JP4116762B2 - High strength spring steel excellent in hydrogen fatigue resistance and method for producing the same - Google Patents
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Description
【０００１】
【産業上の利用分野】
本発明は、自動車等のエンジンの弁ばねや懸架ばね、スタビライザー、トーションバー等に用いられる1700MPa以上の引張強度を有する高強度ばねに関し、特に重要なばね特性である耐水素疲労特性の優れた高強度ばね用鋼、及びその製造方法に関するものである。
【０００２】
【従来の技術】
自動車等に数多く使用されている高強度ばねは、例えばJIS G 3565〜3567及び4801等に規定されているばね用鋼を用いて熱間圧延後、所定の線径まで引き抜き加工し、オイルテンパー処理後にばね加工する、あるいは引き抜き加工後に加熱してばね加工し、焼入れ焼戻しを行う、という方法によって製造される。近年炭酸ガス排出低減などの、環境問題対応のために、自動車には燃費低減のため、軽量化が求められている。その一環として、焼入れ焼戻し後の引張強度を1800MPa以上に高めたばねが求められている。しかしながら一般にばねを高強度化すると、腐食環境下における疲労特性が劣化するため、早期折損が懸念される。腐食疲労特性を劣化させる一因として、腐食反応の進行に伴って発生する水素による脆化があげられ、その改善策としては、種々の合金元素を多量に添加して高強度化を図るという方法が採用されてきたが、この方法では素材のコストが高くなるという問題がある。また、水素疲労特性を抑制する方法としては、結晶粒を微細化させる方法や、微細析出物を生成させる方法が有力と考えられているが、いずれの提案も本発明者らの試験では、大幅な水素脆化特性の改善には至っていない。
【０００３】
以上のように、従来の技術では、耐水素疲労特性を抜本的に向上させた高強度ばねを製造することには限界があった。
【０００４】
【発明が解決しようとする課題】
本発明は上記の如き実状に鑑みなされたものであって、耐水素疲労特性の良好で且つ引張強度が1700MPa以上の高強度ばね用鋼を実現するとともにその製造方法を提供することを目的とするものである。
【０００５】
【課題を解決するための手段】
本発明者らは、まず焼入れ・焼戻し処理によって製造した種々の強度レベルのばね用鋼を用いて、水素疲労挙動を詳細に解析した。その結果、疲労限以下の応力で、疲労寿命が鋼材中の水素によって低下することを明らかにした。また、疲労寿命の低下は、外部環境から鋼材中に侵入し、鋼材中を室温で拡散しうる拡散性水素に起因して発生していることを明らかにした。拡散性水素は、鋼材を１００℃／ｈｏｕｒの速度で加熱した際に得られる温度-鋼材からの水素放出速度の曲線において、約１００℃の温度にピークを有する曲線として測定できる（図１）。従って、環境から侵入した水素を鋼材中の何らかの部分に捕捉することによって拡散しないようにすれば、水素を無害化することが可能になり、疲労寿命低下が抑制される。そこで、耐水素疲労特性について、水素疲労が発生しない「疲労限界水素量」を求めることにより評価した。この方法は、電解水素チャージにより種々のレベルの拡散性水素量を含有させた後、回転曲げ疲労試験中に試料から大気中に水素が抜けることを防止するためにＣｄめっきを施し、その後、大気中で所定の荷重を負荷し、疲労破壊が発生しなくなる拡散性水素量を評価するものである。図２R>２に拡散性水素量と疲労寿命の関係について解析した一例を示す。試料中に含まれる拡散性水素量が少なくなるほど疲労寿命が長くなり、拡散性水素量がある値以下では疲労破壊が発生しなくなる。この水素量を「疲労限界水素量」と定義する。疲労限界水素量が高いほど鋼材の耐水素疲労特性は良好であり、鋼材の成分、熱処理等の製造条件によって決まる鋼材固有の値である。
【０００６】
そこで、高強度ばねの疲労限界水素量を増加させる手段、即ち耐水素疲労特性を上げるべく、オーステナイト結晶粒度、焼入れ焼戻し条件の影響等について検討を重ねた結果、次のことが判明した。
【０００７】
すなわち、熱間加工仕上げ温度を未再結晶温度域である７００℃〜９００℃としこの温度域での圧下率を３０％以上、好ましくは５０％以上とし、加工直後に冷却することによって表層から少なくとも０．５ｍｍ以上の深さまでの旧オーステナイト結晶粒が伸長化され、表層から少なくとも０．５ｍｍの深さまでの旧オーステナイト粒のアスペクト比が２以上、前記の好ましい条件下で４以上であり、かつ面積率最大の層がマルテンサイトであるような組織を得ることができる。このような組織では１７００ＭＰａを超えるような高強度域でも疲労限界水素量が大幅に増加し、耐水素疲労特性が向上するという知見を見出した。
【０００８】
本発明は以上の知見に基づいてなされたものであって、その要旨とするところは、下記の通りである。
(1) 質量%で、C:0.55〜1% Si:0.05〜4% Mn:0.0５〜２％、Al:0.005〜0.1%を含有し、残部がFe及び不可避的不純物よりなり、面積率最大の相が焼戻しマルテンサイトであり、旧オーステナイト粒の長さと幅の比 ( 以後アスペクト比とする )が2以上であり、かつ鋼材の疲労試験を、大気中の疲労限の90%の応力で行う際に、疲労寿命が107回未満に低下しない拡散性水素量(室温から500℃に加熱する際に放出される水素量)の上限値(以後、疲労限界水素量とする)が0.1ppm以上であり、引張強度が1700MPa以上であることを特徴とする耐水素疲労特性の優れた高強度ばね用鋼。
(2) 質量%で、Ti:0.005〜0.5% Cr:0.05〜2%、Mo:0.05〜2% Ni:0.05〜5%、Cu:0.05〜1% V:0.05〜2% Nb:0.005〜0.2% Ta:0.005〜0.5% W:0.05〜0.5%及びB:0.0003〜0.005%の1種または2種以上を含有することを特徴とする上記（1）記載の耐水素疲労特性の優れた高強度ばね用鋼。
(3) 前記(1)または(2)に記載の高強度ばね用鋼を製造する方法であって、700℃〜900℃の温度域で30%以上の圧下率を与える熱間加工工程を経た後、焼入れして面積率最大の相をマルテンサイト組織にし、その後、焼戻処理を行うことを特徴とする耐水素疲特性の優れた高強度ばね用鋼の製造方法。
【０００９】
【発明の実施の形態】
次に、本発明の実施の形態について説明する。
【００１０】
疲労限界水素量：疲労限界水素量が０．１ｐｐｍ未満であると、対疲労特性が不十分なため、０．１ｐｐｍ以上とする。
【００１１】
組織：高強度を得るため、組織は面積率最大の相が焼き戻しマルテンサイトであることが好ましい。本発明において、焼戻しマルテンサイトの面積率は鋼棒のＣ断面ｔ/４部又はボルトのＣ断面ｔ/４を光学顕微鏡で200〜1000倍で10視野観察した場合の平均値である。その他の組織として、残留オーステナイト、ベイナイト、フェライト、パーライトを含有することができる。
【００１２】
アスペクト比：本発明で目的とする高強度ばね用鋼の耐水素疲労特性の向上に対して最も重要な点である旧オーステナイト粒のアスペクト比の限定理由について述べる。図３に焼戻しマルテンサイト組織からなるばねの疲労限界水素量に及ぼす旧オーステナイト粒のアスペクト比の影響について解析した一例を示す。アスペクト比が２未満では疲労限界水素量の向上効果が少ない、即ち耐水素疲労特性向上効果が少ないため、アスペクト比を２以上に限定した。好ましくは３以上である。アスペクト比の上限は特に定めることなく本発明の効果を得ることができるが，遅れ破壊特性以外の良好な機械的性質を得るためには，８以下とすることが好ましい
【００１３】
また、旧オーステナイト粒のアスペクト比は、上記試料において、粒界エッチングを施した後に光学顕微鏡で200〜1000倍で10視野観察した場合の平均値である。
【００１４】
鋼材成分：次に本発明の対象とする鋼の成分の限定理由について述べる。
【００１５】
C:Cはばねの強度を確保する上で必須の元素であるが、0.55%未満では所定の焼戻し温度範囲では所要の強度が得られず、一方1%を越えると靭性を劣化させるために、0.55〜1%の範囲に制限した。
【００１６】
Ｓｉ：Ｓｉは固溶体硬化作用によって強度を高める作用がある。0.05％未満では前記作用が発揮できず、一方、4％を超えると添加量に見合う効果が期待できないために、0.05〜4％の範囲に制限した。
【００１７】
Ｍｎ：Ｍｎは脱酸、脱硫のために必要であるばかりでなく、マルテンサイト組織を得るための焼入性を高めるために有効な元素であるが、0.05％未満では上記の効果が得られず、一方2％を越えるとオーステナイト域加熱時に粒界に偏析し粒界を脆化させるとともに耐遅れ破壊特性を劣化させるために0.05〜2％の範囲に制限した。
【００１８】
以上が本発明の対象とする鋼の基本成分であるが、本発明においては、さらにこの鋼に
Ａｌ：0.005〜0.1％ Ｔｉ：0.005〜0.5％
Ｃｒ：0.05〜2％、 Ｍｏ：0.05〜2％
Ｎｉ：0.05〜5％、 Ｃｕ：0.05〜1％
Ｖ：0.05〜2％ Ｎｂ：0.005〜0.2％
Ｔａ：0.005〜0.5％ Ｗ：0.05〜0.5％
及びＢ：0.0003〜0.005％
の１種または２種以上を含有せしめることができる。
【００１９】
Ａｌ：Ａｌは脱酸および熱処理時においてＡｌＮを形成することによりオーステナイト粒の粗大化を防止する効果とともにＮを固定する効果も有しているが、0.005％未満ではこれらの効果が発揮されず、0.1％を越えても効果が飽和するため0.005〜0.1％の範囲に限定した。
【００２０】
Ｔｉ：ＴｉはＡｌと同様に脱酸および熱処理時においてＴｉＮを形成することによりオーステナイト粒の粗大化を防止する効果とともにＮを固定する効果も有しているが、0.005％未満ではこれらの効果が発揮されず、0.5％を超えると焼入れ時に炭化物を固溶させるために高温に加熱する必要があり、疲労特性を劣化させる脱炭が生じるため0.005〜0.5％の範囲に限定した。
【００２１】
Ｃｒ：Ｃｒは焼入性の向上および焼戻し処理時の軟化抵抗を増加させるために有効な元素であるが、0.05％未満ではその効果が十分に発揮できず、一方2％を超えると靭性の劣化、冷間加工性の劣化を招くために0.05〜2％に限定した。
【００２２】
Ｍｏ：ＭｏはＣｒと同様に強い焼戻し軟化抵抗を有し熱処理後の引張強さを高めるために有効な元素であるが、0.05％未満ではその効果が少なく、一方2％を超えるとその効果は飽和しコストの上昇を招くために0.05〜2％に制限した。
【００２３】
Ｎｉ：Ｎｉは高強度化に伴って劣化する延性を向上させるとともに熱処理時の焼入性を向上させて引張強さを増加させるために添加されるが、0.05％未満ではその効果が少なく、一方5％を越えても添加量にみあう効果が発揮できないため、0.05〜5％の範囲に制限した。
【００２４】
Ｃｕ：Ｃｕは焼戻し軟化抵抗を高めるために有効な元素であるが、0.05％未満では効果が発揮できず、1％を超えると熱間加工性が劣化するため、0.05〜1％に制限した。
【００２５】
Ｖ：Ｖは焼入れ処理時において炭窒化物を生成することによりオーステナイト粒を微細化させる効果があるが、0.05％未満では前記作用の効果が得られず、一方2％を越えても効果が飽和するため0.05〜2％に限定した。
【００２６】
Ｎｂ：Ｎｂは再結晶温度を高め、アスペクト比の大きい旧オーステナイト粒を得るために有効な元素であるが、0.005％未満では上記効果が不十分であり、一方0.2％を越えるとこの効果が飽和するため0.005〜0.2％に制限した。
【００２７】
Ｔａ：ＴａもＮｂと同様にオーステナイト粒の微細化効果を有しているが、0.005％未満では前記の効果が発揮されず、0.5％を越えて添加しても効果が飽和するため、0.005〜0.5％に限定した。
【００２８】
Ｗ：Ｗは高強度ボルトの遅れ破壊特性を向上させるために有効な元素であるが、0.05％未満では前記の効果が発揮されず、一方、0.5％を越えて添加しても効果が飽和するため、0.05〜0.5％の範囲に限定した。
【００２９】
Ｂ：Ｂは粒界破壊を抑制し遅れ破壊特性を向上させる効果がある。更に、Ｂはオーステナイト粒界に偏析することにより焼入性を著しく高めるが、0.0003％未満では前記の効果が発揮されず、0.005％を超えても効果が飽和するため0.0003〜0.005％に制限した。
【００３０】
不純物元素であるＰ、Ｓについては特に制限しないものの、耐水素疲労特性を向上させる観点から、それぞれ0.015％以下が好ましい範囲である。Ｎについては、Ａｌ、Ｖ、Ｎｂ、Ｔｉの窒化物を形成することによって旧オーステナイト粒の微細化、降伏強度の増加の効果があるため、0.002〜0.1％が望ましい範囲である。
【００３１】
本発明の高強度ばねの製造方法では、所定の条件で熱間加工を行った後、直ちに焼入れてマルテンサイト組織にした後、焼戻しを行うものであるが、次にこの製造条件の限定理由について述べる。
【００３２】
熱間加工温度；熱間加工温度が９００℃を超えると熱間加工時の再結晶が顕著になり、アスペクト比が２以上のマルテンサイト組織を得ることが困難である。一方、熱間加工温度が700℃未満では所定のアスペクト比の組織を得るに十分な圧下率を確保できない。従って熱間加工温度を700℃〜９００℃，好ましくは７００〜８５０℃に限定した。
【００３３】
圧下率；アスペクト比が２以上のマルテンサイト組織を得るには未再結晶域で30％以上の圧下率が必要であるため圧下率を30％以上に限定した。
なお、本発明鋼のばね用鋼およびばねの引張強度の上限は特に定めることなく本発明の効果を得られるが、靭性を劣化させないためには、2200MPa以下が望ましい。
【００３４】
【実施例】
以下、実施例により本発明の効果をさらに具体的に説明する。
【００３５】
表１に示す化学組成を有するばね用鋼を焼入れ、表２に示す温度で焼戻しを行い、最大面積率が焼戻しマルテンサイトである組織に調整した。
【００３６】
上記の試料を用いて、機械的性質、組織形態、耐水素疲労特性について評価した結果を表１に示す。水素疲労特性は、前に述べた疲労限界水素量で評価を行い、負荷応力は大気中疲労限の90%の条件で実施した。
【００３７】
【表１】
表２の試験Ｎｏ.１〜１６が本発明例で、その他は比較例である。同表に見られるように本発明例はいずれも熱間加工温度が７００℃〜９００℃で、圧下率が３０％以上であり、旧オーステナイト粒のアスペクト比が２以上であるような、最大面積相が焼戻しマルテンサイトである組織となっている。これらの鋼は疲労限界水素量が従来のばねに比べ高く、耐水素疲労特性の優れたばねが実現されている。
【００３９】
これに対して比較例であるＮｏ．１７は、Ｃ量が低いため、１７００ＭＰａ以上の強度が得られず、高強度ばね用鋼として使用できなかった例である。
【００４０】
比較例であるＮｏ．１８は、圧下率が低かったため、所定のアスペクト比の旧オーステナイト粒が得られず、疲労限界水素量が低かった例である。
【００４１】
比較例であるＮｏ．１９は、熱間加工温度が高かったために、所定のアスペクト比の旧オーステナイト粒が得られず、疲労限界水素量が低かった例である。
【００４２】
比較鋼であるＮｏ．２０は、熱間加工温度が低かったためにフェライトが析出し、所定の強度が得られなかった例である。
【００４３】
比較鋼であるＮｏ．２１はＳｉ含有量が高すぎたために、Ｎｏ．２３は、Ｃ含有量が高すぎるために、Ｎｏ．２４はＭｎ含有量が高すぎるために、いずれも疲労限界水素量が低かった例である。
【００４４】
比較鋼であるＮｏ．２２は、強度が高すぎたため、疲労限界水素量が低かった例である。
【００４５】
【表２】
【発明の効果】
以上の実施例からも明らかなごとく、本発明は旧オーステナイト粒のアスペクト比を特定の値にすることによって、引張強度が１７００ＭＰａ以上の高強度ばねの水素疲労特性を大幅に向上させることを可能にするとともに、鋼の化学成分、熱間加工条件を最適に選択することによって、ばね用鋼及びその製造方法を確立したものであり、産業上の効果は極めて顕著なものがある。
【図面の簡単な説明】
【図１】昇温分析による水素放出曲線と、拡散性水素量を示す図である。
【図２】拡散性水素量と疲労寿命の関係の一例を示す図である。
【図３】旧オーステナイト粒のアスペクト比と、疲労限界水素量の関係を示す図である。[0001]
[Industrial application fields]
The present invention relates to a high-strength spring having a tensile strength of 1700 MPa or more used for a valve spring, a suspension spring, a stabilizer, a torsion bar, etc. of an engine of an automobile or the like, and is particularly excellent in hydrogen fatigue resistance, which is an important spring characteristic. The present invention relates to a steel for strength springs and a manufacturing method thereof.
[0002]
[Prior art]
High-strength springs that are widely used in automobiles, etc. are hot-rolled using, for example, spring steel specified in JIS G 3565-3567 and 4801, etc., drawn to a predetermined wire diameter, and processed with an oil temper. It is manufactured by a method in which the spring is processed later, or after the drawing process is heated and spring processed and quenched and tempered. In recent years, automobiles are required to be lighter in order to reduce fuel consumption in order to deal with environmental problems such as carbon dioxide emission reduction. As part of this, a spring having a tensile strength after quenching and tempering increased to 1800 MPa or more is required. However, in general, when the strength of the spring is increased, the fatigue characteristics in a corrosive environment deteriorate, and there is a concern about early breakage. One of the causes of deterioration of corrosion fatigue characteristics is embrittlement due to hydrogen generated as the corrosion reaction progresses. As a countermeasure, a large amount of various alloy elements are added to increase the strength. However, this method has a problem that the cost of the material becomes high. In addition, as a method for suppressing hydrogen fatigue properties, a method of refining crystal grains and a method of generating fine precipitates are considered promising. The hydrogen embrittlement characteristics have not been improved.
[0003]
As described above, the conventional technique has a limit in manufacturing a high-strength spring having drastically improved hydrogen fatigue resistance.
[0004]
[Problems to be solved by the invention]
The present invention has been made in view of the actual situation as described above, and aims to provide a high strength spring steel having good hydrogen fatigue resistance and a tensile strength of 1700 MPa or more and a method for producing the same. Is.
[0005]
[Means for Solving the Problems]
The present inventors first analyzed hydrogen fatigue behavior in detail using spring steels of various strength levels manufactured by quenching and tempering treatments. As a result, it was clarified that the fatigue life is reduced by hydrogen in the steel at stress below the fatigue limit. It was also clarified that the decrease in fatigue life occurred due to diffusible hydrogen that penetrated into the steel material from the outside environment and could diffuse in the steel material at room temperature. The diffusible hydrogen can be measured as a curve having a peak at a temperature of about 100 ° C. in a temperature-hydrogen release rate curve obtained when the steel is heated at a rate of 100 ° C./hour (FIG. 1). Therefore, if hydrogen that has entered from the environment is prevented from diffusing by trapping in some part of the steel material, hydrogen can be rendered harmless and a reduction in fatigue life is suppressed. Therefore, the hydrogen fatigue resistance was evaluated by determining the “ fatigue limit hydrogen amount” at which hydrogen fatigue does not occur. In this method, after various amounts of diffusible hydrogen are contained by electrolytic hydrogen charging, Cd plating is applied to prevent hydrogen from escaping from the sample into the atmosphere during the rotational bending fatigue test, The amount of diffusible hydrogen at which a predetermined load is applied and fatigue fracture does not occur is evaluated. FIG. 2R> 2 shows an example of analyzing the relationship between the amount of diffusible hydrogen and the fatigue life. As the amount of diffusible hydrogen contained in the sample decreases, the fatigue life becomes longer. When the amount of diffusible hydrogen is below a certain value, fatigue failure does not occur. This amount of hydrogen is defined as “ fatigue limit hydrogen amount”. The higher the fatigue limit hydrogen amount, the better the hydrogen fatigue resistance of the steel material, which is a value inherent to the steel material determined by the manufacturing conditions such as the composition of the steel material and heat treatment.
[0006]
Therefore, as a result of repeated studies on the means of increasing the fatigue limit hydrogen amount of the high-strength spring, that is, the influence of the austenite grain size, quenching and tempering conditions, etc., in order to improve the hydrogen fatigue resistance, the following was found.
[0007]
That is, the hot working finish temperature is set to 700 ° C. to 900 ° C. which is a non-recrystallization temperature range, and the rolling reduction in this temperature range is set to 30% or more, preferably 50% or more. The prior austenite grains up to a depth of 0.5 mm or more are elongated, the aspect ratio of the prior austenite grains from the surface layer to a depth of at least 0.5 mm is 2 or more, 4 or more under the above-mentioned preferred conditions, and the area It is possible to obtain a structure in which the highest rate layer is martensite. In such a structure, the inventors have found that the fatigue limit hydrogen amount is greatly increased even in a high strength region exceeding 1700 MPa, and the hydrogen fatigue resistance is improved .
[0008]
The present invention has been made based on the above findings, and the gist thereof is as follows.
(1) in mass%, C: 0.5 5 ~1% Si: 0.05~4% Mn: 0.05~2%, Al: containing 0.005% to 0.1%, the balance being Fe and unavoidable impurities, the area ratio the largest phase is tempered martensite, the ratio of length to width of prior austenite grains (and hereafter aspect ratio) is equal to or greater than 2, and the fatigue test of the steel material, 90% of the stress of the fatigue limit in air When performing, the upper limit of the amount of diffusible hydrogen (the amount of hydrogen released when heating from room temperature to 500 ° C) that does not decrease the fatigue life to less than 10 7 times (hereinafter referred to as the fatigue limit hydrogen amount) is 0.1 ppm A high-strength spring steel with excellent hydrogen fatigue resistance characterized by a tensile strength of 1700 MPa or higher.
(2) By mass%, Ti: 0.005-0.5% Cr: 0.05-2%, Mo: 0.05-2% Ni: 0.05-5%, Cu: 0.05-1% V: 0.05-2% Nb: 0.005-0.2 % Ta: 0.005-0.5% W: 0.05-0.5% and B: 0.0003-0.005% 1 type or 2 types or more, and high strength with excellent hydrogen fatigue resistance as described in (1) above Spring steel.
(3) A method for producing the high-strength spring steel according to the above (1) or (2), which has undergone a hot working step that gives a rolling reduction of 30% or more in a temperature range of 700 ° C to 900 ° C. A method for producing a high strength spring steel with excellent hydrogen fatigue resistance, comprising quenching to make the phase with the largest area ratio a martensite structure, followed by tempering.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
Next, an embodiment of the present invention will be described.
[0010]
Fatigue limit hydrogen amount: When the fatigue limit hydrogen amount is less than 0.1 ppm, the fatigue resistance characteristic is insufficient, so that it is 0.1 ppm or more.
[0011]
Structure: In order to obtain high strength, it is preferable that the phase having the largest area ratio of the structure is tempered martensite. In the present invention, the area ratio of tempered martensite is an average value when the C cross section t / 4 part of the steel bar or the C cross section t / 4 of the bolt is observed with an optical microscope at 200 to 1000 times in 10 fields. As other structures, residual austenite, bainite, ferrite, and pearlite can be contained.
[0012]
Aspect ratio: The reason for limiting the aspect ratio of prior austenite grains, which is the most important point for improving the hydrogen fatigue resistance of the high-strength spring steel intended in the present invention, will be described. FIG. 3 shows an example in which the influence of the aspect ratio of prior austenite grains on the fatigue limit hydrogen content of a spring having a tempered martensite structure is analyzed. When the aspect ratio is less than 2, the effect of improving the fatigue limit hydrogen amount is small, that is, the effect of improving the hydrogen fatigue resistance is small, so the aspect ratio is limited to 2 or more. Preferably it is 3 or more. Although the upper limit of the aspect ratio is not particularly defined, the effects of the present invention can be obtained, but in order to obtain good mechanical properties other than delayed fracture characteristics, it is preferably 8 or less.
Further, the aspect ratio of the prior austenite grains is an average value when 10 fields of view are observed with an optical microscope at 200 to 1000 times after performing grain boundary etching in the above sample.
[0014]
Steel material components: Next, the reasons for limiting the steel components to be the subject of the present invention will be described.
[0015]
C: C is an essential element in order to ensure the strength of the spring, in order to degrade 0.5 5% prescribed tempering temperature range in less than no required strength is obtained, whereas exceeding 1%, the toughness It was limited to the range of 0.5 5-1%.
[0016]
Si: Si has an effect of increasing strength by a solid solution hardening action. If it is less than 0.05%, the above action cannot be exhibited. On the other hand, if it exceeds 4%, an effect commensurate with the amount of addition cannot be expected.
[0017]
Mn: Mn is not only necessary for deoxidation and desulfurization, but is also an effective element for enhancing the hardenability for obtaining a martensite structure. However, if it is less than 0.05%, the above effect cannot be obtained. On the other hand, if over 2%, it segregates at the grain boundary during heating in the austenite region, embrittles the grain boundary, and limits the range of 0.05 to 2% in order to deteriorate the delayed fracture resistance.
[0018]
The above are the basic components of the steel that is the subject of the present invention. In the present invention, Al: 0.005-0.1% Ti: 0.005-0.5%
Cr: 0.05-2%, Mo: 0.05-2%
Ni: 0.05-5%, Cu: 0.05-1%
V: 0.05-2% Nb: 0.005-0.2%
Ta: 0.005-0.5% W: 0.05-0.5%
And B: 0.0003 to 0.005%
1 type (s) or 2 or more types can be included.
[0019]
Al: Al has an effect of fixing N together with an effect of preventing coarsening of austenite grains by forming AlN at the time of deoxidation and heat treatment, but these effects are not exhibited at less than 0.005%, Since the effect is saturated even if it exceeds 0.1%, it is limited to the range of 0.005 to 0.1%.
[0020]
Ti: Ti has the effect of fixing N as well as the effect of preventing coarsening of austenite grains by forming TiN at the time of deoxidation and heat treatment, as with Al. When it exceeds 0.5%, it is necessary to heat to a high temperature in order to dissolve the carbide at the time of quenching, and decarburization that deteriorates fatigue characteristics occurs, so the content is limited to 0.005 to 0.5%.
[0021]
Cr: Cr is an effective element for improving hardenability and increasing softening resistance during tempering. However, if it is less than 0.05%, the effect cannot be fully exhibited, while if it exceeds 2%, toughness deteriorates. In order to cause deterioration of cold workability, it was limited to 0.05 to 2%.
[0022]
Mo: Mo is an element effective for increasing the tensile strength after heat treatment, having strong tempering softening resistance like Cr. However, if it is less than 0.05%, its effect is small, while if it exceeds 2%, its effect is It was limited to 0.05-2% in order to saturate and increase the cost.
[0023]
Ni: Ni is added to improve the ductility which deteriorates with increasing strength and to improve the hardenability during heat treatment and increase the tensile strength. However, if it is less than 0.05%, the effect is small. Even if it exceeds 5%, the effect of matching the added amount cannot be exhibited, so the content was limited to 0.05 to 5%.
[0024]
Cu: Cu is an effective element for increasing the temper softening resistance. However, if it is less than 0.05%, the effect cannot be exhibited, and if it exceeds 1%, the hot workability deteriorates, so the content is limited to 0.05 to 1%.
[0025]
V: V has the effect of refining austenite grains by forming carbonitrides during the quenching treatment, but if it is less than 0.05%, the above effect cannot be obtained, while if it exceeds 2%, the effect is saturated. Therefore, it was limited to 0.05 to 2%.
[0026]
Nb: Nb is an element effective for increasing the recrystallization temperature and obtaining prior austenite grains having a large aspect ratio. However, if the content is less than 0.005%, the above effect is insufficient, while if it exceeds 0.2%, this effect is saturated. Therefore, it was limited to 0.005 to 0.2%.
[0027]
Ta: Ta also has the effect of refining austenite grains in the same way as Nb. However, if the amount is less than 0.005%, the above effect cannot be exhibited, and the effect is saturated even if added over 0.5%. Limited to 0.5%.
[0028]
W: W is an effective element for improving delayed fracture characteristics of high-strength bolts. However, if the amount is less than 0.05%, the above-mentioned effect cannot be exhibited. On the other hand, even if added over 0.5%, the effect is saturated. Therefore, it was limited to the range of 0.05 to 0.5%.
[0029]
B: B has an effect of suppressing grain boundary fracture and improving delayed fracture characteristics. Furthermore, B segregates at the austenite grain boundary to remarkably improve the hardenability. However, if the content is less than 0.0003%, the above effect cannot be exhibited, and even if it exceeds 0.005%, the effect is saturated, so the content is limited to 0.0003 to 0.005%. .
[0030]
Although there are no particular restrictions on the impurity elements P and S, 0.015% or less is each preferable range from the viewpoint of improving hydrogen fatigue resistance. N is preferably in the range of 0.002 to 0.1% because the formation of nitrides of Al, V, Nb, and Ti has the effect of refining prior austenite grains and increasing yield strength.
[0031]
In the method for producing a high-strength spring according to the present invention, after hot working under predetermined conditions, immediately after quenching to a martensite structure, tempering is performed. State.
[0032]
Hot working temperature: When the hot working temperature exceeds 900 ° C., recrystallization during hot working becomes remarkable, and it is difficult to obtain a martensitic structure having an aspect ratio of 2 or more. On the other hand, when the hot working temperature is less than 700 ° C., it is impossible to secure a reduction ratio sufficient to obtain a structure having a predetermined aspect ratio. Therefore, the hot working temperature is limited to 700 ° C to 900 ° C, preferably 700 to 850 ° C.
[0033]
Reduction ratio: In order to obtain a martensite structure having an aspect ratio of 2 or more, a reduction ratio of 30% or more is required in the non-recrystallized region, so the reduction ratio was limited to 30% or more.
The upper limit of the tensile strength of the spring steel and spring of the steel of the present invention is not particularly defined, but the effect of the present invention can be obtained, but 2200 MPa or less is desirable so as not to deteriorate the toughness.
[0034]
【Example】
Hereinafter, the effects of the present invention will be described more specifically with reference to examples.
[0035]
The spring steel having the chemical composition shown in Table 1 was quenched and tempered at the temperature shown in Table 2 to adjust the structure so that the maximum area ratio was tempered martensite.
[0036]
Table 1 shows the results of evaluating the mechanical properties, structure morphology, and resistance to hydrogen fatigue using the above samples. The hydrogen fatigue characteristics were evaluated using the fatigue limit hydrogen amount described above, and the load stress was 90% of the atmospheric fatigue limit.
[0037]
[Table 1]
Test Nos. 1 to 16 in Table 2 are examples of the present invention, and others are comparative examples. As can be seen from the table, each of the examples of the present invention has a hot working temperature of 700 ° C. to 900 ° C., a rolling reduction of 30% or more, and an aspect ratio of prior austenite grains of 2 or more. It is an organization whose phase is tempered martensite. These steels have higher fatigue limit hydrogen amounts than conventional springs, and springs with excellent hydrogen fatigue resistance are realized.
[0039]
On the other hand, No. as a comparative example. No. 17 is an example in which since the amount of C is low, a strength of 1700 MPa or more could not be obtained and the steel could not be used as a high strength spring steel.
[0040]
No. which is a comparative example. No. 18 is an example in which the austenite grains having a predetermined aspect ratio were not obtained because the rolling reduction was low, and the fatigue limit hydrogen amount was low.
[0041]
No. which is a comparative example. No. 19 is an example in which the austenite grains having a predetermined aspect ratio were not obtained because the hot working temperature was high, and the fatigue limit hydrogen amount was low.
[0042]
No. which is a comparative steel. No. 20 is an example in which a predetermined strength was not obtained because ferrite was precipitated because the hot working temperature was low.
[0043]
No. which is a comparative steel. No. 21 was too high in Si content. No. 23 has a C content that is too high. No. 24 is an example in which the amount of fatigue limit hydrogen was low because the Mn content was too high.
[0044]
No. which is a comparative steel. 22 is an example in which the fatigue limit hydrogen amount was low because the strength was too high.
[0045]
[Table 2]
【The invention's effect】
As is clear from the above examples, the present invention makes it possible to greatly improve the hydrogen fatigue characteristics of a high-strength spring having a tensile strength of 1700 MPa or more by setting the aspect ratio of prior austenite grains to a specific value. At the same time, by optimally selecting the chemical components and hot working conditions of the steel, the spring steel and its manufacturing method have been established, and the industrial effects are extremely remarkable.
[Brief description of the drawings]
FIG. 1 is a diagram showing a hydrogen release curve by temperature analysis and a diffusible hydrogen amount.
FIG. 2 is a diagram showing an example of the relationship between the amount of diffusible hydrogen and fatigue life.
FIG. 3 is a diagram showing the relationship between the aspect ratio of prior austenite grains and the fatigue limit hydrogen content.
Claims (3)
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| US10752969B2 (en) | 2015-03-10 | 2020-08-25 | Nippon Steel Corporation | Steel for suspension spring and method of manufacturing same |
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| US10752969B2 (en) | 2015-03-10 | 2020-08-25 | Nippon Steel Corporation | Steel for suspension spring and method of manufacturing same |
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