JP3661868B2 - Carburizing method - Google Patents

Description

【００３９】
得られた浸炭処理製品について表面から深さ方向に対してビッカース硬さＨ_Ｖの測定を行い、浸炭ガス種の違いによる浸炭挙動（深さ方向に対するビッカース硬さＨ_Ｖの変化）を比較した。
【００４０】
（実施例２／浸炭ガスのガス圧の違いによる浸炭挙動の比較）
実施例１と同じ被浸炭処理材及び浸炭炉を用い、浸炭ガスとして都市ガス（１３Ａ）を３〜９ｋＰａのガス圧範囲且つ６０Ｌ／ｍｉｎの流量（鋼材単位面積当たりの流量：（１０４．９ＮＬ／ｍｉｎ・ｍ^２）で炉内に連続供給しながら浸炭処理を行った。さらに、その後実施例１と同じ条件で拡散処理及び焼入れを行った。
【００４１】
得られた浸炭処理製品について、カーボンがどの程度の深さまで固溶・拡散したかを示す浸炭深さの測定、製品表面のビッカース硬さＨ_Ｖの測定及びその表面におけるビッカース硬さＨ_Ｖ値のばらつきの測定を行った。ここで、ビッカース硬さＨ_Ｖ値のばらつきとは、複数の測定点における最高硬度値と最低硬度値との差を採ったものであり、硬度差が５０以下の場合には実用可能な浸炭製品の許容範囲内であると判断した。
【００４２】
（実施例３／浸炭ガスの流量の違いによる浸炭挙動の比較）
実施例１及び２と同じ被浸炭処理材及び浸炭炉を用い、浸炭ガスとして都市ガス（１３Ａ）を３ｋＰａのガス圧且つ１５〜１２０Ｌ／ｍｉｎの流量範囲内（鋼材単位面積当たりの流量：２６〜２１０ＮＬ／ｍｉｎ・ｍ^２）で炉内に連続供給しながら浸炭処理を行った。さらに、その後実施例１及び２と同じ条件で拡散処理を行った。
【００４３】
得られた浸炭処理製品について表面から深さ方向に対してビッカース硬さＨ_Ｖの測定を行い、浸炭ガスの流量の違いによる浸炭挙動（深さ方向に対するビッカース硬さＨ_Ｖの変化）を比較した。
【００４４】
（実施例４／拡散時間の違いによる浸炭挙動の比較）
実施例１〜３と同じ被浸炭処理材及び浸炭炉を用い、浸炭ガスとして都市ガス（１３Ａ）を９ｋＰａのガス圧且つ６０Ｌ／ｍｉｎの流量（鋼材単位面積当たりの流量：１０４．９ＮＬ／ｍｉｎ・ｍ^２）で炉内に連続供給しながら浸炭処理を行った。浸炭処理後、浸炭ガスの供給を停止して、１ｋＰａ以下となるように炉内を減圧し同温度（９３０℃）に保持した状態で０〜３０ｍｉｎの拡散時間を変化させて拡散処理を行い、さらに一部処理材を除き８５０℃で２０ｍｉｎ焼入れ保持を行った後油焼入れ（急冷）を行った。本実施例の浸炭処理、拡散処理及び焼入れ時の加熱履歴を図４に示す。
【００４５】
得られた浸炭処理製品について表面から深さ方向に対してビッカース硬さＨ_Ｖの測定を行い、拡散処理における拡散時間の違いによる浸炭挙動（深さ方向に対するビッカース硬さＨ_Ｖの変化）を比較した。
【００４６】
図１に実施例１に係る浸炭挙動（深さ方向に対するビッカース硬さＨ_Ｖの変化）に及ぼす浸炭ガス種の影響についての結果を示す。図示したように、アセチレンガスを使用した場合に表面付近のビッカース硬さＨ_Ｖ値が最も高くなったが、他の浸炭ガスとの違いは顕著に認められなかった。すなわち、浸炭ガスとして都市ガス（１３Ａ）を使用した場合にも、従来のアセチレンガスやプロパンガス等と同様の浸炭挙動を示した。また、都市ガス（１３Ａ）を浸炭ガスとした場合には、煤の発生や浸炭ムラの発生がない良質の浸炭処理製品が得られた。
【００４７】
また、図２に実施例２に係る浸炭ガス圧を変化させた場合の浸炭挙動についての結果を示す。図示したように、ガス流量を６０Ｌ／ｍｉｎと一定にしてガス圧を３〜９ｋＰａの範囲内で変化させた場合には、いずれのガス圧下でも浸炭深さが０．６ｍｍ以上且つビッカース硬さＨ_Ｖ値が５５０以上、すなわち、有効硬化層の厚みが０．６ｍｍ以上であり目標値（実用化レベル）に達していた。また、ガス圧が６ｋＰａ以上では、ビッカース硬さＨ_Ｖの最大値と最小値との差が５０以下とビッカース硬さＨ_Ｖ値のばらつきが少なく、煤や浸炭ムラの発生のない良質な浸炭処理材が得られた。なお、ガス圧が３ｋＰａの場合には、硬度差が５０を超えているが、ガス流量を増大させることによって５０以下に抑えることができる。また、ガス圧が３ｋＰａ未満については示していないが、３ｋＰａ未満ではビッカース硬さＨ_Ｖ値、浸炭深さ共に大幅に低下し、さらにビッカース硬さＨ_Ｖ値のバラツキも大きく、浸炭処理が不十分であった。一方、ガス圧が９ｋＰａを超える範囲では、ガス圧の増加による浸炭効果は認められなかった。
【００４８】
また、図３に実施例３に係るガス流量を変化させた場合の浸炭挙動についての結果を示す。図示したように、ガス流量の増加に伴いカーボンの固溶・拡散量も増大する傾向にあり、流量４５Ｌ／ｍｉｎ以上では浸炭ガス圧が３ｋＰａであっても、浸炭深さが約０．６ｍｍの部分までビッカース硬さＨ_Ｖが５５０以上の有効硬化層が形成されていた。なお、流量が６０Ｌ／ｍｉｎを超える範囲では、流量の増加による浸炭効果は認められなかった。
【００４９】
また、図５に実施例４に係る拡散処理における拡散時間の変化させた場合の浸炭挙動についての結果を示す。図示したように、浸炭処理後すぐに油焼入れした場合（焼入れ保持なし）には、鋼材内部へのカーボンの拡散が少なく表面からの０．２〜０．３ｍｍの深さにおいてビッカース硬さＨ_Ｖが最大値となるなどカーボンの濃度ムラが生じた。一方、浸炭処理後、拡散時間を０〜３０ｍｉｎとしその後８５０℃で２０ｍｉｎ焼入れ保持を施した条件では、カーボンが鋼材内部へと拡散し、さらに拡散時間が１５ｍｉｎ以上では有効硬化層が表面から約０．６ｍｍ以上の深さまで形成されると共に、煤や浸炭ムラの発生のない良質の浸炭処理製品が得られた。
【００５０】
以上の結果から、浸炭ガスとして都市ガス（１３Ａ）を使用し、そのガス圧を３〜９ｋＰａの範囲内に制御して連続供給しながら浸炭処理を行うことによって、従来より減圧下での浸炭処理において浸炭ガスとして使用されているアセチレンガス、エチレンガス、プロパンガス等と同様の優れた浸炭効果が得られた。具体的には、一般に浸炭処理材としての実用化レベルとされるビッカース硬さＨ_Ｖが５５０以上の有効硬化層が表面から０．６ｍｍ以上の深さまで形成されるという条件を満たし、かつ、煤や浸炭ムラの発生のない良質の浸炭処理製品を得ることができた。なお、上記実施例では、浸炭ガス圧を３〜９ｋＰａの範囲内で制御しているが、浸炭ガス圧が１０ｋＰａの範囲までは浸炭反応に対するガス圧の効果を確認している。
【００５１】
また、上記の浸炭効果は都市ガス（１３Ａ）を浸炭ガスとして用いた場合だけでなく、天然ガスを用いた場合にも、浸炭ガス圧を３〜１０ｋＰａ、浸炭温度を８５０〜１１００℃、浸炭時間を１５〜１２０ｍｉｎとする浸炭条件下で鋼材を浸炭処理することによって、上記都市ガス（１３Ａ）を用いた場合と同様に有効硬化層が表面から０．６ｍｍ以上の深さまで形成され、かつ、煤や浸炭ムラの発生のない浸炭処理製品を得ることができた。
【００５２】
本発明は上記した実施の形態に何ら限定されるものではなく、本発明の趣旨を逸脱しない範囲で種々の改変が可能である。例えば、浸炭処理並びに拡散処理を行う処理装置は上記のものに限られず種々の形態を有するもので構わない。また、被浸炭処理材として使用する鋼材も上記実施例のものに限られない。さらに、浸炭処理、拡散処理及び焼入れ時の加熱履歴についても上記実施例ものに限られない。
【００５３】
【発明の効果】
本発明の浸炭方法によれば、浸炭ガスとして、従来減圧下での浸炭処理において浸炭用の供給ガスとして有効であるとされてきたエチレンガスやアセチレンガス等よりも安価な都市ガス若しくは天然ガスを使用することによって、浸炭処理コストならびに得られる浸炭処理製品のコストを低減させることができるという効果がある。特に、家庭用等のガスラインとして広く普及している都市ガスを浸炭ガスとして使用することによって、浸炭処理設備への浸炭ガスの供給が極めて容易であり、浸炭処理コスト等をより安価に抑えることができるという効果がある。
【００５４】
また、都市ガスや天然ガスは、比較的浸炭能力の低いメタンが主成分として含まれていることにより、浸炭制御が容易に行え、かつ、煤などの発生のない表面性状に優れた浸炭処理材が得られるという効果がある。また、炭素数２以上の浸炭ガス種の分散性が良好であることから、浸炭時の反応ムラが少なく拡散処理時間ひいては浸炭処理工程全体の短縮化をも図ることができるという効果がある。さらには、常圧浸炭法のように浸炭時に有害なＣＯが一切発生せず、環境面からも優れている。
【００５５】
また、浸炭処理を所定浸炭ガス圧（３〜１０ｋＰａ）、所定温度（８５０〜１１００℃）、所定保持時間（１５〜１２０ｍｉｎ）の条件下で行うことによって、煤や浸炭ムラの発生がなく、鋼材内部までカーボンが十分に固溶・拡散された高品質の浸炭処理製品を提供することができるという効果がある。
【００５６】
また、浸炭処理後に、浸炭処理製品を１ｋＰａ以下の減圧下で所定時間、特に５〜３０ｍｉｎの間保持する拡散処理を行うことによって、浸炭処理によって鋼材の表面部に固溶されたカーボンを処理材内部へと効果的に拡散させることができる。これにより、浸炭ムラの発生が抑えられ、最適な厚みを有する硬化層が形成された良質の浸炭処理製品を提供することができるという効果がある。さらに拡散処理後で所定温度（８００〜９００℃）、所定時間（５〜６０ｍｉｎ）の条件下で行うことによって、カーボン拡散効果がさらに向上しより均一且つ高品質の浸炭処理製品を提供することができるという効果がある。
【００５７】
また、浸炭処理時の流量を所定条件下（鋼材単位面積当たりの流量：２６〜２１０ＮＬ／ｍｉｎ・ｍ^２）に制御することにより、浸炭処理製品の品質等をより高めることができるという効果もある。
【図面の簡単な説明】
【図１】 本発明の第１実施例に係る浸炭処理材における浸炭挙動（深さ方向に対するビッカース硬さＨ_Ｖの変化）に及ぼす浸炭ガス種の影響を示した図である。
【図２】 本発明の第２実施例に係る浸炭処理材における浸炭挙動（深さ方向に対するビッカース硬さＨ_Ｖの変化）に及ぼす浸炭ガス圧の影響を示した図である。
【図３】 本発明の第３実施例に係る浸炭処理材における浸炭挙動（深さ方向に対するビッカース硬さＨ_Ｖの変化）に及ぼす浸炭ガス流量を示した図である。
【図４】 本発明の第４実施例に係る浸炭処理、拡散処理及び焼入れ時の加熱履歴を示した図である。
【図５】 上記実施例に係る浸炭処理材における浸炭挙動（深さ方向に対するビッカース硬さＨ_Ｖの変化）に及ぼす拡散時間の影響を示した図である。[0001]
BACKGROUND OF THE INVENTION
  The present invention relates to a carburizing method, and more particularly to a carburizing method for carburizing with a carburizing gas introduced under reduced pressure while heating the surface of a steel material as a carburized material in a carburizing furnace.
[0002]
[Prior art]
  “Carburization”, which has long been known as a technology for hardening steel surfaces, heats low-carbon steel in a carburizing gas to capture carbon into the steel surface and diffuse the carbon from the steel surface to the inside. This is a method for increasing the carbon concentration in the vicinity of the steel surface. By this carburizing method, a hardened layer that has been martensite formed by rapid cooling from the high-temperature austenite state is formed near the surface of the steel material, and a carburized steel product that retains high toughness while maintaining low carbon inside the steel material is obtained.
[0003]
  As one of the methods known as this carburizing treatment technique, there is an atmospheric pressure carburizing method under an atmospheric pressure gas atmosphere. In this normal pressure carburizing method, for example, carbon monoxide (CO) is used as a carburizing gas, and this is reacted with a carburized material (steel material) at a high temperature and under a normal pressure to react with carbon (C) and carbon dioxide (CO).₂), And the generated carbon is dissolved in the steel surface and further diffused from the steel surface to the inside.
[0004]
  However, in recent years, with the background of growing interest in environmental and resource issues, CO, CO₂While the atmospheric pressure carburizing method that discharges gases such as these is regarded as a problem, the carburizing method that performs carburizing treatment under reduced pressure is attracting attention as a carburizing method that is extremely effective for energy saving, resource saving and pollution prevention.
[0005]
  In this carburizing method, a carburizing process is performed on the surface of a steel material by introducing a hydrocarbon-based gas as a carburizing gas under reduced pressure. In the carburizing process, the hydrocarbon gas reacts with the steel material at a high temperature and under a reduced pressure to cause carbon (C) and hydrogen gas (H₂), And the generated carbon dissolves in the steel and diffuses from the steel surface to the inside. Conventionally, as a carburizing gas in carburizing treatment under reduced pressure, acetylene gas (for example, refer to Patent Document 1), ethylene gas (for example, refer to Patent Document 2), or a mixed gas of ethylene and hydrogen (for example, refer to Patent Document 3). ), A mixed gas of ethylene and acetylene (for example, see Patent Document 4) and the like.
[0006]
[Patent Document 1]
          JP-A-8-325701
[Patent Document 2]
          JP 2002-146512 A
[Patent Document 3]
          JP 2001-262313 A
[Patent Document 4]
          JP 2000-1765 A
[0007]
  According to the carburizing method using these carburizing gases, a high-quality carburized product can be obtained by heat treatment at a high temperature, the waste of heat energy of the carburizing process is eliminated, and the gas consumption is also reduced to the atmospheric pressure carburizing process. Compared to this, there is an advantage in that it requires less and has excellent environmental characteristics since it does not emit carbon dioxide.
[0008]
[Problems to be solved by the invention]
  However, in carburizing treatment using ethylene gas or acetylene gas as the carburizing gas, these carburizing gas species are highly flammable gases composed of unsaturated hydrocarbons, so that soot is easily generated, There is a problem that it is difficult to handle because there is a risk of ignition due to gas leakage from a pipe or the like. Further, since these gases are expensive, there is a problem that the heat treatment cost increases.
[0009]
  In addition, since ethylene gas and acetylene gas have a small amount of gas per unit carbon atom, there is a problem that dispersion and diffusion in the carburizing furnace is difficult and carburizing unevenness is likely to occur.
[0010]
  The problem to be solved by the present invention is that when carburizing steel, CO, CO₂It is an object of the present invention to provide a carburizing method capable of obtaining a high-quality carburized steel that does not generate harmful gases such as, is inexpensive, and does not generate soot and carburizing unevenness.
[0011]
[Means for Solving the Problems]
  In order to solve this problem, the carburizing method of the present invention is a saturated hydrocarbon as the carburizing gas when carburizing a steel material to be carburized while heating in a carburizing furnace supplied with carburizing gas under reduced pressure. While using city gas or natural gas whose main component is a system gas and constantly controlling the gas pressure within the range of 3 to 10 kPa in the carburizing furnace, the steel materialThe diffusion treatment is carried out by heating and holding within a range of heating temperature of 850 to 1100 ° C. and holding time of 15 to 120 min, and then holding for 0 to 120 min under reduced pressure of 1 kPa or less and at the carburizing temperature. Furthermore, after the diffusion treatment, a quenching holding treatment is performed in which the temperature is lowered within a temperature range of 800 to 900 ° C. and held at this temperature for 5 to 60 minutes.This is the gist.
[0012]
  According to this carburizing method, since the city gas and natural gas to be used can be obtained at a lower cost than gases such as ethylene and acetylene, the carburizing cost and the cost of the carburized product obtained can be reduced. In particular, since city gas is widely used as a gas line for home use and the like, supply as a carburizing gas source is extremely easy.
[0013]
  In addition, city gas and natural gas used as carburizing gas are mainly composed of methane, which has a relatively low carburizing ability, and contain a small amount of hydrocarbon-based gas species having 2 or more carbon atoms with a high carburizing ability. A carburized material that can be easily controlled and has excellent surface properties free from soot and the like is obtained. Moreover, since the dispersibility of the carburizing gas species having 2 or more carbon atoms is excellent, the carburizing time can be shortened.
[0014]
  Moreover, since the supply gas pressure of the city gas or natural gas at the time of carburizing is controlled within a range of 3 to 10 kPa, carburization reaction is appropriately performed, and generation of soot and carburizing unevenness is suppressed at the time of carburizing.
[0015]
  Furthermore, the heating temperature in the carburizing furnace is controlled within the range of 850 to 1100 ° C.And soakHolding time in the charcoal furnace1By controlling within the range of 5 to 120 min, the reaction between the carburizing gas and the steel material can be effectively promoted, and an appropriate amount of carbon with no generation of soot can be dissolved.
[0016]
  Moreover, after carburizing the steel material in the carburizing furnace, a diffusion treatment is performed by holding for 0 to 120 minutes under a reduced pressure of 1 kPa or less and the carburizing temperature, and further, the temperature is lowered within a temperature range of 800 to 900 ° C. A good diffusion effect can be obtained by performing a quenching holding treatment for 15 to 60 minutes at this temperature.
[0017]
  Also, the flow rate of carburizing gas that contacts the steel surface in the carburizing furnaceThe, 26-210 NL / min · m per unit surface area of steel²By controlling so that it becomes, it can suppress generation | occurrence | production of a soot and can make an appropriate amount of carbon form a solid solution.
[0018]
DETAILED DESCRIPTION OF THE INVENTION
  Preferred embodiments of the present invention will be described below in detail with reference to the drawings.
[0019]
  First, the carburizing method according to the present invention will be described. When carburizing a steel material using a carburizing furnace, the carburizing gas is supplied to the furnace while being controlled to a predetermined gas pressure, and the furnace is heated to a predetermined temperature. At this time, the carburizing gas is, for example, carbon (C) and hydrogen gas (H₂The carbon produced thereby is taken into the steel material (Fe) (solid solution).
[0020]
[Formula 1]
  <For example, carburizing gas is methane (CH₄If>
    Fe + CH₄(G) → Fe [C] + 2H₂(G)
      (g): represents a gas state.
      [C]: Carbon dissolved in steel (Fe).
[0021]
  When carbon is dissolved in the carburizing reaction as described above, an austenite phase is formed on the surface of the steel material. By soaking it in an oil tank in the oil tank chamber and quenching, the austenite phase on the surface of the steel material is increased. It transforms into a martensitic phase of hardness. By performing the carburizing treatment in this manner, a hardened layer martensite is formed on the surface portion, while a carburized steel product that maintains high toughness while maintaining low carbon inside is obtained.
[0022]
  As the carburizing gas in the carburizing method according to the present invention, city gas or natural gas is used. Both gases are both methane (CH₄) As a main component, and a small amount of hydrocarbon gas having 2 or more carbon atoms such as ethane, propane, butane and the like.
[0023]
  City gas is ethane (C₂H₆): 6% by weight, propane (C₃H₈): 4% by weight, butane (C₄H₁₀): 2% by weight, and the balance: methane. Natural gas is not composed of components, but methane is the main component and the balance is composed of a saturated hydrocarbon gas such as ethane or propane. In the carburizing method according to the present invention, it is preferable to use a natural gas containing 50% by weight or more of methane.
[0024]
  The carburizing gas conventionally used for carburizing under reduced pressure is a gas such as ethylene, acetylene, propane, etc., and city gas and natural gas that are widely used as household gases etc. are used as carburizing gas There is no. This is due to the following reason. That is, carburization occurs when carburizing gas reacts with steel to produce carbon, but methane is a substance with low reaction activity and is inferior in carburizing ability. Therefore, city gas and natural gas, in which most of the gas components are composed of methane, have low carburizing ability and have not been suitable as carburizing gas.
[0025]
  However, as a result of intensive studies by the present inventors, it has been found that even when city gas or natural gas is used as the carburizing gas, the carburizing ability can be sufficiently exhibited and a carburized product at a practical level can be provided. This makes it possible to use city gas and natural gas that are widely used as general-purpose gas instead of conventional expensive gases such as ethylene and acetylene as carburizing gas, and the carburizing cost can be greatly reduced.
[0026]
  Furthermore, carburizing treatment with excellent surface properties that can easily control carburization and does not generate soot by containing a small amount of hydrocarbon-based gas species having 2 or more carbon atoms with high carburizing ability. There is also an advantage that a material can be obtained. The grounds are as follows. In other words, city gas and natural gas supplied to the carburizing furnace contain small amounts of hydrocarbon-based gas types having 2 or more carbon atoms with high carburizing ability, and these gas types come into contact with the surface of the steel material. , Which mainly contributes to carburizing treatment on the surface of steel, but because most of city gas and natural gas are methane gas with relatively low carburizing ability, carbonization with 2 or more carbon atoms such as ethylene and acetylene The carburization reaction proceeds more slowly than when a hydrogen-based gas species is used alone as the carburizing gas. For this reason, there is an advantage that it is extremely easy to control the carburization reaction so that a predetermined amount of carbon is dissolved in the steel. In this case, the main component methane gas is used as a substitute for the carrier gas, and as a result of the hydrocarbon type gas having 2 or more carbon atoms spreading over the entire steel surface, a uniform carburizing treatment can be achieved. It is difficult to cause a situation in which the carburization treatment is biased depending on the location of the steel and a large amount of soot is generated on the steel surface. Thereby, there exists an advantage that the carburized material excellent in surface property can be obtained.
[0027]
  Here, the gas pressure of the carburizing gas at the time of carburizing is preferably in the range of 3 to 10 kPa, and more preferably in the range of 3 to 9 kPa. When the gas pressure of the carburizing gas is less than 3 kPa, there is a problem in that it cannot be dissolved or diffused as a result of shortening the time for contacting a sufficient amount of carbon to the steel surface portion. When the gas pressure exceeds 10 kPa, the time for contacting carbon into the steel material and the time for decomposing into carbon become longer. As a result, solid solution / diffusion is rate-determined and saturated carbon is generated on the surface of the steel material as soot. This is not preferable because of the problem.
[0028]
  Moreover, it is preferable that the heating temperature in the carburizing furnace at the time of carburizing exists in the range of 850-1100 degreeC, and it is more preferable that it exists in the range of 900-1050 degreeC. When the heating temperature is less than 850 ° C., there is a problem that the carburization reaction is not promoted and the carburizing treatment becomes insufficient. On the other hand, when the heating temperature exceeds 1100 ° C., the distortion of the steel material increases. This is not preferable because it is not suitable for practical use.
[0029]
  Moreover, it is preferable that the heat holding time at the time of carburizing is in the range of 15 to 120 min, and more preferably in the range of 15 to 60 min. When the holding time is less than 15 min, there is a problem that the carburizing reaction is not promoted and the carburizing treatment becomes insufficient. On the other hand, when the holding time exceeds 120 min, the carburizing effect due to the elapse of the holding time is obtained. Not only is it not possible, but there is a problem that soot is generated on the surface of the steel material due to excessive supply of carburizing gas, which is not preferable.
[0030]
  In addition, after the heat treatment, a diffusion treatment for holding the carburized material is usually performed for a certain time in the carburizing temperature range. The purpose of this diffusion treatment is to homogenize the carburizing treatment and expand the range of the hardened layer by diffusing carbon dissolved in the surface portion of the carburizing treatment material into the inside of the treatment material by the heat treatment in the previous stage. . The heating temperature in the furnace during the diffusion treatment is preferably the same as the carburizing temperature. Here, the holding time during the diffusion treatment is preferably in the range of 0 to 120 min. If the holding time exceeds 120 min, the carburizing effect due to the elapse of the holding time cannot be obtained, and processing is wasted, which is not preferable.
[0031]
  Further, it is preferable to perform a quenching holding process in which the temperature is lowered to a predetermined temperature range and held for a certain time after the diffusion process. By this quenching holding treatment, the carbon diffusion effect becomes remarkable. Here, the quenching holding treatment temperature is preferably in the range of 800 to 900 ° C, and more preferably 850 ° C. When the holding treatment temperature is less than 800 ° C., the diffusion effect is small. On the other hand, when the holding treatment temperature exceeds 900 ° C., the quenching performance is inferior.
[0032]
  Further, here, the holding time of the quenching holding treatment is preferably in the range of 5 to 60 min, and more preferably in the range of 5 to 30 min. If the holding time is less than 5 min, the carbon diffusion effect due to the quenching holding process cannot be obtained. On the other hand, if the holding time exceeds 60 min, the diffusion effect due to the elapse of the holding time cannot be obtained and is wasted in the processing. This is not desirable because
[0033]
  Also, the flow rate of carburizing gas that contacts the steel surface in the carburizing furnaceIs, 26-210 NL / min · m per unit surface area of steel²It is preferable that Here, the surface area of the steel material means the total surface area of the steel material portion that can come into contact with the carburizing gas when the steel material is installed in the carburizing furnace. Moreover, the flow rate of the carburizing gas here refers to the gas flow rate in a standard state (0 ° C., 1 atm). The flow rate per unit surface area of steel is 26 NL / min · m²If it is less than 1, there is a problem that the carburization reaction is not sufficiently promoted, while the flow rate is 210 NL / min · m.²In the case where it exceeds 1, the effect of promoting the carburization reaction by increasing the flow rate cannot be obtained, which is not preferable.
[0034]
    (Evaluation means for carburized material)
  As an evaluation of the carburized material obtained by the carburizing process, the Vickers hardness H of the carburized material is_VWas measured. This Vickers hardness H_VCorresponds to the carbon concentration in the martensitic hardened layer and serves as an index of how much carbon is dissolved in the carburized material. The measurement was performed in the depth direction of the carburized material. Vickers hardness H_VWas measured according to Japanese Industrial Standard “JIS G 0557”, and the press-fit load was 4.903N. The test was performed at three points at each depth, and the average value was used as the measurement result.
[0035]
  Here, as an evaluation standard of the carburized material, the hardened layer has Vickers hardness H_VIs over 550 and this H_VIt was judged that a cured layer satisfying the condition of ≧ 550 (hereinafter referred to as “effective cured layer”) reaching a depth of 0.6 mm or more from the surface was a carburized product at a practical level.
[0036]
【Example】
  The effects of the present invention will be specifically described with reference to examples.
[0037]
    (Example 1 / Comparison of carburizing behavior depending on carburizing gas type)
  CoveredUsing a steel material (material: SCM415, dimensions: width 400 mm × depth 500 mm × height 300 mm) as a carburizing material, this is inserted into a carburizing furnace (indoor dimensions: width 460 mm, depth 610 mm, height 460 mm), After heating so that the temperature in the furnace was 930 ° C., carburizing treatment was performed for 30 minutes while continuously supplying the carburizing gas so as to have the flow rate and gas pressure shown in Table 1. As carburizing gas, city gas (13A), propane gas (C₃H₈), Acetylene gas (C₂H₂) And ethylene (C₂H₄) And hydrogen (H₂4 types of mixed gas were used. The flow rate of carburizing gas per unit area of steel material is city gas (13A) and ethylene (C₂H₄) And hydrogen (H₂) 104.9 NL / min · m²In propane gas, 41.95 NL / min · m²In acetylene gas, 62.9 NL / min · m²It is. Further, after the carburizing process, the supply of the carburizing gas is stopped, the furnace is depressurized so as to be 1 kPa or less, and the diffusion process is performed for 20 minutes while maintaining the same temperature (930 ° C.). After that, oil quenching was performed.
[0038]
[Table 1]

[0039]
  About the obtained carburized product, Vickers hardness H from the surface to the depth direction_VAnd carburization behavior (Vickers hardness H in the depth direction)_VChange).
[0040]
    (Example 2 / Comparison of carburizing behavior due to difference in gas pressure of carburizing gas)
  Using the same carburized material and carburizing furnace as in Example 1, city gas (13A) as a carburizing gas was in a gas pressure range of 3 to 9 kPa and a flow rate of 60 L / min (flow rate per unit area of steel material: (104.9 NL / min ・ m²The carburization process was performed while continuously feeding into the furnace. Further, diffusion treatment and quenching were then performed under the same conditions as in Example 1.
[0041]
  For the carburized product obtained, measurement of carburization depth indicating how much carbon was dissolved and diffused, Vickers hardness H of the product surface_VMeasurement and Vickers hardness H on the surface_VThe variation of the value was measured. Where Vickers hardness H_VThe value variation is obtained by taking the difference between the maximum hardness value and the minimum hardness value at a plurality of measurement points, and when the hardness difference is 50 or less, it is determined that it is within the allowable range of a practical carburized product. did.
[0042]
    (Example 3 / Comparison of carburizing behavior depending on the flow rate of carburizing gas)
  Using the same carburized material and carburizing furnace as in Examples 1 and 2, city gas (13A) was used as the carburizing gas within a gas pressure of 3 kPa and a flow rate range of 15 to 120 L / min (flow rate per unit area of steel material: 26 to 210NL / min · m²The carburization process was performed while continuously feeding into the furnace. Further, diffusion treatment was then performed under the same conditions as in Examples 1 and 2.
[0043]
  About the obtained carburized product, Vickers hardness H from the surface to the depth direction_VAnd carburizing behavior (Vickers hardness H in the depth direction) due to the difference in the flow rate of carburizing gas._VChange).
[0044]
    (Example 4 / Comparison of carburizing behavior due to difference in diffusion time)
  Using the same carburized material and carburizing furnace as in Examples 1 to 3, city gas (13A) was used as the carburizing gas at a gas pressure of 9 kPa and a flow rate of 60 L / min (flow rate per unit area of steel: 104.9 NL / min. m²The carburization process was performed while continuously feeding into the furnace. After the carburizing treatment, the supply of carburizing gas is stopped, the inside of the furnace is depressurized to 1 kPa or less, and the diffusion time is changed by changing the diffusion time of 0 to 30 min while maintaining the same temperature (930 ° C.). Further, some of the treated materials were removed, and after quenching and holding at 850 ° C. for 20 minutes, oil quenching (rapid cooling) was performed. The heating history at the time of carburizing treatment, diffusion treatment, and quenching in this example is shown in FIG.
[0045]
  About the obtained carburized product, Vickers hardness H from the surface to the depth direction_VAnd carburization behavior (Vickers hardness H in the depth direction) due to the difference in diffusion time in the diffusion treatment_VChange).
[0046]
  FIG. 1 shows carburization behavior (Vickers hardness H with respect to the depth direction) according to Example 1._VThe results on the effect of carburizing gas species on the change in As shown, Vickers hardness H near the surface when acetylene gas is used_VAlthough the value was the highest, the difference from other carburizing gases was not noticeable. That is, even when city gas (13A) was used as the carburizing gas, the same carburizing behavior as that of the conventional acetylene gas or propane gas was exhibited. Moreover, when the city gas (13A) was used as the carburizing gas, a high-quality carburized product without generation of soot and carburizing unevenness was obtained.
[0047]
  Moreover, the result about the carburizing behavior at the time of changing the carburizing gas pressure concerning Example 2 in FIG. 2 is shown. As shown in the figure, when the gas flow rate is kept constant at 60 L / min and the gas pressure is changed within the range of 3 to 9 kPa, the carburization depth is 0.6 mm or more and the Vickers hardness is H._VThe value was 550 or more, that is, the thickness of the effective cured layer was 0.6 mm or more, and the target value (practical use level) was reached. When the gas pressure is 6 kPa or higher, the Vickers hardness H_VThe difference between the maximum and minimum values is 50 or less and Vickers hardness H_VA high-quality carburized material with little variation in value and no occurrence of soot and carburizing unevenness was obtained. Note that when the gas pressure is 3 kPa, the hardness difference exceeds 50, but can be suppressed to 50 or less by increasing the gas flow rate. Further, although the gas pressure is not shown for less than 3 kPa, the Vickers hardness is less than 3 kPa._VBoth value and carburization depth are greatly reduced, and Vickers hardness H_VThe variation in value was large, and the carburizing treatment was insufficient. On the other hand, in the range where the gas pressure exceeds 9 kPa, the carburizing effect due to the increase in gas pressure was not recognized.
[0048]
  Moreover, the result about the carburizing behavior at the time of changing the gas flow rate which concerns on FIG. 3 at Example 3 is shown. As shown in the figure, as the gas flow rate increases, the solid solution / diffusion amount of carbon also tends to increase. At a flow rate of 45 L / min or more, even if the carburizing gas pressure is 3 kPa, the carburizing depth is about 0.6 mm. Vickers hardness H to part_VAn effective cured layer of 550 or more was formed. In addition, the carburizing effect by the increase in the flow rate was not recognized in the range where the flow rate exceeded 60 L / min.
[0049]
  Moreover, the result about the carburizing behavior at the time of changing the diffusion time in the diffusion process which concerns on FIG. 5 at Example 4 is shown. As shown in the figure, when oil quenching is performed immediately after carburizing treatment (without quenching retention), there is little carbon diffusion into the steel material, and Vickers hardness H at a depth of 0.2 to 0.3 mm from the surface._VAs a result, the density of carbon was uneven. On the other hand, after the carburizing treatment, under the condition that the diffusion time is 0 to 30 minutes and then quenching and holding is performed at 850 ° C. for 20 minutes, carbon diffuses into the steel material, and when the diffusion time is 15 minutes or more, the effective hardened layer is about 0% from the surface. A high-quality carburized product that was formed to a depth of 6 mm or more and was free of wrinkles and carburization unevenness was obtained.
[0050]
  From the above results, by using city gas (13A) as the carburizing gas and performing the carburizing process while continuously supplying the gas pressure within the range of 3 to 9 kPa, carburizing process under a reduced pressure than before. The same excellent carburizing effect as acetylene gas, ethylene gas, propane gas and the like used as carburizing gas in the process was obtained. Specifically, Vickers hardness H, which is generally regarded as a practical use level as a carburized material._VSatisfies the condition that an effective hardened layer of 550 or more is formed to a depth of 0.6 mm or more from the surface, and a carburized product having good quality without generation of soot and carburizing unevenness can be obtained. In the above embodiment, the carburizing gas pressure is controlled within the range of 3 to 9 kPa, but the effect of the gas pressure on the carburizing reaction is confirmed up to the carburizing gas pressure of 10 kPa.
[0051]
  Further, the carburizing effect is not only when the city gas (13A) is used as the carburizing gas, but also when natural gas is used, the carburizing gas pressure is 3 to 10 kPa, the carburizing temperature is 850 to 1100 ° C., and the carburizing time. By carburizing the steel under carburizing conditions of 15 to 120 min, an effective hardened layer is formed to a depth of 0.6 mm or more from the surface as in the case of using the city gas (13A), and And carburized product with no occurrence of uneven carburization.
[0052]
  The present invention is not limited to the embodiments described above, and various modifications can be made without departing from the spirit of the present invention. For example, the processing apparatus that performs the carburizing process and the diffusion process is not limited to the above, and may have various forms. Further, the steel material used as the carburized material is not limited to that of the above embodiment. Furthermore, the carburizing process, the diffusion process, and the heating history at the time of quenching are not limited to those in the above embodiment.
[0053]
【The invention's effect】
  According to the carburizing method of the present invention, as a carburizing gas, a city gas or natural gas that is cheaper than ethylene gas, acetylene gas, etc., which has been conventionally effective as a carburizing supply gas in carburizing treatment under reduced pressure, is used. By using it, there is an effect that the carburizing cost and the cost of the carburized product obtained can be reduced. In particular, by using city gas, which is widely used as a gas line for households, as carburizing gas, it is extremely easy to supply carburizing gas to carburizing equipment, and to reduce carburizing cost and other costs. There is an effect that can be.
[0054]
  In addition, city gas and natural gas contain methane, which has a relatively low carburizing capacity, as a main component, so that carburization can be easily controlled and carburizing materials with excellent surface properties that do not generate soot and the like. Is effective. Further, since the dispersibility of the carburized gas species having 2 or more carbon atoms is good, there is an effect that reaction unevenness at the time of carburizing is small and the diffusion treatment time and thus the entire carburizing treatment process can be shortened. Further, unlike the atmospheric pressure carburizing method, no harmful CO is generated at the time of carburizing, which is excellent from the environmental viewpoint.
[0055]
  Further, by performing the carburizing process under the conditions of a predetermined carburizing gas pressure (3 to 10 kPa), a predetermined temperature (850 to 1100 ° C.), and a predetermined holding time (15 to 120 min), there is no occurrence of soot and carburizing unevenness, and steel material There is an effect that it is possible to provide a high-quality carburized product in which carbon is sufficiently dissolved and diffused to the inside.
[0056]
  Further, after the carburizing process, by performing a diffusion process for holding the carburized product under a reduced pressure of 1 kPa or less for a predetermined time, in particular, for 5 to 30 minutes, the carbon that has been dissolved in the surface portion of the steel material by the carburizing process is treated. It can be effectively diffused inside. Thereby, there is an effect that it is possible to provide a high-quality carburized product in which the occurrence of carburizing unevenness is suppressed and a hardened layer having an optimum thickness is formed. Furthermore, by carrying out under the conditions of a predetermined temperature (800 to 900 ° C.) and a predetermined time (5 to 60 min) after the diffusion treatment, the carbon diffusion effect is further improved and a more uniform and high quality carburized product can be provided. There is an effect that can be done.
[0057]
  Moreover, the flow rate at the time of carburizing is set under a predetermined condition (flow rate per unit area of steel material: 26 to 210 NL / min · m).²), The quality of the carburized product can be further improved.
[Brief description of the drawings]
FIG. 1 shows carburization behavior (Vickers hardness H with respect to the depth direction) in a carburized material according to a first embodiment of the present invention._VIt is the figure which showed the influence of the carburizing gas kind which gives to (change of).
FIG. 2 shows carburization behavior (Vickers hardness H with respect to the depth direction) in the carburized material according to the second embodiment of the present invention._VIt is the figure which showed the influence of the carburizing gas pressure which gives to (change of).
FIG. 3 shows carburization behavior (Vickers hardness H with respect to the depth direction) in the carburized material according to the third embodiment of the present invention._VIt is the figure which showed the carburizing gas flow rate which affects to (change).
FIG. 4 is a diagram showing a heating history during carburizing, diffusion, and quenching according to a fourth example of the present invention.
FIG. 5 shows carburizing behavior (Vickers hardness H with respect to the depth direction) in the carburized material according to the embodiment._VIt is the figure which showed the influence of the spreading | diffusion time which gives to the (change of).

Claims (2)

In the carburizing method of carburizing while heating in a carburizing furnace in which the carburizing gas is supplied under reduced pressure to the steel material that is the carburized material,
As the carburizing gas, city gas or natural gas mainly composed of a saturated hydrocarbon gas is used, and the gas pressure is controlled so that the gas pressure is within the range of 3 to 10 kPa. However, the steel material is heated and held within a heating temperature range of 850 to 1100 ° C. and a holding time range of 15 to 120 min , and then carburized, and then 0 to 0 under a reduced pressure of 1 kPa or less and the carburizing temperature. A carburizing method characterized by performing a diffusion treatment for 120 minutes, and further performing a quenching and holding treatment in which the temperature is lowered within a temperature range of 800 to 900 ° C. after the diffusion treatment and is held at this temperature for 5 to 60 minutes . The carburizing method according to claim 1, wherein the flow rate of the carburizing gas contacting the steel material surface in the carburizing furnace is set to 26 to 210 NL / min · m ² per unit surface area of the steel material.

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