JP2013044036A - Method for producing ferrous material - Google Patents
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本発明は、産業機械や自動車等の機械部品に用いられて好適な鉄系材料に関し、特に一部あるいは全部に硬化層を有して高強度を示す鉄系材料の製造方法に関する。 The present invention relates to an iron-based material suitable for use in machine parts such as industrial machines and automobiles, and more particularly to a method for producing an iron-based material having a hardened layer in part or all and exhibiting high strength.
産業機械や自動車等に用いられる機械部品は一般的に、鋼材を切削または塑性加工、あるいはそれらの併用により所定の形状に加工した後、焼入れ焼戻し処理を施すことにより所望の特性を確保するという方法により製造される。
このような機械部品に用いられる鋼材は、機械部品として必要な強度を確保するために、0.3〜0.6mass％程度のＣを含有する。しかしながら、鋼材中に含有されるＣは鋼材の硬度上昇にも寄与するため、切削や鍛造などの冷間加工における加工性を著しく困難にする。
Generally, mechanical parts used in industrial machines, automobiles, etc. are methods of securing desired characteristics by applying a quenching and tempering treatment after processing a steel material into a predetermined shape by cutting or plastic working, or a combination thereof. Manufactured by.
The steel material used for such a machine part contains about 0.3 to 0.6 mass% C in order to ensure the strength required for the machine part. However, since C contained in the steel material also contributes to an increase in the hardness of the steel material, workability in cold working such as cutting and forging becomes extremely difficult.
また、Ｃ含有鋼の焼入れ焼もどしにより得られる焼もどしマルテンサイトは、常温では優れた強度を有するものの、150℃を超える程度の高温に長時間曝されると強度が低下するため、使用環境がこの様な温度に達する用途には必ずしも適合しない。
機械部品を所定の形状に加工する際の冷間加工性と、機械部品に要求される強度という相反する特性をともに満足させる方法として、低C鋼素材に冷間加工を施して所望の形状とした後、浸炭焼入れする方法が、従前行われている。しかしながら、上記方法は、浸炭でＣ濃度を上昇させるといえども、やはり焼もどしマルテンサイトの強度を利用するため、依然として高温環境下での強度低下に関する上記問題は未解決のままであった。
In addition, tempered martensite obtained by quenching and tempering C-containing steel has excellent strength at room temperature, but its strength decreases when exposed to high temperatures exceeding 150 ° C for a long time, so the usage environment is limited. It is not always suitable for applications that reach such temperatures.
As a method of satisfying both the cold workability when machining machine parts into a predetermined shape and the conflicting properties required for machine parts, cold processing is applied to low C steel material to obtain the desired shape. After that, carburizing and quenching has been performed in the past. However, although the above method raises the C concentration by carburizing, it still uses the strength of tempered martensite, and thus the above-mentioned problem relating to strength reduction in a high temperature environment still remains unsolved.
また、上記浸炭焼入れに代えて、窒化処理により表面硬化層を形成する方法も知られている。窒化処理は、処理温度が比較的低温である上、焼入れ工程を必要としないため、発生する熱処理歪みも小さい。そのため、寸法精度が要求される機械部品の強度を確保する方法としては有効である。 A method of forming a hardened surface layer by nitriding treatment instead of the carburizing and quenching is also known. The nitriding process has a relatively low processing temperature and does not require a quenching process, and thus generates less heat distortion. Therefore, it is effective as a method for ensuring the strength of mechanical parts that require dimensional accuracy.
しかしながら、多量の合金添加を行わない鉄系材料に従前の窒化処理を施した、機械部品では、表面硬化層の硬度が不十分であった。例えば、特許文献１〜４には、鋼板を所望の形状に加工した後、窒化処理を施して表面硬化層を形成した機械部品について開示されているが、何れの文献に開示された機械部品においても、その表面硬化層の硬度は最大でもHV400程度である。 However, the hardness of the surface hardened layer is insufficient in the machine parts that have been subjected to the previous nitriding treatment in accordance with the iron-based material without adding a large amount of alloy. For example, Patent Documents 1 to 4 disclose a machine part in which a steel sheet is processed into a desired shape and then subjected to nitriding treatment to form a hardened surface layer. However, the hardness of the hardened surface layer is at most about HV400.
一方、特許文献５には、硬度がHV803の表面硬化層を得る製法が記載されているが、硬化層の厚さが３〜15μmと薄く、面圧の高い部品への適用に課題が残るものであった。 On the other hand, Patent Document 5 describes a production method for obtaining a surface hardened layer having a hardness of HV803, but the thickness of the hardened layer is as thin as 3 to 15 μm, and there remains a problem in application to parts with high surface pressure. Met.
ここに、鋼表層部にHV650を超える高い硬度を付与する為に、窒化時に鋼表層中にAｌやTi等の硬質窒化物を形成させる方法が、従前行われているが、鋼中にAlやTiなどの窒化物形成元素を多量に含有させる必要があり、鋼素材の製造コストを上昇させる等の問題を残していた。 Here, in order to give the steel surface layer a high hardness exceeding HV650, a method of forming a hard nitride such as Al or Ti in the steel surface during nitriding has been conventionally performed. It was necessary to contain a large amount of a nitride-forming element such as Ti, which left problems such as an increase in the manufacturing cost of the steel material.
本発明は、上記の現状を鑑みなされたものであり、必ずしも鋼中に高濃度のＣおよび合金元素を含有させることなく、冷間加工性および最終部品強度を兼備し、さらには高温使用環境における強度にも優れた部品が得られる機械構造用鉄系材料を製造するための方法について提案することを目的とする。 The present invention has been made in view of the above-mentioned present situation, and does not necessarily contain a high concentration of C and alloy elements in steel, has both cold workability and final part strength, and further in a high temperature use environment. The purpose is to propose a method for producing an iron-based material for mechanical structure that can provide a part having excellent strength.
上記目的を達成すべく、本発明者らは、冷間加工性に加え、最終的に高強度を有する機械部品が製造可能である、機械構造用材料を得るための方途について鋭意検討を進めた。その結果、鉄系材料に窒化処理を施すことにより、鉄系材料の少なくとも表層部にオーステナイト形成元素であるＮを高濃度に含有させ、Ｎ高濃度領域をオーステナイト組織とし、窒化処理後に急冷して窒化処理時に形成された上記オーステナイト組織を500℃以下Ms点以上の温度域まで冷却し、これをMs点以上500℃以下の温度域に加熱保持して、上記オーステナイト組織をα（フェライト）とγ´（Fe4N）との微細組織にすることにより、HV650以上の高い硬度が得られ、なおかつ高温に長時間曝された後も高い硬度を維持し得ることを知見した。 In order to achieve the above object, the present inventors have intensively studied a method for obtaining a machine structural material that can finally produce a machine part having high strength in addition to cold workability. . As a result, by nitriding the iron-based material, at least the surface layer portion of the iron-based material contains a high concentration of austenite-forming element N, the N high-concentration region has an austenite structure, and is rapidly cooled after the nitriding treatment. The austenite structure formed at the time of nitriding is cooled to a temperature range of 500 ° C. or lower and a temperature range of Ms point or higher, and this is heated and held in a temperature range of Ms point or higher and 500 ° C. or lower to change the austenite structure to α (ferrite) and γ It has been found that by forming a microstructure with ′ (Fe 4 N), high hardness of HV650 or higher can be obtained, and high hardness can be maintained even after being exposed to high temperature for a long time.
本発明は上記の知見に基づきなされたものであり、その要旨構成は次の通りである。
（１）鉄系素材の少なくとも一部に700℃以上の温度にて窒化処理を施し、該窒化処理部分にＮ：３at％以上８at％未満を含有させた後、500℃以下Ms点以上の温度域まで１℃／s以上の速度で冷却し、その後Ms点以上500℃以下の温度域に10min以上保持してHV650以上の硬質相を、前記窒化処理部分に形成することを特徴とする鉄系材料の製造方法。
This invention is made | formed based on said knowledge, The summary structure is as follows.
(1) At least part of the iron-based material is subjected to nitriding treatment at a temperature of 700 ° C. or more, and N: 3 at% or more and less than 8 at% is contained in the nitriding portion, and then a temperature of 500 ° C. or less Ms point or more The iron system is characterized in that it is cooled at a rate of 1 ° C./s or higher to a temperature range, and then held at a temperature range of Ms point to 500 ° C. for 10 min or longer to form a hard phase of HV650 or higher in the nitriding portion. Material manufacturing method.
（２）前記（１）において、前記鉄系素材は、Ｃ：0.1mass％未満を含み、残部がFeおよび不可避不純物の組成になることを特徴とする鉄系材料の製造方法。 (2) The method for producing an iron-based material according to (1), wherein the iron-based material includes less than C: 0.1 mass%, and the balance has a composition of Fe and inevitable impurities.
（３）前記（２）において、前記鉄系素材は、さらに
Cr：0.05mass％以上3.0mass％以下、
Al：0.005 mass％以上3.0 mass％以下、
Ti：0.0005 mass％以上0.5 mass％以下、
Nb：0.005 mass％以上0.1 mass％以下、
Ｖ：0.02 mass％以上1.0 mass％以下、
Mo：0.02 mass％以上1.0mass％以下、
Mn：0.02 mass％以上2.0 mass％以下、
Si：0.02 mass％以上3.0 mass％以下、
Ni：0.02 mass％以上2.0mass％以下、
Cu：0.02 mass％以上2.0 mass％以下および
Co：0.02 mass％以上2.0 mass％以下
の中から選択される少なくとも１種以上を含有することを特徴とする鉄系材料の製造方法。
(3) In the above (2), the iron-based material further includes
Cr: 0.05 mass% or more and 3.0 mass% or less,
Al: 0.005 mass% to 3.0 mass%,
Ti: 0.0005 mass% or more and 0.5 mass% or less,
Nb: 0.005 mass% or more and 0.1 mass% or less,
V: 0.02 mass% or more and 1.0 mass% or less,
Mo: 0.02 mass% or more and 1.0 mass% or less,
Mn: 0.02 mass% or more and 2.0 mass% or less,
Si: 0.02 mass% or more and 3.0 mass% or less,
Ni: 0.02 mass% or more and 2.0 mass% or less,
Cu: 0.02 mass% to 2.0 mass%
Co: A method for producing an iron-based material comprising at least one selected from 0.02 mass% to 2.0 mass%.
本発明によれば、産業機械や自動車等の機械部品に好適に用いられる機械構造用鉄系材料であって、優れた冷間加工性を有し、寸法精度の高い機械部品が得られる上、HV650以上という、従来にない硬質相を有する鉄系材料が得られる。 According to the present invention, it is a ferrous material for machine structure that is suitably used for machine parts such as industrial machines and automobiles, has excellent cold workability, and can obtain machine parts with high dimensional accuracy. An iron-based material having an unprecedented hard phase of HV650 or higher can be obtained.
本発明の方法では、まず、鉄系素材の少なくとも一部に窒化処理を施して、該窒化処理部分にＮ：３at％以上８at％未満を含有させる。そこで、この窒化処理部分におけるＮ含有量について説明する。
Ｎ：３at％以上８at％未満
Ｎは、本発明において硬質相を形成する上で必須の元素である。先述の通り、本発明においては、鉄系素材に窒化処理を施してその少なくとも一部をオーステナイト組織とし、これを急冷して窒化処理時に形成された上記オーステナイト組織を、α（フェライト）とγ´（Fe4N）との微細分散組織にすることにより、窒化処理部分に硬質相を形成する。そのため、本発明の鉄系材料においては、硬質相を形成すべき部分に、オーステナイト形成元素であり、且つγ´（Fe4N）の構成元素であるＮを所要量含有させる必要がある。すなわち、窒化処理部分のＮ含有量が３at％未満では、窒化処理温度域でオーステナイト組織を得ることが出来ず、また硬化相形成を目的とした保持処理時に十分なα-Fe＋γ´（Fe4N）微細組織が得られないため、HV650以上の硬化層を形成することができない。一方、Ｎ含有量が８at％以上になると、窒化処理に必要な時間が長時間となるため、製造コストが増加する問題がある。以上の理由から、Ｎ含有量を３at％以上８at％未満に規定する。
In the method of the present invention, first, at least a part of the iron-based material is subjected to nitriding treatment, and the nitriding portion contains N: 3 at% or more and less than 8 at%. Therefore, the N content in this nitriding portion will be described.
N: 3 at% or more and less than 8 at% N is an essential element for forming a hard phase in the present invention. As described above, in the present invention, at least a part of an iron-based material is subjected to nitriding treatment to form an austenite structure, and the austenite structure formed at the time of nitriding by rapid cooling is converted into α (ferrite) and γ ′. By forming a finely dispersed structure with (Fe 4 N), a hard phase is formed in the nitriding portion. Therefore, in the iron-based material of the present invention, a required amount of N, which is an austenite forming element and a constituent element of γ ′ (Fe 4 N), needs to be contained in a portion where a hard phase is to be formed. That is, when the N content in the nitriding portion is less than 3 at%, an austenite structure cannot be obtained in the nitriding temperature range, and sufficient α-Fe + γ ′ (Fe 4 N) during the holding treatment for the purpose of forming a hardened phase. ) Since a fine structure cannot be obtained, a hardened layer of HV650 or higher cannot be formed. On the other hand, when the N content is 8 at% or more, the time required for the nitriding treatment becomes long, and there is a problem that the manufacturing cost increases. For the above reasons, the N content is specified to be 3 at% or more and less than 8 at%.
なお、本発明においては必ずしも鉄系材料の全体が上記規定を満足する必要はなく、高い硬度が必要とされる部分についてのみ上記規定を満足させることも可能である。すなわち、鉄系素材の少なくとも一部に窒化処理を施せばよい。ここで、鉄系素材の少なくとも一部とは、高い硬度が必要とされる部分であり、例えば機械構造部品として高い硬度が要求される部位に対応する領域であり、当該部分の少なくとも表層域、具体的には、少なくとも鉄系素材の表面から深さ20μｍ〜200μｍの範囲にわたって窒化処理を施すことが好ましい。 In the present invention, the entire iron-based material is not necessarily required to satisfy the above definition, and it is also possible to satisfy the above specification only for a portion requiring high hardness. That is, nitriding treatment may be performed on at least a part of the iron-based material. Here, at least a part of the iron-based material is a part that requires high hardness, for example, a region corresponding to a part that requires high hardness as a machine structural component, and at least a surface layer region of the part, Specifically, it is preferable to perform nitriding at least over a range of 20 μm to 200 μm in depth from the surface of the iron-based material.
また、前記鉄系素材は、Ｃ：0.1mass％未満を含み、残部がFeおよび不可避不純物の組成になることが、好ましい。
Ｃ：0.1mass％未満
本発明において、Ｃは必須の成分ではない。しかしながら、特に本発明においてHV650以上の硬質相を鉄系材料の表層のみに形成する場合、Ｃは鉄系材料の強度を確保する上で有効な元素であるので、必要に応じて含有する。ただし、その含有量が0.1mass％以上となると、機械部品の寸法精度や冷間加工性に悪影響を及ぼすため、Ｃ含有量を0.1mass％未満とする。
なお、残部は、次に示す添加元素を添加しない場合、Feおよび不可避不純物である。不可避不純物としては、Si、Mn、Ｐ、Ｓ、Cu、Ni、Cr、Mo、Ｖ、Nb、Ti、Al、Ｎ、ＯおよびＢが挙げられる。
Moreover, it is preferable that the said iron-type raw material contains less than C: 0.1 mass%, and the remainder becomes a composition of Fe and an unavoidable impurity.
C: Less than 0.1 mass% In the present invention, C is not an essential component. However, in particular, in the present invention, when a hard phase of HV650 or higher is formed only on the surface of the iron-based material, C is an element effective in securing the strength of the iron-based material, and is contained as necessary. However, if the content is 0.1 mass% or more, the dimensional accuracy and cold workability of machine parts are adversely affected, so the C content is less than 0.1 mass%.
Note that the balance is Fe and inevitable impurities when the following additive elements are not added. Inevitable impurities include Si, Mn, P, S, Cu, Ni, Cr, Mo, V, Nb, Ti, Al, N, O and B.
さらに、必要に応じて、Cr：0.05mass％以上3.0mass％以下、Al：0.005 mass％以上3.0 mass％以下、Ti：0.0005 mass％以上0.5 mass％以下、Nb：0.005 mass％以上0.1 mass％以下、Ｖ：0.02 mass％以上1.0 mass％以下、Mo：0.02 mass％以上1.0mass％以下、Mn：0.02 mass％以上2.0 mass％以下、Si：0.02 mass％以上3.0 mass％以下、Ni：0.02 mass％以上2.0mass％以下、Cu：0.02 mass％以上2.0 mass％以下およびCo：0.02 mass％以上2.0 mass％以下の中から選択される少なくとも一種以上を添加することが出来る。 Furthermore, Cr: 0.05 mass% to 3.0 mass%, Al: 0.005 mass% to 3.0 mass%, Ti: 0.0005 mass% to 0.5 mass%, Nb: 0.005 mass% to 0.1 mass%, as necessary , V: 0.02 mass% to 1.0 mass%, Mo: 0.02 mass% to 1.0 mass%, Mn: 0.02 mass% to 2.0 mass%, Si: 0.02 mass% to 3.0 mass%, Ni: 0.02 mass% More than 2.0 mass%, Cu: 0.02 mass% or more and 2.0 mass% or less and Co: 0.02 mass% or more and 2.0 mass% or less can be added.
すなわち、Cr，Al，Ti，Nb，ＶおよびMoは、いずれも鉄系材料中の窒素と結合して硬質な窒化物を形成し、主に表層において耐摩耗性を向上する作用を有するため、必要に応じて含有させる。含有量が各成分毎の下限に満たない場合は効果が不十分である。一方、各々の上限値を超えて含有してもその効果が飽和するとともに、過剰な窒化物が析出して体積変化をもたらし、寸法精度に悪影響を及ぼす。また、体積変化が生じることにより空隙を含むミクロ組織が形成されるため、鉄系材料の強度が劣化する。 That is, Cr, Al, Ti, Nb, V and Mo all combine with nitrogen in the iron-based material to form a hard nitride, and have the effect of improving wear resistance mainly in the surface layer. It is contained as necessary. When the content is less than the lower limit for each component, the effect is insufficient. On the other hand, even if the content exceeds each upper limit value, the effect is saturated and excessive nitride precipitates to cause volume change, which adversely affects dimensional accuracy. Moreover, since the microstructure containing voids is formed due to the volume change, the strength of the iron-based material is deteriorated.
次に、Mn、Si、Ni、CuおよびCoは、本発明の鉄系材料を製造する上で必要となる、低温でのオーステナイト組織の形成に効果的に作用するため、必要に応じて含有する。含有量が各々の下限値に満たない場合にはその効果が不十分であり、一方、各々の上限値を超えて含有すると、最終的な所望の組織、すなわちαおよびγ´の微細分散組織の形成に悪影響を及ぼす。 Next, Mn, Si, Ni, Cu, and Co are contained as necessary because they effectively act to form an austenite structure at low temperatures, which is necessary for producing the iron-based material of the present invention. . When the content is less than each lower limit, the effect is insufficient. On the other hand, when the content exceeds each upper limit, the final desired structure, that is, the finely dispersed structure of α and γ ′ Adversely affects formation.
本発明の鉄系材料は、上記した組成を有する鉄系素材に、700℃以上の温度で窒化処理を施して該鉄系素材の一部または全体にＮ：３at％以上８at％未満を含有させた後、500℃以下Ms点以上の温度域まで１℃／s以上の速度で冷却し、その後Ms点以上500℃以下の温度域に10min以上保持することによりHV650以上の硬質相を形成する方法により、好適に製造することができる。 In the iron-based material of the present invention, the iron-based material having the above composition is subjected to nitriding treatment at a temperature of 700 ° C. or higher so that a part or the whole of the iron-based material contains N: 3 at% or more and less than 8 at%. After that, it is cooled at a rate of 1 ° C / s or higher to a temperature range of 500 ° C or lower Ms point or higher, and then held at a temperature range of Ms point or higher and 500 ° C or lower for 10 min or longer to form a hard phase of HV650 or higher. Thus, it can be suitably manufactured.
次に、各製造条件について詳述する。
（窒化処理条件）
窒化温度を700℃以上とすることによって、鉄系素材中への十分な窒素の拡散速度を得ることが可能となるとともに、窒化中に安定なオーステナイト相を得ることが可能となる。ただし、極度の高温域での処理は、処理中の窒化進行速度の制御が困難になるとともに、処理中にオーステナイト粒の粗大化を引き起こし、処理後の材料の延性および靭性に悪影響を及ぼす。そのため、窒化処理温度は1000℃以下とすることが好ましい。
Next, each manufacturing condition is explained in full detail.
(Nitriding conditions)
By setting the nitriding temperature to 700 ° C. or higher, it is possible to obtain a sufficient diffusion rate of nitrogen into the iron-based material and to obtain a stable austenite phase during nitriding. However, the treatment in an extremely high temperature region makes it difficult to control the nitriding progress rate during the treatment and causes the austenite grains to become coarse during the treatment, which adversely affects the ductility and toughness of the material after the treatment. Therefore, the nitriding temperature is preferably 1000 ° C. or lower.
なお、上記窒化処理としては、ガス窒化法、ガス軟窒化法、プラズマ窒化法、塩浴窒化法など、公知の方法を適用することができるが、本発明の鉄系材料を製造する上では特に窒化ポテンシャルの制御が比較的容易でかつ処理コストの低廉な、ガス窒化法を適用することが好ましい。また、鉄系材料中の窒素濃度制御の観点から、窒化処理時間は60〜10000minとすることが好ましい。 As the nitriding treatment, a known method such as a gas nitriding method, a gas soft nitriding method, a plasma nitriding method, a salt bath nitriding method, etc. can be applied, but particularly in producing the iron-based material of the present invention. It is preferable to apply a gas nitriding method, in which the control of the nitriding potential is relatively easy and the processing cost is low. Further, from the viewpoint of controlling the nitrogen concentration in the iron-based material, the nitriding time is preferably 60 to 10,000 min.
（冷却条件）
上記の条件に従う窒化処理にて鉄系素材の少なくとも一部には、Ｎ：３at％以上８at％未満を含有するオーステナイト組織が形成される。本発明においては、これを１℃／s以上の冷却速度で500℃以下Ms点以上の温度まで冷却することにより、上記オーステナイト組織を当該温度まで存在させることを可能にする。すなわち、冷却速度が１℃／s未満である場合には、冷却中の組織中にフェライト相が形成してしまい、冷却終了後のオーステナイト含有量が減少するため、その後の熱処理により所望の硬度を有する硬質相が得られない。なお、冷却速度の上限値は特に限定しないが、簡易な冷却方法にて達成するためには、50℃／s以下とすることが好ましい。
一方、冷却停止温度が500℃を超えると、冷却停止後の放冷時に組織中に粗大なフェライト相が形成してしまい、冷却後のオーステナイト含有量が減少する。そのため、500℃以下とする。
(Cooling conditions)
An austenite structure containing N: 3 at% or more and less than 8 at% is formed in at least a part of the iron-based material by nitriding according to the above conditions. In the present invention, the austenite structure can be made to exist up to the temperature by cooling it to a temperature of 500 ° C. or lower and the Ms point or higher at a cooling rate of 1 ° C./s or higher. That is, when the cooling rate is less than 1 ° C./s, a ferrite phase is formed in the structure being cooled, and the austenite content after cooling is reduced. The hard phase which has is not obtained. In addition, although the upper limit of a cooling rate is not specifically limited, In order to achieve with a simple cooling method, it is preferable to set it as 50 degrees C / s or less.
On the other hand, if the cooling stop temperature exceeds 500 ° C., a coarse ferrite phase is formed in the structure at the time of cooling after cooling is stopped, and the austenite content after cooling is reduced. Therefore, it is set to 500 ° C. or less.
また、Ms点未満の温度まで冷却した場合には、該オーステナイトの少なくとも一部にマルテンサイト変態を生じ得る。マルテンサイトそれ自体は硬質な組織であるが、その後の硬質化処理時、および高温使用環境に曝された場合、焼もどしの進行により硬度が低下して所望の強度を得ることが困難となるため、それ以上の冷却完了温度であることが必要である。なお、500℃以下Ms点以上の温度まで１℃／s以上で冷却した後の冷却速度は任意である。 When cooled to a temperature below the Ms point, martensitic transformation can occur in at least a part of the austenite. Martensite itself is a hard structure, but during subsequent hardening treatments and when exposed to high-temperature use environments, the hardness decreases due to the progress of tempering, making it difficult to obtain the desired strength. It is necessary that the cooling completion temperature be higher than that. In addition, the cooling rate after cooling at 1 degree C / s or more to the temperature of 500 degrees C or less Ms point or more is arbitrary.
（硬質相形成処理条件）
上記冷却工程を経た鉄系材料は、少なくとも一部に軟質なオーステナイト組織を有する。しかし、この鉄系材料をMs点以上500℃以下の温度域に保持することにより、上記オーステナイト組織がαとγ´との微細分散組織に変化し、HV650以上の硬質相が形成される。
すなわち、加熱保持温度がMs点未満では、窒化処理により形成したオーステナイトの少なくとも一部にマルテンサイト変態を生じ、所望の硬度を有する硬質相が得られない。一方、該加熱保持温度が500℃を超えると、形成される組織の粗大化を生じるとともに、表層部で脱窒が発生し、やはり硬質相の硬度が不十分となる。
なお、上記温度における保持時間を10min未満とすると上記した組織変化が不十分になることから、鉄系材料を所望の組織とするために10min以上の保持を必要とする。一方、60000minを超えて保持しても、それ以上の硬度の上昇は望めないため、保持時間は60000min以下とすることが望ましい。
(Hard phase formation processing conditions)
The iron-based material that has undergone the cooling step has a soft austenite structure at least partially. However, by holding this iron-based material in a temperature range from the Ms point to 500 ° C., the austenite structure changes to a finely dispersed structure of α and γ ′, and a hard phase of HV650 or higher is formed.
That is, when the heating and holding temperature is lower than the Ms point, martensitic transformation occurs in at least a part of austenite formed by nitriding, and a hard phase having a desired hardness cannot be obtained. On the other hand, when the heating and holding temperature exceeds 500 ° C., the formed structure becomes coarse and denitrification occurs in the surface layer portion, and the hardness of the hard phase becomes insufficient.
Note that if the holding time at the above temperature is less than 10 min, the above-described change in the structure becomes insufficient. Therefore, it is necessary to hold the iron-based material for 10 min or more in order to obtain a desired structure. On the other hand, since the hardness cannot be further increased even if the holding time exceeds 60000 min, the holding time is preferably 60000 min or less.
上記方法においては、フェライトと、γ´（Fe4N）すなわち熱的に安定な相により硬質な相が形成されるため、硬質相形成のための上記保持温度域にて長時間使用された後にも、十分な強度を有する材料となる。従って、比較的高温に曝される環境で長時間使用した後にも、十分な強度を有する部品を得ることが可能となる。なお、本発明に係る鉄系材料を用いて機械部品を製造する際は、窒化処理前の鉄系素材が比較的軟質であり、冷間加工を施す場合であっても容易に所望の形状に成形することができることから、この段階で成形を行うことが有利である。また、一部に硬化層を形成する鉄系材料につき、硬化層以外の部分に冷間加工を施す場合は、硬化層形成後に冷間加工を施すことも可能である。 In the above method, since a hard phase is formed by ferrite and γ ′ (Fe 4 N), that is, a thermally stable phase, after being used for a long time in the holding temperature range for forming the hard phase. Is a material having sufficient strength. Accordingly, it is possible to obtain a component having sufficient strength even after being used for a long time in an environment exposed to a relatively high temperature. When manufacturing machine parts using the iron-based material according to the present invention, the iron-based material before nitriding is relatively soft, and even in the case of cold working, it can be easily formed into a desired shape. Since it can be molded, it is advantageous to perform the molding at this stage. Moreover, about the iron-type material which forms a hardened layer in part, when performing cold work on parts other than a hardened layer, it is also possible to give cold work after hardened layer formation.
表１に示す化学組成の鋼を転炉にて溶製し、連続鋳造によりブルームとした。次いで、ビレット圧延、さらに棒鋼圧延を施して、35mmφの棒鋼とした。こうして得た棒鋼について、素材のビッカース硬さを測定するとともに、以下に示す種々の熱処理に供して特性を調査した。 Steel having the chemical composition shown in Table 1 was melted in a converter and bloomed by continuous casting. Next, billet rolling and further steel bar rolling were performed to obtain a steel bar having a diameter of 35 mm. The steel bars thus obtained were measured for the Vickers hardness of the material and subjected to various heat treatments shown below to investigate the characteristics.
すなわち、表２に示す条件に従って、窒化処理、冷却、その後の硬化熱処理（硬質相形成処理）を行った。その際、まず窒化処理後の鉄系材料について、EPMAを用いて、表面から深さ20μm近傍の窒素濃度（at％）を測定した。また、表面からの深さ20μm部を光学顕微鏡により観察し、構成ミクロ組織の判定を行った。ここで、窒化処理後の冷却完了温度が室温より高いサンプルについては、冷却完了温度から室温まで急冷（水冷）にて組織凍結処理を施した後、EPMAおよびミクロ組織判定を行った。ミクロ組織判定に際しては、凍結組織から凍結前の冷却完了時組織を推定した。次に、硬化熱処理後の鉄系材料については、EPMAを用いて、表面から深さ20μm近傍の窒素濃度（at％）を測定した。また、表面からの深さ20μm部を光学顕微鏡により観察し、構成ミクロ組織の判定を行うとともに、その部分のビッカース硬さを測定した。さらに、20μm間隔で深さ方向にビッカース硬さ測定を行い、硬さがHV650を超える領域の厚さを測定した。
なお、ビッカース硬さの測定はいずれも、荷重25gf（0.245N）、荷重保持時間15ｓの条件にて行った。
That is, according to the conditions shown in Table 2, nitriding treatment, cooling, and subsequent curing heat treatment (hard phase forming treatment) were performed. At that time, the nitrogen concentration (at%) in the vicinity of a depth of 20 μm from the surface of the iron-based material after the nitriding treatment was measured using EPMA. Further, a 20 μm portion from the surface was observed with an optical microscope, and the constituent microstructure was judged. Here, for samples whose cooling completion temperature after nitriding was higher than room temperature, tissue freezing treatment was performed from the cooling completion temperature to room temperature by rapid cooling (water cooling), and then EPMA and microstructure determination were performed. When determining the microstructure, the structure at the completion of cooling before freezing was estimated from the frozen structure. Next, about the iron-type material after hardening heat processing, nitrogen concentration (at%) of the depth vicinity of 20 micrometers from the surface was measured using EPMA. Further, a 20 μm portion from the surface was observed with an optical microscope, the constituent microstructure was judged, and the Vickers hardness of the portion was measured. Furthermore, Vickers hardness measurement was performed in the depth direction at 20 μm intervals, and the thickness of the region where the hardness exceeded HV650 was measured.
The Vickers hardness was measured under the conditions of a load of 25 gf (0.245 N) and a load holding time of 15 s.
また、薄膜TEM観察により、硬化処理後の鉄系材料中に生成したγ´のサイズおよび面積率の測定を行った。サイズ測定は、各実施例について30個以上のγ´を測定し、サイズが300nm以下であったγ´の個数の、全γ´に対する個数比を、微細γ´率として求めた。 Moreover, the size and area ratio of γ ′ produced in the iron-based material after the curing treatment were measured by thin film TEM observation. For size measurement, 30 or more γ ′ were measured for each example, and the number ratio of the number of γ ′ having a size of 300 nm or less to the total γ ′ was obtained as a fine γ ′ rate.
なお、表１中の材料Ｈは、代表的な機械構造用鋼である、JIS−S53Cに相当する。当該鋼は、本発明との比較を目的として特性を調査したものであり、素材としての比較には、上記のとおり、35mmφの棒鋼とした後に焼なましを行ったものを用いた。さらに、表２に示すように、焼入れ焼もどしを行った材料について、表面から20μm部のビッカース硬さ、構成ミクロ組織、硬さがHV650以上の領域の厚さを、上記と同様に測定した。 In addition, the material H in Table 1 corresponds to JIS-S53C, which is a typical machine structural steel. The steel was investigated for characteristics for the purpose of comparison with the present invention. For comparison as a raw material, as described above, steel that was annealed after being made into a 35 mmφ bar steel was used. Further, as shown in Table 2, with respect to the material tempered and tempered, the Vickers hardness, the constituent microstructure, and the thickness of the region having a hardness of HV650 or more from the surface were measured in the same manner as described above.
＜調査特性＞
本発明に従って製造した鉄系材料（実施例No.１，No.10〜17）は何れも、代表的な機械構造用鋼であるJIS−S53C（No.18）の球状化材よりも低い素材硬さを有しており、冷間加工性に優れている。一方で、窒化→硬化熱処理後には、S53Cの焼入れ焼もどし材よりも優れた表層部の硬さを有している。
一方、窒化あるいはその後の冷却の条件が適切でない場合（No.２〜６）には、窒化冷却後における表面から20μm部におけるオーステナイト組織の形成が成されず、その後の硬化処理において十分な硬さが得られなかった。
<Survey characteristics>
The iron-based materials (Examples No. 1 and Nos. 10 to 17) manufactured according to the present invention are all lower materials than the spheroidizing material of JIS-S53C (No. 18), which is a typical steel for machine structural use. It has hardness and excellent cold workability. On the other hand, after the nitriding → curing heat treatment, it has a surface layer hardness superior to the quenching and tempering material of S53C.
On the other hand, when the conditions for nitriding or subsequent cooling are not appropriate (Nos. 2 to 6), the austenite structure is not formed in the 20 μm portion from the surface after cooling by nitriding, and sufficient hardness is obtained in the subsequent hardening process. Was not obtained.
窒化温度が低い場合（No.２）は、窒化時のオーステナイト（γ）相の生成が不十分になり、フェライト（α）相が残存した。窒化時間が短い場合（No.３）は、窒化の進行が不十分であり、十分な窒素濃度が得られなかった。このため、いずれも硬化処理後に十分な硬さが得られなかった。
窒化時間が長い場合（No.４）は、過剰な窒化の進行に伴い、窒素濃度が本発明範囲を超えて、不適な窒化物（ε相）が窒化処理段階で生成し、硬化処理後もこれが残留して硬さに悪影響を及ぼした。
窒化後の冷却速度が遅い場合（No.5）は、冷却途中にフェライト相が生じ、硬化熱処理後は微細γ´の形成が成されず、十分な硬さが得られなかった。
窒化後の冷却完了温度が低い場合（No.6）は、冷却完了後にマルテンサイト（α´）が生じ、硬化熱処理後は焼もどしマルテンサイトとなり、十分な硬さが得られなかった。
When the nitriding temperature was low (No. 2), the austenite (γ) phase was not sufficiently generated during nitriding, and the ferrite (α) phase remained. When the nitriding time was short (No. 3), the progress of nitriding was insufficient, and a sufficient nitrogen concentration could not be obtained. For this reason, in any case, sufficient hardness was not obtained after the curing treatment.
When the nitriding time is long (No. 4), with excessive progress of nitriding, the nitrogen concentration exceeds the range of the present invention, and an inappropriate nitride (ε phase) is generated in the nitriding treatment stage. This remained and adversely affected the hardness.
When the cooling rate after nitriding was slow (No. 5), a ferrite phase was generated during cooling, and fine γ ′ was not formed after the curing heat treatment, and sufficient hardness was not obtained.
When the cooling completion temperature after nitriding was low (No. 6), martensite (α ′) was generated after completion of cooling, and tempered martensite was formed after the curing heat treatment, and sufficient hardness was not obtained.
また、窒化冷却により所望のオーステナイト組織が得られても、硬化熱処理条件が本発明の範囲外である場合（No.７〜９）は、いずれも十分な表層硬さを得ることができなかった。
すなわち、硬化熱処理温度が低い場合（No.７）は、窒化冷却処理で得られたオーステナイトからマルテンサイト（α´）相が生じ、十分な硬さが得られなかった。
硬化熱処理温度が高い場合（No.８）は、オーステナイト相からの組織変化によって形成されたフェライト（α）相およびγ´（Fe4N）相が粗大化し、十分な硬さが得られなかった。
硬化熱処理時間が短い（No.９）場合は、窒化冷却処理で得られたオーステナイトからの組織変化が十分に進行せず、結果として硬化熱処理後、室温での構成ミクロ組織がマルテンサイトとなり、十分な硬さを得られなかった。
Moreover, even if a desired austenite structure was obtained by nitriding cooling, when the curing heat treatment conditions were outside the scope of the present invention (Nos. 7 to 9), none of them could obtain sufficient surface hardness. .
That is, when the curing heat treatment temperature was low (No. 7), a martensite (α ′) phase was generated from austenite obtained by the nitriding cooling treatment, and sufficient hardness was not obtained.
When the curing heat treatment temperature was high (No. 8), the ferrite (α) phase and γ ′ (Fe 4 N) phase formed by the structural change from the austenite phase were coarsened, and sufficient hardness was not obtained. .
When the hardening heat treatment time is short (No. 9), the structural change from the austenite obtained by the nitriding cooling treatment does not proceed sufficiently, and as a result, the structural microstructure at room temperature becomes martensite after the hardening heat treatment. The hardness was not obtained.
産業機械や自動車等の機械部品に好適に用いられる機械構造用鉄系材料を提供する。
Provided is an iron-based material for machine structure that is suitably used for machine parts such as industrial machines and automobiles.
Claims (3)
Cr：0.05mass％以上3.0mass％以下、
Al：0.005 mass％以上3.0 mass％以下、
Ti：0.0005 mass％以上0.5 mass％以下、
Nb：0.005 mass％以上0.1 mass％以下、
Ｖ：0.02 mass％以上1.0 mass％以下、
Mo：0.02 mass％以上1.0mass％以下、
Mn：0.02 mass％以上2.0 mass％以下、
Si：0.02 mass％以上3.0 mass％以下、
Ni：0.02 mass％以上2.0mass％以下、
Cu：0.02 mass％以上2.0 mass％以下および
Co：0.02 mass％以上2.0 mass％以下
の中から選択される少なくとも１種以上を含有することを特徴とする鉄系材料の製造方法。 3. The iron-based material according to claim 2, further comprising:
Cr: 0.05 mass% or more and 3.0 mass% or less,
Al: 0.005 mass% to 3.0 mass%,
Ti: 0.0005 mass% or more and 0.5 mass% or less,
Nb: 0.005 mass% or more and 0.1 mass% or less,
V: 0.02 mass% or more and 1.0 mass% or less,
Mo: 0.02 mass% or more and 1.0 mass% or less,
Mn: 0.02 mass% or more and 2.0 mass% or less,
Si: 0.02 mass% or more and 3.0 mass% or less,
Ni: 0.02 mass% or more and 2.0 mass% or less,
Cu: 0.02 mass% to 2.0 mass%
Co: A method for producing an iron-based material comprising at least one selected from 0.02 mass% to 2.0 mass%.
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| JP2022028931A (en) * | 2018-03-26 | 2022-02-16 | 日本製鉄株式会社 | Steel for nitriding-quenching, nitriding-quenched component and method for manufacturing the same |
| JP7031428B2 (en) | 2018-03-26 | 2022-03-08 | 日本製鉄株式会社 | Steel for soaking and quenching, soaking and quenching parts and their manufacturing methods |
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