JP4971751B2 - Manufacturing method of high-concentration carburized steel - Google Patents

Manufacturing method of high-concentration carburized steel Download PDF

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JP4971751B2
JP4971751B2 JP2006299836A JP2006299836A JP4971751B2 JP 4971751 B2 JP4971751 B2 JP 4971751B2 JP 2006299836 A JP2006299836 A JP 2006299836A JP 2006299836 A JP2006299836 A JP 2006299836A JP 4971751 B2 JP4971751 B2 JP 4971751B2
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temperature
carburizing
carburization
steel
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JP2008115427A (en
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敏之 森田
将臣 猿山
博幸 露崎
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Honda Motor Co Ltd
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Priority to PCT/JP2007/070953 priority patent/WO2008056552A1/en
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    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
    • C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
    • C23C8/20—Carburising
    • C23C8/22—Carburising of ferrous surfaces
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/06—Surface hardening
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/002—Heat treatment of ferrous alloys containing Cr
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium

Description

本発明は、高濃度浸炭鋼の製造方法に関し、さらに詳しくは、浸炭処理によって表面に微細、かつ球状の炭化物を多量に析出させることが可能な高濃度浸炭鋼の製造方法に関する。   The present invention relates to a method for producing high-concentration carburized steel, and more particularly, to a method for producing high-concentration carburized steel capable of precipitating a large amount of fine and spherical carbides on the surface by carburizing treatment.

浸炭とは、鋼を浸炭性雰囲気中で加熱し、表面の炭素濃度を高める処理をいう。浸炭は、一般に低炭素鋼に適用され、浸炭後に焼入れされて使用される。このような浸炭−焼入れ処理された材料は、肌焼鋼あるいは浸炭鋼と呼ばれており、表面が硬く、内部は柔らかいので、軸、軸受け、歯車、ピストンピン、カムなどの機械部品に賞用されている。
浸炭の中でも、材料表面近傍の炭素濃度を高めて炭化物を析出させる処理は、特に「高濃度浸炭」と呼ばれている。高濃度浸炭により得られる材料は、組織中に硬い炭化物が分散しているので、従来より行われている共析浸炭により得られる材料よりも耐摩耗性、面疲労強度が高いという特徴がある。
しかしながら、高濃度浸炭処理材の特性は、炭化物の分散形態の影響を強く受けるので、高い強度を得るためには、炭化物を微細・球状・大量に分散させる必要がある（非特許文献１参照）。特に、粒界に析出した粗大な炭化物は、強度低下の原因となる。 Carburizing refers to a process in which steel is heated in a carburizing atmosphere to increase the carbon concentration on the surface. Carburization is generally applied to low carbon steel and is used after being carburized. Such carburized-quenched material is called case-hardened steel or carburized steel, its surface is hard and the inside is soft, so it is award for mechanical parts such as shafts, bearings, gears, piston pins, cams, etc. Has been.
Among the carburizing processes, the process of increasing the carbon concentration in the vicinity of the material surface to precipitate carbides is called “high-concentration carburizing”. A material obtained by high-concentration carburizing is characterized by higher wear resistance and surface fatigue strength than materials obtained by conventional eutectoid carburizing because hard carbides are dispersed in the structure.
However, since the characteristics of the high-concentration carburized material are strongly influenced by the dispersion form of carbides, it is necessary to disperse the carbides finely, spherically, and in large quantities in order to obtain high strength (see Non-Patent Document 1). . In particular, coarse carbides precipitated at the grain boundaries cause a decrease in strength.

そこでこの問題を解決するために、従来から種々の提案がなされている。
例えば、特許文献１には、
Ｃ：０．０５〜０．４５％を含有する鋼で製造した機械構造部品を、
（イ） ８８０℃以上の温度でプラズマ浸炭することにより、部品表面のＣ濃度を鋼のＡcm以上にして表面近傍に炭化物を析出させる一次浸炭を行い、
（ロ） 徐冷して鋼のＡr1より低い温度に降温し、いったんそこで保持したのち、Ａr1を超える温度に昇温し、
（ハ） 一次浸炭の温度より１０〜６０℃低い温度において、再びプラズマ浸炭による二次浸炭を行い、
（ニ） 直ちに、または拡散処理を施したのち、焼入れ焼戻しをし、
表面Ｃ濃度が１．５％以上であり、浸炭層の炭化物形状がほぼ球形であって、耐摩耗性と耐ピッチング性に優れた部品を得ることからなる鋼の浸炭処理方法
が開示されている。
また、同文献には、
上記工程（ハ）に続いて、
（ホ） 再び徐冷して鋼のＡr1より低い温度に降温し、いったんそこに保持したのち、Ａr1を超える温度に昇温し、
（ヘ） 二次浸炭の温度よりさらに１０〜６０℃低い温度において、再びプラズマ浸炭による三次浸炭を行い、
その後に工程（ニ）を行うことにより、
表面Ｃ濃度１．７％以上で、浸炭層の炭化物形状がほぼ球形であって、耐摩耗性と耐ピッチング性に優れた部品を得ることからなる鋼の浸炭処理方法
が開示されている。 In order to solve this problem, various proposals have heretofore been made.
For example, Patent Document 1 discloses that
C: Machine structural parts manufactured with steel containing 0.05 to 0.45%,
(A) Perform primary carburization by depositing carbide near the surface by making the C concentration on the part surface more than Acm of steel by plasma carburizing at a temperature of 880 ° C. or higher,
(B) Slowly cool, lower the temperature to lower than Ar1 of the steel, hold it there, then raise the temperature to higher than Ar1,
(C) Secondary carburization by plasma carburization is performed again at a temperature lower by 10 to 60 ° C. than the temperature of primary carburization,
(D) Immediately or after diffusion treatment, quenching and tempering,
A carburizing method for steel is disclosed, which has a surface C concentration of 1.5% or more, and the carburized layer has a substantially spherical carbide shape and has a component with excellent wear resistance and pitting resistance. .
In the same document,
Following the above step (c),
(E) Slowly cool again, lower the temperature to lower than Ar1 of the steel, hold it there, then raise the temperature to higher than Ar1,
(F) At a temperature lower by 10-60 ° C. than the temperature of the secondary carburization, again perform the third carburization by plasma carburization,
By performing step (d) after that,
A steel carburizing method is disclosed which comprises obtaining a part having a surface C concentration of 1.7% or more and a carburized layer having a substantially spherical carbide shape and excellent wear resistance and pitting resistance.

同文献には、
（１） 浸炭処理法としてプラズマ浸炭を用いると、高いカーボンポテンシャルにもかかわらず、すすの発生が軽微であるので、浸炭ムラの心配がない点、
（２） 一次浸炭においてＡcmを超える高度の浸炭を行っているので、炭化物をオーステナイト粒界に塊状に析出させることができる点、
（３） 浸炭後、いったん温度をＡr1より低い温度に降温し、再度Ａr1を超える温度に昇温すると、オーステナイト粒界が移動し、はじめに粒界に存在していた炭化物は、新オーステナイト粒内に残存することになる点、
（４） さらに二次浸炭を行うことにより、新オーステナイト粒界に炭化物が析出し、この新しく生成した炭化物と、上記の残存炭化物をあわせて、炭化物分布が好ましい浸炭層が得られる点、
が記載されている。 In the same document,
(1) When plasma carburizing is used as the carburizing method, soot generation is slight despite the high carbon potential, so there is no concern about uneven carburization.
(2) Since carburizing at a high level exceeding Acm is performed in the primary carburizing, the carbide can be precipitated in a mass at the austenite grain boundary,
(3) After carburizing, once the temperature is lowered to a temperature lower than Ar1, and again raised to a temperature exceeding Ar1, the austenite grain boundaries move, and the carbides that were initially present at the grain boundaries are moved into the new austenite grains. The point that will remain,
(4) By further performing secondary carburization, carbide is precipitated at the new austenite grain boundary, and the newly generated carbide and the above remaining carbide are combined to obtain a carburized layer with preferable carbide distribution,
Is described.

さらに、特許文献２には、
浸炭処理により表面炭素濃度を０．８％以上とした鋼製部品をこの浸炭処理後３００℃以下の温度まで０．１℃／ｓｅｃ以上の冷却速度で冷却し、次いで、鋼のＡc1変態温度よりも５０℃高くかつ１５０℃低い温度域に鋼製部品を加熱した後同温度で保持し、更に、１０℃／ｓｅｃ以下の加熱温度で心部がオーステナイト単相ないしはオーステナイトとフェライトの２相であってフェライト面積率が３０％以下となる温度まで昇温して保持した後、直接焼入れを行うかもしくは所定の焼入れ温度まで温度を下げたあと焼入れを行う鋼製部品の浸炭熱処理方法が開示されている。
同文献には、浸炭処理した鋼製部品を鋼のＡc1変態温度より５０℃高くかつ１５０℃低い温度域に保持することによって、微細炭化物を成長させることができる点が記載されている。 Furthermore, Patent Document 2 includes
A steel part having a surface carbon concentration of 0.8% or higher by carburizing treatment is cooled at a cooling rate of 0.1 ° C./sec or higher to a temperature of 300 ° C. or lower after the carburizing treatment, and then from the Ac1 transformation temperature of the steel. The steel part was heated to a temperature range of 50 ° C. and 150 ° C. and kept at the same temperature, and at the heating temperature of 10 ° C./sec or less, the core portion was austenite single phase or two phases of austenite and ferrite. A method of carburizing heat treatment of steel parts is disclosed in which the steel is heated to a temperature at which the ferrite area ratio is 30% or less and held, and then directly quenched or quenched after being lowered to a predetermined quenching temperature. Yes.
This document describes that fine carbides can be grown by maintaining a carburized steel part in a temperature range that is 50 ° C. and 150 ° C. lower than the Ac1 transformation temperature of the steel.

下村哲也、森田敏之、井上幸一郎、電気製鋼、77(2006)、11Tetsuya Shimomura, Toshiyuki Morita, Koichiro Inoue, Electric Steelmaking, 77 (2006), 11 特許第２８０８６２１号公報Japanese Patent No. 2808621 特開平６−１０８２２６号公報JP-A-6-108226

特許文献２に開示されているように、１回の浸炭処理のみによって、微細かつ球状の炭化物を大量に分散させるのは難しい。そのため、従来行われている高濃度浸炭は、１次浸炭及び２次浸炭の２回の浸炭を行うものが多い。１次浸炭は、主として、表面に高濃度の炭素を固溶させ、２次浸炭を行うための再加熱時に微細な炭化物を多量に析出させることを目的として行われる。一方、２次浸炭は、主として、１次浸炭後の再加熱時に生成した微細な炭化物を成長させることを目的として行われる。そのためには、１次浸炭と２次浸炭の温度差が十分あることが望ましい。   As disclosed in Patent Document 2, it is difficult to disperse a fine and spherical carbide in a large amount only by a single carburizing process. For this reason, many high-concentration carburizations that have been performed in the past involve many carburizations of primary carburization and secondary carburization. The primary carburization is mainly performed for the purpose of precipitating a large amount of fine carbides at the time of reheating for carrying out the secondary carburization by dissolving a high concentration of carbon on the surface. On the other hand, the secondary carburization is performed mainly for the purpose of growing fine carbides generated during reheating after the primary carburization. For that purpose, it is desirable that there is a sufficient temperature difference between the primary carburization and the secondary carburization.

しかしながら、１次浸炭と２次浸炭の温度差を大きくするために、１次浸炭の温度を上昇させると、炉の寿命が低下する。また、肌焼鋼は、通常、仕上げ加工することなくそのまま使用されるが、必要以上の高温加熱は、材料の変形を増大させる原因となる。
一方、この問題を解決するために、１次浸炭の温度を下げると同時に２次浸炭の温度を下げると、２次浸炭時の炭素の拡散速度が低下する。そのため、必要量の炭化物を析出させるのに長時間を要し、作業効率が低下する。
さらに、１次浸炭の温度のみを下げ、２次浸炭の温度を高く維持すると、１次浸炭と２次浸炭の温度差が小さくなる。そのため、粒界に片状・粗大な炭化物が析出しやすくなり、組織の再現性が低下する。 However, if the temperature of the primary carburization is increased in order to increase the temperature difference between the primary carburization and the secondary carburization, the life of the furnace is reduced. Further, the case-hardened steel is usually used as it is without finishing, but heating at a higher temperature than necessary causes an increase in deformation of the material.
On the other hand, in order to solve this problem, if the temperature of the secondary carburization is lowered at the same time as the temperature of the primary carburization is lowered, the carbon diffusion rate during the secondary carburization is lowered. For this reason, it takes a long time to precipitate the necessary amount of carbide, and the working efficiency is lowered.
Furthermore, if only the temperature of the primary carburization is lowered and the temperature of the secondary carburization is kept high, the temperature difference between the primary carburization and the secondary carburization becomes small. Therefore, flakes and coarse carbides are likely to precipitate at the grain boundaries, and the reproducibility of the structure is reduced.

本発明が解決しようとする課題は、炉の寿命を低下させることなく、微細かつ球状の炭化物を大量に分散させることが可能な高濃度浸炭鋼の製造方法を提供することにある。
また、本発明が解決しようとする他の課題は、浸炭処理後に大きな変形を生じさせることのない高濃度浸炭鋼の製造方法を提供することにある。
また、本発明が解決しようとする他の課題は、作業効率を低下させることなく、微細かつ球状の炭化物を大量に分散させることが可能な高濃度浸炭鋼の製造方法を提供することにある。
さらに、本発明が解決しようとする他の課題は、粒界に片状・粗大な炭化物が析出せず、組織の再現性が高い高濃度浸炭鋼の製造方法を提供することにある。 The problem to be solved by the present invention is to provide a method for producing high-concentration carburized steel that can disperse a large amount of fine and spherical carbides without reducing the life of the furnace.
Another problem to be solved by the present invention is to provide a method for producing high-concentration carburized steel that does not cause significant deformation after carburizing treatment.
Another problem to be solved by the present invention is to provide a method for producing high-concentration carburized steel that can disperse a large amount of fine and spherical carbides without reducing the working efficiency.
Furthermore, another problem to be solved by the present invention is to provide a method for producing a high-concentration carburized steel in which flakes and coarse carbides do not precipitate at grain boundaries and the structure has high reproducibility.

上記課題を解決するために本発明に係る高濃度浸炭鋼の製造方法は、以下の工程を備えていることを要旨とする。
（イ） Ｃ：０．１５〜０．３０ｍａｓｓ％、Ｓｉ：０．４０〜０．８０ｍａｓｓ％、Ｍｎ：０．３〜０．８ｍａｓｓ％、Ｃｒ：１．２５〜２．００ｍａｓｓ％、残部がＦｅ及び不可避的不純物からなる鋼材を、１１００℃以下である１次浸炭温度Ｔ1（℃）において、その表面炭素濃度ＣがＣeu＜Ｃ≦Ｃ(Ａcm)となるまで浸炭させる１次浸炭工程。
但し、
Ｃeuは、前記鋼材の共析炭素濃度、
Ｃ(Ａcm)は、前記１次浸炭温度Ｔ1における前記鋼材のＡcm線に相当する炭素濃度。
（ロ） 前記１次浸炭工程終了後、前記鋼材を冷却速度１℃／分以上で７００℃以下まで冷却する冷却工程。
（ハ） 前記鋼材を２次浸炭開始温度Ｔ2sまで昇温させ、２次浸炭温度Ｔ2を前記２次浸炭開始温度Ｔ2sに維持したまま前記鋼材を浸炭させる２次浸炭初期工程。
但し、
Ａc1点（℃）≦Ｔ2s（℃）≦１次浸炭温度Ｔ1−１００℃≦２次浸炭開始直後における前記鋼材の表面炭素濃度に相当するＡcm線温度（℃）、
Ｔ2S≦Ｔ2≦前記鋼材の表面炭素濃度に相当するＡcm線温度（℃）。
（ニ） 前記２次浸炭初期工程終了後、引き続き前記鋼材を焼入れ温度Ｔq（℃）まで昇温させ、前記焼入れ温度Ｔqにおいてさらに浸炭させる２次浸炭後期工程。
但し、
Ｔq≦前記鋼材の表面炭素濃度に相当するＡcm線温度（℃）。
（ホ） 前記２次浸炭後期工程終了後、前記鋼材を焼入れする焼入れ工程。 In order to solve the above-described problems, the high-concentration carburized steel manufacturing method according to the present invention includes the following steps.
(A) C: 0.15 to 0.30 mass%, Si: 0.40 to 0.80 mass%, Mn: 0.3 to 0.8 mass%, Cr: 1.25 to 2.00 mass%, the balance being Fe And a primary carburizing step of carburizing a steel material composed of inevitable impurities at a primary carburizing temperature T1 (° C.) of 1100 ° C. or less until the surface carbon concentration C becomes Ceu <C ≦ C (Acm).
However,
Ceu is the eutectoid carbon concentration of the steel material,
C (Acm) is a carbon concentration corresponding to the Acm line of the steel material at the primary carburizing temperature T1.
(B) A cooling step of cooling the steel material to a temperature of 700 ° C. or less at a cooling rate of 1 ° C./min after the completion of the primary carburizing step.
(C) the steel material is warm to the secondary carburization start temperature T2s, the secondary carburization temperature T 2 a secondary carburization initial step of carburizing a pre Symbol steel while maintaining the secondary carburization start temperature T2s.
However,
Ac1 point (° C.) ≦ T 2 s (° C.) ≦ primary carburizing temperature T 1 −100 ° C. ≦ Acm line temperature (° C.) corresponding to the surface carbon concentration of the steel immediately after the start of secondary carburizing,
T2S ≦ T2 ≦ Acm line temperature (° C.) corresponding to the surface carbon concentration of the steel material.
(D) A secondary carburizing late process in which the steel material is continuously heated to the quenching temperature Tq (° C.) after the secondary carburizing initial process is completed, and further carburized at the quenching temperature Tq.
However,
Tq ≦ Acm line temperature (° C.) corresponding to the surface carbon concentration of the steel material.
(E) A quenching step of quenching the steel after the end of the secondary carburization late step.

１次浸炭温度Ｔ1で１次浸炭を行った後、さらに２次浸炭を行う場合において、２次浸炭を焼入れ温度Ｔqより低い２次浸炭温度Ｔ2で浸炭を行う２次浸炭初期工程と、焼入れ温度Ｔqにおいて浸炭を行う２次浸炭後期工程に分割すると、１次浸炭温度Ｔ1が相対的に低温であっても、１次浸炭温度Ｔ1と２次浸炭温度Ｔ2の温度差を十分に取ることができる。そのため、炉の寿命低下や浸炭処理後の大きな変形を生じさせることなく、微細かつ球状の炭化物を大量に分散させることができる。また、２次浸炭後期工程においては、相対的に高温で浸炭が行われるので、作業効率を低下させることもない。さらに、１次浸炭温度Ｔ1と２次浸炭温度Ｔ2の温度差を十分に取ることができるので、材料間に組成のバラツキがあっても、粒界への片状・粗大な炭化物の析出を確実に抑制することができる。   After performing primary carburization at the primary carburizing temperature T1, and further performing secondary carburizing, the secondary carburizing initial process in which the secondary carburizing is performed at the secondary carburizing temperature T2 lower than the quenching temperature Tq, and the quenching temperature. When divided into the secondary carburization late stage in which carburizing is performed at Tq, even if the primary carburizing temperature T1 is relatively low, the temperature difference between the primary carburizing temperature T1 and the secondary carburizing temperature T2 can be sufficiently taken. . Therefore, it is possible to disperse a large amount of fine and spherical carbides without causing a decrease in the life of the furnace or a large deformation after the carburizing process. Further, since the carburizing is performed at a relatively high temperature in the second stage of the second carburizing process, the work efficiency is not lowered. In addition, since the temperature difference between the primary carburizing temperature T1 and the secondary carburizing temperature T2 can be taken sufficiently, even if there is a variation in composition between materials, precipitation of coarse and coarse carbides at grain boundaries is ensured. Can be suppressed.

以下に、本発明の一実施の形態について詳細に説明する。
初めに、本発明に係る高濃度浸炭鋼の製造方法が適用される鋼材について説明する。
本発明に係る方法が適用される鋼材は、以下のような合金元素を含み、残部がＦｅ及び不可避的不純物からなる。合金元素の種類、その成分範囲、及び、その限定理由は、以下の通りである。 Hereinafter, an embodiment of the present invention will be described in detail.
First, steel materials to which the method for producing high-concentration carburized steel according to the present invention is applied will be described.
The steel material to which the method according to the present invention is applied includes the following alloy elements, with the balance being Fe and inevitable impurities. The types of alloy elements, their component ranges, and the reasons for their limitations are as follows.

（１） Ｃ：０．１５〜０．３０ｍａｓｓ％。
Ｃ量が少ないと、心部にフェライトが生成し、強度を低下させる。従って、Ｃ量は、０．１５ｍａｓｓ％以上が好ましい。
一方、Ｃ量が過剰になると、素材の硬さを上げ、製造性（特に、被削性）を低下させる。従って、Ｃ量は、０．３０ｍａｓｓ％以下が好ましい。 (1) C: 0.15 to 0.30 mass%.
When the amount of C is small, ferrite is generated in the core and the strength is lowered. Therefore, the amount of C is preferably 0.15 mass% or more.
On the other hand, when the amount of C is excessive, the hardness of the material is increased and the manufacturability (particularly machinability) is decreased. Therefore, the amount of C is preferably 0.30 mass% or less.

（２） Ｓｉ：０．４０〜０．８０ｍａｓｓ％。
Ｓｉ量が少ないと、マトリックスの焼戻し硬さが低下し、強度を低下させる。従って、Ｓｉ量は、０．４０ｍａｓｓ％以上が好ましい。
一方、Ｓｉ量が過剰になると、炭化物の生成量が低下し、強度を低下させる。また、心部にフェライトが生成し、強度を低下させる。従って、Ｓｉ量は、０．８０ｍａｓｓ％以下が好ましい。 (2) Si: 0.40 to 0.80 mass%.
When the amount of Si is small, the tempering hardness of the matrix is lowered and the strength is lowered. Therefore, the amount of Si is preferably 0.40 mass% or more.
On the other hand, when the amount of Si becomes excessive, the amount of carbide generated decreases and the strength decreases. In addition, ferrite is generated at the core, reducing the strength. Therefore, the amount of Si is preferably 0.80 mass% or less.

（３） Ｍｎ：０．３〜０．８ｍａｓｓ％。
Ｍｎ量が少ないと、マトリックスの焼入れ性が低下し、不完全焼入れにより強度が低下する。従って、Ｍｎ量は、０．３ｍａｓｓ％以上が好ましい。
一方、Ｍｎ量が過剰になると、素材の硬さを上げ、製造性（特に、被削性）を低下させる。従って、Ｍｎ量は、０．８ｍａｓｓ％以下が好ましい。 (3) Mn: 0.3 to 0.8 mass%.
When the amount of Mn is small, the hardenability of the matrix is lowered, and the strength is lowered due to incomplete quenching. Therefore, the amount of Mn is preferably 0.3 mass% or more.
On the other hand, when the amount of Mn becomes excessive, the hardness of the material is increased and manufacturability (particularly machinability) is lowered. Therefore, the amount of Mn is preferably 0.8 mass% or less.

（４） Ｃｒ：１．２５〜２．００ｍａｓｓ％。
Ｃｒ量が少ないと、炭化物の生成量が低下し、強度を低下させる。また、心部にフェライトが生成し、強度を低下させる。従って、Ｃｒ量は、１．２５ｍａｓｓ％以上が好ましい。
一方、Ｃｒ量が過剰になると、素材の硬さを上げ、製造性（特に、被削性）を低下させる。従って、Ｃｒ量は、２．００ｍａｓｓ％以下が好ましい。 (4) Cr: 1.25 to 2.00 mass%.
If the amount of Cr is small, the amount of carbide produced is reduced and the strength is reduced. In addition, ferrite is generated at the core, reducing the strength. Therefore, the Cr amount is preferably 1.25 mass% or more.
On the other hand, when the amount of Cr is excessive, the hardness of the material is increased and the manufacturability (particularly machinability) is decreased. Therefore, the Cr amount is preferably 2.00 mass% or less.

次に、本発明に係る高濃度浸炭鋼の製造方法について説明する。
本発明に係る高濃度浸炭鋼の製造方法は、１次浸炭工程と、冷却工程と、２次浸炭初期工程と、２次浸炭後期工程と、焼入れ工程とを備えている。 Next, the manufacturing method of the high concentration carburized steel which concerns on this invention is demonstrated.
The manufacturing method of the high-concentration carburized steel according to the present invention includes a primary carburizing step, a cooling step, a secondary carburizing initial step, a secondary carburizing late step, and a quenching step.

１次浸炭工程は、上述の組成を有する鋼材を、１次浸炭温度Ｔ1（℃）において、その表面炭素濃度ＣがＣeu＜Ｃ≦Ｃ(Ａcm)となるように浸炭させる工程である。
１次浸炭温度Ｔ1は、少なくとも、後述する２次浸炭開始温度Ｔ2sよりも１００℃以上高い温度であればよい。一般に、１次浸炭温度Ｔ1が高くなるほど、短時間で所定の炭素濃度まで浸炭することができる。１次浸炭温度Ｔ１は、具体的には、９００℃以上が好ましい。
一方、１次浸炭温度Ｔ1が高くなりすぎると、炉の寿命を低下させ、あるいは、浸炭中における鋼材の変形が増大する場合がある。従って、１次浸炭温度Ｔ1は、具体的には、１１００℃以下が好ましく、さらに好ましくは、１０００℃以下である。 The primary carburizing step is a step of carburizing a steel material having the above composition so that the surface carbon concentration C is Ceu <C ≦ C (Acm) at the primary carburizing temperature T1 (° C.).
The primary carburizing temperature T1 may be at least 100 ° C. higher than the secondary carburizing start temperature T2s described later. In general, the higher the primary carburizing temperature T1, the shorter the time required for carburizing to a predetermined carbon concentration. Specifically, the primary carburizing temperature T1 is preferably 900 ° C. or higher.
On the other hand, if the primary carburizing temperature T1 becomes too high, the life of the furnace may be reduced, or the deformation of the steel material during carburizing may increase. Therefore, the primary carburizing temperature T1 is specifically preferably 1100 ° C. or lower, more preferably 1000 ° C. or lower.

また、浸炭は、鋼材の表面炭素濃度ＣがＣeu＜Ｃ≦Ｃ(Ａcm)となるように行う。
ここで、「表面炭素濃度」とは、表面から１０μｍの領域内の平均炭素濃度をいう。
また、「Ｃeu」とは、上述した範囲のＳｉ、Ｍｎ、及びＣｒを含む鋼材の共析炭素濃度をいう。上述した鋼材の場合、いずれも、共析炭素濃度は、０．５ｍａｓｓ％以上となる。
さらに、「Ｃ(Ａcm)」とは、１次浸炭温度Ｔ1における、上述した範囲のＳｉ、Ｍｎ及びＣｒを含む鋼材のＡcm線に相当する炭素濃度をいう。Ｃ≦Ｃ(Ａcm)となるまで浸炭を行うことは、鋼材の表面温度がＡcm線以上となる温度（すなわち、表面がγ相単相となる温度）で１次浸炭を行うことを意味する。 Carburization is performed such that the surface carbon concentration C of the steel material satisfies Ceu <C ≦ C (Acm).
Here, “surface carbon concentration” refers to an average carbon concentration within a region of 10 μm from the surface.
“Ceu” refers to the eutectoid carbon concentration of a steel material containing Si, Mn, and Cr in the above-described range. In the case of the steel materials described above, the eutectoid carbon concentration is 0.5 mass% or more.
Furthermore, “C (Acm)” refers to the carbon concentration corresponding to the Acm line of the steel material containing Si, Mn and Cr in the above-described range at the primary carburizing temperature T1. Carburizing until C ≦ C (Acm) means that the primary carburizing is performed at a temperature at which the surface temperature of the steel material is equal to or higher than the Acm line (that is, the temperature at which the surface becomes a single γ phase).

表面炭素濃度Ｃが少ないと、後述する２次浸炭の昇温中に、マトリックス内に炭化物が析出しない。昇温中にマトリックス内に炭化物が析出しないと、２次浸炭時に粒界に粗大な炭化物が生成する。従って、１次浸炭は、表面炭素濃度ＣがＣeuより大きくなるように行う必要がある。
一方、表面の炭素濃度Ｃが過剰になると、１次浸炭中に粒界に炭化物が生成する。１次浸炭で生成した炭化物は、そのまま残存するため、粗大な炭化物の生成は防止しなければならない。具体的には、長径５μｍ以上の粗大な炭化物が存在しないことが好ましい。従って、１次浸炭は、表面炭素濃度ＣがＣ(Ａcm)以下となるように行う必要がある。
例えば、上述した組成を有する鋼材の場合、１次浸炭温度Ｔ1を９５０〜１０００℃とすると、Ｃ(Ａcm)は、１．２５〜１．４ｍａｓｓ％程度となる。 When the surface carbon concentration C is low, carbides do not precipitate in the matrix during the temperature increase of the secondary carburization described later. If carbides do not precipitate in the matrix during temperature rise, coarse carbides are generated at the grain boundaries during secondary carburization. Therefore, the primary carburization needs to be performed so that the surface carbon concentration C is higher than Ceu.
On the other hand, if the surface carbon concentration C is excessive, carbides are generated at the grain boundaries during the primary carburization. Since the carbide generated by the primary carburization remains as it is, generation of coarse carbides must be prevented. Specifically, it is preferable that there is no coarse carbide having a major axis of 5 μm or more. Accordingly, the primary carburization needs to be performed so that the surface carbon concentration C is C (Acm) or less.
For example, in the case of a steel material having the above-described composition, when the primary carburizing temperature T1 is 950 to 1000 ° C., C (Acm) is about 1.25 to 1.4 mass%.

１次浸炭を行う際の浸炭方法は、特に限定されるものではなく、種々の方法を用いることができる。特に、ガス浸炭及び真空浸炭は、取り扱いが容易で、処理時間も短いので、浸炭方法として好適である。特定の浸炭方法を採用した場合において、浸炭条件を最適化すると、表面の炭素濃度Ｃを上述の範囲に収めることができる。
例えば、ガス浸炭は、浸炭性ガス雰囲気中で鋼材を加熱することにより浸炭を行う。この場合、浸炭量は、浸炭雰囲気のカーボンポテンシャルにより制御することができる。カーボンポテンシャルとは、雰囲気と平衡する純鉄の表面平衡炭素濃度であり、雰囲気中のＣＯ／ＣＯ2比やＨ2Ｏ量に依存する。一般に、カーボンポテンシャルが高くなるほど、及び／又は、１次浸炭温度Ｔ1が高くなるほど、短時間で表面の炭素濃度を高めることができる。
また、例えば、真空浸炭は、鋼材を挿入した炉内を１．３Ｐａ程度に減圧した後、浸炭温度に加熱し、メタン、プロパンなどの炭化水素ガスを炉内に導入することにより浸炭を行う。この場合、浸炭量は、炭化水素ガスの導入時間により制御することができる。なお、真空浸炭を行うと、表面近傍の炭素濃度が高くなりすぎる場合があるので、このような場合には、浸炭後に炭化水素ガスの供給を止め、その状態で保持する拡散処理を行うのが一般的である。 The carburizing method when performing the primary carburizing is not particularly limited, and various methods can be used. In particular, gas carburizing and vacuum carburizing are suitable as carburizing methods because they are easy to handle and the processing time is short. When a specific carburizing method is employed, the carbon concentration C on the surface can be kept within the above range by optimizing the carburizing conditions.
For example, gas carburizing is performed by heating a steel material in a carburizing gas atmosphere. In this case, the carburizing amount can be controlled by the carbon potential of the carburizing atmosphere. The carbon potential is the surface equilibrium carbon concentration of pure iron that is in equilibrium with the atmosphere, and depends on the CO / CO 2 ratio and the amount of H 2 O in the atmosphere. In general, the higher the carbon potential and / or the higher the primary carburizing temperature T1, the higher the surface carbon concentration.
For example, vacuum carburization is performed by depressurizing the inside of the furnace into which the steel material is inserted to about 1.3 Pa, then heating to a carburizing temperature, and introducing hydrocarbon gas such as methane or propane into the furnace. In this case, the carburization amount can be controlled by the introduction time of the hydrocarbon gas. In addition, since the carbon concentration near the surface may become too high when vacuum carburizing is performed, in such a case, the supply of hydrocarbon gas is stopped after carburizing, and diffusion treatment is performed to hold in that state. It is common.

冷却工程は、１次浸炭工程終了後、鋼材を冷却速度１℃／分以上で７００℃以下まで冷却する工程である。
１次浸炭終了後、鋼材は、一端、７００℃以下の温度まで冷却される。７００℃以下の温度まで冷却するのは、２次浸炭の再加熱の際に、粒内に微細な炭化物を析出させるためである。この場合、冷却速度が遅すぎると、冷却中に片状・粗大な炭化物が粒界に析出するので好ましくない。冷却時に生じた粗大な炭化物は、後述の工程においても消滅せず、鋼材の強度を低下させる原因となる。従って、冷却速度は、１℃／分以上が好ましい。冷却速度は、速いほどよい。 The cooling step is a step of cooling the steel material to a temperature of 700 ° C. or lower at a cooling rate of 1 ° C./min or higher after the completion of the primary carburizing step.
After the completion of the primary carburization, the steel material is cooled to a temperature of 700 ° C. or lower at one end. The reason for cooling to a temperature of 700 ° C. or lower is to precipitate fine carbides in the grains during the reheating of the secondary carburization. In this case, if the cooling rate is too slow, it is not preferable because flakes and coarse carbides precipitate at the grain boundaries during cooling. Coarse carbides generated at the time of cooling do not disappear even in a process described later, and cause a reduction in the strength of the steel material. Therefore, the cooling rate is preferably 1 ° C./min or more. The faster the cooling rate, the better.

２次浸炭初期工程は、冷却された鋼材を２次浸炭開始温度Ｔ2sまで昇温させ、２次浸炭温度Ｔ2において鋼材を浸炭させる工程である。
ここで、「２次浸炭開始温度Ｔ2s」とは、次の（１）式の条件を満たす温度をいう。
Ａc1点（℃）≦Ｔ2s（℃）≦１次浸炭温度Ｔ1−１００℃≦２次浸炭開始直後における前記鋼材の表面炭素濃度に相当するＡcm線温度（℃） ・・・（１）
２次浸炭開始温度Ｔ2sと１次浸炭温度Ｔ1との温度差は、１００℃以上が好ましい。両者の温度差が１００℃未満になると、粒界に片状・粗大な炭化物が生成するおそれがある。両者の温度差は、大きいほどよい。
また、２次浸炭開始温度Ｔ2sは、Ａc1点以上であり、かつ、２次浸炭開始直後における鋼材の表面炭素濃度に相当するＡcm線温度以下である必要がある。これは、鋼材の表面温度がＡc1点とＡcm線の間の温度（すなわち、表面がγ＋Ｆｅ3Ｃ相となる温度）で２次浸炭を開始することを意味する。 The secondary carburizing initial step is a step of raising the temperature of the cooled steel material to the secondary carburizing start temperature T2s and carburizing the steel material at the secondary carburizing temperature T2.
Here, the “secondary carburization start temperature T2s” refers to a temperature that satisfies the following equation (1).
Ac1 point (° C) ≤ T2s (° C) ≤ primary carburizing temperature T1-100 ° C ≤ Acm line temperature (° C) corresponding to the surface carbon concentration of the steel immediately after the start of secondary carburizing (1)
The temperature difference between the secondary carburizing start temperature T2s and the primary carburizing temperature T1 is preferably 100 ° C. or higher. If the temperature difference between the two is less than 100 ° C., flaky and coarse carbides may be generated at the grain boundaries. The larger the temperature difference between the two, the better.
Further, the secondary carburization start temperature T2s needs to be not less than the Ac1 point and not more than the Acm line temperature corresponding to the surface carbon concentration of the steel material immediately after the start of the secondary carburization. This means that the secondary carburization starts at a temperature between the Ac1 point and the Acm line (that is, the temperature at which the surface becomes the γ + Fe 3 C phase).

「２次浸炭温度Ｔ2」とは、次の（２）式の条件を満たす温度をいう。
Ｔ2S≦Ｔ2≦前記鋼材の表面の炭素濃度に相当するＡcm線温度（℃）・・・（２）
２次浸炭温度Ｔ2は、２次浸炭開始温度Ｔ2sと同一であっても良く、あるいは、それより高い温度であってもよい。
２次浸炭温度Ｔ2が２次浸炭開始温度Ｔ2sと同一である場合、２次浸炭温度Ｔ2での保持時間は、後述する焼入れ温度Ｔqまで昇温したときに、鋼材の表面温度がＡcm線温度を超えない時間であればよい。一般に、２次浸炭温度Ｔ2での保持時間が長くなるほど、表面の炭素濃度が上昇し、これに応じて表面のＡcm線温度も上昇するので、鋼材の表面温度をＡcm線以下に保ったまま、浸炭を行うことができる。後述する焼入れ温度Ｔqに昇温したときに鋼材の表面温度を確実にＡcm線以下とするためには、２次浸炭温度Ｔ2での保持時間は、１５分以上が好ましい。 The “secondary carburizing temperature T2” refers to a temperature that satisfies the following equation (2).
T2S ≤ T2 ≤ Acm line temperature (° C) corresponding to the carbon concentration on the surface of the steel (2)
The secondary carburizing temperature T2 may be the same as the secondary carburizing start temperature T2s or may be a higher temperature.
When the secondary carburizing temperature T2 is the same as the secondary carburizing start temperature T2s, the holding time at the secondary carburizing temperature T2 is such that when the temperature is raised to the quenching temperature Tq described later, the surface temperature of the steel material becomes the Acm line temperature. It is sufficient if the time does not exceed. In general, the longer the holding time at the secondary carburizing temperature T2, the higher the carbon concentration on the surface, and the Acm line temperature on the surface also increases accordingly. Therefore, the surface temperature of the steel material is kept below the Acm line, Carburization can be performed. In order to ensure that the surface temperature of the steel material is below the Acm line when the temperature is raised to the quenching temperature Tq described later, the holding time at the secondary carburizing temperature T2 is preferably 15 minutes or more.

一方、２次浸炭温度Ｔ2が２次浸炭開始温度Ｔ2sより高い場合、２次浸炭温度Ｔ2は、２次浸炭開始温度Ｔ2sから階段状に温度を上昇させても良く、あるいは、連続的に上昇させてもよい。
「階段状」とは、一定温度で所定時間保持した後、温度を所定の温度幅で上昇させ、さらにその温度で所定時間保持することを繰り返すことをいう。階段状に温度を上昇させる場合であっても、温度の上昇幅及び保持温度での保持時間を最適化すると、鋼材の表面温度をＡcm線以下に保ったまま、浸炭を行うことができる。
また、「連続的」とは、所定の昇温速度で温度を上昇させることをいう。連続的に温度を上昇させる場合であっても、昇温速度を最適化することによって、鋼材の表面温度をＡcm線以下に保ったまま、浸炭を行うことができる。 On the other hand, when the secondary carburizing temperature T2 is higher than the secondary carburizing start temperature T2s, the secondary carburizing temperature T2 may be raised stepwise from the secondary carburizing start temperature T2s or may be continuously increased. May be.
“Staircase” means that after holding at a constant temperature for a predetermined time, the temperature is increased by a predetermined temperature range and then holding at that temperature for a predetermined time is repeated. Even when the temperature is raised stepwise, carburization can be performed while maintaining the surface temperature of the steel material below the Acm line by optimizing the temperature increase range and the holding time at the holding temperature.
“Continuous” refers to increasing the temperature at a predetermined rate of temperature increase. Even when the temperature is continuously increased, carburization can be performed while the surface temperature of the steel material is kept below the Acm line by optimizing the heating rate.

２次浸炭後期工程は、２次浸炭初期工程終了後、引き続き鋼材を焼入れ温度Ｔq（℃）まで昇温させ、焼入れ温度Ｔqにおいてさらに浸炭させる工程である。
但し、Ｔq≦前記鋼材の表面炭素濃度に相当するＡcm線温度（℃）。
２次浸炭後期工程は、単に鋼材の温度を焼入れ温度Ｔqまで上昇させるためだけではなく、より高温において浸炭を行わせ、粒界に片状・粗大な炭化物を析出させることなく、短時間で表面の炭素濃度を目的とする炭素濃度とするための工程でもある。従って、焼入れ温度Ｔqは、鋼材の表面炭素濃度に相当するＡcm線温度以下である必要がある。２次浸炭初期工程の浸炭条件を最適化すると、鋼材の表面温度をＡcm線温度以下に保ったまま、焼入れ温度Ｔqまで昇温することができる。
一般に、焼入れ温度Ｔqが低くなるほど、保持中に、粒界に片状・粗大な炭化物が生成しにくくなる。しかしながら、焼入れ温度Ｔqが低くなりすぎると、炭素の拡散速度が低下するだけでなく、心部の焼入れが不十分となる。従って、焼入れ温度Ｔqは、鋼材の心部がオーステナイト単相となる温度以上とするのが好ましい。 The secondary carburization late process is a process in which the steel material is continuously heated to the quenching temperature Tq (° C.) and further carburized at the quenching temperature Tq after completion of the secondary carburizing initial process.
However, Tq ≦ Acm line temperature (° C.) corresponding to the surface carbon concentration of the steel material.
The second stage of secondary carburizing is not only to raise the temperature of the steel material to the quenching temperature Tq, but also to perform carburizing at a higher temperature and to precipitate the flakes and coarse carbides at the grain boundaries in a short time. It is also a process for making the carbon concentration of the target carbon concentration. Accordingly, the quenching temperature Tq needs to be equal to or lower than the Acm line temperature corresponding to the surface carbon concentration of the steel material. When the carburizing conditions in the initial stage of secondary carburizing are optimized, the temperature can be raised to the quenching temperature Tq while keeping the surface temperature of the steel material below the Acm line temperature.
In general, the lower the quenching temperature Tq, the more difficult it is to form flakes and coarse carbides at the grain boundaries during holding. However, if the quenching temperature Tq is too low, not only the carbon diffusion rate is lowered, but the quenching of the core becomes insufficient. Accordingly, it is preferable that the quenching temperature Tq is equal to or higher than the temperature at which the core of the steel material becomes an austenite single phase.

焼入れ温度Ｔqでの保持時間は、特に限定されるものではなく、鋼材の組成、焼入れ温度Ｔq、鋼材に要求される特性等に応じて、最適な時間を選択する。一般に、保持時間が長くなるほど、鋼材の表面炭素濃度を上昇させることができる。耐摩耗性、面疲労強度に優れた高濃度浸炭鋼を得るためには、焼入れ温度Ｔqでの保持時間（すなわち、浸炭時間）は、１５分以上が好ましい。
なお、２次浸炭初期工程において、段階的又は連続的に２次浸炭温度Ｔ2を上昇させる場合において、焼入れ温度Ｔqに達した時点で十分な浸炭量が得られ、かつ、鋼材の均熱も十分であるときには、焼入れ温度Ｔqに到達後、焼入れ温度Ｔqでの浸炭を実質的に行うことなく、直ちに焼入れを行ってもよい。 The holding time at the quenching temperature Tq is not particularly limited, and an optimum time is selected according to the composition of the steel material, the quenching temperature Tq, the characteristics required for the steel material, and the like. In general, the longer the holding time, the higher the surface carbon concentration of the steel material. In order to obtain a high-concentration carburized steel having excellent wear resistance and surface fatigue strength, the holding time at the quenching temperature Tq (that is, carburizing time) is preferably 15 minutes or more.
When the secondary carburizing temperature T2 is increased stepwise or continuously in the initial stage of secondary carburizing, a sufficient carburizing amount can be obtained when the quenching temperature Tq is reached, and the steel material has sufficient soaking. In this case, after reaching the quenching temperature Tq, quenching may be performed immediately without substantially performing carburization at the quenching temperature Tq.

焼入れ工程は、２次浸炭後期工程終了後、鋼材を焼入れする工程である。
焼入れは、表面の浸炭層及び心部をマルテンサイト変態させるために行われる。そのためには、２次浸炭後期工程終了後の鋼材は、急冷することが好ましい。焼入れ方法としては、具体的には、油焼入れ、ガス焼入れなどがある。 The quenching process is a process of quenching the steel material after the end of the secondary carburization late process.
Quenching is performed in order to transform the carburized layer and the core of the surface into martensite. For that purpose, it is preferable to rapidly cool the steel material after the end of the second stage carburizing process. Specific examples of the quenching method include oil quenching and gas quenching.

次に、本発明に係る高濃度浸炭鋼の製造方法の作用について説明する。
図１（ａ）〜図１（ｄ）に、各種条件で高濃度浸炭した場合における組織変化の模式図を示す。なお、図１（ａ）〜図１（ｄ）には、状態図も併せて示した。
高濃度浸炭は、１次浸炭と２次浸炭の２回の浸炭を行うものが多い。浸炭を２段階に分けて行う従来の高濃度浸炭は、図１（ａ）の状態図に示すように、１次浸炭終了時の表面の炭素濃度は、浸炭温度に対応するＡcm線濃度より低い濃度になっている。すなわち、１次浸炭終了後の表面は、オーステナイト単相状態にある。そのため、この状態から所定の冷却速度で７００℃以下まで冷却すると、鋼材の組織は、図１（ａ）の左図に示すように、粒界に粗大な炭化物が生成していない状態となる。
この状態から鋼材を２次浸炭温度まで昇温すると、図１（ａ）の中図に示すように、２次浸炭の昇温過程で粒内に球状・微細な炭化物が生成する。これは、２次浸炭は、鋼材の表面炭素濃度に相当するＡcm線温度より低い温度（すなわち、表面がγ＋Ｆｅ3Ｃ相となる温度）で行われ、１次浸炭に比べて炭素の拡散速度が遅くなるので、粒界に炭化物が析出しにくくなるためである。
２次浸炭温度に到達後、その温度で２次浸炭を開始すると、図１（ａ）の右図に示すように、昇温過程で生じた微細な炭化物が核となり、炭化物が成長する。 Next, the effect | action of the manufacturing method of the high concentration carburized steel which concerns on this invention is demonstrated.
Fig. 1 (a) to Fig. 1 (d) are schematic diagrams showing the structure change when high-concentration carburization is performed under various conditions. FIGS. 1A to 1D also show state diagrams.
Many of the high-concentration carburizations perform carburization twice, that is, primary carburization and secondary carburization. As shown in the state diagram of FIG. 1A, the conventional high-concentration carburizing performed in two stages of carburizing is such that the surface carbon concentration at the end of the primary carburizing is lower than the Acm line concentration corresponding to the carburizing temperature. It is concentration. That is, the surface after the end of primary carburization is in an austenite single phase state. Therefore, when cooling from this state to 700 ° C. or less at a predetermined cooling rate, the structure of the steel material is in a state in which coarse carbides are not generated at the grain boundaries as shown in the left diagram of FIG.
When the steel material is heated from this state to the secondary carburizing temperature, spherical and fine carbides are generated in the grains during the temperature raising process of the secondary carburizing as shown in the middle diagram of FIG. This is because the secondary carburization is performed at a temperature lower than the Acm line temperature corresponding to the surface carbon concentration of the steel material (that is, the temperature at which the surface becomes the γ + Fe 3 C phase), and the carbon diffusion rate is higher than that of the primary carburization. This is because it becomes slow, and it becomes difficult for carbide to precipitate at the grain boundaries.
After reaching the secondary carburizing temperature, when the secondary carburizing is started at that temperature, as shown in the right figure of FIG. 1A, fine carbides generated in the temperature raising process become nuclei and the carbides grow.

図１（ａ）に示すような組織を得るためには、１次浸炭終了時の温度はＡcm線より高く、２次浸炭開始時の温度はＡcm線より低くなっている必要がある。製造された鋼材にはロット間の成分バラツキがあり、鋼材ごとにＡcm線の位置が多少変動するので、図１（ａ）に示すような組織を確実に得るためには、１次浸炭温度と２次浸炭温度の温度差を十分に取る必要がある。
しかしながら、温度差を十分に取るために、１次浸炭温度を上昇させると、炉の耐久性が低下する。一方、これを回避するために、２次浸炭温度を下げると、２次浸炭時における炭素の拡散速度が低下するので、生産性が大幅に低下する。 In order to obtain a structure as shown in FIG. 1A, the temperature at the end of the primary carburization needs to be higher than the Acm line, and the temperature at the start of the secondary carburization needs to be lower than the Acm line. The manufactured steel has component variations between lots, and the position of the Acm line varies slightly for each steel. Therefore, in order to reliably obtain a structure as shown in FIG. It is necessary to take a sufficient temperature difference between the secondary carburizing temperatures.
However, if the primary carburizing temperature is raised in order to take a sufficient temperature difference, the durability of the furnace is lowered. On the other hand, if the secondary carburizing temperature is lowered in order to avoid this, the carbon diffusion rate at the time of secondary carburizing is lowered, so that the productivity is greatly lowered.

さらに、炉の耐久性と生産性とを両立させるために、１次浸炭温度と２次浸炭温度の温度差を小さくすると、Ａcm線を挟んだ２段階の浸炭処理を再現性良く行うのが困難となる。
例えば、２次浸炭温度を従来と同等の温度に維持し、１次浸炭温度を下げた場合において、１次浸炭温度がＡcm線より上にあるときには、１次浸炭終了後の鋼材の組織は、図１（ｂ）の左図に示すように、粒界に粗大な炭化物が生成していない状態となる。しかしながら、２次浸炭開始温度がＡcm線を越えると、図１（ｂ）の中図に示すように、２次浸炭温度に保持している間に、２次浸炭の昇温過程で粒内に生成した微細な炭化物が再固溶する。粒内には炭化物を成長させるための核が無くなるので、炭化物は、形成エネルギーのより小さい粒界において優先的に生成する。その結果、図１（ｂ）の右図に示すように、粒界に粗大な炭化物が生成する。
一方、２次浸炭温度を従来と同様の温度に維持し、１次浸炭温度を下げた場合において、１次浸炭温度がＡcm線より下にあるときには、１次浸炭終了後の鋼材の組織は、図１（ｃ）の左図に示すように、粒界に片状の炭化物が生成した状態となる。これを２次浸炭開始温度に昇温すると、図１（ｃ）の中図に示すように、昇温過程で粒内に微細な炭化物が生成する。この状態から２次浸炭を行うと、図１（ｃ）の右図に示すように、粒内の微細な炭化物と粒界に生成した片状の炭化物の双方が成長する。
図１（ｂ）及び図１（ｃ）のいずれの場合においても、粒界に生成した片状・粗大な炭化物は、高濃度浸炭鋼の強度を低下させる原因となる。 Furthermore, if the temperature difference between the primary carburizing temperature and the secondary carburizing temperature is made small in order to achieve both durability and productivity of the furnace, it is difficult to perform the two-stage carburizing process with the Acm line between them with good reproducibility. It becomes.
For example, when the primary carburizing temperature is above the Acm line when the secondary carburizing temperature is maintained at a temperature equivalent to the conventional temperature and the primary carburizing temperature is lowered, the structure of the steel material after the completion of the primary carburizing is: As shown in the left diagram of FIG. 1B, coarse carbides are not generated at the grain boundaries. However, when the secondary carburization start temperature exceeds the Acm line, as shown in the middle diagram of FIG. 1B, while maintaining the secondary carburization temperature, the secondary carburization temperature rises within the grains. The fine carbides that are produced re-dissolve. Since there are no nuclei for growing carbides in the grains, the carbides are preferentially generated at grain boundaries with lower formation energy. As a result, coarse carbides are generated at the grain boundaries as shown in the right diagram of FIG.
On the other hand, when the secondary carburizing temperature is maintained at the same temperature as the conventional one and the primary carburizing temperature is lowered, and the primary carburizing temperature is below the Acm line, the structure of the steel material after the completion of the primary carburizing is: As shown in the left figure of FIG.1 (c), it will be in the state which the piece-like carbide | carbonized_material produced | generated in the grain boundary. When the temperature is raised to the secondary carburization start temperature, fine carbides are generated in the grains during the temperature raising process, as shown in the middle diagram of FIG. When secondary carburization is performed from this state, both fine carbides in the grains and flake carbides generated at the grain boundaries grow as shown in the right figure of FIG.
In both cases of FIG. 1B and FIG. 1C, the flakes and coarse carbides generated at the grain boundaries cause the strength of the high-concentration carburized steel to decrease.

これに対し、１次浸炭温度を従来と同等以下にした状態で１次浸炭を終了させると、図１（ｄ）の左図に示すように、１次浸炭終了後の鋼材の組織は、粒界に粗大な炭化物が生成していない状態となる。また、鋼材を冷却後、２次浸炭開始温度に昇温する場合において、２次浸炭開始温度を１次浸炭温度より１００℃以上低い温度にすると、鋼材の表面温度をＡcm線より低い温度に確実に持っていくことができる。そのため、２次浸炭開始温度に到達した時点では、図１（ｄ）の中図に示すように、粒内に微細な炭化物が生成する。 この状態から、２次浸炭開始温度と同一温度で保持し、又は、２次浸炭開始温度から段階的若しくは連続的に温度を上昇させながら一定時間浸炭を行うと、粒界に炭化物が生成することなく、粒内の炭化物が成長する。
また、２次浸炭の進行に伴い、表面の炭素濃度が上昇し、表面のＡcm線温度も上昇する。そのため、２次浸炭初期工程の条件を最適化すれば、鋼材を焼入れ温度に昇温しても、焼入れ温度が表面のＡcm線温度を超えることがない。その結果、図１（ｄ）の右図に示すように、粒界に炭化物を生成させることなく、粒内の炭化物を成長させることができる。 On the other hand, when the primary carburization is finished in a state where the primary carburization temperature is equal to or lower than the conventional one, as shown in the left diagram of FIG. A coarse carbide is not generated in the boundary. In addition, when the temperature is raised to the secondary carburization start temperature after cooling the steel material, if the secondary carburization start temperature is set to a temperature lower by 100 ° C. or more than the primary carburization temperature, the surface temperature of the steel material is surely kept below the Acm line Can be taken to. Therefore, when the secondary carburization start temperature is reached, fine carbides are generated in the grains as shown in the middle diagram of FIG. If this state is maintained at the same temperature as the secondary carburization start temperature, or if carburization is performed for a certain period of time while raising the temperature stepwise or continuously from the secondary carburization start temperature, carbides are generated at the grain boundaries. There is no growth within the grains.
As the secondary carburization proceeds, the surface carbon concentration increases and the surface Acm line temperature also increases. Therefore, if the conditions of the secondary carburizing initial process are optimized, the quenching temperature does not exceed the surface Acm line temperature even if the steel material is heated to the quenching temperature. As a result, as shown in the right figure of FIG. 1D, the carbides in the grains can be grown without generating carbides at the grain boundaries.

粒界に片状・粗大な炭化物の析出を防止するためには、１次浸炭をＡcm線より高い温度で行い、２次浸炭をＡcm線より低い温度で行う必要がある。本発明に係る高濃度浸炭鋼の製造方法は、１次浸炭終了後と２次浸炭開始時の温度差を十分に取ることができるので、鋼材の成分にロット間バラツキがある場合であっても、片状・粗大な炭化物の生成を確実に抑えることができる。また、充分な温度差を取るために１次浸炭温度を上昇させる必要がないので、炉の耐久性を低下させることもない。さらに、２次浸炭開始温度に到達した後、所定時間経過後に焼入れ温度に上昇させて浸炭が続行されるので、表面の炭素濃度を短時間で目的とする濃度に到達させることができる。   In order to prevent precipitation of coarse and coarse carbides at the grain boundaries, it is necessary to perform primary carburization at a temperature higher than the Acm line and perform secondary carburization at a temperature lower than the Acm line. Since the manufacturing method of the high-concentration carburized steel according to the present invention can sufficiently take a temperature difference between the end of the primary carburization and the start of the secondary carburization, even if there are lot-to-lot variations in the components of the steel material Thus, the formation of flakes and coarse carbides can be reliably suppressed. Moreover, since it is not necessary to raise the primary carburizing temperature in order to take a sufficient temperature difference, the durability of the furnace is not lowered. Furthermore, after reaching the secondary carburizing start temperature, the carburization is continued after raising the quenching temperature after a predetermined time has elapsed, so that the surface carbon concentration can reach the target concentration in a short time.

（実施例１〜１５、比較例１〜５）
［１． 試料の作製］
種々の組成を有する鋼材について、種々の条件下で浸炭を行った。なお、浸炭は、いずれも１次浸炭と２次浸炭の２段階で行った。また、２次浸炭は、実施例１５及び比較例１を除き、一定の温度（低温）で一定時間保持する２次浸炭初期工程と、焼入れ温度（高温）に昇温して一定時間保持する２次浸炭後期工程の２段階に分けて行った。図２に、典型的な浸炭処理パターンを示す。 (Examples 1-15, Comparative Examples 1-5)
[1. Preparation of sample]
The steel materials having various compositions were carburized under various conditions. Carburization was performed in two stages, primary carburization and secondary carburization. In addition, except for Example 15 and Comparative Example 1, the secondary carburizing is a secondary carburizing initial step that is maintained at a constant temperature (low temperature) for a certain period of time, and a temperature that is raised to a quenching temperature (high temperature) and maintained for a certain period of time 2 It was divided into two stages of the next carburization late process. FIG. 2 shows a typical carburizing pattern.

浸炭は、１次浸炭、２次浸炭初期及び２次浸炭後期のいずれも、
（１） 総浸炭時間の２％に当たる時間、浸炭ガスを流して浸炭を行う操作、及び
（２） 総浸炭時間の２３％に当たる時間、真空引きをして拡散させる操作、
を合計４回繰り返すことにより行った。
但し、実施例１５については、２次浸炭開始温度を７５０℃とし、焼入れ温度８５０℃に達するまで、浸炭しながら４０分かけて昇温し、焼入れ温度に到達後、直ちに焼入れを行った。
また、比較例２の２次浸炭初期工程は、総浸炭時間の３％に当たる時間、浸炭ガスを流して浸炭する操作と、総浸炭時間の２２％に当たる時間、真空引きをして拡散する操作を、合計４回繰り返した。
さらに、比較例３の２次浸炭初期工程は、総浸炭時間の１％に当たる時間、浸炭ガスを流して浸炭を行い、総浸炭時間の２４％に当たる時間、真空引きをして拡散する操作を、合計４回繰り返した。 Carburization is performed in both primary carburization, initial secondary carburization, and late secondary carburization.
(1) A time corresponding to 2% of the total carburizing time, an operation of performing carburizing by flowing a carburizing gas, and (2) an operation of evacuating and diffusing for a time corresponding to 23% of the total carburizing time,
Was repeated a total of 4 times.
However, in Example 15, the secondary carburization start temperature was set to 750 ° C., the temperature was increased over 40 minutes while carburizing until the quenching temperature reached 850 ° C., and quenching was performed immediately after reaching the quenching temperature.
In addition, the secondary carburizing initial process of Comparative Example 2 includes a time corresponding to 3% of the total carburizing time, a carburizing gas flowing operation, a time corresponding to 22% of the total carburizing time, and an operation of evacuating and diffusing. , Repeated a total of 4 times.
Furthermore, in the secondary carburizing initial step of Comparative Example 3, the carburizing gas is flowed for a time corresponding to 1% of the total carburizing time, the carburizing gas is flown, and the operation corresponding to 24% of the total carburizing time is performed by evacuating and diffusing. Repeated a total of 4 times.

［２． 試験方法］
１次浸炭終了後の表面炭素濃度は、断面の炭素濃度分布をＥＰＭＡにより測定し、表面から１０μｍの領域内の平均炭素濃度を算出した。また、１次浸炭終了後及び焼入れ後の炭化物粒径は、試料断面をピクラルで腐食後、ＳＥＭを用いて写真撮影することにより測定し、１ｍｍ2中に存在する炭化物の粒径の最大値を「炭化物粒径」とした。さらに、焼入れ後の疲労強度は、回転曲げ疲労試験（ＪＩＳ Ｚ ２２７４に準拠）により測定した。 [2. Test method]
The surface carbon concentration after the end of the primary carburization was obtained by measuring the carbon concentration distribution in the cross section with EPMA and calculating the average carbon concentration in the region of 10 μm from the surface. The carbide particle size after the completion of primary carburization and after quenching is measured by taking a photograph using a SEM after the sample cross section is corroded with picral, and the maximum particle size of carbide present in 1 mm 2 is determined. “Carbide particle size”. Furthermore, the fatigue strength after quenching was measured by a rotating bending fatigue test (based on JIS Z 2274).

［３． 結果］
表１に、各種鋼材の成分、浸炭条件、及び試験結果を示す。
比較例１は、焼入れ後において、１０μｍを超える粗大な炭化物が生成した。これは、２次浸炭初期工程を省略し、直ちに８８５℃での２次浸炭後期工程を行っているために、２次浸炭の昇温過程で生じた微細な炭化物が再固溶したためと考えられる。
また、比較例２は、焼入れ後において、１０μｍを超える粗大な炭化物が生成した。これは、１次浸炭が過剰であり、１次浸炭終了時点で粗大な炭化物が既に生成していたためである。
また、比較例３は、６μｍを超える粗大な炭化物が生成した。これは、１次浸炭が不十分であり、表面炭素濃度が共析炭素濃度に満たないために、２次浸炭開始温度到達時点で、粒内に微細な炭化物が生成しなかったためと考えられる。
また、比較例４は、粗大な炭化物はないが、疲労強度が低い。これは、２次浸炭初期温度と２次浸炭後期温度が同一であり、炭素の拡散速度が遅いために、十分な量の炭化物が生成しなかったためと考えられる。
さらに、比較例５は、７μｍを超える粗大な炭化物が生成した。これは、２次浸炭初期温度における保持時間が短いために、２次浸炭後期温度に昇温したときに鋼材の表面温度がＡcm線を越えたためと考えられる。
そのため、比較例１〜５は、いずれも疲労強度が７００ＭＰａ未満であった。 [3. result]
Table 1 shows the components of various steel materials, carburizing conditions, and test results.
In Comparative Example 1, coarse carbides exceeding 10 μm were generated after quenching. This is probably because the initial stage of the second carburization was omitted and the second stage of the second carburization at 885 ° C. was performed immediately, so that fine carbides generated during the temperature raising process of the second carburization were re-dissolved. .
Moreover, the comparative example 2 produced | generated the coarse carbide | carbonized_material over 10 micrometers after hardening. This is because primary carburization is excessive and coarse carbides have already been generated at the end of primary carburization.
Moreover, the comparative example 3 produced | generated the coarse carbide | carbonized_material exceeding 6 micrometers. This is probably because primary carburization is insufficient and the surface carbon concentration is less than the eutectoid carbon concentration, so that fine carbides are not generated in the grains when the secondary carburization start temperature is reached.
In Comparative Example 4, there is no coarse carbide, but the fatigue strength is low. This is presumably because the initial carburizing initial temperature and the secondary carburizing late temperature were the same, and the carbon diffusion rate was slow, so that a sufficient amount of carbide was not generated.
Furthermore, the comparative example 5 produced | generated the coarse carbide | carbonized_material exceeding 7 micrometers. This is thought to be because the surface temperature of the steel material exceeded the Acm line when the temperature was raised to the secondary carburizing late temperature because the holding time at the secondary carburizing initial temperature was short.
Therefore, all of Comparative Examples 1 to 5 had a fatigue strength of less than 700 MPa.

これに対し、実施例１〜１５は、いずれも疲労強度が７００ＭＰａ以上であった。これは、適正な条件下で１次浸炭、２次浸炭初期、及び２次浸炭後期が行われているので、粒界に片状・粗大な炭化物を生成させることなく、微細かつ球状の炭化物を粒内に多量に形成することができたためと考えられる。   On the other hand, Examples 1 to 15 all had a fatigue strength of 700 MPa or more. This is because the primary carburizing, the initial carburizing initial stage, and the secondary carburizing late stage are performed under appropriate conditions, so that fine and spherical carbides can be produced without generating flakes and coarse carbides at the grain boundaries. It is thought that it was possible to form a large amount in the grain.

Figure 0004971751
Figure 0004971751

以上、本発明の実施の形態について詳細に説明したが、本発明は上記実施の形態に何ら限定されるものではなく、本発明の要旨を逸脱しない範囲内で種々の改変が可能である。   Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the scope of the present invention.

本発明に係る高濃度浸炭鋼の製造方法は、軸、軸受け、歯車ピストンピン、カムなどの機械部品の製造方法として使用することができる。   The manufacturing method of the high concentration carburized steel according to the present invention can be used as a manufacturing method of machine parts such as a shaft, a bearing, a gear piston pin, and a cam.

各種条件下で高濃度浸炭した場合における組織変化の模式図、及び、状態図である。It is the schematic diagram of a structure change at the time of carrying out high concentration carburizing on various conditions, and a state diagram. 実施例で用いた典型的な浸炭処理パターンを示す図である。It is a figure which shows the typical carburizing process pattern used in the Example.

Claims (3)

以下の工程を備えた高濃度浸炭鋼の製造方法。
（イ） Ｃ：０．１５〜０．３０ｍａｓｓ％、Ｓｉ：０．４０〜０．８０ｍａｓｓ％、Ｍｎ：０．３〜０．８ｍａｓｓ％、Ｃｒ：１．２５〜２．００ｍａｓｓ％、残部がＦｅ及び不可避的不純物からなる鋼材を、１１００℃以下である１次浸炭温度Ｔ1（℃）において、その表面炭素濃度ＣがＣeu＜Ｃ≦Ｃ(Ａcm)となるまで浸炭させる１次浸炭工程。
但し、
Ｃeuは、前記鋼材の共析炭素濃度、
Ｃ(Ａcm)は、前記１次浸炭温度Ｔ1における前記鋼材のＡcm線に相当する炭素濃度。
（ロ） 前記１次浸炭工程終了後、前記鋼材を冷却速度１℃／分以上で７００℃以下まで冷却する冷却工程。
（ハ） 前記鋼材を２次浸炭開始温度Ｔ2sまで昇温させ、２次浸炭温度Ｔ2を前記２次浸炭開始温度Ｔ2sに維持したまま前記鋼材を浸炭させる２次浸炭初期工程。
但し、
Ａc1点（℃）≦Ｔ2s（℃）≦１次浸炭温度Ｔ1−１００℃≦２次浸炭開始直後における前記鋼材の表面炭素濃度に相当するＡcm線温度（℃）、
Ｔ2S≦Ｔ2≦前記鋼材の表面炭素濃度に相当するＡcm線温度（℃）。
（ニ） 前記２次浸炭初期工程終了後、引き続き前記鋼材を焼入れ温度Ｔq（℃）まで昇温させ、前記焼入れ温度Ｔqにおいてさらに浸炭させる２次浸炭後期工程。
但し、
Ｔq≦前記鋼材の表面炭素濃度に相当するＡcm線温度（℃）。
（ホ） 前記２次浸炭後期工程終了後、前記鋼材を焼入れする焼入れ工程。 The manufacturing method of high concentration carburized steel provided with the following processes.
(A) C: 0.15 to 0.30 mass%, Si: 0.40 to 0.80 mass%, Mn: 0.3 to 0.8 mass%, Cr: 1.25 to 2.00 mass%, the balance being Fe And a primary carburizing step of carburizing a steel material composed of inevitable impurities at a primary carburizing temperature T1 (° C.) of 1100 ° C. or less until the surface carbon concentration C becomes Ceu <C ≦ C (Acm).
However,
Ceu is the eutectoid carbon concentration of the steel material,
C (Acm) is a carbon concentration corresponding to the Acm line of the steel material at the primary carburizing temperature T1.
(B) A cooling step of cooling the steel material to a temperature of 700 ° C. or less at a cooling rate of 1 ° C./min after the completion of the primary carburizing step.
(C) the steel material is warm to the secondary carburization start temperature T2s, the secondary carburization temperature T 2 a secondary carburization initial step of carburizing a pre Symbol steel while maintaining the secondary carburization start temperature T2s.
However,
Ac1 point (° C.) ≦ T 2 s (° C.) ≦ primary carburizing temperature T 1 −100 ° C. ≦ Acm line temperature (° C.) corresponding to the surface carbon concentration of the steel immediately after the start of secondary carburizing,
T2S ≦ T2 ≦ Acm line temperature (° C.) corresponding to the surface carbon concentration of the steel material.
(D) A secondary carburizing late process in which the steel material is continuously heated to the quenching temperature Tq (° C.) after the secondary carburizing initial process is completed, and further carburized at the quenching temperature Tq.
However,
Tq ≦ Acm line temperature (° C.) corresponding to the surface carbon concentration of the steel material.
(E) A quenching step of quenching the steel after the end of the secondary carburization late step. 前記２次浸炭初期工程は、１５分以上浸炭を行うものである請求項１に記載の高濃度浸炭鋼の製造方法。 The method for producing high-concentration carburized steel according to claim 1, wherein the secondary carburizing initial step is carburizing for 15 minutes or more. 前記２次浸炭後期工程は、前記焼入れ温度Ｔqにおいて１５分以上浸炭を行うものである請求項１又は２に記載の高濃度浸炭鋼の製造方法。 3. The method for producing high-concentration carburized steel according to claim 1, wherein the second-stage carburizing late step is for carburizing at the quenching temperature Tq for 15 minutes or longer.
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