CN102812585A - Copper foil for lithium ion battery current collector - Google Patents

Abstract

Disclosed is a copper foil for a lithium-ion battery collector body, the plate thickness of which is highly precise. A copper foil for a lithium-ion battery collector body has an average surface roughness (Ra) in a rolling parallel direction (Raavg) of 0.01-0.15[mu]m, and Delta Ra=Ramax-Ramin is less than or equal to 0.025[mu]m.

Description

Copper foil for lithium ion battery current collector

technical field

The invention relates to a copper foil for a lithium ion battery collector, in particular to a copper foil for a negative electrode collector of a lithium ion secondary battery.

Background technique

Lithium-ion batteries have the characteristics of high energy density and high voltage, and are mostly used in small instruments such as notebook computers, video cameras, digital cameras, and mobile phones. In the future, it is expected to be used as a power source for large-scale equipment such as electric vehicles and distributed power sources in general households.

The electrode body of a lithium-ion battery usually has a stacked structure in which a positive electrode, a separator, and a negative electrode are wound or laminated dozens of times. Usually, the positive electrode is composed of a positive electrode collector made of aluminum foil and a positive electrode active material made of lithium composite oxides such as LiCoO ₂ , LiNiO ₂ and LiMn ₂ O ₄ arranged on its surface, and the negative electrode is made of copper foil. The negative electrode current collector is composed of a negative electrode active material made of carbon or the like coated on its surface.

An important issue of copper foil used as a negative electrode current collector is its adhesion to the negative electrode active material, and research and development of copper foil for current collectors has been conducted with a focus on improving the adhesion. As a general method of improving the adhesiveness with an active material layer, the surface treatment which forms an unevenness|corrugation on the copper foil surface called preliminary roughening process is mentioned. As the method of roughening treatment, there are known methods such as sand blasting, rolling by a rough surface roller, mechanical polishing, electrolytic polishing, chemical polishing and plating of electrodeposited particles, among which plating of electrodeposited particles is particularly Most commonly used. This technology is carried out for the following purposes: using copper sulfate acidic plating bath, a large amount of copper is electrodeposited in the shape of dendrites or small balls on the surface of copper foil to form fine unevenness, and the improvement of adhesion is sought through the anchoring effect; or in When the active material with a large volume change expands, stress is concentrated in the concave portion of the active material layer to form cracks, and it is sought to prevent peeling due to stress concentration at the interface of the current collector (for example, Japanese Patent No. 3733067).

In Japanese Patent No. 3733065, it is described that the preferred surface properties are specifically determined by roughness parameters, and the adhesion between the current collector and the active material is improved by using copper foil with a large value of surface roughness Ra as the current collector. Sex (paragraph 0209). The surface roughness Ra of the current collector is preferably 0.01 μm or more, more preferably 0.01 to 1 μm, and still more preferably 0.05 to 0.5 μm (paragraph 0021, etc.). The surface roughness Ra of the current collector and the average interval S of local peaks preferably have a relationship of 100Ra≥S (paragraph 0022, etc.). The shape of the concavities and convexities on the surface of the current collector is preferably a cone shape (paragraph 0023, etc.).

In addition, it is described that such a surface morphology can be roughened by electrolytically depositing copper on the surface of electrolytic copper foil (paragraph 0044) or rolled copper foil (paragraph 0045), and grinding with sandpaper (paragraph 0045). Paragraph 0205) to obtain.

prior art literature

patent documents

Patent Document 1: Japanese Patent No. 3733067

Patent Document 2: Japanese Patent No. 3733065.

Contents of the invention

The problem to be solved by the invention

Since the battery capacity of a lithium-ion battery changes depending on the coating amount of the negative electrode active material, it is important to control and manage the coating amount of the negative electrode active material in order to stabilize battery characteristics. The amount of coating in the cloth process is managed by the weight including the copper foil after coating. Therefore, if the thickness of the copper foil serving as a current collector is not constant, the amount of the negative electrode active material to be applied cannot be properly managed. Since the specific gravity of copper foil is about 8.92 g/cm ³ and the specific gravity of carbon used as the negative electrode active material is about 0.5 g/cm ³ , the thickness of carbon corresponding to the thickness of 0.1 μm copper foil is 1.78 μm, for example. Therefore, when the copper foil is prepared with a thickness of 10 μm as the target, although the thickness varies only by 0.1 μm (1.00%), the thickness of carbon will have an error of 1.78 μm (equivalent to 4.45% when the target thickness is 40 μm.) . This corresponds to a deviation of 4.45% in a 40 μm thick active material. That is, as a result, the slight thickness variation of the copper foil has a large influence on the thickness of the active material. Therefore, a copper foil excellent in plate thickness accuracy is desired.

However, in the direction of development of copper foils for current collectors so far, control of surface properties from a microscopic point of view for the purpose of improving adhesion to negative electrode active materials is overwhelming. Therefore, the problem of improving the plate thickness accuracy of the copper foil and realizing the capacity stability of the lithium-ion battery remains unsolved from a macroscopic point of view.

Therefore, one subject of the present invention is to provide a copper foil for a current collector of a lithium ion battery with high plate thickness accuracy. Moreover, another subject of this invention is providing the manufacturing method of such copper foil.

means of solving problems

Copper foil is roughly divided into rolled copper foil and electrolytic copper foil. In rolled copper foil, the thickness accuracy is mostly due to the function (capacity) of the rolling machine, but with the existing rolling machine, the thickness accuracy is limited to ±1.6% at the target thickness of 10 μm. Retrofitting or development of rolling mills is also expected as a fundamental measure, but it is difficult to carry out immediately because a large amount of research and development costs are required.

The inventors of the present invention have repeatedly studied to solve the above-mentioned problems under such actual conditions. Since the rolling is mostly a feed-forward thickness control in the production process of rolled copper foil, the thickness accuracy of the product is controlled by the final cold rolling. The deviation of the surface roughness before the final pass is one of the main factors affecting the control of the plate thickness. Focusing on the above situation, it was found that by reducing the surface roughness in the stage before the final pass, the deviation of the surface roughness was reduced. deviation, thereby improving the accuracy of plate thickness. Specifically, it was found that by using work rolls with a small surface roughness for rolling before the final pass and using work rolls with a desired surface roughness in the final pass, it is finally possible to obtain a plate with good thickness accuracy and a desired surface roughness of the copper foil. Copper foil for current collectors needs a certain surface roughness in consideration of the relationship with the adhesiveness of the active material. At the same time have the required surface roughness.

The present invention completed based on the above knowledge is, in one aspect, a copper foil for a current collector of a lithium ion battery, characterized in that the average value of surface roughness Ra (Ra _avg ) in the rolling direction parallel to the rolling direction is 0.01 to 0.15 μm, and ΔRa =Ra _max -Ra _min is 0.025 μm or less.

In one embodiment of the copper foil for a lithium ion battery current collector according to the present invention, the plate thickness of the copper foil is 5 to 20 μm.

In another embodiment of the copper foil for a current collector of a lithium ion battery according to the present invention, the difference between the maximum thickness (t _max ) and the average thickness (t _avg ) or the minimum value (t _min ) of the copper foil is Among the differences from the plate thickness average value (t _avg ), the ratio of any larger value to the plate thickness average value (t _avg ) is 1.3% or less.

In one embodiment of the copper foil for lithium ion battery current collectors according to the present invention, ΔRSm=RSm _max -RSm _min to the ratio of the average value of surface roughness RSm (RSm _avg ) in the direction parallel to rolling (ΔRSm/ RSm _avg ) is 0.5 or less.

In another embodiment of the copper foil for lithium ion battery current collectors concerning this invention, copper foil is for lithium ion secondary battery negative electrode current collectors.

In another aspect, the present invention is a lithium ion battery including the copper foil of the present invention as a current collector.

In yet another aspect, the present invention is a method for preparing copper foil for lithium-ion battery current collectors, which is characterized in that: in the final cold rolling process, the surface roughness Ra of the work rolls used in the final pass is 0.03 μm or more, The surface roughness Ra of the work rolls used in one pass immediately before the final pass is less than 0.03 μm.

The effect of the invention

Since the copper foil according to the present invention is excellent in plate thickness accuracy, it is possible to suppress variations in the coating amount of the negative electrode active material, so that the battery capacity of mass-produced lithium ion batteries can be stabilized.

Embodiment of the invention

The copper foil base material used in this invention is a rolled copper foil. "Copper foil" also includes copper alloy foil. The material of the copper foil is not particularly limited, and may be appropriately selected according to the application or required characteristics. For example, although not limited, in addition to high-purity copper (oxygen-free copper or ductile copper, etc.), copper alloys added with Sn, Ag, Fe, In, Te, etc., copper alloys with Ni, Si, etc. Cu-Ni-Si-based copper alloys, Cu-Zr-based and Cu-Cr-Zr-based copper alloys to which Cr, Zr, etc. are added. Rolled copper foil is excellent in that it has high strength, can cope with environments where vibrations continuously occur, and has high bending resistance.

The thickness of the copper foil is not particularly limited, and may be appropriately selected according to required characteristics. Usually 1~100μm, but when used as a current collector for the negative electrode of a lithium-ion secondary battery, making the copper foil thinner can obtain a higher-capacity battery. From such a viewpoint, the typical thickness is 2 to 50 μm, and the more typical thickness is about 5 to 20 μm.

The copper foil according to the present invention is defined by the average value (Ra _avg ) of the surface roughness Ra in the rolling direction parallel to it, and ΔRa=Ra _max −Ra _min . Ra is the numerical value obtained by dividing the area obtained by the roughness curve and the center line by the reference length L by inflection of the roughness curve from the center line, and is measured in accordance with JIS B0601: 2001. In the present invention, the average value (Ra _avg ) of surface roughness Ra is the average value of arbitrary 10 points, and in the present invention, ΔRa is Ra max as the maximum value and Ra _max as the minimum value among the measured 10 points of Ra. The difference between the Ra _min values. However, the so-called arbitrary 10 points here do not refer to 10 points where the measurement points are adjacent to each other, but, for example, in the case of a coil shape, at least 150 mm in the rolling direction, depending on the obtained length, Ten points are selected at intervals of preferably 400 mm, more preferably at intervals of 1 m or more. The Ra of each measurement point was obtained from the average value of 3 measurements in the vicinity of the measurement point. It should be noted that Ra at the center in the width direction was taken at each measurement point. In addition, when the battery is disassembled, even if it is a sheet in which a plurality of copper foils for negative electrodes are stacked, if a measurement interval of 150 mm or more can be ensured, the surface roughness of the sheet can be measured.

The copper foil according to the present invention is characterized in that the average value of surface roughness Ra (Ra _avg ) in the rolling direction parallel to the rolling direction satisfies 0.01 to 0.15 μm. The reason for the condition of 0.01 μm ≤ Ra ≤ 0.15 μm is that if Ra is less than 0.01 μm, the surface will be smooth and sufficient adhesion to the negative active material cannot be obtained. On the other hand, if it exceeds 0.15 μm, even if it passes through the final pass The rolling before the next pass reduces the roughness so that the variation in surface roughness is small, and variation also occurs in the rolling of the final pass. However, from the viewpoint of stabilizing the appearance quality with few surface defects such as surface flaws, Ra is preferably 0.03 μm or more, and 0.03 μm≦Ra≦0.1 μm is a more preferable range.

In addition, it is also characterized by satisfying ΔRa=Ra _max -Ra _min and being 0.025 μm or less. The reason why ΔRa=Ra _max -Ra _min is 0.025 μm or less is the reason: as a product, if ΔRa of copper foil after final rolling is 0.025 μm or less, it can mean that ΔRa before the final pass of final rolling 0.025 μm or less. If the ΔRa before the final pass of final rolling is 0.025 μm or less, the influence on thickness control due to surface roughness deviation (variation) in the final pass of final rolling is small, and the final pass, that is, the product The plate thickness accuracy is improved. When ΔRa exceeds 0.025 μm, ΔRa before the final pass of final rolling usually exceeds 0.25 μm. The influence of the thickness control of the pass is different, and as a result, the variation in the final rolled thickness under this condition increases. ΔRa is preferably 0.025 μm or less, more preferably 0.020 μm or less.

On the other hand, in rolled copper foil, in addition to the surface roughness determined by the roll mesh, there are many depressions unique to rolled copper foil called oil pits on the surface. The oil pit is a depression produced by pressing the rolling oil into the rolled material, and the density of the oil pit on the surface is different according to the thickness of the oil film of the rolling oil. If the density of the oil pits on the surface is different, it also affects the thickness of the copper foil obtained by the gravimetric method, and becomes a main factor of variation. Therefore, it is desirable that the oil pits are evenly distributed on the surface of the copper foil.

The amount of oil pit generation can be indexed by the surface roughness RSm in the rolling direction. When RSm is large, it means that there are few oil pits on the surface, and when RSm is small, it means that the amount of oil pits is large. Since the determination of plate thickness accuracy _is affected by the deviation of the distribution _of oil pits, the ratio (ΔRSm/ _{RSm avg} ) as the index. The smaller the ΔRSm/RSm _avg , the more uniform the oil pits are distributed on the surface of the copper foil. The reason for dividing by RSm _avg is that even if ΔRSm is large in the distribution deviation, the deviation may not necessarily be large. That is, for example, even with the same ΔRSm, if RSm _avg is large, the influence is small because the variation in distribution is not large, and when RSm _avg is small, the influence is large because the variation in distribution is large.

By increasing the rolling speed, increasing the viscosity of rolling oil, or reducing the compression rate per pass, the amount of oil pits increases and RSm is easy to decrease. On the contrary, by slowing down the rolling speed, reducing the viscosity of rolling oil, or increasing the compression rate per pass, the generation of oil pits is reduced, and RSm is easy to increase.

RSm is the average value of the trough-period interval obtained from the intersection point of the roughness curve and the mean line, and is measured in accordance with JIS B0601: 2001. In the present invention, the average value (RSm _avg ) of surface roughness RSm is the average value of arbitrary 10 points, and ΔRSm is RSm _max which is the maximum value and RSm _min which is the minimum value among Ra measured at 10 points. Difference. However, the so-called arbitrary 10 points here do not refer to 10 points where the measurement points are adjacent to each other, but, for example, in the case of a coil shape, at least 150 mm in the rolling direction, depending on the obtained length, Ten points are selected at intervals of preferably 400 mm, more preferably at intervals of 1 m or more. The RSm of each measurement point was obtained from the average value of three measurements in the vicinity of the measurement point. It should be noted that RSm at the center in the width direction is taken for each measurement point. In addition, when the battery is disassembled, even if it is a sheet in which a plurality of copper foils for negative electrodes are stacked, if a measurement interval of 150 mm or more can be ensured, the surface roughness of the sheet can be measured.

In one preferable embodiment of the copper foil which concerns on this invention, ΔRSm/RSm _avg is 0.5 or less.

In a preferred embodiment of the copper foil related to the present invention, the difference between the maximum thickness (t _max ) and the average thickness (t _avg ) of the copper foil or the minimum value (t min ) and the average thickness (t _min ) of the copper foil ( The ratio of any larger value of the difference between t _avg ) to the plate thickness average value (t _avg ) can be set to 1.3% or less. This ratio may also be preferably set to 1.2% or less, and may be more preferably set to 1.1% or less.

Next, the manufacturing method of the copper foil which concerns on this invention is demonstrated. The control of the surface roughness Ra can be carried out by adjusting the surface roughness of the work roll. For example, if a work roll with a large Ra is used, the Ra of the rolled copper foil obtained will also increase. Conversely, if a work roll with a small Ra is used , the Ra of the obtained rolled copper foil is also reduced. On the other hand, generally, when the average value is large, the deviation value itself also increases. The same is true for the deviation value of the surface roughness Ra. If the average value of the surface roughness Ra is large, the deviation value is also large. Therefore, in order to reduce the deviation value of the surface roughness Ra, it is only necessary to reduce the average value of the surface roughness Ra.

However, in various products, since there is a requirement for surface roughness from the viewpoint of adhesion to the negative electrode active material, etc., it is necessary to finally obtain the required value. In addition, in cold rolling, the surface roughness is preferably rough to some extent from the viewpoint of rolling efficiency that can set a high rolling speed.

Therefore, for example, a work roll with a small surface roughness is used only in the pass immediately before the final pass of the final cold rolling to produce a copper foil with a small surface roughness, that is, a smooth surface, and a work roll with a large surface roughness is used in the final pass. The work roll is made to the required surface roughness Ra.

Thereby, a copper foil having desired surface roughness while obtaining high thickness precision and having good adhesion to an active material can be obtained. That is, it is only necessary to use a roll with a rough surface roughness Ra two passes before the final pass, and use a roll with a roughness smaller than that of the preceding pass and the final pass only one pass immediately before the final pass.

Work rolls with a small surface roughness can be used not only in the pass immediately before the final pass, but also in the pass before it. However, rolls with a small surface roughness cannot increase the rolling speed, so they are not suitable from the viewpoint of productivity. expect. Therefore, the surface roughness of the work rolls used in the pass just before the final pass is generally reduced. However, regardless of the viewpoint of productivity, the effect of reducing surface roughness variation is high by using a roll with a smaller surface roughness in the pass one pass before the final pass. For example, it is also effective to use a roller with a small surface roughness only in the two passes immediately before the final pass.

In the final pass, in order to make the average value of Ra (Ra _avg ) in the rolling direction of the copper foil 0.01~0.15μm, the work roll uses a roll with a surface roughness Ra exceeding 0.01μm, so in order to reduce the surface roughness As for the deviation value, the surface roughness Ra of the work roll used in one pass immediately before the final pass must be smaller than that of the work roll used in the final pass. Therefore, the surface roughness Ra of the work rolls used in one pass immediately before the final pass is preferably 0.01 μm or less.

However, stably producing a roll having a surface roughness Ra of 0.01 μm or less and free from appearance problems such as surface damage requires high technology, resulting in high cost.

Therefore, a more preferable range is that the surface roughness Ra of the work rolls used in the final pass is preferably 0.03 μm or more, so the surface roughness Ra of the work rolls used in one pass immediately before the final pass is preferably less than 0.03 μm.

In order to reduce the variation in surface roughness RSm, it is important to make the distribution of oil pits uniform. In order to make the distribution of oil pits uniform, it is important among several main factors to keep the viscosity of the rolling oil constant during rolling. The viscosity of the rolling oil is basically determined by the type of the rolling oil, but the temperature of the rolling oil gradually rises due to the processing heat during rolling, so that the viscosity decreases. If the degree of rolling oil pressure into the surface of copper foil changes with the viscosity of the rolling oil, it will lead to deviations in the distribution of oil pits.

For example, when the rolling oil is kept at about 25°C during the temperature adjustment before rolling, if the rolling oil is sprayed onto the work rolls during rolling, the heat from the work rolls, etc., which are heated up due to processing heat will be transmitted. Heat to heat up the rolling oil to around 40°C. If this state can be maintained, the distribution variation of the oil pits will be small, and there will be no problem with the shape of the copper foil. However, if the temperature control of the rolling oil is insufficient and the temperature of the rolling oil exceeds 40°C, deviations occur, not only the surface properties of the copper foil tend to vary, but also the shape of the sheet is affected. Therefore, in order to adjust the temperature of the rolling oil to about 40°C during rolling, it is necessary to comprehensively adjust the temperature of the rolling oil before spraying the rolls, the rolling speed, the processing degree, etc.

A lithium ion battery can be produced by a conventional method using a negative electrode composed of a current collector made of the rolled copper foil according to the present invention and an active material layer formed thereon. Lithium-ion batteries include lithium-ion primary batteries and lithium-ion secondary batteries in which lithium ions in the electrolyte conduct electricity. The negative electrode active material is not limited, but examples include carbon, silicon, tin, germanium, lead, antimony, aluminum, indium, lithium, tin oxide, lithium titanate, lithium nitride, tin oxide with indium in solid solution, indium- Tin alloy, lithium-aluminum alloy, lithium-indium alloy, etc.

Example

Examples of the present invention are shown below, but they are provided for better understanding of the present invention and are not intended to limit the present invention.

<Example 1 (Influence of surface roughness Ra deviation)>

[Preparation of rolled copper foil]

After the ductile copper ingot is hot rolled, annealing and cold rolling are repeated, and finally cold rolling is performed to obtain rolled copper foil (No.1~6) with a length of more than 10m in the rolling direction and a set thickness of 10μm. In the final cold rolling, the surface roughness of the work roll used in the pass just before the final pass and the surface roughness of the work roll used in the final pass are shown in Table 1. The viscosity of the rolling oil used is 7.0cSt (40°C), and the temperature of the rolling oil in the final cold rolling is controlled at about 40°C. The surface roughness of the work rolls was measured using a contact surface roughness meter in accordance with JIS B0601: 2001.

The obtained rolled copper foil was placed and fixed on a glass plate, and Ra _avg , ΔRa, RSm _avg , and ΔRSm were calculated based on the above-mentioned measurement method using a confocal microscope HD100D from Razer Tech. The results are shown in Table 1.

[Evaluation of Plate Thickness Accuracy]

The plate thickness of the rolled copper foil is measured according to the gravimetric method (IPC-TM-650). A length in the rolling direction of 10 m was arbitrarily selected from the obtained copper foil, and the plate thickness at 10 points was measured at intervals of 1 m. The plate thickness T at each measurement point is the average value of three measurements. The average value of T at 10 points is taken as T _avg , the maximum value of T at 10 points is taken as T _max , and the minimum value of T at 10 points is taken as T _min . In Table 1, the larger of (T _avg -T _min )/T _avg and (T _max -T _avg) /T _avg is described as "sheet thickness variation (%)".

No.1~No.4 are invention examples, which can suppress the deviation of sheet thickness to 1.3% or less.

In No. 5, ΔRa could not be sufficiently controlled because the surface roughness of the pass just before the final pass was large. Instead of increasing the surface roughness of the work rolls in one pass immediately before the final pass, No. 6 decreased the surface roughness of the work rolls in the final pass, but still could not sufficiently control ΔRa

[Table 1]

。

.

<Example 2 (Influence of oil pit distribution)>

[Preparation of rolled copper foil]

After the ductile copper ingot is hot rolled, annealing and cold rolling are repeated, and finally cold rolling is performed to obtain rolled copper foil (No.7~12) with a length of more than 10m in the rolling direction and a set thickness of 10μm. In the final cold rolling, the surface roughness Ra of the work rolls used up to the final pass was set to 0.010 μm, and the surface roughness Ra of the work rolls used in the final pass was set to 0.050 μm. The viscosity of the rolling oil used is 7.0cSt (40°C), and the invention example adjusts the temperature of the rolling oil in the final cold rolling to about 40°C. Various property evaluations were performed in the same manner as in Example 1. The test results are shown in Table 2.

Inventive examples Nos. 7 to 9 control the rolling oil temperature of the final rolling mill at 40°C, so that the distribution of oil pits is uniform, the deviation is reduced, and the deviation of the plate thickness is as small as less than 1.2%.

Invention Examples No. 10 to 12 were carried out under the same conditions as Invention Examples Nos. 7 to 9 except temperature control of rolling oil in the final cold rolling mill. Among them, since the temperature of the rolling oil in the final cold rolling mill was not sufficiently controlled, it exceeded 40°C and rose to about 45°C. Although it could not be confirmed by the measurement, it is presumed that there are parts exceeding 50°C locally. As a result, the distribution of the oil pits could not be made uniform, and variations in plate thickness exceeding 1.2% were observed.

[Table 2]

。

.

Claims (7)

1. Copper foil for lithium ion battery current collector, characterized in that the average value of surface roughness Ra (Ra _avg ) in the rolling direction parallel to the rolling direction is 0.01~0.15 μm, and ΔRa=Ra _max -Ra _min is 0.025 μm or less. 2. The copper foil for lithium ion battery current collector according to claim 1, characterized in that: the thickness of the copper foil is 5-20 μm. 3. The copper foil for lithium ion battery current collector according to claim 1 or 2, characterized in that: the difference between the maximum thickness (t _max ) and the average thickness (t _avg ) or the minimum value (t _min ) of the copper foil The ratio of any larger value of the difference from the plate thickness average value (t _avg ) to the plate thickness average value (t _avg ) is 1.3% or less. 4. The copper foil for lithium-ion battery current collector according to any one of claims 1 to 3, characterized in that: ΔRSm=RSm _max -RSm _min is the average value of surface roughness RSm (RSm _avg ) relative to the rolling direction parallel The ratio (ΔRSm/RSm _avg ) is 0.5 or less. 5. The copper foil for current collectors of lithium ion batteries according to any one of claims 1 to 4, wherein said copper foil is used for negative electrode current collectors of lithium ion secondary batteries. 6. Lithium ion battery, it has the copper foil any one in the claim 1～5 as current collector. 7. A method for preparing copper foil for lithium-ion battery current collectors, characterized in that: in the final cold rolling process, the surface roughness Ra of the work rolls used in the final pass is 0.03 μm or more, and one month before the final pass The surface roughness Ra of the work rolls used in the pass is less than 0.03 μm.