JP5178156B2 - Ferritic stainless steel material for automobile exhaust gas path members - Google Patents
Ferritic stainless steel material for automobile exhaust gas path members Download PDFInfo
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- JP5178156B2 JP5178156B2 JP2007294930A JP2007294930A JP5178156B2 JP 5178156 B2 JP5178156 B2 JP 5178156B2 JP 2007294930 A JP2007294930 A JP 2007294930A JP 2007294930 A JP2007294930 A JP 2007294930A JP 5178156 B2 JP5178156 B2 JP 5178156B2
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- 239000000463 material Substances 0.000 title claims description 40
- 229910001220 stainless steel Inorganic materials 0.000 title claims description 20
- 229910000831 Steel Inorganic materials 0.000 claims description 30
- 239000010959 steel Substances 0.000 claims description 30
- 229910052750 molybdenum Inorganic materials 0.000 claims description 22
- 229910052758 niobium Inorganic materials 0.000 claims description 21
- 239000000203 mixture Substances 0.000 claims description 9
- 229910052802 copper Inorganic materials 0.000 claims description 8
- 229910052761 rare earth metal Inorganic materials 0.000 claims description 8
- 229910052719 titanium Inorganic materials 0.000 claims description 4
- 229910052721 tungsten Inorganic materials 0.000 claims description 4
- 229910052726 zirconium Inorganic materials 0.000 claims description 4
- 229910052804 chromium Inorganic materials 0.000 claims description 2
- 229910052748 manganese Inorganic materials 0.000 claims description 2
- 229910052759 nickel Inorganic materials 0.000 claims description 2
- 229910052720 vanadium Inorganic materials 0.000 claims description 2
- 238000000137 annealing Methods 0.000 description 20
- 238000001556 precipitation Methods 0.000 description 12
- 238000001816 cooling Methods 0.000 description 10
- 238000000034 method Methods 0.000 description 8
- 239000006104 solid solution Substances 0.000 description 8
- 230000000694 effects Effects 0.000 description 7
- 230000003647 oxidation Effects 0.000 description 6
- 238000007254 oxidation reaction Methods 0.000 description 6
- 239000010935 stainless steel Substances 0.000 description 6
- 238000005728 strengthening Methods 0.000 description 6
- 238000009864 tensile test Methods 0.000 description 5
- 238000005868 electrolysis reaction Methods 0.000 description 4
- 239000011159 matrix material Substances 0.000 description 4
- 239000002244 precipitate Substances 0.000 description 4
- 238000011144 upstream manufacturing Methods 0.000 description 4
- OKKJLVBELUTLKV-UHFFFAOYSA-N Methanol Chemical compound OC OKKJLVBELUTLKV-UHFFFAOYSA-N 0.000 description 3
- 238000005096 rolling process Methods 0.000 description 3
- 239000002436 steel type Substances 0.000 description 3
- 230000007704 transition Effects 0.000 description 3
- 229910000859 α-Fe Inorganic materials 0.000 description 3
- YRKCREAYFQTBPV-UHFFFAOYSA-N acetylacetone Chemical compound CC(=O)CC(C)=O YRKCREAYFQTBPV-UHFFFAOYSA-N 0.000 description 2
- 230000002411 adverse Effects 0.000 description 2
- 229910045601 alloy Inorganic materials 0.000 description 2
- 239000000956 alloy Substances 0.000 description 2
- 230000033228 biological regulation Effects 0.000 description 2
- 230000006866 deterioration Effects 0.000 description 2
- 238000010438 heat treatment Methods 0.000 description 2
- 238000009863 impact test Methods 0.000 description 2
- 238000004519 manufacturing process Methods 0.000 description 2
- 229910017060 Fe Cr Inorganic materials 0.000 description 1
- 229910002544 Fe-Cr Inorganic materials 0.000 description 1
- OKIZCWYLBDKLSU-UHFFFAOYSA-M N,N,N-Trimethylmethanaminium chloride Chemical compound [Cl-].C[N+](C)(C)C OKIZCWYLBDKLSU-UHFFFAOYSA-M 0.000 description 1
- 239000003125 aqueous solvent Substances 0.000 description 1
- 229910001566 austenite Inorganic materials 0.000 description 1
- 230000003197 catalytic effect Effects 0.000 description 1
- UPHIPHFJVNKLMR-UHFFFAOYSA-N chromium iron Chemical compound [Cr].[Fe] UPHIPHFJVNKLMR-UHFFFAOYSA-N 0.000 description 1
- 238000005097 cold rolling Methods 0.000 description 1
- 230000000052 comparative effect Effects 0.000 description 1
- 230000007797 corrosion Effects 0.000 description 1
- 238000005260 corrosion Methods 0.000 description 1
- 239000006185 dispersion Substances 0.000 description 1
- 239000003792 electrolyte Substances 0.000 description 1
- 230000001747 exhibiting effect Effects 0.000 description 1
- 239000000446 fuel Substances 0.000 description 1
- 238000005098 hot rolling Methods 0.000 description 1
- 239000012535 impurity Substances 0.000 description 1
- 229910001068 laves phase Inorganic materials 0.000 description 1
- 229910000734 martensite Inorganic materials 0.000 description 1
- 238000002844 melting Methods 0.000 description 1
- 230000008018 melting Effects 0.000 description 1
- 229910052751 metal Inorganic materials 0.000 description 1
- 239000002184 metal Substances 0.000 description 1
- MWUXSHHQAYIFBG-UHFFFAOYSA-N nitrogen oxide Inorganic materials O=[N] MWUXSHHQAYIFBG-UHFFFAOYSA-N 0.000 description 1
- 238000004445 quantitative analysis Methods 0.000 description 1
- 230000000630 rising effect Effects 0.000 description 1
- 238000002791 soaking Methods 0.000 description 1
- 239000000243 solution Substances 0.000 description 1
- 239000002904 solvent Substances 0.000 description 1
- 230000000087 stabilizing effect Effects 0.000 description 1
- 238000003860 storage Methods 0.000 description 1
- 238000003466 welding Methods 0.000 description 1
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- Exhaust Silencers (AREA)
Description
本発明は、自動車排ガス経路部材に使用するフェライト系ステンレス鋼材であって、特に材料温度が８５０℃を超えるような排ガス経路上流部材、例えばエキゾーストマニホールド、触媒コンバータ、フロントパイプ等に好適な耐熱性、低温靭性に優れる自動車排ガス経路部材用フェライト系ステンレス鋼材に関する。 The present invention is a ferritic stainless steel material used for an automobile exhaust gas path member, and particularly suitable for an exhaust gas path upstream member whose material temperature exceeds 850 ° C., for example, an exhaust manifold, a catalytic converter, a front pipe, etc. The present invention relates to a ferritic stainless steel material for automobile exhaust gas passage members having excellent toughness.
従来、自動車の排ガス経路部材には使用温度域に合わせて大きく２種類のフェライト系鋼種が使い分けられている。１つは材料の最高到達温度が７５０℃レベルの部材に主として適用されるＳＵＳ４２９系鋼に代表される鋼種、もう１つは材料の最高到達温度が８５０℃レベルの部材に主として適用されるＳＵＳ４４４系鋼に代表される鋼種である。 2. Description of the Related Art Conventionally, two types of ferritic steel types are widely used for exhaust gas path members of automobiles according to the operating temperature range. One is a steel type typified by SUS429 steel, which is mainly applied to members having a maximum material temperature of 750 ° C, and the other is SUS444, which is mainly applied to members having a material maximum temperature of 850 ° C. It is a steel type represented by steel.
最近では排ガス規制、燃費規制などに対応するために、排ガス温度が高温化する傾向にあり、今後、排ガス経路の上流部材では材料温度が実際に９００℃程度まで上昇することを想定した材料の要求が高まると予想される。従来のＳＵＳ４４４系鋼（１８Ｃｒ−２Ｍｏ−０.５Ｎｂ系）ではこのような高温に曝される部材に適用するには無理がある。また、このような高温での使用に耐えるためには、単に高温での引張強さが高いだけでは不十分であり、材料が塑性変形を始める応力の指標となる０.２％耐力が高いことが重要となる。 Recently, in order to comply with exhaust gas regulations, fuel efficiency regulations, etc., the exhaust gas temperature tends to increase, and in the future, material requirements assuming that the material temperature will actually rise to about 900 ° C in upstream members of the exhaust gas path. Is expected to increase. Conventional SUS444 series steel (18Cr-2Mo-0.5Nb series) cannot be applied to members exposed to such high temperatures. In order to withstand the use at such a high temperature, it is not sufficient that the tensile strength at a high temperature is simply high, and the material has a high 0.2% proof stress as an index of stress at which the material starts plastic deformation. Is important.
また、エンジンルーム内に搭載される各種装置の増加に伴い、排ガス部材の収容スペースには従来にも増して制約が大きくなっている。このため排ガス経路上流部材には種々の形状に成形加工できる優れた加工性が要求される。特に板だけでなくパイプにおいても複雑形状への厳しい加工に耐えうる優れた加工性が要求されるようになってきた。さらに、排ガス経路部材は低温靭性についても良好である必要がある。 In addition, with the increase of various devices mounted in the engine room, the exhaust gas member storage space is more restricted than in the past. For this reason, the exhaust gas path upstream member is required to have excellent workability that can be molded into various shapes. In particular, not only plates but also pipes have been required to have excellent workability that can withstand severe processing into complex shapes. Further, the exhaust gas path member needs to have good low temperature toughness.
これまでに下記のような耐熱性を向上させたフェライト系ステンレス鋼が種々開発され、実用化されつつある。
特許文献１には９００℃を超える温度域での使用に耐えるために固溶Ｎｂ量を十分に確保し、９５０℃での引張強さが２０ＭＰａを満たすように組成・組織を調整したフェライト系ステンレス鋼が示されている。しかし、０.２％耐力の記載はなく、材料温度が実際に９００℃程度まで上昇した場合の耐久性については定かではない。熱疲労特性や低温靭性に関しても特段の配慮がない。
Various ferritic stainless steels having improved heat resistance as described below have been developed and put into practical use.
Patent Document 1 discloses a ferritic stainless steel in which the amount of solid solution Nb is sufficiently secured to withstand use in a temperature range exceeding 900 ° C., and the composition and structure are adjusted so that the tensile strength at 950 ° C. satisfies 20 MPa. Steel is shown. However, there is no description of 0.2% proof stress, and it is not certain about the durability when the material temperature actually rises to about 900 ° C. There is no special consideration for thermal fatigue properties and low temperature toughness.
特許文献２には９００℃での高温強度に優れ、かつ低温靭性にも優れるフェライト系ステンレス鋼が示されている。しかし、０.２％耐力の記載はなく、材料温度が実際に９００℃程度まで上昇した場合の耐久性を十分に確保するための対策については必ずしも十分とは言えない。 Patent Document 2 discloses a ferritic stainless steel having excellent high temperature strength at 900 ° C. and excellent low temperature toughness. However, there is no description of 0.2% proof stress, and it cannot be said that the measures for ensuring sufficient durability when the material temperature actually rises to about 900 ° C. are necessarily sufficient.
特許文献３には９５０℃での高温強度が高く、加工性も良好なフェライト系ステンレス鋼が記載されている。しかし、０.２％耐力は示されておらず、材料が実際に９００℃程度に曝された場合に耐えうるかどうかは定かでない。低温靭性についても特段の配慮は見られない。 Patent Document 3 describes a ferritic stainless steel having high high-temperature strength at 950 ° C. and good workability. However, 0.2% proof stress is not shown, and it is unclear whether the material can withstand when it is actually exposed to about 900 ° C. There is no special consideration for low temperature toughness.
特許文献４には熱膨張係数の低下を図ったＦｅ−Ｃｒ合金が示されている。しかし、９００℃程度の温度域での高温強度を改善することは意図されていない。 Patent Document 4 discloses an Fe—Cr alloy whose thermal expansion coefficient is reduced. However, it is not intended to improve the high-temperature strength in the temperature range of about 900 ° C.
特許文献５には熱疲労特性に優れ、かつ低温靭性も良好なフェライト系ステンレス鋼が記載されている。しかし、高温強度に関しては６００℃での０.２％耐力で評価されており、材料温度が実際に９００℃程度まで上昇した場合の耐久性については定かではない。 Patent Document 5 describes a ferritic stainless steel having excellent thermal fatigue characteristics and good low-temperature toughness. However, the high temperature strength is evaluated with a 0.2% proof stress at 600 ° C., and the durability when the material temperature actually rises to about 900 ° C. is not clear.
特許文献６には７００℃以上の温度で使用される排ガス系部材に用いるフェライト系ステンレス鋼が示されている。しかし、高温強度に関しては６００℃と８５０℃での引張強さが示されているだけであり、材料が実際に９００℃程度の温度に曝された場合に耐えうるかどうかは定かでない。また、低温靭性についても記載がない。 Patent Document 6 discloses ferritic stainless steel used for an exhaust gas system member used at a temperature of 700 ° C. or higher. However, only the tensile strength at 600 ° C. and 850 ° C. is shown for the high temperature strength, and it is unclear whether the material can withstand when it is actually exposed to temperatures of about 900 ° C. Moreover, there is no description about low temperature toughness.
８５０℃を超える温度で使用された場合に優れた耐久性を呈し、かつ良好な低温靭性および加工性をも同時に具備する材料を安定的に実現させる手法は未だ確立されていない（上記特許文献参照）。
本発明は、８５０℃を超える高温域での０.２％耐力、熱疲労特性、低温靭性、および加工性をともに高レベルで兼ね備える自動車排ガス経路部材用フェライト系ステンレス鋼材を提供することを目的とする。
A technique for stably realizing a material that exhibits excellent durability when used at a temperature exceeding 850 ° C. and also has good low-temperature toughness and workability has not been established yet (see the above-mentioned patent document). ).
An object of the present invention is to provide a ferritic stainless steel material for an automobile exhaust gas path member having a high level of 0.2% yield strength, thermal fatigue characteristics, low temperature toughness, and workability in a high temperature range exceeding 850 ° C. To do.
上記目的は、質量％で、Ｃ：０.０３％以下、Ｓｉ：１％以下、Ｍｎ：０.６％未満、Ｎｉ：０.６％以下、Ｃｒ：１０〜２０％、Ｎｂ：０.３〜０.７％、Ｃｕ：１超え〜２％、Ｍｏ：１〜２.５％、Ａｌ：０.１５％以下、Ｖ：０.０３〜０.２％、Ｎ：０.０３％以下を含有し、必要に応じてＷ、Ｔｉ、Ｚｒの１種以上を合計２％以下の範囲で含有し、あるいはさらにＢ：０.０２％以下、Ｃｏ：２％以下の１種以上を含有し、あるいはさらにＲＥＭ（希土類元素）、Ｃａの１種以上を合計０.１％以下の範囲で含有し、残部Ｆｅおよび不可避的不純物であり、かつ下記（１）式および（２）式を満たす組成を有し、析出相として存在するＮｂとＭｏの総量が０.５質量％以下である組織を有する、耐熱性、低温靭性に優れる自動車排ガス経路部材用フェライト系ステンレス鋼材によって達成される。
１.２Ｎｂ＋５Ｍｏ＋６Ｃｕ≧１１.５ ……（１）
１５Ｎｂ＋２Ｍｏ＋０.５Ｃｕ≧９.５ ……（２）
The purpose is mass%, C: 0.03% or less, Si: 1% or less, Mn: less than 0.6%, Ni: 0.6 % or less, Cr: 10-20%, Nb: 0.3 -0.7%, Cu: more than 1 to 2%, Mo: 1 to 2.5%, Al: 0.15% or less, V: 0.03 to 0.2%, N: 0.03% or less Containing one or more of W, Ti, Zr in a total range of 2% or less as required, or further containing one or more of B: 0.02% or less, Co: 2% or less, Alternatively, a composition containing at least one of REM (rare earth element) and Ca in a total range of 0.1% or less, the balance being Fe and inevitable impurities, and satisfying the following formulas (1) and (2): And has a structure in which the total amount of Nb and Mo existing as a precipitated phase is 0.5% by mass or less, and has excellent heat resistance and low temperature toughness. It is accomplished by the write stainless steel.
1.2 Nb + 5Mo + 6Cu ≧ 11.5 (1)
15Nb + 2Mo + 0.5Cu ≧ 9.5 (2)
ここで、上記（１）式、（２）式の元素記号の箇所には質量％で表された当該元素の含有量の値が代入される。
「自動車排ガス経路部材用フェライト系ステンレス鋼材」とは、自動車排ガス経路部材を製造する過程で９５０℃を超える温度（例えば９５０超え〜１１００℃）に加熱される最終的な焼鈍（以下単に「最終焼鈍」という）を終えた後の鋼材をいう。例えば、鋼板を溶接造管してパイプにした後、成形加工し、その後最終焼鈍を施す場合は、最終焼鈍後のパイプがここでいう自動車排ガス経路部材用フェライト系ステンレス鋼材に該当する。鋼板の段階で最終焼鈍を施す場合は、最終焼鈍後の鋼板や、その後に加工されて得られたパイプ、筒体などが自動車排ガス経路部材用フェライト系ステンレス鋼材に該当する。
上記鋼材のうち、材料温度が８５０℃を超える温度域となる排ガス部材に使用されるものが特に好適な対象となる。
Here, the value of the content of the element expressed in mass% is substituted for the element symbol in the above formulas (1) and (2).
“Ferrite stainless steel material for automobile exhaust gas path member” means final annealing (hereinafter simply referred to as “final annealing”) that is heated to a temperature exceeding 950 ° C. (for example, exceeding 950 to 1100 ° C.) in the process of manufacturing the automobile exhaust gas path member. ") Refers to the steel after finishing. For example, when a steel plate is welded and formed into a pipe, and then molded, and then subjected to final annealing, the pipe after the final annealing corresponds to the ferritic stainless steel material for an automobile exhaust gas path member here. When the final annealing is performed at the stage of the steel plate, the steel plate after the final annealing, and the pipes and cylinders obtained by subsequent processing correspond to the ferritic stainless steel material for the automobile exhaust gas path member.
Among the steel materials, those used for exhaust gas members having a material temperature exceeding 850 ° C. are particularly suitable.
本発明によれば、８５０℃を超える温度に曝された場合に耐えうる高温強度と、良好な熱疲労特性と、良好な加工性と、良好な低温靭性を同時に具備する自動車排ガス経路部材用フェライト系ステンレス鋼材が提供可能になった。この材料は、昨今の排ガス温度の上昇傾向に対応できるものであるとともに、排ガス経路上流部材の設計自由度の拡大をもたらすものである。 According to the present invention, a ferrite for an automobile exhaust gas path member having high temperature strength that can withstand exposure to temperatures exceeding 850 ° C., good thermal fatigue properties, good workability, and good low temperature toughness at the same time. Stainless steel materials can be provided. This material can cope with the recent rising trend of the exhaust gas temperature, and also increases the degree of design freedom of the exhaust gas path upstream member.
本発明においては、６００℃レベルでの高温強度（０.２％耐力）を高く維持しながら、９００℃レベルでの高温強度（０.２％耐力）の向上を図ることが重要である。これらの両温度域にわたって高い強度を呈することが、熱疲労特性を高く維持するために極めて有効である。種々検討の結果、６００℃での０.２％耐力および９００℃での０.２％耐力は、いずれもＳＵＳ４４４系鋼の同温度での耐力値の１.５倍に相当する耐力値以上に高レベルであることが望ましい。具体的には、６００℃での０.２％耐力は２００ＭＰａ以上、９００℃での０.２％耐力は３５ＭＰａ以上であることが好ましい。このような高温強度特性を有する材料は、自動車排ガス経路部材として常温と９００℃程度の温度の間で繰り返しの温度変動を受けた場合に、実用上十分な高温疲労特性を有することがわかった。 In the present invention, it is important to improve the high temperature strength (0.2% yield strength) at the 900 ° C. level while maintaining the high temperature strength (0.2% yield strength) at the 600 ° C. level high. Exhibiting high strength over both these temperature ranges is extremely effective for maintaining high thermal fatigue properties. As a result of various studies, the 0.2% proof stress at 600 ° C and the 0.2% proof stress at 900 ° C are both higher than the proof value corresponding to 1.5 times the proof value of SUS444 steel at the same temperature. A high level is desirable. Specifically, the 0.2% yield strength at 600 ° C. is preferably 200 MPa or more, and the 0.2% yield strength at 900 ° C. is preferably 35 MPa or more. It has been found that a material having such high temperature strength characteristics has practically sufficient high temperature fatigue characteristics when subjected to repeated temperature fluctuations between room temperature and a temperature of about 900 ° C. as an automobile exhaust gas path member.
本発明では６００℃を含む温度域（概ね５００〜８００℃の領域）での高温強度を向上させるために、Ｃｕを利用する。すなわち、Ｃｕを添加することにより６００℃前後の温度域でε−Ｃｕ相が析出し、これがマトリックス中に微細分散することにより析出強化現象を発現する。また、この温度域での高温強度（０.２％耐力）をＳＵＳ４４４系鋼の約１.５倍以上に高く維持するためには、ε−Ｃｕ相の析出に加え、ＮｂおよびＭｏの固溶強化を利用することが必要である。種々検討の結果、Ｎｂ、Ｍｏ、Ｃｕの含有量が（１）式を満たすように成分調整することによって、８００℃以下の領域での高温強度をＳＵＳ４４４系鋼の約１.５倍以上に高めることが可能になる。
１.２Ｎｂ＋５Ｍｏ＋６Ｃｕ≧１１.５ ……（１）
In the present invention, Cu is used in order to improve the high temperature strength in a temperature range including 600 ° C. (approximately 500 to 800 ° C.). That is, by adding Cu, an ε-Cu phase is precipitated in a temperature range of about 600 ° C., and when this is finely dispersed in the matrix, a precipitation strengthening phenomenon is expressed. In order to maintain the high temperature strength (0.2% proof stress) in this temperature range about 1.5 times higher than that of SUS444 steel, in addition to precipitation of ε-Cu phase, solid solution of Nb and Mo It is necessary to take advantage of enhancements. As a result of various studies, by adjusting the components so that the contents of Nb, Mo, and Cu satisfy the formula (1), the high-temperature strength in the region of 800 ° C. or lower is increased to about 1.5 times or more of SUS444 steel. It becomes possible.
1.2 Nb + 5Mo + 6Cu ≧ 11.5 (1)
８００℃を超える温度域になると、ε−Ｃｕ相の固溶化が進み、Ｃｕによる高温強度の向上作用は弱くなる。９００℃での高温強度（０.２％耐力）をＳＵＳ４４４系鋼の約１.５倍以上に高めるためには、ＮｂおよびＭｏの固溶強化を十分に利用することが重要である。固溶Ｃｕも高温強度の向上に有効であるので、それも利用する。種々検討の結果、（２）式を満たすように成分調整する必要があることがわかった。
１５Ｎｂ＋２Ｍｏ＋０.５Ｃｕ≧９.５ ……（２）
（２）式のＮｂの係数はＮｂ：０.１質量％あたりの９００℃での０.２％耐力（ＭＰａ）の上昇量に相当し、ＭｏおよびＣｕの係数はそれぞれＭｏおよびＣｕ：１質量％あたりの９００℃での０.２％耐力（ＭＰａ）の上昇量に相当する。
If it becomes a temperature range exceeding 800 degreeC, solid solution of an epsilon-Cu phase will advance and the improvement effect of the high temperature intensity | strength by Cu will become weak. In order to increase the high-temperature strength at 900 ° C. (0.2% proof stress) to about 1.5 times or more of SUS444 steel, it is important to fully utilize the solid solution strengthening of Nb and Mo. Since solute Cu is also effective in improving high-temperature strength, it is also used. As a result of various studies, it has been found that it is necessary to adjust the components so as to satisfy the expression (2).
15Nb + 2Mo + 0.5Cu ≧ 9.5 (2)
The coefficient of Nb in the formula (2) corresponds to an increase in 0.2% yield strength (MPa) at 900 ° C. per 0.1% by mass of Nb, and the coefficients of Mo and Cu are Mo and Cu: 1 mass respectively. This corresponds to an increase in 0.2% yield strength (MPa) at 900 ° C. per%.
ただし、９００℃という高温での０.２％耐力をＳＵＳ４４４系鋼の１.５倍程度以上に高めるためには、上記（２）式を満たす組成にするだけでは不十分である。詳細な検討によれば、特にＮｂ、Ｍｏの析出物をできるだけ少なくした金属組織とすることが極めて重要であることがわかった。具体的には、最終焼鈍後の状態において、析出相として存在するＮｂとＭｏの総量が０.５質量％以下である組織状態としなければならない。
また、高温強度だけでなく、加工性、低温靭性、溶接性を高く維持するためにも、最終焼鈍後において上記組織状態とすることが極めて有効である。
However, in order to increase the 0.2% proof stress at a high temperature of 900 ° C. to about 1.5 times or more of SUS444 steel, it is not sufficient to have a composition that satisfies the above formula (2). According to a detailed examination, it has been found that it is extremely important to obtain a metal structure with as few Nb and Mo precipitates as possible. Specifically, in the state after the final annealing, the structure state must be such that the total amount of Nb and Mo existing as a precipitation phase is 0.5 mass% or less.
In order to maintain not only high temperature strength but also workability, low temperature toughness, and weldability, it is extremely effective to obtain the above-described microstructure after the final annealing.
「析出相として存在するＮｂとＭｏの総量（質量％）」は、非水溶媒電解液中での定電位電解（ＳＰＥＥＤ法）によって抽出した析出相の残渣について、元素を定量分析し、その残渣中に含まれるＮｂとＭｏの合計質量を、電解により溶解したマトリクスと抽出された析出相の全質量で除して、パーセント表示することにより求めることができる。 “Total amount (% by mass) of Nb and Mo existing as a precipitated phase” is a quantitative analysis of elements of the residue of the precipitated phase extracted by constant potential electrolysis (SPEED method) in a non-aqueous solvent electrolyte. The total mass of Nb and Mo contained therein is divided by the total mass of the matrix dissolved by electrolysis and the extracted precipitated phase, and can be obtained by percentage display.
また、析出相として存在するＮｂとＭｏの総量が０.５質量％以下である組織状態を得るためには、最終焼鈍時の冷却過程において、９５０℃から５００℃までの冷却速度を５℃／ｓｅｃ以上にコントロールする必要がある。例えば溶接造管したパイプを自動車排ガス経路部材に適用する場合は、造管前の鋼板の段階、あるいは造管後、部材として使用に供するまでの段階で少なくとも１回、９５０超え〜１１００℃好ましくは１０００〜１０５０℃で均熱０〜１０分加熱した後、９５０℃から５００℃までの冷却速度が５℃／ｓｅｃ以上となるように冷却する最終焼鈍を施すようにすればよい。なお、自動車排ガス経路部材として使用される前に、一度このような組織状態を得ておけば、その後、自動車排ガス経路部材として９００℃程度の温度に加熱されて使用された際に、ＮｂとＭｏの析出相が必要以上に生じることはなく、高温強度や低温靭性は実用上阻害されない。 In addition, in order to obtain a structure state in which the total amount of Nb and Mo existing as a precipitated phase is 0.5 mass% or less, a cooling rate from 950 ° C. to 500 ° C. is set to 5 ° C./in the cooling process during the final annealing. It is necessary to control over sec. For example, in the case where a welded pipe is applied to an automobile exhaust gas path member, at least once in the stage of the steel plate before pipe forming or the stage after pipe forming until it is used as a member, more than 950 to 1100 ° C, preferably What is necessary is just to give the final annealing which cools so that the cooling rate from 950 degreeC to 500 degreeC may be 5 degrees C / sec or more after heating at 1000-1050 degreeC for 0-10 minutes. In addition, once such a structural state is obtained before being used as an automobile exhaust gas route member, Nb and Mo are then used when heated and used at a temperature of about 900 ° C. as an automobile exhaust gas route member. The precipitate phase does not occur more than necessary, and the high-temperature strength and low-temperature toughness are not impeded practically.
以下、合金成分について説明する。
ＣおよびＮは、一般的にはクリープ強度等の高温強度向上に有効な元素とされるが、過剰に含有すると酸化特性、加工性、低温靱性、溶接性が低下する。本発明ではＣ、Ｎとも０.０３質量％以下に制限する。
Hereinafter, the alloy components will be described.
C and N are generally effective elements for improving high-temperature strength such as creep strength, but if contained excessively, oxidation characteristics, workability, low-temperature toughness, and weldability deteriorate. In the present invention, both C and N are limited to 0.03 mass% or less.
Ｓｉは、高温酸化特性の改善に有効であるが、過剰に添加すると硬さが上昇し、加工性、低温靱性が低下する。本発明ではＳi含有量は１.０質量％以下に制限する。 Si is effective in improving high-temperature oxidation characteristics, but if added excessively, hardness increases and workability and low-temperature toughness decrease. In the present invention, the Si content is limited to 1.0% by mass or less.
Ｍｎは、高温酸化特性、特に耐スケール剥離性を改善する。ただし過剰添加は加工性、溶接性を阻害する。またＭｎはオーステナイト安定化元素であるため、多量に添加するとマルテンサイト相が生成し易くなり、熱疲労特性、加工性の低下要因となる。本発明では、特に加工性および低温靭性の低下を防ぐために、Ｍｎ含有量は０.６質量％以下に制限する。 Mn improves high temperature oxidation properties, particularly scale peel resistance. However, excessive addition hinders workability and weldability. Further, since Mn is an austenite stabilizing element, if added in a large amount, a martensite phase is likely to be generated, which causes a decrease in thermal fatigue characteristics and workability. In the present invention, the Mn content is limited to 0.6% by mass or less in order to prevent deterioration of workability and low temperature toughness.
Ｎｉは、低温靭性の向上に寄与するが、過剰の含有は常温での伸びを低下させる要因となる。本発明ではＮｉ含有量は３質量％まで許容されるが、０.６質量％以下の範囲で含有させることがより好ましい。 Ni contributes to the improvement of low temperature toughness, but excessive inclusion causes a decrease in elongation at room temperature. In the present invention, the Ni content is allowed up to 3% by mass, but it is more preferable to contain it in the range of 0.6% by mass or less.
Ｃｒは、フェライト相を安定化するとともに、高温材料に重視される耐酸化性の改善に寄与する。ただし、過剰のＣｒ含有は鋼材の脆化や加工性劣化を招く。このためＣｒ含有量は１０〜２０質量％とする。 Cr stabilizes the ferrite phase and contributes to the improvement of oxidation resistance, which is important for high temperature materials. However, excessive Cr content causes embrittlement and workability deterioration of the steel material. For this reason, Cr content shall be 10-20 mass%.
Ｎｂは、固溶強化によって６００℃前後の温度域での高温強度を上昇させる作用も有するが、本発明では主として８５０℃を超える高温域での高温強度を確保するためにＮｂの固溶強化作用を活用する。そのためには０.３質量％以上のＮｂ含有量を確保するとともに、前述の（２）式を満たすことが必要である。また、前述のように本発明では析出相として存在するＮｂとＭｏの総量が０.５質量％以下である組織状態とする必要があるが、ＮｂはＣ、Ｎとの親和力が強く、高温強度、低温靭性、加工性等を低下させる要因となる析出物を形成しやすい。このため、Ｎｂ含有量は０.７質量％以下の範囲に制限する。 Nb also has the effect of increasing the high temperature strength in the temperature range around 600 ° C. by solid solution strengthening, but in the present invention, the Nb solid solution strengthening effect mainly in order to ensure the high temperature strength in the high temperature range exceeding 850 ° C. Utilize. For that purpose, it is necessary to ensure the Nb content of 0.3% by mass or more and to satisfy the above-mentioned formula (2). In addition, as described above, in the present invention, it is necessary to have a structural state in which the total amount of Nb and Mo existing as a precipitation phase is 0.5 mass% or less. Nb has a strong affinity for C and N, and has a high temperature strength. It is easy to form precipitates that cause low temperature toughness, workability, and the like. For this reason, Nb content is restrict | limited to the range below 0.7 mass%.
Ｃｕは、本発明において重要な元素である。すなわち、本発明では前述のようにε−Ｃｕ相の微細分散析出現象を利用して６００℃前後（概ね５００〜８５０℃）での強度を高め、熱疲労特性を向上させる。また、８５０℃を超える高温域では、Ｃｕの固溶強化を利用してＮｂおよびＭｏによる高温強度の向上作用を補助する役割を有する。種々検討の結果、これらの効果を十分に引き出すためには、少なくとも１質量％を超えるＣｕ含有が必要となる。ただし過剰のＣｕ含有は加工性、低温靱性、溶接性を低下させるのでＣｕ含有量の上限は２質量％に制限する。 Cu is an important element in the present invention. That is, in the present invention, as described above, the strength at around 600 ° C. (approximately 500 to 850 ° C.) is increased by utilizing the fine dispersion precipitation phenomenon of the ε-Cu phase, and the thermal fatigue characteristics are improved. Moreover, in the high temperature range over 850 degreeC, it has the role which assists the improvement effect of the high temperature strength by Nb and Mo using the solid solution strengthening of Cu. As a result of various studies, in order to sufficiently bring out these effects, it is necessary to contain at least 1% by mass of Cu. However, since excessive Cu content reduces workability, low temperature toughness, and weldability, the upper limit of Cu content is limited to 2% by mass.
Ｍｏは、Ｎｂと同様に固溶強化による高温強度の上昇作用を有する。特に本発明では８５０℃を超える高温域での高温強度を高める必要があることから、１質量％以上のＭｏ添加が必須となる。また、前述のように本発明では析出相として存在するＮｂとＭｏの総量が０.５質量％以下である組織状態とする必要があるが、過剰のＭｏ添加は炭化物やＬａｖｅｓ相（Ｆｅ2Ｍｏ）を形成して高温強度や低温靭性を阻害する要因となる。このためＭｏ含有量は２.５質量％以下の範囲に制限される。 Mo, like Nb, has the effect of increasing the high temperature strength by solid solution strengthening. In particular, in the present invention, since it is necessary to increase the high temperature strength in a high temperature region exceeding 850 ° C., addition of 1% by mass or more of Mo is essential. In addition, as described above, in the present invention, it is necessary to obtain a structural state in which the total amount of Nb and Mo existing as a precipitation phase is 0.5 mass% or less. However, excessive Mo addition may result in a carbide or a Laves phase (Fe 2 Mo ) To inhibit high temperature strength and low temperature toughness. For this reason, Mo content is restrict | limited to the range of 2.5 mass% or less.
Ａｌは、脱酸剤であるとともに、耐高温酸化性を改善する。しかし、多量にＡｌを含有させると表面性状、加工性、溶接性、低温靱性に悪影響を及ぼす。このため、Ａｌは０.１５質量％以下の範囲で添加する。 Al is a deoxidizer and improves high-temperature oxidation resistance. However, when Al is contained in a large amount, the surface properties, workability, weldability, and low temperature toughness are adversely affected. For this reason, Al is added in the range of 0.15% by mass or less.
Ｖは、Ｎｂ、Ｃｕとの複合添加によって高温強度の向上に寄与する。また、Ｎｂとの共存により、加工性、低温靱性、耐粒界腐食感受性、溶接熱影響部の靱性を改善する。これらの作用を十分に得るために、本発明ではＶを０.０３質量％以上含有させる。ただし、Ｖの過剰添加は加工性や低温靱性の低下を招く要因になる。このため、Ｖ含有量は０.２質量％以下の範囲に制限する。 V contributes to the improvement of the high-temperature strength by the combined addition with Nb and Cu. Further, coexistence with Nb improves workability, low temperature toughness, intergranular corrosion resistance, and toughness of the heat affected zone. In order to obtain these effects sufficiently, in the present invention, V is contained in an amount of 0.03 mass% or more. However, excessive addition of V becomes a factor that causes a decrease in workability and low-temperature toughness. For this reason, V content is restrict | limited to the range below 0.2 mass%.
Ｗ、Ｔｉ、Ｚｒは、高温強度の向上に有効な元素であり、必要に応じてこれらの１種以上を添加することができる。ただし、過剰の添加は靭性を低下させるため、Ｗ、Ｔｉ、Ｚｒの１種以上を添加する場合はそれらの合計含有量が２質量％以下の範囲となるようにする。 W, Ti, and Zr are effective elements for improving the high-temperature strength, and one or more of them can be added as necessary. However, since excessive addition reduces toughness, when adding one or more of W, Ti, and Zr, the total content thereof should be in the range of 2% by mass or less.
Ｂ、Ｃｏは、Ｎｉと同様に低温靭性に寄与する元素であり、必要に応じてＢ、Ｃｏの１種または２種を添加することができる。ただし、過剰の添加は常温での伸びを低下させる要因となるので、Ｂ含有量は０.０２質量％以下の範囲、Ｃｏ含有量は２質量％以下の範囲とする。Ｂは０.０００５〜０.０２質量％の含有量を確保することがより効果的である。 B and Co are elements that contribute to low temperature toughness like Ni, and one or two of B and Co can be added as necessary. However, excessive addition causes a decrease in elongation at room temperature, so the B content is in the range of 0.02% by mass or less and the Co content is in the range of 2% by mass or less. It is more effective to secure B in a content of 0.0005 to 0.02% by mass.
ＲＥＭ（希土類元素）、Ｃａは、耐高温酸化特性の改善に寄与する元素であり、必要に応じてこれらの１種以上を添加することができる。ＲＥＭ、Ｃａの合計含有量を０.００１質量％以上とすることがより効果的である。ただし、過剰の添加は製造性に悪影響を及ぼすので、ＲＥＭ、Ｃａの合計含有量は０.１質量％以下の範囲となるようにする。 REM (rare earth element) and Ca are elements that contribute to the improvement of high-temperature oxidation resistance, and one or more of them can be added as necessary. It is more effective to set the total content of REM and Ca to 0.001% by mass or more. However, excessive addition adversely affects manufacturability, so the total content of REM and Ca is set to be in the range of 0.1% by mass or less.
本発明のステンレス鋼材は、以上の組成に調整されたステンレス鋼を通常の手法で溶製したのち、一般的なステンレス鋼板製造プロセスにて所定板厚の鋼板とし、その後、溶接造管、成形加工などの工程に供することによって製造される。その際、９５０超え〜１１００℃好ましくは１０００〜１０５０℃に加熱する最終焼鈍においては、前述のように、９５０℃から５００℃までの冷却速度を５℃／ｓｅｃ以上にコントロールすることが重要である。この冷却条件を外れると、析出相として存在するＮｂとＭｏの総量が０.５質量％以下である組織状態が得られにくくなり、８５０℃を超える高温域での高温強度（０.２％耐力）をＳＵＳ４４４の約１.５倍以上のレベルに安定して向上させることが難しくなる。また、低温靭性の低下を招く要因にもなる。 The stainless steel material of the present invention is obtained by melting a stainless steel adjusted to the above composition by a usual method, and then converting it into a steel plate having a predetermined plate thickness by a general stainless steel plate manufacturing process, and then performing welding pipe forming and forming processing. It is manufactured by subjecting it to such processes. At that time, in the final annealing to be heated to over 950 to 1100 ° C, preferably 1000 to 1050 ° C, it is important to control the cooling rate from 950 ° C to 500 ° C to 5 ° C / sec or more as described above. . If this cooling condition is not satisfied, it becomes difficult to obtain a structural state in which the total amount of Nb and Mo existing as a precipitation phase is 0.5 mass% or less, and high temperature strength (0.2% yield strength) in a high temperature range exceeding 850 ° C. ) To a level of about 1.5 times or more that of SUS444. Moreover, it becomes a factor which causes the fall of low-temperature toughness.
表１に示すフェライト系ステンレス鋼を溶製し、熱間圧延、熱延板焼鈍、冷間圧延、仕上焼鈍の工程を経て板厚２ｍｍの冷延焼鈍鋼板を作製した。上記の仕上焼鈍は、排ガス経路部材用鋼材としての最終焼鈍を模した条件で行った。最終焼鈍条件は、１０００℃×均熱１分の加熱の後、一部の比較例（Ｎｏ.２１）を除き、９５０℃から５００℃までの平均冷却速度が５℃／ｓｅｃ以上となるように冷却した。冷却速度は試料表面に取り付けた熱電対を用いて測定した。このような最終焼鈍を終えて得られた冷延焼鈍鋼板を供試材として排ガス経路部材用鋼材の各種特性を調査した。 A ferritic stainless steel shown in Table 1 was melted, and a cold-rolled annealed steel sheet having a thickness of 2 mm was prepared through the steps of hot rolling, hot-rolled sheet annealing, cold rolling, and finish annealing. Said finish annealing was performed on the conditions which simulated the final annealing as a steel material for exhaust gas path members. As for the final annealing conditions, after heating at 1000 ° C. × soaking for 1 minute, the average cooling rate from 950 ° C. to 500 ° C. is 5 ° C./sec or more except for some comparative examples (No. 21). Cooled down. The cooling rate was measured using a thermocouple attached to the sample surface. Various characteristics of the steel material for exhaust gas passage members were investigated using the cold-rolled annealed steel plate obtained after such final annealing as a test material.
供試材（最終焼鈍を終えたもの）について、析出相として存在するＮｂとＭｏの総量（ここでは「析出Ｎｂ＋析出Ｍｏ量」と表示する）、６００℃での０.２％耐力、９００℃での０.２％耐力、低温靭性、常温での加工性を以下のようにして調べた。 For the test material (after final annealing), the total amount of Nb and Mo existing as a precipitation phase (herein referred to as “precipitation Nb + precipitation Mo amount”), 0.2% proof stress at 600 ° C., 900 ° C. The 0.2% proof stress, the low temperature toughness, and the workability at room temperature were examined as follows.
〔析出Ｎｂ＋析出Ｍｏ量〕
ＳＰＥＥＤ法により、供試材のマトリクスが溶解し、かつ析出相が溶解しない電位での定電位電解を行い、抽出した析出相の残渣について、元素を定量分析した。その残渣中に含まれるＮｂとＭｏの合計質量を、電解により溶解したマトリクスと抽出された析出相の全質量で除して、パーセント表示することにより、析出Ｎｂ＋析出Ｍｏ量を定めた。ＳＰＥＥＤ法においては非水溶媒として、１０％アセチルアセトン＋１％テトラメチルアンモニウムクロライド＋メチルアルコール溶液を用いた。
[Precipitation Nb + Precipitation Mo amount]
By the SPEED method, constant potential electrolysis was performed at a potential at which the matrix of the test material was dissolved and the precipitated phase was not dissolved, and the elements of the extracted precipitated phase residue were quantitatively analyzed. The total mass of Nb and Mo contained in the residue was divided by the total mass of the matrix dissolved by electrolysis and the extracted precipitated phase, and expressed as a percentage to determine the amount of precipitated Nb + precipitated Mo. In the SPEED method, a 10% acetylacetone + 1% tetramethylammonium chloride + methyl alcohol solution was used as a nonaqueous solvent.
〔６００℃、９００℃での０.２％耐力〕
板厚２ｍｍの引張試験片（引張方向が圧延方向に一致するもの）を用いて、ＪＩＳ Ｇ０５６７に準拠して６００℃での引張試験および９００℃での引張試験を行った。６００℃の０.２％耐力はＳＵＳ４４４系鋼の約１.５倍に相当する２００ＭＰａ以上のものを合格、それより低いものを不合格と評価した。９００℃の０.２％耐力はＳＵＳ４４４系鋼の約１.５倍に相当する３５ＭＰａ以上のものを合格、それより低いものを不合格と評価した。
[0.2% yield strength at 600 ° C and 900 ° C]
A tensile test at 600 ° C. and a tensile test at 900 ° C. were performed in accordance with JIS G0567 using a tensile test piece having a thickness of 2 mm (the tensile direction coincides with the rolling direction). The 0.2% proof stress at 600 ° C. was evaluated as acceptable when the pressure was 200 MPa or higher, which is about 1.5 times that of SUS444 steel, and rejected when lower than that. The 0.2% proof stress at 900 ° C. was evaluated as passing a 35 MPa or more equivalent to about 1.5 times that of SUS444 steel, and rejecting a lower one than that.
〔低温靭性〕
板厚２ｍｍの供試材からＶノッチシャルピー衝撃試験片（ハンマーのぶつかる方向が圧延方向に対し平行方向のもの）を作製し、ＪＩＳ Ｚ２２４２に準拠して−７５℃〜２５℃の範囲で２５℃ピッチでシャルピー衝撃試験を行い、延性靭性遷移温度を求めた。遷移温度が−２５℃以下であるものを○（低温靭性；良好）、それより高いものを×（低温靭性；不良）と評価した。
(Low temperature toughness)
A V-notch Charpy impact test piece (having a hammer striking direction parallel to the rolling direction) was prepared from a specimen having a thickness of 2 mm, and 25 ° C. in the range of −75 ° C. to 25 ° C. in accordance with JIS Z2242. A Charpy impact test was performed at the pitch to determine the ductile toughness transition temperature. Those having a transition temperature of −25 ° C. or lower were evaluated as “◯” (low temperature toughness: good), and those having a transition temperature higher than that were evaluated as “x” (low temperature toughness; poor).
〔常温での加工性〕
板厚２ｍｍの供試材から、引張方向が圧延方向に対して０°、４５°、９０°の３種類の引張試験片（ＪＩＳ １３Ｂ号）を作製し、それぞれＪＩＳ ２２４１に準拠して試験数ｎ＝３で引張試験を破断まで行い、破断後の試験片を突き合わせて破断時の伸び（％）を測定し、下記（３）式により平均伸び値ＥＬAを求め、このＥＬAを当該供試材の常温伸びの値とした。
ＥＬA＝（ＥＬL＋ＥＬD＋ＥＬT） ……（３）
ここで、ＥＬLは引張方向が０°の破断時の伸び（ｎ＝３の平均値）、ＥＬDは引張方向が４５°の破断時の伸び（ｎ＝３の平均値）、ＥＬTは引張方向が９０°の破断時の伸び（ｎ＝３の平均値）である。ＥＬAが３０％以上のものを○（常温での加工性；良好）、それより低いものを×（常温での加工性；不良）と評価した。
結果を表２に示す。表２中の「最終焼鈍冷却速度」は、９５０℃から５００℃までの平均冷却速度である。
[Processability at room temperature]
Three types of tensile test pieces (JIS 13B) having a tensile direction of 0 °, 45 °, and 90 ° with respect to the rolling direction were prepared from a specimen having a plate thickness of 2 mm, and the number of tests in accordance with JIS 2241, respectively. subjected to tensile tests at n = 3 to rupture, against the test piece after breaking is measured elongation at break (%), an average elongation value EL a by the following equation (3), the subject of this EL a The room temperature elongation value of the sample was used.
EL A = (EL L + EL D + EL T ) (3)
Here, EL L is the elongation at break when the tensile direction is 0 ° (average value of n = 3), EL D is the elongation at break when the tensile direction is 45 ° (average value of n = 3), and EL T is Elongation at break when the tensile direction is 90 ° (average value of n = 3). Those having EL A of 30% or more were evaluated as ◯ (workability at normal temperature; good), and those lower than that were evaluated as x (workability at normal temperature; poor).
The results are shown in Table 2. “Final annealing cooling rate” in Table 2 is an average cooling rate from 950 ° C. to 500 ° C.
組成および析出Ｎｂ＋析出Ｍｏ量が本発明の規定を満たす本発明例の鋼材は、表２からわかるように、いずれも６００℃の０.２％耐力および９００℃の０.２％耐力がＳＵＳ４４４系鋼の１.５倍程度以上に高く、８５０℃を超える高温域で優れた高温強度を呈すると共に、熱疲労特性も十分に良好であると言える。また、低温靭性、常温での加工性も良好である。 As can be seen from Table 2, the steel materials of the examples of the present invention in which the composition and the amount of precipitated Nb + precipitated Mo satisfy the provisions of the present invention are 0.2% proof stress at 600 ° C and 0.2% proof stress at 900 ° C. It is about 1.5 times higher than steel, exhibits excellent high-temperature strength in a high-temperature region exceeding 850 ° C., and has a sufficiently good thermal fatigue property. Moreover, low temperature toughness and processability at normal temperature are also good.
これに対し、Ｎｏ.２１は、鋼の組成は本発明規定範囲にあるものの、最終焼鈍時の９５０℃から５００℃までの冷却速度が５℃／ｓｅｃより遅かったことにより、その冷却過程でＮｂ、Ｍｏの析出物が多く生成して、析出Ｎｂ＋析出Ｍｏ量が多過ぎる組織状態となったものである。この場合、９００℃での高温強度および低温靭性に劣った。Ｎｏ.２２、２８はＭｏ含有量が低く、（１）式、（２）式を満たさないため、６００℃および９００℃の高温強度が低かった。Ｎｏ.２３はＣｕ含有量が高く（２）式を満たさないため、９００℃での高温強度に劣った。Ｎｏ.２４はＭｏおよびＣｕの含有量が高すぎたもの、またＮｏ.３０はＣｕおよびＮｂの含有量が高すぎたものであり、いずれも析出Ｎｂ＋析出Ｍｏ量が多過ぎる組織状態となった。これらは、低温靭性および常温での加工性に劣った。Ｎｏ.２５はＭｏ含有量が高く、Ｎｂ含有量が低く、（２）式を満たさず、しかも析出Ｎｂ＋析出Ｍｏ量が多過ぎる組織状態のものであり、６００℃および９００℃の高温強度が低く、常温での加工性にも劣った。Ｎｏ.２６はＭｏ含有量が高く、Ｃｕ含有量が低く、析出Ｎｂ＋析出Ｍｏ量が多過ぎる組織状態のものであり、９００℃の高温強度は良好であったものの６００℃の高温強度に劣り、熱疲労特性が十分とは言えない。また、低温靭性、常温での加工性も悪かった。Ｎｏ.２７はＮｂ含有量が低すぎ、また（２）式を満たさないので９００℃での高温強度を十分向上させることができなかった。Ｎｏ.２９はＣｕ含有量が少なく、（１）式を満たさないので６００℃での高温強度が不十分であり、また低温靭性にも劣った。 On the other hand, in No. 21, although the steel composition is within the range specified in the present invention, the cooling rate from 950 ° C. to 500 ° C. during the final annealing was slower than 5 ° C./sec. , A large amount of precipitates of Mo is generated, and the amount of precipitated Nb + precipitated Mo amount is too much. In this case, the high temperature strength and low temperature toughness at 900 ° C. were inferior. Nos. 22 and 28 had a low Mo content and did not satisfy the formulas (1) and (2), so the high-temperature strength at 600 ° C. and 900 ° C. was low. No. 23 was inferior in high temperature strength at 900 ° C. because it had a high Cu content and did not satisfy the formula (2). No. 24 had too high contents of Mo and Cu, and No. 30 had too high contents of Cu and Nb, both of which were in a structure state in which the amount of precipitated Nb + precipitated Mo was too much. . These were inferior in low temperature toughness and processability at room temperature. No. 25 has a high Mo content, a low Nb content, does not satisfy the formula (2), and has a structure in which the amount of precipitated Nb + precipitated Mo is too large, and the high-temperature strength at 600 ° C. and 900 ° C. is low. Also, the processability at room temperature was inferior. No. 26 has a high Mo content, a low Cu content, and a structure in which the amount of precipitated Nb + precipitated Mo is too much. Although the high temperature strength at 900 ° C. was good, it was inferior to the high temperature strength at 600 ° C., Thermal fatigue characteristics are not sufficient. Moreover, the low temperature toughness and the workability at normal temperature were also poor. No. 27 had a too low Nb content and did not satisfy the formula (2), so the high temperature strength at 900 ° C. could not be sufficiently improved. No. 29 has a low Cu content and does not satisfy the formula (1), so the high-temperature strength at 600 ° C. is insufficient and the low-temperature toughness is also inferior.
Claims (5)
１.２Ｎｂ＋５Ｍｏ＋６Ｃｕ≧１１.５ ……（１）
１５Ｎｂ＋２Ｍｏ＋０.５Ｃｕ≧９.５ ……（２） In mass%, C: 0.03% or less, Si: 1% or less, Mn: less than 0.6%, Ni: 0.6 % or less, Cr: 10-20%, Nb: 0.3-0.7 %, Cu: more than 1 to 2%, Mo: 1 to 2.5%, Al: 0.15% or less, V: 0.03 to 0.2%, N: 0.03% or less, remaining Fe and inevitable Heat resistance and low temperature toughness, which have a structure satisfying the following formulas (1) and (2) and having a structure in which the total amount of Nb and Mo present as a precipitated phase is 0.5 mass% or less An excellent ferritic stainless steel material for automobile exhaust gas path members.
1.2 Nb + 5Mo + 6Cu ≧ 11.5 (1)
15Nb + 2Mo + 0.5Cu ≧ 9.5 (2)
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