JP2016079481A - HIGH Si AUSTENITIC STAINLESS STEEL CONTAINING COMPOSITE NONMETALLIC INCLUSION - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high Si austenitic stainless steel exhibiting more stable acid resistance than that in PCT 2013/18629 and further excellent in corrosion resistance under a nitric acid environment at high temperature and high concentration.SOLUTION: There is provided an austenitic stainless steel having a chemical composition containing C:0.04% or less, Si:2.5 to 7.0%, Mn:10% or less, P:0.03% or less, S:0.03% or less, N:0.035% or less, sol.Al:0.03% or less, Cr:7 to 20%, Ni:10 to 22%, Nb:0.05 to 0.7% and the balance Fe with inevitable impurities and having the total amount of a B-based inclusion of 0.03 area% or less, a C-based inclusion which is a composite inclusion of inclusions containing AlO, and Si and Nb and containing Nb and Si of 0.18 at% in the composite inclusion.SELECTED DRAWING: Figure 3

Description

  The present invention relates to a high-Si austenitic stainless steel suitable for use in a high-temperature, high-concentration nitric acid environment.

Stainless steel forms a stable passive film in nitric acid and exhibits excellent corrosion resistance. However, high-temperature, high-concentration nitric acid, for example, temperature: 60 to 90 ° C. and concentration: 98% by mass, is extremely oxidizable and causes general passive corrosion in general stainless steel. Due to the overpassive corrosion, the overall corrosion associated with the elution of Cr ₂ O ₃ forming the passive film proceeds.

As a material having corrosion resistance in this kind of environment, there is a high Si austenitic stainless steel disclosed by Patent Documents 1 and 2. These high-Si austenitic stainless steels exhibit excellent nitric acid corrosion resistance due to the formation of a silicate (SiO ₂ ) film in the overpassive region.

  However, although acid resistance is not a major problem, corrosion may proceed excessively, and the cause and the like are unclear and must be solved.

  Moreover, since high Si austenitic stainless steel has a high Si content, many inclusions and intermetallic compounds are formed in the steel, which causes a decrease in hot workability. In order to solve this, Patent Document 3 discloses that the chemical composition is Al: 0.05% or less (in this specification, “%” relating to the chemical composition means “mass%” unless otherwise specified), O : Limiting to 0.003% or less, and performing hot rolling at 1100 to 1250 ° C. for a long time and / or soaking to eliminate intermetallic compounds, and then performing hot rolling, thereby improving hot workability An improved invention is disclosed. In this invention, inclusions are limited by the total amount, and are not limited by type.

In Patent Document 4, sol. Is disclosed in order to prevent the formation of oxides that impair the processing flow corrosion resistance. Although it is disclosed that the amount of Al is defined, the inclusions generated in the molten steel have not been studied, and there is no description regarding the corrosion resistance deterioration caused by the inclusions. Generally, the amount of inclusions such as Al ₂ O ₃ and sol. Since it is not directly related to the amount of Al, sol. The problem caused by inclusions cannot be sufficiently prevented by merely specifying the amount of Al.

  Patent Document 5 discloses an invention in which inclusions become a starting point of corrosion, and therefore the inclusions are finely dispersed to improve corrosion resistance. However, the present invention only aims at fine dispersion of MnS by regulation of S and regulation of hot rolling conditions, and does not disclose anything about alumina inclusions.

  Patent Document 6 discloses an invention that prevents pitting corrosion by controlling the composition of inclusions to agglomerate the cluster shape and make it water-insoluble. However, such inclusions inhibit the formation of a silicate film necessary for ensuring corrosion resistance at high temperature and high concentration nitrification, and the inclusions themselves can be the starting point of corrosion.

Further, Patent Document 7 discloses a method for producing stainless steel having excellent nitric acid resistance by defining the area ratio of alumina inclusions that are B-based inclusions. This alumina inclusion may be combined with other inclusions to form a C-based inclusion. However, with respect to the C type inclusions, inclusions of "this kind in paragraph 0041 of Patent Document 7 that CaO can not be exhausted by the C type inclusions and apparently distinguished entities, and when B ₁ type inclusions increases to receive more complex / mixture inclusions of the, in the present invention, by limiting the content of B ₁ type inclusions, also will be limited amount of indirectly the composite / mixed inclusions, the purpose It is only stated that the effect of "is achieved." Furthermore, Patent Document 7 aims to thoroughly remove the alumina inclusions by performing thorough removal that is not performed in normal operation. Further, Patent Document 7 does not specifically define anything regarding C-based inclusions in which alumina inclusions are combined with other inclusions, and the Al content is about 0.1%. Since an expensive Fe—Si alloy is used, there is a problem in economy.

Japanese Patent No. 3237132 specification Japanese Patent No. 1119398 JP-A-5-51633 JP-A-6-306548 JP-A-4-202628 Japanese Patent No. 4025170 International Publication No. 2013/18629 Pamphlet

  The object of the present invention is to further stabilize the acid resistance of the high-Si austenitic stainless steel, and to provide an austenitic stainless steel having better corrosion resistance than the invention described in Patent Document 7.

  As a result of examining the reason why the acid resistance of the high-Si austenitic steel is not stable, the present inventors have obtained the following knowledge A to C as described in part in Patent Document 7 and completed the present invention. .

(A) a high temperature, at high concentrations of nitric acid, as is well known, the steel surface becomes excessive passivation corrosion, Cr ₂ O ₃ elutes the passive film, elution of the base material takes place. By containing Si in the steel, it is reprecipitated on the steel surface, and a silicate film (SiO ₂ ) is formed, thereby exhibiting nitric acid corrosion resistance.

At this time, if there are inclusions (B ₁ inclusions, which will be described later) that are difficult to deform by rolling, such as Al ₂ O _{3 in} the steel, the inclusions themselves become the starting point of corrosion and corrosion proceeds. There is a case.

(B) JIS G 0555 (2003) Annex 1 “Microscopic test method for non-metallic inclusions by point calculation” (hereinafter simply referred to as JIS G 0555) is a microscopic test for non-metallic inclusions in steel. A method is defined. Type generally inclusions, A type inclusions was viscous deformation by processing such as hot rolling (A _1-based sulfide, is subdivided into and A ₂ based silicate such as SiO ₂₎ , processing direction (which is subdivided into the oxide-based B ₁ system and carbonitride-based B ₂ system such as alumina) B type inclusions of discontinuously aligned particulate form a population, and was viscous deformation And C-type inclusions such as CaO that are irregularly dispersed.

B ₁ type inclusions such as alumina is formed by oxidation of Al, has high melting point, without also melting in the molten steel refining process is present in a solid. These particles adsorb and agglomerate with each other during the collision of the molten steel during the molten steel treatment, and grow in a cluster shape. However, the shape of the particles may change by being combined with other inclusions.

NbC is classified as B ₂ type inclusions, since not dissolved elevated temperatures, high concentration nitric acid solution, there is no problem that the inclusions is corrosion origin.

Although SiO ₂ is classified as an A-based inclusion, it does not dissolve in a high-temperature, high-concentration nitric acid solution, so that the problem that this inclusion becomes a starting point of corrosion does not occur.

(C) Main inclusions affect the corrosion resistance of high temperature and high concentration of nitric acid is not only _{B 1} type inclusions such as alumina, the _Al 2 _{O 3,} such as _{NbC-Al 2} O 3 -SiO 2 Also included are complex inclusions. For this reason, in order to show nitric acid resistance better than that of the invention described in Patent Document 7, not only the amount of Al ₂ O ₃ which is a B ₁ inclusion is restricted but also Al ₂ O ₃ is included. It is effective to regulate the Nb and Si contents in the composite inclusions. Specifically, by making Nb and Si 0.18 at% or more in the composite inclusions, it is possible to obtain nitric acid resistance that is even better than the invention described in Patent Document 7.

In the present invention, C: 0.04% or less, Si: 2.5-7.0%, Mn: 10% or less, P: 0.03% or less, S: 0.03% or less, N: 0.035 % Or less, sol. Al: 0.03% or less, Cr: 7-20%, Ni: 10-22%, Nb: 0.05-0.7%, the chemical composition consisting of the balance Fe and impurities, JIS G 0555 (2003 ) The total amount of B ₁ inclusions measured by the method described in Appendix 1 “Microscopic test method of non-metallic inclusions by point calculation” is 0.03 area% or less, and Al ₂ O ₃ and Si An austenitic stainless steel having a C-type inclusion which is a composite inclusion of inclusions containing Nb and containing 0.18 at% or more of Nb and Si in the composite inclusion.

  The high-Si austenitic stainless steel according to the present invention exhibits stabilized acid resistance and is excellent in corrosion resistance in a high-temperature, high-concentration nitric acid environment. Therefore, although this stainless steel is suitable as a constituent material of a nitric acid production plant, it can also be used for other applications that require acid resistance.

  Thus, according to the present invention, for example, nitric acid-resistant stainless steel can be provided which is more reliable and economical than the high-Si austenitic stainless steel disclosed in Patent Document 7.

FIG. 1 is an explanatory diagram showing an example of an SEM image of a C-based inclusion before the nitric acid resistance test. FIG. 2 is a graph showing an example of the EDX analysis result of the C-based inclusion before the nitric acid resistance test, and FIG. 2A shows the analysis result of the white inclusion in the SEM image shown in FIG. (B) shows the analysis result of the black inclusion in the SEM image shown in FIG. FIG. 3 is a graph showing the relationship between the corrosion rate and the ratio of Si and Nb elements in the C-based inclusion.

  Hereinafter, the austenitic stainless steel according to the present invention will be described in more detail with reference to the accompanying drawings.

1. Chemical composition [C: 0.04% or less]
C is an element that increases the strength of steel, but it causes the formation of Cr carbide at the grain boundary in the heat-affected zone of the weld and causes sensitization (increased susceptibility to intergranular corrosion). It is. Therefore, the C content is 0.04% or less. The C content is preferably 0.03% or less, and more preferably 0.02% or less.

[Si: 2.5-7.0%]
Si is contained in an amount of 2.5% to 7% in order to improve the corrosion resistance in concentrated nitric acid. In order to form a silicate film that ensures corrosion resistance in nitric acid, the Si content is set to 2.5% or more. On the other hand, when Si is excessively contained, the zero ductility temperature of the stainless steel is lowered, and hot rolling becomes difficult and operation is hindered. Not only the cost is increased, but also weldability is lowered. Therefore, the upper limit of the Si content is 7%. The lower limit of the Si content is preferably 2.7%, more preferably 2.8%. Further, the upper limit of the Si content is preferably 6.8%, and more preferably 6.6%.

[Mn: 10% or less]
Mn is an austenite phase stabilizing element and also acts as a deoxidizer, so it is contained in an amount of 10% or less. If the Mn content exceeds 10%, the corrosion resistance is lowered, hot cracking during welding, and further the workability is lowered. Therefore, the upper limit of the Mn content is 10%. The Mn content is preferably 5% or less, and more preferably 2% or less. In order to reliably obtain the above effect of Mn, the Mn content is preferably 0.5% or more, and more preferably 1.0% or more.

[P: 0.03% or less, S: 0.03% or less]
For both P and S, corrosion resistance, weldability, and S are elements that are particularly harmful to hot workability. The lower the content, the better. When both exceed 0.03%, the harmfulness appears remarkably. . Therefore, the P content is 0.03% or less, and the S content is 0.03% or less.

[N: 0.035% or less]
N has a high affinity with Nb and inhibits the fixation of C by these elements. Therefore, N is preferably as low as possible. If the N content exceeds 0.035%, its harmfulness becomes remarkable. Therefore, the N content is 0.035% or less. The N content is preferably 0.020% or less, and more preferably 0.015% or less.

[Sol. Al: 0.03% or less]
Al is used as a deoxidizing agent and a reducing agent for slag, but is also included in the alloy, so it is mixed when the alloy is added. Al reacts with dissolved oxygen in the molten steel to produce Al ₂ O ₃ . In addition, Al ₂ O ₃ is also generated when Al reduces SiO ₂ inclusions in the molten steel and oxides in the slag.

As disclosed in Patent Document 7, the Al ₂ O ₃ inclusions exposed on the surface layer are water-insoluble and become defects in the silicate film necessary for exhibiting corrosion resistance in nitric acid, and corrosion starting from the inclusions Causes the occurrence. In addition to this, it causes nozzle clogging during casting, appearance defects, cracks and starting points of cracks and corrosion. Therefore, in the present invention, the amount of B ₁ -based inclusions whose main component is Al ₂ O ₃ inclusions is regulated to a certain level or less. Therefore, sol. Al content shall be 0.03% or less. sol. The Al content is preferably 0.02% or less. Reduction of the Al content can be realized by using an alloy having a low Al content.

[Cr: 7-20%]
Cr is a basic element for ensuring the corrosion resistance of stainless steel, and is 7 to 20%. If the Cr content is less than 7%, sufficient corrosion resistance cannot be obtained. On the other hand, if the Cr content is excessive, a two-phase structure in which a large amount of ferrite is precipitated due to the coexistence of Si and Nb results in deterioration of workability and impact resistance, so the upper limit of Cr content is 20%. To do. The lower limit of the Cr content is preferably 10%, and more preferably 15%.

[Ni: 10-22%]
Ni is a stabilizing element of the austenite phase and has an effect of increasing the zero ductility temperature, so it is contained in an amount of 10 to 22%. If the Ni content is less than 10%, it is not sufficient to obtain an austenite single phase. On the other hand, excessive addition of Ni only increases the cost, and a sufficient austenite single phase is obtained at 22% or less. The upper limit of the Ni content is preferably 18%, and more preferably 14%. The lower limit of the Ni content is preferably 11%, and more preferably 12%.

[Nb: 0.05 to 0.7%]
Nb has an effect of fixing C and suppressing a decrease in corrosion resistance due to sensitization, and is an element that is particularly effective in suppressing sensitization of the weld heat affected zone. Further, when C is fixed to become NbC, it is combined with Al ₂ O ₃ to suppress dissolution of inclusions. However, if the Nb content exceeds 0.7%, workability and corrosion resistance are deteriorated. Therefore, the Nb content is 0.05% or more and 0.7% or less.

  In the chemical composition of steel, the balance other than the above elements is Fe and impurities. An impurity here means the thing of the level accept | permitted in the range which can exhibit the effect of this application, for example, Ca, Mg, Mo, Cu etc. are illustrated.

2. Inclusions (2-1) B ₁ inclusions The amounts of B ₁ inclusions and C inclusions defined in the present invention are both measured according to the method described in JIS G 0555. . In addition, the amount (%) of inclusions is area%. In the measurement, 60 visual fields are measured according to the method defined in the above standard, and the average value is calculated as the amount of inclusions.

[Total amount of B ₁ inclusions: 0.03% or less]
In the case of the high Si austenitic stainless steel according to the present invention, most of the B ₁ inclusions are alumina (Al ₂ O ₃ ) in view of the chemical composition. The Al ₂ O ₃ inclusions exposed on the surface layer of the steel material are insoluble in water, can prevent the formation of a silicate film that exhibits corrosion resistance in nitric acid, and can serve as a starting point for corrosion. In addition, Al ₂ O ₃ inclusions in the molten steel cause nozzle clogging and hinder the casting operation. In addition, the inclusions remaining in the slab become wrinkles due to rolling and are not only visually bad, but also become a starting point of cracking during processing and use, so a step of removing wrinkles is necessary. Therefore, in order to improve these, the amount of B ₁ inclusions is set to 0.03% or less. This amount is preferably 0.025% or less.

(2-2) C-based inclusions In the present invention, by defining the Si, Nb ratio in the metal element of the SiO ₂ —Al ₂ O ₃ —NbC composite inclusions detected as C-based inclusions, It also suppresses dissolution of Al ₂ O ₃ in the inclusions.

[Nb, Si, and Al element ratio in C-based inclusions is 0.3 or more]
In the case of the high-Si austenitic stainless steel according to the present invention, as shown in the examples described later, most of the C-based inclusions are alumina (Al ₂ O ₃ ). Al ₂ O ₃ detected as C-based inclusions contains Nb and Si in the inclusions by complexing with NbC, SiO _{2 and the} like. When the element ratio of Si and Nb in the C-based inclusion is as low as less than 0.18 at%, the inclusion itself becomes a corrosion starting point in 98% nitric acid, and the corrosion resistance is deteriorated.

  Therefore, in the present invention, the element ratio of Si and Nb in the C-based inclusion is set to 0.18 at% or more. Thereby, the nitric acid resistance much better than that of the invention described in Patent Document 7 can be obtained.

3. Manufacturing Method A method capable of reliably manufacturing the high Si austenitic stainless steel according to the present invention will be described below. However, as long as the stainless steel according to the present invention specified by the above chemical composition and inclusions can be manufactured, other manufacturing methods can of course be employed.

Al ₂ O ₃ in the molten steel is generated by adding an Fe—Si alloy containing Al or Al in the presence of dissolved oxygen as shown in the formula (1).

2Al + 3O → Al ₂ O ₃ (1)
Further, when an Fe-Si alloy containing Al or Al is introduced in the presence of oxide inclusions generated from an element that is less oxidizable than Al, as shown in formula (2), Al It is reduced to produce Al ₂ O ₃ .

2Al + 3MxO → 3xM + Al ₂ O ₃ (2)
In the case of high Si steel, a large amount of SiO ₂ inclusions are produced in molten steel by adding a large amount of Si. When Al or an Fe-Si alloy containing Al is added thereto, a reduction reaction by Al shown in Formula (2) occurs, and a reaction shown in Formula (3) occurs.

4Al + 3SiO ₂ → 3Si + 2Al ₂ O ₃ (3)
Therefore, in high-Si steel, after a large amount of Si is added, SiO ₂ remains in the steel and the amount of Al is regulated to suppress the formation of Al ₂ O ₃ inclusions by the reaction of the above formula (3). . However, although it is possible to suppress the formation of Al ₂ O ₃ inclusions to some extent by this method, it is insufficient to obtain the required corrosion resistance. Therefore, in addition to limiting the amount of Al, it is necessary to limit the amount of Al ₂ O ₃ inclusions, and for that reason, it is necessary to perform inclusion floating separation treatment.

As a conventional technique for suppressing the generation of Al ₂ O ₃ inclusions by the reaction of the above formula (3), not only the amount of Al input is restricted or added, but also Al alloy used as a Si source is Al. Therefore, it is necessary to select and use an alloy having a low Al content. However, in the present invention, the produced Al ₂ O ₃ inclusions are combined with SiO ₂ and NbC to make them harmless, so that an Fe—Si alloy having an Al content of about 1% can be used. Is more economical than it is.

  The desirable conditions for operation in the steel refining stage when producing the high Si austenitic stainless steel according to the present invention are shown below.

First, scrap and alloy are melted in an electric furnace.
Thereafter, as a refining process, decarburization is first performed in an AOD (argon oxygen decarburization) furnace and then in a VOD (vacuum oxygen decarburization) furnace.

  In decarburization by AOD, oxygen in the molten steel is removed out of the system as CO gas using oxygen gas. At that time, the oxidation of chromium proceeds at the same time, but decarburization is performed while mixing the argon gas and lowering the partial pressure of the CO gas while suppressing the oxidation of chromium.

Still some chromium oxidizes and migrates into the slag as Cr ₂ O ₃ . Since chromium is an expensive element, it is reduced to molten steel using a reducing agent after the treatment is completed. In general, reduction is performed using Al or Fe-Si alloy as a reducing agent. However, in the case of the present invention, in order to suppress the formation of alumina inclusions that deteriorate the corrosion resistance in high-temperature high-concentration nitric acid, it is necessary to limit the input of Al. Therefore, in AOD, reduction is performed using only Fe—Si alloy without using Al.

  The Fe—Si alloy used here does not need to be a low Al alloy as much as possible, and an inexpensive Fe—Si alloy in which about 1% of commonly used Al is mixed may be used in the manufacturing process of the alloy.

Furthermore, the reduced slag contains alumina. In order to avoid that alumina in this slag is reduced in the subsequent steps and contained as Al in the steel, and this Al reduces SiO ₂ inclusions and the like to produce Al ₂ O ₃ inclusions. The alumina in the slag is physically excluded from the system by carefully carrying out the removal after completion of the reduction with AOD.

In normal operation after reduction with AOD, the generated slag is removed until the surface of the bullion is exposed to about 70%, leaving about 30% of the slag. This is to prevent yield deterioration due to loss of bullion discharged out of the system along with removal. However, in the present invention, in order to avoid that the alumina in the slag is reduced as Al to the molten steel and this reacts with the SiO ₂ inclusions to produce Al ₂ O ₃ inclusions, Thorough removal is performed until the face appears on the surface.

  Thereafter, C in the molten steel is removed out of the system as CO gas using oxygen gas to further remove C out of the system by VOD. The system is evacuated and decarburized while reducing the partial pressure of CO gas by reducing the pressure and suppressing oxidation of chromium. Thereafter, in order to ensure corrosion resistance in high-temperature high-concentration nitric acid while also reducing chromium oxide that floats and separates in the slag, an Fe-Si alloy is added for the purpose of adding Si to a predetermined value. . In this case as well, it is not necessary to use a low Al Fe—Si alloy.

After the VOD treatment, the final components and the molten steel temperature are adjusted in the ladle. At the time of this ladle refining, an Fe—Si alloy is introduced in order to adjust to a desired component value. At that time, a small amount of alumina remaining in the slag is reduced by the Fe—Si alloy and dissolved as Al in the steel, and then this Al is reoxidized by reducing inclusions such as SiO ₂ and slag. As a result, formation of Al ₂ O ₃ inclusions occurs. In order to prevent this, a snorkel is used to perform cutting so that the Fe—Si alloy being charged does not directly touch the slag. Compared to molten steel, the Si concentration in the Fe-Si alloy is 10 times higher, and the reducibility by Si is high. Even the Al ₂ O ₃ level in the slag that is not reduced by about 2.5 to 7% Si in the molten steel is reduced by the Fe—Si alloy containing several tens of percent of Si. The reduced Al is reoxidized by slag and inclusions, and harmful Al ₂ O ₃ inclusions are generated. Therefore, to prevent this reoxidation, it is effective to avoid direct contact with the slag when the Fe—Si alloy is charged.

  Then, it casts with CC (continuous casting equipment). Increase the time between the end of ladle refining and the start of casting to promote the floating of inclusions, or reduce the casting speed and use electromagnetic stirring to promote floating separation due to the coarsening of inclusions. However, it is effective for reducing alumina inclusions.

In the present invention, Al ₂ O ₃ —SiO ₂ —NbC having a large Si and Nb content is formed by adjusting the ratio of Al to Si or Nb. That is, by adding an Fe—Si alloy containing about 1% Al in molten steel, the reaction of the formula (3) is advanced to temporarily form Al ₂ O ₃ in the molten steel. since a high affinity for oxygen element, combines with _Al 2 _{O 3} in oxygen to form the _{Al 2 O} 3 -SiO 2 composite inclusions. Forms a molten steel Nb also _{Al 2 O} 3 -SiO 2 -NbO _x composite inclusions by a similar mechanism, since Nb is affinity significantly strong elements of C than oxygen, finally _Al 2 O ₃ -SiO ₂ -NbC next, it is possible to reduce the adverse effect of _Al 2 _{O 3.}

By this production method, sol. Al: 0.03% or less, the total of B ₁ inclusions is 0.03% or less, and Nb and Si are added to the C ₂ inclusions SiO ₂ —Al ₂ O ₃ —NbC composite inclusions in an amount of 0. By containing 18 at% or more, a high Si austenitic stainless steel according to the present invention having stable acid resistance and good corrosion resistance in high temperature and high concentration nitric acid, which has not been obtained so far, is provided. .

  Table 1 shows the compositions of the present invention produced by the above production method and the comparative material produced by the production method of Patent Document 7.

Further, the amount of the B ₁ -based inclusion of the present invention is adjusted to 0.006 to 0.008%, and the metal of the SiO ₂ —Al ₂ O ₃ —NbC composite inclusion detected as the C-based inclusion The Si and Nb ratio in the element was adjusted to 0.18 to 0.24 at%.

  These materials are hot-rolled (1150 ° C.) and annealed (1130 ° C. × 10 min) to create a stretch material, from which SEM observation of cleanliness and inclusions, and a test piece for nitric acid resistance test are cut out, Used for testing.

  Table 2 shows the cleanliness of each material. The cleanliness was examined according to JIS G555.

  An example of an SEM image of these C-based inclusions is shown in FIG. 1, and EDX analysis results of the inclusions are shown in FIGS. 2 (a) and 2 (b). 2A shows the analysis result of white inclusions in the SEM image shown in FIG. 1, and FIG. 2B shows the analysis result of black inclusions in the SEM image shown in FIG.

FIG. 2 (a), the From EDX analysis result of inclusions shown in FIG. 2 (b), as shown in the SEM image of FIG. 1, NbC _{which Al} 2 _{O 3,} which looks black appear white, SiO ₂ composite inclusions It can be seen that is formed.

Usually, Al ₂ O ₃ inclusions are distributed in parallel to the rolling direction and are classified as B ₁ inclusions. As shown in FIGS. 1 and 2, white inclusions containing a large amount of Si and Nb and composite inclusions are included. When an object is formed, it becomes a massive C-based inclusion containing a large amount of Si or Nb in an inclusion containing Al as a main constituent metal element.

  Table 3 shows the metal element ratio of such C-based inclusions. The metal element ratio is an average value obtained by arbitrarily analyzing four or more points by SEM-EDX.

The Si + Nb ratio in the C-based inclusion was calculated from the analysis values shown in Table 3 and the following formula.
(Si + Nb) / (Fe + Si + Mn + Cr + Ni + Al + Nb)
Using materials having different properties of Al ₂ O ₃ inclusions as described above, the nitric acid resistance at high temperature and high concentration was compared.

[Nitric acid resistance test]
In the nitric acid resistance test, N = 2, 60 ° C., 98% nitric acid aqueous solution was immersed for 24 hours, and the corrosion rate was calculated from the corrosion weight loss. Table 4 shows the Si and Nb element ratios in the C-based inclusions together with the results of the nitric acid resistance test. Moreover, those relationships are summarized in FIG.

When the Si and Nb element ratio in the C-based inclusion is 0.18 at% or more, the corrosion rate is reduced. This corresponds to the corrosion behavior of the C-based inclusions themselves. In the example of the present invention (No. 1), Al ₂ O ₃ in the composite inclusions does not become a starting point of corrosion and remains after the nitric acid resistance test. Therefore, the corrosion rate is small.

On the other hand, in the comparative example (No. 6), Al ₂ O ₃ was dissolved, and inclusions containing Al as a main constituent metal element were not observed after the corrosion test. As a result, the corrosion rate increases.

  Thus, by setting Nb and Si in the C-based inclusions to 0.18 at% or more, the corrosion rate is reduced to less than half that of Nb and Si in the C-based inclusions being less than 0.18 at%. Can do.

Claims (1)

In mass%, C: 0.04% or less, Si: 2.5-7.0%, Mn: 10% or less, P: 0.03% or less, S: 0.03% or less, N: 0.035 % Or less, sol. Al: 0.03% or less, Cr: 7-20%, Ni: 10-22%, Nb: 0.05-0.7%, the chemical composition consisting of the balance Fe and impurities, JIS G 0555 (2003 ) The total amount of B ₁ inclusions measured by the method described in Appendix 1 “Microscopic test method of non-metallic inclusions by point calculation” is 0.03 area% or less, and Al ₂ O ₃ and Si An austenitic stainless steel having a C-type inclusion which is a composite inclusion of inclusions containing Nb, and containing 0.18 at% or more of Nb and Si in the composite inclusion.