JP2001300621A - Rolling mill - Google Patents

Abstract

PROBLEM TO BE SOLVED: To perform the hot rolling of a material to be rolled with caliber rolls with high accuracy. SOLUTION: This rolling mill is provided with 1st caliber rolls 20 and 2nd caliber rolls 21, guide rollers 14 for guiding the material A to be rolled to the 2nd caliber rolls between both rolls and supply passages 122 for supplying oil-air for lubrication to the bearings of these rollers 14, and pressure detecting devices 123 are provided for these supply passages 122.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rolling mill, and more particularly, to a rolling mill for hot rolling a wire, a steel bar or the like.

[0002]

2. Description of the Related Art A first caliber roll and a second caliber roll provided downstream of the first caliber roll in the flow direction of a material to be rolled are set as a rolling mill for hot rolling a wire, a steel bar, or the like. A guide roller for guiding (guiding) the material to be rolled to the second caliber roll between the first caliber roll and the second caliber roll;
In this case, it is important to detect the rotation failure of the guide roller. As the detection of the rotation of the guide roller, one described in Japanese Utility Model Laid-Open No. 4-108907 is known.

In this conventional device, a detection piece provided on a roller is detected by a proximity switch, and an electric signal corresponding to the number of rotations is taken out to determine whether or not the rotation is defective. The rotation failure is determined to be failure when the rotation speed of the roller detected by the proximity switch falls below a predetermined value. In order to perform detection only during passing, a passing signal generator is provided.

[0004]

In the prior art, it is not possible to judge whether the rotation is defective or not only by the detected number of rotations of the roller. That is, the determination can be made only by comparing the rotation speed with a predetermined value (reference rotation speed). But,
The passing speed of the material passing through the rollers may be changed, and in that case, the reference rotation speed must be changed, which is complicated. In addition, if the material is not passing, it is not possible to determine that the rotation is poor even if the number of rotations of the roller decreases, so a passing signal generator for monitoring the passage of the material is an essential configuration,
The device becomes complicated.

Further, since the detecting means is a rotation speed detecting sensor such as a photoelectric tube or a proximity switch provided close to the roller, an erroneous detection of the detecting piece or a rotation speed detecting sensor or a detecting piece depends on the use environment of the guide roller. There is a concern that the device may be damaged or the like, and it is not always an accurate detection means.

[0006]

The technical means employed in the present invention to solve such a problem are as follows. That is, the rolling mill according to the present invention includes a first caliber roll and a second caliber roll provided on the downstream side in the flow direction of the material to be rolled of the first caliber roll as a set. A guide roller for guiding the material to be rolled to the second caliber roll between the two caliber rolls; a supply path for oil air supplied to the bearing of the guide roller for lubrication; A pressure detecting device for detecting a change is provided (claim 1).

[0007] By adopting such a configuration, it is possible to detect the rotation failure of the guide roller by a change in the pressure of the lubricating oil-air, and to freely provide the pressure-detecting device at a place where the thermal influence is small. The degree of freedom of design can be improved. In addition, the rotation speed of each of the plurality of guide rollers can be detected by a change in the pressure of the oil air. If the difference between the rotation speeds of the respective guide rollers exceeds a predetermined range, the rotation can be determined to be defective, and the detected rotation speed is determined as the reference speed. There is no need to compare with. Further, in the present invention, in the above-described claim 1, the caliber rolls forming a pair are:
It is preferable that a plurality of sets are provided at intervals in the flow direction of the material to be rolled (claim 2).

Further, in the present invention, the above-mentioned claim 1 or 2 is provided.
In the above, it is recommended that the first caliber roll is a pair of caliber rolls having a square or elliptical caliber, and the second caliber roll is a pair of caliber rolls having a circular caliber (claim 3). ). Also,
In the present invention, it is preferable that in the first to third aspects described above, the material to be rolled is a strip steel, and the caliber roll is a finishing block mill or a subsequent sizing mill (claim 4).

[0009]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a rolling equipment 1 for strip steel (wires, bars, etc.) to which the present invention is applied. The rolling equipment 1 has a heating furnace 2. In order to roll the material to be rolled (workpiece) sent from the heating furnace 2, the heating furnace 2
From the side, a rough row rolling mill 3, one middle row rolling mill 4, and a second middle row rolling mill 5 are arranged. The downstream side of the two intermediate row rolling mills is branched, one is provided with a winding machine 6, the other is provided with a finishing block mill 7, and the sizing mill 8 is provided following the finishing block mill 7. ing. A stellmore conveyor 9 is connected downstream of the sizing mill 8. In addition, the rolling equipment 1 includes a rolling operation room 10 for an operator to monitor and operate a rolling operation. The rolling operation room 10 is installed at a location away from other equipment of the rolling equipment 1.

FIG. 2 shows an example in which the rotating body defect monitoring device 11 according to the present invention is applied to the finishing block mill 7 and the sizing mill 8. The finishing block mill 7 has a main structure in which a plurality of stands are arranged in a housing 13 in a flow direction of a material to be rolled. That is, in FIG. 3, the first having a square or elliptical caliber 20A on the outer periphery is shown.
A pair of caliber rolls (rolling rolls) 20 is provided as a vertical type, and a second caliber roll (rolling roll) 21 having a circular (semicircular) caliber 21A on the outer periphery on the downstream side in the flow direction A1 of the material A to be rolled. Are provided as a Kodaira type, and a mill is formed by combining the pair of first caliber rolls 20 and the pair of second caliber rolls 21. The mills are arranged in four sets (eight stands) in the material flow direction. Have been.

In the mill, the material A to be rolled is guided (guided) to the caliber (hole) 21A of the second caliber roll 21.
For this purpose, a guide roller (sometimes called a roller guide) 14 is provided. That is, the roller guides 14 are provided on the upstream side of the horizontal stand (four in total). These roller guides 14 are for stably supplying hot steel to the horizontal stand. The roller guides 14 are provided with an upper roller guide 14a and a lower roller guide 14b at each position, and are provided in pairs so that the upper and lower roller guides 14a and 14b are vertically opposed to each other across the material to be rolled. . Although this roller guide 14 is not driven, it repeats high-speed rotation by hot steel,
Occasionally, rotation failure such as bearing seizure occurs, leading to poor quality or misroll.

Therefore, the upper and lower roller guides 14a, 14a
A supply pipe (supply path) 122 for supplying oil air for lubrication to a bearing (bearing) that rotatably supports the roller b is provided separately, and each of the supply paths 122 has an upper / lower guide roller 14a. , 14b are separately provided (details will be described later). Also, the sizing mill 8 is provided with one stand on the housing 16. The stand is also provided with a roller guide 14 for stably supplying the material to be rolled, and also supplies oil air to its bearing. A path 122 and a pressure detection device 123 are provided. In the following, among the stands provided in the finishing block mill 7, the stand provided with the roller guide 14 for supplying the material is referred to as a “stand 22st”, a “stand 24s” from the upstream side in the material flow direction.
t "," Stand 26st ", and" Stand 28st ". The stand of the sizing mill 8 is referred to as “stand 30st”. The position of the roller guide is not limited to the above.

FIGS. 4 and 5 show a roller guide 14 provided in the finishing block mill 7 and the sizing mill 8. FIG. The roller guide 14 has a roller 101 on which a caliber for guiding a material to be rolled is formed on an outer peripheral surface, and a brim 102 is formed at a central portion of an inner peripheral surface of the roller 101.
The ball bearing 104 is configured to be accommodated on both sides thereof. The roller shaft 107 of the roller 101 has a bolt shape, and a shoulder 1 for receiving the ball bearing 104 is provided on the head 110 side.
12 is formed, and a screw for screwing the nut 108 is formed at the tip. A supply hole 111 for supplying lubricating oil (oil air) to the ball bearing 104 is formed in the center of the shaft from the head 110 side to the center of the shaft, and communicates with a supply hole 11a formed in the center of the shaft in the radial direction. ing.

The roller 101 is supported by a roller support body 105. The support body 105 has, at one end, roller shaft support members 106, 106 formed in a forked shape having a wider interval than the thickness of the roller 101. A hole 109a for mounting the roller shaft 107 is formed in the support members 106, 106, and the head 110 of the roller shaft 7 is non-rotatably accommodated in the one roller shaft support member 106 from the outer surface side. The other roller shaft 106 has a hole 9b for rotatably receiving the nut 108 from the outer surface side.

[0015] Between the roller 101 and the roller shaft 107,
A spacer ring 116 is provided. The spacer ring 116 is slightly thinner (about 0.5 mm) than the thickness of the flange 102 at the center of the inner peripheral surface of the roller 101 and slightly smaller than the outer diameter of the roller shaft 107. Donut plate-shaped small-diameter spacer ring 117 with a large (about several mm) inner diameter
And a large-diameter spacer ring 118 having an inner diameter slightly larger than the outer diameter of the small-diameter spacer ring 117 (about 0.5 mm) and substantially the same thickness as the thickness of the flange 102 at the center of the inner peripheral surface of the roller 101. In the center of the thickness of the small-diameter spacer ring 117, a through hole 119 is formed in the radial direction.
On the other hand, a groove 120 is formed in the inner periphery of the large-diameter spacer ring 118.

The guide roller having the above structure is assembled in the following procedure. The ball shaft 104 is inserted from both sides of the roller 101 such that the large-diameter spacer ring 118 is non-rotatably fitted to the inner periphery of the collar 102 of the roller 101 and the small-diameter spacer ring 117 is sandwiched between the inner rings of the ball bearing 104. : The roller support body 1 assembled as described above
The roller shaft 107 is inserted between the roller shaft supporting members 106 and 106 of the roller shaft 05, and the roller shaft 107 is inserted through the side surface of the roller shaft supporting member 106 on the hole 109 side. Inner ring and roller shaft support member 1
06 and the bearing holding ring 113 is inserted. : Roller shaft supporting member 1 after inserting roller shaft 107
The spring washer 114 is inserted through the hole 109b of the roller 06 through the roller shaft 107, and the nut 108 is tightened. As a result, the small-diameter spacer ring 117 is fastened between the inner rings of the two ball bearings 104 and is non-rotatably mounted on the roller shaft 107, and has a space 21 communicating with the supply hole 111 a and the through hole 119 between the roller shaft 107. Is formed. : Next, a pipe 122 for supplying oil air is connected to the supply hole 111 of the head 110 of the roller shaft 107, and the pipe 1
The pressure detection device 123 is connected in the middle of the step 22. An oil-air pipe 122 is provided in each of the roller guides 14, and each of the pipes 122 has a pressure detection device 1.
23 are provided. The pressure detection device 123 is provided outside the housings 13 and 16 to avoid the influence of heat. Further, the pressure detecting device 123 is connected to the housing 13,
16 to prevent delay in pressure detection, and the pressure detection device 123 can be provided at an arbitrary position in the supply path 122, thereby increasing design flexibility.

The roller guide 14 constructed as described above
Then, the oil air supplied through the pipe 122 is supplied to the ball bearing 104 through the supply holes 111 and 111a, the space 121, and the through hole 119. When the roller 101 rotates during this supply process, the large-diameter spacer ring 118 rotates with the rotation of the roller 101, whereby the groove 120 formed in the large-diameter spacer ring 118
Is opened and closed. Pressure detector 12
In 3, the change in pressure at the time of opening and closing is detected. Also,
Since the pressure detection devices 123 are provided corresponding to the plurality of roller guides 14,
Pressure fluctuation due to the rotation of.

The pressure fluctuation of the oil air is controlled by the roller guide 1.
4 times per 4 revolutions. FIG. 6 shows a pressure detector 1
23 shows a signal waveform of the pressure fluctuation detected. As shown in FIG. 6A, the groove 120 of the large-diameter spacer ring 118 is used.
When opening the through-hole 119 formed in the small-diameter spacer ring 117, the pressure in the conduit 122 decreases. On the other hand, FIG.
When the through hole 119 is closed as shown in FIG.
The pressure at 22 increases. The signal of the pressure fluctuation changes depending on the rotation angle of the roller guide. That is, this signal has a frequency proportional to the rotation speed of the roller guide 14. In FIG. 6, two cycles of the pressure fluctuation correspond to one rotation of the roller guide 14.

The pressure fluctuation signal output from the pressure detecting device 123 is amplified by the amplifier 35, and is amplified by the control device 37.
Given to. Note that the amplifier 35 is provided on a local device storage panel 36 installed near the finishing block mill 7 or the like. The control device 37 is installed in the rolling cab 10. As shown in FIG. 6, the pressure detecting device 123, the amplifier 35, a high-pass filter 38 described later, and the like constitute an input unit of the monitoring device 11. The control device 37 includes, in addition to the high-pass filter 38, an A / D converter 39 for converting a pressure fluctuation signal, which is an analog signal, into a digital signal, and a computer (general-purpose personal computer) connected to the A / D converter 39 40. The computer 40 includes a display device as a display device 40a. The computer 40 is communicably connected to a rolling line master control device 41 which is a rolling control device 41 for controlling the rolling line, and can exchange information with the rolling control device 41.

The computer 40 includes a low-pass filter unit 4 for filtering the A / D converted data.
3.FF which performs FFT conversion of the filtered data
T conversion unit 44 and rotation failure determining unit 4 for determining rotation failure
5 is held. The low-pass filter unit 43 and the FFT conversion unit 44 include an A / D converter 39
Together, they constitute a signal processing unit. In addition, since the rotation speed (rotation speed) of the roller guide is detected by each function from the pressure detection device 123 to the FFT conversion unit, these functions constitute the rotation speed detection unit 33 according to the present invention. become.

FIG. 8 shows the flow of the monitoring process by the monitoring device 11. First, when the device 11 is started, the system is initialized. Then, the state of the rolling line is determined. The line state determination includes a metal-in determination and a use stand determination. Metal-in determination is a determination of whether or not the material to be rolled is on the line,
The determination result is used to display whether monitoring is in progress or on standby on a monitoring status display screen described later. If there is no material to be rolled, monitoring is unnecessary and it is only necessary to wait.

The determination of the used stand is performed to monitor the used stand and display it on the screen. The stand used is changed depending on the change of the rolling size and the product quality. For example, for one type of rolling, all stands 22st, 24st, 26st, 28s
While t, 30st is used, for other types of rolling,
In some cases, only some of the stands 22st, 24st, 30st are used. For still other types of rolling, some other stands 22st, 24st, 28st may be used. Since the stands that are not used do not have the rolling rollers and the roller guides 14, the monitoring of the roller guides 14 is unnecessary.

The rolling control device 41 holds such a stand information for each rolling method as a data table in its memory, and the monitoring device 11 fetches the used stand information from the rolling control device 41, Recognize the stand to be monitored. Therefore, even if the stand configuration is changed due to the change in the rolling size, only the used stand is automatically diagnosed. When the monitoring of the roller guide 14 is started, the pressure fluctuation analog signal output from the pressure detection device 123 is filtered by the high-pass filter 38, applied to the A / D converter, and converted into digital data. This digital data is stored in the memory of the computer 40. This digital data is filtered by the low-pass filter 43, and is subjected to FFT by the FFT conversion unit.
FT analysis is performed. The rate of change in pressure fluctuation is very high (rotational speed of the roller guide: 20,000 to 30,000 rpm), so it is difficult to handle raw data as it is, but FFT analysis makes it easy to handle.

A frequency component of the pressure fluctuation is obtained by the FFT analysis, and a roller rotation detecting process is performed based on the frequency component. By this processing, the main component (first-order component) frequency of the pressure fluctuation is obtained. Note that the principal component frequency is generally the highest level frequency, and
It can be obtained by creating a peak list and a harmonic list from the data after the T conversion, etc. However, since it is a known method, the description is omitted. When there is no main component, rotation is not monitored. When this processing is performed, the frequency (principal component frequency) of the pressure change of the oil air generated by the rotation of the roller guide 14 as shown in FIG. 9B is obtained from the collected data of the pressure fluctuation as shown in FIG. Is required. This pressure change frequency indicates the rotation state of the roller guide 14, and if the rotation of the roller guide 14 decreases or stops, it appears as a decrease in frequency.

The poor rotation judgment section 45 makes a poor rotation diagnosis based on the frequency component of the pressure fluctuation. Judgment for failure diagnosis is based on a judgment based on the rotational speed difference of each roller guide,
It is performed by using the rotation stop determination based on the frequency distribution together. Is determined by comparing the frequencies of the signals detected by the pressure detection devices 123 provided corresponding to the upper and lower roller guides 14a and 14b, which form a pair across the material to be rolled. This comparison is performed by calculating a difference between the main component frequency obtained for the upper roller guide 14a and the main component frequency obtained for the lower roller guide 14b. The difference is calculated by calculating a difference = (principal component frequency A−principal component frequency B) when the larger one of the two principal component frequencies to be compared is the principal component frequency A and the smaller one is the principal component frequency B. / (Main component frequency) × 100 (%).

For example, as shown in FIG. 10, when the main component frequency of the upper roller guide 14a is 300 Hz and the main frequency component of the lower roller guide 14b is 320 Hz, the difference is calculated as 6.25%. Is done. According to this determination method, it is possible to immediately determine that the rotation is abnormal when any one of the roller guides 14 to be compared with has its rotation speed decreased or stopped. Further, unlike the related art, a reference value corresponding to the passing speed of the material to be rolled is not required. If the calculated difference exceeds a predetermined range, a rotation difference occurs between the upper roller guide 14a and the lower roller guide 14b, and it is determined that the rotation is defective. The predetermined range reference value for determining whether or not the difference is within the allowable range is set in advance in the rotation failure determining unit 45, but it is preferable that the setting can be manually changed to change the accuracy. It is preferable that the setting can be changed for each roller guide or each stand.

The rotation stop determination of the pressure detecting device 123
This is performed based on the frequency component of the signal detected at. Based on the result of the frequency analysis, the frequency components are arranged in order from the one with the highest level, and the one with the highest level (the main component) is compared with the one with the Nth highest (the low-level component). If the ratio of the low-level component to exceeds the predetermined ratio set in advance, it is determined that the rotation of the roller guide 14 is stopped. It has been empirically found that when the rotation of the roller guide 14 stops, the level of the main component decreases and the noise increases. Therefore, it is intended to use this to detect the rotation stop. . According to this determination method, it is not necessary to compare the rotations of the plurality of roller guides 14. Therefore, even if all the roller guides 14 are stopped, it is possible to reliably determine the rotation failure. Therefore,
By combining this with the determination of, the rotation failure can be determined more reliably.

FIG. 11 shows a case where the upper roller guide 14a rotates normally and the lower roller guide 14b stops. In FIG. 10, the frequency components obtained by the FFT transform are arranged in order from the highest frequency level to the lowest frequency level. That is, the component with the highest level located on the left side in FIG. 11 is the principal component frequency,
They are arranged so that the level becomes smaller as going to the right. As shown in FIG. 11A, since the upper roller guide 14a rotates normally, the main component is large and the difference from the Nth (14th in the figure) component is sufficiently large. The level ratio of the Nth frequency component to the main component is 6%. If the ratio is small, it is determined that the rotation is normal.

On the other hand, as shown in FIG. 11B, since the lower roller guide 14b is stopped or stopped, the level of the main component decreases, and conversely, the low-level component increases due to noise. The difference is smaller. Here, the level ratio of the Nth frequency component to the main component is 25%. Since this level ratio is high, it is determined that the rotation of the main component is poor due to a decrease in the level of the main component. In addition, a ratio (predetermined ratio) that is a reference of whether the rotation ratio is good or bad
Is set in advance in the rotation failure determination unit 45, but it is preferable that the setting can be manually changed to change the accuracy. It is preferable that the setting can be changed for each roller guide or each stand. It is also preferable that the value of N can be manually changed.

The result of the rotation failure determination described above is always
It is displayed on the monitor display screen 50 of the display device 40a.
The screen 50 includes a monitoring stand status display section 51 for displaying a stand to be monitored (used stand), and a current monitoring state display section for displaying the current monitoring function in a standby state or a monitoring state. 52, a monitor setting item display unit 53 for displaying the value of a setting item for monitoring, a stand status display unit 54 for displaying a graph of the status of each stand being monitored, a latest abnormality for displaying the latest abnormality information when an abnormality occurs. An information display unit 55 is included.

The monitoring stand status display section 51 displays the currently used stand 14 based on the used stand information. In FIG. 11, all the stands “22, 24, 26, 28, 30” are displayed,
This indicates that all stands are being used and are being monitored. Also, for example, if the used stand is “22s
If it is changed to “t, 24st, 30st”, “22, 24, 30” is displayed on the stand state display section 51. The current monitoring state display section 52 displays the result of the metal-in determination. If monitoring is unnecessary and there is no material to be rolled, "waiting" is displayed as shown in FIG. For example, "monitoring" is displayed.

The stand status display section 54 graphically displays the status of each stand currently used based on the used stand information. In FIG. 11, since all the stands are used, the states of all the stands are displayed. The contents of the displayed graph are the main components of each of the upper roller guide 14a and the lower roller guide 14b. When it is determined by the rotation failure determination unit 45 that the rotation is defective, an abnormality process is performed. The abnormal process includes an abnormal alarm process and an abnormal data storage process. In the abnormality alarm processing, the stand 14 in which the abnormality has occurred is highlighted on the screen 50 and a buzzer sounds to warn the user. The operation of the rotation failure determination process is continued even after the occurrence of the abnormality.

In the abnormal data storage processing, data such as the data after the FFT conversion, the date and time of occurrence of the abnormality, and the number of the stand where the abnormality occurred are stored in a storage unit such as a hard disk of the computer. FIG. 13 shows the case where the upper and lower roller guides 14a and 14b rotate normally, and FIG. 13 shows the lower roller guide 14b.
7 specifically shows the state of the data after the FFT conversion in the case where is stopped. In the case of FIG. 13, the upper roller 14a
Is 180 Hz, and the main component frequency of the lower roller 14b is 183 Hz. Since the difference between the two frequencies is small, it is determined to be normal.

On the other hand, in the case of FIG.
The main component frequency of a is 180 Hz, and the main component frequency of the lower roller 14b is 361 Hz. Since the difference between the two frequencies is large, it is determined that the rotation is defective or stopped. Also, it can be seen that the level of the main component frequency has decreased and the level of the opposite noise has increased.

[0035]

According to the present invention, the material to be rolled by the Gulliver roll can be reliably rolled without any damage.

[Brief description of the drawings]

FIG. 1 is a diagram showing an outline of a rolling facility to which the present invention is applied.

FIG. 2 is a schematic view of an apparatus when the present invention is applied to a finishing block mill and a sizing mill.

FIG. 3 is a view showing a finishing block mill.

FIG. 4 is an explanatory sectional view of a roller guide.

FIG. 5 is a sectional view taken along line XX of FIG. 4;

FIG. 6 is an explanatory diagram showing a relationship between a rotation state of a guide roller and a pressure fluctuation signal.

FIG. 7 is a device schematic diagram including a detailed configuration of a control device.

FIG. 8 is a flowchart showing a flow of processing of the monitoring device.

FIG. 9 is an explanatory diagram illustrating that a frequency of a pressure change is obtained by FFT analysis from a signal detected by the pressure detection device.

FIG. 10 is an explanatory diagram of a rotation failure determination method based on a rotation difference.

FIG. 11 is an explanatory diagram of a rotation failure determination method based on a frequency distribution.

FIG. 12 is an explanatory diagram of a monitoring status display screen.

FIG. 13 shows data after FFT conversion in a normal state.

FIG. 14 shows data after FFT conversion when the lower roller stops.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Rolling equipment 7 Finishing block mill 14 Guide guide roller 20 1st caliber roll 21 2nd caliber roll 122 Oil air supply path 122 Pressure detector

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yukihiro Kanda 2 Nadahama-Higashi-cho, Nada-ku, Kobe City, Hyogo Prefecture Inside the Kobe Steel Works Kobe Works (72) Inventor Tadahiro Shindate 2 Nadahama-Higashi-cho, Nada-ku, Kobe City, Hyogo Prefecture Kobe Steel, Ltd. Kobe Steel F-term (reference) 4E002 AC12 AC14 BA01 BB06 BC08 CA19 CB03 CB09

Claims (4)

[Claims] 1. A first caliber roll and a second caliber roll provided downstream of the first caliber roll in the flow direction of a material to be rolled.
A caliber roll is provided as a set, and a guide roller for guiding a material to be rolled to the second caliber roll is provided between the first caliber roll and the second caliber roll, and supplied to a bearing of the guide roller for lubrication. A rolling mill comprising: an oil-air supply path; and a pressure detection device for detecting a change in pressure of the oil-air in the supply path. 2. The rolling mill according to claim 1, wherein a plurality of sets of caliber rolls are provided at intervals in the flow direction of the material to be rolled. 3. The method according to claim 1, wherein the first caliber roll is a pair of caliber rolls having a square or elliptical caliber, and the second caliber roll is a pair of caliber rolls having a circular caliber. Item 3. The rolling mill according to item 1 or 2. 4. The rolling mill according to claim 1, wherein the material to be rolled is a steel bar, and the caliber roll is a finishing block mill or a sizing mill subsequent thereto.