JP6086079B2 - High strength high yield ratio cold rolled steel sheet and method for producing the same - Google Patents
High strength high yield ratio cold rolled steel sheet and method for producing the same Download PDFInfo
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Description
本発明は、自動車用部材の使途に適した、高強度高降伏比冷延鋼板、特に引張強さ（ＴＳ）が５９０ＭＰａ以上、降伏比が０．８４以上であり、優れた表面性状を兼ね備えた高強度高降伏比冷延鋼板およびその製造方法に関する。 The present invention is a high-strength, high-yield-ratio cold-rolled steel sheet suitable for the use of automobile members, particularly having a tensile strength (TS) of 590 MPa or more and a yield ratio of 0.84 or more, and has excellent surface properties. The present invention relates to a high-strength, high-yield ratio cold-rolled steel sheet and a method for producing the same.
近年地球環境保全の観点から、ＣＯ２排出量削減のため自動車業界全体で自動車の燃費改善が指向されている。自動車の燃費改善には、使用部材の薄肉化（板厚減少）による自動車車体の軽量化が最も有効である。このため、自動車用部材に使用される鋼板については、高強度化して鋼板板厚を減少することが検討されており、軽量化と安全性を両立する高強度冷延鋼板の使用量は年々増加しつつある。このため、高強度冷延鋼板を得る技術として、種々の技術が提案されている。 In recent years, from the viewpoint of protecting the global environment, the automobile industry as a whole has been directed to improving the fuel consumption of automobiles in order to reduce CO 2 emissions. The most effective way to improve the fuel efficiency of automobiles is to reduce the weight of automobile bodies by reducing the thickness of the parts used (reducing the plate thickness). For this reason, with regard to steel plates used for automobile parts, it has been studied to increase the strength and reduce the thickness of the steel plate, and the amount of high-strength cold-rolled steel that achieves both weight reduction and safety increases year by year. I am doing. For this reason, various techniques have been proposed as techniques for obtaining high-strength cold-rolled steel sheets.
例えば、特許文献１には、Ｃ：０．１５〜０．２５％、Ｓｉ：１．０〜２．０％、Ｍｎ：１．８〜２．８％、Ｐ：０．０２０％以下、Ｓ：０．００４０％以下、Ａｌ：０．００５〜０．０８％、Ｎ：０．００８％以下を含有し、残部がＦｅおよび不可避的不純物からなる成分組成を有し、フェライト相とベイナイト相の合計の組織全体に対する面積比率が４０〜７０％、焼戻マルテンサイト相の組織全体に対する面積比率が２０〜４０％で、残留オーステナイト相の量が２〜２０％であり、かつ長軸が３μｍ以上の焼戻マルテンサイト相の個数が２．０×１０４個／ｍｍ２以下とすることで加工性に優れる高降伏比高強度冷延鋼板が得られる技術が提案されている。特許文献１によると、フェライト相、ベイナイト相、焼戻マルテンサイト相の面積比率および残留オーステナイト相の量を制御することで高強度かつ高降伏比の鋼板が得られるとしている。 For example, Patent Document 1 includes C: 0.15 to 0.25%, Si: 1.0 to 2.0%, Mn: 1.8 to 2.8%, P: 0.020% or less, S : 0.0040% or less, Al: 0.005 to 0.08%, N: 0.008% or less, the balance is composed of Fe and unavoidable impurities, the ferrite phase and the bainite phase The area ratio to the total structure is 40 to 70%, the area ratio of the tempered martensite phase to the entire structure is 20 to 40%, the amount of residual austenite phase is 2 to 20%, and the major axis is 3 μm or more. A technique has been proposed in which a high yield ratio high strength cold-rolled steel sheet having excellent workability is obtained when the number of tempered martensite phases is 2.0 × 10 4 pieces / mm 2 or less. According to Patent Document 1, a steel sheet having high strength and high yield ratio can be obtained by controlling the area ratio of ferrite phase, bainite phase, tempered martensite phase and the amount of retained austenite phase.
特許文献２には、鋼板組成を質量％で、Ｃ：０．００１〜０．２％、Ｎ：０．０００１〜０．２％、Ｃ＋Ｎ：０．００２〜０．３％、Ｓｉ：０．００１〜０．１％、Ｍｎ：０．０１〜１％、Ｔｉ：０．００１〜０．１％、Ｎｂ：０．００１〜０．１％を含有し、鋼中に直径１〜１０ｎｍの微細析出物を１×１０１７個／ｃｍ３以上の密度で含むことを特徴とする常温遅時効性と焼付硬化性に優れた薄鋼板を製造する技術が提案されている。特許文献２に提案された技術によると微細析出物としてＣまたはＮを固定し、塗装焼付工程時に固定したＣおよびＮを脱離、拡散させることにより常温遅時効性と焼付硬化性に優れた薄鋼板が得られるとしている。 In Patent Document 2, the composition of the steel sheet is mass%, C: 0.001 to 0.2%, N: 0.0001 to 0.2%, C + N: 0.002 to 0.3%, Si: 0.00. 001 to 0.1%, Mn: 0.01 to 1%, Ti: 0.001 to 0.1%, Nb: 0.001 to 0.1%, fine in diameter of 1 to 10 nm in steel There has been proposed a technique for producing a thin steel sheet excellent in room temperature slow aging and bake hardenability, characterized by containing precipitates at a density of 1 × 10 17 pieces / cm 3 or more. According to the technique proposed in Patent Document 2, C or N is fixed as fine precipitates, and C and N fixed during the coating baking process are desorbed and diffused to achieve a thin film with excellent room temperature slow aging and bake hardenability. It is said that a steel plate is obtained.
特許文献３には、鋼板組成を質量％で、Ｃ：０．０１〜０．１０％、Ｍｎ：０．１０〜３．００％、Ｔｉ：０．０３〜０．１５％を含有し、Ｓｉ：２．５０％以下、Ｎ：０．００６０％以下、Ｎｂ：０．０３％以下、Ｍｏ：０．２５％以下、Ｖ：０．２５％以下に制限し、ＴｉおよびＮｂ、Ｍｏ、Ｖの含有量を調節し、Ｔｉ系炭窒化物の粒子径が１〜５０ｎｍであり、フェライトの面積率が９５％以上であり、該フェライトの平均粒径を２０μｍ以下に制限し、該フェライトに占める該未再結晶フェライトの割合を２５％以下に制限したことを特徴とする析出強化型冷延鋼板を製造する技術が提案されている。特許文献３によると、冷間圧延前にＴｉの固溶を促進し、冷間圧延後の焼鈍時にＴｉの微細な炭窒化物を析出させ、伸びフランジ性が良好な鋼板が得られるとしている。 Patent Document 3 contains a steel plate composition in mass%, C: 0.01 to 0.10%, Mn: 0.10 to 3.00%, Ti: 0.03 to 0.15%, Si : 2.50% or less, N: 0.0060% or less, Nb: 0.03% or less, Mo: 0.25% or less, V: 0.25% or less, Ti and Nb, Mo, V of The content is adjusted, the particle size of the Ti-based carbonitride is 1 to 50 nm, the area ratio of the ferrite is 95% or more, the average particle size of the ferrite is limited to 20 μm or less, A technique for producing a precipitation-strengthened cold-rolled steel sheet characterized by limiting the proportion of unrecrystallized ferrite to 25% or less has been proposed. According to Patent Literature 3, Ti solid solution is promoted before cold rolling, and fine carbonitrides of Ti are precipitated during annealing after cold rolling, thereby obtaining a steel sheet having good stretch flangeability.
さらに、特に自動車用構造部材では、衝突時や使用中に変形すると安全性が著しく損なわれるため、変形を妨げるのに有効な降伏応力を高めた降伏比（＝降伏応力／引張強さ）の高い、高強度高降伏比冷延鋼板への鋼板への要望が高まっている。 Furthermore, especially in the case of automobile structural members, the safety is significantly impaired if it is deformed at the time of a collision or during use. Therefore, the yield ratio (= yield stress / tensile strength) is increased to increase the yield stress effective to prevent deformation. There is a growing demand for steel sheets for cold rolled steel sheets with high strength and high yield ratio.
一方で、自動車部品に使用される高強度鋼板には、鋼板表面の美観やめっき性、化成処理性が良好であることも求められる。自動車部品は厳しい腐食環境下で使用されるものも多く、高強度鋼板を使用することによる部材の薄肉化により、腐食代が少なくなる。そのため、自動車部品用鋼板は、高い強度と良好な耐食性を兼備することも重要となる。良好な耐食性を付与するには、めっき処理を施して鋼板表面にめっき層を備えたり、鋼板表面に化成処理を施し塗装したりすることが有効である。しかしながら、引張強さが５９０ＭＰａ以上の高強度冷延鋼板の多くは、固溶強化元素であるＳｉやＭｎが多量に添加されるため、表面性状が劣化してめっき性や化成処理性が問題となる。 On the other hand, high-strength steel plates used for automobile parts are also required to have good aesthetics, plating properties and chemical conversion properties on the surface of the steel plates. Many automobile parts are used in a severe corrosive environment, and the corrosion allowance is reduced by thinning the member by using a high-strength steel sheet. Therefore, it is also important that the steel sheet for automobile parts has both high strength and good corrosion resistance. In order to give good corrosion resistance, it is effective to provide a plating layer on the surface of the steel sheet by plating or to apply a chemical conversion treatment to the surface of the steel sheet. However, many of the high-strength cold-rolled steel sheets having a tensile strength of 590 MPa or more contain a large amount of solid-solution strengthening elements such as Si and Mn, so that the surface properties deteriorate and plating properties and chemical conversion properties are problematic. Become.
ここで、Ｓｉを多量に添加することなく、鋼板の高強度化を達成する技術としては、例えば特許文献４のような炭化物による強化を利用した技術が開発されている。特許文献４には、鋼板組成を質量％で、Ｃ：０．０１％超〜０．１％、Ｓｉ：０．３％以下、Ｍｎ：０．２〜２．０％、Ｎ：０．００６％以下、Ｔｉ：０．０３〜０．２％を含有し、Ｍｏ：０．５％以下およびＷ：１．０％以下のうち１種以上を含み、組織が実質的にフェライト単相で、原子比で０．５≦Ｃ／（Ｔｉ＋Ｍｏ＋Ｗ）≦１．５を満たす１０ｎｍ未満の炭化物が分散していることを特徴とする加工性に優れた高張力冷延鋼板を製造する技術が提案されている。特許文献４によると、転位密度が低く加工性が良好なフェライト組織に微細に析出するＴｉ、ＭｏおよびＷの１種以上を含む炭化物を分散させることにより、加工性と強度を両立させ、さらに炭化物により強化することで、Ｓｉの多量の添加を不要として、防食のための溶融亜鉛めっきが可能である鋼板が得られるとしている。 Here, as a technique for achieving high strength of a steel sheet without adding a large amount of Si, a technique using strengthening by carbide as in Patent Document 4, for example, has been developed. In Patent Document 4, the steel sheet composition is in mass%, C: more than 0.01% to 0.1%, Si: 0.3% or less, Mn: 0.2 to 2.0%, N: 0.006. %, Containing Ti: 0.03 to 0.2%, including one or more of Mo: 0.5% or less and W: 1.0% or less, the structure is substantially a ferrite single phase, A technique for producing a high-tensile cold-rolled steel sheet with excellent workability, characterized in that carbides of less than 10 nm satisfying an atomic ratio of 0.5 ≦ C / (Ti + Mo + W) ≦ 1.5 is dispersed. Yes. According to Patent Document 4, disperse carbides containing one or more of Ti, Mo, and W that are finely precipitated in a ferrite structure with low dislocation density and good workability, thereby achieving both workability and strength. By strengthening, it is said that a steel sheet capable of hot dip galvanization for corrosion protection can be obtained without adding a large amount of Si.
しかしながら、特許文献１で提案された技術では、多量のＳｉおよびＭｎを含有するため、表面性状に優れた鋼板を得ることは困難である。特許文献２で提案された技術では１５０〜２００℃の塗装焼付温度で析出物を鋼中に溶解し、ＣおよびＮを脱離させる必要があるため、多量にＴｉおよびＮｂを含有させることはできず高強度の鋼板は得られない。特許文献３で提案された技術では、その実施例を参照すると、易酸化性元素であるＳｉ、Ｍｎを多量に含有させている。このため、再結晶焼鈍時に選択的外部酸化反応による表面濃化によりＳｉやＭｎが表面濃化して、良好な鋼板表面性状を有する鋼板を製造することが困難であるという問題がある。さらに、固溶強化を意図してＳｉが０．３％を超える範囲で添加された鋼も散見されるが、後述するようにＳｉは表面にファイヤライトを含む赤スケールを発生させ、表面性状を低下させる。また、特許文献４で提案された技術では、Ｓｉの含有量は少ないものの、その実施例を参照すると、易酸化性元素であるＭｎを多量に含有しているため、良好な鋼板表面性状を有する鋼板を製造することが困難であるという問題がある。 However, since the technique proposed in Patent Document 1 contains a large amount of Si and Mn, it is difficult to obtain a steel sheet having excellent surface properties. In the technique proposed in Patent Document 2, it is necessary to dissolve precipitates in steel at a coating baking temperature of 150 to 200 ° C. and desorb C and N, so that a large amount of Ti and Nb can be contained. Therefore, a high-strength steel sheet cannot be obtained. In the technique proposed in Patent Document 3, referring to the examples, Si and Mn which are easily oxidizable elements are contained in a large amount. For this reason, there is a problem in that it is difficult to manufacture a steel sheet having good steel sheet surface properties due to the surface concentration of Si and Mn due to surface enrichment by selective external oxidation during recrystallization annealing. Furthermore, there are some steels with Si added in a range exceeding 0.3% for the purpose of solid solution strengthening. However, as will be described later, Si generates a red scale containing firelite on the surface, and the surface properties are improved. Reduce. Further, in the technique proposed in Patent Document 4, although the content of Si is small, referring to the examples, since it contains a large amount of Mn which is an easily oxidizable element, it has good steel sheet surface properties. There is a problem that it is difficult to manufacture a steel plate.
さらに、従来の高強度冷延鋼板の多くは、ＳｉやＭｎといった固溶強化元素を多量に添加し、ベイナイト相やマルテンサイト相等の低温変態相を活用して高い強度を得ていた。低温変態相を活用した鋼板では、変態にともない可動転位が導入されるため、降伏応力は低下する傾向にあり、高い降伏比の鋼板を得ることができない。また、前記したように、易酸化性元素であるＳｉおよびＭｎを多量に添加することにより鋼板表面性状も悪化する。 Furthermore, many conventional high-strength cold-rolled steel sheets have obtained high strength by adding a large amount of solid solution strengthening elements such as Si and Mn and utilizing low-temperature transformation phases such as a bainite phase and a martensite phase. In a steel sheet utilizing a low-temperature transformation phase, movable dislocations are introduced along with the transformation, so that the yield stress tends to decrease, and a steel sheet with a high yield ratio cannot be obtained. Further, as described above, the surface properties of the steel sheet are also deteriorated by adding a large amount of easily oxidizable elements Si and Mn.
以上のように、従来技術では低いＳｉ、Ｍｎ含有量で高降伏比を有する高強度冷延鋼板を得ることができず、良好な表面性状を有する高強度冷延鋼板を得ることはできなかった。本発明は、かかる事情を鑑みてなされたものであって、５９０ＭＰａ以上の引張強さを有し、表面性状にも優れた高強度高降伏比冷延鋼板を提供することを目的とする。 As described above, in the prior art, a high-strength cold-rolled steel sheet having a low Si and Mn content and a high yield ratio could not be obtained, and a high-strength cold-rolled steel sheet having good surface properties could not be obtained. . This invention is made | formed in view of this situation, Comprising: It aims at providing the high strength high yield ratio cold-rolled steel plate which has the tensile strength of 590 Mpa or more and was excellent also in surface properties.
上記課題を解決すべく、本発明者らは、固溶強化元素でありかつ易酸化性元素であるＳｉおよびＭｎの添加量を極限まで低減させた成分系で、高強度高降伏比冷延鋼板を得るため、炭化物を利用した析出強化により強化することに着目し、鋭意検討した。その結果、冷延鋼板を製造する際、冷延時に導入したひずみを除去し焼鈍後の鋼板に所望の延性を持たせるため、焼鈍時での回復・再結晶を促進させることが不可欠であるが、同時に炭化物が粗大化し鋼板が軟化する問題があることが判明した。本発明者らは、このような問題について検討を重ねた結果、熱延鋼板中のＴｉを含む炭化物のサイズを制御することで、昇温中にも再結晶が進行し、焼鈍後の冷延鋼板で強度と延性を両立させることができることを明らかとし、加工性を損なうことなく高強度高降伏比鋼板を得ることに成功した。熱延鋼板中の炭化物サイズを制御するには、Ｍｎ量を０．５％未満として、熱間圧延後のランアウトテーブル上での冷却中にγ／α変態を促進させることが有効であり、均一かつ過度に微細化しない微細な炭化物が得られることがわかった。 In order to solve the above problems, the present inventors have developed a high-strength, high-yield-ratio cold-rolled steel sheet with a component system in which the addition amount of Si and Mn, which are solid solution strengthening elements and oxidizable elements, is reduced to the limit In order to obtain this, attention was focused on strengthening by precipitation strengthening using carbide, and intensive studies were conducted. As a result, when manufacturing cold-rolled steel sheets, it is indispensable to promote recovery and recrystallization during annealing in order to remove the strain introduced during cold-rolling and to give the steel sheet after annealing the desired ductility. At the same time, it has been found that there is a problem that the carbide is coarsened and the steel sheet is softened. As a result of repeated studies on such problems, the present inventors have controlled the size of carbides containing Ti in the hot-rolled steel sheet, so that recrystallization proceeds even during temperature rise, and cold rolling after annealing. It was clarified that steel sheet can achieve both strength and ductility, and succeeded in obtaining a high-strength, high-yield ratio steel sheet without impairing workability. In order to control the carbide size in the hot-rolled steel sheet, it is effective to promote the γ / α transformation during cooling on the run-out table after hot rolling by setting the Mn amount to less than 0.5%. And it turned out that the fine carbide | carbonized_material which does not refine excessively is obtained.
本発明は上記の知見に基づき完成されたものであり、その要旨は次のとおりである。 The present invention has been completed based on the above findings, and the gist thereof is as follows.
［１］質量％で、
Ｃ：０．０６％以上０．１４％以下、
Ｓｉ：０．３％以下、
Ｍｎ：０．５％未満、
Ｐ：０．０５％以下、
Ｓ：０．０１％以下、
Ａｌ：０．０８％以下、
Ｎ：０．００８０％以下、
Ｔｉ：０．０８％以上０．１５％以下
を含有し、残部がＦｅおよび不可避的不純物からなる組成と、フェライト相の面積率が９０％以上、前記フェライト相に対する加工フェライトの面積率が１０％以下（０％を含む）、前記フェライト相の結晶粒内のＴｉを含む炭化物の平均粒子径が１５ｎｍ以下である鋼組織を有し、下記式（１）で定めるＳｉ濃化量が１．３以下、下記式（２）で定めるＭｎ濃化量が１．３以下であることを特徴とする、高強度高降伏比冷延鋼板；
Ｓｉ濃化量＝Ｓ１（Ｓｉ）／Ｓ２（Ｓｉ）・・・（１）
Ｍｎ濃化量＝Ｓ１（Ｍｎ）／Ｓ２（Ｍｎ）・・・（２）
ただし、Ｓ１（Ｍ）は、グロー放電発光分析法により得た元素Ｍの濃度プロファイルにおける鋼板表面から０．２μｍの深さ位置までの元素Ｍの濃度の積算値を、Ｓ２（Ｍ）は、グロー放電発光分析法により得た元素Ｍの濃度プロファイルにおける鋼板表面から０．２μｍの深さ位置から０．４μｍの深さ位置までの元素Ｍの濃度の積算値を表す。
[1] By mass%
C: 0.06% or more and 0.14% or less,
Si: 0.3% or less,
Mn: less than 0.5%,
P: 0.05% or less,
S: 0.01% or less,
Al: 0.08% or less,
N: 0.0080% or less,
Ti: 0.08% or more and 0.15% or less, with the balance being Fe and inevitable impurities, the area ratio of the ferrite phase is 90% or more, and the area ratio of the processed ferrite to the ferrite phase is 10% Below (including 0%), it has a steel structure in which the average particle diameter of the carbide containing Ti in the crystal grains of the ferrite phase is 15 nm or less, and the Si concentration determined by the following formula (1) is 1.3. Hereinafter, a high-strength, high-yield ratio cold-rolled steel sheet, characterized in that the Mn concentration determined by the following formula (2) is 1.3 or less;
Si enrichment amount = S1 (Si) / S2 (Si) (1)
Concentration of Mn = S1 (Mn) / S2 (Mn) (2)
However, S1 (M) is the integrated value of the concentration of element M from the steel sheet surface to the 0.2 μm depth position in the concentration profile of element M obtained by glow discharge emission spectrometry, and S2 (M) is the glow The integrated value of the concentration of the element M from the depth position of 0.2 μm to the depth position of 0.4 μm from the steel sheet surface in the concentration profile of the element M obtained by the discharge emission analysis method is represented.
［２］前記組成に加えてさらに、質量％で、Ｖ：０．０１％以上０．１％以下、Ｎｂ：０．０１％以上０．０５％以下の１種または２種を含有することを特徴とする、前記［１］に記載の高強度高降伏比冷延鋼板。 [2] In addition to the above composition, the composition further contains one or two of V: 0.01% to 0.1% and Nb: 0.01% to 0.05% by mass%. The high-strength, high-yield-ratio cold-rolled steel sheet according to [1], characterized in that it is characterized in that
［３］前記組成に加えてさらに、質量％で、Ｃａ、Ｍｇ、ＲＥＭの１種または２種以上を合計で０．０００１％以上０．２％以下を含有することを特徴とする、前記［１］または［２］に記載の高強度高降伏比冷延鋼板。 [3] In addition to the above composition, the composition further contains 0.0001% or more and 0.2% or less of Ca, Mg, or REM in a total of 0.0001% or more by mass%. The high-strength, high-yield ratio cold-rolled steel sheet according to [1] or [2].
［４］前記組成に加えてさらに、質量％で、Ｃｒ：０．０１％以上０．１％以下、Ｎｉ：０．０１％以上０．１％以下、Ｍｏ：０．０１％以上０．０５％以下、Ｗ：０．０１％以上０．０５％以下、Ｈｆ：０．０１％以上０．０５％以下、Ｚｒ：０．０１％以上０．１％以下、Ｃｏ：０．０００１％以上０．１％以下の１種または２種以上を含有することを特徴とする、前記［１］〜［３］のいずれか１つに記載の高強度高降伏比冷延鋼板。 [4] In addition to the above composition, Cr: 0.01% to 0.1%, Ni: 0.01% to 0.1%, Mo: 0.01% to 0.05% by mass % Or less, W: 0.01% to 0.05%, Hf: 0.01% to 0.05%, Zr: 0.01% to 0.1%, Co: 0.0001% to 0 The high-strength, high-yield ratio cold-rolled steel sheet according to any one of the above [1] to [3], comprising 1% or less of 1% or less.
［５］鋼板表面にめっき層を有することを特徴とする、前記［１］〜［４］のいずれか１つに記載の高強度高降伏比冷延鋼板。 [5] The high-strength, high-yield ratio cold-rolled steel sheet according to any one of [1] to [4], wherein the steel sheet surface has a plating layer.
［６］前記めっき層が亜鉛めっき層であることを特徴とする、前記［５］に記載の高強度高降伏比冷延鋼板。 [6] The high-strength, high-yield ratio cold-rolled steel sheet according to [5], wherein the plated layer is a galvanized layer.
［７］前記めっき層が合金化亜鉛めっき層であることを特徴とする、前記［５］に記載の高強度高降伏比冷延鋼板。 [7] The high strength and high yield ratio cold rolled steel sheet according to [5], wherein the plated layer is an alloyed galvanized layer.
［８］鋼素材に、粗圧延と仕上げ圧延からなる熱間圧延を施し、仕上げ圧延終了後、冷却して巻き取り、冷間圧延し、焼鈍することで冷延鋼板とするにあたり、
前記鋼素材を、質量％で、Ｃ：０．０６％以上０．１４％以下、Ｓｉ：０．３％以下、Ｍｎ：０．５％未満、Ｐ：０．０５％以下、Ｓ：０．０１％以下、Ａｌ：０．０８％以下、Ｎ：０．００８０％以下、Ｔｉ：０．０８％以上０．１５％以下を含有し、残部がＦｅおよび不可避的不純物からなる組成とし、前記粗圧延に供する鋼素材の温度を１１００℃以上１３５０℃以下とし、前記仕上げ圧延の仕上げ圧延温度を８２０℃以上とし、前記冷却を仕上げ圧延終了後２秒以内に開始し、前記冷却の平均冷却速度を２０℃／ｓ以上とし、前記巻き取りの巻取り温度を７００℃以下とし、前記冷間圧延の冷間圧延率を３０％以上７５％以下とし、前記焼鈍を、７００℃から焼鈍温度までの平均昇温速度が１０℃／ｓ以下、焼鈍温度が７５０℃以上９００℃以下とすることを特徴とする、フェライト相の面積率が９０％以上、前記フェライト相に対する加工フェライトの面積率が１０％以下（０％を含む）、前記フェライト相の結晶粒内のＴｉを含む炭化物の平均粒子径が１０ｎｍ以下である鋼組織を有し、下記式（１）で定めるＳｉ濃化量が１．３以下、下記式（２）で定めるＭｎ濃化量が１．３以下、引張強さが５９０ＭＰａ以上、降伏比が０．８４以上である高強度高降伏比冷延鋼板の製造方法。
Ｓｉ濃化量＝Ｓ１（Ｓｉ）／Ｓ２（Ｓｉ）・・・（１）
Ｍｎ濃化量＝Ｓ１（Ｍｎ）／Ｓ２（Ｍｎ）・・・（２）
ただし、Ｓ１（Ｍ）は、グロー放電発光分析法により得た元素Ｍの濃度プロファイルにおける鋼板表面から０．２μｍの深さ位置までの元素Ｍの濃度の積算値を、Ｓ２（Ｍ）は、グロー放電発光分析法により得た元素Ｍの濃度プロファイルにおける鋼板表面から０．２μｍの深さ位置から０．４μｍの深さ位置までの元素Ｍの濃度の積算値を表す。
[8] The steel material is subjected to hot rolling consisting of rough rolling and finish rolling, and after finishing rolling, cooled and wound, cold rolled, and annealed to form a cold rolled steel sheet,
The steel material is, in mass%, C: 0.06% or more and 0.14% or less, Si: 0.3% or less, Mn: less than 0.5%, P: 0.05% or less, S: 0.00. 01% or less, Al: 0.08% or less, N: 0.0080% or less, Ti: 0.08 % or more and 0.15% or less, with the balance being composed of Fe and inevitable impurities, The temperature of the steel material used for rolling is set to 1100 ° C. or higher and 1350 ° C. or lower, the finish rolling temperature of the finish rolling is set to 820 ° C. or higher, the cooling is started within 2 seconds after finishing rolling, and the average cooling rate of the cooling is set. 20 ° C./s or more, the winding temperature of the winding is 700 ° C. or less, the cold rolling rate of the cold rolling is 30% or more and 75% or less, and the annealing is an average from 700 ° C. to the annealing temperature. Temperature increase rate is 10 ° C / s or less, annealing temperature is 750 ° C or less Characterized by a 900 ° C. or less, the area ratio of the ferrite phase is 90% or more, (including 0%) 10% or less the area ratio of the deformed ferrite for the ferrite phase, Ti in the crystal grains of the ferrite phase A carbide structure containing a steel structure having an average particle diameter of 10 nm or less, a Si concentration determined by the following formula (1) is 1.3 or less, and a Mn concentration determined by the following formula (2) is 1.3. Hereinafter, a method for producing a high strength, high yield ratio cold rolled steel sheet having a tensile strength of 590 MPa or more and a yield ratio of 0.84 or more .
Si enrichment amount = S1 (Si) / S2 (Si) (1)
Concentration of Mn = S1 (Mn) / S2 (Mn) (2)
However, S1 (M) is the integrated value of the concentration of element M from the steel sheet surface to the 0.2 μm depth position in the concentration profile of element M obtained by glow discharge emission spectrometry, and S2 (M) is the glow The integrated value of the concentration of the element M from the depth position of 0.2 μm to the depth position of 0.4 μm from the steel sheet surface in the concentration profile of the element M obtained by the discharge emission analysis method is represented.
［９］前記鋼素材が、さらに、質量％で、Ｖ：０．０１％以上０．１％以下、Ｎｂ：０．０１％以上０．０５％以下の１種または２種を含有することを特徴とする、前記［８］に記載の高強度高降伏比冷延鋼板の製造方法。 [9] The steel material further contains one or two kinds of V: 0.01% to 0.1% and Nb: 0.01% to 0.05% by mass%. The method for producing a high-strength, high-yield-ratio cold-rolled steel sheet according to [8], characterized in that it is characterized in that
［１０］前記鋼素材が、さらに、質量％で、Ｃａ、Ｍｇ、ＲＥＭの１種または２種以上を合計で０．０００１％以上０．２％以下含有することを特徴とする、前記［８］または［９］に記載の高強度高降伏比冷延鋼板の製造方法。 [10] The above-mentioned [8], wherein the steel material further contains, in mass%, one or more of Ca, Mg, and REM in a total of 0.0001% to 0.2%. ] Or the manufacturing method of the high strength high yield ratio cold-rolled steel sheet according to [9].
［１１］前記鋼素材が、さらに、質量％で、Ｃｒ：０．０１％以上０．１％以下、Ｎｉ：０．０１％以上０．１％以下、Ｍｏ：０．０１％以上０．０５％以下、Ｗ：０．０１％以上０．０５％以下、Ｈｆ：０．０１％以上０．０５％以下、Ｚｒ：０．０１％以上０．１％以下、Ｃｏ：０．０００１％以上０．１％以下の１種または２種以上を含有することを特徴とする、前記［８］〜［１０］のいずれか１つに記載の高強度高降伏比冷延鋼板の製造方法。 [11] The steel material further includes, in mass%, Cr: 0.01% to 0.1%, Ni: 0.01% to 0.1%, Mo: 0.01% to 0.05 % Or less, W: 0.01% to 0.05%, Hf: 0.01% to 0.05%, Zr: 0.01% to 0.1%, Co: 0.0001% to 0 The method for producing a high-strength, high-yield-ratio cold-rolled steel sheet according to any one of the above [8] to [10], comprising 1% or less of 1% or less.
［１２］前記焼鈍温度での焼鈍の後、めっき処理を施すことを特徴とする、前記［８］〜［１１］のいずれか１つに記載の高強度高降伏比冷延鋼板の製造方法。 [12] The method for producing a high-strength, high-yield ratio cold-rolled steel sheet according to any one of [8] to [11], wherein a plating treatment is performed after the annealing at the annealing temperature.
［１３］前記めっき処理が、亜鉛めっき処理であることを特徴とする、前記［１２］に記載の高強度高降伏比冷延鋼板の製造方法。 [13] The method for producing a high-strength, high-yield ratio cold-rolled steel sheet according to [12], wherein the plating treatment is a zinc plating treatment.
［１４］前記めっき処理が、合金化亜鉛めっき処理であることを特徴とする、前記［１２］に記載の高強度高降伏比冷延鋼板の製造方法。 [14] The method for producing a high-strength, high-yield ratio cold-rolled steel sheet according to [12], wherein the plating treatment is an alloyed zinc plating treatment.
本発明によると、引張強さが５９０ＭＰａ以上であり、かつ表面性状に優れた高強度高降伏比冷延鋼板が得られる。本発明の高強度高降伏比冷延鋼板は、自動車の構造部材等の使途に好適であり、かつ自動車部材の軽量化や自動車部材成形を可能とする等の効果を奏する。表面性状も良好であることから、めっき処理や化成処理が可能であり優れた耐食性を付与することができ、産業上格段の効果を奏する。 According to the present invention, a high-strength, high-yield ratio cold-rolled steel sheet having a tensile strength of 590 MPa or more and excellent surface properties can be obtained. The high-strength, high-yield-ratio cold-rolled steel sheet of the present invention is suitable for the use of automobile structural members and the like, and has effects such as reducing the weight of automobile members and forming automobile members. Since the surface property is also good, plating treatment and chemical conversion treatment are possible, excellent corrosion resistance can be imparted, and a remarkable industrial effect can be achieved.
以下、本発明について詳細に説明する。
まず、本発明の冷延鋼板の成分組成の限定理由について説明する。なお、以下の成分組成を表す％は、特に断らない限り質量％を意味するものとする。
Hereinafter, the present invention will be described in detail.
First, the reason for limiting the component composition of the cold-rolled steel sheet of the present invention will be described. In addition,% showing the following component composition shall mean the mass% unless there is particular notice.
Ｃ：０．０６％以上０．１４％以下
Ｃは、Ｔｉと結合し炭化物として鋼板中に微細分散する。また、さらにＶやＮｂを添加した場合、あるいは更にＭｏ、Ｗ、Ｚｒ、Ｈｆを添加した場合、これら元素とも結合し、炭化物として鋼板中に微細分散する。すなわちＣは、微細な炭化物を形成してフェライト組織を著しく強化させる元素である。Ｃは鋼板を強化する上で必須の元素であり、引張強さ５９０ＭＰａ以上を確保するには、Ｃ含有量は０．０６％以上とする必要がある。好ましくは０．０７％以上である。一方、Ｃ含有量が０．１４％を超えると、熱間圧延前のスラブ再加熱工程で粗大なＴｉＣを完全に溶解することができなくなり、強化に対する効果が飽和する。そのため、Ｃ含有量は０．１４％以下とする。好ましくは、０．１２％以下である。
C: 0.06% or more and 0.14% or less C is combined with Ti and finely dispersed in the steel sheet as a carbide. Further, when V or Nb is further added, or when Mo, W, Zr, or Hf is further added, these elements are also combined and finely dispersed in the steel sheet as carbides. That is, C is an element that forms fine carbides and remarkably strengthens the ferrite structure. C is an essential element for strengthening the steel sheet, and in order to ensure a tensile strength of 590 MPa or more, the C content needs to be 0.06% or more. Preferably it is 0.07% or more. On the other hand, if the C content exceeds 0.14%, coarse TiC cannot be completely dissolved in the slab reheating step before hot rolling, and the effect on strengthening is saturated. Therefore, the C content is 0.14% or less. Preferably, it is 0.12% or less.
なお、炭化物形成に関与しなかったＣは、炭化物の熱安定性を向上させて、焼鈍中の炭化物粗大化を抑制する。また、熱延鋼板で形成されたセメンタイトは冷延時にマトリックスとのひずみ勾配を発生させ、蓄積されるひずみエネルギーが増大し、再結晶を促進させる効果も有する。このような観点から、Ｃは炭化物構成元素であるＴｉの含有量、さらにはＶ、Ｎｂ、Ｍｏ、Ｗ、Ｚｒ、Ｈｆを添加した場合には、これら元素の含有量に対し、原子比にして過剰に含有させることが望ましく、下記（３）式を満たすことが好ましい。より好ましくは（３）式の左辺は２．２以上である。なお、ここで、式（３）中の元素記号は、各元素の含有量（質量％）を表す。
（Ｃ／１２）／｛（Ｔｉ／４８）＋（Ｖ／５１）＋（Ｎｂ／９３）＋（Ｍｏ／９６）
＋（Ｗ／１８４）＋（Ｈｆ／１７６）＋（Ｚｒ／９１）｝≧１．８・・・（３）
In addition, C which did not participate in carbide | carbonized_material formation improves the thermal stability of a carbide | carbonized_material, and suppresses the coarsening of the carbide | carbonized_material during annealing. Further, cementite formed of hot-rolled steel sheet has an effect of generating a strain gradient with the matrix during cold rolling, increasing the accumulated strain energy, and promoting recrystallization. From this point of view, C is the content of Ti, which is a carbide constituent element, and when V, Nb, Mo, W, Zr, and Hf are added, the atomic ratio is set to the content of these elements. It is desirable to make it contain excessively, and it is preferable to satisfy | fill following (3) Formula. More preferably, the left side of the formula (3) is 2.2 or more. Here, the element symbol in the formula (3) represents the content (% by mass) of each element.
(C / 12) / {(Ti / 48) + (V / 51) + (Nb / 93) + (Mo / 96)
+ (W / 184) + (Hf / 176) + (Zr / 91)} ≧ 1.8 (3)
Ｓｉ：０．３％以下
Ｓｉは易酸化性元素であり、鋼板表面に濃化し易く、鋼板表面にファイヤライト（Ｆｅ２ＳｉＯ４）を形成する。このファイヤライトは鋼板表面に楔形となって形成し著しく鋼板表面性状を劣化させるが、０．３％まではその影響が小さく許容できる。このため、Ｓｉ含有量は０．３％以下とする。なお、化成処理性の観点からはＳｉ含有量は０．２％以下とすることが好ましく、めっき性の観点からは０．１％未満とすることが好ましい。また、Ｓｉ含有量は０％であってもよい。
Si: 0.3% or less Si is an easily oxidizable element, is easily concentrated on the surface of the steel sheet, and forms firelite (Fe 2 SiO 4 ) on the surface of the steel sheet. This firelite is formed in a wedge shape on the surface of the steel sheet and significantly deteriorates the surface quality of the steel sheet, but the effect is small up to 0.3% and acceptable. For this reason, Si content shall be 0.3% or less. The Si content is preferably 0.2% or less from the viewpoint of chemical conversion properties, and is preferably less than 0.1% from the viewpoint of plating properties. Further, the Si content may be 0%.
Ｍｎ：０．５％未満
ＭｎはＳｉと同様、易酸化性元素であり、鋼板表面に濃化して表面性状を悪化させる。一方で、Ｍｎはγ／α変態を遅延させる元素でもある。本発明では焼鈍工程での回復、再結晶を促進させるため、特に熱延鋼板中の炭化物サイズを制御する。焼鈍での回復、再結晶を促進させるのに適した炭化物サイズにするには、上述したように、熱間圧延後のγ／α変態開始温度が重要である。Ｍｎ含有量を０．５％未満とすることにより、熱間圧延後のγ／α変態を促進して熱間圧延後のγ／α変態開始温度を６８０℃以上とし、過度に微細な炭化物（粒子径が５ｎｍ以下）ではない炭化物が得られる。過度に微細な炭化物が熱延鋼板中に分散した状態であると、冷間圧延後の焼鈍工程での再結晶がＴｉを含む炭化物によって遅延され、良好な加工性を有する鋼板が得られなくなる。本発明では熱延鋼板中の炭化物のサイズを５ｎｍ以上に制御することで再結晶を促進させ、良好な加工性を得るとともに鋼板表面へのＭｎ濃化を低減することで表面性状に優れた鋼板を得る。以上の観点から、Ｍｎ含有量は０．５％未満とする。好ましくは、０．４％以下である。Ｍｎ含有量は、不純物レベルまで低減してもよい。なお、不純物元素であるＳによる加工性低下を抑制するため、Ｍｎ含有量の下限は０．０５％程度とすることが好ましい。
Mn: Less than 0.5% Mn, like Si, is an easily oxidizable element and concentrates on the surface of the steel sheet to deteriorate the surface properties. On the other hand, Mn is also an element that delays the γ / α transformation. In the present invention, in order to promote recovery and recrystallization in the annealing process, the carbide size in the hot-rolled steel sheet is particularly controlled. In order to obtain a carbide size suitable for promoting recovery and recrystallization during annealing, as described above, the γ / α transformation start temperature after hot rolling is important. By setting the Mn content to less than 0.5%, the γ / α transformation after hot rolling is promoted so that the γ / α transformation start temperature after hot rolling is 680 ° C. or more, and an excessively fine carbide ( Carbides with a particle size not greater than 5 nm) are obtained. If excessively fine carbides are dispersed in the hot-rolled steel sheet, recrystallization in the annealing process after cold rolling is delayed by the carbide containing Ti, and a steel sheet having good workability cannot be obtained. In the present invention, a steel sheet excellent in surface properties by promoting recrystallization by controlling the size of carbide in the hot-rolled steel sheet to 5 nm or more, obtaining good workability and reducing Mn concentration on the steel sheet surface. Get. From the above viewpoint, the Mn content is less than 0.5%. Preferably, it is 0.4% or less. The Mn content may be reduced to the impurity level. In addition, in order to suppress the workability fall by S which is an impurity element, it is preferable that the minimum of Mn content shall be about 0.05%.
Ｐ：０．０５％以下
Ｐは粒界に偏析して加工時に粒界割れの起点となり、加工性を劣化させるが、このようなＰの影響は、０．０５％までは許容できる。このため、Ｐ含有量は０．０５％以下とする。好ましくは、Ｐ含有量は０．０３％以下であり、極力低減することが好ましい。Ｐ含有量は０％であってもよい。
P: 0.05% or less P segregates at the grain boundary and becomes the starting point of grain boundary cracking during processing, and deteriorates workability. However, the effect of such P is acceptable up to 0.05%. Therefore, the P content is 0.05% or less. Preferably, the P content is 0.03% or less, and it is preferable to reduce it as much as possible. The P content may be 0%.
Ｓ：０．０１％以下
Ｓは、鋼中でＭｎＳなどの介在物として存在する。この介在物は熱間圧延中に伸展し、伸展した介在物は加工時に割れの起点となるため加工性を低下させるが、このようなＳの影響は、０．０１％までは許容できる。このため、Ｓ含有量は０．０１％以下とする。好ましくは、Ｓ含有量は０．００８％以下であり、低減することが好ましい。Ｓ含有量は０％であってもよい。
S: 0.01% or less S is present as an inclusion such as MnS in steel. This inclusion extends during hot rolling, and the extended inclusion serves as a starting point of cracking during processing, thus reducing workability. However, the influence of S is acceptable up to 0.01%. For this reason, S content shall be 0.01% or less. Preferably, the S content is 0.008% or less, and is preferably reduced. The S content may be 0%.
Ａｌ：０．０８％以下
Ａｌは、脱酸剤として作用する元素である。このような効果を得るためにはＡｌ量は０．０２％以上とすることが好ましい。一方で、Ａｌは酸化物等の介在物を形成し、加工時にボイドの起点となるため加工性を低下させるが、Ａｌ含有量は０．０８％までは許容できる。このため、Ａｌ量の上限を０．０８％とする。好ましくは０．０６％以下である。
Al: 0.08% or less Al is an element that acts as a deoxidizer. In order to obtain such an effect, the Al content is preferably 0.02% or more. On the other hand, Al forms inclusions such as oxides and serves as a starting point for voids during processing, thus reducing workability, but the Al content is acceptable up to 0.08%. For this reason, the upper limit of the Al amount is set to 0.08%. Preferably it is 0.06% or less.
Ｎ：０．００８０％以下
Ｎは製鋼、連続鋳造の段階でＴｉと結合しＴｉＮを形成する。この際析出するＴｉＮは粗大であるため、鋼板の強化に寄与せず、加工時にボイド生成の起点となるため鋼板の加工性に悪影響をもたらす。このため、Ｎは極力低減させることが望ましいが、０．００８０％までは許容できるため、本発明でのＮ含有量の上限を０．００８０％とする。好ましくは０．００６０％以下である。Ｎ含有量は極力低減させることが好ましく、０％であってもよい。
N: 0.0080% or less N is combined with Ti at the stage of steelmaking and continuous casting to form TiN. Since the TiN precipitated at this time is coarse, it does not contribute to the strengthening of the steel sheet, and it becomes a starting point for void generation during processing, thus adversely affecting the workability of the steel sheet. For this reason, although it is desirable to reduce N as much as possible, since it is permissible to 0.0080%, the upper limit of N content in this invention shall be 0.0080%. Preferably it is 0.0060% or less. The N content is preferably reduced as much as possible, and may be 0%.
Ｔｉ：０．０８％以上０．１５％以下
Ｔｉは、Ｃと炭化物を形成して鋼板の高強度化に寄与する元素である。特に本発明では固溶強化元素であるＳｉ、Ｍｎを上記したように低減しているため、所望の鋼板強度を得るにはＴｉを含有させ、Ｔｉを含む微細な炭化物により析出強化して強度を上昇させる必要がある。Ｔｉが０．０８％を下回ると所望の鋼板強度（引張強さ：５９０ＭＰａ以上）が得られなくなるため、Ｔｉ含有量の下限を０．０８％とする。好ましくはＴｉ含有量の下限は０．１０％であり、より好ましくはＴｉ含有量の下限は０．１１％である。一方、Ｔｉ含有量が０．１５％を超えると、鋼板を製造する際、熱間圧延前のスラブ加熱によって粗大なＴｉ炭化物を溶解することができず、高強度化の効果が飽和するため、Ｔｉ含有量の上限を０．１５％とする。好ましくは、Ｔｉ含有量は０．１３％以下である。
Ti: 0.08% or more and 0.15% or less Ti is an element that forms carbide with C and contributes to increasing the strength of the steel sheet. Particularly in the present invention, Si and Mn, which are solid solution strengthening elements, are reduced as described above. Therefore, in order to obtain a desired steel sheet strength, Ti is contained, and precipitation strengthening is performed by fine carbides containing Ti to increase the strength. It needs to be raised. If Ti is less than 0.08%, the desired steel plate strength (tensile strength: 590 MPa or more) cannot be obtained, so the lower limit of the Ti content is 0.08%. Preferably the lower limit of Ti content is 0.10%, more preferably the lower limit of Ti content is 0.11%. On the other hand, when the Ti content exceeds 0.15%, when manufacturing a steel sheet, coarse Ti carbide cannot be dissolved by slab heating before hot rolling, and the effect of increasing strength is saturated. The upper limit of the Ti content is 0.15%. Preferably, the Ti content is 0.13% or less.
以上が、本発明における基本組成であり、残部はＦｅおよび不可避的不純物である。本発明では、上記した基本組成に加えて、さらに目的に応じて、以下の成分を加えてもよい。 The above is the basic composition in the present invention, and the balance is Fe and inevitable impurities. In the present invention, in addition to the basic composition described above, the following components may be added according to the purpose.
Ｖ：０．０１％以上０．１％以下、Ｎｂ：０．０１％以上０．０５％以下の１種または２種
ＶおよびＮｂは、Ｔｉと同様、Ｃと炭化物を形成して鋼板の高強度化に寄与する元素である。このような効果を得るためには、ＶおよびＮｂはそれぞれ０．０１％以上添加する必要がある。一方でＶは炭化物を粗大化させやすく、０．１％を超えて含有しても、強化に対する効果が飽和し、もしくは含有量の増量につれ強度が低下する。このため、Ｖを添加する場合は、Ｖ含有量の上限を０．１％とする。好ましいＶ含有量の上限は０．０８％である。また、Ｎｂは再結晶時にｓｏｌｕｔｅ ｄｒａｇ効果により粒界移動を阻害し、加工フェライト粒を残存させやすい。加工フェライト粒は、鋼板の加工性に悪影響を与える。しかし、Ｎｂ含有量が０．０５％以下であればこのような加工性への悪影響は顕在化しないため、Ｎｂ含有量の上限を０．０５％とする。好ましいＮｂ含有量の上限は０．０４％である。
V: 0.01% or more and 0.1% or less, Nb: 0.01% or more and 0.05% or less, V and Nb, like Ti, form carbides with C to form a high steel plate It is an element that contributes to strengthening. In order to obtain such an effect, it is necessary to add 0.01% or more of V and Nb. On the other hand, V tends to coarsen the carbide, and even if it contains more than 0.1%, the effect on strengthening is saturated, or the strength decreases as the content increases. For this reason, when adding V, the upper limit of V content is made into 0.1%. The upper limit of preferable V content is 0.08%. Nb also inhibits grain boundary migration due to the solution drag effect during recrystallization and tends to leave processed ferrite grains. The processed ferrite grains adversely affect the workability of the steel sheet. However, if the Nb content is 0.05% or less, such an adverse effect on workability does not become obvious, so the upper limit of the Nb content is set to 0.05%. The upper limit of the preferable Nb content is 0.04%.
Ｃａ、Ｍｇ、ＲＥＭの１種または２種以上を合計で０．０００１％以上０．２％以下
Ｃａ、Ｍｇ、ＲＥＭ（ＲＥＭ：スカンジウム（Ｓｃ）、イットリウム（Ｙ）および原子番号５７から７１までのランタノイド元素）は介在物の形態を制御し、介在物から発生するボイド発生を抑制するのに有効な元素である。このような効果を得るにはＣａ、Ｍｇ、ＲＥＭの１種または２種以上を合計で０．０００１％以上添加する必要がある。一方で、これら元素の合計の含有量が０．２％を超えても上記効果が飽和する。このため、Ｃａ、Ｍｇ、ＲＥＭの１種または２種以上の合計量の上限を０．２％とした。好ましい範囲はＣａ、Ｍｇ、ＲＥＭの１種または２種以上を合計で０．０００５％以上０．１％以下である。
One or more of Ca, Mg, and REM in total 0.0001% or more and 0.2% or less Ca, Mg, REM (REM: scandium (Sc), yttrium (Y) and atomic numbers 57 to 71 The lanthanoid element) is an element effective in controlling the form of inclusions and suppressing the generation of voids generated from the inclusions. In order to obtain such an effect, it is necessary to add one or more of Ca, Mg, and REM in a total amount of 0.0001% or more. On the other hand, the above effect is saturated even if the total content of these elements exceeds 0.2%. For this reason, the upper limit of the total amount of one or more of Ca, Mg, and REM is set to 0.2%. A preferable range is 0.0005% or more and 0.1% or less in total of one or more of Ca, Mg, and REM.
Ｃｒ：０．０１％以上０．１％以下、Ｎｉ：０．０１％以上０．１％以下、Ｍｏ：０．０１％以上０．０５％以下、Ｗ：０．０１％以上０．０５％以下、Ｈｆ：０．０１％以上０．０５％以下、Ｚｒ：０．０１％以上０．１％以下、Ｃｏ：０．０００１％以上０．１％以下の１種または２種以上
ＣｒおよびＮｉ、Ｍｏ、Ｗ、Ｈｆ、Ｚｒ、Ｃｏは微量添加で鋼板強度を上昇させるのに有効な元素である。鋼板強度を上昇させるには、Ｃｒ、Ｎｉ、Ｍｏ、Ｗ、Ｈｆ、Ｚｒはそれぞれ０．０１％以上を添加する必要があり、Ｃｏは０．０００１％以上を添加する必要がある。一方で、Ｃｒ、Ｎｉ、Ｚｒ、Ｃｏの含有量がそれぞれ０．１％、Ｍｏ、Ｗ、Ｈｆの含有量がそれぞれ０．０５％を超えるとγ／α変態開始温度が低下し、焼鈍時での回復、再結晶を阻害する微細炭化物を形成させる要因となる。そのためＣｒ、Ｎｉ、Ｚｒ、Ｃｏの上限量はそれぞれ０．１％、ＭｏおよびＷ、Ｈｆの含有量の上限はそれぞれ０．０５％とした。これら元素の中で、Ｍｏ、Ｗ、Ｈｆ、Ｚｒは、再結晶を阻害させやすい元素であるため、Ｍｏ、Ｗ、Ｈｆ、Ｚｒのうちの２種以上を含有させる場合には、Ｍｏ、Ｗ、Ｈｆ、Ｚｒの含有量の合計を０．１％以下とすることが好ましい。
Cr: 0.01% to 0.1%, Ni: 0.01% to 0.1%, Mo: 0.01% to 0.05%, W: 0.01% to 0.05% Hereinafter, Hf: 0.01% or more and 0.05% or less, Zr: 0.01% or more and 0.1% or less, Co: 0.0001% or more and 0.1% or less, or one or more of Cr and Ni , Mo, W, Hf, Zr and Co are effective elements for increasing the steel sheet strength by adding a small amount. In order to increase the steel sheet strength, it is necessary to add 0.01% or more of Cr, Ni, Mo, W, Hf, and Zr, and it is necessary to add 0.0001% or more of Co. On the other hand, if the Cr, Ni, Zr, and Co contents are each 0.1%, and the Mo, W, and Hf contents are each more than 0.05%, the γ / α transformation start temperature decreases, and during annealing, It becomes a factor to form fine carbides that inhibit recovery and recrystallization. Therefore, the upper limit of Cr, Ni, Zr, and Co is 0.1%, respectively, and the upper limit of the contents of Mo, W, and Hf is 0.05%. Among these elements, Mo, W, Hf, and Zr are elements that easily inhibit recrystallization. Therefore, when two or more of Mo, W, Hf, and Zr are contained, Mo, W, The total content of Hf and Zr is preferably 0.1% or less.
次に、本発明鋼板の組織の限定理由について説明する。 Next, the reason for limiting the structure of the steel sheet of the present invention will be described.
フェライト相の面積率が９０％以上
冷延、再結晶焼鈍後の冷延鋼板のマトリックスは、加工性に優れたフェライト単相組織とすることが好ましい。ベイナイト相やマルテンサイト相、残留オーステナイト等のフェライト相と硬度が異なる第二相組織が鋼板組織に混入すると、フェライト相と第二相組織との界面で加工時に応力集中が生じ、割れ等の欠陥が発生する原因となる。また、ベイナイト相やマルテンサイト相は、変態時に変態ひずみを発生させフェライト相に可動転位を導入させる。この可動転位は降伏応力を低下させるため、フェライト相に可動転位が導入されると、高い降伏比を得ることが困難となる。本発明ではフェライト相の面積率を９０％以上とし、フェライト相以外の組織である第二相組織の面積率が１０％以下であれば、上記したような加工性や降伏応力への影響が小さいため、フェライト相の面積率下限を９０％とした。好ましくはフェライト相の面積率は９５％以上である。
The area ratio of the ferrite phase is 90% or more. The matrix of the cold-rolled steel sheet after cold rolling and recrystallization annealing preferably has a ferrite single-phase structure excellent in workability. When a second phase structure with hardness different from the ferrite phase such as bainite phase, martensite phase, and retained austenite is mixed in the steel sheet structure, stress concentration occurs at the interface between the ferrite phase and the second phase structure, resulting in defects such as cracks. Cause the occurrence. Further, the bainite phase and the martensite phase generate transformation strain at the time of transformation and introduce movable dislocations into the ferrite phase. Since this movable dislocation reduces the yield stress, it is difficult to obtain a high yield ratio when the movable dislocation is introduced into the ferrite phase. In the present invention, if the area ratio of the ferrite phase is 90% or more and the area ratio of the second phase structure which is a structure other than the ferrite phase is 10% or less, the influence on the workability and the yield stress as described above is small. Therefore, the lower limit of the area ratio of the ferrite phase is set to 90%. Preferably, the area ratio of the ferrite phase is 95% or more.
フェライト相に対する加工フェライトの面積率が１０％以下（０％を含む）
冷間圧延後には、鋼板全体が加工された組織となる。この組織は粒内に多量の転位を含むため延性が著しく乏しく、加工性を低下させる。加工性に悪影響をもたらさないようにするには、加工フェライトは極力低減する必要があり、フェライト相全体に占める加工フェライトの面積率、すなわち、フェライト相に対する加工フェライトの面積率は１０％以下に制限する必要がある。望ましくは８％以下である。
The area ratio of processed ferrite to ferrite phase is 10% or less (including 0%)
After cold rolling, the entire steel sheet is processed. Since this structure contains a large amount of dislocations in the grains, the ductility is remarkably poor and the workability is lowered. In order not to adversely affect the workability, the processed ferrite needs to be reduced as much as possible, and the area ratio of the processed ferrite in the entire ferrite phase, that is, the area ratio of the processed ferrite to the ferrite phase is limited to 10% or less. There is a need to. Desirably, it is 8% or less.
フェライト相の結晶粒内のＴｉを含む炭化物の平均粒子径が１５ｎｍ以下
本発明鋼では固溶強化元素であるＭｎを低減しているため、粒子分散強化による強化量を最大限高める必要がある。粒子分散強化による強化量は炭化物の析出量の他に炭化物の粒子径が重要な要素となる。炭化物の微細化により鋼板強度は著しく上昇するため、鋼板の引張強さを５９０ＭＰａ以上とするには、フェライト相の結晶粒内のＴｉを含む炭化物の平均粒子径は１５ｎｍ以下とする必要がある。好ましくは１０ｎｍ以下である。本発明では、この微細に析出する炭化物はＴｉを含む組成であるが、Ｔｉの他にＶ、Ｎｂ、Ｍｏ、Ｗ、Ｈｆ、Ｚｒ、Ｎ、Ａｌを含んでいても良い。なお、焼鈍中での再結晶を促進させるため、フェライト相の結晶粒内のＴｉを含む炭化物の平均粒子径は５ｎｍ以上とすることが好ましい。
The average particle diameter of the carbide containing Ti in the ferrite phase crystal grains is 15 nm or less In the steel of the present invention, Mn, which is a solid solution strengthening element, is reduced, and therefore it is necessary to maximize the strengthening amount by particle dispersion strengthening. The amount of strengthening by particle dispersion strengthening is not only the amount of carbide precipitation but also the particle size of carbide. Since the strength of the steel sheet is remarkably increased due to the refinement of the carbide, in order to make the tensile strength of the steel sheet 590 MPa or more, the average particle diameter of the carbide containing Ti in the crystal grains of the ferrite phase needs to be 15 nm or less. Preferably it is 10 nm or less. In the present invention, the finely precipitated carbide has a composition containing Ti, but may contain V, Nb, Mo, W, Hf, Zr, N, and Al in addition to Ti. In order to promote recrystallization during annealing, it is preferable that the average particle diameter of the carbide containing Ti in the ferrite phase crystal grains be 5 nm or more.
Ｓｉ濃化量が１．３以下、Ｍｎ濃化量が１．３以下
ここで、Ｓｉ濃化量は上記式（１）、Ｍｎ濃化量は上記式（２）で定めるものである。なお、Ｓ１（Ｍ）（上記式（１）におけるＳ１（Ｓｉ）、上記式（２）におけるＳ１（Ｍｎ））は、グロー放電発光分析法により得た元素Ｍ（元素Ｍ：上記式（１）ではＳｉ、上記式（２）ではＭｎ）の濃度プロファイルにおける鋼板表面から０．２μｍの深さ位置までの元素Ｍの濃度の積算値を、Ｓ２（Ｍ）（上記式（１）におけるＳ２（Ｓｉ）、上記式（２）におけるＳ２（Ｍｎ））は、グロー放電発光分析法により得た元素Ｍの濃度プロファイルにおける鋼板表面から０．２μｍの深さ位置から０．４μｍの深さ位置までの元素Ｍの濃度の積算値を表す。なおＳ２（Ｍ）は、地鉄の元素Ｍの固溶量に相当する。
Si enrichment amount is 1.3 or less, Mn enrichment amount is 1.3 or less. Here, the Si enrichment amount is determined by the above formula (1), and the Mn enrichment amount is defined by the above formula (2). S1 (M) (S1 (Si) in the above formula (1), S1 (Mn) in the above formula (2)) is the element M obtained by glow discharge emission analysis (element M: the above formula (1) Is the integrated value of the concentration of element M from the steel sheet surface to the 0.2 μm depth position in the concentration profile of Si, and Mn in the above formula (2) is S2 (M) (S2 (Si in the above formula (1) ), S2 (Mn) in the above formula (2) is an element from the depth position of 0.2 μm to the depth position of 0.4 μm from the steel sheet surface in the concentration profile of element M obtained by glow discharge emission spectrometry. It represents the integrated value of the concentration of M. Note that S2 (M) corresponds to the solid solution amount of the element M of the ground iron.
Ｓｉ及びＭｎの表面濃化層は、鋼板表面の濡れ性を著しく低下させ、めっき性や化成処理性を低下させる。したがって、鋼板表面でのＳｉ濃化量およびＭｎ濃化量が増加すると、鋼板の表面性状が劣化する。表面性状の劣化を抑制するためには、Ｓｉ濃化量およびＭｎ濃化量を１．３以下とする必要がある。好ましくは１．２以下である。なおここで、Ｓｉ濃化量およびＭｎ濃化量は、上記した式（１）あるいは式（２）で表されるように、鋼板表面から０．２μｍの深さ位置までのＳｉ濃化量あるいはＭｎ濃化量であり、地鉄のＳｉ固溶量あるいは地鉄のＭｎ固溶量に対して濃化している割合を意味する。 The surface enriched layer of Si and Mn significantly reduces the wettability of the steel sheet surface and lowers the plating property and chemical conversion treatment property. Therefore, when the Si enrichment amount and the Mn enrichment amount on the steel sheet surface increase, the surface properties of the steel sheet deteriorate. In order to suppress the deterioration of the surface properties, the Si concentration and the Mn concentration must be 1.3 or less. Preferably it is 1.2 or less. Here, the Si enrichment amount and the Mn enrichment amount are represented by the above formula (1) or formula (2), the Si enrichment amount from the steel sheet surface to the depth of 0.2 μm, or It is the Mn concentration amount, which means the ratio of concentration with respect to the Si solid solution amount of the base iron or the Mn solid solution amount of the base iron.
ここで、上記したＳｉ濃化量およびＭｎ濃化量は、各々グロー放電発光分析法により得たＳｉの濃度プロファイル、Ｍｎの濃度プロファイルから求めることができる。Ｓｉ濃化量、Ｍｎ濃化量の求め方について、図１を用いて説明する。 Here, the Si enrichment amount and the Mn enrichment amount described above can be obtained from the Si concentration profile and the Mn concentration profile obtained by the glow discharge emission spectrometry, respectively. A method for obtaining the Si concentration and the Mn concentration will be described with reference to FIG.
図１は、グロー放電発光分析法による元素Ｍのスペクトル強度から得られる元素Ｍの濃度プロファイルの模式図である。図１は、鋼板表面からの深さ位置における元素Ｍの濃度を示している。本発明においては、元素Ｍは、ＳｉあるいはＭｎである。図１に示すように、鋼板表面からの０．２μｍの深さ位置までの元素Ｍの濃度の積算値をＳ１（Ｍ）、鋼板表面から０．２μｍの深さ位置から０．４μｍの深さ位置までの元素Ｍの濃度の積算値をＳ２（Ｍ）とすると、本発明においては、元素Ｍの濃化量（鋼板表面から０．２μｍの深さ位置までにおける元素Ｍの濃化量）は、Ｓ１（Ｍ）／Ｓ２（Ｍ）で求めることができる。なお、ここで、鋼板表面から０．２μｍの深さ位置から０．４μｍの深さ位置までの元素Ｍの濃度の積算値Ｓ２（Ｍ）は、地鉄での元素Ｍの固溶量に相当している。 FIG. 1 is a schematic diagram of the concentration profile of element M obtained from the spectral intensity of element M by glow discharge emission spectrometry. FIG. 1 shows the concentration of the element M at a depth position from the steel sheet surface. In the present invention, the element M is Si or Mn. As shown in FIG. 1, the integrated value of the concentration of the element M from the steel plate surface to the 0.2 μm depth position is S1 (M), and the depth from the steel plate surface is 0.2 μm to the 0.4 μm depth. Assuming that the integrated value of the concentration of the element M up to the position is S2 (M), in the present invention, the concentration amount of the element M (concentration amount of the element M from the steel sheet surface to a depth position of 0.2 μm) is , S1 (M) / S2 (M). Here, the integrated value S2 (M) of the concentration of the element M from the depth position of 0.2 μm to the depth position of 0.4 μm from the steel sheet surface corresponds to the solid solution amount of the element M in the base iron. doing.
次に、本発明の冷延鋼板の製造方法について説明する。本発明の冷延鋼板は、鋼素材に、粗圧延と仕上げ圧延からなる熱間圧延を施し、仕上げ圧延終了後、冷却し、巻き取り、冷間圧延し、焼鈍する工程により製造される。 Next, the manufacturing method of the cold rolled steel sheet of this invention is demonstrated. The cold-rolled steel sheet of the present invention is manufactured by a process of subjecting a steel material to hot rolling consisting of rough rolling and finish rolling, and cooling, winding, cold rolling, and annealing after finishing rolling.
本発明は、上記した組成の鋼素材（鋼スラブ）を用い、鋼素材の温度を１１００℃以上１３５０℃以下として、該温度の鋼素材を熱間圧延に供し、仕上げ圧延の仕上げ圧延温度を８２０℃以上として熱間圧延を施し、平均冷却速度が２０℃／ｓ以上の冷却を仕上げ圧延終了後から２秒以内に開始し、巻取り温度７００℃以下で巻取り、冷間圧延率が３０％以上７５％以下の冷間圧延を施し、７００℃から焼鈍温度までの平均昇温速度１０℃／ｓ以下、焼鈍温度７５０〜９００℃の焼鈍を行うことを特徴とする。 In the present invention, the steel material (steel slab) having the above composition is used, the temperature of the steel material is set to 1100 ° C. or more and 1350 ° C. or less, the steel material at the temperature is subjected to hot rolling, and the finish rolling temperature of finish rolling is set to 820. Hot rolling is performed at a temperature of ℃ or higher, cooling at an average cooling rate of 20 ℃ / s or more is started within 2 seconds after finishing rolling, winding is performed at a winding temperature of 700 ℃ or less, and the cold rolling rate is 30%. It is characterized by performing cold rolling of 75% or less and annealing at an average temperature increase rate from 700 ° C. to the annealing temperature of 10 ° C./s or less and an annealing temperature of 750 to 900 ° C.
本発明において、鋼の溶製方法は特に限定されず、転炉、電気炉等、公知の溶製方法を採用することができる。また、真空脱ガス炉にて２次精錬を行ってもよい。その後、生産性や品質上の問題から連続鋳造法によりスラブ（鋼素材）とするのが好ましいが、造塊−分塊圧延法、薄スラブ連鋳法等、公知の鋳造方法でスラブとしても良い。なお、前記したＴｉＮは主に連続鋳造時に析出する。前記したように、ＴｉＮが粗大に析出すると、加工時にボイド生成の起点となり、加工性に悪影響をもたらす。ここで、連続鋳造時の鋳造速度を大きくすることで、ＴｉＮの粒子成長を抑制することができる。例えばＴｉＮのサイズを５μｍ以下に抑制するには、連続鋳造時の鋳造速度を１．０ｍ／ｍｉｎ以上とすることが望ましい。 In the present invention, the method for melting steel is not particularly limited, and a known melting method such as a converter or an electric furnace can be employed. Further, secondary refining may be performed in a vacuum degassing furnace. Thereafter, the slab (steel material) is preferably formed by a continuous casting method from the viewpoint of productivity and quality, but the slab may be formed by a known casting method such as an ingot-bundling rolling method or a thin slab continuous casting method. . The TiN described above is precipitated mainly during continuous casting. As described above, when TiN precipitates coarsely, it becomes a starting point of void generation at the time of processing, and adversely affects workability. Here, TiN particle growth can be suppressed by increasing the casting speed during continuous casting. For example, in order to suppress the size of TiN to 5 μm or less, it is desirable to set the casting speed during continuous casting to 1.0 m / min or more.
鋼素材の温度：１１００℃以上１３５０℃以下
上記の如く得られた鋼素材に、粗圧延および仕上げ圧延からなる熱間圧延を施す。通常、熱間圧延に先立ち鋼素材は加熱され、粗圧延および仕上げ圧延が施される。本発明においては、粗圧延に先立ち鋼素材を加熱して、実質的に均質なオーステナイト相とし、鋼素材中の粗大な炭化物を溶解する必要がある。粗圧延に供する鋼素材の温度、すなわち鋼素材を加熱する場合は鋼素材の加熱温度（以下、単に加熱温度ともいう）が１１００℃未満では、粗圧延前に鋼素材中の粗大な炭化物が溶解せず、冷間圧延、焼鈍後に得られる微細分散する炭化物の量が少なく、鋼板強度が著しく低下する。一方、上記鋼素材の温度（加熱温度）が１３５０℃を超えると、鋼素材表面に生成するスケール量が多く、熱間圧延中にスケールが噛み込みやすく、鋼板表面性状を悪化させる。以上の理由により、粗圧延に供する鋼素材の温度（加熱温度）は、１１００℃以上１３５０℃以下とする。好ましくは１１５０℃以上１３００℃以下である。ただし、鋼素材に熱間圧延を施すに際し、鋳造後の鋼素材が１１５０℃以上１３５０℃以下の温度域にある場合、或いは鋼素材の炭化物が溶解している場合には、鋼素材を加熱することなく直送圧延してもよい。なお、粗圧延条件については特に限定されない。
Temperature of steel material: 1100 ° C. or higher and 1350 ° C. or lower Hot rolling consisting of rough rolling and finish rolling is applied to the steel material obtained as described above. Usually, prior to hot rolling, the steel material is heated and subjected to rough rolling and finish rolling. In the present invention, it is necessary to heat the steel material prior to the rough rolling to obtain a substantially homogeneous austenite phase and dissolve coarse carbides in the steel material. When the temperature of the steel material used for rough rolling, that is, when heating the steel material, the heating temperature of the steel material (hereinafter also simply referred to as the heating temperature) is less than 1100 ° C, the coarse carbides in the steel material are dissolved before the rough rolling. Without, the amount of finely dispersed carbide obtained after cold rolling and annealing is small, and the steel sheet strength is significantly reduced. On the other hand, when the temperature (heating temperature) of the steel material exceeds 1350 ° C., the amount of scale generated on the surface of the steel material is large, the scale is easily bitten during hot rolling, and the steel sheet surface properties are deteriorated. For the above reasons, the temperature (heating temperature) of the steel material used for rough rolling is set to 1100 ° C. or higher and 1350 ° C. or lower. Preferably they are 1150 degreeC or more and 1300 degrees C or less. However, when hot rolling the steel material, the steel material is heated when the steel material after casting is in a temperature range of 1150 ° C. or higher and 1350 ° C. or lower, or when carbide of the steel material is dissolved. Direct rolling may be performed without any problem. The rough rolling conditions are not particularly limited.
仕上げ圧延温度：８２０℃以上
仕上げ圧延温度（以下、仕上げ圧延終了温度ともいう）が８２０℃未満となると、熱間圧延中、鋼板の一部が変態を開始し、鋼板の長手方向および幅方向に対する強度が著しく不均一となる。このような鋼板を冷間圧延すると、鋼板が冷間圧延中に破断したり、形状が著しく不均一になり加工性が低下する問題が生じる。そのため、仕上げ圧延温度は８２０℃以上とする。仕上げ圧延温度の上限は特に定めないが、操業を安定させるには仕上げ圧延温度は１０００℃以下が望ましい。
Finishing rolling temperature: 820 ° C. or more When the finishing rolling temperature (hereinafter also referred to as finishing rolling finishing temperature) is less than 820 ° C., during hot rolling, a part of the steel sheet starts transformation, and the longitudinal direction and the width direction of the steel sheet The strength is extremely uneven. When such a steel sheet is cold-rolled, there arises a problem that the steel sheet breaks during the cold-rolling or the shape becomes extremely non-uniform so that the workability is lowered. Therefore, finish rolling temperature shall be 820 degreeC or more. The upper limit of the finish rolling temperature is not particularly defined, but the finish rolling temperature is desirably 1000 ° C. or lower in order to stabilize the operation.
仕上げ圧延終了後の冷却を開始するまでの時間：２秒以内
仕上げ圧延直後の高温状態の鋼板においては、オーステナイト相に蓄積されたひずみエネルギーが大きいため、ひずみ誘起析出による炭化物が生じる。この炭化物は、高温で析出するため粗大化し易い。本発明では、生成した炭化物は巻取工程ならびに焼鈍工程で粗大化するため、巻取り前には、できる限り粗大な炭化物の生成は抑える必要がある。本発明では、仕上げ圧延終了後なるべく早く強制冷却を開始して、粗大な炭化物の生成を抑制する。このため、仕上げ圧延終了後、少なくとも２秒以内に冷却を開始する。好ましくは１．５秒以内である。
Time until start of cooling after finish rolling: within 2 seconds In a high-temperature steel sheet immediately after finish rolling, carbides are generated due to strain-induced precipitation because the strain energy accumulated in the austenite phase is large. Since this carbide precipitates at a high temperature, it is likely to become coarse. In the present invention, since the generated carbide is coarsened in the winding process and the annealing process, it is necessary to suppress the generation of coarse carbide as much as possible before winding. In the present invention, forced cooling is started as soon as possible after finish rolling to suppress the formation of coarse carbides. For this reason, cooling is started within at least 2 seconds after finish rolling. Preferably, it is within 1.5 seconds.
平均冷却速度：２０℃／ｓ以上
上記したように、仕上げ圧延終了後の鋼板が高温に維持される時間が長いほど、ひずみ誘起析出による炭化物の粗大化が進行し易くなる。このような炭化物の粗大化を回避するため、仕上げ圧延後は急冷する必要があり、２０℃／ｓ以上の平均冷却速度で冷却する必要がある。好ましくは４０℃／ｓ以上である。但し、仕上げ圧延終了後の冷却速度が過剰に大きくなると、巻取り温度の制御が困難となり安定した強度が得られにくくなるという問題が懸念されるため、１５０℃／ｓ以下とすることが好ましい。なお、ここで平均冷却速度は、仕上げ圧延温度終了温度から巻取り温度までの平均冷却速度である。
Average cooling rate: 20 ° C./s or more As described above, the longer the time during which the steel sheet after finish rolling is maintained at a high temperature, the more easily the coarsening of carbides by strain-induced precipitation proceeds. In order to avoid such coarsening of the carbide, it is necessary to rapidly cool after finish rolling, and it is necessary to cool at an average cooling rate of 20 ° C./s or more. Preferably it is 40 degrees C / s or more. However, if the cooling rate after finishing rolling is excessively increased, there is a concern that it is difficult to control the coiling temperature and it is difficult to obtain a stable strength. Here, the average cooling rate is an average cooling rate from the finish rolling temperature end temperature to the winding temperature.
巻取り温度：７００℃以下
本発明において、熱間圧延完了後、巻取工程までに生成せしめたＴｉを含む炭化物の大きさを巻取工程で変化させないことが重要となる。巻取り温度が７００℃を超えるとＴｉを含む炭化物が粗大化し、冷間圧延し焼鈍後の冷延鋼板の強度が５９０ＭＰａを下回る。そのため、巻取温度は７００℃以下とする。好ましくは６６０℃以下である。一方、ランアウトテーブルの長さや水冷設備等の構成上、実質的な巻取温度の下限は２５０℃程度である。
Winding temperature: 700 ° C. or less In the present invention, it is important not to change the size of the carbide containing Ti generated by the winding process after completion of the hot rolling in the winding process. When the coiling temperature exceeds 700 ° C., carbides containing Ti are coarsened, and the strength of the cold-rolled steel sheet after cold rolling and annealing is lower than 590 MPa. Therefore, the winding temperature is set to 700 ° C. or less. Preferably it is 660 degrees C or less. On the other hand, the lower limit of the substantial coiling temperature is about 250 ° C. due to the length of the run-out table and the structure of the water cooling equipment.
冷間圧延率：３０％以上７５％以下
冷間圧延率が３０％未満では、操業上安定せず板形状が不均一になる。また、３０％未満では鋼板がロールに噛み込まず冷間圧延上、トラブル発生の原因にもなる。そのため、冷間圧延率の下限を３０％とする。望ましくは４０％以上である。一方、冷間圧延率が７５％を超えると過度に鋼板が加工硬化し、所望の板厚とすることが困難となったり、冷間圧延中に鋼板が破断したりする可能性があるため、冷間圧延率上限を７５％とする。好ましくは７０％以下である。
Cold rolling rate: 30% or more and 75% or less If the cold rolling rate is less than 30%, the operation is not stable and the plate shape becomes uneven. If it is less than 30%, the steel sheet does not get caught in the roll, and this may cause trouble in cold rolling. Therefore, the lower limit of the cold rolling rate is set to 30%. Desirably, it is 40% or more. On the other hand, if the cold rolling rate exceeds 75%, the steel sheet is excessively work-hardened, and it may be difficult to obtain a desired thickness, or the steel sheet may break during cold rolling, The upper limit of the cold rolling rate is 75%. Preferably it is 70% or less.
７００℃から焼鈍温度までの平均昇温速度：１０℃／ｓ以下
焼鈍における焼鈍温度（最高到達温度）に昇温する際の７００℃から焼鈍温度までの平均昇温速度が１０℃／ｓを超えると、再結晶していない加工フェライトが残存しやすく、加工性が著しく低下する。このため、７００℃から焼鈍温度までの平均昇温速度は１０℃／ｓ以下とする。なお、安定した鋼板の材質を得るには、平均昇温速度は、８℃／ｓ以下とすることが好ましい。なお、ここで、平均昇温速度は７００℃から焼鈍温度までの平均昇温速度である。
Average heating rate from 700 ° C. to annealing temperature: 10 ° C./s or less The average heating rate from 700 ° C. to annealing temperature when heating to the annealing temperature (maximum temperature reached) in annealing exceeds 10 ° C./s Then, processed ferrite that has not been recrystallized tends to remain, and the workability is significantly reduced. For this reason, the average rate of temperature increase from 700 ° C. to the annealing temperature is set to 10 ° C./s or less. In order to obtain a stable steel plate material, the average rate of temperature rise is preferably 8 ° C./s or less. Here, the average temperature increase rate is an average temperature increase rate from 700 ° C. to the annealing temperature.
焼鈍温度：７５０℃以上９００℃以下
冷間圧延で導入された転位を取り除くため、実質的に加工フェライトを残存させず回復、再結晶させる必要がある。完全に再結晶させるには７５０℃以上の焼鈍温度で焼鈍する必要がある。一方で９００℃を超える焼鈍温度では炭化物が粗大化するため、鋼板強度が著しく低下する。したがって、焼鈍温度の上限を９００℃とする。好ましい焼鈍温度の範囲は７６０℃以上８６０℃以下である。Ｔｉを含む炭化物の粗大化、もしくはＴｉを含む炭化物溶解の抑制の観点から焼鈍は７５０℃以上となっている時間が１０分以内であることが望ましく、連続焼鈍ラインもしくは連続溶融めっきラインで製造することが好ましい。なお、ここで焼鈍温度は、焼鈍中の鋼板温度の最高到達温度である。
Annealing temperature: 750 ° C. or more and 900 ° C. or less In order to remove the dislocations introduced in the cold rolling, it is necessary to recover and recrystallize substantially without leaving the processed ferrite. In order to completely recrystallize, it is necessary to anneal at an annealing temperature of 750 ° C. or higher. On the other hand, since the carbides are coarsened at an annealing temperature exceeding 900 ° C., the steel sheet strength is significantly reduced. Therefore, the upper limit of the annealing temperature is set to 900 ° C. A preferable annealing temperature range is 760 ° C. or higher and 860 ° C. or lower. From the viewpoint of coarsening of carbides containing Ti or suppressing dissolution of carbides containing Ti, it is desirable that the annealing time is 750 ° C. or more, and it is desirable that the time is within 10 minutes, and it is produced by a continuous annealing line or a continuous hot dipping line. It is preferable. Here, the annealing temperature is the highest temperature reached during the annealing.
本発明の冷延鋼板は、表面にめっき層を具えたとしても材質変動が極めて小さく鋼板強度や加工性を低下させない。そのため、表面にめっき層を具えることができる。めっき層を付与するには、上記焼鈍温度で焼鈍後、めっき処理を行えば良い。めっき層の種類は特に問わず、電気めっき層、無電解めっき層のいずれも適用可能である。また、めっき層の合金成分も特に問わず、溶融亜鉛めっき等の亜鉛めっき層、合金化溶融亜鉛めっき等の合金化亜鉛めっき層などが好適な例として挙げられる。なお、めっき層の合金成分、めっき層の種類などはこれらに限定されず、従前公知のものがいずれも適用可能である。 Even if the cold-rolled steel sheet of the present invention has a plating layer on the surface, the material fluctuation is extremely small and the steel sheet strength and workability are not lowered. Therefore, a plating layer can be provided on the surface. In order to provide the plating layer, a plating process may be performed after annealing at the annealing temperature. The type of the plating layer is not particularly limited, and any of an electroplating layer and an electroless plating layer can be applied. Further, the alloy component of the plating layer is not particularly limited, and preferable examples include a zinc plating layer such as hot dip galvanizing, and an alloyed galvanizing layer such as galvannealing. In addition, the alloy component of a plating layer, the kind of plating layer, etc. are not limited to these, Any conventionally well-known thing is applicable.
表１に示す組成を有する肉厚２５０ｍｍの鋼素材（鋼スラブ）を、表２に示すスラブ加熱温度に加熱した後、表２に示す熱延条件で熱延鋼板とし、表２に示す条件の冷間圧延を施し、連続焼鈍ラインもしくは連続溶融めっきラインにて冷延鋼板とした。表面にめっき層を具えない“裸材”は連続焼鈍ラインで製造し、溶融亜鉛めっき層を具えた“ＧＩ材”、もしくは合金化溶融亜鉛めっき層を具えた“ＧＡ材”は連続溶融めっきラインにて製造した。連続溶融めっきラインで浸漬するめっき浴（めっき組成：Ｚｎ−０．１３質量％Ａｌ）の温度は４６０℃であり、ＧＡ材はめっき浴に浸漬後、５２０℃で合金化処理を施した。めっき付着量はＧＩ材、ＧＡ材ともに片面当たり４５〜５５ｇ／ｍ２とした。なお、表２に記載の冷却速度は、仕上げ圧延終了温度から巻取り温度までの平均冷却速度であり、平均昇温速度は７００℃から焼鈍温度までの平均昇温速度であり、焼鈍温度は焼鈍中の鋼板温度の最高到達温度である。また、表１に記載したγ／α変態開始温度は、富士電波工機株式会社製サーメックマスターＺを用いて、冷間圧延後の鋼板より採取した試験片に熱伝対をスポット溶接にて着装し、高周波誘導加熱方式にて昇温速度５℃／ｓにて加熱した際の変態膨張曲線から評価した値である。 After heating a steel material (steel slab) having a thickness of 250 mm having the composition shown in Table 1 to a slab heating temperature shown in Table 2, a hot-rolled steel sheet is formed under the hot rolling conditions shown in Table 2, and the conditions shown in Table 2 are satisfied. It cold-rolled and made the cold-rolled steel sheet in the continuous annealing line or the continuous hot dipping line. “Nude material” with no plating layer on the surface is manufactured on a continuous annealing line, “GI material” with a hot dip galvanizing layer or “GA material” with an alloyed hot dip galvanizing layer is a continuous hot dipping line. Manufactured by. The temperature of the plating bath (plating composition: Zn-0.13 mass% Al) immersed in a continuous hot dipping line was 460 ° C., and the GA material was subjected to alloying treatment at 520 ° C. after being immersed in the plating bath. The plating adhesion amount was 45 to 55 g / m 2 per side for both the GI material and the GA material. The cooling rate shown in Table 2 is the average cooling rate from the finish rolling finish temperature to the coiling temperature, the average heating rate is the average heating rate from 700 ° C. to the annealing temperature, and the annealing temperature is annealing. It is the highest temperature reached in the steel plate. In addition, the γ / α transformation start temperature described in Table 1 was obtained by spot welding a thermocouple to a test piece collected from a steel sheet after cold rolling using a cermec master Z manufactured by Fuji Radio Engineering Co., Ltd. It is the value evaluated from the transformation expansion curve when it was dressed and heated at a heating rate of 5 ° C./s by the high frequency induction heating method.
上記により得られた冷延鋼板から試験片を採取し、フェライト相の面積率、フェライト相の平均結晶粒径、加工フェライトの面積率、Ｔｉを含む炭化物の平均粒子径、降伏強度、引張強さ、伸び、ＳｉおよびＭｎ濃化量、めっき性、化成処理性を求めた。試験方法は次のとおりとした。 Test specimens were collected from the cold-rolled steel sheet obtained as described above, and the ferrite phase area ratio, ferrite phase average crystal grain size, processed ferrite area ratio, Ti-containing carbide average particle diameter, yield strength, and tensile strength. , Elongation, Si and Mn concentration, plating properties, and chemical conversion properties were determined. The test method was as follows.
（ｉ）組織観察
フェライト相の面積率は以下の手法により評価した。圧延方向に平行な断面の板厚中心部について、５％ナイタールによる腐食現出組織を走査型光学顕微鏡で４００倍に拡大して１０視野分撮影した。ここで、フェライト相は粒内にラス状の形態やセメンタイトが観察されない形態を有する組織である。また、ポリゴナルフェライト、ベイニティックフェライト、アシキュラーフェライトおよびグラニュラーフェライトをフェライトとして面積率や粒径を求めた。フェライト相の面積率は画像解析によりベイナイト相やマルテンサイト相、パーライト等のフェライト相以外を分離し、観察視野に対するフェライト相の面積率によって求めた。このとき、線状の形態として観察される粒界はフェライト相の一部として計上した。また、伸展された形状で粒内に腐食痕が認められる組織を加工フェライトとみなし、観察視野に占めるフェライト相に対する加工フェライトの面積率を求めた。
(I) Structure observation The area ratio of the ferrite phase was evaluated by the following method. About the central part of the plate thickness having a cross section parallel to the rolling direction, a corrosion appearing structure with 5% nital was magnified 400 times with a scanning optical microscope and photographed for 10 fields of view. Here, the ferrite phase is a structure having a lath-like form or a form in which cementite is not observed in the grains. Further, the area ratio and particle size were determined using polygonal ferrite, bainitic ferrite, acicular ferrite and granular ferrite as ferrite. The area ratio of the ferrite phase was determined from the area ratio of the ferrite phase with respect to the observation field by separating images other than the ferrite phase such as bainite phase, martensite phase, and pearlite by image analysis. At this time, the grain boundary observed as a linear form was counted as a part of the ferrite phase. Moreover, the structure in which corrosion marks were observed in the grains in the extended shape was regarded as processed ferrite, and the area ratio of processed ferrite to the ferrite phase occupying the observation field was determined.
フェライト相の結晶粒内のＴｉを含む炭化物の平均粒子径は、得られた冷延鋼板の板厚中央部から薄膜法によってサンプルを作製し、透過型電子顕微鏡（倍率：１３５０００倍）で観察を行い、１００点以上の析出物粒子径の平均によって求めた。析出物の組成はＴＥＭに付帯するＥＤＸにより分析し、球形および板状、針状である析出物について、Ｔｉが含まれることを確認すれば良い。この析出物粒子径を算出する上で、Ｔｉを含まない粗大なセメンタイトやＣが含まれないＴｉを含む窒化物は含まないものとした。このＴｉを含む窒化物は粒子径が１００ｎｍ以上であり、球形ではなく長方形の形状で観察される。 The average particle diameter of the carbide containing Ti in the ferrite phase crystal grains was prepared by a thin film method from the center of the thickness of the obtained cold-rolled steel sheet, and observed with a transmission electron microscope (magnification: 135,000 times). It was determined by averaging the particle diameters of 100 or more precipitate particles. The composition of the precipitate may be analyzed by EDX attached to the TEM to confirm that Ti is contained in the spherical, plate-like, and needle-like precipitates. In calculating the precipitate particle size, coarse cementite containing no Ti and nitride containing Ti containing no C are excluded. The nitride containing Ti has a particle diameter of 100 nm or more, and is observed in a rectangular shape instead of a spherical shape.
（ｉｉ）引張試験
得られた冷延鋼板から圧延方向に対して垂直方向にＪＩＳ５号引張試験片を作製し、ＪＩＳ Ｚ ２２４１（２０１１）の規定に準拠した引張試験を５回行い、平均の降伏強度（ＹＳ）、引張強さ（ＴＳ）、全伸び（Ｅｌ）を求めた。引張試験のクロスヘッドスピードは１０ｍｍ／ｍｉｎとした。
(Ii) Tensile test A JIS No. 5 tensile test piece was produced from the obtained cold-rolled steel sheet in a direction perpendicular to the rolling direction, a tensile test based on the provisions of JIS Z 2241 (2011) was performed five times, and an average yield was obtained. The strength (YS), tensile strength (TS), and total elongation (El) were determined. The crosshead speed in the tensile test was 10 mm / min.
（ｉｉｉ）ＳｉおよびＭｎ濃度測定
鋼板表面にめっき層が具えられていた場合には、酸洗によってめっき層を剥離した後、鋼板表面をグロー放電発光分析法により濃度プロファイルを測定し、鋼板表面および内部のＳｉおよびＭｎの濃度分布を求めた。Ｓｉの濃化量およびＭｎ濃化量はそれぞれ鋼板表面から深さ０．２μｍ位置までのＳｉの積算値（Ｓ１（Ｓｉ））もしくはＭｎの積算値（Ｓ１（Ｍｎ））と、深さ０．２μｍ位置から０．４μｍ位置までのＳｉの積算値（Ｓ２（Ｓｉ））もしくはＭｎの積算値（Ｓ２（Ｍｎ））を求め、鋼板表面から深さ０．２μｍ位置までの積算値に深さ０．２μｍ位置から０．４μｍ位置までの積算値を割り付けることにより算出した。すなわち、Ｓｉの濃化量はＳ１（Ｓｉ）／Ｓ２（Ｓｉ）にて、Ｍｎの濃化量はＳ１（Ｍｎ）／Ｓ２（Ｍｎ）にて算出した。
(Iii) Si and Mn concentration measurement When a plating layer is provided on the surface of the steel sheet, after peeling the plating layer by pickling, the concentration profile of the steel sheet surface is measured by glow discharge emission spectrometry, The concentration distribution of Si and Mn inside was determined. The Si enrichment amount and the Mn enrichment amount are respectively the Si integrated value (S1 (Si)) or Mn integrated value (S1 (Mn)) and the depth of 0. The integrated value of Si (S2 (Si)) or the integrated value of Mn (S2 (Mn)) from the 2 μm position to the 0.4 μm position is obtained, and the integrated value from the steel plate surface to the depth of 0.2 μm position has a depth of 0. Calculated by assigning integrated values from 2 μm position to 0.4 μm position. That is, the concentration amount of Si was calculated by S1 (Si) / S2 (Si), and the concentration amount of Mn was calculated by S1 (Mn) / S2 (Mn).
（ｉＶ）めっき性調査
コイル長手方向の任意の５カ所から幅方向センター部よりサンプルを採取し、５００ｍｍ×５００ｍｍの範囲での不めっきや合金化不良の有無を目視で調査した。不めっきは斑点状に認められる局部的にめっき層が付与していない不具合であり、最小０．５ｍｍのものまで観察できた。合金化不良部分は適切に合金化された部分よりも明るい銀白色を呈しており、合金化不良は、このような部分に対してＩＣＰ発光分光分析により求めためっき相中に含まれるＦｅの含有量（Ｆｅ％（質量％））が８％未満である不具合であり、合金化処理を施したＧＡ材のみ評価した。不めっきが１０点／ｍ２以上認められた場合、もしくは合金化不良が認められた場合には評価を“×”とし、そうでない場合を“○”とした。
(IV) Plating property investigation Samples were taken from the center portion in the width direction from arbitrary five locations in the longitudinal direction of the coil, and the presence or absence of non-plating or alloying failure in the range of 500 mm x 500 mm was visually examined. Non-plating is a defect in which a plating layer is not locally applied, which is observed in the form of spots, and even a minimum of 0.5 mm can be observed. The poorly alloyed part exhibits a brighter silver white color than the appropriately alloyed part, and the poorly alloyed part contains Fe contained in the plating phase obtained by ICP emission spectroscopic analysis for such part. Only the GA material subjected to the alloying treatment was evaluated because the amount (Fe% (mass%)) was less than 8%. When non-plating was recognized at 10 points / m 2 or more, or when alloying failure was observed, the evaluation was “x”, and when it was not, the evaluation was “◯”.
（Ｖ）化成処理性調査
裸材を対象に化成処理性についても調査した。表面調整液には日本ペイント（株）製サーフファイン５Ｎ−１０、化成処理液には日本ペイント（株）製サーフダインＳＤ２５００を用い、液温４３℃で化成処理を施した。化成処理性の評価は化成処理後の鋼板表面を４００倍で１０視野観察し、化成結晶の空隙部の面積が撮影視野面積に対し１０％以上ある場合には“×”と評し、そうではない場合には“○”とした。
(V) Chemical conversion property investigation It investigated also about chemical conversion property about a bare material. Nippon Paint Co., Ltd. Surffine 5N-10 was used as the surface conditioning solution, and Nippon Paint Co., Ltd. Surfdyne SD2500 was used as the chemical conversion treatment solution. The chemical conversion treatment was evaluated by observing the surface of the steel sheet after chemical conversion at 10Ox with 10 fields of view, and when the area of the void portion of the chemical conversion crystal was 10% or more of the imaging field of view, it was evaluated as “x”. When there was not, it was set as “○”.
また、冷間圧延前の熱延鋼板から試験片を採取し、熱延鋼板の炭化物の平均粒径を求めた。なお、熱延鋼板の炭化物平均粒子径は、上記と同様の透過型電子顕微鏡の撮影写真により求めた。 Moreover, the test piece was extract | collected from the hot-rolled steel plate before cold rolling, and the average particle diameter of the carbide | carbonized_material of a hot-rolled steel plate was calculated | required. In addition, the carbide average particle diameter of the hot-rolled steel sheet was obtained from a photograph taken with a transmission electron microscope similar to the above.
以上により得られた結果を表３に示す。本発明例はいずれも、降伏比（＝ＹＳ／ＴＳ）が０．８４以上、引張強さＴＳ：５９０ＭＰａ以上であり、且つめっき性および化成処理性に優れることから、表面性状に優れた高降伏比冷延鋼板となっている。一方、本発明の範囲を外れる比較例は、所望の強度（引張強さ：５９０ＭＰａ以上）の高強度が確保できていないか、良好なめっき性、化成処理性が得られなかった。 The results obtained as described above are shown in Table 3. In all of the examples of the present invention, the yield ratio (= YS / TS) is 0.84 or more, the tensile strength TS is 590 MPa or more, and the plating property and the chemical conversion treatment property are excellent. It is a specific cold rolled steel sheet. On the other hand, in Comparative Examples that are out of the scope of the present invention, a high strength having a desired strength (tensile strength: 590 MPa or more) cannot be ensured, or good plating properties and chemical conversion properties are not obtained.
Claims (14)
Ｃ：０．０６％以上０．１４％以下、
Ｓｉ：０．３％以下、
Ｍｎ：０．５％未満、
Ｐ：０．０５％以下、
Ｓ：０．０１％以下、
Ａｌ：０．０８％以下、
Ｎ：０．００８０％以下、
Ｔｉ：０．０８％以上０．１５％以下
を含有し、残部がＦｅおよび不可避的不純物からなる組成と、フェライト相の面積率が９０％以上、前記フェライト相に対する加工フェライトの面積率が１０％以下（０％を含む）、前記フェライト相の結晶粒内のＴｉを含む炭化物の平均粒子径が１０ｎｍ以下である鋼組織を有し、下記式（１）で定めるＳｉ濃化量が１．３以下、下記式（２）で定めるＭｎ濃化量が１．３以下、引張強さが５９０ＭＰａ以上、降伏比が０．８４以上であることを特徴とする、高強度高降伏比冷延鋼板；
Ｓｉ濃化量＝Ｓ１（Ｓｉ）／Ｓ２（Ｓｉ）・・・（１）
Ｍｎ濃化量＝Ｓ１（Ｍｎ）／Ｓ２（Ｍｎ）・・・（２）
ただし、Ｓ１（Ｍ）は、グロー放電発光分析法により得た元素Ｍの濃度プロファイルにおける鋼板表面から０．２μｍの深さ位置までの元素Ｍの濃度の積算値を、Ｓ２（Ｍ）は、グロー放電発光分析法により得た元素Ｍの濃度プロファイルにおける鋼板表面から０．２μｍの深さ位置から０．４μｍの深さ位置までの元素Ｍの濃度の積算値を表す。 % By mass
C: 0.06% or more and 0.14% or less,
Si: 0.3% or less,
Mn: less than 0.5%,
P: 0.05% or less,
S: 0.01% or less,
Al: 0.08% or less,
N: 0.0080% or less,
Ti: 0.08% or more and 0.15% or less, with the balance being Fe and inevitable impurities, the area ratio of the ferrite phase is 90% or more, and the area ratio of the processed ferrite to the ferrite phase is 10% Below (including 0%), it has a steel structure in which the average particle diameter of the carbide containing Ti in the ferrite phase crystal grains is 10 nm or less, and the Si concentration determined by the following formula (1) is 1. 3. High strength, high yield ratio cold-rolled steel sheet characterized in that the Mn concentration determined by the following formula (2) is 1.3 or less , the tensile strength is 590 MPa or more, and the yield ratio is 0.84 or more. ;
Si enrichment amount = S1 (Si) / S2 (Si) (1)
Concentration of Mn = S1 (Mn) / S2 (Mn) (2)
However, S1 (M) is the integrated value of the concentration of element M from the steel sheet surface to the 0.2 μm depth position in the concentration profile of element M obtained by glow discharge emission spectrometry, and S2 (M) is the glow The integrated value of the concentration of the element M from the depth position of 0.2 μm to the depth position of 0.4 μm from the steel sheet surface in the concentration profile of the element M obtained by the discharge emission analysis method is represented.
前記鋼素材を、質量％で、Ｃ：０．０６％以上０．１４％以下、Ｓｉ：０．３％以下、Ｍｎ：０．５％未満、Ｐ：０．０５％以下、Ｓ：０．０１％以下、Ａｌ：０．０８％以下、Ｎ：０．００８０％以下、Ｔｉ：０．０８％以上０．１５％以下を含有し、残部がＦｅおよび不可避的不純物からなる組成とし、前記粗圧延に供する鋼素材の温度を１１００℃以上１３５０℃以下とし、前記仕上げ圧延の仕上げ圧延温度を８２０℃以上とし、前記冷却を仕上げ圧延終了後２秒以内に開始し、前記冷却の平均冷却速度を２０℃／ｓ以上とし、前記巻き取りの巻取り温度を７００℃以下とし、前記冷間圧延の冷間圧延率を３０％以上７５％以下とし、前記焼鈍を、７００℃から焼鈍温度までの平均昇温速度が１０℃／ｓ以下、焼鈍温度が７５０℃以上９００℃以下とすることを特徴とする、フェライト相の面積率が９０％以上、前記フェライト相に対する加工フェライトの面積率が１０％以下（０％を含む）、前記フェライト相の結晶粒内のＴｉを含む炭化物の平均粒子径が１０ｎｍ以下である鋼組織を有し、下記式（１）で定めるＳｉ濃化量が１．３以下、下記式（２）で定めるＭｎ濃化量が１．３以下、引張強さが５９０ＭＰａ以上、降伏比が０．８４以上である高強度高降伏比冷延鋼板の製造方法。
Ｓｉ濃化量＝Ｓ１（Ｓｉ）／Ｓ２（Ｓｉ）・・・（１）
Ｍｎ濃化量＝Ｓ１（Ｍｎ）／Ｓ２（Ｍｎ）・・・（２）
ただし、Ｓ１（Ｍ）は、グロー放電発光分析法により得た元素Ｍの濃度プロファイルにおける鋼板表面から０．２μｍの深さ位置までの元素Ｍの濃度の積算値を、Ｓ２（Ｍ）は、グロー放電発光分析法により得た元素Ｍの濃度プロファイルにおける鋼板表面から０．２μｍの深さ位置から０．４μｍの深さ位置までの元素Ｍの濃度の積算値を表す。 The steel material is subjected to hot rolling consisting of rough rolling and finish rolling, and after finishing rolling, cooled and wound, cold rolled, and annealed to make a cold rolled steel sheet,
The steel material is, in mass%, C: 0.06% or more and 0.14% or less, Si: 0.3% or less, Mn: less than 0.5%, P: 0.05% or less, S: 0.00. 01% or less, Al: 0.08% or less, N: 0.0080% or less, Ti: 0.08 % or more and 0.15% or less, with the balance being composed of Fe and inevitable impurities, The temperature of the steel material used for rolling is set to 1100 ° C. or higher and 1350 ° C. or lower, the finish rolling temperature of the finish rolling is set to 820 ° C. or higher, the cooling is started within 2 seconds after finishing rolling, and the average cooling rate of the cooling is set. 20 ° C./s or more, the winding temperature of the winding is 700 ° C. or less, the cold rolling rate of the cold rolling is 30% or more and 75% or less, and the annealing is an average from 700 ° C. to the annealing temperature. Temperature increase rate is 10 ° C / s or less, annealing temperature is 750 ° C or less Characterized by a 900 ° C. or less, the area ratio of the ferrite phase is 90% or more, (including 0%) 10% or less the area ratio of the deformed ferrite for the ferrite phase, Ti in the crystal grains of the ferrite phase A carbide structure containing a steel structure having an average particle diameter of 10 nm or less, a Si concentration determined by the following formula (1) is 1.3 or less, and a Mn concentration determined by the following formula (2) is 1.3. Hereinafter, a method for producing a high strength, high yield ratio cold rolled steel sheet having a tensile strength of 590 MPa or more and a yield ratio of 0.84 or more .
Si enrichment amount = S1 (Si) / S2 (Si) (1)
Concentration of Mn = S1 (Mn) / S2 (Mn) (2)
However, S1 (M) is the integrated value of the concentration of element M from the steel sheet surface to the 0.2 μm depth position in the concentration profile of element M obtained by glow discharge emission spectrometry, and S2 (M) is the glow The integrated value of the concentration of the element M from the depth position of 0.2 μm to the depth position of 0.4 μm from the steel sheet surface in the concentration profile of the element M obtained by the discharge emission analysis method is represented.
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