Method of controlling the wall thickness within a tube elongater by utilizing a screw down control
United States Patent 1 1 1111 3,874,211 Hayashi 1 51 Apr. 1, 1975 METHOD OF CONTROLLING THE WALL 2,780,118 2/1957 Kritscher 72/252 THICKNESS WITHIN A TUBE ELONGATER g g 3;  
 , rien BY UTILIZING A SCREW&#39;DOWN CONTROL 3,645,121 2/1972 Pfeiffer e11 al. 72/205 [75] Inventor: Chihiro Hayashi, Takarazuka, Japan 73 A I S M t H d t d, Primary Examiner-Milton S. Mehr Sslgnee g z f g a n &#34;es &#34;m e Attorney, Agent, or Firm-Watson, Cole, Grindle &amp;  
  Watson [22] Filed: Jan. 22, 1974 [Zl] Appl. No.: 435,509 [57] ABSTRACT This invention relates to a method of controlling the 52 us. 01 72/208 72/19 72/240 Wall thickness of tubes- A Special depressing mecha- 7203i nism is provided on the specific stands of a tube elon- 51 1111. c1 B2lb 17/04, B2l b 37/14 gater a Step Prior to a reducing that, by [58] Field of Search U 72/240 234 8 19 208 opening and tightening control. of this mechanism, the 72/209 54 reduction of the yield by the phenomenon of the ends thickening of a tube in the reducing mill may be pre- [56] References Cited vented. The response to the depression may be quick and thus the efficiency of the production may be re- UNITED STATES PATENTS markably improved 2,209,968 8/1940 Gould et al. 72/209 X 2,767,603 10/1956 Rendcl 72/15 x 4 Claims, 9 Drawing Figures HTENTED W75 3.874.211  
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  a E 0 M N 3 in E ms HN TEETH THUN AWTE wEu 2953mm fiNZ STAND No METHOD OF CONTROLLING THE WALL THICKNESS WITHIN A TUBE ELONGATER BY UTILIZING A SCREW-DOWN CONTROL BACKGROUND OF THE INVENTION This invention relates to a continuous elongation method in the case of producing seamless tubes and more particularly to a method of wall thickness control in a tube elongater to provide a uniform wall thickness distribution in the lengthwise direction of a finished product through both the tube elongator and the reducing mill following it.  
  First of all, the steps for producing a seamless tube with a rolling mill shall be explained. The arrangement of a generally extensively practiced equipment is formed of a piercing mill (Mannesman piercer), tube elongater (mandrel mill) and reducing mill (stretch reducer). A round billet having come out ofa heating furnace is first bored with the piercing mill, the thus rolled hollow piece is short and thick, therefore it is reduced in wall thickness and extended in length with the next tube elongater so as to be shaped into a mother tube to be reduced. Further, a reducing mill is used to determine its outside diameter or to reduce the outside diameter to a smaller size. This invention is to control the screw down of the rolls in the intermediate elongating step in order to obtain a uniform wall thickness distribution in the lengthwise direction of the finished tube in the final reducing step among the above mentioned tube making steps.  
  First of all, the summary of a stretch reducing mill should be explained as an example of the most extensively adopted reducing mill. I  
  Such a mill is formed usually of 8 to 24 stands&#39; and greatly reduces the outside diameter of the mother tube to make a small diameter tube. Generally, ifthe outside diameter of the tube is reduced with caliber rolls, the wall thickness increases at a rate corresponding to about one half the outside diameter reduction rate. Therefore, a considerably large tension (reaching about 85 percent of the resistance to deformation in the extreme case) is given between the respective stands during the rolling so that the wall thickness may be reduced Furthermore, so that the roll r.p.m. on each stand may be freely and accurately varied, the rolls of each stand are controlled individually with a single DC. motor or one electric motor together with a differential gear and hydraulic motor called a Thoma axial piston type pump in each stand.  
  As for the stretch reducing mill, 3-roll type reducing mills have become prevalent and general in recent years compared with the 2-roll type. Irrespective of the use of two or three rolls, as compared with the steady state in which the tube is engaged in all stands of the reducer, when in transient state in which the tube is not engaged in all the stands such as when it is being engaged or is being released at the ends, the level of the tension will be reduce to half. The rolled tube, therefore, will be remarkably thicker near the both ends than in the middle. Therefore, there is a phenomenon of ends thickening near the both ends of the tube. In the final product, such part is cut off as off-gauge and therefore the length of the crop at each end causes a very great loss in the yield.  
 SUMMARY OF THE INVENTION The object of this invention is to provide a controlling method of cancelling the phenomenon of the ends 5 thickening of a tube in a reducing mill by tapering and reducing the wall thickness near each end of the tube in advance in an elongation step prior to a reducing step in order to prevent such phenomenon of the ends thickening of the tube and to greatly improve the yield.  
  That is to say, this invention relates to a method of thinning the ends of a tube in a tube elongater in order to cancel the phenomenon of the ends thickening of a tube in a reducing mill. Here, by taking a mandrel mill as an example of the tube elongater, this invention shall be explained more particularly with reference to a continuous rolling step of a Mannesman piercer (piercing mill), mandrel mill (tube elongater) and stretch reducing mill (reducing mill).  
  The mandrel mill is a rolling mill for elongation of a hollow piece pierced with the Mannesman piercer as inserted with a mandrel bar and is an X-mill formed usually of eight stands inclined by 45 to the horizontal and alternately varied by in the arrangement. The rolls of each stand are driven independently by a DC. motor. The tube is brought into close contact with the mandrel bar with the first one or two stands, is reduced in the wall thickness with the next five stands and is made truly circular in the cross-section with the final stand and, at the same time, a uniform clearance is made between the tube and mandrel bar so that the bar may be easy to strip out. The reduction of the crosssection finishes mostly between the first stand and the sixth stand. The reduction can not be given even with the seventh stand but the seventh stand should be understood to be a sizing stand for making an oval and it is required to make the tube truly circular in crosssection with the eighth stand.  
 BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 A-E is a schematic view showing an example of steps of seamless tube making;  
  FIG. 2 is a view showing the wall thickness distribution in the lengthwise direction of a finished product produced in ordinary tube making steps;  
  FIG. 3 is an&#39;explanatory View showing the essential point of the control of this invention with a depressed area reduction rate;  
  FIG. 4 is an exemplary view showing the wall thickness distribution in the lengthwise direction of a finished product produced by the method of the wall thickness control of this invention;  
  FIG. 5 is a view showing an apparatus of this invention.  
 DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1, A is a rotary bed type heating furnace, B is a Mannesman piercer, C is a mandrel mill, D is a walking beam type reheating furnace and E is a stretch reducing mill.  
  Further, I is a round billet. 2 is a hollow piece, 3 is a mandrel bar and 4 is a mother tube to be reduced.  
  An example of the wall thickness distribution in the lengthwise direction of a tube to be produced in the above tube making steps is shown in FIG. 2. The mother tube entering the reducing mill E in this case is of an outside diameter of 108mm, wall thickness of 5.5mm. and length of 15,700mm. and the finished product tube is of an outside diameter of 27.2mm, wall thickness of 4.5mm. and elongated length of 85,800mm.  
  According to an ordinary reducing step, as evident from the drawings, for 3,000 to 3,500mm. from the tube ends, due to the above described phenomenon of the ends thickening, the tube becomes so remarkably thicker than in the middle part that it can not be used at all as a product and therefore part of the ends of the tube are cut off to remarkably reduce the yield of production.  
  Now, this invention is to control the wall thickness to make thinner the tube ends in the mandrel mill C in the step prior to the reducing mill E to prevent the remarkable reduction of the yield caused by such phenomenon of the thickening. To explain more particularly, as shown in FIG. 3, the screw down as the tube enters and leaves the mill is controlled by using the specific fifth stand and sixth stand. That is to say, the roll gaps of the fifth stand and sixth stand are closed inadvance by a fixed screw down. When a fixed time has passed from the time when the front end of the tube enters the fourth stand, the depressing screws of the fifth stand and sixth stand are elevated at a rising velocity given a fixed acceleration until a fixed roll gap is reached to open the clearance. When a fixed time has passed from the time when the rear end leaves the fourth stand, the depressing screws of the fifth stand and sixth stand are lowered and tightened at a lowering velocity having an acceleration until a fixed roll gap is made to close the clearance. Thus, the wall thickness control to make the tube ends thinner is by the screw down control. In such case, so that the condition of volume velocity constancy may be satisfied in response to the variation of the cross-sectional area of the tube, the control of the roll r.p.m. of the fifth and sixth stands can be simultaneously added. For this purpose, it is easy to particularly add the control of the roll r.p.m. of all the stands as the tube enters and leaves the mill.  
  An example of the wall thickness distribution in the lengthwise direction after the reducing step ofa mother tube having had the wall thickness thus controlled at both entering of the front end and leaving of the rear end of the tube in the step of the mandrel mill C is shown in FIG. 4. In this case. the dimensions of the mother tube entering the reducing mill E are the same as in the case of FIG. 2 and the dimensions of the product are also exactly the same except that the elongated length is slightly different. As evident from this drawing, the phenomenon of the tube ends thickening in the reducing mill E is substantially perfectly cancelled with the ends thinning control ofthe tube thereby providing uniform wall thickness distribution in the lengthwise direction.  
  In the above mentioned exemplification, the case of controlling the screw down of the tube ends by using the fifth and sixth stands of the mandrel mill C as specific stands has been explained but, in the case that the reduction can be applied with the seventh stand, the sixth and seventh stands may be made screw down controlling stands, or the screw down may be made with only one specific stand.  
  It also has been investigated to adopt another hydraulic screw down system instead of the electric screw down system in which a screw down is used to make thinner and control such tube ends. In the hydraulic screw down system, the response to the screw down can be made so quickly that, by its excellent immediate response, it is possible to perfectly cancel the phenomenon of the ends thickening of a tube in the reducing mill with the thinning and controlling of the tube ends in the mandrel mill. However, the hydraulic screw down mechanism so called here is not a well-known conventional mechanism. In the known conventional hydraulic screw down mechanism, as the upper rolls are fixed and the lower rolls are screwed down and controlled, the pass line varies with the adjustment of the depression. Needless to say, even the above described electric screw down system is the same. This causes no trouble at all in the rolling operation of a strip mill but causes a fatal trouble in the case of rolling a tube or particularly elongating a tube.  
  That is to say, the roll gaps of the fifth stand and sixth stand is closed in advance by a fixed screw down and, when a fixed time has passed from the time when the front end of the tube enters the fourth stand, the roll gaps of the fifth stand and sixth stand is opened with a fixed acceleration until a fixed roll gap is reached and, when a fixed time has passed from the time when the rear end of the tube leaves the fourth stand, the roll gaps of the fifth stand and sixth stand is closed with a fixed acceleration until a fixed roll gaps is reached. Thus, the control of the wall thickness to make the tube ends thinner is made by controlling the screw down.  
  In the mandrel mill, as described above, a hollow piece pierced with a Mannesman piercer as inserted with a mandrel bar of a perfect rigid body is elongated and rolled with caliber rolls consisting of about eight stands. Therefore, the pass line must always coincide with the center axis of the mandrel bar. Not only in the stead state in which the tube is engaged in all the stands but also in the transient state in which the tube is entering or leaving them at the rear ends, the rolling must proceed with the pass line unchanged. Therefore, if only the lower rolls are screwed down and controlled with the upper rolls fixed as it is, only the wall thickness on the lower side of the mandrel bar will be varied but the wall thickness on the upper side of the bar will not be controlled. Therefore, in the screw down control of a specific stand in such tube elongation step, both of the upper rolls and lower rolls must be screwed down and controlled always symmetrically to the pass line. An example of hydraulic screw down controlling mechanism relating to this invention is shown in FIG. 5. It shall be explained in the following. This example is an improved development of the hydraulic screw down mill system of I company developed as a strip thickness controlling system of a strip mill. However, a system of H Company or a system developed jointly by the applicant of this invention and M Company may be also used. The principle mechanism of the hydraulic screw down in not to be claimed here.  
 In FIG. 5 illustrating a housing 5 for one side ofa roll I stand of the mandrel mill, 6 is an upper roll, 6&#34; is a lower roll (the marks and on the right shoulders of part numbers represent parts respectively for the upper side and lower side hereinafter) and 7&#39; and 7&#34; are roll chocks which are incorporated in the housing so as to be able to control the roll gap as opposed to each other above and below the pass line as a center. That is to say, the upper surface of the roll chock 7&#39; and the lower surface of the roll chock 7&#34; bearing respectively the rotated and driven roll 6 and 6&#34; are connected respectively with the heads of hydraulically operated cylinder 8 and 8&#34; so as to be slidable up and down within the housing. There are also provided upper compression bars 11 and lower compression bars 11, which are vertically passed on both sides of these roll chocks and are screwed to the housing respectively with screws 9 and 9&#34;. Compression bars 11 and ll are made to contact at the tips 10 and 10&#34; respectively with the roll chocks 7&#39; and 7&#34;.  
  A main load cell 12 is fitted between the upper roll chock 7 and its hydraulic push-up cylinder 8&#39;. The compression bars 11&#39; and 11 are fitted with bar load cells 13&#39; and 13&#34; for the respective adjusting bars. The output signals of these load cells are made inputs respectively of an upper calculation controlling device and lower calculation controlling device 15&#34; through a calculator 14&#39; for the cells 12 and 13&#39; and a summing amplifier 14&#34; for the cells 12 and 13&#34;. Further, roll controlling servo valves 16&#39; and 16&#34; operate respectively the upper cylinder 8&#39; and lower cylinder 8&#34; connected with an oil pressure generating device 17 by the instructions of calculation controlling devices 15&#39; and 15&#34; and control the screw down of the upper roll 6 and lower roll 6&#34; as set symmetrically above and below the pass line. The calculation controlling device controls the loads P and P by comparing the outputs of the main load cell 12 and bar load cell 13&#34; and varies the push-up forces of the oil pressure cylinders 8&#39; and 8&#34; by actuating the servo valves 16 and 16 so as to keep the ratio at a predetermined fixed ratio. Though only the housing on one side is shown in FIG. 5. the housing on the other side is provided also with the same devices. As described later, these controlling devices are connected through an electric circuit so as to operate exactly the same on the right and left hous ings. If the compression bars 11&#39; are moved or the wall thickness, rolling friction coefficient and deformation resistance of the tube material vary, the loads P and P,, will vary, the balance between them will be lost and therefore the calculating devices 15&#39; and 15&#34; will work so that the push-up force P, that is, the roll position may be automatically corrected until the balance is obtained. Thus the roll gap will become constant irrespective of the rolling load, the rolled thickness and outside diameter will be constant irrespective of the rolling conditions and the roll gap will be as set.  
  Now, the thickness adjustment in this type of hydraulic screw down mill can be made by the following two methods. The first of them is to move the compression bars 11&#39; and 11&#34; up and down with the screws 9&#39; and 9&#34; and adjust the lengths projecting out of the roll chocks 7 and 7&#34; oftheir tips 10&#39; and 10&#34;. The second method is to give an electric voltage signal K X proportional to a screw down adjusting amount X to the calculator. The screw down by the first method is used only for the ordinary setting. The ends thinning control of the tube during the rolling is made by the second method. In the latter, the mechanical movement within the rolling mill is only to let in and out the oil pressure oil, all the others are to send out and receive electric signals and therefore a very high speed screw down response is obtained. Here K is a spring constant of the adjusting bar.  
  In the above, a layout using a mandrel mill as a tube elongator has been described. But the idea of the present application can be applied to the case of such other tube elongater as, for example, a plug mill, multi-stand plug mill or Pilger-mill.  
 What is claimed is:  
  1. A method for controlling the wall thickness of a tube in a tube elongater with the utilization of a hydraulilc screw down control mechanism, the method comprising the steps of: passing the tube through a tube elongator after the tube has passed through a piercing mill and prior to the tube passing through a reducing mill; passing the tube through the hydraulic screw down control mechanism arranged within the elongator, such mechanism including upper and lower caliber rolls which may be screwed down and opened symmetrically about a pass line as the tube enters and leaves the elongater; controlling the movement of the upper and lower caliber rolls as the tube passes through the mechanism such that the caliber rolls are temporarily screwed down as the ends of the tube pass through the mechanism and are open when the middle of the tube passes through the mechanism, whereby the ends of the tubes are made thinner within the elongater prior to the tube entering the reducing mill.  
  2. A new method according to claim 1 wherein in the step of thinning the ends of the tube, a screw down screw is utilized in the hydraulic screw down control mechanism.  
  3. A method according to claim 1, wherein the step of thinning the ends of the tube is accomplished by moving compression bars positioned above the upper and lower caliber rolls within the hydraulic screw down mechanism in an up and down direction with the utilization of screws provided in the housing of the tube elongater so as to adjust the portions of the rolls projecting out of their respective roll chocks about the pass line.  
  4. A method according to claim 1, wherein in the step of thinning the ends of the tube, the hydraulic screw down mechanism is electronically controlled with the utilization of a voltage signal proportional to the screw down which is to occur.