JP4173958B2 - Mechanical structural steel with excellent hydrogen fatigue fracture resistance and method for producing the same - Google Patents
Mechanical structural steel with excellent hydrogen fatigue fracture resistance and method for producing the same Download PDFInfo
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Description
【０００１】
【発明の属する技術分野】
本発明は、自動車部品などに用いられる1800MPa以上の引張強度を有し、かつ耐水素疲労特性に優れた機械構造用鋼、及びその製造方法に関するものである。
【０００２】
【従来の技術】
近年、環境問題への対応のため炭酸ガス排出低減や燃費低減を目的に自動車の軽量化が望まれている。自動車の軽量化のためには鋼材の高強度化が有効な手段であり、焼入れ焼戻し後の引張強度を1800MPa以上に高めた機械構造用鋼が要望されている。
【０００３】
しかしながら、一般に鋼材を高強度化すると、切欠き感受性が高まり環境の悪影響を受けやすくなる。特に腐食環境下では表面に腐食ピットが形成されるとこれが応力集中源となり、さらに腐食反応の進行に伴って発生する水素により脆化するため、疲労特性が劣化し早期折損を招くという問題があった。水素による脆化を防止する方法としては、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考えられているが、いずれの方法も本発明者らの試験では大幅な耐水素疲労特性の改善には至っていない。
【０００４】
以上のように、従来の技術では、1800MPa以上の引張強度を有し、かつ耐水素疲労特性に優れた機械構造用鋼を製造することは困難であった。
【０００５】
【発明が解決しようとする課題】
本発明は、上記したような問題点を解決しようとするものであって、1800MPa以上の引張強度を有し、かつ耐水素疲労特性に優れた機械構造用鋼、及びその製造方法を提供することを目的とする。
【０００６】
【課題を解決するための手段】
本発明者らは、係る課題を解決するため、成分の異なる種々の素材に対して後述する疲労限界拡散性水素量を求める手法により耐水素疲労特性について研究を重ねた結果、（Ｍｏ，Ｖ）2Ｃが水素トラップサイトとして非常に有効であり疲労限界拡散性水素量を大幅に高めることを見出した。また、Ｓｉを低減することにより疲労限界拡散性水素量を高めることができることを見出した。さらに研究を進めた結果、ＭｏとＶの添加比率（Ｍｏ／Ｖ）を５〜２０とし、さらにＳｉを０．５％未満とし、さらに粒界強度を低下させるＭｎを０．５％以下とすることにより、１８００MPa以上の引張強度を有しかつ耐水素疲労特性に優れた鋼が得られることを知見した。
【０００７】
本発明はこのような知見に基づいて構成したものであり、その要旨は、
(1) １８００ MPa 以上の引張強度を有する機械構造用鋼であって、質量%で、C:0.35〜0.6%、Si:0.5%未満、Mn:0.1〜0.5%、Mo:0.4〜3.0%、V:0.02〜0.5%、P:0.02%以下、S:0.02以下、Al:0.005〜0.1%、N:0.001〜0.0079%、を含有し、かつ5≦(Mo/V)≦20を満足し、残部がFe及び不可避的不純物からなることを特徴とする耐水素疲労破壊特性に優れた機械構造用鋼。
(2) 前記(1) 記載の成分を含有し、さらに質量%で、Ti:0.005〜0.3%、Nb:0.005〜0.3%、B:0.0003〜0.05%、の1種または2種以上を含有することを特徴とする耐水素疲労破壊特性に優れた機械構造用鋼。
(3) 前記 (2) 記載の成分を含有し、さらに質量％で、 Cr:0.05 〜 3.0% 、 Ni:0.05 〜 5.0% 、 Cu:0.05 〜 2.0% 、の 1 種または 2 種以上を含有することを特徴とする耐水素疲労破壊特性に優れた機械構造用鋼。
(4) 前記(1)〜(3)の何れか1項に記載の成分からなる鋼を焼き入れた後に、500℃以上で焼き戻すことを特徴とする耐水素疲労破壊特性に優れた機械構造用鋼の製造方法。
にある。」
【０００８】
【発明の実施の形態】
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機械構造用鋼を用いて、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、室温下において鋼材中を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、外部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、鋼材中で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。
【０００９】
そこで、耐水素疲労特性について、水素疲労が発生しない「疲労限界拡散性水素量」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「疲労限界拡散性水素量」と定義する。（Ｍｏ，Ｖ）2Ｃ等によってトラップされている水素は疲労特性に悪影響を与えにくいので、このようなトラップサイトを含んだ鋼材は「拡散性水素量」が高くても水素による疲労強度の低下が起こりにくくなる。従って、このような鋼材は疲労限界拡散性水素量が高くなる。実環境で鋼中に侵入する拡散性水素量は環境が厳しいほど多くなるので、疲労限界拡散性水素量が高い鋼材ほどより厳しい環境（拡散性水素量：大）でも水素による疲労強度の低下が起こらないので、疲労限界拡散性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。
【００１０】
以下に、本発明における各要件の意義及び限定理由について具体的に説明する。
【００１１】
本発明者らは、０．５％Ｃ−０．０６％Ｓｉ−０．２％Ｍｎをベース成分としＭｏとＶの添加比率を種々に変えた鋼を焼入れ焼戻し処理により同一強度レベルに調質し、疲労限界拡散性水素量を測定した。ＭｏとＶの添加比率（Ｍｏ／Ｖ）と疲労限界拡散性水素量の関係を図３に示す。図３より、Ｍｏ／Ｖが５以上２０以下のとき疲労限界拡散性水素量が大幅に向上することを知見した。従って、Ｍｏ／Ｖを５以上２０以下とした。
【００１２】
次に、本発明における高強度ばね用鋼の成分限定理由について説明する。
【００１３】
C;Cは鋼の強度を増加させる元素として添加されるものである。0.1%未満では機械構造用鋼に必要な強度の確保が困難であり、0.6%を超える過剰の添加は靭性を著しく劣化させる。従って、C含有量は0.1〜0.6%とした。なお、後述の実施例の記載に基づき、 C 含有量の下限値を 0.35% とした。
【００１４】
Ｓｉ； Ｓｉは脱酸剤として添加されるが、０．５％以上の過剰の添加は疲労限界拡散性水素量を減少させて水素疲労特性を劣化させる。従って、Ｓｉ含有量は少ないほど好ましく、０．５％未満とした。
【００１５】
Ｍｎ； Ｍｎは焼入れ性を高めるために有効な元素であるが、一方で粒界を脆化させ耐水素疲労破壊特性を劣化させる有害な元素である。０．１％未満では焼入れ性を高める効果が発現されず、０．５％を超える過剰の添加は耐水素破壊疲労特性を劣化させる。従って、Ｍｎ含有量は０．１〜０．５％とした。より良好な耐水素疲労破壊特性を得るためには、Ｍｎ含有量を０．３％以下とすることが望ましく、０．２％以下とすればさらに望ましい。
【００１６】
Ｍｏ； ＭｏはＶ、Ｃとともに（Ｍｏ，Ｖ）2Ｃを形成し拡散性水素をトラップすることにより耐水素疲労破壊特性を向上させる必須の元素であるが、０．４％未満ではその効果が発現されず、３．０％を超える過剰の添加は靭性を低下させるため、Ｍｏ含有量は０．４〜３．０％とした。
【００１７】
Ｖ； ＶはＭｏ、Ｃとともに（Ｍｏ，Ｖ）2Ｃを形成し拡散性水素をトラップすることにより耐水素疲労破壊特性を向上させる必須の元素であるが、０．０２％未満ではその効果が発現されず、０．５％を超える過剰の添加は靭性を低下させるため、Ｖ含有量は０．０２〜０．５％とした。
【００１８】
Ｐ； Ｐは粒界に偏析して粒界強度を低下させ、靱性を劣化させる不純物元素であり、可及的低レベルが望ましいが、現状精錬技術の到達可能レベルとコストを考慮して、上限を０．０２％とした。
【００１９】
S;Sは熱間加工性及び靭性を劣化させる不純物元素であり、可及的低レベルが望ましいが、現状精錬技術の到達可能レベルとコストを考慮して、上限を0.02%とした。
Al;Alは脱酸剤として、またAlNを形成し結晶粒粗大化を抑制する効果があるが、0.005%未満ではその効果が発現されず、0.1%を超えて過剰添加すると靭性が劣化するため、Alの含有量を0.005〜0.1%とした。
N;Nは窒化物を形成し結晶粒粗大化を抑制する効果があるが、0.001%未満ではその効果が発現されず、0.05%を超えて添加すると靭性が劣化するため、N含有量を0.001〜0.05%とした。なお、後述の実施例の記載に基づき、 N 含有量の上限値を 0.0079% とした。
【００２０】
以上が本発明の基本成分であり、通常は上記以外はFe及び不可避的不純物からなるが、所望の強度レベルやその他の必要特性に応じて、Cr、Ni、Cu、 Ti、Nb、B、の1種または2種以上を添加しても良い。
【００２１】
Ｃｒ、Ｎｉ、Ｃｕ：Ｃｒ、Ｎｉ、Ｃｕはいずれも耐食性及び強度を向上させる有効な元素である。この効果はそれぞれ０．０５％未満では発現されず、Ｃｒは３％、Ｎｉは５％、Ｃｕは２％を超える過剰添加は靭性を劣化させる。従って、Ｃｒの含有量を０．０５〜３．０％、Ｎｉの含有量を０．０５〜５．０％、Ｃｕの含有量を０．０５〜２．０％、とした。
【００２３】
Ｔｉ； ＴｉはＴｉＮを形成し結晶粒粗大化を抑制する効果があるが、０．００５％未満ではその効果が発現されず、０．３％を超えて過剰添加すると靭性が劣化するため、Ｔｉの含有量を０．００５〜０．３％とした。
【００２４】
Ｎｂ； Ｎｂは微細な炭窒化物を形成し結晶粒粗大化を抑制する効果があるが、０．００５％未満ではその効果が発現されず、０．３％を超えて過剰添加すると靭性が劣化するため、Ｎｂの含有量を０．００５〜０．３％とした。
【００２５】
Ｂ； Ｂは自ら粒界に偏析することにより粒界結合力を向上させるとともにＰ、Ｓ及びＣｕの粒界偏析を抑制し、粒界強度を高め、遅れ破壊特性や靭性を向上させるのに有効な元素であり、また焼入れ性を高めるのに有効な元素でも有る。これらの効果は０．０００３％未満では発現されず、０．０５％を超えて過剰添加すると粒界に粗大な析出物が生成し熱間加工性や靭性が劣化するため、Ｂの含有量を０．０００３〜０．０５％とした。
【００２７】
疲労限界拡散性水素量については０．２ｐｐｍ未満であると、耐水素破壊特性が十分ではなく実際に使用される代表的な環境で水素疲労破壊を生じる場合があるため、０．２ｐｐｍ以上とする。
【００２８】
次に製造条件の限定理由について述べる。
本発明においては、焼入れ焼戻し処理を施す際の焼戻し温度を５００℃以上とすることが重要であり、その他の製造条件は特に制限する必要はない。これは焼戻し温度が５００℃未満では水素トラップサイトとなる（Ｍｏ，Ｖ）2Ｃの析出量が十分に得られないために疲労限界拡散性水素量が低くなるためである。より好ましい条件は５５０℃以上である。焼戻し温度の上限は特に定める必要はないが、焼戻し温度が６５０℃以上になると析出物が粗大化し水素トラップサイトとしての効果が低下するため、６５０℃以下とすることが望ましい。
【００２９】
【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。
【００３０】
表１に示す組成を有する鋼を焼入れた後、表１に示す温度で焼戻しを行った。熱処理後の各鋼片の引張強度を表１に合わせて示す。いずれも１８００ＭＰａ以上の引張強度が得られている。これらの鋼片の耐水素疲労破壊特性について前述した疲労限界拡散性水素量で評価した。なお、疲労試験を行う際の負荷応力は大気中疲労限の９０％の条件で実施した。
【００３１】
【表１】
【００３２】
表1より、本発明例(No.1〜4)ではいずれも疲労限界拡散性水素量が0.2ppm以上であり、耐水素疲労破壊特性が優れている。特に、焼戻し温度が550℃以上のもの(No.1,4)はいずれも疲労限界拡散性水素量が1.0ppm以上であり、耐水素疲労破壊特性が格段に優れている。
【００３３】
一方、Mo量、V量、又は(Mo/V)のいずれか一つ以上が本発明の範囲から逸脱している比較例(No.5,6,7)ではいずれも疲労限界拡散性水素量が0.1ppm以下と低く、耐水素疲労破壊特性に劣ることがわかる。また、Mo量、V量、(Mo/V)は本発明の範囲内にあるがSi量及びMn量が本発明で示した成分範囲から逸脱している比較例(No.8)では疲労限界拡散性水素量が0.1ppm以下と低く、耐水素疲労破壊特性に劣ることがわかる。また、Mo量及び焼戻し温度が本発明の範囲から逸脱している比較例(No.9)では疲労限界拡散性水素量が0.1ppm以下と低く、耐水素疲労破壊特性に劣ることがわかる。
【００３４】
以上より、Ｍｏ、Ｖ、Ｓｉ、Ｍｎの量及びＭｏとＶの添加比率（Ｍｏ／Ｖ）を本発明で示した範囲に特定し、本発明で示した焼戻し条件で製造することにより、１８００MPa以上の引張強度を有しかつ耐水素疲労特性に優れた鋼が得られることが明らかである。
【００３５】
【発明の効果】
以上のように本発明によれば、１８００ＭＰａ以上の引張強度を有し、かつ耐水素疲労特性に優れた機械構造用鋼を得ることができる。
【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】ＭｏとＶの添加比率（Ｍｏ／Ｖ）と疲労限界拡散性水素量の関係を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a steel for machine structure having a tensile strength of 1800 MPa or more used for automobile parts and the like and excellent in hydrogen fatigue resistance, and a method for producing the same.
[0002]
[Prior art]
In recent years, in order to cope with environmental problems, it has been desired to reduce the weight of automobiles for the purpose of reducing carbon dioxide emissions and reducing fuel consumption. In order to reduce the weight of automobiles, increasing the strength of steel is an effective means, and there is a need for steel for machine structures with a tensile strength after quenching and tempering of 1800 MPa or higher.
[0003]
However, in general, when the strength of a steel material is increased, the sensitivity to notch is increased and the steel is easily affected by the environment. Particularly in the corrosive environment, when corrosion pits are formed on the surface, this becomes a stress concentration source and further embrittles due to hydrogen generated as the corrosion reaction progresses, resulting in deterioration of fatigue characteristics and premature breakage. It was. As a method for preventing embrittlement due to hydrogen, a method of refining crystal grains and a method of generating fine precipitates are considered. The property has not been improved.
[0004]
As described above, with the conventional technology, it was difficult to produce a steel for machine structural use having a tensile strength of 1800 MPa or more and excellent hydrogen fatigue resistance.
[0005]
[Problems to be solved by the invention]
The present invention is intended to solve the above-described problems, and provides a steel for machine structural use having a tensile strength of 1800 MPa or more and excellent hydrogen fatigue resistance, and a method for producing the same. With the goal.
[0006]
[Means for Solving the Problems]
In order to solve such problems, the present inventors have conducted research on hydrogen fatigue resistance by a technique for obtaining the fatigue limit diffusible hydrogen content described below for various materials having different components. (Mo, V) It has been found that 2 C is very effective as a hydrogen trap site and greatly increases the fatigue limit diffusible hydrogen content. It was also found that the fatigue limit diffusible hydrogen content can be increased by reducing Si. As a result of further research, the addition ratio of Mo and V (Mo / V) is 5 to 20, Si is less than 0.5%, and Mn, which lowers the grain boundary strength, is 0.5% or less. Thus, it was found that a steel having a tensile strength of 1800 MPa or more and excellent in hydrogen fatigue resistance can be obtained.
[0007]
The present invention is configured based on such knowledge, the gist of which is
(1) Mechanical structural steel having a tensile strength of 1800 MPa or more, in mass%, C: 0.35 to 0.6%, Si: less than 0.5%, Mn: 0.1 to 0.5%, Mo: 0.4 to 3.0%, V: 0.02 to 0.5%, P: 0.02% or less, S: 0.02 or less, Al: 0.005 to 0.1%, N: 0.001 to 0.0079% , and 5 ≦ (Mo / V) ≦ 20 is satisfied, Machine structural steel with excellent hydrogen fatigue fracture resistance, characterized in that the balance consists of Fe and inevitable impurities .
(2) containing components (1) Symbol placement, in addition mass%, Ti: 0.005~0.3%, Nb : 0.005~0.3%, B: containing 0.0003 to 0.05 percent, one or more A machine structural steel with excellent hydrogen fatigue fracture resistance.
(3) containing components of the (2) described in further mass%, Cr: 0.05 ~ 3.0% , Ni: 0.05 ~ 5.0%, Cu: 0.05 ~ 2.0%, containing one or more A machine structural steel with excellent hydrogen fatigue fracture resistance.
(4 ) Mechanical structure excellent in hydrogen fatigue fracture resistance, characterized by quenching the steel comprising the component according to any one of (1) to (3) and then tempering at 500 ° C. or higher. Steel manufacturing method.
It is in. "
[0008]
DETAILED DESCRIPTION OF THE INVENTION
The inventors first analyzed hydrogen fatigue behavior in detail using mechanical structural 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. Diffusible hydrogen can be measured as a curve having a peak at a temperature of about 100 ° C. in the curve of “temperature—hydrogen release rate from steel” obtained when the steel is heated at a rate of 100 ° C./hour (FIG. 1). ). Therefore, by capturing hydrogen that has entered from the external environment in some part of the steel material so that it does not diffuse in the steel material, it becomes possible to render the hydrogen harmless and suppress a decrease in fatigue life.
[0009]
Therefore, the hydrogen fatigue resistance was evaluated by determining the “fatigue limit diffusible hydrogen content” 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. 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 diffusible hydrogen amount”. Since hydrogen trapped by (Mo, V) 2 C, etc. is unlikely to adversely affect fatigue properties, steel materials containing such trap sites have reduced fatigue strength due to hydrogen even if the amount of diffusible hydrogen is high. Is less likely to occur. Therefore, such a steel material has a high fatigue limit diffusible hydrogen content. The amount of diffusible hydrogen that penetrates into steel in a real environment increases as the environment is severer. Therefore, steel materials with higher fatigue limit diffusible hydrogen content have a lower fatigue strength due to hydrogen even in more severe environments (diffusible hydrogen content: large). Since this does not occur, the higher the fatigue limit diffusible hydrogen amount, the better the hydrogen fatigue resistance of the steel material, which is a value inherent to the steel material determined by the production conditions such as the components of the steel material and heat treatment.
[0010]
Below, the meaning of each requirement in this invention and the reason for limitation are demonstrated concretely.
[0011]
The present inventors tempered steel with 0.5% C-0.06% Si-0.2% Mn as a base component and various addition ratios of Mo and V to the same strength level by quenching and tempering treatment. Then, the fatigue limit diffusible hydrogen content was measured. FIG. 3 shows the relationship between the addition ratio of Mo and V (Mo / V) and the fatigue limit diffusible hydrogen content. 3 that the fatigue limit diffusible hydrogen content is greatly improved when Mo / V is 5 or more and 20 or less. Therefore, Mo / V was set to 5 or more and 20 or less.
[0012]
Next, the reasons for limiting the components of the high strength spring steel in the present invention will be described.
[0013]
C; C is added as an element for increasing the strength of steel. If it is less than 0.1%, it is difficult to ensure the strength required for steel for machine structural use, and excessive addition exceeding 0.6% significantly deteriorates toughness. Therefore, the C content is set to 0.1 to 0.6%. In addition, based on description of the below-mentioned Example, the lower limit of C content was 0.35% .
[0014]
Si; Si is added as a deoxidizer, but excessive addition of 0.5% or more reduces the fatigue limit diffusible hydrogen content and degrades the hydrogen fatigue characteristics. Therefore, the smaller the Si content, the better.
[0015]
Mn; Mn is an element effective for improving hardenability, but on the other hand, it is a harmful element that embrittles grain boundaries and deteriorates the resistance to hydrogen fatigue fracture. If it is less than 0.1%, the effect of enhancing the hardenability is not exhibited, and excessive addition exceeding 0.5% deteriorates the hydrogen fracture fatigue resistance. Therefore, the Mn content is set to 0.1 to 0.5%. In order to obtain better hydrogen fatigue fracture resistance, the Mn content is desirably 0.3% or less, and more desirably 0.2% or less.
[0016]
Mo; Mo is an essential element that improves hydrogen fatigue fracture resistance by forming (Mo, V) 2 C together with V and C and trapping diffusible hydrogen, but the effect is less than 0.4%. Since Mo is not expressed and excessive addition exceeding 3.0% reduces toughness, the Mo content is set to 0.4 to 3.0%.
[0017]
V; V is an essential element for improving hydrogen fatigue fracture resistance by forming (Mo, V) 2 C together with Mo and C and trapping diffusible hydrogen. However, if less than 0.02%, V is effective. Since V is not expressed and excessive addition exceeding 0.5% lowers toughness, the V content is set to 0.02 to 0.5%.
[0018]
P; P is an impurity element that segregates at the grain boundary to lower the grain boundary strength and deteriorates toughness, and is preferably as low as possible. However, the upper limit is set in consideration of the reachable level and cost of the current refining technology. Was 0.02%.
[0019]
S; S is an impurity element that degrades hot workability and toughness, and it is desirable that the level be as low as possible. However, the upper limit was set to 0.02% in consideration of the reachable level and cost of current refining technology.
Al; Al is effective as a deoxidizer and suppresses coarsening of grains by forming AlN. However, if less than 0.005%, the effect is not expressed, and if added over 0.1%, toughness deteriorates. The Al content was 0.005 to 0.1%.
N; N has the effect of forming nitrides and suppressing crystal grain coarsening, but if less than 0.001%, the effect is not expressed, and if added over 0.05%, the toughness deteriorates, so the N content is 0.001 ˜0.05%. Note that the upper limit of the N content was set to 0.0079% based on the description in the examples described later .
[0020]
The above is the basic component of the present invention, is usually other than the above consists of Fe and unavoidable impurities, depending on the desired intensity level and other necessary properties, Cr, Ni, Cu, T i, Nb, B, of it may be added alone or in combination of two or more.
[0021]
Cr, Ni, Cu: Cr, Ni, and Cu are all effective elements that improve corrosion resistance and strength. This effect is not manifested at less than 0.05%, and excessive addition of Cr over 3%, Ni over 5% and Cu over 2% degrades toughness. Therefore, the Cr content is set to 0.05 to 3.0%, the Ni content is set to 0.05 to 5.0%, and the Cu content is set to 0.05 to 2.0%.
[0023]
Ti; Ti has the effect of forming TiN and suppressing grain coarsening. However, if less than 0.005%, the effect is not expressed, and if over 0.3% is added, toughness deteriorates. The content of was made 0.005 to 0.3%.
[0024]
Nb; Nb has the effect of forming fine carbonitrides and suppressing crystal grain coarsening, but if less than 0.005%, the effect is not expressed, and if added over 0.3%, toughness deteriorates. Therefore, the Nb content is set to 0.005 to 0.3%.
[0025]
B; B improves segregation at grain boundaries by itself and improves intergranular bonding force, suppresses grain boundary segregation of P, S and Cu, increases grain boundary strength, and is effective in improving delayed fracture characteristics and toughness It is also an effective element for enhancing the hardenability. These effects are not manifested at less than 0.0003%, and excessive addition of more than 0.05% produces coarse precipitates at the grain boundaries and deteriorates hot workability and toughness. It was 0.0003 to 0.05%.
[0027]
If the amount of fatigue limit diffusible hydrogen is less than 0.2 ppm, hydrogen fatigue resistance is not sufficient and hydrogen fatigue failure may occur in typical environments actually used. .
[0028]
Next, the reasons for limiting the manufacturing conditions will be described.
In the present invention, it is important that the tempering temperature at the time of quenching and tempering is 500 ° C. or higher, and other production conditions are not particularly limited. This is because when the tempering temperature is less than 500 ° C., the precipitation amount of (Mo, V) 2 C that becomes hydrogen trap sites cannot be obtained sufficiently, and the fatigue limit diffusible hydrogen amount becomes low. A more preferable condition is 550 ° C. or higher. The upper limit of the tempering temperature is not particularly required. However, when the tempering temperature is 650 ° C. or higher, precipitates are coarsened and the effect as a hydrogen trap site is reduced.
[0029]
【Example】
Hereinafter, the effects of the present invention will be described more specifically with reference to examples.
[0030]
After quenching steel having the composition shown in Table 1, tempering was performed at the temperature shown in Table 1. Table 1 shows the tensile strength of each steel slab after heat treatment. In either case, a tensile strength of 1800 MPa or more is obtained. The hydrogen fatigue fracture resistance of these steel slabs was evaluated by the fatigue limit diffusible hydrogen content described above. In addition, the load stress at the time of performing a fatigue test was implemented on the conditions of 90% of the atmospheric fatigue limit.
[0031]
[Table 1]
[0032]
From Table 1, in all of the inventive examples (Nos. 1 to 4 ), the fatigue limit diffusible hydrogen amount is 0.2 ppm or more, and the hydrogen fatigue fracture resistance is excellent. In particular, those with a tempering temperature of 550 ° C. or higher (Nos. 1 and 4) all have a fatigue limit diffusible hydrogen content of 1.0 ppm or higher, and have excellent resistance to hydrogen fatigue fracture.
[0033]
On the other hand, in the comparative examples (No. 5 , 6 , 7 ) in which at least one of Mo amount, V amount, and (Mo / V) deviates from the scope of the present invention, the fatigue limit diffusible hydrogen amount Is as low as 0.1 ppm or less, indicating that the resistance to hydrogen fatigue fracture is poor. Further, in the comparative example (No. 8 ) in which the Mo amount, V amount, and (Mo / V) are within the scope of the present invention, but the Si amount and Mn amount deviate from the component ranges indicated in the present invention, the fatigue limit It can be seen that the amount of diffusible hydrogen is as low as 0.1 ppm or less and the resistance to hydrogen fatigue fracture is poor. Further, in the comparative example (No. 9 ) in which the Mo amount and the tempering temperature deviate from the scope of the present invention, the fatigue limit diffusible hydrogen amount is as low as 0.1 ppm or less, indicating that the resistance to hydrogen fatigue fracture is poor.
[0034]
From the above, the amount of Mo, V, Si, Mn and the addition ratio of Mo and V (Mo / V) are specified in the range shown in the present invention, and manufactured under the tempering conditions shown in the present invention, 1800 MPa or more It is clear that a steel having a tensile strength of 5 and excellent in hydrogen fatigue resistance can be obtained.
[0035]
【The invention's effect】
As described above, according to the present invention, it is possible to obtain mechanical structural steel having a tensile strength of 1800 MPa or more and excellent hydrogen fatigue resistance.
[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 graph showing the relationship between the addition ratio of Mo and V (Mo / V) and the amount of fatigue limit diffusible hydrogen.
Claims (4)
質量%で、
C:0.35〜0.6%、
Si:0.5%未満、
Mn:0.1〜0.5%、
Mo:0.4〜3.0%、
V:0.02〜0.5%、
P:0.02%以下、
S:0.02%以下、
Al:0.005〜0.1%、
N:0.001〜0.0079%、
を含有し、かつ
5≦(Mo/V)≦20
を満足し、残部がFe及び不可避的不純物からなることを特徴とする耐水素疲労破壊特性に優れた機械構造用鋼。 A mechanical structural steel having a tensile strength of 1800 MPa or more,
% By mass
C: 0.35-0.6%
Si: less than 0.5%,
Mn: 0.1-0.5%
Mo: 0.4-3.0%,
V: 0.02-0.5%
P: 0.02% or less,
S: 0.02% or less,
Al: 0.005-0.1%,
N: 0.001 to 0.0079% ,
And containing
5 ≦ (Mo / V) ≦ 20
Satisfying the requirements, and the balance is made of Fe and inevitable impurities, and has excellent hydrogen fatigue fracture resistance, and is used for machine structural steel.
Ti:0.005〜0.3%、
Nb:0.005〜0.3%、
B:0.0003〜0.05%、
の1種または2種以上を含有することを特徴とする請求項1 記載の耐水素疲労破壊特性に優れた機械構造用鋼。In addition,
Ti: 0.005-0.3%,
Nb: 0.005-0.3%,
B: 0.0003-0.05%
One or characterized by containing two or more claims 1 Symbol mounting resistance to hydrogen fatigue fracture properties superior mechanical structural steel of.
Cr:0.05Cr: 0.05 〜~ 3.0%3.0% 、,
Ni:0.05Ni: 0.05 〜~ 5.0%5.0% 、,
Cu:0.05Cu: 0.05 〜~ 2.0%2.0% 、,
のof 11 種またはSeed or 22 種以上を含有することを特徴とする請求項Claims containing more than species 22 記載の耐水素疲労破壊特性に優れた機械構造用鋼。Machine structural steel with excellent resistance to hydrogen fatigue fracture.
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