JP5239644B2 - Ferritic stainless steel with excellent thermal fatigue properties, high temperature fatigue properties, oxidation resistance and toughness - Google Patents

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 high temperature fatigue properties, oxidation resistance and toughness.

  Components used in exhaust systems such as automobile exhaust manifolds, exhaust pipes, converter cases, and mufflers have thermal fatigue characteristics, high-temperature fatigue characteristics, and oxidation resistance. It is also called “heat resistance”). 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 described in Patent Document 1 to which W is 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.

Examples of materials having excellent heat resistance without using expensive elements such as Mo and W include, for example, ferritic stainless steels disclosed in Patent Documents 2 to 4. These steels are characterized by improving the thermal fatigue characteristics mainly by adding Cu. Patent Document 5 discloses a combustion temperature 1400-1500 ° C. class gas turbine in which oxidation resistance is improved by adding V, N and Cu to a ferritic stainless steel containing a martensite phase of 0-30 vol%. A heat-resistant steel material for exhaust gas passage is disclosed.
JP 2004-018921 A WO2003 / 004714 JP 2006-117985 A JP 2000-297355 A JP 2001-316774 A

  However, studies by the inventors have revealed that Cu is an element that reduces the oxidation resistance of steel itself. Further, it has been found that there is a limit to obtaining thermal fatigue characteristics and high temperature fatigue characteristics exceeding SUS444 only by adding Cu. Furthermore, the conventional steel which does not use Mo and W may have toughness lower than SUS444, and it is desired that the steel is stable and excellent in toughness.

  Accordingly, an object of the present invention is to provide ferrite having excellent thermal fatigue characteristics, high temperature fatigue characteristics and oxidation resistance without using Mo and W, and without causing deterioration of oxidation resistance due to addition of Cu, and also having excellent toughness. Is to provide stainless steel. Here, “excelling in thermal fatigue characteristics and high temperature fatigue characteristics” as used in the present invention specifically means thermal fatigue characteristics when the temperature is raised / decreased at 100 ° C./850° C. and high temperature fatigue characteristics at 750 ° C. Is superior to SUS444, and “excellent in oxidation resistance” means that the oxidation resistance at 950 ° C. is equal to or higher than that of SUS444 (the oxidation increase is equal to or lower). "Means that the toughness is superior to that of SUS444, specifically, the brittle fracture surface ratio when performing a Charpy impact test at -20 ° C is lower than 20% of SUS444 and less than 5%.

  In order to solve the above problems, the inventors based on Cr-containing steel added with Mn of 0.35 mass% or less and Nb of 10 × (C (mass%) + N (mass%)) to 0.60 mass%. Researched earnestly. As a result, in the above steel, N is added in the range of 0.015 to 0.040 mass%, and at the same time, V is in the range of 0.15 to 0.60 mass% and the content of N and V (mass%). After adding the product (V × N) so as to satisfy 0.003 to 0.015, Cu is further added in the range of 0.8 to 1.6 mass% without adding Mo and W, High high temperature strength can be realized in a wide temperature range, thermal fatigue characteristics superior to SUS444, and high temperature fatigue characteristics can be obtained, and further, oxidation resistance reduction due to Cu addition is suppressed and oxidation resistance equal to or higher than SUS444 is achieved. It was found that it can be obtained. Furthermore, in order to stably obtain toughness superior to SUS444, it has been found that it is effective to regulate the upper limit of the N and V contents and the value of (V × N), and the present invention has been completed. It was.

  That is, the present invention includes C: 0.015 mass% or less, Si: 0.5 mass% or less, Mn: 0.35 mass% or less, P: 0.040 mass% or less, S: 0.010 mass% or less, Al: 0. 1 mass% or less, Cr: 16 to 18.5 mass%, Cu: 0.8 to 1.6 mass%, N: 0.015 to 0.030 mass%, V: 0.15 to 0.35 mass%, Nb: 10 × (C (mass%) + N (mass%)) to 0.50 mass%, Ti: 0.01 mass% or less, Zr: 0.01 mass% or less, Ta: 0.01 mass% or less, Ni: 0.4 mass% or less, Mo: 0.1 mass% or less, W: 0.1 mass% or less, and the product of V and N content (mass%) (V × N) is (V × N): 0.003 0.00 Containing meets a ferritic stainless steel having excellent heat resistance and toughness and the balance by having a component composition consisting of Fe and unavoidable impurities.

  In addition to the above component composition, the ferritic stainless steel of the present invention further contains Co: 0.05 to 0.1 mass%, or further contains B: 0.0004 to 0.0030 mass%. Features.

  According to the present invention, it is possible to obtain a ferritic stainless steel having both thermal fatigue characteristics, high temperature fatigue characteristics and toughness superior to SUS444, and oxidation resistance equal to or higher than SUS444 without adding expensive Mo and W. Can do. The ferritic stainless steel of the present invention is suitable for use in automobile exhaust members and the like, and has a remarkable industrial effect.

The basic experiment that triggered the development of the present invention will be described.
First, the following experiments 1 to 3 were performed in order to investigate the effects of the contents of N and V (mass%) and their product (V × N) on the thermal fatigue properties of steel.
(Experiment 1)
C: 0.005-0.011 mass%, Si: 0.14-0.46 mass%, Mn: 0.09-0.31 mass%, Al: 0.035-0.055 mass%, Cr: 17.1 17.6 mass%, Nb: 0.39 to 0.49 mass%, Nb / (C + N): 10.2 to 27.3, Cu: 0.98 to 1.07 mass%, V: 0.20 to 0.25 mass The steel ingot was melted in the laboratory in various ways in the range of 0.008 to 0.043 mass%, and the obtained steel ingot was forged and then heat-treated to obtain 35 mm × A 35 mm square was produced, and a thermal fatigue test piece having the shape and dimensions shown in FIG. 1 was produced from this square. Next, using this thermal fatigue test piece, as shown in FIG. 2, the thermal fatigue life is subjected to a thermal fatigue test in which heating / cooling is performed by raising and lowering the temperature between 100 ° C./850° C. with a constraint ratio of 0.8. Was measured. Here, the above-mentioned “thermal fatigue life” is calculated by dividing the load detected at 100 ° C. by the cross-sectional area of the soaking parallel part of the test piece and calculating the stress from the stress value of (n−1) cycles. It was defined as the first cycle number n in which the stress value was continuously lower. This thermal fatigue life corresponds to the number of cycles in which the test piece was cracked. For comparison, the same thermal fatigue test was performed on SUS444 (18% Cr-2% Mo-0.5% Nb steel).

  FIG. 3 shows the test results. From this figure, the thermal fatigue life of 450 cycles or more, which is superior to SUS444 (380 cycles), is obtained when the N content is in the range of 0.015 to 0.040 mass%. It turns out that it is obtained.

(Experiment 2)
C: 0.004 to 0.009 mass%, Si: 0.08 to 0.38 mass%, Mn: 0.14 to 0.42 mass%, Al: 0.038 to 0.048 mass%, Cr: 17.1 to 17.9 mass%, Nb: 0.38 to 0.45 mass%, Nb / (C + N): 12.7 to 15.7, Cu: 0.99 to 1.07 mass%, N: 0.020 to 0.025 mass After the steel was melted in the laboratory in the same manner as in Experiment 1, it was made into a square bar of 35 mm x 35 mm, with the V content varied in the range of 0.08 to 0.67 mass%. A thermal fatigue test piece having the shape and dimensions shown in Fig. 1 was prepared and subjected to a thermal fatigue test under the conditions shown in Fig. 2 to measure the thermal fatigue life.

  FIG. 4 shows the test results. From this figure, a thermal fatigue life of 450 cycles or more, which is superior to SUS444, was obtained when the V content was in the range of 0.15 to 0.60 mass%. I understand that.

(Experiment 3)
In addition to the results of Experiment 1 and Experiment 2 (FIGS. 3 and 4), C: 0.007 mass%, Si: 0.15 to 0.22 mass%, Mn: 0.13 to 0.21 mass%, Al: 0 0.034 to 0.038 mass%, Cr: 17.4 mass%, Nb: 0.48 mass%, Nb / (C + N): 11.2 to 11.4, Cu: 1.02 to 1.05 mass%, Steels with N content and V content of 0.036 mass%, 0.43 mass%, 0.035 mass%, and 0.49 mass%, respectively, were melted in the laboratory, and in the same manner as in Experiment 1, 30 mm × 30 mm After forming a square bar, a thermal fatigue test piece having the shape and size shown in FIG. 1 was prepared, and subjected to a thermal fatigue test under the conditions shown in FIG.

FIG. 5 shows the relationship between the product of V and N content (mass%) (V × N) and the thermal fatigue life for the above experimental results. From this figure, even if the N content and the V content are within the preferred ranges obtained in Experiments 1 and 2, if the value of (V × N) is not in the range of 0.003 to 0.015, SUS444 It can be seen that a more excellent thermal fatigue life of 450 cycles or more cannot be obtained.
Therefore, from the viewpoint of thermal fatigue characteristics, the N content is in the range of 0.015 to 0.040 mass%, the V content is in the range of 0.15 to 0.60 mass%, and the value of (V × N) is It must be in the range of 0.003 to 0.015.

Next, the effect of Cu addition amount on high temperature fatigue characteristics was investigated.

(Experiment 4)
C: 0.004 to 0.009 mass%, Si: 0.18 to 0.27 mass%, Mn: 0.04 to 0.18 mass%, Al: 0.041 to 0.048 mass%, Cr: 16.9 to 17.4 mass%, Nb: 0.39 to 0.44 mass%, Nb / (C + N): 12.1 to 14.8, N: 0.021 to 0.025 mass%, V: 0.21 to 0.26 mass Steel, in which the content of Cu is varied in the range of 0.02 to 2.11 mass%, is melted in the laboratory, and the resulting steel ingot is heated to 1170 ° C. and then hot rolled. Then, it was hot-rolled sheet annealed, cold-rolled, and finish-annealed to obtain a cold-rolled annealed sheet having a thickness of 2 mm. From this cold-rolled annealed plate, a fatigue test piece having the shape and dimensions shown in FIG. 6 was collected. Thereafter, the test piece was subjected to a Schenk type high-temperature fatigue test (both swings, 1600 Hz) with a bending stress of 110 MPa applied to the plate surface at 750 ° C., and the number of cycles until breakage was measured.

FIG. 7 shows the relationship between the high temperature fatigue characteristics and the Cu content. From this figure, it can be seen that when the Cu content is in the range of 0.8 to 1.6 mass%, the fatigue life is 1.6 × 10 ⁶ cycles or more, which is significantly superior in high temperature fatigue life compared to SUS444. Therefore, in order to obtain high temperature fatigue characteristics superior to SUS444, it was found that the Cu content must be added in the range of 0.8 to 1.6 mass%.

Next, in steel used for exhaust system members, the effect of V content on oxidation resistance, which is an important characteristic along with thermal fatigue characteristics, was investigated.
(Experiment 5)
In addition to the steel obtained in Experiment 2, C: 0.006 mass%, Si: 0.22 mass%, Mn: 0.17 mass%, Al: 0.042 mass%, Cr: 17.6 mass%, Nb: 0.41 mass %, Nb / (C + N): 14.6, N: 0.022 mass%, Cu: 0.99 mass%, and a V content of 0.04 mass% were melted in the laboratory. The steel ingot was hot-rolled, hot-rolled sheet annealed, cold-rolled, and finish-annealed to obtain a cold-rolled annealed sheet having a thickness of 2 mm. A sample for oxidation test having a size 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 a furnace in an air atmosphere maintained at 950 ° C. A 300-hour continuous oxidation test was conducted, and 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.

  The results of the continuous oxidation test are shown in FIG. From this figure, it can be seen that when the V content is 0.15 mass% or more, oxidation resistance equal to or higher than that of SUS444 can be obtained.

Next, the effects of the contents of N and V (mass%) and their product (V × N) on the toughness of Cr-containing steel were investigated.
(Experiment 6)
The steel ingots obtained in the above experiments 1 to 3 are heated to 1170 ° C. and hot-rolled to form a hot-rolled sheet having a thickness of 5 mm. Then, the hot-rolled sheet is subjected to hot-rolled sheet annealing (annealing temperature: 1040 ° C.). Pickling, cold rolling (cold rolling reduction: 60%), finish annealing (annealing temperature: 1040 ° C., average cooling rate: 30 ° C./s), pickling and cold rolling annealing with a plate thickness of 2 mm A board was used.
From this cold-rolled annealed plate, a sub-size Charpy V-notch impact test piece was taken in accordance with JIS Z2202, a Charpy impact test was conducted at a temperature of −20 ° C. in accordance with JIS Z2422, and the fracture surface was observed to be brittle. The fracture surface ratio was measured.

9 to 11 show the effects of N content, V content, and (V × N) on the brittle fracture surface ratio. From this result, in order to make the brittle fracture surface ratio less than 5%, which is superior to the brittle fracture surface ratio (20%) of SUS444, the N content is 0.030 mass% or less, the V content is 0.35 mass% or less, and It can be seen that (V × N) needs to be 0.008 or less.
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 increases the strength of steel, but if it exceeds 0.015 mass%, the toughness and formability are greatly deteriorated, and the addition effect of V, which is a feature of the present invention Cannot be obtained. Therefore, in this invention, C shall be 0.015 mass% or less. In order to further improve the moldability and to maximize the effect of V, the C content is preferably as low as possible, and is preferably 0.010 mass% or less.

Si: 0.5 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. Further, in order to stably obtain excellent low temperature toughness, Si needs to be 0.5 mass% or less. Preferably, it is 0.3 mass% or less.

Mn: 0.35 mass% or less Mn is an element that acts as a deoxidizer. However, excessive addition promotes the formation of a γ phase at a high temperature, and has a large adverse effect on heat resistance, particularly oxidation resistance. Moreover, elongation is reduced and workability is deteriorated. Therefore, in order to eliminate the above adverse effects, Mn is limited to 0.35 mass% or less in the present invention.

P: 0.040 mass% or less P is an element that lowers the toughness and ductility 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: 0.1 mass% or less Al is added as a powerful deoxidizer. It is also an effective element for improving the oxidation resistance of steel. However, if added over 0.1 mass%, the steel is hardened and the workability is lowered, so the upper limit is made 0.1 mass%. Preferably it is the range of 0.03-0.08 mass%.

Cr: 16-18.5 mass%
Cr is an important element in the steel of the present invention that improves the oxidation resistance of the 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, resulting in a decrease in workability. In particular, when it exceeds 18.5 mass%, the workability is greatly lowered. Therefore, Cr is set to a range of 16 to 18.5 mass%.

Cu: 0.8 to 1.6 mass%
Cu is an element effective for improving the thermal fatigue properties and high temperature fatigue properties of steel. In particular, as shown in FIG. 7, there is a remarkable effect in improving high temperature fatigue characteristics. This is due to the effect of fine ε-Cu precipitated at around 750 ° C., and the effect can be obtained by adding 0.8 mass% or more. On the other hand, if added in excess of 1.6 mass%, not only the effect cannot be obtained, but also the high temperature fatigue characteristics become lower than in the case where Cu is not added. This is presumably because the deposited ε-Cu becomes coarse and acts as a starting point for crack generation. Therefore, in this invention, Cu is taken as the range of 0.8-1.6 mass%.

N: 0.015-0.030 mass%, V: 0.15-0.35 mass%, (V × N): 0.003-0.008
N and V are important additive elements in the present invention. As shown in FIGS. 3 to 5, in order to obtain thermal fatigue characteristics superior to SUS444, N: 0.015 to 0.040 mass%, V: 0.15 to 0.60 mass%, and their product (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 (V × N) is less than 0.003, fine VN precipitates that improve thermal fatigue properties are sufficient. Since it is not generated, the desired thermal fatigue characteristics cannot be obtained. On the other hand, when the N content exceeds 0.040 mass%, the V content exceeds 0.60 mass%, or (V × N) exceeds 0.015, the thermal fatigue characteristics are also deteriorated. Such excessive addition of V and N is because fine VN precipitates that contribute to the improvement of thermal fatigue characteristics are coarsened and the desired thermal fatigue characteristics cannot be obtained. Furthermore, as shown in FIG. 8, V has an effect of improving the reduction in oxidation resistance due to the addition of Cu by addition of 0.15 mass% or more, and even if V is added by 0.60 mass% or more. There is no adverse effect on oxidation resistance.
However, as shown in FIGS. 9 to 11, in order to achieve brittleness superior to SUS444, that is, to make the brittle fracture surface ratio less than 5% at −20 ° C., the N content is 0.030 mass% or less, the V content Must be 0.35 mass% or less and (V × N) must be 0.008 or less. Therefore, in this invention, it is set as the range of N: 0.015-0.030 mass%, V: 0.15-0.35 mass%, (V * N): 0.003-0.008.

Nb: 10 × (C (mass%) + N (mass%)) to 0.50 mass%
Nb fixes C and N, suppresses the formation of Cr carbonitride, has the effect of increasing the corrosion resistance of the base metal and the intergranular corrosion resistance of the welded portion, and increases the high-temperature strength and improves the thermal fatigue characteristics. This is an important element in the present invention. In particular, from the viewpoint of preventing sensitization and increasing the intergranular corrosion resistance, it is necessary to contain 10 × (C + N) or more even in steel in which N is positively added as in the present invention. On the other hand, addition of excess Nb suppresses the precipitation of fine VN effective for improving thermal fatigue characteristics. In particular, if added over 0.60 mass%, even if the contents of V and N are in the above ranges, the effect of improving the thermal fatigue characteristics cannot be obtained. On the other hand, if the Nb content exceeds 0.50 mass%, the Laves phase precipitates and becomes brittle, and good low temperature toughness cannot be obtained. Therefore, the Nb content is in the range of 10 × (C + N) to 0.50 mass%.

Ti: 0.01 mass% or less, Zr: 0.01 mass% or less, Ta: 0.01 mass% or less Ti, Zr, and Ta are stronger nitride forming elements than V. Therefore, when these elements are contained in excess of 0.01 mass%, Ti, Zr, and Ta nitrides are formed first, and Nb, V, and the like are precipitated in the nucleus, and thermal fatigue characteristics. As a result, it becomes impossible to obtain fine VN precipitates that effectively act on the improvement. Therefore, each of these elements needs to be limited to 0.01 mass% or less.

Ni: 0.4 mass% or less Ni is an element mixed as an impurity from a steel raw material when melting steel. Since Ni is a strong austenite stabilizing element, when it is excessively mixed, an austenite phase is generated at a high temperature and the oxidation resistance is deteriorated. Therefore, Ni is limited to 0.4 mass% or less.

Mo: 0.1 mass% or less, W: 0.1 mass% or less Mo and W are expensive elements and are not actively added in the present invention for the purpose of developing inexpensive materials. However, it may be contained in an amount of 0.1 mass% or less due to the mixing of the melting raw material from scrap or the like. Therefore, in the present invention, the upper limits of Mo and W are each 0.1 mass%.

The ferritic stainless steel of the present invention can further contain B and / or Co in the following range in addition to the essential components.
Co: 0.05 to 0.1 mass%
Co is an element effective for improving the 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 adding Co, it is set as the range of 0.05-0.1 mass%.

B: 0.0004 to 0.0030 mass%
B is an element effective for improving workability, particularly secondary workability. This effect is obtained by adding 0.0004 mass% or more. However, addition exceeding 0.0030 mass% generates BN and causes deterioration of workability. Therefore, when adding B, it is set as the range of 0.0004-0.0030 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 × 30 mm was formed, and this was forged into a square material of 35 mm × 35 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 100 ° C. and 850 ° C. with a constraint 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 10 ° C./s, the retention time at 850 ° C. was 1 minute, and the retention time at 100 ° C. was 0 minute. The thermal fatigue life is calculated by dividing the load detected at 100 ° C. by the cross-sectional area of the soaking parallel part of the specimen, and calculating the stress value of n cycles from the stress value of (n−1) cycles. Is defined as the first cycle number n that continuously becomes a low value.
As reference examples, the thermal fatigue characteristics of steels (steel Nos. 35-38) and SUS444 (No. 34) having the component compositions disclosed in Patent Documents 2, 3, and 5 were evaluated in the same manner as described above. did.

The other steel ingot obtained in Example 1 was heated to 1170 ° C. and then hot-rolled to form a hot-rolled sheet having a thickness of 5 mm, and this hot-rolled sheet was then subjected to hot-rolled sheet annealing (annealing temperature: 1040 ° C.). , Pickling, cold rolling (rolling rate: 60%), finish annealing (annealing temperature: 1040 ° C., average cooling rate: 20 ° C./s), pickling, and cold-rolled annealed sheet with a thickness of 2 mm It was.
From each cold-rolled annealed plate obtained as described above, a fatigue test piece having the shape and dimensions shown in FIG. 6 was collected and subjected to a high temperature fatigue test. The high temperature fatigue test is performed using a Schenk type high temperature fatigue tester at a temperature of 750 ° C. with a bending stress of 110 MPa applied to the plate surface of the test piece under the condition of double swing and 1600 Hz. The number of cycles until was measured.
As in Example 1, No. 1 was used. For the steels of Reference Examples 34 to 38, the high temperature fatigue properties were evaluated 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 a hole of 4 mmφ was made in the upper part of the sample. The sample was polished and degreased, and then the sample was subjected to an atmospheric continuous oxidation test in which the sample was suspended in an atmospheric furnace heated and maintained at 950 ° C. and held for 300 hours. After the test, the mass of the sample was measured, and the difference from the mass before the test was calculated to obtain the oxidation increase. In addition, the test was implemented twice about each and the oxidation resistance was evaluated by the average value.
As in Example 1, No. 1 was used. For the steels of Reference Examples 34 to 38, the oxidation resistance was evaluated in the same manner as described above.

From the cold-rolled annealed plate having a thickness of 2 mm obtained in Example 2, a sub-size Charpy V-notch impact test piece was collected from this cold-rolled annealed plate in accordance with JIS Z2202, and −20 ° C. in accordance with JIS Z2422. A Charpy impact test was conducted at a temperature of, and the brittle fracture surface ratio was measured by observing the fracture surface.
As in Example 1, No. 1 was used. For the steels of Reference Examples 34 to 38, workability was evaluated in the same manner as described above.

The results of Examples 1 to 4 are shown together in Table 1-2. From these results, all of the steels of the invention examples (Nos. 1 to 11) conforming to the component composition of the present invention have heat fatigue characteristics (thermal fatigue life: 450 cycles or more) exceeding SUS444 and high temperature fatigue characteristics (fatigue). Lifetime: 1.6 × 10 ⁶ cycles or more) and toughness (brittle fracture surface ratio: less than 5%), as well as oxidation resistance (oxidation increase: 28 g / m ² or less) equal to or higher than SUS444 It can be seen that the object of the invention has been achieved. On the other hand, the comparative steels (Nos. 12 to 33) and the prior art steels (Nos. 34 to 38) that do not satisfy the component composition of the present invention are any of heat fatigue characteristics, high temperature fatigue characteristics, toughness, and oxidation resistance. One or more characteristics are inferior to the steel of the present invention.

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

It is a figure explaining the test piece used for the thermal fatigue test. It is a figure explaining the test conditions of a thermal fatigue test. It is a graph which shows the influence of N content which acts on the thermal fatigue characteristic of Cr containing steel. It is a graph which shows the influence of V content which acts on the thermal fatigue characteristic of Cr containing steel. It is a graph which shows the influence of (VxN) which acts on the thermal fatigue characteristic of Cr containing steel. It is a figure explaining the test piece used for the Schenck type fatigue test. It is a graph which shows the influence of Cu content which has on the high temperature fatigue characteristic of Cr containing steel. It is a graph which shows the influence of V content which acts on the oxidation resistance of Cr containing steel. It is a graph which shows the influence of N content which acts on the toughness of Cr containing steel. It is a graph which shows the influence of V content which acts on the toughness of Cr containing steel. It is a graph which shows the influence of (VxN) which acts on the toughness of Cr containing steel.

Claims (3)

C: 0.015 mass% or less,
Si: 0.5 mass% or less,
Mn: 0.35 mass% or less,
P: 0.040 mass% or less,
S: 0.010 mass% or less,
Al: 0.1 mass% or less,
Cr: 16 to 18.5 mass%,
Cu: 0.8 to 1.6 mass%,
N: 0.015-0.030 mass%,
V: 0.15-0.35 mass%,
Nb: 10 × (C (mass%) + N (mass%)) to 0.50 mass%,
Ti: 0.01 mass% or less,
Zr: 0.01 mass% or less,
Ta: 0.01 mass% or less,
Ni: 0.4 mass% or less,
Mo: 0.1 mass% or less,
W: 0.1 mass% or less, and
The product of V and N content (mass%) (V × N)
(V × N): satisfying 0.003 to 0.008,
A ferritic stainless steel excellent in heat resistance and toughness, wherein the balance has a component composition consisting of Fe and inevitable impurities. The ferritic stainless steel according to claim 1, further comprising Co: 0.05 to 0.1 mass% in addition to the component composition. The ferritic stainless steel according to claim 1 or 2, further comprising B: 0.0004 to 0.0030 mass% in addition to the component composition.

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