【０００１】
【発明の属する技術分野】
本発明は主に自動車の排気エンジンバルブに用いられるエンジンバルブ用耐熱合金に関する。
【０００２】
【従来の技術】
従来、排気エンジン用バルブにはＳＵＨ３５等のＦｅ基合金（耐熱鋼）が広く用いられてきたが、一部使用温度の高温化に伴ってＮｉ基合金であるＮＣＦ７５１（Ｎｉ−１５．５Ｃｒ−１Ｎｂ−２．３Ｔｉ−１．２Ａｌ−７Ｆｅ［質量％］）が用いられるようになった。しかし、ＮＣＦ７５１はＮｉを７０質量％も含むため高価であり、ＮＣＦ７５１に近い高温強度と耐熱性を有する省資源型の合金開発が行われてきた。例えば、Ｎｉ含有量を３０〜３５質量％に低減させたＦｅ基耐熱合金が特開平９−２７９３０９号に、Ｎｉ含有量を３０〜４９質量％まで低減させたＦｅ基耐熱合金が特開平７−１０９５３９号に、Ｎｉ含有量を３５〜４５質量％まで低減させたＦｅ基耐熱合金が特開平７−３３２０３５号に、Ｎｉ含有量を５０〜６０質量％まで低減させたＦｅ基耐熱合金が特開平１１−２２９０５９号に開示されている。
【０００３】
【特許文献１】特開平９−２７９３０９号公報
【特許文献２】特開平７−１０９５３９号公報
【特許文献３】特開平７−３３２０３５号公報
【特許文献４】特開平１１−２２９０５９号公報
【０００４】
【発明が解決しようとする課題】
しかしながら、近年、環境問題を背景とするエンジンの更なる高効率化を目的として、バルブ材には従来より高い高温強度が要求されるようになってきた。そのため、ＮｉがＮＣＦ７５１に比べて低い上述の省資源型のＦｅ基耐熱合金では、要求される高温強度を満足することができなくなってきた。またＮＣＦ７５１においては、高温強度は高いものの、Ｎｉを７０質量％以上含有するため高価であり、また省資源化という観点から好ましくない。
本発明の目的は、従来の省資源型Ｆｅ基合金では達し得ない高温強度を有し、かつ良好なバルブ製造性を有するエンジンバルブ用耐熱合金を提供することである。
【０００５】
【課題を解決するための手段】
Ｎｉ基析出強化型合金は、Ｎｉ３（Ａｌ，Ｔｉ，Ｎｂ）からなるγ’相による析出強化とＭｏ、Ｗの固溶強化を利用して高温強度を持つことができる。ただし、単なるγ’相の増加は熱間加工性を悪化させ、バルブへの鍛造が困難となる。また、過度なＭｏ、Ｗの増加は、高温での組織を不安定にし、σ相やα’相を生成させる。そこで、発明者らは、γ’相による析出強化と、Ｍｏ、Ｗ等を増加させてＭｏ，Ｗ等の固溶強化の最適なバランスと、高温強度を劣化させない範囲でのＦｅの積極的な添加を組み合わせることにより、熱間加工性が良好で優れた高温強度を持つ耐熱合金が得られることを新たに知見した。
Ｍｏ、Ｗを高めた場合、α’相またはσ相等の有害相の析出を生じ、高温強度が低下するという問題が生じるので、Ｃｒ、Ｍｏ、Ｗの原子％を適正な値に調整することで熱間加工性と高温疲労強度とを兼備する合金となることを見出し本発明に到達した。
【０００６】
即ち、本発明は、質量％にてＣ：０．０１〜０．１５％、Ｓｉ：１．０％以下（０％を含まず）、Ｍｎ：１．０％以下（０％を含まず）、Ｃｒ：１３％以上２０％未満、Ｍｏ＋１／２Ｗ：５．０％を超えて１０％以下、Ａｌ：０．５〜３．０％、Ｔｉ：１．０〜３．０％、Ｎｂ＋Ｔａ：０．３〜２．０％、Ｆｅ：１０％以下、Ｂ：０．０１５％以下（０％を含まず）を含有し、Ｍｇ：０．０１５％以下およびＣａ：０．０１５％以下のうち一種または二種を含み、残部Ｎｉ及び不可避的不純物からなり、且つ原子％でＣｒ，Ｍｏ，Ｗ量の和を示す下記（１）式を満足するエンジンバルブ用耐熱合金である。
（１）式：２１％≦［Ｃｒ］＋［Ｍｏ］＋［Ｗ］≦２５％
【０００７】
好ましくは、質量％にてＦｅ：５％を超えて８％以下であるエンジンバルブ用耐熱合金である。
本発明において、より高温での耐酸化性を向上させるために、質量％にて、希土類元素を０．０６％以下含んでも良い。
また、本発明において高温疲労強度を向上させるために、上記のＮｉの一部を８％のＣｏで置換しても良い。
【０００８】
【発明の実施の形態】
以下に本発明における成分限定理由について述べる。なお、以下、特に断らない限り、組成％の単位は質量％である。
Ｃは、ＴｉやＮｂと結びついてＭＣ炭化物を形成し、結晶粒の粗大化防止やクリープ破断延性の改善に役立つため、少量添加する必要がある。しかし、０．１５％を超える過度の添加は、長時間加熱時にＭＣ炭化物からＭ２３Ｃ６炭化物への分解反応が多量に生じて、常温における粒界の延性を低下させる。従って０．０１〜０．１５％に限定する。好ましくは、０．０１〜０．０８％である。
Ｓｉ、Ｍｎは、本発明合金において脱酸元素として添加されるが、何れも過度の添加は高温強度の低下を招くため、１．０％以下に限定する。
【０００９】
Ｃｒは、合金に耐酸化性を付与するのに不可欠の元素であり、自動車用等の耐熱性を保証するには最低１３％以上必要であるが、２０％を超えると組織が不安定となり、Ｃｒに富んだα’相またはσ相などの有害脆化相が生成し、クリープ破断強度と常温延性の低下を招くので、Ｃｒは１３％以上２０％未満、好ましくは１５〜１９％である。
【００１０】
ＭｏとＷは同族の元素で、ともにオーステナイト基地を固溶強化し、高温疲労強度と高温クリープ強度を著しく高める効果をもつ。ＷはＭｏの２倍の原子量をもつために、拡散速度がＭｏよりも小さく、同じ原子％の添加（質量比ではＷ／Ｍｏ＝２）では、クリープ強度などにはＷはＭｏよりも有利に働き、また耐食性の点でもＷはＭｏよりも有利である。しかし、質量％の比較では、ＷはＭｏと同等の強度を得るためにはＭｏの２倍近い添加が必要になるので、コストおよび比重の点で不利である。これらの長所と短所を考慮して、ＭｏとＷは必要に応じて一種または二種添加することができ、Ｍｏ＋１／２Ｗの量で規定する。Ｍｏ＋１／２Ｗが５．０％以下であると高温強度が不足し、逆に１０％を超える過度の添加は熱間加工性を害し、Ｃｒと同様にα’相またはσ相等の有害相の析出を生じるため、Ｍｏ＋１／２Ｗは５．０％を超えて１０％以下に限定する。
【００１１】
Ｃｒ，Ｍｏ，Ｗは高温での耐酸化性、引張強度、疲労強度等を高めるために必要であるが、過度の添加は耐熱合金の強度、靭性を害するα’相またはσ相等の有害相を析出させる。このＣｒ，Ｍｏ，Ｗの効果は、原子％に換算したこれら元素量［Ｃｒ］，［Ｍｏ］，［Ｗ］の和で整理することができる。その原子％の和［Ｃｒ］＋［Ｍｏ］＋［Ｗ］が２１％より小さいと所望の耐酸化性、疲労強度が維持できず、一方２５％より大きいと長時間使用中にα’相またはσ相等の有害相の析出が起こる可能性があることから、［Ｃｒ］＋［Ｍｏ］＋［Ｗ］の値は２１〜２５原子％とした。
【００１２】
Ａｌは、安定なγ’相を析出させて所望の高温強度を得るためには不可欠な元素であり、最低０．５％を必要とするが、３．０％を超えると熱間加工性が劣化するので、１．０〜３．０％に限定する。
【００１３】
Ｔｉは、Ｃと結び付いてＭＣ炭化物を形成する一方、Ａｌ、Ｎｂ、ＴａとともにＮｉと結びついてγ’相を形成し、高温強度を向上させる効果があり、１．０％以上の添加が必要である。しかし、Ｔｉを３．０％を超えて添加すると、高温においてγ’相からη相への変態が起こり易くなり高温強度を低下させる。さらに、Ｔｉの過度な添加はγ’相を過度に増加させて熱間加工性を低下させる。従ってＴｉは１．０〜３．０％に限定する。
【００１４】
ＮｂおよびＴａは、Ｔｉと同様Ｃと結び付いてＭＣ炭化物を形成するとともに、γ’相を形成して高温強度を向上させる効果があるが、Ｔｉと比較して高温でγ’相をより安定化させる効果があるため、高温長時間加熱後における高温強度の低下を抑制する。従って、質量％の合計で最低０．３％以上添加することが必要であるが、過度の添加は高温においてγ’相からδ相への変態を起こし易くなるためにＮｂとＴａは合計で０．３〜２．０％に限定する。
【００１５】
Ｆｅは、オーステナイト基地を軟化させ高温強度の点では不利に働くが、熱間加工性、バルブの製造性の点では有利に働く。そのため、Ｆｅは１０％まで添加できるが１０％を超えると極度に高温強度が低下してしまう。よって、Ｆｅは１０％以下に限定する。好ましくは、５％を超えて８％以下である。
【００１６】
Ｂは、本発明において粒界強化作用により高温の強度と延性を高めるのに有効であり、本発明合金に適量添加できる。その効果は少量の添加量から始まるが、０．０１５％を超えると加熱時の初期溶融温度が低下して熱間加工性が低下するので、Ｂは０．０１５％以下に限定する。
【００１７】
ＭｇとＣａは、強力な脱酸・脱硫元素として合金の清浄度を高めるとともに、高温引張やクリープ変形時さらに熱間加工時の延性改善に役立つため、一種または二種を適量添加することが必要である。その効果は少量添加量から始まるが、Ｍｇ、Ｃａがそれぞれ０．０１５％を超えると加熱時の初期溶融温度が低下して熱間加工性が低下するので、Ｍｇ、Ｃａはそれぞれ０．０１５％以下に限定する。上記の効果を得るためには、Ｍｇ，Ｃａは、合計で０．０００１％以上添加するのが好ましい。
【００１８】
希土類元素、より高温で使用される場合にＣｒを主体とする酸化被膜を強固にして耐酸化性を向上させる効果がある元素であり、必要に応じて添加される。ここで希土類元素とは、Ｓｃ，Ｙ，ランタノイドのことをいう。通常は、Ｙ，Ｌａ，Ｃｅ，Ｎｄ，Ｐｒ等を使用することが多いが、同じ効果が得られれば、希土類元素はどの元素を選択してもかまわない。希土類元素は、０．０６％を超えると加熱時の初期溶融温度が低下して熱間加工性が劣化することから、添加する場合は、０．０６％以下とすることが必要である。なお、０．００１％より少ないと十分な効果が得にくいため、より好ましくは、０．００１％以上の添加とする。
【００１９】
Ｃｏはオーステナイト基地に固溶して熱間加工温度域ではγ’相の固溶を促進させて加工性を改善する一方、使用温度域ではγ’相の析出量を増加させて高温強度を高めるため、必要に応じてＮｉと置換して添加される。
なお、不純物元素のうち、下記の元素については以下に示す範囲であれば本発明合金に含まれてもよい。
Ｐ≦０．０４％，Ｓ≦０．０２％，Ｏ≦０．０２％，Ｎ≦０．０５％
【００２０】
【実施例】
本発明合金および比較合金を真空誘導炉にて溶製し１０ｋｇのインゴットを作製後、１１５０℃に加熱して３０ｍｍ角の棒材に鍛伸した。表１に本発明合金Ｎｏ．１〜８，比較合金Ｎｏ．２１〜２４の化学組成を示す。ここで比較合金Ｎｏ．２１はＮＣＦ７５１相当のＮｉ基合金、比較合金Ｎｏ．２２，Ｎｏ．２３は特開平７−１０９５３９号に開示されるＦｅ−Ｎｉ−Ｃｒ超耐熱合金、比較合金Ｎｏ．２４は、特開平１１−２２９０５９号に開示される耐熱合金に相当する合金である。
【００２１】
熱間加工性を評価するため、この鍛伸材から平行部直径８ｍｍ，平行部長さ２４ｍｍの丸棒試験片を採取し、８００〜１２００℃の範囲の種々温度に加熱後、歪速度４．２／ｓｅｃで引張試験を行い、破断絞りが６０％以上となる温度範囲を熱間加工温度範囲とした。また、鍛伸材に１０５０℃×３０分、水冷の固溶化処理後、７５０℃×４時間、空冷の時効処理を行なった。この熱処理後、平行部直径６．３５ｍｍ，標点間距離２５．４ｍｍの丸棒試験片を採取し、ＡＳＴＭ法により８００℃で引張試験を行なった。また、同じく熱処理後に平行部直径８ｍｍの丸棒試験片を採取し、ＪＩＳ Ｚ２２７４に従って、試験温度８００℃、３５００ｒｐｍの回転数で回転曲げ疲労試験を実施し、１０７回での疲労強度を求めた。
【００２２】
表２に各合金の熱間加工温度範囲、８００℃での０．２％耐力、引張強さ、伸び、回転曲げ疲労強度を示す。
【００２３】
【表１】
【００２４】
【表２】
【００２５】
表２からわかるように、本発明合金は、いずれも熱間加工温度範囲が広く、熱間加工性が十分可能であることがわかる。また、本発明合金はいずれも８００℃での０．２％耐力、引張強さが高く、また、８００℃での疲労強度も高いことがわかる。一方、比較合金Ｎｏ．２１，Ｎｏ．２４は本発明合金に比べて熱間加工温度範囲が狭い。また比較合金Ｎｏ．２２，Ｎｏ．２３は、熱間加工温度範囲が広く、熱間加工は比較的容易であるものの、８００℃での引張強度、疲労強度が本発明合金に比べて低い。以上より、本発明合金は良好な熱間加工性を維持しつつ、８００℃での高い高温引張強度と高い高温疲労強度を兼ね備えていることがわかる。
【００２６】
【発明の効果】
以上述べたように本発明合金は、良好な熱間加工性を維持していることから、エンジンバルブへの加工が容易であり、かつ従来の省資源型のＦｅ基耐熱合金に比べて高い高温強度、特に高い高温疲労強度を有していることから、自動車用排気エンジンバルブに使用すれば、エンジン性能の向上に大きく寄与することができる。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a heat-resistant alloy for an engine valve mainly used for an exhaust engine valve of an automobile.
[0002]
[Prior art]
Conventionally, Fe-based alloys (heat-resistant steels) such as SUH35 have been widely used for exhaust engine valves. However, with the use of a higher temperature, the Ni-based alloy NCF751 (Ni-15.5Cr-1Nb) has been used. -2.3Ti-1.2Al-7Fe [% by mass]) has come to be used. However, NCF751 is expensive because it contains 70% by mass of Ni, and resource-saving alloys having high-temperature strength and heat resistance close to NCF751 have been developed. For example, an Fe-based heat-resistant alloy in which the Ni content is reduced to 30 to 35% by mass is disclosed in JP-A-9-279309, and an Fe-based heat-resistant alloy in which the Ni content is reduced to 30 to 49% by mass is disclosed in JP-A-7-279. JP-A-109539 discloses an Fe-based heat-resistant alloy in which the Ni content is reduced to 35 to 45% by mass, and JP-A-7-332035 discloses an Fe-based heat-resistant alloy in which the Ni content is reduced to 50 to 60% by mass. No. 11-229059.
[0003]
[Patent Document 1] Japanese Patent Application Laid-Open No. 9-279309 [Patent Document 2] Japanese Patent Application Laid-Open No. 7-109439 [Patent Document 3] Japanese Patent Application Laid-Open No. 7-332035 [Patent Document 4] Japanese Patent Application Laid-Open No. 11-229059 ]
[Problems to be solved by the invention]
However, in recent years, valve materials have been required to have higher high-temperature strength than before in order to further increase the efficiency of engines against the background of environmental problems. For this reason, the resource-saving Fe-based heat-resistant alloy having a lower Ni content than the NCF751 cannot satisfy the required high-temperature strength. NCF751 has high strength at high temperature, but is expensive because it contains 70% by mass or more of Ni, and is not preferable from the viewpoint of resource saving.
An object of the present invention is to provide a heat-resistant alloy for an engine valve having a high-temperature strength that cannot be attained by a conventional resource-saving Fe-based alloy and having good valve productivity.
[0005]
[Means for Solving the Problems]
The Ni-based precipitation-strengthened alloy can have high-temperature strength by utilizing precipitation strengthening by the γ 'phase composed of Ni 3 (Al, Ti, Nb) and solid solution strengthening of Mo and W. However, a mere increase in the γ 'phase deteriorates hot workability and makes forging into a valve difficult. Excessive increases in Mo and W destabilize the structure at high temperatures and generate a σ phase and an α 'phase. Therefore, the present inventors have proposed an optimum balance between precipitation strengthening by the γ 'phase, solid solution strengthening of Mo, W, etc. by increasing Mo, W, etc., and aggressive use of Fe within a range not deteriorating high-temperature strength. It has been newly found that a combination of the addition provides a heat-resistant alloy having good hot workability and excellent high-temperature strength.
When Mo and W are increased, harmful phases such as α ′ phase and σ phase are precipitated, and a problem that the high-temperature strength is reduced occurs. Therefore, by adjusting the atomic% of Cr, Mo and W to an appropriate value. The present inventors have found that the alloy has both hot workability and high-temperature fatigue strength, and have reached the present invention.
[0006]
That is, in the present invention, C: 0.01 to 0.15%, Si: 1.0% or less (not including 0%), Mn: 1.0% or less (not including 0%) in mass%. , Cr: 13% or more and less than 20%, Mo + 1 / 2W: more than 5.0% and 10% or less, Al: 0.5-3.0%, Ti: 1.0-3.0%, Nb + Ta: 0 0.3 to 2.0%, Fe: 10% or less, B: 0.015% or less (excluding 0%), and one of Mg: 0.015% or less and Ca: 0.015% or less Or, it is a heat-resistant alloy for an engine valve, comprising two kinds, the balance being Ni and unavoidable impurities, and satisfying the following expression (1), which shows the sum of Cr, Mo and W in atomic%.
Formula (1): 21% ≦ [Cr] + [Mo] + [W] ≦ 25%
[0007]
Preferably, it is a heat-resistant alloy for an engine valve in which Fe: more than 5% and 8% or less by mass%.
In the present invention, in order to improve the oxidation resistance at a higher temperature, the rare earth element may be contained in an amount of 0.06% or less by mass%.
Further, in the present invention, in order to improve the high temperature fatigue strength, a part of the above Ni may be replaced with 8% Co.
[0008]
BEST MODE FOR CARRYING OUT THE INVENTION
The reasons for limiting the components in the present invention are described below. Hereinafter, unless otherwise specified, the unit of the composition% is mass%.
C combines with Ti and Nb to form MC carbides, which helps to prevent coarsening of crystal grains and improve creep rupture ductility. Therefore, C needs to be added in a small amount. However, excessive addition exceeding 0.15% causes a large amount of decomposition reaction from MC carbide to M 23 C 6 carbide during heating for a long time, and lowers the ductility of grain boundaries at room temperature. Therefore, it is limited to 0.01 to 0.15%. Preferably, it is 0.01 to 0.08%.
Si and Mn are added as deoxidizing elements in the alloy of the present invention. However, excessive addition of both causes reduction in high-temperature strength, and is therefore limited to 1.0% or less.
[0009]
Cr is an indispensable element for imparting oxidation resistance to the alloy, and at least 13% is necessary to guarantee heat resistance for automobiles and the like, but if it exceeds 20%, the structure becomes unstable, Since a harmful embrittlement phase such as an α 'phase or a σ phase rich in Cr is generated and causes a decrease in creep rupture strength and room-temperature ductility, the Cr content is 13% or more and less than 20%, and preferably 15 to 19%.
[0010]
Mo and W are homologous elements, and both have the effect of solid-solution strengthening the austenite matrix and significantly increasing high-temperature fatigue strength and high-temperature creep strength. Since W has an atomic weight twice that of Mo, the diffusion rate is lower than that of Mo. At the same addition of atomic% (W / Mo = 2 in mass ratio), W is more advantageous than Mo for creep strength and the like. W also has an advantage over Mo in terms of function and corrosion resistance. However, in terms of mass%, W is disadvantageous in terms of cost and specific gravity because it requires addition of almost twice Mo in order to obtain the same strength as Mo. In consideration of these advantages and disadvantages, one or two types of Mo and W can be added as necessary, and are defined by the amount of Mo + ／ W. If Mo + 1 / 2W is 5.0% or less, the high-temperature strength is insufficient, and if it exceeds 10%, excessive addition impairs hot workability and precipitates harmful phases such as α ′ phase or σ phase like Cr. Therefore, Mo + 1 / 2W is limited to more than 5.0% and 10% or less.
[0011]
Cr, Mo, and W are necessary to increase oxidation resistance, tensile strength, fatigue strength, and the like at high temperatures, but excessive addition of harmful phases such as α 'phase or σ phase that impairs the strength and toughness of heat-resistant alloys. Precipitate. The effects of Cr, Mo, and W can be summarized by the sum of the amounts of these elements [Cr], [Mo], and [W] in terms of atomic%. If the sum of the atomic percentages [Cr] + [Mo] + [W] is less than 21%, the desired oxidation resistance and fatigue strength cannot be maintained, while if it exceeds 25%, the α ′ phase or The value of [Cr] + [Mo] + [W] was set to 21 to 25 at% because there is a possibility that a harmful phase such as the σ phase may be precipitated.
[0012]
Al is an indispensable element for precipitating a stable γ 'phase and obtaining a desired high-temperature strength, and requires at least 0.5%. However, if it exceeds 3.0%, hot workability becomes high. Since it deteriorates, it is limited to 1.0 to 3.0%.
[0013]
Ti combines with C to form MC carbides, while it combines with Ni, Al, Nb, and Ta to form a γ 'phase, which has the effect of improving high-temperature strength, and requires an addition of 1.0% or more. is there. However, when Ti is added in excess of 3.0%, transformation from the γ ′ phase to the η phase tends to occur at high temperatures, and the high-temperature strength is reduced. In addition, excessive addition of Ti excessively increases the γ 'phase and reduces hot workability. Therefore, Ti is limited to 1.0 to 3.0%.
[0014]
Nb and Ta combine with C similarly to Ti to form MC carbides and form a γ 'phase, which has the effect of improving high-temperature strength. However, compared to Ti, the γ' phase is more stabilized at high temperatures. Therefore, a decrease in high-temperature strength after high-temperature and long-time heating is suppressed. Therefore, it is necessary to add at least 0.3% or more in total of mass%, but excessive addition easily causes transformation from γ ′ phase to δ phase at high temperature, so that Nb and Ta are added in a total of 0%. Limited to 3 to 2.0%.
[0015]
Fe softens the austenitic matrix and works disadvantageously in terms of high-temperature strength, but works favorably in terms of hot workability and valve manufacturability. For this reason, Fe can be added up to 10%, but if it exceeds 10%, the high-temperature strength is extremely reduced. Therefore, Fe is limited to 10% or less. Preferably, it is more than 5% and 8% or less.
[0016]
B is effective for enhancing the high-temperature strength and ductility by the grain boundary strengthening action in the present invention, and can be added to the alloy of the present invention in an appropriate amount. The effect starts with a small amount of addition, but if it exceeds 0.015%, the initial melting temperature at the time of heating decreases and the hot workability decreases, so B is limited to 0.015% or less.
[0017]
Mg and Ca are powerful deoxidizing and desulfurizing elements that enhance the cleanliness of the alloy and help to improve the ductility during high-temperature tension, creep deformation and hot working. It is. The effect starts with a small amount of addition. However, if the content of Mg and Ca exceeds 0.015%, the initial melting temperature at the time of heating is lowered and the hot workability is reduced. Limited to the following. In order to obtain the above effects, it is preferable to add Mg and Ca in a total amount of 0.0001% or more.
[0018]
Rare earth element, an element having an effect of strengthening an oxide film mainly composed of Cr when used at a higher temperature to improve oxidation resistance, and is added as necessary. Here, the rare earth elements refer to Sc, Y, and lanthanoids. Usually, Y, La, Ce, Nd, Pr, etc. are often used, but any rare earth element may be selected as long as the same effect is obtained. If the rare earth element exceeds 0.06%, the initial melting temperature at the time of heating is lowered and the hot workability is deteriorated. Therefore, when the rare earth element is added, it is necessary to be 0.06% or less. If the content is less than 0.001%, it is difficult to obtain a sufficient effect. Therefore, the content is more preferably 0.001% or more.
[0019]
Co forms a solid solution in the austenite matrix and promotes solid solution of the γ 'phase in the hot working temperature range to improve workability, while increasing the precipitation amount of the γ' phase in the working temperature range to increase the high temperature strength. Therefore, Ni is added in place of Ni as necessary.
Note that, among the impurity elements, the following elements may be included in the alloy of the present invention as long as they are in the following ranges.
P ≦ 0.04%, S ≦ 0.02%, O ≦ 0.02%, N ≦ 0.05%
[0020]
【Example】
The alloy of the present invention and the comparative alloy were melted in a vacuum induction furnace to produce a 10 kg ingot, which was then heated to 1150 ° C. and forged into a 30 mm square bar. Table 1 shows the alloy No. of the present invention. Nos. 1 to 8 and Comparative Alloy Nos. 1 shows the chemical compositions of 21-24. Here, the comparative alloy No. No. 21 is a Ni-base alloy corresponding to NCF751, a comparative alloy No. 21 22, No. No. 23 is a Fe-Ni-Cr super heat-resistant alloy disclosed in Japanese Patent Application Laid-Open No. 24 is an alloy corresponding to the heat-resistant alloy disclosed in JP-A-11-229059.
[0021]
In order to evaluate the hot workability, a round bar specimen having a parallel part diameter of 8 mm and a parallel part length of 24 mm was sampled from this forged material, heated to various temperatures in the range of 800 to 1200 ° C., and then subjected to a strain rate of 4.2. The tensile test was performed at a rate of / sec, and the temperature range in which the drawing reduction was 60% or more was defined as the hot working temperature range. Further, the forged material was subjected to a solution treatment of water cooling at 1050 ° C. for 30 minutes, and then an aging treatment of air cooling at 750 ° C. for 4 hours. After this heat treatment, a round bar test piece having a parallel portion diameter of 6.35 mm and a distance between gauge points of 25.4 mm was sampled and subjected to a tensile test at 800 ° C. by the ASTM method. Further, similarly taken round bar test piece parallel portion diameter 8mm after heat treatment, according to JIS Z2274, test temperature 800 ° C., conducted rotary bending fatigue test at a rotation speed of 3500 rpm, was determined fatigue strength at 10 7 times .
[0022]
Table 2 shows the hot working temperature range of each alloy, 0.2% proof stress at 800 ° C., tensile strength, elongation, and rotational bending fatigue strength.
[0023]
[Table 1]
[0024]
[Table 2]
[0025]
As can be seen from Table 2, all of the alloys of the present invention have a wide hot working temperature range and are sufficiently capable of hot workability. Further, it can be seen that all the alloys of the present invention have high 0.2% proof stress and tensile strength at 800 ° C. and high fatigue strength at 800 ° C. On the other hand, Comparative Alloy No. 21, No. No. 24 has a narrower hot working temperature range than the alloy of the present invention. In addition, the comparative alloy No. 22, No. No. 23 has a wide hot working temperature range and hot working is relatively easy, but the tensile strength and the fatigue strength at 800 ° C. are lower than those of the alloy of the present invention. From the above, it can be seen that the alloy of the present invention has both high high-temperature tensile strength at 800 ° C. and high high-temperature fatigue strength while maintaining good hot workability.
[0026]
【The invention's effect】
As described above, since the alloy of the present invention maintains good hot workability, it can be easily processed into an engine valve and has a higher high temperature than a conventional resource-saving Fe-based heat-resistant alloy. Since it has high strength, particularly high high-temperature fatigue strength, it can greatly contribute to improvement of engine performance when used for exhaust engine valves for automobiles.
