JP3263469B2 - Ferritic stainless steel for exhaust gas flow path member 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 passage member for an automobile muffler and the like, and a method for producing the same.

[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 air and snowmelt in winter. The other is internal corrosion in which corrosion progresses from inside due to condensed water generated by dew condensation of exhaust gas.
The most problematic 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, thereby increasing the corrosiveness. As a result, pitting corrosion generated in the material grows greatly, leading to pitting. As a material resistant to this corrosive environment, a hot-dip Al-plated steel sheet has been conventionally used as an exhaust gas passage member such as a muffler. However, with the switching of the exhaust gas purification system such as the conversion from the oxidation catalyst to the three-way catalyst, the composition and pH of the condensed water changed, and the internal corrosion of the hot-dip Al-plated steel sheet became apparent. Therefore, SUS410
L, SUH409L and other 13Cr stainless steels have come to be used.

[0003] 13Cr stainless steel is a suitable material for a muffler for automobiles because it exhibits excellent corrosion resistance and good heat resistance. However, depending on the grade of the material and the conditions of use, even 13Cr stainless steel may cause corrosion, leading to perforation. In particular, due to the extension of the warranty period of exhaust gas members in recent years, sufficient corrosion resistance may not be expected with 13Cr stainless steel, and the development of materials with even better corrosion resistance has been required.

[0004] Corrosion resistance of stainless steel is improved by increasing the Cr content and adding Mo. But C
With the increase of r, Mo, etc., the stainless steel becomes hard and deteriorates workability such as ductility and toughness. This hardening can be restored by reducing C and N. Reduction of C and N is also effective for improving intergranular corrosion resistance. However, there is naturally a limit to the reduction of C and N, and at the presently commercially and industrially achievable levels of C and N, sufficient softening cannot be expected, and the intergranular corrosion susceptibility is completely reduced. Can not be lost. The adverse effects of C and N on intergranular corrosion include Ti, Nb fixing C and N.
It is known that the problem can be solved by adding a stabilizing element such as In addition, stainless steel in which the Si content is extremely reduced to 0.1% by weight or less has been developed,
This material also fails to meet expectations. Based on these technical backgrounds, SUS43 of low carbon and low nitrogen 19Cr-0.5Cu-Nb steel is used as a material for automobile exhaust systems.
0J1L, 18Cr-1Mo-Ti 18Cr stainless steel and the like have been used.

[0005]

From the investigation and research on the corrosive environment, the ionic species contained in the condensed water generated in the muffler that has passed through the three-way catalyst include chlorine ions, which are conventionally known to exist. In addition to sulfate ions, it has recently been found that there is also a sulfite ion which is a reducing ion. Corrosive environments containing sulfite ions exhibit very severe corrosive properties. Sulfite ions reduce and destroy the passive film of stainless steel, and significantly reduce corrosion resistance. In addition, in North America and the like, automobiles are exposed to an environment where the concentration of sulfate ions, sulfite ions, and the like in condensed water is high due to the high S concentration in gasoline, and in addition, snowmelt containing chlorine ions is sprayed on roads. . Therefore, corrosion also occurs in a muffler made of 18Cr stainless steel such as SUS430J1L. In addition, the exhaust gas temperature has risen with the recent increase in engine output, and the maximum temperature of the muffler has reached 50.
It has reached around 0 ° C. As a result, oxidation inside the muffler is promoted, and the corrosion resistance of the material is substantially reduced due to the deterioration of the passive film, and intergranular corrosion due to Cr carbide precipitated in the weld due to heating is a major problem. Has become.

On the other hand, the exhaust gas passage member is subjected to various processes such as roll forming, caulking, and burring from the viewpoint of construction. In this regard, if the material to be used is hard, processing cracks, springback, and the like become large, and construction becomes difficult. Further, the exhaust gas passage member may be used not only as a shell, a top plate, and an end plate, but also as a pipe such as a front tube, a center tube, and a tail tube. This type of pipe is usually manufactured by high frequency welding or the like. Since various processes such as bending, expansion, and contraction are performed on the pipe, good high-frequency pipe formability that does not cause cracking or necking of the welded portion even after the processing is required. However, the workability of the pipe largely depends on the toughness of the heat affected zone, and generally contradicts the formation of a high alloy for improving corrosion resistance.

Furthermore, the position of exhaust system materials such as mufflers has changed from replacement parts to permanent parts. Also from this point, a material having more excellent corrosion resistance is desired. The present invention has been devised to solve such a problem, and exhibits excellent corrosion resistance even after being heated by exhaust gas, in an exhaust gas dew environment containing corrosive ions such as sulfite ions. It is an object of the present invention to provide a new ferritic stainless steel for exhaust gas flow path members that also satisfies workability and high-frequency pipe forming properties.

[0008]

In order to achieve the object, the ferritic stainless steel for exhaust gas flow passage members of the present invention has a C content of 0.015% by weight or less and a Si content of 0.1 to 0.5.
% By weight, Mn: 0.4% by weight or less, P: 0.04% by weight
S: 0.01% by weight or less, Ni: 0.6% by weight or less, Cr: 18 to 23% by weight, Mo: 1.3 to 2.5% by weight, Cu: 0.1 to 0.3% by weight %, Nb: 0.1 to
0.6% by weight, Ti: 0.05 to 0.3% by weight, Al:
0.01 to 0.3% by weight and N: 0.02% by weight or less, the balance being Fe and unavoidable impurities, and
+ N ≦ 0.03% by weight and Nb + Ti ≧ 7 (C + N) +
0.15 holds, and P = 5Ti + 20 (Al
-0.01) is based on a ferritic stainless steel having a P value of 1.0 or more, and the passivation film formed on the surface of the base material has an Al-enriched layer, The amount of Al in the portion where is concentrated is the amount of A contained in the base material.
It is characterized by being at least twice the amount of l.

The passivation film having the Al-enriched layer has a P content of P = 5Ti + 20 (Al-0.01).
After annealing a ferritic stainless steel base material having a value of 1.0 or more, it is formed by performing an acid treatment including an electrolytic treatment in nitric acid and a dipping treatment in a mixed acid solution of nitric acid and hydrofluoric acid. Is done.

[0010]

The present inventors have studied the effects of alloying elements on the corrosion resistance of materials in an exhaust gas condensing environment containing high concentrations of sulfite ions and chlorine ions and the like and the surface condition, and have also studied the workability, high-frequency tube making and the like. Detailed examination was performed. As a result, the alloy design as described above was reached, and Al
It was clarified that the formation of a passivation film having a concentrated layer significantly improved the corrosion resistance. The exhaust gas flow path is composed of various members such as a manifold, a front tube, a flexible tube, a center tube, and a muffler immediately below an engine in an automobile exhaust system. The muffler is assembled by a shell, a top plate, an end plate, a baffle plate, an inner tube, a tail tube, and the like. These members are usually made of ferritic stainless steel, except for a flexible tube that requires high strength processing.

[0011] The characteristics required of the exhaust gas flow path constituent member are as follows.
It differs slightly depending on the site. However, the properties commonly required are heat resistance, which does not cause deterioration of the material even when heated by exhaust gas, corrosion resistance under conditions where the exhaust gas condenses, various workability such as bending and burring, and even after pipe formation. High-frequency tube-forming properties that can withstand processing. Above all, factors that limit the life extension of recent members include corrosion resistance, particularly after being heated by exhaust gas. Above all,
Attention is paid to corrosion resistance in an environment exposed to exhaust gas condensed water containing highly corrosive ionic species such as sulfite ions. Fig. 1 to simulate the exhaust gas condensation environment
The test specimen was exposed to the environment illustrated in FIG. As a test piece, width 5
A slab having a thickness of 0 mm, a length of 120 mm and a thickness of 1.2 mm was used. With reference to the condensed water collected from an actual vehicle muffler, a test liquid A for promoting corrosion with chlorine ions and a test liquid B for promoting corrosion with sulfite ions as shown in Table 1 were used as the condensed water.

[Table 1]

In the simulation test, a test piece is immersed in a test solution A or B in a semi-immersed state, boiled for 4 hours to concentrate the test solution up to 6 times, and then condensed at a temperature of 30 ° C. and a relative humidity of 80%. The conditions were maintained for 20 hours. After repeating this five times, 500 ° C. simulating heating by exhaust gas
A cycle of heating for 2 hours was repeated twice. In addition, the corrosion test using the test liquid A was equivalent to the test performed based on the knowledge acquired conventionally, and the heating of 500 degreeC x 2 hours was omitted. When a test for corrosion with chlorine ions was performed using test solution A, the degree of corrosion was from the erosion depth to
At 0 mm, it was mild at any Cr level. However, in a test using a test solution B which is heated at 500 ° C. for 2 hours to promote corrosion by sulfite ions,
Corrosion was significantly accelerated. The maximum erosion depth when the boiling / condensation test was performed using the test liquid B changed according to the Cr level as shown in FIG. From FIG. 2, it can be understood that the erosion depth can be reduced by including 18% by weight or more of Cr even in such a corrosive environment. However, a material having a low Al content has a large erosion depth despite a high Cr content.

Further, as shown in FIG. 3, the maximum erosion depth after the boiling / condensation test varied depending on the Mo content. In the corrosion test using the test solution A which is corroded by chlorine ions, even if the material does not contain Mo, the erosion depth is shallow, so that there is no effect or effect due to Mo. However, in the corrosion test using the test solution B which is corroded by sulfite ions, as shown in FIG. 3, Mo extremely effectively acts on the corrosion inhibition. It can be seen that it is significantly reduced. The improvement of corrosion resistance by Mo is C
This is presumed to be due to the synergistic effect of Cr and Mo that occurs when Mo exceeds a certain amount with respect to r. However, in the case of a material having an Al content below the range of the present invention, the effect of Mo addition is not observed. From this, it can be seen that in a corrosive environment due to sulfite ions to which heat is applied, that is, in an exhaust gas dew environment, Al plays an important role in improving corrosion resistance in addition to Cr and Mo, which are general corrosion resistance improving elements.

As shown in FIG. 4, Ti as an immobilizing element affects the erosion depth after the boiling / condensation test using the test solution B. The steel to which Ti is added has a smaller erosion depth than that of Nb alone, and shows excellent corrosion resistance. However, also in this case, the effect of the addition of Ti is not seen in the Al-free steel. From this, it is necessary to improve the corrosion resistance by N
It turns out that the composite addition of b, Ti, and Al is effective. The action and effect of Al in the boiling / condensation test are presumed as follows. In the test of FIG. 1, heating is performed at 500 ° C. for 2 hours to simulate heating of the material by the exhaust gas. At this heating, Fe, Cr, and the like are preferentially oxidized. As a result, the Cr concentration in the passivation film decreases, and the corrosion resistance is impaired. Further, the decrease in corrosion resistance in an environment where sulfite ions are present is considered to be due to the fact that the passive film is destroyed by the reduction action of sulfite ions.
The material having the concentrated layer is not strongly affected by the reducing action of the sulfite ion, and does not affect the Cr of the passive film. That is, the passivation film of Cr plays an important role in the corrosion resistance of stainless steel, but it is presumed that Al plays a useful role in protecting the passivation film of Cr.

The Al-enriched layer in the passivation film is composed of Ti and A
After annealing the stainless steel to which l is added in combination, it is formed by electrolytic treatment in nitric acid and immersion treatment in a mixed acid solution of nitric acid and hydrofluoric acid. If the amount of Al concentrated in the passive film is less than twice the amount of Al contained in the stainless steel as the base material, the effect of improving corrosion resistance is small. Therefore, the ratio of the amount of Al in the portion where Al is concentrated most in the passive film to the amount of Al contained in the stainless steel of the base material (hereinafter, referred to as “the amount of Al”)
This is called an Al concentration ratio) of 2.0 or more. In order to exhibit the effect of Al, it is necessary to include an appropriate amount of alloy component and to perform an appropriate passivation film forming treatment. In addition, Ti and Si, which are elements that easily form oxides, have the same function and effect as Al, but the effect of Al having the smallest free energy for oxide formation is most remarkable. Mo has a solid solution strengthening ability greater than that of Cr, and has a strong effect of increasing proof stress. Therefore,
When considering the workability, the appropriate range of Mo is more strictly regulated than that of Cr.

An inner plate is inserted into the vehicle muffler to partition each room. The inner plate is fixed to the shell by spot welding or the like to form a gap structure. Therefore, in order to evaluate the corrosion resistance of the material in the gap, a test piece in which two plates were overlapped by spot welding was prepared, and a boiling / condensation test using the liquid B was also performed. FIG. 5 shows the erosion depth in the material of 19Cr-2Mo-Nb, Ti, and Al base steels and spot gaps as C.
It is the result arranged by u content. As is clear from FIG. 5, the inclusion of Cu not only improves the corrosion resistance of the material, but also exerts a remarkable improvement effect on the corrosion of the gap. Specifically, by including 0.1% by weight or more of Cu, the erosion depth is significantly reduced.

The reason why Cu is effective in improving the corrosion resistance, particularly the crevice corrosion resistance, is assumed to be as follows. In the gap formed by welding or working, the pH decreases with the hydrolysis of the metal. In the gap, Cl ^- is concentrated in order to maintain electrical neutrality with H ^+, and the solution becomes more corrosive. In this regard, Cu has a high depassivation pH and is preferentially eluted as an ion when the pH is lowered, thereby preventing Cr film from deteriorating. Further, Cu ions cause a precipitation reaction when the potential increases, and hinder the increase in the potential. That is, the potential is maintained constant even in a highly corrosive environment, and corrosion is suppressed. Therefore, it is expected that crevice corrosion resistance is improved by adding Cu. In the present invention, this C
The action and effect of the addition of u were positively utilized.

Next, the results of the study on the high-frequency pipe forming properties will be described. There are two main factors that reduce the high-frequency pipe formability of stainless steel. One of them is Al, T
Oxide-producing elements such as i and Si combine with oxygen in the atmosphere to form oxides, and when a flat test, which is a method for evaluating the workability of pipes, is conducted, defects such as pinholes are formed in the welded portions. Cause it to occur. The other is that the toughness of the material is reduced, the toughness is reduced more than the base metal at the high frequency tube forming bead portion corresponding to the weld heat affected zone, and cracks are generated after the flat test. Defects such as pinholes generated in the welded portion can be improved by changing the gas shielding method, the flow rate of the shielding gas, and the like. However, a decrease in toughness due to the material becomes a more important problem in a high corrosion resistant material having an increased Cr content, Mo content, and the like.

Then, as a result of examining the effect of alloying elements on the Charpy impact toughness, it was found that the transition temperature increases as the Cr content and the Mo content increase. However, in a material in which Nb and Ti as fixing elements and a small amount of Cu are added in combination, a phenomenon in which the transition temperature is remarkably lowered as compared with the Cu-free material and the Nb or Ti-only material was observed. A similar effect is expected for Ni, but Cu is advantageous from the viewpoint of cost. Therefore, a small amount of Cu was added to improve the toughness of the welded portion. When a flat test was performed on an actually formed pipe, the best result was obtained with a steel in which a trace amount of Cu was added to a material in which Nb and Ti were added in combination. The present invention has been completed on the basis of the above-described findings, and has an appropriate range of Cr content and Mo content having both corrosion resistance and workability under an exhaust gas dew condensation environment, and a sulfite ion environment after being heated. A composite addition of Nb, Ti, and Al for improving the corrosion resistance underneath, a composite addition of Nb, Ti, and a trace addition of Cu in consideration of high-frequency tube formability are employed.

Hereinafter, the alloy components contained in the ferritic stainless steel of the present invention and the contents thereof will be described. 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, and the like is reduced, and the weldability and the corrosion resistance of the weld are improved. Therefore, C and N are preferably low, and C + N ≦ 0.0
Under the condition of 3% by weight, the upper limit of the C content was set to 0.015% by weight, and the upper limit of the N content was set to 0.02% by weight. Si: An alloy element added as a deoxidizing agent for stainless steel and effective in improving oxidation resistance. In order to exert the effect of Si, it is necessary to contain 0.1% by weight or more of Si. However, when the Si content exceeds 0.5% by weight, the obtained stainless steel becomes hard,
It causes a reduction in workability and a decrease in toughness of the welded portion. Therefore, in the present invention, the Si content is determined in the range of 0.1 to 0.5% by weight.

Mn: Combines with a small amount of S contained in stainless steel to form soluble sulfide MnS, thereby reducing corrosion resistance. In this regard, the lower the Mn content, the better.
In the present invention, the upper limit of the Mn content is set to 0.4% by weight. P: The lower the P content is, the lower the toughness of the base material and the welded portion is. However, it is difficult to remove phosphorus from the Cr-containing steel, and the extremely low P content increases the production cost. Invite. Therefore, the upper limit of the P content is set to 0.04% by weight. S: A harmful element that has an adverse effect on corrosion resistance and high-temperature cracking of the welded portion. The lower the S content, the better. In the present invention, the upper limit of the S content is set to 0.01% by weight.

Ni: an alloy element effective for improving the toughness of ferritic stainless steel. However, including 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 alloy 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. The effect of adding Cr is also exhibited in an exhaust gas dew condensation environment. Such an effect is insufficient when the Cr content is less than 18% by weight, preferably 18.5% by weight.
It is desirable to contain the above. However, if the Cr content exceeds 23% by weight, the stainless steel becomes hard, and it becomes difficult to process a muffler or the like. In particular, for processing under the same conditions as conventional materials, a Cr content of 21% by weight ≦ is preferred. Therefore, in the present invention, 1
The Cr content was determined in the range of 8 to 23% by weight, preferably 18.5 to 21% by weight.

Mo: An alloy element that effectively acts together with Cr to improve the corrosion resistance of stainless steel. The effect of Mo is further enhanced by the synergistic effect with Cr. Also,
Mo also effectively acts on corrosion resistance in a corrosive environment caused by sulfite ions such as exhaust gas condensed water. These effects are attained by the Mo content of 1.3% by weight or more, preferably 1.
It is remarkably expressed at 5% by weight or more. However, if the Mo content exceeds 2.5% by weight, the stainless steel becomes hard and reduces the toughness of the weld. As a result, muffler material,
In particular, processing as a pipe becomes difficult, and productivity also decreases. Therefore, the Mo content is determined in the range of 1.3 to 2.5% by weight, preferably 1.5 to 2.5% by weight.

Cu: an important alloying element for improving the corrosion resistance of steel, particularly the crevice corrosion resistance of spot welds. It also has the effect of suppressing the toughness deterioration of the base metal and welds,
It improves high-frequency pipe forming properties and works extremely effectively on pipe workability. Such an effect becomes remarkable when the Cu content is 0.1% by weight or more. However, if a large amount of Cu is contained, an intermetallic compound such as a Cu-rich phase is generated when the material for a muffler is heated to around 500 ° C., which is a temperature level reached by the muffler material, and the steel is embrittled. This phenomenon is caused by Cu
It is found at levels above 0.3% by weight. Further, looking at the effect of the Cu content on the proof stress, as the Cu content increases, the proof stress of the material due to solid solution strengthening increases significantly, leading to a reduction in workability. Further, the toughness may decrease with an increase in strength. In the present invention, the processing with a lock seam such as a shell is assumed, and the proof strength of 35 kgf
/ Mm ² or less. For example,
In the steel of 2.0% by weight Mo level, Cu: 0.3% by weight
Yields a strength of about 35 kgf / mm ² . From the above, in the present invention, the Cu content is determined in the range of 0.1 to 0.3% by weight.

[0025] By adding Ti: Al in combination, an Al oxide film is formed on the surface of stainless steel at the time of welding to prevent oxidation loss of Cr. As a result, a decrease in corrosion resistance after heating is prevented. Further, Ti is effective in fixing S to prevent a decrease in pitting corrosion resistance due to generation of MnS, and in fixing C and N to prevent intergranular corrosion. However, excess T
The i-content generates cluster-like inclusions TiN and causes surface flaws when a stainless steel material is subjected to rolling, working, or the like. Further, as the Ti content increases, the toughness of the welded portion becomes poor, and the high-frequency tube formability decreases. Therefore, the lower limit of the Ti content is 0.05% by weight from the viewpoint of corrosion resistance, and the upper limit is 0.3% by weight from the viewpoint of high-frequency tube forming properties.

Nb: In the C-level ferritic stainless steel specified in the present invention, Nb is an alloy element indispensable for preventing intergranular corrosion together with Ti. Nb has a small effect of improving pitting corrosion resistance as compared with Ti, but has a large effect of fixing C and N. Therefore, a range that satisfies both the intergranular corrosion resistance and the high-frequency tube forming property that cannot be obtained by adding Ti alone can be determined by adding Ti and Nb in combination. The effect of the addition of Nb is insufficient if less than 0.1% by weight, and if it exceeds 0.6% by weight, the adverse effect of inhibiting the toughness of the weld is observed. Therefore, 0.1-0.6
The Nb content was determined in the range of weight%. Concentrated in the passivation film by complex addition with Al: Ti and by appropriate annealing and pickling, suppresses oxidation loss of Cr during heating and decrease in repassivation ability in a sulfite ion environment. However, when the Al content is 0.1.
When the Al content is less than 01% by weight, an Al-enriched phase is hardly formed, and when the Al content is more than 0.3% by weight, defects such as pinholes are liable to occur at the time of pipe making. Then, 0.01-0.
The Al content was determined in the range of 3% by weight.

In addition to specifying the contents of the above alloy components, in the present invention, the relational expressions (1) and (2) are further established between the respective alloying elements of C, N, Nb, Ti and Al. . Nb + Ti ≧ 7 (C + N) +0.15 (1) P = 5Ti + 20 (Al−0.01) ≧ 1.0 (2) The relational expression (1) is an experiment by the present inventors. It is an index for obtaining the amount of the fixing element necessary for securing the intergranular corrosion resistance in the welded portion under the exhaust gas dew condensation environment. Nb + Ti is 7 (C + N) +0.1
If it is less than 5, the welded portion becomes sensitized by heating by the exhaust gas during running of the vehicle, and the intergranular corrosion susceptibility increases. In order to prevent this, it is necessary to make Nb + Ti more than 7 (C + N) +0.15.

The relational expression (2) is an index showing the exhaust gas dew condensation environment, that is, the resistance to the heating by the exhaust gas and the corrosion by the condensed water containing the sulfite ion. P = 5Ti +
When the P value defined by 20 (Al-0.01) is less than 1.0, a passivation film having an Al concentration ratio of 2.0 or more is not formed, and a sufficient amount based on the combined addition of Ti and Al is not obtained. Corrosion resistance cannot be secured. In this regard, a P value of 1.0 or more is required. Further, it is preferable to satisfy the relational expression (3). Ti / Nb <0.5 (3) The relational expression (3) is obtained by experimentally determining a preferable component range for simultaneously satisfying the corrosion resistance and the high-frequency tube forming property. That is, from the viewpoint of intergranular corrosion resistance and wet corrosion resistance, it is preferable to include a sufficient amount of Ti, but a large amount of addition impairs high-frequency tube forming properties and causes surface defects. Therefore, high-frequency pipe forming property is ensured by adding Nb, which exhibits the same effect as Ti with respect to intergranular corrosion resistance, in a complex manner.

[0029]

EXAMPLE A stainless steel having the chemical composition shown in Table 2 was melted and hot rolled to produce a hot-rolled sheet having a thickness of 3.5 mm. This hot rolled sheet is cold-rolled to a thickness of 1.2 mm,
After final annealing at 1000 to 1050 ° C., electrolytic treatment at a current density of 2.5 A / dm ² in 5% nitric acid kept at 60 ° C. and mixing of 6% nitric acid-0.5% hydrofluoric acid kept at 55 ° C. An acid washing treatment including an immersion treatment in an acid solution was performed.
Steel Nos. 1 to 4 in Table 2 are stainless steels in the component composition range specified in the present invention, each of which contains Nb and Ti as a fixing element in a complex addition and contains Cu and Al as trace elements. And all the relational expressions (1) to (3) are satisfied.

Steel Nos. 11 to 21 are comparative stainless steels manufactured with the same history as Steel Nos. 1 to 4. Among them, steel Nos. 11 and 12 are stainless steels having a low Cr content, steels Nos. 13 to 15 are stainless steels having a low Mo content, and steels Nos. 16 and 17 are stainless steels free of Ti and having a low Ti content. Stainless steel and steel No. 18 are stainless steel and steel No. 19 to which Ti alone is added without adding Nb.
Are stainless steel with low Al content, steel Nos. 12 and 20
Is stainless steel containing no Cu, and steel No. 21 is a stainless steel containing 0.5% by weight of Cu, which is out of the range of the present invention. Further, the P value is less than 1.0 except for steel Nos. 18 and 20.

[Table 2]

From each stainless steel, a width of 50 mm and a length of 1
A test piece of 20 mm was cut out and subjected to the boiling / condensation test described with reference to FIG. The test results are shown in FIGS. The numbers given to the respective symbols in FIGS. 2 to 4 correspond to the steel No. shown in Table 2. As is clear from FIGS.
By containing a certain amount of Cr, Mo and Ti,
It can be seen that the corrosion resistance improving effect was obtained in a corrosive environment containing sulfite ions. The maximum erosion depth of each of the stainless steels of the present invention is 0.1 mm or less. On the other hand, even when the contents of Cr, Mo and Ti are within the range of the present invention, the steel No. 19 having an Al content of less than 0.01% by weight and a P value of less than 1.0 has a wet corrosion resistance. No improvement effect is seen.

The pickling materials of steel Nos. 1 and 19 were subjected to surface analysis using a glow discharge optical emission spectrometer, and the change in the concentration of each element in the depth direction was examined. Figure 6 showing the survey results
As is clear from the above, in the stainless steel pickling material of steel No. 1 having a P value of 1.0 or more, enrichment of Al was detected in the passive film formed on the surface. However, in the stainless steel pickling material of steel No. 19 having a low Al content and a P value of less than 1.0, no concentration of Al was detected in the passive film. Further, the decrease in the Cr concentration between the passivation film and the base material is smaller in the stainless steel pickling material of steel No. 1 than in steel No. 19, and the formation of a Cr deficient layer is suppressed by Al. It turns out that there is.

Table 3 shows the results of various tests including the results of the boiling and condensation tests shown in FIGS. The presence or absence of surface flaws in Table 3 was determined by visual observation, and those with no surface flaws were evaluated as ，, and those with surface flaws were evaluated as x. In addition, the corrosion resistance was evaluated by grade for the maximum erosion depth of the test pieces subjected to the boiling and dew condensation tests under the conditions of heating shown in FIG. 1 to 0.15mm △, 0.15mm or more ×
Was evaluated. Furthermore, the high-frequency pipe formability was determined by the results of a flat test of the pipe after pipe formation.
A sample in which a pinhole was detected was evaluated as Δ, and a sample in which neither a crack nor a pinhole was evaluated as ○.

[Table 3]

As is evident from Table 3, the stainless steels Nos. 1-4, 18 and 20 having a P value exceeding 1.0 exhibit good corrosion resistance. However, steel No. 18
Since the comparative steel of No. has a high Ti content, and the comparative steels of steel Nos. 12 and 20 do not contain Cu, pinholes or cracks were generated in the flat test after pipe making. In addition, steel No. 1
In Comparative Steel No. 8, not only the high-frequency tube forming property was impaired by excessive addition of Ti, but also surface flaws were observed.
Since the upper limit of the amount of (C + N) in the present invention is 0.03% by weight, assuming that only Ti is added as an element for fixing C and N, 7 (C + N) is required to prevent intergranular corrosion. + 0.15 = 0.395% by weight or more of T
It is necessary to contain i. However, when such a large amount of Ti is contained, surface flaws are easily generated. Therefore, when the Ti content was reduced and the element for fixing with Nb was added in a composite, C and N were fixed without generating surface flaws, and the intergranular corrosion resistance could be improved. In addition, it is found that the case where Cu is not added and the case where Cu is added excessively have an adverse effect on the high-frequency tube forming property, and it is necessary to contain a trace amount and an appropriate amount of Cu.

[0035]

As described above, in the present invention, by controlling the composition of each alloy element, excellent corrosion resistance can be obtained in a solution containing sulfite ions, which is a main corrosive factor in an exhaust gas condensation environment. A ferritic stainless steel for an exhaust gas flow path member exhibiting and satisfying workability and high-frequency pipe forming property can be obtained. In addition, the corrosion resistance in the exhaust gas dew environment
It is further improved by forming a passive film having an Al-enriched layer. This ferritic stainless steel uses the same material for various parts of the muffler where moisture corrosion resistance is a problem and for various pipes that require high-frequency pipe forming,
In addition, it is possible to manufacture with the same history as a conventional material. In addition, since surface flaws are less likely to occur as compared with conventional stainless steel containing a large amount of Ti, the yield in the cold rolling step is high, and it can be manufactured relatively inexpensively.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a corrosion test method simulating an exhaust gas dew condensation environment.

FIG. 2 is a graph showing the effect of the Cr content on the maximum erosion depth with respect to corrosion resistance after corrosion.

FIG. 3 is a graph showing the effect of the Mo content on the maximum erosion depth with respect to corrosion resistance after corrosion.

FIG. 4 is a graph showing the effect of the Ti content on the maximum erosion depth with respect to corrosion resistance after corrosion.

FIG. 5 is a graph showing the effect of Cu content on the maximum pit depth with regard to crevice corrosion.

FIG. 6 is a graph showing the concentration gradient of elements at the surface of a stainless steel pickling material having a passivation film formed thereon.

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Ikuhiro Sugimoto 4976 Nomura Minami-cho, Shinnanyo-shi, Yamaguchi Prefecture Inside Nisshin Steel Corporation Steel Research Laboratory (56) References JP-A-5-70899 (JP, A) JP-A Heihei 5-271880 (JP, A) JP-A-4-17615 (JP, A) JP-A-49-59021 (JP, A) JP-A-63-118011 (JP, A) JP-A-2-115346 (JP, A) A) JP-A-4-80347 (JP, A) JP-A-62-112757 (JP, A) JP-A-49-31523 (JP, A) (58) Fields studied (Int. Cl. ⁷ , DB name ) C22C 38/00-38/60 C21D 9/46-9/48

Claims (2)

(57) [Claims] 1. C: 0.015% by weight or less, Si: 0.
1 to 0.5% by weight, Mn: 0.4% by weight or less, P: 0.
04% by weight or less, S: 0.01% by weight or less, Ni: 0.
6% by weight or less, Cr: 18 to 23% by weight, Mo: 1.3
To 2.5% by weight, Cu: 0.1 to 0.3% by weight, Nb:
0.1 to 0.6% by weight, Ti: 0.05 to 0.3% by weight, Al: 0.01 to 0.3% by weight and N: 0.02% by weight or less, with the balance being Fe and inevitable From impurities
And C + N ≦ 0.03% by weight and Nb + Ti ≧ 7
(C + N) +0.15 holds, and P = 5Ti
The base material is a ferritic stainless steel having a P value defined by +20 (Al-0.01) of 1.0 or more, and the passivation film formed on the surface of the base material has an Al-enriched layer. A ferritic stainless steel for an exhaust gas flow channel member, wherein the amount of Al in a portion where Al is most concentrated is at least twice the amount of Al contained in the base material. 2. C: 0.015% by weight or less, Si: 0.
1 to 0.5% by weight, Mn: 0.4% by weight or less, P: 0.
04% by weight or less, S: 0.01% by weight or less, Ni: 0.
6% by weight or less, Cr: 18 to 23% by weight, Mo: 1.3
To 2.5% by weight, Cu: 0.1 to 0.3% by weight, Nb:
0.1 to 0.6% by weight, Ti: 0.05 to 0.3% by weight, Al: 0.01 to 0.3% by weight and N: 0.02% by weight or less, with the balance being Fe and inevitable From impurities
And C + N ≦ 0.03% by weight and Nb + Ti ≧ 7
(C + N) +0.15 holds, and P = 5Ti
After annealing a base material made of ferritic stainless steel having a P value defined by +20 (Al-0.01) of 1.0 or more, electrolytic treatment in nitric acid and nitric acid-hydrofluoric acid mixed acid solution A method for producing a ferritic stainless steel for an exhaust gas flow channel member, wherein the passivation film according to claim 1 is formed on a surface of the base material by performing pickling including immersion treatment on the base material.