JP5343446B2 - Ferritic stainless steel with excellent thermal fatigue properties, oxidation resistance and high temperature salt corrosion resistance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a ferritic stainless steel which has all of superior thermal fatigue characteristics, oxidation resistance and high-temperature salt-corrosion resistance though Mo, W and Cu are not added thereinto. <P>SOLUTION: The ferritic stainless steel has a component composition containing, by mass%, 0.015% or less C, 1.0% or less Si, 0.5% or less Mn, 0.040% or less P, 0.010% or less S, 0.03 to 0.30% Al, 16 to 20% Cr, 0.5% or less Ni, 0.015 to 0.030% N, 0.30 to 0.60% V, 10(C+N) to 0.60% Nb, 0.01% or less Ti, 0.01% or less Zr, 0.01% or less Ta, 0.1% or less Mo, 0.1% or less W while V and N satisfy the relation of (V&times;N)=0.003 to 0.015, and the balance Fe with unavoidable impurities. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a Cr-containing steel suitable for use in a high-temperature environment such as an exhaust pipe of an automobile or motorcycle, a catalyst outer cylinder, an exhaust duct of a thermal power plant, in particular, excellent thermal fatigue characteristics, The present invention relates to a ferritic stainless steel having both oxidation resistance and high temperature salt damage corrosion resistance.

  Components used in exhaust systems such as automobile exhaust manifolds, exhaust pipes, converter cases, and mufflers have thermal fatigue characteristics and oxidation resistance (hereinafter referred to collectively as “heat resistance”). It is also required to be excellent. For this reason, Cr-containing steel such as Type 429 (14Cr-0.9Si-0.4Nb series) steel to which Nb and Si are added is often used for such applications. However, if the exhaust gas temperature rises with the improvement of engine performance and rises to about 900 ° C., which is higher than the current level, Type 429 steel may have insufficient thermal fatigue characteristics.

  For this problem, a Cr-containing steel in which Nb and Mo are added in combination to improve high-temperature yield strength, such as SUS444 (19Cr-2Mo-0.5Nb) steel defined in JIS G4305, Nb, Mo, Ferritic stainless steel with W added in combination has been developed. However, due to the unusually high price of rare metals such as Mo and W, the development of materials having the same heat resistance without using these elements has been demanded.

  As a technique for obtaining a material having excellent heat resistance without using expensive elements such as Mo and W, for example, there are those disclosed in Patent Documents 1 to 3. These techniques are characterized by improving thermal fatigue characteristics mainly by adding Cu. However, studies by the inventors have revealed that Cu is an element that not only lowers the oxidation resistance of steel itself but also reduces workability. Therefore, in addition to Mo and W, there is an urgent need to design a component that minimizes addition of Cu.

As materials having improved heat resistance without using Mo, W, or Cu, there are steels disclosed in Patent Document 4 and Patent Document 5. These steels are characterized in that heat resistance is improved by utilizing dispersion strengthening of VN particles. Patent Document 6 discloses a method for producing ferritic stainless steel having excellent workability by adding V, and Patent Document 7 discloses a ferritic stainless hot rolled steel sheet having excellent high-temperature fatigue characteristics by adding V. Yes.
WO2003 / 004714 JP 2006-117985 A JP 2000-297355 A JP 2001-316774 A Japanese Patent Laid-Open No. 07-070709 Japanese Patent Laid-Open No. 06-158162 JP 2000-144344 A

  However, according to the researches of the inventors, the steels disclosed in the above Patent Documents 4 to 7 are manufactured according to the disclosed technical contents, and the thermal fatigue characteristics and oxidation resistance aimed by the present invention are also achieved. Was not available, and it became clear that further study was necessary. It has also been clarified that the steel tends to be inferior in high temperature salt damage corrosion resistance as compared with SUS444.

  Therefore, the object of the present invention is to further improve the conventional steel using the dispersion strengthening of VN particles, and all of excellent thermal fatigue properties, oxidation resistance and high temperature salt corrosion resistance without adding Mo, W and Cu. It is to provide a ferritic stainless steel having both. Here, “excellent thermal fatigue characteristics and oxidation resistance” as used in the present invention means that they have the same characteristics as SUS444, specifically, thermal fatigue characteristics at 200 ° C./850° C. and oxidation resistance at 1000 ° C. "Superior high temperature salt corrosion resistance" means that the weight loss by high temperature salt damage corrosion test at 700 ° C is equal to or higher than that of SUS444.

  The inventors have repeated detailed studies on the reason why sufficient heat resistance and high temperature salt damage corrosion resistance cannot be obtained with conventional steel using dispersion strengthening of VN particles. As a result, in steel containing Nb in the range of 10 × (C (mass%) + N (mass%)) to 0.60 mass%, the high temperature strength is increased without adding Mo, W and Cu, and it is equivalent to SUS444. It is necessary to control the contents of Nb, V, and N within an appropriate range in order to achieve the thermal fatigue characteristics of SUS444 and ensure high-temperature salt damage corrosion resistance equal to or higher than SUS444. , N content is controlled to 0.015 to 0.030 mass%, V content is controlled to a range of 0.30 to 0.60 mass%, and the product of V and N content (mass%) (V × N) is controlled to be in the range of 0.003 to 0.015, and it is necessary to effectively utilize the particle dispersion strengthening ability of VN. Further, in order to further improve the thermal fatigue characteristics, Nb, T, which is a nitriding element other than V It has been found that the contents of i, Zr, and Ta also need to be regulated. Moreover, in order to improve the oxidation resistance at 1000 ° C. and make it equivalent to SUS444, the content of Mn is regulated, specifically, it is found that Mn needs to be 0.5 mass% or less, The present invention has been completed.

  That is, the present invention includes C: 0.015 mass% or less, Si: 1.0 mass% or less, Mn: 0.5 mass% or less, P: 0.040 mass% or less, S: 0.010 mass% or less, Al: 0. 03 to 0.30 mass%, Cr: 16 to 20 mass%, Ni: 0.5 mass% or less, N: 0.015 to 0.030 mass%, V: 0.30 to 0.60 mass%, Nb: 10 (C ( mass%) + N (mass%)) to 0.60 mass%, Ti: 0.01 mass% or less, Zr: 0.01 mass% or less, Ta: 0.01 mass% or less, Mo: 0.1 mass% or less, W: 0 0.1 mass% or less, and the product of V and N content (mass%) (V × N) satisfies (V × N): 0.003 to 0.015, and the balance is Fe And a ferritic stainless steel excellent in thermal fatigue characteristics, oxidation resistance and high temperature salt corrosion resistance, characterized by having a component composition consisting of inevitable impurities.

  In addition to the above component composition, the ferritic stainless steel of the present invention further contains one or two selected from B: 0.0004 to 0.0020 mass% and Co: 0.05 to 0.1 mass%. It is characterized by doing.

  According to the present invention, heat resistance (thermal fatigue characteristics and oxidation resistance) equivalent to SUS444 without using expensive Mo or W, and without adding Cu that lowers oxidation resistance, and equivalent to SUS444. The above-described ferritic stainless steel having high temperature salt damage corrosion resistance can be provided at low cost. Therefore, the ferritic stainless steel of the present invention is suitable for an automobile exhaust member, and has a remarkable industrial effect.

The basic experiment that provided the opportunity to obtain the above knowledge of the present invention will be described.
(Experiment 1) First, the influence of the N content on the thermal fatigue properties of 18 mass% Cr-containing steel was investigated.
C: 0.005-0.010 mass%, Si: 0.10-0.30 mass%, Mn: 0.10-0.30 mass%, Al: 0.042-0.048 mass%, Cr: 17-18. Steel containing 5 mass%, Nb: 0.38 to 0.43 mass% and V: 0.19 to 0.22 mass%, and variously changing the N content in the range of 0.008 to 0.048 mass% Was melted in a laboratory, and the obtained steel ingot was forged into a square of 30 mm × 30 mm. After heat treatment, a thermal fatigue test piece having the shape and dimensions shown in FIG. 1 was produced. Next, as shown in FIG. 2, the test piece was subjected to a thermal fatigue test in which the temperature was increased and decreased between 200 ° C. and 850 ° C. with a restraint ratio of 0.8. “Thermal fatigue life” defined by the number of cycles to below 80% was measured. In addition, as a comparative material, also about SUS444 (18Cr-2Mo-0.5Nb steel), the thermal fatigue test piece was produced similarly to the above, and it used for the same thermal fatigue test.

  FIG. 3 shows the test results as a relationship between the thermal fatigue life and the N content. From FIG. 3, it was found that the content of N that provides a thermal fatigue life superior to that of SUS444 is in the range of 0.015 to 0.040 mass%.

(Experiment 2) Next, the effect of the V content on the thermal fatigue properties of 18 mass% Cr-containing steel was investigated.
C: 0.005-0.010 mass%, Si: 0.10-0.30 mass%, Mn: 0.10-0.30 mass%, Al: 0.042-0.048 mass%, Cr: 17-18. Steel containing 5 mass%, Nb: 0.28 to 0.32 mass%, N: 0.019 to 0.022 mass%, and varying the V content in the range of 0.11 to 0.71 mass% 1 was prepared in the laboratory, and then the thermal fatigue test piece shown in FIG. 1 was produced in the same manner as in Experiment 1 above, and subjected to the thermal fatigue test shown in FIG. 2 to measure the thermal fatigue life.

  FIG. 4 shows the results of the above test as the relationship between the thermal fatigue life and the V content. The thermal fatigue life superior to SUS444 in the range where the V content is 0.15 to 0.60 mass%. Was found to be obtained.

(Experiment 3) Next, the influence of the product (V × N) of the contents of V and N (mass%) on the thermal fatigue properties of 18 mass% Cr-containing steel was investigated.
In addition to Experiment 1 and Experiment 2 above, the composition of the components other than V and N is the same as Experiment 1 and Experiment 2, and the value of the product (V × N) of the contents of V and N (mass%) varies The produced steel was melted in the laboratory, and the thermal fatigue test piece shown in FIG. 1 was produced in the same manner as in the above experiment 1 and subjected to the thermal fatigue test under the conditions shown in FIG. It was measured.

  FIG. 5 shows the relationship between the thermal fatigue life and (V × N). From FIG. 5, even when the contents of V and N are within the preferred ranges obtained in Experiments 1 and 2, respectively, when the value of (V × N) is less than 0.003 or more than 0.015, The thermal fatigue life equivalent to SUS444 cannot be obtained. Therefore, in order to obtain the thermal fatigue life equivalent to SUS444, the contents of V and N are controlled within the preferable range obtained in Experiments 1 and 2, and It was found that (V × N) needs to be controlled in the range of 0.003 to 0.015.

  Thus, the reason why there is an optimum range for increasing the thermal fatigue life in the product (V × N) of the content (mass%) of V and N is that when the value of (V × N) is too small, 600 to 800 Since the amount of VN that precipitates finely in the steel at a temperature of ° C. is too small, the effect of improving the strength of the steel and improving the thermal fatigue properties is poor, while if the value of (V × N) becomes too large, This is thought to be because the VN becomes coarse and the thermal fatigue properties are reduced.

(Experiment 4) Next, as with Nb, the effect of Zr, Ti and Ta forming nitrides on the thermal fatigue life of 18 mass% Cr-containing steel was investigated.
C: 0.005-0.010 mass%, Si: 0.19-0.22 mass%, Mn: 0.25-0.29 mass%, Al: 0.042-0.048 mass%, Cr: 17.9- 18.1 mass%, Nb: 0.34 to 0.37 mass%, V: 0.19 to 0.24 mass%, N: 0.0.19 to 0.022 mass%, Zr, Ti and Ta Steels with various amounts varied in the range of 0.003 to 0.020 mass%, 0.003 to 0.014 mass%, and 0.003 to 0.015 mass%, respectively, were melted in the laboratory. Similarly, the thermal fatigue test piece shown in FIG. 1 was produced and subjected to the thermal fatigue test under the conditions shown in FIG. 2, and the thermal fatigue life was measured.

  FIG. 6 shows the influence of the contents of Zr, Ti and Ta on the thermal fatigue life. From FIG. 6, it can be seen that when Zr, Ti and Ta are each contained in an amount exceeding 0.01 mass%, a thermal fatigue life equivalent to that of SUS444 cannot be obtained. The reason for this is considered that when Zr, Ti and Ta are added excessively, the precipitation of VN is suppressed and the effect of dispersion strengthening due to the VN precipitation, which is a feature of the present invention, cannot be obtained. Therefore, in order to improve the thermal fatigue life, not only the above-described contents of V and N and the value of (V × N) are optimized, but also the contents of Zr, Ti and Ta are optimized. Specifically, it has been found that each of Zr, Ti, and Ta must be regulated to 0.01 mass% or less.

(Experiment 5) Next, in steel used for exhaust system members, the influence of the Mn content on the oxidation resistance, which is an important characteristic along with the thermal fatigue characteristics, was investigated.
C: 0.005 to 0.010 mass%, Si: 0.19 to 0.22 mass%, Al: 0.042 to 0.048 mass%, Cr: 17.9 to 18.1 mass%, Nb: 0.29 to 0.43 mass%, V: 0.19 to 0.24 mass%, N: 0.019 to 0.022 mass% N, Mn content varied in the range of 0.13 to 0.97 mass% The obtained steel was melted in a laboratory, and the obtained steel ingot was hot-rolled, cold-rolled and finish-annealed to obtain a cold-rolled annealed plate having a thickness of 2 mm. A sample for an oxidation test having a thickness of 30 mm × 20 mm × thickness was taken from this cold-rolled annealed plate, and the surface of this sample was polished with # 320 emery paper, and then in an oven maintained at 1000 ° C. for 200 hours. The oxidation resistance was evaluated from the mass change (oxidation increase) before and after the oxidation test. In addition, the same continuous oxidation test was done also about SUS444 as a comparative material, and oxidation resistance was evaluated.

  FIG. 7 shows the relationship between the oxidation increase and the Mn content. From FIG. 7, it was found that the Mn content must be limited to 0.5 mass% or less in order to obtain a continuous oxidation resistance equal to or higher than that of SUS444.

(Experiment 6) Next, the influence of the contents of N and V on the high temperature salt corrosion resistance of 18 mass% Cr-containing steel was investigated. This high temperature salt corrosion property means that oxides such as Cr ₂ O ₃ and (Fe, Cr) ₂ O ₃ produced at high temperatures react with Na to form water-soluble compounds, and dissolution / disappearance and oxidation are repeated. This is the corrosion that occurs.
C: 0.005-0.010 mass%, Si: 0.10-0.30 mass%, Mn: 0.10-0.30 mass%, Al: 0.042-0.048 mass%, Cr: 17-18. 5 mass%, Nb: 0.47 to 0.53 mass%, V content is 0.19 to 0.22 mass%, N content is variously changed in the range of 0.009 to 0.048 mass% And steel in which the content of N was varied in the range of 0.11 to 0.71 mass% with the N content being 0.019 to 0.022 mass%, The obtained steel ingot was hot-rolled, hot-rolled sheet annealed, pickled, cold-rolled, and finish-annealed to obtain a cold-rolled annealed sheet having a thickness of 2 mm. A sample of 30 mm × 20 mm × plate thickness was collected from the cold-rolled annealed plate, and the surface of this sample was polished with # 320 emery paper, and then subjected to a Dip & Dry test. In this Dip & Dry test, the above sample was immersed in a saturated saline solution (room temperature) for 5 minutes, heated and maintained at 700 ° C. for 2 hours, and subjected to 10 cycles of corrosion test, and then 10% ammonium citrate. It is a test method in which corrosion weight loss is calculated by immersing in an aqueous solution, brushing with a nylon brush to remove corrosion products on the surface of the test piece, and measuring the mass change before and after the corrosion test.

  9 and 10 show the results of the above Dip & Dry test. From these figures, in order to reduce the corrosion weight loss due to salt damage to less than SUS444, the N content is 0.03 mass% or less, V It was found that the content must be 0.30 mass% or more.

(Experiment 7) Next, the influence of the Al content on the high temperature salt corrosion resistance of 18 mass% Cr-containing steel was investigated.
C: 0.005 to 0.010 mass%, Si: 0.19 to 0.22 mass%, Mn: 0.18 to 0.22 mass%, Cr: 17.9 to 18.1 mass%, Nb: 0.29 to 0.33 mass%, V: 0.33 to 0.35 mass%, N: 0.019 to 0.022 mass%, Al content varied in the range of 0.06 to 0.36 mass% Steel was melted in the laboratory, and the Dip & Dry test was performed in the same manner as in Experiment 6 to measure the corrosion weight loss.

FIG. 11 shows the result of the Dip & Dry test. From this figure, in order to reduce the corrosion weight loss due to salt damage to less than SUS444, the Al content must be 0.03 mass%. all right.
The present invention has been made by further studying the above findings.

Next, the component composition of the ferritic stainless steel of the present invention will be described.
C: 0.015 mass% or less C is an element that enhances the strength of steel, and in order to obtain a desired strength, it is preferable to contain 0.001 mass% or more. However, if the content exceeds 0.015 mass%, the toughness and workability deteriorate significantly, so the upper limit is made 0.015 mass%. In addition, when calculating | requiring workability more, C is so desirable that it is low, and it is preferable that it is 0.008 mass% or less. More preferably, it is the range of 0.002-0.008 mass%.

Si: 1.0 mass% or less Si is an element that improves the oxidation resistance of steel, and is also an element added as a deoxidizer. However, excessive addition reduces processability. Therefore, Si is set to 1.0 mass% or less.

Mn: 0.5 mass% or less Mn is an element having an effect as a deoxidizer. However, excessive addition promotes the formation of a γ phase at a high temperature and reduces heat resistance (oxidation resistance). Therefore, Mn is 0.5 mass% or less. Preferably, it is 0.35 mass% or less.

P: 0.040 mass% or less P is an element that lowers the toughness of steel, and is desirably reduced as much as possible. Therefore, in the present invention, P is set to 0.040 mass% or less. Preferably it is 0.030 mass% or less.

S: 0.010 mass% or less S is an element that lowers the elongation and r value of steel and degrades formability and lowers the corrosion resistance, which is a basic characteristic of stainless steel, and is desirably reduced as much as possible. Therefore, in the present invention, S is limited to 0.010 mass% or less.

Al: 0.03-0.30 mass%
Al is an element effective for improving the oxidation resistance of steel and the salt corrosion resistance at high temperatures. To obtain the salt corrosion resistance equivalent to or higher than SUS444, it is necessary to add 0.03 mass% or more. On the other hand, the effect of Al on the high temperature salt corrosion resistance is almost saturated at about 0.2 mass%, but the addition exceeding 0.3 mass% hardens the steel and causes a decrease in workability. Therefore, Al is set to a range of 0.03 to 0.30 mass%.

Cr: 16-20 mass%
Cr is an important element that improves the oxidation resistance of steel. In order to acquire such an effect, addition of 16 mass% or more is necessary. On the other hand, Cr dissolves in steel and becomes hard and low ductile at room temperature to deteriorate workability. In particular, when it exceeds 20 mass%, the tendency becomes remarkable. Therefore, Cr is set to a range of 16 to 20 mass%. In order to obtain particularly excellent ductility, the Cr content is preferably 18.5 mass% or less.

Ni: 0.5 mass% or less Ni is an element that improves the toughness of steel. However, it is expensive and is a strong γ-phase-forming element, so it promotes the formation of γ-phase at high temperatures and is resistant to oxidation. Reduce. Therefore, Ni is made 0.5 mass% or less.

N: 0.015-0.030 mass%, V: 0.30-0.60 mass%, and (V × N): 0.003-0.015
In the present invention, V and N are important elements for increasing the strength of the steel and improving the thermal fatigue characteristics. As shown in FIGS. 3 to 5, N: 0.015 to 0.040 mass%, V: 0.15 to 0.60 mass%, and the contents of V and N are obtained in order to obtain thermal fatigue characteristics equivalent to or higher than those of SUS444. Product of quantity (mass%) (V × N): It is necessary to satisfy all of 0.003 to 0.015. When the N content is less than 0.015 mass%, the V content is less than 0.15 mass%, or the value of (V × N) is less than 0.003, VN does not precipitate finely in the steel at a temperature of 600 to 800 ° C. For this reason, the effect of improving the thermal fatigue characteristics aimed by the present invention cannot be obtained. On the other hand, when the N content exceeds 0.040 mass% and the V content exceeds 0.60 mass% or (V × N) exceeds 0.015, the finely precipitated VN becomes coarse, and on the other hand, the thermal fatigue characteristics are reduced. Because it will end up.
In addition, in order to obtain good high temperature salt corrosion resistance better than SUS444 at the same time as good thermal fatigue characteristics, as shown in FIGS. 9 and 10, the N content and the V content are 0.03 mass% or less, 0 It is necessary to satisfy 30 mass% or more. Therefore, in this invention, it is set as the range of N: 0.015-0.030 mass% and V: 0.30-0.60 mass%.

Nb: 10 (C (mass%) + N (mass%)) to 0.60 mass%
Nb is an element effective for fixing C and N and enhancing the thermal fatigue properties by increasing the high-temperature strength and having the effect of enhancing the steel's sensitization, formability, and intergranular corrosion of the welded portion. It is. However, if the Nb content is less than 10 times the total content (mass%) of C and N, that is, less than 10 (C + N), the effect of suppressing steel sensitization cannot be obtained. On the other hand, the addition exceeding 0.60 mass% promotes precipitation of the Laves phase and easily causes embrittlement. Further, excessive addition of Nb suppresses the precipitation of VN, which is important in the present invention, and the effect of improving thermal fatigue characteristics cannot be obtained. Therefore, the Nb content is in the range of 10 (C + N) to 0.60 mass%. Preferably, it is in the range of 10 (C + N) to 0.55 mass%.

Ti: 0.01 mass% or less, Zr: 0.01 mass% or less, and Ta: 0.01 mass% or less Ti, Zr and Ta, like Nb and V, fix C and N, corrosion resistance, formability, welding It is an element having the effect of improving the intergranular corrosion property of the part. However, if each of these elements is contained in an amount of 0.01 mass% or more, the precipitation of VN, which plays an important role in the present invention, is suppressed, and the VN precipitation effect cannot be enjoyed, resulting in a decrease in thermal fatigue characteristics. End up. Therefore, in the present invention, these elements are each 0.01% by mass or less.

Mo: 0.1 mass% or less, W: 0.1 mass% or less Mo and W are effective elements for improving high-temperature fatigue characteristics and oxidation resistance, but both are expensive elements and are called inexpensive material development. For the purposes of the present invention, it is not actively added. Therefore, these elements are mixed only from scraps of iron making raw materials, and their content is at most 0.1 mass%. Therefore, in the present invention, the contents of Mo and W are each 0.1 mass% or less.

The ferritic stainless steel of the present invention can further contain B and Co in the following ranges in addition to the essential components.
B: 0.0004 to 0.0020 mass%
B is an element effective for improving workability, particularly secondary workability. This effect is manifested by the addition of 0.0004 mass% or more. However, addition exceeding 0.0020 mass% generates BN and causes deterioration of workability. Therefore, when adding B, it is set as the range of 0.0004-0.0020 mass%.

Co: 0.05 to 0.1 mass%
Co is an element effective for improving the low temperature toughness of steel, and the effect is recognized by addition of 0.05 mass% or more. However, Co is an expensive element, and the above effect is saturated even if it is added in excess of 0.1 mass%. Therefore, when Co is added for the purpose of improving low temperature toughness, it is preferably in the range of 0.05 to 0.1 mass%.
In the ferritic stainless steel of the present invention, components other than those described above are Fe and inevitable impurities.

Next, the manufacturing method of the ferritic stainless steel of this invention is demonstrated.
The method for producing the steel of the present invention is not particularly limited, and any method can be suitably used as long as it is a general method for producing ferritic stainless steel. For example, the above-described steel having a composition suitable for the present invention is melted by applying a secondary refining such as a converter, a smelting furnace such as an electric furnace, or a ladle refining, vacuum refining, etc. It is made into a steel slab (slab) by the ingot-making and ingot rolling method, and then made into a cold-rolled annealed plate through various processes such as hot rolling, hot-rolled sheet annealing, pickling, cold rolling, finish annealing, pickling. Is preferred. In the above method, the cold rolling may be performed once or twice or more with intermediate annealing. Moreover, each process of cold rolling, finish annealing, and pickling may be repeated as needed, and hot-rolled sheet annealing may be omitted. Furthermore, when the gloss of the steel plate surface is required, a skin pass or the like may be applied.

Steel having the composition shown in Table 1-1 and Table 1-2 is melted in a vacuum melting furnace to form a 50 kg steel ingot, divided into two parts, one of the steel ingots is heated to 1170 ° C., and then hot rolled. Then, a hot-rolled sheet bar having a width of 150 mm × 35 mm was formed, which was forged into a square member of 30 mm × 30 mm, and annealed at 1040 ° C. Thereafter, a thermal fatigue test piece having the shape and dimensions shown in FIG. 1 was produced from the square bar by machining, and as shown in FIG. 2, the temperature was repeatedly raised between 200 ° C. and 850 ° C. at a restraint ratio of 0.8. -It was subjected to a thermal fatigue test to lower the temperature, and the thermal fatigue life was measured. The temperature increase / decrease rate was 5 ° C./s, the retention time at 850 ° C. was 1 minute, and the retention time at 200 ° C. was 0 minute. The definition of the thermal fatigue life is defined as the number of cycles until the load detected at 200 ° C. falls below the initial 80%.
Further, as reference examples, the thermal fatigue characteristics of steels (No. 1 to 4) and SUS444 (No. 5) having the component compositions disclosed in Patent Documents 4 to 7 were also evaluated in the same manner as described above.

The other steel ingot obtained in Example 1 was heated to 1170 ° C. and hot-rolled to obtain a hot-rolled sheet having a thickness of 5 mm. Subsequently, this hot-rolled sheet is subjected to hot-rolled sheet annealing (annealing temperature: 1040 ° C.), pickling, cold rolling (cold rolling reduction ratio: 60%), and finish annealing (annealing temperature: 1040 ° C., average cooling). (Speed: 30 ° C./s) and pickled to obtain a cold-rolled annealed plate having a thickness of 2 mm.
Next, from each of the cold-rolled annealed plates obtained as described above, a sample of 30 mm × 20 mm × thickness was cut out, a hole of 4 mmφ was made in the upper part of the sample, and the surface and end face were polished with # 320 emery paper And degreased. Thereafter, the sample was subjected to an atmospheric continuous oxidation test suspended in an atmospheric furnace heated and held at 1000 ° C. and held for 200 hours. After the test, the weight of the sample was measured, and the difference from the weight before the test was calculated to obtain the oxidation increase. The said continuous oxidation test was implemented twice about each cold-rolled annealing board, and oxidation resistance was evaluated by the average value.
Further, as in Example 1, as a reference example, oxidation resistance was also evaluated for conventional steel (No. 1 to 4) and SUS444 (No. 5) in the same manner as described above.

A sample with a thickness of 30 mm × 20 mm × thickness was taken from the cold-rolled annealed plate with a thickness of 2 mm obtained in Example 2, and the surface of this sample was polished with # 320 emery paper, and then the high temperature salt corrosion resistance was obtained. In order to evaluate, it used for the Dip & Dry test. In this Dip & Dry test, the above sample was immersed in a saturated saline solution (room temperature) for 5 minutes, heated and maintained at 700 ° C. for 2 hours, and subjected to 10 cycles of corrosion test, and then 10% ammonium citrate. It is a test method in which corrosion weight loss is calculated by immersing in an aqueous solution, brushing with a nylon brush to remove corrosion products on the surface of the test piece, and measuring the mass change before and after the corrosion test.
As in Example 1, as a reference example, the conventional steel (No. 1 to 4) and SUS444 (No. 5) were also evaluated for high temperature salt corrosion resistance in the same manner as described above.

The results of Examples 1 to 3 are shown together in Table 1-2. From these results, all of the steels of the invention examples (Nos. 32-47) suitable for the component composition of the present invention have the same or better heat fatigue characteristics (thermal fatigue life: 520 cycles or more) and oxidation resistance (SUS 444). It can be seen that both the increase in oxidation: 33 g / m ² or less) and the high temperature salt corrosion resistance (corrosion loss: less than 0.4 kg / m ² ) are combined. On the other hand, comparative steels (No. 6 to 31) that do not satisfy the component composition of the present invention and conventional steels (No. 1 to 4) of reference examples have heat fatigue characteristics, oxidation resistance, and high temperature salt corrosion resistance. Any one or more characteristics are inferior to SUS444 (No. 5), and the target of the present invention is not achieved.

  The ferritic stainless steel of the present invention is not limited to automobile exhaust members, but is used for exhaust path members of thermal power generation systems and solid oxide fuel cells that require the same characteristics as the steel of the present invention. It can also be suitably used as a member.

It is a figure explaining the thermal fatigue test piece used in this invention. It is a figure explaining the thermal fatigue test done in this invention. It is a graph which shows the influence of N content which acts on the thermal fatigue life of 18Cr steel. It is a graph which shows the influence of V content which acts on the thermal fatigue life of 18Cr steel. It is a graph which shows the influence of (VxN) which acts on the thermal fatigue life of 18Cr steel. It is a graph which shows the influence of Zr, Ti, and Ta content which has on the thermal fatigue life of 18Cr steel. It is a graph which shows the influence of Mn content which gives to the oxidation resistance of 18Cr steel. It is a graph which shows the influence of N content which gives to high temperature salt damage test corrosion weight loss of 18Cr steel. It is a graph which shows the influence of the V content which gives to the high temperature salt damage test corrosion weight loss of 18Cr steel. It is a graph which shows the influence of Al content which gives to the high temperature salt damage test corrosion weight loss of 18Cr steel.

Claims (2)

C: 0.015 mass% or less,
Si: 1.0 mass% or less,
Mn: 0.5 mass% or less,
P: 0.040 mass% or less,
S: 0.010 mass% or less,
Al: 0.03-0.30 mass%,
Cr: 16-20 mass%,
Ni: 0.5 mass% or less,
N: 0.015-0.030 mass%,
V: 0.30-0.60 mass%,
Nb: 10 (C (mass%) + N (mass%)) to 0.60 mass%,
Ti: 0.01 mass% or less,
Zr: 0.01 mass% or less,
Ta: 0.01 mass% or less,
Mo: 0.1 mass% or less,
W: 0.1 mass% or less, and
The product (V × N) of the content (mass%) of V and N is
(V × N): containing 0.003 to 0.015,
A ferritic stainless steel excellent in thermal fatigue characteristics, oxidation resistance and high temperature salt corrosion resistance, characterized in that the balance has a component composition consisting of Fe and inevitable impurities. 2. In addition to the said component composition, it further contains 1 type or 2 types chosen from B: 0.0004-0.0020mass% and Co: 0.05-0.1mass%, It is characterized by the above-mentioned. Ferritic stainless steel described in 1.

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