JP2021183708A - Ferritic stainless steel - Google Patents

Abstract

To provide a ferritic stainless steel that has reduced amounts of Cr and Mo added in a steel component and yet has an excellent balance between strength and processability and also has good corrosion resistance.SOLUTION: A ferritic stainless steel contains, in mass%, Si: 0.5% or more to less than 1.4%, Ni: 0.5% or more and 2.0% or less, Cr: 18.0% or more and 21.0% or less, and Mo: 0.3% or more and 1.0% or less. The number of surface defects on the ferritic stainless steel and having a major axis of 5 μm or more in plan view is 5 or less on average per 0.01 mm2 of the surface of the ferritic stainless steel.SELECTED DRAWING: Figure 1

Description

The present invention relates to ferritic stainless steel. More specifically, the present invention relates to a ferritic stainless steel having good corrosion resistance and high strength and workability.

Conventionally, stainless steel is suitably used as a material for products used in applications that require corrosion resistance. Products manufactured by molding and processing stainless steel may have gaps in welds and joints of assembled structures, and in order to improve the corrosion resistance of products, stainless steel with good gap corrosion resistance should be used. It is required to be used as a material. For example, Patent Documents 1 to 4 describe ferritic stainless steels having improved crevice corrosion resistance by adjusting the steel composition.

By the way, in order to reduce the cost of a product, it may be attempted to reduce the thickness and weight of various members constituting the product. In this case, (i) it is required to increase the strength of the member in order to maintain the structure of the product. Further, in order to easily form the (ii) member, good workability is required. In addition, (iii) for example, since the ratio of the pitting depth to the thickness of the member increases in the portion where pitting corrosion occurs, improvement of the corrosion resistance of the member is also required. That is, it is required to increase the strength, workability and corrosion resistance of stainless steel used as a material for products.

Further, as a ferrite-based stainless steel having high corrosion resistance, for example, SUS445J1 (22Cr-1Mo) is known. Then, in order to reduce the cost of the product, a technique has been proposed in which the amount of Cr and the amount of Mo in the steel component is reduced as compared with SUS445J1 while ensuring good corrosion resistance and strength (for example, Patent Document 5).

Japanese Unexamined Patent Publication No. 2007-270226 Japanese Unexamined Patent Publication No. 2008-231542 Japanese Unexamined Patent Publication No. 2005-220249 Japanese Unexamined Patent Publication No. 2005-89828 Japanese Patent No. 6399666

In the techniques described in Patent Documents 1 to 4, the strength and workability of ferritic stainless steel have not been investigated, and in addition to good crevice corrosion resistance, ferritic stainless steel has an excellent balance between strength and workability. There is room for consideration regarding stainless steel. Further, in the technique described in Patent Document 5, further improvement in corrosion resistance is desired.

In view of the current situation, one aspect of the present invention is to provide a ferritic stainless steel having an excellent balance between strength and workability and having good corrosion resistance while reducing the amount of Cr and Mo added to the steel component. The purpose is to provide.

In order to solve the above problems, the ferritic stainless steel according to one aspect of the present invention has Si: 0.5% or more and less than 1.4%, Ni: 0.5% or more and 2.0% in mass%. Hereinafter, Cr: 18.0% or more and 21.0% or less, and Mo: 0.3% or more and 1.0% or less are contained, and the steel extends from the surface of the ferritic stainless steel to cover a part of the surface. It is characterized in that the number of surface defects having a major axis of 5 μm or more when viewed in a plan view is 5 or ^{less per 0.01 mm 2 of the surface.}

Further, the ferritic stainless steel according to the uniform state of the present invention has C: 0.03% or less, Mn: 1.0% or less, P: 0.050% or less, S: 0.020% or less in mass%. , Cu: 2.0% or less, N: 0.030% or less, at least one of Ti and Nb in total of 10 (C + N)% or more and 0.7% or less, Al: 0.15% or less, and B: It may have a component composition containing 0.020% or less and the balance being Fe and unavoidable impurities.

Further, the ferritic stainless steel according to the uniform state of the present invention contains at least one of V, W, and Co in a total of 0.50% or less in terms of mass%, and REM (rare earth element), Mg, and Ca. The component composition may contain at least one kind in a total of 0.1% or less.

According to one aspect of the present invention, it is possible to provide a ferritic stainless steel having an excellent balance between strength and workability and good corrosion resistance while reducing the amount of Cr and Mo added in the steel component. can.

It is a micrograph which shows the surface of the ferritic stainless steel in one Embodiment of this invention. It is a figure which shows the GDS measurement result about Cr in the passive film of a ferritic stainless steel, and is the annealed pickling plate of this invention example which used the salt bath for pickling treatment, and the annealed pickling plate of the comparative example by the conventional method. , And the polishing material of the reference example whose surface has been polished, the vertical axis shows the arbitrary strength of Cr, and the horizontal axis shows the time. It is a micrograph which shows the surface of the annealed pickling plate obtained by the conventional method.

Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. The following description is intended to better understand the gist of the invention, and does not limit the present invention unless otherwise specified. Further, in the present application, "A to B" indicates that it is A or more and B or less.

Further, in the present specification, the term "stainless steel" means a stainless steel material whose specific shape is not limited. Examples of the stainless steel material include steel plates, steel pipes, strips, and the like.

The ferrite-based stainless steel according to the embodiment of the present invention will be described below by exemplifying a ferrite-based stainless steel sheet. Generally, a ferritic stainless steel sheet is manufactured by performing the steps of melting, casting, hot rolling, annealing, pickling, cold rolling, annealing, and pickling in this order.

The present inventors have reduced the amount of Cr and Mo added, added appropriate amounts of Si and Ni, and adjusted the component composition so as to maintain the ferrite phase at room temperature, thereby reducing the cost, strength and processing. An attempt was made to produce a ferritic stainless steel sheet having an excellent balance with properties and having good corrosion resistance. Specifically, a ferritic stainless steel sheet was manufactured by a general manufacturing method using a slab adjusted to a component composition as described later.

It was found that the ferritic stainless steel sheet thus obtained was inferior in actual corrosion resistance to the corrosion resistance assumed from the composition of the components. The present inventors have conducted diligent studies and found the following findings regarding the surface condition of the annealed pickling plate after annealing and pickling.

That is, when the surface of the annealed pickling plate was observed at a magnification of about 200 times with a microscope, for example, there was a surface defect extending from the surface of the ferritic stainless steel and covering a part of the surface. FIG. 3 is a micrograph showing the surface of the annealed pickling plate.

As shown in FIG. 3, the surface defect 10 is formed so as to cover a part of the surface of the annealed pickling plate (ferritic stainless steel plate), in other words, the surface defect 10 is formed on a part of the surface of the ferritic stainless steel. It is formed to cover. The surface defects 10 are an extension portion 11 that covers a part of the surface of the ferritic stainless steel so as to have a gap, and a starting end portion (connection) in which the extension portion extends from the surface substrate of the ferritic stainless steel. Part) 12 and. The extension portion 11 is composed of the same components as the surface substrate of the ferritic stainless steel sheet.

It is considered that such a surface defect 10 is generated as follows. That is, when the scale is removed by the pickling treatment, the surface substrate of the ferritic stainless steel sheet is also partially dissolved. This dissolution reaction proceeds so that the extension portion 11 remains, that is, a gap is formed between the extension portion 11 and the base material of the ferritic stainless steel sheet. In this way, the surface defect 10 is generated in the annealed pickling plate after the pickling treatment.

In such a surface defect (cover) 10, there is a gap between the extending portion 11 and the surface substrate of the ferritic stainless steel sheet. Therefore, in the annealed pickling plate, crevice corrosion may occur due to the surface defect 10. Therefore, it was found that the actual corrosion resistance of the annealed pickling plate deteriorates as described above.

Therefore, the present inventors suppressed the occurrence of crevice corrosion by reducing the number of relatively large surface defects such as those having a major axis length of 5 μm or more among the above surface defects, and ferritic stainless steel. An attempt was made to improve the corrosion resistance of the steel sheet. As a result, as will be described in detail later, the conditions of an appropriate pickling step were found, and the present invention was realized.

If the major axis of the surface defect 10 is shown in FIG. 3, it is L. Since the extension portion 11 in the surface defect 10 can be generated in a distorted shape, for example, an approximate figure (a circle consisting of an ellipse or a curve) is drawn along the outer edge of the extension portion 11 and based on the approximate figure. The length of the long axis L can be obtained.

[Surface condition of ferritic stainless steel]
FIG. 1 is a photomicrograph showing the surface of a ferritic stainless steel sheet (ferritic stainless steel) according to an embodiment of the present invention.

The ferrite-based stainless steel sheet (hereinafter referred to as the present ferrite-based stainless steel sheet) in the present embodiment has a surface defect (hereinafter referred to as a specific surface defect) having a major axis of 5 μm or more when viewed in a plan view. The average number of pieces per 0.01 mm ^{2 surface is 5 or less.} As a result, the ferritic stainless steel sheet suppresses the occurrence of crevice corrosion in the gap between the surface defect and the surface of the ferritic stainless steel, and the corrosion resistance is improved.

^{The range of 0.01 mm 2} for measuring the number of specific surface defects existing on the surface of the ferritic stainless steel sheet may be, for example, a square range of 100 μm × 100 μm.

The portion of the ferritic stainless steel sheet for measuring the number of specific surface defects may be an arbitrarily selected portion and is not particularly limited. For example, 10 points of arbitrary parts on the surface of the ferritic stainless steel sheet are measured, and the measurement results are averaged. This makes it possible to estimate the average number of specific surface defects per ^{0.01 mm 2} of the surface of the ferritic stainless steel sheet. The measurement of the specific surface defect as described above can be performed by magnifying it 200 times using, for example, an optical microscope.

[Chemical composition in steel]
The ferritic stainless steel according to the embodiment of the present invention has a steel component composition of Si: 0.5% or more and less than 1.4%, Ni: 0.5% or more and 2.0% or less, Cr in mass%. : 18.0% or more and 21.0% or less, and Mo: 0.3% or more and 1.0% or less. And, as described above, it has a surface state in which the number of specific surface defects is reduced. This makes it possible to obtain a ferritic stainless steel which is low in cost, has an excellent balance between strength and workability, and has good corrosion resistance. Hereinafter, the significance of the content of each element contained in the ferritic stainless steel will be described.

(Si)
Si (silicon) is preferably 0.5% by mass or more and less than 1.4% by mass. Ferritic stainless steel containing 0.5% by mass or more of Si has high strength and good corrosion resistance. Further, if Si is excessively contained, the ferritic stainless steel has reduced workability and toughness. Therefore, the Si content is defined as less than 1.4% by mass. A ferrite stainless steel having such a Si content has an excellent balance between strength and workability, and has good corrosion resistance.

(Ni)
Ni (nickel) is preferably 0.5% by mass or more and 2.0% by mass or less. Ferritic stainless steel containing 0.5% by mass or more of Ni is excellent in corrosion resistance and toughness of welded portions. Further, if Ni is excessively contained, the ferrite phase becomes unstable and the material cost increases. Therefore, the upper limit of the Ni content is defined as 2.0% by mass.

(Cr)
Cr (chromium) is preferably 18.0% by mass or more and 21.0% by mass or less. Ferritic stainless steel containing 18.0% by mass or more of Cr has excellent corrosion resistance, which is the same in the welded portion and the heat-affected zone. Further, if Cr is excessively contained, the workability is lowered and the material cost is raised. Therefore, the upper limit of the Cr content is defined as 21.0% by mass.

(Mo)
Mo (molybdenum) is preferably 0.3% by mass or more and 1.0% by mass or less, and more preferably 0.4% by mass or more and 0.8% by mass or less. Ferritic stainless steel containing 0.3% by mass or more of Mo has excellent corrosion resistance. Further, if Mo is excessively contained, the processability is lowered and the material cost is raised. Therefore, the upper limit of the Mo content is defined as 1.0% by mass.

Further, the ferritic stainless steel according to the embodiment of the present invention has C: 0.03% or less, Mn: 1.0% or less, P: 0.050% or less, S: 0.020% in mass%. Below, Cu: 2.0% or less, N: 0.030% or less, at least one of Ti and Nb in total is 10 (C + N)% or more and 0.7% or less, Al: 0.15% or less, B: It may contain 0.020% or less. The balance may be iron (Fe) and unavoidable impurities. Further, the balance may contain various other additive elements.

(C)
C (carbon) is preferably 0.030% by mass or less. Ferritic stainless steel containing 0.030% by mass or less of C is excellent in intergranular corrosion resistance (suppression of sensitization) and workability, and can reduce the cost required for refining.

(Mn)
Mn (manganese) is preferably 1.0% by mass or less, and more preferably 0.5% by mass or less. Excessive content of Mn destabilizes the ferrite phase and promotes the generation of corrosion origin (MnS). Therefore, the upper limit of the Mn content is defined as 1.0% by mass.

(P)
P (phosphorus) is preferably 0.050% by mass or less. Excessive content of P can degrade weldability, weld toughness, and workability. Therefore, the upper limit of the P content is defined as 0.050% by mass.

(S)
S (sulfur) is preferably 0.020% by mass or less, and more preferably 0.010% by mass or less. If S is excessively contained, the toughness of the welded portion may be lowered and the generation of corrosion origin may be promoted. Therefore, the upper limit of the S content is defined as 0.020% by mass.

(Cu)
Cu (copper) is preferably 2.0% by mass or less. Cu contributes to the improvement of corrosion resistance, but if Cu is excessively contained, the ferrite phase becomes unstable and the material cost increases. Therefore, the upper limit of the Cu content is defined as 2.0% by mass.

(N)
N (nitrogen) is preferably 0.030% by mass or less. Ferritic stainless steel containing N of 0.030 or less is excellent in intergranular corrosion resistance (suppression of sensitization) and workability, and can reduce the cost required for refining.

(Ti and Nb)
The total amount of Ti (titanium) and Nb (niobium) is preferably 10 (C + N)% or more and 0.7% by mass or less, more preferably Ti is 0.3% by mass or less and Nb is It is 0.4% by mass or less. Ferritic stainless steel containing 10 (C + N) or more of Ti and Nb is excellent in intergranular corrosion resistance (suppression of sensitization). On the other hand, if Ti is excessively contained, the workability and surface quality may deteriorate. Therefore, the upper limit of the Ti content is defined as 0.3% by mass. Further, since the processability and toughness may deteriorate if Nb is excessively contained, the upper limit of the Nb content is defined as 0.4% by mass.

(Al)
Al (aluminum) is preferably 0.15% by mass or less. Ferritic stainless steel containing 0.15% by mass or less of Al is excellent in deacidification and corrosion resistance. On the other hand, if Al is excessively contained, the surface quality may deteriorate. Therefore, the upper limit of the Al content is defined as 0.15% by mass.

(B)
B (boron) is preferably 0.020% by mass or less. If B is excessively contained, the hot workability may be deteriorated. Therefore, the upper limit of the B content is defined as 0.020% by mass.

[Other ingredients]
The ferritic stainless steel according to the embodiment of the present invention further contains at least one of V, W, and Co in a total amount of 0.5% or less in% by mass, and REM (rare earth element), Mg, and Ca. At least one of these may be contained in a total of 0.1% or less.

(V, W, and Co)
At least one of V (vanadium), W (tungsten), and Co (cobalt) is preferably 0.5% by mass or less in total. Ferritic stainless steel containing at least one of V, W, or Co in a total amount of 0.5% by mass or less is excellent in workability and toughness, and can suppress an increase in material cost.

(REM, Mg and Ca)
At least one of REM (rare earth element), Mg (magnesium) and Ca (calcium) is preferably 0.1% by mass or less in total. Ferritic stainless steel containing at least one of REM, Mg and Ca in a total amount of 0.1% by mass or less can suppress an increase in material cost.

[Manufacturing method of ferritic stainless steel]
The ferrite-based stainless steel sheet (ferritic stainless steel) in one embodiment of the present invention has a composition adjusted as described above, and the number of specific surface defects present on the surface is averaged per ^{0.01 mm 2 surface.} It is 5 or less. Such a ferrite stainless steel sheet can be manufactured by the following method.

Generally, a hot-rolled hot-rolled plate or a cold-rolled cold-rolled plate is annealed, and then the annealed plate is pickled as a next step. In this pickling step, after pretreatment with hot sulfuric acid, neutral salt electrolysis, ferric chloride, etc., this treatment is often performed with mixed acid. The annealed pickling plate obtained by using such a conventional method (hereinafter, may be referred to as a conventional method) produces a large number of the above-mentioned specific surface defects on the surface. In order to suppress the occurrence of specific surface defects, the present inventors have an action of dissolving scale and an action of melting a ferrite-based stainless steel substrate in the pretreatment and the main treatment when pickling. I thought it was important to properly adjust the balance of the stainless steel.

Here, in the pretreatment, a method using a salt bath has been conventionally known. Salt bath has a relatively strong action of dissolving the scale generated on the surface of the annealed plate. Therefore, the preliminary treatment can be performed in a relatively short time.

In the conventional method, when, for example, hot sulfuric acid or the like is used for the pretreatment, it may be possible to perform sufficient pretreatment on the scale, for example, by prolonging the treatment time. However, such a long-time pretreatment has a problem that the continuous operation of the line for producing ferritic stainless steel is limited and the production cost increases.

Therefore, in the method for producing ferritic stainless steel in the present embodiment, the scale produced by annealing is effectively modified by using a salt bath. That is, it is reformed to a scale in which Fe and Cr are easily eluted. Then, after performing a preliminary treatment using a salt bath, the main treatment is carried out by immersing in nitric acid electrolysis and nitric acid. By performing the pickling treatment in this way, the scale can be appropriately dissolved and the occurrence of the specific surface defects can be suppressed. That is, it is possible to prevent a dissolution reaction that causes the above-mentioned specific surface defects from occurring in the base material of the ferritic stainless steel. Therefore, the ferrite-based stainless steel according to the present embodiment can be suitably manufactured.

[Physical characteristics of ferritic stainless steel]
The ferrite-based stainless steel preferably has a tensile strength of 450 MPa or more and 550 MPa or less as measured according to JIS Z2291. It can be said that the ferritic stainless steel has good strength when the tensile strength is 450 Mpa or more. Further, if the ferritic stainless steel has a tensile strength of 550 MPa or less, excellent workability can be easily ensured. The lower the tensile strength of the ferritic stainless steel is set within the above range, the easier it is to reduce the thickness and weight of the ferritic stainless steel.

The ferrite-based stainless steel preferably has a forming limit drawing ratio of 2.00 or more in a deep drawing test using a general deep drawing test device (for example, a thin plate forming test device manufactured by Ericssen). The forming limit drawing ratio is an index showing the workability of ferritic stainless steel. It can be said that the ferritic stainless steel has good workability when the forming limit drawing ratio is 2.00 or more.

Further, in this ferritic stainless steel, in the constant potential immersion test, the corrosion generation potential on the surface of the ferritic stainless steel is 0.4 V (vs, Ag / AgCl; hereinafter, unless otherwise specified, the reference for the corrosion generation potential. Is Ag / AgCl) or more, and more preferably 0.45 V or more. Further, in this ferritic stainless steel, when a metal gap is formed on the surface and a constant potential immersion test is performed, the corrosion generation potential is preferably 0.35 V or more, preferably 0.4 V or more. Is more preferable. This ferritic stainless steel shows the above results in a constant potential immersion test and has good corrosion resistance.

Further, the passivation film on the surface of the ferritic stainless steel will be described below with reference to FIG. 2. FIG. 2 is a diagram showing the results of measurement of Cr in the passivation film of ferritic stainless steel using GDS (glow discharge emission spectroscopy). The graphs shown in FIG. 2 show the annealed pickling plate of the present invention using a salt bath for the pickling treatment, the annealed pickling plate of the comparative example by the conventional method, and the abrasive material of the reference example whose surface is polished. The vertical axis represents the arbitrary intensity of Cr, and the horizontal axis represents time. Note that FIG. 2 shows the results of various pickling treatments or polishing treatments using annealed plates manufactured by the same composition and method.

As shown in the examples of the present invention and the comparative example of FIG. 2, it can be seen that the annealed pickling plate of the present invention has a relatively high Cr concentration near the surface as compared with the annealed pickling plate of the comparative example. Further, in the annealed pickling plate of the present invention, the peak of the Cr concentration in the passivation film is substantially the same as the amount of Cr in the passivation film in a relatively deep part from the surface as the surface state after the pickling treatment. .. This is considered to indicate that the effect of the pickling treatment on the surface substrate of the ferritic stainless steel is small. On the other hand, in the annealed pickling plate of the comparative example, the Cr concentration in the passivation film is relatively small, suggesting that the pickling treatment has a large effect on the surface substrate of the ferritic stainless steel. The same can be said for the abrasive material shown in the reference example of FIG. 2 as in the above comparative example which has been pickled by the conventional method.

(Use)
The ferritic stainless steel in the present embodiment has high strength and excellent corrosion resistance while reducing the amount of Cr and Mo added in the steel component, and is suitable as a material for products such as building materials and pipes. Can be used. For example, it can be used as a material for forming products such as roofing materials, wall materials, building material exteriors, ducts, automobile moldings, decorative parts, fences, pipes, water layers, and water storage tanks.

The present invention is not limited to the above-described embodiments, and various modifications can be made within the scope of the claims.

An embodiment of the present invention will be described below.

[Evaluation method for ferritic stainless steel]
[Evaluation of corrosion resistance]
Regarding the corrosion resistance of ferritic stainless steel, the corrosion resistance and the crevice corrosion resistance were evaluated by the following methods.

(Evaluation of corrosion resistance on the material surface of ferritic stainless steel)
A square ferrite stainless steel sheet having a side of 10 mm was used as a test piece, and the corrosion resistance on the material surface of the ferritic stainless steel was evaluated by a constant potential immersion test.

In the constant potential immersion test, the test piece was immersed in a solution at 80 ° C. at a chloride ion concentration of 200 ppm to create an Ar degassing atmosphere. Further, the counter electrode (Pt) and the reference electrode (Ag / AgCl) were immersed and held at a constant potential for 48 hours. Then, the minimum potential at which the current value after 48 hours becomes 1 μA or more was evaluated as the corrosion generation potential (V).

(Evaluation of crevice corrosion resistance in ferritic stainless steel crevices)
A hole was formed in the center of a square ferrite stainless steel plate having a side of 30 mm, and a metal gap was formed by fixing a SUS washer to the hole using a bolt. The constant potential immersion test was used to evaluate the crevice corrosion resistance of ferrite stainless steel in the metal crevices.

In the constant potential immersion test, the test piece was immersed in a solution at 80 ° C. at a chloride ion concentration of 200 ppm to create an Ar degassing atmosphere. Further, the counter electrode (Pt) and the reference electrode (Ag / AgCl) were immersed and held at a constant potential for 48 hours. Then, the minimum potential at which the current value after 48 hours becomes 1 μA or more was evaluated as the corrosion generation potential (V).

[Evaluation of mechanical properties]
(Tensile strength)
In the evaluation of the mechanical properties of the ferritic stainless steel, a tensile test was performed according to JIS Z2291 using the No. 13B test piece specified in JIS Z2241 to evaluate the tensile strength.

(Workability)
The processability (deep drawing property) of the ferrite stainless steel was evaluated using a thin plate forming test device (manufactured by Eriksen). The test piece had a punch diameter: φ40 mm, a punch shoulder R: 6 mm, a die diameter: φ42 mm, and a die shoulder R: 6 mm. Then, the deep drawing property of the ferritic stainless steel was evaluated under the conditions of wrinkle pressing force: 10 kN and punch speed: 20 mm / min. The molding limit drawing ratio A was calculated based on the formula A = B / C, where B is the blank diameter of the test piece and C is the punch diameter.

〔Example〕
Table 1 shows the composition (mass%) of the ferritic stainless steel according to Examples 1 to 12 of the present invention. Stainless steel with each of these alloy compositions is melted by laboratory melting and hot-rolled, annealed, cold-rolled, annealed and pickled to obtain a 0.6 mmt (0.6 mm thick) annealed pickling plate. Manufactured. In the pickling of the ferritic stainless steels according to Examples 1 to 12, pickling was performed by performing salt bath and then dipping in nitric acid electrolysis and nitric acid.

[Comparative example]
In the ferrite-based stainless steels according to Comparative Examples 1 to 10 of the present invention, the content of any one or more of the components is out of the specified range of the present ferrite-based stainless steel. Numerical values outside the specified range are underlined in Table 1.

For the ferrite-based stainless steels according to Comparative Examples 8, 10 and 11 of the present invention, a conventional method was used without using a salt bath in the pickling step. In other words, the ferritic stainless steels according to Comparative Examples 8, 10 and 11 are the same as the ferritic stainless steels according to Examples 1 to 12 except for pickling in the manufacturing process. The composition of the ferrite-based stainless steel according to Comparative Example 11 is the same as that of the ferrite-based stainless steel according to Example 4.

〔result〕
Table 2 shows the test results of the average number of surface defects, tensile strength, workability, and corrosion potential in the gap between materials or metals. The criteria for determining whether or not the ferritic stainless steel has good characteristics, which is intended by the present invention, is as follows. Average number of surface defects: 5 or less, tensile strength: 450 to 550 MPa, molding limit drawing ratio: 2.00 or more, corrosion potential in materials: 0.40 V or more, corrosion potential in metal gaps: 0.45 V or more. Numerical values that deviate from these criteria are underlined in Table 2.

The ferritic stainless steels according to Examples 1 to 12 satisfy the above criteria in terms of the average number of surface defects, tensile strength, workability, and corrosion potential in the gap between materials or metals.

On the other hand, the ferrite stainless steels according to Comparative Examples 1 to 11 have the above-mentioned criteria for one or more of the average number of surface defects, tensile strength, workability, and corrosion potential in the gap between materials or metals. Did not meet. For example, in Comparative Example 2 in which the Si content is less than 0.50% by mass, the corrosion generation potential in the metal gap is less than 0.35 V, and there is a problem in corrosion resistance. Further, in Comparative Example 5 in which the Si content was 1.4% by mass or more, the above criteria were not satisfied with respect to the average number of surface defects, tensile strength, workability, and corrosion potential in the material. ..

From the above results, the ferritic stainless steel having a component composition satisfying the specified range of the present invention and further produced by performing a pickling step using a salt bath has an excellent balance between strength and workability. Moreover, it had good corrosion resistance.

Claims (3)

By mass%, Si: 0.5% or more and less than 1.4%, Ni: 0.5% or more and 2.0% or less, Cr: 18.0% or more and 21.0% or less, and Mo: 0.3%. Contains 1.0% or more and
Surface defects extending from the surface of ferritic stainless steel and covering a part of the surface and having a major axis of 5 μm or more when viewed in a plan view are per ^{0.01 mm 2 of the surface.} Ferritic stainless steel characterized by an average of 5 or less. By mass%, C: 0.03% or less, Mn: 1.0% or less, P: 0.050% or less, S: 0.020% or less, Cu: 2.0% or less, N: 0.030% Hereinafter, at least one of Ti and Nb is contained in total of 10 (C + N)% or more and 0.7% or less, Al: 0.15% or less, and B: 0.020% or less, and the balance is Fe and unavoidable. The ferritic stainless steel according to claim 1, which comprises impurities. Claimed to contain at least one of V, W, and Co in a total of 0.50% or less, and at least one of REM (rare earth element), Mg, and Ca in a total of 0.1% or less in% by mass. Item 2. The ferritic stainless steel according to Item 1 or 2.

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