JP3485591B2 - Ferritic stainless steel for exhaust gas flow path components and manufacturing method - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ferritic stainless steel used as an exhaust gas flow path constituting member for an automobile muffler and the like, and a manufacturing method thereof.

[0002]

2. Description of the Related Art Corrosion of an automobile muffler is roughly divided into two factors. One of them is external salt damage in which corrosion progresses from the outside due to chlorine ions in the atmosphere and snow melting salts in winter. The other is internal corrosion in which corrosion progresses from the inside due to condensed water generated by condensation of exhaust gas.
The most problematic of these is internal corrosion that progresses from inside the muffler. In this internal corrosion, corrosive ions such as chlorine ions and sulfate ions contained in the condensed water are heated and concentrated by the gas while the vehicle is running, and the corrosiveness is enhanced. As a result, the pitting corrosion generated in the material grows significantly, leading to piercing. A hot-dip Al-plated steel sheet has been conventionally used as an exhaust gas flow path constituent member such as a muffler. However, the composition and pH of the condensed water change due to the switching of the exhaust gas purification system, which is seen due to the change from the oxidation catalyst to the three-way catalyst, and internal corrosion becomes apparent even when a hot dip Al plated steel sheet is used. Therefore, 13Cr type stainless steels such as SUS410L and SUH409L, which are more excellent in corrosion resistance, have come to be used.

Since stainless steel has excellent corrosion resistance and heat resistance, it is a suitable material for automobile exhaust gas flow path constituent members. However, depending on the grade of material and the environment in which it is used, corrosion may progress, leading to hole formation. The causes of such corrosion and perforation include the rise of exhaust gas temperature and the composition of exhaust gas passing through the three-way catalyst. Especially in the current situation where the warranty period of exhaust gas components tends to be extended,
The existing 13Cr stainless steel has insufficient corrosion resistance, and it is required to develop a material having excellent corrosion resistance. Exhaust gas temperature tends to rise as engine output increases, and recently the maximum temperature reached by the muffler is 50.
Some even reach around 0 ° C. The muffler exposed to a high temperature atmosphere by heating with exhaust gas is Cr that causes intergranular corrosion.
Carbide tends to precipitate in the weld. As a solution to this problem, the present inventors have developed a steel having good intergranular corrosion resistance of the welded portion even after heating, and disclosed in Japanese Patent Application No. 3-133.
Filed as No. 321.

The rise in exhaust gas temperature promotes the oxidation inside the muffler and causes the substantial corrosion resistance of the material to deteriorate due to the deterioration of the film. That is, Cr in the passivation film formed on the surface of the material is oxidized by heating with the exhaust gas, and the corrosion resistance decreases due to the deficiency of the Cr concentration in the surface layer. Furthermore, in the ion species contained in the condensed water of the exhaust gas that has passed through the three-way catalyst, there are sulfite ions that are reducing ions in addition to conventionally known chlorine ions and sulfate ions. Sulfite ions reduce the breakdown of the passivation film of stainless steel, and significantly deteriorate the corrosion resistance. The present inventors have developed a steel material with improved corrosion resistance by forming a film in which Al is concentrated on the surface layer as a flow path constituent member used in the atmosphere of an exhaust gas purification system incorporating a three-way catalyst, The application was filed as Japanese Patent Application No. 4-101652. The proposed ferritic stainless steel is composed of Nb, Ti,
Combined addition of Al, and annealing and pickling Al on the steel surface
It forms a concentrated layer.

[0005]

[Patent Document 1] Japanese Patent Application No. 4-10165
The No. 2 ferritic stainless steel is softened at a level at which corrosion resistance is not impaired by methods such as limiting the upper limits of Cr and Mo contents in consideration of workability as an exhaust gas member. However, when compared with conventional steel materials, an increase in hardness and proof stress due to high alloying is inevitable. Therefore, in order to avoid the occurrence of defects such as work cracking and springback due to the material becoming hard, it is necessary to perform work using a mold different from the conventional one. Exhaust gas flow path constituent members include not only plate materials such as shells, top plates, and end plates, but also pipe materials such as front tubes, center tubes, and tail tubes.
Since various processing such as bending, expanding, and contracting is performed on the pipe material, good high-frequency pipe forming property that does not cause cracking or necking in the welded part after processing is required. In addition, the workability of the pipe largely depends on the toughness of the heat-affected zone of the welding, and generally tends to conflict with the high alloying for improving the corrosion resistance.

Further, the use of high-grade materials for exhaust system members such as mufflers results in an increase in the manufacturing cost of automobiles, so that lower cost materials are desired. The present invention has been devised to solve such a problem, and by forming a passive film in which Si is concentrated in the surface layer, in an exhaust gas condensation environment containing highly corrosive ions such as sulfite ions. , While exhibiting excellent corrosion resistance even after being heated by exhaust gas,
An object of the present invention is to provide a new ferritic stainless steel for exhaust gas flow path constituent members, which also satisfies high-frequency pipe forming properties, at a relatively low cost.

[0007]

In order to achieve the object, the ferritic stainless steel for exhaust gas flow path constituting members of the present invention is C: 0.02 wt% or less, Si: 0.3-1.
5% by weight, Mn: 1.0% by weight or less, P: 0.01 to
0.2 wt% or less, S: 0.01 wt% or less, Ni:
0.6% by weight or less, Cr: 11 to 23% by weight, Nb:
0.1 to 1.0% by weight, N: 0.02% by weight or less and the balance Fe, and C + N ≦ 0.03% by weight and Nb ≧ 7 (C + N) -0.01 (Cr-12). +0.
The passivation film of Cr satisfying the relationship of 15 and formed on the surface has a Si enriched layer, and the most S of the Si enriched layer is present.
It is characterized in that the Si concentration in the portion where i is concentrated is at least twice the Si content of the material.

This ferritic stainless steel contains Mo: 2.5% by weight or less, Cu: 1.0% by weight or less, Ti: 0.3% by weight or less and Al: 0.3% by weight or less. It may contain one kind or two or more kinds. In this case, Nb + Ti ≧ 7 (C + N) -0.01 (Cr-1
2) Satisfies the condition of +0.15. The passive film in which Si is concentrated has a temperature of 950 to 1050 ° C. and a dew point of −30 to − after rolling or polishing a ferritic stainless steel material.
It is formed by bright annealing finish in the range of 60 ° C.

[0009]

The present inventors have made various studies on the influence of alloying elements and surface finish on the corrosion resistance of materials in the environment of automobile muffler, that is, in the exhaust gas condensation environment containing corrosive ions such as chlorine ions after being heated by exhaust gas. At the same time, a detailed study on workability, high-frequency pipe forming property, etc. was conducted. As a result, it was clarified that the corrosion resistance was significantly improved by reaching the alloy design as described above and forming the passivation film having the Si enriched layer. That is,
By further developing the corrosion resistance improvement by the Al-enriched film of the prior application / Japanese Patent Application No. 4-101652, and by applying bright annealing, a Si-enriched film was formed on the surface layer of the Cr passivation film that constitutes the steel surface. It has been found that when formed, the corrosion resistance after heating is improved, and when a certain amount of P is contained in the specified alloy design, the corrosion resistance is further improved.

Regarding the influence of the surface finish on the pitting potential after heating, the pitting potential after heating the bright annealing finish material and the pickling finish material at each temperature for 2 hours is summarized by the heating temperature and compared with FIG. And show it. The pitting corrosion potential is an index indicating the degree of pitting corrosion generation, and if the steel material is maintained at a potential lower than the pitting corrosion potential, pitting corrosion does not occur. The pickling finishing material used in the conventional exhaust gas flow path constituent members shows a good pitting potential in the condition without heating, as shown by white circles,
The heating significantly reduces the pitting potential. The pitting potential is
Especially, when it is heated to a high temperature of 400 ° C. or higher, it is significantly decreased. On the other hand, the bright annealed finish according to the present invention has a small degree of decrease in the pitting potential after heating, as indicated by black circles. From this, it can be understood that the bright annealed finish material has little deterioration in corrosion resistance even after being heated.

The growth of corrosion is represented by a current density at which active dissolution is maximized in the anodic polarization curve, that is, a maximum current density. Therefore, when the influence of the Cr content on the maximum current density was investigated, it was found that the relationship between them was as shown in FIG. Maximum current density has a Cr content of 1
If it is less than 1% by weight, the stability of the passive film is insufficient,
Remarkably high value. In the region where the Cr content is 11 to 13% by weight, the maximum current density changes greatly. On the other hand, in the region where the Cr content is 13% by weight or more, the maximum current density shows a substantially stable value corresponding to the passivation maintaining current. FIG. 2 also shows the maximum current densities of materials having different P contents at the 12 wt% Cr level. A material with a high P content of 0.05% by weight has a lower maximum current density than a material with a normal P content of 0.025% by weight, and has a bright annealed finish. It can be seen that P makes some contribution to the suppression of corrosion growth in the material.

FIG. 3 shows the results of surface film analysis by GDS for each sample whose pitting potential was measured in FIG. The pickling finish has no elemental concentration on the surface under the condition without heating, but in the heated condition, it is caused by oxidation of Cr.
An r-deficient layer is detected. On the other hand, in the bright annealed finish material, a film in which Si is concentrated is formed on the surface layer without heating, and it is understood that the degree of Cr deficiency is small due to the presence of the Si concentrated layer even after heating. When the influence of the surface finish on the cathode polarization characteristics was investigated, the relationship of FIG. 4 was established between the potential and the cathode current density. From FIG.
It can be seen that the bright annealing finish material according to the present invention has a significantly smaller cathode reaction than the pickling finish material used as a conventional exhaust gas flow path constituent member.

From the above results, the bright annealing finish material not only suppresses the occurrence of pitting corrosion, but also suppresses the growth itself of the generated pitting corrosion, and exhibits the effect of suppressing the corrosion from both aspects of the occurrence and the growth. ing. The corrosion inhibition effect of bright annealing is
It is assumed that this is due to the following reasons. (1) When a steel material is annealed in a non-oxidizing atmosphere, Si is preferentially oxidized, and a film enriched with Si is formed on the surface. The formed film is resistant to heat and prevents the passivation film formed on the stainless steel surface from being destroyed. (2) The Si concentrated layer formed on the surface layer has an insulating property and suppresses the growth of corrosion due to the cathode reaction. (3) The wettability of the surface of the steel material is reduced by the Si concentrated layer, and the effect of preventing dew condensation is exhibited under the condition that the exhaust gas actually condenses.

The effect of each alloying element on the corrosion resistance and workability of the bright annealed finish will be described. As a result of examining the corrosion resistance of various brightly annealed stainless steels for exhaust gas flow path constituent members after heating, it was found that the corrosion resistance is improved by containing a certain amount of P as shown in FIG. The relationship between the addition of P and the improvement of corrosion resistance is not clear at this stage, but P acts as phosphoric acid and is produced in the same way as a film of Si or Al during bright annealing, which is harmful to corrosion resistance. It is presumed that the film is preferentially dissolved. If a large amount of Ti is added, the surface of the steel material is nitrided during bright annealing, and the workability is impaired.

The condition of the bright annealing is set in order to adjust the composition of the film formed on the surface layer. Annealing temperature is 950 ° C
If the temperature is less than or below -30 ° C, a film in which Cr is oxidized is formed on the surface of the steel material, and corrosion resistance deteriorates. On the other hand, if the annealing temperature exceeds 1050 ° C., the crystal grains become coarse and the workability deteriorates. Further, it is practically difficult to set the dew point to −60 ° C. or lower, which causes an increase in manufacturing cost. As described above, in the present invention, the corrosion resistance in the exhaust gas dew condensation environment, that is, the corrosion resistance after being heated is improved by bright annealing, and the lower Cr content and the lower Mn content of the base material are improved according to the improvement of the corrosion resistance. We are aiming to improve workability and reduce costs. Further, the workability after bright annealing is improved by regulating the Cr content and the Ti content, and the corrosion resistance after bright annealing is improved by adding an appropriate amount of P.

The alloying elements contained in the ferritic stainless steel of the present invention and their contents will be described below. C, N: Elements inevitably contained in stainless steel. When the C content and the N content are reduced, the stainless steel becomes soft and the workability is improved. Further, as the C content and the N content are reduced, the generation of carbides, nitrides, etc. is reduced, and the weldability and the corrosion resistance of the welded portion are improved. Therefore, C and N are preferably low, and C + N ≦ 0.0.
The upper limit of the C content is 0.02% by weight under the condition of 3% by weight,
The upper limit of the N content was set to 0.02% by weight, respectively.

Si: An alloying element that plays an important role in the present invention, which forms a Si-enriched film after bright annealing and improves the corrosion resistance after heating. The effect of Si is that the content is 0.
It becomes remarkable at 3% by weight or more. However, the Si content is 1.
If it exceeds 5% by weight, the obtained stainless steel is hardened, which causes deterioration of workability and toughness of the welded portion. Further, the Si concentrated layer is formed deep in the surface layer, and the effect of the Cr passivation film is impaired. Therefore, in the present invention, the Si content is set in the range of 0.3 to 1.5% by weight. Mn: Combines with a small amount of S contained in stainless steel,
It produces a soluble sulfide MnS and reduces corrosion resistance. In addition, it forms a Mn-based film during bright annealing, and inhibits the formation of a Si-enriched film. In this respect, the lower the Mn content, the more preferable. In the present invention, the upper limit of the Mn content is defined as 1.0% by weight.

P: An alloying element effective in improving the corrosion resistance of a steel material having a Si-enriched film formed by bright annealing after heating. The action of P is 0.01% by weight of normal P.
It appears at the level, but it is 0.04 to obtain sufficient effect.
A P content of not less than wt% is preferable. However, 0.2% by weight
If it exceeds, the adverse effect due to the P content appears, and the toughness and workability of the base material and the welded portion deteriorate. Therefore, in the present invention, the P content is set in the range of 0.01 to 0.2% by weight. S: A harmful element that adversely affects the corrosion resistance and the hot cracking of the weld, and the lower the S content, the more preferable. In the present invention, the upper limit of the S content is defined as 0.01% by weight. Ni: An alloying element effective in improving the toughness of ferritic stainless steel. However, containing a large amount of Ni not only increases the cost but also hardens the stainless steel. Therefore, in the present invention, the upper limit is set to 0.6% by weight, which is regulated by ordinary ferritic stainless steel.

Cr: A main alloying element that forms a passivation film on the surface of stainless steel, and significantly improves the corrosion resistance of the base material and the welded portion. This effect is similarly exhibited for the steel material subjected to bright annealing, but the Cr content is 11% by weight.
Less than less. Unlike the case of the raw material, by performing the bright annealing, the corrosion resistance under the sulfite ion environment is maintained even with the Cr content of 19% by weight or less. However,
If a large amount of Cr exceeding 23% by weight is contained, the material becomes hard due to solid solution strengthening, and it becomes difficult to process a muffler or the like. In this respect, a Cr content of 19% by weight or less is preferable for processing under the same conditions as conventional stainless steel. Therefore, the Cr content in the present invention is 11 to 11.
The range was set to 23% by weight, preferably 11 to 19% by weight.

Nb: An alloying element indispensable for preventing intergranular corrosion which is a problem in C-level ferritic stainless steel specified in the present invention. Nb is C and N
It has a strong effect of fixing the steel and improves intergranular corrosion resistance, aluminum plating property, high-frequency pipe forming property, and the like. In this respect, it is difficult for Ti-added steel to define a range that satisfies all of intergranular corrosion resistance, aluminum plating property, and high-frequency pipe forming property. The action of Nb becomes remarkable when the content is 0.1% by weight or more. But,
When a large amount of Nb exceeding 1.0% by weight is contained, there is an adverse effect of impairing the toughness of the welded portion. Therefore, 0.
The Nb content was set in the range of 1 to 1.0% by weight.

In addition to the above alloying elements, the ferritic stainless steel according to the present invention may contain Mo, Cu,
One or more of Ti and Al may be included. It is an alloying element that effectively acts together with Mo: Cr to improve the corrosion resistance of stainless steel, and effectively contributes to the improvement of the corrosion resistance in a corrosive environment containing sulfite ions such as exhaust gas condensed water. However, bright annealed stainless steel is not an essential alloying element but can be treated as an optional component added as necessary. However, M exceeding 2.5% by weight
When the content is o, the stainless steel is hardened and the toughness of the welded portion is reduced. As a result, it becomes difficult to process the muffler material, especially as a pipe, and the productivity is reduced.

Cu: An alloying element that prevents deterioration of the toughness of the base material and the welded portion, improves the high-frequency pipe forming property, and acts extremely effectively on the workability of the pipe. It also has the effect of making the film base and suppressing the formation of a strong passive film. However, if Cu is excessively contained, the toughness of the stainless steel decreases due to the solid solution strengthening ability of Cu. Also,
When the exhaust system member is heated to a temperature level of about 500 ° C., it becomes brittle due to the formation of an intermetallic compound such as a Cu-rich phase. Therefore, when Cu is contained, the upper limit of the content is set to 1.0% by weight.

Since the composite addition of Ti: Al easily forms an Al-based coating effective for preventing Cr oxidation loss on the surface of stainless steel, and exhibits an effect of preventing deterioration of corrosion resistance after heating. -It was an essential alloying element in Japanese Patent Application No. 4-101652. However, in the present invention,
Since the improvement of corrosion resistance depends on the Si concentrated layer after bright annealing, it is not always necessary. Ti fixes S and prevents the deterioration of pitting corrosion resistance due to the formation of MnS, and C,
Fix N to prevent intergranular corrosion. However, excess Ti
The inclusion forms cluster-like inclusions TiN and causes surface defects when the stainless steel material is rolled or processed. Further, as the Ti content increases, the toughness of the weld becomes poor and the high-frequency pipe forming property deteriorates. Therefore, when Ti is contained, the upper limit of the content is set to 0.3% by weight on the assumption that the workability is not deteriorated.

Similar to Al: Ti, prior application / Japanese Patent Application No. 4-10
Although it was an essential alloying element in No. 1652, it is an optional alloying element in the stainless steel of the present invention in which the corrosion resistance is improved by the Si concentrated layer after bright annealing. Al is added because it is effective as a deoxidizing agent. However, if the Al content exceeds 0.3% by weight, defects such as pinholes are likely to occur during pipe forming, and high-frequency pipe forming properties deteriorate. Therefore, when Al is added, the upper limit of the content is set to 0.3% by weight.

In addition to the above-mentioned specification of the content of the alloy component, in the present invention, the relational expression (1) is further established among C, N, Cr and Nb. Nb ≧ 7 (C + N) -0.01 (Cr-12) +0.15 ... (1) The relational expression (1) is obtained by an experiment by the inventors of the present invention and is performed under exhaust gas condensation environment. It is an index for obtaining the amount of the immobilizing element necessary to secure the intergranular corrosion resistance in the welded part. Nb is 7 (C + N) -0.01 (C
If it is less than r-12) +0.15, the weld becomes sensitized by heating with exhaust gas while the vehicle is running, and the susceptibility to intergranular corrosion increases. To prevent this, set Nb to 7 (C + N)-
It is necessary to make it 0.01 (Cr-12) +0.15 or more. The expression (1) is changed to the expression (2) for the ferritic stainless steel containing Ti. Nb + Ti ≧ 7 (C + N) -0.01 (Cr-12) +0.15 ... (2)

[0026]

Example A stainless steel sheet having a chemical composition shown in Table 1 was melted and hot-rolled to produce a hot-rolled sheet having a thickness of 3.5 mm. Cold rolling the hot-rolled sheet to a sheet thickness of 1.0 mm,
Bright annealing was performed at 0 to 1100 ° C. The steel materials of Group A in Table 1 are ferritic stainless steels satisfying the conditions specified in the present invention, and all have Nb as an immobilizing element.
Is included. In addition, some steels contain Mo, Cu, Ti.
And one or more of Al are added. On the other hand, B
The steel materials of the group are ferritic stainless steels that do not satisfy the conditions specified in the present invention. B1 is a pickling finish material for a Ti-only addition steel containing Cr and P in an amount outside the range specified in the present invention. B2 is steel which is bright annealed, although P out of the base material components is out of the conditions specified in the present invention. B3 corresponds to a high-Cr steel in which the corrosion resistance is improved by the base metal component regardless of bright annealing.

[Table 1]

Test pieces were cut out from the annealed material and subjected to various tests. Further, as a comparative material, an electrolytic treatment at a current density of 2.5 A / dm ² in 5% nitric acid held at 60 ° C. and a mixed acid solution of 6% nitric acid-0.5% hydrofluoric acid held at 55 ° C. The one subjected to the pickling treatment including the dipping treatment was used.
The results of investigating the characteristics of each test piece are shown in Table 2. The evaluation of wet corrosion resistance is performed by using an anode polarization curve in a corrosive liquid simulating a liquid composition in which the condensed water in the muffler is concentrated by evaporation, in which the current density when the active dissolution becomes maximum, that is, the maximum current density did.

For workability, the proof stress measured by a tensile test according to JIS Z2241 was used as an index representing the springback of the material. The high-frequency pipe-forming property was determined by subjecting the pipe-formed pipe to a flatness test, and checking for cracks and pinholes. The flatness test was performed under the condition of B flattening in which the welded portion was the bending line. Then, the number of cracks generated per unit length was evaluated.

As is clear from Table 2, all the test pieces of the A group according to the present invention showed good results in all of the corrosion resistance, workability and high-frequency pipe forming property, and the exhaust gas flow path constitution It turns out that it is a suitable material for the member.
On the other hand, steel B1 having low Cr, low P, and high Ti has a problem in wet corrosion resistance and is inferior in high-frequency pipe forming property. Steel B2, which has been subjected to pickling without bright annealing, is inferior in wet corrosion resistance. In addition, steel B3 whose corrosion resistance is improved by making the base material a high alloy is
It has a high yield strength, is difficult to process, and has a problem in high-frequency pipe forming due to insufficient toughness of the material.

As is clear from this comparison, the ferritic stainless steel on which the film having the Si-enriched layer is formed by bright annealing has corrosive ions such as chlorine and sulfurous acid gas even in the region where the Cr content is relatively low. It exhibits excellent corrosion resistance in the environment containing it. Moreover, the brightly annealed ferritic stainless steel has a low degree of deterioration in corrosion resistance even after being heated by exhaust gas in a situation where it is used as an exhaust gas flow path constituent member. Further, it is not necessary to add Ti, and C
Since the r content can be suppressed to be relatively low, good workability is also ensured.

[0031]

As described above, in the present invention, by adjusting the composition of each alloy element and forming the film having the Si concentrated layer by bright annealing, the main corrosion factors in the exhaust gas condensation environment It is possible to obtain a ferritic stainless steel for exhaust gas flow path constituting members, which exhibits excellent corrosion resistance in a solution containing sulfite ions, and which satisfies processability and high-frequency pipe forming property. Corrosion resistance in exhaust gas condensation environment is
It is further improved by forming a passive film having a Si concentrated layer by bright annealing. This ferritic stainless steel uses the same material and is manufactured with the same history as conventional materials for various parts of the muffler where moisture corrosion resistance is a problem and for various pipes that require high-frequency pipe forming properties. It becomes possible. Further, as compared with the conventional stainless steel containing a large amount of Ti, surface defects are less likely to occur,
It has a high yield in the cold rolling process and can be manufactured at a relatively low cost.

[Brief description of drawings]

FIG. 1 is a graph showing the effect of bright annealing on the pitting corrosion resistance of heated ferritic stainless steel.

FIG. 2 is a graph showing the effects of bright annealing and Cr content on the maximum current density at which active dissolution is maximized in a corrosion test.

[Fig. 3] Surface coating analysis result of each test piece subjected to pickling finish and bright annealing finish

FIG. 4 is a graph showing the effect of surface finish on cathode polarization characteristics.

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Claims (3)

(57) [Claims] 1. C: 0.02 wt% or less, Si: 0.3
~ 1.5 wt%, Mn: 1.0 wt% or less, P: 0.0
1 to 0.2% by weight or less, S: 0.01% by weight or less, N
i: 0.6 wt% or less, Cr: 11 to 23 wt%, N
b: 0.1 to 1.0% by weight, N: 0.02% by weight or less and the balance Fe, and C + N ≦ 0.03% by weight and Nb ≧ 7 (C + N) -0.01 (Cr− 12) +
C satisfying the relationship of 0.15 and formed on the surface
An exhaust gas flow path constituting member, wherein the passivation film of r has a Si-enriched layer, and the Si concentration in the most Si-enriched portion of the Si-enriched layer is at least twice the Si content of the material. Ferritic stainless steel for use. 2. C: 0.02 wt% or less, Si: 0.3
~ 1.5 wt%, Mn: 1.0 wt% or less, P: 0.0
1 to 0.2% by weight or less, S: 0.01% by weight or less, N
i: 0.6 wt% or less, Cr: 11 to 23 wt%, N
b: 0.1 to 1.0% by weight, N: 0.02% by weight or less and the balance Fe, Mo: 2.5% by weight or less, Cu: 1.0% by weight or less, Ti: 0 0.3% by weight or less and Al: 0.3% by weight or less and one or more of them are included, and C + N ≦ 0.03% by weight and Nb + Ti ≧ 7.
(C + N) -0.01 (Cr-12) +0.15 is satisfied, and the passivation film of Cr formed on the surface has a Si concentrated layer, and most Si of the Si concentrated layer is concentrated. The ferritic stainless steel for exhaust gas flow path constituent members, characterized in that the Si concentration in the portion in which it is present is at least twice the Si content of the material. 3. A steel material having the composition according to claim 1 or 2 is rolled or polished, and then the temperature is 950 to 1050 ° C.
And a bright annealing finish in the dew point range of -30 to -60 ° C, which is a method for producing a ferritic stainless steel having a Cr passivation film including a Si-enriched layer.