JP2018516175A - Coagulation equipment - Google Patents

Abstract

In one embodiment of the present invention, the correlation between the molten metal surface height fluctuation phenomenon and the traveling speed of the metal plate B can be lowered, the traveling speed of the metal plate is increased, and further, the productivity of the metal plate is increased. A coagulation apparatus is provided to increase the height. A solidification apparatus according to one aspect of the present invention is a solidification apparatus for producing a metal plate by solidifying molten metal, and includes a roller array disposed along a traveling direction of the metal plate, and the roller array includes a plurality of roller arrays. When divided into sections, the average roller pitch, which is the distance between the centers of two rollers adjacent to each other in the traveling direction of the metal plate in a section, is smaller than the average roller pitch in the preceding section.
[Selection] Figure 1

Description

  The present invention relates to a solidification apparatus for producing a steel sheet by continuously solidifying molten metal.

In general, an apparatus for continuously solidifying molten metal includes a mold frame into which molten metal is introduced and a plurality of roller rows arranged side by side on the discharge side of the mold frame. Usually, the plurality of roller rows are provided in segment units.
On the other hand, the molten metal is solidified in the mold and the inside is not solidified, that is, it is discharged in a semi-solid state, and gradually solidifies inside while passing between the roller rows, completely solidifying. And then removed from the device.

  Here, the metal plate formed by solidification of the molten metal is in a semi-solid state for a considerable period of time when passing between the roller rows, so that the molten metal remains as it is inside the metal plate. ing. Therefore, since a considerable pressure is applied to the inside of the metal plate due to the height difference from the molten metal surface in the mold, so as to withstand this load, a roller, a roller bearing, a roller support structure, Is installed.

  Further, as the metal plate moves downward, the pressure increases due to the difference in height from the molten metal surface, so that the diameter of the roller positioned below is usually increased. As the diameter increases, the roller pitch, which is the distance between the centers of two rollers adjacent to each other in the traveling direction of the metal plate, also increases.

  On the other hand, the roller in contact with the surface of the metal plate suppresses the phenomenon that the metal plate expands outside due to the pressure of the molten metal inside. However, when the roller pitch increases at this time, the metal plate and the roller are between the rollers. The section where no contact is made widens, and the surface of the metal plate expands somewhat.

That is, as shown in FIG. 1, in the process of passing between the rollers, the solidified layer of the metal plate expands little by little in the portion where the rollers are not in contact.
Therefore, in order to prevent the solidified layer from expanding, it is preferable to arrange the rollers as finely as possible to make the pitch of the rollers as small as possible.

  On the other hand, when the phenomenon of expansion of the solidified layer occurs, the amount of expansion of the solidified layer may periodically increase or decrease. It is estimated that such a phenomenon causes fluctuations in the height of the molten metal surface. Further, such a phenomenon causes a change in the volume inside the semi-solid metal plate, and the metal plate advances between the rollers in such a state, so that the height of the molten metal surface in the mold is periodically changed. Cause fluctuations.

  It is observed that the period of fluctuation of the height of the melt surface is closely related to the roller pitch interval. Further, it is empirically known that the fluctuation in the height of the molten metal surface tends to increase as the moving speed of the metal plate increases and the roller pitch increases, but its accurate correlation mechanism (Mechanism). Is not clear.

  On the other hand, if the height fluctuation of the molten metal surface exceeds the allowable limit, the phenomenon of non-uniform solidification occurs, which not only adversely affects quality, but in severe cases, the fragile part of the solidified layer may burst. There is a possibility that unsolidified molten metal leaks to the outside and production is interrupted, resulting in serious damage such as damage to production facilities.

  For this reason, in many cases, the work is performed by a method in which the traveling speed of the metal plate is limited within the allowable range of the height fluctuation of the molten metal surface, which causes a reduction in the productivity of the metal plate.

  One embodiment of the present invention eliminates the repeatability of the roller pitch in addition to changing the arrangement structure of the rollers in order to reduce the periodic fluctuations of the molten metal surface, and thereby to the traveling speed of the metal plate. It is an object of the present invention to provide a solidification apparatus that increases the traveling speed of a metal plate and further increases the productivity of the metal plate by relatively reducing the correlation of fluctuations in the height of the molten metal surface.

  A solidification apparatus according to one aspect of the present invention is a solidification apparatus for producing a metal plate by solidifying molten metal, and includes a roller array disposed along a traveling direction of the metal plate, and the roller array is divided into a plurality of sections. When divided, the average of the roller pitch, which is the distance between the centers of two rollers adjacent to each other in the traveling direction of the metal plate in a certain section, is smaller than the average of the roller pitch in the preceding section.

In addition, the average roller pitch in a certain section may be smaller than the average roller pitch in a subsequent section.
The section may be a space having at least four roller pitches.
Moreover, it is preferable that the said area is the space divided including the roller arrange | positioned by the length of 1 m or more.

Further, the section may be a space divided by physical sections.
The physical partition may be a segment unit.
Further, the section may be a space between a certain drive roll and the next drive roll.

The difference between the average roller pitch in the certain section and the average roller pitch in the preceding section is 10 mm or more.
The average difference in roller pitch is preferably 20 mm or more.
The moving speed of the metal plate may be 4 m / min or more.

  Further, the section includes a horizontal section for horizontally transferring the metal plate, and a curved section disposed so as to be curved with a predetermined curvature between the horizontal section and the outlet of the mold from which the molten metal is discharged. Including an inflection section provided at least one of the exit section and the curved section or between the curved section and the horizontal section and arranged with different curvatures. And

  Further, the apparatus may further include a vertical section that is vertically connected to the outlet, vertically transports the metal plate, and is connected to the curved section.

ここで、Ｎは、該当区間で定義されたローラピッチの数であり、ｋは、区間番号であり、ｉは、ｋ区間で示されるローラピッチの一連番号である。 In addition, the average roller pitch in the section can be defined by Equation 1 below.

Here, N is the number of roller pitches defined in the corresponding section, k is a section number, and i is a series number of roller pitches shown in the k section.

  Further, a section where the average roller pitch is smaller than the average roller pitch in the preceding section exists within the length of solidification of the metal plate, and the solidification length of the metal plate is expressed by the following Equation 2. Can be defined.

ここで、Ｌ：金属板の凝固完了長さ（ｍｍ）、Ｖ：金属板の１分あたりの進行速度（ｍｍ／ｍｉｎ）、Ｈ：金属板の厚さ、Ｋ：２０〜３０の凝固定数（ｍｍ／ｍｉｎ^１／２）である。

Here, L: completion length of solidification of metal plate (mm), V: traveling speed per minute of metal plate (mm / min), H: thickness of metal plate, K: solidification constant of 20-30 ( mm / min ^1/2 ).

  Further, there may be two or more sections where the average roller pitch is smaller than the average roller pitch in the preceding section.

  According to an embodiment of the present invention, the correlation between the height fluctuation phenomenon of the molten metal surface in the mold and the traveling speed of the metal plate B can be lowered, thereby increasing the traveling speed of the metal plate. Can do.

It is the conceptual diagram which showed the advancing state of the metal plate simply. It is the schematic of the coagulation apparatus which concerns on one Example of this invention. (A)-(c) is the simple figure which showed the roller row | line | column of the solidification apparatus which concerns on one Example of this invention. It is the graph which showed the average of the roller pitch according to the section of the coagulation apparatus which concerns on one Example of a present Example, and the average of the conventional roller pitch according to a section. 1 is a diagram illustrating a relationship between a solidification length, a speed, and a thickness of a metal plate in an embodiment of the present invention. It is the graph which showed the average of the roller pitch according to section of the solidification apparatus concerning one example of this example.

  Hereinafter, an embodiment of the present invention will be described in detail with reference to the accompanying drawings. Embodiments of the present invention can be modified in various other forms, and the scope of the present invention is not limited to only the embodiments described below. The shape and size of elements in the drawings may be exaggerated for a clearer description, and elements indicated by the same reference numerals in the drawings are the same elements.

FIG. 2 is a schematic view of a coagulation apparatus according to an embodiment of the present invention, and FIG. 3 is a simplified diagram illustrating a roller train of the coagulation apparatus according to an embodiment of the present invention.
As shown in FIGS. 2 and 3, in this embodiment, the solidification apparatus 10 includes a mold 20 that cools the molten metal Y supplied from the tundish 1 and discharges it as a metal plate B at the initial stage of solidification. it can.

  Here, the mold 20 is provided with cooling means inside, whereby the molten metal that contacts the mold 20 starts to be cooled from the surface and is discharged in the form of the metal plate B. Here, the inside of the surface solidified as the metal plate B can be discharged in an unsolidified state.

  Further, the discharge side of the mold 20 can include a row of rollers arranged along the traveling direction of the metal plate to guide the movement of the metal plate B discharged in a semi-solid state. The roller row can constitute the transfer unit 30 in cooperation with the drive unit.

The transfer unit 30 cools and solidifies the semi-solidified metal plate B and transfers it to the subsequent process.
In the present embodiment, the transfer unit 30 can be divided into a plurality of sections according to the average roller pitch of the roller rows.

  Here, as shown in FIG. 3A, the section may be a space having at least four roller pitches. That is, the section can be a space including five rollers, and can be defined as a space having at least four roller pitches, which is the distance between the centers of these rollers. In this embodiment, since the roller pitch is an important factor causing the height fluctuation of the molten metal surface, the section is preferably defined with respect to the roller pitch rather than with respect to the number of rollers.

ここで、Ｎは、該当区間で定義されたローラピッチの数であり、ｋは、区間番号であり、ｉは、ｋ区間で示されるローラピッチの一連番号である。 Further, the average value of the roller pitch in each section can be expressed by the following formula 1.

Here, N is the number of roller pitches defined in the corresponding section, k is a section number, and i is a series number of roller pitches shown in the k section.

  On the other hand, in this embodiment, each section is defined as a space having at least four roller pitches. However, in each section, the roller arrangements of symmetrical sections facing each other need not be the same, but defined as the same section. Therefore, it is preferable that each section is defined as an independent section.

Furthermore, in this embodiment, each section is defined as a space having at least four roller pitches, but the definition of the section is not limited, and other than that, the section is made to correspond to the length. It may be defined.
That is, in the present embodiment, the section can be a space defined by including rollers arranged with a length of 1 m or more.

  In the present embodiment, when the length of each section is less than 1 m, the rollers are arranged in a fairly fine state, and the height fluctuation of the molten metal surface hardly occurs. On the other hand, in the present embodiment, the section can be limited to a space that includes a roller arranged with a sufficient length as long as the equipment condition allows.

On the other hand, the section may be defined as a physically partitioned space.
As an example, the physically partitioned space may be a space partitioned in segment units as shown in FIG. The segments can be provided with a number of rollers that guide the movement of the metal plate, and these roller rows can be physically partitioned so that they can be replaced, replaced, etc. by the segments.

Moreover, as shown in FIG.3 (c), the physically partitioned space can be a space between a certain drive roll and the next drive roll other than a segment.
Typically, a segment can be provided with one drive roll, for example, a drive roll can be provided in the middle of the segment. The segment can include a number of roller rows in addition to the drive roll.

  Here, the space between one drive roll and the next drive roll can be a space between the drive roll of the preceding segment and the drive roll of the subsequent segment. It can be a space defined by spaces between drive rolls of structures provided in various forms, and a large number of roller rows can be arranged in this space.

  In this way, the roller train divided into a plurality of sections has an average roller pitch, which is the distance between the centers of two rollers adjacent to each other in the metal traveling direction in the normal section, than the average roller pitch in the preceding section. It may be large or identical.

  This is because the load increases due to an increase in the height difference between the molten metal surface and the metal plate B as the roller row in which the metal plate B travels goes to the subsequent section, thereby increasing the pressure acting on the roller row. It is because it comes to do.

  On the other hand, in this embodiment, in a certain section of the plurality of sections, the average roller pitch that is the distance between the centers of two rollers adjacent in the traveling direction of the metal plate is greater than the average roller pitch in the preceding section. It may be small. When these roller trains are divided into a plurality of sections, the average roller pitch, which is the distance between the centers of two rollers adjacent to each other in the traveling direction of the metal plate in a section, is greater than the average roller pitch in the preceding section. It may be small.

Preferably, when the average roller pitch that is the distance between the centers of two rollers adjacent in the traveling direction of the metal plate in a certain section is smaller than the average roller pitch in the preceding section,
The average roller pitch in a certain section may be smaller than the average roller pitch in the following section.

  That is, there may be a reverse rotation section in which the average roller pitch in a certain section is smaller than the average roller pitch in the preceding section and the following section.

FIG. 4 is a graph showing the average of the roller pitch by section and the average of the roller pitch by conventional section of the solidification apparatus according to one embodiment of the present embodiment. Here, the x-axis indicates the section, and the y-axis indicates the average roller pitch.
Preferably, the average roller pitch for each section is as shown in the graph of FIG.

As shown in FIG. 4, in this embodiment, it is significant that the casting speed can be increased if the difference between the average roller pitch and the average roller pitch in the preceding section is 10 mm or more with a certain section as a reference. I was able to get a good result.
That is, when the average roller pitch in a certain section was reduced by less than 10 mm from the average roller pitch in the preceding section, the height fluctuation of the molten metal surface in the mold was not affected.

On the other hand, when the average of the roller pitch in a certain section was reduced by 10 mm or more than the average of the roller pitch in the preceding section, fluctuations in the height of the melt surface in the mold could be mitigated.
For example, on the basis of a certain section, when the average of the roller pitch is reduced by about 17 mm compared to the average of the roller pitch in the preceding section, as the fluctuation in the height of the molten metal surface in the mold is alleviated, The casting speed could be improved by about 0.5 m / min compared to the technology. That is, it was found that the casting speed was 6.5 m / min in the prior art, but after applying this technique to reduce the average roller pitch, the casting speed was 7.0 m / min. .

  Preferably, when the average of the roller pitch in a certain section is reduced by 20 mm or more with respect to the average of the roller pitch in the preceding section, the degree of relaxation of the height fluctuation of the molten metal surface in the mold is further greatly improved. Can do.

  For example, when the average of the roller pitch is reduced by about 38 mm compared to the average of the roller pitch in the preceding section with a certain section as a reference, the fluctuation in the height of the molten metal surface in the mold is alleviated. The casting speed could be improved by about 1.5 m / min compared to the prior art. That is, in the prior art, when the casting speed was 6.5 m / min and the average reduction width of the roller pitch was about 1.7 mm, the casting speed was 7.0 m / min, but compared with the average of the roller pitch. It was found that the casting speed could be 8.0 m / min after reducing by about 38 mm.

  Therefore, as shown by the above data, it can be seen that the most preferable result is that the average roller pitch is preferably 20 mm or more with respect to the average roller pitch in the preceding section.

  However, the upper limit may be determined by the allowable load of the equipment to the extent that the average roller pitch can be reduced. In general, increasing the average reduction in roller pitch can add significant loads to the drive structures such as the support structures and bearings that support the rollers, which allows these facilities to withstand the average reduction in roller pitch. The upper limit of the reduction width can be determined by the applied load.

  Thus, it can be seen that the average roller pitch in a certain section may be smaller than the average roller pitch in the preceding section, which can alleviate fluctuations in the height of the melt surface in the mold. Further, after the average roller pitch in a certain section becomes smaller than the average roller pitch in the preceding section, the average roller pitch in the subsequent section can be increased again.

FIG. 5 is a view showing the relationship between the solidification length, speed, and thickness of a metal plate in one embodiment of the present invention.
As shown in FIG. 5, in this embodiment, the length required for the metal plate B to completely solidify can be shortened by increasing the moving speed of the metal plate B.

  The reason why the moving speed of the metal plate B can be increased in this way is that the average fluctuation of the roller pitch and the metal surface B before the metal plate B is completely solidified by a section smaller than the preceding section and the metal surface This is because the correlation with the traveling speed of the plate is relatively low.

  As described above, the section in which the average roller pitch is smaller than the preceding section can exist within the length before the metal plate B is completely solidified, whereby the metal plate B is completely solidified. By reducing the pressure generated by the difference in height from the molten metal surface in the mold before, it is possible to suppress the phenomenon of expanding outward due to the molten metal pressure in the process of moving the metal plate B.

  Therefore, in the present embodiment, the maximum length of the section in which the reverse rotation section where the average roller pitch is smaller than the preceding section is formed is the same as the length necessary for the metal plate B to solidify, and the length thereof. Can be defined by Equation 2 using the moving speed V of the metal plate B as a function.

ここで、Ｌ：金属板の凝固完了長さ（ｍｍ）、Ｖ：金属板の１分あたりの進行速度（ｍｍ／ｍｉｎ）、Ｈ：金属板の厚さ、Ｋ：２０〜３０の凝固定数（ｍｍ／ｍｉｎ^１／２）である。

Here, L: completion length of solidification of metal plate (mm), V: traveling speed per minute of metal plate (mm / min), H: thickness of metal plate, K: solidification constant of 20-30 ( mm / min ^1/2 ).

  Preferably, in the solidification apparatus of the present embodiment, the average roller pitch, which is the distance between the centers of two rollers adjacent in the traveling direction of the metal plate in a certain section, is smaller than the average roller pitch in the preceding section. By doing so, the fluctuation of the height of the molten metal surface in the mold can be relaxed, and thereby the moving speed of the metal plate can be maintained at 4 m / min or more.

Further, in the present embodiment, as shown in FIG. 2, the transfer unit 30 includes a vertical section 100 that is vertically connected to the outlet side of the mold 20, a horizontal section 300 that is connected to the subsequent process, and a vertical section 100. And a curved section 200 arranged so as to be curved between the horizontal section 300 and the horizontal section 300.
At this time, an inflection section 400 that bends at another inclination may be disposed between at least one of the vertical section 100 and the curved section 200 and between the curved section 200 and the horizontal section 300.

  In addition, the transfer unit 30 may be provided with at least one segment in the vertical section 100, and the inflection section 400 disposed between the vertical section 100 and the curved section 200 may have a traveling direction of the metal plate. The average roller pitch that is the distance between the centers of two rollers adjacent to each other can be smaller than the average roller pitch in the preceding section.

  Thus, in the subsequent section after the reverse rotation section where the average roller pitch, which is the distance between the centers of the two rollers adjacent to each other in the traveling direction of the metal plate, is smaller than the average roller pitch in the preceding section, The average pitch can be increased again.

  Meanwhile, in the present embodiment, the transfer unit 30 has been described as being divided into the vertical section 100, the curved section 200, and the horizontal section 300. However, the transfer section 30 may be formed of only the vertical section 100. The curved section 200 without the 100 may be directly connected to the outlet of the mold 20 and may be formed to include the horizontal section 300. In addition, the inflection section 400 is a section in which the bending inclination changes, but if the inclination of the bending section 200 is kept constant, the separate inflection section 400 may not be included.

  Preferably, there may be two or more sections in which the average roller pitch is smaller than the average roller pitch in the preceding section, thereby reducing the height fluctuation of the molten metal surface in the mold. May be further increased.

  The height fluctuation phenomenon of the molten metal surface in the mold 20 increases as the traveling speed of the metal plate B increases, and may increase as the arrangement in which the average roller pitch of the rollers increases constantly is repeated. Accordingly, by increasing the average roller pitch of the rollers and minimizing the repeatability of the arrangement, the correlation between the molten surface height fluctuation phenomenon and the traveling speed of the metal plate B can be lowered. As a result, when the fluctuation in the height of the molten metal surface is maintained at the conventional level, the traveling speed of the metal plate B can be increased as a result.

FIG. 6 is a graph showing the average roller pitch by section of the coagulation apparatus according to one embodiment of the present embodiment. Here, the x-axis indicates the section, and the y-axis indicates the average roller pitch.
As shown in FIG. 6, in this embodiment, the average roller pitch in a certain section is less than 10 mm in decrease relative to the average roller pitch in the preceding preceding section, and the average roller pitch in the following section is slightly increased. However, if the total reduction degree with the average of the roller pitch in the subsequent section is 10 mm or more, the degree of relaxation of the height fluctuation of the molten metal surface in the mold can be improved.

That is, even if the average of each roller pitch within a certain section is not sufficiently reduced to 10 mm or more, if the total of the average roller pitch within a continuous section tends to decrease to 10 mm or more, A significant result was obtained that the fluctuation of the height of the melt surface in the mold was alleviated.
Specifically, the average roller pitch reduction range from D1 to D2 is 8 mm, and after the roller pitch average increases by 4 mm from D2 to D3, the average roller pitch can be reduced by 8 mm from D3 to D4. .

At this time, the average reduction degree of the roller pitch between the sections is less than 10 mm, and the height fluctuation of the melt surface in the mold is alleviated only by the average reduction degree of the roller pitch between the adjacent sections. Can't get meaningful results.
On the other hand, the total average decrease in the roller pitch between D1 and D4 can be 12 mm. As a result, the average decrease in the roller pitch is 10 mm or more as a whole, and the height fluctuation of the melt surface in the mold is changed. Can be relaxed.

  Thus, the average roller pitch can be increased again in the subsequent section (for example, the section after D5) after decreasing in the preceding section (for example, the section between D1 and D4).

  The present invention is not limited by the above-described embodiments and the accompanying drawings, and various forms of substitutions, modifications, and changes are possible without departing from the technical idea of the present invention described in the claims. It is obvious to those who have ordinary knowledge in the art.

The present invention relates to a solidification apparatus that continuously solidifies molten metal to produce a metal plate .

ここで、Ｌ：金属板の凝固完了長さ（ｍｍ）、Ｖ：金属板の１分あたりの進行速度（ｍｍ／ｍｉｎ）、Ｈ：金属板の厚さ（ｍｍ）、Ｋ：２０〜３０の凝固定数（ｍｍ／ｍｉｎ１／２）である。

Here, L: solidification completion length of metal plate (mm), V: traveling speed per minute of metal plate (mm / min), H: thickness of metal plate (mm) , K: 20-30 It is a coagulation constant (mm / min1 / 2).

ここで、Ｌ：金属板の凝固完了長さ（ｍｍ）、Ｖ：金属板の１分あたりの進行速度（ｍｍ／ｍｉｎ）、Ｈ：金属板の厚さ（ｍｍ）、Ｋ：２０〜３０の凝固定数（ｍｍ／ｍｉｎ^１／２）である。

Here, L: solidification completion length of metal plate (mm), V: traveling speed per minute of metal plate (mm / min), H: thickness of metal plate (mm) , K: 20-30 It is a coagulation constant (mm / min ^1/2 ).

Claims (15)

A solidification apparatus for producing a metal plate by solidifying molten metal,
Including a row of rollers arranged along the traveling direction of the metal plate,
When the roller row is divided into a plurality of sections, the average roller pitch that is the distance between the centers of two rollers adjacent in the traveling direction of the metal plate in a section is smaller than the average roller pitch in the preceding section. A coagulation apparatus characterized by that.   The coagulation apparatus according to claim 1, wherein an average of roller pitches in the certain section is smaller than an average of roller pitches in the subsequent section.   The coagulation apparatus according to claim 1, wherein the section is a space having at least four roller pitches.   The coagulation apparatus according to claim 1, wherein the section is a space that includes a roller array arranged with a length of 1 m or more.   The coagulation apparatus according to claim 1, wherein the section is divided by a physical section.   6. The coagulation apparatus according to claim 5, wherein the physical section is a segment unit.   The coagulation apparatus according to claim 1, wherein the section is a space between a certain drive roll and a next drive roll.   The coagulation apparatus according to any one of claims 1 to 7, wherein a difference between an average roller pitch in the certain section and an average roller pitch in the preceding section is 10 mm or more.   9. The coagulation apparatus according to claim 8, wherein the average difference between the roller pitches is 20 mm or more.   The solidification apparatus according to claim 8, wherein the moving speed of the metal plate is 4 m / min or more. The section is
A horizontal section for horizontally transferring the metal plate;
A curved section disposed so as to be curved with a predetermined curvature between the outlet from which the molten metal is discharged and the horizontal section,
The inflection section is provided between at least one of the exit section and the curved section or between the curved section and the horizontal section, and includes inflection sections arranged with different curvatures. 9. The coagulation apparatus according to 8.   The coagulation apparatus according to claim 11, further comprising a vertical section that is vertically connected to the outlet to vertically transfer the metal plate and is connected to the curved section. The coagulation apparatus according to any one of claims 1 to 7, wherein an average of roller pitches in the section is defined by Equation 1 below.

Here, N is the number of roller pitches defined in the corresponding section, k is a section number, and i is a series number of roller pitches shown in the k section. The section where the average of the roller pitch is smaller than the average of the roller pitch in the preceding section exists within the length that the metal plate solidifies,
The solidification apparatus according to claim 1, wherein the solidification length of the metal plate is defined by the following mathematical formula 2.

Here, L: completion length of solidification of metal plate (mm), V: traveling speed per minute of metal plate (mm / min), H: thickness of metal plate, K: solidification constant of 20-30 ( mm / min ^1/2 ).   The coagulation apparatus according to any one of claims 1 to 7, wherein there are two or more sections in which the average roller pitch is smaller than the average roller pitch in the preceding section.

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