JPH10140281A - Steel for machine structural use, excellent in strength recoverability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a steel for machine structural use, having short graphitization time, capable of securing cold workability such as cold forgeability, and also capable of satisfactorily securing strength after refining treatment even in the case of successive short solution heat treatment time, by specifying the chemical composition of a steel. SOLUTION: This steel for machine structural use has a composition consisting of, by mass, 0.1-1.1% C, 0.5-2.0% Si, 0.10-0.9% Mn, <=0.030% P, 0.001-0.03% S, 0.0003-0.015% B, 0.01-0.5% Al, 0.0003-0.015% Ca, 0.01-0.20% Zr, 0.0015-0.010% N, and the balance Fe. In this steel, the steel structure after graphitization treatment is composed essentially of graphite and ferrite and the strength of the steel which is refined after graphitization treatment is >=90% as compared with the strength in the case where refining is performed without graphitization treatment. Moreover, one or more kinds among 0.1-2.0% Cu, 0.1-2.0% Ni, 0.1-2.0% Co, and 0.005-0.30% V or further <=0.020% Ti, <=0.020% Nb, <=0.10% Mo, <=0.10% Cr, and <=0.10% W can further be incorporated into the above composition.

Description

【発明の詳細な説明】DETAILED DESCRIPTION OF THE INVENTION

【０００１】[0001]

【発明の属する技術分野】本発明は、産業機械および自
動車部品等の素材として用いられる機械構造用鋼に関
し、特に連続鋳造法により製造される、冷間鍛造性と調
質処理による強度回復性に優れた機械構造用鋼に関する
ものである。BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel for machine structure used as a material for industrial machines and automobile parts, and more particularly to a cold forging property and a strength recovery property by a tempering treatment, which are manufactured by a continuous casting method. It relates to excellent mechanical structural steel.

【０００２】[0002]

【従来の技術】一般に、冷間鍛造法などの冷間加工は、
熱間鍛造などの熱間加工に比して、生産効率や材料歩留
りが高く、仕上げ寸法精度に優れることから、ボルトや
ナットを始めとして、各種機械部品の製造に広く用いら
れている加工方法である。2. Description of the Related Art Generally, cold working such as cold forging is performed by:
Compared to hot working such as hot forging, production efficiency and material yield are high, and finish dimensional accuracy is excellent, so it is a processing method widely used in the manufacture of various machine parts such as bolts and nuts. is there.

【０００３】この冷間鍛造法による機械部品の製造に適
用される鋼材としては、一般的に、Ｃが０．４５％以下
のＪＩＳ、Ｓ４５Ｃ鋼に球状化焼きなましを施し、鋼を
軟質化させて用いていた。この軟質化により、冷間鍛造
時の加工性や被削性を確保された鋼材は、冷間鍛造後
に、機械部品としての要求強度を確保（回復）するた
め、容体化−焼入れ−焼戻し処理（以後調質処理と称
す）を行う。[0003] As a steel material applied to the production of machine parts by the cold forging method, generally, JIS, S45C steel having a C of 0.45% or less is subjected to spheroidizing annealing to soften the steel. Was used. The steel material, whose workability and machinability at the time of cold forging are ensured by this softening, is subjected to a soaking, quenching, and tempering treatment (to recover (recover)) the required strength as a mechanical part after the cold forging. Hereinafter, referred to as a refining process).

【０００４】これに対して、近年、機械部品としての高
機能、高性能化の要求から、冷間鍛造用鋼の高強度化の
要求が大きく、この高強度化の要求に応えるためには、
前記強度回復のための調質処理時の焼入れ性を確保する
ため、０．４５％以上の高炭素化が必要となっている。
しかし、鋼を高炭素化すると、逆に、前記球状化焼きな
ましだけでは充分な加工性が得られまで軟化させること
ができず、鋼の変形抵抗を増加させ、冷間鍛造用金型の
寿命が低下したり、鍛造荷重の増加を招いたりする、冷
間鍛造性（加工性）の問題が新たに生じることになる。On the other hand, in recent years, there has been a great demand for high strength of cold forging steel due to demands for high performance and high performance as mechanical parts. In order to meet this demand for high strength,
In order to secure the hardenability during the tempering treatment for the strength recovery, it is necessary to increase the carbon content by 0.45% or more.
However, when the steel is made high carbon, conversely, the spheroidizing annealing alone cannot soften until sufficient workability is obtained, increasing the deformation resistance of the steel and increasing the life of the cold forging die. A new problem of cold forgeability (workability), which lowers or causes an increase in forging load, arises.

【０００５】そこで、前記球状化焼きなまし方法に替え
て、機械構造用鋼を黒鉛化処理し、鋼中のセメンタイト
を一旦黒鉛化して、鋼を主として黒鉛およびフェライト
からなる組織とし、冷間加工性を確保する試みがなされ
ている。この黒鉛化処理鋼は、セメンタイトと異なり、
黒鉛が鋼に比べて極めて軟質で潤滑性も有するので、鋼
の強度を上げずに、冷間鍛造性や切削性を改善すること
が可能である。また、冷間鍛造後の調質処理により、黒
鉛を再固溶させ、焼入れることで、強度回復も、前記球
状化焼きなまし鋼と同様に可能である。Therefore, instead of the spheroidizing annealing method, the steel for machine structural use is graphitized, and the cementite in the steel is graphitized once so that the steel has a structure mainly composed of graphite and ferrite. Attempts have been made to secure. This graphitized steel, unlike cementite,
Since graphite is extremely soft and has lubricity as compared with steel, it is possible to improve cold forgeability and machinability without increasing the strength of steel. In addition, graphite can be solid-dissolved again by tempering after cold forging and then quenched, whereby the strength can be recovered as in the case of the spheroidized annealed steel.

【０００６】ただ、鋼中のセメンタイトの黒鉛化処理に
は長時間の熱処理を要すため、従来から、この処理時間
を短縮する試みが種々提案されている。例えば、特開平
６−５７３６９号、特開平６−１１６６７８号公報で
は、鋼組成を１．５〜２．０の高Ｓｉとし、特にＢ、Ａ
ｌを添加すると同時に、Ｃｕ、Ｎｉ、Ｎｂ、Ｔｉなども
添加して、冷間加工時の変形抵抗と変形能を改善し、合
わせてＢＮとＡｌＮを核に黒鉛を生成させることで、鋼
中セメンタイト（Ｃ）の黒鉛化を容易にし、黒鉛化時間
を短縮する方法が開示されている。However, graphitization treatment of cementite in steel requires a long heat treatment, and various attempts to shorten the treatment time have been conventionally proposed. For example, in JP-A-6-57369 and JP-A-6-116678, the steel composition is set to a high Si of 1.5 to 2.0, and particularly, B, A
At the same time as adding l, Cu, Ni, Nb, Ti, etc. are also added to improve the deformation resistance and deformability during cold working, and together with BN and AlN as nuclei to generate graphite, steel A method for facilitating graphitization of cementite (C) and shortening the graphitization time is disclosed.

【０００７】因みに、この種の黒鉛化処理鋼では、調質
処理における鋼の焼入れ性を確保するため、通常添加さ
れる、Ｍｎ、Ｃｒ、Ｔｉ、Ｎｂ等の焼入れ性向上元素
は、セメンタイトを安定化させ黒鉛化を阻害するため、
多量には使えない。これに対し、前記Ｂは、これら元素
と異なり、黒鉛生成の核となるＢＮを形成して黒鉛化を
促進するとともに、焼入れ性にも寄与する。[0007] Incidentally, in this type of graphitized steel, in order to secure the hardenability of the steel in the tempering treatment, the quenchability improving elements such as Mn, Cr, Ti, Nb, etc., which are usually added, stabilize cementite. To inhibit graphitization,
Not usable in large quantities. On the other hand, B, unlike these elements, forms BN which is a nucleus for graphite generation to promote graphitization and also contributes to hardenability.

【０００８】[0008]

【発明が解決しようとする課題】しかし、前記従来の黒
鉛化処理鋼では、黒鉛化処理の時間短縮はそれなりに図
れるものの、冷間鍛造後の調質処理、特に溶体化処理に
おいて、鋼中に黒鉛を再溶解させるために時間を要する
とともに、焼入れ性にも寄与するＢを含んでなお、調質
後の各種機械部品としての強度を確保できない問題を残
していた。However, in the conventional graphitized steel, although the time required for the graphitizing process can be shortened, the tempering process after cold forging, particularly in the solution treatment, causes the steel to be in the steel. It takes a long time to re-dissolve graphite and contains B, which also contributes to hardenability, and still leaves the problem that the strength of various mechanical parts after tempering cannot be ensured.

【０００９】例えば、通常この種の調質処理は、８７０
℃×３０分の加熱、焼入れ、５５０℃×１ｈｒの焼戻し
の条件で行われる。しかし、この調質処理を、前記従来
のＢ入り黒鉛化処理鋼に施しても、調質処理後の強度
が、黒鉛化処理せずに調質処理した鋼の強度に比して９
０％未満程度しか回復せず、前記機械部品としての高機
能、高性能化の要求に対応する強度を確保できない。[0009] For example, this kind of tempering process is usually 870
The heating and quenching are performed at 550 ° C. for 1 hour. However, even if this tempering treatment is performed on the conventional graphitized steel containing B, the strength after the tempering treatment is 9 times higher than the strength of the steel tempered without the graphitization treatment.
It recovers only less than about 0%, and cannot secure the strength corresponding to the demand for high performance and high performance as the mechanical parts.

【００１０】黒鉛化処理した鋼の調質後の強度が、黒鉛
化処理せずに調質処理した鋼の強度に比して９０％未満
に低下する（強度回復性が悪い）のは、黒鉛が鋼中に再
溶解しにくく、鋼中に再溶解させるために時間を要する
ためである。前記従来のＢ入り黒鉛化処理鋼でも、Ｂの
添加により、ＢＮなどの析出物を利用して、黒鉛を微細
に析出させるから、Ｂ無添加鋼よりは、黒鉛の鋼中への
再溶解が促進しているはずである。しかしながら、前記
のように短い容体化処理時間では、従来のＢ入り黒鉛化
処理鋼でも、充分に黒鉛が溶解するほど黒鉛を微細化で
きないため、依然として充分な強度が確保できないのが
実情である。[0010] The reason why the tempered strength of the graphitized steel decreases to less than 90% (poor strength recovery) compared to the strength of the tempered steel without the graphitization treatment is that of graphite. Is difficult to re-dissolve in steel and it takes time to re-dissolve in steel. Even in the conventional graphitized steel containing B, graphite is finely precipitated using BN or the like by the addition of B, so that the graphite is re-dissolved in the steel more than the steel without B added. It should be promoting. However, with the short encapsulation treatment time as described above, even in the conventional graphitized steel containing B, graphite cannot be refined enough to sufficiently dissolve graphite, so that sufficient strength cannot be ensured yet.

【００１１】これに対し、容体化処理時間を延ばして黒
鉛の鋼中への再溶解時間を確保する手法も考えられる
が、機械構造部品製造工程の効率や製造コストの面か
ら、実用的な方法とは言いがたい。[0011] On the other hand, a method of increasing the time for the soaking process and securing the time for re-dissolving graphite in steel is also conceivable. It is hard to say.

【００１２】従って、本発明は、前記従来技術の問題点
に鑑み、黒鉛化時間を短縮する、言い換えると、短時間
の黒鉛化処理であっても黒鉛化が完了し、冷間鍛造など
の冷間加工性を確保することは勿論、その後の短い容体
化処理時間であっても、調質処理後の強度を充分確保で
きる機械構造用鋼を提供することを目的とする。Accordingly, the present invention has been made in view of the above-mentioned problems of the prior art, and therefore, the graphitization time is shortened. In other words, the graphitization is completed even in a short time graphitization process, and the cold graphing process such as cold forging is performed. It is an object of the present invention to provide a steel for machine structural use capable of ensuring sufficient strength after tempering, not to mention ensuring inter-workability, but even after a short encapsulation time.

【００１３】[0013]

【課題を解決するための手段】この目的のために、本発
明では、機械構造用鋼の化学成分組成を、質量％にて、
Ｃ；０．１〜１．１％、Ｓｉ；０．５〜２．０％、Ｍ
ｎ；０．１０〜０．９％、Ｐ；≦０．０３０％、Ｓ；
０．００１〜０．０３％、Ｂ；０．０００３〜０．０１
５％、Ａｌ；０．０１〜０．５％、Ｃａ；０．０００３
〜０．０１５％、Ｚｒ；０．０１〜０．２０％、Ｎ；
０．００１５〜０．０１０％を含有し、残部がＦｅおよ
び不可避的不純物からなるものとする。For this purpose, according to the present invention, the chemical composition of the steel for machine structural use is expressed as
C: 0.1 to 1.1%, Si: 0.5 to 2.0%, M
n; 0.10 to 0.9%, P; ≤ 0.030%, S;
0.001-0.03%, B; 0.0003-0.01
5%, Al; 0.01 to 0.5%, Ca; 0.0003
0.01 to 0.20%, N;
0.0015 to 0.010%, with the balance being Fe and unavoidable impurities.

【００１４】そして、この化学成分組成とすることによ
り、鋼の特性として、短時間の黒鉛化処理であっても黒
鉛化が完了し、鋼組織が主として黒鉛およびフェライト
からなり、冷間鍛造性を確保する。また、その後の調質
処理における、短い溶体化処理時間であっても、鋼中へ
の黒鉛の再溶解が完了し、調質処理された鋼の強度が、
黒鉛化処理せずに調質処理のみを施した鋼の強度に比し
て９０％以上に回復するものとできる。これらの特性
は、実施例にて後述する通り、同じ黒鉛の微細化鋼であ
っても、Ｂのみを添加、あるいは、ＣａやＺｒのみを添
加した鋼の場合よりも高い。[0014] By adopting this chemical composition, as a characteristic of the steel, the graphitization is completed even with a short time graphitization treatment, the steel structure is mainly composed of graphite and ferrite, and the cold forgeability is improved. Secure. Further, in the subsequent tempering treatment, even if the solution treatment time is short, the re-dissolution of graphite in the steel is completed, and the strength of the tempered steel is
The strength can be recovered to 90% or more compared to the strength of the steel subjected to the tempering treatment without the graphitization treatment. As described later in the examples, these characteristics are higher than the case of the same graphite refined steel in which only B is added, or only Ca or Zr is added.

【００１５】また、本発明鋼では、冷間加工後の調質処
理における焼入れ性向上のために、Ｃｕ、Ｎｉ、Ｃｏ、
Ｖの内から一種または二種以上を添加することを含み、
また、Ｎｂ、Ｔｉ、Ｍｏ、Ｃｒ、Ｗの内から一種または
二種以上を、黒鉛化を阻害しない範囲で含有することを
許容する。In the steel of the present invention, Cu, Ni, Co,
Including the addition of one or more of V
Further, one or more of Nb, Ti, Mo, Cr, and W are allowed to be contained in a range that does not inhibit graphitization.

【００１６】[0016]

【発明の実施の形態】まず、ＣａとＺｒとを複合添加す
ることによる、黒鉛化時間の短縮効果について説明す
る。図１は、０．４５％Ｃ−０．２０％Ｍｎ−１．５５
％Ｓｉ−０．０１％Ｓｉ−０．０１％Ｓ−０．００２％
Ｂ−０．０２％Ａｌからなる、Ｂを添加した低Ｍｎ高Ｓ
ｉ鋼１（○印）と、このベース鋼にＣａとＺｒとを各々
単独および複合添加した鋼〔Ｃａ添加鋼２（△印）、Ｚ
ｒ添加鋼３（□印）、ＣａとＺｒとを複合添加した本発
明鋼４（◇印）〕（各々後述する表１の番号に対応）に
ついて、黒鉛化率と黒鉛化処理時間（但し、７００℃で
の保持時間）との関係を示している。First, the effect of shortening the graphitization time by adding Ca and Zr in combination will be described. FIG. 1 shows 0.45% C-0.20% Mn-1.55.
% Si-0.01% Si-0.01% S-0.002%
B-added low Mn high S made of 0.02% Al
i steel 1 (marked with ○) and a steel obtained by adding Ca and Zr individually and in combination to the base steel [Ca-added steel 2 (marked with △), Z
r-added steel 3 (indicated by □), inventive steel 4 in which Ca and Zr are combined and added (indicated by ◇)] (corresponding to the numbers in Table 1 described below), respectively, for the graphitization rate and graphitization treatment time (however, (Retention time at 700 ° C.).

【００１７】同図から、Ｂ添加低Ｍｎ高Ｓｉ鋼１や、こ
れにＣａとＺｒとを各々単独添加した鋼２、３に比し
て、Ｂ添加低Ｍｎ高Ｓｉ鋼にＣａとＺｒとを複合添加し
た本発明鋼４の黒鉛化が速まっていることが分かる。Ｃ
ａとＺｒとを複合添加することにより黒鉛化処理時間を
短縮できる理由は、ＣａとＺｒとを複合添加することに
より、ＣａとＺｒとの複合硫化物が析出し、この硫化物
を核に析出するＢＮの数が増加し、更にこのＢＮを核に
して黒鉛化が加速されるものと推定される。[0017] From the figure, it can be seen that Ca and Zr are added to the B-added low Mn high Si steel in comparison with the B-added low Mn high Si steel 1 and the steels 2 and 3 to which Ca and Zr are individually added. It can be seen that the graphitization of the steel 4 of the present invention to which composite addition has been accelerated. C
The reason why the graphitization treatment time can be shortened by the complex addition of a and Zr is that the complex sulfide of Ca and Zr precipitates by the complex addition of Ca and Zr, and this sulfide precipitates on the nucleus. It is presumed that the number of BNs to be increased increases, and further the graphitization is accelerated using the BNs as nuclei.

【００１８】そして、ＣａとＺｒとの複合硫化物自体が
微細に析出しやすい性質を持っているのか、ある環境下
で微細に析出しやすいのかは、未だ不明であるが、Ｃａ
とＺｒとの複合硫化物が微細に析出することにより、析
出するＢＮも微細となり、更にこのＢＮを核にした黒鉛
も微細に析出する。そしてこのことによって、黒鉛化処
理における、セメンタイトの黒鉛化に必要な時間も短縮
されると考えられる。It is not yet known whether the complex sulfide itself of Ca and Zr has a property of easily precipitating finely, or whether it is likely to precipitate finely under a certain environment.
By finely precipitating the composite sulfide of Zr and Zr, the BN to be precipitated becomes fine, and graphite having BN as a nucleus also finely precipitates. It is considered that this also shortens the time required for graphitizing cementite in the graphitization treatment.

【００１９】一方、ＣａとＺｒとを複合添加しない鋼
２、３では、硫化物として鋼中に存在するのはＭｎＳの
み乃至ＣａとＺｒと各々の硫化物しかなく、しかも黒鉛
化処理用のベース鋼は、一般的に黒鉛化促進のために、
黒鉛化を阻害するＭｎ量を減じており、ＭｎＳ自体の量
も少ない。したがって、結果ＢＮの数が少なく、黒鉛化
も遅くなるものと推定される。On the other hand, in steels 2 and 3 in which Ca and Zr are not combined, only MnS or only Ca and Zr and each sulfide are present as sulfides in the steel. Steel is generally used to promote graphitization,
The amount of Mn that inhibits graphitization is reduced, and the amount of MnS itself is also small. Therefore, it is estimated that the number of resulting BNs is small and the graphitization is also slow.

【００２０】図２は、図１の場合と同じ鋼を用いて、黒
鉛化処理後に、調質処理（容体化処理は高周波で１００
０℃、６０秒間加熱）した場合の引っ張り強さを示した
ものである。同図から、Ｂ添加鋼１や、Ｂ添加鋼にＣａ
とＺｒとを各々単独添加した鋼２、３の強度に比して、
Ｂ添加鋼にＣａとＺｒとを複合添加した鋼４の、調質処
理後の強度が最も高いことが分かる。この理由も、前記
黒鉛化の場合と同様、ＣａとＺｒとにより、析出黒鉛が
微細となるため、容体化処理において、黒鉛が再溶解し
やすいものと考えられる。FIG. 2 shows that the same steel as in FIG. 1 is used, and after the graphitizing treatment, the refining treatment (the materializing treatment is performed at a high frequency of 100%).
It shows the tensile strength when heated at 0 ° C. for 60 seconds. From the figure, B-added steel 1 and B-added steel
And Zr, respectively, compared to the strength of steels 2 and 3 to which each was independently added,
It is understood that the strength of the steel 4 in which Ca and Zr are added to the B-added steel in a complex manner is the highest after the tempering treatment. This is also because, as in the case of graphitization, the precipitated graphite becomes finer due to Ca and Zr, and it is considered that the graphite is easily redissolved in the embedding treatment.

【００２１】以下に、本発明における化学成分の限定理
由について説明する。Ｃは、切削部品として本発明鋼の
最低要求強度６０ｋｇｆ／ｍｍ² （５９０Ｎ／ｍｍ² ）
以上を確保するための必須の元素であり、その効果を発
揮するためには、０．１％以上の含有が必要である。し
かし、１．１％を超えて過多に含有すると、鋼がロック
ウエル硬度でＨＲＣ３２（Ｃスケール）以上に硬くなり
すぎ、冷間鍛造性を劣化させる。したがって、Ｃの含有
量は０．１〜１．１％の範囲とする。The reasons for limiting the chemical components in the present invention will be described below. C is the minimum required strength of the steel of the present invention of 60 kgf / mm ² (590 N / mm ² ) as a cutting part.
It is an indispensable element for securing the above, and in order to exhibit its effect, the content of 0.1% or more is necessary. However, if the content exceeds 1.1%, the steel becomes too hard with a Rockwell hardness of HRC32 (C scale) or more, deteriorating the cold forgeability. Therefore, the content of C is set in the range of 0.1 to 1.1%.

【００２２】Ｓｉは、鋼溶製時に脱酸剤として作用し、
また、黒鉛化促進と高強度化のために必須の元素であ
る。その効果を発揮するためには、０．５％以上の含有
を必要とする。しかしながら、２．０％を超えて過多に
含有すると、鋼がロックウエル硬度でＨＲＣ３２以上に
硬くなりすぎ、冷間鍛造性を劣化させる。したがって、
Ｓｉの含有量は０．５〜２．０％の範囲とする。Si acts as a deoxidizing agent at the time of smelting steel,
Further, it is an essential element for promoting graphitization and increasing strength. In order to exhibit the effect, the content needs to be 0.5% or more. However, if the content exceeds 2.0%, the steel becomes too hard in Rockwell hardness of HRC32 or more, and deteriorates cold forgeability. Therefore,
The content of Si is in the range of 0.5 to 2.0%.

【００２３】Ｍｎは、Ｓと結合して鋼の被削性の保持に
最低必要なＭｎＳを生成させるとともに、鋼の強度を確
保する上で有用な元素である。これらの効果を発揮する
ためには、０．１０％以上の含有が必要であるが、０．
９％を超えて含有すると、鋼の靱性の劣化を生じる。し
たがって、Ｍｎの含有量は０．１０〜０．９％の範囲と
する。Mn is an element that combines with S to generate MnS, which is the minimum necessary for maintaining the machinability of steel, and is useful in ensuring the strength of steel. In order to exhibit these effects, the content of 0.10% or more is necessary.
If the content exceeds 9%, the toughness of the steel deteriorates. Therefore, the Mn content is in the range of 0.10 to 0.9%.

【００２４】Ｐは、黒鉛化を阻害する不純物であり、極
力少ない方が好ましいので、許容量の上限を０．０３０
％とする。P is an impurity that inhibits graphitization, and it is preferable that P be as small as possible.
%.

【００２５】Ｓは、Ｍｎと結合して鋼の被削性の保持に
最低必要なＭｎＳを生成させるとともに、ＣａとＺｒと
の複合硫化物を形成するために重要な元素である。０．
００１％未満では硫化物の絶対量が不足し、被削性向上
や黒鉛化促進および調質処理後の強度回復効果が無くな
り、逆に０．０３％を超えて含有すると、鋼の靱性が低
下し、鋼材の熱間圧延で脆化を起こし加工困難となる。
したがって、Ｓの含有量は０．００１〜０．０３％とす
る。S is an important element for combining with Mn to form MnS, which is the minimum necessary for maintaining the machinability of steel, and to form a composite sulfide of Ca and Zr. 0.
If the content is less than 001%, the absolute amount of sulfide is insufficient, the machinability is not improved, the graphitization is promoted, and the strength recovery effect after the tempering treatment is lost. Conversely, if the content exceeds 0.03%, the toughness of the steel decreases. However, the hot rolling of the steel material causes embrittlement and makes working difficult.
Therefore, the content of S is set to 0.001 to 0.03%.

【００２６】Ｂは、微量の添加により鋼の焼入れ性を向
上させるとともに、ＢＮを形成し、黒鉛生成の核となる
元素である。その効果を得るためには、０．０００３％
以上の含有が必要であり、０．０１５％を超えて添加し
ても効果が飽和する。したがって、Ｂの含有量は０．０
００３〜０．０１５％とする。B is an element that improves the hardenability of steel by adding a trace amount thereof, forms BN, and becomes a nucleus for graphite formation. To obtain the effect, 0.0003%
The above content is necessary, and the effect is saturated even if it exceeds 0.015%. Therefore, the content of B is 0.0
003 to 0.015%.

【００２７】Ａｌは、鋼溶製時に脱酸剤として作用する
ととともに、黒鉛化を促進する効果もある。この効果を
発揮するためには、０．０１％以上の含有を必要とする
が、一方で０．５％を超えて含有すると、鋼中にＡｌ₂
Ｏ₃ が多量に生成して、却って加工性や被削性に悪影響
を与える。したがって、Ａｌの含有量は０．０１〜０．
５％の範囲とする。[0027] Al acts as a deoxidizing agent at the time of smelting steel and has an effect of promoting graphitization. In order to exhibit this effect, the content of 0.01% or more is required. On the other hand, if the content exceeds 0.5%, Al ₂
O ₃ is generated in large amounts, which adversely affects workability and machinability. Therefore, the content of Al is 0.01 to 0.1.
The range is 5%.

【００２８】ＣａとＺｒとは、前記した通り本発明鋼の
重要な元素であり、複合添加することによりＣａとＺｒ
との複合硫化物が析出し、この硫化物を核に析出するＢ
Ｎの数が増加し、更にこのＢＮを核にして微細な黒鉛化
が進む。この結果、黒鉛化処理時間を短縮するととも
に、容体化処理において黒鉛が再溶解しやすく、調質処
理後の強度が高く、黒鉛化処理前の９０％以上の強度を
有するようになる。この効果を発揮させるためには、Ｃ
ａで０．０００３％以上、Ｚｒで０．０１％以上の含有
を必要とする。一方でこの効果は、Ｃａで０．０１５
％、Ｚｒで０．２０％を超えると飽和する。したがっ
て、含有量は各々、Ｃａは０．０００３〜０．０１５
％、Ｚｒは０．０１〜０．２０％の範囲とする。As described above, Ca and Zr are important elements of the steel of the present invention.
B, which precipitates complex sulfide with
The number of N increases, and further graphitization proceeds with this BN as a nucleus. As a result, the graphitization treatment time is shortened, the graphite is easily redissolved in the embedding treatment, the strength after the tempering treatment is high, and the strength becomes 90% or more of that before the graphitization treatment. To achieve this effect, C
The content of a is required to be 0.0003% or more, and the content of Zr is required to be 0.01% or more. On the other hand, this effect is 0.015 in Ca.
% And Zr, the content is saturated when it exceeds 0.20%. Therefore, the content of Ca is 0.0003 to 0.015, respectively.
% And Zr are in the range of 0.01 to 0.20%.

【００２９】Ｎは、ＢＮを形成し、黒鉛生成の核となり
黒鉛化を促進する元素である。０．００１５％未満で
は、ＢＮ析出量が少なすぎて黒鉛化に時間がかかり、
０．０１０％を超えると、固溶Ｎ低減のためのＢやＴ
ｉ、Ｚｒを多量に添加する必要が生じ、コスト的にも不
利である。したがって、Ｎの含有量は０．００１５〜
０．０１０％とする。しかし一方で、固溶Ｎは、逆に黒
鉛化を阻害するので、ＮはＢと当量以下を含有するのが
望ましい。N is an element that forms BN, becomes a nucleus for graphite production, and promotes graphitization. If it is less than 0.0015%, the amount of BN deposited is too small, and it takes time to graphitize,
If it exceeds 0.010%, B or T for reducing solid solution N
It is necessary to add a large amount of i and Zr, which is disadvantageous in cost. Therefore, the content of N is 0.0015 to
0.010%. On the other hand, on the other hand, solid solution N inhibits graphitization, and therefore, it is desirable that N contains an equivalent amount of B or less.

【００３０】以上の成分の他に、本発明では、強度向上
の目的で、更に、Ｃｕ、Ｎｉ、Ｃｏ、Ｖ、Ｔｉ、Ｎｂ、
Ｍｏ、Ｃｒ、Ｗの内から、一種または二種以上を含有す
ることができる。但し、これらの元素は、本発明で重要
な黒鉛化にたいする影響が異なるので、以下述べるよう
に含有量には注意することが必要である。In addition to the above components, the present invention further comprises Cu, Ni, Co, V, Ti, Nb,
One, two or more of Mo, Cr and W can be contained. However, since these elements have different effects on graphitization, which is important in the present invention, it is necessary to pay attention to the contents thereof as described below.

【００３１】Ｃｕ、Ｎｉ、Ｃｏ、Ｖは、ＭｎやＴｉ、Ｎ
ｂ、Ｍｏ、Ｃｒ、Ｗと異なり、黒鉛化を抑制せず、逆に
黒鉛化を促進する効果も有し、しかも微量の添加で焼入
れ性を向上させ、母材強度を上昇させることができる。
したがって、鋼の更なる高強度化を図る場合には、これ
らの元素を含有させるが、含有量が多すぎても効果は飽
和し、製造コストの上昇を招き経済的でもないので、Ｃ
ｕで０．１〜２．０％、Ｎｉで０．１〜２．０％、Ｃｏ
で０．１〜２．０％、Ｖで０．００５〜０．３０％の含
有量とする。Cu, Ni, Co, V are Mn, Ti, N
Unlike b, Mo, Cr, and W, it does not suppress graphitization, but also has the effect of promoting graphitization, and can improve hardenability and increase base material strength with a small amount of addition.
Therefore, in order to further increase the strength of steel, these elements are contained. However, if the content is too large, the effect is saturated and the production cost is increased, which is not economical.
0.1 to 2.0% for u, 0.1 to 2.0% for Ni, Co
At 0.1 to 2.0% and V at 0.005 to 0.30%.

【００３２】Ｔｉ、Ｎｂ、Ｍｏ、Ｃｒ、Ｗも、微量の添
加で焼入れ性を向上させ、母材強度を上昇させることが
できる。また、Ｔｉには、単窒化物を形成し、オーステ
ナイト粒の粗大化を抑制し、ＢＮを微細に析出させる効
果もある。しかし、これらの元素には、鋼中のセメンタ
イトを安定化し、セメンタイトの黒鉛化を抑制する作用
があるので、添加は避けるべきである。しかし、どうし
ても添加する必要がある場合あるいは不純物として含ま
れてくる場合には、各々黒鉛化を抑制しない含有量以下
にすべきである。その含有量の上限は各々、Ｔｉで０．
０２０％、Ｎｂで０．０２０％、Ｍｏで０．１０％、Ｃ
ｒで０．１０％、Ｗで０．１０％である。言い換える
と、鋼の黒鉛化を抑制しないために、Ｔｉ、Ｎｂ、Ｍ
ｏ、Ｃｒ、Ｗは、前記上限値までの含有は許容されると
いうことである。Ti, Nb, Mo, Cr and W can also improve the hardenability and increase the strength of the base material by adding a small amount. Further, Ti has an effect of forming mononitride, suppressing coarsening of austenite grains, and precipitating BN finely. However, these elements have an effect of stabilizing cementite in steel and suppressing graphitization of cementite, and therefore, their addition should be avoided. However, when it is absolutely necessary to add it or when it is included as an impurity, the content should be not more than the content that does not suppress graphitization. The upper limits of the contents are each set to 0.1 in Ti.
020%, 0.020% for Nb, 0.10% for Mo, C
r is 0.10% and W is 0.10%. In other words, in order not to suppress the graphitization of steel, Ti, Nb, M
It means that the contents of o, Cr and W are allowed up to the upper limit.

【００３３】なお、本発明鋼では、Ｐｂ、Ｂｉ、Ｔｅ等
の鋼の被削性をより向上させる元素は添加しない。言い
換えると、これらの人体に有害な又は高価な元素を添加
せずとも、加工性なり、被削性を向上できる点が本発明
鋼の利点でもある。In the steel of the present invention, no element such as Pb, Bi, Te or the like for further improving the machinability of the steel is added. In other words, the advantage of the steel of the present invention is that the workability and machinability can be improved without adding any harmful or expensive element to the human body.

【００３４】本発明鋼は、通常のＣ、Ｍｎ鋼と同じよう
な製造方法で作ることができる。即ち、従来公知の転
炉、電気炉などで溶製したのち、連続鋳造法または造塊
・分塊法によってスラブやビレットとし、ついで熱間圧
延により所定の形状の条鋼や鋼板などにする。この際、
熱間圧延における圧延終了温度は、Ａｒ₃ 点以上、例え
ば８００℃以上とすることが好ましい。前記被削性向上
元素や焼入れ性向上元素を含む場合には、鋼の熱間加工
性が低下しがちで、加熱温度や熱間圧延温度を高めとす
る必要があるが、本発明にはその必要が無いことも利点
の一つである。The steel of the present invention can be manufactured by the same manufacturing method as that of ordinary C and Mn steels. That is, after smelting in a conventionally known converter, electric furnace, or the like, a slab or a billet is formed by a continuous casting method or an ingot-bulking method, and then a strip or a steel sheet of a predetermined shape is formed by hot rolling. On this occasion,
The rolling end temperature in the hot rolling is preferably set to ₃ points or more of Ar, for example, 800 ° C. or more. When containing the machinability improving element or the quenchability improving element, the hot workability of the steel tends to decrease, it is necessary to increase the heating temperature or hot rolling temperature, but the present invention does not One of the advantages is that there is no need.

【００３５】次に、前記所定形状の鋼を、冷間鍛造など
の冷間加工する前に、焼きなまし処理などにより黒鉛化
処理し、鋼中のセメンタイトを一旦黒鉛化して、鋼を主
として黒鉛およびフェライトからなる組織とし、冷間加
工性を確保する。焼きなまし処理（黒鉛化処理）条件
は、黒鉛化の程度により異なるため一概に言えないが、
一般的には６００〜７５０℃の温度範囲で５〜２０時間
保持される。しかし、本発明の目的の一つである黒鉛化
処理時間の短縮からは、１０時間以内の保持が好まし
い。Next, prior to cold working such as cold forging, the steel having the predetermined shape is graphitized by annealing or the like, and the cementite in the steel is graphitized once. And ensure cold workability. Annealing treatment (graphitization treatment) conditions vary depending on the degree of graphitization, but cannot be said unconditionally.
Generally, it is kept at a temperature in the range of 600 to 750 ° C. for 5 to 20 hours. However, from the viewpoint of shortening the graphitization treatment time, which is one of the objects of the present invention, it is preferable to keep the time within 10 hours.

【００３６】黒鉛化処理された鋼は、冷間鍛造や切削加
工などの冷間加工により機械（構造用）部品とされる。
そして、冷間加工された機械部品は、容体化−焼入れ−
焼戻し処理の調質処理を施され、黒鉛化処理により一旦
低下した強度の回復を行い、要求強度を確保される。前
記した通り、通常この種の調質処理は、まず溶体化処理
のために、Ａ₃ 点以上、例えば８７０℃×３０分間加熱
してしたのち、水焼入れし、その後低温での脆化域を避
けて、目的強度に応じて、例えば５５０℃×１時間で焼
戻しされるなどの条件で行われる。この通常の調質処理
により目標の強度が得られる（強度回復性が優れる）点
が、本発明の大きな利点である。The graphitized steel is turned into a machine (structural) part by cold working such as cold forging or cutting.
Then, the cold-worked machine parts are condensed-quenched-
Tempering treatment of tempering treatment is performed, and strength temporarily reduced by the graphitization treatment is recovered, thereby securing required strength. As described above, the normal refining process of this type, first for the solution treatment, A ₃ or more points, after heated for example 870 ° C. × 30 minutes, then water quenched, followed embrittlement zone at a low temperature Avoiding is performed under conditions such as tempering at 550 ° C. × 1 hour, for example, according to the target strength. It is a great advantage of the present invention that a target strength can be obtained by this normal refining treatment (excellent strength recovery).

【００３７】[0037]

【実施例】以下、具体的な実施例を用いて、本発明を更
に説明する。表１〜３に示した化学成分を有する鋼を８
０ｔ転炉により溶製し、脱ガス処理を施した後に、連続
鋳造して鋳片を得、これを、加熱温度１１５０℃、で熱
間圧延し、５０ｍｍφの棒鋼とした。棒鋼の組織は、い
ずれも、セメンタイトとフェライトとが混在した組織で
あった。The present invention will be further described below with reference to specific examples. Steels having the chemical components shown in Tables 1 to 3
After being melted by a 0t converter and subjected to a degassing treatment, it was continuously cast to obtain a slab, which was hot-rolled at a heating temperature of 1150 ° C. to obtain a 50 mmφ steel bar. Each of the steel bars had a structure in which cementite and ferrite were mixed.

【００３８】この棒鋼を適当な長さに切断し、黒鉛化処
理を７００℃で実施し、自己潤滑作用による冷間加工性
確保の目安である、セメンタイトの７０％が黒鉛化する
までの時間を測定した。黒鉛化するセメンタイトの率を
１００％とせず、７０％としたのは、あとの溶体化処理
における黒鉛溶解の難易を考慮したためである。因み
に、黒鉛化させるセメンタイトの率は、確保する冷間加
工性の程度により異なるため一概には規定しにくい。本
発明において、黒鉛化処理後の鋼組織を、主として黒鉛
およびフェライトからなる組織と規定し、黒鉛化するセ
メンタイトの率が１００％未満のものも含むようにした
のは、このためである。This steel bar is cut into an appropriate length, graphitization treatment is performed at 700 ° C., and the time until 70% of cementite is graphitized, which is a measure for securing cold workability by self-lubricating action, is set. It was measured. The reason why the ratio of cementite to be graphitized is not set to 100% but set to 70% is in consideration of the difficulty of graphite dissolution in the subsequent solution treatment. Incidentally, the ratio of the cementite to be graphitized varies depending on the degree of the cold workability to be secured, so that it is generally difficult to specify the ratio. In the present invention, it is for this reason that the steel structure after the graphitization treatment is defined as a structure mainly composed of graphite and ferrite, and the ratio of the cementite to be graphitized is less than 100%.

【００３９】前記セメンタイトの７０％が黒鉛化した棒
鋼を、高周波加熱により、８７０℃で３０分間加熱して
溶体化処理したのち、水焼入れし、５５０℃で１時間焼
戻し後放冷する調質処理を施し、鋼の引っ張り強さ（Ｔ
Ｓ２）を測定した。比較のために、圧延後の同じ棒鋼
を、黒鉛化処理せずに前記調質処理のみを施したものの
引っ張り強さ（ＴＳ１）も測定した。その結果を、表４
〜６に示す。表１〜３の鋼番号は、表４〜６の鋼番号
に、各々対応している。本実施例では、黒鉛化処理後の
冷間加工の、鋼強度に対する影響は、調質処理により無
くなるため、冷間加工工程を省略している。The steel bar in which 70% of the cementite is graphitized is subjected to solution treatment by heating at 870 ° C. for 30 minutes by high frequency heating, then water quenched, tempered at 550 ° C. for 1 hour, and then cooled down. And the tensile strength of the steel (T
S2) was measured. For comparison, the tensile strength (TS1) of the same steel bar after rolling, which had been subjected to only the tempering treatment without graphitization treatment, was also measured. Table 4 shows the results.
To # 6. The steel numbers in Tables 1 to 3 correspond to the steel numbers in Tables 4 to 6, respectively. In the present embodiment, the effect of the cold working after the graphitization treatment on the steel strength is eliminated by the tempering treatment, and thus the cold working step is omitted.

【００４０】表４〜６から分かる通り、ＣａとＺｒとを
複合添加している本発明鋼４、６、７、１０、１１、１
３、１５、１６、１９、２０、２３、２４、２７〜２
９、３２〜３４、３７、３８、４１、４２、４４〜４
７、４９、５１、５３、５５、５７は、黒鉛化処理時間
が１０時間以内と短縮されるとともに、調質処理後の強
度が高く、黒鉛化処理せずに前記調質処理のみを施した
鋼に対し、９０％以上の強度を有する。As can be seen from Tables 4 to 6, the steels 4, 6, 7, 10, 11, 1 and 1 according to the present invention in which Ca and Zr are added in combination.
3, 15, 16, 19, 20, 23, 24, 27-2
9, 32 to 34, 37, 38, 41, 42, 44 to 4
7, 49, 51, 53, 55, and 57 have the graphitization time reduced to 10 hours or less, have high strength after tempering, and have been subjected to only the tempering without graphitization. It has a strength of 90% or more with respect to steel.

【００４１】これに対し、比較鋼は、本発明鋼に比し
て、黒鉛化処理時間が比較的長くなるとともに、調質処
理後の強度も低く、黒鉛化処理せずに前記調質処理のみ
を施した鋼に対し、９０％以上の強度を確保できていな
い。因みに、比較鋼１〜３はＣａとＺｒとが複合添加さ
れておらず、比較鋼５はＣ量が少なく、強度回復率が低
い。比較鋼８はＣ量が多すぎ、また、比較鋼２１はＳ量
が多すぎるため、熱間圧延自体が困難である。比較鋼９
はＳｉ量が少なく黒鉛化が遅く、逆に比較鋼１２はＳｉ
量が多すぎるため冷間鍛造性が劣化する。On the other hand, the comparative steel has a relatively longer graphitization time than the steel of the present invention, has a lower strength after the temper treatment, and has only the temper treatment without the graphitization treatment. 90% or more of the strength of the steel subjected to the above. Incidentally, in Comparative Steels 1 to 3, Ca and Zr were not added in a complex manner, and Comparative Steel 5 had a small C content and a low strength recovery rate. Since the comparative steel 8 has too much C content, and the comparative steel 21 has too much S content, the hot rolling itself is difficult. Comparative steel 9
Is low in Si content and graphitization is slow.
Since the amount is too large, the cold forgeability deteriorates.

【００４２】比較鋼１４はＭｎ量、比較鋼１７はＰ量
が、各々多すぎ、比較鋼１８はＳ量、比較鋼２２はＢ
量、比較鋼２６はＡｌ量、比較鋼３１はＣａ量、比較鋼
３６はＺｒ量、比較鋼４０はＮ量が、各々少なすぎ、反
対に比較鋼４８はＮｂ量、比較鋼５０はＴｉ量、比較鋼
５２はＶ量、比較鋼５４はＭｏ量、比較鋼５６はＣｒ
量、比較鋼５８はＷ量が、各々多すぎるため、全て黒鉛
化が遅く、黒鉛化処理時間が長くなっている。The comparative steel 14 has too much Mn content, the comparative steel 17 has too much P content, the comparative steel 18 has an S content, and the comparative steel 22 has a B content.
Comparative steel 26 has an Al amount, comparative steel 31 has a Ca amount, comparative steel 36 has a Zr amount, comparative steel 40 has an N amount, and comparative steel 40 has an excessively small N amount. Conversely, comparative steel 48 has an Nb amount and comparative steel 50 has a Ti amount. , Comparative steel 52 is V amount, Comparative steel 54 is Mo amount, Comparative steel 56 is Cr amount.
Since the W amount and the comparative steel 58 each have an excessively large W amount, graphitization is slow in all cases, and the graphitization treatment time is long.

【００４３】比較鋼２５はＢ量が多いにも関わらず、Ｂ
量が比較的少ない本明鋼２４と材質的に大差なくコスト
アップになる。比較鋼３０はＡｌ量が多すぎ、被削性が
悪くなる。比較鋼３５はＣａ量が多いにも関わらず、Ｃ
ａ量が比較的少ない発明鋼３４と材質的に大差なくコス
トアップになる。比較鋼３９はＺｒ量が多いにも関わら
ず、Ｚｒ量が比較的少ない発明鋼３８と材質的に大差な
くコストアップになる。比較鋼４３はＮ量が多いにも関
わらず、Ｎ量が比較的少ない発明鋼４２と材質的に大差
ないし、また、鋼材が硬くなりすぎ、熱間や冷間での加
工性を阻害する問題がある。Although the comparative steel 25 has a large amount of B,
The cost is substantially the same as that of the present steel 24 with a relatively small amount, and the cost is increased without much difference. The comparative steel 30 has too much Al content, resulting in poor machinability. Although the comparative steel 35 has a large amount of Ca,
The cost is increased without much difference in material from the invention steel 34 having a relatively small amount of a. Although the comparative steel 39 has a large amount of Zr, the cost is substantially the same as that of the inventive steel 38 having a relatively small amount of Zr. Although the comparative steel 43 has a large N content, the material is not much different from the inventive steel 42 having a relatively small N content, and the steel material is too hard, which impairs hot or cold workability. There is.

【００４４】[0044]

【表１】 [Table 1]

【００４５】[0045]

【表２】 [Table 2]

【００４６】[0046]

【表３】 [Table 3]

【００４７】[0047]

【表４】 [Table 4]

【００４８】[0048]

【表５】 [Table 5]

【００４９】[0049]

【表６】 [Table 6]

【００５０】[0050]

【発明の効果】本発明によれば、短時間の黒鉛化処理で
あっても黒鉛化が完了し、冷間鍛造などの冷間加工性を
確保することは勿論、その後短い容体化処理時間であっ
ても調質処理後の強度を充分確保できる機械構造用鋼を
提供することができる。According to the present invention, graphitization is completed even in a short time graphitization treatment, and it is possible to ensure cold workability such as cold forging, etc. Even if there is, it is possible to provide a steel for machine structural use that can sufficiently secure the strength after the tempering treatment.

【図面の簡単な説明】[Brief description of the drawings]

【図１】図１は、Ｂを添加した従来の低Ｍｎ高Ｓｉ鋼
と、この従来鋼にＣａとＺｒとを各々単独および複合添
加した発明鋼について、黒鉛化率と黒鉛化処理時間との
関係を示す、説明図である。BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a graph showing the graphitization ratio and graphitization time of a conventional low Mn high Si steel to which B is added and an inventive steel in which Ca and Zr are individually and compositely added to this conventional steel. It is explanatory drawing which shows a relationship.

【図２】図２は、図１同じ鋼を用いて、黒鉛化処理後
に、調質処理した場合の強度を示す、説明図である。FIG. 2 is an explanatory diagram showing strength when tempering treatment is performed after graphitization treatment using the same steel as in FIG. 1;

Claims (3)

【特許請求の範囲】[Claims] 【請求項１】 質量％にて、Ｃ；０．１〜１．１％、Ｓ
ｉ；０．５〜２．０％、Ｍｎ；０．１０〜０．９％、
Ｐ；≦０．０３０％、Ｓ；０．００１〜０．０３％、
Ｂ；０．０００３〜０．０１５％、Ａｌ；０．０１〜
０．５％、Ｃａ；０．０００３〜０．０１５％、Ｚｒ；
０．０１〜０．２０％、Ｎ；０．００１５〜０．０１０
％を含有し、残部がＦｅおよび不可避的不純物からな
り、黒鉛化処理後の鋼組織が主として黒鉛およびフェラ
イトからなるとともに、黒鉛化処理後に調質処理された
鋼の強度が、黒鉛化処理せずに調質処理を施した場合の
強度に比して、９０％以上であることを特徴とする強度
回復性に優れた機械構造用鋼。C. 0.1 to 1.1% by mass%, S
i: 0.5 to 2.0%, Mn: 0.10 to 0.9%,
P: ≦ 0.030%, S: 0.001 to 0.03%,
B: 0.0003 to 0.015%, Al: 0.01 to
0.5%, Ca; 0.0003 to 0.015%, Zr;
0.01 to 0.20%, N; 0.0015 to 0.010
%, The balance being Fe and unavoidable impurities, the steel structure after the graphitization treatment is mainly composed of graphite and ferrite, and the strength of the temper-treated steel after the graphitization treatment is not graphitized. A steel for machine structural use excellent in strength recovery, characterized in that it has a strength of 90% or more as compared with the strength when subjected to a tempering treatment. 【請求項２】 質量％にて、Ｃ；０．１〜１．１％、Ｓ
ｉ；０．５〜２．０％、Ｍｎ；０．１０〜０．９％、
Ｐ；≦０．０３０％、Ｓ；０．００１〜０．０３％、
Ｂ；０．０００３〜０．０１５％、Ａｌ；０．０１〜
０．５％、Ｃａ；０．０００３〜０．０１５％、Ｚｒ；
０．０１〜０．２０％、Ｎ；０．００１５〜０．０１０
％を含有するとともに、Ｃｕ；０．１〜２．０％、Ｎ
ｉ；０．１〜２．０％、Ｃｏ；０．１〜２．０％、Ｖ；
０．００５〜０．３０％の内から一種または二種以上を
含有し、残部がＦｅおよび不可避的不純物からなり、黒
鉛化処理後の鋼組織が主として黒鉛およびフェライトか
らなるとともに、黒鉛化処理後に調質処理された鋼の強
度が、黒鉛化処理せずに調質処理を施した場合の強度に
比して、９０％以上であることを特徴とする強度回復性
に優れた機械構造用鋼。2. In mass%, C: 0.1 to 1.1%, S
i: 0.5 to 2.0%, Mn: 0.10 to 0.9%,
P: ≦ 0.030%, S: 0.001 to 0.03%,
B: 0.0003 to 0.015%, Al: 0.01 to
0.5%, Ca; 0.0003 to 0.015%, Zr;
0.01 to 0.20%, N; 0.0015 to 0.010
%; Cu; 0.1-2.0%, N
i; 0.1 to 2.0%, Co; 0.1 to 2.0%, V;
One or more of 0.005 to 0.30%, the balance being Fe and unavoidable impurities, the steel structure after the graphitization treatment is mainly composed of graphite and ferrite, and after the graphitization treatment, A steel for machine structural use excellent in strength recovery, characterized in that the tempered steel has a strength of 90% or more as compared with the strength when tempered without graphitization. . 【請求項３】 前記化学成分の他、Ｔｉ；０．０２０％
以下、Ｎｂ；０．０２０％以下、Ｍｏ；０．１０％以
下、Ｃｒ；０．１０％以下、Ｗ；０．１０％以下まで含
むことを許容した請求項１または２に記載の強度回復性
に優れた機械構造用鋼。3. In addition to the chemical components, Ti: 0.020%
The strength recovery property according to claim 1 or 2, wherein Nb: 0.020% or less, Mo: 0.10% or less, Cr: 0.10% or less, W: 0.10% or less. Excellent machine structural steel.