JP4692259B2 - High-strength steel sheet with excellent formability and shape freezeability - Google Patents
High-strength steel sheet with excellent formability and shape freezeability Download PDFInfo
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- 229910000831 Steel Inorganic materials 0.000 title claims description 89
- 239000010959 steel Substances 0.000 title claims description 89
- 229910000859 α-Fe Inorganic materials 0.000 claims description 44
- 229910001566 austenite Inorganic materials 0.000 claims description 29
- 238000000137 annealing Methods 0.000 claims description 25
- 230000000717 retained effect Effects 0.000 claims description 18
- 238000005096 rolling process Methods 0.000 claims description 16
- 239000013078 crystal Substances 0.000 claims description 15
- 239000000463 material Substances 0.000 claims description 13
- 229910000734 martensite Inorganic materials 0.000 claims description 11
- 238000004519 manufacturing process Methods 0.000 claims description 10
- 239000000203 mixture Substances 0.000 claims description 9
- 230000009467 reduction Effects 0.000 claims description 9
- 229910001563 bainite Inorganic materials 0.000 claims description 8
- 238000007710 freezing Methods 0.000 claims description 6
- 230000008014 freezing Effects 0.000 claims description 6
- 229910052804 chromium Inorganic materials 0.000 claims description 5
- 229910052802 copper Inorganic materials 0.000 claims description 5
- 229910052750 molybdenum Inorganic materials 0.000 claims description 5
- 229910052759 nickel Inorganic materials 0.000 claims description 5
- 239000000126 substance Substances 0.000 claims description 5
- 238000005098 hot rolling Methods 0.000 claims description 4
- 239000012535 impurity Substances 0.000 claims description 4
- 229910052758 niobium Inorganic materials 0.000 claims description 4
- 229910052748 manganese Inorganic materials 0.000 claims description 2
- 238000004804 winding Methods 0.000 claims 1
- 230000035882 stress Effects 0.000 description 24
- 230000000694 effects Effects 0.000 description 14
- 229910000794 TRIP steel Inorganic materials 0.000 description 13
- 238000011156 evaluation Methods 0.000 description 10
- 230000015572 biosynthetic process Effects 0.000 description 9
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- 238000000034 method Methods 0.000 description 7
- XEEYBQQBJWHFJM-UHFFFAOYSA-N iron Substances [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 description 6
- 238000012545 processing Methods 0.000 description 6
- 229910052761 rare earth metal Inorganic materials 0.000 description 5
- 238000005097 cold rolling Methods 0.000 description 4
- 230000006866 deterioration Effects 0.000 description 4
- 238000000465 moulding Methods 0.000 description 4
- 238000005728 strengthening Methods 0.000 description 4
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- 150000001247 metal acetylides Chemical class 0.000 description 2
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- 238000007747 plating Methods 0.000 description 2
- 230000001737 promoting effect Effects 0.000 description 2
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- 239000006104 solid solution Substances 0.000 description 2
- 229910052720 vanadium Inorganic materials 0.000 description 2
- 241000219307 Atriplex rosea Species 0.000 description 1
- 229910001335 Galvanized steel Inorganic materials 0.000 description 1
- 239000004809 Teflon Substances 0.000 description 1
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- 239000000446 fuel Substances 0.000 description 1
- 239000008397 galvanized steel Substances 0.000 description 1
- 238000000227 grinding Methods 0.000 description 1
- 238000010438 heat treatment Methods 0.000 description 1
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- 229910052747 lanthanoid Inorganic materials 0.000 description 1
- 150000002602 lanthanoids Chemical class 0.000 description 1
- 230000001050 lubricating effect Effects 0.000 description 1
- 239000010687 lubricating oil Substances 0.000 description 1
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P10/00—Technologies related to metal processing
- Y02P10/20—Recycling
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- Heat Treatment Of Sheet Steel (AREA)
Description
本発明は、自動車や電気機器等の分野で使用される成形性および成形後の形状凍結性に優れる高強度鋼板に関するものである。 The present invention relates to a high-strength steel sheet that is excellent in formability and shape freezing property after forming used in the fields of automobiles and electrical equipment.
近年、地球環境を保護する観点から、自動車の燃費向上が重要な課題となっている。そこで、車体に用いられている鋼板を高強度化して薄肉化し、車体を軽量化することが進められている。また、自動車の安全性向上の観点からも、自動車への高強度鋼板の適用が進められている。 In recent years, from the viewpoint of protecting the global environment, improving the fuel efficiency of automobiles has become an important issue. In view of this, efforts have been made to reduce the thickness of the vehicle body by increasing the strength and thickness of the steel plate used in the vehicle body. In addition, from the viewpoint of improving the safety of automobiles, the application of high-strength steel sheets to automobiles is being promoted.
しかし、鋼板の高強度化は、一般に成形性の低下を招くことから、高強度と高加工性とを兼ね備えた材料の開発が望まれている。このような要求を満たす材料として、残留オーステナイトの変態誘起超塑性（TRansformation Induced Plasticity）を利用したＴＲＩＰ鋼が注目を浴びており、これまで、種々の鋼板が開発されている。 However, increasing the strength of a steel sheet generally leads to a decrease in formability, and therefore development of a material having both high strength and high workability is desired. As a material satisfying such requirements, TRIP steel utilizing transformation induced superplasticity of retained austenite has attracted attention, and various steel sheets have been developed so far.
例えば、特許文献１には、化学成分および鋼板中の残留オーステナイト（γ）量を制御したプレス成形性に優れる鋼板が、そして、特許文献２には、その製造方法が開示されている。また、特許文献３には、５％以上の残留γを含む局部延性に優れる鋼板が開示されている。また、特許文献４には、３％以上の残留γを含みかつその平均軸比を３〜２０とし、母相の平均硬度を２７０Ｈｖ以下とした伸びフランジ性と張出し性を兼備する鋼板が開示されている。 For example, Patent Document 1 discloses a steel sheet excellent in press formability with controlled chemical components and the amount of retained austenite (γ) in the steel sheet, and Patent Document 2 discloses a manufacturing method thereof. Further, Patent Document 3 discloses a steel plate having excellent local ductility including 5% or more of residual γ. Further, Patent Document 4 discloses a steel sheet that has both stretch flangeability and stretchability, including 3% or more of residual γ, an average axial ratio of 3 to 20, and an average hardness of the matrix phase of 270 Hv or less. ing.
さらに、特許文献５、特許文献６には、焼戻しマルテンサイトまたは焼戻しベイナイトを５０％以上含有し、３％以上の残留γを含有することにより、高延性と高伸びフランジ性を両立した鋼板が開示されている。また、特許文献７には、残留γの体積分率とそのＣ含有量およびフェライト（α）相のアスペクト比を適性化し、予加工を施した後の成形性を改善した鋼板とその製造方法が開示されている。 Further, Patent Documents 5 and 6 disclose steel sheets that have both high ductility and high stretch flangeability by containing 50% or more of tempered martensite or tempered bainite and containing 3% or more of residual γ. Has been. Further, Patent Document 7 discloses a steel sheet and a method for manufacturing the same, in which the volume fraction of residual γ, its C content, and the aspect ratio of the ferrite (α) phase are optimized and formability after pre-processing is improved. It is disclosed.
また、特許文献８には、３％以上の残留γを含有し、かつその粒の７０％以上のアスペクト比が２．５〜５．０であることを特徴とする強度−伸びバランスおよび疲労特性に優れる高張力溶融亜鉛めっき鋼板が開示されている。さらに、特許文献９には、特許文献８の鋼板における低温変態相中のマルテンサイトの比率を２０％以下とし、かつ低温変態相中のベイナイトと主相のフェライトの硬度比を２．６以下とすることにより、穴拡げ性にも優れた鋼板を得る技術が開示されている。
しかしながら、従来開発されたＴＲＩＰ鋼は、成形性は良好であるものの、部品に成形した後のスプリングバックが大きいため、形状凍結性が悪く、部品の寸法精度を所定の範囲内に収めることが困難であるという問題を抱えていた。一般に、スプリングバックは、材料強度の上昇とともに大きくなるが、特にＴＲＩＰ鋼では、スプリングバックが大きくなる傾向がある。その原因は、ＴＲＩＰ鋼の優れた成形性は、残留オーステナイトがマルテンサイトに変態することで得られるが、その際、成形後の鋼板（部品）内に、マルテンサイト変態を起こした部分と起こさない部分とが混在する組織の不均一が発生し、その結果、成形部品内部に大きな残留応力が発生するためと推測される。 However, although TRIP steel developed in the past has good formability, it has a large spring back after being formed into parts, so its shape freezeability is poor, and it is difficult to keep the dimensional accuracy of parts within a predetermined range. Had the problem of being. In general, the springback increases as the material strength increases, but the springback tends to increase particularly in TRIP steel. The cause is that the excellent formability of TRIP steel is obtained by transforming residual austenite to martensite, but at that time, it does not occur in the steel sheet (part) after forming with the part that has undergone martensite transformation. It is presumed that the structure in which the portion is mixed is uneven and, as a result, a large residual stress is generated inside the molded part.
したがって、ＴＲＩＰ鋼の適用範囲をさらに拡大するためには、スプリングバックを小さくする、すなわち、成形後の部品内部に発生する残留応力を低減することが必要であると考えられる。しかしながら、上記特許文献１〜９に代表される従来技術は、成形性の向上に関する技術ばかりであり、スプリングバックを小さくすることに関する検討はなされていない。 Therefore, in order to further expand the application range of TRIP steel, it is considered necessary to reduce the springback, that is, to reduce the residual stress generated in the molded part. However, the conventional techniques represented by Patent Documents 1 to 9 are only techniques related to improvement of moldability, and no studies have been made regarding reducing the springback.
そこで、本発明の目的は、ＴＲＩＰ鋼の高成形性を維持しつつ、成形に伴い発生する部品内部の残留応力を小さくし、スプリングバックの低減を図ることができる高強度鋼板とその製造方法を提案することにある。なお、本発明における高強度鋼板とは、熱延鋼板、冷延鋼板および表面処理鋼板のいずれをも含むものである。 Accordingly, an object of the present invention is to provide a high-strength steel sheet and a method for manufacturing the same, which can reduce the residual stress inside the part generated by forming and reduce the springback while maintaining the high formability of TRIP steel. It is to propose. In addition, the high strength steel plate in the present invention includes any of a hot rolled steel plate, a cold rolled steel plate, and a surface-treated steel plate.
発明者らは、ＴＲＩＰ鋼の高成形性を維持しつつ、成形後の部品内部の残留応力を低減することについて検討を重ねた。その結果、高強度鋼板の組織を、体積分率にして２０〜９７％のフェライト相と３％以上のオーステナイト相から主としてなるものとすると共に、フェライト相以外の部分における結晶粒のアスペクト比２．５以下のものの割合を５０〜９５％とすることにより、ＴＲＩＰ鋼の特徴である高成形性を有したまま、従来よりも成形後の残留応力を低減し、ひいてはスプリングバックの小さい高強度鋼板を得ることができることを知見し、本発明を完成するに至った。ここで、「フェライト相以外の部分における結晶粒」とは、ミクロ組織をナイタールでエッチングし、ＳＥＭで観察したときに、フェライト粒以外の１つの粒として見える領域を言う。 Inventors repeated examination about reducing the residual stress inside the component after shaping | molding, maintaining the high moldability of TRIP steel. As a result, the structure of the high-strength steel sheet is mainly composed of 20 to 97% ferrite phase and 3% or more austenite phase in volume fraction, and the aspect ratio of crystal grains in the portion other than the ferrite phase. By making the ratio of 5 or less 50 to 95%, while maintaining the high formability that is characteristic of TRIP steel, the residual stress after forming is reduced as compared with the conventional one, and as a result, a high strength steel plate with a small spring back is obtained. As a result, the present invention has been completed. Here, “crystal grains in a portion other than the ferrite phase” refers to a region that appears as one grain other than ferrite grains when the microstructure is etched with nital and observed with SEM.
すなわち、本発明は、Ｃ：０．０５〜０．３０ｍａｓｓ％、Ｓｉ：２．０ｍａｓｓ％以下、Ｍｎ：０．８〜３．０ｍａｓｓ％、Ｐ：０．００３〜０．１ｍａｓｓ％、Ｓ：０．０１ｍａｓｓ％以下、Ａｌ：０．０１〜２．５０ｍａｓｓ％、Ｎ：０．００７ｍａｓｓ％以下を含有し、ＳｉとＡｌがＳｉ＋Ａｌ≧０．５０ｍａｓｓ％の関係を満たし、残部がＦｅおよび不可避的不純物からなる成分組成を有し、体積分率にして２０〜９７％のフェライト相と３％以上の残留オーステナイト相と、残部がマルテンサイト相とベイナイト相からなり、フェライト相以外の部分における結晶粒のアスペクト比２．５以下のものの割合が５０〜９５％であることを特徴とする成形性および形状凍結性に優れる高強度鋼板である。 That is, the present invention is : C: 0.05-0.30 mass%, Si: 2.0 mass% or less, Mn: 0.8-3.0 mass%, P: 0.003-0.1 mass%, S: 0 .01 mass% or less, Al: 0.01-2.50 mass%, N: 0.007 mass% or less, Si and Al satisfy the relationship of Si + Al ≧ 0.50 mass%, and the balance is Fe and inevitable impurities With a volume fraction of 20-97% ferrite phase, 3% or more retained austenite phase , the balance consisting of martensite phase and bainite phase , and the aspect ratio of the crystal grains in the portion other than the ferrite phase It is a high-strength steel sheet excellent in formability and shape freezing property, characterized in that the ratio of those having a ratio of 2.5 or less is 50 to 95%.
また、本発明の高強度鋼板は、上記成分組成に加えてさらに、下記Ａ〜Ｄ群のうちの少なくとも１群の成分を含有することを特徴とする。
Ａ群；Ｖ：０．００５〜２．０ｍａｓｓ％、Ｍｏ：０．００５〜２．０ｍａｓｓ％、Ｎｉ：０．００５〜２．０ｍａｓｓ％、Ｃｕ：０．００５〜２．０ｍａｓｓ％およびＣｒ：０．００５〜２．０ｍａｓｓ％のうちから選ばれる１種または２種以上
Ｂ群；Ｔｉ：０．０１〜０．２ｍａｓｓ％、Ｎｂ：０．０１〜０．１ｍａｓｓ％のうちから選ばれる１種または２種
Ｃ群；Ｂ：０．０００２〜０．００５ｍａｓｓ％
Ｄ群；Ｃａ：０．００１〜０．００５ｍａｓｓ％、ＲＥＭ：０．００１〜０．００５ｍａｓｓ％のうちから選ばれる１種または２種
The high-strength steel sheet of the present invention is characterized by further containing at least one component of the following groups A to D in addition to the above component composition.
Group A; V: 0.005-2.0 mass%, Mo: 0.005-2.0 mass%, Ni: 0.005-2.0 mass%, Cu: 0.005-2.0 mass%, and Cr: 0 One or two or more B groups selected from 0.005 to 2.0 mass%; Ti: 0.01 to 0.2 mass%, Nb: one selected from 0.01 to 0.1 mass% or Group 2 C; B: 0.0002 to 0.005 mass%
Group D; Ca: 0.001~0.005mass%, REM: 0.001~0.005mass% of the inner shell one or two elements selected
また、本発明は、上記いずれかの成分組成を有する鋼スラブを、仕上圧延における１０００℃以下の温度での累積圧下率を２０％以下として熱間圧延し、コイルに巻き取った後、圧下率が６５％以下で冷間圧延し、その後、７００〜９００℃の温度で１５〜６００秒保持後、５５０℃以下まで１０℃／秒以上の速度で冷却してから再度７００〜９００℃の温度で１５〜６００秒保持し、冷却し、３５０〜５５０℃の温度で３０秒以上保持する焼鈍を行い、体積分率にして２０〜９７％のフェライト相と３％以上の残留オーステナイト相と、残部がマルテンサイト相とベイナイト相からなり、フェライト相以外の部分における結晶粒のアスペクト比２．５以下のものの割合が５０〜９５％である鋼板組織とすることを特徴とする成形性および形状凍結性に優れる高強度鋼板の製造方法を提案する。 In addition, the present invention is a steel slab having any one of the above component compositions, hot rolled with a cumulative rolling reduction at a temperature of 1000 ° C. or lower in finish rolling at 20% or lower, wound on a coil, and then rolled down. Is cold-rolled at 65% or less, then held at a temperature of 700 to 900 ° C. for 15 to 600 seconds, cooled to 550 ° C. or less at a rate of 10 ° C./second or more, and then again at a temperature of 700 to 900 ° C. hold 15 to 600 seconds, it cooled, and 350 to 550 have rows annealing holding temperature at 30 seconds or longer ° C., 20 to 97% of ferrite phase by volume fraction and at least 3% of retained austenite phase and the balance formability and form but consists martensite and bainite phase, the ratio of crystal grains having an aspect ratio of 2.5 or less as in the portion other than the ferrite phase, characterized in that the steel sheet microstructure is 50% to 95% We propose a method for manufacturing high strength steel sheet excellent in fixability.
本発明によれば、成形性に優れ、かつ成形後のスプリングバックの小さい高強度鋼板を提供することができる。したがって、本発明の高強度鋼板は、自動車車体や電機機器の素材として用いた場合には、製品の軽量化や生産効率の向上に大いに寄与する。 ADVANTAGE OF THE INVENTION According to this invention, it is excellent in a moldability and can provide the high strength steel plate with a small spring back after shaping | molding. Therefore, the high-strength steel sheet of the present invention greatly contributes to the reduction of product weight and the improvement of production efficiency when used as a material for automobile bodies and electrical equipment.
発明者らは、種々のＴＲＩＰ鋼を用いて、成形後の部品内に残留する内部応力とスプリングバックの大きさとの関係について調査を行った。その結果、鋼板をカップ形状にプレス加工した後のスプリングバックの大きさと、下記の（１）式および（２）式で定義される残留応力評価指数δとの間に極めてよい相関関係が認められることを見出した。そしてさらに、スプリングバックが小さく、優れた形状凍結性を得るためには、加工後の部品内部の残留応力評価指数δを０．３５以下に制御することが必要であることを見出した。
δ＝（σｃ／ρ）／ＴＳ ・・・（１）
σｃ＝｛Ｅ／（１−ν２）｝×ｔ×（１／Ｄ０−１／Ｄ１） ・・・（２）
ここで、Ｅは、鋼のヤング率（ＭＰａ）、νは、鋼のポアソン比、ｔは、鋼板の板厚（ｍｍ）、ＴＳは、鋼板の引張強度（ＭＰａ）を表す。また、Ｄ０は、絞り比ρ（＝ブランク径／ポンチ径）で鋼板をカップ成形した時のカップ外径（ｍｍ）、Ｄ１は、カップの側面部からリング試料を切り出し、鋼板の圧延方向に切れ目を入れてリング試料を開口した時の、圧延方向に対して直角方向のリング外径（ｍｍ）を表す。
ここで、σｃは、従来から知られている残留応力の指標となる数値である（例えば、「残留応力−発生・影響・測定・対策」，共立出版，１９６３年，Ｐ．６４、（３・４４）式）。残留応力は、加工歪量、材料強度の影響を受けることから、発明者らは、このσｃを加工量（絞り比ρ）および材料強度（ＴＳ）で規格化したδで部品内部の残留応力を評価することとした。
The inventors investigated the relationship between the internal stress remaining in the molded part and the size of the springback using various TRIP steels. As a result, a very good correlation is recognized between the size of the springback after the steel plate is pressed into a cup shape and the residual stress evaluation index δ defined by the following equations (1) and (2). I found out. Furthermore, it has been found that in order to obtain a small spring back and excellent shape freezing property, it is necessary to control the residual stress evaluation index δ inside the processed part to 0.35 or less.
δ = (σ c / ρ) / TS (1)
σ c = {E / (1-ν 2 )} × t × (1 / D 0 −1 / D 1 ) (2)
Here, E is the Young's modulus (MPa) of steel, ν is the Poisson's ratio of steel, t is the plate thickness (mm) of the steel plate, and TS is the tensile strength (MPa) of the steel plate. D 0 is the cup outer diameter (mm) when the steel plate is cup-formed with a drawing ratio ρ (= blank diameter / punch diameter), D 1 is a ring sample cut out from the side surface of the cup, and the rolling direction of the steel plate Represents the outer diameter (mm) of the ring in the direction perpendicular to the rolling direction when a ring sample is opened.
Here, σ c is a numerical value that serves as a conventionally known index of residual stress (for example, “Residual Stress—Generation / Influence / Measurement / Countermeasure”, Kyoritsu Shuppan, 1963, P.64, (3 (44) Formula). Since the residual stress is affected by the amount of processing strain and material strength, the inventors have determined that σ c is the residual stress inside the part by δ normalized by the processing amount (drawing ratio ρ) and material strength (TS). It was decided to evaluate.
さらに、発明者らは、ＴＲＩＰ鋼の特徴である高加工性を維持しつつ、上記残留応力評価指数δを０．３５以下に制御するためには、鋼板自体がどのような要件を具備すべきか検討した。その結果、高強度鋼板の組織を、体積分率にして２０〜９７％のフェライト相と３％以上のオーステナイト相から主としてなるものとした上で、フェライト相以外の部分における結晶粒のアスペクト比２．５以下のものの割合を５０〜９５％の範囲に制御することが必要であることを新規に見出した。本発明は上記知見に基くものである。 Furthermore, the inventors should have what requirements the steel sheet itself should have in order to control the residual stress evaluation index δ to 0.35 or less while maintaining the high workability characteristic of TRIP steel. investigated. As a result, the structure of the high-strength steel sheet is mainly composed of a ferrite phase of 20 to 97% and an austenite phase of 3% or more in terms of volume fraction. It has been newly found out that it is necessary to control the ratio of those of 5 or less to 50 to 95%. The present invention is based on the above findings.
次に、本発明が対象とする高強度鋼板の組織の特徴について説明する。
フェライト相分率：体積分率で２０〜９７％
フェライトは、強度と伸び特性に大きく影響する相である。フェライト相の分率が、体積分率で２０％未満では、熱処理後の残留γとフェライト相以外の相分率が多くなり過ぎるため良好な成形性が得られない。一方、９７％を超えると、残留γ粒の相分率が不十分となるため、良好な成形性が得られない。したがって、フェライト相分率は、体積分率で２０〜９７％の範囲に制御する必要がある。好ましくは４０〜８０％の範囲である。
Next, the characteristics of the structure of the high-strength steel plate targeted by the present invention will be described.
Ferrite phase fraction: 20-97% in volume fraction
Ferrite is a phase that greatly affects strength and elongation characteristics. If the fraction of the ferrite phase is less than 20% in terms of volume fraction, the residual γ after the heat treatment and the phase fraction other than the ferrite phase become too large, so that good formability cannot be obtained. On the other hand, if it exceeds 97%, the phase fraction of the residual γ grains becomes insufficient, so that good moldability cannot be obtained. Therefore, it is necessary to control the ferrite phase fraction in the range of 20 to 97% in terms of volume fraction. Preferably it is 40 to 80% of range.
残留オーステナイト相分率：体積分率で３％以上
残留オーステナイトの量（相分率）は、特に、伸び特性に関係する。その相分率が、体積分率で３％未満では、残留γによりＴＲＩＰ効果が不十分となるため良好な成形性が得られないので、３％以上確保する必要がある。好ましくは５％以上である。
なお、本発明の高強度鋼板は、フェライト相および残留オーステナイト相以外の残部は、マルテンサイト相、ベイナイト相などからなる。
Residual austenite phase fraction: 3% or more in volume fraction The amount of retained austenite (phase fraction) is particularly related to elongation characteristics. If the phase fraction is less than 3% in terms of volume fraction, the TRIP effect becomes insufficient due to residual γ, and good moldability cannot be obtained. Therefore, it is necessary to ensure 3% or more. Preferably it is 5% or more.
In the high-strength steel sheet of the present invention, the balance other than the ferrite phase and the retained austenite phase is composed of a martensite phase, a bainite phase, and the like.
フェライト相以外の部分における結晶粒のアスペクト比２．５以下のものの個数割合：５０〜９５％
フェライト相以外の部分、すなわち、残留オーステナイト相、マルテンサイト相、ベイナイト相などの部分における結晶粒のアスペクト比２．５以下の粒の個数割合が５０％未満では、フェライト相以外の部分に不均一な変形が起こるため、加工後の残留応力が大きくなり、δの値を０．３５以下とすることができない。一方、アスペクト比２．５以下のものの割合が９５％を超えると、鋼板の引張強度ＴＳに対する降伏応力ＹＳの比である降伏比ＹＲ（＝ＹＳ／ＴＳ）が急激に低下する。特に、ＴＲＩＰ鋼においては、ＹＲが０．５０未満となると、自動車の衝突時の鋼板強度が不足し、耐衝突特性が劣化する。したがって、ＴＲＩＰ鋼の耐衝突特性を確保するためには、フェライト相以外の部分における結晶粒のアスペクト比２．５以下の粒の個数割合を９５％以下とする必要がある。好ましくは６６〜８０％の範囲である。
Number ratio of crystal grains having an aspect ratio of 2.5 or less in a portion other than the ferrite phase: 50 to 95%
If the number ratio of grains having an aspect ratio of 2.5 or less in the portion other than the ferrite phase, that is, the retained austenite phase, martensite phase, bainite phase, etc. is less than 50%, the portion other than the ferrite phase is not uniform. Therefore, the residual stress after processing increases, and the value of δ cannot be set to 0.35 or less. On the other hand, when the ratio of those having an aspect ratio of 2.5 or less exceeds 95%, the yield ratio YR (= YS / TS), which is the ratio of the yield stress YS to the tensile strength TS of the steel sheet, rapidly decreases. In particular, in TRIP steel, when YR is less than 0.50, the steel plate strength at the time of automobile collision is insufficient, and the collision resistance is deteriorated. Therefore, in order to ensure the impact resistance characteristics of TRIP steel, the number ratio of grains having an aspect ratio of 2.5 or less in the portion other than the ferrite phase needs to be 95% or less. Preferably it is 66 to 80% of range.
なお、上述した本発明の高強度鋼板が有する高成形性とは、引張試験で得られた引張強度ＴＳと全伸びＥｌとの積（ＴＳ×Ｅｌ）が１８０００（ＭＰａ・％）以上であることを意味する。 The high formability of the above-described high-strength steel sheet of the present invention means that the product (TS × El) of the tensile strength TS and total elongation El obtained in the tensile test is 18000 (MPa ·%) or more. Means.
本発明の鋼板は、上記組織が得られれば、所期した特性をえることができる。そのためには、下記の成分組成とすることが好ましい。
Ｃ：０．０５〜０．３０ｍａｓｓ％
Ｃは、オーステナイト相を安定化させる元素であり、残留オーステナイト相やその他のフェライト相以外の相を生成させるために必要な元素である。Ｃ量が０．０５ｍａｓｓ％未満では、製造条件を最適化しても、鋼板強度を確保すると同時に、残留オーステナイト相の量を確保し、もって高成形性を得ることが難しくなる。一方、Ｃ量が０．３０ｍａｓｓ％を超えると、溶接した時の溶接部および熱影響部の硬化が大きく、溶接性が劣化する。よって、Ｃの含有量は、０．０５〜０．３０ｍａｓｓ％の範囲とするのが好ましい。より好ましくは、０．０８〜０．２０ｍａｓｓ％である。
If the said structure is obtained, the steel plate of this invention can obtain the expected characteristic. For that purpose, it is preferable to set it as the following component composition.
C: 0.05-0.30 mass%
C is an element that stabilizes the austenite phase, and is an element necessary for generating a phase other than the retained austenite phase and other ferrite phases. If the amount of C is less than 0.05 mass%, even if the production conditions are optimized, it is difficult to secure the strength of the steel sheet and at the same time to secure the amount of retained austenite phase, thereby obtaining high formability. On the other hand, when the amount of C exceeds 0.30 mass%, the welded portion and the heat-affected zone are hardened when welded, and the weldability deteriorates. Therefore, the C content is preferably in the range of 0.05 to 0.30 mass%. More preferably, it is 0.08-0.20 mass%.
Ｓｉ：２．０ｍａｓｓ％以下
Ｓｉは、鋼の高強度化に有効な元素である。また、フェライト生成元素である。オーステナイト相中へのＣの濃化促進および炭化物の生成を抑制し、残留オーステナイト相の生成を促進する働きがあるので、複合組織鋼およびＴＲＩＰ鋼に添加されることが多い。このような効果は、Ｓｉ＋Ａｌ≧０．５０ｍａｓｓ％以上で得られる。一方、Ｓｉの過剰な添加は、フェライト相中への固溶量の増加による成形性の低下や靭性の劣化、赤スケール等の発生による表面性状の低下、さらに溶融めっきを施す場合には、めっき付着性や密着性の低下を引き起こす。よって、Ｓｉの添加量は、２．０ｍａｓｓ％以下とするのが好ましい。より好ましくは、０．０２〜１．５ｍａｓｓ％の範囲である。
Si: 2.0 mass% or less Si is an element effective for increasing the strength of steel. It is also a ferrite-forming element. Since it has a function of promoting the concentration of C in the austenite phase and suppressing the formation of carbides and promoting the formation of the retained austenite phase, it is often added to the composite structure steel and TRIP steel. Such an effect is obtained when Si + Al ≧ 0.50 mass% or more. On the other hand, excessive addition of Si is caused by a decrease in formability and toughness due to an increase in the amount of solid solution in the ferrite phase, a decrease in surface properties due to the occurrence of red scale, etc. It causes a decrease in adhesion and adhesion. Therefore, the amount of Si added is preferably 2.0 mass% or less. More preferably, it is the range of 0.02-1.5 mass%.
Ｍｎ：０．８〜３．０ｍａｓｓ％
Ｍｎは、鋼の高強度化に有効な元素である。また、オーステナイト相を安定化させる元素であり、マルテンサイト相や残留オーステナイト相の体積分率の確保に必要な元素である。この効果は、Ｍｎが０．８ｍａｓｓ％以上で得られる。一方、Ｍｎを３．０ｍａｓｓ％を超えて過剰に添加すると、フェライト以外の相分率の過大や固溶強化による強度上昇が大きくなり、残留オーステナイト相の生成が難しくなるため成形性が低下する。従って、Ｍｎの含有量は、０．８〜３．０ｍａｓｓ％の範囲とするのが好ましい。より好ましくは、１．０〜２．３ｍａｓｓ％の範囲である。
Mn: 0.8 to 3.0 mass%
Mn is an element effective for increasing the strength of steel. Moreover, it is an element which stabilizes an austenite phase, and is an element required for ensuring the volume fraction of a martensite phase and a retained austenite phase. This effect is obtained when Mn is 0.8 mass% or more. On the other hand, when Mn is added excessively exceeding 3.0 mass%, the strength increase due to excessive phase fraction other than ferrite and solid solution strengthening becomes large, and the formation of the retained austenite phase becomes difficult, so that the formability is lowered. Therefore, the Mn content is preferably in the range of 0.8 to 3.0 mass%. More preferably, it is the range of 1.0-2.3 mass%.
Ｐ：０．００３〜０．１ｍａｓｓ％
Ｐは、鋼の強化に有効な元素であり、この効果は０．００３ｍａｓｓ％以上の添加で得られる。しかし、０．１ｍａｓｓ％を超えて過剰に添加すると、粒界偏析による脆化を引き起こし、耐衝撃性を劣化させる。よって、Ｐの含有量は０．００３〜０．１ｍａｓｓ％とするのが好ましい。
P: 0.003-0.1 mass%
P is an element effective for strengthening steel, and this effect can be obtained by adding 0.003 mass% or more. However, excessive addition exceeding 0.1 mass% causes embrittlement due to grain boundary segregation and degrades impact resistance. Therefore, the content of P is preferably 0.003 to 0.1 mass%.
Ｓ：０．０１ｍａｓｓ％以下
Ｓは、ＭｎＳなどの介在物となって鋼中に存在し、耐衝撃性の劣化や溶接部のメタルフローに沿った割れの原因となるので、極力低い方が好ましい。しかし、Ｓの過度の低減は製造コストの上昇を招くので、上限は０．０１ｍａｓｓ％以下とするのが好ましい。
S: 0.01 mass% or less S is an inclusion such as MnS and is present in the steel, causing deterioration in impact resistance and cracking along the metal flow of the weld. Therefore, it is preferable that S be as low as possible. . However, since excessive reduction of S causes an increase in manufacturing cost, the upper limit is preferably set to 0.01 mass% or less.
Ａｌ：０．０１〜２．５ｍａｓｓ％
Ａｌは、フェライト生成元素であり、オーステナイト相中へのＣの濃化促進および炭化物の生成を抑制することから、残留オーステナイト相の生成を促進する働きがあり、この効果は、Ｓｉ＋Ａｌ≧０．５０ｍａｓｓ％以上で得られる。また、Ａｌは、Ｓｉによるめっき性、鋼板の表面性状およびめっき表面性状の低下を抑制する働きがあり、この効果はＡｌ≧０．０１ｍａｓｓ％で得られる。このような理由から、Ａｌは、複合組織鋼およびＴＲＩＰ鋼に多量に添加される場合があるが、過剰な添加は、フェライト相の脆化を招き、材料の強度−延性バランスを低下させることになる。また、２．５ｍａｓｓ％を超える含有は、鋼板中の介在物が多くなり延性をも劣化させる。よって、Ａｌの含有量は０．０１〜２．５ｍａｓｓ％の範囲とするのが好ましい。より好ましくは、０．０２〜１．２ｍａｓｓ％の範囲である。
Al: 0.01-2.5 mass%
Al is a ferrite-forming element, and since it promotes the concentration of C in the austenite phase and suppresses the formation of carbides, it functions to promote the formation of residual austenite phase. This effect is obtained by adding Si + Al ≧ 0.50 mass. % Or more. In addition, Al has a function of suppressing deterioration of the plating property due to Si, the surface property of the steel sheet, and the plating surface property, and this effect is obtained when Al ≧ 0.01 mass%. For these reasons, Al may be added in a large amount to the composite structure steel and TRIP steel. However, excessive addition leads to embrittlement of the ferrite phase and lowers the strength-ductility balance of the material. Become. Further, when the content exceeds 2.5 mass%, inclusions in the steel sheet increase and ductility is deteriorated. Therefore, the Al content is preferably in the range of 0.01 to 2.5 mass%. More preferably, it is the range of 0.02-1.2 mass%.
Ｎ：０．００７ｍａｓｓ％以下
Ｎは、鋼の耐時効性を最も大きく劣化させる元素であり、その含有量は少ないほどよい。特に、０．００７ｍａｓｓ％を超えると、耐時効性の劣化が顕著となるので、０．００７ｍａｓｓ％以下とするのが好ましい。
N: 0.007 mass% or less N is an element that most deteriorates the aging resistance of steel, and the smaller the content, the better. In particular, when it exceeds 0.007 mass%, the deterioration of aging resistance becomes remarkable, and therefore it is preferably 0.007 mass% or less.
本発明の高強度鋼板は、上記成分の他に、Ｖ，Ｍｏ，Ｎｉ，ＣｕおよびＣｒの中から選ばれる１種または２種以上を必要に応じて以下の範囲で含有することができる。
Ｖ：０．００５〜２．０ｍａｓｓ％、Ｍｏ：０．００５〜２．０ｍａｓｓ％、Ｎｉ：０．００５〜２．０ｍａｓｓ％、Ｃｕ：０．００５〜２．０ｍａｓｓ％およびＣｒ：０．００５〜２．０ｍａｓｓ％
Ｖ，Ｍｏ，Ｎｉ，ＣｕおよびＣｒは、いずれも、焼鈍温度からの冷却時にパーライト相が生成するのを抑制し、残留オーステナイト相を生成し易くする効果を有する。この効果は、それぞれ０．００５ｍａｓｓ％以上の添加で得られる。しかし、２．０ｍａｓｓ％を超えると、フェライト相の量が過少となり、成形性の低下を招く。よって、上記成分は、０．００５〜２．０ｍａｓｓ％の範囲で添加することが好ましい。
The high-strength steel sheet of the present invention can contain one or more selected from V, Mo, Ni, Cu and Cr in addition to the above components in the following ranges as necessary.
V: 0.005-2.0 mass%, Mo: 0.005-2.0 mass%, Ni: 0.005-2.0 mass%, Cu: 0.005-2.0 mass%, and Cr: 0.005- 2.0 mass%
V, Mo, Ni, Cu and Cr all have the effect of suppressing the formation of a pearlite phase during cooling from the annealing temperature and facilitating the formation of a retained austenite phase. This effect can be obtained by adding 0.005 mass% or more. However, if it exceeds 2.0 mass%, the amount of the ferrite phase becomes too small, leading to a decrease in formability. Therefore, it is preferable to add the said component in 0.005-2.0 mass%.
さらに、本発明の高強度鋼板は、Ｔｉ、Ｎｂのうちから選ばれる１種または２種を以下の範囲で含有することができる。
Ｔｉ：０．０１〜０．２ｍａｓｓ％、Ｎｂ：０．０１〜０．１ｍａｓｓ％
Ｔｉは、鋼の強化に有効な元素であり、この効果は０．０１ｍａｓｓ％以上の添加で得られる。しかし、０．２ｍａｓｓ％を超えると、成形性および形状凍結性が低下する。よって、０．０１〜０．２ｍａｓｓ％の範囲で添加するのが好ましい。また、Ｎｂも、鋼の強化に有効な元素であり、この効果は０．０１ｍａｓｓ％以上の添加で得られる。しかし、０．１ｍａｓｓ％を超えると、成形性および形状凍結性が低下する。よって、０．０１〜０．１ｍａｓｓ％の範囲で添加するのが好ましい。
Furthermore, the high-strength steel sheet of the present invention can contain one or two selected from Ti and Nb in the following ranges.
Ti: 0.01-0.2 mass%, Nb: 0.01-0.1 mass%
Ti is an element effective for strengthening steel, and this effect can be obtained by addition of 0.01 mass% or more. However, if it exceeds 0.2 mass%, the moldability and the shape freezing property are lowered. Therefore, it is preferable to add in the range of 0.01 to 0.2 mass%. Nb is also an element effective for strengthening steel, and this effect can be obtained by addition of 0.01 mass% or more. However, if it exceeds 0.1 mass%, the moldability and the shape freezeability are lowered. Therefore, it is preferable to add in the range of 0.01 to 0.1 mass%.
さらに、本発明の高強度鋼板は、Ｂを以下の範囲で含有することができる。
Ｂ：０．０００２〜０．００５ｍａｓｓ％
Ｂは、オーステナイト粒界からのフェライト相の生成を抑制する作用を有する。その効果は０．０００２ｍａｓｓ％以上の添加で得られる。しかし、０．００５ｍａｓｓ％を超えると、フェライト量が過少となり、成形性が低下する。よって、０．０００２〜０．００５ｍａｓｓ％の範囲で添加するのが好ましい。
Furthermore, the high-strength steel sheet of the present invention can contain B in the following range.
B: 0.0002 to 0.005 mass%
B has an action of suppressing the formation of a ferrite phase from the austenite grain boundary. The effect is acquired by addition of 0.0002 mass% or more. However, if it exceeds 0.005 mass%, the amount of ferrite becomes too small, and the formability deteriorates. Therefore, it is preferable to add in the range of 0.0002 to 0.005 mass%.
さらに、本発明の高強度鋼板は、ＣａおよびＲＥＭの中から選ばれる１種または２種を以下の範囲で添加することができる。
Ｃａ：０．００１〜０．００５ｍａｓｓ％、ＲＥＭ：０．００１〜０．００５ｍａｓｓ％
Ｃａ，ＲＥＭは、いずれも局部延性を向上させることにより、伸びを向上する元素である。その効果は０．００１ｍａｓｓ％以上で得られ、０．００５ｍａｓｓ％で飽和する。よって、０．００１〜０．００５ｍａｓｓ％の範囲で添加するのが好ましい。なお、上記ＲＥＭとは、Ｓｃ，Ｙおよび原子番号が５７〜７１までのランタノイドの１７元素の総称のことである。
Furthermore, 1 type or 2 types chosen from Ca and REM can be added to the high strength steel plate of this invention in the following ranges.
Ca: 0.001 to 0.005 mass%, REM: 0.001 to 0.005 mass%
Ca and REM are both elements that improve elongation by improving local ductility. The effect is obtained at 0.001 mass% or more, and is saturated at 0.005 mass%. Therefore, it is preferable to add in the range of 0.001 to 0.005 mass%. The REM is a generic name for 17 elements of Sc, Y and lanthanoids having atomic numbers of 57 to 71.
本発明の高強度鋼板は、上記成分以外の残部は、Ｆｅおよび不可避的不純物からなる。ただし、本発明の作用効果を損なわない範囲であれば、上記成分以外の成分を不可避的不純物のレベルを超えて含有することを妨げるものではない。 In the high-strength steel sheet of the present invention, the balance other than the above components consists of Fe and inevitable impurities. However, as long as the effects of the present invention are not impaired, it does not preclude containing components other than the above components beyond the level of inevitable impurities.
次に、本発明の高強度鋼板の製造方法について説明する。
本発明の高強度鋼板は、上記好適な成分組成に調整された鋼スラブを、熱間圧延し、酸洗し、冷間圧延し、その後、仕上焼鈍することにより製造される。以下、各工程の製造条件について説明する。
熱間圧延
熱間圧延では、仕上圧延における１０００℃以下の温度域での累積圧下率を２０％以下とする必要がある。累積圧下率が２０％を超えると、仕上焼鈍後の鋼板組織におけるフェライト相以外の部分における結晶粒のアスペクト比２．５以下のものの割合が５０％未満となり、製品加工後の残留応力が大きくなるからである。好ましくは１０％以下である。
Next, the manufacturing method of the high strength steel plate of this invention is demonstrated.
The high-strength steel sheet of the present invention is manufactured by hot-rolling, pickling, cold-rolling, and then finishing annealing a steel slab adjusted to the above-mentioned suitable component composition. Hereinafter, the manufacturing conditions of each process will be described.
Hot rolling In hot rolling, the cumulative rolling reduction in the temperature range of 1000 ° C. or less in finish rolling needs to be 20% or less. When the cumulative rolling reduction exceeds 20%, the ratio of the crystal grains having an aspect ratio of 2.5 or less in the portion other than the ferrite phase in the steel sheet structure after the finish annealing becomes less than 50%, and the residual stress after product processing increases. Because. Preferably it is 10% or less.
冷間圧延
冷間圧延における圧下率は、６５％以下とする必要がある。冷延圧下率が６５％を超えると、仕上焼鈍後の鋼板組織におけるフェライト相以外の部分における結晶粒のアスペクト比２．５以下のものの割合が５０％未満となり、製品加工後の残留応力が大きくなるからである。好ましくは５０％以下である。
Cold rolling The rolling reduction in cold rolling needs to be 65% or less. When the cold rolling reduction ratio exceeds 65%, the ratio of the crystal grains having an aspect ratio of 2.5 or less in the portion other than the ferrite phase in the steel sheet structure after finish annealing is less than 50%, and the residual stress after product processing is large. Because it becomes. Preferably it is 50% or less.
仕上焼鈍：一次焼鈍
仕上焼鈍は、一次焼鈍と二次焼鈍の２段階からなる。一次焼鈍は、７００〜９００℃の温度に加熱後、１５〜６００秒保持し、その後、５５０℃以下まで１０℃／秒以上の速度で冷却する必要がある。焼鈍温度が７００℃未満、もしくは保持時間が１５秒未満では、次に行われる二次焼鈍後の鋼板の、フェライト相以外の部分における結晶粒のアスペクト比２．５以下のものの割合が５０％未満となり、成形後の鋼板内部の残留応力が増大し、形状凍結性が低下するからである。一方、焼鈍温度が９００℃を超えると、二次焼鈍後の鋼板の、フェライト相以外の部分における結晶粒のアスペクト比が２．５以下のものの割合が９５％を超えるため、耐衝突特性が低下する。また、保持時間を６００秒超えとしても、フェライト相分率等の変化が小さく、材質もほとんど変化しなくなるため、６００秒以下とする。均熱後の冷却は、パーライト相が生成し、成形性が低下するのを防止するため、５５０℃以下まで１０℃／秒以上の速度で冷却する必要がある。なお、好ましい一次焼鈍の温度は７８０〜８８０℃の範囲である。
Finish annealing: Primary annealing Finish annealing consists of two stages: primary annealing and secondary annealing. In the primary annealing, it is necessary to heat at a temperature of 700 to 900 ° C., hold for 15 to 600 seconds, and then cool to 550 ° C. or less at a rate of 10 ° C./second or more. When the annealing temperature is less than 700 ° C. or the holding time is less than 15 seconds, the ratio of the crystal grains having an aspect ratio of 2.5 or less in the portion other than the ferrite phase of the steel sheet after the secondary annealing to be performed next is less than 50%. This is because the residual stress inside the steel sheet after forming increases and the shape freezing property decreases. On the other hand, when the annealing temperature exceeds 900 ° C., the ratio of the crystal grains having an aspect ratio of 2.5 or less in the portion other than the ferrite phase of the steel sheet after the secondary annealing exceeds 95%, so that the collision resistance is deteriorated. To do. Even when the holding time exceeds 600 seconds, the change in the ferrite phase fraction and the like is small and the material hardly changes. Cooling after soaking requires cooling at a rate of 10 ° C./second or more to 550 ° C. or less in order to prevent formation of a pearlite phase and deterioration of moldability. In addition, the temperature of a preferable primary annealing is the range of 780-880 degreeC.
仕上焼鈍：二次焼鈍
一次焼鈍に続く二次焼鈍は、７００〜９００℃の温度に再加熱し、１５〜６００秒保持し、その後、３５０〜５５０℃の温度まで冷却し、その温度域で３０秒以上保持することが必要である。再加熱温度が７００℃未満もしくは保持時間が１５秒未満では、残留オーステナイト相の分率が、体積分率で３％未満となるため、成形性が低下する。一方、再加熱温度が９００℃を超えると、フェライト相分率が２０％未満となるため、成形性が低下する。また、保持時間を６００秒超えとしても、フェライト相分率の変化が小さく材質はほとんど変化しない。二次焼鈍後は、残留オーステナイトを３％以上確保するため、３５０〜５５０℃の温度域で３０秒以上保持することが必要である。より成形性を向上させるためには、６０秒以上の保持が好ましい。なお、二次焼鈍の好ましい焼鈍温度は７２０〜８６０℃の範囲である。
Finish annealing: secondary annealing Secondary annealing following primary annealing is reheated to a temperature of 700 to 900 ° C., held for 15 to 600 seconds, then cooled to a temperature of 350 to 550 ° C., and 30 in that temperature range. It is necessary to hold for more than a second. If the reheating temperature is less than 700 ° C. or the holding time is less than 15 seconds, the fraction of the retained austenite phase is less than 3% in terms of volume fraction, so that the moldability is lowered. On the other hand, when the reheating temperature exceeds 900 ° C., the ferrite phase fraction becomes less than 20%, so that the moldability is lowered. Even when the holding time exceeds 600 seconds, the change in the ferrite phase fraction is small and the material hardly changes. After the secondary annealing, in order to secure 3% or more of retained austenite, it is necessary to hold at a temperature range of 350 to 550 ° C. for 30 seconds or more. In order to further improve the moldability, holding for 60 seconds or more is preferable. In addition, the preferable annealing temperature of secondary annealing is the range of 720-860 degreeC.
表１に示した化学成分を有する記号Ａ〜ＡＡの鋼を溶製し、連続鋳造して得た鋼スラブを、表２に示した各種条件で熱間圧延し、冷間圧延して、板厚が１．２ｍｍの冷延板とし、この冷延板を同じく表２に示した条件で仕上焼鈍し、次いで、圧下率０．３％の調質圧延を施してＮｏ．１〜４０の各種高強度鋼板を得た。これらの高強度鋼板から、供試材を採取し、以下の試験に供した。
＜鋼板組織調査＞
鋼板の圧延（Ｌ）方向断面を研摩し、３％ナイタール溶液で組織を現出させて、板厚の１／４位置を、走査型電子顕微鏡（ＳＥＭ）を用いて３０００倍で３視野の組織写真を撮影し、フェライト相以外の部分における結晶粒のアスペクト比（長辺／短辺）を測定した。また、それらの組織写真を画像処理して、フェライト相の分率を測定した。また、残留オーステナイト相の分率は、鋼板を板厚の１／４位置まで平面研削後、化学研磨によりさらに０．１ｍｍ研磨した面について、ＣｏのＫα線を用いたＸ線回折装置で、ｂｃｃ鉄の（２００）、（２１１）、（２２０）各面の回折強度に対するｆｃｃ鉄の（２００）、（２２０）、（３１１）各面の回折強度の比を求め、それらの平均値を残留オーステナイト相の体積率とした。
＜引張試験＞
上記供試材から、引張方向が圧延直角（Ｃ）方向となるようにＪＩＳ５号試験片を採取し、これをＪＩＳ Ｚ ２２４１に準拠して引張試験を行い、降伏応力ＹＳ、引張強度ＴＳおよび全伸びＥｌを測定し、降伏比ＹＲ（＝ＹＳ／ＴＳ）および強度−延性バランス（＝ＴＳ×Ｅｌ）を求めた。なお、特性の評価は、ＹＲ≧０．５０、ＴＳ×Ｅｌ≧１８０００（ＭＰａ・％）を良好と判定した。
＜残留応力評価指数δの測定＞
図１に示したように、板厚が１．２ｍｍの上記供試材からブランク径が６８ｍｍの鋼板を打ち抜き、径が３３ｍｍのポンチを用いて、ダイ肩Ｒが２．５ｍｍ、しわ押さえ力が１０ｋＮ、絞り速度が１０ｍｍ／ｍｉｎ、絞り比（ブランク径／ポンチ径）が２．０６の条件で、ダイ側を潤滑油およびテフロン（登録商標）で潤滑してカップ成形し、得られたカップ底部から高さ８ｍｍの位置の上部より、幅１０ｍｍのリング試料を放電加工により切り出し、鋼板の圧延方向に切れ目を入れて開口した。この開口前後における圧延方向に対して直角方向のリングの外径Ｄ０およびＤ１を測定し、前述した（１）式および（２）式から残留応力評価指数δを求めた。この際、ヤング率Ｅは２１０ＧＰａ（＝２１０×１０３ＭＰａ）、ポアソン比νは０．３を用いた。
Steel slabs obtained by melting and continuously casting steels of symbols A to AA having chemical components shown in Table 1 were hot-rolled under various conditions shown in Table 2, cold-rolled, and plate A cold-rolled sheet having a thickness of 1.2 mm was prepared, and the cold-rolled sheet was subjected to finish annealing under the conditions shown in Table 2 and then subjected to temper rolling with a rolling reduction of 0.3%. 1 to 40 various high strength steel plates were obtained. Sample materials were collected from these high-strength steel plates and subjected to the following tests.
<Steel sheet structure investigation>
The rolling (L) direction cross section of the steel sheet is polished, the structure is revealed with a 3% nital solution, and the 1/4 position of the sheet thickness is 3000 times as large as 3000 structures using a scanning electron microscope (SEM). A photograph was taken, and the aspect ratio (long side / short side) of the crystal grains in the portion other than the ferrite phase was measured. Moreover, those structural photographs were subjected to image processing, and the fraction of the ferrite phase was measured. Further, the fraction of retained austenite phase was determined by bcc with an X-ray diffractometer using Co Kα rays on a surface obtained by subjecting a steel sheet to surface grinding to ¼ position and further polished by 0.1 mm by chemical polishing. The ratio of the diffraction intensity of each surface of (200), (220), (311) of fcc iron to the diffraction intensity of each surface of (200), (211), (220) of iron is obtained, and the average value thereof is determined as retained austenite. The volume ratio of the phase was used.
<Tensile test>
A JIS No. 5 test piece was sampled from the above test material so that the tensile direction was the direction perpendicular to the rolling direction (C), and this was subjected to a tensile test according to JIS Z 2241, yield stress YS, tensile strength TS and total The elongation El was measured, and the yield ratio YR (= YS / TS) and the strength-ductility balance (= TS × El) were determined. In the evaluation of characteristics, it was determined that YR ≧ 0.50 and TS × E1 ≧ 18000 (MPa ·%) were good.
<Measurement of residual stress evaluation index δ>
As shown in FIG. 1, a steel plate with a blank diameter of 68 mm is punched out of the above test material with a plate thickness of 1.2 mm, a die shoulder R is 2.5 mm, and a wrinkle holding force is obtained using a punch with a diameter of 33 mm. The cup bottom obtained by cup molding by lubricating the die side with lubricating oil and Teflon (registered trademark) under the conditions of 10 kN, drawing speed 10 mm / min, drawing ratio (blank diameter / punch diameter) 2.06 A ring sample having a width of 10 mm was cut out from the upper portion at a height of 8 mm by electric discharge machining, and a cut was made in the rolling direction of the steel sheet and opened. The outer diameters D 0 and D 1 of the ring in the direction perpendicular to the rolling direction before and after the opening were measured, and the residual stress evaluation index δ was obtained from the above-described equations (1) and (2). At this time, Young's modulus E was 210 GPa (= 210 × 10 3 MPa), and Poisson's ratio ν was 0.3.
上記測定の結果を表３に示した。また、図２に、フェライト相以外の部分における結晶粒のアスペクト比が２．５以下のものの割合と残留応力評価指標δとの関係を示した。これらの結果から、本発明の要件を満たす鋼板は、フェライト相以外の部分における結晶粒のアスペクト比が２．５以下のものの割合が５０％以上で、残留応力評価指標δが０．３５以下となり、成形性に優れると同時に成形後の残留応力も小さいことがわかる。 The measurement results are shown in Table 3. FIG. 2 shows the relationship between the ratio of the crystal grains having an aspect ratio of 2.5 or less in the portion other than the ferrite phase and the residual stress evaluation index δ. From these results, in the steel sheet satisfying the requirements of the present invention, the ratio of the crystal grains having an aspect ratio of 2.5 or less in the portion other than the ferrite phase is 50% or more, and the residual stress evaluation index δ is 0.35 or less. It can be seen that the moldability is excellent and the residual stress after molding is small.
本発明の効果は、上述した本発明の鋼板組織を有するものであればいずれの鋼板においても得ることができる。 The effect of the present invention can be obtained in any steel sheet as long as it has the above-described steel sheet structure of the present invention.
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| JP5323563B2 (en) | 2009-03-31 | 2013-10-23 | 株式会社神戸製鋼所 | High-strength cold-rolled steel sheet with excellent workability and shape freezeability |
| CN101928875A (en) * | 2009-06-22 | 2010-12-29 | 鞍钢股份有限公司 | High-strength cold-rolled plate with favorable forming property and preparation method thereof |
| ES2705232T3 (en) | 2010-01-29 | 2019-03-22 | Nippon Steel & Sumitomo Metal Corp | Steel sheet and method for manufacturing steel sheet |
| CN103703157B (en) | 2011-07-29 | 2015-12-02 | 新日铁住金株式会社 | The high tensile steel plate of shape-holding property excellence, high strength galvanized steel plate and their manufacture method |
| JP5252142B1 (en) * | 2011-07-29 | 2013-07-31 | 新日鐵住金株式会社 | High-strength steel sheet excellent in formability, high-strength galvanized steel sheet, and production method thereof |
| CN102952998B (en) * | 2011-08-19 | 2015-05-06 | 鞍钢股份有限公司 | 800MPa-grade hot-rolling transformation induced plasticity steel plate and its manufacturing method |
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| CN105506476A (en) * | 2014-09-26 | 2016-04-20 | 鞍钢股份有限公司 | 600 MPa-grade highly-chambered steel plate for automotive chassis and manufacturing method thereof |
| US10954578B2 (en) * | 2014-10-30 | 2021-03-23 | Jfe Steel Corporation | High-strength steel sheet and method for manufacturing same |
| US10626485B2 (en) | 2015-02-17 | 2020-04-21 | Jfe Steel Corporation | Thin high-strength cold-rolled steel sheet and method of producing the same |
| BR112017017134A2 (en) * | 2015-02-24 | 2018-04-03 | Nippon Steel & Sumitomo Metal Corporation | cold rolled steel sheet and method of manufacturing it |
| CN104762599A (en) * | 2015-04-15 | 2015-07-08 | 京东方科技集团股份有限公司 | Vapor deposition method and vapor deposition device |
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