WO2018135028A1 - フェライト系ステンレス鋼及び自動車排ガス経路部材用フェライト系ステンレス鋼 - Google Patents
フェライト系ステンレス鋼及び自動車排ガス経路部材用フェライト系ステンレス鋼 Download PDFInfo
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- 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%
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- 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
- C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
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- 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
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- 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
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- 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
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- 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
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- 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
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- 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
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- 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
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- 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
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- 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
- C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
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- 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
- C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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- 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
- C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
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- 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
- C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
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- 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
- C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
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- 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
- C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
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- 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
- C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
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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
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- 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 and ferritic stainless steel for automobile exhaust gas path members.
- Ferritic stainless steel has a smaller thermal expansion coefficient than austenite, and is excellent in thermal fatigue characteristics and high-temperature oxidation characteristics. Therefore, it is used in heat-resistant applications where thermal strain is a problem.
- Typical applications include automobile exhaust path members such as exhaust manifolds, front pipes, catalyst carrier outer cylinders, center pipes, mufflers, and tail pipes.
- Recent automobile engines tend to raise the exhaust gas temperature for the purpose of improving exhaust gas purification efficiency and output, and especially high heat resistance (such as exhaust manifold, front pipe, catalyst carrier outer cylinder) High temperature strength and high temperature oxidation resistance) are required.
- the shape of the exhaust gas path member tends to be complicated.
- the exhaust manifold and the catalyst carrier outer cylinder are formed into complicated shapes by various methods such as mechanical press molding, servo press molding, spinning processing, and hydroforming.
- thermal strain associated with starting and stopping of the engine is concentrated at one place, and thermal fatigue failure is likely to occur, and the material temperature is locally increased and abnormal oxidation is likely to occur. Therefore, heat resistance cannot be sacrificed in improving moldability.
- SUH409L and SUS430J1L are known as ferritic stainless steels with high heat resistance.
- SUH409L has good processability and is often used for exhaust gas path members.
- SUS430J1L has excellent heat resistance that can be used at 900 ° C.
- Patent Document 1 proposes a technique based on a SUS429-based steel composition that improves workability by adding no Nb and suppresses deterioration of thermal fatigue characteristics by adding Cu. However, if it is held for a long time in the Cu precipitation temperature range, the Cu precipitates aggregate and become coarse, and the effect of improving the high temperature strength becomes small. For this reason, this ferritic stainless steel may have reduced thermal fatigue characteristics.
- Patent Document 2 is based on a SUS429-based steel composition, improves thermal fatigue characteristics by adding Nb and Cu, and increases ⁇ max to leave martensite in the slab and improve slab toughness. suggest. However, since this ferritic stainless steel has a high ⁇ max, when it is heated to a high temperature such as welding, a martensite phase is generated, which may reduce the thermal fatigue characteristics.
- ferritic stainless steel used for applications such as automobile exhaust gas path members can be processed into complex shapes by various molding methods, and can contribute to the expansion of the design flexibility of members. There has been a demand for high workability. Further, since ferritic stainless steel used for applications such as automobile exhaust gas path members needs to have excellent thermal fatigue characteristics and oxidation characteristics even at high temperatures, it is not desirable that heat resistance is lowered. However, as can be seen from the above patent documents, the present situation is that a ferritic stainless steel in which excellent workability and excellent heat resistance are simultaneously improved has not yet been obtained.
- An object of the present invention is to provide a ferritic stainless steel excellent in workability and heat resistance and having a good surface quality, and a ferritic stainless steel for automobile exhaust gas passage members.
- the present invention includes C: 0.03% by mass or less, Si: 0.1 to 0.8% by mass, Mn: 1.0% by mass or less, P: 0.04% by mass or less, S: 0.01 % By mass or less, Ni: 0.5% by mass or less, Cr: 12.0 to 15.0% by mass, N: 0.03% by mass or less, Nb: 0.1 to 0.5% by mass, Cu: 0. Ferritic stainless steel containing 8 to 1.5% by mass, Al: 0.1% by mass or less, the balance being Fe and inevitable impurities, and ⁇ max represented by the following formula (1) being 55 or less .
- ⁇ max 420C-11.5Si + 7Mn + 23Ni-11.5Cr + 470N + 9Cu-52Al + 1189 (1)
- C, Si, Mn, Ni, Cr, N, Cu, and Al mean mass% of the element.
- C 0.03% by mass or less
- Si 0.1 to 0.8% by mass
- Mn 1.0% by mass or less
- P 0.04% by mass or less
- S 0.01 %
- Ni 0.5% by mass or less
- Cr 12.0 to 15.0% by mass
- N 0.03% by mass or less
- Nb 0.1 to 0.5% by mass
- Cu 0. Ferrite for automobile exhaust gas path member containing 8 to 1.5 mass%
- Al 0.1 mass% or less
- the balance being Fe and inevitable impurities
- ⁇ max represented by the following formula (1) being 55 or less Stainless steel.
- ⁇ max 420C-11.5Si + 7Mn + 23Ni-11.5Cr + 470N + 9Cu-52Al + 1189 (1)
- C, Si, Mn, Ni, Cr, N, Cu, and Al mean mass% of the element.
- ferritic stainless steel and ferritic stainless steel for automobile exhaust gas path members that are excellent in workability and heat resistance and have good surface quality.
- the ferritic stainless steel of the present invention contains C, Si, Mn, P, S, Ni, Cr, N, Nb, Cu and Al, with the balance being Fe and inevitable impurities. Moreover, this ferritic stainless steel may further contain one or more selected from the group consisting of Ti, Mo, V, Zr, W, Co, and B as an optional component. Here, in this specification, it shows that it can contain to an unavoidable impurity level about content of the element which does not prescribe
- C and N are generally effective elements for improving high temperature strength such as creep strength.
- C and N are excessively contained, a martensite phase is easily generated, and thermal fatigue characteristics, oxidation characteristics, and workability are deteriorated.
- Nb is added as an element for fixing C and N as carbonitrides
- it is necessary to add Nb in an amount corresponding to the C and N concentration so the cost of ferritic stainless steel increases.
- C and N are significantly reduced, the burden on steelmaking becomes excessive, leading to an increase in cost.
- both C and N are regulated to 0.03 mass% or less. In consideration of oxidation characteristics and workability, it is desirable that both C and N be 0.015% by mass or less.
- Cr is restricted to 12.0 to 15.0% by mass.
- Mn is an alloy element that improves the high-temperature oxidation characteristics of ferritic stainless steel, particularly scale peelability, but excessive addition of Mn degrades workability. Moreover, since it is an austenite phase stabilizing element, when Mn is excessively added to a steel type with a small amount of Cr, a martensite phase is likely to be generated, resulting in deterioration of thermal fatigue characteristics and workability. Therefore, Mn is regulated to 1.0% by mass or less, preferably 0.8% by mass or less.
- P and S adversely affect high-temperature oxidation resistance and hot-rolled sheet toughness, they are preferably reduced as much as possible. Therefore, P is regulated to 0.04 mass% or less, and S is regulated to 0.01 mass% or less.
- Ni is an element effective for improving low-temperature toughness.
- Ni is an austenite phase stabilizing element, when Ni is excessively added to a steel type having a low Cr content, a martensite phase is generated in the same manner as Mn, and thermal fatigue characteristics and workability are deteriorated.
- the Ni content is regulated to 0.5% by mass or less.
- the minimum of Ni content is not specifically limited, Preferably it exceeds 0 mass%, More preferably, it is 0.01 mass% or more.
- Nb fixes C and N as carbonitrides, and the remaining solid solution Nb to which carbonitrides are fixed exhibits the effect of increasing the high-temperature strength.
- the Nb content is regulated to 0.1 to 0.5% by mass, preferably 0.2 to 0.4% by mass.
- Cu is an element that improves high temperature strength. In order to obtain the required high temperature strength, a Cu content of 0.8% by mass or more is necessary. However, as the Cu content increases, the workability and high temperature oxidation resistance deteriorate. Therefore, the Cu content is regulated to 0.8 to 1.5% by mass, preferably 0.9 to 1.3% by mass.
- Al is added as a deoxidizer during steelmaking and also exhibits an effect of improving high-temperature oxidation resistance.
- excessive addition of Al deteriorates surface properties and adversely affects workability. Therefore, the smaller the Al content, the better, and the content is regulated to 0.1% by mass or less, preferably 0.05% by mass or less.
- Ti is an element that improves the ductility and workability by fixing solute C and N in steel as carbonitride. Ti can also be expected to have the effect of suppressing the grain boundary precipitation of Cr carbide and improving the corrosion resistance. However, when an excessive amount of Ti is added, the surface properties of the steel material deteriorate due to the formation of TiN, which adversely affects weldability and low temperature toughness. Therefore, Ti may be added as necessary at 0.20 mass% or less, preferably 0.1 mass% or less.
- Mo, V, Zr, W, and Co are elements that improve high-temperature strength and heat fatigue resistance by solid solution strengthening or precipitation strengthening.
- Mo, Zr, W, and Co may each be 0.5 mass% or less, and V may be 0.1 mass% or less, if necessary.
- B is an element that improves the secondary workability of steel and suppresses cracking during multistage forming. However, when B is added excessively, manufacturability and weldability deteriorate. Therefore, B may be added as necessary at 0.01 mass% or less.
- Equations (1) and (2) indicate ⁇ max and are an austenite phase generation index. If ⁇ max is too high, a martensite phase is likely to be formed, but if a martensite phase is present, thermal fatigue properties are reduced. Therefore, ⁇ max is regulated to 55 or less so as not to generate a martensite phase.
- (1) Formula is (gamma) max when Mo or Ti which is an arbitrary component is not included, (2) Formula is (gamma) max when Mo or Ti which is an arbitrary component is included.
- ⁇ max 420C-11.5Si + 7Mn + 23Ni-11.5Cr + 470N + 9Cu-52Al + 1189
- ⁇ max 420C-11.5Si + 7Mn + 23Ni-11.5Cr + 470N + 9Cu-12Mo-49Ti-52Al + 189
- C, Si, Mn, Ni, Cr, N, Cu, Al, Mo, and Ti mean mass% of the element.
- the method for producing the ferritic stainless steel of the present invention is not particularly limited.
- the slab cast by a predetermined method is heated to 1000 to 1250 ° C. for 1 to 3 hours, A step of rolling, a step of annealing at a temperature of 900 to 1100 ° C., a step of pickling and cold rolling by a predetermined method, and a step of annealing and annealing at a temperature of 900 to 1100 ° C. are sequentially performed. It may be manufactured by.
- the ferritic stainless steel of the present invention thus produced, even if the slab heating temperature is lowered, the Fe-based oxide scale is uniformly generated, and the surface quality during hot rolling is good. Moreover, this ferritic stainless steel is excellent in workability and heat resistance. Therefore, the ferritic stainless steel of the present invention is suitable for heat resistance, particularly for automobile exhaust gas path members.
- a method for confirming the generation state of oxide scale when the slab heating temperature is lowered will be described.
- the ingot is cut into 5 mmt x 25 mmw x 35 mmL, the surface is polished with a # 120 polishing belt, and the furnace is heated at 1000 ° C. for 2 hours in an electric furnace that reproduces the same amount of oxygen and water vapor as the hot rolling furnace.
- the generation state of oxide scale was confirmed by cross-sectional observation. Those in which the oxide scale mainly composed of Fe was uniformly generated were evaluated as good (O: the same applies below), and those not locally generated or generated were evaluated as defective (X: the same applies below).
- a cold-rolled annealed plate having a thickness of 1.5 mm was subjected to a high temperature oxidation test and workability evaluation.
- a test piece having a size of 25 mm ⁇ 35 mm was prepared, and a continuous oxidation test of 875 ° C. ⁇ 200 h in the furnace was carried out in an air atmosphere in an electric furnace, and then the weight of the test piece was measured. .
- the weight change was 5 mg / cm 2 or less compared with the weight before the test, the case where the weight change exceeded 5 mg / cm 2 was evaluated as x.
- About workability evaluation it evaluated by the normal temperature tension test.
- a JIS No. 13 B test piece was prepared and the elongation at break in the rolling direction was measured. Evaluation was made with a breaking elongation of 35% or more as ⁇ and a elongation less than 35% as x.
- a thermal fatigue test piece was prepared from the round bar annealed material and subjected to a thermal fatigue test.
- the range of the minimum temperature of 200 ° C and the maximum temperature of 750 ° C is heated and cooled at 3 ° C / second with a high-frequency heating device, and the holding time at the minimum and maximum temperatures is 30 seconds each, and this is one cycle It was.
- the restraint rate was set to 25%.
- the number of cycles in which the maximum stress per cycle was reduced by 25% from the steady-state value was defined as thermal fatigue life, and those with a thermal fatigue life of 1600 cycles or more were evaluated as ⁇ and those with less than 1600 cycles as x.
- the ferritic stainless steels of Comparative Example 21 that does not contain Nb, Comparative Example 24 in which Nb is less than the lower limit, and Comparative Example 28 in which Cu is less than the lower limit are insufficient in high-temperature strength, so that they have thermal fatigue characteristics. Decreased. Furthermore, since the ferritic stainless steel of Comparative Example 28 had an excessive Cr content, the workability was lowered and an Fe-based oxide scale was generated nonuniformly when heated at 1000 ° C. for 2 hours. In the ferritic stainless steels of Comparative Examples 22 and 23, since ⁇ max exceeds the upper limit value, a martensite phase is easily generated, and the thermal fatigue characteristics are deteriorated.
- the ferritic stainless steel of Comparative Example 23 has a high C content, workability was also insufficient.
- the Ni content and ⁇ max exceeded the upper limit values, so the thermal fatigue characteristics were lowered, and the Cr content was small, so the high temperature oxidation characteristics were insufficient.
- the ferritic stainless steel of Comparative Example 25 Since the ferritic stainless steel of Comparative Example 25 has a high Si content, an Fe-based oxide scale is not uniformly formed when heated at 1000 ° C. for 2 hours, and since the Si and Nb contents are high, workability is increased. Also declined. In the ferritic stainless steel of Comparative Example 26, since N and Al were excessive, workability was lowered. Since the ferritic stainless steel of Comparative Example 29 has a low Si content, the high-temperature oxidation characteristics deteriorated. Since the ferritic stainless steel of Comparative Example 30 has excessive Mn and Cu contents, workability deteriorated along with high-temperature oxidation characteristics.
- the ferritic stainless steel according to the present invention is excellent in surface quality, high temperature oxidation characteristics, workability and thermal fatigue characteristics, and various internal combustion engines including automobiles such as exhaust manifolds, front pipes, center pipes and catalytic converter outer cylinders. It is suitable for use in the exhaust gas flow path member.
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Abstract
Description
そこで、以下のようなフェライト系ステンレス鋼が開発されている。
γmax=420C-11.5Si+7Mn+23Ni-11.5Cr+470N+9Cu-52Al+189・・・(1)
式中、C、Si、Mn、Ni、Cr、N、Cu及びAlは、当該元素の質量%を意味する。
γmax=420C-11.5Si+7Mn+23Ni-11.5Cr+470N+9Cu-52Al+189・・・(1)
式中、C、Si、Mn、Ni、Cr、N、Cu及びAlは、当該元素の質量%を意味する。
ここで、本明細書において、下限の規定がない元素の含有量については、不可避的不純物レベルまで含み得ることを示す。
以下に各元素の限定理由について説明する。
γmax=420C-11.5Si+7Mn+23Ni-11.5Cr+470N+9Cu-52Al+189・・・(1)
γmax=420C-11.5Si+7Mn+23Ni-11.5Cr+470N+9Cu-12Mo-49Ti-52Al+189・・・(2)
ここで、(1)及び(2)式において、C、Si、Mn、Ni、Cr、N、Cu、Al、Mo及びTiは、当該元素の質量%を意味する。
このようにして製造される本発明のフェライト系ステンレス鋼は、スラブ加熱温度を低下させても、Fe主体の酸化スケールが均一に生成し、熱間圧延時の表面品質は良好である。また、このフェライト系ステンレス鋼は、加工性及び耐熱性にも優れている。そのため、本発明のフェライト系ステンレス鋼は、耐熱用、特に、自動車排ガス経路部材用として適している。
表1の鋼組成をもつ各種フェライト系ステンレス鋼を真空溶解炉で溶製し、30kgのインゴットに鋳造した。インゴット(スラブ)を1100℃×2h加熱した後、熱間圧延、焼鈍、冷間圧延及び仕上げ焼鈍を順次行うことによって板厚1.5mmの冷延焼鈍板を製造した。また、インゴットを鍛造及び焼鈍して丸棒焼鈍材も製造した。表中、No.1~20は本発明鋼、No.21~30は比較鋼である。その中でも、No.21は特許文献1に相当する鋼、No.22は特許文献2に相当する鋼である
インゴットを5mmt×25mmw×35mmLに切り出し、表面を#120の研磨ベルトにて研磨し、熱延加熱炉と同様な酸素量及び水蒸気量を再現した電気炉にて、炉内加熱1000℃×2h加熱した後、断面観察により酸化スケールの生成状態を確認した。Fe主体の酸化スケールが均一に生成しているものを良好(○:以下同様)、局所的に生成又は生成していないものを不良(×:以下同様)として評価した。
高温酸化試験については、25mm×35mmの大きさの試験片を作製し、電気炉にて、炉内加熱875℃×200hの連続酸化試験を大気雰囲気で実施した後、試験片の重量を測定した。酸化増量の測定結果は、試験前の重量と比較し、重量変化が5mg/cm2以下のものを○、5mg/cm2を超える重量変化があったものを×として評価した。
加工性評価については、常温引張試験にて評価した。JIS13号B試験片を作製して圧延方向破断伸びを測定した。破断延びが35%以上のものを○、35%未満のものを×として評価した。
比較例22及び23のフェライト系ステンレス鋼は、γmaxが上限値を上回るため、マルテンサイト相が生成し易く、熱疲労特性が低下した。さらに、比較例23のフェライト系ステンレス鋼は、Cの含有量が多いため、加工性も不十分であった。
比較例27のフェライト系ステンレス鋼は、Ni含有量及びγmaxが上限値を上回るため熱疲労特性が低下するとともに、Cr含有量が少ないため高温酸化特性も不十分であった。
比較例26のフェライト系ステンレス鋼は、N及びAlが過剰であるため加工性が低下した。
比較例29のフェライト系ステンレス鋼は、Si含有量が少ないため高温酸化特性が低下した。
比較例30のフェライト系ステンレス鋼は、Mn及びCu含有量が過剰であるため高温酸化特性とともに加工性が低下した。
Claims (5)
- C:0.03質量%以下、
Si:0.1~0.8質量%、
Mn:1.0質量%以下、
P:0.04質量%以下、
S:0.01質量%以下、
Ni:0.5質量%以下、
Cr:12.0~15.0質量%、
N:0.03質量%以下、
Nb:0.1~0.5質量%、
Cu:0.8~1.5質量%、
Al:0.1質量%以下
を含有し、残部がFe及び不可避的不純物からなり、かつ下記(1)式で示されるγmaxが55以下であるフェライト系ステンレス鋼。
γmax=420C-11.5Si+7Mn+23Ni-11.5Cr+470N+9Cu-52Al+189・・・(1)
式中、C、Si、Mn、Ni、Cr、N、Cu及びAlは、当該元素の質量%を意味する。 - Ti:0.20質量%以下、Mo:0.5質量%以下、V:0.1質量%以下、Zr:0.5質量%以下、W:0.5質量%以下、Co:0.5質量%以下、B:0.01質量%以下からなる群から選択される1種以上をさらに含有し、かつ下記(2)式で示されるγmaxが55以下である請求項1に記載のフェライト系ステンレス鋼。
γmax=420C-11.5Si+7Mn+23Ni-11.5Cr+470N+9Cu-12Mo-49Ti-52Al+189・・・(2)
式中、C、Si、Mn、Ni、Cr、N、Cu、Mo、Ti及びAlは、当該元素の質量%を意味する。 - 耐熱用である請求項1又は2に記載のフェライト系ステンレス鋼。
- C:0.03質量%以下、
Si:0.1~0.8質量%、
Mn:1.0質量%以下、
P:0.04質量%以下、
S:0.01質量%以下、
Ni:0.5質量%以下、
Cr:12.0~15.0質量%、
N:0.03質量%以下、
Nb:0.1~0.5質量%、
Cu:0.8~1.5質量%、
Al:0.1質量%以下
を含有し、残部がFe及び不可避的不純物からなり、かつ下記(1)式で示されるγmaxが55以下である自動車排ガス経路部材用フェライト系ステンレス鋼。
γmax=420C-11.5Si+7Mn+23Ni-11.5Cr+470N+9Cu-52Al+189・・・(1)
式中、C、Si、Mn、Ni、Cr、N、Cu及びAlは、当該元素の質量%を意味する。 - Ti:0.20質量%以下、Mo:0.5質量%以下、V:0.1質量%以下、Zr:0.5質量%以下、W:0.5質量%以下、Co:0.5質量%以下、B:0.01質量%以下からなる群から選択される1種以上をさらに含有し、かつ下記(2)式で示されるγmaxが55以下である請求項4に記載の自動車排ガス経路部材用フェライト系ステンレス鋼。
γmax=420C-11.5Si+7Mn+23Ni-11.5Cr+470N+9Cu-12Mo-49Ti-52Al+189・・・(2)
式中、C、Si、Mn、Ni、Cr、N、Cu、Mo、Ti及びAlは、当該元素の質量%を意味する。
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