JP4306497B2 - High-strength cold-rolled steel sheet with excellent workability and post-coating corrosion resistance and method for producing the same - Google Patents

High-strength cold-rolled steel sheet with excellent workability and post-coating corrosion resistance and method for producing the same Download PDF

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JP4306497B2
JP4306497B2 JP2004066755A JP2004066755A JP4306497B2 JP 4306497 B2 JP4306497 B2 JP 4306497B2 JP 2004066755 A JP2004066755 A JP 2004066755A JP 2004066755 A JP2004066755 A JP 2004066755A JP 4306497 B2 JP4306497 B2 JP 4306497B2
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steel sheet
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ferrite
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英尚 川辺
健二 河村
靖 田中
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JFE Steel Corp
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Description

本発明は、優れた加工性および塗装後耐食性が要求される自動車用部品の強度部材等に好適な、高強度冷延鋼板およびその製造方法に関するものである。   The present invention relates to a high-strength cold-rolled steel sheet suitable for a strength member of an automotive part that requires excellent workability and post-coating corrosion resistance, and a method for producing the same.

近年、自動車の構造部品は省資源、省エネルギーの立場から、より減量な部品が求められるようになっている。その一方で、衝突安全性の向上を図るため、引張強度が780MPa以上の高強度冷延鋼板が補強部材を中心に積極的に活用されている。一般的に、780MPa以上の高強度冷延鋼板では、絞り成形や張出し成形といった軟鋼板で適用される成形手法を適用することは難しく、成形手法としては曲げ成形および伸びフランジ成形が主体となる。したがって、自動車の構造部品として高強度冷延鋼板を用いる場合、加工性として延性(伸び)だけでなく、伸びフランジ性および曲げ性をバランス良く備えることが重要となる。   In recent years, structural parts of automobiles are required to have more reduced parts from the standpoint of resource saving and energy saving. On the other hand, high-strength cold-rolled steel sheets with a tensile strength of 780 MPa or more are actively used mainly for reinforcing members in order to improve collision safety. In general, in a high strength cold rolled steel sheet of 780 MPa or more, it is difficult to apply a forming method applied to a mild steel plate such as drawing or stretch forming, and bending methods and stretch flange forming are mainly used as forming methods. Therefore, when a high-strength cold-rolled steel sheet is used as a structural part of an automobile, it is important to provide not only ductility (elongation) but also stretch flangeability and bendability in a balanced manner as workability.

また自動車用鋼板はプレス、溶接組み立て、化成処理、電着塗装工程を経ることが一般的であるため、優れた加工性とともに優れた塗装後耐食性を有することが要求される。加工性に関しては、軟質なフェライトと硬質なマルテンサイトから構成される強度と加工性とを同時に高めた複合組織鋼が知られており、広く用いられている。しかし、この複合組織鋼は、確かに延性は良好であるものの曲げ性は不足し、厳しい曲げ加工を経て製造される部品への適用は不向きである。また伸びフランジ性についても成形過程において軟質相と硬質相の界面でボイド生成、亀裂進展し、最終的に破断にいたるため厳しい伸びフランジ加工される部品への適用は不向きである。   Further, since steel plates for automobiles are generally subjected to pressing, welding assembly, chemical conversion treatment, and electrodeposition coating processes, they are required to have excellent post-coating corrosion resistance as well as excellent workability. Regarding the workability, a composite structure steel that is made of soft ferrite and hard martensite and has simultaneously improved strength and workability is known and widely used. However, this composite steel has good ductility, but lacks bendability, and is not suitable for application to parts manufactured through severe bending. As for stretch flangeability, void formation and crack growth occur at the interface between the soft phase and the hard phase in the molding process, leading to final breakage, making it unsuitable for use in parts subjected to severe stretch flange processing.

ところで、鋼板の曲げ加工においては、曲げ外周表層部に円周方向に大きな引張応力が、また、曲げ内周表層部に大きな圧縮応力がかかるため、高強度冷延鋼板の曲げ性には表層部の状態も大きく影響し、表層に軟質層を有することで、曲げ加工時に鋼板表面に生じる引張応力、圧縮応力を緩和し、曲げ性が改善されることがわかっている。このような表層に軟質層を有する高強度鋼板に関しては、特許文献1〜4に以下のような鋼板および製造方法が開示されている。
特許文献1では、曲げ加工性とスポット溶接性を改善することを目的とし、表層を脱炭焼鈍し、表層に10vol%の軟質層と内層に10vol%以上の残留オーステナイトを含む硬質中心層を有する高強度鋼板およびその製造方法が開示されている。
By the way, in bending of a steel sheet, a large tensile stress is applied to the bending outer peripheral surface layer in the circumferential direction, and a large compressive stress is applied to the bending inner peripheral surface layer. It is known that the state of the above has a great influence, and having a soft layer on the surface layer relaxes the tensile stress and compressive stress generated on the surface of the steel sheet during bending and improves the bendability. Regarding such a high-strength steel sheet having a soft layer as a surface layer, Patent Documents 1 to 4 disclose the following steel sheets and manufacturing methods.
In Patent Document 1, for the purpose of improving bending workability and spot weldability, the surface layer is decarburized and annealed, and the surface layer has a hard center layer containing 10 vol% soft layer and the inner layer containing 10 vol% or more retained austenite. A high-strength steel sheet and a method for manufacturing the same are disclosed.

特許文献2には、表層にC:0.1wt%以下の軟質層を両面に3〜15%有し、残部を10%未満の残留オーステナイトと低温変態相あるいはフェライトとの複合組織とする冷延鋼板および製造方法が記載されている。
特許文献3には、表層10μm〜200μmの部分がフェライト主体からなり、内層部分が、ベイナイト、マルテンサイトを主体とする冷延鋼板およびその製造方法が記載されている。
Patent Document 2 describes a cold-rolled steel sheet having a soft layer of C: 0.1 wt% or less on the surface layer of 3 to 15% on both surfaces and the balance being a composite structure of less than 10% of retained austenite and a low-temperature transformation phase or ferrite. And manufacturing methods are described.
Patent Document 3 describes a cold-rolled steel sheet having a surface layer of 10 μm to 200 μm mainly composed of ferrite, and an inner layer portion mainly composed of bainite and martensite, and a manufacturing method thereof.

特許文献4には、表層10μm以内を除き、金属組織が実質的にマルテンサイト単相とした、伸びフランジ性に優れた冷延鋼板および製造方法が記載されている。   Patent Document 4 describes a cold-rolled steel sheet excellent in stretch flangeability and a manufacturing method in which the metal structure is substantially a martensite single phase except for a surface layer of 10 μm or less.

また、塗装後耐食性については化成処理前の鋼板表面状態の影響が大きいことが知られており、例えば、特許文献5には、鋼板表面に極微量の鉄被覆層を有することにより塗装後の耐食性が優れることが記載されている。
特開平2-175839号公報 特開平5-195149号公報 特開平10-130782号公報 特開2002-161336号公報 特開平5-320952号公報
In addition, it is known that the corrosion resistance after coating is greatly influenced by the surface state of the steel sheet before chemical conversion treatment.For example, Patent Document 5 discloses that the corrosion resistance after coating by having a very small iron coating layer on the surface of the steel sheet. Is described as being excellent.
Japanese Patent Laid-Open No. 2-175839 Japanese Patent Laid-Open No. 5-195149 JP 10-130782 A JP 2002-161336 A JP-A-5-320952

しかしながら、特許文献1では、中心層に残留オーステナイトを10vol%以上も含有させるため、成形時にマルテンサイトを形成し、軟質なフェライトと硬質相の界面でボイドを生成し、亀裂発生、亀裂の伝播が容易に起こるため、伸びフランジ性に悪影響を及ぼすことになる。
また、特許文献2および特許文献3では、脱炭処理により表層軟質層を形成しているが、連続焼鈍で製造するにあたり、炉雰囲気を酸素含有や高露点に制御する必要性があり、脱炭処理操業終了後の次コイル以降でCなどのロールピックアップが発生する危険性が考えられる。
特許文献4では、厚さが10μm以内の軟質層が生成することがある、と記載されているが、表層軟質層を積極的に生成させ、生成量を制御し加工性を向上するという技術思想ではないため、曲げ性が不充分である。
However, in Patent Document 1, since the central layer contains 10 vol% or more of retained austenite, martensite is formed during molding, voids are generated at the interface between the soft ferrite and the hard phase, and crack generation and crack propagation occur. Since it occurs easily, the stretch flangeability is adversely affected.
Further, in Patent Document 2 and Patent Document 3, the surface soft layer is formed by decarburization treatment, but it is necessary to control the furnace atmosphere to contain oxygen or to have a high dew point when manufacturing by continuous annealing. There is a risk that roll pick-up such as C will occur after the next coil after the processing operation ends.
Patent Document 4 describes that a soft layer with a thickness of 10 μm or less may be generated, but the technical idea of positively generating a surface soft layer and controlling the generation amount to improve workability Therefore, the bendability is insufficient.

特許文献5は、高加工性確保の点で不十分あり、めっき工程が必須であるため高コストであり経済上の課題もある。   Patent Document 5 is insufficient in terms of ensuring high workability, and a plating process is indispensable, resulting in high costs and economic problems.

本発明は、上記問題点を解決するためになされたもので、加工性と塗装後耐食性に優れる高強度冷延鋼板およびその製造方法を提供することを目的とする。   The present invention has been made to solve the above problems, and an object of the present invention is to provide a high-strength cold-rolled steel sheet excellent in workability and post-coating corrosion resistance, and a method for producing the same.

本発明者らは、上記の課題を解決すべく、鋭意研究した。その結果、鋼板表層と中心部の組織に着目し、鋼板表層にフェライト体積率90%以上で厚さが10〜100μmの軟質層を有し、中心部の組織は焼戻しマルテンサイト体積率が30〜80%であり、0.5≦(焼戻マルテンサイト中のSi濃度)/(フェライト中のSi濃度)≦1.5とすることにより、加工性および塗装後耐食性が向上することを見出した。さらに、徐冷と急冷とを組み合わせた2段冷却を行った後に連続焼鈍することにより、上記組織を有した高強度冷延鋼板が得られることも見出した。   The present inventors have intensively studied to solve the above problems. As a result, paying attention to the steel sheet surface layer and the structure of the central part, the steel sheet surface layer has a soft layer having a ferrite volume ratio of 90% or more and a thickness of 10 to 100 μm, and the structure of the central part has a tempered martensite volume ratio of 30 to It was found that the workability and post-coating corrosion resistance were improved by 80% and 0.5 ≦ (Si concentration in tempered martensite) / (Si concentration in ferrite) ≦ 1.5. Furthermore, the present inventors have also found that a high-strength cold-rolled steel sheet having the above structure can be obtained by performing continuous annealing after performing two-stage cooling combining slow cooling and rapid cooling.

本発明は、以上の知見に基づきなされたもので、その要旨は以下のとおりである。   The present invention has been made based on the above findings, and the gist thereof is as follows.

[1]mass%で、C:0.03〜0.18%、Si:0.01〜1.5%、Mn:0.5〜3.0%、P:0.001〜0.1%、S:0.0001〜0.008%、Sol.Al:0.01〜0.1%、N:0.0001〜0.008%を含有し、残部はFeおよび不可避不純物からなり、鋼板表層にフェライト体積率90%以上で厚さが10〜100μmの軟質層を有し、中心部の組織は焼戻マルテンサイト体積率が30〜80%であり、0.5≦(焼戻マルテンサイト中のSi濃度)/(フェライト中のSi濃度)≦1.5であることを特徴とする加工性および塗装後耐食性に優れる高強度冷延鋼板。 [1] In mass%, C: 0.03-0.18%, Si: 0.01-1.5%, Mn: 0.5-3.0%, P: 0.001-0.1%, S: 0.0001-0.008%, Sol.Al: 0.01-0.1% , N: 0.0001 to 0.008%, the balance consists of Fe and inevitable impurities , the steel sheet has a soft layer with a ferrite volume ratio of 90% or more and a thickness of 10 to 100 μm, and the structure in the center is tempered The martensite volume fraction is 30-80%, and 0.5 ≦ (Si concentration in tempered martensite) / (Si concentration in ferrite) ≦ 1.5 Strength cold-rolled steel sheet.

[2]上記[1]において、さらに、mass%で、Cu:0.01〜1%、Ni:0.01〜1%、Mo:0.01〜1%、Cr:0.01〜1%、B:0.0001〜0.005%の1種または2種以上を含有することを特徴とする加工性および塗装後耐食性に優れる高強度冷延鋼板。   [2] In the above [1], further, in mass%, Cu: 0.01 to 1%, Ni: 0.01 to 1%, Mo: 0.01 to 1%, Cr: 0.01 to 1%, B: 0.0001 to 0.005% A high-strength cold-rolled steel sheet excellent in workability and post-coating corrosion resistance, characterized by containing one or more.

[3]上記[1]または[2]において、さらに、mass%で、Ti:0.001〜0.1%、Nb:0.001〜0.1%、V:0.01〜0.5%の1種または2種以上を含有することを特徴とする加工性および塗装後耐食性に優れる高強度冷延鋼板。   [3] In the above [1] or [2], the material further contains one or more of mass: Ti: 0.001 to 0.1%, Nb: 0.001 to 0.1%, V: 0.01 to 0.5%. High-strength cold-rolled steel sheet with excellent workability and post-coating corrosion resistance.

[4]上記[1]ないし[3]において、さらに、mass%で、Ca:0.0001〜0.005%を含有することを特徴とする加工性と塗装後耐食性に優れる高強度冷延鋼板。   [4] A high-strength cold-rolled steel sheet excellent in workability and post-coating corrosion resistance, characterized in that, in [1] to [3] above, it further contains mass: Ca: 0.0001 to 0.005%.

[5]上記[1]ないし[4]のいずれかに記載の成分組成を有する鋼スラフ゛を鋳造後、熱間圧延、酸洗、冷間圧延を施し鋼板とし、該鋼板を連続焼鈍するにあたり、750〜950℃の再結晶焼鈍温度、10〜1200秒の焼鈍時間で熱処理し、次いで、鋼板表層において、ア)550〜750℃の冷却開始温度から、5〜50℃/秒の冷却速度、0.05〜5秒の冷却時間で冷却後、イ)100℃以下の冷却停止温度まで、100〜2000℃/秒の冷却速度で冷却の2段冷却を行い、次いで、150〜500℃の温度で焼戻しすることを特徴とする加工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。   [5] After casting the steel slab having the composition described in any one of [1] to [4] above, hot rolling, pickling, and cold rolling are performed to obtain a steel plate, and the steel plate is subjected to continuous annealing. Heat treatment was performed at a recrystallization annealing temperature of 750 to 950 ° C. and an annealing time of 10 to 1200 seconds, and then, in the steel sheet surface layer, a) a cooling start temperature of 550 to 750 ° C., a cooling rate of 5 to 50 ° C./second, 0.05 After cooling with a cooling time of ~ 5 seconds, a) perform two-stage cooling at a cooling rate of 100-2000 ° C / second to a cooling stop temperature of 100 ° C or lower, and then temper at a temperature of 150-500 ° C A method for producing a high-strength cold-rolled steel sheet having excellent processability and post-coating corrosion resistance.

お、本明細書において、鋼の成分を示す%は、すべてmass%である。 Contact name herein,% indicating the components of the steel are all mass%.

また、本発明において、高強度薄鋼板とは、例えば自動車の構造部品として好適な引張強度が780MPa以上の薄鋼板である。   In the present invention, the high-strength thin steel sheet is a thin steel sheet having a tensile strength of 780 MPa or more suitable as, for example, an automobile structural component.

また、本発明において、中心部とは、板厚のt/4〜3t/4の部分を意味する。   In the present invention, the central portion means a portion having a thickness t / 4 to 3t / 4.

本発明によれば、加工性および塗装後耐食性に優れる高強度冷延鋼板を得ることができる。このように、本発明の鋼板は高強度であり、なおかつ加工性に優れているので、従来、高強度鋼板の適用が困難であった例えば自動車構造部材等の難成形部材として適用することが可能となる。さらに、塗装後耐食性にも優れており、バンパー、インパクトビームなど耐衝突部材に加えホワイトボディへの適用も十分可能である。   According to the present invention, a high-strength cold-rolled steel sheet having excellent workability and post-coating corrosion resistance can be obtained. Thus, since the steel sheet of the present invention has high strength and excellent workability, it can be applied as a difficult-to-form member such as an automobile structural member that has conventionally been difficult to apply to a high-strength steel sheet. It becomes. In addition, it has excellent post-painting corrosion resistance, and can be applied to white bodies in addition to bumpers, impact beams and other impact-resistant members.

さらに、自動車構造部品として本発明の高強度冷延鋼板を用いた場合、自動車の軽量化、安全性向上などに寄与し、産業上極めて有益である。   Furthermore, when the high-strength cold-rolled steel sheet of the present invention is used as an automobile structural part, it contributes to reducing the weight of the automobile, improving safety, and the like, which is extremely useful industrially.

また、家電および建築など厳しい加工性が必要とされる分野でも好適に使用することが可能となる。   Also, it can be suitably used in fields where strict processability is required, such as home appliances and architecture.

本発明の高強度冷延鋼板は、下記に示す成分に規定し、さらに鋼板表層にフェライト体積率90%以上で厚さが10〜100μmの軟質層を有し、中心部の組織は焼戻しマルテンサイト体積率が30〜80%であり、0.5≦(焼戻マルテンサイト中のSi濃度)/(フェライト中のSi濃度)≦1.5である複合組織とすることを特徴とする。これらは本発明において最も重要な要件であり、上記のように成分、表層および中心部の組織を最適化することにより、加工性および塗装後耐食性に優れた高強度冷延鋼板を得ることができる。また、上記高強度冷延鋼板は、熱間圧延、酸洗、冷間圧延を行い得られた鋼板を、750〜950℃の再結晶焼鈍温度、10〜1200秒の焼鈍時間で熱処理し、次いで、鋼板表層において、ア)550〜750℃の冷却開始温度から、5〜50℃/秒の冷却速度、0.05〜5秒の冷却時間で冷却後、イ)100℃以下の冷却停止温度まで、100〜2000℃/秒の冷却速度で冷却の2段冷却を行い、次いで、150〜500℃の温度で焼戻しすることにより製造が可能となる。   The high-strength cold-rolled steel sheet of the present invention is defined by the following components, and further has a soft layer with a ferrite volume ratio of 90% or more and a thickness of 10 to 100 μm on the surface layer of the steel sheet, and the structure at the center is tempered martensite. The volume ratio is 30 to 80%, and the composite structure is 0.5 ≦ (Si concentration in tempered martensite) / (Si concentration in ferrite) ≦ 1.5. These are the most important requirements in the present invention, and by optimizing the composition of the components, surface layer and central portion as described above, a high-strength cold-rolled steel sheet excellent in workability and post-coating corrosion resistance can be obtained. . In addition, the high-strength cold-rolled steel sheet is obtained by heat-treating a steel sheet obtained by hot rolling, pickling, and cold rolling at a recrystallization annealing temperature of 750 to 950 ° C. and an annealing time of 10 to 1200 seconds, In the surface layer of the steel sheet, a) after cooling at a cooling start temperature of 550 to 750 ° C., with a cooling rate of 5 to 50 ° C./second and a cooling time of 0.05 to 5 seconds, and a) to a cooling stop temperature of 100 ° C. or less, 100 Manufacturing is possible by performing two-stage cooling at a cooling rate of ˜2000 ° C./sec and then tempering at a temperature of 150 ° C. to 500 ° C.

以下、本発明を詳細に説明する。   Hereinafter, the present invention will be described in detail.

まず、本発明における鋼の化学成分の限定理由は以下の通りである。   First, the reasons for limiting the chemical components of steel in the present invention are as follows.

C:0.03〜0.18%
Cは焼入れ焼戻しによって得られる組織である焼戻しマルテンサイトを強化するために重要な元素である。Cが0.03%未満では所望の強度確保が困難となる。一方、Cが0.18%を超えると、良好な溶接性が得られず、また冷間圧延時の負荷も増加するので0.18%以下とする。以上より、Cは0.03%以上0.18%以下とする。また、強度を安定的に確保し、かつ加工性とのバランスを考慮すると0.05%以上0.15%以下がより好ましい。
C: 0.03-0.18%
C is an important element for strengthening tempered martensite, which is a structure obtained by quenching and tempering. If C is less than 0.03%, it is difficult to ensure the desired strength. On the other hand, if C exceeds 0.18%, good weldability cannot be obtained, and the load during cold rolling also increases, so the content is made 0.18% or less. From the above, C is 0.03% or more and 0.18% or less. Further, it is more preferably 0.05% or more and 0.15% or less when the strength is stably secured and the balance with workability is taken into consideration.

Si: 0.01〜1.5%
Siは固溶強化元素であり、高強度で高伸びの鋼板を得るために有効な元素である。その効果は0.01%以上添加することにより発揮するが、1.5%を超えると鋼板表面にSi酸化物を多量に形成し、化成処理性を劣化させ、塗装後の耐食性も劣る。以上より、Siは0.01%以上1.5%以下とする。より好ましくは0.2%以上1.4%以下である。
Si: 0.01-1.5%
Si is a solid solution strengthening element and is an effective element for obtaining a steel plate having high strength and high elongation. The effect is exhibited by adding 0.01% or more. However, if it exceeds 1.5%, a large amount of Si oxide is formed on the surface of the steel sheet, the chemical conversion property is deteriorated, and the corrosion resistance after coating is also inferior. From the above, Si is made 0.01% to 1.5%. More preferably, it is 0.2% or more and 1.4% or less.

Mn: 0.5〜3.0%
Mnは固溶強化によって鋼を強化するだけではなく、鋼の焼入れ性を向上させて低温変態相の生成を促進させる効果があるが、その効果は0.5%未満では発揮されない。一方、3.0%を超えるとフェライトが生成せず軟質相が存在しないので加工性が劣化する。また連続鋳造工程でスラブ割れが発生する。以上より、Mnは0.5%以上3.0%以下とする。より好ましくは1%以上2.5%以下である。
Mn: 0.5-3.0%
Mn not only strengthens the steel by solid solution strengthening but also improves the hardenability of the steel and promotes the formation of a low temperature transformation phase, but the effect is not exhibited at less than 0.5%. On the other hand, if it exceeds 3.0%, ferrite does not form and the soft phase does not exist, so the workability deteriorates. In addition, slab cracking occurs in the continuous casting process. From the above, Mn is set to 0.5% to 3.0%. More preferably, it is 1% or more and 2.5% or less.

P:0.001〜0.1%
Pは固溶強化元素として高強度化に寄与するが、0.1%を超えて過度に含有すると溶接性に悪影響を及ぼす。また溶接性、耐2次加工脆性の観点からは低いほうが好ましいが、過度の低減は製鋼コストをいたずらに増加させるだけである。以上より、Pは、0.001%以上0.1%以下とする。好ましくは0.01%以上0.05%以下である。
P: 0.001 to 0.1%
P contributes to high strength as a solid solution strengthening element, but if it exceeds 0.1%, it has an adverse effect on weldability. Further, from the viewpoint of weldability and secondary work brittleness resistance, a lower value is preferable, but excessive reduction only increases the steelmaking cost unnecessarily. From the above, P is set to 0.001% or more and 0.1% or less. Preferably they are 0.01% or more and 0.05% or less.

S:0.0001〜0.008%
SはMnSなど介在物として存在し、加工性に悪影響を及ぼすため、鋼板の加工性を考慮した場合、極めて低いほうが好ましいが、0.008%以下であれば許容できる。一方、P同様に過度の低減は製鋼コストをいたずらに増加させるだけである。以上より、Sは0.0001%以上0.008%以下とする。好ましくは0.0005%以上0.003%以下である。
S: 0.0001 to 0.008%
Since S exists as inclusions such as MnS and adversely affects workability, when considering the workability of the steel sheet, it is preferably extremely low, but 0.008% or less is acceptable. On the other hand, like P, excessive reduction only increases the steelmaking cost. From the above, S is made 0.0001% or more and 0.008% or less. Preferably it is 0.0005% or more and 0.003% or less.

Sol.Al:0.01〜0.1%
Alは脱酸材として使用され、非金属介在物をスラグ中へ分離除去することにより鋼板の加工性を向上させるが、その効果は0.01%未満では十分に得られない。一方、0.1%超えではAl添加の効果は飽和し合金コストの上昇を招くだけである。以上より、Sol.Alは0.01%以上0.1%以下とする。より好ましくは0.02%以上0.06%以下である。
Sol.Al: 0.01-0.1%
Al is used as a deoxidizing material and improves the workability of the steel sheet by separating and removing non-metallic inclusions into the slag, but the effect is not sufficiently obtained if it is less than 0.01%. On the other hand, if it exceeds 0.1%, the effect of Al addition is saturated and only increases the alloy cost. Therefore, Sol.Al is set to 0.01% or more and 0.1% or less. More preferably, it is 0.02% or more and 0.06% or less.

N:0.0001〜0.008%以下
Nは固溶強化鋼の場合、歪時効の原因となることが懸念されるが、本発明のように組織強化鋼の場合、歪時効は問題とならない。しかしながらNの含有量が過度に多くなると窒化物を多数形成し、成形時のボイド生成の起点となり破断しやすくなる。したがって成形性の観点から0.008%以下に制限する必要がある。しかしながら、Nを必要以上に低減すると製鋼コストが上昇するので、実質的に無害となる0.0001%以上とする。以上より、Nは0.0001%以上0.008%以下とする。より好ましくは0.002%以上0.005%以下である。
N: 0.0001 to 0.008% or less
N is feared to cause strain aging in the case of solid solution strengthened steel, but in the case of structure strengthened steel as in the present invention, strain aging is not a problem. However, when the N content is excessively large, a large number of nitrides are formed, which becomes a starting point for void formation during molding and is easily broken. Therefore, it is necessary to limit it to 0.008% or less from the viewpoint of moldability. However, if N is reduced more than necessary, the steelmaking cost increases, so the content is made 0.0001% or more, which is substantially harmless. Therefore, N is set to 0.0001% or more and 0.008% or less. More preferably, it is 0.002% or more and 0.005% or less.

本発明の鋼板は、上記の必須添加元素で目的とする特性が得られるが、所望の特性に応じて以下の元素を含有することができる。   The steel sheet of the present invention can achieve the desired characteristics with the above-mentioned essential additive elements, but can contain the following elements depending on the desired characteristics.

Cu:0.01〜1%、Ni:0.01〜1%、Mo:0.01〜1%、Cr:0.01〜1%、B:0.0001〜0.005%の1種または2種以上
鋼板の高強度化のため、必要に応じて添加される。Cu、Ni、Mo、Cr については0.01%以上の添加により効果を発現するが、過度に含有してもその効果は飽和する傾向にあり、1%以下の含有量で添加することが好ましい。またいずれも強化元素であるSi、Mn、Pなどと比較すると非常に高価な元素であり、過度に含有してもコストアップするだけである。以上より、Cu、Ni、MoおよびCrを含有する場合は、Cuは0.01%以上1%以下、Niは0.01%以上1%以下、Moは0.01%以上1%以下、Crは0.01%以上1%以下とする。好ましくはCu、Ni、MoおよびCr 各々、0.1%以上0.5%以下である。また、Bについては鋼の焼き入れ性を向上させ、低温変態相の生成を促進させる効果によって高強度化に寄与する。0.0001%以上の添加で効果を発揮するが、0.005%超の含有では効果は飽和し、含有量に見合う効果は得られない。以上より、Bを含有する場合は0.0001%以上0.005%以下とする。より好ましくは0.0002%以上0.0015%以下である。
One or more of Cu: 0.01 to 1%, Ni: 0.01 to 1%, Mo: 0.01 to 1%, Cr: 0.01 to 1%, B: 0.0001 to 0.005% Necessary for increasing the strength of steel sheets Depending on the addition. For Cu, Ni, Mo and Cr, the effect is exhibited by addition of 0.01% or more, but the effect tends to be saturated even if contained excessively, and it is preferable to add at a content of 1% or less. All of them are very expensive elements as compared with Si, Mn, P, etc., which are strengthening elements, and even if they are contained excessively, they only increase the cost. From the above, when Cu, Ni, Mo and Cr are contained, Cu is 0.01% to 1%, Ni is 0.01% to 1%, Mo is 0.01% to 1%, Cr is 0.01% to 1% The following. Preferably, Cu, Ni, Mo and Cr are each 0.1% or more and 0.5% or less. Further, B contributes to high strength by the effect of improving the hardenability of steel and promoting the formation of a low temperature transformation phase. The effect is exerted with addition of 0.0001% or more. However, when the content exceeds 0.005%, the effect is saturated, and an effect commensurate with the content cannot be obtained. From the above, when B is contained, the content is made 0.0001% or more and 0.005% or less. More preferably, it is 0.0002% or more and 0.0015% or less.

Ti:0.001〜0.1%、Nb:0.001〜0.1%、 V:0.01〜0.5%の1種または2種以上
炭窒化物を形成するため多量に含有するのは好ましくない。しかし、適量であれば結晶粒微細化による組織の均一化、強度調整に寄与し、成形性を向上させ、Tiが0.001%以上、Nbが0.001%以上、 Vが0.01%以上で効果を発揮する。以上よりTi、NbおよびVを含有する場合は、Tiは0.001%以上0.1%以下、Nbは0.001以上0.1%以下、Vは0.01%以上0.5%以下とする。好ましくはTi、Nb各々、0.005%以上0.05%以下、Vは0.05%以上0.3%以下である。
One or more of Ti: 0.001 to 0.1%, Nb: 0.001 to 0.1%, V: 0.01 to 0.5% It is not preferable to contain a large amount in order to form a carbonitride. However, if it is an appropriate amount, it contributes to the homogenization of the structure by grain refinement and the strength adjustment, improves the formability, and is effective when Ti is 0.001% or more, Nb is 0.001% or more, and V is 0.01% or more. . From the above, when Ti, Nb and V are contained, Ti is 0.001% to 0.1%, Nb is 0.001 to 0.1%, and V is 0.01% to 0.5%. Preferably, Ti and Nb are each 0.005% or more and 0.05% or less, and V is 0.05% or more and 0.3% or less.

Ca:0.0001〜0.005%
CaはMnSなど介在物の形状制御により伸びフランジ性や曲げ性向上に寄与する効果を有している。かかる効果は0.0001%以上の含有で発現される。しかしながら過度に含有しても効果が飽和するばかりか、製鋼-連続鋳造工程においてCa単体のまま存在し、欠陥となるため0.005%以下の含有とする。以上より、Caを含有する場合は0.0001%以上0.005%以下とする。より好ましくは0.0005%以上0.0025%以下である。
Ca: 0.0001 to 0.005%
Ca has the effect of contributing to the improvement of stretch flangeability and bendability by controlling the shape of inclusions such as MnS. Such an effect is manifested with a content of 0.0001% or more. However, even if contained excessively, the effect is saturated, and in the steelmaking-continuous casting process, Ca remains as it is and becomes a defect, so the content is made 0.005% or less. As mentioned above, when it contains Ca, it is 0.0001% or more and 0.005% or less. More preferably, it is 0.0005% or more and 0.0025% or less.

なお、上記以外の残部はFeおよび不可避不純物からなIncidentally, balance other than the above Ru Tona Fe and unavoidable impurities.

次に本発明の製造方法について説明する。   Next, the manufacturing method of this invention is demonstrated.

以上の化学成分範囲に調整された溶鋼から、連続鋳造または造塊でスラブを溶製する。次いで、得られたスラブを一旦冷却後再加熱するか、あるいはそのまま熱間圧延を行う。熱間圧延における最終圧延温度は、延性(伸び)および伸びフランジ性を向上させるため850℃以上が望ましい。850℃より低い最終圧延温度では、最終圧延の段階で二相組織となるためフェライト粒の著しい粗大化が起こり、不均一な組織となったり、圧延温度の低下にともない圧延負荷が増大し、ロールと鋼板表面の面圧が上昇し、鋼板表面が荒れるため冷延、焼鈍を行っても加工性の良い鋼板が得られない場合がある。   From the molten steel adjusted to the above chemical composition range, a slab is melted by continuous casting or ingot forming. Subsequently, the obtained slab is once cooled and then reheated or hot rolled as it is. The final rolling temperature in hot rolling is desirably 850 ° C. or higher in order to improve ductility (elongation) and stretch flangeability. At a final rolling temperature lower than 850 ° C., a two-phase structure is formed at the final rolling stage, so that the ferrite grains are markedly coarsened. As a result, the rolling load increases as the rolling temperature decreases. Since the surface pressure of the steel sheet surface increases and the steel sheet surface becomes rough, a steel sheet with good workability may not be obtained even if cold rolling or annealing is performed.

次いで、酸洗後、冷間圧延により所望の板厚とする。このときの冷間圧延率は、伸びおよび伸びフランジ性を向上させるため50%以上が望ましい。   Next, after pickling, the sheet thickness is set to a desired thickness by cold rolling. The cold rolling rate at this time is preferably 50% or more in order to improve elongation and stretch flangeability.

次いで、上記により得られた鋼板に対して1)熱処理、2)2段冷却および3)焼戻処理を行う。特に熱処理後の2段冷却は本発明の効果を得るために、最も重要な要件であり、本発明の特徴でもある。以下に製造条件を詳細に説明する。   Next, 1) heat treatment, 2) two-stage cooling, and 3) tempering treatment are performed on the steel sheet obtained as described above. In particular, two-stage cooling after heat treatment is the most important requirement for obtaining the effects of the present invention, and is also a feature of the present invention. The manufacturing conditions will be described in detail below.

1)750〜950℃の再結晶焼鈍温度、10〜1200秒の焼鈍時間で熱処理
再結晶焼鈍温度が750℃未満では、焼鈍過程において加熱昇温、均熱保持中に十分な量のオーステナイトが得られないため、急冷後にマルテンサイトが得られず、所望の強度が達成できない。一方、950℃を超える温度で加熱すると、焼鈍中にオーステナイト粒径が粗大化するため、最終組織が粗大化し、均一微細な組織が得られず、加工性が低下する。
1) Heat treatment at a recrystallization annealing temperature of 750 to 950 ° C and an annealing time of 10 to 1200 seconds. When the recrystallization annealing temperature is less than 750 ° C, a sufficient amount of austenite is obtained during heating and heating soaking in the annealing process. Therefore, martensite cannot be obtained after rapid cooling and the desired strength cannot be achieved. On the other hand, when heated at a temperature exceeding 950 ° C., the austenite grain size becomes coarse during annealing, so that the final structure becomes coarse, a uniform fine structure cannot be obtained, and workability deteriorates.

したがって再結晶焼鈍温度は750℃以上950℃以上とする。高温焼鈍によりオーステナイト体積率は増加、オーステナイト単相組織となった後には粗大結晶粒化する傾向にある。したがって、より安定して所望の強度を得ることと、より微細組織化を達成することを両立するためには、好ましくは、780℃以上880℃以下である。焼鈍時間については、10秒未満では未溶解炭化物が存在する可能性が高くなり、オーステナイト相の存在量が少なくなる可能性があり強度確保が困難となる。1200秒を超えるとオーステナイト粒径が粗大化し、加工性が低下する。したがって焼鈍時間は10秒以上1200秒以下とする。一般に長時間焼鈍により結晶粒は成長し粗大化する傾向にある。したがって、より微細な組織を達成することと、また焼鈍前の組織の影響を小さくし均一な組織を得ることを両立するためには、好ましくは、20秒以上500秒以下である。再結晶焼鈍後、急冷開始温度までの冷却は、特に限定されず、例えば、放冷、ガスジェット冷却、ミスト冷却等の手段を用いることができる。  Therefore, the recrystallization annealing temperature is 750 ° C. or higher and 950 ° C. or higher. The austenite volume fraction increases due to high temperature annealing, and tends to become coarse grains after becoming an austenite single phase structure. Therefore, the temperature is preferably 780 ° C. or higher and 880 ° C. or lower in order to achieve a desired strength more stably and achieve a finer structure. As for the annealing time, if it is less than 10 seconds, there is a high possibility that undissolved carbide is present, and the abundance of the austenite phase may be reduced, making it difficult to ensure strength. If it exceeds 1200 seconds, the austenite grain size becomes coarse and the workability deteriorates. Therefore, the annealing time is 10 seconds or more and 1200 seconds or less. In general, crystal grains tend to grow and become coarse due to long-term annealing. Therefore, in order to achieve both the achievement of a finer structure and the reduction of the influence of the structure before annealing to obtain a uniform structure, the time is preferably from 20 seconds to 500 seconds. The cooling to the rapid cooling start temperature after the recrystallization annealing is not particularly limited, and for example, means such as cooling, gas jet cooling, mist cooling, etc. can be used.

2)鋼板表層において、ア)550〜750℃の冷却開始温度から、5〜50℃/秒の冷却速度、0.05〜5秒の冷却時間で冷却後、イ)100℃以下の冷却停止温度まで、100〜2000℃/秒の冷却速度で冷却の2段冷却。   2) In the steel plate surface layer, a) after cooling at a cooling start temperature of 550 to 750 ° C., with a cooling rate of 5 to 50 ° C./second and a cooling time of 0.05 to 5 seconds, and a) to a cooling stop temperature of 100 ° C. or less, Two-stage cooling at a cooling rate of 100-2000 ° C / sec.

一定の冷却速度では、鋼板板厚方向に均一な組織となるので、本発明の特徴とする表層(軟質層)と中心部(複合組織)で組織が異なる鋼板を得るために、2段冷却は重要である。   At a constant cooling rate, a uniform structure is formed in the thickness direction of the steel sheet. In order to obtain a steel sheet having a different structure in the surface layer (soft layer) and the central part (composite structure), which is a feature of the present invention, two-stage cooling is performed. is important.

ア)550〜750℃の冷却開始温度から、5〜50℃/秒の冷却速度、0.05〜5秒の冷却時間で冷却。   A) Cooling from a cooling start temperature of 550 to 750 ° C., with a cooling rate of 5 to 50 ° C./second and a cooling time of 0.05 to 5 seconds.

冷却開始温度が550℃未満では、冷却開始時点におけるオーステナイト量が少なくなることから冷却後に十分な量のマルテンサイトが得られず、またフェライトが過度に生成するため所望の強度確保が困難となる。一方、750℃超えではマルテンサイト単相組織となりフェライトが得られず、延性が低下する。したがって、冷却開始温度は550℃以上750℃以下とする。冷却開始温度が低ければ低いほどフェライトの生成が多く、低TS高El化する傾向にある。強度を確実に確保し、かつ加工性、より高い伸びを得るためには、好ましくは、600℃以上700℃以下である。表層の冷却速度が50℃/秒超えになると、フェライトの析出が起きず、軟質層が存在しない、もしくはその厚さが非常に薄くなり、本発明の効果が得られない。一方、5℃/秒未満では、過度にフェライトが生成するため強度が低下する。したがって1段目の冷却における冷却速度は5℃/秒以上50℃/秒以下とする。冷却速度が遅いほどフェライトがより多く生成し低TS化する傾向にある。強度を確実に確保し、かつ表層部においてフェライト層を確保するには、好ましくは、10℃/秒以上40℃/秒以下である。冷却時間が5秒より長いとやはり過度にフェライトが生成するため強度確保が困難である。0.05秒より短いと冷却効果がなく、所望の厚さの表層軟質相が得られない。したがって冷却時間は0.05秒以上5秒以内とする。冷却時間が長いほどフェライトがより多く生成し低TS化する傾向にある。強度を確実に確保し、かつ表層部においてフェライト層を確保するには、好ましくは、0.10秒以上2秒以下である。また、冷却速度を50℃/s以下と低くする設備手段としては、例えば、噴流水槽の直前で鋼板表面に水を噴射する等が挙げられる。   If the cooling start temperature is less than 550 ° C., the amount of austenite at the start of cooling decreases, so that a sufficient amount of martensite cannot be obtained after cooling, and ferrite is excessively generated, making it difficult to ensure the desired strength. On the other hand, if it exceeds 750 ° C., it becomes a martensite single phase structure, ferrite cannot be obtained, and ductility decreases. Therefore, the cooling start temperature is set to 550 ° C. or higher and 750 ° C. or lower. The lower the cooling start temperature, the more ferrite is generated, and the lower the TS and the higher El. In order to ensure the strength and to obtain processability and higher elongation, the temperature is preferably 600 ° C. or higher and 700 ° C. or lower. When the cooling rate of the surface layer exceeds 50 ° C./second, ferrite does not precipitate, the soft layer does not exist, or the thickness thereof becomes very thin, and the effect of the present invention cannot be obtained. On the other hand, when the temperature is less than 5 ° C./second, ferrite is excessively generated and the strength decreases. Therefore, the cooling rate in the first stage cooling is 5 ° C./second or more and 50 ° C./second or less. The slower the cooling rate, the more ferrite is generated and the lower the TS. In order to ensure the strength and to secure the ferrite layer in the surface layer portion, it is preferably 10 ° C./second or more and 40 ° C./second or less. If the cooling time is longer than 5 seconds, it is difficult to secure the strength because ferrite is excessively generated. If it is shorter than 0.05 seconds, there is no cooling effect, and a surface soft layer having a desired thickness cannot be obtained. Therefore, the cooling time is 0.05 seconds or more and 5 seconds or less. The longer the cooling time, the more ferrite is generated and the lower the TS. In order to ensure the strength and secure the ferrite layer in the surface layer portion, it is preferably from 0.10 seconds to 2 seconds. Examples of the facility means for reducing the cooling rate to 50 ° C./s or less include, for example, injecting water onto the steel sheet surface immediately before the jet water tank.

イ)100℃以下の冷却停止温度まで、100〜2000℃/秒の冷却速度で冷却。   B) Cooling at a cooling rate of 100 to 2000 ° C./second up to a cooling stop temperature of 100 ° C. or lower.

2段目の冷却速度が100℃/秒未満では、冷却中に過度にフェライトが生成し強度確保が困難となる。冷却速度が100℃/秒以上であれば、フェライトの過度の生成を抑制可能であり、容易に所望の強度確保ができる。また冷却速度が100℃/s未満では、パーライトやベイナイトの析出、またマルテンサイト変態が起こらない可能性があり、高強度を得るには合金添加量を多くしなければならない等新たな問題が起きてしまう。冷却速度の上限は2000℃/s超えで効果は飽和し、冷却設備などコスト上の観点から過度に速くする必要性は無く、2000℃/s以下とした。以上より2段目の冷却における冷却速度は100℃/s以上2000℃/s以下とする。より好ましくは、500〜1000℃/sである。   If the cooling rate of the second stage is less than 100 ° C./sec, ferrite is excessively generated during cooling, and it is difficult to ensure the strength. When the cooling rate is 100 ° C./second or more, excessive generation of ferrite can be suppressed, and desired strength can be easily secured. Also, if the cooling rate is less than 100 ° C / s, pearlite and bainite may not precipitate and martensite transformation may occur, and new problems such as increasing the amount of alloy must be obtained to obtain high strength. End up. The upper limit of the cooling rate exceeds 2000 ° C / s, and the effect is saturated. There is no need to make it too fast from the viewpoint of cost such as cooling equipment, and it was set to 2000 ° C / s or less. From the above, the cooling rate in the second stage cooling is set to 100 ° C./s or more and 2000 ° C./s or less. More preferably, it is 500-1000 degrees C / s.

冷却停止温度が100℃より高い場合、焼き入れが不十分であり、十分な量の低温変態相が得られない。一方、冷却停止温度は低ければ低いほうが好ましいが、冷却設備などコスト上の観点から過度に低温度にする必要性はなく、10℃程度で十分である。冷却手段については特に限定するものではないが、板幅方向、圧延長手方向の材質変動を抑制するためには、噴流水中に焼入れることが最も望ましい。さらに、この噴流水中の焼入れにより、冷却速度100℃/秒以上、急冷終了温度100℃以下の冷却を容易に達成することができる。   When the cooling stop temperature is higher than 100 ° C., quenching is insufficient and a sufficient amount of the low temperature transformation phase cannot be obtained. On the other hand, if the cooling stop temperature is low, it is preferable to be low, but it is not necessary to make the temperature excessively low from the viewpoint of cost such as cooling equipment, and about 10 ° C. is sufficient. Although it does not specifically limit about a cooling means, In order to suppress the material fluctuation | variation of a plate width direction and a rolling longitudinal direction, it is most desirable to quench into jet water. Further, by quenching in the jet water, cooling at a cooling rate of 100 ° C./second or more and a quenching end temperature of 100 ° C. or less can be easily achieved.

2段冷却を実施する場合の一実施態様を図1に示す。図1において、1は鋼板、2はシールロール、3は徐冷スプレー帯、4は噴流水槽、5はシンクロールである。図1によれば、徐冷スプレー帯3で鋼板1の表層に水を噴射することで5〜50℃/sの冷却速度、0.05〜5秒の冷却時間が達成される。冷却時間は鋼板搬送速度により調節される。その後、噴流水槽4への浸漬により100℃/s〜2000℃/sの冷却速度での2段目の冷却が行われる。噴流設備の無い水槽の場合、鋼板1の表層に生成される蒸気膜のため、冷却速度が低くなるが、高圧で噴流水を吹き付けることでその膜が壊れ、高い冷却速度が可能となる。   FIG. 1 shows an embodiment in which two-stage cooling is performed. In FIG. 1, 1 is a steel plate, 2 is a seal roll, 3 is a slow cooling spray zone, 4 is a jet water tank, and 5 is a sink roll. According to FIG. 1, a water cooling rate of 5 to 50 ° C./s and a cooling time of 0.05 to 5 seconds are achieved by spraying water onto the surface layer of the steel sheet 1 in the slow cooling spray zone 3. The cooling time is adjusted according to the steel plate conveyance speed. Thereafter, second-stage cooling is performed at a cooling rate of 100 ° C./s to 2000 ° C./s by immersion in the jet water tank 4. In the case of a water tank without a jet facility, the cooling rate is low because of the vapor film generated on the surface layer of the steel plate 1, but by blowing the jet water at a high pressure, the membrane is broken and a high cooling rate is possible.

3)150〜500℃の温度で焼戻し
焼戻温度が150℃未満ではオーステナイトを急冷して得られたマルテンサイトが十分焼戻されず、延性が低い。一方、焼戻温度が500℃超えでは、焼戻処理を行った場合、マルテンサイトがフェライトとセメンタイトに分解し過度に軟化するため強度の急激な低下が起こる。
以上より、加工性および塗装後耐食性に優れる高強度冷延鋼板が得られる。そして、本発明の超高強度冷延鋼板の組織は、鋼板表層にフェライト体積率90%以上で厚さが10〜100μmの軟質層を有し、中心部の組織は焼戻マルテンサイト体積率が30〜80%であり、0.5≦(焼戻マルテンサイト中のSi濃度)/(フェライト中のSi濃度)≦1.5となる。以下に本発明の高強度冷延鋼板の特徴である鋼板の組織について説明する。
3) Tempering at a temperature of 150 to 500 ° C. When the tempering temperature is less than 150 ° C., martensite obtained by quenching austenite is not sufficiently tempered and ductility is low. On the other hand, when the tempering temperature exceeds 500 ° C., when tempering is performed, martensite is decomposed into ferrite and cementite and excessively softened, resulting in a rapid drop in strength.
From the above, a high-strength cold-rolled steel sheet having excellent workability and post-coating corrosion resistance can be obtained. The microstructure of the ultra-high-strength cold-rolled steel sheet of the present invention has a soft layer having a ferrite volume ratio of 90% or more and a thickness of 10 to 100 μm on the steel sheet surface layer, and the structure of the central part has a tempered martensite volume ratio 30 to 80%, and 0.5 ≦ (Si concentration in tempered martensite) / (Si concentration in ferrite) ≦ 1.5. The structure of the steel sheet, which is a feature of the high-strength cold-rolled steel sheet of the present invention, will be described below.

鋼板表層にフェライト体積率90%以上で厚さが10〜100μmの軟質層
表層軟質層の厚さは10μm未満では曲げに対し十分な効果が得られず、また、100μmより厚いと強度低下量が大きく、780MPa以上の強度を得るためには多量の合金を添加する必要が出てくる。よって、表層の軟質層の厚さは10μm以上100μm以下とする。なお、表層の軟質層の厚さとは、最表層からフェライト分率90%のところまでの厚さであり、上記のように2段冷却(主に冷却速度)の制御により、所望の軟質層の厚みが達成される。
A soft layer with a ferrite volume ratio of 90% or more and a thickness of 10 to 100 μm on the surface of the steel plate. If the thickness of the surface soft layer is less than 10 μm, a sufficient effect on bending cannot be obtained. In order to obtain a large strength of 780 MPa or more, it is necessary to add a large amount of alloy. Therefore, the thickness of the surface soft layer is set to 10 μm or more and 100 μm or less. The thickness of the surface soft layer is the thickness from the outermost layer to the ferrite fraction of 90%. As described above, the desired soft layer thickness is controlled by controlling the two-stage cooling (mainly the cooling rate). Thickness is achieved.

鋼板表層に軟質層を有することは、塗装後耐食性にも重要な作用を示す。一般的な複合組織ではフェライトと焼戻マルテンサイトのSi濃度差、(Si濃度ムラ)が大きく、そのため局所的にSi濃度が高いと脱脂-表面調整-化成処理工程後に鋼板表面に形成される化成結晶が微細均一に緻密でなく、電着塗装後の耐食性が低下する。一方、Si濃度ムラがない状態を達成するには単相組織化するしかない。これに対し、本発明では表層のみフェライト単相に近いフェライト90%以上である軟質相とすることで通常のフェライトと硬質な低温変態相から構成される複合組織鋼よりも塗装後耐食性が向上する。   Having a soft layer on the steel sheet surface layer has an important effect on the corrosion resistance after coating. In a general composite structure, the difference in Si concentration between ferrite and tempered martensite (Si concentration unevenness) is large. Therefore, if the Si concentration is locally high, the chemical conversion formed on the steel sheet surface after the degreasing-surface conditioning-chemical conversion treatment process The crystals are not fine and uniform and the corrosion resistance after electrodeposition coating is reduced. On the other hand, to achieve a state without Si concentration unevenness, there is no choice but to make a single phase structure. On the other hand, in the present invention, the corrosion resistance after coating is improved more than the composite steel composed of normal ferrite and a hard low-temperature transformation phase by making the surface layer a soft phase that is 90% or more of ferrite close to a ferrite single phase. .

中心部の組織は焼戻マルテンサイト体積率が30〜80%であり、0.5≦(焼戻マルテンサイト中のSi濃度)/(フェライト中のSi濃度)≦1.5
中心部の焼戻マルテンサイトの体積率が80%超では延性が低下し、30%未満では強度確保が困難である。延性と伸びフランジ性をバランス良く確保するには体積率30〜80%の焼戻マルテンサイトが必要である。中心部の焼戻マルテンサイトの体積率も表層組織同様、2段冷却(主に冷却速度かつ冷却開始温度)の制御より達成される。なお、残部はフェライトであり、その他として残留オーステナイトが5%以下、存在してもよい。
The center structure has a tempered martensite volume fraction of 30-80%, 0.5 ≦ (Si concentration in tempered martensite) / (Si concentration in ferrite) ≦ 1.5
If the volume ratio of the tempered martensite in the center exceeds 80%, the ductility decreases, and if it is less than 30%, it is difficult to ensure the strength. In order to secure a good balance between ductility and stretch flangeability, tempered martensite with a volume ratio of 30 to 80% is required. The volume ratio of tempered martensite at the center is also achieved by controlling two-stage cooling (mainly the cooling rate and the cooling start temperature), similar to the surface layer structure. The balance is ferrite, and in addition, 5% or less of retained austenite may be present.

また、塗装後耐食性の向上は、表層のSi濃度ムラの低減、平滑化、かつ表層部をフェライト単相組織に近づけることに起因する。焼戻マルテンサイト中のSi濃度/フェライト中のSi濃度の値が1より高ければ高いほど、表層にある軟質相を構成するフェライト中のSi濃度が相対的に低下し、マルテンサイト中のSi濃度との差が拡大しSi成分の分布が不均一な組織となり、上記値が1.5を超えると塗装後耐食性は劣化する。一方、上記値が1より小さい場合、Si濃度差が拡大することに加え、表層フェライト中のSi濃度が相対的に高いことになり、塗装後耐食性が劣化する傾向にあり、0.5未満では塗装後耐食性の確保は困難である。したがって0.5≦(焼戻マルテンサイト中のSi濃度)/(フェライト中のSi濃度)≦1.5とする。より表層のSi濃度ムラを低減、平滑化し、かつ表層部をフェライト単相組織にできるかぎり近づけ塗装後耐食性を向上させるためには、好ましくは、0.75以上1.25以下である。   Moreover, the improvement in the corrosion resistance after coating is caused by the reduction of the Si concentration unevenness of the surface layer, smoothing, and bringing the surface layer portion closer to the ferrite single phase structure. The higher the Si concentration in the tempered martensite / Si concentration in the ferrite, the higher the Si concentration in the ferrite constituting the soft phase in the surface layer, and the lower the Si concentration in the martensite. The difference between the difference and the distribution of Si components becomes uneven, and if the above value exceeds 1.5, the corrosion resistance after coating deteriorates. On the other hand, if the above value is smaller than 1, the Si concentration difference will increase and the Si concentration in the surface ferrite will be relatively high, and the corrosion resistance after painting tends to deteriorate. It is difficult to ensure corrosion resistance. Therefore, 0.5 ≦ (Si concentration in tempered martensite) / (Si concentration in ferrite) ≦ 1.5. In order to further reduce and smooth the Si concentration unevenness of the surface layer and improve the corrosion resistance after coating as close as possible to the ferrite single phase structure, the surface layer portion is preferably 0.75 or more and 1.25 or less.

Si濃度制御に関しては、再結晶焼鈍-冷却開始温度までのオーステナイトとフェライト中へのSiの分配、および焼鈍、冷却過程におけるフェライトの生成量が重要であり、焼戻マルテンサイト量が少ないとフェライト中のSi濃度が相対的に高くなり、一方、焼戻マルテンサイトが多すぎると焼戻マルテンサイト中のSi濃度が相対的に増加し、Si濃度分布が不均一になる。上記のように焼鈍、冷却条件を所定の範囲とし、焼戻マルテンサイト体積率を制御することにより、0.5≦(焼戻マルテンサイト相のSi濃度)/(フェライト相のSi濃度)≦1.5とすることが可能となる。   Regarding the Si concentration control, recrystallization annealing-distribution of Si into austenite and ferrite up to the cooling start temperature, and the amount of ferrite produced in the annealing and cooling processes are important. If the amount of tempered martensite is small, On the other hand, if the Si concentration in the tempered martensite is too much, the Si concentration in the tempered martensite increases relatively and the Si concentration distribution becomes non-uniform. As described above, the annealing and cooling conditions are set within a predetermined range, and the volume ratio of tempered martensite is controlled to satisfy 0.5 ≦ (Si concentration of tempered martensite phase) / (Si concentration of ferrite phase) ≦ 1.5. It becomes possible.

表1に示す化学成分を有するスラブを用い、スラブ加熱温度:1200℃、仕上げ圧延温度:880℃、巻取り温度:610℃、冷延圧下率:50%の条件で、加熱、熱間圧延、酸洗、冷間圧延を行い、次いで、表2に示す条件で連続焼鈍を行い冷延鋼板を製造した。   Using slabs having the chemical components shown in Table 1, slab heating temperature: 1200 ° C, finish rolling temperature: 880 ° C, winding temperature: 610 ° C, cold rolling reduction: 50%, heating, hot rolling, Pickling and cold rolling were performed, and then continuous annealing was performed under the conditions shown in Table 2 to produce a cold-rolled steel sheet.

得られた冷延鋼板対して、以下に示す各試験を行い材料特性を評価した。得られた結果を表3に示す。   The obtained cold-rolled steel sheet was subjected to the following tests to evaluate the material properties. The results obtained are shown in Table 3.

鋼板の組織:圧延方向断面を光学顕微鏡または走査型電子顕微鏡で観察することにより調査した。倍率1000倍の断面組織写真を用いて、画像解析により任意に設定した100mm四方の正方形領域内に存在する該当相の占有面積を求め、該当相の体積率とした。軟質相以外の中心層は板厚1/4面位置を代表として観察した。   Steel sheet structure: The cross-section in the rolling direction was examined by observing with an optical microscope or a scanning electron microscope. Using a cross-sectional structure photograph at a magnification of 1000 times, the area occupied by the corresponding phase in a 100 mm square area set arbitrarily by image analysis was determined, and the volume ratio of the corresponding phase was obtained. The central layer other than the soft phase was observed with the thickness of 1/4 plane as a representative.

Si濃度比:オージェ電子分光(AES)測定により求めた。スパッタ速度40Å/minで120秒スパッタ後のSi強度を用いて、各相のSiピーク強度比の値をSi濃度比とした。測定位置は表層軟質相中の任意の場所におけるフェライトと焼き戻しマルテンサイトである。   Si concentration ratio: determined by Auger electron spectroscopy (AES) measurement. Using the Si intensity after sputtering for 120 seconds at a sputtering rate of 40 Å / min, the value of the Si peak intensity ratio of each phase was defined as the Si concentration ratio. The measurement positions are ferrite and tempered martensite at an arbitrary position in the surface soft phase.

引張特性:圧延方向と90°の方向を長手方向(引張方向)とするJISZ2201の5号試験片を用い、JISZ2241準拠した引張試験を行い評価した。なお、引張特性の伸びの評価基準は15%以上を良好とした。   Tensile properties: Using a JISZ2201 No. 5 test piece with the rolling direction and 90 ° as the longitudinal direction (tensile direction), a tensile test based on JISZ2241 was conducted and evaluated. In addition, the evaluation standard of elongation of the tensile properties was 15% or more as good.

穴拡げ率:日本鉄鋼連盟規格JFST1001に基づき実施した。初期直径d0=10mmの穴を打抜き、60°の円錐ポンチを上昇させ穴を拡げた際に、亀裂が板厚貫通したところでポンチ上昇を止め、亀裂貫通後の打抜き穴径dを測定し、穴拡げ率λ(%)=((d- d0)/ d0)×100として算出した。N=3で試験し、単純平均値で求めた。なお、穴拡げ率の評価基準は60%以上を良好とした。 Hole expansion rate: Implemented based on the Japan Iron and Steel Federation Standard JFST1001. When a hole with an initial diameter d 0 = 10 mm was punched, the cone punch of 60 ° was raised and the hole was expanded, the punch was stopped when the crack penetrated the plate thickness, and the punched hole diameter d after crack penetration was measured, The hole expansion rate was calculated as λ (%) = ((d−d 0 ) / d 0 ) × 100. Tested at N = 3 and determined by simple average. The evaluation standard for the hole expansion rate was 60% or more.

曲げ特性:圧延方向を長手方向とする40mm幅×200mm長さの試験片を用い、JISZ2248に準拠した曲げ試験を行いN=3で評価した。なお、曲げ特性の評価基準は限界曲げ半径1mm以下を良好とした。   Bending characteristics: Using a test piece of 40 mm width × 200 mm length with the rolling direction as the longitudinal direction, a bending test based on JISZ2248 was conducted and evaluated at N = 3. The evaluation criteria for the bending characteristics was good when the critical bending radius was 1 mm or less.

化成電着塗装後耐食性:市販の液を用いて、板厚×75mm×150mmの試験片にリン酸亜鉛による化成処理を施し、厚さ25μmになるように電着塗装し、ついでカッターナイフで、試験片に長さ100mm、2本の切り込みを入れ、5%NaCl、50℃の溶液中に240時間浸漬したのち、粘着テープを切り込み上に貼って剥がした後の、塗膜の剥離巾を測定して評価した。最大剥離全巾が5.0mm以下であれば、化成電着塗装後の耐食性は良好と判断した。   Corrosion resistance after chemical conversion electrodeposition: Using a commercially available solution, a test specimen of plate thickness x 75 mm x 150 mm was subjected to chemical conversion treatment with zinc phosphate, electrodeposition was applied to a thickness of 25 μm, and then with a cutter knife, Measure the peel width of the coating film after cutting it into a test piece with a length of 100mm and two pieces, immersing it in a solution of 5% NaCl, 50 ° C for 240 hours, and then sticking the adhesive tape on the cut and peeling it off. And evaluated. If the maximum peel width was 5.0 mm or less, the corrosion resistance after chemical electrodeposition coating was judged to be good.

表3より、本発明例では、強度はTSが780Mpa以上と高く、その他の機械的特性にも優れ、加工性および塗装後耐食性に優れた高強度冷延鋼板が得られていることがわかる。   From Table 3, it can be seen that in the present invention example, a high strength cold-rolled steel sheet having a high TS of 780 MPa or more, excellent in other mechanical properties, excellent in workability and corrosion resistance after coating is obtained.

一方、No2、5〜7の比較例では、焼鈍温度、冷却時間、冷却速度、冷却停止温度のいずれかが本発明の範囲外のため、表層軟質層の厚みもしくは中心部の焼戻マルテンサイト体積率のいずれか一つ以上が本発明の範囲外となり、結果として強度および穴拡げ率が劣っている。No8の比較例では、焼戻し温度が本発明の範囲外のため、強度および穴拡げ率が劣っている。
No3、16の比較例では、冷却開始温度もしくは成分(Mn)が本発明の範囲外のため、表層軟質層が形成されず本発明の範囲外となり、結果として伸びおよび曲げ特性が劣っている。No4の比較例では、冷却開示温度と冷却速度が本発明の範囲外のため、表層および中心部の鋼板組織が本発明の範囲外となり、結果として伸び、曲げ特性および化成電着塗装後の耐食性が劣っている。No17の比較例は成分(Si)が本発明の範囲外のため、化成電着塗装後の耐食性が劣っている。
On the other hand, in the comparative examples of No2 and 5-7, since any of the annealing temperature, cooling time, cooling rate, and cooling stop temperature is outside the scope of the present invention, the thickness of the surface soft layer or the tempered martensite volume at the center portion Any one or more of the rates are outside the scope of the present invention, resulting in poor strength and hole expansion rate. In the comparative example No. 8, since the tempering temperature is outside the range of the present invention, the strength and the hole expansion rate are inferior.
In the comparative examples No. 3 and 16, the cooling start temperature or the component (Mn) is outside the scope of the present invention, so the surface soft layer is not formed and is outside the scope of the present invention. As a result, the elongation and bending properties are inferior. In the comparative example of No4, the cooling disclosure temperature and the cooling rate are outside the scope of the present invention, so the surface layer and the central steel sheet structure are outside the scope of the present invention, resulting in elongation, bending characteristics, and corrosion resistance after chemical electrodeposition coating. Is inferior. In Comparative Example No. 17, since the component (Si) is outside the scope of the present invention, the corrosion resistance after chemical electrodeposition coating is poor.

自動車構造部品以外の家電および建築など厳しい加工性が必要とされる分野でも好適である。   It is also suitable in fields where severe workability is required, such as home appliances and buildings other than automobile structural parts.

2段冷却を実施する場合の一実施態様を示す図である。It is a figure which shows one embodiment in the case of implementing two-stage cooling.

符号の説明Explanation of symbols

1 鋼板
2 シールロール
3 徐冷スプレー帯
4 噴流水槽
5 シンクロール
1 Steel plate
2 Seal roll
3 Slow cooling spray zone
4 Jet tank
5 Sync roll

Claims (5)

mass%で、C:0.03〜0.18%、Si:0.01〜1.5%、Mn:0.5〜3.0%、P:0.001〜0.1%、S:0.0001〜0.008%、Sol.Al:0.01〜0.1%、N:0.0001〜0.008%を含有し、残部はFeおよび不可避不純物からなり、鋼板表層にフェライト体積率90%以上で厚さが10〜100μmの軟質層を有し、中心部の組織は焼戻マルテンサイト体積率が30〜80%であり、0.5≦(焼戻マルテンサイト中のSi濃度)/(フェライト中のSi濃度)≦1.5であることを特徴とする加工性および塗装後耐食性に優れる高強度冷延鋼板。 In mass%, C: 0.03-0.18%, Si: 0.01-1.5%, Mn: 0.5-3.0%, P: 0.001-0.1%, S: 0.0001-0.008%, Sol.Al: 0.01-0.1%, N: Containing 0.0001-0.008%, the balance consists of Fe and inevitable impurities , the steel layer has a soft layer with a ferrite volume fraction of 90% or more and a thickness of 10-100 μm, and the structure of the center is tempered martensite volume 30% to 80%, 0.5 ≦ (Si concentration in tempered martensite) / (Si concentration in ferrite) ≦ 1.5 steel sheet. さらに、mass%で、Cu:0.01〜1%、Ni:0.01〜1%、Mo:0.01〜1%、Cr:0.01〜1%、B:0.0001〜0.005%の1種または2種以上を含有することを特徴とする請求項1に記載の加工性および塗装後耐食性に優れる高強度冷延鋼板。 Furthermore, in mass%, Cu: 0.01 to 1%, Ni: 0.01 to 1%, Mo: 0.01 to 1%, Cr: 0.01 to 1%, B: 0.0001 to 0.005%, or one or more of them are contained. 2. The high-strength cold-rolled steel sheet having excellent workability and post-coating corrosion resistance according to claim 1. さらに、mass%で、Ti:0.001〜0.1%、Nb:0.001〜0.1%、V:0.01〜0.5%の1種または2種以上を含有することを特徴とする請求項1または2に記載の加工性および塗装後耐食性に優れる高強度冷延鋼板。 The processing according to claim 1 or 2, further comprising one or more of Ti: 0.001 to 0.1%, Nb: 0.001 to 0.1%, and V: 0.01 to 0.5% in mass%. -Strength cold-rolled steel sheet with excellent corrosion resistance and corrosion resistance after painting. さらに、mass%で、Ca:0.0001〜0.005%を含有することを特徴とする請求項1ないし3に記載の加工性と塗装後耐食性に優れる高強度冷延鋼板。 The high-strength cold-rolled steel sheet having excellent workability and post-coating corrosion resistance according to claim 1, further comprising, in mass%, Ca: 0.0001 to 0.005%. 請求項1ないし4のいずれかに記載の成分組成を有する鋼スラフ゛を鋳造後、熱間圧延、酸洗、冷間圧延を施し鋼板とし、該鋼板を連続焼鈍するにあたり、750〜950℃の再結晶焼鈍温度、10〜1200秒の焼鈍時間で熱処理し、次いで、鋼板表層において、ア)550〜750℃の冷却開始温度から、5〜50℃/秒の冷却速度、0.05〜5秒の冷却時間で冷却後、イ)100℃以下の冷却停止温度まで、100〜2000℃/秒の冷却速度で冷却の2段冷却を行い、次いで、150〜500℃の温度で焼戻しすることを特徴とする加工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。 A steel slab having the composition according to any one of claims 1 to 4 is cast, and then subjected to hot rolling, pickling, and cold rolling to obtain a steel plate. When the steel plate is continuously annealed, Heat treatment was performed at a crystal annealing temperature and an annealing time of 10 to 1200 seconds, and then on the surface layer of the steel sheet a) From a cooling start temperature of 550 to 750 ° C., a cooling rate of 5 to 50 ° C./second, a cooling time of 0.05 to 5 seconds 2) Cooling at the temperature of 100-2000 ° C / sec., Then tempering at a temperature of 150-500 ° C. For producing high-strength cold-rolled steel sheets with excellent corrosion resistance and post-coating corrosion resistance.
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