JP2007009269A - ULTRAHIGH STRENGTH COLD ROLLED STEEL SHEET HAVING HIGH DUCTILITY, EXCELLENT CHEMICAL CONVERSION TREATABILITY AND >=780 MPa TENSILE STRENGTH, AND ITS MANUFACTURING METHOD - Google Patents
ULTRAHIGH STRENGTH COLD ROLLED STEEL SHEET HAVING HIGH DUCTILITY, EXCELLENT CHEMICAL CONVERSION TREATABILITY AND >=780 MPa TENSILE STRENGTH, AND ITS MANUFACTURING METHOD Download PDFInfo
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本発明は、厳しい形状にプレス成形される自動車部品などに好適な、高延性で、化成処理性に優れる780〜1180MPa級の引張強度TSを有する超高強度冷延鋼板およびその製造方法に関する。 The present invention relates to an ultra-high-strength cold-rolled steel sheet having a tensile strength TS of 780 to 1180 MPa class, which has a high ductility and excellent chemical conversion properties, and is suitable for automobile parts that are press-formed into a severe shape.
自動車部品などに用いられるTSが780〜1180MPa級の超高強度冷延鋼板は、その用途の特徴から高強度に加え、伸びフランジ性などの延性が高く、化成処理性に優れていることが必要である。 Super high strength cold-rolled steel sheets with a TS of 780 to 1180 MPa used for automobile parts and the like need to have high ductility such as stretch flangeability and excellent chemical conversion properties in addition to high strength due to their application characteristics. It is.
高延性な高強度冷延鋼板に関しては、例えば、C:0.05〜0.35%、Si:0.5〜3.0%、Mn:0.5〜3.0%、P:0.05%以下、S:0.005%以下、Al:0.10%未満、N:0.0020〜0.0100%、Ti:0.001〜0.20%、Caおよび/またはREM:0.0010〜0.010%を含有し、体積率で30%以上のフェライト相、体積率で2%以上の残留オーステナイト相および低温変態相からなる複合組織を有し、かつフェライト粒径が15μm以下で、鋼板表層から10nm深さまでのGDS分析によるSiとFeの強度比が6.0以下である塗膜の2次密着性に優れた高強度高延性冷延鋼板が開示されている(特許文献1)。また、C:0.06〜0.25%、Si:2.5%以下、Mn:0.5〜3.0%、P:0.1%以下、S:0.03%以下、Al:0.1〜2.5%、Ti:0.003〜0.08%、N:0.01%以下を含有し、かつ(48/14)N≦Ti≦(48/14)N+(48/32)S+0.01を満足し、体積率で5%以上の残留オーステナイト相を有する表面性状並びに衝撃吸収性に優れた高延性型高張力冷延鋼板が開示されている(特許文献2)。さらに、C:0.04〜0.14%、Si:0.2〜2.0%、Mn:0.5〜2.0%、P:0.008%以下、B:0.0003〜0.0050%、Al:0.010〜2.00%を含有し、かつ面積率で4〜15%以上の残留オーステナイト相とフェライト相、ベイナイト相の複合組織からなる加工性と溶接性に優れた高強度冷延鋼板が開示されている(特許文献3)。 For high-strength, high-strength cold-rolled steel sheets, for example, C: 0.05 to 0.35%, Si: 0.5 to 3.0%, Mn: 0.5 to 3.0%, P: 0.05% or less, S: 0.005% or less, Al: 0.10% Less than N, 0.0020 to 0.0100%, Ti: 0.001 to 0.20%, Ca and / or REM: 0.0010 to 0.010%, ferrite phase of 30% or more by volume ratio, residual austenite phase of 2% or more by volume ratio In addition, the secondary adhesion of the coating film has a composite structure consisting of a low-temperature transformation phase, a ferrite grain size of 15 μm or less, and a strength ratio of Si and Fe of 6.0 or less by GDS analysis from the steel sheet surface layer to a depth of 10 nm. An excellent high strength and high ductility cold rolled steel sheet is disclosed (Patent Document 1). Also, C: 0.06-0.25%, Si: 2.5% or less, Mn: 0.5-3.0%, P: 0.1% or less, S: 0.03% or less, Al: 0.1-2.5%, Ti: 0.003-0.08%, N: Surface properties containing 0.01% or less and satisfying (48/14) N ≦ Ti ≦ (48/14) N + (48/32) S + 0.01 and having a retained austenite phase of 5% or more by volume and A high ductility type high-tensile cold-rolled steel sheet excellent in shock absorption is disclosed (Patent Document 2). Furthermore, it contains C: 0.04-0.14%, Si: 0.2-2.0%, Mn: 0.5-2.0%, P: 0.008% or less, B: 0.0003-0.0050%, Al: 0.010-2.00%, and in area ratio A high-strength cold-rolled steel sheet excellent in workability and weldability composed of a composite structure of 4 to 15% or more of retained austenite phase, ferrite phase, and bainite phase is disclosed (Patent Document 3).
一方、化成処理性に関しては、必ずしも高強度冷延鋼板を対象としてないが、Ni酸化物やNi水酸化物を表面に付着させた化成処理性に優れた表面処理鋼板が開示されている(特許文献4)。また、C:0.001〜0.25%、Si:0.1〜1.5%、Mn:0.2〜3.0%、およびCu:0.5〜1.5%、Ni:0.5〜1.5%、Mo: 0.2〜1.0%、Cr:0.1〜2.5%、V:0.01〜0.1%、B:0.0005〜0.0030%、Ti:0.01〜0.20%、Nb:0.01〜0.20%のうちから選ばれた少なくとも一つの元素を含み、表面に鉄被覆層が形成された塗装後耐食性に優れた高強度冷延鋼板が開示されている(特許文献5)。さらに、C:0.001〜0.25%、Si:0.1〜1.5%、Mn:0.2〜3.0%、およびCu:0.5〜1.5%、Ni:0.5〜1.5%、Mo: 0.2〜1.0%、Cr:0.1〜2.5%、V:0.01〜0.1%、B:0.0005〜0.0030%、Ti:0.01〜0.20%、Nb:0.01〜0.20%のうちから選ばれた少なくとも一つの元素を含み、表面にニッケル金属粒子からなるニッケル金属層が形成されたリン酸塩処理性に優れた高強度冷延鋼板が開示されている(特許文献6)。また、Siの分布については、Si基酸化物の表面被覆率について開示されている(特許文献7)。さらに、TRIP鋼について、表面Si濃化量の平均値、表面Si濃度分布に占める鋼中Si濃度に対する濃度比が10以上である部位の面積率について開示されている(特許文献8)。
しかしながら、特許文献1〜3に記載された高延性の高強度冷延鋼板では、化成処理性が不十分である。特に、特許文献1に記載の高強度冷延鋼板では、酸洗処理やブラシ処理によってSiとFeの強度比を6.0以下にして塗膜の2次密着性の向上を図っているが、十分に優れた化成処理性が得られず、鋼板表面に不均一な化成処理皮膜が形成される。また、特許文献3に記載された高強度冷延鋼板では、780MPa以上のTSが得られない。 However, the high ductility, high-strength cold-rolled steel sheets described in Patent Documents 1 to 3 have insufficient chemical conversion properties. In particular, in the high-strength cold-rolled steel sheet described in Patent Document 1, the strength ratio of Si and Fe is reduced to 6.0 or less by pickling or brushing to improve the secondary adhesion of the coating film. An excellent chemical conversion treatment property is not obtained, and a non-uniform chemical conversion treatment film is formed on the steel sheet surface. Further, with the high-strength cold-rolled steel sheet described in Patent Document 3, a TS of 780 MPa or more cannot be obtained.
特許文献4〜6に記載された表面処理鋼板や高強度冷延鋼板では、Ni酸化物、Ni水酸化物、鉄被覆層、ニッケル金属層などの特殊なめっき処理を行う必要があり、高コストで生産性が劣る。また、鋼板の成分しか開示されておらず、製造条件によっては高延性で、780MPa以上のTSを有する超高強度冷延鋼板が得られない。 The surface-treated steel sheets and high-strength cold-rolled steel sheets described in Patent Documents 4 to 6 require special plating treatments such as Ni oxides, Ni hydroxides, iron coating layers, nickel metal layers, and are expensive. Productivity is poor. Further, only the components of the steel sheet are disclosed, and depending on the production conditions, an ultra-high strength cold-rolled steel sheet having a high ductility and a TS of 780 MPa or more cannot be obtained.
特許文献7に記載された鋼板では、リン酸亜鉛結晶を隙間なく生成させることを図っているが、Si濃度を制御していないので、十分に優れた塗装後の耐食性が得られない。特許公報8に記載された鋼板は、TRIP特性を有するので延性のみに優れ、伸びフランジ性に劣る。加えて、表面Si平均濃化レベルが11〜42と極めて高く、鋼中Si濃度に対する濃度比が10以上である部位が存在しており、酸洗2回、研削、デスケーリングなど通常より高コストな工程を経ているにもかかわらず、十分にSi濃化の減少を達成できていない。焼鈍前に鋼板表面Siを低減しても、焼鈍中に鋼板表面にSi酸化物が生成してしまうことが大きな課題である。 In the steel sheet described in Patent Document 7, zinc phosphate crystals are generated without gaps, but since the Si concentration is not controlled, a sufficiently excellent corrosion resistance after coating cannot be obtained. Since the steel sheet described in Patent Publication 8 has TRIP characteristics, it is excellent only in ductility and inferior in stretch flangeability. In addition, the surface Si average concentration level is extremely high at 11 to 42, and there are sites where the concentration ratio to the Si concentration in the steel is 10 or more. In spite of going through various processes, Si concentration has not been sufficiently reduced. Even if the steel sheet surface Si is reduced before annealing, it is a big problem that Si oxides are generated on the steel sheet surface during annealing.
本発明は、安価に製造可能な、高延性で、化成処理性に優れるTSが780〜1180MPa級の超高強度冷延鋼板およびその製造方法を提供することを目的とする。 An object of the present invention is to provide an ultra-high-strength cold-rolled steel sheet having a TS of 780 to 1180 MPa class, which has high ductility and excellent chemical conversion properties, and can be manufactured at low cost, and a method for manufacturing the same.
本発明者らが、安価に製造可能な、高延性で、化成処理性に優れるTSが780〜1180MPaの超高強度冷延鋼板について検討を進めたところ、以下の知見が得られた。 When the inventors proceeded to study a super-high strength cold-rolled steel sheet having a TS of 780 to 1180 MPa, which can be manufactured at a low cost and has high ductility and excellent chemical conversion property, the following knowledge was obtained.
i)高延性化とTSが780MPa以上の高強度化を図るには、成分組成を適正化し、ミクロ組織をフェライト相と焼戻マルテンサイト相からなる複合組織とし、かつ焼戻マルテンサイト相の体積率を20〜70%にする必要がある。 i) In order to achieve high ductility and high strength of TS of 780 MPa or more, the composition of the components should be optimized, the microstructure should be a composite structure composed of a ferrite phase and a tempered martensite phase, and the volume of the tempered martensite phase. The rate should be 20-70%.
ii)優れた化成処理性を図るには、鋼板表面のSi濃度Si(0)と鋼板表面から0.1mm深さのところのSi濃度Si(0.1)の比、すなわち[Si(0)/Si(0.1)]を1.0〜1.6に、かつその標準偏差を0.50以下にする必要がある。 ii) In order to achieve excellent chemical conversion, the ratio of Si concentration Si (0) on the steel sheet surface to Si concentration Si (0.1) at a depth of 0.1 mm from the steel sheet surface, that is, [Si (0) / Si ( 0.1)] should be 1.0 to 1.6, and the standard deviation should be 0.50 or less.
iii)上記のようなミクロ組織や表面Si濃度のコントロールは、例えば、連続焼鈍により焼鈍を行って、焼鈍条件を適正化するとともに、焼鈍後に酸洗処理とアルカリ処理を行うことにより初めて可能となる。 iii) The microstructure and surface Si concentration as described above can be controlled for the first time by, for example, performing annealing by continuous annealing to optimize the annealing conditions and performing pickling treatment and alkali treatment after annealing. .
本発明は、このような知見に基づきなされたもので、質量%で、C:0.05〜0.2%、Si:0.5〜2.0%、Mn:1.5〜3.5%、P:0.001〜0.05%、S:0.0001〜0.005%、Al:0.005〜0.08%、N:0.001〜0.01%、および残部がFeおよび不可避的不純物からなり、フェライト相と焼戻マルテンサイト相からなる複合組織を有し、焼戻マルテンサイト相の体積率が20〜70%であり、かつ鋼板表面のSi濃度Si(0)と鋼板表面から0.1mm深さのところのSi濃度Si(0.1)との比[Si(0)/Si(0.1)]が1.0〜1.6で、この比の標準偏差が0.50以下であることを特徴とする高延性で、化成処理性に優れる780MPa以上の引張強度を有する超高強度冷延鋼板を提供する。 The present invention has been made based on such findings, and in mass%, C: 0.05 to 0.2%, Si: 0.5 to 2.0%, Mn: 1.5 to 3.5%, P: 0.001 to 0.05%, S: 0.0001 ~ 0.005%, Al: 0.005 ~ 0.08%, N: 0.001 ~ 0.01%, the balance consists of Fe and inevitable impurities, has a composite structure consisting of ferrite phase and tempered martensite phase, tempered martensite phase The ratio of the Si concentration Si (0) on the steel sheet surface to the Si concentration Si (0.1) at a depth of 0.1 mm from the steel sheet surface [Si (0) / Si (0.1 )] Is 1.0 to 1.6, and the standard deviation of this ratio is 0.50 or less, and it provides an ultra-high strength cold-rolled steel sheet having a tensile strength of 780 MPa or more, which is excellent in chemical conversion processability.
本発明の超高強度冷延鋼板には、さらに、質量%で、Ti:0.001〜0.04%、Nb:0.001〜0.04%、V:0.001〜0.2%のうちから選ばれた少なくとも1種の元素を含有できる。 The ultra-high-strength cold-rolled steel sheet of the present invention further contains at least one element selected from Ti: 0.001 to 0.04%, Nb: 0.001 to 0.04%, and V: 0.001 to 0.2% by mass%. Can be contained.
また、本発明の超高強度冷延鋼板には、さらに、質量%で、Cu:0.01〜1%、Ni:0.01〜1%、Mo:0.01〜1%、Cr:0.01〜1%、B:0.0001〜0.005%のうちから選ばれた少なくとも1種の元素やCa:0.0001〜0.005%を含有することもできる。 Further, the ultra-high-strength cold-rolled steel sheet of the present invention further includes, in mass%, Cu: 0.01 to 1%, Ni: 0.01 to 1%, Mo: 0.01 to 1%, Cr: 0.01 to 1%, B: It can also contain at least one element selected from 0.0001 to 0.005% and Ca: 0.0001 to 0.005%.
本発明の超高強度冷延鋼板は、質量%で、C:0.05〜0.2%、Si:0.5〜2.0%、Mn:1.5〜3.5%、P:0.001〜0.05%、S:0.0001〜0.005%、Al:0.005〜0.08%、N:0.001〜0.01%、および残部がFeおよび不可避的不純物からなる
鋼スラブを熱間圧延後、冷間圧延して鋼板とし、この鋼板を、連続焼鈍炉にて、700〜900℃の温度に加熱後、550〜800℃の温度から500℃/秒以上の冷却速度で急冷し、150〜500℃の温度で100〜1400秒間再加熱し、引き続き、0〜4のpHで、10〜100℃の温度で5〜150秒間酸洗処理後、10〜14のpHで、10〜100℃の温度で2〜50秒間アルカリ処理を行い、0.2〜1.5%の伸長率で調質圧延を行うことを特徴とする高延性で、化成処理性に優れる780MPa以上の引張強度を有する超高強度冷延鋼板の製造方法により製造できる。
The ultra-high-strength cold-rolled steel sheet of the present invention is mass%, C: 0.05 to 0.2%, Si: 0.5 to 2.0%, Mn: 1.5 to 3.5%, P: 0.001 to 0.05%, S: 0.0001 to 0.005%, After hot rolling a steel slab consisting of Al: 0.005-0.08%, N: 0.001-0.01% and the balance Fe and unavoidable impurities, cold rolled into a steel plate, this steel plate, in a continuous annealing furnace, After heating to a temperature of 700-900 ° C, it is rapidly cooled from a temperature of 550-800 ° C at a cooling rate of 500 ° C / second or more, re-heated at a temperature of 150-500 ° C for 100-1400 seconds, and subsequently 0-4 After pickling treatment at a pH of 10 to 100 ° C. for 5 to 150 seconds, an alkali treatment is performed at a pH of 10 to 14 at a temperature of 10 to 100 ° C. for 2 to 50 seconds, and an elongation rate of 0.2 to 1.5%. It can be produced by a method for producing an ultra-high strength cold-rolled steel sheet having a tensile strength of 780 MPa or more, which has high ductility and is excellent in chemical conversion treatment, characterized by temper rolling.
本発明の製造方法の鋼スラブには、さらに、質量%で、Ti:0.001〜0.04%、Nb:0.001〜0.04%、V:0.001〜0.2%のうちから選ばれた少なくとも1種の元素を含有できる。 The steel slab of the production method of the present invention further contains at least one element selected from Ti: 0.001 to 0.04%, Nb: 0.001 to 0.04%, and V: 0.001 to 0.2% by mass%. it can.
また、本発明の製造方法の鋼スラブには、さらに、質量%で、Cu:0.01〜1%、Ni:0.01〜1%、Mo:0.01〜1%、Cr:0.01〜1%、B:0.0001〜0.005%のうちから選ばれた少なくとも1種の元素やCa:0.0001〜0.005%を含有することもできる。 In addition, the steel slab of the production method of the present invention further includes, in mass%, Cu: 0.01 to 1%, Ni: 0.01 to 1%, Mo: 0.01 to 1%, Cr: 0.01 to 1%, B: 0.0001. It is also possible to contain at least one element selected from ˜0.005% and Ca: 0.0001 to 0.005%.
本発明により、高延性で、化成処理性に優れるTSが780〜1180MPa級の超高強度冷延鋼板を安価に製造できるようになった。本発明の超高強度冷延鋼板は、厳しい形状にプレス成形される自動車部品などに好適である。 According to the present invention, an ultra-high strength cold-rolled steel sheet having a TS of 780 to 1180 MPa class, which has high ductility and excellent chemical conversion property, can be produced at low cost. The ultra-high-strength cold-rolled steel sheet of the present invention is suitable for automobile parts that are press-formed into a strict shape.
以下に、本発明の超高強度冷延鋼板およびその製造方法について詳細に説明する。 Below, the ultra-high-strength cold-rolled steel sheet and the manufacturing method thereof according to the present invention will be described in detail.
1)成分
C: Cはマルテンサイト相などの低温変態相を利用して鋼を強化するために必要不可欠である。一般に、低温変態相の強度はC量に比例する傾向にある。その量が0.05%未満では、ミクロ組織がフェライト相とマルテンサイト相からなる複合組織にならず、780MPa以上のTSが得られず、TS-Elバランスも低い。ここで、Elは引張試験で得られる伸びを表す。さらに、780MPa以上のTSを得るにはその量が多いほうが好ましいが、0.2%を超えるとスポット溶接性が著しく劣化したり、マルテンサイト相が過度に硬質化して延性の低下を招く傾向にある。したがって、C量は0.05〜0.2%、好ましくは0.06〜0.16%とする。
1) ingredients
C: C is indispensable for strengthening steel using low-temperature transformation phase such as martensite phase. In general, the strength of the low temperature transformation phase tends to be proportional to the C content. If the amount is less than 0.05%, the microstructure does not become a composite structure composed of a ferrite phase and a martensite phase, a TS of 780 MPa or more cannot be obtained, and the TS-El balance is low. Here, El represents the elongation obtained by the tensile test. Further, in order to obtain TS of 780 MPa or more, it is preferable that the amount is large. However, if it exceeds 0.2%, the spot weldability is remarkably deteriorated, or the martensite phase is excessively hardened and the ductility tends to be lowered. Therefore, the C content is 0.05 to 0.2%, preferably 0.06 to 0.16%.
Si: Siは固溶強化により高強度化に寄与する元素である。また、フェライト相を強化することによりマルテンサイト相との硬度差を低減し、伸びフランジ成形時にフェライト相とマルテンサイト相の境界での割れ発生や亀裂伝播を抑制して均一変形を促進し、伸びフランジ性を向上させる効果もある。そのため、その量は0.5%以上にする必要がある。一方、その量が2.0%を越えるとこうした効果が飽和するばかりではなく、以下のような問題が起こる。すなわち、過度に強化されフェライト相の延性が大きく低下する。スポット溶接性が低下する。熱間圧延時に難剥離性のスケールが生成し表面欠陥の原因となる。鋼板表面にSi酸化物が高濃度に偏在すると、塗装前処理段階である表面調整時にTiコロイドや帯電したリン酸塩微粒子の吸着を阻害したり、化成処理性を劣化させて、塗装後耐食性を低下させる。したがって、Si量は0.5〜2.0%、好ましくは0.8〜1.6%とする。 Si: Si is an element that contributes to strengthening by solid solution strengthening. Also, by strengthening the ferrite phase, the hardness difference from the martensite phase is reduced, and at the time of stretch flange molding, cracking and crack propagation at the boundary between the ferrite phase and the martensite phase are suppressed to promote uniform deformation and elongation. There is also an effect of improving flangeability. Therefore, the amount needs to be 0.5% or more. On the other hand, when the amount exceeds 2.0%, not only is this effect saturated, but the following problems occur. That is, it is strengthened excessively and the ductility of the ferrite phase is greatly reduced. Spot weldability decreases. A hard-to-peel scale is generated during hot rolling, causing surface defects. If the Si oxide is unevenly distributed on the surface of the steel sheet, it will inhibit the adsorption of Ti colloids and charged phosphate particles during surface preparation, which is the pre-coating treatment stage, and will deteriorate the chemical conversion treatment performance, thereby improving the post-coating corrosion resistance. Reduce. Therefore, the Si content is 0.5 to 2.0%, preferably 0.8 to 1.6%.
Mn: Mnは複合組織の形成に必要な元素であり、鋼の焼入れ性を高め硬質なマルテンサイト相を形成し、TS-Elバランスを向上させる。また、焼鈍時の加熱、冷却開始におけるフェライト相とオーステナイト相の体積分率制御により最終的に得られる焼戻マルテンサイト分率に影響を与える。したがって、所望の材料特性、組織を得るためには、その量は1.5%以上にする必要がある。一方、その量が3.5%を越えるとスポット溶接性が低下したり、Mnの偏析に起因したフェライト相とマルテンサイト相の層状組織が形成され、延性が低下する。したがって、Mn量は1.5〜3.5%、好ましくは1.7〜2.4%とする。 Mn: Mn is an element necessary for the formation of a composite structure. It enhances the hardenability of steel and forms a hard martensite phase, improving the TS-El balance. It also affects the final tempered martensite fraction by controlling the volume fraction of the ferrite and austenite phases at the start of heating and cooling during annealing. Therefore, in order to obtain desired material characteristics and structure, the amount needs to be 1.5% or more. On the other hand, if the amount exceeds 3.5%, spot weldability is reduced, or a layered structure of ferrite phase and martensite phase is formed due to segregation of Mn, and ductility is reduced. Therefore, the Mn content is 1.5 to 3.5%, preferably 1.7 to 2.4%.
P: Pは固溶強化元素であるが、粒界への偏析により粒界の結合力を低下させ成形性やスポット溶接性を低下させる。そのため、その量は0.05%以下にする必要がある。一方、その量を0.001%未満に低減するには製造コストが著しく増加する。したがって、P量は0.001〜0.05%、好ましくは0.001〜0.02%とする。 P: P is a solid solution strengthening element. However, segregation to the grain boundary lowers the bond strength of the grain boundary and lowers formability and spot weldability. Therefore, the amount needs to be 0.05% or less. On the other hand, reducing the amount to less than 0.001% significantly increases the manufacturing cost. Therefore, the P content is 0.001 to 0.05%, preferably 0.001 to 0.02%.
S: Sは鋼板中に板状の介在物MnSとして存在し、鋼板の極限変形能を低下させ、伸びフランジ性を低下させる。そのため、その量は0.005%以下にする必要がある。一方、その量を0.0001%未満に低減するには生産性の低下やコストの増加が伴う。したがって、S量は0.0001〜0.005%、好ましくは0.0001〜0.002%とする。 S: S exists in the steel sheet as a plate-like inclusion MnS, which reduces the ultimate deformability of the steel sheet and reduces stretch flangeability. Therefore, the amount needs to be 0.005% or less. On the other hand, reducing the amount to less than 0.0001% is accompanied by a decrease in productivity and an increase in cost. Therefore, the S content is 0.0001 to 0.005%, preferably 0.0001 to 0.002%.
Al: Alは鋼の脱酸剤として有効であり、局部延性を低下させる非金属介在物をスラグ中へ除去する効果もある。また、化成処理性に悪影響を及ぼすSiよりも酸化しやすく、鋼板表面にSiが局在したり、過剰に濃化するのを抑制する効果も有する。さらに、Alはフェライト相中に一部固溶し、フェライト相を強化し、フェライト相と硬質なマルテンサイト相の硬度差を低減して伸びフランジ性の向上にも寄与する。そのため、その量は0.005%以上にする必要である。一方、その量が0.08%を超えると、鋼板表面が過度にAlの不動態膜で覆われて、化成処理性が低下し、耐食性も劣化する。また、溶接性も低下する。したがって、Al量は0.005〜0.08%、好ましくは0.01〜0.05%とする。 Al: Al is effective as a deoxidizer for steel, and also has the effect of removing non-metallic inclusions that reduce local ductility into the slag. In addition, it is easier to oxidize than Si, which adversely affects chemical conversion properties, and has the effect of suppressing Si from being localized or excessively concentrated on the steel sheet surface. Furthermore, Al partially dissolves in the ferrite phase, strengthens the ferrite phase, reduces the hardness difference between the ferrite phase and the hard martensite phase, and contributes to the improvement of stretch flangeability. Therefore, the amount needs to be 0.005% or more. On the other hand, if the amount exceeds 0.08%, the steel plate surface is excessively covered with an Al passivated film, the chemical conversion property is lowered, and the corrosion resistance is also deteriorated. Also, the weldability is reduced. Therefore, the Al content is 0.005 to 0.08%, preferably 0.01 to 0.05%.
N: Nは歪時効を引き起こす不純物元素ではあるが、複合組織鋼板においてはその影響は大きくなく歪時効が問題となることはない。また、Nは窒化物を形成し、スラブの表面割れを抑制する作用を有する。そのため、その量は0.001%以上にする必要がある。一方、その量が0.01%を超えるとスラブ表面割れ抑制の効果は飽和する傾向にある。したがって、N量は0.001〜0.01%、好ましくは0.001〜0.0050%とする。 N: N is an impurity element that causes strain aging, but in a composite steel sheet, the effect is not so great that strain aging does not become a problem. N forms nitrides and has the effect of suppressing surface cracking of the slab. Therefore, the amount needs to be 0.001% or more. On the other hand, when the amount exceeds 0.01%, the effect of suppressing slab surface cracking tends to be saturated. Therefore, the N content is 0.001 to 0.01%, preferably 0.001 to 0.0050%.
残部は、Feおよび不可避的不純物である。 The balance is Fe and inevitable impurities.
本発明の目的を達成するには上記の成分で十分であるが、以下の理由により、Ti:0.001〜0.04%、Nb:0.001〜0.04%、V:0.001〜0.2%のうちから選ばれた少なくとも1種の元素が含有されることが好ましい。すなわち、Tiが0.04%を超えて含有されるとフェライト相中にTiの炭窒化物が析出し、フェライト相の延性を低下させたり、焼鈍中にオーステナイト相へ濃化すべきC量を減少させ、マルテンサイト相の体積率を低下させる。一方、Tiが0.001%以上含有されると結晶粒の微細化により組織が均一化され、延性を向上させる。特に、熱間圧延に先立つスラブ加熱時に粒成長を抑制し、その後の組織の微細化に効果的である。また、Tiは、スラブの冷却時に高温で炭窒化物や硫化物として析出し、比較的低温で起こるAlNの析出やNbやVの炭化物の粒界析出を抑制して、スラブの表面割れを防止する上でも有効な元素である。したがって、Ti量は0.001〜0.04%、より好ましくは0.001〜0.02%とする。NbおよびVは、Tiと同様、0.001%以上含有されるとNbC、VCなどの炭化物として析出し、焼鈍時におけるフェライト相の成長を抑え、組織を微細均一化して伸びフランジ性を著しく向上させるのに有効な元素である。一方、Nbが0.04%を超えて含有されると、また、Vが0.2%を超えて含有されると、析出強化によりYSが上昇し、加工性が低下したり、マルテンサイト相が減少して延性を低下させる。したがって、Nb量は0.001〜0.04%、より好ましくは0.001〜0.02%とし、V量は0.001〜0.2%、より好ましくは0.001〜0.01%とする。 The above components are sufficient to achieve the object of the present invention, but at least selected from Ti: 0.001 to 0.04%, Nb: 0.001 to 0.04%, and V: 0.001 to 0.2% for the following reasons. It is preferable that one element is contained. That is, when Ti is contained in excess of 0.04%, Ti carbonitride precipitates in the ferrite phase, reduces the ductility of the ferrite phase, or reduces the amount of C to be concentrated to the austenite phase during annealing, Reduce the volume fraction of the martensite phase. On the other hand, when Ti is contained in an amount of 0.001% or more, the structure becomes uniform due to the refinement of crystal grains and the ductility is improved. In particular, grain growth is suppressed during slab heating prior to hot rolling, which is effective for subsequent refinement of the structure. Ti also precipitates as carbonitrides and sulfides at high temperatures during slab cooling, preventing the precipitation of AlN and the precipitation of Nb and V carbides at relatively low temperatures to prevent surface cracks in the slab. It is also an effective element for doing so. Therefore, the Ti content is 0.001 to 0.04%, more preferably 0.001 to 0.02%. Nb and V, like Ti, precipitate as carbides such as NbC and VC when contained over 0.001%, suppress the growth of the ferrite phase during annealing, and make the structure finer and homogenously improve stretch flangeability. Is an effective element. On the other hand, when Nb exceeds 0.04%, and when V exceeds 0.2%, YS increases due to precipitation strengthening, workability decreases, and the martensite phase decreases. Reduce ductility. Therefore, the Nb amount is 0.001 to 0.04%, more preferably 0.001 to 0.02%, and the V amount is 0.001 to 0.2%, more preferably 0.001 to 0.01%.
さらに、以下の理由により、Cu:0.01〜1%、Ni:0.01〜1%、Mo:0.01〜1%、Cr:0.01〜1%、B:0.0001〜0.005%のうちから選ばれた少なくとも1種の元素が含有されることが好ましい。すなわち、Cu、Ni、Mo、Crが0.01%以上含有されるとマルテンサイト相の生成が促進されるとともに、マルテンサイト相自体が強化され、また、Bが0.0001%以上含有されると焼入れ性が高くなりマルテンサイト相の生成が促進され、高強度化に寄与する。一方、Cu、Ni、Mo、Crが1%を超えて含有されると、また、Bが0.005%を超えて含有されると、焼入性が高くなり過ぎてフェライト相の生成が抑制されたり、マルテンサイト相が過度に硬化して加工性を低下させる。またコスト的にも不利となる。したがって、Cu量は0.01〜1%、より好ましくは0.01〜0.5%とし、Ni量は0.01〜1%、より好ましくは0.01〜0.5%とし、Mo量は0.01〜1%、より好ましくは0.01〜0.5%とし、Cr量は0.01〜1%、より好ましくは0.01〜0.5%とし、B量は0.0001〜0.005%、より好ましくは0.0001〜0.002%とする。 Furthermore, at least one selected from Cu: 0.01 to 1%, Ni: 0.01 to 1%, Mo: 0.01 to 1%, Cr: 0.01 to 1%, B: 0.0001 to 0.005% for the following reasons These elements are preferably contained. That is, when containing 0.01% or more of Cu, Ni, Mo, Cr, the formation of the martensite phase is promoted, the martensite phase itself is strengthened, and when B is contained by 0.0001% or more, the hardenability is increased. It becomes higher and the formation of martensite phase is promoted, which contributes to higher strength. On the other hand, if Cu, Ni, Mo, Cr is contained in excess of 1%, and if B is contained in excess of 0.005%, the hardenability becomes too high and the formation of the ferrite phase is suppressed. , The martensite phase is excessively cured and the workability is lowered. It is also disadvantageous in terms of cost. Therefore, the Cu amount is 0.01 to 1%, more preferably 0.01 to 0.5%, the Ni amount is 0.01 to 1%, more preferably 0.01 to 0.5%, and the Mo amount is 0.01 to 1%, more preferably 0.01 to 0.5%. %, The Cr content is 0.01 to 1%, more preferably 0.01 to 0.5%, and the B content is 0.0001 to 0.005%, more preferably 0.0001 to 0.002%.
さらにまた、Caが0.0001〜0.005%、より好ましくは0.0001〜0.002%含有されると、MnSなど硫化物の形状を制御して伸びフランジ性などの延性を向上させる。 Furthermore, when Ca is contained in an amount of 0.0001 to 0.005%, more preferably 0.0001 to 0.002%, the shape of sulfide such as MnS is controlled to improve ductility such as stretch flangeability.
なお、Caと同様な効果を有するREMや鋼板表層の結晶を整粒にする作用を有するSbなどを0.0001〜0.1%の範囲内で含有させることもできる。これにより、化成処理性が大きく損なわれることはない。 In addition, REM having the same effect as Ca, Sb having an effect of regulating the crystal of the steel sheet surface layer, and the like can be contained in a range of 0.0001 to 0.1%. Thereby, chemical conversion property is not impaired significantly.
2)ミクロ組織
上述したように、高延性化とTSが780MPa以上の高強度化を図るには、ミクロ組織をフェライト相と焼戻マルテンサイト相の複合組織とし、かつ焼戻マルテンサイト相の体積率をコントロールする必要がある。焼戻マルテンサイト相は、オーステナイト相をマルテンサイト変態温度Ms点以下まで水焼入れなどにより急冷して得られる低温変態相であるマルテンサイト相を再加熱して得られる組織であり、フェライト相より硬質なため強度に寄与する。780MPa以上のTSを得るためには、この焼戻マルテンサイト相の体積率を20%以上にする必要がある。一方、焼戻マルテンサイト相の体積率が70%を超えると軟質なフェライト相の体積率が低下して過度に高強度化され、TS-Elバランスが低下する。したがって、焼戻マルテンサイト相の体積率は20〜70%、好ましくは30〜65%とする。
2) Microstructure As described above, in order to achieve high ductility and high strength of TS of 780 MPa or more, the microstructure is a composite structure of ferrite phase and tempered martensite phase, and the volume of tempered martensite phase. The rate needs to be controlled. The tempered martensite phase is a structure obtained by reheating the martensite phase, which is a low temperature transformation phase obtained by quenching the austenite phase to the martensite transformation temperature Ms point or less by water quenching, etc., and is harder than the ferrite phase Therefore, it contributes to strength. In order to obtain TS of 780 MPa or more, the volume ratio of the tempered martensite phase needs to be 20% or more. On the other hand, when the volume fraction of the tempered martensite phase exceeds 70%, the volume fraction of the soft ferrite phase is lowered and excessively strengthened, and the TS-El balance is lowered. Therefore, the volume ratio of the tempered martensite phase is 20 to 70%, preferably 30 to 65%.
ここで、焼戻マルテンサイト相の体積率は、鋼板の圧延方向に平行な板厚断面の板厚1/4の位置を光学顕微鏡あるいは走査型電子顕微鏡により倍率1000で観察し、100mm四方の領域に存在する焼戻マルテンサイト相の占有面積率を画像処理によって求め、体積率とした。 Here, the volume ratio of the tempered martensite phase was measured at a magnification of 1000 with an optical microscope or a scanning electron microscope at the position of the thickness 1/4 of the thickness cross section parallel to the rolling direction of the steel plate. The area ratio occupied by the tempered martensite phase present in the film was obtained by image processing and used as the volume ratio.
3)[Si(0)/Si(0.1)]
上述したように、優れた化成処理性を図るには、鋼板表面のSi濃度Si(0)と鋼板表面から0.1mm深さのところのSi濃度Si(0.1)の比[Si(0)/Si(0.1)]およびその標準偏差をコントロールする必要がある。Siは易酸化元素であり、SiO2として過度に鋼板表面に存在すると鋼板表面に絶縁体が点在することになり、塗装前処理段階である表面調整時にTiコロイドや帯電したリン酸塩微粒子の吸着を阻害する。また、鋼板表面のSi酸化物は、電着塗装の下地処理として行われるリン酸亜鉛などによる化成処理皮膜の形成初期に、鋼板のエッチング性を阻害する。さらに、Si酸化物が局所的に高濃度で存在している場合には、エッチングにムラが生じ、高Si濃度の場所では化成結晶の形成が阻害される。すなわち化成処理皮膜が鋼板表面に十分に均一微細かつ緻密に形成されず、電着塗装後に塩温水のような劣悪な環境下に曝されると塗膜の2次密着性が著しく劣化する。そこで、鋼板表面のSi濃度を最適化するために、上記の比[Si(0)/Si(0.1)]と化成処理性の関係を検討したところ、[Si(0)/Si(0.1)]を1.0〜1.6、好ましくは1.0〜1.4とすれば優れた化成処理性が得られることがわかった。この比が1.6を超えると化成処理性が著しく低下する。なお、この比が1.0のとき、すなわち表面のSi濃度と0.1mm深さのところのSi濃度が同じとなり、優れた化成処理性が得られる。
3) [Si (0) / Si (0.1)]
As described above, in order to achieve excellent chemical conversion, the ratio of Si concentration Si (0) on the steel sheet surface to Si concentration Si (0.1) at a depth of 0.1 mm from the steel sheet surface [Si (0) / Si (0.1)] and its standard deviation need to be controlled. Si is an easily oxidizable element. If SiO 2 is excessively present on the surface of the steel sheet, insulators will be scattered on the surface of the steel sheet. Inhibits adsorption. Further, the Si oxide on the surface of the steel sheet inhibits the etching property of the steel sheet at the initial stage of formation of the chemical conversion film with zinc phosphate or the like, which is performed as a base treatment for electrodeposition coating. Further, when the Si oxide is locally present at a high concentration, unevenness occurs in etching, and formation of a chemical conversion crystal is hindered at a location with a high Si concentration. That is, the chemical conversion treatment film is not sufficiently finely and densely formed on the surface of the steel sheet, and when exposed to a poor environment such as salt warm water after electrodeposition coating, the secondary adhesion of the coating film is remarkably deteriorated. Therefore, in order to optimize the Si concentration on the steel sheet surface, the relationship between the above ratio [Si (0) / Si (0.1)] and chemical conversion treatment was examined. [Si (0) / Si (0.1)] It was found that excellent chemical conversion treatment properties can be obtained by setting the ratio to 1.0 to 1.6, preferably 1.0 to 1.4. When this ratio exceeds 1.6, the chemical conversion treatment performance is remarkably lowered. When this ratio is 1.0, that is, the Si concentration at the surface is the same as the Si concentration at a depth of 0.1 mm, and excellent chemical conversion property is obtained.
鋼板表面のSi濃度に加えて、Siの分布も極めて重要である。これは、同じSi濃度であっても、高Si濃度の領域が多く存在する場合もあれば、Si濃度が均一である場合もあるからである。そこで、[Si(0)/Si(0.1)]の標準偏差と塗装後耐食性の関係について検討したところ、この標準偏差が0.50を超えると局所的にSi濃度の高い領域が存在して塗装後耐食性が急激に劣化する傾向にあることがわかった。したがって、この標準偏差は0.50以下、好ましくは0.30以下とする。
In addition to the Si concentration on the steel sheet surface, the distribution of Si is extremely important. This is because even if the Si concentration is the same, there may be many regions having a high Si concentration, or the Si concentration may be uniform. Therefore, when the relationship between the standard deviation of [Si (0) / Si (0.1)] and the corrosion resistance after painting was examined, if this standard deviation exceeded 0.50, there was a local region with high Si concentration, and the corrosion resistance after painting. Was found to tend to deteriorate rapidly. Therefore, this standard deviation is 0.50 or less, preferably 0.30 or less.
Siが平均的に存在していれば、エッチング性は悪いが、均一に反応が進み、より均一微細で緻密な化成結晶が生成する。それゆえ、この標準偏差が小さい、すなわち鋼板表面でSi濃度が均一であればあるほど好ましい。 If Si is present on average, the etching property is poor, but the reaction proceeds uniformly and more uniform fine and dense chemical crystals are formed. Therefore, it is preferable that the standard deviation is small, that is, the Si concentration is uniform on the steel plate surface.
ここで、Si(0)、Si(0.1)は次のようにして測定した。すなわち、日本電子(株)社製のEPMA、JXA8600MXを用い、分析エリア：200μm×200μm、電子線加速エネルギー：15kV、電流：1.2×10-7A、分析時間：50msec/pointの条件で、Si元素分布を調査し、Siの分布を強度レベルに応じてSiカウント0(MIN)〜500(MAX)のレンジで測定した。そして、鋼板表面のSiカウントの平均をSi(0)とし、鋼板表面から0.1mm深さまで研削した面のSiカウントの平均をSi(0.1)とした。なお、Si(0)とSi(0.1)の比の標準偏差は、Si(0)に対するSiカウントの分布から求めた。 Here, Si (0) and Si (0.1) were measured as follows. That is, using EPMA JXA8600MX manufactured by JEOL Ltd., analysis area: 200 μm × 200 μm, electron beam acceleration energy: 15 kV, current: 1.2 × 10 −7 A, analysis time: 50 msec / point, Si The element distribution was investigated, and the Si distribution was measured in the range of Si count 0 (MIN) to 500 (MAX) according to the strength level. The average Si count on the steel sheet surface was Si (0), and the average Si count on the surface ground to a depth of 0.1 mm from the steel sheet surface was Si (0.1). The standard deviation of the ratio between Si (0) and Si (0.1) was obtained from the distribution of Si counts relative to Si (0).
4)製造方法
本発明の超高強度冷延鋼板は、例えば、上記した成分組成の鋼スラブを熱間圧延後、冷間圧延して鋼板とし、この鋼板を、連続焼鈍炉にて、700〜900℃の温度に加熱後、550〜800℃の温度から500℃/秒以上の冷却速度で急冷し、150〜500℃の温度で100〜1400秒間再加熱し、引き続き、0〜4のpHで、10〜100℃の温度で5〜150秒間酸洗処理後、10〜14のpHで、10〜100℃の温度で2〜50秒間アルカリ処理を行い、0.2〜1.5%の伸長率で調質圧延を行うことにより製造できる。
4) Manufacturing method The ultra-high-strength cold-rolled steel sheet of the present invention is, for example, hot-rolled a steel slab having the above-described component composition, and then cold-rolled into a steel sheet. After heating to a temperature of 900 ° C, quench from a temperature of 550-800 ° C at a cooling rate of 500 ° C / second or more, reheat at a temperature of 150-500 ° C for 100-1400 seconds, and then at a pH of 0-4 After pickling treatment at a temperature of 10 to 100 ° C. for 5 to 150 seconds, an alkali treatment is carried out at a pH of 10 to 14 at a temperature of 10 to 100 ° C. for 2 to 50 seconds, and tempered at an elongation rate of 0.2 to 1.5%. It can be manufactured by rolling.
本発明において、鋼スラブは連続鋳造または造塊と分塊圧延により製造され、製造後のスラブは冷却後再加熱されて、あるいはそのまま熱間圧延され熱延板となる。熱延板は冷却し巻取られ、酸洗、冷間圧延されて所望の板厚の冷延板となる。この時、熱間圧延から冷間圧延までの条件は特に限定されないが、熱間圧延における仕上温度は、熱延板の組織を均一化し、曲げ性などの加工性を向上させるため、850℃以上が望ましく、熱間圧延後の巻取温度は、冷間変形抵抗を低減して冷間圧延性を向上させるため450℃〜650℃が望ましく、冷間圧延率は、フェライト相の再結晶を促進させて延性を向上させるため、30%以上が望ましい。次いで、冷間圧延後の冷延板は連続焼鈍炉で焼鈍されるが、以下に示す連続焼鈍条件は、本発明に必要な鋼板のミクロ組織や鋼板表面のSi濃度を達成する上で、極めて重要である。 In the present invention, the steel slab is manufactured by continuous casting or ingot-making and split rolling, and the manufactured slab is reheated after cooling or hot-rolled as it is to form a hot-rolled sheet. The hot-rolled sheet is cooled and wound, pickled, and cold-rolled to obtain a cold-rolled sheet having a desired thickness. At this time, the conditions from hot rolling to cold rolling are not particularly limited, but the finishing temperature in hot rolling is 850 ° C. or higher in order to make the structure of the hot rolled sheet uniform and improve workability such as bendability. The coiling temperature after hot rolling is preferably 450 ° C to 650 ° C in order to reduce cold deformation resistance and improve cold rolling properties, and the cold rolling rate promotes recrystallization of the ferrite phase. In order to improve the ductility, 30% or more is desirable. Next, the cold-rolled sheet after cold rolling is annealed in a continuous annealing furnace, but the continuous annealing conditions shown below are extremely important for achieving the microstructure of the steel sheet and the Si concentration of the steel sheet surface necessary for the present invention. is important.
4-1)焼鈍温度：焼鈍温度が700℃より低いと、冷間圧延により展伸された組織からバンド状の不均一な組織が形成され、伸びフランジ性や曲げ性が劣化する。また、加熱時にオーステナイト相がほとんど存在しないためマルテンサイト相が形成されず、高強度化、高延性化を図れない。一方、焼鈍温度が900℃より高いと、結晶粒が過度に粗大化するとともに、フェライト相の生成量も減少して、伸びフランジ性などの延性が低下する。したがって、焼鈍温度は700〜900℃とする。 4-1) Annealing temperature: When the annealing temperature is lower than 700 ° C., a band-like non-uniform structure is formed from the structure expanded by cold rolling, and stretch flangeability and bendability deteriorate. Further, since there is almost no austenite phase at the time of heating, a martensite phase is not formed, and high strength and high ductility cannot be achieved. On the other hand, when the annealing temperature is higher than 900 ° C., the crystal grains are excessively coarsened, the amount of ferrite phase formed is also reduced, and ductility such as stretch flangeability is lowered. Accordingly, the annealing temperature is 700 to 900 ° C.
4-2)急冷開始温度：上記の焼鈍温度で加熱後の鋼板は、高延性化と高強度化を図るために、放冷後、所望の急冷開始温度から急冷されて、フェライト相とマルテンサイト相の複合組織が形成される。急冷開始温度が550℃未満では、マルテンサイト相の体積率が少なく、780MPa以上のTSが得られない。一方、急冷開始温度が800℃を越えるとマルテンサイト相の体積率が増大し、延性の低下を招く。したがって、急冷開始温度は550〜800℃とする。 4-2) Rapid cooling start temperature: In order to increase the ductility and strength of the steel sheet after heating at the above annealing temperature, after cooling, the steel sheet is rapidly cooled from the desired rapid cooling start temperature, and the ferrite phase and martensite A phase complex is formed. When the quenching start temperature is less than 550 ° C., the volume ratio of the martensite phase is small, and a TS of 780 MPa or more cannot be obtained. On the other hand, when the quenching start temperature exceeds 800 ° C., the volume ratio of the martensite phase increases, leading to a decrease in ductility. Therefore, the rapid cooling start temperature is 550 to 800 ° C.
4-3)急冷時の冷却速度：本発明ではフェライト安定化元素であるSiが添加されているため、冷却速度が500℃より低いとフェライト相が過度に生成し、マルテンサイト相の体積率が低下し、780MPa以上のTSが得られない。したがって、急冷時の冷却速度は500℃/秒以上、好ましくは1000℃/秒超えとする。なお、この冷却速度は、冷却開始温度から100℃までの平均冷却速度である。また、冷却は、水冷が好ましいが、放冷、ガス冷却、ミスト冷却、ロール冷却などを用いて組み合わせて冷却を行うことも可能である。 4-3) Cooling rate during quenching: In the present invention, since the ferrite stabilizing element Si is added, if the cooling rate is lower than 500 ° C., the ferrite phase is excessively formed, and the volume ratio of the martensite phase is reduced. Decreases and TS above 780MPa cannot be obtained. Therefore, the cooling rate at the time of quenching is 500 ° C./second or more, preferably 1000 ° C./second or more. This cooling rate is an average cooling rate from the cooling start temperature to 100 ° C. The cooling is preferably water cooling, but it is also possible to perform cooling by combining using natural cooling, gas cooling, mist cooling, roll cooling, or the like.
4-4)再加熱条件：再加熱温度が150℃より低いと、マルテンサイト相が十分に焼戻されずTS-Elバランスが低下する。また、硬質相が多くなるので、フェライト相との硬度差も大きくなり、伸びフランジ性や曲げ性が劣化する。一方、再加熱温度が500℃を越えると、焼戻が過度に急激に進行してマルテンサイト相がフェライト相と炭化物に分解し、軟化するため780MPa以上のTSが得られない。したがって、再加熱温度は150〜500℃、好ましくは200〜400℃とする。再加熱時間が100秒に満たないと、マルテンサイト相の焼戻が不十分となり、過度に高強度化し、TS-Elバランスや伸びフランジ性の低下を招く。一方、再加熱時間が1400秒を越えるとその効果は飽和する傾向にあるばかりではなく、焼戻が過度に進行し780MPa以上のTSが得られない。したがって、再加熱時間は100〜1400秒、好ましくは300〜1000秒とする。なお、再加熱後室温までの冷却は、空冷、炉冷却、ガス冷却、ミスト冷却、水冷などで行える。 4-4) Reheating conditions: When the reheating temperature is lower than 150 ° C, the martensite phase is not tempered sufficiently and the TS-El balance is lowered. In addition, since the hard phase increases, the hardness difference from the ferrite phase also increases, and stretch flangeability and bendability deteriorate. On the other hand, when the reheating temperature exceeds 500 ° C., tempering proceeds excessively rapidly, the martensite phase decomposes into a ferrite phase and carbide, and softens, so a TS of 780 MPa or more cannot be obtained. Therefore, the reheating temperature is 150 to 500 ° C, preferably 200 to 400 ° C. If the reheating time is less than 100 seconds, the tempering of the martensite phase is insufficient, the strength is excessively increased, and the TS-El balance and stretch flangeability are deteriorated. On the other hand, when the reheating time exceeds 1400 seconds, not only does the effect tend to saturate, but tempering proceeds excessively, and a TS of 780 MPa or more cannot be obtained. Therefore, the reheating time is 100 to 1400 seconds, preferably 300 to 1000 seconds. In addition, cooling to room temperature after reheating can be performed by air cooling, furnace cooling, gas cooling, mist cooling, water cooling, or the like.
4-5)酸洗処理条件：酸洗処理の目的は、鋼板表層に生成したFe系酸化物いわゆるスケールを除去して鋼板表面を清浄にすること、塗装前処理段階である表面調整時にTiコロイドや帯電したリン酸塩微粒子の吸着を阻害したり、化成処理の薬液に浸漬時に化成結晶の核生成を阻害するSiとMnの複合酸化物などを除去することである。そのためには、4以下のpHで、10℃以上の温度で5秒以上の間酸洗処理を行う必要がある。一方、処理温度が100℃を超えたり、処理時間が150秒を超えると、その効果は飽和するだけでなく、過酸洗となり鋼板表面に凹凸が生じるため化成電着塗装後の耐食性が低下する。なお、pHが0に近ければ近いほど反応は早く起こる。したがって、酸洗処理は、0〜4のpHで、10〜100℃の温度で5〜150秒間の処理条件で行う。酸洗は、塩酸、硫酸、硝塩酸などで行うのが好ましい。 4-5) Conditions for pickling treatment: The purpose of pickling treatment is to remove the Fe-based oxide so-called scale formed on the surface layer of the steel plate to clean the steel plate surface, and to colloid Ti during surface conditioning at the pre-painting treatment stage. It is to remove the complex oxide of Si and Mn that inhibits adsorption of charged phosphate fine particles or inhibits nucleation of chemical crystals when immersed in chemical solution for chemical conversion treatment. For this purpose, it is necessary to carry out pickling treatment at a pH of 4 or less and a temperature of 10 ° C. or more for 5 seconds or more. On the other hand, when the treatment temperature exceeds 100 ° C or the treatment time exceeds 150 seconds, the effect is not only saturated, but also the perforation and the surface of the steel sheet become uneven, resulting in reduced corrosion resistance after chemical electrodeposition coating. . The closer the pH is to 0, the faster the reaction takes place. Accordingly, the pickling treatment is performed at a pH of 0 to 4 and at a temperature of 10 to 100 ° C. for 5 to 150 seconds. The pickling is preferably performed with hydrochloric acid, sulfuric acid, nitric hydrochloric acid or the like.
4-6)アルカリ処理条件：アルカリ処理は、酸洗後に残っている難酸溶性のSiO2などを除去するために行われる。pH-電位図によると、SiO2は0〜4のpH領域では不活性であり、酸洗処理では除去されない。そのため、従来の技術では、SiO2の上にNiめっきなどを施して化成塗装後の耐食性を確保している。本発明者らは、酸洗後にアルカリ処理を行うことにより熱延または焼鈍中に生成し、酸洗後に残っている難酸溶性のSiO2を完全に除去できることを見出した。そのためには、10以上のpHで、10℃以上の温度で2秒以上の間アルカリ処理を行う必要がある。一方、処理温度が100℃を超えたり、処理時間が50秒を超えると、その効果は飽和する。なお、pHが14に近ければ近いほど反応は早く起こる。したがって、アルカリ処理は、10〜14のpHで、10〜100℃の温度で2〜50秒間の処理条件で行う。アルカリ薬液種は問わないが、苛性ソーダ、縮合リン酸塩系などが好ましい。 4-6) Alkali treatment conditions: The alkali treatment is carried out to remove the hardly acid-soluble SiO 2 remaining after pickling. According to the pH-potential diagram, SiO 2 is inactive in the pH range of 0-4 and is not removed by pickling. Therefore, in the conventional technique, Ni plating or the like is applied on SiO 2 to ensure the corrosion resistance after chemical conversion coating. The present inventors have found that it is possible to completely remove the hardly acid-soluble SiO 2 produced during hot rolling or annealing by performing an alkali treatment after pickling and remaining after pickling. For this purpose, it is necessary to perform an alkali treatment at a pH of 10 or higher and a temperature of 10 ° C. or higher for 2 seconds or longer. On the other hand, when the processing temperature exceeds 100 ° C. or the processing time exceeds 50 seconds, the effect is saturated. The closer the pH is to 14, the faster the reaction takes place. Accordingly, the alkali treatment is performed at a pH of 10 to 14 and at a temperature of 10 to 100 ° C. for 2 to 50 seconds. The alkaline chemical liquid type is not limited, but caustic soda, condensed phosphate, and the like are preferable.
4-7)調質圧延の伸長率：一般に、調質圧延は降伏伸びなどを除去する目的で実施されるが、化成処理性にも影響を及ぼす。調質圧延の伸長率が0.2%未満だと、前処理段階である表面調整時のTiコロイドや帯電したリン酸塩微粒子の吸着のサイトの一つとなる格子欠陥数が少なく、その後の化成処理工程において微細緻密な化成結晶の生成が困難となる。また、鋼板表面には凹凸が残り、均一に化成結晶を生成させることが難しくなる。一方、伸長率を1.5%以上にすると、こうした効果が飽和する傾向にあるとともに、Elの低下が認められる。したがって、調質圧延の伸長率は0.2〜1.5%とする。 4-7) Elongation ratio of temper rolling: In general, temper rolling is performed for the purpose of removing yield elongation, but it also affects the chemical conversion processability. If the elongation rate of temper rolling is less than 0.2%, the number of lattice defects that become one of the sites of adsorption of Ti colloid and charged phosphate particles during surface preparation, which is the pretreatment stage, is small, and the subsequent chemical conversion treatment process In this case, it becomes difficult to produce fine and dense chemical crystals. Moreover, unevenness | corrugation remains on the steel plate surface, and it becomes difficult to produce a chemical conversion crystal uniformly. On the other hand, when the elongation rate is 1.5% or more, such effects tend to saturate and a decrease in El is observed. Therefore, the elongation rate of temper rolling is set to 0.2 to 1.5%.
表1に示す成分を有する鋼A〜Kのスラブを、1250℃に加熱後、880℃の仕上温度で熱間圧延し、620℃の巻取温度で巻取り、50%の冷延圧下率で冷間圧延を行い、板厚1.8mmの鋼板とした。次いで、冷間圧延された鋼板に対し、表2に示す条件で連続焼鈍、酸洗処理、アルカリ処理、および調質圧延を行い冷延鋼板1〜18を作製した。そして、上記の方法によりミクロ組織や[Si(0)/Si(0.1)]を調査するとともに、下記の方法により引張特性、伸びフランジ性、化成処理性および化成電着塗装後の耐食性についても検討を行った。また、フェライト平均結晶粒径をJIS Z 0522に準拠して測定した。 Slabs of steels A to K having the components shown in Table 1 were heated to 1250 ° C, hot-rolled at a finishing temperature of 880 ° C, wound at a winding temperature of 620 ° C, and with a cold rolling reduction ratio of 50%. Cold rolling was performed to obtain a steel plate having a thickness of 1.8 mm. Subsequently, continuous annealing, pickling treatment, alkali treatment, and temper rolling were performed on the cold-rolled steel sheet under the conditions shown in Table 2 to produce cold-rolled steel sheets 1-18. In addition to investigating the microstructure and [Si (0) / Si (0.1)] by the above methods, the following methods are also used to examine tensile properties, stretch flangeability, chemical conversion properties, and corrosion resistance after chemical electrodeposition coating. Went. Further, the ferrite average crystal grain size was measured according to JIS Z 0522.
引張特性：鋼板の圧延方向と90°の方向を長手方向とするJIS Z 2201の5号試験片を用い、JIS Z 2241準拠した引張試験を行いTSとElを測定した。ここで、TS×El≧17500MPa・%のとき優れたTS-Elバランスであると判定した。 Tensile properties: Using a JIS Z 2201 No. 5 test piece with the rolling direction of the steel sheet and the 90 ° direction as the longitudinal direction, a tensile test based on JIS Z 2241 was performed to measure TS and El. Here, when TS × El ≧ 17500 MPa ·%, it was determined that the TS-El balance was excellent.
伸びフランジ性：日本鉄鋼連盟規格JFS T1001に基づき、鋼板に直径10mmの穴を打抜き、60°の円錐ポンチを用いてこの穴を拡げ、亀裂が板厚貫通したときの穴径d(mm)を測定し、穴拡げ率λ=[(d-10)/10]×100を求めた。なお、同様な試験を3回行い、その平均値で評価した。ここで、TS×λ≧70000MPa・%のとき優れたTS-λバランスであると判定した。 Stretch flangeability: Based on the Japan Iron and Steel Federation standard JFS T1001, punch a 10mm diameter hole into a steel plate, expand this hole using a 60 ° conical punch, and determine the hole diameter d (mm) when the crack penetrates the plate thickness. The hole expansion ratio λ = [(d-10) / 10] × 100 was determined. A similar test was performed three times, and the average value was evaluated. Here, when TS × λ ≧ 70000 MPa ·%, it was determined that the TS-λ balance was excellent.
化成処理性および化成電着塗装後の耐食性：日本ペイント(株)社製の表面調整薬品(5N-10)と化成処理液(SD2800)を用いて、75mm×150mmの試験片にリン酸亜鉛による化成処理後、厚さ25μmの電着塗装(塗料：V-50ブラック)を施し、カッターナイフで長さ100mmの2本の切り込みを入れ、50℃の5%NaCl溶液中に240時間浸漬後、粘着テープを切り込み上に貼って剥がし、塗膜の剥離巾を測定した。そして、化成結晶粒径が2〜10μm、皮膜重量が1.8〜2.6g/m2、最大剥離巾が2.5mm以下のとき、化成処理性および電着塗装後の耐食性が良好と判定した。ここで、皮膜重量は化成処理後に化成皮膜を溶解して、溶解前後の重量測定により求めた。また、化成結晶粒径は、SEMにより1000倍で組織観察し、切断法で測定した。 Chemical conversion treatment and corrosion resistance after chemical electrodeposition coating: Using a surface conditioning chemical (5N-10) manufactured by Nippon Paint Co., Ltd. and chemical conversion solution (SD2800), a test piece of 75 mm x 150 mm is coated with zinc phosphate. After chemical conversion treatment, apply electrodeposition coating (paint: V-50 black) with a thickness of 25μm, make two cuts with a length of 100mm with a cutter knife, immerse in 50% 5% NaCl solution for 240 hours, The adhesive tape was cut and pasted on the cut, and the peel width of the coating film was measured. When the chemical conversion grain size was 2 to 10 μm, the coating weight was 1.8 to 2.6 g / m 2 , and the maximum peel width was 2.5 mm or less, it was determined that the chemical conversion treatment property and the corrosion resistance after electrodeposition coating were good. Here, the film weight was obtained by dissolving the chemical film after the chemical conversion treatment and measuring the weight before and after dissolution. The chemical crystal grain size was measured by a cutting method after observing the structure at 1000 times with SEM.
結果を表3に示す。本発明例である鋼板1、2、10〜17は、TS×El≧17500MPa・%、TS×λ≧70000MPa・%、最大剥離巾が2.5mm以下となっており、高延性で、化成処理性に優れるTSが780〜1180MPaの超高強度冷延鋼板であることがわかる。酸洗処理、アルカリ処理、調質圧延の伸長率が本発明範囲外である鋼板3、4、5、6、7、8、9およびSi成分が本発明範囲外である鋼板18は、[Si(0)/Si(0.1)]およびその標準偏差が本発明範囲外となっており、化成処理性および塗装後の耐食性に劣っている。 The results are shown in Table 3. Steel plates 1, 2, 10 to 17 as examples of the present invention are TS × El ≧ 17500 MPa ·%, TS × λ ≧ 70000 MPa ·%, maximum peel width is 2.5 mm or less, high ductility, chemical conversion treatment property It can be seen that this is a super-high-strength cold-rolled steel sheet having an excellent TS of 780 to 1180 MPa. Steel plate 3, 4, 5, 6, 7, 8, 9 whose elongation ratio of pickling treatment, alkali treatment and temper rolling is outside the scope of the present invention and steel plate 18 whose Si component is outside the scope of the present invention are: (0) / Si (0.1)] and its standard deviation are outside the scope of the present invention, and are inferior in chemical conversion treatment and corrosion resistance after coating.
本発明の超高強度冷延鋼板は、自動車部品に限らず、建築材料や家電製品の分野で厳しい曲げ加工や化成処理性が必要とされる部材にも好適である。
The ultra-high-strength cold-rolled steel sheet of the present invention is suitable not only for automobile parts but also for members that require severe bending and chemical conversion properties in the fields of building materials and home appliances.
Claims (8)
鋼スラブを熱間圧延後、冷間圧延して鋼板とし、前記鋼板を、連続焼鈍炉にて、700〜900℃の温度に加熱後、550〜800℃の温度から500℃/秒以上の冷却速度で急冷し、150〜500℃の温度で100〜1400秒間再加熱し、引き続き、0〜4のpHで、10〜100℃の温度で5〜150秒間酸洗処理後、10〜14のpHで、10〜100℃の温度で2〜50秒間アルカリ処理を行い、0.2〜1.5%の伸長率で調質圧延を行うことを特徴とする高延性で、化成処理性に優れる780MPa以上の引張強度を有する超高強度冷延鋼板の製造方法。 In mass%, C: 0.05 to 0.2%, Si: 0.5 to 2.0%, Mn: 1.5 to 3.5%, P: 0.001 to 0.05%, S: 0.0001 to 0.005%, Al: 0.005 to 0.08%, N: 0.001 to After hot rolling a steel slab consisting of 0.01% and the balance Fe and unavoidable impurities, cold rolled into a steel plate, the steel plate was heated to a temperature of 700 to 900 ° C. in a continuous annealing furnace, 550 Quench from a temperature of ~ 800 ° C at a cooling rate of 500 ° C / sec or more, reheat at a temperature of 150-500 ° C for 100-1400 seconds, and then continue at 5 at a temperature of 10-100 ° C at a pH of 0-4. After pickling treatment for ~ 150 seconds, alkali treatment is performed at a pH of 10 to 14 at a temperature of 10 to 100 ° C. for 2 to 50 seconds, and temper rolling is performed at an elongation rate of 0.2 to 1.5%. A method for producing an ultra-high strength cold-rolled steel sheet having a tensile strength of 780 MPa or more that is ductile and has excellent chemical conversion properties.
Further, the steel slab containing Ca: 0.0001 to 0.005% by mass% is used, and the tensile strength of 780 MPa or more which is excellent in chemical conversion treatment property with high ductility according to any one of claims 5 to 7. A method for producing an ultra-high strength cold-rolled steel sheet having strength.
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