JP3602603B2

JP3602603B2 - Method for producing fine graphite uniformly dispersed steel for cold working with excellent toughness

Info

Publication number: JP3602603B2
Application number: JP08961695A
Authority: JP
Inventors: 真小此木; 房男石川; 稔彦高橋
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 1995-04-14
Filing date: 1995-04-14
Publication date: 2004-12-15
Anticipated expiration: 2019-12-15
Also published as: JPH08283843A

Description

【０００１】
【産業上の利用分野】
本発明は冷間加工（鍛造、切削）後に焼入れ・焼戻しして使用される鋼材の製造方法に係わり、特に黒鉛を微細かつ均一に分散させることにより、焼入れ・焼戻し後の靱性に優れた微細黒鉛均一分散鋼の製造方法に係わるものである。
【０００２】
【従来の技術】
中炭素鋼中のフェライト＋パーライト組織をフェライト＋黒鉛の２相組織にするとその硬さはＨｖ１６０からＨｖ１１０程度までに減少する。そのために黒鉛率を上げると冷間鍛造性は硫黄快削鋼のそれを上回ることが、例えば日本金属学会誌、Ｖｏｌ．５３（１９８９），ｐ．２０６の研究論文に報告されている。工業的にも特公昭６３−９５８０号公報に見られるようにミクロ組織をフェライト＋黒鉛の２相組織にすると冷間鍛造性は著しく向上することが紹介されている。
【０００３】
一方、黒鉛分散鋼は冷間鍛造性は優れているものの、特開平２−１１１８４２号公報によると、焼入れ加熱時に黒鉛の分散速度が遅く十分にオーステナイト中に溶解しないために焼入れ硬さが不足する欠点があるとされている。この欠点を解決するためには、黒鉛を微細化すればよいこと、その具体的な方法としてＢＮを黒鉛の析出核として利用することが有効であることが同じく特開平２−１１１８４２号公報に開示されている。
【０００４】
また、黒鉛分散鋼を得るための製造方法として、炭素過飽和状態（マルテンサイト組織）とマルテンサイト変態歪を利用する方法が有効であることが特開昭４９−６７８１７号公報に開示されている。これによると、特定の化学成分を有する鋼を、７５０〜９５０℃に加熱して焼入れしてマルテンサイト変態させ、これを再度加熱して６００〜７５０℃で焼鈍することにより黒鉛分散鋼が得られている。
【０００５】
【発明が解決しようとする課題】
しかしながら、よく知られているように、ＢＮは通常は結晶粒界に析出しやすいために、ＢＮを核生成サイトとする黒鉛も同様にフェライト粒界あるいは旧オーステナイト粒界に偏析して不均一に分散する。またマルテンサイト変態させた場合にも黒鉛の生成サイトは変態歪の集中する旧オーステナイト粒界あるいはマルテンサイトブロック境界に限られており、黒鉛は不均一に分散する。
【０００６】
粒界に黒鉛が不均一分散していると黒鉛間距離の変動が大きくなりその最大値も大きくなる。一方、黒鉛は周囲の炭素原子を吸収しながら成長する。従って、黒鉛間距離が大きな場所では、その分黒鉛粒子一個あたりが周囲の炭素を吸収しうる範囲も大きくなり、最終的に得られる黒鉛粒径が大きくなるものと考えられる。このために、現状技術で得られている黒鉛寸法は、微細化はされてはいるものの５〜１０μｍ程度で、焼入れ加熱の際に黒鉛すなわち炭素が拡散して黒鉛の存在していた箇所が、５〜１０μｍ程度の空孔となって残存する。この空孔が原因で破壊靱性値（衝撃値）が低いという欠点を有しており、未だ十分な靱性を保証するには至っていないのが現状である。
【０００７】
本発明は結晶粒界に留まらずフェライト粒内にも黒鉛を均一に分散させ、かつ、黒鉛の平均粒径（焼入れ後の空孔の平均寸法）を極力小さくして靱性を高めた冷間加工用超微細黒鉛均一分散鋼を提供しようとするものである。
【０００８】
【課題を解決するための手段】
本発明者らは黒鉛の生成サイトとなるＢＮの微細分散に関して種々検討を重ねた結果、圧延前の加熱温度をＢＮ析出温度以上に規定し、急冷によってＢとＮを鋼中に過飽和に固溶させた後、ＢＮが迅速に析出しはじめる特定の温度域にて熱間圧延を行い加工転位を多数導入し、その直後より徐冷させることにより転位上にＢＮを均一に析出させ、これらのＢＮを起点として微細黒鉛の均一分散が可能であるとの新規な知見を得た。
【０００９】
本発明者らはこのような知見に基づいて、従来困難であった黒鉛の平均粒径（焼入れ後の空孔の平均寸法）の超微細化を実現し、靱性を高めた冷間加工用微細黒鉛均一分散鋼の製造方法を提示するに至った。
（１）すなわち第一の本発明は、質量％で、
Ｃ：０．３０〜１．０％、
Ｓｉ：０．６０〜１．３％、
Ｍｎ：０．４０〜１．０％、
Ｐ：０．０２％以下、
Ｓ：０．０１５〜０．０５５％、
Ａｌ：０．０１０〜０．０３５％、
Ｂ：０．００１〜０．００４％、
Ｎ：０．００２〜０．００８％
を含有し、残部がＦｅおよび不可避的不純物からなる鋼を、１１５０℃以上に加熱した後に、冷却速度１０℃／ｓｅｃ以上で熱延開始温度まで冷却し、熱延開始温度、熱延終了温度ともに８５０〜１０００℃の範囲内とし、熱延終了後から急冷開始温度までの平均冷却速度を０．１〜１．０℃／ｓｅｃとし、Ａｒ_３点〜８５０℃の範囲内の急冷開始温度よりＭ_ｓ点以下の温度に冷却速度３０〜１００℃／ｓｅｃの条件で冷却後に自然冷却し、次いで加熱温度６００〜７２０℃にて黒鉛化処理することにより、平均粒径２．０μｍ以下の黒鉛０．３０〜１．０質量％を有することを特徴とする靱性に優れる冷間加工用微細黒鉛均一分散鋼の製造方法である。
（２）第二の本発明は、鋼材の成分として、さらに、質量％で、
Ｍｏ：０．０５〜０．２０％
を含有することを特徴とする（１）記載の靱性に優れる冷間加工用微細黒鉛均一分散鋼の製造方法である。
【００１０】
【作用】
次に本発明における鋼材化学成分、黒鉛の粒径と含有率の限定理由を詳細に説明する。
Ｃは焼入後の強度を確保するために、その下限値を０．３０％とした。一方、冷間加工後の熱処理における焼割れを防止するために上限値を１．０％とした。
【００１１】
Ｓｉは黒鉛化を促進する有力な元素であることから、その下限値を０．６０％とした。ただしＳｉ量が多くなるとフェライト相が固溶硬化し冷間加工性の劣化を招くので、上限値を１．３％とした。
ＭｎはＳと結合してＭｎＳ介在物として存在し、黒鉛あるいはＢＮの生成サイトとなることから必要な元素である。また焼入れ性を増加させ強度を確保する上でも必要な元素でもあることから、その下限値を０．４０％とした。ただしＭｎ量が大きくなると黒鉛化を著しく阻害するので上限値は１．０％とした。
【００１２】
Ｐは鋼中で粒界偏析や中心偏析を起こし靱性劣化の原因となるので、その上限を０．０２％とした。
ＳはＭｎと結合してＭｎＳ介在物とし、黒鉛あるいはＢＮの生成サイトとして有効な元素であるため、下限値を０．０１５％とした。ただし多すぎると冷間加工性を劣化させるため、上限値を０．０５５％とした。
【００１３】
Ａｌは鋼中酸素を酸化物系介在物として除去する。また結晶粒を調整するために０．０１０％以上の添加が必要である。一方多量添加すると脱酸の効果は飽和するため、上限値を０．０３５％とした。
ＢとＮはＢＮを形成して黒鉛の生成サイトを与えることにより黒鉛化を促進する。十分な黒鉛化促進効果を得るには０．００１％以上のＢを添加しなければならない。Ｂが０．００４％を越えると黒鉛化促進効果は飽和するので、その上限値を０．００４％とした。Ｎは０．００１〜０．００４％ＢをＢＮとするために必要な量、すなわち０．００２〜０．００８％である。
【００１４】
Ｍｏは焼入れ性を確保するために必要に応じて添加される。焼入れ性増加の効果を十分に得るために、下限値を０．０５％とした。また多量添加は経済性の点で好ましくないため、その上限値を０．２０％とした。
黒鉛の平均粒径は靱性（衝撃特性）の点からその上限を２．０μｍとした。２．０μｍを越えると、焼入れ加熱の際に黒鉛のあった箇所が２．０μｍ超の粗大な空孔となって残存し、衝撃値を低下させる。
【００１５】
鋼中Ｃはそのほぼ全量が黒鉛化するので、黒鉛の量はＣ含有量にほぼ等しい。従って、黒鉛の限定理由はＣの限定理由で述べた場合と全く同じである。
次に本発明における製造条件の限定理由を詳細に説明する。
熱間圧延前の加熱温度は本発明の範囲のＢ，Ｎ量で析出しうるＢＮを完全にマトリックス中に固溶させるために１１５０℃以上とした。圧延開始温度および終了温度はＢＮ析出前にその生成サイトとなる加工転位を多数導入するためにＢＮが迅速に析出する温度域、すなわち８５０〜１０００℃の範囲内とした。熱間圧延後に加工転位上にＢＮを十分に析出させるために、熱延終了後から急冷開始温度までの平均冷却速度を規定した。その上限値はＢＮの析出促進をはかるために１．０℃／ｓｅｃとし、下限値は生産性を考慮して０．１℃／ｓｅｃとした。熱間圧延後の冷却開始温度はＢＮの加工転位上への析出がほぼ完了する８５０℃を上限値とし、急冷後マルテンサイト組織を得ることを考慮してＡｒ_３点を下限値とした。マルテンサイト組織を得るために急冷終了温度はＭ_ｓ点以下でなければならない。平均冷却速度の下限値を３０℃／ｓｅｃとしたのは十分なマルテンサイト組織を得るためであり、その上限値を１００℃／ｓｅｃとしたのはこれ以上急冷してもマルテンサイト変態量は増加しないためである。
【００１６】
黒鉛化処理の温度の下限値を６００℃、上限値を７２０℃の限定したのはこの範囲における黒鉛化時間が最も短いためである。
【００１７】
【実施例】
以下に、本発明の効果を実施例によりさらに具体的に記す。
本実施例に使用した素材は直径１９ｍｍφ以上の直棒またはバーインコイルである。熱延ラインの延長線上に設置した水冷装置により棒鋼表面の全面に単位面積あたり０．３〜０．５トンの冷却水を均一に散水することに冷却した。冷却装置は長さ２０ｍで、円周上に多数の冷却水を供給するための孔を有するパイプで、棒鋼はパイプの中心線上を通過する際に冷却される。冷却開始温度、冷却終了温度は鋼材の表面温度を放射温度計で測定した値であり、冷却平均速度は冷却開始温度と冷却終了温度との差を冷却時間で除すことにより求めた。その後、焼鈍炉にて黒鉛化処理が行なわれた。
【００１８】
黒鉛化率は次式により算出した。
（鋼中黒鉛含有量／鋼の炭素含有量）×１００（％）
ここで鋼の炭素含有量は化学分析により定量した。また黒鉛含有量は黒鉛平均粒径、密度、および黒鉛粒子数より算出した。
黒鉛平均粒径は次の方法によった。黒鉛粒子に電子線を照射し反射電子線の強度を二値化することによりＳＥＭ画面上に黒鉛を結像させて解析システムを利用して粒径を測定・解析した。一視野の面積は１００μｍ×１００μｍで、視野数は２５であり、測定総面積は０．２５ｍｍ^２である。
【００１９】
黒鉛間最大距離は倍率２００倍の光学顕微鏡写真上で測定した。写真上に黒鉛の存在しない箇所のみを含む円弧を描きその直径の最大値を黒鉛間最大距離とした。
衝撃吸収エネルギーは以下のようにして求めた。すなわち、黒鉛化させた棒鋼を、焼入れ（８５０℃×３０分→水冷）、および焼戻し（６００℃×６０ｍｉｎ→水冷）を行った後、２ｍｍＵノッチシャルピー衝撃試験片（ＪＩＳＺ２２０２
Ｎｏ．３）を採取した。シャルピー衝撃試験の試験温度は２０℃である。
【００２０】
表１には化学成分、黒鉛析出状況および黒鉛化後に焼入れ・焼戻した場合の靱性を示した。
【００２１】
【表１】

【００２２】
なお、表１に用いた鋼材は全て以下の製造条件によって黒鉛化された。すなわち、２００ｍｍ角のビレットを１２５０℃に加熱した後、冷却速度１５℃／ｓｅｃで９８０℃まで冷却した。その後９８０℃より熱間で圧延を開始し８８０℃で圧延を終え直径２０ｍｍφの丸棒とした。その後、０．５℃／ｓｅｃで冷却し、７５０℃より１００℃まで急冷を行った。この間の平均冷却速度は６０℃／ｓｅｃであった。引続き、焼鈍温度７００℃、焼鈍時間１２時間で黒鉛化処理が行われた。表１において本発明はいずれも黒鉛化率が１００％と高いのに比べ、比較例の黒鉛化率は５０％程度と低い。また、本発明は黒鉛平均粒径、黒鉛間最大距離ともに比較例に比べ小さい。従って本発明の焼入れ・焼戻し後の靱性を表すシャルピー衝撃吸収エネルギーも比較例のそれに対して著しく高い。
【００２３】
表２および表３には表１中において本発明の成分範囲を満足する鋼Ｎｏ．１〜５を用いて、製造条件を変化させた場合の黒鉛化状況ならびに靱性を示す。本発明に従う製造方法では黒鉛平均粒径、黒鉛間最大距離ともに比較例に比べ小さくなっており、本発明の製造方法により得た黒鉛鋼の焼入れ・焼戻し後のシャルピー衝撃吸収エネルギーすなわち靱性も比較例により得た黒鉛鋼のそれに対して著しく高い。
【００２４】
【表２】

【００２５】
【表３】

【００２６】
【発明の効果】
以上の実施例からも明らかなように本発明によれば、靱性の優れた微細黒鉛均一分散鋼の製造方法を提供することが可能であり、産業上の効果は極めて顕著なるものがある。[0001]
[Industrial applications]
The present invention relates to a method for producing a steel material which is used after quenching and tempering after cold working (forging, cutting), and in particular, fine graphite excellent in toughness after quenching and tempering by dispersing graphite finely and uniformly. The present invention relates to a method for producing uniformly dispersed steel.
[0002]
[Prior art]
When the ferrite + pearlite structure in the medium carbon steel is changed to a two-phase structure of ferrite + graphite, its hardness decreases from Hv160 to about Hv110. Therefore, when the graphite ratio is increased, the cold forgeability exceeds that of the free-cutting steel, for example, the journal of the Japan Institute of Metals, Vol. 53 (1989), p. It has been reported in 206 research papers. It is introduced industrially that cold forgeability is significantly improved when the microstructure is made into a two-phase structure of ferrite and graphite as seen in Japanese Patent Publication No. 63-9580.
[0003]
On the other hand, although graphite-dispersed steel is excellent in cold forgeability, according to JP-A-2-111842, the quenching hardness is insufficient because the dispersion speed of graphite is slow at the time of quenching and heating and the graphite is not sufficiently dissolved in austenite. It is said to have drawbacks. To solve this drawback, it is disclosed in Japanese Patent Application Laid-Open No. 2-111842 that it is only necessary to make graphite finer, and that it is effective to use BN as a precipitation nucleus of graphite as a specific method. Have been.
[0004]
JP-A-49-67817 discloses that a method utilizing a carbon supersaturated state (martensite structure) and a martensitic transformation strain is effective as a production method for obtaining graphite-dispersed steel. According to this, a steel having a specific chemical composition is heated to 750 to 950 ° C. and quenched to transform into martensite, which is heated again and annealed at 600 to 750 ° C. to obtain a graphite dispersed steel. ing.
[0005]
[Problems to be solved by the invention]
However, as is well known, BN usually precipitates easily at crystal grain boundaries, so that graphite having BN as a nucleation site similarly segregates at ferrite grain boundaries or former austenite grain boundaries and becomes uneven. Spread. In the case of martensite transformation as well, the formation site of graphite is limited to the former austenite grain boundary or martensite block boundary where the transformation strain is concentrated, and the graphite is unevenly dispersed.
[0006]
If the graphite is unevenly dispersed in the grain boundaries, the variation in the distance between the graphites increases and the maximum value also increases. On the other hand, graphite grows while absorbing the surrounding carbon atoms. Therefore, in a place where the distance between graphites is large, it is considered that the range in which one graphite particle can absorb the surrounding carbon becomes large, and the graphite particle size finally obtained becomes large. For this reason, the graphite size obtained by the state of the art is about 5 to 10 μm, although finer, and graphite, ie, carbon, diffused during quenching and heating, where graphite was present. Voids of about 5 to 10 μm remain. Due to the voids, they have a drawback that the fracture toughness value (impact value) is low, and at present, sufficient toughness has not yet been guaranteed.
[0007]
The present invention is a cold working method in which graphite is uniformly dispersed not only in crystal grain boundaries but also in ferrite grains, and the average grain size of graphite (average size of pores after quenching) is minimized to increase toughness. It is an object of the present invention to provide an ultrafine graphite uniformly dispersed steel for use.
[0008]
[Means for Solving the Problems]
The inventors of the present invention have conducted various studies on the fine dispersion of BN, which serves as a graphite generation site. As a result, the heating temperature before rolling was specified to be equal to or higher than the BN precipitation temperature, and B and N were dissolved in steel in a supersaturated manner by rapid cooling. After that, hot rolling is performed in a specific temperature range in which BN starts to precipitate rapidly, and a number of processing dislocations are introduced. Immediately after that, BN is uniformly precipitated on the dislocations by being gradually cooled. From the above, a new finding was obtained that fine graphite can be uniformly dispersed.
[0009]
Based on such knowledge, the present inventors have realized the ultrafine refining of the average particle size of graphite (average size of pores after quenching), which has been difficult in the past, and improved the toughness of fine particles for cold working. A method for producing graphite uniformly dispersed steel has been proposed.
(1) or first of the present invention, in mass%,
C: 0.30 to 1.0%,
Si: 0.60 to 1.3%,
Mn: 0.40 to 1.0%,
P: 0.02% or less,
S: 0.015 to 0.055%,
Al: 0.010-0.035%,
B: 0.001 to 0.004%,
N: 0.002 to 0.008%
, The balance consisting of Fe and unavoidable impurities is heated to 1150 ° C or higher, and then cooled to a hot-rolling start temperature at a cooling rate of 10 ° C / sec or more. 850-1000 ° C., the average cooling rate from the end of hot rolling to the quenching start temperature is 0.1-1.0 ° C./sec, and Ar is M from the quenching start temperature in the range of _3- point-850 ° C. _After cooling to a temperature below the _s point at a cooling rate of 30 to 100 ° C./sec, the mixture is naturally cooled, and then graphitized at a heating temperature of 600 to 720 ° C. to obtain a graphite having an average particle size of 2.0 μm or less. This is a method for producing a fine graphite uniformly dispersed steel for cold working, which is excellent in toughness, characterized by having 30 to 1.0% by mass .
(2) The second present invention further provides, as a component of steel material ,
Mo: 0.05 to 0.20%
(1) A method for producing a fine graphite homogeneously dispersed steel for cold working having excellent toughness as described in (1).
[0010]
[Action]
Next, the reasons for limiting the chemical composition of steel and the particle size and content of graphite in the present invention will be described in detail.
C has a lower limit of 0.30% in order to secure strength after quenching. On the other hand, the upper limit was set to 1.0% in order to prevent burning cracks in the heat treatment after cold working.
[0011]
Since Si is a powerful element that promotes graphitization, its lower limit is set to 0.60%. However, when the amount of Si increases, the ferrite phase solid-solution hardens and causes deterioration in cold workability. Therefore, the upper limit is set to 1.3%.
Mn is an element necessary because it is present as MnS inclusions by combining with S and serves as a site for generating graphite or BN. Further, since it is also an element necessary for increasing the hardenability and securing the strength, the lower limit value is set to 0.40%. However, if the amount of Mn is large, graphitization is significantly inhibited, so the upper limit is set to 1.0%.
[0012]
Since P causes grain boundary segregation and center segregation in steel and causes toughness deterioration, the upper limit is set to 0.02%.
Since S is combined with Mn to form MnS inclusions and is an effective element for generating graphite or BN, the lower limit is set to 0.015%. However, when the content is too large, the cold workability is deteriorated. Therefore, the upper limit is set to 0.055%.
[0013]
Al removes oxygen in steel as oxide inclusions. Further, in order to adjust the crystal grains, it is necessary to add 0.010% or more. On the other hand, if a large amount is added, the effect of deoxidation is saturated, so the upper limit was made 0.035%.
B and N promote graphitization by forming BN to provide a site for producing graphite. To obtain a sufficient graphitization promoting effect, 0.001% or more of B must be added. If B exceeds 0.004%, the graphitization promoting effect is saturated, so the upper limit was made 0.004%. N is 0.001 to 0.004%, which is an amount necessary to convert B into BN, that is, 0.002 to 0.008%.
[0014]
Mo is added as necessary to ensure hardenability. In order to sufficiently obtain the effect of increasing hardenability, the lower limit is set to 0.05%. Further, since the addition of a large amount is not preferable in terms of economy, the upper limit is set to 0.20%.
The upper limit of the average particle size of graphite was set to 2.0 μm from the viewpoint of toughness (impact characteristics). If it exceeds 2.0 μm, the portion where graphite was present during quenching and heating will remain as coarse pores exceeding 2.0 μm, lowering the impact value.
[0015]
Since almost all of C in steel is graphitized, the amount of graphite is substantially equal to the C content. Therefore, the reason for limiting graphite is exactly the same as that described for the reason for limiting C.
Next, the reasons for limiting the manufacturing conditions in the present invention will be described in detail.
The heating temperature before the hot rolling was set to 1150 ° C. or higher in order to completely dissolve BN which can be precipitated in the amounts of B and N within the range of the present invention in the matrix. The rolling start temperature and the ending temperature were set in a temperature range in which BN rapidly precipitates in order to introduce a large number of processing dislocations serving as generation sites before BN precipitation, that is, 850 to 1000 ° C. In order to sufficiently precipitate BN on working dislocations after hot rolling, the average cooling rate from the end of hot rolling to the quenching start temperature was specified. The upper limit was set to 1.0 ° C./sec in order to promote the precipitation of BN, and the lower limit was set to 0.1 ° C./sec in consideration of productivity. The upper limit of the cooling start temperature after hot rolling was 850 ° C. at which BN almost completely precipitated on the working dislocation, and the lower limit was Ar ₃ in consideration of obtaining a martensite structure after rapid cooling. The quenching end temperature must be below the _Ms point to obtain a martensitic structure. The lower limit of the average cooling rate was set to 30 ° C./sec in order to obtain a sufficient martensite structure, and the upper limit was set to 100 ° C./sec. This is because they do not.
[0016]
The lower limit of the graphitization temperature is set to 600 ° C. and the upper limit to 720 ° C. because the graphitization time in this range is the shortest.
[0017]
【Example】
Hereinafter, the effects of the present invention will be described more specifically with reference to examples.
The material used in this embodiment is a straight bar or a burn-in coil having a diameter of 19 mmφ or more. Cooling was performed by uniformly spraying 0.3 to 0.5 tons of cooling water per unit area over the entire surface of the steel bar using a water cooling device installed on an extension of the hot rolling line. The cooling device is a pipe having a length of 20 m and having holes for supplying a large number of cooling water on the circumference, and the steel bar is cooled as it passes on the center line of the pipe. The cooling start temperature and the cooling end temperature are values obtained by measuring the surface temperature of the steel material with a radiation thermometer, and the average cooling speed was obtained by dividing the difference between the cooling start temperature and the cooling end temperature by the cooling time. Thereafter, a graphitization treatment was performed in an annealing furnace.
[0018]
The graphitization rate was calculated by the following equation.
(Graphite content in steel / Carbon content of steel) × 100 (%)
Here, the carbon content of the steel was determined by chemical analysis. The graphite content was calculated from graphite average particle size, density, and number of graphite particles.
The average particle size of graphite was determined by the following method. By irradiating the graphite particles with an electron beam and binarizing the intensity of the reflected electron beam, the graphite was imaged on an SEM screen, and the particle size was measured and analyzed using an analysis system. The area of one visual field is 100 μm × 100 μm, the number of visual fields is 25, and the total measured area is 0.25 mm ² .
[0019]
Graphite inter maximum distance was measured on an optical microscope photograph of 200 magnifications. The maximum value of the diameter an arc containing only nonexistent location of graphite was graphite inter maximum distance on the photograph.
The impact absorption energy was determined as follows. That is, the graphitized steel bars are quenched (850 ° C. × 30 minutes → water cooling) and tempered (600 ° C. × 60 minutes → water cooling), and then subjected to a 2 mm U notch Charpy impact test piece (JIS Z2202).
No. 3) was collected. The test temperature of the Charpy impact test is 20 ° C.
[0020]
Table 1 shows the chemical components, the state of graphite precipitation, and the toughness when quenched and tempered after graphitization.
[0021]
[Table 1]

[0022]
In addition, all the steel materials used in Table 1 were graphitized under the following manufacturing conditions. That is, a 200 mm square billet was heated to 1250 ° C., and then cooled to 980 ° C. at a cooling rate of 15 ° C./sec. Thereafter, hot rolling was started at 980 ° C., and rolling was completed at 880 ° C. to obtain a round bar having a diameter of 20 mmφ. Thereafter, the mixture was cooled at a rate of 0.5 ° C./sec and rapidly cooled from 750 ° C. to 100 ° C. The average cooling rate during this time was 60 ° C./sec. Subsequently, the graphitization treatment was performed at an annealing temperature of 700 ° C. and an annealing time of 12 hours. In Table 1, the graphitization rate of the present invention is as high as 100%, whereas the graphitization rate of the comparative example is as low as about 50%. Further, in the present invention, both the average particle size of graphite and the maximum distance between graphites are smaller than those of Comparative Examples. Therefore, the Charpy impact absorption energy indicating the toughness after quenching / tempering of the present invention is significantly higher than that of the comparative example.
[0023]
Tables 2 and 3 show that, in Table 1, steel No. satisfying the component range of the present invention. 1 to 5 show the graphitization state and toughness when the production conditions were changed. In the production method according to the present invention, both the graphite average particle size and the maximum distance between graphites are smaller than those of the comparative example, and the Charpy impact absorption energy after quenching and tempering of the graphite steel obtained by the production method of the present invention, that is, the toughness is also comparative example. Significantly higher than that of the graphite steel obtained by the method described above.
[0024]
[Table 2]

[0025]
[Table 3]

[0026]
【The invention's effect】
As is clear from the above examples, according to the present invention, it is possible to provide a method for producing fine graphite uniformly dispersed steel having excellent toughness, and the industrial effect is extremely remarkable.

Claims

In mass%,
C: 0.30 to 1.0%,
Si: 0.60 to 1.3%,
Mn: 0.40 to 1.0%,
P: 0.02% or less,
S: 0.015 to 0.055%,
Al: 0.010-0.035%,
B: 0.001 to 0.004%,
N: 0.002 to 0.008%
, The balance consisting of Fe and unavoidable impurities is heated to 1150 ° C or higher, and then cooled to a hot-rolling start temperature at a cooling rate of 10 ° C / sec or more. 850-1000 ° C., the average cooling rate from the end of hot rolling to the quenching start temperature is 0.1-1.0 ° C./sec, and Ar is M from the quenching start temperature in the range of _3- point-850 ° C. _After cooling at a cooling rate of 30 to 100 ° C./sec to a temperature of not more than the _s point, the mixture is naturally cooled, and then subjected to a graphitization treatment at a heating temperature of 600 to 720 ° C. to obtain a graphite having an average particle size of 2.0 μm or less. A method for producing a fine graphite uniformly dispersed steel for cold working having excellent toughness, characterized by having 30 to 1.0% by mass .

As a component of steel materials ,
Mo: 0.05 to 0.20%
The method for producing a fine graphite homogeneously dispersed steel for cold working having excellent toughness according to claim 1, characterized by comprising: