JPH06256841A - Production of ferritic stainless steel having excellent high-temperature salt damage resistance - Google Patents

Abstract

PURPOSE:To develop the ferritic stainless steel having high-temp. salt damage resistance by annealing and pickling the ferritic stainless steel contg. specific elements under specific conditions. CONSTITUTION:The hot rolled material of the ferritic stainless steel composed of the compsn. contg., by weight%, <0.02% C, <0.02% N, 0.1 to 2% Si, 0.1 to 1% Mn, 0.01 to 0.1% P, <0.01% S, 11 to 20% Cr, 0.01 to 0.3% Al, contg. one or >=2 kinds of 0.1 to 0.5% Nb, 0.01 to 0.5% Zr and 0.01 to 0.5% Ti so as to satisfy C/12+N/14<=Nbh93+Ti/48+Zn/91<=2.0, further, contg. one or >=2 kinds of 0.1 to 2.0% Mo and 0.1 to 2.0% W so as to satisfy 0.1<=Mo+W<=3.0 is annealed at 930 to 1000 deg.C in an atmosphere of >2vol.% residual oxygen concn. The ferritic stainless steel having the excellent high-temp. salt damage resistance is produced even if the rolled material is subjected to pickling treatments in order of salt pickling, nitric acid electrolying and mixture of hydrofluoric acid with nitric acid.

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 damage environment such as an exhaust pipe and a center pipe of an automobile and an insulator of an exhaust manifold, and which is excellent in high temperature salt damage resistance. The present invention relates to a method for manufacturing steel.

[0002]

2. Description of the Related Art In recent years, it has been desired to improve the fuel efficiency and output of automobiles. In addition, purification of exhaust gas is strongly demanded due to stricter 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, thermal fatigue properties and high-temperature fatigue properties, and these structures tend to be thinned from the viewpoint of weight reduction.

Further, in coastal areas and North America, the corrosiveness against salt at high temperatures has become one of the important characteristics in addition to the high temperature strength due to the spraying of snow melting materials. Hot salt corrosion is generally general corrosion, which leads to a total reduction in plate thickness. It can be said that insulators of the exhaust manifold, front pipes, center pipes and the like are typical examples of the members in such a use environment. Conventionally, AISI409 and SUS4 have been used for these applications.
30 JIL and these Al plated materials are used.
Since the Al-plated material itself has a low melting point, it can be used only up to about 600 ° C. as a high-temperature member, and cannot withstand a future rise in exhaust gas temperature. Also, A
The bare materials of ISI409 and SUS430JIL also do not have sufficient high temperature salt damage resistance properties, and are currently used as existing materials from the viewpoint of high temperature strength and fatigue strength.

In the future, since the exhaust gas temperature is becoming higher and the wall thickness is becoming thinner, it is necessary to suppress not only the high temperature strength but also the total amount of metal thinning due to high temperature salt damage, and the high temperature salt damage resistance is higher than that of existing materials. Materials are needed. Most of the publicly known examples of such high temperature salt damage resistance particularly relate to materials for automobile flexible tubes, for example, Materials and Processes (1991) vol. 4, p. 1808-1815
Is shown in. However, all of these publicly known examples relate to austenitic stainless steels, and there is almost nothing related to ferritic stainless steels.

[0005]

As described above, in the conventional steels and the known examples, there are few examples regarding the ferritic stainless steel and the manufacturing method thereof in which the high temperature salt damage resistance is particularly taken into consideration. The present inventors have been paying attention to the high temperature salt damage resistance of ferritic stainless steels, and have filed a patent application No. 4-0891.
As shown in the specification No. 21, a ferritic stainless steel having excellent high temperature salt damage resistance has been found.

The present invention relates to a manufacturing method for further improving high temperature salt damage resistance, and more particularly to a manufacturing method of ferritic stainless steel capable of coping with weight reduction (thinning) and high heat resistance of exhaust system parts. provide.

[0007]

A feature of the present invention is that, in% by weight, C: 0.02 or less, N: 0.02 or less,
Si: 0.1 to 2 or less, Mn: 0.1 to 1, P: 0.0
1 to 0.1, S: 0.01 or less, Cr: 11 to 20, A
1: 0.01-0.3, and Nb: 0.1-
0.5, Zr: 0.01 to 0.5, Ti: 0.01 to
0.5 of 1 type, and in the case of 2 or more types, C / 12 + N /
14 ≦ Nb / 93 + Ti / 48 + Zr / 91 ≦ 2.0, and Mo: 0.1-2.0, W:
0.1 to 2.0, and if two, 0.1 ≦ M
The residual oxygen concentration in the steel contained in the range of o + W ≦ 3.0 is 2
Annealing is performed in a temperature range of 930 to 1000 ° C in an atmosphere of vol.% or more, and then salt pickling, nitric acid electrolysis, and nitric acid are pickled in this order to obtain ferritic stainless steel with excellent high temperature salt damage resistance. In production.

The present invention will be described in detail below. The present inventors have found that Mo, W, Al and Si are mainly effective in improving the high temperature salt damage resistance of heat resistant ferritic stainless steel, and further determine the surface state of the final product during the manufacturing process. By optimizing the conditions of the final annealing process and the pickling process, we succeeded in creating a surface condition that was more resistant to high temperature salt damage.

As chemical components, Si and Al are added in such an amount that does not deteriorate the normal temperature ductility, and with respect to Mo and W as selective elements, excessive addition deteriorates the high temperature salt damage resistance, so an optimum value range is set. The ingredients
It is considered that the addition of these elements improves the high temperature salt damage resistance because it strengthens the oxide film on the surface particularly at moderate to high temperatures and improves the adhesion to the base metal.

As a manufacturing condition, a ferritic stainless steel slab having the above-mentioned composition range is hot-rolled or a continuously cast slab having a thickness almost equal to that of a hot-rolled sheet is obtained, and the residual oxygen concentration of this steel sheet is 2 vol. 930 ° C to 1000 in an atmosphere of.% Or more
The thickness of the oxide film before pickling and the Cr-deficient layer accompanying the film formation is controlled by annealing in the temperature range of ℃, and then the surface is excellent in high temperature salt damage resistance in the following pickling step. It builds the condition. Pickling step (3 steps of pickling performed in the order of →→): Salt pickling: Alkali concentration 30-40% (wt%), Water content 50-60%, Temperature 350-450 ° C Nitric acid electrolysis: Nitric acid concentration 80- 120g / 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. As described above, the present invention is resistant to high temperature salt damage. From the viewpoint of improving the property, the manufacturing process conditions for steel in which the addition ranges of Si, W, Mo, Al, etc. are set are presented.

[0011]

The chemical composition of the steel used in the present invention will be described below. C: Taking the intergranular corrosion resistance from low temperature to high temperature into consideration, and from the viewpoint of improving workability and hot-rolled sheet toughness, the upper limit is set to 0.
It was set to 02%. N: From the viewpoint of improving workability and toughness of hot-rolled sheet, the upper limit was made 0.02%. Si: An element that is effective as a deoxidizer and improves oxidation resistance and high temperature salt damage resistance. On the other hand, in order to reduce the workability and weldability, the range is 0.1 to 2%. Mn: Since it is a deoxidizing element, at least 0.1% is necessary.
Further, it is an austenite forming element and the upper limit was set to 1% in order to prevent martensitic transformation. P: It is useful for high temperature strengthening (solid solution strengthening), but it deteriorates weldability, so it was made 0.01 to 0.1% in range. S: An element forming MnS, which is 0.01% or less in order to reduce the corrosion resistance which is a basic characteristic of stainless steel. Cr: Effective in improving oxidation resistance and high temperature salt damage resistance,
In order to secure the oxidation resistance at 800 ° C or higher and the corrosion resistance which is a basic characteristic of stainless steel, it is set to 11% or higher,
The addition of more than 20% reduces the effect of improving the oxidation resistance and corrosion resistance, so the upper limit was made 20%. Al: An element that is effective as a deoxidizing agent and improves oxidation resistance and high temperature salt damage resistance. Since it is an element effective for high temperature salt damage resistance, 0.01% or more is necessary.
Further, it is an additional element necessary from the viewpoint of improving the oxidation resistance, particularly, the oxidation resistance in exhaust gas. On the other hand, it is set to 0.3% or less in order to deteriorate the workability and the weld bead shape. Nb: an additive element effective in preventing grain growth in the welded portion and the weld affected zone, and ensuring intergranular corrosion resistance and high temperature strength. However, it has a strong affinity with C, N and Fe and forms a precipitate during use, so that the effect of suppressing intergranular corrosion and the high temperature strengthening effect by solid solution strengthening can be more effectively exerted in a range of 0.1 to 0.5. %, And when compounded with Ti and / or Zr, C / 12 + N / 14 ≦ Ti / 48 + Nb /
93 + Zr / 91 ≦ 2.0. Ti: C + N is an element that is effective for fixing workability, improving workability, intergranular corrosion resistance, and ensuring long-term stability of the metallic phase structure. Since Ti has a stronger affinity for C and N than W, Mo and Nb, it has a function of suppressing precipitation of carbonitrides of Nb, Mo and W during use. As a result, the solid solution W, Mo and Nb can be secured during use, and the high temperature strength and high temperature salt damage resistance during use can be secured. In order to fix C and N that do not form a solid solution in the mother phase, the minimum addition amount is 0.01%.
And Further, since part of the high temperature strength before use is supported by Ti carbonitride, addition of Ti in excess of 0.5% coarsens the carbonitride and therefore lowers the high temperature strength before use. Therefore, the upper limit is set to 0.5%. Also, Nb
And / or C / 12 + N / 1 when combined with Zr
4 ≦ Ti / 48 + Nb / 93 + Zr / 91 ≦ 2.0. Zr: An element effective for fixing C + N, improving workability, intergranular corrosion resistance, and ensuring long-term stability of the metallic phase structure. Since Zr has a stronger affinity for C and N than W, Mo and Nb, it has a function of suppressing precipitation of carbonitrides of Nb, Mo and W during use. As a result, the solid solution W, Mo and Nb can be secured during use, and the high temperature strength and high temperature salt damage resistance during use can be secured. In order to fix C and N that do not form a solid solution in the mother phase, the minimum addition amount is 0.01%.
And Further, since a part of the high temperature strength before use is supported by the carbonitride of Zr, addition of Zr in excess of 0.5% coarsens the carbonitride and therefore lowers the high temperature strength before use. Therefore, the upper limit is set to 0.5%. Also, Ti
And / or C / 12 + N / 1 when combined with Nb
4 ≦ Ti / 48 + Nb / 93 + Zr / 91 ≦ 2.0. W: An additive element that enhances high temperature strength and high temperature salt damage resistance. Further, compared to Nb, it is less likely to precipitate, so that the amount of solid solution can be secured even during use, which is effective in maintaining high temperature strength and high temperature salt damage resistance during use. Therefore, the lower limit is 0.1%
And However, since excessive addition deteriorates the high temperature salt damage resistance, the upper limit was 2% alone and 3% in combination with Mo. Further, W is one of the elements that raises the recrystallization temperature, and is an element in which an intermetallic compound with Fe and carbonitride are easily deposited. Specified Mo: An additive element that enhances high-temperature strength and high-temperature salt damage resistance, and is also effective in ensuring the corrosion resistance, which is a basic characteristic of stainless steel. Therefore, the lower limit value is set to 0.1%. Further, compared to Nb, it is less likely to precipitate, so that the amount of solid solution can be secured even during use, which is effective in maintaining high temperature strength and high temperature salt damage resistance during use. However, addition of a large amount deteriorates high temperature salt damage resistance, so 2% alone, 3% in combination with W
Was set as the upper limit. Further, Mo is one of the elements that raises the recrystallization temperature like W, and since an intermetallic compound with Fe and carbonitrides are easily deposited, considering the range where it is difficult to deposit a large amount of these, The range was set.

Next, the manufacturing conditions of the present invention will be described.
A slab is formed from molten steel having the above-mentioned chemical composition, subjected to heat treatment, and then hot rolled to obtain a desired sheet thickness, or a cast piece having a thickness corresponding to a hot rolled sheet is cast by continuous casting. After that, if necessary, annealing is performed and then cold rolling is performed, and then such steel sheet is subjected to final annealing. Since this annealing produces an oxide film and a Cr-deficient layer that are easily modified before pickling, the residual oxygen concentration is 930 to 1000 ° C in an atmosphere of 2 vol.% Or more.
Perform within the temperature range. The oxide film formed by this final annealing is in a state of being easily modified during salt pickling. That is, the formed oxide film contained 6 in Cr in a strong alkali (in salt).
It can be inferred that the price is likely to be high. Therefore, the descaling property in the subsequent pickling becomes extremely good.

The pickling step is performed in the order of →→ under the following conditions. This makes it possible to secure a sufficient amount of fusing and to make the surface state excellent in high temperature salt damage resistance. Salt pickling: Alkali concentration 30-40% (wt%), Water content 50-60%, Temperature 350-450 ° C Nitric acid electrolysis: Nitric acid concentration 90-120g / 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. Descaling mechanism under the above conditions is not clear. However, it is considered as follows.

In the salt pickling, it is considered that the oxide film formed by annealing is modified to have a good descaling property. That is, it is currently estimated that the oxide, which is an oxide film forming element, is oxidized from trivalent to hexavalent to enhance the descaling property in the subsequent step. In the nitric acid electrolytic pickling, after the salt, the surface film modified to have a good descaling property is removed. Furthermore, the Cr-deficient layer near the outermost surface is dissolved, and Cr near the surface is dissolved.
Reduce the area of the depletion layer.

In the nitric acid pickling, part of the remaining C
Completely remove the r-deficient layer. As described above, by performing the above-mentioned pickling treatment step, the descaling was improved, and by almost removing the Cr deficient layer, it became possible to form a surface having a uniform surface Cr concentration. Also, it can be estimated that the surface roughness is effective for high temperature salt damage resistance, and for these reasons, the manufacturing method of the present invention enabled the production of a surface excellent in high temperature salt damage resistance.

[0016]

EXAMPLE A sample steel having the chemical composition shown in Table 1 was melted into a slab shape by vacuum melting, and then slab heating-hot rolling-
A 4 mm plate was produced through annealing-pickling. This annealing is LN
The residual oxygen concentration was set to 2 to 7 vol.% In the G combustion gas atmosphere, a high temperature salt damage test was conducted using this plate, and the relationship between various characteristics and pickling conditions is shown in Table 2.

From Table 2, when the annealing temperature exceeds 1000 ° C., as shown in the examples of A-4 and C-4, the high temperature salt damage resistance tends to deteriorate even if the pickling conditions are satisfied, and When the annealing temperature was lower than 930 ° C., the high temperature salt damage resistance tended to deteriorate even if the pickling conditions were satisfied as shown in the example of B-5. A-3, B-4 and C-4, which are annealing and pickling conditions satisfying the claims of the present invention, have good high temperature salt damage resistance, and a condition that lacks any one of the three stages of pickling conditions is: It was found that the high temperature salt damage resistance deteriorates.

[0018]

[Table 1]

[0019]

[Table 2]

[0020]

INDUSTRIAL APPLICABILITY The present invention provides a ferritic stainless steel which is exposed to a salt corrosion environment at high temperature, especially as an automobile exhaust system material, considering the use environment and having excellent high temperature salt corrosion resistance. With this, it will be possible to sufficiently cope with future improvements in fuel efficiency, output, and exhaust gas purification performance of automobiles.

Claims (1)

[Claims] 1. C .: 0.02 or less N: 0.02 or less Si: 0.1-2 or less Mn: 0.1-1 P: 0.01-0.1 S: 0.01 The following Cr: 11-20 Al: 0.01-0.3 is contained, and Nb: 0.1-0.5, Zr: 0.01-0.5, T
i: 0.01 to 0.5, and in the case of 2 or more, C / 12 + N / 14 ≦ Nb / 93 + Ti / 48 + Zr /
91 ≦ 2.0, and Mo: 0.1
.About.2.0, W: 0.1 to 2.0, and in the case of 2 kinds, contained in the range of 0.1.ltoreq.Mo + W.ltoreq.3.0 with the balance remaining in the steel consisting essentially of Fe. High temperature salt damage resistance, characterized by annealing in a temperature range of 930 to 1000 ° C in an atmosphere with an oxygen concentration of 2 vol.% Or more, and then performing pickling in order of salt pickling, nitric acid electrolysis, and nitric acid. Excellent ferritic stainless steel manufacturing method.