JP2009052139A - High-strength steel sheet - Google Patents

High-strength steel sheet Download PDF

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JP2009052139A
JP2009052139A JP2008194546A JP2008194546A JP2009052139A JP 2009052139 A JP2009052139 A JP 2009052139A JP 2008194546 A JP2008194546 A JP 2008194546A JP 2008194546 A JP2008194546 A JP 2008194546A JP 2009052139 A JP2009052139 A JP 2009052139A
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steel sheet
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precipitates
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JP5326403B2 (en
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Koichi Nakagawa
功一 中川
Takeshi Yokota
毅 横田
Nobuyuki Nakamura
展之 中村
Kazuhiro Seto
一洋 瀬戸
Tetsushi Jodai
哲史 城代
Katsumi Yamada
克美 山田
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JFE Steel Corp
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
    • C21D8/0426—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/004—Dispersions; Precipitations
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength steel sheet excellent in stretch-flange characteristics after working and in corrosion resistance after coating. <P>SOLUTION: A steel sheet which has a composition containing by mass, C: 0.02 to 0.20%, Si: 0.3% or below, Mn: 0.5 to 2.5%, P: 0.06% or below, S: 0.01% or below, Al: 0.1% or below, Ti: 0.05 to 0.25%, and V: 0.05 to 0.25% with the balance consisting of Fe and unavoidable impurities and a substantially ferrite single-phase structure wherein the contents of Ti, V, and solid-soluted V in precipitates of less than 20 nm in size are 200 to 1,750 mass ppm, 150 to 1,750 mass ppm, and 200 to less than 1,750 mass ppm, respectively. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

本発明は、加工後の伸びフランジ特性および塗装後耐食性に優れた高強度鋼板に関するものである。   The present invention relates to a high-strength steel sheet excellent in stretch flange characteristics after processing and corrosion resistance after coating.

自動車の足回り部材やトラック用フレーム等といった部品には成形性（主に伸びおよび伸びフランジ特性）が必要とされており、従来は引張強度が590ＭＰａ級の鋼が使用されてきた。しかし、近年、自動車の環境負荷低減や衝撃特性向上の観点から、自動車用鋼板の高強度化が推進されており、引張強度が780ＭＰａ級の鋼の使用が検討され始めている。   Parts such as automobile undercarriage members and truck frames are required to have formability (mainly stretch and stretch flange characteristics). Conventionally, steel having a tensile strength of 590 MPa has been used. However, in recent years, from the viewpoint of reducing the environmental impact of automobiles and improving the impact characteristics, the strength of automobile steel sheets has been increased, and the use of steel having a tensile strength of 780 MPa class has begun to be studied.

鉄鋼材料は、一般に、強度が上昇するに伴い加工性が低下する。そのため、高強度かつ高加工性を有する鋼板についての研究がなされてきた。伸びおよび伸びフランジ特性を向上させる技術として、例えば、特許文献１〜6が挙げられる。   In general, the workability of steel materials decreases as the strength increases. Therefore, research has been made on steel sheets having high strength and high workability. Examples of techniques for improving the stretch and stretch flange characteristics include Patent Documents 1 to 6.

特許文献１には、実質的にフェライト単相組織であり、平均粒径10nm未満のＴｉおよびＭｏを含む炭化物が分散析出していることを特徴とする、引張強度が590ＭＰａ以上の加工性に優れた高張力鋼板に関する技術が開示されている。   Patent Document 1 is excellent in workability with a tensile strength of 590 MPa or more, characterized by having a ferrite single-phase structure, and carbides containing Ti and Mo having an average particle size of less than 10 nm are dispersed and precipitated. Further, a technique related to a high-tensile steel plate is disclosed.

特許文献２には、質量で、Ｃ：0.08〜0.20%、Ｓｉ：0.001%以上0.2%未満、Ｍｎ：1.0%超3.0%以下、Ａｌ：0.001〜0.5%、Ｖ：0.1%超0.5%以下、Ｔｉ：0.05%以上0.2%未満およびＮｂ：0.005%〜0.5%を含有し、かつ、（式１）（Ｔｉ/48＋Ｎｂ/93）×Ｃ/12≦4.5×10−５、（式２）0.5≦（Ｖ/51＋Ｔｉ/48＋Ｎｂ/93）/（Ｃ/12）≦1.5、（式３）Ｖ＋Ｔｉ×2＋Ｎｂ×1.4＋Ｃ×2＋Ｍｎ×0.1≧0.80の3式を満たし、残部Ｆｅおよび不可避的不純物からなり、平均粒径5μｍ以下で硬度が250Ｈｖ以上のフェライトを70体積%以上含有する鋼組織を有し、880ＭＰａ以上の強度と降伏比0.80以上を有する高強度熱延鋼板に関する技術が開示されている。 In Patent Document 2, by mass, C: 0.08 to 0.20%, Si: 0.001% to less than 0.2%, Mn: more than 1.0% to 3.0% or less, Al: 0.001 to 0.5%, V: more than 0.1% to 0.5% or less, Ti: 0.05% or more and less than 0.2% and Nb: 0.005% to 0.5%, and (Formula 1) (Ti / 48 + Nb / 93) × C / 12 ≦ 4.5 × 10 −5 , (Formula 2) 0.5 ≦ (V / 51 + Ti / 48 + Nb / 93) / (C / 12) ≦ 1.5, (Formula 3) V + Ti × 2 + Nb × 1.4 + C × 2 + Mn × 0.1 ≧ 0.80 is satisfied, consisting of the balance Fe and inevitable impurities, average A technique relating to a high-strength hot-rolled steel sheet having a steel structure containing 70% by volume or more of ferrite having a particle size of 5 μm or less and a hardness of 250 Hv or more, having a strength of 880 MPa or more and a yield ratio of 0.80 or more is disclosed.

特許文献３には、質量で、Ｃ：0.05〜0.2%、Ｓｉ：0.001%〜3.0%、Ｍｎ：0.5〜3.0、Ｐ：0.001〜0.2%、Ａｌ：0.001〜3%、Ｖ：0.1%を超えて1.5%までを含み、残部はＦｅ及び不可避的不純物からなり、組織は平均粒径1〜5μmのフェライトを主相とし、フェライト粒内に平均粒径が50nm以下のＶの炭窒化物が存在することを特徴とする熱延鋼板に関する技術が開示されている。   In Patent Document 3, by mass, C: 0.05 to 0.2%, Si: 0.001% to 3.0%, Mn: 0.5 to 3.0, P: 0.001 to 0.2%, Al: 0.001 to 3%, V: over 0.1% Up to 1.5%, the balance is made of Fe and inevitable impurities, the structure is mainly composed of ferrite with an average particle size of 1 to 5 μm, and V carbonitride with an average particle size of 50 nm or less exists in the ferrite grains. A technique related to a hot-rolled steel sheet is disclosed.

特許文献４には、鋼組織中に炭化物を析出させてなる熱的安定性に優れた高強度薄鋼板が開示されている。この薄鋼板は、炭化物がMを金属元素とした場合にMＣで表されるNaＣl型の結晶構造を有し、金属元素Mは2種以上の金属からなり、かつこれら2種以上の金属が結晶格子内で規則的に配列した超格子構造を形成していることを特徴とする。   Patent Document 4 discloses a high-strength thin steel sheet having excellent thermal stability by depositing carbides in a steel structure. This thin steel sheet has a NaCl-type crystal structure represented by MC when the carbide is M as a metal element. The metal element M is composed of two or more metals, and these two or more metals are crystallized. A superlattice structure regularly arranged in the lattice is formed.

特許文献５には、以下の熱延鋼板が開示されている。成分組成は、質量%で、C：0.0002〜0.25%、Si：0.003〜3.0%、Mn：0.003~3.0%及びAl：0.002〜2.0%を含有し、残部はFe及び不可避的不純物からなり、不可避的不純物中のPは0.15%以下、Sは0.05%以下、Nは0.01%以下である。そして、面積割合で金属組織の70%以上がフェライト相で、その平均結晶粒径が20μm以下、アスペクト比が3以下である。さらに、フェライト粒界の70%以上が大角粒界からなり、大角粒界で形成されたフェライト相のうち、最大径が30μm以下、最小径が5nm以上である析出物の面積割合が金属組織の2%以下である。さらに、フェライト相と析出物とを除く残部相のなかで面積割合が最大である第二相の平均結晶粒径が20μm以下であり、最も近い第二相間にフェライト相の大角粒界が存在する。   Patent Document 5 discloses the following hot-rolled steel sheet. Ingredient composition is% by mass, C: 0.0002 to 0.25%, Si: 0.003 to 3.0%, Mn: 0.003 to 3.0% and Al: 0.002 to 2.0%, the balance consists of Fe and inevitable impurities, unavoidable P in the target impurities is 0.15% or less, S is 0.05% or less, and N is 0.01% or less. In terms of area ratio, 70% or more of the metal structure is the ferrite phase, the average crystal grain size is 20 μm or less, and the aspect ratio is 3 or less. Furthermore, 70% or more of the ferrite grain boundaries are composed of large-angle grain boundaries, and among the ferrite phases formed at the large-angle grain boundaries, the area ratio of precipitates having a maximum diameter of 30 μm or less and a minimum diameter of 5 nm or more is the metal structure. 2% or less. Furthermore, the average crystal grain size of the second phase, which has the largest area ratio among the remaining phases excluding the ferrite phase and precipitates, is 20 μm or less, and there is a large-angle grain boundary of the ferrite phase between the closest second phases. .

特許文献6には、質量%にて、C：0.01〜0.1%、S≦0.03%、N≦0.005%、Ti：0.05~0.5%を含み、さらにTi-48/12C-48/14N-48/32S≧0%を満たす範囲でTiを含有し、残部がFe及び不可避的不純物からなり、少なくとも板厚の1/2厚における板面の{100}<011>〜{223}<110>方位群のX線ランダム強度比の平均値が3以上、かつ、{554}<225>、{111}<112>および{111}<110>の3方位のX線ランダム強度比の平均値が3.5以下であり、少なくとも一方の鋼板表面の算術平均粗さRaが1〜3.5である鋼板に、潤滑効果のある組成物が塗布されていることを特徴とする形状凍結性に優れる絞り可能なバーリング性高強度薄鋼板について、開示されている。
登録特許第３５９１５０２号公報 特開２００６−１６１１１２号公報 特開２００４−１４３５１８号公報 特開２００３−３２１７４０号公報 特開２００３−２９３０８３号公報 特開２００３−１６０８３６号公報 Patent Document 6 contains, in mass%, C: 0.01 to 0.1%, S ≦ 0.03%, N ≦ 0.005%, Ti: 0.05 to 0.5%, and Ti-48 / 12C-48 / 14N-48 / {100} <011> to {223} <110> orientation group of the plate surface at least 1/2 of the plate thickness, containing Ti in the range satisfying 32S ≧ 0%, the balance being Fe and inevitable impurities The average value of the X-ray random intensity ratio of 3 or more, and the average value of the X-ray random intensity ratio of the three directions of {554} <225>, {111} <112> and {111} <110> is 3.5 or less A squeezable burring property having excellent shape freezing property, wherein a composition having a lubricating effect is applied to a steel plate having an arithmetic mean roughness Ra of 1 to 3.5 on the surface of at least one steel plate. A strength thin steel sheet is disclosed.
Japanese Patent No. 3591502 JP 2006-161112 A JP 2004-143518 A JP 2003-321740 A JP 2003-293083 A JP 2003-160836 A

しかしながら、上述の従来技術には、次のような問題がある。
特許文献１および４では、Ｍｏを含有しているため、近年のＭｏの原材料価格の高騰に絡んで、著しいコスト増加を招く問題がある。
さらに、自動車産業のグローバル化が進み、自動車に使用される鋼板は、厳しい腐食環境下において使用されるようになり、鋼板に対してより高い塗装後耐食性が必要とされている。これに対して、Ｍｏの添加は化成結晶の生成または成長を阻害するため、鋼板の塗装後耐食性を低下させ、上記要求に対応することができない。すなわち、特許文献1および特許文献4に記載の鋼では、近年の自動車産業の要求を満たす塗装後耐食性が得られない。 However, the above prior art has the following problems.
In Patent Documents 1 and 4, since Mo is contained, there is a problem that a significant cost increase is caused in connection with a recent increase in the raw material price of Mo.
Furthermore, with the globalization of the automobile industry, steel sheets used in automobiles are used in severe corrosive environments, and higher post-paint corrosion resistance is required for steel sheets. On the other hand, since addition of Mo inhibits the formation or growth of chemical crystals, the corrosion resistance after painting of the steel sheet is lowered, and the above-mentioned demand cannot be met. That is, the steels described in Patent Document 1 and Patent Document 4 cannot provide post-coating corrosion resistance that satisfies the demands of the automobile industry in recent years.

一方、近年のプレス技術の進歩により、ドロー（絞りおよび張り出し）→トリム(穴抜き)→リストライク（穴広げ）のような加工工程が採用され始めており、このような加工工程を経て成形される鋼板の伸びフランジ部位には、ドロー・トリム後、すなわち加工後の伸びフランジ特性が必要とされる。しかし、特許文献２、３、４では、780ＭＰａ以上のＴＳを得ようとすると、必ずしも十分な加工後の伸びフランジ特性が得られない。特許文献３に添加されるＮｂは、熱間圧延後のオーステナイトの再結晶を抑制する働きが高い。そのため、鋼板に未再結晶粒を残存させ、加工性を低下させる問題がある。また、熱間圧延時の圧延荷重を増加させる問題がある。   On the other hand, due to the recent progress in press technology, processing processes such as drawing (drawing and overhanging) → trimming (hole punching) → restriking (hole expansion) have begun to be adopted, and molding is performed through such processing processes. The stretch flange portion of the steel plate is required to have stretch flange characteristics after draw trimming, that is, after processing. However, in Patent Documents 2, 3, and 4, if an attempt is made to obtain a TS of 780 MPa or more, sufficient stretch flange characteristics after processing cannot always be obtained. Nb added to Patent Document 3 has a high function of suppressing recrystallization of austenite after hot rolling. Therefore, there is a problem that unrecrystallized grains remain in the steel sheet and the workability is lowered. There is also a problem of increasing the rolling load during hot rolling.

特許文献５では、引張強度ＴＳが422MPaまでのフェライト単相鋼板（例えば、実施例の表6、試験番号1から5および実施例の表8、試験番号45）と、引張強度ＴＳが780MPa以上のフェライト相と第二相で構成される複合組織鋼板（例えば、実施例表6、試験番号33から36および実施例表8、試験番号49）が開示されている。これら特許文献5の鋼板では、主に、SiまたはMnによる固溶強化と、硬質な第二相を利用した変態組織強化が活用されている。そのため、鋼板は、仕上げ圧延終了後2秒以内に平均冷却速度30℃/s以上で600〜800℃の温度範囲まで冷却され、次いで3〜15秒の間空冷された後、さらに平均冷却速度30℃/秒以上で水冷されて巻き取られる必要がある。これにより、フェライト変態時の二相分離が促進され、鋼板の組織はフェライト相と第二相による複合組織となる。また仕上げ圧延温度が、(Ae3点+100℃)〜Ae3点と、後述する本発明を製造するにあたり好適と考える温度範囲よりも低い温度範囲に設定されている。例えば、引張強度ＴＳが780MPa以上の複合組織鋼板（実施例の表6、試験番号33から36）では、仕上げ圧延温度が871℃〜800℃であった。仕上げ圧延温度が低い場合、オーステナイト相中のＴｉなどの炭化物形成元素の固溶限が低下し、かつ、圧延による加工によって析出サイトが導入されるため、20nm以上の析出物が生成する。この現象は歪誘起析出と呼ばれる。特許文献５における鋼板および製造方法では、歪誘起析出が生じるために、大きさ20nm以上の析出物の生成量が多くなってしまう。
さらに、特許文献５では、鋼組成のＣ含有量を非常に少なくし、かつオーステナイト安定化元素であるＭｎの含有量を少なくすることで、フェライト単相組織の製造を実現した技術が開示されている（参照：実施例の表２、鋼番AAからAE）。しかし、この場合は、固溶強化元素でもあるＭｎの含有量が減るので、固溶強化量が低下する。また、Ｃ含有量の減少により析出強化に効果のあるTiやNbなどの炭化物の析出量が減少し、析出強化量も減少する。その結果、固溶強化量および析出強化量を合わせても、フェライト単相組織鋼板の場合は、780MPa以上の強度が出せないということになる（参照：実施例の表6、試験番号1から5および実施例の表8、試験番号45）。
以上の理由から、特許文献５の技術においては、本発明が目的とする、組織が実質的にフェライト単相で引張強度が780MPa以上であり他の特性も有する鋼板は製造できない。 In Patent Document 5, a ferrite single-phase steel sheet having a tensile strength TS of up to 422 MPa (for example, Table 6 of Examples, Test Nos. 1 to 5 and Table 8 of Examples, Test No. 45), and a tensile strength TS of 780 MPa or more. A composite structure steel plate composed of a ferrite phase and a second phase (for example, Example Table 6, Test Nos. 33 to 36 and Example Table 8, Test No. 49) is disclosed. These steel sheets of Patent Document 5 mainly utilize solid solution strengthening by Si or Mn and transformation structure strengthening using a hard second phase. Therefore, the steel sheet is cooled to a temperature range of 600 to 800 ° C. at an average cooling rate of 30 ° C./s or more within 2 seconds after finishing rolling, and then air-cooled for 3 to 15 seconds, and then an average cooling rate of 30 It is necessary to wind up after being cooled with water at a temperature of ° C / second or more. Thereby, two-phase separation at the time of ferrite transformation is promoted, and the structure of the steel sheet becomes a composite structure of the ferrite phase and the second phase. Further, the finish rolling temperature is set to a temperature range (Ae3 point + 100 ° C.) to Ae3 point, which is lower than the temperature range considered suitable for manufacturing the present invention described later. For example, the finish rolling temperature was 871 ° C. to 800 ° C. in the composite structure steel plate having the tensile strength TS of 780 MPa or more (Table 6 of Example, test numbers 33 to 36). When the finish rolling temperature is low, the solid solubility limit of carbide-forming elements such as Ti in the austenite phase is lowered, and precipitation sites are introduced by processing by rolling, so precipitates of 20 nm or more are generated. This phenomenon is called strain induced precipitation. In the steel sheet and the manufacturing method in Patent Document 5, strain-induced precipitation occurs, so that the amount of precipitates having a size of 20 nm or more increases.
Furthermore, Patent Document 5 discloses a technique that realizes the production of a ferrite single-phase structure by reducing the C content of the steel composition very much and reducing the content of Mn, which is an austenite stabilizing element. (Reference: Table 2 of Examples, Steel No. AA to AE). However, in this case, since the content of Mn, which is also a solid solution strengthening element, is reduced, the solid solution strengthening amount is lowered. Moreover, the amount of precipitation of carbides, such as Ti and Nb which is effective in precipitation strengthening, decreases by decreasing the C content, and the amount of precipitation strengthening also decreases. As a result, even if the solid solution strengthening amount and the precipitation strengthening amount are combined, the strength of 780 MPa or more cannot be obtained in the case of a ferritic single-phase steel sheet (see: Table 6 of Examples, Test Nos. 1 to 5). And Table 8 of Examples, Test No. 45).
For the reasons described above, the technique disclosed in Patent Document 5 cannot produce a steel sheet having a structure that is substantially a ferrite single phase, has a tensile strength of 780 MPa or more, and has other characteristics.

特許文献6では、引張強度σBが780MPa以上の鋼板（例えば、実施例表中の鋼記号A-4、A-8、A-10、C、E、H）が開示されているが、これらの鋼板のＹＲ（単位％、ここでＹＲとは、σY/σB×100）は69%から74%と低いことから、これらの鋼板はマルテンサイト相などの硬質な第二相を含んでいることが推察される。
これより、特許文献６における780MPa以上の鋼板の設計思想は、特許文献５と同様に、主に、ＳｉまたはＭｎによる固溶強化と、硬質な第二相を利用した変態組織強化を活用するものと考えられる。そのため、特許文献５と同様に、後述する本発明を製造するにあたり好適と考える温度範囲よりも低い仕上げ圧延温度（Ar3点+100℃以下）で、合計圧化率25%以上の圧延を行なう必要がある。例えば、特許文献６の実施例によれば、引張強度σBが780MPa以上の鋼板の仕上げ圧延温度は800℃から890℃であった。特許文献６における鋼板および製造方法では、特許文献５と同様に、歪誘起析出が生じて大きさ20nm以上の析出物の生成量が多くなり、結果として本発明が目的とする、組織が実質的にフェライト単相で引張強度が780MPa以上および他の特性も有する鋼板は製造できない。 Patent Document 6 discloses steel sheets having a tensile strength σ B of 780 MPa or more (for example, steel symbols A-4, A-8, A-10, C, E, and H in the examples). YR (unit%, where YR is σ Y / σ B × 100) is as low as 69% to 74%, so these steel sheets contain a hard second phase such as martensite phase. It is inferred that
From this, the design philosophy of the steel plate of 780 MPa or more in Patent Document 6 mainly utilizes solid solution strengthening by Si or Mn and transformation structure strengthening using a hard second phase, as in Patent Document 5. it is conceivable that. Therefore, as in Patent Document 5, it is necessary to perform rolling at a total compression ratio of 25% or more at a finish rolling temperature (Ar3 point + 100 ° C. or lower) lower than the temperature range considered suitable for manufacturing the present invention described later. There is. For example, according to the example of Patent Document 6, the finish rolling temperature of a steel sheet having a tensile strength σ B of 780 MPa or more was 800 ° C. to 890 ° C. In the steel sheet and the manufacturing method in Patent Document 6, as in Patent Document 5, strain-induced precipitation occurs, and the amount of precipitates having a size of 20 nm or more increases, resulting in the substantial structure of the present invention. In addition, a steel sheet having a ferrite single phase, a tensile strength of 780 MPa or more and other characteristics cannot be produced.

本発明は、かかる事情に鑑み、加工後の伸びフランジ特性に優れ、かつ、塗装後耐食性に優れた高強度鋼板を提供することを目的とする。   In view of such circumstances, an object of the present invention is to provide a high-strength steel sheet that is excellent in stretch flange characteristics after processing and excellent in corrosion resistance after coating.

本発明者等は、加工後の伸びフランジ特性および塗装後耐食性に優れ、引張強度が780ＭＰａ以上である高強度熱延鋼板を得るべく検討したところ、以下の知見を得た。
i)高強度と塗装後耐食性に優れた鋼板を得るためには、析出物を微細化（大きさ20nm未満）し、微細な析出物（大きさ20nm未満）の割合を高め必要がある。そして、析出物を微細なまま維持するには析出物としてＴｉ−Ｍｏを含むもの、または、Ｔｉ−Ｖを含むものが挙げられるが、塗装後耐食性を向上させる観点からはＴｉとＶの複合析出が有用である。
ii)加工後の伸びフランジ性の向上にはＶの固溶が重要であり、特性向上に最適なＶの固溶量が存在する。
本発明は、以上の知見に基づきなされたもので、その要旨は以下のとおりである。
［１］mass%で、C：0.02%以上0.20%以下、Si：0.3%以下、Mn：0.5%以上2.5%以下、P：0.06%以下、S：0.01%以下、Al：0.1%以下、Ti：0.05%以上0.25%以下、V：0.05%以上0.25%以下を含有し、残部がFeおよび不可避的不純物からなる成分組成と、実質的にフェライト単相組織であり、前記フェライト単相組織中には、大きさが20nm未満の析出物に含まれるTiが200mass ppm以上1750mass ppm以下、Vが150 mass ppm以上1750 mass ppm以下であり、固溶Vが200 mass ppm以上1750 mass ppm未満である組織を有することを特徴とする高強度鋼板。
［２］前記［１］において、mass%で、さらに、Cr：0.01%以上0.5%以下、W:0.005%以上0.2%以下、Zr：0.0005%以上0.05%以下のいずれか1種または2種以上を含有することを特徴とする高強度鋼板。
［３］前記［１］または［２］において、引張強度ＴＳが780ＭＰａ以上であることを特徴とする高強度鋼板。
［４］前記［１］〜［３］のいずれかにおいて、塩温水浸漬試験におけるテープ剥離試験後の片側最大剥離幅が3.0mm以下であることを特徴とする高強度鋼板。
［５］前記［１］〜［４］のいずれかにおいて、伸張率10%での圧延後の伸びフランジ特性λ10が60%以上であることを特徴とする高強度鋼板。
なお、本明細書において、鋼の成分を示す%、ppmは、すべてmass%、mass ppmである。また、本発明における高強度鋼板とは、引張強度（以下、ＴＳと称する場合もある）が780ＭＰａ以上の鋼板であり、熱延鋼板、さらには、これらの鋼板に例えばめっき処理等の表面処理を施した表面処理鋼板も対象とする。
さらに、本発明の目標とする特性は、伸張率10%で圧延後の伸びフランジ特性（λ10）≧60%、後述する塩温水浸漬試験(ＳＤＴ)におけるテープ剥離試験後の片側最大剥離幅≦3.0mmである。 The present inventors have studied to obtain a high-strength hot-rolled steel sheet that is excellent in stretch flange characteristics after processing and corrosion resistance after coating, and has a tensile strength of 780 MPa or more, and has obtained the following knowledge.
i) In order to obtain a steel sheet with high strength and excellent corrosion resistance after coating, it is necessary to refine the precipitates (size less than 20 nm) and increase the proportion of fine precipitates (size less than 20 nm). In order to keep the precipitate fine, the precipitate includes Ti-Mo or Ti-V. From the viewpoint of improving the corrosion resistance after coating, the combined precipitation of Ti and V Is useful.
ii) The solid solution of V is important for improving stretch flangeability after processing, and there is an optimum amount of solid solution of V for improving the characteristics.
The present invention has been made based on the above findings, and the gist thereof is as follows.
[1] In mass%, C: 0.02% to 0.20%, Si: 0.3% or less, Mn: 0.5% to 2.5%, P: 0.06% or less, S: 0.01% or less, Al: 0.1% or less, Ti : 0.05% or more and 0.25% or less, V: 0.05% or more and 0.25% or less, with the remaining component composition consisting of Fe and inevitable impurities, and substantially a ferrite single-phase structure, Is a structure in which Ti contained in precipitates with a size of less than 20 nm is 200 mass ppm or more and 1750 mass ppm or less, V is 150 mass ppm or more and 1750 mass ppm or less, and solid solution V is 200 mass ppm or more and less than 1750 mass ppm. A high-strength steel sheet characterized by comprising:
[2] In [1], in mass%, Cr: 0.01% to 0.5%, W: 0.005% to 0.2%, Zr: 0.0005% to 0.05%, one or more A high-strength steel plate characterized by containing.
[3] A high-strength steel sheet according to [1] or [2], wherein the tensile strength TS is 780 MPa or more.
[4] The high-strength steel sheet according to any one of [1] to [3], wherein a one-side maximum peel width after a tape peel test in a salt warm water immersion test is 3.0 mm or less.
[5] The high strength steel sheet according to any one of the above [1] to [4], wherein the stretch flange characteristic λ 10 after rolling at an elongation rate of 10% is 60% or more.
In this specification, “%” and “ppm” indicating the components of steel are mass% and mass ppm, respectively. The high-strength steel plate in the present invention is a steel plate having a tensile strength (hereinafter sometimes referred to as TS) of 780 MPa or more, a hot-rolled steel plate, and further, a surface treatment such as plating treatment is applied to these steel plates. The surface-treated steel sheets that have been applied are also targeted.
Furthermore, the target properties of the present invention are: stretch flange characteristics after rolling at an elongation ratio of 10% (λ 10 ) ≧ 60%, maximum peel width on one side after a tape peel test in a salt warm water immersion test (SDT) described later ≦ 3.0mm.

本発明によれば、加工後の伸びフランジ特性および塗装後耐食性に優れ、ＴＳが７８０ＭＰａ以上である高強度熱延鋼板が得られる。さらに、本発明では、Ｍｏを添加せずとも上記効果が得られるので、コスト削減がはかれることになる。
そして、例えば、本発明の高強度熱延鋼板を自動車の足回り部材やトラック用フレームなどに用いることにより、板厚減少が可能となり、自動車の環境負荷が低減され、衝撃特性が大きく向上することが期待される。 According to the present invention, a high-strength hot-rolled steel sheet having excellent stretch flange characteristics after processing and corrosion resistance after coating and having a TS of 780 MPa or more can be obtained. Furthermore, in the present invention, the above effect can be obtained without adding Mo, so that the cost can be reduced.
For example, by using the high-strength hot-rolled steel sheet of the present invention for automobile underbody members, truck frames, etc., it becomes possible to reduce the plate thickness, reduce the environmental load of the automobile, and greatly improve the impact characteristics. There is expected.

以下、本発明を詳細に説明する。
１）まず、本発明における鋼の化学成分（成分組成）の限定理由について説明する。
Ｃ：0.02%以上0.20%以下
Ｃは、ＴｉやＶと炭化物を形成しフェライト中に析出することで、鋼板の強度化に寄与する元素である。TSを780ＭＰａ以上とするためには、Ｃ量を0.02%以上とする必要がある。一方、Ｃ量が0.20%を超えると析出物の粗大化や第二相組織の形成により加工後の伸びフランジ特性が低下する。以上より、Ｃ量は0.02%以上0.20%以下、好ましくは、0.03%以上0.15%以下とする。 Hereinafter, the present invention will be described in detail.
1) First, the reasons for limiting the chemical composition (component composition) of steel in the present invention will be described.
C: 0.02% to 0.20% C is an element that contributes to strengthening of the steel sheet by forming carbides with Ti and V and precipitating in ferrite. In order to set TS to 780 MPa or more, the C content needs to be 0.02% or more. On the other hand, if the amount of C exceeds 0.20%, the stretch flange characteristics after processing deteriorate due to coarsening of precipitates and formation of a second phase structure. Accordingly, the C content is 0.02% to 0.20%, preferably 0.03% to 0.15%.

Ｓｉ：0.3%以下
Ｓｉは固溶強化に寄与する元素であるが、0.3%を超えて添加すると粒界にセメンタイトが生成し、加工後の伸びフランジ特性が低下する。よって、Ｓｉ量は0.3%以下とする。好ましくは、0.001%以上0.2%以下とする。 Si: 0.3% or less Si is an element that contributes to solid solution strengthening, but if added over 0.3%, cementite is generated at the grain boundaries, and the stretch flange characteristics after processing deteriorate. Therefore, the Si amount is set to 0.3% or less. Preferably, the content is 0.001% or more and 0.2% or less.

Ｍｎ：0.5%以上2.5%以下
Ｍｎは固溶強化に寄与する元素である。しかしながら、その量が0.5%に満たないと780ＭＰａ以上のＴＳが得られない。一方、Ｍｎ量が2.5%を越えて添加すると、溶接性を著しく低下させる。以上より、Ｍｎ量は0.5%以上2.5%以下、好ましくは0.6%以上2.0%以下である。 Mn: 0.5% to 2.5% Mn is an element contributing to solid solution strengthening. However, if the amount is less than 0.5%, TS of 780 MPa or more cannot be obtained. On the other hand, if the Mn content exceeds 2.5%, weldability is remarkably lowered. From the above, the amount of Mn is 0.5% to 2.5%, preferably 0.6% to 2.0%.

Ｐ：0.06%以下
Ｐは旧オーステナイト粒界に偏析するため、低温靭性劣化と加工性低下を招く。そのため、Ｐ量は極力低減することが好ましく、0.06%以下とする。好ましくは、0.001%以上0.055%以下とする。 P: 0.06% or less P segregates at the prior austenite grain boundaries, causing low temperature toughness degradation and workability degradation. Therefore, it is preferable to reduce the amount of P as much as possible, and set it to 0.06% or less. Preferably, the content is 0.001% or more and 0.055% or less.

Ｓ：0.01%以下
Ｓは旧オーステナイト粒界に偏析したりＭｎＳとして多量に析出すると、低温靭性を低下させる。また、加工の有無に関わらず伸びフランジ性を著しく低下させる。そのため、Ｓ量は極力低減することが好ましく、0.01%以下とする。好ましくは、0.0001%以上、0.005%以下とする。 S: 0.01% or less When S segregates at the prior austenite grain boundaries or precipitates in a large amount as MnS, the low temperature toughness is lowered. Further, the stretch flangeability is remarkably lowered regardless of the presence or absence of processing. Therefore, the amount of S is preferably reduced as much as possible, and is 0.01% or less. Preferably, the content is 0.0001% or more and 0.005% or less.

Ａｌ：0.1%以下
Ａｌは、鋼の脱酸剤として添加され、鋼の清浄度を向上させるのに有効な元素である。この効果を得るためには0.001%以上含有させることが好ましい。しかし、0.1%を超えると介在物が多量に発生し、鋼板の疵の原因になる。よって、Ａｌ量は0.1%以下とする。好ましくは、0.01%以上0.04%以下である。 Al: 0.1% or less Al is an element that is added as a deoxidizer for steel and is effective in improving the cleanliness of the steel. In order to acquire this effect, it is preferable to make it contain 0.001% or more. However, if it exceeds 0.1%, a large amount of inclusions are generated, which causes the steel sheet to become wrinkled. Therefore, the Al content is 0.1% or less. Preferably, it is 0.01% or more and 0.04% or less.

Ｔｉ：0.05%以上0.25%以下
Ｔｉは、フェライトを析出強化する上で非常に重要な元素であり、本発明の効果を得る上で、重要な要件となる。Ｔｉ量が0.05%未満では必要な強度を確保することが困難である。一方、0.25%を超えるとその効果は飽和し、コストアップとなるだけである。よって、Ｔｉ量は0.05%以上0.25%以下、好ましくは0.08%以上0.20%以下とする。 Ti: 0.05% or more and 0.25% or less Ti is an extremely important element for precipitation strengthening of ferrite, and is an important requirement for obtaining the effects of the present invention. If the amount of Ti is less than 0.05%, it is difficult to ensure the required strength. On the other hand, if it exceeds 0.25%, the effect is saturated and only the cost is increased. Therefore, the Ti content is 0.05% or more and 0.25% or less, preferably 0.08% or more and 0.20% or less.

Ｖ：0.05%以上0.25%以下
Ｖは、析出強化または固溶強化として強度の向上に寄与する元素であり、上記Ｔｉと並んで、本発明の効果を得る上で、重要な要件となる。適量をＴｉとともに複合添加することで、粒径（以下、「大きさ」と称することもある）20nm未満の微細なＴｉ−Ｖ炭化物として析出する傾向にあり、かつ、Ｍｏのように塗装後耐食性を低下させることはない。Ｖ量が0.05%未満では、上記添加効果が乏しい。一方、Ｖ量が0.25%超えでは、その効果は飽和し、コストアップとなるだけである。よって、Ｖ量は0.05%以上0.25%以下、好ましくは、0.06%以上0.20%以下とする。 V: 0.05% or more and 0.25% or less V is an element that contributes to the improvement of strength as precipitation strengthening or solid solution strengthening, and along with Ti, is an important requirement for obtaining the effects of the present invention. By adding a proper amount together with Ti, there is a tendency to precipitate as fine Ti-V carbide with a particle size of less than 20 nm (hereinafter also referred to as “size”), and corrosion resistance after painting like Mo. Will not be reduced. If the amount of V is less than 0.05%, the effect of addition is poor. On the other hand, if the V amount exceeds 0.25%, the effect is saturated and only the cost is increased. Therefore, the V content is 0.05% or more and 0.25% or less, preferably 0.06% or more and 0.20% or less.

以上の必須添加元素で、本発明鋼は目的とする特性が得られるが、上記の必須添加元素に加えて、以下の理由により、さらにＣｒ：0.01%以上0.5%以下、Ｗ:0.005%以上0.2%以下、Ｚｒ：0.0005%以上0.05%以下のいずれか1種または2種以上を添加してもよい。
Ｃｒ：0.01%以上0.5%以下、Ｗ:0.005%以上0.2%以下、Ｚｒ：0.0005%以上0.05%以下
Ｃｒ、ＷおよびＺｒは、Ｖと同様、析出物を形成してあるいは固溶状態でフェライトを強化する働きを有する。Ｃｒ量が0.01%未満、Ｗ量が0.005%未満、あるいはＺｒ量が0.0005%未満では高強度化にほとんど寄与しない。一方、Ｃｒ量が0.5%超え、Ｗ量が0.2%超え、あるいはＺｒ量が0.05%超えでは加工性が劣化する。したがって、Ｃｒ、Ｗ、Ｚｒのいずれか1種または2種以上を添加する場合、その添加量はＣｒ：0.01%以上0.5%以下、Ｗ:0.005%以上0.2%以下、Ｚｒ：0.0005%以上0.05%以下とする。好ましくは、Ｃｒ：0.03%以上0.3%以下、Ｗ：0.01%以上0.18%以下、Ｚｒ：0.001%以上0.04%以下である。 With the above essential additive elements, the steel of the present invention can achieve the desired characteristics. In addition to the above essential additive elements, Cr: 0.01% to 0.5%, W: 0.005% to 0.2% for the following reasons. % Or less, Zr: Any one or more of 0.0005% or more and 0.05% or less may be added.
Cr: 0.01% or more and 0.5% or less, W: 0.005% or more and 0.2% or less, Zr: 0.0005% or more and 0.05% or less Cr, W, and Zr, like V, form ferrite in solid solution or form precipitates. Has a strengthening function. If the Cr content is less than 0.01%, the W content is less than 0.005%, or the Zr content is less than 0.0005%, it hardly contributes to increasing the strength. On the other hand, if the Cr content exceeds 0.5%, the W content exceeds 0.2%, or the Zr content exceeds 0.05%, workability deteriorates. Therefore, when adding one or more of Cr, W, and Zr, the amount added is Cr: 0.01% to 0.5%, W: 0.005% to 0.2%, Zr: 0.0005% to 0.05% The following. Preferably, Cr: 0.03% to 0.3%, W: 0.01% to 0.18%, Zr: 0.001% to 0.04%.

なお、上記以外の残部はＦｅ及び不可避的不純物からなる。不可避的不純物として、例えば、Ｏは非金属介在物を形成し品質に悪影響を及ぼすため、0.003%以下に低減するのが望ましい。また、本発明では、本発明の作用効果を害さない微量元素として、Ｃｕ、Ｎｉ、Ｓｎ、Ｓｂを0.1%以下の範囲で含有してもよい。   The remainder other than the above consists of Fe and inevitable impurities. As an unavoidable impurity, for example, O forms non-metallic inclusions and adversely affects quality, so it is desirable to reduce it to 0.003% or less. In the present invention, Cu, Ni, Sn, and Sb may be contained in a range of 0.1% or less as trace elements that do not impair the effects of the present invention.

２)次に、本発明の高強度鋼板の組織について説明する。   2) Next, the structure of the high-strength steel sheet of the present invention will be described.

実質的にフェライト単相組織
TSが780MPa以上で、かつ、加工後の伸びフランジ性の向上には、転位密度の低いフェライトが有効であり、かつ、単相組織とすることが有効である。特に、延性に富むフェライト単相組織とすることで、加工後の伸びフランジ性の向上効果が顕著となる。ただし、必ずしも完全にフェライト単相組織でなくてもよく、実質的にフェライト単相組織であれば上記効果は十分に得られる。ここで、実質的にフェライト単相組織とは、本発明の炭化物以外に、微量の他の相ないしは析出物を許容することであり、好ましくはフェライトの体積率が95%以上である。また、体積率が5%までの範囲であれば、セメンタイト、パーライト、ベイナイトの組織を含んでも、本発明の特性に影響ない。
なお、フェライトの体積率は、圧延方向に平行な板厚断面のミクロ組織を3%ナイタールで現出し、走査型電子顕微鏡（ＳＥＭ）を用いて1500倍で板厚1/4位置を観察し、例えば、住友金属テクノロジー株式会社製の画像処理ソフト「粒子解析II」を用いてフェライト面積率を測定することで求められる。 Substantial ferrite single phase structure
In order to improve the stretch flangeability after processing with TS of 780 MPa or more, ferrite having a low dislocation density is effective and it is effective to have a single phase structure. In particular, by using a ferrite single phase structure rich in ductility, the effect of improving stretch flangeability after processing becomes significant. However, it is not always necessary to have a ferrite single-phase structure. If the ferrite single-phase structure is substantially obtained, the above effect can be sufficiently obtained. Here, the ferrite single-phase structure substantially means that a small amount of other phases or precipitates are allowed in addition to the carbide of the present invention, and preferably the ferrite volume fraction is 95% or more. Further, if the volume fraction is in the range of up to 5%, the structure of cementite, pearlite, and bainite will not be affected by the characteristics of the present invention.
In addition, the volume ratio of the ferrite, the microstructure of the plate thickness cross section parallel to the rolling direction appears in 3% nital, using a scanning electron microscope (SEM), observe the plate thickness 1/4 position at 1500 times, For example, it is obtained by measuring the ferrite area ratio using image processing software “particle analysis II” manufactured by Sumitomo Metal Technology.

フェライト単相組織中には、大きさ20nm未満の析出物に含まれるＴｉが200ppm以上1750ppm以下、Ｖが150ppm以上1750ppm以下
本発明の高強度鋼板において、Ｔｉおよび/またはＶを含む析出物は、主に炭化物としてフェライト中に析出している。これは、フェライトにおけるＣの固溶限が小さく、過飽和のＣがフェライト中に炭化物として析出しやすいためと考えられる。そして、このような析出物により軟質のフェライトが硬質化（高強度化）し、780ＭＰａ以上のＴＳが得られることになる。また、YSが高くなり、83%以上のYR(=YS/YR)が得られることになる。 In the ferrite single-phase structure, Ti contained in precipitates having a size of less than 20 nm is 200 ppm or more and 1750 ppm or less, and V is 150 ppm or more and 1750 ppm or less. In the high-strength steel sheet of the present invention, precipitates containing Ti and / or V are: It is precipitated in ferrite mainly as carbides. This is presumably because the solid solubility limit of C in ferrite is small, and supersaturated C tends to precipitate as carbide in the ferrite. And such a precipitate hardens (strengthens) soft ferrite, and a TS of 780 MPa or more is obtained. In addition, YS increases and YR (= YS / YR) of 83% or more is obtained.

高強度鋼板を得るためには、上述したように、析出物は微細化（大きさ20nm未満）し、この微細な析出物（大きさ20nm未満）の割合を高めることが重要である。析出物の大きさが20nm以上では、転位の移動を抑制する効果が小さく、フェライトを十分に硬質化できないため、強度が低下する場合がある。
さらに、検討した結果、塗装後耐食性については、析出物サイズが微細であることが重要であることが明らかとなった。従来のＴｉ系(Ｔｉ単独添加)ＨＳＬＡ鋼においては、Ｔｉの添加量が増すに伴い、析出物は粗大化し易い傾向にある。そのため、このような鋼板では強度低下に伴い塗装後耐食性も低下する傾向にあった。析出物の粗大化に伴う塗装後耐食性の低下の理由は明らかではないが、粗大な析出物は化成結晶の生成、または、成長を阻害するためと考えられる。
以上より、析出物の大きさは20nm未満とすることが好ましい。この20nm未満の微細な析出物は、ＴｉとＶを共に添加することにより達成される。Ｖは主にＴｉと複合炭化物を形成する。理由は明らかではないが、これらの析出物は、本発明範囲の巻取温度内の高温長時間下において、安定的に微細なままで存在することがわかった。 In order to obtain a high-strength steel plate, as described above, it is important to refine the precipitates (less than 20 nm in size) and increase the proportion of the fine precipitates (less than 20 nm in size). If the size of the precipitate is 20 nm or more, the effect of suppressing the movement of dislocations is small, and the ferrite cannot be hardened sufficiently, so that the strength may decrease.
Furthermore, as a result of examination, it became clear that it is important for the post-coating corrosion resistance to have a fine precipitate size. In conventional Ti-based (Ti alone added) HSLA steel, precipitates tend to be coarsened as the amount of Ti added increases. Therefore, in such a steel plate, the corrosion resistance after coating tends to decrease as the strength decreases. The reason for the decrease in post-coating corrosion resistance associated with the coarsening of the precipitate is not clear, but it is thought that the coarse precipitate inhibits the formation or growth of chemical crystals.
From the above, the size of the precipitate is preferably less than 20 nm. This fine precipitate of less than 20 nm is achieved by adding both Ti and V. V mainly forms composite carbide with Ti. Although the reason is not clear, it was found that these precipitates exist stably and finely under a high temperature and a long time within the coiling temperature within the range of the present invention.

さらに、大きさが20nm未満の析出物に含まれるＴｉ量およびＶ量の制御が重要となる。20nm未満の析出物に含まれるＴｉ量が200ppm未満、また、Ｖ量が150ppm未満であると、析出物の数密度が小さくなり、各析出物の間隔が広くなるため、転位の移動を抑制する効果が小さくなることがわかった。そのため、フェライトを十分に硬質化できないため、TSが780ＭＰａ以上の強度が得られなくなる。また、20nm未満の析出物に含まれるＴｉ量が200ppm以上で、20nm未満の析出物に含まれるＶ量が150ppm未満の時は、析出物は粗大化し易い傾向にあるため、ＴＳが780ＭＰａ以上の強度が得られなくなる場合がある。また、20nm未満の析出物に含まれるＴｉ量が200ppm未満で、20nm未満の析出物に含まれるＶ量が150ppm以上の時は、Ｖの析出効率が悪くなるため、ＴＳが780ＭＰａ以上の強度が得られなくなる場合がある。一方、20nm未満の析出物に含まれるＴｉ量が1750ppmを越え、または、Ｖ量が1750ppmを越えて析出すると、塗装後耐食性が低下し、目標の特性が得られなくなる。これは、多量の微細析出物は鋼板表面において、化成結晶の生成、または、成長を阻害するためと考えられる。よって、大きさが20nm未満の析出物に含まれる析出Ｔｉ量および析出Ｖ量は共に満足する必要がある。
さらに大きさ20nm未満の析出物に含まれるTi量とV量の比が0.4≦(Ti/48)/(V/51)≦2.5であるとき、785MPa以上のTSが得られ、より好適な状態となることがわかった。理由は明らかではないが、TiとVの組成比が最適化されることによって、熱的な安定性が向上したためと考えられる。 Furthermore, it is important to control the amount of Ti and the amount of V contained in the precipitate having a size of less than 20 nm. When the amount of Ti contained in the precipitates of less than 20 nm is less than 200 ppm and the amount of V is less than 150 ppm, the number density of the precipitates is reduced and the interval between the precipitates is widened. I found it smaller. Therefore, since the ferrite cannot be hardened sufficiently, a strength of TS of 780 MPa or more cannot be obtained. Further, when the amount of Ti contained in the precipitate of less than 20 nm is 200 ppm or more and the amount of V contained in the precipitate of less than 20 nm is less than 150 ppm, the precipitate tends to be coarsened, so TS is 780 MPa or more. Strength may not be obtained. Further, when the amount of Ti contained in the precipitate of less than 20 nm is less than 200 ppm and the amount of V contained in the precipitate of less than 20 nm is 150 ppm or more, the precipitation efficiency of V deteriorates, so that the strength of TS is 780 MPa or more. It may not be obtained. On the other hand, if the amount of Ti contained in the precipitate of less than 20 nm exceeds 1750 ppm, or if the amount of V exceeds 1750 ppm, the corrosion resistance after coating decreases, and the target characteristics cannot be obtained. This is probably because a large amount of fine precipitates inhibits formation or growth of chemical conversion crystals on the steel sheet surface. Therefore, both the amount of precipitated Ti and the amount of precipitated V contained in the precipitate having a size of less than 20 nm must be satisfied.
Furthermore, when the ratio of Ti amount and V amount contained in precipitates with a size of less than 20 nm is 0.4 ≦ (Ti / 48) / (V / 51) ≦ 2.5, a TS of 785 MPa or more is obtained, and a more suitable state I found out that The reason is not clear, but it is thought that the thermal stability was improved by optimizing the composition ratio of Ti and V.

以上より、大きさが20nm未満の析出物に含まれるＴｉ量は200ppm以上1750ppm以下、Ｖ量は150ppm以上1750ppm以下とする。さらには、大きさ20nm未満の析出物に含まれるTi量とV量の比は0.4≦(Ti/48)/(V/51)≦2.5が好ましい。
なお、析出物及び／又は介在物をまとめて析出物等と称する場合がある。 From the above, the amount of Ti contained in the precipitate having a size of less than 20 nm is set to 200 ppm to 1750 ppm, and the amount of V is set to 150 ppm to 1750 ppm. Furthermore, the ratio of the Ti amount and the V amount contained in the precipitate having a size of less than 20 nm is preferably 0.4 ≦ (Ti / 48) / (V / 51) ≦ 2.5.
Note that precipitates and / or inclusions may be collectively referred to as precipitates.

なお、上記Ｔｉ量およびＶ量は、巻取温度によって制御することができる。この時の巻取温度は500℃以上700℃以下が好ましい。巻取温度が700℃を超えると析出物の粗大化が起こり、20nm未満の析出物に含まれるＴｉおよびＶの析出量がそれぞれ200ppm未満、150ppm未満となり、780ＭＰａ以上のＴＳが得られない。また、巻取温度が500℃未満でも、20nm未満の析出物に含まれるＴｉおよびＶの析出量がそれぞれ200ppm、150ppm未満となる。これは、巻取温度が低いため、ＴｉおよびＶの拡散が不十分なためであると考えられる。   The Ti amount and V amount can be controlled by the coiling temperature. The coiling temperature at this time is preferably 500 ° C. or higher and 700 ° C. or lower. When the coiling temperature exceeds 700 ° C., the precipitate is coarsened, and the precipitation amounts of Ti and V contained in the precipitate of less than 20 nm are less than 200 ppm and less than 150 ppm, respectively, and a TS of 780 MPa or more cannot be obtained. Even when the coiling temperature is less than 500 ° C., the precipitation amounts of Ti and V contained in the precipitates of less than 20 nm become 200 ppm and less than 150 ppm, respectively. This is thought to be because the winding temperature is low and the diffusion of Ti and V is insufficient.

大きさが20nm未満の析出物に含まれるＴｉ量およびＶ量は、以下の方法により確認することができる。
試料を電解液中で所定量電解した後、試料片を電解液から取り出して分散性を有する溶液中に浸漬する。次いで、この溶液中に含まれる析出物を、孔径20nmのフィルタを用いてろ過する。この孔径20nmのフィルタをろ液と共に通過した析出物が大きさ20nm未満である。次いで、ろ過後のろ液に対して、誘導結合プラズマ（ＩＣＰ）発光分光分析法、ＩＣＰ質量分析法、および原子吸光分析法等から適宜選択して分析し、大きさ20nm未満の析出物に含まれるＴｉ量およびＶ量を求める。 The amount of Ti and the amount of V contained in the precipitate having a size of less than 20 nm can be confirmed by the following method.
After the sample is electrolyzed in a predetermined amount in the electrolytic solution, the sample piece is taken out of the electrolytic solution and immersed in a solution having dispersibility. Next, the precipitate contained in the solution is filtered using a filter having a pore diameter of 20 nm. The precipitate that has passed through the filter having a pore diameter of 20 nm together with the filtrate has a size of less than 20 nm. Next, the filtrate after filtration is analyzed by appropriately selecting from inductively coupled plasma (ICP) emission spectroscopy, ICP mass spectrometry, atomic absorption spectrometry, etc., and contained in precipitates having a size of less than 20 nm The amount of Ti and V are determined.

固溶Ｖ量が200ppm以上1750ppm未満の組織
本発明において、固溶Ｖは最も重要な要件である。加工後の伸びフランジ特性向上には、Ｖの固溶が重要である。固溶Ｖが200ppm未満ではその効果に乏しく、上記効果を得るためには固溶Ｖ量は200 ppm以上必要である。一方、固溶Ｖ量が1750ppm以上では、その効果が飽和するために、上限値とした。
以上より、固溶Ｖ量は200ppm以上1750ppm未満とする。なお、本発明鋼も強度の上昇に伴い、加工性が若干低下するが、大きさが20nm未満の析出物に含まれるＴｉ量が1750ppm以下、Ｖ量が1750ppm以下の範囲内では、固溶Ｖ量を200ppm以上とすることで、目標とする加工後の伸びフランジ特性が十分確保される。
なお、固溶Ｖ量が200ppm以上1750ppm未満については、例えば、以下の方法により確認することができる。
試料を非水溶媒系電解液中で所定量だけ電解した後、電解液を分析溶液とし、元素分析を行う。分析方法としては、誘導結合プラズマ（ＩＣＰ）発光分光分析法、ＩＣＰ質量分析法、又は原子吸光分析法等が挙げられる。 Structure in which the amount of solid solution V is 200 ppm or more and less than 1750 ppm In the present invention, solid solution V is the most important requirement. In order to improve stretch flange characteristics after processing, solid solution of V is important. If the solid solution V is less than 200 ppm, the effect is poor. To obtain the above effect, the solid solution V amount needs to be 200 ppm or more. On the other hand, when the amount of solute V is 1750 ppm or more, the effect is saturated, so the upper limit is set.
From the above, the solid solution V amount is 200 ppm or more and less than 1750 ppm. In addition, although the steel of the present invention also has a slight decrease in workability as the strength increases, a solid solution V is contained within a range where the amount of Ti contained in precipitates having a size of less than 20 nm is 1750 ppm or less and the amount of V is 1750 ppm or less. By setting the amount to 200 ppm or more, sufficient stretch flange characteristics after processing are ensured.
In addition, about the amount of solid solution V being 200 ppm or more and less than 1750 ppm, it can confirm with the following method, for example.
After the sample is electrolyzed by a predetermined amount in a non-aqueous solvent electrolyte solution, elemental analysis is performed using the electrolyte solution as an analysis solution. Examples of the analysis method include inductively coupled plasma (ICP) emission spectroscopy, ICP mass spectrometry, atomic absorption spectrometry, and the like.

3）次に、本発明の高強度鋼板の製造方法について説明する。
本発明の高強度鋼板は、例えば、上記化学成分範囲に調整された鋼スラブを、1150℃以上1350℃以下に加熱したのち、仕上げ圧延温度を850℃以上1100℃以下として熱間圧延を行ない、その後、500℃から700℃で巻き取ることにより得られる。これらの好適条件について以下に詳細に説明する。 3) Next, the manufacturing method of the high strength steel plate of this invention is demonstrated.
The high-strength steel sheet of the present invention is, for example, a steel slab adjusted to the above chemical composition range, heated to 1150 ° C. or more and 1350 ° C. or less, and then hot-rolled with a finish rolling temperature of 850 ° C. or more and 1100 ° C. or less, Thereafter, it is obtained by winding at 500 ° C. to 700 ° C. These preferred conditions will be described in detail below.

鋼スラブ加熱温度：1150℃以上1350℃以下
ＴｉあるいはＶなどの炭化物形成元素は、鋼スラブ中ではほとんどが析出物として存在している。熱間圧延後にフェライト組織中に目標どおりに析出させるためには熱間圧延前に炭化物として析出している析出物を一旦溶解させる必要がある。そのためには1150℃以上で加熱する必要がある。
1150℃未満では、析出強化や塗装後耐食性に寄与しない20nm以上の大きさの炭化物が残存するため、本発明の効果を得るために必要な大きさ20nm未満の微細な析出物の生成に関わるTi量およびV量が減少し、後述する巻取時に大きさ20nm未満の析出物の量が目標どおり得られない。さらに本発明の鋼板の製造方法では、スラブ加熱時と仕上げ圧延時にはＴｉやＶを含む炭化物は溶解させたままで、仕上げ圧延後の巻取時にＴｉやＶを含む微細な炭化物として析出させるのが、最も望ましい形態である。そのため、当該炭化物がほぼ完全に溶解する温度として、加熱温度は1170℃以上とするのがより好ましい。
一方、1350℃を超えて加熱すると、結晶粒径が粗大になりすぎて加工後の伸びフランジ特性、伸び特性ともに劣化する。さらに、この後にかかる熱処理条件を考慮すると、1300℃以下とすれば、結晶粒径の粗大化はほぼ完全に防ぐことができる。
よって、スラブ加熱温度は、1150℃以上1350℃以下が好ましい。より好ましくは1170℃以上1300℃以下である。 Steel slab heating temperature: 1150 ° C. or higher and 1350 ° C. or lower Most carbide forming elements such as Ti or V are present as precipitates in the steel slab. In order to precipitate as desired in the ferrite structure after hot rolling, it is necessary to once dissolve precipitates that have precipitated as carbides before hot rolling. For this purpose, it is necessary to heat at 1150 ° C or higher.
If the temperature is lower than 1150 ° C., carbides having a size of 20 nm or more that do not contribute to precipitation strengthening and post-coating corrosion resistance remain, so Ti related to the formation of fine precipitates having a size of less than 20 nm necessary for obtaining the effects of the present invention. The amount of V and the amount of V decrease, and the amount of precipitates having a size of less than 20 nm cannot be obtained as intended during winding as described later. Furthermore, in the method for producing a steel sheet of the present invention, carbides containing Ti and V are dissolved during slab heating and finish rolling, and precipitated as fine carbides containing Ti and V during winding after finish rolling. This is the most desirable form. Therefore, the heating temperature is more preferably 1170 ° C. or higher as the temperature at which the carbide is almost completely dissolved.
On the other hand, when heated above 1350 ° C., the crystal grain size becomes too coarse, and both the stretch flange characteristics and the stretch characteristics after processing deteriorate. Further, considering the heat treatment conditions thereafter, if the temperature is 1300 ° C. or less, the coarsening of the crystal grain size can be prevented almost completely.
Therefore, the slab heating temperature is preferably 1150 ° C. or higher and 1350 ° C. or lower. More preferably, it is 1170 ° C. or higher and 1300 ° C. or lower.

熱間圧延における仕上げ圧延温度：850℃以上1100℃以下
本発明における大きさ20nm未満の析出物に含まれるＴｉ量およびＶ量を得るためには、仕上げ圧延温度の制御が重要となる。加工後の鋼スラブを、熱間圧延の終了温度である仕上げ圧延温度850℃〜1100℃で熱間圧延するのが好ましい。仕上げ圧延温度が850℃未満では、フェライト+オーステナイトの領域で圧延され、展伸したフェライト組織となるため、加工後の伸びフランジ特性や伸び特性が劣化する場合がある。また、鋼スラブ加熱温度を1150℃以上で行い圧延前の炭化物として析出している析出物が一旦溶解されたとしても、仕上げ圧延温度が850℃未満の場合は、歪誘起析出により、ＴｉやＶを含んだ炭化物が析出してしまう。そのため、本発明の効果に必要な大きさ20nm未満の微細な析出物の生成にかかわるＴｉ量およびＶ量が減少し、後述する巻取時にて大きさ20nm未満の析出物の量が目標どおり得られない。つまり、前述のスラブ加熱時に一旦溶解したＴｉやＶを含む炭化物が、この仕上げ圧延においてもなるべく溶解した状態のまま、次の巻取工程に進むのが重要となる。そのため、炭化物が溶解した状態を保つには、仕上げ圧延温度は935℃以上とするのがより好ましい。
一方、仕上げ圧延温度が1100℃を超えると、フェライト粒が粗大化するため、780ＭＰａのＴＳが得られない場合がある。フェライト粒の粗大化を防ぐには、990℃以下とするのがより好ましい。
よって、仕上げ圧延温度は850℃以上1100℃以下が好ましい。より好ましくは、935℃以上990℃以下である。 Finish rolling temperature in hot rolling: 850 ° C. or higher and 1100 ° C. or lower In order to obtain the Ti amount and the V amount contained in the precipitate having a size of less than 20 nm in the present invention, it is important to control the finish rolling temperature. The steel slab after processing is preferably hot-rolled at a finish rolling temperature of 850 ° C. to 1100 ° C., which is the end temperature of hot rolling. If the finish rolling temperature is less than 850 ° C., the ferrite and austenite regions are rolled into a stretched ferrite structure, and thus the stretch flange characteristics and stretch characteristics after processing may be deteriorated. In addition, even if the precipitate precipitated as carbide before rolling at a steel slab heating temperature of 1150 ° C or higher is once dissolved, if the finish rolling temperature is less than 850 ° C, strain induced precipitation causes Ti and V Carbide containing is deposited. Therefore, the amount of Ti and the amount of V related to the formation of fine precipitates having a size of less than 20 nm necessary for the effect of the present invention are reduced, and the amount of precipitates having a size of less than 20 nm can be obtained as intended at the time of winding described later. I can't. That is, it is important that the carbide containing Ti or V once dissolved during the slab heating proceeds to the next winding process while being dissolved as much as possible even in the finish rolling. Therefore, in order to keep the carbide dissolved, the finish rolling temperature is more preferably 935 ° C. or higher.
On the other hand, when the finish rolling temperature exceeds 1100 ° C., ferrite grains are coarsened, so that a TS of 780 MPa may not be obtained. In order to prevent the coarsening of the ferrite grains, the temperature is more preferably 990 ° C. or lower.
Therefore, the finish rolling temperature is preferably 850 ° C. or higher and 1100 ° C. or lower. More preferably, it is 935 ° C. or higher and 990 ° C. or lower.

巻取温度：500℃以上700℃以下
本発明における大きさ20nm未満の析出物に含まれるＴｉ量およびＶ量を得るためには、巻取温度の制御が重要となる。前述したとおり、この巻取工程にて、析出サイトが多数形成され、この析出サイトから炭化物が析出し、かつ、当該炭化物の粒成長が大きさ20nm以上とならないように抑制されることが、最も望ましい製造形態だからである。組織を実質的にフェライト単相組織とし、本発明の特性を得るためには、巻取温度は500℃以上700℃以下が好ましい。
本発明では、巻取温度が500℃未満では、Ｔｉおよび/またはＶを含む炭化物の析出量が不十分となり、強度低下を招く場合がある。また、ベイナイト相が生成し、フェライト単相組織が得られない場合がある。
析出サイトが多数形成され、かつこの析出サイトから炭化物を生成させるには、温度はより高い方が好ましく、550℃以上であるのがより好ましい条件となる。
一方、巻取温度が700℃を超えると、析出した炭化物の粗大化が起こり、強度低下を招く場合がある。また、パーライト相が生成しやすくなり、加工後の伸びフランジ性の低下を招く場合がある。650℃以下とすれば、確実に析出した炭化物の粗大化が防げるのでより好ましい。
よって、巻取温度は500℃以上700℃以下が好ましく、より好ましくは、550℃以上650℃以下である。 Winding temperature: 500 ° C. or more and 700 ° C. or less In order to obtain the Ti amount and the V amount contained in the precipitate having a size of less than 20 nm in the present invention, it is important to control the winding temperature. As described above, in this winding process, a large number of precipitation sites are formed, carbides are precipitated from the precipitation sites, and the grain growth of the carbides is suppressed so as not to exceed 20 nm in size. This is because it is a desirable manufacturing form. The coiling temperature is preferably 500 ° C. or higher and 700 ° C. or lower in order to make the structure substantially a ferrite single phase structure and obtain the characteristics of the present invention.
In the present invention, when the coiling temperature is less than 500 ° C., the amount of precipitation of carbides containing Ti and / or V becomes insufficient, and the strength may be reduced. In addition, a bainite phase may be generated and a ferrite single phase structure may not be obtained.
In order to form a large number of precipitation sites and to generate carbides from the precipitation sites, the temperature is preferably higher, and more preferably 550 ° C. or higher.
On the other hand, when the coiling temperature exceeds 700 ° C., the precipitated carbide is coarsened, which may cause a decrease in strength. In addition, a pearlite phase is likely to be generated, which may cause a reduction in stretch flangeability after processing. If it is 650 degrees C or less, since the coarsening of the carbide | carbonized_material which precipitated reliably can be prevented, it is more preferable.
Therefore, the coiling temperature is preferably 500 ° C. or higher and 700 ° C. or lower, and more preferably 550 ° C. or higher and 650 ° C. or lower.

本発明の鋼板には、表面に表面処理や表面被覆処理を施したものを含む。特に、本発明の鋼板には溶融亜鉛系めっき皮膜を形成し、溶融亜鉛めっき系鋼板としたものに好適に適用できる。すなわち、本発明の鋼板は良好な加工性を有することから、溶融亜鉛系めっき皮膜を形成しても良好な加工性を維持できる。ここで、溶融亜鉛系めっきとは、亜鉛および亜鉛を主体とした(すなわち約90%以上を含有する)溶融めっきであり、亜鉛のほかにＡｌ、Ｃｒなどの合金元素を含んだものも含む、また、溶融亜鉛系めっきを施したままでも、めっき後に合金化処理を行なってもかまわない。   The steel sheet of the present invention includes those having a surface subjected to surface treatment or surface coating treatment. In particular, the steel sheet of the present invention can be suitably applied to a steel sheet obtained by forming a hot-dip galvanized coating film on the hot-dip galvanized steel sheet. That is, since the steel sheet of the present invention has good workability, good workability can be maintained even when a hot dip galvanized film is formed. Here, the hot dip galvanizing is hot dip plating mainly composed of zinc and zinc (that is, containing about 90% or more), including those containing alloy elements such as Al and Cr in addition to zinc. Moreover, even if hot dip galvanizing is performed, alloying treatment may be performed after plating.

なお、鋼の溶製方法は特に限定されず、公知の溶製方法の全てを適応することができる。例えば、溶製方法としては、転炉、電気炉等で溶製し、真空脱ガス炉にて2次精錬を行なう方法が好適である。鋳造方法は、生産性、品質上の観点から、連続鋳造が好ましい。また、鋳造後、直ちに、または補熱を目的とする加熱を施した後に、そのまま熱間圧延を行なう直送圧延を行なっても、本発明の効果に影響はない。さらに、粗圧延後に、仕上圧延前で、熱延材を加熱してもよく、粗圧延後に圧延材を接合して行なう連続熱延を行なっても、さらには、圧延材の加熱材の加熱と連続圧延を同時に行なっても、本発明の効果は損なわれない。   In addition, the melting method of steel is not specifically limited, All the known melting methods can be applied. For example, as the melting method, a method of melting in a converter, electric furnace or the like and performing secondary refining in a vacuum degassing furnace is suitable. The casting method is preferably continuous casting from the viewpoint of productivity and quality. Further, the effect of the present invention is not affected even if the direct feed rolling, in which the hot rolling is performed as it is, immediately after casting or after heating for the purpose of supplementary heating is performed. Furthermore, after the rough rolling, before the finish rolling, the hot rolled material may be heated, or even if the continuous hot rolling is performed by joining the rolled material after the rough rolling, and further, the heating material of the rolled material is heated. Even if continuous rolling is performed simultaneously, the effect of the present invention is not impaired.

表1に示す組成の鋼を転炉で溶製し、連続鋳造により鋼スラブとした。次いで、これらの鋼スラブに対して、表2に示す条件で加熱、熱間圧延、巻取りを施し板厚2.0mmの熱延鋼板を作製した。   Steels having the compositions shown in Table 1 were melted in a converter and steel slabs were obtained by continuous casting. Subsequently, these steel slabs were heated, hot-rolled and wound under the conditions shown in Table 2 to produce hot-rolled steel sheets having a thickness of 2.0 mm.

Figure 2009052139
Figure 2009052139

得られた熱延鋼板に対して、以下に示す方法でミクロ組織を解析し、20nm未満の析出物に含まれるＴｉ量およびＶ量と、固溶Ｖの量を求めた。また、以下に示す方法で引張強度：ＴＳ、加工後の伸びフランジ特性：λ10および塗装後耐食性：ＳＤＴ片側最大剥離幅を求め、評価した。 With respect to the obtained hot-rolled steel sheet, the microstructure was analyzed by the following method, and the amounts of Ti and V contained in precipitates of less than 20 nm and the amount of solute V were determined. Further, tensile strength: TS, stretch flange characteristic after processing: λ 10 and corrosion resistance after coating: SDT one-side maximum peel width were determined and evaluated by the following methods.

ミクロ組織の解析
上記により得られた熱延鋼板を適当な大きさに切断し、10％AA系電解液（10vol％アセチルアセトン-1mass％塩化テトラメチルアンモニウム-メタノール）中で、約0.2gを電流密度20mA/cm2で定電流電解した。 Microstructure analysis The hot-rolled steel sheet obtained as described above is cut to an appropriate size, and about 0.2 g of current density is obtained in 10% AA electrolyte (10 vol% acetylacetone-1 mass% tetramethylammonium chloride-methanol). Constant current electrolysis was performed at 20 mA / cm 2 .

大きさ20nm未満の析出物に含まれるＴｉ量およびＶの量の測定
電解後の、表面に析出物が付着している試料片を電解液から取り出して、ヘキサメタリン酸ナトリウム水溶液（500mg/l）（以下、SHMP水溶液と称す）中に浸漬し、超音波振動を付与して、析出物を試料片から剥離しSHMP水溶液中に抽出した。次いで、析出物を含むSHMP水溶液を、孔径20nmのフィルタを用いてろ過し、ろ過後のろ液に対してICP発光分光分析装置を用いて分析し、ろ液中のＴｉとＶの絶対量を測定した。次いで、ＴｉとＶの絶対量を電解重量で除して、大きさ20nm未満の析出物に含まれるＴｉ量およびＶ量を得た。なお、電解重量は、析出物剥離後の試料に対して重量を測定し、電解前の試料重量から差し引くことで求めた。 Measurement of the amount of Ti and V contained in precipitates having a size of less than 20 nm After the electrolysis, a sample piece with precipitates attached to the surface was taken out from the electrolyte solution, and an aqueous solution of sodium hexametaphosphate (500 mg / l) ( (Hereinafter referred to as “SHMP aqueous solution”), ultrasonic vibration was applied, and the precipitate was peeled from the sample piece and extracted into the SHMP aqueous solution. Next, the SHMP aqueous solution containing the precipitate is filtered using a filter with a pore size of 20 nm, and the filtrate after filtration is analyzed using an ICP emission spectroscopic analyzer, and the absolute amounts of Ti and V in the filtrate are determined. It was measured. Next, the absolute amounts of Ti and V were divided by the electrolytic weight to obtain the Ti amount and the V amount contained in the precipitate having a size of less than 20 nm. In addition, the electrolysis weight was calculated | required by measuring a weight with respect to the sample after deposit peeling, and subtracting from the sample weight before electrolysis.

固溶Ｖの量の測定
電解後の電解液を分析溶液とし、ＩＣＰ質量分析法を用いてＶおよび比較元素としてFeの液中濃度を測定した。得られた濃度を基に、Ｆｅに対するＶの濃度比を算出し、さらに、試料中のＦｅの含有率を乗じることで、固溶状態にあるＶの量を求めた。なお、試料中のＦｅの含有率は、Ｆｅ以外の組成値の合計を100%から減算することで求めることができる。 Measurement of the amount of the solid solution V The electrolytic solution after electrolysis was used as an analysis solution, and ICP mass spectrometry was used to measure the concentration of V as a comparative element and Fe in the solution. Based on the obtained concentration, the concentration ratio of V to Fe was calculated, and further, the amount of V in a solid solution state was determined by multiplying by the Fe content in the sample. In addition, the content rate of Fe in a sample can be calculated | required by subtracting the sum total of composition values other than Fe from 100%.

ＴＳ
圧延方向を引張り方向としてＪＩＳ5号試験片を用いてＪＩＳ Ｚ 2241に準拠した方法で引張り試験を行ない、ＴＳを求めた。 TS
A tensile test was performed by a method based on JIS Z 2241 using a JIS No. 5 test piece with the rolling direction as the tensile direction, and TS was obtained.

加工後の伸びフランジ特性：λ10
伸張率10%で圧延後、鉄連規格ＪＦＳＴ 1001に準じて穴広げ試験を行ない、λ10を求めた。 Stretch flange characteristics after processing: λ 10
After rolling at an elongation rate of 10%, a hole expansion test was performed in accordance with the iron standard JFST 1001, and λ 10 was obtained.

塗装後耐食性：ＳＤＴ片側最大剥離幅
化成処理は、日本ペイント（株）製の脱脂剤；サーフクリーナーＥＣＯ９０、表面調整剤；サーフファイン５Ｎ−１０、化成処理剤；サーフダインＳＤ２８００用い、それぞれの温度や濃度条件は標準条件より劣悪な条件で実施した。標準条件の1例として、脱脂工程は濃度16g/l、処理温度42〜44℃、処理時間120s、スプレー脱脂、表面調整工程は、全アルカリ度1.5〜2.5ポイント、遊離酸度0.7〜0.9ポイント、促進剤濃度2.8〜3.5ポイント、処理温度44℃、処理時間120sとした。劣悪条件としては、化成処理工程での処理温度を38℃に低下させた。その後、日本ペイント社製の電着塗装剤；Ｖ-50を使用して電着塗装を行った。化成処理皮膜の付着量は2〜2.5g/m2、電着塗装は膜厚25μmを狙いとした。
塗装後耐食性の評価は、塩温水浸漬試験(ＳＤＴ)で行なった。化成処理、電着塗装を施した試料にカッターにてクロスカット疵を付与し、塩温水(5%NaCl：55℃)に10日間浸漬したのち、水洗、乾燥し、カット疵部についてテープ剥離を行い、カット疵部左右の最大剥離幅を測定した。片側最大剥離幅が3.0mm以下であれば、塗装後耐食性は良好といえる。
以上により得られた結果を表２に製造条件と併せて示す。 Corrosion resistance after coating: SDT one-side maximum peel width chemical conversion treatment was performed using a degreasing agent manufactured by Nippon Paint Co., Ltd .; Surf Cleaner ECO90, Surface Conditioning Agent; Concentration conditions were performed under conditions worse than the standard conditions. As an example of standard conditions, the degreasing process has a concentration of 16 g / l, the processing temperature is 42 to 44 ° C., the processing time is 120 s, the spray degreasing, and the surface conditioning process has a total alkalinity of 1.5 to 2.5 points and a free acidity of 0.7 to 0.9 points. The agent concentration was 2.8 to 3.5 points, the treatment temperature was 44 ° C., and the treatment time was 120 s. As an inferior condition, the treatment temperature in the chemical conversion treatment step was lowered to 38 ° C. Thereafter, electrodeposition coating was performed using an electrodeposition coating agent; V-50 manufactured by Nippon Paint Co., Ltd. The amount of chemical conversion coating was 2 to 2.5 g / m 2 , and electrodeposition coating was aimed at a film thickness of 25 μm.
The corrosion resistance after coating was evaluated by a salt warm water immersion test (SDT). After applying chemical conversion treatment and electrodeposition coating, a cross-cut wrinkle was applied with a cutter, immersed in salt warm water (5% NaCl: 55 ° C) for 10 days, then washed with water and dried, and the tape was peeled off from the cut wrinkle. The maximum peel width on the left and right sides of the cut collar was measured. If the maximum peel width on one side is 3.0 mm or less, it can be said that the corrosion resistance after coating is good.
The results obtained as described above are shown in Table 2 together with the production conditions.

Figure 2009052139
Figure 2009052139

表２より、本発明例では、ＴＳが780ＭＰａ以上、λ10が60%以上であり、ＳＤＴ片側最大剥離幅3.0mm以下となっており、加工後の伸びフランジ特性および塗装後耐食性に優れた熱延鋼板であることがわかる。 From Table 2, in the present invention example, TS is 780 MPa or more, λ 10 is 60% or more, SDT one-side maximum peel width is 3.0 mm or less, and heat excellent in stretch flange characteristics after processing and corrosion resistance after painting. It turns out that it is a rolled steel sheet.

一方、比較例は、ＴＳ（強度）、λ10（加工後の伸びフランジ姓）、ＳＤＴ片側最大剥離幅（塗装後耐食性）のいずれか1つ以上が劣っている。 On the other hand, the comparative example is inferior in any one or more of TS (strength), λ 10 (elongated flange after processing), and SDT one-side maximum peel width (corrosion resistance after painting).

表３に示す組成の鋼を転炉で溶製し、連続鋳造により鋼スラブとした。次いで、これらの鋼スラブに対して、表４に示す条件で加熱、熱間圧延、巻取りを施し板厚2.0mmの熱延鋼板を作製した。   Steel having the composition shown in Table 3 was melted in a converter, and a steel slab was formed by continuous casting. Subsequently, these steel slabs were heated, hot-rolled and wound under the conditions shown in Table 4 to produce hot-rolled steel sheets having a thickness of 2.0 mm.

Figure 2009052139
Figure 2009052139

得られた熱延鋼板に対して、実施例１と同様の方法でミクロ組織を解析し、20nm未満の析出物に含まれるＴｉ量およびＶ量と、固溶Ｖの量を求めた。また、実施例１と同様の方法で引張強度：ＴＳ、加工後の伸びフランジ特性：λ10および塗装後耐食性：ＳＤＴ片側最大剥離幅を求め、評価した。
以上により得られた結果を表４に示す。 The microstructure of the obtained hot-rolled steel sheet was analyzed in the same manner as in Example 1, and the amounts of Ti and V contained in precipitates of less than 20 nm and the amount of solute V were determined. Further, tensile strength: TS, stretched flange characteristics after processing: λ 10 and corrosion resistance after coating: SDT one side maximum peel width were determined and evaluated in the same manner as in Example 1.
Table 4 shows the results obtained as described above.

Figure 2009052139
Figure 2009052139

表４より、本発明例では、ＴＳが780ＭＰａ以上、λ10が60%以上であり、ＳＤＴ片側最大剥離幅3.0mm以下となっており、加工後の伸びフランジ特性および塗装後耐食性に優れた熱延鋼板であることがわかる。
さらに、鋼板No.1（表2）に比べて、Ｃｒ、ＷやＺｒを添加した鋼板No25〜28、35〜37においては、ＴＳがより向上していることがわかる。 From Table 4, in the present invention example, TS is 780 MPa or more, λ 10 is 60% or more, SDT one-side maximum peel width is 3.0 mm or less, and heat excellent in stretch flange characteristics after processing and corrosion resistance after coating. It turns out that it is a rolled steel sheet.
Furthermore, it can be seen that TS is further improved in steel plates Nos. 25 to 28 and 35 to 37 to which Cr, W and Zr are added, compared to steel plate No. 1 (Table 2).

本発明の鋼板は高強度であり、かつ、優れた加工後の伸びフランジ特性および塗装後耐食性を有するので、例えば、自動車やトラック用のフレーム等、伸びおよび伸びフランジ特性を必要とする部品として最適である。   The steel sheet of the present invention has high strength and excellent stretch flange characteristics after processing and corrosion resistance after coating, so it is ideal for parts that require stretch and stretch flange characteristics, such as frames for automobiles and trucks. It is.

Claims (5)

mass%で、C：0.02%以上0.20%以下、Si：0.3%以下、Mn：0.5%以上2.5%以下、P：0.06%以下、S：0.01%以下、Al：0.1%以下、Ti：0.05%以上0.25%以下、V：0.05%以上0.25%以下を含有し、残部がFeおよび不可避的不純物からなる成分組成と、実質的にフェライト単相組織であり、前記フェライト単相組織中には、大きさが20nm未満の析出物に含まれるTiが200mass ppm以上1750mass ppm以下、Vが150 mass ppm以上1750 mass ppm以下であり、固溶Vが200 mass ppm以上1750 mass ppm未満である組織を有することを特徴とする高強度鋼板。   In mass%, C: 0.02% to 0.20%, Si: 0.3% or less, Mn: 0.5% to 2.5%, P: 0.06% or less, S: 0.01% or less, Al: 0.1% or less, Ti: 0.05% 0.25% or less, V: 0.05% or more and 0.25% or less, with the balance being a component composition consisting of Fe and inevitable impurities, and substantially a ferrite single-phase structure, It has a structure in which Ti contained in precipitates with a thickness of less than 20 nm is 200 mass ppm or more and 1750 mass ppm or less, V is 150 mass ppm or more and 1750 mass ppm or less, and solid solution V is 200 mass ppm or more and less than 1750 mass ppm. High strength steel plate characterized by mass%で、さらに、Cr：0.01%以上0.5%以下、W:0.005%以上0.2%以下、Zr：0.0005%以上0.05%以下のいずれか1種または2種以上を含有することを特徴とする請求項１に記載の高強度鋼板。   It is mass%, and further contains Cr: 0.01% or more and 0.5% or less, W: 0.005% or more and 0.2% or less, Zr: 0.0005% or more and 0.05% or less. Item 5. A high-strength steel sheet according to Item 1. 引張強度ＴＳが780ＭＰａ以上であることを特徴とする請求項１または２に記載の高強度鋼板。   The high-strength steel sheet according to claim 1 or 2, wherein the tensile strength TS is 780 MPa or more. 塩温水浸漬試験におけるテープ剥離試験後の片側最大剥離幅が3.0mm以下であることを特徴とする請求項１〜３のいずれかに記載の高強度鋼板。   The high-strength steel sheet according to any one of claims 1 to 3, wherein a one-side maximum peel width after a tape peel test in a salt warm water immersion test is 3.0 mm or less. 伸張率10%での圧延後の伸びフランジ特性λ10が60%以上であることを特徴とする請求項１〜４のいずれかに記載の高強度鋼板。 The high-strength steel sheet according to any one of claims 1 to 4, wherein the stretch flange characteristic λ 10 after rolling at an elongation rate of 10% is 60% or more.
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