JP3798251B2

JP3798251B2 - Manufacturing method of undercarriage forgings for automobiles

Info

Publication number: JP3798251B2
Application number: JP2001064407A
Authority: JP
Inventors: 直樹岩間; 英久加藤; 昭二岩城; 元秀森; 剛河本
Original assignee: Toyota Motor Corp; Aichi Steel Corp
Current assignee: Toyota Motor Corp; Aichi Steel Corp
Priority date: 2001-03-08
Filing date: 2001-03-08
Publication date: 2006-07-19
Anticipated expiration: 2021-03-08
Also published as: JP2002266023A

Description

【０００１】
【技術分野】
本発明は，自動車等の足廻り部品に用いられる鍛造品であって，熱間鍛造後に冷間加工を実施し，冷間加工により導入された歪により特別な熱処理を施すことなく歪時効効果を得て，優れた曲げ降伏強度の得られる自動車用足廻り用鍛造品の製造方法に関するものである。
【０００２】
【従来技術】
自動車の足廻り部品であるロアーアーム，アッパーアーム，キャンバーコントロール，トレーリングアーム等は，高強度と高い曲げ耐力が要求されるために，従来は炭素鋼又は低合金鋼を熱間鍛造後焼入焼もどし処理を実施した後，機械加工して使用されてきた。しかしながら，最近では，ＣＯ２削減，製造コスト低減の観点から一部で非調質鋼も利用されてきている。
【０００３】
一方，自動車の燃費向上のため，足廻り部品の軽量化要求が高く材料を鋼からアルミ合金等の軽金属へ置き換える試みがなされているが，材料コストが増加する問題が生じる。そこで，高強度の鋼材を用いて足廻り部品を薄肉軽量化する検討が盛んに進められている。
【０００４】
特に，最近では化学成分の最適化に加え熱間鍛造後の冷間加工を利用して鍛造品の高強度化を図る試みがなされている。例えば，特開平６−２４８３４１号，特開平１０−１５２７４９号，特開平１０−１５２７５０号，特開平１１−１３１１３４号等の出願がされている。これらの公報に記載の発明はいずれも熱間加工後の冷間加工を利用して引張強さ，降伏比等の強度の高い鋼の提供を可能とし，部品の軽量化を図ることを目的とするものである。
【０００５】
【発明が解決しようとする課題】
しかしながら，十分な軽量化効果を得るまでに鋼を高強度化しようとした場合に，強度向上のために硬さの増加が避けられず，工具寿命の低下，切削に必要な時間の延長，切削工具交換周期の短時間化等の被削性の低下をまねき，コスト的に高くなるという問題がある。また，鍛造後に時効処理を施すことで硬さを増加させずに０．２％耐力を向上させる手法が発明されている（特開平５−３０２１１６号，特開平５−３０２１１７号）が，時効のための再加熱処理が必要であり，コストが高くなるという問題がある。
【０００６】
また，前記した先願公報には，熱間鍛造品に冷間加工を加えて，引張強度，引張耐力の向上が達成できたとする記載はされているが，足廻り部品に必要な特性である曲げ０．２％耐力と被削性について全く記載がなく，高強度と優れた被削性をいかにして両立させるかという点について全く検討されていない。
そこで，十分な軽量化効果を得る足廻り部品を低コストに製造する方策について，発明者等は種々の検討を試みた結果，以下の知見を得て本発明を得た。
【０００７】
本発明は，高い曲げ０．２％耐力を確保しつつ優れた被削性を得ることのできる自動車用足廻り部品の製造方法を提供することを目的とする。
【０００８】
【課題の解決手段】
第一の発明は，質量％で，Ｃ：０．１０超〜０．３５％，Ｓｉ：０．０５〜２．００％，Ｍｎ：１．５〜３．０％，Ｃｒ：０．５０〜３．００％，Ｎ：０．０１２〜０．０３０％，Ｐ：０．０３５％以下，Ａｌ：０．０２０％以下，Ｏ：０．００２０％未満を含有すると共に，不純物として不可避に含有されるＴｉ，Ｎｂ，Ｂが，Ｔｉ：０．０１％未満，Ｎｂ：０．０１％未満，Ｔｉ＋Ｎｂ≦０．０１％，Ｂ：０．０００５％未満であり，残部がＦｅ及び不可避的不純物から成り，かつ，固溶Ｎが０．００４〜０．０２０％である鋼を，
温度１１５０〜１３００℃にて熱間鍛造し，８００〜４００℃の温度範囲を平均冷却速度：ＣＶ（℃／ｍｉｎ）が４０℃／ｍｉｎ〜３００℃／ｍｉｎとなる条件にて冷却してベイナイト組織又はベイナイト＋フェライト組織とし，得られた硬さがＨｖ３２０以下とした熱間鍛造品に，曲げ降伏強度が必要な部位に圧縮方向の冷間加工率で１２〜５０％までの冷間加工を加えることを特徴とする自動車用足廻り鍛造品の製造方法である（請求項１）。
【０００９】
本発明によれば，まず第１に，従来から足廻り部品で使用されているフェライト・パーライト型非調質鋼に比べＣ量を低くしＭｎ，Ｃｒ量を高めること及び熱間鍛造後の冷却速度を一定範囲内にすることでベイナイト又はベイナイト＋フェライト組織を得つつ，Ｈｖ３２０以下の硬さとした場合，優れた被削性が確保できる。
第２に，窒化物形成元素であるＡｌ，Ｔｉ，Ｎｂ，Ｂを極力低減して固溶Ｎ量を一定以上確保し，かつ熱間鍛造後高い曲げ強度を必要とする箇所を特定範囲の加工率で冷間加工して部品内部に歪を導入すると，その後特別な処理を施さなくても，大きな歪時効効果が得られ，熱間鍛造後の硬さがＨｖ３２０以下であるにもかかわらず，優れた曲げ０．２％耐力を確保できる。
【００１０】
以下に本発明において鋼材の成分組成及び製造条件を限定した理由を明らかにする。
【００１１】
Ｃ（炭素）：０．１０超〜０．３５％
Ｃは，機械構造用鋼としての強度を確保するための元素である。しかし，少なすぎると炭素量の微少なバラツキによる硬さ変化が大きく，結果として引張強さのバラツキが極めて大きくなり安定した強度が得られないため，Ｃを０．１０％超とした。又，炭素量が多すぎると熱間鍛造後の硬さをＨｖ３２０以下にすることが困難となり，被削性が悪化するためＣを０．３５％以下とした。
【００１２】
Ｓｉ（珪素）：０．０５〜２．００％
Ｓｉは，製鋼時の脱酸剤として不可欠であるためＳｉを０．０５％以上とする。しかし，多量に含有させると，被削性が悪化するためＳｉを２．００％以下とした。
【００１３】
Ｍｎ（マンガン）：１．５〜３．０％
Ｍｎは，熱間鍛造後の冷却過程でベイナイト組織を得るために必要な元素であり，ベイナイト組織を安定して得るためにはＭｎは少なくとも１．５％以上必要である。しかし，多すぎると熱間鍛造後の冷却時にマルテンサイト組織等が発生し，被削性に優れたベイナイト組織を得られなくなるためＭｎを３．０％以下とした。
【００１４】
Ｃｒ（クロム）：０．５０〜３．００％
ＣｒはＭｎと同じく，熱間鍛造後の冷却過程でベイナイト組織を得るために必要な元素であり，ベイナイト組織を安定して得るためにはＣｒは少なくとも０．５０％以上必要である。しかし，多すぎると熱間鍛造後の冷却時にマルテンサイト組織等が発生し，被削性に優れたベイナイト組織を得られなくなるためＣｒを３．００％以下とした。
【００１５】
Ｎ（窒素）：０．０１２〜０．０３０％
Ｎは冷間加工により鋼中に導入された転位を固着することにより，歪時効効果を得て高い曲げ０．２％耐力を確保するために必要な元素である。また，結晶粒の粗大化を防止する効果も有する。前記効果を十分に得るためには，Ｎを０．０１２％以上の含有が必要である。しかし，多すぎると製鋼精練時での成分調整が困難になるためＮを０．０３０％以下とする。
【００１６】
また，本発明では前記した歪時効効果を十分に得るために必要な固溶Ｎの量を，Ｎと化合物を生成する元素をできるだけ低減（後述）することにより確保している。前記効果を得るために，固溶Ｎは０．００４０％以上，好ましくは０．０８０％以上含有させることが望ましい。しかし，多量の含有は成分調整が困難となるので，固溶Ｎを０．０２００％以下とするのがより望ましい。
【００１７】
Ｐ（リン）：０．０３５％以下
Ｐは冷間加工時の限界加工率を阻害する元素である。熱間圧延，熱間鍛造時の割れを防ぐため上限を０．０３５％とする。
【００１８】
Ａｌ（アルミニウム）：０．０２０％以下，Ｔｉ（チタン）：０．０１％未満，
Ｎｂ（ニオブ）：０．０１％未満，Ｂ（ホウ素）：０．０００５％未満
Ａｌ，Ｔｉ，Ｎｂ，ＢはＮと化合物を形成し，歪時効効果を得るために必要な固溶Ｎの量を減らすため，歪時効効果を減少させて冷間加工後の曲げ０．２％耐力を低下させる原因となる元素である。Ａｌは脱酸のため少量の添加は必要であるが，最低限の量に抑える必要があり，また他の３つの元素は製造時に添加しなくても電気炉溶解する際に使用するスクラップやスラグから混入する場合があり，本発明のように少量の含有が問題となる場合には，この３元素が不純物として多量に含有されないように注意して製造する必要がある。従って，４元素共極力低減する必要があり，Ａｌ，Ｔｉ，Ｎｂ，Ｂをそれぞれ，０．０２０％以下，０．０１％未満，０．０１％未満，０．０００５％未満とした。
【００１９】
Ｔｉ＋Ｎｂ≦０．０１％
Ｔｉ，Ｎｂは，特にＮと化合物を形成しやすく，必要な歪時効効果を得るためには特にその含有量の上限を厳しく管理する必要がある。従って，合計含有率の上限を０．０１％とした。好ましくは上限を０．００５％とするのが良い。
【００２０】
Ｏ（酸素）：０．００２０％未満
Ｏは鋼中で酸化物系介在物となって存在し，冷間加工時の限界加工率を低くする原因となる元素である。特に変形方向の冷間加工率が４０％以上の高い冷間加工を加える場合には，酸素が多量に含有すると有害な割れを引き起こす可能性が高くなるため，Ｏを０．００２０％未満とした。
その他，本発明の鋼には，Ｆｅ，不可避的不純物を含む。
【００２１】
次に本発明の製造条件限定理由について説明する。
熱間鍛造時の加熱温度を１１５０〜１３００℃に限定した理由は，加熱温度が１１５０℃未満になると，鍛造加熱時にＡｌＮ等の窒化物が十分に鋼中に固溶せず，固溶Ｎが減少しその後の歪時効効果が十分に得られず，必要な曲げ０．２％耐力が得られないからである。また１３００℃を超えるとオーステナイト粒が粗大化し，靭性が低下するとともに，硬さが上昇し，被削性が低下するため，上限を１３００℃とした。本願発明では特に歪時効効果を重視しているため，その効果を得るために下限を１２００℃を超える高い温度に設定するのがより好ましい。
【００２２】
熱間鍛造後８００〜４００℃の平均冷却速度ＣＶ（℃／ｍｉｎ）を４０〜３００℃／ｍｉｎとしたのは，冷却が遅すぎるとフェライトの割合が増加し，必要な曲げ０．２％耐力が得られなくなるためであり，冷却速度が３００℃／ｍｉｎを超えると熱間鍛造後の冷却時にマルテンサイト組織が現れ，熱間加工後の硬さＨｖ３２０を超え機械加工の工具寿命が悪くなるからである。また，この温度範囲においては同一の冷却方法（例えば空冷）を施すのが好ましく，前記温度範囲の途中から徐冷する等冷却速度が大きく変化する方法を施すと，規定された平均冷却速度の範囲内であっても狙いとする組織が得られなくなる場合があるので注意が必要である。温度範囲の下限を４００℃としたのは，熱間鍛造後の冷却中にベイナイト組織の出現を完了させるためである。４００℃に冷却されるまでに組織の出現が完了するため，４００℃未満の温度における冷却速度は特に特定範囲に限定する必要はない。
【００２３】
熱間加工後の硬さをＨｖ３２０以下としたのは，Ｈｖ３２０を超すとドリルに代表される機械加工時の工具寿命が著しく悪くなるためである。
【００２４】
曲げ降伏強度が必要な部位に，冷間加工率で１２〜５０％までの冷間加工を加えるのは，冷間加工率が１２％未満であると加工硬化及び歪時効による強度向上効果が小さくなって，必要な曲げ０．２％耐力が得られないためである。しかし，必要以上の冷間加工を加えると十分な歪時効効果は得られるが，割れ発生の可能性が高くなるので，冷間加工率を５０％以下とした。
なお，冷間加工率とは，圧縮方向の高さ変化率のことを示す。例えば，図１の場合には，冷間加工率（％）＝（（Ｌ−Ｌ２）／Ｌ）×１００となる。
【００２５】
また，本発明では熱間鍛造後の鋼の組織をベイナイト又はベイナイト＋フェライトとしている。この鋼の主な組織はベイナイトであって，フェライトの割合は１５％以内，好ましくは１０％以下である。これは，高い曲げ耐力と優れた被削性を得るのに最も適した組織であるからである。
もし，マルテンサイト組織が生成されると，硬さが高くなり，割れやすくなるとともに，被削性が低下する。また，パーライト組織が生成されると曲げ耐力が低下する。従って，マルテンサイト組織，パーライト組織は極力少ないことが望ましく，共に０％であることが最適である。
【００２６】
以上説明した条件を満足するように製造することによって，優れた被削性と曲げ０．２％耐力９５０ＭＰａ以上の確保が可能となる。
本発明の自動車用足廻り鍛造品としては，たとえば，ロアーアーム，アッパーアーム，キャンバーコントロール，トレーリングアーム，ステアリングナックル，タイロッドなどがある。
【００２７】
第二の発明は，請求項１に記載の鋼に加え，さらに質量％で，Ｓ：０．０４〜０．２０％，Ｐｂ：０．０１〜０．３０％，Ｂｉ：０．０１〜０．３０％，Ｃａ：０．０００５〜０．０１％，Ｍｇ：０．０００３〜０．０１％
，ＲＥＭ：０．０１〜０．２０％から選択した１種または２種以上を含有させた鋼を温度１１５０〜１３００℃にて熱間鍛造し，８００〜４００℃の温度範囲を平均冷却速度：ＣＶ（℃／ｍｉｎ）が４０℃／ｍｉｎ〜３００℃／ｍｉｎとなる条件にて冷却してベイナイト組織又はベイナイト＋フェライト組織とし，得られた硬さがＨｖ３２０以下とした熱間鍛造品に，曲げ降伏強度が必要な部位に圧縮方向の冷間加工率で１２〜５０％までの冷間加工を加えることを特徴とする自動車用足廻り鍛造品の製造方法である（請求項２）。
【００２８】
第二発明によれば，上記第一発明の効果に加え，更に，切削抵抗を低減でき刃具の寿命を延ばすことができるという効果を得ることができる。
【００２９】
Ｓ（硫黄）：０．０４〜０．２０％，Ｐｂ（鉛）：０．０１〜０．３０％，Ｂｉ（ビスマス）：０．０１〜０．３０％，Ｃａ（カルシウム）：０．０００５〜０．０１％，Ｍｇ（マグネシウム）：０．０００３〜０．０１％，ＲＥＭ：０．０１〜０．２０％の１種または２種以上
Ｓ，Ｐｂ，Ｂｉ，Ｃａ，Ｍｇ，ＲＥＭは旋削，ドリル穿孔時に切削抵抗を低減し，刃具の寿命向上に有効な元素であり，必要に応じて添加されるものである。前記効果を得るためには，Ｓは０．０４％以上，Ｐｂは０．０１％以上，Ｂｉは０．０１％以上，Ｃａは０．０００５％以上，Ｍｇは０．０００３％以上，ＲＥＭは０．０１％以上の含有が必要である。しかし多量に含有すると，コスト増加を招くとともに，非金属介在物が増し，異方性の増大，疲労強度の低下がある為，Ｓは０．２０％以下，Ｐｂは０．３０％以下，Ｂｉは０．３０％以下，Ｃａは０．０１％以下，Ｍｇは０．０１％以下，ＲＥＭは０．２０％以下とした。
【００３０】
第三の発明は，請求項１または２に記載の鋼に加え，さらに質量％で，Ｎｉ：３．０％以下，Ｍｏ：０．３０％以下，Ｖ：０．２０％以下から選択した１種または２種以上を含有させた鋼を温度１１５０〜１３００℃にて熱間鍛造し，８００〜４００℃の温度範囲を平均冷却速度：ＣＶ（℃／ｍｉｎ）が４０℃／ｍｉｎ〜３００℃／ｍｉｎとなる条件にて冷却してベイナイト組織又はベイナイト＋フェライト組織とし，得られた硬さがＨｖ３２０以下とした熱間鍛造品に，曲げ降伏強度が必要な部位に圧縮方向の冷間加工率で１２〜５０％までの冷間加工を加えることを特徴とする自動車用足廻り鍛造品の製造方法である（請求項３）。
【００３１】
第三発明によれば，上記第一，第二の発明の効果に加え，更に，ベイナイト組織の微細化と曲げ０．２％耐力の上昇という効果を得ることができる。
【００３２】
Ｎｉ（ニッケル）：３．０％以下，Ｍｏ（モリブデン）：０．３０％以下，Ｖ（バナジウム）：０．２０％以下
Ｎｉ，Ｍｏ，Ｖはベイナイト組織の微細化と曲げ０．２％耐力の上昇に効果があり，必要に応じ添加することが可能である。しかし，必要以上の添加はベイナイトミクロ組織中に残留オーステナイトを増加させ，逆に曲げ０．２％耐力を低下させるとともに，コスト増加を招くため，Ｎｉは３．０％以下，Ｍｏは０．３０％以下，Ｖは０．２０％以下とした。
【００３３】
【発明の実施の形態】
本発明の実施形態にかかる自動車用足廻り鍛造品の供試材，及び比較例の供試材を作製した。表１及び表２は，供試材の化学成分を示すものである。
【００３４】
【表１】

【００３５】
【表２】

【００３６】
上記表に示す供試材のうち，１〜１９鋼は第一，第二，第三発明鋼であり，２０〜３０鋼は一部の成分が上記本発明の範囲外である比較鋼である。また，３１〜３５鋼は従来鋼であって，３１，３２，３５鋼は焼入れ焼もどしを実施しない足廻り部品に既に適用されているベイナイト型非調質鋼であり，３３鋼はＢを含有した従来のベイナイト型非調質鋼であり，３４鋼は従来のフェライトパーライト型非調質鋼である。３５鋼は，Ｔｉ，Ｎｂの低減対策がされていない従来のベイナイト型非調質鋼である。
以上説明した供試材を使用して，自動車用ロアアーム鍛造品を作製し，部品形状での曲げ試験，被削性試験を実施した。
【００３７】
表１に示す成分組成の鋼を３０ｋｇ溶解装置で溶解後造塊し，φ４０ｍｍに熱間鍛造しその後ロアアーム熱間鍛造品を作製した。その後熱間鍛造品について表面を磁粉探傷し，１５００ｔｏｎプレスを用いて強度が必要な部分に冷間加工を実施した。冷間加工は，図１に示すごとく，一対のダイ２，３で供試材１を挟み，上下方向から加圧した。加圧前の供試材１の高さＬと加圧後の供試材１の高さＬ２を求め，これらの値を（（Ｌ−Ｌ２）／Ｌ）×１００％の算出式に代入して，冷間加工率（％）を得た。
【００３８】
以上の方法で製造した供試材を用い，後述する方法で組織観察，硬さ測定，曲げ試験，被削性の評価を実施した。
硬さ試験は断面切断後研磨しビッカース硬さ試験機で荷重１０ｋｇで実施した。
【００３９】
ミクロ組織観察については，硬さ試験後，研磨したものを試料として用い，光学顕微鏡にて観察し，鍛造後の組織について調査した。
【００４０】
静的三点曲げ試験は，ロアアームをスパン６０ｍｍ，ポンチ速度１ｍｍ／分で実施した。曲げ０．２％耐力はゲージ長さ１ｍｍの歪ゲージにて測定した。
【００４１】
被削性の評価は各熱間鍛造品でドリル穿孔評価を実施し，ドリルがφ６ｍｍのストレートシャンク，ドリルの材質はＳＫＨ５１，ドリル回転数は９６６ｒ．ｐ．ｍ．，潤滑油なし，荷重７５ｋｇの条件で実施した。測定した結果は従来鋼を使用して製造した鍛造品である３２鋼の鍛造品の穿孔距離を１００とし，それぞれの穿孔距離を整数比で表３に示した。
【００４２】
【表３】

【００４３】
【表４】

【００４４】
各種試験評価結果を表３，表４に示す。ここに示すように，各合金成分が本発明範囲内である１〜１９鋼はいずれも熱間鍛造後の組織がベイナイトを主体とするベイナイト＋フェライト組織となっており，冷間加工後の割れもなく，硬さ，曲げ耐力，被削性の全てについて従来鋼に比べ優れた特性を示している。
【００４５】
これに対して２０鋼は，Ｃ量が低いために冷間加工後の曲げ０．２％耐力が９５０ＭＰａ未満である。また２１鋼は逆にＣ量が高いために，マルテンサイト組織が現れ，硬さがＨｖ３２０を超え被削性指数が１００以下で従来鋼より劣る。
【００４６】
２２鋼についてはＭｎ量が低いため冷間加工後の曲げ０．２％耐力が９５０ＭＰａ未満である。２３鋼については，逆にＭｎ量が高いために，マルテンサイト組織が現れ，硬さがＨｖ３２０を超え，被削性指数が１００以下で従来鋼より劣る。２４鋼についてはＣｒ量が低いため冷間加工後の曲げ０．２％耐力が９５０ＭＰａ未満である。２５鋼については逆にＣｒ量が高いために，マルテンサイト組織が現れ，硬さがＨｖ３２０を超え被削性指数が１００以下で従来鋼より劣る。２６鋼についてはＡｌ量が高いため固溶Ｎによる歪時効効果が少なく冷間加工後の曲げ０．２％耐力が９５０ＭＰａ未満である。２７鋼についてはＮ量が低いため，歪時効効果量が少なく冷間加工後の曲げ０．２％耐力が９５０ＭＰａ未満である。２８鋼についてはＴｉ量が高いため固溶Ｎによる歪時効効果量が少なく冷間加工後の曲げ０．２％耐力が９５０ＭＰａ未満である。２９鋼はＯが多いため冷間加工後に割れが発生する。
【００４７】
また，従来品である３１〜３５鋼は全て冷間加工後の曲げ０．２％耐力が本願発明鋼を用いた鍛造品に比べ著しく劣るものである。
【００４８】
次に製造条件の影響，すなわち鍛造加熱温度，鍛造後の冷却条件，冷間加工量，影響を調査した実施例を示す。
【００４９】
表１に示す成分組成のうち，本発明の成分の条件を満足する鋼種２について，製造条件を変化させ，前述の試験方法と同様に試験を実施した。結果を表５に示す。
【００５０】
【表５】

【００５１】
各種試験評価結果を表５に示す。Ａ〜Ｉは製造条件が本願発明範囲内であり，全てについて優れた被削性，曲げ０．２％耐力が得られている。
【００５２】
Ｊは加熱温度が１０５０℃未満であり窒化物が十分に固溶しないために冷間加工による歪時効の効果が少なく曲げ０．２％耐力が９５０ＭＰａ未満である。Ｋは加熱温度が１３００℃を超え，硬さが硬いため，被削性が従来品より悪い。Ｌは冷却速度が遅くパーライト組織が現出し曲げ０．２％耐力が９５０ＭＰａ未満である。Ｍは冷却速度が速くマルテンサイト組織が現出し硬さが高いため被削性が従来品より悪い。Ｎは７００℃までを空冷し，その後徐冷（５℃／ｍｉｎ）した実施例であるが，ベイナイト変態が完了する前に冷却速度が急変し，徐冷されるため，パーライト組織が現出し，曲げ０．２％耐力が低いものである。なお，表５に示した冷却速度は，空冷中のみの平均冷却速度である。Ｏは熱間鍛造後の冷間加工率が１２％未満であり，十分に大きな歪時効効果が得られず曲げ０．２％耐力が低いものであり，Ｐは逆に冷間加工率が高いため，冷間鍛造割れを呈す。以上の実施例から明らかなように，化学成分が本発明で規定した条件範囲内であっても，製造条件（鍛造加熱温度，鍛造後の冷却条件，冷間加工率）のいずれか１項目でも満足しない場合には，得られる特性が劣るものである。
【００５３】
【発明の効果】
以上の説明で明らかなように，自動車等の足廻り部品に用いられる鍛造品で，熱間鍛造後に冷間加工を実施し，一定量以上の固溶Ｎ量を確保して歪時効効果を十分に得ることにより，冷間加工後に熱処理を施すことなく優れた曲げ耐力と被削性の両立できる自動車用足廻り用鍛造品を得ることができる。従って，ＣＯ２発生を抑制しつつ自動車用足廻り鍛造品の大幅な軽量化を達成することができる。
【図面の簡単な説明】
【図１】本発明の実施形態における冷間加工方法を説明する説明図（ａ），（ｂ）。
【符号の説明】
１．．．供試材，
２，３．．．ダイ，[0001]
【Technical field】
The present invention is a forged product used for undercarriage parts of automobiles, etc., which is subjected to cold working after hot forging, and exhibits strain aging effect without applying special heat treatment due to strain introduced by cold working. In particular, the present invention relates to a method for manufacturing a forging for an undercarriage for automobiles that can obtain excellent bending yield strength.
[0002]
[Prior art]
Lower arm, upper arm, camber control, trailing arm, etc., which are parts for automobiles, require high strength and high bending strength. Conventionally, carbon steel or low alloy steel is quenched and hardened after hot forging. It has been used after being machined after the return processing. However, recently, non-tempered steel has been used in part from the viewpoint of CO2 reduction and manufacturing cost reduction.
[0003]
On the other hand, in order to improve the fuel efficiency of automobiles, there is a high demand for weight reduction of undercarriage parts, and attempts have been made to replace materials from steel to light metals such as aluminum alloys, but there is a problem that the material cost increases. Therefore, studies are underway to reduce the thickness and weight of suspension parts using high-strength steel.
[0004]
In particular, in recent years, attempts have been made to increase the strength of forgings by using cold working after hot forging in addition to optimization of chemical components. For example, Japanese Patent Application Laid-Open Nos. 6-248341, 10-152749, 10-152750, and 11-131134 have been filed. The inventions described in these publications all aim to reduce the weight of parts by making it possible to provide steel with high strength such as tensile strength and yield ratio using cold working after hot working. To do.
[0005]
[Problems to be solved by the invention]
However, when trying to increase the strength of the steel before obtaining a sufficient lightening effect, an increase in hardness is inevitable to improve the strength, reducing the tool life, extending the time required for cutting, cutting There is a problem that the machinability such as shortening of the tool change cycle is lowered and the cost is increased. In addition, methods for improving 0.2% proof stress without increasing the hardness by applying an aging treatment after forging have been invented (Japanese Patent Laid-Open Nos. 5-302116 and 5-302117). For this reason, there is a problem that the reheating treatment is necessary and the cost becomes high.
[0006]
In addition, in the above-mentioned prior application publication, although it is stated that the hot forging product has been cold worked to improve the tensile strength and tensile strength, it is a necessary characteristic for the suspension parts. There is no description of 0.2% proof stress and machinability at all, and no consideration has been given to how to achieve both high strength and excellent machinability.
Accordingly, as a result of various studies on the measures for manufacturing a suspension part that achieves a sufficient weight reduction effect at low cost, the inventors obtained the following knowledge and obtained the present invention.
[0007]
An object of this invention is to provide the manufacturing method of the suspension part for motor vehicles which can acquire the outstanding machinability, ensuring high bending 0.2% yield strength.
[0008]
[Means for solving problems]
1st invention is the mass%, C: more than 0.10 to 0.35%, Si: 0.05-2.00%, Mn: 1.5-3.0%, Cr: 0.50 3.00%, N: 0.012-0.030%, P: 0.035% or less, Al: 0.020% or less , O: Less than 0.0020% , and unavoidably contained as impurities Ti, Nb, and B are Ti: less than 0.01%, Nb: less than 0.01%, Ti + Nb ≦ 0.01%, B: less than 0.0005% , and the balance is composed of Fe and inevitable impurities. And steel having a solute N of 0.004 to 0.020% ,
Hot forging at a temperature of 1150 to 1300 ° C., and cooling in a temperature range of 800 to 400 ° C. under the condition that the average cooling rate: CV (° C./min) is 40 ° C./min to 300 ° C./min. Alternatively, a hot working product with a bainite + ferrite structure and a hardness of Hv320 or less is subjected to cold working up to 12-50% in the compressive cold working rate at the site where bending yield strength is required. This is a method for producing an undercarriage forging for automobiles (claim 1).
[0009]
According to the present invention, first of all, lowering the amount of C and increasing the amount of Mn and Cr and cooling after hot forging compared to the ferrite and pearlite type non-heat treated steel conventionally used for undercarriage parts. By obtaining a bainite or bainite + ferrite structure by keeping the speed within a certain range, excellent machinability can be ensured when the hardness is Hv320 or less.
Second, the nitride forming elements Al, Ti, Nb, and B are reduced as much as possible to ensure a certain amount of solute N, and processing in a specific range where high bending strength is required after hot forging. When strain is introduced into the part by cold working at a high rate, a large strain aging effect can be obtained without special treatment thereafter, and the hardness after hot forging is Hv320 or less. Excellent bending 0.2% proof stress can be secured.
[0010]
The reason why the composition of steel materials and the production conditions are limited in the present invention will be clarified below.
[0011]
C (carbon): more than 0.10 to 0.35%
C is an element for ensuring the strength as mechanical structural steel. However, if the amount is too small, the change in hardness due to slight variations in the carbon content is large, and as a result, the variation in tensile strength becomes extremely large and a stable strength cannot be obtained. Therefore, C is set to exceed 0.10%. Further, if the amount of carbon is too large, it becomes difficult to reduce the hardness after hot forging to Hv320 or less, and the machinability deteriorates, so C is set to 0.35% or less.
[0012]
Si (silicon): 0.05 to 2.00%
Since Si is indispensable as a deoxidizer during steelmaking, Si is made 0.05% or more. However, if contained in a large amount, the machinability deteriorates, so Si was made 2.00% or less.
[0013]
Mn (manganese): 1.5-3.0%
Mn is an element necessary for obtaining a bainite structure in the cooling process after hot forging, and Mn is required to be at least 1.5% or more in order to obtain a bainite structure stably. However, if the amount is too large, a martensite structure or the like is generated during cooling after hot forging, and a bainite structure excellent in machinability cannot be obtained, so Mn is set to 3.0% or less.
[0014]
Cr (chromium): 0.50 to 3.00%
Cr, like Mn, is an element necessary for obtaining a bainite structure in the cooling process after hot forging. In order to obtain a bainite structure stably, Cr is required to be at least 0.50% or more. However, if the amount is too large, a martensite structure or the like is generated during cooling after hot forging, and a bainite structure excellent in machinability cannot be obtained, so Cr was made 3.00% or less.
[0015]
N (nitrogen): 0.012-0.030%
N is an element necessary for securing a high strain 0.2% proof stress by obtaining a strain aging effect by fixing dislocations introduced into steel by cold working. It also has the effect of preventing crystal grain coarsening. In order to sufficiently obtain the above effects, N must be contained in an amount of 0.012% or more. However, if the amount is too large, it becomes difficult to adjust the components during steel refining, so N is set to 0.030% or less.
[0016]
Further, in the present invention, the amount of solute N necessary for sufficiently obtaining the strain aging effect described above is ensured by reducing the elements that form N and compounds as much as possible (described later). In order to acquire the said effect, it is desirable to contain solid solution N 0.0040% or more, preferably 0.080% or more. However, since it is difficult to adjust the components if a large amount is contained, it is more preferable that the solid solution N is 0.0200% or less.
[0017]
P (phosphorus): 0.035% or less P is an element that hinders the critical working rate during cold working. In order to prevent cracking during hot rolling and hot forging, the upper limit is made 0.035%.
[0018]
Al (aluminum): 0.020% or less, Ti (titanium): less than 0.01%,
Nb (niobium): less than 0.01%, B (boron): less than 0.0005% Al, Ti, Nb, B forms a compound with N, and the amount of solid solution N necessary to obtain a strain aging effect In order to reduce the aging effect, it is an element that reduces the strain aging effect and decreases the 0.2% yield strength after cold working. Al needs to be added in a small amount for deoxidation, but it must be kept to a minimum, and the other three elements are scrap and slag used for melting in the electric furnace even if they are not added during production. In the case where a small amount is a problem as in the present invention, it is necessary to manufacture carefully so that these three elements are not contained in large amounts as impurities. Therefore, it is necessary to reduce the 4-element copolarity, and Al, Ti, Nb, and B are set to 0.020% or less, less than 0.01%, less than 0.01%, and less than 0.0005%, respectively.
[0019]
Ti + Nb ≦ 0.01%
Ti and Nb are particularly easy to form a compound with N, and in order to obtain a necessary strain aging effect, it is necessary to strictly control the upper limit of the content thereof. Therefore, the upper limit of the total content is set to 0.01%. Preferably, the upper limit is 0.005%.
[0020]
O (oxygen): less than 0.0020% O is present as an oxide inclusion in the steel, and is an element that causes a reduction in the critical working rate during cold working. In particular, when high cold working with a cold working rate of 40% or more in the deformation direction is added, there is a high possibility that harmful cracking will occur if oxygen is contained in a large amount. Therefore, O is set to less than 0.0020%. .
In addition, the steel of the present invention contains Fe and inevitable impurities.
[0021]
Next, the reasons for limiting the production conditions of the present invention will be described.
The reason for limiting the heating temperature during hot forging to 1150 to 1300 ° C. is that when the heating temperature is less than 1150 ° C., nitrides such as AlN are not sufficiently dissolved in the steel during forging heating, so This is because the strain aging effect is reduced and the subsequent strain aging effect cannot be obtained sufficiently, and the required 0.2% proof stress cannot be obtained. When the temperature exceeds 1300 ° C., the austenite grains become coarse, the toughness decreases, the hardness increases, and the machinability decreases. Therefore, the upper limit is set to 1300 ° C. In the present invention, since the strain aging effect is particularly emphasized, it is more preferable to set the lower limit to a high temperature exceeding 1200 ° C. in order to obtain the effect.
[0022]
The average cooling rate CV (° C./min) of 800 to 400 ° C. after hot forging was set to 40 to 300 ° C./min. If the cooling was too slow, the ratio of ferrite increased and the required bending 0.2% yield strength If the cooling rate exceeds 300 ° C./min, a martensite structure will appear during cooling after hot forging, and the tool life of machining will deteriorate due to exceeding the hardness Hv320 after hot working. It is. Further, in this temperature range, it is preferable to apply the same cooling method (for example, air cooling). If a method in which the cooling rate is greatly changed, such as slow cooling from the middle of the temperature range, is applied, the specified average cooling rate range is set. It is necessary to be careful because the target organization may not be obtained even if it is within. The reason why the lower limit of the temperature range is 400 ° C. is to complete the appearance of the bainite structure during cooling after hot forging. Since the appearance of the tissue is completed before being cooled to 400 ° C., the cooling rate at a temperature lower than 400 ° C. need not be limited to a specific range.
[0023]
The reason why the hardness after hot working is set to Hv320 or less is that if it exceeds Hv320, the tool life at the time of machining represented by a drill is remarkably deteriorated.
[0024]
The cold working rate of 12 to 50% is added to the part where bending yield strength is required. If the cold working rate is less than 12%, the effect of improving the strength by work hardening and strain aging is small. This is because the required 0.2% yield strength cannot be obtained. However, although sufficient strain aging effect can be obtained by adding more cold work than necessary, the possibility of cracking increases, so the cold work rate was set to 50% or less.
The cold working rate indicates the rate of change in height in the compression direction. For example, in the case of FIG. 1, the cold working rate (%) = ((L−L2) / L) × 100.
[0025]
In the present invention, the steel structure after hot forging is bainite or bainite + ferrite. The main structure of this steel is bainite, and the proportion of ferrite is within 15%, preferably 10% or less. This is because it is the most suitable structure for obtaining high bending strength and excellent machinability.
If a martensite structure is generated, the hardness becomes high, the crack becomes easy, and the machinability deteriorates. In addition, when the pearlite structure is generated, the bending strength decreases. Accordingly, it is desirable that the martensite structure and the pearlite structure are as small as possible, and it is optimal that both are 0%.
[0026]
By manufacturing so as to satisfy the conditions described above, it is possible to ensure excellent machinability and a bending 0.2% proof stress of 950 MPa or more.
Examples of the undercarriage for automobiles according to the present invention include a lower arm, an upper arm, a camber control, a trailing arm, a steering knuckle, and a tie rod.
[0027]
The second invention is, in addition to the steel according to claim 1, in further mass%, S: 0.04~0.20%, Pb : 0.01~0.30%, Bi: 0.01~0 .30%, Ca: 0.0005 to 0.01%, Mg: 0.0003 to 0.01%
, REM: A steel containing one or more selected from 0.01 to 0.20% is hot forged at a temperature of 1150 to 1300 ° C, and an average cooling rate in a temperature range of 800 to 400 ° C: Bending into a hot forged product having a CV (° C./min) of 40 ° C./min to 300 ° C./min to form a bainite structure or a bainite + ferrite structure and having a hardness of Hv 320 or less. A method for manufacturing an undercarriage forging for an automobile, wherein cold working up to 12 to 50% is performed at a cold working rate in a compression direction on a portion requiring yield strength (Claim 2).
[0028]
According to the second invention, in addition to the effect of the first invention, it is possible to obtain an effect that the cutting resistance can be further reduced and the life of the blade can be extended.
[0029]
S (sulfur): 0.04 to 0.20%, Pb (lead): 0.01 to 0.30%, Bi (bismuth): 0.01 to 0.30%, Ca (calcium): 0.0005 -0.01%, Mg (magnesium): 0.0003-0.01%, REM: 0.01-0.20%, one or more types S, Pb, Bi, Ca, Mg, REM are turned It is an element that reduces cutting resistance during drilling and is effective in improving the tool life, and is added as needed. In order to obtain the effect, S is 0.04% or more, Pb is 0.01% or more, Bi is 0.01% or more, Ca is 0.0005% or more, Mg is 0.0003% or more, REM is It is necessary to contain 0.01% or more. However, if it is contained in a large amount, the cost increases, nonmetallic inclusions increase, anisotropy increases, and fatigue strength decreases. Therefore, S is 0.20% or less, Pb is 0.30% or less, Bi 0.30% or less, Ca 0.01% or less, Mg 0.01% or less, and REM 0.20% or less.
[0030]
According to a third aspect of the present invention, in addition to the steel according to

claim

1 or 2, 1 % selected from mass% , Ni: 3.0% or less, Mo: 0.30% or less, V: 0.20% or less The steel containing seeds or two or more kinds is hot forged at a temperature of 1150 to 1300 ° C., and an average cooling rate: CV (° C./min) is 40 ° C./min to 300 ° C./800° C. in the temperature range of 800 to 400 ° C. It is cooled under the condition of min to a bainite structure or a bainite + ferrite structure, and the obtained hardness is Hv320 or less in a hot forged product with a cold working rate in the compression direction at a site where bending yield strength is required. A method for producing an undercarriage forging for automobiles, characterized in that a cold working of 12 to 50% is applied (Claim 3).
[0031]
According to the third invention, in addition to the effects of the first and second inventions, it is possible to obtain the effects of further refinement of the bainite structure and increase of 0.2% proof stress.
[0032]
Ni (nickel): 3.0% or less, Mo (molybdenum): 0.30% or less, V (vanadium): 0.20% or less Ni, Mo, and V are refinement of bainite structure and bending 0.2% yield strength It is effective in increasing the amount of water and can be added as needed. However, excessive addition increases residual austenite in the bainite microstructure, conversely lowers the bending 0.2% proof stress and increases costs, so Ni is 3.0% or less and Mo is 0.30. % Or less and V is 0.20% or less.
[0033]
DETAILED DESCRIPTION OF THE INVENTION
A test material for an undercarriage for automobile according to an embodiment of the present invention and a test material for a comparative example were prepared. Tables 1 and 2 show the chemical components of the test materials.
[0034]
[Table 1]

[0035]
[Table 2]

[0036]
Among the test materials shown in the above table, steels 1 to 19 are first, second, and third invention steels, and steels 20 to 30 are comparative steels having some components outside the scope of the present invention. . Steels 31 to 35 are conventional steels. Steels 31, 32 and 35 are bainite-type non-tempered steels already applied to suspension parts not subjected to quenching and tempering, and steel 33 contains B. The conventional bainite-type non-tempered steel, and No. 34 steel is a conventional ferritic pearlite-type non-tempered steel. Steel No. 35 is a conventional bainite-type non-tempered steel that does not take measures to reduce Ti and Nb.
Using the specimens described above, automobile lower arm forgings were produced, and bending tests and machinability tests were performed on the parts.
[0037]
Steel with the component composition shown in Table 1 was melted with a 30 kg melting apparatus and then ingot, hot forged to φ40 mm, and then a lower arm hot forged product was produced. Thereafter, the surface of the hot forged product was subjected to magnetic particle flaw detection, and cold working was performed on a portion requiring strength using a 1500 ton press. In the cold working, as shown in FIG. 1, the specimen 1 was sandwiched between a pair of dies 2 and 3 and pressed from above and below. Obtain the height L of the specimen 1 before pressurization and the height L2 of the specimen 1 after pressurization, and substitute these values into the formula of ((L−L2) / L) × 100%. Thus, the cold working rate (%) was obtained.
[0038]
Using the specimens manufactured by the above method, microstructure observation, hardness measurement, bending test, and machinability evaluation were performed by the methods described later.
The hardness test was performed after cutting the cross section and polishing with a Vickers hardness tester with a load of 10 kg.
[0039]
Regarding the microstructure observation, after the hardness test, the polished one was used as a sample, observed with an optical microscope, and the microstructure after forging was investigated.
[0040]
The static three-point bending test was performed with the lower arm having a span of 60 mm and a punch speed of 1 mm / min. The 0.2% yield strength was measured with a strain gauge having a gauge length of 1 mm.
[0041]
Evaluation of machinability was conducted by drilling with each hot forged product. The drill was a straight shank with a diameter of 6 mm, the drill material was SKH51, and the drill rotation speed was 966 r. p. m. The test was carried out under the conditions of no lubrication oil and a load of 75 kg. As a result of the measurement, the drilling distance of a forged product of 32 steel, which is a forged product manufactured using conventional steel, was set to 100, and each drilling distance was shown in Table 3 as an integer ratio.
[0042]
[Table 3]

[0043]
[Table 4]

[0044]
Tables 3 and 4 show the results of various test evaluations. As shown here, each of the 1-19 steels whose alloy components are within the scope of the present invention has a bainite + ferrite structure mainly composed of bainite after hot forging, and cracks after cold working. However, it has superior properties compared to conventional steel in terms of hardness, bending strength and machinability.
[0045]
On the other hand, 20 steel has a low C content, so the 0.2% yield strength after cold working is less than 950 MPa. In contrast, Steel No. 21 has a high C content, so a martensitic structure appears, the hardness exceeds Hv320, and the machinability index is 100 or less, which is inferior to that of conventional steel.
[0046]
For Steel No. 22, since the Mn content is low, the 0.2% yield strength after cold working is less than 950 MPa. On the other hand, Steel No. 23 is inferior to the conventional steel because the Mn content is high and a martensitic structure appears, the hardness exceeds Hv320, and the machinability index is 100 or less. For Steel No. 24, since the Cr content is low, the 0.2% yield strength after cold working is less than 950 MPa. On the other hand, for steel No. 25, the Cr content is high, so a martensitic structure appears, the hardness exceeds Hv320, and the machinability index is 100 or less, which is inferior to conventional steel. For Steel No. 26, since the Al content is high, the strain aging effect due to solute N is small, and the 0.2% yield strength after cold working is less than 950 MPa. Since No. 27 steel has a low N content, the strain aging effect is small, and the 0.2% yield strength after cold working is less than 950 MPa. For Steel No. 28, the amount of Ti is high, so the amount of strain aging effect due to solute N is small, and the 0.2% yield strength after cold working is less than 950 MPa. Since steel No. 29 has a lot of O, cracking occurs after cold working.
[0047]
In addition, the conventional steels 31 to 35 are all inferior to the forged products using the invention steel of the present invention in 0.2% proof stress after cold working.
[0048]
Next, an example in which the influence of manufacturing conditions, that is, the forging heating temperature, the cooling condition after forging, the amount of cold work, and the influence are investigated will be shown.
[0049]
Of the component compositions shown in Table 1, for steel type 2 that satisfies the conditions of the components of the present invention, the production conditions were changed, and the test was performed in the same manner as the test method described above. The results are shown in Table 5.
[0050]
[Table 5]

[0051]
Various test evaluation results are shown in Table 5. A to I have production conditions within the scope of the present invention, and excellent machinability and 0.2% proof stress are obtained for all.
[0052]
J has a heating temperature of less than 1050 ° C. and nitrides are not sufficiently dissolved, so that the effect of strain aging by cold working is small, and the bending 0.2% proof stress is less than 950 MPa. K has a machinability worse than that of the conventional product because the heating temperature exceeds 1300 ° C. and the hardness is hard. L has a slow cooling rate and a pearlite structure appears, and the bending 0.2% proof stress is less than 950 MPa. Since M has a high cooling rate and a martensite structure appears and the hardness is high, the machinability is worse than the conventional product. N is an example of air cooling up to 700 ° C., followed by gradual cooling (5 ° C./min), but the pearlite structure appears because the cooling rate suddenly changes and gradually cools before the bainite transformation is completed, Bending 0.2% yield strength is low. The cooling rates shown in Table 5 are average cooling rates only during air cooling. O has a cold working rate after hot forging of less than 12%, a sufficiently large strain aging effect cannot be obtained, and 0.2% bending strength is low. P, on the contrary, has a high cold working rate. Therefore, it exhibits cold forging cracks. As is clear from the above examples, even if the chemical composition is within the condition range defined in the present invention, any one of the manufacturing conditions (forging heating temperature, cooling condition after forging, cold work rate) If not satisfied, the obtained characteristics are inferior.
[0053]
【The invention's effect】
As is clear from the above explanation, forged products used for undercarriage parts of automobiles, etc., cold working is performed after hot forging, ensuring a solute N amount of a certain amount or more and sufficient strain aging effect. Thus, it is possible to obtain an automotive undercarriage forging that can achieve both excellent bending strength and machinability without performing heat treatment after cold working. Therefore, it is possible to achieve a significant weight reduction of the automobile undercarriage forging while suppressing the generation of CO2.
[Brief description of the drawings]
BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is an explanatory diagram (a) and (b) for explaining a cold working method in an embodiment of the present invention.
[Explanation of symbols]
1. . . Specimen,
2,3. . . Die,

Claims

% By mass, C: more than 0.10 to 0.35%, Si: 0.05 to 2.00%, Mn: 1.5 to 3.0%, Cr: 0.50 to 3.00%, N : 0.012 to 0.030%, P: 0.035% or less, Al: 0.020% or less , O: Less than 0.0020% and Ti, Nb, B inevitably contained as impurities but, Ti: less than 0.01% Nb: less than 0.01%, Ti + Nb ≦ 0.01 %, B: less than 0.0005%, the balance Ri consists of Fe and unavoidable impurities, and solid solution Steel whose N is 0.004 to 0.020% ,
Hot forging at a temperature of 1150 to 1300 ° C., and cooling in a temperature range of 800 to 400 ° C. under the condition that the average cooling rate: CV (° C./min) is 40 ° C./min to 300 ° C./min. Alternatively, a hot working product with a bainite + ferrite structure and a hardness of Hv320 or less is subjected to cold working up to 12-50% in the compressive cold working rate at the site where bending yield strength is required. A method for producing an undercarriage forging for automobiles.

In addition to the steel according to claim 1, in further mass%, S: 0.04~0.20%, Pb : 0.01~0.30%, Bi: 0.01~0.30%, Ca: A steel containing one or more selected from 0.0005 to 0.01%, Mg: 0.0003 to 0.01%, REM: 0.01 to 0.20% at a temperature of 1150 to 1300 ° C. Bainite structure or bainite + ferrite structure by cooling in a temperature range of 800 to 400 ° C. under the condition that the average cooling rate is 40 ° C./min to 300 ° C./min. The hot forgings obtained with a hardness of Hv320 or less are subjected to cold working up to 12 to 50% in the compressive cold working rate at the parts where bending yield strength is required. Manufacturing method for undercarriage for automobiles.

In addition to the steel according to claim 1 or 2, one or more selected from mass% , Ni: 3.0% or less, Mo: 0.30% or less, V: 0.20% or less The steel contained was hot forged at a temperature of 1150 to 1300 ° C., and a temperature range of 800 to 400 ° C. under the condition that the average cooling rate: CV (° C./min) was 40 ° C./min to 300 ° C./min. It is cooled to a bainite structure or a bainite + ferrite structure, and the obtained hardness is Hv320 or less. For hot forgings where bending yield strength is required, the cold working rate in the compression direction is 12 to 50%. A method for producing an automotive undercarriage forging characterized by adding cold working.