Rolled plate sectional profile control rolling method and rolling mill

Abstract: A rolling mill comprises a pair of crown work rolls (1.1′) each having, at both ends of a drum, tapered ends ground at different taper angles (5,5′), respectively. The work rolls are located one above the other with one tapered end of one work roll being in opposition to one tapered end having a different taper angle of the other work roll. The work rolls are movable in axial directions, such that edges of the plates are rolled by one tapered end of one work roll and a drum of the other work roll, and between tapered ends of both the work rolls. The method and rolling mills are capable of controlling the crown and edge drop reduction and simultaneously preventing local protrusions such as high spots and edge built-up to produce flat rolled plates having no difference in thickness and further capable of controlling the crown and the edge drop according to the material, thickness and width of the plates.

Description: This invention relates to a method of controlling the sectional profiles of plates, such as steel plates, during thick plate rolling and hot and cold rolling by means of two, four, five and six-high-mills and to a rolling mill including particularly constructed rolls for controlling the sectional profiles of the plates to be rolled. When considering the configuration and quality of rolled products, it is important to eliminate four defects, that is, (a) wave deformations resulting from waving phenomenon (problems of flatness), (b) crown formation due to differences in thickness between the edges and the centres, (c) edge dropping due to metal flow occurring particularly at the edges and (d) local protrusions (high spots, edge build-ups, etc.). In general, when a material is being rolled to reduce its thickness, the sectional profile of the material in the width direction depends on the deformation of the axes of the work rolls, flattened deformations of the rolls, and thermal crown formation and wear of the rolls caused by the rolling. Thus the sectional profile of the plate to be rolled needs to be controlled. In order to uniformly control the above configuration and quality of the rolled products, i.e. the flatness and thickness profiles, various methods have been proposed such as the roll bending method, the rolling schedule changing method (Japanese Laid-open Patent Application No. 55-92,215), the method of combining a six-high HC mill shifting method and four-high work roll shifting method with the roll bending method (Japanese Patent Application Publication No. 7,635/76) and the method of combining a four-high work roll shifting method with a pair of working rolls having a single tapered ground end and a drum portion (Japanese Laid-open Patent Application No. 55-77,903). In the latter case, the tapered end of each work roll is opposite the drum portion of the other work roll. Patents Abstracts of Japan Vol.8 No. 12 (M-269) [1449] 19th January 1984 and Japanese Laid-open Patent Application No. 58-176002 describe modifications of the method described in Japanese laid-open Patent Application No. 55-77903 wherein the single tapered end of the work rolls includes both a more steeply tapered portion and a more gently tapered portion. However, this proposal does not provide an entirely satisfactory solution to the problem of sectional profile control because it enables only limited control of the thickness of the plate edges since the edges of the plates are rolled between the tapered end of one work roll and the drum portion of the other work roll. EP-A2-0153849 which document is cited under Art 54(3) EPC also describes a modification of Japanese Laid-open Patent Application No. 58-176002. The work rolls similarly have a single tapered end only but in this case the rolls are cyclically shifted with the plate edges remaining adjacent the tapered end. Although this prevents irregular wear of the work rolls, it still does not enable adequate control of the sectional profile to be obtained. In order to prevent waving, control the crown of the material and reduce edge drops, there has hitherto been no effective method other than by carefully carrying out the rolling operation from the cold rolling to the hot rolling. Although the roll bending method or apparatus has been mainly used and is effective to control the flatness of the material to a certain extent, it is hardly effective to control the crown or reduce edge dropping. Moreover, the rolling schedule changing method is not effective to control the edge dropping, although it is effective to control the crown so as to make it constant. In a six-high HC mill, intermediate rolls are shifted in dependence upon the width of the material to be rolled and the roll bending action is combined therewith. In this case, if the intermediate rolls are further shifted inwardly, excess surface pressure occurs on the surfaces of the rolls which causes spalling to an extent such that the further inward shifting of the intermediate rolls cannot be actually realized. Accordingly, the crown-controlling performance is decreased and the method is not effective to reduce the edge drops. Moreover, the construction and reconstruction costs are expensive. Work rolls having tapered ground ends, so-called "trapezoidal crown" rolls make it possible to control the crowns and to control edge dropping. Such work rolls are effective to prevent waving if they are combined with a roll bending apparatus because this improves the control of the crowns and edge dropping reduction. However, when the widths of the plates to be rolled are changed, the control effect correspondingly changes and local protrusions cannot be prevented. Since local protrusions such as high spots, edge build-ups and the like are due to extraordinary wear of the work rolls which occurs at constant distances from the edges of the material in the width directions, prevention of the local protrusions is difficult in rolling mills whose work rolls assume constant positions. More particularly, as the edge build-ups are caused by the extraordinary wear occurring at the edges of the material which contact the tapered ground ends of the work rolls and whose temperature is lower than that of the center of the material edge build-up tends to occur when plates of the same width are continuously rolled. Accordingly, edge build-ups occur more coniderably when rolling with trapezoidal crown rolls which are required to maintain the widths of the plates to be rolled at a substantially constant value since the tapered ground ends of the work rolls contact the material at substantially the same location of the material. In using trapezoidal crown rolls, the edge build-ups and edge drops tend to increase when the quality or hardness of the material to be rolled is changed. Rolling by means of a rolling mill including work rolls having a single tapered ground end according to the four-high work roll shifting method, on the other hand, is effective to control the crown and the edge drops. However, once the configuration of the single tapered ground end of the work rolls has been determined, this is not necessarily satisfactory when the quality and thickness of the material to be rolled are changed. Particularly, the control of the edge drop is insufficient and is required to be more improved. It is an object of the present invention to provide a rolling method and a rolling mill, particularly for steel plates, which are capable of controlling the crown and edge dropping and simultaneously preventing local protrusions such as high spots and edge build-ups so as to produce flat rolled plates having no difference in thickness and further which are capable of controlling the crown and the edge dropping according to the material, thickness and width of the plates. In general, in order to roll a steel plate having uniform thickness in the width directions, it is important to keep uniform surfaces of the work rolls in contact with the plate and to keep a uniform clearance between the upper and lower work rolls in the width direction. It is therefore possible to produce rolled plates which are superior in flatness and sectional profile in the width direction by effecting a rolling operation which fulfils the above conditions as far as possible. To this end, it is necessary to remove the extra bending moment occurring at the ends of the drums of the work rolls caused by the back-up rolls which are in contact therewith so as to reduce the deformation of the roll axes. It is also necessary to mitigate the rapid change in the flat deformation of the work roll at the edges of the rolled plates to eliminate the metal flow at the edges and further to prevent the extraordinary wear which locally occurs on the work rolls. The present invention enables the above functions to be applicable to steel plates having any widths. According to one aspect of the present invention there is provided a rolling mill comprising a mill housing, a pair of axially moveable work rolls mounted in the housing one above the other, wherein each work roll includes a drum portion and a tapered end, and back-up rolls mounted in the housing for backing-up the work rolls characterised in that the work rolls are crown work rolls each having, at both ends of the drum portion, tapered ends ground at different taper angles with the tapered ends of each work roll being in opposition to the tapered ends of the other roll which are of different taper angle. According to another aspect of the present invention there is provided a method of rolling a plate comprising (i) arranging a pair of tapered work rolls one above the other in a rolling mill, each work roll including a drum portion and a tapered end, (ii) moving the work rolls in opposite directions along their axes in dependence on the thickness, width and material of the plate and (iii) rolling the plate between the work rolls characterised in that each work roll has two tapered ends ground at different taper angles with the tapered ends of each work roll being in opposition to the tapered ends of the other roll which are of different taper angle and the edges of the plate are rolled between the tapered ends of the work rolls. In a preferred embodiment of the invention the tapered ends of the work rolls are conical. In carrying out the invention, the ratio of the steep taper to the gentle taper of said tapered ends is preferably larger than one but not larger than ten. For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:- Fig. 1 is a schematic view of a four-high rolling mill in accordance with the present invention; Fig. 2a is a schematic view in accordance with the prior art (Japanese Laid-Open Patent Application No. 55-77903); Fig. 2b is a schematic view of a rolling mill in accordance with the present invention Fig. 3 illustrates examples of the profiles of rolled plates rolled by the work roll shifting method using work rolls having single tapered ground ends; Fig. 4a-4b illustrate defects in the shape and quality of rolled products; Fig. 5 is a schematic view illustrating the elastic deformation of work rolls and the sectional profile of the material being rolled; Fig. 6 is a graph illustrating the reduction of crown formation and edge dropping according to the present invention; and Fig. 7 is a schematic view illustrating the sectional profile of a product rolled according to the present invention. Fig. 1 illustrates a four-high rolling mill to which the present invention is applied. This rolling mill comprises work rolls 1 and 1', back-up rolls 2 and 2' and a mill housing 3 and is for rolling a material 4. Each work roll 1 or 1' has two tapered ends which are ground. One tapered end t₁ is steeper than the other tapered end t₂. These work rolls 1 and 1' are incorporated in the mill such that the tapered ends having different taper angles are arranged one above the other and such that they are shiftable in axial directions relative to the mill housing 3 as shown by the arrows in Fig. 1. The work rolls 1 and 1' have ground surfaces 5 and 5' at their tapered ends and are supported in bearing chocks 6 and 7, respectively. The work rolls 1 and 1' further have spindles 8 and 9 which are splined for torque transmission. Driving means (not shown) for moving the upper and lower work rolls 1 and 1' in their axial directions may be arranged in the proximities of the bearing chocks 6 and 7 or at the extended ends of the spindle 8 and 9. The driving system of the driving means may be hydraulic, electrical or magnetic. Reference numeral 10 denotes a balancing or roll bending device for increasing the bending action acting upon the work rolls 1 and 1'. Numeral 11 denotes a roll bending device for decreasing the bending action. The back-up rolls are supported by chocks 12 and 13 including bearings 14 and are urged downwardly by screws 15. Although the work rolls are driven in this embodiment, the back-up rolls may be driven. Moreover, although the more steeply tapered end of the upper work roll is on the right side as viewed in Fig. 4, it may be on the left side. Fig. 3 illustrates typical profiles of rolled materials rolled in a four-high rolling mill including work rolls having single tapered ground ends according to the work roll shifting method as shown in Fig. 2a. As can be seen from Fig. 3, the thicknesses of the edges of the rolled materials are considerably decreased. This variation in thickness is not linear. Moreover, the profiles of the thickness are greatly different depending upon the thicknesses of the finished plates. If the quality and thickness of the material to be rolled are changed, the shapes of the edge drops are also changed. In view of the results of Fig. 3, in order to effect complete crown and edge drop controls, it is necessary to prepare a plurality of profiles for the tapered ends of the work rolls to be subjected to the roll shifting method in order to deal with complicated changes in thickness of the material in the vicinity of its edges. As shown in Fig. 2b, according to the invention, the work rolls 1 and 1' have steep tapered ends and gentle tapered ends, so that the following controls can be effected in dependence upon the shifted distances of the work rolls as shown in Fig. 2b in the order from the top to the bottom. Thus the crown formation and edge dropping can be controlled by (1) only the gentle tapered portions t₂, (2) the gentle and steep tapered portions t₁ and t₂ (with hard and soft materials which tend to considerably decrease their thickness at the edges), and (3) only the steep tapered portions t₁. In other words, by adjusting the shifted distances of the work rolls according to the quality, thickness and width of the material to be rolled, effective control of crown formation and edge dropping can be carried out. According to the invention, both ends of the work rolls are ground to form different tapers and the work rolls are arranged with their differently tapered ends alternately arranged so that the contact pressure between the drum part of the work rolls and the back-up rolls becomes small without any extra bending moment acting upon the work rolls, with the result that the deformations of the axes of the work rolls decrease to ensure the prevention of waving and the control of the crown. Moreover, the upper and lower work rolls 1 and 1' are moved reversely relative to each other according to the thickness, width and quality of the material 4 to be rolled so that the edges of the material are located adjacent one of the tapered ground ends or both of the tapered ground ends. Accordingly, the contact pressure of the work rolls 1 and 1' with the edges of the material to be rolled is decreased so as to mitigate the rapid change in deformation of the work rolls as they tend to flatten the edges of the material. Thus excessive metal flow of the material at its edges is eliminated and this effectively controls the edge drop. Moreover, the upper and lower work rolls 1 and 1' can be moved in the axial directions, so that extraordinary local wear is also mitigated which would otherwise occur with conventional work rolls and thus local protrusions are also effectively eliminated. In other words, even if extraordinary local wear occurs on the roll surfaces, the work rolls are moved in the axial direction to distribute the wear over all the straight ground surfaces of the work rolls so that the high spots caused by the straight ground surfaces can be effectively mitigated. Moreover, as can be seen from an embodiment later described, the contact position of the material with the tapered ground ends need not be limited to one point and thus can be varied over an allowable range. Accordingly, edge build-up can be effectively prevented by changing the contact position of the material within the allowable range (for example, -50 to +50 mm). When the quality of the material 4 is changed, for example, from a hard material to a soft material, edge drop and edge build-up can be effectively prevented by finely adjusting the shifted distances of the work rolls in a manner such as to make small the length of the edges of the material to be rolled by the ground surfaces in addition to making the adjustment for the change in width of the material. The ratio of the gentle tapered angle to the steep tapered angle is determined in dependence upon the quality, thickness and width of the material in the same rolling cycle. From the typical profiles shown in Fig. 3, the following relationship is desirable. 1 < the steep taper/the gentle taper ≦ 10 Moreover, the length of the tapered ground portions of the work rolls 1 and 1' in the axial directions is preferably 2-500 mm. Figs. 4a-4d illustrate (a) the defect in flatness due to the waving phenomenon, (b) the crown resulting from the differene in thickness between the edges and the center, (c) edge drop due to excessive metal flowing at the edges, and (d) high spots and edge build-up due to the local wear of the work rolls. Fig. 5 illustrates (b) the deformation of the roll axis, (f) the flat deformation and (e) edge drop due to these deformations. Kind of steel Width (mm) Thickness on entry side (mm) Thickness on delivery side (mm) EL EH EL1 (mm) EL2 (mm) EH1 (µm) EH2 (µm) A Low carbon steel 800 4.5 3.2 150 50 150 100 B Low carbon steel 1,000 4.5 3.2 250 100 250 200 C Low carbon steel 800 4.5 3.8 200 75 200 150 D High carbon steel 800 4.5 3.2 200 100 250 200 Steep taper/Gentle taper = 2 Four kinds of steel were hot-rolled by three rolling methods viz. the conventional rolling method using axially fixed work rolls, the work roll shifting method using work rolls having single tapered ground ends, and the rolling method according to the invention using the rolling mill of the invention. Positional relations between the work rolls and the material to be rolled are indicated by EL and EH which are defined as shown in Figs. 2a and 2b. Crowns (Ch 50 = hc - h 50 2 ) and edge drops (Eh 100 = h 100 - h 10 2 ) of the rolled products are shown in Fig. 6 in which the present invention is compared with the prior art. In this case, hc is the thickness of the material 4 at the middle of the width and h₁₀₀ is the thickness of the material at locations spaced 100 mm from the edges of the material. Moreover, h₅₀ and h₁₀ are thicknesses at locations spaced 50 mm and 10 mm from the edges of the material, respectively. Fig. 7 illustrates a profile of the thickness of the A material (low carbon steel, 800 mm width and 3.2 mm thickness on the delivery side) rolled by the method according to the invention. As can be seen from Figs. 6 and 7, the crowns and edge drops of the products rolled according to the invention are smaller than those rolled by the prior art. Moreover, according to the invention, the rolled products have preferred profiles without any high spots and edge build-up. The tapered ends of the work rolls which are shown are conical, but they may be part-sine curved or arcuate. Furthermore, the present invention enables the flatness of the folled plates to be controlled with the aid of a roll bending apparatus. As can be seen from the above description, the present invention is very effective to control crown formation, edge dropping, and local protrusions The invention is applicable to two, four, five and six-high rolling mills and cluster mills including slabbing mills and series of roughing and finishing mills for hot and cold rolling. Moreover, the application of the invention is simple and easy as is the conversion of existing mills so that the cost of installation is inexpensive which is advantageous. As a uniform wear of work rools can be achieved, the number of rolled coils per single rolling cycle can be increased. Moreover, the schedule for rolling materials having various widths is not limited, so that the working efficiency can be remarkably improved and the service period of the rolls to be used can be considerably prolonged. While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that changes in form and details can be made therein without departing from the scope of the invention as defined by the claims.

Claims: A rolling mill comprising a mill housing (3), a pair of axially moveable work rolls (1,1') mounted in the housing (3) one above the other, wherein each work roll includes a drum portion and a tapered end, and back-up rolls (2) mounted in the housing (3) for backing-up the work rolls (1,1') characterised in that the work rolls are crown work rolls each having, at both ends of the drum portion, tapered ends (5,5') ground at different taper angles with the tapered ends of each work roll being in opposition to the tapered ends of the other roll which are of different taper angle. A rolling mill as claimed in claim 1, wherein said tapered ends of the work rolls are conical. A rolling mill as claimed in claim 1, wherein the ratio of the more steeply tapered end to the more gently tapered end is larger than one but not larger than ten. A method of rolling a plate comprising (i) arranging a pair of tapered work rolls (1,1') one above the other in a rolling mill, each work roll including a drum portion and a tapered end, (ii) moving the work rolls in opposite directions along their axes in dependence on the thickness, width and material of the plate and (iii) rolling the plate between the work rolls characterised in that each work roll has two tapered ends (5,5') ground at different taper angles with the tapered ends of each work roll being in opposition to the tapered ends of the other roll which are of different taper angle and the edges of the plate are rolled between the tapered ends of the work rolls. A method of rolling a plate as claimed in claim 4, wherein said tapered ends of the work rolls are conical. A method of rolling a plate as claimed in claim 4 or 5, wherein the ratio of the steeper taper to the gentler taper is larger than one but not larger than ten.

Inventors: KITAHAMA MASANORI, SASAKI TORU, YARITA IKUO, KITAHAMA, MASANORI, SASAKI, TORU, YARITA, IKUO, KITAHAMA, MASANORI C/O TECHNICAL RESEARCH DIVISON, SASAKI, TORU C/O TECHNICAL RESEARCH DIVISON, YARITA, IKUO C/O TECHNICAL RESEARCH DIVISON
Applicants: KAWASAKI STEEL CO, KAWASAKI STEEL CORPORATION
Application year: 1985
Main classification: B21B13
IPC codes: B21B13, B21B1, B21B37, B21B27