JP2006016669A - Austenitic stainless steel for inner side of dual structure exhaust manifold - Google Patents
Austenitic stainless steel for inner side of dual structure exhaust manifold Download PDFInfo
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- 229910000963 austenitic stainless steel Inorganic materials 0.000 title claims abstract description 16
- 230000009977 dual effect Effects 0.000 title abstract description 6
- 229910052804 chromium Inorganic materials 0.000 claims abstract description 7
- 229910052791 calcium Inorganic materials 0.000 claims abstract description 4
- 229910052802 copper Inorganic materials 0.000 claims abstract description 3
- 239000012535 impurity Substances 0.000 claims abstract description 3
- 229910052750 molybdenum Inorganic materials 0.000 claims abstract description 3
- 229910052759 nickel Inorganic materials 0.000 claims abstract description 3
- 229910052761 rare earth metal Inorganic materials 0.000 claims abstract description 3
- 229910052727 yttrium Inorganic materials 0.000 claims abstract description 3
- 229910052726 zirconium Inorganic materials 0.000 claims abstract description 3
- 229910052758 niobium Inorganic materials 0.000 claims description 6
- 229910052757 nitrogen Inorganic materials 0.000 claims description 4
- 229910052720 vanadium Inorganic materials 0.000 claims description 4
- 229910052799 carbon Inorganic materials 0.000 claims description 2
- 229910052748 manganese Inorganic materials 0.000 claims description 2
- 229910052698 phosphorus Inorganic materials 0.000 claims description 2
- 229910001220 stainless steel Inorganic materials 0.000 abstract description 4
- 239000010935 stainless steel Substances 0.000 abstract description 3
- 229910052721 tungsten Inorganic materials 0.000 abstract description 2
- 229910000831 Steel Inorganic materials 0.000 description 47
- 239000010959 steel Substances 0.000 description 47
- 239000000463 material Substances 0.000 description 19
- 238000012360 testing method Methods 0.000 description 19
- 230000003647 oxidation Effects 0.000 description 16
- 238000007254 oxidation reaction Methods 0.000 description 16
- 239000007789 gas Substances 0.000 description 12
- 238000010438 heat treatment Methods 0.000 description 11
- 238000003466 welding Methods 0.000 description 10
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- 238000000746 purification Methods 0.000 description 6
- 230000009467 reduction Effects 0.000 description 6
- 238000001816 cooling Methods 0.000 description 5
- 229910000734 martensite Inorganic materials 0.000 description 5
- 229910001566 austenite Inorganic materials 0.000 description 4
- 230000007423 decrease Effects 0.000 description 4
- 239000000203 mixture Substances 0.000 description 4
- 238000012545 processing Methods 0.000 description 4
- 238000005096 rolling process Methods 0.000 description 4
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- 238000009864 tensile test Methods 0.000 description 4
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- 230000007774 longterm Effects 0.000 description 2
- 150000001247 metal acetylides Chemical class 0.000 description 2
- 230000008569 process Effects 0.000 description 2
- 239000006104 solid solution Substances 0.000 description 2
- 230000000087 stabilizing effect Effects 0.000 description 2
- 238000000137 annealing Methods 0.000 description 1
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- 230000033228 biological regulation Effects 0.000 description 1
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- 238000005098 hot rolling Methods 0.000 description 1
- 239000011810 insulating material Substances 0.000 description 1
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- 238000011835 investigation Methods 0.000 description 1
- 150000004767 nitrides Chemical class 0.000 description 1
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Abstract
Description
本発明は、自動車エンジンの排気ガス経路部材である二重構造エキゾーストマニホールドの内側を構成する材料に用いるオーステナイト系ステンレス鋼に関する。 The present invention relates to an austenitic stainless steel used as a material constituting the inside of a dual structure exhaust manifold which is an exhaust gas passage member of an automobile engine.
近年、自動車エンジンおよび排気ガス処理システムには、厳しい排ガス規制をクリアする浄化性能が求められている。排ガス浄化手段としては排ガス経路に触媒コンバーターを設けるのが一般的であるが、エンジン始動直後は浄化装置の温度が低く通常運転時よりも浄化効率が低下するため、このときにできるだけ高効率で作動させることが重要となる。その対策として浄化装置をエキゾーストマニホールド直下に追加設置すること、あるいは燃焼ガス温度そのものを上昇させることなどが有効であり、種々検討されてきたが、これらにも限界がある。 In recent years, automobile engines and exhaust gas treatment systems have been required to have purification performance that satisfies strict exhaust gas regulations. As a means for purifying exhaust gas, it is common to install a catalytic converter in the exhaust gas path. However, immediately after starting the engine, the temperature of the purification device is low and the purification efficiency is lower than in normal operation. Is important. As countermeasures, it is effective to additionally install a purification device directly under the exhaust manifold or to raise the combustion gas temperature itself. Various studies have been made, but these have limitations.
その後、エキゾーストマニホールドを二重構造にする方法が提案され、既に一部で実用化されている。これによると従来の単構造パイプよりも部品単価は高くなるものの、燃焼ガスの保温効果が非常に高いので浄化効率が高まり、断熱材，加熱装置，更なる浄化装置等を付加する必要がなく、部品点数削減によるコスト低減メリットが生じる。 Thereafter, a method of making the exhaust manifold into a double structure has been proposed and has already been partially put into practical use. According to this, although the unit cost is higher than the conventional single structure pipe, the heat insulation effect of combustion gas is very high, so the purification efficiency is increased, and there is no need to add heat insulating material, heating device, further purification device, etc. Benefits of cost reduction by reducing the number of parts.
単構造のエキゾーストマニホールドでは加熱・冷却の繰り返しによる熱疲労破壊を避けるために、オーステナイト系よりも熱膨張係数の小さいフェライト系鋼種が使用される。一方、二重構造では、外側の管（外管）はやはり拘束された状態で加熱冷却の繰り返しを受けるため単管と同様にフェライト系鋼種を使用することが望ましい。しかし内側の管（内管）は、肉厚が１ｍｍ以下と薄いため外管より一層優れた加工性が要求され、また、材料が拘束されないように設計することが可能であることから、オーステナイト系鋼種を使用する方が有利な場合が多くなる。 In order to avoid thermal fatigue failure due to repeated heating and cooling, a single structure exhaust manifold uses a ferritic steel grade that has a smaller coefficient of thermal expansion than austenitic. On the other hand, in the double structure, since the outer tube (outer tube) is repeatedly restrained by heating and cooling, it is desirable to use a ferritic steel type like the single tube. However, the inner tube (inner tube) has a thickness of 1 mm or less, and therefore requires better workability than the outer tube and can be designed so that the material is not constrained. In many cases, it is more advantageous to use a steel grade.
エキゾーストマニホールドの内管は排ガスに直接曝されるため、材料温度は排ガスと同程度の８００〜１０００℃に達する。この温度域で酸化増量の少ない鋼種を使用する必要があるが、例えば代表的なオーステナイト系ステンレス鋼であるＳＵＳ３０４では基本的にこの特性が不十分である。また、一般にオーステナイト系ステンレス鋼は、フェライト系ステンレス鋼よりも酸化スケールの密着性が劣るため、繰り返し加熱冷却における耐スケール剥離性には特に注意を要する。 Since the inner pipe of the exhaust manifold is directly exposed to the exhaust gas, the material temperature reaches 800 to 1000 ° C., which is similar to that of the exhaust gas. Although it is necessary to use a steel type having a small amount of oxidation increase in this temperature range, for example, SUS304, which is a typical austenitic stainless steel, basically has insufficient properties. In general, austenitic stainless steel is inferior in the adhesion of oxide scale to ferritic stainless steel, and therefore special attention is required for resistance to scale peeling during repeated heating and cooling.
さらに、エキゾーストマニホールドの内管用材料としては、高温強度，加工性，溶接性に優れることも要求される。すなわち、高温強度については、材料が拘束されないよう設計することで加熱冷却の繰り返しによる熱疲労破壊は回避し得るものの、エンジンの振動による疲労が問題となってくる。このため高温高サイクル疲労特性に優れることが望まれる。加工性については、プレス成形，バルジ成形，フランジ成形など種々の加工が想定されるため、延性に優れることが重要となる。溶接性については、ＴＩＧ溶接，ＭＩＧ溶接等における溶接割れ感受性の低い材料が好ましい。 Further, the material for the inner pipe of the exhaust manifold is required to be excellent in high temperature strength, workability and weldability. That is, with respect to the high temperature strength, thermal fatigue failure due to repeated heating and cooling can be avoided by designing the material not to be constrained, but fatigue due to engine vibration becomes a problem. For this reason, it is desired to be excellent in high temperature and high cycle fatigue characteristics. As for workability, various processes such as press molding, bulge molding, and flange molding are assumed, and it is important to have excellent ductility. Regarding the weldability, a material having low weld cracking sensitivity in TIG welding, MIG welding, or the like is preferable.
耐熱性オーステナイト系鋼種については従来から種々の鋼種が開発されている（特許文献１〜１２）。なかでも特許文献１２には、エキゾーストマニホールドの内管に適したオーステナイト系ステンレス鋼が提案されている。
Various steel types have been developed for heat-resistant austenitic steel types (
特許文献１２のエキゾーストマニホールド内管用オーステナイト系ステンレス鋼は排ガス温度８００〜１０００℃を想定したものであり、特性としては、１０００℃で１００サイクルの断続加熱において優れた耐酸化特性を呈するものである。また、成形性や溶接性を配慮した成分設計となっている。 The austenitic stainless steel for exhaust manifold inner pipe of Patent Document 12 assumes an exhaust gas temperature of 800 to 1000 ° C., and exhibits excellent oxidation resistance in 100 cycles of intermittent heating at 1000 ° C. In addition, the component design takes into account formability and weldability.
しかしながら、昨今では自動車の長期信頼性を向上させる取り組みが各自動車メーカーで行われ、断続加熱に対する耐久性に関しては特許文献１２で行っている１００サイクル程度の試験では足りず、１０００サイクル以上、好ましくは２０００サイクルの耐久試験において優れた耐久性、特に耐スケール剥離性を示す性能が望まれるようになってきた。 However, in recent years, efforts to improve the long-term reliability of automobiles have been carried out by each automobile manufacturer, and the durability against intermittent heating is not sufficient for the test of about 100 cycles performed in Patent Document 12, and 1000 cycles or more, preferably In the endurance test of 2000 cycles, excellent durability, in particular, performance showing scale peeling resistance has been desired.
一方で、車種によって最高排ガス温度は多様化しており、それに応じて、エキゾーストマニホールドに用いる材料にも最適レベルの性能を有するものが求められる。つまり、材料側にも最高排ガス温度に応じて性能の多様化が求められており、性能不足の懸念がある材料や過剰性能を有する材料の使用は従来にも増して許容されなくなってきた。 On the other hand, the maximum exhaust gas temperature is diversified depending on the vehicle type, and accordingly, the material used for the exhaust manifold is required to have an optimum level of performance. That is, diversification of performance is demanded on the material side in accordance with the maximum exhaust gas temperature, and the use of a material having a fear of insufficient performance or a material having excessive performance has become unacceptable.
現状においては、最高排ガス温度８５０〜９００℃で使用するエキゾーストマニホールドの需要が多々あるにもかかわらず、その限られた温度域で二重構造の内管として最適な特性を有する鋼は見当たらない。例えば特許文献１２の鋼の場合、１０００℃での酸化を抑制する性能をもつが、逆に８５０〜９００℃レベルでの長期耐久性、例えば２０００サイクルの繰り返し加熱試験に耐え得る「耐スケール剥離性」について見れば、必ずしも十分満足できるレベルにあるとは限らない。また、使用温度が９００℃以下の場合、１０００℃程度まで昇温される場合に比べσ脆化が生じやすい。さらに、特許文献１２の鋼は８５０〜９００℃での使用を前提とした耐σ脆化について十分な配慮がなされていない。 At present, even though there is a great demand for an exhaust manifold used at a maximum exhaust gas temperature of 850 to 900 ° C., there is no steel having optimum characteristics as a double structure inner pipe in the limited temperature range. For example, in the case of the steel of Patent Document 12, it has the ability to suppress oxidation at 1000 ° C., but conversely, it can withstand long-term durability at a level of 850 to 900 ° C., for example, a repeated heating test of 2000 cycles. ", It is not always at a sufficiently satisfactory level. Further, when the operating temperature is 900 ° C. or lower, σ embrittlement is more likely to occur than when the temperature is raised to about 1000 ° C. Furthermore, the steel of Patent Document 12 does not give sufficient consideration to σ embrittlement resistance premised on use at 850 to 900 ° C.
また、特許文献１２の鋼は優れた穴拡げ性を呈するように成分設計されているが、複雑形状への曲げ加工性や穴拡げ性を考慮したとき、必ずしも十分な延性を有しているとは言えない。 In addition, the steel of Patent Document 12 is designed so as to exhibit excellent hole expansibility, but when considering bending workability and hole expansibility to complex shapes, the steel does not necessarily have sufficient ductility. I can't say that.
本発明は、このような現状に鑑み、材料温度が８５０〜９００℃となるような環境で使用される二重構造エキゾーストマニホールドの内側材（内管）に求められる高温強度，耐σ脆化性，長期繰り返しにおける耐スケール剥離性を具備し、かつ延性および溶接性にも優れた鋼を開発し提供しようというものである。 In view of such a current situation, the present invention provides high temperature strength and σ embrittlement resistance required for the inner material (inner pipe) of a dual structure exhaust manifold used in an environment where the material temperature is 850 to 900 ° C. , It is intended to develop and provide a steel that is resistant to scale peeling over a long period of time and has excellent ductility and weldability.
本発明で提供する鋼は、質量％で、Ｃ：０.０８％以下，Ｓi：１.５〜３％，Ｍn：２％以下，Ｐ：０.０４％以下，Ｓ：０.０１％以下，Ｎi：８〜１０％，Ｃr：１７〜１９％，Ｎ：０.２％以下，Ｎb：０.３％以下，Ａl：０.０８％未満であり、必要に応じてＴi：０.０５〜０.５％，Ｍo：０.０５〜０.５％，Ｃu：０.０３〜０.５％，Ｖ：０.０５〜０.５，Ｗ：０.０５〜０.５％，Ｚr：０.０５〜０.５％の１種または２種以上を含有し、また必要に応じてＲＥＭ，Ｙ，Ｃaの合計含有量が０.００５〜０.１％であり、残部Ｆeおよび不可避的不純物であり、
ただし、ＮおよびＮbについては、Ｎ：０.０８超え〜０.２％，Ｎb：０.０５〜０.３％のうちいずれか一方または両方を満たし、
かつ下記(1)〜(3)式を満たす二重構造エキゾーストマニホールドの内側用オーステナイト系ステンレス鋼である。
１８≦Ｃr＋０.５Ｓi＜２０ ……(1)
−８０≦５５１−４６２(Ｃ＋Ｎ)−９.２Ｓi−８.１Ｍn−２９(Ｎi＋Ｃu)−１３.７Ｃr−１８.５Ｍo＜−４０ ……(2)
７≦(Ｃr＋１.５Ｓi＋０.５Ｎb＋Ｍo)−(Ｎi＋０.５Ｍn＋０.３Ｃu＋３０Ｃ＋３０Ｎ)≦１０ ……(3)
The steel provided in the present invention is, by mass%, C: 0.08% or less, Si: 1.5-3%, Mn: 2% or less, P: 0.04% or less, S: 0.01% or less , Ni: 8 to 10%, Cr: 17 to 19%, N: 0.2% or less, Nb: 0.3% or less, Al: less than 0.08%, Ti: 0.05 if necessary ~ 0.5%, Mo: 0.05-0.5%, Cu: 0.03-0.5%, V: 0.05-0.5, W: 0.05-0.5%, Zr : 0.05 to 0.5% of one or more, and if necessary, the total content of REM, Y, Ca is 0.005 to 0.1%, the balance Fe and inevitable Impurities,
However, N and Nb satisfy one or both of N: more than 0.08 to 0.2%, Nb: 0.05 to 0.3%,
And it is an austenitic stainless steel for the inner side of the double structure exhaust manifold that satisfies the following formulas (1) to (3).
18 ≦ Cr + 0.5Si <20 (1)
−80 ≦ 551-462 (C + N) −9.2Si−8.1Mn−29 (Ni + Cu) −13.7Cr−18.5Mo <−40 (2)
7 ≦ (Cr + 1.5Si + 0.5Nb + Mo) − (Ni + 0.5Mn + 0.3Cu + 30C + 30N) ≦ 10 (3)
(1)〜(3)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が代入される。 The content of the element expressed in mass% is substituted for the element symbol in the formulas (1) to (3).
本発明によれば、８５０〜９００℃の温度域で長期間繰り返し使用したときに優れた耐久性、特に優れた耐スケール剥離性を呈し、かつ耐σ脆化についても十分配慮した鋼が実現された。この鋼は延性，高温強度にも優れ、エキゾーストマニホールドの特に二重構造の内管に好適な特性を有する。したがって本発明は、最高排ガス温度が８５０〜９００℃の自動車において、コストメリットの高い鋼材を提供することで、エキゾーストマニホールドの信頼性向上およびコスト低減に寄与するものである。 According to the present invention, a steel exhibiting excellent durability when used repeatedly for a long time in a temperature range of 850 to 900 ° C., particularly excellent scale peeling resistance, and sufficiently considering σ embrittlement resistance is realized. It was. This steel is excellent in ductility and high-temperature strength, and has characteristics suitable for an exhaust manifold, particularly for a double structure inner pipe. Therefore, the present invention contributes to the improvement of the reliability of the exhaust manifold and the cost reduction by providing a steel material having a high cost merit in an automobile having a maximum exhaust gas temperature of 850 to 900 ° C.
本発明の鋼は、８５０〜９００℃の温度に曝される二重構造エキゾーストマニホールドの内管に求められる高温強度，耐σ脆化性，長期繰り返しにおける耐スケール剥離性、および良好な延性を実現すべく、以下のような成分設計を行ったものである。 The steel of the present invention achieves the high temperature strength, σ embrittlement resistance, scale peeling resistance, and good ductility required for the inner pipe of a dual structure exhaust manifold exposed to temperatures of 850 to 900 ° C. Therefore, the following component design was performed.
Ｃは、オーステナイト系ステンレス鋼の高温強度向上に有効である。しかし、過剰に含有させるとエキゾーストマニホールドとして使用中にＣr炭化物を形成して靱性が劣化するとともに、耐高温酸化性の向上に有効な固溶Ｃr量が減少する。このためＣ含有量は０.０８質量％以下に制限される。好ましいＣ含有量の範囲は０.０２〜０.０８質量％である。 C is effective for improving the high temperature strength of austenitic stainless steel. However, if excessively contained, Cr carbides are formed during use as an exhaust manifold and the toughness deteriorates, and the amount of solid solution Cr effective in improving high-temperature oxidation resistance decreases. For this reason, C content is restrict | limited to 0.08 mass% or less. The range of preferable C content is 0.02-0.08 mass%.
Ｓiは、高温酸化特性の改善に非常に有効である。およそ１.５質量％以上の含有により、８５０〜９００℃の温度域でＳi濃化皮膜をＣr酸化物の内層に形成させ、耐スケール剥離性の向上が可能になる。しかし、Ｓiの多量添加はσ脆化感受性を高め、使用中にσ脆化を誘発する。このためＳi含有量の上限は３質量％に制限される。より好ましいＳi含有量範囲は２超え〜３質量％である。 Si is very effective in improving high temperature oxidation characteristics. When the content is about 1.5% by mass or more, an Si concentrated film is formed on the inner layer of the Cr oxide in the temperature range of 850 to 900 ° C., and the scale peeling resistance can be improved. However, a large amount of Si increases the σ embrittlement sensitivity and induces σ embrittlement during use. For this reason, the upper limit of Si content is restrict | limited to 3 mass%. A more preferable Si content range is more than 2 to 3% by mass.
Ｍnは、オーステナイト安定化元素であり、本発明では主として相バランス調整のために添加される。しかし、過剰なＭn添加は耐高温酸化性の低下を招くので２質量％以下に制限される。 Mn is an austenite stabilizing element and is added mainly for adjusting the phase balance in the present invention. However, excessive Mn addition causes a decrease in high-temperature oxidation resistance, so it is limited to 2% by mass or less.
Ｐは、オーステナイト系ステンレス鋼の熱間加工性を損なう元素であり、可能な限り低減することが望ましい。このためＰ含有量は０.０４質量％以下に制限される。 P is an element that impairs the hot workability of austenitic stainless steel, and is desirably reduced as much as possible. For this reason, P content is restrict | limited to 0.04 mass% or less.
Ｓは、Ｐと同様にオーステナイト系ステンレス鋼の熱間加工性を損なう元素である。鋼の製造歩留りを低下させないため、可能な限り低いことが望ましい。このためＳ含有量は０.０１質量％以下に制限される。 S, like P, is an element that impairs the hot workability of austenitic stainless steel. In order not to reduce the production yield of steel, it is desirable that it be as low as possible. For this reason, S content is restrict | limited to 0.01 mass% or less.
Ｎiは、オーステナイト安定化元素であり、オーステナイトバランス調整のため８〜１０質量％含有させる。 Ni is an austenite stabilizing element and is contained in an amount of 8 to 10% by mass for adjusting the austenite balance.
Ｃrは、高温でのスケール生成を抑制する基本元素であり、本発明では１７質量％以上の含有量が必要である。ただし過剰のＣr含有はσ脆化を招くので１９質量％以下に制限される。 Cr is a basic element that suppresses scale formation at high temperatures, and in the present invention, a content of 17% by mass or more is necessary. However, since excessive Cr content causes σ embrittlement, it is limited to 19% by mass or less.
Ｎは、固溶強化により高温強度の向上に寄与する。９００℃まで昇温されるエキゾーストマニホールド内管としては０.０８質量％を超えるＮ含有量を確保することが望ましい。ただ、後述のＮb添加によっても高温強度の改善が可能であるため、Ｎbを所定量含有させる場合は必ずしもＮを含有させなくてもよい。Ｎの過剰添加はＣr窒化物の形成により鋼の靱性を低下させるため、Ｎ含有量の上限は０.２質量％に制限される。 N contributes to the improvement of high temperature strength by solid solution strengthening. As an exhaust manifold inner pipe heated to 900 ° C., it is desirable to ensure an N content exceeding 0.08 mass%. However, since the high-temperature strength can be improved also by adding Nb, which will be described later, when Nb is contained in a predetermined amount, N may not necessarily be contained. Since excessive addition of N decreases the toughness of the steel due to the formation of Cr nitride, the upper limit of the N content is limited to 0.2% by mass.
Ｎbは、Ｃr23Ｃ6型炭化物を微細分散析出させる作用があり、これによって高温強度の向上に寄与する。この効果を十分に発揮させるには０.０５質量％以上のＮb含有が望まれる。しかし、Ｎbを過剰に添加すると鋼製造のいずれかの工程または材料昇温時にＮb炭窒化物を生成してしまうため、Ｃr炭化物の微細析出による高温強度向上作用が希釈され、また靱性低下を招くようになる。このため、Ｎb含有量は０.３質量％以下に制限される。 Nb has the effect of finely dispersing and precipitating Cr 23 C 6 type carbide, thereby contributing to the improvement of high temperature strength. In order to fully exhibit this effect, 0.05 mass% or more of Nb content is desired. However, if Nb is added excessively, Nb carbonitrides are produced at any stage of steel production or when the temperature of the material is raised, so the effect of improving the high-temperature strength due to fine precipitation of Cr carbides is diluted and the toughness is reduced. It becomes like this. For this reason, Nb content is restrict | limited to 0.3 mass% or less.
ただし、前述のようにＮ添加によって高温強度の向上が可能であるため、Ｎを０.０８質量％を超えて含有させる場合は必ずしもＮbを含有させる必要はない。具体的には、Ｎ：０.０８超え〜０.２質量％を含有させる場合は、Ｎｂは無添加または０.３質量％以下の範囲で含有させればよい。Ｎ含有量が０.０８質量％以下の場合は、Ｎbを０.０５〜０.３質量％の範囲で含有させる必要がある。 However, since the high temperature strength can be improved by adding N as described above, Nb is not necessarily contained when N is contained in an amount exceeding 0.08% by mass. Specifically, when N: more than 0.08 to 0.2% by mass is contained, Nb may be added without addition or within a range of 0.3% by mass or less. When N content is 0.08 mass% or less, it is necessary to contain Nb in the range of 0.05-0.3 mass%.
Ａlは、耐高温酸化性の向上に有効である。ただし多量に含有させると鋼が硬質化し、原料コストも高くなる。このため、Ａlは０.０８質量％未満の範囲で含有させる。０.０１〜０.０７質量％のＡl含有量とすることが好ましく、０.０３〜０.０７質量％とすることが一層好ましい。 Al is effective in improving high-temperature oxidation resistance. However, if contained in a large amount, the steel becomes hard and the raw material cost increases. For this reason, Al is contained in a range of less than 0.08% by mass. The Al content is preferably 0.01 to 0.07% by mass, and more preferably 0.03 to 0.07% by mass.
以上の元素に加え、本発明では以下の元素を選択的に含有させることができる。
Ｔi，ＶおよびＷは、高温強度の向上に有効である。しかし、多量に添加すると鋼が硬質になり、また原料コストも高くなる。このため添加量の上限はいずれも０．５質量％に制限される。好ましい含有量範囲は、Ｔi：０.０５〜０.５質量％，Ｖ：０.０５〜０.５質量％，Ｗ：０.０５〜０．５質量％である。これらは単独で添加しても複合で添加しても構わない。
In addition to the above elements, the following elements can be selectively contained in the present invention.
Ti, V and W are effective in improving the high temperature strength. However, if added in a large amount, the steel becomes hard and the raw material costs increase. For this reason, the upper limit of the addition amount is limited to 0.5% by mass. Preferable content ranges are Ti: 0.05-0.5 mass%, V: 0.05-0.5 mass%, W: 0.05-0.5 mass%. These may be added alone or in combination.
Ｍoは、フェライト生成元素であり、高温強度の改善に有効である。しかし、過剰のＭo添加はσ脆化を招き、鋼の靱性を損なう。このため、Ｍoを添加する場合は０.５質量％以下の範囲で行う必要がある。好ましいＭo含有量範囲は０.０５〜０.５質量％である。 Mo is a ferrite-forming element and is effective in improving high temperature strength. However, excessive Mo addition causes σ embrittlement and impairs the toughness of the steel. For this reason, when adding Mo, it is necessary to carry out in the range of 0.5 mass% or less. A preferable Mo content range is 0.05 to 0.5 mass%.
Ｃuは、オーステナイト生成元素であり、これも高温強度の向上に有効である。このため、オーステナイトバランスの調整を兼ねて積極添加することができる。しかし、Ｃuの多量添加は耐高温酸化性の低下を招く。したがって、Ｃuを添加する場合は０.５質量％以下の範囲で行う必要がある。好ましいＣu含有量範囲は０.０.０３〜０.５質量％、さらに好ましいＣu含有量範囲は０.０５〜０.５質量％ である。 Cu is an austenite-forming element, which is also effective for improving high temperature strength. For this reason, it can add positively also adjusting the austenite balance. However, the addition of a large amount of Cu causes a decrease in high temperature oxidation resistance. Therefore, when adding Cu, it is necessary to carry out in the range of 0.5 mass% or less. A preferable Cu content range is 0.03 to 0.5 mass%, and a more preferable Cu content range is 0.05 to 0.5 mass%.
Ｚrは、高温強度の向上に有効であるとともに、微量の添加で耐高温酸化性も改善される。しかし、多量のＺr添加はσ脆化を招き、鋼の靱性を損なう。このためＺrを添加する場合は０.５質量％以下の範囲で行う必要があり、０.０５〜０.５質量％の範囲で含有させることが好ましい。 Zr is effective for improving the high-temperature strength, and the high-temperature oxidation resistance is also improved by adding a small amount. However, a large amount of Zr addition causes σ embrittlement and impairs the toughness of the steel. For this reason, when adding Zr, it is necessary to carry out in the range of 0.5 mass% or less, and it is preferable to make it contain in the range of 0.05-0.5 mass%.
ＲＥＭ，ＹおよびＣaは、耐高温酸化性の向上に有効であり、その効果を十分に発揮させるためにはこれらの元素の１種または２種以上を添加することによりその合計含有量を０.００５質量％以上とすることが望ましい。ただし多量に含有させると鋼が硬質化し、原料コストも高くなる。このため、これらの元素の合計含有量は０.１質量％以下に制限される。 REM, Y, and Ca are effective in improving high-temperature oxidation resistance, and in order to fully exhibit their effects, the total content of these elements is reduced to 0.1 by adding one or more of these elements. It is desirable to set it to 005 mass% or more. However, if contained in a large amount, the steel becomes hard and the raw material cost increases. For this reason, the total content of these elements is limited to 0.1% by mass or less.
本発明では、８５０〜９００℃レベルでの繰り返しの使用に長期間耐え得る優れた耐スケール剥離性を付与することを重要な課題としている。具体的には、後述の実施例で説明する９００℃，２０００サイクルの高温酸化試験において、板厚０.８ｍｍの材料で減肉率２０％未満となるような優れた特性を具備させる。その手法として前述のようにＣrおよびＳiを含有させる。その一方で、Ｃr，Ｓi添加のいわば副作用であるσ脆化の問題を解決しなければならない。特に８５０〜９００℃で使用する場合は、９００〜１０００℃程度の高温で使用する場合と比べ、σ脆化が起こりやすい。このため本発明では、耐スケール剥離性と耐σ脆化の兼ね合いでＣrおよびＳi含有量を厳しくコントロールする必要がある。発明者らはこの点を考慮して多くの実験を行ってきた。 In the present invention, it is an important subject to provide excellent scale peel resistance that can withstand repeated use at a level of 850 to 900 ° C. for a long period of time. Specifically, in a high-temperature oxidation test at 900 ° C. and 2000 cycles, which will be described later in Examples, a material having a plate thickness of 0.8 mm is provided with excellent characteristics such that the thickness reduction rate is less than 20%. As the method, Cr and Si are contained as described above. On the other hand, the problem of σ embrittlement, which is a side effect of adding Cr and Si, must be solved. In particular, when used at 850 to 900 ° C., σ embrittlement is more likely to occur than when used at a high temperature of about 900 to 1000 ° C. For this reason, in the present invention, it is necessary to strictly control the Cr and Si contents in view of balance between scale peel resistance and σ embrittlement resistance. The inventors have conducted many experiments in consideration of this point.
その実験結果を図１に例示する。図１は板厚０.８ｍｍまたは２.０ｍｍのＣr−６〜１１％Ｎiオーステナイト系ステンレス鋼板について、耐スケール剥離性と耐σ脆化に及ぼすＳi含有量，Ｃr含有量の影響を示してある。横軸がＳi含有量、縦軸がＣr含有量である。耐スケール剥離性は、後述実施例に示す９００℃，２０００サイクルの試験（板厚０.８ｍｍ材使用）において減肉率が２０％未満のものを良好（○または□）、それ以上のものを不良（●または■）とした。耐σ脆化は、板厚２.０ｍｍ材を用いて９００℃，３００時間の加熱を行ったのち、ＪＩＳ Ｚ ２２４２のＶノッチシャルピー衝撃試験を室温にて行い、シャルピー衝撃値が１００Ｊ／ｃｍ2以上のものを良好（○または●）、１００Ｊ／ｃｍ2未満のものを不良（□または■）とした。 The experimental results are illustrated in FIG. FIG. 1 shows the effects of Si content and Cr content on scale peel resistance and σ embrittlement resistance of Cr-6 to 11% Ni austenitic stainless steel sheet having a thickness of 0.8 mm or 2.0 mm. . The horizontal axis represents the Si content, and the vertical axis represents the Cr content. The scale peel resistance is good (◯ or □) when the thinning rate is less than 20% in a test at 900 ° C. and 2000 cycles (using a plate thickness of 0.8 mm) shown in the examples below, and more than that. Defective (● or ■). For σ embrittlement resistance, a 2.0 mm thick material was heated at 900 ° C. for 300 hours, and then a JIS Z 2242 V-notch Charpy impact test was performed at room temperature. The Charpy impact value was 100 J / cm 2. The above were evaluated as good (◯ or ●), and less than 100 J / cm 2 were evaluated as poor (□ or ■).
図１からわかるように、上記の優れた耐スケール剥離性を付与するためにはＳi≧１.５，Ｃr≧１７，かつＣr＋０.５Ｓi≧１８を満たす必要がある。一方、良好な耐σ脆性を確保するにはＣr≦１９，Ｓi≦３，かつＣr＋１.５Ｓi＜２０を満たす必要がある。したがって本発明では、ＣrおよびＳiの含有量に関し、それぞれ前記の含有量範囲であって、かつ下記(1)式を満足することを要件とする。
１８≦Ｃr＋０.５Ｓi＜２０ ……(1)
As can be seen from FIG. 1, it is necessary to satisfy Si ≧ 1.5, Cr ≧ 17, and Cr + 0.5Si ≧ 18 in order to impart the above-described excellent scale peel resistance. On the other hand, in order to ensure good σ brittleness resistance, it is necessary to satisfy Cr ≦ 19, Si ≦ 3, and Cr + 1.5Si <20. Therefore, in the present invention, regarding the Cr and Si contents, it is a requirement that they are within the above-mentioned content ranges and satisfy the following formula (1).
18 ≦ Cr + 0.5Si <20 (1)
本発明では、複雑形状のエキゾーストマニホールドへの加工を行いやすくするため、良好な延性を付与する。種々検討の結果、昨今の自動車で要求される複雑形状のエキゾーストマニホールド内管への加工性を満足させるためには、造管前の素材鋼板において、圧延方向に直角方向の常温での伸びが５０％以上という良好な延性を付与する必要があると判断された。この延性をこの成分系において実現するには、加工時に加工誘起マルテンサイト相が誘起されない組成域にコントロールする必要があることがわかった。二重構造エキゾーストマニホールドの内管の加工度を考慮すると、下記Ｍ値が−４０より小さい値になるように組成コントロールする必要がある。
Ｍ＝５５１−４６２(Ｃ＋Ｎ)−９.２Ｓi−８.１Ｍn−２９(Ｎi＋Ｃu)−１３.７Ｃr−１８.５Ｍo
In the present invention, favorable ductility is imparted in order to facilitate processing into an exhaust manifold having a complicated shape. As a result of various investigations, in order to satisfy the workability of the exhaust manifold inner pipe having a complicated shape required in recent automobiles, the material steel plate before pipe forming has an elongation at room temperature in the direction perpendicular to the rolling direction of 50. It was judged that it was necessary to impart good ductility of at least%. In order to realize this ductility in this component system, it has been found that it is necessary to control the composition range in which no work-induced martensite phase is induced during processing. Considering the degree of processing of the inner pipe of the dual structure exhaust manifold, it is necessary to control the composition so that the following M value is smaller than −40.
M = 551-462 (C + N) -9.2 Si-8.1 Mn-29 (Ni + Cu) -13.7 Cr-18.5 Mo
Ｍ値が−４０以上になると加工誘起マルテンサイトの多量生成に起因して厳しい曲げ加工時に割れが生じる恐れがある。Ｍ値が小さくなるほど加工誘起マルテンサイト相が生成し難くなり、若干の加工誘起マルテンサイトの生成により延性は向上するが、あまりＭ値が小さくなると加工誘起マルテンサイトが常温加工で全く生成しなくなり、かえって延性が低下するので、Ｍ値の下限は−８０とする。したがって本発明では前記(2)式を満たすことが要件となるのである。 When the M value is -40 or more, there is a risk that cracking may occur during severe bending due to a large amount of work-induced martensite generation. The smaller the M value, the harder the work-induced martensite phase is produced, and the ductility is improved by the formation of some work-induced martensite. However, if the M value is too small, the work-induced martensite is not produced at all at room temperature processing. On the contrary, the ductility is lowered, so the lower limit of the M value is -80. Therefore, in the present invention, it is a requirement to satisfy the expression (2).
さらに本発明では、溶接性にも十分配慮する。エキゾーストマニホールドを製造する際には、造管時に溶接を行うか、プレス成形後に溶接を行う。発明者らはオーステナイト系ステンレス鋼の溶接高温割れ感受性を抑制する成分組成について多くの実験結果を有しており、本発明では下記Ｄ値が７以上に調整された鋼を用いる必要があると判断された。
Ｄ＝(Ｃr＋１.５Ｓi＋０.５Ｎb＋Ｍo)−(Ｎi＋０.５Ｍn＋０.３Ｃu＋３０Ｃ＋３０Ｎ)
なお、Ｄ値が１０を超えると溶接高温割れ感受性が高くなるとともに、加工性の低下が問題となる場合がある。したがって本発明では前記(3)式を満たすことが要件となるのである。
Furthermore, in the present invention, sufficient consideration is given to weldability. When manufacturing an exhaust manifold, welding is performed at the time of pipe making or welding is performed after press forming. The inventors have many experimental results on the component composition that suppresses the weld hot cracking susceptibility of austenitic stainless steel, and in the present invention, it is judged that it is necessary to use steel having the following D value adjusted to 7 or more. It was done.
D = (Cr + 1.5Si + 0.5Nb + Mo)-(Ni + 0.5Mn + 0.3Cu + 30C + 30N)
In addition, when D value exceeds 10, while a welding hot crack sensitivity becomes high, the fall of workability may become a problem. Therefore, in the present invention, it is a requirement to satisfy the expression (3).
以上のように成分調整した鋼は、通常のステンレス鋼板製造設備を用いて例えば板厚０.８ｍｍ程度の鋼板とし、溶接造管によりエキゾーストマニホールド用の管に成形される。 The steel whose components have been adjusted as described above is formed into a steel plate having a thickness of, for example, about 0.8 mm using a normal stainless steel plate manufacturing facility, and is formed into a pipe for an exhaust manifold by welding pipe making.
表１に示す鋼を溶製し、通常のステンレス鋼板製造条件にしたがって、熱間圧延→焼鈍酸洗→冷間圧延→焼鈍酸洗の工程により板厚２.０ｍｍの鋼板を作り、さらに冷間圧延と焼鈍酸洗を行って板厚０.８ｍｍの鋼板を得た。 The steel shown in Table 1 is melted, and a steel plate having a thickness of 2.0 mm is made by a process of hot rolling → anneal pickling → cold rolling → anneal pickling according to the normal stainless steel plate manufacturing conditions, Rolling and annealing pickling were performed to obtain a steel plate having a thickness of 0.8 mm.
板厚０.８ｍｍの各鋼板から圧延方向に直角方向の引張試験片（ＪＩＳ １３Ｂ号）を切り出し、ＪＩＳ Ｚ ２２４１に準拠して常温での引張試験を行い、延性を評価するために伸びを測定した。また、板厚２.０ｍｍの各鋼板から圧延方向に平行方向の高温引張試験片を切り出し、ＪＩＳ Ｇ ０５６７に準拠して高温引張試験を９００℃で行い、高温強度の指標として９００℃における０.２％耐力を求めた。 A tensile test piece (JIS 13B) perpendicular to the rolling direction is cut out from each steel sheet having a thickness of 0.8 mm, a tensile test is performed at room temperature in accordance with JIS Z 2241, and the elongation is measured to evaluate ductility. did. Further, a high-temperature tensile test piece parallel to the rolling direction was cut out from each steel plate having a thickness of 2.0 mm, and a high-temperature tensile test was conducted at 900 ° C. according to JIS G 0567. 2% yield strength was determined.
また、板厚０.８ｍｍの各鋼板から２５×３５ｍｍの高温酸化試験片を切り出し、ＪＩＳ Ｚ ２２８２に準拠して「大気中９００℃×５分→５分間の空冷」を１サイクルとする２０００サイクル繰り返しの高温酸化試験に供した。高温酸化試験前後の重量変化、および試験後最も板厚が減少した箇所の減肉率を求めた。減肉率は次式により算出される。
減肉率＝（試験前板厚−試験後板厚）／試験前板厚×１００
また、板厚２.０ｍｍの各鋼板を用いて９００℃，３００時間の加熱を行った後、ＪＩＳ Ｚ ２２４２のＶノッチシャルピー衝撃試験を室温にて実施し、シャルピー衝撃値によって耐σ脆化を評価した。シャルピー衝撃値が１００Ｊ／ｃｍ2以上のものを耐σ脆性が良好であると判定した。
In addition, a 25 × 35 mm high-temperature oxidation test piece is cut out from each steel plate having a thickness of 0.8 mm, and 2000 cycles in which “air cooling in the atmosphere 900 ° C. × 5 minutes → 5 minutes” is one cycle in accordance with JIS Z 2282. Subjected to repeated high temperature oxidation tests. The weight change before and after the high temperature oxidation test and the thickness reduction rate at the point where the plate thickness decreased most after the test were obtained. The thickness reduction rate is calculated by the following equation.
Thinning rate = (plate thickness before test−plate thickness after test) / plate thickness before test × 100
In addition, after heating at 900 ° C. for 300 hours using each steel plate having a thickness of 2.0 mm, a V-notch Charpy impact test of JIS Z 2242 was performed at room temperature, and σ embrittlement resistance was reduced by the Charpy impact value. evaluated. Those having a Charpy impact value of 100 J / cm 2 or more were judged to have good σ brittleness resistance.
さらに、各鋼板について溶接高温割れ感受性の指標となる「臨界ひずみ(％)」を以下のようにして求めた。すなわち、板厚０.８ｍｍ，幅４０ｍｍ，長さ２００ｍｍの試験片を用意し、この試験片の長手方向に最大３００Ｎ／ｍｍ2までの範囲で一定の引張荷重を加え、この状態で試験片長手方向にＴＩＧのなめづけ溶接をビード長さが１００ｍｍとなるように施した後、予め設定した標点間５０ｍｍ内に生じた割れ個数を測定する。また、溶接後の試験片の標点間距離を測定し、「溶接時に付与されるひずみ」を求める。このような試験を引張荷重を段階的に変えて行い、割れが発生し始めるときのひずみ値をその材料の臨界ひずみとする。臨界ひずみが８％以上のものを、耐溶接高温割れ性が良好であると判定した。
これらの結果を表２に示す。
Furthermore, “critical strain (%)”, which is an index of weld hot cracking susceptibility, was determined for each steel sheet as follows. That is, a test piece having a thickness of 0.8 mm, a width of 40 mm, and a length of 200 mm is prepared, and a constant tensile load is applied in the longitudinal direction of the test piece up to 300 N / mm 2 , and in this state, the length of the test piece is long. After performing TIG tanning welding in the direction so that the bead length is 100 mm, the number of cracks generated within 50 mm between the preset gage points is measured. In addition, the distance between the test marks of the test piece after welding is measured to obtain “strain applied during welding”. Such a test is performed by changing the tensile load step by step, and the strain value when cracks start to occur is defined as the critical strain of the material. Those having a critical strain of 8% or more were judged to have good weld hot cracking resistance.
These results are shown in Table 2.
表２からわかるように、本発明で規定の化学組成を満たす材料は、２０００サイクルの高温酸化試験後の減肉率が２０％未満であり、９００℃での繰り返し加熱において優れた耐スケール剥離性を呈した。つまり、本発明鋼は８５０〜９００℃域での繰り返し加熱に曝した場合に優れた耐久性を安定して呈することが確認された。加えて、酸化試験前後の重量変化も少なく、耐σ脆化にも優れていた。延性（常温での伸び），高温強度（９００℃での０.２％耐力），耐溶接高温割れ性（臨界ひずみ）も十分であった。 As can be seen from Table 2, the material satisfying the specified chemical composition in the present invention has a thickness reduction rate of less than 20% after a high-temperature oxidation test of 2000 cycles, and has excellent scale peeling resistance in repeated heating at 900 ° C. Was presented. That is, it was confirmed that the steel of the present invention stably exhibits excellent durability when exposed to repeated heating in the range of 850 to 900 ° C. In addition, the weight change before and after the oxidation test was small, and the σ embrittlement resistance was excellent. Ductility (elongation at normal temperature), high temperature strength (0.2% proof stress at 900 ° C.), and weld hot cracking resistance (critical strain) were sufficient.
これに対し、鋼Ｎｏ.１６はＣr＋０.５Ｓiが低すぎ前記(1)式を満たさないため、また鋼Ｎｏ.１７はＳi含有量が低すぎるため、また鋼Ｎｏ.１８はＣr含有量が低すぎるため、これらは耐スケール剥離性に劣った。鋼Ｎｏ.１９はＣr＋０.５Ｓiが高すぎ前記(1)式を満たさないため、また鋼Ｎｏ.２０はＳi含有量が高すぎるため、また鋼Ｎｏ.２１はＣr含有量が高すぎるためいずれも耐σ脆化に劣った。鋼Ｎｏ.２２はＮ含有量およびＮb含有量がともに低すぎるため、高温強度が低かった。鋼Ｎｏ.２３はＭ値が高く、鋼Ｎｏ.２４はＭ値が低く、いずれも(2)式を満たさないため、延性（常温での伸び）が低かった。鋼Ｎｏ.２５はＤ値が高く(3)式を満たさないため、耐溶接高温割れ性（臨界ひずみ）に劣った。
なお、鋼Ｎｏ.２６は特許文献１２の発明鋼に相当する比較鋼であるが、Ｍ値が高く(2)式を満たさないため、常温での伸びに劣った。
In contrast, Steel No. 16 has too low Cr + 0.5Si and does not satisfy the above formula (1), Steel No. 17 has too low Si content, and Steel No. 18 has low Cr content. Therefore, they were inferior in scale peeling resistance. Steel No. 19 has too high Cr + 0.5Si and does not satisfy the above formula (1), Steel No. 20 has too high Si content, and Steel No. 21 has too high Cr content. Inferior to σ embrittlement resistance. Steel No. 22 had a low high-temperature strength because both the N content and the Nb content were too low. Steel No. 23 had a high M value, Steel No. 24 had a low M value, and none of the formula (2) was satisfied, so the ductility (elongation at room temperature) was low. Steel No. 25 had a high D value and did not satisfy the formula (3), and therefore was inferior in welding hot crack resistance (critical strain).
Steel No. 26 is a comparative steel corresponding to the invented steel of Patent Document 12, but has a high M value and does not satisfy the formula (2), so it is inferior in elongation at room temperature.
Claims (3)
ただし、ＮおよびＮbについては、Ｎ：０.０８超え〜０.２％，Ｎb：０.０５〜０.３％のうちいずれか一方または両方を満たし、
かつ下記(1)〜(3)式を満たす二重構造エキゾーストマニホールドの内側用オーステナイト系ステンレス鋼。
１８≦Ｃr＋０.５Ｓi＜２０ ……(1)
−８０≦５５１−４６２(Ｃ＋Ｎ)−９.２Ｓi−８.１Ｍn−２９(Ｎi＋Ｃu)−１３.７Ｃr−１８.５Ｍo＜−４０ ……(2)
７≦(Ｃr＋１.５Ｓi＋０.５Ｎb＋Ｍo)−(Ｎi＋０.５Ｍn＋０.３Ｃu＋３０Ｃ＋３０Ｎ)≦１０ ……(3) In mass%, C: 0.08% or less, Si: 1.5-3%, Mn: 2% or less, P: 0.04% or less, S: 0.01% or less, Ni: 8-10%, Cr: 17 to 19%, N: 0.2% or less, Nb: 0.3% or less, Al: less than 0.08%, the balance Fe and inevitable impurities,
However, N and Nb satisfy one or both of N: more than 0.08 to 0.2%, Nb: 0.05 to 0.3%,
And austenitic stainless steel for the inner side of double structure exhaust manifold that satisfies the following formulas (1) to (3).
18 ≦ Cr + 0.5Si <20 (1)
−80 ≦ 551-462 (C + N) −9.2Si−8.1Mn−29 (Ni + Cu) −13.7Cr−18.5Mo <−40 (2)
7 ≦ (Cr + 1.5Si + 0.5Nb + Mo) − (Ni + 0.5Mn + 0.3Cu + 30C + 30N) ≦ 10 (3)
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| JP2009012070A (en) * | 2007-06-07 | 2009-01-22 | Jfe Steel Kk | Weld metal of stainless steel weld joint, and its forming method |
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