JP6759842B2 - Steel manufacturing method - Google Patents
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本発明は、少なくとも浸炭工程を含む鋼材の製造方法に関する。 The present invention relates to a method for producing a steel material including at least a carburizing step.
歯車等を鋼材で製造する際に、鋼材表面の強度を向上させるために、浸炭処理が行われる。また、表面の馴染み性を確保する等の目的で、浸炭処理とともに浸窒処理を行うことも、特許文献１に記載のように、知られている。 When a gear or the like is manufactured of a steel material, a carburizing treatment is performed in order to improve the strength of the steel material surface. Further, as described in Patent Document 1, it is also known that a carburizing treatment is performed together with a carburizing treatment for the purpose of ensuring the familiarity of the surface.
歯車のようにエッジ形状部位を有する鋼材の場合、浸炭処理時に、エッジ形状部位はエッジ形状を有さない部位に対して浸炭が促進しやすい傾向にあり、セメンタイト（鉄炭化物）が析出しやすく、部分的に鋼材強度が低下する可能性がある。また、本発明者らの経験では、鋼材に対して浸炭工程を行った後に、浸窒工程を実施する場合において、表面粗さが大きくなり、また、馴染み性が低下することを経験した。 In the case of a steel material having an edge-shaped portion such as a gear, during the carburizing treatment, the edge-shaped portion tends to promote carburizing with respect to the portion having no edge shape, and cementite (iron carbide) is likely to precipitate. Steel strength may be partially reduced. Further, in the experience of the present inventors, when the carburizing step is carried out on the steel material and then the nitrification step is carried out, the surface roughness is increased and the familiarity is lowered.
本発明は、上記の事情に鑑みてなされたものであり、浸炭処理を行った後に、浸窒工程を行うようにした鋼材の製造方法において、鋼材表面の強度の低下と馴染み性の低下の双方を回避できるようにした、鋼材の製造方法を提供することを課題とする。 The present invention has been made in view of the above circumstances, and in a method for producing a steel material in which a carburizing treatment is performed and then a nitrification step is performed, both the strength of the steel material surface and the compatibility are reduced. It is an object of the present invention to provide a method for manufacturing a steel material, which can avoid the above.
本発明による鋼材の製造方法は、少なくとも浸炭工程を含む鋼材の製造方法において、前記製造方法は、前記浸炭工程後に浸炭ガスを含まない浸窒ガス雰囲気下で浸窒処理を行う浸窒工程をさらに含み、前記浸窒工程は、浸窒ガスを雰囲気中に供給しながら鋼材の温度を９５０℃以上に高めて浸窒処理を行う浸窒過程と、前記浸窒過程の後に、窒素を鋼材に拡散させるための期間を挟まずに、鋼材の冷却を開始する冷却過程を含む、ことを特徴とする。 The method for producing a steel material according to the present invention is at least a method for producing a steel material including a carburizing step, wherein the manufacturing method further comprises a carburizing step of performing a carburizing treatment in a carburizing gas-free atmosphere after the carburizing step. Including, the carburizing step includes a carburizing process in which the temperature of the steel material is raised to 950 ° C. or higher to perform a carburizing treatment while supplying a carburizing gas into the atmosphere, and after the carburizing process, nitrogen is diffused into the steel material. It is characterized by including a cooling process in which cooling of the steel material is started without a period for causing the steel material to be cooled.
本発明の製造方法によれば、浸窒工程において、浸窒過程の後に、窒素を鋼材に拡散させるための期間を挟むことなく、直ちに鋼材の冷却を開始するようにしている。そのために、浸炭工程で鋼材内部に浸入した炭素が、浸窒工程中に鋼材表面に戻ってくるのを阻止することができ、結果、セメンタイトの析出による鋼材表面の強度の低下が抑制され、かつ、浸窒中の脱炭によって柔らかくなった表面をそのまま維持することができることで、表面の馴染み性も向上した鋼材を得ることができる。 According to the production method of the present invention, in the nitrification step, cooling of the steel material is started immediately after the nitrification process without a period for diffusing nitrogen into the steel material. Therefore, it is possible to prevent the carbon that has entered the inside of the steel material in the carburizing process from returning to the surface of the steel material during the carburizing process, and as a result, the decrease in the strength of the steel material surface due to the precipitation of cementite is suppressed, and Since the surface softened by decarburization during carburizing can be maintained as it is, a steel material having improved surface compatibility can be obtained.
以下、本発明の実施の形態を本発明者が得た新たな知見とともに説明する。
最初に、鋼材に施される一般的な浸炭工程について図１（ａ）を参照して説明する。図１（ａ）に示すように、処理室内で鋼材は所要の温度にまで昇温され、その温度を維持した状態で、炭素を含む処理ガス（浸炭ガス）の所要流量を適数回に分けて供給することで、浸炭過程が進行する。浸炭後、処理ガスの供給を停止した状態で、炭素を鋼材に拡散させる拡散過程が行われ、その後、冷却処理が施される。そのような一連の浸炭工程を終えた後の鋼材表面近傍に含まれる炭素の濃度は、図１（ｂ）に示すように、表面から深さ方向に向けて次第に減少した傾向となる。
Hereinafter, embodiments of the present invention will be described together with new findings obtained by the present inventor.
First, a general carburizing process applied to a steel material will be described with reference to FIG. 1 (a). As shown in FIG. 1A, the temperature of the steel material is raised to the required temperature in the processing chamber, and the required flow rate of the processing gas containing carbon (carburized gas) is divided into appropriate times while maintaining that temperature. The carburizing process proceeds by supplying the gas. After carburizing, the diffusion process of diffusing carbon into the steel material is performed with the supply of the processing gas stopped, and then the cooling process is performed. As shown in FIG. 1B, the concentration of carbon contained in the vicinity of the surface of the steel material after the completion of such a series of carburizing steps tends to gradually decrease from the surface toward the depth.
図１に示した方法で浸炭処理を施した鋼材に対して、一般的な浸窒工程を施すときの処理の進行状態が図２（ａ）に示される。図２（ａ）に示すように、処理室内に浸炭処理後の鋼材をセットし、該鋼材を所要の温度にまで昇温した後、その温度を維持した状態で、窒素を含む処理ガス（浸窒ガス）の所要流量を適数回に分けて供給することで、浸窒過程が進行する。浸窒後、浸窒ガスの供給を停止した状態で、窒素を鋼材に拡散させる拡散過程を行い、その後、冷却処理が施される。 FIG. 2A shows the progress of the treatment when a general carburizing step is applied to the steel material which has been carburized by the method shown in FIG. As shown in FIG. 2A, the steel material after the carburizing treatment is set in the treatment chamber, the temperature of the steel material is raised to a required temperature, and then the treatment gas containing nitrogen (immersion) is maintained at that temperature. The carburizing process proceeds by supplying the required flow rate of nitrogen gas) in appropriate several times. After the immersion, the diffusion process of diffusing the nitrogen into the steel material is performed with the supply of the immersion gas stopped, and then the cooling treatment is performed.
上記のようにして、浸炭処理と浸窒処理を行った鋼材について、鋼材表面近傍に含まれる窒素と炭素の濃度を測定したところ、図２（ｂ）に実線で示すようであった。また、その表面の成分分析と表面粗さの測定を行ったところ、表面からセメンタイトが析出されるとともに、表面粗さも図４（ａ）に示すように、＋−方向にブレの大きなものであった。また、周知の馴染み性評価試験後での表面粗さも、ほぼ同じ状態であり、馴染み性の点で、満足できるものではなかった。 When the concentrations of nitrogen and carbon contained in the vicinity of the surface of the steel material were measured for the steel material subjected to the carburizing treatment and the nitrification treatment as described above, it was shown by a solid line in FIG. 2 (b). Further, when the component analysis of the surface and the measurement of the surface roughness were performed, cementite was precipitated from the surface, and the surface roughness also showed a large blur in the +-direction as shown in FIG. 4 (a). It was. In addition, the surface roughness after the well-known familiarity evaluation test was almost the same, and the familiarity was not satisfactory.
その原因を見つけるべく、さらに分析を行った。その結果、図２（ａ）に示す浸窒過程直後の鋼材においては、浸窒により、表面近傍の炭素濃度は、図２（ｂ）に点線で示すように大きく減少していたものが、拡散過程を終えた時点では、図２（ｂ）に実線で示すように濃度が増大したことが分かった。この原因は、より深いところに浸透していた炭素が、濃度勾配に起因して、拡散過程中に、より表面側部位に移動してきたものと考えた。そして、雰囲気ガス（処理ガス）である浸窒ガスは炭素を含まないのにもかかわらず、移動してきた炭素によって、鋼材の表面にセメンタイトが形成されやすくなり、表面粗さが大きくなるとともに、表面に十分な柔らかさが得られず、馴染み性評価試験後での表面粗さにも変化が見られなかったものと考えた。 Further analysis was conducted to find the cause. As a result, in the steel material immediately after the nitrification process shown in FIG. 2 (a), the carbon concentration in the vicinity of the surface was significantly reduced due to nitrification as shown by the dotted line in FIG. 2 (b), but diffused. At the end of the process, it was found that the concentration increased as shown by the solid line in FIG. 2 (b). It is considered that the cause of this is that the carbon that had penetrated deeper moved to the surface side part during the diffusion process due to the concentration gradient. Although the nitrifying gas, which is an atmospheric gas (treatment gas), does not contain carbon, the transferred carbon makes it easier for cementite to be formed on the surface of the steel material, increasing the surface roughness and surface. It was considered that sufficient softness was not obtained and there was no change in the surface roughness after the familiarity evaluation test.
その仮定のもとに、浸炭工程の後に行う浸窒工程において、通常の浸窒処理工程における前記「拡散過程」の段階を省略して、「浸窒過程」の直後に「冷却」を行って鋼材を製造した。図３（ａ）は、実験で行った浸窒工程の時間経過であり、図３（ｂ）は、製造された鋼材での表面近傍における深さ方向での炭素と窒素の濃度を示している。 Based on that assumption, in the carburizing step performed after the carburizing step, the step of the "diffusion process" in the normal carburizing process is omitted, and "cooling" is performed immediately after the "carburizing process". Manufactured steel. FIG. 3A shows the time course of the nitriding step performed in the experiment, and FIG. 3B shows the concentration of carbon and nitrogen in the depth direction in the vicinity of the surface of the manufactured steel material. ..
図３（ａ）に示すように、ここでは、浸窒過程の終了直後に、直ちに、鋼材の冷却を行っている。なお、浸窒過程は、鋼材を９５０℃以上に高めて行った。その結果、冷却後の鋼材の炭素の濃度は、図３（ｂ）に示すように、先に図２（ｂ）に点線で示した、従来の浸窒処理における浸窒過程直後の鋼材における炭素濃度とほぼ同じ状態であり、炭素濃度の回復は観察されなかった。結果、セメンタイトの析出もなかった。また、表面粗さは、従来の浸窒処理を施した鋼材の表面粗さとほぼ同じであったが、周知の馴染み性評価試験後での表面粗さは、図４（ｂ）に示すように、ブレの小さいものとなっており、高い馴染み性も得られた。これは、浸窒工程において、表面への炭素の回復が回避された結果であると考えられる。 As shown in FIG. 3A, here, the steel material is cooled immediately after the end of the nitrification process. The nitrogen immersion process was carried out by raising the temperature of the steel material to 950 ° C. or higher. As a result, as shown in FIG. 3 (b), the carbon concentration of the steel material after cooling is as shown by the dotted line in FIG. 2 (b). It was almost the same as the concentration, and no recovery of carbon concentration was observed. As a result, there was no precipitation of cementite. Further, the surface roughness was almost the same as the surface roughness of the conventional steel material subjected to the distilling treatment, but the surface roughness after the well-known familiarity evaluation test is as shown in FIG. 4 (b). , The blur is small, and high familiarity is also obtained. This is considered to be the result of avoiding the recovery of carbon to the surface in the nitrification step.
本発明は、上記の実験結果に基づいており、前記したように、本発明による鋼材の製造方法は、少なくとも浸炭工程を含む鋼材の製造方法において、前記製造方法は、前記浸炭工程後に浸炭ガスを含まない浸窒ガス雰囲気下で浸窒処理を行う浸窒工程をさらに含み、前記浸窒工程は、浸窒ガスを雰囲気中に供給しながら鋼材の温度を９５０℃以上に高めて浸窒処理を行う浸窒過程と、前記浸窒過程の後に、窒素を鋼材に拡散させるための期間を挟まずに、鋼材の冷却を開始する冷却過程を含む、ことを特徴とする。 The present invention is based on the above experimental results, and as described above, the method for producing a steel material according to the present invention is at least a method for producing a steel material including a carburizing step, and the manufacturing method uses a carburized gas after the carburizing step. The carburizing step further includes a carburizing process in which the carburizing gas is not contained, and the carburizing step raises the temperature of the steel material to 950 ° C. or higher while supplying the carburizing gas into the atmosphere. It is characterized by including a carburizing process to be performed and a cooling process in which cooling of the steel material is started without a period for diffusing nitrogen into the steel material after the carburizing process.
特に制限されないが、本発明の製造方法の具体的な一例として、浸炭工程において表面炭素濃度を０．６〜０．８％まで上昇させた後の鋼材に対して、９５０℃〜１１００℃で１０分〜１時間浸窒処理を施し、表面窒素濃度を０．２〜０．６％まで上昇させた後、拡散工程を設けることなく、直ちに冷却処理（焼き入れ処理）を行うようにする。それにより、浸窒工程での安定した表面脱炭ができ、かつ馴染み性が良好な歯車等を製造することができる。浸窒工程において、拡散工程をなくすことで、前記のように、表面への炭素の回復を抑制できるとともに、製造の短時間化も可能となる。なお、必要な浸窒深さは、高温（９５０℃以上）での処理のため、拡散工程を設けなくても、確保することができる。 Although not particularly limited, as a specific example of the production method of the present invention, 10 at 950 ° C to 1100 ° C with respect to the steel material after raising the surface carbon concentration to 0.6 to 0.8% in the carburizing step. After carburizing for 1 minute to 1 hour to raise the surface nitrogen concentration to 0.2 to 0.6%, the cooling treatment (quenching treatment) is immediately performed without providing a diffusion step. As a result, stable surface decarburization in the nitrification step can be performed, and gears and the like having good familiarity can be manufactured. By eliminating the diffusion step in the nitrification step, as described above, the recovery of carbon to the surface can be suppressed and the production time can be shortened. Since the treatment is performed at a high temperature (950 ° C. or higher), the required immersion depth can be secured without providing a diffusion step.
［実施例］
鋼材（ＳＣＭ４２０）に対して、従来法により浸炭処理を行った。浸炭ガスにはアセチレンガスを用い、浸炭工程は、１１００℃、７００Ｐａの条件下で、１２ｍｉｎ行った。
浸炭処理後の鋼材を冷却した後、その鋼材に対して、図３（ａ）に示す手順で、すなわち、拡散過程を行わないで、浸窒過程後に直ちに冷却する浸窒工程を行った。浸窒ガスとしてアンモニアガスを用いた。温度は１１００℃、圧力は７００Ｐａに維持し、１０ｍｉｎの処理を行った。
製造後の鋼材における表面からの深さ方向での、炭素濃度と窒素濃度を図５に示した。図５に示すように、表面近傍での炭素濃度の回復は起こらなかった。また、その炭素濃度分布は、浸窒過程直後の濃度分布とほぼ同じであった。
[Example]
The steel material (SCM420) was carburized by a conventional method. Acetylene gas was used as the carburizing gas, and the carburizing step was carried out for 12 minutes under the conditions of 1100 ° C. and 700 Pa.
After cooling the steel material after the carburizing treatment, the steel material was subjected to a nitriding step of immediately cooling after the nitriding process according to the procedure shown in FIG. 3A, that is, without performing the diffusion process. Ammonia gas was used as the nitriding gas. The temperature was maintained at 1100 ° C. and the pressure was maintained at 700 Pa, and the treatment was carried out for 10 minutes.
The carbon concentration and nitrogen concentration in the depth direction from the surface of the manufactured steel material are shown in FIG. As shown in FIG. 5, the recovery of the carbon concentration near the surface did not occur. Moreover, the carbon concentration distribution was almost the same as the concentration distribution immediately after the inking process.
［比較例］
浸窒工程において、浸窒後に、１５ｍｉｎの拡散工程を行った以外は、実施例と同様にして鋼材を製造した。製造後の鋼材における表面からの深さ方向での炭素濃度を図６に示した。図６に示すように、表面近傍での炭素濃度は０．８％弱の値であり、この値は、浸炭処理後の鋼材の炭素濃度の値とほぼ同じであった。
[Comparison example]
In the nitriding step, a steel material was produced in the same manner as in Examples except that a diffusion step of 15 minutes was performed after nitriding. The carbon concentration in the depth direction from the surface of the manufactured steel material is shown in FIG. As shown in FIG. 6, the carbon concentration in the vicinity of the surface was a value of less than 0.8%, and this value was almost the same as the value of the carbon concentration of the steel material after the carburizing treatment.
［評価］
実施例と比較例からわかるように、本発明の製造方法で製造された鋼材は、その表面の炭素濃度が、従来法で製造したものと比較して低下している。結果、表面の柔らかさを確保することができ、馴染み性の点で優れた鋼材が得られる。また、表面の強度低下も回避できる。
[Evaluation]
As can be seen from Examples and Comparative Examples, the carbon concentration on the surface of the steel material produced by the production method of the present invention is lower than that produced by the conventional method. As a result, the softness of the surface can be ensured, and a steel material having excellent compatibility can be obtained. In addition, a decrease in surface strength can be avoided.
Claims (1)
前記製造方法は、前記浸炭工程後に浸炭ガスを含まない雰囲気下で浸窒ガスを供給することで浸窒処理を行う浸窒工程をさらに含み、
前記浸炭工程は、
前記浸炭ガスを複数回に分けて雰囲気中に断続的に供給することにより浸炭処理を行う浸炭過程と、
前記浸炭過程の後に、炭素を鋼材に拡散させる拡散過程とを含み、
前記浸炭過程は、前記浸炭ガスを雰囲気中に供給する複数の供給過程と、前記供給過程同士の間に設けられ前記浸炭ガスの供給を中断する中断過程とを含み、
前記拡散過程は、前記中断過程よりも長く、
前記浸窒工程は、
前記浸窒ガスを複数回に分けて雰囲気中に断続的に供給しながら鋼材の温度を９５０℃以上に高めて浸窒処理を行う浸窒過程と、
前記浸窒ガスの最後の供給を停止した後に、窒素を鋼材に拡散させるための期間を挟まずに、鋼材の冷却を開始する冷却過程を含み、
前記浸窒ガスの最初の供給時間は、前記浸窒ガスの２回目以降の供給時間よりも長いことを特徴とする鋼材の製造方法。 At least in the method of manufacturing steel including the carburizing process
The manufacturing method further includes a carburizing step of performing a carburizing treatment by supplying a carburizing gas in an atmosphere containing no carburizing gas after the carburizing step.
The carburizing step
A carburizing process in which the carburizing gas is intermittently supplied to the atmosphere in a plurality of times, and a carburizing process.
After the carburizing process, a diffusion process of diffusing carbon into the steel material is included.
The carburizing process includes a plurality of supply processes for supplying the carburized gas into the atmosphere, and an interruption process provided between the supply processes to interrupt the supply of the carburized gas.
The diffusion process is longer than the interruption process
The nitrogenation step is
The nitriding process in which the temperature of the steel material is raised to 950 ° C. or higher and the nitriding treatment is performed while intermittently supplying the nitriding gas into the atmosphere in a plurality of times.
After stopping the last supply of the nitriding gas, nitrogen with no intervening period for diffusing the steel, saw including a cooling process to start cooling of the steel,
A method for producing a steel material, wherein the first supply time of the nitriding gas is longer than the second and subsequent supply times of the nitriding gas .
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