JP5146869B2 - Method for producing high-strength molded article, high-strength molded article and high-strength machine screw obtained thereby - Google Patents

Method for producing high-strength molded article, high-strength molded article and high-strength machine screw obtained thereby Download PDF

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JP5146869B2
JP5146869B2 JP2006547991A JP2006547991A JP5146869B2 JP 5146869 B2 JP5146869 B2 JP 5146869B2 JP 2006547991 A JP2006547991 A JP 2006547991A JP 2006547991 A JP2006547991 A JP 2006547991A JP 5146869 B2 JP5146869 B2 JP 5146869B2
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screw
head
strength
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steel
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JPWO2006057430A1 (en
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史郎 鳥塚
榮次郎 村松
寿 長井
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National Institute for Materials Science
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21K—MAKING FORGED OR PRESSED METAL PRODUCTS, e.g. HORSE-SHOES, RIVETS, BOLTS OR WHEELS
    • B21K1/00—Making machine elements
    • B21K1/56—Making machine elements screw-threaded elements
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/0093—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for screws; for bolts
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
    • F16B—DEVICES FOR FASTENING OR SECURING CONSTRUCTIONAL ELEMENTS OR MACHINE PARTS TOGETHER, e.g. NAILS, BOLTS, CIRCLIPS, CLAMPS, CLIPS OR WEDGES; JOINTS OR JOINTING
    • F16B35/00—Screw-bolts; Stay-bolts; Screw-threaded studs; Screws; Set screws
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
    • F16B—DEVICES FOR FASTENING OR SECURING CONSTRUCTIONAL ELEMENTS OR MACHINE PARTS TOGETHER, e.g. NAILS, BOLTS, CIRCLIPS, CLAMPS, CLIPS OR WEDGES; JOINTS OR JOINTING
    • F16B5/00—Joining sheets or plates, e.g. panels, to one another or to strips or bars parallel to them
    • F16B5/02—Joining sheets or plates, e.g. panels, to one another or to strips or bars parallel to them by means of fastening members using screw-thread
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P10/00—Technologies related to metal processing
    • Y02P10/20—Recycling

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Forging (AREA)
  • Heat Treatment Of Articles (AREA)
  • Heat Treatment Of Steel (AREA)

Description

【技術分野】
【０００１】
本発明は、頭部の厚さが薄い高強度小ねじのように、その最薄肉厚部の厚さＱが薄い成形品であって、高強度で且つ耐遅れ破壊特性にも優れた鋼製成形品乃至ねじ部品に関するものであり、特に上記高強度小ねじについていえば、ワーク（後出の図１３中、符号１７参照）にねじ込まれたねじの頭部をワークからできるだけ突出さないようにするために、ねじの頭部の厚さをできるだけ薄くし、しかも、ねじとしてのねじ込み締結機能に優れ、遅れ破壊特性にも優れ、しかもねじの外径（めねじの場合は内径）ができるだけ小さいものまでを対象とする技術に関するものである。
【背景技術】
【０００２】
最近の電化製品及び情報機器製品においては、持ち運びが便利で且つ性能が優れたもの、例えば、電子手帳、ノート型パソコン、ＨＤＤ、携帯電話、デジタルカメラ等の電子機器が急速に広く普及している。このような製品においては、小型化・コンパクト化のために、製品厚さが薄いことも要請され、これを組み立てるためのねじ部品も必然的に小径のもの、具体的には呼び径が２ｍｍ（ねじの呼びでＭ２）以下の小径の小ねじが使用されている。そして一層の小型化・コンパクト化、薄肉製品化のためには、この小ねじを更に小さくする必要がある。その際、ねじの頭部の厚さを薄くして、頭部がワークから突出しないようにし、美観を損なうことなく、また製品使用時にこのねじの頭部による引っ掛かりが発生しないようにし、しかもねじ本来の基本性能を十分に備えたねじ製品が要求されている。
【０００３】
一般に、ねじの頭部にはドライバビットに係合させるための係合溝を形成させる必要があるので、上記要求に対応してねじ部品の頭部の厚さを薄くした場合には、この係合溝の形状・寸法を保持する限り、係合溝の底部先端位置がねじの軸部にまで食い込んでくことを避けることができない。そのため、係合溝の底部領域の内壁と頭部座面への最近接距離、又は係合溝の底部領域の内壁とねじ部（頭部の首下部及び不完全ねじ部を含む。これはＪＩＳ Ｂ０１０１ 番号２１０２ の規定による）の表面との間の最短距離（以下、本明細書において「頭部首下付近の最薄肉厚部の厚さ」という）が小さくなる。こうして、頭部の厚さを相当に薄くした場合には、（１）上記頭部首下付近の最薄肉厚部の厚さが０となる領域が発生して孔が開いたり、あるいは（２）この部分の構造が弱体化してねじ込みの標準トルク値が確保できなくなる。また、（３）頭部の厚さを薄くした上に、係合溝の深さを浅くして、頭部の首下付近の最薄肉厚部の厚さを所要値に確保した場合には、ねじ込み時に頭部に形成された係合溝がなめる（壊れる）ということも発生し、有効なトルク値が著しく低下する。
【０００４】
このように頭部の厚さを薄くした場合に発生するねじに関する上記問題は、更に、呼び径が小さくなるほど一層重大な問題となり、ねじ本来の基本性能を損なうので、上記問題の解決は一層困難となる。
【０００５】
上述した状況に対応して、従来、ねじの頭部の厚さを薄くした薄頭付きねじが提案されている。そして、この薄頭付きねじにおいては、頭部を薄くしたことによる上述した問題を解決するために、頭部の首下部に「補強部」と称するねじ無しの部分を形成させている。
【０００６】
この補強部に関しては、例えば特許文献１には、次の技術が開示されている。即ち、図９は、その薄頭付きねじの一例を説明するための正面図であり、図１０は、図９の部分切欠き断面を伴なう要部の拡大正面図であり、図１１は、図９のねじを組み込んだ使用状態を示す縦断面図である。
【０００７】
これらの図において、１が従来の薄頭付きねじであり、この薄頭付きねじ１は、頭部２と、ねじ山３を有する軸部４と、首下部に形成されたねじ無し部の補強部１０とで構成されている。このねじ１の頭部２には十字形状の係合溝（十字穴）５が形成されており、この係合溝５は先端が軸部４にまで食い込んで達している。ここで、頭部２は、その厚みが通常のねじの頭部よりかなり薄く形成されており、この頭部２の厚さはこのねじ１の呼び径に対して、その１／３程度あるいはそれ以下に設定されている。即ち、例えば、呼び径が２ｍｍのＭ２においては、頭部２の厚さは０．６７ｍｍ程度乃至これ以下、呼び径が１．２ｍｍのＭ１．２においては、頭部２の厚さは０．４ｍｍ程度乃至これ以下となることになる。
【０００８】
また、頭部２の座面７と軸部４のねじ山３との間に形成された補強部１０は、その形状は例えば図１０に示すように、縦断面における外形が比較的大きな円弧形状を呈し、上端部の横断面の直径は下端面のそれよりも大きく形成されており、この補強部１０の上端側が頭部２の座面７に接続する位置Ｐ１０の軌跡円の直径は、ねじの外径Ｄ０よりも僅かに大きくなっている。また、係合溝５の内壁８と頭部２の頂面６とが交叉する軌跡円の直径ｍを、ねじの谷径Ｄより小さく設定している（ｍ＜Ｄ）。更には、この係合溝５は頭部２の頂面６から軸部４にかけて深くなるにつれて軸部４の軸心方向に傾斜している。こうして、係合溝５の内壁８と補強部１０との間には、ねじ込み時に駆動力をねじ１に伝達するのに十分な厚みが確保されている。
【０００９】
特許文献２には、更に、上述した補強部１０における係合溝５の内壁８との間の肉厚部の厚みδを更に厚くして、ねじの最小破壊トルク値を向上させるために、図１２に示すように、補強部１０が頭部２の座面７に接続する位置Ｐ１０の軌跡円の直径を、ねじの外径Ｄ０よりも大きく形成すること、そして、ねじ締結時に補強部１０の下端部に応力集中が発生するのを防ぐために、補強部１０が軸部４側に接続する部分の形状を、不完全ねじ部９に対して滑らかな曲面形状で連続する弧状部１１に形成することが、開示されている。図１３は、図１２のねじを組み込んだ使用状態を示す縦断面図である。１７は被締結部材であり、１８はワークである。
【００１０】
このように、従来技術においては、ねじ１の頭部２の厚さを薄くするために、頭部２の首下にねじ無しの補強部１０を形成させると共に、補強部１０の形状及び寸法、並びに係合溝５の形状及び寸法を適切に設計することにより、頭部首下付近の最薄肉厚部の厚さを確保して、ねじ１のねじ込み時にねじ本来の締結機能が発揮されるように工夫されている。また、ねじによる締結時に、首下部への応力集中の緩和が図られるようにも工夫されている。
【００１１】
【特許文献１】
：特開２００２−０５４６１７
【特許文献２】
：特開２００２−１３９０１０
【００１２】
上述した従来技術の提案によれば、ねじの頭部の厚さが薄くなったので、製品におけるねじの頭部の出っ張りが小さくなり、また製品の薄肉化・軽量化に寄与する。更に、締結時の首下部への応力集中の緩和により、耐遅れ破壊特性の向上が図られている。
【００１３】
しかしながら、上述したねじ頭部の薄肉化の従来技術によっても、高性能な携帯用電子機器製品の一層の小型化・コンパクト化のためには、下記問題点が残されている。即ち、頭部の首下に補強部が形成されているが、これにはねじが形成されていないので、この補強部は直接的な締結機能を有しない。そして、この補強部の高さの分だけ、被締結部材（図１３中の符号１８）の厚さが厚くなるのは避けられない。そのため、機器製品全体としての厚さの低減効果が若干損なわれる。更に、補強材の上端側部の直径はねじの外径よりも大きいので、被締結部材の下穴の径をねじの外径以上にしなければならず、機器製品のコンパクト化効果が損なわれる。
【００１４】
次に、一般のねじ部品に関する場合と同様、情報機器製品等において使用される小ねじにおいても、その製造工程は、素材の予備成形及び頭部成形のための冷間圧造、並びに、次いで行われるねじ部成形のための転造を行なうために、長時間を要する球状化焼なましに代表される軟化処理を施して、冷間における成形性を向上させる必要がある。次いで、こうして得られた成形体に対して、ねじ部表面層の硬さの向上のために、浸炭焼入れ・焼戻しあるいは窒化処理等の表面硬化処理が施されている。しかしながら、このような表面硬化の熱処理を施された小ねじにあっては、当該表面硬化層領域については、その硬さ及び強度は優れているが、一方、この領域は脆く、靭性に劣っているという問題点がある。
【発明の開示】
【発明が解決しようとする課題】
【００１５】
上述した問題点に鑑み、本発明の課題は、（１）薄頭付き小ねじの頭部の首下に形成されるねじ無しの補強部形成による被締結部材の下穴の径の縮小化を図ること、及び、（２）小ねじの成形素材に対する球状化焼なまし等の軟化処理が不要であり、更に、小ねじの成形体に対して表面硬化のための熱処理を施す必要のない、薄頭付きの強靭な小ねじを製造することである。
【００１６】
従って、本発明の目的は、上記課題を解決することにより、頭部の厚さの薄い小ねじであって、表面硬化のための熱処理が不要である小ねじを製造し、携帯用電子機器等の小型化・コンパクト化の向上に寄与し、更にこの製造技術の応用範囲を拡大して、小ねじに制限されず、薄肉部を有する成形品において当該薄肉部の強度及び靱性の向上により当該成形品の用途が拡大され、またねじ部品を包含する成形品の高強度化・高靱性化が可能となり、しかも成形体の素材の軟化処理工程、及び成形体の強靭化処理工程を省略可能な製造技術を提供し、もって産業の発達に寄与することにある。
【課題を解決するための手段】
【００１７】
上記課題を解決するために、本発明者等は鋭意研究を重ねた。本発明者等は、先ず、本発明により製造すべき成形品、ねじ類又はねじに使用するための素材とすべき鋼は、その機械的性質の内、引張強さＴＳが８００ＭＰａの水準を超えること、望ましく１０００ＭＰａ以上であって、絞りＲＡが７０％の水準を超えることを目標とした。このような高強度で且つ高延性を有するものを得るためには、本発明者等の研究成果に基づく本出願人による特許出願（特願２００４−１１６１６８及び特願２００４−１１６２４２）における知見より、得ようとする鋼の素材に対して適切な温度範囲における温間加工を施すことにより、相変態による強化機構を実質的に利用せずに鋼の結晶粒径を所定値以下に微細化し、次いで、こうして得られた微細粒組織鋼に対して更に、所要の冷間加工を施す。こうすることにより、結晶粒が冷間加工方向に伸びたフェライトが主相のバンブーストラクチャーを呈する鋼を調製し、これにより、当該加工方向に垂直な断面における粒径を上記温間圧延後の材料におけるよりも更に小さくすることにより、強度が確実に上昇すると共に、冷間加工性の低下量は上記温間圧延後における状態の絞りＲＡを指標として、これに比較して極めて小さく抑えることが可能であり、上記引張強さＴＳ及び絞りＲＡを同時に満足することを見出している。更に、上記研究において、この現象は、鋼の化学成分として特別な元素を添加しなくても得られることもわかった。
【００１８】
本発明者等は、上述した製造法により得られた鋼が、７００ＭＰａ程度以上の高強度フェライト鋼であるにもかかわらず、絞りＲＡが７０％以上の高水準を備えているという予想外の新しい知見に基づき、この鋼に冷間圧造ないし転造等の成形加工等を施すに際して、当該鋼に対して球状化焼なましによる軟化処理を施すことなく使用することに着眼し、特に頭部の厚さの薄い小ねじ（「薄頭付きねじ」という）に成形した。その結果、成形性が良好であると共に、更に、当該成形された小ねじに対して焼入処理、焼戻し処理、浸炭処理、及び浸炭窒化処理の内のいずれの熱処理を施さなくても、薄頭付きねじに必要な基本性能を有していることを見出した。
【００１９】
本明細書において本願発明における成形品とは、ねじ、ボルト、ナット、シャフト、リベット、ピン、スタッドボルト、ファスナー類、歯車、軸類、バネ、その他機械構造部品（日本鉄鋼協会発行、渡辺敏幸著 機械用構造用鋼 Ｐ４６、Ｐ９７）等であって、これらの素材の形状は、棒鋼、鋼線材又は鋼線のいずれでもよく、また、薄板を素材とした成形品であってもよい。
また、最薄肉厚部には、ねじ部品のねじ山の頂付近は含まないものとする。
【００２０】
本明細書において本願発明におけるねじ部品とは、ＪＩＳ Ｂ０１０１ ねじ用語の番号２１０１で規定された通りのボルト、ナット等のように、その一部にねじをもった部品である。
【００２１】
本願の第１の発明に係る高強度成形品は、最薄肉厚部の厚さＱが０．２５ｍｍ以下である肉厚部分を有する成形品であって、この最薄肉厚部は、その肉厚部領域における少なくとも１方向の断面における平均結晶粒径が２００ｎｍ以下のフェライト組織を有する超微細組織鋼で構成されており、しかも焼入処理、焼戻し処理、浸炭処理、及び浸炭窒化処理の内いずれの熱処理をも施されていないものであると共に、前記成形品の成形素材に対する球状化焼なまし等の軟化処理が不要であることに特徴を有するものである。
【００２２】
本願の第２の発明に係る高強度成形品は、第１の発明において、上記成形品が、その一部にねじ部が形成されているねじ部品であることに特徴を有するものである。
なお、ここで、最薄肉厚部には、上記ねじ部のねじ山は含まないものとする。
【００２３】
本願の第３の発明に係る高強度成形品は、第１又は２の発明において、上記成形品の化学成分組成の内、Ｃ含有量を規定するものであって、そのＣ含有量が０．０１０質量％以下であるものであることに特徴を有するものである。
【００２４】
以下は、本願発明に係る高強度薄頭小ねじに関するものである。
本願の第４の発明に係る高強度薄頭小ねじは、回転駆動力を伝達するドライバビットが係合する係合溝が形成された頭部と、ねじ部が形成された軸部とが一体化して形成された小ねじであって、次の通り構成されたものである。即ち、上記係合溝の内壁と上記頭部の座面との最短距離をＱｂｓとし、また上記係合溝の内壁と上記軸部のねじ部（頭部の首下部及び不完全ねじ部を含む）の表面との最短距離をＱｓｈとしたとき、これら最短距離Ｑｂｓ及びＱｓｈの内、少なくともいずれか一方は０．２０ｍｍ以下であり、またねじ頭部の厚さｋが、ｋ≦０．２５ｍｍであり、しかも焼入処理、焼戻し処理、浸炭処理、及び浸炭窒化処理の内いずれの熱処理をも施されることなく、前記最短距離Ｑｂｓ又はＱｓｈが０．２０ｍｍ以下である部位は、当該部位における少なくとも１方向の断面における平均結晶粒径が２００ｎｍ以下のフェライト組織を有する超微細組織鋼で構成されていると共に、前記小ねじの成形素材に対する球状化焼なまし等の軟化処理が不要であることに特徴を有するものである。
【００２５】
なお、本明細書において本願発明における小ねじとは、ＪＩＳ Ｂ０１０１ ねじ用語の番号２４０２で規定された通りの比較的軸径の小さい頭付きのねじ、及びこれよりも軸径の小さい頭付きのねじ、並びにこれらに準じるものである。
【００２６】
本願の第５の発明に係る高強度薄頭小ねじは、第４の発明において、上記頭部と上記軸部との間には、ねじが形成されていない補強部が形成されており、この補強部の上端が上記頭部の座面に接続する軌跡円の直径Ｄｒｉｆは、ねじの外径Ｄ０よりも小さいか、又は同じであるものであることに特徴を有するものである。
【００２７】
本願の第６の発明に係る高強度薄頭小ねじは、第４または５のいずれかの発明において、上記係合溝の翼と上記頭部の頂面とが交叉する軌跡円の直径である穴の翼長さｍが、上記ねじ部に形成されたねじの谷の径Ｄよりも小さいか又は同一であることが付加されているものであることに特徴を有するものである。
【００２８】
本願の第７の発明に係る高強度薄頭小ねじは、第４から第６のいずれかの発明において、上記ねじの外径Ｄ０は、Ｄ０≦１．０ｍｍであるものであることに特徴を有するものである。
【００２９】
本願の第８の発明に係る高強度薄頭小ねじは、第４から第７のいずれかの発明において、ビッカース硬さＨＶが２５０以上のものであることに特徴を有するものである。
【００３０】
なお、本明細書において本願発明におけるねじ部の表層部とは、呼び径が指定されたときに行なわれる通常の浸炭処理により、表層部が硬化される程度の厚さを意味し、ねじ部の表面から凡そ０．１ｍｍ程度の深さまでの領域をいう。
【００３１】
本願の第９の発明に係る高強度薄頭小ねじは、第４から第８のいずれかの発明において、上記係合溝の形態が、十字穴であるか、又は上記頭部の軸芯を中心とする円周上に等間隔でｎ等分されて（但し、ｎは、ｎ＝３又はｎ≧５の自然数であって）放射状に形成された穴であるものであることに特徴を有するものである。
【００３２】
本願の第１０の発明に係る高強度薄頭小ねじは、第４から第９のいずれかの発明において、上記ねじの化学成分組成が、そのＣ含有量が０．０１０質量％以下であるものであることに特徴を有するものである。
【図面の簡単な説明】
【００３３】
【図１】圧延条件パラメータＺと平均フェライト粒径との関係を例示するグラフである。
【図２】本発明に係る薄頭付きねじの実施の形態を示す正面図である。
【図３】実施例１の小ねじの軸芯を通る縦断研磨面の３％ナイタール腐食の拡大写真である。
【図４】実施例２の小ねじの軸芯を通る縦断研磨面の３％ナイタール腐食の拡大写真である。
【図５】実施例３の小ねじの軸芯を通る縦断研磨面の３％ナイタール腐食の拡大写真である。
【図６】実施例１の概観の拡大写真である。
【図７】実施例２の概観の拡大写真である。
【図８】実施例３の概観の拡大写真である。
【図９】従来技術における薄頭付きねじの一例を説明するための正面図である。
【図１０】図９の部分切欠き断面を伴なう要部の拡大正面図である。
【図１１】図９のねじを組み込んだ使用状態を示す縦断面図である。
【図１２】他の従来技術における薄頭付きねじの一例を説明するための正面図である。
【図１３】図１２のねじを組み込んだ使用状態を示す縦断面図である。
【符号の説明】
【００３４】
なお、図中の符号は次のものを示す。
１ ねじ
２ 頭部
３ ねじ山
４ 軸部
５ 係合溝
６ 頂面
７ 座面
８ 係合溝の内壁
９ 不完全ねじ部
１０ 補強部
１１ 弧状部
１２ 十字穴（クロスリセス）
１３ 翼
１５ 小ねじ
１６ 首下部
１７ 被締結部材
１８ ワーク
１９ 下穴（取付け穴）
【発明を実施するための最良の形態】
【００３５】
本発明は前述した通りの構成とそれにかかわる特徴を有するものである。そこで、次に、本発明の基本的実施形態の限定理由、及び当該実施形態における態様の限定理由について述べる。
【００３６】
（１）本発明品の基本的製造方法（熱間加工又は温間加工＋冷間加工の組合せからなる工程の規定）
本発明に係る高強度成形品及び高強度小ねじの製造方法において、根幹を成す構成要件は、所定の粒径以下のフェライト組織を有する超微細組織鋼を素材として、これを用いて所望の形態の成形品又は小ねじを成形することにある。ここで、素材の所定の結晶粒径は、鋼塊、鋳片、鋼片又は鋼材半成品に温間加工を施し、次いで更に冷間加工を施し、この冷間加工の最終加工方向に垂直な断面における平均結晶粒径が５００ｎｍ以下、望ましくは２００ｎｍ以下とする。
このように素材の粒径を規定する理由は、かかる素材を用いれば、（１）この素材に球状化焼なまし等の軟化処理を施すことなく、小ねじ等の成形体に加工することが可能であること、及び（２）得られた成形体に表面硬化熱処理等の強靭化熱処理を施さなくても、小ねじ等成形品本来の基本性能が具備された製品が得られること、を知見したからである。ここで、上記小ねじ等成形品の形状・寸法には重要な規定をする。即ち、当該成形品の最も肉厚が薄い部位の厚さ（本願発明において「最薄肉厚部の厚さ」といい、Ｑで表わす）が、０．２５ｍｍ以下であることを規定する。
【００３７】
更に、上記熱間加工又は温間加工により調製される材料の結晶粒径は、これを規定することが必要であり、当該熱間加工又は温間加工の最終加工方向に垂直な断面における平均結晶粒径が３μｍ以下とすることを規定する。かかる熱間加工又は温間圧延材料に対して、適切な条件下での冷間加工を施すと言うものであり、この冷間加工により、Ｃ方向断面における結晶粒が、一層微細化された組織鋼を得るというものであり、その粒径として、冷間加工工程の最終加工方向に垂直な断面において、５００ｎｍ以下に超微細化する。そして望ましくは更に微細化して、２００ｎｍ以下にする。
【００３８】
上記において、熱間加工又は温間加工後の材料を上記の通り規定された微細粒組織鋼調製し、これを用いて冷間加工を施すと、材料強度が著しく上昇するにもかかわらず、極めて好都合なことには、加工性の低下が極めて小さいことが見出された。従来予想が困難であったこの新規知見は、本願発明の完成に重要な地位を占める。
このように、上記素材を用いれば、当該素材の球状化焼なまし等の軟化処理を施す必要が無くなり、しかも、成形体の表面硬化熱処理等の強靭化熱処理を施す必要が無くなる理由は、当該素材が機械的性質において、引張強さＴＳと絞りＲＡとが共に優れている、即ち、両特性値が良好なバランスを備えつつ両特性値の水準が優れているからである。
【００３９】
（２）鋼の成分組成
本発明品の製造方法において、温間加工の素材として用いる鋼塊、鋳片、鋼片又は鋼材半成品の化学成分組成は、炭素鋼成分及び低合金鋼のいずれであっても、鋼の標準組織において主相がフェライト又はフェライトのみであれることが必要であり、Ｃ含有量が０．４５質量％以下であればよい。本発明においては、前述した通り、上記鋼材半成品等に対して適切な温間加工を施すことにより、相変態による強化機構を実質的に利用せずに鋼の結晶粒径を所定値以下に微細化し、次いで、こうして得られた微細粒組織鋼に対して更に、所要の冷間加工を施すことにより、一層微細な超微細組織鋼を調製することにより、高強度且つ高延性を有する素材を得る。
【００４０】
本発明においては、焼入れ処理により、マルテンサイト変態を起こさない化学成分組成であってもよいことが重要な特徴である。その理由は、本願発明の他の構成要件を満たしていれば、熱間加工又は温間加工及び冷間加工後の材料の目標とする引張強さ８００ＭＰａ以上、望ましくは９００ＭＰａ以上、更に望ましくは１２００ＭＰａ以上、そして更に望ましくは１５００ＭＰａ以上が得られ、しかも当該引張強さに応じて絞りＲＡも高水準に維持された鋼が得られるからである。
特に、Ｃ含有量が０．０１０質量％以下、更に望ましくは、０．００２質量％未満の極低炭素鋼成分であっても、高強度が得られる。ここで、Ｃ含有量が０．００２質量％未満であれば、冷間加工後の材料に対する焼鈍が不要となる。
高強度で且つ高延性という両者のバランスに優れた機械的特性が得られるのは、冷間加工性を劣化させる要因である硬質なセメンタイトが実質的に生成していない点に大きく依存している。
【００４１】
なお、本発明に係る成形品又は小ねじにおいて、実質的にセメンタイトフリーであるか否かの判断は、実際問題としては必ずしも容易ではない。そこで、日常操業において実際的なＣ含有量の定量分析値により精度よく推定することができる。そこで、金相学的判断からＣ含有量がＡｅ１点におけるフェライト相中の炭素の固溶限以下であることと規定した。更には、通常の低合金鋼ないし炭素鋼の成分系においては、セメンタイトが生成しないと考えられるＣ含有量の範囲として、０．０１０質量％以下、更に望ましくは、上記焼鈍不要の観点から０．００２質量％未満に規定した。
上記において、Ａｅ１点におけるフェライト相中の固溶Ｃ濃度（質量％）以下となっているために、セメンタイトフリーの組織となっている。炭素鋼及び低合金鋼のいずれにおいても、このセメンタイトフリーが得られるＣ濃度（質量％）は、例えば計算ソフトＴｈｅｒｍｏ−ｃａｌｃを用いて計算することができる。
【００４２】
このように、本発明においては、セメンタイトフリーのフェライト組織を有する鋼材において、上述したような高強度を有し、且つ冷間加工性にも優れている材料（強度と加工性とのバランスに優れた高強度鋼）の設計が可能となった。従来、かかる成分設計による冷間加工性に優れた高強度鋼線又は棒鋼が実現された例は見当たらない。
【００４３】
本発明においては、上述した通り高強度特性を得るために、セメンタイトフリーの鋼であることを重視するものである。そこで、上記化学成分組成の規定に際しても、合金元素の添加に依存させることは、望ましくは不要である。そこで、焼入れ性向上を促進させる元素、例えばＣｒやＭｏ、その他の同類元素の添加、並びに、固溶強化元素であるＣｕやＮｉ、その他の同類元素の添加は敢えて添加する必要が無い。そればかりか、上記合金元素は製造コスト低減上からも添加しないことが望ましい。従って、上記元素はいずれも、鋼の精錬・溶製工程において不可避的に混入する以上の含有量は、無い方が望ましい。更に、本発明では特に規定するものではないが、析出強化に有効な元素であるＴｉやＮｂ、その他の合金元素も添加するには及ばない。本発明のセメンタイトフリーの成分系により、十分な引張強さを確保することができるからであり、製造コストの低減にも役立つ。
【００４４】
なお、上述した化学成分の内、Ｃ含有量に関しては、本発明における本発明品の製造方法を実施する限り、０．０１０質量％超えであっても類似した作用・効果が得られる。この観点から、Ｃ含有量は０．４５質量％以下であればよい。
また、上述したＣをはじめとする各種の化学成分組成は、本願発明における鋼塊、鋳片、鋼片又は鋼材半成品から高強度成形品又は小ねじまでの製造工程において、その含有量は実質的に変化しないとみなすことができる。
【００４５】
（３）熱間加工又は、温間加工及び冷間加工における温度及びひずみ条件
（３−１）熱間加工又は温間加工条件：上記高強度成形品又は小ねじの製造工程の実施の形態として、まず所定の鋼塊、鋳片又は鋼片乃至鋼材に対する一方、温間加工の望ましいは条件は、加工温度が３５０〜８００℃の範囲内とすべきである。更に、その際に材料中へ導入されて残留する塑性ひずみを確保すべきである。この塑性ひずみ量は、公知の３次元有限要素法による計算で求めることができる。その値を「ε」で表記すると、ε≧０．７以上であることが望ましい。このような温間加工条件を採用したのは、相変態による強化機構を実質的に利用せずに鋼の高強度、且つ高延性を実現する方法として、Ｃ方向断面における平均結晶粒径を３μｍ以下にするためである。
【００４６】
なお、上記熱間加工又は温間加工条件において、ひずみとしてεを指標とする代わりに、操業上比較的簡便に求めることができる材料のひずみ（この出願の発明明細書において「工業的ひずみ」といい、「ｅ」で表記する）により、実用的に代替することができる。工業的ひずみｅは、材料の総減面率Ｒの関数であり、下記（１）式：
ｅ＝−ｌｎ（１−Ｒ／１００）‥‥‥‥‥‥‥‥‥‥（１）
で表わされる。但し、Ｒは下記（２）式：
Ｒ＝｛（Ｓ０−Ｓ）／Ｓ０｝×１００‥‥‥‥‥‥‥‥（２）
但し、Ｒ：鋼塊、鋳片又は鋼片乃至鋼材に対して施される総減面率（％）
Ｓ０：熱間加工又は温間加工開始直前の鋼塊、鋳片又は鋼片乃至鋼材のＣ方向断面積
Ｓ：熱間加工又は温間加工終了後に得られる材料のＣ方向断面積
で表わされる総減面率Ｒである。
上記（１）式及び（２）式を用いて、ε≧０．７に相当するＲの値を計算すると、Ｒ≧５０％が得られる。従って、熱間加工又は温間加工においては、上記塑性ひずみε≧０．７の代わりに、材料の総減面率Ｒ≧５０％を採用してもよい。
【００４７】
更に、一方、本発明者等は、温間強加工（温間における１パスによる大ひずみ加工）によって形成される超微細粒の平均粒径は、加工温度とひずみ速度に依存することに着眼し、圧延条件パラメータとして、下記（３）式：
Ｚ＝ｌｏｇ［（ε／ｔ）ｅｘｐ｛Ｑ／（８．３１（Ｔ＋２７３））｝］
‥‥‥‥‥‥‥‥‥（３）
但し、ε：平均塑性ひずみ
ｔ：圧延開始から終了までの時間（Ｓ）
Ｑ：定数（結晶組織がｂｃｃのとき、２５４０００Ｊ／ｍｏｌ）
Ｔ：圧延温度（℃）、多パス圧延の場合は各パスの圧延温度を平均した温度
で表わされるＺｅｎｅｒ−Ｈｏｌｌｏｍｏｎ ｐａｒａｍｅｔｅｒを導入し（但し、対数形式で表記）、結晶粒径は、圧延条件パラメータＺの増加につれて微細化することを見出した。図１に、圧延条件パラメータＺと平均フェライト粒径との関係を例示する。即ち、図１は、Ｚ≧１１となるように圧延を制御することにより、平均フェライト粒径が１μｍ以下の微細粒組織が得られることを示している。従って、温間圧延温度をＺ≧１１を満たすように制御することにより、素材の平均フェライト粒径を３μｍ未満にすることが可能となる。
【００４８】
更に、温間加工法としては、温間圧延及び温間鍛造のいずれを採用してもよく、その際、複数バス（温間鍛造の場合は、複数回の鍛造スケジュールとする）により複数方向に加工することにより、材料内への塑性ひずみの均一化が図られるので、望ましい。
【００４９】
（３−２）冷間加工条件：次に、上記の通り温間加工により調製された微細粒組織を有し、高強度で且つ加工性に優れている材料に対して、予め施すべき望ましい冷間加工条件は、冷間加工温度が３５０℃未満であることが望ましい。加工発熱により、冷間加工中にこれよりも高い温度に達すると、引張強さの上昇度合いが低下して望ましくない。次に、冷間加工により材料中へ導入される残留ひずみを、所望する引張強さに応じて確保することが必要である。このような観点から、３次元有限要素法により求められる塑性ひずみεが、少なくとも０．０５以上となるように冷間加工を施すことが望ましい。これにより結晶の冷間加工組織は加工方向に延伸した形態を呈し、加工方向に対するＣ方向断面における粒径も細粒化されて、引張強さの上昇が確保される。その際、絞りＲＡの低下量は小さく抑えられる。
【００５０】
上記冷間加工条件において、加工量としてεを指標とする代わりに、前記（１）式により説明した「工業的ひずみ」であるｅを媒介することにより、ε≧０．０５に相当する材料の総減面率Ｒを計算すると、Ｒ≧５％が得られる。従って、冷間加工においては、上記塑性ひずみε≧０．０５の代わりに、材料の総減面率Ｒ≧５％を採用してもよい。
なお、上記冷間加工においては、公知の冷間伸線法及び冷間圧延法のいずれを採用してもよい。
【００５１】
（４）最薄肉厚部の厚さ及び結晶粒径
本発明に係る成形品においては、その最薄肉厚部の厚さが０．２５ｍｍ以下であるものを対象としている。このように肉厚が薄い部分を有する成形品であって、この部分においても高強度を必要とするものにあって、焼入処理、焼戻し処理、あるいは浸炭焼入・焼戻し又は窒化処理を施さないでもよいものとして、当該最薄肉厚部における少なくとも１方向の断面における平均結晶粒径が５００ｎｍ以下のフェライト組織となっていればよい。そして、このような超微細組織鋼で構成された成形品は延性にも優れている。この平均結晶粒径は２００ｎｍ以下であれば最薄肉厚部の強度が一層上昇して望ましい。ここで、そのような成形品として、ねじ部品のように肉厚が薄い部分を有する物に対して望ましい条件であり、更に、小ねじの内でも特にその頭部の厚さが薄いものの場合には望ましい条件である。なお、このような高強度で且つ高延性を有する成形体乃至小ねじは、上記（１）〜（３）に記載した製造技術に基づく本発明に係る製造方法により得ることができる。
【００５２】
（５）薄頭付きの小ねじ
図２に、本発明に係る薄頭付きねじの実施の形態を示す正面図である。
図２において、１５は小ねじであって、頭部２とこの頭部と一体をなしねじ山３を有する軸部４とからなっている。この小ねじ１５の頭部２には軸部４の軸芯線上に回転駆動力を伝達するドライバビットが係合することができる係合溝５として十字円錐形状の十字穴（クロスリセス）１２が形成され、この係合溝５（１１）は先端が軸部４に達している。また、この係合溝５の内壁８は、頭部２の頂面６から軸部４にかけて深くなるにつれて軸部４の軸芯方向に傾斜している。
上記十字穴からなる係合溝５の内壁８と頭部２の座面７との最短距離Ｑｂｓ、及び、係合溝５の内壁８と軸部４のねじ部（頭部２の首下部１６及び不完全ねじ部９を含む）の表面との最短距離Ｑｓｈの内、少なくともいずれか一方は０．２０ｍｍ以下に形成されている。
【００５３】
また、頭部２は、その厚みが通常の小ねじの頭部よりかなり薄く形成され、頭部２の厚さ（ｋで表わす）は、ねじの外径（Ｄ０で表わす）の１／５〜１／６程度である。従って、例えば、ねじの外径Ｄ０＝１．２〜１．４ｍｍの小ねじ１５（Ｍ１．２〜Ｍ１．４）の場合、頭部の厚さｋ≒０．２４ｍｍ程度であり、ねじの外径Ｄ０≦１．０ｍｍとなると、頭部の厚さｋ≦０．２ｍｍとなる。また、頭部２の外径（ｄｋで表わす）は、ねじ１の外径Ｄ０の１．７〜２．２倍程度である。従って、Ｍ１．２の場合、頭部２の外径ｄｋ≒２．５ｍｍ程度、Ｍ１．４の場合、頭部２の外径ｄｋ≒２．７ｍｍ程度である。
【００５４】
上記の通り、上記最短距離Ｑｂｓ又はＱｓｈが０．２０ｍｍ以下と、係合溝５の最小肉厚部が極めて薄いので、ねじ締付け時の最小破壊トルク値の確保が重要となり、また、頭部の厚さが薄いので、ねじ締付け時に係合溝がなめる（破壊する）のを避ける必要がある。これに対して、本発明に係る小ねじは、焼入処理、焼戻し処理、浸炭処理、及び浸炭窒化処理の内いずれの熱処理をも施されていないが、当該各部位を含めて、少なくとも１方向の断面における平均結晶粒径が５００ｎｍ以下の、更に望ましくは２００ｎｍ以下のフェライト組織を有する超微細組織鋼で構成しているので、上記最小破壊トルク値の確保及びなめりの防止がなされる。
【００５５】
次に、頭部２と軸部４との間には、ねじが形成されていない補強部１０が形成されている。そして、この補強部１０は、僅かに上広がりの円錐台形状を呈し、その上端が頭部２の座面７に接続する軌跡円の直径Ｄｒｉｆは、ねじの外径Ｄ０以下となるように、補強部１０は形成されている。このように、Ｄｒｉｆ≦Ｄ０とすることにより、頭部２の座面７の下側に接して締結される被締結部材（図示せず。但し、図１３の符号１７参照）に開けるべき下穴（取付け穴）（図１３の符号１９参照）の径を大きくする必要がなくなる。また、補強部１０の高さは、できるだけ低くしてある。即ち、頭部の厚さｋを薄くしたので、係合溝５の翼１３が頭部２の頂面６と交叉する軌跡円の直径、即ち穴の翼長さｍを短くすることにより、係合溝５の内壁８の最小厚さである前記Ｑｂｓ、及びＱｓｈが、薄くなり過ぎるのを防止して、係合溝５に係合されたドライバビットの回転駆動力をねじの軸部４に十分に伝達するためである。この際、上記穴の翼長さｍを、ねじの谷の径Ｄ以下となるように形成することにより、上記Ｑｂｓ、及びＱｓｈの大きさが確保され、一層望ましい。
【００５６】
本発明に係る小ねじの外径Ｄ０は、携帯用電子機器等製品の小型化・コンパクト化の要請の観点から小さいことが望ましい。上述した小ねじにおいて、Ｄ０を１．０ｍｍ以下に設計することにより、上記要請に対して一層好ましい。小ねじの外径Ｄ０を小さくするほど、前記Ｑｂｓ、及びＱｓｈが薄くなるので、一層高強度の小ねじが必要となる。本発明に係る小ねじはこの肉薄部における超微細組織鋼の構成により、このような高強度化の要請を満たすものである。
そして、本発明に係る小ねじの係合溝の形態は、ねじ締付け時に穴が一層なめらないようにするために、十字形状に限定されることなく、三叉形状の穴とすることができ、その他円周を４以上に等分した放射状穴を形成することもできる。こうすることにより、機器製品の開放を防ぐ防犯対策にもなる。
【実施例】
【００５７】
以下、この出願の発明を実施例により更に詳しく説明する。
表１に示した化学成分組成を有する成分Ｎｏ．１及び２の鋼を真空溶解炉を用いて溶製し、鋼塊に鋳造した。
【表１】
【００５８】
得られた各鋼塊を熱間鍛造により１２ｍｍφの棒鋼に成形した。これから温間圧延用素材を採取し、温間における多方向の多パスカリバー圧延により１２ｍｍφに成形し、水冷して棒鋼を調製した。この温間圧延の条件は、５５０℃に加熱した後、圧延温度４５０〜５３０℃の範囲内において、６ｍｍφに成形した。この間、圧延の各パス毎に材料を長さ方向軸芯の周りに回転させて圧下方向を変化させ、多方向の多パス圧延を行なった。温間圧延用素材である１２ｍｍφからこの６ｍｍφへの総減面率は７５％である。なお、このときの平均塑性ひずみεは、前記（１）及び（２）式を参照して、ε＝２．９とみなすことができる。そして、この温間圧延された６ｍｍφの線材のフェライト粒径は、Ｃ方向断面において、０．５μｍとなっていた。
【００５９】
次に、上記温間圧延により調製された成分Ｎｏ．１、２及び３の６ｍｍφの線材のそれぞれを冷間伸線により、１．１５ｍｍφ、１．０ｍｍφ及び０．８ｍｍφの３サイズの鋼線に調製した。なお、このときの伸線率はそれぞれ９６％、９７％及び９８％であり、このときの平均塑性ひずみεは、前記（１）及び（２）式を参照して、ε＝３．２、ε＝３．５及びε＝３．９とみなすことができる。そして、上記成分Ｎｏ．１から冷間伸線された１．１５ｍｍφ及び０．８ｍｍφのフェライト平均粒径は、Ｃ方向断面においてそれぞれ１０５ｎｍ及び８８ｎｍであり、そして成分Ｎｏ．２から冷間伸線された１．０ｍｍφのフェライト平均粒径は、Ｃ方向断面において９５ｎｍであった。
【００６０】
次に、これら３種の鋼線の内、１．１５ｍｍφの鋼線（成分Ｎｏ．１）からＭ１．４（呼び径１．４ｍｍ）の薄頭付きの小ねじを、０．８ｍｍφの鋼線（成分Ｎｏ．１）からＭ１．０（呼び径１．０ｍｍ）の薄頭付きの小ねじを製造した。また、１．０ｍｍφの鋼線（成分Ｎｏ．２）からＭ１．２（呼び径１．２ｍｍ）の薄頭付きの小ねじを製造した。これらの小ねじを順番にそれぞれ、実施例１、実施例３及び実施例２という。表２及び表３に、製造された実施例である各小ねじの各部位の寸法を示す。実施例１〜３において、係合溝の内壁と前記頭部の座面との最短距離Ｑｂｓ、又は係合溝の内壁と前記軸部のねじ部の表面との最短距離Ｑｓｈであって、０．１２〜０．２０ｍｍの範囲にある。これに対して、ねじの頭部の厚さｋは、０．２２〜０．２６ｍｍの範囲にある。即ち、実施例１〜３においてはいずれも、頭部の厚さｋよりもＱｂｓ又はＱｓｈの方が薄く、最薄肉厚部の厚さＱは、実施例１では０．１２ｍｍ、実施例２では０．２０ｍｍ、そして実施例３では０．１８ｍｍという極めて薄い肉厚部を有するねじである。また、表２には、更に、係合溝の内壁と頭部の座面との最短距離Ｑｂｓ、及び係合溝の内壁と軸部のねじ部（前記頭部の首下部及び不完全ねじ部を含む）の表面との最短距離Ｑｓｈの内、短い距離の方の部位に相当する位置（「係合溝の最薄肉部」と略称する）における断面の平均結晶粒径（フェライト粒径）の値を併記した。
【００６１】
【表２】
【００６２】
【表３】
【００６３】
次に、実施例１、２及び３のそれぞれの小ねじにつき、その軸芯を通る縦断面の研磨面につき、３％ナイタール腐食面のそれぞれの拡大写真を図３（実施例１）、図４（実施例２）及び図５（実施例３）に示し、また、それぞれの概観の拡大写真を図６、図７及び図８に示す。なお、係合溝の穴翼の形態は、実施例１及び実施例２においては十字形状であり、実施例３においては三叉形状である。
【００６４】
一方、実施例１〜３のそれぞれの小ねじにつき、次の位置のビッカース硬さＨＶを測定した。即ち、（１）表２に記載した「係合溝の最薄肉部」の係合溝の内壁直近部、外周表面直近部及び肉厚中心部、（２）係合溝の底面直近部、（３）ねじ山表面直近部及びねじ山中間部、並びに（４）ねじの長さ中央部における軸芯部において測定した。その結果、実施例１〜３の全てにつき、上記測定値の全ての位置において、ビッカース硬さＨＶは２５０以上であり、係合部表面は３００以上あった。
【００６５】
上述した各実施例の小ねじにつき、係合溝の穴（リセス）がなめらないかどうか（破壊しないかどうか）のリセス強度試験、首下部の破壊トルク試験、及びねじ締め状態における遅れ破壊試験を行なった。その結果、全ての実施例につき、良好な結果が得られた。
【発明の効果】
【００６６】
本発明は以上述べた通りの構成を有するので、次の効果が発揮される。先ず、薄頭付きの高強度小ねじをはじめとする高強度成形品の製造において、成形体に加工するための素材として、超微細結晶を有するフェライト組織鋼を使用する。この素材は高強度であるにもかかわらず、延性にも優れているので、その成形体への加工に先立って、従来行なわれている極めて長時間を要する球状化焼なまし等の軟化処理をその素材に施す必要がない。更に、加工後のこの成形体には、上記素材の材料特性である高強度が引き継がれ、さらに延性も強度とバランスよく引き継がれる。従って、加工後の成形体に対して、従来行なわれている表面硬化熱処理や焼入処理、焼戻し処理等の強靭化のための熱処理を施す必要がない。そして、得られる薄頭付きの高強度小ねじをはじめとする成形品は、強度及び靭性に優れている。特に、肉厚が極めて薄い部分を有する成形品の製造において、当該部分の強靭性向上のために行なわれる表面硬化熱処理の内、浸炭焼入・焼戻し処理のように、その硬化層の厚さを所定範囲内に薄く制御しなければならないという、困難な技術を要する場合であっても、本発明品であれば、そのような熱処理も一切不要である。従って、また低コストで製造され、特に薄頭付きの小ねじのような製品については、安定した高品質の成形品が製造され得る。具体的には、薄頭付きねじであって、現状のＪＩＳでは規定されていないような軸径の小さい頭付きの小ねじ（マイクロねじ）であっても、ねじの基本性能が発揮される製品が得られる。即ち、ねじの頭部の厚さを薄くし、ねじの外径を小さくしても、ねじの締付け時に係合溝の穴（リセス）が破壊されず、締付けによるねじの破壊トルク値が十分であり、しかも締結後の遅れ破壊が起こらない。こうして、特に携帯用等の精密電子機器の小型化・コンパクト化及び軽量化に寄与しうる。【Technical field】
[0001]
The present invention is a molded product in which the thickness Q of the thinnest part is thin, such as a high-strength machine screw with a thin head, and is made of steel having high strength and excellent delayed fracture resistance. The present invention relates to a molded product or a screw component, and particularly with respect to the high-strength small screw, the head of a screw screwed into a workpiece (see reference numeral 17 in FIG. 13 described later) should not protrude from the workpiece as much as possible. In order to achieve this, the thickness of the screw head is made as thin as possible, the screw fastening function as a screw is excellent, the delayed fracture characteristics are excellent, and the outer diameter of the screw (in the case of female threads) is as small as possible It relates to technology that covers everything.
[Background]
[0002]
In recent electrical appliances and information equipment products, electronic devices such as electronic notebooks, notebook computers, HDDs, mobile phones, digital cameras, etc., which are convenient to carry and have excellent performance, are rapidly spreading. . In such products, in order to reduce the size and size, it is also required that the product thickness is thin, and the screw parts for assembling the product are inevitably small in diameter, specifically, the nominal diameter is 2 mm ( A small screw with a diameter smaller than M2) is used. In order to further reduce the size and size of the product and make it thinner, it is necessary to further reduce the size of the machine screw. At that time, reduce the thickness of the head of the screw so that the head does not protrude from the workpiece, so that the appearance of the screw is not impaired, and that the screw head does not get caught during use of the product. There is a demand for screw products with sufficient basic performance.
[0003]
In general, since it is necessary to form an engagement groove for engaging with the driver bit on the screw head, if the thickness of the head of the screw part is reduced in response to the above requirement, this engagement is required. As long as the shape and dimensions of the mating groove are maintained, it is inevitable that the bottom end position of the engaging groove bites into the shaft portion of the screw. For this reason, the closest distance between the inner wall of the bottom region of the engaging groove and the head seat surface, or the inner wall and threaded portion of the bottom region of the engaging groove (including the head neck and incomplete threaded portion. This includes JIS. The shortest distance from the surface of the B0101 number 2102 (hereinafter referred to as “thickness of the thinnest thick portion near the head neck” in the present specification) is reduced. Thus, when the thickness of the head is considerably reduced, (1) a region where the thickness of the thinnest thick portion near the head neck is 0 is generated and a hole is opened or (2 ) The structure of this part is weakened and the standard torque value for screwing cannot be secured. (3) In the case where the thickness of the thinnest portion near the neck of the head is secured to the required value by reducing the depth of the engaging groove while reducing the thickness of the head. When the screw is screwed in, the engaging groove formed in the head is licked (broken), and the effective torque value is significantly reduced.
[0004]
In this way, the above-mentioned problem relating to the screw that occurs when the thickness of the head is reduced becomes more serious as the nominal diameter becomes smaller, and the basic performance of the screw is impaired. It becomes.
[0005]
In response to the above-described situation, a screw with a thin head in which the thickness of the head of the screw is reduced has been proposed. And in this thin headed screw, in order to solve the above-mentioned problem caused by making the head thinner, a screwless portion called a “reinforcing portion” is formed at the lower neck portion of the head.
[0006]
For example, Patent Document 1 discloses the following technique regarding the reinforcing portion. That is, FIG. 9 is a front view for explaining an example of the thin-headed screw, FIG. 10 is an enlarged front view of a main part with a partially cutaway section of FIG. 9, and FIG. FIG. 10 is a longitudinal sectional view showing a use state in which the screw of FIG. 9 is incorporated.
[0007]
In these drawings, reference numeral 1 denotes a conventional screw with a thin head, and the screw with a thin head 1 reinforces a head portion 2, a shaft portion 4 having a thread 3 and a screwless portion formed at a lower portion of the neck. Part 10. A cross-shaped engagement groove (cross hole) 5 is formed in the head portion 2 of the screw 1, and the engagement groove 5 reaches the shaft portion 4 with its tip bite. Here, the head 2 is formed so that the thickness thereof is considerably thinner than that of a normal screw head. The thickness of the head 2 is about 1/3 of the nominal diameter of the screw 1 or more. It is set as follows. That is, for example, in M2 having a nominal diameter of 2 mm, the thickness of the head 2 is about 0.67 mm or less, and in M1.2 having a nominal diameter of 1.2 mm, the thickness of the head 2 is 0. It will be about 4 mm or less.
[0008]
Further, the reinforcing portion 10 formed between the seating surface 7 of the head 2 and the thread 3 of the shaft portion 4 has an arc shape whose outer shape in a longitudinal section is relatively large as shown in FIG. 10, for example. The diameter of the trajectory circle at the position P10 where the upper end side of the reinforcing portion 10 is connected to the seat surface 7 of the head 2 is the screw diameter. It is slightly larger than the outer diameter D0. In addition, the diameter m of the locus circle where the inner wall 8 of the engagement groove 5 and the top surface 6 of the head 2 intersect is set smaller than the root diameter D of the screw (m <D). Furthermore, the engaging groove 5 is inclined in the axial direction of the shaft portion 4 as it becomes deeper from the top surface 6 of the head 2 to the shaft portion 4. Thus, a sufficient thickness is secured between the inner wall 8 of the engaging groove 5 and the reinforcing portion 10 to transmit the driving force to the screw 1 when screwed.
[0009]
In Patent Document 2, in order to further increase the thickness δ of the thick portion between the reinforcing portion 10 and the inner wall 8 of the engaging groove 5 in order to improve the minimum breaking torque value of the screw, FIG. 12, the diameter of the locus circle of the position P10 where the reinforcing portion 10 is connected to the seating surface 7 of the head 2 is formed larger than the outer diameter D0 of the screw, and the reinforcing portion 10 is tightened when the screw is fastened. In order to prevent stress concentration from occurring at the lower end portion, the shape of the portion where the reinforcing portion 10 is connected to the shaft portion 4 side is formed as an arc-shaped portion 11 that is continuous with a smooth curved surface shape with respect to the incomplete screw portion 9. It is disclosed. FIG. 13 is a longitudinal sectional view showing a use state in which the screw of FIG. 12 is incorporated. 17 is a member to be fastened, and 18 is a workpiece.
[0010]
Thus, in the prior art, in order to reduce the thickness of the head 2 of the screw 1, the non-threaded reinforcing portion 10 is formed under the neck of the head 2, and the shape and size of the reinforcing portion 10 are as follows: In addition, by appropriately designing the shape and dimensions of the engagement groove 5, the thickness of the thinnest wall thickness portion near the head neck is secured, so that the original fastening function of the screw is exhibited when the screw 1 is screwed. Has been devised. In addition, it has been devised so that stress concentration on the lower part of the neck can be relaxed when fastening with screws.
[0011]
[Patent Document 1]
: JP 2002-054617 A
[Patent Document 2]
: JP 2002-139010 A
[0012]
According to the above-described proposal of the prior art, since the thickness of the screw head is reduced, the protrusion of the screw head in the product is reduced, and the product is reduced in thickness and weight. Furthermore, the delayed fracture resistance is improved by alleviating the stress concentration at the bottom of the neck during fastening.
[0013]
However, even with the above-described conventional technology for thinning the screw head, the following problems remain in order to further reduce the size and size of high-performance portable electronic device products. That is, a reinforcing portion is formed under the neck of the head, but since no screw is formed on this, the reinforcing portion does not have a direct fastening function. And it is inevitable that the thickness of the member to be fastened (reference numeral 18 in FIG. 13) increases by the height of the reinforcing portion. Therefore, the effect of reducing the thickness of the entire device product is slightly impaired. Furthermore, since the diameter of the upper end side portion of the reinforcing material is larger than the outer diameter of the screw, the diameter of the prepared hole of the member to be fastened must be equal to or larger than the outer diameter of the screw, and the effect of downsizing the equipment product is impaired.
[0014]
Next, as in the case of general screw parts, also in small screws used in information equipment products, the manufacturing process is performed by cold heading for preforming and head molding of the material, and then In order to perform rolling for forming the threaded portion, it is necessary to improve the cold formability by performing a softening treatment represented by spheroidizing annealing that requires a long time. Next, the molded body thus obtained is subjected to surface hardening treatment such as carburizing and tempering or nitriding treatment in order to improve the hardness of the thread surface layer. However, in the machine screw subjected to such surface hardening heat treatment, the surface hardened layer region is excellent in hardness and strength, but this region is brittle and inferior in toughness. There is a problem that.
DISCLOSURE OF THE INVENTION
[Problems to be solved by the invention]
[0015]
In view of the above-described problems, the object of the present invention is to (1) reduce the diameter of the pilot hole of the member to be fastened by forming a screwless reinforcing portion formed below the neck of the head of the thin screw. And (2) softening treatment such as spheroidizing annealing on the molding material of the small screw is unnecessary, and further, it is not necessary to perform heat treatment for surface hardening on the compacted body of the small screw. It is to produce a strong machine screw with a thin head.
[0016]
Accordingly, an object of the present invention is to solve the above-mentioned problems, and to produce a small screw having a thin head and which does not require heat treatment for surface hardening. In addition to expanding the range of application of this manufacturing technology, the molding is not limited to small screws, and the molded part having a thin part is improved by improving the strength and toughness of the thin part. The use of products is expanded, and it is possible to increase the strength and toughness of molded products including threaded parts, and the process of softening the material of the molded body and the process of toughening the molded body can be omitted. To provide technology and contribute to industrial development.
[Means for Solving the Problems]
[0017]
In order to solve the above-mentioned problems, the present inventors have conducted extensive research. First of all, the inventors of the present invention are steels to be used for molded articles, screws or screws to be manufactured according to the present invention. Among the mechanical properties, the tensile strength TS exceeds the level of 800 MPa. In other words, it was desirably 1000 MPa or more and the aperture RA exceeded 70%. In order to obtain such a material having high strength and high ductility, based on the findings in patent applications (Japanese Patent Application Nos. 2004-116168 and 2004-116242) by the present applicant based on the research results of the present inventors, By subjecting the steel material to be obtained to warm working in an appropriate temperature range, the crystal grain size of the steel is refined to a predetermined value or less without substantially using a strengthening mechanism by phase transformation, Then, the required cold working is further performed on the fine grain structure steel thus obtained. In this way, a steel having a ferrite structure in which the crystal grains extend in the cold working direction exhibits a bamboo structure of the main phase is prepared, whereby the grain size in the cross section perpendicular to the working direction is set to the material after the warm rolling. By making it even smaller than in the above, the strength is surely increased, and the amount of decrease in cold workability can be suppressed to an extremely small value by using the drawing RA in the state after the warm rolling as an index. It has been found that the above-mentioned tensile strength TS and aperture RA are satisfied simultaneously. Furthermore, in the above research, it was also found that this phenomenon can be obtained without adding a special element as a chemical component of steel.
[0018]
The inventors of the present invention have an unexpected new fact that the drawing RA has a high level of 70% or more, even though the steel obtained by the above-described manufacturing method is a high strength ferritic steel of about 700 MPa or more. Based on the knowledge, when carrying out forming processing such as cold forging or rolling on this steel, we focused on using it without subjecting the steel to softening treatment by spheroidizing annealing. It was formed into a thin screw having a small thickness (referred to as “thin headed screw”). As a result, the moldability is good, and further, the thin head can be formed without any heat treatment among the quenching treatment, tempering treatment, carburizing treatment, and carbonitriding treatment on the machined screw. It has been found that it has the basic performance necessary for a threaded screw.
[0019]
BookIn the specification, the molded product in the present invention means screws, bolts, nuts, shafts, rivets, pins, stud bolts, fasteners, gears, shafts, springs, and other mechanical structural parts (issued by the Japan Iron and Steel Institute, written by Toshiyuki Watanabe) Structural steel P46, P97), etc., and the shape of these materials may be any of steel bar, steel wire, or steel wire, or may be a molded product made of a thin plate.
  In addition, the thinnest thick portion does not include the vicinity of the top of the thread of the screw part.
[0020]
BookIn the specification, the threaded part in the present invention is a part having a threaded part, such as a bolt and a nut as defined in JIS B0101 thread term number 2101.
[0021]
  The high-strength molded product according to the first invention of the present application is a molded product having a thick portion where the thickness Q of the thinnest thick portion is 0.25 mm or less, and the thinnest thick portion has its thickness In the cross section in at least one direction in the partial region, and is composed of ultrafine structure steel having a ferrite structure of 200 nm or less, and any of quenching, tempering, carburizing, and carbonitriding It has not been heat-treatedIn addition, a softening process such as spheroidizing annealing is not required for the molding material of the molded product.It has a special feature.
[0022]
No. of this application2The high-strength molded product according to the invention is1In the invention, the molded product is characterized in that it is a threaded part in which a threaded portion is formed.
  Here, the thinnest thick part does not include the thread of the thread part.
[0023]
No. of this application3The high-strength molded product according to the invention is1 or 2In the invention, the C content is defined in the chemical component composition of the molded article, and the C content is 0.010% by mass or less.
[0024]
  The following relates to the high-strength thin head machine screw according to the present invention.
  A high-strength thin head machine screw according to a fourth invention of the present application is such that a head portion in which an engagement groove with which a driver bit for transmitting rotational driving force is engaged is formed with a shaft portion in which a screw portion is formed. A small screw formed as described above and configured as follows. That is, the shortest distance between the inner wall of the engaging groove and the seating surface of the head is defined as QbsAnd Q is the shortest distance between the inner wall of the engagement groove and the surface of the threaded portion of the shaft portion (including the head neck and incomplete threaded portion).shThe shortest distance QbsAnd QshAnd at least one of them is 0.20 mm or less, and the thickness k of the screw head is k ≦ 0.25 mm, and among quenching, tempering, carburizing, and carbonitriding The shortest distance Q without any heat treatmentbsOr QshIs 0.20 mm or less, and is composed of ultra-fine structure steel having a ferrite structure with an average crystal grain size in a cross section in at least one direction of the part being 200 nm or less.In addition, a softening process such as spheroidizing annealing is not required for the molding material of the small screw.It has a special feature.
[0025]
In the present specification, the small screw in the present invention means a screw with a head having a relatively small shaft diameter as defined by JIS B0101 screw term number 2402, and a screw with a head having a smaller shaft diameter. As well as these.
[0026]
No. of this application5High strength according to the inventionThin headMachine screw4In this invention, a reinforcing portion not formed with a screw is formed between the head portion and the shaft portion, and the diameter of the locus circle connecting the upper end of the reinforcing portion to the seating surface of the head portion. DrifIs the outer diameter D of the screw0It is characterized by being smaller or the same.
[0027]
No. of this application6High strength according to the inventionThin headMachine screw4 or 5In any one of the inventions, the blade length m of the hole, which is the diameter of the locus circle where the blade of the engagement groove and the top surface of the head intersect, is the diameter of the valley of the screw formed in the screw portion. It is characterized by being added to be smaller than or equal to D.
[0028]
No. of this application7High strength according to the inventionThin headMachine screw4To the second6In any one of the inventions, the outer diameter D of the screw0D0≦ 1.0 mmFeatureIt is what has.
[0029]
No. of this application8High strength according to the inventionThin headMachine screw4To the second7In any of the inventions, the Vickers hardness HV is 250 or more.
[0030]
In the present specification, the surface layer portion of the screw portion in the present invention means a thickness to which the surface layer portion is hardened by a normal carburizing process performed when a nominal diameter is specified, An area from the surface to a depth of about 0.1 mm.
[0031]
No. of this application9High strength according to the inventionThin headMachine screw4To the second8In any of the inventions, the shape of the engagement groove is a cross hole or is equally divided into n on the circumference centered on the axis of the head (where n is It is a natural number of n = 3 or n ≧ 5) and is characterized by being radially formed holes.
[0032]
No. of this application10High strength according to the inventionThin headMachine screw4To the second9In any one of the inventions, the chemical component composition of the screw is characterized in that its C content is 0.010% by mass or less.
[Brief description of the drawings]
[0033]
FIG. 1 is a graph illustrating the relationship between rolling condition parameter Z and average ferrite grain size.
FIG. 2 is a front view showing an embodiment of a thin-headed screw according to the present invention.
3 is an enlarged photograph of 3% nital corrosion of a longitudinally polished surface passing through the axis of a machine screw of Example 1. FIG.
4 is an enlarged photograph of 3% nital corrosion of a longitudinally polished surface passing through the axis of a machine screw of Example 2. FIG.
5 is an enlarged photograph of 3% nital corrosion of a longitudinally polished surface passing through the axis of a machine screw of Example 3. FIG.
6 is an enlarged photograph of an overview of Example 1. FIG.
7 is an enlarged photograph of an overview of Example 2. FIG.
8 is an enlarged photograph of an overview of Example 3. FIG.
FIG. 9 is a front view for explaining an example of a screw with a thin head in the prior art.
10 is an enlarged front view of a main part with a partially cutaway cross section of FIG. 9. FIG.
11 is a longitudinal sectional view showing a use state in which the screw of FIG. 9 is incorporated.
FIG. 12 is a front view for explaining an example of a thin-headed screw in another conventional technique.
13 is a longitudinal sectional view showing a use state in which the screw of FIG. 12 is incorporated.
[Explanation of symbols]
[0034]
In addition, the code | symbol in a figure shows the following.
  1 screw
  2 head
  3 Thread
  4 Shaft
  5 engaging groove
  6 Top surface
  7 Seat
  8 Inner wall of engaging groove
  9 Incomplete thread
  10 Reinforcing part
  11 Arc-shaped part
  12 Cross hole
  13 Wings
  15 Machine screw
  16 Lower neck
  17 Fastened member
  18 work
  19 Pilot hole (Mounting hole)
BEST MODE FOR CARRYING OUT THE INVENTION
[0035]
The present invention has the configuration as described above and the characteristics related thereto. Therefore, next, the reason for limiting the basic embodiment of the present invention and the reason for limiting the aspect in the embodiment will be described.
[0036]
(1) Basic manufacturing method of the product of the present invention (regulation of processes consisting of a combination of hot working or warm working + cold working)
  In the method for producing a high-strength molded product and a high-strength machine screw according to the present invention, the essential constituent elements are ultra-fine structure steel having a ferrite structure of a predetermined grain size or less as a raw material, and a desired form is used using this. Forming a molded product or a machine screw. Here, the predetermined crystal grain size of the raw material is a cross section perpendicular to the final processing direction of this cold working, in which a steel ingot, cast slab, steel slab or semi-finished steel product is subjected to warm working and then further cold working. The average crystal grain size is 500 nm or less, preferably 200 nm or less.
  The reason for defining the particle size of the material in this way is that, if such a material is used, (1) the material can be processed into a compact such as a small screw without subjecting the material to a softening process such as spheroidizing annealing. Knowledge that (2) it is possible to obtain a product that has the original basic performance of molded products such as machine screws without subjecting the resulting molded product to toughening heat treatment such as surface hardening heat treatment. Because. Here, an important rule is given to the shape and size of the molded product such as the machine screw. That is, it is defined that the thickness of the thinnest portion of the molded product (referred to as “thickness of the thinnest thickness portion” in the present invention, expressed by Q) is 0.25 mm or less.
[0037]
Furthermore, it is necessary to define the crystal grain size of the material prepared by the above hot working or warm working, and the average crystal in the cross section perpendicular to the final working direction of the hot working or warm working. The particle size is specified to be 3 μm or less. Such hot working or warm rolled material is said to be cold worked under appropriate conditions, and by this cold working, the crystal grains in the cross section in the C direction are further refined. Steel is obtained, and its grain size is ultrafinened to 500 nm or less in a cross section perpendicular to the final working direction of the cold working process. Then, it is desirably further refined to 200 nm or less.
[0038]
In the above, the material after hot working or warm working is prepared as described above as fine-grained steel, and when cold working is performed using this steel, the material strength is remarkably increased. Fortunately, it has been found that the processability degradation is very small. This new knowledge, which has been difficult to predict in the past, occupies an important position for the completion of the present invention.
  In this way, if the material is used, it is not necessary to perform a softening treatment such as spheroidizing annealing of the material, and the reason for eliminating the need for a toughening heat treatment such as a surface hardening heat treatment of the molded body is This is because the tensile strength TS and the drawing RA are both excellent in mechanical properties of the material, that is, the level of both characteristic values is excellent while providing a good balance between both characteristic values.
[0039]
(2) Component composition of steel
In the manufacturing method of the present invention product, the chemical composition of the steel ingot, cast slab, steel slab or semi-finished steel product used as a material for warm working is a standard structure of steel regardless of whether it is a carbon steel component or a low alloy steel. In this case, the main phase must be ferrite or only ferrite, and the C content may be 0.45% by mass or less. In the present invention, as described above, by applying appropriate warm working to the steel semi-finished product and the like, the crystal grain size of the steel is reduced to a predetermined value or less without substantially using a strengthening mechanism by phase transformation. Next, by subjecting the fine-grained steel thus obtained to further required cold working, a finer ultrafine-structured steel is prepared, thereby obtaining a material having high strength and high ductility. .
[0040]
In the present invention, it is an important feature that a chemical component composition that does not cause martensitic transformation by quenching may be used. The reason is that if the other constituent requirements of the present invention are satisfied, the target tensile strength of the material after hot working or warm working and cold working is 800 MPa or more, desirably 900 MPa or more, more desirably 1200 MPa. This is because, as described above, and more desirably, 1500 MPa or more can be obtained, and a steel in which the drawing RA is also maintained at a high level according to the tensile strength can be obtained.
  In particular, high strength can be obtained even with an ultra-low carbon steel component having a C content of 0.010% by mass or less, and more preferably less than 0.002% by mass. Here, if C content is less than 0.002 mass%, annealing to the material after cold working becomes unnecessary.
The mechanical properties excellent in the balance between high strength and high ductility are largely dependent on the fact that hard cementite, which is a factor that deteriorates cold workability, is not substantially formed. .
[0041]
Note that it is not always easy to determine whether or not the molded article or the small screw according to the present invention is substantially cementite-free. Therefore, it can be accurately estimated by a quantitative analysis value of C content practical in daily operations. Therefore, it was stipulated that the C content is not more than the solid solubility limit of carbon in the ferrite phase at the point Ae1 from the metal phase judgment. Furthermore, in a normal low alloy steel or carbon steel component system, the C content range that is considered not to produce cementite is 0.010% by mass or less, and more preferably, from the viewpoint of not requiring annealing. Less than 002% by mass
  In the above, since it is below the solid solution C concentration (mass%) in the ferrite phase at the point Ae1, it is a cementite-free structure. In any of carbon steel and low alloy steel, the C concentration (mass%) at which this cementite free is obtained can be calculated using, for example, calculation software Thermo-calc.
[0042]
Thus, in the present invention, the steel material having a cementite-free ferrite structure has the above-described high strength and excellent cold workability (excellent balance between strength and workability). High strength steel) can be designed. Conventionally, there has been no example in which a high-strength steel wire or steel bar excellent in cold workability by such a component design has been realized.
[0043]
In the present invention, in order to obtain high strength characteristics as described above, importance is attached to cementite-free steel. Therefore, it is not necessary to depend on the addition of the alloy element when defining the chemical component composition. Therefore, it is not necessary to add the elements that promote the improvement of hardenability, such as Cr and Mo, and other similar elements, and the addition of the solid solution strengthening elements such as Cu and Ni, and other similar elements. In addition, it is desirable not to add the alloy elements from the viewpoint of reducing the manufacturing cost. Therefore, it is desirable that none of the above elements has a content exceeding that inevitably mixed in the steel refining / melting process. Furthermore, although not specifically defined in the present invention, it is not necessary to add Ti, Nb, and other alloy elements which are effective elements for precipitation strengthening. This is because the cementite-free component system of the present invention can ensure a sufficient tensile strength, which also helps to reduce manufacturing costs.
[0044]
In addition, regarding C content among the chemical components mentioned above, as long as the manufacturing method of the product of the present invention in the present invention is carried out, similar actions and effects can be obtained even if it exceeds 0.010% by mass. In this respect, the C content may be 0.45% by mass or less.
In addition, in the production process from the steel ingot, cast slab, steel slab or semi-finished steel product to high-strength molded product or small screw in the present invention, the various chemical composition compositions including C described above are substantially the same in content. It can be regarded as not changing.
[0045]
(3) Temperature and strain conditions in hot working or warm working and cold working
(3-1) Hot working or warm working conditions: As an embodiment of the manufacturing process of the high-strength molded product or small screw, first, warm working on a predetermined steel ingot, slab or steel slab or steel material Desirable conditions should be such that the processing temperature is in the range of 350-800 ° C. Furthermore, the plastic strain introduced into the material and remaining there should be ensured. The amount of plastic strain can be obtained by calculation using a known three-dimensional finite element method. When the value is expressed as “ε”, it is desirable that ε ≧ 0.7 or more. Such warm working conditions were adopted because the average grain size in the cross section in the C direction was 3 μm as a method for realizing high strength and high ductility of steel without substantially using a strengthening mechanism by phase transformation. This is to make the following.
[0046]
Note that, in the above hot working or warm working conditions, instead of using ε as an index as a strain, a strain of a material that can be obtained relatively easily in operation (“industrial strain” in the specification of the invention of this application) It can be practically replaced by “e”. The industrial strain e is a function of the total area reduction ratio R of the material, and the following equation (1):
e = -ln (1-R / 100) (1)
It is represented by However, R is the following formula (2):
R = {(S0-S) / S0} × 100 (2)
However, R: the total area reduction (%) applied to the steel ingot, slab or steel slab or steel material
S0： C direction cross-sectional area of steel ingot, cast slab or steel slab or steel just before start of hot working or warm working
S: Cross-sectional area in the C direction of the material obtained after completion of hot working or warm working
Is the total area reduction ratio R.
  When the value of R corresponding to ε ≧ 0.7 is calculated using the above equations (1) and (2), R ≧ 50% is obtained. Therefore, in the hot working or warm working, instead of the plastic strain ε ≧ 0.7, the total area reduction ratio R ≧ 50% of the material may be adopted.
[0047]
On the other hand, the present inventors have focused on the fact that the average particle size of ultrafine grains formed by warm strong processing (large strain processing by one pass in warm) depends on the processing temperature and strain rate. As a rolling condition parameter, the following formula (3):
Z = log [(ε / t) exp {Q / (8.31 (T + 273))}]
…………………………………………………………………… (3)
Where ε is the average plastic strain
t: Time from the start to the end of rolling (S)
Q: Constant (when the crystal structure is bcc, 254000 J / mol)
T: Rolling temperature (° C.), in the case of multi-pass rolling, a temperature obtained by averaging the rolling temperature of each pass
A Zener-Holomon parameter represented by the following formula was introduced (however, expressed in a logarithmic form), and the crystal grain size was found to become finer as the rolling condition parameter Z increased. FIG. 1 illustrates the relationship between the rolling condition parameter Z and the average ferrite grain size. That is, FIG. 1 shows that a fine grain structure having an average ferrite grain size of 1 μm or less can be obtained by controlling the rolling so that Z ≧ 11. Therefore, by controlling the warm rolling temperature so as to satisfy Z ≧ 11, the average ferrite grain size of the material can be made less than 3 μm.
[0048]
Furthermore, as the warm working method, any of warm rolling and warm forging may be adopted, and in that case, a plurality of baths (in the case of warm forging, a plurality of forging schedules are used in plural directions). By processing, the plastic strain in the material can be made uniform, which is desirable.
[0049]
(3-2) Cold working conditions: Next, a desirable cold to be applied in advance to a material having a fine grain structure prepared by warm working as described above and having high strength and excellent workability. The cold working temperature is preferably a cold working temperature of less than 350 ° C. If a higher temperature is reached during cold working due to processing heat generation, the degree of increase in tensile strength decreases, which is undesirable. Next, it is necessary to ensure the residual strain introduced into the material by cold working according to the desired tensile strength. From such a viewpoint, it is desirable to perform cold working so that the plastic strain ε obtained by the three-dimensional finite element method is at least 0.05 or more. As a result, the cold-worked structure of the crystal exhibits a form stretched in the working direction, the grain size in the cross section in the C direction with respect to the working direction is also refined, and an increase in tensile strength is ensured. At that time, the amount of reduction of the aperture RA is kept small.
[0050]
In the cold working conditions described above, instead of using ε as an index as a processing amount, by mediating e which is an “industrial strain” explained by the above equation (1), a material corresponding to ε ≧ 0.05 is obtained. When the total area reduction ratio R is calculated, R ≧ 5% is obtained. Therefore, in the cold working, instead of the plastic strain ε ≧ 0.05, the total area reduction ratio R ≧ 5% may be adopted.
  In the cold working, any of the known cold wire drawing method and cold rolling method may be employed.
[0051]
(4) The thickness and crystal grain size of the thinnest wall thickness part
The molded product according to the present invention is intended for a product having the thinnest thickness portion of 0.25 mm or less. Such a molded product having a thin portion, which also requires high strength, and is not subjected to quenching treatment, tempering treatment, carburizing quenching / tempering or nitriding treatment However, it is sufficient that the ferrite crystal structure has an average crystal grain size of 500 nm or less in a cross section in at least one direction in the thinnest thick portion. And the molded article comprised with such ultra-fine structure steel is excellent also in ductility. If the average crystal grain size is 200 nm or less, it is desirable that the strength of the thinnest wall portion is further increased. Here, it is a desirable condition for such a molded product having a thin part such as a screw part. Further, in the case of a small screw having a particularly thin head. Is a desirable condition. In addition, the molded object thru | or small screw which has such a high intensity | strength and high ductility can be obtained with the manufacturing method based on this invention based on the manufacturing technique described in said (1)-(3).
[0052]
(5) Machine screw with thin head
FIG. 2 is a front view showing an embodiment of a screw with a thin head according to the present invention.
In FIG. 2, reference numeral 15 denotes a machine screw, which is composed of a head portion 2 and a shaft portion 4 that is integral with the head portion and has a screw thread 3. The head 2 of the machine screw 15 is formed with a cross-cone-shaped cross hole 12 as an engaging groove 5 on which the driver bit for transmitting the rotational driving force can be engaged on the axis of the shaft 4. The tip of the engaging groove 5 (11) reaches the shaft portion 4. Further, the inner wall 8 of the engagement groove 5 is inclined in the axial direction of the shaft portion 4 as it becomes deeper from the top surface 6 of the head portion 2 to the shaft portion 4.
  The shortest distance Qbs between the inner wall 8 of the engaging groove 5 and the seat surface 7 of the head 2 formed by the cross hole, and the threaded portion of the inner wall 8 of the engaging groove 5 and the shaft portion 4 (the lower neck portion 16 of the head 2). And at least one of them is formed to be 0.20 mm or less.
[0053]
The head 2 is formed so that its thickness is considerably thinner than that of a normal machine screw, and the thickness of the head 2 (represented by k) is the outer diameter of the screw (D01/5) to 1/6. Thus, for example, the outer diameter D of the screw0In the case of a small screw 15 (M1.2 to M1.4) of 1.2 to 1.4 mm, the head thickness k is about 0.24 mm, and the outer diameter D of the screw0When ≦ 1.0 mm, the head thickness k ≦ 0.2 mm. The outer diameter of head 2 (denoted by dk) is the outer diameter D of screw 1.0Is about 1.7 to 2.2 times. Therefore, in the case of M1.2, the outer diameter dk of the head 2 is about 2.5 mm, and in the case of M1.4, the outer diameter dk of the head 2 is about 2.7 mm.
[0054]
As above, the shortest distance QbsOr QshIs less than 0.20 mm, the minimum thickness of the engagement groove 5 is extremely thin, so it is important to secure the minimum breaking torque value when tightening the screw, and the thickness of the head is thin. It is necessary to avoid licking (breaking) the groove. On the other hand, the machine screw according to the present invention is not subjected to any heat treatment among the quenching process, the tempering process, the carburizing process, and the carbonitriding process, but includes at least one direction including the respective parts. Since the average crystal grain size in the cross section is made of ultra-fine structure steel having a ferrite structure of 500 nm or less, more preferably 200 nm or less, the minimum fracture torque value is ensured and licking is prevented.
[0055]
Next, between the head 2 and the shaft portion 4, a reinforcing portion 10 in which no screw is formed is formed. The reinforcing portion 10 has a truncated conical shape that slightly expands, and a diameter D of a trajectory circle whose upper end is connected to the seating surface 7 of the head 2.rifIs the outer diameter D of the screw0The reinforcement part 10 is formed so that it may become the following. Thus, Drif≦ D0Thus, a pilot hole (attachment hole) (FIG. 13) to be opened in a member to be fastened (not shown, but refer to reference numeral 17 in FIG. 13) to be fastened in contact with the lower side of the seating surface 7 of the head 2. No need to increase the diameter of the reference numeral 19). Moreover, the height of the reinforcement part 10 is made as low as possible. That is, since the thickness k of the head is reduced, the diameter of the locus circle where the wing 13 of the engagement groove 5 intersects the top surface 6 of the head 2, that is, the wing length m of the hole is shortened. Q which is the minimum thickness of the inner wall 8 of the groove 5bsAnd QshThis is to prevent the driver bit engaged with the engagement groove 5 from being sufficiently thin and to sufficiently transmit the rotational driving force of the driver bit to the shaft portion 4 of the screw. At this time, by forming the blade length m of the hole to be equal to or less than the diameter D of the valley of the screw, the QbsAnd QshThis is more desirable.
[0056]
Outer diameter D of the machine screw according to the present invention0Is preferably small from the viewpoint of the demand for downsizing and downsizing of products such as portable electronic devices. In the machine screw described above, D0Is more preferably 1.0 mm or less with respect to the above requirement. Small screw outer diameter D0The smaller Q is, the more QbsAnd QshTherefore, a higher strength machine screw is required. The machine screw according to the present invention satisfies such a demand for high strength by the configuration of the ultrafine structure steel in the thin portion.
And the form of the engagement groove of the machine screw according to the present invention is not limited to the cross shape so that the hole is not further licked at the time of screw tightening, and can be a three-pronged hole, In addition, it is possible to form a radial hole in which the circumference is equally divided into four or more. This also serves as a security measure to prevent the opening of equipment products.
【Example】
[0057]
Hereinafter, the invention of this application will be described in more detail with reference to examples.
Ingredient No. having the chemical composition shown in Table 1. Steels 1 and 2 were melted using a vacuum melting furnace and cast into a steel ingot.
[Table 1]
[0058]
Each obtained steel ingot was formed into a 12 mmφ bar steel by hot forging. From this, a raw material for warm rolling was collected, formed into 12 mmφ by multi-directional multi-pass caliber rolling in warm, and cooled with water to prepare a steel bar. The warm rolling was performed at a temperature of 550 ° C. and then formed into 6 mmφ within a rolling temperature range of 450 to 530 ° C. During this time, the rolling direction was changed by rotating the material around the longitudinal axis for each rolling pass, and multi-directional multi-pass rolling was performed. The total area reduction from 12 mmφ which is a material for warm rolling to 6 mmφ is 75%. The average plastic strain ε at this time can be regarded as ε = 2.9 with reference to the equations (1) and (2). The ferrite grain size of the 6 mmφ wire rod that was hot-rolled was 0.5 μm in the cross section in the C direction.
[0059]
Next, component No. prepared by the above warm rolling was used. Each of 1, 2 and 3 6 mmφ wire rods was prepared by cold drawing into three size steel wires of 1.15 mmφ, 1.0 mmφ and 0.8 mmφ. The drawing ratios at this time are 96%, 97%, and 98%, respectively, and the average plastic strain ε at this time is expressed as ε = 3.2, referring to the equations (1) and (2). It can be considered that ε = 3.5 and ε = 3.9. And said component No. The ferrite average particle diameters of 1.15 mmφ and 0.8 mmφ cold drawn from 1 are 105 nm and 88 nm, respectively, in the cross section in the C direction. The ferrite average particle diameter of 1.0 mmφ that was cold-drawn from 2 was 95 nm in the cross section in the C direction.
[0060]
Next, among these three types of steel wires, a 1.15 mmφ steel wire (component No. 1) to an M1.4 (nominal diameter 1.4 mm) thin headed screw with a 0.8 mmφ steel wire A machine screw with a thin head of M1.0 (nominal diameter 1.0 mm) was produced from (Component No. 1). Also, a machine screw with a thin head of M1.2 (nominal diameter 1.2 mm) was manufactured from a 1.0 mmφ steel wire (component No. 2). These machine screws are referred to as Example 1, Example 3, and Example 2, respectively. Tables 2 and 3 show the dimensions of each part of each machine screw which is a manufactured example. In Examples 1 to 3, the shortest distance Q between the inner wall of the engagement groove and the seating surface of the headbsOr the shortest distance Q between the inner wall of the engaging groove and the surface of the threaded portion of the shaft portionshIn the range of 0.12 to 0.20 mm. On the other hand, the thickness k of the screw head is in the range of 0.22 to 0.26 mm. That is, in each of the first to third embodiments, the Q is more than the head thickness k.bsOr QshThe thickness Q of the thinnest part is 0.12 mm in Example 1, 0.20 mm in Example 2, and 0.18 mm in Example 3, and is a screw having an extremely thin part. . Table 2 further shows the shortest distance Q between the inner wall of the engaging groove and the seating surface of the head.bs, And the shortest distance Q between the inner wall of the engagement groove and the surface of the thread portion of the shaft portion (including the neck portion of the head and the incomplete thread portion).shAmong these, the value of the average crystal grain size (ferrite grain size) of the cross section at the position corresponding to the part of the shorter distance (abbreviated as “the thinnest part of the engaging groove”) is also shown.
[0061]
[Table 2]
[0062]
[Table 3]
[0063]
Next, for each of the small screws of Examples 1, 2, and 3, enlarged photographs of the 3% nital corrosion surface are shown in FIG. 3 (Example 1) and FIG. (Example 2) and FIG. 5 (Example 3) are shown in FIG. 6, FIG. 7, and FIG. In addition, the shape of the hole blade of the engagement groove is a cross shape in the first and second embodiments, and a three-pronged shape in the third embodiment.
[0064]
On the other hand, the Vickers hardness HV at the following position was measured for each of the small screws of Examples 1 to 3. That is, (1) The inner wall nearest portion of the engagement groove, the outermost surface nearest portion and the thickness center portion of “the thinnest portion of the engagement groove” described in Table 2, (2) the nearest portion of the bottom surface of the engagement groove, ( 3) Measured at the immediate vicinity of the thread surface and the middle part of the thread, and (4) the shaft core at the center of the screw length. As a result, for all of Examples 1 to 3, the Vickers hardness HV was 250 or more and the engagement portion surface was 300 or more at all the positions of the measured values.
[0065]
With respect to the small screw of each of the above-described embodiments, the recess strength test of whether the engagement groove hole (recess) is not licked (whether it is not broken), the fracture torque test of the lower neck, and the delayed fracture test in the screw tightened state Was done. As a result, good results were obtained for all examples.
【Effect of the invention】
[0066]
Since the present invention has the configuration as described above, the following effects are exhibited. First, in the manufacture of high-strength molded products including thin-headed high-strength small screws, ferritic steel having ultrafine crystals is used as a material for processing into a molded body. Despite its high strength, this material is also excellent in ductility. Therefore, prior to processing the molded body, conventional softening treatment such as spheroidizing annealing, which takes a very long time, is performed. There is no need to apply to the material. Furthermore, high strength which is the material characteristic of the raw material is inherited in the molded body after processing, and ductility is inherited in a balanced manner with strength. Therefore, it is not necessary to perform heat treatment for toughening such as surface hardening heat treatment, quenching treatment, and tempering treatment, which has been conventionally performed, on the molded body after processing. And the molded article including the high strength machine screw with a thin head obtained is excellent in strength and toughness. In particular, in the manufacture of a molded product having a part with a very thin wall thickness, the thickness of the hardened layer can be set as in carburizing quenching / tempering treatment among surface hardening heat treatments performed to improve the toughness of the part. Even in the case where a difficult technique of having to control thinly within a predetermined range is required, such heat treatment is not necessary at all in the case of the product of the present invention. Therefore, a stable and high-quality molded product can be manufactured at a low cost, particularly for products such as machine screws with a thin head. Specifically, it is a thin-headed screw product that demonstrates the basic performance of a screw even if it is a small head screw (micro screw) with a small shaft diameter that is not specified in the current JIS. Is obtained. That is, even if the screw head thickness is reduced and the outer diameter of the screw is reduced, the engagement groove hole (recess) is not destroyed when the screw is tightened, and the screw breaking torque value by tightening is sufficient. And there is no delayed destruction after fastening. In this way, it can contribute to miniaturization, compactness, and weight reduction of precision electronic devices such as portable devices.

Claims (10)

最薄肉厚部の厚さＱが０．２５ｍｍ以下である肉厚部分を有する成形品であって、焼入れ、焼戻し、浸炭処理、及び浸炭窒化処理の内いずれの熱処理をも施されることなく、前記最薄肉厚部は、当該肉厚部領域における少なくとも１方向の断面における平均結晶粒径が２００ｎｍ以下のフェライト組織を有する超微細組織鋼で構成されていると共に、前記成形品の成形素材に対する球状化焼なまし等の軟化処理が不要であることを特徴とする、高強度成形品。It is a molded article having a thick portion where the thickness Q of the thinnest thick portion is 0.25 mm or less, without being subjected to any heat treatment among quenching, tempering , carburizing treatment, and carbonitriding treatment , The thinnest thick portion is made of ultrafine structure steel having a ferrite structure with an average crystal grain size in a cross section in at least one direction in the thick portion region of 200 nm or less, and spherical with respect to a molding material of the molded product. A high-strength molded product characterized in that it does not require softening treatment such as chemical annealing . 前記成形品は、その一部にねじ部が形成されたねじ部品であることを特徴とする、請求項１に記載の高強度成形品。  The high-strength molded article according to claim 1, wherein the molded article is a screw part in which a screw portion is formed. 前記成形品の化学成分組成は、そのＣ含有量が０．０１０質量％以下であることを特徴とする、請求項１または２に記載の高強度成形品。  3. The high-strength molded article according to claim 1, wherein the chemical component composition of the molded article has a C content of 0.010 mass% or less. 回転駆動力を伝達するドライバビットが係合する係合溝が形成された頭部と、ねじ部が形成された軸部とが一体化して形成された小ねじにおいて、
前記係合溝の内壁と前記頭部の座面との最短距離Ｑｂｓ、及び前記係合溝の内壁と前記軸部のねじ部（前記頭部の首下部及び不完全ねじ部を含む）の表面との最短距離Ｑｓｈの内、少なくともいずれか一方は０．２０ｍｍ以下であり、またねじ頭部の厚さｋが、ｋ≦０．２５ｍｍであり、焼入れ、焼戻し、浸炭処理、及び浸炭窒化処理の内いずれの熱処理をも施されることなく、
前記最短距離Ｑｂｓ又はＱｓｈが０．２０ｍｍ以下である部位は、当該部位における少なくとも１方向の断面における平均結晶粒径が２００ｎｍ以下のフェライト組織を有する超微細組織鋼で構成されていると共に、
前記小ねじの成形素材に対する球状化焼なまし等の軟化処理が不要であることを特徴とする、高強度薄頭小ねじ。In a machine screw formed integrally with a head portion formed with an engagement groove with which a driver bit for transmitting a rotational driving force is engaged and a shaft portion with a thread portion formed therein,
The shortest distance Q bs between the inner wall of the engaging groove and the seat surface of the head, and the threaded portion of the inner wall of the engaging groove and the shaft portion (including the lower neck portion of the head and the incomplete threaded portion) At least one of the shortest distances Q sh to the surface is 0.20 mm or less, and the thickness k of the screw head is k ≦ 0.25 mm. Quenching, tempering , carburizing treatment, and carbonitriding Without any heat treatment of the treatment ,
The portion where the shortest distance Q bs or Q sh is 0.20 mm or less is composed of ultrafine structure steel having a ferrite structure with an average crystal grain size in a cross section in at least one direction at the portion being 200 nm or less ,
A high-strength thin head machine screw characterized by not requiring a softening process such as spheroidizing annealing on the molding material of the machine screw. 前記頭部と前記軸部との間には、ねじが形成されていない補強部が形成されており、前記補強部の上端が前記頭部の座面に接続する軌跡円の直径Ｄｒｉｆは、ねじの外径Ｄ０よりも小さいか、又は同じであることを特徴とする、請求項４に記載の高強度薄頭小ねじ。Between the head part and the shaft part, a reinforcing part not formed with a screw is formed, and a diameter D rif of a locus circle connecting the upper end of the reinforcing part to the seating surface of the head part is wherein the or smaller than the outer diameter D 0 of the screw, or the same, high-strength thin head machine screw according to claim 4. 更に、前記係合溝の翼と前記頭部の頂面とが交叉する軌跡円の直径である穴の翼長さｍが、前記ねじ部に形成されたねじの谷の径Ｄよりも小さいか又は同一であることを特徴とする、請求項４又は５に記載の高強度薄頭小ねじ。  Further, whether the blade length m of the hole, which is the diameter of the locus circle where the blade of the engagement groove and the top surface of the head intersect, is smaller than the diameter D of the screw valley formed in the screw portion, The high-strength thin head machine screw according to claim 4 or 5, characterized in that they are the same. 前記ねじの外径Ｄ０は、Ｄ０≦１．０ｍｍであることを特徴とする、請求項４ないし６のいずれかに記載の高強度薄頭小ねじ。7. The high-strength thin head screw according to claim 4, wherein an outer diameter D 0 of the screw is D 0 ≦ 1.0 mm. ビッカース硬さＨＶは２５０以上であることを特徴とする、請求項４ないし７のいずれかに記載の高強度薄頭小ねじ。  The high-strength thin head machine screw according to any one of claims 4 to 7, wherein the Vickers hardness HV is 250 or more. 前記係合溝の形態は、十字穴であるか、又は前記頭部の軸芯を中心とする円周上に等間隔でｎ等分されて（但し、ｎは、ｎ＝３又はｎ≧５の自然数であって）放射状に形成された穴であることを特徴とする、請求項４ないし８のいずれかに記載の高強度薄頭小ねじ。  The form of the engaging groove is a cross hole or is equally divided into n on the circumference centered on the axis of the head (where n is n = 3 or n ≧ 5) The high-strength thin-head machine screw according to any one of claims 4 to 8, characterized in that the holes are radially formed. 前記ねじの化学成分組成は、そのＣ含有量が０．０１０質量％以下であることを特徴とする、請求項４ないし９のいずれかに記載の高強度薄頭小ねじ。  The high-strength thin head machine screw according to any one of claims 4 to 9, wherein the chemical component composition of the screw has a C content of 0.010% by mass or less.
JP2006547991A 2004-11-24 2005-11-24 Method for producing high-strength molded article, high-strength molded article and high-strength machine screw obtained thereby Expired - Fee Related JP5146869B2 (en)

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JP2001220650A (en) * 1999-11-30 2001-08-14 Sumitomo Electric Ind Ltd Steel wire, spring and producing method therefor
JP2001355045A (en) * 2000-06-12 2001-12-25 Nippon Steel Corp Rail excellent in wear resistance
JP2002285278A (en) * 2001-03-27 2002-10-03 Japan Science & Technology Corp High strength and high ductility steel sheet with hyperfine crystal grain structure obtainable by subjecting plain low carbon steel to low strain working and annealing and production method therefor
JP2004060046A (en) * 2002-06-05 2004-02-26 National Institute For Materials Science Molded part and method for producing the same

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