JP7188432B2 - Case-hardening steel for warm forging and forged rough shape manufactured using the same - Google Patents

Case-hardening steel for warm forging and forged rough shape manufactured using the same Download PDF

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JP7188432B2
JP7188432B2 JP2020212173A JP2020212173A JP7188432B2 JP 7188432 B2 JP7188432 B2 JP 7188432B2 JP 2020212173 A JP2020212173 A JP 2020212173A JP 2020212173 A JP2020212173 A JP 2020212173A JP 7188432 B2 JP7188432 B2 JP 7188432B2
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ferrite
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JP2022098655A (en
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孝佳 杉浦
亮太 高尾
友明 湯谷
岳幸 島田
秀平 横井
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Aichi Steel Corp
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21J—FORGING; HAMMERING; PRESSING METAL; RIVETING; FORGE FURNACES
    • B21J5/00—Methods for forging, hammering, or pressing; Special equipment or accessories therefor
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/06—Surface hardening
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/32—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for gear wheels, worm wheels, or the like
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron

Description

本発明は、温間鍛造用肌焼鋼及びこれを用いて製造した鍛造粗形材に関する。 TECHNICAL FIELD The present invention relates to a case-hardened steel for warm forging and a forged blank manufactured using the same.

歯車に代表されるトランスミッション用の鋼部品等は、熱間鍛造によって得た鍛造粗形材に、機械加工及び表面硬化処理を施して製造されることが多い。熱間鍛造は、鍛造加熱温度が高いためエネルギー消費が比較的大きく、表面にスケールが発生することにより歩留まりが悪く、寸法精度確保が困難という課題がある。また、熱間鍛造後の鍛造粗形材は、硬さの向上によってそのままでは機械加工性が良くないため、機械加工前に硬度を低下させる熱処理を施すことが必須となり、それによるエネルギー消費も問題である。 Steel parts for transmissions, typified by gears, are often manufactured by applying machining and surface hardening treatment to forged blanks obtained by hot forging. Hot forging involves relatively large energy consumption due to the high forging heating temperature, and has the problem of poor yield due to the formation of scales on the surface and difficulty in ensuring dimensional accuracy. In addition, since the forged blanks after hot forging do not have good machinability due to the increased hardness, it is essential to apply heat treatment to lower the hardness before machining, and the resulting energy consumption is also a problem. is.

熱間鍛造を選択した場合の上記課題の一部は、鍛造温度が低い温間鍛造に代替することによって解消できる可能性がある。すなわち、温間鍛造は、熱間鍛造と比べ鍛造温度が低いため、エネルギー消費量が低く、またスケール発生量も少ないため歩留まりが良く、さらには寸法精度が良く、次工程の取り代が少ないといった利点がある。 Some of the above problems when hot forging is selected may be solved by substituting for warm forging with a lower forging temperature. In other words, since warm forging has a lower forging temperature than hot forging, it consumes less energy and produces less scale. There are advantages.

しかしながら、従来の肌焼鋼を用いた場合、温間鍛造を選択した場合であっても、機械加工前の熱処理を省略することは困難である。例えば、ＳＣＭ４２０やＳＣｒ４２０などの通常の肌焼鋼を用いて温間鍛造すると、熱間鍛造の場合よりは程度は低いが、切削加工性を悪化させる程度まで鍛造後の硬さが増加してしまう。そのため、これらの通常の肌焼鋼を用いた温間鍛造後の鍛造粗形材をそのまま切削すると、切り屑処理性の悪化、工具摩耗の増大を招いてしまう。したがって、従来の通常の肌焼鋼を用いる限りは、温間鍛造を選択することによるメリットは限られており、機械加工前の焼準や焼鈍などの熱処理を必須とすることによるエネルギー消費、スケール発生による歩留まり低下等の不具合の解消は困難である。 However, when conventional case-hardened steel is used, it is difficult to omit heat treatment before machining even when warm forging is selected. For example, if normal case-hardening steel such as SCM420 or SCr420 is used for warm forging, the hardness after forging increases to such an extent that machinability deteriorates, although the degree is lower than in the case of hot forging. . Therefore, if these forged blanks after warm forging using these ordinary case-hardened steels are cut as they are, chip disposability deteriorates and tool wear increases. Therefore, as long as conventional ordinary case-hardened steel is used, the merits of choosing warm forging are limited. It is difficult to eliminate problems such as a decrease in yield due to the occurrence.

また、従来、温間鍛造用の肌焼鋼としては、例えば、後述する特許文献１～３に記載の技術が開示されている。特許文献１には、温間鍛造を行うこと及び切削性に関する言及はあるものの、温間鍛造の後に切削を行う場合の切削性向上についての記載はない。また、特許文献２には、温間鍛造と切削性に関する記載があるものの、温間鍛造後の切削性の評価はなされておらず、かつ、化学成分からみて最終的な高強度化が見込めない鋼しか記載がない。また、特許文献３には、温間鍛造の記載はあるものの、その後の切削性の評価はなされていない。したがって、特許文献１～３からは、温間鍛造後に熱処理を加えることなく切削性を向上させるために、鋼の化学成分組成をどのように工夫することが必要かを導くことはできない。 Conventionally, as case-hardened steel for warm forging, for example, techniques described in Patent Documents 1 to 3, which will be described later, have been disclosed. Although Patent Document 1 mentions warm forging and machinability, there is no description of improvement in machinability when cutting is performed after warm forging. In addition, although Patent Document 2 describes warm forging and machinability, the machinability after warm forging is not evaluated, and the final increase in strength cannot be expected from the chemical composition. Only steel is mentioned. Further, although Patent Document 3 describes warm forging, it does not evaluate machinability after that. Therefore, from Patent Documents 1 to 3, it is not possible to derive how the chemical composition of steel should be devised in order to improve machinability without applying heat treatment after warm forging.

特開２００７－３２１２１１号公報JP 2007-321211 A 特開２００１－１３１６８６号公報Japanese Patent Application Laid-Open No. 2001-131686 特開昭６０－２６２９４１号公報JP-A-60-262941

本発明は、かかる背景に鑑みてなされたものであり、温間鍛造後の切削性に優れた肌焼鋼及びこれを用いて温間鍛造を施した切削性に優れた鍛造粗形材を提供しようとするものである。 The present invention has been made in view of such a background, and provides a case-hardened steel having excellent machinability after warm forging and a forged blank having excellent machinability obtained by warm forging using the same. I am trying to.

本発明の一態様は、鍛造温度が８５０℃～１１００℃である温間鍛造用の肌焼鋼であって、
質量％において、Ｃ：０．１５～０．２３％、Ｓｉ：０．６０～０．９５％、Ｍｎ：０．６０～１．２０％、Ｐ：０．０３５％以下、Ｓ：０．０３５％以下、Ｃｒ：１．５０％以下、Ａｌ：０．０５０％以下、Ｔｉ：０．０１～０．０５％、Ｂ：０．０００５～０．００５０％、Ｎ：０．００２０～０．０２００％を含み、
任意元素としてＭｏ：０．２０％以下、任意元素としてＮｂ：０．０１～０．０５％を含み、
残部がＦｅ及び不可避的不純物からなる化学成分組成を有し、
式１：９０≧－１２０＊Ｃ＋２０．１＊Ｓｉ－５．３＊Ｍｎ－８．５＊Ｍｏ＋９６≧８０、及び、
式２：１６１≧４０＊Ｓｉ＋３９＊Ｍｎ＋１０＊Ｃｒ＋３０＊Ｍｏ＋８４≧１４１、
（ただし、式１及び式２における元素記号は、各元素の含有率（質量％）を意味する。）を満足する、温間鍛造用肌焼鋼にある。 One aspect of the present invention is a case hardening steel for warm forging with a forging temperature of 850° C. to 1100° C.,
In mass %, C: 0.15 to 0.23%, Si: 0.60 to 0.95%, Mn: 0.60 to 1.20%, P: 0.035% or less, S: 0.035 % or less, Cr: 1.50% or less , Al: 0.050% or less, Ti: 0.01-0.05%, B: 0.0005-0.0050%, N: 0.0020-0.0200 including %,
Mo: 0.20% or less as an optional element, Nb: 0.01 to 0.05% as an optional element,
Having a chemical composition with the balance being Fe and unavoidable impurities,
Formula 1: 90≧−120*C+20.1*Si−5.3*Mn−8.5*Mo+96≧80, and
Equation 2: 161 ≧40*Si+39*Mn+10*Cr+30*Mo+84≧ 141 ,
(wherein the symbols of the elements in the formulas 1 and 2 mean the contents (% by mass) of the respective elements), the case-hardened steel for warm forging satisfies.

本発明の他の態様は、上記温間鍛造用肌焼鋼を用いて得られた鍛造粗形材であって、
表面硬さが２００ＨＶ以下であり、
フェライト率が、８０％～９０％である金属組織を有し、かつ、
フェライト硬さが、マイクロビッカース硬さにおいて、１４０ｍＨＶ～１６０ｍＨＶである、鍛造粗形材にある。 Another aspect of the present invention is a forged blank obtained using the case hardening steel for warm forging,
The surface hardness is 200 HV or less,
Having a metal structure with a ferrite rate of 80% to 90%, and
The ferrite hardness in the micro Vickers hardness is 140 mHV to 160 mHV in the forged blank.

鋭意検討の結果、温間鍛造後の被削性を確保するためには、マクロ的な硬さの抑制に加え、フェライト率およびフェライト硬さを最適な範囲に制御することが重要であることが見出された。また、温間鍛造を採用して組織が微細化した場合には、切り屑が伸びる傾向があるが、Ｓｉ含有率とフェライト組織（率、硬さ）の調整により、切り屑分断性向上に有効なＳ含有率を特別に増加させずとも、切り屑分断性を向上できることを見出した。そして、Ｓｉ含有率の最適化を図れば、Ｓ含有率の増加を必要としないことから、Ｓ含有率増加に起因して生じうる温間鍛造による割れ発生の懸念を抑えることも可能となる。さらには、フェライト安定化元素であるＳｉとＭｏに着目し、温間鍛造後のフェライト率とフェライト硬さの傾向を調査したところ、上記化学成分組成の具備を基本として、さらに、式１及び式２を満たすことにより、温間鍛造後のフェライト率及びフェライト硬さを最適な範囲内に制御することができ、これにより被削性が確保できることが見い出された。 As a result of intensive studies, in order to ensure machinability after warm forging, it is important to control the ferrite ratio and ferrite hardness within the optimum range, in addition to suppressing the macroscopic hardness. Found. In addition, when warm forging is used to refine the structure, chips tend to elongate, but adjusting the Si content and ferrite structure (ratio, hardness) is effective in improving chip separation. It has been found that the chip splittability can be improved without particularly increasing the S content. If the Si content is optimized, it is not necessary to increase the S content, so it is possible to suppress the concern that cracks may occur due to warm forging due to an increase in the S content. Furthermore, focusing on Si and Mo, which are ferrite stabilizing elements, the tendency of ferrite ratio and ferrite hardness after warm forging was investigated. It was found that by satisfying 2, the ferrite ratio and ferrite hardness after warm forging can be controlled within an optimum range, thereby ensuring machinability.

以上のように上記の基本の化学成分組成の範囲内であって、かつ、上記式１及び式２を具備する特定の化学成分組成からなる肌焼鋼は、温間鍛造を施した後における熱処理を省略しても製造上問題のない切削性を確保できる優れた鍛造粗形材を得ることが可能となる。 As described above, the case-hardened steel having a specific chemical composition within the range of the basic chemical composition and having the above formulas 1 and 2 is subjected to heat treatment after warm forging. Even if is omitted, it is possible to obtain an excellent forged rough shape material that can ensure machinability that does not cause any problems in manufacturing.

上記温間鍛造用肌焼鋼は、鍛造温度が８５０℃～１１００℃である温間鍛造を施すことが予定された鋼である。温間鍛造の鍛造温度は、低すぎると鍛造時の変形抵抗が大きくなって、目的とする形状への成形が難しくなるため、８５０℃以上とし、一方、高すぎると熱間鍛造に比較した省エネ効果が低下してくるため、１１００℃以下とする。 The case-hardened steel for warm forging is intended to be subjected to warm forging at a forging temperature of 850°C to 1100°C. If the forging temperature of warm forging is too low, the deformation resistance during forging will increase, making it difficult to form into the desired shape. Since the effect decreases, the temperature is set to 1100° C. or less.

この温間鍛造用肌焼鋼は、基本的な化学成分組成として、質量％において、Ｃ：０．１５～０．２３％、Ｓｉ：０．６０～０．９５％、Ｍｎ：０．６０～１．２０％、Ｐ：０．０３５％以下、Ｓ：０．０３５％以下、Ｃｒ：１．５０％以下（０％は除く）、Ａｌ：０．０５０％以下、Ｔｉ：０．０１～０．０５％、Ｂ：０．０００５～０．００５０％、Ｎ：０．００２０～０．０２００％を含み、残部がＦｅ及び不可避的不純物からなる化学成分組成を有する。 This case-hardening steel for warm forging has a basic chemical composition in mass% of C: 0.15 to 0.23%, Si: 0.60 to 0.95%, Mn: 0.60 to 1.20%, P: 0.035% or less, S: 0.035% or less, Cr: 1.50% or less (excluding 0%), Al: 0.050% or less, Ti: 0.01-0 0.05%, B: 0.0005-0.0050%, N: 0.0020-0.0200%, and the balance being Fe and unavoidable impurities.

Ｃ：０．１５～０．２３％；
Ｃ（炭素）は、焼き入れ後の必要な強度確保、及び、切り屑処理性悪化防止のために０．１５％以上含有させる。一方、Ｃ含有率が高すぎると、マクロ硬さが高くなりすぎて鍛造後に行う機械加工性が低下するおそれがあるため、０．２３％以下とする。 C: 0.15-0.23%;
C (carbon) is contained in an amount of 0.15% or more to ensure necessary strength after quenching and to prevent deterioration of chip disposability. On the other hand, if the C content is too high, the macro-hardness becomes too high and the machinability after forging may deteriorate.

Ｓｉ：０．６０～０．９５％；
Ｓｉ（ケイ素）は被削性確保に必要な元素である。Ｓｉ含有率が低すぎるとフェライト硬さが低くなり、切り屑処理性が悪化して、工具の摩耗が促進されるおそれがあるため、０．６０％以上含有させる。一方、Ｓｉ含有率が高すぎると、硬さが増加しすぎて鍛造後に行う機械加工性が低下するおそれがあるため、０．９５％以下とする。 Si: 0.60-0.95%;
Si (silicon) is an element necessary for ensuring machinability. If the Si content is too low, the ferrite hardness will be low, chip disposability will be deteriorated, and there is a risk that tool wear will be accelerated. On the other hand, if the Si content is too high, the hardness may increase too much and the machinability after forging may deteriorate.

Ｍｎ：０．６０～１．２０％；
Ｍｎ（マンガン）は、浸炭後の内部硬さ強度を確保するために０．６０％以上含有させる。一方、Ｍｎ含有率が高すぎると、残留オーステナイトが増加して浸炭層の硬さ低下の懸念が生じるとともに、鍛造後の硬さが上昇し被削性の劣化を招くおそれがあるため、１．２０％以下とする。 Mn: 0.60-1.20%;
Mn (manganese) is contained in an amount of 0.60% or more in order to ensure internal hardness strength after carburizing. On the other hand, if the Mn content is too high, the amount of retained austenite increases and there is concern that the hardness of the carburized layer may decrease, and the hardness after forging may increase, resulting in deterioration of machinability. 20% or less.

Ｐ：０．０３５％以下；
Ｐ（リン）は、含有率が高すぎると、粒界に偏析して疲労強度低下の原因となるため、０．０３５％以下とする。 P: 0.035% or less;
If the content of P (phosphorus) is too high, it segregates at grain boundaries and causes a decrease in fatigue strength, so the content is made 0.035% or less.

Ｓ：０．０３５％以下；
Ｓ（硫黄）は、含有率が高すぎると、硫化物系介在物が増加して疲労強度低下の原因となるため、０．０３５％以下とする。 S: 0.035% or less;
If the content of S (sulfur) is too high, sulfide-based inclusions increase and cause a decrease in fatigue strength, so the content is made 0.035% or less.

Ｃｒ：１．５０％以下（０％は除く）；
Ｃｒ（クロム）は、焼入れ性の向上による内部硬さの確保に有効であるが、含有率が高すぎると、温間鍛造後の硬さが上昇し、被削性低下するおそれがあるため、１．５０％以下とする。 Cr: 1.50% or less (excluding 0%);
Cr (chromium) is effective in ensuring internal hardness by improving hardenability. 1.50% or less.

Ａｌ：０．０５０％以下；
Ａｌ（アルミニウム）は、含有率が高すぎると、ＡｌＮの粗大な析出物が増加して靭性が悪化するおそれがあるため、０．０５０％以下とする。 Al: 0.050% or less;
If the content of Al (aluminum) is too high, coarse precipitates of AlN may increase and toughness may deteriorate, so the content is made 0.050% or less.

Ｔｉ：０．０１～０．０５％；
Ｔｉ（チタン）は、ＮがＢと結びつくのを防止するためＴｉＮとしてＮを消費する作用、いわゆるＮキル作用を得るのに有効であるため、０．０１％以上含有させる。一方、Ｔｉは、含有率が高すぎると、ＴｉＮ生成による強度低下の懸念、及び、切削時の工具の異常摩耗が早くなるおそれがあるため、０．０５％以下とする。 Ti: 0.01-0.05%;
Ti (titanium) is effective in obtaining an effect of consuming N as TiN to prevent bonding of N with B, that is, a so-called N-kill effect, so it is contained in an amount of 0.01% or more. On the other hand, if the Ti content is too high, there is a concern that the strength may decrease due to the formation of TiN and abnormal wear of the tool during cutting may be accelerated.

Ｂ：０．０００５～０．００５０％；
Ｂ（ホウ素）は、粒界強化による強度向上効果を得るため、０．０００５％以上含有させる。一方、Ｂ含有率が高くなりすぎても、前述の効果が飽和するため、上限を０．００５０％とする。 B: 0.0005 to 0.0050%;
B (boron) is contained in an amount of 0.0005% or more in order to obtain an effect of improving strength by strengthening grain boundaries. On the other hand, if the B content is too high, the above-mentioned effects are saturated, so the upper limit is made 0.0050%.

Ｎ：０．００２０～０．０２００％；
Ｎ（窒素）は、ＡｌＮとなって、ピン止め効果により結晶粒粗大化を抑制する効果があるため、０．００２０％以上含有させる。一方、Ｎ含有率が高すぎると、ＡｌＮの粗大な析出物が増加して靭性が悪化するおそれがあるため、０．０２００％以下とする。 N: 0.0020 to 0.0200%;
N (nitrogen) turns into AlN and has the effect of suppressing grain coarsening due to the pinning effect, so it is contained in an amount of 0.0020% or more. On the other hand, if the N content is too high, coarse precipitates of AlN may increase and the toughness may deteriorate.

任意元素としてのＭｏ：０．２０％以下；
Ｍｏ（モリブデン）は、任意添加元素であり、積極的に含有させる必要はなく、含有率０％でもよいが、不純物として少量含有する場合もある。そして、Ｍｏは、その含有により、焼入れ性向上に有効な元素であるので、必要に応じ少量添加することができる。一方、Ｍｏ含有率が高すぎると、コストアップ及び切削加工性劣化のおそれがあるため、０．２０％以下に制限する。 Mo as an optional element: 0.20% or less;
Mo (molybdenum) is an optional additive element, and it is not necessary to actively contain it, and the content may be 0%, but it may be contained in a small amount as an impurity. And, since Mo is an element effective in improving hardenability due to its inclusion, it can be added in a small amount as necessary. On the other hand, if the Mo content is too high, there is a risk of an increase in cost and deterioration of machinability, so it is limited to 0.20% or less.

任意元素としてのＮｂ：０．０１～０．０５％；
Ｎｂ（ニオブ）は、任意添加元素であり、積極的に含有させる必要はないが、０．０１％以上含有することによって結晶粒微細化の効果を得ることができる。一方、Ｎｂ含有率が高すぎると、浸炭性が劣化するおそれがあるため、０．０５％以下に制限する。 Nb as an optional element: 0.01-0.05%;
Nb (niobium) is an optional additive element, and it is not necessary to actively contain it. On the other hand, if the Nb content is too high, the carburizability may deteriorate, so it is limited to 0.05% or less.

次に、上記の基本的な化学成分組成を具備することを前提として、次の式１及び式２の両方を具備するように、化学成分を調整することが重要である。式１及び式２を満たすことにより、温間鍛造後のフェライト率及びフェライト硬さを最適な範囲内に制御することができ、これにより被削性が確保できる。 Next, on the premise of having the above basic chemical composition, it is important to adjust the chemical composition so as to satisfy both the following formulas 1 and 2. By satisfying Formulas 1 and 2, the ferrite ratio and ferrite hardness after warm forging can be controlled within the optimum range, thereby ensuring machinability.

式１：９０≧－１２０＊Ｃ＋２０．１＊Ｓｉ－５．３＊Ｍｎ－８．５＊Ｍｏ＋９６≧８０；
式１は、温間鍛造後における金属組織中のフェライト率の推定に有効な関係式である。式の値がそのままフェライト率と完全に一致するわけではないが、式１の値が大きいほどフェライト率が高くなる傾向となり、その値が８０以上９０以下の範囲にある場合に、温間鍛造後のフェライト率を最適な範囲に制御することが容易となる。 Formula 1: 90≧−120*C+20.1*Si−5.3*Mn−8.5*Mo+96≧80;
Formula 1 is a relational expression effective for estimating the ferrite ratio in the metal structure after warm forging. Although the value of the formula does not completely match the ferrite ratio as it is, the larger the value of Formula 1, the higher the ferrite ratio. It becomes easy to control the ferrite ratio of the to the optimum range.

式２：１６１≧４０＊Ｓｉ＋３９＊Ｍｎ＋１０＊Ｃｒ＋３０＊Ｍｏ＋８４≧１４１；
式２は、温間鍛造後における金属組織中のフェライト硬さの推定に有効な関係式である。式の値がそのままフェライト硬さと完全に一致するわけではないが、式２の値が大きいほどフェライト硬さが高くなる傾向となり、その値が１４５以上１６０以下の範囲にある場合に、温間鍛造後のフェライト硬さを最適な範囲に制御することが容易となる。 Formula 2: 161 ≧40*Si+39*Mn+10*Cr+30*Mo+84≧ 141 ;
Equation 2 is a relational expression effective for estimating the ferrite hardness in the metal structure after warm forging. Although the value of the formula does not completely match the ferrite hardness as it is, the larger the value of Formula 2, the higher the ferrite hardness. It becomes easy to control the later ferrite hardness in the optimum range.

次に、上記温間鍛造用肌焼鋼を用いて鍛造温度が８５０℃～１１００℃の温間鍛造を施して得られた鍛造粗形材は、表面硬さが２００ＨＶ以下であり、フェライト率が、８０％～９０％である金属組織を有し、かつ、フェライト硬さが、１４０ｍＨＶ～１６０ｍＨＶという特性にすることができる。 Next, the forged coarse material obtained by performing warm forging at a forging temperature of 850 ° C. to 1100 ° C. using the case hardening steel for warm forging has a surface hardness of 200 HV or less and a ferrite rate of , a metal structure of 80% to 90%, and a ferrite hardness of 140 mHV to 160 mHV.

上記鍛造粗形材の表面硬さ、すなわちマクロ硬さを２００ＨＶ以下とすることによって、鍛造後において熱処理を施すことなく切削加工を行うことが可能となる。 By setting the surface hardness, that is, the macro hardness, of the forged blank to 200 HV or less, it becomes possible to carry out cutting without heat treatment after forging.

また、上記鍛造粗形材の金属組織におけるフェライト率を８０％～９０％の範囲とし、かつ、フェライト硬さを１４０ｍＨＶ～１６０ｍＨＶの範囲とすることにより、切り屑処理性の確保、工具摩耗量悪化の抑制等による切削性向上効果が得られる。 In addition, by setting the ferrite ratio in the metal structure of the forged rough material to the range of 80% to 90% and the ferrite hardness to the range of 140 mHV to 160 mHV, the chip disposability is secured and the amount of tool wear is increased. It is possible to obtain an effect of improving machinability by suppressing the

一方、フェライト率が上記下限値を下回る場合にはパーライト面積率が上がり、マクロ硬さ（表面硬さ）が高くなって、熱間鍛造から温間鍛造に変更したことによる工具摩耗量悪化抑制効果が低下するおそれがある。また、フェライト率が上記上限値を上回る場合にはマクロ硬さ（表面硬さ）が低くなりすぎて切り屑処理性が悪化するおそれがある。 On the other hand, when the ferrite ratio is below the above lower limit, the pearlite area ratio increases, the macro hardness (surface hardness) increases, and the change from hot forging to warm forging has the effect of suppressing deterioration of tool wear. may decrease. On the other hand, if the ferrite ratio exceeds the above upper limit, the macro hardness (surface hardness) may become too low and chip disposability may deteriorate.

また、フェライト硬さが上記下限値を下回る場合には切り屑処理性が悪化するおそれがあり、フェライト硬さが上記上限値を上回る場合には工具摩耗量悪化抑制効果が低下するおそれがある。 Further, if the ferrite hardness is below the above lower limit, chip disposability may deteriorate, and if the ferrite hardness is above the above upper limit, there is a risk that the effect of suppressing deterioration of tool wear may be reduced.

（実施例１）
本例の温間鍛造用肌焼鋼及び鍛造粗形材に係る実施例について説明する。
本例では、表１及び表２に示すごとく、化学成分が異なる２９種類の鋼材（鋼種１～２９）を用いて鍛造粗形材を作製し、各種評価を実施した。表１、表２に示す鋼のうち、鋼種１～１６が、本発明の条件を満足する実施例、鋼種１７～２８が、一部の条件を満足しない比較例、鋼種２９が従来鋼であるＪＩＳのＳＣｒ４２０である。 (Example 1)
Examples relating to the case hardening steel for warm forging and the forged blank of this example will be described.
In this example, as shown in Tables 1 and 2, 29 types of steel materials (steel grades 1 to 29) with different chemical compositions were used to produce forged crude bars, and various evaluations were performed. Of the steels shown in Tables 1 and 2, steel types 1 to 16 are examples that satisfy the conditions of the present invention, steel types 17 to 28 are comparative examples that do not partially satisfy the conditions, and steel type 29 is conventional steel. It is SCr420 of JIS.

各鍛造粗形材の製造は、各種鋼材を電気炉溶解して得られた鋼塊を鍛伸して直径６５ｍｍφのビレットを作製し、それぞれ後述する表３に記載の鍛造温度にて、鋼種１～２８は温間鍛造を施し、鍛造粗形材を得た。温間鍛造した鍛造粗形材（鋼種１～２８）には、鍛造後に熱処理は行わなかった。また、鋼種２９は、温間鍛造用に適した成分最適化＋温間鍛造の適用による効果を確認するために比較として準備したものである。具体的には、従来鋼であるＳＣｒ４２０に対し、従来行われていた熱間鍛造を行い、その後加工性向上のため、９００℃に１時間保持する熱処理を行ったものである。なお、今回行った実施例では、溶解母材の関係上、任意添加元素であるＭｏを不純物として少量含有していた。従って、表１、表２には、不純物として含有していたＭｏの分析値も合わせて記載した。 For the production of each forged bulk material, a steel ingot obtained by melting various steel materials in an electric furnace is forged to produce a billet with a diameter of 65 mmφ, and each is heated at the forging temperature shown in Table 3 below. ~28 was subjected to warm forging to obtain a forged rough shape. The warm forged forged blanks (steel grades 1 to 28) were not subjected to heat treatment after forging. In addition, steel type 29 was prepared for comparison in order to confirm the effect of application of composition optimization suitable for warm forging and application of warm forging. Specifically, the conventional steel SCr420 was subjected to hot forging, which has been conventionally performed, and then to heat treatment at 900° C. for 1 hour in order to improve workability. In addition, in the examples conducted this time, a small amount of Mo, which is an optional additive element, was contained as an impurity due to the dissolution base material. Therefore, Tables 1 and 2 also show analytical values of Mo contained as an impurity.

Figure 0007188432000001
Figure 0007188432000001

Figure 0007188432000002
Figure 0007188432000002

＜フェライト（α）率及びフェライト（α）硬さ＞
鍛造後の歯車相当部分の機械加工を想定して、歯車相当部分の位置に相当する上記鍛造粗形材の表面近傍の断面をナイタール腐食させた後、光学顕微鏡を用いて観察し、フェライトの面積率を、画像解析により求め、この値をフェライト率とした。フェライト硬さとしては、上記断面のフェライト組織部分において測定したマイクロビッカース硬さの値とした。 <Ferrite (α) rate and ferrite (α) hardness>
Assuming machining of the gear-equivalent part after forging, the cross-section near the surface of the forged raw material corresponding to the position of the gear-equivalent part is subjected to nital corrosion, and then observed using an optical microscope. The ratio was determined by image analysis, and this value was taken as the ferrite ratio. As the ferrite hardness, the value of the micro Vickers hardness measured at the ferrite structure portion of the cross section was used.

＜マクロ硬さ＞
鍛造後の歯車相当部分の機械加工を想定して、歯車相当部分の位置に相当する上記鍛造粗形材の表面近傍の断面において測定したビッカース硬さをマクロ硬さとした。 <Macro hardness>
Assuming machining of the gear-equivalent portion after forging, the Vickers hardness measured in the cross section near the surface of the forged raw material corresponding to the position of the gear-equivalent portion was defined as the macro hardness.

＜切削性（工具摩耗量及び切り屑）の評価＞
上記鍛造粗形材の表面を以下の条件で切削して旋削性評価を行った。
・切削速度：２５０ｍ／ｍｉｎ
・切込：０．８ｍｍ
・送り：０．４ｍｍ／ｒｅｖ
・潤滑：ｗｅｔ <Evaluation of machinability (tool wear amount and chips)>
The surface of the above-mentioned forged coarse material was cut under the following conditions to evaluate the machinability.
・Cutting speed: 250m/min
・Cut: 0.8mm
・Feed: 0.4mm/rev
・Lubrication: wet

工具摩耗量の評価は、切削工具の逃げ面の摩耗量を測定して行った。工具摩耗量が、基準とするＳＣｒ２０相当の鋼種２９（熱間鍛造後熱処理追加処理品）の工具摩耗量の結果を１００％として、１１０％以下の場合を「合格（○）」とし、１１０％を超える場合を「不合格（×）」とした。切り屑評価は、切り屑長さが、基準である上記鋼種２９の結果と比較して同等以下のものを「良好」、基準よりも長いものを「悪化」と評価した。 The amount of tool wear was evaluated by measuring the amount of wear on the flank of the cutting tool. If the tool wear amount is 110% or less, the result of the tool wear amount of steel type 29 (additional heat treatment after hot forging) corresponding to SCr20 as a reference is set as 100%, and if it is 110% or less, it is "passed (○)". was set as "failed (x)". Chips were evaluated as "good" when the chip length was equal to or less than the standard steel type 29, and "worse" when the chip length was longer than the standard.

＜浸炭後強度＞
上記鍛造粗形材と同じ製造方法により、試験片用粗形材を作製し、その後機械加工にて１２角×長さ１１０の試験片を作製（試験片中央に深さ２ｍｍ、角度６０度、ノッチ底Ｒ１．０のノッチ付き）し、これに浸炭熱処理を実施した後、ノッチ側の面を０．２ｍｍ研磨する表面の仕上げ加工をすることにより、試験片を作製した。浸炭熱処理条件は、浸炭温度：９５０℃×１５０ｍｉｎ、Ｃｐ：０．８５の条件で浸炭処理した後、油冷して焼入れし、その後、１５０℃×１Ｈｒの焼き戻し処理を行う条件とした。 <Strength after carburizing>
A test piece blank is produced by the same manufacturing method as the forging blank, and then a test piece of 12 angles × length 110 is produced by machining (depth 2 mm at the center of the test piece, angle 60 degrees, A notch bottom R1.0) was subjected to carburizing heat treatment, and then the surface on the notch side was polished by 0.2 mm to finish the surface to prepare a test piece. The carburizing heat treatment conditions were as follows: carburizing temperature: 950°C x 150 min, Cp: 0.85, followed by oil cooling and quenching, followed by tempering at 150°C x 1 Hr.

浸炭後強度評価試験は、３点曲げ疲労試験により行った。疲労試験は、周波数１Ｈｚの条件で行い、繰り返し数１００回で破壊する低サイクル曲げ疲労強度を求めることにより、評価した。そして、基準である上記鋼種２９の結果と比較して同等以上の場合を「合格（○）」とし、基準よりも低いものを「不合格（×）」と評価した。 The post-carburizing strength evaluation test was performed by a three-point bending fatigue test. The fatigue test was performed under the condition of a frequency of 1 Hz, and evaluation was made by determining the low-cycle bending fatigue strength at which failure occurs at 100 repetitions. When compared with the results of steel type 29, which is the standard, the results were evaluated as "acceptable (○)" when they were equal to or higher than the standard, and "failed (x)" when they were lower than the standard.

上記評価結果を表３に示す。工具摩耗量、切り屑、浸炭後強度の結果が、それぞれ、「合格（〇）」、「良好」、「合格（〇）」の場合を、総合的な判定として「合格（〇）」とし、それ以外を「不合格（×）」とした。 Table 3 shows the above evaluation results. If the results of tool wear amount, chips, and strength after carburizing are "pass (〇)", "good", and "pass (〇)", the overall judgment is "pass (〇)". Other than that, it was set as "failed (x)".

Figure 0007188432000003
Figure 0007188432000003

表３から理解できるように、鋼種１～１６については、鍛造温度を９００～１０５０℃とした温間鍛造を選択し、かつ、その後の熱処理を省略しても、切削性及び浸炭後強度において、従来の熱間鍛造後熱処理を付与した鋼種２９と同等以上の特性が得られた。 As can be understood from Table 3, for steel types 1 to 16, even if warm forging with a forging temperature of 900 to 1050 ° C. is selected and the subsequent heat treatment is omitted, machinability and strength after carburizing are Properties equal to or better than those of steel type 29 to which conventional heat treatment was applied after hot forging were obtained.

一方、鋼種１７は、炭素（Ｃ）含有率が低いために浸炭後強度が低くなりすぎた。 On the other hand, steel type 17 had a low carbon (C) content, so the strength after carburization was too low.

鋼種１８は、炭素（Ｃ）含有率が高いためにマクロ硬さが高くなりすぎ、工具摩耗量が悪化した。 Since steel type 18 had a high carbon (C) content, the macro hardness became too high, and the tool wear amount worsened.

鋼種１９は、ケイ素（Ｓｉ）含有率が低いためにフェライト硬さが低くなりすぎ、切り屑処理性が悪化した。 Steel type 19 had a low silicon (Si) content, so the ferrite hardness was too low, and the chip disposability deteriorated.

鋼種２０は、ケイ素（Ｓｉ）含有率が高いためにマクロ硬さ高くなりすぎ、工具摩耗量が悪化した。 Steel type 20 had a high silicon (Si) content, so the macro hardness was too high, and the amount of tool wear was worse.

鋼種２１は、マンガン（Ｍｎ）含有率が低いために浸炭焼き入れ後の硬さ不足となり、浸炭後強度が不合格となった。 Steel type 21 had a low manganese (Mn) content, resulting in insufficient hardness after carburizing and quenching, and failed in strength after carburizing.

鋼種２２は、炭素（Ｃ）及びマンガン（Ｍｎ）含有率が高いためにマクロ硬さが増加し、工具摩耗量が悪化すると共に、Ｍｎ含有率が高いことによる残留オーステナイトの増加に起因する浸炭層の硬さ低下によって、浸炭後強度が不合格となった。 Steel type 22 has a high carbon (C) and manganese (Mn) content, so the macro hardness increases, the amount of tool wear increases, and the carburized layer due to the increase in retained austenite due to the high Mn content. The strength after carburization failed due to a decrease in hardness.

鋼種２３は、モリブデン（Ｍｏ）含有率が高いためにマクロ硬さが増加し、工具摩耗量が悪化した。 Since steel type 23 had a high molybdenum (Mo) content, the macro hardness increased and the tool wear amount worsened.

鋼種２４は、クロム（Ｃｒ）含有率が高いためにマクロ硬さが増加し、工具摩耗量が悪化した。 Steel type 24 had a high chromium (Cr) content, resulting in an increase in macro hardness and deterioration in tool wear.

鋼種２５は、化学成分組成が式１を満たさず下限を外れたためにフェライト率が低くなり、マクロ硬さが増加し、工具摩耗量が悪化した。 In steel type 25, the chemical composition did not satisfy Formula 1 and fell outside the lower limit, so the ferrite ratio decreased, the macro hardness increased, and the tool wear amount worsened.

鋼種２６は、化学成分組成が式２を満たさず下限を外れたためにフェライト硬さが低くなり、切り屑処理性が悪化した。 Steel type 26 had a low ferrite hardness and poor chip disposability because the chemical composition did not satisfy Formula 2 and fell outside the lower limit.

鋼種２７は、化学成分組成が式１を満たさず上限を外れたためにフェライト率が低くなると共に、マクロ硬さが低くなり、切り屑処理性が悪化した。 In steel type 27, the chemical composition did not satisfy the formula 1 and exceeded the upper limit, so the ferrite ratio was low, the macro hardness was low, and the chip disposability was deteriorated.

鋼種２８は、化学成分組成が式２を満たさず上限を外れたために、フェライト硬さが増加し、工具摩耗量が悪化した。 In steel type 28, the chemical composition did not satisfy the formula 2 and exceeded the upper limit, so the ferrite hardness increased and the tool wear amount worsened.

Claims (2)

鍛造温度が８５０℃～１１００℃である温間鍛造用の肌焼鋼であって、
質量％において、Ｃ：０．１５～０．２３％、Ｓｉ：０．６０～０．９５％、Ｍｎ：０．６０～１．２０％、Ｐ：０．０３５％以下、Ｓ：０．０３５％以下、Ｃｒ：１．５０％以下、Ａｌ：０．０５０％以下、Ｔｉ：０．０１～０．０５％、Ｂ：０．０００５～０．００５０％、Ｎ：０．００２０～０．０２００％を含み、
任意元素としてＭｏ：０．２０％以下、任意元素としてＮｂ：０．０１～０．０５％を含み、
残部がＦｅ及び不可避的不純物からなる化学成分組成を有し、
式１：９０≧－１２０＊Ｃ＋２０．１＊Ｓｉ－５．３＊Ｍｎ－８．５＊Ｍｏ＋９６≧８０、及び、
式２：１６１≧４０＊Ｓｉ＋３９＊Ｍｎ＋１０＊Ｃｒ＋３０＊Ｍｏ＋８４≧１４１、
（ただし、式１及び式２における元素記号は、各元素の含有率（質量％）を意味する。）を満足する、温間鍛造用肌焼鋼。 A case-hardened steel for warm forging with a forging temperature of 850°C to 1100°C,
In mass %, C: 0.15 to 0.23%, Si: 0.60 to 0.95%, Mn: 0.60 to 1.20%, P: 0.035% or less, S: 0.035 % or less, Cr: 1.50% or less, Al: 0.050% or less, Ti: 0.01 to 0.05%, B: 0.0005 to 0.0050%, N: 0.0020 to 0.0200 including %,
Mo: 0.20% or less as an optional element, Nb: 0.01 to 0.05% as an optional element,
Having a chemical composition in which the balance is Fe and unavoidable impurities,
Formula 1: 90≧−120*C+20.1*Si−5.3*Mn−8.5*Mo+96≧80, and
Formula 2: 161≧40*Si+39*Mn+10*Cr+30*Mo+84≧141,
A case-hardening steel for warm forging that satisfies (wherein the element symbols in formulas 1 and 2 mean the contents (% by mass) of the respective elements). 請求項１に記載の温間鍛造用肌焼鋼を用いて得られた鍛造粗形材であって、
表面硬さが２００ＨＶ以下であり、
フェライト率が、８０％～９０％である金属組織を有し、かつ、
フェライト硬さが、マイクロビッカース硬さにおいて、１４０ｍＨＶ～１６０ｍＨＶである、
鍛造粗形材。 A forged blank obtained using the case hardening steel for warm forging according to claim 1,
The surface hardness is 200 HV or less,
Having a metal structure with a ferrite rate of 80% to 90%, and
The ferrite hardness is 140 mHV to 160 mHV in micro Vickers hardness ,
Forged rough bars.
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Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2012072427A (en) 2010-09-28 2012-04-12 Kobe Steel Ltd Case hardened steel and method for manufacturing the same
WO2012046779A1 (en) 2010-10-06 2012-04-12 新日本製鐵株式会社 Case hardened steel and method for producing the same
JP2015134949A (en) 2014-01-17 2015-07-27 Ｊｆｅ条鋼株式会社 Case hardened steel and machine structural component
JP2021154387A (en) 2020-03-25 2021-10-07 愛知製鋼株式会社 Manufacturing method for forging material for carburization

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2012072427A (en) 2010-09-28 2012-04-12 Kobe Steel Ltd Case hardened steel and method for manufacturing the same
WO2012046779A1 (en) 2010-10-06 2012-04-12 新日本製鐵株式会社 Case hardened steel and method for producing the same
JP2015134949A (en) 2014-01-17 2015-07-27 Ｊｆｅ条鋼株式会社 Case hardened steel and machine structural component
JP2021154387A (en) 2020-03-25 2021-10-07 愛知製鋼株式会社 Manufacturing method for forging material for carburization

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