JP4336020B2 - Cold-rolled steel sheet that prevents material deterioration in welds and weld heat-affected zones - Google Patents
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Description
【０００１】
【発明の属する技術分野】
本発明は、溶接部および溶接熱影響部の材質劣化を防止した鋼板に関するものである。
【０００２】
【従来の技術】
近年、自動車用鋼板に要求される材料特性は年々高度になっている。特に自動車のフェンダーやオイルパン等については、極めて過酷なプレス成形がなされることから、深絞り性および延性についてはより一層の向上が期待されてきた。折から、ＲＨやＤＨなどの真空脱ガス技術の発展にともない鋼中の固溶元素を低減し成形性を向上させた極低炭素鋼が開発され、それまで使用されてきた低炭素Ａｌキルド鋼に代わって用いられるようになった。さらに最近ではＴｉ、Ｎｂ等によって鋼中の固溶Ｃ、Ｎをｓｃａｖｅｎｇｉｎｇすることで飛躍的に成形性を向上させた鋼として、特開平１−２２５７２７号公報、特開平２−３４７２２号公報等に開示されているＩｎｔｅｒｓｔｉｔｉａｌ Ｆｒｅｅ ｓｔｅｅｌ（以下ＩＦ鋼）が、広く用いられるようになっている。
【０００３】
一方、サスペンションアーム、ロードホイール等の足廻り部品やサイドメンバー、クロスメンバー等の内板構造部材においては、成形性、静的強度などに加えて耐久性が強く求められるのは当然のことながら、フロア、ダッシュ等の内板パネルおよびフードアウター、トランクリッドアウター等の外板部品についても、走行時の振動によって外力が繰返し加えられるため、疲労耐久性等が求められている。
【０００４】
このような内外板部品の接合にはスポット溶接、部材間の接合にはレーザー溶接やアーク溶接が多用され、例えば乗用車一台のスポット溶接の打点数は数千点にも及ぶものもある。こうした溶接継手部分はその形状から応力集中が起こりやすく、自動車走行時の振動による疲労破壊の起点になる危険性がある。一般に疲労特性は、材料の引張強度や降伏強度に比例しているが、溶接部の疲労特性は、その形状および、溶接によって受ける局部的な温度履歴のため、静的強度による整理が困難であり、その材料設計には特別な配慮が必要である。しかし、このような溶接継手部の疲労特性について述べられているものは極めて少ない。
【０００５】
例えば、特公平３−５６３０１号公報には、スポット溶接前の原板中に未再結晶組織を５〜３０％残留させることでスポット溶接後の溶接継手部近傍の硬度分布を最適化しスポット溶接継手部の疲労強度を向上させる方法が開示されている。しかしながら、未再結晶組織は、深絞り性および張出し性などの成形性には有害であるため、高度なプレス成形性を要求される自動車の内外板部品への適用には不適当である。
【０００６】
また、特公平５−５７３３０号公報に開示されている技術は、Ｔｉ、ＮｂおよびＢを複合添加しスポット溶接部を細粒化することで疲労強度の改善を狙ったものである。当該技術によれば、疲労限については従来の低炭素鋼と同等以上の特性が得られているものの、１００万回以下の低サイクルにおける時間強度を向上させるためには、さらにスキンパス圧延率を上げことが必須とされている。しかしながら、そのように焼鈍後に過度の冷間圧延を施すことは伸びなどの材質劣化をもたらすので成形性にとって好ましくない。
【０００７】
さらに、特公平７−５６０５４公報に開示されている技術は、Ｏ量およびＡｌ（％）／Ｎ（％）の値を規定し、特にＡｌ／Ｎ≧３０とすることで溶接部の組織を改善し、疲労強度を向上させることを特徴としている。Ａｌ／Ｎ≧３０を満たすためには、Ｎを少なくするか、Ａｌの添加量を増やさなければならない。Ｎを極端に少なくすることは、製鋼コストの増加につながる。一方、Ａｌの増加は、表面性状の劣化につながるのでいずれも好ましくない。
【０００８】
また、ＩＦ鋼のスポット溶接の疲労特性については、特開平１１−２７９６８９号公報にあるように、溶接継ぎ手の疲労強度がＡｌキルド鋼に比べ低い原因が、両鋼種の間で疲労破壊の起点が異なる事になることが見出されている。溶接熱影響部（Ｈｅａｔ Ａｆｆｅｃｔｅｄ Ｚｏｎｅ：以下ＨＡＺ）の金属組織が両者で大きく異なり、ＩＦ鋼では疲労破壊の起点が粗粒化したＨＡＺ部であるのに対し、低炭素Ａｌキルド鋼では、ＨＡＺ部は粗粒化せずにより強度の低い母材が疲労破壊の起点となる。これに対して、ＭｇとＴｉの酸化物および／または複合酸化物を予め分散させておくとＨＡＺのフェライト粒の粒成長を抑制することを見出している。しかしながら、これらは入熱の低いスポット溶接に限った技術である。
【０００９】
一方で、自動車の軽量化や低コスト化のニーズに伴い、各部位の一体成形（例えばハイドロフォーム 特開平１０−１７５０２６号公報参照）や強度の異なる鋼板同士の接合・成形（例えばテーラードブランク、特開平１０−１８０４７０号公報参照）などの成形加工方法が実際に採用されつつある。これらの新しい成形手法は、部品のモジュール化から、大入熱による連続溶接が採用される。これによりスポット溶接以上に結晶粒の粗大化が著しく促進されることで、溶接部での成形性や疲労特性はさらに劣化する。このような比較的入熱の大きい溶接時においても薄鋼板に於いて粒の粗大化を抑制する技術はない。
【００１０】
【発明が解決しようとする課題】
本発明の目的は、良好な溶接継手の機械的特性を必要とする自動車部品等への適用を目指すため、成形性を損なうことなく溶接継手の材質劣化を防止した鋼板を提供することである。
【００１１】
【課題を解決するための手段】
本発明者らは、溶接継手の疲労特性等の機械的特性を向上させるための長年にわたる研究の成果として、溶接部または溶接熱影響部で生じる金属組織の粗大化が材質劣化の主たる原因であることを見出し、これらをＭｇを活用することで抑制する手法を見出すに至った。スポット溶接などの比較的入熱が低い場合には、特開平１１−２７９６８９号公報にあるように、フェライト粒の粒成長を抑制することが重要である。しかし、アークやレーザー溶接の様な比較的入熱の大きい溶接時には、熱影響部のみならず溶接部も粗粒化する。このような溶接に於いては、フェライト粒成長の抑制のみを考慮するだけでは、溶接部および溶接熱影響部双方の粗粒化を防止できない。すなわち、フェライト粒成長に加えて、▲１▼冷却時のオーステナイト粒径を小さくすること、▲２▼オーステナイトからフェライトへの変態時の変態界面の移動の抑制および▲３▼変態核生成の促進の３項目を実施することで初めて溶接部および溶接熱影響部双方での粗粒化防止が達成でき、Ｍｇの酸化物および／又は硫化物がこれに大きな効果を持つことを明らかにした。
【００１２】
すなわち、本発明は、上記知見により構成したもので、その要旨は、以下の通りである。
（１）質量％にて、Ｃ：０．０００１〜０．２０％、Ｓｉ：０．０１〜２．５％、Ｍｎ：０．０１〜３．０％、Ｐ：０．００１〜０．２％、Ｓ：０．０００１〜０．０２％、Ａｌ：０．００８〜０．０５％、Ｍｇ：０．００２１〜０．６％、Ｏ：０．００１〜０．１０％、Ｎ：０．００２４〜０．０１％を含有し、かつ、Ｍｇを含有した酸化物および／又は硫化物を含み、残部がＦｅ及び不可避的不純物から成り、前記Ｍｇを含有した酸化物および／又は硫化物のサイズが０．０００５〜０．０５μｍの粒子を１０5個／ｍｍ2以上１０7個／ｍｍ2以下かつ０．０５μｍ超１μｍ以下の粒子を１０4個／ｍｍ2以上１０7個／ｍｍ2以下含み、溶接部および溶接熱影響部の最大結晶粒径と母材の平均結晶粒径の比が３．０以下であることを特徴とする溶接部および溶接熱影響部の材質劣化を防止した冷延鋼板。
【００１３】
（２）Ｔｉ、Ｔａ、Ｚｒ、Ｖ及びＮｂの１種又は２種以上を合計で０．００１〜０．５質量％含む事を特徴とした前記（１）記載の溶接部および溶接熱影響部の材質劣化を防止した冷延鋼板。
【００１４】
（３）Ｂを０．０００１〜０．０１質量％以下含むことを特徴とした前記（１）又は（２）に記載の溶接部および溶接熱影響部の材質劣化を防止した冷延鋼板。
（４）Ｃｒ、Ｃｕ、Ｎｉ、Ｃｏ、Ｗ及びＭｏの１種又は２種以上を合計で０．００１〜１．５質量％含む事を特徴とした前記（１）〜（３）の何れか１項に記載の溶接部および溶接熱影響部の材質劣化を防止した冷延鋼板。
（５）Ｃａ及び希土類元素の１種又は２種を合計で０．０００１〜０．５質量％含む事を特徴とした前記（１）〜（４）の何れか１項に記載の溶接部および溶接熱影響部の材質劣化を防止した冷延鋼板である。
また、上記の本発明は、溶融亜鉛めっき鋼板にも適用可能である。
【００１５】
【発明の実施の形態】
以下、本発明の構成要素について説明する。まず、本発明の化学成分の限定理由について説明する。
Ｃ：Ｃは冷延時に固溶状態で鋼中に存在していると、後の焼鈍時に深絞り性にとって好ましくない方位が生成するため特に成形性を重要視するためには低いことが望ましい。また、強化元素でもある。このことから０．２０％以下、成形性が特に必要な場合には質量％で０．００３％以下がよい。一方では、極低炭素化は製鋼コストの上昇を伴うため、成形性に悪影響が無くなる範囲として０．０００１％以上とした。
【００１６】
Ｓｉ：Ｓｉは固溶強化元素であり０．０１％以上添加することとした。一方で過剰添加は、固溶硬化が著しくなり加工用に不適当になるばかりでなく、ランクフォード値を劣化させかつ溶融亜鉛めっきの密着性を悪くするため２．５％以下とした。また、特に成形性が必要な場合には質量％で０．０５％以下がよい。
Ｍｎ：Ｍｎは３．０％超添加すると延性が著しく劣化し、また成形性も低下するためこれを上限とした。一方で、強化元素であり０．０１％以上の添加とした。
【００１７】
Ｐ：Ｐは粒界に偏析し粒界脆化を起こし、二次加工割れの原因となる元素であるため０．２％以下とした。一方では、成形性にあまり悪影響を及ぼさない強化元素であり０．００１％以上とした。
Ｓ：Ｓは多すぎると熱間圧延時の割れを引き起こすばかりでなく、ランクフォード値の劣化を起こすので極力低減させるべきであるが０．０２％以下ならば許容できる範囲である。一方では、Ｍｇと硫化物を形成して、溶接部及び溶接熱影響部の粗粒化防止に有効であるため０．０００１％以上の添加とした。
【００１８】
Ａｌ：溶鋼脱酸のために添加するが、０．０５％超添加するとＭｇ添加の効果を阻害するために０．０５％以下とする。一方では、Ｍｇとスピネル酸化物を形成することで、溶接部及び溶接熱影響部の粗粒化防止に寄与することから０．００１％以上の添加とした。なお、Ａｌ量の下限値は、本発明の実施例に基づき、０．００８％以上とする。
Ｍｇ：Ｍｇは、酸素及び／または硫黄と結合していろいろなサイズの酸化物及び／または硫化物を形成する。鋼中に分散したこれらのＭｇ含有の粒子はピニング効果によってオーステナイト及びフェライト域での結晶粒の粗大化を抑制に寄与する。また、変態時の界面移動を抑制し、変態の核生成サイトとしてのも有効な粒子を形成して、最終的な粒径の粗大化抑制に大きく寄与する。このために０．０００１％以上の添加とした。しかし、０．６％超添加してもその効果は飽和してしまうばかりでなく、製鋼技術上非常に難しい。従ってＭｇの添加量は０．０００１％以上０．６％以下とする。なお、Ｍｇ量の下限値は、本発明の実施例に基づき、０．００２１％以上とする。
【００１９】
Ｏ：Ｏは、Ｍｇと酸化物を形成して溶接時に結晶粒の粗大化を抑制するため、０．００１％以上必要である。しかし、０．１０％超では、酸化物が粗大化し、その効果が低下するばかりか、成形性に好ましくないＣ系介在物が増加する。従ってＯの含有量は０．００１％以上０．０１％以下とする。
Ｎ：Ｎは、Ｔｉとの窒化物および複合窒化物を形成し、溶接熱影響部の粗粒化防止に有効であるため、０．００１％以上含有する必要がある。しかし、延性向上やランクフォード値向上にとって好ましくない元素であるので、Ｎの含有量は０．００１％以上０．０１％以下とする。
【００２０】
Ｔｉ、Ｔａ、Ｚｒ、Ｖ、Ｎｂ：また、必要に応じて添加するＴｉ、Ｔａ、Ｚｒ、Ｖ、Ｎｂは、炭化物、窒化物もしくは炭窒化物を形成することによって鋼材を高強度化する事が出来るうえ、Ｍｇ含有の酸化物又は硫化物と溶接部および溶接熱影響部の粗粒化防止にも特に効果的であるので合計の含有量として０．００１質量％以上添加する。また、一方、その合計が０．５％を越えた場合には母相であるフェライト粒内もしくは粒界に多量の炭化物、窒化物もしくは炭窒化物として析出して延性を劣化させる場合があり、上限を０．５質量％とした。
【００２１】
Ｂ：また、必要に応じて添加するＢは、粒界の強化や鋼材の高強度化に有効ではあるので０．０００１質量％以上添加する。一方、その添加量が０．０１質量％を越えるとその効果が飽和するばかりでなく、必要以上に鋼板強度を上昇させ、加工性も低下させることから、上限を０．０１質量％とした。
Ｃｒ、Ｃｕ、Ｎｉ、Co、Ｗ、Ｍｏ：必要に応じて添加するＣｒ、Ｃｕ、Ｎｉ、Co、ＷおよびＭｏは強化元素であり、溶接性の観点からＣの添加量が制限される場合には、この様な強化元素を適量添加する。合計で０．００１質量％以上添加とし、一方では、合計が１．５質量％を越える場合には、母相であるフェライトの硬質化を招くためこれを上限とした。
【００２２】
Ｃａ、Ｒｅｍ：必要に応じて添加するＣａ，ＲＥＭは介在物制御やＭｇ含有の酸化物および硫化物制御に有効な元素で、適量添加は熱間加工性を向上させるうえ溶接部及び溶接熱影響部の粒粗大化防止にも寄与するので合計で０．０００１質量％以上添加するが、過剰添加は逆に熱間脆化を助長させたりＭｇの酸化物及び硫化物の生成を阻害しするため上限を０．５質量％ととした。
なお、本発明において上記以外の成分はＦｅとなるがスクラップ等の溶製原料（ＣｕやＳｎ等）から混入する不可避的不純物の含有は許容される。
【００２３】
次に、本発明の鋼板に含まれるＭｇ含有の酸化物および／又は硫化物の存在状態について説明する。
ここで、Ｍｇ含有の酸化物および／又は硫化物とはＭｇＯやＭｇＳのようなＭｇ単独およびＴｉ2 Ｏ3 のようなＴｉ単独の酸化物粒子およびそれらの複合酸化物粒子だけでなく、それら以外の酸化物、複合酸化物（ＭｇおよびＡｌを含有したスピネル型酸化物など）、窒化物（ＴｉＮなど）および硫化物（ＭｎＳなど）の析出物が含まれるような複合粒子のことを指す。その粒子径が０．０５μｍ超であると冷却時のオーステナイト粒やフェライト粒の細粒化および変態界面の移動速度の抑制による溶接時の粗粒化を抑制することが不十分になり、溶接継手の疲労特性や他の機械的特性の向上が望めない。
【００２４】
粒界や変態界面の移動を妨げるには微細な粒子ほど効果的である一方で、０．００５μｍ未満の超微細粒子になると再結晶焼鈍時の粒成長が過度に妨げられ良好な延性やランクフォード値が得られないので、０．０００５μｍ以上とする。これらの面密度は、１０5 個／ｍｍ2 未満では、これら粒子の数が少なく溶接時のフェライト粒の粒成長を抑制することが不十分になり、１０7 個／ｍｍ2 超になると延性劣化を招くため、これを上限とした。
【００２５】
さらに、変態核生成を促進させ、核生成サイトを多く導入することで、溶接部および溶接熱影響部の粗大粒防止を図った。変態核として粒子が作用するためには０．０５μｍ以下では不十分な大きさであり、１μｍ超では延性を劣化させるための粒子径を０．０５μｍ超１μｍ以下とした。また、面密度については１０4 個／ｍｍ2 未満では細粒化に寄与するに十分な核生成サイトを確保できず、１０7 個／ｍｍ2 超では加工性を劣化させるため、０．０５μｍ超１μｍ以下の粒子を１０4 個／ｍｍ2 以上１０7 個／ｍｍ2 以下を含むこととした。
【００２６】
上記の微細粒子の測定には透過電子顕微鏡を用いる。１万倍から１０万倍にて少なくとも１０００μｍ2 以上の範囲を測定することとする。また、粒子の組成の同定は、主にＥＤＳ（Ｅｎｅｒｇｙ Ｄｉｓｐｅｒｓｉｖｅ Ｘ−ｒａｙ Ｓｐｅｃｔｒｏｓｃｏｐｙ）やＥＥＬＳ（Ｅｌｅｃｔｒｏｎ Ｅｎｅｒｇｙ Ｌｏｓｓ Ｓｐｅｃｔｒｏｓｃｏｐｙ）を用いて行う。また、特に５０ｎｍ以下の非常に微細な粒子に関しては電解放射型電子銃を搭載した透過電子顕微鏡を用いて微細領域を分析する必要がある。
本発明に係るＭｇを含有する酸化物及び／又は硫化物は製鋼〜焼鈍の各段階で生成・成長する。また、溶接時の溶融部では、これらの製造工程で生成したＭｇ含有の酸化物および／又は硫化物が一旦解けて冷却時に生成する事が考えられるが、上記溶接前の鋼板に於いて粒子の大きさ及び面密度を満足すれば何ら本発明の効果を損なうものではない。
【００２７】
また、上述のとおり、溶接部および溶接熱影響部の材質劣化は粒の粗大化がその大きな原因である。溶接部および溶接熱影響部の最大粒径と母材の粒径の比が３．０を越えると継ぎ手の疲労耐久性や加工性等が劣化するためこれを上限とした。また、下限は設けないが、母材よりも溶接部及び熱影響部が細粒になってしまうと母材／溶接部間又は母材／熱影響部間での硬度差等の特性が大きくなってしまいこれも継ぎ手材質として好ましくなく、母材相当の粒径が溶接部および熱影響部で得られること、すなわち、溶接部および最大粒径と母材の平均粒径の比が０．８以上が継ぎ手材質として好ましい。粒径は光学顕微鏡により圧延方向および板面に垂直な面（Ｃ断面）を１００倍〜１０００倍で観察して切断法で平均粒径を求める。最大粒径は、１０断面以上の断面を観察して、画像解析により各々の円相当径を求めて最大のものを最大径とする。
【００２８】
【実施例】
以下に、実施例により本発明をさらに説明する。
表１に示す成分の鋼を、実験室的規模で溶製して、その鋳片を加熱温度１２００℃で再加熱し、９１０℃〜９３０℃の仕上げ圧延の温度範囲で６．０ｍｍに圧延した後７１０℃で巻取った。酸洗後０．８ｍｍまで冷間圧延を施し、各鋼の再結晶温度を測定して、その再結晶温度〜再結晶温度＋５０℃の範囲（概ね８００〜１０００℃）で焼鈍し、スキンパス圧延を圧延率１％で行った。
その後、アーク溶接をビードオンプレートで板厚を貫通する条件で行い、溶接部および溶接熱影響部の最大粒径と母材の平均粒径を圧延方向に垂直な断面で光学顕微鏡にて測定した。また、同じ断面から、透過電子顕微鏡用のカーボン抽出レプリカを作製して、１万倍から１０万倍で観察をして、各粒子を各々１００視野観察することで面密度を測定した。粒子の組成はＴＥＭに付属のＥＤＳおよびＥＥＬＳにより分析した。また、機械的性質として継ぎ手の引張り試験を行い、評価した。その結果を表２に示す。
【００２９】
【表１】
【表２】
Ｄ１〜Ｄ１９は、溶接部および溶接熱影響部の最大粒径と母材の平均粒径の比が大きくとも３程度におさまっており良好な継ぎ手引張り特性の強度延性バランスを得ていることが判る。
一方で、酸素および酸素と結合力の強いＭｇ，Ａｌ，Ｔｉ，Ｚｒ，ＣａおよびＲｅｍの添加量が本発明の範囲を満たさないＣ２〜Ｃ６およびＣ９は、各粒子の面密度の低く、溶接部又は溶接熱影響部に母材平均粒径よりも５倍以上大きい粒が認められ、一部の鋼種（Ｃ９）は継ぎ手引張りの強度延性バランスも低い。また、面密度の大きな値をとるＣ１や高ＢのＣ７、高ＭｏのＣ８は、溶接部および溶接熱影響部の最大粒径と母材の平均粒径の比は良好なものの、継ぎ手引張りの強度延性バランスがＴＳ×Ｅｌ＜１１０００と低い。
【００３２】
【発明の効果】
本発明は、上述したように溶接部および溶接熱影響部の結晶粒の粗大化を抑制して継手特性および成形性に優れた冷延鋼板を与えるもので、これらの鋼板を用いることで、自動車の部品等における耐久性の大幅な改善が期待できる。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a steel plate in which material deterioration of a welded portion and a weld heat affected zone is prevented.
[0002]
[Prior art]
In recent years, the material properties required for automotive steel sheets have become more advanced year by year. In particular, automobile fenders, oil pans, and the like are subjected to extremely severe press molding, so that further improvement in deep drawability and ductility has been expected. Occasionally, along with the development of vacuum degassing technologies such as RH and DH, ultra-low carbon steel with improved formability by reducing solid solution elements in steel has been developed, and the low-carbon Al-killed steel that has been used so far Has been used instead of. More recently, steels having dramatically improved formability by scavenging solute C and N in steel with Ti, Nb, etc. are disclosed in JP-A-1-225727, JP-A-2-34722, etc. The disclosed Interstitial Free steel (hereinafter referred to as IF steel) has been widely used.
[0003]
On the other hand, it is a matter of course that durability in addition to formability, static strength, etc. is strongly demanded for suspension parts such as suspension arms, road wheels, and inner plate structural members such as side members and cross members. Since external forces are repeatedly applied to the inner plate panels such as the floor and the dash and the outer plate parts such as the hood outer and the trunk lid outer, the fatigue durability is required.
[0004]
Spot welding is often used for joining the inner and outer plate parts, and laser welding and arc welding are often used for joining the members. For example, the number of spot welding points for a single passenger car can reach several thousand. Such welded joints are subject to stress concentration due to their shape, and there is a danger of becoming a starting point for fatigue failure due to vibrations during vehicle travel. In general, fatigue properties are proportional to the tensile strength and yield strength of the material, but the fatigue properties of welds are difficult to organize by static strength because of their shape and local temperature history experienced by welding. The material design requires special consideration. However, very little is described about the fatigue characteristics of such welded joints.
[0005]
For example, Japanese Patent Publication No. 3-56301 discloses a spot welded joint portion that optimizes the hardness distribution in the vicinity of the welded joint portion after spot welding by leaving an unrecrystallized structure 5 to 30% in the original plate before spot welding. A method for improving the fatigue strength of steel is disclosed. However, since the non-recrystallized structure is harmful to formability such as deep drawability and stretchability, it is unsuitable for application to automobile inner and outer plate parts that require high press formability.
[0006]
In addition, the technique disclosed in Japanese Patent Publication No. 5-57330 aims to improve fatigue strength by adding Ti, Nb, and B in combination to refine the spot weld. According to the technology, although the fatigue limit has the same or better characteristics as the conventional low carbon steel, the skin pass rolling rate is further increased in order to improve the time strength in the low cycle of 1 million times or less. It is essential. However, excessive cold rolling after annealing as described above is not preferable for formability because it causes material deterioration such as elongation.
[0007]
Furthermore, the technique disclosed in Japanese Patent Publication No. 7-56054 defines the amount of O and the value of Al (%) / N (%), and particularly improves the microstructure of the weld by setting Al / N ≧ 30. And improving fatigue strength. In order to satisfy Al / N ≧ 30, N must be reduced or the amount of Al added must be increased. Reducing N extremely leads to an increase in steelmaking costs. On the other hand, an increase in Al leads to deterioration of surface properties, which is not preferable.
[0008]
Further, regarding the fatigue characteristics of spot welding of IF steel, as disclosed in Japanese Patent Laid-Open No. 11-279589, the cause of low fatigue strength of welded joints compared to Al killed steel is the origin of fatigue failure between both steel types. It has been found to be different. The metal structure of the heat affected zone (Heat Affected Zone: HAZ) is greatly different between the two, and the origin of fatigue fracture in the IF steel is a coarse HAZ part, whereas in the low carbon Al killed steel, the HAZ part. The base material of lower strength does not coarsen and becomes the starting point of fatigue failure. On the other hand, it has been found that when Mg and Ti oxides and / or composite oxides are dispersed in advance, grain growth of ferrite grains of HAZ is suppressed. However, these techniques are limited to spot welding with low heat input.
[0009]
On the other hand, in accordance with the needs for reducing the weight and cost of automobiles, each part is integrally formed (see, for example, Hydroform JP-A-10-175026), and steel sheets having different strengths are joined and formed (for example, tailored blanks, special features). A molding method such as Kaihei 10-180470 is actually being adopted. These new forming methods employ modular welding and continuous welding with high heat input. As a result, the coarsening of the crystal grains is significantly promoted more than the spot welding, so that the formability and fatigue characteristics at the welded portion are further deteriorated. There is no technique for suppressing grain coarsening in thin steel sheets even during welding with such a relatively large heat input.
[0010]
[Problems to be solved by the invention]
An object of the present invention is to provide a steel sheet that prevents deterioration of the material quality of a welded joint without impairing formability in order to aim at application to automobile parts and the like that require good mechanical characteristics of the welded joint.
[0011]
[Means for Solving the Problems]
As a result of many years of research to improve mechanical properties such as fatigue properties of welded joints, the present inventors are the main cause of material deterioration due to coarsening of the metal structure that occurs in the welded part or welded heat-affected zone. As a result, they have come up with a technique for suppressing these by utilizing Mg. In the case where the heat input is relatively low such as spot welding, it is important to suppress the grain growth of ferrite grains as disclosed in JP-A No. 11-279589. However, at the time of welding with relatively large heat input such as arc or laser welding, not only the heat affected zone but also the weld zone becomes coarse. In such welding, it is not possible to prevent coarsening of both the welded portion and the weld heat affected zone only by considering only the suppression of ferrite grain growth. In other words, in addition to ferrite grain growth, (1) reducing the austenite grain size during cooling, (2) suppressing migration of the transformation interface during transformation from austenite to ferrite, and (3) promoting transformation nucleation. For the first time, it was clarified that the prevention of coarsening in both the weld zone and the weld heat-affected zone was achieved by carrying out the three items, and that Mg oxides and / or sulfides had a great effect on this.
[0012]
That is, this invention is comprised by the said knowledge, The summary is as follows.
(1) In mass%, C: 0.0001 to 0.20%, Si: 0.01 to 2.5%, Mn: 0.01 to 3.0%, P: 0.001 to 0.2 %, S: 0.0001 to 0.02%, Al: 0.008 to 0.05%, Mg: 0.0021 to 0.6%, O: 0.001 to 0.10%, N: 0.0. containing 0024 to 0.01%, and comprises an oxide and / or sulphide containing Mg, the size of the balance of Fe and unavoidable impurities, oxides and / or sulfides containing the Mg includes but particles of 0.0005～0.05Myuemu 10 5 pieces / mm 2 or more 10 7 / mm 2 or less and a 0.05μm ultra 1μm particles below 10 4 / mm 2 or more 10 7 / mm 2 or less The ratio of the maximum crystal grain size of the weld zone and the weld heat affected zone to the average crystal grain size of the base material is 3.0 or less, Welds and cold rolled steel sheet to prevent the material deterioration of the weld heat affected zone that.
[0013]
(2) Ti, Ta, Zr , V and one or the that characterized by containing 0.001 to 0.5 wt% of two or more in total (1) Symbol placement of welds and weld heat affected the Nb Cold-rolled steel sheet that prevents material deterioration of parts.
[0014]
( 3 ) A cold-rolled steel sheet which prevents deterioration of the material of the welded part and the welded heat-affected zone according to (1) or (2), wherein B is contained in an amount of 0.0001 to 0.01% by mass or less.
( 4 ) Any one of (1) to ( 3 ) above, characterized in that it contains 0.001 to 1.5 mass% in total of one or more of Cr, Cu, Ni, Co, W and Mo. A cold-rolled steel sheet in which the material deterioration of the welded part and the weld heat-affected part described in item 1 is prevented.
( 5 ) The welded portion according to any one of (1) to ( 4 ) above, wherein one or two of Ca and rare earth elements are included in a total amount of 0.0001 to 0.5 mass%. This is a cold-rolled steel sheet that prevents material deterioration of the weld heat affected zone.
Moreover, said invention is applicable also to a hot dip galvanized steel plate.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the components of the present invention will be described. First, the reasons for limiting the chemical components of the present invention will be described.
C: When C is present in the steel in the form of a solid solution during cold rolling, an orientation that is not preferable for deep drawability is generated during subsequent annealing, so that it is desirable that C be low particularly in view of formability. In addition, is also the strengthening elements. For this reason, it is preferably 0.20% or less, and 0.003% or less by mass% when moldability is particularly necessary. On the other hand, since extremely low carbon accompanies an increase in steelmaking costs, the range in which there is no adverse effect on formability is set to 0.0001% or more.
[0016]
Si: Si is a solid solution strengthening element, and 0.01% or more is added. On the other hand, excessive addition not only makes the solid solution hardening remarkably unsuitable for processing, but also degrades the Rankford value and deteriorates the adhesion of hot dip galvanizing, so that it is made 2.5% or less. Further, when formability is particularly required, 0.05% or less by mass% is preferable.
Mn: When Mn exceeds 3.0%, the ductility is remarkably deteriorated and the moldability is also lowered. On the other hand, it is a strengthening element and added in an amount of 0.01% or more.
[0017]
P: P is an element that segregates at the grain boundary and causes embrittlement at the grain boundary, and causes secondary work cracking. On the other hand, it is a strengthening element that does not adversely affect the moldability and is made 0.001% or more.
S: If S is too much, not only cracking during hot rolling will be caused, but also the Rankford value will be deteriorated. Therefore, it should be reduced as much as possible, but if it is 0.02% or less, it is an acceptable range. On the other hand, since Mg and sulfide are formed and effective in preventing coarsening of the welded portion and the weld heat affected zone, the addition amount is 0.0001% or more.
[0018]
Al: added for deoxidation of molten steel, but if added over 0.05%, the effect of adding Mg is inhibited to 0.05% or less. On the other hand, by forming Mg and spinel oxide, it contributes to preventing coarsening of the welded part and the weld heat affected zone, so 0.001% or more was added. Note that the lower limit of the amount of Al is set to 0.008% or more based on the example of the present invention.
Mg: Mg combines with oxygen and / or sulfur to form oxides and / or sulfides of various sizes. These Mg-containing particles dispersed in the steel contribute to the suppression of crystal grain coarsening in the austenite and ferrite regions by the pinning effect. In addition, it suppresses interface movement during transformation and forms effective particles as a nucleation site for transformation, which greatly contributes to suppression of coarsening of the final particle size. For this reason, the addition amount is 0.0001% or more. However, even if added over 0.6%, the effect is not only saturated, but also very difficult in steelmaking technology. Therefore, the addition amount of Mg is set to 0.0001% or more and 0.6% or less. Note that the lower limit of the Mg amount is 0.0021% or more based on the embodiment of the present invention.
[0019]
O: O is required to be 0.001% or more in order to form an oxide with Mg and suppress coarsening of crystal grains during welding. However, if it exceeds 0.10%, the oxide becomes coarse and the effect thereof decreases, and C-based inclusions which are undesirable for moldability increase. Therefore, the content of O is set to 0.001% or more and 0.01% or less.
N: N forms nitrides and complex nitrides of T i, since it is effective to coarsen the prevention of weld heat affected zone, it is necessary to contain 0.001% or more. However, since it is an element which is not preferable for improving ductility and Rankford value, the N content is set to 0.001% or more and 0.01% or less.
[0020]
Ti, Ta, Zr, V, and Nb: Ti, Ta, Zr, V, and Nb added as necessary may increase the strength of the steel material by forming carbide, nitride, or carbonitride. In addition, it is particularly effective for preventing coarsening of the Mg-containing oxide or sulfide and the welded portion and the weld heat-affected zone, so 0.001% by mass or more is added as the total content. On the other hand, if the total exceeds 0.5%, it may precipitate as a large amount of carbide, nitride or carbonitride in the ferrite grain or grain boundary as the parent phase, and deteriorate ductility. The upper limit was 0.5% by mass.
[0021]
B: Further, B added if necessary is effective for strengthening grain boundaries and increasing the strength of steel, so 0.0001% by mass or more is added. On the other hand, when the addition amount exceeds 0.01% by mass, not only the effect is saturated but also the steel sheet strength is increased more than necessary, and the workability is also decreased, so the upper limit was made 0.01% by mass.
Cr, Cu, Ni, Co, W, Mo: Cr, Cu, Ni, Co, W, and Mo added as necessary are reinforcing elements, and when the amount of addition of C is limited from the viewpoint of weldability Add an appropriate amount of such reinforcing elements. In total, 0.001% by mass or more is added. On the other hand, if the total exceeds 1.5% by mass, the upper limit is set because it causes hardening of the ferrite as a parent phase.
[0022]
Ca, Rem: Ca and REM added as necessary are elements effective for inclusion control and Mg-containing oxide and sulfide control. Adding appropriate amounts improves hot workability and affects welds and welding heat. Addition of 0.0001% by mass or more in total, but excessive addition promotes hot embrittlement and inhibits the formation of Mg oxides and sulfides. The upper limit was 0.5% by mass.
In the present invention, components other than those described above are Fe, but inclusion of inevitable impurities mixed from melting raw materials such as scrap (Cu, Sn, etc.) is allowed.
[0023]
Next, the presence state of Mg-containing oxide and / or sulfide contained in the steel sheet of the present invention will be described.
Here, Mg-containing oxides and / or sulfides include not only Mg single particles such as MgO and MgS, and single oxide particles of Ti such as Ti 2 O 3 and composite oxide particles thereof, but also other than these. This refers to composite particles containing precipitates of oxides, composite oxides (such as spinel oxides containing Mg and Al), nitrides (such as TiN), and sulfides (such as MnS). If the particle diameter exceeds 0.05 μm, it becomes insufficient to suppress the coarsening during welding by reducing the austenite grains and ferrite grains during cooling and by controlling the moving speed of the transformation interface. The improvement of fatigue properties and other mechanical properties cannot be expected.
[0024]
While finer particles are more effective in preventing the movement of grain boundaries and transformation interfaces, ultrafine particles of less than 0.005 μm excessively hinder the grain growth during recrystallization annealing and provide good ductility and rankford. Since a value cannot be obtained, it is set to 0.0005 μm or more. If the areal density is less than 10 5 pieces / mm 2 , the number of these particles is small, and it is insufficient to suppress the growth of ferrite grains during welding, and if it exceeds 10 7 pieces / mm 2 , ductility deteriorates. This is the upper limit.
[0025]
Furthermore, the transformation nucleation was promoted and a large number of nucleation sites were introduced to prevent coarse grains in the welded part and the weld heat affected zone. In order for the particles to act as transformation nuclei, 0.05 μm or less is insufficient, and if it exceeds 1 μm, the particle diameter for deteriorating ductility is set to more than 0.05 μm and 1 μm or less. Further, if the surface density is less than 10 4 pieces / mm 2 , sufficient nucleation sites cannot be secured to contribute to fine graining, and if it exceeds 10 7 pieces / mm 2 , the workability deteriorates, so that it exceeds 0.05 μm. particles below 1μm was to contain 10 4 / mm 2 or more 10 7 / mm 2 or less.
[0026]
A transmission electron microscope is used to measure the fine particles. A range of at least 1000 μm 2 or more is measured at 10,000 to 100,000 times. The identification of the composition of the particles is mainly performed using EDS (Energy Dispersive X-ray Spectroscopy) or EELS (Electron Energy Loss Spectroscopy). In particular, for very fine particles of 50 nm or less, it is necessary to analyze a fine region using a transmission electron microscope equipped with an electrolytic emission electron gun.
The oxide and / or sulfide containing Mg according to the present invention is generated and grown at each stage of steelmaking to annealing. In addition, it is considered that the Mg-containing oxides and / or sulfides generated in these manufacturing processes are once dissolved in the molten part at the time of welding, and are generated at the time of cooling. If the size and the surface density are satisfied, the effect of the present invention is not impaired.
[0027]
Further, as described above, the material deterioration of the welded portion and the weld heat affected zone is largely caused by the coarsening of the grains. When the ratio of the maximum particle size of the welded part and the weld heat affected zone to the particle size of the base material exceeds 3.0, the fatigue durability and workability of the joint deteriorate, so this is set as the upper limit. Although there is no lower limit, if the welded part and heat-affected zone become finer than the base metal, characteristics such as hardness difference between the base metal / welded part or between the base material / heat-affected part will increase. This is also not preferable as a joint material, and a grain size equivalent to the base material can be obtained at the welded part and the heat affected zone, that is, the ratio of the welded part and the maximum grain size to the average grain size of the base material is 0.8 or more. Is preferable as a joint material. The particle size is determined by observing the rolling direction and a plane (C cross section) perpendicular to the plate surface with an optical microscope at a magnification of 100 to 1000 times by a cutting method. The maximum particle size is observed by observing 10 or more cross-sections, obtaining the equivalent circle diameter by image analysis, and taking the maximum one as the maximum diameter.
[0028]
【Example】
The following examples further illustrate the present invention.
Steels having the components shown in Table 1 were melted on a laboratory scale, and the slab was reheated at a heating temperature of 1200 ° C. and rolled to 6.0 mm in the temperature range of finish rolling from 910 ° C. to 930 ° C. Thereafter, the film was wound at 710 ° C. Cold-roll to 0.8 mm after pickling, measure the recrystallization temperature of each steel, anneal in the range of the recrystallization temperature to the recrystallization temperature + 50 ° C (approximately 800 to 1000 ° C), and perform skin pass rolling. The rolling was performed at 1%.
Then, arc welding was performed under conditions that penetrate the plate thickness with a bead-on-plate, and the maximum grain size of the welded part and the weld heat-affected zone and the average grain size of the base material were measured with an optical microscope in a cross section perpendicular to the rolling direction. . Further, from the same cross section, a carbon extraction replica for a transmission electron microscope was prepared, observed at 10,000 to 100,000 times, and the surface density was measured by observing each particle 100 times. The composition of the particles was analyzed by EDS and EELS attached to the TEM. Moreover, the tensile test of the joint was performed and evaluated as a mechanical property. The results are shown in Table 2.
[0029]
[Table 1]
[Table 2]
D1 to D19 show that the ratio of the maximum particle size of the welded portion and the weld heat affected zone and the average particle size of the base material is at most about 3 and that a good ductility balance of good joint tensile properties is obtained. .
On the other hand, C2 to C6 and C9, in which the addition amount of oxygen and Mg, Al, Ti, Zr, Ca, and Rem, which have a strong binding force with oxygen, do not satisfy the scope of the present invention, have a low areal density of each particle, and a weld zone Or the grain 5 times or more larger than a base material average particle diameter is recognized by the welding heat affected zone, and some steel types (C9) have a low strength ductile balance of joint tension. Also, C1 with a high surface density, C7 with high B, and C8 with high Mo have a good ratio of the maximum particle size of the welded portion and the weld heat affected zone to the average particle size of the base material, but the joint tension The strength ductility balance is as low as TS × El <11000.
[0032]
【The invention's effect】
As described above, the present invention provides a cold-rolled steel sheet excellent in joint characteristics and formability by suppressing the coarsening of crystal grains in the welded part and the weld heat-affected zone. By using these steel sheets, an automobile It can be expected that the durability of these parts will be greatly improved.
Claims (5)
Ｃ ：０．０００１〜０．２０％、
Ｓｉ：０．０１〜２．５％、
Ｍｎ：０．０１〜３．０％、
Ｐ ：０．００１〜０．２％、
Ｓ ：０．０００１〜０．０２％、
Ａｌ：０．００８〜０．０５％、
Ｍｇ：０．００２１〜０．６％、
Ｏ ：０．００１〜０．１０％、
Ｎ ：０．００２４〜０．０１％、
を含有し、かつ、Ｍｇを含有した酸化物および／又は硫化物を含み、残部がＦｅ及び不可避的不純物から成り、前記Ｍｇを含有した酸化物および／又は硫化物のサイズが０．０００５〜０．０５μｍの粒子を１０5個／ｍｍ2以上１０7個／ｍｍ2以下かつ０．０５μｍ超１μｍ以下の粒子を１０4個／ｍｍ2以上１０7個／ｍｍ2以下含み、溶接部および溶接熱影響部の最大結晶粒径と母材の平均結晶粒径の比が３．０以下であることを特徴とする溶接部および溶接熱影響部の材質劣化を防止した冷延鋼板。% By mass
C: 0.0001 to 0.20%
Si: 0.01 to 2.5%,
Mn: 0.01 to 3.0%,
P: 0.001 to 0.2%,
S: 0.0001 to 0.02%,
Al: 0.008 to 0.05%,
Mg: 0.0021 to 0.6%,
O: 0.001 to 0.10%,
N: 0.0024 to 0.01%
And oxides and / or sulfides containing Mg, the balance consisting of Fe and inevitable impurities, and the size of the oxides and / or sulfides containing Mg is 0.0005 to 0 particles .05Myuemu 10 5 particles / mm comprising 2 to 10 7 particles / mm 2 or less and a 0.05μm ultra 1μm particles below 10 4 / mm 2 or more 10 7 / mm 2 or less, welds and weld heat A cold-rolled steel sheet in which the ratio of the maximum crystal grain size of the affected zone and the average crystal grain size of the base material is 3.0 or less, preventing deterioration of the weld zone and the heat affected zone of the weld zone.
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| JP2000103144A JP4336020B2 (en) | 2000-04-05 | 2000-04-05 | Cold-rolled steel sheet that prevents material deterioration in welds and weld heat-affected zones |
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| JP4424485B2 (en) * | 2004-06-24 | 2010-03-03 | 住友金属工業株式会社 | Steel with excellent cold cracking resistance and steel for high-strength structures |
| JP5079795B2 (en) | 2007-04-11 | 2012-11-21 | 新日本製鐵株式会社 | Hot-dip hot-dip steel sheet for press working with excellent low-temperature toughness and method for producing the same |
| JP5652321B2 (en) * | 2011-05-13 | 2015-01-14 | 新日鐵住金株式会社 | Steel sheet for hot stamping excellent in hot composite formability and delayed fracture resistance of punched parts, and its manufacturing method |
| WO2014091604A1 (en) * | 2012-12-13 | 2014-06-19 | 新日鐵住金株式会社 | Steel material for welding |
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