JP2002322533A - Thin steel sheet for automobile excellent in notch fatigue strength and production method therefor - Google Patents

Thin steel sheet for automobile excellent in notch fatigue strength and production method therefor

Info

Publication number
JP2002322533A
JP2002322533A JP2001247306A JP2001247306A JP2002322533A JP 2002322533 A JP2002322533 A JP 2002322533A JP 2001247306 A JP2001247306 A JP 2001247306A JP 2001247306 A JP2001247306 A JP 2001247306A JP 2002322533 A JP2002322533 A JP 2002322533A
Authority
JP
Japan
Prior art keywords
steel sheet
fatigue strength
thin steel
rolling
temperature
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Granted
Application number
JP2001247306A
Other languages
Japanese (ja)
Other versions
JP3927384B2 (en
Inventor
Tatsuo Yokoi
龍雄 横井
Natsuko Sugiura
夏子 杉浦
Naoki Yoshinaga
直樹 吉永
Koichi Dobashi
浩一 土橋
Takehiro Nakamoto
武広 中本
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Nippon Steel Corp
Original Assignee
Nippon Steel Corp
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority to JP2001247306A priority Critical patent/JP3927384B2/en
Application filed by Nippon Steel Corp filed Critical Nippon Steel Corp
Priority to EP02700640A priority patent/EP1362930A4/en
Priority to KR1020037010529A priority patent/KR100572762B1/en
Priority to CA002438393A priority patent/CA2438393A1/en
Priority to CNB028054024A priority patent/CN1221680C/en
Priority to US10/468,945 priority patent/US20040069382A1/en
Priority to PCT/JP2002/001498 priority patent/WO2002066697A1/en
Publication of JP2002322533A publication Critical patent/JP2002322533A/en
Application granted granted Critical
Publication of JP3927384B2 publication Critical patent/JP3927384B2/en
Anticipated expiration legal-status Critical
Expired - Fee Related legal-status Critical Current

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Classifications

    • 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
    • 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
    • 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
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—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
    • 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
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0273—Final recrystallisation annealing
    • 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
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing 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
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
    • C23C2/0224—Two or more thermal pretreatments
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C23C2/024—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by cleaning or etching
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/34—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the shape of the material to be treated
    • C23C2/36—Elongated material
    • C23C2/40—Plates; Strips

Abstract

PROBLEM TO BE SOLVED: To provide a thin steel sheet for an automobile excellent in notch fatigue strength, and to provide a production method therefor. SOLUTION: The thin steel sheet for automobile having excellent notch fatigue strength consists of steel having a composition containing 0.01 to 0.3% C, 0.01 to 2% Si, 0.05 to 3% Mn, <=0.1% P, <=0.01% S and 0.005 to 1% Al, and the balance Fe with inevitable impurities. The average value of the X-ray random intensity ratios in the orientation groups of 100}<011> to 223}<110> in the sheet surface thereof at an arbitrary depth till 0.5 mm from the outermost surface in the sheet thickness direction is >=2, and also, the average value of the X-ray random intensity ratios in the three orientations of 554}<225>, 111}<112> and 111}<110> is <=4. The steel sheet has a sheet thickness of 0.5 to 12 mm. In the method for producing the same steel sheet, the steel having the above composition is rolled in the temperature region of the Ar3 transformation temperature +100 deg.C or lower at a total rolling reduction rate of >=25%.

Description

【発明の詳細な説明】DETAILED DESCRIPTION OF THE INVENTION

【０００１】[0001]

【発明の属する技術分野】本発明は、切り欠き疲労強度
に優れる自動車用薄鋼板およびその製造方法に関するも
のであり、特に、打ち抜き加工部や溶接部等の応力集中
部からの疲労き裂の進展が問題となるような自動車足廻
り部品等の素材として好適な、切り欠き疲労強度に優れ
る自動車用薄鋼板およびその製造方法に関するものであ
る。BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin steel sheet for automobiles having excellent notch fatigue strength and a method for producing the same, and more particularly, to the development of fatigue cracks from stress-concentrated parts such as punched parts and welded parts. The present invention relates to a thin steel sheet for an automobile having excellent notch fatigue strength, which is suitable as a material for an automobile undercarriage part or the like in which the above problem occurs, and a method for producing the same.

【０００２】[0002]

【従来の技術】近年、自動車の燃費向上などのために軽
量化を目的として、Ａｌ合金等の軽金属や高強度鋼板の
自動車部材への適用が進められている。ただし、Ａｌ合
金等の軽金属は比強度が高いという利点があるものの、
鋼に比較して著しく高価であるためその適用は特殊な用
途に限られている。従ってより広い範囲で自動車の軽量
化を推進するためには、安価な高強度鋼板の適用が強く
求められている。2. Description of the Related Art In recent years, the application of light metals such as Al alloys and high-strength steel sheets to automobile members has been promoted for the purpose of weight reduction in order to improve fuel efficiency of automobiles. However, although light metals such as Al alloys have the advantage of high specific strength,
Its application is limited to special applications because it is significantly more expensive than steel. Therefore, in order to promote weight reduction of automobiles in a wider range, there is a strong demand for the use of inexpensive high-strength steel sheets.

【０００３】このような高強度化の要求に対して、これ
までは車体重量の１／４程度を占めるホワイトボティー
やパネル類に使用される冷延鋼板の分野において、強度
と深絞り性を兼ね備えた鋼板や焼付け硬化性のある鋼板
等の開発が進められ、車体の軽量化に寄与してきた。と
ころが現在、軽量化の対象は車体重量の約２０％を占め
る構造部材や足廻り部材にシフトしてきており、これら
の部材に用いる高強度薄鋼板の開発が急務となってい
る。[0003] In response to such demands for high strength, in the field of cold rolled steel sheets used for white bodies and panels that occupy about 1/4 of the body weight, they have both strength and deep drawability. Development of steel sheets and bake-hardening steel sheets has been promoted, which has contributed to weight reduction of vehicle bodies. However, at present, the object of weight reduction is shifting to structural members and undercarriage members occupying about 20% of the vehicle body weight, and there is an urgent need to develop high-strength thin steel sheets used for these members.

【０００４】ただし、高強度化は一般的に成形性（加工
性）等の材料特性を劣化させるため、材料特性を劣化さ
せずに如何に高強度化を図るかが高強度鋼板開発のカギ
になる。特に構造部材や足廻り部材用鋼板に求められる
特性として、伸びはもちろんのことせん断や打ち抜き加
工性、バーリング加工性、疲労耐久性および耐食性等が
重要であり、高強度とこれら特性を如何に高次元でバラ
ンスさせるかが重要である。例えばサスペンションアー
ム等の部品は、せん断や打ち抜き加工によりブランキン
グや穴開けを行った後にプレス成形し、部材によっては
さらに溶接して部品にする。このような部品において
は、せん断加工された端面や溶接部近傍からき裂が進展
し疲労破壊に至る場合が少なくない。すなわち、せん断
加工された端面や溶接部が切り欠きのような応力集中部
となり、そこから疲労き裂が進展する。However, since high strength generally deteriorates material properties such as formability (workability), how to achieve high strength without deteriorating material properties is the key to the development of high strength steel sheets. Become. In particular, not only elongation but also shear and punching workability, burring workability, fatigue durability and corrosion resistance are important as properties required for steel sheets for structural members and suspension members. It is important to balance by dimension. For example, a component such as a suspension arm is blanked or punched by shearing or punching and then press-formed, and depending on the member, is further welded into a component. In such a component, cracks often propagate from the sheared end face or the vicinity of the welded portion, resulting in fatigue failure. That is, the sheared end face or the welded portion becomes a stress concentration portion such as a notch, from which a fatigue crack propagates.

【０００５】一方、一般的に材料の疲労限は切り欠きが
鋭くなると低下する。しかし、ある程度切り欠きが鋭く
なると疲労限はそれ以上低下しなくなる現象が起こる。
これは、疲労限がき裂発生限界からき裂進展限界へと遷
移するためである。材料を高強度化すると、き裂発生限
界は向上するが、き裂進展限界は向上しないため、疲労
限がき裂発生限界からき裂進展限界へと遷移するポイン
トが、切り欠きの鋭い側に移動する。従って、材料を高
強度化しても切り欠きによる疲労限の低下が著しくな
り、切り欠きが鋭い場合の疲労限は高強度のメリットを
享受できない。すなわち、高強度化すると切り欠きに対
する感受性が高くなる。On the other hand, the fatigue limit of a material generally decreases as the notch becomes sharp. However, when the notch is sharpened to some extent, a phenomenon occurs in which the fatigue limit does not decrease any more.
This is because the fatigue limit changes from the crack initiation limit to the crack growth limit. When the material is strengthened, the crack initiation limit is improved, but the crack growth limit is not improved. . Therefore, even if the strength of the material is increased, the fatigue limit is significantly reduced due to the notch, and the advantage of high strength cannot be enjoyed in the fatigue limit when the notch is sharp. That is, the higher the strength, the higher the sensitivity to the notch.

【０００６】現在、これら自動車足廻り用薄鋼板として
３４０〜４４０ＭＰａ級の鋼板が用いられているが、こ
れら部材用鋼板に要求される強度レベルは５９０〜７８
０ＭＰａ級へとさらなる高強度化へ向かいつつある。従
ってこれらの要求に応えてゆくためには、鋭い切り欠き
が存在する場合でも高強度化のメリットが享受できるよ
うな鋼板の開発が不可欠である。At present, steel sheets of 340 to 440 MPa class are used as the thin steel sheets for undercarriage of automobiles.
We are heading for even higher strength toward the 0 MPa class. Therefore, in order to meet these demands, it is essential to develop a steel sheet that can enjoy the advantage of high strength even when a sharp notch exists.

【０００７】打ち抜きやせん断加工端面が存在する場合
の疲労強度を向上させる方法は、大きく分けて二つ考え
られる。一つは打ち抜きやせん断加工端面に発生するバ
リのような鋭い切り欠きを無くしてしまうこと、もう一
つはそのような鋭い切り欠きが存在してもき裂進展に対
する抵抗を高めることである。[0007] There are roughly two methods for improving the fatigue strength when there is a punched or sheared end face. One is to eliminate sharp notches such as burrs generated on the end face of punching or shearing, and the other is to increase resistance to crack propagation even in the presence of such sharp notches.

【０００８】前者に属する発明として、例えば特開平５
−５１６９５号公報には、Ｓｉの添加量を少なくし、Ｔ
ｉ，Ｎｂ，Ｖの析出物で破断伸びを小さくすることでバ
リの発生を抑えて、打ち抜きやせん断加工ままでの疲労
強度を向上させる技術が開示されている。また特開平５
−１７９３４６号公報には、圧延仕上げ温度の上限を規
定することでベイナイトの体積分率の上限を限定して、
打ち抜きやせん断加工ままでの疲労強度を向上させる技
術が開示されている。また特開平８−１３０３３号公報
には、圧延後の冷却速度を規定しマルテンサイトの生成
を抑えることによって、打ち抜きやせん断加工ままでの
疲労強度を向上させる技術が開示されている。The invention belonging to the former is disclosed in, for example,
No. 5,516,955 discloses that the amount of added Si is reduced,
A technique has been disclosed in which the generation of burrs is suppressed by reducing the elongation at break by the precipitates of i, Nb, and V, and the fatigue strength as it is punched or sheared is improved. Also, Japanese Patent Application Laid-Open
In the -179346 publication, the upper limit of the volume fraction of bainite is limited by defining the upper limit of the rolling finishing temperature,
There is disclosed a technique for improving the fatigue strength as it is punched or sheared. Further, Japanese Patent Application Laid-Open No. 8-13033 discloses a technique in which the cooling rate after rolling is regulated and the generation of martensite is suppressed, thereby improving the fatigue strength in punching and shearing.

【０００９】また特開平８−３０２４４６号公報には、
複合組織鋼において第二相の硬さをフェライトの１．３
倍以上と規定して打ち抜きやせん断加工時のひずみエネ
ルギーを小さくし、打ち抜きやせん断加工ままでの疲労
強度を向上させる技術が開示されている。また特開平９
−１７００４８号公報には、粒界セメンタイトの長さを
規定して打ち抜きやせん断加工時にバリを少なくし、打
ち抜きやせん断加工ままでの疲労強度を向上させる技術
が開示されている。さらに特開平９−２０２９４０号公
報には、Ｔｉ，Ｎｂ，Ｃｒの添加量で整理したパラメー
タを規定することで打ち抜き性を改善し、打ち抜きまま
での疲労強度を向上させる技術が開示されている。Japanese Patent Application Laid-Open No. 8-302446 discloses that
In the dual-structure steel, the hardness of the second phase was set to 1.3 of ferrite.
A technique has been disclosed in which the strain energy during punching or shearing is reduced by defining the number as twice or more, and the fatigue strength as it is punched or sheared is improved. Japanese Patent Application Laid-Open No. Hei 9
Japanese Patent Application Publication No. -170048 discloses a technique in which the length of grain boundary cementite is defined to reduce burrs during punching or shearing, and to improve the fatigue strength as it is punched or sheared. Furthermore, Japanese Patent Application Laid-Open No. 9-202940 discloses a technique for improving punching properties by defining parameters arranged by the added amounts of Ti, Nb, and Cr, and improving fatigue strength as punched.

【００１０】一方、後者に属する発明として、特開平６
−８８１６１号公報には、表層における圧延面に平行な
集合組織の（１００）面強度が１．５以上と規定して疲
労き裂伝播速度を低下させる技術が開示されている。ま
た特開平８−１９９２８６号公報および特開平１０−１
４７８４６号公報には、Ｘ線で測定した板厚方向の（２
００）回折強度比を２．０〜１５．０に規定し、回復ま
たは再結晶フェライトの面積率を１５〜４０％とするこ
とで、疲労き裂伝播速度を低下させる技術が開示されて
いる。On the other hand, the invention belonging to the latter is disclosed in
JP-A-88161 discloses a technique for reducing the fatigue crack propagation speed by defining the (100) plane strength of the texture parallel to the rolled surface in the surface layer as 1.5 or more. Also, JP-A-8-199286 and JP-A-10-1
No. 47846 discloses that (2) in the thickness direction measured by X-rays.
00) A technique for reducing the fatigue crack propagation speed by setting the diffraction intensity ratio to 2.0 to 15.0 and setting the area ratio of the recovered or recrystallized ferrite to 15 to 40% is disclosed.

【００１１】しかし、前記特開平５−５１６９５号、同
５−１７９３４６号、同８−１３０３３号、同８−３０
２４４６号、同９−１７００４８号および同９−２０２
９４０号等の公報に開示されている、打ち抜きやせん断
加工端面に発生するバリのような鋭い切り欠きを低減す
る技術は、発生するバリの程度が打ち抜きやせん断加工
時のクリアランスによって大きく変化するので、どのよ
うな条件下でも適用できる技術ではなく、切り欠き疲労
強度に優れる鋼板としては不十分であると言わざるを得
ない。However, JP-A-5-51695, JP-A-5-179346, JP-A-8-13033, and JP-A-8-30
Nos. 2446, 9-170048 and 9-202
The technology disclosed in Japanese Patent Publication No. 940 or the like for reducing sharp notches such as burrs generated on the end face of punching or shearing is because the degree of generated burrs varies greatly depending on the clearance at the time of punching or shearing. However, it is not a technique that can be applied under any conditions, and it cannot be said that it is insufficient as a steel sheet having excellent notch fatigue strength.

【００１２】一方、特開平６−８８１６１号公報、同８
−１９９２８６号公報および同１０−１４７８４６号公
報に開示されている、集合組織を制御してき裂進展に対
する抵抗を高める技術は、主に建設機械、船舶、橋梁等
の大型構造物用の鋼を対象とした発明であり、本発明の
ように自動車用薄鋼板を対象としていない。また上記技
術は、主に溶接止端部より進展する疲労き裂の破壊力学
で言うところのＰＡＲＩＳ域でのき裂伝播速度を制御す
るというものであり、自動車用薄鋼板のように板厚が薄
いゆえにＰＡＲＩＳ域でのき裂伝播領域がほとんど存在
しない場合における技術としては不十分である。また、
薄鋼板用として用いられる平面げ疲労試験法で、図１
（ｂ）に示す試験片を用いて切り欠き疲労特性を評価し
た発明は、これまで見あたらない。On the other hand, Japanese Patent Application Laid-Open Nos. 6-88161 and 8
The techniques disclosed in JP-A-199286 and JP-A-10-147846 to increase the resistance to crack propagation by controlling the texture are mainly intended for steel for large structures such as construction machines, ships, and bridges. The present invention is not directed to a thin steel sheet for automobiles as in the present invention. In addition, the above-mentioned technology mainly controls the crack propagation speed in the PARIS region in terms of the fracture mechanics of a fatigue crack that propagates from the weld toe. Because of its thinness, it is insufficient as a technique in the case where there is almost no crack propagation region in the PARIS region. Also,
Fig. 1 is a flat fatigue test method used for thin steel sheets.
An invention in which the notch fatigue property was evaluated using the test piece shown in (b) has not been found so far.

【００１３】[0013]

【発明が解決しようとする課題】そこで本発明は、自動
車用薄鋼板において、打ち抜きやせん断加工端面のよう
な切り欠きから進展する疲労き裂を、打ち抜きやせん断
加工時のクリアランス等の条件によらず、集合組織を制
御してき裂進展に対する抵抗を高めることによって改善
する技術に関する。すなわち本発明は、切り欠き疲労強
度に優れる自動車用薄鋼板、およびその鋼板を安価に安
定して製造できる製造方法を提供することを目的とす
る。SUMMARY OF THE INVENTION Accordingly, the present invention provides a thin steel sheet for automobiles that is capable of reducing fatigue cracks that develop from notches such as punched or sheared end faces depending on conditions such as clearance during punching and shearing. The present invention relates to a technique for improving the texture by controlling the texture and increasing the resistance to crack propagation. That is, an object of the present invention is to provide a thin steel sheet for automobiles having excellent notch fatigue strength, and a method of manufacturing the steel sheet stably at low cost.

【００１４】[0014]

【課題を解決するための手段】本発明者らは、現在通常
に採用されている製造設備により工業的規模で生産され
ている薄鋼板の製造プロセスを念頭において、自動車用
薄鋼板の切り欠き疲労強度の向上を達成すべく鋭意研究
を重ねた。その結果、最表面から板厚方向に０．５ｍｍ
までの任意深さにおける板面の｛１００｝＜０１１＞〜
｛２２３｝＜１１０＞方位群のＸ線ランダム強度比の平
均値が２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４以下であり、板厚が０．５
ｍｍ以上１２ｍｍ以下であることが、切り欠き疲労強度
向上に非常に有効であることを新たに見出し、本発明を
なしたものである。DISCLOSURE OF THE INVENTION The inventors of the present invention have considered notch fatigue of a steel sheet for automobiles, taking into account the manufacturing process of a steel sheet which is produced on an industrial scale by the manufacturing equipment which is usually employed at present. We worked diligently to improve the strength. As a result, 0.5 mm from the outermost surface in the thickness direction
{100} <011> ~ of the plate surface at any depth up to
The average value of the X-ray random intensity ratio of the {223} <110> orientation group is 2 or more, and {554} <225>, {111}
The average value of the X-ray random intensity ratios in three directions of <112> and {111} <110> is 4 or less, and the plate thickness is 0.5
The present invention has been found out that a thickness of not less than 12 mm and not more than 12 mm is very effective in improving notch fatigue strength, and the present invention has been made.

【００１５】即ち、本発明の要旨は以下の通りである。 （１） 最表面から板厚方向に０．５ｍｍまでの任意深
さにおける板面の｛１００｝＜０１１＞〜｛２２３｝＜
１１０＞方位群のＸ線ランダム強度比の平均値が２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４以下であり、板厚が０．５ｍｍ以上１２
ｍｍ以下であることを特徴とする切り欠き疲労強度に優
れる自動車用薄鋼板。 （２） 前記（１）に記載の鋼板のミクロ組織が、体積
分率最大の相をベイナイト，またはフェライトおよびベ
イナイトの複合組織、であることを特徴とする、切り欠
き疲労強度に優れる自動車用薄鋼板。 （３） 前記（１）に記載の鋼板のミクロ組織が、体積
分率５％以上２５％以下の残留オーステナイトを含み、
残部が主にフェライト、ベイナイトからなる複合組織で
あることを特徴とする、切り欠き疲労強度に優れる自動
車用薄鋼板。 （４） 前記（１）に記載の鋼板のミクロ組織が、体積
分率最大の相をフェライトとし、第二相を主にマルテン
サイトとする複合組織であることを特徴とする、切り欠
き疲労強度に優れる自動車用薄鋼板。 （５） 質量％で、 Ｃ ：０．０１〜０．３％、Ｓｉ：０．０１〜２％、 Ｍｎ：０．０５〜３％、 Ｐ ≦０．１％、 Ｓ ≦０．０１％、 Ａｌ：０．００５〜１％ を含み、残部がＦｅ及び不可避的不純物からなる鋼であ
ることを特徴とする、前記（１）〜（４）のいずれか１
項に記載の切り欠き疲労強度に優れる自動車用薄鋼板。 （６） 鋼成分が、さらに質量％で、Ｃｕ：０．２〜２
％を含有することを特徴とする、前記（５）に記載の切
り欠き疲労強度に優れる自動車用薄鋼板。 （７） 鋼成分が、さらに質量％で、Ｂ：０．０００２
〜０．００２％を含有することを特徴とする、前記
（５）または（６）に記載の切り欠き疲労強度に優れる
自動車用薄鋼板。 （８） 鋼成分が、さらに質量％で、Ｎｉ：０．１〜１
％を含有することを特徴とする、前記（５）ないし
（７）のいずれか１項に記載の切り欠き疲労強度に優れ
る自動車用薄鋼板。 （９） 鋼成分が、さらに質量％で、Ｃａ：０．０００
５〜０．００２％、ＲＥＭ：０．０００５〜０．０２％
の一種または二種を含有することを特徴とする、前記
（５）ないし（８）のいずれか１項に記載の切り欠き疲
労強度に優れる自動車用薄鋼板。 （１０） 鋼成分が、さらに質量％で、 Ｔｉ：０．０５〜０．５％、 Ｎｂ：０．０１〜０．５％、 Ｍｏ：０．０５〜１％、 Ｖ ：０．０２〜０．２％、 Ｃｒ：０．０１〜１％、 Ｚｒ：０．０２〜０．２％ の一種または二種以上を含有することを特徴とする、前
記（５）ないし（９）のいずれか１項に記載の切り欠き
疲労強度に優れる自動車用薄鋼板。 （１１） 前記（１）ないし（１０）のいずれか１項に
記載の自動車用薄鋼板に亜鉛めっきが施されていること
を特徴とする、切り欠き疲労強度に優れる自動車用薄鋼
板。That is, the gist of the present invention is as follows. (1) {100} <011> to {223} <of the plate surface at an arbitrary depth from the outermost surface to 0.5 mm in the plate thickness direction
The average value of the X-ray random intensity ratios in the three orientations of {554} <225>, {111} <112>, and {111} <110> is 2 or more and the average value of the X-ray random intensity ratios of the 110> orientation group is 2 or more. Is 4 or less, and the plate thickness is 0.5 mm or more and 12
mm, which is excellent in notch fatigue strength. (2) The microstructure of a steel sheet according to (1), wherein the phase having the largest volume fraction is bainite or a composite structure of ferrite and bainite, and the thin steel sheet for automobiles having excellent notch fatigue strength. steel sheet. (3) The microstructure of the steel sheet according to (1) includes retained austenite having a volume fraction of 5% or more and 25% or less,
A thin steel sheet for automobiles having excellent notch fatigue strength, characterized in that the remainder has a composite structure mainly composed of ferrite and bainite. (4) Notch fatigue strength, wherein the microstructure of the steel sheet according to (1) is a composite structure in which the phase having the largest volume fraction is ferrite and the second phase is mainly martensite. Excellent steel sheet for automobiles. (5) In mass%, C: 0.01 to 0.3%, Si: 0.01 to 2%, Mn: 0.05 to 3%, P ≦ 0.1%, S ≦ 0.01%, Al: 0.005 to 1%, the balance being steel consisting of Fe and unavoidable impurities, any one of the above (1) to (4),
A thin steel sheet for automobiles having excellent notch fatigue strength according to the item. (6) The steel component is further in mass%, and Cu: 0.2 to 2
%, Which is excellent in notch fatigue strength according to the above (5). (7) The steel component is further represented by mass%, B: 0.0002.
The thin steel sheet for automobiles having excellent notch fatigue strength according to the above (5) or (6), characterized by containing 0.002% to 0.002%. (8) The steel component is further in mass%, Ni: 0.1 to 1
%. The automotive thin steel sheet having excellent notch fatigue strength according to any one of the above (5) to (7), characterized by containing (9) The steel composition further contains Ca: 0.000% by mass.
5 to 0.002%, REM: 0.0005 to 0.02%
The thin steel sheet for automobiles having excellent notch fatigue strength according to any one of the above (5) to (8), characterized by containing one or two of the following. (10) Steel component in further mass%, Ti: 0.05 to 0.5%, Nb: 0.01 to 0.5%, Mo: 0.05 to 1%, V: 0.02 to 0 2%, Cr: 0.01 to 1%, Zr: 0.02 to 0.2%, any one of the above (5) to (9). A thin steel sheet for automobiles having excellent notch fatigue strength according to the item. (11) A thin steel sheet for an automobile having excellent notch fatigue strength, wherein the thin steel sheet for an automobile according to any one of the above (1) to (10) is galvanized.

【００１６】（１２） 前記（５）ないし（１０）のい
ずれか１項に記載の成分を有する薄鋼板を得るための熱
間圧延する際に、該成分を有する鋼片を粗圧延後にＡｒ
3 変態点温度＋１００℃以下の温度域で鋼板厚の合計圧
下率２５％以上の仕上圧延をすることを特徴とする、切
り欠き疲労強度に優れる自動車用薄鋼板の製造方法。 （１３） 前記（５）ないし（１０）のいずれか１項に
記載の成分を有する薄鋼板を得るため熱間圧延する際
に、該成分を有する鋼片を粗圧延後にＡｒ3 変態点温度
＋１００℃以下の温度域で鋼板厚の合計圧下率２５％以
上の仕上圧延をし、その後、２０℃／ｓ以上の冷却速度
で冷却して、４５０℃以上の巻取温度で巻き取ることを
特徴とする、切り欠き疲労強度に優れる自動車用薄鋼板
の製造方法。 （１４） 前記（５）ないし（１０）のいずれか１項に
記載の成分を有する薄鋼板を得るため熱間圧延する際
に、該成分を有する鋼片を粗圧延後にＡｒ3 変態点温度
＋１００℃以下の温度域で鋼板厚の合計圧下率２５％以
上の仕上圧延をし、その後Ａｒ1 変態点温度以上Ａｒ3
変態点温度以下の温度域で１〜２０秒間滞留し、その後
さらに、２０℃／ｓ以上の冷却速度で冷却して、３５０
℃超４５０℃未満の温度域の巻取温度で巻き取ることを
特徴とする、切り欠き疲労強度に優れる自動車用薄鋼板
の製造方法。 （１５） 前記（５）ないし（１０）のいずれか１項に
記載の成分を有する薄鋼板を得るため熱間圧延する際
に、該成分を有する鋼片を粗圧延後にＡｒ3 変態点温度
＋１００℃以下の温度域で鋼板厚の合計圧下率２５％以
上の仕上圧延をし、その後Ａｒ1 変態点温度以上Ａｒ3
変態点温度以下の温度域で１〜２０秒間滞留し、その後
さらに、２０℃／ｓ以上の冷却速度で冷却して、３５０
℃以下の巻取温度で巻き取ることを特徴とする、切り欠
き疲労強度に優れる自動車用薄鋼板の製造方法。 （１６） 熱間圧延に際し、粗圧延後の仕上圧延におい
て潤滑圧延を施すことを特徴とする、前記（１２）ない
し（１５）のいずれか１項に記載の切り欠き疲労強度に
優れる自動車用薄鋼板の製造方法。 （１７） 前記（１２）または（１６）に記載の熱間圧
延に際し、粗圧延終了後、デスケーリングを行うことを
特徴とする、切り欠き疲労強度に優れる自動車用薄鋼板
の製造方法。(12) At the time of hot rolling to obtain a thin steel sheet having the component described in any one of the above (5) to (10), the steel slab having the component is subjected to Ar rolling after rough rolling.
(3) A method for producing a thin steel sheet for automobiles having excellent notch fatigue strength, comprising performing finish rolling at a total reduction rate of 25% or more of the steel sheet thickness in a temperature range of a transformation point temperature + 100 ° C or less. (13) When hot rolling is performed to obtain a thin steel sheet having the component described in any one of the above (5) to (10), the steel slab having the component is roughly rolled, and then the Ar3 transformation point temperature is + 100 ° C. In the following temperature range, finish rolling is performed at a rolling reduction of 25% or more of the total thickness of the steel sheet, and then cooling is performed at a cooling rate of 20 ° C./s or more, and winding is performed at a winding temperature of 450 ° C. or more. , A method of manufacturing a thin steel sheet for automobiles having excellent notch fatigue strength. (14) When hot rolling is performed to obtain a thin steel sheet having the component described in any one of the above (5) to (10), the slab having the component is roughly rolled, and then the Ar3 transformation point temperature is + 100 ° C. In the following temperature range, finish rolling with a total reduction of 25% or more of the thickness of the steel sheet is performed, and then Ar3
It stays for 1 to 20 seconds in a temperature range not higher than the transformation point temperature, and is further cooled at a cooling rate of 20 ° C./s or more,
A method for manufacturing a thin steel sheet for automobiles having excellent notch fatigue strength, wherein the winding is performed at a winding temperature in a temperature range of more than 450C and less than 450C. (15) When hot rolling is performed to obtain a thin steel sheet having the component described in any one of the above (5) to (10), the slab having the component is roughly rolled, and then the Ar3 transformation point temperature is + 100 ° C. In the following temperature range, finish rolling with a total reduction of 25% or more of the thickness of the steel sheet is performed, and then Ar3
It stays for 1 to 20 seconds in a temperature range not higher than the transformation point temperature, and is further cooled at a cooling rate of 20 ° C./s or more,
A method for producing a thin steel sheet for automobiles having excellent notch fatigue strength, characterized by winding at a winding temperature of not more than ° C. (16) In the hot rolling, the lubricating rolling is performed in the finish rolling after the rough rolling, and the thin steel sheet for automobiles having excellent notch fatigue strength according to any one of the above (12) to (15), Steel sheet manufacturing method. (17) A method for producing a thin steel sheet for automobiles having excellent notch fatigue strength, wherein descaling is performed after rough rolling in the hot rolling according to (12) or (16).

【００１７】（１８） 前記（５）ないし（１０）のい
ずれか１項に記載の成分を有する薄鋼板を得るため熱間
圧延する際に、該成分を有する鋼片を熱間圧延後、続く
酸洗、鋼板厚圧下率８０％未満の冷間圧延後、回復温度
以上Ａｃ3 変態点温度＋１００℃以下の温度域で５〜１
５０秒間保持し、冷却する工程の回復または再結晶焼鈍
を行うことを特徴とする、切り欠き疲労強度に優れる自
動車用薄鋼板の製造方法。 （１９） 前記（５）ないし（１０）のいずれか１項に
記載の成分を有する薄鋼板を得るための熱間圧延する際
に、該成分を有する鋼片を熱間圧延後、続く酸洗、鋼板
厚圧下率８０％未満の冷間圧延後、Ａｃ1 変態点温度以
上Ａｃ3 変態点温度＋１００℃以下の温度域で５〜１５
０秒間保持し、その後に冷却する工程の熱処理を行うこ
とを特徴とする、切り欠き疲労強度に優れる自動車用薄
鋼板の製造方法。 （２０） 前記（５）ないし（１０）のいずれか１項に
記載の成分を有する薄鋼板を得るための熱間圧延する際
に、該成分を有する鋼片を熱間圧延後、続く酸洗、鋼板
厚圧下率８０％未満の冷間圧延後、Ａｃ1 変態点温度以
上Ａｃ3 変態点温度＋１００℃以下の温度域で５〜１５
０秒間保持し、２０℃／ｓ以上の冷却速度で３５０℃超
４５０℃未満の温度域まで冷却し、その後さらにその温
度域で５〜６００秒間保持し、５℃／ｓ以上の冷却速度
で２００℃以下の温度域まで冷却する工程の熱処理を行
うことを特徴とする、切り欠き疲労強度に優れる自動車
用薄鋼板の製造方法。 （２１） 前記（５）ないし（１０）のいずれか１項に
記載の成分を有する薄鋼板を得るための熱間圧延する際
に、該成分を有する鋼片を熱間圧延後、続く酸洗、鋼板
厚圧下率８０％未満の冷間圧延後、Ａｃ1 変態点温度以
上Ａｃ3 変態点温度＋１００℃以下の温度域で５〜１５
０秒間保持し、２０℃／ｓ以上の冷却速度で３５０℃以
下の温度域まで冷却する工程の熱処理をすることを特徴
とする、切り欠き疲労強度に優れる自動車用薄鋼板の製
造方法。 （２２） 前記（１２）ないし（１７）のいずれか１項
に記載の製造方法において、熱間圧延後に亜鉛めっき浴
中に浸漬させて鋼板表面を亜鉛めっきすることを特徴と
する、切り欠き疲労強度に優れる自動車用薄鋼板の製造
方法。 （２３） 前記（１８）ないし（２１）のいずれか１項
に記載の製造方法において、回復または再結晶焼鈍終了
後、亜鉛めっき浴中に浸漬させて鋼板表面を亜鉛めっき
することを特徴とする、切り欠き疲労強度に優れる自動
車用薄鋼板の製造方法。 （２４） 亜鉛めっき浴中に浸漬して亜鉛めっき後、合
金化処理することを特徴とする、前記（２２）または
（２３）に記載の切り欠き疲労強度に優れる自動車用薄
鋼板の製造方法。(18) When hot rolling is performed to obtain a thin steel sheet having the component described in any one of the above (5) to (10), the steel slab having the component is hot-rolled and then heated. After pickling and cold rolling at a steel sheet thickness reduction of less than 80%, 5-1 in the temperature range from the recovery temperature to the Ac3 transformation point temperature + 100 ° C or less.
A method for producing a thin steel sheet for automobiles having excellent notch fatigue strength, wherein recovery of a step of cooling for 50 seconds and cooling or recrystallization annealing are performed. (19) When hot rolling is performed to obtain a thin steel sheet having the component described in any one of the above (5) to (10), the steel slab having the component is hot-rolled and then pickled. After cold rolling at a steel sheet thickness reduction rate of less than 80%, 5 to 15 in the temperature range from the Ac1 transformation point temperature to the Ac3 transformation point temperature + 100 ° C or less.
A method for producing a thin steel sheet for automobiles having excellent notch fatigue strength, wherein the heat treatment is performed in a step of holding for 0 second and thereafter cooling. (20) When hot rolling is performed to obtain a thin steel sheet having the component according to any one of the above (5) to (10), the steel slab having the component is hot-rolled and then pickled. After cold rolling at a steel sheet thickness reduction rate of less than 80%, 5 to 15 in the temperature range from the Ac1 transformation point temperature to the Ac3 transformation point temperature + 100 ° C or less.
Hold for 0 seconds, cool at a cooling rate of 20 ° C./s or more to a temperature range of more than 350 ° C. and less than 450 ° C., and further hold for 5 to 600 seconds at that temperature range, and cool at a cooling rate of 5 ° C./s or more A method for producing a thin steel sheet for automobiles having excellent notch fatigue strength, comprising performing a heat treatment in a step of cooling to a temperature range of not more than ° C. (21) When hot rolling is performed to obtain a thin steel sheet having the component according to any one of the above (5) to (10), the steel slab having the component is hot-rolled and then pickled. After cold rolling at a steel sheet thickness reduction rate of less than 80%, 5 to 15 in the temperature range from the Ac1 transformation point temperature to the Ac3 transformation point temperature + 100 ° C or less.
A method for producing a thin steel sheet for automobiles having excellent notch fatigue strength, wherein the heat treatment is performed in a step of holding for 0 second and cooling at a cooling rate of 20 ° C / s or more to a temperature range of 350 ° C or less. (22) The manufacturing method according to any one of (12) to (17), wherein the steel sheet surface is galvanized by dipping in a zinc plating bath after hot rolling. A method for manufacturing automotive thin steel sheets with excellent strength. (23) The manufacturing method according to any one of (18) to (21), wherein after the recovery or recrystallization annealing is completed, the steel sheet surface is galvanized by dipping in a zinc plating bath. , A method of manufacturing a thin steel sheet for automobiles having excellent notch fatigue strength. (24) The method for producing a thin steel sheet for an automobile having excellent notch fatigue strength according to the above (22) or (23), wherein the steel sheet is immersed in a galvanizing bath, galvanized, and then alloyed.

【００１８】[0018]

【発明の実施の形態】まず、本発明に至った基礎研究結
果について以下に説明する。一般に疲労き裂は表面より
発生する。これは切り欠きのような応力集中部が存在す
る場合も例外ではない。また、打ち抜きやせん断加工端
面が存在する場合においても、面外曲げ方向の荷重モー
ドが含まれる繰り返し荷重下では、鋼板表面端部より疲
労き裂が進展することが多く観察されている。従ってこ
のような場合でも、鋼板最表面もしくは結晶粒数個程度
の深さまでのき裂進展抵抗の増加が、切り欠き疲労強度
向上に有効なことは明らかである。また、板厚中心部に
おいてき裂進展抵抗を増加させたとしても、既にき裂を
停留させることは難しい。ゆえに本発明では、疲労強度
向上に有効な集合組織の範囲を最表面から板厚方向に
０．５ｍｍまでに限定する。望ましくは０．１ｍｍまで
である。DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the results of basic research that led to the present invention will be described below. Generally, fatigue cracks occur from the surface. This is not an exception even when a stress concentration portion such as a notch exists. Further, even when a punched or sheared end face is present, it is often observed that a fatigue crack propagates from the steel sheet surface end under a repeated load including a load mode in an out-of-plane bending direction. Therefore, even in such a case, it is clear that increasing the crack propagation resistance to the outermost surface of the steel sheet or to a depth of about several crystal grains is effective for improving the notch fatigue strength. Even if the crack propagation resistance is increased at the center of the thickness, it is difficult to stop the crack already. Therefore, in the present invention, the range of the texture effective for improving the fatigue strength is limited to 0.5 mm in the thickness direction from the outermost surface. It is preferably up to 0.1 mm.

【００１９】切り欠き疲労強度に及ぼす最表面から板厚
方向に０．５ｍｍまでの任意深さにおける、板面の｛１
００｝＜０１１＞〜｛２２３｝＜１１０＞方位群のＸ線
ランダム強度比の平均値および、｛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の影響を調査し
た。そのための供試材は、次のようにして準備した。す
なわち、０．０８％Ｃ−０．９％Ｓｉ−１．２％Ｍｎ−
０．０１％Ｐ−０．００１％Ｓ−０．０３％Ａｌに成分
調整し溶製した鋳片を、Ａｒ3 変態点温度以上のいずれ
かの温度で板厚が３．５ｍｍになるように熱間仕上圧延
を終了した後、巻き取った。The effect of notch on the plate surface at an arbitrary depth from the outermost surface to 0.5 mm in the plate thickness direction on the notch fatigue strength
The average of the X-ray random intensity ratios of the 00｝ <011> to {223} <110> orientation groups and {554} <225
>, {111} <112> and {111} <110>
The effect of the average value of the X-ray random intensity ratio in three directions was investigated. The test material for that was prepared as follows. That is, 0.08% C-0.9% Si-1.2% Mn-
The slab was melted by adjusting the composition to 0.01% P-0.001% S-0.03% Al so that the thickness became 3.5 mm at any temperature above the Ar3 transformation point temperature. After finishing the finish rolling, it was wound up.

【００２０】このようにして得られた鋼板の最表面から
板厚方向に０．５ｍｍまでの任意深さにおける、板面の
｛１００｝＜０１１＞〜｛２２３｝＜１１０＞方位群の
Ｘ線ランダム強度比の平均値および、｛５５４｝＜２２
５＞、｛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を求めるため
に、板幅の１／４Ｗもしくは３／４Ｗ位置より３０ｍｍ
φに切り取った試片の、最表層より０．０５ｍｍ程度の
深さまで三山仕上の研削を行い、次いで化学研磨または
電解研磨によって歪みを除去して作製した。The X-rays of the {100} <011> to {223} <110> orientation group of the plate surface at an arbitrary depth from the outermost surface of the obtained steel plate to 0.5 mm in the plate thickness direction. Average value of random intensity ratio and {554} <22
5>, {111} <112> and {111} <110
30 mm from the 1/4 W or 3/4 W position of the plate width to obtain the average value of the X-ray random intensity ratio in the three directions
The sample cut into φ was ground to a depth of about 0.05 mm from the outermost layer, and then subjected to three-step finish grinding, and then the strain was removed by chemical polishing or electrolytic polishing.

【００２１】なお、｛ｈｋｌ｝＜ｕｖｗ＞で表される結
晶方位とは、板面の法線方向が＜ｈｋｌ＞に平行で、圧
延方向が＜ｕｖｗ＞と平行であることを示している。Ｘ
線による結晶方位の測定は、例えば「新版カリティＸ線
回折要論」（１９８６年発行、松村源太郎訳、株式会社
アグネ）２７４〜２９６頁に記載の方法に従った。Note that the crystal orientation represented by {hkl} <uvw> indicates that the normal direction of the plate surface is parallel to <hkl> and the rolling direction is parallel to <uvw>. X
The measurement of the crystal orientation by a line was performed according to the method described in, for example, “New Version of Curity X-ray Diffraction” (published in 1986, translated by Gentaro Matsumura, Agne Co., Ltd.), pages 274 to 296.

【００２２】ここで、｛１００｝＜０１１＞〜｛２２
３｝＜１１０＞方位群のＸ線ランダム強度比の平均値と
は、この方位群に含まれる主な方位、｛１００｝＜０１
１＞、｛１１６｝＜１１０＞、｛１１４｝＜１１０＞、
｛１１３｝＜１１０＞、｛１１２｝＜１１０＞、｛３３
５｝＜１１０＞および｛２２３｝＜１１０＞のＸ線回折
強度を、｛１１０｝極点図に基づきベクトル法により計
算した３次元集合組織、または｛１１０｝、｛１０
０｝、｛２１１｝、｛３１０｝極点図のうち複数の極点
図（望ましくは３つ以上）を用いて級数展開法で計算し
た３次元集合組織から求めた。Here, {100} <011> to {22}
The average value of the X-ray random intensity ratio of the 3｝ <110> orientation group is the main orientation included in this orientation group, {100} <01
1>, {116} <110>, {114} <110>,
{113} <110>, {112} <110>, $ 33
X-ray diffraction intensities of 5} <110> and {223} <110> were calculated by the vector method based on the {110} pole figure, or {110}, {10}
It was obtained from a three-dimensional texture calculated by a series expansion method using a plurality of pole figures (preferably three or more) of the pole maps of {0}, {211}, and {310}.

【００２３】例えば、後者の方法における上記各結晶方
位のＸ線ランダム強度比は、３次元集合組織のφ２＝４
５゜断面における（００１）［１−１０］、（１１６）
［１−１０］、（１１４）［１−１０］、（１１３）
［１−１０］、（１１２）［１−１０］、（３３５）
［１−１０］、（２２３）［１−１０］の強度をそのま
ま用いればよい。ただし｛１００｝＜０１１＞〜｛２２
３｝＜１１０＞方位群のＸ線ランダム強度比の平均値と
は、上記の各方位の相加平均である。For example, in the latter method, the X-ray random intensity ratio of each of the above crystal orientations is represented by φ2 = 4 in the three-dimensional texture.
(001) [1-10], (116) in 5 ° section
[1-10], (114) [1-10], (113)
[1-10], (112) [1-10], (335)
[1-10], (223) The intensity of [1-10] may be used as it is. However, {100} <011>-$ 22
The average value of the X-ray random intensity ratio of the 3｝ <110> azimuth group is an arithmetic mean of each azimuth described above.

【００２４】上記全ての方位の強度を得ることができな
い場合には、｛１００｝＜０１１＞、｛１１６｝＜１１
０＞、｛１１４｝＜１１０＞、｛１１２｝＜１１０＞、
｛２２３｝＜１１０＞の各方位の相加平均で代替しても
よい。次に｛５５４｝＜２２５＞、｛１１１｝＜１１２
＞および｛１１１｝＜１１０＞の３方位のＸ線ランダム
強度比の平均値とは、上記の方法と同様に計算した３次
元集合組織から求めればよい。When it is not possible to obtain the intensity in all the above directions, {100} <011>, {116} <11
0>, {114} <110>, {112} <110>,
The arithmetic mean of each direction of {223} <110> may be substituted. Next, {554} <225>, {111} <112
> And {111} <110> may be obtained from the three-dimensional texture calculated in the same manner as the above method.

【００２５】次に、上記鋼板の切り欠き疲労強度を調査
するために、板幅の１／４Ｗもしくは３／４Ｗ位置から
圧延方向が長辺になるように、図１（ｂ）に示す形状の
疲労試験片を採取し疲労試験に供した。ここで図１
（ａ）に記載の疲労試験片が一般的な素材の疲労強度を
得るための平滑試験片であるのに対して、図１（ｂ）に
記載の疲労試験片は、切り欠き疲労強度を得るために作
製された切り欠き試験片である。ただし、疲労試験片に
は最表層から０．０５ｍｍ程度の深さまで三山仕上の研
削を施した。疲労試験は電気油圧サーボ型疲労試験機を
用い、試験方法はＪＩＳ Ｚ ２２７３−１９７８およ
びＪＩＳ Ｚ ２２７５−１９７８に準じた。Next, in order to investigate the notch fatigue strength of the steel sheet, the shape shown in FIG. 1 (b) was set so that the rolling direction became the longer side from the 1 / 4W or 3 / 4W position of the sheet width. Fatigue test pieces were collected and subjected to a fatigue test. Here, FIG.
While the fatigue test piece shown in (a) is a smooth test piece for obtaining the fatigue strength of a general material, the fatigue test piece shown in FIG. 1 (b) obtains a notched fatigue strength. Is a notched test piece prepared for the test. However, the fatigue test specimen was subjected to three-step finish grinding from the outermost layer to a depth of about 0.05 mm. The fatigue test used an electrohydraulic servo-type fatigue tester, and the test method conformed to JIS Z 2273-1978 and JIS Z 2275-1978.

【００２６】切り欠き疲労強度に及ぼす｛１００｝＜０
１１＞〜｛２２３｝＜１１０＞方位群のＸ線ランダム強
度比の平均値および、｛５５４｝＜２２５＞、｛１１
１｝＜１１２＞および｛１１１｝＜１１０＞の３方位の
Ｘ線ランダム強度比の平均値の影響を調査した結果を図
２に示す。ここで○中の数字は、図１（ｂ）に示す形状
の切り欠き疲労試験片を用いて行った疲労試験より得ら
れる疲労限（１０7 回での時間強度）であり、以下切り
欠き疲労強度とする。The effect of {100} <0 on notch fatigue strength
11> to {223} <110> average value of the X-ray random intensity ratio of the orientation group, and {554} <225>, {11}
FIG. 2 shows the results of investigating the influence of the average value of the X-ray random intensity ratio in three directions of 1｝ <112> and {111} <110>. The numbers here in ○ are views 1 (b) to show the shape of the notch fatigue test piece obtained from fatigue tests conducted with fatigue limit (time strength at 10 7 times), following notch fatigue Strength.

【００２７】｛１００｝＜０１１＞〜｛２２３｝＜１１
０＞方位群のＸ線ランダム強度比の平均値および、｛５
５４｝＜２２５＞、｛１１１｝＜１１２＞および｛１１
１｝＜１１０＞の３方位のＸ線ランダム強度比の平均値
と切り欠き疲労強度との間には強い相関があり、それぞ
れの平均値が２以上かつ４以下で著しく切り欠き疲労強
度が向上することが示された。{100} <011> to {223} <11
0> the average value of the X-ray random intensity ratio of the orientation group and $ 5
54 {<225>, {111} <112> and {11
There is a strong correlation between the average value of the X-ray random intensity ratios in three directions of 1｝ <110> and the notch fatigue strength. When the average value is 2 or more and 4 or less, the notch fatigue strength is significantly improved. It was shown to be.

【００２８】本発明者らは、これらの実験結果を詳細に
検討した結果、切り欠き疲労強度を向上させるために
は、最表面から板厚方向に０．５ｍｍまでの任意深さに
おける板面の、｛１００｝＜０１１＞〜｛２２３｝＜１
１０＞方位群のＸ線ランダム強度比の平均値が２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４以下であることが非常に重要であると新た
に知見するに至った。The present inventors have examined these experimental results in detail, and as a result, in order to improve the notch fatigue strength, the plate surface at an arbitrary depth from the outermost surface to a thickness of 0.5 mm in the plate thickness direction was examined. , {100} <011> to {223} <1
10> The average value of the X-ray random intensity ratio of the orientation group is 2 or more, and the average value of the X-ray random intensity ratio of the three orientations {554} <225>, {111} <112>, and {111} <110> It is newly found that it is very important that the value is 4 or less.

【００２９】ただし、切り欠きだけでなく平滑での疲労
き裂発生抵抗も向上させるためには、最表面から板厚方
向に０．５ｍｍまでの任意深さにおける板面の、｛１０
０｝＜０１１＞〜｛２２３｝＜１１０＞方位群のＸ線ラ
ンダム強度比の平均値が４以上かつ、｛５５４｝＜２２
５＞、｛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２．５以下
であること望ましい。However, in order to improve not only the notch but also the smooth fatigue crack initiation resistance, the surface of the sheet at an arbitrary depth of 0.5 mm from the outermost surface to the sheet thickness direction must be reduced by 10%.
The average of the X-ray random intensity ratios of the 0 <011> to {223} <110> orientation groups is 4 or more and {554} <22
5>, {111} <112> and {111} <110
It is desirable that the average value of the X-ray random intensity ratio in the three directions of> is 2.5 or less.

【００３０】このメカニズムは必ずしも明らかではない
が以下のように推測される。一般的に、鋭い切り欠きが
存在する場合の疲労限は、き裂進展限界、すなわちき裂
を停留させるためのき裂進展抵抗の大小によって決ま
る。疲労き裂の進展は切り欠き底もしくは応力集中箇所
における小規模な塑性変形の繰り返しであるが、き裂長
さが比較的短く、結晶粒程度の大きさの範囲でその塑性
変形が起こる場合においては、結晶学的なすべり面及び
すべり方向の影響が大きいと推測される。従って、き裂
進展方位およびき裂面に対して、き裂進展抵抗が高いす
べり面及びすべり方向を持つ結晶の割合が多ければ、疲
労き裂の進展が抑制される。Although this mechanism is not always clear, it is assumed as follows. In general, the fatigue limit in the presence of a sharp notch is determined by the crack growth limit, that is, the magnitude of the crack growth resistance for stopping the crack. Fatigue crack growth is a repetition of small-scale plastic deformation at the notch bottom or stress concentration point. It is presumed that the influence of the crystallographic slip plane and the slip direction is large. Therefore, if the ratio of the slip surface having a high crack growth resistance and the crystal having the slip direction is large with respect to the crack growth direction and the crack surface, the growth of the fatigue crack is suppressed.

【００３１】次に、本発明における鋼板の板厚の限定理
由について説明する。板厚が０．５ｍｍ未満では、応力
集中の程度に関わらず小規模降伏条件を満足することが
できないため、モノトニックな延性破壊に至る危険性が
ある。また、き裂停留という観点からは十分な塑性拘束
が必要であるため、平面ひずみ状態を保つためには少な
くとも１．２ｍｍ以上の板厚であることが望ましい。一
方、板厚が１２ｍｍ超では、板厚効果（寸法効果）によ
る疲労強度の低下が顕著になる。また板厚が８ｍｍ超で
あると、切り欠き疲労強度向上に有効な集合組織を得る
ための熱間もしくは冷間圧延条件を達成するためには、
設備に過大な荷重負荷がかかる恐れがあることから、８
ｍｍ以下が望ましい。従って本発明において、その板厚
は０．５ｍｍ以上１２ｍｍ以下と限定する。望ましくは
１．２ｍｍ以上８ｍｍ以下である。Next, the reason for limiting the thickness of the steel sheet in the present invention will be described. If the plate thickness is less than 0.5 mm, the small-scale yield condition cannot be satisfied regardless of the degree of stress concentration, and there is a risk of causing monotonic ductile fracture. In addition, since sufficient plastic restraint is required from the viewpoint of crack arrest, it is desirable that the thickness be at least 1.2 mm or more in order to maintain a plane strain state. On the other hand, if the plate thickness exceeds 12 mm, the fatigue strength is significantly reduced due to the plate thickness effect (size effect). When the sheet thickness is more than 8 mm, in order to achieve hot or cold rolling conditions for obtaining a texture effective for improving notch fatigue strength,
Since there is a possibility that excessive load may be applied to the equipment, 8
mm or less is desirable. Therefore, in the present invention, the plate thickness is limited to 0.5 mm or more and 12 mm or less. Desirably, it is 1.2 mm or more and 8 mm or less.

【００３２】次に、本発明における鋼板のミクロ組織に
ついて説明する。本発明において、その切り欠き疲労強
度を向上させるという目的のために鋼板のミクロ組織を
特に限定する必要はなく、通常の鋼が呈するフェライ
ト、ベイナイト、パーライト、マルテンサイト組織にお
いて本発明範囲の集合組織（本発明範囲のＸ線ランダム
強度比）が得られていれば、本発明の切り欠き疲労強度
を向上させるという効果は得られるので、他の必要特性
に応じてミクロ組織を規定することが好ましい。ただ
し、特定のミクロ組織、例えば体積分率５％以上２５％
以下の残留オーステナイトを含み、残部が主にフェライ
ト、ベイナイトからなる複合組織、または体積分率最大
の相をフェライトとし、第二相を主にマルテンサイトと
する複合組織等においてはこの効果を更に高めることが
できる。Next, the microstructure of the steel sheet according to the present invention will be described. In the present invention, the microstructure of the steel sheet does not need to be particularly limited for the purpose of improving the notch fatigue strength, and the ferrite, bainite, pearlite, and martensite structures exhibited by ordinary steel have a texture within the scope of the present invention. If the (X-ray random intensity ratio in the range of the present invention) is obtained, the effect of improving the notch fatigue strength of the present invention can be obtained. Therefore, it is preferable to define the microstructure according to other necessary characteristics. . However, a specific microstructure, for example, a volume fraction of 5% or more and 25% or more
This effect is further enhanced in a composite structure containing the following retained austenite, the balance being mainly ferrite, bainite, or a composite structure in which the phase having the largest volume fraction is ferrite and the second phase is mainly martensite. be able to.

【００３３】なお、ここで言うベイナイトとは、ベイニ
ティックフェライトおよびアシュキュラーフェライト組
織も含む。ただし、二相以上の複合組織において残留オ
ーステナイト等の結晶構造がｂｃｃでないものを含む場
合は、それ以外の組織の体積分率で換算したＸ線ランダ
ム強度比が本発明の範囲内であれば差し支えない。ま
た、粗大な炭化物を含むパーライトは疲労き裂の発生サ
イトになり極端に疲労強度を低下させる恐れがあるの
で、粗大な炭化物を含むパーライトの体積分率は１５％
以下が望ましい。さらに良好な疲労特性を確保するため
には、粗大な炭化物を含むパーライトの体積分率は５％
以下が望ましい。The term "bainite" as used herein includes bainitic ferrite and ashular ferrite structures. However, when the crystal structure such as retained austenite is not bcc in the composite structure of two or more phases, if the X-ray random intensity ratio converted by the volume fraction of the other structure is within the range of the present invention, there is no problem. Absent. Further, since pearlite containing coarse carbides may become a fatigue crack initiation site and extremely reduce fatigue strength, the volume fraction of pearlite containing coarse carbides is 15%.
The following is desirable. In order to secure better fatigue properties, the volume fraction of pearlite containing coarse carbide should be 5%.
The following is desirable.

【００３４】なお、ここで、フェライト、ベイナイト、
パーライト、マルテンサイトおよび残留オーステナイト
の体積分率とは、鋼板板幅の１／４Ｗもしくは３／４Ｗ
位置より切出した試料を圧延方向断面に研磨し、ナイタ
ール試薬および／または特開平５−１６３５９０号公報
で開示されている試薬を用いてエッチングし、光学顕微
鏡を用い２００〜５００倍の倍率で観察された板厚の１
／４ｔにおけるミクロ組織の面積分率で定義される。た
だし、残留オーステナイトは上記試薬によるエッチング
では容易に判別できない場合もあるので、下記の手法に
て体積分率を算出してもよい。すなわち、オーステナイ
トはフェライトと結晶構造が違うため結晶学的に容易に
識別できる。従って、残留オーステナイトの体積分率は
Ｘ線回折法によっても実験的に求めることができる。す
なわち、ＭｏのＫα線を用いてオーステナイトとフェラ
イトとの反射面強度の違いより次式を用いてその体積分
率を簡便に求める方法である。 Ｖγ＝（２／３）｛１００／（０．７×α（２１１）／
γ（２２０）＋１）｝＋（１／３）｛１００／（０．７
８×α（２１１）／γ（３１１）＋１）｝ ただし、α（２１１）、γ（２２０）およびγ（３１
１）は、それぞれフェライト（α）オーステナイト
（γ）のＸ線反射面強度である。Here, ferrite, bainite,
The volume fraction of pearlite, martensite and retained austenite is 1 / 4W or 3 / 4W of the steel sheet width.
A sample cut out from the position is polished to a cross section in the rolling direction, etched using a nital reagent and / or a reagent disclosed in JP-A-5-163590, and observed at a magnification of 200 to 500 times using an optical microscope. Plate thickness 1
It is defined by the area fraction of the microstructure at / 4t. However, since the retained austenite may not be easily distinguished by etching with the above reagent, the volume fraction may be calculated by the following method. That is, since austenite has a different crystal structure from ferrite, it can be easily identified crystallographically. Therefore, the volume fraction of retained austenite can also be experimentally determined by the X-ray diffraction method. That is, a method of easily obtaining the volume fraction of the austenite and ferrite from the difference in the reflection surface intensity between austenite and ferrite using the Mo Kα ray and the following equation. Vγ = (2/3) ｛100 / (0.7 × α (211) /
γ (220) +1)｝ + (１ ／) ｛100 / (0.7
8 × α (211) / γ (311) +1) where α (211), γ (220) and γ (31
1) is the X-ray reflection surface intensity of ferrite (α) austenite (γ), respectively.

【００３５】本発明において、切り欠き疲労強度の向上
の他に良好なバーリング加工性を付与するためには、そ
のミクロ組織を体積分率最大の相をベイナイト，または
フェライトおよびベイナイトの複合組織、とする。ただ
し、不可避的なマルテンサイト、残留オーステナイトお
よびパーライトを含むことを許容するものである。良好
なバーリング加工性（穴拡げ値）を得るためには、硬質
な残留オーステナイトおよびマルテンサイトを合わせた
体積分率は５％未満が望ましい。また、ベイナイトの体
積分率は３０％以上が望ましい。さらに、良好な延性を
得るためにはベイナイトの体積分率は７０％以下が望ま
しい。In the present invention, in order to provide good burring workability in addition to improving the notch fatigue strength, the microstructure of the present invention is defined as a phase having a maximum volume fraction of bainite or a composite structure of ferrite and bainite. I do. However, the inclusion of unavoidable martensite, retained austenite and pearlite is permitted. In order to obtain good burring workability (hole expansion value), the combined volume fraction of hard retained austenite and martensite is desirably less than 5%. The volume fraction of bainite is desirably 30% or more. Further, in order to obtain good ductility, the volume fraction of bainite is desirably 70% or less.

【００３６】また、本発明において切り欠き疲労強度の
向上の他に良好な延性を付与するためには、そのミクロ
組織を体積分率５％以上２５％以下の残留オーステナイ
トを含み、残部が主にフェライト、ベイナイトからなる
複合組織とする。ただし、合わせて５％未満の不可避的
なマルテンサイトおよびパーライトを含むことを許容す
るものである。さらに、本発明において切り欠き疲労強
度の向上の他に良好な形状凍結性を得るための低降伏比
を付与するためには、そのミクロ組織を体積分率最大の
相をフェライトとし、第二相を主にマルテンサイトとす
る複合組織とする。ただし、合わせて５％未満の不可避
的なベイナイト、残留オーステナイトおよびパーライト
を含むことを許容するものである。なお、７０％以下の
低降伏比を確保するためには、フェライトの体積分率は
５０％以上が望ましい。In order to impart good ductility in addition to improving the notch fatigue strength in the present invention, the microstructure of the present invention contains retained austenite having a volume fraction of 5% or more and 25% or less, and the balance is mainly The composite structure is composed of ferrite and bainite. However, the inclusion of less than 5% of inevitable martensite and pearlite is permitted. Furthermore, in order to provide a low yield ratio for obtaining a good shape freezing property in addition to the improvement of notch fatigue strength in the present invention, the microstructure of the microstructure is defined as a phase having a maximum volume fraction of ferrite, and the second phase is defined as a ferrite. Is mainly composed of martensite. However, the inclusion of less than 5% of inevitable bainite, retained austenite, and pearlite is permitted. In order to secure a low yield ratio of 70% or less, the volume fraction of ferrite is desirably 50% or more.

【００３７】続いて、本発明の化学成分の限定理由につ
いて説明する。Ｃは、所望のミクロ組織を得るのに必要
な元素である。ただし、０．３％超含有していると加工
性が劣化するので、０．３％以下とする。また、０．２
％超含有すると溶接性が劣化する傾向があるので、好ま
しくは０．２％以下が望ましい。一方、０．０１％未満
であると強度が低下するので、０．０１％以上とする。
また、良好な延性を得るための十分な残留オーステナイ
ト量を安定的に得るためには好ましくは０．０５％以上
が望ましい。Next, the reasons for limiting the chemical components of the present invention will be described. C is an element necessary for obtaining a desired microstructure. However, if the content exceeds 0.3%, the workability deteriorates, so the content is set to 0.3% or less. Also, 0.2
%, The weldability tends to deteriorate. Therefore, the content is preferably 0.2% or less. On the other hand, if it is less than 0.01%, the strength is reduced.
In order to stably obtain a sufficient amount of retained austenite for obtaining good ductility, the content is preferably 0.05% or more.

【００３８】Ｓｉは、固溶強化元素として強度上昇に有
効である。所望の強度を得るためには０．０１％以上含
有する必要がある。しかし、２％超含有すると加工性が
劣化する。そこでＳｉの含有量は０．０１〜２％とす
る。Si is effective in increasing the strength as a solid solution strengthening element. In order to obtain a desired strength, it is necessary to contain 0.01% or more. However, if the content exceeds 2%, the workability deteriorates. Therefore, the content of Si is set to 0.01 to 2%.

【００３９】Ｍｎは、固溶強化元素として強度上昇に有
効である。所望の強度を得るためには０．０５％以上必
要である。また、Ｍｎ以外にＳによる熱間割れの発生を
抑制するＴｉなどの元素が十分に添加されない場合に
は、質量％でＭｎ／Ｓ≧２０となるＭｎ量を添加するこ
とが望ましい。さらに、Ｍｎはオーステナイト安定化元
素であり、良好な延性を得るための十分な残留オーステ
ナイト量を安定的に得るためその添加量は０．１％以上
が望ましい。一方、３％超添加するとスラブ割れを生ず
るため、３％以下とする。Mn is effective for increasing strength as a solid solution strengthening element. To obtain the desired strength, 0.05% or more is required. When an element such as Ti that suppresses the occurrence of hot cracking due to S is not sufficiently added in addition to Mn, it is desirable to add an Mn amount that satisfies Mn / S ≧ 20 by mass%. Further, Mn is an austenite stabilizing element, and its addition amount is desirably 0.1% or more in order to stably obtain a sufficient amount of retained austenite for obtaining good ductility. On the other hand, if added over 3%, slab cracks occur, so the content is set to 3% or less.

【００４０】Ｐは、不純物であり低いほど望ましく、
０．１％超含有すると加工性や溶接性に悪影響を及ぼす
とともに疲労特性も低下させるので、０．１％以下とす
る。P is an impurity and is preferably as low as possible.
If the content exceeds 0.1%, the workability and the weldability are adversely affected and the fatigue characteristics are also reduced.

【００４１】Ｓは、不純物であり低いほど望ましく、多
すぎると局部延性やバーリング加工性を劣化させるＡ系
介在物を生成するので、極力低減させるべきであるが、
０．０１％以下ならば許容できる範囲である。S is an impurity and is preferably as low as possible. If it is too large, A-based inclusions which deteriorate local ductility and burring workability are generated.
If it is 0.01% or less, it is within an acceptable range.

【００４２】Ａｌは、溶鋼脱酸のために０．００５％以
上添加する必要があるが、コストの上昇を招くためその
上限を１．０％とする。また、あまり多量に添加すると
非金属介在物を増大させ伸びを劣化させるので、望まし
くは０．５％以下とする。Al needs to be added in an amount of 0.005% or more for deoxidation of molten steel, but the upper limit is set to 1.0% because it causes an increase in cost. Further, if added in an excessively large amount, nonmetallic inclusions increase and elongation deteriorates. Therefore, the content is desirably 0.5% or less.

【００４３】Ｃｕは、固溶状態で疲労特性を改善する効
果があるので必要に応じ添加する。ただし、０．２％未
満ではその効果が少なく、２％を超えて含有しても効果
が飽和する。そこでＣｕの含有量は０．２〜２％の範囲
とする。ただし、巻取温度が４５０℃以上の場合は、
１．２％を超えて含有すると巻取り後に析出して加工性
を著しく劣化させる恐れがあので、１．２％以下とする
ことが望ましい。Since Cu has an effect of improving fatigue characteristics in a solid solution state, Cu is added as necessary. However, if the content is less than 0.2%, the effect is small, and if the content exceeds 2%, the effect is saturated. Therefore, the content of Cu is set in the range of 0.2 to 2%. However, if the winding temperature is 450 ° C or higher,
If the content exceeds 1.2%, it is likely to precipitate after winding to significantly degrade the workability.

【００４４】Ｂは、Ｃｕと複合添加されることによって
疲労限を上昇させる効果があるので、必要に応じ添加す
る。ただし、０．０００２％未満ではその効果を得るた
めに不十分であり、０．００２％超添加するとスラブ割
れが起こる。よって、Ｂの添加は０．０００２〜０．０
０２％とする。B has an effect of increasing the fatigue limit by being added in combination with Cu, so B is added as necessary. However, if it is less than 0.0002%, it is insufficient to obtain the effect, and if it exceeds 0.002%, slab cracking occurs. Therefore, the addition of B is 0.0002 to 0.0
02%.

【００４５】Ｎｉは、Ｃｕ含有による熱間脆性防止のた
めに必要に応じ添加する。ただし、０．１％未満ではそ
の効果が少なく、１％を超えて添加してもその効果が飽
和するので、０．１〜１％とする。Ni is added as necessary to prevent hot brittleness due to the inclusion of Cu. However, if the content is less than 0.1%, the effect is small, and if the content exceeds 1%, the effect is saturated. Therefore, the content is set to 0.1 to 1%.

【００４６】ＣａおよびＲＥＭは、破壊の起点となった
り、加工性を劣化させる非金属介在物の形態を変化させ
て無害化する元素である。ただし、それぞれ０．０００
５％未満添加してもその効果がなく、Ｃａならば０．０
０２％超、ＲＥＭならば０．０２％超添加してもその効
果が飽和するので、Ｃａ：０．０００５〜０．００２
％、ＲＥＭ：０．０００５〜０．０２％添加することが
望ましい。Ca and REM are elements that become the starting point of destruction and change the form of nonmetallic inclusions that degrade workability and render them harmless. However, each 0.000
There is no effect even if less than 5% is added.
If the content exceeds 02%, and if REM exceeds 0.02%, the effect is saturated, so Ca: 0.0005 to 0.002
%, REM: 0.0005 to 0.02%.

【００４７】さらに、強度を付与するために、Ｔｉ，Ｎ
ｂ，Ｍｏ，Ｖ，Ｃｒ，Ｚｒの析出強化もしくは固溶強化
元素の一種または二種以上を添加してもよい。ただし、
それぞれ０．０５％、０．０１％、０．０５％、０．０
２％、０．０１％、０．０２％未満ではその効果を得る
ことができない。また、それぞれ０．５％、０．５％、
１％、０．２％、１％、０．２％を超え添加してもその
効果は飽和する。Further, in order to impart strength, Ti, N
One, two or more of b, Mo, V, Cr, Zr precipitation strengthening or solid solution strengthening elements may be added. However,
0.05%, 0.01%, 0.05%, 0.0
If it is less than 2%, 0.01% or 0.02%, the effect cannot be obtained. Also, 0.5%, 0.5%,
Even if it exceeds 1%, 0.2%, 1%, and 0.2%, the effect is saturated.

【００４８】なお、これらを主成分とする鋼にＳｎ，Ｃ
ｏ，Ｚｎ，Ｗ，Ｍｇを合計で１％以下含有しても構わな
い。しかしながらＳｎは熱間圧延時に疵が発生する恐れ
があるので、０．０５％以下が望ましい。It should be noted that Sn, C
o, Zn, W, and Mg may be contained in a total of 1% or less. However, since Sn may cause flaws during hot rolling, 0.05% or less is desirable.

【００４９】次に、本発明の製造方法の限定理由につい
て、以下に詳細に述べる。本発明は、鋳造後、熱間圧延
後冷却ままもしくは熱間圧延後に冷却・酸洗し冷延した
後に焼鈍、あるいは熱延鋼板もしくは冷延鋼板を溶融め
っきラインにて熱処理を施したまま、更にはこれらの鋼
板に別途表面処理を施すことによっても得られる。Next, the reasons for limiting the production method of the present invention will be described in detail below. The present invention, after casting, hot rolling after cooling or pickling after cold rolling or hot rolling, annealing after annealing, or hot-rolled steel sheet or cold-rolled steel sheet while subjected to heat treatment in a hot-dip plating line, Can also be obtained by subjecting these steel sheets to a separate surface treatment.

【００５０】本発明において、熱間圧延に先行する製造
方法は特に限定するものではない。すなわち、高炉や電
炉等による溶製に引き続き各種の２次製錬で目的の成分
含有量になるように成分調整を行い、次いで通常の連続
鋳造、インゴット法による鋳造の他、薄スラブ鋳造など
の方法で鋳造すればよい。原料にはスクラップを使用し
ても構わない。連続鋳造よって得たスラブの場合には、
高温鋳片のまま熱間圧延機に直送してもよいし、室温ま
で冷却後に加熱炉にて再加熱した後に熱間圧延してもよ
い。In the present invention, the production method prior to hot rolling is not particularly limited. In other words, following smelting in a blast furnace or electric furnace, the components are adjusted in the various secondary smelting processes so that the desired component content is obtained. It may be cast by a method. Scrap may be used as a raw material. For slabs obtained by continuous casting,
The hot slab may be sent directly to a hot rolling mill as it is, or may be cooled to room temperature, reheated in a heating furnace, and then hot rolled.

【００５１】再加熱温度については特に制限はないが、
１４００℃以上であると、スケールオフ量が多量になり
歩留まりが低下するので、再加熱温度は１４００℃未満
が望ましい。また、１０００℃未満の加熱はスケジュー
ル上操業効率を著しく損なうため、再加熱温度は１００
０℃以上が望ましい。熱間圧延工程は、粗圧延を終了
後、仕上げ圧延を行うが、粗圧延終了後にデスケーリン
グを行う場合は、鋼板表面での高圧水の衝突圧Ｐ（ＭＰ
ａ）×流量Ｌ（リットル／ｃｍ2 ）≧０．００２５の条
件を満たすことが望ましい。The reheating temperature is not particularly limited,
If the temperature is 1400 ° C. or more, the scale-off amount becomes large and the yield decreases, so the reheating temperature is desirably less than 1400 ° C. Further, since the heating at less than 1000 ° C. significantly impairs the operation efficiency according to the schedule, the reheating temperature is set at 100 ° C.
0 ° C. or higher is desirable. In the hot rolling step, finish rolling is performed after the rough rolling is completed, but when descaling is performed after the rough rolling is completed, the collision pressure P (MP
It is desirable that the condition of a) × flow rate L (liter / cm 2 ) ≧ 0.0025 is satisfied.

【００５２】鋼板表面での高圧水の衝突圧Ｐは以下のよ
うに記述される（「鉄と鋼」１９９１、ｖｏｌ．７７、
Ｎｏ．９、ｐ１４５０参照）。 Ｐ（ＭＰａ）＝５．６４×Ｐ0 ×Ｖ／Ｈ2 ただし、 Ｐ0 （ＭＰａ）：液圧力 Ｖ（リットル／ｍｉｎ）：ノズル流液量 Ｈ（ｃｍ）：鋼板表面とノズル間の距離The collision pressure P of the high-pressure water on the steel plate surface is described as follows ("Iron and Steel", 1991, vol. 77,
No. 9, p. 1450). P (MPa) = 5.64 × P 0 × V / H 2 where P 0 (MPa): liquid pressure V (liter / min): nozzle flow H (cm): distance between steel plate surface and nozzle

【００５３】流量Ｌは以下のように記述される。 Ｌ（リットル／ｃｍ2 ）＝Ｖ／（Ｗ×ｖ） ただし、 Ｖ（リットル／ｍｉｎ）：ノズル流液量 Ｗ（ｃｍ）：ノズル当たり噴射液が鋼板表面に当たって
いる幅 ｖ（ｃｍ／ｍｉｎ）：通板速度 衝突圧Ｐ×流量Ｌの上限は、本発明の効果を得るために
は特に定める必要はないが、ノズル流液量を増加させる
とノズルの摩耗が激しくなる等の不都合が生じるため、
０．０２以下とすることが望ましい。The flow rate L is described as follows. L (liter / cm 2 ) = V / (W × v), where V (liter / min): Nozzle flow amount W (cm): Width of jet liquid per nozzle hitting steel sheet surface v (cm / min): Passing speed The upper limit of the collision pressure P × the flow rate L does not need to be particularly determined in order to obtain the effects of the present invention. However, increasing the flow rate of the nozzle causes inconveniences such as intense wear of the nozzle.
It is desirable to set it to 0.02 or less.

【００５４】さらに、仕上げ圧延後の鋼板の最大高さＲ
ｙが１５μｍ（１５μｍＲｙ，ｌ２．５ｍｍ，ｌｎ１
２．５ｍｍ）以下であることが望ましい。これは、例え
ば「金属材料疲労設計便覧」、日本材料学会編、８４頁
に記載されている通り、熱延または酸洗ままの鋼板の疲
労強度は、鋼板表面の最大高さＲｙと相関があることか
ら明らかである。また、その後の仕上げ圧延はデスケー
リング後に再びスケールが生成してしまうのを防ぐため
に、５秒以内に行うのが望ましい。また、粗圧延後また
はそれに続くデスケーリング後にシートバーを接合し、
連続的に仕上げ圧延をしてもよい。その際に粗バーを一
旦コイル状に巻き、必要に応じて保温機能を有するカバ
ーに格納し、再度巻き戻してから接合を行ってもよい。Further, the maximum height R of the steel sheet after the finish rolling is performed.
y is 15 μm (15 μm Ry, 12.5 mm, ln1
2.5 mm) or less. This is because, as described in, for example, “Handbook for Fatigue Design of Metallic Materials”, edited by The Society of Materials Science, Japan, p. It is clear from that. Further, the subsequent finish rolling is desirably performed within 5 seconds in order to prevent scale from being formed again after descaling. Also, after rough rolling or subsequent descaling, join the sheet bar,
Finish rolling may be performed continuously. At that time, the coarse bar may be temporarily wound in a coil shape, stored in a cover having a heat retaining function, if necessary, and re-wound before joining.

【００５５】仕上げ圧延は、熱延鋼板として最終製品に
する場合においては、その仕上げ圧延後半にＡｒ3 変態
点温度＋１００℃以下の温度域で合計圧下率２５％以上
の圧延を行う必要がある。ここでＡｒ3 変態点温度と
は、例えば以下の計算式により鋼成分との関係で簡易的
に示される。すなわち、 Ａｒ3 ＝９１０−３１０×％Ｃ＋２５×％Ｓｉ−８０×
％ＭｎIn the final rolling, when a final product is formed as a hot-rolled steel sheet, it is necessary to perform rolling at a total rolling reduction of 25% or more in the temperature range of the Ar3 transformation point temperature + 100 ° C. or lower in the latter half of the finishing rolling. Here, the Ar3 transformation point temperature is simply shown in relation to the steel composition by the following calculation formula, for example. That is, Ar3 = 910-310 *% C + 25 *% Si-80 *
% Mn

【００５６】Ａｒ3 変態点温度＋１００℃以下の温度域
での合計圧下率２５％未満であると、圧延されたオース
テナイトの集合組織が十分に発達しないために、この
後、如何様な冷却を施したとしても本発明の効果が得ら
れない。よりシャープな集合組織を得るためには、Ａｒ
3 変態点温度＋１００℃以下の温度域での合計圧下率を
３５％以上とすることが望ましい。If the total rolling reduction in the temperature range of the Ar3 transformation point temperature + 100 ° C. or less is less than 25%, the texture of the rolled austenite does not sufficiently develop, so that any cooling was performed thereafter. However, the effect of the present invention cannot be obtained. To obtain a sharper texture, Ar
3 It is desirable that the total rolling reduction in the temperature range of the transformation point temperature + 100 ° C or less is 35% or more.

【００５７】また、合計圧下率２５％以上の圧延を行う
温度域の下限は特に限定しないが、Ａｒ3 変態点温度未
満であると、圧延中に析出したフェライトに加工組織が
残留して延性が低下してしまい加工性が劣化するため、
合計圧下率２５％以上の圧延を行う温度域の下限はＡｒ
3 変態点温度以上が望ましい。ただし、この温度がＡｒ
3 変態点温度未満であっても、後の巻取処理もしくは巻
取処理後の熱処理により回復または再結晶がある程度進
行している場合はこの限りではない。本発明では、Ａｒ
3 変態点温度＋１００℃以下の温度域での合計圧下率の
上限を特に限定しないが、この圧下率合計が９７．５％
を超えると、圧延荷重が増大し圧延機の剛性を過剰に高
める必要があり、経済上のデメリットを生じるため、望
ましくは９７．５％以下とする。The lower limit of the temperature range in which the rolling with a total draft of 25% or more is performed is not particularly limited. And the workability deteriorates.
The lower limit of the temperature range in which rolling at a total draft of 25% or more is Ar
3 Transformation point temperature or higher is desirable. However, this temperature is Ar
3 Even if the temperature is lower than the transformation point temperature, this does not apply to the case where recovery or recrystallization has progressed to some extent by a subsequent winding process or a heat treatment after the winding process. In the present invention, Ar
3 The upper limit of the total rolling reduction in the temperature range of the transformation point temperature + 100 ° C or lower is not particularly limited, but the total rolling reduction is 97.5%.
If it exceeds, the rolling load increases and the rigidity of the rolling mill needs to be excessively increased, which causes economical disadvantages. Therefore, the content is desirably 97.5% or less.

【００５８】ここで、Ａｒ3 変態点温度＋１００℃以下
の温度域での熱間圧延時の熱間圧延ロールと鋼板との摩
擦が大きい場合には、鋼板表面近傍における板面に｛１
１０｝面を主とする結晶方位が発達し、切り欠き疲労強
度が劣化するため、熱間圧延ロールと鋼板との摩擦を低
減するために必要に応じて潤滑を施す。Here, when the friction between the hot rolling roll and the steel sheet during hot rolling in the temperature range of the Ar3 transformation point temperature + 100 ° C. or less is large, the sheet surface near the steel sheet surface is reduced by ｛1.
Since the crystal orientation mainly consisting of the 10 ° plane develops and the notch fatigue strength deteriorates, lubrication is performed as necessary to reduce the friction between the hot rolling roll and the steel sheet.

【００５９】本発明において熱間圧延ロールと鋼板との
摩擦係数の上限は特に限定しないが、０．２超では｛１
１０｝面を主とする結晶方位の発達が顕著になり、切り
欠き疲労強度が劣化するので、Ａｒ3 変態点温度＋１０
０℃以下の温度域での熱間圧延時における少なくとも１
パスについて、熱間圧延ロールと鋼板との摩擦係数を
０．２以下とすることが望ましい。さらに望ましくは、
Ａｒ3 変態点温度＋１００℃以下の温度域での熱間圧延
時における全パスについて、熱間圧延ロールと鋼板との
摩擦係数を０．１５以下とする。ここで熱間圧延ロール
と鋼板との摩擦係数とは、先進率、圧延荷重、圧延トル
ク等の値より圧延理論に基づいて計算により求めた値で
ある。In the present invention, the upper limit of the coefficient of friction between the hot rolling roll and the steel sheet is not particularly limited.
Since the development of the crystal orientation mainly of the 10 ° plane becomes remarkable and the notch fatigue strength is deteriorated, the temperature of the Ar3 transformation point +10
At least 1 at the time of hot rolling in a temperature range of 0 ° C. or less.
For the pass, it is desirable that the coefficient of friction between the hot rolling roll and the steel sheet be 0.2 or less. More preferably,
The friction coefficient between the hot rolling roll and the steel sheet is set to 0.15 or less for all passes during hot rolling in the temperature range of the Ar3 transformation point temperature + 100 ° C or less. Here, the coefficient of friction between the hot rolling roll and the steel sheet is a value obtained by calculation based on a rolling theory from values such as an advanced ratio, a rolling load, and a rolling torque.

【００６０】仕上げ圧延の最終パス温度（ＦＴ）につい
ては特に限定しないが、仕上げ圧延の最終パス温度（Ｆ
Ｔ）はＡｒ3 変態点温度以上で終了することが望まし
い。これは、熱間圧延中に圧延温度がＡｒ3 変態点温度
未満であると、圧延前もしくは圧延中に析出したフェラ
イトに加工組織が残留して延性が低下してしまい、加工
性が劣化するためである。ただし、仕上げ圧延の最終パ
ス温度（ＦＴ）がＡｒ3変態点温度未満であっても、後
の巻取処理もしくは巻取処理後に回復、再結晶させるた
めの熱処理を施す場合はこの限りではない。The final pass temperature (FT) of the finish rolling is not particularly limited.
T) is desirably terminated at the Ar3 transformation point temperature or higher. This is because if the rolling temperature during the hot rolling is lower than the Ar3 transformation point temperature, the processed structure remains in the ferrite precipitated before or during the rolling and the ductility is reduced, thereby deteriorating the workability. is there. However, even if the final pass temperature (FT) of the finish rolling is lower than the Ar3 transformation point temperature, this is not limited to the case where a subsequent winding process or a heat treatment for recovery and recrystallization after the winding process is performed.

【００６１】一方、仕上げ温度の上限については特に上
限を設けないが、Ａｒ3 変態点温度＋１００℃超では、
Ａｒ3 変態点温度＋１００℃以下の温度域で合計圧下率
２５％以上の圧延を行うことが事実上不可能であるの
で、仕上げ温度の上限はＡｒ3変態点温度＋１００℃以
下が望ましい。On the other hand, although there is no particular upper limit for the finishing temperature, when the Ar3 transformation point temperature exceeds + 100 ° C.,
Since it is practically impossible to perform rolling at a total reduction of 25% or more in a temperature range of Ar3 transformation point temperature + 100 ° C. or less, the upper limit of the finishing temperature is desirably Ar3 transformation point temperature + 100 ° C. or less.

【００６２】本発明において、その切り欠き疲労強度を
向上させるという目的のためだけに鋼板のミクロ組織を
特に限定する必要はないので、仕上圧延を終了した後、
所定の巻取温度にて巻取るまでの冷却工程については特
に定めないが、所定の巻取温度で巻き取るためもしくは
ミクロ組織を制御するために必要に応じて冷却を行う。
冷却速度の上限は特に限定しないが、熱ひずみによる板
反りが懸念されることから、３００℃／ｓ以下とするこ
とが望ましい。さらに、あまりにもこの冷却速度が早い
と冷却終了温度を制御できず、オーバーシュートして所
定の巻取温度以下まで過冷却されてしまう可能性がある
ので、ここでの冷却速度は１５０℃／ｓ以下が望まし
い。また、冷却速度の下限は特に定めないが、冷却を行
わない場合の空冷速度は５℃／ｓ以上である。In the present invention, it is not necessary to particularly limit the microstructure of the steel sheet only for the purpose of improving the notch fatigue strength.
The cooling step until winding at a predetermined winding temperature is not particularly limited, but cooling is performed as necessary for winding at a predetermined winding temperature or controlling the microstructure.
Although the upper limit of the cooling rate is not particularly limited, it is preferably 300 ° C./s or less because there is a concern about warpage due to thermal strain. Further, if the cooling rate is too high, the cooling end temperature cannot be controlled, and there is a possibility that the overcooling will result in overcooling to a predetermined winding temperature or less, so the cooling rate here is 150 ° C./s. The following is desirable. Although the lower limit of the cooling rate is not particularly defined, the air cooling rate when no cooling is performed is 5 ° C./s or more.

【００６３】本発明において、切り欠き疲労強度の向上
の他に良好なバーリング加工性を付与する目的でミクロ
組織の体積分率最大の相をベイナイト，またはフェライ
トおよびベイナイトの複合組織、とするために仕上圧延
を終了した後、所定の巻取温度にて巻取るまでの工程に
ついては、その間の冷却速度以外は特に定めないが、バ
ーリング性をそれほど劣化させずに延性との両立を目指
す場合は、Ａｒ3 変態点からＡｒ1 変態点までの温度域
（フェライトとオーステナイトの二相域）で１〜２０秒
間滞留させてもよい。ここでの滞留は、二相域でフェラ
イト変態を促進させるために行うが、１秒未満では、二
相域におけるフェライト変態が不十分なため、十分な延
性が得られず、２０秒超では、パーライトが生成し、目
的とする体積率最大のミクロ組織として、ベイナイト，
またはフェライトおよびベイナイトの複合組織、が得ら
れない。In the present invention, in order to improve notch fatigue strength and to impart good burring workability, the phase having the maximum volume fraction of the microstructure is made bainite or a composite structure of ferrite and bainite. After finishing the finish rolling, the process up to winding at a predetermined winding temperature is not particularly defined except for the cooling rate during that time, but if aiming for compatibility with ductility without significantly deteriorating the burring property, It may be retained for 1 to 20 seconds in a temperature range from the Ar3 transformation point to the Ar1 transformation point (two-phase region of ferrite and austenite). The retention here is performed to promote ferrite transformation in the two-phase region, but if it is less than 1 second, the ferrite transformation in the two-phase region is insufficient, so that sufficient ductility cannot be obtained. The pearlite is formed, and bainite,
Or, a composite structure of ferrite and bainite cannot be obtained.

【００６４】また、１〜２０秒間の滞留をさせる温度域
は、フェライト変態を容易に促進させるためにはＡｒ1
変態点以上８００℃以下が望ましい。さらに、１〜２０
秒間の滞留時間は生産性を極端に低下させないために
は、１〜１０秒間とすることが望ましい。また、これら
の条件を満たすためには、仕上げ圧延終了後２０℃／ｓ
以上の冷却速度で当該温度域に迅速に到達させることが
必要である。冷却速度の上限は特に定めないが、冷却設
備の能力上３００℃／ｓ以下が妥当な冷却速度である。
さらに、あまりにもこの冷却速度が早いと冷却終了温度
を制御できず、オーバーシュートしてＡｒ1 変態点以下
まで過冷却されてしまう可能性があり、延性改善の効果
が失われるので、ここでの冷却速度は１５０℃／ｓ以下
が望ましい。The temperature range in which the stagnation is carried out for 1 to 20 seconds is set to Ar1 to facilitate the ferrite transformation.
The temperature is preferably from the transformation point to 800 ° C. In addition, 1-20
The residence time in seconds is desirably 1 to 10 seconds in order not to significantly reduce the productivity. Further, in order to satisfy these conditions, after finishing rolling, 20 ° C./s
It is necessary to quickly reach the temperature range at the above cooling rate. Although the upper limit of the cooling rate is not particularly defined, 300 ° C./s or less is an appropriate cooling rate in view of the capacity of the cooling equipment.
Further, if the cooling rate is too high, the cooling end temperature cannot be controlled, and there is a possibility that the overshooting will cause overcooling to the Ar1 transformation point or lower, and the effect of improving ductility is lost. The speed is desirably 150 ° C./s or less.

【００６５】次に、その温度域から巻取温度（ＣＴ）ま
では２０℃／ｓ以上の冷却速度で冷却するが、２０℃／
ｓ未満の冷却速度では、パーライトもしくは炭化物を含
むベイナイトが生成してしまい、目的とする体積率最大
のミクロ組織として、ベイナイト，またはフェライトお
よびベイナイトの複合組織、が得られない。巻取温度ま
での冷却速度の上限は特に定めることなく本発明の効果
を得ることができるが、熱ひずみによる板そりが懸念さ
れることから、３００℃／ｓ以下とすることが望まし
い。Next, cooling is performed at a cooling rate of 20 ° C./s or more from the temperature range to the winding temperature (CT).
If the cooling rate is less than s, bainite containing pearlite or carbide is generated, and bainite or a composite structure of ferrite and bainite cannot be obtained as the intended microstructure having the maximum volume fraction. The effect of the present invention can be obtained without any particular upper limit of the cooling rate up to the winding temperature.

【００６６】また本発明において、切り欠き疲労強度の
向上の他に良好な延性を付与する目的で、ミクロ組織を
体積分率５％以上２５％以下の残留オーステナイトを含
み、残部が主にフェライト、ベイナイトからなる複合組
織とするために、仕上圧延を終了した後の工程は、ま
ず、Ａｒ3 変態点温度からＡｒ1 変態点温度までの温度
域（フェライトとオーステナイトの二相域）で１〜２０
秒間滞留する。ここでの滞留は、二相域でフェライト変
態を促進させるために行うが、１秒未満では、二相域に
おけるフェライト変態が不十分なため、十分な延性が得
られず、２０秒超では、パーライトが生成し、目的とす
る体積分率５％以上２５％以下の残留オーステナイトを
含み、残部が主にフェライト、ベイナイトからなるミク
ロ組織が得られない。In the present invention, for the purpose of imparting good ductility in addition to the improvement of notch fatigue strength, the microstructure contains a retained austenite having a volume fraction of 5% or more and 25% or less, with the balance being mainly ferrite, In order to obtain a composite structure composed of bainite, the process after finishing rolling is performed in a temperature range from the Ar3 transformation temperature to the Ar1 transformation temperature (two-phase region of ferrite and austenite).
Stay for a second. The retention here is performed to promote ferrite transformation in the two-phase region, but if it is less than 1 second, the ferrite transformation in the two-phase region is insufficient, so that sufficient ductility cannot be obtained. A pearlite is formed, containing a target austenite with a volume fraction of 5% or more and 25% or less, and a microstructure whose balance is mainly composed of ferrite and bainite cannot be obtained.

【００６７】また、１〜２０秒間の滞留をさせる温度域
はフェライト変態を容易に促進させるため、Ａｒ1 変態
点温度以上８００℃以下が望ましい。さらに、１〜２０
秒間の滞留時間は生産性を極端に低下させないためには
１〜１０秒間とすることが望ましい。また、これらの条
件を満たすためには、仕上げ圧延終了後２０℃／ｓ以上
の冷却速度で当該温度域に迅速に到達させることが必要
である。冷却速度の上限は特に定めないが、冷却設備の
能力上３００℃／ｓ以下が妥当な冷却速度である。さら
に、あまりにもこの冷却速度が早いと冷却終了温度を制
御できず、オーバーシュートしてＡｒ1 変態点温度以下
まで過冷却されてしまう可能性があるので、ここでの冷
却速度は１５０℃／ｓ以下が望ましい。The temperature range in which the stagnation is maintained for 1 to 20 seconds is desirably not lower than the Ar1 transformation point temperature and not higher than 800 ° C. in order to easily promote ferrite transformation. In addition, 1-20
The residence time per second is desirably 1 to 10 seconds in order not to significantly reduce the productivity. In order to satisfy these conditions, it is necessary to quickly reach the temperature range at a cooling rate of 20 ° C./s or more after finishing rolling. Although the upper limit of the cooling rate is not particularly defined, 300 ° C./s or less is an appropriate cooling rate in view of the capacity of the cooling equipment. Further, if the cooling rate is too high, the cooling end temperature cannot be controlled, and there is a possibility of overshoot and overcooling to a temperature lower than the Ar1 transformation point temperature. Is desirable.

【００６８】次に、その温度域から巻取温度（ＣＴ）ま
では２０℃／ｓ以上の冷却速度で冷却するが、２０℃／
ｓ未満の冷却速度では、パーライトもしくは炭化物を含
むベイナイトが生成してしまい、十分な残留オーステナ
イトが得られず、目的とする体積分率５％以上２５％以
下の残留オーステナイトを含み、残部が主にフェライ
ト、ベイナイトからなるミクロ組織が得られない。巻取
温度までの冷却速度の上限は特に定めることなく本発明
の効果を得ることができるが、熱ひずみによる板そりが
懸念されることから、３００℃／ｓ以下とすることが望
ましい。Next, cooling is performed at a cooling rate of 20 ° C./s or more from the temperature range to the winding temperature (CT).
If the cooling rate is less than s, bainite containing pearlite or carbide is generated, and sufficient retained austenite cannot be obtained. A microstructure consisting of ferrite and bainite cannot be obtained. The effect of the present invention can be obtained without any particular upper limit of the cooling rate up to the winding temperature.

【００６９】さらに、本発明において切り欠き疲労強度
の向上の他に良好な形状凍結性を得るための低降伏比を
付与する目的で、ミクロ組織の体積分率最大の相をフェ
ライトとし、第二相を主にマルテンサイトとする複合組
織とするために、仕上圧延を終了した後の工程は、ま
ず、Ａｒ3 変態点温度からＡｒ1 変態点温度までの温度
域（フェライトとオーステナイトの二相域）で１〜２０
秒間滞留する。ここでの滞留は、二相域でフェライト変
態を促進させるために行うが、１秒未満では、二相域に
おけるフェライト変態が不十分なため、十分な延性が得
られず、２０秒超では、パーライトが生成し、目的とす
る体積分率最大の相をフェライトとし、第二相を主にマ
ルテンサイトとする複合組織が得られない。Further, in the present invention, for the purpose of imparting a low yield ratio for obtaining a good shape freezing property in addition to the improvement of the notch fatigue strength, the phase having the maximum volume fraction of the microstructure is defined as ferrite. In order to obtain a composite structure in which the phase is mainly martensite, the step after finish rolling is first performed in a temperature range from the Ar3 transformation temperature to the Ar1 transformation temperature (two-phase region of ferrite and austenite). 1-20
Stay for a second. The residence here is performed in order to promote ferrite transformation in the two-phase region, but if it is less than 1 second, the ferrite transformation in the two-phase region is insufficient, so that sufficient ductility cannot be obtained. A pearlite is formed, and a composite structure in which a phase having a desired maximum volume fraction is ferrite and a second phase is mainly martensite cannot be obtained.

【００７０】また、１〜２０秒間の滞留をさせる温度域
は、フェライト変態を容易に促進させるためＡｒ1 変態
点温度以上８００℃以下が望ましい。さらに１〜２０秒
間の滞留時間は、生産性を極端に低下させないためには
１〜１０秒間とすることが望ましい。また、これらの条
件を満たすためには、仕上げ圧延終了後２０℃／ｓ以上
の冷却速度で当該温度域に迅速に到達させることが必要
である。冷却速度の上限は特に定めないが、冷却設備の
能力上３００℃／ｓ以下が妥当な冷却速度である。さら
に、あまりにもこの冷却速度が早いと冷却終了温度を制
御できず、オーバーシュートしてＡｒ1 変態点温度以下
まで過冷却されてしまう可能性があるので、ここでの冷
却速度は１５０℃／ｓ以下が望ましい。The temperature range in which the stagnation is carried out for 1 to 20 seconds is desirably from the Ar1 transformation point temperature to 800 ° C. in order to facilitate the ferrite transformation. Further, the residence time for 1 to 20 seconds is desirably 1 to 10 seconds in order not to significantly reduce the productivity. In order to satisfy these conditions, it is necessary to quickly reach the temperature range at a cooling rate of 20 ° C./s or more after finishing rolling. Although the upper limit of the cooling rate is not particularly defined, 300 ° C./s or less is an appropriate cooling rate in view of the capacity of the cooling equipment. Further, if the cooling rate is too high, the cooling end temperature cannot be controlled, and there is a possibility of overshoot and overcooling to a temperature lower than the Ar1 transformation point temperature. Is desirable.

【００７１】次に、その温度域から巻取温度（ＣＴ）ま
では２０℃／ｓ以上の冷却速度で冷却するが、２０℃／
ｓ未満の冷却速度では、パーライトもしくはベイナイト
が生成してしまい、十分なマルテンサイトが得られず、
目的とするフェライトを体積分率最大の相とし、マルテ
ンサイトを第二相とするミクロ組織が得られない。巻取
温度までの冷却速度の上限は特に定めることなく本発明
の効果を得ることができるが、熱ひずみによる板そりが
懸念されることから、３００℃／ｓ以下とすることが望
ましい。Next, cooling is performed at a cooling rate of 20 ° C./s or more from the temperature range to the winding temperature (CT).
If the cooling rate is less than s, pearlite or bainite is generated, and sufficient martensite cannot be obtained.
A microstructure having the intended ferrite as the phase having the largest volume fraction and martensite as the second phase cannot be obtained. The effect of the present invention can be obtained without any particular upper limit of the cooling rate up to the winding temperature.

【００７２】本発明において、その切り欠き疲労強度を
向上させるという目的のためだけに鋼板のミクロ組織を
特に限定する必要はないので、巻取温度の上限について
は特に定めないが、Ａｒ3 変態点温度＋１００℃以下の
温度域で合計圧下率２５％以上の圧延で得られたオース
テナイトの集合組織を遺伝させるためには、下記に示す
巻取温度Ｔ0 以下で巻き取ることが望ましい。ただし、
Ｔ0 は室温以下にする必要はない。このＴ0 は、オース
テナイトと、オーステナイトと同一成分のフェライトが
同一の自由エネルギーを持つ温度として熱力学的に定義
される温度で、Ｃ以外の成分の影響も考慮して、下記の
式を用いて簡易的に計算することができる。 Ｔ0 ＝−６５０．４×％Ｃ＋ＢIn the present invention, it is not necessary to specifically limit the microstructure of the steel sheet only for the purpose of improving the notch fatigue strength. In order to inherit the texture of austenite obtained by rolling at a total draft of 25% or more in a temperature range of + 100 ° C. or less, it is desirable to wind at a winding temperature T 0 or less shown below. However,
T 0 does not need to be below room temperature. This T 0 is a temperature that is thermodynamically defined as a temperature at which austenite and ferrite having the same component as austenite have the same free energy. It can be easily calculated. T 0 = −650.4 ×% C + B

【００７３】ここで、Ｂは下記のように決定される。 Ｂ＝−５０．６×Ｍｎｅｑ＋８９４．３ また、ここでＭｎｅｑとは下記に示す含有元素の質量％
より決定される。 Ｍｎｅｑ＝％Ｍｎ＋０．２４×％Ｎｉ＋０．１３×％Ｓ
ｉ＋０．３８×％Ｍｏ＋０．５５×％Ｃｒ＋０．１６％
Ｃｕ−０．５０×％Ａｌ−０．４５×％Ｃｏ＋０．９０
×％Ｖ なお、Ｔ0 に及ぼす本発明で規定した上記以外の成分の
質量％の影響はそれほど大きくないので、ここでは無視
できる。Here, B is determined as follows. B = −50.6 × Mneq + 894.3 Here, Mneq is the mass% of the contained element shown below.
Determined by Mneq =% Mn + 0.24 ×% Ni + 0.13 ×% S
i + 0.38 ×% Mo + 0.55 ×% Cr + 0.16%
Cu-0.50x% Al-0.45x% Co + 0.90
×% V Since mass% of the effect of components other than the above as defined in the present invention on T 0 is not so large, where negligible.

【００７４】また巻取温度の下限値は、その切り欠き疲
労強度を向上させるという目的のためだけに鋼板のミク
ロ組織を特に限定する必要はないので、特に限定する必
要はないが、コイルが長時間水濡れの状態にあると錆に
よる外観不良が懸念されるため、５０℃以上が望まし
い。本発明において、切り欠き疲労強度の向上の他に良
好なバーリング加工性を付与する目的で、ミクロ組織の
体積分率最大の相をベイナイト，またはフェライトおよ
びベイナイトの複合組織、とするためには、巻取温度が
４５０℃未満では、バーリング性に有害と考えられてい
る残留オーステナイトまたはマルテンサイトが多量に生
成する恐れがあり、目的とする体積率最大のミクロ組織
であるベイナイト，またはフェライトおよびベイナイト
からなる複合組織が得られないため、巻取温度は４５０
℃以上と限定する。さらに、巻取り後の冷却速度は特に
限定しないが、Ｃｕを１．２％以上添加した場合、巻取
り後にＣｕが析出して加工性が劣化するばかりでなく、
疲労特性向上に有効な固溶状態のＣｕが失われる恐れが
あるので、巻取り後の冷却速度は２００℃までを３０℃
／ｓ以上とすることが望ましい。The lower limit of the winding temperature does not need to be particularly limited for the purpose of improving the notch fatigue strength of the steel sheet only. If it is wet for a long time, the appearance may be poor due to rust. In the present invention, for the purpose of imparting good burring workability in addition to improving the notch fatigue strength, in order to make the phase having the maximum volume fraction of the microstructure into bainite or a composite structure of ferrite and bainite, If the winding temperature is lower than 450 ° C., a large amount of retained austenite or martensite, which is considered to be detrimental to the burring property, may be formed. The winding temperature is 450
Limit to ℃ or higher. Furthermore, the cooling rate after winding is not particularly limited, but when Cu is added in an amount of 1.2% or more, not only Cu precipitates after winding but also deteriorates workability,
The cooling rate after winding may be up to 200 ° C. to 30 ° C.
/ S or more is desirable.

【００７５】また、本発明において切り欠き疲労強度の
向上の他に良好な延性を付与する目的で、ミクロ組織を
体積分率５％以上２５％以下の残留オーステナイトを含
み、残部が主にフェライト、ベイナイトからなる複合組
織とするためには、巻取温度が４５０℃以上では、炭化
物を含むベイナイトが生成して十分な残留オーステナイ
トが得られず、目的とする体積分率５％以上２５％以下
の残留オーステナイトを含み、残部が主にフェライト、
ベイナイトからなるミクロ組織が得られないため、巻取
温度は４５０℃未満と限定する。また巻取温度が３５０
℃以下では、マルテンサイトが多量に生成して十分な残
留オーステナイトが得られず、目的とする体積分率５％
以上２５％以下の残留オーステナイトを含み、残部が主
にフェライト、ベイナイトからなるミクロ組織が得られ
ないため、巻取温度は３５０℃超と限定する。さらに、
巻取り後の冷却速度は特に限定しないが、Ｃｕを１％以
上添加した場合、巻取り後にＣｕが析出して加工性が劣
化するばかりでなく、疲労特性向上に有効な固溶状態の
Ｃｕが失われる恐れがあるので、巻取り後の冷却速度は
２００℃までを３０℃／ｓ以上とすることが望ましい。In the present invention, for the purpose of imparting good ductility in addition to improving the notch fatigue strength, the microstructure contains a retained austenite having a volume fraction of 5% or more and 25% or less, with the balance being mainly ferrite, In order to form a composite structure composed of bainite, if the winding temperature is 450 ° C. or higher, bainite containing carbides is not generated and sufficient residual austenite cannot be obtained, and the intended volume fraction of 5% or more and 25% or less is obtained. Including retained austenite, the remainder is mainly ferrite,
Since a microstructure consisting of bainite cannot be obtained, the winding temperature is limited to less than 450 ° C. The winding temperature is 350
If the temperature is lower than 0 ° C., a large amount of martensite is formed, and sufficient retained austenite cannot be obtained.
The winding temperature is limited to more than 350 ° C. because it contains retained austenite of 25% or less and a microstructure composed mainly of ferrite and bainite cannot be obtained. further,
Although the cooling rate after winding is not particularly limited, when Cu is added in an amount of 1% or more, not only Cu precipitates after winding and the workability is deteriorated, but also Cu in a solid solution state effective for improving the fatigue property is obtained. It is desirable that the cooling rate after winding up to 200 ° C. be 30 ° C./s or more, since there is a risk of loss.

【００７６】さらに、本発明において切り欠き疲労強度
の向上の他に良好な形状凍結性を得るための低降伏比を
付与する目的で、ミクロ組織の体積分率最大の相をフェ
ライトとし、第二相を主にマルテンサイトとする複合組
織とするためには、巻取温度が３５０℃超では、ベイナ
イトが生成して十分なマルテンサイトが得られず、目的
とするフェライトを体積分率最大の相とし、マルテンサ
イトを第二相とするミクロ組織が得られないため、巻取
温度は３５０℃以下と限定する。また、巻取温度の下限
値は特に限定する必要はないが、コイルが長時間水濡れ
の状態にあると錆による外観不良が懸念されるため、５
０℃以上が望ましい。熱間圧延工程終了後は必要に応じ
て酸洗し、その後インラインまたはオフラインで、圧下
率１０％以下のスキンパスまたは圧下率４０％程度まで
の冷間圧延を施しても構わない。Further, in order to improve the notch fatigue strength in the present invention and to provide a low yield ratio for obtaining a good shape freezing property, the phase having the maximum volume fraction of the microstructure is defined as ferrite. If the winding temperature is higher than 350 ° C., bainite is formed and sufficient martensite cannot be obtained, and the desired ferrite is formed into a phase having the maximum volume fraction in order to obtain a composite structure mainly composed of martensite. Since a microstructure having martensite as the second phase cannot be obtained, the winding temperature is limited to 350 ° C. or less. Further, the lower limit of the winding temperature does not need to be particularly limited. However, if the coil is in a wet state for a long time, there is a fear of poor appearance due to rust.
0 ° C. or higher is desirable. After the hot rolling step, pickling may be performed, if necessary, followed by in-line or off-line skin pass with a rolling reduction of 10% or less or cold rolling to a rolling reduction of about 40%.

【００７７】次に、冷延鋼板として最終製品にする場合
であるが、熱間での仕上げ圧延条件は特に限定しない。
ただし、より良好な切り欠き疲労強度を得るためには、
Ａｒ3 変態点温度＋１００℃以下の温度域での合計圧下
率が２５％以上であることが望ましい。また、仕上げ圧
延の最終パス温度（ＦＴ）はＡｒ3 変態点温度未満で終
了しても差し支えないが、その場合は、圧延前もしくは
圧延中に析出したフェライトに強い加工組織が残留する
ため、続く巻取処理または加熱処理により回復、再結晶
させることが望ましい。Next, there is a case where a final product is formed as a cold-rolled steel sheet, but the hot finish rolling conditions are not particularly limited.
However, in order to obtain better notch fatigue strength,
It is desirable that the total rolling reduction in the temperature range of the Ar3 transformation point temperature + 100 ° C or less is 25% or more. Although the final pass temperature (FT) of the finish rolling may be completed below the Ar3 transformation point temperature, in this case, a strong work structure remains in the ferrite precipitated before or during the rolling, so It is desirable to recover and recrystallize by removing or heating.

【００７８】続く酸洗後の冷間圧延の合計圧下率は８０
％未満とする。これは、冷間圧延の合計圧下率は８０％
以上であると、一般的な冷間圧延−再結晶集合組織であ
る板面に平行な結晶面の｛１１１｝面や｛５５４｝面の
Ｘ線回折積分面強度比が高くなるためである。また、望
ましくは７０％以下である。冷間圧延率の下限は特に定
めることなく本発明の効果を得ることができるが、結晶
方位の強度を適当な範囲に制御するためには３％以上と
することが望ましい。The total rolling reduction of the cold rolling after the subsequent pickling was 80.
%. This is because the total reduction of cold rolling is 80%
With the above, the X-ray diffraction integral plane intensity ratio of the {111} plane and the {554} plane of the crystal plane parallel to the sheet plane, which is a general cold-rolled recrystallization texture, is increased. Further, it is desirably 70% or less. The effect of the present invention can be obtained without any particular lower limit of the cold rolling reduction, but is preferably 3% or more in order to control the strength of the crystal orientation in an appropriate range.

【００７９】この様に冷間圧延された鋼板の熱処理は連
続焼鈍工程を前提としている。まず、Ａｃ3 変態点温度
＋１００℃以下の温度域で５〜１５０秒間行う。この熱
処理温度の上限がＡｃ3 変態点温度＋１００℃超では、
再結晶によって生成したフェライトがオーステナイトへ
変態し、オーステナイトの粒成長によっての集合組織が
ランダム化され、最終的に得られるフェライトの集合組
織もランダム化されてしまうので、熱処理の上限温度Ａ
ｃ3 変態点温度＋１００℃以下とする。ここでＡｃ1 変
態点温度およびＡｃ3 変態点温度とは、例えば「レスリ
ー鉄鋼材科学」（１９８５年発行、熊井浩・野田龍彦
訳、丸善株式会社）２７３頁に記載の計算式により、鋼
成分との関係で示される。一方、この熱処理温度の下限
は、その切り欠き疲労強度を向上させるという目的のた
めに鋼板のミクロ組織を特に限定する必要はないので、
回復温度以上で構わないが、回復温度未満の場合には加
工組織が残留し成形性を著しく劣化させるので、熱処理
の下限温度は回復温度以上とする。また、この温度域で
の保持時間は、５秒未満では、セメンタイトが完全に再
固溶するのに不十分であり、一方、１５０秒超の熱処理
を行ってもその効果が飽和するばかりでなく生産性を低
下させるので、保持時間は５〜１５０秒間とする。The heat treatment of the cold-rolled steel sheet is based on a continuous annealing process. First, the heat treatment is performed for 5 to 150 seconds in a temperature range not higher than the Ac3 transformation point temperature + 100 ° C. If the upper limit of the heat treatment temperature is higher than the Ac3 transformation point temperature + 100 ° C,
The ferrite generated by the recrystallization is transformed into austenite, the texture of the austenite grains is randomized, and the texture of the finally obtained ferrite is also randomized.
c3 Transformation point temperature + 100 ° C or less. Here, the Ac1 transformation point temperature and the Ac3 transformation point temperature are defined, for example, by the calculation formulas described in "Leslie Iron and Steel Science" (published in 1985, by Hiroshi Kumai and Tatsuhiko Noda, Maruzen Co., Ltd.), p. Indicated by the relationship. On the other hand, the lower limit of the heat treatment temperature is not particularly limited to the microstructure of the steel sheet for the purpose of improving the notch fatigue strength,
The temperature may be higher than the recovery temperature, but if the temperature is lower than the recovery temperature, the processed structure remains and the formability is significantly deteriorated. If the holding time in this temperature range is less than 5 seconds, the cementite is insufficient to completely re-dissolve the cementite. To reduce the productivity, the holding time is set to 5 to 150 seconds.

【００８０】その後の冷却条件については特に限定しな
いが、ミクロ組織を制御するために、必要に応じて以下
の冷却または任意温度での保持および冷却を行ってもよ
い。本発明において、切り欠き疲労強度の向上の他に良
好なバーリング加工性を付与する目的で、ミクロ組織の
体積分率最大の相をベイナイト，またはフェライトおよ
びベイナイトの複合組織、とするためには、その熱処理
温度の下限温度をＡｃ1 変態点温度以上とする。この下
限温度がＡｃ1 変態点温度未満の場合には、目的とする
体積分率最大の相をベイナイト，またはフェライトおよ
びベイナイトの複合組織、が得られない。ここで、バー
リング性をそれほど劣化させずに延性との両立を目指す
場合は、フェライトの体積分率を増加させるために、そ
の温度域をＡｃ1 変態点温度以上Ａｃ3 変態点温度以下
（フェライトとオーステナイトの二相域）の温度域とす
る。また、更に良好なバーリング性を得るためには、ベ
イナイトの体積分率を増加させるために、Ａｃ3 変態点
温度以上Ａｃ3 変態点温度＋１００℃以下の温度域が望
ましい。The subsequent cooling conditions are not particularly limited. However, in order to control the microstructure, the following cooling or holding and cooling at an arbitrary temperature may be performed as necessary. In the present invention, for the purpose of imparting good burring workability in addition to improving the notch fatigue strength, in order to make the phase having the maximum volume fraction of the microstructure into bainite or a composite structure of ferrite and bainite, The lower limit temperature of the heat treatment temperature is set to be higher than the Ac1 transformation point temperature. When the lower limit temperature is lower than the Ac1 transformation point temperature, bainite or a composite structure of ferrite and bainite cannot be obtained in the phase having the maximum desired volume fraction. Here, when aiming at compatibility with ductility without significantly deteriorating the burring property, in order to increase the volume fraction of ferrite, the temperature range is set to a temperature from the Ac1 transformation point temperature to an Ac3 transformation point temperature (the ferrite and austenite temperature). (Two-phase region). Further, in order to obtain a better burring property, a temperature range from the Ac3 transformation point temperature to the Ac3 transformation point temperature + 100 ° C. or less is desirable in order to increase the volume fraction of bainite.

【００８１】次に、冷却工程については本発明で特に定
めないが、前記熱処理温度がＡｃ1変態点温度以上Ａｃ3
変態点温度以下の場合においては、２０℃／ｓ以上の
冷却速度で３５０℃超前記Ｔ0 温度以下の温度域まで冷
却することが望ましい。これは、冷却速度が２０℃／ｓ
未満では、炭化物を多量に含むベイナイトもしくはパー
ライト変態のノーズにかかる恐れがあるためである。ま
た、冷却終了温度は、３５０℃以下ではバーリング性に
有害と考えられているマルテンサイトが多量に生成する
恐れがあり、目的とする体積率最大のミクロ組織である
ベイナイト，またはフェライトおよびベイナイトからな
る複合組織が得られないため、３５０℃超が望ましい。
さらに、前工程までに得られた集合組織を遺伝させるた
めにはＴ 0 以下が望ましい。Next, the cooling step is particularly defined in the present invention.
However, the heat treatment temperature is higher than the Ac1 transformation point temperature.
 When the temperature is lower than the transformation point temperature, the temperature is 20 ° C./s or more.
At a cooling rate of over 350 ° C.0Cool down to the temperature range below the temperature
It is desirable to dismiss it. This is because the cooling rate is 20 ° C / s
If less, bainite or par
This is because there is a risk that the nose of light transformation will occur. Ma
When the cooling end temperature is 350 ° C or less,
Generates large amounts of martensite, which is considered harmful
There is a fear that it is the microstructure with the maximum target volume fraction
Bainite or ferrite and bainite
Since a complex structure cannot be obtained, the temperature is preferably higher than 350 ° C.
In addition, the inheritance of the texture obtained by the previous process was inherited.
T for 0The following is desirable.

【００８２】最後に冷却工程の終了温度までの冷却速度
は、２０℃／ｓ以上では冷却中にバーリング性に有害と
考えられているマルテンサイトが多量に生成する恐れが
あり、目的とする体積率最大のミクロ組織であるベイナ
イト，またはフェライトおよびベイナイトからなる複合
組織が得られない恐れがあるので、２０℃／ｓ未満とす
ることが望ましい。また冷却工程の終了温度は、２００
℃超では時効性が劣化する恐れがあるので、２００℃以
下とすることが望ましい。また下限は、水冷もしくはミ
ストで冷却する場合コイルが長時間水濡れの状態にある
と、錆による外観不良が懸念されるため、５０℃以上が
望ましい。一方、前記熱処理温度がＡｃ3 変態点温度超
Ａｃ3 変態点温度＋１００℃以下の場合においては、２
０℃／ｓ以上の冷却速度で２００℃以下の温度まで冷却
することが望ましい。これは、２０℃／ｓ以上では、炭
化物を多量に含むベイナイトもしくはパーライト変態の
ノーズにかかる恐れがあるためである。また冷却終了温
度は、２００℃超では時効性が劣化する恐れがあるの
で、２００℃以下が望ましい。下限は、水冷もしくはミ
ストで冷却する場合、コイルが長時間水濡れの状態にあ
ると錆による外観不良が懸念されるため、５０℃以上が
望ましい。Finally, if the cooling rate to the end temperature of the cooling step is 20 ° C./s or more, a large amount of martensite considered to be detrimental to the burring property may be generated during cooling, and the desired volume ratio Since there is a possibility that bainite, which is the largest microstructure, or a composite structure composed of ferrite and bainite may not be obtained, the temperature is preferably set to less than 20 ° C / s. The temperature at the end of the cooling step is 200
If the temperature exceeds ℃, the aging property may be deteriorated. The lower limit is desirably 50 ° C. or higher, because when the coil is cooled with water or mist, if the coil is in a wet state for a long time, the appearance may be deteriorated due to rust. On the other hand, when the heat treatment temperature is higher than the Ac3 transformation point temperature and the Ac3 transformation point temperature is 100 ° C. or less, 2
It is desirable to cool to a temperature of 200 ° C. or less at a cooling rate of 0 ° C./s or more. This is because at a temperature of 20 ° C./s or more, there is a possibility that a nose of bainite or pearlite transformation containing a large amount of carbide may occur. If the cooling end temperature is higher than 200 ° C., the aging property may be deteriorated. The lower limit is desirably 50 ° C. or higher because when the coil is cooled with water or mist, if the coil is in a wet state for a long time, the appearance may be poor due to rust.

【００８３】また、本発明において切り欠き疲労強度の
向上の他に良好な延性を付与する目的で、ミクロ組織を
体積分率５％以上２５％以下の残留オーステナイトを含
み、残部が主にフェライト、ベイナイトからなる複合組
織とするためには、前記同様にＡｃ1 変態点温度以上Ａ
ｃ3 変態点温度＋１００℃以下の温度域で５〜１５０秒
間行う。このとき、その温度域内でも低温すぎると、熱
延板段階でセメンタイトが析出していた場合、セメンタ
イトが再固溶するのに時間がかかりすぎ、高温すぎると
オーステナイトの体積率が大きくなりすぎて、オーステ
ナイト中のＣ濃度が低下し炭化物を多量に含むベイナイ
トもしくはパーライト変態のノーズにかかりやすくなる
ため、７８０℃以上８５０℃以下で加熱するのが好まし
い。保持後の冷却速度が２０℃／ｓ未満では、炭化物を
多量に含むベイナイトもしくはパーライト変態のノーズ
にかかる恐れがあるため、２０℃／ｓ以上の冷却速度と
する。In the present invention, for the purpose of imparting good ductility in addition to improving the notch fatigue strength, the microstructure contains a retained austenite having a volume fraction of 5% or more and 25% or less, with the balance being mainly ferrite, In order to obtain a composite structure composed of bainite, the temperature must be higher than the Ac1 transformation point
c3 The treatment is carried out for 5 to 150 seconds in a temperature range of + 100 ° C. or lower. At this time, if the temperature is too low even in that temperature range, if cementite is precipitated in the hot-rolled sheet stage, it takes too much time for cementite to re-dissolve, and if the temperature is too high, the volume fraction of austenite becomes too large, Heating at 780 ° C. or higher and 850 ° C. or lower is preferable because the C concentration in austenite decreases and the nose of bainite or pearlite transformation containing a large amount of carbides tends to occur. If the cooling rate after the holding is less than 20 ° C./s, the nose of bainite or pearlite transformation containing a large amount of carbide may be applied, so the cooling rate is set to 20 ° C./s or more.

【００８４】次に、ベイナイト変態を促進し必要な量の
残留オーステナイトを安定化する工程であるが、冷却終
了温度が４５０℃以上では、残留したオーステナイトが
炭化物を多量に含むベイナイトまたはパーライトに分解
してしまい、目的とする体積分率５％以上２５％以下の
残留オーステナイトを含み、残部が主にフェライト、ベ
イナイトからなるミクロ組織が得られない。また３５０
℃未満では、マルテンサイトが多量に生成する可能性が
あり十分な残留オーステナイトが得られず、目的とする
体積分率５％以上２５％以下の残留オーステナイトを含
み、残部が主にフェライト、ベイナイトからなるミクロ
組織が得られないため、３５０℃超の温度域まで冷却す
る。Next, a step of accelerating bainite transformation and stabilizing a necessary amount of retained austenite. When the cooling end temperature is 450 ° C. or higher, the remaining austenite is decomposed into bainite or pearlite containing a large amount of carbide. As a result, the desired microstructure containing retained austenite having a volume fraction of 5% or more and 25% or less and the remainder mainly composed of ferrite and bainite cannot be obtained. Also 350
If the temperature is lower than 0 ° C., a large amount of martensite may be generated, and sufficient retained austenite cannot be obtained. Since no microstructure can be obtained, cooling is performed to a temperature range higher than 350 ° C.

【００８５】さらに、その温度域での保持時間である
が、５秒未満では残留オーステナイトを安定化するため
のベイナイト変態が不十分であり、不安定な残留オース
テナイトが続く冷却終了時にマルテンサイト変態する恐
れがあり、目的とする体積分率５％以上２５％以下の残
留オーステナイトを含み、残部が主にフェライト、ベイ
ナイトからなるミクロ組織が得られない。また６００秒
超ではベイナイト変態が促進しすぎて、必要な量の安定
した残留オーステナイトを得ることができず、目的とす
る体積分率５％以上２５％以下の残留オーステナイトを
含み、残部が主にフェライト、ベイナイトからなるミク
ロ組織が得られない。従って、その温度域での保持時間
は５秒以上６００秒以下とする。Further, when the holding time in the temperature range is less than 5 seconds, bainite transformation for stabilizing retained austenite is insufficient, and martensitic transformation occurs at the end of cooling when unstable retained austenite continues. There is a possibility that the desired microstructure including retained austenite having a volume fraction of 5% or more and 25% or less and a balance mainly composed of ferrite and bainite cannot be obtained. If it exceeds 600 seconds, bainite transformation is excessively promoted, so that a required amount of stable retained austenite cannot be obtained. A microstructure consisting of ferrite and bainite cannot be obtained. Therefore, the holding time in that temperature range is set to 5 seconds or more and 600 seconds or less.

【００８６】最後に冷却終了までの冷却速度は、５℃／
ｓ未満では冷却中にベイナイト変態が促進しすぎる可能
性があり、必要な量の安定した残留オーステナイトを得
ることができず、目的とする体積分率５％以上２５％以
下の残留オーステナイトを含み、残部が主にフェライ
ト、ベイナイトからなるミクロ組織が得られない恐れが
あるので、５℃／ｓ以上とする。また冷却終了温度は、
２００℃超では時効性が劣化する恐れがあるので、２０
０℃以下とする。冷却終了温度の下限については特に限
定しないが、水冷もしくはミストで冷却する場合、コイ
ルが長時間水濡れの状態にあると、錆による外観不良が
懸念されるため、５０℃以上が望ましい。Finally, the cooling rate until the end of cooling is 5 ° C. /
If it is less than s, bainite transformation may be excessively promoted during cooling, and a required amount of stable retained austenite cannot be obtained. Since the remainder may not be able to obtain a microstructure mainly composed of ferrite and bainite, the temperature is set to 5 ° C./s or more. The cooling end temperature is
If the temperature exceeds 200 ° C., the aging property may deteriorate.
0 ° C or less. The lower limit of the cooling end temperature is not particularly limited. However, when cooling with water or mist, if the coil is in a wet state for a long time, the appearance may be poor due to rust.

【００８７】さらに、本発明において切り欠き疲労強度
の向上の他に良好な形状凍結性を得るための低降伏比を
付与する目的で、ミクロ組織の体積分率最大の相をフェ
ライトとし、第二相を主にマルテンサイトとする複合組
織とするために、前記同様にＡｃ1 変態点温度以上Ａｃ
3 変態点温度＋１００℃以下の温度域で５〜１５０秒間
行う。このとき、その温度範囲内でも低温すぎると、熱
延板段階でセメンタイトが析出していた場合、セメンタ
イトが再固溶するのに時間がかかりすぎ、高温すぎると
オーステナイトの体積率が大きくなりすぎて、オーステ
ナイト中のＣ濃度が低下し、炭化物を多量に含むベイナ
イトもしくはパーライト変態のノーズにかかりやすくな
るため、７８０℃以上８５０℃以下で加熱するのが好ま
しい。Further, in order to improve the notch fatigue strength in the present invention and to provide a low yield ratio for obtaining good shape freezing property, the phase having the maximum volume fraction of the microstructure is defined as ferrite, In order to obtain a composite structure in which the phase is mainly martensite, the temperature is higher than the Ac1 transformation point temperature as described above.
3 Perform for 5 to 150 seconds in the temperature range of transformation point + 100 ° C or lower. At this time, if the temperature is too low even within that temperature range, if cementite is precipitated in the hot-rolled sheet stage, it takes too much time for cementite to re-dissolve, and if the temperature is too high, the volume fraction of austenite becomes too large. In addition, since the C concentration in austenite decreases and the nose of bainite or pearlite transformation containing a large amount of carbides tends to occur, it is preferable to heat at 780 ° C. or more and 850 ° C. or less.

【００８８】保持後の冷却速度は、２０℃／ｓ未満では
炭化物を多量に含むベイナイトもしくはパーライト変態
のノーズにかかる恐れがあるため、２０℃／ｓ以上の冷
却速度とする。冷却終了温度が３５０℃超では、目的と
するフェライトを体積分率最大の相とし、マルテンサイ
トを第二相とするミクロ組織が得られないので、３５０
℃以下の温度域まで冷却する。冷却工程の終了温度の下
限については特に限定しないが、水冷もしくはミストで
冷却する場合、コイルが長時間水濡れの状態にあると錆
による外観不良が懸念されるため、５０℃以上が望まし
い。さらにその後、必要に応じてスキンパス圧延を施し
てもよい。酸洗後の熱延鋼板、または上記の再結晶焼鈍
終了後の冷延鋼板に亜鉛めっきを施すためには、亜鉛め
っき浴中に浸漬し、必要に応じて合金化処理してもよ
い。If the cooling rate after the holding is lower than 20 ° C./s, there is a possibility that the nose of bainite or pearlite transformation containing a large amount of carbides may occur. If the cooling end temperature is higher than 350 ° C., a microstructure having the target ferrite as the phase having the maximum volume fraction and martensite as the second phase cannot be obtained.
Cool down to a temperature below ℃. The lower limit of the end temperature of the cooling step is not particularly limited. However, in the case of cooling with water or mist, if the coil is in a wet state for a long time, the appearance may be poor due to rust. Thereafter, skin pass rolling may be performed as necessary. In order to galvanize the hot-rolled steel sheet after pickling or the cold-rolled steel sheet after the recrystallization annealing, the steel sheet may be immersed in a galvanizing bath and subjected to an alloying treatment if necessary.

【００８９】[0089]

【実施例】（実施例１）以下に、実施例１により本発明
をさらに説明する。表１に示す化学成分を有するＡ〜Ｌ
の鋼は、転炉にて溶製して、連続鋳造後、再加熱し、粗
圧延後に続く仕上げ圧延で１．２〜５．５ｍｍの板厚に
した後に巻き取った。ただし、表中の化学組成について
の表示は質量％である。次に製造条件の詳細を表２に示
す。ここで、「ＳＲＴ」はスラブ加熱温度、「ＦＴ」は
最終パス仕上げ圧延温度、「圧延率」とはＡｒ3 変態点
温度＋１００℃以下の温度域での圧下率の合計を示す。
ただし、後に冷延工程にて圧延を行う場合はこのような
制限の限りではないので「―」とした。また、「潤滑」
はＡｒ3 変態点温度＋１００℃以下の温度域での潤滑の
有無を示した。さらに「巻取」とは、巻取温度（ＣＴ）
がＴ0 以下ならば「○」、Ｔ0 超の場合には「×」とし
た。ただし、冷延鋼板の場合は製造の条件として特に限
定する必要がないので「―」とした。EXAMPLES (Example 1) The present invention will be further described with reference to Example 1. AL having the chemical components shown in Table 1
Was melted in a converter, continuously cast, reheated, rolled after finishing rough rolling to a sheet thickness of 1.2 to 5.5 mm, and then wound. However, the indication of the chemical composition in the table is% by mass. Next, the details of the manufacturing conditions are shown in Table 2. Here, "SRT" indicates the slab heating temperature, "FT" indicates the final pass finishing rolling temperature, and "rolling ratio" indicates the total rolling reduction in the temperature range of Ar3 transformation point temperature + 100 ° C or lower.
However, when rolling is performed later in the cold rolling process, such a limitation is not applied, so "-" is used. Also, "lubrication"
Indicates the presence or absence of lubrication in the temperature range of the Ar3 transformation point temperature + 100 ° C or lower. Further, "winding" means a winding temperature (CT).
Is smaller than T 0, “○”, and above T 0, “×”. However, in the case of a cold-rolled steel sheet, it is not necessary to particularly limit the manufacturing conditions, and thus “−” is used.

【００９０】次に、一部については熱間圧延後、酸洗、
冷延、焼鈍を行った。板厚は０．７〜２．３ｍｍであ
る。ここで、「冷延率」とは合計冷間圧延率、「Ｔｉｍ
ｅ」は焼鈍時間、「焼鈍」とは、焼鈍温度が回復温度以
上Ａｃ3 変態点温度＋１００℃以下の温度域に含まれて
いれば「○」、外れていれば「×」とした。なお、鋼Ｌ
については粗圧延後に衝突圧２．７ＭＰａ、流量０．０
０１リットル／ｃｍ2 の条件でデスケーリングを施し
た。一方、上記鋼板のうち鋼Ｇおよび鋼Ｆ−５について
は、亜鉛めっきを施した。このようにして得られた熱延
板の引張試験は、供試材を、まず、ＪＩＳ Ｚ２２０１
記載の５号試験片に加工し、ＪＩＳ Ｚ ２２４１記載
の試験方法に従って行った。表２に降伏強度（σY ）、
引張強度（σB ）、破断伸び（Ｅｌ）を併せて示す。Next, for a part, after hot rolling, pickling,
Cold rolling and annealing were performed. The plate thickness is 0.7 to 2.3 mm. Here, the “cold rolling reduction” is the total cold rolling reduction, “Tim
"e" is the annealing time, and "annealing" is "o" if the annealing temperature is included in the temperature range from the recovery temperature to the Ac3 transformation point temperature + 100 ° C or less, and "x" if it is outside. In addition, steel L
About the following, after rough rolling, the collision pressure was 2.7 MPa and the flow rate was 0.0
The descaling was performed under the condition of 01 liter / cm 2 . On the other hand, among the above steel sheets, steel G and steel F-5 were galvanized. In the tensile test of the hot-rolled sheet thus obtained, first, the test material was subjected to JIS Z2201.
The test piece was processed into the No. 5 test piece described in accordance with the test method described in JIS Z 2241. Table 2 shows the yield strength (σY),
The tensile strength (σB) and elongation at break (El) are also shown.

【００９１】さらに、板幅の１／４Ｗもしくは３／４Ｗ
位置より３０ｍｍφに切り取った試片の、最表層より
０．０５ｍｍ程度の深さまで三山仕上の研削を行い、次
いで化学研磨または電解研磨によって歪みを除去して作
製し、「新版カリティＸ線回折要論」（１９８６年発
行、松村源太郎訳、株式会社アグネ）２７４〜２９６頁
に記載の方法に従ってＸ線回折強度の測定を行った。こ
こで｛１００｝＜０１１＞〜｛２２３｝＜１１０＞方位
群のＸ線ランダム強度比の平均値とは、この方位群に含
まれる主な方位、｛１００｝＜０１１＞、｛１１６｝＜
１１０＞、｛１１４｝＜１１０＞、｛１１３｝＜１１０
＞、｛１１２｝＜１１０＞、｛３３５｝＜１１０＞およ
び｛２２３｝＜１１０＞のＸ線回折強度を、｛１１０｝
極点図に基づきベクトル法により計算した３次元集合組
織、または｛１１０｝、｛１００｝、｛２１１｝、｛３
１０｝極点図のうち複数の極点図（望ましくは３つ以
上）を用いて級数展開法で計算した３次元集合組織から
求めた。Further, 1/4 W or 3/4 W of the plate width
A sample cut to 30 mmφ from the position was ground to a depth of about 0.05 mm from the outermost layer, and then ground to a depth of about 0.05 mm. Then, the strain was removed by chemical polishing or electrolytic polishing. (Published in 1986, translated by Gentaro Matsumura, Agne Co., Ltd.), pages 274 to 296, and the X-ray diffraction intensity was measured. Here, the average values of the X-ray random intensity ratios of the {100} <011> to {223} <110> directions are the main directions included in this direction group, {100} <011>, {116} <
110>, {114} <110>, {113} <110
>, {112} <110>, {335} <110>, and {223} <110>,
3D texture calculated by vector method based on pole figure, or {110}, {100}, {211}, {3
It was obtained from a three-dimensional texture calculated by a series expansion method using a plurality of pole figures (preferably three or more) among the 10 ° pole figures.

【００９２】例えば、後者の方法における上記各結晶方
位のＸ線ランダム強度比は、３次元集合組織のφ２＝４
５゜断面における（００１）［１−１０］、（１１６）
［１−１０］、（１１４）［１−１０］、（１１３）
［１−１０］、（１１２）［１−１０］、（３３５）
［１−１０］、（２２３）［１−１０］の強度をそのま
ま用ればよい。ただし｛１００｝＜０１１＞〜｛２２
３｝＜１１０＞方位群のＸ線ランダム強度比の平均値と
は、上記の各方位の相加平均である。上記全ての方位の
強度を得ることができない場合には、｛１００｝＜０１
１＞、｛１１６｝＜１１０＞、｛１１４｝＜１１０＞、
｛１１２｝＜１１０＞、｛２２３｝＜１１０＞の各方位
の相加平均で代替してもよい。次に｛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とは、上記の方
法と同様に計算した３次元集合組織から求めればよい。For example, in the latter method, the X-ray random intensity ratio of each of the crystal orientations described above is expressed as φ2 = 4 in the three-dimensional texture.
(001) [1-10], (116) in 5 ° section
[1-10], (114) [1-10], (113)
[1-10], (112) [1-10], (335)
[1-10], (223) The intensity of [1-10] may be used as it is. However, {100} <011>-$ 22
The average value of the X-ray random intensity ratio of the 3｝ <110> azimuth group is an arithmetic mean of each azimuth described above. If the intensity cannot be obtained in all the above directions, {100} <01
1>, {116} <110>, {114} <110>,
The arithmetic mean of each orientation of {112} <110> and {223} <110> may be used instead. Next, {554} <225
>, {111} <112> and {111} <110>
The average value of the X-ray random intensity ratios in the three directions may be obtained from the three-dimensional texture calculated in the same manner as in the above method.

【００９３】表２において、Ｘ線ランダム強度比のうち
「強度比１」とは、｛１００｝＜０１１＞〜｛２２３｝
＜１１０＞方位群のＸ線ランダム強度比の平均値、「強
度比２」とは｛５５４｝＜２２５＞、｛１１１｝＜１１
２＞および｛１１１｝＜１１０＞の３方位のＸ線ランダ
ム強度比の平均値である。In Table 2, the “intensity ratio 1” of the X-ray random intensity ratio is {100} <011> to {223}.
The average value of the X-ray random intensity ratio of the <110> orientation group, “intensity ratio 2” is {554} <225>, {111} <11
2> and {111} <110> are average values of X-ray random intensity ratios in three directions.

【００９４】次に、上記鋼板の切り欠き疲労強度を調査
するために、板幅の１／４Ｗもしくは３／４Ｗ位置から
圧延方向が長辺になるように、図１（ｂ）に示す形状の
疲労試験片を採取し疲労試験に供した。ただし、疲労試
験片には最表層より０．０５ｍｍ程度の深さまで三山仕
上の研削を施した。疲労試験は電気油圧サーボ型疲労試
験機を用い、試験方法はＪＩＳ Ｚ ２２７３−１９７
８およびＪＩＳ Ｚ２２７５−１９７８に準じた。表２
に切り欠き疲労限（σWK）、切り欠き疲労限度比（σWK
／σB ）を併せて示す。Next, in order to investigate the notch fatigue strength of the steel sheet, the shape shown in FIG. 1 (b) was set so that the rolling direction became the longer side from the 1 / 4W or 3 / 4W position of the sheet width. Fatigue test pieces were collected and subjected to a fatigue test. However, the fatigue test piece was subjected to three-side finish grinding to a depth of about 0.05 mm from the outermost layer. The fatigue test uses an electrohydraulic servo-type fatigue tester, and the test method is JIS Z 2273-197.
8 and JIS Z2275-1978. Table 2
Notch fatigue limit (σWK), notch fatigue limit ratio (σWK
/ ΣB).

【００９５】本発明に沿うものは、鋼Ａ，Ｅ，Ｆ−１，
Ｆ−２，Ｆ−５，Ｇ，Ｈ，Ｉ，Ｊ，Ｋ，Ｌの１１鋼であ
り、所定の量の鋼成分を含有し、最表面から板厚方向に
０．５ｍｍまでの任意深さにおける板面の｛１００｝＜
０１１＞〜｛２２３｝＜１１０＞方位群のＸ線ランダム
強度比の平均値が２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４以下であり、板
厚が０．５ｍｍ以上１２ｍｍ以下であることを特徴とす
る切り欠き疲労強度に優れる自動車用薄鋼板が得られて
おり、従って、本発明記載の方法によって評価した従来
鋼の疲労限度比０．２〜０．３を上回っている。According to the present invention, steels A, E, F-1,
11 steels of F-2, F-5, G, H, I, J, K and L, containing a predetermined amount of steel components, and an arbitrary depth from the outermost surface to 0.5 mm in the thickness direction {100} <
011> to {223} <110>, the average value of the X-ray random intensity ratio of the orientation group is 2 or more, and {554} <225>;
{111} <112> and {111} <110>
An average value of the X-ray random intensity ratio of the orientation is 4 or less, and a thin steel sheet for automobiles having excellent notch fatigue strength characterized in that the thickness is 0.5 mm or more and 12 mm or less has been obtained. It exceeds the fatigue limit ratio of the conventional steel evaluated by the method according to the present invention of 0.2 to 0.3.

【００９６】上記以外の鋼は、以下の理由によって本発
明の範囲外である。すなわち、鋼Ｂは、Ｃの含有量が本
願請求項５の範囲外であるので、十分な強度（σB ）が
得られていない。鋼Ｃは、Ｐの含有量が本願請求項５の
範囲外であるので、十分な切り欠き疲労強度（σWK／σ
B ）が得られていない。鋼Ｄは、Ｓの含有量が本願請求
項５の範囲外であるので、十分な伸び（Ｅｌ）が得られ
ていない。鋼Ｆ−３は、Ａｒ3 変態点温度＋１００℃以
下の温度域での合計圧下率が本願請求項１２の範囲外で
あるので、請求項１記載の目的とする集合組織が得られ
ず、十分な切り欠き疲労強度（σWK／σB ）が得られて
いない。Other steels are outside the scope of the present invention for the following reasons. That is, the steel B does not have a sufficient strength (σB) because the content of C is out of the range defined in claim 5 of the present application. In steel C, since the content of P is out of the range of claim 5 of the present application, sufficient notch fatigue strength (σWK / σ) is obtained.
B) is not obtained. Steel D has a sufficient elongation (El) because the content of S is outside the range of claim 5 of the present application. Since steel F-3 has a total rolling reduction in a temperature range of Ar3 transformation point temperature + 100 ° C. or less out of the range of claim 12 of the present application, the intended texture described in claim 1 cannot be obtained, and sufficient steel can be obtained. Notch fatigue strength (σWK / σB) was not obtained.

【００９７】鋼Ｆ−４は、仕上圧延終了温度（ＦＴ）が
本願請求項１２の範囲外で、かつ巻取温度も本願明細書
記載の範囲外でかつ巻取温度も本願発明範囲外であるの
で、請求項１記載の目的とする集合組織が得られず、十
分な切り欠き疲労強度（σWK／σB ）が得られていな
い。鋼Ｆ−６は、冷延率が本願請求項１８の範囲外であ
るので、請求項１記載の集合組織が得られず、十分な切
り欠き疲労強度（σWK／σB ）が得られていない。鋼Ｆ
−７は、焼鈍温度が本願請求項１８の範囲外であるの
で、請求項１記載の目的とする集合組織が得られず、十
分な切り欠き疲労強度（σWK／σB ）が得られていな
い。鋼Ｆ−８は、焼鈍時間が本願請求項１８の範囲外で
あるので、請求項１記載の集合組織が得られず、十分な
切り欠き疲労強度（σWK／σB ）が得られていない。Steel F-4 has a finish rolling end temperature (FT) outside the scope of claim 12 of the present application, a winding temperature outside the range described in the present specification, and a winding temperature outside the present invention. Therefore, the desired texture as described in claim 1 cannot be obtained, and a sufficient notch fatigue strength (σWK / σB) cannot be obtained. Since the steel F-6 has a cold rolling ratio outside the range of the eighteenth aspect of the present invention, the texture described in the first aspect cannot be obtained, and a sufficient notch fatigue strength (σWK / σB) cannot be obtained. Steel F
In the case of -7, since the annealing temperature is out of the range of the eighteenth aspect of the present invention, the target texture described in the first aspect cannot be obtained, and sufficient notch fatigue strength (σWK / σB) cannot be obtained. Since the annealing time of steel F-8 is out of the range of claim 18 of the present application, the texture described in claim 1 cannot be obtained, and sufficient notch fatigue strength (σWK / σB) has not been obtained.

【００９８】（実施例２）次に、実施例２により本発明
をさらに詳しく説明する。表１に示す化学成分を有する
Ｇ、Ｈの２鋼を表３に示す加熱温度で再加熱し、粗圧延
後に続く仕上げ圧延で１．２〜５．５ｍｍの板厚にした
後に巻き取った。また、表３に示すようにいくつかにつ
いては、粗圧延後に衝突圧２．７ＭＰａ、流量０．００
１リットル／ｃｍ2 の条件でデスケーリングを施した。
製造条件の詳細を表３に示す。ここで、「ＳＲＴ」はス
ラブ加熱温度、「ＦＴ」は最終パス仕上げ圧延温度、
「圧延率」とはＡｒ3 変態点温度＋１００℃以下の温度
域での圧下率の合計を示す。ただし、後に冷延工程にて
圧延を行う場合はこのような制限の限りではないので
「―」とした。また、「潤滑」はＡｒ3 変態点温度＋１
００℃以下の温度域での潤滑の有無を示した。さらに
「ＣＴ」とは巻取温度を示している。ただし、冷延鋼板
の場合は製造の条件として特に限定する必要がないので
「―」とした。次に、一部については熱間圧延後、酸
洗、冷延、熱処理を行った。板厚は０．７〜２．３ｍｍ
である。「冷延率」とは合計冷間圧延率、「ＳＴ」と
は、熱処理温度、「Ｔｉｍｅ」は熱処理時間である。な
お、上記鋼板のうちいくつかについては、亜鉛めっきを
施した。(Embodiment 2) Next, the present invention will be described in more detail by Embodiment 2. Two steels of G and H having the chemical components shown in Table 1 were reheated at the heating temperature shown in Table 3, and were rolled after having a sheet thickness of 1.2 to 5.5 mm by finish rolling following rough rolling. In addition, as shown in Table 3, for some, after rough rolling, the collision pressure was 2.7 MPa and the flow rate was 0.00.
The descaling was performed under the condition of 1 liter / cm 2 .
Table 3 shows the details of the manufacturing conditions. Here, “SRT” is the slab heating temperature, “FT” is the final pass finish rolling temperature,
"Rolling ratio" indicates the total rolling reduction in the temperature range of the Ar3 transformation point temperature + 100 ° C or lower. However, when rolling is performed later in the cold rolling process, such a limitation is not applied, so "-" is used. "Lubrication" is the Ar3 transformation point temperature + 1
The presence or absence of lubrication in a temperature range of 00 ° C. or less is shown. Further, “CT” indicates a winding temperature. However, in the case of a cold-rolled steel sheet, it is not necessary to particularly limit the manufacturing conditions, and thus “−” is used. Next, for a part, after hot rolling, pickling, cold rolling and heat treatment were performed. The board thickness is 0.7 to 2.3mm
It is. “Cold rolling reduction” is the total cold rolling reduction, “ST” is the heat treatment temperature, and “Time” is the heat treatment time. In addition, some of the above steel sheets were galvanized.

【００９９】このようにして得られた熱延板および冷延
板の引張試験は、上記同様な方法にて実施した。表４に
降伏強度（σY ）、引張強度（σB ）、破断伸び（Ｅ
ｌ）および降伏比（ＹR ）、強度−延性バランス（σB
×Ｅｌ）を示す。一方、バーリング加工性（穴拡げ性）
については、日本鉄鋼連盟規格ＪＦＳ Ｔ １００１−
１９９６記載の穴拡げ試験方法に従って評価した。表４
に穴拡げ率（λ）を示す。さらにミクロ組織についても
表４に示す。ここで、その他とはパーライト、および／
または表４に個別に示すフェライト、ベイナイト、残留
オーステナイト、マルテンサイト以外の組織である。鋼
板のミクロ組織において、フェライト、ベイナイト、残
留オーステナイト、パーライト、マルテンサイトの体積
分率とは、鋼板板幅の１／４Ｗもしくは３／４Ｗ位置よ
り切出した試料を圧延方向断面に研磨し、ナイタール試
薬および特開平５−１６３５９０号公報で開示されてい
る試薬を用いてエッチングし、光学顕微鏡を用い２００
〜５００倍の倍率で観察された板厚の１／４ｔにおける
ミクロ組織の面積分率で定義される。The tensile test of the hot rolled sheet and the cold rolled sheet thus obtained was carried out in the same manner as described above. Table 4 shows the yield strength (σY), tensile strength (σB) and elongation at break (E
l) and yield ratio (YR), strength-ductility balance (σB
× El). On the other hand, burring workability (hole expanding property)
About Japan Iron and Steel Federation Standard JFS T 1001-
The evaluation was performed according to the hole expansion test method described in 1996. Table 4
Shows the hole expansion ratio (λ). Table 4 also shows the microstructure. Here, the others are perlite, and / or
Alternatively, it is a structure other than ferrite, bainite, retained austenite, and martensite individually shown in Table 4. In the microstructure of a steel sheet, the volume fractions of ferrite, bainite, retained austenite, pearlite, and martensite are as follows. And etching using a reagent disclosed in Japanese Patent Application Laid-Open No. 5-163590.
It is defined as the area fraction of the microstructure at 1 / 4t of the plate thickness observed at a magnification of ~ 500 times.

【０１００】一方、オーステナイトはフェライトと結晶
構造が違うため結晶学的に容易に識別できる。従って、
残留オーステナイトの体積分率はＸ線回折法によっても
実験的に求めることができる。すなわち、ＭｏのＫα線
を用いてオーステナイトとフェライトとの反射面強度の
違いより次式を用いてその体積分率を簡便に求める方法
である。 Ｖγ＝（２／３）｛１００／（０．７×α（２１１）／
γ（２２０）＋１）｝＋（１／３）｛１００／（０．７
８×α（２１１）／γ（３１１）＋１）｝ ただし、α（２１１）、γ（２２０）およびγ（３１
１）は、それぞれフェライト（α）オーステナイト
（γ）のＸ線反射面強度である。残留オーステナイトの
体積分率は、光学顕微鏡観察およびＸ線回折法のいずれ
の方法を用いてもほぼ一致した値が得られたので、いず
れの測定値を用いても差し支えない。さらに、前記と同
様な方法に従ってＸ線回折強度の測定、疲労試験を行っ
た。また、疲労試験は前記と同様な方法に従って行っ
た。表４に切り欠き疲労限（σWK）、切り欠き疲労限度
比（σWK／σB ）を示す。On the other hand, austenite has a different crystal structure from ferrite and can be easily identified crystallographically. Therefore,
The volume fraction of retained austenite can also be experimentally determined by an X-ray diffraction method. That is, a method of easily obtaining the volume fraction of the austenite and ferrite from the difference in the reflection surface intensity between austenite and ferrite using the Mo Kα ray and the following equation. Vγ = (2/3) ｛100 / (0.7 × α (211) /
γ (220) +1)｝ + (１ ／) ｛100 / (0.7
8 × α (211) / γ (311) +1) where α (211), γ (220) and γ (31
1) is the X-ray reflection surface intensity of ferrite (α) austenite (γ), respectively. Regarding the volume fraction of retained austenite, almost the same value was obtained by using either the method of optical microscope observation or the X-ray diffraction method, and any measured value may be used. Further, the measurement of the X-ray diffraction intensity and the fatigue test were performed in the same manner as described above. The fatigue test was performed according to the same method as described above. Table 4 shows the notch fatigue limit (σWK) and the notch fatigue limit ratio (σWK / σB).

【０１０１】本発明に沿うものは、鋼ｇ−１、ｇ−２、
ｇ−３、ｇ−５、ｇ−６、ｇ−７、ｈ−１、ｈ−２、ｈ
−３の９鋼であり、所定の量の鋼成分を含有し、最表面
から板厚方向に０．５ｍｍまでの任意深さにおける板面
の｛１００｝＜０１１＞〜｛２２３｝＜１１０＞方位群
のＸ線ランダム強度比の平均値が２以上かつ、｛５５
４｝＜２２５＞、｛１１１｝＜１１２＞および｛１１
１｝＜１１０＞の３方位のＸ線ランダム強度比の平均値
が４以下、かつ板厚が０．５ｍｍ以上１２ｍｍ以下であ
り、かつ、体積分率最大の相をベイナイト，またはフェ
ライトおよびベイナイトの複合組織、または、体積分率
５％以上２５％以下の残留オーステナイトを含み、残部
が主にフェライト、ベイナイトからなる複合組織、また
は、体積分率最大の相をフェライトとし、第二相を主に
マルテンサイトとする複合組織であることを特徴とする
切り欠き疲労強度に優れる自動車用薄鋼板が得られてお
り、従って、本発明記載の方法によって評価した従来鋼
の疲労限度比２０〜３０％に対して有意差が認められ
る。According to the present invention, steel g-1, g-2,
g-3, g-5, g-6, g-7, h-1, h-2, h
-3, a steel containing a predetermined amount of a steel component, and {100} <011> to {223} <110> of the plate surface at an arbitrary depth from the outermost surface to 0.5 mm in the plate thickness direction. The average value of the X-ray random intensity ratio of the azimuth group is 2 or more and $ 55
4 {225}, {111} <112> and {11
The average of the X-ray random intensity ratios in three directions of 1｝ <110> is 4 or less, the plate thickness is 0.5 mm or more and 12 mm or less, and the phase having the maximum volume fraction is bainite or ferrite and bainite. A composite structure or a composite structure containing retained austenite with a volume fraction of 5% or more and 25% or less, with the balance being mainly ferrite or bainite, or the phase with the largest volume fraction as ferrite, and the second phase as main A thin steel sheet for automobiles having excellent notch fatigue strength, which is characterized by having a composite structure of martensite, has been obtained. Therefore, the fatigue limit ratio of the conventional steel evaluated by the method according to the present invention is reduced to 20 to 30%. There is a significant difference.

【０１０２】上記以外の鋼は、以下の理由によって本発
明の範囲外である。すなわち、鋼ｇ−４は、仕上圧延終
了温度（ＦＴ）およびＡｒ3 変態点温度＋１００℃以下
の温度域での合計圧下率が本発明請求項１２の範囲外で
あるので、請求項１記載の目的とする集合組織が得られ
ず、十分な切り欠き疲労強度（σWKｋ／σB ）が得られ
ていない。鋼ｇ−８は、冷延率が本発明請求項１３の範
囲外であるので、請求項１記載の目的とする集合組織が
得られず、十分な切り欠き疲労強度（σWK／σB ）が得
られていない。鋼ｈ−４は、仕上圧延終了温度（ＦＴ）
およびＡｒ3 変態点温度＋１００℃以下の温度域での合
計圧下率が本発明請求項１２の範囲外であるので、請求
項１記載の目的とする集合組織が得られず、十分な切り
欠き疲労強度（σWK／σB ）が得られていない。Steels other than those described above are outside the scope of the present invention for the following reasons. That is, steel g-4 has a finish rolling finish temperature (FT) and a total rolling reduction in a temperature range of not higher than the Ar3 transformation point temperature + 100 ° C. are outside the scope of claim 12 of the present invention. Is not obtained, and sufficient notch fatigue strength (σWKk / σB) is not obtained. Since steel g-8 has a cold rolling reduction outside the scope of claim 13 of the present invention, the desired texture described in claim 1 cannot be obtained, and sufficient notch fatigue strength (σWK / σB) can be obtained. Not been. Steel h-4 has a finish rolling end temperature (FT)
Since the total rolling reduction in the temperature range of not more than 100 ° C. and the Ar 3 transformation point temperature is out of the range of claim 12 of the present invention, the desired texture described in claim 1 cannot be obtained, and the notch fatigue strength is sufficient. (ΣWK / σB) is not obtained.

【０１０３】[0103]

【表１】 [Table 1]

【０１０４】[0104]

【表２】 [Table 2]

【０１０５】[0105]

【表３】 [Table 3]

【０１０６】[0106]

【表４】 [Table 4]

【０１０７】[0107]

【発明の効果】以上詳述したように、本発明は、切り欠
き疲労強度に優れる自動車用薄鋼板およびその製造方法
に関するものであり、これらの薄鋼板を用いることによ
り、打ち抜き加工部や溶接部等の応力集中部からの疲労
き裂の進展が問題となるような、自動車足廻り部品等の
耐久性が求められる部材における重要な特性の一つであ
る切り欠き疲労強度の大幅な改善が期待できるため、工
業的価値が高い発明である。As described in detail above, the present invention relates to a thin steel sheet for automobiles having excellent notch fatigue strength and a method for producing the same. It is expected that the notch fatigue strength, which is one of the important properties of parts requiring durability, such as automobile undercarriage parts, where the growth of fatigue cracks from stress concentration parts such as stress becomes a problem, is greatly improved. It is an invention with high industrial value because it can be made.

【図面の簡単な説明】[Brief description of the drawings]

【図１】疲労試験片の形状を説明する図であり、（ａ）
は平滑疲労試験片、（ｂ）は切り欠き疲労試験片を示
す。FIG. 1 is a view for explaining the shape of a fatigue test piece, and FIG.
Indicates a smooth fatigue test specimen, and (b) indicates a notched fatigue test specimen.

【図２】本発明に至る予備実験の結果を、｛１００｝＜
０１１＞〜｛２２３｝＜１１０＞方位群のＸ線ランダム
強度比の平均値および、｛５５４｝＜２２５＞、｛１１
１｝＜１１２＞および｛１１１｝＜１１０＞の３方位の
Ｘ線ランダム強度比の平均値と切り欠き疲労強度（１０
7 回での時間強度：疲労限）の関係において示す図であ
る。FIG. 2 shows the results of preliminary experiments leading to the present invention, {100} <
011>-{223} <110> average value of the X-ray random intensity ratio of the orientation group, and {554} <225>, {11}
1} <112> and {111} <110> mean values of X-ray random intensity ratios in three directions and notch fatigue strength (10
It is a figure which shows in the relationship of time intensity | strength in seven times: fatigue limit).

───────────────────────────────────────────────────── フロントページの続き (51)Int.Cl.7 識別記号 ＦＩ テーマコート゛(参考） Ｃ２２Ｃ 38/58 Ｃ２２Ｃ 38/58 (72)発明者 吉永 直樹 富津市新富20−１ 新日本製鐵株式会社技 術開発本部内 (72)発明者 土橋 浩一 大分市大字西ノ洲１番地 新日本製鐵株式 会社大分製鐵所内 (72)発明者 中本 武広 大分市大字西ノ洲１番地 新日本製鐵株式 会社大分製鐵所内 Ｆターム(参考） 4E002 AD04 BC05 4K032 AA01 AA02 AA04 AA05 AA08 AA11 AA14 AA15 AA16 AA17 AA19 AA22 AA23 AA27 AA29 AA31 AA32 AA35 AA36 AA39 BA01 CA02 CA03 CB01 CB02 CD02 CD03 CD05 CG01 CG02 CJ02 CJ03 4K037 EA01 EA02 EA05 EA06 EA09 EA11 EA13 EA15 EA16 EA17 EA19 EA20 EA23 EA25 EA27 EA28 EA31 EA32 EA35 EA36 EB06 EB07 EB08 EB09 EC01 FA02 FA03 FB01 FC04 FE03 FH01 FJ05 FJ06 FK02 FK03 HA02 ──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. 7 Identification code FI Theme coat ゛ (Reference) C22C 38/58 C22C 38/58 (72) Inventor Naoki Yoshinaga 20-1 Shintomi, Futtsu Nippon Steel Corporation Within the Technology Development Headquarters (72) Inventor Koichi Dobashi 1 Nishinosu, Oita, Oita City Nippon Steel Corporation (72) Inventor Takehiro Nakamoto 1 Nishinosu, Oita City, Oita Nippon Steel Corporation Oita In-house F-term (reference) 4E002 AD04 BC05 4K032 AA01 AA02 AA04 AA05 AA08 AA11 AA14 AA15 AA16 AA17 AA19 AA22 AA23 AA27 AA29 AA31 AA32 AA35 AA36 AA39 BA01 CA02 CA03 CB01 CB02 CB01 CB02 CB02 CB02 CB02 EA13 EA15 EA16 EA17 EA19 EA20 EA23 EA25 EA27 EA28 EA31 EA32 EA35 EA36 EB06 EB07 EB08 EB09 EC01 FA02 FA03 FB01 FC04 FE03 FH01 FJ05 FJ06 FK02 FK03 HA02

Claims (24)

【特許請求の範囲】[Claims] 【請求項１】 最表面から板厚方向に０．５ｍｍまでの
任意深さにおける板面の｛１００｝＜０１１＞〜｛２２
３｝＜１１０＞方位群のＸ線ランダム強度比の平均値が
２以上かつ、｛５５４｝＜２２５＞、｛１１１｝＜１１
２＞および｛１１１｝＜１１０＞の３方位のＸ線ランダ
ム強度比の平均値が４以下であり、板厚が０．５ｍｍ以
上１２ｍｍ以下であることを特徴とする切り欠き疲労強
度に優れる自動車用薄鋼板。1. {100} <011> to {22} of the plate surface at an arbitrary depth from the outermost surface to 0.5 mm in the plate thickness direction
The average value of the X-ray random intensity ratio of the 3｝ <110> orientation group is 2 or more, and {554} <225>, {111} <11
An automobile having excellent notch fatigue strength, wherein the average value of X-ray random intensity ratios in three directions of 2> and {111} <110> is 4 or less, and the plate thickness is 0.5 mm or more and 12 mm or less. For thin steel sheet. 【請求項２】 請求項１に記載の鋼板のミクロ組織が、
体積分率最大の相をベイナイト，またはフェライトおよ
びベイナイトの複合組織、であることを特徴とする、切
り欠き疲労強度に優れる自動車用薄鋼板。2. The microstructure of the steel sheet according to claim 1,
A thin steel sheet for automobiles having excellent notch fatigue strength, characterized in that the phase having the largest volume fraction is bainite or a composite structure of ferrite and bainite. 【請求項３】 請求項１に記載の鋼板のミクロ組織が、
体積分率５％以上２５％以下の残留オーステナイトを含
み、残部が主にフェライト、ベイナイトからなる複合組
織であることを特徴とする、切り欠き疲労強度に優れる
自動車用薄鋼板。3. The microstructure of the steel sheet according to claim 1,
A thin steel sheet for automobiles having excellent notch fatigue strength, comprising a retained austenite having a volume fraction of 5% or more and 25% or less, and having a composite structure mainly composed of ferrite and bainite. 【請求項４】 請求項１に記載の鋼板のミクロ組織が、
体積分率最大の相をフェライトとし、第二相を主にマル
テンサイトとする複合組織であることを特徴とする、切
り欠き疲労強度に優れる自動車用薄鋼板。4. The microstructure of the steel sheet according to claim 1,
A thin steel sheet for automobiles having excellent notch fatigue strength, characterized by having a composite structure in which the phase having the largest volume fraction is ferrite and the second phase is mainly martensite. 【請求項５】 質量％で、 Ｃ ：０．０１〜０．３％、 Ｓｉ：０．０１〜２％、 Ｍｎ：０．０５〜３％、 Ｐ ≦０．１％、 Ｓ ≦０．０１％、 Ａｌ：０．００５〜１％ を含み、残部がＦｅ及び不可避的不純物からなる鋼であ
ることを特徴とする、請求項１ないし４のいずれか１項
に記載の切り欠き疲労強度に優れる自動車用薄鋼板。5. Mass%, C: 0.01 to 0.3%, Si: 0.01 to 2%, Mn: 0.05 to 3%, P ≦ 0.1%, S ≦ 0.01 %, Al: 0.005 to 1%, the balance being steel comprising Fe and unavoidable impurities, the notch fatigue strength according to any one of claims 1 to 4, being excellent. Automotive thin steel sheet. 【請求項６】 鋼成分が、さらに質量％で、 Ｃｕ：０．２〜２％ を含有することを特徴とする、請求項５に記載の切り欠
き疲労強度に優れる自動車用薄鋼板。6. The thin steel sheet for automobiles having excellent notch fatigue strength according to claim 5, wherein the steel component further contains Cu: 0.2 to 2% by mass%. 【請求項７】 鋼成分が、さらに質量％で、 Ｂ ：０．０００２〜０．００２％ を含有することを特徴とする、請求項５または６に記載
の切り欠き疲労強度に優れる自動車用薄鋼板。7. The thin steel sheet for automobiles having excellent notch fatigue strength according to claim 5, wherein the steel component further contains B: 0.0002 to 0.002% by mass%. steel sheet. 【請求項８】 鋼成分が、さらに質量％で、 Ｎｉ：０．１〜１％ を含有することを特徴とする、請求項５ないし７のいず
れか１項に記載の切り欠き疲労強度に優れる自動車用薄
鋼板。8. The notch fatigue strength according to claim 5, wherein the steel component further contains Ni: 0.1 to 1% by mass%. Automotive thin steel sheet. 【請求項９】 鋼成分が、さらに質量％で、 Ｃａ：０．０００５〜０．００２％、 ＲＥＭ：０．０００５〜０．０２％ の一種または二種を含有することを特徴とする、請求項
５ないし８のいずれか１項に記載の切り欠き疲労強度に
優れる自動車用薄鋼板。9. The steel composition further comprises one or two types of Ca: 0.0005 to 0.002% and REM: 0.0005 to 0.02% by mass%. Item 9. An automotive thin steel sheet having excellent notch fatigue strength according to any one of items 5 to 8. 【請求項１０】 鋼成分が、さらに質量％で、 Ｔｉ：０．０５〜０．５％、 Ｎｂ：０．０１〜０．５％、 Ｍｏ：０．０５〜１％、 Ｖ ：０．０２〜０．２％、 Ｃｒ：０．０１〜１％、 Ｚｒ：０．０２〜０．２％ の一種または二種以上を含有することを特徴とする、請
求項５ないし９のいずれか１項に記載の切り欠き疲労強
度に優れる自動車用薄鋼板。10. The steel composition further includes, by mass%, Ti: 0.05 to 0.5%, Nb: 0.01 to 0.5%, Mo: 0.05 to 1%, and V: 0.02. 10% to 0.2%, Cr: 0.01% to 1%, Zr: 0.02% to 0.2%. Automotive thin steel sheet having excellent notch fatigue strength according to 1. 【請求項１１】 請求項１ないし１０のいずれか１項に
記載の自動車用薄鋼板に亜鉛めっきが施されていること
を特徴とする、切り欠き疲労強度に優れる自動車用薄鋼
板。11. An automotive thin steel sheet having excellent notch fatigue strength, wherein the automotive thin steel sheet according to any one of claims 1 to 10 is galvanized. 【請求項１２】 請求項５ないし１０のいずれか１項に
記載の成分を有する薄鋼板を得るため熱間圧延する際
に、該成分を有する鋼片を粗圧延後にＡｒ3 変態点温度
＋１００℃以下の温度域で鋼板厚の合計圧下率２５％以
上の仕上圧延することを特徴とする、切り欠き疲労強度
に優れる自動車用薄鋼板の製造方法。12. When hot rolling is performed to obtain a thin steel sheet having the component according to any one of claims 5 to 10, the slab having the component is roughly 100 ° C. or lower after the rough rolling. A method for producing a thin steel sheet for automobiles having excellent notch fatigue strength, wherein the steel sheet is finish-rolled at a total reduction rate of 25% or more of the steel sheet thickness in the temperature range described above. 【請求項１３】 請求項５ないし１０のいずれか１項に
記載の成分を有する薄鋼板を得るため熱間圧延する際
に、該成分を有する鋼片を粗圧延後にＡｒ3 変態点温度
＋１００℃以下の温度域で鋼板厚の合計圧下率２５％以
上の仕上圧延をし、その後、２０℃／ｓ以上の冷却速度
で冷却して、４５０℃以上の巻取温度で巻き取ることを
特徴とする、切り欠き疲労強度に優れる自動車用薄鋼板
の製造方法。13. A hot rolling process for obtaining a thin steel sheet having the component according to any one of claims 5 to 10, wherein a slab having the component is subjected to rough rolling and then to an Ar3 transformation point temperature of + 100 ° C. or lower. Finishing rolling in a temperature range of 25% or more of the total reduction of the thickness of the steel sheet, and then cooling at a cooling rate of 20 ° C / s or more and winding at a winding temperature of 450 ° C or more. A method for manufacturing thin steel sheets for automobiles with excellent notch fatigue strength. 【請求項１４】 請求項５ないし１０のいずれか１項に
記載の成分を有する薄鋼板を得るため熱間圧延する際
に、該成分を有する鋼片を粗圧延後にＡｒ3 変態点温度
＋１００℃以下の温度域で鋼板厚の合計圧下率２５％以
上の仕上圧延をし、その後Ａｒ1 変態点温度以上Ａｒ3
変態点温度以下の温度域で１〜２０秒間滞留し、その後
さらに、２０℃／ｓ以上の冷却速度で冷却して、３５０
℃超４５０℃未満の温度域の巻取温度で巻き取ることを
特徴とする、切り欠き疲労強度に優れる自動車用薄鋼板
の製造方法。14. When hot rolling is performed to obtain a thin steel sheet having the component according to any one of claims 5 to 10, the slab having the component is subjected to roughing-rolling followed by Ar3 transformation point temperature of + 100 ° C. or lower. Finish rolling at a total reduction of 25% or more of the steel sheet thickness in the temperature range of
It stays for 1 to 20 seconds in a temperature range not higher than the transformation point temperature, and is further cooled at a cooling rate of 20 ° C./s or more,
A method for manufacturing a thin steel sheet for automobiles having excellent notch fatigue strength, wherein the winding is performed at a winding temperature in a temperature range of more than 450C and less than 450C. 【請求項１５】 請求項５ないし１０のいずれか１項に
記載の成分を有する薄鋼板を得るため熱間圧延する際
に、該成分を有する鋼片を粗圧延後にＡｒ3 変態点温度
＋１００℃以下の温度域で鋼板厚の合計圧下率２５％以
上の仕上圧延をし、その後Ａｒ1 変態点温度以上Ａｒ3
変態点温度以下の温度域で１〜２０秒間滞留し、その後
さらに２０℃／ｓ以上の冷却速度で冷却して、３５０℃
以下の巻取温度で巻き取ることを特徴とする、切り欠き
疲労強度に優れる自動車用薄鋼板の製造方法。15. When hot rolling is performed to obtain a thin steel sheet having the component according to any one of claims 5 to 10, the slab having the component is roughly 100 ° C. or lower after the rough rolling. Finish rolling at a total reduction of 25% or more of the steel sheet thickness in the temperature range of
It stays for 1 to 20 seconds in the temperature range below the transformation point temperature, and then cools at a cooling rate of 20 ° C./s or more,
A method for producing a thin steel sheet for automobiles having excellent notch fatigue strength, characterized by winding at the following winding temperature. 【請求項１６】 熱間圧延に際し、粗圧延後の仕上圧延
において潤滑圧延を施すことを特徴とする、請求項１２
ないし１５のいずれか１項に記載の切り欠き疲労強度に
優れる自動車用薄鋼板の製造方法。16. The hot rolling according to claim 12, wherein lubricating rolling is performed in finish rolling after rough rolling.
16. The method for producing a thin steel sheet for automobiles having excellent notch fatigue strength according to any one of items 15 to 15. 【請求項１７】 請求項１２または１６に記載の熱間圧
延に際し、粗圧延終了後、デスケーリングを行うことを
特徴とする、切り欠き疲労強度に優れる自動車用薄鋼板
の製造方法。17. A method for producing a thin steel sheet for automobiles having excellent notch fatigue strength, wherein descaling is performed after rough rolling in hot rolling according to claim 12 or 16. 【請求項１８】 請求項５ないし１０のいずれか１項に
記載の成分を有する薄鋼板を得るための熱間圧延する際
に、該成分を有する鋼片を熱間圧延後、続く酸洗、鋼板
厚圧下率８０％未満の冷間圧延後、回復温度以上Ａｃ3
変態点温度＋１００℃以下の温度域で５〜１５０秒間保
持し、冷却する工程の回復または再結晶焼鈍を行うこと
を特徴とする、切り欠き疲労強度に優れる自動車用薄鋼
板の製造方法。18. Hot-rolling for obtaining a thin steel sheet having the component according to any one of claims 5 to 10, after hot-rolling a slab having the component, followed by pickling, After cold rolling at a steel sheet thickness reduction of less than 80%, the recovery temperature is higher than Ac3
A method for producing a thin steel sheet for automobiles having excellent notch fatigue strength, wherein the steel sheet is held in a temperature range of a transformation point temperature + 100 ° C. or lower for 5 to 150 seconds and a cooling step is recovered or recrystallization annealing is performed. 【請求項１９】 請求項５ないし１０のいずれか１項に
記載の成分を有する薄鋼板を得るための熱間圧延する際
に、該成分を有する鋼片を熱間圧延後、続く酸洗、鋼板
厚圧下率８０％未満の冷間圧延後、Ａｃ1 変態点温度以
上Ａｃ3 変態点温度＋１００℃以下の温度域で５〜１５
０秒間保持し、その後に冷却する工程の熱処理を行うこ
とを特徴とする、切り欠き疲労強度に優れる自動車用薄
鋼板の製造方法。19. Hot rolling to obtain a thin steel sheet having the component according to any one of claims 5 to 10, after hot rolling the steel slab having the component, followed by pickling, After cold rolling at a steel sheet thickness reduction of less than 80%, 5 to 15 in the temperature range from the Ac1 transformation point temperature to the Ac3 transformation point temperature + 100 ° C or less.
A method for producing a thin steel sheet for automobiles having excellent notch fatigue strength, wherein the heat treatment is performed in a step of holding for 0 second and thereafter cooling. 【請求項２０】 請求項５ないし１０のいずれか１項に
記載の成分を有する薄鋼板を得るための熱間圧延する際
に、該成分を有する鋼片を熱間圧延後、続く酸洗、鋼板
厚圧下率８０％未満の冷間圧延後、Ａｃ1 変態点温度以
上Ａｃ3 変態点温度＋１００℃以下の温度域で５〜１５
０秒間保持し、２０℃／ｓ以上の冷却速度で３５０℃超
４５０℃未満の温度域まで冷却し、その後さらにその温
度域で５〜６００秒間保持し、５℃／ｓ以上の冷却速度
で２００℃以下の温度域まで冷却する工程の熱処理を行
うことを特徴とする、切り欠き疲労強度に優れる自動車
用薄鋼板の製造方法。20. When hot rolling is performed to obtain a thin steel sheet having the component according to any one of claims 5 to 10, after hot rolling a steel slab having the component, followed by pickling, After cold rolling at a steel sheet thickness reduction of less than 80%, 5 to 15 in the temperature range from the Ac1 transformation point temperature to the Ac3 transformation point temperature + 100 ° C or less.
Hold for 0 seconds, cool at a cooling rate of 20 ° C./s or more to a temperature range of more than 350 ° C. and less than 450 ° C., and further hold for 5 to 600 seconds at that temperature range, and cool at a cooling rate of 5 ° C./s or more A method for producing a thin steel sheet for automobiles having excellent notch fatigue strength, comprising performing a heat treatment in a step of cooling to a temperature range of not more than ° C. 【請求項２１】 請求項５ないし１０のいずれか１項に
記載の成分を有する薄鋼板を得るための熱間圧延する際
に、該成分を有する鋼片を熱間圧延後、続く酸洗、鋼板
厚圧下率８０％未満の冷間圧延後、Ａｃ1 変態点温度以
上Ａｃ3 変態点温度＋１００℃以下の温度域で５〜１５
０秒間保持し、２０℃／ｓ以上の冷却速度で３５０℃以
下の温度域まで冷却する工程の熱処理をすることを特徴
とする、切り欠き疲労強度に優れる自動車用薄鋼板の製
造方法。21. When hot rolling is performed to obtain a thin steel sheet having the component according to any one of claims 5 to 10, the steel slab having the component is hot-rolled, and then pickled, After cold rolling at a steel sheet thickness reduction of less than 80%, 5 to 15 in the temperature range from the Ac1 transformation point temperature to the Ac3 transformation point temperature + 100 ° C or less.
A method for producing a thin steel sheet for automobiles having excellent notch fatigue strength, wherein the heat treatment is performed in a step of holding for 0 second and cooling at a cooling rate of 20 ° C / s or more to a temperature range of 350 ° C or less. 【請求項２２】 請求項１２ないし１７のいずれか１項
に記載の製造方法において、熱間圧延後に亜鉛めっき浴
中に浸漬させて鋼板表面を亜鉛めっきすることを特徴と
する、切り欠き疲労強度に優れる自動車用薄鋼板の製造
方法。22. The notch fatigue strength according to claim 12, wherein after hot rolling, the steel sheet surface is galvanized by dipping in a galvanizing bath. Method for manufacturing thin steel sheets for automobiles that excels in quality. 【請求項２３】 請求項１８ないし２１のいずれか１項
に記載の製造方法において、回復または再結晶焼鈍終了
後、亜鉛めっき浴中に浸漬させて鋼板表面を亜鉛めっき
することを特徴とする、切り欠き疲労強度に優れる自動
車用薄鋼板の製造方法。23. The method according to claim 18, wherein after the recovery or recrystallization annealing is completed, the steel sheet is immersed in a galvanizing bath to galvanize the surface of the steel sheet. A method for manufacturing thin steel sheets for automobiles with excellent notch fatigue strength. 【請求項２４】 亜鉛めっき浴中に浸漬して亜鉛めっき
後、合金化処理することを特徴とする、請求項２２また
は２３に記載の切り欠き疲労強度に優れる自動車用薄鋼
板の製造方法。24. The method for producing a thin steel sheet for automobiles having excellent notch fatigue strength according to claim 22 or 23, wherein the steel sheet is immersed in a galvanizing bath, galvanized, and then alloyed.
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