".
c + 0.1 × (ac) ≧ b ・ ・ ・ (1)

Further, in the present invention, "excellent in corrosion resistance" means that after polishing a steel sheet to No. 400 with emery abrasive paper, a 5.0 mass% NaCl aqueous solution is sprayed (2 hours, 35 ° C.) in accordance with JASO M609-91. , 98% RH), dry (4 hours, 60 ° C, 30% RH), wet (2 hours, 50 ° C, 95% RH or more) as one cycle, and a corrosion test of 5 cycles was performed. Indicates that is 20% or less.

Further, in the present invention, "having a 0.2% proof stress equivalent to AISI439" means that the steel sheet has a rolling direction (L direction), a 45 degree direction (D direction) with respect to the rolling direction, and a rolling direction. As a result of collecting JIS No. 13B test pieces and conducting a tensile test so that each of them is longitudinal in the perpendicular direction (C direction), it is found that all of the obtained 0.2% proof stress is 230 MPa or more and 300 MPa or less. Point.

In response to the above problems, the present inventors have a Cr content of less than 15.0% by mass, are excellent in productivity and corrosion resistance, and further have a ferritic stainless steel sheet having a 0.2% proof stress equivalent to AISI439. It was investigated. As a result, the following findings were obtained.

That is, in terms of mass%, C: 0.004 to 0.020%, Si: 0.05 to 0.90%, Mn: 0.05 to 0.60%, P: 0.050% or less, S: 0. .030% or less, Al: 0.001 to 0.100%, Cr: 13.0% or more and less than 15.0%, Ti: 0.15 to 0.35%, Nb: 0.030 to 0.090% , V: 0.010 to 0.200%, and N: 0.004 to 0.020%, the balance is composed of Fe and unavoidable impurities, and the average cross-sectional area of the crystal grains is 200 to 400 μm. ^By using a ferrite-based stainless steel plate having a structure of 2 and having a 0.2% resistance in the L, D, and C directions of 230 to 300 MPa, the productivity and corrosion resistance are excellent, and AISI439 is used. A ferrite-based stainless steel plate having the same 0.2% strength can be obtained.

The mechanism is considered as follows.

In a ferritic stainless steel sheet having a Cr content lower than that of AISI439, increasing its 0.2% proof stress to make it equivalent to AISI439 is to make the crystal grains of the cold-rolled annealed sheet finer and to provide an appropriate solid solution in the steel. This is achieved by including a ferritic element.

Here, in order to refine the crystal grains of the cold-rolled annealed plate, it is effective to refine the crystal grains of the hot-rolled annealed plate, which is an intermediate material obtained during production. Further, the grain refinement of the hot-rolled annealed plate is achieved by making the conditions of hot rolling and hot-rolled annealed suitable. Further, by cold-rolling a hot-rolled annealed plate having fine crystal grains and then performing finish annealing under appropriate conditions, a cold-rolled annealed plate having fine crystal grains can be obtained, and a 0.2% proof stress can be obtained. Can be improved.

Further, as a solid solution strengthening element for increasing the 0.2% proof stress of the cold-rolled annealed plate, Nb was selected from the viewpoint of not causing a decrease in corrosion resistance. However, when Nb is contained, the recrystallization temperature of the cold rolled plate rises. To this end, we have found a method of suppressing an increase in recrystallization temperature by setting an appropriate upper limit on the Nb content and compoundly containing Nb and an appropriate amount of V in the steel. The increase in the recrystallization temperature due to Nb is due to the fact that some Nb are precipitated as fine NbC, which causes dislocations and pinning effects at grain boundaries. On the other hand, when V is contained in the steel, the precipitated NbC is mainly precipitated as a composite precipitate with coarse TiN ((Nb, V) C is precipitated on the surface of the coarse TiN) and the recrystallization temperature. Is thought to be suppressed. By combining the solid solution strengthening by Nb and the above-mentioned grain refinement, a ferritic stainless steel sheet having excellent productivity and corrosion resistance and 0.2% proof stress equivalent to AISI439 has been realized. can do.

The present invention is based on the above findings, and its gist structure is as follows.
[1] By mass%,
C: 0.004 to 0.020%,
Si: 0.05 to 0.90%,
Mn: 0.05 to 0.60%,
P: 0.050% or less,
S: 0.030% or less,
Al: 0.001 to 0.100%,
Cr: 13.0% or more and less than 15.0%,
Ti: 0.15-0.35%,
Nb: 0.030 to 0.090%,
A component composition containing V: 0.010 to 0.200% and N: 0.004 to 0.020%, the balance of which is Fe and unavoidable impurities.
It has a structure in which the average cross-sectional area of the crystal grains is 200 to 400 μm ^2.
A ferritic stainless steel sheet having a 0.2% proof stress in the L, D and C directions of 230 to 300 MPa.
[2] The composition of the components is further increased by mass%.
Ni: 0.01-0.60%,
Cu: 0.01 to 0.80%,
Co: 0.01-0.50%,
Mo: 0.01 to 1.00%, and W: 0.01 to 0.50%
The ferrite-based stainless steel sheet according to [1], which contains one or more selected from the above.
[3] The composition of the components is further increased by mass%.
Zr: 0.01-0.50%,
B: 0.0003 to 0.0030%,
Mg: 0.0005-0.0100%,
Ca: 0.0003 to 0.0030%,
Y: 0.01 to 0.20%,
REM (rare earth metal): 0.01-0.10%,
The ferrite stainless steel according to [1] or [2], which contains one or more selected from Sn: 0.01 to 0.50% and Sb: 0.01 to 0.50%. Steel plate.
[4] The ferritic stainless steel sheet according to any one of [1] to [3], which is used for automobile exhaust system members.
[5] The method for producing a ferritic stainless steel sheet according to any one of [1] to [4] above.
A hot rolling step in which a steel material having the above-mentioned composition is held at a temperature of 1100 to 1250 ° C. for 10 minutes or more, then hot-rolled to obtain a hot-rolled plate, and then wound at a winding temperature of 500 to 600 ° C. When,
A hot-rolled plate annealing step of obtaining a hot-rolled annealed plate by subjecting the hot-rolled plate after the hot-rolling step to annealing the hot-rolled plate at a temperature of 940 to 1000 ° C. for 5 to 180 seconds.
The hot-rolled annealed plate after the hot-rolled plate annealing step is cold-rolled to obtain a cold-rolled plate, and then cold-rolled annealed at a temperature of 880 to 900 ° C. for 5 to 180 seconds to obtain a cold-rolled annealed plate. A method for manufacturing a ferrite-based stainless steel plate, which comprises a cold-rolled sheet annealing step.

According to the present invention, there is provided a ferritic stainless steel sheet having a Cr content of less than 15.0% by mass, excellent productivity and corrosion resistance, and a 0.2% proof stress equivalent to AISI439, and a method for producing the same. can do.

Hereinafter, the present invention will be specifically described.

First, the reason for limiting the component composition in the present invention will be described. In addition, "%" indicating the content of the component of a steel sheet means mass% unless otherwise specified.

C: 0.004 to 0.020%
C is an element effective for increasing the 0.2% proof stress of steel. This effect can be obtained by setting the C content to 0.004% or more. However, if the C content exceeds 0.020%, the steel becomes hard and the moldability is lowered, or the corrosion resistance is lowered. Therefore, the C content is set to 0.004 to 0.020%. Preferably, the C content is 0.006% or more. More preferably, the C content is 0.008% or more. Further, preferably, the C content is 0.015% or less. More preferably, the C content is 0.012% or less.

Si: 0.05 to 0.90%
Si has a deoxidizing effect. This effect can be obtained by setting the Si content to 0.05% or more. However, when the Si content exceeds 0.90%, the steel becomes hard and the 0.2% proof stress increases excessively. Therefore, the Si content is set to 0.05 to 0.90%. Preferably, the Si content is 0.07% or more. More preferably, the Si content is 0.10% or more. More preferably, the Si content is 0.15% or more. Even more preferably, the Si content is 0.22% or more. Further, preferably, the Si content is 0.80% or less. More preferably, the Si content is 0.60% or less.

Mn: 0.05 to 0.60%
Mn has a deoxidizing effect. This effect can be obtained by setting the Mn content to 0.05% or more. However, when the Mn content exceeds 0.60%, precipitation and coarsening of MnS are promoted, and this MnS becomes a starting point of corrosion and the corrosion resistance of the steel sheet is lowered. Therefore, the Mn content is set to 0.05 to 0.60%. Preferably, the Mn content is 0.15% or more. Further, preferably, the Mn content is 0.30% or less.

P: 0.050% or less P is an element that reduces corrosion resistance. Further, P is segregated at the grain boundaries to reduce hot workability. Therefore, the P content is preferably as low as possible, and is 0.050% or less. Preferably, the P content is 0.040% or less. More preferably, the P content is 0.030% or less.

S: 0.030% or less S forms MnS as a precipitate with Mn. This MnS becomes a starting point of corrosion and lowers corrosion resistance. Therefore, the S content is preferably as low as 0.030% or less. Preferably, the S content is 0.020% or less.

Al: 0.001 to 0.100%
Al has a deoxidizing effect. This effect is obtained when the Al content is 0.001% or more. However, when the Al content exceeds 0.100%, the steel becomes hard and the moldability is lowered, and the corrosion resistance is lowered. Therefore, the Al content is set to 0.001 to 0.100%. Preferably, the Al content is 0.030% or more. Further, preferably, the Al content is 0.060% or less.

Cr: 13.0% or more and less than 15.0% Cr is an element that forms a passivation film on the surface to enhance corrosion resistance. If the Cr content is less than 13.0%, sufficient corrosion resistance cannot be obtained. On the other hand, when the Cr content is 15.0% or more, the raw material cost and the manufacturing cost increase. Therefore, the Cr content is set to 13.0% or more and less than 15.0%. Preferably, the Cr content is 13.5% or more. Further, preferably, the Cr content is 14.5% or less. Preferably, the Cr content is 14.0% or less.

Ti: 0.15-0.35%
Ti is an element that fixes C and N by forming a carbonitride and suppresses the occurrence of sensitization. This effect can be obtained by setting the Ti content to 0.15% or more. However, when the Ti content exceeds 0.35%, the steel becomes hard and the formability deteriorates. Therefore, the Ti content is set to 0.15 to 0.35%. Preferably, the Ti content is 0.20% or more. Further, preferably, the Ti content is 0.30% or less.

Nb: 0.030 to 0.090%
Nb is an element effective for increasing the 0.2% proof stress of steel by being dissolved in the steel of the cold-rolled annealed sheet. This effect is obtained by setting the Nb content to 0.030% or more. However, when the Nb content exceeds 0.090%, the recrystallization temperature of steel rises even if the effect of suppressing the rise in recrystallization temperature due to V, which will be described later, is obtained, and an annealing line for both ordinary steel and stainless steel is formed. When manufactured using it, the softening of the steel becomes insufficient, or the crystal grains become excessively fine, and the 0.2% proof stress increases. Therefore, the Nb content is set to 0.030 to 0.090%. Preferably, the Nb content is 0.035% or more. More preferably, the Nb content is 0.040% or more. Further, preferably, the Nb content is 0.080% or less. More preferably, the Nb content is 0.070% or less.

V: 0.010 to 0.200%
V is an element that enhances productivity by suppressing an increase in the recrystallization temperature of steel due to Nb. This effect can be obtained by setting the V content to 0.010% or more. On the other hand, if V is excessively contained, V carbonitride is excessively precipitated, the recrystallization temperature rises, and the productivity of steel is lowered. Therefore, the V content is set to 0.010 to 0.200%. Preferably, the V content is 0.020% or more. More preferably, the V content is 0.030% or more. Further, preferably, the V content is 0.150% or less. More preferably, the V content is 0.100% or less.

N: 0.004 to 0.020%
N is an element effective in increasing the 0.2% proof stress of steel. This effect can be obtained by setting the N content to 0.004% or more. However, when the N content exceeds 0.020%, the steel becomes hard and the moldability is lowered, or the corrosion resistance is lowered. Therefore, the N content is set to 0.004 to 0.020%. Preferably, the N content is 0.005% or more. More preferably, the N content is 0.007% or more. Further, preferably, the N content is 0.015% or less. More preferably, the N content is 0.012% or less.

The rest other than the above components are Fe and unavoidable impurities.

In the present invention, in addition to the above-mentioned components, one or two selected from the following groups A and B may be contained.
(Group A) Ni: 0.01 to 0.60%, Cu: 0.01 to 0.80%, Co: 0.01 to 0.50%, Mo: 0.01 to 1.00%, and W. : One or more selected from 0.01 to 0.50% (Group B) Zr: 0.01 to 0.50%, B: 0.0003 to 0.0030%, Mg: 0 0005 to 0.0100%, Ca: 0.0003 to 0.0030%, Y: 0.01 to 0.20%, REM (rare earth metal): 0.01 to 0.10%, Sn: 0.01 ~ 0.50%, and Sb: one or more selected from 0.01 to 0.50%

Ni: 0.01 to 0.60%
Ni enhances the corrosion resistance of steel by suppressing the active melting of steel in a low pH environment. On the other hand, if an excessive amount of Ni is contained, the component cost and the manufacturing cost of the steel are increased, and the steel is hardened and the formability is lowered. Therefore, when Ni is contained, the Ni content is set to 0.01 to 0.60%. Preferably, the Ni content is 0.10% or more. Further, preferably, the Ni content is 0.25% or less.

Cu: 0.01 to 0.80%
Cu is an element that improves the corrosion resistance of stainless steel. On the other hand, if Cu is contained in an excessive amount, the component cost and the manufacturing cost of the steel are increased, and ε-Cu is likely to be precipitated, so that the corrosion resistance is lowered. Therefore, when Cu is contained, the Cu content is set to 0.01 to 0.80%. Preferably, the Cu content is 0.30% or more. More preferably, the Cu content is 0.40% or more. Further, preferably, the Cu content is 0.50% or less. More preferably, the Cu content is 0.45% or less. Even more preferably, the Cu content is 0.42% or less.

Co: 0.01-0.50%
Co is an element that improves the corrosion resistance of stainless steel. On the other hand, if Co is excessively contained, the steel becomes hard and the 0.2% proof stress increases excessively. Therefore, when Co is contained, the Co content is set to 0.01 to 0.50%. Preferably, the Co content is 0.03% or more. More preferably, the Co content is 0.05% or more. Further, preferably, the Co content is 0.30% or less. More preferably, the Co content is 0.10% or less.

Mo: 0.01-1.00%
Mo has the effect of improving the corrosion resistance of stainless steel. On the other hand, if Mo is contained in an excessive amount, the component cost and the manufacturing cost of the steel are increased, and the steel is hardened and the 0.2% proof stress is excessively increased. Therefore, when Mo is contained, the Mo content is set to 0.01 to 1.00%. Preferably, the Mo content is 0.03% or more. More preferably, the Mo content is 0.05% or more. Further, preferably, the Mo content is 0.50% or less. More preferably, the Mo content is 0.30% or less.

W: 0.01 to 0.50%
W is an element that improves the corrosion resistance of stainless steel. On the other hand, if W is excessively contained, the steel becomes hard and the 0.2% proof stress increases excessively. Therefore, when W is contained, the W content is set to 0.01 to 0.50%. Preferably, the W content is 0.03% or more. More preferably, the W content is 0.05% or more. Further, preferably, the W content is 0.30% or less. More preferably, the W content is 0.10% or less.

Zr: 0.01-0.50%
Zr is an element that fixes C and N by forming a carbonitride and improves the corrosion resistance of steel. On the other hand, if Zr is excessively contained, the carbonitride is excessively precipitated, and the corrosion resistance of the steel is lowered. Therefore, when Zr is contained, the Zr content is set to 0.01 to 0.50%. Preferably, the Zr content is 0.03% or more. More preferably, the Zr content is 0.05% or more. Further, preferably, the Zr content is 0.40% or less. More preferably, the Zr content is 0.30% or less.

B: 0.0003 to 0.0030%
B has the effect of improving the strength of steel. On the other hand, if B is excessively contained, the steel becomes hard and the 0.2% proof stress increases excessively. Therefore, when B is contained, the B content is set to 0.0003 to 0.0030%. Preferably, the B content is 0.0010% or more. Further, preferably, the B content is 0.0025% or less.

Mg: 0.0005 to 0.0100%
Mg acts as an antacid. On the other hand, if Mg is contained in excess, surface defects increase. Therefore, when Mg is contained, the Mg content is set to 0.0005 to 0.0100%. Preferably, the Mg content is 0.0010% or more. Further, preferably, the Mg content is 0.0050% or less. More preferably, the Mg content is 0.0030% or less.

Ca: 0.0003 to 0.0030%
Ca acts as an antacid. On the other hand, if Ca is contained in excess, surface defects increase. Therefore, when Ca is contained, the Ca content is set to 0.0003 to 0.0030%. Preferably, the Ca content is 0.0005% or more. More preferably, the Ca content is 0.0007% or more. Further, preferably, the Ca content is 0.0025% or less. More preferably, the Ca content is 0.0015% or less.

Y: 0.01 to 0.20%
Y is an element that improves the cleanliness of steel. On the other hand, if Y is contained in excess, surface defects increase. Therefore, when Y is contained, the Y content is set to 0.01 to 0.20%. Preferably, the Y content is 0.03% or more. Further, preferably, the Y content is 0.10% or less.

REM (Rare Earth Metals): 0.01-0.10%
REM (rare earth metal: an element having atomic numbers 57 to 71 such as La, Ce, and Nd) is an element that improves the cleanliness of steel. On the other hand, if REM is excessively contained, surface defects will increase. Therefore, when REM is contained, the REM content is set to 0.01 to 0.10%. Preferably, the REM content is 0.02% or more. Further, preferably, the REM content is 0.05% or less. The REM content in the present invention is the total content of one or more elements selected from the above-mentioned REM.

Sn: 0.01 to 0.50%
Sn is an element effective in suppressing rough processed skin. On the other hand, if Sn is contained in an excessive amount, the hot workability of the steel is lowered. Therefore, when Sn is contained, the Sn content is set to 0.01 to 0.50%. Preferably, the Sn content is 0.03% or more. Further, preferably, the Sn content is 0.20% or less.

Sb: 0.01 to 0.50%
Similar to Sn, Sb is an element effective in suppressing rough processed skin. On the other hand, if Sb is contained in excess, surface defects increase. Therefore, when Sb is contained, the Sb content is set to 0.01 to 0.50%. Preferably, the Sb content is 0.03% or more. Further, preferably, the Sb content is 0.20% or less.

When the contents of Ni, Cu, Co, Mo, W, Zr, B, Mg, Ca, Y, REM (rare earth metal), Sn, and Sb described as the above optional components are less than the lower limit, the components are It shall be contained as an unavoidable impurity.

Average cross-sectional area of crystal grains: 200-400 μm ²
In the present invention, in addition to controlling the content of various elements such as Nb, by controlling the average cross-sectional area of the crystal grains in the structure within a predetermined range, it is possible to produce with excellent productivity. , Ferritic stainless steel having 0.2% proof stress equivalent to AISI439 can be obtained. Here, the average cross-sectional area of the crystal grains affects the 0.2% proof stress of the steel. If the average cross-sectional area of the crystal grains ^{is less than 200 μm 2} , the 0.2% proof stress of the steel becomes high, and the 0.2% proof stress equivalent to AISI439 cannot be obtained. Further, when the average cross-sectional area of the crystal grains ^{exceeds 400 μm 2} , the 0.2% proof stress of the steel becomes low, and the 0.2% proof stress equivalent to AISI439 cannot be obtained. Therefore, the average cross-sectional area of the crystal grains in the structure is set to 200 to 400 μm ² . Preferably, the average cross-sectional area of the crystal grains is 240 μm ² or more. Further, preferably, the average cross-sectional area of the crystal grains is 360 μm ² or less. The average cross-sectional area of the crystal grains can be controlled by a production method described later.

The average cross-sectional area of the crystal grains can be evaluated by the following method. A test piece for microstructure observation having a width of 10 mm and a length of 15 mm is cut out from a ferritic stainless steel plate, embedded in a resin so that the cross section in the length direction becomes the observation surface, and then the observation surface is mirror-polished. Then, the observation surface is etched with a picric acid hydrochloric acid solution (100 mL ethanol-1 g picric acid-5 mL hydrochloric acid) to reveal grain boundaries, and then the structure is photographed with an optical microscope at a magnification of 500 times. A circle with a radius of 100 μm (a circle with a radius of 50 mm when the observation image is printed at a magnification of 500 times) is drawn with respect to the obtained observation image in the actual field of view, and the crystal grains completely contained in the circle. The average cross-sectional area A (μm) of the crystal grains given by measuring the number as n ₁ and the number of crystal grains cut by the circumference as n _{2 and substituting the obtained measurement results into the following formula (2)} ² ) is evaluated.
A = 31400 / (n ₁ + 0.6 × n ₂ ) ・ ・ ・ (2)

0.2% proof stress in the L direction: 230-300 MPa
0.2% proof stress in D direction: 230-300 MPa
0.2% proof stress in C direction: 230-300 MPa
In order to have 0.2% proof stress equivalent to AISI439 and to obtain the same springback amount as AISI439 when processed, 0.2% in the L, C and D directions of ferritic stainless steel sheets. It is necessary that the yield strength is in the range of 230 to 300 MPa. When the 0.2% proof stress in any direction is less than 230 MPa, the steel is processed so that the direction perpendicular to the direction in which the 0.2% proof stress is less than 230 MPa is the bending ridge line, as compared with AISI439. Then, the amount of springback becomes smaller. Further, when the 0.2% proof stress in any direction exceeds 300 MPa, the steel is processed so that the direction perpendicular to the direction in which the 0.2% proof stress exceeds 300 MPa becomes the bending ridge line, as compared with AISI439. Then, the amount of springback increases. Therefore, the 0.2% proof stress in the L direction, the D direction, and the C direction is set to 230 to 300 MPa. Preferably, the 0.2% proof stress is 240 MPa or more. Further, preferably, the 0.2% proof stress is 290 MPa or less.

In order to keep the springback amount of the ferritic stainless steel sheet, which has excellent corrosion resistance and can be manufactured with high productivity, within the appropriate range as described above, and to keep the 0.2% proof stress of the steel sheet within the appropriate range as described above. It is necessary to adjust the production method as described later so that the content of each element is within the above range and the average cross-sectional area of the crystal grains is within the above range.

Next, a suitable manufacturing method of the ferritic stainless steel sheet of the present invention will be described. A steel having the above-mentioned composition is melted by a known method such as a converter or an electric furnace, and then made into a steel material (steel slab) by a continuous casting method or an ingot-integration method. After holding this steel material at a temperature of 1100 to 1250 ° C. for 10 minutes or more, it is hot-rolled to obtain a hot-rolled plate, and then the hot-rolled plate is wound at a winding temperature of 500 ° C. or higher and 600 ° C. or lower to be hot-rolled. It is a plate coil. At this time, it is preferable to hot-roll the hot-rolled plate so that the plate thickness is 2.0 to 5.0 mm. The hot-rolled plate thus produced is annealed by holding it at a temperature of 940 to 1000 ° C. for 5 to 180 seconds to obtain a hot-rolled annealed plate. The atmosphere of hot-rolled sheet annealing is preferably an atmospheric atmosphere. The scale is then removed by pickling. Next, cold rolling is performed to obtain a cold-rolled plate, and then the cold-rolled plate is annealed at a temperature of 880 to 900 ° C. for 5 to 180 seconds to obtain a cold-rolled annealed plate. After annealing the cold rolled plate, pickling or surface grinding is performed to remove the scale. Skin pass rolling may be performed on the cold-rolled annealed plate from which the scale has been removed. However, if the reduction rate of skin pass rolling exceeds 2%, not only the 0.2% proof stress becomes excessively high, but also the moldability deteriorates. Therefore, when skin pass rolling is performed, the reduction rate is set to 2% or less. It is preferable to do so.

First, a method for controlling the average cross-sectional area of crystal grains in the above-mentioned suitable production method will be described below. By casting steel having the above-mentioned components, it is possible to obtain a steel slab in which carbonitrides typified by TiN, TiC, NbC, VC and the like are deposited in the steel. By heating the steel slab before hot rolling to 1100 ° C. or higher, solid solution of TiN, TiC, NbC, and VC into the steel proceeds. After hot rolling of the steel slab, the hot-rolled plate is cooled and wound at a winding temperature of 500 ° C. or higher and 600 ° C. or lower to form a hot-rolled plate coil. A hot-rolled plate with a small amount of solid solution C and solid solution N can be obtained by precipitating nitride. Since the obtained hot-rolled sheet has strain remaining in the steel and has a small amount of solid solution C and N, recrystallization can occur even by annealing at a relatively low temperature of 940 ° C. or higher and 1000 ° C. or lower. Occur. Further, by setting the annealing temperature to a relatively low temperature, a hot-rolled annealed plate having relatively small crystal grains can be obtained.

Next, a hot-rolled annealed plate having relatively small crystal grains is cold-rolled to obtain a cold-rolled plate, and then the cold-rolled sheet is annealed at a temperature of 880 ° C. or higher and 900 ° C. or lower to cool the desired crystal grain size. An annealing plate can be obtained.

By the above-mentioned process, a cold-rolled annealed sheet having an average cross-sectional area of 200 to 400 μm ² is obtained, and a ferritic stainless steel sheet having a desired 0.2% proof stress is obtained.

Hereinafter, the reason why the conditions are set within the above-mentioned range in each step of the above-mentioned suitable production method will be described in more detail.

A process in which a steel slab is held at a temperature of 1100 ° C. or higher and 1250 ° C. or lower for 10 minutes or longer, then hot-rolled to obtain a hot-rolled plate, and then wound at a winding temperature of 500 to 600 ° C. (hot rolling step).
If the heating temperature of the steel slab is less than 1100 ° C., NbC in the steel is not sufficiently solidified, and the effect of increasing the 0.2% proof stress by Nb on the cold-rolled annealed sheet cannot be obtained. 0.2% proof stress decreases. Further, if the heating time of the steel slab is less than 10 minutes, NbC in the steel is not sufficiently solid-solved, and the effect of increasing the 0.2% proof stress by Nb on the cold-rolled annealed plate cannot be obtained, and the cold-rolled annealing is not obtained. The 0.2% proof stress of the board is reduced. Further, when the heating temperature of the steel slab exceeds 1250 ° C., the steel slab is deformed and the manufacturability of the hot-rolled plate in the hot rolling process is lowered. Therefore, in the present invention, it is preferable to hold the steel slab at 1100 ° C. or higher and 1250 ° C. or lower for 10 minutes or longer and then hot-roll it to obtain a hot-rolled plate. More preferably, the heating temperature of the steel slab is 1150 ° C. or higher. The heating time is more preferably 30 minutes or more. Further, more preferably, the heating temperature of the steel slab is 1200 ° C. or lower. Further, since heating and holding the steel slab for an excessively long time causes deformation of the steel slab and lowers the manufacturability of the hot-rolled plate in the hot rolling process, the heating time of the steel slab is preferably 2 hours or less. ..

Further, if the winding temperature of the hot-rolled plate is less than 500 ° C., the precipitation of Cr carbonitride in the steel becomes insufficient, and the amount of solid solution C and solid solution N contained in the hot-rolled plate becomes excessive. As a result, the recrystallization temperature of the hot-rolled sheet becomes high. In that case, even if the hot-rolled plate is annealed at a temperature described later, the hot-rolled plate does not recrystallize. In the cold rolling of a hot-rolled sheet having a recrystallized structure, a locally high-strain field with locally high lattice strain is formed near the grain boundaries, and these are recrystallized nuclei in the annealing process of the cold-rolled sheet. This contributes to the refinement of the crystal grains of the cold-rolled annealed plate. On the other hand, when a hot-rolled annealed plate having an unrecrystallized structure is cold-rolled, it is difficult to form a local high strain field as a recrystallization nucleus in the steel during the annealing process of the cold-rolled plate. The crystal grains of the plate become coarse, and the 0.2% resistance of the cold-rolled annealed plate decreases. When the winding temperature of the hot-rolled plate exceeds 600 ° C., the strain introduced into the hot-rolled plate in the hot rolling step is recovered, and the recrystallization temperature of the hot-rolled plate becomes high. In that case, even if the hot-rolled plate is annealed at a temperature described later, the hot-rolled plate does not recrystallize, and the crystal grains of the cold-rolled annealed plate become coarse, and the 0.2% proof stress of the cold-rolled annealed plate decreases. do. Therefore, in the present invention, it is preferable to wind the hot-rolled plate after hot rolling at a winding temperature of 500 ° C. or higher and 600 ° C. or lower to obtain a hot-rolled plate coil.

A process of annealing a hot-rolled plate by holding the hot-rolled plate at a temperature of 940 ° C. or higher and 1000 ° C. or lower for 5 to 180 seconds to obtain a hot-rolled annealed plate (hot-rolled plate annealing step).
If the hot-rolled sheet annealing temperature is less than 940 ° C., the hot-rolled sheet does not recrystallize, and accordingly, the crystal grains of the cold-rolled annealed plate become coarse, and the 0.2% proof stress of the cold-rolled annealed plate decreases. When the hot-rolled annealed temperature exceeds 1000 ° C., the crystal grains of the hot-rolled annealed plate become coarse, the crystal grains of the cold-rolled annealed plate become coarse, and the 0.2% proof stress of the cold-rolled annealed plate decreases. Further, if the holding time of the hot-rolled annealed plate is less than 5 seconds, the hot-rolled plate does not recrystallize, and the crystal grains of the cold-rolled annealed plate become coarse, and the 0.2% proof stress of the cold-rolled annealed plate becomes coarse. Decreases. When the holding time of the hot-rolled annealed plate exceeds 180 seconds, the crystal grains of the hot-rolled annealed plate become coarse, the crystal grains of the cold-rolled annealed plate become coarse, and the 0.2% proof stress of the cold-rolled annealed plate decreases. do. Therefore, in the present invention, it is preferable that the hot-rolled plate is annealed by holding the hot-rolled plate at a temperature of 940 ° C. or higher and 1000 ° C. or lower for 5 to 180 seconds to obtain a hot-rolled annealed plate. More preferably, the annealing temperature range of the hot-rolled plate is 950 ° C. or higher and 980 ° C. or lower. Moreover, the above-mentioned holding time is more preferably 10 seconds or more. Moreover, the above-mentioned holding time is more preferably 30 seconds or less.

Next, the hot-rolled annealed sheet after the hot-rolled sheet annealing step is cold-rolled to obtain a cold-rolled sheet. At this time, the cold reduction rate is preferably 50% or more. More preferably, it is 65% or more.

A step of annealing a cold-rolled plate in which the cold-rolled plate is held at a temperature of 880 ° C. or higher and 900 ° C. or lower for 5 to 180 seconds to obtain a cold-rolled annealed plate (cold-rolled plate annealing step).
When the cold rolled sheet annealing temperature is less than 880 ° C., the crystal grains of the steel become excessively fine and the 0.2% proof stress becomes excessively high. On the other hand, cold-rolled plate annealing exceeding 900 ° C. cannot be performed on a highly productive ordinary steel-stainless steel combined annealing line. Further, if the holding time of the cold rolled sheet annealing is less than 5 seconds, the crystal grains of the steel become excessively fine and the 0.2% proof stress becomes excessively high. On the other hand, when the holding time of the cold rolled sheet annealing exceeds 180 seconds, the crystal grains of the steel become coarse and the 0.2% proof stress becomes excessively low. Therefore, in the present invention, it is preferable to perform cold-rolled plate annealing in which the cold-rolled plate is held at 880 ° C. or higher and 900 ° C. or lower for 5 to 180 seconds. More preferably, the annealing temperature range of the cold rolled plate is 890 ° C. or higher. Moreover, the above-mentioned holding time is more preferably 10 seconds or more. Moreover, the above-mentioned holding time is more preferably 120 seconds or less.

[Example 1]
After melting a ferritic stainless steel having the component composition shown in Table 1-1 into a 100 kg ingot (steel material), each slab shown in Table 1-2 is heated at each slab heating temperature shown in Table 1-2. After holding the heating time, hot rolling was performed to obtain a hot-rolled plate having a plate thickness of 3.0 mm. Immediately after the final pass of hot rolling is completed, the hot-rolled plate is air-cooled to each winding temperature shown in Table 1-2, and then the hot-rolled plate is inserted into an electric furnace and held at each winding temperature for 1 hour. After that, it was cooled in an electric furnace. In the process of inserting this hot-rolled plate into an electric furnace, holding it at each winding temperature for 1 hour, and then cooling it in the electric furnace, the hot-rolled plate after hot rolling is placed at each winding temperature in the actual production line. This is a simulation of the temperature history in which the product is wound into a coil and then slowly cooled.

The obtained hot-rolled plate was held at each hot-rolled plate annealing temperature shown in Table 1-2 for the annealing time of each hot-rolled plate shown in Table 1-2, and then air-cooled to obtain a hot-rolled annealed plate. This hot-rolled annealed plate was pickled with a sulfuric acid solution followed by a mixed solution of hydrofluoric acid and nitric acid to be used as a material for cold rolling, and then cold-rolled to a plate thickness of 1.0 mm to obtain a cold-rolled plate. .. A part of the obtained cold-rolled plate was held at the annealing temperature of each cold-rolled plate shown in Table 1-2 for the annealing time of each cold-rolled plate shown in Table 1-2, and then air-cooled, and then the front and back surfaces were cooled. The surface was ground to remove the surface scale to obtain a cold-rolled annealed plate. The obtained cold-rolled plate and cold-rolled annealed plate were subjected to the following evaluation.

(1) Evaluation of Productivity The hardness a of the cold-rolled plate obtained under the above manufacturing conditions and the hardness of the cold-rolled annealed plate obtained by annealing the cold-rolled plate at 900 ° C. for 20 s. By comparing b with the hardness c of the cold-rolled annealed plate that has been annealed for 20 s at 1050 ° C. as an index when it is sufficiently softened, the hardness change of the cold-rolled plate due to annealing. Was evaluated. Specifically, three test pieces having a length of 15 mm and a width of 20 mm were cut out from the cold-rolled plate, and the Vickers hardness (HV) of the cross section of one of the test pieces was measured and used as the above-mentioned hardness a. .. Further, the remaining two test pieces were annealed at 900 ° C. for 20 s and at 1050 ° C. for 20 s, respectively, and then cut into a size of 15 mm in length × 10 mm in width, and the Vickers hardness of the cross section of the cut test pieces ( HV) was measured and set to the above hardnesses b and c, respectively. After embedding the test piece with resin, the test surface was mirror-polished and used for the test. The measurement conditions for Vickers hardness were a test force of 9.8 N and a holding time of 15 seconds. The measured hardnesses a, b, and c were evaluated as "○ (pass)" when they satisfied the formula (1) and as "▲ (fail)" when they did not meet the equation (1). If the evaluation is ◯, it can be evaluated that the cold rolled sheet can be annealed on the annealing line for both ordinary steel and stainless steel, and the productivity is excellent.
c + 0.1 × (ac) ≧ b ・ ・ ・ (1)

(2) Evaluation of average cross-sectional area of crystal grains A test piece for microstructure observation having a width of 10 mm and a length of 15 mm is cut out from the cold-rolled annealed plate obtained under the above manufacturing conditions, and the cross section in the length direction becomes the observation surface. After embedding in the resin as described above, the observation surface was mirror-polished. Then, the observation surface was etched with a picric acid hydrochloric acid solution (100 mL ethanol-1 g picric acid-5 mL hydrochloric acid) to reveal grain boundaries, and then the structure was photographed with an optical microscope at a magnification of 500 times. A circle with a radius of 100 μm (a circle with a radius of 50 mm when the observation image is printed at a magnification of 500 times) is drawn with respect to the obtained observation image in the actual field of view, and the crystal grains completely contained in the circle. The average cross-sectional area A (μm) of the crystal grains given by measuring the number as n ₁ and the number of crystal grains cut by the circumference as n _{2 and substituting the obtained measurement results into the following formula (2)} ² ) was evaluated.
A = 31400 / (n ₁ + 0.6 × n ₂ ) ・ ・ ・ (2)

(3) Evaluation of 0.2% proof stress From the cold-rolled annealed sheet obtained under the above manufacturing conditions, with respect to the rolling direction (L direction), the 45 degree direction (D direction) with respect to the rolling direction, and the rolling direction. JIS No. 13B test pieces were collected and subjected to a tensile test so that each of them was longitudinal in the perpendicular direction (C direction). The tensile test was carried out in accordance with JIS Z 2241, and the 0.2% proof stress of each test piece obtained was evaluated.

(4) Evaluation of Corrosion Resistance A test piece having a length of 80 mm and a width of 60 mm was cut out from a cold-rolled annealed plate obtained under the above manufacturing conditions by shearing. The surface of the test piece was polished to No. 400 with emery paper, degreased with acetone, and then subjected to a corrosion test to evaluate the corrosion resistance. Corrosion tests were carried out in accordance with JASO M609-91. One cycle is 5.0 mass% NaCl aqueous solution spray (35 ° C., relative humidity 98%) 2h → dry (60 ° C., relative humidity 30%) 4h → wet (50 ° C., relative humidity 95% or more) 2h, 5 cycles Corrosion test was carried out. After the test, the rusting area ratio was measured by image analysis in a region of 30 mm × 30 mm in the center of the surface of the test piece from a photograph of the surface of the test piece. Then, those having a rust area ratio of 20% or less were evaluated as "○ (pass)", and those having a rust area ratio of more than 20% were evaluated as "▲ (fail)". If the evaluation is ◯, it can be evaluated that the corrosion resistance is excellent.

The results obtained are shown in Table 1-2.

The ferritic stainless steel plate (Test Nos. 1-1 to 1-9) of the example of the present invention has a productivity evaluation of "○", an average cross-sectional area of crystal grains of 200 μm ² or more and 400 μm ² or less, and L. The 0.2% proof stress in all three directions of the direction, the D direction and the C direction is 230 MPa or more and 300 MPa or less, and the evaluation of the corrosion resistance is "○", and the 0.2% proof stress equivalent to AISI439 is obtained. It was found that it was excellent in productivity and corrosion resistance.

Test No. In the comparative example of 1-10, the slab heating temperature was lower than the range of the present invention, and the proof stress in the L direction and the C direction was lower than the range of the present invention by 0.2%.

Test No. In the comparative example of 1-11, the slab heating time was shorter than the range of the present invention, and the 0.2% proof stress in the L direction was lower than the range of the present invention.

Test No. In the comparative example of 1-12, the hot-rolled plate winding temperature is higher than the range of the present invention, the average cross-sectional area of the crystal grains is larger than the range of the present invention, and the L direction and the C direction are larger than the range of the present invention. 0.2% proof stress was low.

Test No. In the comparative example of 1-13, the hot-rolled plate winding temperature is lower than the range of the present invention, the average cross-sectional area of the crystal grains is larger than the range of the present invention, and the L direction, the D direction and the range of the present invention are higher than the range of the present invention. The 0.2% proof stress was low in all three directions in the C direction.

Test No. In the comparative example of 1-14, the hot-rolled plate annealing temperature is lower than the range of the present invention, the average cross-sectional area of the crystal grains is larger than the range of the present invention, and the L direction, the D direction and C are larger than the range of the present invention. The yield strength was low by 0.2% in all three directions.

Test No. In the comparative example of 1-15, the hot-rolled plate annealing temperature is higher than the range of the present invention, the average cross-sectional area of the crystal grains is larger than the range of the present invention, and the L direction, the D direction and C are larger than the range of the present invention. The yield strength was low by 0.2% in all three directions.

Test No. In the comparative example of 1-16, the hot-rolled plate annealing time is shorter than the range of the present invention, the average cross-sectional area of the crystal grains is larger than the range of the present invention, and the L direction, the D direction and C are larger than the range of the present invention. The yield strength was low by 0.2% in all three directions.

Test No. In the comparative example of 1-17, the hot-rolled plate annealing time is longer than the range of the present invention, the average cross-sectional area of the crystal grains is larger than the range of the present invention, and the L direction, the D direction and the C are larger than the range of the present invention. The yield strength was low by 0.2% in all three directions.

Test No. In the comparative example of 1-18, the cold-rolled plate annealing temperature was lower than the range of the present invention, the average cross-sectional area of the crystal grains was smaller than the range of the present invention, and the L direction, the D direction and C were smaller than the range of the present invention. The 0.2% proof stress was high in all three directions.

Test No. In the comparative example of 1-19, the cold-rolled plate annealing time was shorter than the range of the present invention, the average cross-sectional area of the crystal grains was smaller than the range of the present invention, and 0.2% in the D direction than the range of the present invention. The bearing capacity was high.

Test No. In the comparative examples of 1-20, the cold-rolled plate annealing time was longer than the range of the present invention, the average cross-sectional area of the crystal grains was larger than the range of the present invention, and the L direction, the D direction and the C were larger than the range of the present invention. The yield strength was low by 0.2% in all three directions.

[Example 2]
Ferritic stainless steel having the composition shown in Table 2 is melted into a 100 kg ingot (steel material), heated at a temperature of 1160 ° C. for 1 hour, hot-rolled, and hot-rolled to a plate thickness of 3.0 mm. It was made into a board. Immediately after the final pass of hot rolling is completed, the hot-rolled plate is air-cooled to 550 ° C, then the hot-rolled plate is inserted into an electric furnace set at 550 ° C and held for 1 hour, and then cooled in the electric furnace. bottom. The obtained hot-rolled plate was held at 980 ° C. for 20 seconds and then air-cooled to obtain a hot-rolled annealed plate. The hot-rolled annealed plate was pickled with a sulfuric acid solution followed by a mixed solution of hydrofluoric acid and nitric acid to be used as a material for cold rolling, and then cold-rolled to a plate thickness of 1.0 mm to obtain a cold-rolled plate. A part of the obtained cold-rolled plate was held at 900 ° C. for 100 seconds, then air-cooled, and then the front and back surfaces were ground to remove the surface scale to obtain a cold-rolled annealed plate. The obtained cold-rolled plate and cold-rolled annealed plate were subjected to the above-mentioned evaluation. In addition, the test No. 2-32 and 2-33 are reference examples, and the above-mentioned test No. No. 2-32 is a component composition of SUH409L standard, and the above-mentioned test No. 2-33 is a component composition of AISI439 standard.

The results obtained are shown in Table 2.

The ferritic stainless steel plate (Test No. 2-1 to 2-25) of the example of the present invention has a productivity evaluation of "○", an average cross-sectional area of crystal grains of 200 μm ² or more and 400 μm ² or less, and L. The 0.2% proof stress in all three directions of the direction, the D direction and the C direction is 230 MPa or more and 300 MPa or less, and the evaluation of the corrosion resistance is "○", and the 0.2% proof stress equivalent to AISI439 is obtained. It was found that it was excellent in productivity and corrosion resistance.

Test No. In the comparative example of 2-26, the Nb content was lower than the component range of the present invention, and the proof stress was lower than the range of the present invention by 0.2% in all three directions of the L direction, the D direction and the C direction.

Test No. In the comparative example of 2-27, since the content of Nb is higher than the component range of the present invention, the productivity is inferior, and the average cross-sectional area of the crystal grains is smaller than the range of the present invention, which is smaller than the range of the present invention. The 0.2% proof stress was high in all three directions of the L direction, the D direction and the C direction.

Test No. In the comparative example of 2-28, since the V content is lower than the component range of the present invention, the productivity is inferior, and the average cross-sectional area of the crystal grains is smaller than the range of the present invention, which is smaller than the range of the present invention. The 0.2% proof stress was high in all three directions of the L direction, the D direction and the C direction.

Test No. In the comparative example of 2-29, the V content is higher than the component range of the present invention, the productivity is inferior, and the average cross-sectional area of the crystal grains is smaller than the range of the present invention, which is larger than the range of the present invention. The 0.2% proof stress was high in all three directions of the direction, the D direction and the C direction.

Test No. In the comparative example of 2-30, the Si content was higher than the component range of the present invention, and the proof stress was higher than the range of the present invention by 0.2% in all three directions of the L direction, the D direction and the C direction.

Test No. In the comparative example of 2-31, the Cr content was lower than the component range of the present invention, so that the corrosion resistance was inferior.

Test No. 2-32 is a reference example having a component composition of SUH409L standard. Test No. In 2-32, the desired corrosion resistance and 0.2% proof stress cannot be obtained.

Test No. 2-33 is a reference example having a component composition of AISI439 standard. Test No. Since 2-33 contains Cr of 15.0% by mass or more, the raw material cost and the manufacturing cost are high.

Since the ferrite-based stainless steel sheet of the present invention has excellent corrosion resistance and 0.2% proof stress equivalent to AISI439, it is used for automobile exhaust system members, rockers, parts for home appliances, building materials, kitchen equipment, railway vehicles, and electric appliances. It is suitable for parts and the like, and is particularly suitable for automobile exhaust system members such as automobile exhaust pipes, converter cases, front pipes, center pipes, mufflers, and muffler cutters. The ferritic stainless steel sheet of the present invention is particularly suitable as an inexpensive alternative steel for the members used in AISI439.

Claims (5)

By mass%
C: 0.004 to 0.020%,
Si: 0.05 to 0.90%,
Mn: 0.05 to 0.60%,
P: 0.050% or less,
S: 0.030% or less,
Al: 0.001 to 0.100%,
Cr: 13.0% or more and less than 15.0%,
Ti: 0.15-0.35%,
Nb: 0.030 to 0.090%,
A component composition containing V: 0.010 to 0.200% and N: 0.004 to 0.020%, the balance of which is Fe and unavoidable impurities.
It has a structure in which the average cross-sectional area of the crystal grains is 200 to 400 μm ^2.
A ferritic stainless steel sheet having a 0.2% proof stress in the L, D and C directions of 230 to 300 MPa. The component composition is further increased by mass%.
Ni: 0.01-0.60%,
Cu: 0.01 to 0.80%,
Co: 0.01-0.50%,
Mo: 0.01 to 1.00%, and W: 0.01 to 0.50%
The ferrite-based stainless steel sheet according to claim 1, which contains one or more selected from the above. The component composition is further increased by mass%.
Zr: 0.01-0.50%,
B: 0.0003 to 0.0030%,
Mg: 0.0005-0.0100%,
Ca: 0.0003 to 0.0030%,
Y: 0.01 to 0.20%,
REM (rare earth metal): 0.01-0.10%,
The ferrite-based stainless steel sheet according to claim 1 or 2, which contains one or more selected from Sn: 0.01 to 0.50% and Sb: 0.01 to 0.50%. The ferritic stainless steel sheet according to any one of claims 1 to 3, which is used for an automobile exhaust system member. The method for producing a ferritic stainless steel sheet according to any one of claims 1 to 4.
A hot rolling step in which a steel material having the above-mentioned composition is held at a temperature of 1100 to 1250 ° C. for 10 minutes or more, then hot-rolled to obtain a hot-rolled plate, and then wound at a winding temperature of 500 to 600 ° C. When,
A hot-rolled plate annealing step of obtaining a hot-rolled annealed plate by subjecting the hot-rolled plate after the hot-rolling step to annealing the hot-rolled plate at a temperature of 940 to 1000 ° C. for 5 to 180 seconds.
The hot-rolled annealed plate after the hot-rolled plate annealing step is cold-rolled to obtain a cold-rolled plate, and then cold-rolled annealed at a temperature of 880 to 900 ° C. for 5 to 180 seconds to obtain a cold-rolled annealed plate. A method for manufacturing a ferrite-based stainless steel plate, which comprises a cold-rolled sheet annealing step.