JP2009221553A - Stainless steel for low nickel springs excellent in settling resistance and bendability - Google Patents

Stainless steel for low nickel springs excellent in settling resistance and bendability Download PDF

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JP2009221553A
JP2009221553A JP2008067940A JP2008067940A JP2009221553A JP 2009221553 A JP2009221553 A JP 2009221553A JP 2008067940 A JP2008067940 A JP 2008067940A JP 2008067940 A JP2008067940 A JP 2008067940A JP 2009221553 A JP2009221553 A JP 2009221553A
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Seiichi Isozaki
誠一 磯崎
Satoshi Suzuki
聡 鈴木
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Nisshin Steel Co Ltd
日新製鋼株式会社
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Abstract

<P>PROBLEM TO BE SOLVED: To provide an austenitic stainless steel which exhibits excellent bendability while suppressing Ni to a necessary minimum content, and has settling resistance regarded to be essential as a spring made of a high-strength stainless steel and corrosion resistance in combination. <P>SOLUTION: The stainless steel for a low-Ni spring which is excellent in settling resistance and bendability contains by mass%, 0.10≤C+0.5N≤0.25% (where C<.05%, N>0.05%), Si≤1.5%, 0.5%≤Mn<3.0%, P≤0.06%, S≤0.05%, 1.5%≤Ni<5.0%, 15.0%≤Cr≤19.0%, 0.8%≤Cu≤0.8 to 4.0%, and the balance Fe and inevitable impurities, wherein the austenite stability index Md<SB>30</SB>expressed by formula (1): Md<SB>30</SB>=551-462(C+N)-9.2Si-8.1Mn-29(Ni+Cu)-13.7Cr is 0 to 60 and the lamination defect energy generation index SFE expressed by formula (2): SFE=6.2Ni+18.6Cu+0.7Cr+3.2Mn-53 is ≥0 and <40, and working induced martensite phase is contained by 5 to 50 vol.%, the balance an austenite phase. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

本発明は、Niを節減するため、必要最小限の含有量に抑制しつつも優れた曲げ加工性を発現し、高強度ステンレス製ばねとして必須とされる耐へたり性および耐食性をも兼備する高強度ばね用ステンレス鋼ならびに高強度ステンレスばねに関する。   In order to save Ni, the present invention expresses excellent bending workability while suppressing to the minimum necessary content, and has both sag resistance and corrosion resistance that are essential as a high-strength stainless steel spring. The present invention relates to stainless steel for high strength springs and high strength stainless springs.
SUS301およびSUS304に代表される加工硬化型の準安定オーステナイト系ステンレス鋼は、冷間加工により高強度が得られ、高強度ステンレスばね用素材として多用されている。   Work hardening type metastable austenitic stainless steel represented by SUS301 and SUS304 has high strength by cold working and is frequently used as a material for high strength stainless springs.
しかしながら、昨今のNi原料高騰により、Niを6%以上含有するSUS301や8%以上含有するSUS304などでは、コスト的に活用できない用途も散見されるに至っている。これに対応すべく、近年、以下の特許文献1−4に記される、いわゆる200系ステンレス鋼をベースとした鋼が、300系ステンレス鋼の代替材として提供されつつある。これらの鋼は、ばね性に関連があると思われる材料特性、例えば比例限界応力、耐力、ばね限界値、常温ならびに高温耐へたり性について優れたレベルが得られることを特徴としている。
これらの鋼はNiに代わるオーステナイト形成元素として多くは約4%以上のMnを含有させ、Ni含有量を低減させてコスト節減を図る素材である。
However, due to the recent rise in Ni raw materials, SUS301 containing 6% or more of Ni, SUS304 containing 8% or more of Ni, and the like are often used in a cost-effective manner. In order to cope with this, in recent years, steel based on the so-called 200 series stainless steel described in Patent Documents 1-4 below is being provided as an alternative to the 300 series stainless steel. These steels are characterized by excellent levels of material properties that may be related to spring properties, such as proportional limit stress, yield strength, spring limit, room temperature and high temperature sag resistance.
These steels are materials that reduce the Ni content by reducing the Ni content by containing about 4% or more of Mn as an austenite forming element instead of Ni.
一方、ばね用を上述した文献までの大量のMnを含有させずとも、Niを節減したオーステナイト系ステンレス鋼の技術も提示されている(特許文献5および6)。   On the other hand, a technique of austenitic stainless steel with reduced Ni is also proposed without including a large amount of Mn up to the above-mentioned literature for the spring (Patent Documents 5 and 6).
特開2006−111932号公報JP 2006-111932 A 特開2007−197806号公報JP 2007-197806 A 特開平11−241145号公報JP-A-11-241145 特開平7一70700号公報Japanese Patent Laid-Open No. 7-170700 特公昭60−33186号公報Japanese Patent Publication No. 60-33186 特開2006−22369号公報JP 2006-22369 A
4%以上のMnを含有する技術では、その製鋼、精錬の際に有害なMn酸化物のダストを生成し、環境保全の観点から課題が多い。さらに、ステンレス鋼をリサイクルする際に、従来は非磁性であれば300系スクラップとして処理して来たが、高Mn含有鋼も非磁性であるために、Niを多く含有する有用なスクラップとNiが少なくMnを多量に含有する鋼とを区分することが困難となり、スクラップ市場の混乱を招くことが懸念される。また、Mn含有量が高いことで表面品質が低下し、焼鈍酸洗性や光輝焼鈍などの生産性を損ない、Niを低減したにも関わらず、これらの生産性低下によりその効果が総コスト面で相殺されてしまうという課題があった。   In the technology containing 4% or more of Mn, harmful Mn oxide dust is generated during steelmaking and refining, and there are many problems from the viewpoint of environmental protection. Furthermore, when stainless steel is recycled, conventionally, it has been treated as 300 series scrap if it is non-magnetic. However, since high Mn content steel is also non-magnetic, useful scrap containing a large amount of Ni and Ni Therefore, it is difficult to distinguish from a steel containing a small amount of Mn and a large amount of Mn. In addition, the high Mn content deteriorates the surface quality, impairs the productivity of annealing pickling and bright annealing, etc., and despite the reduction of Ni, the effect of this reduction in productivity is the total cost. There was a problem that it was offset by.
一方、Mnを抑制したNi低減鋼である技術(特許文献5)では、高強度ばねとして優れた耐へたり性を発現させるのに必要とされる加工度が高いため、良好な曲げ性を得ることが困難である。また、特許文献6の技術においては高強度ばねとしての十分な強度が得られないという課題があった。   On the other hand, in the technique (Patent Document 5) which is a Ni-reduced steel in which Mn is suppressed, a high degree of work required to express excellent sag resistance as a high-strength spring is obtained, and thus good bendability is obtained. Is difficult. Moreover, in the technique of patent document 6, there existed a subject that sufficient intensity | strength as a high intensity | strength spring was not obtained.
本発明は、Niを節減しつつもステンレスばね用素材として優れた耐へたり性および曲げ性を有するオーステナイト系ステンレス鋼を提供するものである。   The present invention provides an austenitic stainless steel having excellent sag resistance and bendability as a material for a stainless spring while reducing Ni.
上記課題は、質量%で、0.10%≦C+0.5N≦0.25%(但しC>0.05%、N>0.05%)、Si≦1.5%、0.5%≦Mn<3.0%、P≦0.06%、S≦0.005%、1.5%≦Ni<5.0%、15.0%≦Cr≦19.0%、0.8%≦Cu≦4.0%、残部がFe及び不可避的不純物からなり、下記(1)式で示されるオーステナイト安定度指標Md30が0〜60、下記(2)式で示される積層欠陥エネルギー生成指標SFEが0〜40未満であって、加工誘起マルテンサイト相を5〜50体積%、残部がオーステナイト相からなる、耐へたり性および曲げ性に優れた低Niばね用ステンレス鋼によって達成される。
この低Niばね用ステンレス鋼にはさらに150〜500℃の温度範囲で時効処理が施されていても良い。
Md30=551−462(C+N)−9.2Si−8.1Mn−29(Ni+Cu)−13.7Cr…(1)
SFE=6.2Ni+18.6Cu+0.7Cr+3.2Mn−53 ‥・(2)
The above-mentioned problems are, in mass%, 0.10% ≦ C + 0.5N ≦ 0.25% (where C> 0.05%, N> 0.05%), Si ≦ 1.5%, 0.5% ≦ Mn <3.0%, P ≦ 0.06%, S ≦ 0.005%, 1.5% ≦ Ni <5.0%, 15.0% ≦ Cr ≦ 19.0%, 0.8% ≦ Cu ≦ 4.0%, the balance being Fe and inevitable impurities, austenite stability index Md 30 represented by the following formula (1) is 0 to 60, and stacking fault energy generation index SFE represented by the following formula (2) Is 0 to less than 40, and is achieved by a stainless steel for low Ni springs that is excellent in sag resistance and bendability, in which a work-induced martensite phase is 5 to 50% by volume and the balance is an austenite phase.
The low Ni spring stainless steel may be further subjected to an aging treatment in a temperature range of 150 to 500 ° C.
Md 30 = 551-462 (C + N ) -9.2Si-8.1Mn-29 (Ni + Cu) -13.7Cr ... (1)
SFE = 6.2Ni + 18.6Cu + 0.7Cr + 3.2Mn-53 (2)
ここで、上記(1)および(2)式の元素記号の箇所には質量%で表されたそれぞれの元素の含有値が代入される。   Here, the content value of each element represented by mass% is substituted for the element symbol in the above formulas (1) and (2).
本発明によれば、Ni含有量を5.0質量%未満に節減しつつもMn含有量の多量添加を回避し、ステンレスばねとして優れた耐へたり性と曲げ性、耐食性を兼ね備えたオーステナイト系ステンレス鋼が提供される。
この鋼を用いて製造されるばねは、素材が300系ステンレス鋼である高強度ばねに代替できる。したがって、本発明はコスト面および品質面でNi原料の高騰に対応し得るものである。
According to the present invention, while reducing the Ni content to less than 5.0% by mass, the addition of a large amount of Mn content is avoided, and the austenite system has excellent sag resistance, bendability, and corrosion resistance as a stainless spring. Stainless steel is provided.
A spring manufactured using this steel can be replaced with a high-strength spring made of 300 series stainless steel. Therefore, the present invention can cope with the soaring Ni raw material in terms of cost and quality.
本発明者らは、Ni含有量を5.0質量%未満に抑制したオーステナイト系ステンレス鋼において、上記課題を達成すべく鋭意研究し、以下の知見を得るに至った。   The present inventors diligently studied to achieve the above-mentioned problems in the austenitic stainless steel in which the Ni content is suppressed to less than 5.0% by mass, and have obtained the following knowledge.
まず、ばねとして優れた耐久性を発現させるために、その素材は優れた耐へたり性を有していることが前提となり、それは素材を高強度化することで達成される。オーステナイト系ステンレス鋼をベースに高強度を得る手段として最も有効な手段は、冷間圧延などの加工を付与してオーステナイトを加工硬化させるとともに、オーステナイトの一部を硬質な加工誘起変態させる、いわゆる加工誘起変態塑性(TRIP)現象を利用することである。ばねへの成形工程でも加工ひずみが付与された部分ではTRIP現象により硬化し、このTRIP現象の起こりやすさはオーステナイト安定度により左右される。高強度化すればするほど耐へたり性は向上するものの、ばねとしての加工性、例えば後述の曲げ性は低下する。ばね素材として優れた加工性を維持しつつ、ばねとして良好な耐へたり性を得る上で、オーステナイト安定度およびばね素材の加工誘起マルテンサイト相の量を調整する必要があることが明らかとなった。   First, in order to develop excellent durability as a spring, it is premised that the material has excellent sag resistance, which is achieved by increasing the strength of the material. The most effective means for obtaining high strength based on austenitic stainless steel is the so-called processing, in which processing such as cold rolling is applied to harden austenite and hard-induced transformation of a part of austenite. It is to use the induced transformation plasticity (TRIP) phenomenon. Even in the forming process to the spring, the portion to which processing strain is applied is cured by the TRIP phenomenon, and the susceptibility of the TRIP phenomenon depends on the austenite stability. As the strength is increased, the sag resistance is improved, but the workability as a spring, for example, the bendability described later is lowered. It is clear that it is necessary to adjust the austenite stability and the amount of work-induced martensite phase of the spring material in order to obtain good sag resistance as a spring while maintaining excellent workability as a spring material. It was.
一方、曲げ性は引張試験などで評価される伸びとある程度相関があり、伸びにはTRIP現象と加工誘起マルテンサイトの強度を左右する固溶強化元素であるC,N含有量が深く関与していることが分かった。すなわち、オーステナイト安定度ならびにC,N量を適正範囲に調整することが必須とされた。ただし、これのみでは安定して優れた曲げ性が得られないことが分かった。そこで、本発明者らはさらに鋭意研究を重ねた結果、積層欠陥エネルギーの生成指標であるSFEを適正範囲に調整することで安定して良好な曲げ性が得られることが明らかとなった。この理由として、SFEが大きいとオーステナイトの加工硬化が小さくなるために加工誘起マルテンサイトとオーステナイトとの硬度差が大きくなること、逆にSFEが小さいとオーステナイトの加工硬化が大きくなるためにオーステナイトの延性が低下し、このいずれも曲げ性を低下させる要因となるためであると推定される。   On the other hand, the bendability has a certain correlation with the elongation evaluated by a tensile test or the like, and the elongation is caused by the content of C and N, which are solid solution strengthening elements that influence the strength of the TRIP phenomenon and the processing-induced martensite. I found out. That is, it has been essential to adjust the austenite stability and the amounts of C and N to an appropriate range. However, it was found that only this could not provide a stable and excellent bendability. Therefore, as a result of further earnest studies, the present inventors have found that stable bendability can be obtained stably by adjusting SFE, which is a generation index of stacking fault energy, to an appropriate range. The reason for this is that when SFE is large, the work hardening of austenite becomes small, so that the hardness difference between work-induced martensite and austenite becomes large, and conversely, when SFE is small, the work hardening of austenite becomes large and ductility of austenite becomes large. It is presumed that this is due to the fact that both of these are factors that reduce bendability.
「成分元素」
以下、本発明鋼に含まれる合金成分ならびに含有範囲限定理由について説明する。
1)CおよびN
C,Nは、加工誘起マルテンサイト(α’)相を固溶強化するために有用な元素である。本発明鋼においてはCに対するNの固溶強化の寄与はおおよそ半分であり、α’相の生成の際、TRIPによる十分な延性を発現させるためには、C質量%+0.5×N質量%(以下、C+0.5Nと略記)を0.10質量%以上とする必要がある。また、C,Nとも0.05質量%を越える含有量を確保することが顕著な延性向上作用を安定して得るために重要である。一方、C、Nの含有量が多くなりすぎると過度に硬質化し、加工性を阻害する要因となる。この傾向は(C+0.5N)が0.25質量%を越えると顕著に現れるため、これ以下となるように調整する必要がある。より好ましくは、C含有量が0.12質量%以下、N含有量が0.18質量%以下で調整されるのが良い。
"Constituent elements"
Hereinafter, the alloy components contained in the steel of the present invention and the reasons for limiting the content range will be described.
1) C and N
C and N are useful elements for strengthening the solution-induced martensite (α ′) phase by solid solution strengthening. In the steel of the present invention, the contribution of the solid solution strengthening of N to C is approximately half, and in order to develop sufficient ductility by TRIP when the α ′ phase is formed, C mass% + 0.5 × N mass% (Hereinafter abbreviated as C + 0.5N) needs to be 0.10% by mass or more. Further, it is important to secure a content exceeding 0.05% by mass for both C and N in order to stably obtain a remarkable ductility improving effect. On the other hand, if the contents of C and N are too large, the content becomes excessively hard, which becomes a factor that hinders workability. This tendency appears prominently when (C + 0.5N) exceeds 0.25% by mass, and it is necessary to adjust the tendency to be less than this. More preferably, the C content is adjusted to 0.12% by mass or less and the N content is adjusted to 0.18% by mass or less.
2)Si
Siは、製鋼での脱酸に有用な元素であるが、1.5質量%を越えて過剰に含有させると鋼が硬質化し加工性を損なう要因となる。また、Siはフェライト生成元素であるため、過剰添加は高温域でのδフェライト相の多量生成を招き、熱間加工性を阻害する。したがって、Si含有量は1.5質量%以下に制限される。
2) Si
Si is an element useful for deoxidation in steelmaking. However, if it is contained in excess of 1.5% by mass, the steel becomes hard and impairs workability. Further, since Si is a ferrite-forming element, excessive addition causes a large amount of δ-ferrite phase to be generated at a high temperature range, thereby impairing hot workability. Therefore, the Si content is limited to 1.5% by mass or less.
3)Mn
MnはNiに比べて安価で、Niの機能を代替できる有用なオーステナイト形成元素である。本発明においてその機能を活用するために0.5%以上のMn含有量を確保する必要がある。一方、Mn含有量が過剰となると、製鋼工程における環境保全の問題が生じやすくなる。また、表面性状に起因する生産性の低下ならびにMnSなどの介在物生成に起因する曲げ性の劣化を引き起こす要因となる。このため、Mn含有量は3.0質量%未満、好ましくは2.5質量%未満に制限される。
3) Mn
Mn is a useful austenite-forming element that is less expensive than Ni and can substitute for the function of Ni. In order to utilize the function in the present invention, it is necessary to secure a Mn content of 0.5% or more. On the other hand, if the Mn content is excessive, environmental conservation problems in the steel making process are likely to occur. Moreover, it becomes a factor which causes the fall of productivity resulting from the surface property, and the deterioration of bendability resulting from inclusion production | generations, such as MnS. For this reason, the Mn content is limited to less than 3.0% by mass, preferably less than 2.5% by mass.
4)PおよびS
PおよびSは不可避的不純物として混入するが、その含有量は低いほど望ましく、曲げ性その他の材料特性や製造性に多大な悪影響を与えない範囲として、Pについては0.06質量%以下、Sは0.005質量%以下に規定した。
4) P and S
P and S are mixed as unavoidable impurities, but the content is preferably as low as possible. As a range that does not have a great adverse effect on bendability and other material properties and manufacturability, P is 0.06% by mass or less, S Was defined as 0.005 mass% or less.
5)Ni
Niはオーステナイト系ステンレス鋼に必須の元素であるが、本発明ではコスト低減の観点からNi含有量を極力低く抑える成分設計を行っており、上限を5.0質量%未満に規定する。ただし、上記Mn含有量の範囲で製造性や加工性を兼備させる成分バランスを実現させるためには1.5質量%以上のNi含有量を確保する必要がある。
5) Ni
Ni is an essential element for austenitic stainless steel. However, in the present invention, a component design is performed to keep the Ni content as low as possible from the viewpoint of cost reduction, and the upper limit is specified to be less than 5.0% by mass. However, in order to realize a component balance that combines manufacturability and workability within the range of the Mn content, it is necessary to secure a Ni content of 1.5% by mass or more.
6)Cr
Crはステンレス鋼の耐食性を担保する不動態皮膜の形成に必須の元素である。Cr含有量が15.0質量%未満であると、本発明の代替対象となる従来の300系オーステナイト系ステンレス鋼に要求される耐食性が十分に確保できない場合がある。ただし、Crはフェライト生成元素であるため、過度のCr含有は高温域でのδフェライト相の多量生成を招き、熱間加工性を損なう要因となるため好ましくない。種々検討の結果、本発明では19.0質量%までCrを含有させることができる。したがって、Cr含有量は15.0〜19.0質量%に規定される。
6) Cr
Cr is an essential element for forming a passive film that ensures the corrosion resistance of stainless steel. When the Cr content is less than 15.0% by mass, the corrosion resistance required for the conventional 300 series austenitic stainless steel that is an alternative object of the present invention may not be sufficiently secured. However, since Cr is a ferrite-forming element, excessive Cr content is not preferable because it causes a large amount of δ-ferrite phase to be generated in a high temperature range and causes a deterioration in hot workability. As a result of various studies, the present invention can contain Cr up to 19.0% by mass. Therefore, Cr content is prescribed | regulated to 15.0-19.0 mass%.
7)Cu
Cuはオーステナイト生成元素であることから、Cu含有量の増加に応じてNi含有量の設定自由度が拡大し、Niを抑制した成分設計が容易になる。また、加工誘起マルテンサイト相の生成に起因する加工硬化が抑制されるとともに、SFE値を高める上で有効な元素でもある。これらの作用を有効に得るためには0.8質量%以上のCu含有量を確保する必要がある。ただし、4.0質量%を越える多量のCu含有は熱間加工性を阻害しやすい。このため、Cu含有量は0.8〜4.0質量%に規定される。
7) Cu
Since Cu is an austenite-generating element, the degree of freedom in setting the Ni content increases with an increase in Cu content, and component design that suppresses Ni becomes easy. Moreover, it is an element effective in raising the SFE value while suppressing the work hardening resulting from the production | generation of a process induction martensite phase. In order to effectively obtain these effects, it is necessary to secure a Cu content of 0.8% by mass or more. However, a large amount of Cu exceeding 4.0% by mass tends to hinder hot workability. For this reason, Cu content is prescribed | regulated to 0.8-4.0 mass%.
本発明鋼は、上記の成分に加えて、熱間加工性確保を目的としたB、Caの1種あるいは2種、耐食性向上を目的としたMoを含有することができる。ただし含有される場合には、BあるいはCaは総量で0.0070質量%以下、Moは1.5質量%以下で含有されるのが望ましい。   In addition to the above components, the steel of the present invention can contain one or two of B and Ca for ensuring hot workability, and Mo for improving corrosion resistance. However, when contained, it is desirable that B or Ca is contained in a total amount of 0.0070 mass% or less and Mo is contained in 1.5 mass% or less.
(1)式で表されるオーステナイト安定度指標Md30が大きいほどオーステナイトからα’相への変態が起こり易いことから、軽度の冷延ひずみの付与で高強度が得られるとともに、優れた延性を確保することができる。また、ばねへの成形工程においても曲げ部など加工ひずみが付与された部分はTRIP現象によりさらに高強度化し、優れた耐へたり性を発現しやすい。このような効果はMd30が0以上で顕著に現れる。ただし、Md30が60を越えて大きくなると、曲げ加工を施した部分におけるα’生成量が多くなり過ぎるために割れが誘発され、曲げ性が劣化する。したがって、Md30は0〜60の範囲に規定した。 Since the transformation from austenite to α ′ phase is more likely to occur as the austenite stability index Md 30 represented by the formula (1) is larger, high strength can be obtained by applying mild cold rolling strain, and excellent ductility can be obtained. Can be secured. In addition, even in the forming process of the spring, a portion to which processing strain is applied, such as a bent portion, is further increased in strength by the TRIP phenomenon and easily exhibits excellent sag resistance. Such an effect is prominent when Md 30 is 0 or more. However, if Md 30 exceeds 60, the amount of α ′ generated in the portion subjected to the bending process becomes excessive, so that cracks are induced and the bendability deteriorates. Therefore, Md 30 was specified in the range of 0-60.
(2)式で表される積層欠陥エネルギー指標SFEは、良好な曲げ性を安定して得る上で0〜40未満の範囲に規定した。この理由として、前述の通り、SFEが40以上となるとオーステナイトの加工硬化が小さくなるためにα’相とオーステナイト相との硬度差が大きくなり、曲げ加工時にはα’相/オーステナイト界面近傍でき裂が生じやすくためと考えられる。また、逆にSFEが0未満の場合にはオーステナイトの加工硬化が大きくなることによる延性低下が顕著に起こるようになるためと推定される。   The stacking fault energy index SFE represented by the formula (2) is defined within a range of 0 to less than 40 in order to stably obtain good bendability. The reason for this is that, as described above, when the SFE is 40 or more, the work hardening of austenite becomes small, so the hardness difference between the α ′ phase and the austenite phase becomes large, and cracking occurs near the α ′ phase / austenite interface during bending. It is thought that it is easy to occur. On the other hand, when SFE is less than 0, it is presumed that a decrease in ductility due to an increase in work hardening of austenite occurs remarkably.
さらに、加工誘起マルテンサイトを5〜50体積%含有し、他はオーステナイト相となるように金属組織が調整される。この加工誘起マルテンサイトは、最終焼鈍後に行われる冷間あるいは温間での調質圧延や加工により導入される。本発明鋼では、加工誘起マルテンサイトが5体積%以上となるようにひずみが付与されることで優れた耐へたり性を発現するようになる。耐へたり性は強度が高いほど、つまり加工誘起マルテンサイト量が多いほど優れるが、その反面、曲げ成形時における割れ発生頻度が増し、ばね成形素材の加工誘起マルテンサイトが50体積%を超えると曲げ加工時の割れ発生が顕著となる。なお、オーステナイト、加工誘起マルテンサイト以外に2体積%以下のフェライトを含んでいても良い。   Further, the metal structure is adjusted so that 5 to 50% by volume of processing-induced martensite is contained and the others are in the austenite phase. This work-induced martensite is introduced by cold or warm temper rolling or work performed after the final annealing. In the steel of the present invention, excellent sag resistance is exhibited by applying strain so that the work-induced martensite is 5% by volume or more. The higher the strength, that is, the greater the amount of work-induced martensite, the better. However, on the other hand, the frequency of occurrence of cracks during bending is increased, and the work-induced martensite of the spring forming material exceeds 50% by volume. Cracks during bending are prominent. In addition to austenite and work-induced martensite, it may contain 2% by volume or less of ferrite.
以上のように化学成分および金属組織が調整された本発明鋼にさらに時効処理を施しても良い。時効処理を施すことにより、調質圧延などで付与されたひずみが一部回復すると同時に、ひずみ時効強化あるいはCuによる析出強化が起こるため、高強度を維持した状態で曲げ性を向上させることが可能である。このような効果は、時効温度が150℃で現れる。一方、時効処理温度が500℃を超えるとひずみの回復が大きくなるとともにひずみ時効強化や析出強化の効果が小さくなるために、強度が急激に低下する。したがって、時効処理温度は150〜500℃に規定した。時効処理時間は特に規定しないが、均熱時間が0s〜5hであることが望ましい。   As described above, the steel of the present invention in which the chemical composition and the metal structure are adjusted may be further subjected to an aging treatment. By applying an aging treatment, strain applied by temper rolling partially recovers, and at the same time, strain aging strengthening or precipitation strengthening by Cu occurs, so it is possible to improve bendability while maintaining high strength. It is. Such an effect appears at an aging temperature of 150 ° C. On the other hand, when the aging treatment temperature exceeds 500 ° C., the recovery of strain increases and the effects of strain aging strengthening and precipitation strengthening decrease, so that the strength rapidly decreases. Therefore, the aging treatment temperature was regulated to 150 to 500 ° C. Although the aging treatment time is not particularly defined, it is desirable that the soaking time is 0 s to 5 h.
本発明鋼は、一般的なオーステナイト系ステンレス鋼板の製造プロセスにより製造可能である。熱間圧延以降の中間焼鈍あるいは仕上焼鈍は1050〜1100℃の範囲で行うことが望ましい。また、仕上焼鈍後は目標硬さに応じた調質圧延が施され、例えば板厚0.1〜3mmの調質圧延鋼板とすることができる。その後、形状矯正や前述の時効温度範囲における連続時効処理が適宜実施されても良い。   The steel of the present invention can be manufactured by a general austenitic stainless steel sheet manufacturing process. It is desirable to perform intermediate annealing or finish annealing after hot rolling in the range of 1050 to 1100 ° C. Moreover, after finish annealing, the temper rolling according to target hardness is given, for example, it can be set as the temper rolled steel plate of plate thickness 0.1-3mm. Thereafter, shape correction and continuous aging treatment in the above-described aging temperature range may be appropriately performed.
(実施例1)
表1の組成をもつ各種ステンレス鋼A〜Lを溶製した。表1において、A1〜A11が本発明で規定する化学成分を有する発明対象鋼、B1〜8が比較鋼、C1、2はそれぞれ従来鋼SUS301、SUS304である。なお、B1およびB2はSFE、B3およびB4はMd30、B5およびB6はC+0.5Nの値、B7はMn含有量、B8はS含有量が本発明で規定する範囲を外れる。
Example 1
Various stainless steels A to L having the compositions shown in Table 1 were melted. In Table 1, A1 to A11 are invention steels having chemical components defined in the present invention, B1 to 8 are comparative steels, and C1 and 2 are conventional steels SUS301 and SUS304, respectively. B1 and B2 are SFE, B3 and B4 are Md 30 , B5 and B6 are C + 0.5N, B7 is Mn content, and B8 is outside the range defined by the present invention.
各鋼とも100kgの鋼塊を得た後に、抽出温度1230℃で熱間圧延することにより板厚3mmの熱延鋼帯を製造した。熱延鋼帯に1080℃で均熱1分の焼鈍を施した後、冷間圧延、焼鈍を繰り返すことにより、硬さが450±3HV5、板厚が0.50±0.003mmの調質圧延鋼帯を得た。なお、調質圧延後の硬さが450HV5となる調質圧延率をそれぞれの鋼についてあらかじめ調べておき、その調質圧延率をもとに仕上焼鈍時の板厚を設定し、その板厚まで冷間圧延を行った後に1080℃で均熱1分の仕上焼鈍を実施した。その後の板厚0.5mmまでの調質圧延では、板温が70℃となるよう加温した上で7〜10パスで行った。   After obtaining a steel ingot of 100 kg for each steel, a hot-rolled steel strip having a thickness of 3 mm was manufactured by hot rolling at an extraction temperature of 1230 ° C. After subjecting the hot-rolled steel strip to annealing at 1080 ° C for 1 minute, temper rolling with a hardness of 450 ± 3HV5 and a thickness of 0.50 ± 0.003mm by repeating cold rolling and annealing A steel strip was obtained. In addition, the temper rolling rate at which the hardness after temper rolling becomes 450 HV5 is examined in advance for each steel, and the plate thickness at the time of finish annealing is set based on the temper rolling rate, up to the plate thickness After cold rolling, finish annealing was performed at 1080 ° C. for 1 minute. Subsequent temper rolling to a sheet thickness of 0.5 mm was performed in 7 to 10 passes after the sheet temperature was raised to 70 ° C.
上記の板厚0.5mmの調質圧延材を用いて、加工誘起マルテンサイト相量の測定および耐へたり性、曲げ性の調査を行った。加工誘起マルテンサイト量は、径5mmの円盤を採取後、エッジをリン酸硫酸中にて電解研磨したサンプルを用い、4枚重ね合わせて振動試料型磁力計により測定した。耐へたり量は、60mm各の試験片の中央に径28mmの円孔を打ち抜いた後、金型を用いて荷重50kNで突起を成形した。図1に耐へたり性評価試験片の外観模式図を示す。突起を成形後常温において荷重18〜21kNで平押し加工することにより、各供試材の突起高さを212±1μmに調整した。その後、50kNの荷重を負荷し、4サイクル繰り返した後の突起高さを45°おきに8点測定し平均化した値を平均突起高さとした。
この実験方法によれば、実際のばね形状に成形加工し、セッチングと呼ばれる工程を経て実使用された際の耐へたり性をシミュレートすることが可能であり、平均突起高さが高いほど耐へたり性に優れると評価される。
Using the temper rolled material having a thickness of 0.5 mm, the work-induced martensite phase amount was measured and the sag resistance and bendability were investigated. The amount of work-induced martensite was measured with a vibrating sample type magnetometer using four samples obtained by collecting a disk having a diameter of 5 mm and then electrolytically polishing the edges in phosphoric acid sulfuric acid. With respect to the amount of sag resistance, a circular hole having a diameter of 28 mm was punched out at the center of each 60 mm test piece, and then a protrusion was formed with a load of 50 kN using a mold. FIG. 1 is a schematic external view of a test piece for evaluating sag resistance. The protrusion height of each test material was adjusted to 212 ± 1 μm by flat pressing with a load of 18 to 21 kN at room temperature after forming the protrusion. Thereafter, a load of 50 kN was applied, and the height of the protrusion after repeating 4 cycles was measured at 8 points every 45 ° and averaged to obtain the average protrusion height.
According to this experimental method, it is possible to simulate sag resistance when it is molded into an actual spring shape and actually used through a process called setting. It is evaluated as having excellent sagability.
曲げ性は、幅25mm、長さ50mmの短冊片を、長さ方向が圧延方向と直角となるように採取し、90°のV曲げ試験を行った。曲げ稜線が圧延方向となるように試験片をセットし、先端のRが0.8〜1.5mmの金型を用いて20kNの荷重を負荷して曲げを行った後、曲げ部外周側をマイクロスコープを用いて150倍で観察し、割れによる開口が認められなかった最大のRを曲げ限界R(m)とした。
表2に各鋼の加工誘起マルテンサイト量、平均突起高さおよび曲げ限界Rを示す。
As for bendability, strips having a width of 25 mm and a length of 50 mm were collected so that the length direction was perpendicular to the rolling direction, and a 90 ° V-bending test was performed. Set the test piece so that the bending ridge line is in the rolling direction, and after bending by applying a load of 20 kN using a die having a tip R of 0.8 to 1.5 mm, Observation with a microscope at a magnification of 150 times, the maximum R in which no opening due to cracking was observed was defined as the bending limit R (m).
Table 2 shows the amount of work-induced martensite, average protrusion height, and bending limit R of each steel.
本発明鋼は加工誘起マルテンサイト量が14〜48体積%であり、平均突起高さ90μm以上、曲げ限界Rは1.0mm以下と、いずれも優れた耐へたり性および曲げ性を有している。一方、比較鋼のB3、B5、B7および現行材C1、C2の平均突起高さは86μm以下であり、本発明鋼に比べ耐へたり性に劣る。
曲げ限界Rについては比較鋼、従来鋼とも1.2mm以上であり、本発明鋼に比べ曲げ性に劣る。本実施例のように、調質圧延により高強度化した鋼にて優れた耐へたり性と曲げ性を両立させるには本発明で規定した化学成分に調整する必要があることが確認された。
The steel of the present invention has a work-induced martensite amount of 14 to 48% by volume, an average protrusion height of 90 μm or more, and a bending limit R of 1.0 mm or less, both having excellent sag resistance and bendability. Yes. On the other hand, the average protrusion heights of the comparative steels B3, B5, and B7 and the current materials C1 and C2 are 86 μm or less, which is inferior to the steel of the present invention in terms of sag resistance.
The bending limit R is 1.2 mm or more for both the comparative steel and the conventional steel, which is inferior in bendability compared to the steel of the present invention. As in this example, it was confirmed that it was necessary to adjust to the chemical components defined in the present invention in order to achieve both excellent sag resistance and bendability in steel strengthened by temper rolling. .
(実施例2)
本発明鋼の鋼No.A1およびA7について、仕上板厚は0.5mmとしつつ、調質圧延率を変化させることで加工誘起マルテンサイト相量を変化させた鋼の造り込みを行い、実施例1に示す実験方法により平均突起高さおよび曲げ限界Rを測定した。表3に測定結果を示す。なお、試験番号X3およびX11はそれぞれ表2に記載の鋼A1およびA7の測定値と同一である。
(Example 2)
Steel No. of the present invention steel. For A1 and A7, steel with a work-induced martensite phase amount changed by changing the temper rolling ratio while the finished plate thickness was 0.5 mm was averaged by the experimental method shown in Example 1. The protrusion height and the bending limit R were measured. Table 3 shows the measurement results. The test numbers X3 and X11 are the same as the measured values of steels A1 and A7 described in Table 2, respectively.
本発明例ではいずれも加工誘起マルテンサイト相量が6〜44体積%であり、平均突起高さ90μm以上、曲げ限界Rは1.0mm以下と、いずれも優れた耐へたり性および曲げ性を有している。一方、比較例の試験番号X1、X2およびX8の平均突起高さは82μm以下であり、本発明鋼に比べ耐へたり性に劣る。これは加工誘起マルテンサイト相量が少ないために突起部の強度が低かったためであると推定される。
一方、試験番号X7、X12の曲げ限界Rは1.4mm以上であり、本発明例に比べ曲げ性に劣る。これは、過剰のマルテンサイトを生成させたために曲げ割れ感受性が高まったためと思われる。本実施例より、本発明で規定した範囲に加工誘起マルテンサイト相量を調整する必要があることが確認された。
In all of the examples of the present invention, the processing-induced martensite phase amount is 6 to 44% by volume, the average protrusion height is 90 μm or more, and the bending limit R is 1.0 mm or less, both having excellent sag resistance and bendability. Have. On the other hand, the average protrusion heights of the test numbers X1, X2 and X8 of the comparative examples are 82 μm or less, which is inferior in sag resistance compared to the steel of the present invention. This is presumably because the strength of the protrusions was low due to the small amount of work-induced martensite phase.
On the other hand, the bending limit R of test numbers X7 and X12 is 1.4 mm or more, which is inferior to the bendability as compared with the examples of the present invention. This seems to be due to the increased sensitivity to bending cracks caused by the formation of excess martensite. From this example, it was confirmed that it was necessary to adjust the amount of work-induced martensite phase within the range defined by the present invention.
(実施例3)
本発明鋼の鋼No.A1の、加工誘起マルテンサイト相量が32体積%である鋼(実施例2表3中の試験番号X5について、50〜600℃で在炉30minの時効処理を施し、実施例1に示す実験方法により平均突起高さおよび曲げ限界Rを測定した。表4に測定結果を示す。なお、表中には時効処理後にマルテンサイト量を調査した結果も付した。
(Example 3)
Steel No. of the present invention steel. A1 steel having a work-induced martensite phase amount of 32% by volume (Example 2 Test method X5 in Table 3 was subjected to aging treatment at 50 to 600 ° C. for 30 minutes in the furnace, and the experimental method shown in Example 1 The average protrusion height and the bending limit R were measured by the following, and the measurement results are shown in Table 4. In the table, the results of investigating the amount of martensite after aging treatment were also attached.
時効処理温度が50℃である試験番号Y1は、時効処理なしの場合と同一の平均突起高さおよび曲げ限界Rであり、時効処理の効果が現れない。100℃以上の時効処理により曲げ限界Rが小さくなり、曲げ性が向上することが認められる。時効処理温度が高くなるにともない平均突起高さが小さくなる傾向を示す。その傾向は時効処理温度が500℃を越える領域で特に顛著となり、平均突起硬さは83μm以下となる。以上より、本発明で規定した範囲で時効処理を施すことにより、耐へたり性を損なうことなく曲げ性をさらに向上し得ることが確認された。   Test number Y1 having an aging treatment temperature of 50 ° C. has the same average protrusion height and bending limit R as those without aging treatment, and the effect of aging treatment does not appear. It is recognized that the bending limit R is reduced by aging treatment at 100 ° C. or higher, and the bendability is improved. The average protrusion height tends to decrease as the aging treatment temperature increases. This tendency is particularly remarkable in the region where the aging treatment temperature exceeds 500 ° C., and the average protrusion hardness is 83 μm or less. From the above, it was confirmed that the bendability can be further improved without impairing the sag resistance by performing the aging treatment within the range defined in the present invention.
実施例で用いた耐へたり性試験片の外観模式図を示す。The external appearance schematic diagram of the sag resistance test piece used in the Example is shown.

Claims (2)

  1. 質量%で、0.10%≦C+0.5N≦0.25%(但しC>0.05%、N>0.05%)、Si≦1.5%、0.5%≦Mn<3.0%、P≦0.06%、S≦0.005%、1.5%≦Ni<5.0%、15.0%≦Cr≦19.0%、0.8%≦Cu≦4.0%を含み、残部がFe及び不可避的不純物からなり、
    下記(1)式で示されるオーステナイト安定度指標Md30が0〜60、下記(2)式で示される積層欠陥エネルギー生成指標SFEが0〜40未満であって、加工誘起マルテンサイト相を5〜50体積%、残部がオーステナイト相からなる、耐へたり性および曲げ性に優れた低Niばね用ステンレス鋼。
    Md30=551−462(C+N)−9.2Si−8.1Mn−29(Nl+Cu)−13.7Cr・‥(1)
    SFE=6.2Ni+18.6Cu+0.7Cr+3.2Mn−53‥・(2)
    0.10% ≦ C + 0.5N ≦ 0.25% (where C> 0.05%, N> 0.05%), Si ≦ 1.5%, 0.5% ≦ Mn <3. 0%, P ≦ 0.06%, S ≦ 0.005%, 1.5% ≦ Ni <5.0%, 15.0% ≦ Cr ≦ 19.0%, 0.8% ≦ Cu ≦ 4. Containing 0%, the balance consisting of Fe and inevitable impurities,
    The austenite stability index Md 30 represented by the following formula (1) is 0 to 60, the stacking fault energy generation index SFE represented by the following formula (2) is 0 to less than 40, and the work-induced martensite phase is 5 to 5. Stainless steel for low Ni springs, 50% by volume, the balance being an austenitic phase and excellent in sag resistance and bendability.
    Md 30 = 551-462 (C + N ) -9.2Si-8.1Mn-29 (Nl + Cu) -13.7Cr · ‥ (1)
    SFE = 6.2Ni + 18.6Cu + 0.7Cr + 3.2Mn-53 (2)
  2. 請求項1に記載のステンレス鋼に、更に150〜500℃の温度域で時効処理が施された、耐へたり性および曲げ性に優れた低Niばね用ステンレス鋼。   A stainless steel for low Ni springs having excellent sag resistance and bendability, wherein the stainless steel according to claim 1 is further subjected to an aging treatment in a temperature range of 150 to 500 ° C.
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JP2014185367A (en) * 2013-03-22 2014-10-02 Nippon Steel & Sumikin Stainless Steel Corp Stainless steel wire excellent in twisting processability and manufacturing method therefor, and stainless steel wire and manufacturing method therefor
JP2014189800A (en) * 2013-03-26 2014-10-06 Nisshin Steel Co Ltd LOW Ni AUSTENITIC STAINLESS STEEL SHEET AND MOLDED ARTICLE THEREOF
JP2014189802A (en) * 2013-03-26 2014-10-06 Nisshin Steel Co Ltd LOW Ni AUSTENITIC STAINLESS STEEL SHEET EXCELLENT IN AGE HARDENING PROPERTY AND METHOD OF PRODUCING THE SAME
CN104136645A (en) * 2012-03-29 2014-11-05 新日铁住金不锈钢株式会社 High-strength stainless steel wire having excellent heat deformation resistance, high-strength spring, and method for manufacturing same

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JP2010189719A (en) * 2009-02-18 2010-09-02 Nisshin Steel Co Ltd Age-hardening type stainless steel sheet for spring
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CN104136645A (en) * 2012-03-29 2014-11-05 新日铁住金不锈钢株式会社 High-strength stainless steel wire having excellent heat deformation resistance, high-strength spring, and method for manufacturing same
JP2014185367A (en) * 2013-03-22 2014-10-02 Nippon Steel & Sumikin Stainless Steel Corp Stainless steel wire excellent in twisting processability and manufacturing method therefor, and stainless steel wire and manufacturing method therefor
JP2014189800A (en) * 2013-03-26 2014-10-06 Nisshin Steel Co Ltd LOW Ni AUSTENITIC STAINLESS STEEL SHEET AND MOLDED ARTICLE THEREOF
JP2014189802A (en) * 2013-03-26 2014-10-06 Nisshin Steel Co Ltd LOW Ni AUSTENITIC STAINLESS STEEL SHEET EXCELLENT IN AGE HARDENING PROPERTY AND METHOD OF PRODUCING THE SAME

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