Apparatus for producing internally grooved tube

Abstract

An apparatus for producing an internally grooved seamless tube by rolling. The apparatus has pressing rolls 3 and back-up rolls 20, which work with a constant load under control by the load detector 21 to form uniform grooves without breaking the mother tube.

Description

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an apparatus for producing an internally grooved seamless tube. More particularly, the present invention relates to an apparatus for producing an internally grooved seamless tube by pressing rolls (having inwardly rounded periphery) against the outside of a tube, with a grooved plug placed therein.

[0003] 2. Description of the Prior Art

[0004] Internally grooved tubes are conventionally classified into two major categories—inner grooved tubes by rolling process and welded tubes. The former is produced by rolling and the latter is produced by welding. The existing rolling machine to produce grooved tubes has the disadvantage of requiring a mother tube of annealed soft material, because rolling hardly makes grooves on a mother tube of hard material with poor workability. This leads to high production cost. In addition, the rolling machine is limited in production rate to about 40-60 m/min. Any attempt to increase the production rate ends up with tube breakage. This leads to low productivity.

[0005] On the other hand, internally grooved tubes produced by welding need a raw material in the form of thin strip. This leads to high production cost. Such a raw material varies in welding performance depending on the shape of edges or the butting pressure of edges. It is difficult to completely eliminate imperfect welding. Imperfectly welded tubes would suffer coolant leakage when used for heat exchange. Such heat exchange tubes are poor in reliability. In addition, welding gives rise to spatter (fine metal particles scattered from the weld zone) which remains in the tube. While such tubes are being used for an air conditioner, the spatter is carried to the compressor by the coolant and the compressor is damaged. This deteriorates the reliability and life of air conditioners.

[0006] In order to address the above-mentioned problem associated with internally grooved tubes, there has been proposed a seamless grooved tube produced by a rolling machine. This rolling machine is constructed of a grooved plug placed in a mother tube and a pair of rolls (having inwardly rounded periphery) which are placed at a position corresponding to the grooved plug. In operation, the rolls press the mother tube against the grooved plug so as to reduce the diameter of the mother tube and transfer the shape of the grooved plug to the inside of the stock tube. The rolling machine employs the above-mentioned rolls in placed of rolling balls, thereby pressing the mother tube against the grooved plug. This rolling machine can process a hard mother tube in the above-mentioned manner. This eliminates the necessity of annealed mother tube and hence contributes to reduced production cost. In addition, this rolling machine is superior in productivity, with a rolling speed in excess of 100 m/min. In addition, the abovementioned rolling process without welding ensures high reliability as in the case of conventional rolling process.

[0007] However, the above-mentioned rolling machine to produce a seamless rolled tube has the disadvantage of generating heat due to rolling because the mother tube undergoes reduction in diameter and grooving inside at the same time at the part where the rolls are mounted, in the case where the positions of the rolls are established before operation according to the size of the mother tube. As the rolling time elapses, the rolls get hot and expand, decreasing the distance between the two rolls. This leads to deformed grooves, excess drafts, and breakage during rolling.

SUMMARY OF THE INVENTION

[0008] The present invention was completed in view of the foregoing. Thus, it is an object of the present invention to provide an apparatus for producing an internally grooved seamless tube, said apparatus being characterized in keeping a constant pressing load without changing the groove shape and causing the breakage of mother tube.

[0009] The present invention is directed to an apparatus for producing an internally grooved tube which is characterized by a rolling-grooving unit comprising: a plug shaft placed in a mother tube, a grooved plug rotatably supported by said plug shaft placed in the mother tube; a pair of pressing rolls with inwardly rounded periphery rolling on said mother tube which are placed outside said mother tube at a position corresponding to said grooved plug, with their axes being parallel to each other and perpendicular to the axial direction of said mother tube; a drive mechanism to rotate said pressing rolls in the drawing direction of the mother tube; a pair of back-up rolls to press said pressing rolls in the direction in which said pressing rolls press the mother tube; a load detector to detect the load applied to at least either of said pressing rolls and said back-up rolls; and a control unit to keep constant the pressing load applied to at least either of said pressing rolls and said back-up rolls according to the values measured by said load detector.

[0010] The above-mentioned apparatus for producing an internally grooved tube may be so modified as to have more than one rolling-grooving unit. The number of the rolling-grooving unit may be two. In this case, a reducing die may be placed between two rolling-grooving units.

[0011] The above-mentioned two rolling-grooving units may be mounted such that the one placed downstream in the drawing direction is rotatable through an angle (&thgr;) from 0° to 90° which is made between the vertical direction and the axis of the pressing rolls in the plane perpendicular to the axial direction of the mother tube.

[0012] According to the present invention, the apparatus for producing an internally grooved seamless tube is operated in such a way that the pressing rolls work under a constant load during operation. Thus, the fin height between the grooves transferred to the tube internal surface becomes constant. Consequently, it is possible to produce the internally grooved tube having the groove shape (the depth of the groove, the groove bottom wall thickness) which has become stable in the lengthwise direction of the tube. In addition, the apparatus of the present invention produces internally grooved tubes without breaking the mother tube.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIG. 1 is a diagram showing the apparatus for producing an internally grooved tube which is demonstrated in Example 1.

[0014] FIG. 2 is a diagram showing the structure of the pressing rolls and back-up rolls mounted in the apparatus shown in FIG. 1.

[0015] FIG. 3 is a diagram showing the apparatus for producing an internally grooved tube which is demonstrated in Example 2.

[0016] FIG. 4 is a diagram illustrating the inclined axis of the pressing roll in the second rolling-grooving unit mounted in the apparatus of the present invention.

[0017] FIG. 5 is a diagram illustrating the inclined axis of the pressing roll in the second rolling-grooving unit mounted in the apparatus of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] The invention will be described in more detail with reference to the accompanying drawings. FIG. 1 is a schematic diagram showing the apparatus for producing an internally grooved tube which is demonstrated in Example 1. This apparatus has one rolling-grooving unit, which consists of a first reducing die 2, a pair of pressing rolls 3, and a second reducing die 6, which are arranged in the direction in which the mother tube 1 advances. Inside the mother tube 1 is arranged a floating plug 7 at the position corresponding to the reducing die 2. Also inside the mother tube 1 is arranged a grooved plug 9 at the position corresponding to the paired pressing rolls 3. The floating plug 7 consists of a large cylindrical part 7a and a tapered part 7b. The outside diameter of the reduced diameter part 7b becomes small in the plug axis direction in the same ratio as the inside diameter of the reducing diameter die 2, so that the floating die 7 is held in position by the reducing die 2. The grooved plug 9 is rotatably supported by the plug shaft 8. (Hence the grooved plug 9 is connected to the floating plug 7 through the plug shaft 8.) Behind the pressing rolls 3 are arranged the back-up rolls 20, so that the pressing rolls 3 are positioned apart as prescribed. The load applied to the pressing roll 3 is detected by the load detector 21.

[0019] FIG. 2 is a diagram showing the structure of the pressing rolls 3 and back-up rolls 20. The pressing rolls 3 and back-up rolls 20 are built into a square sub-frame 22, which is attached to a square main frame 23. Between the sub-frame 22 and the main frame 23 is placed the load detector 21 to detect the load applied to the pressing rolls 3. To the main frame 23 is fixed a unit 24 to apply rolling loads. This load applying unit 24 applies rolling loads to the mother tube 1 through the sub-frame 22, the back-up rolls 20, and the pressing rolls 3. The load applying unit 24 is provided with a servo-motor which applies loads to the sub-frame 22 as it rotates. The reactive force acting on the pressing rolls 3 or back-up rolls 20 is transmitted to the load detector 21 through the sub-frame 22.

[0020] The rolling load detected by the load detector 21 is fed back to a load controller (not shown) which controls the load applying unit 24 so that it applies a rolling load that agrees with a set value.

[0021] The apparatus constructed as mentioned above works in the following way. First, the mother tube 1 has its diameter reduced by the reducing die 2. Second, the mother tube 1 is drawn toward the grooved plug 9 by the pressing rolls 3. Third, the mother tube 1 has its inside grooved by transfer of the helical groove pattern 9a formed on the periphery of the grooved plug 9. Thus there is obtained the internally grooved tube 12, which has its diameter reduced by the second reducing die 6. In this way there is obtained the internally grooved tube 14 with desired dimensions.

[0022] The mother tube 1 receives a prescribed rolling load which is detected by the load detector 21 and controlled by the load applying unit 24.

[0023] The rolling load is controlled as follows. First, an initial value for control is set. The rolling load (A) to be applied to the pressing rolls should be 900-500 N, depending on the diameter of the mother tube and the dimensions of the pressing rolls. The tolerance for load variation (B) should be 1-3% of the rolling load (A). That is, if the load which has been measured by the load detector 21 is within the range of A±B, the rolling load of the roll is regarded as the set value, and control is not carried out. This B is set usually to the value of about 1-3% of the roll press down set load A.

[0024] The load controller calculates the lower limit of rolling load (D1=A−B) and the upper limit of rolling load (D2=A+B).

[0025] The load detector measures the rolling load (C). It should preferably be one capable of making measurements as frequently as 20-200 times a second, so that the object of the present invention is achieved.

[0026] The control unit calculates the average rolling load (C) from n measurements. In other words, C=(C1+C2+C3 + . . . Cn)/n, where n is 5 to 20.

[0027] The control unit compares the average rolling load (C) with the tolerance D1 and D2 and performs control as follows according to the difference obtained by comparison.

[0028] If D1≦C≦D2 (or the rolling load is within the tolerance), the control unit issues no signals.

[0029] If C<D1 (or the rolling load is smaller than the lower limit), the control unit issues a signal instructing the load applying unit 24 to increase the rolling load to the preset value (A). The result is a decrease in distance between the pressing rolls.

[0030] If C>D2 (or the rolling load is larger than the upper limit), the control unit issues a signal instructing the load applying unit 24 to decrease the rolling load to the preset value (A). The result is an increase in distance between the pressing rolls.

[0031] Then, the load detector 21 measures the rolling load at prescribed intervals and sends the results of measurements to the load applying unit 24 for its control.

[0032] According to the embodiment mentioned above, the rolling-grooving unit applies a constant load during operation so that the resulting grooved tube has uniform grooves (in terms of groove depth, fin height, and bottom wall thickness).

[0033] Incidentally, it is not always necessary to mount the load detector between the sub-frame and the main frame. It may also be mounted on the axle of the pressing roll or back-up roll or on the frame which supports the pressing rolls and back-up rolls. The load detector sends the measured values to the control unit (not shown). The control unit compares the measured values with the preset values. If there is any difference, the control unit issues signals to the load applying unit to adjust the rolling load to the preset value. In response to the signals, the load applying unit applies a prescribed load to the back-up rolls, thereby keeping the rolling load constant during operation.

[0034] Incidentally, the embodiment shown in FIG. 1 is constructed such that the load detector 21 is mounted between the sub-frame and the main frame and the load applying unit is mounted so that rolling load is applied through the sub-frame. This construction may be changed such that the load detector is mounted on the axle of the back-up roll or the pressing roll.

[0035] FIG. 3 is a schematic diagram showing the apparatus for producing an internally grooved tube which has two rolling-grooving units arranged in tandem in the drawing direction. In FIGS. 1 and 3, the same parts are indicated by the same reference numerals, and the description of the same parts is omitted. It is to be noted in FIG. 3 that additional paired pressing rolls 5 are mounted between the paired pressing rolls 3 and the reducing die 6 and that an additional grooved plug 11 is mounted in the mother tube 1 at a position corresponding to the position of the pressing rolls 5. This grooved plug 11 is also rotatably supported by the plug shaft 10. Between the paired pressing rolls 3 and the paired pressing rolls 5 is mounted a die 4. This die 4, because the cross section shape of the mother tube which has been rolled by the first rolling-grooving unit becomes elliptic, if rolling is carried out as such by the second rolling-grooving unit, it is impossible to roll into the correct cross section shape, and it becomes impossible to form the predetermined groove shape. Therefore, one corrects the cross section shape of the mother tube 1 to a circle by the die 4.

[0036] The second rolling-grooving unit is also constructed of pressing rolls, back-up rolls, load detector, and axis controller (not shown), which are arranged in the same way as in the first rolling-grooving unit. It is also provided with the control mechanism as mentioned above.

[0037] The apparatus in tandem structure works as follows. The first rolling-grooving unit forms grooves inside the mother tube as the first grooved plug turns. This grooving action slightly twists the mother tube. For example, if it is assumed that the grooved plug 9 in FIG. 3 turns clockwise (as viewed from upstream), then the mother tube 1 twists clockwise (as viewed from upstream) until it reaches the second rolling-grooving unit.

[0038] Usually, the first rolling-grooving unit performs grooving on the upper half (region from 10 to 2 o'clock) and lower half (region from 4 to 8 o'clock) of the mother tube. The second rolling-grooving unit is intended to form grooves in the region where grooves were not formed by the first rolling-grooving unit. If the tube does not twist, the second rolling-grooving unit may be simply shifted by 90° with respect to the first rolling-grooving unit.

[0039] In actual, however, the mother tube twists while it travels from the first rolling-grooving unit to the second rolling-grooving unit. For the second rolling-grooving unit to form grooves exactly in the regions where grooves were not formed by the first rolling-grooving unit, it is necessary that the second rolling-grooving unit be additionally inclined through an angle corresponding to the twist.

[0040] The magnitude of tube twist varies depending on the shape (depth, apex angle, and lead angle) of the grooves formed by the first and second rolling-grooving units, the rolling rate, the draft, and the mechanical properties of the mother tube. It is necessary to previously determine how much the second rolling-grooving unit should be inclined with respect to the first rolling-grooving unit. It is assumed that the first rolling-grooving unit forms grooves in the upper and lower parts of the mother tube (the right and left grooves are not yet formed) and the second rolling-grooving unit forms grooves in the left and right parts of the mother tube, as shown in FIG. 4 (which is a sectional view on a plane perpendicular to the axial direction of the mother tube). If the mother tube twists clockwise (as viewed from upstream) after it has been rolled on its upper and lower parts by the first rolling-grooving unit (as shown in FIG. 4), then the grooves formed by the first rolling-grooving unit are shifted by an angle of &thgr; (with respect to the vertical line) when the mother tube reaches the second rolling-grooving unit (as shown in FIG. 5). Thus, for the second rolling-grooving unit to form grooves, thereby producing the grooved tube as shown in FIG. 4, it is necessary that the second rolling-grooving unit should be turned through an angle of &thgr; (with respect to the vertical line). As mentioned above, the angle of &thgr; varies depending on the shape (depth, apex angle, and lead angle) of the grooves formed by the first and second rolling-grooving units, the rolling rate, the draft, the mechanical properties of the mother tube, and the distance between the first and second rolling-grooving units. The second rolling-grooving unit would work satisfactorily under any condition if it is rotatable through an angle from 0° to 90°.

[0041] The apparatus for producing an internally grooved seamless tube may be applied to a mother tube produced from a cast billet which is cut in the predetermined length by hot extrusion and subsequent cold rolling and drawing. The thus obtained mother tube can be used as such without annealing for softening. The material of the mother tube includes oxygen-free copper, phosphorus deoxidized copper, copper alloys (such as Cu—Sn—P and Cu—Sn—Zn—P), aluminum, and aluminum alloys. Any other materials can also be used so long as they permit rolling.

[0042] The above-mentioned pressing rolls and back-up rolls may be made of tool steel, cemented carbide, ceramics (silicon nitride and sialon), etc., alone or in combination, which are used for rolling machines for ordinary steel sheets and nonferrous metal sheets. The load detector 21 may be a load cell for accurate measurement. The load applying unit 24 may be so designed as to apply an adequate load to the rolls by means of a servo motor which is driven according to signals from the control circuit. For example, it may be designed as shown in FIG. 2. That is, the servo motor is fixed to the main frame, and as the servo motor turns according to signals from the control circuit, it turns gears to adjust pressure between the main frame and the sub-frame. In this way it is possible to adjust the rolling pressure applied to the pressing rolls.

EXAMPLES

[0043] The invention will be described in more detail with reference to the following Examples and Comparative Examples.

Example 1

[0044] This example demonstrates the production of an internally grooved tube by the apparatus shown in FIG. 1 which has one rolling-grooving unit. The sample obtained in this example has a cross section (perpendicular to the axial direction) as specified in Table 1. The mother tube used in this example has the properties shown in Table 2. The working conditions are shown in Table 3. 1 TABLE 1 Angle of circumference on which Outside Lead Fin Bottom wall Apex grooves are diameter angle height thickness angle formed 7 mm 5° 0.10 mm 0.25 mm 100° 120° +120°

[0045] 2 TABLE 2 Material Outside diameter Wall thickness Tensile strength C1220 10 mm 0.35 mm 400 N/mm2

[0046] 3 TABLE 3 Outside Outside diameter diameter Circumferential of of Rate of Rolling speed of pressing back-up Condition drawing load pressing roll roll roll 1  60 m/min 200 kgf 116 m/min 30 mm 80 mm 2 100 m/min 200 kgf 194 m/min 30 mm 80 mm

[0047] In Table 3, the outside diameter of the pressing roll is the smallest value of the outside diameter measured at the part that comes into contact with the copper mother tube. The circumferential speed of the pressing roll is that which is measured at the above-defined outside diameter.

[0048] Each operation under conditions 1 and 2 was carried out with or without load control.

[0049] An internally grooved tube, 3000 m long, was produced under the specified conditions, and the sample was examined for fin height and bottom wall thickness at both ends (start and finish). The difference between two measurements was calculated.

[0050] The bottom wall thickness is defined as follows. Since the finished tube has two identical grooved zones, measurements are made for each zone. The bottom wall thickness is measured at both sides of a fin with the maximum total wall thickness in each grooved zone, and measured values are averaged. An average of the average values in the two grooved zones is regarded as the bottom wall thickness.

[0051] The fin height is defined as follows. Since the finished tube has two identical grooved zones, measurements are made for each zone. The maximum total wall thickness and the bottom wall thickness are measured for each zone. The fin height is the difference between the maximum total wall thickness and the bottom wall thickness. The measured values are averaged for each zone. An average of the average values in the two grooved zones is regarded as the fin height.

[0052] The maximum total wall thickness is defined as the sum of the bottom wall thickness and the fin height. Since the finished tube has two identical grooved zones, measurements are made for each zone. The largest value in measurements for the two grooved zones is regarded as the maximum total wall thickness (bottom wall thickness+fin height).

[0053] The results are shown in Table 4. 4 TABLE 4 Difference Difference in bottom Breakage Run Load Rate of in fin wall during Class No. control drawing height thickness working Example 1 1 yes  60 m/min 0.003 mm −0.011 mm none 2 yes 100 m/min 0.013 mm −0.017 mm none Comparative 3 no  60 m/min 0.024 mm −0.033 mm none Example 1 4 no 100 m/min — — * * Broken after drawing about 450 m.

[0054] Operation in Example 1 (Run Nos. 1 and 2) was successful without breakage over the entire length of the tube. The finished tube has a variation smaller than 0.02 mm in fin height and bottom wall thickness measured at both ends (start and finish). This performance meets requirements of heat transfer tube manufacturers.

[0055] By contrast, operation in Comparative Example 1 was unsuccessful, with breakage (in Run No. 3) or with large variation in fin height and bottom wall thickness (in Run No. 4). This poor performance is far from getting satisfaction of heat transfer tube manufacturers.

Example 2

[0056] This example demonstrates drawing with the apparatus in tandem structure as shown in FIG. 3. The finished grooved tube has a cross section (perpendicular to the axial direction) as specified in Table 5. Note that it has two grooved zones which have different lead angles. 5 TABLE 5 Angle of cir- cumference Bottom on which Outside Grooved Lead Fin wall Apex grooves are diameter zone angle height thickness angle formed 7 mm 1 +10° 0.10 0.25 mm 100° 90° +90° mm 2  −5° 0.10 0.25 mm 100° 90° +90° mm

[0057] In Table 5, the grooved zone 1 is that which is formed by the first rolling-grooving unit, and the grooved zone 2 is that which is formed by the second rolling-grooving unit.

[0058] The mother tube used in this example has the properties shown in Table 6. 6 TABLE 6 Material Outside diameter Wall thickness Tensile strength C1220 10 mm 0.35 mm 400 N/mm2

[0059] The mother tube was drawn and grooved under the conditions shown in Table 7. 7 TABLE 7 Circum- Outside Outside ferential diameter diameter speed of of of Con- Rate of Rolling Rolling pressing pressing back-up dition drawing unit load roll roll roll 1  60 m/min 1st unit 180 kgf 104 m/min 30 mm 80 mm 2nd unit 200 kgf 116 m/min 30 mm 80 mm 2 100 m/min 1st unit 180 kgf 172 m/min 30 mm 80 mm 2nd unit 200 kgf 194 m/min 30 mm 80 mm

[0060] The mother tube was worked under the conditions 1 and 2, each with or without load control, as shown in Table 7. In Table 7, the outside diameter of the pressing roll is the smallest value of the outside diameter measured at the part that comes into contact with the copper mother tube. The circumferential speed of the pressing roll is that which is measured at the above-defined outside diameter. The apparatus used in this example has a die between the first and second rolling-grooving units, which correct the cross section of the intermediately drawn tube. Since the copper tube is twisted after rolling by the first rolling-grooving unit 1, the second rolling-grooving unit 2 is so positioned as to compensate this twist (or to prevent the overlapping of grooving).

[0061] An internally grooved tube, 3000 m long, was produced under the specified conditions, and the sample was examined for fin height and bottom wall thickness at both ends (start and finish). The difference between two measurements was calculated.

[0062] The bottom wall thickness is defined as follows. Since the finished tube has two identical grooved zones, measurements are made for each zone. The bottom wall thickness is measured at both sides of a fin with the maximum total wall thickness in each grooved zone, and measured values are averaged. An average of the average values in the two grooved zones is regarded as the bottom wall thickness.

[0063] The fin height is defined as follows. Since the finished tube has two identical grooved zones, measurements are made for each zone. The maximum total wall thickness and the bottom wall thickness are measured for each zone. The fin height is the difference between the maximum total wall thickness and the bottom wall thickness. The measured values are averaged for each zone. An average of the avers age values in the two grooved zones is regarded as the fin height.

[0064] The maximum total wall thickness is defined as the sum of the bottom wall thickness and the fin height. Since the finished tube has two identical grooved zones, measurements are made for each zone. The largest value in measurements for the two grooved zones is regarded as the maximum total wall thickness (bottom wall thickness+fin height).

[0065] The results are shown in Table 8. 8 TABLE 8 Difference Difference in bottom Breakage Run Load Rate of Grooved in fin wall during Class No. control drawing zone height thickness working Example 1 5 yes  60 m/min 1 0.006 mm −0.004 mm none 2 0.005 mm −0.008 mm 6 yes 100 m/min 1 −0.007 mm 0.013 mm none 2 −0.009 mm 0.013 mm Comparative 7 no  60 m/min 1 0.018 mm −0.036 mm none Example 1 2 0.018 mm −0.035 mm 8 no 100 m/min 1 — — * 2 — — * Broken after drawing about 300 m.

[0066] Operation in Example 2 (Run Nos. 5 and 6) was successful without breakage over the entire length of the tube. The finished tube has a variation smaller than 0.02 mm in fin height and bottom wall thickness measured at both ends (start and finish). This performance meets requirements of heat transfer tube manufacturers.

[0067] By contrast, operation in Comparative Example 2 was unsuccessful, with breakage (in Run No. 8) or with large variation in fin height and bottom wall thickness (in Run No. 7). This poor performance is far from getting satisfaction of heat transfer tube manufacturers.

Claims

1. An apparatus for producing an internally grooved tube comprising a rolling-grooving unit, said rolling-grooving unit comprising:

a plug shaft placed in a mother tube;

a grooved plug placed in the mother tube, said grooved plug being rotatably supported by said plug shaft;

a pair of pressing rolls, with inwardly rounded periphery, rolling on said mother tube, said pressing rolls being placed outside said mother tube at a position corresponding to said grooved plug, with their axes being parallel to each other and perpendicular to the axial direction of said mother tube;

a drive mechanism to rotate said pressing rolls in the drawing direction of the mother tube;

a pair of back-up rolls to press said pressing rolls in the direction in which said pressing rolls press the mother tube;

a load detector to detect the load applied to at least either of said pressing rolls and said back-up rolls; and a control unit to keep constant the pressing load applied to at least either of said pressing rolls and said back-up rolls according to the values measured by said load detector.

2. The apparatus for producing an internally grooved tube as defined in claim 1, wherein the number of said rolling-grooving unit is more than one.

3. The apparatus for producing an internally grooved tube as defined in claim 2, wherein the number of said rolling-grooving unit is two.

4. The apparatus for producing an internally grooved tube as defined in claim 3, further comprising a correcting die between said two rolling-grooving units.

5. The apparatus for producing an internally grooved tube as defined in claim 3, wherein at least one of said two rolling-grooving units which is mounted downstream in the drawing direction is rotatable through an angle (&thgr;) which is made between the vertical direction and the axis of the pressing rolls in the plane perpendicular to the axial direction of the mother tube.

6. The apparatus for producing an internally grooved tube as defined in claim 3, wherein said angle (&thgr;) ranges from 0° to 90°.