US 3577754 A
Description (OCR text may contain errors)
United States Patent Inventor Albert H. Calmes 4, Avenue DuJaman, Lausanne, Switzerland Appl. No. 728,938 Filed May 14, 1968 Division of Ser. No. 421,845 Dec. 29,1964, Pat. No. 3,394,568 Patented May 4, 1971 Priority Sept. 9, 1964 Great Britain 36,988/64 PROCESS AND APPARATUS FOR ROLLING SEAMLESS TUBES 6 Claims, 15 Drawing Figs.
US. Cl 72/45, 72/69, 72/209, 72/236, 72/370 Int. Cl BZlb 45/02, 1321b 27/10, B211) 17/04 Field of Search 72/41, 43,
Primary Examiner-Richard J. Herbst Assistant Examiner-E. M. Combs Attorney-Brown, Critchlow, Flick and Peckham ABSTRACT: A hollow metal blank is rolled forward upon an advancing hollow tapered mandrel to reduce the wall thickness of the blank in longitudinally-spaced steps and thereby elongate it and form gaps around the mandrel between those steps. Lubricant is forced into the mandrel and discharged laterally from it into at least one of said gaps to fill it. The lubricant is prevented from flowing from the gaps back into the mandrel as the gaps are shortened by the advancing mandrel, so the shortening of the gaps with their restricted rear ends increases the pressure on the lubricant above the supply pressure.
PATENTED m men 3.577754 SHEET 1 [1E6 SHEET 2 UF 6 I ,Q 7 Q PATENTEU AY 4 797:
PATENTEB MAY 41971 I 33577754 //vu/v AL 5527' HENRI CAL 5 BY PATENTEU HAY 4 l97| SHEET U UF 6 wa N M INVENTOAQ. ALBERT HEWR/(ALMES ATTORNEYS.
PATENTEDBAY 4:971 3577.754,
' I sum 5 0F 6 lA/VENTOR. ALBERT HENRI CALMES W.M m%%m ATTORNEYS.
PROCESS AND APPARATUS FOR ROLLING SEAMLESS TUBES This application is a division of my copending patent application, Ser. No. 421,845, filed Dec. 29, 1964 now US. Pat. No. 3,394,568.
It is known that seamless tubes of small diameters can be rolled in a continuousmill on a free floating cylindrical mandrel, which is afterwards extracted from the tubes. Thereafter, the tubes are reheated and reduced in a tube-reducing mill to the desired finished smaller size. Because it is necessary to permit free flow of the steel on the cylindrical mandrel, the roll grooves must be oval in transverse section with large openings at the roll flanks, and thus the rolled products tend to have unequal wall thicknesses and different wall thicknesses at each end.
Tubes of larger sizes can be rolled in a one stand two-high plug mill in two or three passes, using a fixed plug held in the roll pass by an axially compressed bar. Such tubes tend to have uneven walls and inside scratches, so they are subsequently expanded by cross-rolling in a tube reeler over an opposed plug held by an axially compressed bar, in order to iron out the unevenness and the scratches.
There also are known other seamless tube finishing mills, viz., the pilger mill and the push bench, which have lower output capacities. The present some inconveniences and some advantages relative to the above-mentioned processes.
Multistand plug mills for manufacturing seamless tubes have also been proposed that are provided with plug mandrels held under tension or with plug mandrels and plain mandrels having controlled floating advance in the direction of rolling. It has been proposed to cool the mandrels from the outside, and also from the inside by conducting cooling water through an inner pipe to the forward end of the mandrel and then returning it along the inside of the mandrel to its inlet end. Using the latter method, the inside scale of the hollow body or blank around the mandrel is not eliminated and lubrication tends to be inefficient, being effected by spraying lubricant onto the outside of the mandrel or by the previous introduction of mineral salt or borax into the bore of the blank to be rolled. The result is that wear on the mandrel and the inner surfaces of the finished products is excessive.
Furthermore, it has been proposed to conduct a lubricant through an inner bore and radial ducts in a plug mandrel to the gap between the hollow blank and mandrel in order to create a layer of lubricant betweenthem. This proposal also provides for conducting water under pressure outside a concentric pipe in the inner bore of the mandrel and out through radial ducts ahead of the first rolling plug at the moment when the first stand of rolls reduces the hollow blank for the purpose of eliminating the scale separating from the inner wall of the blank. The cooling of the rolling plug is effected subsequently, after the rolling, from outside. Such a method leads to loss of time for the outside cooling.
It has also been proposed to conduct a lubricant and cooling water alternately through an inside bore in a plug mandrel for lubricating and cooling the mandrel, and to deliver water In the manufacture of thin-wall seamless tubes, the working conditions needed for a single hollow mandrel during the rolling operation in order to conserve a smooth surface on the mandrel during its working life and to reduce its wear, thereby producing a high hourly output, are the following:
1. Above all, between the inner surface of the hollow body or blank and the mandrel there should exist a complete, con tinuous, separating and lubricating film, in order to avoid heat transmission that would overheat the mandrel, to eliminate adherence of the steel to the mandrel surface, and to decrease efficiently frictional resistance between them.
2. Using a single mandrel, it has to be cooled continuously and intensively from the inside during the whole rolling cycle in order to avoid creation of high temperatures at its surface and in its mass, which soon would lead to heat tensions and consequently to cracks.
3. During the rolling of thin-wall tubes, the inner surface of the hollow blank mustnot come in contact with cool water, because this contact, and especially the formation of steam, would withdraw from the blank important quantities of heat, leading to an inadmissible loss of temperature of the hollow body during the rolling operation.
4. lt is important for the rolling of thin-wall tubes, when high rolling pressures and temperatures increases from friction are created, that the working surfaces of the mandrel be gradually changed in order to keep thesurface temperatures below the critical values, and that-an efficient lubricating and cooling take place during the whole rolling operation. I
5. The scale adhering to the inside wall of the hollow blank being rolled, which increases wear of the mandrel and creates inside scratches through local accumulations on the inside of the tube, has to be detached and eliminated through a gas jet or it must be transformed before the rolling operation into a harmless plastic or liquid substance.
6. In order to achieve in all the rolling grooves of the mill equal deformation of the work, without dangerous stresses of the steel, and to obtain uniform wall thickness, it is necessary to roll the hollow blank in grooves that are as round as possible with limited roll flank openings. Therefore it is required, or
, highly desirable, that the plug or mandrel diameters be under pressure for internal descaling ahead of the first plug through a separate concentric duct. That system results in reduction of the hourly output of tubes and leads to abnormal consumption of the lubricants.
it further has been proposed to cool the inside of the plugs, to lubricate them and to descale the inside of the surrounding hollow blanks by conducting pressure water in a single duct system through the mandrel to the plugs. By such a method, the mandrel is intensively cooled from the inside, but the blanks also are intensively cooled. Moreover, lubricants of the plugs by water is inefficient.
All of these proposals permit thick-wall hollow shells to be rolled into tubes if the mandrel bar is replaced frequently, but they do not meet the working conditions necessary for manufacturing thin-wall tubes on a multistand continuous mill, for minimizing the wear of the plugs or the mandrel bars, and for economically manufacturing thin-wall tubes with smooth inside surfaces.
tapered forward in the rolling direction. This provision also leadsto equal and wall thicknesses. The last two roll passes should not have ovaling and rounding roll grooves for the loosening of the tube from the mandrel, but only circular grooves for deformation purposes, in order to achieve a,high total elongation in as few rolling stands as possible. The tapering of the mandrel favors its extraction from the tube, notwithstanding the high degree of circumferential contact caused by the circular roll passes. I
7. The dimensions and resistance properties of the hollow mandrel have to be adequate for the fulfilling of the abovementioned working conditions, in order that it can withstand with a high degree of security the stress and pressure created through the rolling forces and heat tensions.
It is among the objects of this invention, when rolling thinwall seamless tubes in a multistand continuous mill on a single, tapered, hollow, tension-fixed stepped mandrel or on a hollow plan mandrel advancing at a controlled speed in a rolling direction, to achieve the foregoing seven working requirements in order to manufacture such tubes with uniform wall thickness and to maintain the mandrel and the inside of the finished tubes smooth, with a most economical consumption of mandrels and a high hourly output of tubes. I
The invention is illustrated in the accompanying drawings, in which:
FIG. 1 is a view of a five stand continuous mill, showing the v mandrel and the hollow blank at the start of a rolling cycle;
F IG. 2 is an enlarged fragmentary longitudinal section of the mandrel and workpiece starting to be rolled in rolling grooves represented schematically by the inner diameters of the grooves, shown for convenience as lying in a common plane;
FIG. 3 is a view similar tov FIG. 2, showing the hollowblank as it starts to pass between the second pair of rolls; i
FIG. 4 is a view similar to FIG. 2, showing completion of the rolling operation and beginning of the mandrel-extracting operation;
FIGS. 5, 6 and 7 are enlarged cross sections taken on the lines V-V, VI-VI and VII-VII, respectively, of FIG. 3;
FIG. 8 is a view similar to FIG. 3, but showing'a modificatron;
FIG. 9 is a view similar to FIG. 8, but showing the operation further along;
FIGS. 10 and 11 are cross sections taking on the lines X-X and Xl-Xl, respectively, of FIG. 8;
FIG. 12 is a view similar to FIG. 9, but illustrating another embodiment of the invention;
FIG. 13 is a view similar to FIG. 12, but after completion of the rolling and removal of the tube from the mandrel;
FIG. 14 is an enlarged longitudinal section of a still further modification of the mandrel, showing a hollow body about to enter between the first pair of rolls; and
FIG. 15 is a cross section taken on the line XV-XV of FIG. 14.
Referring to FIG. 1 of the drawings, a continuous tube mill is shown consisting, for example, of five two-high stands of driven rolls 1, 2, 3, 4 and 5, with the axes of the rolls in the second and fourth stands perpendicular to those of the other stands. However, there can be more than two rolls per stand if desired. Each stand of rolls is provided with roll grooves that form a circular roll pass. At the entrance end of the mill a heated tubular, cylindrical body or blank 6 is disposed, ready to be fed into the mill. Spaced a considerable distance behind the blank is a carriage 7 that can be moved toward and away from the mill by any suitable means, such as by a fluid pressure cylinder 8. Secured to the front of the carriage and extending toward the mill is a mandrel 9. The rear portion of the mandrel is encircled by a cylindrical pusher or ram 10 that is engaged by the carriage. The mandrel is small enough to be projected through the hollow blank 6, and the ram is large enough to engage the rear end of the blank. If the cylindrical body or blank is taken directly from the mill shown in my copending US. Pat. application, Ser. No. 379,834, from which the body exits at high temperature, no reheating of the body before entering the present mill vwill be required. Also, the concentric sidewalls and even ends of the cylindrical body produced in the other mill are desirable features for the bodies fed to the mill disclosed herein.
Attached to the entrance end of the mill at opposite sides of the blank 7 are double-acting hydraulic cylinders 12, from which plungers 13 extend back toward carriage 7. The outer ends of the plungers are connectedby a crosshead 14 provided with an opening '15 large enough to receive the ram, but too small to permit the carriage to pass through it. After the carriage has been moved forward against the crosshead, the speed of the continued forward movement of the carriage is controlled by the rate of escape of fluid from the inner ends of the two cylinders through suitable throttling valves 16. Therefore, the cylinders control the advance of the mandrel through the mill and actually compel it to move at a very slow speed compared to that of the front end of the elongating blank.
Also, by delivering fluid under pressure to the front ends of 60 cylinders 12 through wide-open valves 16, the crosshead will push the carriage backward away from the mill and thereby withdraw the mandrel from the mill.
The mandrel is hollow, as shown in FIGS. 2 to 7, having a tubular body 17 closed at its front end, and a concentric pipe 18 extends forward through the body to a point near its front end. The outer or rear end of the pipe may be connected with a flexible drain hose 19 FIG. 1. Encircling this center pipe is another pipe 20 that is concentric with the first one and that is spaced from it and from the inner surface of the mandrel body. The annular space between the front ends of the two pipes is closed by a suitable sealing ring 21. The passage formed between the outer pipe and the mandrel body is connected at its rear end with a flexible hose 22, through which The water flows to the front end of the mandrel body and then returns through the center pipe to the drain hose, thereby continuously cooling the mandrel.
The rear end of the passage formed between'the two pipes in the mandrel body is connected to a flexible hose 24 (FIG. I), through which a suitable lubricant'is supplied under high pressure. The lubricant can be a mixture of graphite, wood sawdust and heavy oil, or graphite, wood sawdust and cheap waste grease. In such a mixture the graphite is the real lubricant, the sawdust the filling substance, and the heavy oil or grease the transport element. Other examples of lubricants are mineral salt, borax, specific glass powder, silicates or metal oxides with a low fusion point and high protective and lubricating properties. To allow the lubricant to reach the outer surface of the mandrel, the sidewall of the mandrel body is provided at predetermined axially spaced locations with outlet openings 25 that are connected by valves with ports in the side of the outer pipe 20.. As shown in FIG. 6, these valves normally are closed by movable valve members 26 pressed outwardly against their seats by the pressure of the lubricant behind them. The valve stems are provided with heads 27 that extend outwardly beyond the periphery of the mandrel body. When these heads .are pushed inwardly, as shown in FIG. 5, the valves are opened to permit lubricant to be forced out through the side of the mandrel. The mandrel decreases in size from its'rear end to its front end and therefore is tapered. The taper may take the form of a gradual uniform size reduction, or in steps, or a combination of the two. The tapered mandrel shown in FIGS. 1 to 4 is stepped down forward to form, in effect, a series of long plugs connected by radial or annular steps. All but the rearmost valve is shown located at the base of a step. 1
To form the heated cylindrical tubular blank 6 shown in FIG. I into a thin-walled seamless tube 30 (FIG. 4), the mandrel carriage 7 is moved forward rapidly by cylinder 8 to pass the mandrel through the blank until the front end of ram 10 engages the back of the blank and starts to move the blank forward with the mandrel. Continued advance of the ram will .1, which thereupon grip the blank and contract it upon the mandrel. as shown in. FIG. 2. These rolls can be designed to sink the blank onto the mandrel or merely to reduce its diameter to the point where there is only a thin annular gap between the blank and the mandrel. Reduction of the wall thickness of the blank is not the purpose of this first stand of rolls.
The blank continues to move forward on and with the mandrel in order to enter the roll pass formed by the'second stand of rolls 2, as shown in FIG. 3, which is the first pair of wall reducing rolls. At each following stand of rolls the blank is reduced further, whereby its wall thickness is reduced in iongitudinally spaced steps with an annular space around the mandrel between the steps. At the time the blank enters the pass at rolls 2, the mandrel carriage 7 engages the crosshead 14 of the feed-limiting device for the mandrel, which thereafter reduces and controls the speed of advance of the mandrel through the mill so that by the time the rear end of the tube 30 rolled in the mill leaves the last roll pass between rolls 5, each valve head 27 will have moved forward to the next roll pass. However, the advancing valves do not move so far forward as to enter the roll passes during rolling, as that would result in the tube being rolled onto the valves, which must not occur. The controlled advance of the mandrel during rolling changes continuously the surface of the mandrel in each roll pass where the rolls are working the metal being rolled therein, and this change avoids excessive surface temperatures on the mandrel which would alter its composition and reduce its resistance to wear.
After'the tube, supported by a runout table 31 or the like,
leaves the last pass, cylinder valves 16 are opened wide to permit the mandrel to be advanced rapidly a short distance until the rear end of the tube has been carried by the mandrel pasta pair of spaced stripper plates 32. The mandrel then stops, the
cooling water under high pressure is supplied to the mandrel. stripper plates are moved toward each other by any suitable now push the blank into the grooves of the first stand of rolls the stripper plates, the tube could be loosened from the mandrel by passing the tube through two final roll stands provided with' oval and round passes that I do not reduce the wall thickness of the tube in reducing its diameter. As soon as the mandrel has been withdrawn from the tube, the rear end of the tube can be cropped if desired.
During the rolling operation, continuous and intensive colling of the inside of the mandrel is carried out by'conducting water under high pressure through flexible hose 22 into the rearend of the mandrel passage that encircles both pipes inside of the mandrel body. The water flows forward-through this passage against the inner surface of the mandrel body until it reaches the front end of central pipe 18. The water then returns through this pipe to the rear end of the mandrel and the drain hose 19. The water maintains a constant low average temperature of the mandrel and thereby permits the use of a single mandrel. The cooling prevents temperature buildup in the mandrel, which would lead to outside surface cracks. It will be noted that none of the cooling water comes in contact with the inside of the hollow blank or the tube.
Simultaneously with this cooling, lubricant is delivered to the rear end of the passage between pipes 18 and 20 and flows forward to the various discharge openings in the wall of the mandrel. As soon as the front end of the blank leaves the paxcs through the first roll stand, to the rear end of the hollow blank and out of it. The graphite, and the carbon formed from the sawdust and cracking of the heavy oil, are deposited on the inner surface of the hollow blank and on the first mandrel plug for complete and efficient lubrication of the plug.
During further advance of the blank through the roll stands, the successive lubricant-discharge valves are opened by the. front end of the advancing tubeand they distribute the lubricant under high pressure, in the way just described, between the blank and the mandrel. Therefore, all of the spaces between the blank and the mandrel plugs are filledcompletely with the lubricant to provide layers of lubrication and thermal protection for the long tapered plugs. Flow of lubricant from the gaps back through the feeding system is prevented by a conventional check valve in the system. Between each pair of reducing stands the lubricant-filled gap around the mandrel is shortened by the advancing mandrel shoulder at the rear end of the gap, and since the metal rolledin against the mandrel around the shoulder restricts flow of lubricant out of the rear end of the gap, shortening of the gap increases the pressure on the lubricant in the gap above the pressure at which it is supplied to the gap. This helps to force the lubricant forward into the roll pass to createa more effective continuous separating and lubricating film. As the rear end of the tube being rolled passes each valve, that valve is closed automatically by the outward pressure of the lubricant against the valve member 26. Through this automatic opening and closing of the valves, 3 most efficient lubrication is obtained with economical consumption of lubricant. The speed of advance of the blank between the first and second roll stands is related to the chemical and physical properties of the lubricant, because there should be enough time for the lubricant that has been introduced into the gap between mandrel and blank to attain the desired fusion temperature and the related properties of lubrication before it reaches the second stand.
By using outwardly opening spring-closed valve members,
they could be opened by increasing the lubricant-supply pressure to a predetermined value instead of by the leading end of the blank depressing heads 27.
instead of the examples of lubricants given above, the lubricants may be formed from many other substances, as long as they provide the necessary thermal protection, lubrication and the transformation of the solid scale in a'short contact with the hot hollow blank. The efficiency of the lubricant can be enhanced by chrome-plating the outer surface of the mandrel, or by applying a coating of ceramic, plastic or similar substances with lower friction coefficients than steel mandrels have. This measure contributes not only to a smoother inner surface of the tube being rolled, but also provides a smoother outer surface because the reduction of the coefficient of friction of the mandrel allows a proportionately lower coefiicient of friction of the rolls, which permits the use of smoother roll surfaces.
This apparatus allows seamless tubes with smooth inner surfaces to be manufactured at low costs for tools, power and labor. Long tubes can be produced for introduction into a stretching mill without further heating. By using the continuous mill with a tapered mandrel and a stripper that permits the mandrel to be extracted from the tube, the rolls can form substantially closed places that will, deform, without harm cheap raw material, such as square continuously cast blooms.
The modified mandrel shown in FIGS. 8 to 11 is advanced slowly between the rolls during rolling inthe same manner as the mandrel first described. herein. The modified mandrel body 40 likewise is tapered, but it is provided with only a sininner end of these openings connect with a passage formed around a pipe 45 spaced from the inner surface of the mandrel body'and joined at its front end to the shoulder. The rear end of this passage is connected to a source of supply of lubricant under pressure. Disposed inside pipe 45 and spacedtherefrom is a center pipe 46 that extends forward in the mandrel to a point near its front end. While the mandrel is being advanced slowly through the mill, efficient and intensive inside cooling is achieved by introducing cooling water under pressure into the rear end of the passage between the two pipes so that it .will flow forward in the mandrel to cool it and then back through the center pipe to a suitable drain or cooler and recirculator.
As soon as the hollow blank 48 has been contracted onto the mandrel behind the lubricant openings by the first stand of rolls49, lubricant is forced out through the openings under the control'of any suitable automatic timer (notshown), so that the lubricant will fill the gap formed between the advancing tubular blank and the mandrel. This gap is completely filled because it is closed at its rear end at step 41, and is closed at its front end because the second stand of rolls 50 (the first reducing stand) rolls the front end of the blank against the mandrel. Of course, the lubricant feeding system is provided with a check valve to prevent backflow of the lubricant put under high pressure. As the mandrel advances, the
lubricant-filled gap is gradually shortened as shown in FIG. 9,
so that the lubricant is pushed along by the mandrel step 41 and rolled into a layer between the hollow blank and mandrel for the thermal protection and lubrication of the mandrel. The inside wall of the blank will not be descaled by a gas current, but existing scale is made innocuous by introducing in any suitable manner mineral salt, borax and coal dust or similar material into the hollow blank before it is charged into the mill.
The external shape of the hollow tapered mandrel shown in FIGS. 12 and 13 are substantially the same as the one just described, except that the front end of the body 55 of the mandrel is not closed by a fixed plug. Instead, it is provided with a forwardly facing valve seat 56 (P10. 13), in front of which there is a socket 57 containing a valve. A movable valve member 58 is pressed against the seat by a coil spring 59. Just in front of the inner ends of the lubricant openings 61 in the i sidewall of the mandrel body there is a radial shoulder 62 connecting the two inside diameters of the body. Joined to this shoulder is the front end of the pipe 63 that extends rearwardly through the mandrel body in spaced relation therewith. Lubricant under pressure is supplied to the outer or rear end of the passage between the pipe and the surrounding inner surface of the mandrel body. The lubricant ,is ejected from openings 61 into the gap between the mandrel and tubular blank 64 in the same way and with the same results as explained in connection with FIGS. 8 and 9. However, cooling of the mandrel is different.
Thus, cooling water is supplied to the outer end of pipe 63, but at a pressure low enough to prevent it from opening the valve at the front end of the mandrel. The mandrel therefore is filled with water throughout the rolling operation, but during that period the water does not circulate into and out of the mandrel. As soon as the rolling operation has been completed and the mandrel has been retracted to extract it from the rolled tube, the pressure of the cooling water is increased sufficiently to cause it to force valve member 58 off its seat and allow'the water to flow out of the front end of the mandrel while the mandrel is being returned to its starting position and while it is waiting to start the next rolling operation.
The main reason for this design of mandrel is to allow the use of small diameter mandrels having outside diameters of 2 to 3 inches for the manufacture of small, especially thick wall 7 tubes. The construction of this mandrel can be of a high strength notwithstanding the separate water cooling and lubricating systems within it.
FIGS. 14 and 15 illustrate a tapered mandrel that is provided with its own seal for closing the rear end of the gap that extends forward between the tubular blank 66 and the mandrel from the radial lubricant-discharging openings 67 in the wall of the mandrel body 68. The seal is formed by machining an annular groove 69 in the outside of the mandrel body and mounting an elastic ring 70 in the groove. The ring projects from the mandrel body far enough to snugly engage the inner surface of the tubular blank when the latter is telescoped over i the mandrel and pushed forward by a ram 71 encircling the mandrel. The seal prevents flow of the lubricant backward along the mandrel during rolling of the tube, and the seal also forces the lubricant forward between the blank and mandrel timed in such a manner that filling of the gap will be 'completed at substantially the same time the front end of the blank is reduced by rolls 72 and the gap thereby is closed. The
lubricant is delivered to the inner ends of openings 67 through lustrated in FIGS. 8 to 13. Cooling water is forced through the passage between the two pipes toward the front end of the mandrel, and leaves through the center pipe 74.
I claim: 7
1. A process for producing seamless tubes in a continuous tube-rolling mill, comprising advancing the front end of a hollow tapered mandrel through the mill at a'controlled speed, rolling upon theadvancing mandrel in the mill an encircling metal blank to reduce its wall thickness and cause it to elongate forward alongthe mandrel, conducting a stream of lubricant through the advancing mandrel and discharging it laterally therefrom between the mandrel and the encircling blank at a predetermined point to lubricate the outer surface of the mandrel in front of said discharge point, restricting appreciable flow of the lubricant backward along the outside of the mandrel from said point, and simultaneously preventing flow of the lubricant backward through the mandrel.
2. A process according to claim 1, in which said advancing speed is such that the lubricant attains a predetermined temperature within a predetermined time after it is discharged from the mandrel at said point.
3. A process according to claim 1, m which the mandrel is advanced rapidly a predetermined distance as soon as rolling has been completed.
4. A process according to claim 1, in which said backward flow restricting is accomplished by filling the space between the blank and mandrel behind said discharge point with a sealing ring.
5. A process for producing seamless tubes in a continuous tube-rolling mill provided with a plurality of roll stands, comprising placing a hollow metal blank around a hollow tapered mandrel, advancing the front end of the mandrel through the mill at a controlled speed, rolling upon the advancing mandrel in the mill the encircling metal blank to reduce its wall thickness in longitudinally spaced steps and thereby cause it to elongate forward along the mandrel and to form annular gaps around the mandrel between said steps, forcing lubricant into the advancing mandrel and discharging it laterally therefrom into a said gap to fill it, and preventing flow of the lubricant fromthe filled gap back into the mandrel as pressure on the lubricant in the gap is increased by shortening said gap as the mandrel advances through said roll stands.
6. A process for producing seamless tubes according to claim 5, in which saidadvancingspeed is such that the lubricant attains a predetermined temperature within a predetermined time after it is discharged from the mandrel at said point.