JP2010185103A - Steel for machine structure for cold working, method for producing the same, and component for machine structure - Google Patents
Steel for machine structure for cold working, method for producing the same, and component for machine structure Download PDFInfo
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本発明は、冷間加工により機械構造用部品に製造される機械構造用鋼に関する。 The present invention relates to a machine structural steel manufactured into a machine structural component by cold working.
自動車等の機械構造用部品であるボルト・ナット、ピニオンギヤ、ステアリングシャフト、バルブリフター、コモンレール等は、近年、軽量化のために高強度化が要求されている。 Bolts / nuts, pinion gears, steering shafts, valve lifters, common rails, and the like, which are parts for machine structures such as automobiles, have recently been required to have high strength for weight reduction.
一方、自動車用変速機や差動装置等の各種歯車伝達装置に利用されるクランクシャフト、コンロッド、トランスミッションギヤ等の機械構造用部品は、一般に、鋼材に鍛造等の熱間加工を施した後、切削加工を施すことによって最終形状に仕上げられる。このような機械構造用部品においても、製造工程におけるＣＯ2の排出量削減のため、熱間加工に代えて冷間加工による鍛造で製造することが要求されている。 On the other hand, crankshafts, connecting rods, transmission gears, and other mechanical structural parts used in various gear transmissions such as automobile transmissions and differentials are generally subjected to hot working such as forging on steel. It is finished to the final shape by cutting. Such mechanical structural parts are also required to be manufactured by forging by cold working instead of hot working in order to reduce CO 2 emission in the manufacturing process.
冷間鍛造は、熱間鍛造と異なり高温下での工程ではないため、冷却による形状、寸法の変化が小さく、これらの精度がよいという利点がある。一方、熱間鍛造に比べて変形抵抗が高く変形能が小さいため、加工時に鋼材や金型に割れが発生し易いという難点がある。冷間加工により製造される機械構造用部品の強度向上に際しては、このような加工性を低下させないことが必要である。 Since cold forging is not a process under high temperature unlike hot forging, there is an advantage that changes in shape and dimensions due to cooling are small, and these precisions are good. On the other hand, since the deformation resistance is high and the deformability is small compared to hot forging, there is a problem that cracks are likely to occur in the steel material and the mold during processing. In order to improve the strength of machine structural parts manufactured by cold working, it is necessary not to deteriorate such workability.
鋼材の高強度化の方法の一つとして、結晶粒の微細化が有効であることが知られている。すなわち、結晶粒径が小さくなるにしたがい降伏強度が大きくなるというHall-Petchの法則によるものである。結晶粒の微細化には、冷間加工や温間加工によって加工ひずみを付与して結晶粒を直接伸張させたり動的再結晶させる方法、加熱後の急冷や加熱冷却を繰り返す相変態熱処理を利用する方法等が用いられる。 As one of the methods for increasing the strength of steel materials, it is known that refinement of crystal grains is effective. That is, according to Hall-Petch's law that the yield strength increases as the crystal grain size decreases. For crystal grain refinement, a method of directly stretching or dynamically recrystallizing a crystal grain by applying a working strain by cold working or warm working, and a phase transformation heat treatment that repeats rapid cooling and heating cooling after heating are used. Or the like is used.
例えば、特許文献１は、平均結晶粒径を、温間加工により３μｍ以下、さらに冷間加工により５００ｎｍ以下にまで極微細化したフェライト相を主組織とすることにより高強度とした鋼線または棒鋼を開示している。また、特許文献２は、パーライト組織とした高炭素鋼に伸線加工を施すことにより、パーライトラメラ間隔を微細化かつ均一化して引張強度を向上させた極細鋼線を開示している。また、特許文献３は、マルテンサイト組織とした鋼を熱処理と加工を同時に施すことによって動的逆変態を生じさせて極微細な等軸オーステナイト粒を形成し、直ちに冷却することによってサブミクロンオーダーの微細組織として強度と靭性を向上させた鋼材を製造する方法を開示している。 For example, Patent Document 1 discloses a steel wire or steel bar having an average crystal grain size of 3 μm or less by warm working and high strength by using a ferrite phase as a main structure that has been refined to 500 nm or less by cold working. Is disclosed. Further, Patent Document 2 discloses an ultrafine steel wire in which tensile strength is improved by performing a wire drawing process on a high carbon steel having a pearlite structure so that the pearlite lamella spacing is refined and uniformized. Further, Patent Document 3 discloses that a steel having a martensitic structure is subjected to heat treatment and processing simultaneously to cause dynamic reverse transformation to form ultrafine equiaxed austenite grains, which are immediately cooled to have a submicron order. A method for producing a steel material with improved strength and toughness as a microstructure is disclosed.
しかしながら、特許文献１，２に開示された微細組織の鋼材とするためには、総減面率の大きな温間加工や伸線加工を施す必要があり、装置の制約上、加工前の鋼は大きさに限界があるため、得られる鋼材は大幅に縮小された結果、線状や棒状になる。また、特許文献３に開示された逆変態を生じさせる処理方法は、１パスの加工率が極めて大きく、工業的に製造が困難である。 However, in order to make the steel material of the fine structure disclosed in Patent Documents 1 and 2, it is necessary to perform warm processing and wire drawing processing with a large total area reduction rate. Since the size is limited, the resulting steel material is greatly reduced, resulting in a linear or bar shape. Further, the processing method for causing reverse transformation disclosed in Patent Document 3 has a very high processing rate for one pass and is difficult to manufacture industrially.
本発明は、前記問題点に鑑みてなされたものであり、冷間加工により製造でき、強度に優れ、かつクランクシャフト等のような大きな部品とすることもできる機械構造用部品、ならびにこのような機械構造用部品に製造できる冷間加工性を有する機械構造用鋼を提供することを目的とする。 The present invention has been made in view of the above problems, and can be manufactured by cold working, has excellent strength, and can be a large part such as a crankshaft. It is an object of the present invention to provide a machine structural steel having cold workability that can be manufactured into a machine structural component.
前記課題を解決するために、本発明者らは、機械構造用鋼の組織の構成について検討した。一般的に鋼材の強度向上に有効とされるマルテンサイト組織等の硬質相を、冷間加工性を保持するための軟質相であるフェライト相に分布させた構成では、冷間加工されると、変形時に軟質相と硬質相との界面が応力集中の起点となり、割れが発生し易い。また、軟質相に動的ひずみ時効が集中し、硬質相にひずみが入り易くなって付与するひずみ以上に硬質相が加工硬化するため、静的ひずみ時効による強度向上は見込めるものの、それ以上に変形抵抗が増加して冷間加工性が劣化してしまう。そこで、冷間加工性を付与するために、軟質のフェライト相を主組織とし、また、ある程度の大きさの機械構造用鋼を得るために、極度に大きな加工率とせずに冷間加工で得られる数ミクロンオーダーの結晶粒径の鋼材とした。一方で、この程度の大きさの結晶粒径でかつフェライト単相であっても、冷間加工にて機械構造用部品とした後の強度が向上するように、固溶させたＮ（窒素）により冷間加工時に強度を増加させることに至った。 In order to solve the above-mentioned problems, the present inventors examined the structure of the structure of steel for machine structural use. In a configuration in which a hard phase such as a martensite structure, which is generally effective for improving the strength of a steel material, is distributed in a ferrite phase that is a soft phase for maintaining cold workability, At the time of deformation, the interface between the soft phase and the hard phase becomes the starting point of stress concentration, and cracks are likely to occur. In addition, dynamic strain aging concentrates in the soft phase, and the hard phase hardens more than the strain imparted because the hard phase becomes more easily strained. Resistance increases and cold workability deteriorates. Therefore, in order to provide cold workability, a soft ferrite phase is used as the main structure, and in order to obtain a mechanical structural steel of a certain size, it can be obtained by cold working without extremely high working rate. A steel material having a crystal grain size on the order of several microns was obtained. On the other hand, even with a crystal grain size of this size and a ferrite single phase, N (nitrogen) dissolved so that the strength after forming into a machine structural part by cold working is improved. As a result, the strength was increased during cold working.
すなわち、本発明に係る機械構造用鋼は、Ｃ：０．００５〜０．０４５質量％、Ｓｉ：０．００５〜０．０５質量％、Ｍｎ：０．４〜１．０質量％、Ａｌ：０．０１〜０．０６質量％、Ｓ：０．００５〜０．０５質量％、Ｐ：０．０５質量％以下、Ｎ：０．００９〜０．０２質量％を含有し、残部がＦｅおよび不可避的不純物からなる組成を有し、Ｎ固溶量が０．００８５質量％以上であり、フェライト相の組織分率が９０％以上であり、フェライト結晶粒の平均粒径が７μｍ以下であり、引張強度が５００ＭＰａ以上であり、前記フェライト結晶粒の平均粒径（μｍ）をｄ、前記引張強度（ＭＰａ）をＴＳとしてそれぞれ表したとき、０≦ＴＳ−１２００／√ｄ≦３００を満足することを特徴とする。 That is, the steel for machine structure according to the present invention has C: 0.005 to 0.045% by mass, Si: 0.005 to 0.05% by mass, Mn: 0.4 to 1.0% by mass, Al: 0.01 to 0.06 mass%, S: 0.005 to 0.05 mass%, P: 0.05 mass% or less, N: 0.009 to 0.02 mass%, with the balance being Fe and It has a composition consisting of inevitable impurities, the N solid solution amount is 0.0085% by mass or more, the ferrite fraction has a structure fraction of 90% or more, and the average grain size of ferrite crystal grains is 7 μm or less, When the tensile strength is 500 MPa or more and the average grain size (μm) of the ferrite crystal grains is expressed as d and the tensile strength (MPa) is expressed as TS, 0 ≦ TS-1200 / √d ≦ 300 is satisfied. It is characterized by.
このように、低炭素鋼とすることにより、主組織をフェライト相として冷間加工性を付与することができる。そして、溶製中の脱酸、脱硫のためにＭｎを添加し、一方、脱酸元素であるＳｉは冷間加工性を低下させないように、同じく脱酸元素であるＡｌはＮの固溶量を低減させないように、それぞれ微量の添加とする。また、Ｎを十分に固溶させることで、軟質のフェライト組織を、冷間加工後に加工硬化分以上に強度を増加させることができる。また、フェライト結晶粒径を所定値以下として引張強度を確保し、かつ引張強度に対して小さくなりすぎないように制御することにより、冷間加工性を劣化させないことができる。 Thus, by using low-carbon steel, cold workability can be imparted with the main structure as the ferrite phase. Then, Mn is added for deoxidation and desulfurization during melting, while Al, which is also a deoxidizing element, is a solid solution amount of N so that Si, which is a deoxidizing element, does not deteriorate cold workability. In order not to reduce the amount, each is added in a small amount. Further, by sufficiently dissolving N, the strength of the soft ferrite structure can be increased more than the work-hardened content after cold working. Moreover, cold workability can be prevented from deteriorating by controlling the ferrite crystal grain size to be equal to or smaller than a predetermined value to ensure the tensile strength and not to be too small with respect to the tensile strength.
また、本発明に係る機械構造用鋼において、前記組成がさらに、Ｃｒ：２質量％以下、およびＭｏ：２質量％以下のうち１種以上を含有してもよい。Ｃｒ，Ｍｏを添加することにより、機械構造用鋼の冷間加工性および冷間加工後の硬さを向上させることができる。 Moreover, in the steel for machine structure according to the present invention, the composition may further contain one or more of Cr: 2% by mass or less and Mo: 2% by mass or less. By adding Cr and Mo, the cold workability of the steel for machine structure and the hardness after cold work can be improved.
また、本発明に係る機械構造用鋼において、前記組成がさらに、Ｔｉ：０．２質量％以下、Ｎｂ：０．２質量％以下、およびＶ：０．２質量％以下のうち１種以上を含有してもよい。Ｔｉ，Ｎｂ，Ｖを添加することにより、これらの窒素化合物が形成されて機械構造用鋼の冷間加工後の靭性を高くして耐割れ性を向上させることができる。 In the steel for machine structure according to the present invention, the composition further includes at least one of Ti: 0.2% by mass or less, Nb: 0.2% by mass or less, and V: 0.2% by mass or less. You may contain. By adding Ti, Nb, and V, these nitrogen compounds are formed, and the toughness after cold working of the steel for mechanical structure can be increased to improve the crack resistance.
また、本発明に係る機械構造用鋼において、前記組成がさらに、Ｂ：０．００５質量％以下を含有してもよい。Ｂを添加することにより、不可避的に含有されるＰのフェライト粒界偏析による粒界強度の低下を抑制することができる。 In the steel for machine structure according to the present invention, the composition may further contain B: 0.005% by mass or less. By adding B, it is possible to suppress a decrease in grain boundary strength due to segregation of P ferrite grain boundaries unavoidably contained.
また、本発明に係る機械構造用鋼において、前記組成がさらに、Ｃｕ：５質量％以下、Ｎｉ：５質量％以下、およびＣｏ：５質量％以下のうち１種以上を含有してもよい。Ｃｕ，Ｎｉ，Ｃｏを添加することにより、機械構造用鋼のひずみ時効を促進させて冷間加工後の強度を向上させることができる。 In the steel for machine structure according to the present invention, the composition may further contain one or more of Cu: 5% by mass or less, Ni: 5% by mass or less, and Co: 5% by mass or less. By adding Cu, Ni and Co, the strain aging of the mechanical structural steel can be promoted and the strength after cold working can be improved.
また、本発明に係る機械構造用鋼において、前記組成がさらに、Ｃａ：０．０５質量％以下、ＲＥＭ：０．０５質量％以下、Ｍｇ：０．０２質量％以下、Ｌｉ：０．０２質量％以下、Ｐｂ：０．５質量％以下、およびＢｉ：０．５質量％以下のうち１種以上を含有してもよい。これらの元素を添加することにより、機械構造用鋼の冷間加工性および冷間加工後の被削性を向上させることができる。 In the mechanical structural steel according to the present invention, the composition further includes Ca: 0.05% by mass or less, REM: 0.05% by mass or less, Mg: 0.02% by mass or less, Li: 0.02% by mass. % Or less, Pb: 0.5% by mass or less, and Bi: 0.5% by mass or less. By adding these elements, the cold workability of the machine structural steel and the machinability after cold work can be improved.
また、本発明に係る機械構造用鋼の製造方法は、前記組成の鋼を１０００℃以上に加熱した後、熱間加工し、この熱間加工後、１０００℃から２００℃以下まで冷却速度１．５℃／ｓｅｃ以上で冷却する熱間加工工程と、開始温度２００℃未満、ひずみ量０．３以上で冷間加工する冷間加工工程と、を行うことを特徴とする。熱間加工において所定以上の温度に加熱することでＮを鋼中に固溶させ、十分なひずみ量を付与して冷間加工することにより、結晶粒を微細化して強度を向上させることができる。 In addition, in the method for manufacturing steel for machine structural use according to the present invention, the steel having the above composition is heated to 1000 ° C. or higher and then hot-worked, and after this hot working, a cooling rate of 1.degree. A hot working process for cooling at 5 ° C./sec or more and a cold working process for cold working at a start temperature of less than 200 ° C. and a strain amount of 0.3 or more are performed. By heating to a temperature higher than a predetermined temperature in hot working, N can be dissolved in steel, and by applying a sufficient strain amount and cold working, the crystal grains can be refined and the strength can be improved. .
本発明に係る機械構造用部品は、前記機械構造用鋼を、開始温度２００℃未満で冷間加工して製造される。このような冷間加工によりさらに強度が増加される。 The machine structural component according to the present invention is manufactured by cold working the machine structural steel at a start temperature of less than 200 ° C. Such cold working further increases the strength.
本発明に係る機械構造用鋼は、冷間加工性を十分に有し、またある程度の大きさの鋼材に容易に製造可能である。そして、本発明に係る機械構造用鋼の製造方法によれば、前記の機械構造用鋼を安定して製造することができる。また、このような機械構造用鋼を冷間加工して製造された本発明に係る機械構造用部品は、強度が向上されて部品の軽量化を可能とするものである。 The steel for machine structure according to the present invention has sufficient cold workability and can be easily manufactured into a steel material having a certain size. And according to the manufacturing method of steel for machine structure concerning the present invention, the steel for machine structure can be manufactured stably. In addition, the mechanical structural component according to the present invention manufactured by cold working such mechanical structural steel has improved strength and enables weight reduction of the component.
〔機械構造用鋼〕
以下、本発明に係る機械構造用鋼を実施するための形態について説明する。
本発明に係る機械構造用鋼の組成は、Ｃ：０．００５〜０．０４５質量％、Ｓｉ：０．００５〜０．０５質量％、Ｍｎ：０．４〜１．０質量％、Ａｌ：０．０１〜０．０６質量％、Ｓ：０．００５〜０．０５質量％、Ｐ：０．０５質量％以下、Ｎ：０．００９〜０．０２質量％を含有し、残部がＦｅおよび不可避的不純物からなる。
以下に、本発明に係る機械構造用鋼の組成の各成分の含有量の数値範囲およびその数値範囲の限定理由について説明する。
[Mechanical structural steel]
Hereinafter, the form for implementing the steel for machine structure concerning the present invention is explained.
The composition of the steel for machine structure according to the present invention is as follows: C: 0.005 to 0.045 mass%, Si: 0.005 to 0.05 mass%, Mn: 0.4 to 1.0 mass%, Al: 0.01 to 0.06 mass%, S: 0.005 to 0.05 mass%, P: 0.05 mass% or less, N: 0.009 to 0.02 mass%, with the balance being Fe and Consists of inevitable impurities.
Below, the numerical range of content of each component of the composition of the steel for machine structure according to the present invention and the reason for limiting the numerical range will be described.
（Ｃ：０．００５〜０．０４５質量％）
Ｃは、機械構造用鋼を軟質のフェライト単相とするために極力低減する必要がある。ただし、Ｃ含有量が極端に少ないと溶製中の脱酸が困難になり、０．００５質量％未満では溶製時にガス欠陥が発生し易くなって歩留りが低下する。したがって、Ｃ含有量は０．００５質量％以上とし、０．０１質量％以上が好ましく、０．０１５質量％以上がさらに好ましい。一方、Ｃ含有量が０．０４５質量％を超えると、セメンタイトが硬質のパーライトを形成するようになり、フェライト−パーライトの複相組織となって冷間加工性を劣化させる。したがって、Ｃ含有量は０．０４５質量％以下とし、０．０４３質量％以下が好ましく、０．０４質量％以下がさらに好ましい。
(C: 0.005-0.045% by mass)
C needs to be reduced as much as possible in order to make mechanical structural steel a soft ferrite single phase. However, if the C content is extremely low, deoxidation during melting becomes difficult, and if it is less than 0.005% by mass, gas defects are likely to occur during melting and yield decreases. Therefore, the C content is 0.005% by mass or more, preferably 0.01% by mass or more, and more preferably 0.015% by mass or more. On the other hand, if the C content exceeds 0.045% by mass, cementite will form hard pearlite, which will become a ferrite-pearlite multiphase structure and deteriorate the cold workability. Therefore, the C content is 0.045% by mass or less, preferably 0.043% by mass or less, and more preferably 0.04% by mass or less.
（Ｓｉ：０．００５〜０．０５質量％）
Ｓｉは脱酸効果を有し、含有量が０．００５質量％未満では溶製時にガス欠陥が発生し易くなる。したがって、Ｓｉ含有量は０．００５質量％以上とし、０．００７質量％以上が好ましく、０．０１質量％以上がさらに好ましい。一方、Ｓｉはフェライト相を固溶強化させるため、含有量が０．０５質量％を超えると、変形抵抗の増大すなわち冷間加工性の低下を生じさせる。したがって、Ｓｉ含有量は０．０５質量％以下とし、０．０４質量％以下が好ましく、０．０３質量％以下がさらに好ましい。
(Si: 0.005 to 0.05 mass%)
Si has a deoxidizing effect, and if the content is less than 0.005% by mass, gas defects are likely to occur during melting. Therefore, the Si content is 0.005% by mass or more, preferably 0.007% by mass or more, and more preferably 0.01% by mass or more. On the other hand, since Si strengthens the ferrite phase by solid solution, when the content exceeds 0.05 mass%, an increase in deformation resistance, that is, a decrease in cold workability is caused. Therefore, the Si content is 0.05% by mass or less, preferably 0.04% by mass or less, and more preferably 0.03% by mass or less.
（Ｍｎ：０．４〜１．０質量％）
Ｍｎは、脱酸、脱硫効果を有し、また、Ｓと結合してＭｎＳとして析出して機械構造用鋼の変形能を向上させる。含有量が０．４質量％未満ではこれらの効果が不十分である。したがって、Ｍｎ含有量は０．４質量％以上とし、０．４２質量％以上が好ましく、０．４５質量％以上がさらに好ましい。一方、Ｍｎはフェライト相を固溶強化させるため、含有量が１．０質量％を超えると、変形抵抗の顕著な増大すなわち冷間加工性の低下を生じさせる。したがって、Ｍｎ含有量は１．０質量％以下とし、０．９８質量％以下が好ましく、０．９５質量％以下がさらに好ましい。
(Mn: 0.4 to 1.0% by mass)
Mn has a deoxidation and desulfurization effect, and combines with S to precipitate as MnS to improve the deformability of the steel for machine structural use. If the content is less than 0.4% by mass, these effects are insufficient. Therefore, the Mn content is 0.4% by mass or more, preferably 0.42% by mass or more, and more preferably 0.45% by mass or more. On the other hand, since Mn strengthens the ferrite phase by solid solution, when the content exceeds 1.0% by mass, the deformation resistance is remarkably increased, that is, the cold workability is decreased. Therefore, the Mn content is 1.0% by mass or less, preferably 0.98% by mass or less, and more preferably 0.95% by mass or less.
（Ａｌ：０．０１〜０．０６質量％）
Ａｌは、強い脱酸効果を有して機械構造用鋼の内部品質を向上させ、含有量が０．０１質量％未満では溶製時にガス欠陥が発生し易くなる。したがって、Ａｌ含有量は０．０１質量％以上とし、０．０１５質量％以上が好ましく、０．０２質量％以上がさらに好ましい。一方、Ａｌは鋼中のＮと結合してＡｌＮを形成するためＮの固溶量を低下させ、含有量が０．０６質量％を超えると固溶量を不足させる。したがって、Ａｌ含有量は０．０６質量％以下とし、０．０５５質量％以下が好ましく、０．０５質量％以下がさらに好ましい。
(Al: 0.01-0.06 mass%)
Al has a strong deoxidizing effect and improves the internal quality of steel for machine structural use. If the content is less than 0.01% by mass, gas defects are likely to occur during melting. Therefore, the Al content is 0.01% by mass or more, preferably 0.015% by mass or more, and more preferably 0.02% by mass or more. On the other hand, Al combines with N in the steel to form AlN, so the solid solution amount of N is reduced, and when the content exceeds 0.06% by mass, the solid solution amount becomes insufficient. Accordingly, the Al content is 0.06% by mass or less, preferably 0.055% by mass or less, and more preferably 0.05% by mass or less.
（Ｓ：０．００５〜０．０５質量％）
Ｓは、鋼に不可避的に含まれる元素であり、被削性を向上させる効果を有し、含有量が０．００５質量％未満では被削性が劣化する。したがって、Ｓ含有量は０．００５質量％以上とし、０．００７質量％以上が好ましく、０．０１質量％以上がさらに好ましい。一方で、Ｓは、Ｆｅと結合してＦｅＳとして粒界上に膜状に析出して、冷間加工性を劣化させる。したがって、Ｓはその全量をＭｎと結合させ、ＭｎＳとして析出させる必要があるが、ＭｎＳの析出量が過剰になるとやはり冷間加工性が劣化する。したがって、Ｓ含有量は０．０５質量％以下とし、０．０４質量％以下が好ましく、０．０３質量％以下がさらに好ましい。
(S: 0.005 to 0.05 mass%)
S is an element inevitably contained in steel and has an effect of improving machinability. When the content is less than 0.005% by mass, the machinability deteriorates. Therefore, the S content is 0.005% by mass or more, preferably 0.007% by mass or more, and more preferably 0.01% by mass or more. On the other hand, S is combined with Fe and deposited as a film on the grain boundary as FeS, thereby deteriorating cold workability. Therefore, the entire amount of S must be combined with Mn and precipitated as MnS. However, when the amount of MnS precipitated becomes excessive, cold workability is deteriorated. Therefore, the S content is 0.05% by mass or less, preferably 0.04% by mass or less, and more preferably 0.03% by mass or less.
（Ｐ：０．０５質量％以下）
Ｐは鋼に不可避的に含まれる元素（不純物）である。Ｐは、フェライト粒界に偏析して冷間加工性を劣化させ、また、フェライトを固溶強化させて変形抵抗を増大させるので、可能な限り低減されることが好ましい。したがって、Ｐ含有量は０．０５質量％以下とし、０．０４質量％以下が好ましく、０．０３質量％以下がさらに好ましい。
(P: 0.05% by mass or less)
P is an element (impurity) inevitably contained in steel. P segregates at the ferrite grain boundaries to deteriorate the cold workability, and strengthens the solid solution to increase the deformation resistance. Therefore, P is preferably reduced as much as possible. Therefore, the P content is 0.05% by mass or less, preferably 0.04% by mass or less, and more preferably 0.03% by mass or less.
（Ｎ：０．００９〜０．０２質量％）
本発明に係る機械構造用鋼において、Ｎ（窒素）は鋼中に固溶して、後記するように機械構造用鋼を冷間加工（冷間鍛造）した後の強度を向上させる。Ｎ含有量が０．００９質量％未満ではこのＮ固溶量を十分に得られないため、Ｎ含有量は０．００９質量％以上とし、０．００９５質量％以上が好ましく、０．０１質量％以上がさらに好ましい。一方、Ｎ含有量が０．０２質量％を超えると、Ｎ固溶量が過剰になって冷間加工性を劣化させる。したがって、Ｎ含有量は０．０２質量％以下とし、０．０１８質量％以下が好ましく、０．０１６質量％以下がさらに好ましい。なお、Ｎは鋼の溶融工程で大気中から不可避的に混入するため、精錬工程で調整してＮ含有量を制御することができる。また、成分として含有される金属元素（例えばＭｎ）の窒素化合物を添加してもよい。
(N: 0.009 to 0.02 mass%)
In the steel for machine structure according to the present invention, N (nitrogen) is dissolved in the steel and improves the strength after cold working (cold forging) of the steel for machine structure as described later. If the N content is less than 0.009% by mass, this N solid solution cannot be obtained sufficiently. Therefore, the N content is set to 0.009% by mass or more, preferably 0.0095% by mass or more, and 0.01% by mass. The above is more preferable. On the other hand, if the N content exceeds 0.02% by mass, the N solid solution amount becomes excessive and the cold workability is deteriorated. Therefore, the N content is 0.02% by mass or less, preferably 0.018% by mass or less, and more preferably 0.016% by mass or less. In addition, since N is inevitably mixed from the atmosphere in the steel melting step, the N content can be controlled by adjusting in the refining step. Moreover, you may add the nitrogen compound of the metal element (for example, Mn) contained as a component.
本発明に係る機械構造用鋼の組成において、さらに、Ｃｒ：２質量％以下、Ｍｏ：２質量％以下、Ｔｉ：０．２質量％以下、Ｎｂ：０．２質量％以下、Ｖ：０．２質量％以下、Ｂ：０．００５質量％以下、Ｃｕ：５質量％以下、Ｎｉ：５質量％以下、Ｃｏ：５質量％以下、Ｃａ：０．０５質量％以下、ＲＥＭ：０．０５質量％以下、Ｍｇ：０．０２質量％以下、Ｌｉ：０．０２質量％以下、Ｐｂ：０．５質量％以下、Ｂｉ：０．５質量％以下、のうちの１種以上を含有してもよい。 In the composition of the steel for machine structural use according to the present invention, Cr: 2% by mass or less, Mo: 2% by mass or less, Ti: 0.2% by mass or less, Nb: 0.2% by mass or less, V: 0.0. 2 mass% or less, B: 0.005 mass% or less, Cu: 5 mass% or less, Ni: 5 mass% or less, Co: 5 mass% or less, Ca: 0.05 mass% or less, REM: 0.05 mass% % Or less, Mg: 0.02 mass% or less, Li: 0.02 mass% or less, Pb: 0.5 mass% or less, Bi: 0.5 mass% or less, Good.
（Ｃｒ，Ｍｏ：各２質量％以下）
ＣｒやＭｏは、機械構造用鋼の冷間加工性および冷間加工後の硬さを向上させる。この効果を得るために、Ｃｒ含有量は０．１質量％以上が好ましく、０．２質量％以上がより好ましく、０．３質量％以上がさらに好ましい。同様に、Ｍｏ含有量は０．０４質量％以上が好ましく、０．１２質量％以上がさらに好ましい。一方、Ｃｒ，Ｍｏはどちらも過剰に添加されると、変形抵抗が増大して却って冷間加工性が劣化する。したがって、Ｃｒ，Ｍｏの各含有量は２質量％以下とし、１．５質量％以下が好ましく、１質量％以下がさらに好ましい。
(Cr, Mo: 2 mass% or less each)
Cr and Mo improve the cold workability of machine structural steel and the hardness after cold working. In order to obtain this effect, the Cr content is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, and further preferably 0.3% by mass or more. Similarly, the Mo content is preferably 0.04% by mass or more, and more preferably 0.12% by mass or more. On the other hand, if both Cr and Mo are added excessively, the deformation resistance increases and the cold workability deteriorates. Therefore, each content of Cr and Mo is 2% by mass or less, preferably 1.5% by mass or less, and more preferably 1% by mass or less.
（Ｔｉ，Ｎｂ，Ｖ：各０．２質量％以下）
Ｔｉ，Ｎｂ，Ｖは、それぞれが鋼中のＮと結合して窒素化合物を形成して結晶粒を微細化し、冷間加工後の靭性を高くして耐割れ性を向上させる。この効果を得るために、Ｔｉ，Ｎｂ，Ｖの各含有量は０．００１質量％以上が好ましく、０．００２質量％以上がより好ましく、０．００３質量％以上がさらに好ましい。一方、Ｔｉ，Ｎｂ，ＶはＮとの親和力が強いため、いずれも過剰に添加されるとＮの固溶量を低下させる。したがって、Ｔｉ，Ｎｂ，Ｖの各含有量は０．２質量％以下とし、０．１５質量％以下が好ましく、０．１質量％以下がさらに好ましい。
(Ti, Nb, V: each 0.2 mass% or less)
Each of Ti, Nb, and V combines with N in steel to form a nitrogen compound to refine crystal grains, increase toughness after cold working, and improve crack resistance. In order to acquire this effect, each content of Ti, Nb, and V is preferably 0.001% by mass or more, more preferably 0.002% by mass or more, and further preferably 0.003% by mass or more. On the other hand, Ti, Nb, and V have a strong affinity with N, and if any of them is added excessively, the solid solution amount of N is reduced. Therefore, each content of Ti, Nb, and V is 0.2% by mass or less, preferably 0.15% by mass or less, and more preferably 0.1% by mass or less.
（Ｂ：０．００５質量％以下）
Ｂは、フェライト粒界に集まる傾向があり、Ｐのフェライト粒界偏析による粒界強度の低下を抑制する効果がある。この効果を得るために、Ｂ含有量は０．０００２質量％以上が好ましく、０．０００４質量％以上がより好ましく、０．０００６質量％以上がさらに好ましい。一方、ＢはＮとの親和力が強く、鋼中のＮと結合してＢＮを形成するため、Ｂが過剰に添加されると、ＢＮがフェライト粒界に過剰に偏析して粒界強度を低減させ、またＮの固溶量を低下させる。したがって、Ｂ含有量は０．００５質量％以下とし、０．００３５質量％以下が好ましく、０．００２質量％以下がさらに好ましい。
(B: 0.005 mass% or less)
B tends to gather at ferrite grain boundaries, and has an effect of suppressing a decrease in grain boundary strength due to P ferrite grain boundary segregation. In order to obtain this effect, the B content is preferably 0.0002% by mass or more, more preferably 0.0004% by mass or more, and further preferably 0.0006% by mass or more. On the other hand, B has a strong affinity with N and combines with N in the steel to form BN. Therefore, when B is added excessively, BN excessively segregates at the ferrite grain boundaries, reducing the grain boundary strength. In addition, the solid solution amount of N is reduced. Therefore, the B content is 0.005% by mass or less, preferably 0.0035% by mass or less, and more preferably 0.002% by mass or less.
（Ｃｕ，Ｎｉ，Ｃｏ：各５質量％以下）
Ｃｕ，Ｎｉ，Ｃｏは、それぞれが機械構造用鋼のひずみ時効を促進させて冷間加工後の強度を向上させる。この効果を得るために、Ｃｕ，Ｎｉ，Ｃｏの各含有量は０．１質量％以上が好ましく、０．２質量％以上がより好ましく、０．３質量％以上がさらに好ましい。一方、これらの元素が５質量％を超えて添加されても前記効果は飽和する上、割れも促進される。したがって、Ｃｕ，Ｎｉ，Ｃｏの各含有量は５質量％以下とし、４質量％以下が好ましく、３質量％以下がさらに好ましい。
(Cu, Ni, Co: each 5% by mass or less)
Each of Cu, Ni and Co promotes the strain aging of the machine structural steel and improves the strength after cold working. In order to acquire this effect, each content of Cu, Ni, Co is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, and further preferably 0.3% by mass or more. On the other hand, even if these elements are added in excess of 5% by mass, the above effects are saturated and cracking is promoted. Therefore, each content of Cu, Ni and Co is 5% by mass or less, preferably 4% by mass or less, and more preferably 3% by mass or less.
（Ｃａ：０．０５質量％以下、ＲＥＭ：０．０５質量％以下、Ｍｇ：０．０２質量％以下、Ｌｉ：０．０２質量％以下、Ｐｂ：０．５質量％以下、Ｂｉ：０．５質量％以下）
Ｃａ，ＲＥＭ（希土類金属元素），Ｍｇ，Ｌｉは、それぞれがＭｎＳ等の硫化物系介在物を球状化させて機械構造用鋼の冷間加工性を向上させ、また被削性を向上させる元素である。これらの効果を得るために、Ｃａ，ＲＥＭの各含有量は０．０００５質量％以上が好ましく、０．００１質量％以上がより好ましく、０．００１５質量％以上がさらに好ましい。同様に、Ｍｇ，Ｌｉの各含有量は０．０００１質量％以上が好ましく、０．０００３質量％以上がより好ましく、０．０００５質量％以上がさらに好ましい。なお、希土類金属元素として具体的に、Ｃｅ，Ｌａ，Ｎｄ等の元素が挙げられ、本明細書におけるＲＥＭの含有量とは、これらのすべての希土類金属元素の含有量の合計を指す。一方、これらの元素が過剰に添加されても前記効果は飽和するため、コスト高となる。したがって、Ｃａ，ＲＥＭの各含有量は０．０５質量％以下とし、０．０３質量％以下が好ましく、０．０１質量％以下がさらに好ましい。同様に、Ｍｇ，Ｌｉの各含有量は０．０２質量％以下とし、０．０１質量％以下が好ましく、０．００５質量％以下がさらに好ましい。
(Ca: 0.05 mass% or less, REM: 0.05 mass% or less, Mg: 0.02 mass% or less, Li: 0.02 mass% or less, Pb: 0.5 mass% or less, Bi: 0.0. 5% or less)
Ca, REM (rare earth metal element), Mg, and Li are elements that spheroidize sulfide inclusions such as MnS to improve the cold workability of machine structural steel and improve machinability. It is. In order to obtain these effects, the content of Ca and REM is preferably 0.0005% by mass or more, more preferably 0.001% by mass or more, and further preferably 0.0015% by mass or more. Similarly, each content of Mg and Li is preferably 0.0001% by mass or more, more preferably 0.0003% by mass or more, and further preferably 0.0005% by mass or more. Specific examples of rare earth metal elements include elements such as Ce, La, and Nd, and the content of REM in this specification refers to the total content of all these rare earth metal elements. On the other hand, even if these elements are added excessively, the effect is saturated, resulting in an increase in cost. Therefore, each content of Ca and REM is 0.05% by mass or less, preferably 0.03% by mass or less, and more preferably 0.01% by mass or less. Similarly, each content of Mg and Li is 0.02% by mass or less, preferably 0.01% by mass or less, and more preferably 0.005% by mass or less.
Ｐｂ，Ｂｉは被削性を向上させる元素であり、各含有量は０．０１質量％以上が好ましく、０．０３質量％以上がより好ましく、０．０５質量％以上がさらに好ましい。一方、Ｐｂ，Ｂｉはどちらも過剰に添加されると、圧延疵等の製造上の問題を生じる。したがって、Ｐｂ，Ｂｉの各含有量は０．５質量％以下とし、０．４質量％以下が好ましく、０．３質量％以下がさらに好ましい。 Pb and Bi are elements that improve the machinability, and each content is preferably 0.01% by mass or more, more preferably 0.03% by mass or more, and further preferably 0.05% by mass or more. On the other hand, if both Pb and Bi are added excessively, problems in manufacturing such as rolling mills occur. Therefore, each content of Pb and Bi is set to 0.5% by mass or less, preferably 0.4% by mass or less, and more preferably 0.3% by mass or less.
本発明に係る機械構造用鋼は、前記組成の鋼からなり、Ｎ固溶量は０．００８５質量％以上であり、フェライト相の組織分率は９０％以上であり、フェライト結晶粒の平均粒径は７μｍ以下である。そして、本発明に係る機械構造用鋼は、引張強度が５００ＭＰａ以上であり、この引張強度（ＭＰａ）をＴＳ、前記平均粒径（μｍ）をｄとしてそれぞれ表したとき、０≦ＴＳ−１２００／√ｄ≦３００を満足する。
以下に、本発明に係る機械構造用鋼を構成する各要素について説明する。
The steel for machine structure according to the present invention is made of steel having the above composition, the N solid solution amount is 0.0085% by mass or more, the structure fraction of the ferrite phase is 90% or more, and the average grain size of the ferrite crystal grains The diameter is 7 μm or less. The mechanical structural steel according to the present invention has a tensile strength of 500 MPa or more. When the tensile strength (MPa) is expressed as TS and the average particle size (μm) is expressed as d, 0 ≦ TS-1200 / √d ≦ 300 is satisfied.
Below, each element which comprises the steel for machine structure which concerns on this invention is demonstrated.
（Ｎ固溶量：０．００８５質量％以上）
機械構造用鋼中に固溶したＮ（固溶Ｎ）は、冷間加工時に発生する動的ひずみ時効により多くの転位を導入させる。そして冷間加工後には、この導入された転位が加工発熱によって動き易くなった固溶Ｎによって固着されることで、静的ひずみ時効分の強化が付与され、加工硬化分以上に強度を増加させる。Ｎ固溶量が０．００８５質量％未満では、静的ひずみ時効による強度増加の効果を十分に得ることができない。したがって、Ｎ固溶量は０．００８５質量％以上とし、０．００９質量％以上が好ましく、０．００９５質量％以上がさらに好ましい。一方、Ｎ固溶量が過剰になると、静的ひずみ時効よりも動的ひずみ時効の影響が顕著になり、変形抵抗が増大するため、冷間加工性が劣化する。Ｎ固溶量は前記組成におけるＮ含有量以下となるので、Ｎ固溶量の上限値は前記Ｎ含有量の上限値すなわち０．０２質量％に収束される。このようなＮ固溶量は、前記のＮ含有量およびＡｌ含有量のそれぞれの制限を満足し、かつ後記するように製造時の熱間加工工程（圧延、鍛造）における温度等を制御することにより、制御される。なお、本発明におけるＮ（窒素）固溶量の値は、ＪＩＳ Ｇ １２２８に準拠し、機械構造用鋼の全窒素量（Ｎ含有量）から窒素化合物における窒素量を減じた差とする。以下に、鋼中のそれぞれの窒素量を測定する方法を説明する。
(N solid solution amount: 0.0085% by mass or more)
N (solid solution N) dissolved in machine structural steel introduces many dislocations due to dynamic strain aging that occurs during cold working. After the cold working, the introduced dislocations are fixed by the solid solution N that has become easy to move due to the heat generated by the processing, thereby strengthening the static strain aging and increasing the strength more than the work hardening. . If the amount of N solid solution is less than 0.0085% by mass, the effect of increasing the strength by static strain aging cannot be obtained sufficiently. Accordingly, the N solid solution amount is 0.0085% by mass or more, preferably 0.009% by mass or more, and more preferably 0.0095% by mass or more. On the other hand, when the amount of N solid solution is excessive, the influence of dynamic strain aging becomes more prominent than static strain aging, and deformation resistance increases, so that cold workability deteriorates. Since the N solid solution amount is equal to or less than the N content in the composition, the upper limit value of the N solid solution amount is converged to the upper limit value of the N content, that is, 0.02% by mass. Such N solid solution amount satisfies the above-mentioned restrictions on N content and Al content, and controls the temperature in the hot working process (rolling, forging) at the time of manufacture as described later. It is controlled by. In addition, the value of the N (nitrogen) solid solution amount in the present invention is a difference obtained by subtracting the nitrogen amount in the nitrogen compound from the total nitrogen amount (N content) of the steel for machine structural use according to JIS G 1228. Below, the method to measure each nitrogen amount in steel is demonstrated.
鋼中の全窒素量は、不活性ガス融解法−熱伝導度法により測定できる。この方法は、供試鋼材から切り出された試料をるつぼに投入し、この試料を不活性ガス気流中で融解して窒素を含めたガスを抽出し、このガスを熱伝導度セルに搬送して熱伝導度の変化を測定して、窒素の量を求めるものである。 The total amount of nitrogen in the steel can be measured by an inert gas melting method-thermal conductivity method. In this method, a sample cut from a test steel material is put into a crucible, the sample is melted in an inert gas stream, a gas containing nitrogen is extracted, and the gas is conveyed to a thermal conductivity cell. The amount of nitrogen is determined by measuring the change in thermal conductivity.
鋼中の窒素化合物における窒素量は、アンモニア蒸留分離インドフェノール青吸光光度法により測定できる。この方法は以下の通りである。まず、供試鋼材から切り出された約０．５ｇの試料を、１０％ＡＡ系電解液（鋼材の表面に不動態皮膜を生成させない非水溶媒系の電解液であり、具体的には、１０％アセチルアセトン、１０％塩化テトラメチルアンモニウム、残部：メタノール）中での定電流電解により溶解する。この溶解した試料（と電解液）をメッシュサイズ０．１μｍのポリカーボネート製フィルタでろ過し、不溶解残渣（窒素化合物）とろ液とに分離する。不溶解残渣を硫酸、硫酸カリウム、および純Ｃｕチップ中で加熱、分解した後、前記ろ液に混合する。この混合された溶液を、水酸化ナトリウムでアルカリ化した後、水蒸気蒸留して、留出したアンモニウムを希硫酸に吸収させる。溶液にフェノール、次亜塩素酸ナトリウム、およびペンタシアノニトロシル鉄（III）酸ナトリウムを加えて青色錯体を生成させる。この青色錯体の吸光度を光度計を用いて測定して、この吸光度から窒素化合物中の窒素の量を求めるものである。 The amount of nitrogen in the nitrogen compound in the steel can be measured by ammonia distillation separation indophenol blue absorptiometry. This method is as follows. First, about 0.5 g of a sample cut out from the test steel material was converted into a 10% AA electrolyte solution (a non-aqueous solvent electrolyte solution that does not generate a passive film on the surface of the steel material. % Acetylacetone, 10% tetramethylammonium chloride, balance: methanol). The dissolved sample (and electrolyte solution) is filtered through a polycarbonate filter having a mesh size of 0.1 μm to separate into an insoluble residue (nitrogen compound) and the filtrate. The insoluble residue is heated and decomposed in sulfuric acid, potassium sulfate, and pure Cu chips, and then mixed with the filtrate. The mixed solution is alkalized with sodium hydroxide and then steam distilled to absorb the distilled ammonium in dilute sulfuric acid. To the solution is added phenol, sodium hypochlorite, and sodium pentacyanonitrosyl iron (III) to form a blue complex. The absorbance of this blue complex is measured using a photometer, and the amount of nitrogen in the nitrogen compound is determined from this absorbance.
（フェライト相の組織分率：９０％以上）
本発明に係る機械構造用鋼は、冷間加工性を付与するために軟質のフェライト相を主組織とする。フェライト単相とすることで、機械構造用鋼を冷間加工して機械構造用部品を製造する際に、組織全体が同時にかつ均一に変形、硬化するので、全体として変形抵抗の上昇が抑えられ、冷間加工性が劣化しない。また、検討の結果、必ずしも完全なフェライト単相組織でなくてもよいことが判り、フェライト相の全組織に対する面積率（組織分率）が９０％以上であればよい。一部粒界にセメンタイトが析出していても、それが球状化していれば冷間加工性を劣化させないためである。フェライト相の組織分率が９０％未満になると、フェライトとセメンタイトとの界面が割れの起点となり易く、冷間加工性が劣化する。フェライト相の組織分率は、好ましくは９３％以上、さらに好ましくは９５％以上である。組織を判別する方法としては、光学顕微鏡での観察が一例として挙げられる。また、組織を観察する位置としては、機械構造用鋼の表面から機械構造用鋼を製造する際の冷間加工方向（圧縮方向）の長さ（縮径して円柱形状に加工した場合は当該円柱の直径）の１／４の深さの位置が好ましく、その近傍の複数視野（例えば５視野）を観察して、得られた組織分率の平均で判定することができる。具体的には、機械構造用鋼を、前記観察位置を切断面に含むように切り出して、切断面を鏡面に研磨した後、ナイタール液（３％硝酸エタノール溶液）で腐食させ、腐食面を光学顕微鏡にて１００倍程度で観察し、白く見える領域がフェライト相である。組織分率を求めるには、例えば、光学顕微鏡写真上からランダムに複数点（例えば１００点）を選び、各点の組織を判別して、フェライト相の点数の全点数に対する百分率を算出すればよい。あるいは、光学顕微鏡写真を市販の画像解析ソフトで処理して白い領域の面積率を求めてもよい。
(Structure fraction of ferrite phase: 90% or more)
The steel for machine structural use according to the present invention has a soft ferrite phase as a main structure in order to impart cold workability. By using a single-phase ferrite, when manufacturing machine structural parts by cold working machine structural steel, the entire structure is deformed and hardened simultaneously and uniformly, so the increase in deformation resistance as a whole can be suppressed. Cold workability does not deteriorate. Further, as a result of the examination, it is found that the ferrite single phase structure is not necessarily required, and the area ratio (structure fraction) of the entire structure of the ferrite phase may be 90% or more. This is because even if cementite is precipitated at some grain boundaries, cold workability is not deteriorated if it is spheroidized. If the structure fraction of the ferrite phase is less than 90%, the interface between ferrite and cementite tends to be the starting point of cracking, and cold workability is deteriorated. The structural fraction of the ferrite phase is preferably 93% or more, more preferably 95% or more. An example of a method for discriminating a tissue is observation with an optical microscope. In addition, as a position for observing the structure, the length in the cold working direction (compression direction) when manufacturing the steel for machine structural use from the surface of the machine structural steel (when the diameter is reduced and processed into a cylindrical shape A position having a depth of ¼ of the diameter of the cylinder) is preferable, and a plurality of visual fields (for example, five visual fields) in the vicinity thereof are observed, and the average of the obtained tissue fractions can be determined. Specifically, machine structural steel is cut out so that the observation position is included in the cut surface, the cut surface is polished to a mirror surface, and then corroded with a nital solution (3% nitric acid ethanol solution) to optically corrode the corroded surface. Observed with a microscope at a magnification of about 100, the white area is the ferrite phase. In order to obtain the structure fraction, for example, a plurality of points (for example, 100 points) are randomly selected from the optical micrograph, the structure of each point is discriminated, and the percentage of the total number of points of the ferrite phase may be calculated. . Or you may process an optical micrograph with commercially available image analysis software, and may obtain | require the area ratio of a white area | region.
（フェライト結晶粒の平均粒径：７μｍ以下）
本発明に係る機械構造用鋼は、機械構造用部品を製造する際の冷間加工によって強度を増加させるが、冷間加工前すなわち機械構造用鋼の段階での強度が不足していては、冷間加工後に十分な強度とならない。そして、金属材料の強度は、一般的に結晶粒径の平方根に反比例することがHall-Petchの法則として知られている。すなわち、結晶粒径が小さくなるにしたがい強度が大きくなるので、前記の強度を得るため、機械構造用鋼におけるフェライト結晶粒の平均粒径を７μｍ以下とする。好ましくは６．５μｍ以下、さらに好ましくは５．５μｍ以下である。このような結晶粒とするためには、後記するように、機械構造用鋼を製造する際の冷間加工において、ひずみ量を所定値以上とする。フェライト結晶粒は、前記の組織の判別と同じ位置の複数視野を観察位置として、組織の判別と同様にナイタール液で腐食させた切断面を光学顕微鏡にて１０００倍程度で観察することによって検出することができる。結晶粒径を求めるには、例えば、光学顕微鏡写真に直線を引き、この直線と交差する結晶粒界の数をカウントし、この結晶粒界の数で直線の長さを割れば、当該光学顕微鏡写真上の結晶粒の平均粒径を算出できる。
(Average grain size of ferrite crystal grains: 7 μm or less)
The machine structural steel according to the present invention increases the strength by cold working when manufacturing a machine structural component, but the strength before cold working, that is, at the mechanical structural steel stage, is insufficient. Not enough strength after cold working. It is known as the Hall-Petch law that the strength of a metal material is generally inversely proportional to the square root of the crystal grain size. That is, since the strength increases as the crystal grain size decreases, in order to obtain the above strength, the average grain size of ferrite crystal grains in the steel for machine structure is set to 7 μm or less. Preferably it is 6.5 micrometers or less, More preferably, it is 5.5 micrometers or less. In order to obtain such crystal grains, as will be described later, the strain amount is set to a predetermined value or more in cold working when manufacturing steel for machine structural use. The ferrite crystal grains are detected by observing the cut surface corroded with the nital liquid at about 1000 times with an optical microscope in the same manner as in the structure discrimination with a plurality of visual fields at the same position as the above-mentioned structure discrimination as the observation position. be able to. In order to obtain the crystal grain size, for example, a straight line is drawn on an optical microscope photograph, the number of crystal grain boundaries crossing the straight line is counted, and the length of the straight line is divided by the number of crystal grain boundaries. The average grain size of the crystal grains on the photograph can be calculated.
（引張強度：５００ＭＰａ以上）
本発明に係る機械構造用鋼は、前記した通り、機械構造用鋼の段階である程度の強度を有する必要がある。したがって、機械構造用鋼の引張強度は５００ＭＰａ以上とし、好ましくは５５０ＭＰａ以上、さらに好ましくは６００ＭＰａ以上である。さらに、本発明に係る機械構造用鋼は、その結晶粒の微細さに対する強度が高く、フェライト結晶粒の平均粒径（μｍ）をｄとして表したとき、引張強度は（１２００／√ｄ）ＭＰａ以上とする。好ましくは（２０＋１２００／√ｄ）ＭＰａ以上、さらに好ましくは（５０＋１２００／√ｄ）ＭＰａ以上である。結晶粒径に対してこのような引張強度とするためには、Ｎの固溶量を制御する。すなわち、機械構造用鋼を製造する際の冷間加工において、結晶粒の微細化と共に、固溶Ｎにより強度を増加させる。そして、機械構造用部品を製造する際の冷間加工時には、同様に固溶Ｎにより強度がさらに増加して、機械構造用部品の強度を向上させることができる。一方、結晶粒の微細さに対する強度が過剰に高くなると、冷間加工性が劣化するため、引張強度は（３００＋１２００／√ｄ）ＭＰａ以下とする。好ましくは（２８０＋１２００／√ｄ）ＭＰａ以下、さらに好ましくは（２５０＋１２００／√ｄ）ＭＰａ以下である。したがって、引張強度（ＭＰａ）をＴＳとして表したとき、引張強度は下式にしたがう。
０≦ＴＳ−１２００／√ｄ≦３００
機械構造用鋼の引張強度は、例えばＪＩＳ Ｚ ２２４１による引張試験により測定する。
(Tensile strength: 500 MPa or more)
As described above, the steel for machine structure according to the present invention needs to have a certain level of strength at the stage of steel for machine structure. Accordingly, the tensile strength of the mechanical structural steel is 500 MPa or more, preferably 550 MPa or more, and more preferably 600 MPa or more. Furthermore, the steel for machine structural use according to the present invention has high strength against the fineness of the crystal grains, and when the average grain size (μm) of the ferrite crystal grains is expressed as d, the tensile strength is (1200 / √d) MPa. That's it. Preferably, it is (20 + 1200 / √d) MPa or more, more preferably (50 + 1200 / √d) MPa or more. In order to obtain such a tensile strength with respect to the crystal grain size, the solid solution amount of N is controlled. That is, in cold working when manufacturing steel for machine structural use, the strength is increased by solute N along with the refinement of crystal grains. And at the time of the cold working at the time of manufacturing the machine structural part, the strength is further increased by the solid solution N, and the strength of the machine structural part can be improved. On the other hand, when the strength against the fineness of the crystal grains becomes excessively high, the cold workability deteriorates, so the tensile strength is set to (300 + 1200 / √d) MPa or less. Preferably it is (280 + 1200 / √d) MPa or less, more preferably (250 + 1200 / √d) MPa or less. Therefore, when the tensile strength (MPa) is expressed as TS, the tensile strength follows the following formula.
0 ≦ TS-1200 / √d ≦ 300
The tensile strength of machine structural steel is measured by, for example, a tensile test according to JIS Z 2241.
本発明に係る機械構造用鋼の製造方法は、前記成分の組成を有する鋼を公知の方法で溶製したものを１０００℃以上に加熱した後、熱間加工し、１０００℃から２００℃以下まで冷却速度１．５℃／ｓｅｃ以上で冷却する熱間加工工程と、開始温度２００℃未満、ひずみ量０．３以上で冷間加工する冷間加工工程と、を行う。以下に、製造方法における各要素について説明する。 In the method for manufacturing steel for machine structural use according to the present invention, a steel having the above composition is melted by a known method, heated to 1000 ° C. or higher, and then hot worked, from 1000 ° C. to 200 ° C. or lower. A hot working process for cooling at a cooling rate of 1.5 ° C./sec or more and a cold working process for cold working at a start temperature of less than 200 ° C. and a strain amount of 0.3 or more are performed. Below, each element in a manufacturing method is demonstrated.
〔熱間加工工程〕
（加熱温度：１０００℃以上）
機械構造用鋼の製造における熱間加工の際には、まず、鋼（溶製したもの）を１０００℃以上に加熱する。これは、溶製時等に生成したＡｌＮを分解して、Ｎを鋼中に固溶させるためである。好ましくは１０２０℃以上、さらに好ましくは１０５０℃以上である。一方、過剰に高く加熱しても、前記効果が飽和して製造コストが増大するので、好ましくは１２００℃以下、より好ましくは１１８０℃以下、さらに好ましくは１１５０℃以下である。このような温度に加熱された後、速やかに熱間鍛造や熱間圧延により所望の形状および大きさとする。
[Hot working process]
(Heating temperature: 1000 ° C or higher)
In the case of hot working in the manufacture of steel for machine structural use, the steel (melted) is first heated to 1000 ° C. or higher. This is for decomposing AlN produced at the time of smelting and the like, so that N is dissolved in steel. Preferably it is 1020 degreeC or more, More preferably, it is 1050 degreeC or more. On the other hand, even if the heating is excessively high, the above effect is saturated and the production cost increases, so the temperature is preferably 1200 ° C. or lower, more preferably 1180 ° C. or lower, and further preferably 1150 ° C. or lower. After being heated to such a temperature, the desired shape and size are quickly obtained by hot forging or hot rolling.
（冷却速度：１．５℃／ｓｅｃ以上）
熱間加工後の冷却が緩やかであると、鋼中にＡｌＮが析出するため、１０００℃から２００℃以下になるまでは冷却速度１．５℃／ｓｅｃ以上で冷却する。好ましくは２℃／ｓｅｃ℃以上、さらに好ましくは２．５℃／ｓｅｃ以上である。冷却速度の上限については特に規制されず、設備能力や熱間加工材の形状に応じて設定すればよく、好ましくは１５℃／ｓｅｃ℃以下、さらに好ましくは１０℃／ｓｅｃ以下である。
(Cooling rate: 1.5 ° C / sec or more)
If the cooling after hot working is gentle, AlN precipitates in the steel, so that the cooling is performed at a cooling rate of 1.5 ° C./sec or more until the temperature falls from 1000 ° C. to 200 ° C. or less. It is preferably 2 ° C./sec° C. or higher, more preferably 2.5 ° C./sec or higher. The upper limit of the cooling rate is not particularly restricted, and may be set according to the equipment capacity and the shape of the hot work material, and is preferably 15 ° C./sec° C. or less, more preferably 10 ° C./sec or less.
〔冷間加工工程〕
（開始温度：２００℃未満）
機械構造用鋼の製造における冷間加工は、開始温度を２００℃未満とする。本発明に係る機械構造用鋼はＮの固溶量が多いため、２００℃以上で加工されると、変形時の動的ひずみ時効によって加工性が劣化し、さらに温度が高いと転位が増殖し難くなって強化が不十分になるからである。
[Cold working process]
(Starting temperature: less than 200 ° C)
Cold working in the manufacture of steel for machine structural use has a starting temperature of less than 200 ° C. Since the steel for machine structural use according to the present invention has a large solid solution amount of N, when processed at 200 ° C. or higher, workability deteriorates due to dynamic strain aging during deformation, and dislocation grows when the temperature is higher. This is because it becomes difficult and strengthening becomes insufficient.
（冷間加工ひずみ量：０．３以上）
冷間加工においては、ひずみ量を０．３以上とする。冷間加工ひずみ量が０．３未満では、結晶粒が微細化されず、またこの冷間加工による強化も十分に得られない。
(Cold working strain: 0.3 or more)
In cold working, the amount of strain is 0.3 or more. When the amount of cold working strain is less than 0.3, the crystal grains are not refined, and the strengthening by this cold working cannot be sufficiently obtained.
〔機械構造用部品〕
本発明に係る機械構造用部品は、前記機械構造用鋼を開始温度２００℃未満で冷間加工（冷間鍛造）して製造される。機械構造用鋼を製造する際の冷間加工と同様、２００℃以上で加工されると、強化が不十分になるからである。冷間加工後は、切削等、公知の方法で所望の形状に仕上げる。
[Mechanical structural parts]
The machine structural component according to the present invention is manufactured by cold working (cold forging) the machine structural steel at a start temperature of less than 200 ° C. It is because strengthening becomes insufficient when it is processed at 200 ° C. or higher as in the case of cold working when manufacturing steel for machine structural use. After the cold working, it is finished into a desired shape by a known method such as cutting.
以上のようにして得られる機械構造用部品は、機械構造用鋼の元の強度に対して、冷間加工における通常の加工硬化に加え、固溶Ｎの作用によりさらに強化されたものとなる。具体的には、機械構造用鋼を、冷間鍛造として１６００ｔプレスで圧縮率８０％まで圧縮してなる機械構造用部品のビッカース硬さ（冷間加工後硬さＨ）が、（１）３３０Ｈｖ以上、あるいは、前記冷間鍛造において最大の変形抵抗（ＭＰａ）をＤＲとして表したとき（２）(ＤＲ＋２００)／２．５以上、となる。変形抵抗ＤＲは、機械構造用鋼の元の強度である引張強度ＴＳに略比例するので、前記（２）の基準を満足することは、引張強度５００ＭＰａ以上である機械構造用鋼が冷間加工により大幅に強化されたことを表す。また、これは同時に、変形抵抗が小さく、冷間加工性に優れている割に高強度の機械構造用部品になることを表す。なお、圧縮率は、圧縮前（機械構造用鋼）の圧縮方向長をＨ0、圧縮後（機械構造用部品）の圧縮方向長をＨとして表したとき、(Ｈ0−Ｈ)／Ｈ0×１００で算出される。 The machine structural component obtained as described above is further strengthened by the action of solute N in addition to the usual work hardening in cold working with respect to the original strength of the steel for machine structural use. Specifically, the Vickers hardness (hardness H after cold working) of a machine structural part obtained by compressing mechanical structural steel as a cold forging by a 1600 t press to a compression rate of 80% is (1) 330 Hv. When the maximum deformation resistance (MPa) in the cold forging is expressed as DR, (2) (DR + 200) /2.5 or more. The deformation resistance DR is substantially proportional to the tensile strength TS, which is the original strength of the mechanical structural steel. Therefore, satisfying the criterion (2) is that the mechanical structural steel having a tensile strength of 500 MPa or more is cold worked. This means that it has been greatly enhanced. At the same time, this indicates that the machine structural component has a high strength even though the deformation resistance is small and the cold workability is excellent. The compression rate is expressed as (H 0 −H) / H 0 when the compression direction length before compression (machine structural steel) is H 0 , and the compression direction length after compression (machine structural component) is H. Calculated with x100.
以上、本発明を実施するための形態について述べてきたが、以下に、本発明の効果を確認した実施例を、本発明の要件を満たさない比較例と対比して具体的に説明する。なお、本発明はこの実施例によって制限を受けるものではなく、請求項に示した範囲で適当に変更を加えて実施することも可能であり、それらはいずれも本発明の技術的範囲に包含される。 As mentioned above, although the form for implementing this invention has been described, the Example which confirmed the effect of this invention is demonstrated concretely compared with the comparative example which does not satisfy | fill the requirements of this invention below. It should be noted that the present invention is not limited by this embodiment, and can be implemented with appropriate modifications within the scope of the claims, all of which are included in the technical scope of the present invention. The
〔供試材作製〕
表１および表２に示す化学成分組成の鋼１５０ｋｇを真空誘導炉で溶解して、上面：φ２４５ｍｍ、下面：φ２１０ｍｍ×高さ４８０ｍｍのインゴットに鋳造した。このインゴットを、１１５０〜１２５０℃で３ｈｒのソーキングの後、１５５ｍｍ角の四角材に熱間鍛造して、長さ６００ｍｍ程度に切断した。
[Sample preparation]
150 kg of steel having the chemical composition shown in Tables 1 and 2 was melted in a vacuum induction furnace and cast into an ingot having an upper surface of φ245 mm and a lower surface of φ210 mm × height 480 mm. The ingot was soaked at 1150 to 1250 ° C. for 3 hours, hot forged into a 155 mm square material, and cut to a length of about 600 mm.
（熱間加工工程）
表１に示す供試材Ｎｏ．１〜３３については、前記四角材をダミービレットに溶接し、１０００〜１２００℃に加熱して、φ８０ｍｍの丸棒材に熱間圧延し、冷却速度２℃／ｓｅｃで２００℃以下に冷却して、長さ９００ｍｍ程度に切断した。また、表２に示す供試材Ｎｏ．３４〜４７については、前記四角材を１０００〜１２００℃に加熱して、φ８０ｍｍの丸棒材に熱間鍛造し、冷却速度２℃／ｓｅｃで２００℃以下に冷却して、長さ９００ｍｍ程度に切断した。
(Hot processing process)
Specimen No. shown in Table 1 For 1-33, the square material is welded to a dummy billet, heated to 1000-1200 ° C., hot-rolled to a round bar of φ80 mm, and cooled to 200 ° C. or less at a cooling rate of 2 ° C./sec. And cut to a length of about 900 mm. In addition, the test material No. About 34-47, the said square material is heated to 1000-1200 degreeC, it hot-forges to a φ80mm round bar, it cools to 200 degrees C or less with a cooling rate of 2 degrees C / sec, and length is about 900 mm. Disconnected.
（冷間加工工程）
前記それぞれの丸棒材（熱間圧延材、熱間鍛造材）を、冷間加工にて表１および表２に示すひずみ量となるようにドロー加工して、φ７５ｍｍ〜φ１０．５ｍｍに縮径された丸棒状（円柱形状）の機械構造用鋼の供試材を作製した。得られた供試材について、Ｎ固溶量、フェライト組織分率、フェライト結晶粒の平均粒径、および引張強度を測定し、表１および表２に示す。
(Cold working process)
Each of the round bars (hot rolled material, hot forged material) is drawn by cold working so as to have the strain amounts shown in Tables 1 and 2, and reduced in diameter to φ75 mm to φ10.5 mm. A round bar-like (cylindrical shape) machine structural steel specimen was prepared. About the obtained test material, N solid solution amount, a ferrite structure fraction, the average particle diameter of a ferrite crystal grain, and tensile strength were measured, and it shows in Table 1 and Table 2.
〔機械構造用鋼の測定、評価〕
（Ｎ固溶量）
供試材から切り出したサンプルで、前記ＪＩＳ Ｇ １２２８に準拠する不活性ガス融解法−熱伝導度法およびアンモニア蒸留分離インドフェノール青吸光光度法にてＮ固溶量を測定した。
[Measurement and evaluation of machine structural steel]
(N solid solution amount)
A sample cut out from the test material was used to measure the amount of N solid solution by an inert gas melting method-thermal conductivity method and ammonia distillation separation indophenol blue absorptiometry based on the above JIS G 1228.
（フェライト組織分率、フェライト結晶粒径）
供試材の表面から円柱の直径の１／４の深さの位置かつ軸方向中央近傍を観察できるように、供試材を円柱の軸に沿って（半円柱形状に）切断して樹脂に埋め込み、切断面をエメリー紙およびダイヤモンドバフで鏡面に研磨し、ナイタール液（３％硝酸エタノール溶液）で腐食させた。腐食面を光学顕微鏡で観察して構成組織および結晶粒を判別した。組織解析は、１００倍で５箇所（５視野）の写真を撮影し、これらの写真に対して、画像解析ソフト（Image Pro Plus、Media Cybernetics社製）を用いて画像を２値化して、白色の領域をフェライト相として各写真の面積率を算出し、５視野の平均値をフェライト組織分率とした。結晶粒径の測定は、１０００倍で５箇所（５視野）の写真を撮影し、写真に直線を引き、この直線と交差する結晶粒界の数をカウントして結晶粒径の平均値を算出し、さらに５視野の平均値を平均粒径とした。
(Ferrite structure fraction, ferrite crystal grain size)
Cut the specimen along the axis of the cylinder (in a semi-cylindrical shape) so that it can be observed from the surface of the specimen at a depth of 1/4 of the diameter of the cylinder and near the center in the axial direction. The embedded and cut surfaces were polished to a mirror surface with emery paper and a diamond buff and corroded with a nital solution (3% nitric acid ethanol solution). The corroded surface was observed with an optical microscope to determine the structure and crystal grains. Tissue analysis was taken at 5 times (5 fields of view) at 100x, and these images were binarized using image analysis software (Image Pro Plus, Media Cybernetics) to produce white The area ratio of each photograph was calculated using the above region as the ferrite phase, and the average value of the five fields of view was defined as the ferrite structure fraction. The crystal grain size is measured by taking five photographs (5 fields of view) at 1000 times, drawing a straight line on the photograph, and counting the number of crystal grain boundaries that intersect the straight line to calculate the average value of the crystal grain size. Further, the average value of the five fields of view was defined as the average particle diameter.
（引張強度）
供試材の中心部（円柱の軸近傍）からＪＩＳ１４Ａ号の引張試験片（標点φ８ｍｍの棒状試験片）を切り出し、ＪＩＳ Ｚ ２２４１による引張試験を実施して引張強度を測定した。また、この引張強度（ＭＰａ）ＴＳと前記フェライト結晶粒の平均粒径（μｍ）ｄとから算出した（ＴＳ−１２００／√ｄ）を表１および表２に併記する。なお、供試材の中心部から試験片を切り出すとは、供試材の円柱の軸と棒状試験片の円柱の軸とが略一致するということである。
(Tensile strength)
A tensile test piece of JIS No. 14A (bar-shaped test piece with a gauge of φ8 mm) was cut out from the center of the specimen (near the axis of the cylinder), and a tensile test was performed according to JIS Z 2241 to measure the tensile strength. Tables 1 and 2 also show (TS-1200 / √d) calculated from the tensile strength (MPa) TS and the average grain size (μm) d of the ferrite crystal grains. In addition, cutting out a test piece from the center part of a test material means that the axis | shaft of the cylinder of a test material and the axis | shaft of a cylinder of a rod-shaped test piece substantially correspond.
〔冷間鍛造材作製〕
表１に示す供試材Ｎｏ．１〜３３の中心部から、φ１０ｍｍ×１５ｍｍの試験片を切り出した。この試験片を、１６００ｔプレスを用い、端面を拘束した状態で、表３に示す開始温度で、ひずみ速度１０／ｓｅｃの冷間鍛造により試験片の軸方向に圧縮率８０％まで圧縮して、機械構造用部品の供試材（冷間鍛造材）を作製した。この冷間鍛造における冷間加工性、および得られた冷間鍛造材の強度を評価し、表３に示す。
(Cold forging production)
Specimen No. shown in Table 1 A test piece of φ10 mm × 15 mm was cut out from the central part of 1-33. This test piece was compressed to 80% in the axial direction of the test piece by cold forging with a strain rate of 10 / sec at the start temperature shown in Table 3 in a state where the end face was constrained using a 1600 t press, Test materials (cold forging materials) for machine structural parts were produced. The cold workability in this cold forging and the strength of the obtained cold forging are evaluated and are shown in Table 3.
表２に示す供試材Ｎｏ．３４〜４７の中心部からφ１０ｍｍ×１５ｍｍの試験片を、また前記供試材の一部について、中心部からさらにφ２０ｍｍ×３０ｍｍ、φ３０ｍｍ×４５ｍｍの試験片をそれぞれ切り出した。これらの試験片を、１６００ｔプレスを用い、端面を拘束した状態で、開始温度２０℃で、ひずみ速度１０／ｓｅｃの冷間鍛造により試験片の軸方向に圧縮率８０％まで圧縮して、機械構造用部品の供試材（冷間鍛造材）を作製した。この冷間鍛造における冷間加工性、および得られた冷間鍛造材の強度を評価し、表４に示す。 Specimen No. shown in Table 2 A test piece of φ10 mm × 15 mm was cut out from the center part of 34 to 47, and a test piece of φ20 mm × 30 mm and φ30 mm × 45 mm was cut out from the center part of each of the test materials. These test pieces were compressed to a compressibility of 80% in the axial direction of the test pieces by cold forging at a starting temperature of 20 ° C. and a strain rate of 10 / sec using a 1600 t press with the end face restrained. Test materials (cold forging materials) for structural parts were produced. The cold workability in this cold forging and the strength of the obtained cold forging are evaluated and are shown in Table 4.
〔機械構造用部品の測定、評価〕
（冷間加工性）
冷間鍛造時に、１６００ｔプレスに付属のロードセルと変位計を用いて、変位抵抗−変位曲線を記録し、この曲線における変形抵抗の最大値を冷間加工における変形抵抗ＤＲとして表３および表４に示す。さらに、この変形抵抗（ＭＰａ）ＤＲから算出した（（ＤＲ＋２００）／２．５）を表３および表４に併記する。また、冷間鍛造により割れの発生した冷間鍛造材を冷間加工性が不良であるとして「×」、割れのない冷間鍛造材を冷間加工性が良好であるとして「○」と示した。
[Measurement and evaluation of machine structural parts]
(Cold workability)
At the time of cold forging, a displacement resistance-displacement curve was recorded using a load cell attached to a 1600 t press and a displacement meter, and the maximum value of the deformation resistance in this curve is shown in Tables 3 and 4 as the deformation resistance DR in cold working. Show. Further, ((DR + 200) /2.5) calculated from the deformation resistance (MPa) DR is also shown in Tables 3 and 4. In addition, “X” indicates that cold forging with cracks caused by cold forging has poor cold workability, and “○” indicates that cold forging without cracks has good cold workability. It was.
（冷間加工後強度）
冷間加工後の強度として、冷間鍛造材のビッカース硬さを測定した。冷間鍛造材の円柱の軸（冷間鍛造前の試験片の軸と同）に沿って切断して、樹脂に埋め込んで試料として調整した。この試料について、冷間鍛造材の円柱の軸方向中央における直径の１／４位置の両側３点ずつの計６点のビッカース硬さを測定し、６点の平均値を冷間加工後硬さＨとした。冷間加工後強度の合格基準は、冷間加工後硬さＨが（１）３３０Ｈｖ以上、および前記の（２）(ＤＲ＋２００)／２．５以上、の少なくとも一方とする。
(Strength after cold working)
The Vickers hardness of the cold forged material was measured as the strength after cold working. The sample was cut along a cylindrical axis of the cold forged material (same as the axis of the test piece before cold forging), embedded in resin, and prepared as a sample. For this sample, measure the Vickers hardness of a total of 6 points, 3 points on each side of the 1/4 position of the diameter in the axial center of the cylinder of the cold forging material, and the average value of 6 points is the hardness after cold working H. The acceptance criterion for the strength after cold working is at least one of the hardness H after cold working of (1) 330 Hv or more and (2) (DR + 200) /2.5 or more.
（評価）
表１および表２に示すように、機械構造用鋼の供試材Ｎｏ．１〜２２，３４〜４３は、その組成における成分および冷間加工条件が本発明の範囲であるので、Ｎ固溶量、フェライト相の組織分率、結晶粒径および引張強度ならびに両者の相関（以下、これらをまとめて機械構造用鋼の特性という）が、本発明の機械構造用鋼の要件を満足した。さらに、これらの供試材のうち、表３および表４に示すように、開始温度２００℃未満で冷間鍛造を施した供試材Ｎｏ．１〜２１，３４〜４３は、いずれも割れを生じることなく良好な冷間加工性を示し、かつ良好な冷間加工後強度が得られた。このうち供試材Ｎｏ．１においては、機械構造用鋼の引張強度ＴＲが本発明の範囲内（５００ＭＰａ以上）で低かったために冷間加工後硬さＨが３３０Ｈｖに達しなかったが、それ以外の供試材Ｎｏ．２〜２１，３４〜４３においては冷間加工後硬さＨが３３０Ｈｖ以上の高強度が得られた。さらに、供試材Ｎｏ．２，９，１３，１８，３４〜３６は、冷間加工後硬さＨに対して変形抵抗ＤＲが低く、特に優れた冷間加工性を示した。また、これらの効果は、供試材Ｎｏ．３４〜３９，４２，４３の結果が示すように、冷間鍛造前の試験片の大きさに依存しない。これに対して、供試材Ｎｏ．２２は、機械構造用鋼としては本発明の要件を満足するが、開始温度２００℃以上で鍛造を施したため、強度が十分に増加しなかった。
(Evaluation)
As shown in Table 1 and Table 2, the test material No. 1 to 22, 34 to 43, since the components and cold working conditions in the composition are within the scope of the present invention, the amount of N solid solution, the structure fraction of the ferrite phase, the crystal grain size and the tensile strength, and the correlation between them ( Hereinafter, these are collectively referred to as the characteristics of steel for machine structural use) and satisfy the requirements for steel for machine structural use of the present invention. Furthermore, among these test materials, as shown in Tables 3 and 4, the test material No. 1 was subjected to cold forging at a starting temperature of less than 200 ° C. Nos. 1 to 21 and 34 to 43 showed good cold workability without causing cracks, and good strength after cold work was obtained. Of these, the test material No. In No. 1, since the tensile strength TR of the steel for machine structural use was low within the range of the present invention (500 MPa or more), the hardness H after cold working did not reach 330 Hv. In 2-21 and 34-43, high strength with a hardness H of 330 Hv or more after cold working was obtained. Furthermore, the test material No. Nos. 2, 9, 13, 18, 34 to 36 had a low deformation resistance DR with respect to hardness H after cold working, and exhibited particularly excellent cold workability. In addition, these effects are obtained from the test material No. As the results of 34 to 39, 42 and 43 show, it does not depend on the size of the test piece before cold forging. On the other hand, the test material No. No. 22 satisfies the requirements of the present invention as steel for machine structural use, but the strength was not sufficiently increased because forging was performed at a starting temperature of 200 ° C. or higher.
供試材Ｎｏ．２３，４４はＣ含有量が過剰なため、機械構造用鋼としてのフェライト相の組織分率は満足しているが、硬質のパーライトが形成されたことにより、冷間加工性が劣化した。さらに供試材Ｎｏ．３３，４７は一般的な炭素鋼で、Ｃ，Ｓｉ含有量が過剰で、かつＮ含有量が不足しているため、フェライト相の組織分率が低く、さらにＮ固溶量が不足して結晶粒径に対する引張強度が低く、その結果、冷間加工性および冷間加工後の強度が得られなかった。供試材Ｎｏ．２４はＳｉ含有量が不足しているため、機械構造用鋼の特性は満足しているが、冷間加工性が劣化した。一方、供試材Ｎｏ．２５は、Ｓｉ含有量が過剰なため冷間加工性が劣化し、また冷間加工時のひずみ量が不足したため結晶粒が微細化されず、かつ引張強度が不足した。 Specimen No. Nos. 23 and 44 satisfy the structure fraction of the ferrite phase as steel for machine structural use because of the excessive C content, but the cold workability deteriorated due to the formation of hard pearlite. Furthermore, sample No. Nos. 33 and 47 are general carbon steels. Since the C and Si contents are excessive and the N content is insufficient, the structure fraction of the ferrite phase is low, and the amount of N solid solution is insufficient. The tensile strength with respect to the particle size was low, and as a result, cold workability and strength after cold work were not obtained. Specimen No. Since No. 24 is insufficient in Si content, the properties of steel for machine structural use are satisfactory, but the cold workability is deteriorated. On the other hand, the test material No. In No. 25, since the Si content was excessive, the cold workability deteriorated, and the amount of strain during cold working was insufficient, so the crystal grains were not refined and the tensile strength was insufficient.
供試材Ｎｏ．２６，２７，４５はＭｎ含有量が本発明の範囲外であり、また供試材Ｎｏ．２８はＰ含有量が、供試材Ｎｏ．２９はＳ含有量がそれぞれ過剰なため、いずれも機械構造用鋼の特性は満足しているが、冷間加工性が低下した。 Specimen No. Nos. 26, 27 and 45 have Mn contents outside the scope of the present invention. No. 28 has a P content of no. No. 29 has an excessive S content, so that all the properties of steel for machine structural use are satisfied, but the cold workability is lowered.
供試材Ｎｏ．３０はＡｌ含有量が過剰なため、供試材Ｎｏ．３１，４６はＮ含有量が不足しているため、それぞれＮ固溶量が不足した結果、結晶粒径に対する引張強度が不足し、さらに冷間加工後の強度が十分に増加しなかった。一方、供試材Ｎｏ．３２はＮ含有量が過剰なため、固溶Ｎも過剰となり、結晶粒径に対して引張強度が高くなりすぎた結果、冷間加工性が劣化した。 Specimen No. No. 30 has an excessive Al content. Nos. 31 and 46 were insufficient in N content. As a result, the tensile strength against the crystal grain size was insufficient as a result of insufficient N solid solution, and the strength after cold working was not sufficiently increased. On the other hand, the test material No. Since No. 32 has an excessive N content, solid solution N also became excessive, and as a result of the tensile strength becoming too high with respect to the crystal grain size, the cold workability deteriorated.
Claims (8)
Ｎ固溶量は０．００８５質量％以上であり、
フェライト相の組織分率は９０％以上であり、
フェライト結晶粒の平均粒径は７μｍ以下であり、
引張強度は５００ＭＰａ以上であり、
前記フェライト結晶粒の平均粒径（μｍ）をｄ、前記引張強度（ＭＰａ）をＴＳとしてそれぞれ表したとき、０≦ＴＳ−１２００／√ｄ≦３００を満足することを特徴とする機械構造用鋼。 C: 0.005-0.045 mass%, Si: 0.005-0.05 mass%, Mn: 0.4-1.0 mass%, Al: 0.01-0.06 mass%, S: 0.005 to 0.05% by mass, P: 0.05% by mass or less, N: 0.009 to 0.02% by mass, with the balance being composed of Fe and inevitable impurities,
N solid solution amount is 0.0085 mass% or more,
The structure fraction of the ferrite phase is 90% or more,
The average grain size of the ferrite crystal grains is 7 μm or less,
The tensile strength is 500 MPa or more,
The steel for mechanical structures, wherein 0 ≦ TS-1200 / √d ≦ 300 is satisfied, where d is the average grain size (μm) of the ferrite crystal grains and TS is the tensile strength (MPa). .
前記組成の鋼を、１０００℃以上に加熱した後、熱間加工し、この熱間加工後、１０００℃から２００℃以下まで冷却速度１．５℃／ｓｅｃ以上で冷却する熱間加工工程と、
開始温度２００℃未満、ひずみ量０．３以上で冷間加工する冷間加工工程と、を行うことを特徴とする機械構造用鋼の製造方法。 A method for producing a steel for machine structure according to any one of claims 1 to 6,
The steel having the above composition is heated to 1000 ° C. or higher, and then hot worked, and after this hot working, a hot working step of cooling from 1000 ° C. to 200 ° C. or lower at a cooling rate of 1.5 ° C./sec or more;
And a cold working step in which cold working is performed at a starting temperature of less than 200 ° C. and a strain amount of 0.3 or more.
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| JP2001226744A (en) * | 2000-02-15 | 2001-08-21 | Kawasaki Steel Corp | High tensile strength for rolled steel sheet excellent in backing hardenability and impact resistance and producing method therefor |
| JP2002053935A (en) * | 2000-02-29 | 2002-02-19 | Kawasaki Steel Corp | High tensile strength cold-rolled steel sheet excellent in strain age-hardening characteristic and its production method |
| JP2004204315A (en) * | 2002-12-26 | 2004-07-22 | Jfe Steel Kk | Method for producing formed body using steel plate |
| JP2008163410A (en) * | 2006-12-28 | 2008-07-17 | Kobe Steel Ltd | Steel for high-speed cold working and method for production thereof, and method for producing part formed by high-speed cold working |
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| JP2001226744A (en) * | 2000-02-15 | 2001-08-21 | Kawasaki Steel Corp | High tensile strength for rolled steel sheet excellent in backing hardenability and impact resistance and producing method therefor |
| JP2002053935A (en) * | 2000-02-29 | 2002-02-19 | Kawasaki Steel Corp | High tensile strength cold-rolled steel sheet excellent in strain age-hardening characteristic and its production method |
| JP2004204315A (en) * | 2002-12-26 | 2004-07-22 | Jfe Steel Kk | Method for producing formed body using steel plate |
| JP2008163410A (en) * | 2006-12-28 | 2008-07-17 | Kobe Steel Ltd | Steel for high-speed cold working and method for production thereof, and method for producing part formed by high-speed cold working |
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