JP4847681B2 - Ti-containing case-hardened steel - Google Patents

Ti-containing case-hardened steel Download PDF

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JP4847681B2
JP4847681B2 JP2004030993A JP2004030993A JP4847681B2 JP 4847681 B2 JP4847681 B2 JP 4847681B2 JP 2004030993 A JP2004030993 A JP 2004030993A JP 2004030993 A JP2004030993 A JP 2004030993A JP 4847681 B2 JP4847681 B2 JP 4847681B2
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陽介 新堂
安部  聡
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Kobe Steel Ltd
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    • CCHEMISTRY; METALLURGY
    • C23COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23CCOATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/06Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
    • C23C8/08Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
    • C23C8/20Carburising
    • C23C8/22Carburising of ferrous surfaces
    • CCHEMISTRY; METALLURGY
    • C23COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23CCOATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/06Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
    • C23C8/08Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
    • C23C8/24Nitriding
    • C23C8/26Nitriding of ferrous surfaces
    • CCHEMISTRY; METALLURGY
    • C23COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23CCOATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/06Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
    • C23C8/28Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases more than one element being applied in one step
    • C23C8/30Carbo-nitriding
    • C23C8/32Carbo-nitriding of ferrous surfaces

Description

本発明は、自動車、建築機械、その他産業機械などの分野において、表面硬化熱処理される部品(例えば、歯車、シャフト類、軸受、ミクロネジなど)を製造するのに有用な鋼材に関するものである。   The present invention relates to a steel material useful for manufacturing parts (for example, gears, shafts, bearings, micro screws, etc.) subjected to surface hardening heat treatment in the fields of automobiles, construction machines, and other industrial machines.

自動車、建築機械、その他産業機械において高強度が要求される部品は、従来、浸炭処理、窒化処理、浸炭窒化処理などの表面硬化熱処理が適用されている。このような用途には、通常、SCr鋼、SCM鋼、SNCM鋼などのJISで定められた肌焼き鋼が使用され、鍛造や切削などの機械加工によって所定の部品形状に成形した後、表面硬化熱処理を施し、次いで研磨などを行って製品(部品)となる。   Conventionally, surface hardening heat treatments such as carburizing, nitriding, and carbonitriding have been applied to parts that require high strength in automobiles, construction machinery, and other industrial machines. For such applications, case-hardened steel defined by JIS, such as SCr steel, SCM steel, SNCM steel, etc., is usually used. After forming into a predetermined part shape by machining such as forging or cutting, surface hardening is performed. A heat treatment is performed, followed by polishing or the like to obtain a product (part).

近年、これら部品の製造原価の低減、リードタイムの短縮などが望まれてきており、浸炭や浸炭窒化処理を高温化することによって熱処理時間の短縮が進められている。しかし高温化すると、素材の結晶粒の粗大化が進行し、熱処理歪量が増大する。   In recent years, it has been desired to reduce the manufacturing cost and lead time of these parts, and the heat treatment time has been shortened by increasing the temperature of carburizing and carbonitriding. However, when the temperature rises, the coarsening of the crystal grains of the material proceeds and the amount of heat treatment strain increases.

結晶粒の粗大化を防止するため、鋼材中に、Al、Nb、Tiなどの元素を添加し、これらの炭化物、窒化物、又は炭窒化物によるピンニング効果によって結晶粒の粗大化を抑制する技術が開発されてきた(例えば特許文献1〜2参照)。特にNb添加鋼は結晶粒の粗大化防止特性に優れており、数多くのNb添加鋼が実用化されてきている。   Technology to prevent coarsening of crystal grains by adding elements such as Al, Nb, Ti, etc. to steel materials and preventing pinning by these carbides, nitrides, or carbonitrides to prevent grain coarsening Have been developed (see, for example, Patent Documents 1 and 2). In particular, Nb-added steel has excellent crystal grain coarsening prevention properties, and many Nb-added steels have been put into practical use.

しかしTi添加鋼は、結晶粒の粗大化防止特性に優れているものの、鋼材の硬度が上昇し、加工性(特に冷間鍛造性)が低下するために実用化された例が少ない。Tiは、前述した結晶粒の粗大化防止特性の他、耐遅れ破壊特性を向上させるのにも有用であり(例えば特許文献3〜4参照)、また焼入性向上などのためにBを添加した場合にはBの添加効果を阻害するBNの生成を抑制するのにも有用であり(例えば特許文献1〜2参照)、極めて優れた元素であるため、実用化を促進するためにも冷間鍛造性の向上が極めて重要となる。
特開平10−81938号公報 特開2000−63983号公報 特開平11−92863号公報 特開平11−293392号公報
However, although Ti-added steel is excellent in crystal grain coarsening prevention properties, there are few examples of practical use because the hardness of the steel material is increased and workability (particularly cold forgeability) is lowered. Ti is useful for improving delayed fracture resistance in addition to the above-described crystal grain coarsening prevention characteristics (see, for example, Patent Documents 3 to 4), and B is added to improve hardenability. In this case, it is useful for suppressing the formation of BN that inhibits the effect of addition of B (see, for example, Patent Documents 1 and 2), and is an extremely excellent element. Improvement of the forgeability is extremely important.
Japanese Patent Laid-Open No. 10-81938 JP 2000-63983 A JP-A-11-92863 JP-A-11-293392

本発明は上記の様な事情に着目してなされたものであって、その目的は、Ti含有肌焼き鋼の冷間鍛造性を向上させることにある。   This invention is made paying attention to the above situations, and the objective is to improve the cold forgeability of Ti containing case hardening steel.

本発明者らは、前記課題を解決するために鋭意研究を重ねた結果、以下のことを見出した。すなわち肌焼き鋼は被削性を確保するために所定量以上のSを含有しているものが多く、Tiを添加しない場合には、鋼中のSは軟質のMn硫化物を形成し、冷間鍛造性に殆ど悪影響を与えないが、Ti添加鋼では硬質のTi硫化物やTi炭硫化物を形成し、冷間鍛造性を阻害していたのである。そして本発明者らは、Tiを添加する際にCaも複合添加すると、硫化物は軟質のMn・Ca系硫化物(例えばMn硫化物、Ca硫化物、Caを一部固溶したMn硫化物など)が主体となり、冷間鍛造性が顕著に改善されることを見出し、本発明を完成した。   As a result of intensive studies to solve the above problems, the present inventors have found the following. That is, many case-hardened steels contain a predetermined amount or more of S in order to ensure machinability. When Ti is not added, S in the steel forms soft Mn sulfides and is cooled. Although there is almost no adverse effect on cold forgeability, Ti-added steel forms hard Ti sulfide and Ti carbosulfide to inhibit cold forgeability. When the present inventors add Ca together when adding Ti, the sulfide is a soft Mn · Ca-based sulfide (for example, Mn sulfide, Ca sulfide, Mn sulfide in which Ca is partly dissolved). And the like, and the cold forgeability was remarkably improved, and the present invention was completed.

すなわち、本発明に係るTi含有肌焼き鋼は、質量%で、
C :0.05〜0.3%、
Si:2.0%以下(0%を含まない)、
Mn:2.0%以下(0%を含まない)、
P :0.03%以下(0%を含まない)、
S :0.005〜0.2%、
Cr:2.0%以下(0%を含まない)、
Ti:0.01〜0.2%、
Ca:0.0001〜0.02%、
N :0.03%以下(0%を含まない)、及び
O :0.002%以下(0%を含まない)
を含有し、残部はFe及び不可避不純物である点に要旨を有するものである。
That is, the Ti-containing case-hardening steel according to the present invention is in mass%,
C: 0.05 to 0.3%
Si: 2.0% or less (excluding 0%),
Mn: 2.0% or less (excluding 0%),
P: 0.03% or less (excluding 0%),
S: 0.005 to 0.2%,
Cr: 2.0% or less (excluding 0%),
Ti: 0.01-0.2%
Ca: 0.0001 to 0.02%,
N: 0.03% or less (not including 0%), and O: 0.002% or less (not including 0%)
And the balance is Fe and inevitable impurities.

前記Ti含有肌焼き鋼は、さらにAl:0.2%以下(0%を含まない)、V:0.5%以下(0%を含まない)、Ni:3%以下(0%を含まない)、Cu:0.5%以下(0%を含まない)、Mo:1.0%以下(0%を含まない)、B:0.005%以下(0%を含まない)、Pb:0.1%以下(0%を含まない)、Bi:0.1%以下(0%を含まない)、Mg:0.01%以下(0%を含まない)、REM:0.1%以下(0%を含まない)などを適宜添加してもよい。   The Ti-containing case-hardened steel is further Al: 0.2% or less (not including 0%), V: 0.5% or less (not including 0%), Ni: 3% or less (not including 0%) ), Cu: 0.5% or less (not including 0%), Mo: 1.0% or less (not including 0%), B: 0.005% or less (not including 0%), Pb: 0 0.1% or less (not including 0%), Bi: 0.1% or less (not including 0%), Mg: 0.01% or less (not including 0%), REM: 0.1% or less ( And the like may be added as appropriate.

本発明によれば、Caを添加しているため、Ti含有肌焼き鋼の冷間鍛造性を向上できる。   According to the present invention, since Ca is added, the cold forgeability of the Ti-containing case-hardened steel can be improved.

本発明の肌焼き鋼は、C:0.05〜0.3%(質量%の意。以下、同じ)、Si:2.0%以下(0%を含まない)、Mn:2.0%以下(0%を含まない)、P:0.03%以下(0%を含まない)、S:0.005〜0.2%、Cr:2.0%以下(0%を含まない)、Ti:0.01〜0.2%、Ca:0.0001〜0.02%、N:0.03%以下(0%を含まない)、及びO:0.002%以下(0%を含まない)を含有している点に特徴を有する。以下、各成分の限定理由について説明する。   The case-hardened steel of the present invention has C: 0.05 to 0.3% (meaning mass%, hereinafter the same), Si: 2.0% or less (excluding 0%), Mn: 2.0% Or less (excluding 0%), P: 0.03% or less (not including 0%), S: 0.005 to 0.2%, Cr: 2.0% or less (not including 0%), Ti: 0.01 to 0.2%, Ca: 0.0001 to 0.02%, N: 0.03% or less (not including 0%), and O: 0.002% or less (including 0%) Is not included). Hereinafter, the reason for limitation of each component is demonstrated.

Cは本発明の肌焼き鋼を表面硬化熱処理して部品としたときに必要な芯部硬さを確保すると共に、部品としての静的強度を確保する上で重要な元素である。従ってCは、0.05%以上、好ましくは0.10%以上、さらに好ましくは0.15%以上、特に0.17%以上とする。一方、Cが過剰になると硬くなりすぎて、冷間鍛造性や被削性が低下する。従ってCは、0.3%以下、好ましくは0.25%以下、さらに好ましくは0.23%以下とする。   C is an important element for securing the core hardness required when the case-hardened steel of the present invention is subjected to surface hardening heat treatment to obtain a part, and ensuring the static strength of the part. Accordingly, C is 0.05% or more, preferably 0.10% or more, more preferably 0.15% or more, and particularly 0.17% or more. On the other hand, when C is excessive, it becomes too hard, and cold forgeability and machinability deteriorate. Therefore, C is 0.3% or less, preferably 0.25% or less, more preferably 0.23% or less.

Siは脱酸剤として使用される元素であり、鋼中に必ず残存するが、Siが多くなりすぎると鋼材が硬くなりすぎて冷間鍛造性や被削性が低下する。従ってSiは、2.0%以下、好ましくは1.5%以下、さらに好ましくは1.0%以下、特に0.5%以下とする。一方、Siは焼戻し処理時の軟化を抑制する作用を有しており、本発明の肌焼き鋼を表面硬化熱処理したときの表層硬さを確保するのに有効であるため、前記上限量以下の範囲で積極的に多くしてもよい。かかる効果はSi量が多くなるにつれて顕著となるが、好ましくは0.05%以上、さらに好ましくは0.10%以上、特に0.2%以上とすることが推奨される。   Si is an element used as a deoxidizer, and it always remains in the steel. However, if the amount of Si is excessive, the steel material becomes too hard and cold forgeability and machinability deteriorate. Therefore, Si is 2.0% or less, preferably 1.5% or less, more preferably 1.0% or less, and particularly 0.5% or less. On the other hand, Si has an action of suppressing softening during tempering treatment, and is effective in securing the surface layer hardness when the case hardening steel of the present invention is subjected to surface hardening heat treatment. You may actively increase the range. Such an effect becomes more prominent as the amount of Si increases, but it is recommended that 0.05% or more, more preferably 0.10% or more, particularly 0.2% or more be recommended.

Mnも酸化物系介在物量を低減して鋼材の内部品質を高めるために脱酸剤として添加される元素であり、鋼中に必ず残存するが、Mnが多くなりすぎると鋼材の硬さが上昇するため冷間鍛造性が低下する。また中心偏析が顕著となって逆に内部品質が劣化するとともに縞状組織も顕著となり、材質のバラツキが大きくなる結果、耐衝撃特性が低下する。従ってMnは、2.0%以下、好ましくは1.5%以下、さらに好ましくは1.0%以下とする。一方、Mnは表面硬化熱処理における焼入れ(例えば、浸炭焼入れ、浸炭窒化焼入れなど)の時の焼入性を著しく向上させる効果がある。かかる効果はMn量が多くなるにつれて顕著となるが、好ましくは0.1%以上、さらに好ましくは0.3%以上、特に0.5%以上とすることが推奨される。   Mn is an element added as a deoxidizer to reduce the amount of oxide inclusions and improve the internal quality of steel, and it always remains in the steel, but if Mn is too much, the hardness of the steel increases. Therefore, the cold forgeability is reduced. In addition, the center segregation becomes conspicuous, the internal quality deteriorates, and the stripe structure becomes conspicuous. As a result, the material variation increases, resulting in a decrease in impact resistance. Therefore, Mn is 2.0% or less, preferably 1.5% or less, more preferably 1.0% or less. On the other hand, Mn has an effect of remarkably improving the hardenability during quenching (for example, carburizing quenching, carbonitriding quenching, etc.) in the surface hardening heat treatment. Such an effect becomes more prominent as the amount of Mn increases, but it is preferably 0.1% or more, more preferably 0.3% or more, and particularly preferably 0.5% or more.

Pは鋼材中に不可避的に含まれる元素であり、結晶粒界に偏析して部品の耐衝撃特性を低下させる。従ってPは、0.03%以下、好ましくは0.02%以下、さらに好ましくは0.015%以下とする。   P is an element inevitably contained in the steel material, and segregates at the grain boundary to lower the impact resistance of the part. Therefore, P is 0.03% or less, preferably 0.02% or less, more preferably 0.015% or less.

SはMnと結合してMnS系介在物を生成し、鋼の被削性を改善するために有用である。従ってSは0.005%以上、好ましくは0.010%以上、さらに好ましくは0.015%以上とする。しかしSが過剰になると、粗大なMnSが生成することにより冷間鍛造性が悪化し、また耐衝撃特性も低下する。従ってSは、0.2%以下、好ましくは0.1%以下、さらに好ましくは0.06%以下、特に0.03%以下とする。   S combines with Mn to produce MnS inclusions and is useful for improving the machinability of steel. Therefore, S is 0.005% or more, preferably 0.010% or more, more preferably 0.015% or more. However, when S is excessive, coarse MnS is generated, so that cold forgeability is deteriorated and impact resistance is also deteriorated. Therefore, S is 0.2% or less, preferably 0.1% or less, more preferably 0.06% or less, and particularly 0.03% or less.

Crは炭化物に固溶して硬さを向上させる作用があり、耐摩耗性の向上に有効であるため、本発明では積極的に添加するものとする。Crは、好ましくは0.1%以上、さらに好ましくは0.2%以上、特に0.3%以上とすることが推奨される。しかしCrが過剰になると、鋼材の硬さが上昇するため冷間鍛造性が悪化する。従ってCrは、2.0%以下、好ましくは1.5%以下、さらに好ましくは1.2%以下とする。   Since Cr has the effect of improving the hardness by dissolving in a carbide, and is effective in improving the wear resistance, it is positively added in the present invention. It is recommended that Cr is 0.1% or more, more preferably 0.2% or more, and particularly 0.3% or more. However, when Cr is excessive, the hardness of the steel material increases, so that cold forgeability deteriorates. Therefore, Cr is 2.0% or less, preferably 1.5% or less, more preferably 1.2% or less.

Tiは鋼中のNやCと結びついて、微細な炭化物、窒化物、又は炭窒化物を生成し、表面硬化熱処理における加熱工程での結晶粒成長を抑制する効果がある。また微細なTi炭化物は、水素をトラップするため、耐遅れ破壊特性を向上させるのにも有用である。さらには、焼入性向上などのためにBを添加した場合には、Bの添加効果を阻害するBNの生成を抑制するのにも有用である。従ってTiは、0.01%以上、好ましくは0.02%以上、さらに好ましくは0.03%以上とする。なおTiは硫化物や炭硫化物を生成して冷間鍛造性を低下させる特性を有しているが、後述するように、本発明ではCaによってかかるTiの悪影響を抑制している。しかしTiが過剰になると硫化物や炭硫化物の生成を抑制しきれず、冷間鍛造性が悪化してしまう。また粗大な窒化物や酸化物を生成し、鋼の靭性を低下させる。従ってTiは、0.2%以下、好ましくは0.1%以下、さらに好ましくは0.08%以下とする。   Ti combines with N and C in steel to produce fine carbides, nitrides, or carbonitrides, and has the effect of suppressing crystal grain growth in the heating step in the surface hardening heat treatment. Fine Ti carbides are also useful for improving delayed fracture resistance because they trap hydrogen. Furthermore, when B is added for improving hardenability, it is also useful for suppressing the formation of BN that inhibits the effect of adding B. Therefore, Ti is 0.01% or more, preferably 0.02% or more, more preferably 0.03% or more. Ti has the property of producing sulfides and carbon sulfides and lowering the cold forgeability. However, as described later, in the present invention, the adverse effect of Ti is suppressed by Ca. However, when Ti is excessive, generation of sulfides and carbon sulfides cannot be suppressed, and cold forgeability is deteriorated. In addition, coarse nitrides and oxides are produced, and the toughness of the steel is lowered. Therefore, Ti is 0.2% or less, preferably 0.1% or less, more preferably 0.08% or less.

Caは、所定量以上のTiとSを含有する本発明鋼にとって極めて重要である。すなわちTiやSには、それぞれ、上述したように優れた効果があるものの、該TiとSが共存すると、Ti硫化物を形成し、冷間鍛造性を低下させてしまうという弊害がある。ところがここにCaも共存させると硫化物を、Ti硫化物ではなく、軟質のMn・Ca系硫化物(例えばMn硫化物、Ca硫化物、Caを一部固溶したMn硫化物など)主体のものとすることができ、冷間鍛造性を顕著に改善することができる。Caは0.0001%以上、好ましくは0.0003%以上、さらに好ましくは0.0005%以上である。なおCaが過剰になると粗大なCa系介在物(CaOなど)が生成して疲労強度が大幅に低下する。従ってCaは、0.02%以下、好ましくは0.01%以下、さらに好ましくは0.006%以下、特に0.003%以下とする。   Ca is extremely important for the steel of the present invention containing a predetermined amount or more of Ti and S. That is, Ti and S have excellent effects as described above, but when Ti and S coexist, Ti sulfide is formed and cold forgeability is deteriorated. However, when Ca also coexists here, the sulfide is not Ti sulfide but soft Mn · Ca-based sulfide (for example, Mn sulfide, Ca sulfide, Mn sulfide partially dissolved in Ca, etc.). The cold forgeability can be remarkably improved. Ca is 0.0001% or more, preferably 0.0003% or more, and more preferably 0.0005% or more. When Ca is excessive, coarse Ca-based inclusions (CaO or the like) are generated, and the fatigue strength is greatly reduced. Therefore, Ca is 0.02% or less, preferably 0.01% or less, more preferably 0.006% or less, and particularly 0.003% or less.

Nは鋼材に不可避的に含まれる元素であるが、Nが過剰になると熱間加工性が低下する。従ってNは0.03%以下、好ましくは0.025%以下、さらに好ましくは0.020%以下とする。一方、Nは前述のTiや、後述の任意添加元素であるAl、Vなどと結びついて窒化物や炭窒化物を形成し、表面硬化熱処理時の加熱工程でのオーステナイト粒成長を抑制する効果がある。従ってNは、例えば、0.001%以上、好ましくは0.002%以上、さらに好ましくは0.003%以上となっているのが望ましい。   N is an element inevitably contained in the steel material, but when N is excessive, hot workability is reduced. Therefore, N is 0.03% or less, preferably 0.025% or less, more preferably 0.020% or less. On the other hand, N forms nitrides and carbonitrides in combination with the above-mentioned Ti and optional additive elements described later, such as Al and V, and has the effect of suppressing austenite grain growth in the heating process during surface hardening heat treatment. is there. Therefore, N is desirably 0.001% or more, preferably 0.002% or more, and more preferably 0.003% or more.

Oも鋼材に不可避的に含まれる元素であるが、過剰になると粗大なCaOやTiO2などの介在物が生成して疲労強度が低下する。従ってOは、0.002%以下、好ましくは0.0015%以下、さらに好ましくは0.001%以下とする。 O is also an element inevitably contained in the steel material, but if it is excessive, coarse inclusions such as CaO and TiO 2 are generated and the fatigue strength is lowered. Therefore, O is 0.002% or less, preferably 0.0015% or less, more preferably 0.001% or less.

本発明の肌焼き鋼は、上述した各成分が必須元素であるが、必要に応じてさらに他の元素を含有していてもよい。任意元素としては、結晶粒粗大化抑制元素(Al、Vなど)、耐食性向上元素(Ni、Cuなど)、焼入性向上元素(Mo、Bなど)、被削性向上元素(Pb、Biなど)、耐衝撃特性向上元素(Mg、REMなど)などが挙げられる。これら任意元素は、適宜組み合わせて添加してもよい。なおZrも前記結晶粒粗大化抑制元素として知られているが、ZrSを生成して冷間鍛造性が低下するため、使用しない方が望ましい。   In the case-hardened steel of the present invention, each component described above is an essential element, but may further contain other elements as necessary. As optional elements, grain coarsening suppression elements (Al, V, etc.), corrosion resistance improving elements (Ni, Cu, etc.), hardenability improving elements (Mo, B, etc.), machinability improving elements (Pb, Bi, etc.) ), Impact resistance improving elements (Mg, REM, etc.). These optional elements may be added in combination as appropriate. Zr is also known as an element for suppressing grain coarsening, but it is preferable not to use ZrS because it generates ZrS and cold forgeability deteriorates.

結晶粒粗大化抑制元素であるAl、Vなどは、いずれも、微細介在物(炭化物、窒化物、炭窒化物など)を生成するため、表面硬化熱処理時の加熱工程において結晶粒の成長を抑制することができる。かかる効果はこれらの元素の量が多くなるにつれて顕著となるが、例えば、Alは0.01%以上(好ましくは0.02%以上)、Vは0.02%以上(好ましくは0.05%以上)とすることが推奨される。しかしAlが過剰になると粗大で硬質な非金属介在物(Al23)が生成し、冷間鍛造性が低下すると共に、衝撃強度や転動疲労寿命も低下する。またVが過剰になると、鋼の靭性が低下する。従ってAlは0.2%以下(好ましくは0.1%以下、さらに好ましくは0.04%以下)、Vは0.5%以下(好ましくは0.3%以下)とする。 Al, V, etc., which are crystal grain coarsening inhibiting elements, all generate fine inclusions (carbides, nitrides, carbonitrides, etc.), and therefore suppress the growth of crystal grains in the heating process during surface hardening heat treatment. can do. Such effects become more prominent as the amount of these elements increases. For example, Al is 0.01% or more (preferably 0.02% or more), and V is 0.02% or more (preferably 0.05%). It is recommended that However, when Al is excessive, coarse and hard non-metallic inclusions (Al 2 O 3 ) are generated, cold forgeability is lowered, and impact strength and rolling fatigue life are also lowered. Moreover, when V becomes excessive, the toughness of steel will fall. Therefore, Al is 0.2% or less (preferably 0.1% or less, more preferably 0.04% or less), and V is 0.5% or less (preferably 0.3% or less).

上記Al、Vなどは、単独で添加してもよく、適宜組み合わせて添加してもよい。ただしAlは溶鋼の脱酸剤としても使用することが多いため、この脱酸剤由来のAlを有効利用することが推奨される。従って、好ましくはAlを優先的に使用し、必要に応じてさらVを添加する。   The above Al, V, etc. may be added alone or in appropriate combination. However, since Al is often used as a deoxidizer for molten steel, it is recommended to effectively use Al derived from this deoxidizer. Therefore, preferably, Al is used preferentially and further V is added as necessary.

耐食性向上元素であるNi、Cuなどは、これらの元素の量が多くなるにつれてその耐食効果が顕著となるが、例えば、Niは0.01%以上(好ましくは0.1%以上)、Cuは0.01%以上(好ましくは0.03%以上)とすることが推奨される。これらNi、Cuなどは、単独で又は適宜組み合わせて添加できる。特にNiは鋼の耐衝撃特性を向上させる効果もあり、有用である。しかしNiを過剰に添加しても効果が飽和して、コスト上昇を招くだけである。またCuを過剰に添加すると、硬質なε−Cu相が生成することから冷間鍛造性が低下し、さらには熱間延性も低下する。従ってNiは3%以下(好ましくは1.5%以下)、Cuは0.5%以下(好ましくは0.3%以下、さらに好ましくは0.05%以下)とする。   Corrosion resistance improving elements such as Ni and Cu become more prominent as the amount of these elements increases. For example, Ni is 0.01% or more (preferably 0.1% or more), Cu is It is recommended that the content be 0.01% or more (preferably 0.03% or more). These Ni, Cu and the like can be added alone or in appropriate combination. Ni is particularly useful because it has the effect of improving the impact resistance of steel. However, even if Ni is added excessively, the effect is saturated and only the cost is increased. Moreover, when Cu is added excessively, since a hard ε-Cu phase is generated, cold forgeability is lowered, and hot ductility is also lowered. Therefore, Ni is 3% or less (preferably 1.5% or less), and Cu is 0.5% or less (preferably 0.3% or less, more preferably 0.05% or less).

焼入性向上元素であるMo、Bなどは、これらの元素の量が多くなるにつれてその焼入れ性向上効果が顕著となるが、例えば、Moは0.01%以上(好ましくは0.05%以上、さらに好ましくは0.10%以上)、Bは0.0005%以上(好ましくは0.0008%以上)とすることが推奨される。これらMo、Bなどは、単独で又は適宜組み合わせて添加できる。なおMoは焼戻し処理時の軟化を抑制して、表面硬化熱処理部品の表層硬さを確保するのに有効であり、また耐水素脆性の向上にも有効である。一方Bは結晶粒界を強化して衝撃強度を高める作用がある。これら焼入性向上以外の効果を考慮して、MoやBを選択的に添加してもよい。ただしMoを過剰に添加すると、鋼が硬くなりすぎて冷間鍛造性が低下し、また被削性も低下する。Bを過剰に添加すると、冷間鍛造性が低下し、熱間延性も低下する。従ってMoは1.0%以下(好ましくは0.4%以下)、Bは0.005%以下(好ましくは0.002%以下)とする。   As for the hardenability improving elements Mo, B, etc., the effect of improving the hardenability becomes remarkable as the amount of these elements increases. For example, Mo is 0.01% or more (preferably 0.05% or more). It is recommended that B be 0.0005% or more (preferably 0.0008% or more). These Mo, B and the like can be added alone or in appropriate combination. Mo is effective in suppressing the softening during the tempering process, ensuring the surface hardness of the surface-hardened heat-treated component, and effective in improving the hydrogen embrittlement resistance. On the other hand, B has an effect of strengthening the crystal grain boundary and increasing the impact strength. In consideration of effects other than these hardenability improvements, Mo or B may be selectively added. However, when Mo is added excessively, the steel becomes too hard and cold forgeability is lowered, and machinability is also lowered. When B is added excessively, cold forgeability will fall and hot ductility will also fall. Therefore, Mo is 1.0% or less (preferably 0.4% or less), and B is 0.005% or less (preferably 0.002% or less).

被削性向上元素であるPb、Biなどは、これらの元素が多くなるにつれてその被削性向上効果が顕著となるが、例えば、Pbは0.01%以上(好ましくは0.02%以上)、Biは0.01%以上(好ましくは0.02%以上)とすることが推奨される。しかしこれらの元素が過剰になると、材料強度が低下する。従ってPbは0.1%以下(好ましくは0.06%以下)、Biは:0.1%以下(好ましくは0.06%以下)とする。これらPb及びBiは、単独で又は組み合わせて添加できる。   The machinability improving elements Pb, Bi and the like become more prominent as the amount of these elements increases. For example, Pb is 0.01% or more (preferably 0.02% or more). , Bi is recommended to be 0.01% or more (preferably 0.02% or more). However, when these elements become excessive, the material strength decreases. Therefore, Pb is 0.1% or less (preferably 0.06% or less), and Bi is 0.1% or less (preferably 0.06% or less). These Pb and Bi can be added alone or in combination.

MgやREM[希土類元素(Rare Earth Metal)。例えば、La、Ce、Nd、Prなど]は、鋼材中の硫化物の展伸を抑制することによって、耐衝撃特性を向上させる。これらの元素が多くなるにつれてその効果が顕著となるが、例えば、Mgは0.0001%以上(好ましくは0.0003%以上)、REMは0.01%以上(好ましくは0.02%以上)とする。しかしこれらMgやREMが過剰となると、粗大な介在物(硫化物、酸化物など)を生成し、部品の疲労強度の低下を招く。従ってMgは0.01%以下(好ましくは0.006%以下)、REMは0.1%以下(好ましくは0.06%以下)とする。   Mg and REM [rare earth metal]. For example, La, Ce, Nd, Pr, etc.] improve the impact resistance by suppressing the extension of sulfides in the steel. The effect becomes remarkable as the amount of these elements increases. For example, Mg is 0.0001% or more (preferably 0.0003% or more), and REM is 0.01% or more (preferably 0.02% or more). And However, if these Mg and REM are excessive, coarse inclusions (sulfides, oxides, etc.) are generated, and the fatigue strength of the parts is reduced. Accordingly, Mg is 0.01% or less (preferably 0.006% or less), and REM is 0.1% or less (preferably 0.06% or less).

本発明の肌焼き鋼では、残部の成分はFe及び不可避不純物である。   In the case-hardened steel of the present invention, the remaining components are Fe and inevitable impurities.

肌焼き鋼は、冷間鍛造、切削などによって適宜所定形状に加工した後、表面硬化熱処理に供することによって鋼製品(部品)とする。本発明の肌焼き鋼は、Sによって被削性が改善されており、またTiによって表面硬化熱処理時の結晶粒の粗大化が抑制されており、しかもCaによってTiSの生成を抑制して冷間鍛造性が改善されているため、前述したような冷間鍛造、切削、表面硬化熱処理などを組み合わせて製造される鋼製品(例えば、歯車、軸受、シャフト類、ミクロネジなど)を製造するのに極めて有用である。   The case-hardened steel is processed into a predetermined shape as appropriate by cold forging, cutting, etc., and then subjected to surface hardening heat treatment to obtain a steel product (part). In the case-hardened steel of the present invention, the machinability is improved by S, and the coarsening of crystal grains during the surface hardening heat treatment is suppressed by Ti, and further, the formation of TiS is suppressed by Ca and cold. Due to improved forgeability, it is extremely useful for manufacturing steel products (for example, gears, bearings, shafts, micro screws, etc.) manufactured by combining cold forging, cutting, surface hardening heat treatment, etc. as described above. Useful.

なお前記表面硬化熱処理には、浸炭処理、窒化処理、浸炭窒化処理などの化学的表面硬化処理が含まれ、該化学的表面硬化処理は焼入れ、焼戻し処理を伴うものも含まれる。特に浸炭処理、浸炭窒化処理は比較的高温で長時間に亘って加熱されるため、結晶粒が粗大化しやすい処理となっているが、本発明ではTiによって結晶粒の粗大化を抑制しているため、問題なく部品を製造できる。   The surface hardening heat treatment includes a chemical surface hardening treatment such as a carburizing treatment, a nitriding treatment, and a carbonitriding treatment, and the chemical surface hardening treatment includes those accompanied by quenching and tempering treatment. In particular, the carburizing treatment and the carbonitriding treatment are heated at a relatively high temperature for a long time, so that the crystal grains are likely to be coarsened. In the present invention, the grain coarsening is suppressed by Ti. Therefore, parts can be manufactured without problems.

以下、実施例を挙げて本発明をより具体的に説明するが、本発明はもとより下記実施例によって制限を受けるものではなく、前・後記の趣旨に適合し得る範囲で適当に変更を加えて実施することも勿論可能であり、それらはいずれも本発明の技術的範囲に包含される。   EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

実験例1
下記表1〜4に示す化学成分の鋼材を小型溶製炉によって溶製し、直径50mmの棒状体に熱間鍛造した後、溶体化処理(1250℃×1時間)及び焼ならし処理(900℃×1時間)を行い、球状化焼鈍(750℃で5時間加熱した後、10℃/時間の速度で650℃まで冷却し、650℃に到達した時点で空冷に切り替えた)を行った。
Experimental example 1
Steel materials having chemical components shown in Tables 1 to 4 below are melted in a small melting furnace and hot forged into a rod-shaped body having a diameter of 50 mm, and then subjected to solution treatment (1250 ° C. × 1 hour) and normalizing treatment (900 And spheroidizing annealing (heating at 750 ° C. for 5 hours, cooling to 650 ° C. at a rate of 10 ° C./hour, and switching to air cooling when reaching 650 ° C.).

得られた鋼材の特性を以下のようにして評価した。   The properties of the obtained steel materials were evaluated as follows.

[冷間鍛造性]
切削加工によって図1に示すような直径20mm×高さ30mmの概略円筒状の試験片を作成した。なお該試験片には、側面に軸方向に沿った2つのV字型切り欠きが形成されており、このV字型切り欠きは図2に示すように、底部のRが0.05mm、角度60°、斜辺部の長さL=0.3〜0.05mmとなっている。
[Cold forgeability]
A substantially cylindrical test piece having a diameter of 20 mm and a height of 30 mm as shown in FIG. 1 was prepared by cutting. The test piece is formed with two V-shaped notches along the axial direction on the side surface. As shown in FIG. 2, the V-shaped notch has a bottom R of 0.05 mm and an angle. It is 60 ° and the length L of the hypotenuse part is 0.3 to 0.05 mm.

1)試験片を圧縮率[=(圧縮前高さ−圧縮後高さ)/圧縮前高さ]が50%となるように軸方向に冷間で圧縮した。試験数を10とし、1つでも割れが観察された場合には、試験を終了した。   1) The test piece was cold compressed in the axial direction so that the compression ratio [= (height before compression−height after compression) / height before compression] was 50%. The test was terminated when the number of tests was 10 and even one crack was observed.

2)割れが全く観察されなかった場合には、圧縮率をさらに2.5%増加して、前記1)と同様に試験した。これを繰り返し、割れなく圧縮できる最大の圧縮率(割れ発生限界)を調べた。   2) When no cracks were observed, the compression rate was further increased by 2.5%, and the test was performed in the same manner as in 1) above. This was repeated, and the maximum compression ratio (cracking limit) that could be compressed without cracking was investigated.

[結晶粒の粗大化防止特性]
切り欠きがない以外は冷間鍛造性の場合と同様の試験片を圧縮率70%で冷間鍛造した後、温度950℃、975℃、1000℃、または1025℃で3時間保持し、水冷後の結晶粒度を調べた。3時間保持しても、結晶粒が粗大(JISに規定する粒度番号が4番以下)とならなかった最高の温度を求めた。該温度が高いほど、結晶粒の粗大化防止特性に優れている。なお975℃は、通常の浸炭処理の最高温度程度である。
[Characteristics for preventing grain coarsening]
A test piece similar to the case of cold forgeability except that there is no notch is cold forged at a compression rate of 70%, then held at a temperature of 950 ° C., 975 ° C., 1000 ° C., or 1025 ° C. for 3 hours, and after water cooling The crystal grain size of was examined. The highest temperature at which the crystal grains did not become coarse (the grain size number specified in JIS was 4 or less) even after holding for 3 hours was determined. The higher the temperature, the better the crystal grain coarsening prevention property. In addition, 975 degreeC is about the highest temperature of a normal carburizing process.

結果を表1〜4に示す。   The results are shown in Tables 1-4.

表3から明らかなように、Caを含有しないNo.28〜35は、Tiによって結晶粒の粗大化は抑制されているものの、冷間鍛造性が悪い。これに対して表1〜2から明らかなように、Caを添加すると、結晶粒の粗大化抑制能は維持したまま、冷間鍛造性が向上する(No.1〜27)。なおCaを添加しても、他の成分量が不適切であると、不具合が生じる(表4参照)。すなわちC、Si、Mn、S、Cr、Ti、Al、Cu、Mo、又はBが過剰な鋼では、冷間鍛造性が低下してしまう(No.37〜39、41〜43、46〜49)。またCが不足しているNo.36では強度が不十分となる。Pが過剰なNo.40では、耐衝撃特性が低下する。Caが過剰なNo.44では、疲労強度が低下する。Oが過剰なNo.45でも、疲労強度が低下する。さらにNo.39(Mn過剰)、No.42(Cr過剰)、及びNo.48(Mo過剰)な例では、過冷組織が生成して組織の均一性が低下し、結晶粒が粗大化する。   As is apparent from Table 3, No. containing no Ca. In Nos. 28 to 35, although coarsening of crystal grains is suppressed by Ti, cold forgeability is poor. On the other hand, as apparent from Tables 1-2, when Ca is added, the cold forgeability is improved while maintaining the ability to suppress coarsening of crystal grains (Nos. 1-27). Even if Ca is added, problems occur if the amounts of other components are inappropriate (see Table 4). That is, cold forgeability deteriorates in steels with excess C, Si, Mn, S, Cr, Ti, Al, Cu, Mo, or B (No. 37 to 39, 41 to 43, 46 to 49). ). In addition, no. At 36, the strength is insufficient. No. with excessive P At 40, the impact resistance is reduced. No. with excessive Ca. In 44, the fatigue strength decreases. No. with excessive O Even at 45, fatigue strength decreases. Furthermore, no. 39 (Mn excess), no. 42 (Cr excess), and no. In an example of 48 (Mo excess), a supercooled structure is generated, the uniformity of the structure is lowered, and the crystal grains are coarsened.

図1は実施例で使用した冷間鍛造試験片の形状を示す概略斜視図である。FIG. 1 is a schematic perspective view showing the shape of a cold forging test piece used in the examples. 図2は図1の試験片の切り欠き部を示す部分拡大正面図である。FIG. 2 is a partially enlarged front view showing a notch portion of the test piece of FIG.

Claims (5)

質量%で、
C :0.05〜0.3%、
Si:2.0%以下(0%を含まない)、
Mn:2.0%以下(0%を含まない)、
P :0.03%以下(0%を含まない)、
S :0.005〜0.1%、
Cr:2.0%以下(0%を含まない)、
Ti:0.02〜0.08%、
Ca:0.0001〜0.02%、
N :0.03%以下(0%を含まない)、及び
O :0.002%以下(0%を含まない)
を含有し、残部はFe及び不可避不純物であるTi含有肌焼き鋼。
% By mass
C: 0.05 to 0.3%
Si: 2.0% or less (excluding 0%),
Mn: 2.0% or less (excluding 0%),
P: 0.03% or less (excluding 0%),
S: 0.005 to 0.1%,
Cr: 2.0% or less (excluding 0%),
Ti: 0.02 to 0.08%,
Ca: 0.0001 to 0.02%,
N: 0.03% or less (not including 0%), and O: 0.002% or less (not including 0%)
Ti-containing case-hardened steel, the balance being Fe and inevitable impurities.
さらにAl:0.2%以下(0%を含まない)、及びV:0.5%以下(0%を含まない)から選択された少なくとも1種を含有する請求項1に記載のTi含有肌焼き鋼。   The Ti-containing skin according to claim 1, further comprising at least one selected from Al: 0.2% or less (not including 0%) and V: 0.5% or less (not including 0%). Baked steel. さらにNi:3%以下(0%を含まない)、及びCu:0.5%以下(0%を含まない)から選択された少なくとも1種を含有する請求項1又は2に記載のTi含有肌焼き鋼。   The Ti-containing skin according to claim 1 or 2, further comprising at least one selected from Ni: 3% or less (not including 0%) and Cu: 0.5% or less (not including 0%). Baked steel. さらにMo:1.0%以下(0%を含まない)、及びB:0.002%以下(0%を含まない)から選択された少なくとも1種を含有する請求項1〜3のいずれかに記載のTi含有肌焼き鋼。 Furthermore, at least 1 sort (s) selected from Mo: 1.0% or less (excluding 0%) and B: 0.002% or less (not including 0%) is contained in any one of Claims 1-3 The Ti-containing case-hardening steel described. さらにMg:0.01%以下(0%を含まない)、及びREM:0.1%以下(0%を含まない)から選択された少なくとも1種を含有する請求項1〜4のいずれかに記載のTi含有肌焼き鋼。   Furthermore, Mg: 0.01% or less (not including 0%) and REM: at least one selected from 0.1% or less (not including 0%) The Ti-containing case-hardening steel described.
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