JP4299377B2

JP4299377B2 - Method for producing alloyed hot-dip galvanized steel sheet with increased heat treatment performance after forming

Info

Publication number: JP4299377B2
Application number: JP13163597A
Authority: JP
Inventors: 武志西脇; 一正山崎
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 1997-05-07
Filing date: 1997-05-07
Publication date: 2009-07-22
Anticipated expiration: 2017-05-07
Also published as: JPH10310824A

Description

【０００１】
【発明の属する技術分野】
本発明は、建設部材、機械構造用部品、自動車の構造用部品など、構造上の強度、特に変形時の強度及び又は剛性が必要とされる箇所に適用されるに好適な、プレス等による加工成形後に所定温度域で強度上昇熱処理がなされる成形体の素鋼板として用いられる合金化溶融亜鉛めっき鋼板の製造方法に関するものである。
【０００２】
【従来の技術】
薄鋼板からなるプレス成形体を製造するに際し、プレス成形前は軟質でプレス成形しやすく、プレス成形後に硬化させ部品強度を高める方法としては、２００℃未満で塗装焼付する方法などがある。この塗装焼付用鋼板としてＢＨ鋼板が開発された。
【０００３】
例えば、特開昭５５−１４１５２６号公報、特開昭５５−１４１５５５号公報の如くＮｂ添加鋼において、鋼中のＣ、Ｎ、ＡＩ含有量に応じてＮｂを添加してａｔ％でＮｂ／（固溶Ｃ＋固溶Ｎ）を特定範囲内に制限し、さらに、焼鈍後の冷却速度を制御することにより、鋼板中の固溶Ｃ、固溶Ｎを調整する方法や特公昭６１−４５６８９号公報の如くＴｉとＮｂの複合添加によって焼付硬化性を向上することが開示されている。
【０００４】
しかしながら、前述のような鋼板は、深絞り性に優れる材質とする為、鋼板の強度は低く、構造用材料としてとしては必ずしも十分ではない。
【０００５】
また、さらに、特開平５−２５５４９号公報の如く鋼にＷ、Ｃｒ、Ｍｏの単独または複合添加によって焼付硬化性を向上することが開示されている。しかしながら、焼付硬化により強度が上昇するのは、鋼板中に含まれる固溶Ｃ、固溶Ｎを利用する為、例えば図１の破線のＢＨ鋼板の応力―歪み曲線に模式的に示すように、材料の降伏強さのみを上昇させる（図１のＢＨ分の応力上昇）だけであり、引張強さ（引張強度）を上昇させるものではない。このため、部材または部品の変形の開始応力を高める効果しかなく、部材または部品の変形開始から変形終了まで変形中全域にわたって変形に要する応力（以下、変形強度特性と記す）を高める効果については、必ずしも十分ではない。
【０００６】
プレス成形体の塗装焼付以外の硬化方法としては、プレス成形後に軟窒化処理による方法がある。例えば、特開平２−８０５３９号公報の如く窒化処理により強度が高まるようにＣｒ、Ａｌ、Ｖ等の窒化物形成元素を鋼中に含有させる方法や、特開平３−１２２２５５号公報の如く窒化処理の熱を利用して、Ｃｕを析出硬化させ部材の硬さを高める方法などが開示されている。しかしながら、これらの方法では、４５０℃超と加熱温度が高く、耐食性を高めようとして通常の亜鉛めっきを用いると、めっき層が蒸散し耐食性の良いものが得られ難いという欠点を有していた。
【０００７】
【発明が解決しようとする課題】
本発明は、上記のような問題点を解決するべく、加工成形前は強度レベルが３００Ｍｐａ級、４００Ｍｐａ級、５００Ｍｐａ級、６００Ｍｐａ級、７００Ｍｐａ級で比較的軟質の高強度鋼でプレス成形等の加工成形がしやすく、プレス成形等の成形加工後に強度上昇を目的とした比較的低温での短時間熱処理を行うことで、引張強さ又は硬さが上昇し部材や部品の変形強度特性を高めるか、あるいは剛性をたかめることが可能な素鋼板としての合金化溶融亜鉛めっき鋼板の製造方法を提供することを課題とする。
【０００８】
【課題を解決するための手段】
本発明者らは、合金化溶融亜鉛めっきを施した薄鋼板からなる各種成形材料や部品の形状を成形する上での加工性、部材や部品を熱処理することで硬化させる熱処理方法、および該鋼板からなる部品としてのプレス成形体の変形強度特性など鋭意研究を行った。その結果、鋼組成において適量のＣとＷ、Ｍｏを複合添加することにより、更には鋼のミクロ組織を特定し、溶融亜鉛めっき及び加熱合金化処理を施すことによって、成形加工後に２００〜４５０℃の温度域に加熱した際に短時間で高い引張強さ上昇（又は変形強度特性上昇）が得られること新たに発見した。本発明はこの発見に基づいて成し遂げたものである。
【０００９】
その発明の要旨は、次のとおりである。
【００１１】
（１）重量％にて
Ｃ：０．０１〜０．０８％、
Ｓｉ：０．００５〜１．０％、
Ｍｎ：０．０１〜３．０％、
Ｐ：０．００１〜０．１５％、
Ｓ：０．００１〜０．０２％、
Ａｌ：０．００１〜０．１％、
Ｎ：０．０００２〜０．０１％
および
Ｗ、Ｍｏの１種または２種を、合計量が０．０５〜３．０％
含有し、残部が鉄および不可避的不純物からなる成分を有する鋼を熱間圧延した後に、調質圧延率で３〜１６％の調質圧延を施し、次いで６５０〜９００℃の温度で焼鈍を施し、その後溶融亜鉛めっきを行い、加熱合金化処理を施したことを特徴とする、塑性ひずみで２％以上のひずみが加わる成形加工を施した後、２００〜４５０℃の温度範囲での１分〜３０分間の短時間保持することで、加工成形前後の引張強さを比較して６０ＭＰａ以上引張強さ向上可能な成形後強度上昇熱処理性能を有する合金化溶融亜鉛めっき鋼板の製造方法。
【００１２】
（２）鋼組成として、更に重量％で、
Ｔｉ：０．００５〜０．１％、
Ｖ：０．００５〜０．１％
の１種または２種を含有せしめたことを特徴とする、請求項１に記載の塑性ひずみで２％以上のひずみが加わる成形加工を施した後、２００〜４５０℃の温度範囲での１分〜３０分間の短時間保持することで、加工成形前後の引張強さを比較して６０ＭＰａ以上引張強さ向上可能な成形後強度上昇熱処理性能を有する合金化溶融亜鉛めっき鋼板の製造方法。
【００１３】
【発明の実施の形態】
本発明者らは、部材や部品のプレス成形性等の加工成形性を確保しつつ部材や部品に変形強度特性を付与する方法として、鋼板組成、鋼板製法、熱処理方法、成形性（特にプレス成形性）について鋭意研究を行ったところ、Ｃと親和性の弱い炭化物形成元素であるＷ、Ｍｏを含有する鋼板に溶融亜鉛めっきと加熱合金化処理を施した後で、２％以上の歪みを与えるプレス成形法で加工し、さらにその後に２００〜４５０℃の熱処理を施すことにより、耐食性を損なうことなく合金化亜鉛めっき鋼板の変形強度特性が著しく向上することを見出した。また、さらに鋼組成として、Ｔｉ、Ｖを複合添加することで前記変形強度特性の上昇量（ΔＴＳ）が著しいことを見出した。
【００１４】
以下に本発明を詳細に説明する。
【００１５】
まず、以下に鋼の成分を限定する理由について述べる。
【００１６】
Ｃは、鋼の加工性に影響を及ぼす元素であり、含有量が多くなると、加工性は劣化する。従って０．０８％以下とする。また、０．０１％未満では、２００〜４５０℃の熱処理時に炭化物として析出する量が少なく、熱処理の際、強度を上昇させる効果が少ないので、０．０１％を下限とする。
【００１７】
Ｓｉは、０．００５％未満では、製造コストが飛躍的に上がり経済的でなくなるので、０．００５％を下限とし、１．０％を越えると加工性は劣化するとともに、亜鉛が付着しにくく亜鉛めっきの密着性を損なうので、１．０％を上限とする。
【００１８】
Ｍｎは、０．０１％未満では、製造コストが飛躍的に上がり経済的でなくなるので、０．０１％を下限とし、３．０％を越えると加工性は劣化するので、３．０％を上限とする。
【００１９】
Ｐは、深絞り性を損なわずに強度を上げられる元素であり、強度レベルに応じて添加するが、０．００１％未満にするには製造コストが飛躍的に上がり経済的でなくなるので、０．００１％を下限とし、０．１５％を越えると二次加工脆性の問題が発生してくるので、０．１５％を上限とする。
【００２０】
Ｓは、本来、鋼中に存在することが無意味な元素であるため少ない方が良いが、０．００１％未満では製造コストが飛躍的に上がり経済的でなくなるので、０．００１％を下限とし、０．０２％を超えると熱間圧延時に赤熱脆性を起こし、表面で割れる、いわゆる、熱間脆性を起こすため、０．０２％を上限とする。
【００２１】
Ａｌは、通常、脱酸成分として添加し、ブローホール等の欠陥が発生を防止するため、０．００１％以上添加する必要がある。０．００１％未満では十分脱酸できないため、０．００１％を下限とする。また、０．１％を越えると脱酸の効果は飽和するので、０．１％を上限とする。また、更にＴｉを添加して介在物を形態制御して、プレス割れを少なくするには、Ａｌは０．００１〜０．００５％が好ましい。
【００２２】
Ｎは、加工性を確保するためには少ない方が良いが、０．０００２％未満では製造コストが飛躍的に上がり経済的でなくなるので、０．０００２％を下限とし、０．０１％を越えると加工性が劣化してくるので、０．０１％を上限とする。
【００２３】
加工成形後の所定の低温熱処理の際に、引張強さを上昇させる効果を発揮するには、前記元素に加えて必須元素として、更にＭｏ、Ｗの１種または２種を、Ｍｏ、Ｗの合計量が０．０５〜３．０％鋼に含有せしめる。合計量が０．０５％未満では、前記の熱処理を施しても、引張強さの上昇が十分に期待できない。また、合計量が３．０％を超えるとＭｏ、Ｗが鋼を強化しすぎて、加工成形前の強度が高くなりすぎ加工性を損ったり、前記の加工後熱処理を施しても引張強さを上昇させる効果が小さいか又は飽和してしまうために、経済的に不都合となるためである。
【００２４】
Ｗ、Ｍｏを含有する鋼板に２％以上の歪みを付与するようなプレス成形等の加工成形を行い、２００〜４５０℃の熱処理を施すと、鋼板の引張強さが上昇する。鋼へＷ、Ｍｏを所定量添加してこの熱処理を施した際の引張強さの上昇理由は明らかではないが、２％以上の歪みの付与によって、鋼板中のフェライト部に相当量の転位が導入され、Ｃ、Ｗ、Ｍｏの鋼中での拡散は低温でも飛躍的に高まり、またさらに導入された転位を核として、転位上にＷ、Ｍｏ炭化物の析出が生じるため、低温短時間で炭化物が析出して部材や部品の引張強さが高まると本発明者らは考えている。
【００２５】
また、さらに加工成形後熱処理の際、引張強さを上昇させるには、前記のＷ、Ｍｏに加えて、選択元素としてＴｉ、Ｖを添加することが出来る。Ｔｉ、Ｖを含有した部材又は部品の引張強さや硬さを上昇させる効果が高まる理由は明らかではないが、Ｔｉ、Ｖを添加すると、前記のＷ、Ｍｏ添加による低温熱処理での作用と相乗作用を発揮し、鋼板中で微細な炭化物を形成し、この微細炭化物は、プレス時に付与する歪みに対して、転位を効果的に増殖させ、歪み量を増やしたような効果が現れるためと本発明者らは考ている。特に、Ｖを添加し、更にＴｉ添加が加工成形後絶処理を短縮できるので好ましい。
【００２６】
Ｔｉは熱処理の際、強度を上昇させる効果を高める元素であり、０．００５％未満では、その効果が小さいので、０．００５％を下限としする。また、Ｔｉは鋼の強度を高める元素であり、０．１％を超えると加工性が劣化するので、０．１％を上限とする。
【００２８】
Ｖは熱処理の際、強度を上昇させる効果を高める元素であり、０．００５％未満では、その効果が小さいので、０．００５％を下限としする。また、Ｖは鋼の強度を高める元素であり、０．１％を超えると加工性が劣化するので、０．１％を上限とする。
【００２９】
以上のように成分を調整するが、成形後熱処理の際、強度を上昇させる効果を高めるためには、鋼板中のＣ量を熱処理温度で、固溶状態にしておく事が望ましいので、炭化物形成元素であるＴｉ、ＶをＴｉ量で｛（４８／１２）×Ｃ［％］＋（４８／１４）×Ｎ［％］｝以下、もしくはＶ量を｛（５１×４／１２×３）×Ｃ［％］＋（５１／１４）×Ｎ［％］｝以下、もしくはＴｉ、Ｖを複合添加する場合では、｛Ｔｉ［％］×１２／４８＋Ｖ［％］×１２×３／５１×４｝＜Ｃ［％］＋Ｎ［％］×１２／１４を満足するように添加することが望ましい。
【００３０】
ついで本発明の製造方法で得られる鋼ミクロ組織の好ましい態様についてのべる。
【００３１】
鋼のミクロ組織は、フェライトまたはフェライトを主体（鋼のミクロ組織のフェライトの組織分率を６０％以上）とすることが好ましい。フェライトは軟質で加工性に優れる上に、加工を加えたときに結晶粒内に多くの転位を蓄積することができる。また、歪みを加えたときに転位が均一に入り、鋼板全体の強度を均一に高めることができるので、鋼のミクロ組織をフェライトまたはフェライトを主体とする。またさらに、フェライト主体とする組織のフェライトでない残部の組織をパーライト及び又はベイナイトとするとフェライトと残部組織の界面に応力集中が高まり、効果的にフェライトに転位を付与することができる。そのうえ、パーライト及び又はベイナイト組織はマルテンサイト組織ほど硬質でないので、パーライト及び又はベイナイト組織自身も変形し、鋼板全体として転位量が増加するので熱処理の際鋼の強度が効果的に上昇する。
【００３２】
本発明の成形後強度上昇熱処理性能とは、塑性相当ひずみで２％以上のひずみが加わる成形加工を施した後、２００〜４５０℃（更に好ましくは２２０〜３７０℃）の温度範囲での１分〜３０分間の短時間保持（冷間成形後の加熱または温間成形後の温度保持など）において、加工成形前後の引張強さを比較した引張強さΔＴＳ（＝加工後熱処理後ＴＳ−加工前ＴＳ）で６０ＭＰａ以上（更に好ましくはΔＴＳで９０Ｍｐ以上）強度向上可能な、またはビッカース硬さΔＨｖ（＝加工後熱処理後ΔＨｖ−加工前ΔＨｖ）で１８以上（更に好ましくはΔＨｖで２７以上）熱処理後に上昇可能な熱処理を示す。但し、この熱処理は窒化処理等のように成形体に外部から硬化元素（例えば窒素等）を積極的に侵入せしめる必要がない。本発明の加工成形後強度（引張強さ）上昇熱処理素材としての、高強度合金化溶融亜鉛めっき鋼板としては、熱延鋼板でも、冷延鋼板でもかまわず、板厚も限定されるものではないが、０．４〜６ｍｍで特に有効である。
【００３３】
本発明鋼の製造に際しては、上記成分に調整された溶鋼を連続鋳造法にて鋳片又は鋼片となすか造塊法にて鋼片となし、高温のまま加熱することなく又は加熱後に熱間圧延を施す。熱間圧延や巻取り条件に関しては特段の制限はなく、常法に従い実施する。
【００３４】
熱間圧延後、脱スケール処理を施し、調質圧延率で３〜１６％の調質圧延を施して鋼板とする。その後、焼鈍・溶融亜鉛めっきを行い、その後に加熱合金化処理を施し高強度合金化溶融亜鉛めっき鋼板とする。この際の焼鈍温度は、６５０〜９００℃とする。６５０℃未満では、再結晶が完了せず、十分な伸びが得られないので６５０℃を下限として焼鈍を施す。９００℃超では、伸びを良くする効果が飽和するだけでなく、粗大粒が発生しかえって加工性を劣化させるので９００℃を上限とする。加熱合金化処理の加熱方式は特に限定あれるものではなく、燃焼ガスによる直接加熱や、誘導加熱、直接通電加熱等、を適宜選択出来る。
【００３５】
高強度合金化溶融亜鉛めっき鋼板となした後、加工性の向上や、加工後の外観のために調質圧延を施した鋼板（ダル仕上げ鋼板、ブライト仕上げ鋼板、表面に特定形状のパターンを転写された鋼板等）、表面に防錆油、潤滑油などの油膜層を有する鋼板など、通常に薄鋼板として用いられる表面の処理を施したいずれの鋼板においても、本発明の成分範囲の鋼板であれば本発明の効果を十分に享受することができる。
【００３６】
ついで、上記化学成分の鋼板を用いて加工成形、例えば絞り加工などのプレス加工を行う。プレス加工を施すにあたっては、鋼板に適当な量の転位を与えるために、強度（引張強さ）や硬度が必要とされる部位に、２％以上の塑性相当ひずみが加えられる成形を施す。歪み量が少ない場合には、後熱処理を施しても本願発明の強度上昇の効果が発揮できないので、プレス時に加える歪み量は２％以上、好ましくは５％以上とする。また、プレス成形法は、２％超の歪みを付与する方法であれば、特に規定するものではなく、絞り加工、張り出し加工、曲げ加工、しごき加工、打ち抜き加工等を加えても何等差し支えない。図２にプレス成形時の歪み量とプレス成形および熱処理後の引張り強さの上昇量（ΔＴＳ）の関係を示す。２％以上、好ましくは５％以上の歪みで、引張強さの上昇量が著しいことが分かる。
【００３７】
プレス成形後、低温での熱処理を施す。この際、熱処理温度が２００℃未満では、本願発明の効果が発現できないので２００℃を下限とし、４５０℃を越えると亜鉛めっき層の鉄・亜鉛合金化反応が進行して、めっき層中の鉄濃度が異常に高まり、できあがった部品の耐食性を損ねることになるので、４５０℃を上限とする。
【００３８】
２００〜４５０℃の温度に加熱する熱処理方法としては、特に規定するものではなく、部分高周波加熱、通電加熱、温浴熱処理、赤外線加熱、熱風加熱など、少なくとも歪み付与部を２００〜４５０℃の温度に加熱する方法であれば、いずれでもかまわない。図３に熱処理温度と熱処理後の引張り強さの上昇量（ΔＴＳ）の関係を示す。成形後ΔＴＳが更に剛性上好ましい９０ＭＰａ以上となる成形後熱処理としては、好ましくは温度が２２０〜３７０℃の範囲である。
【００３９】
【実施例】
以下に、本発明を実施例に基づいて具体的に説明する。表１に示す成分の鋼を溶製し、常法に従い連続鋳造でスラブとした。そして、加熱炉中で１２００℃まで加熱し、８８０℃の仕上げ温度で、熱間圧延を行い、５５０℃で巻取り、ついで、酸洗を施し熱延鋼板とした。また、熱延鋼板の一部に、表１で示す圧延率が３〜１６％の調質圧延を施した。調質圧延を施さない（調質圧延率が０％の）ものを含め被めっき鋼板が板厚０．８〜５．１ｍｍの熱延鋼板を製造した。
【００４０】
更に、熱延鋼板の一部には、比較例として、表１で示す圧延率が７８％の冷間圧延を行い、板厚０．８ｍｍの冷延鋼板を製造した。鋼板は溶融亜鉛めっき前に、表１の温度で６０秒の焼鈍後、８３０℃×６０秒の再結晶焼鈍を行い、冷延鋼板となした。
【００４１】
得られた熱延鋼板、冷延鋼板をＪＩＳ５号引張試験片に加工し、機械的特性値（熱処理なし）の評価を行った。
【００４２】
また、別途、該鋼板をプレスにて成形し、図４に示されるハット型のプレス成形品となした。このとき、しわ押さえ圧を調整し、たて壁部Ａに平均で５％、平坦部Ｂに２％の塑性相当ひずみを加えた。該部品を雰囲気が２５０℃に保たれた炉に１０分間入れ、その後空冷し、熱を加えた。該部品のたて壁部Ａと平坦部Ｂから引張試験片を切り出し、引張強さを測定した。プレス加工後の引張試験では、真の応力−歪み関係を測定することになるので、公称応力での上昇代を見るために、プレス加工前の板厚を試験片板厚とし換算して、公称応力とした。
【００４３】
以上の結果を表１に併記する。
【００４４】
表１から明らかなように、本発明の製造方法による鋼板の方が、熱処理硬化性に優れていることが分かる。
【００４５】
【表１】

【００４６】
【発明の効果】
本発明によれば、加工成形前は強度レベルが３００Ｍｐａ級、４００Ｍｐａ級、５００Ｍｐａ級、６００Ｍｐａ級、あるいは７００Ｍｐａ級で比較的軟質の高強度鋼でプレス成形等の加工成形がしやすく、そして、プレス成形等の成形加工後には比較的低温での短時間熱処理を行うことで、引張強さ又は硬さが上昇し部材や部品の変形強度を高めるか、あるいは剛性を高めることが可能な合金化溶融亜鉛めっき鋼板を製造することができる。
【図面の簡単な説明】
【図１】本発明法と従来方法による鋼板の応力ー歪み曲線を説明する模式図である。
【図２】本発明法による鋼板にプレス成形で付与した歪み量と熱処理硬化量の関係を示す図である。
【図３】成形後熱処理温度と熱処理後の引張強さ上昇量との関係を示す図である。
【図４】ハット型のプレス成形品の形状を示す模式図である。
【符号の説明】
Ａ壁部
Ｂ平坦部[0001]
BACKGROUND OF THE INVENTION
INDUSTRIAL APPLICABILITY The present invention is a processing by a press or the like that is suitable for application to a location where structural strength, particularly deformation strength and / or rigidity is required, such as construction members, machine structural components, and automotive structural components. The present invention relates to a method for producing an alloyed hot-dip galvanized steel sheet used as a base steel sheet of a molded body that is subjected to heat treatment for increasing strength in a predetermined temperature range after forming.
[0002]
[Prior art]
When manufacturing a press-formed body made of a thin steel plate, before press forming, it is soft and easy to press-form, and as a method of hardening after press forming to increase the strength of the part, there is a method of baking at less than 200 ° C. A BH steel sheet has been developed as a steel sheet for paint baking.
[0003]
For example, in Nb-added steels as disclosed in JP-A-55-141526 and JP-A-55-141555, Nb is added in accordance with the C, N, and AI contents in the steel and Nb / (at%. The method of adjusting the solid solution C and solid solution N in the steel sheet by limiting the solid solution C + solid solution N) within a specific range and controlling the cooling rate after annealing, and Japanese Patent Publication No. 61-45689. Thus, it is disclosed that the bake hardenability is improved by the combined addition of Ti and Nb.
[0004]
However, since the steel plate as described above is made of a material having excellent deep drawability, the strength of the steel plate is low and is not necessarily sufficient as a structural material.
[0005]
Furthermore, as disclosed in JP-A-5-25549, it is disclosed that bake hardenability is improved by adding W, Cr, or Mo alone or in combination to steel. However, the increase in strength due to bake hardening is due to the use of solute C and solute N contained in the steel sheet, for example, as schematically shown in the stress-strain curve of the broken BH steel sheet in FIG. Only the yield strength of the material is increased (stress increase due to BH in FIG. 1), and the tensile strength (tensile strength) is not increased. For this reason, there is only an effect of increasing the starting stress of deformation of the member or part, and the effect of increasing the stress required for deformation throughout the entire deformation from the start of deformation to the end of deformation of the member or part (hereinafter referred to as deformation strength characteristics) Not always enough.
[0006]
As a curing method other than press baking of the press-molded body, there is a method by soft nitriding after press molding. For example, a method in which a nitride-forming element such as Cr, Al, V or the like is included in steel so that the strength is increased by nitriding as in JP-A-2-80539, or a nitriding treatment as in JP-A-3-122255. A method of increasing the hardness of a member by precipitation-hardening Cu using the heat of the above is disclosed. However, these methods have the disadvantage that when heating at a temperature higher than 450 ° C. and using ordinary galvanizing to improve corrosion resistance, the plating layer evaporates and it is difficult to obtain a product with good corrosion resistance.
[0007]
[Problems to be solved by the invention]
In order to solve the above-mentioned problems, the present invention is a process such as press molding with a relatively soft high-strength steel having a strength level of 300 Mpa class, 400 Mpa class, 500 Mpa class, 600 Mpa class, 700 Mpa class before processing and forming. Is it easy to form and after heat treatment at a relatively low temperature for the purpose of increasing strength after press forming or other forming processes, will the tensile strength or hardness increase and improve the deformation strength characteristics of members and parts? Alternatively, an object of the present invention is to provide a method for producing an alloyed hot-dip galvanized steel sheet as a base steel sheet capable of increasing rigidity.
[0008]
[Means for Solving the Problems]
The inventors of the present invention have provided a process for forming shapes of various molding materials and parts made of alloyed hot dip galvanized thin steel sheets, a heat treatment method for curing members and parts by heat treatment, and the steel sheets. We conducted intensive research on the deformation strength characteristics of press-formed bodies as parts made of As a result, by adding a suitable amount of C, W, and Mo in the steel composition, further specifying the microstructure of the steel, and performing hot dip galvanizing and heat alloying treatment, it is 200 to 450 ° C. after forming. It was newly discovered that a high increase in tensile strength (or an increase in deformation strength characteristics) can be obtained in a short time when heated to a temperature range of. The present invention has been accomplished based on this discovery.
[0009]
The gist of the invention is as follows.
[0011]
( 1 ) C: 0.01 to 0.08% by weight%,
Si: 0.005 to 1.0%,
Mn: 0.01 to 3.0%,
P: 0.001 to 0.15%,
S: 0.001 to 0.02%,
Al: 0.001 to 0.1%,
N: 0.0002 to 0.01%
And one or two of W and Mo in a total amount of 0.05 to 3.0%
Contained and hot-rolled steel having a component consisting of iron and inevitable impurities, and then subjected to temper rolling at a temper rolling rate of 3 to 16% , followed by annealing at a temperature of 650 to 900 ° C. performs subsequent galvanizing, characterized in that subjected to heat alloying treatment, was subjected to molding the strain 2% or more is applied in plastic strain, 1 minute to the temperature range of 200 to 450 ° C. by short hold 30 minutes, the production method of the galvannealed steel sheet having a comparison and 60MPa or more and tensile strength capable of improving molding strength after elevated heat treatment performance tensile strength after pre-processing forming.
[0012]
( 2 ) As a steel composition, in further weight percent,
Ti: 0.005 to 0.1%,
V: 0.005 to 0.1%
Characterized in that for the additional inclusion of one or two, after subjected to plastic strain of 2% or more strain molded applied in the

claim

1, 1 minute in the temperature range of 200 to 450 ° C. by short hold 30 minutes, the production method of the galvannealed steel sheet having a comparison and 60MPa or more and tensile strength capable of improving molding strength after elevated heat treatment performance tensile strength after pre-processing forming.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
As a method of imparting deformation strength characteristics to a member or part while ensuring processability such as press formability of the member or part, the present inventors have adopted a steel sheet composition, a steel sheet manufacturing method, a heat treatment method, formability (particularly press molding). As a result of intensive research on the properties of steel, the steel sheet containing W and Mo , which are carbide-forming elements having a weak affinity with C, is subjected to hot dip galvanizing and heat alloying treatment, giving a strain of 2% or more. It has been found that the deformation strength characteristics of the alloyed galvanized steel sheet are remarkably improved without loss of corrosion resistance by processing by a press forming method and then performing a heat treatment at 200 to 450 ° C. Furthermore, it has been found that the amount of increase in deformation strength (ΔTS) is remarkable by adding Ti and V as steel compositions.
[0014]
The present invention is described in detail below.
[0015]
First, the reason for limiting the steel components will be described below.
[0016]
C is an element that affects the workability of steel, and the workability deteriorates as the content increases. Therefore, it is 0.08% or less. If it is less than 0.01%, the amount precipitated as carbide during heat treatment at 200 to 450 ° C. is small, and the effect of increasing the strength during heat treatment is small, so 0.01% is made the lower limit.
[0017]
If Si is less than 0.005%, the manufacturing cost will increase drastically, making it less economical, so 0.005% is the lower limit, and if it exceeds 1.0%, the workability deteriorates and zinc does not adhere easily. Since the adhesion of galvanizing is impaired, 1.0% is made the upper limit.
[0018]
If Mn is less than 0.01%, the manufacturing cost will be drastically increased and it will not be economical. Therefore, 0.01% is the lower limit, and if it exceeds 3.0%, the workability deteriorates. The upper limit.
[0019]
P is an element that can increase the strength without impairing the deep drawability, and is added according to the strength level. However, if it is less than 0.001%, the manufacturing cost is drastically increased and it is not economical. 0.001% is the lower limit, and if it exceeds 0.15%, secondary work brittleness will occur, so 0.15% is the upper limit.
[0020]
S is essentially an element that is meaningless to be present in steel, so it is better to be less, but if it is less than 0.001%, the manufacturing cost will increase dramatically and it will not be economical, so 0.001% is the lower limit. If it exceeds 0.02%, red hot embrittlement occurs during hot rolling, and cracks occur on the surface, so-called hot brittleness, so 0.02% is made the upper limit.
[0021]
Al is usually added as a deoxidizing component, and in order to prevent the occurrence of defects such as blow holes, it is necessary to add 0.001% or more. If it is less than 0.001%, it cannot be sufficiently deoxidized, so 0.001% is made the lower limit. Further, if it exceeds 0.1%, the effect of deoxidation is saturated, so 0.1% is made the upper limit. Further, in order to control the form of inclusions by further adding Ti to reduce press cracks, Al is preferably 0.001 to 0.005%.
[0022]
N is preferable to be small in order to ensure workability, but if it is less than 0.0002%, the manufacturing cost increases dramatically and is not economical, so 0.0002% is the lower limit and exceeds 0.01%. Since the workability deteriorates, 0.01% is made the upper limit.
[0023]
During processing molding predetermined low temperature heat treatment after, to exert an effect of increasing the tensile strength, as essential elements, in addition to the element, further Mo, one or two of W, Mo, the W The total amount is 0.05 to 3.0% in steel. If the total amount is less than 0.05%, the tensile strength cannot be sufficiently increased even if the heat treatment is performed. On the other hand, if the total amount exceeds 3.0%, Mo and W strengthen the steel too much, the strength before work forming becomes too high, and the workability is deteriorated. This is because the effect of increasing the thickness is small or saturated, which is economically inconvenient.
[0024]
When the steel sheet containing W or Mo is subjected to work forming such as press forming that gives a strain of 2% or more and subjected to heat treatment at 200 to 450 ° C., the tensile strength of the steel sheet increases. The reason why the tensile strength is increased when a predetermined amount of W or Mo is added to the steel and this heat treatment is performed is not clear, but by applying a strain of 2% or more, a considerable amount of dislocations are generated in the ferrite portion in the steel sheet. Introduced, the diffusion of C, W, and Mo in steel dramatically increases even at low temperatures, and further, the introduction of dislocations into the core causes precipitation of W and Mo carbides on the dislocations. The present inventors believe that the tensile strength of members and parts increases as a result of precipitation.
[0025]
Further, in order to increase the tensile strength during the heat treatment after work forming , in addition to W and Mo , Ti and V can be added as selective elements. The reason why the effect of increasing the tensile strength and hardness of a member or part containing Ti or V is not clear, but when Ti and V are added, the effect of the low-temperature heat treatment due to the addition of W and Mo described above is synergistic. In the present invention, fine carbides are formed in the steel sheet, and this fine carbide has an effect of increasing the amount of strain by effectively multiplying dislocations with respect to the strain applied during pressing. They are thinking. In particular, addition of V and further addition of Ti are preferable because they can shorten the processing after processing and forming.
[0026]
Ti is an element that enhances the effect of increasing the strength during heat treatment. If less than 0.005%, the effect is small, so 0.005% is made the lower limit. Ti is an element that increases the strength of the steel, and if it exceeds 0.1%, workability deteriorates, so 0.1% is made the upper limit.
[0028]
V is an element that enhances the effect of increasing the strength during heat treatment, and if less than 0.005%, the effect is small, so 0.005% is made the lower limit. V is an element that increases the strength of the steel. If it exceeds 0.1%, workability deteriorates, so 0.1% is made the upper limit.
[0029]
Although the components are adjusted as described above, in order to increase the effect of increasing the strength during the heat treatment after forming, it is desirable to keep the amount of C in the steel sheet in a solid solution state at the heat treatment temperature. element and a Ti, and V in Ti weight {(48/12) × C [% ] + (48/14) × N [%]} below, the even properly V amount {(51 × 4/12 × 3) x C [%] + (51/14) x N [%]} or less, or when Ti and V are added in combination, {Ti [%] x 12/48 + V [%] x 12 x 3/51 It is desirable to add so as to satisfy x4} <C [%] + N [%] x12 / 14.
[0030]
Next, preferred embodiments of the steel microstructure obtained by the production method of the present invention will be described.
[0031]
The steel microstructure is preferably mainly composed of ferrite or ferrite (the ferrite fraction of the steel microstructure is 60% or more). Ferrite is soft and excellent in workability, and can accumulate many dislocations in crystal grains when processed. Further, since dislocations uniformly enter when strain is applied and the strength of the entire steel sheet can be increased uniformly, the microstructure of the steel is mainly composed of ferrite or ferrite. Furthermore, if the remaining structure of the ferrite-based structure that is not ferrite is pearlite and / or bainite, stress concentration is increased at the interface between the ferrite and the remaining structure, and dislocations can be effectively imparted to the ferrite. In addition, since the pearlite and / or bainite structure is not as hard as the martensite structure, the pearlite and / or bainite structure itself is deformed and the amount of dislocation increases as a whole of the steel sheet, so that the strength of the steel is effectively increased during heat treatment.
[0032]
The post-molding strength-increasing heat treatment performance of the present invention is 1 minute in a temperature range of 200 to 450 ° C. (more preferably 220 to 370 ° C.) after performing a molding process in which a strain equivalent to 2% or more is applied as a plastic equivalent strain. Tensile strength ΔTS (= TS after post-processing heat treatment-TS before processing) in comparison with the tensile strength before and after processing molding in short-time holding for 30 minutes (heating after cold forming or temperature holding after warm forming, etc.) (TS) 60 MPa or more (more preferably, ΔTS is 90 Mp or more) The strength can be improved, or Vickers hardness ΔHv (= post-treatment heat treatment ΔHv−pre-treatment ΔHv) 18 or more (more preferably ΔHv is 27 or more) after heat treatment The heat treatment which can be raised is shown. However, this heat treatment does not require a hardening element (for example, nitrogen or the like) to actively enter the molded body from the outside like nitriding treatment or the like. The high strength alloyed hot-dip galvanized steel sheet as a heat treatment material for increasing strength (tensile strength) after work forming of the present invention may be either a hot-rolled steel sheet or a cold-rolled steel sheet, and the thickness is not limited. Is particularly effective at 0.4 to 6 mm.
[0033]
In the production of the steel of the present invention, the molten steel adjusted to the above components is formed into a slab or steel slab by a continuous casting method or a steel slab by a granulation method, and heated without heating or after heating. to facilities between rolling. There are no particular restrictions on the hot rolling and winding conditions, and it is carried out in accordance with conventional methods.
[0034]
After hot rolling, subjected to a descaling process, the steel plate subjected to temper rolling 3 to 16% in the temper rolling rate. Then, annealing and hot dip galvanizing are performed, and then heat alloying treatment is performed to obtain a high-strength galvannealed steel sheet. The annealing temperature at this time shall be 650-900 degreeC. If the temperature is lower than 650 ° C., recrystallization is not completed and sufficient elongation cannot be obtained. Therefore, annealing is performed with 650 ° C. being the lower limit. If it exceeds 900 ° C., the effect of improving elongation is saturated, and coarse grains are generated and the workability is deteriorated, so 900 ° C. is set as the upper limit. The heating method of the heat alloying treatment is not particularly limited, and direct heating by combustion gas, induction heating, direct current heating, or the like can be appropriately selected.
[0035]
Steel sheet that has been temper rolled for improved workability and appearance after processing after being made into a high-strength galvannealed steel sheet. In any steel sheet that has been subjected to surface treatment that is usually used as a thin steel sheet, such as a steel sheet having an oil film layer such as rust preventive oil or lubricating oil on the surface, the steel sheet having the component range of the present invention is used. If so, the effects of the present invention can be fully enjoyed.
[0036]
Next, press working such as drawing is performed using the steel plate having the above chemical components. In performing the press working, in order to give an appropriate amount of dislocation to the steel sheet, forming that applies a plastic equivalent strain of 2% or more is performed on a portion where strength (tensile strength) and hardness are required. When the amount of strain is small, the effect of increasing the strength of the present invention cannot be exhibited even if post-heat treatment is performed. Therefore, the amount of strain applied during pressing is 2% or more, preferably 5% or more. The press molding method is not particularly limited as long as it gives a strain of more than 2%, and there is no problem even if drawing, overhanging, bending, ironing, punching, or the like is added. FIG. 2 shows the relationship between the amount of strain during press molding and the amount of increase in tensile strength (ΔTS) after press molding and heat treatment. It can be seen that the increase in tensile strength is significant at strains of 2% or more, preferably 5% or more.
[0037]
After press molding, heat treatment is performed at a low temperature. At this time, if the heat treatment temperature is less than 200 ° C., the effect of the present invention cannot be exhibited. Therefore, the lower limit is 200 ° C., and if it exceeds 450 ° C., the iron-zinc alloying reaction of the galvanized layer proceeds, and the iron in the plated layer Since the concentration increases abnormally and the corrosion resistance of the finished part is impaired, 450 ° C. is set as the upper limit.
[0038]
The heat treatment method for heating to a temperature of 200 to 450 ° C. is not particularly specified, and at least the strain imparting part is set to a temperature of 200 to 450 ° C., such as partial high-frequency heating, current heating, warm bath heat treatment, infrared heating, hot air heating, and the like. Any heating method may be used. FIG. 3 shows the relationship between the heat treatment temperature and the amount of increase in tensile strength (ΔTS) after heat treatment. As the post-molding heat treatment in which ΔTS after molding is more preferably 90 MPa or more in terms of rigidity, the temperature is preferably in the range of 220 to 370 ° C.
[0039]
【Example】
The present invention will be specifically described below based on examples. Steels having the components shown in Table 1 were melted and slabs were obtained by continuous casting according to a conventional method. And it heated to 1200 degreeC in the heating furnace, hot-rolled with the finishing temperature of 880 degreeC, wound up at 550 degreeC, and then pickled, and it was set as the hot-rolled steel plate. Moreover, temper rolling with a rolling rate of 3 to 16 % shown in Table 1 was applied to a part of the hot-rolled steel sheet. Plated steel sheets including those not subjected to temper rolling (the temper rolling ratio was 0%) produced hot rolled steel sheets having a plate thickness of 0.8 to 5.1 mm.
[0040]
Further, as a comparative example , a part of the hot-rolled steel sheet was subjected to cold rolling with a rolling rate of 78 % shown in Table 1 to produce a cold-rolled steel sheet having a thickness of 0.8 mm. Prior to hot dip galvanization, the steel sheet was annealed for 60 seconds at the temperature shown in Table 1 and then recrystallized at 830 ° C. for 60 seconds to form a cold-rolled steel sheet.
[0041]
The obtained hot-rolled steel sheet and cold-rolled steel sheet were processed into JIS No. 5 tensile test pieces, and mechanical property values (no heat treatment) were evaluated.
[0042]
Separately, the steel sheet was formed by pressing to form a hat-shaped press-formed product shown in FIG. At this time, the wrinkle pressure was adjusted, and an average plastic strain of 5% was applied to the vertical wall portion A and 2% to the flat portion B. The parts were placed in a furnace maintained at 250 ° C. for 10 minutes, then air cooled and heated. Tensile test pieces were cut out from the vertical wall part A and flat part B of the part, and the tensile strength was measured. In the tensile test after pressing, the true stress-strain relationship is measured, so to see the rise in nominal stress, the plate thickness before pressing is converted to the test piece plate thickness, Stress was used.
[0043]
The above results are also shown in Table 1.
[0044]
As can be seen from Table 1, the steel sheet produced by the production method of the present invention is superior in heat treatment curability.
[0045]
[Table 1]

[0046]
【The invention's effect】
According to the present invention, before processing, the strength level is 300 Mpa class, 400 Mpa class, 500 Mpa class, 600 Mpa class, or 700 Mpa class, which is relatively soft high-strength steel and can be easily processed by press molding. Alloying and melting that can increase the tensile strength or hardness and increase the deformation strength of members and parts, or increase the rigidity by performing short-term heat treatment at a relatively low temperature after forming processing such as forming. A galvanized steel sheet can be manufactured.
[Brief description of the drawings]
FIG. 1 is a schematic diagram for explaining stress-strain curves of a steel sheet according to the method of the present invention and a conventional method.
FIG. 2 is a graph showing the relationship between the amount of strain imparted to a steel sheet by press forming and the amount of heat treatment hardening according to the present invention.
FIG. 3 is a graph showing the relationship between the heat treatment temperature after molding and the amount of increase in tensile strength after heat treatment.
FIG. 4 is a schematic diagram showing the shape of a hat-type press-formed product.
[Explanation of symbols]
A Wall B Flat part

Claims

C: 0.01 to 0.08% by weight%,
Si: 0.005 to 1.0%,
Mn: 0.01 to 3.0%,
P: 0.001 to 0.15%,
S: 0.001 to 0.02%,
Al: 0.001 to 0.1%,
N: 0.0002 to 0.01%
And one or two of W and Mo in a total amount of 0.05 to 3.0%
Contained and hot-rolled steel having a component consisting of iron and inevitable impurities, and then subjected to temper rolling at a temper rolling rate of 3 to 16% , followed by annealing at a temperature of 650 to 900 ° C. performs subsequent galvanizing, characterized in that subjected to heat alloying treatment, was subjected to molding the strain 2% or more is applied in plastic strain, 1 minute to the temperature range of 200 to 450 ° C. by short hold 30 minutes, the production method of the galvannealed steel sheet having a comparison and 60MPa or more and tensile strength capable of improving molding strength after elevated heat treatment performance tensile strength after pre-processing forming.

As a steel composition, the weight percentage
Ti: 0.005 to 0.1%,
V: 0.005-0.1%
Characterized in that for the additional inclusion of one or two, after subjected to plastic strain of 2% or more strain molded applied in the claim 1, 1 minute in the temperature range of 200 to 450 ° C. by short hold 30 minutes, the production method of the galvannealed steel sheet having a comparison and 60MPa or more and tensile strength capable of improving molding strength after elevated heat treatment performance tensile strength after pre-processing forming.