JP5098486B2 - Manufacturing method of carburized parts - Google Patents
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本発明は、浸炭時の粒径の粗大化や、粗大粒の発生を抑制した、粒径粗大化防止特性に優れた浸炭部品用鋼を素材とする浸炭部品の製造方法に関するものである。
The present invention relates to a method for manufacturing a carburized part made of steel for carburized parts, which has excellent particle size coarsening prevention characteristics and suppresses the coarsening of the grain size during carburization and the generation of coarse grains.
歯車、軸受部品、転動部品、シャフト、等速ジョイント部品は、通常、例えばＪＩＳ Ｇ ４０５２、ＪＩＳ Ｇ ４１０４、ＪＩＳ Ｇ ４１０５、ＪＩＳ Ｇ ４１０６などに規定されている中炭素の機械構造用合金鋼を使用し、熱間鍛造、冷間鍛造、転造、切削により所定の形状に加工された後、浸炭処理などの熱処理を施して製造される。 Gears, bearing parts, rolling parts, shafts, constant velocity joint parts are usually made of medium carbon alloy steel for machine structure as defined in JIS G 4052, JIS G 4104, JIS G 4105, JIS G 4106, etc. After being used and processed into a predetermined shape by hot forging, cold forging, rolling, and cutting, it is manufactured by performing heat treatment such as carburizing treatment.
冷間鍛造は、熱間鍛造と比較して、製造コストが低く、また、製品の表面肌、寸法精度も良好である。そのため、熱間鍛造で製造されていた部品のうち、冷間鍛造後、浸炭して製造される浸炭部品が増加している。鋼材を成形後、特に冷間鍛造後、浸炭して浸炭部品を製造する際の大きな課題として、熱処理歪みの低減が挙げられる。 Cold forging is lower in production cost than hot forging, and also has good surface texture and dimensional accuracy of the product. For this reason, among the parts manufactured by hot forging, carburized parts manufactured by carburizing after cold forging are increasing. A major problem when carburized parts are manufactured by carburizing after forming a steel material, particularly after cold forging, is to reduce heat treatment strain.
熱処理歪みは、成形後、例えば冷間鍛造を施した後、熱処理時に生じる部品の変形の総称である。例えば、冷間鍛造で成形したシャフトを浸炭処理する場合、高温に加熱して焼入れを行った際に偏心すると、シャフトとしての機能が損なわれてしまう。また、歯車や等速ジョイント部品では、浸炭処理による熱処理歪みが大きければ、騒音や振動の原因となる。 Heat treatment distortion is a general term for deformation of parts that occurs during heat treatment after forming, for example, cold forging. For example, when carburizing a shaft formed by cold forging, if it is eccentric when heated to a high temperature and quenched, the function as the shaft is impaired. Further, in gears and constant velocity joint parts, if the heat treatment distortion due to the carburizing process is large, it may cause noise and vibration.
このような浸炭処理における熱処理歪みの最大の原因は、加熱時に発生する粗大粒である。そのため、従来は、例えば、鋼材を冷間鍛造した後、浸炭処理を施す前に焼鈍を行い、粗大粒の発生を抑制していた。しかし、近年、製造コストの削減を目的として、焼鈍が省略されるようになり、浸炭時に粗大粒を生じない鋼材が要求されている。 The largest cause of heat treatment distortion in such a carburizing process is coarse particles generated during heating. Therefore, conventionally, for example, after cold forging a steel material, annealing was performed before carburizing treatment to suppress the generation of coarse particles. However, in recent years, annealing has been omitted for the purpose of reducing manufacturing costs, and a steel material that does not generate coarse grains during carburization is required.
また、歯車、軸受部品、転動部品のなどの高面圧が負荷される部品は、耐摩耗性を向上させるために浸炭層を深くすることが必要であり、高深度浸炭が行われるようになっている。しかし、通常の浸炭温度である９３０℃程度で高深度浸炭を行うと、十数時間から数十時間という長時間を要する。浸炭時間を短縮するには、浸炭温度の高温化が有効であるが、１０００℃以上の温度域で、いわゆる高温浸炭を行った場合、部品が変形して必要な疲労特性、転動疲労特性等が得られなくなるという問題がある。この原因も、浸炭処理を高温で行った際に発生する粗大粒である。 Also, parts that are subjected to high surface pressure such as gears, bearing parts, and rolling parts need to have a deep carburized layer in order to improve wear resistance, so that deep carburization is performed. It has become. However, if deep carburization is performed at a normal carburizing temperature of about 930 ° C., it takes a long time of several tens of hours to several tens of hours. To shorten the carburizing time, it is effective to increase the carburizing temperature. However, if so-called high-temperature carburizing is performed in the temperature range of 1000 ° C or higher, the parts are deformed and necessary fatigue characteristics, rolling fatigue characteristics, etc. There is a problem that cannot be obtained. This is also due to the coarse particles generated when the carburizing process is performed at a high temperature.
以上のことから、冷間鍛造後に浸炭処理を行う場合や、高温浸炭を行う場合でも粗大粒が発生しない浸炭部品用鋼、浸炭部品の製造方法が要求されている。このような問題に対して、微細なＭｎＳを分散させて、粒成長を抑制する、いわゆるピンニング効果を発現するピン止め粒子として活用し、粗大粒の生成を防止した肌焼鋼が提案されている（例えば、特許文献１〜３）。
In view of the above, there is a need for a carburized part steel and a carburized part manufacturing method that does not generate coarse grains even when carburizing is performed after cold forging or when high temperature carburizing is performed. For such a problem, a case-hardened steel is proposed in which fine MnS is dispersed to suppress grain growth, and is used as pinning particles that express a so-called pinning effect, thereby preventing the formation of coarse grains. (For example,
しかしながら、この肌焼鋼は、快削性を向上させるためにＳ量を多く含有させると、連続鋳造により鋼片を製造する際に表面疵を生じることがあった。また、浸炭温度の高温化や、靭性の向上などのために、更に優れた粗大粒防止特性が要求されている。 However, when this case-hardened steel contains a large amount of S in order to improve free-cutting properties, surface flaws may be produced when steel pieces are produced by continuous casting. Further, in order to increase the carburizing temperature and improve toughness, further excellent coarse grain prevention characteristics are required.
本発明は、連続鋳造により鋼片を製造する際に表面疵を生じることがなく、浸炭時の粒径の粗大化、粗大粒の発生を抑制した、粒径粗大化防止特性に優れた浸炭部品用鋼を素材とし、靭性など特性に優れた浸炭部品の製造方法の提供を目的とするものである。
The present invention is a carburized part that has excellent particle size prevention properties without producing surface flaws when producing steel slabs by continuous casting, and suppressing the coarsening of the particle size and the generation of coarse particles during carburizing. The purpose is to provide a method for manufacturing carburized parts made of steel for steel and having excellent properties such as toughness.
本発明は、微細析出させたＭｎを主体とする硫化物の生成が、粗大粒防止特性の向上に有効であり、また、連続鋳造時の表面疵の発生の抑制や、靭性の向上には、ＭｎとＳのモル比、（Ｍｎ／５５）／（Ｓ／３２）を適正な範囲に制御することが有効であるという知見に基づいてなされたものであり、その要旨は、以下のとおりである。 In the present invention, the generation of sulfides mainly composed of finely precipitated Mn is effective in improving the coarse grain prevention characteristics, and the suppression of the occurrence of surface defects during continuous casting, and the improvement of toughness, This is based on the knowledge that it is effective to control the molar ratio of Mn and S, (Mn / 55) / (S / 32), to the proper range. .
（１） 質量％で、
Ｃ ：０．１０〜０．４０％、
Ｓｉ：０．０５〜１．００％、
Ｍｎ：０．０５〜０．４０％、
Ｓ ：０．００５〜０．１００％
を含有し、
Ｐ ：０．０２５％以下、
Ａｌ：０．０５％以下、
Ｎ ：０．０２０％以下
に制限し、残部がＦｅ及び不可避的不純物からなり、Ｍｎの含有量とＳの含有量とが、
（Ｍｎ／５５）／（Ｓ／３２）：６．１〜１１．６
を満足する浸炭部品用鋼を、１１５０〜１３００℃の範囲内で加熱し、熱間加工を施して冷却し、熱間加工の加熱温度におけるＭｎＳの平衡モル分率と、浸炭処理温度における平衡ＭｎＳのモル分率との差が０．０１００ｍｏｌ％以上となるように、９５０℃超から１１３０℃以下までの温度域で浸炭処理を行うことを特徴とする浸炭部品の製造方法。
（２） 前記浸炭部品用鋼が、１２２５℃におけるＭｎＳの平衡モル分率と、１０１０℃におけるＭｎＳの平衡モル分率との差が、０．０１００ｍｏｌ％以上であることを特徴とする請求項１に記載の浸炭部品の製造方法。
（３） 前記浸炭部品用鋼が、質量％で、さらに、
Ｖ ：０．０５〜０．２０％、
Ｔｉ：０．００５〜０．２００％、
Ｎｂ：０．０１〜０．０６％
のうち１種又は２種以上を含有することを特徴とする上記（１）または（２）に記載の浸炭部品の製造方法。
（４） 前記浸炭部品用鋼が、質量％で、さらに、
Ｃｒ：３．０％以下、
Ｍｏ：０．５０％以下、
Ｎｉ：１．０％以下、
Ｃｕ：３．０％以下、
Ｂ ：０．００２０％以下
のうち１種又は２種以上を含有することを特徴とする上記（１）〜（３）のいずれか１項に記載の浸炭部品の製造方法。
( 1 ) In mass%,
C: 0.10 to 0.40%,
Si: 0.05-1.00%,
Mn: 0.05 to 0.40%,
S: 0.005 to 0.100%
Containing
P: 0.025% or less,
Al: 0.05% or less,
N: 0.020% or less
The balance consists of Fe and inevitable impurities, the content of Mn and the content of S,
(Mn / 55) / (S / 32): 6.1 to 11.6
The steel for carburized parts satisfying the above conditions is heated within a range of 1150 to 1300 ° C., cooled by hot working, and the equilibrium molar fraction of MnS at the heating temperature of hot working and the equilibrium MnS at the carburizing temperature. A method for manufacturing a carburized part, characterized in that carburizing treatment is performed in a temperature range from more than 950 ° C. to 1130 ° C. or less so that the difference from the mole fraction of the above becomes 0.0100 mol% or more.
(2) The carburized part steel has a difference between the equilibrium molar fraction of MnS at 1225 ° C and the equilibrium molar fraction of MnS at 1010 ° C being 0.0100 mol% or more. The manufacturing method of carburized components as described in 2.
(3) The steel for carburized parts is in mass%,
V: 0.05-0.20%,
Ti: 0.005 to 0.200%,
Nb: 0.01 to 0.06%
1 or 2 types or more are contained, The manufacturing method of the carburized component as described in said (1) or (2) characterized by the above-mentioned.
(4) The steel for carburized parts is in mass%,
Cr: 3.0% or less,
Mo: 0.50% or less,
Ni: 1.0% or less,
Cu: 3.0% or less,
B: 0.0020% or less
1 or 2 types or more are contained, The manufacturing method of the carburized components of any one of said (1)-(3) characterized by the above-mentioned.
（５） 熱間加工後、冷間加工によって成形し、浸炭処理を行うことを特徴とする上記（１）〜（４）のいずれか１項に記載の浸炭部品の製造方法。
( 5 ) The method for producing a carburized part according to any one of (1) to (4) , wherein the carburizing treatment is performed by cold working after hot working.
（６） 熱間加工後、球状化焼鈍を施して冷間加工によって成形し、浸炭処理を行うことを特徴とする上記（１）〜（４）のいずれか１項に記載の浸炭部品の製造方法。
( 6 ) Manufacture of carburized parts according to any one of the above (1) to (4), characterized by performing spheroidizing annealing after hot working, forming by cold working, and carburizing treatment. Method.
（７） 熱間加工によって成形し、浸炭処理を行うことを特徴とする上記（１）〜（４）のいずれか１項に記載の浸炭部品の製造方法。
( 7 ) The method for producing a carburized part according to any one of (1) to (4) , wherein the carburized treatment is performed by hot working.
本発明によれば、連続鋳造により鋼片を製造する際に表面疵を生じることがない、浸炭時の粒径の粗大化、粗大粒の発生を抑制した、粒径粗大化防止特性に優れた浸炭部品用鋼、更に、その浸炭部品用鋼を素材とし、靭性など特性に優れた浸炭部品の製造方法の提供が可能になり、鋼の合金コストの削減による低コスト化、浸炭温度の高温化による浸炭処理の効率化など、産業上の貢献が極めて顕著である。 According to the present invention, when producing steel slabs by continuous casting, surface flaws are not generated, the particle size is coarsened during carburizing, and the generation of coarse particles is suppressed, and the particle size coarsening prevention property is excellent. It is possible to provide carburized parts steel, and also to manufacture carburized parts with excellent properties such as toughness using the carburized parts steel as a raw material, lowering the cost by reducing the alloy cost of the steel, raising the carburizing temperature. Industrial contributions such as increasing the efficiency of carburizing treatment by LNG are extremely remarkable.
本発明者は、浸炭部品用鋼を浸炭処理する際に、結晶粒の粗大化の防止に作用するピン止め粒子として、ＭｎＳを利用する方法を検討した。その結果、次のような知見を得た。
（ａ）ピン止め粒子として有効に作用するＭｎＳは、熱間圧延、熱間鍛造などの熱間加工の加熱時に溶解して鋼中に固溶したＭｎ及びＳが、熱間加工後、浸炭処理時に再析出したものである。
（ｂ）一方、鋼中に液相で晶出した粗大なＭｎＳや、鋳造時に固相で析出したＭｎＳのうち、加熱時に溶解せずに残存したものは粗大であり、ピン止め粒子としては有効ではない。
The present inventor examined a method of using MnS as pinning particles that act to prevent coarsening of crystal grains when carburizing steel for carburized parts. As a result, the following knowledge was obtained.
(A) MnS that effectively acts as pinning particles is melted during hot working such as hot rolling and hot forging, and Mn and S dissolved in the steel are carburized after hot working. Sometimes re-deposited.
(B) On the other hand, coarse MnS crystallized in the liquid phase in steel and MnS precipitated in the solid phase during casting remain undissolved during heating and are effective as pinning particles. is not.
図１は、浸炭処理時のピン止め粒子として作用するＭｎＳの析出挙動を模式的に示したものであり、横軸は温度、縦軸はＭｎＳのモル分率である。図１の平衡析出曲線１よりも下の領域ではＭｎＳが生成し、鋼の融点２以上ではＭｎＳは晶出し、融点２未満ではＭｎＳは析出する。即ち、図１の融点２と平衡曲線１の交点以下のＭｎＳモル分率は晶出ＭｎＳ量である。また、領域５は、凝固後、浸炭処理までの工程を行った後、鋼中に析出しているＭｎＳの合計量（析出ＭｎＳ量）である。
FIG. 1 schematically shows the precipitation behavior of MnS acting as pinning particles during carburizing treatment, with the horizontal axis representing temperature and the vertical axis representing the molar fraction of MnS. In the region below the
鋳造後、熱間加工を行う際、例えば熱間圧延時に、図１の熱間加工加熱温度３に加熱されると、熱間加工加熱温度３と平衡析出曲線１の交点と、鋼の融点２と平衡析出曲線１の交点の間のＭｎＳが析出し、熱間加工加熱温度３と平衡析出曲線１の交点よりも過剰なＭｎＳは固溶する。その後、熱間加工を行い、冷却し、更に、浸炭温度４に加熱すると、浸炭温度４と平衡析出曲線１の交点と、熱間加工加熱温度３と平衡析出曲線１の交点との間のＭｎＳ、即ち、再析出するＭｎＳのモル分率６に相当する量のＭｎＳ（再析出ＭｎＳ量）が微細に分散する。この再析出するＭｎＳは、浸炭時にピン止め粒子として作用する微細なＭｎＳである。
When performing hot working after casting, for example, during hot rolling, when heated to hot working
鋼中に再析出したＭｎＳは、高倍率の電子顕微鏡を使用すれば、観察することができる。しかし、再析出したＭｎＳは極めて微細であり、粒径、密度を、高倍率の電子顕微鏡を用いた直接的なミクロ観察によって精度良く評価し、信頼性を確保するには、多大なデータの蓄積が要求される。 MnS re-precipitated in steel can be observed by using a high magnification electron microscope. However, the re-deposited MnS is extremely fine, and a large amount of data is needed to accurately evaluate the particle size and density by direct micro observation using a high-magnification electron microscope and to ensure reliability. Is required.
そのため、簡易な代替方法として、熱力学平衡計算によって、再析出ＭｎＳ量を推定することができる。具体的には、熱力学平衡計算ソフトウェアであるサーモカルク（登録商標）により、熱力学データベースＴＣＦＥ２を用いて、所定の成分を入力し、鉄の相、即ち、面心立方晶（ＦＣＣ）、体心立方晶（ＢＣＣ）、セメンタイト（Ｃｅｍｅｎｔｉｔｅ）、液相（Ｌｉｑｕｉｄ）に加えて、ＭｎＳ相を選択して浸炭処理温度及び熱間加工加熱温度での平衡計算を実施する方法である。この方法によれば、浸炭処理温度及び熱間加工加熱温度でのＭｎＳの平衡モル分率をそれぞれ計算し、熱間加工の加熱温度でのＭｎＳ平衡モル分率と浸炭処理温度でのＭｎＳの平衡モル分率との差として再析出ＭｎＳ量を求めることができる。 Therefore, as a simple alternative method, the amount of reprecipitated MnS can be estimated by thermodynamic equilibrium calculation. Specifically, a thermodynamic database TCFE2 is used to input predetermined components by thermocalculus (registered trademark) thermodynamic equilibrium calculation software, and an iron phase, that is, face-centered cubic (FCC), body center In this method, an MnS phase is selected in addition to cubic (BCC), cementite (Centite), and liquid phase (Liquid), and equilibrium calculation is performed at the carburizing temperature and hot working heating temperature. According to this method, the equilibrium mole fraction of MnS at the carburizing temperature and the hot working heating temperature is calculated, respectively, and the MnS equilibrium mole fraction at the hot working heating temperature and the MnS equilibrium at the carburizing temperature are calculated. The amount of reprecipitated MnS can be determined as the difference from the molar fraction.
図２に、Ｍｎ量、Ｍｎ／Ｓモル比率と、熱力学平衡計算によって評価した再析出ＭｎＳ量との関係を示す。図２の縦軸の再析出ＭｎＳ量は、成分組成と、仮想の熱間加工の加熱温度及び浸炭処理温度から平衡熱力学計算によって求めた、熱間加工の加熱温度におけるＭｎＳの平衡モル分率と浸炭処理温度におけるＭｎＳの平衡モル分率の差である。図２の横軸は、ＭｎとＳの量の比をモル比率で示したものであるが、これはＭｎとＳの含有量により、（Ｍｎ／５５）／（Ｓ／３２）として求めることができる。 FIG. 2 shows the relationship between the amount of Mn, the Mn / S molar ratio, and the amount of reprecipitated MnS evaluated by thermodynamic equilibrium calculation. The amount of re-deposited MnS on the vertical axis in FIG. 2 is the equilibrium molar fraction of MnS at the hot working heating temperature, which is obtained by equilibrium thermodynamic calculation from the component composition, the virtual hot working heating temperature and the carburizing temperature. And the difference in the equilibrium molar fraction of MnS at the carburizing temperature. The horizontal axis of FIG. 2 shows the ratio of the amount of Mn and S in terms of a molar ratio, which can be obtained as (Mn / 55) / (S / 32) based on the contents of Mn and S. it can.
なお、熱力学平衡計算は、仮想の熱間加工の加熱温度を代表的な温度である１２２５℃とし、仮想の浸炭処理温度を通常よりも高温である１０１０℃として行った。また、成分組成は、質量％で、Ｃ：０．１０〜０．４０％、Ｓｉ：０．０５〜１．００％、Ｍｎ：０．０５〜０．４０％、Ｓ：０．００５〜０．１００％、Ｐ：０．０２５％以下、Ａｌ：０．０５％以下、Ｎ：０．０２０％以下の範囲とした。 In the thermodynamic equilibrium calculation, the heating temperature of the virtual hot working was set to 1225 ° C., which is a typical temperature, and the virtual carburizing temperature was set to 1010 ° C., which is higher than usual. Moreover, a component composition is the mass%, C: 0.10-0.40%, Si: 0.05-1.00%, Mn: 0.05-0.40%, S: 0.005-0 100%, P: 0.025% or less, Al: 0.05% or less, N: 0.020% or less.
図２に示したように、再析出ＭｎＳ量は、Ｍｎ／Ｓモル比率及びＭｎ量の増加とともに減少する。これは、Ｍｎ／Ｓモル比率及びＭｎ量が多いと、ＭｎＳが安定になり、熱間加工の加熱時にＭｎＳが固溶し難くなるためである。図２に示したように再析出ＭｎＳ量を確保するには、Ｍｎ量の上限を０．４％以下とし、更に、Ｍｎ／Ｓモル比率の上限を１５．０以下にすることが必要である。 As shown in FIG. 2, the amount of reprecipitated MnS decreases as the Mn / S molar ratio and the amount of Mn increase. This is because when the Mn / S molar ratio and the amount of Mn are large, MnS becomes stable, and MnS hardly dissolves during heating in hot working. As shown in FIG. 2, in order to secure the amount of re-precipitated MnS, it is necessary to set the upper limit of the Mn amount to 0.4% or less and further to set the upper limit of the Mn / S molar ratio to 15.0 or less. .
更に、本発明者は、再析出ＭｎＳ量の確保による浸炭処理時の粒粗大化抑制効果について検討を行った。まず、種々の成分組成を有する鋼を溶製し、円柱形状の鋼片とし、１２２５℃に加熱し、熱間鍛造を施し、円柱形状に成形した。なお、熱間鍛造は９００〜１０００℃の範囲内で鍛錬比を５０％として行った。また、円柱形状の鋼片を１２２５℃に加熱し、熱間圧延後、冷却し、一部の鋼材には球状化処理を施し、残りの鋼材はそのまま、室温で鍛錬比を５０％として冷間鍛造を行い、円柱形状に成形した。ここで、鍛錬比は、成形前の鋼材の高さと成形後の部材の高さとの差を成形前の鋼材の高さで除した値を、百分率として表した数値である。 Furthermore, this inventor examined the grain coarsening inhibitory effect at the time of the carburizing process by ensuring the amount of reprecipitation MnS. First, steel having various component compositions was melted to form a cylindrical steel piece, heated to 1225 ° C., hot forged, and formed into a cylindrical shape. The hot forging was performed at a forging ratio of 50% within a range of 900 to 1000 ° C. In addition, a columnar steel slab is heated to 1225 ° C., cooled after hot rolling, and a part of the steel is subjected to spheroidization treatment, and the remaining steel is left as it is at a room temperature of 50%. Forging was performed to form a cylindrical shape. Here, the forging ratio is a numerical value expressed as a percentage obtained by dividing the difference between the height of the steel material before forming and the height of the member after forming by the height of the steel material before forming.
次に、熱間鍛造及び冷間鍛造によって得られた円柱形状の部材を、浸炭処理を想定して９００〜１１００℃に加熱し、急冷した。その後、円柱長手方向の断面を観察面として、試料を採取し、ＪＩＳ Ｇ ０５５１に準拠して、旧オーステナイト結晶粒界を現出させた。光学顕微鏡により、部材の特に中心部と角部を、５０〜２００倍で観察し、最大のオーステナイト粒径を測定し、粗大粒が観察される温度を限界浸炭処理温度とした。また、成分組成から、熱間加工の代表的な加熱温度である１２２５℃と、通常よりも高温の浸炭温度である１０１０℃におけるＭｎＳの平衡モル分率を平衡熱力学計算によって求めた。 Next, the cylindrical member obtained by hot forging and cold forging was heated to 900 to 1100 ° C. and rapidly cooled assuming carburizing treatment. Then, the sample was extract | collected by making the cross section of a cylindrical longitudinal direction into an observation surface, and the former austenite grain boundary was made to appear based on JISG0551. The center and corners of the member were observed with an optical microscope at 50 to 200 times, the maximum austenite grain size was measured, and the temperature at which coarse grains were observed was defined as the limit carburizing temperature. Further, from the component composition, the equilibrium molar fraction of MnS at 1225 ° C., which is a typical heating temperature for hot working, and 1010 ° C., which is a carburizing temperature higher than usual, was determined by equilibrium thermodynamic calculation.
図３は、１２２５℃におけるＭｎＳの平衡モル分率と、１０１０℃におけるＭｎＳの平衡モル分率との差、即ち、析出ＭｎＳ量に対して、限界浸炭温度を示したものである。図３において、○は熱間圧延後、更に冷間鍛造したもの、△は熱間鍛造まま、□は熱間圧延後、球状化処理を施して冷間鍛造したものである。図３に示したように、１２２５℃におけるＭｎＳモル分率と、１０１０℃におけるＭｎＳモル分率の差が、０．０１００％以上になると、限界浸炭温度が１０００℃以上になる。したがって、１２２５℃におけるＭｎＳモル分率と、１０１０℃におけるＭｎＳモル分率の差を、０．０１００％以上にすることが好ましい。 FIG. 3 shows the limit carburizing temperature with respect to the difference between the equilibrium molar fraction of MnS at 1225 ° C. and the equilibrium molar fraction of MnS at 1010 ° C., that is, the amount of precipitated MnS. In FIG. 3, ◯ is after hot rolling and further cold forging, Δ is hot forging, and □ is hot forging and cold forging after hot rolling. As shown in FIG. 3, when the difference between the MnS mole fraction at 1225 ° C. and the MnS mole fraction at 1010 ° C. is 0.0100% or more, the critical carburizing temperature is 1000 ° C. or more. Therefore, the difference between the MnS molar fraction at 1225 ° C. and the MnS molar fraction at 1010 ° C. is preferably 0.0100% or more.
更に、本発明の浸炭部品用鋼を素材として、浸炭部品を製造する場合、素材の製造における熱間加工の加熱温度と、浸炭処理の温度の条件を、ＭｎＳのモル分率に応じて決定することが好ましい。これは、以下の検討によって得られた知見に基づくものである。 Furthermore, when manufacturing a carburized part using the steel for carburized parts of the present invention as a raw material, the heating temperature for hot working in the manufacture of the raw material and the temperature conditions for the carburizing treatment are determined according to the molar fraction of MnS. It is preferable. This is based on the knowledge obtained by the following examination.
質量％で、Ｃ：０．１０〜０．４０％、Ｓｉ：０．０５〜１．００％、Ｍｎ：０．０５〜０．４０％、Ｓ：０．００５〜０．１００％を含有し、Ｐ：０．０２５％以下、Ａｌ：０．０５％以下、Ｎ：０．０２０％以下に制限し、残部がＦｅ及び不可避的不純物からなる、種々の成分組成を有する鋼を溶製し、加熱温度を変化させて熱間鍛造を行った。また、同様の成分組成を有する鋼材に、加熱温度を変化させて熱間圧延を施して冷却し、一部の鋼材には、必要に応じて球状化処理を施した後、冷間鍛造を行った。熱間鍛造は９００〜１０００℃の範囲内で鍛錬比を５０％として行い、冷間鍛造は室温で５０％の鍛錬比で行った。得られた部材に、浸炭処理を想定し、９００〜１１００℃に加熱して急冷する熱処理を施した。 In mass%, C: 0.10 to 0.40%, Si: 0.05 to 1.00%, Mn: 0.05 to 0.40%, S: 0.005 to 0.100% , P: 0.025% or less, Al: 0.05% or less, N: 0.020% or less, the balance is composed of Fe and inevitable impurities, steel having various composition, Hot forging was performed while changing the heating temperature. In addition, steel materials having the same component composition are subjected to hot rolling by changing the heating temperature and cooled, and some steel materials are subjected to spheroidizing treatment as necessary, and then cold forging is performed. It was. Hot forging was performed at a forging ratio of 50% within the range of 900 to 1000 ° C., and cold forging was performed at a forging ratio of 50% at room temperature. The obtained member was subjected to a heat treatment that was rapidly cooled by heating to 900 to 1100 ° C. assuming carburization.
熱処理後、光学顕微鏡を用いて旧オーステナイト粒径を測定し、限界浸炭処理温度を求めた。また、成分組成から、熱間圧延又は熱間鍛造の加熱温度と、限界浸炭処理温度におけるＭｎＳの平衡モル分率を平衡熱力学計算によって求めた。 After the heat treatment, the prior austenite grain size was measured using an optical microscope, and the critical carburizing temperature was determined. Further, from the component composition, the heating temperature of hot rolling or hot forging and the equilibrium molar fraction of MnS at the critical carburizing temperature were determined by equilibrium thermodynamic calculation.
図４は、熱間加工加熱温度におけるＭｎＳの平衡モル分率と、限界浸炭処理温度におけるＭｎＳの平衡モル分率の差に対して、限界浸炭温度を示したものであり、●は熱間圧延後、更に冷間鍛造したもの、▲は熱間鍛造まま、■は熱間圧延後、球状化処理を施して冷間鍛造したものである。図４に示したように、熱間加工加熱温度におけるＭｎＳの平衡モル分率と、浸炭処理温度におけるＭｎＳの平衡モル分率の差が、０．０１００％以上になると、限界浸炭温度が１０００℃以上になることがわかる。 FIG. 4 shows the critical carburizing temperature with respect to the difference between the equilibrium molar fraction of MnS at the hot working heating temperature and the equilibrium molar fraction of MnS at the critical carburizing temperature, and ● indicates hot rolling. After cold forging, ▲ indicates hot forging, and ■ indicates hot forging and spheroidizing after cold rolling. As shown in FIG. 4, when the difference between the equilibrium molar fraction of MnS at the hot working heating temperature and the equilibrium molar fraction of MnS at the carburizing temperature is 0.0100% or more, the limit carburizing temperature is 1000 ° C. It turns out that it becomes the above.
以下、本発明について詳細に説明する。 Hereinafter, the present invention will be described in detail.
まず、本発明において、圧延に供する鋼片の化学成分を限定した理由について説明する。なお、％は、質量％を意味する。 First, the reason why the chemical composition of the steel slab used for rolling is limited in the present invention will be described. In addition,% means the mass%.
Ｃは、鋼の強度を上昇させる元素であり、浸炭部品に要求される引張強さを確保するため、０．１０％以上を添加することが必要である。一方、Ｃを過剰に添加すると硬さが上昇し、０．４０％を超えると加工性が劣化し、浸炭部品の靭性が劣化する。したがって、Ｃの含有量は、０．１０〜０．４０％の範囲内にすることが必要である。 C is an element that increases the strength of steel, and in order to ensure the tensile strength required for carburized parts, it is necessary to add 0.10% or more. On the other hand, if C is added excessively, the hardness increases, and if it exceeds 0.40%, the workability deteriorates and the toughness of the carburized parts deteriorates. Therefore, the C content needs to be in the range of 0.10 to 0.40%.
Ｓｉは、鋼の脱酸剤として使用される元素であり、本発明では、鋼の強度の上昇、焼入れ性の向上、更には焼戻し軟化抵抗の向上に有効な元素である。本発明の浸炭部品用鋼に要求される特性を得るには、Ｓｉを０．０５％以上添加することが必要である。また、Ｓｉは粒界強化の増加に有効な元素であり、更に、例えば、軸受部品、転動部品においては、転動疲労過程での組織変化、材質劣化の抑制による高寿命化に有効な元素である。この効果を有効に活用するには、Ｓｉ量の下限を０．２０％以上にすることが好ましい。 Si is an element used as a deoxidizer for steel. In the present invention, Si is an element effective for increasing the strength of steel, improving hardenability, and further improving resistance to temper softening. In order to obtain the characteristics required for the steel for carburized parts of the present invention, it is necessary to add Si by 0.05% or more. In addition, Si is an element effective for increasing grain boundary strengthening. Further, for example, in bearing parts and rolling parts, it is an element effective for extending the life by suppressing structural changes and material deterioration during the rolling fatigue process. It is. In order to effectively utilize this effect, the lower limit of the Si amount is preferably set to 0.20% or more.
一方、Ｓｉ量が１．００％を超えると硬さの上昇を招き、加工性が劣化するため、上限を１．００％とすることが必要である。また、Ｓｉは、浸炭時の内部酸化を助長する元素であり、内部酸化によって生成したＳｉの酸化物が疲労き裂の起点となることがあるため、上限を０．５０％以下にすることが好ましい。 On the other hand, if the amount of Si exceeds 1.00%, the hardness increases and the workability deteriorates, so the upper limit needs to be 1.00%. Si is an element that promotes internal oxidation during carburization, and the oxide of Si generated by internal oxidation may become the starting point of fatigue cracks, so the upper limit should be made 0.50% or less. preferable.
Ｍｎは、微細に析出するＭｎＳによる結晶粒のピン止め効果を得るために、本発明において極めて重要な元素である。本発明者は、Ｍｎ量を低減させると、ＭｎＳが分解して鋼中に固溶する温度、即ちＭｎＳの溶体化温度が低下することを見出した。熱間加工の加熱時にＭｎＳの固溶を促進させ、微細なＭｎＳを析出させるためには、上限を０．４０％以下とすることが必要である。また、Ｍｎは、ＭｎＳの生成挙動に大きく影響する元素であり、溶鋼中でのＭｎＳの晶出による粗大なＭｎＳの残存に起因する靭性の低下を防止するためには、上限を０．２５％以下とすることが好ましい。一方、Ｍｎは、ＭｎＳを生成してＳを固定する元素であり、ＭｎＳの生成量を確保し、鋳造疵の発生を抑制するには、０．０５％以上添加することが必要である。 Mn is an extremely important element in the present invention in order to obtain a crystal grain pinning effect due to finely precipitated MnS. The present inventor has found that when the amount of Mn is reduced, the temperature at which MnS decomposes and forms a solid solution in the steel, that is, the solution temperature of MnS decreases. In order to promote the solid solution of MnS and to precipitate fine MnS during heating in the hot working, the upper limit needs to be 0.40% or less. Further, Mn is an element that greatly affects the formation behavior of MnS. In order to prevent a decrease in toughness due to coarse MnS remaining due to crystallization of MnS in molten steel, the upper limit is 0.25%. The following is preferable. On the other hand, Mn is an element that generates MnS and fixes S, and it is necessary to add 0.05% or more in order to secure the amount of MnS generated and suppress the occurrence of cast iron.
Ｓも、ＭｎＳの析出に関与し、本発明において極めて重要な元素である。本発明者は、ＭｎＳの析出量がＭｎ及びＳの濃度に依存するものの、Ｓは、Ｍｎとは異なり、ＭｎＳの溶体化温度には殆ど影響を及ぼさないことを見出した。したがって、結晶粒のピン止めに活用する微細なＭｎＳの再析出量はＳの濃度により調整することが可能である。Ｓ量が０．００５％未満では再析出ＭｎＳ量が不十分である。一方、０．１００％を超えるＳを添加すると、粒界に偏析して、粒界脆化を招く。したがって、Ｓの含有量を０．００５〜０．１００％の範囲内にする必要がある。なお、軸受部品、転動部品においてはＭｎＳが転動疲労寿命を劣化させるため、Ｓの上限を０．０１０％以下にすることが好ましい。 S also participates in the precipitation of MnS and is an extremely important element in the present invention. The present inventor has found that although the amount of MnS deposited depends on the concentrations of Mn and S, S has little influence on the solution temperature of MnS, unlike Mn. Therefore, the reprecipitation amount of fine MnS utilized for pinning the crystal grains can be adjusted by the concentration of S. If the amount of S is less than 0.005%, the amount of reprecipitated MnS is insufficient. On the other hand, when S exceeding 0.100% is added, it segregates at the grain boundary and causes embrittlement at the grain boundary. Therefore, the S content needs to be in the range of 0.005 to 0.100%. In bearing parts and rolling parts, MnS degrades the rolling fatigue life, so the upper limit of S is preferably 0.010% or less.
Ｍｎの含有量とＳの含有量から、（Ｍｎ／５５）／（Ｓ／３２）によって計算されるＭｎ／Ｓモル比率は、上述のように、本発明においては極めて重要である。浸炭処理時の再析出ＭｎＳ量を確保するには、（Ｍｎ／５５）／（Ｓ／３２）を１５．０以下にすることが必要である。一方、（Ｍｎ／５５）／（Ｓ／３２）の下限は、Ｓに起因する鋳造疵の発生を防止するため、１．５以上にすることが必要であるが、本発明では（Ｍｎ／５５）／（Ｓ／３２）の範囲を実施例に記載した６．１〜１１．６に限定した。
As described above, the Mn / S molar ratio calculated from (Mn / 55) / (S / 32) from the Mn content and the S content is extremely important in the present invention. In order to secure the amount of reprecipitated MnS during the carburizing process, it is necessary to set (Mn / 55) / (S / 32) to 15.0 or less. On the other hand, the lower limit of (Mn / 55) / (S / 32) is required to be 1.5 or more in order to prevent the occurrence of cast iron due to S. In the present invention, (Mn / 55) ) / (S / 32) was limited to 6.1 to 11.6 described in the examples.
Ｐは、不純物であり、浸炭部品の結晶粒界を脆化させて疲労強度を劣化させるため、０．０２５％以下に制限する必要がある。Ｐは、浸炭部品の靭性等にも悪影響を及ぼすため、０．０１０％以下とすることが好ましい。なお、製鋼コストの観点から許容されるＰの下限値は、通常、０．００１％である。 P is an impurity and needs to be limited to 0.025% or less in order to embrittle the grain boundary of the carburized component and deteriorate the fatigue strength. P has an adverse effect on the toughness of the carburized parts, and therefore is preferably 0.010% or less. In addition, the lower limit of P permitted from the viewpoint of steelmaking cost is usually 0.001%.
Ａｌは、脱酸元素であり、介在物の生成を抑制するため、０．０５％を上限として添加する。また、Ａｌ量が０．０３％を超えると、粗大なＡｌＮが析出し、靭性を損なうことがあるため、上限を０．０３％以下にすることが好ましい。一方、Ａｌは、浸炭加熱の際に、ＡｌＮを形成し、浸炭処理時の結晶粒粗大化の抑制に寄与することもあり、０．０１％以上を添加することが好ましい。 Al is a deoxidizing element, and 0.05% is added as an upper limit in order to suppress the formation of inclusions. Further, if the Al content exceeds 0.03%, coarse AlN precipitates and the toughness may be impaired, so the upper limit is preferably made 0.03% or less. On the other hand, Al forms AlN during carburizing heating, which may contribute to the suppression of crystal grain coarsening during the carburizing treatment, and is preferably added in an amount of 0.01% or more.
Ｎは、不純物であり、鋼を脆化させるため、上限を０．０２０％以下とすることが必要である。また、鋳造、圧延時の割れ、キズの発生を抑制するには、上限を０．１５０％とすることが好ましい。一方、Ｎは、ＡｌＮやＮｂ（ＣＮ）等の析出により、浸炭時の結晶粒粗大化の抑制に寄与することもあり、０．００３％以上を含有させることが好ましい。
次に本発明において選択的に添加する合金元素について説明する。
N is an impurity, and in order to embrittle the steel, the upper limit needs to be 0.020% or less. Moreover, in order to suppress generation | occurrence | production of the crack at the time of casting and rolling, and a crack, it is preferable to make an upper limit into 0.150%. On the other hand, N may contribute to suppression of crystal grain coarsening during carburization by precipitation of AlN, Nb (CN), etc., and is preferably contained in an amount of 0.003% or more.
Next, the alloy elements selectively added in the present invention will be described.
Ｖ、Ｔｉ、Ｎｂは、鋼中のＣ、Ｎと炭窒化物を形成し、浸炭時の結晶粒粗大化の抑制に寄与する元素であり、１種又は２種以上を含有させることが好ましい。なお、粒径粗大化防止の効果を得るには、Ｖ、Ｔｉ、Ｎｂを、それぞれ、０．０５％以上、０．００５％以上、０．０１％以上添加することが好ましい。一方、Ｖ、Ｔｉ、Ｎｂの含有量が、それぞれ、０．２０％、０．２００％、０．０６％を超えると、硬さの上昇を招き加工性が劣化することがある。また、Ｖ、Ｔｉ、Ｎｂは、強度の向上にも有効な元素であり、下限を、それぞれ、０．０７％、０．０２０％、０．０２％とすることが更に好ましい。 V, Ti, and Nb are elements that form carbonitrides with C and N in steel and contribute to the suppression of crystal grain coarsening during carburization, and are preferably contained in one or more. In order to obtain the effect of preventing the coarsening of the particle size, it is preferable to add V, Ti, Nb, respectively, 0.05% or more, 0.005% or more, 0.01% or more. On the other hand, if the contents of V, Ti, and Nb exceed 0.20%, 0.200%, and 0.06%, respectively, the hardness may increase and the workability may deteriorate. V, Ti, and Nb are effective elements for improving the strength, and the lower limits are more preferably 0.07%, 0.020%, and 0.02%, respectively.
Ｃｒ、Ｍｏ、Ｎｉ、Ｃｕ、Ｂは、鋼材の焼入れ性を向上させる元素であり、浸炭部品に必要な強度を確保するために、１種又は２種以上を含有させることが好ましい。 Cr, Mo, Ni, Cu, and B are elements that improve the hardenability of the steel material, and it is preferable to include one or more of them in order to ensure the strength required for carburized parts.
Ｃｒ、Ｍｏは、軸受部品、転動部品として使用される際には、転動疲労過程での組織変化及び材質劣化を抑制し、高寿命化を図るためにも有効な元素である。効果を得るためには、Ｃｒ、Ｍｏを、それぞれ、０．８％以上、０．０５％以上添加することが好ましい。一方、３．０％超のＣｒを添加すると、硬さが上昇し、加工性が劣化することがある。したがって、Ｃｒ量の上限は、３．０％以下にすることが好ましい。なお、Ｃｒ量の好ましい上限は２．５％である。また、Ｍｏは、０．５０％超を含有させても効果が飽和するため、上限を０．５０％以下とすることが好ましい。 When used as a bearing part or rolling part, Cr and Mo are effective elements for suppressing structural changes and material deterioration in the rolling fatigue process and extending the life. In order to acquire an effect, it is preferable to add Cr and Mo 0.8% or more and 0.05% or more, respectively. On the other hand, when more than 3.0% of Cr is added, the hardness increases and the workability may deteriorate. Therefore, the upper limit of the Cr content is preferably 3.0% or less. In addition, the preferable upper limit of Cr amount is 2.5%. Moreover, since the effect is saturated even if Mo is contained in excess of 0.50%, the upper limit is preferably made 0.50% or less.
Ｎｉ及びＣｕは、靭性の向上にも有効な元素であり、それぞれ、０．４％以上、０．３％以上を添加することが好ましい。一方、１．０％超のＮｉを添加しても効果が飽和する。なお、好ましいＮｉ量の上限は０．９％である。Ｃｕは過剰に添加すると、加熱時に粒界に濃化し、表面性状を劣化させることがあるため、３．０％を上限とすることが好ましい。更に、Ｃｕによる表面性状の劣化を抑制するには、Ｎｉとの複合添加が好ましく、好適範囲はＮｉ濃度の３倍以下である。Ｎｉを添加しない場合は、Ｃｕの含有量を０．６％以下とすることが好ましい。 Ni and Cu are effective elements for improving toughness, and it is preferable to add 0.4% or more and 0.3% or more, respectively. On the other hand, the effect is saturated even if Ni over 1.0% is added. In addition, the upper limit of preferable Ni amount is 0.9%. If Cu is added excessively, it may be concentrated at the grain boundary during heating and the surface properties may be deteriorated. Therefore, the upper limit is preferably 3.0%. Furthermore, in order to suppress the deterioration of the surface properties due to Cu, combined addition with Ni is preferable, and the preferable range is 3 times or less of the Ni concentration. When Ni is not added, the Cu content is preferably 0.6% or less.
Ｂは、鋼材の焼入れ性を著しく向上させる元素であり、０．０００５％以上の添加が好ましい。一方、０．００２０％を超えるＢを添加すると、焼入れ性の向上によって靭性が損なわれることがある。なお、Ｂの含有量の好適な範囲は、０．００１０〜０．００１８％である。 B is an element that remarkably improves the hardenability of the steel material, and 0.0005% or more is preferably added. On the other hand, when B exceeding 0.0020% is added, toughness may be impaired due to improvement in hardenability. In addition, the suitable range of content of B is 0.0010 to 0.0018%.
次に、本発明の浸炭部品用鋼及び浸炭部品の製造方法について説明する。
常法によって、鋼を溶製、鋳造し、得られた鋼片、鋼塊を熱間加工し、鋼材を得る。この熱間加工は、熱間圧延、熱間鍛造であり、複数回行っても良く、熱間圧延と熱間鍛造を組み合わせても良い。
Next, the steel for carburized parts and the method for producing carburized parts of the present invention will be described.
By a conventional method, steel is melted and cast, and the obtained steel slab and steel ingot are hot worked to obtain a steel material. This hot working is hot rolling or hot forging, and may be performed a plurality of times, or hot rolling and hot forging may be combined.
なお、熱間加工として、複数回の熱間圧延や、熱間圧延と熱間鍛造を組み合わせて行う場合は、浸炭処理の直前に行った熱間加工の加熱温度が極めて重要である。これは、浸炭処理の直前に行った加熱によって固溶し、浸炭処理時に析出した微細なＭｎＳが、粒径の粗大化の防止に大きく寄与するためである。一方、浸炭処理の直前に行った加熱によって固溶しないＭｎＳは粗大化するため、粒界をピンニングする効果が小さい。したがって、本発明の熱間加工の加熱温度は、浸炭処理の直前に行った熱間加工の加熱温度とする。 In addition, when performing hot rolling several times or combining hot rolling and hot forging as hot working, the heating temperature of the hot working performed immediately before the carburizing process is extremely important. This is because the fine MnS that is dissolved by heating immediately before the carburizing process and precipitated during the carburizing process greatly contributes to the prevention of the coarsening of the particle size. On the other hand, since MnS that does not dissolve by heating performed just before the carburizing treatment is coarsened, the effect of pinning the grain boundaries is small. Therefore, the heating temperature of the hot working of the present invention is the heating temperature of the hot working performed immediately before the carburizing process.
熱間加工の加熱温度は、ＭｎＳを固溶させ、熱間加工から浸炭処理までに析出するＭｎＳ量を確保するため、１１５０℃以上にすることが必要である。なお、加熱温度が高いほど、析出ＭｎＳ量が増加するため、１２００℃以上に加熱することが好ましい。一方、加熱温度が１３００℃を超えると、３０分以上保持した場合、粒径が粗大化して、熱間加工性が低下する。 The heating temperature for hot working is required to be 1150 ° C. or higher in order to dissolve MnS and secure the amount of MnS precipitated from hot working to carburizing treatment. In addition, since the amount of precipitation MnS increases, so that heating temperature is high, it is preferable to heat at 1200 degreeC or more. On the other hand, when the heating temperature exceeds 1300 ° C., the particle size becomes coarse and the hot workability deteriorates when held for 30 minutes or more.
また、熱間加工の加熱温度を１１５０℃以上にして、ＭｎＳを固溶させるには、保持時間を３０分以上にすることが好ましい。加熱を、例えば、高周波加熱によって１２５０℃以上の高温で行う場合は、保持時間は３０分未満でも良い。保持時間の上限は規定しないが、１２０時間を超えて保持すると生産性を損なうことがある。
Moreover, in order to make the heating temperature of
浸炭部品用鋼を、熱間加工後、成形して浸炭処理を施し、浸炭部品を製造する。なお、成形は、熱間鍛造などの熱間加工によって行っても良く、熱間加工後、冷間鍛造などの冷間加工を施して成形しても良い。また、浸炭処理の前後に切削加工を施しても良い。製造コストの観点から、熱間加工後の素材に冷間鍛造などの冷間加工による成形を施すことが好ましい。熱間鍛造、冷間鍛造で部品を成形する際の加工率は、生産性の観点から、３０％以上とすることが好ましい。 Carburized parts steel is hot-worked and then molded and carburized to produce carburized parts. Note that the forming may be performed by hot working such as hot forging, or may be formed by performing cold working such as cold forging after hot working. Further, cutting may be performed before and after the carburizing process. From the viewpoint of manufacturing cost, it is preferable to form the material after hot working by cold working such as cold forging. From the viewpoint of productivity, the processing rate when forming a part by hot forging or cold forging is preferably 30% or more.
更に、冷間加工を施す前には、成形性を向上させるために、球状化焼鈍を行っても良い。球状化焼鈍は、７００〜８００℃の温度範囲内で行うことが好ましい。 Furthermore, before cold working, spheroidizing annealing may be performed in order to improve formability. The spheroidizing annealing is preferably performed within a temperature range of 700 to 800 ° C.
従来、浸炭処理は９５０℃以下で行うことが多いが、生産性の観点から、高深度浸炭として９５０℃超で浸炭処理を行う。本発明の浸炭部品用鋼は、９５０℃超で浸炭処理を高温で行っても、粗大粒の発生が抑制される。これにより、高深度浸炭の生産性を向上することが可能になる。また、熱間加工の加熱温度を下限である１１５０℃とする場合は、再析出ＭｎＳ量を確保するために、浸炭処理を１１３０℃以下で行うことが必要である。浸炭処理温度は、１０１０℃以上の高温で行うことが好ましいが、処理温度が高すぎると設備への負担が大きい。したがって、浸炭処理温度は１１００℃以下とすることが好ましい。 Conventionally, the carburizing process is often performed at 950 ° C. or less, but from the viewpoint of productivity, the carburizing process is performed at a temperature higher than 950 ° C. as deep carburizing. The steel for carburized parts of the present invention suppresses the generation of coarse particles even when carburizing is performed at a high temperature above 950 ° C. Thereby, it becomes possible to improve the productivity of deep carburization. When the heating temperature for hot working is set to 1150 ° C., which is the lower limit, it is necessary to perform carburizing treatment at 1130 ° C. or lower in order to secure the amount of reprecipitated MnS. The carburizing temperature is preferably 1010 ° C. or higher, but if the processing temperature is too high, the burden on the equipment is large. Accordingly, the carburizing temperature is preferably 1100 ° C. or lower.
更に、熱間加工の加熱温度におけるＭｎＳの平衡モル分率と、浸炭処理温度における平衡ＭｎＳのモル分率との差を、０．０１００％以上にすると、浸炭処理前後のＭｎ、Ｓの固溶量の差が大きくなり、微細なＭｎＳの析出量が増加する。これにより、浸炭処理時の粗大粒発生の防止に寄与する微細な再析出ＭｎＳ量を確保することができる。加熱温度及び浸炭処理温度におけるＭｎＳの平衡モル分率は、汎用の熱力学平衡計算ソフトウェアによって計算することができる。 Further, if the difference between the equilibrium molar fraction of MnS at the heating temperature in hot working and the molar fraction of equilibrium MnS at the carburizing temperature is 0.0100% or more, the solid solution of Mn and S before and after the carburizing treatment The difference in amount increases, and the amount of fine MnS deposited increases. Thereby, the fine reprecipitation MnS amount which contributes to prevention of the generation | occurrence | production of the coarse grain at the time of a carburizing process can be ensured. The equilibrium molar fraction of MnS at the heating temperature and carburizing temperature can be calculated by general-purpose thermodynamic equilibrium calculation software.
表１に示した化学成分を有する鋼片を、表２に示した温度に加熱して６０分保持し、表２に示した条件で、棒又は線材に熱間圧延した。なお、Ｍｎ／Ｓモル比は、Ｍｎ量及びＳ量から、（Ｍｎ／５５）／（Ｓ／３２）として計算した。Ｎｏ．１９は、Ｍｎ／Ｓモル比が小さく、熱間圧延時にキズが発生し、鍛造素材を得ることができなかった例である。 A steel slab having the chemical composition shown in Table 1 was heated to the temperature shown in Table 2 and held for 60 minutes, and was hot-rolled to a bar or wire under the conditions shown in Table 2. The Mn / S molar ratio was calculated as (Mn / 55) / (S / 32) from the amount of Mn and the amount of S. No. No. 19 is an example in which the Mn / S molar ratio was small, scratches were generated during hot rolling, and a forged material could not be obtained.
したがって、Ｎｏ．１〜１８の鋼材から円柱状の素材を採取し、冷間鍛造による成形を行い、円柱型試料を採取した。なお、冷間鍛造を行う前には、必要に応じて球状化処理を施した。球状化処理は、最高温度を７６０℃として行った。加工率は、冷間鍛造前の素材の高さと冷間鍛造後の試料の高さの差を、冷間鍛造前の素材の高さで除して求め、百分率で示した。 Therefore, no. A cylindrical material was sampled from 1 to 18 steel materials, formed by cold forging, and a cylindrical sample was collected. In addition, before performing cold forging, the spheroidization process was performed as needed. The spheroidizing treatment was performed at a maximum temperature of 760 ° C. The processing rate was obtained by dividing the difference between the height of the material before cold forging and the height of the sample after cold forging by the height of the material before cold forging, and expressed as a percentage.
得られた成形品を、浸炭処理を想定して９００〜１１００℃に加熱し、５時間保時し、急冷した。その後、歪み量が最大の部位となる中央部近傍の断面を観察面として、試料を採取し、ＪＩＳ Ｇ ０５５１に準拠して、旧オーステナイト結晶粒界を現出させた。光学顕微鏡により、５０〜２００倍で観察し、最大のオーステナイト粒径を測定し、粗大粒が観察される温度を限界浸炭処理温度とした。したがって、Ｎｏ．１、３、４、６、９、１０の本発明の部材は、９５０℃超から限界浸炭温度以下で浸炭処理、即ち高温浸炭を行っても粗大粒が発生しない。一方、Ｎｏ．１１〜１９は、本発明の範囲外の成分であり、限界浸炭温度が９５０℃以下であり、高温浸炭を行った場合は、粒径の粗大化が問題になる。なお、Ｎｏ．２、５、７、８は、参考例である。
The obtained molded product was heated to 900-1100 ° C. assuming carburization treatment, held for 5 hours, and rapidly cooled. Thereafter, a sample was taken using the cross section in the vicinity of the central portion where the amount of strain was the maximum as an observation surface, and a prior austenite grain boundary was revealed in accordance with JIS G 0551. The maximum austenite particle size was measured with an optical microscope at 50 to 200 times, and the temperature at which coarse particles were observed was defined as the critical carburizing temperature. Therefore, no. 1, 3,4,6, 9, 10 member of the present invention, carburizing limit carburization temperature below the 950 ° C. greater, i.e. coarse grains even if the high-temperature carburization do not want to occur. The other hand, No. 11 to 19 are components outside the range of the present invention, the limit carburizing temperature is 950 ° C. or less, and when high-temperature carburizing is performed, coarsening of the particle size becomes a problem. In addition, No. 2, 5, 7, and 8 are reference examples.
更に、それぞれの鋼の１２２５℃、１０１０℃、熱間加工の加熱温度及び限界浸炭処理温度におけるＭｎＳの平衡析出モル分率を、熱力学平衡計算ソフトウェアであるサーモカルク（登録商標）により計算した。表２のＭｎＳ再析出量の１０１０℃は、１２２５℃におけるＭｎＳ平衡モル分率と１０１０℃におけるＭｎＳ平衡析出モル分率との差を意味する。また、表２のＭｎＳ再析出量の浸炭限界温度は、熱間圧延加熱温度におけるＭｎＳ平衡モル分率と限界浸炭温度におけるＭｎＳ平衡析出モル分率との差を意味する。表２のＮｏ．２は限界浸炭温度が１１３０℃を超えたため、限界浸炭温度を１１３０℃としてＭｎＳ平衡析出モル分率を計算した。 Further, the equilibrium precipitation mole fraction of MnS at 1225 ° C., 1010 ° C., the hot working heating temperature and the critical carburizing temperature of each steel was calculated by Thermocalc (registered trademark) which is thermodynamic equilibrium calculation software. The MnS reprecipitation amount of 1010 ° C. in Table 2 means the difference between the MnS equilibrium molar fraction at 1225 ° C. and the MnS equilibrium precipitation molar fraction at 1010 ° C. Further, the carburization limit temperature of the amount of MnS reprecipitation in Table 2 means the difference between the MnS equilibrium mole fraction at the hot rolling heating temperature and the MnS equilibrium precipitation mole fraction at the limit carburization temperature. No. in Table 2 In No. 2, since the limit carburizing temperature exceeded 1130 ° C., the MnS equilibrium precipitation mole fraction was calculated by setting the limit carburizing temperature to 1130 ° C.
表２から明らかなように、本発明鋼は微細なＭｎＳの析出量を示すＭｎＳ再析出量が０．０１ｍｏｌ％以上であり、浸炭限界温度が１０００℃を超える。一方、本発明の範囲外の成分では、限界浸炭温度が９５０℃以下であり、高温浸炭を行うことができない。 As is apparent from Table 2, the steel of the present invention has a MnS reprecipitation amount of 0.01 mol% or more indicating a fine MnS precipitation amount, and the carburization limit temperature exceeds 1000 ° C. On the other hand, with components outside the scope of the present invention, the critical carburizing temperature is 950 ° C. or lower, and high temperature carburizing cannot be performed.
表３に示した化学成分を有する円柱形状の鋼片を、表４に示した温度に加熱して６０分保持し、表４に示した条件で、熱間鍛造を行った。なお、加工率は、鋼片の高さと熱間鍛造後の試料の高さの差を、鋼片の高さで除して求め、百分率で示した。 The columnar steel slab having the chemical composition shown in Table 3 was heated to the temperature shown in Table 4 and held for 60 minutes, and hot forging was performed under the conditions shown in Table 4. The processing rate was obtained by dividing the difference between the height of the steel slab and the height of the sample after hot forging by the height of the steel slab, and expressed as a percentage.
得られた成形品を、浸炭処理を想定して９００〜１１００℃に加熱し、５時間保時し、急冷した。その後、歪み量が最大の部位となる中央部近傍の断面を観察面として、試料を採取し、実施例１と同様にして限界浸炭処理温度を求めた。したがって、Ｎｏ．２０の本発明の部材は、９５０℃超から限界浸炭温度以下で浸炭処理を行っても粗大粒が発生しない。Ｎｏ.２１〜２５は参考例である。なお、Ｎｏ．２５は、限界浸炭温度が１１３０℃を超えている。一方、Ｎｏ．２６〜２８は、本発明の範囲外の成分であり、限界浸炭温度が９５０℃以下であり、高温浸炭を行った場合は、粒径の粗大化が問題になる。
The obtained molded product was heated to 900-1100 ° C. assuming carburization treatment, held for 5 hours, and rapidly cooled. Thereafter, a sample was taken with the cross section in the vicinity of the central portion where the amount of strain was the maximum as the observation surface, and the limit carburizing temperature was determined in the same manner as in Example 1. Therefore, no. 2 0 of the members of the present invention, the limit carburization temperature following coarse be performed carburization grains have such generated from 950 ° C. greater. Nos. 21 to 25 are reference examples. In addition, No. 25, limit carburizing temperature that exceeds the 1130 ℃. The other hand, No. Nos. 26 to 28 are components outside the scope of the present invention, and the limit carburizing temperature is 950 ° C. or lower. When high-temperature carburizing is performed, coarsening of the particle size becomes a problem.
更に、実施例１と同様にして、熱力学平衡計算を行い、１２２５℃におけるＭｎＳ平衡モル分率と１０１０℃におけるＭｎＳ平衡析出モル分率との差、熱間圧延加熱温度におけるＭｎＳ平衡モル分率と限界浸炭温度におけるＭｎＳ平衡析出モル分率との差を求めた。表４のＮｏ．２５は限界浸炭温度が１１３０℃を超えたため、限界浸炭温度を１１３０℃としてＭｎＳ平衡析出モル分率を計算した。 Further, the thermodynamic equilibrium calculation was performed in the same manner as in Example 1, and the difference between the MnS equilibrium mole fraction at 1225 ° C. and the MnS equilibrium precipitation mole fraction at 1010 ° C., the MnS equilibrium mole fraction at the hot rolling heating temperature. And the difference between the MnS equilibrium precipitation mole fraction at the critical carburizing temperature. No. in Table 4 For No. 25, since the limit carburizing temperature exceeded 1130 ° C., the MnS equilibrium precipitation mole fraction was calculated by setting the limit carburizing temperature to 1130 ° C.
表４から明らかなように、本発明鋼は微細なＭｎＳの析出量を示すＭｎＳ再析出量が０．０１ｍｏｌ％以上であり、浸炭限界温度が１０００℃を超える。一方、本発明の範囲外の成分では、限界浸炭温度が９５０℃以下であり、高温浸炭を行うことができない。 As is apparent from Table 4, the steel of the present invention has a MnS reprecipitation amount of 0.01 mol% or more indicating a fine MnS precipitation amount, and the carburization limit temperature exceeds 1000 ° C. On the other hand, with components outside the scope of the present invention, the critical carburizing temperature is 950 ° C. or lower, and high temperature carburizing cannot be performed.
１ ＭｎＳの平衡析出曲線
２ 鋼の融点
３ 熱間加工の加熱温度
４ 浸炭処理温度
５ 析出ＭｎＳ量
６ 再析出ＭｎＳ量
1 MnS
Claims (7)
Ｃ ：０．１０〜０．４０％、
Ｓｉ：０．０５〜１．００％、
Ｍｎ：０．０５〜０．４０％、
Ｓ ：０．００５〜０．１００％
を含有し、
Ｐ ：０．０２５％以下、
Ａｌ：０．０５％以下、
Ｎ ：０．０２０％以下
に制限し、残部がＦｅ及び不可避的不純物からなり、Ｍｎの含有量とＳの含有量とが、
（Ｍｎ／５５）／（Ｓ／３２）：６．１〜１１．６
を満足する浸炭部品用鋼を、１１５０〜１３００℃の範囲内で加熱し、熱間加工を施して冷却し、熱間加工の加熱温度におけるＭｎＳの平衡モル分率と、浸炭処理温度における平衡ＭｎＳのモル分率との差が０．０１００ｍｏｌ％以上となるように、９５０℃超から１１３０℃以下までの温度域で浸炭処理を行うことを特徴とする浸炭部品の製造方法。 % By mass
C: 0.10 to 0.40%,
Si: 0.05-1.00%,
Mn: 0.05 to 0.40%,
S: 0.005 to 0.100%
Containing
P: 0.025% or less,
Al: 0.05% or less,
N: 0.020% or less
The balance consists of Fe and inevitable impurities, the content of Mn and the content of S,
(Mn / 55) / (S / 32): 6.1 to 11.6
The steel for carburized parts satisfying the above conditions is heated within a range of 1150 to 1300 ° C., cooled by hot working, and the equilibrium molar fraction of MnS at the heating temperature of hot working and the equilibrium MnS at the carburizing temperature. A method for manufacturing a carburized part, characterized in that carburizing treatment is performed in a temperature range from more than 950 ° C. to 1130 ° C. or less so that the difference from the mole fraction of the above becomes 0.0100 mol% or more.
Ｖ ：０．０５〜０．２０％、V: 0.05-0.20%,
Ｔｉ：０．００５〜０．２００％、Ti: 0.005 to 0.200%,
Ｎｂ：０．０１〜０．０６％Nb: 0.01 to 0.06%
のうち１種又は２種以上を含有することを特徴とする請求項１または２に記載の浸炭部品の製造方法。1 or 2 types or more are contained among these, The manufacturing method of the carburized component of Claim 1 or 2 characterized by the above-mentioned.
Ｃｒ：３．０％以下、Cr: 3.0% or less,
Ｍｏ：０．５０％以下、Mo: 0.50% or less,
Ｎｉ：１．０％以下、Ni: 1.0% or less,
Ｃｕ：３．０％以下、Cu: 3.0% or less,
Ｂ ：０．００２０％以下B: 0.0020% or less
のうち１種又は２種以上を含有することを特徴とする請求項１〜３のいずれか１項に記載の浸炭部品の製造方法。The manufacturing method of the carburized component of any one of Claims 1-3 characterized by containing 1 type, or 2 or more types.
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