JP4084462B2 - Free-cutting hot-worked steel and its manufacturing method - Google Patents

Description

【００６５】
【表２】

【００６６】
鋼Ｎo.１〜２３は本発明鋼、鋼Ｎo.２４〜４５は比較鋼、そして、鋼Ｎo.４６〜５０は従来鋼である。従来鋼の内、鋼Ｎo.４６はＳＵＭ２４Ｌ、鋼Ｎo.４７はＳ４５ＣへのＰｂ添加鋼、鋼Ｎo.４８はＳ４８ＣにＶを０．１２％、Ｐｂを０．２１％添加した非調質鋼、鋼Ｎo.４９は球状黒鉛鋳鉄、鋼Ｎo.５０はＳＣＭ８２２である。これらの成分の供試鋼を、１３０トン電気炉で溶製後、連続鋳造又は造塊法によりそれぞれ鋳片又は鋼塊に鋳造した。鋳片又は鋼塊を分塊圧延して所定寸法の鋼片に圧延し、次いで所定寸法の棒鋼に熱間圧延した。棒鋼を次の通り使用した。
【００６７】
下記試験１においては、棒鋼を切削加工して、ピストンピンに機械加工して仕上げた。試験２においては、棒鋼を熱間鍛造して、クランクシャフトの熱間鍛造材を作り、これを切削加工してクランクシャフトに機械加工仕上げした。試験３においては、棒鋼を熱間鍛造して、デファレンシャルドライブギアの熱間鍛造材を作り、これを切削加工してデファレンシャルドライブギアに仕上げ、それぞれ目的の製品を製造した。但し、鋼Ｎo.４９の球状黒鉛鋳鉄のみは、直接、製品形状品に鋳造し、目的の製品を製造した。
【００６８】
〔試験１〕
試験には、表１及び表２に示した鋼Ｎo.１〜２３の本発明鋼、鋼Ｎo.２４〜４５の比較鋼、及び鋼Ｎo.４６〜４７の従来鋼を用いた。本発明の範囲内の試験である実施例としては、本発明鋼の鋼Ｎo.１〜２０を用いた実施例１−１〜１−２０、本発明の範囲外の試験である比較例としては、本発明鋼の鋼Ｎo.２１〜２３を用いた比較例１−２０〜１−２３、比較鋼の鋼Ｎo.２４〜４５を用いた比較例１−２４〜１−４５、並びに従来鋼の鋼Ｎo.４６、４７を用いた比較例１−４６、１−４７を行なった。各鋼Ｎo.の鋳片又は鋼塊を分塊圧延して、１６０ｍｍ角の鋼片を製造し、鋼片加熱炉にて８２０〜１２００℃の間の温度に加熱して、直径３２ｍｍφの棒鋼に熱間圧延した。熱間圧延後の棒鋼にはカバーをかけて徐冷した。但し、鋼Ｎo.２３の棒鋼のみは熱間圧延後、放冷した。製造した棒鋼を切削により、ブレーキの油圧部品であるピストンピンに機械加工した。
【００６９】
表３及び表４に、実施例及び比較例の試験条件を示す。
【００７０】
【表３】

【００７１】
【表４】

【００７２】
実施例及び比較例について、下記内容の試験を行なった。
▲１▼棒鋼について、表面を目視で疵の判定をした。また、黒鉛の析出状態及び金属組織を光学顕微鏡により調査した。
【００７３】
▲２▼棒鋼の切削性試験として、切り屑処理性及びハイス工具の寿命試験をした。切り屑処理性の判定は図２に示すように、切り屑が２巻き以下で分断しているものを「良好」としてランク１、切り屑が３〜６巻で分断しているものを「普通」としてランク２、そして切り屑が８巻以上につながっているものを「劣る」としてランク３と位置づけた。工具寿命の試験は、ハイス工具で切削速度１５０ｍ／ｍｉｎ、送り０．２０ｍｍ／ｒｅｖにて切削油をかけた状態で切削し、刃先が溶損して切削不能になるまでの時間を測定し、工具寿命とした。
【００７４】
試験結果は次の通りである。
実施例１−１〜１−２０は、鋼片の化学成分及び加熱温度、並びに熱間圧延後の棒鋼の冷却速度（圧延後７００℃に下がるまでの冷却時間）共に、本発明の範囲内の条件を満たしている。そして、熱間圧延棒鋼の黒鉛粒の大きさは１．０〜２５μｍの間となっており、黒鉛粒の数もすべて５０個／ｍｍ² 以上の十分に多数存在していた。また、金属組織はフェライトまたはフェライト＋パーライトの比較的硬度の低い組織になっていた。なお、一例として、図３に実施例１−５における熱間圧延棒鋼の金属組織（倍率：６００）を示す。その結果、棒鋼の表面に割れの発生はなかった。また、棒鋼の切り屑は、全て２巻以下に小さく分断した良好な形状を呈して、切り屑処理性は優れていた。また、切削工具の寿命も全て２０分以上と長かった。こうした良好な状態で、棒鋼をブレーキの油圧部品であるピストンピンに機械加工することができた。
【００７５】
これに対して、本発明の範囲外の比較例では、次の通り、棒鋼の表面性状及び切削性において何らかの問題があった。
●比較例１−２１は、成分組成は本発明の範囲内であったが、加熱温度が本発明の範囲より高かったため、熱間延性が不足して、棒鋼に割れが生じた。また比較例１−２２も成分組成は本発明の範囲内であったが、加熱温度が逆に本発明の範囲より低かったため、熱間延性が不足して、棒鋼に割れが生じた。
【００７６】
●比較例１−２３は、成分組成は本発明の範囲内であったが、熱間圧延後７００℃までの冷却時間が本発明の範囲より短かった。このため黒鉛が成長する時間がなく、黒鉛粒が０．６μｍと小さく、また黒鉛の廻りにフェライトの発生もなく、組織がパーライトのみであり、硬さの高いものであった。このため切り屑処理性がやや劣り、また工具寿命も６分と短いものであった。
【００７７】
●比較例１−２４は、Ｃ含有率が本発明の範囲を外れて低く、黒鉛の析出は見られなかった。そのため、切り屑処理性が悪く、工具寿命も短かった。比較例１−２５は逆に、Ｃ含有率が本発明を外れて高く、熱間延性が不足して、棒鋼に大きな割れが発生した。
【００７８】
●比較例１−２６は、Ｓｉ含有率が本発明の範囲を外れて低く、このため黒鉛化指数ＣＥが小さくなり、黒鉛の析出は見られず、切り屑が長くつながってしまった。このため機械を停止して切り屑を除去する必要があった。比較例１−２７は、Ｓｉ含有率が本発明の範囲を外れて高く、このため熱間延性が不足して、棒鋼に割れを生じた。
【００７９】
●比較例１−２８はＭｎ含有率が本発明の範囲より高く、黒鉛の析出は見られなかった。そのため、切り屑処理性が悪く、工具寿命も短かった。比較例１−２９は、Ｐ含有率が本発明の範囲より高く、延性不足で、棒鋼に割れが発生した。比較例１−３０は、Ｓ含有率が本発明の範囲より高いため、やはり熱間延性が不足して、割れが発生したのみならず、Ｓの過剰添加が悪影響を及ぼして見かけの黒鉛化指数ＣＥは高いものの、黒鉛の析出がみられなかった。そのため、切り屑処理性が悪く、工具寿命も短かった。
【００８０】
●比較例１−３１は、Ｃｕ含有率が本発明の範囲より高く、鋼片加熱中にＣｕが表面に濃化して粒界に侵入し、圧延棒鋼に割れが発生した。比較例１−３２は、Ｃｒ含有率が本発明の範囲より高く、このため熱間延性が不足して、棒鋼に割れが生じた。比較例１−３３は、Ｎｉ含有率が本発明の範囲より高く、このため熱間延性が不足して、棒鋼に割れが生じた。比較例１−３４は、Ｃｏ、Ｍｏ及びＯ含有率が本発明の範囲より高く、やはり棒鋼に割れを生じた。
【００８１】
●比較例１−３５は、Ｂ及びＮ含有率が本発明の範囲より高く、多量のＢＮが析出して延性不足から割れを生じた。比較例１−３６は、Ｔｉ含有率が本発明の範囲より高く、熱間延性が不足して、棒鋼に割れが生じた。比較例１−３７は、Ｚｒ含有率が、比較例１−３８は、Ｖ含有率が、比較例１−３９はＡｌ含有率が、比較例１−４０は、Ｎｂ含有率が、いずれも本発明の範囲より高く、このため延性不足で棒鋼に割れが生じた。
【００８２】
●比較例１−４１は、Ｃａ含有率が、比較例１−４２は、Ｍｇ含有率が、比較例１−４３は、ＲＥＭ含有率が、いずれも本発明の範囲より高く、このため酸化物系介在物を多量に巻き込み、これが圧延疵の原因となり、棒鋼に割れが発生した。
【００８３】
●比較例１−４４及び１−４５は、化学成分の個々の含有率は本発明の範囲内であるが、黒鉛化指数ＣＥが１．３０より低かったために黒鉛が析出しなかった。そのため、いずれも切り屑処理性が悪く、工具寿命も短かった。
【００８４】
●比較例１−４６は、従来のＳＵＭ２４Ｌを用いたものであり、良好な被削性を有していた。しかし耐摩耗性を向上させるためＳＵＭ２４Ｌにおいては、９３０℃×５ｈｒの浸炭焼入れ、１７０℃×３０分焼戻しを施す必要があった。これに対して、実施例１−１〜１−２０においては、簡便な高周波焼入れで耐摩耗性を向上させることができた。また、比較例１−４７は、Ｓ４５ＣへのＰｂ添加鋼を用いたものであり、切り屑処理性は良好であったが、工具寿命がやや短い。上記比較例１−４６及び１−４７は、Ｐｂ添加快削鋼であり、地球環境保護の観点から、使用を控える方向で部品を製造することが現在求められている。
【００８５】
以上述べた通り、本発明によれば、従来の硫黄・鉛複合快削鋼に匹敵する被削性を有する鋼製品の製造が可能であり、その工具寿命は鉛添加機械構造用炭素鋼材を上回る熱間圧延棒鋼を製造することができる。
【００８６】
〔試験２〕
試験には、表１に示した鋼Ｎo.１及び１８の本発明鋼、並びに、鋼Ｎo.４８及び４９の従来鋼を用いた。本発明の範囲内の試験である実施例としては、本発明鋼の鋼Ｎo.１及び１８を用いた実施例２−１及び２−１８、本発明の範囲外の試験である比較例としては、従来鋼の鋼Ｎo.４８及び４９を用いた比較例２−４８及び２−４９を行なった。
【００８７】
実施例２−１及び２−１８では、鋳片又は鋼塊を分塊圧延して、１６０ｍｍ角の鋼片を製造し、鋼片を直径９５ｍｍφの棒鋼に熱間圧延した。上記熱間圧延棒鋼を用いて、１０００℃に加熱後、クランクシャフト形状に熱間鍛造した。鍛造後は黒鉛の析出と機械的性質とを両立させるため、クランクシャフト鍛造材をコンベア上で扇風機により弱冷した。即ち、黒鉛の析出を促進するためには、ゆっくり冷却した方がよいが、黒鉛の成長につれて、パーライトの量が少なくなり、所望とする強度が確保できなくなる。そこで、冷却速度を最適に調整する必要がある。こうして、鍛造後のクランクシャフトの９００℃から７００℃までの冷却時間を５分とした。クランクシャフトの黒鉛の大きさは、５〜７μｍで、数は２０００〜３０００個であり、組織はフェライト＋パーライトであった。図４に、実施例２−１におけるクランクシャフト鍛造材の金属組織（倍率：６００）を示す。次いで、クランクシャフトの外周を切削したのち、小径深穴ドリルにより３ｍｍ径の油穴を明けた。
【００８８】
比較例２−４８では、鋳片を分塊圧延して、１６０ｍｍ角の鋼片を製造し、鋼片を直径９５ｍｍφの棒鋼に熱間圧延した。上記熱間圧延棒鋼を用いて、１２５０℃に加熱後、実施例２−１及び２−１８と同一形状のクランクシャフトに熱間鍛造し、以降、前記同様、コンベア上で扇風機により弱冷し、鍛造後のクランクシャフトの９００℃から７００℃までの冷却時間を５分とした。次いで、クランクシャフトの外周を切削したのち、小径深穴ドリルにより３ｍｍ径の油穴を明けた。
【００８９】
比較例２−４９では、従来の球状黒鉛鋳鉄を上記と同一形状のクランクシャフトに直接鋳造して、凝固させた。そして、上記と同じく、クランクシャフトの外周を切削したのち、小径深穴ドリルにより３ｍｍ径の油穴を明けた。
【００９０】
次いで、上記いずれのクランクシャフトも、曲げ疲労試験に供した。
実施例及び比較例の試験結果は次の通りである。
▲１▼クランクシャフトの油穴明け時の切り屑処理性は、実施例及び比較例共いずれの場合も、２巻き以下の細かく分断した良好な切り屑であった。
【００９１】
▲２▼曲げ疲労試験における疲労強度について、実施例２−１は５１０Ｎ／ｍｍ² 、実施例２−１８は５２０Ｎ／ｍｍ² であり、比較例２−４８の５００Ｎ／ｍｍ² と同等であり、良好な疲労強度を有していた。これに対して比較例２−４９の球状黒鉛鋳鉄材は、４１０Ｎ／ｍｍ² の疲労強度しかなかった。これは、鋳鉄ではヤング率が低いこと、及び小さい気泡が疲労の起点となり、疲労限を低下させたためと考えられる。
【００９２】
以上述べた通り、本発明によれば、無鉛で被削性に優れた非調質の快削鋼部品の製造が可能であり、これは、被削性については、鉛快削鋼や球状黒鉛鋳鉄と同等であり、またその疲労特性については、従来の球状黒鉛鋳鉄部品より優れており、従来の非調質鋼部品と同等の高い疲労強度を有していることがわかる。
【００９３】
〔試験３〕
試験には、表１に示した鋼Ｎo.３及び６の本発明鋼、並びに、表２に示した鋼Ｎo.４９及び５０の従来鋼を用いた。本発明の範囲内の試験である実施例としては、本発明鋼の鋼Ｎo.３及び６を用いた実施例３−３及び３−６、並びに、本発明の範囲外の試験である比較例としては、従来鋼の鋼Ｎo.４９及び５０を用いた比較例３−４９及び３−５０を行なった。
【００９４】
試験方法は次の通りである。
実施例３−３及び３−６では、２００ｍｍ角の小断面鋳片を分塊圧延することなく直接直径１３０ｍｍφの棒鋼に熱間圧延した。上記熱間圧延棒鋼を用いて、外径３２０ｍｍのデファレンシャルドライブギアの粗形材に熱間鍛造した。鍛造加熱温度は１０８０℃とし、９００℃から７００℃まで下がる冷却時間を３．５分に調整した。このため上記粗形材の金属組織は８％のフェライトを含むパーライトであり、黒鉛分布は２〜３μｍ径のものが、５０００〜７０００個／ｍｍ² 析出していた。上記粗形材を、そのままホブ盤にてデファレンシャルドライブギア（歯車）に切削加工し、その後５７０℃、５時間のガス軟窒化を施して表面を硬化させた。
【００９５】
比較例３−５０でも、２００ｍｍ角の小断面鋳片を分塊圧延することなく直接直径１３０ｍｍφの棒鋼に熱間圧延した。上記熱間圧延棒鋼を用いて、外径３２０ｍｍのデファレンシャルドライブギアの粗形材に熱間鍛造した。鍛造加熱温度は１２３０℃とし、熱間鍛造後コンベアにて搬送し空冷した。上記鍛造ままの粗形材の金属組織はベイナイトであり、硬いのでそのまま切削加工することは困難であった。そこで、９１０℃×３時間加熱後、６５０℃×１時間保持のサイクル焼鈍をして軟化させた後、ホブ盤にてデファレンシャルドライブギア（歯車）に切削加工した。切削加工した後、表面を硬化せさるため、９３０℃×５時間の浸炭後、８５０℃×３０分保持の焼入れ処理を行なって表面を硬化させた。
【００９６】
比較例３−４９では、鋼種が従来球状黒鉛鋳鉄であるため、上記デファレンシャルドライブギアと同一形状のギア砂型に直接鋳込んだ。鋳込材を型から取り出して、直接、切削加工した後、９００℃×１時間加熱後、２８０℃×４時間保持のソルト浴浸漬のオーステンパー処理を施した。
【００９７】
試験結果は次の通りである。
ホブ切り加工においては、いずれも良好な切り屑処理性を示し、また工具の摩耗も少なく、切削面のむしれもなく、良好な切削状態であった。
【００９８】
各熱処理を施したデファレンシャルドライブギアを疲労試験に供した。実施例３−３及び３−６のガス軟窒化ギアの歯元曲げ疲労強度はそれぞれ、４６０Ｎ／ｍｍ² 及び４９０Ｎ／ｍｍ² であった。一方、ＳＣＭ８２２を用い、浸炭焼入れをした比較例３−５０のギアの歯元曲げ疲労強度は４７０Ｎ／ｍｍ² であった。しかしながら、球状黒鉛鋳鉄のオーステンパー処理材である比較例３−４９においては、３３０Ｎ／ｍｍ² と低いものであった。
【００９９】
次に、各熱処理後のギアの変形は、歯車かみ合い時の騒音の原因となる。そこで、各ギアのドライブ側のプレッシャ−アングルの変形量を測定した。
図５に、ギアのプレッシャ−アングルの変形量の説明図を示す。図中、１は歯車、２は角度変位を示す。浸炭焼入れ材の比較例３−５０では、アングルのずれは１６分（１分は１°の６０分の１）であったが、軟窒化材の実施例３−３及び３−６では、アングルのずれは１分であり、殆んど変形のないものであった。また、オーステンパー材の比較例３−４９では、熱処理直後の変形は４分と比較的変形の小さいものであったが、１０００回の疲労回数を越えると２０分と変形の大きいものであった。これは、オーステンパー処理によって組織内に留められた残留オーステナイトが、マルテンサイトに変態したために、変形量が大きくなったものと考えられる。
【０１００】
以上説明したように、本発明にかかるギアは、軟化焼鈍を施さなくても、被削性が良好であり、疲労強度も球状黒鉛鋳鉄より高く、従来のＳＣＭ鋼の浸炭焼入れギアに匹敵する高い強度を有し、且つ歪みが小さく、騒音の発生の小さいものであることが確認された。
【０１０１】
また本発明鋼の鋼Ｎo.１〜２０の熱間圧延棒鋼を用いて、コネクチングロッド、ナックルスピンドル、カムシャフト、エンジンギア、ピニオンギア及びシャフトギア等、各種の製品を本発明の条件内で製造したが、すべて被削性が良好で、耐疲労性に優れた特性を有していた。
【０１０２】
【発明の効果】
以上述べたように、この発明によれば、有毒なＰｂを用いることなく、被削性及び疲労特性共に優れた鋼の熱間加工製品の製造が可能であり、非調質の快削鋼部品や低歪みで高い疲労強度を有する歯車を製造することが可能となるといった工業上有用な効果がもたらされる。
【図面の簡単な説明】
【図１】本発明鋼の鋼Ｎo.５の５０ｍｍ厚さ鋼片を１０５０℃に加熱し、３０ｍｍ厚さに鍛造した後、９００℃から７００℃まで温度を下げるまでの冷却時間と鍛造材の硬さとの関係を示すグラフである。
【図２】部品材切削時の切り屑処理性のランクと切り屑形態との対応関係を説明する図である。
【図３】実施例１−５における熱間圧延棒鋼の金属組織（倍率：６００）を示す図である。
【図４】実施例２−１におけるクランクシャフト鍛造材の金属組織（倍率：６００）を示す図である。
【図５】歯車のプレッシャアングルの歪みの説明図である。
【符号の説明】
１ 歯車
２ 角度変位[0001]
BACKGROUND OF THE INVENTION
The present invention relates to parts for automobiles and industrial machinery such as crankshafts and differential gears, and has fine graphite as it is hot-worked, and is comparable to conventional lead free-cutting steel without performing precipitation heat treatment of graphite. The present invention relates to a technology for producing a lead-free hot-worked product having excellent machinability.
[0002]
[Prior art]
When machining parts used for automobiles and industrial machines by directly cutting the steel bars, for example, in the cutting work of piston rods or the like, the steel bars are required to have excellent machinability. In addition, when a steel bar is hot forged to produce a rough shape, and this is machined by cutting to produce a part, for example, in the processing of a connecting rod, crankshaft or differential gear that is a part around an engine of an automobile. However, excellent machinability is required for the forged rough profile before cutting.
[0003]
The quality of the above-described machinability is determined by the long life of the cutting tool and the processability of chips generated during cutting, that is, whether or not the chips are finely divided into appropriate sizes. In particular, these machines are often machined unattended by automatic lathes, and if the chips become long and tangled, it becomes necessary to stop the cutting machine or perform extra work to remove the chips, which increases productivity. Will be reduced.
[0004]
In addition, the connecting rod and crankshaft are provided with several small diameter holes for supplying lubricating oil. However, since these holes are deep, chips are finely divided during drilling. Therefore, it is necessary to discharge from the drill hole without any problem. That is, chips that are difficult to divide are not discharged from the hole, and the chips are clogged into the hole and cause breakage of the drill.
[0005]
Therefore, in machining parts such as the above, lead free-cutting steel with 0.05 to 0.30% of lead, a free-cutting element, is widely used to improve tool life and chip disposal. Has been. Since the melting point of lead is as low as about 327 ° C., it is easily melted by the heat of machining, the ductility of the steel is lowered, and the chips are divided into an appropriate size. This extends the tool life.
[0006]
Further, among free-cutting steels currently widely used, the most excellent machinability is sulfur-lead composite free-cutting steel (JIS G 4804, SUM24L) to which sulfur and lead are added in combination. However, this steel material is machined and used in parts that place emphasis on machinability, such as piston pins, which are hydraulic parts of brakes, and caps of water taps.
[0007]
The above steel is a free-cutting steel that makes the best use of the chip cutting effect of lead. However, since lead is toxic, lead-free free-cutting steel is strongly demanded with the recent increase in global environmental protection.
[0008]
In addition to Pb, elements such as S, Ca, Bi, Se, and Te are known as elements that improve machinability. However, these elements have at least one of the following disadvantages: (1) the machinability improvement effect is not as good as lead, (2) expensive, and (3) toxic. It cannot be an element.
[0009]
On the other hand, graphite is a substance that greatly improves machinability as seen in cast iron. However, in steel, when a large amount of carbon is added to precipitate graphite, cementite precipitates and it is not easy to obtain graphite. In the case of steel having carbon of 0.10 to 1.5% in the conventional invention, for example, in JP-A-2-107742 and JP-A-3-140411, several hours at a temperature of 600 to 800 ° C. A steel material in which graphite is deposited by annealing for as long as ˜200 hours or a method for producing the same is disclosed.
[0010]
JP-A-49-67816 and JP-A-49-67817 disclose graphite free-cutting steel in which graphite is formed by quenching at 750 to 950 ° C. and tempering at 600 to 750 ° C. Yes.
[0011]
As described above, in all of the conventional disclosure examples, it is necessary to perform graphitization heat treatment for obtaining graphite. Therefore, the cost becomes extremely high. In addition, the metal structure becomes ferrite by the graphitization heat treatment. For this reason, it is limited to the manufacture of low-strength parts and small parts that can be manufactured by cold forging, and cannot be applied to the manufacture of large forged parts such as crankshafts and connecting rods.
[0012]
On the other hand, it is well known that cast iron and cast steel having a carbon content of around 3.8% can easily obtain spheroidal graphite as cast by inoculation with Ca, Mg, etc., and have good machinability. However, since these cast irons and cast steels are used as cast, there is a drawback that the toughness such as elongation, drawing and impact value is low, although there is a degree of freedom in shape.
[0013]
In recent years, the toughness has been improved by making the base structure bainite by austempering. For example, Japanese Patent Application Laid-Open No. 61-243121 discloses a method for manufacturing a crankshaft in which spheroidal graphite cast iron is austempered. A method for manufacturing a grod is disclosed. However, these cast products have lower Young's modulus and inferior fatigue strength than forged products of non-tempered steel containing S48C as a basic component and V of about 0.10%. Also, toughness is not as good as that of forged products. Further, in these cast products, a casting nest of about 0.1 mm may be generated. This is a starting point of fatigue failure, so that it is inferior in reliability. Therefore, strict attention must be paid to the casting method and the ultrasonic inspection of the product. Therefore, it also contributes to the cost increase.
[0014]
[Problems to be solved by the invention]
The object of the present invention is comparable to conventional lead free-cutting steel even in the manufacturing process of steel parts for automobiles and industrial machinery, having fine graphite as it is hot-worked and without performing precipitation heat treatment of graphite. The aim is to develop a manufacturing technology for lead-free hot-worked products with excellent machinability. In order to achieve this object, the above-described prior art has the following problems.
[0015]
{Circle around (1)} By adding Pb to the steel, the free cutting property of the steel material is remarkably improved. However, Pb free cutting steel has a problem from the viewpoint of eliminating the toxicity of Pb.
(2) When the effect of improving the machinability of graphite is exhibited in C: 0.1 to 1.5% steel, it is necessary to perform graphitization heat treatment, and the cost becomes remarkably high. As a result, the metal structure becomes ferrite, so that large forged parts have insufficient mechanical properties and fatigue properties and cannot be manufactured.
[0016]
(3) The machinability improvement effect of graphite can be exhibited in cast iron and cast steel, and the material can be improved by austempering. And it is excellent in the point of freedom of shape. However, even with such improvements, the mechanical properties and fatigue properties are insufficient and cannot be used for required parts.
[0017]
Therefore, the biggest problem of the present invention is to solve such problems and achieve the above-mentioned object by improving the machinability of the steel by appropriately controlling the size and amount of graphite. The purpose is to develop a technology to produce hot-worked steel or rough profile.
[0018]
[Means for Solving the Problems]
The present inventors have conducted intensive research in consideration of the above-described background, and the greatest knowledge is as follows. That is, it was found that the addition of a small amount of Ti significantly promotes the precipitation of graphite and is effective in improving hot ductility. Then, it has been found that a free-cutting hot-worked product in which graphite is deposited can be produced directly only by performing appropriate gentle cooling after hot working. In this way, we have developed a technology for manufacturing hot-worked products with excellent machinability comparable to cast iron without adding lead.
[0019]
  The present invention has been made on the basis of the above findings and has the following characteristics.
  The invention described in claim 1quality%: C: 0.70 to 1.50%, Si: 0.70 to 3.00%, Mn: 0.01 to 2.00%, P: 0.050% or less, S: 0.10 %, Ti: 0.001 to 0.100%, O: 0.0050% or less, and N: 0.020% or less, having a chemical component consisting of the balance Fe and inevitable impurities, 1) 50 graphite / mm of graphite having an average particle size of 1.0 μm or more with a graphitization index CE obtained by the formula of 1.30 or more²It has the above, and has a feature that the metal structure is ferrite or ferrite + pearlite.
  CE = C + Si / 3-Mn / 12 + Ti / 3 ------------------ (1)
  However, the element symbols in the above formula arequalityRepresents the amount%.
[0020]
  Claim 2inventionThe invention of claim 1In, Mn content is in the range of 0.01 to 0.35%.
[0021]
  Invention of Claim 3 is invention of Claim 1InThe Mn content is more than 1.00 to 2.00%.
[0022]
  The invention according to claim 4 is the invention according to any one of claims 1 to 3,SaidIn addition to the chemical component composition, at least one selected from the group consisting of the following six chemical component compositions is added and contained:qualityCu: 0.01 to 2.0%, Ni: 0.01 to 2.0%, Co: 0.01 to 0.50%, Cr: 0.01 to 1.0%, Mo: 0.01 to 0.50%, B: 0.0005 to 0.010%, and the following formula (2) is used as a calculation formula for the graphitization index CE.
  CE = C + Si / 3 + Ti / 3-Mn / 12 + Cu / 9 + Ni / 9 + Co / 9
        -Cr / 9-Mo / 9 + B --------------------------- (2)
  However, the element symbols in the above formula arequalityRepresents the amount%.
[0023]
  The invention according to claim 5 is the invention according to any one of claims 1 to 4,SaidIn addition to the chemical component composition, at least one selected from the group consisting of the following four chemical component compositions is added and contained,quality%: Al: 0.001 to 0.50%, Zr: 0.005 to 0.10%, V: 0.01 to 0.50%, and Nb: 0.01 to 0.50%, And it has the characteristics in using the following (3) Formula as a formula for calculating the graphitization index CE.
  CE = C + Si / 3 + Ti / 3-Mn / 12 + Cu / 9 + Ni / 9 + Co / 9
        -Cr / 9-Mo / 9 + B + Al / 6 + Zr / 3-V / 3-Nb / 3
                              ---------------------------- (3)
  However, the element symbols in the above formula arequalityRepresents the amount%.
[0024]
  The invention according to claim 6 is the invention according to any one of claims 1 to 5,SaidIn addition to the chemical component composition, at least one selected from the group consisting of the following three chemical component compositions is added and contained:qualityAs a calculation formula for Ca: 0.0010 to 0.010%, Mg: 0.0010 to 0.10%, and REM: 0.0010 to 0.10%, and graphitization index CE, It is characterized by using the following formula (4).
  CE = C + Si / 3 + Ti / 3-Mn / 12 + Cu / 9 + Ni / 9 + Co / 9
        -Cr / 9-Mo / 9 + B + Al / 6 + Zr / 3-V / 3-Nb / 3
        +0.07 ----- (4)
  However, the element symbols in the above formula arequalityRepresents the amount%.
The invention according to claim 7 is mass%, C: 0.70 to 1.50%, Si: 0.70 to 3.00%, Mn: 0.01 to 2.00%, P: 0.050. %: S: 0.10% or less, Ti: 0.001 to 0.100%, O: 0.0050% or less, and N: 0.020% or less, and the balance is Fe and inevitable impurities. A hot-rolled steel material having a chemical component and having a graphitization index CE determined by the following formula (1) of 1.30 or more is 800 ° C. or higher and the solidus temperature of the hot-rolled steel material is −50 ° C. or lower. Heated to a temperature in between, hot worked, and slowly cooled down to 700 ° C. over a period of 1 minute or more, and 50 graphite / mm of average particle size of 1.0 μm or more ² It is characterized in that it is deposited as described above and the metal structure is ferrite or ferrite + pearlite.
CE = C + Si / 3-Mn / 12 + Ti / 3 ---------------------------- (1)
However, the element symbol in the above formula represents mass% of each element.
The invention described in claim 8 is characterized in that, in the invention described in claim 7, the Mn content is in the range of 0.01 to 0.35%.
The invention according to claim 9 is characterized in that, in the invention according to claim 7, the Mn content is more than 1.00 to 2.00%.
The invention according to claim 10 is the invention according to any one of claims 7 to 9, further comprising at least one selected from the group consisting of the following six kinds of chemical component compositions, and by mass%, Cu: 0.01-2.0%, Ni: 0.01-2.0%, Co: 0.01-0.50%, Cr: 0.01-1.0%, Mo: 0.01-0. 50% and B: 0.0005 to 0.010%, and the following formula (2) is used as a formula for calculating the graphitization index CE.
CE = C + Si / 3 + Ti / 3-Mn / 12 + Cu / 9 + Ni / 9 + Co / 9
-Cr / 9-Mo / 9 + B ---------------------------- (2)
However, the element symbol in the above formula represents mass% of each element.
Invention of Claim 11 contains at least 1 sort (s) chosen from the group which consists of the following 4 types of chemical component composition in any one invention of Claims 7-10, and is Al: 0.001 to 0.50%, Zr: 0.005 to 0.10%, V: 0.01 to 0.50%, and Nb: 0.01 to 0.50%, of the graphitization index CE As a calculation formula, the following formula (3) is used.
CE = C + Si / 3 + Ti / 3-Mn / 12 + Cu / 9 + Ni / 9 + Co / 9
-Cr / 9-Mo / 9 + B + Al / 6 + Zr / 3-V / 3-Nb / 3
---------------------------- (3)
However, the element symbol in the above formula represents mass% of each element.
Invention of Claim 12 contains at least 1 sort (s) chosen from the group which consists of the following 3 types of chemical component composition in any one invention of Claims 7-11, and is Ca: 0.0010 to 0.010%, Mg: 0.0010 to 0.10%, and REM: 0.0010 to 0.10%, The following formula (4) is used as a calculation formula of the graphitization index CE. It has a special feature.
CE = C + Si / 3 + Ti / 3-Mn / 12 + Cu / 9 + Ni / 9 + Co / 9
-Cr / 9-Mo / 9 + B + Al / 6 + Zr / 3-V / 3-Nb / 3
+0.07 ------------------------------------------ (4)
However, the element symbol in the above formula represents mass% of each element.
[0027]
DETAILED DESCRIPTION OF THE INVENTION
  Next, the configuration requirements of the present invention and the reasons for limitation will be described.In the following,% indicating the ratio of the chemical component composition is all mass%.
  (1) Carbon (C)
  C is an important element for precipitating graphite and ensuring strength. C is required to be 0.70% or more in order to precipitate graphite while being hot worked. However, if the C content exceeds 1.50%, the hot ductility is greatly reduced, and the generation of surface defects increases during processing. Moreover, the graphite grain which precipitates after hot processing becomes coarse and reduces toughness. Therefore, the C content is limited to a range of 0.70 to 1.50%.
[0028]
(2) Silicon (Si)
Si is an element that plays an important role in the present invention. That is, Si is an element that promotes graphitization of cementite. It is an element effective for strengthening ferrite and enhancing toughness. However, if it is less than 0.70%, the effect is small. On the other hand, when Si exceeds 3.00%, non-metallic inclusions increase to cause a decrease in toughness, and decarburization increases in heating during hot working. Accordingly, the Si content is limited to the range of 0.70 to 3.00%.
[0029]
(3) Manganese (Mn)
Mn fixes S in steel in the form of MnS, detoxifies S and improves hot ductility. For that purpose, it is necessary to add at least 0.01% or more. In addition, Mn improves hardenability, refines pearlite, and strengthens steel. In order to exhibit this effect more fully, it is necessary to add Mn in an amount exceeding 1.00%. However, since Mn is an element that inhibits the precipitation of graphite, if it is added in a large amount exceeding 2.00%, it becomes necessary to add a large amount of Si to eliminate the inhibition of the Mn graphite precipitation. For this reason, hot ductility is reduced. In order to avoid this, the Mn content needs to be 2.00% or less. On the other hand, it is desirable to suppress the Mn content to 0.35% or less in order to keep the toughening effect of steel by Mn sparingly and to eliminate the Mn graphite precipitation inhibiting action as much as possible.
[0030]
Therefore, when aiming at a component whose main purpose is to obtain a desired strength and toughness by performing a heat treatment operation after machining, Mn: 0.01 to 2.00%,
By slow cooling after hot working, as much graphite as possible is precipitated, softening the base iron, and when aiming at a part that emphasizes machinability, Mn: 0.01 to 0.35%,
In addition, when only an appropriate amount of graphite is precipitated by only slow cooling after hot working, and when the main purpose is a part having desired strength and toughness for the base iron, Mn: more than 1.00 to 2.00% And
[0031]
(4) Phosphorus (P)
P is an element that promotes graphitization, segregates at the grain boundary, reduces hot ductility, and promotes the generation of surface defects on the steel material. In order to suppress this, the P content is limited to 0.050% or less. Desirably, it is 0.030% or less.
[0032]
(5) Sulfur (S)
S is an element that combines with Mn to form MnS and improves machinability, but is also an element that inhibits graphitization. If the amount of S exceeds 0.10%, it is necessary to add a large amount of a graphitization accelerating element such as Si, resulting in a decrease in hot ductility. Therefore, the S content is limited to 0.10% or less. Desirably, it is made into 0.050% or less.
[0033]
(6) Titanium (Ti)
Ti combines with N and renders N graphitization inhibitory action harmless. Moreover, fine TiN acts as a nucleus for graphite precipitation, and promotes the precipitation of graphite by these effects. For this purpose, Ti requires addition of 0.001% or more. However, if it exceeds 0.100%, a large amount of TiN and TiC are formed, which are hard and promote wear of the cutting tool. In addition, the cleanliness of the steel is lowered to reduce the hot ductility, which causes cracks during hot working. Therefore, the Ti content is within the range of 0.001 to 0.100%. Further, the optimum range of Ti content for maximizing the precipitation of graphite is between 0.005 and 0.050%, and the effect decreases as the addition amount is increased. Therefore, by adding an appropriate amount of Ti, the amount of C and Si can be reduced, and the hot ductility can be improved accordingly. Further, it is possible to add Mn as much as the amount of C and Si is reduced, and it is possible not only to improve hot ductility but also to adjust mechanical properties such as strength and ductility.
[0034]
(7) Oxygen (O)
O is an element that lowers the cleanliness of steel and inhibits graphitization, and should be kept as low as possible. However, up to 0.0050% is allowed, so the upper limit was made 0.0050%.
[0035]
(8) Nitrogen (N)
N, when present alone in steel, inhibits graphitization. If it exceeds 0.020%, it becomes difficult to precipitate graphite, and a large number of blow holes are formed by nitrogen gas, which may cause surface defects after rolling. Therefore, the N content is set to 0.020% or less.
[0036]
The following Cu, Ni, Co, Cr, Mo, and B all have a common effect from the viewpoint of improving the steel material characteristics in the present invention in that they all have the effect of improving the hardenability of the steel.
[0037]
(9) Copper (Cu)
Cu is an element that promotes the precipitation of graphite and improves the hardenability. In this invention, Cu is added for this purpose, and addition of 0.01% or more is required. However, if Cu exceeds 2.0%, Cu is concentrated on the surface of the steel at the time of heating before rolling and before hot working, thereby reducing hot ductility. Therefore, it is desirable to contain Cu within the range of 0.01 to 2.0%.
[0038]
(10) Nickel (Ni)
Ni, like Cu, is an element that promotes precipitation of graphite and improves hardenability. In this invention, Ni is added for this purpose, and addition of 0.01% or more is required. However, adding Ni in excess of 2.0% not only saturates the effect, but also increases the cost. Therefore, it is desirable to contain Ni in the range of 0.01 to 2.0%.
[0039]
(11) Cobalt (Co)
Co, like Cu and Ni, is an element that promotes the precipitation of graphite and improves the hardenability. In this invention, Co is added for this purpose, and addition of 0.01% or more is required. However, Co is an expensive element, and if it exceeds 0.50%, it becomes impossible to produce a steel bar that is inexpensive enough for practical use. Therefore, it is desirable to contain Co in the range of 0.01 to 0.50%.
[0040]
(12) Chrome (Cr)
Cr, like Mn, is an element that greatly improves hardenability and refines pearlite. In this invention, Cr is added for this purpose, and addition of 0.01% or more is required. However, Cr has a greater effect of inhibiting graphitization than Mn, and if it exceeds 1.0%, a large amount of graphitization accelerating element is required and the cost is increased. Therefore, it is desirable to contain Cr within a range of 0.01 to 1.0%.
[0041]
(13) Molybdenum (Mo)
Mo is also an element that enhances the hardenability of steel, and its effect is small at less than 0.01%. However, Mo is an element that inhibits graphitization like Mn and Cr. If it exceeds 0.50%, a large amount of graphitization promoting elements is required. Therefore, it is desirable to contain Mo within a range of 0.01 to 0.50%.
[0042]
(14) Boron (B)
B is an element that enhances hardenability by adding a small amount. Moreover, B fixes N in steel as BN, and reduces the graphitization inhibitory action of N. In this invention, B is used for this purpose, and the addition of 0.0005% or more is required. However, adding more than 0.010% B not only saturates the effect, but also reduces hot ductility. Therefore, it is desirable to contain B within the range of 0.0005 to 0.010%.
[0043]
The following Al, Zr, V and Nb all have a common effect from the viewpoint of improving the steel material characteristics in the present invention in that they all have the effect of refining the crystal grains of the steel.
[0044]
(15) Aluminum (Al)
Al is an element that is important as a deoxidizing material, and is an element that combines with N to precipitate AlN to refine crystal grains. Moreover, it is an element which accelerates | stimulates graphitization similarly to Si. In this invention, Al is used for this purpose, and the addition of 0.001% or more is required. However, when it is added in a large amount exceeding 0.50%, the amount of oxide inclusions is increased, the cleanliness of the steel is lowered, and cracks are generated during forging. Therefore, it is desirable to contain Al in the range of 0.001 to 0.50%.
[0045]
(16) Zirconium (Zr)
Zr, like Ti and Al, precipitates nitrides and carbides, refines crystal grains, and promotes precipitation of graphite. In this invention, Al is used for this purpose, and if the added amount is less than 0.005%, the effect is small. On the other hand, if it is added in a large amount exceeding 0.10%, the wear of the cutting tool increases due to hard nitrides and carbides. . Moreover, the cleanliness is lowered and the hot ductility is lowered. Therefore, it is desirable to contain Zr in the range of 0.005 to 0.10%.
[0046]
(17) Vanadium (V)
V, like Ti, Al, and Zr, precipitates nitrides and carbides and refines the crystal grains. Moreover, since the precipitate is fine, the yield stress of steel is increased and the fatigue limit stress is improved. In this invention, Al is added for this purpose, and the effect is small if it is less than 0.01%. On the other hand, V is an element that inhibits graphitization, and when it is added in a large amount exceeding 0.50%, not only a large amount of a graphitization accelerating element needs to be added, but also hot ductility is lowered. Therefore, it is desirable to contain V in the range of 0.01 to 0.50%.
[0047]
(18) Niobium (Nb)
Nb, like V and the like, precipitates nitrides and carbides, refines crystal grains and promotes precipitation of graphite. Nb carbonitride does not dissolve in steel even at a high temperature of 1150 ° C., prevents coarsening of austenite grains, refines grains after forging, and improves toughness. In this invention, Nb is added for this purpose, and the effect is small when the addition amount is less than 0.01%. On the other hand, if added over 0.50%, it not only inhibits the precipitation of graphite, but also reduces the hot ductility. Therefore, it is desirable to contain Nb within a range of 0.01 to 0.50%.
[0048]
The following Ca, Mg, and REM all have a common effect from the viewpoint of improving the steel material characteristics in the present invention in that they have an action of promoting precipitation of graphite in the steel material.
[0049]
(19) Calcium (Ca)
Ca is used as an inoculum in cast iron and promotes graphitization. This is because the vapor pressure of Ca is high, and during the casting, the vapor of Ca forms minute cavities in the iron, which becomes the core of graphite precipitation and precipitates spherical graphite. And in steel as well as cast iron, Ca facilitates graphite precipitation after hot working. Further, when Ca is present as an oxide inclusion, it is a desirable additive element for free-cutting steel because it has a great effect of forming a bellows in cutting a carbide tool and extending the tool life. In this invention, Ca is added for this purpose, and it is necessary to add 0.0010% or more for that purpose. However, even if added over 0.010%, the effect is saturated. Therefore, it is desirable to contain Ca in the range of 0.0010 to 0.010%.
[0050]
(20) Magnesium (Mg)
Mg, like Ca, is used as an inoculum in cast iron to promote graphitization and also facilitates precipitation of graphite after hot working in steel. In this invention, Mg is added for this purpose, and therefore the effect is small at less than 0.0010%. On the other hand, even if it is added in a large amount exceeding 0.10%, the effect is saturated. Therefore, it is desirable to contain Mg in the range of 0.0010 to 0.10%.
[0051]
(21) REM (rare earth element)
REMs such as Ce and La also promote the precipitation of graphite after forging. In the present invention, REM is added for this purpose, and therefore the effect is small at less than 0.0010%. On the other hand, even if it is added in a large amount exceeding 0.10%, the effect is saturated. Therefore, it is desirable to contain REM within the range of 0.0010 to 0.10%.
[0052]
In addition to the above, the steel in this invention may contain elements inevitably mixed such as Sn and As. In addition, if the environmental problems are small, it is possible to add a small amount of free cutting elements such as Bi, Se, Te and the like.
[0053]
(22) Graphitization index
Next, in the process of manufacturing a free-cutting hot-worked product in the present invention, when the hot-worked rough shaped material is processed into the product by cutting, the machinability of the rough shaped material may be good. is important. On the other hand, as a factor for improving the machinability, an appropriate graphite distribution in the rough shape material is effective, and in particular, it is effective for improving the chip disposability during cutting. Here, in order to promote the precipitation of graphite in the steel material, it is important to pay attention to the graphitization index CE of the steel. The graphitization index CE is expressed by the following formula for the main elements.
[0054]
CE = C + Si / 3-Mn / 12 + Ti / 3 + Cu / 9 + Ni / 9 + Co / 9-Cr / 9-Mo / 9 + B + Al / 6 + Zr / 3-V / 3-Nb / 3
However, the element symbol in the above formula represents the weight% of each element. When at least one of Ca, Mg, and REM is included in an amount of 0.0010% or more, 0.07 is added to the right side of the above formula.
That is, the precipitation of graphite depends on the heating temperature, the degree of processing, and the cooling rate, and therefore is not uniquely determined by CE. However, if CE is not 1.3 or more, it is difficult to precipitate graphite under practical conditions unless heat treatment for precipitating graphite such as annealing is performed. Therefore, the graphitization index CE is limited to 1.30 or more. In addition, about the appropriate conditions of the heating temperature of a steel material, a workability, and a cooling rate which are related also to the precipitation tendency of graphite, the relationship with the other characteristic of steel materials was considered, and it prescribed | regulated as follows.
[0055]
(23) Heating temperature
Hot working temperature is an important factor for promoting the precipitation of graphite. If the chemical composition of the steel is appropriate and the heating temperature is appropriate, fine graphite precipitates while the steel is in a high temperature state. In addition, by allowing a large amount of lattice defects introduced by hot working to remain, the precipitation of graphite during subsequent cooling is facilitated. However, if held at an excessively high temperature for a long time, the graphite once precipitated re-dissolves during the holding at the high temperature, and the number of graphite grains obtained after processing decreases.
[0056]
The heating temperature of the steel material is the solidus temperature T of the steel._SWhen it exceeds -50 ° C, the hot ductility is drastically lowered, cracking occurs in the forged material, and flaws occur in the rolled steel bar. Therefore, the heating temperature is the solidus temperature T_SMust be −50 ° C. or lower. On the other hand, when the heating temperature at the time of processing is less than 800 ° C., which is higher than the eutectoid temperature of steel (about 780 ° C.), the deformation resistance of the material increases and the life of the forging tool is shortened. In addition, the deformability is insufficient, causing forging cracks. Therefore, the heating temperature of the steel is 800 ° C. or higher, and the solidus temperature T_SLimited to temperatures between −50 ° C. and below. The appropriate temperature at which the precipitation of graphite is promoted and forging is performed more smoothly is the solidus temperature T of the steel._SIt is around -200 ° C.
[0057]
When the heating temperature of the steel material is determined as described above, the upper limit is the solidus temperature of the steel (the temperature at which the liquid phase begins to appear when the steel is heated) T_SDepends on. This solidus temperature T_SIs determined by the chemical composition of the steel and decreases as the C content and Si content increase. C and Si content is solidus temperature T_SThe effect on is roughly the following formula:
T_S(° C.) = 1420−250 (C−0.5) −20Si
It is represented by For example, the solidus temperature T of 1.2% C-1.5% Si steel_SIs 1215 ° C. from the above equation, the upper limit of the heating temperature is 1165 ° C., which is 50 ° C. lower than this. If this temperature is exceeded, the hot ductility will drop rapidly. And a crack generate | occur | produces in a hot-rolled steel bar, or a crack generate | occur | produces in a hot forging product. Therefore, as described above, the upper limit of the heating temperature is the solidus temperature T of the steel._SSet to −50 ° C.
[0058]
Now, the normal 0.5% C medium carbon steel T_SIs about 1420 ° C., T of the steel according to the present invention, for example, the above 1.2% C-1.5% Si steel._S(1250 ° C) is about 200 ° C lower. This suggests that even at a heating temperature as low as 200 ° C., it has deformation resistance and deformability equivalent to those of conventional mechanical structural steel, and can be said to be a preferable steel material from the viewpoint of energy saving.
[0059]
(24) Cooling rate after hot working
The cooling rate after hot working greatly affects the precipitation of graphite and the hardness of steel. In FIG. 1, a 50 mm thick steel slab having a chemical composition (steel No. 5 in Table 1 described later) within the scope of the present invention is heated to 1050 ° C. and forged to 30 mm thickness, and then from 900 ° C. The relationship between the cooling time until the temperature is lowered to 700 ° C. and the hardness of the forged material is shown. The smaller the cooling rate, the easier the graphite precipitates, and the carbon present as pearlite in the parted iron precipitates as graphite and the amount of ferrite increases, so the hardness decreases. Precipitation / growth of graphite is most active around 800 ° C., and it is necessary to cool around this temperature at an appropriate cooling rate. When the steel is cooled to 700 ° C. in less than 1 minute after hot working, the graphite particle size is small and the metal structure becomes pearlite. Therefore, after the hot working, the cooling time until the temperature drops to 700 ° C. is set to 1 minute or more. Typically, it is desirable to control the cooling time from 900 ° C. to 700 ° C. to about 5 to 10 minutes. Considering productivity and workability, it is not preferable that the cooling time is long, and the practically allowable time is about 180 minutes.
[0060]
(25) Graphite particle size
As for the particle size of graphite suitable for improving the machinability of the rough profile, if the average particle size of the graphite deposited in a granular form is less than 1.0 μm, the effect of crushing chips small during cutting is small, cutting The contribution to improvement is small. Therefore, the average particle diameter of graphite is 1.0 μm or more. On the other hand, the upper limit of the average particle diameter is not particularly limited, but if a large number of graphite exceeding 30 μm precipitates, it causes a decrease in toughness, and is desirably 30 μm or less. The shape of graphite in the present invention is generally expressed as a lump, but it may be spherical, granular or ellipsoidal, especially if the average length / thickness ratio is 5 or less. There is no problem.
[0061]
(26) Number of graphite
The number of graphite suitable for improving the machinability of the rough profile will be described. The number of graphite per unit area deposited in the form of particles is important for breaking up the chips into small pieces. The number is 50 / mm²If it is less than 1, the effect of improving chip disposal is small, so the number of graphite is 50 / mm.²That's it. The number of graphite depends on the size of the graphite, and it decreases as the grain size increases and increases as it decreases. In the present invention, when graphite having a diameter of 10 to 25 μm is deposited, the number is about 100 to 1000 pieces / mm.²When graphite having a diameter of 1.0 to 5 μm is deposited, the number is about 3000 to 50000 pieces / mm.²To reach.
[0062]
(27) Processed product organization
In addition to containing graphite, the hot-rolled steel bar and the hot-forged crankshaft and other rough shaped materials need to have a metal structure of ferrite or ferrite + pearlite. The reason for this is to improve the machinability of semi-finished products such as the above-mentioned steel bars and rough sections. Graphite grows when fine graphite particles agglomerate surrounding carbon during slow cooling. Accordingly, ferrite is formed around the graphite. When graphite agglomerates carbon in the vicinity sufficiently, the structure becomes graphite and ferrite. When the cooling rate is slightly high and the carbon cannot be sufficiently agglomerated, it becomes a structure of graphite and ferrite + pearlite. When the metal structure is only pearlite, the size of the graphite is not sufficient, and it becomes higher than the desired strength, resulting in poor machinability. Therefore, the metal structure is ferrite or ferrite + pearlite.
[0063]
【Example】
Next, the present invention will be described in more detail with reference to examples.
Tables 1 and 2 show the chemical component composition, graphitization index CE, and solidus temperature T of the test materials used in the test._SA value of −50 ° C. is indicated. In this specification, when paying attention to the chemical composition including the value of the graphitization index CE, the steel within the scope of the present invention is referred to as “the present invention steel”, and the steel other than the present invention steel, This is referred to as “comparative steel”. However, among the comparative steels, a known steel is referred to as “conventional steel”.
[0064]
[Table 1]

[0065]
[Table 2]

[0066]
Steel Nos. 1 to 23 are invention steels, Steel Nos. 24 to 45 are comparative steels, and Steels Nos. 46 to 50 are conventional steels. Among conventional steels, steel No. 46 is SUM24L, steel No. 47 is Pb-added steel to S45C, and steel No. 48 is non-tempered steel with 0.12% V and 0.21% Pb added to S48C. Steel No. 49 is spheroidal graphite cast iron, and Steel No. 50 is SCM822. Test steels of these components were melted in a 130-ton electric furnace and then cast into slabs or steel ingots by a continuous casting or ingot forming method, respectively. The slab or the steel ingot was divided and rolled into a steel slab of a predetermined size, and then hot-rolled to a steel bar of a predetermined size. Steel bars were used as follows.
[0067]
In Test 1 below, steel bars were cut and machined into piston pins for finishing. In Test 2, a steel bar was hot forged to produce a hot forged material for the crankshaft, which was machined into a crankshaft. In Test 3, a steel bar was hot forged to produce a hot forged material for a differential drive gear, which was then cut into a differential drive gear to produce the desired product. However, only the spheroidal graphite cast iron of steel No. 49 was directly cast into a product shape product to produce the desired product.
[0068]
[Test 1]
In the test, the steels No. 1 to 23 according to the present invention, the steel No. 24 to 45 comparative steel, and the steel No. 46 to 47 conventional steel shown in Table 1 and Table 2 were used. Examples that are tests within the scope of the present invention include Examples 1-1 to 1-20 using steel Nos. 1 to 20 of the steel of the present invention, and Comparative Examples that are tests outside the scope of the present invention. Comparative Examples 1-20 to 1-23 using Steel Nos. 21 to 23 of the present invention steel, Comparative Examples 1-24 to 1-45 using Steel Nos. 24 to 45 of the comparative steel, and conventional steels Comparative Examples 1-46 and 1-47 using steel Nos. 46 and 47 were performed. Each steel No. slab or steel ingot is divided and rolled to produce a 160 mm square steel slab, which is heated to a temperature between 820 and 1200 ° C. in a steel slab heating furnace to form a steel bar having a diameter of 32 mmφ. Hot rolled. The steel bar after hot rolling was gradually cooled with a cover. However, only the steel No. 23 steel bar was allowed to cool after hot rolling. The manufactured steel bar was machined into a piston pin, which is a hydraulic part of the brake, by cutting.
[0069]
Tables 3 and 4 show test conditions for the examples and comparative examples.
[0070]
[Table 3]

[0071]
[Table 4]

[0072]
About the Example and the comparative example, the test of the following content was done.
(1) For steel bars, the surface was visually judged for wrinkles. Further, the precipitation state of graphite and the metal structure were examined by an optical microscope.
[0073]
(2) As a machinability test for steel bars, chip disposal and a life test for a high-speed tool were performed. As shown in FIG. 2, the determination of the chip disposability is defined as “good” when the chips are divided by 2 or less turns, and “normal” when the chips are divided by 3 to 6 turns. ”And rank 3 as“ inferior ”if chips are connected to more than 8 volumes. The tool life test was performed by cutting with a high-speed tool at a cutting speed of 150 m / min and a feed oil of 0.20 mm / rev while cutting oil was applied, and measuring the time until the cutting edge melted and became incapable of cutting. The life is assumed.
[0074]
The test results are as follows.
In Examples 1-1 to 1-20, both the chemical composition and heating temperature of the steel slab, and the cooling rate of the steel bar after hot rolling (cooling time until the temperature decreases to 700 ° C. after rolling) are within the scope of the present invention. The condition is met. And the size of the graphite grains of the hot rolled steel bar is between 1.0 and 25 μm, and the number of graphite grains is all 50 / mm.²A sufficient number of these were present. Further, the metal structure was a structure of relatively low hardness of ferrite or ferrite + pearlite. As an example, FIG. 3 shows the metal structure (magnification: 600) of the hot-rolled steel bar in Example 1-5. As a result, no cracks occurred on the surface of the steel bar. Further, all the steel bar chips had a good shape, which was divided into two or less volumes, and the chip disposal was excellent. Moreover, all the cutting tool lifespans were as long as 20 minutes or more. In this good condition, the steel bar could be machined into a piston pin, the hydraulic part of the brake.
[0075]
On the other hand, in the comparative example outside the scope of the present invention, there were some problems in the surface properties and cutting properties of the steel bars as follows.
In Comparative Example 1-21, the component composition was within the range of the present invention, but the heating temperature was higher than the range of the present invention, so the hot ductility was insufficient and cracking occurred in the steel bar. In Comparative Example 1-22, the component composition was within the range of the present invention. However, since the heating temperature was lower than the range of the present invention, the hot ductility was insufficient and the steel bar was cracked.
[0076]
In Comparative Example 1-23, the component composition was within the range of the present invention, but the cooling time to 700 ° C. after hot rolling was shorter than the range of the present invention. For this reason, there was no time for the graphite to grow, the graphite grains were as small as 0.6 μm, no ferrite was generated around the graphite, the structure was only pearlite, and the hardness was high. For this reason, chip disposal was slightly inferior and the tool life was as short as 6 minutes.
[0077]
In Comparative Example 1-24, the C content was low outside the range of the present invention, and no precipitation of graphite was observed. Therefore, chip disposal was poor and the tool life was short. On the other hand, Comparative Example 1-25 had a high C content that deviated from the present invention, lacked hot ductility, and generated large cracks in the steel bar.
[0078]
In Comparative Example 1-26, the Si content was low outside the scope of the present invention, and as a result, the graphitization index CE was decreased, no precipitation of graphite was observed, and chips were connected for a long time. For this reason, it was necessary to stop the machine and remove the chips. In Comparative Example 1-27, the Si content was high outside the range of the present invention, and therefore, the hot ductility was insufficient and cracked steel bars.
[0079]
● Comparative Example 1-28 had a Mn content higher than the range of the present invention, and no precipitation of graphite was observed. Therefore, chip disposal was poor and the tool life was short. In Comparative Example 1-29, the P content was higher than the range of the present invention, the ductility was insufficient, and cracking occurred in the steel bar. In Comparative Example 1-30, since the S content is higher than the range of the present invention, not only the hot ductility is insufficient and cracks are generated, but also the excessive addition of S has an adverse effect and the apparent graphitization index. Although CE was high, no precipitation of graphite was observed. Therefore, chip disposal was poor and the tool life was short.
[0080]
In Comparative Example 1-31, the Cu content was higher than the range of the present invention, and during the slab heating, Cu concentrated on the surface and entered the grain boundaries, and cracks occurred in the rolled steel bar. In Comparative Example 1-32, the Cr content was higher than the range of the present invention, so that the hot ductility was insufficient and cracking occurred in the steel bar. In Comparative Example 1-33, the Ni content was higher than the range of the present invention. Therefore, the hot ductility was insufficient, and the steel bar was cracked. In Comparative Example 1-34, the Co, Mo, and O contents were higher than the range of the present invention, and cracks were also generated in the steel bar.
[0081]
In Comparative Example 1-35, the B and N contents were higher than the range of the present invention, and a large amount of BN precipitated and cracked due to insufficient ductility. In Comparative Example 1-36, the Ti content was higher than the range of the present invention, the hot ductility was insufficient, and cracking occurred in the steel bar. Comparative Example 1-37 has a Zr content, Comparative Example 1-38 has a V content, Comparative Example 1-39 has an Al content, and Comparative Example 1-40 has an Nb content. It was higher than the scope of the invention, and cracking occurred in the steel bar due to insufficient ductility.
[0082]
Comparative Example 1-41 has a Ca content, Comparative Example 1-42 has a Mg content, and Comparative Example 1-43 has a REM content that is higher than the range of the present invention. A large amount of system inclusions were involved, which caused rolling defects and cracked the steel bar.
[0083]
In Comparative Examples 1-44 and 1-45, the individual contents of the chemical components are within the scope of the present invention, but no graphite was precipitated because the graphitization index CE was lower than 1.30. Therefore, in all cases, chip disposal was poor and the tool life was short.
[0084]
-Comparative Example 1-46 uses conventional SUM24L and has good machinability. However, in order to improve wear resistance, SUM24L had to be carburized and quenched at 930 ° C. × 5 hr and tempered at 170 ° C. for 30 minutes. On the other hand, in Examples 1-1 to 1-20, wear resistance could be improved by simple induction hardening. In addition, Comparative Example 1-47 uses Pb-added steel to S45C, and the chip disposal is good, but the tool life is slightly short. Comparative Examples 1-46 and 1-47 are Pb-added free-cutting steels, and from the viewpoint of protecting the global environment, it is currently required to manufacture parts in a direction to refrain from use.
[0085]
As described above, according to the present invention, it is possible to manufacture a steel product having machinability comparable to that of a conventional sulfur / lead composite free-cutting steel, and its tool life exceeds that of a carbon steel material for lead-added mechanical structures. Hot rolled steel bars can be manufactured.
[0086]
[Test 2]
In the test, steels No. 1 and 18 of the present invention shown in Table 1 and conventional steels Nos. 48 and 49 were used. Examples that are tests within the scope of the present invention include Examples 2-1 and 2-18 using steel Nos. 1 and 18 of the steel of the present invention, and Comparative Examples that are tests outside the scope of the present invention. Comparative Examples 2-48 and 2-49 using conventional steel Nos. 48 and 49 were performed.
[0087]
In Examples 2-1 and 2-18, a slab or a steel ingot was divided and rolled to produce a 160 mm square steel slab, and the steel slab was hot-rolled into a steel bar having a diameter of 95 mmφ. Using the hot rolled steel bar, the steel was heated to 1000 ° C. and then hot forged into a crankshaft shape. After forging, the crankshaft forging material was slightly cooled by a fan on the conveyor in order to achieve both precipitation of graphite and mechanical properties. That is, in order to promote the precipitation of graphite, it is better to cool slowly, but as the graphite grows, the amount of pearlite decreases and the desired strength cannot be ensured. Therefore, it is necessary to optimally adjust the cooling rate. Thus, the cooling time from 900 ° C. to 700 ° C. of the crankshaft after forging was set to 5 minutes. The size of graphite of the crankshaft was 5 to 7 μm, the number was 2000 to 3000, and the structure was ferrite + pearlite. In FIG. 4, the metal structure (magnification: 600) of the crankshaft forging material in Example 2-1 is shown. Next, after cutting the outer periphery of the crankshaft, a 3 mm diameter oil hole was drilled with a small diameter deep hole drill.
[0088]
In Comparative Example 2-48, the slab was batch-rolled to produce a 160 mm square steel slab, and the steel slab was hot-rolled into a steel bar having a diameter of 95 mmφ. Using the above hot-rolled steel bar, after heating to 1250 ° C., hot forging into a crankshaft having the same shape as in Examples 2-1 and 2-18, and thereafter, using a fan on the conveyor as described above, The cooling time from 900 ° C. to 700 ° C. of the crankshaft after forging was set to 5 minutes. Next, after cutting the outer periphery of the crankshaft, a 3 mm diameter oil hole was drilled with a small diameter deep hole drill.
[0089]
In Comparative Example 2-49, conventional spheroidal graphite cast iron was directly cast on a crankshaft having the same shape as above and solidified. And as above, after cutting the outer periphery of the crankshaft, a 3 mm diameter oil hole was drilled with a small diameter deep hole drill.
[0090]
Next, any of the above crankshafts was subjected to a bending fatigue test.
The test results of the examples and comparative examples are as follows.
{Circle around (1)} The chip disposability at the time of drilling an oil hole in the crankshaft was a good chip that was finely divided into two or less turns in both the examples and the comparative examples.
[0091]
(2) Regarding the fatigue strength in the bending fatigue test, Example 2-1 is 510 N / mm²Example 2-18 is 520 N / mm²500 N / mm of Comparative Example 2-48²And had good fatigue strength. On the other hand, the spheroidal graphite cast iron material of Comparative Example 2-49 is 410 N / mm.²There was only fatigue strength. This is presumably because cast iron has a low Young's modulus, and small bubbles have become the starting point of fatigue, reducing the fatigue limit.
[0092]
As described above, according to the present invention, it is possible to produce a non-tempered free-cutting steel part that is lead-free and excellent in machinability. It is equivalent to cast iron, and its fatigue characteristics are superior to conventional spheroidal graphite cast iron parts, and it can be seen that it has high fatigue strength equivalent to conventional non-tempered steel parts.
[0093]
[Test 3]
In the test, steels No. 3 and 6 of the present invention shown in Table 1 and steel Nos. 49 and 50 of conventional steel shown in Table 2 were used. Examples which are tests within the scope of the present invention include Examples 3-3 and 3-6 using steel Nos. 3 and 6 of the present invention steel, and Comparative Examples which are tests outside the scope of the present invention. As Comparative Examples 3-49 and 3-50 using conventional steels Nos. 49 and 50 were performed.
[0094]
The test method is as follows.
In Examples 3-3 and 3-6, a 200 mm square small cross-section slab was directly hot-rolled into a steel bar having a diameter of 130 mmφ without performing ingot rolling. Using the hot-rolled steel bar, hot forging was performed on a rough shaped member of a differential drive gear having an outer diameter of 320 mm. The forging heating temperature was 1080 ° C., and the cooling time for lowering from 900 ° C. to 700 ° C. was adjusted to 3.5 minutes. For this reason, the metal structure of the above rough shaped material is pearlite containing 8% ferrite, and a graphite distribution having a diameter of 2 to 3 μm is 5000 to 7000 pieces / mm.²It was precipitated. The rough shaped material was directly cut into a differential drive gear (gear) with a hobbing machine, and then subjected to gas soft nitriding at 570 ° C. for 5 hours to cure the surface.
[0095]
Also in Comparative Example 3-50, a 200 mm square small section slab was directly hot-rolled into a steel bar having a diameter of 130 mmφ without performing ingot rolling. Using the hot-rolled steel bar, hot forging was performed on a rough shaped member of a differential drive gear having an outer diameter of 320 mm. The forging heating temperature was 1230 ° C., and after hot forging, it was conveyed by a conveyor and air-cooled. The metal structure of the as-forged rough shaped material is bainite and is hard, so it was difficult to perform cutting as it is. Therefore, after heating at 910 ° C. × 3 hours and softening by cycle annealing at 650 ° C. × 1 hour, it was cut into a differential drive gear (gear) with a hobbing machine. After cutting, in order to harden the surface, after carburizing at 930 ° C. for 5 hours, the surface was cured by quenching at 850 ° C. for 30 minutes.
[0096]
In Comparative Example 3-49, since the steel type was conventionally spheroidal graphite cast iron, it was directly cast into a gear sand mold having the same shape as the differential drive gear. The cast material was taken out of the mold and directly cut, then heated at 900 ° C. for 1 hour, and then subjected to an austemper treatment in a salt bath soaked at 280 ° C. for 4 hours.
[0097]
The test results are as follows.
In the hobbing process, all showed good chip disposal, little tool wear, no cutting surface, and good cutting conditions.
[0098]
Each heat-treated differential drive gear was subjected to a fatigue test. The root bending fatigue strength of the gas soft nitriding gears of Examples 3-3 and 3-6 is 460 N / mm, respectively.²And 490 N / mm²Met. On the other hand, the tooth root bending fatigue strength of the gear of Comparative Example 3-50 carburized and quenched using SCM822 is 470 N / mm.²Met. However, in Comparative Example 3-49, which is an austempered material of spheroidal graphite cast iron, 330 N / mm²It was low.
[0099]
Next, the deformation of the gear after each heat treatment causes noise during gear meshing. Therefore, the deformation amount of the pressure angle on the drive side of each gear was measured.
FIG. 5 shows an explanatory diagram of the deformation amount of the pressure angle of the gear. In the figure, 1 is a gear and 2 is an angular displacement. In Comparative Example 3-50 of the carburized quenching material, the angle shift was 16 minutes (1 minute is 1 / 60th of 1 °), but in Examples 3-3 and 3-6 of soft nitriding material, the angle shift was The deviation was 1 minute, and there was almost no deformation. Further, in Comparative Example 3-49 of the austemper material, the deformation immediately after the heat treatment was a relatively small deformation of 4 minutes, but when the fatigue number of 1000 times was exceeded, the deformation was as large as 20 minutes. . This is presumably because the retained austenite retained in the structure by the austemper treatment was transformed into martensite, and the deformation amount was increased.
[0100]
As described above, the gear according to the present invention has good machinability and higher fatigue strength than spheroidal graphite cast iron even without soft annealing, and is comparable to conventional carburized and quenched gears of SCM steel. It was confirmed to have strength, small distortion, and low noise generation.
[0101]
In addition, various products such as connecting rods, knuckle spindles, camshafts, engine gears, pinion gears, and shaft gears are manufactured within the conditions of the present invention by using hot rolled steel bars of steel No. 1-20 of the present invention steel. However, all had good machinability and excellent fatigue resistance.
[0102]
【The invention's effect】
  As described above, according to the present invention, it is possible to manufacture a hot-worked steel product having excellent machinability and fatigue characteristics without using toxic Pb, and a non-tempered free-cutting steel part. Can produce gears with high fatigue strength at low strainAnd becomeAn industrially useful effect is brought about.
[Brief description of the drawings]
FIG. 1 A steel No. 5 50 mm-thick steel slab of the present invention is heated to 1050 ° C. and forged to a thickness of 30 mm, and the cooling time until the temperature is lowered from 900 ° C. to 700 ° C. It is a graph which shows the relationship with hardness.
FIG. 2 is a diagram for explaining a correspondence relationship between a chip disposal rank at the time of cutting a part material and a chip form;
FIG. 3 is a view showing a metal structure (magnification: 600) of a hot-rolled steel bar in Example 1-5.
4 is a view showing a metal structure (magnification: 600) of a crankshaft forging material in Example 2-1. FIG.
FIG. 5 is an explanatory diagram of distortion of a pressure angle of a gear.
[Explanation of symbols]
1 gear
2 Angular displacement

Claims (12)

% By mass
C: 0.70 to 1.50%,
Si: 0.70 to 3.00%,
Mn: 0.01 to 2.00%
P: 0.050% or less,
S: 0.10% or less,
Ti: 0.001 to 0.100%,
O: 0.0050% or less, and
N: 0.020% or less, having a chemical component consisting of the balance Fe and inevitable impurities, and having a graphitization index CE calculated by the following formula (1) of 1.30 or more and an average particle diameter of 1.0 μm or more A free-cutting hot-worked steel material having a graphite of 50 pieces / mm ² or more and a metal structure of ferrite or ferrite + pearlite.
CE = C + Si / 3-Mn / 12 + Ti / 3 --------------------------- (1)
However, the element symbol in the above formula represents mass% of each element.   The free-cutting hot-worked steel material according to claim 1, wherein the Mn content is in the range of 0.01 to 0.35%.   The free-cutting hot-worked steel material according to claim 1, wherein the Mn content is more than 1.00 to 2.00%. The invention according to any one of claims 1 to 3, further comprising at least one selected from the group consisting of the following six chemical component compositions added to the chemical component composition:
% By mass
Cu: 0.01 to 2.0%,
Ni: 0.01 to 2.0%,
Co: 0.01 to 0.50%
Cr: 0.01 to 1.0%,
Mo: 0.01 to 0.50%, and
B: 0.0005 to 0.010%,
And the following (2) formula is used as a formula for calculating the graphitization index CE, a free-cutting hot-worked steel material.
CE = C + Si / 3 + Ti / 3-Mn / 12 + Cu / 9 + Ni / 9 + Co / 9
-Cr / 9-Mo / 9 + B --------------------------- (2)
However, the element symbol in the above formula represents mass% of each element. The invention according to any one of claims 1 to 4, further comprising at least one selected from the group consisting of the following four chemical component compositions added to the chemical component composition:
% By mass
Al: 0.001 to 0.50%,
Zr: 0.005 to 0.10%,
V: 0.01-0.50% and
Nb: 0.01 to 0.50%,
A free-cutting hot-worked steel material using the following formula (3) as a formula for calculating the graphitization index CE.
CE = C + Si / 3 + Ti / 3-Mn / 12 + Cu / 9 + Ni / 9 + Co / 9
-Cr / 9-Mo / 9 + B + Al / 6 + Zr / 3-V / 3-Nb / 3
---------------------------- (3)
However, the element symbol in the above formula represents mass% of each element. In any one of Claims 1-5, at least 1 sort (s) chosen from the group which consists of the following 3 types of chemical component composition is further added to the said chemical component composition, and is contained.
% By mass
Ca: 0.0010 to 0.010%,
Mg: 0.0010 to 0.10%, and
REM: 0.0010 to 0.10%
And the following (4) formula is used as a formula for calculating the graphitization index CE, a free-cutting hot-worked steel material.
CE = C + Si / 3 + Ti / 3-Mn / 12 + Cu / 9 + Ni / 9 + Co / 9
-Cr / 9-Mo / 9 + B + Al / 6 + Zr / 3-V / 3-Nb / 3
+0.07 ------------------------------------------ (4)
However, the element symbol in the above formula represents mass% of each element. % By mass
C: 0.70 to 1.50%,
Si: 0.70 to 3.00%,
Mn: 0.01 to 2.00%
P: 0.050% or less,
S: 0.10% or less,
Ti: 0.001 to 0.100%,
O: 0.0050% or less, and
N: A hot rolled steel material containing 0.020% or less, having a chemical component consisting of the balance Fe and inevitable impurities, and having a graphitization index CE calculated by the following formula (1) of 1.30 or more, 800 Heat to a temperature between ℃ ℃ and the solidus temperature of the hot-rolled steel material -50 ℃ or less, hot work, and slowly cool down to 700 ℃ over 1 minute. A method for producing a free-cutting hot-worked steel material, wherein 50 / mm ² or more of graphite having an average particle size of 1.0 μm or more is precipitated and the metal structure is ferrite or ferrite + pearlite.
CE = C + Si / 3-Mn / 12 + Ti / 3 --------------------------- (1)
However, the element symbol in the above formula represents mass% of each element.   The method for producing a free-cutting hot-worked steel material according to claim 7, wherein the Mn content is in the range of 0.01 to 0.35%.   The method for producing a free-cutting hot-worked steel material according to claim 7, wherein the Mn content is more than 1.00 to 2.00%. The invention according to any one of claims 7 to 9, further comprising at least one selected from the group consisting of the following six chemical component compositions,
% By mass
Cu: 0.01 to 2.0%,
Ni: 0.01 to 2.0%,
Co: 0.01 to 0.50%
Cr: 0.01 to 1.0%,
Mo: 0.01 to 0.50%, and
B: 0.0005 to 0.010%,
The following formula (2) is used as a calculation formula for the graphitization index CE, a method for producing a free-cutting hot-worked steel material.
CE = C + Si / 3 + Ti / 3-Mn / 12 + Cu / 9 + Ni / 9 + Co / 9
-Cr / 9-Mo / 9 + B --------------------------- (2)
However, the element symbol in the above formula represents mass% of each element. In any one invention of Claims 7-10, it contains at least 1 sort (s) further selected from the group which consists of the following 4 types of chemical component composition,
% By mass
Al: 0.001 to 0.50%,
Zr: 0.005 to 0.10%,
V: 0.01-0.50% and
Nb: 0.01 to 0.50%,
The following formula (3) is used as a calculation formula for the graphitization index CE, a method for producing a free-cutting hot-worked steel material.
CE = C + Si / 3 + Ti / 3-Mn / 12 + Cu / 9 + Ni / 9 + Co / 9
-Cr / 9-Mo / 9 + B + Al / 6 + Zr / 3-V / 3-Nb / 3
---------------------------- (3)
However, the element symbol in the above formula represents mass% of each element. In any one invention of Claims 7-11, it contains at least 1 sort (s) further selected from the group which consists of the following 3 types of chemical component composition,
% By mass
Ca: 0.0010 to 0.010%,
Mg: 0.0010 to 0.10%, and
REM: 0.0010 to 0.10%
The following formula (4) is used as a formula for calculating the graphitization index CE, a method for producing a free-cutting hot-worked steel material.
CE = C + Si / 3 + Ti / 3-Mn / 12 + Cu / 9 + Ni / 9 + Co / 9
-Cr / 9-Mo / 9 + B + Al / 6 + Zr / 3-V / 3-Nb / 3
+0.07 ------------------------------------------ (4)
However, the element symbol in the above formula represents mass% of each element.

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