JP4974341B2 - High-strength composite steel sheet with excellent formability, spot weldability, and delayed fracture resistance - Google Patents

High-strength composite steel sheet with excellent formability, spot weldability, and delayed fracture resistance Download PDF

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JP4974341B2
JP4974341B2 JP2006156442A JP2006156442A JP4974341B2 JP 4974341 B2 JP4974341 B2 JP 4974341B2 JP 2006156442 A JP2006156442 A JP 2006156442A JP 2006156442 A JP2006156442 A JP 2006156442A JP 4974341 B2 JP4974341 B2 JP 4974341B2
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steel sheet
ferrite
delayed fracture
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JP2007321237A (en
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道治 中屋
陽一 向井
公一 杉本
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Kobe Steel Ltd
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Priority to US12/303,566 priority patent/US20100221138A1/en
Priority to PCT/JP2007/061301 priority patent/WO2007142197A1/en
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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
    • 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
    • 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
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • 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
    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
    • 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
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • 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
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
    • 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
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
    • 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
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • 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
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese

Description

本発明は、例えば９８０ＭＰａ級以上の引張強度を有すると共に、成形性と耐遅れ破壊性、更にはスポット溶接性に優れ、自動車用構造部材（ピラー、メンバー、リンフォース類などのボディー骨格部材、バンパー、ドアガードバー、シート部品、足回り部品その他の強化部材）などとして有用な高強度の複合組織鋼板に関するものである。   The present invention has a tensile strength of, for example, a 980 MPa class or more, is excellent in formability, delayed fracture resistance, and spot weldability, and is a structural member for automobiles (body skeleton members such as pillars, members, and reinforcements, bumpers) , Door guard bars, seat parts, underbody parts, and other reinforcing members).

近年、自動車などの車体重量の軽量化による燃費の軽減や、衝突時の安全性確保などを目的として高強度鋼板の需要はますます増大している。それに伴って、鋼板の引張強度に対する要望も、従来の５９０ＭＰａ級から９８０ＭＰａ級以上が求められる様になっている。しかし、９８０ＭＰａ級以上の高強度鋼板になると成形性の低下が避けられず、複雑形状の部品には適用し難いため用途面の制約を受ける。   In recent years, the demand for high-strength steel sheets has been increasing for the purpose of reducing fuel consumption by reducing the weight of automobile bodies and the like and ensuring safety in the event of a collision. In connection with it, the request | requirement with respect to the tensile strength of a steel plate is also calculated | required from the conventional 590 MPa class to the 980 MPa class or more. However, when it becomes a high-strength steel plate of 980 MPa class or higher, a decrease in formability is unavoidable, and it is difficult to apply it to a component having a complicated shape.

また、引張強度が９８０ＭＰａ級以上の高強度鋼板になると、プレス成形時に生じる残留応力も大きくなって遅れ破壊の危険性が高まる。即ち遅れ破壊は、特に高強度の鋼板において、腐食環境もしくは雰囲気中の水素が鋼材組織中の転位や空孔、粒界などに拡散・集積して材料を脆化させ、応力が付加された時に破壊を起こす現象であり、鋼材の延性や靭性に重大な影響をもたらす。   Further, when a high-strength steel sheet having a tensile strength of 980 MPa or higher is obtained, the residual stress generated during press forming is increased and the risk of delayed fracture is increased. In other words, delayed fracture occurs especially in high-strength steel sheets when hydrogen in a corrosive environment or atmosphere diffuses and accumulates in dislocations, vacancies, and grain boundaries in the steel structure, embrittles the material, and stress is applied. It is a phenomenon that causes fracture, and has a significant effect on the ductility and toughness of steel.

従って、上述した様な高強度化の要請に応えるには、強度に加えて成形性（即ち、伸びと伸びフランジ性）と耐遅れ破壊性の改善が極めて重要となる。   Therefore, in order to meet the demand for higher strength as described above, improvement in formability (ie, elongation and stretch flangeability) and delayed fracture resistance in addition to strength is extremely important.

ところで、優れた成形性を示す高強度鋼板としては、金属組織中に残留オーステナイトを含む種々の鋼板が実用化されている。   By the way, as a high-strength steel sheet exhibiting excellent formability, various steel sheets containing retained austenite in a metal structure have been put into practical use.

例えば、非特許文献１には、金属組織がベイニティックフェライト主体でラス状残留オーステナイトを有する複合組織とすることで、高強度を確保しつつ穴拡げ性（即ち、伸びフランジ性）を高めた鋼板が開示されている。しかしこの鋼板は、引張強度（ＴＳ）が９８０ＭＰａ級以上になると、強度（ＴＳ）・延性（Ｅｌ）の指標となるＴＳ×Ｅｌでせいぜい９０００〜１０３００を示すに止まり、満足し得るものとはいえない。   For example, in Non-Patent Document 1, the metal structure is a composite structure mainly composed of bainitic ferrite and having lath-like retained austenite, so that the hole expandability (that is, stretch flangeability) is improved while ensuring high strength. A steel sheet is disclosed. However, when the tensile strength (TS) becomes 980 MPa class or more, this steel sheet can only satisfy 9000 to 10300 in TS × El, which is an index of strength (TS) and ductility (El), and can be said to be satisfactory. Absent.

また、連続焼鈍炉を用いた実操業の量産ラインでの最高加熱温度は９００℃程度、加熱時間は５分以下とされているが、この文献に開示されている製造条件では、９５０℃で１２００秒の焼鈍の後、ソルトバスで３５０〜４００℃まで冷却することを求めており、実操業にそぐわない。   Moreover, the maximum heating temperature in the mass production line of the actual operation using the continuous annealing furnace is about 900 ° C., and the heating time is 5 minutes or less. However, in the manufacturing conditions disclosed in this document, 1200 at 950 ° C. is 1200. After annealing for 2 seconds, it is required to cool to 350 to 400 ° C. with a salt bath, which is not suitable for actual operation.

また特許文献１には、母相をベイニティックフェライト主体の組織とし、３％以上の残留オーステナイトを含有させることで、９８０ＭＰａ級以上の引張強度を確保しつつ、伸び（Ｅｌ）で２０％程度、伸びフランジ性（λ）で５５％レベルを得ている。しかしこの技術では、高価な合金元素であるＭｏやＮｉ，Ｃｕなどの添加が不可欠であり、コスト的に改善の余地を残している。   Patent Document 1 discloses that the matrix is composed mainly of bainitic ferrite and contains 3% or more of retained austenite, thereby securing a tensile strength of 980 MPa or more and an elongation (El) of about 20%. Further, 55% level is obtained in stretch flangeability (λ). However, in this technique, addition of expensive alloy elements such as Mo, Ni, and Cu is indispensable, leaving room for improvement in terms of cost.

更に特許文献２には、母相組織を焼戻しマルテンサイトおよびフェライトとし、残留オーステナイトを占積率で５〜３０％とすることで、高レベルの伸びと伸びフランジ性を得ている。しかしこの技術で求める金属組織を得るには、焼鈍前のミクロ組織が重要となるため、熱延工程で低温巻取りを行うことで適正な金属組織を取込んだ後に連続焼鈍を行うか、連続焼鈍を２回以上行う必要がある。ところが、熱延工程で低温巻取りを行った場合は、その後の冷間圧延率を低く抑えないと焼鈍前の組織が崩れて意図する金属組織が得られないため、板厚や板厚公差に著しい制約が加わる。また連続焼鈍を２回行う場合は、板厚などの制約は受けないものの、常法に比べて工程数が増えるためコストアップが避けられない。   Furthermore, Patent Document 2 obtains a high level of elongation and stretch flangeability by setting the matrix structure to tempered martensite and ferrite and the retained austenite to be 5 to 30% in space factor. However, since the microstructure before annealing is important for obtaining the metal structure required by this technique, continuous annealing is performed after the proper metal structure is taken in by performing low-temperature winding in the hot rolling process, or continuous It is necessary to perform annealing twice or more. However, when low-temperature winding is performed in the hot rolling process, if the subsequent cold rolling rate is not kept low, the structure before annealing will collapse and the intended metal structure will not be obtained, so the thickness and thickness tolerance will be Significant restrictions are added. Further, when continuous annealing is performed twice, although there are no restrictions such as plate thickness, the number of processes is increased as compared with the conventional method, and thus an increase in cost is inevitable.

更に特許文献３には、母相組織を焼戻しベイナイト主体とすることで全伸びと伸びフランジ性を高めた鋼板が開示されている。しかしこの鋼種は、引張強度で９００ＭＰａ級以下を中心に検討されているため、９８０ＭＰａ級以上で特に問題となる遅れ破壊については十分考慮されていない。
ISIJ International,Vol.40(2000),No.9.p920-926 特開２００４−３３２０９９号公報 特開２００３−１７１７３５号公報 特開２００２−３０９３３４号公報 Further, Patent Document 3 discloses a steel sheet having a total phase structure and stretch flangeability improved by using a matrix structure mainly composed of tempered bainite. However, since this steel type has been studied mainly with a tensile strength of 900 MPa class or less, delayed fracture, which is a particular problem at 980 MPa class or more, has not been sufficiently considered.
ISIJ International, Vol.40 (2000), No.9.p920-926 JP 2004-332099 A JP 2003-171735 A JP 2002-309334 A

本発明は上記の様な従来技術に鑑みてなされたものであり、その目的は、Ｍｏ，Ｎｉ，Ｃｕといった高価な合金元素を添加することなく、自動車用構造部品などとして有用な９８０ＭＰａ級の引張強度を有し、且つ優れた成形性（伸び−伸びフランジ性）を有する他、スポット溶接性や耐遅れ破壊性にも優れた高強度鋼板を提供することにある。   The present invention has been made in view of the prior art as described above. The purpose of the present invention is to provide a tensile of 980 MPa class useful as a structural component for automobiles without adding expensive alloy elements such as Mo, Ni, and Cu. Another object of the present invention is to provide a high-strength steel sheet having strength and excellent formability (elongation-stretch flangeability), as well as excellent spot weldability and delayed fracture resistance.

上記課題を解決することのできた本発明の高強度複合組織鋼板とは、Ｃ：０．１２〜０．２５％、Ｓｉ：１．０〜３．０％、Ｍｎ：１．５〜３．０％、Ｐ：０．１５％以下（０％を含まない）、Ｓ：０．０２％以下（０％を含まない）、Ａｌ：０．４％以下（０％を含まない）、を満足し、残部が鉄および不可避不純物よりなる鋼からなり、上記Ｓｉ，Ａｌ，Ｍｎの含有量が下記式（Ｉ）の関係を満たし、
（Ｓｉ＋Ａｌ）／Ｍｎ：０．７４〜１．２６……（Ｉ）
或は、Ｃ：０．１２〜０．２５％、Ｓｉ：１．０〜３．０％、Ｍｎ：１．５〜３．０％、Ｃｒ：１．０％以下（０％を含まない）、Ｐ：０．１５％以下（０％を含まない）、Ｓ：０．０２％以下（０％を含まない）、Ａｌ：０．４％以下（０％を含まない）、を満足し、残部が鉄および不可避不純物よりなる鋼からなり、上記Ｓｉ，Ａｌ，Ｍｎ，Ｃｒの含有量が下記式（II）の関係を満たし、
（Ｓｉ＋Ａｌ）／（Ｍｎ＋Ｃｒ）：０．７４〜１．２６……（II）
なお且つ、縦断面のミクロ組織が、全組織に対する占積率で、
１）ベイニティックフェライト：５０％以上、
２）ポリゴナルフェライト：５〜３５％、
３）ポリゴナルフェライトの平均粒径：１０μｍ以下、
４）残留オーステナイト：５％以上
であることを特徴とする、成形性と耐遅れ破壊性優れた高強度複合組織鋼板である。 The high-strength composite steel sheet according to the present invention that has solved the above problems is C: 0.12-0.25%, Si: 1.0-3.0%, Mn: 1.5-3.0. %, P: 0.15% or less (not including 0%), S: 0.02% or less (not including 0%), Al: 0.4% or less (not including 0%) The balance is made of steel consisting of iron and inevitable impurities, and the content of Si, Al, Mn satisfies the relationship of the following formula (I)
(Si + Al) / Mn: 0.74 to 1.26 (I)
Or, C: 0.12-0.25%, Si: 1.0-3.0%, Mn: 1.5-3.0%, Cr: 1.0% or less (excluding 0%) , P: 0.15% or less (not including 0%), S: 0.02% or less (not including 0%), Al: 0.4% or less (not including 0%), The balance is made of steel consisting of iron and inevitable impurities, and the content of Si, Al, Mn, Cr satisfies the relationship of the following formula (II),
(Si + Al) / (Mn + Cr): 0.74 to 1.26 (II)
In addition, the microstructure of the longitudinal section is the space factor for the entire structure,
1) Bainitic ferrite: 50% or more
2) Polygonal ferrite: 5-35%,
3) Average particle size of polygonal ferrite: 10 μm or less,
4) Residual austenite: A high-strength steel sheet with excellent formability and delayed fracture resistance, characterized by being 5% or more.

本発明に係る上記複合組織鋼板は、必要により上記元素以外に、Ｔｉ：０．１５％以下（０％を含まない）および／またはＮｂ：０．１％以下（０％を含まない）が含まれていてもよく、あるいは更に他の元素として、Ｃａ：３０ｐｐｍ以下（０％を含まない）および／またはＲＥＭ：３０ｐｐｍ以下（０％を含まない）を含むものであってもよい。   The composite structure steel plate according to the present invention includes Ti: 0.15% or less (not including 0%) and / or Nb: 0.1% or less (not including 0%), if necessary, in addition to the above elements. Alternatively, as another element, Ca: 30 ppm or less (not including 0%) and / or REM: 30 ppm or less (not including 0%) may be included.

本発明の高強度複合組織鋼板は、その優れた強度をより有効に生かすため、引張強さで９８０ＭＰａ以上を有するものが特に好ましい。   The high-strength composite steel sheet of the present invention preferably has a tensile strength of 980 MPa or more in order to make effective use of its excellent strength.

本発明によれば、上記の様に鋼材の化学成分を特定し、特に（Ｓｉ＋Ａｌ）／Ｍｎ比または（Ｓｉ＋Ａｌ）／（Ｍｎ＋Ｃｒ）比を特定範囲に制御すると共に、金属組織をベイニティックフェライト（ＢＦ）主体で、ポリゴナルフェライト（ＰＦ）と残留オーステナイト（残留γ）を含む複合組織とすることで、引張強さで例えば９８０ＭＰａレベル以上を確保しつつ、なお且つ成形性（伸び−伸びフランジ性）が良好で、しかもスポット溶接性や耐遅れ破壊性にも優れた複合組織鋼板を安価に提供できる。   According to the present invention, the chemical composition of the steel material is specified as described above, and in particular, the (Si + Al) / Mn ratio or the (Si + Al) / (Mn + Cr) ratio is controlled to a specific range, and the metal structure is bainitic ferrite ( By forming a composite structure containing mainly BF) and polygonal ferrite (PF) and retained austenite (residual γ), for example, a tensile strength of, for example, a level of 980 MPa or more is secured, and the formability (stretch-stretch flangeability) ) And a composite structure steel plate having excellent spot weldability and delayed fracture resistance can be provided at low cost.

本発明者らは前述した様な解決課題の下で、ベイニティックフェライトを母相とする９８０ＭＰａ級以上のＴＲＩＰ（TRansformation Induced Plasticity：変態誘起塑性）鋼板に焦点を絞り、その成形性やスポット溶接性、耐遅れ破壊特性を更に改善すべく、金属組織中の第２相の形態と、化学成分、特にＳｉ，Ａｌ，Ｍｎ（あるいは更にＣｒ）に着目して改質研究を重ねた結果、次の様な知見を得た。   Under the above-mentioned problems, the present inventors focused on TRIP (TRansformation Induced Plasticity) steel sheets having bainitic ferrite as a matrix and having a 980 MPa class or higher, and their formability and spot welding. As a result of repeated modification studies focusing on the morphology of the second phase in the metal structure and chemical components, especially Si, Al, Mn (or even Cr), in order to further improve the properties and delayed fracture resistance The following knowledge was obtained.

１）その第１は、ベイニティックフェライト主体の組織に微細なポリゴナルフェライトを所定量混入させると、伸びが著しく改善されることである。しかも、混入させるフェライトが微細なものであれば、強度や伸びフランジ性の低下が抑えられ、なお且つ該組織は耐遅れ破壊特性においても優れた性能を示すこと。   1) The first is that when a predetermined amount of fine polygonal ferrite is mixed in the structure mainly composed of bainitic ferrite, the elongation is remarkably improved. In addition, if the ferrite to be mixed is fine, deterioration of strength and stretch flangeability can be suppressed, and the structure also exhibits excellent performance in delayed fracture resistance.

２）鋼の化学成分のうち、（Ｓｉ＋Ａｌ）とＭｎまたは（Ｍｎ＋Ｃｒ）が所定比率となる様に調整すれば、スポット溶接性の低下を抑えつつ、９８０ＭＰａ級以上の強度をもった所望の組織が得られること。   2) Among the chemical components of steel, (Si + Al) and Mn or (Mn + Cr) are adjusted so as to have a predetermined ratio, and a desired structure having a strength of 980 MPa or higher is obtained while suppressing deterioration of spot weldability. To be obtained.

そこでこうした知見を生かし、鋼成分中のＳｉ，Ａｌ，Ｍｎ，Ｃｒ含量と金属組織が、当該鋼板の強度や成形性、更にはスポット溶接性や遅れ破壊特性に及ぼす影響を主体にして研究を重ねてきた。その結果、前述した如く特定成分組成の鋼材を使用することを前提として、金属組織中に占めるベイニティックフェライトの占積率を制御すると共に、ポリゴナルフェライトと残留γの占積率、更にはポリゴナルフェライトの平均粒径を特定値以下に制御してやれば、上記目的に叶う高性能の高強度複合組織鋼板が得られることを確認し、本発明に想到した。   Therefore, by making use of these findings, research was conducted mainly on the effects of the Si, Al, Mn, Cr content and metal structure in steel components on the strength and formability of the steel sheet, as well as spot weldability and delayed fracture characteristics. I came. As a result, on the premise of using a steel material with a specific component composition as described above, the space factor of bainitic ferrite in the metal structure is controlled, and the space factor of polygonal ferrite and residual γ, It was confirmed that if the average grain size of polygonal ferrite was controlled to a specific value or less, a high-performance, high-strength composite steel sheet meeting the above purpose could be obtained, and the present invention was conceived.

以下、鋼材の化学成分および金属組織を定めた理由を追って、本発明の具体的な構成を明らかにしていく。   Hereinafter, the specific configuration of the present invention will be clarified following the reasons for determining the chemical composition and the metal structure of the steel material.

まず、鋼材の化学成分を定めた理由について説明する。   First, the reason for determining the chemical composition of the steel material will be described.

Ｃ：０．１０％以上、０．２５％以下
Ｃは、高強度を保障すると共に残留γを確保する上でも欠くことのできない元素であり、γ相中に十分な量のＣを含有せしめ、室温でも所望量のγ相を残留させるために重要な元素である。こうした作用を有効に発揮させるには、Ｃを０．１０％以上含有させることが必要であり、好ましくは０．１２％以上、より好ましくは０．１５％以上含有させるのがよい。但し、Ｃ量が多過ぎるとスポット溶接性に顕著な悪影響が現れてくるので、スポット溶接性確保の観点から上限を０．２５％とした。好ましくは０．２３％以下、より好ましくは０．２０％以下である。 C: 0.10% or more and 0.25% or less C is an element indispensable for ensuring high strength and securing residual γ, and a sufficient amount of C is contained in the γ phase, It is an important element for leaving a desired amount of γ phase even at room temperature. In order to exhibit such an action effectively, it is necessary to contain 0.10% or more of C, preferably 0.12% or more, more preferably 0.15% or more. However, if the amount of C is too large, a remarkable adverse effect appears on spot weldability, so the upper limit was made 0.25% from the viewpoint of securing spot weldability. Preferably it is 0.23% or less, More preferably, it is 0.20% or less.

Ｓｉ：１．０〜３．０％
Ｓｉは、固溶強化元素として有効に作用する他、残留γが分解して炭化物が生成するのを抑える上でも必須の元素であり、これらの作用を有効に発揮させるには、１．０％以上含有させねばならず、好ましくは１．２％以上含有させるのがよい。但し、それらの効果は３．０％で飽和し、それ以上になると、スポット溶接性の劣化や熱間脆性を起こすなどの障害を招くので、多くとも３．０％以下、望ましくは２．５％以下に抑えるのがよい。 Si: 1.0-3.0%
Si effectively acts as a solid solution strengthening element, and is an essential element for suppressing the formation of carbides by decomposition of residual γ, and 1.0% is required to effectively exhibit these effects. It should be contained above, preferably 1.2% or more. However, those effects saturate at 3.0%, and if it exceeds this, it causes troubles such as deterioration of spot weldability and hot brittleness, so at most 3.0% or less, preferably 2.5. It is good to keep it below%.

Ｍｎ：１．５〜３．０％
Ｍｎは、過度のポリゴナルフェライトの生成を抑えてベイニティックフェライト主体の組織とするために必要な元素である。また、γを安定化し所望の残留γを確保する上でも重要な元素であり、少なくとも１．５％以上、好ましくは２．０％以上含有させるのがよい。 Mn: 1.5 to 3.0%
Mn is an element necessary for suppressing the formation of excessive polygonal ferrite and forming a structure mainly composed of bainitic ferrite. Further, it is an important element for stabilizing γ and securing a desired residual γ, and it is preferable to contain at least 1.5% or more, preferably 2.0% or more.

しかし、過度の添加はスポット溶接性や耐遅れ破壊特性を劣化させるので、多くとも３．０％以下、好ましくは２．５％以下に抑えるのがよい。   However, excessive addition degrades spot weldability and delayed fracture resistance, so it should be suppressed to 3.0% or less, preferably 2.5% or less at most.

Ｐ：０．１５％以下、Ｓ：０．０２％以下
これらの元素は、鋼内に不可避的に混入してくる元素であるが、含有量が多くなると加工性やスポット溶接性を劣化させるので、それぞれ上限値以下に抑えねばならない。 P: 0.15% or less, S: 0.02% or less These elements are inevitably mixed in the steel, but if the content increases, workability and spot weldability deteriorate. Each must be kept below the upper limit.

Ａｌ：０．４％以下
Ａｌは、Ｓｉと同様に炭化物の生成を抑えて残留γを確保するうえで有用な元素であるが、多過ぎるとポリゴナルフェライトが生成し易くなるので、多くとも０．４％以下、好ましくは０．２％以下に抑えるべきである。 Al: 0.4% or less Al, like Si, is an element useful for suppressing the formation of carbides and securing residual γ, but if it is too much, polygonal ferrite is likely to be formed, so at most 0 .4% or less, preferably 0.2% or less.

Ｃｒ：１．０％以下
Ｃｒは、ポリゴナルフェライトの生成を抑えて強度を高める作用を有しているので、必要に応じて添加することができる。しかし、過度に添加すると、本発明で狙いとする金属組織の生成に悪影響を及ぼす恐れがあるので、多くとも１．０％以下に抑えるべきである。 Cr: 1.0% or less Cr has the effect of suppressing the formation of polygonal ferrite and increasing the strength, and can be added as necessary. However, if excessively added, there is a possibility of adversely affecting the formation of the metal structure targeted in the present invention, so it should be suppressed to 1.0% or less at most.

（Ｓｉ＋Ａｌ）／Ｍｎ（またはＭｎ＋Ｃｒ）：０．７４〜１．２６（質量比）
本発明で意図する金属組織を得るには、過度のポリゴナルフェライト（ＰＦ）の生成を抑えてベイニティックフェライト（ＢＦ）変態を促進させる必要がある。しかも、ベイニティックフェライトから排出されるＣ（炭素）はラス状の残留γ中に濃縮されるため、ベイニティックフェライト変態の促進はラス状残留γを得る上でも重要となる。 (Si + Al) / Mn (or Mn + Cr): 0.74 to 1.26 (mass ratio)
In order to obtain the metal structure intended in the present invention, it is necessary to suppress the formation of excessive polygonal ferrite (PF) and promote the bainitic ferrite (BF) transformation. In addition, since C (carbon) discharged from bainitic ferrite is concentrated in the lath-like residual γ, promotion of bainitic ferrite transformation is important in obtaining the lath-like residual γ.

また本発明では、金属組織中に微細なフェライトを適量分散させることが必要であるが、そのためには、フェライト生成促進元素であるＳｉやＡｌと、フェライト抑制元素であるＭｎ（またはＭｎ＋Ｃｒ）の含有比率が一定の関係を満たす様に制御することが極めて有効であることが分かった。しかも、これらのフェライト生成促進元素／抑制元素の含有比率を制御することは、耐遅れ破壊特性を高める上でも有効であることが確認された。   In the present invention, it is necessary to disperse an appropriate amount of fine ferrite in the metal structure. For this purpose, Si and Al that are ferrite formation promoting elements and Mn (or Mn + Cr) that is a ferrite suppressing element are included. It was found that it is extremely effective to control the ratio so as to satisfy a certain relationship. Moreover, it has been confirmed that controlling the content ratio of these ferrite formation promoting elements / inhibiting elements is also effective in improving the delayed fracture resistance.

ちなみに、（Ｓｉ＋Ａｌ）／Ｍｎ（またはＭｎ＋Ｃｒ）比が０．７４未満では、適度のポリゴナルフェライトの確保が困難になるばかりでなく、ベイニティックフェライトすらも十分に確保できなくなる。しかも、残留γ中にＣを十分濃縮できず、残留γの安定性が低下して伸びが劣化し、更には、マルテンサイト量が増大して伸びフランジ性も悪化する。   Incidentally, when the (Si + Al) / Mn (or Mn + Cr) ratio is less than 0.74, it is difficult to secure an adequate polygonal ferrite, and even bainitic ferrite cannot be sufficiently secured. Moreover, C cannot be sufficiently concentrated in the residual γ, the stability of the residual γ is lowered and the elongation is deteriorated, and further, the amount of martensite is increased and the stretch flangeability is also deteriorated.

更に加えて、上記元素の比率を適正範囲にコントロールすることで耐遅れ破壊性も改善される。この理由の詳細は不明であるが、次の様なことが考えられる。即ちＭｎは、粒界偏析により粒界強度を低下させることで遅れ破壊を助長する他、上述した様にマルテンサイトの生成を通じて加工時に遅れ破壊の起点となるボイド生成を促進するのに対し、ＳｉとＡｌは、遅れ破壊を誘発する水素の許容量を増加させる効果があるため、両者の比により遅れ破壊性が変化するものと考えられる。   In addition, the delayed fracture resistance is also improved by controlling the ratio of the above elements within an appropriate range. Although the details of this reason are unknown, the following can be considered. That is, Mn promotes delayed fracture by reducing the grain boundary strength by grain boundary segregation, and also promotes void formation that becomes the starting point of delayed fracture during processing through the formation of martensite as described above. Since Al and Al have the effect of increasing the allowable amount of hydrogen that induces delayed fracture, the delayed fracture property is considered to change depending on the ratio of the two.

一方、上記（Ｓｉ＋Ａｌ）／Ｍｎ（またはＭｎ＋Ｃｒ）比が１．２６を超えると、ポリゴナルフェライトの生成が過度に促進されてその占積率が過大となる上に、フェライトの粒径も１０μｍを超え易くなり、強度、伸びフランジ性が共に低下してくる。また、フェライト粒径が粗大化するとフェライト粒界が減少するため、耐遅れ破壊性も低下する。   On the other hand, if the (Si + Al) / Mn (or Mn + Cr) ratio exceeds 1.26, the formation of polygonal ferrite is excessively promoted and the space factor is excessive, and the ferrite grain size is also reduced to 10 μm. The strength and stretch flangeability both decrease. Moreover, since the ferrite grain boundary decreases when the ferrite grain size becomes coarse, the delayed fracture resistance also decreases.

よって本発明では、（Ｓｉ＋Ａｌ）／Ｍｎ（またはＭｎ＋Ｃｒ）比が０．７４〜１．２６の範囲となる様に成分調整することが重要であり、より好ましくは０．８４以上、１．１６以下に調整するのがよい。   Therefore, in the present invention, it is important to adjust the components so that the (Si + Al) / Mn (or Mn + Cr) ratio is in the range of 0.74 to 1.26, more preferably 0.84 or more and 1.16 or less. It is good to adjust to.

Ｎｂ：０．１％以下、Ｔｉ：０．１５％以下
これらの元素は、いずれも金属組織を微細化して靭性を高める作用があるため、必要に応じて少量添加することができる。しかし、上限値を超えて添加してもそれ以上の効果は得られず、コストアップを招くだけであるので無駄である。 Nb: 0.1% or less, Ti: 0.15% or less All of these elements have the effect of increasing the toughness by refining the metal structure, and can be added in small amounts as necessary. However, adding more than the upper limit is useless because no further effect can be obtained and only the cost is increased.

Ｃａ：０．０１％以下、ＲＥＭ：０．０１％以下
ＣａとＲＥＭは、少量の添加で何れも伸びフランジ性を高める作用を有しているため、必要によっては少量添加してもよいが、その効果は各々０．０１%程度で飽和するので、それ以上の添加は全く無駄である。 Ca: 0.01% or less, REM: 0.01% or less Ca and REM both have the effect of increasing stretch flangeability with a small amount of addition, and if necessary, a small amount may be added. Since the effect is saturated at about 0.01%, addition beyond that is completely useless.

Ｍｏ，Ｃｕ，Ｎｉ：各々０．１％程度以下
これらの元素は、従来技術でも記載した様に強度や耐遅れ破壊性の向上に有効であるが、本発明では、これらの元素を添加せずとも十分に優れた性能を確保することができ、またこれらの元素は高価でコストアップを招くので、あえて添加する必要はない。しかし、不純物レベルでの混入まで規制する理由はなく、それぞれ０．１％程度までの添加は許容範囲である。 Mo, Cu, Ni: each about 0.1% or less These elements are effective in improving strength and delayed fracture resistance as described in the prior art, but in the present invention, these elements are not added. In both cases, sufficiently excellent performance can be ensured, and these elements are expensive and incur costs, so there is no need to add them. However, there is no reason to restrict the contamination at the impurity level, and the addition up to about 0.1% is acceptable.

次に金属組織の限定理由について説明する。   Next, the reason for limiting the metal structure will be described.

ベイニティックフェライト≧５０％
ベイニティックフェライトは、転位密度がある程度高く、高強度を容易に達成できるばかりか、第２相との硬度差を低減して伸びフランジ性を高める作用を発揮する。また、耐遅れ破壊性を高める上でも有用な組織であり、これは、遅れ破壊の亀裂起点となるセメンタイトがないか極めて少なく、また転位が多いため水素吸蔵効果も高いためと考えられる。これらの効果を有効に発揮させるには、ベイニティックフェライトを５０％以上存在させる必要がある。より好ましくは６０％以上である。 Bainitic ferrite ≧ 50%
Bainitic ferrite not only has a high dislocation density and can easily achieve high strength, but also exhibits the effect of reducing the hardness difference from the second phase and enhancing stretch flangeability. It is also a useful structure for enhancing delayed fracture resistance, which is thought to be due to the fact that there is no or very little cementite as a crack initiation point for delayed fracture and that the hydrogen storage effect is high due to many dislocations. In order to exhibit these effects effectively, it is necessary to make bainitic ferrite 50% or more. More preferably, it is 60% or more.

なお、このベイニティックフェライトは、組織内に炭化物を有していない点で、ベイナイト組織とは明らかに異なり、また、転位がないかあるいは極めて少ない下部組織を有するポリゴナルフェライト組織や、細かいサブグレイン等の下部組織を持った準ポリゴナルフェライト組織とも異なっており、これらの違いはＴＥＭ（透過型電子顕微鏡）観察などによって容易に識別できる。   This bainitic ferrite is clearly different from the bainite structure in that it does not have carbides in the structure, and is a polygonal ferrite structure having no or few dislocations or a fine substructure. It is also different from a quasi-polygonal ferrite structure having a substructure such as a grain, and these differences can be easily identified by TEM (transmission electron microscope) observation.

ポリゴナルフェライト（ＰＦ）：５〜３５％
ベイニティックフェライト（ＢＦ）を母相とする引張強度９８０ＭＰａ級以上の鋼板に、後述する様な平均粒径のポリゴナルフェライトを所定量含有させると、伸びが更に向上する。こうした効果を有効に発揮させるには、ポリゴナルフェライトを５％以上含有させねばならない。但し、ポリゴナルフェライトが多過ぎると、引張強度や伸びフランジ性の確保が困難となるので、多くとも３５％以下に抑えるべきである。ポリゴナルフェライトのより好ましい占積率は１０％以上、３０％以下である。 Polygonal ferrite (PF): 5 to 35%
When a predetermined amount of polygonal ferrite having an average particle diameter as described later is contained in a steel sheet having a tensile strength of 980 MPa or more having bainitic ferrite (BF) as a matrix, elongation is further improved. In order to exert such an effect effectively, 5% or more of polygonal ferrite must be contained. However, if there is too much polygonal ferrite, it will be difficult to ensure tensile strength and stretch flangeability, so it should be kept at most 35%. The more preferable space factor of polygonal ferrite is 10% or more and 30% or less.

ポリゴナルフェライトの平均粒径：１０μｍ以下
ポリゴナルフェライトの平均粒径は１０μｍ以下にしなければならない。フェライトを微細化することで第２相としての分散を均一にし、伸びフランジ性、強度を共に高めると共に、耐遅れ破壊性をも向上させるためである。これは、ポリゴナルフェライトを微細化することで増加するフェライト粒界に水素がトラップされ、危険部位への水素の集中が抑制されるためと考えられる。 The average particle diameter of polygonal ferrite: 10 μm or less The average particle diameter of polygonal ferrite must be 10 μm or less. This is because by making the ferrite finer, the dispersion as the second phase is made uniform, the stretch flangeability and strength are both improved, and the delayed fracture resistance is also improved. This is presumably because hydrogen is trapped at the ferrite grain boundaries that increase as the polygonal ferrite is refined, and the concentration of hydrogen at the hazardous site is suppressed.

残留γ≧５％
残留γは、材料が歪を受けて変形する際にマルテンサイトに変態することで変形部の硬化を促し、歪の集中を防ぐ効果がある（ＴＲＩＰ効果）。こうした効果を有効に発揮させるには、残留γを５％以上含有させる必要がある。残留γ量に上限は存在しないが、過度の残留γを生成させるには多量のＣが必要になるため、スポット溶接性との両立が困難になり、また加工性、特に伸びフランジ性が低下傾向となるので、好ましくは３０％程度以下に抑えるのがよい。 Residual γ ≧ 5%
Residual γ has an effect of accelerating hardening of the deformed portion by transforming into martensite when the material is deformed due to strain and preventing concentration of strain (TRIP effect). In order to exert such an effect effectively, it is necessary to contain 5% or more of residual γ. There is no upper limit to the amount of residual γ, but a large amount of C is required to generate excessive residual γ, making it difficult to achieve compatibility with spot weldability, and workability, especially stretch flangeability, tends to decrease Therefore, it is preferable to keep it to about 30% or less.

本発明の複合組織鋼板では、上記以外の残部組織としてマルテンサイト、ベイナイト、パーライトなどが存在し得るが、これら他の組織は、上述した作用効果に悪影響を及ぼさない様、５％以下に抑えることが望ましい。   In the composite structure steel sheet of the present invention, martensite, bainite, pearlite and the like may exist as the remaining structure other than the above, but these other structures should be suppressed to 5% or less so as not to adversely affect the above-described effects. Is desirable.

次に、本発明で定める上記の金属組織を得るための製造条件に格別の制限はなく、一般的な鋼板の製造手順、例えば、連続鋳造→熱間圧延→酸洗→冷間圧延→連続焼鈍の中で、加熱温度や昇温速度、保持温度、冷却開始温度や冷却速度などを適正に制御すればよく、また溶融亜鉛めっき鋼板や合金化溶融亜鉛めっき鋼板の場合は、連続溶融亜鉛めっきラインを含めて適正な温度制御を行なえばよいが、上記金属組織を得る上で最も重要となるのは、連続焼鈍ラインにおける熱処理条件であるので、以下、連続焼鈍ラインにおける好ましい熱処理条件を主体にして説明を加える。   Next, there is no particular limitation on the production conditions for obtaining the above-described metal structure defined in the present invention, and a general steel sheet production procedure, for example, continuous casting → hot rolling → pickling → cold rolling → continuous annealing Among them, the heating temperature, heating rate, holding temperature, cooling start temperature, cooling rate, etc. can be controlled appropriately. In the case of hot-dip galvanized steel sheets and galvannealed steel sheets, continuous hot-dip galvanizing lines However, since it is the heat treatment condition in the continuous annealing line that is most important in obtaining the above metal structure, the preferred heat treatment condition in the continuous annealing line will be mainly described below. Add a description.

焼鈍時の加熱温度：Ａｃ３＋１０℃以上
ベイニティックフェライトリッチの金属組織を得るため、ポリゴナルフェライトの生成を抑えるには、焼鈍時の加熱温度を「Ａｃ３＋１０℃以上」とするのがよい。ちなみに、Ａｃ３点以下で連続焼鈍を行うと、その後の冷却過程で、残存するフェライトを核にしてポリゴナルフェライトが生成し易くなり、本発明で意図する金属組織が得られ難くなる。より好ましい加熱温度は「Ａｃ３＋３０℃以上」である。 Heating temperature during annealing: Ac 3 + 10 ° C. or higher In order to suppress the formation of polygonal ferrite in order to obtain a metal structure rich in bainitic ferrite, the heating temperature during annealing is set to “Ac 3 + 10 ° C. or higher”. Good. Incidentally, if continuous annealing is performed at Ac 3 points or less, polygonal ferrite is likely to be generated with the remaining ferrite as a nucleus in the subsequent cooling process, and the metal structure intended in the present invention is difficult to obtain. A more preferable heating temperature is “Ac 3 + 30 ° C. or higher”.

焼鈍後の冷却速度：
焼鈍後の冷却速度は、ポリゴナルフェライトの生成を抑制するため速い方が好ましいが、設備的な制約や温度コントロールの困難性を考えると、個別の成分系に応じてポリゴナルフェライトを一定量以下に抑え得る様、好ましくは２５℃／ｓｅｃ以上、より好ましくは３０℃／ｓｅｃ以上とするのがよい。 Cooling rate after annealing:
The cooling rate after annealing is preferably faster in order to suppress the formation of polygonal ferrite, but considering the restrictions on equipment and the difficulty of temperature control, the amount of polygonal ferrite is less than a certain amount depending on the individual component system. The temperature is preferably 25 ° C./sec or more, more preferably 30 ° C./sec or more.

焼鈍後の急冷停止温度：
焼鈍後の急冷を停止する温度は、微細ポリゴナルフェライトが生成する温度以下にすべきであり、好ましくは６５０℃以下、より好ましくは６００℃以下にするのがよい。急冷停止温度が高くなるとポリゴナルフェライトが粗大となり、本発明の目的が果たせなくなる。但し、あまりに低温になると十分な量のポリゴナルフェライトが得られなくなるので、３６０℃程度以上、より好ましくは４００℃までにすべきである。 Rapid cooling stop temperature after annealing:
The temperature at which the rapid cooling after annealing is stopped should be lower than the temperature at which fine polygonal ferrite is generated, preferably 650 ° C. or lower, more preferably 600 ° C. or lower. If the quenching stop temperature increases, polygonal ferrite becomes coarse, and the object of the present invention cannot be achieved. However, since a sufficient amount of polygonal ferrite cannot be obtained when the temperature is too low, it should be about 360 ° C. or higher, more preferably 400 ° C. or higher.

冷却後の保持温度：
上記冷却後には、一定温度に保持することでベイニティックフェライト変態が進行し、また、オーステナイトへのＣの濃縮が進んで残留γとなるので、冷却後の保持温度を適正にコントロールすることも重要である。本発明の金属組織を得る上で好ましい保持温度は３６０〜４４０℃の範囲である。この温度は、上記冷却停止温度と同じである必要はない。好ましい保持時間は１分以上である。 Holding temperature after cooling:
After the cooling, the bainitic ferrite transformation proceeds by holding at a constant temperature, and the concentration of C to austenite proceeds to become residual γ, so that the holding temperature after cooling can be controlled appropriately. is important. A preferable holding temperature for obtaining the metal structure of the present invention is in the range of 360 to 440 ° C. This temperature need not be the same as the cooling stop temperature. A preferable holding time is 1 minute or more.

本発明の高強度複合組織鋼板は、上記の様に化学成分の特定された鋼材を使用し、且つ冷却条件や保持条件などを含めて適正な熱処理条件を採用することで所定の金属組織を確保することにより、９８０ＭＰａ級以上の高強度を有すると共に、成形性が良好でスポット溶接性や耐遅れ破壊特性にも優れた複合組織鋼板を安価に提供できる。   The high-strength composite structure steel sheet of the present invention uses a steel material with a specified chemical composition as described above, and secures a predetermined metal structure by adopting appropriate heat treatment conditions including cooling conditions and holding conditions. By doing so, a composite structure steel plate having high strength of 980 MPa class or more, good formability, and excellent spot weldability and delayed fracture resistance can be provided at low cost.

以下、実験例を挙げて本発明をより具体的に説明するが、本発明はもとより下記実験例によって制限を受けるものではなく、前・後記の趣旨に適合し得る範囲で適当に変更を加えて実施することも可能であり、それらは何れも本発明の技術的範囲に含まれる。   Hereinafter, the present invention will be described more specifically with reference to experimental examples.However, the present invention is not limited by the following experimental examples, but may be appropriately modified within a range that can meet the purpose described above and below. It is also possible to implement, and they are all included in the technical scope of the present invention.

実験例
表１に示す成分組成の鋼材を溶製し、連続鋳造の後、下記の条件で熱間圧延、酸洗、冷間圧延を行い、次いで、表２（図１も参照）に示す条件で熱処理（焼鈍）することにより冷延鋼板を得た。 Experimental Example Steel materials having the composition shown in Table 1 were melted, and after continuous casting, hot rolling, pickling and cold rolling were performed under the following conditions, and then the conditions shown in Table 2 (see also FIG. 1). A cold-rolled steel sheet was obtained by heat treatment (annealing).

［熱間圧延］
加熱温度：１２００℃×６０分
仕上温度：８８０℃
冷却：４０℃／秒で７２０℃まで冷却し、１０秒間空冷した後４０℃／秒で５００℃まで冷却し、その後５００℃で６０分間保持してから炉冷した。
仕上げ板厚：３．２ｍｍ。 [Hot rolling]
Heating temperature: 1200 ° C x 60 minutes Finishing temperature: 880 ° C
Cooling: Cooled to 720 ° C. at 40 ° C./second, air-cooled for 10 seconds, then cooled to 500 ° C. at 40 ° C./second, held at 500 ° C. for 60 minutes, and then cooled in the furnace.
Finished plate thickness: 3.2 mm.

［酸洗、冷間圧延］
酸洗後、板厚１．２ｍｍまで冷間圧延した。 [Pickling, cold rolling]
After pickling, it was cold rolled to a plate thickness of 1.2 mm.

［熱処理（焼鈍）］
表２に示す如く、所定の焼鈍温度に加熱して１８０秒間保持した後、所定の速度で所定の冷却停止温度まで冷却し、所定温度で６分間保持してから炉冷した。 [Heat treatment (annealing)]
As shown in Table 2, after heating to a predetermined annealing temperature and holding it for 180 seconds, it was cooled to a predetermined cooling stop temperature at a predetermined speed, held at the predetermined temperature for 6 minutes, and then cooled in the furnace.

得られた冷延鋼板の金属組織を下記の方法で確認すると共に、各供試鋼板について引張試験、穴拡げ試験、スポット溶接試験、耐遅れ破壊試験を行い、表３に一括して示す結果を得た。   While confirming the metal structure of the obtained cold-rolled steel sheet by the following method, each test steel sheet was subjected to a tensile test, a hole expansion test, a spot welding test, and a delayed fracture resistance test. Obtained.

［金属組織］
組織同定方法：
Ａ： レペラー腐食による光学顕微鏡観察(１０００倍)、１視野。
Ｂ：ＳＥＭ観察（４０００倍）、４視野。 [Metal structure]
Tissue identification method:
A: Observation with an optical microscope (1000 times) due to repeller corrosion, 1 field of view.
B: SEM observation (4000 times), 4 fields of view.

ポリゴナルフェライト（ＰＦ）：
上記Ａにより撮影した写真から算出する。残留γとマルテンサイトの白に対し、ＰＦはグレーに腐食されるため識別できる。 Polygonal ferrite (PF):
Calculated from the photograph taken by A above. The residual γ and the white martensite are identifiable because PF corrodes gray.

残留γ、マルテンサイト（Ｍ）、ベイナイト（Ｂ）：
透過型電子顕微鏡（ＴＥＭ：倍率１５０００）によって残留γ、Ｍ、Ｂであることを確認した上で、占積率は、上記Ｂにより撮影した写真から算出した。 Residual γ, martensite (M), bainite (B):
After confirming the residual γ, M, and B with a transmission electron microscope (TEM: 15000 magnification), the space factor was calculated from the photograph taken by B above.

ベイニティックフェライト（ＢＦ）：
透過型電子顕微鏡（ＴＥＭ：倍率１５０００）により、ベイナイトや擬フェライトの如き他の組織でないことを確認した上で、占積率は、１００％からポリゴナルフェライト量と上記残留γ量および残部のマルテンサイト（Ｍ）とベイナイト（Ｂ）を減じた値とした。 Bainitic ferrite (BF):
After confirming by transmission electron microscope (TEM: magnification 15000) that the structure is not other structures such as bainite and pseudoferrite, the space factor is from 100% to the amount of polygonal ferrite, the amount of residual γ, and the remaining martensite. It was set as the value which reduced the site (M) and the bainite (B).

［性能評価試験］
引張試験：ＪＩＳ ５号引張試験片によって測定した。
穴広げ試験：鉄鋼連盟規格ＪＦＳＴ １００１に準じて実施した。
スポット溶接性：
下記の条件でスポット溶接を行い、ナゲット径５√ｔの時の延性比が０．２５以上あれば、スポット溶接性が良好（○）とした。 [Performance evaluation test]
Tensile test: Measured with a JIS No. 5 tensile test piece.
Hole expansion test: It was carried out in accordance with the Steel Federation standard JFST 1001.
Spot weldability:
Spot welding was performed under the following conditions, and if the ductility ratio at a nugget diameter of 5√t was 0.25 or more, the spot weldability was good (◯).

＜溶接条件 ＞
供試材厚さ：１．２ｍｍ、
電極：ドームラジアス型(先端直径６ｍｍ)、
加圧力：３７５ｋｇ、
アップスロープ；１サイクル、通電時間；１２サイクル、ホールド；１サイクル(６０Ｈｚ)、
ナゲット径の調整：溶接電流により調整、
延性比：十字引張強度／せん断引張強度。 <Welding conditions>
Test material thickness: 1.2 mm,
Electrode: Dome radius type (tip diameter 6mm),
Applied pressure: 375 kg,
Upslope; 1 cycle, energization time; 12 cycles, hold; 1 cycle (60 Hz),
Nugget diameter adjustment: Adjust by welding current
Ductility ratio: Cross tensile strength / shear tensile strength.

［耐遅れ破壊性］
Ｒ＝３ｍｍの６０°Ｖブロックを用いてＶ字曲げを施した後、曲げ部に１５００ＭＰａの応力を負荷して５％塩酸水溶液中に浸漬し、割れ発生までの時間を測定した。４８時間で割れなかったものを耐遅れ破壊良好（○）とした。 [Delayed fracture resistance]
After performing V-shaped bending using a 60 ° V block of R = 3 mm, a stress of 1500 MPa was applied to the bent portion and immersed in a 5% hydrochloric acid aqueous solution, and the time until cracking was measured. Those that did not crack in 48 hours were regarded as good delayed fracture resistance (◯).

Figure 0004974341
Figure 0004974341

Figure 0004974341
Figure 0004974341

Figure 0004974341
Figure 0004974341

表１〜３より、次の様に考えることができる。   From Tables 1 to 3, it can be considered as follows.

実験Ｎｏ．１〜１０は、本発明の規定要件を全て満たす実施例であり、何れも９８０ＭＰａ級以上の引張強度を有すると共に、強度×伸び特性、強度×伸びフランジ特性によって評価される成形性が良好であり、更にスポット溶接性や耐遅れ破壊性においても良好な結果が得られている。   Experiment No. Nos. 1 to 10 are examples that satisfy all of the prescribed requirements of the present invention, all of which have a tensile strength of 980 MPa class or higher, and good formability evaluated by strength × elongation characteristics and strength × stretch flange characteristics. In addition, good results have been obtained in spot weldability and delayed fracture resistance.

これらに対し実験Ｎｏ．１１は、鋼材のＣ含量が不足し、金属組織中のベイニティックフェライト量も不足するため、強度不足で、強度×伸び特性、強度×伸びフランジ特性によって評価される成形性も悪い。また実験Ｎｏ．１２は、用いた鋼材のＳｉ含量が不足すると共に（Ｓｉ＋Ａｌ）／（Ｍｎ＋Ｃｒ）比が規定範囲を外れるため、金属組織中に残留γが存在せず、強度×伸び特性、強度×伸びフランジ特性によって評価される成形性が劣悪で耐遅れ破壊性も悪い。   For these, Experiment No. No. 11 has an insufficient C content in the steel material and an insufficient amount of bainitic ferrite in the metal structure. Therefore, the strength is insufficient, and the formability evaluated by strength × elongation characteristics and strength × elongation flange characteristics is also poor. In addition, Experiment No. No. 12, because the Si content of the steel material used is insufficient and the (Si + Al) / (Mn + Cr) ratio is outside the specified range, there is no residual γ in the metal structure, and the strength × elongation characteristics and strength × elongation flange characteristics The formability evaluated is poor and the delayed fracture resistance is also poor.

実験Ｎｏ．１３は、Ｍｎ量が規定範囲を外れると共に（Ｓｉ＋Ａｌ）／Ｍｎ比が規定範囲を超えているため、ポリゴナルフェライトが増え過ぎてベイニティックフェライト量が大幅に少なくなっており、強度不足で加工性も劣悪である。実験Ｎｏ．１４は、個々の含有元素量は規定要件を満たしているが、（Ｓｉ＋Ａｌ）／Ｍｎ比が規定範囲を超えており、ポリゴナルフェライトが粗大で平均粒径が規定値を超えているため、強度不足で加工性も悪く、且つスポット溶接性もよくない。   Experiment No. In No. 13, since the amount of Mn is out of the specified range and the (Si + Al) / Mn ratio exceeds the specified range, the amount of polygonal ferrite is excessively increased, and the amount of bainitic ferrite is greatly reduced. Sex is also poor. Experiment No. No. 14, the content of each element satisfies the specified requirements, but the (Si + Al) / Mn ratio exceeds the specified range, the polygonal ferrite is coarse, and the average particle size exceeds the specified value. Insufficient workability and poor spot weldability.

実験Ｎｏ．１５は、鋼成分は規定要件を満たしているが、熱処理条件が不適切であるため組織中のポリゴナルフェライト量が過多となってベイニティックフェライト量が不足し、強度×伸び特性、強度×伸びフランジ特性によって評価される成形性が劣悪で、耐遅れ破壊性も悪い。   Experiment No. No. 15, the steel component satisfies the prescribed requirements, but the heat treatment conditions are inappropriate, so the amount of polygonal ferrite in the structure becomes excessive, the amount of bainitic ferrite is insufficient, and the strength × elongation characteristics, strength × Formability evaluated by stretch flange characteristics is poor, and delayed fracture resistance is also poor.

実験例で採用した熱処理のヒートパターンを示す説明図である。It is explanatory drawing which shows the heat pattern of the heat processing employ | adopted by the experiment example.

Claims (5)

Ｃ：０．１２〜０．２５％（化学成分の場合は質量％を表す、以下同じ）、
Ｓｉ：１．０〜３．０％、
Ｍｎ：１．５〜３．０％、
Ｐ：０．１５％以下（０％を含まない）、
Ｓ：０．０２％以下（０％を含まない）、
Ａｌ：０．４％以下（０％を含まない）、
を満足し、残部が鉄および不可避不純物よりなる鋼からなり、上記Ｓｉ，Ａｌ，Ｍｎの含有量が下記式（Ｉ）の関係を満たし、
（Ｓｉ＋Ａｌ）／Ｍｎ：０．７４〜１．２６……（Ｉ）
且つ、縦断面のミクロ組織が、全組織に対する占積率で、
１）ベイニティックフェライト：５０％以上、
２）ポリゴナルフェライト：５〜３５％、
３）ポリゴナルフェライトの平均粒径：１０μｍ以下、
４）残留オーステナイト：５％以上
であり、
引張強さＴＳが９８０ＭＰａ以上であり、且つ、引張強さＴＳ（ＭＰａ）×伸びＥｌ（％）が１９３０４以上を満足することを特徴とする、成形性、スポット溶接性、および耐遅れ破壊性に優れた高強度複合組織鋼板。 C: 0.12 to 0.25% (in the case of chemical components, represents mass%, the same shall apply hereinafter),
Si: 1.0-3.0%,
Mn: 1.5-3.0%
P: 0.15% or less (excluding 0%),
S: 0.02% or less (excluding 0%),
Al: 0.4% or less (excluding 0%),
And the balance is made of steel consisting of iron and inevitable impurities, the content of Si, Al, Mn satisfies the relationship of the following formula (I),
(Si + Al) / Mn: 0.74 to 1.26 (I)
And the microstructure of the longitudinal section is the space factor for the whole structure,
1) Bainitic ferrite: 50% or more
2) Polygonal ferrite: 5-35%,
3) Average particle size of polygonal ferrite: 10 μm or less,
4) retained austenite: Ri der 5% or more,
Tensile strength TS of not less than 980 MPa, and the tensile strength TS (MPa) × elongation El (%) is characterized that you meet the above 19304, formability, spot weldability, and resistance to delayed fracture Excellent high-strength composite steel sheet. Ｃ：０．１２〜０．２５％、
Ｓｉ：１．０〜３．０％、
Ｍｎ：１．５〜３．０％、
Ｃｒ：１．０％以下（０％を含まない）、
Ｐ：０．１５％以下（０％を含まない）、
Ｓ：０．０２％以下（０％を含まない）、
Ａｌ：０．４％以下（０％を含まない）、
を満足し、残部が鉄および不可避不純物よりなる鋼からなり、上記Ｓｉ，Ａｌ，Ｍｎ，Ｃｒの含有量が下記式（II）の関係を満たし、
（Ｓｉ＋Ａｌ）／（Ｍｎ＋Ｃｒ）：０．７４〜１．２６……（II）
且つ、縦断面のミクロ組織が、全組織に対する占積率で、
１）ベイニティックフェライト：５０％以上、
２）ポリゴナルフェライト：５〜３５％、
３）ポリゴナルフェライトの平均粒径：１０μｍ以下、
４）残留オーステナイト：５％以上
であり、
引張強さＴＳが９８０ＭＰａ以上であり、且つ、引張強さＴＳ（ＭＰａ）×伸びＥｌ（％）が１９３０４以上を満足することを特徴とする、成形性、スポット溶接性、および耐遅れ破壊性に優れた高強度複合組織鋼板。 C: 0.12-0.25%,
Si: 1.0-3.0%,
Mn: 1.5-3.0%
Cr: 1.0% or less (excluding 0%),
P: 0.15% or less (excluding 0%),
S: 0.02% or less (excluding 0%),
Al: 0.4% or less (excluding 0%),
And the balance is made of steel consisting of iron and inevitable impurities, the content of Si, Al, Mn, Cr satisfies the relationship of the following formula (II),
(Si + Al) / (Mn + Cr): 0.74 to 1.26 (II)
And the microstructure of the longitudinal section is the space factor for the whole structure,
1) Bainitic ferrite: 50% or more
2) Polygonal ferrite: 5-35%,
3) Average particle size of polygonal ferrite: 10 μm or less,
4) retained austenite: Ri der 5% or more,
Tensile strength TS of not less than 980 MPa, and the tensile strength TS (MPa) × elongation El (%) is characterized that you meet the above 19304, formability, spot weldability, and resistance to delayed fracture Excellent high-strength composite steel sheet. Ｃは０．１４％以上である請求項１または２に記載の高強度複合組織鋼板。The high-strength composite steel sheet according to claim 1 or 2, wherein C is 0.14% or more. 鋼が、他の元素として、
Ｔｉ：０．１５％以下（０％を含まない）および／またはＮｂ：０．１％以下（０％を含まない）を含有するものである請求項１〜３のいずれかに記載の高強度複合組織鋼板。 Steel is another element
The high strength according to any one of claims 1 to 3, which contains Ti: 0.15% or less (not including 0%) and / or Nb: 0.1% or less (not including 0%). Composite steel sheet. 鋼が、更に他の元素として、
Ｃａ：３０ｐｐｍ以下（０％を含まない）および／またはＲＥＭ：３０ｐｐｍ以下（０％を含まない）を含むものである請求項１〜４のいずれかに記載の高強度複合組織鋼板。 Steel is another element,
The high-strength composite steel sheet according to any one of claims 1 to 4 , which contains Ca: 30 ppm or less (not including 0%) and / or REM: 30 ppm or less (not including 0%).
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