Note: Descriptions are shown in the official language in which they were submitted.
~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
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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
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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
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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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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-
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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,
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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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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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Table 1
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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
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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
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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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