CA1046308A - Press roll piercing method - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A press roll piercing method for manufacturing a hollow shell is disclosed wherein a polygonal metal billet is pushed in its axial direct-ion between a pair of rolling rolls mounted one above the other and having semicircular grooves so as to be rolled into circular or almost circular shape, and the piercing is effected in a core portion of the billet by means of a piercing plug supported between the rolling rolls. The improvement resides in selecting a pushing force of a predetermined value which corresponds to the stress above the yield stress of the billet at the start of the piercing although the actual stress occurring in the billet at the start of the piercing may or may not exceed the yield stress;
selecting the pushing speed and/or the peripheral speed of the rolls to be of such values that the volume of the billet to be pushed in per unit time and the volume of the hollow shell to be discharged from the rolling rolls become substantially equal, and biting the billet between the rolling rolls at said set values.

Description

~0~630~3 The present invention relates to a piercing method for the manufac-ture of seamless metal pipe from a polygonal billet.
As the conventional method of manufacturing hollow metal bodies (such as seamlcss steel pipc) there are counted the press piercing method using such an apparatus as a crank press or a hydraulic press and the roll piercing method using such an apparatus as a Mannesman piercer or a three-roll piercer; and as a method in this category besides these conventional methods, public attention has been paid to a press roll piercing method, which can make a hollow shell of a low-priced square or rectangular billet through one piercing operation. The press roll piercing method involves a technique of feeding a square billet between a pair of drive rolls mounted one above the other with the application of power in its axial direction and of piercing the core portion of the billet while rolling the billet into round shape.
When the press roll piercing mill is designed and manufactured, in order to determine the strength and structure of the piercing mill proper, the rolling load is an important factor and in order to determine the capacity of the motor for driving the rolls and the strength and structure of the drive transmitting system, accurate determination of the roll torque is required, but no proposal for a reliable calculating formula has been presented thus far.
An object of the present invention is to provide a press roll pierc-ing method making it possible to design the installation of proper size with minimization of load such as rolling load, roll drive torque and stabilization of such load.
Thus, according to the present invention there is provided in a press roll piercin~ method for manufacturing a seamless pipe wherein a polygonal metal billet is pushed in its axial direction between a pair of grooved drive rolls mounted one above the other and piercing is effected in the core portion of the billet by means of a piercing plug extending between the rolls, the improvement which comprises setting the pushing force to be of a predetermined value which corresponds to the stress above the yield stress of the billet at the commencement of piercing; setting the pushing speed and~or the peripheral speed of the rolls to such values that the volume of the billet being pushed 0,., ~. .

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~046308 in per unit time and the volume of the pipe being discharged from the rolls per unit time become substantially equal, and biting the billet between the rolls at said set values, thereby forcing the billet to be bitten between the rolls to start piercing operation, said process being carried out in such man-ncr that when the billet hesitates under the interference of the plug, pushing force exceeds the yielding stress of the material of the billet, causing the respective portions near both ends of the billet to bulge so as to take the shape of a dog bone and, at the same time, causing both the contact area and the contact pressure between the billet and the rolls to increase so as to enhance pulling force of the rolls, the so enhanced pulling force together with the increasing pushing force causing the billet to be effectively bitten by the rolls.
In the accompanying drawings:
Figure 1 is a cross section showing an example of a press roll pierc-ing mill for carrying out the method of the present invention;
Figure 2 is a graph showing the relationship between the set value of pushing stress and the rate of occurrence of inferior biting;
Figure 3 is a graph showing the relationship between time and reac-tion stress to pusher;
Figure 4 is an electromagnetic oscillograph chart showing a case where control of a fixed speed pushing method is not carried out;
Figure 5 is an electromagnetic oscillograph chart showing the case where the control is carried out with the rolling load as a desired value;
Figure 6 is an electromagnetic oscillograph chart showing the case where the control is carried out with the roll torque as a desired value; and Pigure 7 is a block diagram explaining the control method of the ; present invention.
As shown in Figure 1, a square billet 3-1 is pushed between rolls 4,4', which are rotated by a motor 20, by means of a pusher rod 2 with a push-ing cylinder 1. In front and in rear of the rolls 4,4', an inlet guide 5 and an outlet guide 6 are disposed to keep the material 3, namely, the square billet 3-1 and the pipe 3-2 on the mill center line X-X.

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The above-mentioned rolls 4,4' are formed with circular grooves and a piercing plug 7 is supported by mandrel 8 in the center of the pass formed by the rolls 4,4'.
The square billet 3-1 in the rolling stage is made to advance by the force applied by means of the pushing cylinder 1 and the force applied by means of the rolls 4,4' and the core portion of the square billet is . J~ , ~ ` -: - . .
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10~6308 pierced and expanded to pipe form by means of the piercing plug 7. The outer surface is continuously formed into circular shape by means of the roll grooves and the working is substantially complete at the center line of the rolls, Y-Y.
Th0 pipo 3-2 and the mandrel 8 are supported by guide rolls 10.
We have developed two kinds of pushing methods in an effort to put the press roll piercing method into practical use. One of the two methods is a fixed pressure pushing method, and is a method of piercing the square billet by keeping the pushing force applied in the axial direction of the square bil-let constant during the piercing operation, wherein the pushing stress p (=pushing force/material cross sectional area) is set to be almost equal to yield stress ~m (at permanent strain 1%) of the material. The other method is a fixed speed pushing method wherein the pushing force Fo is set by pre-suming c-p > ~m. The pushing speed is set so that the volume of the billet to be pushed in per unit time and the volume of the hollow shell to be discharged per unit time from the rolling rolls become substantially equal. The pushing speed Vp is determined by the following formula.

Ll Vp = Kl K2 L VR .................... .... (1) wherein, Kl is a correction constant depending upon the method of manufacture of the material, and for example, in the case of rolled billet, Kl is 1.0, and in the case of continuous cast billet, it is properly set at 1.06, but, even in the case where the billet has a particularly large center cavity, Kl does not exceed 1.1. Accordingly, Kl _ is the reciprocal number of the substan-tial elongating ratio. K2 is the ratio of the roll peripheral speed to the outlet speed of the material, and the roll peripheral speed VR is obtained by the following formula wherein the roll groove bottom peripheral speed is VMIN
- and the roll groove bottom diameter is DMIN, and the groove diameter is d, . .

V V MIN
MIN

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and the value of K2 is the constant of 0.95 - 1.20. VR can be determined at a location such as a maximum diameter portion of the roll where the calculation can be easily made, and the roll peripheral speed of the optional diameter por-tion of the roll is V~R " and the constant K'2corresponding to the constant K2 is determined with K'2 = K2 VR/V'R,. Also, the fixed speed pushing method spe-cifies only the relationship between the roll peripheral speed and the pushing speed as indicated in the formula (1), and as a result, Vp, VR may be changed while holding this relationship, but Kl and K2 to be determined by the kind of billet material and rolling conditions are kept constant during rolling opera-tion. In the press roll piercing method, inferior biting tends to occur fre-quently. Thus, the rear end face of the billet is pushed by the pusher, and the front end portion of the billet is urged against the piercing plug support-ed in the center of the rolls, and the core portion of the billet is pierced by the piercing plug with substantially the pushing force of the pusher only, and only when the front end portion passes between the rolls and leaves the rolls does the advancing force exerted by the rolls begin to reach the constant value. The pushing force necessary for completing the biting frequently ex-ceeds the yield stress of the material. After the billet is extracted from the heating furnace, the temperature drop in the vicinity of the front end por-tion is greater as compared with that of the mid portion in proportion to the time intervening until the start of piercing and, as a result, a difference occurs in the deformation resistance. The pushing force necessary for com-pleting the biting is variable depending on the conditions of the rolls, plug, and other accessories of the piercing mill or the conditions of the billet, but ` it is approximated to the yield stress of the material.
The relationship between the pushing stress set valueCp and rate of occurrence of inferior biting is shown in Figure 2. WhenCrp~- 'm is esta-blished, and the biting does not start easily due to unfavorable conditions of some kind, the end faces of the billet bulge initially, the billet becomes dog -bone-like in shape, and the contact area with the rolls is increased. In the .

104~308 meantime, the center portion bulges at a speed lower than that of the end por-tions. This condition continues until the biting is completed. Where ~p~m is prcsont, this tendency is not observed, and inferior biting tends to occur.
the case where the pushing stress set value ~p is set sufficiently greater than ~m, as shown in Figure 3, the reacting stress (aQ ) (=reaction force to the pusher/material cross sectional area) to the pusher occurs. Of course the reaction stress laQ ) can not exceed the pushing stress set value (ap).
Refer now to the curves shown in Figure 3.
A curve: The piercing cannot be continued due to too high pushing speed. The billet has jammed in front of the rolls during the piercing operation and the piercing is not desirable.
B curve: This curve tends to occur in the case where the temperature drop in the front end face of the billet is greater or depending on the conditions of the rolls, plug or other accessories. '~ reaches peak va-lue of P~ greater than crm, and after the completion of biting, it drops below' m. If ~p is set to ~1 which is a value smaller than ~m~CJ~can only be elevated to Q point which is equal to 1, and the inferior biting occurs. t In the B curve, in the time where ~Z~ 0m is present, bulging in the transverse direction occurs).
C curve: In the case where the temperature drop in the front end ~ face of the billet is small, and also the setting of the piercing mill is made satisfactorily, the operation becomes like the C curve even if the setting is made at ~p>~m, and even at the highest point Pc, the value is smaller thanm. In this case, p = ~l~rm is tentatively set from the beginning (provided that 1 is greater than the Pc point), the biting is carried out successfully, and the curve becomes identical with the C
curve.
In actual operation, the B curve and C curve are present in mixed forms, and unless the pushing force is set at a value above the maximum pushing force (Pa, Pb, Pc points) necessary for the completion of biting, the advancement of the A~

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104tj308 material stops and the material will be scrapped.
In the fixed pressure pushing method, the reason for setting ~p and ~m aln~ost equal is that when ~p~ ~m is established, the biting becomes remark-a~ly difficult, and the rate of inferior biting reaches 16% with ~p = 0.8 m, and, as the pushing speed is not limited, with ~p ~'m, the material is com-pressed during the piercing operation, and the material jams before the rolls, and finally piercing becomes impossible. Accordingly, in the case of the fixed pressure pushing method ~p must be set accurately to a value slightly smaller than ~m, but it is extremely difficult to cope with the fluctuation of -m due to the difference in the temperature, composition or cross sectional dimension of each billet, and such a setting is not practical industrially.
In the fixed speed pushing method, mis-rolling by the biting due to bulging of the material during the piercing operation or inferior biting in-volved in the fixed pressure pushing method is sharply decreased, and in the case where the inlet guide is the container guide, it is decreased to 4.5% and in the case where the inlet guide is the roller guide, it is decreased to 2.2%.
However, in the fixed speed pushing method, when compared with the fixed pres-sure pushing method in satisfactory condition, the load fluctuation in the p~ercing operation becomes great, and a greater safety factor must be provided in the designing and manufacturing of the installation. For example, the rolling loat and the torque become 1.2 to 2.6 times and 1.1 - 1.6 times res-pectively in the vicinity of the rear end portion as compared with the vicinity of the front end portion, ant m~reover a difference occurs in the deformation of the pipe, and particularly, over-fill is caused in the vicinity of the rear end portion, which is not satisfactory.
In the press roll piercing machine, the present invention stabilizes the load characteristics to low level by controlling the relationship between the pushing speed or peripheral speed of the rolls, rolling load, torque, and electric current of motor, which makes the design of a proper size installa-tion possible, and prevents mis-rolling in the piercing, and improves the pro-duct quality of the pipe by carrying out the deformation without undue stress.
In the fixed speed pushing method, great fluctuation of load charac-teristics during the piercing operation result from the fluctuation of pushing force, bulging of the material during the piercing, which is accompanied by fluctuation in the contact area in the roll groove, pressure distribution, metal flow and moreover the difference in the deformation resistance resulting from the temperature difference in the longitudinal direction of the material, and therefore forecasting of the total effect on the load characteristics is difficult.
The present inventors have studied the relationship of the pushing speed and the load characteristics in detail in the fixed speed pushing method, and discovered the preferred conditions, but even in the preferred conditions, there is a limit in coping sufficiently with the variety of factors involved.
Now, the present invention will be described more in detail in the following, wherein the relationship between the pushing force before the start of piercing and the pushing speed and the roll rotating speed is set in accord-ance with the fixed speed pushing method mentioned above, and thus inferior biting is prevented and smooth piercing is started. Thereafter, the rolling load, roll torque or current of roll drive motor are measured, and the pushing speed or the peripheral speed of the roll or both of them are manipulated to perform feedback control.
The reason for setting the pushing force to ~p> ~m in accordance with the fixed speed pushing method is that the pushing speed and the roll peripher-al speed can be independently controlled, besides the prevention of inferior biting. The choice of pushing speed for the start of piercing according to formula tl) is made because formula (1) is extremely close to the desired val-ue and also it can be done with small control amount.
The measurement of the roll load can be carried out with a pressure transducer and dynamic strain meter, and the roll torque is measured with a strain gauge and torqu~ transmitter, torque receiver, and dynamic strain meter, ~;'`

`. : . ~ - : ' 104~;308 and the current is generally measured with an ammeter. The torque is pre-ferably measured not on the roll shaft but between the roll and the reduction gear or between the reduction gear and the motor. When the desired value for control is set, the established technique can be used of scanning the value for each micro time as to whether or not the value has reached the constant value, and in order to do it simply, the non-constant portion at the start of piercing is 0.3 - 0.5 sec with the pushing speed of 270 mm/sec, and the value may be set at the time thereafter or the value after one second, from the time when the load characteristics start to fluctuate. Among the various characteristics, what to select as the control target may be changed according to the requirement from time to time. For example, when the strength of the rolling mill or the mill rigidity is presented as a problem, the fixed rolling load is preferred, and when the capacity of the roll drive motor or the strength of the drive transmitting unit is presented as a problem, the fixed torque or current is preferred, and in the case where uniform deformation of the material is desired, the fixed rolling load produces high precision. Prin-cipal load characteristics other than the rolling load, roll torque, current of the roll trive motor thereinafter referred to briefly as rolling load etc.) csn be enumerated as the pushing force, pushing speed, roll thrust, mandrel thrust, etc. but the interrelation with the rolling load etc. is not clear, ant they cannot be used independently as single factors. In the case of starting the piercing with fixed speed pushing, the rolling load etc. is such, as shown in Figure 4, that there is a constant portion after a non-constant portion at the biting timel and thereafter there is a rising range and a fall-ing range. When the rolling load is in the rising range, the pushing speed is tecreased or the roll peripheral speed is increased. The change of the roll peripheral speed is within + 20% which is sufficient, and the change in the deformation change is far smaller from the effect exerted on the rolling load by the relationship between the roll peripheral speed and the pushing speed in ` 30 the practical range. However, the acceleration during the piercing operation :' .
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104~308 should be limited to a minimum and it is preferable to combine it with a de-crease of the pushing speed. When the rolling load is in the falling range, the countermeasure is the opposite to that employed in the rising range. Thus the roll peripheral speed is decreased or the roll peripheral speed is de-creased together with an increase in the pushing speed. In this case, the in-crement of the pushing speed only results in increased load on the pushing de-vice, and therefore, such an increment will be carried out in a range where there is room for installation, and it is not advisable to design the install-ation capacity for such purpose. In the case where the roll torque or the current for the roll drive motor (hereinafter referred as torque etc.) are given emphasis, situations change somewhat. In the case where the torque etc.
is in the rising range, the pushing speed is lowered or the pushing speed is lowered and the roll peripheral speed is lowered at a rate smaller than the drop in pushing speed. In the case where the torque etc. is in the falling range, the pushing speed is elevated or the roll peripheral speed is elevated at a ratio smaller than the pushing speed.
The foregoing controls are described as examples with respect to the case where the target values are fixed values, but when consideration is given to the facts that the temperature of the square billet drops in accordance with the lapse of time, and the deformation resistance is greater towards the rear portion, and similarly the amount to be worked becomes bigger on account of the bulging at the rolls or plug, and if the emphasis is placed on the uniform deformation of the pipe, the desired value for control is preferably like the specific curve which increases during the rolling operation, and is ; contained in the present invention where the rolling load etc. is the subject for control. However, this curve can achieve the object sufficiently in the range of 0 ~~10% with respect to the initial desired value.
Now, the embodiment of the present invention will be described in the following, and Table 1 shows the experimental conditions.

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104~308 Table 1 _ ,_ _ Square billet dimension (mm) 800 x 1300 Piercing dimension (mm) 91~ x 22.7 x 1700 Kinds of steel Low carbon steel ~rolled) Heating temperature (C) 1280 Roll groove dimension (mm) 91.O~
Roll maximum diameter (mm) 452 Plug diameter (mm) 45.5~
Plug tip position 40 mm to the inlet from roll center Reference roll diameter peripheral speed (VR) (mm/sec) 300 Pushing initial speed (Vp) (mm/sec) 270 Set pushing force (ton) 38 Figure 4 is an electromagnetic oscillation chart of the rolling load and roll torque in case the conditions of Table 1 are carried out from the first to the last, and towards the rear portion, the load becomes bigger, and the rolling load is 2.2 times that at the steady range portion and the torque is 1.5 times. However, the set pushing force is 38 ton, and when it is translated into the set pushing stress, it becomes 6 kg/mm2 and it is about three times the yield stress of the material at the time, but actually, the reaction merely exceeds the yield stress slightly. The cross section of the square billet keeps bulging during the piercing operation, and the four sides are bulged by 2 to 5 mm. Figure S shows the embodiment wherein the roll peri-pheral speed is set constant, and the rolling load a' is set at the desired ~alue and the pushing speed is manipulated, whereby the rolling load is with-~ in 1.4 times of the desired value, and the roll torque is within 1.2 times.
! Figure 6 shows the embodiment wherein the roll peripheral speed is set con-`' 30 stant, and the roll torque b" is set at the desired value and the pushing speed , , A

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104ti308 is manipulated, whereby the rolling load is 1.2 times of the desired value, and the roll torque is less than 1.1 times of the desired value, and is sta-bilized remarkably at low level when compared with the embodiment in Figure 4.
In the foregoing Figure 1, the arrangement for controlling the push-ing speed and the peripheral speed of the rolls which are described in the foregoing is illustrated.
The pushing force generated on the billet and the pushing speed of the billet are measured by the load cell (12) and the speedometer (13) res-pectively, and the measuring signals are transmitted to the control computer (11) through amplifiers (21) and t22). Also, the rolling load and the rolling torque are measured by the load cell (14) and the torque meter (15), and the measuring signals are transmitted to the computer~(ll) through the amplifiers (16) and (17).
The computer (11) transmits the control signals to the flow con-troller (18) and motor controller (19) on the basis of the signals. The pis-ton speed of the cylinder for pushing (1) is controlled by the flow controller (18), whereby the pushing speed of the billet is controlled, and the motor (20) is controlled by the motor controller (19), whereby the peripheral speed of the rolls is controlled.
Figure 7 is a block diagram showing the speed control of the pushing cylinter. The piston speed of the hydraulic cylinder is changed by manipu- ;~
lating the discharge amount of an axial plunger type variable discharge pump with the use of a servo pump, and the control is carried out so that the roll load coincides with the target value.
Thus, the roll load is detected by the load cell (14) and is am-plified by the dynamic strain amplifier (16). The signals from the amplifier ` (16) are feedback signals and are compared with the desired value, and the ; control signals are transmitted to the flow controller (18). In the flow con-troller (18), the inclination amount of the link mechanism (26) is detected by the inclination amount detector (23), and the feedback signals from the 104~308 detector (23) are compared with the-control signals, and the signal of dif-ference between them is transmitted to the servo amplifier (24). The signal from the servo amplifier (24) is transmitted to the control cylinder 25 and the link mechanism (26) is inclined according to the operation of the control cylinder (25). The discharge amount of the axial plunger type variable de-livery pump (27) changes according to the inclination of the link mechanism (26), and the piston speed of the hydraulic cylinder (1) changes, and the roll load is controlled so as to coincide with the desired value.
The pushing device need not be hydraulic and, for example, a method of pushing the rear end of the square billet using the rotating motion of a motor converted to linear motion by means of rack and pinion can be used.
Furthermore, the ratio of the pushing speed and the roll peripheral speed is `
of relative type and a similar effect can be obtained by manipulating either of the two as described above.
The present invention can be applied not only to steel but also to metal that can be plastically deformed. Also, the cross sectional shape of the billet is not limited to square or rectangular shape. Moreover, the cross sectional shape of the pipe is not limited to the circular shape and may be of oval shape or polygonal shape such as one having 80 angles, which is allaost of circular shape.

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Claims (14)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a press roll piercing method for manufacturing a seamless pipe wherein a polygonal metal billet is pushed in its axial direction between a pair of grooved drive rolls mounted one above the other and piercing is effected in the core portion of the billet by means of a piercing plug extend-ing between the rolls, the improvement which comprises setting the pushing force to be of a predetermined value which corresponds to the stress above the yield stress of the billet at the commencement of piercing; setting the pushing speed and/or the peripheral speed of the rolls to such values that the volume of the billet being pushed in per unit time and the volume of the pipe being discharged from the rolls per unit time become substantially equal, and biting the billet between the rolls at said set values.

2. A method as set forth in the claim 1 wherein the pushing speed Vp is determined by the following, where K1 = 1.0 - 1.1, correction constant determined by the method of manu-facture of the material of the billet, K2 = ratio 0.95 - 1.20 of roll peripheral speed VR against the out-let speed of material, L1 = Length of material, L2 = Length after piercing, VR = Roll peripheral speed calculated by the following formula:

where VMIN = peripheral speed of bottom portion of roll groove, DMIN = diameter of bottom portion of roll groove, d = Diameter of roll groove.

3. A method as set forth in the claim 1 wherein the piercing is carried out after the rolls bite the billet.

4. A method as set forth in the claim 3 wherein the billet is continuously pushed from the start of rolling to the completion thereof at pushing speed Vp, where K1 = 1.0 - 1.1, correction constant determined by the method of manufacture of the billet material, K2 = ratio 0.95 - 1.20 of roll peripheral speed VR against the outlet speed of material, L1 = Length of material, L2 = Length after piercing, VR = Roll peripheral speed calculated by the following formula:
VMIN = peripheral speed of bottom portion of roll groove, DMIN = diameter of bottom portion of roll groove, d = diameter of roll groove.

5. A method as set forth in the claim 3 wherein the piercing of the billet is effected while controlling the pushing speed and/or the peripheral speed of the rolls.

6. A method as set forth in the claim 5 wherein the controlling comprises the steps of rotating the rolls at predetermined speed, measuring the rolling load of the rolls, and selecting the pushing speed such that the rolling load becomes substantially equal to a desired value.

7. A method as set forth in the claim 5 wherein the controlling compri-ses the steps of pushing the billet between the rolls at the set pushing speed, measuring the rolling load of the rolls, and setting the peripheral speed of the rolls such that the rolling load becomes substantially equal to a desired value.

8. A method as set forth in the claim 5 wherein the controlling comprises the steps of measuring the rolling load of the rolls, and setting the pushing speed and/or the peripheral speed of the rolls such that the rolling load be-comes substantially equal to a desired value.

9. A method as set forth in the claim 5 wherein the controlling compris-es the steps of rotating the rolls at set speed, measuring the torque of the rolls, and setting the pushing speed such that the torque becomes substan-tially equal to a desired value.

10. A method as set forth in the claim 5 wherein the controlling compris-es the steps of rotating the rolls at set speed, measuring the electric cur-rent of a motor driving the rolls, and setting the pushing speed such that the electric current becomes substantially equal to a desired value.

11. A method as set forth in the claim 5 wherein the controlling com-prises the steps of pushing the billet between the rolls at the set speed, measuring the torque of the rolls, and setting the peripheral speed of the rolls such that the torque becomes substantially equal to a desired value.

12. A method as set forth in the claim 5 wherein the controlling com-prises the steps of pushing the billet between the rolls at the set pushing speed, measuring the electric current of a motor driving the rolls, and setting the peripheral speed of the rolls such that the electric current becomes sub-stantially equal to a desired value.

13. A method as set forth in the claim 5 wherein the controlling com-prises the steps of measuring the torque of the rolls, and setting the pushing speed and/or the peripheral speed of the rolls such that the torque becomes substantially equal to a desired value.

14. A method as set forth in the claim 5 wherein the controlling com-prises the steps of measuring the electric current of a motor driving the rolls, and setting the pushing speed and/or the peripheral speed of the rolls such that the electric current becomes substantially equal to a desired value.