JP6086080B2 - High-strength cold-rolled steel sheet and manufacturing method thereof - Google Patents
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Description
本発明は、自動車用部材の使途に有用な、引張強さ（ＴＳ）が４４０ＭＰａ以上の高強度と優れた加工性、特に優れた曲げ性を兼ね備えた高強度冷延鋼板およびその製造方法に関する。 TECHNICAL FIELD The present invention relates to a high-strength cold-rolled steel sheet having high tensile strength (TS) of not less than 440 MPa and excellent workability, particularly excellent bendability, and a method for producing the same, which are useful for the use of automobile members.
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。自動車車体の軽量化と強化を同時に満たすには、剛性が問題とならない範囲で部材素材を薄肉化することが有効であり、薄肉した部材素材の安全性を保証するには鋼板の高強度化が有効である。軽量化効果は、使用する鋼板が高強度であるほど大きくなる。 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. In order to satisfy the weight reduction and strengthening of the automobile body at the same time, it is effective to reduce the thickness of the material so long as rigidity does not matter. To ensure the safety of the thinner material, it is necessary to increase the strength of the steel sheet. It is valid. The weight reduction effect increases as the steel sheet used has higher strength.
一方、鋼板を素材とする自動車部品の多くは、プレス加工によって成形される。鋼板の高強度化にともない、割れ、しわ、寸法精度といった成形性が劣位となる。割れやしわは鋼板の延性や表面性状等に関係し、寸法精度はスプリングバック量と相関が強い降伏強度に関係がある。割れやしわを抑制できる加工性の良い高強度冷延鋼板が求められている。 On the other hand, many automobile parts made of steel plates are formed by press working. As the strength of the steel sheet increases, the formability such as cracks, wrinkles, and dimensional accuracy becomes inferior. Cracks and wrinkles are related to the ductility and surface properties of the steel sheet, and the dimensional accuracy is related to the yield strength, which has a strong correlation with the amount of springback. A high-strength cold-rolled steel sheet with good workability that can suppress cracks and wrinkles is required.
また、近年のＣＡＥ（Ｃｏｍｐｕｔｅｒ Ａｓｓｉｓｔｅｄ Ｅｎｇｉｎｅｅｒｉｎｇ）技術の発達により高強度鋼板の成形技術が向上してきた。しかしながら、鋼板の材質ばらつきが大きい場合にはＣＡＥによる予測精度が低下し、寸法精度良く成形することができなくなる。このような背景から、加工性に優れるとともに、材質安定性にも優れた高強度冷延鋼板が求められている。 Further, with the recent development of CAE (Computer Assisted Engineering) technology, the forming technology of high-strength steel sheets has been improved. However, when the material variation of the steel sheet is large, the prediction accuracy by CAE is lowered and it becomes impossible to form with high dimensional accuracy. From such a background, there is a demand for a high-strength cold-rolled steel sheet that is excellent in workability and excellent in material stability.
以上から、高強度冷延鋼板を自動車部品等に適用するうえでは、強度と加工性、材質安定性とを両立させることが必須である。高強度化や成形性を向上させる技術は現在までに様々なものが提案されている。 From the above, when applying a high-strength cold-rolled steel sheet to automobile parts and the like, it is essential to achieve both strength, workability, and material stability. Various techniques for increasing the strength and improving the moldability have been proposed so far.
例えば、特許文献１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１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。 For example, Patent Document 1 discloses that the steel sheet composition is mass%, C: 0.001 to 0.2%, N: 0.0001 to 0.2%, C + N: 0.002 to 0.3%, Si: 0.001 to 0.1%, Mn: 0.01 to 1%, Ti: 0.001 to 0.1%, Nb: 0.001 to 0.1%, diameter 1 to 10 nm in steel A technology for producing a thin steel sheet excellent in room temperature slow aging and bake hardenability, characterized in that it contains a fine precipitate of 1 × 10 17 particles / cm 3 or more. According to the technique proposed in Patent Document 1, C or N is fixed as a fine precipitate, and C and N fixed during the coating baking process are desorbed and diffused, thereby being excellent in normal temperature slow aging and bake hardenability. It is said that a thin steel plate can be obtained.
また、特許文献２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２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た鋼板が得られるとしている。 Moreover, in patent document 2, steel plate composition is the mass%, C: more than 0.01%-0.1%, Si: 0.3% or less, Mn: 0.2-2.0%, N: 0 0.006% or less, Ti: 0.03 to 0.2%, Mo: 0.5% or less, and W: 1.0% or less, and 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 are dispersed is proposed. Has been. According to Patent Document 2, a steel sheet that has both workability and strength can be obtained by dispersing 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. It is supposed to be 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.
しかしながら、特許文献１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。 However, in the technique proposed in Patent Document 1, 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 is contained. Cannot be obtained.
特許文献２で提案された技術では、実施例を参照すると、加工性を低下させるＭｎを多量に含有しているため、本発明で求める加工性を安定的に発現させるのが困難であるという問題がある。さらに、焼鈍工程での再結晶を阻害させるＭｏおよびＷを多量に含むため、加工フェライト組織が残存しやすく加工性が低下するという問題もある。特許文献３で提案された技術でも特許文献２と同様に、加工性を低下させるＭｎを多量に含有している。 In the technique proposed in Patent Document 2, referring to the examples, since Mn is contained in a large amount to reduce workability, it is difficult to stably develop the workability required in the present invention. There is. Furthermore, since it contains a large amount of Mo and W that inhibit recrystallization in the annealing process, there is also a problem that the work ferrite structure tends to remain and the workability is lowered. Similarly to Patent Document 2, the technique proposed in Patent Document 3 contains a large amount of Mn that reduces workability.
以上のように、従来技術では低いＭｎ含有量で高強度の冷延鋼板を得ることができず、良好な加工性を有する高強度冷延鋼板を得ることができない。本発明はかかる事情を鑑みてなされたものであって、４４０ＭＰａ以上の引張強さとともに、優れた加工性、特に優れた曲げ加工性を有し、材質安定性に優れた高強度冷延鋼板を提供することを目的とする。 As described above, the conventional technique cannot obtain a high-strength cold-rolled steel sheet with a low Mn content, and cannot obtain a high-strength cold-rolled steel sheet having good workability. The present invention has been made in view of such circumstances, and a high-strength cold-rolled steel sheet having excellent workability, particularly excellent bending workability, and excellent material stability along with a tensile strength of 440 MPa or more. The purpose is to provide.
本発明者らは、加工性と材質安定性を両立させるべく軟質なフェライト単一組織に着目した。その結果、本発明者らは、安定的に加工性を良好なものとするには、固溶強化および偏析により延性を低下させるＭｎ含有量を制限する必要があることを知見した。 The inventors focused on a soft ferrite single structure in order to achieve both workability and material stability. As a result, the present inventors have found that it is necessary to limit the Mn content that lowers the ductility by solid solution strengthening and segregation in order to stably improve the workability.
従来、高強度鋼板を得るにあたっては、Ｍｎを多量に添加し、Ｍｎの固溶強化により強度を得ていた。したがって、Ｍｎ含有量を従来に比べて低く制限するには、高強度化が課題であった。発明者らはこのような課題に対し、フェライト粒内に炭化物を微細分散させて強度を得る、粒子分散強化機構に着目して鋭意検討した。 Conventionally, when obtaining a high-strength steel sheet, a large amount of Mn is added and strength is obtained by solid solution strengthening of Mn. Therefore, in order to limit the Mn content to be lower than before, increasing the strength has been a problem. The inventors have intensively studied to solve such a problem by paying attention to a particle dispersion strengthening mechanism in which carbide is finely dispersed in ferrite grains to obtain strength.
微細な炭化物が分散した鋼であると、低い焼鈍温度では、粒界のピン止め効果により再結晶組織を得ることができず、加工性が著しく損なわれる。一方で、高い焼鈍温度では、炭化物が粗大化するため所望の高強度鋼板を得ることが困難である。この問題を解決するため、焼鈍中の炭化物粗大化挙動に着目して検討を行ったところ、逆変態したオーステナイト中での炭化物の粗大化速度は、フェライト中の粗大化速度よりも大きいことが判明した。焼鈍温度に対する回復、再結晶挙動を詳細に調査した結果、５００℃以上であれば、転位密度が減少し始め、回復、再結晶が開始されることを見出した。そこで、次の２点により材質の安定性と加工性の両立を図った。 In a steel in which fine carbides are dispersed, at a low annealing temperature, a recrystallized structure cannot be obtained due to the pinning effect of the grain boundaries, and the workability is significantly impaired. On the other hand, it is difficult to obtain a desired high-strength steel sheet at a high annealing temperature because carbides are coarsened. In order to solve this problem, we examined the coarsening behavior of carbides during annealing, and found that the coarsening rate of carbides in reverse transformed austenite was larger than the coarsening rate in ferrite. did. As a result of detailed investigation of the recovery and recrystallization behavior with respect to the annealing temperature, it was found that if it is 500 ° C. or higher, the dislocation density starts to decrease and the recovery and recrystallization are started. Therefore, both the stability of the material and workability were achieved by the following two points.
１）微細炭化物の粗大化を抑制するためには、逆変態前のフェライト中で回復、再結晶を促進させることが望ましい。そのため、焼鈍工程での昇温速度を遅くすることで逆変態前に回復、再結晶を促進させることが可能となる。転位の消滅は５００℃以上から開始されるため、特に５００℃以上での昇温速度の制御が重要となる。 1) In order to suppress coarsening of fine carbides, it is desirable to promote recovery and recrystallization in ferrite before reverse transformation. Therefore, it is possible to recover and promote recrystallization before reverse transformation by slowing the rate of temperature increase in the annealing process. Since the disappearance of dislocation starts from 500 ° C. or higher, it is particularly important to control the rate of temperature rise at 500 ° C. or higher.
２）さらに逆変態前で回復、再結晶させるには、Ａｃ１点を上昇させることも有効である。Ａｃ１点上昇させるにはＳｉおよびＣｒの添加が有効である。 2) It is also effective to raise the Ac1 point in order to recover and recrystallize before reverse transformation. Addition of Si and Cr is effective for increasing the Ac1 point.
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：０．０３％以上０．１５％以下、
Ｓｉ：１．５％以下、
Ｍｎ：０．６％以下、
Ｐ：０．０５％以下、
Ｓ：０．０１％以下、
Ａｌ：０．０８％以下、
Ｎ：０．００８０％以下、
Ｔｉ：０．０４％以上０．１８％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を有し、フェライト相の面積率が９０％以上、コイル面内における該フェライト相の面積率のばらつきが３％以下、前記フェライト相に対する加工フェライトの面積率が１５％以下、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、含有するＴｉ量に対しマトリックス中に固溶状態として存在するＴｉ量の割合が１０％未満であることを特徴とする、高強度冷延鋼板。
[1] By mass%
C: 0.03% to 0.15%,
Si: 1.5% or less,
Mn: 0.6% or less,
P: 0.05% or less,
S: 0.01% or less,
Al: 0.08% or less,
N: 0.0080% or less,
Ti: 0.04% or more and 0.18% or less, with the balance being composed of Fe and inevitable impurities, the ferrite phase area ratio being 90% or more, the ferrite phase area ratio in the coil plane 3% or less, the area ratio of the processed ferrite with respect to the ferrite phase is 15% or less, the average particle size of the carbide containing Ti in the crystal grains of the ferrite phase is 10 nm or less, and the amount of Ti contained in the matrix A high-strength cold-rolled steel sheet, characterized in that the proportion of Ti present as a solid solution is less than 10%.
［２］前記組成に加えてさらに、質量％で、Ｃｒ：０．０１％以上１．０％以下を含有することを特徴とする、前記［１］に記載の高強度冷延鋼板。 [2] The high-strength cold-rolled steel sheet according to [1], further containing Cr: 0.01% to 1.0% by mass% in addition to the composition.
［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］に記載の高強度冷延鋼板。 [3] In addition to the above composition, the composition further contains one or more of Ca, Mg, and REM in a mass percentage of 0.0001% to 0.2% in total [1] ] Or a high-strength cold-rolled steel sheet according to [2].
［４］前記組成に加えてさらに、質量％で、Ｖ：０．０１％以上０．２％以下、Ｎｂ：０．０１％以上０．０５％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］ないし［３］のいずれか１項に記載の高強度冷延鋼板。 [4] In addition to the above composition, V: 0.01% to 0.2%, Nb: 0.01% to 0.05%, 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 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］ないし［４］のいずれか１項に記載の高強度冷延鋼板。 [5] The high-strength 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］に記載の高強度冷延鋼板。 [6] The high-strength 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］に記載の高強度冷延鋼板。 [7] The high-strength 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：０．０３％以上０．１５％以下、Ｓｉ：１．５％以下、Ｍｎ：０．６％以下、Ｐ：０．０５％以下、Ｓ：０．０１％以下、Ａｌ：０．０８％以下、Ｎ：０．００８０％以下、Ｔｉ：０．０４％以上０．１８％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４００℃以上７００℃以下とし、前記冷間圧延の冷間圧延率を３０％以上９０％以下とし、前記焼鈍を、５００℃から最高到達温度までの平均昇温速度を７℃／ｓ以下、焼鈍温度を７００℃以上８５０℃以下とすることを特徴とする、高強度冷延鋼板の製造方法。 [8] The steel material is subjected to hot rolling consisting of rough rolling and finish rolling, and after completion of finish rolling, the steel material is cooled, wound, cold-rolled, and annealed to form a cold-rolled steel sheet. In mass%, C: 0.03% or more and 0.15% or less, Si: 1.5% or less, Mn: 0.6% or less, P: 0.05% or less, S: 0.01% or less Al: 0.08% or less, N: 0.0080% or less, Ti: 0.04% or more and 0.18% or less, with the balance being composed of Fe and inevitable impurities, and subjected to the rough rolling The temperature of the steel material is set to 1100 ° C. or more and 1350 ° C. or less, the finish rolling temperature of the finish rolling is set to 820 ° C. or more, the cooling is started within 2 seconds after finishing rolling, and the average cooling rate of the cooling is 20 ° C. / s or more, and the winding temperature of the winding is 400 ° C. or more and 70 The average rolling rate from 500 ° C. to the highest temperature is 7 ° C./s or less, and the annealing temperature is 700 ° C. A method for producing a high-strength cold-rolled steel sheet, characterized by being 850 ° C. or lower.
［９］前記鋼素材が、前記組成に加えてさらに、質量％で、Ｃｒ：０．０１％以上１．０％以下を含有する組成とすることを特徴とする、前記［８］に記載の高強度冷延鋼板の製造方法。 [9] The steel material according to [8], wherein the steel material further contains, in addition to the composition, Cr: 0.01% to 1.0% in mass%. Manufacturing method of high-strength cold-rolled steel sheet.
［１０］前記鋼素材が、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］に記載の高強度冷延鋼板の製造方法。 [10] The steel material further includes 0.0001% or more and 0.2% or less in total of one or more of Ca, Mg, and REM in mass% in addition to the composition. The method for producing a high-strength cold-rolled steel sheet according to [8] or [9].
［１１］前記鋼素材が、前記組成に加えてさらに、質量％で、Ｖ：０．０１％以上０．２％以下、Ｎｂ：０．０１％以上０．０５％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］ないし［１０］のいずれか１項に記載の高強度冷延鋼板の製造方法。 [11] In addition to the above composition, the steel material further includes, in mass%, V: 0.01% to 0.2%, Nb: 0.01% to 0.05%, Mo: 0.01 % To 0.05%, W: 0.01% to 0.05%, Hf: 0.01% to 0.05%, Zr: 0.01% to 0.1%, Co: 0 The method for producing a high-strength cold-rolled steel sheet according to any one of the above [8] to [10], comprising one or more of 0.0001% or more and 0.1% 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］ないし［１１］のいずれか１項に記載の高強度冷延鋼板の製造方法。 [12] The method for producing a high-strength 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］に記載の高強度冷延鋼板の製造方法。 [13] The method for producing a high-strength 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］に記載の高強度冷延鋼板の製造方法。 [14] The method for producing a high-strength 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：４４０ＭＰａ以上であり且つ加工性、材質安定性に優れた高強度冷延鋼板が得られ、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。また、加工性を兼ね備えた引張強さ：４４０ＭＰａ以上の高強度冷延鋼板が得られることから、高強度冷延鋼板の更なる用途展開が可能となり、産業上格段の効果を奏する。 According to the present invention, a high-strength cold-rolled steel sheet having a tensile strength of 440 MPa or more and excellent workability and material stability can be obtained, suitable for use as a structural member of an automobile, and weight reduction of an automobile member. And effects such as enabling molding of automobile parts. Moreover, since a high strength cold-rolled steel sheet having a workability and a tensile strength of 440 MPa or more can be obtained, further application development of the high-strength cold-rolled steel sheet becomes possible, and a remarkable industrial effect is 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：０．０３％以上０．１５％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４４０ＭＰａ以上を確保するには、Ｃ含有量は０．０３％以上とする必要があり、０．０４％以上とすることが好ましい。さらに、炭化物形成に関与しなかったＣは、熱延板ではパーライトを形成し再結晶を促進させる効果もある。このような効果を得るには、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、下記式（１）を満たすことが好ましい。より好ましくは、式（１）の左辺は１．５以上である。なお、式（１）中の元素記号は、各元素の含有量（質量％）を表す。一方、Ｃ含有量が０．１５％を超えると、フェライト粒界に、フィルム状のセメンタイトの他に加工性を著しく低下させる粒状のセメンタイトやパーライトが生成する。このため、Ｃ含有量は０．１５％以下とする。好ましくは０．１４％以下である。
（Ｃ／１２）／｛（Ｔｉ／４８）＋（Ｖ／５１）＋（Ｎｂ／９３）＋（Ｍｏ／９６）
＋（Ｗ／１８４）＋（Ｈｆ／１７６）＋（Ｚｒ／９１）｝≧１．２・・・（１）
C: 0.03% or more and 0.15% 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 440 MPa or more, the C content needs to be 0.03% or more, and preferably 0.04% or more. . Furthermore, C that was not involved in carbide formation also has the effect of promoting crystallization by forming pearlite in the hot-rolled sheet. In order to obtain such an effect, C is the content of Ti which is a carbide constituent element, and further, when V, Nb, Mo, W, Zr, and Hf are added, It is desirable to make it contain excessively, and it is preferable to satisfy | fill following formula (1). More preferably, the left side of Formula (1) is 1.5 or more. In addition, the element symbol in Formula (1) represents content (mass%) of each element. On the other hand, when the C content exceeds 0.15%, granular cementite and pearlite that remarkably lower the workability in addition to the film-like cementite are generated at the ferrite grain boundaries. For this reason, C content shall be 0.15% or less. Preferably it is 0.14% or less.
(C / 12) / {(Ti / 48) + (V / 51) + (Nb / 93) + (Mo / 96)
+ (W / 184) + (Hf / 176) + (Zr / 91)} ≧ 1.2 (1)
Ｓｉ：１．５％以下
Ｓｉは、冷間圧延後、焼鈍工程での逆変態温度（Ａｃ１点）を上昇させ、逆変態で生じるオーステナイト相の生成を抑制する。オーステナイト中での炭化物は粗大化しやすいため、Ｓｉは焼鈍工程での炭化物粗大化抑制に寄与し、高強度化させる元素として有効である。この効果を得るためには、Ｓｉ量を０．１％以上とすることが好ましい。一方で、Ｓｉは、鋼板表面に濃化し易く、鋼板表面にファイヤライト（Ｆｅ２ＳｉＯ４）を形成し、鋼板の表面性状を低下させる。このファイヤライトは鋼板表面に楔形となって形成するため、鋼板の加工性をさせ、特に曲げ加工時の亀裂発生の起点となり、曲げ加工性を低下させる。したがって、本発明ではＳｉ含有量の上限を１．５％とする必要がある。好ましくは、１．０％以下である。
Si: 1.5% or less Si, after cold rolling, raises the reverse transformation temperature (Ac1 point) in the annealing process and suppresses the formation of the austenite phase generated by the reverse transformation. Since carbides in austenite are easily coarsened, Si contributes to the suppression of carbide coarsening in the annealing process and is effective as an element for increasing the strength. In order to obtain this effect, the Si amount is preferably 0.1% or more. On the other hand, Si is easy to concentrate on the surface of the steel sheet, forms firelite (Fe 2 SiO 4 ) on the surface of the steel sheet, and lowers the surface properties of the steel sheet. Since this firelite is formed in a wedge shape on the surface of the steel sheet, the workability of the steel sheet is increased, and in particular, it becomes a starting point of crack generation during bending, and the bending workability is lowered. Therefore, in the present invention, the upper limit of the Si content needs to be 1.5%. Preferably, it is 1.0% or less.
Ｍｎ：０．６％以下
Ｍｎは固溶強化元素として鋼板を強化する一方で、延性を低下させる元素である。さらには、不可避的に生じる板厚中央付近での偏析により著しく加工性を低下させる。本発明では、安定的に良好な加工性を得るために、極力Ｍｎ量を低減する。安定的に加工性を良好なものとするには、Ｍｎ量は０．６％以下とする必要があり、好ましくは０．５％未満である。なお、Ｍｎ含有量は不純物レベルまで低減してもよい。
Mn: 0.6% or less Mn is an element that lowers ductility while strengthening a steel sheet as a solid solution strengthening element. Furthermore, workability is significantly reduced by segregation in the vicinity of the center of the plate thickness which inevitably occurs. In the present invention, the amount of Mn is reduced as much as possible in order to stably obtain good workability. In order to stably improve the workability, the amount of Mn needs to be 0.6% or less, preferably less than 0.5%. Note that the Mn content may be reduced to the impurity level.
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含有量は不純物レベルまで低減してもよい。
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 reduced to the impurity level.
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、極力低減することが好ましく、Ｓ含有量は不純物レベルまで低減してもよい。
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 as much as possible. The S content may be reduced to the impurity level.
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、Ａｌ量は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, the Al content 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、Ｎ含有量は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, the N content is 0.0060% or less. It is preferable to reduce the N content as much as possible.
Ｔｉ：０．０４％以上０．１８％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Ｍｎを低減しているため、所望の鋼板強度を得るにはＴｉを添加して、Ｔｉを含む炭化物を微細に分散させる必要がある。Ｔｉ含有量が０．０４％を下回ると所望の鋼板強度（引張強さ：４４０ＭＰａ以上）が得られなくなるため、Ｔｉ含有量の下限を０．０４％とする。好ましくは、Ｔｉ含有量は０．０５％以上である。一方、Ｔｉ含有量が０．１８％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ばかりか、粗大なＴｉ炭化物は曲げ加工時にボイドの起点となり、加工性が低下する。このため、Ｔｉ含有量の上限を０．１８％とする。好ましくは、Ｔｉ含有量は０．１７％以下である。したがって、Ｔｉ含有量は０．０４％以上０．１８％以下とし。好ましくは０．０５％以上０．１７％以下である。特に、引張強さ：５９０ＭＰａ以上の鋼板を得る場合には、Ｔｉ含有量は０．０９％以上０．１８％以下とすることが望ましい。
Ti: 0.04% or more and 0.18% or less Ti is an element that contributes to increasing the strength of the steel sheet by forming carbides with C. In particular, in the present invention, Mn, which is a solid solution strengthening element, is reduced. Therefore, in order to obtain a desired steel sheet strength, it is necessary to add Ti and finely disperse the carbide containing Ti. If the Ti content is less than 0.04%, the desired steel plate strength (tensile strength: 440 MPa or more) cannot be obtained, so the lower limit of the Ti content is set to 0.04%. Preferably, the Ti content is 0.05% or more. On the other hand, when the Ti content exceeds 0.18%, when manufacturing a steel sheet, coarse Ti carbide cannot be dissolved by slab heating before hot rolling, and not only the effect of increasing the strength is saturated. Coarse Ti carbide becomes the starting point of voids during bending, and the workability decreases. For this reason, the upper limit of the Ti content is set to 0.18%. Preferably, the Ti content is 0.17% or less. Therefore, the Ti content is 0.04% or more and 0.18% or less. Preferably they are 0.05% or more and 0.17% or less. In particular, when obtaining a steel sheet having a tensile strength of 590 MPa or more, the Ti content is preferably 0.09% or more and 0.18% 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.
Ｃｒ：０．０１％以上１．０％以下
ＣｒはＳｉと同様、Ａｃ１点を上昇させ、炭化物の粗大化の抑制に寄与する元素である。この効果を得るためには、少なくとも０．０１％以上添加する必要があり、０．０５％以上添加することが好ましい。さらに好ましくは、Ｃｒ量は０．１％以上である。Ｃｒを多量添加すると耐食性が低下し、孔食による不具合の原因となる。そのため、Ｃｒ量の上限は１．０％とした。好ましくは、Ｃｒ量は０．８％以下である。
Cr: 0.01% or more and 1.0% or less Cr, like Si, is an element that increases the Ac1 point and contributes to suppression of coarsening of carbides. In order to acquire this effect, it is necessary to add at least 0.01% or more, and it is preferable to add 0.05% or more. More preferably, the Cr amount is 0.1% or more. When a large amount of Cr is added, the corrosion resistance is lowered, which causes problems due to pitting corrosion. Therefore, the upper limit of Cr content is set to 1.0%. Preferably, the Cr content is 0.8% or less.
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少なくともＣａ、Ｍｇ、ＲＥＭの１種または２種以上を合計で０．０００１％以上を添加する必要がある。一方で、これらの元素の含有量の合計が０．２％を超えると、上記効果が飽和するため、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 voids generated from the inclusions. In order to obtain such an effect, it is necessary to add at least 0.0001% or more of at least one of Ca, Mg, and REM. On the other hand, when the total content of these elements exceeds 0.2%, the above effect is saturated, so the upper limit of the total amount of one or more of Ca, Mg, and REM is 0.2%. did. A preferable range is 0.0005% or more and 0.1% or less in total of one or more of Ca, Mg, and REM.
Ｖ：０．０１％以上０．２％以下、Ｎｂ：０．０１％以上０．０５％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
ＶおよびＮｂ、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、ＶおよびＮｂ、Ｍｏ、Ｗ、Ｈｆ、Ｚｒはそれぞれ０．０１％以上、Ｃｏは０．０００１％以上を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０．２％を超える含有は強化に対する効果が飽和、もしくは含有量の増量につれ強度が低下する。以上からＶ含有量の上限を０．２％とする。好ましいＶ含有量の上限は０．１５％である。Ｎｂは再結晶時にｓｏｌｕｔｅ ｄｒａｇ効果により粒界移動を阻害させ加工フェライト粒が残存しやすくなり、この加工フェライト粒は加工性を低下させることとなる。０．０５％以下のＮｂ含有量であれば上記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。望ましいＮｂ含有量の上限は０．０４％である。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％を超えると加工性に悪影響をもたらすため、Ｚｒ、Ｃｏの含有量の上限はそれぞれ０．１％、Ｍｏ、Ｗ、Ｈｆの含有量の上限はそれぞれ０．０５％とした。特に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を２種以上含有させる場合には、これらの元素の含有量を合計で０．１％以下とすることが好ましい。
V: 0.01% to 0.2%, Nb: 0.01% to 0.05%, Mo: 0.01% to 0.05%, W: 0.01% to 0.05% Hereinafter, Hf: 0.01% to 0.05%, Zr: 0.01% to 0.1%, Co: 0.0001% to 0.1%, one or two or more V and Nb , 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 strength of the steel sheet, it is necessary to add 0.01% or more of V and Nb, Mo, W, Hf, and Zr, and 0.0001% or more of Co. On the other hand, V tends to coarsen the carbide, and if the content exceeds 0.2%, the effect on strengthening is saturated, or the strength decreases as the content increases. From the above, the upper limit of V content is 0.2%. A preferable upper limit of the V content is 0.15%. Nb inhibits grain boundary movement due to the solution drag effect at the time of recrystallization, and the processed ferrite grains tend to remain, and the processed ferrite grains lower the workability. If the Nb content is 0.05% or less, the above-described adverse effect does not become obvious, so the upper limit of the Nb content is set to 0.05%. A desirable upper limit of Nb content is 0.04%. If the Zr and Co contents are each 0.1% and the Mo, W, and Hf contents are more than 0.05%, the workability is adversely affected. The upper limit of the content of 1%, Mo, W, and Hf was 0.05%. In particular, since Mo, W, Hf, and Zr are elements that easily inhibit recrystallization, when two or more of Mo, W, Hf, and Zr are contained, the total content of these elements is 0.1%. The following is preferable.
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、フェライト以外の組織であ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こととなる。また、ベイナイト相やマルテンサイト相は延性に乏しいためベイナイト相もしくはマルテンサイト相の単相組織では所望の加工性が得られない。本発明鋼においてフェライト相の面積率は９０％以上とすることで、優れた加工性を得ることができるため、フェライト相の面積率下限を９０％とした。好ましくは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. If the second phase structure, which is a structure other than ferrite and has a 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. As a result, defects such as cracks occur. In addition, since the bainite phase and the martensite phase are poor in ductility, a desired workability cannot be obtained with a single phase structure of the bainite phase or the martensite phase. In the steel of the present invention, when the area ratio of the ferrite phase is 90% or more, excellent workability can be obtained, so the lower limit of the area ratio of the ferrite phase is 90%. Preferably it is 95% or more.
コイル面内におけるフェライト相の面積率のばらつきが３％以下
強度や延性といった材質を安定化させるには、コイル面内での金属組織を安定化させることが有効となる。フェライト相の面積率のばらつきが大きい場合には、硬質な第二相の面積率のばらつきが大きくなり、強度や延性にばらつきが生じる。所望の特性を得るには、該フェライト相の面積率のばらつきは３％以下である必要がある。好ましくは、２％以下である。ここで、フェライト相の面積率のばらつきは、コイル面内におけるフェライト相の面積率のばらつきであり、次のようにして求める。
すなわち、面積率のばらつきを評価する材料のコイル位置（評価する材料の採取位置）は、コイル状に巻き取った鋼板のコイル内周および外周の１巻き分を除き、鋼板の長手方向に対し２０等分割し、幅方向に対し１／４、１／２、３／４の位置とする。このような位置から採取した各々の材料について、フェライト相の面積率を求める。コイル面内におけるフェライト相の面積率のばらつきは、このようにして求めたフェライト相の面積率の標準偏差に２を乗じた値とする。
In order to stabilize materials such as strength and ductility, it is effective to stabilize the metal structure within the coil surface. When the variation in the area ratio of the ferrite phase is large, the variation in the area ratio of the hard second phase becomes large, and the strength and ductility vary. In order to obtain desired characteristics, the variation in the area ratio of the ferrite phase needs to be 3% or less. Preferably, it is 2% or less. Here, the variation in the area ratio of the ferrite phase is a variation in the area ratio of the ferrite phase in the coil surface, and is obtained as follows.
That is, the coil position of the material for which the variation in the area ratio is evaluated (the sampling position of the material to be evaluated) is 20 with respect to the longitudinal direction of the steel sheet, except for the coil inner circumference and one turn of the outer circumference of the steel sheet wound in a coil shape. Equally divided and set to 1/4, 1/2, and 3/4 positions in the width direction. For each material sampled from such a position, the area ratio of the ferrite phase is obtained. The variation in the area ratio of the ferrite phase in the coil surface is a value obtained by multiplying the standard deviation of the area ratio of the ferrite phase thus obtained by 2.
なお、フェライト相の平均結晶粒径は、１０μｍ以下とすることが好ましい。再結晶後のフェライト粒径は鋼中に分散する炭化物の大きさと相関を持つ。フェライト相の平均結晶粒径が１０μｍを超える場合には、炭化物が過度に粗大化しピン止め効果が低下したことを意味する。所望の炭化物の平均粒子径を得るには、該フェライト相の平均結晶粒径が１０μｍ以下であることが望ましい。さらに望ましくは、８μｍ以下である。 Note that the average crystal grain size of the ferrite phase is preferably 10 μm or less. The ferrite grain size after recrystallization has a correlation with the size of carbides dispersed in the steel. When the average crystal grain size of the ferrite phase exceeds 10 μm, it means that the carbide is excessively coarsened and the pinning effect is reduced. In order to obtain an average particle size of the desired carbide, it is desirable that the average crystal particle size of the ferrite phase is 10 μm or less. More desirably, it is 8 μm or less.
フェライト相に対する加工フェライトの面積率が１５％以下
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。加工フェライトの面積率は１５％以下であれば加工性への悪影響が顕在化せず、延性のばらつきが許容される範囲となる。望ましくは１０％以下である。なお、ここで加工フェライトの面積率は、フェライト相に対する加工フェライトの面積率であり、フェライト相全体に占める加工フェライトの面積率である。
The area ratio of the processed ferrite with respect to the ferrite phase is 15% or less After the cold rolling, the entire steel sheet has a processed structure. Since this structure contains a large amount of dislocations in the grains, the ductility is remarkably poor and the workability is lowered. If the area ratio of the processed ferrite is 15% or less, the adverse effect on the workability does not become obvious, and a variation in ductility is allowed. Desirably, it is 10% or less. Here, the area ratio of the processed ferrite is the area ratio of the processed ferrite with respect to the ferrite phase, and is the area ratio of the processed ferrite in the entire ferrite phase.
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
固溶強化元素であるＭｎを低減したため、本発明鋼では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、所望の鋼板強度を得るには、フェライト相の結晶粒内の炭化物平均粒子径は１０ｎｍ以下とする必要がある。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。
The average particle diameter of the carbide containing Ti in the ferrite phase crystal grains is 10 nm or less. Since Mn, which is a solid solution strengthening element, is reduced, it is necessary to maximize the strengthening amount by particle dispersion strengthening in the steel of the present invention. 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, the carbide average particle diameter in the ferrite phase crystal grains needs to be 10 nm or less in order to obtain a desired steel sheet strength. 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.
含有するＴｉ量に対しマトリックス中に固溶状態として存在するＴｉ量の割合が１０％未満
前述のように、粒子分散強化による強化量は炭化物の析出量にも関係する。焼鈍後の冷延鋼板中に強化に寄与しない固溶状態にあるＴｉ量が多量に残存した状態であると、鋼板強度が低下する。含有するＴｉ量に対しマトリックス中に固溶状態として存在するＴｉ量の割合が１０％未満であれば、上記の悪影響が顕在化しない。このため、含有するＴｉ量に対しマトリックス中に固溶状態として存在するＴｉ量の割合は１０％未満とする。望ましくは５．５％以下である。
The proportion of Ti present as a solid solution in the matrix with respect to the amount of Ti contained is less than 10%. As described above, the strengthening amount due to particle dispersion strengthening is also related to the amount of precipitated carbide. If the Ti content in a solid solution state that does not contribute to strengthening remains in the cold-rolled steel sheet after annealing, the steel sheet strength is reduced. If the ratio of the Ti amount existing as a solid solution in the matrix is less than 10% with respect to the contained Ti amount, the above-described adverse effect is not manifested. For this reason, the ratio of the amount of Ti existing as a solid solution state in the matrix with respect to the amount of Ti contained is less than 10%. Desirably, it is 5.5% or less.
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、仕上げ圧延終了後から２秒以内に平均冷却速度が２０℃／ｓ以上の冷却を開始し、巻取り温度を４００℃以上７００℃以下として巻取り、冷間圧延率が３０％以上９０％以下の冷間圧延を施し、５００℃から最高到達温度までの平均昇温速度を７℃／ｓ以下、焼鈍温度を７００℃以上８５０℃以下とする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 is started at an average cooling rate of 20 ℃ / s or more within 2 seconds after finishing rolling, winding is performed at a coiling temperature of 400 ℃ or more and 700 ℃ or less, and cold rolling is performed. Cold rolling at a rate of 30% or more and 90% or less, annealing at an average temperature increase rate from 500 ° C. to the highest temperature of 7 ° C./s or less, and an annealing temperature of 700 ° C. or more and 850 ° C. or less Features.
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。 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. .
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１１００℃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, if the steel material after casting is in a temperature range of 1100 ° C. or more and 1350 ° C. or less, or if the carbide of the steel material is dissolved, the steel material is heated. 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、コイル面内の強度、すなわち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 strength in the coil plane, that is, the steel sheet The strength with respect to the longitudinal direction and the width direction 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一方であるため、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：４００℃以上７００℃以下
熱延板の組織はフェライト相とパーライトの混在組織とし、パーライトにより冷間圧延時にひずみエネルギーを上昇させ再結晶を促進させる必要がある。巻取り温度が４００℃を下回ると、熱延板に十分にパーライトが生成せず、再結晶促進効果が得られない。一方、巻取温度が７００℃を超えると析出した炭化物が粗大化し、所望の炭化物の粒子径ならびに鋼板強度が得られなくなる。したがって、巻取温度の範囲は４００℃以上７００℃以下とする。好ましくは５００℃以上６７０℃以下である。
Winding temperature: 400 ° C. or more and 700 ° C. or less The structure of the hot rolled sheet is a mixed structure of ferrite phase and pearlite, and it is necessary to increase the strain energy during cold rolling by pearlite and promote recrystallization. When the coiling temperature is lower than 400 ° C., pearlite is not sufficiently generated on the hot-rolled sheet, and the recrystallization promotion effect cannot be obtained. On the other hand, when the coiling temperature exceeds 700 ° C., the precipitated carbide becomes coarse, and the desired carbide particle diameter and steel plate strength cannot be obtained. Therefore, the range of coiling temperature shall be 400 degreeC or more and 700 degrees C or less. Preferably they are 500 degreeC or more and 670 degrees C or less.
冷間圧延率：３０％以上９０％以下
冷間圧延率が３０％を下回ると、操業上安定せず板形状が不均一になる。不均一な板形状であると加工性が低下し、材質ばらつきが増大するため、冷間圧延率は３０％以上とする。好ましくは４０％以上である。一方、冷間圧延率が９０％を超えると、過度に鋼板が加工硬化し所望の板厚が得られなくなるため、冷間圧延率の上限は９０％とする。好ましくは冷間圧延率は８０％以下である。
Cold rolling rate: 30% or more and 90% or less If the cold rolling rate is less than 30%, the operation is not stable and the plate shape becomes uneven. If the plate shape is not uniform, the workability decreases and the material variation increases, so the cold rolling rate is set to 30% or more. Preferably it is 40% or more. On the other hand, if the cold rolling rate exceeds 90%, the steel sheet is excessively work-hardened and a desired plate thickness cannot be obtained, so the upper limit of the cold rolling rate is 90%. Preferably, the cold rolling rate is 80% or less.
５００℃から最高到達温度までの平均昇温速度：７℃／ｓ以下
本発明鋼では５００℃以上の温度で冷間圧延時に導入された転位が回復、再結晶を開始する。極力低い温度で回復、再結晶させれば、炭化物の粗大化を抑制しつつ高い延性を保つことができる。さらに、極力低い温度から回復、再結晶を制御することにより、コイル面内での金属組織のばらつきを抑制することが可能である。このような効果を得るには５００℃から最高到達温度までの平均昇温速度を７℃／ｓ以下に抑えることが必要である。より材質安定性を向上させるには、５℃／ｓ以下が望ましい。下限は特に設けないが、製造工程での効率の観点から、０．１５℃／ｓ以上とすることが望ましい。
Average heating rate from 500 ° C. to the highest temperature: 7 ° C./s or less In the steel of the present invention, the dislocations introduced during cold rolling at a temperature of 500 ° C. or higher are recovered and recrystallization starts. If recovered and recrystallized at the lowest possible temperature, high ductility can be maintained while suppressing coarsening of the carbide. Furthermore, by controlling recovery and recrystallization from the lowest possible temperature, it is possible to suppress variations in the metal structure within the coil plane. In order to obtain such an effect, it is necessary to suppress the average rate of temperature increase from 500 ° C. to the maximum temperature reached to 7 ° C./s or less. In order to further improve the material stability, 5 ° C./s or less is desirable. Although there is no particular lower limit, it is preferably 0.15 ° C./s or higher from the viewpoint of efficiency in the manufacturing process.
焼鈍温度：７００℃以上８５０℃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完全に再結晶させる必要がある。実質的に加工フェライトを残存させず完全に再結晶させるためには、７００℃以上で焼鈍する必要がある。好ましくは、７２０℃以上である。一方で、上記したように昇温速度を制御したうえで、焼鈍温度が８５０℃を超えると、炭化物が粗大化し固溶Ｔｉ量が増加するため、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８５０℃とし、好ましくは、８３０℃とする。以上により、焼鈍温度は７００℃以上８５０℃以下とする。好ましい焼鈍温度の範囲は７２０℃以上８３０℃以下である。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: 700 ° C. or higher and 850 ° C. or lower In order to remove dislocations introduced by cold rolling, it is necessary to completely recrystallize without substantially leaving the processed ferrite. In order to recrystallize completely without substantially leaving the processed ferrite, it is necessary to anneal at 700 ° C. or higher. Preferably, it is 720 ° C or higher. On the other hand, when the temperature rise rate is controlled as described above and the annealing temperature exceeds 850 ° C., the carbide is coarsened and the amount of solute Ti is increased, so that the steel sheet strength is remarkably lowered. Therefore, the upper limit of the annealing temperature is 850 ° C., preferably 830 ° C. As described above, the annealing temperature is set to 700 ° C. or higher and 850 ° C. or lower. A preferable annealing temperature range is 720 ° C. or higher and 830 ° C. or lower. 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４５〜６５ｇ／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５００℃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。 After heating a steel material (steel slab) with a thickness of 250 mm having the composition shown in Table 1 to the slab heating temperature shown in Table 2, the hot-rolled sheet shown in Table 2 is used as a hot-rolled sheet. 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 65 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 500 ° C. to the annealing temperature, and the annealing temperature is annealing. It is the highest temperature reached in the steel plate.
上記により得られた冷延鋼板から試験片を採取し、組織観察、引張試験を行い、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、限界曲げ半径等を求めた。試験方法は次のとおりとした。 Samples are taken from the cold-rolled steel sheet obtained as described above, and the structure is observed and subjected to a tensile test. The area ratio of the ferrite phase, the average crystal grain size of the ferrite phase, the area ratio of the processed ferrite, and the average particle of carbide containing Ti Diameter, yield strength, tensile strength, elongation, critical bending radius, etc. 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。冷延、焼鈍後のコイル状に巻き取った冷延鋼板を巻き戻し、長手方向に対し２０等分割し、幅方向に対し１／４、１／２、３／４の位置から評価に供するサンプルを６３枚採取した。ただし、長手方向の評価位置において、コイル状に巻き取った鋼板のコイル内周および外周の１巻き分は含まない。各々のサンプルの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。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。得られたフェライト相の面積率の平均値およびコイル面内におけるフェライト面積率のばらつきを表３に示す。面積率のばらつきは、６３サンプルに対する標準偏差に２を乗じた値とした。
(I) Structure observation The area ratio of the ferrite phase was evaluated by the following method. Cold-rolled and annealed cold-rolled steel sheet wound into a coil shape is rewound, divided into 20 equal parts in the longitudinal direction, and samples used for evaluation from 1/4, 1/2, 3/4 positions in the width direction 63 were collected. However, at the evaluation position in the longitudinal direction, one turn of the coil inner circumference and outer circumference of the steel sheet wound in a coil shape is not included. About the central part of the plate thickness of the cross section parallel to the rolling direction of each sample, the corrosion appearance structure by 5% nital was magnified 400 times with a scanning optical microscope and photographed for 10 fields of view. 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. Table 3 shows the average value of the area ratio of the obtained ferrite phase and the variation of the ferrite area ratio in the coil plane. The variation of the area ratio was a value obtained by multiplying the standard deviation for 63 samples by 2.
フェライト平均結晶粒径は、上記４００倍に拡大して撮影した代表的な写真３枚について、水平線および垂直線をそれぞれ１０本ずつ引きＡＳＴＭ Ｅ １１２−１０に準拠した切断法によって求め、最終的に３枚の平均値を１つのサンプルの平均結晶粒径とした。表３には６３サンプルに対する平均値を記した。 The ferrite average crystal grain size was determined by drawing 10 horizontal lines and 10 vertical lines for each of the three representative photographs taken at 400 times magnification, and finally, by a cutting method in accordance with ASTM E 112-10. The average value of three sheets was taken as the average crystal grain size of one sample. Table 3 shows the average values for 63 samples.
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。すなわち、加工フェライトの面積率としては、フェライト相全体を母集合とした上で、加工フェライトの面積率を求めた。６３サンプルについて求めた加工フェライトの面積率の平均値を表３に記した。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、Ｔｉが含まれることを確認した。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。これは析出物粒子径を算出するうえで含めない。マトリックス中に含まれる固溶Ｔｉ量の分析は、得られた冷延鋼板を用い、１０％ＡＡ系電解液（１０ｖｏｌ％アセチルアセトン−１ｍａｓｓ％塩化テトラメチルアンモニウム-メタノール）中で、約０．２ｇを電流密度２０ｍＡ／ｃｍ２で定電流電解した後、電解液中に含まれるＴｉ量を分析し、含有量に対する割合をマトリックス中に固溶状態にあるＴｉ量の割合として算出した。 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. That is, as the area ratio of the processed ferrite, the area ratio of the processed ferrite was obtained after setting the entire ferrite phase as a mother set. Table 3 shows the average area ratio of the processed ferrite obtained for 63 samples. 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 was analyzed by EDX attached to TEM, and it was confirmed that Ti was contained. 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. This is not included in calculating the precipitate particle size. For analysis of the amount of dissolved Ti contained in the matrix, about 0.2 g of 10% AA-based electrolyte (10 vol% acetylacetone-1 mass% tetramethylammonium chloride-methanol) was used using the obtained cold-rolled steel sheet. After constant current electrolysis at a current density of 20 mA / cm 2 , the amount of Ti contained in the electrolytic solution was analyzed, and the ratio to the content was calculated as the ratio of the amount of Ti in a solid solution state in the matrix.
（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。組織観察と同様の手順で、平均値と降伏強度のばらつき（ΔＹＳ）を求め、表３に示した。降伏強度のばらつきはフェライト相の面積率とそのばらつき、Ｔｉを含む炭化物の平均粒子径、マトリックス中に固溶状態として存在するＴｉ量の割合によって変動する。そこで、本発明で求める材質が安定した冷延鋼板が得られているかを確認するため、ΔＹＳを求め、評価した。
(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, and a tensile test in accordance with the provisions of JIS Z 2241 (2011) was conducted five times to obtain an average yield. The strength (YS), tensile strength (TS), and total elongation (El) were determined. The crosshead speed in the tensile test was 10 mm / min. The average value and the variation in yield strength (ΔYS) were determined by the same procedure as in the structure observation, and are shown in Table 3. The variation in yield strength varies depending on the area ratio of the ferrite phase and its variation, the average particle size of carbides containing Ti, and the proportion of Ti present as a solid solution in the matrix. Therefore, ΔYS was determined and evaluated in order to confirm whether or not a cold-rolled steel sheet having a stable material required in the present invention was obtained.
（ｉｉｉ）曲げ試験（曲げ性評価）
得られた冷延鋼板のコイル長手方向および幅方向に対し中央部から、試験に供する短冊状の試験片（１００ｍｍＷ（幅）×３５ｍｍＬ（長さ））をせん断加工によって３枚採取した。このとき、試験片端面は、せん断加工ままで曲げ試験を実施したが、せん断面と破断面が４つの辺を持つ短冊状の試験片端面の全てで同一の方向となるよう、試験片端面は同じ方向からせん断加工を施した。ＪＩＳ Ｚ ２２４８に準拠したＶブロック法による曲げ試験を３回行い、試験後サンプルの湾曲部外側を肉眼もしくは１０倍の拡大鏡で観察し、裂けや疵等の欠点がないものを合格した。押金具の内側半径（Ｒ）に対し、合格となった最小のＲと板厚（t）との商を下式に示す限界曲げ半径とした。なお、Ｒ、ｔともに、単位はｍｍである。
（限界曲げ半径）＝（合格となった押金具の最小内側半径）／（鋼板板厚）
限界曲げ半径は小さい値であるほど良い結果であることを示す。限界曲げ半径が２．０以下の場合に曲げ性が良好であると評価した。なお、曲げ性は押金具の内側半径と、鋼板板厚に左右される。そのため、限界曲げ半径は鋼板板厚の影響を除した指標で評価した。
(Iii) Bending test (bendability evaluation)
Three strip-shaped test pieces (100 mmW (width) × 35 mmL (length)) to be used for the test were collected from the center with respect to the coil longitudinal direction and width direction of the obtained cold-rolled steel sheet by shearing. At this time, the end face of the test piece was subjected to a bending test while being sheared, but the end face of the test piece was such that all the end faces of the strip-shaped test piece having four sides of the shear face and the fracture surface were in the same direction. Shearing was performed from the same direction. The bending test by the V-block method in accordance with JIS Z 2248 was performed three times, and the outside of the curved portion of the sample after the test was observed with the naked eye or a 10-fold magnifier, and passed without any defects such as tears and wrinkles. For the inner radius (R) of the metal fitting, the quotient of the minimum R and the plate thickness (t) that passed was used as the limit bending radius shown in the following equation. The unit of both R and t is mm.
(Limit bending radius) = (Minimum inner radius of the accepted metal fitting) / (Steel plate thickness)
A smaller limit bending radius indicates a better result. It was evaluated that the bendability was good when the critical bending radius was 2.0 or less. The bendability depends on the inner radius of the metal fitting and the steel plate thickness. Therefore, the critical bending radius was evaluated by an index excluding the influence of the steel plate thickness.
以上により得られた結果を表３に示す。なお、得られた結果について、引張強さ(ＴＳ)が４４０ＭＰａ以上、降伏強度のばらつきが３０ＭＰａ以下、かつ限界曲げ半径が１．０以下であれば”○”、それ以外は”×”として評価し、その結果も表３に示している。本発明例はいずれも、引張強さＴＳ：４４０ＭＰａ以上であり且つ曲げ性にも優れることから強度と加工性を兼備した冷延鋼板となっている。また、本発明例では、組織のばらつきが小さいため、ＴＳが４４０ＭＰａ以上かつΔＹＳが３０ＭＰａ以下の加工性が良好な冷延鋼板が得られている。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：４４０ＭＰａ以上）の高強度が確保できていないか、良好な曲げ性が得られておらず加工性が乏しいことがわかる。また、例えば鋼板Ｎｏ．２はフェライト面積率のばらつきが大きく降伏強度のばらつきが３０ＭＰａを上回り、材質が不安定なものであった。 The results obtained as described above are shown in Table 3. The results obtained were evaluated as “◯” if the tensile strength (TS) was 440 MPa or more, the yield strength variation was 30 MPa or less, and the critical bending radius was 1.0 or less, and “x” otherwise. The results are also shown in Table 3. Each of the examples of the present invention is a cold-rolled steel sheet having both strength and workability because it has a tensile strength of TS: 440 MPa or more and is excellent in bendability. Moreover, in the example of the present invention, since the variation of the structure is small, a cold-rolled steel sheet having good workability with TS of 440 MPa or more and ΔYS of 30 MPa or less is obtained. On the other hand, it can be seen that the comparative example out of the scope of the present invention does not ensure a high strength of a desired strength (tensile strength: 440 MPa or more), or does not have good bendability and has poor workability. . For example, steel plate No. No. 2 had a large variation in ferrite area ratio, a variation in yield strength exceeding 30 MPa, and the material was unstable.
Claims (14)
Ｃ：０．０３％以上０．１５％以下、
Ｓｉ：１．５％以下、
Ｍｎ：０．６％以下、
Ｐ：０．０５％以下、
Ｓ：０．０１％以下、
Ａｌ：０．０８％以下、
Ｎ：０．００８０％以下、
Ｔｉ：０．０４％以上０．１８％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を有し、フェライト相の面積率が９０％以上、コイル面内における該フェライト相の面積率のばらつきが３％以下、前記フェライト相に対する加工フェライトの面積率が６％以下、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、含有するＴｉ量に対しマトリックス中に固溶状態として存在するＴｉ量の割合が１０％未満であることを特徴とする、高強度冷延鋼板。 % By mass
C: 0.03% to 0.15%,
Si: 1.5% or less,
Mn: 0.6% or less,
P: 0.05% or less,
S: 0.01% or less,
Al: 0.08% or less,
N: 0.0080% or less,
Ti: 0.04% or more and 0.18% or less, with the balance being composed of Fe and inevitable impurities, the ferrite phase area ratio being 90% or more, the ferrite phase area ratio in the coil plane 3% or less, the area ratio of processed ferrite with respect to the ferrite phase is 6 % or less, the average particle diameter of carbides containing Ti in the crystal grains of the ferrite phase is 10 nm or less, and the amount of Ti contained in the matrix A high-strength cold-rolled steel sheet, characterized in that the proportion of Ti present as a solid solution is less than 10%.
前記鋼素材を、質量％で、Ｃ：０．０３％以上０．１５％以下、Ｓｉ：１．５％以下、Ｍｎ：０．６％以下、Ｐ：０．０５％以下、Ｓ：０．０１％以下、Ａｌ：０．０８％以下、Ｎ：０．００８０％以下、Ｔｉ：０．０４％以上０．１８％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４００℃以上７００℃以下とし、前記冷間圧延の冷間圧延率を３０％以上９０％以下とし、前記焼鈍を、５００℃から最高到達温度までの平均昇温速度を７℃／ｓ以下、焼鈍温度を７００℃以上８５０℃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、コイル面内における該フェライト相の面積率のばらつきが３％以下、前記フェライト相に対する加工フェライトの面積率が６％以下、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、含有するＴｉ量に対しマトリックス中に固溶状態として存在するＴｉ量の割合が１０％未満である高強度冷延鋼板の製造方法。 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: 0.03% or more and 0.15% or less, Si: 1.5% or less, Mn: 0.6% or less, P: 0.05% or less, S: 0.00. 01% or less, Al: 0.08% or less, N: 0.0080% or less, Ti: 0.04% or more and 0.18% 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 400 ° C. or more and 700 ° C. or less, the cold rolling rate of the cold rolling is 30% or more and 90% or less, and the annealing reaches the maximum from 500 ° C. The average rate of temperature increase up to 7 ° C / s, annealing temperature Characterized by a 700 ° C. or higher 850 ° C. or less, the area ratio of the ferrite phase is 90% or more, the variation of the area ratio of the ferrite phase in the coil plane is 3% or less, the area ratio of the deformed ferrite for the ferrite phase Is 6% or less, the average particle diameter of the carbide containing Ti in the ferrite phase crystal grains is 10 nm or less, and the proportion of Ti present as a solid solution in the matrix with respect to the amount of Ti contained is less than 10%. Manufacturing method of high-strength cold-rolled steel sheet.
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