JP4698971B2 - High-strength cold-rolled steel sheet with excellent coating film adhesion and workability - Google Patents
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本発明は、塗膜密着性と加工性に優れた高強度冷延鋼板に関するものであり、殊に、優れた塗膜密着性を有すると共に、引張強度が３９０ＭＰａ以上で優れた加工性を発揮する自動車部品用鋼板等として最適な冷延鋼板に関するものである。 The present invention relates to a high-strength cold-rolled steel sheet excellent in coating film adhesion and workability, and in particular, has excellent coating film adhesion and exhibits excellent workability at a tensile strength of 390 MPa or more. The present invention relates to an optimal cold-rolled steel sheet as a steel sheet for automobile parts.
自動車の燃費向上や軽量化を背景に鋼材の高強度化が求められており、冷延鋼板の分野でもハイテン化（高強度化）が進んでいる。一方、冷延鋼板は部品製造時にプレス成形が施されるため、伸び等の延性を十分確保することが前提となる。高強度化を図るには合金元素の添加が有効であるが、該合金元素量の増加に伴い、延性は低下する傾向にある。 Steel materials are required to have high strength against the background of improving fuel economy and weight reduction, and high strength (high strength) is also progressing in the field of cold-rolled steel sheets. On the other hand, since cold-rolled steel sheets are press-formed at the time of component production, it is assumed that sufficient ductility such as elongation is secured. Addition of an alloy element is effective for increasing the strength, but the ductility tends to decrease as the amount of the alloy element increases.
しかし上記合金元素の中でも、Ｓｉは延性低下の比較的小さい元素であり、延性を確保しつつ高強度化を図るのに有効な元素である。ところがＳｉ含有量が増加すると、化成処理性が劣化して塗装後の塗膜密着性が低下する。そのため化成処理性が重視される場合にはＳｉ含有量の低減を余儀なくされていた。またＳｉ含有量が増加すると、鋼板表面に生成するＳｉ含有粒界酸化物を原因とするクラックが発生し易くなり、これが塗膜密着性を劣化させる要因となっていた。 However, among the above alloy elements, Si is an element having a relatively small ductility reduction, and is an element effective for achieving high strength while ensuring ductility. However, when the Si content is increased, the chemical conversion processability is deteriorated and the coating film adhesion after coating is lowered. For this reason, when the chemical conversion treatment is important, the Si content has to be reduced. Moreover, when Si content increased, it became easy to generate | occur | produce the crack resulting from the Si containing grain-boundary oxide produced | generated on the steel plate surface, and this became a factor which deteriorates coating-film adhesiveness.
これまで機械的特性と化成処理性を両立させる技術としては、クラッド材を鋼板表面に被覆し、鋼板表面に低Ｓｉ濃度層を設けることで化成処理性を高め、内部の高Ｓｉ濃度層で機械的特性を確保する技術がある（例えば特許文献１）。しかしクラッド構造としなければならないため、製造工程が複雑になりコストアップにつながるという問題点がある。また上記クラッド材は、化成処理性の確保を優先して成分設計が行われるため機械的特性の確保が難しく、鋼板全体の機械的特性を十分に高めたものとは言い難い。 Until now, as a technology to achieve both mechanical properties and chemical conversion treatment, the steel sheet surface is coated with a clad material, and a low Si concentration layer is provided on the steel sheet surface to improve chemical conversion treatment. There is a technique for ensuring the target characteristics (for example, Patent Document 1). However, since the clad structure is required, there is a problem that the manufacturing process is complicated and the cost is increased. In addition, since the component design is performed with priority given to the chemical conversion processability, it is difficult to ensure the mechanical characteristics of the clad material, and it is difficult to say that the mechanical characteristics of the entire steel sheet are sufficiently enhanced.
化成処理性を阻害するＳｉが表面に濃化しないよう特殊な合金元素を添加する従来技術もある（例えば特許文献２や特許文献３）。この方法では、ＮｉやＣｕを添加することで鋼板表層へのＳｉ濃化を抑制し、化成処理性を確保している。しかし該方法では、高価なＮｉやＣｕを使用するためコストアップを招くという問題がある。 There is also a conventional technique in which a special alloy element is added so that Si that inhibits chemical conversion treatment does not concentrate on the surface (for example, Patent Document 2 and Patent Document 3). In this method, by adding Ni or Cu, concentration of Si on the steel sheet surface layer is suppressed, and chemical conversion processability is ensured. However, this method has a problem that the cost is increased because expensive Ni or Cu is used.
特許文献４では、ＮｂＣを析出させ、これをりん酸亜鉛結晶の核生成サイトとして活用することで化成処理性を確保している。しかしこの技術は、０．０２％以下の低Ｃ濃度域で集合組織を制御することによって深絞り性や機械的特性を確保した所謂ＩＦ鋼と同様の技術である。 In Patent Document 4, NbC is precipitated, and this is used as a nucleation site for zinc phosphate crystals to ensure chemical conversion treatment. However, this technique is the same technique as so-called IF steel that ensures deep drawability and mechanical characteristics by controlling the texture in a low C concentration range of 0.02% or less.
特許文献５では、表層のＳｉＯ２／Ｍｎ２ＳｉＯ４比率を規定することで化成処理性を確保した残留オーステナイト含有鋼板が提案されている。この技術では、表層酸化物を制御したりＳｉ／Ｆｅの元素比率を制御するため、連続焼鈍後の表面を酸洗またはブラシ処理してＳｉ酸化物を除去するか、またはＡｃ１変態点以上の温度で露点を−３０℃以上に調整し、Ｓｉ酸化物の生成量を抑える必要がある。 Patent Document 5 proposes a retained austenite-containing steel sheet that ensures chemical conversion processability by defining the SiO 2 / Mn 2 SiO 4 ratio of the surface layer. In this technique, in order to control the surface layer oxide or to control the element ratio of Si / Fe, the surface after continuous annealing is pickled or brushed to remove the Si oxide, or the Ac 1 transformation point or higher. It is necessary to adjust the dew point to −30 ° C. or higher with temperature to suppress the amount of Si oxide produced.
しかし上記酸洗やブラシ処理を行うと、工程数の増大により製造コストの上昇を招く。また露点制御は、連続焼鈍炉内で行われるが、文献に示された実施例を見る限り、該露点を制御したとしても最表層におけるＳｉＯ２／Ｍｎ２ＳｉＯ４比率は１．０程度であり、化成処理皮膜結晶の生成を阻害するＳｉＯ２がＭｎ２ＳｉＯ４と同程度生じていることから、化成処理性が十分に改善されているとは言い難い。 However, when the pickling or brush treatment is performed, the manufacturing cost increases due to an increase in the number of steps. Although the dew point control is performed in a continuous annealing furnace, as long as the examples shown in the literature are seen, even if the dew point is controlled, the SiO 2 / Mn 2 SiO 4 ratio in the outermost layer is about 1.0. Further, since SiO 2 that inhibits the formation of the chemical conversion film is generated to the same extent as Mn 2 SiO 4 , it is difficult to say that the chemical conversion property is sufficiently improved.
特許文献６では、ＸＰＳで鋼板表面を観察し、酸化物を構成するＳｉとＭｎの比（Ｓｉ／Ｍｎ）を１以下に抑えて化成処理性を高める技術が提案されている。 Patent Document 6 proposes a technique for observing the surface of a steel sheet by XPS and suppressing the ratio of Si to Mn (Si / Mn) constituting the oxide to 1 or less to improve chemical conversion property.
Ｓｉ／Ｍｎ比が１以下である鋼として、例えばＳｉ量がほぼゼロの軟鋼やＳｉ量が０．１％以下の鋼板が化成処理性に優れていることは一般に知られている。しかし上述の通り、強度と延性を共に高めるにはＳｉをある程度含有させる必要があり、Ｓｉ量を低減してＳｉ／Ｍｎ比を１以下にするには限界がある。また適量のＳｉ量を確保しつつＭｎ量を制御してＳｉ／Ｍｎ比を１以下にした場合でも、良好な化成処理性を発揮する鋼板が安定して得られるとは限らない。
本発明は上記事情に鑑みてなされたものであって、その目的は、優れた塗膜密着性を有すると共に、引張強度が３９０ＭＰａ以上で優れた加工性を発揮する冷延鋼板を提供することにある。 The present invention has been made in view of the above circumstances, and its object is to provide a cold-rolled steel sheet having excellent coating film adhesion and exhibiting excellent workability at a tensile strength of 390 MPa or more. is there.
本発明に係る高強度冷延鋼板とは、質量％で（化学成分について以下同じ）、Ｃ：０．０３〜１％、Ｍｎ：０．２〜３％、Ｓｉ：０．１〜１％、Ｓｉ／Ｍｎ≦０．４を満たし、
金属組織が、占積率で（金属組織について以下同じ）、フェライト：６０％以上、ベイナイトおよび／または炭化物：５％以上、該フェライトとベイナイトおよび／または炭化物との合計：９０％以上、マルテンサイト：３％以下（０％含む）、残留オーステナイト：３％以下（０％含む）を満たす複合組織鋼板であって、
引張強度が３９０ＭＰａ以上で、かつ引張強度（ＴＳ：単位MPa）と伸び（Ｅｌ：単位％）が下記式（１）を満たすと共に、
（I）鋼板表面（平面視する場合をいう）において、ＭｎとＳｉの原子比（Ｍｎ／Ｓｉ）が０．５以上である長径０．０１μｍ以上５μｍ以下のＭｎ−Ｓｉ複合酸化物が１０個／１００μｍ２以上存在し、かつＳｉを主体とする酸化物の鋼板表面被覆率が１０％以下であるところに特徴を有する（以下「本発明鋼板１」ということがある）。
ＴＳ×Ｅｌ≧１４０００ …（１）
尚、上記Ｓｉを主体とする酸化物とは、酸化物を構成する酸素以外の元素のうちＳｉが原子比で６７％超を占めるものをいう。また当該酸化物は、分析の結果、非晶質であると考えられる。
The high-strength cold-rolled steel sheet according to the present invention is in mass% (the same applies to chemical components below), C: 0.03 to 1%, Mn: 0.2 to 3%, Si: 0.1 to 1%, Satisfying Si / Mn ≦ 0.4,
Metal structure is a space factor (the same applies to the metal structure hereinafter), ferrite: 60% or more, bainite and / or carbide: 5% or more, total of ferrite and bainite and / or carbide: 90% or more, martensite : Composite steel sheet satisfying 3% or less (including 0%), retained austenite: 3% or less (including 0%),
The tensile strength is 390 MPa or more, and the tensile strength (TS: unit MPa) and elongation (El: unit%) satisfy the following formula (1),
(I) 10 Mn—Si composite oxides having a major axis of 0.01 μm or more and 5 μm or less with an atomic ratio (Mn / Si) of Mn and Si of 0.5 or more on the surface of the steel sheet (when viewed in plan) / 100 μm 2 or more, and the feature is that the steel sheet surface coverage of the oxide mainly composed of Si is 10% or less (hereinafter sometimes referred to as “the steel sheet 1 of the present invention”).
TS × El ≧ 14000 (1)
The oxide mainly composed of Si refers to an element other than oxygen constituting the oxide in which Si accounts for more than 67% by atomic ratio. Further, the oxide is considered to be amorphous as a result of analysis.
Ｓｉを主体とする酸化物の鋼板表面被覆率は、後述する実施例で示す通り、抽出レプリカ法で処理したサンプルをＴＥＭ（transmission electron microscope）で観察し、ＥＤＸ（Energy Dispersive X-ray）分析でＳｉ、Ｏ（酸素）、Ｍｎ、Ｆｅのマッピングおよび定量分析を行い、このデータを用いて画像解析法により求めた。尚、抽出レプリカのＴＥＭ観察が煩雑であれば、ＡＥＳ（auger electron spectroscopy）を用いて倍率：２０００〜５０００倍でＳｉ、Ｏ、ＭｎおよびＦｅについて表面マッピングを行い、そのデータを画像解析してもよい。 As shown in the examples to be described later, the surface coverage of the oxide mainly composed of Si is obtained by observing a sample processed by the extraction replica method with a transmission electron microscope (TEM) and analyzing with an EDX (Energy Dispersive X-ray) analysis. Mapping and quantitative analysis of Si, O (oxygen), Mn, and Fe were performed, and the data was obtained by an image analysis method. If TEM observation of the extracted replica is complicated, surface mapping is performed on Si, O, Mn, and Fe at a magnification of 2000 to 5000 using AES (auger electron spectroscopy), and the data is subjected to image analysis. Good.
上記課題を解決し得た本発明の別の鋼板は、Ｃ：０．０３〜１％、Ｍｎ：０．２〜３％、Ｓｉ：０．１〜１％を満たし、金属組織が、フェライト：６０％以上、ベイナイトおよび／または炭化物：５％以上、該フェライトとベイナイトおよび／または炭化物との合計：９０％以上、マルテンサイト：３％以下（０％含む）、残留オーステナイト：３％以下（０％含む）を満たす複合組織鋼板であって、
引張強度が３９０ＭＰａ以上で、かつ引張強度（ＴＳ：単位MPa）と伸び（Ｅｌ：単位％）が上記式（１）を満たすと共に、
（II）ＳＥＭ（scanning electron microscope）を用いて２０００倍で鋼板表面近傍の断面を観察したときに、任意の１０視野において幅３μｍ以下で深さ５μｍ以上のクラックが存在しないところに特徴を有している（以下「本発明鋼板２」ということがある）。
Another steel plate of the present invention that has solved the above problems satisfies C: 0.03 to 1%, Mn: 0.2 to 3%, Si: 0.1 to 1%, and the metal structure is ferrite: 60% or more, bainite and / or carbide: 5% or more, total of the ferrite and bainite and / or carbide: 90% or more, martensite: 3% or less (including 0%), residual austenite: 3% or less (0 % Including the composite structure steel plate,
The tensile strength is 390 MPa or more, and the tensile strength (TS: unit MPa) and elongation (El: unit%) satisfy the above formula (1),
(II) When a cross section near the surface of the steel sheet is observed at a magnification of 2000 using a scanning electron microscope (SEM), it is characterized in that there are no cracks with a width of 3 μm or less and a depth of 5 μm or more in any 10 fields of view. (Hereinafter sometimes referred to as “the present steel plate 2”).
尚、上記クラックの幅および深さとは、ＳＥＭ（日立製作所製 Ｓ−４５００）を用いて２０００倍で鋼板断面の表面近傍を観察したときの、図１（鋼板断面概略図）に示す部分をいうものとする。 The width and depth of the cracks refer to the portion shown in FIG. 1 (schematic cross section of steel plate) when the vicinity of the surface of the steel plate cross section is observed at 2000 times using SEM (S-4500 manufactured by Hitachi, Ltd.). Shall.
上記課題を解決し得た本発明の更に別の鋼板は、Ｃ：０．０３〜１％、Ｍｎ：０．２〜３％、Ｓｉ：０．１〜１％、Ｓｉ／Ｍｎ≦０．４を満たし、金属組織が、フェライト：６０％以上、ベイナイトおよび／または炭化物：５％以上、該フェライトとベイナイトおよび／または炭化物との合計：９０％以上、マルテンサイト：３％以下（０％含む）、残留オーステナイト：３％以下（０％含む）を満たす複合組織鋼板であって、
引張強度が３９０ＭＰａ以上で、かつ引張強度（ＴＳ：単位MPa）と伸び（Ｅｌ：単位％）が上記式（１）を満たすと共に、上記要件（I）および（II）を満たすところに特徴を有している（以下「本発明鋼板３」ということがある）。
Still another steel sheet of the present invention that can solve the above problems is C: 0.03 to 1%, Mn: 0.2 to 3%, Si: 0.1 to 1%, Si / Mn ≦ 0.4. And the metal structure is ferrite: 60% or more, bainite and / or carbide: 5% or more, the total of the ferrite and bainite and / or carbide: 90% or more, martensite: 3% or less (including 0%) Residual austenite: a composite structure steel plate satisfying 3% or less (including 0%),
It is characterized in that the tensile strength is 390 MPa or more, and the tensile strength (TS: unit MPa) and elongation (El: unit%) satisfy the above formula (1) and the above requirements (I) and (II). (Hereinafter sometimes referred to as “the present steel plate 3”).
上記本発明の鋼板は、更に他の元素として、Ｎｂ：０．１％以下（０％含まない）、Ｔｉ：０．２％以下（０％含まない）およびＶ：０．２％以下（０％含まない）よりなる群から選択される１種以上や、Ｃｒ：０．５％以下（０％含まない）、Ｍｏ：０．５％以下（０％含まない）およびＷ：０．５％以下（０％含まない）よりなる群から選択される１種以上を含んでいてもよい。 In the steel sheet of the present invention, as other elements, Nb: 0.1% or less (not including 0%), Ti: 0.2% or less (not including 0%), and V: 0.2% or less (0 Or more selected from the group consisting of Cr: 0.5% or less (not including 0%), Mo: 0.5% or less (not including 0%), and W: 0.5% One or more selected from the group consisting of the following (not including 0%) may be included.
本発明によれば、優れた塗膜密着性を発揮すると共に、引張強度が３９０ＭＰａ以上で優れた加工性を発揮する自動車用に最適な鋼板を、クラッドを構成したり高価な元素を添加することなく効率良く実現できる。 According to the present invention, a steel plate optimal for automobiles exhibiting excellent coating film adhesion and exhibiting excellent workability at a tensile strength of 390 MPa or more is configured as a clad or added with an expensive element. Can be realized efficiently.
塗膜密着性と加工性に優れた引張強度が３９０ＭＰａ以上の鋼板を得るべく検討したところ、特に、優れた塗膜密着性を確保するには、下記要件（I）および／または（II）を満足させればよいことを見出し本発明に想到した。更にこれらの要件を満足させると共に、３９０ＭＰａ以上の引張強度において優れた加工性を確保するための成分組成や製造条件についても検討を行った。 In order to obtain a steel sheet having a tensile strength of 390 MPa or more excellent in coating film adhesion and workability, the following requirements (I) and / or (II) are particularly required to ensure excellent coating film adhesion: The present inventors have found that it should be satisfied and have come up with the present invention. Furthermore, while satisfying these requirements, the component composition and manufacturing conditions for ensuring excellent workability at a tensile strength of 390 MPa or more were also examined.
（I）鋼板表面（平面視する場合をいう）において、
(i）ＭｎとＳｉの原子比（Ｍｎ／Ｓｉ）が０．５以上である長径０．０１μｍ以上５μｍ以下のＭｎ−Ｓｉ複合酸化物を１０個／１００μｍ２以上存在させ、かつ
(ii）Ｓｉを主体とする酸化物（酸化物を構成する酸素以外の元素のうちＳｉが原子比で６７％超を占める酸化物）の鋼板表面被覆率を１０％以下とする。
(I) On the steel plate surface (when viewed in plan)
(i) Mn—Si composite oxide having a major axis of 0.01 μm or more and 5 μm or less having an atomic ratio (Mn / Si) of Mn to Si of 0.5 or more is present at 10 pieces / 100 μm 2 or more, and
(ii) The steel sheet surface coverage of an oxide mainly composed of Si (an oxide other than oxygen constituting the oxide in which Si accounts for more than 67% by atomic ratio) is set to 10% or less.
（II）ＳＥＭを用いて２０００倍で鋼板表面近傍の断面を観察したときに、任意の１０視野において、幅３μｍ以下で深さ５μｍ以上のクラックが存在しないようにする。 (II) When observing a cross section in the vicinity of the steel sheet surface at a magnification of 2000 using an SEM, no cracks having a width of 3 μm or less and a depth of 5 μm or more are present in any 10 visual fields.
以下、まず上記要件（I)，(II）を規定した理由について詳述する。 The reason why the requirements (I) and (II) are specified will be described in detail below.
＜鋼板表面におけるＭｎとＳｉの原子比（Ｍｎ／Ｓｉ）が０．５以上である長径０．０１〜５μｍのＭｎ−Ｓｉ複合酸化物：１０個／１００μｍ２以上＞
本発明者らは、塗膜密着性に優れた高強度鋼板を得るべく以前から研究を進めており、Ｓｉを比較的多く含む鋼板の化成処理性向上技術について、既に提案している（特願２００３−１０６１５２号）。この技術は、焼鈍雰囲気を制御することで、化成処理性に悪影響を及ぼす非晶質のＳｉ酸化物を細かく分散させることにより化成処理性の向上を図ったものである。しかしＳｉ濃度の比較的低い領域では、主な酸化物として、非晶質のＳｉ酸化物ではなくＭｎ−Ｓｉ複合酸化物が生成する。この複合酸化物も、非晶質のＳｉ酸化物と同様に塗膜密着性を低下させると考えられる。そこで、該Ｍｎ−Ｓｉ複合酸化物を化成処理性の向上に積極的に活用することはできないかと考え、その線に沿って研究を進めてきた。
<Mn-Si composite oxide having a major axis of 0.01 to 5 μm with an atomic ratio of Mn to Si (Mn / Si) of 0.5 or more on the steel sheet surface: 10/100 μm 2 or more>
The present inventors have been researching for a long time to obtain a high-strength steel sheet excellent in coating film adhesion, and have already proposed a chemical conversion treatment improving technique for a steel sheet containing a relatively large amount of Si (Japanese Patent Application). 2003-106152). This technique aims to improve chemical conversion treatment by finely dispersing amorphous Si oxide that adversely affects chemical conversion treatment by controlling the annealing atmosphere. However, in the region where the Si concentration is relatively low, not the amorphous Si oxide but the Mn—Si composite oxide is generated as the main oxide. This composite oxide is also considered to reduce the adhesion of the coating film in the same manner as the amorphous Si oxide. Therefore, the Mn—Si composite oxide is considered to be actively used for improving chemical conversion treatment, and research has been advanced along that line.
その結果、鋼板表層部に形成される鉄系酸化物基地中に、該Ｍｎ−Ｓｉ複合酸化物を微細分散させて、後述する通り、りん酸亜鉛結晶の核生成サイトとして作用する「酸化物界面の電気化学的不均一場」を形成することで、化成処理性を高めることができた。本発明で規定するＭｎ−Ｓｉ複合酸化物が、りん酸亜鉛結晶の生成核に有効である理由は明確ではないが、次の様に考えられる。 As a result, the Mn-Si composite oxide is finely dispersed in the iron-based oxide matrix formed on the surface layer portion of the steel sheet, and as described later, the “oxide interface that acts as a nucleation site for zinc phosphate crystals. The chemical conversion processability could be improved by forming an "electrochemical heterogeneous field". The reason why the Mn—Si composite oxide defined in the present invention is effective for the nuclei of zinc phosphate crystals is not clear, but is considered as follows.
化成処理工程において、りん酸亜鉛結晶は、例えば結晶粒界や予め表面調整処理時に鋼板表面に付着させたＴｉコロイド周辺などに形成される「電気化学的不均一場」に生成し易いことが知られている。そして本発明においても、Ｍｎ−Ｓｉ複合酸化物の周辺に電気化学的な不均一場が形成されることで、化成処理時にりん酸亜鉛結晶が付着しやすくなり、良好な化成処理性が発揮されるものと考えられる。 In the chemical conversion treatment process, it is known that zinc phosphate crystals are likely to be generated in the “electrochemical inhomogeneous field” formed around the grain boundaries and around the Ti colloid previously deposited on the steel plate surface during the surface conditioning treatment. It has been. Also in the present invention, an electrochemical heterogeneous field is formed around the Mn-Si composite oxide, so that zinc phosphate crystals are easily attached during chemical conversion treatment, and good chemical conversion treatment performance is exhibited. It is thought that.
化成処理後のりん酸亜鉛結晶は、塗膜密着性の観点から数μｍ以下であることが好ましいとされている。よって上述の電気化学的不均一場も、数μｍオーダーまたはそれ以下であることが望ましいと考えられる。そこでＭｎとＳｉの原子比（Ｍｎ／Ｓｉ）が０．５以上である長径０．０１μｍ以上５μｍ以下のＭｎ−Ｓｉ複合酸化物を１００μｍ２に１０個以上存在させて（平均して１０μｍ２に１個以上存在させて）、該複合酸化物粒子の平均粒子間隔が数μｍとなるようにし、上記サイズの電気化学的不均一場が形成されやすい状態とした。 It is said that the zinc phosphate crystal after the chemical conversion treatment is preferably several μm or less from the viewpoint of coating film adhesion. Therefore, it is considered that the above-mentioned electrochemical non-uniform field is desirably on the order of several μm or less. Therefore, 10 or more Mn—Si composite oxides having a major axis of 0.01 μm or more and 5 μm or less having an atomic ratio of Mn to Si (Mn / Si) of 0.5 or more are present in 100 μm 2 (on average 10 μm 2) . One or more particles were present), and the average particle spacing of the composite oxide particles was set to several μm so that an electrochemical heterogeneous field having the above size was easily formed.
尚、存在する全てのＭｎ−Ｓｉ複合酸化物において、電気化学的不均一場が有効に形成されるとは限らないので、好ましくは１００μｍ２あたり５０個以上、より好ましくは１００個以上、さらに好ましくは１５０個以上の上記Ｍｎ−Ｓｉ複合酸化物を存在させるのがよい。該Ｍｎ−Ｓｉ複合酸化物としては、例えばＭｎ２ＳｉＯ４が挙げられる。 Note that, in all the Mn—Si composite oxides present, the electrochemical heterogeneous field is not necessarily formed effectively, so preferably 50 or more per 100 μm 2 , more preferably 100 or more, even more preferably. It is preferable that 150 or more of the above Mn—Si composite oxide exist. Examples of the Mn—Si composite oxide include Mn 2 SiO 4 .
＜Ｓｉを主体とする酸化物の鋼板表面被覆率：１０％以下＞
りん酸亜鉛結晶の生成核として有効なＭｎ−Ｓｉ複合酸化物を適量存在させても、化成処理を阻害するその他の物質が存在すれば、優れた化成処理性は発揮されず、結果として塗膜密着性に劣るものとなる。
<Stainless steel sheet surface coverage of oxide mainly composed of Si: 10% or less>
Even if an appropriate amount of a Mn-Si composite oxide effective as a zinc phosphate crystal nucleus is present, if there is another substance that inhibits the chemical conversion treatment, the excellent chemical conversion treatment performance will not be exhibited. It becomes inferior to adhesiveness.
上述した様に、Ｓｉを主体とする酸化物（酸化物を構成する酸素以外の元素のうちＳｉが原子比で６７％超を占める酸化物）が鋼板表面に存在すると、当該部位には、りん酸亜鉛結晶が生成せず化成処理性が著しく低下する。そこで、Ｓｉを主体とする酸化物の鋼板表面被覆率を１０％以下とした。 As described above, when an oxide mainly composed of Si (an oxide other than oxygen constituting the oxide in which Si accounts for more than 67% by atomic ratio) is present on the surface of the steel sheet, the region contains phosphorus. Zinc acid crystals are not formed, and the chemical conversion treatment performance is significantly reduced. Therefore, the steel sheet surface coverage of the oxide mainly composed of Si is set to 10% or less.
尚、本発明者らは、上述の通りＳｉを主体とする酸化物を細かく分散させて化成処理性を高める技術を提案しているが、Ｍｎ−Ｓｉ複合酸化物の前記作用を活用する本発明においては、Ｓｉを主体とする酸化物を極力存在させない方が好ましいことがわかった。よってＳｉを主体とする酸化物の鋼板表面被覆率は、５％以下に抑えることがより好ましく、最も好ましくは０％である。 The inventors of the present invention have proposed a technique for finely dispersing an oxide mainly composed of Si as described above to improve the chemical conversion treatment property. However, the present invention utilizes the above-described action of the Mn—Si composite oxide. In the above, it was found that it is preferable that an oxide mainly composed of Si is not present as much as possible. Therefore, the steel sheet surface coverage of the oxide mainly composed of Si is more preferably suppressed to 5% or less, and most preferably 0%.
＜ＳＥＭを用いて2000倍で鋼板表面近傍の断面を観察したときに、任意の10視野において、幅３μｍ以下で深さ５μｍ以上のクラックが存在しないこと＞
鋼板表面に鋭利なクラックが存在すると、化成処理時に当該部位にりん酸亜鉛結晶が付着せず、その結果、当該部位の腐食が進行しやすくなり、塗膜密着性が低下すると考えられる。つまり塗膜密着性を高めるには、りん酸亜鉛結晶の付着しない鋭利なクラックを極力抑制することが重要となる。
<When observing a cross section near the steel sheet surface at 2000 times using SEM, there should be no cracks with a width of 3 μm or less and a depth of 5 μm or more in any 10 fields of view>
If sharp cracks are present on the surface of the steel plate, it is considered that zinc phosphate crystals do not adhere to the site during the chemical conversion treatment, and as a result, corrosion of the site is likely to proceed, resulting in a decrease in coating film adhesion. That is, in order to improve the adhesion of the coating film, it is important to suppress as much as possible sharp cracks to which zinc phosphate crystals do not adhere.
本発明者らは、既に、Ｓｉと酸素を含む線状化合物（幅３００ｎｍ以下）の存在深さを１０μｍ以下にすることで塗膜密着性を高める技術を提案している。該技術では、連続焼鈍後に酸洗を施さないことを前提としているが、鋼板にはむしろ連続焼鈍後に酸洗を施す場合の方が多く、その場合には、線状酸化物が除去されてクラックが生じる。 The present inventors have already proposed a technique for improving the adhesion of a coating film by setting the existing depth of a linear compound (width: 300 nm or less) containing Si and oxygen to 10 μm or less. In this technique, it is assumed that pickling is not performed after continuous annealing, but the steel sheet is more often subjected to pickling after continuous annealing, in which case, the linear oxide is removed and cracks occur. Occurs.
クラック深さと線状酸化物の定量的な関係は明確でないが、線状酸化物が、上記の通り酸溶解されるか、又は機械的に脱落してクラックが生じると考えられ、上記線状酸化物が除去されたあとも、酸等によりクラック部分の溶解が進むので、線状酸化物の存在深さよりも該酸化物の除去後に形成されるクラックの方が深いと考えられる。 Although the quantitative relationship between the crack depth and the linear oxide is not clear, it is considered that the linear oxide is dissolved in the acid as described above, or mechanically dropped to cause cracks, and the linear oxidation described above. It is considered that cracks formed after removal of the oxide are deeper than the existence depth of the linear oxide because dissolution of the crack portion proceeds by acid or the like even after the material is removed.
そこで本発明では、上記提案済の技術のように線状酸化物の存在深さを規定するよりも、クラックを制御する方が塗膜密着性をより確実に高めることができると考え、制御すべきクラックの形態について調べたところ、クラックの幅が、りん酸亜鉛結晶粒径と同程度かそれ以下であると、該クラックにりん酸亜鉛結晶が付着し難く、また、特に深さが５μｍ以上のクラックにはりん酸亜鉛結晶が付着し難いことから、幅３μｍ以下でかつ深さが５μｍ以上のクラックを抑制の対象とした。 Therefore, in the present invention, it is considered that controlling the cracks can more reliably improve the adhesion of the coating film than controlling the depth of existence of the linear oxide as in the proposed technique. When the shape of the power crack was examined, if the crack width was the same as or smaller than the zinc phosphate crystal grain size, it was difficult for the zinc phosphate crystal to adhere to the crack, and the depth was particularly 5 μm or more. Since zinc phosphate crystals are difficult to adhere to the cracks, cracks having a width of 3 μm or less and a depth of 5 μm or more were targeted for suppression.
そして上記クラックが、ＳＥＭを用いて２０００倍で鋼板表面近傍の断面を観察したときに、任意の１０視野において存在しないことを要件とした。 And when the said crack observed the cross section near the steel plate surface by 2000 times using SEM, it made it a requirement that it did not exist in arbitrary 10 visual fields.
本発明では、上記Ｍｎ−Ｓｉ複合酸化物を効率良く析出させると共に規定するクラックを抑制し、また高強度鋼板としての特性を備えるため化学成分を下記の通り規定した。 In the present invention, the chemical components are defined as follows in order to efficiently precipitate the Mn—Si composite oxide and to suppress the specified cracks and to provide the characteristics as a high-strength steel sheet.
＜Ｓｉ(質量％)／Ｍｎ(質量％)≦０．４＞
上述の通り、Ｓｉを主体とする酸化物は、化成処理性に悪影響を及ぼすため、該酸化物を細かく分散させるよりも極力抑制する方が好ましい。そこで本発明者らは、鋼中Ｓｉ含有量（質量％）と鋼中Ｍｎ含有量の比率（Ｓｉ／Ｍｎ）を０．４以下とすることで、Ｓｉを主体とする酸化物を抑制し、化成処理性を高めることとした。Ｓｉ／Ｍｎは好ましくは０．３以下である。
<Si (mass%) / Mn (mass%) ≦ 0.4>
As described above, since an oxide mainly composed of Si adversely affects chemical conversion properties, it is preferable to suppress it as much as possible rather than finely dispersing the oxide. Therefore, the inventors suppress the oxide mainly composed of Si by setting the ratio of Si content (mass%) in steel to Mn content in steel (Si / Mn) to 0.4 or less, The chemical conversion processability was improved. Si / Mn is preferably 0.3 or less.
＜Ｃ：０．０３〜１％＞
Ｃは、強度確保に寄与する炭化物（主にセメンタイトであり、ＮｂやＴｉ、Ｖ等の添加によるこれらの炭化物を含む）の形成に必要な元素であり、０．０３％以上、好ましくは０．０５％以上含有させる。しかしＣが過剰に存在すると、溶接性が低下するので１％以下に抑える。好ましくは０．２３％以下、より好ましくは０．１５％以下である。
<C: 0.03 to 1%>
C is an element necessary for forming carbides (mainly cementite, including these carbides by addition of Nb, Ti, V, etc.) contributing to securing strength, and is 0.03% or more, preferably 0.8. Add 0.5% or more. However, if C is present excessively, the weldability deteriorates, so it is suppressed to 1% or less. Preferably it is 0.23% or less, More preferably, it is 0.15% or less.
＜Ｍｎ：０．２〜３％＞
Ｍｎは、鋼の脆化を招くＳ（硫黄）をＭｎＳとして固定するのに有効な元素であり、このような効果を発揮させるためには０．２％以上、好ましくは０．３％以上含有させる。しかし過剰になると延性と溶接性が共に劣化するため、３％以下、好ましくは２％以下に抑える。
<Mn: 0.2-3%>
Mn is an element effective for fixing S (sulfur) that causes embrittlement of steel as MnS, and in order to exert such an effect, it is 0.2% or more, preferably 0.3% or more. Let However, if it is excessive, both ductility and weldability deteriorate, so the content is suppressed to 3% or less, preferably 2% or less.
＜Ｓｉ：０．１〜１％＞
Ｓｉは、優れた強度−延性バランスを確保するのに有効な元素であり、この様な効果を十分に発揮させるには、Ｓｉを０．１％以上、好ましくは０．２％以上含有させる。一方、Ｓｉ含有量が過剰になると、固溶強化作用が過大となって圧延負荷が増大するため１％以下に抑える。好ましくは０．８％以下である。
<Si: 0.1 to 1%>
Si is an element effective for ensuring an excellent strength-ductility balance. In order to sufficiently exhibit such an effect, Si is contained in an amount of 0.1% or more, preferably 0.2% or more. On the other hand, if the Si content is excessive, the solid solution strengthening action becomes excessive and the rolling load increases, so the content is suppressed to 1% or less. Preferably it is 0.8% or less.
本発明で規定する含有元素は上記の通りであり、残部成分は実質的にＦｅであるが、鋼中に、原料、資材、製造設備等の状況によって持ち込まれる元素として、０．０１％以下のＮ（窒素）、０．０１％以下のＯ（酸素）、０．１％以下のＡｌ等の不可避不純物が含まれることが許容されるのは勿論のこと、前記本発明の作用に悪影響を与えない範囲で、更に他の元素として下記の元素を積極的に含有させることも可能である。 The contained elements specified in the present invention are as described above, and the remaining component is substantially Fe, but as an element brought into the steel depending on the situation of raw materials, materials, manufacturing equipment, etc., 0.01% or less Needless to say, inevitable impurities such as N (nitrogen), O (oxygen) of 0.01% or less, and Al of 0.1% or less are allowed to be adversely affected. In addition, it is possible to positively contain the following elements as other elements within the range.
＜Ｎｂ：０．１％以下（０％含まない）、
Ｔｉ：０．２％以下（０％含まない）および
Ｖ：０．２％以下（０％含まない）よりなる群から選択される１種以上＞
これらの元素は、炭化物を形成し析出強化により鋼の強度を高めるのに有効な元素である。この様な効果を発揮させるには、Ｎｂを０．０１％以上、Ｔｉを０．０１％以上、Ｖを０．０１％以上含有させることが好ましい。しかし過剰に添加すると延性が低下し易くなるので、Ｎｂは０．１％以下、Ｔｉは０．２％以下、Ｖは０．２％以下（より好ましくは０．０５％以下）の範囲で含有させるのがよい。
<Nb: 0.1% or less (excluding 0%),
Ti: 0.2% or less (excluding 0%) and V: one or more selected from the group consisting of 0.2% or less (not including 0%)>
These elements are effective elements for forming carbides and increasing the strength of steel by precipitation strengthening. In order to exhibit such an effect, it is preferable to contain Nb 0.01% or more, Ti 0.01% or more, and V 0.01% or more. However, if added in excess, the ductility tends to decrease, so Nb is 0.1% or less, Ti is 0.2% or less, and V is 0.2% or less (more preferably 0.05% or less). It is good to let them.
＜Ｃｒ：０．５％以下（０％含まない）、
Ｍｏ：０．５％以下（０％含まない）、および
Ｗ：０．５％以下（０％含まない）よりなる群から選択される１種以上＞
これらの元素は、固溶強化に有効な元素であり、また上記Ｔｉ、Ｎｂ、Ｖと複合析出物を形成して鋼板の強度を高めるのにも有効な元素である。この様な効果を発揮させるには、Ｃｒ：０．０５％以上、Ｍｏ：０．０５％以上、Ｗ：０．０５％以上添加するのがよいが、過剰に添加すると加工性の低下を招くため、Ｃｒ、ＭｏおよびＷはいずれも０．５％以下に抑えることが好ましい。
<Cr: 0.5% or less (excluding 0%),
Mo: 0.5% or less (0% not included), and W: 0.5% or less (0% not included)
These elements are effective elements for solid solution strengthening, and are also effective elements for increasing the strength of the steel sheet by forming composite precipitates with Ti, Nb, and V. In order to exert such an effect, it is preferable to add Cr: 0.05% or more, Mo: 0.05% or more, W: 0.05% or more, but if added excessively, workability will be lowered. For this reason, Cr, Mo, and W are all preferably suppressed to 0.5% or less.
本発明は、金属組織がフェライトとベイナイトおよび／または炭化物を主体とし、マルテンサイトや残留オーステナイト（γＲ）の抑制された鋼板を対象とするものである。 The present invention is directed to a steel sheet whose metal structure is mainly composed of ferrite and bainite and / or carbide and in which martensite and retained austenite (γ R ) are suppressed.
本発明における「フェライト」とは、ポリゴナルフェライト、即ち、転位密度の少ないフェライトを意味する。該フェライトを６０％以上（好ましくは７０％以上）確保することで優れた延性を発揮でき、延性向上の観点からフェライトが多いほど好ましいが、後述する通り、強度向上のためにベイナイトや炭化物を確保するにはフェライトを９５％以下に抑えるのがよい。 The “ferrite” in the present invention means polygonal ferrite, that is, ferrite having a low dislocation density. It is possible to exhibit excellent ductility by securing 60% or more (preferably 70% or more) of the ferrite, and more ferrite is preferable from the viewpoint of improving ductility. However, as will be described later, bainite and carbide are ensured for strength improvement. For this purpose, the ferrite content should be suppressed to 95% or less.
一方、ベイナイトおよび／または炭化物は、強度確保に寄与する組織であり、該炭化物としては、主にセメンタイトが挙げられ、ＮｂやＴｉ、Ｖを含む場合にはこれらの炭化物を含む。ベイナイトおよび／または炭化物を少なくとも５％（好ましくは８％以上）存在させることによって強度向上効果が表れるが、延性確保の観点からは、これらの組織を４０％以下に抑えるのがよい。 On the other hand, bainite and / or carbide is a structure that contributes to securing the strength. As the carbide, mainly cementite is included, and when Nb, Ti, and V are included, these carbides are included. The presence of at least 5% (preferably 8% or more) of bainite and / or carbide exhibits an effect of improving the strength, but from the viewpoint of ensuring ductility, it is preferable to suppress these structures to 40% or less.
これらフェライトとベイナイトおよび／または炭化物との複合組織とすることによって、高強度でありながら、強度と延性のバランスに優れた鋼板を達成できる。 By making a composite structure of these ferrite and bainite and / or carbide, a steel sheet having a high balance between strength and ductility can be achieved while having high strength.
本発明の鋼板は、上記該フェライトとベイナイトおよび／または炭化物からなるものの他、製造過程で必然的に残存し、本発明の作用を損なわない範囲で含まれる組織として、マルテンサイトと残留オーステナイトが挙げられるが、これらの組織はそれぞれ３％以下に抑えるのがよく、好ましくは０％である。マルテンサイトや残留オーステナイト（加工を受けるとマルテンサイトに加工誘起変態する）が混入すると、伸びフランジ性（穴拡げ性）が低下し、またＹＲ（降伏比）が低くなり、部品や部材の衝突特性に悪影響を及ぼす場合があるからである。 In addition to the ferrite and bainite and / or carbide, the steel sheet of the present invention inevitably remains in the production process and includes martensite and retained austenite as a structure included within a range not impairing the function of the present invention. However, these structures should be suppressed to 3% or less, preferably 0%. When martensite and retained austenite (processing-induced transformation occurs in martensite when subjected to processing), stretch flangeability (hole expandability) decreases and YR (yield ratio) decreases, resulting in impact characteristics of parts and members. This is because it may adversely affect the situation.
本発明の鋼板は、前記基本成分を満たす鋼板であって、前記金属組織を有し、かつ特性として、
・引張強度が３９０ＭＰａ以上（特に、５５０ＭＰａ以上）でかつ、
・引張強度（ＴＳ：単位MPa）と伸び（Ｅｌ：単位％）が下記式（１）を満足する。下記式（１）の右辺が特に１６０００以上であるものは、強度と加工性のバランスに優れており好ましい。
ＴＳ×Ｅｌ≧１４０００ …（１）
The steel plate of the present invention is a steel plate satisfying the basic components, has the metal structure, and as a characteristic,
-Tensile strength is 390 MPa or more (particularly 550 MPa or more), and
-Tensile strength (TS: unit MPa) and elongation (El: unit%) satisfy the following formula (1). The right side of the following formula (1) is particularly preferably 16000 or more because it is excellent in the balance between strength and workability.
TS × El ≧ 14000 (1)
化成処理性を高めるべく、上記要件（I）として規定する通り鋼板表面に析出する酸化物の形態を制御するには、成分組成を満足させる他、製造工程において、熱間圧延後に、液温が７０〜９０℃で１〜１８質量％の塩酸に４０秒間以上（好ましくは６０秒間以上）浸漬し、かつ連続焼鈍時の露点を−４０℃以下（好ましくは−４５℃以下）に抑えることが有効である。尚、塩酸への浸漬時間は、塩酸浴が複数設置され、断続的に浸漬する場合には、浸漬時間の合計が４０秒間以上であればよい。 In order to enhance the chemical conversion treatment property, in order to control the form of the oxide deposited on the steel sheet surface as specified in the above requirement (I), in addition to satisfying the component composition, in the production process, after the hot rolling, the liquid temperature is It is effective to immerse in 1 to 18% by mass hydrochloric acid at 70 to 90 ° C. for 40 seconds or more (preferably 60 seconds or more) and suppress the dew point during continuous annealing to −40 ° C. or less (preferably −45 ° C. or less). It is. In addition, as for the immersion time in hydrochloric acid, when a plurality of hydrochloric acid baths are installed and intermittent immersion is performed, the total immersion time may be 40 seconds or more.
また上記要件（II）として規定する通り、クラックを発生させないようにするには、成分組成を満足させる他、製造工程において、熱間圧延の巻き取り温度を５００℃以下（好ましくは４８０℃以下）とし、かつ熱間圧延後、液温が７０〜９０℃で１〜１８質量％の塩酸に４０秒間以上（好ましくは６０秒間以上）浸漬し、連続焼鈍時の露点を−４０℃以下（好ましくは−４５℃以下）とし、更に連続焼鈍時の焼き入れ開始温度（「徐冷終点温度」ということがある。）を５５０℃以下（好ましくは４００〜４５０℃）にすることが有効である。 Further, as specified in the above requirement (II), in order not to generate cracks, in addition to satisfying the component composition, in the manufacturing process, the hot rolling coiling temperature is 500 ° C. or lower (preferably 480 ° C. or lower). And after hot rolling, the liquid temperature is 70 to 90 ° C. and immersed in 1 to 18% by mass of hydrochloric acid for 40 seconds or more (preferably 60 seconds or more), and the dew point during continuous annealing is −40 ° C. or less (preferably It is effective to set the quenching start temperature during continuous annealing (sometimes referred to as “annealing end point temperature”) to 550 ° C. or less (preferably 400 to 450 ° C.).
本発明は、その他の製造条件まで規定するものでなく、通常行われている通り、溶製後に鋳造し熱間圧延を行えばよい。また後述する実施例では連続焼鈍後に酸洗を行っているが、該酸洗の有無も問わない。 The present invention is not limited to other production conditions, and may be cast after melting and hot-rolled as usual. Moreover, in the Example mentioned later, although pickling is performed after continuous annealing, the presence or absence of this pickling is not ask | required.
以下、実施例を挙げて本発明をより具体的に説明するが、本発明はもとより下記実施例によって制限を受けるものではなく、前・後記の趣旨に適合し得る範囲で適当に変更を加えて実施することも可能であり、それらはいずれも本発明の技術的範囲に含まれる。 EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. It is also possible to implement, and they are all included in the technical scope of the present invention.
表１に示す化学成分組成の鋼材を溶製し、鋳造して得られたスラブを用いて熱間圧延を行いその後酸洗を行った。製造条件を表２に示す。尚、酸洗は、温度が７０〜９０℃で濃度が１〜１８質量％の塩酸水溶液を用いて行った。その後、冷間圧延を行い、１．４ｍｍ厚の鋼板を得た。そして図２に示す方法で得られた鋼板に連続焼鈍を施した。連続焼鈍における均熱・徐冷後の冷却は、汽水（ミスト）冷却、ガス吹き付けによる冷却（ＧＪ）、水冷ロール抜熱による冷却（ＲＱ）のいずれかの方法で行い、上記冷却後には図２に示す通り焼き戻しを行った。尚、汽水（ミスト）冷却の場合は、焼き戻し後に、液温：５０℃で濃度：５質量％の塩酸に５秒間浸漬（酸洗）した。表２の加熱温度、徐冷終点温度、焼き戻し温度は図２に示す箇所での温度を示している。また露点は連続焼鈍炉の雰囲気露点である。 A steel material having the chemical composition shown in Table 1 was melted and cast using a slab obtained by casting, and then pickled. The manufacturing conditions are shown in Table 2. The pickling was performed using an aqueous hydrochloric acid solution having a temperature of 70 to 90 ° C. and a concentration of 1 to 18% by mass. Thereafter, cold rolling was performed to obtain a steel plate having a thickness of 1.4 mm. And the continuous annealing was given to the steel plate obtained by the method shown in FIG. Cooling after soaking and gradual cooling in continuous annealing is performed by any one of brackish water (mist) cooling, gas spray cooling (GJ), and water cooling roll heat removal (RQ). Tempering was performed as shown in FIG. In the case of brackish water (mist) cooling, after tempering, it was immersed (pickled) for 5 seconds in hydrochloric acid having a liquid temperature of 50 ° C. and a concentration of 5% by mass. The heating temperature, annealing end point temperature, and tempering temperature in Table 2 indicate the temperatures at the locations shown in FIG. The dew point is the atmospheric dew point of the continuous annealing furnace.
フェライト（Ｆ）、ベイナイト（Ｂ）及び／又は炭化物、マルテンサイト（Ｍ）、残留オーステナイト（γＲ）の面積率を次の様にして調べた。即ち、鋼板をレペラー腐食し、光学顕微鏡（倍率４００倍）観察により組織を同定した後、光学顕微鏡写真（倍率４００倍）におけるフェライトと、ベイナイト及び／又は炭化物の面積率を算出した。残留オーステナイトはＸ線回折によりその含有量を求めた。マルテンサイトは、ナイタール腐食させた鋼板をＳＥＭ（倍率１５００倍）で観察してその存在を確認した上で、上記フェライト、ベイナイト、残留オーステナイトの残部組織としてその割合を計算により求めた。 The area ratio of ferrite (F), bainite (B) and / or carbide, martensite (M), and retained austenite (γ R ) was examined as follows. That is, after the steel plate was repeller-corroded and the structure was identified by observation with an optical microscope (400 times magnification), the area ratio of ferrite and bainite and / or carbide in the optical microscope photograph (400 times magnification) was calculated. The content of residual austenite was determined by X-ray diffraction. Martensite was obtained by observing the steel plate subjected to nital corrosion with SEM (magnification 1500 times) and confirming its presence, and then calculating the ratio as the remaining structure of the ferrite, bainite, and retained austenite by calculation.
また得られた鋼板を用いて、機械的特性および塗膜密着性を評価した。機械的特性は、ＪＩＳ５号試験片を採取して測定し、引張強度（ＴＳ）、Ｅｌ（全伸び）および降伏点（ＹＰ）を求め、引張強度（ＴＳ）が３９０ＭＰａ以上で、引張強度と伸びの積（ＴＳ×Ｅｌ）が１４０００以上の場合を、機械的特性に優れていると評価した。 Moreover, the mechanical characteristics and coating-film adhesiveness were evaluated using the obtained steel plate. Mechanical properties were measured by taking a JIS No. 5 test piece and determining the tensile strength (TS), El (total elongation) and yield point (YP). The tensile strength (TS) was 390 MPa or more, and the tensile strength and elongation. When the product (TS × El) was 14000 or more, it was evaluated that the mechanical properties were excellent.
塗膜密着性として、化成処理性とクラックの有無を調べた。化成処理性は、鋼板表面の酸化物の状態を下記の様にして調べ、かつ下記条件で化成処理を行って化成処理後の鋼板表面を１０００倍でＳＥＭ観察し、１０視野のりん酸亜鉛結晶の付着状態を調べた。そして１０視野全てにおいてりん酸亜鉛結晶が均一に付着している場合を「○」、りん酸亜鉛結晶の付着していない部分が１視野でも存在する場合を「×」と評価した。その結果を表３に示す。
・化成処理液：日本パーカライジング社製 パルボンド Ｌ ３０２０
・化成処理工程：脱脂 → 水洗 → 表面調整 → 化成処理
As the coating film adhesion, chemical conversion property and presence of cracks were examined. For the chemical conversion treatment, the state of the oxide on the surface of the steel sheet was examined as follows, and the chemical conversion treatment was performed under the following conditions, and the steel sheet surface after the chemical conversion treatment was observed by SEM at 1000 times, and 10 phosphate crystals of zinc phosphate were observed. The adhesion state of was investigated. The case where the zinc phosphate crystals were uniformly attached in all 10 fields of view was evaluated as “◯”, and the case where the portion where no zinc phosphate crystals were adhered was present as “x”. The results are shown in Table 3.
・ Chemical conversion treatment liquid: Palbond L 3020 manufactured by Nihon Parkerizing Co., Ltd.
・ Chemical conversion treatment process: degreasing → washing → surface adjustment → chemical conversion treatment
Ｍｎ−Ｓｉ酸化物の個数は、鋼材表面の抽出レプリカ膜を作製し、これを１５０００倍でＴＥＭ観察し（日立製作所製 Ｈ−８００）、任意の２０視野の平均個数（１００μｍ２あたり）を調べた。 As for the number of Mn-Si oxides, an extraction replica film on the surface of a steel material was prepared, and this was observed with a TEM at 15000 times (H-800, manufactured by Hitachi, Ltd.), and the average number (per 100 μm 2 ) of 20 arbitrary fields was examined. It was.
Ｓｉを主体とする酸化物の鋼板表面被覆率は、抽出レプリカ法で処理したサンプルをＴＥＭで観察し、画像解析法で被覆率を求めた。尚、抽出レプリカ法は、下記（ａ）〜（ｄ）の手順に添って行った。 As for the steel sheet surface coverage of the oxide mainly composed of Si, the sample treated by the extraction replica method was observed by TEM, and the coverage was determined by an image analysis method. The extraction replica method was performed according to the following procedures (a) to (d).
（ａ）鋼材の表面にカーボンを蒸着させる。
（ｂ）サンプル平面上に２〜３ｍｍ角の碁盤目状の切れ目を入れる。
（ｃ）１０％アセチルアセトン−９０％メタノールエッチング液で腐食させてカーボンを浮上させる。
（ｄ）アルコール中に保存して観察に用いる。
(A) Carbon is vapor-deposited on the surface of the steel material.
(B) A grid-like cut of 2 to 3 mm square is made on the sample plane.
(C) The carbon is levitated by being corroded with 10% acetylacetone-90% methanol etching solution.
(D) Store in alcohol and use for observation.
この様に処理したサンプルを用いてＴＥＭにて、倍率１５０００倍で１０視野分の写真（１３ｃｍ×１１ｃｍ）を撮影し、Ｓｉを主体とする酸化物（酸化物を構成する酸素以外の元素のうちＳｉが原子比で６７％超を占めるもの）の面積を測定し、Ｓｉを主体とする酸化物の被覆率を求めた。 Using the sample processed in this manner, a TEM image of 10 fields of view (13 cm × 11 cm) was taken at a magnification of 15000 times, and an oxide mainly composed of Si (among elements other than oxygen constituting the oxide) The area of Si (over 67% by atomic ratio) was measured, and the coverage of the oxide mainly composed of Si was determined.
またクラックの有無は、ＳＥＭ（日立製作所製 Ｓ−４５００）を用いて２０００倍で、鋼板断面の表面近傍における任意の１０視野（１視野：１３ｃｍ×１１ｃｍ）を観察して調べた。これらの測定結果を表３に示す。 The presence or absence of cracks was examined using an SEM (S-4500, manufactured by Hitachi, Ltd.) at a magnification of 2000 and by observing any 10 visual fields (1 visual field: 13 cm × 11 cm) near the surface of the cross section of the steel sheet. These measurement results are shown in Table 3.
表１〜３から、以下の様に考察できる（尚、下記Ｎｏ．は実験Ｎｏ．を示す）。即ちＮ
ｏ．３０は、本発明鋼板１としての規定要件を満たしているため化成処理性に優れており、塗膜密着性に優れている。該実施例において、クラックを抑制してより優れた塗膜密着性を確保するには、製造条件として特に巻取温度や徐冷終了温度を制御するのがよいことがわかる。
From Tables 1 to 3, the following can be considered (the following No. indicates the experiment No.). Ie N
o. No. 30 satisfies the prescribed requirements as the steel sheet 1 of the present invention, and therefore has excellent chemical conversion treatment properties and excellent coating film adhesion. In this example, it can be seen that, in order to suppress cracks and ensure better coating film adhesion, it is particularly preferable to control the coiling temperature and the annealing end temperature as production conditions.
Ｎｏ．２２、２３は、本発明鋼板２として規定する要件を満たしているため、クラックが発生しておらず、塗膜密着性に優れた鋼板が得られている。該実施例において、化成処理性を確保して塗膜密着性をより高めるには、成分組成を制御して鋼板表面に析出する酸化物の形態を規定の通りにするのがよい。 No. Since Nos. 22 and 23 satisfy the requirements defined as the steel plate 2 of the present invention, cracks are not generated, and a steel plate having excellent coating film adhesion is obtained. In this example, in order to ensure the chemical conversion treatment and further improve the adhesion of the coating film, it is preferable to control the component composition so that the form of the oxide deposited on the steel sheet surface is as specified.
またＮｏ．１〜５、７、９、１０、１２〜２０、２７、２８、３１は、本発明鋼板３で規定する要件（即ち、本発明鋼板１および本発明鋼板２で規定する要件）を満足しているため、優れた化成処理性を確保でき、かつクラックの発生が抑制されて優れた塗膜密着性を発揮する。 No. 1 to 5, 7, 9, 10, 12 to 20, 27, 28, 31 satisfy the requirements defined by the steel plate 3 of the present invention (that is, requirements defined by the steel plate 1 of the present invention and the steel plate 2 of the present invention). Therefore, excellent chemical conversion treatment properties can be secured, and the occurrence of cracks is suppressed and excellent coating film adhesion is exhibited.
これらに対し、Ｎｏ．６、８、１１、２１、２４〜２６、２９は、本発明鋼板１〜３の要件をいずれも満たしておらず、塗膜密着性に優れていないか、強度−延性バランスに優れておらず、高強度でかつ優れた加工性を発揮するものが得られていない。 In contrast, no. 6, 8, 11, 21, 24 to 26, 29 do not satisfy any of the requirements of the steel plates 1 to 3 of the present invention, and are not excellent in coating film adhesion or in strength-ductility balance. However, a material having high strength and excellent workability has not been obtained.
Ｎｏ．６、８、１１、２１は、本発明で規定する成分組成を満足しないため、機械的特性に劣るか塗膜密着性に劣る結果となった。即ち、Ｎｏ．６、１１はＳｉ量が過剰であり、Ｓｉ／Ｍｎ比も上限を超えているため、規定する鋼板表面とならず、塗膜密着性に劣る結果となった。Ｎｏ．８はＳｉ量が少ないため、強度−延性バランスに劣るものとなった。またＮｏ．２１は、Ｃ量が少なすぎるため、規定の強度を確保できていない。 No. Since 6, 8, 11, and 21 do not satisfy the component composition prescribed | regulated by this invention, it resulted in inferior to a mechanical characteristic or inferior coating-film adhesiveness. That is, no. Nos. 6 and 11 had an excessive amount of Si, and the Si / Mn ratio exceeded the upper limit, so that the surface of the steel sheet was not defined and the coating film adhesion was poor. No. No. 8 was inferior in strength-ductility balance due to a small amount of Si. No. No. 21 cannot secure the prescribed strength because the amount of C is too small.
Ｎｏ．２４〜２６、２９は、推奨する条件で製造せず、本発明で規定する酸化物の形態でないため化成処理性に劣っており、またクラックも発生して塗膜密着性に劣っている。 No. 24 to 26 and 29 are not manufactured under the recommended conditions, and are not in the form of oxides defined in the present invention, so that they are inferior in chemical conversion treatment, and cracks are also generated, resulting in poor coating film adhesion.
即ち、Ｎｏ．２４は、熱間圧延後の巻取温度が高いため熱延でのＳｉ表面濃化が助長され、Ｎｏ．２５は、酸洗時間が短いため濃化Ｓｉ層の除去が不足し、またＮｏ．２６は露点が高いため焼鈍段階でＳｉの表面濃化が促進されて、いずれもＳｉ主体の酸化物が多量に存在し、また粒界にもＳｉ酸化物が生成して酸洗後にクラックが発生し、塗膜密着性に劣る結果となった。 That is, no. No. 24 has a high coiling temperature after hot rolling, which facilitates Si surface concentration in hot rolling. No. 25 has a short pickling time, so that removal of the concentrated Si layer is insufficient. No. 26 has a high dew point, so that the surface concentration of Si is promoted in the annealing stage, and in each case, a large amount of Si-based oxides are present, and Si oxides are also generated at the grain boundaries, causing cracks after pickling. As a result, the coating film adhesion was inferior.
Ｎｏ．２９では焼鈍後にミスト冷却を行っており、この様に高温で水蒸気雰囲気に曝すことで、表面と粒界のどちらにもＳｉ主体の酸化物が多量に生成したため化成処理性に劣る結果となった。また、その後の酸洗工程で上記酸化物が溶解してクラックが発生したため塗膜密着性に著しく劣るものとなった。 No. In No. 29, mist cooling was performed after annealing, and exposure to a steam atmosphere at a high temperature in this manner resulted in a large amount of Si-based oxides on both the surface and grain boundaries, resulting in poor chemical conversion treatment performance. . Moreover, since the said oxide melt | dissolved in the subsequent pickling process and the crack generate | occur | produced, it became inferior to coating-film adhesiveness significantly.
参考までに、本実施例で得られた鋼板の抽出レプリカをＴＥＭ観察した顕微鏡写真、及び化成処理後の鋼板表面のＳＥＭ観察写真を示す。図３は、比較例であるＮｏ．１１の鋼板表面におけるＴＥＭ観察写真であるが、この図３から、鋼板表層領域がＳｉ主体の酸化物層で覆われていることがわかる。 For reference, a micrograph obtained by TEM observation of an extracted replica of the steel sheet obtained in this example and a SEM observation photograph of the steel sheet surface after chemical conversion treatment are shown. FIG. FIG. 3 shows that the steel sheet surface layer region is covered with an Si-based oxide layer.
また図４は、上記鋼板を化成処理した後の表面をＳＥＭで観察した顕微鏡写真である。該図４から、Ｎｏ．１１ではりん酸亜鉛結晶が密に形成されていないことがわかる。 FIG. 4 is a photomicrograph of the surface of the steel sheet after chemical conversion treatment observed with an SEM. From FIG. 11 shows that zinc phosphate crystals are not densely formed.
これに対し図５は、本発明例であるＮｏ．３の鋼板表面におけるＴＥＭ観察写真であるが、鋼板表層領域に上記Ｎｏ．１１の様な層は形成されておらず、代わりにＭｎ主体のＭｎ−Ｓｉ複合酸化物が微細に分散している。つまり、図５から、Ｎｏ．３の鋼板表層領域には、化成処理性を低下させるＳｉ主体の酸化物はほとんどなく、化成処理性の向上に有効なＭｎ−Ｓｉ複合酸化物が多数存在していることを確認できる。 On the other hand, FIG. 3 is a TEM observation photograph on the steel plate surface. 11 is not formed, and instead, Mn-based Mn—Si composite oxide is finely dispersed. That is, from FIG. In the steel plate surface layer region 3, there is almost no Si-based oxide that lowers the chemical conversion processability, and it can be confirmed that there are many Mn—Si composite oxides effective for improving the chemical conversion processability.
図６は、上記鋼板を化成処理した後の表面をＳＥＭで観察した顕微鏡写真であるが、該図６から、Ｎｏ．３ではりん酸亜鉛結晶が小さく隙間がないことがわかる。 FIG. 6 is a photomicrograph of the surface of the steel sheet after chemical conversion treatment, which was observed with an SEM. 3 indicates that the zinc phosphate crystals are small and have no gaps.
Claims (5)
Ｃ ：０．０３〜１％、
Ｍｎ：０．２〜３％、
Ｓｉ：０．１〜１％、
Ｓｉ／Ｍｎ≦０．４
を満たし、残部が鉄および不可避不純物であり、
金属組織が、占積率で（金属組織について以下同じ）、
フェライト：６０％以上、
ベイナイトおよび／または炭化物：５％以上、
該フェライトとベイナイトおよび／または炭化物との合計：９０％以上、
マルテンサイト：３％以下（０％含む）、
残留オーステナイト：３％以下（０％含む）を満たす複合組織鋼板であって、
鋼板表面において、ＭｎとＳｉの原子比（Ｍｎ／Ｓｉ）が０．５以上である長径０．０１μｍ以上５μｍ以下のＭｎ−Ｓｉ複合酸化物が１０個／１００μｍ2以上存在すると共に、Ｓｉを主体とする酸化物の鋼板表面被覆率が１０％以下であり、
引張強度が３９０ＭＰａ以上で、かつ引張強度（ＴＳ：単位MPa）と伸び（Ｅｌ：単位
％）が下記式（１）を満たすことを特徴とする塗膜密着性と加工性に優れた高強度冷延鋼板。
ＴＳ×Ｅｌ≧１４０００ …（１） % By mass (the same applies to chemical components)
C: 0.03 to 1%,
Mn: 0.2-3%,
Si: 0.1 to 1%,
Si / Mn ≦ 0.4
The balance is iron and inevitable impurities,
The metal structure is the space factor (the same applies to the metal structure below)
Ferrite: 60% or more,
Bainite and / or carbide: 5% or more,
Total of the ferrite and bainite and / or carbide: 90% or more,
Martensite: 3% or less (including 0%),
Residual austenite: a composite structure steel plate satisfying 3% or less (including 0%),
In the steel sheet surface, with the atomic ratio of Mn and Si (Mn / Si) is 5μm or less of Mn-Si composite oxide major diameter 0.01μm or more and 0.5 or more is present 10/100 [mu] m 2 or more, mainly of Si The steel sheet surface coverage of the oxide is 10% or less,
A high-strength cold with excellent coating film adhesion and workability, characterized by having a tensile strength of 390 MPa or more and a tensile strength (TS: unit MPa) and elongation (El: unit%) satisfy the following formula (1): Rolled steel sheet.
TS × El ≧ 14000 (1)
Ｍｎ：０．２〜３％、
Ｓｉ：０．１〜１％
を満たし、残部が鉄および不可避不純物であり、
金属組織が、
フェライト：６０％以上、
ベイナイトおよび／または炭化物：５％以上、
該フェライトとベイナイトおよび／または炭化物との合計：９０％以上、
マルテンサイト：３％以下（０％含む）、
残留オーステナイト：３％以下（０％含む）を満たす複合組織鋼板であって、
ＳＥＭを用いて２０００倍で鋼板表面近傍の断面を観察したときに、任意の１０視野において幅３μｍ以下で深さ５μｍ以上のクラックが存在せず、
引張強度が３９０ＭＰａ以上で、かつ引張強度（ＴＳ：単位MPa）と伸び（Ｅｌ：単位
％）が下記式（１）
ＴＳ×Ｅｌ≧１４０００ …（１）
を満たし、
下記（ａ）〜（ｃ）の工程を行なうことによって、前記クラックを発生させないようにすることを特徴とする塗膜密着性と加工性に優れた高強度冷延鋼板。
（ａ）熱間圧延の巻き取り温度を５００℃以下とし、
（ｂ）熱間圧延後、液温が７０〜９０℃で１〜１８質量％の塩酸に４０秒間以上浸漬し、
（ｃ）連続焼鈍時の露点を−４０℃以下とする。 C: 0.03 to 1%,
Mn: 0.2-3%,
Si: 0.1 to 1%
The balance is iron and inevitable impurities,
The metal structure is
Ferrite: 60% or more,
Bainite and / or carbide: 5% or more,
Total of the ferrite and bainite and / or carbide: 90% or more,
Martensite: 3% or less (including 0%),
Residual austenite: a composite structure steel plate satisfying 3% or less (including 0%),
When observing a cross section in the vicinity of the steel sheet surface at 2000 times using SEM, there are no cracks having a width of 3 μm or less and a depth of 5 μm or more in any 10 fields of view,
The tensile strength is 390 MPa or more, and the tensile strength (TS: unit MPa) and elongation (El: unit%) are represented by the following formula (1).
TS × El ≧ 14000 (1)
Meet the,
A high-strength cold-rolled steel sheet excellent in coating film adhesion and workability , wherein the cracks are not generated by performing the following steps (a) to (c) .
(A) The hot rolling coiling temperature is 500 ° C. or less,
(B) After hot rolling, the liquid temperature is 70-90 ° C. and immersed in 1-18 mass% hydrochloric acid for 40 seconds or more,
(C) The dew point at the time of continuous annealing is set to −40 ° C. or lower.
Ｍｎ：０．２〜３％、
Ｓｉ：０．１〜１％、
Ｓｉ／Ｍｎ≦０．４
を満たし、残部が鉄および不可避不純物であり、
金属組織が、
フェライト：６０％以上、
ベイナイトおよび／または炭化物：５％以上、
該フェライトとベイナイトおよび／または炭化物との合計：９０％以上、
マルテンサイト：３％以下（０％含む）、
残留オーステナイト：３％以下（０％含む）を満たす複合組織鋼板であって、
（I）鋼板表面において、ＭｎとＳｉの原子比（Ｍｎ／Ｓｉ）が０．５以上である長径
０．０１μｍ以上５μｍ以下のＭｎ−Ｓｉ複合酸化物が１０個／１００μｍ2以上存在すると共に、Ｓｉを主体とする酸化物の鋼板表面被覆率が１０％以下であり、かつ
（II）ＳＥＭを用いて２０００倍で鋼板表面近傍の断面を観察したときに、任意の１０視野において幅３μｍ以下で深さ５μｍ以上のクラックが存在せず、
引張強度が３９０ＭＰａ以上で、かつ引張強度（ＴＳ：単位MPa）と伸び（Ｅｌ：単位
％）が下記式（１）を満たすことを特徴とする塗膜密着性と加工性に優れた高強度冷延鋼板。
ＴＳ×Ｅｌ≧１４０００ …（１） C: 0.03 to 1%,
Mn: 0.2-3%,
Si: 0.1 to 1%,
Si / Mn ≦ 0.4
The balance is iron and inevitable impurities,
The metal structure is
Ferrite: 60% or more,
Bainite and / or carbide: 5% or more,
Total of the ferrite and bainite and / or carbide: 90% or more,
Martensite: 3% or less (including 0%),
Residual austenite: a composite structure steel plate satisfying 3% or less (including 0%),
In (I) the steel sheet surface, with the atomic ratio of Mn and Si (Mn / Si) is 5μm or less of Mn-Si composite oxide major diameter 0.01μm or more and 0.5 or more is present 10/100 [mu] m 2 or more, The steel plate surface coverage of the oxide mainly composed of Si is 10% or less, and (II) When the cross section near the steel plate surface is observed at 2000 times using SEM, the width is 3 μm or less in any 10 fields of view. There are no cracks with a depth of 5 μm or more,
A high-strength cold with excellent coating film adhesion and workability, characterized by having a tensile strength of 390 MPa or more and a tensile strength (TS: unit MPa) and elongation (El: unit%) satisfy the following formula (1): Rolled steel sheet.
TS × El ≧ 14000 (1)
Ｎｂ：０．１％以下（０％含まない）、
Ｔｉ：０．２％以下（０％含まない）、および
Ｖ ：０．２％以下（０％含まない）よりなる群から選択される１種以上を含有する請求項１〜３のいずれかに記載の高強度冷延鋼板。 As other elements,
Nb: 0.1% or less (excluding 0%),
Ti: 0.2% or less (not including 0%), and V: to any one of claims 1 to 3 containing at least one selected from the group consisting of 0.2% or less (not including 0%) The high-strength cold-rolled steel sheet described.
Ｃｒ：０．５％以下（０％含まない）、
Ｍｏ：０．５％以下（０％含まない）、および
Ｗ ：０．５％以下（０％含まない）よりなる群から選択される１種以上を含有する請求項１〜４のいずれかに記載の高強度冷延鋼板。 As other elements,
Cr: 0.5% or less (excluding 0%),
Mo: 0.5% or less (not including 0%), and W: to any one of claims 1 to 4 containing at least one selected from the group consisting of 0.5% or less (not including 0%) The high-strength cold-rolled steel sheet described.
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| JP5399681B2 (en) | 2008-10-08 | 2014-01-29 | Ｊｆｅスチール株式会社 | High workability and high strength steel pipe excellent in chemical conversion and process for producing the same |
| JP5853683B2 (en) | 2011-01-25 | 2016-02-09 | Ｊｆｅスチール株式会社 | Method for producing cold-rolled steel sheet with excellent chemical conversion and post-coating corrosion resistance |
| JP5929498B2 (en) * | 2012-05-21 | 2016-06-08 | 新日鐵住金株式会社 | Aluminum plated steel sheet for fuel tank with excellent corrosion resistance and method for producing the same |
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| US20210395895A1 (en) | 2018-11-09 | 2021-12-23 | Jfe Steel Corporation | Cold rolled steel sheet for zirconium-based chemical conversion treatment, method for producing same, zirconium-based chemical conversion-treated steel sheet, and method for producing same |
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