JP2004018879A

JP2004018879A - Steel for cold forging superior in swarf treatment property

Info

Publication number: JP2004018879A
Application number: JP2002171169A
Authority: JP
Inventors: Hiroshi Momozaki; 百▲崎▼　寛; Masami Somekawa; 染川　雅実; Masato Shikaiso; 鹿礒　正人
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 2002-06-12
Filing date: 2002-06-12
Publication date: 2004-01-22
Anticipated expiration: 2022-06-12
Also published as: JP4146167B2

Abstract

<P>PROBLEM TO BE SOLVED: To establish a technology for improving a machinability after being cold worked, particularly a swarf treatment property, which particularly aims at a steel to be machined after being cold worked. <P>SOLUTION: A steel for cold forging superior in the swarf treatment property after being cold worked comprises 0.001-0.01% B (% means mass% hereafter), 0.002-0.01% N and 0.005-0.10% Bi, and has 15 or more precipitates of BN and B-containing Bi with diameters of 0.7 μm or larger, per visual field of 0.5 mm×0.5 mm in a cross section of the steel. <P>COPYRIGHT: (C)2004,JPO

Description

【０００１】
【発明の属する技術分野】
本発明は、冷間鍛造（冷間圧造を含む、以下、本明細書において同じ）により所定の形状に冷間加工した後に切削加工される冷間鍛造用鋼に関し、特に、冷間加工後に切削加工を行なう際の切屑処理性が大幅に改善された冷間鍛造用鋼に関するものである。
【０００２】
【従来の技術】
冷間加工は、熱間加工に比べて生産性が高いうえに鋼材の歩留まりも良好であることから、ボルト、ナット、ねじ等の機械部品や電装部品などを製造するための加工法として汎用されている。
【０００３】
また、近年における冷間加工技術の向上はめざましいものがあり、ニアネットシェイプやネットシェイプによって仕上げ切削加工の手数を省略乃至軽減しようとする試みもなされている。しかし、最終製品に求められる精度や表面品位の要求を満たすには、仕上げ切削加工に頼らざるを得ないことも多く、そのため、冷間加工性と被削性の両立も重要な要求特性とされている。
【０００４】
また、冷間加工技術の進歩に伴って切削加工時の切削代は減少する傾向が見られる。その結果として切り込みが小さくなると、切屑は伸び易くなって絡まり易くなり、それが原因となって切削製品の表面品質が劣化したり、更には自動切削運転の停止を余儀なくさせられる、といった切屑処理性の問題が生じてくる。しかも、冷間加工により加工硬化した後では、切削加工時の剪断角が大きくなって切屑は更に薄くなり、切屑処理性の低下が一層顕著になってくる。そのため、焼入焼戻し等の熱処理を施さずに冷間加工してから切削加工を行なう場合には、切屑処理性の改善が大きな課題となる。
【０００５】
他方、被削性改善手段として、鋼中に硫黄を添加して鋼中にＭｎＳなどの硫化物を生成させ、或いは鉛を添加することが有効であることはよく知られている。しかし硫黄や鉛を多量添加すると、冷間加工性が低下するという問題が生じてくる。
【０００６】
こうした問題の解決策として、硫化物の形態制御を行なう方法があり、本件出願人も既に特開昭４９−５８０１９号、特開昭５０−７７１７号、特公昭５９−４７０２４号公報などに開示の技術を提案している。
【０００７】
即ち上記特開昭４９−５８０１９号や特開昭５０−７７１７号公報では、鋼中にＳと共にＺｒを含有させ、ＭｎＳ中にＺｒを固溶させて（Ｍｎ，Ｚｒ）Ｓとすることにより硫化物の変形能を低下させ、硫化物を丸く制御することで冷間加工性の改善を図っている。そして、Ｓを０．０４〜０．０９％と通常より多めに添加することで被削性を高めることを提案した。また特公昭５９−４７０２４号公報では、Ｃａを添加することによってＭｎＳを（Ｍｎ，Ｃａ）Ｓとし、上記Ｚｒと同様の作用により冷間加工性を改善すると共に、ＳとＣａの積極添加によって硬質酸化物（Ａｌ_２Ｏ_３）を低減し、それにより被削性を改善する方法を提案した。
【０００８】
この場合に添加されるＳ量は０．０１〜０．１５質量％の範囲であり、被削性の若干の改善ならば、この程度の少ないＳ添加で目的を果たすことができる。ところが、被削性の要求程度が高くなるとＳを多量添加しなければならず、それに伴う冷間加工性の低下が軽視できなくなる。
【０００９】
他方、ＢＮを被削性の向上に利用した例も幾つかあり、例えば特許第２７３３９８９号や、特開平１１−１７４６号、特開平３−２４０９３１号などが知られている。しかしこれらは、いずれも冷間加工後の切削加工時における切屑処理性の改善を意図するものではなく、上記特許第２７３３９８９号や特開平１１−１７４６号は、熱間圧延や熱間鍛造などの熱間加工性と被削性の両立を目的としている。そして、これらに開示された成分の規定だけでは、冷間加工の用途に適用しても十分な効果は得られない。また特開平３−２４０９３１号では、被削性向上のためＢＮを多量析出させることに主眼を置いてＮを０．０１％以上含有させているが、Ｎを多量に含有させると固溶Ｎ量の増大によって歪み時効が抑制できなくなり、冷間加工時の加工硬化が顕著になるばかりでなく、冷間加工前の強度も上昇するため、冷間加工性に及ぼす悪影響が問題になってくる。
【００１０】
【発明が解決しようとする課題】
本発明は上記の様な事情に着目してなされたものであって、特に冷間鍛造後に切削加工される鋼を対象とし、冷間加工後の被削性、特に切屑処理性を向上させることのできる技術を確立することにある。
【００１１】
【課題を解決するための手段】
上記課題を解決することのできた本発明に係る切屑処理性に優れた冷間鍛造用鋼とは、Ｂ：０．００１〜０．０１０％（質量％を表わす、以下同じ）とＮ：０．００２〜０．０１０％およびＢｉ：０．００５〜０．１０％を含む鋼からなり、当該鋼の横断面０．５ｍｍ×０．５ｍｍの視野面積当たりに、直径（平均直径：短径と長径の平均値を意味する、以下同じ）０．７μｍ以上のＢＮと、Ｂを含むＢｉとが総計で１５個以上存在するところに特徴を有している。
【００１２】
従って、本発明に係る切屑処理性に優れた冷間鍛造用鋼の特徴は、本質的に鋼中のＢ，ＮおよびＢｉの各含有率と、当該鋼断面内に存在するＢＮとＢｉの存在形態を特定した点に存在するが、その特徴が実用鋼として有効に発揮されるのは、当該鋼が、Ｃ：０．００５〜０．５０％を含み、且つ、Ｓｉ：１％以下、Ｍｎ：２％以下、Ｐ：０．０３％以下、Ｓ：０．０７％以下、Ａ１：０．１％以下、Ｃｒ：１．６％以下にそれぞれ制限され、残部が実質的にＦｅからなるものであり、あるいは必要により、他の元素として
１）Ｃｕ：２．０％以下、Ｎｉ：２．０％以下、Ｍｏ：１．０％以下のうち１種または２種以上を含有させることにより、鋼としての強度を高めたもの、
２）Ｐｂ：０．１％以下を含有させて被削性を更に高めたもの、
３）Ｍｇ：０．０１％以下、Ｃａ：０．０１％以下、Ｚｒ：０．２％以下、Ｔｅ：０．１％以下のうち１種または２種以上を含有させ、硫化物形態を丸く形態制御することで冷間加工性を更に高めたもの、
等が、実用的な好ましい冷間鍛造用鋼として推奨される。
【００１３】
【発明の実施の形態】
本発明の冷間鍛造用鋼は、冷間加工性と被削性を両立せしめ得るものであるが、被削性の中でも特に切屑処理性に主眼をおいている。そして本発明者らは、前述した従来例の如く、被削性向上を快削成分の添加のみに頼るのではなく、冷間加工により加工硬化した後で鋼を切削加工すると切屑処理性が著しく低下することに注目した。そして、冷間加工時の加工硬化を低減させるため、加工硬化を助長する固溶ＮをＢ添加によりＢＮとして固定し、更には、適量のＢｉを含有させると共に、該ＢｉはＢを含んだ状態で鋼母材中に析出させることで、加工硬化を抑制しつつ切屑処理性を向上させることに成功したものである。
【００１４】
ところで、ＢＮが鋼中に微細析出して被削性向上に有効に作用することは以前から知られているが、冷間鍛造用鋼として冷間加工後に切削加工される際の切屑処理性を高めるには、このＢＮを比較的粗大なものとして存在させることがより有効となることを突き止めた。また、適量のＢｉを含有させることで被削性が向上することも知られているが、該Ｂｉについては、その中にＢを含有させた状態で析出させると共に、そのサイズも比較的粗大なものとして存在させることが有効であることを見出し、適切な圧延条件の制御によってこれを達成したものである。
【００１５】
先ず本発明者らは、冷間加工後の被削性、特に切屑処理性の向上に好適な鋼の開発を期して鋭意研究を進めてきた。その結果、Ｂ添加により切屑処理性に優れた冷間鍛造用鋼が得られることを確認した。
【００１６】
鋼の切屑処理性が、ＳやＰｂの如き快削成分の添加によって向上することはよく知られている。しかし、これらの元素を添加し過ぎると冷間加工性が低下するので、冷間鍛造用鋼への積極的な添加は好ましくない。そこで、ＳやＰｂの添加以外の解決策について検討を進めた。
【００１７】
ところで、同一成分の鋼であっても、硬質になるほど切屑処理性は低下する。このため、冷間鍛造用鋼の如く冷間鍛造による冷間加工後に切削加工される鋼材では、冷間加工時の加工硬化によって硬質化し切屑処理性が低下する。そこで、切削加工性を改善するには、加工硬化による硬さ上昇を抑えることが有効であると考え、その線に沿って研究を進めたところ、Ｂ添加によりＮをＢＮとして析出させれば、加工硬化に影響を及ぼす固溶Ｎが減少し、加工硬化による硬さの上昇が抑えられて切屑処理性を向上せしめ得ることが確認された。
【００１８】
また、Ｂｉを添加した鋼の場合、Ｂｉは母相内に固溶しないため母相内に個別に存在することになるが、Ｂｉは融点の低い元素であるため、単に添加して圧延すると、展伸した状態で母相中に存在することになる。このため被削性は向上するものの冷間鍛造性を劣化させる。ところが、ＢとＮを含む鋼材中にＢｉを複合添加すると、ＢＮの生成と共にＢｉがＢを含んだ状態で析出し、冷間鍛造性を劣化させることなく切屑処理性の向上に寄与することが確認された。
【００１９】
但しこれだけでは、冷間加工率を高めた場合に十分な効果が得られないことから、更なる切屑処理性向上対策について検討を重ねた結果、固溶Ｎを固定するために析出させたＢＮ析出物と上記Ｂ含有Ｂｉ析出物の存在形態を適正に制御することが極めて有効であるとの知見を得た。
【００２０】
ＢＮやＢｉが被削性の改善に好結果をもたらすことは知られているが、ＢＮ析出物やＢｉは一般に非常に微細であり、単に析出させただけでは、冷間加工後の切屑処理性に十分な作用を与えることはできない。そこで、圧延条件を主体にして更に追究を重ねた結果、ＢＮおよびＢ含有Ｂｉよりなる直径０．７μｍ以上の析出物が、鋼材横断面０．５ｍｍ×０．５ｍｍの視野面積当たりに総計で１５個以上存在するものは、安定して優れた切屑処理性を示すことが確認された。
【００２１】
上記ＢＮ析出物とＢ含有Ｂｉのサイズと個数を規定した理由は、直径０．７μｍ以上のＢＮ析出物やＢ含有Ｂｉだけが被削性に寄与するということではなく、冷間加工後の鋼の切屑処理性に及ぼすＢＮ析出物とＢ含有Ｂｉ析出物の影響を飛躍的に発揮させるため、制御圧延なしでは微細で冷間加工後の切屑処理性向上への寄与が小さいＢＮやＢ含有Ｂｉを全体的に粗大化させることの目安として、直径０．７μｍ以上のサイズのＢＮとＢ含有Ｂｉ析出物を標準的に選択し、その数を鋼材横断面０．５ｍｍ×０．５ｍｍの視野当たり１５個以上と定めているのである。
【００２２】
ちなみに後記図８は、直径０．７μｍ以上のＢＮおよびＢ含有Ｂｉ析出物の横断面０．５ｍｍ×０．５ｍｍ視野当たりに存在する個数が切屑処理性に与える影響を整理して示したグラフであり、このグラフからも、直径０．７μｍ以上のＢＮおよびＢ含有Ｂｉ析出物の個数が切屑処理性と密接な相関性を有していることを確認できる。そして本発明者らは、冷間鍛造用鋼として実用上満足の行く切屑処理性を有していると評価できるのは、切屑処理性指数で１０超あればよいことを確認しており、この値を満たすには、上記サイズのＢＮおよびＢ含有Ｂｉ析出物数の総計で、横断面０．５ｍｍ×０．５ｍｍ視野当たり１５個以上を確保すればよいことが分かる。
【００２３】
次に、本発明で鋼の化学成分を定めた理由を明確にする。
【００２４】
「Ｂ：０．００１〜０．０１０％」
Ｂは、本発明のポイントとなる元素で、ＢＮ析出物を生成し、固溶Ｎによる歪み時効を抑制して加工硬化による硬さの上昇を抑えると共に、Ｂｉ析出物中にも混入した状態で存在し、冷間鍛造性を劣化させることなく切屑処理性の向上に寄与する。こうした作用を有効に発揮させるには０．００１％以上含有させねばならない。但し、Ｂ含有量が多くなり過ぎると熱間延性を著しく低下させるので、その上限を０．０１０％と定めている。Ｂのより好ましい含有量は０．００１５％以上、０．００８％以下である。
【００２５】
「Ｎ：０．００２〜０．０１０％」
Ｎは、上記の様に鋼中に固溶Ｎとして存在することで冷間加工性に悪影響を及ぼす元素であるが、本発明では、Ｂとの反応によりＢＮを析出させて切屑処理性を高めるのに重要な元素であり、少なくとも０．００２％以上含有させなければならない。しかし多過ぎると、過剰分は固溶Ｎとして残存し冷間加工性を劣化させるので、０．０１０％以下に抑えるべきである。Ｎのより好ましい含有量は０．００３％以上、０．００８５％以下である。
【００２６】
「Ｂｉ：０．００５〜０．１０％」
Ｂ，Ｎと共にＢｉを含有させる場合、Ｂｉ添加による被削性向上効果は０．００５％以上の添加で有為に発揮される。しかしＢｉ添加量が多過ぎると、Ｂの共存にも関わらず粒状化しないで展伸し、冷間鍛造性を著しく害する傾向が現れてくる。よってＢｉの添加量は０．００５％以上、０．１０％以下にすべきであり、より好ましくは０．０１％以上、０．０８％以下とすることが望ましい。
【００２７】
上記の様に本発明の基本思想は、Ｂ、ＮおよびＢｉの各含有量を規定すると共に、横断面中に存在するＢＮとＢ含有Ｂｉの析出物のサイズと個数を定めたところに特徴を有しているが、これらの特徴を実用鋼材として有効に発揮させるには、鋼材自体の基本成分や許容される合金元素なども規定しておくことが望ましいので、以下、それらの元素についても説明する。
【００２８】
「Ｃ：０．００５〜０．５０％」
Ｃは、最終鍛造製品の強度を確保するために有用な元素で、０．００５％未満では強度不足になることがあり、一方０．５０％を超えると強度が高くなり過ぎて冷間加工性が劣化する。Ｃのより好ましい範囲は０．００７％以上、０．４５％以下である。
【００２９】
「Ｓｉ：１％以下（０％を含む）」
Ｓｉは、脱酸剤として有用な元素であるが、過剰量になると固溶強化により冷間加工性や被削性を劣化させるので、１％以下、より好ましくは０．７％以下に抑えるべきである。
【００３０】
「Ｍｎ：２％以下（０％を含む）」
Ｍｎは、強化元素として作用する他、鋼中のＳと結合してＭｎＳを形成し被削性の向上にも寄与する有用な元素である。しかし多過ぎると、強度が高くなり過ぎて冷間加工性を害するので、２％以下、より好ましくは１．５％以下に抑えるのがよい。
【００３１】
「Ｐ：０．０３％以下（０％を含む）」
Ｐは、粒界に偏析して鋼の延性を低下させる有害な元素であり、少ない方が望ましい。しかし、ある程度以上は不可避的に混入してくるので、０．０３％以下、より好ましくは０．０２％以下に抑えるべきである。
【００３２】
「Ｓ：０．０７％以下（０％を含む）」
Ｓは、被削性向上に有効な元素であるが、多過ぎると冷間加工性を著しく劣化させる。本発明では、被削性向上にＳを積極的に活用しないので、その有害作用を排除するため０．０７％以下、より好ましくは０．０６％以下に抑えるのがよい。但し、少量の積極添加は被削性の一層の向上に有効であり、好ましくは０．００３％以上、更に好ましくは０．００５％以上含有させることが望ましい。
【００３３】
「Ｃｒ：１．６％以下（０％を含む）」
Ｃｒは、鍛造製品に所定の強度を与えるのに有用な元素であるが、多過ぎると鋼が硬質化して冷間加工性や被削性に悪影響を及ぼすので、１．６％以下、より好ましくは１．２％以下、更に好ましくは０．５％以下に抑えるのがよい。
【００３４】
「Ａｌ：０．１％以下（０％を含む）」
Ａｌは脱酸剤として有用な元素であるが、過剰量になると酸化物系の介在物源となって冷間加工性を劣化させるので、０．１％以下、より好ましくは０．０５％以下に抑えるのがよい。
【００３５】
「Ｃｕ：２．０％以下（０％を含まない）、Ｎｉ：２．０％以下（０％を含まない）およびＭｏ：１．０％以下（０％を含まない）から選ばれる１種以上」
Ｃｕ，Ｎｉ，Ｍｏは、共に強度向上元素として有用な元素であるが、多過ぎると被削性などに悪影響を及ぼすので、鍛造製品の用途や要求特性に応じて適宜選択して適量添加することが望ましい。何れにしても、被削性などに与える悪影響を抑えるには、Ｃｕは２．０％以下、Ｎｉは２．０％以下、Ｍｏは１．０％以下にそれぞれ抑えることが望ましい。
【００３６】
「Ｐｂ：０．１％以下（０％を含まない）」
Ｐｂは、前記Ｂｉと同様に代表的な低融点金属であり、被削性の向上に有効に作用する。従って、被削性がより重視される場合には積極的に添加することも有効である。しかし、過多に添加すると冷間加工性を劣化させるので、０．１％以下に抑えるべきである。また最近では公害防止の観点からＰｂフリー鋼の要望が強いため、今後はむしろ忌避されるものと考えている。
【００３７】
「Ｍｇ：０．０１％以下（０％を含まない）、Ｃａ：０．０１％以下（０％を含まない）、Ｚｒ：０．２％以下（０％を含まない）、Ｔｅ：０．１％以下（０％を含まない）から選択される１種以上」
Ｍｇ，Ｃａ，Ｚｒ，Ｔｅは、硫化物の形態制御のため補完的に添加することができる。但し、過度に添加してもその効果は飽和するためそれぞれ上限値までの添加に止めるべきである。
【００３８】
ところで、本発明における最大の特徴である前記ＢＮやＢ含有Ｂｉ析出物の形態は、上述した如き適切な化学成分の鋼片を適切な条件で圧延・冷却・巻取りを行なうことによって確保できるので、好ましくは８５０〜１０５０℃の範囲まで加熱し、７２５〜１０００℃の範囲で所定の線径まで圧延した後、水流等によって６００〜６０００℃／分の冷却速度で７５０〜９５０℃まで急冷し、引き続き６００℃までを１℃／秒以下の冷却速度で徐冷することが好ましい。以下、それら各条件を推奨する理由を説明する。
【００３９】
「鋼片の加熱温度：８５０〜１０５０℃」
鋼片の加熱温度が高過ぎると、固溶Ｎ量が増大してＢＮの析出が制限される他、Ｂ含有Ｂｉも微分散し過ぎて切屑処理性向上効果が有効に発揮されなくなるので、１０５０℃以下に抑えるのがよく、より好ましくは１０００℃以下である。但し、加熱温度が低過ぎると圧延時の圧延ロール負荷が増大するなどの障害が現れてくるので、８５０℃以上が好ましい。より好ましい加熱温度は９００℃以上である。
【００４０】
「圧延温度：７２５〜１０００℃」
圧延温度は、窒化物の固溶を防止するために設定したもので、あまりに高温で行なうことは避けるべきである。また、圧延ロールの負荷増大、寸法精度の低下、表面疵の発生等を防止するという観点も考慮して、実用上は７５０℃以上、１０００℃以下に制御することが望ましい。より好ましい圧延温度は７７５℃以上、９７５℃以下である。
【００４１】
「巻取り温度：７５０〜９５０℃」
最終圧延後の巻取りに当たっては、代表的には水を冷媒とし６００〜６０００℃／分の冷却速度で７５０〜９５０℃まで冷却するのがよく、巻取り温度が９５０℃を超えると、ＢＮの析出が遅くなる。一方、巻取り温度が低過ぎると、鋼材組織が硬くて脆くとなり、冷間加工に適さなくなる。実操業レベルでのより好ましい巻取り温度は、８００℃以上、９２５℃以下である。
【００４２】
「冷却速度：１℃／秒以下（６００℃まで）」
本発明で採用される製造条件で最も重要となるのが冷却速度であり、冷却工程でＢＮやＢ含有Ｂｉを粗大な析出物として生成させるには冷却速度を遅くすることが望ましく、６００℃までを１℃／秒以下、より好ましくは０．５℃／秒以下、更に好ましくは０．４℃／秒以下に抑えることが望まれる。
【００４３】
本発明は以上の様に構成されており、鋼中に適正量のＢ，Ｎ，Ｂｉを含有させると共に、該鋼中に比較的粗大なＢＮやＢ含有Ｂｉを多量存在させることによって、冷間鍛造後においても優れた切屑処理性を示す冷間鍛造用鋼を提供し得ることになった。
【００４４】
【実施例】
以下、実施例を挙げて本発明をより具体的に説明するが、本発明はもとより下記実施例によって制限を受けるものではなく、前・後記の趣旨に適合し得る範囲で適当に変更を加えて実施することも可能であり、それらはいずれも本発明の技術的範囲に包含される。
【００４５】
実施例１
高周波溶解炉を使用し、表１に示す化学成分の鋼材を溶製した。発明鋼１と比較鋼１はＳ１０Ｃをベースとし、発明鋼２と比較鋼２はＳ２０Ｃをベース、発明鋼３と比較鋼３はＳ３５Ｃをベースにしたものである。各鋼材を溶製してから鋳造し、各鋳片を熱間鍛造により１５５ｍｍ角の鋼片とした後、さらに加熱してから熱間圧延を行って線材に圧延した。圧延は、各鋼種とも直径８．５ｍｍ、直径９．５ｍｍ、直径１２．５ｍｍの３つのサイズに圧延し、その後、直径８ｍｍに冷間引き抜きを行なって磨棒を作製し、各々について自動切削盤を用いて切屑処理性の評価を行なった。
【００４６】
【表１】

【００４７】
本発明の特徴は、冷間鍛造や冷間圧造などにより冷間加工を付与した後の切削加工時の切屑処理性を高めたところにあり、実用上は冷間鍛造や冷間圧造などによって成形された部品に対し切削加工を施す際にその特徴が有効に発揮される。ただし、実際の部品では歪み分布が一様でなく、切削部位の違いによる影響や部品形状の影響も受けるため、切削加工性を定量的に評価することは難しい面がある。このため本実施例では、定量的な効果をより明確にするため、圧延後、線径を細くする冷間引き抜きによって一様な歪みを与えた磨棒の状態で切削試験を行なった。また、冷間加工率の影響を把握するため、磨棒の線径は直径８ｍｍに統一し、冷間加工前の圧延径を直径８．５ｍｍ〜１２．５ｍｍに変化させたときの影響も評価した。ちなみに、圧延径の直径が８．５ｍｍであるときの冷間加工率は１２％、直径９．５ｍｍであるときの冷間加工率は３０％、直径１２．５ｍｍであるときの冷間加工率は６０％となる。
【００４８】
表２に圧延条件と切削試験結果を示す。圧延は、発明鋼、比較鋼とも本発明で推奨する前記要件を満たす条件で実施した。また、ＢＮとＢ含有Ｂｉ析出物の個数の測定法は下記の様にして行なった。
【００４９】
すなわち、圧延後の線材の横断面を走査型電子顕微鏡で０．５ｍｍ×０．５ｍｍの視野を観察し、直径０．７μｍ以上のＢＮとＢ含有Ｂｉについて、ＥＰＭＡを用いて組成分析を行い、測定視野内のＢＮとＢ含有Ｂｉ析出物の個数を求めた。
【００５０】
結果は表２に示す通りであり、本発明鋼では、０．５ｍｍ×０．５ｍｍの視野面積当たりに直径０．７μｍ以上のＢＮとＢ含有Ｂｉ析出物が１５個以上存在している。なお切削試験は、直径８ｍｍの磨棒を自動盤にて切削し、切屑処理性を評価した。切削条件は表３に示す通りで、冷間鍛造後の仕上げ切削を想定して切り込みを０．５ｍｍと小さくし、送り速度を４水準変化させた。そして各条件で切屑を採取し、各切屑片の形態により図１に示す評価点に基づいて、４条件の評価点の合計を切屑処理性指数とし、この値の大小で被削性の良否を判断した。
【００５１】
【表２】

【００５２】
【表３】

【００５３】
図２〜図４は、Ｓ１０Ｃ，Ｓ２０Ｃ，Ｓ３５Ｃを夫々ベース鋼とした冷間加工率と硬さの関係を示したものである。
【００５４】
発明鋼は、歪み時効抑制により加工硬化が抑えられているため、比較鋼よりも硬さが低減していることが分かる。図５〜図７はＳ１０Ｃ，Ｓ２０Ｃ，Ｓ３５Ｃを夫々ベース鋼とした冷間加工率と切屑処理性指数の関係を示したもので、発明鋼は加工硬化抑制とＢＮおよびＢ含有Ｂｉ析出物の制御により切屑処理性が向上していることを確認できる。
【００５５】
実施例２
表４に示すＳ２０Ｃベース鋼を溶製・鋳造後、直径９．５ｍｍに圧延してから、冷間加工率３０％で引き抜いて直径８ｍｍの磨棒を作製し、上記実施例１と同様の評価を行なった。鋼Ｎｏ．４〜６は、Ｂ添加量を変化させてＢＮおよびＢ含有Ｂｉ析出物の量を変動させている。鋼Ｎｏ．７はＣｕ，Ｎｉを含有させたもの、鋼Ｎｏ．８はＭｇ、鋼Ｎｏ．９はＣａを含有させたものである。
【００５６】
圧延条件と切削試験および冷間鍛造性試験の結果を表５に示す。鋼Ｎｏ．５は本発明で推奨する適正範囲内の条件を採用したもの（５−１）、冷却速度が適正範囲を外れるもの（５−２）、加熱温度が適正範囲を外れるもの（５−３）で、これら以外は前記適正範囲内の条件で圧延した。
【００５７】
尚、冷間鍛造性は次の様にして求めた。すなわち図９に示す如く、直径８ｍｍの磨棒を長さ１２ｍｍ（ｈ_０）に切断して試験片とし、これを縦方向に圧縮変形させ、圧縮片が割れを起こすまでの限界高さ（ｈ_１）を求め、これらの値から［圧縮率（％）＝１００×（Ｈ_０−Ｈ_１）／Ｈ_０］によって限界圧縮率を算出する。
【００５８】
【表４】

【００５９】
【表５】

【００６０】
表４，５において（５−２）は、圧延後の冷却速度が速いためＢＮおよびＢ含有Ｂｉ析出物を大きく制御できていない。（５−３）は、加熱温度が高過ぎたため固溶Ｎが多くなり十分な数のＢＮおよびＢ含有Ｂｉ析出物が析出していない。
【００６１】
表５の結果をまとめて図８に示す。この図からも明らかな様に、切屑処理性に及ぼすＢＮおよびＢ含有Ｂｉ析出物の影響は、直径０．７μｍ以上のＢＮおよびＢ含有Ｂｉ析出物の数が多いほど顕著に表われている。また、Ｂ添加量が多いほどＢＮおよびＢ含有Ｂｉ析出物量は多くなる（表４，５）。
【００６２】
鋼Ｎｏ．４，（５−１），６は、ＢＮおよびＢ含有Ｂｉ析出物量でほぼ整理でき、ＢＮおよびＢ含有Ｂｉ析出物量が多くなるほど切屑処理性は明らかに向上している。これらに対し鋼Ｎｏ．（５−２），（５−３）は、ＢＮおよびＢ含有Ｂｉ析出物が適正に制御されていないため切屑処理性が悪い。また、少量のＭｇやＣａを添加した鋼Ｎｏ．８，９も、十分量のＢＮおよびＢ含有Ｂｉ析出物量が生成しているため高い切屑処理性を示している。
【００６３】
なお図１０，１１は、上記表４に示した鋼Ｎｏ．４の線材の横断面を研磨してから観察した走査型電子顕微鏡写真（２５００倍）、図１２，１３，１４は、上記図１０，１１における符号Ａ，Ｂ，Ｃの析出物について、それぞれＥＰＭＡによる組成分析を行なった結果を示すチャートである。また図１５は、同じく表４に示した鋼Ｎｏ．８の横断面を示す走査型電子顕微鏡写真、図１６は、図１５に符号Ｄとして現れる析出物のＥＰＭＡ組成分析チャートであり、これらの図からも明らかな様に、母相に不溶物として析出するＢｉ粒子の中には全て少量のＢが含まれていることを確認できる。
【００６４】
【発明の効果】
本発明は以上のように構成されており、冷間加工後に切削加工される鋼材に対し、冷間加工に伴う切屑処理性の低下を未然に解決することができる。よって、この鋼材を使用すれば、冷間加工後の切削加工の自動化をより安定的に実施することができ、生産性の向上に大きくな貢献できる。
【図面の簡単な説明】
【図１】実験で採用した切屑処理性評価点の基準を示す図である。
【図２】実施例で得た鋼の冷間加工率と硬さの関係を示すグラフである。
【図３】同じく、実施例で得た鋼の冷間加工率と硬さの関係を示すグラフである。
【図４】同じく、実施例で得た鋼の冷間加工率と硬さの関係を示すグラフである。
【図５】実施例で得た冷間加工率と切屑処理性指数の関係を示すグラフである。
【図６】同じく、実施例で得た冷間加工率と切屑処理性指数の関係を示すグラフである。
【図７】同じく、実施例で得た冷間加工率と切屑処理性指数の関係を示すグラフである。
【図８】供試鋼横断面の０．５ｍｍ×０．５ｍｍ視野当たりに観察される直径０．７μｍ以上のＢＮおよびＢ含有Ｂｉ析出物の個数が切屑処理性に与える影響を整理して示したグラフである。
【図９】各供試剤の冷間鍛造性評価試験法を説明するための図である。
【図１０】実験で得た発明鋼の横断面走査型電子顕微鏡写真である。
【図１１】
同じく実験で得た発明鋼の横断面走査型電子顕微鏡写真である。
【図１２】
図１０に符合Ａとして示した析出物のＥＰＭＡ分析チャートである。
【図１３】
図１０に符合Ｂとして示した析出物のＥＰＭＡ分析チャートである。
【図１４】
図１１に符合Ｃとして示した析出物のＥＰＭＡ分析チャートである。
【図１５】
実験で得た他の発明鋼の横断面走査型電子顕微鏡写真である。
【図１６】
図１５に符合Ｄとして示した析出物のＥＰＭＡ分析チャートである。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a steel for cold forging that is cold-worked into a predetermined shape by cold forging (including cold-forging, hereinafter the same in the present specification), and in particular, is cut after cold-working. The present invention relates to a steel for cold forging, which has significantly improved chip disposability during processing.
[0002]
[Prior art]
Cold working is generally used as a processing method for manufacturing mechanical parts such as bolts, nuts, screws, etc., and electrical parts because it has higher productivity and better yield of steel materials than hot working. ing.
[0003]
In recent years, there has been a remarkable improvement in cold working technology, and attempts have been made to reduce or reduce the number of finish cutting processes by using near net shape or net shape. However, in order to satisfy the demands for accuracy and surface quality required for final products, it is often necessary to rely on finish cutting, and therefore, both cold workability and machinability are important characteristics. ing.
[0004]
Also, with the progress of cold working technology, the cutting allowance during cutting tends to decrease. As a result, when the depth of cut is small, the chips are easy to grow and become entangled, which causes the surface quality of the cutting products to deteriorate and also causes the automatic cutting operation to be stopped. The problem arises. Moreover, after work hardening by cold working, the shear angle at the time of cutting becomes large, the chips become thinner, and the reduction in chip processing becomes more remarkable. Therefore, in the case where cutting is performed after cold working without performing heat treatment such as quenching and tempering, improvement of chip disposability becomes a major issue.
[0005]
On the other hand, it is well known that as a means of improving machinability, it is effective to add sulfur to steel to generate sulfides such as MnS in steel or to add lead. However, when a large amount of sulfur or lead is added, there arises a problem that the cold workability is reduced.
[0006]
As a solution to such a problem, there is a method of controlling the form of sulfide. The present applicant has already disclosed in Japanese Patent Application Laid-Open Nos. 49-58019, 50-7717 and 59-47024. Propose technology.
[0007]
That is, in the above-mentioned JP-A-49-58019 and JP-A-50-7717, sulfide is contained by adding Zr together with S in steel and dissolving Zr in MnS to form (Mn, Zr) S. The cold workability is improved by reducing the deformability of the material and controlling the sulfide to be round. Then, it has been proposed that the machinability is increased by adding S in a larger amount than usual, such as 0.04 to 0.09%. In Japanese Patent Publication No. 59-47024, MnS is changed to (Mn, Ca) S by adding Ca, cold workability is improved by the same action as that of Zr, and hardness is increased by positive addition of S and Ca. Oxide (Al ₂ O ₃ ) Have been proposed to reduce the consequent machinability.
[0008]
The amount of S added in this case is in the range of 0.01 to 0.15% by mass, and if the machinability is slightly improved, the purpose can be achieved with such a small amount of S addition. However, when the required degree of machinability increases, a large amount of S must be added, and the accompanying decrease in cold workability cannot be neglected.
[0009]
On the other hand, there are some examples in which BN is used for improving machinability, and for example, Japanese Patent No. 2733989, Japanese Patent Application Laid-Open No. 11-1746, and Japanese Patent Application Laid-Open No. 3-249931 are known. However, none of these is intended to improve the chip disposability at the time of cutting after cold working, and the above-mentioned Japanese Patent No. 2733989 and Japanese Patent Application Laid-Open No. 11-1746 disclose hot rolling and hot forging. The purpose is to achieve both hot workability and machinability. And, only by defining the components disclosed therein, a sufficient effect cannot be obtained even when applied to cold working applications. In Japanese Patent Application Laid-Open No. 3-249931, N is contained in an amount of 0.01% or more for the purpose of precipitating a large amount of BN in order to improve machinability. Due to the increase in strain, strain aging cannot be suppressed, and not only work hardening during cold working becomes remarkable, but also strength before cold working increases, so that adverse effects on cold workability become a problem.
[0010]
[Problems to be solved by the invention]
The present invention has been made in view of the above-described circumstances, and is particularly directed to steel that is machined after cold forging, and to improve the machinability after cold working, particularly the chip disposability. It is to establish the technology which can do it.
[0011]
[Means for Solving the Problems]
The steel for cold forging according to the present invention which can solve the above-mentioned problems and has excellent chip controllability includes B: 0.001 to 0.010% (expressing mass%, the same applies hereinafter) and N: 0. 002 to 0.010% and Bi: 0.005 to 0.10%, and the diameter (average diameter: minor axis and major axis) per visual area of 0.5 mm × 0.5 mm in cross section of the steel. (The same applies to the following, the same applies hereinafter). A feature is that a total of 15 or more BNs of 0.7 μm or more and Bis containing B exist.
[0012]
Therefore, the characteristics of the steel for cold forging having excellent chip controllability according to the present invention are essentially the contents of B, N and Bi in the steel, and the existence of BN and Bi existing in the steel cross section. Although it is present at the point where the form is specified, the characteristic is exhibited effectively as a practical steel because the steel contains C: 0.005 to 0.50%, Si: 1% or less, Mn : 2% or less, P: 0.03% or less, S: 0.07% or less, A1: 0.1% or less, Cr: 1.6% or less, with the balance being substantially Fe Or, if necessary, as other elements
1) one containing at least one of Cu: 2.0% or less, Ni: 2.0% or less, and Mo: 1.0% or less, thereby increasing the strength as steel;
2) Pb: containing 0.1% or less to further enhance machinability,
3) One or more of Mg: 0.01% or less, Ca: 0.01% or less, Zr: 0.2% or less, Te: 0.1% or less are contained, and the sulfide form is rounded. The one that further improves cold workability by controlling the shape,
Etc. are recommended as a practical preferred cold forging steel.
[0013]
BEST MODE FOR CARRYING OUT THE INVENTION
The steel for cold forging of the present invention can achieve both cold workability and machinability, but particularly focuses on chip disposability among machinability. The present inventors, as in the conventional example described above, do not rely only on the addition of free-cutting components to improve machinability, but when the steel is cut and hardened by cold working, the chip disposability becomes remarkable. Note that it drops. Then, in order to reduce work hardening during cold working, solid solution N that promotes work hardening is fixed as BN by adding B, and further, an appropriate amount of Bi is contained, and the Bi contains B. Thus, by precipitating in the steel base material, it has succeeded in improving the chip disposability while suppressing work hardening.
[0014]
By the way, it has long been known that BN is finely precipitated in steel and effectively acts to improve machinability. However, it is necessary to improve the chip disposability when cutting after cold working as a steel for cold forging. It has been found that it is more effective to increase the BN by making this BN exist as a relatively coarse substance. It is also known that the machinability is improved by containing an appropriate amount of Bi. However, the Bi is precipitated in a state where B is contained therein, and its size is relatively coarse. The present inventors have found that it is effective to make them exist, and have achieved this by controlling rolling conditions appropriately.
[0015]
First, the inventors of the present invention have made intensive researches on the development of steel suitable for improving the machinability after cold working, especially the chip controllability. As a result, it was confirmed that a steel for cold forging having excellent chip controllability was obtained by adding B.
[0016]
It is well known that the chip controllability of steel is improved by the addition of free-cutting components such as S and Pb. However, when these elements are added excessively, the cold workability is deteriorated, so that the positive addition to the steel for cold forging is not preferable. Therefore, studies were made on solutions other than the addition of S and Pb.
[0017]
By the way, even with steel having the same composition, the harder the steel, the lower the chip disposability. For this reason, in a steel material that is cut after cold working by cold forging, such as steel for cold forging, work hardening at the time of cold working hardens and the chip disposability decreases. Therefore, in order to improve the machinability, it was considered effective to suppress the increase in hardness due to work hardening, and research was conducted along that line. If N was precipitated as BN by adding B, It was confirmed that the amount of solid solution N affecting work hardening was reduced, and an increase in hardness due to work hardening was suppressed, so that chip controllability could be improved.
[0018]
Further, in the case of steel to which Bi is added, Bi does not form a solid solution in the parent phase, and therefore exists individually in the parent phase. However, since Bi is an element having a low melting point, when Bi is simply added and rolled, It will be present in the mother phase in a stretched state. Therefore, the machinability is improved, but the cold forgeability is deteriorated. However, when Bi is added to a steel material containing B and N in a combined manner, Bi precipitates in a state containing B together with the generation of BN, and contributes to the improvement of the chip disposability without deteriorating the cold forgeability. confirmed.
[0019]
However, this alone does not provide a sufficient effect when the cold working rate is increased. Therefore, as a result of further study on measures for improving the chip disposability, it was found that BN deposited to fix the solute N was deposited. It has been found that it is extremely effective to appropriately control the presence form of the precipitate and the B-containing Bi precipitate.
[0020]
It is known that BN and Bi have good results in improving machinability, but BN precipitates and Bi are generally very fine, and if they are simply precipitated, the chip disposability after cold working is poor. Can not give enough effect. Therefore, as a result of further investigation mainly on rolling conditions, precipitates composed of BN and B-containing Bi having a diameter of 0.7 μm or more were found in a total of 15 mm per 0.5 mm × 0.5 mm cross-sectional area of the steel material. It was confirmed that the presence of more than one piece stably exhibited excellent chip disposal.
[0021]
The reason why the size and the number of the BN precipitate and the B-containing Bi are specified is that not only the BN precipitate and the B-containing Bi having a diameter of 0.7 μm or more contribute to the machinability, but the steel after cold working. BN and B-containing Bi with a small contribution to the improvement of the chip disposability after cold working are small without controlled rolling in order to make the effect of the BN precipitates and the B-containing Bi precipitates significantly exert on the chip disposability of the steel. As a standard for coarsening the whole, BN and B-containing Bi precipitates having a diameter of 0.7 μm or more are selected as a standard, and the number thereof is determined per steel field cross section of 0.5 mm × 0.5 mm. It is determined to be 15 or more.
[0022]
Incidentally, FIG. 8 to be described later is a graph in which the influence of the number of BN and B-containing Bi precipitates having a diameter of 0.7 μm or more per visual field having a cross section of 0.5 mm × 0.5 mm on the chip disposability is arranged and shown. From this graph, it can also be confirmed that the number of BN and B-containing Bi precipitates having a diameter of 0.7 μm or more has a close correlation with the chip disposability. The present inventors have confirmed that it is only necessary that the chip disposability index should be more than 10 to be evaluated as having practically satisfactory chip disposability as a cold forging steel. It can be seen that in order to satisfy the value, it is sufficient to secure 15 or more per 0.5-mm × 0.5-mm cross-sectional visual field in total of the numbers of BN and B-containing Bi precipitates of the above size.
[0023]
Next, the reason for determining the chemical composition of steel in the present invention will be clarified.
[0024]
"B: 0.001 to 0.010%"
B is an element that is a point of the present invention, and forms a BN precipitate, suppresses the strain aging due to solid solution N, suppresses the increase in hardness due to work hardening, and is mixed with the Bi precipitate. It exists and contributes to the improvement of the chip disposability without deteriorating the cold forgeability. In order to effectively exert such an effect, it must be contained at 0.001% or more. However, if the B content is too large, the hot ductility is significantly reduced, so the upper limit is set to 0.010%. The more preferable content of B is 0.0015% or more and 0.008% or less.
[0025]
"N: 0.002 to 0.010%"
N is an element that adversely affects cold workability by being present as solid solution N in steel as described above, but in the present invention, BN is precipitated by reaction with B to enhance chip disposability. Is an important element, and must be contained at least 0.002% or more. However, if it is too large, the excess will remain as solid solution N and deteriorate the cold workability, so it should be suppressed to 0.010% or less. The more preferable content of N is 0.003% or more and 0.0085% or less.
[0026]
"Bi: 0.005 to 0.10%"
When Bi is added together with B and N, the effect of improving the machinability due to the addition of Bi is significantly exerted by adding 0.005% or more. However, if the added amount of Bi is too large, it spreads without granulation in spite of the coexistence of B, and tends to significantly impair cold forgeability. Therefore, the amount of Bi added should be 0.005% or more and 0.10% or less, and more preferably 0.01% or more and 0.08% or less.
[0027]
As described above, the basic idea of the present invention is characterized in that the respective contents of B, N and Bi are defined, and the size and the number of the precipitates of BN and B-containing Bi existing in the cross section are determined. However, in order to effectively exhibit these characteristics as a practical steel material, it is desirable to specify the basic components of the steel material itself and the allowable alloying elements, etc. I do.
[0028]
"C: 0.005 to 0.50%"
C is a useful element for securing the strength of the final forged product. If the content is less than 0.005%, the strength may be insufficient, while if it exceeds 0.50%, the strength becomes too high and the cold workability is increased. Deteriorates. The more preferable range of C is 0.007% or more and 0.45% or less.
[0029]
"Si: 1% or less (including 0%)"
Si is a useful element as a deoxidizing agent, but if it is excessive, it deteriorates cold workability and machinability due to solid solution strengthening. Therefore, it should be suppressed to 1% or less, more preferably 0.7% or less. It is.
[0030]
"Mn: 2% or less (including 0%)"
Mn is a useful element that acts as a strengthening element and also combines with S in steel to form MnS and contribute to improvement in machinability. However, if it is too large, the strength becomes too high and impairs the cold workability. Therefore, the content is preferably suppressed to 2% or less, more preferably 1.5% or less.
[0031]
"P: 0.03% or less (including 0%)"
P is a harmful element that segregates at the grain boundaries and lowers the ductility of steel, and it is desirable that P be small. However, since a certain amount or more is unavoidably mixed, it should be suppressed to 0.03% or less, more preferably 0.02% or less.
[0032]
"S: 0.07% or less (including 0%)"
S is an element effective for improving machinability, but if it is too large, cold workability is significantly deteriorated. In the present invention, since S is not actively utilized for improving machinability, it is preferable to suppress the content to 0.07% or less, more preferably 0.06% or less, in order to eliminate its harmful effects. However, the active addition of a small amount is effective for further improving the machinability, and it is desirable that the content is preferably 0.003% or more, more preferably 0.005% or more.
[0033]
"Cr: 1.6% or less (including 0%)"
Cr is an element useful for giving a predetermined strength to a forged product, but if too much, the steel becomes hard and adversely affects cold workability and machinability, so 1.6% or less is more preferable. Is preferably suppressed to 1.2% or less, more preferably 0.5% or less.
[0034]
"Al: 0.1% or less (including 0%)"
Al is an element useful as a deoxidizing agent, but in an excessive amount, it becomes a source of oxide-based inclusions and deteriorates cold workability. Therefore, 0.1% or less, more preferably 0.05% or less It is better to suppress.
[0035]
"Cu: at least one selected from 2.0% or less (excluding 0%), Ni: 2.0% or less (excluding 0%), and Mo: 1.0% or less (excluding 0%) that's all"
Cu, Ni, and Mo are all useful elements as strength-enhancing elements. However, if they are too much, they have an adverse effect on machinability and the like. Therefore, they should be appropriately selected and added in an appropriate amount according to the use and required characteristics of the forged product. Is desirable. In any case, in order to suppress adverse effects on machinability and the like, it is desirable that Cu is suppressed to 2.0% or less, Ni is 2.0% or less, and Mo is 1.0% or less.
[0036]
"Pb: 0.1% or less (excluding 0%)"
Pb is a typical low-melting-point metal like Bi, and effectively acts to improve machinability. Therefore, when the machinability is more important, it is effective to positively add the material. However, excessive addition deteriorates cold workability, so that the content should be suppressed to 0.1% or less. In recent years, there has been a strong demand for Pb-free steel from the viewpoint of pollution prevention, and it is believed that it will be avoided in the future.
[0037]
“Mg: 0.01% or less (excluding 0%), Ca: 0.01% or less (excluding 0%), Zr: 0.2% or less (excluding 0%), Te: 0. At least one selected from 1% or less (not including 0%) "
Mg, Ca, Zr, and Te can be added complementarily for controlling the form of sulfide. However, the effect is saturated even if it is added excessively, so it is necessary to stop the addition to the respective upper limit values.
[0038]
By the way, the form of the BN or B-containing Bi precipitate, which is the greatest feature of the present invention, can be ensured by rolling, cooling, and winding a slab of an appropriate chemical composition under appropriate conditions as described above. Preferably, after heating to a range of 850 to 1050 ° C. and rolling to a predetermined wire diameter in a range of 725 to 1000 ° C., quenching to 750 to 950 ° C. at a cooling rate of 600 to 6000 ° C./min by a water flow or the like, Subsequently, it is preferable to gradually cool to 600 ° C. at a cooling rate of 1 ° C./sec or less. The reason for recommending each of these conditions will be described below.
[0039]
"Heating temperature of slab: 850 to 1050 ° C"
If the heating temperature of the steel slab is too high, the amount of solid solution N increases and the precipitation of BN is limited, and the B-containing Bi is also finely dispersed, so that the effect of improving the chip controllability is not effectively exhibited. The temperature is preferably controlled to not more than 1000C, more preferably not more than 1000C. However, if the heating temperature is too low, an obstacle such as an increase in the load of a rolling roll during rolling appears, so that the temperature is preferably 850 ° C. or higher. A more preferred heating temperature is 900 ° C. or higher.
[0040]
"Rolling temperature: 725-1000 ° C"
The rolling temperature is set in order to prevent solid solution of the nitride, and should not be performed at an excessively high temperature. Further, it is desirable to control the temperature to 750 ° C. or more and 1000 ° C. or less in practical use in consideration of the viewpoint of preventing an increase in the load of the rolling roll, a decrease in dimensional accuracy, and the occurrence of surface flaws. A more preferable rolling temperature is 775 ° C or more and 975 ° C or less.
[0041]
"Winding temperature: 750-950 ° C"
In the winding after the final rolling, typically, it is preferable to cool to 750 to 950 ° C. at a cooling rate of 600 to 6000 ° C./min using water as a coolant, and when the winding temperature exceeds 950 ° C., BN Precipitation slows down. On the other hand, if the winding temperature is too low, the steel structure becomes hard and brittle, and is not suitable for cold working. A more preferable winding temperature at the actual operation level is 800 ° C. or more and 925 ° C. or less.
[0042]
"Cooling rate: 1 ° C / sec or less (up to 600 ° C)"
It is the cooling rate that is most important in the production conditions employed in the present invention. In order to generate BN and B-containing Bi as coarse precipitates in the cooling step, it is desirable to reduce the cooling rate, and to 600 ° C. Is controlled to 1 ° C./sec or less, more preferably 0.5 ° C./sec or less, and still more preferably 0.4 ° C./sec or less.
[0043]
The present invention is configured as described above. In addition to containing an appropriate amount of B, N, and Bi in steel and causing a large amount of relatively coarse BN and B-containing Bi in the steel, the present invention provides a It has become possible to provide a steel for cold forging exhibiting excellent chip controllability even after forging.
[0044]
【Example】
Hereinafter, the present invention will be described more specifically with reference to Examples. However, the present invention is not limited to the following Examples, and may be appropriately modified within a range that can be adapted to the purpose of the preceding and the following. The present invention can be implemented, and all of them are included in the technical scope of the present invention.
[0045]
Example 1
Using a high-frequency melting furnace, steel materials having the chemical components shown in Table 1 were produced. Invention steel 1 and comparison steel 1 are based on S10C, invention steel 2 and comparison steel 2 are based on S20C, and invention steel 3 and comparison steel 3 are based on S35C. Each steel material was melted and cast, and each slab was formed into a 155 mm square steel slab by hot forging, and then further heated and then hot-rolled into a wire. For rolling, each steel type is rolled into three sizes of 8.5 mm, 9.5 mm, and 12.5 mm in diameter, and then cold drawn to 8 mm in diameter to produce a polished bar. Was used to evaluate the chip disposability.
[0046]
[Table 1]

[0047]
The feature of the present invention resides in that the chip disposability at the time of cutting after imparting cold working by cold forging or cold forging is enhanced, and practically, forming by cold forging, cold forging, or the like. The feature is effectively exhibited when performing the cutting process on the finished part. However, in actual parts, the strain distribution is not uniform, and is affected by the difference in the cut portion and the shape of the part. Therefore, it is difficult to quantitatively evaluate the cutting workability. Therefore, in this example, in order to clarify the quantitative effect, after the rolling, a cutting test was performed in a state of a polished bar to which uniform strain was given by cold drawing for reducing the wire diameter. In addition, in order to grasp the effect of the cold working rate, the wire diameter of the polishing rod was unified to 8 mm in diameter, and the effect when the rolling diameter before cold working was changed to a diameter of 8.5 mm to 12.5 mm was also evaluated. did. Incidentally, when the rolling diameter is 8.5 mm, the cold working rate is 12%, when the diameter is 9.5 mm, the cold working rate is 30%, and when the rolling diameter is 12.5 mm, the cold working rate is 12.5 mm. Is 60%.
[0048]
Table 2 shows rolling conditions and cutting test results. The rolling was performed under the conditions satisfying the requirements recommended in the present invention for both the inventive steel and the comparative steel. The number of BN and B-containing Bi precipitates was measured as follows.
[0049]
That is, the cross section of the wire after rolling is observed in a 0.5 mm × 0.5 mm field of view with a scanning electron microscope, and for BN and B-containing Bi having a diameter of 0.7 μm or more, a composition analysis is performed using EPMA, The number of BN and B-containing Bi precipitates in the measurement visual field was determined.
[0050]
The results are as shown in Table 2. In the steel of the present invention, 15 or more BN and B-containing Bi precipitates having a diameter of 0.7 μm or more exist per 0.5 mm × 0.5 mm viewing area. In the cutting test, a polishing rod having a diameter of 8 mm was cut by an automatic lathe to evaluate chip disposability. The cutting conditions are as shown in Table 3. The cut was reduced to 0.5 mm and the feed rate was changed by four levels, assuming finish cutting after cold forging. Chips were collected under each condition, and the total of the evaluation points of the four conditions was used as a chip disposability index based on the evaluation points shown in FIG. 1 according to the form of each chip piece, and the machinability was determined by the magnitude of this value. It was judged.
[0051]
[Table 2]

[0052]
[Table 3]

[0053]
2 to 4 show the relationship between the cold working ratio and the hardness using S10C, S20C, and S35C as base steels, respectively.
[0054]
It can be seen that the invention steel has a lower hardness than the comparative steel because work hardening is suppressed by the suppression of strain aging. 5 to 7 show the relationship between the cold work rate and the chip disposability index using S10C, S20C, and S35C as base steels, respectively, and the invention steel suppresses work hardening and controls BN and B-containing Bi precipitates. It can be confirmed that the chip disposability is improved.
[0055]
Example 2
After smelting and casting the S20C base steel shown in Table 4, it was rolled to a diameter of 9.5 mm, and then drawn at a cold working rate of 30% to produce a polished rod having a diameter of 8 mm, which was evaluated in the same manner as in Example 1 above. Was performed. Steel No. Nos. 4 to 6 change the amounts of BN and B-containing Bi precipitates by changing the amount of B added. Steel No. No. 7 containing Cu and Ni, steel No. 7 No. 8 is Mg, steel No. Numeral 9 contains Ca.
[0056]
Table 5 shows the rolling conditions and the results of the cutting test and the cold forgeability test. Steel No. Reference numeral 5 denotes a case in which the conditions within the proper range recommended in the present invention are adopted (5-1), a condition in which the cooling rate is outside the proper range (5-2), and a condition in which the heating temperature is outside the proper range (5-3). Other than these, rolling was performed under conditions within the above-mentioned appropriate range.
[0057]
In addition, cold forgeability was calculated | required as follows. That is, as shown in FIG. 9, a polishing rod having a diameter of 8 mm was ₀ ) Is cut into a test piece, which is compressed and deformed in the vertical direction, and the critical height (h) until the compressed piece cracks. ₁ ), And [Compression rate (%) = 100 × (H ₀ -H ₁ ) / H ₀ ] To calculate the critical compression ratio.
[0058]
[Table 4]

[0059]
[Table 5]

[0060]
In Tables 4 and 5, (5-2) indicates that the BN and B-containing Bi precipitates could not be controlled significantly because the cooling rate after rolling was high. In (5-3), since the heating temperature was too high, solute N increased, and a sufficient number of BN and B-containing Bi precipitates were not deposited.
[0061]
The results in Table 5 are summarized in FIG. As is apparent from this figure, the influence of the BN and B-containing Bi precipitates on the chip disposition is more remarkable as the number of BN and B-containing Bi precipitates having a diameter of 0.7 μm or more increases. In addition, as the amount of B added increases, the amounts of BN and B-containing Bi precipitates increase (Tables 4 and 5).
[0062]
Steel No. Nos. 4, (5-1) and 6 can be almost arranged by the amounts of BN and B-containing Bi precipitates, and the chip disposability is clearly improved as the amounts of BN and B-containing Bi precipitates increase. On the other hand, steel No. In (5-2) and (5-3), the BN and B-containing Bi precipitates are not properly controlled, so that the chip disposability is poor. In addition, steel No. to which a small amount of Mg or Ca was added was used. Nos. 8 and 9 also show high chip disposability because a sufficient amount of BN and B-containing Bi precipitates are generated.
[0063]
10 and 11 show the steel No. shown in Table 4 above. Scanning electron micrographs (2500 times) observed after polishing the cross section of the wire rod of No. 4 and FIGS. 12, 13 and 14 show EPMAs for the deposits A, B and C in FIGS. 6 is a chart showing the results of a composition analysis performed by the method shown in FIG. FIG. 15 also shows the steel Nos. 8 is a scanning electron micrograph showing the cross section of FIG. 8, and FIG. 16 is an EPMA composition analysis chart of the precipitate appearing as the symbol D in FIG. 15, and as is clear from these figures, the precipitate was found to be insoluble in the parent phase. It can be confirmed that all of the Bi particles contain a small amount of B.
[0064]
【The invention's effect】
The present invention is configured as described above, and it is possible to solve the problem of the chip controllability due to the cold working with respect to the steel material cut after the cold working. Therefore, if this steel material is used, automation of cutting after cold working can be performed more stably, and can greatly contribute to improvement in productivity.
[Brief description of the drawings]
BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a diagram showing a criterion of a chip disposal evaluation point adopted in an experiment.
FIG. 2 is a graph showing the relationship between the cold work rate and the hardness of the steel obtained in the examples.
FIG. 3 is a graph showing the relationship between the cold work ratio and the hardness of the steel obtained in the example.
FIG. 4 is a graph showing the relationship between the cold work ratio and the hardness of the steel obtained in the example.
FIG. 5 is a graph showing the relationship between the cold working ratio and the chip disposability index obtained in the examples.
FIG. 6 is a graph showing the relationship between the cold working rate and the chip disposability index similarly obtained in the examples.
FIG. 7 is a graph showing the relationship between the cold working ratio and the chip disposability index obtained in the example.
FIG. 8 summarizes the influence of the number of BN and B-containing Bi precipitates having a diameter of 0.7 μm or more observed per 0.5 mm × 0.5 mm field of view of the cross section of the test steel on chip disposability. FIG.
FIG. 9 is a diagram for explaining a cold forgeability evaluation test method for each test agent.
FIG. 10 is a cross-sectional scanning electron micrograph of the invention steel obtained in the experiment.
FIG. 11
It is a cross-sectional scanning electron microscope photograph of the invention steel similarly obtained by the experiment.
FIG.
FIG. 11 is an EPMA analysis chart of the precipitate shown as symbol A in FIG.
FIG. 13
FIG. 11 is an EPMA analysis chart of the precipitate shown as symbol B in FIG.
FIG. 14
12 is an EPMA analysis chart of the precipitate shown as a symbol C in FIG.
FIG.
It is a cross section scanning electron micrograph of other invention steel obtained by experiment.
FIG.
FIG. 16 is an EPMA analysis chart of the precipitate shown as symbol D in FIG.

Claims

B: a steel containing 0.001 to 0.010% (representing mass%, the same applies hereinafter), N: 0.002 to 0.010% and Bi: 0.005 to 0.10%, A total of 15 or more BNs with a diameter of 0.7 μm or more (Bi including B) and Bis containing B are present in a visual field area of 0.5 mm × 0.5 mm in cross section. Cold forging steel with excellent chip controllability characterized by the following characteristics.

The steel contains C: 0.005 to 0.50%, and Si: 1% or less, Mn: 2% or less, P: 0.03% or less, S: 0.07% or less, A1: 0. 2. The steel for cold forging excellent in chip disposal according to claim 1, wherein the steel is limited to 1% or less and Cr: 1.6% or less, and the balance is substantially Fe.

The chip treatment according to claim 2, wherein the steel further contains one or more of Cu: 2.0% or less, Ni: 2.0% or less, and Mo: 1.0% or less as other elements. Cold forging steel with excellent heat resistance.

The steel for cold forging excellent in chip disposability according to claim 2 or 3, wherein the steel further contains Pb: 0.1% or less as another element.

The steel further contains one or more of Mg: 0.01% or less, Ca: 0.01% or less, Zr: 0.2% or less, and Te: 0.1% or less as other elements. The steel for cold forging excellent in chip disposability according to any one of claims 2 to 4, wherein