ROLLING METHOD MAKING USE OF WORK ROLL SHIFT ROLLING MILL

Abstract: A tandem rolling mill is operated such that roll stands (F1 to F3) of an upstream stage of the mill adjacent to a strip inlet thereof are positioned relative to each other to effect a strip crown control or an edge drop control, while roll stands (F4 to F7) of a downstream stage of the mill adjacent to a strip outlet thereof are operated to axially shift the downstream stage work rolls cyclically to distribute roll wear and thermal crown in the axial direction thereof.

Description: The invention relates to a method for rolling a metal strip in a tandem rolling mill according to the first portion of claim 1 and to a tandem rolling mill for using that method. DESCRIPTION OF THE PRIOR ART The US-A 3,857,268 discloses in Fig. 2 a tandem roll mill in which upper and lower work rolls of each stand are shiftable in the axial direction of these rolls so as to vary the axial length over which each work roll and an associated backup roll contact with each other, thereby effecting crown control of a rolled strip. The JP-A 55-77903 discloses a rolling mill in which one of upper and lower work rolls is tapered at its one axial end such that the diameter is progressively reduced towards the outer end extremity, while the other of the work rolls is similarly tapered at its end opposite to the tapered end of the first-mentioned work roll. In operation, the strip to be rolled and the work rolls are located relative to each other such that the both side edges of the strip are positioned in the vicinities of the tapered ends of the work rolls, thereby reducing the tendency of occurrence of edge drop. The rolling method which makes use of this type of rolling mill will be referred to as one-sided taper roll position control method. The JP-A 55-77903 discloses a pair roll cross rolling mill in which each of upper and lower work rolls together with its associated backup roll is angularly movable in a horizontal plane so that the angle formed between the axes of both work rolls is controllable. The JP-A 56-30014 discloses a rolling mill in which upper and lower work rolls of each stand are provided with point-symmetric profiles such as S-shaped or sine-curve-shaped profiles and are axially movable relative to each other. These two types of rolling mills have both been developed for the purpose of control of the strip crown. A method called as "cycle-shift method" has been reported in HITACHI REVIEW, Vol. 34, No. 4, August 1985. In this method, work rolls are cyclically shifted in the axial direction so as to uniformly distribute any roll wear and thermal crown in the axial direction. This method, when applied to rolling of strips by any of the roll mills shown in Fig. 2 of the above-mentioned US-Patent and the three JP-Publications, is not suited to axially uniformly distribute the roll wear and thermal crown, although the control of strip crown or edge drop can be achieved appreciably well. Figs. 8 and 9 of the above-mentioned US-Patent discloses a rolling mill of the type in which an intermediate roll is interposed between each work roll and an associated backup roll and is axially shiftable together with the associated work roll in accordance with the width of the strip to be rolled. When the above-mentioned cycle shift method is carried out with this type of rolling mill, both the wear of the work rolls and thermal crown are uniformly distributed along the axes of the work rolls and the strip crown is also improved because the intermediate roll is shiftable. Unfortunately, however, the construction of this type of rolling mill is complicated due to the addition of the shiftable intermediate rolls and the installation and running costs are raised accordingly. Accordingly, an object of the present invention is to provide a rolling method which makes use of a rolling mill having no intermediate roll between each work roll and the backup roll and which makes it possible to uniformly distribute the roll wear and the thermal crown, while improving the strip crown or edge drop. This object will be solved by the features of the claim 1. In general, work rolls in roll stands on the material inlet end of a tandem rolling mill (such stands will be referred to as "upstream stands") are made of adamite or High-chromium. As a result of contact with the material at a high temperature, the surfaces of these rolls are oxidized to form oxide films, so that the wear is to a very small extent on these rolls. On the other hands, rolls on roll stands near the outlet end of the mill (such stands will be referred to as "downstream stands") are usually made of nickel grain and exhibit heavy wear. This fact will be more clearly seen in Fig. 10 which is a graph in which the axis of abscissa represents the No. of the roll stands, while the axis of ordinate represents the amount a of wear of work rolls in terms of roll diameter, copying coefficient β which represents the coefficient of copy of the roll wear to the rolled product, and the amount or height of step (α x β) formed on the work roll and to be transferred to the strip. From this figure, it will be understood that the height of step (α x β) greatly increases from the inlet end towards the outlet end of the rolling mill. In view of the above facts, according to the rolling method of the present invention which makes use of a tandem rolling mill, the strip crown or edge drop is controlled so as to improve the strip crown or to eliminate edge drop in upstream stand or stands which suffers from only slight wear of work rolls, whereas, in the downstream roll stand or stands which suffer from heavy wear of work rolls, the work rolls are reciprocally and, preferably, cyclically moved in the axial direction regardless of the width of the material, thereby axially distributing the wear of the work rolls as well as thermal crown. With advantage, the shifting movement of the work rolls of the downstream stage is always in counterdirection and is a setting movement before or after the rolling operation of a predetermined number of strips. In praxis, the minimum and maximum lengths of the shifting movement (setting movement) before or after rolling of a predetermined number (from one to several) coils are about 20 mm (min) and about 400 mm (max), respectively, as indicated by A in Fig. 11, whereas the minium and maximum lengths of reciprocal movements of the downstream stage work rolls are 140 mm and 400 mm, respectively, as indicated by B in Fig. 11 and 12. The above and other objects, features and advantages of the present invention will become more clear from the following description of preferred embodiments with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic illustration of a tandem rolling mill and a controlling system suitable for use in carrying out the rolling method of the present invention. Fig. 2 is a schematic front elevational view of an example of arrangement of work rolls and backup rolls in one of a plurality of upstream roll stands of the tandem rolling mill shown in Fig. 1; Figs. 2A and 2B are similar to Fig. 2 but show work rolls and backup rolls in a downstream roll stand of the rolling mill shown in Fig. 1; Fig. 3 is a diagram illustrating work roll local wear and work roll wear profile in relation to Case A (work rolls are not shifted pattern both in crown control and Case B (roll profile is controlled); Fig. 4 is a somewhat exaggerated illustration of the profile of a strip rolled in accordance with a first embodiment of the method of the present invention; Fig. 5 is a somewhat exaggerated illustration of the profile of a strip rolled without cycle shift method; Fig. 6 is a schematic front elevational view of another example of the work rolls and backup rolls in an upstream roll stand of the tandem rolling mill shown in Fig 1; Fig. 7 is a schematic perspective view of a still another example of the work rolls and backup rolls in an upstream roll stand of the tandem rolling mill shown in Fig. 1; Fig. 8 is a schematic front elevational view of work rolls and backup rolls in an upstream roll stand of the tandem rolling mill shown in Fig. 1 used in carrying out a second embodiment of the method in accordance with the present invention; Fig. 9 is a somewhat exaggerated illustration of a strip profile of a strip rolled in accordance with the second embodiment of the method of the present invention; and Fig. 10 is a graph showing the relationship between roll wear in terms of roll diameter, coefficient of copy of roll wear to strip and stepped roll wear to be copied to the strip in each of roll stands. Fig. 11 and 12 are graphs of the shifting movements of the workrolls of the downstream stage. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Fig. 1, a tandem rolling mill has seven roll stands F1 to F7 each of which has upper and lower work rolls 1 and backup rolls 3 which back up the respective work rolls. In each of the roll stands F1 to F7, the upper and lower work rolls are axially shiftable in the opposite axial directions by means of work roll shifting means which is shown as being hydraulic cylinders 5 associated with the respective work rolls. The amount of shift of each work roll is indicated by δ with a suffix representing the No. of the roll stand. The series of roll stands F1 to F7 are so constructed that roll bender forces P1 to P7 are exerted onto the work rolls so as to urge the work rolls away from each other by benders 6 which are indicated by double-headed arrows. The work roll shifting means 5 for axially displacing the work rolls and the roll bender 6 have been known and disclosed, for example, in the aforementioned US-A-3 857 268, so that detailed description thereof is omitted in this specification. Although work roll shifting means 5 associated with the first and the seventh roll stands F1 and F7 are shown, it will be obvious to those skilled in the art that similar work roll shifting means are provided also for other rolls stands F2 to F6. The rolling mill is equipped with a control unit 7 which is capable of controlling the amounts δ of shift of the work rolls 1 as well as the roll bender forces P by the control of the work roll shifting means 5 and the roll benders 6 of all the roll stands F1 to F7 in accordance with rolling information concerning the rolling conditions such as the kind of the strip 2 to be rolled, temperature of the material, thicknesses and widths of the strips before and after the rolling and the rolling speed. The crown of the strip 2 delivered from the final roll stand F7 is detected by a crown detector 8 and the result is fed back to the control unit 7, whereby a desired crown is attained on the strip 2 rolled by the method of the present invention. Such a feedback control system is well known to those skilled in the art, so that no further explanation will be needed. The series of roll stands of the tandem rolling mill is divided into two stages or groups: namely, an upstream stage adjacent to the inlet for the strip 2 and constituted by three upstream roll stands F1 to F3 and a downstream stage adjacent to the outlet for the strip 2 and constituted by four downstream roll stands F4 to F7. First Embodiment In the first embodiment of the rolling method of the present invention, the roll stands F1 to F3 of the upstream stage conduct the rolling in accordance with the crown control mode, whereas the roll stands F4 to F7 of the downstream stage conduct the rolling in accordance with the roll profile control method. More specifically, in each of the roll stands F1 to F3 of the upstream stage, the upper and lower work rolls 1 are axially shifted in opposite directions in an amount according to the width of the strip 2 to positions optimum for the crown control rolling, as shown in Fig. 6. Alternatively, one end of the upper roll is positioned in the vicinity of the corresponding edge of the strip 2 while the end of the lower work roll opposite to the above-mentioned end is located in the vicinity of the other edge of the strip, as shown in Fig. 2. This control is effected by a crown control means 7a in the control unit 7 which sets the amount δ₁ to δ₃ of axial roll shifts in the respective roll stands F1 to F3 to be optimum for the control of the crown. A similar control can be effected for the cross mill shown in Fig. 7 in which the upper and lower work rolls in each roll stand are angularly movable in horizontal planes. Simultaneously, the roll bender forces P1 to P3 applied by the roll benders 6 to the work rolls 1 in the respective roll stands F1 to F3 are controlled by the crown control means 7a such that bender forces optimum for the crown control are obtained in the respective roll stands. In the roll stands F4 to F7 in the downstream stage of the tandem rolling mill, the work rolls 1 are reciprocally and cyclically moved in the axial direction, as shown in Fig. 2A. This reciprocal shift is conducted at predetermined intervals regardless of the widths of the strips 2. At the same time, the roll bender forces P4 to P7 are set at levels which are optimum for the roll profile control. The control of the work roll shifting means 5 and the setting of the roll bender forces P4 to P7 in the respective stages F4 - F7 are performed by a roll profile control means 7b in the control unit 7. As will be understood from the foregoing description, in the first embodiment of the present invention, the profile, i.e., cross-sectional shape, of the rolled strip 2 is controlled and regulated by virtue of the crown control mode of rolling operation performed by the roll stands F1 to F3 of the upstream stage, whereas, in the roll stands F4 to F7 of the downstream stage, the roll wear and thermal crown are substantially uniformly distributed along the length of each work roll so as to eliminate any local concentration of wear because these roll stands F4 to F7 are operated in the cycle shift mode. Experimental Test 1 A test operation was conducted using the 7-stand work roll shift type tandem rolling mill as shown in Figs. 1 to 2A. In this test, 140 pieces of strips of 1000 mm wide were continuously rolled by making use of this roll mill. Throughout the test, the work rolls 1 of the roll stands F1 to F3 of the upstream stage were positioned as shown in Fig. 2 with respect to the strip 2, while the work rolls of the roll stands F4 to F7 of the downstream stage were cyclically shifted for each coil in accordance with the shift pattern which is shown in "Shift Pattern" of the "Case B" in Fig. 3. The profile of the strip produced by this test rolling is shown in Fig. 4 in a somewhat exaggerated manner. The state of wear caused on the work rolls 1 throughout the rolling process is shown in "Work Roll Wear Profile" of the "Case B" in Fig. 3. A comparison test was conducted for the purpose of evaluating the test result shown in Fig. 4. In this comparison test, 140 pieces of strips 2 and width the same as those of the strips produced in the above-mentioned test operation were rolled by making use of a rolling mill of the same construction as that of the rolling mill shown in Figs. 1 and 2, with the work rolls 1 of the all stands F1 to F7 fixedly set at the optimum crown control positions shown in Fig. 2. In this case, the shift pattern of the work rolls 1 is as shown in "Shift Pattern" of the "Case A" in Fig. 3 because none of the work rolls 1 is shifted during the rolling operation. The state of wear caused on the work rolls 1 is shown in "Work Roll Wear Profile" of the "Case A" in Fig. 3. The profile of the rolled strip is shown in Fig. 5 in a somewhat exaggerated manner. A comparison between the work roll wear profiles in "Case A" and "Case B" in Fig. 3 as well as between the strip profiles shown in Figs. 4 and 5 clearly shows that, in the "Case A" in which the rolling was conducted with the work rolls 1 of all the stands F1 to F7 fixed at predetermined positions, each work roll was heavily worn over an axial region corresponding to the width of the strip 2 and that this heavy wear of the roll was copied to the rolled strip 2 so that keen projection of a height of about 165 microns was formed on each lateral or side edge of the strip 2. In contrast, in "Case B", the influence caused on the rolled strip 2 by the wear of the work rolls 1 in the roll stands F1 to F3 of the upstream stage was effectively cancelled by virtue of the cyclical shift of the work rolls 1 in the roll stands F4 to F7 of the downstream stage, so that the projection formed on each lateral or side edge of the strip 2 was as low as 10 microns or less which is practically negligible. In this case, however, a local thinning known as "edge drop" was caused on each lateral edge 2a of the strip. Experimental Test 2 Another test operation was conducted by executing the operation modes of "Case A" and "Case B" by making use of a work roll shift mill in which the work rolls 1 and the backup rolls 3 of the roll stands F1 to F3 of the upstream stage were of the structure shown in Fig. 6 while the work rolls 1 and the backup rolls 3 of the downstream stage roll stands F4 to F7 were of the structure shown in Fig. 2A. The upper and lower work rolls 1 shown in Fig. 6 were of the type disclosed in the aforementioned JP-A-56-30,014. Namely, the upper and lower work rolls 1 had profiles defined by curves symmetrical with each other with respect to a point. The result obtained by the operation in "Case A" mode was similar to that obtained in "Case A" in the Experimental Test 1; namely, the strip 2 showed a profile as shown in Fig. 5. Similarly, the result obtained by the operation in "Case B" mode was substantially the same as that obtained in "Case B" in the Experimental Test 1; namely, the strip 2 showed a profile as shown in Fig. 4. Experimental Test 3 A further test operation was conducted both in the "Case A" mode and "Case B" mode as in Experimental Test 1 by employing a rolling mill in which the upstream roll stands F1 to F3 were of pair roll cross rolling type shown in Fig. 7. This type of rolling mill is disclosed in the aforementioned JP-A-55-77903. The downstream roll stands F4 to F7 were of the structure shown in Fig. 2A. In the test operation in the "Case A" mode, the work rolls 1 in all of the rolls stands where fixedly held at positions optimum for the crown control throughout the test, whereas, in the "Case B" mode, the work rolls of the roll stands F4 to F7 in the downstream stage were cyclically shifted for a predetermined number of coils. The results obtained from the test operation in the "Case A" mode and in the "Case B" mode were substantially the same as those obtained in "Case A" and "Case B" in the Experimental Test 1 described before. Second Embodiment In the second embodiment of the present invention, the rolling by the roll stands F1 to F3 of the upstream stage of the rolling mill is conducted in accordance with the one-sided taper roll position control method, whereas, the roll stands F4 to F7 of the downstream stage are controlled in accordance with the roll profile control method. More specifically, the second embodiment of the rolling method in accordance with the invention employs roll stands of the type shown in Fig. 8. Throughout the rolling operation, the work rolls 1 of the roll stands F1 to F3 are fixedly held at positions optimum for the edge drop control as shown in Fig. 8, while the work rolls in the roll stands F4 to F7 are cyclically shifted. In this embodiment, the strip 2 rolled through the roll stands F1 to F3 of the upstream stage exhibits such a thickness distribution that the thickness is greater at both side edge portions than at the mid portion of the strip. The strip having such a thickness distribution is then rolled through the roll stands F4 to F7 of the downstream stage, so that the final rolled strip exhibits a smaller edge drop at the edge portions 2a than in the case of the strip in accordance with the first embodiment of the method of the invention. Experimental Test 4 A still further test rolling was conducted with a work roll shift mill having an upstream stage composed of three roll stands F1 to F3 of the type shown in Fig. 8 and a downstream stage composed of four roll stands F4 to F7 of the type shown in Fig. 2A. The roll stand shown in Fig. 8 is the same one as one disclosed in the JP-A-55-77903 referred to above. In this roll stand, the upper work roll 1 is tapered at its one axial end (right end as viewed in Fig. 8) such that the diameter is progressively decreased towards the outer end extremity, while the lower work roll 1 is similarly tapered at its end which is on the left side as viewed in Fig. 8. The strip 2 was located with respect to these work rolls such that both edges of the strip 2 were registered with the adjacent tapered portions of the upper and lower work rolls 1. The roll stands F4 to F7 of the downstream stage of the mill were the same as those shown in Fig. 1. The work rolls of these stands were cyclically shifted in accordance with the shift pattern of the "Case B" in the Experimental Test 1. Rolling test was conducted in the same way as in the preceding test operations. The result is shown in Fig. 9. As will be understood from the strip profile shown in Fig. 9, the rolling in accordance with the second embodiment of the present invention causes an edge drop which is much smaller than that caused by the first embodiment of the method of the present invention. As will be understood from the foregoing description, the rolling method of the invention remarkably improves the strip crown or the edge drop as compared with those caused in the prior art rolling methods and provides substantially uniform distributions of roll wear and thermal crown in the axial direction of the work rolls. This enables the work rolls of the downstream stage to withstand a greatly increased number of rolling operations.

Claims: Method for rolling a metal strip in a tandem rolling mill comprising at least two roll stands (F₁ to F₇) with upper and lower backup rolls (3) and horizontally adjustable upper and lower work rolls (1) equipped with bending means (6) for controlling the shape of the rolled strip (2), characterized in that for controlling the crown and/or the edge drop of the strip (2) the horizontal displacement δ of the work rolls (1) and the bending forces (P) acting on said work rolls (1) of an upstream group (F₁ to F₃) of the roll stands (F₁ to F₇) will be adjusted in accordance with the rolling conditions including the width of the strips (2) and that for controlling the wear and the thermal crown of the work rolls (1) in a downstream group (F₄ to F₇) of the roll stands said work rolls (1) will be reciprocally shifted at predetermined intervalls regardless to the width of the strip (2). Rolling method according to claim 1, characterized in that the work rolls (1) of the roll stands (F₁ to F₇) in the downstream group (F₄ to F₇) will be shifted cyclically before or after a predetermined number of passes of the strip (2) in the axial opposite direction. Rolling method according to claim 1 or 2, characterized in that the work rolls (1) of the roll stands (F₁ to F₇) in the upstream group (F₁ to F₃) will be fixedly set in the axial opposite directions to a predetermined position before starting the rolling operation. Rolling method according to claim 1 or 2, characterized in that the work rolls (1) of the roll stands (F₁ to F₇) in the upstream group (F₁ to F₃) will be angularly adjusted. Tandem roll mill comprising a plurality of roll stands (F₁ to F₇), each having upper and lower backup rolls (3), upper and lower work rolls (1), means (5) for displacing said work rolls (1) in the horizontal directions, means (6) for bending said work rolls (1) and a control unit (7) connected with a strip shape detector (8) for controlling the displacement δ and the bending force (P) of the work rolls (1) in the roll stands (F₁ to F₇), characterized in that the control unit (7) includes a strip crown control (7a) for adjusting the displacement δ₁ to δ₃ and the bending force (P₁ to P₃) of the work rolls (1) in a upstream group (F₁ to F₃) of the roll stands (F₁ to F₇) in accordance with the rolling conditions including the widths of the strip (2) and a work roll profile control (7b) for reciprocally shifting the upper and lower cylindrical work rolls (1) of a downstream group (F₄ to F₇) of the roll stands (F₁ to F₇) in axially directions regardless of the width of the strip (2). Rolling mill according to claim 5, characterized in that the work rolls (1) of the upstream group (F₁ to F₃) of the roll stands (F₁ to F₇) have tapered end portions and are axially movable and that the strip crown control (7b) adjusts the axial displacement δ₁ to δ₃ of said work rolls (1) such that both edges of the strip (2) are registered with the adjacent tapered end portions of the upper and lower work rolls (1). Rolling mill according to claim 5, characterized in that the work rolls (1) of the upstream group (F₁ to F₃) of the roll stands (F₁ to F₇) are angularly adjustable in a horizontal plane by the action of the strip crown control (7b). Rolling mill according to claim 5, characterized in that the work rolls (1) of the upstream group (F₁ to F₃) of the roll stands (F₁ to F₇) are provided with substantially S-shaped profiles defined by curves symmetrical with each other with respect to a point and are axially movable relative to each other by the action of the strip crown control (7b).

Inventors: OCHIAI, TSUNEO
Applicants: HITACHI, LTD.
Application year: 1988
Main classification: B21B37
IPC codes: B21B13, B21B1, B21B37