JP2019058866A - Production method of crushed product by roll crusher and roll crusher - Google Patents

Abstract

To provide a production method of a crushed product by a roll crusher constantly crushing sorted and screened semolina to a preliminarily stipulated grain size.SOLUTION: A production method of a crushed product by a roll crusher includes: a crushing step passing raw particulate grains in a clearance between a pair of rotating rolls, crushing the raw particulate grains and obtaining the crushed product; a sampling step sampling the crushed product; a measurement step measuring the particle size distribution of a sample sampled in the sampling step; and an adjustment step adjusting the clearance between the pair of rolls so that the characteristic value of the sample calculated on the basis of the particle size distribution of the sample measured in the measurement step is within a predetermined range.SELECTED DRAWING: Figure 4

Description

  The present invention relates to a method for producing a ground product by a roll crusher and a roll crusher, in which a raw material powder is crushed by passing through a gap between a pair of rotating rolls to obtain a ground material.

  In the crushing step which is the step of grinding wheat in the milling step of milling wheat to make wheat flour, first, breaking of selected wheat is performed. For breaking, a roll crusher equipped with a brake roll, which is a dressing roll, is used, and breaking gives wheat pieces (semolina) of various particle sizes. Next, the semolina obtained by breaking is sieved by a sifter and grading is performed to separate from the skin. In addition, since a considerable endosperm part adheres to the peel, it is crushed again by a roll crusher equipped with a brake roll of the next stage. Next, the skin that could not be removed by grading is purified from the semolina.

  Next, the reduced and refined semolina purified by purification is finely ground to a flour of the required particle size. For reduction, use is made of a roll crusher equipped with a smooth roll which is a smooth and smooth surface. The remaining peel is then screened from the flour which has been ground to the required particle size in the reduction. In addition, the roll type crusher which grind | pulverizes wheat is disclosed by patent document 1 etc. FIG.

Japanese Patent Application Publication No. 2002-500084

  By the way, in reduction, it is necessary to crush semolina through stages so as not to deteriorate the quality of semolina. Therefore, a roll crusher equipped with smooth rolls is used in a plurality of stages, and the optimum semolina quality obtained by the roll crusher equipped with each smooth roll is specified.

  In the past, workers collected the semolina crushed by a roll crusher equipped with a smooth roll, and manually checked the smooth roll gap of the roll crusher while observing the roll crusher operating conditions such as the roll temperature. It was adjusting.

  However, when the worker determines the characteristic value regarding the particle size of the crushed semolina and manually adjusts the roll gap of the roll crusher based on the characteristic value regarding this particle diameter, the workload of the worker is increased, It was difficult to adjust the roll gap frequently. In addition, there is a problem that the adjustment accuracy of the roll gap varies among the workers.

  An object of the present invention is to provide a method and a roll crusher for producing a ground material by a roll crusher which always crushes sorted and screened semolina to a predetermined particle size.

  In the method for producing a pulverized material by a roll crusher according to the present invention, a raw powder particle is passed through a gap between a pair of rotating rolls, and the raw powder particle is pulverized to obtain a pulverized product. The characteristic value of the sample calculated in advance based on the sampling step for sampling, the measurement step for measuring the particle size distribution of the sample sampled in the sampling step, and the particle size distribution of the sample measured in the measurement step And adjusting the gap between the pair of rolls so as to be within the specified range.

  Further, in the method of producing a pulverized material by a roll crusher according to the present invention, in the sampling step, a first sampling which is sampling of the pulverized material in a predetermined range on one side of the pulverizing region of the pair of rolls And second sampling which is sampling of the crushed material in a predetermined range on the other side of the grinding region of the pair of rolls, and in the measuring step, the particle size of the first sample sampled in the first sampling The first particle size distribution, which is a distribution, and the second particle size distribution, which is the particle size distribution of a second sample sampled in the second sampling, are measured, and in the adjusting step, based on the first particle size distribution. Adjust the gap on one side of the pair of rolls so that the characteristic value of the first sample calculated in the above becomes within a predetermined range , And adjusting the gap between the other side of the second granularity said second sample said pair of rolls so that the characteristic value is within a predetermined range of which is calculated based on the distribution.

  In the method of producing a pulverized material by a roll crusher according to the present invention, in the adjusting step, when the characteristic value of the first sample is larger than the target value of the characteristic value, the gap on one side of the pair of rolls Is adjusted to reduce the characteristic value, and when the characteristic value of the first sample is smaller than the target value of the characteristic value, the gap on one side of the pair of rolls is adjusted to increase the characteristic value; When the characteristic value of the second sample is larger than the target value of the characteristic value, the gap on the other side of the pair of rows is adjusted to decrease the characteristic value, and the characteristic value of the second sample is the characteristic value. When the value is smaller than the target value, the gap on the other side of the pair of rolls is adjusted to increase the characteristic value.

  In the method for producing a pulverized material by a roll crusher according to the present invention, the characteristic value of the sample is determined in advance even when the adjustment limit is set to the lower limit or the upper limit of the adjustment limit of the pair of rolls in the adjusting step. It is characterized by emitting an alarm when it does not fall within the range.

  The method for producing a pulverized material by a roll crusher according to the present invention is characterized in that the pair of rolls is a smooth roll used in a reduction step of pulverizing semolina to a required particle size.

  The method for producing a pulverized material by a roll crusher according to the present invention is characterized in that the pair of rolls is the smooth roll disposed on the upper side among the plurality of smooth rolls used in the reduction step. Do.

  The roll crusher according to the present invention includes a crusher for passing the raw material powdery particles through a gap between a pair of rotating rolls and crushing the raw material powdery particles to obtain a crusher, and the crusher crushed by the crusher. Of the sample, the measuring device for measuring the particle size distribution of the sample sampled in the sampling mechanism, and the characteristic value of the sample calculated in advance based on the particle size distribution of the sample measured in the measuring device And an adjusting mechanism for adjusting the gap between the pair of rolls so as to fall within a predetermined range.

  In the roll crusher according to the present invention, a first sampling mechanism for sampling the crushed material in a predetermined range on one side of the crushing area of the pair of rolls, and the crushing of the pair of rolls A second sampling mechanism for sampling the crushed material in a predetermined range on the other side of the region, wherein the measuring device is a first particle size distribution of the first sample sampled by the first sampling mechanism And a second measuring device for measuring a second particle size distribution, which is a particle size of the second sample sampled by the second sampling mechanism, wherein the adjusting mechanism comprises the first measuring device for measuring the distribution, and the adjusting mechanism is the first measuring device. The gap on one side of the pair of rolls is adjusted so that the characteristic value of the first sample calculated based on the particle size distribution of Adjusting the gap on the other side of the pair of rolls so that the characteristic value of the second sample calculated based on the first adjusting mechanism and the second particle size falls within a predetermined range. It is characterized by having an adjustment mechanism.

  The roll crusher of the present invention is characterized in that the first measuring device and the second measuring device are one device.

  In the roll crusher according to the present invention, when the characteristic value of the first sample is larger than the target value of the characteristic value, the first adjustment mechanism makes the gap on one side of the pair of rolls smaller in characteristic value. And the gap on one side of the pair of rolls is adjusted to increase the characteristic value when the characteristic value of the first sample is smaller than the target value of the characteristic value, and the second adjustment mechanism is adjusted. When the characteristic value of the second sample is larger than the target value of the characteristic value, the gap on the other side of the pair of rolls is adjusted to decrease the characteristic value, and the characteristic value of the second sample is characteristic When the value is smaller than the target value, the gap on the other side of the pair of rolls is adjusted to increase the characteristic value.

  In the roll crusher according to the present invention, the characteristic value of the sample does not fall within the predetermined range even when the adjustment mechanism adjusts the lower limit or the upper limit of the adjustment limit of the pair of rolls. It is characterized by including an alarm unit that issues an alarm.

  The roll crusher according to the present invention is characterized in that the pair of rolls is a smooth roll used in a reduction step of grinding the semolina to a required particle size.

  The roll crusher according to the present invention is characterized in that the pair of rolls is the smooth roll disposed on the upper side among the plurality of smooth rolls used in the reduction step.

  According to the present invention, it is possible to provide a method for producing a ground product by a roll crusher and a roll crusher which always grinds a sorted and sieved semolina to a predetermined particle size.

It is a figure which shows the crushing process in the wheat flour concerning embodiment. (a) is the schematic diagram seen from the upper surface which shows the structure of the roll type grinder provided with the smooth roll used at the crushing process which concerns on embodiment, (b) is the schematic diagram seen from the side. It is a figure showing the system configuration of the roll crusher provided with the smooth roll concerning an embodiment. It is a flowchart which shows particle size distribution control of semolina crushed with the roll type crusher provided with the smooth roll which concerns on embodiment.

  Hereinafter, with reference to drawings, the crushing process in the flour milling of the wheat which concerns on embodiment is demonstrated. FIG. 1 is a view showing a crushing process in wheat flour according to an embodiment. First, as shown in FIG. 1, braking of the wheat selected in the braking step is performed. That is, the wheat which has been selected is crushed by a roll crusher (1B roll) equipped with a brake roll which is a setting roll to obtain wheat shreds (semolina) of various particle sizes. Next, in a grading step, semolina obtained by breaking with a 1B roll is sieved by a sifter and separated from the skin. Since a considerable endosperm portion is attached to the skin, the skin is returned to the breaking process and is crushed again by a roll crusher (2B roll) equipped with a brake roll of the next stage.

  Similarly, in the braking process, semolina obtained by breaking with the 2B roll is sifted by the sifter in the grading process and separated from the skin. The separated skins are returned to the breaking process and crushed again in a roll crusher (3B roll) equipped with the next stage brake roll. In the braking step, semolina obtained by breaking with the 3B roll is sieved by the sifter in the grading step and separated from the skin. The separated skin is returned to the breaking process and re-crushed in a roll crusher (4B roll) equipped with the next stage brake roll. In the braking step, semolina obtained by breaking with the 4B roll is sifted by the sifter in the grading step and separated from the skin. The skin screened by this grading is taken out as coarse bran or small bran.

  In the grading step, the semolina is separated from the skin in the breaking step, and the semolina is classified by the shifter.

  Next, in the purification step, for each of the semolinas classified according to particle size in the grading step, the peel which could not be removed by grading is removed. The removed skin is taken out as small bran.

  Next, in the reduction step, the semolina that has been purified and screened in the puri? Cation step is comminuted into wheat flour of the required particle size. In the reduction step, multiple stages of roll mills 10, 11, 12, 13, 14, 15, 16 are used, and the semolina is finely crushed sequentially to obtain wheat flour of the required particle size. The roll crushers 10, 11, 12, 13, 14, 15, 16 are roll crushers provided with smooth rolls which are smooth smooth rolls, and the roll crushers 10, 11, 12, 13, 14, 15, 16 In order to finely grind semolina into flour of the required particle size sequentially, the flour milled for each roll type 10, 11, 12, 13, 14, 15, 16 A target particle size distribution, which is a target value of particle size distribution, is defined.

  In the reduction step, the flour pulverized to the required particle size by the roll mills 10, 11, 12, 13, 14, 15, 16 is sieved to remove the skin as a small amount of bran. In addition, the screened wheat flour is reground by each roll crusher disposed downstream according to the particle size.

  Fig.2 (a), (b) is a figure which shows the structure of the roll type grinder 10 provided with the smooth roll which concerns on embodiment. The roll crusher 10 has a pair of smooth rolls 10a and 10b and passes semolina, which is a raw material powder, through a gap between a pair of rotating smooth rolls 10a and 10b, and crushes the semolina to obtain a flour as a ground product. It is a roll crusher.

  The roll crusher 10 is provided with a first sampling mechanism 20a for sampling a predetermined range of wheat flour (first sample) on one side of a grinding area of a pair of smooth rolls 10a and 10b, and a pair of smooth rolls 10a, A second sampling mechanism 20b is provided for sampling the predetermined range of wheat flour (second sample) on the other side of the grinding area 10b. In addition, the first particle size distribution measuring apparatus 21a for measuring the first particle size distribution which is the particle size distribution of the first sample sampled by the first sampling mechanism 20a, and the particle size of the second sample sampled by the second sampling mechanism 20b. A second particle size distribution measuring device 21b for measuring the second particle size distribution, which is a distribution, is provided.

  Furthermore, one of the pair of smooth rolls 10a and 10b is set so that the characteristic value of the first sample calculated based on the first particle size distribution measured by the first particle size distribution measuring device 21a falls within a predetermined range. The characteristic values of the second sample calculated based on the second particle size distribution measured by the first gap adjusting mechanism 22a for adjusting the gap on the side and the second particle size distribution measuring device 21b fall within a predetermined range. The second gap adjusting mechanism 22b is provided to adjust the gap on the other side of the pair of smooth rolls 10a and 10b.

  Here, the characteristic values are (1) median diameter by volume in light diffraction method, (2) median diameter by particle diameter in light diffraction method, and (3) sieve when sieved with a predetermined sieve. The lower weight, (4) the volume under the sieve when sieving with a predetermined sieve, (5) the most frequent particle diameter when sieving with a plurality of sieves with different predetermined openings, (6 ) Separated for each particle size area, measure the density, particle size of the highest density area, (7) Separate each particle size area, measure the density, particle size of the lowest density area, (8) Separate for each particle size area, measure the surface area, particle size with the largest surface area per unit weight, (9) Separate for each particle size area, measure the surface area, most per unit weight Such as particle size with small surface area.

  FIG. 3 is a view showing a part of the system configuration of the roll crusher 10 according to the embodiment. The roll crusher 10 is provided with a control unit 30 that controls the entire apparatus in an integrated manner. The control unit 30 stores an input unit 32 for inputting a target characteristic value which is a target value of the characteristic value of wheat flour which is a pulverized material pulverized by the roll crusher 10, and stores the target characteristic value inputted by the input unit 32. A storage unit 34 is connected. In addition, even when the gap between the pair of smooth rolls 10a and 10b is adjusted to the upper limit or the lower limit of the adjustment limit, the control unit 30 issues an alarm when the characteristic value of the flour which is the ground product does not reach the target characteristic value. The alarm unit 33 and the display unit 31 for displaying the value of the target characteristic value inputted by the input unit 32 and the alarm by the alarm unit are connected.

  Furthermore, a first sampling mechanism 20a, a second sampling mechanism 20b, a first particle size distribution measuring device 21a, a second particle size distribution measuring device 21b, a first gap adjusting mechanism 22a, and a second gap adjusting mechanism 22b are connected. Here, the first sampling mechanism 20a, the second sampling mechanism 20b, the first particle size distribution measuring device 21a, the second particle size distribution measuring device 21b, the first gap adjusting mechanism 22a, the second gap adjusting mechanism 22b of the roll crusher 10 The control unit 30, the display unit 31, the input unit 32, the alarm unit 33, and the storage unit 34 constitute a gap adjusting unit that adjusts the gap between the smooth rolls 10a and 10b.

  FIG. 4 is a flowchart showing a method of producing a pulverized material by a roll crusher in which semolina is crushed to a required particle size using a roll crusher according to the embodiment. When the semolina is crushed by the roll crusher 10, the first sampling mechanism 20a performs sampling of wheat flour which is a crushed material of a predetermined range on one side of the crushing region of the pair of smooth rolls 10a and 10b 1) is performed (step S11). Next, the particle size distribution (first particle size distribution) of the first sample sampled by the first sampling mechanism 20a is measured by the first particle size distribution measuring device 21a (step S12), and the first particle size distribution is determined by the controller 30. Send to

  Further, the second sampling mechanism 20b performs sampling (second sampling) of wheat flour which is a crushed material in a predetermined range on the other side of the crushing region of the pair of smooth rolls 10a and 10b (step S13).

  Next, the particle size distribution (second particle size distribution) of the second sample sampled by the second sampling mechanism 20b is measured by the second particle size distribution measuring apparatus 21b, and the second particle size distribution is transmitted to the control unit 30. (Step S14).

  Next, the control unit 30 causes the characteristic value of the first sample calculated based on the first particle size distribution measured by the first particle size distribution measuring device 21a to be the target characteristic value stored in the storage unit 34. A first adjustment value for adjusting the gap on one side of the pair of smooth rolls 10a and 10b is calculated (step S15). Further, the control unit 30 causes the characteristic value of the second sample calculated based on the second particle size distribution measured by the second particle size distribution measuring device 21b to be the target characteristic value stored in the storage unit 34. A second adjustment value for adjusting the gap on the other side of the pair of smooth rolls 10a and 10b is calculated (step S16).

  Next, the control unit 30 determines whether or not the first adjustment value and the second adjustment value are within the upper limit or the lower limit of the adjustment limit of the gap between the pair of smooth rolls 10a, 10b (step S17). If the alarm unit 33 exceeds the threshold, the alarm unit 33 issues an alarm (step S20). That is, even when adjusting to the lower limit or the upper limit of the adjustment limit of the gap between the pair of smooth rolls 10a and 10b, an alarm is issued when the characteristic value of the first sample or the characteristic value of the second sample does not become the target characteristic value.

  If the first adjustment value and the second adjustment value are within the upper limit or the lower limit of the adjustment limit of the gap between the pair of smooth rolls 10a and 10b, the first adjustment value is transmitted to the first gap adjustment mechanism 22a. The gap adjustment mechanism 22a adjusts the gap on one side of the pair of smooth rolls 10a and 10b based on the first adjustment value (step S18).

  Here, when the characteristic value of the first sample is larger than the target value of the characteristic value, the first gap adjustment mechanism 22a adjusts the clearance on one side of the pair of smooth rolls 10a and 10b so that the characteristic value becomes smaller. When the characteristic value of the first sample is smaller than the target value of the characteristic value, the gap on one side of the pair of smooth rolls 10a and 10b is adjusted so as to increase the characteristic value.

  The second adjustment value is sent to the second gap adjustment mechanism 22b, and the second gap adjustment mechanism 22b adjusts the gap on the other side of the pair of smooth rolls 10a, 10b based on the second adjustment value (step S19).

  Here, when the characteristic value of the second sample is larger than the target value of the characteristic value, the second gap adjustment mechanism 22b adjusts the other side of the pair of smooth rolls 10a and 10b so that the characteristic value becomes smaller. When the characteristic value of the second sample is smaller than the target value of the characteristic value, the gap on the other side of the pair of smooth rolls 10a and 10b is adjusted so as to increase the characteristic value. Then, the process returns to step S11, and the processes of steps S11 to S20 are repeated.

  The configurations of the roll type crushers 11, 12, 13, 14, 15, 16 are the same as the configuration of the roll type crusher 10, even if they are provided with a gap adjusting section, they are not provided with a gap adjusting section. Although it may be, it is preferable that the roll type crusher installed in the upper stage side among roll type crushers 11, 12, 13, 14, 15, 16 is provided with the gap adjustment part.

  According to the method of manufacturing a pulverized material by the roll crusher and the roll crusher according to this embodiment, the roll gap of the roll crusher is repeatedly adjusted at fixed time intervals without increasing the burden on the worker. As it can be done, the flour produced can be of high quality. Moreover, the problem that dispersion | variation arises in adjustment precision of a roll gap for every worker can also be eliminated.

  In the above embodiment, although the first particle size distribution measuring device 21a and the second particle size distribution measuring device 21b are provided, the first sampling mechanism 20a samples the first particle size distribution measuring device 21a. The particle size distribution (first particle size distribution) of the sample may be measured, and further, the particle size distribution (second particle size distribution) of the second sample sampled by the second sampling mechanism 20b may be measured. .

  10, 11, 12, 13, 14, 15, 16 ... roll type crusher, 10a, 10b ... smooth roll, 20a ... first sampling mechanism, 20b ... second sampling mechanism, 21a ... first particle size distribution measuring device, 21b ... 2nd particle size distribution measuring device, 22a ... 1st gap adjustment mechanism, 22b ... 2nd gap adjustment mechanism, 30 ... control unit, 31 ... display unit, 32 ... input unit, 33 ... alarm unit, 34 ... storage unit

Claims (13)

A step of passing raw material particles through a gap between a pair of rotating rolls and grinding the raw material particles to obtain a ground product;
A sampling step of sampling the crushed material;
Measuring the particle size distribution of the sample sampled in the sampling step;
Adjusting the gap between the pair of rolls such that the characteristic value of the sample calculated based on the particle size distribution of the sample measured in the measuring step falls within a predetermined range;
A method of producing a pulverized material by a roll crusher characterized by comprising: In the sampling step, a first sampling which is a sampling of the crushed material in a predetermined range on one side of the crushing area of the pair of rolls, and a predetermined value on the other side of the crushing area of the pair of rolls Perform a second sampling, which is a sampling of said ground material in a range
In the measurement step, a first particle size distribution that is a particle size distribution of the first sample sampled in the first sampling, and a second particle size distribution that is the particle size distribution of the second sample sampled in the second sampling Measure the
In the adjusting step, the gap on one side of the pair of rolls is adjusted so that the characteristic value of the first sample calculated based on the first particle size distribution falls within a predetermined range, The gap on the other side of the pair of rolls is adjusted so that the characteristic value of the second sample calculated based on the second particle size distribution falls within a predetermined range. Method of producing a pulverized material by a roll crusher. In the adjustment step,
When the characteristic value of the first sample is larger than the target value of the characteristic value, the gap on one side of the pair of rolls is adjusted to decrease the characteristic value, and the characteristic value of the first sample is the characteristic value. When it is smaller than the target value, the gap on one side of the pair of rolls is adjusted to increase the characteristic value,
When the characteristic value of the second sample is larger than the target value of the characteristic value, the gap on the other side of the pair of rows is adjusted to decrease the characteristic value, and the characteristic value of the second sample is the characteristic value. 3. The method for producing a pulverized material by a roll crusher according to claim 2, wherein when the value is smaller than the target value, the gap on the other side of the pair of rolls is adjusted to increase the characteristic value.   In the adjustment step, an alarm is issued when the characteristic value of the sample does not fall within the predetermined range even when the adjustment limit of the gap between the pair of rolls is adjusted to the lower limit or the upper limit. The manufacturing method of the ground material by the roll type grinder of 1 statement.   The method for producing a pulverized material by a roll crusher according to any one of claims 1 to 4, wherein the pair of rolls is a smooth roll used in a reduction step of pulverizing semolina to a required particle size. .   The said pair of rolls is the said smooth roll installed in the upper stage side among several smooth rolls used at the said reduction process, The grind | pulverized thing by the roll type crusher of Claim 5 characterized by the above-mentioned Production method. A pulverizing device for passing raw material particles through a gap between a pair of rotating rolls and pulverizing the raw material particles to obtain a pulverized material;
A sampling mechanism for sampling the pulverized material pulverized by the pulverizing apparatus;
A measuring device for measuring a particle size distribution of a sample sampled by the sampling mechanism;
An adjusting mechanism that adjusts the gap between the pair of rolls so that the characteristic value of the sample calculated based on the particle size distribution of the sample measured by the measuring device falls within a predetermined range;
A roll crusher characterized by comprising: The sampling mechanism is a first sampling mechanism for sampling the crushed material in a predetermined range on one side of the crushing area of the pair of rolls, and a predetermined of the other side of the crushing area of the pair of rolls A second sampling mechanism for sampling the crushed material in the range;
The measuring device measures a first particle size distribution, which is a particle size distribution of a first sample sampled by the first sampling mechanism, and a second sample sampled by the second sampling mechanism. A second measuring device for measuring a second particle size distribution, which is the particle size,
The adjustment mechanism adjusts a gap on one side of the pair of rolls such that the characteristic value of the first sample calculated based on the first particle size distribution falls within a predetermined range. A mechanism, and a second adjusting mechanism for adjusting the gap on the other side of the pair of rolls so that the characteristic value of the second sample calculated based on the second particle size falls within a predetermined range The roll crusher according to claim 7, characterized in that:   The roll crusher according to claim 8, wherein the first measuring device and the second measuring device are one device. The first adjustment mechanism adjusts the gap on one side of the pair of rolls so that the characteristic value becomes smaller when the characteristic value of the first sample is larger than the target value of the characteristic value, and the first sample When the characteristic value of is smaller than the target value of the characteristic value, the gap on one side of the pair of rolls is adjusted so as to increase the characteristic value,
When the characteristic value of the second sample is larger than the target value of the characteristic value, the second adjustment mechanism adjusts the gap on the other side of the pair of rolls so that the characteristic value becomes smaller, and the second sample 10. The roll crusher according to claim 8, wherein the gap on the other side of the pair of rolls is adjusted to increase the characteristic value when the characteristic value of the characteristic value is smaller than the target value of the characteristic value. .   The adjustment mechanism includes an alarm unit that issues an alarm when the characteristic value of the sample does not fall within the predetermined range even when the adjustment limit of the gap between the pair of rolls is adjusted to the lower limit or the upper limit. The roll crusher according to claim 7.   The roll type crusher according to any one of claims 7 to 11, wherein the pair of rolls are smooth rolls used in a reduction step of grinding the semolina to a required particle size. The roll type crusher according to claim 12, wherein the pair of rolls is the smooth roll disposed on the upper side among the plurality of smooth rolls used in the reduction step.

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