JP5987821B2 - Ferritic stainless steel - Google Patents

Description

  The present invention relates to a ferritic stainless steel capable of imparting excellent corrosion resistance and low temperature toughness to a welded portion. Especially this invention is suitable for the water heater can used in a cold region.

  Cans for hot water storage of electric water heaters (hereinafter sometimes referred to as “water heater cans” in this specification) have high corrosion resistance that does not cause corrosion even when immersed in hot tap water for a long time. Is required. Speaking of high corrosion resistance material, stainless steel can be mentioned. However, a water heater can body manufactured using a general austenitic stainless steel may cause stress corrosion cracking during use. For this reason, general austenitic stainless steel is not suitable as a material for a water heater can body. Therefore, ferritic stainless steel having low corrosion corrosion sensitivity and high corrosion resistance has been conventionally used as a material for water heater can bodies.

  The water heater can has a vertically long cylindrical shape with rounded ends. A general water heater can body is manufactured by TIG welding a bowl-shaped steel member called a mirror and a cylindrical steel member called a body plate. When a water heater can is manufactured using a steel member obtained by processing ferritic stainless steel by this TIG welding method, the crystal grains become coarse at the TIG welded portion of the water heater can, and therefore, compared with the base material. As a result, low temperature toughness decreases.

  Further, a joint (SUS316L) connecting the water supply / drainage pipe and the water heater can body is also joined to the can body by TIG welding. In the welding of ferritic stainless steel and SUS316L, Cr carbonitride generated in the weld zone causes intergranular corrosion and reduces corrosion resistance. Since the corrosion part of this intergranular corrosion becomes a notch shape, stress concentrates on the front-end | tip of intergranular corrosion, and the low temperature toughness of a welding part falls further. Therefore, in cold regions, the corrosion resistance and toughness of the welded portion are insufficient, and cracks may occur in the welded portion.

  As a method for improving the toughness of the welded portion, for example, Patent Document 1 discloses ferritic stainless steel that is excellent in the toughness of the weld heat affected zone. The steel of Patent Document 1 is obtained by improving the toughness of the heat affected zone by adding Nb and V. In Patent Document 1, the toughness of the welded part of the weld without using a welding wire that causes a problem in the water heater can body. Is not mentioned.

  Patent Document 2 discloses a ferritic stainless steel for fusion welding that can impart excellent toughness to a weld. This steel secures the toughness of the weld by generating a martensite phase of 7% or more in the weld bead. However, in the weld bead, when a martensite phase is formed and a two-phase structure with a ferrite phase is formed, macrocells are formed and the corrosion resistance of the welded portion is lowered. For this reason, the steel of patent document 2 cannot be applied to the water heater can body with a severe corrosive environment.

  Patent Document 3 discloses a high corrosion resistance low-strength stainless steel capable of imparting excellent workability and toughness to a welded portion, and a welded joint manufactured using the steel. This steel has insufficient corrosion resistance due to its low Cr content. Patent Document 3 does not mention any sensitization by welding steel and SUS316L, and the steel described in Patent Document 3 is unsuitable for use as a material for a water heater can body.

Japanese Examined Patent Publication No. 63-66378 Japanese Patent No. 2733786 Japanese Patent No. 3975882

  In view of the above-mentioned problems of the prior art, the present invention is a ferritic stainless steel suitable as a material for products used by welding in cold regions, and when products are manufactured by welding, An object of the present invention is to provide a ferritic stainless steel capable of imparting excellent corrosion resistance and low temperature toughness to a welded portion.

  In order to solve the above-mentioned problems, the present inventors performed butt TIG welding on various ferritic stainless steels, investigated the corrosion resistance and low-temperature toughness of the welds, and obtained the following results.

When the fracture surface of the Charpy specimen at the weld was examined in detail, it was confirmed that coarse inclusions of TiN and Al ₂ O ₃ were present at the fracture origin and fracture surface. Addition of Co suppressed aggregation of these inclusions and improved low temperature toughness.

  By suppressing the contents of Si and Al, the coarsening of crystal grains in the weld metal was suppressed, the crystal grain size in the plate thickness direction in the weld metal was reduced, and the low temperature toughness was improved.

  The addition of Ti refined the crystal grains of the weld bead and improved the low temperature toughness of the weld. However, when Ti was contained excessively, the low temperature toughness was conversely reduced.

  About suppression of the intergranular corrosion of a welding part, the suppression effect was confirmed because content of Ti, Nb, and V became a fixed value or more in total.

  The present invention is configured based on the above results. That is, the present invention is summarized as follows.

[1] C: 0.001 to 0.020% by mass%, Si: 0.03 to 0.30%, Mn: 0.05 to 0.30%, P: 0.05% or less, S: 0 0.01% or less, Cr: more than 22.0% to 28.0%, Ni: 0.01% to less than 0.30%, Mo: 0.2 to 3.0%, Al: 0.01 to 0. 15%, Ti: 0.20 to 0.40%, Nb: 0.001 to 0.10%, V: 0.02 to 0.20%, Co: 0.01 to 0.3%, N: 0 0.001 to 0.020%, O: less than 0.0050%, satisfying the following formulas (1) to (4), the balance being Fe and inevitable impurities, ferritic stainless steel steel.
Al × O ≦ 0.0005 (1)
Ti × N ≦ 0.005 (2)
Co / Ti ≧ 0.05 (3)
Ti + Nb + V ≧ 0.30 (4)
The element symbols in the formulas (1) to (4) mean numerical values when the contents of these elements are shown in mass%.

  [2] One or two selected from Cu: 1.0% or less, Zr: 0.5% or less, W: 1.0% or less, REM: 0.1% or less, and B: 0.01% or less Ferritic stainless steel according to [1], which contains more than seeds.

  The ferritic stainless steel of the present invention can impart excellent corrosion resistance and low temperature toughness to a welded part of a product produced by welding. That is, a welded part of a product manufactured by welding using the ferritic stainless steel of the present invention is excellent in corrosion resistance and low-temperature toughness, so that the welded part is hardly cracked even in a cold region.

  Hereinafter, embodiments of the present invention will be described. In addition, this invention is not limited to the following embodiment.

  The ferritic stainless steel of the present invention is C: 0.001 to 0.020% in mass%, Si: 0.03 to 0.30%, Mn: 0.05 to 0.30%, P: 0.05. %: S: 0.01% or less, Cr: more than 22.0% to 28.0%, Ni: 0.01% to less than 0.30%, Mo: 0.2 to 3.0%, Al: 0.01 to 0.15%, Ti: 0.20 to 0.40%, Nb: 0.001 to 0.10%, V: 0.02 to 0.20%, Co: 0.01 to 0. 3%, N: 0.001 to 0.020%, O: less than 0.0050%, Al, O, Ti, N, Co, so as to satisfy the specific formulas (1) to (4) The content of Nb and V is adjusted, and the balance is Fe and inevitable impurities. First, the content of each component will be described. In the description of the content of each component, “%” means “% by mass”.

C: 0.001 to 0.020%
C is an element inevitably contained in steel. When the C content is large, the strength of the steel is improved. In order to obtain steel having sufficient strength, the C content needs to be 0.001% or more. However, when there is too much content of C, Cr carbide which reacts with Cr in a welding part may precipitate, and the corrosion resistance fall by local Cr deficiency may be caused. Moreover, when there is too much content of C, C may form inclusions, such as coarse Nb (C, N), and may reduce the low temperature toughness of a welding part. Therefore, the content of C is suitably 0.020% or less. As described above, the C content was 0.001 to 0.020%. More preferably, it is 0.002 to 0.015%.

Si: 0.03-0.30%
Si is an element useful for deoxidation. Further, Si is an element that concentrates in a temper color formed by welding to improve the protective property of the oxide film, and improves the corrosion resistance of the welded portion in a state where the temper color is formed. These effects can be obtained by making the Si content 0.03% or more. However, Si is an element that coarsens the ferrite grain size. If Si is excessively contained, the crystal grain size of the welded part is coarsened, and the low temperature toughness of the welded part is lowered. When the Si content exceeds 0.30%, the low temperature toughness of the welded portion is significantly lowered. Therefore, the Si content is set to 0.03% to 0.30%. More preferably, it is 0.05% to 0.20%.

Mn: 0.05-0.30%
Mn has the effect of increasing the strength of the steel. The effect is acquired by making Mn content 0.05% or more. However, when the Mn content is excessive, precipitation of MnS, which is a starting point of corrosion, is promoted, and the corrosion resistance of the steel is lowered. For this reason, the Mn content is suitably 0.30% or less. Therefore, the Mn content is set to 0.05 to 0.30%. More preferably, it is 0.08% to 0.25%.

P: 0.05% or less P is an element inevitably contained in steel. When P is contained excessively, weldability is lowered, and P segregated at the grain boundary promotes the cathode reaction and easily causes intergranular corrosion. In the present invention, the content of P is preferably small, and may be 0%. Therefore, in the present invention, the P content is set to 0.05% or less. More preferably, it is 0.03% or less.

S: 0.01% or less S is an element inevitably contained in steel. If the S content exceeds 0.01%, formation of water-soluble sulfides such as CaS and MnS is promoted, and the corrosion resistance of the steel is lowered. In the present invention, the content of S is preferably small and may be 0%. Therefore, the content of S is set to 0.01% or less.

Cr: more than 22.0% to 28.0%
Cr is the most important element for ensuring the corrosion resistance of stainless steel. When the Cr content is 22.0% or less, the surface layer Cr is reduced by oxidation due to welding, so that sufficient corrosion resistance cannot be obtained in the weld bead and its periphery. Here, the surface layer refers to a region of 1 to 5 μm in the depth direction from the surface. On the other hand, when Cr is contained excessively, workability and manufacturability of steel are lowered. For this reason, the content of Cr is suitably 28.0% or less. Therefore, in the present invention, the Cr content is set to more than 22.0% to 28.0%. More preferably, it is 22.2 to 26.0%.

Ni: 0.01% to less than 0.30% Ni is an element that improves the corrosion resistance of stainless steel. Ni is an element that suppresses the progress of corrosion in a corrosive environment where a passive film cannot be formed and active dissolution occurs. The effect can be obtained by making the Ni content 0.01% or more. However, when the Ni content is 0.30% or more, the workability of the steel decreases. Further, when the content of Ni increases, the manufacturing cost increases because Ni is an expensive element. Therefore, the Ni content is set to 0.01 to less than 0.30%. More preferably, it is 0.03% to 0.24%.

Mo: 0.2-3.0%
Mo is an element that promotes repassivation of the passive film and improves the corrosion resistance of stainless steel. The effect becomes more remarkable by containing Mo together with Cr. The effect of improving the corrosion resistance by Mo can be obtained by setting the Mo content to 0.2% or more. However, if the Mo content exceeds 3.0%, the strength of the steel increases excessively, and the rolling load increases, so the productivity decreases. Therefore, the Mo content is set to 0.2 to 3.0%. More preferably, it is 0.6 to 2.4%.

Al: 0.01 to 0.15%
Al is an element useful for deoxidation. In the present invention, Al is an element that forms AlN to suppress coarsening of TiN and imparts good low temperature toughness to the welded portion. This effect is obtained when the Al content is 0.01% or more. However, Al is an element that coarsens the ferrite crystal grain size together with Si. If the Al content exceeds 0.15%, the ferrite crystal grain size of the welded portion increases excessively, and the low temperature toughness of the welded portion decreases. Furthermore, it combines with O in the steel to form coarse Al ₂ O ₃ inclusions, and lowers the low temperature toughness of the weld. Therefore, the content of Al is set to 0.01 to 0.15%. More preferably, it is 0.03 to 0.12%.

Ti: 0.20 to 0.40%
Ti is an element that binds preferentially to C and N and suppresses a decrease in corrosion resistance due to precipitation of Cr carbonitride. Ti is an important element together with Nb and V in order to obtain the necessary corrosion resistance in the weld. In completing the present invention, the relatively fine needle-like Ti (C, N) precipitated after solidification of the melted part has the effect of suppressing the coarsening of crystal grains and improving the low temperature toughness of the welded part. It was issued. The effect is acquired by making content of Ti 0.20% or more. However, when the Ti content exceeds 0.40%, coarse TiN that precipitates before solidification of the melted part lowers the low temperature toughness of the welded part. Therefore, the Ti content is set to 0.20 to 0.40%. More preferably, it is 0.22 to 0.35%.

Nb: 0.001 to 0.10%
Nb is an element that preferentially binds to C and N, suppresses a decrease in corrosion resistance due to precipitation of Cr carbonitride, and improves the corrosion resistance of the welded portion. Nb combines with C and N at a temperature lower than Ti to form inclusions. Since the cooling rate after welding increases as the temperature increases, the cooling rate is too high in the Ti precipitation temperature range, and it is difficult to make the C and N dissolved in solid form harmless as an inclusion. Therefore, in order to completely suppress the intergranular corrosion due to the precipitation of Cr carbonitride by adding only Ti, it is necessary to add an excessive amount of Ti. When an excessive amount of Ti is added, coarse TiN is formed in the weld and low temperature toughness is reduced. Therefore, in the present invention, the amount of Ti added is within an appropriate range, a small amount of Nb is added, and C and N in the weld zone are divided into two temperature ranges, a Ti precipitation temperature region and a Nb precipitation temperature region. Efficiently detoxifying, achieving both corrosion resistance and low temperature toughness of the weld. The effect is acquired by making Nb content 0.001% or more. If the Nb content exceeds 0.10%, the Nb carbonitride that adheres to TiN and precipitates increases, and TiN, which tends to become coarse, becomes coarser precipitates and lowers the low-temperature toughness of the weld. Let Therefore, the Nb content is set to 0.001 to 0.10%. More preferably, it is 0.01 to 0.08%.

V: 0.02 to 0.20%
V is an important element that improves the low temperature toughness of the weld. In addition, when V forms N and VN, the amount of N bonded to Ti is reduced, and the formation of coarse TiN is suppressed. The effect is obtained when the V content is 0.02% or more. However, if the V content exceeds 0.20%, the workability decreases. Therefore, the content of V is set to 0.02 to 0.20% or less. More preferably, it is 0.03 to 0.15%.

Co: 0.01 to 0.3%
Co is an important element that suppresses aggregation of coarse inclusions such as TiN and improves the low temperature toughness of the weld. The effect can be obtained by setting the Co content to 0.01% or more. If the Co content exceeds 0.3%, the productivity decreases. Therefore, the content of Co is set to 0.01 to 0.3%. More preferably, it is 0.02 to 0.1%.

N: 0.001 to 0.020%
N is an element that is inevitably contained in steel like C, and has the effect of increasing the strength of the steel by solid solution strengthening. The effect is acquired by making N content 0.001% or more. However, Cr nitride precipitated by containing N excessively may reduce the corrosion resistance of the weld. Further, the excessive N content promotes the coarsening of TiN and lowers the low temperature toughness of the welded portion. For this reason, the N content is suitably 0.020% or less. Therefore, in the present invention, the content of N is set to 0.001 to 0.020%. More preferably, it is 0.002 to 0.015%.

O: Less than 0.0050% O is an element inevitably contained in steel. In the present invention, O combines with Al in the steel to form coarse Al ₂ O ₃ inclusions and lowers the low temperature toughness of the weld. The effect becomes significant when the O content is 0.0050% or more. Therefore, the content of O is set to less than 0.0050%. More preferably, it is less than 0.0030%.

  The balance other than the above components is Fe and inevitable impurities.

In the ferritic stainless steel of the present invention containing the above essential components, the contents of Al, O, Ti, N, Co, Nb and V are adjusted so as to satisfy the following formulas (1) to (4). .
Al × O ≦ 0.0005 (1)
Ti × N ≦ 0.005 (2)
Co / Ti ≧ 0.05 (3)
Ti + Nb + V ≧ 0.30 (4)
The element symbols in the formulas (1) to (4) mean numerical values when the contents of these elements are shown in mass%.

Al × O ≦ 0.0005, Ti × N ≦ 0.005
A variety of ferritic stainless steels were subjected to Charpy tests on welds, and the fracture surfaces of Charpy specimens were investigated in detail for those with excellent and inferior low-temperature toughness. It became clear that coarse inclusions of TiN and Al ₂ O ₃ were present at the starting point and fracture surface. Therefore, it was considered that these inclusions caused the low temperature toughness of the welded portion. Since both TiN and Al ₂ O ₃ start to precipitate from the molten steel stage, they become coarse inclusions with a particle size of several μm. In order to suppress the precipitation of this coarse inclusion, it is important to suppress the precipitation in the molten steel as much as possible by setting the product of the concentration of Ti and N and the product of the concentration of Al and O to a certain value or less, respectively. . When Al × O ≦ 0.0005 for Al ₂ O _{3 and} Ti × N ≦ 0.005 for TiN, the number of coarse inclusions decreased, and the low temperature toughness of the welded portion was greatly improved. Therefore, Al × O ≦ 0.0005 and TiN ≦ 0.005. More preferably, Al × O ≦ 0.0003 and Ti × N ≦ 0.003.

Co / Ti ≧ 0.05
When the fracture surface of the material inferior to the low temperature toughness of the welded portion was observed in more detail, a tendency for the fracture surface unit to increase relatively was confirmed in the TiN aggregated region. This is probably because coarse TiN aggregates into clusters, which further facilitates the propagation of cracks, thus increasing the number of fracture surface units. Therefore, it is considered that low temperature toughness is improved by suppressing aggregation of TiN. Although the tendency to eliminate the aggregation of TiN was confirmed by the addition of Co, the steel added with a large amount of Ti tended to have a small effect in eliminating the aggregation of TiN when the addition amount of Co was small. Therefore, when the coagulation state of TiN was investigated with respect to the Co content and the Ti content, when Co / Ti ≧ 0.05, the aggregation of TiN was eliminated and the number of TiN clusters decreased. Therefore, Co / Ti ≧ 0.05. More preferably, Co / Ti ≧ 0.07.

Ti + Nb + V ≧ 0.30
The low temperature toughness of the welded portion is greatly reduced when intergranular corrosion occurs in the welded portion. This is because the intergranular corrosion portion has a notch shape, causing stress concentration and facilitating breakage. Therefore, in order to ensure low temperature toughness, it is preferable that intergranular corrosion of the welded portion does not occur. The occurrence of intergranular corrosion in the weld is caused by the precipitation of Cr carbonitride due to the combination of Cr, C, and N. Ti, Nb, and V are elements that have an affinity for C and N that is stronger than that of Cr, and bind to C and N to suppress the formation of Cr carbonitride. By adding these elements, intergranular corrosion of the weld can be suppressed. On the other hand, when these elements are added excessively, the low temperature toughness of the welded portion is lowered. Therefore, the upper limit of the content of each element is determined from the request for low temperature toughness, but it is necessary to contain a certain amount of Ti, Nb, and V in total in order to suppress intergranular corrosion of the weld. . As a result of the investigation, an effect of suppressing the intergranular corrosion of the welded portion was obtained when Ti + Nb + V ≧ 0.30. Therefore, Ti + Nb + V ≧ 0.30. More preferably, Ti + Nb + V ≧ 0.35.

  In the ferritic stainless steel of the present invention, the desired characteristics are obtained by adjusting the content of the specific essential elements so as to satisfy the formulas (1) to (4), including the above essential elements. . Further, the ferritic stainless steel of the present invention may contain the following elements as optional components according to desired characteristics.

Cu: 1.0% or less Cu is an element that improves the corrosion resistance of stainless steel, and the effect can be obtained by making the Cu content 0.01% or more. However, if Cu is contained excessively, the passive maintenance current is increased to make the passive film unstable, and the corrosion resistance of the steel may be lowered. Therefore, when Cu is contained, the content of Cu is preferably 1.0% or less.

Zr: 0.5% or less Zr combines with C and N and has an effect of suppressing sensitization. The effect is obtained by making the Zr content 0.01% or more. However, when Zr is contained excessively, the workability of the steel is lowered. Further, since Zr is an extremely expensive element, an increase in the content of Zr causes an increase in cost. Therefore, the Zr content is preferably 0.5% or less.

W: 1.0% or less W, like Mo, has the effect of improving corrosion resistance. The effect is obtained by making the W content 0.01% or more. However, when W is contained excessively, the strength of the steel increases and the productivity decreases. Therefore, the W content is preferably 1.0% or less.

REM: 0.1% or less REM improves the oxidation resistance of steel, suppresses the formation of oxide scale, and suppresses the formation of a Cr-deficient region immediately below the temper collar generated during welding. Among REMs, La and Ce are particularly effective. The effect of suppressing the formation of the Cr-deficient region can be obtained by setting the REM content to 0.0001% or more. However, when REM is contained excessively, productivity, such as pickling property, will fall. Moreover, since REM is expensive, when the content of REM increases, cost increases. Therefore, the content of REM is preferably 0.1% or less.

B: 0.01% or less B is an element that improves secondary work brittleness, and in order to obtain the effect, the content of B is suitably 0.0001% or more. However, when B is contained excessively, ductility is lowered due to solid solution strengthening. Therefore, the content of B is preferably 0.01% or less.

  The stainless steel of the present invention may be manufactured using any manufacturing method, but an example of a preferable manufacturing method is shown below.

  After heating the slab (plate thickness 180-300 mm) having the above composition to 1100 ° C.-1300 ° C., the finishing temperature is 700 ° C.-1000 ° C., the winding temperature is 400 ° C.-850 ° C., and the plate thickness is 2.0 mm- Hot rolling is performed to 5.0 mm. The hot-rolled steel sheet thus produced is annealed (hot-rolled sheet annealing) at a temperature of 800 ° C. to 1000 ° C., pickled, then cold-rolled, and cold-rolled sheet annealed at a temperature of 700 ° C. to 1000 ° C. I do. After cold-rolled sheet annealing, pickling is performed to remove scale. The cold-rolled steel sheet from which the scale has been removed may be subjected to skin pass rolling.

  Hereinafter, the present invention will be described based on examples.

  A 200 mm thick slab obtained by vacuum melting stainless steel having the composition shown in Table 1 is heated to 1200 ° C, hot rolled at a finishing temperature of 800 ° C and a coiling temperature of 450 ° C, at 950 ° C. Hot-rolled sheet annealing was performed, and the scale was removed by pickling. Furthermore, it cold-rolled to plate thickness 0.8mm, cold-rolled sheet annealing was performed at 900 degreeC, pickled, and it was set as the test material.

  The produced specimen was butt welded with SUS316L. Welding was TIG welding, the welding current was 90 A, and the welding speed was 60 cm / min. No welding wire is used. The welding position was adjusted so that the penetration ratio of SUS316L into the weld bead was approximately 50% by mass. As the shielding gas, 100% Ar gas was used for both the front side (torch side) and the back side, and the flow rate was 15 L / min on the front side and 10 L / min on the back side. The width of the front side weld bead was approximately 4 to 5 mm.

A Charpy impact test piece having a 2 mm V-notch in the center of the weld bead of the manufactured weld was prepared, and a Charpy impact test in accordance with JIS Z 2242 was performed. The test temperature was −25 ° C. The results are shown in Table 1. When the Charpy impact value at −25 ° C. was 15 J / cm ² or more, it was determined to be very good low temperature toughness at 10 J / cm ² or more. No. which is an example of the present invention. 2, 3, 4, 6, 7, 9, 12, 13, 15, 16 and comparative example No. No. 18, the low temperature toughness of the welded portion was very good. No. 1, 5, 8, 10, 11, 14 and No. of Comparative Examples. No. 26, the low temperature toughness of the welded portion was good. No. In 17, 19 to 25, 27, and 28, the formula represented by the component range or component content deviated from the scope of the present invention, and the low-temperature toughness was below the standard.

  A test piece of 60 × 80 mm including the welded portion was collected, the unevenness of the scale and weld bead formed by welding was removed by polishing, finish-polished using # 600 emery polishing paper, and conforming to JIS H8502 The corrosion resistance of the weld was evaluated by a neutral salt spray cycle test. Table 1 shows the case where corrosion occurred in the 10-cycle test as “x” and the case where corrosion did not occur as “◯”. No. Corrosion occurred at 18, 19, 23 and 26, which failed. In either case, the expression expressed by the component or component content deviated from the scope of the present invention, so that the corrosion resistance of the welded portion became insufficient.

  If the ferritic stainless steel of the present invention is used, excellent corrosion resistance and low temperature toughness can be imparted to a welded part of a product manufactured by welding that can be used even in a cold region. The ferritic stainless steel obtained by the present invention is suitable for application to an outdoor use, for example, a hot water storage can material for an electric water heater, etc., in which a structure is produced by welding.

Claims (2)

C: 0.001 to 0.020% by mass%, Si: 0.03 to 0.30%, Mn: 0.05 to 0.30%, P: 0.05% or less, S: 0.01% Hereinafter, Cr: more than 22.0% to 28.0%, Ni: 0.01% to less than 0.30%, Mo: 0.2 to 3.0%, Al: 0.01 to 0.15%, Ti: 0.20 to 0.40%, Nb: 0.001 to 0.10%, V: 0.02 to 0.20%, Co: 0.01 to 0.3%, N: 0.001 A ferritic stainless steel containing 0.020%, O: less than 0.0050%, satisfying the following formulas (1) to (4), and the balance being Fe and inevitable impurities.
Al × O ≦ 0.0005 (1)
Ti × N ≦ 0.005 (2)
Co / Ti ≧ 0.05 (3)
Ti + Nb + V ≧ 0.30 (4)
The element symbols in the formulas (1) to (4) mean numerical values when the contents of these elements are shown in mass%.   One or more selected from Cu: 1.0% or less, Zr: 0.5% or less, W: 1.0% or less, REM: 0.1% or less, and B: 0.01% or less The ferritic stainless steel according to claim 1, which is contained.