JP3951282B2 - Hot-dip galvanized steel sheet and manufacturing method thereof - Google Patents
Hot-dip galvanized steel sheet and manufacturing method thereof Download PDFInfo
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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に適用され、とくに板のまま又は成形後に、溶接により構造部材として組み立てられる用途に使用されるのに適した高強度溶融亜鉛メッキ鋼板に関するものである。
【0002】
【従来技術】
引張強度が440MPaを超える高強度溶融亜鉛メッキ鋼板は、その優れた防錆性と高い耐力を利点とし、建設部材、機械構造用部品、自動車の構造用部品などに広く適用されている。このため、高強度溶融亜鉛メッキ鋼板に係る発明は非常に多く開示されている。特に、適用範囲が拡大する中で、加工性に対する要求特性が高まっているため、特開平5−311244号公報や特開平7−54051号公報等、加工性に優れた高強度溶融亜鉛メッキ鋼板に関する発明が数多く開示されている。
【0003】
【発明が解決しようとする課題】
近年、製造されたままの鋼板の加工性に対する要求特性が高まる一方で、適用技術の拡大に伴い、テーラードブランク材などのように、溶接部を含んだ状態で加工されたり、あるいは、溶接部を含んだ構造部材の高速変形挙動に対する要求特性が厳しくなるなど、溶接部の特性が製品の要求特性として着目されつつある。
【0004】
しかしながら、前記のような従来の加工性に優れた高強度溶融亜鉛メッキ鋼板は、一般にその主たる強化機構がオーステナイト相の急冷により得られるマルテンサイトやベイナイトといった低温変態相を利用しているため、溶接時にHAZが軟化してしまうという大きな弱点を有する。このような溶接時のHAZ軟化は、例えば、テーラードブランク材では成形性の劣化につながり、その他の用途に使用する場合にも、変形強度、破断強度、高速変形強度など構造部材としての性能を劣化させる原因ともなってしまうという問題点を有する。
【0005】
本発明はこのような事情に鑑みてなされたもので、レーザー溶接、マッシュシーム溶接あるいはアーク溶接といった、幅広い溶接法下において、HAZ部の硬度変化が極めて小さいという性質を有する、新規な高強度溶融亜鉛メッキ鋼板、及びその製造方法を提供することを課題とする。
【0006】
前記課題を解決するための第1の手段は、重量%で、C:0.04%〜0.13%、Si:0.5%以下、Mn:1〜2%、P:0.05%以下、S:0.01%以下(0を含む)、sol.Al:0.05%以下、N:0.007%以下(0を含む)、Mo:0.05〜0.5%、Cr:0.2%以下(0を含む)を含有し、残部が Feおよび不可避不純物からなり、かつ平均粒径20μm以下のフェライトと体積率5〜40%のマルテンサイトが体積率で 95 %を超える組織からなることを特徴とする溶融亜鉛メッキ鋼板(請求項1)である。
【0007】
前記課題を解決するための第2の手段は、前記第1の手段の成分に加え、さらにV:0.02〜0.2%を含有し、平均粒径20μm以下のフェライトと体積率5〜40%のマルテンサイトが体積率で 95 %を超える組織からなることを特徴とする溶融亜鉛メッキ鋼板(請求項2)である。
【0008】
前記課題を解決するための第3の手段は、前記第1の手段又は第2の手段に係る溶融亜鉛メッキ鋼板の製造方法であって、それぞれ前記第1の手段又は第2の手段に記載した成分を有する鋼を、鋳造後に熱延鋼帯とし、酸洗後、必要に応じて40%以上の圧下率で冷間圧延し、続く連続溶融亜鉛メッキラインにおいて、750〜850℃で均熱した後、1〜50℃/secの冷却速度で600℃以下の温度域まで冷却して亜鉛メッキを行い、必要に応じてさらに合金化処理を行い、その後400〜600℃での滞留時間が200秒以内となるような状態で冷却を行うことを特徴とするもの(請求項3)である。
【0009】
なお、本明細書及び図面において、鋼の成分を示す%は、特に断らない限り重量%を意味する。
【0010】
(発明に至る経緯とMo、V、Cr及び組織の限定理由)
本発明者らは、前記課題を解決するため、溶接部の強度変化に及ぼす鋼成分と組織の影響について鋭意検討した結果、ある限定されたC、Si、Mn等の基本成分を有する鋼に適量のMoを含有させ、かつ、組織を平均粒径20μm以下のフェライトと、体積率が5〜40%に限定されたマルテンサイトを主体とした組織とすることで、HAZ部の硬度低下が極めて小さい高強度溶融亜鉛メッキ鋼板を製造できることを見出した。また、この効果は適量のVを含有させることによりさらに高まることをも見出した。
【0011】
一般に、マルテンサイトやベイナイトといったオーステナイト相の急冷により得られる低温変態相は、400〜800℃といった高温に保持されると、短時間でも容易に焼戻されたり、炭化物が粗大化したりして急激に強度が低下することが知られている。本発明者らは、このような低温変態強化型高強度鋼の溶接時の硬度変化について、鋼成分と微視組織の影響を詳細に検討した結果、以下の制御が、強度の低下を防ぐのに有効であることを見出した。
【0012】
▲1▼ 転位密度の高いマルテンサイトを硬質相とし、2次析出強化を利用することで、短時間での昇温においては硬質相の強度低下が下げられる。このためには、Mo又はVを含有させることが有効であるが、これらの含有量が多くなると、かえってHAZ部が部分的に母材よりも硬度上昇してしまい好ましくない。また、Mo、Vと同様に2次析出強化元素として知られているCrは、短時間昇温での析出が早いので、HAZ部の硬度変化が大きくなってしまうので、Cr含有量を多くするのは好ましくない。
【0013】
▲2▼ 溶接時の硬度変化が大きいマルテンサイト相の体積率は40%以下に制限し、残部はフェライトとすることで、全体としての硬度変化は小さくできる。ただし、マルテンサイトの体積率が小さくなりすぎると、逆にマルテンサイト相の2次析出強化の効果をHAZ軟化抵抗に有効に活用できないため、下限を5%に規定する。
【0014】
▲3▼ さらに、フェライト粒径の制御も重要で、平均粒径を20μm以下として粒界面積を増加させることで、短時間昇温時に粒界でのオーステナイトの析出が促進される。これにより、最もマルテンサイト相の硬度低下が大きくなるAc3変態点の上昇が避けられ、マルテンサイト相の硬度低下が抑制される。
【0015】
以下、Mo、V、Crの限定理由について説明する。
Mo:0.05〜0.5%
Moは、本発明の効果を得るために必須の元素である。上述したように、これは、溶接時にHAZ部での昇温によるマルテンサイト相の焼戻し軟化が、Moの炭化物の析出で抑制されるためである。このため効果が発現する0.05%を下限として含有させる。しかし、過剰に含有させると、逆にHAZ部で硬度上昇が大きくなり、HAZ部の硬度変化は大きくなる。このことから、上限を0.5%に規定する。
【0016】
Cr:0.2%以下(0を含む)
本発明を行うに際しては、Mo含有をベースにその他のマルテンサイト相の焼戻し軟化抵抗に有効と思われる元素、具体的にはV、Crについても検討を加えた。この結果、溶接時のHAZ部のように短時間の昇温においては元素の種類による影響が異なり、Crは微量の含有でもHAZ部での硬度上昇が大きくなり、0.2%を越えて含有させるとHAZ部の硬度変化が大きくなることが明らかとなった。このため、本発明ではCrの含有量は0.2%以下(0を含む)に制限する。
【0017】
V:好ましくは0.02〜0.2%
本検討で注目されたのはVであり、MoとVの複合含有でHAZ部の硬度変化が極めて小さくなった。これは、マルテンサイト相の短時間の昇温時のV炭化物による析出強化がそれほど大きくなく、しかも、Mo炭化物が析出する温度と異なるため、HAZ部の広い熱履歴域において、均一な焼戻し軟化抵抗が得られるためと考えられた。このような効果を得るためのV量の下限は0.02%であり、過剰に含有させるとやはりHAZ部での硬度上昇が大きくなるので、上限は0.2%に規定する。なお、第2の手段(請求項2)において、Vの下限を0.02%に限定する理由は以上のようなものであるので、第1の手段(請求項1)は、Vの含有量が0.02%未満であるものを排除するものではない。
【0018】
図2に、上述したMo、V、Cr含有量の過不足によるHAZ部での硬度変化を模式的に示す。(a)は、Mo、Vの含有量が適正量未満の場合であり、HAZ部の最軟化部と母材の硬さとの差ΔHvが大きくなっている。(b)は、Mo、V、Crの含有量が適正量を超える場合であり、HAZ部での軟化量は小さくなるが、母材も硬化するので、結局ΔHvは大きくなる。(c)は、Mo、V、Crの含有量本発明に範囲の場合であり、ΔHvは小さくなる。
【0019】
(その他の成分の限定理由)
C:0.04〜0.13%
Cは、所望の強度を確保するために必須の元素であるが、含有量が多くなるとマルテンサイト体積率が増加しすぎて、HAZ部の硬度低下が大きくなる。すなわち、下限は強度を確保するための最低限量として、また、上限はHAZ部の硬度低下が大きくなるマルテンサイト体積率が40%を超えないために前記のように規定する。
【0020】
Si:0.5%以下,
Siは、フェライト+マルテンサイト2相組織を安定して得るためには必須の元素であるが、含有量が多くなると亜鉛メッキの密着性や表面外観が著しく劣化するので、上限を0.5%に規定する。
Mn:1〜2%
MnはC同様、所望の強度を確保するために必須の元素である。所望の強度を得るため1%が下限として必要であるが、過剰に含有させるとマルテンサイト体積率が増大してHAZ部の硬度低下が大きくなるので、上限を2%に規定する。
【0021】
P:0.05%以下
Pは、Siと同様にフェライト+マルテンサイト2相組織を安定して得るためには必須の元素であるが、含有量が多くなると溶接部の靭性が劣化するので、上限を0.05%に規定する。
S:0.01%以下
Sは不純物であり、含有量が高いとPと同様に溶接部の靭性が劣化する。このため上限を0.01%に規定する。
【0022】
sol.Al:0.05%以下
sol.Alは、通常の鋼に含有される量であれば本発明の効果を損なわず、0.05%以下であれば問題無いので上限を0.05%に規定する。
N:0.007%以下(0を含む)
Nも、通常の鋼に含有される量であれば本発明の効果を損なわず、0.007%以下であれば問題無いので上限を0.007%に規定する。
【0023】
その他、言及していない元素については、極端に多く含有しなければ、とくに本発明の効果を損なわない。例えば、鋼の高強度化あるいは微細化を目的としてNbやTiを添加する場合、0.05%以内であれば問題はない。
【0024】
(製造方法)
次に、本発明溶融亜鉛メッキ鋼板の製造法に関して説明する。
本発明鋼板を得るには、各成分元素を上記のごとく限定したうえ、平均粒径20μm以下のフェライトと体積率5〜40%のマルテンサイトを主体とした組織に制御する必要がある。
【0025】
まず、所定成分を有する鋼を鋳造後、熱延鋼帯とし、酸洗後、必要に応じてさらに40%以上の圧下率で冷間圧延してメッキ下地を準備する。熱間圧延条件はとくに規定していない。仕上げ熱延がAr3変態点を下回ったり、熱延終了後の冷却速度が10℃/sec以下と緩冷却であるなど、熱延板粒径が著しく大きくなるような圧延方法でなければ特に問題は生じない。逆に、熱延終了後に1秒以内に100〜300℃/secといった大冷却を活用したり、これにさらに仕上げ熱延大圧下を組み合わせるなど、熱延板粒径を小さくする行為は、本発明の効果を阻害しない。冷間圧延の際の圧下率を40%以上に規定しているのは、これ以下だと焼鈍で粒径が大きくなり易くなるためである。
【0026】
続く連続溶融亜鉛メッキラインにおいて、750〜850℃で均熱した後、1〜50℃/secの冷却速度で600℃以下の温度域まで冷却し、400〜600℃での滞留時間が200秒以内となるよう亜鉛メッキを行い、必要に応じてさらに合金化処理を行う。均熱温度は、安定してオーステナイト相を得るため750℃以上が必要であるが、850℃を超えると粒径が大きくなり所望の特性が得られなくなるのでこれを上限とする。この後、1〜50℃/secの冷却速度で600℃以下の温度域まで冷却するが、これはパーライトを生じさせず、かつ微細なフェライトを所望の体積率析出させるためで、冷速の下限はこれ以下ではパーライトが生じたりフェライト粒径が大きくなるため規定する。冷却速度の上限は、これを超えるとフェライトが十分析出しなばかりかマルテンサイト体積率が40%を超えて大きくなるので規定する。
【0027】
酸洗板あるいは冷延板は、600℃以下の温度域まで冷却された後、亜鉛メッキされ、必要に応じてさらに合金化処理を施される、最終的に室温まで冷却される。本発明者らの検討によれば、室温までの冷却過程において、400〜600℃での滞留時間が組織形成に大きく影響を及ぼすことが明らかとなった。すなわち、滞留時間が長くなると、オーステナイトからのセメンタイトの析出が著しくなり、マルテンサイト相の体積率が低下して強度が低下するばかりか、MoやVの析出によるHAZ軟化抵抗効果が得られなくなる。本発明者らの検討結果に基づき、この滞留時間の上限は200秒に規定する。
【0028】
ここで、本発明においては、組織をフェライトと体積率5〜40%のマルテンサイトの他に、体積率で5%以内の、セメンタイト、ベイナイト、あるいは残留オーステナイトといった組織を含んだものとしても、本発明の効果は損なわれない。
【0029】
なお、その他、特に言及していないが、造塊又は連続鋳造等、スラブ製造法や、熱延での粗熱延バー接続による連続熱延、熱延過程でのインダクションヒーターを利用した200℃以内の昇温などは、本発明の効果に対して影響を及ぼさない。
【0030】
【実施例】
以下、本発明の実施例と比較例について説明する。
表1に示すような成分を有する本発明成分鋼A〜Xと、本発明の成分から外れた範囲の成分を有する比較成分鋼a〜mを転炉で製造し、連続鋳造によりスラブとした。これらのスラブを表2に示す加熱温度と熱延巻取り条件で熱延鋼帯とし、酸洗し、その一部を冷延率65%で冷間圧延して、メッキ下地を準備した。続いて、連続溶融亜鉛メッキラインにて、表3に示す条件で溶融亜鉛メッキ鋼板又は合金化溶融亜鉛メッキ鋼板を製造した。なお、連続溶融亜鉛メッキラインでの熱サイクルは、請求項3に示した好ましい範囲とした。
【0031】
表3には、これらの鋼板の組織、引張強度およびレーザー溶接によるHAZ部の硬度変化量ΔHv(図2で定義)を評価した結果を示す。なお、表2の鋼番と表3の鋼番は対応している。レーザー溶接条件は、出力5kW、速度2m/minであり、溶接速度は特に遅くして、HAZ軟化の生じやすい条件とした。
【0032】
図1は、表3に示した鋼のΔHvを○(ΔHv≦10)、△(10<ΔHv≦20)、×(ΔHv>20)の三段階評価にして、MoとVの含有量で整理した図である。図1から分かるように、Moおよびその他の元素の含有量を本発明規定する範囲内にすることで、ΔHv≦20と優れた耐HAZ軟化特性が得られており、さらにVを請求項2に記載の範囲とすることで、ΔHv≦10のものが得られるようになっている(なお、図1においては、表3の鋼番26、27のようにCが本発明の範囲を外れているものと、鋼番36〜38のようにCrが本発明の範囲を外れているものは除外してある)。
【0033】
【表1】
【0034】
【表2】
【0035】
【表3】
【0036】
表4は、本発明成分鋼Hについて、とくに連続溶融亜鉛メッキラインでの熱サイクル条件を変化させて、特性の変化を検討した結果を示している。鋼番1と鋼番5では均熱温度が、鋼番6や11では冷却速度が適切でないため、また、鋼番16では400〜600℃での滞留時間が長すぎるため、本発明の規定する組織が得られておらず、所望の耐HAZ軟化特性が得られていない。これに対し、請求項3に記載する製造条件で製造した本発明鋼においては、請求項1に記載する組織が得られ、いずれも、ΔHv≦20と優れた耐HAZ軟化特性が得られている。
【0037】
【表4】
【0038】
【発明の効果】
以上説明したように、本発明によれば、HAZ部の硬度変化が小さい高強度溶融亜鉛メッキ鋼板が得られる。すなわち、本発明に係る溶融亜鉛メッキ鋼板をテーラードブランク材に使用した場合には成形性の劣化が防止される。また、本発明に係る溶融亜鉛メッキ鋼板は、溶接で構造体となされたとき、部材の変形強度、破断強度、高速変形強度などの性能の劣化が極めて小さい。よって、本発明に係る溶融亜鉛メッキ鋼板は、テーラードブランク材用、及び溶接された構造体用として用いるのに極めて好適である。
【図面の簡単な説明】
【図1】 Mo、V含有量とΔHvとの関係を示す図である。
【図2】 Mo、V、Cr含有量の過不足によるHAZ部での硬度変化を模式的に示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention is applied to construction members, machine structural parts, automobile structural parts, and the like, and is particularly suitable for use as a structural member that is assembled as a structural member by welding as a plate or after molding. The present invention relates to a plated steel sheet.
[0002]
[Prior art]
High-strength hot-dip galvanized steel sheets with a tensile strength exceeding 440 MPa have the advantage of excellent rust prevention and high proof stress, and are widely applied to construction members, machine structural parts, automotive structural parts, and the like. For this reason, very many inventions related to high-strength hot-dip galvanized steel sheets have been disclosed. In particular, since the required characteristics for workability are increasing as the application range is expanded, it relates to a high-strength hot-dip galvanized steel sheet excellent in workability, such as JP-A-5-31244 and JP-A-7-54051. A number of inventions have been disclosed.
[0003]
[Problems to be solved by the invention]
In recent years, while the required characteristics for workability of as-manufactured steel sheets have increased, with the expansion of applied technology, it has been processed in a state including a welded part, such as a tailored blank material, or The characteristics of the welded part have been attracting attention as the required characteristics of the product, such as the required characteristics for the high-speed deformation behavior of the included structural members become severe.
[0004]
However, the conventional high-strength hot-dip galvanized steel sheet with excellent workability as described above generally uses a low-temperature transformation phase such as martensite or bainite, whose main strengthening mechanism is obtained by rapid cooling of the austenite phase. There is a big weakness that HAZ is sometimes softened. Such softening of HAZ during welding leads to deterioration of formability in, for example, tailored blank materials, and deteriorates performance as a structural member such as deformation strength, breaking strength, and high-speed deformation strength even when used for other applications. It has the problem of becoming a cause.
[0005]
The present invention has been made in view of such circumstances, and is a novel high-strength melt having the property that the hardness change of the HAZ part is extremely small under a wide range of welding methods such as laser welding, mash seam welding or arc welding. It is an object of the present invention to provide a galvanized steel sheet and a manufacturing method thereof.
[0006]
The first means for solving the above-mentioned problems is, by weight, C: 0.04% to 0.13%, Si: 0.5% or less, Mn: 1 to 2%, P: 0.05% or less, S: 0.01% or less ( Sol.Al: 0.05% or less, N: 0.007% or less (including 0), Mo: 0.05 to 0.5%, Cr: 0.2% or less (including 0), the balance being Fe and inevitable A hot-dip galvanized steel sheet comprising an impurity and having a structure in which ferrite having an average particle diameter of 20 μm or less and martensite having a volume ratio of 5 to 40% exceed 95 % by volume .
[0007]
The second means for solving the above-mentioned problems is that in addition to the components of the first means, V: 0.02 to 0.2%, ferrite having an average particle diameter of 20 μm or less, and martensite having a volume ratio of 5 to 40%. A hot-dip galvanized steel sheet characterized in that a site is composed of a structure exceeding 95 % by volume .
[0008]
The 3rd means for solving the above-mentioned subject is a manufacturing method of the hot dip galvanized steel sheet concerning the 1st means or the 2nd means, and was indicated in the 1st means or the 2nd means, respectively. The steel having the components is made into a hot-rolled steel strip after casting, and after pickling, it is cold-rolled at a reduction rate of 40% or more as necessary, and is soaked at 750 to 850 ° C. in a continuous hot-dip galvanizing line. After that, it is cooled to a temperature range of 600 ° C. or lower at a cooling rate of 1 to 50 ° C./sec, galvanized, further alloyed as necessary, and then a residence time at 400 to 600 ° C. for 200 seconds. The cooling is performed in such a state that it is within the range (Claim 3).
[0009]
In the specification and drawings,% indicating the components of the steel, the meaning taste weight unless otherwise specified.
[0010]
(Background to the invention and reasons for limitation of Mo, V, Cr and structure)
In order to solve the above-mentioned problems, the present inventors have intensively studied the influence of the steel composition and the structure on the strength change of the welded portion. As a result, the present invention has an appropriate amount for a steel having a certain basic component such as C, Si, Mn. The hardness reduction of the HAZ part is extremely small by making the structure mainly composed of ferrite having an average particle diameter of 20 μm or less and martensite whose volume ratio is limited to 5 to 40%. It has been found that a high-strength hot-dip galvanized steel sheet can be produced. It has also been found that this effect is further enhanced by containing an appropriate amount of V.
[0011]
In general, the low-temperature transformation phase obtained by rapid cooling of austenitic phases such as martensite and bainite, when kept at a high temperature such as 400 to 800 ° C., is easily tempered even in a short time, and the carbides become coarse, resulting in a rapid increase. It is known that the strength decreases. As a result of detailed examination of the effects of the steel composition and the microstructure on the hardness change during welding of such a low-temperature transformation strengthened high-strength steel, the present inventors have confirmed that the following control prevents a decrease in strength. Found to be effective.
[0012]
{Circle around (1)} By using martensite with a high dislocation density as a hard phase and utilizing secondary precipitation strengthening, the strength reduction of the hard phase can be reduced at a short temperature rise. For this purpose, it is effective to contain Mo or V. However, if these contents increase, the HAZ part partially increases in hardness compared to the base material, which is not preferable. In addition, Cr, which is known as a secondary precipitation strengthening element, like Mo and V, is precipitating quickly at a high temperature, so the hardness change in the HAZ part becomes large, so the Cr content is increased. Is not preferred.
[0013]
(2) The volume ratio of the martensite phase, which has a large hardness change during welding, is limited to 40% or less, and the balance is ferrite, so that the overall hardness change can be reduced. However, if the volume fraction of martensite becomes too small, the effect of secondary precipitation strengthening of the martensite phase cannot be effectively utilized for the HAZ softening resistance, so the lower limit is defined as 5%.
[0014]
{Circle around (3)} Further, control of the ferrite grain size is also important. By increasing the grain boundary area by setting the average grain size to 20 μm or less, precipitation of austenite at the grain boundary is promoted when the temperature is raised for a short time. As a result, an increase in Ac3 transformation point at which the most significant decrease in the hardness of the martensite phase is avoided, and a decrease in the hardness of the martensite phase is suppressed.
[0015]
Hereinafter, the reasons for limitation of Mo, V, and Cr will be described.
Mo: 0.05-0.5%
Mo is an essential element for obtaining the effects of the present invention. As described above, this is because the temper softening of the martensite phase due to the temperature rise in the HAZ part during welding is suppressed by precipitation of Mo carbides. Therefore, 0.05% at which the effect is manifested is contained as the lower limit. However, if it is excessively contained, the hardness increase is greatly increased in the HAZ part, and the hardness change of the HAZ part is increased. For this reason, the upper limit is defined as 0.5%.
[0016]
Cr: 0.2% or less (including 0)
In carrying out the present invention, other elements that are considered effective for temper softening resistance of the martensite phase based on the Mo content, specifically, V and Cr were also examined. As a result, the effect of the type of element is different when the temperature is raised for a short time like the HAZ part during welding, and even if Cr is contained in a trace amount, the hardness increase in the HAZ part is large, and if it is contained exceeding 0.2% It became clear that the hardness change of the HAZ part became large. For this reason, in the present invention, the Cr content is limited to 0.2% or less (including 0).
[0017]
V: Preferably 0.02 to 0.2%
It was V that attracted attention in this study, and the hardness change in the HAZ part became extremely small due to the combined inclusion of Mo and V. This is because precipitation strengthening due to V carbide at the time of temperature rise in the martensite phase is not so large, and is different from the temperature at which Mo carbide is precipitated, and therefore, uniform temper softening resistance in a wide thermal history region of the HAZ part. It was thought that it was obtained. The lower limit of the V amount for obtaining such an effect is 0.02%. If excessively contained, the increase in hardness at the HAZ part will also increase, so the upper limit is defined as 0.2%. The reason for limiting the lower limit of V to 0.02% in the second means (Claim 2) is as described above. Therefore, the first means (Claim 1) has a V content of 0.02%. It does not exclude what is less than%.
[0018]
FIG. 2 schematically shows a change in hardness at the HAZ portion due to the excess or deficiency of the Mo, V, and Cr contents described above. (a) is a case where the contents of Mo and V are less than appropriate amounts, and the difference ΔHv between the softest part of the HAZ part and the hardness of the base material is large. (b) is a case where the contents of Mo, V, and Cr exceed appropriate amounts, and the softening amount in the HAZ portion is small, but the base material is also cured, so ΔHv eventually increases. (c) is the content of Mo, V, Cr within the range of the present invention, and ΔHv becomes small.
[0019]
(Reason for limitation of other ingredients)
C: 0.04-0.13%
C is an essential element for securing a desired strength. However, when the content increases, the martensite volume fraction increases excessively, and the hardness of the HAZ part decreases greatly. That is, the lower limit is defined as the minimum amount for securing the strength, and the upper limit is defined as described above so that the martensite volume ratio at which the hardness reduction of the HAZ part is large does not exceed 40%.
[0020]
Si: 0.5% or less,
Si is an indispensable element for obtaining a ferrite + martensite two-phase structure stably. However, as the content increases, the adhesion and surface appearance of galvanizing deteriorate significantly, so the upper limit is specified to 0.5%. To do.
Mn: 1-2%
Mn, like C, is an essential element for securing a desired strength. In order to obtain a desired strength, 1% is necessary as the lower limit. However, if excessively contained, the martensite volume ratio increases and the hardness of the HAZ part decreases greatly, so the upper limit is defined as 2%.
[0021]
P: 0.05% or less P is an essential element in order to stably obtain a ferrite + martensite two-phase structure as in Si. However, as the content increases, the toughness of the welded portion deteriorates. Set to 0.05%.
S: 0.01% or less S is an impurity, and if the content is high, the toughness of the welded portion deteriorates as in the case of P. Therefore, the upper limit is specified as 0.01%.
[0022]
sol.Al: 0.05% or less
If the amount of sol.Al is an amount contained in ordinary steel, the effect of the present invention is not impaired, and if it is 0.05% or less, there is no problem, so the upper limit is specified as 0.05%.
N: 0.007% or less (including 0)
If N is an amount contained in ordinary steel, the effect of the present invention is not impaired, and if it is 0.007% or less, there is no problem, so the upper limit is defined as 0.007%.
[0023]
In addition, elements not mentioned are not particularly impaired unless they are contained in an extremely large amount. For example, when Nb or Ti is added for the purpose of increasing the strength or refining of steel, there is no problem as long as it is within 0.05%.
[0024]
(Production method)
Next, the manufacturing method of the hot dip galvanized steel sheet of the present invention will be described.
In order to obtain the steel sheet of the present invention, it is necessary to limit each component element as described above and to control the structure mainly composed of ferrite having an average particle size of 20 μm or less and martensite having a volume ratio of 5 to 40%.
[0025]
First, after casting a steel having a predetermined component, a hot-rolled steel strip is formed, and after pickling, cold rolling is further performed at a reduction rate of 40% or more as necessary to prepare a plating base. Hot rolling conditions are not specified. There is a particular problem unless the rolling method is such that the hot-rolled sheet particle size becomes significantly large, such as when the finish hot rolling is below the Ar3 transformation point or the cooling rate after hot rolling is 10 ° C / sec or less Does not occur. On the contrary, the action of reducing the hot-rolled plate particle size, such as utilizing large cooling such as 100 to 300 ° C./sec within 1 second after the end of hot-rolling, and further combining this with hot rolling under finish, is the present invention. Does not hinder the effects of The reason why the rolling reduction during cold rolling is specified to be 40% or more is that if the rolling reduction is less than this, the grain size tends to increase due to annealing.
[0026]
In the continuous hot dip galvanizing line, after soaking at 750 to 850 ° C, it is cooled to a temperature range of 600 ° C or less at a cooling rate of 1 to 50 ° C / sec, and the residence time at 400 to 600 ° C is within 200 seconds. Zinc plating is performed, and further alloying treatment is performed as necessary. The soaking temperature needs to be 750 ° C. or higher in order to stably obtain the austenite phase, but if it exceeds 850 ° C., the particle size becomes large and desired characteristics cannot be obtained, so this is the upper limit. After that, it is cooled to a temperature range of 600 ° C. or less at a cooling rate of 1 to 50 ° C./sec. This does not cause pearlite and precipitates the desired volume fraction of fine ferrite. Is defined below because pearlite is produced and the ferrite grain size is increased. The upper limit of the cooling rate is specified because not only ferrite is sufficiently precipitated but also the martensite volume fraction exceeds 40%.
[0027]
The pickled plate or cold-rolled plate is cooled to a temperature range of 600 ° C. or lower, galvanized, and further subjected to alloying treatment as necessary, and finally cooled to room temperature. According to the study by the present inventors, it has been clarified that the residence time at 400 to 600 ° C. greatly affects the formation of the structure in the cooling process to room temperature. That is, when the residence time becomes longer, precipitation of cementite from austenite becomes remarkable, the volume fraction of the martensite phase decreases and the strength decreases, and the HAZ softening resistance effect due to precipitation of Mo and V cannot be obtained. Based on the results of the study by the present inventors, the upper limit of this residence time is defined as 200 seconds.
[0028]
Here, in the present invention, in addition to ferrite and martensite having a volume ratio of 5 to 40%, the structure may include cementite, bainite, or retained austenite having a volume ratio of 5% or less. The effect of the invention is not impaired.
[0029]
In addition, although not specifically mentioned, within 200 ° C using an induction heater in the slab manufacturing method such as ingot forming or continuous casting, continuous hot rolling by hot-rolling rough hot-rolling bar connection, hot-rolling process The temperature rise or the like does not affect the effects of the present invention.
[0030]
【Example】
Examples of the present invention and comparative examples will be described below.
Invention component steels A to X having components as shown in Table 1 and comparative component steels a to m having components in a range deviating from the components of the present invention were produced in a converter and made into slabs by continuous casting. These slabs were made into hot-rolled steel strips under the heating temperatures and hot-rolling conditions shown in Table 2, pickled, and a part of the slabs were cold-rolled at a cold rolling rate of 65% to prepare a plating base. Then, the hot dip galvanized steel plate or the alloyed hot dip galvanized steel plate was manufactured on the conditions shown in Table 3 in the continuous hot dip galvanizing line. In addition, the heat cycle in the continuous hot dip galvanizing line was made into the preferable range shown in Claim 3.
[0031]
Table 3 shows the results of evaluating the microstructure, tensile strength, and hardness change amount ΔHv (defined in FIG. 2) of the HAZ part by laser welding. The steel numbers in Table 2 correspond to the steel numbers in Table 3. The laser welding conditions were an output of 5 kW and a speed of 2 m / min. The welding speed was particularly slow so that HAZ softening was likely to occur.
[0032]
Figure 1 shows the three-stage evaluation of ΔHv of steel shown in Table 3 as ○ (ΔHv ≦ 10), Δ (10 <ΔHv ≦ 20), and × (ΔHv> 20). FIG. As can be seen from FIG. 1, by setting the contents of Mo and other elements within the range prescribed in the present invention, ΔHv ≦ 20 and excellent HAZ softening resistance can be obtained. By setting it as the range of description, the thing of (DELTA) Hv <= 10 can be obtained now. (In FIG. 1, C is outside the range of this invention like the steel numbers 26 and 27 of Table 3.) And those in which Cr is outside the scope of the present invention, such as steel numbers 36 to 38, are excluded).
[0033]
[Table 1]
[0034]
[Table 2]
[0035]
[Table 3]
[0036]
Table 4 shows the results of examining changes in properties of the steel H of the present invention, particularly by changing the heat cycle conditions in the continuous hot dip galvanizing line. Since the soaking temperature is not appropriate for steel numbers 1 and 5 and the cooling rate is not appropriate for steel numbers 6 and 11, and the residence time at 400 to 600 ° C. is too long for steel numbers 16 and 11, the present invention stipulates. The structure is not obtained, and the desired HAZ softening resistance is not obtained. On the other hand, in the steel of the present invention manufactured under the manufacturing conditions described in claim 3, the structure described in claim 1 is obtained, and in all cases, excellent HAZ softening resistance characteristics such as ΔHv ≦ 20 are obtained. .
[0037]
[Table 4]
[0038]
【The invention's effect】
As described above, according to the present invention, a high-strength hot-dip galvanized steel sheet having a small hardness change in the HAZ part can be obtained. That is, when the hot-dip galvanized steel sheet according to the present invention is used for a tailored blank material, deterioration of formability is prevented. Moreover, when the hot dip galvanized steel sheet according to the present invention is formed into a structure by welding, the deterioration of performance such as deformation strength, breaking strength, and high-speed deformation strength of the member is extremely small. Therefore, the hot dip galvanized steel sheet according to the present invention is extremely suitable for use as a tailored blank material and a welded structure.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between Mo and V content and ΔHv.
FIG. 2 is a diagram schematically showing a change in hardness in the HAZ part due to excess or deficiency of Mo, V, and Cr contents.
Claims (3)
Priority Applications (7)
Application Number | Priority Date | Filing Date | Title |
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JP2000019616A JP3951282B2 (en) | 2000-01-28 | 2000-01-28 | Hot-dip galvanized steel sheet and manufacturing method thereof |
DE60116765T DE60116765T2 (en) | 2000-01-24 | 2001-01-23 | FIREPLATED STEEL PLATE AND METHOD OF MANUFACTURING THEREOF |
PCT/JP2001/000403 WO2001053554A1 (en) | 2000-01-24 | 2001-01-23 | Hot dip zinc plated steel sheet and method for producing the same |
DE60133493T DE60133493T2 (en) | 2000-01-24 | 2001-01-23 | Hot-dip galvanized steel sheet and process for its production |
EP04006816A EP1443124B1 (en) | 2000-01-24 | 2001-01-23 | Hot-dip galvanized steel sheet and method for producing the same |
EP01942682A EP1227167B1 (en) | 2000-01-24 | 2001-01-23 | Hot dip zinc plated steel sheet and method for producing the same |
US09/953,788 US6440584B1 (en) | 2000-01-24 | 2001-09-17 | Hot-dip galvanized steel sheet and method for producing the same |
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JP2000019616A JP3951282B2 (en) | 2000-01-28 | 2000-01-28 | Hot-dip galvanized steel sheet and manufacturing method thereof |
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