JP2019112709A - Ferritic stainless steel - Google Patents

Abstract

To provide a ferritic stainless steel excellent in high temperature salt damage resistance and corrosion resistance.SOLUTION: A ferritic stainless steel contains C:0.030 mass% or less, Si:1.40 mass% to 2.00 mass%, Mn:1.00 mass% or less, P:0.050 mass% or less, S:0.020 mass% or less, Ni:0.50 mass% to 2.00 mass%, Cr:18.00 mass% to 21.00 mass%, Mo:0.40 mass% to 2.50 mass%, N:0.030 mass% or less, at least one kind of Ti and Nb of total 10(C+N) mass% to 0.70 mass%, and the balance Fe with inevitable impurities. In a range of the alloy composition, 7Mo+3 Ni+5Si≥12.0 and Cr+3(Mo+Si)+Ni≥25.5 are satisfied.SELECTED DRAWING: None

Description

  The present invention relates to a ferritic stainless steel excellent in high temperature salt resistance and corrosion resistance.

  Conventionally, in order to ensure corrosion resistance due to exhaust gas condensed water and salt damage resistance derived from snow melting salt and the like, SUS409, SUS439 and SUS436 etc. are used as the exhaust gas passage member according to the material temperature.

  Such an exhaust gas passage member may have an exhaust gas temperature at the time of reaching the catalyst lowered by the improvement of the combustion efficiency of the engine, and may be covered with a heat retaining cover, a heat insulating material or the like for early activation of the catalyst.

  However, when the exhaust gas passage member is covered with a heat retaining cover, a heat insulating material, etc., the material temperature rises more than before, and it is exposed to a high temperature environment including snow melting salt. It is assumed that gender is required.

  In addition, since the interior of the heat retaining cover, the heat insulating material and the like becomes a wet environment containing salt when the engine is stopped, etc., not only high temperature resistance but also corrosion resistance is important.

  Although austenitic stainless steels containing Si and Mo are known as stainless steels excellent in high-temperature salt resistance, there are concerns about corrosion resistance deterioration due to sensitization and stress corrosion cracking as compared to ferritic stainless steels.

  Further, since it is necessary to add a bellows-processed flexible tube to the exhaust gas passage member as a measure against thermal fatigue, the disadvantages of these austenitic stainless steels become a serious problem.

  On the other hand, as a ferritic stainless steel applied to an exhaust gas passage member, for example, as disclosed in Patent Document 1 or the like, a material that secures workability to a flexible tube and is excellent in high-temperature salt resistance is known.

  Moreover, low-cost ferritic stainless steel which simulates the use condition of an exhaust gas path | route member like the patent document 2 grade | etc., And satisfies the reference | standard of corrosion resistance after 400 degreeC-8 h heating is known.

Japanese Patent Application Publication No. 2007-16305 JP, 2014-162964, A

  However, in the ferritic stainless steel of the above-mentioned patent documents 1, it is not examined about corrosion resistance, but it is unclear whether it can be applied under the environment where high temperature salt damage and wet corrosion are repeated.

  Moreover, in the ferritic stainless steel of patent document 2, it is evaluation of only the wet corrosion after 400 degreeC-8 h heating, and the high temperature salt resistance property in a high temperature environment is not evaluated, but high temperature salt damage and wet corrosion are repeated. It is unclear if it can be applied under the

  Therefore, a ferritic stainless steel excellent in high temperature salt resistance and corrosion resistance has been required so that it can be applied to use in an environment where high temperature salt damage and wet corrosion are repeated.

  The present invention has been made in view of these points, and it is an object of the present invention to provide a ferritic stainless steel excellent in high temperature salt resistance and corrosion resistance.

  The ferritic stainless steel described in claim 1 has C: 0.030 mass% or less, Si: 1.40 mass% or more and 2.00 mass% or less, Mn: 1.00 mass% or less, P: 0. 050% by mass or less, S: 0.020% by mass or less, Ni: 0.50% by mass to 2.00% by mass, Cr: 18.00% by mass to 21.00% by mass, Mo: 0.40% by mass % And 2.50 mass% or less and N: 0.030 mass% or less, containing at least one of Ti and Nb in total at 10 (C + N) mass% or more and 0.70 mass% or less, with the balance being It consists of Fe and unavoidable impurities, satisfies 7Mo + 3Ni + 5Si ≧ 12.0, and satisfies Cr + 3 (Mo + Si) + Ni ≧ 25.5.

  The ferritic stainless steel described in claim 2 is the ferritic stainless steel according to claim 1, wherein Al: 0.15 mass% or less, B: 0.0020 mass% or less, Cu: 2.00 mass % Or less and at least one of V, W and Co at a total of 1.00% by mass and REM and Ca at a total of at least one of 0.10% by mass or less It is a thing.

  According to the present invention, 7Mo + 3Ni + 5Si ≧ 12.0 is satisfied and Cr + 3 (Mo + Si) + Ni ≧ 25.5 is satisfied in the range of the specified predetermined alloy composition, so that high-temperature salt damage resistance and corrosion resistance are good.

  Hereinafter, the configuration of an embodiment of the present invention will be described in detail.

  The ferritic stainless steel according to one embodiment of the present invention has C (carbon): 0.030 mass% or less, Si (silicon): 1.40 mass% or more and 2.00 mass% or less, Mn (manganese): 1.00 mass% or less, P (phosphorus): 0.050 mass% or less, S (sulfur): 0.020 mass% or less, Ni (nickel): 0.50 mass% to 2.00 mass%, Cr (Chromium): 18.00% by mass or more and 21.00% by mass or less, Mo (molybdenum): 0.40% by mass or more and 2.50% by mass or less, and N (nitrogen): 0.030% by mass or less And at least one of Ti (titanium) and Nb (niobium) in total at 10 (C + N) mass% or more and 0.70 mass% or less, with the balance being Fe (iron) and unavoidable impurities.

  Moreover, Al (aluminum): 0.15% by mass or less, B (boron): 0.0020% by mass or less, Cu (copper): 2.00% by mass or less, V (vanadium), if necessary. And at least one of W (tungsten) and Co (cobalt) in total 1.00% by mass or less, and at least one of REM (rare earth element) and Ca (calcium) in total: 0.10% by mass or less And at least one of them.

  And in the range of content of each element of the above-mentioned alloy composition, it satisfies (1) Formula of 7Mo + 3Ni + 5Si> = 12.0, and satisfies (2) Formula of Cr + 3 (Mo + Si) + Ni> 25.5.

  In addition, each elemental symbol of Formula (1) and (2) shows content of each element which the ferritic stainless steel contains, the value (mass%) of the content is substituted, and it is additive-free. 0 is substituted for.

  C is an element which is unavoidably contained in steel and is an element which reduces intergranular corrosion resistance (sensitivity to suppress sensitization) and processability. Therefore, it is preferable to suppress the content, but the content is excessively reduced. If done, the cost of refining will rise more than necessary. Therefore, the content of C is set to 0.030% by mass or less.

  Si is an element effective for improving high-temperature salt resistance and corrosion resistance after heating, and in order to exert these effects, it is necessary to contain 1.40% by mass or more. On the other hand, if Si is contained in excess of 2.00% by mass, there is a possibility that the workability and weld zone toughness may be reduced. Therefore, the content of Si is set to 1.40% by mass or more and 2.00% by mass or less.

  Mn is useful as a deoxidizing element, but when it is contained in excess of 1.00% by mass, it tends to form MnS, which is a starting point of corrosion, and may destabilize the ferrite phase. Therefore, the content of Mn is 1.00% by mass or less, preferably 0.50% by mass or less.

  Since P is an element that reduces weldability, weld zone toughness and processability, it is preferable to reduce the content, but if it is reduced excessively, the refining cost will increase more than necessary. Therefore, the content of P is set to 0.050% by mass or less.

  S is an element that lowers the toughness of the welded portion and at the same time it is preferable to suppress the content to generate MnS which becomes a starting point of corrosion, but if it is reduced excessively, the refining cost will rise more than necessary. Therefore, the content of S is 0.020 mass% or less, preferably 0.010 mass% or less.

  Ni is an element effective for improving high-temperature salt resistance, corrosion resistance after heating, and processability (brittle cracking suppressing action), and in order to exhibit these actions, it is necessary to contain 0.50 mass% or more. On the other hand, when Ni is contained in excess of 2.00% by mass, the ferrite phase may be destabilized, and the material cost is increased more than necessary. Therefore, the content of Ni is 0.50% by mass or more and 2.00% by mass or less, preferably 1.00% by mass or less.

  Cr is an important element in securing corrosion resistance after heating, and in order to exhibit this function, it is necessary to contain 18.00 mass% or more. On the other hand, if Cr is contained in excess of 21.00% by mass, the processability may be lowered, and the material cost is increased more than necessary. Therefore, the content of Cr is set to 18.00% by mass or more and 21.00% by mass or less, preferably 18.50% by mass or more and 20.50% by mass or less.

  Mo is an element effective for improving high-temperature salt resistance and corrosion resistance after heating, and in order to exert this function, it is necessary to contain 0.40% by mass or more. On the other hand, when Mo is contained in excess of 2.50% by mass, the processability may be lowered, and the material cost is increased more than necessary. Therefore, the content of Mo is 0.40 mass% or more and 2.50 mass% or less, preferably 1.10 mass% or less, and more preferably 0.90 mass% or less.

  Since N is an element that reduces intergranular corrosion resistance (sensitivity to suppress sensitization) and processability, it is preferable to reduce the content, but if it is reduced excessively, the refining cost rises more than necessary. Therefore, the content of N is set to 0.030% by mass or less.

  Ti and Nb are elements that improve intergranular corrosion resistance (sensitization suppression action), and in order to exert this function, Ti and Nb content that lowers intergranular corrosion resistance, Ti And Nb in a total content of 10 (C + N) mass% or more. On the other hand, when the total content of Ti and Nb exceeds 0.70% by mass, the processability may be reduced. Therefore, the total content of Ti and Nb is 10 (C + N) mass% or more and 0.70 mass% or less. If Ti is contained in an amount of more than 0.30% by mass, the processability and the surface quality of the product may be degraded. If Nb is contained in an amount of more than 0.40% by mass, the formability and Since the toughness may decrease, the content of Ti is preferably 0.30% by mass or less, and the content of Nb is preferably 0.40% by mass or less.

  Al acts as a deoxidizing element, but if it is contained in excess of 0.15% by mass, it may be the cause of deterioration of the surface quality of the product. Therefore, when it contains Al, the content is made into 0.15 mass% or less.

  B is an element that improves the secondary workability, but if it is contained in excess of 0.0020 mass%, the hot workability may be reduced. Therefore, when it contains B, the content is made into 0.0020 mass% or less.

  Cu is an element improving the corrosion resistance, but if it is contained in excess of 2.00% by mass, the ferrite phase may be destabilized, and the material cost is increased more than necessary. Therefore, the content of Cu is 2.00% by mass or less, preferably 1.00% by mass, and more preferably 0.50% by mass or less.

  V, W and Co are elements that improve corrosion resistance and V and W are elements that improve toughness, but when the total content of these exceeds 1.00 mass%, workability and toughness are increased There is a possibility of lowering and material cost increases more than necessary. Therefore, when at least one of V, W and Co is contained, the total content of V, W and Co is 1.00 mass% or less.

  Although REM and Ca are useful as deoxidizing elements, if the total content of these exceeds 0.10% by mass, the material cost increases more than necessary. Therefore, when any of REM and Ca is contained, the total content of REM and Ca is 0.10 mass% or less.

  Here, Si, Ni and Mo are effective in improving the high temperature salt resistance of the above-mentioned alloy components, and the contribution of each component was examined based on the high temperature salt resistance evaluation at 650 ° C. 7Mo + 3Ni + 5Si It was derived that the high temperature salt resistance can be improved by adjusting the alloy components so as to satisfy the equation (1) shown by 112.0.

  Further, since the corrosion resistance is most reduced when an oxide film is formed at 500 ° C., it is important to be able to ensure the corrosion resistance even after heating to 500 ° C.

  Therefore, among the above alloy components, Si, Ni, Cr, and Mo are effective in improving the corrosion resistance at the time of heating, so Cr + 3 (Mo + Si) + Ni 2 25.5 based on the corrosion resistance evaluation at 500 ° C. It was derived that the corrosion resistance at the time of heating can be secured by adjusting the alloy components so as to satisfy the equation (2).

  Next, the operation and effects of the above-described embodiment will be described.

  According to the above ferritic stainless steel, while adjusting each element to the above-mentioned range, high temperature salt damage resistance can be improved by satisfying the relation of formula (1) of 7Mo + 3Ni + 5Si ≧ 12.0, and Cr + 3 (Mo + Si) + Ni Corrosion resistance at the time of heating can be secured by satisfying the relationship of expression (2) of 225.5.

  That is, even when exposed to a high temperature and wet environment containing salt, corrosion due to high temperature salt damage and corrosion due to wet corrosion can be suppressed.

  Therefore, the ferritic stainless steel excellent in high-temperature salt resistance and corrosion resistance as described above can be applied to applications used in an environment where high-temperature salt damage and wet corrosion are repeated, such as exhaust gas passage members.

  Hereinafter, the present embodiment and the comparative example will be described.

  First, a stainless steel of each alloy composition shown in Table 1 is melted, subjected to hot rolling, annealing / pickling, cold rolling, annealing / pickling to form a 1.2 mm cold rolled annealed sheet, The test was conducted to evaluate high temperature salt resistance, corrosion resistance and processability.

  In the evaluation of high temperature resistance to salt damage, each cold rolled annealed sheet is cut out to 35 mm (length) × 25 mm (width), the surface is polished with # 400, and the end face is polished with # 600. It was a test piece. In addition, the dimensions (length, width and thickness) and weight of each test piece were measured before the high temperature salt resistance test.

  In the high temperature salt resistance test, the test piece is immersed in saturated saline solution (26% NaCl aqueous solution) for 5 minutes at normal temperature, heated for 2 hours at 650 ° C in air, and air cooled to room temperature for 10 cycles as one cycle. Carried out.

  After the high temperature salt resistance test, the corrosion products adhering to the surface of the test piece were removed, and the weight was measured to calculate the corrosion loss per unit area.

And when the corrosion weight loss is 0.03 g / cm ² or less, it is evaluated that the high temperature resistance to salt damage is good and shown by ○ in Table 1, and when the corrosion weight loss exceeds 0.03 g / cm ^{2 the} high temperature resistance It was evaluated that salt damage was low and indicated by x in Table 1.

  In the corrosion resistance evaluation, the corrosion resistance after heating at 500 ° C. for 2 minutes was evaluated from the results of the preliminary test.

Specifically, as a preliminary test, a cold rolled annealed sheet of SUS 436 is washed with acetone, heated for 2 minutes at a temperature of 400 to 900 ° C., and in accordance with JIS G 0577, 1 M by potentiodynamic method using potentio studs Anodic polarization was carried out in a NaCl solution, and a potential exceeding 10 μA / cm ² was used as a pitting potential.

  In this preliminary test, since the pitting potential was lowest in the case of heating at 500 ° C. for 2 minutes, each cold rolled annealed sheet was washed with acetone, and a corrosion resistance test was conducted after heating at 500 ° C. for 2 minutes.

  In the corrosion resistance test, salt spray (35 ° C., 5% NaCl, 15 minutes), drying (60 ° C., 30% RH, 60 minutes), and wetting (50 ° C., 95% RH, 3 hours) are one cycle. 30 cycles were performed.

  After this corrosion resistance test, the corrosion products adhering to the test piece were removed, and the maximum corrosion depth was determined by the microscopic focal depth method.

  The corrosion resistance is evaluated as good when the maximum corrosion depth is 50 μm or less and indicated by ○ in Table 1, and when the maximum corrosion depth exceeds 50 μm is evaluated as low when the corrosion resistance is indicated by × in Table 1. The

  In the evaluation of workability (bending) of the welded portion, each cold rolled annealed sheet was subjected to TIG welding with a bead on plate under conditions of a welding current of 30 to 60 A and a welding speed of 20 to 40 cm / min, and then a bending test was performed.

  In the bending test, V-block bending at a bending radius of 2 mm and a bending angle of 90 degrees was performed at 0 ° C. perpendicular to the welding direction. After that, bending was performed at 0 ° C. until adhesion was further made, and the presence or absence of cracking at room temperature was observed.

  Then, those in which no cracks were observed were evaluated as good in processability and indicated by ○ in Table 1, and those in which cracks were confirmed were evaluated as poor in processability and indicated by × in Table 1.

  As shown in Table 1, in any of the examples, high temperature salt resistance, corrosion resistance and processability were all good.

  On the other hand, each comparative example which does not satisfy the condition of the range of the above-mentioned alloy composition, the relation of (1) Formula, and the relation of (2) Formula is high temperature salt resistance, corrosion resistance, and at least any of workability. Did not meet the criteria.

  In particular, Comparative Example 10, which does not satisfy the relationship of Formula (2) within the range of the alloy composition, does not meet the criteria of corrosion resistance, and does not meet the relationship of Formula (1) within the range of the alloy composition. The comparative example 11 did not satisfy | fill the standard of high-temperature salt damage resistance.

The ferritic stainless steel described in claim 1 has C: 0.030 mass% or less, Si: 1.40 mass% or more and 2.00 mass% or less, Mn: 1.00 mass% or less, P: 0. 050 mass% or less, S: 0.020 mass% or less, Ni: 0.50 mass% or more and 2.00 mass% or less, Cr: 18. 50 mass% or more and 21.00 mass% or less, Mo: 0.40 mass% or more and 2.50 mass% or less , N: 0.030 mass% or less, and Cu: 0.02 mass% or more and 2.00 mass% or less Containing at least one of Ti and Nb in total at 10 (C + N) mass% or more and 0.70 mass% or less, the balance being composed of Fe and unavoidable impurities, satisfying 7Mo + 3Ni + 5Si ≧ 12.0, Cr + 3 Mo + Si) + Ni ≧ 25.5 is satisfied.

The ferritic stainless steel described in claim 2 is the ferritic stainless steel according to claim 1, wherein Al: 0.15% by mass or less, B: 0.0020% by mass or less , V 2 , W and Co. It contains at least one of 1.00% by mass or less in total of at least one, and at least 0.10% by mass or less in total of at least one of REM and Ca.

The ferritic stainless steel according to one embodiment of the present invention has C (carbon): 0.030 mass% or less, Si (silicon): 1.40 mass% or more and 2.00 mass% or less, Mn (manganese): 1.00 mass% or less, P (phosphorus): 0.050 mass% or less, S (sulfur): 0.020 mass% or less, Ni (nickel): 0.50 mass% to 2.00 mass%, Cr (Chrome): 18. 50 mass% or more and 21.00 mass% or less, Mo (molybdenum): 0.40 mass% or more and 2.50 mass% or less , N (nitrogen): 0.030 mass% or less , and Cu (copper): 0. 02 wt% to 2.00 mass% and less, contained in Ti (titanium) and Nb (niobium) of at least one of 0.70 wt% 10 (C + N)% by weight or more in total less, is the balance It consists of Fe (iron) and unavoidable impurities.

In addition, if necessary, Al (aluminum): 0.15% by mass or less, B (boron): 0.0020% by mass or less, and at least one of V (vanadium), W (tungsten) and Co (cobalt) The seed contains at least one of 1.00% by mass or less in total, and at least one of REM (rare earth element) and Ca (calcium) in total: 0.10% by mass or less.

Cr is an important element in securing corrosion resistance after heating, and it is necessary to exert this effect. It is necessary to contain 50 % by mass or more. On the other hand, if Cr is contained in excess of 21.00% by mass, the processability may be lowered, and the material cost is increased more than necessary. Therefore, the content of Cr is 18. The content is 50 % by mass or more and 21.00% by mass or less, preferably 18.50% by mass or more and 20.50% by mass or less.

Claims (2)

C: 0.030% by mass or less, Si: 1.40% by mass to 2.00% by mass, Mn: 1.00% by mass or less, P: 0.050% by mass or less, S: 0.020% by mass or less Ni: 0.50% by mass or more and 2.00% by mass or less Cr: 18.00% by mass or more and 21.00% by mass or less Mo: 0.40% by mass or more and 2.50% by mass or less N: 0. 030% by mass or less, containing at least one of Ti and Nb in a total amount of 10 (C + N)% by mass or more and 0.70% by mass or less, with the balance being Fe and unavoidable impurities,
Satisfy 7Mo + 3Ni + 5Si ≧ 12.0,
A ferritic stainless steel characterized by satisfying Cr + 3 (Mo + Si) + Ni ≧ 25.5. Al: 0.15% by mass or less, B: 0.0020% by mass or less, Cu: 2.00% by mass or less, and at least one of V, W and Co in total 1.00% by mass or less The ferritic stainless steel according to claim 1, characterized in that it contains at least one of REM and Ca and 0.10% by mass or less in total.