JP2012031458A - High-strength cold-rolled thin steel sheet superior in formability and method of manufacturing the same - Google Patents
High-strength cold-rolled thin steel sheet superior in formability and method of manufacturing the same Download PDFInfo
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本発明は、主として建材、家電、自動車等の分野で使用する部品用として好適な、高強度冷延薄鋼板およびその製造方法に係り、とくに、成形性の向上に関する。なお、ここでいう「鋼板」には、鋼板、鋼帯を含むものとする。また、ここでいう「高強度薄鋼板」とは、板厚：3.0mm以下で、引張強さ：400MPa以上700MPa未満の強度を有する鋼板をいうものとする。 The present invention relates to a high-strength cold-rolled thin steel sheet suitable for parts mainly used in the fields of building materials, home appliances, automobiles, and the like, and more particularly to improvement of formability. The “steel plate” here includes a steel plate and a steel strip. The “high-strength thin steel plate” here refers to a steel plate having a thickness of 3.0 mm or less and a tensile strength of 400 MPa or more and less than 700 MPa.
電機分野や建材分野では、近年、販売競争の激化に伴い、コスト低減のために、安価な素材が強く要求されてきた。またさらに、運送費の低減のために、素材および製品の軽量化も要求されている。また、自動車分野では、コスト低減に加えて、地球環境の保全という観点から、自動車の燃費向上が強く要求され、最近では、自動車車体の軽量化が進められている。このような要求に対する有効な対策としては、素材である鋼板を薄肉化し軽量化を図るために、高強度化した高張力鋼板を使用すること、およびコスト低減のために安価な高張力鋼板を使用することが、まず挙げられる。 In recent years, in the electric field and building material field, with the intensification of sales competition, inexpensive materials have been strongly demanded for cost reduction. Furthermore, weight reduction of materials and products is also required to reduce transportation costs. In the automobile field, in addition to cost reduction, there is a strong demand for improving the fuel efficiency of automobiles from the viewpoint of protecting the global environment, and recently, the weight reduction of automobile bodies has been promoted. Effective measures to meet these requirements include the use of high-strength steel sheets with increased strength in order to reduce the thickness and weight of the steel sheet, and use inexpensive high-tensile steel sheets to reduce costs. First of all.
鋼板を高強度化する手段としては、加工硬化、固溶強化、析出強化、組織強化等の強化方法が知られている。しかし、固溶強化、析出強化による方法では、多量の合金元素を含有させる必要があり、また組織強化による方法ではさらに急冷等の熱処理を施す必要があり、鋼板製造コストの高騰を伴うという問題がある。
このような問題に対し、コスト低減要求の強い電機分野や建材分野向け鋼板では、加工硬化による鋼板の高強度化が有利と考えられている。しかし、加工硬化による高強度化は、他の強化方法に比べて、延性の低下を伴うという問題がある。
Strengthening methods such as work hardening, solid solution strengthening, precipitation strengthening, and structure strengthening are known as means for increasing the strength of steel sheets. However, in the method using solid solution strengthening and precipitation strengthening, it is necessary to contain a large amount of alloy elements, and in the method using structure strengthening, it is necessary to further perform heat treatment such as rapid cooling, resulting in an increase in steel sheet manufacturing costs. is there.
In order to solve such problems, it is considered advantageous to increase the strength of the steel sheet by work hardening in the steel sheet for the electrical machinery field and the building material field, which are strongly demanded for cost reduction. However, the increase in strength by work hardening has a problem in that the ductility is lowered as compared with other strengthening methods.
例えば特許文献１には、連続焼鈍工程を省略し、冷間圧延ままで用いることができ、コストを削減できる缶用鋼板の製造方法が記載されている。特許文献１に記載された技術は、主として、３ピース缶用素材を目的として、Ｃ：0.015％以下、Si：0.020％以下、Mn：0.10％以下、Ｐ：0.010％以下、Ｓ：0.005％以下、Ｎ：0.0030％以下、Al：0.150％以下を含み、さらに、Cr：0.020〜0.500％、Nb：0.0020〜0.0200％、Ti：0.0050〜0.0200％、Ｂ：0.0002〜0.0020％の１種または２種以上を含む連続鋳造スラブを1050℃以下に再加熱した後に熱延し、仕上げ圧延機入側温度を950℃以下とし、そこでの合計圧下率を40％以上、かつ、最終圧下率を25％以上とし、最終の熱延母板厚みを1.2mm以下として、500〜750℃の温度で巻取りを行い、通常の酸洗の後、圧下率50〜98％の冷間圧延を行う缶用鋼板の製造方法である。 For example, Patent Document 1 describes a method for producing a steel plate for a can that can be used as it is in cold rolling without using a continuous annealing step and can reduce costs. The technology described in Patent Document 1 is mainly intended for three-piece can materials, C: 0.015% or less, Si: 0.020% or less, Mn: 0.10% or less, P: 0.010% or less, S: 0.005% or less N: 0.0030% or less, Al: 0.150% or less, Cr: 0.020-0.500%, Nb: 0.0020-0.0200%, Ti: 0.0050-0.0200%, B: 0.0002-0.0020% The continuous cast slab containing the above is re-heated to 1050 ° C or less and then hot-rolled, the finish rolling mill entry side temperature is set to 950 ° C or less, and the total rolling reduction is 40% or more, and the final rolling reduction is 25% or more. And the final hot-rolled mother board thickness is 1.2 mm or less, the steel sheet is wound at a temperature of 500 to 750 ° C., and after normal pickling, the steel sheet for cans is subjected to cold rolling at a reduction rate of 50 to 98%. It is a manufacturing method.
また、例えば、特許文献２には、成形時には軟質であるが、成形後の塗装焼付け処理時に生じる歪時効硬化を利用して硬質化する、引張強さ：590MPa未満の比較的低強度の溶融亜鉛めっき冷延鋼板の製造方法が記載されている。特許文献２に記載された技術は、mass％で、Ｃ：0.01〜0.2％、Si：0.4％以下、Mn：0.2〜2.0％、Ｐ：0.05％以下、Al：0.01〜0.1％、Ｎ：0.005〜0.02％を含有し、固溶Ｎの含有量が50ppm以上である組成を有する冷延鋼板に、650℃超え900℃以下の温度域に加熱した後、少なくとも当該加熱温度から650℃までの温度域を平均冷却速度：５〜50℃/sで冷却する、加熱−冷却処理を施し、引き続きめっき処理を施す溶融亜鉛めっき冷延鋼板の製造方法である。なお、この加熱−冷却処理では、Ｎ含有量とAl含有量の特定関係で定義されるＮslが0.005未満の場合には、650℃以上の温度域における処理時間を、加熱温度、冷間圧下率、Ｎ含有量の特定関係で定義されるｔgl秒以内に規制するとしている。 Further, for example, Patent Document 2 discloses a relatively low-strength molten zinc having a tensile strength of less than 590 MPa, which is soft at the time of molding but is hardened by using strain age hardening that occurs during paint baking after molding. A method for producing a plated cold-rolled steel sheet is described. The technology described in Patent Document 2 is mass%, C: 0.01 to 0.2%, Si: 0.4% or less, Mn: 0.2 to 2.0%, P: 0.05% or less, Al: 0.01 to 0.1%, N: 0.005 A cold-rolled steel sheet having a composition containing ˜0.02% and a solid solution N content of 50 ppm or more is heated to a temperature range of 650 ° C. to 900 ° C. and at least the temperature from the heating temperature to 650 ° C. This is a method for producing a hot-dip galvanized cold-rolled steel sheet in which the region is cooled at an average cooling rate of 5 to 50 ° C./s, subjected to a heating-cooling process and subsequently subjected to a plating process. In this heating-cooling treatment, when Nsl defined by the specific relationship between the N content and the Al content is less than 0.005, the treatment time in the temperature range of 650 ° C. or higher is defined as the heating temperature and the cold reduction rate. , The regulation is made within tgl seconds defined by the specific relationship of the N content.
特許文献１に記載された技術によれば、製造された冷延鋼板は、降伏強さYSが570〜690MPa（58〜70 kg/mm２）程度の高強度を有しているが、伸びが５〜７％と低く、同一強度の冷延鋼板と比べ加工性が低下しているうえ、スラブ組成を高純度化し、さらに熱延後の板厚を1.2mm以下と薄くする必要があるため、製造コストが高騰するという問題がある。
また、特許文献２に記載された技術によれば、引張強さ：590MPa未満の比較的低強度のめっき鋼板の製造が可能となる。しかし、特許文献２に記載された技術では、高強度を得るために、成形後焼付け塗装処理を施す必要があり、最終製品までのコスト上昇が避けられないうえ、特許文献２に記載された技術では、引張強さ：590MPa以上の高強度を有し、かつ成形性に優れた高強度鋼板を製造できるまでに至っていないという、問題がある。
According to the technique described in Patent Document 1, the produced cold-rolled steel sheet has a high strength with a yield strength YS of about 570 to 690 MPa (58 to 70 kg / mm 2 ), but the elongation is low. 5-7% is low, workability is lower than cold-rolled steel sheets of the same strength, and the slab composition needs to be highly purified, and the thickness after hot rolling needs to be as thin as 1.2 mm or less. There is a problem that the manufacturing cost increases.
Moreover, according to the technique described in Patent Document 2, it is possible to produce a relatively low strength plated steel sheet having a tensile strength of less than 590 MPa. However, in the technology described in Patent Document 2, it is necessary to perform post-molding baking coating processing in order to obtain high strength, and the cost up to the final product is unavoidable, and the technology described in Patent Document 2 However, there is a problem that a high strength steel sheet having a high tensile strength of 590 MPa or more and excellent formability has not yet been produced.
このような従来技術の問題に鑑み、本発明は、引張強さ：400MPa以上700MPa未満の高強度と、優れた成形性とを兼備する安価な、成形性に優れた高強度冷延薄鋼板およびその製造方法を提供することを目的とする。 In view of such a problem of the prior art, the present invention provides an inexpensive, high-strength cold-rolled thin steel sheet having excellent formability, which has both high strength of tensile strength: 400 MPa to less than 700 MPa and excellent formability, and It aims at providing the manufacturing method.
本発明者らは、上記した目的を達成するために、合金元素量を極限まで低減した冷延薄鋼板において、強度と成形性とに及ぼす各種要因について鋭意研究した。
その結果、本発明者らは、まず、高強度と延性とを兼備させるために、冷間圧延と、フェライトの回復は起こるが再結晶は生じない比較的低温度域での焼鈍処理とを組み合せて施すことを思い付いた。そこでまず、電機分野で汎用的な組成である、質量％で、0.019％Ｃ−0.0020％Ｎ−0.01％Si−0.15％Mn−0.02％Ｐ−0.01％Ｓ−0.038％Al−残部Feからなる組成の鋼板（板厚：2.6mm）に、圧下率：70〜80％の冷間圧延と、焼鈍温度：720℃以下の温度域での焼鈍処理とを施し冷延鋼板とし、得られた冷延鋼板から、圧延方向が引張方向となるように試験片（JIS 13号 B引張試験片：GL：25ｍｍ）を採取し、JIS Z 2241の規定に準拠して、引張試験を実施し、引張特性（引張強さTS、全伸びT-El、局部伸びL-El）を求めた。
In order to achieve the above-mentioned object, the present inventors diligently studied various factors affecting strength and formability in a cold-rolled thin steel sheet with the amount of alloying elements reduced to the limit.
As a result, the inventors first combined cold rolling and annealing treatment in a relatively low temperature range in which ferrite recovery occurs but recrystallization does not occur in order to combine high strength and ductility. I came up with something to do. Therefore, first, a composition composed of 0.019% C-0.0020% N-0.01% Si-0.15% Mn-0.02% P-0.01% S-0.038% Al-balance Fe, which is a general composition in the electric field. Steel sheet (sheet thickness: 2.6 mm) was subjected to cold rolling with a rolling reduction of 70-80% and annealing treatment at an annealing temperature of 720 ° C. or less to obtain a cold-rolled steel sheet. A test piece (JIS No. 13 B tensile test piece: GL: 25 mm) is taken from the steel plate so that the rolling direction is the tensile direction, and a tensile test is performed in accordance with the provisions of JIS Z 2241 to obtain tensile properties ( Tensile strength TS, total elongation T-El, and local elongation L-El) were determined.
また、得られた鋼板について、Ｘ線回折により、αの（２２０）面の回折ピークの半価幅を測定し、加工歪の残存の有無を調査した。なお、半価幅が0.30°未満の場合を加工歪無とした。
得られた鋼板のうち、冷間加工で導入された加工歪が残存する、加工歪有りの鋼板について、引張強さTS（MPa）とT-El全伸び（％）との関係を、図１に示す。図１から、上記した組成の鋼板に冷間加工と、720℃以下の比較的低い温度域での焼鈍処理と、を施したのちの、TSとT-Elとの関係は、次（１）式
TS（MPa）＝−10×（T-El（％））＋700 ‥‥（ａ）
で近似的に表されることがわかる。また、再結晶が生ぜず回復のみが生じている冷延鋼板の場合には、L-Elは0.5（T-El）以上となることも見出した。
Further, with respect to the obtained steel sheet, the half-value width of the diffraction peak of the (220) plane of α was measured by X-ray diffraction, and the presence or absence of residual processing strain was investigated. The case where the half width was less than 0.30 ° was regarded as no processing strain.
Fig. 1 shows the relationship between tensile strength TS (MPa) and total elongation of T-El (%) for steel plates with work strain, in which work strain introduced by cold working remains among the obtained steel plates. Shown in From FIG. 1, the relationship between TS and T-El after cold working and annealing at a relatively low temperature range of 720 ° C. or less is as follows: formula
TS (MPa) = -10 x (T-El (%)) + 700 (a)
It can be seen that It was also found that L-El is 0.5 (T-El) or more in the case of a cold-rolled steel sheet in which recrystallization does not occur and only recovery occurs.
しかし、図１の結果から、冷間加工と比較的低い温度域での焼鈍処理を組み合わせて施しても、上記した組成の鋼板では、T-El：10％程度以上の高い延性を維持しつつ、かつ、TS：600MPa以上の高強度を安定して確保することは難しいことがわかる。
さらに、高い延性を維持しつつ、更なる高強度化、すなわち引張強さTSの増加、例えば（ａ）式で表されるTSに比べ、少なくとも50MPaの増加、が達成できれば、従来と同レベルの剛性を維持しつつ、鋼板板厚の減少が可能となる。例えば、引張強さが450MPaから500MPaへ増加することで板厚0.8mmの鋼板では、５％程度の更なる鋼板板厚の低減が可能となり、部材の軽量化に寄与できることになる。
However, from the results shown in FIG. 1, even when cold working and annealing at a relatively low temperature range are combined, the steel sheet having the above composition maintains a high ductility of about 10% or more. In addition, it can be seen that it is difficult to stably secure a high strength of TS: 600 MPa or more.
Furthermore, if a further increase in strength, that is, an increase in tensile strength TS, for example, an increase of at least 50 MPa as compared with TS expressed by the formula (a) can be achieved while maintaining high ductility, the same level as before. The steel plate thickness can be reduced while maintaining the rigidity. For example, when the tensile strength is increased from 450 MPa to 500 MPa, a steel plate with a thickness of 0.8 mm can further reduce the steel plate thickness by about 5%, which can contribute to weight reduction of the member.
そこで、本発明者らは、引張強さ増加のために、固溶Ｎによる固溶強化および歪時効硬化を利用することに着目し、所定量以上の固溶Ｎを有する状態の鋼板に冷間加工を施し、そしてさらに、所望の熱処理を施すことに想到した。これにより、延性を損なわずに従来より高強度な冷延鋼板を製造できることを知見した。
すなわち、0.0040質量％以上の固溶Ｎを有する組成の鋼板に、冷間加工とさらに回復のみが生じるような条件の熱処理である焼鈍、あるいは亜鉛めっき処理を施すと、固溶強化と加工硬化に加えて、さらに、歪時効硬化および回復が生起され、所望の高強度を、著しい延性の低下を伴うことなく確保できる。そして、圧延方向の引張強さTSとT-Elとの関係が、（ａ）式で示される関係より、同一の全伸びT-El値で比較してTSで50MPa以上高い関係となる、次（１）式
TS（MPa）≧ −10×（T-El（％））＋750 ‥‥（１）
を満足する、成形性に優れた高強度薄鋼板を、安価に、しかも安定して製造できることを見出した。また、更なる検討により、Ti、Nb、Ｂ、Ｖ等の再結晶抑制元素を大幅なコスト上昇を伴わない程度に、少量含有する組成の鋼板とすることにより、さらに容易に安定して高強度と、高延性（優れた成形性）とを兼備した鋼板とすることができることも見出した。
Therefore, the present inventors pay attention to the use of solid solution strengthening and strain age hardening by solid solution N in order to increase the tensile strength, and cold-work the steel sheet in a state having a predetermined amount or more of solid solution N. It was conceived that processing was performed and further desired heat treatment was performed. As a result, it has been found that a cold-rolled steel sheet having higher strength than before can be produced without impairing ductility.
In other words, when annealing or galvanizing treatment is performed on a steel sheet having a composition having a solid solution N of 0.0040% by mass or more, only cold working and further recovery occur, solid solution strengthening and work hardening can be achieved. In addition, strain age hardening and recovery occur, and the desired high strength can be ensured without significant reduction in ductility. And, the relationship between the tensile strength TS and T-El in the rolling direction is higher than TS in the same total elongation T-El value by 50 MPa or more compared to the relationship expressed by the equation (a). (1) Formula
TS (MPa) ≧ −10 × (T-El (%)) + 750 (1)
It was found that a high-strength thin steel sheet excellent in formability that satisfies the above requirements can be manufactured at low cost and stably. In addition, by further study, steel sheets with a composition containing a small amount of recrystallization-inhibiting elements such as Ti, Nb, B, V, etc. to a level that does not significantly increase the cost, make it easier and more stable and high strength. It has also been found that a steel sheet having both high ductility (excellent formability) can be obtained.
本発明は、かかる知見に基づき、さらに検討を加えて完成されたものである。すなわち、本発明の要旨は次のとおりである。
（１）質量％で、Ｃ：0.010〜0.060％、Si：0.3％以下、Mn：0.1〜0.5％、Ｐ：0.05％以下、Ｓ：0.05％以下、Al：0.05％以下、Ｎ：0.0060〜0.0200％を、次（３−ａ）式
Ｎef＝Ｎ ・・・（３−ａ）
（ここで、Ｎ：Ｎの含有量（質量％））
で定義される有効Ｎ量ＮefとAl含有量との比、Ｎef／Alが0.2以上を満足するように含有し、さらに固溶Ｎを0.0040％以上含み、残部Feおよび不可避的不純物からなる組成と、フェライト相を主相とする組織とを有し、圧延方向の引張強さTS：400MPa以上700MPa未満で、かつ圧延方向の引張強さTSと全伸びT-Elとの関係が次（１）式
TS（MPa）≧ −10×（T-El（％））＋750 ‥‥（１）
（ここで、T-El：圧延方向の全伸び（％）、TS：圧延方向の引張強さ（MPa））
を、全伸びT-Elと局部伸びL-Elとの関係が次（２）式
L-El／T-El ≧ 0.5 ‥‥‥‥‥‥（２）
（ここで、T-El：圧延方向の全伸び（％）、L-El：圧延方向の局部伸び（％））
を、それぞれ満足することを特徴とする成形性に優れた高強度冷延薄鋼板。
The present invention has been completed based on such findings and further studies. That is, the gist of the present invention is as follows.
(1) By mass%, C: 0.010 to 0.060%, Si: 0.3% or less, Mn: 0.1 to 0.5%, P: 0.05% or less, S: 0.05% or less, Al: 0.05% or less, N: 0.0060 to 0.0200 % In the following formula (3-a) N ef = N (3-a)
(N: N content (mass%))
The ratio of the effective N amount N ef to the Al content defined by the above formula, N ef / Al is contained so as to satisfy 0.2 or more, and further contains solid solution N 0.0040% or more, and the balance is Fe and inevitable impurities. It has a composition and a structure having a ferrite phase as the main phase, tensile strength TS in the rolling direction: 400 MPa or more and less than 700 MPa, and the relationship between the tensile strength TS in the rolling direction and the total elongation T-El is as follows ( 1) Formula
TS (MPa) ≧ −10 × (T-El (%)) + 750 (1)
(Where T-El: total elongation in rolling direction (%), TS: tensile strength in rolling direction (MPa))
The relationship between total elongation T-El and local elongation L-El is the following equation (2)
L-El / T-El ≧ 0.5 (2)
(Where T-El: total elongation in rolling direction (%), L-El: local elongation in rolling direction (%))
A high-strength cold-rolled thin steel sheet with excellent formability characterized by satisfying
（２）（１）において、前記組成に加えてさらに、Ti、Ｂのうちから選ばれた１種または２種を含有し、前記（３−ａ）式に代えて、次（３−ｂ）式
Ｎef＝Ｎ−14×（Ti／48＋Ｂ／11） ‥‥‥‥（３−ｂ）
（ここで、N、Ti、B：各要素の含有量（質量％））
で定義される有効Ｎ量Ｎefが0.0060〜0.0200％を満足する組成とすることを特徴とする高強度冷延薄鋼板。
(2) In (1), in addition to the above composition, further contains one or two selected from Ti and B, and instead of the formula (3-a), the following (3-b) Formula N ef = N-14 × (Ti / 48 + B / 11) (3-b)
(Here, N, Ti, B: Content of each element (mass%))
A high-strength cold-rolled thin steel sheet characterized by having a composition satisfying an effective N amount N ef defined by the formula of 0.0060 to 0.0200%.
（３）（１）または（２）において、前記組成に加えてさらに、Nb、Ｖのうちから選ばれた１種または２種を合計で、質量％で、0.05％未満で、かつ前記有効Ｎ量ＮefとNb含有量の比、Ｎef／Nbが0.7以上、前記有効Ｎ量ＮefとＶ含有量の比、Ｎef／Ｖが0.4以上、を満足するように、含有する組成とすることを特徴とする高強度冷延薄鋼板。
（４）鋼素材に、該鋼素材を加熱し熱間圧延を施し熱延板とする熱延工程と、該熱延板に冷間圧延を施し冷延板とする冷延工程と、該冷延板に焼鈍処理を施し冷延焼鈍板とする焼鈍工程と、を順次施し、高強度冷延薄鋼板とする高強度冷延薄鋼板の製造方法において、前記鋼素材を、質量％で、Ｃ：0.010〜0.060％、Si：0.3％以下、Mn：0.1〜0.5％、Ｐ：0.05％以下、Ｓ：0.05％以下、Al：0.05％以下、Ｎ：0.0060〜0.0200％を、次（３−ａ）式
Ｎef＝Ｎ ・・・（３−ａ）
（ここで、Ｎ：Ｎの含有量（質量％））
で定義される有効Ｎ量ＮefとAl含有量との比、Ｎef／Alが0.2以上を満足するように含有し、残部Feおよび不可避的不純物からなる組成を有する鋼素材とし、前記熱延工程を、前記鋼素材を1000℃以上の温度に加熱し、粗圧延を施しシートバーとしたのち、該シートバーに、仕上圧延出側温度：フェライト変態開始温度以上とする仕上圧延を施し熱延板とし、ついで、次（４）式
ＴＡ ＝ 700−10（Al／Ｎef） ‥‥（４）
（ここで、Al：Alの含有量（質量％）、Ｎef：有効Ｎ量（質量％））
で定義されるＴＡ℃以下の温度で巻き取る工程とし、前記冷延工程を、前記熱延板に、圧下率が30〜80％となる冷間圧延を施す工程とし、前記焼鈍工程を、焼鈍温度：250℃以上、次（４）式
ＴＡ ＝ 700−10（Al／Ｎef） ‥‥（４）
（ここで、Al：Alの含有量（質量％）、Ｎef：有効Ｎ量（質量％））
で定義されるＴＡ℃以下で、かつフェライト再結晶開始温度未満の温度に加熱し、該温度で１〜600ｓ間保持する焼鈍処理を施す工程とすることを特徴とする成形性に優れた高強度冷延薄鋼板の製造方法。
(3) In (1) or (2), in addition to the above composition, one or two selected from Nb and V are added in a total amount of less than 0.05% by mass, and the effective N The composition is contained so as to satisfy the ratio of the amount N ef to the Nb content, N ef / Nb is 0.7 or more, and the ratio of the effective N amount N ef to the V content, N ef / V is 0.4 or more. A high-strength cold-rolled thin steel sheet characterized by
(4) A hot rolling process in which the steel material is heated and hot-rolled to form a hot-rolled sheet; a cold-rolling process in which the hot-rolled sheet is cold-rolled to form a cold-rolled sheet; In the method for producing a high-strength cold-rolled thin steel sheet that is subjected to an annealing process to obtain a cold-rolled annealed sheet by sequentially subjecting the steel sheet to a cold-rolled annealed sheet, : 0.010 to 0.060%, Si: 0.3% or less, Mn: 0.1 to 0.5%, P: 0.05% or less, S: 0.05% or less, Al: 0.05% or less, N: 0.0060 to 0.0200%, (3-a ) Formula N ef = N (3-a)
(N: N content (mass%))
The ratio of the effective N amount N ef to the Al content defined by the above, a steel material containing N ef / Al so that N ef / Al satisfies 0.2 or more, and having the balance Fe and inevitable impurities, The steel material is heated to a temperature of 1000 ° C. or higher, subjected to rough rolling to obtain a sheet bar, and then the sheet bar is subjected to finish rolling at a finish rolling outlet temperature: a ferrite transformation start temperature or higher to perform hot rolling. And then the following equation (4) T A = 700-10 (Al / N ef ) (4)
(Where Al: Al content (mass%), N ef : effective N quantity (mass%))
And in being defined T A ° C. wound at a temperature below steps, the cold rolling step, the hot-rolled sheet, reduction ratio and the step of performing cold rolling to be 30% to 80%, the annealing step, Annealing temperature: 250 ° C. or higher, Formula (4) T A = 700-10 (Al / N ef ) (4)
(Where Al: Al content (mass%), N ef : effective N quantity (mass%))
A high temperature excellent in formability, characterized in that it is a step of heating to a temperature below T A ° C. defined below and lower than the ferrite recrystallization start temperature, and subjecting it to an annealing treatment for 1 to 600 s at that temperature. A manufacturing method of high strength cold-rolled thin steel sheet.
（５）（４）において、前記仕上圧延が、仕上圧延出側温度：フェライト変態開始温度未満とする熱間圧延であり、前記冷間圧延が、次（５）式
TR（％）＝［１−（１−ｐ／100）（１−CR／100）］×100 ‥‥（５）
（ここで、熱延板のX線回析における（220）面からの回析ピークの半価値が0.30°以上である場合、ｐ：仕上圧延最終圧延スタンドにおける圧下率（％）、0.30°未満である場合、ｐ：０（％）、CR：冷間圧延における累積圧下率（％））
で定義されるTR（％）が30〜80％となるように、冷間圧延の圧下率CRを調整する圧延であることを特徴とする高強度冷延薄鋼板の製造方法。
(5) In (4), the finish rolling is hot rolling with a finish rolling outlet temperature: less than the ferrite transformation start temperature, and the cold rolling is expressed by the following formula (5)
TR (%) = [1- (1-p / 100) (1-CR / 100)] × 100 (5)
(Here, when the half value of diffraction peak from (220) plane in X-ray diffraction of hot-rolled sheet is 0.30 ° or more, p: reduction ratio (%) in final rolling final rolling stand, less than 0.30 ° , P: 0 (%), CR: cumulative rolling reduction in cold rolling (%))
A method for producing a high-strength cold-rolled thin steel sheet, characterized in that the rolling is performed by adjusting the rolling reduction CR of the cold rolling so that TR (%) defined in the above is 30 to 80%.
（６）（４）または（５）において、前記焼鈍工程に代えて、加熱温度：450℃以上、前記（４）式で定義されるＴＡ℃以下で、かつフェライト再結晶開始温度未満の温度に加熱し、該温度で１〜600ｓ間保持する熱処理を施したのち、500℃以下の温度まで冷却し、ついで溶融亜鉛めっき浴に浸漬し、溶融亜鉛めっき層を形成し、その後冷却する溶融亜鉛めっき処理を施す、溶融亜鉛めっき処理工程とすることを特徴とする高強度冷延鋼板の製造方法。 (6) In (4) or (5), in place of the annealing step, a heating temperature: 450 ° C. or higher, T A ° C or lower defined by the formula (4), and a temperature lower than the ferrite recrystallization start temperature , Then heat treatment is performed for 1 to 600 s at that temperature, and then cooled to a temperature of 500 ° C. or lower, then immersed in a hot dip galvanizing bath to form a hot dip galvanized layer, and then cooled. A method for producing a high-strength cold-rolled steel sheet, characterized in that a hot-dip galvanizing process is performed.
（７）（６）において、前記溶融亜鉛めっき処理工程に引続き、470℃以上550℃以下でかつフェライト再結晶開始温度未満の温度に加熱し、前記溶融亜鉛めっき層を合金化する合金化処理を行なう合金化処理工程を施すことを特徴とする高強度冷延鋼板の製造方法。
（８）（４）ないし（７）のいずれかにおいて、前記組成に加えてさらに、質量％で、Ti、Ｂのうちから選ばれた１種または２種を含有し、前記（３−ａ）式に代えて、次（３−ｂ）式
Ｎef＝Ｎ−14×（Ti／48＋Ｂ／11） ‥‥‥‥（３−ｂ）
（ここで、N、Ti、B：各要素の含有量（質量％））
で定義される有効Ｎ量Ｎefが質量％で、0.0060〜0.0200％を満足する組成とすることを特徴とする高強度冷延薄鋼板の製造方法。
(7) In (6), subsequent to the hot dip galvanizing process, heating to a temperature not lower than 470 ° C. and lower than 550 ° C. and lower than the ferrite recrystallization start temperature to alloy the hot dip galvanized layer. A method for producing a high-strength cold-rolled steel sheet, comprising performing an alloying process.
(8) In any one of (4) to (7), in addition to the above composition, the composition further contains one or two selected from Ti and B by mass%, and the above (3-a) Instead of the formula, the following formula (3-b)
N ef = N-14 × (Ti / 48 + B / 11) (3-b)
(Here, N, Ti, B: Content of each element (mass%))
A method for producing a high-strength cold-rolled steel sheet, characterized in that the effective N amount N ef defined by the formula (1) is a mass% and satisfies 0.0060 to 0.0200%.
（９）（４）ないし（８）のいずれかにおいて、前記組成に加えてさらに、質量％で、Nb、Ｖのうちから選ばれた１種または２種を合計で0.05％未満で、かつ前記有効Ｎ量ＮefとNb含有量の比、Ｎef／Nbが0.7以上、前記有効Ｎ量ＮefとＶb含有量の比、Ｎef／Ｖが0.4以上を満足するように、調整して含有する組成とすることを特徴とする高強度冷延薄鋼板の製造方法。 (9) In any one of (4) to (8), in addition to the above-mentioned composition, the total amount of one or two selected from Nb and V is less than 0.05% by mass%, and The ratio of effective N amount N ef to Nb content, N ef / Nb is 0.7 or more, and the ratio of effective N amount N ef to Vb content, N ef / V is adjusted to satisfy 0.4 or more. The manufacturing method of the high intensity | strength cold-rolled thin steel plate characterized by setting it as a composition to do.
本発明によれば、高価な合金元素を多量含有することなく、また、特別な焼付け塗装処理を施す必要もなく、引張強さ：400MPa以上700MPa未満の高強度を有し、かつ優れた成形性を兼備する、高強度冷延薄鋼板を安価に、しかも安定して製造でき、産業上格段の効果を奏する。また、本発明によれば、冷間圧延工程を省略して製造することも可能で、大幅なコスト削減が可能となるという効果もある。また、本発明によれば、建材、家電、あるいは自動車用等として十分な特性を有する部品を安価に提供できるという効果もある。 According to the present invention, it does not contain a large amount of expensive alloy elements, does not require special baking coating treatment, has a high tensile strength: 400 MPa or more and less than 700 MPa, and has excellent formability. The high-strength cold-rolled thin steel sheet can be manufactured inexpensively and stably, and has a remarkable industrial effect. In addition, according to the present invention, it is possible to manufacture by omitting the cold rolling step, and there is an effect that significant cost reduction is possible. In addition, according to the present invention, there is an effect that it is possible to provide inexpensively a component having sufficient characteristics for building materials, home appliances, automobiles and the like.
以下、本発明について具体的に説明する。
本発明の冷延薄鋼板は、圧延方向の引張強さTS：400MPa以上700MPa未満で、かつ圧延方向の引張強さTSと全伸びT-Elとの関係が次（１）式
TS（MPa）≧ −10×（T-El（％））＋750 ‥‥（１）
（ここで、T-El：圧延方向の全伸び（％）、TS：圧延方向の引張強さ（MPa））
を、また、全伸びT-Elと局部伸びL-Elとの関係が次（２）式
L-El／T-El ≧ 0.5 ‥‥‥‥‥‥（２）
（ここで、T-El：圧延方向の全伸び（％）、L-El：圧延方向の局部伸び（％））
を、それぞれ満足する特性を有する、成形性に優れた高強度冷延薄鋼板である。なお、ここで、全伸びT-El（％）、局部伸びL-El（％）は、標点間距離GL：25mmのJIS 13号B試験片を用いて求めた値を使用するものとする。
Hereinafter, the present invention will be specifically described.
The cold-rolled thin steel sheet of the present invention has a tensile strength TS in the rolling direction of 400 MPa or more and less than 700 MPa, and the relationship between the tensile strength TS in the rolling direction and the total elongation T-El is expressed by the following formula (1):
TS (MPa) ≧ −10 × (T-El (%)) + 750 (1)
(Where T-El: total elongation in rolling direction (%), TS: tensile strength in rolling direction (MPa))
Also, the relationship between total elongation T-El and local elongation L-El is the following equation (2)
L-El / T-El ≧ 0.5 (2)
(Where T-El: total elongation in rolling direction (%), L-El: local elongation in rolling direction (%))
Is a high-strength cold-rolled thin steel sheet having excellent formability and characteristics satisfying the above. Here, the total elongation T-El (%) and the local elongation L-El (%) are values obtained using a JIS No. 13 B test piece with a distance between gauge points GL: 25 mm. .
TSとT-Elが、（１）式を満足することにより、従来の高強度冷延鋼板に比べて、顕著に強度−延性バランスが向上したTS：400MPa以上700MPa未満有する高強度冷延鋼板となる。また、本発明鋼板はフェライト相が加工フェライトとなり、局部伸びと全伸びが、（２）式を満足することにより、所望の高延性を維持しながら、所望の高強度を確保できる。 TS and T-El satisfy the formula (1), and the strength-ductility balance is significantly improved compared to conventional high-strength cold-rolled steel sheets. TS: High-strength cold-rolled steel sheets having 400 MPa or more and less than 700 MPa Become. In the steel sheet of the present invention, the ferrite phase becomes processed ferrite, and the local elongation and the total elongation satisfy the formula (2), whereby a desired high strength can be secured while maintaining a desired high ductility.
まず、本発明冷延薄鋼板の組成限定理由について説明する。以下、とくに断わらないかぎり質量％は、単に％で記す。
Ｃ：0.010〜0.060％
Ｃは、鋼板の強度を増加させる作用を有する元素であり、所望の高強度を確保するために、本発明では0.010％以上の含有を必要とする。一方、0.060％を超える含有は、炭化物量が多くなりすぎて強度が高くなりすぎ、所望の延性が確保できにくくなる。このため、Ｃは0.010〜0.060％の範囲に限定した。なお、好ましくは0.010〜0.040％である。
First, the reasons for limiting the composition of the cold-rolled thin steel sheet of the present invention will be described. Hereinafter, unless otherwise specified, mass% is simply expressed as%.
C: 0.010-0.060%
C is an element having an action of increasing the strength of the steel sheet, and in the present invention, it is necessary to contain 0.010% or more in order to ensure a desired high strength. On the other hand, if the content exceeds 0.060%, the amount of carbide becomes too large and the strength becomes too high, making it difficult to ensure the desired ductility. For this reason, C was limited to the range of 0.010 to 0.060%. In addition, Preferably it is 0.010 to 0.040%.
Si：0.3％以下
Siは、脱酸剤として作用するとともに、鋼中に固溶して鋼板の強度を増加させる固溶強化能が高く、とくに降伏強さを大きく増加させる作用を有する元素である。このような効果は、0.01％以上の含有で認められるが、0.3％を超えて含有すると、所望の延性の確保ができにくくなる。このため、Siは0.3％以下に限定した。なお、好ましくは0.2％以下である。
Si: 0.3% or less
Si is an element that acts as a deoxidizer and has a high solid-solution strengthening ability to form a solid solution in steel and increase the strength of the steel sheet, and in particular has a function to greatly increase the yield strength. Such an effect is recognized when the content is 0.01% or more. However, when the content exceeds 0.3%, it becomes difficult to secure desired ductility. For this reason, Si was limited to 0.3% or less. In addition, Preferably it is 0.2% or less.
Mn：0.1〜0.5％
Mnは、Ｓと結合し硫化物を形成し、Ｓによる熱間脆性を抑制するとともに、鋼中に固溶して鋼板の強度を増加させる固溶強化能が高く、とくに降伏強さを大きく増加させる作用を有する元素である。このような効果を得るためには0.1％以上の含有を必要とする。一方、0.5％を超える含有は、延性を低下させる。このため、Mnは0.1〜0.5％の範囲に限定した。なお、好ましくは0.4％以下である。
Mn: 0.1-0.5%
Mn combines with S to form sulfides, suppresses hot brittleness due to S, and has a high solid solution strengthening ability to dissolve in steel and increase the strength of the steel sheet, especially greatly increasing the yield strength. It is an element which has the effect | action to make. In order to obtain such an effect, a content of 0.1% or more is required. On the other hand, the content exceeding 0.5% lowers the ductility. For this reason, Mn was limited to the range of 0.1 to 0.5%. In addition, Preferably it is 0.4% or less.
Ｐ：0.08％以下
Ｐは、鋼中に固溶して鋼板の強度を増加させる固溶強化能が高い固溶強化元素であり、所望の強度に応じて0.01％以上程度含有することが望ましいが、0.08％を超える多量の含有は延性を著しく低下させる。このため、Ｐは0.08％以下に限定した。なお、好ましくは0.05％以下である。
P: 0.08% or less P is a solid solution strengthening element having a high solid solution strengthening ability for increasing the strength of a steel sheet by dissolving in steel, and it is desirable to contain about 0.01% or more depending on the desired strength. In addition, a large content exceeding 0.08% significantly lowers the ductility. For this reason, P was limited to 0.08% or less. In addition, Preferably it is 0.05% or less.
Ｓ：0.05％以下
Ｓは、鋼中では硫化物として存在し、鋼板の延性、成形性、とくに伸びフランジ成形性を低下させる元素であるため、できるだけ低減することが望ましい。しかし、過度の低減は精錬コストの高騰を招く。Ｓを0.05％まで低減すれば、成形性への悪影響が無視できることから、Ｓは0.05％以下に限定した。なお、好ましくは0.03％以下である。
S: 0.05% or less S is present in the steel as sulfides, and is an element that lowers the ductility and formability of the steel sheet, particularly the stretch flange formability. However, excessive reduction leads to an increase in refining costs. If S is reduced to 0.05%, the adverse effect on formability can be ignored, so S was limited to 0.05% or less. In addition, Preferably it is 0.03% or less.
Al：0.05％以下
Alは、脱酸剤として作用し、鋼の清浄度を向上させるとともに、結晶粒を微細化する作用を有する元素であり、このような効果を得るためには0.001％以上含有することが望ましい。一方、Alは、Ｎとの親和力が強く、多量の含有は、本発明で重要な、強度増加の役割を担う固溶Ｎ量の低減に繋がる。このため、Alは0.05％以下に限定した。なお、好ましくは0.001〜0.04％である。
Al: 0.05% or less
Al is an element that acts as a deoxidizer, improves the cleanliness of steel, and has the effect of refining crystal grains. In order to obtain such an effect, Al is desirably contained in an amount of 0.001% or more. On the other hand, Al has a strong affinity for N, and inclusion of a large amount leads to a decrease in the amount of solid solution N, which plays an important role in increasing the strength in the present invention. For this reason, Al was limited to 0.05% or less. In addition, Preferably it is 0.001 to 0.04%.
Ｎ：0.0060〜0.0200％
Ｎは、固溶して鋼の強度を増加させるとともに、歪時効硬化に寄与し所望の鋼板強度の確保に寄与する元素であり、このような効果は0.0060％以上の含有で安定して得られる。一方、0.0200％を超える多量の含有は、連続鋳造時のスラブ割れなどの発生が顕著になる。このため、Ｎは0.0060〜0.0200％の範囲に限定した。なお、好ましくは0.0060〜0.0170％である。
N: 0.0060-0.0200%
N is an element that increases the strength of the steel by solid solution and contributes to strain age hardening and contributes to securing the desired strength of the steel sheet, and such an effect can be stably obtained with a content of 0.0060% or more. . On the other hand, when the content exceeds 0.0200%, the occurrence of slab cracking during continuous casting becomes remarkable. For this reason, N was limited to the range of 0.0060 to 0.0200%. In addition, Preferably it is 0.0060-0.0170%.
Ｎef／Al：0.2以上
上記したように、Alは、強力にＮを固定する作用を有する元素であり、所望の固溶Ｎ量を確保する観点から、本発明では、有効Ｎ量ＮefとAl含有量の比、Ｎef／Alを0.2以上に限定した。これにより、0.0040％以上の固溶Ｎ量を安定して確保でき、所望の優れた成形性を確保できる。なお、Ｎef／Alは、好ましくは0.3以上である。ここで、Ti、Bを含有しない場合、Ｎefは次（３−ａ）式
Ｎef＝Ｎ ・・・（３−ａ）
（ここで、Ｎ：Ｎの含有量（質量％））
で定義される。
N ef / Al: 0.2 or more As described above, Al is an element having an action of strongly fixing N. From the viewpoint of securing a desired amount of dissolved N, in the present invention, effective N amount N ef is The ratio of Al content, N ef / Al, was limited to 0.2 or more. Thereby, the solid solution N amount of 0.0040% or more can be stably secured, and desired excellent moldability can be secured. N ef / Al is preferably 0.3 or more. Here, when Ti and B are not contained, N ef is expressed by the following formula (3-a): N ef = N (3-a)
(N: N content (mass%))
Defined by
一方、Ti、Bを含有する場合はTi、ＢとＮの結合力は強く、Ti、Ｂと結合したＮは強度増加（固溶強化および歪時効硬化）に有効なＮとして作用しなくなるため、Ｎef は、N含有量から、TiNおよびBNの形成に必要なＮ量を差引いた値であると定義され、有効Ｎ量Ｎef は次（３−ｂ）式
Ｎef＝Ｎ−14×（Ti／48＋Ｂ／11） ‥‥‥‥（３−ｂ）
（ここで、Ｎ、Ti、Ｂ：各元素の含有量（質量％））
で定義される。なお、（３−ｂ）式中のTi、Ｂは、TiあるいはＢを含有しない場合には零として計算するものとする。
On the other hand, when Ti and B are contained, the bonding strength between Ti, B and N is strong, and N combined with Ti and B does not act as effective N for increasing the strength (solid solution strengthening and strain age hardening). N ef is defined as a value obtained by subtracting the N amount necessary for the formation of TiN and BN from the N content, and the effective N amount N ef is expressed by the following equation (3-b): N ef = N-14 × ( Ti / 48 + B / 11) (3-b)
(N, Ti, B: content of each element (mass%))
Defined by Note that Ti and B in the formula (3-b) are calculated as zero when Ti or B is not contained.
固溶Ｎ：0.0040％以上
固溶状態のＮは、フェライト相の強化に寄与する。冷延鋼板として所望の高強度を確保するためには、固溶状態のＮを0.0040％以上確保する必要がある。なお、安定して高強度を確保するためには、固溶状態のＮは0.0060％以上とすることが好ましい。
ここで、固溶状態のＮは、鋼中の全Ｎ量から、析出Ｎ量を差し引いた値とする。析出Ｎ量は、定電位電解法を用いた電解抽出による溶解法を適用して求めた値を用いる。なお、電解抽出の電解液としては、アセチルアセトン系電解液を用いることが好ましい。定電位電解法を用いた電解抽出による溶解法にて抽出した残渣を化学分析して、残渣中のＮ量を求め、これを析出Ｎ量とした。
Solid solution N: 0.0040% or more N in a solid solution state contributes to strengthening of the ferrite phase. In order to secure a desired high strength as a cold-rolled steel sheet, it is necessary to secure N in a solid solution state of 0.0040% or more. In addition, in order to ensure high strength stably, it is preferable that N of a solid solution state shall be 0.0060% or more.
Here, N in a solid solution state is a value obtained by subtracting the amount of precipitated N from the total amount of N in the steel. As the amount of precipitated N, a value obtained by applying a dissolution method by electrolytic extraction using a constant potential electrolysis method is used. In addition, as an electrolytic solution for electrolytic extraction, it is preferable to use an acetylacetone-based electrolytic solution. The residue extracted by the dissolution method by electrolytic extraction using a constant potential electrolysis method was chemically analyzed to determine the amount of N in the residue, and this was defined as the amount of precipitated N.
上記した成分が基本の組成であるが、基本の組成に加えてさらに、Ti、Ｂのうちから選ばれた１種または２種、および／または、Nb、Ｖのうちから選ばれた１種または２種を合計で0.05％未満、含有することができる。
Ti、Ｂのうちから選ばれた１種または２種
Ti、Ｂはいずれも、冷間加工されたフェライトの再結晶開始温度を高め、焼鈍時に鋼板が軟質化するのを抑制する作用を有する元素であり、必要に応じて１種または２種を選択して、少量含有できる。しかし、Ti、Ｂは、いずれもＮとの親和力が非常に強く、高温で窒化物を形成し、室温（冷間圧延時）での固溶Ｎ量を著しく低減させる。また、Tiは固溶強化能が強い元素であり、過剰に含有すると延性が低下する。そのため、含有する場合には、Ti、Ｂのうちから選ばれた１種または２種を、Ｎ含有量との関係で、上記した（３−ｂ）式で定義される有効Ｎ量Ｎefが0.0060〜0.0200％の範囲となるように含有させることが好ましい。なお、Ti、Ｂを含有する場合には、所望の固溶Ｎ量を確保する観点から、本発明では、有効Ｎ量ＮefとAl含有量の比、Ｎef／Alを0.2以上に限定することが好ましい。
The above-mentioned component is a basic composition, but in addition to the basic composition, one or two selected from Ti and B, and / or one selected from Nb and V, or Two kinds can be contained in a total of less than 0.05%.
One or two selected from Ti and B
Ti and B are both elements that have the effect of increasing the recrystallization start temperature of cold-worked ferrite and suppressing the softening of the steel sheet during annealing. Select one or two as required. And can be contained in a small amount. However, both Ti and B have a very strong affinity with N, and nitrides are formed at a high temperature, and the amount of solute N at room temperature (during cold rolling) is remarkably reduced. Further, Ti is an element having a strong solid solution strengthening ability, and if contained excessively, ductility is lowered. Therefore, when it contains, the effective N amount Nef defined by the above-mentioned (3-b) formula is 0.0060 in relation to N content for 1 type or 2 types selected from Ti and B. It is preferable to make it contain so that it may become -0.0200% of range. In the case where Ti and B are contained, from the viewpoint of securing a desired solute N amount, in the present invention, the ratio of the effective N amount N ef to the Al content, N ef / Al is limited to 0.2 or more. It is preferable.
Nb、Ｖのうちから選ばれた１種または２種：合計で0.05％未満
Nb、Ｖはいずれも、冷間加工されたフェライトの再結晶開始温度を高め、焼鈍時に鋼板が軟質化するのを抑制する作用を有する元素であり、必要に応じて１種または２種を選択して、合計で0.05％未満含有できる。しかし、Nb、Ｖはいずれも、Ｎとの親和力が強く、室温（冷間圧延時）での固溶Ｎ量を低減させる。また、Nb、Ｖは，固溶強化能が強く、過剰に含有すると延性が低下するとともに、材料コストの高騰を招く。そのため、含有する場合には、Nb、Ｖのうちから選ばれた１種または２種は、合計で0.05％未満に限定することが好ましい。
One or two selected from Nb and V: less than 0.05% in total
Nb and V are both elements that have the effect of increasing the recrystallization start temperature of cold-worked ferrite and suppressing the softening of the steel sheet during annealing. Select one or two as required. And it can contain less than 0.05% in total. However, both Nb and V have a strong affinity for N and reduce the amount of solute N at room temperature (during cold rolling). Moreover, Nb and V have a strong solid solution strengthening ability, and if contained excessively, the ductility is lowered and the material cost is increased. Therefore, when it contains, it is preferable to limit 1 type or 2 types chosen from Nb and V to less than 0.05% in total.
なお、Nbおよび／またはＶを含有する場合には、上記した含有量の範囲内でかつ、所望の固溶Ｎ量を確保する観点から、Nbおよび／またはＶの含有量をＮ量との関係で、（３−ａ）、（３−ｂ）式で定義される有効Ｎ量Ｎefと、Nb含有量の比、Ｎef／Nbが0.7以上、有効Ｎ量ＮefとＶ含有量の比、Ｎef／Ｖが0.4以上、を満足するように、調整することが好ましい。 When Nb and / or V is contained, the content of Nb and / or V is related to the N amount within the above-described content range and from the viewpoint of securing a desired solute N amount. The ratio of the effective N amount N ef defined by the formulas (3-a) and (3-b) and the Nb content, N ef / Nb is 0.7 or more, the ratio of the effective N amount N ef to the V content N ef / V is preferably adjusted so as to satisfy 0.4 or more.
上記した成分以外の残部は、Feおよび不可避的不純物からなる。
つぎに、本発明冷延薄鋼板の組織限定理由について説明する。
本発明冷延薄鋼板は、フェライト相を主相とする組織を有する。ここでいう「主相」とは、組織全体に対する体積率で92％以上、好ましくは95％以上である場合をいう。主相以外の第二相は、セメンタイトやパーライト等である。第二相は、体積率で10％以下とする。第二相が10％を超えて多くなると、延性、とくに局部延性の低下が著しくなり、高度な加工性が要求される部品向けとしては問題となる。さらに良好な延性が必要とされる用途では、５％未満とすることが好ましく、さらに好ましくは４％以下である。なお、本発明では、主相であるフェライト相は、冷間圧延により加工硬化したフェライト相であり、例えば、Ｘ線回折によるフェライト相の（220）面の回折ピークの半価幅が0.30°〜0.60°となる相である。
The balance other than the components described above consists of Fe and inevitable impurities.
Next, the reason for limiting the structure of the cold-rolled thin steel sheet of the present invention will be described.
The cold-rolled thin steel sheet of the present invention has a structure having a ferrite phase as a main phase. The “main phase” as used herein refers to a case where the volume ratio of the whole structure is 92% or more, preferably 95% or more. The second phase other than the main phase is cementite or pearlite. The second phase is 10% or less by volume. When the amount of the second phase exceeds 10%, the ductility, particularly the local ductility, is remarkably deteriorated, which becomes a problem for parts that require high workability. In applications where further good ductility is required, the content is preferably less than 5%, more preferably 4% or less. In the present invention, the ferrite phase that is the main phase is a ferrite phase that is work-hardened by cold rolling. For example, the half-value width of the diffraction peak of the (220) plane of the ferrite phase by X-ray diffraction is 0.30 ° to The phase is 0.60 °.
つぎに、本発明の冷延薄鋼板の好ましい製造方法について説明する。
本発明では、鋼素材に、該鋼素材を加熱し熱間圧延を施し熱延板とする熱延工程と、該熱延板に冷間圧延を施し冷延板とする冷延工程と、該冷延板に焼鈍処理を施し冷延焼鈍板とする焼鈍工程と、を順次施し、高強度冷延薄鋼板とする。
鋼素材は、上記した鋼板の組成と、固溶Ｎ量以外の成分含有量が同じである組成を有する鋼素材を用いる。
Below, the preferable manufacturing method of the cold-rolled thin steel plate of this invention is demonstrated.
In the present invention, a hot rolling process in which the steel material is heated and subjected to hot rolling to form a hot rolled sheet, a cold rolling process in which the hot rolled sheet is subjected to cold rolling to form a cold rolled sheet, An annealing process is performed on the cold-rolled sheet to form a cold-rolled annealed sheet, and a high-strength cold-rolled steel sheet is obtained.
As the steel material, a steel material having a composition in which the composition of the steel sheet described above and the component content other than the solute N content are the same is used.
鋼素材の製造方法は、とくに限定する必要はないが、上記した組成の溶鋼を転炉等の常用の溶製方法で溶製し、連続鋳造法などの常用の鋳造方法でスラブ等の鋼素材とすることが好ましい。鋼素材の鋳造方法は、連続鋳造法以外の、造塊法、薄スラブ鋳造法としてもなんら問題はない。
得られた鋼素材は、ついで、熱延工程を施される。熱間圧延のための加熱は、いったん室温まで冷却し、その後再加熱する方法に加えて、室温まで冷却しないで、温片のままで加熱炉に装入する、あるいはわずかの保熱を行った後に直ちに圧延する直送圧延・直接圧延などの省エネルギープロセスも問題なく適用できる。
The manufacturing method of the steel material is not particularly limited, but the molten steel having the above composition is melted by a conventional melting method such as a converter, and a steel material such as a slab by a conventional casting method such as a continuous casting method. It is preferable that There is no problem in the steel material casting method as the ingot-making method and the thin slab casting method other than the continuous casting method.
The obtained steel material is then subjected to a hot rolling process. In addition to the method of once cooling to room temperature and then reheating, the heat for hot rolling was not cooled to room temperature, but was charged in a heating furnace as it was, or a little heat was retained. Energy saving processes such as direct feed rolling and direct rolling, which are rolled immediately afterwards, can be applied without any problem.
本発明における熱延工程では、鋼素材を加熱温度：1000℃以上に加熱し、粗圧延してシートバーとし、ついで該シートバーに仕上圧延出側温度：フェライト変態開始温度以上とする仕上圧延を施し、巻取り温度：ＴＡ℃以下で巻き取り熱延板とする。なお、ＴＡは、次（４）式
ＴＡ ＝ 700−10（Al／Ｎef） ‥‥（４）
（ここで、Al：Alの含有量（質量％）、Ｎef：有効Ｎ量（質量％））
で定義される値である。なお、Ｎef：有効Ｎ量は、上記した（３−ａ）、（３−ｂ）式で定義される値である。
In the hot rolling process of the present invention, the steel material is heated to a heating temperature of 1000 ° C. or higher, roughly rolled into a sheet bar, and then the finish rolling to the sheet bar is finished rolling to a temperature higher than the ferrite transformation start temperature. Application and winding temperature: It is set as a rolled hot-rolled sheet at T A or lower. Note that T A is the following equation (4) T A = 700-10 (Al / N ef ) (4)
(Where Al: Al content (mass%), N ef : effective N quantity (mass%))
It is a value defined by. Note that N ef : effective N amount is a value defined by the above equations (3-a) and (3-b).
加熱温度：1000℃以上
熱間圧延のための加熱温度は、1000℃以上とすることが好ましい。加熱温度が1000℃未満では、Ｎが未固溶のままとなり、冷延板での所望の固溶Ｎを確保することが困難となると共に、結晶粒が十分に粗大化せず硬質化し、熱間圧延時の圧延荷童の増加が著しくなり、圧延が困難となる場合がある。なお、加熱温度の上限はとくに限定する必要はないが、1280℃を超えて高温になると、酸化層が厚く形成され、酸化に伴うロスが増大し、歩留りが低下するとともに、酸洗処理の負荷が増大する。このようなことから、熱間圧延のための加熱温度は1000℃以上、好ましくは1280℃以下である。
Heating temperature: 1000 ° C. or higher The heating temperature for hot rolling is preferably 1000 ° C. or higher. If the heating temperature is less than 1000 ° C., N remains undissolved, making it difficult to secure the desired solid solution N in the cold-rolled sheet, and the crystal grains are hardened without being sufficiently coarsened, There are cases where the increase in the number of rolled goods during the hot rolling becomes significant and the rolling becomes difficult. The upper limit of the heating temperature is not particularly limited. However, when the temperature exceeds 1280 ° C, the oxide layer is formed thicker, the loss due to oxidation increases, the yield decreases, and the load of pickling treatment Will increase. Therefore, the heating temperature for hot rolling is 1000 ° C. or higher, preferably 1280 ° C. or lower.
仕上圧延出側温度：フェライト変態開始温度以上またはフェライト変態開始温度未満
仕上圧延は、均一な組織を有する熱延板を確保するという観点から、Ａr3変態点以上のオーステナイト単相域で行うことが好ましい。というのは、フェライトが析出した温度域での圧延は、熱延板の組織を不均一にし、得られる製品の特性を不安定にする場合が多いという問題に加えて、フェライトが析出した温度域での圧延は熱延板の強度を増加させ、その後の冷間圧延が困難になる場合が多いからである。しかし、本発明では、仕上圧延の最終圧延スタンドでは、オーステナイト（γ）−フェライト（α）二相域、フェライト（α）単相域あるいはフェライト（α）−セメンタイト（θ）二相域での圧延を施してもよい。γ−α二相域、α単相域、あるいはα−θ二相域での圧延は、熱延板に加工歪を残留させないという点からは、仕上圧延出側温度をフェライト変態開始温度以上とすることが好ましいが、仕上圧延出側温度をフェライト変態開始温度未満としてもよい。なお、フェライト変態開始温度（Ar3変態点）は次式
Ar3（℃）＝901−325C−92Mn＋33Si＋278P＋40Al
ここに、C、Mn、Si、P、Al：各元素の含有量（質量％）
により求めることができる。
Finishing rolling exit temperature: Ferrite transformation start temperature or higher or lower than ferrite transformation start temperature Finish rolling may be performed in the austenite single phase region above the Ar3 transformation point from the viewpoint of securing a hot rolled sheet having a uniform structure. preferable. This is because rolling in the temperature range where the ferrite is precipitated often makes the structure of the hot-rolled sheet non-uniform and unstable the properties of the resulting product, in addition to the temperature range where the ferrite is precipitated. This is because the rolling at 1 increases the strength of the hot-rolled sheet and the subsequent cold rolling is often difficult. However, in the present invention, in the final rolling stand of finish rolling, rolling in the austenite (γ) -ferrite (α) two-phase region, ferrite (α) single-phase region or ferrite (α) -cementite (θ) two-phase region May be applied. Rolling in the γ-α two-phase region, α single-phase region, or α-θ two-phase region does not leave processing strain in the hot-rolled sheet, and the finish rolling exit temperature is set to the ferrite transformation start temperature or higher. However, the finish rolling outlet temperature may be lower than the ferrite transformation start temperature. The ferrite transformation start temperature (Ar 3 transformation point) is
Ar 3 (℃) = 901−325C−92Mn + 33Si + 278P + 40Al
Here, C, Mn, Si, P, Al: Content of each element (mass%)
It can ask for.
なお、仕上圧延出側温度の上限はとくに限定する必要はないが、過度に高い仕上圧延出側温度で圧延した場合には、スケール疵などが発生しやすくなるため、仕上圧延出側温度の上限は概ね950℃程度以下とすることが好ましい。
また、仕上圧延出側温度をフェライト変態開始温度未満とし、少なくとも最終圧延スタンドにおける圧下パスをフェライト変態開始温度未満の圧下とすることにより、少なくとも最終圧延パス時に導入される加工歪が熱延板に蓄積されやすくなり、熱延板の強度が増加し、その後の冷間圧延における圧下率を軽減または省略することが可能となり、製造コストの削減に繋がるという利点がある。この場合、本発明では、熱延板をＸ線回折し、（220）面からの回折ピークの半価幅が0.30°以上の場合には、最終圧延スタンドでの圧下率：ｐ％を、フェライト変態開始温度未満での圧下率とみなし、一方、熱延板の（220）面からの回折ピークの半価幅が0.30°未満の場合には、フェライト変態開始温度未満の圧下率は０％であるとする。
The upper limit of the finish rolling exit temperature is not particularly limited. However, when rolling at an excessively high finish rolling exit temperature, scale wrinkles are likely to occur, so the upper limit of the finish rolling exit temperature. Is preferably about 950 ° C. or less.
Also, by setting the finish rolling exit temperature below the ferrite transformation start temperature and at least reducing the rolling pass at the final rolling stand below the ferrite transformation start temperature, at least the processing strain introduced during the final rolling pass is applied to the hot rolled sheet. Accumulation is facilitated, the strength of the hot-rolled sheet is increased, and the reduction ratio in the subsequent cold rolling can be reduced or omitted, leading to a reduction in manufacturing cost. In this case, in the present invention, the hot-rolled plate is subjected to X-ray diffraction, and when the half-value width of the diffraction peak from the (220) plane is 0.30 ° or more, the rolling reduction ratio at the final rolling stand: p% When the half-width of the diffraction peak from the (220) plane of the hot-rolled sheet is less than 0.30 °, the reduction ratio below the ferrite transformation start temperature is 0%. Suppose there is.
仕上圧延終了後、熱延板は巻取り温度まで冷却され、コイル状に巻き取られる。冷却は、所望の固溶Ｎ量を確保する観点からは水冷等の急冷を施すことが好ましいが、過度の急冷は組織の不均一を生じやすくため、本発明では、板厚中心位置の平均冷却速度で20℃／ｓ以下の加速冷却を施すことが好ましい。
巻取り温度：ＴＡ℃以下
巻取り温度は、ＴＡ℃以下とすることが好ましい。なお、ＴＡは、次（４）式
ＴＡ ＝ 700−10（Al／Ｎef） ‥‥（４）
（ここで、Al：Alの含有量（質量％）、Ｎef：有効Ｎ量（質量％））
で定義される値であり、また、有効Ｎ量Ｎefは、Ｎ含有量から、TiNおよびBNの形成に必要なＮ量を差引いた値であり、上記したようにTi、Bを含有しない場合には（３−ａ）式で示されるようにN含有量と等しいと定義され、Ti、Bを含有する場合には（３−ｂ）式（＝Ｎ−14×（Ti／48＋Ｂ／11））で定義される値である。（４）式の関係式は、所望の固溶Ｎ量を確保するために本発明者らが種々検討して得た実験式である。巻取り温度が、ＴＡを超えて高温となると、各種窒化物が析出し、所望の固溶Ｎ量を確保できなくなる。巻取り温度を200℃未満とすると、鋼板形状の乱れが顕著となり、実際の使用に当たり不具合を生じる危険性が増大する。また、材質の不均一性も増大する傾向となる。このため、巻取り温度は200℃以上とすることが好ましい。
After finishing rolling, the hot-rolled sheet is cooled to the coiling temperature and wound into a coil. Cooling is preferably performed by rapid cooling such as water cooling from the viewpoint of securing a desired solid solution N amount. However, excessive rapid cooling tends to cause unevenness of the structure. Therefore, in the present invention, the average cooling at the plate thickness center position is performed. It is preferable to perform accelerated cooling at a rate of 20 ° C./s or less.
Winding temperature: T A C or lower The winding temperature is preferably T A C or lower. Note that T A is the following equation (4) T A = 700-10 (Al / N ef ) (4)
(Where Al: Al content (mass%), N ef : effective N quantity (mass%))
In addition, the effective N amount N ef is a value obtained by subtracting the N amount necessary for forming TiN and BN from the N content, and when Ti and B are not contained as described above. Is defined to be equal to the N content as shown by the formula (3-a), and in the case of containing Ti and B, the formula (3-b) (= N-14 × (Ti / 48 + B / 11) ) Is a value defined by The relational expression (4) is an experimental expression obtained by various studies by the present inventors in order to secure a desired amount of dissolved N. Coiling temperature, when a high temperature beyond the T A, various nitrides are precipitated, it can not be ensured a desired amount of dissolved N. When the coiling temperature is less than 200 ° C., the shape of the steel plate is significantly disturbed, and the risk of causing problems in actual use increases. In addition, the material non-uniformity tends to increase. For this reason, the winding temperature is preferably 200 ° C. or higher.
上記した熱延工程を経た熱延板は、好ましくは酸洗処理を施され、ついで、冷延工程を施される。酸洗処理は、常用の方法に準じて行えばよく、特に限定する必要はない。なお、スケールが極めて薄い場合には酸洗を行うことなく直接、冷延工程を行ってもよい。
冷延工程では、熱延板に、圧下率：30〜80％の冷間圧延を施し、所望の板厚の冷延板とする。
The hot-rolled sheet that has undergone the above-described hot-rolling step is preferably subjected to pickling treatment and then subjected to a cold-rolling step. The pickling treatment may be performed according to a commonly used method, and is not particularly limited. If the scale is extremely thin, the cold rolling process may be performed directly without pickling.
In the cold rolling process, the hot-rolled sheet is subjected to cold rolling with a reduction ratio of 30 to 80% to obtain a cold-rolled sheet having a desired thickness.
本発明では、冷間圧延による加工硬化と、その後の熱処理による歪時効硬化とにより、高強度化を図るため、冷間圧延の圧下率（冷延圧下率）CRは30％以上とする。冷間圧延により、転位が導入され、冷延板強度が増加し、さらに全伸びT-Elに対する局部伸びL-Elの割合が著しく増加する。冷延圧下率CRが、30％未満では、所望の高強度を確保できないうえ、焼鈍後の組織が混粒となり、延性が著しく低下する。一方、冷延圧下率CRが、80％を超えて高くなると、延性の低下が著しくなるとともに、冷間圧延時間が長くなり冷延工程コストの大幅なアップが避けられない。 In the present invention, the cold rolling reduction ratio (cold rolling reduction ratio) CR is set to 30% or more in order to increase the strength by work hardening by cold rolling and subsequent strain aging hardening by heat treatment. By cold rolling, dislocations are introduced, the cold-rolled sheet strength is increased, and the ratio of local elongation L-El to total elongation T-El is remarkably increased. If the cold rolling reduction ratio CR is less than 30%, the desired high strength cannot be ensured, and the structure after annealing becomes a mixed grain, and the ductility is significantly reduced. On the other hand, when the cold rolling reduction ratio CR is higher than 80%, the ductility is remarkably deteriorated, and the cold rolling time is increased, so that the cold rolling process cost is inevitably increased.
なお、熱延工程で、仕上圧延出側温度がフェライト変態開始温度未満となる仕上圧延が施され、熱延板の（220）面からの回折ピークの半価幅が0.30°以上となる場合には、仕上圧延最終圧延スタンドにおける圧下率：ｐ（％）をフェライト変態開始温度未満での圧下率：ｐ％として、冷間圧延においては、次（５）式
TR（％）＝［１−（１−ｐ／100）（１−CR／100）］×100 ‥‥（５）
（ここで、ｐ：仕上圧延最終圧延スタンドにおける圧下率（％）、CR：冷間圧延における累積圧下率（％））
で定義されるトータル圧下率TR（％）が30〜80％を満足するように、冷延圧下率CRを調整することができる。トータル圧下率TRが、30％未満では、所望の高強度を確保できないうえ、焼鈍後の組織が混粒となり、延性が著しく低下する。一方、トータル圧下率TRが、80％を超えて高くなると、延性の低下が著しくなるとともに、冷間圧延時間が長くなり冷延工程コストの大幅なアップが避けられない。
In the hot rolling process, when the finish rolling exit temperature is lower than the ferrite transformation start temperature, the half width of the diffraction peak from the (220) plane of the hot rolled sheet is 0.30 ° or more. In the cold rolling, the reduction ratio: p (%) in the final rolling stand for final rolling is set as the reduction ratio: p% below the ferrite transformation start temperature.
TR (%) = [1- (1-p / 100) (1-CR / 100)] × 100 (5)
(Where p: rolling reduction (%) in the final rolling stand, CR: cumulative rolling reduction (%) in cold rolling)
The cold rolling reduction ratio CR can be adjusted so that the total rolling reduction ratio TR (%) defined in (1) satisfies 30 to 80%. If the total rolling reduction TR is less than 30%, a desired high strength cannot be ensured, and the structure after annealing becomes a mixed grain, and the ductility is significantly reduced. On the other hand, when the total rolling reduction TR is higher than 80%, the ductility is remarkably deteriorated, and the cold rolling time is increased, so that the cold rolling process cost is inevitably increased.
上記した冷延工程を経た冷延板は、ついで焼鈍工程を施され冷延焼鈍板とされる。
焼鈍工程は、冷延板に、焼鈍温度：250℃以上、ＴＡ℃以下で、かつフェライト再結晶開始温度未満の温度に加熱し、該温度で１〜600ｓ間保持する焼鈍処理を施す工程とする。
焼鈍温度：250℃以上、ＴＡ℃以下でかつフェライト再結晶開始温度未満
冷延板に施す焼鈍処理では、焼鈍温度は、250℃以上、上記した（４）式で定義されるＴＡ℃以下とすることが好ましい。焼鈍温度が250℃未満では、フェライト相の回復が進まず、所望の延性増加が得られない。一方、ＴＡ℃を超えて高温となると、窒化物が形成され、固溶Ｎによる強化が十分に発揮されなくなる。また、焼鈍温度が、フェライト再結晶開始温度を超えて高温となると、再結晶が進行して、過度に軟質化し、所望の高強度を確保できなくなる。なお、上記した範囲の焼鈍温度で、600ｓを超える時間保持すると、強度の低下が著しくなり、所望の高強度を確保できなくなる。一方、保持時間が１ｓ未満では、加工組織が十分に回復せず、所望の延性を確保できない。このため、上記した範囲の焼鈍温度での保持時間は、１〜600ｓの範囲とすることが好ましい。
The cold-rolled sheet that has undergone the above-described cold-rolling process is then subjected to an annealing process to form a cold-rolled annealed sheet.
The annealing step is a step of heating the cold-rolled sheet to an annealing temperature: 250 ° C. or higher, T A ° C. or lower and lower than the ferrite recrystallization start temperature, and subjecting the cold-rolled plate to an annealing treatment for 1 to 600 seconds. To do.
Annealing temperature: 250 ° C. or more and T A ° C. or less and less than the ferrite recrystallization start temperature In the annealing treatment applied to the cold-rolled sheet, the annealing temperature is 250 ° C. or more and T A ° C or less defined by the above-described equation (4). It is preferable that If the annealing temperature is less than 250 ° C., recovery of the ferrite phase does not proceed and a desired increase in ductility cannot be obtained. On the other hand, when the temperature is higher than T A ° C., a nitride is formed, and the strengthening by the solid solution N is not sufficiently exhibited. Further, when the annealing temperature becomes a high temperature exceeding the ferrite recrystallization start temperature, the recrystallization proceeds and becomes excessively soft, and a desired high strength cannot be secured. In addition, if the annealing temperature is in the above-described range and the holding time is longer than 600 s, the strength is remarkably lowered, and a desired high strength cannot be secured. On the other hand, if the holding time is less than 1 s, the processed structure is not sufficiently recovered and desired ductility cannot be ensured. For this reason, it is preferable that the holding time at the annealing temperature in the above-described range is in the range of 1 to 600 s.
なお、ここでいう「フェライト再結晶開始温度」はつぎのようにして求めた温度を言うものとする。圧下率：60〜70％の冷間圧延を施した各鋼板に、20℃間隔の加熱温度で焼鈍処理を施し、焼鈍板としたのち、該各焼鈍板について、室温で引張強さまたは硬さ（強度）を測定し、図２に示すような強度−焼鈍温度の関係を求め、強度（引張強さまたは硬さ）と焼鈍温度との関係が、直線から外れ始めた焼鈍温度を、各鋼板のフェライト再結晶開始温度と定義する。 The “ferrite recrystallization start temperature” here refers to the temperature determined as follows. Reduction ratio: Each steel plate subjected to cold rolling at 60 to 70% is annealed at a heating temperature of 20 ° C. to obtain an annealed plate, and each annealed plate has a tensile strength or hardness at room temperature. (Strength) was measured, the relationship between strength and annealing temperature as shown in FIG. 2 was obtained, and the annealing temperature at which the relationship between strength (tensile strength or hardness) and annealing temperature began to deviate from the straight line was determined for each steel plate. Of ferrite recrystallization.
なお、上記した焼鈍工程に代えて、連続溶融亜鉛めっきラインを利用して、熱処理と溶融亜鉛めっき処理とを連続して行う溶融亜鉛めっき処理工程を施しても良い。溶融亜鉛めっき処理工程は、冷延板を、加熱温度：450℃以上、ＴＡ℃以下で、かつフェライト再結晶開始温度未満の温度に加熱し、該温度で１〜600ｓ間保持する熱処理を施したのち、500℃以下の温度まで冷却し、ついで溶融亜鉛めっき浴に浸漬し、溶融亜鉛めっき層を形成し、その後冷却する溶融亜鉛めっき処理を施す工程としても良い。 In addition, it may replace with the above-mentioned annealing process and may perform the hot dip galvanization process process which performs a heat processing and a hot dip galvanization process continuously using a continuous hot dip galvanization line. In the hot dip galvanizing process, the cold-rolled sheet is heated to a temperature not lower than 450 ° C. and not higher than T A ° C. and lower than the ferrite recrystallization start temperature, and subjected to a heat treatment for 1 to 600 s. After that, it may be cooled to a temperature of 500 ° C. or lower, then immersed in a hot dip galvanizing bath to form a hot dip galvanized layer, and then subjected to a hot dip galvanizing treatment for cooling.
溶融亜鉛めっき処理は、通常、460℃程度に保持された溶融亜鉛めっき浴中に鋼板を浸漬して行うため、溶融亜鉛めっき処理前の熱処理としては、溶融亜鉛めっき処理に悪影響を及ぼさない範囲の温度である、450℃を下限とする熱処理とした。一方、熱処理の加熱温度が上記した（４）式で定義されるＴＡ℃を超えて高温となると、窒化物が形成されるため、ＴＡ℃を上限とした。また、加熱温度が、フェライト再結晶開始温度を超えて高温となると、再結晶が進行して、過度に軟質化し、所望の高強度を確保できなくなる。このため、溶融亜鉛めっき処理前の熱処理は、加熱温度：450℃以上、ＴＡ℃以下で、かつフェライト再結晶開始温度未満の温度に加熱し、保持する処理に限定することが好ましい。なお、上記した範囲の加熱温度で、600ｓを超える時間保持すると、熱処理時間が長くなりすぎ、鋼板製造コストの高騰を招く。一方、保持時間が１ｓ未満では、加工組織が十分に回復せず、所望の延性を確保できない。このため、上記した範囲の加熱温度での保持時間は、１〜600ｓの範囲とすることが好ましい。なお、局部伸びの向上のためには、保持時間は、10ｓ以上とすることがより好ましい。上記した熱処理後、500℃以下の温度まで冷却したのち、溶融亜鉛めっき浴に浸漬し、溶融亜鉛めっき層を形成し、その後冷却する溶融亜鉛めっき処理を施す。なお、熱処理後に500℃以下の温度に冷却するのはめっき浴の温度を安定化させるためである。溶融亜鉛めっき処理工程で行う溶融亜鉛めっき処理は、通常の溶融亜鉛めっき処理がそのまま適用できる。 The hot dip galvanizing process is usually performed by immersing the steel sheet in a hot dip galvanizing bath maintained at about 460 ° C. The heat treatment was performed at a temperature of 450 ° C. as a lower limit. On the other hand, when the heating temperature of the heat treatment exceeds T A ° C defined by the above formula (4) and becomes a high temperature, nitrides are formed, so T A ° C was set as the upper limit. Further, when the heating temperature exceeds the ferrite recrystallization start temperature and becomes a high temperature, the recrystallization proceeds and becomes too soft, and the desired high strength cannot be secured. For this reason, it is preferable that the heat treatment before the hot dip galvanizing treatment is limited to a treatment in which the heating temperature is 450 ° C. or higher and T A ° C. or lower and heated to a temperature lower than the ferrite recrystallization start temperature. In addition, if the heating temperature in the above-described range is maintained for a time exceeding 600 s, the heat treatment time becomes too long and the steel sheet manufacturing cost increases. On the other hand, if the holding time is less than 1 s, the processed structure is not sufficiently recovered and desired ductility cannot be ensured. For this reason, it is preferable that the holding time at the heating temperature in the above-described range is in the range of 1 to 600 s. In order to improve local elongation, the holding time is more preferably 10 s or longer. After the heat treatment described above, after cooling to a temperature of 500 ° C. or lower, it is immersed in a hot dip galvanizing bath to form a hot dip galvanized layer, followed by a hot dip galvanizing treatment for cooling. The reason for cooling to a temperature of 500 ° C. or lower after the heat treatment is to stabilize the temperature of the plating bath. As the hot dip galvanizing process performed in the hot dip galvanizing process, a normal hot dip galvanizing process can be applied as it is.
また、熱処理−溶融亜鉛めっき処理に引続いて、470℃以上550℃以下の温度に加熱し、めっき層を合金化する合金化処理を連続して行う、合金化溶融亜鉛めっき処理工程としても良い。
焼鈍工程、あるいは溶融亜鉛めっき処理工程に続いて、鋼板形状の矯正等を目的として、調質圧延、レベリング等を施しても良い。
Moreover, it is good also as an alloying hot dip galvanization process process which heats to the temperature of 470 degreeC or more and 550 degrees C or less following the heat treatment-hot dip galvanization process, and performs the alloying process which alloys a plating layer continuously. .
Subsequent to the annealing process or the hot dip galvanizing process, temper rolling, leveling, or the like may be performed for the purpose of correcting the shape of the steel sheet.
以下、さらに実施例に基づいて、さらに詳細に本発明について説明する。 Hereinafter, the present invention will be described in more detail based on examples.
表１に示す組成の溶鋼を転炉で溶製し、連続鋳造法で鋼素材（スラブ：肉厚250mm）とした。これら鋼素材を、表２に示す条件の熱延工程で、板厚4.0mmの熱延鋼帯(熱延板)とし、コイル状に巻き取った。なお、一部の熱延板では、仕上圧延終了後、仕上圧延出側温度から巻取温度までの平均冷却速度で20または50℃/sの加速冷却を行った。得られた熱延板について、Ｘ線回折法により、フェライト相αの(220)面の回折ピークを測定し、その半価幅を求めた。使用したＸ線は、Co−Ｋα線（波長：1.79Å）とした。半価幅が0.30°未満の場合を、フェライト変態開始温度以下の圧延は「無し」とし、半価幅が0.30°以上の場合を、フェライト変態開始温度未満の圧延が「有り」として、最終圧延スタンドにおける圧下パスの圧下率をフェライト変態開始温度未満の圧下率ｐ（％）とした。 Molten steel having the composition shown in Table 1 was melted in a converter and made into a steel material (slab: thickness 250 mm) by a continuous casting method. These steel materials were wound into coils in a hot-rolled steel strip (hot-rolled sheet) having a thickness of 4.0 mm in the hot-rolling process under the conditions shown in Table 2. Some hot-rolled sheets were subjected to accelerated cooling at an average cooling rate of 20 or 50 ° C./s from the finish rolling exit temperature to the coiling temperature after finishing rolling. About the obtained hot-rolled sheet, the diffraction peak of the (220) plane of the ferrite phase α was measured by the X-ray diffraction method, and the half width was obtained. The X-ray used was a Co-Kα ray (wavelength: 1.79 mm). When the half width is less than 0.30 °, the rolling below the ferrite transformation start temperature is “None”, and when the half width is 0.30 ° or more, the rolling below the ferrite transformation start temperature is “Yes”. The reduction rate of the reduction path in the stand was defined as the reduction rate p (%) below the ferrite transformation start temperature.
また、「フェライト再結晶開始温度」は、各鋼種の熱延板に、圧下率：60〜70％の冷間圧延を施し、ついで、20℃間隔の加熱温度で熱処理を施したのち、室温で硬さを測定し、硬さ−加熱温度の関係を求め、硬さと加熱温度との関係が、直線から外れ始めた温度を、各鋼のフェライト再結晶開始温度とした。
ついで、これら熱延板に、酸洗処理を施したのち、表２に示す冷延圧下率の冷間圧延を施す冷延工程で板厚1.2〜2.4mmの冷延板とした。なお、仕上圧延でフェライト変態開始温度未満の圧延が「有り」の場合には、フェライト変態開始温度未満の圧下率ｐを配慮してトータル圧下率TRが所定の範囲内となるように、冷延圧下率CRを調整した。なお、一部では比較として冷間圧延を行わなかった。
“Ferrite recrystallization start temperature” is determined by subjecting hot rolled sheets of each steel type to cold rolling at a reduction ratio of 60 to 70%, followed by heat treatment at a heating temperature of 20 ° C., and then at room temperature. The hardness was measured, the relationship between hardness and heating temperature was determined, and the temperature at which the relationship between hardness and heating temperature began to deviate from the straight line was defined as the ferrite recrystallization start temperature of each steel.
Next, these hot-rolled sheets were pickled, and then cold-rolled with a cold rolling reduction ratio shown in Table 2 to obtain cold-rolled sheets having a thickness of 1.2 to 2.4 mm. In addition, in the case where the finish rolling is less than the ferrite transformation start temperature and “Yes”, cold rolling is performed so that the total reduction rate TR is within a predetermined range in consideration of the reduction rate p less than the ferrite transformation start temperature. The rolling reduction CR was adjusted. In some cases, cold rolling was not performed for comparison.
得られた冷延板に、連続焼鈍ライン、または、連続溶融亜鉛めっきラインを用いて、表２に示す条件で焼鈍工程、溶融亜鉛めっき処理工程、あるいは、合金化溶融亜鉛めっき処理工程を施し、冷延焼鈍板、あるいはめっき板とした。なお、溶融亜鉛めっき処理は、熱処理に引続いて、めっき浴の温度：460℃の溶融亜鉛めっき浴に連続的に浸漬する処理とした。また、合金化溶融亜鉛めっき処理は、溶融亜鉛めっき処理後にさらに溶融亜鉛めっき層に表２に示す合金化処理温度で合金化処理を施す処理とした。焼鈍工程、溶融亜鉛めっき処理工程、あるいは、合金化溶融亜鉛めっき処理工程後に、鋼板形状の矯正のために、伸び率：0.5％の調質圧延を施した。 The obtained cold-rolled sheet is subjected to an annealing process, a hot dip galvanizing process, or an alloying hot dip galvanizing process under the conditions shown in Table 2 using a continuous annealing line or a continuous hot dip galvanizing line, A cold-rolled annealed plate or a plated plate was used. In addition, the hot dip galvanizing process was set as the process immersed continuously in the hot dip galvanizing bath of the temperature of a plating bath: 460 degreeC following heat processing. Further, the alloying hot dip galvanizing treatment was a treatment in which the hot dip galvanized layer was further subjected to alloying treatment at the alloying treatment temperature shown in Table 2 after the hot dip galvanizing treatment. After the annealing process, the hot dip galvanizing process, or the alloying hot dip galvanizing process, temper rolling with an elongation of 0.5% was performed to correct the shape of the steel sheet.
得られた冷延焼鈍板（めっき板）から、試験片を採取し、組織観察、引張試験、析出Ｎ量の測定を実施した。試験方法は次のとおりとした。
（１）組織観察
得られた冷延焼鈍板およびめっき板から、圧延方向に平行な断面（Ｌ断面）が観察面となるように組織観察用試験片を採取し、研磨しナイタール腐食して、光学顕微鏡（倍率：400倍）または走査型電子顕微鏡（倍率：1000倍）を用いて、組織を構成する各相の同定を行うとともに、該各相の組織分率を、画像解析装置を用いて、測定した。なお、フェライト相については、Ｘ線回折法で（220）面のピークを求め、その半価幅を求めた。
（２）引張試験
得られた冷延鋼板およびめっき鋼板から、試験方向が圧延方向となるように、J1S 13号Ｂサイズ試験片を採取し、必要に応じて研削加工を施し、板厚を1.2mmとしたのち、J1S Z 2241の規定に準拠して引張試験を行い、引張特性(引張強さTS、全伸び(ElＴ)、局部伸び(ElＬ))を求めた。
（３）固溶Ｎ量の測定
また、得られた冷延鋼板およびめっき鋼板から分析用試材を採取し、固溶Ｎ量を測定した。固溶Ｎ量は、全Ｎ量から、析出Ｎ量を差し引いた値とした。析出Ｎ量は、アセチルアセトン系電解液による定電位電解法を用いた電解抽出による溶解法を適用して抽出した残渣を化学分析して、残渣中のＮ量を求めた値を用いた。
A test piece was collected from the obtained cold-rolled annealed plate (plated plate), and a structure observation, a tensile test, and a measurement of the amount of precipitated N were performed. The test method was as follows.
(1) Microstructure observation From the obtained cold-rolled annealed plate and plated plate, a microstructural specimen was taken so that the cross section parallel to the rolling direction (L cross section) became the observation surface, polished, and subjected to nital corrosion. Using an optical microscope (magnification: 400 times) or a scanning electron microscope (magnification: 1000 times), each phase constituting the tissue is identified, and the tissue fraction of each phase is determined using an image analyzer. ,It was measured. In addition, about the ferrite phase, the peak of (220) plane was calculated | required with the X ray diffraction method, and the half value width was calculated | required.
(2) Tensile test J1S No. 13 B size test specimen was taken from the obtained cold-rolled steel sheet and plated steel sheet so that the test direction was the rolling direction, and subjected to grinding as necessary to obtain a sheet thickness of 1.2 After setting to mm, a tensile test was performed in accordance with the provisions of J1S Z 2241 to determine tensile properties (tensile strength TS, total elongation (El T ), local elongation (El L )).
(3) Measurement of solid solution N amount Moreover, the analytical sample was extract | collected from the obtained cold-rolled steel plate and plated steel plate, and solid solution N amount was measured. The solute N amount was a value obtained by subtracting the precipitated N amount from the total N amount. As the amount of precipitated N, a value obtained by chemically analyzing a residue extracted by applying a dissolution method by electrolytic extraction using a constant potential electrolysis method with an acetylacetone-based electrolytic solution to obtain an N amount in the residue was used.
得られた結果を表３に示す。 The obtained results are shown in Table 3.
本発明例は、いずれも、高価な合金元素を多量含有することなく、所望の高強度（引張強さ400MPa以上700MPa未満）を有し、かつ次（１）式
TS（MPa）≧ −10×（T-El（％））＋750 ‥‥（１）
（ここで、T-El：圧延方向の全伸び（％）、TS：圧延方向の引張強さ（MPa））
を満足する強度と延性のバランス、を有し、成形性に優れた高強度冷延薄鋼板および高強度溶融亜鉛めっき薄鋼板となっている。一方、本発明範囲を外れる比較例は、強度が不足するかあるいは強度が高すぎるか、延性が低下するかして、強度と延性のバランスが低下している。
Each of the examples of the present invention has a desired high strength (tensile strength of 400 MPa or more and less than 700 MPa) without containing a large amount of expensive alloy elements, and the following formula (1)
TS (MPa) ≧ −10 × (T-El (%)) + 750 (1)
(Where T-El: total elongation in rolling direction (%), TS: tensile strength in rolling direction (MPa))
Has a balance between strength and ductility that satisfies the requirements, and is a high-strength cold-rolled steel sheet and a high-strength hot-dip galvanized sheet steel excellent in formability. On the other hand, in the comparative example that is out of the scope of the present invention, the balance between strength and ductility is lowered because the strength is insufficient, the strength is too high, or the ductility is lowered.
Claims (9)
Ｃ：0.010〜0.060％、 Si：0.3％以下、
Mn：0.1〜0.5％、 Ｐ：0.05％以下、
Ｓ：0.05％以下、 Al：0.05％以下、
Ｎ：0.0060〜0.0200％
を、下記（３−ａ）式で定義される有効Ｎ量ＮefとAl含有量との比、Ｎef／Alが0.2以上を満足するように含有し、さらに固溶Ｎを0.0040％以上含み、残部Feおよび不可避的不純物からなる組成と、フェライト相を主相とする組織とを有し、さらに固溶Ｎを0.0040％以上含み、残部Feおよび不可避的不純物からなる組成と、フェライト相を主相とする組織とを有し、圧延方向の引張強さTS：400MPa以上700MPa未満で、かつ圧延方向の引張強さTSと全伸びT-Elとの関係が下記（１）式を、全伸びT-Elと局部伸びL-Elとの関係が下記（２）式を、それぞれ満足することを特徴とする成形性に優れた高強度冷延薄鋼板。
記
TS（MPa）≧ −10×（T-El（％））＋750 ‥‥（１）
L-El／T-El ≧ 0.5 ・・・・（２）
ここで、TS：圧延方向の引張強さ（MPa）
T-El：圧延方向の全伸び（％）、
L-El：圧延方向の局部伸び（％）
Ｎef＝Ｎ ‥‥（３−ａ）
ここで、Ｎ：Ｎの含有量（質量％） % By mass
C: 0.010 to 0.060%, Si: 0.3% or less,
Mn: 0.1 to 0.5%, P: 0.05% or less,
S: 0.05% or less, Al: 0.05% or less,
N: 0.0060-0.0200%
Is a ratio of the effective N amount N ef defined by the following formula (3-a) to the Al content, N ef / Al is included so as to satisfy 0.2 or more, and further includes solid solution N of 0.0040% or more. And a composition comprising the balance Fe and inevitable impurities and a structure having a ferrite phase as a main phase, and further containing 0.0040% or more of solid solution N, and a composition consisting of the balance Fe and inevitable impurities and a ferrite phase. The tensile strength TS in the rolling direction is 400 MPa or more and less than 700 MPa, and the relationship between the tensile strength TS in the rolling direction and the total elongation T-El is expressed by the following formula (1). A high-strength cold-rolled steel sheet with excellent formability, characterized in that the relationship between T-El and local elongation L-El satisfies the following formula (2).
Record
TS (MPa) ≧ −10 × (T-El (%)) + 750 (1)
L-El / T-El ≧ 0.5 (2)
Where TS: Tensile strength in the rolling direction (MPa)
T-El: Total elongation in rolling direction (%)
L-El: Local elongation in rolling direction (%)
N ef = N (3-a)
Here, N: N content (mass%)
記
Ｎef＝Ｎ−14×（Ti／48＋Ｂ／11） ‥‥（３−ｂ）
ここで、Ｎ、Ti、Ｂ：各元素の含有量（質量％）、 In addition to the above composition, Ti or B further contains one or two kinds selected from the above (3-a) formula, and an effective N amount defined by the following formula (3-b) The high-strength cold-rolled thin steel sheet according to claim 1, wherein N ef is a composition satisfying 0.0060 to 0.0200% by mass.
Record
N ef = N-14 × (Ti / 48 + B / 11) (3-b)
Here, N, Ti, B: Content (mass%) of each element,
前記鋼素材を、質量％で、
Ｃ：0.010〜0.060％、 Si：0.3％以下、
Mn：0.1〜0.5％、 Ｐ：0.05％以下、
Ｓ：0.05％以下、 Al：0.05％以下、
Ｎ：0.0060〜0.0200％
を、下記（３−ａ）式で定義される有効Ｎ量ＮefとAl含有量との比、Ｎef／Alが0.2以上を満足するように含有し、残部Feおよび不可避的不純物からなる組成を有する鋼素材とし、
前記熱延工程を、前記鋼素材を1000℃以上の温度に加熱し、粗圧延を施しシートバーとしたのち、該シートバーに、仕上圧延出側温度：フェライト変態開始温度以上とする仕上圧延を施し熱延板とし、ついで、下記（４）式で定義されるＴＡ℃以下の温度で巻き取る工程とし、
前記冷延工程を、前記熱延板に、圧下率が30〜80％となる冷間圧延を施す工程とし、
前記焼鈍工程を、焼鈍温度：250℃以上、下記（４）式で定義されるＴＡ℃以下で、かつフェライト再結晶開始温度未満の温度に加熱し、該温度で１〜600ｓ間保持する焼鈍処理を施す工程と、
することを特徴とする成形性に優れた高強度冷延薄鋼板の製造方法。
記
Ｎef＝Ｎ ‥‥（３−ａ）
ここで、Ｎ：Ｎの含有量（質量％）
ＴＡ ＝ 700−10（Al／Ｎef） ‥‥（４）
ここで、Al：Alの含有量（質量％）、Ｎef：有効Ｎ量（質量％） A hot rolling process in which the steel material is heated and rolled into a hot rolled sheet, a cold rolling process in which the hot rolled sheet is cold rolled into a cold rolled sheet, and the cold rolled sheet In the manufacturing method of the high strength cold-rolled thin steel sheet, which is subjected to an annealing process and subjected to an annealing process to be a cold-rolled annealed sheet in order, and to be a high-strength cold-rolled thin steel sheet
The steel material in mass%,
C: 0.010 to 0.060%, Si: 0.3% or less,
Mn: 0.1 to 0.5%, P: 0.05% or less,
S: 0.05% or less, Al: 0.05% or less,
N: 0.0060-0.0200%
Is a ratio of the effective N amount N ef defined by the following formula (3-a) to the Al content, N ef / Al is satisfied so as to satisfy 0.2 or more, and the composition comprising the balance Fe and inevitable impurities A steel material having
In the hot rolling step, the steel material is heated to a temperature of 1000 ° C. or higher, subjected to rough rolling to form a sheet bar, and then the finish rolling is performed on the sheet bar at a finish rolling outlet temperature: a ferrite transformation start temperature or higher. A hot-rolled sheet, and then a step of winding at a temperature equal to or lower than T A ° C defined by the following formula (4):
The cold rolling step is a step of subjecting the hot rolled sheet to cold rolling with a rolling reduction of 30 to 80%,
The annealing step is performed by heating to an annealing temperature: 250 ° C. or higher, T A ° C or lower defined by the following formula (4), and lower than the ferrite recrystallization start temperature, and maintaining the temperature for 1 to 600 seconds. A process of processing,
A method for producing a high-strength cold-rolled thin steel sheet excellent in formability, characterized by:
Record
N ef = N (3-a)
Here, N: N content (mass%)
T A = 700-10 (Al / N ef ) (4)
Here, Al: Al content (% by mass), N ef : Effective N amount (% by mass)
記
TR（％）＝［１−（１−ｐ／100）（１−CR／100）］×100 ‥‥（５）
ここで、熱延板のX線回析における（220）面からの回析ピークの半価値が、0.30°以上である場合は、ｐ：仕上圧延最終圧延スタンドにおける圧下率（％）、0.30°未満である場合は、ｐ：０（％）、CR：冷間圧延における圧下率（％） The finish rolling is hot rolling at a finish rolling exit temperature: less than the ferrite transformation start temperature, and the cold rolling has a TR (%) defined by the following formula (5) of 30 to 80%. Thus, it is rolling which adjusts the rolling reduction CR of cold rolling, The manufacturing method of the high-strength cold-rolled thin steel plate of Claim 4 characterized by the above-mentioned.
Record
TR (%) = [1- (1-p / 100) (1-CR / 100)] × 100 (5)
Here, when the half value of the diffraction peak from the (220) plane in the X-ray diffraction of the hot-rolled sheet is 0.30 ° or more, p: reduction ratio (%) in the final rolling final rolling stand, 0.30 ° If less than, p: 0 (%), CR: reduction ratio in cold rolling (%)
記
Ｎef＝Ｎ−14×（Ti／48＋Ｂ／11） ‥‥（３−ｂ）
ここで、Ｎ、Ti、Ｂ：各元素の含有量（質量％） In addition to the above composition, Ti or B further contains one or two kinds selected from the above (3-a) formula, and an effective N amount defined by the following formula (3-b) N ef is in mass%, the method of producing a high strength cold rolled thin steel sheet according to any one of claims 4 to 7, characterized in that the composition satisfies 0.0060 to 0.0200 percent.
Record
N ef = N-14 × (Ti / 48 + B / 11) (3-b)
Here, N, Ti, B: Content of each element (mass%)
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