JP4801799B2 - Method for producing spheroidal graphite cast iron with excellent strength, elongation and machinability - Google Patents

Description

【００４９】
【表２】

【００５０】
また比較例１では、銅を０．４１％と多めとすることで、球状黒鉛鋳鉄の組織のパーライト化（ラメラパーライト）を促進させているものの、Ａ１変態点付近で加熱保持する加熱保持工程は行われていないため、粒状パーライトの積極生成処理は行われておらず、α＝０％であった。比較例２、３は、従来技術で述べた公報に係る文献データである。比較例２、３では粒状パーライトの積極生成処理は行われていない。表１、表２は比較例についても示す。
【００５１】
表１に示すように、実施例１〜実施例７の球状黒鉛鋳鉄では黒鉛球状化率は８１〜８８％であり、パーライト面積率は４５〜８６％であり、α=４１〜７８％の範囲であった。殊に、実施例３に係る球状黒鉛鋳鉄はαが最も低く、α=４１％とされている。実施例４に係る球状黒鉛鋳鉄はαが最も高く、α=７８％とされている。
【００５２】
黒鉛球状化の測定については、ＪＩＳ Ｇ５５０２に基づいた。パーライト面積率の測定については、球状黒鉛鋳鉄の組織を顕微鏡で観察し、画像処理を利用し、黒鉛を除いた組織の面積を１００％としたとき、パーライト相（ラメラパーライト、粒状パーライトを含む）が占める面積を求め、パーライト相が占める面積割合をパーライト面積率（％）とした。αの測定については、画像処理を利用し、パーライト相の面積を１００％とし、ラメラパーライトの面積を求め、残部を粒状パーライトの面積とみなし、全パーライト相において粒状パーライトが占める面積割合をα（％）とした。
【００５３】
表２から理解できるように、実施例１〜実施例７に係る球状黒鉛鋳鉄は、引張強度、耐力、伸び、曲げ変位、曲げ荷重、衝撃値に優れていた。具体的には、表２に示すように、実施例１に係る球状黒鉛鋳鉄によれば、引張強度６００ＭＰａ以上（６０２ＭＰａ）、伸び１８％以上（１８．５％）、衝撃値１０Ｊ／ｃｍ^２以上（１０．１Ｊ／ｃｍ^２）の試験結果が得られた。実施例２に係る球状黒鉛鋳鉄によれば、引張強度６４０ＭＰａ以上（６４６ＭＰａ）、伸び１３％以上（１３．１％）、衝撃値７Ｊ／ｃｍ^２以上（７．５Ｊ／ｃｍ^２）の試験結果が得られた。実施例３に係る球状黒鉛鋳鉄によれば、引張強度７１０ＭＰａ以上（７１３ＭＰａ）、伸び１２％以上（１２．３％）、衝撃値５Ｊ／ｃｍ^２以上（５．６Ｊ／ｃｍ^２）の試験結果が得られた。
【００５４】
実施例４に係る球状黒鉛鋳鉄によれば、引張強度５６０ＭＰａ以上（５６４ＭＰａ）、伸び１９％以上（１９．６％）、衝撃値１１Ｊ／ｃｍ^２以上（１１．３Ｊ／ｃｍ^２）の試験結果が得られた。実施例５に係る球状黒鉛鋳鉄によれば、引張強度５８０ＭＰａ（５８８ＭＰａ）以上、伸び１６％以上（１６．８％）、衝撃値１２Ｊ／ｃｍ^２以上（１２．４Ｊ／ｃｍ^２）の試験結果が得られた。
【００５５】
実施例６に係る球状黒鉛鋳鉄によれば、引張強度６５０ＭＰａ以上（６５３ＭＰａ）、伸び１３％以上（１３．７％）、衝撃値７Ｊ／ｃｍ^２以上（７．５Ｊ／ｃｍ^２）の試験結果が得られた。実施例７に係る球状黒鉛鋳鉄によれば、引張強度６４０ＭＰａ以上（６４９ＭＰａ）、伸び１３％以上（１３．８％）、衝撃値７Ｊ／ｃｍ^２以上（７．８Ｊ／ｃｍ^２）の試験結果が得られた。
【００５６】
これに対して比較例２、比較例３に係る球状黒鉛鋳鉄については、引張強度、耐力、伸びは必ずしも充分ではない。比較例のなかでも比較例１に係る球状黒鉛鋳鉄（パーライト促進元素である銅含有量が多い）については、引張強度、耐力が良好であるものの、伸びがやや低く、曲げ変位が３．１ｍｍであり、測定したなかでは最も低く、更に衝撃値も３．１Ｊ／ｃｍ^２であり、測定したなかでは最も低かった。即ち、比較例１に係る球状黒鉛鋳鉄と実施例２に係る球状黒鉛鋳鉄については、引張強度及び耐力がほぼ同程度であるものの、αが６５％である実施例２に係る球状黒鉛鋳鉄の衝撃値は７．５Ｊ／ｃｍ^２であったが、αが０％である比較例１に係る球状黒鉛鋳鉄の衝撃値は３．１Ｊ／ｃｍ^２であり、実施例２に比べて約４０％（３．１／７．５=０．４１）とかなり低かった。更に比較例１に係る球状黒鉛鋳鉄は、伸び及び衝撃値が低いばかりか、パーライト促進元素である銅が０．４１％と多量に含有されているため、球状黒鉛鋳鉄をリサイクルする際における制約が大きくなる不具合がある。
【００５７】
更に各実施例に係る球状黒鉛鋳鉄によれば、上記した公報技術に係る球状黒鉛鋳鉄と異なり、強度低下の要因ともなり得るフェライトリッチの表層を積極的に形成せずとも良いため、量産化が容易であり、適用範囲の拡大に貢献できる。
【００５８】
更に球状黒鉛鋳鉄の被削性についても測定した。この場合、実施例Ａに係る球状黒鉛鋳鉄の溶湯を鋳型（砂型）に注湯し、円筒形の試験片（外径:１２０ｍｍ、内径：７０ｍｍ、軸長２００ｍｍ）を鋳造で形成し、図４に示す熱履歴パターンに基づいて、常温から昇温させた後に９００℃で２時間加熱してオーステナイト化を図った。その後、第１冷却工程として、−１．７℃／秒の平均冷却速度で６００℃まで強制冷却（強制空冷）し、ラメラパーライトを生成した。更に常温まで冷却した。その後、昇温工程を経て加熱保持工程に移行し、加熱保持工程において７００℃で２０分間加熱し、組織を調整し、ラメラパーライトの粒状化を進行させた。そして実施例Ａに係る球状黒鉛鋳鉄で形成した円筒形の試験片について、被削性試験を行ない、切削工具の刃先磨耗量を測定した。比較例Ａに係る球状黒鉛鋳鉄でも、同様な円筒形の試験片を形成し、被削性試験を行なった。表３は実施例Ａに係る球状黒鉛鋳鉄、比較例Ａに係る球状黒鉛鋳鉄の組成、特性を示す。表３に示すように、実施例Ａに係る球状黒鉛鋳鉄、比較例Ａに係る球状黒鉛鋳鉄共に、パーライト面積率、硬さはほぼ同程度であった。しかし実施例Ａに係る球状黒鉛鋳鉄では粒状パーライトが生成しており、α=５８％であった。これに対して、比較例Ａに係る球状黒鉛鋳鉄ではラメラパーライトだけであり、粒状パーライトが生成しておらず、α=０％であった。
【００５９】
【表３】

【００６０】
上記した被削性試験においては、図８に示す試験片と、切削工具（超硬合金、ＵＣ５００５）とを用い、周速が１５０ｍｍ/分、送りが０．３ｍ/ｒｅｖ、切り込みが０．５ｍｍとし、試料をそれぞれ２個ずつ用い、１試料で切削加工距離２０００ｍまで切削し、２つの試料を連続して切削した。図８に示す試験片では、黒皮の影響を無くすため、被削性試験に先だって前加工で黒皮の外周面部分を２ｍｍ予め切除し、外径１１６ｍｍとした。切削装置へのチャック部分を考慮して、試験片の評価部分の軸長を１５０ｍｍとした。
【００６１】
被削性試験の結果を図９に示す。図９に示すように、１試料目において、比較例Ａに係る球状黒鉛鋳鉄では切削工具の刃先磨耗量は１０００μｍを越えていたが、実施例Ａに係る球状黒鉛鋳鉄では切削工具の刃先磨耗量はそれよりも少なく６５０μｍ程度であった。２試料目において、比較例Ａに係る球状黒鉛鋳鉄では切削工具の刃先磨耗量は１３００μｍを越えていたが、実施例Ａに係る球状黒鉛鋳鉄では切削工具の刃先磨耗量はそれよりも少なく１０００μｍ程度であった。上記した被削性試験の結果から、実施例Ａに係る球状黒鉛鋳鉄は、比較例Ａに係る球状黒鉛鋳鉄に対して、パーライト面積率、硬さがほぼ同程度であるものの、被削性がかなり優れており、切削工具の刃先の磨耗量は約６０〜８０％程度（６５０／１０００=０．６５、１０００／１３００≒０．７７）に抑え得ることがわかる。
【００６２】
以上の説明から理解できるように本実施例に係る球状黒鉛鋳鉄によれば、球状黒鉛と、球状黒鉛の回りに生成したフェライト相と、隣設するフェライト相間に生成されたパーライト相とを有しており、パーライト相は、粒状パーライトとラメラパーライトとが混在して形成されている。このため本実施例に係る球状黒鉛鋳鉄は強度、伸び及び被削性に優れている。従って、強度及び伸びの双方が要請される足回り部品等に代表される車両部品、工作機械等の産業機器部品等の鋳鉄部品に広く適用することができる。殊に、足回り部品等に代表される鋳鉄部品では、高強度化を図ると共に衝突時における衝撃を吸収し易い衝撃吸収促進モードで設計される鋳鉄部品に適用することができる。勿論、高強度化又は軽量化を図るモードで設計される鋳鉄部品に適用することもできる。
【００６３】
また本実施例によれば、加熱保持工程をＡ１変態点の直下の温度領域で行うことにしているため、ラメラパーライトの粒状化が過剰に速く進行せず、適切な速度で進行するため、工業的生産においても加熱保持工程の時間の制御を容易になし得る利点が得られる。従って球状黒鉛鋳鉄の実際のαの値を、αの目標範囲に設定するのに貢献できる。
【００６４】
（その他）
上記した各実施例によれば、加熱保持工程はＡ１変態点の直下の温度領域で行うようにしているが、これに限らず、Ａ１変態点の直上の温度領域で行っても、球状黒鉛鋳鉄のラメラパーライトを粒状化させることができる。また、Ａ１変態点の直上及び直下の繰返しで行っても、球状黒鉛鋳鉄のラメラパーライトを粒状化させることができる。その他、本発明は上記した実施例のみに限定されるものではなく、要旨を逸脱しない範囲内で適宜変更して実施できるものである。
【００６５】
（付記）上記した記載から次の技術的思想も把握できる。
［付記項１］各請求項において、引張強度が６５０ＭＰａ以上、伸びが１０％以上であることを特徴とする強度、伸び及び被削性に優れた球状黒鉛鋳鉄。
［付記項２］各請求項において、引張強度が６５０ＭＰａ以上、伸びが１０％以上、衝撃値が６Ｊ／ｃｍ^２以上であることを特徴とする強度、伸び及び被削性に優れた球状黒鉛鋳鉄。
［付記項３］各請求項において、引張強度が７００ＭＰａ以上、伸びが１０％以上、衝撃値が４Ｊ／ｃｍ^２以上であることを特徴とする強度、伸び及び被削性に優れた球状黒鉛鋳鉄及びその製造方法。
［付記項４］各請求項において、引張強度が５５０ＭＰａ以上、伸びが１８％以上、衝撃値が９Ｊ／ｃｍ^２以上であることを特徴とする強度、伸び及び被削性に優れた球状黒鉛鋳鉄及びその製造方法。
［付記項５］各請求項において、引張強度が６００ＭＰａ以上、伸びが１５％以上、衝撃値が８Ｊ／ｃｍ^２以上であることを特徴とする強度、伸び及び被削性に優れた球状黒鉛鋳鉄及びその製造方法。
［付記項６］各請求項において、重量比で、炭素：３．４０〜３．９０％、シリコン：１．９〜３．４％、Ｍｎ：０．５％以下、リン：０．０８％以下、イオウ：０．０３％以下、マグネシウム：０．０２〜０．２０％、残部が不可避不純物及び鉄の組成を有することを特徴とする強度、伸び及び被削性に優れた球状黒鉛鋳鉄及びその製造方法。
［付記項７］各請求項において、重量比で、炭素：３．４０〜３．９０％、シリコン：１．９〜３．４％、Ｍｎ：０．５％以下、リン：０．０８％以下、イオウ：０．０３％以下、マグネシウム：０．０２〜０．２０％、銅及びスズの少なくとも一方：０．４０％以下、残部が不可避不純物及び鉄の組成を有することを特徴とする強度、伸び及び被削性に優れた球状黒鉛鋳鉄及びその製造方法。
［付記項８］球状黒鉛と、前記球状黒鉛の回りに生成したフェライト相と、隣設する前記フェライト相間に生成されたパーライト相とを有すると共に、前記パーライト相は、粒状パーライトとラメラパーライトとを主体としている強度、伸び及び被削性に優れた球状黒鉛鋳鉄で形成された車両鋳鉄部品。
［付記項９］球状黒鉛鋳鉄の溶湯を鋳型に注湯する工程と、パーライトを生成する冷却速度で球状黒鉛鋳鉄を冷却する第１冷却工程と、前記球状黒鉛鋳鉄をＡ１変態点の直下または直上の温度領域に加熱保持する加熱保持工程と、加熱保持後に前記球状黒鉛鋳鉄を冷却する第２冷却工程とを含む、強度、伸び及び被削性に優れた球状黒鉛鋳鉄で形成された車両鋳鉄部品の製造方法。
［付記項１０］球状黒鉛と、前記球状黒鉛の回りに生成したフェライト相と、隣設する前記フェライト相間に生成されたパーライト相とを有する球状黒鉛鋳鉄において、前記パーライト相は、粒状パーライトとラメラパーライトとを主体としていることを特徴とする強度、伸び及び被削性に優れた球状黒鉛鋳鉄。
［付記項１１］付記項１０において、前記パーライト相は、面積比で、（粒状パーライト／全パーライト）×１００％をαとしたとき、α=２０〜８５％に設定されていることを特徴とする強度、伸び及び被削性に優れた球状黒鉛鋳鉄。
［付記項１２］付記項１０または１１において、引張強度が６００ＭＰａ以上、伸びが１０％以上であることを特徴とする強度、伸び及び被削性に優れた球状黒鉛鋳鉄。
［付記項１３］付記項１０〜１２のうちの一項において、衝撃値が５Ｊ／ｃｍ ^２ 以上であり、耐衝撃性にも優れていることを特徴とする強度、伸び及び被削性に優れた球状黒鉛鋳鉄。
［付記項１４］付記項１０〜１３のうちの一項において、重量比で、炭素：３．４０〜３．９０％、シリコン：１．９〜３．４％、Ｍｎ：０．５％以下、リン：０．０８％以下、イオウ：０．０３％以下、マグネシウム：０．０２〜０．２０％、残部鉄および不可避の不純物からなることを特徴とする強度、伸び及び被削性に優れた球状黒鉛鋳鉄。
【００６６】
【発明の効果】
以上説明したように本発明に係る製造方法で製造された球状黒鉛鋳鉄によれば、パーライト相は、粒状パーライトとラメラパーライトとが混在して形成されているため、強度、伸び及び被削性に優れている。本発明に係る球状黒鉛鋳鉄の製造方法によれば、パーライトを生成する冷却速度で球状黒鉛鋳鉄を冷却する第１冷却工程と、球状黒鉛鋳鉄をＡ１変態点の直下または直上の温度領域に加熱保持する加熱保持工程とを実施するため、上記した粒状パーライトとラメラパーライトとが混在するパーライト相を形成することができる。故に強度、伸び及び被削性に優れた球状黒鉛鋳鉄を容易に製造することができる。
【００６７】
殊に加熱保持工程をＡ１変態点の直下の温度領域で行う場合には、ラメラパーライトの粒状化の速度を抑えることができるため、工業的生産においても加熱保持工程の時間の制御を容易になし得る利点が得られ、球状黒鉛鋳鉄の実際のαの値を、αの目標範囲に設定するのに貢献できる。
【図面の簡単な説明】
【図１】各加熱保持工程における球状黒鉛鋳鉄の温度履歴を模式的に示すグラフである。
【図２】第１冷却工程、加熱保持工程、第２冷却工程を連続的に行う場合における球状黒鉛鋳鉄の温度履歴を模式的に示すグラフである。
【図３】第１冷却工程と加熱保持工程とを非連続的に行う場合における球状黒鉛鋳鉄の温度履歴を模式的に示すグラフである。
【図４】別の形態に係り、第１冷却工程と加熱保持工程とを非連続的に行う場合における球状黒鉛鋳鉄の温度履歴を模式的に示すグラフである。
【図５】引張試験片を模式的に示す側面図である。
【図６】（Ａ）は曲げ試験片を模式的に示す平面図であり、（Ｂ）は曲げ試験片を模式的に示す側面図である。
【図７】曲げ試験装置で曲げ試験を行っている状態を示す側面図である。
【図８】被削性試験で用いる試験片を模式的に示す斜視図である。
【図９】被削性試験の試験結果を示すグラフである。
【図１０】第１冷却工程、加熱保持工程及び第２冷却工程を経た実施例４に係る球状黒鉛鋳鉄の顕微鏡写真（倍率４００倍）である。
【図１１】第１冷却工程、加熱保持工程及び第２冷却工程を経た実施例４に係る球状黒鉛鋳鉄の顕微鏡写真（倍率６００倍）である。
【図１２】第１冷却工程、加熱保持工程及び第２冷却工程を経た実施例４に係る球状黒鉛鋳鉄における粒状パーライトが生成している部分を更に拡大した顕微鏡写真である。
【図１３】比較例１に係る顕微鏡写真（倍率４００倍）である。
【符号の説明】
図中、１００は支持部、１１０は加圧部を示す。[0001]
BACKGROUND OF THE INVENTION
  The present invention is a spheroidal graphite cast excellent in strength, elongation and machinability.ironIt relates to a manufacturing method. The present invention can be applied to parts that require strength, elongation (ductility), and machinability, such as vehicle suspension arms and steering knuckles.
[0002]
[Prior art]
  The industry provides spheroidal graphite cast iron. Spheroidal graphite cast iron has less notch effect due to graphite and superior mechanical properties such as strength, elongation and impact value compared to flake graphite cast iron. However, in recent years, the mechanical properties required for vehicle parts, industrial equipment parts and the like have become increasingly severe. For this reason, even with spheroidal graphite cast iron having excellent mechanical properties, strength and elongation are mutually contradictory properties, so it is easy to fully satisfy recent severe requirements in both strength and elongation. It wasn't.
[0003]
  That is, spheroidal graphite cast iron with much pearlite, such as FCD700, shows high tensile strength but low elongation. On the other hand, spheroidal graphite cast iron with many ferrites like FCD400, FCD450, etc. shows low elongation but high elongation. However, the above-mentioned spheroidal graphite cast iron cannot always satisfy recent strict requirements in both strength and elongation.
[0004]
  In particular, cast iron parts typified by undercarriage parts of vehicles are not only designed in a mode to increase rigidity and strength, but also to increase strength and easily absorb the impact at the time of collision. It may be designed in shock absorption acceleration mode. When designing in such a shock absorption promotion mode and simultaneously reducing the weight, the above-mentioned spheroidal graphite cast irons such as FCD700, FCD450, and FCD400 cannot always cope with severe requirements.
[0005]
  Further, as a spheroidal graphite cast iron with improved strength and elongation, Japanese Patent Application Laid-Open No. 6-17186 discloses that a ferrite layer is made of pearlite and ferrite with a ferrite layer rate of 70% or more, and the ferrite layer rate is 15% lower than the surface layer. Spheroidal graphite cast iron is disclosed. According to this gazette technique, after the solidification of the molten metal is completed, the mold is released from the mold when the casting is above the A1 transformation point, and then the temperature difference between the surface layer and the inside of the casting becomes 40 to 60 ° C. At that time, the cast product is placed in a heat treatment furnace maintained at 750 to 900 ° C., and then the cast product is cooled at a cooling rate of 15 to 100 ° C./min.
[0006]
  In addition, as a spheroidal graphite cast iron with improved bendability and impact resistance, Japanese Patent Application Laid-Open No. 9-296215 discloses a spherical shape having a surface layer having a ferritization rate of 60% or more and an interior in which most of the base is made of pearlite. Graphite cast iron is disclosed. According to this publication technique, the casting product is heated and held at a temperature at which the entire base of the spheroidal graphite cast iron is austenitized, and then the cooling rate (5 to 20 ° C.) where the ferritization of the surface layer portion occurs before the internal pearlite is formed. / Min), then the casting is cooled to a temperature (730 to 750 ° C.) at which the base of the surface layer remains ferrite and the pearlite of the internal matrix occurs (730 to 750 ° C.). To cool down.
[0007]
[Problems to be solved by the invention]
  According to the spheroidal graphite cast iron according to the above-mentioned publication technique, the formation of a crack in the maximum stress portion at the time of bending is suppressed by forming a surface layer with a large amount of ferrite and high elongation. However, even in the spheroidal graphite cast iron according to the publication technique having such an effect, it is not always sufficient to satisfy strict required characteristics in both strength and elongation.
[0008]
  That is, according to the spheroidal graphite cast iron according to the above-mentioned publication technique, in order to compensate for the insufficient strength of the ferrite-rich surface layer, it is necessary to make the inside of the spheroidal graphite cast iron have higher strength (that is, lower elongation). For this reason, it is assumed that cracks are likely to occur from the internal high-strength portion having low elongation. In particular, when spheroidal graphite cast iron is applied to a product whose both ends are joined to the mounting part in various forms, both ends of the product are subject to some restraint, so that when a load is applied to the product, the axial direction If the tensile load is applied to the high-strength portion forming the interior of the product as a component force and the elongation in the axial length direction is low, the internal high-strength portion may break due to the displacement. Furthermore, according to the spheroidal graphite cast iron according to the above-mentioned publication technique, the ferrite-rich surface layer has good machinability, but the interior occupying most of the spheroidal graphite cast iron is occupied by the lamellar pearlite of the high strength portion. The machinability of spheroidal graphite cast iron is not always good.
[0009]
  The present invention has been made in view of the above-described circumstances, and improves the strength and elongation, and at the same time obtains good machinability, and has excellent strength, elongation and machinability.ironIt is an object to provide a manufacturing method.
[0010]
[Means for Solving the Problems]
  The present inventor has developed a spheroidal graphite cast iron excellent in strength, elongation and machinability.Manufacturing methodWe have been working hard on development for many years. And the present inventor, in spheroidal graphite cast iron having spheroidal graphite, a ferrite phase generated around the spheroidal graphite, and a pearlite phase generated between adjacent ferrite phases, the pearlite phase, granular pearlite and lamellar pearliteMixedIf it is a phase, it is found that a spheroidal graphite cast iron excellent in strength, elongation and machinability can be obtained, confirmed by a test, and the spheroidal graphite cast iron according to the present inventionManufacturing methodDeveloped. Here, lamellar perlite means layered perlite.
[0011]
  Furthermore, in order to obtain the above-mentioned spheroidal graphite cast iron, the present inventorPouring molten metal of spheroidal graphite cast iron into a mold formed of sand mold,
A first cooling step of cooling the spheroidal graphite cast iron at a cooling rate for generating a lamellar pearlite as a structure in the spheroidal graphite cast iron by taking out the solid spheroidal graphite cast iron from the mold and air cooling after solidification of the molten metal; The spheroidal graphite cast iron produced with a temperature range immediately below the A1 transformation point (including the temperature range from the A1 transformation point to ΔK1 (60 ° C) lower, including the A1 transformation point), or the temperature range immediately above (A1 transformation point) In the temperature range from A to ΔK2 (30 ° C.), including the A1 transformation point), or in the repeated temperature range immediately above and immediately below the A1 transformation point, after a heating and holding step for 5 to 50 minutes. A second cooling step for cooling the spheroidal graphite cast iron (except for surface quenching after quenching and cooling after heating to 850 ° C. or higher)As a result, it has been found that a spheroidal graphite cast iron excellent in strength, elongation and machinability can be produced, and the method for producing a spheroidal graphite cast iron according to the present invention has been completed.
[0012]
  Perlite phase with granular perlite and lamellar perliteMixedThe reason why a spheroidal graphite cast iron excellent in strength, elongation and machinability is obtained when it is used as a phase is that the continuity of lamellar pearlite is interrupted, and the direction to fracture and elongation in the structure is reduced. It is assumed that
[0013]
  That is, according to the present inventionManufactured by manufacturing methodSpheroidal graphite cast iron excellent in strength, elongation and machinability is a spheroidal graphite cast iron having spheroidal graphite, a ferrite phase generated around the spheroidal graphite, and a pearlite phase generated between the adjacent ferrite phases. The pearlite phase is mainly composed of granular pearlite and lamellar pearlite. The main component means that granular pearlite and lamellar pearlite occupy most (90% or more) of the pearlite phase in the structure of spheroidal graphite cast iron. Here, when the pearlite phase in the spheroidal graphite cast iron is 100%, the proportion of granular pearlite occupies 20 to 85% in terms of area ratio, and the remainder can be substantially lamella (layered) pearlite. In other words, when the pearlite phase in the spheroidal graphite cast iron is 100%, the ratio of granular pearlite occupies 20 to 85% in area ratio, and lamellar (layered) pearlite occupies 80 to 15% in area ratio. it can.
[0014]
  The granular pearlite is pearlite that is granular and spherical, and can include elongated granular particles (length / width = 50 or less) that are separated from each other in the longitudinal direction and the laminating direction by lamellar pearlite. It is assumed that the elongated granular pearlite is in the middle of transition from lamellar pearlite to fully granular pearlite.
[0015]
  According to the present inventionManufactured by manufacturing methodAccording to spheroidal graphite cast iron with excellent strength, elongation and machinability, the pearlite phase consists of granular pearlite and lamellar pearlite.Formed in a mixed mannerThe For this reason, elongation is improved, ensuring intensity | strength. Further, the granular pearlite is improved in elongation while securing strength, and also has an advantage of good machinability as shown in a machinability test described later because it has less directionality to fracture.
[0016]
  The manufacturing method of spheroidal graphite cast iron excellent in strength, elongation and machinability according to the present invention is as follows:(I) a step of pouring the molten graphite graphite iron into a mold formed of a sand mold, and after solidification of the molten metal, the solidified spherical graphite cast iron is taken out of the mold and air-cooled, whereby the lamellar pearlite is A first cooling step of cooling the spheroidal graphite cast iron at a cooling rate that causes the spheroidal graphite cast iron to form as a structure;
The spheroidal graphite cast iron in which lamellar pearlite is generated is either in a temperature region immediately below the A1 transformation point (a temperature region from the A1 transformation point to ΔK1 (60 ° C.) lower) or just above the temperature region (A1 transformation point to ΔK2 ( 30 ° C.) high temperature range), or a heating and holding step of heating and holding for 5 to 50 minutes in a repeated temperature range immediately above and below the A1 transformation point
A second cooling step for cooling the spheroidal graphite cast iron after heating and holding (except for a surface quenching step for quenching and cooling after heating to 850 ° C. or higher), in order,
(Ii) By weight, carbon: 3.40-3.90%, silicon: 1.9-3.4%, Mn: 0.5% or less, phosphorus: 0.08% or less, sulfur: 0.03 % Or less, magnesium: 0.02 to 0.20%, balance iron and inevitable impurities,
It has spherical graphite, a ferrite phase generated around the spherical graphite, and a pearlite phase generated between the adjacent ferrite phases, and the pearlite phase is formed by mixing granular pearlite and lamellar pearlite. The pearlite phase is set to α = 20 to 85% in terms of area ratio, where (granular pearlite / total pearlite) × 100% is α.
It is characterized by producing spheroidal graphite cast iron excellent in strength, elongation and machinability with a tensile strength of 564 MPa or more and an elongation of 10% or more.Is. As the first cooling process, the solidified spheroidal graphite cast iron is used.Formed with sand moldTake out from moldYeah.
[0017]
  According to the method for producing spheroidal graphite cast iron excellent in strength, elongation and machinability according to the present invention, the spheroidal graphite cast iron is cooled at a cooling rate that generates pearlite.With air coolingSince the 1st cooling process is performed, the area ratio of the pearlite phase in the base of spheroidal graphite cast iron can be secured, and the strength of spheroidal graphite cast iron can be secured. Furthermore, in order to perform a heating and holding step of heating and holding the spheroidal graphite cast iron in a temperature region immediately below or immediately above the A1 transformation point,LamellaThe cementite in the pearlite phase is granulated, and the pearlite phase is divided into granular pearlite and lamellae.(Layered)It can be a phase mainly composed of pearlite. For this reason, spheroidal graphite cast iron with improved elongation can be obtained while ensuring strength. Furthermore, since granular pearlite has little directionality to fracture, there is also an advantage that machinability of spheroidal graphite cast iron is good.
[0018]
DETAILED DESCRIPTION OF THE INVENTION
  The spheroidal graphite cast iron according to the present invention has spheroidal graphite, a ferrite phase generated around the spheroidal graphite, and a pearlite phase generated between adjacent ferrite phases. The spheroidization rate of the spherical graphite can be generally 75% or more, particularly 80% or more. Although the average particle diameter of spheroidal graphite varies depending on the use of spheroidal graphite cast iron, the thickness of spheroidal graphite cast iron, etc., it can be exemplified by 20-100 μm, especially 20-60 μm.
[0019]
  ・ The pearlite phase of spheroidal graphite cast iron consists of granular pearlite and lamellae.(Layered)Mainly pearlite. This can be achieved by heating and maintaining a spheroidal graphite cast iron having a pearlite structure in a lamellar shape in a temperature region immediately below or just above the A1 transformation point. In this way, lamellar pearlite and granular pearlite on the ferrite phase side of the pearlite phase.MixedMixed phases can be formed. Generally, it becomes a gradient composition in which granular pearlite gradually increases as it goes from lamellar pearlite to the ferrite phase. With such a gradient composition, it is presumed that the directionality in the pearlite phase is reduced and the dispersibility of the load stress in the pearlite phase is improved.
[0020]
  -In the pearlite phase, when the area ratio is (granular pearlite / total pearlite) x 100%, α is set to α = 20 to 85%.TheAs described above, the lower limit value and the upper limit value of α can be selected depending on the use of the spheroidal graphite cast iron, the required mechanical properties, the heat treatment cost in the heating and holding step for obtaining granular pearlite, and the like. Accordingly, examples of α include 25 to 85%, 30 to 85%, 35 to 75%, 35 to 70%, and 40 to 70%. In general, in spheroidal graphite cast iron, which has a strong demand for high strength, α can be 25 to 60%, particularly 25 to 50%. In spheroidal graphite cast iron, which has a strong demand for high elongation, α can be 40 to 85%, particularly 60 to 85%.
[0021]
  In order to change α, the temperature and heating time in the heating and holding step may be adjusted. As described above, the lower limit value and the upper limit value of α can be selected depending on the use of the spheroidal graphite cast iron, the required mechanical properties, the heat treatment cost in the heating and holding step for obtaining granular pearlite, and the like. Examples of the lower limit of α include 26%, 30%, 33%, 36%, and 39%. Examples of the upper limit value of α that can be combined with the above lower limit value include 83%, 78%, 75%, 72%, and 70%.
[0022]
  -Spheroidal graphite cast iron can have a tensile strength of 600 MPa or more and an elongation of 10% or more. In this case, the impact value is 5 J / cm.²As described above, the spheroidal graphite cast iron having excellent impact resistance in addition to strength and elongation can be obtained. Even in spheroidal graphite cast iron, when the pearlite phase is formed of lamellar pearlite and does not include granular pearlite, if the tensile strength is 600 MPa or more, the elongation is 10% and the impact value is 5 J / cm.²It is hard to get.
[0023]
  The composition of spheroidal graphite cast iron is selected according to the use of spheroidal graphite cast iron, the required mechanical properties, and the like. Examples of the composition of the spheroidal graphite cast iron include the following compositions. That is, by weight ratio, carbon: 3.40-3.90%, silicon: 1.9-3.4%, Mn: 0.5% or less, phosphorus: 0.08% or less, sulfur: 0.03% Magnesium: 0.02 to 0.20%, Balance iron and inevitable impuritiesThe composition. However, it is not limited to this composition and can be changed as necessary. In this case, if necessary, at least one of copper and tin, which are pearlite promoting elements, may be contained in an amount of 0.005 to 0.40%, particularly 0.01 to 0.35%.
[0024]
  Furthermore, composition of spheroidal graphite cast ironExplanationTo do. Carbon is necessary for securing castability, but if it is excessive, the tensile strength is lowered. Therefore, the carbon can be 3.40 to 3.90%, in particular 3.5 to 3.8%. Silicon is necessary for securing castability and stabilizing the structure, but if it is excessive, pearlite formation can be suppressed. Therefore, silicon can be 1.9 to 3.4%, especially 2.4 to 2.9%. Manganese is a pearlite-promoting element, but if it is excessive, impact value and elongation are lowered. Therefore, manganese can be 0.05-0.5%, especially 0.05-0.35%. Phosphorus is a spheronization inhibiting element, and is preferably 0.08% or less. Sulfur is a spheroidizing inhibitory element and is preferably 0.03% or less. Magnesium is necessary for the spheroidization of graphite, and can be 0.02 to 0.20%, in particular 0.025 to 0.15%. % Means weight ratio.
[0025]
  -The method for producing the spheroidal graphite cast iron according to the present invention is as follows:The process of pouring the molten graphite iron into a mold made of sand mold, and after solidifying the molten metal, the solidified spherical graphite cast iron is taken out of the mold and air-cooled, so that the lamellar pearlite is formed into the structure of the spherical graphite cast iron. The first cooling step of cooling the spheroidal graphite cast iron at a cooling rate to be generated, and the spheroidal graphite cast iron on which lamellar pearlite is generated are in a temperature region immediately below the A1 transformation point (temperature from the A1 transformation point to ΔK1 (60 ° C.) Area), or a temperature range immediately above (temperature range from the A1 transformation point to ΔK2 (30 ° C.) higher), or a repeated temperature range immediately above and immediately below the A1 transformation point, for heating and holding for 5 to 50 minutes A holding step and a second cooling step for cooling the spheroidal graphite cast iron after heating and holding (excluding surface quenching for quenching and cooling after heating to 850 ° C. or higher).Including. As molds, sand molds such as fresh sand mold and shell moldAdoptTo do. In the first cooling process, pearlite is placed on the base of spheroidal graphite cast iron.(LaCool at a cooling rate that produces melaperlite). When the cooling rate in the first cooling step is slow, good pearlite is hardly generated. Considering the formation of pearlite, the average cooling rate in the first cooling step is generally 1.0 ° C./second to 3.0 ° C./second, particularly 1.3 ° C./second to 2.0 ° C. Per second.
[0026]
  In the case of a mold such as a sand mold that has a limited increase in cooling rate, the first cooling step is to take out the spheroidal graphite cast iron from the mold after the molten spheroidal graphite cast iron has solidified in the mold. Preferably it is done. The temperature of the spheroidal graphite cast iron from which the spheroidal graphite cast iron after solidification is taken out from the mold can be set to a temperature region that is austenitized in a temperature region that exceeds the A1 transformation point. Specifically, the temperature may be, for example, 820 to 1050 ° C., particularly 850 to 980 ° C., although it varies depending on the size, composition, mold material, and the like of the spheroidal graphite cast iron. The first cooling step can be performed by air cooling. As the air cooling, forced air cooling can be adopted for the generation of pearlite, but natural air cooling may be used, and depending on the case, a cooling mode using another heat medium may be used.
[0027]
  In the heating and holding step, the spheroidal graphite cast iron is heated and held in a temperature region immediately below or just above the A1 transformation point. In this way, the lamellar pearlite can be made into granular pearlite. The heating and holding time in the heating and holding step is generally 5 to 50 minutes, although it varies depending on the size and composition of the spheroidal graphite cast iron. When the heating and holding time is excessively short, it is difficult to generate granular pearlite. If the heating and holding time is excessively long, the production of granular pearlite and ferrite becomes excessive, and the material tends to have a low tensile strength. Examples of the heating atmosphere in the heating and holding step include an air atmosphere and a reducing atmosphere. The average cooling rate in the second cooling step after the heating and holding step is generally 1 ° C./min to 3.0 ° C./sec, particularly 0.1 ° C. in consideration of the formation of ferrite due to residual heat. It can be set to ° C / second to 1.0 ° C / second.
[0028]
  -If the temperature in the heating and holding step is high, the production of granular pearlite and ferrite becomes excessive, so it is preferable to shorten the heating and holding time. If the temperature in the heating and holding step is low, it takes time for the pearlite to change to granular pearlite, so it is preferable to increase the heating and holding time.
[0029]
  If the time for the heating and holding step is excessively short, it is not easy to control the time required for the heating and holding step in an industrial production line. For this reason, when the controllability in an industrial production line is taken into consideration, it is preferable that the heating and holding step be performed for a predetermined time or longer. Considering these points, the heating and holding step can be performed under any of the following conditions (a) to (c). The temperature of the spheroidal graphite cast iron is based on the inside of the spheroidal graphite cast iron, not the surface layer of the spheroidal graphite cast iron.
(A) Temperature: 700 to 760 ° C., heating time: 3 to 20 minutes
(B) Temperature: 670-730 ° C., heating time: 7-30 minutes
(C) Temperature: 670-730 ° C., heating time: 20-50 minutes
  The heating and holding step is a step of heating and holding the spheroidal graphite cast iron in a temperature region immediately below or immediately above the A1 transformation point as described above. Heating and maintaining the spheroidal graphite cast iron in the temperature region immediately below or just above the A1 transformation point is the following (1), (2)It can be either. The temperature of the spheroidal graphite cast iron is based on the temperature inside the spheroidal graphite cast iron, not the surface layer of the spheroidal graphite cast iron.
(1) Directly below the A1 transformation point
(2)Repeat directly above and below the A1 transformation point
  Here, as shown in FIG. 1A, immediately below the A1 transformation point means within a temperature region from the A1 transformation point to a temperature lower by ΔK1 (60 ° C.), and within a temperature region within the ΔK1 from the A1 transformation point. Any A1 transformation point itself may be included. Therefore, the temperature may be 10 ° C. or 20 ° C. lower than the A1 transformation point.
[0031]
  FIG.(B) (C)As shown in FIG. 4, the repetitions immediately above and immediately below the A1 transformation point are within a temperature range between a temperature ΔK3 (30 ° C.) higher than the A1 transformation point and a temperature ΔK4 (60 ° C.) lower than the A1 transformation point. It means that the temperature rise and the temperature fall or the temperature fall and the temperature rise are repeated. Therefore, the temperature increase and decrease, or the temperature decrease and increase may be repeated within a temperature range from + 20 ° C. to the A1 transformation point and −20 ° C. from the A1 transformation point.
[0032]
  The heating and holding time can be 5 to 50 minutes as described above, but when the heating temperature is low, it is preferable to increase the heating time.
[0033]
  -According to the method for producing spheroidal graphite cast iron according to the present invention, FIG.(D)As illustrated in Fig. 5, a step of pouring a molten graphite graphite cast iron into a mold is performed. Next, when the temperature of the spheroidal graphite cast iron after solidification reaches T1 (T1: 840 to 990 ° C.), the spheroidal graphite cast iron is taken out from the mold, and at a cooling rate that generates pearlite (generally lamellar perlite), The 1st cooling process which cools spheroidal graphite cast iron to temperature T3 below temperature T2 of a heating maintenance process is performed. The average cooling rate in the first cooling step can be 1.0 ° C./second to 3.0 ° C./second. Thereafter, a heating step is performed to raise the spheroidal graphite cast iron to a temperature T2 immediately below the A1 transformation point. Furthermore, a heating and holding step of heating and holding at the temperature T2 is performed. Further, a second cooling step is performed in which the spheroidal graphite cast iron is cooled to a normal temperature region after being heated and held. When the temperature T2 in the heating and holding step is 730 ° C. (1003K), it can be 5 to 10 minutes, when it is 700 ° C. (973K), it can be 10 to 20 minutes, and when it is 680 ° C. (953K), it can be 20 to 40 minutes. The average cooling rate in the second cooling step can be 1 ° C./min to 3.0 ° C./sec.
[0034]
  -The form which performs the above-mentioned 1st cooling process, a heating holding process, and a 2nd cooling process continuously can be employ | adopted. In this case, after casting the molten metal in the mold, the first cooling step, the heating and holding step, and the second cooling step can be performed continuously. As described above, when the first cooling step, the heating and holding step, and the second cooling step are performed continuously, it is not necessary to reheat the spheroidal graphite cast iron, so that energy saving can be achieved.
[0035]
  -Moreover, the form which performs the above-mentioned 1st cooling process and heating holding process discontinuously is employable. In this case, in the first cooling step, the spheroidal graphite cast iron is once cooled to room temperature or near room temperature. Further, prior to the heating and holding step, a temperature raising step is performed in which the spheroidal graphite cast iron at or near room temperature is reheated and the temperature is raised to the temperature range of the heating and holding step. Thus, after performing a temperature rising process, the form which performs a heating holding process is employable. As described above, when the first cooling step and the heating and holding step are performed discontinuously, the spheroidal graphite cast iron can be collectively transferred to the heating and holding step even in the case of small-lot production.
[0036]
【Example】
  Hereinafter, embodiments embodying the present invention will be described in detail with reference to FIGS. First, a melting material blended so as to have a target composition was melted in a melting furnace to obtain a spheroidal graphite cast iron melt according to each example. The molten metal was spheroidized with a magnesium-based spheroidizing agent. The spheroidization temperature was 1480-1500 ° C. The spheroidized molten metal was poured into a Y-type block mold (sand mold) to form a Y-type block as a casting material. In the Y-type block mold, the specimen collection part has a thickness of 25 mm.
[0037]
  Composition of spheroidal graphite cast iron according to each example(Including copper)Is shown in Table 1. In Example 1, spheroidal graphite cast iron was manufactured based on the thermal history pattern shown in FIG. That is, as shown in FIG. 2, an in-mold cooling step was performed after pouring to solidify the molten metal in the mold and cool the spheroidal graphite cast iron to 920 ° C. in the mold. Immediately after the mold was disassembled (high-temperature dispersion), the solidified spheroidal graphite cast iron was taken out of the mold. The sand adhering to the surface layer of the spheroidal graphite cast iron was dropped, the spheroidal graphite cast iron was cooled by forced cooling (forced air cooling), and the first cooling step was performed. The average cooling rate in the first cooling step was −1.5 ° C./second. This makes the structure of spheroidal graphite cast ironInPerlite (lamellar perlite) was precipitated. In the first cooling step, the spheroidal graphite cast iron was cooled to 680 ° C. (a temperature region immediately below the A1 transformation point). The structure of the spheroidal graphite cast iron that has undergone the first cooling step has spheroidal graphite, a ferrite phase generated around the spheroidal graphite, and a pearlite phase (lamellar perlite) generated between adjacent ferrite phases. It is considered a bullseye type organization.
[0038]
  When the first cooling step is completed, the spheroidal graphite cast iron is charged into a heat treatment furnace (set temperature: 700 ° C.), and heated and held at 700 ° C. (temperature region immediately below the A1 transformation point) for 15 minutes. Was done. As a result, the structure of the spheroidal graphite cast iron was adjusted, and the lamella perlite was granulated. When such a heating and holding step was completed, the spheroidal graphite cast iron was taken out from the heat treatment furnace, allowed to cool (air-cooled), and the second cooling step was performed.
[0039]
  When the structure of the spheroidal graphite cast iron after the second cooling step was observed with an optical microscope, the structure of the spheroidal graphite cast iron according to Example 1 was found to be spheroidal graphite, a ferrite phase generated around the spheroidal graphite, and the adjacent structure. And a pearlite phase formed between the ferrite phases. The perlite phase consists of granular perlite and lamellar perlite.MixedIt was. A large amount of granular pearlite was produced on the ferrite side. Therefore, the mixed phase in which granular pearlite and lamellar pearlite are mixed is formed on the ferrite phase side.
[0040]
  The production conditions of Examples 2 to 7 were basically the same as those of Example 1. In the following, the manufacturing conditions of Example 2 to Example 7 will be described with a focus on differences from Example 1. In Example 2, the average cooling rate in the first cooling step was faster than that in Example 1, and was set to -2.3 ° C / second instead of -1.5 ° C / second. This promotes pearlite formation.
[0041]
  In Example 3, while using the molten metal according to Example 1, a large amount (0.21%) of copper functioning as a pearlite promoting element was added. This promotes pearlite formation. In Example 4, the set temperature of the heat treatment furnace was set to 710 ° C. (temperature region immediately below the A1 transformation point) instead of 700 ° C.
[0042]
  In Example 5, the silicon content of the composition of the spheroidal graphite cast iron of Example 1 was lowered by 0.3%, and silicon: 2.38%. The reason why the silicon is lowered is to promote the pearlite formation in the first cooling step and to improve the impact value. In Example 6, spheroidal graphite cast iron was manufactured based on the thermal history pattern shown in FIG. That is, in Example 6, the first cooling step and the heating and holding step are performed discontinuously. In Example 6, after the molten metal was poured into the mold, the in-mold cooling step was performed to solidify the molten metal in the mold (sand mold), and the spheroidal graphite cast iron was cooled to 920 ° C. in the mold. Then, as a first cooling step, the mold is disassembled, the solidified spheroidal graphite cast iron is taken out from the mold, and the spheroidal graphite cast iron is cooled to 700 ° C. by forced air cooling at an average cooling rate of −1.5 ° C./second, An appropriate amount of perlite (lamellar perlite) was deposited on the tissue. Thereafter, the spheroidal graphite cast iron was naturally cooled to room temperature. Thereafter, after appropriately performing post-treatment and physical distribution processes on the spheroidal graphite cast iron, the spheroidal graphite cast iron is charged into a heat treatment furnace, a temperature raising process is performed, and the spheroidal graphite cast iron is reheated. Heating and holding at 10 ° C. (directly below the A1 transformation point) for 10 minutes, a heating and holding step was performed, the structure of the spheroidal graphite cast iron was adjusted, and granulation of lamellar pearlite was promoted.
[0043]
  In Example 7, spheroidal graphite cast iron was manufactured based on the thermal history pattern shown in FIG. That is, in Example 7, a molten spheroidal graphite cast iron was poured into a mold and then solidified, and then cooled to near room temperature in the mold. Next, the spheroidal graphite cast iron taken out from the mold was heated to 900 ° C. and heated at 900 ° C. for 1 hour to form austenite. After that, as the first cooling step, pearlite normalization was performed in which forced cooling (forced air cooling) to 600 ° C. was performed at an average cooling rate of −1.3 ° C./second to precipitate pearlite (lamellar pearlite). Thereafter, it was naturally cooled to near room temperature. Furthermore, a heating process is performed in which the spheroidal graphite cast iron is reheated to 700 ° C. (just below the A1 transformation point), and the heating and holding process is performed by heating and holding at 700 ° C. for 20 minutes to adjust the structure of the spheroidal graphite cast iron, The pearlite was granulated. Then, the 2nd cooling process was performed and it naturally left to cool to normal temperature vicinity.
[0044]
  In addition, each temperature regarding the above-mentioned spheroidal graphite cast iron was based on the temperature inside spheroidal graphite cast iron, not the surface layer of spheroidal graphite cast iron.
[0045]
  10 and 11 show optical micrographs of spheroidal graphite cast iron (after nital corrosion) according to Example 4. FIG. As shown in FIGS. 10 and 11, the structure of spheroidal graphite cast iron is adjacent to spheroidal graphite expressed as a black lump, and a ferrite phase expressed in white around the spheroidal graphite and expressed in white. And a pearlite phase generated between the ferrite phases. This perlite phase consists of granular perlite and lamellar perlite.Mixedis doing. FIG. 12 is an enlarged view of the above-described photograph. As shown in FIG. 12, many fine granular pearlites are generated on the ferrite side. In other words, on the ferrite phase side of the pearlite phase, a large number of fine granular pearlite and lamellar pearlite (layered)MixedA mixed phase is formed. According to FIG. 12, the granular pearlite has a size of 10 μm or less and is much smaller than the spherical graphite. Similar structures were obtained for spheroidal graphite cast irons according to other examples.
[0046]
  FIG. 13 shows an optical micrograph of spheroidal graphite cast iron (after nital corrosion) according to Comparative Example 1. As shown in FIG. 13, the structure of the spheroidal graphite cast iron according to Comparative Example 1 is composed of spherical graphite expressed as a black lump, a ferrite phase expressed in white around the spherical graphite, and expressed in white. And a pearlite phase generated between the ferrite phases to be provided. The boundary between the ferrite phase and the pearlite phase is clear. When the structure | tissue which concerns on the comparative example 1 was observed, only the lamellar pearlite was produced | generated in the pearlite phase, and the production | generation of granular pearlite was not recognized.
[0047]
  A tensile test piece (JIS Z2201 No. 4), an impact test piece (JIS Z2202 No. 3, with a U-notch having a depth of 2 mm), and a bending test piece were formed from the above Y-shaped block. FIG. 5 shows a tensile test piece. In FIG. 5, the diameter D of the measurement part was 15 mm, and the score distance L of the measurement part was 50 mm. In the tensile test, the above-described tensile test piece was used, and the load and the deformation amount until breakage were measured by the crosshead movement amount with a 250KN Shimadzu autograph. 6A and 6B show a bending test piece. The bending test piece was a plate-like test piece having a target size of width 20 × length 100 mm × thickness 7 mm. In the bending test, as shown in FIG. 7, with the bending test piece set on the two support parts 100 of the bending test apparatus (the dimension display in FIG. 7 is in millimeters), the pressure unit 110 is directed toward the bending test piece. And pressurizing in the direction of arrow PA. In this case, the relationship between the bending stress and the amount of deformation was determined using a 250KN Shimadzu autograph. The test results are shown in Tables 1 and 2.
[0048]
[Table 1]

[0049]
[Table 2]

[0050]
  Further, in Comparative Example 1, although the copper content is increased to 0.41%, the pearlite (lamellar pearlite) structure of the spheroidal graphite cast iron is promoted, but the heating and holding step of heating and holding near the A1 transformation point is Since it was not performed, the active processing for generating granular pearlite was not performed, and α = 0%. Comparative Examples 2 and 3 are document data related to the gazette described in the related art. In Comparative Examples 2 and 3, the aggressive generation process of granular pearlite is not performed. Tables 1 and 2 also show comparative examples.
[0051]
  As shown in Table 1, in the spheroidal graphite cast iron of Examples 1 to 7, the graphite spheroidization ratio is 81 to 88%, the pearlite area ratio is 45 to 86%, and the range of α = 41 to 78%. Met. In particular, the spheroidal graphite cast iron according to Example 3 has the lowest α and α = 41%. The spheroidal graphite cast iron according to Example 4 has the highest α, and α = 78%.
[0052]
  The measurement of graphite spheroidization was based on JIS G5502. For the measurement of the pearlite area ratio, the structure of the spheroidal graphite cast iron is observed with a microscope, and image processing is used. When the area of the structure excluding graphite is 100%, the pearlite phase (including lamellar pearlite and granular pearlite) The area ratio occupied by the pearlite phase was defined as the pearlite area ratio (%). For the measurement of α, image processing is used, the area of the pearlite phase is taken as 100%, the area of the lamellar pearlite is obtained, the remainder is regarded as the area of the granular pearlite, and the area ratio occupied by the granular pearlite in the total pearlite phase is expressed as α ( %).
[0053]
  As can be understood from Table 2, the spheroidal graphite cast iron according to Examples 1 to 7 was excellent in tensile strength, yield strength, elongation, bending displacement, bending load, and impact value. Specifically, as shown in Table 2, according to the spheroidal graphite cast iron according to Example 1, the tensile strength is 600 MPa or more (602 MPa), the elongation is 18% or more (18.5%), and the impact value is 10 J / cm.²Or more (10.1 J / cm²) Test results were obtained. According to the spheroidal graphite cast iron according to Example 2, the tensile strength is 640 MPa or more (646 MPa), the elongation is 13% or more (13.1%), and the impact value is 7 J / cm.²(7.5 J / cm²) Test results were obtained. According to the spheroidal graphite cast iron according to Example 3, the tensile strength is 710 MPa or more (713 MPa), the elongation is 12% or more (12.3%), and the impact value is 5 J / cm.²(5.6 J / cm²) Test results were obtained.
[0054]
  According to the spheroidal graphite cast iron according to Example 4, the tensile strength is 560 MPa or more (564 MPa), the elongation is 19% or more (19.6%), and the impact value is 11 J / cm.²(11.3 J / cm²) Test results were obtained. According to the spheroidal graphite cast iron according to Example 5, the tensile strength is 580 MPa (588 MPa) or more, the elongation is 16% or more (16.8%), and the impact value is 12 J / cm.²(12.4 J / cm²) Test results were obtained.
[0055]
  According to the spheroidal graphite cast iron according to Example 6, the tensile strength is 650 MPa or more (653 MPa), the elongation is 13% or more (13.7%), and the impact value is 7 J / cm.²(7.5 J / cm²) Test results were obtained. According to the spheroidal graphite cast iron according to Example 7, the tensile strength is 640 MPa or more (649 MPa), the elongation is 13% or more (13.8%), and the impact value is 7 J / cm.²(7.8 J / cm²) Test results were obtained.
[0056]
  On the other hand, the spheroidal graphite cast irons according to Comparative Example 2 and Comparative Example 3 are not necessarily sufficient in tensile strength, yield strength, and elongation. Among the comparative examples, the spheroidal graphite cast iron according to Comparative Example 1 (having a high copper content as a pearlite-promoting element) has good tensile strength and yield strength, but has a slightly low elongation and a bending displacement of 3.1 mm. Yes, it is the lowest measured, and the impact value is 3.1 J / cm²It was the lowest measured. That is, for the spheroidal graphite cast iron according to Comparative Example 1 and the spheroidal graphite cast iron according to Example 2, the impact of the spheroidal graphite cast iron according to Example 2 in which α is 65%, although the tensile strength and the proof stress are almost the same. The value is 7.5 J / cm²However, the impact value of the spheroidal graphite cast iron according to Comparative Example 1 in which α is 0% is 3.1 J / cm.²Compared to Example 2, it was considerably low at about 40% (3.1 / 7.5 = 0.41). Furthermore, the spheroidal graphite cast iron according to Comparative Example 1 not only has low elongation and impact values, but also contains a large amount of copper, which is a pearlite-promoting element, of 0.41%. Therefore, there are restrictions in recycling spheroidal graphite cast iron. There is a bug that gets bigger.
[0057]
  Furthermore, according to the spheroidal graphite cast iron according to each example, unlike the spheroidal graphite cast iron according to the above-mentioned publication technique, it is not necessary to actively form a ferrite-rich surface layer that can cause a decrease in strength, so that mass production is possible. It is easy and can contribute to the expansion of the application range.
[0058]
  Furthermore, the machinability of spheroidal graphite cast iron was also measured. In this case, the molten spheroidal graphite cast iron according to Example A is poured into a mold (sand mold), and a cylindrical test piece (outer diameter: 120 mm, inner diameter: 70 mm, shaft length: 200 mm) is formed by casting, and FIG. On the basis of the heat history pattern shown in FIG. 2, the temperature was raised from room temperature and then heated at 900 ° C. for 2 hours to achieve austenite. Thereafter, as a first cooling step, forced cooling (forced air cooling) was performed to 600 ° C. at an average cooling rate of −1.7 ° C./sec to generate lamellar perlite. Furthermore, it cooled to normal temperature. After that, the temperature rising process was passed to the heating and holding process. In the heating and holding process, heating was performed at 700 ° C. for 20 minutes, the structure was adjusted, and granulation of lamella perlite was advanced. And about the cylindrical test piece formed with the spheroidal graphite cast iron which concerns on Example A, the machinability test was done and the cutting edge wear amount of the cutting tool was measured. With the spheroidal graphite cast iron according to Comparative Example A, a similar cylindrical test piece was formed and a machinability test was performed. Table 3 shows the composition and characteristics of the spheroidal graphite cast iron according to Example A and the spheroidal graphite cast iron according to Comparative Example A. As shown in Table 3, both the spheroidal graphite cast iron according to Example A and the spheroidal graphite cast iron according to Comparative Example A had substantially the same pearlite area ratio and hardness. However, in the spheroidal graphite cast iron according to Example A, granular pearlite was generated, and α = 58%. On the other hand, the spheroidal graphite cast iron according to Comparative Example A had only lamellar pearlite, no granular pearlite was generated, and α = 0%.
[0059]
[Table 3]

[0060]
  In the machinability test described above, the test piece shown in FIG. 8 and a cutting tool (hard metal, UC5005) were used, the peripheral speed was 150 mm / min, the feed was 0.3 m / rev, and the cut was 0.5 mm. Two samples were used, and one sample was cut to a cutting distance of 2000 m, and two samples were cut continuously. In the test piece shown in FIG. 8, in order to eliminate the influence of the black skin, 2 mm of the outer peripheral surface portion of the black skin was preliminarily cut prior to the machinability test to have an outer diameter of 116 mm. Considering the chuck portion to the cutting device, the axial length of the evaluation portion of the test piece was set to 150 mm.
[0061]
  The results of the machinability test are shown in FIG. As shown in FIG. 9, in the first sample, the spheroidal graphite cast iron according to Comparative Example A had a cutting edge wear amount of more than 1000 μm, whereas the spheroidal graphite cast iron according to Example A had a cutting edge wear amount. Was less than that and about 650 μm. In the second sample, the cutting edge wear amount of the cutting tool exceeded 1300 μm in the spheroidal graphite cast iron according to Comparative Example A, whereas the cutting edge wear amount of the cutting tool in the spheroidal graphite cast iron according to Example A was less than about 1000 μm. Met. From the results of the machinability test described above, the spheroidal graphite cast iron according to Example A has approximately the same pearlite area ratio and hardness as the spheroidal graphite cast iron according to Comparative Example A, but has a machinability. It can be seen that the wear amount of the cutting edge of the cutting tool can be suppressed to about 60 to 80% (650/1000 = 0.65, 1000 / 1300≈0.77).
[0062]
  As can be understood from the above description, the spheroidal graphite cast iron according to the present embodiment has spheroidal graphite, a ferrite phase generated around the spheroidal graphite, and a pearlite phase generated between adjacent ferrite phases. The perlite phase consists of granular perlite and lamellar perlite.Formed in a mixeding. For this reason, the spheroidal graphite cast iron according to the present example is excellent in strength, elongation and machinability. Therefore, the present invention can be widely applied to cast iron parts such as vehicle parts typified by undercarriage parts and the like that require both strength and elongation, and industrial equipment parts such as machine tools. In particular, cast iron parts typified by undercarriage parts can be applied to cast iron parts that are designed in a shock absorption acceleration mode in which the strength is increased and the impact at the time of collision is easily absorbed. Of course, the present invention can also be applied to cast iron parts designed in a mode for increasing the strength or reducing the weight.
[0063]
  In addition, according to the present embodiment, since the heating and holding step is performed in the temperature region immediately below the A1 transformation point, the granulation of lamella perlite does not proceed excessively fast, but proceeds at an appropriate speed. Also in the production, there is an advantage that the time of the heating and holding process can be easily controlled. Therefore, it is possible to contribute to setting the actual value of α of the spheroidal graphite cast iron to the target range of α.
[0064]
  (Other)
According to each of the embodiments described above, the heating and holding step is performed in the temperature region immediately below the A1 transformation point. However, the present invention is not limited to this, and even if performed in the temperature region immediately above the A1 transformation point, the spheroidal graphite cast iron The lamellar perlite can be granulated. Moreover, the lamellar pearlite of spheroidal graphite cast iron can be granulated even if it is repeated directly above and below the A1 transformation point. In addition, the present invention is not limited to the above-described embodiments, and can be implemented with appropriate modifications within a range not departing from the gist.
[0065]
  (Supplementary note) The following technical idea can be grasped from the above description.
[Additional Item 1] Spheroidal graphite cast iron excellent in strength, elongation and machinability, characterized in that the tensile strength is 650 MPa or more and the elongation is 10% or more in each claim.
[Appendix 2] In each claim, the tensile strength is 650 MPa or more, the elongation is 10% or more, and the impact value is 6 J / cm.²Spheroidal graphite cast iron excellent in strength, elongation and machinability characterized by the above.
[Appendix 3] In each claim, the tensile strength is 700 MPa or more, the elongation is 10% or more, and the impact value is 4 J / cm.²A spheroidal graphite cast iron excellent in strength, elongation and machinability, characterized by the above, and a method for producing the same.
[Appendix 4] In each claim, the tensile strength is 550 MPa or more, the elongation is 18% or more, and the impact value is 9 J / cm.²A spheroidal graphite cast iron excellent in strength, elongation and machinability, characterized by the above, and a method for producing the same.
[Appendix 5] In each claim, the tensile strength is 600 MPa or more, the elongation is 15% or more, and the impact value is 8 J / cm.²A spheroidal graphite cast iron excellent in strength, elongation and machinability, characterized by the above, and a method for producing the same.
[Appendix 6] In each claim, by weight, carbon: 3.40 to 3.90%, silicon: 1.9 to 3.4%, Mn: 0.5% or less, phosphorus: 0.08% Spherical graphite cast iron excellent in strength, elongation and machinability, characterized in that sulfur: 0.03% or less, magnesium: 0.02 to 0.20%, the balance having the composition of inevitable impurities and iron Its manufacturing method.
[Additional Item 7] In each claim, by weight, carbon: 3.40 to 3.90%, silicon: 1.9 to 3.4%, Mn: 0.5% or less, phosphorus: 0.08% Hereinafter, sulfur: 0.03% or less, magnesium: 0.02 to 0.20%, at least one of copper and tin: 0.40% or less, the balance having the composition of inevitable impurities and iron Spheroidal graphite cast iron excellent in elongation and machinability and a method for producing the same.
[Additional Item 8] Spherical graphite, a ferrite phase generated around the spherical graphite, and a pearlite phase generated between the adjacent ferrite phases, and the pearlite phase includes granular pearlite and lamellar pearlite. Vehicle cast iron parts made of spheroidal graphite cast iron with excellent strength, elongation and machinability.
[Additional Item 9] A step of pouring a melt of spheroidal graphite cast iron into a mold, a first cooling step of cooling the spheroidal graphite cast iron at a cooling rate for generating pearlite, and the spheroidal graphite cast iron immediately below or immediately above the A1 transformation point. A cast iron part formed of spheroidal graphite cast iron having excellent strength, elongation, and machinability, including a heating and holding step for heating and holding in the temperature range, and a second cooling step for cooling the spheroidal graphite cast iron after heating and holding Manufacturing method.
[Additional Item 10] Spheroidal graphite cast iron having spheroidal graphite, a ferrite phase generated around the spheroidal graphite, and a pearlite phase generated between the adjacent ferrite phases, the pearlite phase includes granular pearlite and lamellae. Spheroidal graphite cast iron excellent in strength, elongation and machinability characterized by being mainly composed of pearlite.
[Additional Item 11] In Additional Item 10, the pearlite phase is set to α = 20 to 85% in terms of area ratio, where (granular pearlite / total pearlite) × 100% is α. Spheroidal graphite cast iron with excellent strength, elongation and machinability.
[Additional Item 12] Spheroidal graphite cast iron excellent in strength, elongation and machinability according to Additional Item 10 or 11, wherein tensile strength is 600 MPa or more and elongation is 10% or more.
[Additional Item 13] In one of Additional Items 10 to 12, the impact value is 5 J / cm. ² Spheroidal graphite cast iron excellent in strength, elongation and machinability characterized by being excellent in impact resistance.
[Additional Item 14] In one of Additional Items 10 to 13, the weight ratio of carbon is 3.40 to 3.90%, silicon is 1.9 to 3.4%, and Mn is 0.5% or less. , Phosphorus: 0.08% or less, sulfur: 0.03% or less, magnesium: 0.02 to 0.20%, balance iron and inevitable impurities, excellent strength, elongation and machinability Spheroidal graphite cast iron.
[0066]
【The invention's effect】
  As described above, according to the present inventionManufactured by manufacturing methodAccording to spheroidal graphite cast iron, the pearlite phase consists of granular pearlite and lamellar pearlite.Are mixed and formedTherefore, it is excellent in strength, elongation and machinability. According to the method for producing spheroidal graphite cast iron according to the present invention, the first cooling step of cooling the spheroidal graphite cast iron at a cooling rate for generating pearlite, and heating and holding the spheroidal graphite cast iron in a temperature region immediately below or immediately above the A1 transformation point. In order to carry out the heating and holding step, the above-described granular pearlite and lamellar pearliteMixedPearlite phase can be formed. Therefore, spheroidal graphite cast iron excellent in strength, elongation and machinability can be easily produced.
[0067]
  In particular, when the heating and holding step is performed in a temperature region immediately below the A1 transformation point, the rate of granulation of lamellar perlite can be suppressed, so that the time of the heating and holding step can be easily controlled even in industrial production. The obtained advantage can be obtained and can contribute to setting the actual value of α of the spheroidal graphite cast iron to the target range of α.
[Brief description of the drawings]
FIG. 1 is a graph schematically showing a temperature history of spheroidal graphite cast iron in each heating and holding step.
FIG. 2 is a graph schematically showing a temperature history of spheroidal graphite cast iron when the first cooling step, the heating and holding step, and the second cooling step are continuously performed.
FIG. 3 is a graph schematically showing a temperature history of spheroidal graphite cast iron when the first cooling step and the heating and holding step are performed discontinuously.
FIG. 4 is a graph schematically showing a temperature history of spheroidal graphite cast iron when the first cooling step and the heating and holding step are discontinuously performed according to another embodiment.
FIG. 5 is a side view schematically showing a tensile test piece.
6A is a plan view schematically showing a bending test piece, and FIG. 6B is a side view schematically showing the bending test piece.
FIG. 7 is a side view showing a state in which a bending test is performed by a bending test apparatus.
FIG. 8 is a perspective view schematically showing a test piece used in a machinability test.
FIG. 9 is a graph showing a test result of a machinability test.
10 is a micrograph (magnification 400 times) of spheroidal graphite cast iron according to Example 4 that has undergone the first cooling step, the heating and holding step, and the second cooling step. FIG.
FIG. 11 is a micrograph (magnification 600 times) of spheroidal graphite cast iron according to Example 4 that has undergone a first cooling step, a heating and holding step, and a second cooling step.
FIG. 12 is a photomicrograph further enlarging a portion where granular pearlite is generated in spheroidal graphite cast iron according to Example 4 which has undergone the first cooling step, the heating and holding step, and the second cooling step.
13 is a micrograph (magnification 400 times) according to Comparative Example 1. FIG.
[Explanation of symbols]
  In the figure, reference numeral 100 denotes a support part, and 110 denotes a pressure part.

Claims (5)

(I) pouring molten metal of spheroidal graphite cast iron into a mold formed of a sand mold;
A first cooling step of cooling the spheroidal graphite cast iron at a cooling rate that causes the spheroidal graphite cast iron to be formed as a structure by taking out the spheroidal graphite cast iron after solidification from the mold and air-cooling after the solidification of the molten metal. When,
The spheroidal graphite cast iron in which lamellar pearlite is produced is a temperature region immediately below the A1 transformation point (a temperature region from the A1 transformation point to ΔK1 (60 ° C.) lower, including the A1 transformation point), or a temperature region immediately above ( A temperature range from the A1 transformation point to ΔK2 (30 ° C.) higher, including the A1 transformation point), or a heating and holding step of heating and holding for 5 to 50 minutes in a repeated temperature range immediately above and immediately below the A1 transformation point;
A second cooling step for cooling the spheroidal graphite cast iron after heating and holding (except for a surface quenching step for quenching and cooling after heating to 850 ° C. or higher), in order,
(Ii) By weight, carbon: 3.40-3.90%, silicon: 1.9-3.4%, Mn: 0.5% or less, phosphorus: 0.08% or less, sulfur: 0.03 % Or less, magnesium: 0.02 to 0.20%, balance iron and inevitable impurities,
It has spherical graphite, a ferrite phase generated around the spherical graphite, and a pearlite phase generated between the adjacent ferrite phases, and the pearlite phase is formed by mixing granular pearlite and lamellar pearlite. The pearlite phase is set to α = 20 to 85% in terms of area ratio, where (granular pearlite / total pearlite) × 100% is α.
A method for producing spheroidal graphite cast iron excellent in strength, elongation and machinability, characterized by producing spheroidal graphite cast iron excellent in strength, elongation and machinability having tensile strength of 564 MPa or more and elongation of 10% or more . 2. The method for producing spheroidal graphite cast iron excellent in strength, elongation and machinability according to claim 1, wherein the spheroidal graphite cast iron contains 0.005 to 0.21% of copper. 3. The method for producing spheroidal graphite cast iron having excellent strength, elongation and machinability according to claim 1, wherein the impact value is 5 J / cm ² or more and the impact resistance is also excellent. In any one of Claims 1-3, when the structure | tissue of the said nodular graphite cast iron is observed with a microscope and the area of the structure | tissue except graphite is made into 100%, a pearlite phase (A lamellar pearlite and a granular pearlite are included. ), And the area ratio occupied by the pearlite phase is the pearlite area ratio (%), and the pearlite area ratio is 45 to 86%. The spherical shape having excellent strength, elongation and machinability A method for producing graphite cast iron. In any one of claims 1 to 4, wherein in the first cooling step, the spherical graphite cast iron is cooled to near room temperature or ambient temperature, the heating and holding process, the spherical graphite near ambient or room temperature A method for producing spheroidal graphite cast iron excellent in strength, elongation and machinability, which is performed after performing a temperature raising step of raising the temperature of the cast iron to the temperature range of the heating and holding step.

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