JP4066903B2 - Case-hardened steel and carburized parts that can be carburized in a short time - Google Patents
Case-hardened steel and carburized parts that can be carburized in a short time Download PDFInfo
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Description
【０００１】
【産業上の利用分野】
本発明は、肌焼鋼と、それを使用した浸炭部品に関する。本発明の肌焼鋼は短時間で浸炭可能であるから、とくにプラズマ浸炭法、真空浸炭法により浸炭処理を施される機械構造用部品、たとえば自動車のトランスミッションギアなど、疲労強度が要求される部品の製造に本発明は好適である。
【０００２】
【従来の技術】
トランスミッションギアのような機械構造用部品は、耐衝撃特性と疲労強度にすぐれていることが要求されるため、熱間・温間の鍛造および切削加工により所定の部品形状に成形後、浸炭処理を施して部品とするのが普通である。
【０００３】
一方、部品の製作には、生産コストを低減し作業環境を改善するという意図から、歩留まりが高く寸法精度のよい冷間加工が採用されている。肌焼鋼に対する十分な浸炭深さを得るため、浸炭工程では、部品を高温に加熱し、長時間その温度に保持する必要があり、この作業が浸炭部品のコストを高いものにしていた。
【０００４】
現在、肌焼鋼としてはＪＩＳ−ＳＣＲ４２０が広く用いられている。この鋼は部品への製造性はすぐれているものの、浸炭に長時間を要するので、それを短縮することが望まれていた。そこで、十分な浸炭深さを得ながら、浸炭時間を短縮することができる肌焼鋼の開発が進められている。その代表として、Ｃ含有量を増加させた鋼棒が提供されているが、Ｃは素材硬さを高めて部品への製造性を低下させるため、高Ｃの肌焼鋼は、あまり有利な材料ではない。
【０００５】
【発明が解決しようとする課題】
本発明の目的は、短縮された時間で浸炭可能な肌焼鋼を提供するとともに、その肌焼鋼を用いて、すぐれた物性をもった機械構造用の浸炭部品を有利に製造する方法を提供することにある。
【０００６】
【課題を解決するための手段】
本発明の短時間で浸炭可能な肌焼鋼は、質量％で、Ｃ：０．１０〜０．３５％、Ｓｉ：０．５〜２．０％、Ｍｎ：０．３〜１．０％、Ｃｒ：０．３〜２．０％、Ａｌ：０．０１〜０．１０％およびＮ：０．００１〜０．０２０％に加えて、さらに、Ｎｂ：０．０１〜０．１０％、Ｔｉ：０．０１〜０．２０％、Ｂ：０．０００１〜０．０１０％、Ｎｉ：０．０５〜３．００％およびＭｏ：０．０５〜１．００％を含有し、残部Ｆｅおよび不可避な不純物からなる合金組成を有し、式１により定義される炭素拡散の活性化エネルギーＱの値を３４０００(kca1)以下に選んだ、短時間で浸炭可能な肌焼鋼である。
Ｑ＝３４１４０−６０５［％Ｓｉ］＋１８３［％Ｍｎ］
＋１３６［％Ｃｒ］＋１２２［％Ｍｏ］ 式１
【０００７】
本発明の浸炭部品は、上記の肌焼鋼を部品形状に成形し、プラズマ浸炭法または真空浸炭法により、浸炭時間Ｔｃと拡散時間Ｔｄとの比Ｔｃ／Ｔｄが０．４〜０．７となる条件で浸炭を行なって得た浸炭部品である。
【０００８】
【発明の実施形態】
短縮された時間で一定量の浸炭を実現するには、炭素拡散の活性化エネルギーを下げ、拡散定数を増大させることが必要であって、このような観点から発明者が採択したのは、肌焼鋼の合金組成を選択するに当たり、拡散定数を増大させる成分、具体的にはＳｉを増量するともに、拡散定数を減少させる成分、具体的にはＭｎ，ＣｒおよびＭｏは低減する。
【００１０】
本発明の肌焼鋼において、合金組成を上記のように選択した理由を、以下に説明する。
【００１１】
Ｃ：０．１０〜０．３５％
Ｃは、部品の芯部の強度を確保するために必要であり、前記したトランスミッションギアのような部品に要求される芯部強度を得るには、下限の０．１０％以上のＣが必要である。一方、多量のＣの存在は靭性を低下させ、そのことはまたこの種部品にとって致命的な問題になるから、上限の０．３５％までの添加に止める。
【００１２】
Ｓｉ：０．５〜２．０％
Ｓｉは脱酸剤として鋼に加えられるが、部品の強度および焼入れ性の確保にも役立つ。前述のように、Ｓｉは炭素拡散の活性化エネルギーを増大させるので、積極的に添加したい。しかし、多量にすぎると加工性が低くなり、肌焼鋼として好ましくない。これらのバランスを考えて、Ｓｉの添加量は、０．５〜２．０％の範囲で決定する。
【００１３】
Ｍｎ：０．３〜１．０％
Ｍｎもまた脱酸剤であるが、Ｓｉと同様に、部品の強度および焼入れ性の確保にも役立つ。これも前述したように、Ｍｎは炭素拡散の活性化エネルギーを減少させる方向に働くので、多量に添加することはできない。０．３〜１．０％の範囲の添加量を選ぶ。
【００１４】
Ｃｒ：０．３〜２．０％
Ｃｒの役割は、部品の強度を確保するため、焼入れ性を高めることにある。この効果を得るためには、少なくとも０．３％のＣｒの添加を要する。多量になると加工性が低下するから、２．０％以内の添加に止める。
【００１５】
Ａｌ：０．０１〜０．１０％
Ａｌは脱酸剤として添加されるが、結晶粒の粗大化を抑制するはたらきもある。そこで、０．０１％以上を添加するが、過大な添加は、やはり加工性を損なうから、０．１０％までの添加量を選ぶのがよい。
【００１６】
Ｎ：０．００１〜０．０２０％
Nは結晶粒の粗大化を防止する作用があるので、少なくとも０．００１％が存在するようにする。この効果は０．０２０％程度で飽和するので、この限界を超えて存在させる意味はない。
【００１７】
Ｎｉ：０．０５〜３．００％およびＭｏ：０．０５〜１．００％の１種または２種
これらの成分の役割は、焼入れ性を高めて部品の強度を増大させることにある。この効果を得るためには、少なくとも、Ｎｉで０．０５％、Ｍｏでも０．０５％の添加を要する。多量に加えると加工性が低下するから、Ｎｉは３．００％、Ｍｏは１．００％以内の添加に止める。
【００１８】
Ｎｂ：０．０１〜０．１０％、Ｔｉ：０．０１〜０．２０％およびＢ：０．００１〜０．０１０％の１種または２種以上
これらの元素は、いずれも結晶粒の粗大化を抑制する効果を有するから、下限値である、Ｎｂ：０．０１％、Ｔｉ：０．０１％、Ｂ：０．００１％以上の１種または２種以上を添加する。この効果は、上限値である、Ｎｂ：０．１０％、Ｔｉ：０．２０％、Ｂ：０．０１０％あたりで飽和するから、それを超える添加は適切でない。
【００１９】
炭素拡散の活性化エネルギーＱ≦３４０００(kca1)
Ｑ＝３４１４０−６０５［％Ｓｉ］＋１８３［％Ｍｎ］
＋１３６［％Ｃｒ］＋１２２［％Ｍｏ］ 式１
いうまでもなく、炭素の拡散に要する活性化エネルギーが低い方が、浸炭が進みやすいので、処理に要する時間が短縮できる。この効果は、Ｑの値が３４０００を下回ると顕著になる。前述のように、ＳｉはＱを低下させ、Ｍｎ，ＣｒおよびＭｏは上昇させ、それぞれの影響は式１に表されるとおりである。
【００２０】
式２により定義される（％は質量％）硬さ指数Ｙの値：９０ＨＲＢ以下
Ｙ＝５５．６＋２８．６［％Ｃ］＋６．９２［％Ｓｉ］＋１４．９［％
Ｍｎ］＋６．４３［％Ｎｉ］＋５．３５［％Ｃｒ］＋２５．２４［％
Ｍｏ］ 式２
肌焼鋼の加工性は、式２で表される硬さ指数Ｙによって決定される。Ｙの値が９０ＨＲＢ以下であれば、浸炭部品への製造が容易である。
【００２１】
浸炭時間Ｔｃと拡散時間Ｔｄとの比Ｔｃ／Ｔｄ：０．４〜０．７
本発明の肌焼鋼は、プラズマ浸炭法または真空浸炭法により、上記の条件で浸炭を行なうことが好ましい。それにより、表面炭素濃度および表面から内部に向かっての炭素濃度プロファイルが適切なものとなる。
【００２２】
表面炭素濃度は、できるだけ高くして、炭素濃度プロファイルを底上げすることが望まれるが、浸炭製品の表面において粒界炭化物を生成させないことも肝要である。表面から深さ０．１mmまでの平均炭素濃度Ｘの値にして、０．７５〜０．８５（質量％）の範囲が、こうした観点から適切である。
【００２３】
【実施例】
［肌焼鋼の製造］
表１に掲げた合金成分を有する肌焼鋼を溶解し、５０ｋｇのインゴットに鋳造した。各インゴットを、１２５０℃×２時間の加熱の後、鍛造して直径３２mmの丸棒とした。鍛造後は空冷し、焼きならし処理した。
［浸炭試験］
上記の丸棒素材から下記の試験片を採取し、表２に示す条件で浸炭処理した。試験片：直径２５mm、長さ１３０mmの浸炭性試験片
浸炭条件：ＥＣＤ（有効浸炭深さmm）＝０．８となるように設定した。
【００２４】
各肌焼鋼のＱおよびＹの値、浸炭部品の表面硬さおよび芯部硬さ、ＥＣＤ、３５％硬さおよび表面Ｃ濃度（％）を算出または測定し、表２にまとめて示した。
【００２５】
【発明の効果】
本発明に従う特定の合金組成を有し、炭素拡散の活性化エネルギーＱの値を、式１を満たすように一定値以下に抑えた肌焼鋼は、従来の肌焼鋼にくらべて、はるかに短縮された熱処理時間で浸炭処理することが可能である。それによって、浸炭部品の製造コストを低減することができる。特定量のＮｂ、ＴｉおよびＢの１種または２種以上を添加したことにより、本発明の肌焼鋼は結晶粒の粗大化が抑制されるので、靱性の高い浸炭部品を与える。
【００２８】
好ましい実施態様に従って、特定量のＮｉまたはＭｏを添加した合金組成の肌焼鋼は、焼入れ性が高く、部品の強度を増大できるという利益が得られる。式２により定義される硬さ指数Ｙを一定値以下に抑えた肌焼鋼は、浸炭部品への製造性がすぐれている。
【００２９】
このような肌焼鋼を浸炭処理するに適した本発明の浸炭方法は、プラズマ浸炭法または真空浸炭法を採用するとともに、浸炭時間と拡散時間とのバランスを適切に選択することによって、表面炭素濃度および炭素濃度プロファイルが好適な浸炭製品を与える。[0001]
[Industrial application fields]
The present invention relates to case-hardened steel and carburized parts using the steel. Since the case-hardened steel of the present invention can be carburized in a short time, parts for machine structures that are subjected to carburizing by plasma carburizing or vacuum carburizing, such as parts that require fatigue strength, such as automobile transmission gears. The present invention is suitable for the production of
[0002]
[Prior art]
Machine structural parts such as transmission gears are required to have excellent impact resistance and fatigue strength, so they are carburized after being molded into a predetermined part shape by hot and warm forging and cutting. It is common to make parts.
[0003]
On the other hand, in the production of parts, cold working with a high yield and good dimensional accuracy is adopted with the intention of reducing the production cost and improving the working environment. In order to obtain a sufficient carburizing depth for the case-hardened steel, it is necessary to heat the parts to a high temperature and maintain the temperature for a long time in the carburizing process, and this work increases the cost of the carburized parts.
[0004]
Currently, JIS-SCR420 is widely used as case hardening steel. Although this steel has excellent manufacturability to parts, it takes a long time to carburize, and it has been desired to shorten it. Therefore, development of case-hardened steel capable of shortening the carburizing time while obtaining a sufficient carburizing depth is underway. As a representative example, a steel bar having an increased C content is provided. However, since C increases the material hardness and decreases the manufacturability of parts, a high C case-hardened steel is a less advantageous material. is not.
[0005]
[Problems to be solved by the invention]
An object of the present invention is to provide a case-hardened steel that can be carburized in a shortened time, and to provide a method for advantageously producing carburized parts for machine structures having excellent physical properties using the case-hardened steel. There is to do.
[0006]
[Means for Solving the Problems]
The case-hardened steel that can be carburized in a short time of the present invention is in mass%, C: 0.10 to 0.35%, Si: 0.5 to 2.0%, Mn: 0.3 to 1.0%. Cr: 0.3-2.0%, Al: 0.01-0.10% and N: 0.001-0.020%, Nb: 0.01-0.10%, Ti: 0.01 to 0.20% , B: 0.0001 to 0.010%, Ni: 0.05 to 3.00% and Mo: 0.05 to 1.00% , the balance Fe and It is a case-hardened steel that has an alloy composition of inevitable impurities and that can be carburized in a short time, with the activation value Q of carbon diffusion defined by Equation 1 set to 34000 (kca1) or less.
Q = 34140-605 [% Si] +183 [% Mn]
+136 [% Cr] +122 [% Mo] Formula 1
[0007]
The carburized part of the present invention is formed by molding the above case-hardened steel into a part shape, and the ratio Tc / Td between the carburizing time Tc and the diffusion time Td is 0.4 to 0.7 by plasma carburizing or vacuum carburizing. Carburized parts obtained by carburizing under the following conditions.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
In order to achieve a certain amount of carburization in a shortened time, it is necessary to lower the activation energy of carbon diffusion and increase the diffusion constant. From this viewpoint, the inventor adopted the skin. In selecting the alloy composition of the hardened steel, a component that increases the diffusion constant, specifically, Si is increased, while a component that decreases the diffusion constant, specifically, Mn, Cr, and Mo are reduced.
[0010]
The reason why the alloy composition is selected as described above in the case hardening steel of the present invention will be described below.
[0011]
C: 0.10 to 0.35%
C is necessary to secure the strength of the core part of the component, and in order to obtain the core strength required for the component such as the transmission gear described above, the lower limit of 0.10% or more C is necessary. is there. On the other hand, the presence of a large amount of C reduces toughness, which is also a fatal problem for this type of part, so it is limited to the upper limit of 0.35%.
[0012]
Si: 0.5 to 2.0%
Si is added to steel as a deoxidizer, but it also helps to ensure the strength and hardenability of the parts. As described above, Si increases the activation energy of carbon diffusion, so it is desirable to add it actively. However, if the amount is too large, the workability is lowered, which is not preferable as case hardening steel. Considering these balances, the addition amount of Si is determined in the range of 0.5 to 2.0%.
[0013]
Mn: 0.3 to 1.0%
Mn is also a deoxidizer, but it helps to ensure the strength and hardenability of parts as well as Si. As described above, since Mn works in a direction to reduce the activation energy of carbon diffusion, it cannot be added in a large amount. An addition amount in the range of 0.3 to 1.0% is selected.
[0014]
Cr: 0.3-2.0%
The role of Cr is to increase the hardenability in order to ensure the strength of the parts. In order to obtain this effect, it is necessary to add at least 0.3% of Cr. When the amount is too large, the workability is lowered, so the addition is limited to 2.0% or less.
[0015]
Al: 0.01-0.10%
Al is added as a deoxidizer, but it also serves to suppress coarsening of crystal grains. Therefore, 0.01% or more is added, but excessive addition also impairs workability, so it is preferable to select an addition amount of up to 0.10%.
[0016]
N: 0.001 to 0.020%
Since N has an effect of preventing coarsening of crystal grains, at least 0.001% should be present. Since this effect is saturated at about 0.020%, it does not make it exist beyond this limit.
[0017]
One or two of Ni: 0.05 to 3.00% and Mo: 0.05 to 1.00% The role of these components is to increase hardenability and increase the strength of the part. In order to obtain this effect, at least 0.05% of Ni and 0.05% of Mo are required. If a large amount is added, the workability deteriorates, so Ni should be added within 3.00% and Mo within 1.00%.
[0018]
One or more of Nb: 0.01 to 0.10%, Ti: 0.01 to 0.20% and B: 0.001 to 0.010% These elements are all coarse grains Therefore, the lower limit values of Nb: 0.01%, Ti: 0.01%, and B: 0.001% or more are added. This effect is saturated around the upper limit values of Nb: 0.10%, Ti: 0.20%, and B: 0.010%, so addition exceeding this is not appropriate.
[0019]
Activation energy of carbon diffusion Q ≦ 34000 (kca1)
Q = 34140-605 [% Si] +183 [% Mn]
+136 [% Cr] +122 [% Mo] Formula 1
Needless to say, the lower the activation energy required for carbon diffusion, the easier the carburization proceeds, so the time required for the treatment can be shortened. This effect becomes significant when the Q value is less than 34,000. As described above, Si decreases Q and Mn, Cr, and Mo increase, and the influence of each is as expressed in Equation 1.
[0020]
Value of hardness index Y defined by Formula 2 (% is% by mass): 90HRB or less Y = 55.6 + 28.6 [% C] +6.92 [% Si] +14.9 [%
Mn] +6.43 [% Ni] +5.35 [% Cr] +25.24 [%
Mo] Formula 2
The workability of the case-hardened steel is determined by the hardness index Y expressed by Equation 2. If the value of Y is 90 HRB or less, manufacture to carburized parts is easy.
[0021]
Ratio Tc / Td between carburizing time Tc and diffusion time Td: 0.4 to 0.7
The case-hardened steel of the present invention is preferably carburized under the above conditions by plasma carburizing or vacuum carburizing. Thereby, the surface carbon concentration and the carbon concentration profile from the surface toward the inside become appropriate.
[0022]
Although it is desirable to increase the surface carbon concentration as much as possible to raise the carbon concentration profile, it is important not to generate grain boundary carbides on the surface of the carburized product. The range of 0.75 to 0.85 (mass%) as the value of the average carbon concentration X from the surface to a depth of 0.1 mm is appropriate from this viewpoint.
[0023]
【Example】
[Manufacture of case-hardened steel]
Case-hardened steel having the alloy components listed in Table 1 was melted and cast into a 50 kg ingot. Each ingot was forged after heating at 1250 ° C. for 2 hours to form a round bar having a diameter of 32 mm. After forging, it was air-cooled and processed normally.
[Carburization test]
The following specimens were collected from the above round bar material and carburized under the conditions shown in Table 2. Test piece: Carburizing test piece having a diameter of 25 mm and a length of 130 mm Carburizing condition: ECD (effective carburizing depth mm) was set to 0.8.
[0024]
The values of Q and Y of each case-hardened steel, the surface hardness and core hardness, ECD, 35% hardness and surface C concentration (%) of the carburized parts were calculated or measured, and are summarized in Table 2.
[0025]
【The invention's effect】
The case-hardening steel having a specific alloy composition according to the present invention and having the activation energy Q value of carbon diffusion suppressed to a certain value or less so as to satisfy Equation 1 is far more than the conventional case-hardening steel. Carburizing can be performed with a shortened heat treatment time. Thereby, the manufacturing cost of carburized parts can be reduced. By adding one or more of specific amounts of Nb, Ti and B, the case-hardened steel of the present invention suppresses the coarsening of crystal grains, thus giving a carburized part with high toughness.
[0028]
According to a preferred embodiment, the case-hardened steel having an alloy composition to which a specific amount of Ni or Mo is added has the advantage of high hardenability and increased component strength. Case-hardened steel in which the hardness index Y defined by Equation 2 is kept below a certain value has excellent manufacturability to carburized parts.
[0029]
The carburizing method of the present invention suitable for carburizing such case-hardened steel employs a plasma carburizing method or a vacuum carburizing method, and by appropriately selecting a balance between carburizing time and diffusion time, Concentration and carbon concentration profiles provide a suitable carburized product.
Claims (3)
Ｑ＝３４１４０−６０５［％Ｓｉ］＋１８３［％Ｍｎ］
＋１３６［％Ｃｒ］＋１２２［％Ｍｏ］ 式１In mass%, C: 0.10 to 0.35%, Si: 0.5 to 2.0%, Mn: 0.3 to 1.0%, Cr: 0.3 to 2.0%, Al: In addition to 0.01 to 0.10% and N: 0.001 to 0.020% , Nb: 0.01 to 0.10%, Ti: 0.01 to 0.20% , B: 0 0.0001 to 0.010%, Ni: 0.05 to 3.00% and Mo: 0.05 to 1.00% , having an alloy composition consisting of the remainder Fe and inevitable impurities, A case-hardening steel that can be carburized in a short time and has a defined carbon diffusion activation energy Q of 34000 (kca1) or less.
Q = 34140-605 [% Si] +183 [% Mn]
+136 [% Cr] +122 [% Mo] Formula 1
Ｙ＝５５．６＋２８．６［％Ｃ］＋６．９２［％Ｓｉ］＋１４．９［％Ｍｎ］
＋６．４３［％Ｎｉ］＋５．３５［％Ｃｒ］＋２５．２４［％Ｍｏ］ 式２The case-hardened steel according to claim 1, which is excellent in manufacturability to carburized parts, wherein the value of hardness index Y defined by Formula 2 (% is% by mass) is 90 HRB or less.
Y = 55.6 + 28.6 [% C] +6.92 [% Si] +14.9 [% Mn]
+6.43 [% Ni] +5.35 [% Cr] +25.24 [% Mo] Equation 2
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| WO2013046678A1 (en) | 2011-09-30 | 2013-04-04 | Ｊｆｅスチール株式会社 | Ingot for bearing and production process |
| US9139887B2 (en) | 2010-08-31 | 2015-09-22 | Jfe Steel Corporation | Bearing steel and ingot material for bearing having excellent rolling contact fatigue life characteristics and method for manufacturing the same |
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| JP5121123B2 (en) * | 2005-03-14 | 2013-01-16 | 山陽特殊製鋼株式会社 | High-temperature carburizing steel with excellent grain resistance and its manufacturing method, and high-temperature carburizing shaped product and its carburizing and quenching method |
| JP5530763B2 (en) | 2009-05-13 | 2014-06-25 | 新日鐵住金株式会社 | Carburized steel parts with excellent low cycle bending fatigue strength |
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Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
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| US9139887B2 (en) | 2010-08-31 | 2015-09-22 | Jfe Steel Corporation | Bearing steel and ingot material for bearing having excellent rolling contact fatigue life characteristics and method for manufacturing the same |
| WO2013046678A1 (en) | 2011-09-30 | 2013-04-04 | Ｊｆｅスチール株式会社 | Ingot for bearing and production process |
| KR20140073506A (en) | 2011-09-30 | 2014-06-16 | 제이에프이 스틸 가부시키가이샤 | Ingot for bearing and production process |
| KR20190049908A (en) | 2011-09-30 | 2019-05-09 | 제이에프이 스틸 가부시키가이샤 | Ingot for bearing and production process |
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