JP2907673B2 - Ferritic stainless steel excellent in high-temperature salt damage resistance and its 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 which is used as a high-temperature or medium-temperature member exposed to a high-temperature and salt-hazardous environment, such as an exhaust pipe and a center pipe of an automobile, and an insulator of an exhaust manifold, and has excellent high-temperature salt damage resistance. It relates to steel and its manufacturing method.

[0002]

2. Description of the Related Art In recent years, there has been a demand for improved fuel efficiency and higher output of automobiles. Also, purifying exhaust gas has been strongly demanded due to the strengthening of pollution regulations. Against this background, the maximum exhaust gas temperature of automobiles has risen to around 900 ° C. The heat-resistant materials used for these thin plate structures are required to have high high-temperature strength, high thermal fatigue characteristics, and high-temperature fatigue characteristics. Further, these structures tend to be thin from the viewpoint of weight reduction.

[0003] In coastal areas and North America, the corrosiveness to salt at high temperatures due to the spraying of snow-melting material and the like has become one of the important characteristics in addition to the high-temperature strength. High-temperature salt damage corrosion is generally a general corrosion, which leads to a total reduction in the thickness of the plate. Typical examples of the members in such a use environment include an insulator of an exhaust manifold, a front pipe, a center pipe, and the like. Conventionally, AISI409 or SUS4
30JIL and these Al plating materials are used.
Since the Al plating material itself has a low melting point, it can only be used as a high-temperature member up to about 600 ° C., and cannot withstand a future rise in exhaust gas temperature. Also, A
Even bare materials such as ISI409 and SUS430JIL do not have sufficient high-temperature salt damage resistance, and are currently used as existing materials from the viewpoint of high-temperature strength and fatigue strength.

[0004] In the future, since the exhaust gas temperature tends to become thinner in addition to the high temperature, it is necessary to suppress not only the high-temperature strength but also the total thickness loss due to the high-temperature salt damage, and the high-temperature salt damage resistance is higher than that of existing materials. Materials are needed. Most of the known examples of such high-temperature salt damage resistance particularly relate to materials for automobile flexible tubes. For example, "Materials and Processes" (1991) vol. 1808-1815
Is shown in However, all of these known examples relate to austenitic stainless steel, and at present there is almost nothing related to ferritic stainless steel.

[0005]

As described above, there are few examples of ferritic stainless steels and methods for producing ferritic stainless steels in which special consideration is given to high-temperature salt damage resistance among conventional steels and known examples. The present inventors have paid much attention to the high-temperature salt damage resistance of ferritic stainless steel, and have filed Japanese Patent Application No. 4-0891.
As shown in the specification of Japanese Patent No. 21, a ferritic stainless steel excellent in high-temperature salt damage resistance has been found.

The present invention relates to a component and a production method for further improving high-temperature salt damage resistance, and provides a ferritic stainless steel capable of coping with a reduction in the weight (thickness) and a high heat resistance of exhaust system components. .

[0007]

The feature of the present invention is that the weight%
And C: 0.003 to 0.015, N: 0.02 or less,
C + N: 0.03 or less, Si: more than 0.5 to 2 or less, M
n: 0.1 to 1, P: 0.01 to 0.1, S: 0.01
Hereinafter, Cr: less than 13 to 17, Al: 0.02 to 0.1.
3, Mg: 0.001-0.05, Nb: 0.1-0.
5 including Zr: 0.01 to 0.5 or Ti:
One kind of 0.01 to 0.5, or 0.1 in the case of two kinds.
01 ≦ Ti + Zr ≦ 0.5.
o: 0.1 to 2.0 or W: 0.1 to 2.0, or in the case of two, in the range of 0.1 ≦ Mo + W ≦ 3.0, and the balance substantially from Fe The ferrite stainless steel excellent in high-temperature salt damage resistance and the ferritic stainless steel having the above-mentioned chemical components are annealed in an atmosphere having a residual oxygen concentration of 2 vol. And then pickling in the order of salt pickling, nitric acid electrolysis and nitric acid to produce a ferritic stainless steel excellent in high-temperature salt damage resistance.

Hereinafter, the present invention will be described in detail. The present inventors have found that Mo, W and Si are mainly effective in improving the high-temperature salt damage resistance of a heat-resistant ferritic stainless steel, and furthermore, determine the surface condition of the final product in the manufacturing process. By optimizing the conditions of the annealing step and the pickling step, we succeeded in creating a surface state with even better high-temperature salt damage resistance.

As the chemical components, Mo, W and Si are added in such an amount that the ductility at room temperature is not deteriorated.
As for W and W, an excessive addition deteriorates the high-temperature salt damage resistance, so that an optimum value range is provided. Mo, W and S
In order to further improve the high-temperature salt damage resistance without hindering the effect of improving the high-temperature salt damage resistance of i, Al and Mg were added in such an amount that the room-temperature ductility was not deteriorated. It is considered that the addition of these elements enhances the high-temperature salt damage resistance in order to strengthen the oxide film on the surface particularly at a medium temperature to a high temperature and improve the adhesion to the base metal.

The production conditions are as follows. A ferritic stainless steel slab having the above-mentioned composition range is hot-rolled, or a continuous cast slab having a thickness substantially equal to that of a hot-rolled sheet is obtained, and if necessary, annealed. After washing and cold rolling, the final annealing is carried out in an atmosphere having a residual oxygen concentration of 2 vol.
Anneal in a temperature range of 1000 ° C., thereby controlling the thickness of the oxide film before pickling and the thickness of the Cr-deficient layer accompanying the film formation. It creates excellent surface conditions. Pickling process (three stages of pickling performed in the order of →→): Salt pickling: alkali concentration 30-40% (weight%), water content 50-60%, temperature 350-450 ° C. Nitric acid electrolysis: nitric acid concentration 80- 120 g / l, temperature 45
-75 ° C., current 3-60 mA / cm ² nitric acid: nitric acid concentration 30-100 g / l, hydrofluoric acid concentration 10-80 g / l, temperature 40-70 ° C. From the viewpoint of improving Si, W, Mo, Mg, and Al are set, and the manufacturing process conditions are presented.

[0011]

The chemical components of the steel of the present invention will be described below. C: This steel supports a part of high-temperature strength by carbonitride of Ti or Zr, and 0.003% or more is necessary. However, considering the intergranular corrosion resistance from low temperature to high temperature, it is processed. The upper limit is 0.015 from the viewpoint of the improvement of the ductility and toughness of the hot rolled sheet.
%. In addition, N + C was set to 0.03%. Si: an element that is effective as a deoxidizing material and improves oxidation resistance and high-temperature salt damage resistance. In particular, it is an element effective for high-temperature salt damage resistance, so that it needs to exceed 0.5%.
Further, since the steel of the present invention has a lower Cr content than SUS430J1L, it is an additional element necessary for maintaining oxidation resistance, particularly for ensuring oxidation resistance in exhaust gas. On the other hand, the content is set to 2% or less to reduce workability and weldability. Mn: Since it is a deoxidizing element, at least 0.1% is required.
The upper limit is set to 1% in order to prevent martensitic transformation as an austenite forming element. P: Useful for increasing the strength at high temperatures (solid solution strengthening), but deteriorates the weldability, so it was made 0.01 to 0.1%. S: An element forming MnS, and is set to 0.01% or less to reduce corrosion resistance, which is a basic characteristic of stainless steel. Cr: effective for improving oxidation resistance and high-temperature salt damage resistance,
The content is set to 13% or more in order to secure oxidation resistance around 900 ° C. In addition, the maximum operating temperature of this steel is 900 ° C.
Considering the vicinity, addition of 17% or more is not very effective, so the upper limit was set to less than 17%. Nb: an additional element for preventing grain growth and ensuring high-temperature strength in a weld and a weld-affected zone. However, C,
Since the affinity for N and Fe is strong and precipitates are formed during use, the content of Nb is set to 0.1 to less than 0.5% as a range in which the effect of solid solution strengthening works more effectively. Ti: An element necessary for fixing C + N and improving workability and ensuring long-term stability of the matrix structure. Ti
Has a stronger affinity for C and N than W, Mo and Nb, and thus has the function of suppressing the precipitation of carbonitrides of Nb, Mo and W during use. Thereby, the solid solution W, Mo in use is used.
And Nb can be secured, and high-temperature strength and high-temperature salt damage resistance during use can be secured. In order to fix C and N which do not form a solid solution in the mother phase, the minimum addition amount was 0.01%. Further, since a part of the high-temperature strength before use is supported by the carbonitride of Ti, the addition of Ti exceeding 0.5% lowers the high-temperature strength before use to coarsen the carbonitride. Therefore, the upper limit is set to 0.5%. Further, in the case of a composite with Zr, 0.01 ≦ Zr + Ti ≦ 0.5. Zr: An element necessary for fixing C + N and improving workability and ensuring long-term stability of the matrix structure. Zr
Has a stronger affinity for C and N than W, Mo and Nb, and thus has the function of suppressing the precipitation of carbonitrides of Nb, Mo and W during use. Thereby, the solid solution W, Mo in use is used.
And Nb can be secured, and high-temperature strength and high-temperature salt damage resistance during use can be secured. In order to fix C and N which do not form a solid solution in the mother phase, the minimum addition amount was 0.01%. Further, since a part of the high-temperature strength before use is supported by the carbonitride of Zr, the addition of Zr exceeding 0.5% lowers the high-temperature strength before use to coarsen the carbonitride. Therefore, the upper limit is set to 0.5%. In the case of a composite with Ti, the relation is 0.01 ≦ Zr + Ti ≦ 0.5. W: an additive element that enhances high-temperature strength and high-temperature salt damage resistance, and needs to be added in an amount of 0.1% or more. Further, since it is harder to precipitate than Nb, the amount of solid solution can be ensured even during use, which is effective for maintaining high-temperature strength and high-temperature salt damage resistance during use. However, excessive addition degrades high-temperature salt damage resistance, so the upper limit is 2% alone, and the upper limit is 3% in combination with Mo. In addition, W is one of the elements that raise the recrystallization temperature, and is an element in which an intermetallic compound or a carbonitride with Fe is easily precipitated. It was decided. Mo: an additive element that enhances high-temperature strength and high-temperature salt damage resistance. Since the steel of the present invention has low Cr, addition of 0.1% or more is necessary in order to ensure corrosion resistance, which is a basic characteristic of stainless steel. Further, since it is harder to precipitate than Nb, the amount of solid solution can be ensured even during use, which is effective for maintaining high-temperature strength and high-temperature salt damage resistance during use. However, the addition of a large amount deteriorates high-temperature salt damage resistance by 2% alone,
And the upper limit of 3% in the composite. In addition, Mo is one of the elements that raise the recrystallization temperature, and is an element in which an intermetallic compound or a carbonitride with Fe is easily precipitated. It was decided. Al: An element that is effective as a deoxidizing material and improves oxidation resistance and high-temperature salt damage resistance. 0.02% or more is necessary because it is an element effective for high-temperature salt damage resistance.
In addition, since the present steel has a lower Cr content than SUS430J1L, it is an additional element necessary from the viewpoint of improving oxidation resistance, particularly from the viewpoint of improving oxidation resistance in exhaust gas. On the other hand, the content is set to 0.3% or less in order to deteriorate workability and deteriorate the shape of a weld bead. Mg: an element that improves high-temperature salt damage resistance, and thus 0.0
01% or more is necessary. On the other hand, the content is set to 0.05% or less in order to deteriorate ductility and toughness. N: The steel of the present invention supports a part of the high temperature strength by the carbonitride of Ti or Zr. From the viewpoint, the upper limit is set to 0.02%. In addition, C + N ≦ 0.03 together with C.

Next, the manufacturing conditions of the present invention will be described.
A slab is formed from molten steel having the above-described chemical components, and a heat treatment is performed and then hot-rolled to obtain a desired sheet thickness, or a thin steel sheet having a thickness corresponding to a hot-rolled sheet by continuous casting. After casting, if necessary, annealing is performed, cold rolling is performed, and such a steel sheet is subjected to final annealing. This annealing forms an oxide film and a Cr-depleted layer which are easily modified before pickling, so that the residual oxygen concentration is 930 to 1000 in an atmosphere of 2 vol.% Or more.
Perform in the temperature range of ° C. It is considered that annealing under these conditions forms an oxide film and a Cr-deficient layer having excellent descalability. Although this mechanism has not been elucidated, it can be assumed that the oxide film exists in a state where the oxide film is easily modified at the time of salt pickling, that is, in a strong alkali (in the salt), Cr is likely to become hexavalent. For this reason, the descaling property in the subsequent pickling becomes extremely good. The pickling process is performed in the order of →→ according to the following conditions. As a result, a sufficient cutting amount can be ensured and a surface state excellent in high-temperature salt damage resistance can be obtained. Salt pickling: alkali concentration 30 to 40% (% by weight), water content 50 to 60%, temperature 350 to 450 ° C Nitric acid electrolysis: nitric acid concentration 90 to 120 g / l, temperature 4
5 to 75 ° C, current 3 to 60 mA / cm ² nitric acid: nitric acid concentration 30 to 100 g / l, hydrofluoric acid concentration 10 to 80 g / l, temperature 40 to 70 ° C The mechanism of descaling under the above conditions is apparent. No, but it is considered as follows.

[0013] In the salt pickling, it is considered that the oxide film formed by the annealing is modified to have a good descaling property. That is, it is currently estimated that the descalability in the subsequent process is enhanced by oxidizing Cr, which is an oxide film forming element, from trivalent to hexavalent. In the nitric acid electrolytic pickling, after the salt is removed, the surface film modified to have a good descaling property is removed. Further, the Cr-deficient layer near the outermost surface is dissolved, and the Cr near the surface is dissolved.
The area of the depletion layer is reduced.

[0014] In the pickling with nitric acid, the remaining C
Completely remove the r-deficient layer. As described above, it is considered that descaling is improved by performing the processing step according to claim (2), and a surface having a uniform surface Cr concentration can be formed by substantially removing the Cr-depleted layer. It is considered that the surface is excellent in high-temperature salt damage.

[0015]

EXAMPLES Test steels having the chemical components shown in Table 1 were melted by vacuum melting to form slabs, and then subjected to slab heating-hot rolling-final annealing-pickling to produce 4 mm plates. Also, K
Samples 1, 3 and 5 were subjected to slab heating-hot rolling-shot or pickling-cold rolling-final annealing-pickling to produce 2 mm cold-rolled annealed sheets. A tensile test and a high-temperature salt damage test were performed, and various characteristics are shown in Table 2. Table 3 shows Kl, K
3 and K5 show the relationship between the production conditions and the high-temperature salt damage resistance.

According to Table 2, K1 to 6 to which Mg was added were Mg
High temperature salt damage resistance is better than H2 without addition. Low H1, A1 with Si and low H2 with Mg, high H with Mo
3. H4 having a high W is inferior in high-temperature salt damage resistance as compared with the steel of the present invention. On the other hand, H5 has high resistance to high-temperature salt damage due to the addition of a large amount of Mg, but has a low value of room-temperature ductility.

From Table 3, when the annealing temperature exceeds 1000 ° C., as can be seen from the example of K1-4, the high-temperature salt damage resistance tends to deteriorate even if the pickling conditions are satisfied. Further, even if the annealing temperature is lower than 930 ° C., as shown in the example of K5-1, even if the pickling condition is satisfied, the high-temperature salt damage resistance tends to be deteriorated. The annealing and pickling conditions K1-3, K3-3 and K5-4 satisfying the claims of the present invention were found to have good resistance to high-temperature salt damage, and the conditions lacking any one of the three stages of pickling conditions. Has poor high-temperature salt damage resistance.

Table 2 shows that the cold-rolled sheets of K1C, K3C and K5C have the same high-temperature salt damage resistance characteristics as the hot-rolled sheets, and Table 3 shows that the annealing temperature was 930-1000 ° C. It can be understood that the high temperature salt damage resistance characteristics can be improved by setting the temperature to.

[0019]

[Table 1]

[0020]

[Table 2]

[0021]

[Table 3]

[0022]

According to the present invention, there is provided a ferrite stainless steel which is exposed to a salt corrosion environment at a high temperature, particularly a material for an automobile exhaust system, in consideration of a use environment and excellent in high temperature salt damage resistance. Thus, it is possible to sufficiently cope with the future improvement of heat consumption, high output, and exhaust gas purification performance of automobiles.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-6-248394 (JP, A) JP-A-5-125491 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C22C 38/00 302 C21D 6/00 102 C21D 8/02 C22C 38/34

Claims (4)

(57) [Claims] 1. In weight%, C: 0.003 to 0.015 N: 0.02 or less C + N: 0.03 or less Si: more than 0.5 to 2 Mn: 0.1 to 1 P: 0. 01 to 0.1 S: 0.01 or less Cr: 13 to less than 17 Al: 0.02 to 0.3 Mg: 0.001 to 0.05 Nb: 0.1 to less than 0.5, Zr: 0.01 to 0.5, Ti: 1 of 0.01 to 0.5
Species and, in the case of two, 0.01 ≦ Ti + Zr ≦ 0.5
And Mo: 0.1 to 2.0, W: 0.1 to 2.0,
A ferritic stainless steel excellent in high-temperature salt damage resistance, wherein two types are included in the range of 0.1 ≦ Mo + W ≦ 3.0, and the balance is substantially composed of Fe. 2. The ferritic stainless steel slab having the chemical composition according to claim 1 is hot-rolled, and then the hot-rolled sheet is heated to 930 in an atmosphere having a residual oxygen concentration of 2 vol.
A method for producing a ferritic stainless steel excellent in high-temperature salt damage resistance, comprising annealing in a temperature range of up to 1000 ° C., and thereafter pickling in the order of salt pickling, nitric acid electrolysis, and nitric acid. 3. The hot-rolled sheet is annealed if necessary, pickled, cold-rolled, and then annealed in an atmosphere having a residual oxygen concentration of 2 vol. 3. The production method according to claim 2, wherein acid pickling is performed in the order of salt pickling, nitric acid electrolysis, and nitric acid. 4. A continuous cast slab having the chemical composition and having a thickness substantially equal to that of a hot-rolled plate, if necessary, is subjected to pickling and then cold-rolled. The manufacturing method as described.