JP5233346B2 - High-strength cold-rolled steel sheet excellent in chemical conversion treatment and post-coating corrosion resistance and method for producing the same - Google Patents

High-strength cold-rolled steel sheet excellent in chemical conversion treatment and post-coating corrosion resistance and method for producing the same Download PDF

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JP5233346B2
JP5233346B2 JP2008070530A JP2008070530A JP5233346B2 JP 5233346 B2 JP5233346 B2 JP 5233346B2 JP 2008070530 A JP2008070530 A JP 2008070530A JP 2008070530 A JP2008070530 A JP 2008070530A JP 5233346 B2 JP5233346 B2 JP 5233346B2
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勇樹 田路
浩平 長谷川
淳一郎 平澤
佳久 澤田
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    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
    • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
    • C23G1/02—Cleaning or pickling metallic material with solutions or molten salts with acid solutions
    • C23G1/08—Iron or steel
    • C23G1/085—Iron or steel solutions containing HNO3
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
    • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
    • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
    • C23G1/02—Cleaning or pickling metallic material with solutions or molten salts with acid solutions
    • C23G1/08—Iron or steel
    • C23G1/081—Iron or steel solutions containing H2SO4
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
    • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
    • C23G1/02—Cleaning or pickling metallic material with solutions or molten salts with acid solutions
    • C23G1/08—Iron or steel
    • C23G1/086—Iron or steel solutions containing HF

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  • Heat Treatment Of Sheet Steel (AREA)
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Description

本発明は、自動車車体の強度部材や補強部材等に用いて好適な、引張強さが７８０ＭＰａ以上で加工性に優れかつ化成処理性および化成電着塗装後の耐食性にも優れる高強度冷延鋼板とその製造方法に関するものである。   The present invention is a high-strength cold-rolled steel sheet suitable for use as a strength member or reinforcing member of an automobile body, having a tensile strength of 780 MPa or more, excellent workability, and excellent chemical conversion property and corrosion resistance after chemical electrodeposition coating. And its manufacturing method.

近年、地球環境を保護する観点から、自動車車体を軽量化し、燃費を改善することが要求されている。また、衝突時における乗員の安全性を確保する観点から、自動車車体の強度を向上することが要求されている。自動車車体の軽量化と強度向上を共に実現するには、素材を高強度化して薄肉化し、軽量化することが効果的である。そこで、自動車車体には、板厚を低減した高強度鋼板の採用が積極的に進められており、最近では、自動車車体の強度部材や補強部材には、引張強さが７８０ＭＰａ以上の高強度薄鋼板が使用され始めている。   In recent years, from the viewpoint of protecting the global environment, it has been required to reduce the weight of an automobile body and improve fuel efficiency. In addition, from the viewpoint of ensuring the safety of passengers in the event of a collision, it is required to improve the strength of the automobile body. In order to achieve both weight reduction and strength improvement of an automobile body, it is effective to increase the strength of the material to make it thinner and lighter. Therefore, the adoption of high-strength steel sheets with reduced plate thickness has been actively promoted for automobile bodies. Recently, high-strength thin sheets with a tensile strength of 780 MPa or more are used for the strength members and reinforcing members of automobile bodies. Steel plates are starting to be used.

自動車車体の強度部材等は、一般に、プレス加工等の成形加工により製造されるため、その素材には、高強度であること以外に、加工性に優れることが要求される。素材となる鋼板を高強度化するには、ＳｉやＭｎ等の合金元素を添加して固溶強化したり結晶粒を微細化したりする方法や、Ｎｂ，Ｔｉ、Ｖ等の析出物形成元素を添加して析出強化する方法、マルテンサイト相等の硬質な変態組織を生成させて強化する方法等が有効であり、既に実用化されている。   Since a strength member or the like of an automobile body is generally manufactured by a molding process such as a press process, the material is required to have excellent workability in addition to high strength. In order to increase the strength of a steel sheet as a raw material, a method of adding a solution element such as Si or Mn to strengthen the solution or refining crystal grains, or a precipitate forming element such as Nb, Ti, or V is used. A method of adding and strengthening precipitation and a method of strengthening by generating a hard transformation structure such as a martensite phase are effective, and have already been put into practical use.

一般に、合金元素を添加し、鋼板を高強度化しようとすると、加工性が低下するのが普通である。しかし、Ｓｉは、加工性の低下を抑制しつつ高強度化できるため、鋼板の強度と加工性とを両立させる上で有用な合金元素であることが知られている。ところが、Ｓｉは、酸化され易い元素であるため、含有量が増加すると、通常の冷延鋼板の製造工程で実施されている還元性雰囲気下での焼鈍中に、Ｓｉが優先的に酸化されて鋼板表面近傍に濃化し、自動車用冷延鋼板に要求される特性の一つである化成処理性や化成電着塗装後の耐食性を低下させることも知られている。そのため、Ｓｉを多量に含有する鋼板の自動車車体への適用は、化成電着塗装後の耐食性が要求されない部位に限定されていた。   In general, when an alloy element is added to increase the strength of a steel sheet, the workability usually decreases. However, Si is known to be an alloy element useful for achieving both the strength and workability of a steel sheet because it can increase the strength while suppressing a decrease in workability. However, since Si is an easily oxidizable element, when the content is increased, Si is preferentially oxidized during annealing in a reducing atmosphere that is performed in a normal cold-rolled steel sheet manufacturing process. It is also known to concentrate near the surface of the steel sheet to reduce the chemical conversion processability and the corrosion resistance after chemical electrodeposition coating, which are one of the characteristics required for cold rolled steel sheets for automobiles. For this reason, application of steel plates containing a large amount of Si to automobile bodies has been limited to sites where corrosion resistance after chemical electrodeposition coating is not required.

Ｓｉ含有鋼板、特に０．８ｍａｓｓ％以上の高いＳｉを含有する冷延鋼板の化成処理性および化成電着塗装後の耐食性を改善する方法については、従来から多くの提案がなされている。それらの技術は、Ｓｉ含有鋼の化成処理性や化成電着塗装後の耐食性が劣る原因は、概略すると、以下の２つの理由によると考えており、それぞれに対して異なる改善策が提案されている。   Conventionally, many proposals have been made on methods for improving the chemical conversion property of a Si-containing steel sheet, particularly a cold-rolled steel sheet containing a high Si content of 0.8 mass% or more and the corrosion resistance after chemical electrodeposition coating. These technologies are considered to be due to the following two reasons for the reason that the chemical conversion processability of the Si-containing steel and the corrosion resistance after chemical electrodeposition coating are inferior, and different improvement measures have been proposed for each. Yes.

まず、考えられる第一の理由は、Ｓｉを主体とする酸化物が鋼板表面を被覆することにより、化成結晶が鋼板表面へ生成するのが阻害されて、化成結晶にスケが生じ、電着塗膜の密着性が低下するというものである。これに対する改善策としては、例えば、特許文献１には、鋼板表層酸化物のＳｉ／Ｍｎ比を１以下に制御し、化成処理性に悪影響を及ぼすＳｉ酸化物の比率を下げることにより、化成処理性を改善する技術が提案されている。また、特許文献２〜４には、鋼板表層酸化物のＳｉとＭｎの比に加えて、酸化物の存在状態（大きさ、密度、被覆率）を制御することにより、化成処理性、塗膜密着性を改善する技術が提案されている。また、特許文献５には、連続焼鈍中の露点を０℃〜−２０℃に制御し、かつ連続焼鈍後に濃塩酸または濃硫酸で表層のＳｉ酸化物を除去することにより、Ｓｉ酸化物による鋼板表面被覆率およびＳｉ酸化物の大きさを制御し、化成処理性を改善する技術が提案されている。また、特許文献６には、鋼板表面から０．１〜１００μｍの深さまでの結晶粒界および／または結晶粒内にＳｉ等の酸化物を形成させることで、鋼板表面へのＳｉ等の濃化を抑制し、化成処理性を改善する技術が提案されている。また、特許文献７には、連続焼鈍後に鋼板表面を２．０ｇ／ｍ２以上研削したのち塩酸酸洗し、あるいはその後さらに０．１〜０．３ｇ／ｍ２の研削を行うことにより、化成処理性を改善する技術が提案されている。さらに、特許文献８には、連続焼鈍後に０〜４のｐＨ、１０〜１００℃の温度で５〜１５０秒間の酸洗処理し、その後さらに、１０〜１４のｐＨで、１０〜１００℃の温度で２〜５０秒間のアルカリ処理を行うことにより、化成処理性を改善する技術が提案されている。これらの技術は、いずれも、鋼板表層の酸化物の組成、存在状態を適正に制御することにより、化成処理性を改善し、化成電着塗装後の耐食性を改善しようとするものである。 First, the first possible reason is that the oxide mainly composed of Si coats the surface of the steel sheet, thereby preventing the formation of chemical crystals on the surface of the steel sheet, resulting in the formation of scale crystals in the chemical conversion crystal. The adhesion of the film is lowered. As an improvement measure against this, for example, in Patent Document 1, the Si / Mn ratio of the steel sheet surface layer oxide is controlled to 1 or less, and the chemical conversion treatment is performed by reducing the ratio of the Si oxide that adversely affects the chemical conversion treatment property. Techniques for improving the performance have been proposed. In addition, in Patent Documents 2 to 4, in addition to the ratio of Si and Mn of the steel sheet surface layer oxide, by controlling the presence state (size, density, coverage) of the oxide, chemical conversion processability, coating film Techniques for improving adhesion have been proposed. Patent Document 5 discloses a steel plate made of Si oxide by controlling the dew point during continuous annealing to 0 ° C. to −20 ° C. and removing Si oxide on the surface layer with concentrated hydrochloric acid or concentrated sulfuric acid after continuous annealing. There has been proposed a technique for improving the chemical conversion treatment property by controlling the surface coverage and the size of the Si oxide. Patent Document 6 discloses that Si or the like is concentrated on the surface of the steel sheet by forming an oxide such as Si in the grain boundary and / or in the crystal grain from the steel sheet surface to a depth of 0.1 to 100 μm. A technique has been proposed that suppresses the above and improves the chemical conversion processability. Further, Patent Document 7 discloses that after continuous annealing, the steel plate surface is ground at 2.0 g / m 2 or more and then pickled with hydrochloric acid, or thereafter, further 0.1 to 0.3 g / m 2 is ground. Techniques for improving the processability have been proposed. Furthermore, in Patent Document 8, pickling treatment is performed at a pH of 0 to 4 at a temperature of 10 to 100 ° C. for 5 to 150 seconds after continuous annealing, and then at a pH of 10 to 14 at a temperature of 10 to 100 ° C. A technique for improving the chemical conversion processability by performing an alkali treatment for 2 to 50 seconds is proposed. All of these techniques are intended to improve the chemical conversion processability and improve the corrosion resistance after chemical conversion electrodeposition by appropriately controlling the composition and presence state of the oxide on the surface layer of the steel sheet.

次に、考えられる第二の理由は、鋼板表面近傍の結晶粒界に生成したＳｉを含有する酸化物が、酸洗などで除去されることにより表層に微細な凹凸またはクラックが生成され、その内部においては、化成結晶の生成が起こり難いため、該部分で腐食が促進して耐食性が劣化するというものである。これに対する改善策としては、例えば、特許文献２〜４および特許文献９に、鋼板表面から１０μｍ程度の範囲内に存在する粒界酸化物や表層クラックの存在状態を制御することで、塗膜密着性を改善する技術が提案されている。
特開平０４−２７６０６０号公報 特開２００５−１８７８６３号公報 特開２００５−２９０４４０号公報 特開２００６−２８３１３０号公報 特開２００４−３２３９６９号公報 特許第３３８６６５７号公報 特開２００３−２２６９２０号公報 特開２００７−０００２６９号公報 特開２００４−２４４６９８号公報 Next, the second possible reason is that the oxide containing Si generated at the grain boundary near the steel sheet surface is removed by pickling or the like, so that fine irregularities or cracks are generated in the surface layer, In the interior, formation of chemical crystals is unlikely to occur, so that corrosion is accelerated at this portion and the corrosion resistance deteriorates. As an improvement measure against this, for example, in Patent Documents 2 to 4 and Patent Document 9, by controlling the presence of grain boundary oxides and surface layer cracks within a range of about 10 μm from the surface of the steel sheet, coating film adhesion Techniques for improving the performance have been proposed.
Japanese Patent Laid-Open No. 04-276060 JP 2005-187863 A JP 2005-290440 A JP 2006-283130 A JP 2004-323969 A Japanese Patent No. 3386657 JP 2003-226920 A JP 2007-000269 A JP 2004-244698 A

しかしながら、特許文献１〜４に記載の方法は、表層酸化物中のＳｉとＭｎの比や存在状態を制御する技術であるため、必然的に鋼中に含有するＳｉとＭｎの比が制限され、添加元素の自由度が低下し、十分な強度と加工性を得ることが難しい。また表層酸化物の組成や存在状態は、焼鈍時の露点や水素濃度等の影響を受けやすく、優れた化成処理性を安定的に得ることは難しいという問題があった。また、特許文献２〜４に記載の塗膜密着性を改善し、化成電着塗装後の耐食性の向上を図る方法は、熱間圧延の巻取温度が５００℃以下、連続焼鈍時の露点が−４０℃以下、連続焼鈍時の焼入開始温度が５５０℃以下等と、製造条件が極めて限定されているため、所望の強度と加工性を得ることが難しいという問題もある。   However, since the methods described in Patent Documents 1 to 4 are techniques for controlling the ratio and existence state of Si and Mn in the surface layer oxide, the ratio of Si and Mn contained in the steel is inevitably limited. The degree of freedom of the additive element is reduced, and it is difficult to obtain sufficient strength and workability. In addition, the composition and presence state of the surface oxide are easily affected by the dew point during annealing, the hydrogen concentration, and the like, and it is difficult to stably obtain excellent chemical conversion properties. Moreover, the method of improving the coating-film adhesiveness of patent documents 2-4, and aiming at the improvement of the corrosion resistance after a chemical conversion electrodeposition is that the coiling temperature of hot rolling is 500 degrees C or less, and the dew point at the time of continuous annealing is Since manufacturing conditions are extremely limited, such as −40 ° C. or lower and a quenching start temperature during continuous annealing of 550 ° C. or lower, there is a problem that it is difficult to obtain desired strength and workability.

また、特許文献５に記載の方法は、表層のＳｉ酸化物の低減により、化成処理性はある程度改善されるが、塗装後の耐食性については何ら検討されていない。同様に、特許文献６に記載の方法も、表層へのＳｉ等の濃化を抑制することにより化成処理性は改善されるが、化成電着塗装後の耐食性の改善には至っていない。また、特許文献７に記載の方法は、機械的研削を２度も行う必要があるため、製造コストが上昇する。また、制御対象が鋼板表層に限られており、塗装後耐食性については触れられていない。また、特許文献８に記載の方法は、酸洗処理のあとにアルカリ処理を施す必要があり、コストアップを招くほか、アルカリ処理では鋼板表面のＳｉ含有酸化物は溶解できるが、鋼板内部に存在するＳｉ含有酸化物を除去することができないため、優れた塗装後の耐食性を安定して得ることは困難である。また、特許文献９に記載の方法は、耐食性不良は、焼鈍炉から水焼入用の水槽への移動中の酸化によるＳｉとＯ由来の線状化合物が原因であるとしており、耐食性を向上するには、水槽を非酸化ガスで充填したチャンバーで囲う必要があり、設備費が高くなる。さらに、後述するように、特許文献２〜４や特許文献９に記載の方法は、粒界酸化物や表層クラックの存在状態を制御しても、優れた耐食性を安定して得ることは困難であった。   In the method described in Patent Document 5, the chemical conversion treatment property is improved to some extent by reducing the Si oxide in the surface layer, but no investigation is made on the corrosion resistance after coating. Similarly, in the method described in Patent Document 6, the chemical conversion property is improved by suppressing the concentration of Si or the like on the surface layer, but the corrosion resistance after chemical electrodeposition coating has not been improved. Moreover, since the method described in Patent Document 7 needs to perform mechanical grinding twice, the manufacturing cost increases. Moreover, the object to be controlled is limited to the steel sheet surface layer, and no mention is made of the corrosion resistance after painting. In addition, the method described in Patent Document 8 requires an alkali treatment after the pickling treatment, resulting in an increase in cost. In the alkali treatment, the Si-containing oxide on the surface of the steel sheet can be dissolved, but is present inside the steel sheet. Since it is impossible to remove the Si-containing oxide, it is difficult to stably obtain excellent post-coating corrosion resistance. In addition, the method described in Patent Document 9 says that the corrosion resistance failure is caused by the linear compounds derived from Si and O due to oxidation during the transfer from the annealing furnace to the water quenching water tank, and improves the corrosion resistance. In this case, it is necessary to enclose the water tank with a chamber filled with non-oxidizing gas, which increases the equipment cost. Furthermore, as described later, the methods described in Patent Documents 2 to 4 and Patent Document 9 are difficult to stably obtain excellent corrosion resistance even if the presence state of grain boundary oxides or surface layer cracks is controlled. there were.

以上のように、Ｓｉ含有鋼板、特にＳｉを０．８ｍａｓｓ％以上含有する高強度冷延鋼板の化成処理性や化成電着塗装後の耐食性が劣るという問題点を解決するために、従来から様々な検討がなされてきているが、上記問題点を抜本的に解決した技術は今のところ見出されていないのが実情である。   As described above, in order to solve the problems that the chemical conversion processability of Si-containing steel sheets, particularly high-strength cold-rolled steel sheets containing 0.8 mass% or more of Si and the corrosion resistance after chemical electrodeposition coating are inferior, However, a technology that drastically solves the above problems has not been found so far.

そこで、本発明の目的は、従来技術が抱える上記問題点を解決し、高強度で加工性に優れると共に、化成処理性および化成電着塗装後の耐食性に優れる高強度冷延鋼板とその製造方法を提案することにある。   Therefore, the object of the present invention is to solve the above-mentioned problems of the prior art, high strength cold rolled steel sheet having high strength and excellent workability, and excellent chemical conversion property and corrosion resistance after chemical electrodeposition coating, and its manufacturing method Is to propose.

発明者らは、上記の課題を解決すべく、Ｃ：０．０３〜３．０ｍａｓｓ％、Ｓｉ：０．１〜３．０ｍａｓｓ％、Ｍｎ：１．０〜３．５ｍａｓｓ％の範囲で種々に変化させた鋼を実機にて溶製し、連続鋳造して鋼スラブとし、熱間圧延し、冷間圧延し、その後、種々の焼鈍条件で仕上焼鈍して板厚が１．６ｍｍの冷延焼鈍板とし、この冷延焼鈍板について、機械的特性、化成処理性および化成電着塗装後の耐食性（以降、「塗装後耐食性」ともいう）を、後述する実施例の条件で調査した。   In order to solve the above-mentioned problems, the inventors variously in the range of C: 0.03-3.0 mass%, Si: 0.1-3.0 mass%, Mn: 1.0-3.5 mass%. The changed steel is melted in the actual machine, continuously cast into a steel slab, hot-rolled, cold-rolled, and then finish-annealed under various annealing conditions and cold-rolled with a thickness of 1.6 mm As the annealed plate, the cold rolled annealed plate was examined for mechanical properties, chemical conversion treatment, and corrosion resistance after chemical electrodeposition coating (hereinafter also referred to as “corrosion resistance after coating”) under the conditions of the examples described later.

その結果、所望とする強度（引張強さ：７８０ＭＰａ以上）と加工性（強度−延性バランス（ＴＳ×Ｅｌ≧１８０００ＭＰａ・％））を両立させるには、０．８ｍａｓｓ％以上のＳｉの添加が必要であることがわかった。しかし、Ｓｉ含有量が０．８ｍａｓｓ％以上となると、図１（ａ）〜（ｃ）に模式的に示したように、鋼板表面および／または表面近傍の鋼板内部に、Ｓｉを含有する酸化物（以降、「Ｓｉ含有酸化物」ともいう）が必ず存在し、化成処理性や塗装後耐食性の低下原因となっていることがわかった。   As a result, in order to achieve both the desired strength (tensile strength: 780 MPa or more) and workability (strength-ductility balance (TS × El ≧ 18000 MPa ·%)), it is necessary to add 0.8 mass% or more of Si. I found out that However, when the Si content is 0.8 mass% or more, as schematically shown in FIGS. 1A to 1C, an oxide containing Si is present on the steel sheet surface and / or in the vicinity of the steel sheet. (Hereinafter, also referred to as “Si-containing oxide”) is inevitably present, and it has been found that the chemical conversion treatment property and the post-coating corrosion resistance are reduced.

また、化成処理性や塗装後耐食性を改善するために、図１（ｃ）に示すように、鋼板表面のＳｉ酸化物のみを完全に除去しても、化成結晶のスケは改善できるが、塗装後耐食性が劣化する場合があることがわかった。さらに、鋼板内部の粒界酸化物や表層クラックの存在形態を、従来技術に従って適正に制御しても、優れた塗装後耐食性を安定して得られない場合があることがわかった。   In addition, in order to improve the chemical conversion property and the corrosion resistance after coating, as shown in FIG. 1 (c), even if only the Si oxide on the surface of the steel sheet is completely removed, the scale of the chemical conversion crystal can be improved, but the coating can be improved. It was found that post-corrosion resistance may deteriorate. Furthermore, it has been found that even after the presence of grain boundary oxides and surface layer cracks in the steel sheet is appropriately controlled in accordance with the prior art, excellent post-coating corrosion resistance may not be obtained stably.

そこで、各鋼板の表層部断面を、走査電子顕微鏡（ＳＥＭ）を用いて観察し、塗装後耐食性が低下する原因について詳細な調査を行った。その結果、鋼板表面にＳｉを主体とする酸化物が存在しなくとも、表面近傍の鋼板内部に、どのような形態であれ、Ｓｉを含有する酸化物が存在する場合には、塗装後耐食性が劣化すること、したがって、図１（ｄ）に示すように、鋼板表面および表面近傍の鋼板内部に存在するＳｉを含有する酸化物を完全に除去することによってのみ、安定して優れた化成処理性と塗装後耐食性を得ることができることを見出し、本発明を完成させた。   Therefore, the surface layer cross section of each steel plate was observed using a scanning electron microscope (SEM), and a detailed investigation was conducted on the cause of the decrease in corrosion resistance after coating. As a result, even if there is no oxide mainly composed of Si on the surface of the steel sheet, if there is an oxide containing Si in any form inside the steel sheet in the vicinity of the surface, the corrosion resistance after coating is reduced. Therefore, as shown in FIG. 1 (d), stable and excellent chemical conversion treatment can be achieved only by completely removing the Si-containing oxide present in the steel sheet surface and in the vicinity of the steel sheet. And found that corrosion resistance after painting can be obtained, and the present invention has been completed.

すなわち、本発明は、Ｃ：０．０５〜０．３０ｍａｓｓ％、Ｓｉ：０．８〜３．０ｍａｓｓ％、Ｍｎ：１．５〜３．０ｍａｓｓ％、Ｐ：０．１０ｍａｓｓ％以下、Ｓ：０．０１ｍａｓｓ％以下、Ａｌ：０．０１〜０．１ｍａｓｓ％、Ｎ：０．００５ｍａｓｓ％以下を含有し、残部がＦｅおよび不可避的不純物からなる成分組成を有し、鋼板表面および表面から深さ１μｍの範囲の鋼板内部におけるＳｉ濃度の最大値Ｐ２が、板厚１／４におけるＳｉ濃度Ｐ１の１．３倍以下であることを特徴とする化成処理性および塗装後耐食性に優れる高強度冷延鋼板である。 That is, the present invention is C: 0.05-0.30 mass%, Si: 0.8-3.0 mass%, Mn: 1.5-3.0 mass%, P: 0.10 mass% or less, S: 0 .01 mass% or less, Al: 0.01 to 0.1 mass%, N: 0.005 mass% or less, with the balance being composed of Fe and inevitable impurities, with a steel plate surface and a depth of 1 μm from the surface. high strength cold maximum P 2 Si concentration in the steel inside the range of, excellent in chemical conversion treatability and corrosion resistance after coating, characterized in that is less than 1.3 times the Si concentration P 1 in the sheet thickness 1/4 It is a rolled steel sheet.

また、本発明は、Ｃ：０．０５〜０．３０ｍａｓｓ％、Ｓｉ：０．８〜３．０ｍａｓｓ％、Ｍｎ：１．５〜３．０ｍａｓｓ％、Ｐ：０．１０ｍａｓｓ％以下、Ｓ：０．０１ｍａｓｓ％以下、Ａｌ：０．０１〜０．１ｍａｓｓ％、Ｎ：０．００５ｍａｓｓ％以下を含有し、残部がＦｅおよび不可避的不純物からなる成分組成を有するスラブを熱間圧延し、冷間圧延し、連続焼鈍して冷延鋼板を製造する方法において、上記連続焼鈍後、酸洗して鋼板表面を片面当たり１μｍ以上除去することを特徴とする化成処理性および塗装後耐食性に優れる高強度冷延鋼板の製造方法を提案する。   In the present invention, C: 0.05 to 0.30 mass%, Si: 0.8 to 3.0 mass%, Mn: 1.5 to 3.0 mass%, P: 0.10 mass% or less, S: 0 .01 mass% or less, Al: 0.01 to 0.1 mass%, N: 0.005 mass% or less, and a slab having a composition composed of Fe and unavoidable impurities in the remainder is hot-rolled and cold-rolled In the method of producing a cold-rolled steel sheet by continuous annealing, the steel sheet is pickled after the continuous annealing, and the steel sheet surface is removed by 1 μm or more per side. A method for producing rolled steel sheets is proposed.

本発明の上記製造方法は、酸洗に、１〜５ｍａｓｓ％の硝酸水溶液、２〜２０ｍａｓｓ％の硝酸と０．１〜５ｍａｓｓ％の塩酸からなる混酸水溶液および０．５〜５ｍａｓｓ％の硝酸と０．１〜５ｍａｓｓ％の弗酸からなる混酸水溶液のいずれかを用いることを特徴とする。   In the above production method of the present invention, 1-5 mass% nitric acid aqueous solution, 2-20 mass% nitric acid and 0.1-5 mass% hydrochloric acid aqueous solution, 0.5-5 mass% nitric acid and 0 are used for pickling. Any one of a mixed acid aqueous solution comprising 1 to 5 mass% hydrofluoric acid is used.

本発明によれば、高強度で加工性に優れるだけでなく、化成処理性や塗装後耐食性にも優れる高強度冷延鋼板を安定して製造することができる。したがって、本発明の高強度冷延鋼板は、自動車の高強度部材等の素材に要求される強度、加工性、化成処理性および塗装後耐食性の全ての要件を満たしており、自動車用部品の素材として好適に用いることができる。   According to the present invention, it is possible to stably produce a high-strength cold-rolled steel sheet that not only has high strength and excellent workability but also has excellent chemical conversion properties and post-coating corrosion resistance. Therefore, the high-strength cold-rolled steel sheet of the present invention satisfies all requirements for strength, workability, chemical conversion property and post-coating corrosion resistance required for materials such as high-strength members of automobiles. Can be suitably used.

まず、本発明の高強度冷延鋼板は、引張強さが７８０ＭＰａ以上のものを対象とする。本発明の鋼板は、高い強度が要求される自動車車体の強度部材や補強部材および自動車シートの骨格部等への適用を意図したものであるからである。また、引張強さが７８０ＭＰａ未満では、Ｓｉを０．８ｍａｓｓ％以上添加することなく、高強度と加工性を兼ね備えた特性を有する鋼板を容易に得ることができるからでもある。なお、本発明における加工性に優れるとは、伸び特性に優れることを言うが、強度とのバランスから、引張強さＴＳと伸びＥｌとの積である強度−延性バランス（ＴＳ×Ｅｌ）が１８０００ＭＰａ・％以上であることをいう。   First, the high-strength cold-rolled steel sheet of the present invention is intended for those having a tensile strength of 780 MPa or more. This is because the steel sheet of the present invention is intended for application to a strength member or reinforcing member of an automobile body that requires high strength, a skeleton part of an automobile seat, or the like. In addition, if the tensile strength is less than 780 MPa, a steel sheet having characteristics having both high strength and workability can be easily obtained without adding Si by 0.8 mass% or more. Note that excellent workability in the present invention means excellent elongation characteristics, but from the balance with strength, the strength-ductility balance (TS × El), which is the product of tensile strength TS and elongation El, is 18000 MPa.・ It means% or more.

次に、本発明の上記鋼板が有すべき成分組成について説明する。
Ｃ：０．０５〜０．３０ｍａｓｓ％
Ｃは、所望の鋼板強度を確保するために必要な元素であり、本発明では、加工性を確保しつつ引張強さ：７８０ＭＰａ以上を得るために、０．０５ｍａｓｓ％以上の添加を必要とする。一方、０．３０ｍａｓｓ％を超える添加は、溶接性を著しく劣化させる。よって、Ｃは０．０５〜０．３０ｍａｓｓ％の範囲で添加する。より優れた溶接性が要求される場合には、Ｃは０．２０ｍａｓｓ％以下が好ましく、０．１５ｍａｓｓ％以下であることがより好ましい。 Next, the component composition that the steel sheet of the present invention should have will be described.
C: 0.05-0.30 mass%
C is an element necessary for securing a desired steel plate strength. In the present invention, 0.05 mass% or more is required to obtain a tensile strength of 780 MPa or more while ensuring workability. . On the other hand, addition exceeding 0.30 mass% significantly deteriorates weldability. Therefore, C is added in the range of 0.05 to 0.30 mass%. When more excellent weldability is required, C is preferably 0.20 mass% or less, and more preferably 0.15 mass% or less.

Ｓｉ：０．８〜３．０ｍａｓｓ％
Ｓｉは、固溶強化により、鋼板の強度を高める元素であり、特に、加工性の低下を抑制しつつ強度を上昇させることができる有効な元素である。ただし、Ｓｉは、先述したように、化成処理性や塗装後耐食性を害するが、本発明では、鋼板表面のＳｉ酸化物層を完全に除去することにより、その弊害を回避することができる。そこで、本発明では、所望の強度と加工性を確保するため、Ｓｉを０．８ｍａｓｓ％以上添加する。しかし、３．０ｍａｓｓ％を超える添加は、上記の効果が飽和し、却って、加工性の低下をもたらすため、３．０ｍａｓｓ％以下とする。好ましくは、１．０〜２．５ｍａｓｓ％の範囲である。 Si: 0.8-3.0 mass%
Si is an element that increases the strength of the steel sheet by solid solution strengthening, and is particularly an effective element that can increase the strength while suppressing a decrease in workability. However, as described above, Si impairs chemical conversion properties and post-coating corrosion resistance, but in the present invention, the adverse effects can be avoided by completely removing the Si oxide layer on the steel sheet surface. Therefore, in the present invention, Si is added in an amount of 0.8 mass% or more in order to ensure desired strength and workability. However, the addition exceeding 3.0 mass% saturates the above effect and, on the other hand, causes a decrease in workability. Preferably, it is in the range of 1.0 to 2.5 mass%.

Ｍｎ：１．５〜３．０ｍａｓｓ％
Ｍｎは、固溶強化により鋼板の強度向上に寄与するほか、焼入れ性を高める効果を有するため、強度を安定して確保するのに有効な元素である。このような効果を得るためには、１．５ｍａｓｓ％以上の添加が必要である。一方、３．０ｍａｓｓ％を超える添加は、加工性の低下をもたらす。よって、Ｍｎは１．５〜３．０ｍａｓｓ％の範囲で添加する。好ましくは、１．８〜２．５ｍａｓｓ％の範囲である。 Mn: 1.5 to 3.0 mass%
Mn contributes to improving the strength of the steel sheet by solid solution strengthening, and also has an effect of improving the hardenability, so that Mn is an effective element for ensuring the strength stably. In order to obtain such an effect, addition of 1.5 mass% or more is necessary. On the other hand, the addition exceeding 3.0 mass% causes a decrease in workability. Therefore, Mn is added in the range of 1.5 to 3.0 mass%. Preferably, it is in the range of 1.8 to 2.5 mass%.

Ｐ：０．１０ｍａｓｓ％以下
Ｐは、鋼を強化する作用があり、鋼板の強度レベルに応じて添加することができるが、０．１０ｍａｓｓ％を超えて添加すると、溶接性や靭性が低下する。よって、Ｐは０．１０ｍａｓｓ％以下とする。より優れた溶接性や靭性が要求される場合には、０．０５ｍａｓｓ％以下とするのが好ましい。 P: 0.10 mass% or less P has an effect of strengthening steel, and can be added according to the strength level of the steel sheet. However, if it exceeds 0.10 mass%, weldability and toughness are lowered. Therefore, P is 0.10 mass% or less. When more excellent weldability and toughness are required, it is preferably 0.05 mass% or less.

Ｓ：０．０１ｍａｓｓ％以下
Ｓは、鋼中に硫化物系介在物として存在し、伸びフランジ性を低下させる有害元素である。そのため、Ｓはできるだけ低減するのが好ましく、０．０１ｍａｓｓ％以下とする必要がある。より優れた伸びフランジ性を要求される場合には、０．００５ｍａｓｓ％以下とするのが好ましい。 S: 0.01 mass% or less S is a harmful element that exists as sulfide inclusions in steel and reduces stretch flangeability. Therefore, S is preferably reduced as much as possible, and needs to be 0.01 mass% or less. When more excellent stretch flangeability is required, the content is preferably 0.005 mass% or less.

Ａｌ：０．０１〜０．１ｍａｓｓ％
Ａｌは、鋼の脱酸元素として添加され、鋼の清浄度を向上する効果を有し、また、鋼組織を微細化する効果もあるため、本発明では、Ａｌを０．０１ｍａｓｓ％以上添加する。しかし、Ａｌの過剰な添加は、表面性状の悪化をもたらすため、上限を０．１ｍａｓｓ％とする。 Al: 0.01-0.1 mass%
Al is added as a deoxidizing element for steel and has the effect of improving the cleanliness of the steel and also has the effect of refining the steel structure. Therefore, in the present invention, Al is added in an amount of 0.01 mass% or more. . However, excessive addition of Al causes deterioration of the surface properties, so the upper limit is made 0.1 mass%.

Ｎ：０．００５ｍａｓｓ％以下
Ｎは、０．００５ｍａｓｓ％を超える含有は、強度のバラツキの原因となる。よって、Ｎは０．００５ｍａｓｓ％以下に制限する。 N: 0.005 mass% or less The content of N exceeding 0.005 mass% causes variation in strength. Therefore, N is limited to 0.005 mass% or less.

本発明の鋼板は、上記成分組成を満たすことで、目的とする強度と加工性を得ることができるが、要求される強度等に応じて、上記成分に加えてさらに、Ｔｉ：０．００５〜０．１ｍａｓｓ％、Ｎｂ：０．００５〜０．１ｍａｓｓ％、Ｖ：０．００５〜０．１ｍａｓｓ％、Ｃｒ：０．０１〜０．５ｍａｓｓ％、Ｍｏ：０．０１〜０．５ｍａｓｓ％およびＢ：０．０００５〜０．００５ｍａｓｓ％のうちから選ばれる１種または２種以上を添加することができる。   The steel sheet of the present invention can obtain the intended strength and workability by satisfying the above component composition, but depending on the required strength and the like, in addition to the above components, Ti: 0.005 0.1 mass%, Nb: 0.005-0.1 mass%, V: 0.005-0.1 mass%, Cr: 0.01-0.5 mass%, Mo: 0.01-0.5 mass%, and B : One or more selected from 0.0005 to 0.005 mass% can be added.

Ｔｉ，ＮｂおよびＶは、いずれも鋼中で炭化物を形成し、析出強化したり、結晶粒を微細化したりすることにより、強度を高める効果を有する。このような効果を得るためには、それぞれ０．００５ｍａｓｓ％以上添加する必要がある。一方、０．１ｍａｓｓ％を超える添加は、加工性を著しく低下させる。よって、Ｔｉ，Ｎｂ，Ｖは、それぞれ０．００５〜０．１ｍａｓｓ％の範囲で添加するのが好ましい。より好ましくは、０．０１〜０．０５ｍａｓｓ％の範囲である。   Ti, Nb, and V all have the effect of increasing strength by forming carbides in the steel, strengthening precipitation, and refining crystal grains. In order to acquire such an effect, it is necessary to add 0.005 mass% or more, respectively. On the other hand, the addition exceeding 0.1 mass% significantly reduces workability. Therefore, Ti, Nb, and V are preferably added in the range of 0.005 to 0.1 mass%, respectively. More preferably, it is the range of 0.01-0.05 mass%.

また、Ｃｒ，ＭｏおよびＢは、焼入れ性を高めて強度を高める効果を有しており、必要に応じて添加することができる。このような効果を得るには、Ｃｒ，Ｍｏは０．０１ｍａｓｓ％以上、Ｂは０．０００５ｍａｓｓ％以上の添加が必要である。一方、これら元素の多量の添加は、いずれも加工性を低下させる。よって、Ｃｒ，Ｍｏは０．０１〜０．５ｍａｓｓ％、Ｂは０．０００５〜０．００５ｍａｓｓ％の範囲で添加するのが好ましい。より好ましくは、Ｃｒ，Ｍｏは０．０５〜０．４ｍａｓｓ％、Ｂは０．００１〜０．００４ｍａｓｓ％の範囲である。   Further, Cr, Mo and B have the effect of increasing the hardenability and increasing the strength, and can be added as necessary. In order to obtain such an effect, it is necessary to add Cr and Mo in an amount of 0.01 mass% or more and B in an amount of 0.0005 mass% or more. On the other hand, the addition of a large amount of these elements decreases the workability. Therefore, Cr and Mo are preferably added in the range of 0.01 to 0.5 mass% and B in the range of 0.0005 to 0.005 mass%. More preferably, Cr and Mo are in the range of 0.05 to 0.4 mass%, and B is in the range of 0.001 to 0.004 mass%.

本発明の鋼板は、上記成分以外の残部は、Ｆｅおよび不可避的不純物からなる。上記不可避的不純物としては、例えば、上述した選択添加元素であるＴｉ，Ｎｂ，Ｖ，Ｃｒ，Ｍｏ，Ｂの他に、Ｎｉ，Ｃｕ，Ｍｇ，Ｃａ，Ｚｒ，ＲＥＭ等が挙げられるが、これらの元素は、通常、含有される不純物の範囲内でかつ本発明の効果を害しない範囲であれば含有を拒むものではない。また、その他の不可避的不純物として、Ｓｂ，Ｓｎ，Ｚｎ，Ｃｏ等の不純物も挙げられるが、これらは、Ｓｂ：０．０１ｍａｓｓ％以下、Ｓｎ：０．１ｍａｓｓ％以下、Ｚｎ：０．０１ｍａｓｓ％以下、Ｃｏ：０．１ｍａｓｓ％以下であれば許容され得る。   In the steel sheet of the present invention, the balance other than the above components consists of Fe and inevitable impurities. Examples of the inevitable impurities include Ni, Cu, Mg, Ca, Zr, and REM in addition to Ti, Nb, V, Cr, Mo, and B, which are the selective additive elements described above. In general, elements are not rejected as long as they are within the range of impurities contained and do not impair the effects of the present invention. Other inevitable impurities include impurities such as Sb, Sn, Zn, and Co. These are Sb: 0.01 mass% or less, Sn: 0.1 mass% or less, Zn: 0.01 mass% or less. Co: 0.1 mass% or less is acceptable.

次に、本発明の高強度冷延鋼板の表面および表面近傍の鋼板内部について説明する。
本発明の高強度冷延鋼板は、高強度で加工性に優れる他、化成処理性および化成電着塗装後の耐食性にも優れる。そのためには、本発明の鋼板は、上述した成分組成を有すると共に、鋼板表面および表面から深さ１μｍの範囲の鋼板内部に、Ｓｉを含有する酸化物が実質的に存在しないことが必要である。ここで、上記「Ｓｉを含有する酸化物が実質的に存在しない」とは、鋼板表面をグロー放電分光分析装置（ＧＤＳ）で測定したときの鋼板表面および表面から深さ１μｍの範囲の鋼板内部におけるＳｉ強度の最大値Ｐ２が、板厚１／４まで研削した鋼板表面のＳｉ強度Ｐ１の１．３倍以下（Ｐ２／Ｐ１≦１．３）であることを意味する。なお、上記Ｓｉ強度は、Ｓｉ濃度を意味する。 Next, the surface of the high-strength cold-rolled steel sheet of the present invention and the inside of the steel sheet near the surface will be described.
The high-strength cold-rolled steel sheet of the present invention has high strength and excellent workability, as well as excellent chemical conversion property and corrosion resistance after chemical electrodeposition coating. For this purpose, the steel sheet of the present invention has the above-described component composition, and it is necessary that substantially no Si-containing oxide is present in the steel sheet surface and in the steel sheet having a depth of 1 μm from the surface. . Here, “the oxide containing Si substantially does not exist” means that the inside of the steel sheet within a range of 1 μm in depth from the steel sheet surface and the surface when the steel sheet surface is measured by a glow discharge spectrometer (GDS). maximum value P 2 of Si intensity at it is meant that 1.3 times the Si intensity P 1 of grinding the steel sheet surface below (P 2 / P 1 ≦ 1.3) to a thickness of 1/4. In addition, the said Si intensity means Si density | concentration.

上述したように、鋼板表面にＳｉを主体とする酸化物が存在すると、化成結晶にスケができ、電着塗膜の密着性を低下させる。また、鋼板表面から深さ１μｍの範囲の鋼板内部にＳｉを含有する酸化物が存在すると、鋼板表面にＳｉを含有する酸化物が存在しない場合でも、十分な塗装後の耐食性を得ることができない。鋼板内部にＳｉ含有酸化物が存在する場合に塗装後の耐食性が低下する理由は十分に解明されていないが、塗装後の鋼板表面に傷が入った場合、傷表面に露出するＳｉを含有する酸化物と地鉄との間の電位差によって、地鉄の腐食が促進されるためであると発明者らは考えている。したがって、優れた化成処理性および塗装後耐食性を安定して得るためには、鋼板表面および表面から深さ１μｍの範囲の鋼板内部にＳｉを含有する酸化物が実質的に存在しないことが必要である。   As described above, when an oxide mainly composed of Si is present on the surface of the steel sheet, the conversion crystal can be scaled and the adhesion of the electrodeposition coating film is lowered. In addition, when an oxide containing Si is present inside the steel sheet having a depth of 1 μm from the steel sheet surface, sufficient corrosion resistance after coating cannot be obtained even when no oxide containing Si is present on the steel sheet surface. . The reason why the corrosion resistance after painting decreases when Si-containing oxide is present inside the steel sheet has not been fully elucidated, but if the steel sheet surface after painting is scratched, it contains Si exposed on the scratched surface The inventors consider that this is because corrosion of the ground iron is promoted by the potential difference between the oxide and the ground iron. Therefore, in order to stably obtain excellent chemical conversion treatment properties and corrosion resistance after coating, it is necessary that substantially no oxide containing Si is present in the steel sheet surface and in the steel sheet within a range of 1 μm depth from the surface. is there.

次に、本発明の化成処理性および塗装後耐食性に優れる高強度冷延鋼板の製造方法について説明する。
本発明の鋼板は、上記に説明した本発明に適合する成分組成を有する鋼を常法の製鋼プロセスで溶製してスラブとし、このスラブを熱間圧延し、冷間圧延し、連続焼鈍して製造する。 Next, the manufacturing method of the high intensity | strength cold-rolled steel plate excellent in the chemical conversion property of this invention and the corrosion resistance after coating is demonstrated.
The steel sheet of the present invention is a slab obtained by melting a steel having a composition suitable for the present invention described above into a slab by a conventional steelmaking process. The slab is hot-rolled, cold-rolled, and continuously annealed. Manufactured.

上記スラブの製造方法は、連続鋳造法または造塊−分塊圧延法のいずれを用いてもよいが、スラブ内のマクロ偏析を防止し、材質を安定させる観点からは、連続鋳造法で製造するのが好ましい。また、薄スラブ鋳造法を用いてもよい。   The slab manufacturing method may use either a continuous casting method or an ingot-bundling rolling method. From the viewpoint of preventing macro segregation in the slab and stabilizing the material, the slab is manufactured by a continuous casting method. Is preferred. Further, a thin slab casting method may be used.

続く、熱間圧延は、一旦室温まで冷却したスラブを加熱炉で１０００℃以上の温度に再加熱してから行うのが通常であるが、スラブ製造後（連続鋳造後）、再加熱することなく直ちに圧延する直送圧延（直接圧延）する方法や、室温まで冷却することなく温片状態で加熱炉に装入し、軽加熱もしくは保温を行ってから圧延する省エネルギープロセスを採用してもよい。   The subsequent hot rolling is usually performed after reheating the slab once cooled to room temperature to a temperature of 1000 ° C. or higher in a heating furnace, but without reheating after slab production (after continuous casting). An energy saving process in which rolling is performed after direct heating rolling (direct rolling) or by charging in a heating furnace in a warm piece state without cooling to room temperature and performing light heating or heat retention may be employed.

上記スラブを再加熱する場合、スラブ加熱温度は１０００℃以上とするのが好ましい。上限は、特に限定されないが、１３００℃を超えると酸化重量の増加に伴いスケールロスが増大したり、表面欠陥が発生したりする原因となることから、１３００℃を上限とするのが好ましい。また、上記省エネルギープロセスを採用する場合も、スラブ温度は１０００℃以上とするのが好ましい。   When reheating the slab, the slab heating temperature is preferably 1000 ° C. or higher. The upper limit is not particularly limited, but if it exceeds 1300 ° C., scale loss increases as the oxidized weight increases or surface defects occur, and therefore, the upper limit is preferably 1300 ° C. Moreover, also when employ | adopting the said energy saving process, it is preferable that slab temperature shall be 1000 degreeC or more.

また、熱間圧延は、必要に応じて粗圧延を行った後、仕上圧延終了温度を８００℃以上とする仕上圧延を行い熱延板とするのが好ましい。仕上圧延終了温度が８００℃を下回ると、鋼板組織が不均一となり、加工性を低下させる。一方、仕上圧延終了温度の上限は、特に限定されないが、過度に高い温度で圧延すると、スケール痕などの表面欠陥の原因となるので、１０００℃以下とするのが好ましい。熱間圧延後は、６５０℃以下の温度で巻き取るのが好ましい。巻取温度が６５０℃を超えると、巻き取り後に多量のスケールが生成し、冷間圧延前の酸洗負荷が大きくなる。   In the hot rolling, it is preferable that hot rolling is performed by performing rough rolling as necessary and then finishing rolling with a finish rolling finishing temperature of 800 ° C. or higher. When the finish rolling finish temperature is lower than 800 ° C., the steel sheet structure becomes non-uniform and the workability is lowered. On the other hand, the upper limit of the finish rolling finish temperature is not particularly limited, but if it is rolled at an excessively high temperature, it causes surface defects such as scale marks. After hot rolling, it is preferable to wind at a temperature of 650 ° C. or lower. When the winding temperature exceeds 650 ° C., a large amount of scale is generated after winding, and the pickling load before cold rolling becomes large.

次いで、上記のようにして得た熱延板は、酸洗して脱スケール後、冷間圧延する。この冷間圧延は、所望の寸法・形状の冷延板を得ることができれば特に限定されないが、表面の平坦度や組織の均一性の観点からは、圧下率２０％以上の圧延を施すことが好ましい。なお、冷間圧延前の酸洗は、熱延板の表面スケールが極めて薄い場合には、省くこともできる。   Next, the hot-rolled sheet obtained as described above is pickled, descaled, and then cold-rolled. The cold rolling is not particularly limited as long as a cold-rolled sheet having a desired size and shape can be obtained. From the viewpoint of the flatness of the surface and the uniformity of the structure, rolling with a rolling reduction of 20% or more may be performed. preferable. In addition, pickling before cold rolling can be omitted when the surface scale of the hot-rolled sheet is extremely thin.

冷間圧延後の冷延板は、その後、所望の強度と加工性を付与するため連続焼鈍ラインで焼鈍を施す。この連続焼鈍における焼鈍は、７５０〜９００℃の温度域に加熱保持することが好ましい。加熱保持温度が７５０℃未満では、十分に再結晶が起こらず、加工性が低下する。一方、９００℃超えでは、組織が粗大化し、強度−延性バランスが低下する。また、上記温度に保持する時間は、３０ｓｅｃ以上が好ましく、鋼板の材質を均一化するためには、６０ｓｅｃ以上であることがより好ましい。さらに好ましくは１２０ｓｅｃ以上である。一方、保持時間の上限は、生産性を考慮し、６００ｓｅｃ以下であることが好ましい。また、加熱保持後の冷却は、平均冷却速度３０℃／ｓｅｃ以上で３００℃以下まで急冷するのが好ましい。冷却速度が３０℃／ｓｅｃ未満では、変態組織による強度向上が図れないため、所望の強度を得るために、合金元素を多量に添加する必要があり、原料コストが上昇する。また、上記急冷後はそのままとするか、あるいは、室温近くまで冷却後、再加熱し、１００〜４５０℃の温度に３〜３０ｍｉｎ間保持するのが好ましい。   The cold-rolled sheet after cold rolling is then annealed in a continuous annealing line in order to impart desired strength and workability. The annealing in this continuous annealing is preferably heated and held in a temperature range of 750 to 900 ° C. When the heating and holding temperature is less than 750 ° C., recrystallization does not occur sufficiently, and workability decreases. On the other hand, if it exceeds 900 ° C., the structure becomes coarse and the strength-ductility balance decreases. The time for maintaining the temperature is preferably 30 seconds or longer, and more preferably 60 seconds or longer in order to make the steel plate material uniform. More preferably, it is 120 sec or more. On the other hand, the upper limit of the holding time is preferably 600 sec or less in consideration of productivity. Moreover, it is preferable that the cooling after heating is rapidly cooled to 300 ° C. or less at an average cooling rate of 30 ° C./sec or more. If the cooling rate is less than 30 ° C./sec, the strength cannot be improved by the transformation structure. Therefore, in order to obtain a desired strength, it is necessary to add a large amount of alloy elements, and the raw material cost increases. Moreover, it is preferable to leave it as it is after the rapid cooling or to reheat it after cooling it to near room temperature and hold it at a temperature of 100 to 450 ° C. for 3 to 30 minutes.

また、本発明では、上記連続焼鈍における加熱保持中の露点は−２０℃以下とすることが好ましい。露点が−２０℃を超えると、鋼板表層における脱炭が顕著になり、材質に悪影響を及ぼす。より好ましくは−２５℃以下である。   Moreover, in this invention, it is preferable that the dew point during the heating holding in the said continuous annealing shall be -20 degrees C or less. When the dew point exceeds −20 ° C., decarburization in the steel sheet surface layer becomes remarkable, which adversely affects the material. More preferably, it is −25 ° C. or lower.

さらに、本発明では、本発明の優れた化成処理性および塗装後耐食性を付与するために、上記連続焼鈍後の鋼板に、片面当たり１μｍ以上酸洗して、鋼板表面および鋼板表面から１μｍ範囲の鋼板内部のＳｉ酸化物を完全に除去する酸洗処理を施す必要がある。酸洗による除去量が１μｍ未満では、優れた化成処理性や塗装後の耐食性を安定して得ることができない。好ましくは１．５μｍ以上である。上限は鋼板歩留まりの観点から、３μｍ以下であることが好ましい。   Furthermore, in the present invention, in order to provide the excellent chemical conversion treatment property and post-coating corrosion resistance of the present invention, the steel plate after the continuous annealing is pickled at least 1 μm per side, and the steel plate surface and the steel plate surface are in the range of 1 μm. It is necessary to perform a pickling treatment that completely removes the Si oxide inside the steel plate. If the removal amount by pickling is less than 1 μm, excellent chemical conversion properties and corrosion resistance after coating cannot be stably obtained. Preferably it is 1.5 micrometers or more. The upper limit is preferably 3 μm or less from the viewpoint of steel plate yield.

酸洗により除去量を上記範囲に限定する理由は、以下の理由による。
本発明の製造方法における連続焼鈍では、上記のように、露点が−２０℃以下の還元性雰囲気下で、７５０〜９００℃の温度で３０〜６００ｓｅｃの条件で焼鈍を行うのが好ましいが、この条件で鋼板内部に形成されるＳｉ含有酸化物層の深さは、通常、０．５μｍ程度であり、最大でも１μｍである。そこで、本発明では、酸洗によるＳｉ含有酸化物層の除去量を、鋼板の片面当たり１μｍ以上とする。また、ＧＤＳでのＳｉ濃度分布の測定深さは、酸洗除去量があることを加味すれば、焼鈍後の鋼板にＳｉ含有酸化物層が存在する最大深さの１μｍとすれば、十分である。 The reason why the removal amount is limited to the above range by pickling is as follows.
In continuous annealing in the production method of the present invention, as described above, it is preferable to perform annealing at a temperature of 750 to 900 ° C. for 30 to 600 seconds in a reducing atmosphere having a dew point of −20 ° C. or less. The depth of the Si-containing oxide layer formed inside the steel sheet under conditions is usually about 0.5 μm, and at most 1 μm. Therefore, in the present invention, the removal amount of the Si-containing oxide layer by pickling is set to 1 μm or more per one surface of the steel plate. In addition, the measurement depth of the Si concentration distribution in GDS should be 1 μm, which is the maximum depth at which the Si-containing oxide layer is present in the steel sheet after annealing, considering that there is a pickling removal amount. is there.

次に、本発明の製造方法において、上記酸洗に用いる酸は、生産性の観点から、硝酸水溶液、あるいは、硝塩酸や硝弗酸等の硝酸を含む混酸水溶液であることが好ましい。片面当たり１μｍ以上の酸洗除去を、工業的に生産性や経済性を阻害しない時間内（概ね３０ｓｅｃ以内）で行うためには、濃塩酸、濃硫酸等では溶解速度が小さいため、酸洗処理槽を長くしたり、ライン速度を遅くしたりする必要があり非経済的である。また、Ｓｉ酸化物の溶解を弗酸のみや濃アルカリで行う場合、極表層のＳｉ酸化物の除去は可能であるが、鋼板内部のＳｉ酸化物を除去することは困難である。   Next, in the production method of the present invention, the acid used for the pickling is preferably a nitric acid aqueous solution or a mixed acid aqueous solution containing nitric acid such as nitric hydrochloric acid or nitric hydrofluoric acid from the viewpoint of productivity. In order to perform pickling removal of 1 μm or more per side within a time period that does not impede industrial productivity or economy (approximately within 30 sec), the concentration rate of concentrated hydrochloric acid, concentrated sulfuric acid, etc. is low, so pickling treatment It is not economical because it is necessary to lengthen the tank or slow down the line speed. Further, when the Si oxide is dissolved only with hydrofluoric acid or concentrated alkali, it is possible to remove the Si oxide on the extreme surface layer, but it is difficult to remove the Si oxide inside the steel sheet.

上記酸洗に用いる酸は、硝酸水溶液のみを用いる場合は、濃度を１〜５ｍａｓｓ％の範囲とするのが好ましい。濃度が１ｍａｓｓ％未満では、３０ｓｅｃ以内で所望の除去量を得ることは難しい。一方、５ｍａｓｓ％を超えると、溶解速度が大きくなり過ぎ、除去量の制御が難しくなる。   The acid used for the pickling preferably has a concentration in the range of 1 to 5 mass% when only a nitric acid aqueous solution is used. If the concentration is less than 1 mass%, it is difficult to obtain a desired removal amount within 30 seconds. On the other hand, if it exceeds 5 mass%, the dissolution rate becomes too high and the removal amount becomes difficult to control.

なお、除去量を制御する観点からは、上記酸洗に用いる酸は、硝酸と塩酸の混酸水溶液を用いることが好ましい。硝酸水溶液に塩酸を加えることで、酸洗速度が低下し、除去量の制御性が向上すると共に、酸洗による鋼板の表面荒れを抑制することができる。この場合の、硝酸と塩酸の濃度は、硝酸は２〜２０ｍａｓｓ％、塩酸は０．１〜５ｍａｓｓ％の範囲とするのが好ましい。硝酸の酸洗速度は、塩酸を加えることにより、硝酸単独の場合に比べて低下するため、硝酸濃度の下限は２ｍａｓｓ％とする。一方、硝酸濃度が２０ｍａｓｓ％を超えると、混酸にした場合でも、溶解速度が大きくなり過ぎ、酸洗速度を制御することが難しくなる。また、塩酸濃度が、０．１ｍａｓｓ％未満では、硝酸の溶解速度を低減する効果は得られず、一方、５ｍａｓｓ％を超えると、溶解速度が低下し過ぎて、３０ｓｅｃ以内で所望の除去量を得ることができない。   From the viewpoint of controlling the removal amount, the acid used for the pickling is preferably a mixed acid aqueous solution of nitric acid and hydrochloric acid. By adding hydrochloric acid to the aqueous nitric acid solution, the pickling speed is reduced, the controllability of the removal amount is improved, and surface roughness of the steel sheet due to pickling can be suppressed. In this case, the concentration of nitric acid and hydrochloric acid is preferably in the range of 2 to 20 mass% for nitric acid and 0.1 to 5 mass% for hydrochloric acid. Since the pickling speed of nitric acid is reduced by adding hydrochloric acid as compared with the case of nitric acid alone, the lower limit of the nitric acid concentration is 2 mass%. On the other hand, if the nitric acid concentration exceeds 20 mass%, even when mixed acid is used, the dissolution rate becomes too high and it becomes difficult to control the pickling rate. Also, if the hydrochloric acid concentration is less than 0.1 mass%, the effect of reducing the dissolution rate of nitric acid cannot be obtained. On the other hand, if the concentration exceeds 5 mass%, the dissolution rate is too low and the desired removal amount can be reduced within 30 seconds. Can't get.

また、Ｓｉ酸化物の除去性の観点からは、硝酸と弗酸の混酸水溶液を用いることが好ましい。弗酸は、Ｓｉ酸化物そのものを溶解可能であるため、硝酸単独の場合に比べて、より効率よく除去することができる。硝酸と弗酸の混酸水溶液を用いる場合、硝酸は０．５〜５ｍａｓｓ％、弗酸は０．１〜５ｍａｓｓ％の範囲とするのが好ましい。硝酸濃度が０．５ｍａｓｓ％未満では、３０秒以内で所望の除去量を得ることができない。一方、５ｍａｓｓ％を超えると、溶解速度が大きくなり過ぎ、除去量の制御が難しくなる。また、弗酸濃度が０．１ｍａｓｓ％未満では、十分なＳｉ酸化物の除去効率向上効果が得られず、一方、５ｍａｓｓ％を超えると、効果が飽和し、経済性が悪くなる。   Further, from the viewpoint of removal of Si oxide, it is preferable to use a mixed acid aqueous solution of nitric acid and hydrofluoric acid. Since hydrofluoric acid can dissolve the Si oxide itself, it can be removed more efficiently than nitric acid alone. When a mixed acid aqueous solution of nitric acid and hydrofluoric acid is used, nitric acid is preferably in the range of 0.5 to 5 mass%, and hydrofluoric acid is preferably in the range of 0.1 to 5 mass%. If the nitric acid concentration is less than 0.5 mass%, a desired removal amount cannot be obtained within 30 seconds. On the other hand, if it exceeds 5 mass%, the dissolution rate becomes too high and the removal amount becomes difficult to control. If the hydrofluoric acid concentration is less than 0.1 mass%, a sufficient Si oxide removal efficiency improvement effect cannot be obtained. On the other hand, if it exceeds 5 mass%, the effect is saturated and the economy is deteriorated.

なお、酸洗に用いる上記酸水溶液の温度は、３０〜８０℃の範囲であることが好ましい。３０℃を下回ると、液温を保持するために連続焼鈍後の鋼板を過度に冷却することが必要となり、一方、８０℃を超えると、溶解速度の促進効果が飽和するだけでなく、鋼板表面の荒れが顕著となる。   In addition, it is preferable that the temperature of the said acid aqueous solution used for pickling is the range of 30-80 degreeC. If the temperature is below 30 ° C, it is necessary to excessively cool the steel sheet after continuous annealing in order to maintain the liquid temperature. On the other hand, if the temperature exceeds 80 ° C, the effect of promoting the dissolution rate is not only saturated, but also the steel plate surface. Roughness becomes remarkable.

また、本発明の製造方法においては、上記酸洗処理後の鋼板は、水洗し、中和処理を施すことが好ましい。さらに、化成処理性および塗装後耐食性をより向上するために、上記酸洗処理後、化成結晶の生成、成長を促進したり、均一微細化したりする効果のあるＮｉ等を５〜５０ｍｇ／ｍ２程度付着させる処理を施してもよい。さらに、上記焼鈍後の鋼板に、形状矯正や表面粗度を調整するためあるいは材質を制御するため、伸び率５％以下の調質圧延を施してもよい。 Moreover, in the manufacturing method of this invention, it is preferable to wash with water and to neutralize the steel plate after the said pickling process. Furthermore, in order to further improve the chemical conversion treatment property and the corrosion resistance after coating, 5-50 mg / m 2 of Ni or the like, which has the effect of promoting the formation and growth of chemical conversion crystals and uniform refinement after the above pickling treatment. You may perform the process made to adhere to some extent. Further, the annealed steel sheet may be subjected to temper rolling with an elongation of 5% or less in order to adjust the shape correction, surface roughness, or control the material.

表１に示した成分組成を有するＡ〜Ｉの鋼を常法の製鋼プロセスで溶製し、連続鋳造してスラブとし、次いで、このスラブを１２５０℃に再加熱後、仕上圧延終了温度を８５０℃、巻取温度を６００℃とする熱間圧延し、板厚３．０ｍｍの熱延板とした。この熱延板を、酸洗後、冷間圧延して板厚１．６ｍｍの冷延板とし、次いで、露点−５０〜−２０℃に制御した雰囲気下で、７８０〜８６０℃の温度に３００ｓｅｃ間保持する焼鈍後、５５０〜７２０℃まで徐冷し、室温まで水冷し、１５０〜４００℃で３００〜１２００ｓｅｃ間の焼戻し処理する連続焼鈍を施した。次いで、同じく表２に示した条件で酸洗処理を施して鋼板表面のＳｉ含有酸化層を除去した後、あるいは酸洗処理を施さずに、伸び率０．３％の調質圧延を施して高強度冷延鋼板とした。なお、各実施例における連続焼鈍後の酸洗による表面除去量（板厚減量）は、予め行ったラボ実験によって、各酸洗条件における酸洗減量を測定し、その減量値を試験片の表面積および鉄の密度で割って板厚に換算して酸洗条件と板厚減量との関係を求めておくことにより見積もった。   A to I steels having the composition shown in Table 1 were melted by a conventional steelmaking process, continuously cast into a slab, and then the slab was reheated to 1250 ° C., and the finish rolling finish temperature was 850. C. and hot rolled to a coiling temperature of 600.degree. C. to obtain a hot rolled sheet having a thickness of 3.0 mm. This hot-rolled sheet is pickled and cold-rolled to obtain a cold-rolled sheet having a thickness of 1.6 mm, and then in an atmosphere controlled at a dew point of −50 to −20 ° C. to a temperature of 780 to 860 ° C. for 300 sec. After annealing, the steel was gradually cooled to 550 to 720 ° C., cooled to room temperature, and subjected to continuous annealing for tempering at 150 to 400 ° C. for 300 to 1200 seconds. Next, after performing the pickling process under the conditions shown in Table 2 to remove the Si-containing oxide layer on the steel sheet surface, or without performing the pickling process, temper rolling with an elongation of 0.3% was performed. A high-strength cold-rolled steel sheet was used. In addition, the amount of surface removal (sheet thickness reduction) by pickling after continuous annealing in each example was measured by pickling reduction under each pickling condition by a laboratory experiment conducted in advance, and the reduction value was determined as the surface area of the test piece. It was estimated by calculating the relationship between the pickling condition and the plate thickness reduction by dividing by the iron density and converting to the plate thickness.

Figure 0005233346
Figure 0005233346

上記のようにして得た各種冷延鋼板について、機械的性質、鋼板表面および表面層内のＳｉ含有酸化物の生成状況、化成処理性および化成電着塗装後の耐食性を、下記の要領で調査した。
＜機械的性質＞
上記酸洗後の各冷延鋼板から、圧延方向に直角する方向からＪＩＳ５号引張試験片を採取し、ＪＩＳ Ｚ２２４１の規定に準拠して引張試験を行い、引張強さ（ＴＳ：ＭＰａ）、伸び（Ｅｌ：％）を測定し、強度−延性バランス（ＴＳ×Ｅｌ：ＭＰａ・％）を求めた。なお、本実施例では、ＴＳ：７８０ＭＰａ以上、ＴＳ×Ｅｌが１８０００ＭＰａ・％以上を機械的特性が良好であると評価した。
＜鋼板表面および表面層内のＳｉ含有酸化物の生成状況＞
上記酸洗後の各冷延鋼板の表面および鋼板断面（鋼板表面および鋼板表面から深さ１μｍの範囲の鋼板内部）を、ＳＥＭを用いて１０００倍で５視野を観察し、ＥＤＸにより元素分析を行い、Ｓｉ含有酸化物の存在有無を調べた。なお、Ｓｉ含有酸化物の有無は、ＥＤＸによる分析でＳｉとＯが検出されるＳｉ含有酸化物が全く観察されないものを「無」、一つでも観察された場合は「有」とした。
また、鋼板表面から深さ１μｍの範囲における深さ方向のＳｉの濃度分布を、ＧＤＳを用いて測定し、板厚１／４まで研削した試料表面のＳｉ強度（Ｐ１）と、表面から１μｍの範囲におけるＳｉ強度の最大値（Ｐ２）との比（Ｐ２／Ｐ１）を求めた。
＜化成処理性＞
上記酸洗後の各冷延鋼板に、市販の化成処理薬剤（日本パーカライジング社製、パルボンドＰＢ−Ｌ３０２０）を用いて、浴温：４３℃、処理時間：１２０ｓｅｃの条件で化成処理を施し、化成処理後の鋼板表面をＳＥＭを用いて倍率５００倍で５視野観察し、５視野全てにおいて面積率９５％以上で均一な化成結晶が生成している場合を化成処理性が良好「○」、１視野でも面積率５％超えのスケが認められた場合を化成処理性が劣化「×」と評価した。
＜化成電着塗装後の耐食性＞
上記化成処理後の鋼板から、１５０ｍｍ×７０ｍｍの試験片を各２枚ずつ採取し、これらの試験片に、市販の電着塗料（関西ペイント社製、ＧＴ−１０ＨＴ）を用いて、塗膜厚が２０〜２５μｍになるように電着塗装を施した後、１７０℃×２０分の焼付処理を施した。次いで、この試験片の表面に、カッターナイフで、深さが約２０μｍで長さが１００ｍｍの切れ込みを２本Ｘ状に入れてから、５ｍａｓｓ％ＮａＣｌの５０℃の溶液中に２４０時間浸漬した。その後、上記試験片の切り込み上に粘着テープを貼り付けて、剥がした時の塗膜の剥離幅を測定する塗膜剥離試験を行い、全ての試験片において最大剥離幅が５．０ｍｍ以下であれば、化成電着塗装後の耐食性が良好「○」、１枚でも５．０ｍｍ超えであれば化成電着塗装後の耐食性が劣化「×」と評価した。 For the various cold-rolled steel sheets obtained as described above, the mechanical properties, the formation status of Si-containing oxides on the steel sheet surface and surface layer, the chemical conversion treatment property and the corrosion resistance after chemical electrodeposition coating were investigated in the following manner. did.
<Mechanical properties>
From each cold-rolled steel sheet after pickling, a JIS No. 5 tensile test piece is taken from the direction perpendicular to the rolling direction, and subjected to a tensile test in accordance with the provisions of JIS Z2241, tensile strength (TS: MPa), elongation. (El:%) was measured, and a strength-ductility balance (TS × El: MPa ·%) was determined. In this example, TS: 780 MPa or more and TS × El of 18000 MPa ·% or more were evaluated as having good mechanical properties.
<Generation of Si-containing oxides on steel sheet surface and surface layer>
The surface of each cold-rolled steel sheet and the cross-section of the steel sheet after pickling (the steel sheet surface and the inside of the steel sheet within a range of 1 μm in depth from the steel sheet surface) are observed 1000 times using SEM, and five elements are analyzed by EDX. The presence or absence of Si-containing oxide was examined. The presence or absence of the Si-containing oxide was determined to be “None” when no Si-containing oxide in which Si and O were detected by analysis by EDX was observed, and “Yes” when even one was observed.
In addition, the Si concentration distribution in the depth direction in the range of 1 μm depth from the steel sheet surface was measured using GDS, and the Si strength (P 1 ) of the sample surface ground to a thickness of ¼, and 1 μm from the surface The ratio (P 2 / P 1 ) with the maximum value (P 2 ) of the Si intensity in the range was determined.
<Chemical conversion processability>
Each cold-rolled steel sheet after the pickling is subjected to chemical conversion treatment using a commercially available chemical conversion chemical (Nippon Parkerizing Co., Ltd., Palbond PB-L3020) under conditions of bath temperature: 43 ° C. and processing time: 120 sec. The treated steel sheet surface is observed with 5 fields of view at 500 times magnification using SEM, and the chemical conversion processability is good when the area ratio is 95% or more and uniform conversion crystals are formed in all 5 fields. The chemical conversion treatment property was evaluated as “x” when the scale with an area ratio exceeding 5% was observed even in the visual field.
<Corrosion resistance after chemical conversion electrodeposition>
Two pieces of 150 mm × 70 mm test pieces were sampled from each of the steel sheets after the chemical conversion treatment, and a coating thickness was obtained using a commercially available electrodeposition paint (manufactured by Kansai Paint Co., Ltd., GT-10HT). Was subjected to electrodeposition coating so as to be 20 to 25 μm, followed by baking at 170 ° C. for 20 minutes. Next, two notches having a depth of about 20 μm and a length of 100 mm were put in an X shape on the surface of the test piece with a cutter knife, and then immersed in a solution of 5 mass% NaCl at 50 ° C. for 240 hours. Thereafter, an adhesive tape is applied on the cut of the test piece, and a film peel test is performed to measure the peel width of the paint film when peeled off. If the maximum peel width is 5.0 mm or less in all the test pieces, For example, if the corrosion resistance after chemical electrodeposition coating is good “◯”, even if one sheet exceeds 5.0 mm, the corrosion resistance after chemical electrodeposition coating was evaluated as “x”.

Figure 0005233346
Figure 0005233346

上記試験の結果を表２にまとめて示した。
この結果から、本発明に適合する成分組成を有し、本発明に適合する製造条件で製造された発明例の鋼板は、いずれも引張強さＴＳが７８０ＭＰａ以上で、強度延性バランス（ＴＳ×Ｅｌ）が１８０００ＭＰａ・％以上であり、強度と加工性を両立している。さらに、板厚１／４におけるＳｉ強度（Ｐ１）に対して、鋼板表面および表面から深さ１μｍの範囲におけるＳｉ強度の最大値（Ｐ２）が１．３倍以下（Ｐ２／Ｐ１≦１．３）となっており、その結果、化成処理性および塗装後耐食性がいずれも優れていることが分かる。
これに対して、Ｓｉ含有量が本発明の下限を外れるＮｏ．３の鋼板は、ＴＳ×Ｅｌが１８０００ＭＰａ・％未満であり、加工性が十分ではない。また、Ｎｏ．７〜１１の鋼板は、酸洗しないか、酸洗が不十分なため、鋼板表面にＳｉ含有酸化物層が残存し、Ｐ２／Ｐ１＞１．３となっており、その結果、化成処理性および塗装後耐食性が要求特性を満たしていない。また、酸洗していないＮｏ．１３の鋼板は、表面に加えて、表面から深さ１μｍの範囲の鋼板内部にもＳｉ含有酸化物が存在しており、化成処理性および塗装後耐食性が要求特性を満たしていない。さらに、Ｎｏ．１４および１６の鋼板は、表面にはＳｉ含有酸化物が存在しないため、化成処理性には優れるが、酸洗しないか、酸洗が不十分なため、鋼板内部にＳｉ含有酸化物が存在しているため、塗装後耐食性に劣っている。また、Ｎｏ．１５の鋼板は、塩酸酸洗後、強アルカリでＳｉ含有酸化物除去処理を行った例であるが、除去量が１μｍ未満であり、鋼板内部に生成したＳｉ含有酸化物を除去できていないため、塗装後耐食性に劣っている。 The results of the above tests are summarized in Table 2.
From these results, the steel sheets of the inventive examples having the component composition suitable for the present invention and manufactured under the production conditions suitable for the present invention all have a tensile strength TS of 780 MPa or more and a strength ductility balance (TS × El ) Is 18000 MPa ·% or more, and both strength and workability are achieved. Furthermore, with respect to the Si strength (P 1 ) at a thickness of 1/4, the maximum value (P 2 ) of the Si strength in the range of 1 μm depth from the steel sheet surface and the surface is 1.3 times or less (P 2 / P 1 ≦ 1.3), and as a result, it can be seen that both the chemical conversion property and the corrosion resistance after coating are excellent.
On the other hand, No. in which Si content deviates from the lower limit of the present invention. The steel plate No. 3 has TS × El of less than 18000 MPa ·%, and the workability is not sufficient. No. Since the steel plates 7 to 11 were not pickled or were not pickled sufficiently, the Si-containing oxide layer remained on the steel plate surface, and P 2 / P 1 > 1.3. Processability and post-coating corrosion resistance do not meet the required characteristics. In addition, no pickling No. In Steel No. 13, in addition to the surface, the Si-containing oxide is also present inside the steel plate having a depth of 1 μm from the surface, and the chemical conversion property and the corrosion resistance after painting do not satisfy the required characteristics. Furthermore, no. The steel plates 14 and 16 are excellent in chemical conversion treatment because there is no Si-containing oxide on the surface, but they are not pickled or insufficiently pickled, so there are Si-containing oxides inside the steel plate. Therefore, it is inferior in corrosion resistance after painting. No. Steel plate No. 15 is an example in which Si-containing oxide removal treatment was performed with strong alkali after hydrochloric acid pickling, but the removal amount was less than 1 μm, and the Si-containing oxide generated inside the steel plate could not be removed. Inferior in corrosion resistance after painting.

本発明の鋼板は、高強度で加工性に優れ、しかも化成処理性および化成電着塗装後の耐食性にも優れているので、自動車車体用としてだけでなく、例えば、家電製品や建築分野などで、同様の特性が求められる分野にも好適に用いることができる。   The steel sheet of the present invention has high strength and excellent workability, and also has excellent chemical conversion treatment and corrosion resistance after chemical electrodeposition coating, so that it is not only used for automobile bodies, for example, in home appliances and construction fields. It can also be suitably used in fields where similar characteristics are required.

Ｓｉ含有鋼板表面および鋼板表面近傍のＳｉ含有酸化物の存在状態と、化成処理性および化成電着塗装後の耐食性との関係を模式的に説明する図である。It is a figure which illustrates typically the relation between the existence state of Si content oxide near the Si content steel plate surface and the steel plate surface, and the chemical conversion treatment nature and the corrosion resistance after chemical electrodeposition coating.

Claims (5)

Ｃ：０．０５〜０．３０ｍａｓｓ％、Ｓｉ：０．８〜３．０ｍａｓｓ％、Ｍｎ：１．５〜３．０ｍａｓｓ％、Ｐ：０．１０ｍａｓｓ％以下、Ｓ：０．０１ｍａｓｓ％以下、Ａｌ：０．０１〜０．１ｍａｓｓ％、Ｎ：０．００５ｍａｓｓ％以下を含有し、残部がＦｅおよび不可避的不純物からなる成分組成を有し、鋼板表面および表面から深さ１μｍの範囲の鋼板内部におけるＳｉ濃度の最大値Ｐ２が、板厚１／４におけるＳｉ濃度Ｐ１の１．３倍以下であることを特徴とする化成処理性および塗装後耐食性に優れる高強度冷延鋼板。 C: 0.05 to 0.30 mass%, Si: 0.8 to 3.0 mass%, Mn: 1.5 to 3.0 mass%, P: 0.10 mass% or less, S: 0.01 mass% or less, Al : 0.01 to 0.1 mass%, N: 0.005 mass% or less, with the balance being a component composition consisting of Fe and inevitable impurities, in the steel sheet surface and in the steel sheet within a depth range of 1 μm from the surface maximum value P 2 of the Si concentration is high strength cold rolled steel sheet excellent in chemical conversion treatability and corrosion resistance after coating, characterized in that is less than 1.3 times the Si concentration P 1 in the sheet thickness 1/4. Ｃ：０．０５〜０．３０ｍａｓｓ％、Ｓｉ：０．８〜３．０ｍａｓｓ％、Ｍｎ：１．５〜３．０ｍａｓｓ％、Ｐ：０．１０ｍａｓｓ％以下、Ｓ：０．０１ｍａｓｓ％以下、Ａｌ：０．０１〜０．１ｍａｓｓ％、Ｎ：０．００５ｍａｓｓ％以下を含有し、残部がＦｅおよび不可避的不純物からなる成分組成を有するスラブを熱間圧延し、冷間圧延し、連続焼鈍して冷延鋼板を製造する方法において、上記連続焼鈍後、酸洗して鋼板表面を片面当たり１μｍ以上除去することを特徴とする化成処理性および塗装後耐食性に優れる高強度冷延鋼板の製造方法。 C: 0.05 to 0.30 mass%, Si: 0.8 to 3.0 mass%, Mn: 1.5 to 3.0 mass%, P: 0.10 mass% or less, S: 0.01 mass% or less, Al : A slab containing 0.01 to 0.1 mass%, N: 0.005 mass% or less, with the balance being composed of Fe and inevitable impurities, hot-rolled, cold-rolled, and continuously annealed In the method for producing a cold-rolled steel sheet, a method for producing a high-strength cold-rolled steel sheet having excellent chemical conversion property and post-coating corrosion resistance, characterized in that after the above-described continuous annealing, pickling is performed to remove the steel sheet surface by 1 μm or more per side. 上記酸洗に１〜５ｍａｓｓ％の硝酸水溶液を用いることを特徴とする請求項２に記載の、化成処理性および塗装後耐食性に優れる高強度冷延鋼板の製造方法。 The method for producing a high-strength cold-rolled steel sheet having excellent chemical conversion properties and post-coating corrosion resistance according to claim 2, wherein a 1 to 5 mass% nitric acid aqueous solution is used for the pickling. 上記酸洗に２〜２０ｍａｓｓ％の硝酸と０．１〜５ｍａｓｓ％の塩酸からなる混酸水溶液を用いることを特徴とする請求項２に記載の化成処理性および塗装後耐食性に優れる高強度冷延鋼板の製造方法。 The high-strength cold-rolled steel sheet having excellent chemical conversion property and post-coating corrosion resistance according to claim 2, wherein a mixed acid aqueous solution comprising 2 to 20 mass% nitric acid and 0.1 to 5 mass% hydrochloric acid is used for the pickling. Manufacturing method. 上記酸洗に０．５〜５ｍａｓｓ％の硝酸と０．１〜５ｍａｓｓ％の弗酸からなる混酸水溶液を用いることを特徴とする請求項２に記載の化成処理性および塗装後耐食性に優れる高強度冷延鋼板の製造方法。 The high strength excellent in chemical conversion treatment property and corrosion resistance after coating according to claim 2, wherein a mixed acid aqueous solution comprising 0.5 to 5 mass% nitric acid and 0.1 to 5 mass% hydrofluoric acid is used for the pickling. A method for producing a cold-rolled steel sheet.
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