JP2004346361A - Method for producing cold-rolled thin steel strip excellent in deep-drawability, and cold-rolled thin steel strip - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a cold-rolled thin steel strip having higher r value than that of the conventional strip and little variation in the r value and Δr in the whole range of the steel strip and especially having excellent deep-drawability. <P>SOLUTION: A hot-rolling is applied to a steel blank to be formed into a hot-rolled sheet, the steel blank containing ≤0.008% C, ≤0.2% Si, ≤1.0% Mn and further, one or two elements between Ti and Nb so as to simultaneously satisfy (Ti/48+Nb/93)>(C/12+N/14+S/32) and Ti/48>N/14 (wherein, Ti, Nb, C, N, S: content (mass%) of each element). In such a case, as a finish rolling in the hot-rolling, a joined rolling is performed after joining the preceding steel sheet to the following steel sheet to form a hot-rolled steel strip, thereafter repeated bending/bending-back working is applied to the hot-rolled steel strip with a leveller in the temperature range of 500-900°C to give 0.05-2.0 cumulative strain on the surface layer, then this steel strip is wound and annealing and the cold-rolling are applied thereto and further, a recrystallizing-annealing is applied. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【００５９】
【表２】

【００６０】
【表３】

【００６１】
【表４】

【００６２】
【表５】

【００６３】
同一鋼種、同一熱間仕上圧延条件で比較すれば、所定温度範囲においてレベラでの繰返し曲げ・曲げ戻し加工を施すことにより、高いｒ値、小さいΔｒが得られ、所定温度範囲においてレベラでの繰返し曲げ・曲げ戻し加工が深絞り性向上に効果があることがわかる。
特に、熱間仕上圧延をα域でかつ潤滑を施しつつ行い、熱間仕上圧延後レベラでの繰返し曲げ・曲げ戻し加工を施す本発明例（鋼帯Ｎｏ． ９〜１２ ） では、鋼帯長さ方向全域において、ｒ値が３．１ 以上、Δｒが０．２ 以下という非常に優れた深絞り性を有する鋼帯となることがわかる。
【００６４】
一方、レベラでの繰返し曲げ・曲げ戻し加工の加工温度が本発明の範囲を外れる比較例（鋼帯Ｎｏ．１５）や、与える歪量が本発明の範囲を外れる比較例（鋼帯Ｎｏ．１６）では、本発明範囲内の本発明例（鋼帯Ｎｏ．１、Ｎｏ． ２）に比べ、ｒ値、Δｒとも低下している。
また、本発明例はいずれも、高い（２．０ 以上の）ｒ値と、小さい（０．７ 以下）Δｒ を有する鋼帯となっている。
【００６５】
【発明の効果】
以上、説明したように、本発明によれば、優れた深絞り性を有する冷延鋼帯を鋼板形状、鋼板寸法などが従来に比べて劣ることなく、また、長さ方向および幅方向全域において特性変動の少ない鋼帯を製造することが可能となり、産業上格段の効果を奏する。
【図面の簡単な説明】
【図１】本発明において適用するのに好適な、熱間圧延ラインの設備列を模式的に示す説明図である。
【図２】レベラにおける鋼帯の加工状況を模式的に示す説明図である。
【図３】本発明で適用するのに好適な潤滑油の噴射手段を模式的に示す説明図である。
【符号の説明】
１ 鋼帯
２ 粗圧延機列
３ 仕上圧延機列
３ａ ワークロール
３ｂ バックアップロール
３１ ヘッダ
３２ 噴射用ノズル
４ 第一の冷却装置
５ レベラ
５ａ レベラワークロール
５ｂ レベラバックアップロール
６ 第二の冷却装置
７ 巻取り装置
８ 切断装置
１０ コイルボックス
１１ クロップシャ
１２ 接合装置
１３ バリ取り装置
１４ 接合部冷却装置
５０ 制御装置
７０ プロセスコンピュータ
１００ メジャーリングロール
１０２ 温度計
１０３ 温度計
１０４ 温度計
Ｓ 鋼素材
ＳＢ シートバー[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a cold-rolled thin steel strip, and more particularly to a method for producing a cold-rolled thin steel strip excellent in deep drawability, which is suitable for applications such as automotive steel sheets.
[0002]
[Prior art]
In recent years, material properties required for steel sheets for automobiles have been increasing more and more. In particular, since steel sheets for inner and outer panels of automobiles are subjected to press forming under severe conditions, they are required to have excellent deep drawability. An r value (Rankford value) is used as an index of the deep drawability, and the larger the r value, the better the deep drawability. The r-value of a steel sheet is closely related to its texture, and the more the {111} texture and the smaller the {001} texture, the higher the r-value and the better the deep drawability. Are known.
[0003]
As a steel sheet having a high r-value, there is a so-called IF steel (Interstitial atoms Free Steel) having a composition in which a strong carbonitride forming element such as Ti or Nb is added to an ultra-low carbon steel. For IF steel, the C and N contents must be reduced to the ultra-low carbon region and ultra-low nitrogen region of 0.003 mass% or less at the steel making stage, and then Ti, Nb, etc. are added to fix the solute C and N. Thereby, the solid solution elements in the steel are reduced to almost zero. This facilitates the development of {111} texture during recrystallization annealing of the cold rolled sheet.
[0004]
Also, various proposals have been made as a measure for further increasing the r-value of the IF steel. For example, Patent Literature 1 discloses that an ultra-low carbon steel containing C: 0.0030 wt% or less and N: 0.0040 wt% or less contains a predetermined amount or more of Ti defined in relation to the amounts of C, N, and S. A cold-rolled steel sheet having a composition containing Nb in a small amount relative to the amount of Ti and defined in relation to the amount of Ti has been proposed. The cold-rolled steel sheet described in Patent Document 1 has an r value of 2.8 or more and a work hardening index n value of 0.26 or more, and has extremely excellent deep drawability and stretchability. .
[0005]
To improve the r-value, various measures from a rolling method and an annealing method have been proposed in addition to the above-described measures from the aspect of composition. For example, Patent Literature 2 discloses that extremely low C and N, Ti: 0.035 to 0.20 wt%, Nb: 0.001 to 0.015 wt%, Ti and Nb content and C, The steel material containing Ti and Nb to satisfy the specific relationship with the N and S contents is 950 ° C. to Ar₃Hot rough rolling performed in the temperature range of the transformation point;₃A method for producing a cold-rolled steel sheet for ultra deep drawing, which performs hot finish rolling at a rolling reduction of 80% or more while performing lubrication in a temperature range from the transformation point to 600 ° C., and then performs cold rolling and recrystallization annealing. Proposed. In the technology described in Patent Document 2, hot finish rolling is performed in a ferrite (hereinafter, also referred to as α) region and lubricated rolling, whereby an excellent deep drawability with an r value of 2.7 to 2.9 is obtained. It is said that a cold-rolled steel sheet having
[0006]
In a normal method in which hot finish rolling is performed in an austenite (hereinafter also referred to as γ) region, since the rolled texture accumulated by hot finish rolling is randomized at the time of γ → α transformation at the time of cooling, ｛ 111｝ texture could not be fully developed. On the other hand, in the technique described in Patent Literature 2, hot finish rolling is performed in the α region, so that the rolling texture accumulated by hot finish rolling is not randomized due to transformation. It is stated that by performing crystal annealing, a developed {111} texture can be obtained, and excellent deep drawability can be obtained. Further, in the technology described in Patent Document 2, if hot finish rolling is performed without lubrication, {110} texture that is not preferable for deep drawability is preferentially formed in the surface layer portion of the plate thickness. It is necessary.
[0007]
Patent Document 3 discloses that an ultra-low carbon steel containing 0.0015 wt% or less of C, 0.010 to 0.060 wt% of Ti, and 0.0003 to 0.0025 wt% of B is hot-rolled. And rolling reduction: 65% or more of cold rolling, and recrystallization temperature or more Ac₃There has been proposed a method for producing a high-strength cold-rolled steel sheet having excellent deep drawability by performing annealing at a temperature equal to or lower than the transformation point. However, the r-value of the cold-rolled steel sheet manufactured by the technique described in Patent Document 3 is at most 2.5, and the technique described in Patent Document 3 shows that the steel sheet has an No consideration is given to the fluctuation of the r value and Δr at each position.
[0008]
Patent Document 4 discloses that C: 0.0005 to 0.01% by weight, N: 0.007% by weight or less, Ti: 0.01 to 0.1% by weight, and / or Nb: 0.01 to 0%. A steel slab having a composition containing 0.1% by weight is heated and the finish rolling temperature is adjusted to Ar.₃Secondary cold rolling at a total cold rolling rate of 50 to 95% of hot rolling at a transformation point or higher, primary cold rolling at a cold rolling rate of 5 to 50%, and primary cold rolling at a cold rolling rate of 20% or more. A method for producing a cold-rolled steel sheet having excellent workability by performing cold rolling and annealing at a temperature of not less than a recrystallization temperature and not more than 900 ° C. is described. However, the r-value of a cold-rolled steel sheet manufactured by the technique described in Patent Document 4 is at most 2.4, and the technique described in Patent Document 4 shows that the steel sheet when viewed over the entire area of the steel sheet is used. No consideration is given to the fluctuation of the r value and Δr at each position.
[0009]
Further, Patent Document 5 contains 0.0005 to 0.002 wt% of C, 0.0005 to 0.003 wt% of N, and 0.01 to 0.03 wt% of Ti. When hot finish rolling is performed on a steel slab containing₃In the temperature range from the transformation point to 1000 ° C., rolling with a total draft of 50% or more is performed under lubrication, and the finishing temperature is set to Ar.₃A method for producing a cold-rolled steel sheet excellent in formability in which hot finish rolling at a transformation point or higher is performed, and then cold rolling with a total reduction of 88 to 95% and recrystallization annealing are performed. The r value of the cold-rolled steel sheet manufactured by the technique described in Patent Document 5 is about 3.0 to 3.3, but the r value at each position in the steel sheet, Δr when viewed over the entire steel sheet. No consideration is given to fluctuations in
[0010]
[Patent Document 1]
JP-A-5-279797
[Patent Document 2]
Japanese Patent Publication No. 7-107179
[Patent Document 3]
JP-A-6-346149
[Patent Document 4]
JP-A-9-296225
[Patent Document 5]
JP-A-10-330844
[0011]
[Problems to be solved by the invention]
In recent years, as the design of automobiles has become more complicated, parts having complicated shapes have been required, and it has been increasingly difficult to perform press forming with conventional steel sheets for deep drawing. For this reason, there is a demand for a steel sheet for automobiles which can be press-formed without causing a sheet crack or the like even in a complicated shape and which is more excellent in deep drawability than in the past.
[0012]
The more complicated the shape of the part, the more the steel plate used must have a higher r value, and such a high r value is maintained uniformly over the entire length, that is, the entire steel plate (steel strip). This is preferable for preventing troubles such as cracks and holes during press forming, and is also preferable from the viewpoint of ensuring high steel sheet yield and high forming yield.
In view of such demands, the present invention advantageously solves the above-described problems of the prior art, has a higher r value and a lower Δr over the entire steel plate (steel strip), and has a small fluctuation thereof. It is an object of the present invention to provide a method for producing a cold-rolled thin steel strip having excellent deep drawability. The “thin steel strip” in the present invention refers to a steel strip having a thickness of 5 mm or less.
[0013]
[Means for Solving the Problems]
The present inventors have studied various factors affecting the r value in order to solve the above-described problem. As described above, it is known that it is effective to accumulate strain in a steel sheet during hot rolling by hot finish rolling in the α region and to develop a rolling texture, in order to improve the r value. I have. However, since the thickness of the final slab and the thickness of the final product are determined, and due to the problem of the rolling capacity of the finishing mill train, there is a limit to the strain that can be given during hot finish rolling, and , There was a limit to the improvement of the r value.
[0014]
Thus, the present inventors further studied various factors affecting the r value. As a result, the present inventors performed a steel sheet after hot finish rolling in the α region, further subjected to repeated bending and bending back processing by a leveler in the α region, and accumulate more strain in the steel band than before. , {111} texture was developed, and the r value was further improved. In addition, it has been found that the same r-value improving effect can be obtained even if the steel strip after hot finish rolling in the γ region is subjected to the repeated bending and bending back processing by the leveler in the same manner. The bending process can accumulate strain in the steel strip without changing the sheet thickness, so that strain can be accumulated in the steel strip more than before.
[0015]
Furthermore, the present inventors set the composition of the steel material to extremely low C, set the Ti and Nb contents to the composition specified by the relational expression with the C, N, and S contents, and then joined the preceding material and the succeeding material. After performing hot finish rolling by joining rolling to roll, then, in an appropriate temperature range, after performing repeated bending and bending back processing with a leveler and winding, then cold rolling, recrystallization annealing, It has been found that a cold-rolled thin steel strip having a high r value of 2.0 or more, preferably 3.1 or more, and a small Δr of 0.7 or less, preferably 0.2 or less can be obtained in the entire steel strip. .
[0016]
The present invention has been completed based on the above findings, with further investigations. That is, the gist of the present invention is as follows.
(1) In% by mass, C: 0.008% or less, Si: 0.2% or less, Mn: 1.0% or less, and one or two of Ti and Nb are further described in the following (1). Equation and Equation (2)
(Ti / 48 + Nb / 93)> (C / 12 + N / 14 + S / 32) (1)
Ti / 48> N / 14 (2)
(Here, Ti, Nb, C, N, S: content of each element (% by mass))
In order to form a hot-rolled steel strip by subjecting a steel material containing B to 0.001% or less and preferably having a composition comprising the balance of Fe and unavoidable impurities to hot rolling, The finish rolling of the hot rolling is performed to form a hot-rolled steel strip as a joining rolling performed after joining a preceding material and a subsequent material, and then the hot-rolled steel strip is subjected to a temperature range of 500 ° C to 900 ° C, The surface layer of the hot-rolled steel strip is subjected to repeated bending and bending-back processing to impart a cumulative strain of 0.05 or more and 2.0 or less, and then wound, and then subjected to hot-rolled steel strip annealing and cold rolling. And producing a cold-rolled steel strip, and further performing recrystallization annealing on the cold-rolled steel strip.
(2) In (1), the finish rolling of the hot rolling is performed at 600 ° C.₃A method for producing a cold-rolled thin steel sheet, wherein the method is performed while lubricating in a temperature range below the transformation point.
(3) In mass%, C: 0.008% or less, Si: 0.2% or less, Mn: 1.0% or less, and one or two of Ti and Nb are further described in (1) Equation and Equation (2)
(Ti / 48 + Nb / 93)> (C / 12 + N / 14 + S / 32) (1)
Ti / 48> N / 14 (2)
(Here, Ti, Nb, C, N, S: content of each element (% by mass))
And a composition consisting of the balance of Fe and unavoidable impurities, and the following formula (3)
r = (r₀+ 2r₄₅+ R₉₀) / 4 ............ (3)
(Where r₀: R value (Rankford value) in the rolling direction (steel strip length direction), r₄₅: R value (Rankford value) in the 45 ° direction from the rolling direction in the plane of the steel strip, r₉₀: R value in the direction perpendicular to the rolling direction in the plane of the steel strip (Rankford value)
Is greater than or equal to 3.1 and the following equation (4)
Δr = | r₀-2r₄₅+ R₉₀| / 2 ……… (4)
(Where r₀: R value (Rankford value) in the rolling direction (steel strip length direction), r₄₅: R value (Rankford value) in the 45 ° direction from the rolling direction in the plane of the steel strip, r₉₀: R value in the direction perpendicular to the rolling direction in the plane of the steel strip (Rankford value)
A cold-rolled thin steel strip excellent in deep drawability, characterized in that Δr defined by is 0.2 or less.
(4) The cold-rolled thin steel strip according to (3), further containing B: 0.001% or less by mass% in addition to the composition.
[0017]
BEST MODE FOR CARRYING OUT THE INVENTION
First, the reasons for limiting the composition of the steel material used in the present invention will be described. Hereinafter, mass% is simply described as%.
C: 0.008% or less
C is an element that forms an interstitial solid solution, increases strength, and reduces ductility and deep drawability, and is preferably reduced as much as possible in the present invention. If the content is reduced to 0.008% or less, good deep drawability can be secured. Therefore, in the present invention, C is limited to 0.008% or less. Incidentally, the content is preferably 0.003% or less.
[0018]
Si: 0.2% or less,
Si is an element that increases the strength. In the present invention, it is desirable to contain Si in an amount of 0.01% or more according to the desired strength. However, if it exceeds 0.2%, the workability is reduced. For this reason, Si was limited to 0.2% or less. In addition, from the viewpoint of improving the deep drawability, the content is preferably 0.05% or less.
[0019]
Mn: 1.0% or less
Mn is an element that increases the strength. In the present invention, it is desirable to contain Mn in an amount of 0.1% or more according to the desired strength. However, if it exceeds 1.0%, the workability is reduced. For this reason, Mn was limited to 1.0% or less. Note that, from the viewpoint of improving the deep drawability, the content is preferably 0.5% or less.
[0020]
(Ti / 48 + Nb / 93)> (C / 12 + N / 14 + S / 32) (1)
Ti / 48> N / 14 (2)
(Here, Ti, Nb, C, N, S: content of each element (% by mass))
Ti and Nb are elements that combine with C and N to form a carbonitride and have an effect of reducing solid solution C and N in steel. Further, Ti and Nb also combine with S to form a sulfide and have an effect of reducing solid solution S. In order to fix the solid solution C, N and S almost completely as a precipitate and to easily form a desired texture, the present invention satisfies the expression (1) according to the amounts of C, N and S so as to satisfy the formula (1). The content of Ti and Nb in the steel material is adjusted. If the Ti and Nb contents do not satisfy the expression (1), further improvement of the r value and further reduction of Δr cannot be expected.
[0021]
Further, in a high temperature range, in order to precipitate and fix solid solution N, the Ti content in the steel material is adjusted so as to satisfy the expression (2). When the Ti content does not satisfy the expression (2), the r value decreases.
As described above, Ti and Nb are contained so as to satisfy the formulas (1) and (2) in order to reduce the solid solution C, N and S in the steel and to easily form a desired texture. However, when the content of Ti exceeds 0.2% and the content of Nb exceeds 0.2%, the effect saturates, the recrystallization temperature increases, and the annealing temperature must be set high. It is not preferable because it causes soaring. For these reasons, it is preferable to limit the content to Ti: 0.2% or less and Nb: 0.2% or less, respectively. More preferably, Ti is more than 0.03% and 0.15% or less, and Nb is 0.15% or less.
[0022]
In addition to the above basic components, B may further contain 0.001% or less.
B: 0.001% or less
B is an effective element that segregates at the crystal grain boundaries and improves the brittleness in secondary processing, and can be contained as necessary in the present invention. If the content exceeds 0.001%, the above-mentioned effect is saturated, and an effect commensurate with the content cannot be expected, which is economically disadvantageous. For this reason, B is preferably set to 0.001% or less.
[0023]
Further, in addition to the above components, P: 0.1% or less may be further contained.
P is an element that increases the strength, and for this purpose, it is desirable to contain P by 0.05% or more. If the content exceeds 0.1%, grain boundary segregation may cause grain boundary cracking, which is a starting point of fatigue fracture. Therefore, P is preferably set to 0.1% or less. When it is not necessary to increase the strength, it is preferable to set the unavoidable impurity to 0.02% or less.
[0024]
The balance other than the components described above consists of Fe and inevitable impurities. As the inevitable impurities, it is preferable that S: 0.02% or less and N: 0.008% or less.
Since the deep drawability improves as the S content decreases, it is preferable that S be suppressed to 0.02% or less in the present invention. More preferably, it is 0.005% or less.
Since N is dissolved in an interstitial form, which increases the strength and decreases ductility and deep drawability, in the present invention, N as an impurity is preferably 0.008% or less, and is preferably reduced as much as possible. Since the lower the N content, the better the deep drawability, the upper limit of the present invention is preferably set to 0.008%. More preferably, it is 0.003% or less.
[0025]
The molten steel having the above-described composition is smelted by a known smelting method such as a converter or an electric furnace, and is made into a steel material such as a slab by a known casting method such as a continuous casting method. In the present invention, the method for producing the steel material is not particularly limited. Any of the usual methods can be preferably applied.
Next, the steel material is heated and subjected to hot rolling to form a hot-rolled steel strip.
The heating temperature of the steel material is preferably in the range of 1000 to 1200C. When the temperature of the steel material is equal to or higher than the temperature at which hot rolling can be performed, heating may be performed without heating or slightly. The heated steel material is converted into a sheet bar by hot rough rolling, and then subjected to hot finish rolling to form a hot-rolled steel strip having a predetermined thickness. When a sheet bar or a thin slab is used as a material, it goes without saying that rough rolling may be omitted.
[0026]
The rough rolling in the hot rolling in the present invention is not particularly limited as long as a sheet bar having a predetermined size and shape can be obtained, and the rolling conditions are not particularly limited, but the total rolling reduction is 3 to 9 passes in a temperature range of 900 to 1100 ° C. The ratio is preferably set to 70 to 90%. Thereby, a coarse cast structure can be refined by recrystallization, and improvement in deep drawability can be expected.
The finish rolling of the hot rolling in the present invention is a joining rolling performed after joining a preceding material (preceding sheet bar) and a following material (succeeding sheet bar) before finish rolling. By joining the preceding material and the succeeding material, it is possible to perform stable rolling at both the leading end and the trailing end except for the first leading end and the last trailing end of the joined rolling, and to obtain a thin steel strip excellent in deep drawability. The yield is improved. From the viewpoint of further improving the yield, it is preferable that the number of joints is large. For joining the preceding material and the succeeding material, any of a known method of combining induction heating and pressure welding, a method of using a laser, and the like can be preferably applied.
[0027]
The conditions of the finish rolling in the hot rolling are not particularly limited, but it is preferable that the total draft is 70 to 98% in 5 to 9 passes in a temperature range of 600 to 1000 ° C. In finish rolling, the finish rolling end temperature is set to Ar₃Although normal γ-range rolling may be performed at a temperature not lower than the transformation point, in order to obtain a large r value, the finish rolling temperature range is set to 600 ° C. or more and Ar₃It is preferable to use lubricated rolling in which the rolling is performed in an α + γ or α region below the transformation point and while lubricating. When lubricating rolling is applied in the finish rolling of hot rolling, problems such as poor biting and meandering occur at the leading end of the first joint rolling and the end of the final book. It is better not to perform lubrication rolling.
[0028]
The type of lubricating oil in the lubricating rolling, the method of lubrication is not particularly limited, but a preferable lubricating method is, for example, a method of mixing a lubricating oil based on mineral oil with water and directly injecting it into a work roll. Can be For the purpose of minimizing the fluctuation of the r value or Δr in the width direction of the steel strip, it is necessary to distribute the lubricating oil uniformly in the width direction between the steel strip during hot finish rolling and the work roll of the rolling mill. is there. To this end, as shown in FIG. 3 which is commonly used, a plurality of injection nozzles 32 are arranged by setting the injection angle and nozzle interval of each nozzle so as to cover the entire area in the steel strip width direction. It is preferable that the lubricating oil is injected toward the work roll 3a from the plurality of injection nozzles using the provided header 31.
[0029]
After finishing the hot finish rolling, the steel strip is preferably rapidly cooled to a predetermined temperature range, that is, a temperature range of 500 to 900 ° C.
The hot-rolled steel strip cooled to the predetermined temperature range is subjected to repeated bending / bending-back processing with a leveler at a temperature range of 500 ° C or more and 900 ° C or less, so that the surface layer of the hot-rolled steel strip is 0.05 or more 2 After giving a distortion of 0.0 or less, winding is performed. Cooling to the processing temperature by the leveler is preferably rapid cooling at a cooling rate of 5 to 50 ° C / s. The most important thing in the present invention is to subject the steel strip after the finish rolling of hot rolling to repeated bending / bending back processing with a leveler.
[0030]
Cooling to the processing temperature by the leveler is preferably rapid cooling at a cooling rate of 5 to 50 ° C / s. The temperature at which repeated bending / bending back processing is performed with a leveler is a temperature in a temperature range of 500 ° C. or more and 900 ° C. or less. If the processing temperature is lower than 500 ° C., the deformation resistance of the steel strip increases, the torque required to rotate the leveler work roll (hereinafter, referred to as WR) increases, and the steel sheet cannot be driven. become unable. On the other hand, when the processing temperature exceeds 900 ° C., the recovery of the strain occurs remarkably, and it becomes difficult to accumulate the strain. The processing temperature range of the leveler has a strain accumulation effect regardless of the γ range and the α range.₃In the case of the γ region at or above the transformation point, the crystal orientation is randomized by the γ → α transformation upon subsequent cooling, and the strain accumulation effect is reduced. For this reason, processing with a leveler requires Ar₃It is preferably performed in a two-phase region of γ + α below the transformation point, more preferably in an α region.
[0031]
FIG. 2 shows a bending state of the steel strip 1 in one bending operation by a leveler. The cumulative strain ε applied to the surface layer of the steel strip (the outermost surface in the thickness direction) by processing with a leveler is
ε = (N−2) × 2tδ / L²
(Where, t: thickness of steel strip (mm), δ: leveler tightening amount (mm), 2L: center axis interval of leveler WR (mm), N: number of levelers WR)
Is approximately represented by Note that the leveler tightening amount δ is defined as a distance in which the leveler WR is tightened from a state where the steel plate is sandwiched between the upper and lower levelers WR.
[0032]
If the cumulative strain applied to the surface layer of the hot-rolled steel strip by the processing with the leveler is less than 0.05, the effect of strain accumulation is not sufficient, while the strain ε exceeding 2.0 is actually applied. It will be difficult. For example, when δ = 19 mm, 2L = 180 mm, and t = 4 mm, according to the equation (1), when N is increased to 109 or more, ε can be increased to more than 2.0, but the equipment length is 9.1. It is too long, 9 m, which is not realistic. As is clear from FIG. 2, the leveler tightening amount δ is limited due to the arrangement of the work rolls, and it is difficult to increase ε beyond 2.0. Further, when δ = 7 mm, N = 29, and t = 4 mm, according to the equation (1), ε exceeds 2.0 by reducing the leveler WR center axis interval 2L to 55 mm or less. Although it is possible, the WR diameter is reduced, the deflection of the WR is large, and the shape of the steel sheet is deteriorated.
[0033]
In the processing by the leveler, it is preferable not to perform the processing by the leveler at the leading end of the first and the end of the final joint due to problems such as sticking to the leveler WR and drawing. Therefore, in order to obtain a steel strip in which the variation of the r value and Δr in the entire steel strip is small, it is necessary to incorporate the target sheet bar from the second to the last −1st (one immediately before the last) of the joining rolling. preferable. In principle, both the force received from the roll during rolling and the force received from the leveler WR during the repeated bending and unbending processes are almost uniform in the width direction of the steel strip. In most cases, cooling water is sprayed so as to cover the entire steel strip width direction using a header in which a plurality of spray nozzles are arranged at intervals in the strip width direction. Non-uniform processing conditions that cause fluctuations in the value and Δr are excluded in principle. For this reason, in order to reduce the fluctuation of the r value and Δr in the entire steel strip, it is important to perform the processing by the leveler using the joining rolling which can reduce the unevenness of the processing in the longitudinal direction of the steel strip.
[0034]
The hot-rolled steel strip that has been repeatedly bent and bent back by a leveler and wound is then subjected to hot-rolled steel strip annealing.
The hot-rolled steel strip annealing is preferably recrystallization annealing in a temperature range of 600 to 950 ° C. The hot-rolled steel strip annealing may be either continuous annealing or box annealing. The annealing time is not particularly limited, but is preferably 30 s or more and 10 h or less. If the processing end temperature at the leveler is high and the winding can be performed at a temperature of 600 ° C. or more, annealing is not necessarily required. In this case, the recrystallization can be completed by self-annealing in the wound state.
[0035]
The hot-rolled steel strip subjected to the hot-rolled steel strip annealing is then subjected to cold rolling to be a cold-rolled steel strip. The rolling reduction of the cold rolling is preferably set to 50 to 95%. If the rolling reduction is less than 50%, the effect of improving the r-value is small, while if it exceeds 95%, the r-value is reduced.
The cold-rolled steel strip is then subjected to recrystallization annealing to obtain a product (cold-rolled thin steel strip). The recrystallization annealing of the cold-rolled steel strip is preferably continuous annealing. It goes without saying that box annealing may be used. The annealing temperature of the recrystallization annealing is desirably in the range of 600 to 950 ° C. If the annealing temperature is lower than 600 ° C., recrystallization is not completed, so that the r value decreases. On the other hand, when the annealing temperature exceeds 950 ° C., although depending on the chemical composition, α → γ transformation occurs and the texture is randomized, so that the r value decreases. Further, the annealing time is preferably set to 20 s or more and 5 h or less. If the annealing time is less than 20 s, recrystallization is not completed, while if it exceeds 5 h, the effect is saturated and energy is lost.
[0036]
In the hot rolling in the present invention, for example, the hot rolling line shown in FIG.ApplicationIs preferred.
The steel material S is heated in a heating furnace (not shown), or directly fed in a hot state from an upstream process, and hot-rolled by the row of rough rolling mills 2 to form a sheet bar SB. In the sheet bar SB, a preceding material and a succeeding material are joined by using a coil box 10, a cropper 11, and a joining device 12 provided between the rough rolling mill row 2 and the finishing rolling mill row 3, and finishing is performed. Hot finish rolling is performed by the rolling mill train 3 to form a hot-rolled steel strip 1 having a predetermined size. At the time of joining, a deburring device 13 may be used in order to ensure the uniformity of the surface properties with the steel strip peripheral portion, and to ensure the uniformity of the temperature and material with the steel strip peripheral portion. The joint cooling device 14 may be used. Reference numeral 3a in the finishing mill train 3 denotes a work roll, and 3b denotes a backup roll.
[0037]
A first cooling device 4, a leveler 5 and a second cooling device 6 are arranged downstream of the first cooling device 4 in this order on the output side of the finishing mill train 3, and a winding device 7 is further provided. A cutting device 8 for cutting the joined steel strip is provided on the outlet side of the second cooling device 6. In addition, although not shown, a large number of table rollers are arranged between the main facilities to transport the steel material S and the hot-rolled steel strip 1.
[0038]
The leveler 5 includes three or more work rolls 5a arranged in a zigzag pattern, or a backup roll 5b for backing up the work rolls 5a. In addition, it is preferable that the work roll diameter of this leveler be 300 mm or less, because the strain that can be imparted by the bending / unbending process by the leveler can be increased.
As a production line other than the hot rolling line, any of a normal cold rolling line, a continuous annealing line, and the like can be used.
[0039]
A cold-rolled thin steel strip manufactured by sequentially subjecting a steel material having the above-described composition to hot rolling, leveling, hot-rolled steel strip annealing, cold rolling, and recrystallization annealing under the above-described conditions has a mass% of C : 0.008% or less, Si: 0.2% or less, Mn: 1.0% or less, and one or two of Ti and Nb are represented by the following formulas (1) and (2).
(Ti / 48 + Nb / 93)> (C / 12 + N / 14 + S / 32) (1)
Ti / 48> N / 14 (2)
(Here, Ti, Nb, C, N, S: content of each element (% by mass))
Or further contains B: 0.001% or less, has a composition consisting of the balance of Fe and unavoidable impurities, and has an r value of 2.0 or more, preferably 3 in the entire steel strip. .1 and not more than 0.7, preferably not more than 0.2, resulting in a cold-rolled thin steel strip having excellent deep drawability.
[0040]
The r value is calculated by the following equation (3).
r = (r₀+ 2r₄₅+ R₉₀) / 4 ............ (3)
(Where r₀: R value (Rankford value) in the rolling direction (steel strip length direction), r₄₅: R value (Rankford value) in the 45 ° direction from the rolling direction in the plane of the steel strip, r₉₀: R value in the direction perpendicular to the rolling direction in the plane of the steel strip (Rankford value)
Is defined by Δr is given by the following equation (4).
Δr = | r₀-2r₄₅+ R₉₀| / 2 ……… (4)
(Where r₀: R value (Rankford value) in the rolling direction (steel strip length direction), r₄₅: R value (Rankford value) in the 45 ° direction from the rolling direction in the plane of the steel strip, r₉₀: R value in the direction perpendicular to the rolling direction in the plane of the steel strip (Rankford value)
Is defined by
[0041]
In addition, "the whole area | region of a steel strip" mentioned above means that it extends over the whole area of both directions of a steel strip length direction and a width direction. The steel strip width direction is literally the entire area in the width direction. On the other hand, the length direction of the steel strip is the entire area excluding the unsteady part accompanying the joining. In the present invention, the "unsteady portion associated with the joining" refers to the surrounding portion, the portion where the material is different due to the different temperature, and the remaining of the molten steel splash or the weld bead due to the joining, or the pressure welding It means a portion having different dimensions due to a bump or the like. When hot rolling is applied by joining rolling, the entire length of the steel strip in the length direction is from the front end of the steel strip in the length direction to the center, from the position of 10 m in terms of finish rolling to the center from the tail end in the length direction after finish rolling. It is an area up to a position of 10 m toward the camera. On the other hand, when hot rolling is not applied by joining rolling, both the leading end and the tail end of the steel strip extend from the end to the center over one hundred and several tens of meters, due to differences in temperature, tension, etc. during finish rolling. As a result, an unsteady portion is formed. By applying the hot rolling to the joining rolling, the yield of the steel strip is remarkably improved as compared with the non-application.
[0042]
Here, a method of tracking the joint will be described with reference to FIG.
In the present invention, the injection of the lubricating oil is started and stopped before and after the timing at which the joint tracked by the control device 50 passes through each rolling mill of the finishing mill train 3. Also, after the joint passes through the finishing mill train 3 and before it reaches the winding device 7 from the exit side of the finishing mill train 3, the leveler 5 is tightened before or after the timing when the joint passes. In some cases, the cooling water may be opened or the injection of the cooling water may be temporarily started or stopped in the first cooling device 4 or the second cooling device 6, or the flow rate of the cooling water may be adjusted.
[0043]
If the finishing rolling in hot rolling is applied by joining rolling, the steel strip will be rolled and transported without interruption, so tracking of the joining portion will be performed for the next three periods.
(1) Finish rolling of the first steel strip to the arrival of the winding device
(2) After joining the subsequent steel strip joints from the second joint to the arrival of the first stand of the finishing mill row
(3) From the arrival of the first stand of the finishing mill to the occurrence and cutting of the scheduled cutting section of the subsequent steel strip joint
It is important to do this separately.
[0044]
Hereinafter, a method of tracking the joint at these times will be sequentially described.
(1) Finish rolling of the first steel strip to the arrival of the winding device
The distal end of the first steel strip to be joined is provided with a thermometer 102 provided on the exit side of the joining device 12, a thermometer 103 provided on the entry side of the finishing mill row 3, and a thermometer 103 provided on the exit side of the finishing mill row 3. The temperature can be intermittently detected by the thermometer 104. After the tip leaves the finishing mill train 3, tracking of the tip position is performed by a drive device (not shown) connected to a roll motor of the final stand of the finishing mill train 3 to a predetermined circumference (accurately, This value can be obtained by counting down the mechanical distance to the winding device 7 with a value obtained by multiplying the pulse generated at each rolling by the advance rate, by multiplying the predetermined rotation angle of the roll by the roll radius. The end of the first steel strip joined to the final stand of the third row of finishing mills is replaced by the arrival of the tip of the first steel strip at the thermometer 104 disposed on the exit side of the third row of finishing mills. The arrival may be used. This can be done by raising the rolling load.
[0045]
In addition, the above-described sensors such as thermometers may be replaced with other sensors such as a sensor for measuring the thickness and width of the material to be rolled and a torque meter attached to the electric motor of each finishing mill. Good.
(2) After joining the subsequent steel strip joints from the second joint to the arrival of the first stand of the finishing mill row
The tracking of the succeeding steel strip joint is performed after the joining until the joint arrives at the first stand of the finishing mill train 3, starting from the running end position of the joining device 12 and rotating with the transport of the material to be rolled therefrom. The control unit 50 receives and counts a pulse emitted from the measuring roll 100 every predetermined circumferential length of conveyance. It is important that the measuring roll 100 is pressed against the material to be rolled and starts rotating before joining.
(3) From the arrival of the first stand of the finishing mill row at the subsequent steel strip joint to the occurrence and cutting of the part to be cut
The tracking of the subsequent steel strip joint from the time when the joint arrives at the first stand of the finishing mill row 3 until it reaches the final stand is performed as follows.
[0046]
The product of the mechanical distance between each stand and the thickness of the material to be rolled at the exit side of each stand is accumulated for all the stands, and the calculated material volume in the finishing mill row is calculated as the finish rolling mill. A drive device (not shown) connected to the roll motor shaft of the last stand in the row provides a pulse generated at every rolling of a predetermined circumference (more precisely, a value obtained by multiplying a predetermined rotation angle of the roll by a roll radius). The control unit 50 counts down the value obtained by multiplying the ratio of the final stand exit side plate thickness of the finishing rolling mill row by the ratio and converting it to the transport volume distance.
[0047]
Further, tracking of the subsequent steel strip joining portion from when the joining portion arrives at the final stand of the finishing mill row 3 until the joining portion is cut is performed as follows.
Note that, in the present invention, the scheduled cutting section is generated at a final rolling mill row final stand position, for example, after the joining section reaches a final stand of the finishing mill row 3 and then rolls a predetermined distance. It is virtually generated on the upstream side or downstream side in the transport direction near the joining portion within 50.
[0048]
A drive device (not shown) connected to the roll motor shaft of the final stand of the finishing mill row is used for every predetermined circumference (more precisely, a value obtained by multiplying the roll radius by a predetermined rotation angle of the roll). The control device 50 receives the pulse, counts it, and further multiplies it by the advance rate to convert it to the rolling distance of the material to be rolled, and counts down the mechanical distance from the final stand of the finishing mill row to the cutting device 8. Thus, tracking of the subsequent steel strip joint is performed.
[0049]
Note that the roll radius and the advance rate used in the above tracking may be given to the control device 50 as follows.
Immediately after polishing a roll regularly, the roll radius is measured manually or automatically, and the measured value is manually or automatically input to a process computer 70, and the calculated value is obtained by calculation in the process computer 70. What is necessary is just to transmit from the computer 70 to the control apparatus 50. The advance rate is stored as a predetermined value in a table corresponding to the material, product dimensions, and the like by the process computer 70, and during the finish rolling, a predetermined value is stored in the table according to the material, product dimensions, and the like of the material to be rolled. The value is retrieved and transmitted to the control device 50. Instead of such a table formula, a model formula corresponding to the material of the material to be rolled, the product dimensions, the draft, etc. may be used.
[0050]
Hereinafter, the present invention will be described in more detail with reference to Examples.
[0051]
【Example】
Molten steel having the chemical components shown in Table 1 was smelted in a converter and made into a steel material (slab) by a continuous casting method. The slab thickness was 200 mm.
These slabs (steel materials) were heated to 1100 ° C, and then hot-rolled to form a 30 mm-thick sheet bar with 7 passes in a temperature range of 1000 to 1100 ° C. After joining these sheet bars by butt, induction heating and pressure welding, they are sequentially joined, and in 7 passes, the finishing side thickness: 4 mm, the finishing rolling side speed: 720 m / min, and the finishing rolling temperature shown in Table 2 Hot finish rolling was performed to obtain a hot-rolled steel strip. In the joining rolling, three sheet bars were joined and rolled, and the second sheet was evaluated. A part of the finish rolling was lubricated. In the lubricating rolling, the lubricating oil (mineral oil VG 46: 50%, mixed fatty acid ester of dometerol propane: 45%, sulfide oil: 4.9%, antioxidant: 0.1%) is mixed with water to form a work. Direct injection to the roll. When the joining rolling was applied, lubrication was performed so as to cover the entire steel strip from the end to the tail end of the joined second steel strip in terms of finish rolling. Even if an error occurs in the tracking of the tip of the second steel strip, the center of the first steel strip is converted from the tail end of the first steel strip in terms of finish rolling so that the lubrication can be reliably performed from the actual tip of the second steel strip. Lubricating oil was injected from a target position of 20 m. Also, in order to ensure that the actual tail end of the second steel strip is lubricated, the injection of lubricating oil should be stopped at a target position 20 m closer to the center of the third steel strip in terms of finish rolling from the tip of the third steel strip. I made it. In the case where the joining rolling is not applied, at least the lubricating oil is sprayed on the tip 150 m in terms of the finish after rolling to prevent slip at the tip and at one end of the work roll at the tail end to prevent slip of the next material. Stopped. In addition, in order to burn out the lubricating oil adhering to the work roll, the injection of the lubricating oil was stopped at a tail end of 100 m in terms of the value after finish rolling.
[0052]
After the hot finish rolling, the steel sheet was cooled to a cooling stop temperature shown in Table 2 at a cooling rate shown in Table 2.
Then, using the cooling stop temperature shown in Table 2 as a processing temperature, repeated bending and bending back processing was performed with a leveler, and the cumulative strain shown in Table 2 was imparted to the surface layer of the hot-rolled steel strip. In addition, the repetitive bending / bending process with the leveler was performed in the whole area from the tip end to the tail end of the joined second steel strip in terms of the finish after rolling, similarly to the injection of the lubricating oil.
[0053]
The leveler used had a leveler WR diameter of 170 mm, a center axis interval of the leveler WR: 180 mm, and the number of levelers WR: 29, and the amount of strain applied was changed by variously changing the leveler tightening amount.
After the processing, it was cooled to a winding temperature at a cooling rate shown in Table 2, and was wound. When the winding temperature is 600 ° C. or higher, self-annealing in the wound state is performed. In other cases, the steel strip after winding is recrystallized by box annealing again at the temperature shown in Table 2. Was given.
[0054]
Next, the hot-rolled steel strip was subjected to pickling treatment, and then subjected to five-pass cold rolling (cold rolling reduction: 80%) to obtain a 0.8-mm-thick cold-rolled steel strip.
Next, the cold-rolled steel strip was subjected to recrystallization annealing at 750 ° C. × 40 s.
From the obtained cold-rolled steel strip (joined second steel strip), a JIS No. 5 test piece was sampled in accordance with JIS Z 2201 and a tensile test was performed in accordance with JIS Z 2241, and elongation was performed. , R value and Δr were determined.
[0055]
The test piece was converted to a position 10 m from the tip toward the center, a center in the length direction, and a position 10 m from the tail end toward the center, after conversion to hot strip, for each steel strip. The angle between the steel strip length direction and the test piece length direction is 0 ° from a total of three areas, that is, an area of ± 50 mm at the center in the steel strip width direction and 100 to 200 mm from both width ends. (Rolling direction), 45 ° (rolling direction and 45 ° direction), and 90 ° (direction perpendicular to the rolling direction) were collected at nine locations in total.
[0056]
The r value in each direction was determined by giving a 15% tensile strain to the test piece, measuring three points of strain in the width direction, and averaging them. From the r value in each of these directions, the r value (average r value) and Δr (in-plane anisotropy) at each position were calculated by Expressions (3) and (4).
r = (r₀+ 2r₄₅+ R₉₀) / 4 ............ (3)
Δr = | r₀-2r₄₅+ R₉₀| / 2 ……… (4)
(Where r₀: R value in the rolling direction (steel strip length direction), r₄₅: R value in the direction of 45 ° from the rolling direction in the plane of the steel strip, r₉₀: R value in the direction perpendicular to the rolling direction in the plane of the steel strip)
Further, at each position of the obtained steel strip, the r value and Δr in each steel strip in the length direction were determined from the r value and Δr in the region of ± 50 mm at the center in the width direction of the steel strip.
[0057]
Table 3 shows the obtained results. Note that the r value and Δr obtained in each region of 100 to 200 mm from both width ends of the steel strip hardly differed between them, so the value at the end on the operator side was used as the representative value of the width end. The results are shown in Table 3.
[0058]
[Table 1]

[0059]
[Table 2]

[0060]
[Table 3]

[0061]
[Table 4]

[0062]
[Table 5]

[0063]
When compared under the same steel type and the same hot finish rolling conditions, a high r value and a small Δr can be obtained by performing repeated bending and bending back processing with a leveler in a predetermined temperature range. It can be seen that bending and unbending are effective in improving deep drawability.
In particular, in the present invention example (steel strip Nos. 9 to 12) in which hot finish rolling is performed in the α range and while lubricating, and after hot finish rolling, repeated bending / bending back processing is performed with a leveler, the steel strip length is large. It can be seen that a steel strip having a very excellent deep drawability with an r value of 3.1 or more and Δr of 0.2 or less is obtained in the entire area in the length direction.
[0064]
On the other hand, a comparative example (steel strip No. 15) in which the processing temperature of the repeated bending / bending back processing with the leveler is out of the range of the present invention, and a comparative example (steel strip No. 16) in which the applied strain amount is out of the range of the present invention. ), Both the r value and Δr are lower than those of the invention examples (steel strip No. 1 and No. 2) within the scope of the invention.
In addition, each of the examples of the present invention is a steel strip having a high (2.0 or more) r value and a small (0.7 or less) Δr.
[0065]
【The invention's effect】
As described above, according to the present invention, according to the present invention, a cold-rolled steel strip having excellent deep drawability has a steel sheet shape, steel sheet dimensions and the like which are not inferior to those of the related art, and in the entire length and width directions. It is possible to manufacture a steel strip with little characteristic fluctuation, and it has a remarkable industrial effect.
[Brief description of the drawings]
FIG. 1 is an explanatory view schematically showing an equipment row of a hot rolling line suitable for application in the present invention.
FIG. 2 is an explanatory view schematically showing a processing state of a steel strip in a leveler.
FIG. 3 is an explanatory view schematically showing a lubricating oil injection unit suitable for application in the present invention.
[Explanation of symbols]
1 Steel strip
2 Rough rolling mill row
3 Finishing mill row
3a Work roll
3b Backup roll
31 header
32 Nozzle for injection
4 First cooling device
5 Leveler
5a Leveler work roll
5b Leveler backup roll
6 Second cooling device
7 Winding device
8 Cutting device
10 Coil box
11 Kropsha
12 Joining equipment
13 Deburring device
14 Joint cooling device
50 Control device
70 Process computer
100 measuring roll
102 thermometer
103 thermometer
104 thermometer
S steel material
SB seat bar

Claims (4)

In mass%,
C: 0.008% or less, Si: 0.2% or less,
Mn: 1.0% or less, and further hot-rolled into a steel material having a composition containing one or two of Ti and Nb so as to satisfy the following formulas (1) and (2). To form a hot-rolled steel strip, the finish rolling of the hot rolling is performed as a hot-rolled steel strip as joining rolling performed after joining a preceding material and a following material, and then the hot-rolled steel strip is subjected to 500 In a temperature range of not less than 900 ° C. and not more than 900 ° C., the steel sheet is subjected to repeated bending / returning by a leveler to impart a cumulative strain of not less than 0.05 to not more than 2.0 to the surface layer of the hot-rolled steel strip, and then wound. A method for producing a cold-rolled thin steel strip excellent in deep drawability, characterized by subjecting a cold-rolled steel strip to cold-rolled steel strip by annealing and cold-rolling, and then subjecting the cold-rolled steel strip to recrystallization annealing.
(Ti / 48 + Nb / 93)> (C / 12 + N / 14 + S / 32) (1)
Ti / 48> N / 14 (2)
Here, Ti, Nb, C, N, S: content of each element (% by mass) Method for producing a cold-rolled thin steel sheet according to claim 1, characterized in that said finish rolling of hot rolling, while lubricating alms at a temperature range ₃ transformation point 600 ° C. or higher Ar. In mass%,
C: 0.008% or less, Si: 0.2% or less,
Mn: not more than 1.0%, and one or two of Ti and Nb are contained so as to satisfy the following formulas (1) and (2), and the balance consists of Fe and unavoidable impurities. It has a composition, and the r value defined by the following formula (3) is 3.1 or more and Δr defined by the following formula (4) is 0.2 or less over the entire steel strip. Cold rolled thin steel strip with excellent deep drawability.
(Ti / 48 + Nb / 93)> (C / 12 + N / 14 + S / 32) (1)
Ti / 48> N / 14 (2)
r = (r ₀ + 2r ₄₅ + r ₉₀ ) / 4 (3)
Δr = | r ₀ −2r ₄₅ + r ₉₀ | / 2 (4)
Here, Ti, Nb, C, N, S: content of each element (% by mass)
r _0: r value in the rolling direction (steel strip length direction) (Lankford value)
r ₄₅ : r value in the direction of 45 ° from the rolling direction in the plane of the steel strip (Rankford value)
r ₉₀ : r value in the direction perpendicular to the rolling direction in the plane of the steel strip (Rankford value) The cold-rolled thin steel strip according to claim 3, further comprising, by mass%, B: 0.001% or less in addition to the composition.

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