JP2007277585A - High-strength hot-rolled steel sheet excellent in chemical convertibility - Google Patents
High-strength hot-rolled steel sheet excellent in chemical convertibility Download PDFInfo
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本発明は、高強度を有すると共に、リン酸塩処理などの化成処理性に優れた熱延鋼板に関するものである。 The present invention relates to a hot-rolled steel sheet having high strength and excellent chemical conversion treatment properties such as phosphate treatment.
最近、自動車などの軽量化に伴う燃費向上、更には排ガス低減の観点から、鋼材の一層の高強度化が求められており、特に冷延鋼板についてはハイテン化（高強度化）が急速に進んできている。しかし半面、衝突安全性の観点から剛性強化用として厚物高強度鋼板に対する要求も少なくない。こうした要望に対しては、設備面やコスト面から冷延鋼板による対応が困難であり、熱延鋼板で対応することが必要となる。 Recently, from the viewpoint of improving fuel economy and reducing exhaust gas due to the weight reduction of automobiles and the like, there has been a demand for further strengthening of steel materials, especially for cold-rolled steel plates, which are rapidly becoming high-tensile (high-strength). is made of. However, on the other hand, from the viewpoint of collision safety, there are many requests for thick high-strength steel sheets for strengthening rigidity. It is difficult to deal with such demands with cold-rolled steel sheets in terms of equipment and costs, and it is necessary to deal with hot-rolled steel sheets.
そして熱延鋼板を使用する場合でも、製品形状に加工する際には冷延鋼板と同様にプレス成形されるので、高強度化を指向するにしても伸びなどの延性を軽視することはできない。しかし、熱延鋼板に限らず素材一般の性質として、高強度化すればするほど延性が低下し加工性が悪くなる。そこで鉄鋼材では、延性を低下させることなく強度を高めることのできる合金元素の添加が検討されている。 Even when a hot-rolled steel sheet is used, since it is press-formed in the same way as a cold-rolled steel sheet when processed into a product shape, ductility such as elongation cannot be neglected even if the strength is increased. However, not only hot-rolled steel sheets, but as general properties of materials, the higher the strength, the lower the ductility and the worse the workability. Therefore, for steel materials, the addition of alloying elements that can increase the strength without reducing ductility has been studied.
中でもＭｏは、延性をそれほど低下させることなく強度を高める有用な元素として注目されている。しかもＭｏは、熱延終了後の冷却過程で生じるフェライト組織の生成を抑えて高強度化に寄与するベイナイト組織の生成を助長し、熱延前のスラブ加熱温度を高めたり低温巻取りを採用したりするといったプロセス制御の必要もないことから、高強度熱延鋼板用の添加合金元素としても注目されている。 Among these, Mo is attracting attention as a useful element that increases strength without significantly reducing ductility. Moreover, Mo promotes the formation of a bainite structure that contributes to high strength by suppressing the formation of a ferrite structure that occurs during the cooling process after the end of hot rolling, and increases the slab heating temperature before hot rolling and employs low-temperature winding. Therefore, it is also attracting attention as an additive alloy element for high-strength hot-rolled steel sheets.
ところが、合金元素としてＭｏを添加すると鋼板の化成処理性が悪化し、ひいては電着塗装後の塗膜の密着不良などによって仕上り製品の外観や耐食性などに悪影響を及ぼす。 However, when Mo is added as an alloy element, the chemical conversion property of the steel sheet is deteriorated, and as a result, the adhesion of the coated film after electrodeposition coating is adversely affected and the appearance and corrosion resistance of the finished product are adversely affected.
他方、鋼板の化成処理性を改善するため、鋼板素材の表面性状（たとえば、微視的凹凸パターンなど）を改善する方法は幾つか提案されている。 On the other hand, in order to improve the chemical conversion treatment properties of steel plates, several methods for improving the surface properties (for example, microscopic unevenness pattern) of the steel plate materials have been proposed.
例えば特許文献１には、鋼板表面の微視的形状を規定することによって化成処理性を高めた熱延酸洗鋼板が開示されている。この技術は、高エネルギー密度ビームで鋼板表面に凹凸模様を形成したロール（ダルロール）を用いてスキンパス圧延し、ロール表面の該凹凸模様を鋼板表面に転写することにより表面性状を整えるものである。しかしこの方法は、ロールのダル加工やスキンパス圧延といった工程付加によるコストアップが避けられず、しかも、特にＭｏ添加鋼に対しては満足のいく効果が得られない。 For example, Patent Document 1 discloses a hot-rolled pickled steel sheet that has improved chemical conversion properties by defining a microscopic shape of the steel sheet surface. In this technique, the surface texture is adjusted by performing skin pass rolling using a roll (dull roll) having a concavo-convex pattern formed on the surface of a steel sheet with a high energy density beam and transferring the concavo-convex pattern on the surface of the roll to the surface of the steel sheet. However, this method cannot avoid a cost increase due to additional processes such as roll dull processing and skin pass rolling, and in particular, a satisfactory effect cannot be obtained for Mo-added steel.
また特許文献２には、Ｔｉ添加熱延高張力鋼板の平均結晶粒径を３．０μｍ以下に抑えると共に、表面粗さ（Ｒａ）を１．５μｍ以下に抑えることで化成処理性を高める方法が開示されている。しかしこの方法も、Ｍｏ添加鋼に対しては意図する様な効果が得られない。 Patent Document 2 discloses a method for improving the chemical conversion treatment property by suppressing the average crystal grain size of the Ti-added hot-rolled high-tensile steel sheet to 3.0 μm or less and suppressing the surface roughness (Ra) to 1.5 μm or less. It is disclosed. However, this method also cannot achieve the intended effect on Mo-added steel.
更に特許文献３には、鋼板表面の微視的凹凸パターンを制御する技術が開示されている。しかしこの技術は、鋼板の塗装鮮映性とプレス加工性の向上を目的とするもので、その凹凸パターンは、凸部の直径を５０〜２００μｍの範囲に制御するものであって、本発明で化成処理性に影響を及ぼす因子として注目するリン酸亜鉛の結晶サイズである数μｍよりもはるかに大きいため、化成処理性の向上には殆ど寄与しない。
本発明は上記の様な事情に着目してなされたものであって、その目的は、Ｍｏを含まない高強度熱延鋼板は勿論のこと、高強度化を期してＭｏを添加した熱延鋼板であっても、安定して優れた化成処理性を発揮し得る様な高強度熱延鋼板を提供することにある。 The present invention has been made by paying attention to the above-described circumstances, and its purpose is not only high-strength hot-rolled steel sheet not containing Mo, but also hot-rolled steel sheet to which Mo is added for higher strength. Even so, an object is to provide a high-strength hot-rolled steel sheet that can stably exhibit excellent chemical conversion properties.
上記課題を解決することのできた本発明に係る高強度熱延鋼板は、３９０ＭＰａレベル以上、更には７８０ＭＰａレベル以上の引張強度を有する高強度熱延鋼板であって、該鋼板表面に存在する凹凸の最大深さ（Ｒｙ）が１０μｍ以上、該凹凸の平均間隔（Ｓｍ）が３０μｍ以下という要件を満たす他、
１）表面凹凸の負荷長さ率（ｔｐ４０）が２０％以下、および
２）表面凹凸の負荷長さ率（ｔｐ６０）と同（ｔｐ４０）の差が６０％以上、
という２つの要件のうちいずれか１方の要件を満たし、より好ましくは、これら１），２）の要件を同時に満たす化成処理性に優れた高強度熱延鋼板である。
The high-strength hot-rolled steel sheet according to the present invention that has solved the above-mentioned problems is a high-strength hot-rolled steel sheet having a tensile strength of 390 MPa level or higher, and further 780 MPa level or higher, and has unevenness present on the steel sheet surface. In addition to satisfying the requirement that the maximum depth (Ry) is 10 μm or more and the average interval (Sm) of the irregularities is 30 μm or less,
1) The load length ratio (tp40) of the surface unevenness is 20% or less, and 2) The difference between the load length ratio (tp60) of the surface unevenness (tp40) is 60% or more,
It is a high-strength hot-rolled steel sheet that satisfies any one of the two requirements, and more preferably has excellent chemical conversion properties that simultaneously satisfies the requirements 1) and 2).
本発明に係る上記鋼板の成分組成は、要求強度に応じて任意に変更できるが、好ましいのは、基本成分としてＣ：０．０３〜１．０％、Ｓｉ：２％以下（０％を含む）、Ｍｎ：０．３〜４．０％、Ａｌ：０．００５〜０．５％を満たし、好ましくは更に、高強度化のため、Ｍｏ：０．０５〜１．０％を含み、あるいは、必要に応じて更にＣｒ：１．５％以下（０％を含まない）、Ｔｉ：０．２％以下（０％を含まない）、Ｎｂ：０．１％以下（０％を含まない）、Ｖ：０．１％以下（０％を含まない）、Ｃｕ：１．０％以下（０％を含まない）、Ｎｉ：１．０％以下（０％を含まない）、Ｂ：０．００２％以下（０％を含まない）、Ｃａ：０．００５％以下（０％を含まない）よりなる群から選択される少なくとも１種の元素を含むもので、残部は鉄と不可避不純物からなる鋼である。 The component composition of the steel sheet according to the present invention can be arbitrarily changed according to the required strength, but preferably, C: 0.03 to 1.0%, Si: 2% or less (including 0%) as basic components ), Mn: 0.3 to 4.0%, Al: 0.005 to 0.5%, preferably further Mo: 0.05 to 1.0% for higher strength, or Further, if necessary, Cr: 1.5% or less (not including 0%), Ti: 0.2% or less (not including 0%), Nb: 0.1% or less (not including 0%) V: 0.1% or less (not including 0%), Cu: 1.0% or less (not including 0%), Ni: 1.0% or less (not including 0%), B: 0.0. It contains at least one element selected from the group consisting of 002% or less (not including 0%), Ca: 0.005% or less (not including 0%), Parts are steel consisting of iron and inevitable impurities.
また、本発明に係る高強度熱延鋼板の強度レベルは、用途・目的によっても変わってくるので一律に定めることはできないが、汎用的な強度レベルは引張強度で３９０ＭＰａ以上を有するものである。また、最近における鋼板の高強度化の要請に応えるには、７８０ＭＰａ以上の引張強度を有するものが好ましく、この場合は、Ｍｏを０．０５〜１．０％含有させると共に、Ｃｒを１．５％以下の含有率で含有させるのがよく、更に９００ＭＰａ以上の引張強度を有する高強度鋼板を得るには、Ｍｏ含量を０．０５〜１．０％、Ｃｒ含量を０．３〜１．５％とし、且つ金属組織の８５％以上をベイナイトとするのがよい。 In addition, the strength level of the high strength hot rolled steel sheet according to the present invention cannot be determined uniformly because it varies depending on the application and purpose, but the general strength level has a tensile strength of 390 MPa or more. Further, in order to meet the recent demand for higher strength of steel sheets, those having a tensile strength of 780 MPa or more are preferable. In this case, 0.05 to 1.0% of Mo is contained and 1.5% of Cr is added. In order to obtain a high-strength steel sheet having a tensile strength of 900 MPa or more, the Mo content is 0.05 to 1.0%, and the Cr content is 0.3 to 1.5. And 85% or more of the metal structure is preferably bainite.
本発明によれば、熱延鋼板の表面に存在する凹凸の最大深さ（Ｒｙ）と該凹凸の平均間隔（Ｓｍ）を規定すると共に、当該表面凹凸の負荷長さ率（ｔｐ４０）および／または該負荷長さ率（ｔｐ４０）と同（ｔｐ６０）の差を特定することによって、化成処理性を著しく改善することができ、Ｍｏを含まない熱延鋼板はもとより、化成処理性を劣化させるＭｏを高強度化のため適量含有させた高強度鋼板であっても優れた化成処理性を保証し、強度と化成処理性を兼ね備えた高強度熱延鋼板を安価に提供できる。 According to the present invention, the maximum depth (Ry) of the unevenness existing on the surface of the hot-rolled steel sheet and the average interval (Sm) of the unevenness are defined, and the load length ratio (tp40) of the surface unevenness and / or By specifying the difference between the load length ratio (tp40) and the same (tp60), the chemical conversion processability can be remarkably improved, and not only hot rolled steel sheets not containing Mo but also Mo which deteriorates the chemical conversion processability. Even a high-strength steel sheet containing an appropriate amount for high strength can guarantee excellent chemical conversion treatment, and can provide a high-strength hot-rolled steel sheet having both strength and chemical conversion treatment at low cost.
本発明者らは前述した様な解決課題の下で、特に高強度化の手段としてＭｏを添加した熱延鋼板を対象として、Ｍｏ添加による化成処理性低下の問題を改善すべく鋭意研究を進めてきた。 Under the above-described problems, the present inventors have made extensive research to improve the problem of chemical conversion treatment deterioration due to the addition of Mo, particularly for hot-rolled steel sheets to which Mo has been added as a means for increasing the strength. I came.
その結果、熱延鋼板表面の凹凸の最大深さ（Ｒｙ）を「１０μｍ以上」、該凹凸の平均間隔（Ｓｍ）を「３０μｍ以下」に特定すると共に、表面凹凸の負荷長さ率（ｔｐ４０）を２０％以下に、および／または該凹凸の負荷長さ率（ｔｐ６０）と同（ｔｐ４０）の差［（ｔｐ６０）−（ｔｐ４０）］を６０％以上に調整してやれば、Ｍｏを含まない熱延鋼板はもとより、適量のＭｏが添加された熱延鋼板であっても化成処理性の低下が可及的に抑えられ、優れた化成処理性と強度を兼ね備えた熱延鋼板が得られることをつき止めた。 As a result, the maximum depth (Ry) of the unevenness on the surface of the hot-rolled steel sheet is specified as “10 μm or more”, the average interval (Sm) of the unevenness is specified as “30 μm or less”, and the load length ratio of the surface unevenness (tp40) Is adjusted to 20% or less and / or the difference [(tp60) − (tp40)] between the load length ratio (tp60) and the same (tp40) of the unevenness to 60% or more, the hot rolling not including Mo Not only steel sheets, but also hot-rolled steel sheets to which an appropriate amount of Mo has been added, a decrease in chemical conversion property is suppressed as much as possible, and a hot-rolled steel sheet having excellent chemical conversion property and strength can be obtained. stopped.
本発明で規定する表面凹凸の上記最大深さ（Ｒｙ）とは、例えば図１に示す如く表面粗さ曲線の最高山頂（Ｒｔ）と最深谷底（Ｒｂ）との間隔を意味し、該凹凸の平均間隔（Ｓｍ）とは、例えば図２に示す如く表面粗さ曲線における平均線の山から谷に変わる点を変化点として、変化点から次の変化点までの間隔（Ｓ１，Ｓ２……Ｓｎ）の平均値を意味する。また負荷長さ率（ｔｐ）とは、例えば図３に示す如く表面粗さ曲線をある切断線レベル（ｐ）で切断したときの切断部分長さ（ｌ１，ｌ２……ｌｎ）の測定長さ（Ｌ）に対する百分率を意味し、上記切断線レベル（ｐ）が最高山頂（Ｒｔ）であるものは０（ゼロ）で（ｔｐ０）、最深谷底（Ｒｂ）であるものは１００で（ｔｐ１００）と表わされる。そして、該切断線レベル（ｐ）が「４０」または「６０」であるときの上記切断部分長さ（ｌ１＋ｌ２＋ｌ３＋……ｌｎ）の測定長さ（Ｌ）に対する百分率が、（ｔｐ４０）または（ｔｐ６０）で表わされる値である。 The above-mentioned maximum depth (Ry) of the surface irregularities defined in the present invention means the distance between the highest peak (Rt) and the deepest valley bottom (Rb) of the surface roughness curve as shown in FIG. For example, as shown in FIG. 2, the average interval (Sm) is a point (S 1 , S 2 ...) From a change point to the next change point, with a change point from a peak to a valley of the average line in the surface roughness curve. ... means the average value of S n ). The load length ratio (tp) is, for example, the length of the cut portion (l 1 , l 2 ... L n ) when the surface roughness curve is cut at a certain cutting line level (p) as shown in FIG. It means the percentage with respect to the measurement length (L), and when the cutting line level (p) is the highest peak (Rt), it is 0 (zero) and (tp0) is the deepest valley (Rb). tp100). The percentage of the cut portion length (l 1 + l 2 + l 3 +... N n ) when the cut line level (p) is “40” or “60” with respect to the measured length (L) is It is a value represented by (tp40) or (tp60).
そして、上記表面凹凸の最大深さ（Ｒｙ）が「１０μｍ以上」、平均間隔（Ｓｍ）が「３０μｍ以下」で、且つ、上記表面凹凸の負荷長さ率（ｔｐ４０）が２０％以下、および／または同負荷長さ率（ｔｐ６０）と同（ｔｐ４０）の差［（ｔｐ６０）−（ｔｐ４０）］が６０％以上であるものは、Ｍｏを含まない鋼材は勿論のこと、適量のＭｏを含有する熱延鋼板であっても安定して優れた化成処理性を発揮し得ることが確認されたのである。 And the maximum depth (Ry) of the surface irregularities is “10 μm or more”, the average interval (Sm) is “30 μm or less”, and the load length ratio (tp40) of the surface irregularities is 20% or less, and / or Alternatively, the difference [(tp60) − (tp40)] between the same load length ratio (tp60) and (tp40) is 60% or more includes not only steel materials not containing Mo but also an appropriate amount of Mo. It has been confirmed that even hot-rolled steel sheets can stably exhibit excellent chemical conversion properties.
本発明においては、上記の様に表面凹凸の最大深さ（Ｒｙ）が相対的に深く、且つ該凹凸の平均間隔（Ｓｍ）が相対的に小さいほど、表面凹凸が微細で且つ深くリン酸亜鉛結晶の核生成サイトとしての機能が高まり、全面にリン酸亜鉛結晶が生成、成長し易くなって化成処理性が高まるものと考えられる。 In the present invention, as described above, as the maximum depth (Ry) of the surface unevenness is relatively deep and the average interval (Sm) of the unevenness is relatively small, the surface unevenness is fine and deep zinc phosphate. It is considered that the function as a nucleation site of the crystal is enhanced, and the zinc phosphate crystal is easily formed and grown on the entire surface, so that the chemical conversion treatment property is enhanced.
また上記表面凹凸の負荷長さ率（ｔｐ４０）が「２０％以下」（即ち、相対的に小さい）ということは、表面に突出した凸部よりも窪んだ凹部の領域（面積）が相対的に多いことを意味しており、該凹部が同様にリン酸亜鉛結晶の核生成サイトとなってリン酸亜鉛結晶の生成、成長を促し、更に、上記負荷長さ率（ｔｐ６０）と同（ｔｐ４０）の差［（ｔｐ６０）−（ｔｐ４０）］が「６０％以上」（即ち、ｔｐ６０とｔｐ４０の差が相対的に大きい）ということは、凸部の頂部から凹部の底部に渡る斜面が、底部方向に直線状の傾斜面を有しているのではなく湾上に窪んでいることを表わしており、該湾状に窪んだ斜面部分が結晶析出サイトとして機能することでリン酸亜鉛結晶の生成、成長を促し、化成処理性の一層の向上に寄与しているものと考えられる。 Further, the load length ratio (tp40) of the surface unevenness is “20% or less” (that is, relatively small), which means that the area (area) of the recessed portion recessed from the protruding portion protruding on the surface is relatively This means that the concave portion similarly becomes a nucleation site of the zinc phosphate crystal, promotes the formation and growth of the zinc phosphate crystal, and further has the same load length ratio (tp60) (tp40). Difference [(tp60) − (tp40)] is “60% or more” (that is, the difference between tp60 and tp40 is relatively large) means that the slope extending from the top of the convex portion to the bottom of the concave portion is in the bottom direction. It represents that it is depressed on the bay instead of having a linear inclined surface, and the formation of zinc phosphate crystals by the sloped portion recessed in the bay functioning as a crystal precipitation site, Promotes growth and contributes to further improvement in chemical conversion processability It is considered that.
いずれにしても本発明では、後記実施例でも明らかにする如く、上記表面凹凸の最大深さ（Ｒｙ）を「１０μｍ以上」、平均間隔（Ｓｍ）を「３０μｍ以下」にする他、これまでは化成処理性の観点から全く認識されたことのない負荷長さ率（ｔｐ４０）を「２０％以下」、および／または同負荷長さ率（ｔｐ６０）と同（ｔｐ４０）の差［（ｔｐ６０）−（ｔｐ４０）］を「６０％以上」に定めることによって、安定して優れた化成処理性を得ることができたのである。 In any case, in the present invention, the maximum depth (Ry) of the surface irregularities is set to “10 μm or more” and the average interval (Sm) is set to “30 μm or less”, as will be clarified in the examples described later. The load length rate (tp40), which has never been recognized from the viewpoint of chemical conversion treatment, is “20% or less” and / or the difference between the load length rate (tp60) and the same (tp40) [(tp60) − By setting (tp40)] to “60% or more”, it was possible to stably obtain excellent chemical conversion properties.
化成処理性を高める上でより好ましいのは、平均間隔（Ｓｍ）が２０μｍ以下、負荷長さ率（ｔｐ４０）が１５％以下、負荷長さ率の差［（ｔｐ６０）−（ｔｐ４０）］が７０％以上のものである。尚、付加長さ率（ｔＰ６０）の値は特に規定しないが、化成処理性を高める上で好ましいのは６０％以上、より好ましくは７０％以上である。 More preferably, the average interval (Sm) is 20 μm or less, the load length ratio (tp40) is 15% or less, and the difference in load length ratio [(tp60) − (tp40)] is 70 in order to improve the chemical conversion processability. % Or more. Although the value of the additional length ratio (tP60) is not particularly defined, it is preferably 60% or more, more preferably 70% or more in order to improve the chemical conversion treatment property.
上記の様な表面性状とすることで、化成処理によって鋼板表面に析出するリン酸塩結晶はより微細なものとなり、またリン酸塩の健全性の指標であるＰ比、即ちPhosphophyllite（ホスフォフェライト：Ｐ）とHopeite（ホパイト：Ｈ）の比（Ｐ／Ｐ＋Ｈ）はより１に近づき、化成処理性が向上する。またＭｏ添加鋼では、化成処理液中で自然電位が貴な方向に進むため化成処理性が低下するが、上記の様な表面性状にしてやれば、Ｍｏによる化成処理性の劣化を補って余りある優れた化成処理性を得ることができる。 By adopting the surface properties as described above, the phosphate crystals deposited on the steel sheet surface by the chemical conversion treatment become finer, and the P ratio, which is an indicator of the soundness of phosphate, namely Phosphophyllite (phosphoferrite) : P) and Hopeite (Hopeite: H) ratio (P / P + H) is closer to 1, and chemical conversion treatment is improved. In addition, in Mo-added steel, the chemical potential deteriorates because the natural potential proceeds in a noble direction in the chemical conversion solution. However, if the surface properties as described above are used, the deterioration of chemical conversion properties due to Mo is more than compensated. Excellent chemical conversion processability can be obtained.
上記の様な表面性状の熱延鋼板を得るための方法は特に制限されないが、本発明者らの実験によれば、酸洗処理時間を厳しくすることで上記表面性状に近づけることができることを確認している。即ち熱間圧延工程で鋼板表面に生成した酸化物（所謂スケール）を除去するための酸洗は、通常１０〜２０％程度の塩酸水溶液を用いて５０〜８５℃程度で１０〜３０秒程度行なわれるが、本発明で意図する上記表面性状を得るには、酸洗液の塩酸濃度を高めに、酸洗温度を高めに、あるいは酸洗時間を長めに設定することで達成できる。より具体的には、酸洗液の塩酸濃度をＡ（％）、酸洗温度をＢ（℃）、酸洗時間（浸漬時間）をＣ（秒）とした時、これらが下記（Ｉ）式の関係
（Ａ／１００）×Ｂ２×Ｃ≧４００００……（Ｉ）
を満たす様に制御（例えば、１１％ＨＣｌ−７５℃−８０秒、１５％ＨＣｌ−８０℃−５０秒、１６％ＨＣｌ−８５℃−４０秒など）し、より好ましくは更に、酸洗浴中を走行する鋼板の表面に１．０〜５．０ｍ／秒程度の流速で酸洗液を供給し、あるいは酸洗液をノズルから吹き込み、鋼板表面で酸洗液を高速乱流状態としてやれば、前述した様な表面性状が得られ易くなることを確認している。
The method for obtaining the hot-rolled steel sheet having the surface texture as described above is not particularly limited, but according to the experiments by the present inventors, it was confirmed that the surface texture can be approximated by tightening the pickling time. is doing. That is, pickling for removing oxides (so-called scales) generated on the surface of the steel sheet in the hot rolling process is usually performed at about 50 to 85 ° C. for about 10 to 30 seconds using about 10 to 20% hydrochloric acid aqueous solution. However, the above-mentioned surface properties intended in the present invention can be achieved by increasing the concentration of hydrochloric acid in the pickling solution, increasing the pickling temperature, or setting the pickling time longer. More specifically, when the hydrochloric acid concentration of the pickling solution is A (%), the pickling temperature is B (° C.), and the pickling time (immersion time) is C (seconds), these are the following formula (I) (A / 100) × B 2 × C ≧ 40000 …… (I)
(For example, 11% HCl-75 ° C.-80 seconds, 15% HCl-80 ° C.-50 seconds, 16% HCl-85 ° C.-40 seconds, etc.), more preferably further in the pickling bath If the pickling solution is supplied to the surface of the traveling steel plate at a flow rate of about 1.0 to 5.0 m / second, or the pickling solution is blown from the nozzle, and the pickling solution is brought into a high-speed turbulent state on the steel plate surface, It has been confirmed that the surface texture as described above can be easily obtained.
次に、本発明で使用する鋼材の好ましい成分組成を定めた理由は下記の通りである。 Next, the reason why the preferable component composition of the steel material used in the present invention is determined is as follows.
Ｃ：０．０３〜１．０％
Ｃは、熱延鋼板の強度を高める上で重要な元素であり、０．０３％未満ではＣの大部分がフェライトに固溶してしまうため、高強度化に寄与する炭化物（基本的には鉄の炭化物であるセメンタイト、あるいは、必要に応じて添加されることのあるＮｂ，Ｔｉ，Ｖなどの炭化物）の生成が不十分で、本発明で意図するレベルの強度が得られ難くなる。より好ましくは０．０５％以上含有させるのがよい。しかし多過ぎると、成形加工性が悪くなるほか溶接性にも悪影響が現れてくるので、多くとも１．０％以下、より好ましくは０．２３％以下に抑えるのがよい。
C: 0.03-1.0%
C is an important element for increasing the strength of the hot-rolled steel sheet, and if it is less than 0.03%, most of C is dissolved in ferrite. The formation of cementite, which is an iron carbide, or carbides such as Nb, Ti, V, etc., which may be added as necessary, is insufficient, and it is difficult to obtain the strength intended by the present invention. More preferably, it is 0.05% or more. However, if the amount is too large, the moldability is deteriorated and the weldability is adversely affected. Therefore, it is preferable to keep it at most 1.0% or less, more preferably 0.23% or less.
Ｓｉ：２％以下（０％を含む）
Ｓｉは、鋼を溶製する際に脱酸性元素として有効に作用する他、鋼材の高強度化にも寄与する元素であるが、多過ぎると成形加工性を劣化させるばかりでなく、表面欠陥を生じ易くなり、酸洗性や塗装性にも悪影響を及ぼす様になるので、多くとも２％以下、好ましくは１．５％以下に抑えるのがよい。
Si: 2% or less (including 0%)
Si is an element that effectively acts as a deoxidizing element when melting steel, and also contributes to increasing the strength of steel, but if too much, not only deteriorates formability but also causes surface defects. Since it tends to occur and adversely affects pickling properties and paintability, it is preferable to keep it at most 2%, preferably 1.5% or less.
Ｍｎ：０．３〜４．０％
Ｍｎは、強度を高める効果を有する他、鋼中に混入して脆化要因となるＳをＭｎＳとして固定する上でも重要な元素である。これらの作用を有効に発揮させるには、少なくとも０．３％以上、好ましくは０．５％以上含有させるのがよい。しかし多過ぎると、延性を低下させて加工性に悪影響を及ぼすばかりでなく溶接性も低下させるので、多くとも４．０％以下、好ましくは２．５％以下に抑えるのがよい。
Mn: 0.3 to 4.0%
Mn has an effect of increasing strength, and is also an important element for fixing S, which is mixed into steel and causes embrittlement, as MnS. In order to effectively exhibit these actions, it is preferable to contain at least 0.3% or more, preferably 0.5% or more. However, if the amount is too large, not only the ductility is lowered and workability is adversely affected, but also the weldability is lowered. Therefore, it is preferable to keep it at most 4.0%, preferably 2.5% or less.
Ａｌ：０．００５〜０．５％
Ａｌは脱酸元素として不可欠の元素であり、その効果を有効に発揮させるには０．００１％以上含有させねばならず、好ましくは０．００５％以上含有させるのがよい。しかし多過ぎると、酸化物系介在物量の増大により靭性が劣化する他、表面欠陥を生じ易くなるので、多くとも０．５％以下、好ましくは０．３％以下に抑えるのがよい。
Al: 0.005 to 0.5%
Al is an indispensable element as a deoxidizing element, and in order to exert its effect effectively, it must be contained in an amount of 0.001% or more, preferably 0.005% or more. However, if the amount is too large, the toughness deteriorates due to an increase in the amount of oxide inclusions, and surface defects are liable to occur. Therefore, it is preferable to keep it at most 0.5%, preferably 0.3% or less.
Ｍｏ：１．０％以下
Ｍｏは、固溶強化による熱延鋼板の高強度化を進めるうえで重要な元素であり、その効果は０．０５％以上含有させることで有効に発揮される。但し、要求強度が３９０ＭＰａレベル未満の場合は、敢えてＭｏを含有させるまでもない。Ｍｏ量は要求される熱延鋼板の強度レベルにもよるが、その効果がより確実に発揮されるのは０．１％以上である。しかし、１．０％を超えると、高強度化への寄与以上に延性（加工性）に与える悪影響が顕著で、強度−伸びバランスが急激に悪くなるので、上限を１．０％と定めた。より好ましくは０．５％以下に抑えるのがよい。なお本発明は、先にも述べた様にＭｏ添加により劣化する化成処理性を表面性状の改善で補うところに最大の特徴を有するものであるが、表面性状による化成処理性改善効果は、Ｍｏを含まない高強度熱延鋼板についても有効に発揮される。
Mo: 1.0% or less Mo is an important element for increasing the strength of a hot-rolled steel sheet by solid solution strengthening, and the effect is effectively exhibited by containing 0.05% or more. However, when the required strength is less than the 390 MPa level, it is not necessary to include Mo. Although the amount of Mo depends on the required strength level of the hot-rolled steel sheet, it is 0.1% or more that the effect is more reliably exhibited. However, if it exceeds 1.0%, the adverse effect on ductility (workability) is more significant than the contribution to increasing the strength, and the strength-elongation balance deteriorates rapidly, so the upper limit was set to 1.0%. . More preferably, it should be suppressed to 0.5% or less. As described above, the present invention has the greatest feature in that the chemical conversion treatment performance deteriorated by the addition of Mo is supplemented by the improvement of the surface properties. High strength hot rolled steel sheet that does not contain steel is also effectively exhibited.
Ｃｒ：１．５％以下
Ｃｒは、少ない添加量で熱延鋼板の強度を高める作用を有しており、特に７８０ＭＰａレベル以上の引張強度が求められる場合は、少なくとも０．１％程度以上含有させるのがよく、９００ＭＰａレベル以上の引張強度が求められる場合は０．３％以上含有させるのがよい。しかし、Ｃｒ含量が多過ぎると、Ｍｏと同様に高強度化への寄与以上に延性（加工性）が大幅に劣化するので、多くとも１．５％以下、好ましくは１．０％以下に抑えるのがよい。
Cr: 1.5% or less Cr has an effect of increasing the strength of the hot-rolled steel sheet with a small addition amount, and particularly when a tensile strength of 780 MPa level or more is required, it is contained at least about 0.1% or more. In the case where a tensile strength of 900 MPa level or more is required, it is preferable to contain 0.3% or more. However, if the Cr content is too large, ductility (workability) is greatly deteriorated more than the contribution to high strength like Mo, so at most 1.5% or less, preferably 1.0% or less. It is good.
尚、求められる引張強度が３９０〜７８０ＭＰａレベルの範囲であれば、上記元素のうちＣ，Ｓｉ，Ｍｎ，Ｍｏの含有率を調整するだけで、Ｃｒを添加せずとも目標の引張強度を得ることができる。しかしその様な強度レベルの熱延鋼板を得る場合でも、Ｃｒ添加量を微調整するだけで強度を容易にコントロールできるので、Ｃｒの添加は実用上極めて有効である。その様な観点から、より好ましいＣｒ含量は０．１％以上、１．５％以下である。 If the required tensile strength is in the range of 390 to 780 MPa, the target tensile strength can be obtained without adding Cr by simply adjusting the content of C, Si, Mn, and Mo among the above elements. Can do. However, even when a hot-rolled steel sheet having such a strength level is obtained, the addition of Cr is extremely effective in practice because the strength can be easily controlled only by finely adjusting the Cr addition amount. From such a viewpoint, a more preferable Cr content is 0.1% or more and 1.5% or less.
本発明で使用する鋼の構成元素は上記の通りであり、残部は実質的にＦｅである。ここで「実質的に」とは、鋼原料もしくはその製造工程で混入し得る不可避不純物元素の含有を許容し、或いは前述した各成分元素の作用効果を阻害しない範囲で、更に他の元素が少量含まれていてもよいことを意味する。その様な不可避不純物元素としては、例えばＰ，Ｓ，Ｎ，Ｏなどが挙げられ、またその他の元素としては、Ｔｉ，Ｎｂ，Ｖ，Ｃｕ，Ｎｉ，Ｂ，Ｃａなどが例示される。しかしこれらの元素は、多過ぎると大なり小なり延性や表面性状を劣化させ、化成処理性にも悪影響を及ぼすので、Ｔｉは０．２％以下、Ｎｂは０．１％以下、Ｖは０．１％以下、Ｃｕは１．０％以下、Ｎｉは１．０％以下、Ｂは０．００２％以下、Ｃａは０．００５％以下に、夫々抑えるべきである。 The constituent elements of the steel used in the present invention are as described above, and the balance is substantially Fe. Here, “substantially” means that the inclusion of inevitable impurity elements that can be mixed in the steel raw material or its manufacturing process is allowed, or the amount of other elements is small in a range that does not hinder the action and effect of each component element described above. It may be included. Examples of such inevitable impurity elements include P, S, N, and O, and examples of other elements include Ti, Nb, V, Cu, Ni, B, and Ca. However, if these elements are too large, the ductility and surface properties are deteriorated to a greater or lesser extent and adversely affect the chemical conversion properties. Therefore, Ti is 0.2% or less, Nb is 0.1% or less, and V is 0. .1% or less, Cu is 1.0% or less, Ni is 1.0% or less, B is 0.002% or less, and Ca is 0.005% or less.
尚、本発明に係る熱延鋼板の強度は、用途に応じてＣ，Ｓｉ，Ｍｎ，Ｍｏ，Ｃｒなどの含有率を変えることで３９０ＭＰａレベル以上、７８０ＭＰａレベル以上、更には９００ＭＰａレベル以上の任意の強度に調整できるが、９００ＭＰａレベル以上の高強度熱延鋼板を得たい場合は、Ｍｏ以外の強化元素としてＣｒも必須的に添加し、且つ熱処理条件を工夫する（例えば、熱間圧延の仕上げ温度を［Ａｃ３点］以上とし、その後の冷却速度を３０℃／秒以上とし、３５０〜５５０℃で巻き取るなど）ことによって、鋼組織をベイナイトリッチ（好ましくは８５％以上）にするのがよい。 The strength of the hot-rolled steel sheet according to the present invention can be set at any level of 390 MPa level or higher, 780 MPa level or higher, or 900 MPa level or higher by changing the content of C, Si, Mn, Mo, Cr, etc. according to the application. Although it is possible to adjust the strength, in order to obtain a high strength hot rolled steel sheet of 900 MPa level or higher, Cr is also essentially added as a strengthening element other than Mo, and heat treatment conditions are devised (for example, hot rolling finishing temperature) (Ac 3 points) or more, and then the cooling rate is 30 ° C./second or more, and the steel structure is bainite rich (preferably 85% or more). .
本発明は以上の様に構成されるが、高強度鋼板を対象として化成処理性を改善し、特に強化元素として有用なＭｏを添加した高強度熱延鋼板であっても、表面性状を適正にコントロールすることで、Ｍｏ添加に伴う現実的な問題点として指摘されていた化成処理性の劣化を防止し、高強度と優れた化成処理性を兼ね備えた熱延鋼板を提供し得ることになった。 Although the present invention is configured as described above, the chemical conversion treatment property is improved for high-strength steel sheets, and the surface properties are appropriately adjusted even for high-strength hot-rolled steel sheets to which Mo useful as a strengthening element is added. By controlling, it was possible to prevent the deterioration of chemical conversion treatment, which had been pointed out as a practical problem with the addition of Mo, and to provide a hot-rolled steel sheet having both high strength and excellent chemical conversion treatment. .
以下、実施例を挙げて本発明を更に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勿論可能であり、それらは何れも本発明の技術的範囲に含まれる。 EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited by the following examples as a matter of course, and appropriate modifications are made within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.
実施例
下記表１に示す化学成分の鋼材１〜１５を溶製し、鋳造によりスラブを製造した。このスラブをＡｃ３点以上に再加熱し、表２に示す条件で熱間圧延を行って厚さ３．２ｍｍの熱延鋼板を得た。得られた熱延鋼板の機械的特性と縦断面組織中のベイナイト面積率を表２に併記した。
Examples Steel materials 1 to 15 having chemical components shown in Table 1 below were melted and slabs were produced by casting. This slab was reheated to Ac 3 points or more, and hot rolled under the conditions shown in Table 2 to obtain a hot rolled steel sheet having a thickness of 3.2 mm. Table 2 shows the mechanical properties of the obtained hot-rolled steel sheet and the bainite area ratio in the longitudinal sectional structure.
得られた各熱延鋼板を表３，４に示す条件で酸洗し（なお酸洗液の流速とは、酸洗槽の出入り口に配設したスリット状の噴流ノズルから鋼板表面に向けて吹き込む酸洗液の流速を意味する）、得られた酸洗鋼板の表面性状を、レーザー顕微鏡（レーザーテック社製、型番「１ＬＭ２１Ｗ」）により５０倍の対物レンズを用いて観察し、無作為に選択した１０箇所について、１箇所当り０．１６ｍｍ×０．２２ｍｍの面積を走査することによって、表面凹凸の平均間隔（Ｓｍ）、最大深さ（Ｒｙ）、負荷長さ率（ｔｐ４０）および（ｔｐ６０）の値とその差を各々平均値として求めると共に、下記の方法で化成処理性を評価した。なお一部の試料については、酸洗処理の後、所定の圧下率でスキンパスを行ってから化成処理性を評価した。結果を表３，４に一括して示す。 Each obtained hot-rolled steel sheet is pickled under the conditions shown in Tables 3 and 4 (note that the flow rate of the pickling solution is blown toward the steel sheet surface from a slit-like jet nozzle disposed at the entrance and exit of the pickling tank. The surface properties of the pickled steel sheet obtained were observed with a laser microscope (model number “1LM21W” manufactured by Lasertec Co., Ltd.) using a 50 × objective lens and randomly selected. By scanning an area of 0.16 mm × 0.22 mm per location for 10 locations, the average spacing (Sm), maximum depth (Ry), load length ratio (tp40) and (tp60) of surface irregularities While calculating | requiring each value and its difference as an average value, the chemical conversion treatment property was evaluated by the following method. In addition, about some samples, after the pickling process, the skin pass was performed with the predetermined rolling reduction, and chemical conversion treatment property was evaluated. The results are collectively shown in Tables 3 and 4.
化成処理性：
各供試鋼板の表面を下記の条件で化成処理した後、鋼板表面を１０００倍でＳＥＭ観察し、無作為に選択した１０視野についてリン酸亜鉛結晶の付着状況を調べ、下記の基準で化成処理性を評価した。
化成処理液…日本パーカライジング社製の化成処理液「パルボンドＬ３０２０」を使用
化成処理工程…脱脂（日本パーカライジング社製の脱脂液「ファインクリーナー」を用い、４５℃で１２０秒）→水洗（３０秒）→表面調整（日本パーカライジング社製の表面調整液「プレパレンＺ」に１５秒浸漬）→化成処理（上記化成処理液に４３℃で１２０秒浸漬）
Chemical conversion:
After chemical conversion treatment of the surface of each test steel plate under the following conditions, the steel plate surface was observed by SEM at a magnification of 1000 times, and the adhesion status of zinc phosphate crystals was examined for 10 randomly selected fields, and chemical conversion treatment was performed according to the following criteria. Sex was evaluated.
Chemical conversion treatment liquid: Uses Palbond L3020 chemical conversion treatment liquid manufactured by Nihon Parkerizing Co., Ltd. Chemical conversion treatment process: Degreasing (120 degree at 45 ° C. using degreasing liquid “Fine Cleaner” manufactured by Nihon Parkerizing Co., Ltd.) → Water washing (30 seconds) → Surface adjustment (immersion for 15 seconds in surface preparation liquid “preparen Z” manufactured by Nihon Parkerizing Co., Ltd.) → Chemical conversion treatment (immersion in the above chemical conversion liquid at 43 ° C. for 120 seconds)
評価基準
スケ：１０視野全てにおいて均一に付着しているもの：◎、１０視野中で５％以下のスケが認められたものが３視野以下であるもの：（○）、それ以外：（×）。
粒径：各視野の中から大きなものを１０個選択し、その平均径で評価する。
１０μｍ以上：×、７μｍ以上〜１０μｍ未満：○、
４μｍ以上〜７μｍ未満：◎、４μｍ未満：●。
Ｐ比：化成処理後の鋼板表面をＸ線回折によりホスホフェライト（ｐ）とホパイト（Ｈ）に相当するピークを測定し、その比（Ｐ／Ｐ＋Ｈ）（ｎ＝５の平均値）によって評価する。Ｐ比＝Ｐ／（Ｐ＋Ｈ）で、０．８５未満：×、０．８５以上〜０．９３未満：○、０．９３以上〜０．９６未満：◎、０．９６以上：●。
Evaluation Criteria Skete: Uniformly adhered in all 10 fields of view: ◎ Skews of 5% or less in 10 fields of view were observed in 3 fields or less: (◯), others: (x) .
Particle size: 10 large particles are selected from each field of view, and the average particle size is evaluated.
10 μm or more: ×, 7 μm or more and less than 10 μm: ○,
4 μm or more to less than 7 μm: ◎ <4 μm: ●.
P ratio: Peaks corresponding to phosphoferrite (p) and hopite (H) are measured by X-ray diffraction on the steel sheet surface after chemical conversion treatment, and evaluated by the ratio (P / P + H) (average value of n = 5). . P ratio = P / (P + H), less than 0.85: x, 0.85 or more and less than 0.93: ◯, 0.93 or more and less than 0.96: ◎, 0.96 or more: ●.
判定は、上記スケと粒径およびＰ比から、下記の様に総合評価した。
スケが◎、粒径が●、Ｐ比が●であるものは、総合で●（ベスト）、
スケが◎、粒径とＰ比が◎以上で、上記以外のものは、総合で◎（優）、
スケ、粒径、Ｐ比が○以上で上記以外のものは、総合で○（良）、
スケ、粒径、Ｐ比のどれか１つでも×であるものは、総合で×（不良）。
Judgment was comprehensively evaluated from the above-mentioned scale, particle size and P ratio as follows.
If the scale is ◎, the particle size is ●, and the P ratio is ●, the total is ● (best),
Scale is ◎, particle size and P ratio is more than ◎, other than the above, ◎ (excellent),
Suke, particle size, P ratio is more than ○, and other than the above, ○ (good),
If any of the scale, particle size, and P ratio is x, it is x (bad) in total.
上記表１〜４から次の様に考えることができる。 From the above Tables 1 to 4, it can be considered as follows.
試料Ｎｏ．１，５，９，１４は、表面性状が本発明の規定要件を外れる比較例であり、いずれも化成処理性が悪い。また試料Ｎｏ．１３は、試料Ｎｏ．１１の酸洗処理後にスキンパス処理を施したものであるが、スキンパスによって表面性状が本発明の規定範囲外になったため化成処理性が悪くなっている。 Sample No. Nos. 1, 5, 9, and 14 are comparative examples in which the surface properties deviate from the requirements of the present invention, and all have poor chemical conversion properties. Sample No. 13 is sample No. Although the skin pass treatment was performed after the pickling treatment of No. 11, the surface treatment was out of the specified range of the present invention by the skin pass, so that the chemical conversion treatment property was deteriorated.
試料Ｎｏ．１６は、酸洗条件がやや緩慢であるため本発明で規定する表面性状が得られておらず、化成処理性が十分とは言えない。 Sample No. No. 16 has a slightly mild pickling condition, so the surface properties defined in the present invention are not obtained, and the chemical conversion treatment property cannot be said to be sufficient.
試料Ｎｏ．２０は、酸洗温度が相対的に低く且つ浸漬時間が相対的に短い上に、鋼板に向けて噴射する酸洗液の流速が相対的に遅く表面性状が十分に改善されていないため、化成処理性がやや不十分である。 Sample No. No. 20 has a relatively low pickling temperature and a relatively short immersion time, and the flow rate of the pickling liquid sprayed toward the steel sheet is relatively slow and the surface properties are not sufficiently improved. The processability is slightly insufficient.
試料Ｎｏ．２４は、酸洗処理後のスキンパスで最大深さ（Ｒｙ）が規定範囲外となったため、化成処理性がやや不足気味となっている。 Sample No. In No. 24, since the maximum depth (Ry) was out of the specified range in the skin pass after the pickling treatment, the chemical conversion treatment performance was slightly insufficient.
試料Ｎｏ．２８は、酸洗処理後のスキンパスで特に表面凹凸の平均間隔（Ｓｍ）が好適範囲を外れる様になった例であり、化成処理性が劣悪である。 Sample No. No. 28 is an example in which the average interval (Sm) of the surface irregularities is particularly out of the preferred range in the skin pass after the pickling treatment, and the chemical conversion treatment property is poor.
試料Ｎｏ．３１は、酸洗処理条件が不適切であるためか適正な表面性状が得られておらず、化成処理性がやや不足気味になっている。 Sample No. No. 31 does not have an appropriate surface property because the pickling treatment conditions are inappropriate, and the chemical conversion treatment is slightly insufficient.
試料Ｎｏ．３６は、酸洗処理性が適切で表面性状も良好であるが、用いた鋼材が規定値を超える量のＭｏを含んでいるため、化成処理性が悪い。 Sample No. No. 36 is suitable for pickling treatment and has good surface properties, but because the steel material used contains Mo in an amount exceeding the specified value, the chemical conversion treatment property is poor.
これらに対し試料Ｎｏ．２〜４，６〜８，１０〜１２，１５，１７〜１９，２１〜２３，２５〜２７，２９，３０，３２〜３５，３７は、いずれも本発明の規定要件を満足する実施例であり、いずれも優れた化成処理性が得られている。 In contrast, sample no. 2 to 4, 6 to 8, 10 to 12, 15, 17 to 19, 21 to 23, 25 to 27, 29, 30, 32 to 35, and 37 are examples that satisfy the prescribed requirements of the present invention. In all cases, excellent chemical conversion properties are obtained.
なお試料Ｎｏ．１６，２０，２４，３１は、表面の負荷長さ率に関する要件は本発明の規定要件を満たしていないが、鋼中にＭｏが含まれていないため、それなりに良好な化成処理性が得られている。 Sample No. Nos. 16, 20, 24, and 31 do not satisfy the requirements of the present invention regarding the load length ratio of the surface, but Mo is not contained in the steel, so that a good chemical conversion treatment property can be obtained. ing.
Claims (8)
Ｃｒ：１．５％以下（０％を含まない）、
Ｔｉ：０．２％以下（０％を含まない）、
Ｎｂ：０．１％以下（０％を含まない）、
Ｖ：０．１％以下（０％を含まない）、
Ｃｕ：１．０％以下（０％を含まない）、
Ｎｉ：１．０％以下（０％を含まない）、
Ｂ：０．００２％以下（０％を含まない）、
Ｃａ：０．００５％以下（０％を含まない）、
よりなる群から選択される少なくとも１種の元素を含むものである請求項４〜６のいずれかに記載の化成処理性に優れた高強度熱延鋼板。 Steel is another element,
Cr: 1.5% or less (excluding 0%),
Ti: 0.2% or less (excluding 0%),
Nb: 0.1% or less (excluding 0%),
V: 0.1% or less (excluding 0%),
Cu: 1.0% or less (excluding 0%),
Ni: 1.0% or less (excluding 0%),
B: 0.002% or less (excluding 0%),
Ca: 0.005% or less (excluding 0%),
The high-strength hot-rolled steel sheet excellent in chemical conversion property according to any one of claims 4 to 6, comprising at least one element selected from the group consisting of:
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