JP4178490B2 - Maraging steel with high fatigue strength and maraging steel strip using it - Google Patents
Maraging steel with high fatigue strength and maraging steel strip using it Download PDFInfo
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- JP4178490B2 JP4178490B2 JP16048699A JP16048699A JP4178490B2 JP 4178490 B2 JP4178490 B2 JP 4178490B2 JP 16048699 A JP16048699 A JP 16048699A JP 16048699 A JP16048699 A JP 16048699A JP 4178490 B2 JP4178490 B2 JP 4178490B2
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- 229910001240 Maraging steel Inorganic materials 0.000 title claims description 37
- 229910052757 nitrogen Inorganic materials 0.000 claims description 17
- 229910052750 molybdenum Inorganic materials 0.000 claims description 14
- 229910052799 carbon Inorganic materials 0.000 claims description 13
- 239000012535 impurity Substances 0.000 claims description 10
- 229910052717 sulfur Inorganic materials 0.000 claims description 10
- 229910052748 manganese Inorganic materials 0.000 claims description 7
- 229910052759 nickel Inorganic materials 0.000 claims description 7
- 229910052760 oxygen Inorganic materials 0.000 claims description 7
- 229910052698 phosphorus Inorganic materials 0.000 claims description 7
- 238000005121 nitriding Methods 0.000 description 26
- 230000035882 stress Effects 0.000 description 17
- 229910000831 Steel Inorganic materials 0.000 description 16
- 239000010959 steel Substances 0.000 description 16
- ATJFFYVFTNAWJD-UHFFFAOYSA-N Tin Chemical compound [Sn] ATJFFYVFTNAWJD-UHFFFAOYSA-N 0.000 description 12
- 230000032683 aging Effects 0.000 description 10
- 238000005728 strengthening Methods 0.000 description 10
- 230000000052 comparative effect Effects 0.000 description 8
- 230000005540 biological transmission Effects 0.000 description 6
- 230000007423 decrease Effects 0.000 description 5
- 230000000694 effects Effects 0.000 description 5
- 229910000765 intermetallic Inorganic materials 0.000 description 5
- 238000001556 precipitation Methods 0.000 description 4
- 229910001566 austenite Inorganic materials 0.000 description 3
- 229910052742 iron Inorganic materials 0.000 description 3
- 229910000734 martensite Inorganic materials 0.000 description 3
- 238000002844 melting Methods 0.000 description 3
- 230000008018 melting Effects 0.000 description 3
- 239000000203 mixture Substances 0.000 description 3
- 238000004519 manufacturing process Methods 0.000 description 2
- 150000004767 nitrides Chemical class 0.000 description 2
- 239000000126 substance Substances 0.000 description 2
- 229910018072 Al 2 O 3 Inorganic materials 0.000 description 1
- NINIDFKCEFEMDL-UHFFFAOYSA-N Sulfur Chemical compound [S] NINIDFKCEFEMDL-UHFFFAOYSA-N 0.000 description 1
- 230000002411 adverse Effects 0.000 description 1
- 238000005275 alloying Methods 0.000 description 1
- 230000015572 biosynthetic process Effects 0.000 description 1
- 238000006243 chemical reaction Methods 0.000 description 1
- 238000005097 cold rolling Methods 0.000 description 1
- 230000006835 compression Effects 0.000 description 1
- 238000007906 compression Methods 0.000 description 1
- 239000013078 crystal Substances 0.000 description 1
- 238000005098 hot rolling Methods 0.000 description 1
- 230000006698 induction Effects 0.000 description 1
- 230000000977 initiatory effect Effects 0.000 description 1
- 239000000463 material Substances 0.000 description 1
- 239000011159 matrix material Substances 0.000 description 1
- 239000002184 metal Substances 0.000 description 1
- 229910052751 metal Inorganic materials 0.000 description 1
- 150000001247 metal acetylides Chemical class 0.000 description 1
- 238000000034 method Methods 0.000 description 1
- 238000007670 refining Methods 0.000 description 1
- 230000002787 reinforcement Effects 0.000 description 1
- 239000007787 solid Substances 0.000 description 1
- 239000011593 sulfur Substances 0.000 description 1
- 230000009466 transformation Effects 0.000 description 1
Landscapes
- Heat Treatment Of Sheet Steel (AREA)
- Solid-Phase Diffusion Into Metallic Material Surfaces (AREA)
Description
【０００１】
【発明の属する技術分野】
本発明は、自動車用無段変速機等に使用される動力伝達用ベルトのような高疲労強度が要求される部材に使用されるのに適した高疲労強度を有するマルエージング鋼に関するものである。
【０００２】
【従来の技術】
マルエージング鋼は、2000MPa前後の非常に高い引張強さをもつため、高強度が要求される部材、例えば、ロケット用部品、遠心分離機部品、航空機部品、自動車エンジンの無段変速機用部品、金型、等種々の用途に使用されている。その代表的な組成には、18%Ni-8%Co-5%Mo-0.4%Ti-0.1%Al-bal.Feが挙げられる。
そして、マルエージング鋼は、強化元素として、Mo、Tiを適量含んでおり、時効処理を行うことによって、Ni3Mo、Ni3Ti、Fe2Mo等の金属間化合物を析出させて高強度を得ることのできる鋼である。
【０００３】
【発明が解決しようとする課題】
しかし、マルエージング鋼は、非常に高引張強度が得られる一方、疲労強度に関しては必ずしも高くない。疲労強度は一般に、硬さ、引張強さに比例して上昇する傾向があるが、硬さは約400HV以上、引張強さが約1200MPa以上の高強度材では、硬さ、引張強さが上昇しても疲労強度は上昇しなくなり、マルエージング鋼も例外ではない。そこで高い疲労強度が得られるマルエージング鋼が望まれていた。
本発明は、高疲労強度を有するマルエージング鋼ならびに該マルエージング鋼からなるマルエージング鋼帯を提供することを目的とするものである。
【０００４】
【課題を解決するための手段】
従来の高強度鋼では、例えば日本機械学会論文集Ａ６４巻２５３６〜２５４１頁に開示されるように、低サイクル域で疲労破壊する場合には、疲労破壊は表面を起点としたき裂発生、伝播によって起こることが知られている。また、従来、疲労限と考えられていた１０の７乗回を超える超高サイクル域においては、疲労破壊は表面を起点とせず、内部の介在物を起点として起こることが知られている。
表面起点の破壊による疲労強度は、表面に圧縮残留応力を与えることによって改善することができ、また、内部起点の破壊による疲労強度は介在物を微細化することで改善できると考えられる。
【０００５】
本発明者は上述した問題を解決すべく、鋭意研究を行なった結果、表面起点の疲労強度向上には、適切な窒化処理を施し、表面に大きな圧縮残留応力与えることが有効であると判断した。
また、本発明者は従来のマルエージング鋼の内部起点の疲労破壊の起点を詳細に分析を行った結果、起点となった箇所に介在物の存在を確認し、その介在物はTiN(またはTi(C、N))であることを知見した。この結果、TiN(またはTi(C、N))の介在物を無くすことが疲労強度向上に有効であると判断した。TiNを無くすには、TiまたはNを低減することが有効であるが、極端なNの低減は量産溶解設備では限界があり、また製造コストも大きく上昇する可能性がある。
【０００６】
一方、Tiを大幅に低減すればTiNを低減でき、TiN量の減少、微細化が達成できると考えられる。しかし、Tiはマルエージング鋼の重要な強化元素であり、単純にTi量を低下させると、強度が大きく低下してしまう。Tiを低減したマルエージング鋼としては、特開平10-152759号に開示される靭性に優れたマルエージング鋼、特開平1-142052号に開示される継目無金属ベルト及びその製造方法が知られている。
しかし、特開平10-152759号では、Nを靭性向上のために0.005〜0.03%の範囲で、むしろ積極的に添加している。また、特開平1-142052号では、Mo量が3〜6%の範囲を提案しており、既存のTiを含むマルエージング鋼と同レベルである。
【０００７】
本発明者は、疲労強度向上に有害な介在物TiN低減のためにTi、Nをともに低く抑え、かつTi低減による引張強度低下をMoを増加させること、およびCo/3+Mo+4Tiの値を適正範囲に限定することによって大幅な合金元素の増加なしに補うことができることを見出した。
また、Ti量は窒化処理後の表面硬さに対してあまり影響を及ぼさないが、窒化による表面圧縮残留応力の絶対値はTi量が少ない方が、またMo量が多い方が大きくなることを新規に見出し、本発明に到ったものである。
【０００８】
すなわち、本発明の第１発明は、重量％にて、C:0.008%以下、Si:0.1%以下、Mn:0.1%以下、P:0.010%以下、S:0.005%以下、Ni:16〜20%、Mo:6.0%を超え9.0%以下、Co:7.0以上11.0%未満、Ti:0.01%以下、Co/3＋Mo＋4Ti:8.0〜13.0、Al:0.2%以下、N:0.005%未満、O:0.003%以下、残部はFe 及び不可避的不純物からなる高疲労強度を有するマルエージング鋼である。
【０００９】
第２発明は、重量％にて、C:0.008%以下、Si:0.1%以下、Mn:0.1%以下、P:0.010%以下、S:0.005%以下、Ni:17.5を超え19.0%以下、Mo:6.5〜9.0%、Co:7.0%以上11.0%未満、Ti:0.01%以下、Co/3＋Mo＋4Ti:8.0〜11.0、Al:0.2%以下、N:0.004%以下、O:0.003%以下、残部はFe 及び不可避的不純物からなる高疲労強度を有するマルエージング鋼である。
【００１０】
また、第３発明は、重量％にて、C:0.008%以下、Si:0.1%以下、Mn:0.1%以下、P:0.010%以下、S:0.005%以下、Ni:17.5を超え19.0%以下、Mo:6.0%を超え9.0%以下、Co:7.0%以上9.0%未満、Ti:0.01%以下、Co/3＋Mo＋4Ti:8.0〜11.0、Al:0.2%以下、N:0.004%以下、O:0.003%以下、残部はFe 及び不可避的不純物からなる高疲労強度を有するマルエージング鋼である。
また、本発明のマルエージング鋼は、重量％にて、B:0.01%以下を含むことができる。また、上述したマルエージング鋼を用いてなる本発明のマルエージング鋼帯は、適正な窒化処理によって表面に窒化層を形成させ、表面に圧縮残留応力を付与することができる。
【００１１】
【発明の実施の形態】
以下に本発明における各元素の作用について述べる。
Cは、Ti、Moと炭化物、炭窒化物を形成して、析出すべき金属間化合物を減少させて強度を低下させるため、低く抑える必要がある。このような理由からCは0.008%以下とした。
Si、Mnは、O、S等と結合して介在物を形成し、疲労強度を低下させることから、いずれも0.10%以下に抑える。
【００１２】
P、Sは、旧オーステナイト粒界に偏析したり、介在物を形成したりすることで、マルエージング鋼を脆化させ、疲労強度を低下させる有害な元素であるため、Pは0.01%以下、Sは0.005%以下とした。
Niは、マルエージング鋼の基地組織である低Cマルテンサイト組織を形成させるため、少なくとも16%は必要であるが、20%を超えるとオーステナイト組織が安定化し、マルテンサイト変態を起こしにくくなることから、Niは16〜20%とした。さらに望ましくは17.5%を超え19.0%以下がよい。
【００１３】
Moは、時効処理時にNi3Mo、Fe2Mo等の微細な金属間化合物を形成し、析出強化に寄与する重要な元素である。Tiも同様に時効析出によって強化に寄与する元素であるが、Tiを低く抑えると、Tiによる引張強度の低下分をMoの添加量を増すことによって補う必要がある。
また、Moは窒化による表面の硬さおよび圧縮残留応力を大きくするために有効な元素である。このためのMoは、6.0%以下では引張強度低下分を補うには不十分であり、一方、9.0%より多いとFe、Moを主要元素とする粗大な金属間化合物を形成しやすくなるため、Moは6.0%を超え、9.0%以下とした。望ましくは、6.5〜9.0%がよい。
【００１４】
Coは、マトリックスのマルテンサイト組織の安定性に大きく影響することなく、時効析出温度域でのMoの固溶度を低下させることによって微細なMoを含む金属間化合物の析出を促進し、時効強化に寄与する重要な元素である。Coは7.0%より少ないと十分な効果が得られず、一方、11.0%以上では強度が高くなり過ぎて靭性が低下する傾向があることから、Coは7.0%以上11.0%未満とした。望ましくは、7.0%以上9.0%未満がよい。
【００１５】
Tiは、本来、マルエージング鋼における重要な強化元素の一つであるが、同時に介在物であるTiNまたはTi(C、N)を形成して、特に超高サイクル域での疲労強度を低下させる有害元素でもあるので、疲労強度を重視する場合には、不純物として低く抑える必要がある。
また、Tiは表面に薄くて安定な酸化膜を形成しやすく、この酸化膜が形成されると窒化反応を阻害するため、十分な窒化表面の圧縮残留応力が得られにくくなる。窒化を容易に行うために、また窒化後の表面の圧縮残留応力を大きくするために、Tiは有害な不純物元素であり、低く抑える必要がある。Tiは、0.01%より多いとTiNまたはTi(C、N)の低減に十分な効果が得られず、また安定な酸化膜を表面に形成しやすくなることから、Tiは0.01%以下とした。望ましくは0.005%以下がよい。
【００１６】
Co、MoおよびTiは、ともにマルエージング鋼における主要な強化元素であるが、その強化への寄与は同じではなく、CoおよびTiによる強化分はMoによる強化分のそれぞれ1/3および4である。
したがって、Co、Moによる強化はCo/3＋Mo＋4Tiで整理できる。Co/3＋Mo＋4Tiの値が8.0%より少ないと強度が十分でなく、一方、13.0%を超えると強度が高くなりすぎ、靭性低下の恐れがあることから、Co/3＋Mo＋4Tiは、8.0〜13.0%とした。望ましくは、8.0〜11.0%がよい。
【００１７】
Alは、脱酸のために少量添加されるが、0.2%より多いとAl2O3介在物を多く形成して疲労強度を低下させるので、Alは0.2%以下とした。
Nは、Tiと結合してTiNまたはTi(C、N)の介在物を形成して、特に超高サイクル域での疲労強度を低下させる不純物元素である。Tiを含むマルエージング鋼では、粗大なTiNまたはTi(C、N)の形成を防ぐため、Nを大幅に低く抑える必要がある。しかし、Tiをほとんど含まないマルエージング鋼ではNは通常の真空溶解で混入する量でも悪影響が少ないことから、0.005%未満とした。望ましくは、0.004%以下がよい。さらに望ましくは、0.002%以下がよい。
【００１８】
Oは、酸化物系介在物を形成して靭性、疲労強度を低下させる不純物元素であるので、0.003%以下に制限した。
Bは、旧オーステナイト結晶粒を微細化して強化に寄与する元素であり、必要に応じて添加する。Bが0.01%より多いと靭性が低下することから、Bは0.01%以下とした。
【００１９】
本マルエージング鋼は、窒化を阻害する可能性のある安定な酸化膜を表面に形成するTiをほとんど含まないため、通常のガス窒化、ガス軟窒化、浸硫窒化、イオン窒化、等の種々の窒化処理が容易にできる。
また、上述の本発明で規定する化学組成範囲内に調整されたマルエージング鋼を、例えば自動車エンジンの無段変速機用部品に適用できるように、帯状に形成し、本マルエージング鋼帯に適当な条件で窒化処理を行うと、窒化物をほとんど形成することなく表面に20〜40μｍ程度の薄い窒化層を形成でき、表面に大きな圧縮残留応力を付与でき、十分な疲労強度を得ることができる。
なお、表面の圧縮残留応力は高い方が好ましいが、そのコントロールは窒化層の厚みを適宜調整することで可能である。
【００２０】
【実施例】
本発明鋼および比較鋼を真空誘導溶解炉で溶解し、10kgのインゴットを作製し、熱間鍛造した。さらに熱間圧延、冷間圧延によって約0.3mm厚さの帯材を作製した。その後、825℃で固溶化処理を行ない、さらに490℃で時効処理を行なった後に、450〜460℃において窒化深さが20〜40μmとなるような条件でイオン窒化およびガス軟窒化を行った。
表１に本発明鋼No.1〜9、比較鋼No.21〜24の化学組成を示す。また、表２に各試料を時効した後の内部硬さ、窒化処理後の表面硬さ、および窒化処理後の表面の残留応力を示す。ここで、表２中の残留応力の符号は、＋が引張、−が圧縮を表しており、全て圧縮残留応力である。
なお、表には示さないが、上記の本発明鋼および比較鋼の断面にて、電子顕微鏡とエックス線分析装置を用いて、微細介在物の観察、分析を行い、比較鋼No.22を除いた全ての試験片でTiNやTi(C、N)の介在物の量が極めて少ない量であったことを確認した。
【００２１】
【表１】
【表２】
表２より、本発明鋼No.1〜9はいずれも時効後の内部硬さが500HV以上であり、マルエージング鋼として十分な強度をもっており、かつ、イオン窒化、ガス軟窒化のいずれの窒化のよっても高い表面硬さと大きな表面圧縮残留応力をもつことがわかる。一方、MoとCo/3＋Mo＋4Tiの値が低い比較鋼No.21は、時効後の内部硬さおよび窒化処理後の圧縮残留応力が小さく、またCoとCo/3＋Mo＋4Tiの値が低い比較鋼No.24は、時効後の内部硬さが400HV台であり、強度がやや不十分である。
また、Mo、CoおよびCo/3＋Mo＋4Tiの値がともに高い比較鋼No.23およびTiが高い比較鋼No.22は、窒化処理後の圧縮残留応力が小さく、大きな圧縮残留応力を得ることが難しいことがわかる。
【００２４】
【発明の効果】
以上説明したように本発明のマルエージング鋼は、高強度と窒化処理後の表面の高硬度および大きな圧縮残留応力を得ることができることから、自動車用無段変速機等に使用される動力伝達用ベルトのような高疲労強度が要求される部材に使用されると、長い疲労寿命を有することができる等、工業上顕著な効果をもつことが予想される。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to maraging steel having high fatigue strength suitable for use in a member requiring high fatigue strength, such as a power transmission belt used in a continuously variable transmission for automobiles. .
[0002]
[Prior art]
Since maraging steel has a very high tensile strength of around 2000 MPa, members that require high strength, such as rocket parts, centrifuge parts, aircraft parts, automobile engine continuously variable transmission parts, It is used for various applications such as molds. A typical composition is 18% Ni-8% Co-5% Mo-0.4% Ti-0.1% Al-bal.Fe.
And maraging steel contains appropriate amounts of Mo and Ti as strengthening elements, and by performing an aging treatment, intermetallic compounds such as Ni 3 Mo, Ni 3 Ti and Fe 2 Mo are precipitated to increase the strength. It is a steel that can be obtained.
[0003]
[Problems to be solved by the invention]
However, maraging steel can obtain very high tensile strength, but is not necessarily high in terms of fatigue strength. Fatigue strength generally tends to increase in proportion to hardness and tensile strength, but hardness and tensile strength increase for high-strength materials with hardness of about 400HV or higher and tensile strength of about 1200MPa or higher. Even so, fatigue strength does not increase, and maraging steel is no exception. Therefore, maraging steel that can provide high fatigue strength has been desired.
An object of the present invention is to provide a maraging steel having high fatigue strength and a maraging steel strip made of the maraging steel.
[0004]
[Means for Solving the Problems]
In conventional high-strength steels, as disclosed in, for example, the Japan Society of Mechanical Engineers, Vol. A, Vol. 64, pages 2536 to 2541, when fatigue fracture occurs in a low cycle range, fatigue fracture is caused by crack initiation and propagation starting from the surface. Is known to occur. Further, it is known that fatigue fracture does not start from the surface but starts from internal inclusions in an ultra-high cycle region exceeding 10 7 times, which was conventionally considered the fatigue limit.
It is considered that the fatigue strength due to the fracture at the surface origin can be improved by applying a compressive residual stress to the surface, and the fatigue strength due to the fracture at the internal origin can be improved by making the inclusions finer.
[0005]
As a result of diligent research to solve the above-mentioned problems, the present inventor has determined that it is effective to give a large compressive residual stress to the surface by applying an appropriate nitriding treatment to improve the fatigue strength at the surface origin. .
In addition, as a result of detailed analysis of the origin of fatigue fracture at the internal origin of conventional maraging steel, the present inventor confirmed the presence of inclusions at the location of the origin, and the inclusion was TiN (or TiN (C, N)). As a result, it was determined that eliminating the inclusion of TiN (or Ti (C, N)) was effective in improving fatigue strength. In order to eliminate TiN, it is effective to reduce Ti or N. However, extreme reduction of N has a limit in mass production melting equipment, and there is a possibility that the manufacturing cost will greatly increase.
[0006]
On the other hand, if Ti is drastically reduced, TiN can be reduced, and the reduction and refinement of the TiN amount can be achieved. However, Ti is an important strengthening element of maraging steel, and if the Ti amount is simply reduced, the strength is greatly reduced. Known maraging steels with reduced Ti include maraging steels with excellent toughness disclosed in JP-A-10-152759, seamless metal belts disclosed in JP-A-1-42052 and methods for producing the same. Yes.
However, in JP-A-10-152759, N is added positively in the range of 0.005 to 0.03% to improve toughness. Japanese Patent Application Laid-Open No. 1-142052 proposes a Mo content in the range of 3 to 6%, which is the same level as the existing maraging steel containing Ti.
[0007]
The present inventor has found that Ti and N are both kept low to reduce inclusion TiN, which is harmful to fatigue strength improvement, and that the tensile strength decrease due to Ti reduction is increased by Mo, and the value of Co / 3 + Mo + 4Ti It was found that by limiting to a proper range, it can be compensated without a significant increase in alloying elements.
In addition, the amount of Ti does not significantly affect the surface hardness after nitriding, but the absolute value of the surface compressive residual stress due to nitriding is larger when the amount of Ti is smaller and the amount of Mo is larger. The present invention has been newly found and reached the present invention.
[0008]
That is, according to the first invention of the present invention, by weight%, C: 0.008% or less, Si: 0.1% or less, Mn: 0.1% or less, P: 0.010% or less, S: 0.005% or less, Ni: 16-20 %, Mo: Over 6.0% and 9.0% or less, Co: 7.0 to less than 11.0%, Ti: 0.01% or less, Co / 3 + Mo + 4Ti: 8.0 to 13.0, Al: 0.2% or less, N: Less than 0.005%, O: 0.003% Hereinafter, the balance is maraging steel having high fatigue strength composed of Fe and inevitable impurities .
[0009]
According to the second invention, by weight%, C: 0.008% or less, Si: 0.1% or less, Mn: 0.1% or less, P: 0.010% or less, S: 0.005% or less, Ni: 17.5 and 19.0% or less, Mo : 6.5 to 9.0%, Co: 7.0% or more and less than 11.0%, Ti: 0.01% or less, Co / 3 + Mo + 4Ti: 8.0 to 11.0, Al: 0.2% or less, N: 0.004% or less, O: 0.003% or less, balance is Fe And a maraging steel having high fatigue strength composed of inevitable impurities .
[0010]
In addition, the third invention is, by weight%, C: 0.008% or less, Si: 0.1% or less, Mn: 0.1% or less, P: 0.010% or less, S: 0.005% or less, Ni: 17.5 and 19.0% or less , Mo: Over 6.0% to 9.0%, Co: 7.0% to less than 9.0%, Ti: 0.01% or less, Co / 3 + Mo + 4Ti: 8.0 to 11.0, Al: 0.2% or less, N: 0.004% or less, O: 0.003% Hereinafter, the balance is maraging steel having high fatigue strength composed of Fe and inevitable impurities .
In addition, the maraging steel of the present invention can contain B: 0.01% or less by weight%. Moreover, the maraging steel strip of the present invention using the above-described maraging steel can form a nitrided layer on the surface by an appropriate nitriding treatment and impart compressive residual stress to the surface.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
The operation of each element in the present invention will be described below.
C forms Ti and Mo with carbides and carbonitrides, reduces the intermetallic compounds to be precipitated and lowers the strength, so it needs to be kept low. For these reasons, C is set to 0.008% or less.
Si and Mn combine with O, S and the like to form inclusions and reduce fatigue strength, so both are suppressed to 0.10% or less.
[0012]
P, S is a harmful element that segregates or forms inclusions in the prior austenite grain boundaries, embrittles the maraging steel and lowers fatigue strength, so P is 0.01% or less, S was 0.005% or less.
Ni forms a low-C martensite structure, which is the base structure of maraging steel, so at least 16% is necessary, but if it exceeds 20%, the austenite structure stabilizes and martensite transformation is difficult to occur. , Ni was 16 to 20%. More desirably, it is more than 17.5% and not more than 19.0%.
[0013]
Mo is an important element that contributes to precipitation strengthening by forming fine intermetallic compounds such as Ni 3 Mo and Fe 2 Mo during aging treatment. Ti is also an element that contributes to strengthening by aging precipitation, but if Ti is kept low, it is necessary to compensate for the decrease in tensile strength due to Ti by increasing the amount of Mo added.
Mo is an effective element for increasing the surface hardness and compressive residual stress due to nitriding. For this reason, Mo is less than 6.0%, which is insufficient to make up for the decrease in tensile strength.On the other hand, when it exceeds 9.0%, it becomes easy to form a coarse intermetallic compound containing Fe and Mo as main elements. Mo exceeds 6.0% and is 9.0% or less. Desirably, 6.5-9.0% is good.
[0014]
Co promotes the precipitation of fine intermetallic compounds containing Mo by reducing the solid solubility of Mo in the aging precipitation temperature range without greatly affecting the stability of the martensitic structure of the matrix, and strengthens aging It is an important element that contributes to If Co is less than 7.0%, a sufficient effect cannot be obtained. On the other hand, if it is 11.0% or more, the strength tends to be too high and the toughness tends to decrease, so Co was made 7.0% or more and less than 11.0%. Desirably, it is 7.0% or more and less than 9.0%.
[0015]
Ti is inherently one of the important strengthening elements in maraging steel, but at the same time it forms inclusions TiN or Ti (C, N), reducing fatigue strength especially in the ultra-high cycle range. Since it is also a harmful element, it is necessary to keep it low as an impurity when emphasizing fatigue strength.
In addition, Ti easily forms a thin and stable oxide film on the surface. When this oxide film is formed, the nitriding reaction is inhibited, so that it is difficult to obtain a sufficient compressive residual stress on the nitrided surface. In order to easily perform nitriding and to increase the compressive residual stress on the surface after nitriding, Ti is a harmful impurity element and needs to be kept low. If Ti is more than 0.01%, a sufficient effect for reducing TiN or Ti (C, N) cannot be obtained, and a stable oxide film can be easily formed on the surface, so Ti was made 0.01% or less. Preferably it is 0.005% or less.
[0016]
Co, Mo and Ti are all major strengthening elements in maraging steel, but their contributions to strengthening are not the same, and the strengthening by Co and Ti is 1/3 and 4 respectively by the strengthening by Mo .
Therefore, the reinforcement by Co and Mo can be organized by Co / 3 + Mo + 4Ti. If the value of Co / 3 + Mo + 4Ti is less than 8.0%, the strength is not sufficient. On the other hand, if it exceeds 13.0%, the strength becomes too high and the toughness may be reduced, so Co / 3 + Mo + 4Ti is set to 8.0-13.0% . Preferably, the content is 8.0 to 11.0%.
[0017]
Al is added in a small amount for deoxidation, but if it exceeds 0.2%, a large amount of Al 2 O 3 inclusions are formed and the fatigue strength is lowered, so Al was made 0.2% or less.
N is an impurity element that combines with Ti to form inclusions of TiN or Ti (C, N), and lowers fatigue strength particularly in the ultra-high cycle region. In maraging steel containing Ti, N needs to be significantly reduced in order to prevent the formation of coarse TiN or Ti (C, N). However, in maraging steel containing almost no Ti, N is less than 0.005% because there is little adverse effect even if mixed in by normal vacuum melting. Desirably, it is 0.004% or less. More desirably, it is 0.002% or less.
[0018]
O is an impurity element that reduces the toughness and fatigue strength by forming oxide inclusions, so it was limited to 0.003% or less.
B is an element that contributes to strengthening by refining prior austenite crystal grains, and is added as necessary. If B is more than 0.01%, the toughness decreases, so B was made 0.01% or less.
[0019]
This maraging steel contains almost no Ti that forms a stable oxide film on the surface that may inhibit nitriding, so various gas nitriding, gas soft nitriding, sulfur nitriding, ion nitriding, etc. Nitriding can be easily performed.
Further, the maraging steel adjusted within the above-mentioned chemical composition range defined in the present invention is formed into a strip shape so that it can be applied to, for example, a continuously variable transmission part of an automobile engine, and is suitable for this maraging steel strip. When nitriding is performed under various conditions, a thin nitride layer of about 20 to 40 μm can be formed on the surface with almost no nitride, a large compressive residual stress can be applied to the surface, and sufficient fatigue strength can be obtained. .
In addition, although the one where the surface compressive residual stress is higher is preferable, the control is possible by adjusting the thickness of a nitrided layer suitably.
[0020]
【Example】
The steel of the present invention and the comparative steel were melted in a vacuum induction melting furnace to produce a 10 kg ingot, and hot forged. Further, a strip with a thickness of about 0.3 mm was produced by hot rolling and cold rolling. Thereafter, a solution treatment was performed at 825 ° C., an aging treatment was further performed at 490 ° C., and then ion nitriding and gas soft nitriding were performed at 450 to 460 ° C. under a condition that the nitriding depth was 20 to 40 μm.
Table 1 shows the chemical compositions of the inventive steels Nos. 1 to 9 and the comparative steels Nos. 21 to 24. Table 2 shows the internal hardness after aging each sample, the surface hardness after nitriding, and the residual stress on the surface after nitriding. Here, as for the sign of the residual stress in Table 2, + indicates tension and-indicates compression, and all are compressive residual stresses.
Although not shown in the table, in the cross section of the steel of the present invention and the comparative steel, observation and analysis of fine inclusions were performed using an electron microscope and an X-ray analyzer, and comparative steel No. 22 was excluded. It was confirmed that the amount of inclusions of TiN and Ti (C, N) was extremely small in all the test pieces.
[0021]
[Table 1]
[Table 2]
From Table 2, steels Nos. 1 to 9 of the present invention all have an internal hardness after aging of 500 HV or more, have sufficient strength as maraging steel, and are of any nitriding of ion nitriding or gas soft nitriding. Therefore, it can be seen that it has high surface hardness and large surface compressive residual stress. On the other hand, Comparative Steel No. 21 with low values of Mo and Co / 3 + Mo + 4Ti has low internal hardness after aging and compressive residual stress after nitriding treatment, and Comparative Steel No. 24 with low values of Co and Co / 3 + Mo + 4Ti. The internal hardness after aging is in the 400 HV range, and the strength is slightly insufficient.
Also, comparative steel No.23 with high values of Mo, Co and Co / 3 + Mo + 4Ti and comparative steel No.22 with high Ti have small compressive residual stress after nitriding treatment, and it is difficult to obtain large compressive residual stress. I understand.
[0024]
【The invention's effect】
As described above, the maraging steel of the present invention can obtain high strength, high hardness of the surface after nitriding treatment, and large compressive residual stress. Therefore, it is used for power transmission used in continuously variable transmissions for automobiles. When used for a member that requires a high fatigue strength such as a belt, it is expected to have a significant industrial effect such as having a long fatigue life.
Claims (5)
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP16048699A JP4178490B2 (en) | 1999-06-08 | 1999-06-08 | Maraging steel with high fatigue strength and maraging steel strip using it |
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| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP16048699A JP4178490B2 (en) | 1999-06-08 | 1999-06-08 | Maraging steel with high fatigue strength and maraging steel strip using it |
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| JP2000345302A JP2000345302A (en) | 2000-12-12 |
| JP4178490B2 true JP4178490B2 (en) | 2008-11-12 |
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Families Citing this family (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE60033772T2 (en) | 1999-12-24 | 2007-10-31 | Hitachi Metals, Ltd. | Martensitic hardening steel with high fatigue strength and martensitic hardening steel strip |
| FR2816959B1 (en) * | 2000-11-17 | 2003-08-01 | Imphy Ugine Precision | PROCESS FOR MANUFACTURING A STRIP OR A CUT PIECE IN A COLD-ROLLED MARAGING STEEL STRIP |
| CN102605281A (en) * | 2007-07-11 | 2012-07-25 | 日立金属株式会社 | Maraging steel and maraging steel used for metal belt |
| KR20120078757A (en) * | 2007-07-11 | 2012-07-10 | 히타치 긴조쿠 가부시키가이샤 | Maraging steel and maraging steel for metallic belt |
| US20140230968A1 (en) * | 2011-09-30 | 2014-08-21 | Hitachi Metals, Ltd. | Maraging steel |
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