JP6321062B2 - Ferritic stainless steel with excellent intergranular corrosion resistance - Google Patents

Description

  The present invention relates to ferritic stainless steel having excellent intergranular corrosion resistance, and more specifically, ferrite that is exposed to high temperatures for a long time, such as automobile exhaust system parts, thermal power generation equipment parts, nuclear power generation equipment parts, fuel cell parts, etc. The present invention relates to a ferritic stainless steel excellent in intergranular corrosion resistance that can be used to prevent intergranular corrosion of a stainless steel.

  Intergranular corrosion in stainless steel is a major cause of shortening the lifetime of structures, and research to prevent this has been ongoing for a long time. In recent years, as a main cause of intergranular corrosion of stainless steel, chromium (Cr) carbides are caused by reaction of carbon (C) contained in stainless steel and chromium (Cr) which is a main alloy element of stainless steel. It has become clear that intergranular corrosion occurs due to the chromium (Cr) deficiency formed around the chromium (Cr) carbide due to the generation at the boundary.

  Therefore, in order to prevent intergranular corrosion of stainless steel, various techniques for suppressing the formation of chromium (Cr) carbide have been studied. Typical common techniques for preventing the formation of chromium (Cr) carbides are titanium (Ti) and niobium, which can produce carbides before chromium (Cr), because their affinity for carbon is greater than chromium (Cr). In this method, a so-called “carbide stabilizing element” such as (Nb) is added to stainless steel.

  This method uses a stabilizing element (M) such as titanium (Ti) or niobium (Nb) in an amount about 8 to 20 times greater than the content of carbon (C) and nitrogen (N) contained in stainless steel. Added, and through a hot rolling and annealing process at a temperature of about 1000 ° C., a stabilizing element (M) -carbide or a stabilizing element (M) —generated earlier at a temperature of about 800 ° C. or higher. By preferentially producing carbonitrides, it suppresses the production of chromium (Cr) carbides that are mainly produced at temperatures of about 800 ° C. or less, and is widely used as a technique for preventing intergranular corrosion. .

  During the welding of the ferritic stainless steel material, the temperature of the heat-affected zone around the welded portion rises to 1300 ° C or higher. At this time, the stabilizing element (M) -carbide or the stabilizing element (M) -carbonitride generated during the production of stainless steel is decomposed and dissolved in the stainless steel in the cooling process after welding. It comes to exist. When welded stainless steel parts and structures are used at temperatures of about 400 to 700 ° C. such as automobile steel plates, thermal power generation equipment parts, nuclear power generation equipment parts, fuel cell parts, etc., they are fixed in the stainless steel. Dissolved carbon (C) and titanium (Ti) and / or niobium (Nb) and chromium (Cr) diffuse into the grain boundary, and titanium (Ti) and niobium (Nb) preferentially take carbon at the grain boundary. To form a stabilizing element (M) -carbide (e.g. TiC) or a stabilizing element (M) -carbonitride (e.g. Ti (C, N)) again to form chromium (Cr) carbide This is the principle that intergranular corrosion is prevented because a chromium (Cr) -deficient layer is not formed.

  However, according to the recent research results on the intergranular corrosion of ferritic stainless steel to which a stabilizing element is added (Non-patent Documents 1 and 2), unlike the conventional intergranular corrosion reaction mechanism theory, it is formed at the grain boundary. The chromium (Cr) -deficient layer is formed by the diffusion of chromium (Cr) itself into the grain boundary without reacting with carbon (C) while chromium (Cr) diffuses into the grain boundary. It was revealed that intergranular corrosion was induced. Further, chromium (Cr) segregation occurs around the stabilizing element (M) -carbide or the stabilizing element (M) -carbonitride generated at the grain boundary of the ferritic stainless steel material to which the stabilizing element is added. It has been reported that this occurs (Non-patent Document 3).

Therefore, only the conventional method of adding a carbide stabilizing element having a strong affinity for carbon (C) such as titanium (Ti) or niobium (Nb) is more suitable for stainless steel used in welding heat affected zone and high temperature environment. Specifically, there is a limit in preventing intergranular corrosion of low chromium (Cr) stainless steel.
In this regard, in the conventional patent (Patent Document 1), in order to prevent intergranular corrosion after welding of stainless steel to which titanium (Ti) as a stabilizing element is added, 600 to 700 ° C. after welding. In which the chromium (Cr) -deficient layer is eliminated by diffusing chromium (Cr) from the inside of the grains to the chromium (Cr) -deficient layer at the grain boundary. However, since this method requires a separate heat treatment after welding, the welding process is complicated and not only increases the manufacturing cost of the welded structure, but in the case of a large structure, 600 to 700 after welding. There is a problem that it cannot be applied to a portion that is difficult to be heat-treated at ° C.

Korean Published Patent No. 2003-50212 JP 2013-76153 A

J. et al. K. Kim, Y. et al. H. Kim, S.M. H. Uhm, J. et al. S. Lee, K.M. Y. Kim, Corros. Sci. 51 (2009) 2716. J. et al. K. Kim, Y. et al. H. Kim, B.I. H. Lee, K.M. Y. Kim, Electrochimica Acta. 56 (2011) 1701. J. et al. H. Park, J. et al. K. Kim, B.I. H. Lee, K.M. Y. Kim, Scripter Mater. 68 (2013) 237.

  An object of the present invention is to provide a ferritic stainless steel excellent in the effect of suppressing intergranular corrosion of ferritic stainless steel without adding a carbide stabilizing element having a higher affinity for carbon than chromium (Cr). That is.

In the present invention, chromium (Cr): 10 to 14% by mass, carbon (C): 0.02% by mass or less, nitrogen (N): 0.02% by mass or less, phosphorus (P): 0.04% by mass or less , Sulfur (S): 0.01% by mass or less, molybdenum (Mo): 0.05 to 2.0% by mass, silicon (Si): 0.2 to 1.5% by mass, manganese (Mn): 0. 1 to 1.0% by mass, and the value of the relational expression of 6 {(Mo−0.05) × (Si−0.2) × (Mn−0.18)} / (C + N) is 1 or more. Consisting of the remaining iron (Fe) and inevitable impurities,
Molybdenum (Mo) -silicon (Si) -manganese (Mn) -carbon (C) composite intermetallic compounds are produced at and around the grain boundaries of ferritic stainless steel.
(However, Mo, Si, Mn, C and N in the above relational expression mean mass% of each component.)

  The ferritic stainless steel having excellent intergranular corrosion resistance according to the present invention can be obtained without adding a carbide stabilizing element having a strong affinity for carbon such as titanium (Ti) and niobium (Nb). Add a small amount of molybdenum (Mo), silicon (Si), and manganese (Mn), which are elements with a weaker affinity with carbon, to generate intermetallic compounds at the grain boundaries and their surroundings, and form solid solutions inside the steel. By stabilizing the generated carbon, chromium (Cr) is prevented from diffusing toward the grain boundaries, the formation of chromium (Cr) carbide is prevented, and the generation of the chromium (Cr) -deficient layer is prevented by , It has the effect of preventing intergranular corrosion. In particular, the present invention can effectively prevent intergranular corrosion of the weld heat affected zone.

It is a graph which shows the carbide | carbonized_material formation energy of a main metal element, Comprising: It is a graph which shows the Gibbs free energy in each temperature of each metal element. It is a photograph which shows the result of having performed the intergranular corrosion test, after heat-processing the stainless steel added with the component composition of the comparative example of Example 1 of the Example of this invention, and an Example at 500 degreeC. The result of having observed the degree of intergranular corrosion after carrying out modified-Strauss evaluation of Comparative Examples 5 and 6 and Examples 1 and 2 with the optical microscope is shown. The result of having analyzed the crystal grain boundary of the stainless steel of the comparative example 1 which added stabilization elements, such as titanium (Ti) 20 times or more than carbon (C) + nitrogen (N) content, by 3DAP is shown. It is a graph which shows the density | concentration according to a position by analyzing the elemental distribution in the grain boundary of the comparative example 1 which added the stabilizing element by 3DAP. The result of having observed the intermetallic compound produced | generated along the grain boundary of Example 2 which concerns on this invention with the scanning electron microscope is shown. It is the photograph which showed the result of having carried out the solution treatment of Example 2 for 10 minutes at 1300 degreeC, and then carrying out the sensitization heat processing at 500 degreeC for 2 hours, extracting the deposit by the carbon replica analysis technique, and observing with a transmission electron microscope. It is the graph which analyzed distribution according to element in the grain boundary of comparative example 6 by 3DAP. It is the graph which analyzed distribution according to the element in the grain boundary of Example 2 by 3DAP. It is a graph which shows the relationship between the area reduction rate of the steel materials after the intergranular corrosion experiment of the comparative example of an Example of this invention, and an Example, and the result value of a relational expression.

  The present invention relates to a ferritic stainless steel excellent in intergranular corrosion resistance that can prevent intergranular corrosion of a ferritic stainless steel exposed to a weld heat affected zone or a high temperature for a long time.

FIG. 1 is a graph showing the carbide formation energy of the main metal element, and shows the Gibbs free energy of each metal element at each temperature.
As shown in FIG. 1, the carbon affinity of an element that generates carbide is determined by the degree of generation energy of various carbides, and the lower the carbide generation energy, the higher the affinity with carbon.
Conventionally, in order to prevent intergranular corrosion, by adding a strong carbide forming element having a higher carbon affinity than chromium (Cr), such as titanium (Ti) or niobium (Nb), chromium is added. (Cr) tried to prevent the formation of carbides, but intergranular corrosion of ferritic stainless steel can be prevented by the addition of strong carbide forming elements such as titanium (Ti) or niobium (Nb). There wasn't. The present invention is completely different from the mechanism of the prior art, and molybdenum (Mo), silicon (Si), and manganese (weak carbide formers) having a carbon affinity lower than that of chromium (Cr). Mn) is added in combination to produce intermetallic compounds at and around the grain boundaries, thereby preventing intergranular corrosion of stainless steel.

  In the present invention, the “carbide stabilizing element” is an element having a higher affinity for carbon than chromium (Cr), such as titanium (Ti) and niobium (Nb), and is added to stainless steel. This means that the element can prevent the formation of chromium (Cr) carbide since this element reacts with carbon to produce carbide preferentially.

  Further, in the present invention, “not containing a carbide stabilizing element” means that titanium (Ti) or niobium having a stronger affinity for carbon than chromium (Cr), which has been conventionally used as a stabilizing element, in stainless steel. This means that no element such as (Nb) is contained, and molybdenum (Mo), silicon (Si), and manganese (Mn), which have a lower affinity for carbon than chromium (Cr), are substantially carbides. It means that it does not act as a stabilizing element.

  The inventors of the present invention have made molybdenum (Mo), silicon (Si) more than the case of adding an element that forms carbide prior to chromium (Cr), such as titanium (Ti) and niobium (Nb), to stainless steel. ) And manganese (Mn) were found to be able to more effectively prevent intergranular corrosion of the weld heat affected zone.

  Hereinafter, the ferritic stainless steel excellent in intergranular corrosion resistance of the present invention will be described in detail.

  The ferritic stainless steel of the present invention having excellent intergranular corrosion resistance is chromium (Cr): 10 to 14% by mass, carbon (C): 0.02% by mass or less, and nitrogen (N): 0.02% by mass. % Or less, phosphorus (P): 0.04 mass% or less, sulfur (S): 0.01 mass% or less, molybdenum (Mo): 0.05 to 2.0 mass%, silicon (Si): 0.2 -1.5 mass%, manganese (Mn): 0.1-1.0 mass%, and the balance of iron (Fe) and inevitable impurities, 6 {(Mo-0.05) × (Si-0. 2) satisfying that the value of the relational expression of × (Mn−0.18)} / (C + N) is 1 or more (where, Mo, Si, Mn, C and N in the relational expression are the mass of each component) % Means).

  Hereinafter, the reasons for limiting the components and the composition range of the present invention will be described in detail.

Chromium (Cr): 10-14% by mass
Chromium (Cr) is a basic component that imparts corrosion resistance to stainless steel, and it is necessary to add a large amount in order to improve corrosion resistance. Here, when it is less than 10% by mass, the corrosion resistance is very deteriorated, so it is limited to 10% by mass or more, and when it exceeds 14% by mass, intergranular corrosion phenomenon due to chromium (Cr) concentration in the metal state. However, it has a low influence on the corrosion resistance of steel, and it is possible to prevent intergranular corrosion only by adding a conventional stabilizing element. Therefore, the ferritic stainless steel excellent in intergranular corrosion resistance proposed in this patent is proposed. The range of the chromium (Cr) content in is preferably 14% by mass or less.

Carbon (C): 0.02% by mass or less Carbon (C) is a component that deteriorates weldability and workability, and when added in excess of 0.02% by mass, weldability and workability become very poor. Therefore, it is limited to 0.02% by mass or less.

Nitrogen (N): 0.02% by mass or less Nitrogen (N) is a component that deteriorates weldability and workability, and if added over 0.02% by mass, weldability and workability become very poor. Therefore, it is limited to 0.02% by mass or less.

Phosphorus (P): 0.04% by mass or less Phosphorus (P) is a component contained in steel as an inevitable impurity, and the smaller the amount, the better the toughness and workability of the steel. On the other hand, if added over 0.04% by mass, the toughness and workability are very deteriorated, so the amount is limited to 0.04% by mass or less.

Sulfur (S): 0.01% by mass or less Sulfur (S) is a component contained in steel as an inevitable impurity, and the smaller the amount, the better the hot workability of the steel. On the other hand, if added over 0.01% by mass, the hot workability is greatly reduced in the hot rolling step, so the content is limited to 0.01% by mass or less.

Molybdenum (Mo): 0.05 to 2.0 mass%
Molybdenum (Mo) is an element effective in suppressing the corrosion of ferritic stainless steel. Molybdenum (Mo) is particularly suitable as an intergranular corrosion preventing element proposed in the present invention because it is concentrated at the grain boundaries of the ferritic stainless steel material and easily generates secondary phases. However, when molybdenum (Mo) is added in an amount exceeding 2.0% by mass, the s phase is precipitated at a temperature of 650 ° C. or more to reduce the impact resistance and corrosion resistance. It is preferable. Moreover, when it is less than 0.05% by mass, molybdenum (Mo) -silicon (Si) -manganese (Mn) -carbon (C) -based intermetallic compound for preventing intergranular corrosion at the grain boundary and its surroundings. Since it is hard to produce | generate, Preferably it adds in 0.05-2.0 mass%.

Silicon (Si): 0.2 to 1.5% by mass
Silicon (Si) is an effective deoxidizer and a powerful ferrite-forming element. Further, since it is easily concentrated at the grain boundaries of the steel material, it is preferably added in an amount of 0.2% by mass or more to prevent intergranular corrosion. When a large amount of silicon (Si) is added, the steelmaking and pickling steps Therefore, it is preferable to add at 1.5% by mass or less.

Manganese (Mn): 0.1 to 1.0% by mass
Manganese (Mn) acts as a deoxidation and desulfurization agent and is used effectively for austenite stabilization, and reduces sulfur (S) that dissolves in stainless steel to reduce grain boundary segregation of sulfur (S). In order to suppress (segregation), since it plays the role which prevents the crack generation by sulfur (S) at the time of hot rolling, it is preferable to add 0.1 mass% or more for such an effect. Further, manganese (Mn) may deteriorate toughness, corrosion resistance and oxidation resistance when added in a large amount to ferritic steel, and is preferably limited to 1.0% by mass or less. Therefore, it is preferable to add manganese (Mn) in the range of 0.1 to 1.0% by mass.

Titanium (Ti) and niobium (Nb)
In the present invention, titanium (Ti), niobium (Nb) and the like can be contained, and these titanium (Ti) and niobium (Nb) do not affect the improvement of intergranular corrosion resistance.

  The remaining component of the present invention is iron (Fe). However, in a normal manufacturing process, an unintended impurity may be inevitably mixed from the raw material or the surrounding environment, so this cannot be excluded. Since these impurities can be understood by any engineer using a normal manufacturing process, the entire contents thereof are not particularly described in this specification.

  The present invention satisfies the above component range and satisfies the following relational expression 1 so that molybdenum (Mo) -silicon (Si) -manganese (Mn) -carbon (C) -based metal is present at and around the grain boundaries of the steel material. Intergranular corrosion can be prevented by generating intermetallic compounds.

[Relational expression 1]
6 {(Mo−0.05) × (Si−0.2) × (Mn−0.18)} / (C + N) ≧ 1

  In order to produce molybdenum (Mo) -silicon (Si) -manganese (Mn) -carbon (C) -based intermetallic compounds at the grain boundaries of steel materials and in the vicinity thereof, molybdenum (Mo), silicon (Si), and manganese (Mn) must be added to the steel material in a certain amount or more, and if the content of one element is deficient even if the content of the other two elements is sufficiently high, molybdenum (Mo) -silicon ( Si) -manganese (Mn) -carbon (C) -based intermetallic compounds cannot be produced sufficiently. Relational expression 1 is between molybdenum (Mo) -silicon (Si) -manganese (Mn) -carbon (C) metal for preventing intergranular corrosion due to the content of carbon (C) and nitrogen (N) in the steel. The necessary conditions for the molybdenum (Mo), silicon (Si), and manganese (Mn) contents necessary for the formation of the compound are derived. Even in the case of the example satisfying the relational expression 1 as shown in the following example, it can be confirmed that no intergranular corrosion is exhibited as shown in Table 2 and FIG. 10, and the relational expression 1 is satisfied. In the case of no comparative example, the effect of the generation of molybdenum (Mo) -silicon (Si) -manganese (Mn) -carbon (C) intermetallic compound and the resulting improvement in intergranular corrosion resistance does not appear, It can be seen that in order to improve the intergranular corrosion resistance, the content of the alloy element added to the steel material must satisfy the relational expression 1.

  In the present invention, molybdenum (Mo), silicon (Si), and manganese (Mn), which are the weak carbide forming elements described above, are added in combination to the grain boundary of the microstructure of the steel material and the periphery thereof. An attempt is made to prevent intergranular corrosion by producing a molybdenum (Mo) -silicon (Si) -manganese (Mn) -carbon (C) intermetallic compound. The production of the intermetallic compound serves to stabilize carbon dissolved in the steel material by restraining carbon in the intermetallic compound.

Here, it is preferable that the molybdenum (Mo) -silicon (Si) -manganese (Mn) -carbon (C) -based intermetallic compound is CMn ₄ MoSi. Molybdenum (Mo), silicon (Si), manganese (Mn), and carbon (C) are easily concentrated thermodynamically at the grain boundaries inside the steel material. When the content of each element is a certain amount or more, the grain boundaries and Molybdenum (Mo) -silicon (Si) -manganese (Mn) -carbon (C) -based intermetallic compounds can be generated along the periphery, and in the following examples, the grain boundary inside the steel material and its periphery It was shown that the molybdenum (Mo) -silicon (Si) -manganese (Mn) -carbon (C) -based intermetallic compound produced in (1) was precipitated mainly in the form of CMn ₄ MoSi.

  Further, in the grain boundary of the ferritic stainless steel of the present invention, the deviation between the maximum content of chromium (Cr) concentration and the minimum content of chromium (Cr) -deficient layer is preferably 10 atomic% or less. The cause of intergranular corrosion in ferritic stainless steel materials is that electrochemical polarization occurs due to the difference in chromium (Cr) concentration at the grain boundary and chromium (Cr) concentration in the depleted layer. Therefore, the ferritic stainless steel with excellent intergranular corrosion resistance is controlled by suppressing the deviation between the maximum concentration of chromium (Cr) at the grain boundary of the ferritic stainless steel and the minimum content of the deficient layer to 10 atomic% or less. Can be provided.

  Here, the maximum content of the chromium (Cr) concentration is 20 atomic% or less, and the minimum content of the chromium (Cr) -deficient layer is more preferably 9.5 atomic% or more. If the minimum content is maintained, intergranular corrosion of ferritic stainless steel can be more effectively suppressed.

  Hereinafter, the present invention will be described in detail based on preferred embodiments of the present invention with reference to the accompanying drawings. Note that the configurations shown in the following embodiments and drawings are only preferred embodiments of the present invention and do not show all the technical ideas of the present invention, and therefore various equivalents which can be substituted at the time of the present application. There may be modified examples, and the scope of the present invention is not limited to the examples described later.

  Table 1 below shows the compositions of Comparative Examples and Examples of the present invention.

  A ferritic stainless steel material having the component contents shown in Table 1 below and containing 0.001% by mass or less of phosphorus (P) and sulfur (S) as impurities is subjected to solution treatment at 1300 ° C. for 10 minutes, and then 500 ° C. For 2 hours. Such heat treatment is for reproducing the processing environment after welding of a commercial steel material.

  When ferritic stainless steel is welded, it is heated to near the melting temperature of stainless steel (about 1300 ° C or higher), and when a stainless steel structure including such a weld is used in a temperature range of 400 to 700 ° C, carbide stabilization is achieved. In the case of a stainless steel material to which titanium (Ti) and niobium (Nb), which are chemical elements, are added, sensitization progresses, causing intergranular corrosion.

The ferritic stainless steel materials having the component contents of the respective comparative examples and examples were subjected to the solution treatment and the sensitization heat treatment, and then the degree of sensitization was evaluated by the “modified-Strauss test method”.
The above-mentioned “Modified-Strauss test method” is a method in which a solution having a volume of 300 ml containing 6% by mass of copper sulfate (CuSO ₄₎ and 0.5% by mass of sulfuric acid (H ₂ SO ₄₎ in distilled water is brought to a temperature of 105 ° C. This is a method of evaluating by measuring the area reduction rate and intergranular corrosion of the steel material after maintaining and electrochemically connecting a copper ball with a test piece and dipping for 20 hours.

  In addition, after subjecting the ferritic stainless steel materials having the component contents of the respective comparative examples and examples to the solution treatment and the sensitizing heat treatment, the microstructure of the steel materials is subjected to a scanning electron microscope (SEM), a transmission electron microscope (TEM). ) And three-dimensional atomic microscope (3DAP), the concentration and depletion of metal elements in the vicinity of grain boundaries and the precipitation behavior of carbides, nitrides and intermetallic compounds were observed.

  The results of the above measurements are summarized in Table 2 below. The value of the relational expression in Table 2 is the mass of each component in the relational expression of “6 {(Mo−0.05) × (Si−0.2) × (Mn−0.18)} / (C + N)”. It means the result of substituting% value.

FIG. 2 shows the surfaces of the steel materials of the comparative examples and the examples after the modified-Strauss evaluation by the method as described above.
As shown in FIG. 2, it can be confirmed that the intergranular corrosion has progressed in the case of Comparative Example 1 and Comparative Example 2 in which Ti which is a carbide stabilizing element is added. In addition, as a result of observing the influence of the content of weak carbide forming elements on the degree of intergranular corrosion, intergranular corrosion was observed in the test pieces of Comparative Examples 3 and 4 to which molybdenum (Mo) was not added. Large intergranular corrosion also occurs in Comparative Example 5 where the silicon (Si) content is insufficient, Comparative Example 6 where the manganese (Mn) is insufficient, and Comparative Example 7 where the manganese (Mn) and molybdenum (Mo) are insufficient. did.
However, no intergranular corrosion occurred in Examples 1 and 2, which are test pieces to which a proper amount of weak carbide forming element was added. In addition, intergranular corrosion was also applied to Examples 3 and 4, which are test pieces in which appropriate amounts of molybdenum (Mo), silicon (Si), and manganese (Mn), which are weak carbide forming elements, were added and titanium (Ti) was added simultaneously. Did not occur. From the above results, it is confirmed that when titanium (Ti) is added to the component composition of the present invention, it has no effect on the improvement of intergranular corrosion and has the same effect regardless of the addition of titanium (Ti). did it.

FIG. 3 is a photograph showing the result of observing the degree of intergranular corrosion with a light microscope after Comparative-Strauss evaluation of Comparative Examples 5 and 6 and Examples 1 and 2.
As shown in FIG. 3, Comparative Examples 5 and 6 clearly show the grain drop-out phenomenon due to intergranular corrosion. However, no intergranular corrosion occurred in the photographs of Examples 1 and 2. From these experiments, even when titanium (Ti) is not added, when appropriate amounts of molybdenum (Mo), silicon (Si), and manganese (Mn), which are weak carbide generating elements, are added, ferritic stainless steel It was confirmed that the intergranular corrosion of the steel can be completely prevented.

  According to ASTM standard A 240 / A 240M-08, low chromium ferritic stainless steels in general environments have a titanium (Ti) content that is more than eight times greater than the carbon (C) + nitrogen (N) content. It describes that intergranular corrosion is prevented. However, according to recent research results, in the heat affected zone of steel used at 400 ° C. to 700 ° C., stabilizing elements such as titanium (Ti) and niobium (Nb) are carbon (C) + nitrogen (N). Despite the addition of more than 20 times the content, chromium (C) r diffuses to the grain boundaries in the temperature range, and chromium (Cr) deficient layers are formed due to segregation of chromium (C) r, resulting in intergranular corrosion. Progressed.

  The intergranular corrosion test results of Comparative Examples 1 and 2 above show that the addition of a stabilizing element cannot completely prevent intergranular corrosion of low chromium stainless steel.

FIG. 4 shows the result of 3DAP analysis of the grain boundary of stainless steel of Comparative Example 1 in which a stabilizing element such as titanium (Ti) is added 20 times or more than the carbon (C) + nitrogen (N) content. is there.
As shown in FIG. 4, even if chromium (Cr) cannot generate precipitates at the grain boundaries due to the reaction between the stabilizing element and C under the above conditions, it concentrates around the grain boundary precipitates and at the grain boundaries. It has been confirmed that this phenomenon occurs.

FIG. 5 is a graph showing the concentration by position by analyzing the distribution by element at the grain boundary of Comparative Example 1 to which a stabilizing element is added by 3DAP, or the distribution of chromium (Cr) at the grain boundary of Comparative Example 1 It is shown.
As shown in FIG. 5, the chromium (Cr) concentration phenomenon that can be confirmed from FIG. 4 increases the chromium (Cr) concentration at the grain boundary by 35 atomic% or more, and in the peripheral portion of the concentrated chromium (Cr), chromium (Cr It can be confirmed that deficiency occurred and the chromium (Cr) concentration was reduced to 5.3 atomic% or less. This is because even in the low chromium stainless steel material, even if a stabilizing element is added to suppress the formation of chromium (Cr) precipitates, chromium (Cr) concentration at the grain boundary and chromium (Cr) deficiency around the grain boundary and This means that intergranular corrosion due to this cannot be prevented.

  Referring to FIG. 2, Comparative Examples 3 and 4 to which 0.6% by mass and 0.7% by mass of silicon (Si) are added, respectively, and Comparative Example to which 0.1% by mass of molybdenum (Mo) is added. In the case of 5, intergranular corrosion occurred, but it can be confirmed that the intergranular corrosion rate was significantly reduced as compared with Comparative Examples 1 and 2. In the case of Comparative Examples 6 and 7 in which the contents of molybdenum (Mo) and silicon (Si) are higher than those of Comparative Examples 3 to 5, but the manganese (Mn) content is low, the intergranular corrosion resistance is rather comparative Example 3. It was confirmed that it was lower than ˜5.

  On the other hand, in Examples 1-4 in which molybdenum (Mo), silicon (Si), and manganese (Mn) were added in combination, it can be confirmed that no intergranular corrosion occurred. From the above results, the intergranular corrosion of stainless steel that cannot be prevented by the addition of titanium (Ti), which is a carbide stabilizing element having a higher carbon affinity than chromium (Cr), is proposed in the present invention. It was confirmed that this can be prevented by a method in which elements having lower carbon affinity are added in combination.

  The above results indicate that when molybdenum (Mo), silicon (Si), and manganese (Mn) are added in combination, the sensitization and intergranular corrosion of ferritic stainless steel materials are completely eliminated when the respective concentrations are above a certain mass. It means that it can be prevented.

  FIG. 6 shows a typical steel grade in which intergranular corrosion did not occur. Example 2 was subjected to solution treatment at 1300 ° C. for 10 minutes, followed by sensitizing heat treatment at 500 ° C. for 2 hours, and the grain boundaries were observed by SEM analysis. The result is shown. As a result of the observation, it was confirmed that the intermetallic compound was uniformly generated and located along the grain boundary.

FIG. 7 shows the results obtained by subjecting Example 2 to solution treatment at 1300 ° C. for 10 minutes, followed by sensitizing heat treatment at 500 ° C. for 2 hours, extracting precipitates by a carbon replica analysis technique, and observing with a transmission electron microscope. It is a photograph. In the case of Example 2, molybdenum (Mo), silicon (Si), and manganese (Mn) -based intermetallic compounds are generated along the grain boundaries, and CMn ₄ MoSi, which is an intermetallic compound, is analyzed by an analysis pattern (Diffraction pattern) analysis. Was observed to be produced. A particularly noteworthy phenomenon is that the possibility of carbide formation is reduced by the solid solution of C in the CMn ₄ MoSi intermetallic compound.

  FIG. 8 shows a graph obtained by analyzing the elemental distribution in the grain boundary of Comparative Example 6 by 3DAP. Comparative Example 6 is a test piece in which manganese (Mn) is adjusted to 0.22% by mass, which is lower than the proper amount, and the intergranular corrosion has occurred greatly. The grain boundary of this test piece is observed with 3DAP. As a result, the chromium (Cr) concentration increased to 23 atomic% along the grain boundary, chromium (Cr) deficiency occurred at the periphery of the concentrated chromium (Cr), and the chromium (Cr) concentration was 7.8 atoms. % Can be confirmed. This is because, in the case of Comparative Example 6, since the content of added molybdenum (Mo) -silicon (Si) -manganese (Mn) is not sufficient, the concentration and depletion phenomenon of chromium (Cr) occurred. It means that the field corrosion cannot be prevented.

FIG. 9 is a graph obtained by analyzing the elemental distribution at the grain boundaries in Example 2 by 3DAP.
As a result of observing FIG. 9, the chromium (Cr) concentration increased to 18.2 atomic% along the grain boundary, but this value was significantly lower than Comparative Example 1 in FIG. 5 and Comparative Example 6 in FIG. The minimum content of Cr in the vicinity of the part is 9.9 atomic%, which is similar to the chromium (Cr) content in the grains. Therefore, in the case of Example 2, chromium (Cr) deficiency did not occur along the grain boundaries, and intergranular corrosion was effectively prevented.

  Table 3 shows the concentrations of chromium (Cr) and carbon (C) that have the most significant effect on the intergranular corrosion among the 3DAP analysis results of Comparative Example 1, Comparative Example 6 and Example 2, along with the intergranular corrosion test results. The illustrated embodiment is shown. As described above, the intergranular corrosion of stabilized ferritic stainless steel is caused by the concentration of carbon atoms dissolved in the grain boundary after solution treatment, thereby concentrating and depleting the chromium (Cr) grain boundary. Occurs when Comparing the carbon enrichment amounts in Table 3, the carbon enrichment amount in Comparative Example 6 was reduced to less than half compared to the carbon enrichment amount in Comparative Example 1, thereby reducing the grain boundary enrichment and deficiency phenomenon of chromium (Cr). However, damage due to intergranular corrosion still occurs. In the case of Example 2, the amount of carbon enrichment was significantly reduced as compared with Comparative Examples 1 and 6, thereby reducing the chromium (Cr) enrichment phenomenon at the grain boundary, and no chromium (Cr) deficiency phenomenon occurred. . Therefore, as a result of the reduction of the carbon concentration in Example 2, intergranular corrosion was prevented.

  More specifically, the reason why intergranular corrosion does not occur in ferritic stainless steel to which molybdenum (Mo) -silicon (Si) -manganese (Mn) is added in a sufficient amount is due to the following two phenomena. is there.

1) While a CMn ₄ MoSi intermetallic compound is formed in an alloy in which molybdenum (Mo) -silicon (Si) -manganese (Mn), which is a weak carbide forming element, is added, a solid solution is formed inside the steel material. Chromium (Cr) is prevented from diffusing due to carbon grain boundary concentration by fixing the carbon inside the intermetallic compound to enhance the stability of the carbon.

  2) A coarse intermetallic compound is formed in the periphery of the grain boundary to prevent chromium (Cr) from diffusing into the grain boundary. Further, by forming an intermetallic compound that does not contain chromium (Cr) around the grain boundary, chromium (Cr) distributed at the position where the intermetallic compound is formed becomes chromium (Cr) near the grain boundary. It diffuses in the deficient layer to alleviate the chromium (Cr) deficiency.

  That is, the stainless steel according to the present invention is stabilized in a state where the solid solution carbon is trapped by the molybdenum (Mo) -silicon (Si) -manganese (Mn) intermetallic compound in a high-temperature environment after production and welding. Therefore, it is possible to block the production of chromium carbide produced by the reaction between solid solution carbon and chromium (Cr), thereby preventing the induction of a chromium (Cr) deficient layer, so that stainless steel used in high temperature environments In particular, intergranular corrosion in the weld heat affected zone can be effectively prevented.

  On the other hand, conventional intergranular corrosion prevention technology, which has a carbon affinity better than chromium (Cr) and preferentially produces carbides over chromium (Cr), adds titanium (Ti) carbide or niobium produced. Metal carbide such as (Nb) carbide is decomposed in the welding heat-affected zone heated to a high temperature during the welding process, carbon is re-dissolved in the welding heat-affected zone, and the re-dissolved carbon is used in a high-temperature environment. Occasionally, a chromium (Cr) -deficient layer is formed, making it difficult to prevent intergranular corrosion at the weld heat affected zone of the high temperature environment stainless steel part.

  FIG. 10 is a graph showing the area reduction rate of the steel material after the intergranular corrosion experiment of the comparative example and the example of the present invention by the result value of the relational expression. A steel material having a relational expression value of 1 or more on which coarse molybdenum (Mo) -silicon (Si) -manganese (Mn) -based intermetallic compound is deposited has a grain boundary corrosion loss rate of 0, and intergranular corrosion does not occur. .

  As shown in the above examples, chromium (Cr): 10 to 14% by mass, carbon (C): 0.02% by mass or less, nitrogen (N): 0.02% by mass or less, phosphorus (P): 0.04 mass% or less, sulfur (S): 0.01 mass% or less, molybdenum (Mo): 0.05 to 2.0 mass%, silicon (Si): 0.2 to 1.5 mass%, manganese (Mn): 0.1 to 1.0% by mass, and the balance of iron (Fe) and inevitable impurities, and the relational expression “6 {(Mo−0.05) × (Si−0.2) × (Mn) It was confirmed that the ferritic stainless steel satisfying a result value of −0.18)} / (C + N) ”of 1 or more effectively suppresses intergranular corrosion.

As described above, the exemplary embodiments of the present invention have been described with reference to the drawings. However, various modifications and other embodiments can be implemented by skilled engineers in the field. Such modifications and other embodiments are all within the scope of the appended claims and do not depart from the true spirit and scope of the present invention.

Claims (5)

Chromium (Cr): 10 to 14% by mass, carbon (C): 0.02% by mass or less, nitrogen (N): 0.02% by mass or less, phosphorus (P): 0.04% by mass or less, sulfur (S ): 0.01 mass% or less, molybdenum (Mo): 0.05 to 2.0 mass%, silicon (Si): 0.2 to 1.5 mass%, manganese (Mn): 0.1 to 1. 0% by mass, and further satisfying that the value of the relational expression of 6 {(Mo−0.05) × (Si−0.2) × (Mn−0.18)} / (C + N) is 1 or more, and the rest Consisting of iron (Fe) and inevitable impurities,
Intergranular corrosion resistance, characterized in that molybdenum (Mo) -silicon (Si) -manganese (Mn) -carbon (C) composite intermetallic compound is formed at and around the grain boundaries of ferritic stainless steel Excellent ferritic stainless steel.
(However, Mo, Si, Mn, C and N in the above relational expression mean mass% of each component.) The ferritic stainless steel excellent in intergranular corrosion resistance according to claim 1 , wherein the intermetallic compound is CMn ₄ MoSi. 3. The resistance against resistance according to claim 1 , wherein a deviation between a maximum content of chromium (Cr) concentration and a minimum content of chromium (Cr) -deficient layer is 10 atomic% or less at the grain boundary of the ferritic stainless steel. Ferritic stainless steel with excellent intergranular corrosion. The ferritic stainless steel excellent in intergranular corrosion resistance according to claim 3 , wherein the maximum content of the chromium (Cr) concentration is 20 atomic% or less. The ferritic stainless steel excellent in intergranular corrosion resistance according to claim 3 , wherein the minimum content of the chromium (Cr) -deficient layer is 9.5 atomic% or more.

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