GB1602088A

GB1602088A - Rolling method and apparatus

Info

Publication number: GB1602088A
Application number: GB22382/78A
Authority: GB
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 1977-05-28
Filing date: 1978-05-25
Publication date: 1981-11-04
Also published as: CA1086105A; DE2823071C2; FR2391785A1; LU79718A1; BR7803380A; AU3652678A; SE7806058L; AU520817B2; SE432542B; IT7823928A0; IT1094862B; DE2823071A1; FR2391785B1

Description

(54) ROLLING METHOD AND APPARATUS (71) We, NIPPON STEEL CORPORATION, a Japanese Company, of 6-3, Otemachi 2-chome, Chiyoda-ku, Tokyo, Japan, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a method and apparatus for rolling a workpiece in the form of plate, billet, bar or rod.

In a conventional rolling operation in which the reduction in cross-sectional area of the workpiece per pass is from about 20 to 30%, the contact angle 0 between the workpiece and a pair of flat rolls must be considered in determining the maximum reduction in cross-sectional area.Thus under any conditions inter alia in respect of rolling speed, roll configuration and workpiece, in order to perform steady-state rolling, the contact angle 0 must be in the relation H < tan~ly (in which y denotes a coefficient of friction between the workpiece and rolls biting on the workpiece), and it is common practice to determine a maximum level of reduction of workpiece cross-sectional area within the range of permissible contact angle 0 defined by the relation between 0 and Ju. For grooved rolls a similar relationship applies to the equivalent contact angle 01.

On the other hand, with respect to the coefficient of friction between the workpiece and workrolls, assuming that the coefficient of friction in the steady rolling condition after the initial biting of the rolls on the workpiece is completed is ,u', the following relation is well known: tan-IC1 > tan-1C1 in which ,u' is the coefficient of friction between the rolls and the workpiece when the workpiece is completely bitten by, ie engaged between, the rolls.The relation shows that, in the steady rolling condition, it is possible to perform the rolling operation with large contact angle B as compared with the contact angle when the rolls begin to bite the workpiece, so that rolling may be performed with a large reduction in cross-sectional area by using contact angles 0 or ol defined by the expression o or flztan-',u'.

A method of rolling a metal workpiece with high degree of reduction in area is disclosed for example in US patent application Serial No. 759,868. This method is to roll the metal workpiece with a rolling mill wherein the workrolls are supported so that the rolls do not shift in the direction of the forward feed movement of the workpiece, and is characterised in that the gap between the workrolls is so adjusted as to keep the contact angle 0 in the relation of 8) tan-'y, and rolling is performed while the workpiece is being continuously driven between the rolls adjusted to the foregoing gap, with a thrust of magnitude such that a neutral point in respect of the rolling operation is produced in the plane where the workrolls and the workpiece are in contact.However, this method requires a device such as a large-scale thruster to thrust the workpiece an entire length of the workpiece between the workrolls continuously and steadily and this method therefore has the drawback that it is very difficult to effect continuous rolling with a large reduction in workpiece cross-sectional area per pass, with a plurality of roll stands. It is possible for example to use a master-slave thruster system to remedy this drawback. In this system the workpiece is pushed into the first roll stand by a master thruster, and the workpiece is pushed into the second stand by the-first stand. However, this system cannot be used to thrust or drive the workpiece through the workrolls continuously in more than two passes, and accordingly, this imposes a limit in regard to making the rolling mill train of a compact size by employing high-reduction rolling.

Another form of workpiece pushing method comprises driving the workpiece into the roll gap by pinch rolls or by using one roll stand to drive the workpiece into the next following roll stand, and such a method remedies the foregoing problems arising from the use of the thruster apparatus. However, when the thrust becomes zero between the pinch rolls and the rolling mill or between the rolling mills, the workpiece is rolled under unsteady or fluctuating conditions. Because of this phenomonon, there is a danger of defective biting of the rolls on the workpiece, and in addition there is often substantial fluctuation in the width of the rolled workpiece, which impairs the efficiency of the rolling operation in regard to output and product quality.In order to solve the problems, rolling may be performed by connecting the preceding and succeeding workpieces by means of welding, but for this operation, another installation becomes necessary, which gives rise to further complication in respect of operation and installation. Also, the thrust applied to the workpiece influences the deformation of the workpiece between the workrolls, and this results in increased spreading of the workpiece in the direction of its width, over the entire length of the workpiece, thus resulting in a drop in the efficiency of workpiece deformation.

Another method of rolling a workpiece with the high reduction in workpiece section, where the contact angle is 0 > tan-',u, is disclosed in US Patent No.

3,553,997. This rolling method comprises performing continuous inline reduction wherein a front end of the workpiece is made to pass between the workrolls to a certain extent by opening the gap of the workrolls sufficiently, so that the workpiece is gripped by the workrolls. The roll gap is subsequently reduced to perform the rolling operation with a large contact angle. In this rolling method, it is possible to roll the workpiece with a high reduction in cross-sectional area, but the tip portion of the workpiece becomes off gauge, lowering the output of rolled product. This method has other problems, for example that the roll gap has to be changed for every fresh workpiece to be fed into the roll stand, and the rolling installations become complicated and of large size.Furthermore, in this rolling method, the cross-section of the tip portion of the workpiece is larger than the cross-section of the succeeding part, and also it changes rapidly whereby a dynamic movable guide is required at the entry side of the rolling stand of the next downstream stage, and the rolling speed has to be changed according to the change in the workpiece cross-section, thus requiring a complicated control mechanism.

Also, in this rolling method, in the case where grooved rolls are being used, the leading end or tip portion of the workpiece, whose cross-section is larger, does not fit the roll groove and so rolling of a billet, bar and rod workpiece is almost impossible.

Methods of assisting the biting of the workpiece by the rolls have been previously proposed, which comprise cutting the leading end of the workpiece in a wedge form, or driving the workpiece between the workrolls by causing another cold or hot workpiece to collide with the trailing end of the first workpiece.

However, these methods are employed in an ordinary rolling operation wherein the contact angle 8 is in the relation of B < tan-'. Moreover, in these methods, there are also many problems for example with respect to output of rolled product, maintenance of the installation and steadiness of the rolling operation.

According to the invention there is provided a method of rolling a metal workpiece to produce a high degree of reduction in its cross sectional area by a rolling mill having flat rolls (in which case the contact angle between the workpiece and the rolls is represented by 0) or grooved rolls (in which case the equivalent contact angle Ol is given by 81=tan' (Cos a, tan 0) where 0 represents the contact angle between the workpiece and the rolls at the bottom of the roll groove and a represents the angle of inclination of the groove sides), which method comprises feeding the workpiece through a first rolling stand and/or pinch roll means towards at least one further rolling stand downstream of said first rolling stand or pinch roll means and producing in the workpiece a compressive stress which is below the yield stress at the time of rolling and acts in a feed direction along the rolling line between the first rolling stand or the pinch roll means and the further rolling stand thereby enabling the workrolls of the further rolling stand to bite the workpiece;; engaging the workpiece in the workrolls of the further roll stand with the gap of the workrolls of said further roll stand set such that tanz or Olitan-',u where y is the coefficient of friction between the workpiece and the workrolls at the time of engagement of the workpiece into the roll gap and ,u' represents the coefficient of friction between the workpiece and the workrolls when the workpiece is completely engaged between the workrolls and rolling is in progress:: when the workpiece is completely engaged in the workrolls of the further rolling stand maintaining the workrolls of said further rolling stand at their previously set roll gap, and readjusting the peripheral speed of the workrolls of the first rolling stand and/or the pinch roll means and continuing the rolling operation while there is substantially no stress in the workpiece acting in a direction along the rolling line between the further rolling stand and the first rolling stand or pinch roll means.

The invention also provides apparatus for rolling a workpiece to produce a high degree of reduction in its cross-sectional area, comprising a rolling mill having flat rolls (in which case the contact angle between the workpiece and the rolls is represented by 0) or grooved rolls (in which case the equivalent contact angle Ol is given by Bl=tan-1 (cos a, tan 0) where B represents the contact angle between the workpiece and the rolls at the bottom of the roll groove and a represents the angle of inclination of the groove sides comprising:: a first rolling stand wherein the roll gap is so set that the contact angle o or the equivalent contact angle OlAtan-1/ where y denotes the coefficient of friction between the workpiece and the workrolls at the time of engagement of the workpiece into the roll gap; at least one second rolling stand disposed downstream of said first rolling stand and having a roll gap so set that the contact angle B or the equivalent contact angle BI is such that tan-',u' > tan-1,u (wherein ,u' denotes the coefficient of friction between the workpiece and the workrolls when the workpiece is completely engaged between the workrolls and rolling is in progress);; speed detecting means for detecting the peripheral speed of the workrolls of the second rolling stand; means for detecting the completion of engagement of the leading end portion of the workpiece between the rolls of the second rolling stand; and means for controlling the peripheral speed of the workrolls of the first rolling stand on the basis of a signal from the speed detecting means so that a compressive stress of above 1% and below 100% of the yield stress is generated in the workpiece at the upstream or entry side of the second rolling stand until the workpiece is completely engaged between the rolls of the second rolling stand, and for adjusting the peripheral speed of the rolls of the first and/or the second rolling stand on the basis of a signal from the engagement completion detecting means so that said stress is not generated in the workpiece at the upstream or entry side of the second rolling stand after the workpiece is completely engaged between the rolls of the second rolling stand.

A method and apparatus according to the present invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a graph showing the relationship between the thrust or pushing force on the workpiece and the equivalent contact angle between the workpiece and the rolls, Figures 2a and 2b are diagrammatic views illustrating the equivalent contact angle, Figure 2a showing the inclined angle of the roll groove, and Figure 2b showing the contact angle at the bottom of the roll groove, Figure 3 is a graph schematically showing changes in the rolling load versus time, Figure 4 is a graph showing the relationship between the rolling speed and the equivalent contact angle, Figure 5 is a diagrammatic view of one embodiment of the rolling apparatus, Figures 6a to 6c show alternative forms of the guide rolls of the Figure 5 apparatus, Figure 7 is an explanatory diagram of the groove arrangement in a diamond system, Figure 8 is an explanatory diagram of the groove arrangement in box-like or square and oval systems, Figure 9 is a diagrammatic view showing a conventional representative train of rolling mills, Figures 9a-c showing a train of billet mills, and Figures 9d and e showing a train of continuous rough rolling rod mills, Figure 10 is a diagrammatic view showing an example of a train of rolling mills according to this invention, Figure l0a showing a train of billet mills, Figure l0b showing a train of bar mills, and Figure l0c showing a train of continuous rough rolling rod mills Figure 11 is a block diagram of control arrangement in the train of rolling mills according to this invention, Figure 12 is a diagrammatic view for explanation of definition of grooves, Figure 13 is a view of an example of roll grooves employed in the rolling of a bar or rod workpiece.

Referring to Figure 1, in the graph shown, the thrust applied to the workpiece is represented by the ratio (risk between the compressive stress a generated in the workpiece while it is under thrust, and the yield stress K of the workpiece at the time of rolling. The contact angle between the workpiece and the rolls which in this instance are grooved is represented by the equivalent contact angle Ol which is influenced or changed by the angle of inclination of the walls of the groove in the workroll.As shown in Figure 2a, the angle formed by the working surface 3 of the groove 2 of the workrolls 1, with a line I which is directed parallel to the roll axis, is shown as a, and as shown in Figure 2b, the contact angle at the bottom diameter Do of the roll groove 2 is shown as 0, so that the equivalent contact angle Ol is represented as follows: BI=tan-' (cos ax tan 0) It will be appreciated that the groove in the workrolls may be of oval cross-section instead of the angular groove of Figure 2a and the angle of inclination of the side of the angular groove of Figure 2a will be replaced by the angle of inclination of the oval groove.The cross-sectional area of the substituted angular groove and the longer diagonal line thereof are respectively replaced by the area of the crosssection of the oval groove and the long axis thereof; in the case of rolling by a flat roll, since v0, cos a=l, and the equivalent contact angle Blbecomes equal to Oso that the present parameters can describe rolling with both flat and grooved workrolls.

In the meantime, curve I shown in Figure 1 is a line showing the equivalent contact angle at the limit of initial engagement or biting of the workpiece by the rolls. Point A represents the equivalent contact angle Ol at the limit of initial engagement of biting of the workpiece between the workrolls without applied thrust, and this angle is, as described in the foregoing, BI=tan-'CL. Curve II is a line showing the limit of rolling of the workpiece in the steady condition after biting of the workpiece between the workrolls, namely, the limit of generating slip between the workpiece and the workrolls.For example, if it is desired to perform a rolling operation with a high reduction in workpiece sectional area, operation at a thrust ratio b will enable the workpiece to be driven between the workrolls at a maximum equivalent contact angle Ol of D. In this case, the curve II shows that after the workpiece is initially engaged, or bitten, into the workrolls, rolling can be performed in the steady state with a lesser thrust ratio b'. Also the point B shows the equivalent contact angle at the limit of rolling of the workpiece in the steady state after the workpiece is bitten into the workrolls and without applied thrust and this angle is, as described in the foregoing, Ol=tan~',u'.

The range wherein the equivalent contact angle BI is from 0 to A is a range where the workpiece is bitten in the workrolls, under no applied thrust and within this range the workpiece can only be rolled at the hitherto conventional contact angle o or Ol to bring about a conventionally obtainable reduction in cross-sectional area. Also, the range B-C is a range wherein the workpiece is bitten by the workrolls under an applied thrust within the yield stress at the moment of rolling of the workpiece, and even after completion of the biting step, the thrust is applied continuously during the rolling operation; the foregoing high-reduction rolling method falls within this range. Furthermore, at a thrust ratio of 1.0, initial biting of the workpiece by the rolls can be effected with the equivalent contact angle of point C. At this time, the minimum required thrust for continuing steady-state rolling corresponds to the contact angle B or Ol represented by point C at the intersection between the line extending perpendicularly to the ordinate axis from the point B' on the curve II, and the ordinate axis. It will be understood that, when the thrust ratio exceeds 1.0, the workpiece at the time of biting is subject to a thrust above the yield stress, and is deformed just before the rolling mill; the range of the equivalent contact angle above the point C corresponds to this condition.In this range, for example, a high-reduction extrusion method is employed in which a heated workpiece housed in a container is shaped by being thrust through a forming aperture portion or die by thrust applied by a pushing device.

The rolling method of this invention uses the range of equivalent contact angle A-B shown in Figure 1, and the value of the equivalent contact angle is not comparable to those of the high-reduction rolling method and the high-reduction extrusion method, but it exceeds the reduction in work-piece area achieved in a conventional method. But neither the high-reduction extrusion method nor the high-reduction rolling method can operate continuously with more than two passes, whereas the rolling method of this invention permits the rolling of a workpiece continuously in more than two passes.

In general, in the rolling of a workpiece in the form of plate, billet, bar or rod, normally the total factor of elongation from the starting material to the rolled product is about 400--500, which is frequently used; and there is also the following relationship between the total elongation A total and the elongation Ai for each pass: A total=Ao,Al,A2 n Now, a major technical problem is how large the elongation for each pass should be made or how the plural rolling steps with high elongation may be effected.

The thrust ratio employed in this rolling method of this invention as described herein is basically in the range of O--a in Figure 1; in this case, the range of the reduction in workpiece cross-sectional area that can be achieved is in the range of A-B, in terms of the equivalent contact angle as described in the foregoing, namely, tan-'c < B or OeAtan~',a'. Accordingly, when the workpiece is bitten into the workrolls, the equivalent contact angle B can be achieved at a thrust ratio a.

Also, after biting of the workpiece into the workrolls is complete, rolling is continued with no induced compressive stress in the workpiece in the direction of the rolling line. The time when the leading end- or tip portion of the workpiece reaches the roll centre line m (Figure 2b), over the entire cross-section of the workpiece, is the time when biting of the workpiece into the rolls is completed; even if a local portion of the tip of the workpiece (for example a neck or crop portion) reaches the roll centre line m, biting of the workpiece is still not completed until the entire workpiece section reaches the line m.

Figure 3 shows diagrammatically the change in the rolling load during rolling.

Point e shows the start of workpiece biting, namely the initial contact between the workpiece and the workrolls, pointf shows completion of biting, point g shows the point where the compressive stress in the forward feed direction along the rolling line is no longer applied to the workpiece, point h shows start of release of the workpiece from the workrolls and point i shows completion of workpiece release respectively. In Figure 3, the portion between the point e and the point g is a section of unsteady rolling where the rolling load fluctuates considerably, while the portion between the point g and the point h is a section in which the rolling load is a substantially fixed value F, and in which rolling is performed in a steady state.After the rolling operation reaches the steady state, even if the thrust is made zero, the equivalent contact angle in Figure 1 is within the range below the curve II, and therefore there is no slip between the workpiece and the workrolls.

It will be appreciated that attainment of the condition indicated by the pointf can be detected by a detector such as a load cell, or hot metal detector. Also, the point g is shifted by adjustment in the peripheral speed of the rolls, but, for example, the following method of timing can be used. The average rolling load F in the steady rolling state is known from the past rolling performance; the rolling load F rises only after the workpiece is bitten into the workrolls, and, when the force F reaches 0.8 F, a timer is operated. After the elapse of the proper time, the peripheral speed of the roll is adjusted to eliminate the stress or force acting on the workpiece in the direction along the rolling line.

As described in the foregoing, the magnitude of the thrust ratio is basically in the range Wer in Figure 1. However, in order to obtain a stable thrust, the magnitude of the applied thrust is desirably such that the compressive stress generated in the workpiece by the thrust is above 1% of the yield stress. Also, the thrust may be of a magnitude higher than that at the point a, to ensure satisfactory biting of the workpiece by the rolls. However, before the workpiece is bitten, in order to ensure that the stress generated in the workpiece does not exceed the yield stress and that plastic deformation does not occur, it is necessary that the compressive stress generated in the workpiece is below the yield stress of the workpiece at the time of rolling, namely, a/k in Figure 1 must be below 1.0.

As described in the foregoing, after biting of the workpiece by the rolls is completed, steady-state rolling is performed; the peripheral speed of the rolls is the speed that does not generate slip between the workpiece and the workrolls which are set with a roll gap wherein the equivalent contact angle Oe is tan-',u < 0eAtan~',u'.

As shown in Figure 4, there is a close relationship between the rolling speed Vm and the equivalent contact angle Oe that can be obtained. The result of Figure 4 is obtained from a hot steel workpiece.

The curve III of Figure 4 is ascertained from the results of various experiments; in the case where the thrust on the workpiece is zero, the curve Ill shows the limit of generation of slip between the workpiece and the workrolls in the steady rolling condition. The region K above the curve Ill is a region where continuous rolling is difficult, due to slip. Curve IV shows the result of experiment conducted by the applicants, comprising hot rolling by means of workrolls with a rough roll surface, and it shows the limit of biting where the applied thrust is zero.

The region L above curve IV represents the region where biting is difficult unless a thrust is applied to the workpiece, and the region M below the curve IV is a region where biting is possible without applying thrust. This curve IV perfectly coincides with therresult disclosed by W Tafel (Literature 'Stahl und Eisen' 1921). The curve V is a curve showing the equivalent contact angle at the maximum level employed in hot rolling, and it can be said that the contact angle has a wide range. The curve VI causes no problem from the point of view of slip but in the region N above the curve VI, due to dropping or buckling of the workpiece, scars or flaws occur. Curve VI thus represents an upper limit above which there is no practical significance in regard to the rolling operation.

In Figure 4, the relationship between rolling speed and contact angle Oe is defined by the region enclosed by the curves III, IV and VI. At a speed within this range, in the steady rolling state, as will be understood from the foregoing, there is no slip between the workpiece and the workrolls. It will be understood that, when the rolling speed, such as in the actual billet mill or the rod rolling mill train, is adjusted to a permitted value derived from in Figure 4, it is in a range of below approximately 2.5 M/S. Namely, in Figure 4, the rolling speed range adopted matches perfectly with the actual rolling installation for a billet, bar and rod.

Conversely, the rolling method of this invention shows its effectiveness particularly in the rough rolling of a billet, bar or rod where rolling speeds are relatively low.

Furthermore, at the higher end of the rolling speed range, the curves III and IV approach each other, and no substantially significant difference is to be seen between the two curves. This rolling speed is generally 2.5 M/S.

Reference is now made to Figure 5 which shows an example of a rolling mill or apparatus for working the method as described above. The roll gap of workrolls 6 of a first stand S is set similarly to a conventional stand, namely, the equivalent contact angle Oe becomes Oe < tan-',u, but the roll gap of the workrolls 6a and 6b of the second and third stands Sa and Sb are pre-set so that the equivalent contact angle Oe becomes: tan-',u < Oe < tan-',a'.

Initially, the workpiece is supplied to the first stand 5 by the usual means, such as a roller table assembly, but since the roll gap is set as described in the foregoing, the workpiece is engaged into or bitten by the workrolls 6 of the first stand 5 without requiring any special means for that purpose. The workpiece passes through the first stand S and reaches the second stand Sa, being guided by a buckling preventing device 16 with rollers 17. Until the leading end or tip of the workpiece reaches the second stand 5a, no stress in the direction along the rolling line is generated in the workpiece.However, when the tip of the workpiece reaches the second stand Sa, since the roll gap of the workrolls 6a of the second stand 5a is pre-set as described above, the workpiece does not immediately bite into the workrolls 6a, and a compressive stress in the direction along the rolling line is generated in the workpiece between the first stand 5 and the second stand Sa. In this condition, as the workpiece is continuously discharged from the first stand S, the compressive stress in the workpiece gradually rises until finally, it exceeds the value shown in the curve I of Figure 1, and the workpiece is engaged into the gap between workrolls 6a of the second stand Sa.

In the condition where the workpiece is completely bitten in the workrolls 6a of the second stand 5, the pointf of the rolling load graph shown in Figure 3 can be found by detecting the rolling load by means of a load cell 8a; the detector signal is transmitted to a tension or tensile stress detecting signal amplifier 1 lea.

Additionally, load cells 9 and 10 are provided at the first stand S and load cells 9a and 10a are provided at the second stand Sa to detect any tension or tensile stress in the workpiece between the first stand 5 and the second stand Sa, ie, the tensile stress in the workpiece in a direction along the rolling line, a negative value of such stress thus representing a compressive stress. The detector signal is amplified by the amplifier 1 la, and is transmitted to the tension control device 12. At a comparator 13, the output signal from the control device 12 is compared with the output signal from a comparator 13a. Also the rotary speed of drive motor 7 is detected by a tachometer generator 15, and the output signal of 15 is applied, together with the signal from the comparator 13, to an aromatic speed control device 14.The automatic speed control device 14 controls the speed of the drive motor 7 so that there is no tension in the workpiece between the first stand 5 and the second stand 5a.

The workpiece is discharged from the second stand 5a and is bitten into the third stand Sb. When rolling is continued, the control of tension in the workpiece between the stands is performed by controlling the speed of the drive motor 7a, by means such as the tension detecting signal amplifier 1 lb, tension control device 12a, tachometer generator lSa and the like, on the basis of signals from the load cells 8b, 10a and 9b. In this case, the tension in the workpiece between the first and second stands changes, and in order to minimise the change, the output signal that matches the output signal from the comparator 13a is transmitted to the comparator 13 to control the speed of the drive motor 7a; at the same time, the speed of the drive motor 7 is controlled.As described in the foregoing, if the speed of an upstream stand is changed, the speeds of all the stands downstream from said stand are changed successively. Figure S shows only up to a third stand 5a, but if further rolling stands are successively disposed downstream of those illustrated, the tension in the workpiece between the third stand Sb and the next stand is controlled by controlling the speed of the drive motor 7b, by the load cell 10b, control devices 12b, 14b, comparator 13b and tachometer generator lSb and the like, similarly as described in the foregoing. Although the foregoing description relates to a control method based on a known upstream control system, this invention is not limited to such an upstream control system but may also employ a control method based on a downstream system.In these controls, each device is known to those skilled in the art, and also the rotary speed of the workrolls may be adjusted by manual operation so that the workpiece tension is made zero, on the basis of the detected tension.

When the workpiece is engaged into the workrolls 6a of the second stand 5a, the peripheral speed of the workrolls 6a of the second stand Sa is approximately set with VR2 < AW2-VR1 and during the normal rolling operation, a slight compressive force is generated in the workpiece. In the above equation, sR, and vR2 denote the respective peripheral speeds, corresponding to the working diameter, of the workrolls of the first stand and the second stand in the zerotension condition of the workpiece and i2 denotes the elongation at the second stand in the zero-tension condition.

It will be appreciated that, even if the ratio of the peripheral speeds of the workrolls of both stands is set to vR2=A2.vR, wherein a substantially zero-tension and zero-compression condition is produced during the normal rolling operation, the tip of the workpiece, as described in the foregoing, is finally engaged into the workrolls as a result of generation of the compressive stress. In either case, when the workpiece is bitten into the workrolls, temporarily between the two stands, a fixed condition of mass flow is not established. Also, when the peripheral speeds of the workrolls 6 and 6a are made to be equal to the peripheral speed in the steady state or to a value similar to that speed, a minimum correction of the speeds of the workrolls 6 and 6a is required after completion of the biting of the workpiece into the roll gap.

Each buckling preventing device 16 is provided with a plurality of pairs of rotatable guide rolls 17, and these guide rolls 17 extend to a position just upstream of the workrolls 6a and 6b, as viewed in the direction of feed movement of the workpiece. As the workpiece is guided by these guide rolls 17, no buckling occurs upstream of the stands 5a and 5b, and also the workpiece is properly guided to the roll gap between the workrolls 6a and 6b. The cross-sectional shape of the path formed by the guide rolls 17 is preferably similar to the cross-sectional shape of the workpiece as shown in Figure 6b, but as shown in Figure 6c, these two crosssectional shapes may be different from each other, as long as the bucklingpreventing and guiding functions are maintained.

In the rolling stand producing the high reduction in workpiece cross-sectional area, the lower limit of the proper equivalent contact angle Oe is normally selected to have a certain safety margin, as compared with the value at point B shown in Figure 1. The reason for this is that it is necessary to apply the pushing force, even though it is present in the stand immediately downstream, except for the final stand, and also to avoid trouble resulting from a sudden disturbance during rolling.

Furthermore, instead of the first stand S, pinch rolls may be used to feed the workpiece into the second stand Sa. Also, instead of detecting the completion of biting of the workpiece by means of the load cells 8a and 8b, the detection operation may be performed by employing a well-known hot metal detector.

In the case of grooved rolls, the roll grooves may be formed in the rolls by a lathe, but a grooving system of diamond-diamond form or diamond-square form as shown in Figure 7 is advantageous from the viewpoint of operation and obtaining a high quality product or achieving the high degree of workpiece elongation desired.

In addition, a box-box system or oval-square system as shown in Figure 8 may achieve the high degree of elongation desired but there is the danger of the workpiece dropping or sagging or the generation of wrinkles or flaws in the groove so such systems require a high degree of operating technology and skill and may be accompanied by considerable difficulties. When the final cross-sectional shape of the workpiece is a square cross-section, such as a billet, a square groove, which provides the usual degree of cross-sectional area reduction to be achieved in a conventional rolling operation, may be disposed downstream of the diamond groove, while in case of a round steel bar, a square groove and a round groove, with the normal degree of reduction employed in a conventional rolling operation, may be disposed downstream of the diamond groove.

As will be obvious from the foregoing detailed description, the described method and apparatus provide that high-reduction continuous rolling can be performed, similarly to continuous rolling with the conventional or normal reduction in area, without applying tensile or compressive force to the workpiece and under conditions where difficulties in carrying out the rolling do not arise. This means the elimination of rolling under unsteady conditions, which is advantageous from the point of view of securing the necessary dimensional accuracy and output of rolled product. Also, since rolling can be performed continuously with a high degree of reduction in workpiece cross-sectional area, the above-described method is advantageous from the standpoint of improving the total elongation of the workpiece.

Furthermore, if a high degree of reduction in the workpiece area is to be achieved, rolling with the high reduction in area can be made possible by decreasing the equivalent contact angle, with workrolls of a relatively larger diameter. However, in this case, the installations become unnecessarily big and are therefore not really practical. On this point, with the present method and apparatus, rolling can be performed with the required high degree of reduction in area by a compact but high efficient rolling installation which has the required minimum diameter of workrolls.

A comparison will now be made between the above-described method of this invention and a conventional method, from the standpoint of installations for the rolling of a product measuring 100 mm square, from 230 mm square steel starting material.

Table 1 shows a comparison between the diameters of the workrolls of the rolling mill train for rolling the product.

TABLE 1

Method of this invention Conventional method Roll dia. Equivalent contact Roll dia. Equivalent contact e Rolling Stand No. (mm) angle (degree) (mm) angle (degree) speed (m/s) 1 596 30.0 988 20.0 0.45 2 651 38.0 1678 20.0 0.70 3 697 33.6 1599 20.0 1.12 4 608 30.6 1187 20.0 1.66 5 446 20.0 446 20.0 2.00 In Table 1, the diameter of the rolls of the method of this invention is determined by the curve III of Figure 4, and the diameter of the rolls of the conventional method is also determined by the curve V. It should be noted that stand No. S of this invention performs a rolling step with the conventional reduction in workpiece area.As will be obvious from Table 1, in the method of this invention, the rolls are of very small diameter as compared with those of the conventional method. In the conventional method, larger-diameter workrolls such as described above in Table 1 are generally not actually used because of their excessive size; and it is a common practice for the diameter of the rolls to be less than 800 mm and, to compensate for the reduced diameter, for the number of stands to be set at 7 to 8 units.

The foregoing description relates to the case where the workpiece is steel, but it is obvious from the constitution, operation and effect of the method and apparatus that the method and apparatus of this invention can be applied to the workpiece made of materials other than steel for example, aluminium alloy or copper alloy.

Reference will now be made to a continuous rolling mill, for working the above-described method of this invention, by comparison with a conventional mill.

In Figures 9a to 9e, showing a conventional representative rolling mill, 21 denotes a heating furnace, M denotes a workpiece, and 23 denotes a breakdown mill. In Figures 9a, 9b and 9c in order to obtain a total reduction in workpiece cross-sectional area of about 85%, 6 to 8 units of continuous billet mills 24, 24a are required, while in order to obtain a similar total reduction in area in Figures 9d and 9e, almost the same number of units of rough rod rolling mills 25 and 25a are required. References 26 and 26a show mills downstream of the intermediate mills 25 and 25a.

In contrast, Figure 10 shows a train of rolling mill for working the abovedescribed method of this invention.

Figures 10a, 10b and 10c show an example of a layout for manufacturing respectively a billet, a round bar and a rod from an ingot, bloom or continuous cast bloom.

The workpiece M which is heated to or held at a predetermined temperature by the heating furnace 21 is extracted by an extracting device (not shown) and is caused to pass through a rolling mill or a pinch roll assembly 27 for supplying a pushing force sufficient for rolling with the high degree of reduction in workpiece area after the next stand. The mill 27 has only a low degree of reduction in workpiece area, or in other words basically the starting material M is made to pass through a rolling mill 27 having a normal degree of reduction in area of about 1 > 30% which allows biting of the workpiece, easily and without the auxiliary pushing force, by the following high-elongation rolling mill train 28 having a high degree of reduction in workpiece area.The reason for including the word basically above is that it means that a device may be provided for carrying the workpiece to the first rolling mill 27 and allowing the workpiece to slightly collide with the rolls, such as a roller table or simple pinch rolls which are frequently employed for this purpose in rolling processes.

The number of units in the high-elongation rolling mill train is determined by the total reduction of area from the starting material to the rolled product and the effective maximum speed at the exit end of the group of rolling mills, namely which is in the effective speed range of the present high-elongation rolling method, as shown in Figure 4. The workpiece is then passed through the train of rolling mills as at 29 (more than one unit of finishing rolling mills having an extremely low degree of reduction in workpiece area is included in this group, for the purpose of adjusting the workpiece shape and dimension). The rolling mill train 29 provides a normal reduction of about 1030%, whereby the workpiece is finished in the desired product shape.

In the actual rolling operation, the workpiece becomes the final product through the shearing machine or cooling device, not shown. The embodiment will be described concretely, in the following description.

Reference will now be made to Figure 11 which shows an embodiment of a control arrangement. In Figure 11, 31 and 61 show rolling stands in which 016tan-' and 41 and 51 show rolling stands in which tan-t,u < 02 < tan-1,u'.

The final stand 51 for rolling with the contact angle fl2 is in the form of a pivot stand, the speed of revolution of the workrolls of which is the reference speed set within the range L shown in Figure 4. The reason for this is that the limit of rolling with the contact angle 02 as shown in Figure 4 is subject to restriction by the rolling speed, and therefore, a method of controlling the speed of revolution of the workrolls (rolling speed) of the other upstream and downstream rolling mills by monitoring the maximum speed of rotation of the workrolls (rolling speed) of the final stand 51 which has the highest rolling speed is preferable.

The control arrangement shown in Figure 11, uses the zero-tensile stress and zero-compressive stress control system of a current memory system, as an example.

In Figure 11, 32, 42, 52 and 62 are respectively current detecting devices, 33, 43, 53 and 63 are current memories, 34, 44, 54 and 64 are revolution control devices, 45 and 55 are rolling speed monitoring devices, 46 and 56 are input signals defining the limits of the permitted rolling speed range, 48 and 58 are pilot generators and 49 and 59 are hot metal detectors.

The workpiece M is bitten by the rolling stand 31, and at this moment, the constant current value, excluding the leading portion, is detected by the device 32, and stored in the memory device 33. The workpiece is then bitten by the rolling stand 41, after receiving an auxiliary pushing force from the rolling stand 31. At this time, between the rolling stand 31 and rolling stand 41, a compressive force is generated temporarily and the current of the drive motor of the rolling stand 31 increases, but upon completion of the biting stage, it reaches a fixed current value, which is then detected and compared with the previously stored current value to control the speed of rotation of the rolls of the rolling stand 31 by means of the control device 34, thus producing the zero-tensile stress and zero-compressive stress condition.In this case, confirmation of completion of the biting stage is effected by the hot metal detectors 41 and 51; it is possible to start the control action by using those signals and also, if trouble occurs, such as defective biting, it is possible to use those signals as a countermeasure for preventing continuation of the process.

The workpiece M is then bitten by the rolling stand 51 by being subjected to the auxiliary pushing force resulting from the biting of the rolling stand 41, but the control between the rolling stands 51 and 41 is performed in an entirely similar manner to the control between the rolling stand 31 and the rolling stand 41.

However, before entering the control of the rolling stands 41 and 51, it is advantageous to complete the control between the rolling stands 31 and 41.

It should be noted that, with the control of the rolling stands 41 and 51, the speed of rotation of the workrolls of the rolling stand 31 is changed by the successive-control action if the speed of rotation of the workrolls of the rolling stand 41 changes.

Next, the workpiece is bitten into the rolling stand 61, but in this case, since the rolling stand has lAtan-,a, the auxiliary pushing force for the biting stage becomes unnecessary and the control action to produce the zero-tensile stress and zero-compressive stress condition in the steady rolling state is performed as described above.

However, in this case, the rolling stand 51 is the pivot stand, so that control of an improper speed of rotation is compensated, by controlling the speed of rotation of the workrolls of the rolling stand 61.

A similar control is applied to the succeeding stands. It should be noted that the rolling speed of the pivot stand 51 is constantly monitored by the rolling speed monitoring device 55, and is compared with the speed standard value 56 to be determined from the relationship of the rolling speed Vm and the equivalent contact angle Oe in Figure 4, and is controlled within the appropriate range. In this case, for example, if the rolling speed of the rolling stand 51 is required to be controlled, a method of changing the entire line speed is adopted.

The foregoing describes an example of the current memory system, but in the operating condition where uniform heating is sufficiently performed in a normal walking beam heating furnace, the foregoing controlling method is sufficiently effective. However, if the control action is performed in a condition where a skid mark occurs or the uniformity of heating is extremely poor, it is preferable to use a known current memory load correcting system wherein the load value, in addition to the current value, is employed to change the ratio, or a system of completing the control between the rolling stands 31 and 41 if a control action is performed between the rolling stands 41 and 51 as described in the foregoing; however, other systems may be employed, for example when the workpiece is bitten in a group of stands comprising more than two units, it is possible to employ a known full-length control system wherein control is effected over the entire length, or it is possible to use a known control method wherein the tensile stress generated between the stands is directly detected and then is reduced to zero.

Figure 10a shows a layout for producing a billet of 120W (in mm) from a 300m continuously cast bloom, and an example of a billet mill, wherein a total elongation 6.25 (hereinafter this degree of elongation is used for the purposes of this description, since it means 84 when converted to the reduction in cross-sectional area) is required. In this case, the number of units in the total rolling mill train is 4 to 5 stands. The degree of elongation in each of the rolling stands is as shown in the following Table 2.

Table 2 shows that a small degree of elongation is produced in the first stand, the rolling mill being far more compact than a conventional mill, and its primary purpose being to engage the workpiece into the roll gap of the second stand.

TABLE 2

Stand No. 1 No.2 No. 3 No. 4 No. 5 Total Elongation 1.10 1.85 1.85 1.50 1.11 6.25 Groove DS D D DS S Remarks: The symbol representing the roll groove denotes the following shapes, each shape being shown in Figure 2: D: diamond S: square DS: diamond resembling square Stands No. 2 and No. 3 produce a high degree of elongation greatly exceeding the conventional level. In Stand No. 4, although a high degree of elongation is produced, in order to obtain a workpiece resembling the final desired crosssectional square shape, the stand has a diamond groove with a ratio between its axes of close to 1.0.Accordingly, in the final stand No. 5 only a small degree of elongation is required, and the dimensional fluctuation due to the variation in the degree of workpiece spreading at the exit of Stand No. 4 is wholly absorbed. As a result, the product has a high degree of dimensional accuracy and an extremely good shape. Moreover, the rolling mill at Stand No. 5 can be of compact size. This layout provides an extremely compact billet mill when considered as the rolling mill group, since Stand No. 1 and Stand No. 5 can be of extremely coinpact size, and therefore substantially the entire workpiece elongation can be obtained with three units or stands.

Although the rolling speed can be selected in relation to the size of the mills, in mills with outputs of 50,000 ton/month to 200,000/ton month with a normal rolling operation factor, the rolling speed can be sufficiently within the range of the effective rolling speed shown in Figure 4 and can be freely selected.

Table 3 shows that the degrees of elongation of Stand No. 1 and Stand No. 4 are set at the conventional values, and the high degree of elongation is produced by Stand No. 2 and No. 3. Dimensional accuracy is slightly inferior to that of the Table 2, and also, in Stands No. 1 and No. 4, the mills are of conventional mill size.

However, on the whole, a set of four stands is sufficient to perform the rolling operation, and is a layout with a high level of merit such as simplification of mill line length and installation.

TABLE 3

Stand No. 1 No. 2 No. 3 No. 4 Total Elongation 1.26 1.95 1.95 1.30 6.25 Groove D D D D S Remarks: Symbols for the roll groove are the same in Table 1.

It might be noted for reference that, in a group of rolling mills operating according to the conventional method, in order to manufacture a product by reducing from 300W to 120a1, about eight passes are needed, provided for example by a combination mill train of one unit in the form of a breakdown mill and four units in the form of continuous mills, or continuous mills forming seven to eight stands, so that it is common for the conventional mill group to form an extremely large installation as compared with the method and apparatus of the present invention.

Reference is now made to Figure 10b showing a layout for manufacturing a round bar. The workpiece of square cross-section is formed from a bloom by employing the process shown in Figure 10a, and then at least two rolling mills providing normal degrees of workpiece reduction are employed in the succeeding process, to produce the round bar.

However, the required size of round bar in general varies by several mm and ranges over many kinds, and therefore depending on the range of sizes, it becomes necessary to increase the number of rolling stands.

Also, in case of a round bar whose cross-section resembles the cross-section of the starting material, there is no need to produce a very high degree of elongation, and in this case, in Figure 10a, the groove for forming of the round bar is incorporated in the rolls of the rolling stands.

For the manufacture of round bar, the groove in the latter half of the manufacturing process should be oval and round, and there two types as shown in Figure 13a and 13b, either one of which may be adopted. It should be observed however that, with respect to producing the high degree of elongation in one pass.

it is at a disadvantage, as compared with the above-mentioned diamond or square groove.

An embodiment will be further described with reference to the case of the rolling line for rolling the bar and rod in Figure 10c.

In the bar and rod rolling line, as will be obvious from the example of reducing from a normal starting material of 120# to a rolled product of 5.5W, the total elongation requires an extremely high elongation factor such as about 500.

Accordingly, the number of units in the high-elongation rolling mill train, such as rolling stands 28, will be controlled in this case by the size of the workpiece and the maximum rolling speed.

Representative examples of the bar and rod, produced by a rough rolling mill train, wherein 20 mm bar steel is made from 200 mm square and 5.5 mm round is made from 120 mm square, with a finishing speed of 20 m/sec (1200 m/min), rod 60 m/sec (3600 m/min) respectively, are shown in Table 4 and Table 5.

TABLE 4

Rolling stands Starting material No. 1 No.2 2 No.3 No.4 No. S Final Size(mm) 200 (182.5) (136.1) (101.4) (75.6) (56.4) 20 Elongation - 1.20 1.80 1.80 1.80 1.80 1.10 Speed at exit of each rolling stand (m/s) 0.16 0.19 0.34 0.61 1.10 1.98 20 Bracketed numerals represent the size converted correspondingly to square crosssection.

TABLE 5

Rolling stands Starting material No. 1 No.2 No. 3 No.4 No.5 No.6 Final Size (mm) 120 (109.5) (81.6) (60.8) (45.3) (33.8) (25.6) 5.5 Elongation - 2 1.80 1.80 1.80 1.80 1.80 1.10 Speed at exit of each rolling stand (m/s) 0.10 0.12 0.21 0.38 0.69 1.25 2.24 60 Bracketed numerals represent the size converted correspondingly to square crosssection.

Table 4 shows an example of a bar steel rolling line, wherein the rolling mills producing a high degree of elongation are disposed as Stands Nos 2 to 5. The reason for disposing the high-elongation rolling mills as the stands up to No. 5 is that the rolling speed after stand No. 5 will exceed the effective rolling speed range shown in Figure 4. Accordingly, in the stand after Stand No. 6, conventional rolling is performed in the range of the equivalent contact angle as indicated by curve IV.

Also, Table 5 shows an example of a rod rolling line, wherein the rolling mills producing a high degree of elongation are disposed as stands Nos 2 to 6. In this case too, at the stand after stand No. 7, there the rolling speed will exceed the effective rolling speed range shown in Figure 4.

In the Table 4, there are four stands producing high degrees of elongation of 1.8 and in the Table 5, there are five units, and the total elongation with the train of rough rolling stands is 1.2x1.84=12.6 in Table 4, and in Table 5, 1.2x1.85=22.7. In the conventional rolling mill train whose degree of elongation per pass is 1.25, where such a degree of total elongation is effected by rough rolling mills, 11 to 14 stand units will be necessary. On the contrary, in the method of this invention, the number of stands in the rough rolling mill train is five to six units, and therefore, six to eight units are eliminated. Accordingly, in this invention this is a very good layout permitting large-scale reductions in the costs related to the rolling installation, including a reduction in the length of the rolling line.

In the foregoing embodiments, the rolling efficiency and reduction in size of the rolling mills are taken into consideration, and rolling with the maximum contact angle as shown in Figure 4 is employed as the basic principle, and although the diameter of the rolls is made relatively smaller, as in the whole installation, it has an extremely high level of efficiency as described in the foregoing.

WHAT WE CLAIM IS: 1. A method of rolling a metal workpiece to produce a high degree of reduction in its cross-sectional area by a rolling mill having flat rolls (in which case the contact angle between the workpiece and the rolls is represented by 0) or grooved rolls (in which case the equivalent contact angle Ol is given by 0l=tan-1 (Cos a, tan 8) where 0 represents the contact angle between the workpiece and the rolls at the bottom of the roll groove and a represents the angle of inclination of the groove slides), which method comprises:: feeding the workpiece through a first rolling stand and/or pinch roll means towards at least one further rolling stand downstream of said first rolling stand or pinch roll means and producing in the workpiece a compressive stress which is below the yield stress at the times of rolling and acts in a feed direction along the rolling line between the first rolling stand or the pinch roll means and the further rolling stand thereby enabling the workrolls of the further rolling stand to bite the workpiece;; engaging the workpiece in the workrolls of the further roll stand with the gap of the workrolls of said further roll stand set such that tanw or ol tan-'u where ,u is the coefficient of friction between the workpiece and the workrolls at the time of engagement of the workpiece into the roll gap and ' represents the coefficient of friction between the workpiece and the workrolls when the workpiece is completely engaged between the workrolls and rolling is in progress; when the workpiece is completely engaged in the workrolls of the further rolling stand maintaining the workrolls of said further rolling stand at their previously set roll gap; and readjusting the peripheral speed of the workrolls of the first rolling stand and/or the pinch roll means and continuing the rolling operation while there is substantially no stress in the workpiece acting in a direction along the rolling line between the further rolling stand and the first rolling stand or pinch roll means.

2. A method according to Claim I wherein the rolling mill comprises two or more of said further rolling stands.

3. A method according to Claim 1 or Claim 2 wherein the magnitude of the compressive stress generated in the workpiece is at least 1% of the yield stress.

4. A method according to Claim 1, Claim 2 or Claim 3 wherein the peripheral speed of the workrolls is below 2.5 m/s.

5. A method of rolling a workpiece, substantially as hereinbefore described with reference to Figures 1 to 8 and Figures 10 to 13 of the accompanying drawings.

6. Apparatus for rolling a workpiece to produce a high degree of reduction in its cross-sectional area, comprising a rolling mill having flat rolls (in which case the contact angle between the workpiece and the rolls is represented by 0) or grooved

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. Table 4 shows an example of a bar steel rolling line, wherein the rolling mills producing a high degree of elongation are disposed as Stands Nos 2 to 5. The reason for disposing the high-elongation rolling mills as the stands up to No. 5 is that the rolling speed after stand No. 5 will exceed the effective rolling speed range shown in Figure 4. Accordingly, in the stand after Stand No. 6, conventional rolling is performed in the range of the equivalent contact angle as indicated by curve IV. Also, Table 5 shows an example of a rod rolling line, wherein the rolling mills producing a high degree of elongation are disposed as stands Nos 2 to 6. In this case too, at the stand after stand No. 7, there the rolling speed will exceed the effective rolling speed range shown in Figure 4. In the Table 4, there are four stands producing high degrees of elongation of 1.8 and in the Table 5, there are five units, and the total elongation with the train of rough rolling stands is 1.2x1.84=12.6 in Table 4, and in Table 5, 1.2x1.85=22.7. In the conventional rolling mill train whose degree of elongation per pass is 1.25, where such a degree of total elongation is effected by rough rolling mills, 11 to 14 stand units will be necessary. On the contrary, in the method of this invention, the number of stands in the rough rolling mill train is five to six units, and therefore, six to eight units are eliminated. Accordingly, in this invention this is a very good layout permitting large-scale reductions in the costs related to the rolling installation, including a reduction in the length of the rolling line. In the foregoing embodiments, the rolling efficiency and reduction in size of the rolling mills are taken into consideration, and rolling with the maximum contact angle as shown in Figure 4 is employed as the basic principle, and although the diameter of the rolls is made relatively smaller, as in the whole installation, it has an extremely high level of efficiency as described in the foregoing. WHAT WE CLAIM IS:

1. A method of rolling a metal workpiece to produce a high degree of reduction in its cross-sectional area by a rolling mill having flat rolls (in which case the contact angle between the workpiece and the rolls is represented by 0) or grooved rolls (in which case the equivalent contact angle Ol is given by 0l=tan-1 (Cos a, tan 8) where 0 represents the contact angle between the workpiece and the rolls at the bottom of the roll groove and a represents the angle of inclination of the groove slides), which method comprises:: feeding the workpiece through a first rolling stand and/or pinch roll means towards at least one further rolling stand downstream of said first rolling stand or pinch roll means and producing in the workpiece a compressive stress which is below the yield stress at the times of rolling and acts in a feed direction along the rolling line between the first rolling stand or the pinch roll means and the further rolling stand thereby enabling the workrolls of the further rolling stand to bite the workpiece;; engaging the workpiece in the workrolls of the further roll stand with the gap of the workrolls of said further roll stand set such that tanw or ol tan-'u where ,u is the coefficient of friction between the workpiece and the workrolls at the time of engagement of the workpiece into the roll gap and ' represents the coefficient of friction between the workpiece and the workrolls when the workpiece is completely engaged between the workrolls and rolling is in progress; when the workpiece is completely engaged in the workrolls of the further rolling stand maintaining the workrolls of said further rolling stand at their previously set roll gap; and readjusting the peripheral speed of the workrolls of the first rolling stand and/or the pinch roll means and continuing the rolling operation while there is substantially no stress in the workpiece acting in a direction along the rolling line between the further rolling stand and the first rolling stand or pinch roll means.

rolls (in which case the equivalent contact angle ol is given by 81=tan' (Cos a, tan 0) where a represents the contact angle between the workpiece and the rolls at the bottom of the roll groove and a represents the angle of inclination of the groove sides comprising:: a first rolling stand wherein the roll gap is so set that the contact angle 0 or the equivalent contact angle alAtan-ty, where 1 denotes the coefficient of friction between the workpiece and the workrolls at the time of engagement of the workpiece into the roll gap; at least one second rolling stand disposed downstream of said first rolling stand and having a roll gap so set that the contact angle 0 or the equivalent contact angle Ol is such that tan~t,a' > 8ktan-t,a (wherein K denotes the coefficient of friction between the workpiece and the workrolls when the workpiece is completely engaged between the workrolls and rolling is in progress);; speed detecting means for detecting the peripheral speed of the workrolls of the second rolling stand; means for detecting the completion of engagement of the leading end portion of the workpiece between the rolls of the second rolling stand; and means for controlling the peripheral speed of the workrolls of the first rolling stand on the basis of a signal from the speed detecting means so that a compressive stress of above 1% and below 100% of the yield stress is generated in the workpiece at the upstream or entry side of the second rolling stand until the workpiece is completely engaged between the rolls of the second rolling stand, and for adjusting the peripheral speed of the rolls of the first and/or the second rolling stand on the basis of a signal from the engagement completion detecting means so that said stress is not generated in the workpiece at the upstream or entry side of the second rolling stand after the workpiece is completely engaged between the rolls of the second rolling stand.

7. Apparatus according to Claim 6, further comprising an arithmetic operation processing means for storing the slip limit curve in the relationship between the equivalent contact angle and the rolling speed in the second rolling stand and for comparing the speed signal from the speed detecting means and the slipping limit speed and supplying a speed correction parameter as an output; and means for controlling the peripheral speed of the rolls of the second rolling stand on the basis of said engagement-completion signal and said speed correction parameter signal.

8. Apparatus according to Claim 6 having two or more of said second rolling stands.

9. Apparatus for rolling a workpiece, substantially as hereinbefore described with reference to Figure 5, or Figures 6a to 6c, or Figure 7, or Figure 8, or Figures 10a to 10c, or Figure 11, or Figure 12, or Figure 13 of the accompanying drawings.