JP5504643B2 - High-strength hot-dip galvanized steel sheet excellent in workability and manufacturing method thereof - Google Patents
High-strength hot-dip galvanized steel sheet excellent in workability and manufacturing method thereof Download PDFInfo
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- 229910001335 Galvanized steel Inorganic materials 0.000 title claims description 23
- 239000008397 galvanized steel Substances 0.000 title claims description 23
- 238000004519 manufacturing process Methods 0.000 title claims description 11
- 229910000831 Steel Inorganic materials 0.000 claims description 47
- 239000010959 steel Substances 0.000 claims description 47
- 229910000734 martensite Inorganic materials 0.000 claims description 45
- 238000001816 cooling Methods 0.000 claims description 37
- 229910000859 α-Fe Inorganic materials 0.000 claims description 25
- 229910001566 austenite Inorganic materials 0.000 claims description 22
- 238000005246 galvanizing Methods 0.000 claims description 11
- 239000000203 mixture Substances 0.000 claims description 8
- 230000000717 retained effect Effects 0.000 claims description 8
- 230000009466 transformation Effects 0.000 claims description 8
- 239000012535 impurity Substances 0.000 claims description 6
- 238000009864 tensile test Methods 0.000 claims description 5
- 229910052750 molybdenum Inorganic materials 0.000 claims description 3
- 229910052720 vanadium Inorganic materials 0.000 claims description 3
- 229910052799 carbon Inorganic materials 0.000 claims description 2
- 229910052804 chromium Inorganic materials 0.000 claims description 2
- 229910052748 manganese Inorganic materials 0.000 claims description 2
- 229910052698 phosphorus Inorganic materials 0.000 claims description 2
- 230000000694 effects Effects 0.000 description 22
- 238000000137 annealing Methods 0.000 description 21
- 230000001965 increasing effect Effects 0.000 description 11
- 238000005097 cold rolling Methods 0.000 description 10
- 238000005096 rolling process Methods 0.000 description 9
- 238000005275 alloying Methods 0.000 description 7
- 238000000034 method Methods 0.000 description 7
- 229910045601 alloy Inorganic materials 0.000 description 6
- 239000000956 alloy Substances 0.000 description 6
- 230000008569 process Effects 0.000 description 6
- 238000005728 strengthening Methods 0.000 description 6
- XEEYBQQBJWHFJM-UHFFFAOYSA-N iron Substances [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 description 5
- 238000007747 plating Methods 0.000 description 5
- 229920006395 saturated elastomer Polymers 0.000 description 5
- 238000002791 soaking Methods 0.000 description 5
- 239000000126 substance Substances 0.000 description 5
- 230000015572 biosynthetic process Effects 0.000 description 4
- 238000005098 hot rolling Methods 0.000 description 4
- 238000003303 reheating Methods 0.000 description 4
- 238000005496 tempering Methods 0.000 description 4
- HCHKCACWOHOZIP-UHFFFAOYSA-N Zinc Chemical compound [Zn] HCHKCACWOHOZIP-UHFFFAOYSA-N 0.000 description 3
- 238000005266 casting Methods 0.000 description 3
- 230000007423 decrease Effects 0.000 description 3
- 238000010438 heat treatment Methods 0.000 description 3
- 229910001562 pearlite Inorganic materials 0.000 description 3
- 239000002244 precipitate Substances 0.000 description 3
- 238000012360 testing method Methods 0.000 description 3
- 229910052725 zinc Inorganic materials 0.000 description 3
- 239000011701 zinc Substances 0.000 description 3
- 230000002411 adverse Effects 0.000 description 2
- 238000005452 bending Methods 0.000 description 2
- 229910001567 cementite Inorganic materials 0.000 description 2
- 230000007797 corrosion Effects 0.000 description 2
- 238000005260 corrosion Methods 0.000 description 2
- 230000006866 deterioration Effects 0.000 description 2
- 239000000463 material Substances 0.000 description 2
- 229910052758 niobium Inorganic materials 0.000 description 2
- 238000005554 pickling Methods 0.000 description 2
- 238000010791 quenching Methods 0.000 description 2
- 230000000171 quenching effect Effects 0.000 description 2
- 230000009467 reduction Effects 0.000 description 2
- 238000005204 segregation Methods 0.000 description 2
- 239000006104 solid solution Substances 0.000 description 2
- 229910052718 tin Inorganic materials 0.000 description 2
- 229910000885 Dual-phase steel Inorganic materials 0.000 description 1
- 229910000794 TRIP steel Inorganic materials 0.000 description 1
- ATJFFYVFTNAWJD-UHFFFAOYSA-N Tin Chemical compound [Sn] ATJFFYVFTNAWJD-UHFFFAOYSA-N 0.000 description 1
- 238000002441 X-ray diffraction Methods 0.000 description 1
- 229910052787 antimony Inorganic materials 0.000 description 1
- 229910001563 bainite Inorganic materials 0.000 description 1
- 230000008901 benefit Effects 0.000 description 1
- 238000006243 chemical reaction Methods 0.000 description 1
- 239000010960 cold rolled steel Substances 0.000 description 1
- 239000002131 composite material Substances 0.000 description 1
- 238000007796 conventional method Methods 0.000 description 1
- 229910052802 copper Inorganic materials 0.000 description 1
- 230000003247 decreasing effect Effects 0.000 description 1
- 230000003111 delayed effect Effects 0.000 description 1
- 238000011161 development Methods 0.000 description 1
- 238000005516 engineering process Methods 0.000 description 1
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- 238000011156 evaluation Methods 0.000 description 1
- 230000002349 favourable effect Effects 0.000 description 1
- 239000000446 fuel Substances 0.000 description 1
- 238000010191 image analysis Methods 0.000 description 1
- 230000006872 improvement Effects 0.000 description 1
- 238000011835 investigation Methods 0.000 description 1
- 229910052742 iron Inorganic materials 0.000 description 1
- KSOKAHYVTMZFBJ-UHFFFAOYSA-N iron;methane Chemical compound C.[Fe].[Fe].[Fe] KSOKAHYVTMZFBJ-UHFFFAOYSA-N 0.000 description 1
- 239000007788 liquid Substances 0.000 description 1
- 238000002844 melting Methods 0.000 description 1
- 230000008018 melting Effects 0.000 description 1
- 239000002184 metal Substances 0.000 description 1
- 229910052751 metal Inorganic materials 0.000 description 1
- 238000000465 moulding Methods 0.000 description 1
- 229910052759 nickel Inorganic materials 0.000 description 1
- 230000006911 nucleation Effects 0.000 description 1
- 238000010899 nucleation Methods 0.000 description 1
- 238000005498 polishing Methods 0.000 description 1
- 229910001568 polygonal ferrite Inorganic materials 0.000 description 1
- 238000012545 processing Methods 0.000 description 1
- 238000011084 recovery Methods 0.000 description 1
- 238000001953 recrystallisation Methods 0.000 description 1
- 230000004044 response Effects 0.000 description 1
- 229910052721 tungsten Inorganic materials 0.000 description 1
- 238000003466 welding Methods 0.000 description 1
- 238000004804 winding Methods 0.000 description 1
- 229910052726 zirconium Inorganic materials 0.000 description 1
Description
本発明は、自動車、電機等の産業分野で使用される加工性に優れた高強度溶融亜鉛めっき鋼板およびその製造方法に関する。 The present invention relates to a high-strength hot-dip galvanized steel sheet excellent in workability used in industrial fields such as automobiles and electric machines, and a method for producing the same.
近年、地球環境保全の見地から、自動車の燃費向上が重要な課題となっている。このため、車体材料の高強度化により薄肉化を図り、車体そのものを軽量化しようとする動きが活発である。しかしながら、鋼板の高強度化は成形加工性の低下を招くことから、高強度と高加工性を併せ持つ材料の開発が望まれている。 In recent years, improving the fuel efficiency of automobiles has become an important issue from the viewpoint of global environmental conservation. For this reason, efforts are being made to reduce the thickness of the vehicle body by increasing the strength of the vehicle body material and to reduce the weight of the vehicle body itself. However, the development of a material having both high strength and high workability is desired since the increase in strength of the steel sheet causes a decrease in forming workability.
このような要求に対して、これまでフェライト−マルテンサイト二相鋼（Dual-Phase（ＤＰ）鋼）や残留オーステナイトの変態誘起塑性を利用したＴＲＩＰ鋼など、種々の複合組織鋼板が開発されてきた。例えば、特許文献１では、化学成分および鋼板中の残留オーステナイト量を制御することによるプレス成形性に優れた鋼板が開示されている。また、特許文献２では、化学成分と焼鈍温度からの冷却条件を制御し、マルテンサイトおよび残留オーステナイトの存在位置および体積率と最大径を規定することでプレス加工性に優れた鋼板が得られることが開示されている。特許文献３では、化学成分の規定と焼鈍工程における急速冷却と焼戻し熱処理とによりプレス成形性に優れたＤＰ組織鋼板が得られることが開示されている。さらに、特許文献４では、焼鈍工程においてＭＳ点以下までの急速冷却とその後の再加熱処理によりプレス成形性に優れたＤＰ組織鋼板が得られることが開示されている。 In response to such demands, various steel sheets with various microstructures have been developed, such as ferritic-martensitic dual-phase steel (Dual-Phase (DP) steel) and TRIP steel utilizing transformation-induced plasticity of retained austenite. . For example, Patent Document 1 discloses a steel sheet excellent in press formability by controlling the chemical components and the amount of retained austenite in the steel sheet. Moreover, in patent document 2, the cooling conditions from a chemical component and annealing temperature are controlled, and the steel plate excellent in press workability is obtained by prescribing the location, volume ratio, and maximum diameter of martensite and retained austenite. Is disclosed. Patent Document 3 discloses that a DP-structure steel sheet having excellent press formability can be obtained by defining chemical components, rapid cooling in an annealing process, and tempering heat treatment. Furthermore, Patent Document 4 discloses that a DP-structure steel sheet having excellent press formability can be obtained by rapid cooling to the MS point or lower and subsequent reheating treatment in the annealing process.
しかしながら、これら従来技術は、その大半が延性の向上を図るために開発されたものであり、高強度鋼板の成形時における重要な加工性である伸びフランジ性や曲げ性とのバランスの取れた成形性の確保に対しては十分な考慮がなされていない。また、考慮されている場合においても、その効果は十分とはいえなかった。 However, most of these conventional technologies were developed to improve ductility, and forming with a balance with stretch flangeability and bendability, which are important workability when forming high-strength steel sheets. Sufficient consideration has not been given to ensuring sex. Moreover, even when considered, the effect was not sufficient.
例えば、特許文献１では、ＴＲＩＰ効果の活用により延性は十分に得られるものの、伸びフランジ性に関してはフェライト−マルテンサイト二相鋼よりも劣る。特許文献２では、伸びフランジ性については全く考慮されていない。冷却中に生成したマルテンサイトは硬質であると推測されるため、伸びフランジ性が劣位であることが予想される。特許文献３では焼入れにより生成されたマルテンサイトを焼戻すことでマルテンサイトを軟質化し、フェライトとの硬度差を低減することで伸びフランジ性を改善しているが、一旦室温近くまで水冷（ＷＱ）によって急速冷却し、再加熱を実施しており、特殊な設備が必要である。特許文献４ではＭＳ点以下まで、ＬｎＣＲ＝１．１Ｍｎｅｑ＋１．８７（ただし、Ｍｎｅｑ＝Ｍｎ＋１．５２Ｍｏ＋１．１０Ｃｒ＋１．４１Ｖ＋１００Ｂ）の式で示される臨界冷却速度ＣＲ（℃／ｓ）以上の冷却速度にて冷却後、再加熱処理を施すことでマルテンサイトを軟化し、伸びフランジ性を改善している。しかしながら、一般的な連続溶融亜鉛めっきラインでは、鋼板を亜鉛浴に入れる際、鋼板温度は亜鉛浴の温度以上でなければならず、ＭＳ点以下まで冷却する場合には再加熱処理が可能な特殊設備が必要である。 For example, in Patent Document 1, although the ductility can be sufficiently obtained by utilizing the TRIP effect, the stretch flangeability is inferior to that of the ferrite-martensite duplex steel. In Patent Document 2, stretch flangeability is not considered at all. Since the martensite produced during cooling is assumed to be hard, it is expected that the stretch flangeability is inferior. In Patent Document 3, martensite is softened by tempering martensite generated by quenching, and stretch flangeability is improved by reducing the hardness difference from ferrite. However, it is once water-cooled to near room temperature (WQ). It is rapidly cooled and reheated by a special equipment. In Patent Document 4, cooling is performed at a cooling rate equal to or higher than the critical cooling rate CR (° C./s) represented by the formula LnCR = 1.1 Mneq + 1.87 (where Mneq = Mn + 1.52Mo + 1.10Cr + 1.41V + 100B) to the MS point or lower. Later, by reheating, the martensite is softened and the stretch flangeability is improved. However, in a general continuous hot dip galvanizing line, when a steel sheet is put into a zinc bath, the steel sheet temperature must be higher than the temperature of the zinc bath, and when it is cooled below the MS point, it can be reheated. Equipment is required.
実際のプレス成形等において、優れた成形性を確保するためには、延性に優れるのみでなく伸びフランジ性とのバランスが非常に重要となる。しかしながら、上述したように従来技術ではこれらの両立が十分ではないか、または、両立するために冷却後に冷却停止温度以上に再加熱可能な特殊な装置を具備している必要がある。 In actual press molding or the like, in order to ensure excellent formability, it is very important to balance not only excellent ductility but also stretch flangeability. However, as described above, in the conventional technique, both of these are not sufficient, or in order to achieve both, it is necessary to have a special device that can be reheated to a cooling stop temperature or higher after cooling.
本発明は、かかる事情に鑑みてなされたものであって、冷却後に再加熱処理を行わずに良好な延性および伸びフランジ性を得ることができる、優れた加工性を有する高強度溶融亜鉛めっき鋼板およびその製造方法を提供することを目的とする。 The present invention has been made in view of such circumstances, and is a high-strength hot-dip galvanized steel sheet having excellent workability capable of obtaining good ductility and stretch flangeability without performing reheating treatment after cooling. And it aims at providing the manufacturing method.
上記課題を解決するため、本発明は以下の（１）〜（８）を提供する。
（１）質量％で、Ｃ：０．０３〜０．１７％、Ｓｉ：０．０１〜０．７５％、Ｍｎ：１．５〜２．５％、Ｐ：０．０８０％以下、Ｓ：０．０１０％以下、ｓｏｌ．Ａｌ：０．０１〜１．２０％、Ｃｒ：０．３〜１．３％を含有し、残部がＦｅおよび不可避不純物からなり、鋼組織が、体積率で３０〜７０％のフェライト、３％未満の残留オーステナイト、および残部のマルテンサイトからなり、マルテンサイトのうちの２０％以上が焼戻しマルテンサイトである下地鋼板上に溶融亜鉛めっき層を有することを特徴とする加工性に優れた高強度溶融亜鉛めっき鋼板。
（２）質量％でＰ：０．０４０％以下を含有することを特徴とする（１）に記載の加工性に優れた高強度溶融亜鉛めっき鋼板。
（３）引張試験における局部伸びの均一伸びに対する比が０．８〜１．２であることを特徴とする（１）または（２）に記載の加工性に優れた高強度溶融亜鉛めっき鋼板。
（４）質量％で、Ｎｂ：０．００５〜０．０５％、Ｖ：０．００５〜０．０５％、Ｔｉ：０．００５〜０．０５％のうちの１種または２種以上をさらに含有することを特徴とする（１）から（３）のいずれかに記載の加工性に優れた高強度溶融亜鉛めっき鋼板。
（５）質量％で、Ｂ：０．０００５〜０．００３％、Ｍｏ：０．０１〜０．１５％のうちの１種または２種をさらに含有することを特徴とする（１）〜（４）のいずれかに記載の加工性に優れた高強度溶融亜鉛めっき鋼板。
（６）上記（１）〜（５）のいずれかに記載の成分組成を有する鋼板を、７００〜９００℃の温度域で５０〜５００秒間保持し、引き続いて溶融亜鉛めっき浴に浸漬し冷却するにあたり、オーステナイトからマルテンサイトへの変態を開始する温度をＭＳ（℃）とした場合に、ＭＳ〜ＭＳ−１００℃の温度範囲における平均冷却速度が５℃／ｓ以下であることを特徴とする加工性に優れた高強度溶融亜鉛めっき鋼板の製造方法。
（７）上記（１）〜（５）のいずれかに記載の成分組成を有する鋼板を、７００〜９００℃の温度域で５０〜５００秒間保持し、引き続いて溶融亜鉛めっき浴に浸漬し、その後合金化処理を施した後、冷却するにあたり、オーステナイトからマルテンサイトへの変態を開始する温度をＭＳ（℃）とした場合に、ＭＳ〜ＭＳ−１００℃の温度範囲における平均冷却速度が５℃／ｓ以下であることを特徴とする加工性に優れた高強度溶融亜鉛めっき鋼板の製造方法。
（８）ＭＳ〜ＭＳ−１００℃の温度範囲における平均冷却速度が３℃／ｓ以下であることを特徴とする（６）または（７）に記載の加工性に優れた高強度溶融亜鉛めっき鋼板の製造方法。
In order to solve the above problems, the present invention provides the following (1) to (8).
(1) By mass%, C: 0.03-0.17%, Si: 0.01-0.75%, Mn: 1.5-2.5%, P: 0.080% or less, S: 0.010% or less, sol. Al: 0.01 to 1.20%, Cr: 0.3 to 1.3%, the balance is Fe and inevitable impurities, the steel structure is 30 to 70% ferrite by volume, 3% Fewer than less retained austenite and the remaining martensite, 20% or more of the martensite has a hot-dip galvanized layer on the base steel plate that is tempered martensite, and has high workability with excellent workability Galvanized steel sheet.
(2) The high-strength hot-dip galvanized steel sheet with excellent workability as described in (1), which contains P: 0.040% or less by mass%.
(3) The high-strength hot-dip galvanized steel sheet having excellent workability as described in (1) or (2), wherein the ratio of local elongation to uniform elongation in a tensile test is 0.8 to 1.2.
(4) By mass%, Nb: 0.005-0.05%, V: 0.005-0.05%, Ti: 0.005-0.05% A high-strength hot-dip galvanized steel sheet excellent in workability according to any one of (1) to (3), characterized by comprising.
(5) In mass%, B: 0.0005-0.003%, Mo: 0.01-0.15% of 1 type or 2 types are further contained, (1)-( 4) A high-strength hot-dip galvanized steel sheet excellent in workability.
(6) A steel plate having the component composition described in any one of (1) to (5) above is held at a temperature range of 700 to 900 ° C. for 50 to 500 seconds, and subsequently immersed in a hot dip galvanizing bath and cooled. In this case, when the temperature at which transformation from austenite to martensite is started is MS (° C.), the average cooling rate in the temperature range of MS to MS-100 ° C. is 5 ° C./s or less. For producing high-strength hot-dip galvanized steel sheets with excellent properties.
(7) The steel plate having the component composition according to any one of (1) to (5) above is held at a temperature range of 700 to 900 ° C. for 50 to 500 seconds, and subsequently immersed in a hot dip galvanizing bath. When the temperature at which transformation from austenite to martensite is started is MS (° C.) when cooling after the alloying treatment, the average cooling rate in the temperature range of MS to MS-100 ° C. is 5 ° C. / The manufacturing method of the high intensity | strength hot-dip galvanized steel plate excellent in workability characterized by being below s.
(8) High-strength hot-dip galvanized steel sheet with excellent workability as described in (6) or (7), wherein the average cooling rate in the temperature range of MS to MS-100 ° C is 3 ° C / s or less Manufacturing method.
本発明によれば、冷却後に再加熱処理を行わずに良好な延性および伸びフランジ性を得ることができる、優れた加工性を有する高強度溶融亜鉛めっき鋼板およびその製造方法が提供される。このため、従来、高強度鋼板の適用が困難であった例えば自動車構造部材等の難成形の部材として適用することが可能となる。さらに、自動車構造部材として本発明の高強度鋼板を用いた場合、自動車の軽量化、安全性向上などに寄与し、産業上極めて有益である。 ADVANTAGE OF THE INVENTION According to this invention, the high strength hot-dip galvanized steel plate which has the outstanding workability which can obtain favorable ductility and stretch flangeability, without performing a reheating process after cooling, and its manufacturing method are provided. For this reason, it becomes possible to apply as a difficult-to-form member such as an automobile structural member, which has heretofore been difficult to apply a high-strength steel sheet. Furthermore, when the high-strength steel sheet of the present invention is used as an automobile structural member, it contributes to reducing the weight of the automobile, improving safety, and the like, which is extremely useful industrially.
以下、本発明について詳細に説明する。
まず、本発明の溶融亜鉛めっき鋼板の下地鋼板の成分組成について説明する。以下の説明において、特にことわらない限り、％表示は質量％である。
Hereinafter, the present invention will be described in detail.
First, the component composition of the base steel sheet of the hot dip galvanized steel sheet of the present invention will be described. In the following description, unless otherwise specified, the% display is mass%.
Ｃ：０．０３〜０．１７％
Ｃは、鋼の強化および焼入れ性を向上させるためには重要な元素であり、フェライトとマルテンサイトおよびベイナイト等からなる複合組織を得るのに不可欠である。引張強度（ＴＳ）７５０ＭＰａ以上を得るために０．０３％以上必要とする。一方、含有量が多くなると、セメンタイトなどの鉄系炭化物の粗大化が起こりやすくなって局部成形性が劣化するばかりか、溶接後の硬さ上昇が著しくなる。このため、Ｃ含有量の上限を０．１７％とする。
C: 0.03-0.17%
C is an important element for improving the strengthening and hardenability of steel, and is indispensable for obtaining a composite structure composed of ferrite, martensite, bainite and the like. In order to obtain a tensile strength (TS) of 750 MPa or more, 0.03% or more is required. On the other hand, when the content is increased, iron carbide such as cementite is likely to be coarsened and local formability is deteriorated, and the hardness after welding is remarkably increased. For this reason, the upper limit of the C content is set to 0.17%.
Ｓｉ：０．０１〜０．７５％
Ｓｉは、鋼の加工性を低下させることなく強度を上昇させるのに好ましい元素である。しかし、その含有量が０．０１％未満では、伸びフランジ性に有害なパーライト組織を形成しやすくなる上、フェライトの固溶強化能の低下で、形成される組織間の硬度差が大きくなり、伸びフランジ性の劣化を招く。一方、Ｓｉ含有量が０．７５％を超えると、鋼板表面に生成するＳｉ酸化物によりめっき性の低下を生じる。このため、Ｓｉ含有量を０．０１〜０．７５％とする。
Si: 0.01 to 0.75%
Si is a preferable element for increasing the strength without decreasing the workability of the steel. However, if its content is less than 0.01%, it becomes easy to form a pearlite structure that is harmful to stretch flangeability, and the solid solution strengthening ability of ferrite decreases, resulting in a large difference in hardness between the formed structures. It causes deterioration of stretch flangeability. On the other hand, if the Si content exceeds 0.75%, the plating property is lowered by the Si oxide generated on the steel sheet surface. For this reason, Si content shall be 0.01 to 0.75%.
Ｍｎ：１．５〜２．５％
Ｍｎは、鋼の強化に有効であることに加え、焼入れ性強化に有効な元素である。Ｍｎ含有量が１．５％未満では強度が不十分なものとなるばかりか、焼入れ性の低下により、焼鈍後の冷却中に延性を劣化させるパーライトが形成されやすくなる。また、Ｍｎ含有量が２．５％を超えると、溶製された鋼をスラブに鋳造する際、スラブ表面やコーナー部に割れが生じやすくなる。さらに、鋳造時にＭｎ偏析が発生しやすく、焼鈍後の組織においてもこの偏析に起因したバンド状組織が発達し、焼鈍工程以降もこのバンド状組織が残存し、伸びフランジ性に悪影響を及ぼす可能性がある。このため、Ｍｎ含有量を１．５〜２．５％の範囲とする。また、熱間圧延および冷間圧延荷重が高くなることを防止する観点からは２．３％以下が好ましい。
Mn: 1.5 to 2.5%
Mn is an element effective for strengthening hardenability in addition to being effective for strengthening steel. If the Mn content is less than 1.5%, not only the strength becomes insufficient, but also the pearlite that deteriorates the ductility during cooling after annealing is likely to be formed due to a decrease in hardenability. On the other hand, if the Mn content exceeds 2.5%, when the molten steel is cast into a slab, cracks are likely to occur on the slab surface and corner portions. Furthermore, Mn segregation is likely to occur during casting, and a band-like structure resulting from this segregation develops even in the structure after annealing, and this band-like structure may remain after the annealing process, which may adversely affect stretch flangeability. There is. For this reason, Mn content is taken as 1.5 to 2.5% of range. Further, from the viewpoint of preventing an increase in hot rolling and cold rolling load, 2.3% or less is preferable.
Ｐ：０．０８０％以下
Ｐは固溶強化元素としてフェライトの強度を上昇させるのに好ましい元素であるが、加工性およびめっき密着性の点では低いほうが好ましく、そのため本発明では０．０８０％以下に限定する。好ましくは０．０４０％以下であり、さらに好ましくは０．０１５％以下である。
P: 0.080% or less P is a preferable element for increasing the strength of ferrite as a solid solution strengthening element. However, it is preferably lower in terms of workability and plating adhesion. Therefore, in the present invention, 0.080% or less. Limited to. Preferably it is 0.040% or less, More preferably, it is 0.015% or less.
Ｓ：０．０１０％以下
Ｓは鋼の延性を著しく劣化させるため、できるだけ少ない方が好ましく、そのため本発明では０．０１０％以下に限定する。好ましくは０．００５％以下である。
S: 0.010% or less Since S significantly deteriorates the ductility of steel, it is preferably as small as possible. Therefore, in the present invention, it is limited to 0.010% or less. Preferably it is 0.005% or less.
ｓｏｌ．Ａｌ：０．０１〜１．２０％
Ａｌは鋼の脱酸のために使用される。このような効果を有効に発揮させるためには、ｓｏｌ．Ａｌとして０．０１％添加する必要がある。また、炭化物の生成を抑制する効果や、Ａｃ３点を大幅に高める効果もあり、この効果はＤＰ組織を形成する上で有効である。ただし、ｓｏｌ．Ａｌが１．２０％を超えてもＡｌ添加の効果が飽和し、不経済となる。このため、ｓｏｌ．Ａｌを０．０１〜１．２０％とする。
sol. Al: 0.01-1.20%
Al is used for deoxidation of steel. In order to effectively exhibit such an effect, sol. It is necessary to add 0.01% as Al. In addition, there is an effect of suppressing the generation of carbide and an effect of greatly increasing the Ac 3 point, and this effect is effective in forming the DP structure. However, sol. Even if Al exceeds 1.20%, the effect of addition of Al is saturated, which is uneconomical. For this reason, sol. Al is 0.01 to 1.20%.
Ｃｒ：０．３〜１．３％
Ｃｒは本発明において重要な元素であり、焼入れ性を高める効果がある他、焼鈍時に所定の温度域を所定の冷却速度で冷却することにより、軟質なマルテンサイト相形成に寄与する効果を有する。このような効果を得るためには、Ｃｒ含有量を０．３％以上とすることが必要である。一方、１．３％を超えて含有すると、合金コストが増加する。このため、Ｃｒ含有量の範囲を０．３〜１．３％とする。好ましくは、０．５〜０．８％である。
Cr: 0.3 to 1.3%
Cr is an important element in the present invention and has the effect of enhancing the hardenability, and also has the effect of contributing to the formation of a soft martensite phase by cooling a predetermined temperature range at a predetermined cooling rate during annealing. In order to obtain such an effect, the Cr content needs to be 0.3% or more. On the other hand, if the content exceeds 1.3%, the alloy cost increases. For this reason, the range of Cr content shall be 0.3 to 1.3%. Preferably, it is 0.5 to 0.8%.
Ｎｂ：０．００５〜０．０５％、Ｖ：０．００５〜０．０５％、Ｔｉ：０．００５〜０．０５％のうちの１種または２種以上
Ｎｂは、微細な炭窒化物を形成し、再結晶フェライトの粒成長抑制、焼鈍時のオーステナイト核生成サイトの増加を期待することができ、結果として焼鈍後の延性を向上させることができるため、必要に応じて添加する。このような効果を得るためには０．００５％以上含有することが好ましい。一方、０．０５％を超えて含有すると、炭窒化物が多く析出し、この析出物が延性を劣化させる。さらに、合金コストの増加を招くばかりか熱間圧延および冷間圧延負荷が増大し、圧延能率が低下し製造コストが増加する問題もある。したがって、Ｎｂ含有量を０．００５〜０．０５％とする。好ましくは０．０１〜０．０３％である。
Ｖは、焼入れ性を高める効果があるため、必要に応じて添加する。この効果は０．００５％以上で発現するが、０．０５％を超えるとこの効果が飽和するばかりか、合金コストの増加を招く。したがって、Ｖ含有量を０．００５〜０．０５％とする。好ましくは０．０１〜０．０３％である。
Ｔｉは、鋳造時の表面割れの原因であるＡｌＮ生成を抑制し、ＮをＴｉＮとして固定するため、必要に応じて添加する。この効果は０．００５％以上で発現するが、０．０５％を超えると焼鈍後の延性が著しく劣化する。したがって、Ｔｉ含有量を０．００５〜０．０５％とする。好ましくは０．０１〜０．０３％である。
One or more of Nb: 0.005 to 0.05%, V: 0.005 to 0.05%, Ti: 0.005 to 0.05% Nb is a fine carbonitride. It is possible to suppress the grain growth of the recrystallized ferrite and increase the austenite nucleation site during annealing, and as a result, the ductility after annealing can be improved. In order to acquire such an effect, it is preferable to contain 0.005% or more. On the other hand, if the content exceeds 0.05%, a large amount of carbonitride precipitates, and this precipitate deteriorates ductility. Furthermore, there is a problem that not only the alloy cost is increased but also the hot rolling and cold rolling loads are increased, the rolling efficiency is lowered, and the manufacturing cost is increased. Therefore, the Nb content is set to 0.005 to 0.05%. Preferably it is 0.01 to 0.03%.
V has an effect of improving hardenability, and is added as necessary. This effect is manifested at 0.005% or more, but if it exceeds 0.05%, this effect is saturated and the alloy cost is increased. Therefore, the V content is set to 0.005 to 0.05%. Preferably it is 0.01 to 0.03%.
Ti is added as necessary in order to suppress the formation of AlN that causes surface cracks during casting and to fix N as TiN. This effect is manifested at 0.005% or more, but if it exceeds 0.05%, the ductility after annealing is significantly deteriorated. Therefore, the Ti content is set to 0.005 to 0.05%. Preferably it is 0.01 to 0.03%.
Ｂ：０．０００５〜０．００３％、Ｍｏ：０．０１〜０．１５％のうちの１種または２種
Ｂは、オーステナイトからフェライトへの変態を抑制し、硬質なマルテンサイトの生成を促進し、鋼板の強度上昇に寄与するため、必要に応じて添加する。このような効果は０．０００５％以上で発現する。しかし、０．００３％を超えると、焼入れ向上効果が飽和するばかりか、鋼板表面においてＢの酸化物形成により、化成処理や溶融亜鉛めっき性を悪化させる。したがって、Ｂ含有量を０．０００５〜０．００３％とする。好ましくは０．０００７〜０．００２％である。
Ｍｏは、焼き入れ性強化に有効な元素であり、フェライト、パーライト変態のノーズを長時間側に移行させるため、焼鈍後冷却中のマルテンサイト生成に有効な元素であり、必要に応じて添加する。その効果を得るためには０．０１％以上含有させる必要があるが、０．１５％を超えるとその効果は飽和し、さらに合金コストが増加する。したがって、Ｍｏ含有量を０．０１〜０．１５％とする。
B: 0.0005 to 0.003%, Mo: One or two of 0.01 to 0.15% B suppresses transformation from austenite to ferrite and promotes the formation of hard martensite In order to contribute to an increase in the strength of the steel sheet, it is added as necessary. Such an effect appears at 0.0005% or more. However, if it exceeds 0.003%, not only the quenching improvement effect is saturated, but also the chemical conversion treatment and hot dip galvanizing property are deteriorated by the formation of B oxide on the steel sheet surface. Therefore, the B content is set to 0.0005 to 0.003%. Preferably it is 0.0007 to 0.002%.
Mo is an element effective for strengthening hardenability, and is an element effective for martensite generation during cooling after annealing in order to shift the nose of ferrite and pearlite transformation to a long time side, and is added as necessary. . In order to acquire the effect, it is necessary to contain 0.01% or more, but when it exceeds 0.15%, the effect is saturated and the alloy cost further increases. Therefore, the Mo content is set to 0.01 to 0.15%.
なお、上記以外の残部はＦｅおよび不可避的不純物からなる。不可避的不純物として、例えば、Ｏは非金属介在物を形成し、品質に悪影響をおよぼすため、０．００３％以下に低減するのが望ましい。また、他の不純物としてＣｕ、Ｎｉ、Ｗ、Ｚｒ、Ｓｎ、Ｓｂ等を挙げることができ、これらは通常、不純物として許容される範囲で含有してもよい。 The balance other than the above consists of Fe and inevitable impurities. As an unavoidable impurity, for example, O forms non-metallic inclusions and adversely affects the quality, so it is desirable to reduce it to 0.003% or less. Moreover, Cu, Ni, W, Zr, Sn, Sb etc. can be mentioned as another impurity, These may be contained in the range normally accept | permitted as an impurity.
次に、本発明の鋼板の金属組織について説明する。
本発明では、引張強度（ＴＳ）７５０ＭＰａ以上、かつ、高延性を達成するために、フェライトとマルテンサイトを主相とする。具体的には、体積率で３０〜７０％のフェライト、３％未満の残留オーステナイト、および残部のマルテンサイトからなり、マルテンサイトのうちの２０％以上が焼戻しマルテンサイトである。ここでいうフェライトは、ポリゴナルフェライト、ベイニティックフェライトを指す。
Next, the metal structure of the steel sheet of the present invention will be described.
In the present invention, the tensile strength (TS) is 750 MPa or more, and ferrite and martensite are used as main phases in order to achieve high ductility. Specifically, it comprises 30 to 70% ferrite by volume, less than 3% retained austenite, and the remaining martensite, and 20% or more of the martensite is tempered martensite. The ferrite here refers to polygonal ferrite and bainitic ferrite.
フェライト体積率：３０〜７０％
延性を確保する観点から、フェライト体積率は３０％以上とする。また、引張強度を７５０ＭＰａ以上とするには、フェライト体積率は７０％以下であることが必要である。したがって、フェライト体積率を３０〜７０％とする。
Ferrite volume ratio: 30-70%
From the viewpoint of ensuring ductility, the ferrite volume fraction is set to 30% or more. Moreover, in order to make the tensile strength 750 MPa or more, the ferrite volume fraction needs to be 70% or less. Therefore, the ferrite volume fraction is set to 30 to 70%.
残留オーステナイト体積率：３％未満
鋼板組織中にオーステナイトが残存すると二次加工脆性や遅れ破壊特性が悪化するため、残留オーステナイトは少ないことが望ましい、残留オーステナイト体積率が３％未満の場合には、これらの悪化の程度は少なく許容できる範囲であるから、残留オーステナイト体積率を３％未満とする。好ましくは１％以下である。
Residual austenite volume fraction: less than 3% When austenite remains in the steel sheet structure, secondary work brittleness and delayed fracture characteristics deteriorate, so it is desirable that the retained austenite content is small. Since the extent of these deteriorations is small and acceptable, the residual austenite volume fraction is set to less than 3%. Preferably it is 1% or less.
マルテンサイト：残部
焼戻しマルテンサイト：マルテンサイトの２０％以上
引張強度７５０ＭＰａ以上を確保する観点から、残部をマルテンサイトとする。７５０ＭＰａ以上の引張強度と優れた加工性を両立するためには、マルテンサイトは焼戻しされたものと焼戻しされていないものが共存する必要がある。そして、優れた伸びフランジ性を達成するためには、マルテンサイト全体のうち２０％以上は焼戻しマルテンサイトとする。焼戻しマルテンサイトが２０％未満では伸びフランジ性向上効果および曲げ性向上効果が発現しない。焼戻しマルテンサイトが９０％を超えると、以下に述べる局部伸びの均一伸びに対する比が１．２より高くなり張出し性が低下する。そのため、焼戻しマルテンサイトはマルテンサイトの３０％以上９０％以下が好ましい。ここで、「焼戻し」とは、マルテンサイト中に過飽和に固溶していたＣの一部が炭化物として析出する現象を指す。
Martensite: Remainder Tempered martensite: 20% or more of martensite From the viewpoint of securing a tensile strength of 750 MPa or more, the remainder is martensite. In order to achieve both a tensile strength of 750 MPa or more and excellent workability, martensite must be tempered and non-tempered. In order to achieve excellent stretch flangeability, 20% or more of the entire martensite is tempered martensite. If the tempered martensite is less than 20%, the stretch flangeability improving effect and the bendability improving effect are not exhibited. If the tempered martensite exceeds 90%, the ratio of the local elongation to the uniform elongation described below is higher than 1.2, and the stretchability is lowered. Therefore, tempered martensite is preferably 30% or more and 90% or less of martensite. Here, “tempering” refers to a phenomenon in which a part of C, which is supersaturated in martensite, precipitates as carbide.
本発明において、引張試験における局部伸びの均一伸びに対する比が０．８〜１．２であることが好ましい。伸びフランジ性は局部伸びとよい相関があり、優れた伸びフランジ性を得るためには、引張試験における局部伸びの均一伸びに対する比は０．８以上が好ましい。また、０．８未満の場合には曲げ性も低下する。一方、局部伸びの均一伸びに対する比が１．２より高い場合では、張出し性が低下する。 In the present invention, the ratio of local elongation to uniform elongation in the tensile test is preferably 0.8 to 1.2. Stretch flangeability has a good correlation with local elongation, and in order to obtain excellent stretch flangeability, the ratio of local elongation to uniform elongation in a tensile test is preferably 0.8 or more. If it is less than 0.8, the bendability is also lowered. On the other hand, when the ratio of the local elongation to the uniform elongation is higher than 1.2, the overhanging property is lowered.
本発明においては、以上の下地鋼板上に溶融亜鉛めっき層を形成して溶融亜鉛めっき鋼板とする。その目付量は、要求される耐食性の程度により適宜決定すればよく、特に限定されないが、自動車構造部材に使用される鋼板では、３０〜６０ｇ／ｍ２の範囲が好ましい。また、上述したように溶融亜鉛めっき層は溶融亜鉛めっき処理後、必要に応じて合金化処理を施した合金化溶融亜鉛めっき層であってもよい。 In the present invention, a hot-dip galvanized steel sheet is formed on the above base steel sheet. The basis weight may be appropriately determined depending on the required degree of corrosion resistance, and is not particularly limited. However, in the case of a steel sheet used for an automobile structural member, a range of 30 to 60 g / m 2 is preferable. Further, as described above, the hot-dip galvanized layer may be an alloyed hot-dip galvanized layer subjected to an alloying treatment as necessary after the hot-dip galvanizing treatment.
次に、本発明の溶融亜鉛めっき鋼板の製造工程について説明する。
本発明では、上述の成分組成の鋼を溶製し、鋳造後、熱間圧延する。熱間圧延は、鋳造後直ちに行ってもよいし、一旦冷却し、再び加熱してから行ってもよい。仕上げ圧延終了温度は８００℃以上が好ましい。仕上げ圧延終了温度が８００℃未満の場合、圧延荷重負荷が増大するばかりでなく、最終圧延の段階で二相組織となり、フェライト粒の著しい粗大化が起こり、冷延、焼鈍を行っても加工性の良い鋼板が得られない場合がある。巻取り温度は冷間圧延時の負荷や酸洗性の観点から４００〜７００℃が好ましい。
Next, the manufacturing process of the hot dip galvanized steel sheet of the present invention will be described.
In the present invention, steel having the above-described composition is melted and cast and then hot rolled. Hot rolling may be performed immediately after casting, or may be performed after cooling and heating again. The finish rolling finish temperature is preferably 800 ° C. or higher. When the finish rolling finish temperature is less than 800 ° C., not only the rolling load is increased, but also a two-phase structure is formed at the final rolling stage, and the ferrite grains are markedly coarsened. May not be obtained. The coiling temperature is preferably 400 to 700 ° C. from the viewpoint of load during cold rolling and pickling properties.
次いで、冷間圧延を施す。冷間圧下率は、所望とする冷延板の板厚に応じ、適宜決定することができる。しかし、冷間圧延率が３０％未満になる場合には、冷延板に導入される歪が少なく、焼鈍時のフェライトの再結晶粒が大きくなり、延性が低下する。そのため、冷間圧下率は３０％以上とするのが好ましい。なお、冷間圧延を施す前に、酸洗を行い、熱延鋼板の表面に形成されているスケールを除去することが好ましい。 Next, cold rolling is performed. The cold rolling reduction can be appropriately determined according to the desired thickness of the cold rolled sheet. However, when the cold rolling rate is less than 30%, the strain introduced into the cold-rolled sheet is small, the recrystallized grains of ferrite at the time of annealing become large, and the ductility is lowered. Therefore, the cold rolling reduction is preferably 30% or more. In addition, before performing cold rolling, it is preferable to perform pickling and to remove the scale formed on the surface of the hot-rolled steel sheet.
冷間圧延した後、焼鈍処理を施す。焼鈍処理は、焼鈍均熱温度７００〜９００℃の温度域で５０〜５００ｓｅｃの時間滞留させることにより行う。引き続いて溶融亜鉛浴に浸漬して溶融亜鉛めっきを施し、その後冷却する。この際に、オーステナイトからマルテンサイトへの変態を開始する温度をＭＳ（℃）とした場合に、ＭＳ〜ＭＳ−１００℃の温度範囲における平均冷却速度を５℃／ｓ以下とする。好ましくは３℃／ｓ以下である。ＭＳ（℃）は下記（１）式を用いて求めることができる。
ＭＳ（℃）＝５４０−３５０×｛［Ｃ％］／（１−〔α％〕／１００）｝−４０×［Ｍｎ％］＋３０×［Ａｌ％］−２０×［Ｃｒ％］−３５×［Ｖ％］−１０×［Ｍｏ％］ ……（１）
ただし、［Ｘ％］は合金元素Ｘの質量％（［Ａｌ％］はｓｏｌ．Ａｌの質量％）、〔α％〕はフェライトの体積分率（％）を意味する。フェライト分率は、焼鈍後の冷却終了時に、例えばフェライト分率を非破壊で測定することができるセンサーを用いて測定することができる。もしくは、フェライト分率と焼鈍条件との相関を求めた結果より推定してもよい。
After cold rolling, an annealing treatment is performed. The annealing treatment is performed by retaining for 50 to 500 seconds in a temperature range of annealing soaking temperature of 700 to 900 ° C. Subsequently, it is immersed in a hot dip galvanizing bath to apply hot dip galvanizing, and then cooled. At this time, when the temperature at which transformation from austenite to martensite is started is MS (° C.), the average cooling rate in the temperature range of MS to MS-100 ° C. is 5 ° C./s or less. Preferably it is 3 degrees C / s or less. MS (° C.) can be obtained using the following equation (1).
MS (° C.) = 540−350 × {[C%] / (1− [α%] / 100)} − 40 × [Mn%] + 30 × [Al%] − 20 × [Cr%] − 35 × [ V%]-10 × [Mo%] (1)
However, [X%] means mass% of alloy element X ([Al%] is mass% of sol.Al), and [α%] means the volume fraction (%) of ferrite. The ferrite fraction can be measured at the end of cooling after annealing using, for example, a sensor that can measure the ferrite fraction nondestructively. Or you may estimate from the result of calculating | requiring the correlation with a ferrite fraction and annealing conditions.
溶融亜鉛めっき浴に浸漬した後、必要に応じて合金化処理を施し、その後に上記（１）式で得られるＭＳ〜ＭＳ−１００℃の温度範囲における平均冷却速度を５℃／ｓ以下、好ましくは３℃／ｓ以下として冷却してもよい。 After immersing in a hot dip galvanizing bath, an alloying treatment is performed as necessary, and then the average cooling rate in the temperature range of MS to MS-100 ° C. obtained by the above formula (1) is preferably 5 ° C./s or less, preferably May be cooled to 3 ° C./s or less.
焼鈍均熱温度は、再結晶が起きかつオーステナイト中へのＣ濃化を促進させるため、オーステナイト＋フェライトの二相域の温度以上の７００℃以上にする必要がある。一方、焼鈍温度が９００℃を超えると、オーステナイト粒径が著しく粗大化し、延性が低下する。このため、焼鈍均熱温度を７００〜９００℃の範囲とする。好ましくは７５０〜８５０℃である。
均熱時間は、５０秒未満だと、冷延時のひずみ回復が不十分で所望の特性が得られない。一方、５００秒を超えると、効果が飽和し、コストアップの要因となる。このため、均熱時間は５０〜５００秒とする。
The annealing soaking temperature needs to be 700 ° C. or higher, which is higher than the temperature of the two-phase region of austenite + ferrite, in order to cause recrystallization and promote C concentration in the austenite. On the other hand, when the annealing temperature exceeds 900 ° C., the austenite grain size is remarkably coarsened and ductility is lowered. For this reason, annealing soaking temperature shall be the range of 700-900 degreeC. Preferably it is 750-850 degreeC.
If the soaking time is less than 50 seconds, strain recovery during cold rolling is insufficient and desired characteristics cannot be obtained. On the other hand, if it exceeds 500 seconds, the effect is saturated, which causes an increase in cost. For this reason, the soaking time is set to 50 to 500 seconds.
溶融亜鉛めっきの条件は特に限定されない。また、その目付量は、上述したように、要求される耐食性の程度により適宜決定すればよく、特に限定されないが、自動車構造部材に使用される鋼板では、３０〜６０ｇ／ｍ２の範囲が好ましい。また、溶融亜鉛めっき処理後に必要に応じて行われる合金化処理は、４５０〜５８０℃の温度域に保持することにより合金化することが好ましい。これは、合金化処理温度が５８０℃を超えて高温となると、めっき層中のＦｅ含有量が１５％を超え、めっき密着性や加工性の確保が困難となる傾向にあり、一方、４５０℃未満では、合金化の進行が遅く、生産性が低下するからである。 The conditions for hot dip galvanizing are not particularly limited. Further, as described above, the basis weight may be appropriately determined depending on the required degree of corrosion resistance, and is not particularly limited. However, in the case of a steel sheet used for an automobile structural member, a range of 30 to 60 g / m 2 is preferable. . Moreover, it is preferable to alloy by the alloying process performed as needed after the hot dip galvanization process by hold | maintaining in the temperature range of 450-580 degreeC. This is because when the alloying treatment temperature exceeds 580 ° C. and the temperature becomes high, the Fe content in the plating layer exceeds 15%, and it is difficult to ensure plating adhesion and workability, while 450 ° C. If the ratio is less than 1, the progress of alloying is slow, and the productivity is lowered.
溶融亜鉛めっき後の冷却の際に、ＭＳ〜ＭＳ−１００（℃）の範囲の冷却速度を規定したのは、この範囲はオーステナイトからマルテンサイトに変態する温度であるばかりでなく、生成したマルテンサイトが焼戻される温度域でもあり、優れた伸びフランジ性および曲げ性を得るためにはこの温度範囲の冷却速度が極めて重要となるからである。この範囲での冷却速度を平均冷却速度で５℃／ｓ以下としたのは、５℃／ｓ超えの場合には、生成したマルテンサイトのうち焼戻しが進行したマルテンサイトの割合が少なく、伸びフランジ性および曲げ性が低いものとなるからである。ＭＳ−１００℃未満の冷却は放冷、急冷のいずれでもよい。 During cooling after hot dip galvanization, the cooling rate in the range of MS to MS-100 (° C.) was defined not only in the temperature at which austenite transforms into martensite but also in the martensite produced. This is because the cooling rate in this temperature range is extremely important in order to obtain excellent stretch flangeability and bendability. The cooling rate in this range was set to 5 ° C./s or less in terms of average cooling rate. When the average cooling rate exceeded 5 ° C./s, the proportion of martensite that had undergone tempering in the generated martensite was small and the stretch flange This is because the properties and bendability are low. The cooling below MS-100 ° C. may be either cooling or rapid cooling.
以下、本発明の実施例について説明する。
表１に示す組成の鋼を真空溶解炉で溶製し、小型鋼塊とし、次いで１２５０℃に加熱（保持１ｈ）した後、熱間圧延を施して、板厚３．０ｍｍの熱延板とした。なお、仕上げ圧延終了温度は８９０℃とした。熱間圧延後、平均２０℃／ｓの冷却速度で鋼板を冷却し、６００℃での巻き取りに相当する６００℃×１ｈの熱処理を施した。次に、これら熱延板を酸洗し、板厚１．４ｍｍまで冷間圧延した。この冷延鋼板に、還元性雰囲気（５％Ｈ２−Ｎ２）で焼鈍処理を施し、溶融亜鉛中に浸漬し、付着量（片面あたり）５０ｇ／ｍ２に調整した後、そのまま冷却するかまたは５００℃で合金化処理を行った。焼鈍処理条件を表２に示す。このようにして得られた鋼板サンプルについて、引張特性、伸びフランジ特性、曲げ性を評価し、さらに組織調査を行った。これらの結果を表２に併記する。
Examples of the present invention will be described below.
Steel having the composition shown in Table 1 was melted in a vacuum melting furnace to form a small steel ingot, then heated to 1250 ° C. (holding 1 h), and then hot-rolled to obtain a hot-rolled sheet having a thickness of 3.0 mm did. The finish rolling finish temperature was 890 ° C. After the hot rolling, the steel sheet was cooled at an average cooling rate of 20 ° C./s and subjected to a heat treatment of 600 ° C. × 1 h corresponding to winding at 600 ° C. Next, these hot-rolled sheets were pickled and cold-rolled to a thickness of 1.4 mm. Is this cold-rolled steel sheet annealed in a reducing atmosphere (5% H 2 -N 2 ), immersed in molten zinc and adjusted to an adhesion amount (per one side) of 50 g / m 2 and then cooled as it is? Alternatively, alloying treatment was performed at 500 ° C. Table 2 shows the annealing treatment conditions. The steel plate samples thus obtained were evaluated for tensile properties, stretch flange properties, and bendability and further subjected to a structure investigation. These results are also shown in Table 2.
これら評価および調査の内容を以下に示す。
・引張特性：ＪＩＳ Ｚ ２２０１に規定のＪＩＳ５号試験片を圧延直角方向が引張方向になるように試験片を採取し、ＪＩＳ Ｚ ２２４１に準じた引張試験を実施して、引張特性を評価した。
・伸びフランジ特性：日本鉄鋼連盟規格ＪＦＳＴ１００１−１９９６に準拠して穴広げ試験を実施し、穴広げ率（％）で伸びフランジ性を評価した。
・曲げ性：ＪＩＳ Ｚ ２２４８に基づき、圧延方向と垂直に短冊試験片を切り出し、曲げ半径を変えて１８０°Ｕ曲げを行い、割れの発生しない限界曲げ半径Ｒ（ｍｍ）で評価した。
・鋼板組織：圧延方向に平行な板厚断面の組織を走査型顕微鏡（ＳＥＭ）にて観察・撮影した組織写真を用いて画像解析し、フェライトおよびマルテンサイトの体積率を線分法により測定した。残留オーステナイト量については、板厚の１／４の深さに相当する面まで化学研磨した後、この研磨面をＸ線回折により調査した。マルテンサイト中の焼戻しマルテンサイトの分率は、１％ナイタール液でエッチングした後、走査型電子顕微鏡（ＳＥＭ）を用いて３０００倍で板厚１／４部の組織観察を行い、焼戻された部分を同定し、画像処理により分率測定を行った。
The contents of these evaluations and surveys are shown below.
-Tensile properties: JIS No. 5 test pieces defined in JIS Z 2201 were collected so that the direction perpendicular to the rolling direction was the tensile direction, and a tensile test according to JIS Z 2241 was performed to evaluate the tensile properties.
Stretch flange characteristics: A hole expansion test was performed in accordance with Japan Iron and Steel Federation standard JFST1001-1996, and the stretch flangeability was evaluated by the hole expansion ratio (%).
-Bendability: Based on JIS Z 2248, a strip test piece was cut out perpendicular to the rolling direction, bent at a bending radius of 180 ° U, and evaluated with a limit bending radius R (mm) at which no cracks occurred.
Steel sheet structure: The structure of the plate thickness section parallel to the rolling direction was observed and photographed with a scanning microscope (SEM) and image analysis was performed, and the volume fraction of ferrite and martensite was measured by the line segment method. . Regarding the amount of retained austenite, after chemical polishing to a surface corresponding to ¼ of the plate thickness, the polished surface was examined by X-ray diffraction. The tempered martensite fraction in the martensite was tempered after etching with 1% nital liquid, and using a scanning electron microscope (SEM) to observe the structure of the plate thickness 1/4 part at 3000 times. The part was identified and the fraction was measured by image processing.
表２に示すように、本発明で規定する要件を満足する鋼板は、強度−延性バランス、伸びフランジ性および曲げ性に優れた特性が得られ、めっき性も良好であることが確認された。 As shown in Table 2, it was confirmed that the steel sheet satisfying the requirements defined in the present invention was excellent in strength-ductility balance, stretch flangeability and bendability, and had good plating properties.
本発明の高強度鋼板は、高強度に加え、優れた延性、伸びフランジ性および曲げ性を有しているため、厳しい伸びおよび伸びフランジ加工部、ならびに曲げ加工部に適用することができ、自動車用はもとより、家電および建築など、厳しい加工性が必要とされる分野に好適に使用可能である。 Since the high-strength steel sheet of the present invention has excellent ductility, stretch flangeability and bendability in addition to high strength, it can be applied to severely stretched and stretched flanged parts and bent parts. It can be suitably used not only for applications but also in fields that require strict processability, such as home appliances and architecture.
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