EP2628814B1 - Ferritic stainless steel excellent in heat resistance and workability - Google Patents
Ferritic stainless steel excellent in heat resistance and workability Download PDFInfo
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- EP2628814B1 EP2628814B1 EP11832650.3A EP11832650A EP2628814B1 EP 2628814 B1 EP2628814 B1 EP 2628814B1 EP 11832650 A EP11832650 A EP 11832650A EP 2628814 B1 EP2628814 B1 EP 2628814B1
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- 229910001220 stainless steel Inorganic materials 0.000 title claims description 21
- 239000000203 mixture Substances 0.000 claims description 17
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Images
Classifications
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
- F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
- F01N13/00—Exhaust or silencing apparatus characterised by constructional features ; Exhaust or silencing apparatus, or parts thereof, having pertinent characteristics not provided for in, or of interest apart from, groups F01N1/00 - F01N5/00, F01N9/00, F01N11/00
- F01N13/16—Selection of particular materials
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/002—Heat treatment of ferrous alloys containing Cr
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0226—Hot rolling
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0236—Cold rolling
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
- C21D8/0273—Final recrystallisation annealing
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/30—Ferrous alloys, e.g. steel alloys containing chromium with cobalt
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
Definitions
- the present invention relates to ferritic stainless steel having high heat resistance (a thermal fatigue property, oxidation resistance and a high-temperature fatigue property) and formability which can be ideally used for the parts of an exhaust system which are used in a high temperature environment such as an exhaust pipe and a catalyst outer cylinder (also called a converter case) of an automobile and a motorcycle and an exhaust air duct of a thermal electric power plant.
- high heat resistance a thermal fatigue property, oxidation resistance and a high-temperature fatigue property
- formability which can be ideally used for the parts of an exhaust system which are used in a high temperature environment such as an exhaust pipe and a catalyst outer cylinder (also called a converter case) of an automobile and a motorcycle and an exhaust air duct of a thermal electric power plant.
- the parts of an exhaust system such as an exhaust manifold, an exhaust pipe, a converter case and a muffler which are used in the environment of the exhaust system of an automobile are required to be excellent in a thermal fatigue property, a high-temperature fatigue property and oxidation resistance (hereinafter, these properties are collectively called heat resistance).
- a thermal fatigue property a high-temperature fatigue property and oxidation resistance
- heat resistance oxidation resistance
- the parts such as an exhaust manifold are subjected to heating and cooling due to the repetition of start and stop of engine operation in a state in which they are restrained by the surrounding parts, the thermal expansion and contraction of the material of the parts are restricted, which results in the occurrence of thermal strain.
- the fatigue phenomenon due to this thermal strain is thermal fatigue.
- the parts are continuously subjected to vibration while they are heated during engine operation.
- the fatigue phenomenon due to the accumulation of strain caused by this vibration is high-temperature fatigue.
- the former is low-cycle fatigue and the latter is high-cycle fatigue and both are completely
- Type429 containing 14Cr-0.9Si-0.4Nb
- an exhaust gas temperature has become higher than 900°C with the improvement of engine performance, the thermal fatigue property of Type429 has become unsatisfactory.
- Patent Literature 2 discloses ferritic stainless steel to be used for the parts of an exhaust gas flow channel of an automobile.
- Nb 0.50 mass% or less
- Cu 0.8 mass% or more and 2.0 mass% or less
- V 0.03 mass% or more and 0.20 mass% or less are added to steel having a Cr content of 10 mass% or more and 20 mass% or less.
- Patent Literature 3 discloses ferritic stainless steel excellent in a thermal fatigue property.
- Patent Literatures 3 Ti: 0.05 mass% or more and 0.30 mass% or less, Nb: 0.10 mass% or more and 0.60 mass% or less, Cu: 0.8 mass% or more and 2.0 mass% or less and B: 0.0005 mass% or more and 0.02 mass% or less are added to steel having a Cr content of 10 mass% or more and 20 mass% or less.
- Patent Literature 4 discloses ferritic stainless steel to be used for the parts of an exhaust gas flow channel of an automobile.
- Cu: 1 mass% or more and 3 mass% or less is added to steel having a Cr content of 15 mass% or more and 25 mass% or less.
- Patent Literature 5 discloses a ferritic stainless steel sheet with excellent thermal fatigue properties, suitable for use in an automotive exhaust-gas path member, including an exhaust manifold, a catalyst converter, a front pipe, or a center pipe.
- the present invention has been completed in view of the situation described above, and an object of the present invention is to provide ferritic stainless steel excellent in heat resistance (oxidation resistance, a thermal fatigue property and a high-temperature fatigue property) and formability, while preventing a decrease in oxidation resistance due to Cu, without adding expensive chemical elements such as Mo and W.
- the meaning of "excellent in heat resistance” is that oxidation resistance, a thermal fatigue property and a high-temperature fatigue property are equivalent to or better than those of SUS444. Specifically, it means that oxidation resistance at a temperature of 950°C is equivalent to or better than that of SUS444, that a thermal fatigue property when temperature fluctuations repeatedly occur between the temperatures of 100°C and 850°C is equivalent to or better than that of SUS444 and that a high-temperature fatigue property at a temperature of 850°C is equivalent to or better than that of SUS444.
- the meaning of "excellent in formability" according to the present invention is that a mean elongation in the three directions at room temperature is 36% or more.
- the present inventors diligently conducted investigations in order to develop ferritic stainless steel excellent in oxidation resistance and a thermal fatigue property by preventing a decrease in oxidation resistance due to Cu which occurs in the conventional methods without adding expensive chemical elements such as Mo or W.
- a high strength in high-temperature can be achieved by adding the combination of Nb: 0.3 mass% or more and 0.65 mass% or less and Cu: 1.0 mass% or more and 2.5 mass% or less.
- a thermal fatigue property can be improved in a wide temperature range.
- the present inventors found that a decrease in oxidation resistance due to the addition of Cu can be prevented by adding an appropriate amount of Al (0.2 mass% or more and 1.0 mass% or less).
- the present inventors found that, therefore, heat resistance (a thermal fatigue property and oxidation resistnace) equivalent to or better than that of SUS444 can be achieved only by controlling the contents of Nb, Cu and Al to the appropriate range as described above without adding Mo or W.
- the present inventors diligently conducted investigations regarding a method for improving oxidation resistance in an environment containing water vapor which is assumed in the case where the ferritic stainless steel is practically used for an exhaust manifold and the like, and found that oxidation resistance in an atmosphere containing water vapor (hereinafter, called water vapor oxidation resistance) also becomes equivalent to or better than that of SUS444 by adjusting a Si content (0.4 mass% or more and 1.0 mass% or less) .
- a fatigue resistance property against vibration in practical service conditions of the parts of the exhaust system of an automobile such as an exhaust manifold is also important. Therefore, the present inventors diligently conducted investigations regarding a method for improving a high-temperature fatigue property, and found that a high-temperature fatigue property also becomes equivalent to or better than that of SUS444 by adjusting the balance of a Si content and an Al content (Si ⁇ Al).
- the present invention provides ferritic stainless steel excellent in heat resistance and formability having a chemical composition consisting of, by mass%, C: 0.015% or less, Si: 0.4 % or more and 1.0% or less, Mn: 1.0% or less, P: 0.040% or less, S: 0.010% or less, Cr: 12% or more and less than 16%, N: 0.015% or less, Nb: 0.3% or more and 0.65% or less, Ti: 0.15% or less, Mo: 0.1% or less, W: 0.1% or less, Cu: 1.0% or more and 2.5 % or less and Al: 0.2% or more and 1.0% or less, while the relationship Si>Al is satisfied, and optionally B: 0.003% or less, REM: 0.080% or less, Zr: 0.50% or less, V: 0.50% or less, Co: 0.5% or less, Ni: 0.5% or less, O: 0.010% or less, Sn: 0.005% or less, Mg: 0.005% or less and Ca : 0.005% or less
- ferritic stainless steel having heat resistance (a thermal fatigue property, oxidation resistance and a high-temperature fatigue property) equivalent to or better than that of SUS444 (JIS G 4305) and excellent formability can be obtained without adding expensive Mo or W. Therefore, the steel according to the present invention can be ideally used for the parts of the exhaust system of an automobile.
- the thermal fatigue life of the specimen was observed by performing a thermal cycle heat treatment in which a restraint ratio was 0.30 and in which heating and cooling were repeated so that temperature fluctuations repeatedly occurred between 100°C and 850°C as illustrated in Fig. 2 .
- the thermal fatigue life represents as the number of cycles at which the stress first started to continuously decrease from that in the previous cycle.
- the stress was derived by calculated as the quotient of the load detected at 100°C divided by the cross section area of the soaked parallel portion of a test specimen indicated in Fig. 1 . This number of cycles corresponded to that at which a crack occurred in the test specimen.
- SUS444 (19%Cr-2%Mo-0.5%Nb steel) for comparison.
- Fig. 4 illustrates the influence of Cu content on thermal fatigue life in the thermal fatigue test described above. This figure indicates that thermal fatigue life equivalent to or longer than that of SUS444 (about 1350 cycles) can be achieved by setting the Cu content to be 1.0% or more. Therefore, it is necessary that the Cu content be 1.0% or more in order to improve a thermal fatigue property.
- test specimen of 30 mm ⁇ 20 mm was cut out of the cold rolled steel sheet obtained as described above, then a hole of 4 mm ⁇ was punched in the upper part of the test specimen, then the surface and the edge face of the specimen was polished with a #320 emery paper, then degreased and then used in a continuous oxidation test in air described below.
- test specimen described above was held in a furnace in air at a temperature of 950°C for duration of 200 hours, and then an increase in weight per unit area due to oxidation (g/m 2 ) was derived from the observed difference in the mass of the test specimen before and after the heating test.
- Fig. 5 illustrates the influence of Al content on the increase in weight due to oxidation in the continuous oxidation test in air described above. This figure indicates that an oxidation resistance equivalent to or better than that of SUS444 (increase in weight due to oxidation: 19 g/m 2 or less) can be achieved by setting the Al content to be 0.2% or more.
- the steel ingot was subjected to hot rolling, hot rolled annealing, cold rolling and finishing annealing and made into a cold rolled and annealed steel sheet having a thickness of 2 mm.
- a test specimen of 30 mm ⁇ 20 mm was cut out of the cold rolled steel sheet obtained as described above. Then a hole of 4 mm ⁇ was punched in the upper part of the test specimen, then the surface and the edge face of the specimen was polished with a #320 emery paper. Then degreased and then used in a continuous oxidation test in water vapor atmosphere described below.
- test specimen described above was held in a furnace in a water vapor atmosphere in which a gas of 10 vol%CO 2 -20 vol%H 2 O-5 vol%O 2 -bal. N 2 was blown at a rate of 0.5 L/min, and then an increase in weight per unit area due to oxidation (g/m 2 ) was derived from the observed difference in the mass of the specimen before and after the heating test.
- Fig. 6 illustrates the influence of the Si content on the increase in weight due to oxidation in the oxidation test in water vapor atmosphere described above. This figure indicates that water vapor oxidation resistance equivalent to that of SUS444 (increase in weight due to oxidation: 37g/m 2 or less) cannot be achieved, unless the Si content is set to be 0.4% or more.
- the high-temperature fatigue property of the test specimen described above was evaluated by using a Schenck type fatigue testing machine and by performing reversed vibration of 22 Hz (1300 rpm) at a temperature of 850°C.
- a bending stress of 70 MPa was exerted on the surface of the steel sheet during the test, and the fatigue property was evaluated in terms of a number of cycles until fracture occurred.
- Fig. 7 illustrates the influence of Si - Al on the number of cycles in the high-temperature fatigue test described above. This figure indicates that it is necessary to satisfy the relationship Si ⁇ Al in order to achieve a high-temperature fatigue property equivalent to that of SUS444 (24 ⁇ 10 5 cycles).
- the steel ingot was subjected to hot rolling, hot rolled annealing, cold rolling and finishing annealing and made into a cold rolled and annealed steel sheet having a thickness of 2 mm.
- a test specimen of 30 mm ⁇ 20 mm was cut out from the cold rolled steel sheet obtained as described above, then a hole of 4 mm ⁇ was punched in the upper part of the test specimen.
- the surface and the edge face of the specimen was polished with a #320 emery paper, then degreased and then used in the oxidation test in water vapor atmosphere described above.
- Fig. 8 illustrates the influence of the Cr content on the increase in weight due to oxidation in the oxidation test in water vapor atmosphere described above. This figure indicates that water vapor oxidation resistance equivalent to that of SUS444 (increase in weight due to oxidation: 37 g/m 2 or less) can be achieved in the case where the Cr content is 12% or more.
- Fig. 9 illustrates the influence of the Cr content on the mean value of elongations in the three directions (L, C and D directions) in the tensile test. This figure indicates that excellent formability in terms of the mean elongation in the three directions (L, C and D directions) of 36% or more can be achieved in the case where the Cr content is less than 16%.
- the present invention has been completed by conducting further investigations on the basis of the results of the fundamental experiments described above.
- the C content is set to be 0.015% or less.
- the C content it is preferable that the C content be as small as possible from the viewpoint of achieving formability and that the carbon content be 0.008% or less.
- the C content it is preferable that the C content be 0.001% or more in order to achieve strength which is required of the parts of an exhaust system, more preferably 0.002% or more and 0.008% or less.
- Si 0.4% or more and 1.0% or less
- Si is a chemical element which is important for improving oxidation resistance in a water vapor atmosphere. As Fig. 6 indicates, it is necessary to set Si content to be 0.4% or more in order to achieve water vapor oxidation resistance equivalent to that of SUS444. On the other hand, there is a significant decrease in formability in the case where the Si content is more than 1.0%. Therefore, the Si content is set to be 0.4% or more and 1.0% or less, preferably 0.5% or more and 0.9% or less.
- the mechanism through which water vapor oxidation resistance is increased in the case where the Si content is 0.4% or more is not clear, it is considered that a dense continuous oxide layer is formed on the surface of a steel sheet in the case where the Si content is 0.4% or more and the penetration of gas elements from outside is prevented, which results in an increase in water vapor oxidation resistance. It is preferable that the Si content be 0.5% or more in the case where there is a necessity of oxidation resistance in a severer environment.
- the Mn content is set to be 1.0% or less, preferably 0.7% or less. It is preferable that the Mn content be 0.05% or more in order to realize the effect of increasing strength and deoxidation.
- the P content is set to be 0.040% or less, preferably 0.030% or less.
- S is a harmful chemical element which causes a decrease in elongation and an r value.
- S has a negative influence on formability and S causes a decrease in corrosion resistance which is the basic property of stainless steel. It is preferable that S content be as small as possible. Therefore, the S content is set to be 0.010% or less, preferably 0.005% or less.
- Cr is a chemical element which is effective for increasing corrosion resistance and oxidation resistance, which are the characteristics of stainless steel. Sufficient oxidation resistance cannot be achieved in the case where Cr content is less than 12%.
- Cr is a chemical element which causes an increase in hardness and a decrease in ductility of steel at room temperature by solid solution strengthening, in particular, these negative influences become significant in the case where the Cr content is 16% or more. Therefore, the Cr content is set to be 12% or more and less than 16%, preferably 12% or more and 15% or less.
- N is a chemical element which causes a decrease in the ductility and the formability of steel, and these negative influences are significant in the case where N content is more than 0.015%. Therefore, the N content is set to be 0.015% or less. Incidentally, it is preferable that the N content be as small as possible from the viewpoint of achieving ductility and formability and that the N content be less than 0.010%.
- Nb 0.3% or more and 0.65% or less
- Nb is a chemical element which is effective for increasing corrosion resistance, formability and intergranular corrosion resistance at welds by forming carbide, nitride and carbonitride in combination with C and N.
- Nb is effective for increasing a thermal fatigue property by increasing high-temperature strength. These effects can be realized by setting Nb content to be 0.3% or more.
- a Laves phase (Fe 2 Nb) tends to be precipitated in the case where the Nb content is more than 0.65%, which results in the acceleration of embrittlement. Therefore, the Nb content is set to be 0.3% or more and 0.65% or less, preferably 0.4% or more and 0.55% or less.
- Mo is an expensive chemical element, additionally in view of the purpose of the present invention, Mo is not added positively.
- Mo may be mixed in from the material of steel such as scrap in the range of 0.1% or less. Therefore, Mo content is set to be 0.1% or less.
- W is an expensive chemical element like Mo, additionally in view of the purpose of the present invention, W is not added positively.
- W may be mixed in from the material of steel such as scrap in the range of 0.1% or less. Therefore, W content is set to be 0.1% or less.
- Cu is a chemical element which is very effective for improving a thermal fatigue property.
- Fig. 3 indicates, it is necessary that Cu content be 1.0% or more in order to achieve a thermal fatigue property equivalent to or better than that of SUS444.
- ⁇ -Cu is precipitated when cooling is performed after a heat treatment, which results in an increase in the hardness of steel and results in embrittlement tending to occur when hot work is performed.
- a thermal fatigue property is improved by adding Cu, contrarily the oxidation resistance of steel is decreased, which results in the deterioration of the overall heat resistance. The reason for this has not been fully identified.
- the Cu content is set to be 1.0% or more and 2.5% or less, preferably 1.1% or more and 1.8% or less.
- Ti is effective for improving corrosion resistance, formability and intergranular corrosion resistance of a welded part by fixing C and N like Nb does.
- this effect saturates and there is an increase in the hardness of steel in the case where Ti content is more than 0.15% in the present invention in which Nb is contained. Therefore, the Ti content is set to be 0.15% or less.
- Ti has higher affinity for N than Nb does, Ti tends to form TiN of a large size. Since TiN of a large size tends to become the origin of a crack and causes a decrease in toughness, it is preferable that the Ti content be 0.01% or less in the case where the toughness of a hot rolled steel sheet is necessary.
- the lower limit of the Ti content includes 0%.
- Al is a chemical element which is essential for increasing the oxidation resistance of Cu containing steel as Fig. 5 indicates.
- Al is effective as a solid solution strengthening element and, in particular, is effective for increasing high-temperature strength at a temperature of higher than 800°C
- Al is a chemical element which is important for improving a high-temperature fatigue property in the present invention. It is necessary that Al content be 0.2% or more in order to achieve oxidation resistance equivalent to or better than that of SUS444. On the other hand, there is a decrease in formability due to an increase in the hardness of steel in the case where the Al content is more than 1.0%. Therefore, the Al content is set to be 0.2% or more and 1.0% or less, preferably 0.3% or more and 1.0% or less, more preferably 0.3% or more and 0.5% or less.
- Al is effective as a solid solution strengthening element and, in particular, effective for increasing high-temperature strength at a high temperature of higher than 800°C
- Al is a chemical element which is important for improving a high-temperature fatigue property in the present invention as described above
- Si is a chemical element which is important for effectively utilizing this effect of solid solution strengthening of Al.
- the amount of Si is less than that of Al
- there is a decrease in the amounts of solid solution Al because Al preferentially forms oxides and nitrides at high temperature, which decreases the contribution of Al to strengthening.
- Si is preferentially oxidized and forms a dense continuous oxide layer on the surface of a steel sheet.
- One, two or more chemical elements selected from among B, REM, Zr, V, Co and Ni may be further contained in the ferritic stainless steel according to the present invention in addition to the chemical composition described above.
- B is a chemical element which is effective for improving formability, in particular, secondary formability.
- B content is set to be 0.003% or less. Since the effect described above is realized in the case where the B content is 0.0004% or more, it is more preferable that the B content be 0.0004% or more and 0.003% or less.
- REM rare earth elements
- Zr are chemical elements which are effective for improving oxidation resistance and may be added as needed in the present invention.
- the content of REM is more than 0.080%
- the steel become easier to occur the embrittle crack
- Zr content is more than 0.50%
- the steel also become easier to occur the embrittle crack due to the precipitation of a Zr intermetallic compound. Therefore, in the case where REM is contained, the content of REM is set to be 0.080% or less, and, in the case where Zr in contained, the Zr content is set to be 0.50% or less.
- the content of REM is 0.01% or more and in the case where the Zr content is 0.0050% or more, it is preferable that the content of REM be 0.001% or more and 0.080% or less and that the Zr content be 0.0050% or more and 0.50% or less.
- V 0.50% or less
- V is a chemical element which is effective for improving formability and oxidation resistance.
- V content is set to be 0.50% or less. It is preferable that the V content be 0.15% or more and 0.50% or less in order to realize the effect of improving formability and oxidation resistance, more preferably 0.15% or more and 0.4% or less.
- Co is a chemical element which is effective for improving toughness.
- Co is an expensive chemical element and the effect of Co saturates in the case where Co content is more than 0.5%. Therefore, in the case where Co is contained, the Co content is set to be 0.5% or less. Since the effect described above is effectively realized in the case where the Co content is 0.02% or more, it is preferable that the Co content be 0.02% or more and 0.5% or less, more preferably 0.02% or more and 0.2% or less.
- Ni is a chemical element which improves toughness.
- Ni is expensive and a chemical element which strongly forms a ⁇ phase
- Ni causes a decrease in oxidation resistance by forming a ⁇ phase at a high temperature in the case where Ni content is more than 0.5%. Therefore, in the case where Ni is contained, the Ni content is set to be 0.5% or less. Since the effect described above is effectively realized in the case where the Ni content is 0.05% or more, it is preferable that the Ni content be 0.05% or more and 0.5% or less, more preferably 0.05% or more and 0.4% or less.
- the remainder of the chemical composition consists of Fe and inevitable impurities.
- the O content is 0.010% or less
- the Sn content is 0.005% or less
- the Mg content is 0.005% or less
- the Ca content is 0.005% or less
- the O content is 0.005% or less
- the Sn content is 0.003% or less
- the Mg content is 0.003% or less
- the Ca content is 0.003% or less.
- the stainless steel according to the present invention may be manufactured in a common method for manufacturing ferritic stainless steel and there is no particular limitation on manufacturing conditions.
- ideal manufacturing methods include smelting steel by using a well-known melting furnace such as a steel converter or an electric furnace, further, optionally, making the steel have the chemical composition according to the present invention described above by performing secondary refining such as ladle refining or vacuum refining, then making a slab of the steel by using a continuous casting method or an ingot casting-blooming rolling method, and then making the slab a cold rolled and annealed steel sheet through the processes such as hot rolling, hot rolled annealing, pickling, cold rolling, finishing annealing, pickling and so forth.
- the cold rolling described above may be performed one time or repeated two times or more with process annealing in between, and the processes of cold rolling, finishing annealing and pickling may be performed repeatedly.
- hot rolled annealing may be omitted, and skin pass rolling may be performed after cold rolling or finishing annealing in the case where brightness of a steel sheet is required.
- a hot rolling process and a cold rolling process it is preferable that some of the conditions of a hot rolling process and a cold rolling process be specified.
- a steel making process it is preferable to smelt molten steel having the essential chemical composition described above and the optional chemical elements to be added as needed and to perform secondary refining by using a VOD method (Vacuum Oxygen Decurbarization method).
- VOD method Vauum Oxygen Decurbarization method
- the smelted molten steel may be made into a steel material by using a well-known method, it is preferable to use a continuous casting method from the viewpoint of productivity and material quality.
- the steel material obtained through a continuous casting process is heated up to a temperature of, for example, from 1000°C or higher and 1250°C or lower, and then made into a hot rolled steel sheet having a specified thickness.
- the steel material may be made into a material of a shape other than a sheet.
- This hot rolled steel sheet is subjected to, as needed, batch annealing at a temperature of 600°C or higher and 800°C or lower or continuous annealing at a temperature of 900°C or higher and 1100°C or lower, and then made into a hot rolled sheet product after being descaled by performing pickling or the like.
- descaling may be performed by using a shot blasting method before pickling being performed.
- the hot rolled and annealed steel sheet obtained as described above is made into a cold rolled steel sheet through a cold rolling process.
- cold rolling may be performed two times or more with process annealing in between as needed.
- the total rolling ratio of the cold rolling process, in which cold rolling is performed for one, two or more times, is set to be 60% or more, preferably 70% or more.
- the cold rolled steel sheet is subjected to continuous annealing (finishing annealing) at a temperature of 900°C or higher and 1150°C or lower, preferably 950°C or higher and 1120°C or lower, and pickling, and then made into a cold rolled and annealed steel sheet.
- finishing annealing continuous annealing
- the shape of and the material quality of the steel sheet may be adjusted by performing rolling with a light reduction ratio such as skin pass rolling after cold rolled annealing being performed.
- the hot rolled sheet product or cold rolled and annealed sheet product obtained as described above are formed into the exhaust pipe of an automobile or a motor bicycle, a material to be used for a catalyst outer cylinder, the exhaust air duct of a thermal electric power plant, or a part related to a fuel cell (such as a separator, an inter connector or a reformer) by performing bending work or other kinds of work in accordance with use application.
- welding methods for assembling these parts and common arc welding methods such as MIG (Metal Inert Gas), MAG (Metal Active Gas) and TIG (Tungsten Inert Gas), resistance welding methods such as spot welding and seam welding, high-frequency resistance welding methods such as electric resistance welding and high-frequency induction welding methods may be applied.
- Each of the steel No. 1 through 23 having chemical compositions given in Table 1 was smelted by using a vacuum melting furnace and made into steel ingot of 50 kg, then the steel ingot was subjected to forging, and then the forged ingot was divided into two pieces.
- each of the divided ingots was heated up to a temperature of 1170°C, then subjected to hot rolling and made into a hot rolled steel sheet having a thickness 5 mm, then subjected to hot rolled annealing, pickling, cold rolling with a rolling ratio of 60%, finishing annealing at a temperature of 1040°C, cooling at a cooling rate of 5°C /sec, pickling and then made into a cold rolled and annealed steel sheet having a thickness of 2 mm.
- Each of the steel No. 1 through 11 is an example in the range according to the present invention
- each of the steel No. 12 through 23 is a comparative example out of the range according to the present invention. Incidentally, among the comparative examples, steel No.
- the cold rolled steel sheets No. 1 through 23 were used in the two kinds of continuous oxidation tests, a high-temperature fatigue test and a tensile test at room temperature as described below.
- a sample of 30 mm ⁇ 20 mm was cut out of each of the cold rolled and annealed steel sheet obtained as described above, then a hole of 4 mm ⁇ was punched in the upper part of the sample, then the surface and the edge face of the sample was polished with a #320 emery paper, then degreased and then the sample was suspended in a furnace heated up to a temperature of 950°C in air for a holding time of 200 hours. After the test, the mass of the sample was observed, and then an increase in weight due to oxidation (g/m 2 ) was calculated by deriving the difference between the mass observed before and after the test. Incidentally, the test was repeated two times, and oxidation resistance in air was evaluated by using the mean value of the difference in mass.
- a sample of 30 mm ⁇ 20 mm was cut out from each of the cold rolled and annealed steel sheet obtained as described above. Then a hole of 4 mm ⁇ was punched in the upper part of the sample, then the surface and the edge face of the sample was polished with a #320 emery paper and then degreased. Thereafter, the sample was held in a furnace heated up to a temperature of 950°C in a water vapor atmosphere in which a gas of 10 vol%CO 2 -20 vol%H 2 O-5 vol%O 2 -bal.
- N 2 was blown at a rate of 0.5 L/min for a holding time of 200 hours, then, after the test, the mass of the sample was observed, and then an increase in weight due to oxidation (g/m 2 ) was calculated by deriving the difference between the mass observed before and after the test.
- a test specimen illustrated in Fig. 3 was cut out from the cold rolled and annealed steel sheet obtained as described above was subjected to reversed vibration of 1300 rpm (22 Hz) at a temperature of 850°C by using a Schenck type fatigue testing machine. Incidentally, a bending stress of 70 MPa was exerted on the surface of the steel sheet during the test, and evaluation was done in terms of a number of cycles (cycle) until fracture occurred.
- a tensile test piece conforming to JIS No. 13B which had the directions of tension respectively in the rolling direction (L direction), in the direction at right angle to the rolling direction (C direction) and in the direction at 45° angles to the rolling direction (D direction) was cut out from the cold rolled and annealed steel sheet described above. Then tensile tests in these directions were conducted at room temperature, then breaking elongations were observed and then a mean elongation was derived by using the equation below.
- Mean elongation El % E L + 2 E D + E C / 4, where E L : El (%) in L direction, E D : El (%) in D direction and E C : El (%) in C direction.
- thermal fatigue life was observed by repeatedly heating and cooling the test specimen between the temperatures of 100°C and 850°C at a restraint ratio of 0.30.
- a heating rate and a cooling rate were both 10°C/sec
- a holding time at a temperature of 100°C was 2 minutes
- a holding time at a temperature of 850°C was 5 minutes.
- the thermal fatigue life represents as the number of cycles at which the stress first started to continuously decrease from that in the previous cycle.
- the stress was derived by calculated as the quotient of the load detected at 100°C divided by the cross section area of the soaked parallel portion of a test specimen indicated in Fig. 1 .
- the steel of the comparative example which is out of the range according to the present invention is poor in either of oxidation resistance, thermal fatigue resistance, a high-temperature fatigue property or formability, which means it has been confirmed that the steel does not satisfy the object of the present invention.
- the steel according to the present invention can be ideally used not only for the parts of an exhaust system of an automobile but also the parts of an exhaust system of a thermal electric power system and the parts of a solid-oxide fuel cell for which similar properties as that of the parts of an exhaust system of an automobile are required.
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PCT/JP2011/073980 WO2012050226A1 (ja) | 2010-10-14 | 2011-10-12 | 耐熱性と加工性に優れるフェライト系ステンレス鋼 |
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JP5304935B2 (ja) * | 2011-10-14 | 2013-10-02 | Jfeスチール株式会社 | フェライト系ステンレス鋼 |
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ES2706305T3 (es) * | 2014-02-05 | 2019-03-28 | Jfe Steel Corp | Lámina de acero inoxidable ferrítico laminada en caliente y recocida, procedimiento para producir la misma, y lámina de acero inoxidable ferrítico laminada en frío y recocida |
EP3118341B1 (en) * | 2014-05-14 | 2019-12-18 | JFE Steel Corporation | Ferritic stainless steel |
CA2964055C (en) * | 2014-10-31 | 2020-06-30 | Nippon Steel & Sumikin Stainless Steel Corporation | Ferrite-based stainless steel plate, steel pipe, and production method therefor |
CN105220074A (zh) * | 2015-10-22 | 2016-01-06 | 山西太钢不锈钢股份有限公司 | 一种锅炉吊管托块用中铬铁素体耐热钢制作方法 |
JP6665936B2 (ja) * | 2016-12-21 | 2020-03-13 | Jfeスチール株式会社 | フェライト系ステンレス鋼 |
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DE60204323T2 (de) | 2001-07-05 | 2006-01-26 | Nisshin Steel Co., Ltd. | Ferritischer nichtrostender stahl für ein element einer abgasstrompassage |
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JP4468137B2 (ja) | 2004-10-20 | 2010-05-26 | 日新製鋼株式会社 | 熱疲労特性に優れたフェライト系ステンレス鋼材および自動車排ガス経路部材 |
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US20080279712A1 (en) * | 2007-05-11 | 2008-11-13 | Manabu Oku | Ferritic stainless steel sheet with excellent thermal fatigue properties, and automotive exhaust-gas path member |
KR20090052954A (ko) * | 2007-11-22 | 2009-05-27 | 주식회사 포스코 | 내식성 및 장출성형성이 우수한 저크롬 페라이트계스테인리스강 및 그 제조방법 |
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JP4624473B2 (ja) * | 2008-12-09 | 2011-02-02 | 新日鐵住金ステンレス株式会社 | 耐銹性に優れた高純度フェライト系ステンレス鋼およびその製造方法 |
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