CONTINUOUS ANNEALING APPARATUS

Abstract: This vertical continuous annealing apparatus for steel strip (60) is provided with cylindrical rolls (70) in contact with the steel strip (60) at positions close to hearth rolls (50) in the high-temperature zone of the furnace. The cylindrical rolls correct wrinkles in the steel strip caused by the shape of convex hearth rolls, and thus prevent buckling which is likely to occur in the steel strip.

Description: CONTINUOUS ANNEALING APPARATUS BACKGROUND OF THE INVENTION This invention relates to a vertical continuous annealing apparatus for steel strip. A continuous annealing furnace comprises a heating zone, a soaking zone, and a cooling zone arranged in that order from the upstream side toward the downstream side. In the continuous annealing furnace, steel strip is subjected to appropriate heat treatment as it passes through the heating zone, the soaking zone, and the cooling zone in sequence. In a vertical continuous annealing furnace, a plurality of hearth rolls are provided in parallel in the upper and lower parts of the furnace, and the arrangement is such that the steel strip moves up and down vertically between the hearth rolls to pass through each zone of the annealing furnace. However, a problem with such an apparatus can be that the steel strip passing through the apparatus will meander during the operation as it winds around the succession of hearth rolls. Meandering of the steel strip refers to a movement wherein the center line of the steel strip deviates horizontally from the center line of the hearth rolls in a lateral direction. To prevent the steel strip from meandering, one or two steering rolls for adjusting the meandering of the steel strip are provided in each zone in the vertical continuous annealing apparatus, but this is still not a satisfactory method of preventing the steel strip from meandering. For this reason, such a vertical continuous annealing apparatus usually has hearth rolls that have a convex shape to prevent the steel strip from meandering. The same principle as that which prevents a transmission belt coming off a belt pulley is applied in this way. Since the transmission belt and the belt pulley rotate together due to the friction between them, a force acts to move the belt toward the center part of the pulley face, where its diameter is greatest. Similarly, the drum of each hearth roll is tapered. That is, the drum has a cylindrical center section with both ends tapered to form a conical shape. Or alternatively, the drum is formed to have a curved crown. Thus the hearth rolls act to pull the steel strip toward the center part of the hearth roll. Since the hearth roll drums are convex and not flat cylinders, the distribution of longitudinal tensile stress within the steel strip wound around the hearth rolls is uneven across its width. In particular, when the angle of taper of the hearth rolls is large, or the degree of curvature is high, or when the thermal expansion of the center parts of each hearth roll is larger than that of other parts due to the radiated heat in the furnace, or when the longitudinal tension in the steel strip increases, in any of these situations, unevenness in the tension in the steel strip increases. Such an unevenly distributed longitudinal tensile stress may give rise to compressive stress in the material. Buckling will occur in the steel strip as a result. When the hearth rolls are tapered as described above, for example, buckling can occur at the places where the steel strip comes into contact with the shoulder where the tapers begin from the cylindrical part of each hearth roll, that is to say the boundaries between the cylindrical and conical sections. Such buckling can not only reduce the quality of the product, but may also cause a rupture in the steel strip when severe. This rupture would cause tremendous trouble to the whole system of which the annealing furnace is a part. Buckling can be avoided by forming the hearth rolls as flat cylinders without crowns, however, the steel strip then meanders easily, as described above. Thus, in order to solve the problem prevailing hitherto, it is necessary to develop a technique in the art which prevents both meandering and buckling. Recently there has been a large demand for coldrolled steel in a variety of sizes and strengths. In the vertical continuous annealing furnace, it is very difficult, simply by altering only the shape of the hearth rolls, to provide appropriately shaped hearth rolls that will prevent both meandering and buckling simultaneously in all kinds of cold-rolled steel of different sizes and heat treatment conditions, and thus another method of solving these problems is required. SUMMARY OF THE INVENTION This invention has been developed to solve the problems described above. An object of this invention is to provide an annealing apparatus which is capable of efficiently preventing the buckling which arises in steel strip due to convex hearth rolls in a vertical continuous heat treatment furnace. Another object of this invention is to provide an annealing apparatus capable of heat-treating various kinds and sizes of material. A further object of this invention is to provide an annealing apparatus by which a fault-free product is obtainable. To achieve these objects, cylindrical rolls for correcting deformations in the steel strip are provided at position close to the hearth rolls in a hightemperature region to prevent buckling occurring in the steel strip. The above and other objects and features of this invention will become apparent from the following description taken in conjunction with the accompanying drawings. The drawings are intended for explanation only, and do not limit the scope of this invention. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a fragmentary longitudinal section through a vertical annealing furnace to which the invention has been applied; Fig. 2 is a front view of a hearth roll tapered at both ends; Fig. 3 is a front view of a hearth roll with a curved crown; Fig. 4 is a sectional side view of a hearth roll wound around with the steel strip; Fig. 5 is a rear view of Fig. 4 which shows the steel strip on the side receding from the hearth roll; Fig. 6 is a front view of Fig. 4; Fig. 7 is a section through the steel strip taken along the line IV-IV of Fig. 6; Fig. 8 shows wrinkles formed on the back of the steel strip. This is a rear elevation of Fig. 4; Fig. 9 is a schematic illustration of an embodiment of this invention with an auxiliary roll positioned behind the steel strip on the advancing side in relation to the hearth roll; Fig. 10 is a schematic illustration of an embodiment of this invention with auxiliary rolls positioned behind the steel strip on the advancing and receding sides in relation to the hearth roll; Fig. 11 is a schematic illustration of an embodiment of this invention with an auxiliary roll positioned in front of the steel strip on the advancing side relative to the hearth roll; Fig. 12 is a schematic illustration of an embodiment of this invention with auxiliary rolls positioned in front of the steel strip on the advancing and receding sides relative to the hearth roll; ; Fig. 13 is a schematic illustration of an embodiment of this invention with auxiliary rolls posi tioned in front of and behind the steel strip on the advancing and receding sides relative to the hearth roll; Fig. 14 is a front view of Fig. 9; Fig. 15 is a section through the steel strip taken along the line V-V of Fig. 14; Fig. 16 and Fig. 17 are stress-strain graphs for steel at room temperature and the annealing temperature, respectively; Fig. 18 and Fig. 19 are graphs indicating the critical tension for buckling of steel strip on the sides advancing toward and receding from the hearth rolls, respectively. DETALLED DESCRIPTION OF THE INVENTION Fig. 1 is a schematic longitudinal section of portions of a heating zone 30 and a soaking zone 40 of a vertical continuous annealing furnace. A steel strip 60 fed into the heating zone 30 from the left in Fig. 1 runs upward and downward vertically wound alternately between upper hearth rolls 50 and lower hearth rolls 52 disposed in parallel and above each other in the heating zone 30, and is then heated using a heater (not illustrated ). The steel strip 60 next enters the soaking zone 40 from the heating zone 30, and is kept at a soaking temper ature while vertical movement is maintained in the same way as in the soaking zone 40. The steel strip 60 then advances toward a cooling zone (not illustrated) to the right. Auxiliary rolls 70 are provided for the three uper hearth rolls 50 at the rear end of the heating zone 30 and also for the three upper hearth rolls 50 of the soaking zone 40. These auxiliary rolls 70 are the subject of this invention. To prevent the steel strip 60 from meandering, rolls 54 with tapered surfaces or rolls 56 with convex surfaces, as shown in Fig. 2 and Fig. 3, are used as the hearth rolls 50. Fig. 5 shows the surface of the steel strip 60 on the side receding from the tapered hearth roll 54 of Fig. 2, indicating buckling 62 occurring. in the parts of the steel strip 60 in contact with the shoulders of the hearth roll 54. The inventors have carried out experiments in which the shape of the hearth rolls 50, and the width and thickness of the steel strip 60 were changed in various ways to determine the mechanism by which buckling is produced in the steel strip 60. As a result, they discovered that buckling occurs immediately before the steel strip 60 winds around the hearth roll 50, and again immediately after. Fig. 6 is a front view of the steel strip 60 advancing toward the hearth roll 50 immediately before it winds around it. The steel strip 60 forms wrinkles 62 at a position close to the hearth roll 50. The nature of the wrinkles can be seen easily in Fig. 7 which is a section taken along the IV-IV of Fig. 6. As will be apparent from Fig. 6 and Fig. 7, when tension is applied longitudinally to the steel strip 60, the steel strip 60 is subjected to compressive membrane stress in the lateral direction in the places at which the steel strip comes into contact with the taper shoulders of the hearth roll 50, and large wrinkles 62 are produced in the steel strip 60 by this compressive stress immediately before it winds around the hearth roll 50. The steel strip 60 winds around the hearth roll in this state. If the wrinkles in the steel strip 60 are not removed by the surface of the hearth roll 50, buckling together with intermittently-creased lines, as shown in Fig. 5, may occur in the steel strip 60 being subjected to bending and unbending by subsequent hearth rolls 50. This situation which has been experienced in actual operation is unacceptable. Since there are no countermeasures provided against wrinkles produced by tension in a conventional type of annealing apparatus, as described above, the critical region in which buckling occurs covers a considerably wide range. The inventors have discovered that buckling can be effectively prevented from occurring in the steel strip 60 by providing an apparatus that is capable of correcting deformations in the steel strip 60 at the places where these deformations occur. This invention has been developed in response to this finding. This invention relates to a vertical continuous annealing apparatus wherein one or more cylindrical rolls 70 placed parallel to the hearth roll shafts, are arrayed at positions where the steel strip 60 is close to the hearth rolls 50 so that they are in contact with the steel strip 60, thereby correcting wrinkles occurring within the steel strip 60. Fig. 9 to Fig. 13 show preferred embodiments of this invention. Fig. 9 shows an embodiment wherein an auxiliary roll 70 is positioned so that it is brought into contact firmly with the surface of the steel strip which comes in contact with the hearth roll, and at the part of the steel strip 60 which is advancing toward the hearth roll 50, or a position immediately before the steel strip 60 winds around the hearth roll 50. Any wrinkles occurring in the steel strip 60 can be corrected smoothly by keeping the cylindrical roll 70 firmly in contact with the steel strip 60, thus solving the problem mentioned above. A view from the direction III-III of Fig. 9 showing the steel strip thus corrected is given in Fig. 14. Fig. 14 shows the appearance of the wrinkles produced in the surface of the steel strip in this case. A section taken along the line V-V thereof is given in Fig. 15. The wrinkles in the steel strip 60 immediately before it winds around the hearth roll 50 can be corrected by the action of the cylindrical roll 70. Its section becomes even and thus buckling can be prevented. The description above relates to buckling arising on the side of the steel strip 60 advancing toward the hearth roll, however, wrinkles may occur on the side of the steel strip 60 receding from the hearth roll 50 in the same way as in the advancing side. Fig. 8 is a rear elevation of Fig. 6, showing wrinkles 64 produced in the steel strip 60 receding from the hearth roll 50. The wrinkles 64 pruduced in the steel strip 60 receding from the hearth roll 50 reach the next hearth roll and are subjected to bending and rebending de formation there. The wrinkles then develop into intermittently-creased lines 62 shown in Fig. 5 which is accompanied by the creases as described before. Therefore, correction df the wrinkles occurring in the steel strip 60 receding from the hearth roll 50 is also necessary to prevent buckling. Buckling is produced by a lower tensile stress on the side of the steel strip 60 advancing toward the hearth roll 50 than on the receding side. As shown in Fig. 9 and Fig. 11, therefore, first consideration must be given to the correction of wrinkles occurring on the side of the steel strip 60 advancing toward the hearth roll 50. For a steel strip 60 with thin thickness, has a low yield stress, and fits easily around the hearth roll 50 and has a low rigidity against buckling, cylindrical rolls for correcting the wrinkles must be positioned on the steel strip 60 on the side receding from the hearth roll 50, as shown in Fig. 6, Fig. 12, and Fig. 13. The smaller the diameter of the cylindrical roll, the greater the longitudinal bending stress acting on the surface of the steel strip 60 in contact with the roll. If the stress exceeds a certain level when the steel strip 60 has the relationship between stress and strain shown in Fig. 16, creases perpendicular to the longitudinal direction may arise in the steel strip 60. However, in the heating and soaking zones where buckling is particularly likely to occur in the annealing furnace, the temperature of the steel strip 60 is between 700 to 850"C. The stress-strain relationship under these conditions is shown by the smooth curve given in Fig. 17, and thus the creases are not likely to occur in the steel strip 60. For safety's sake, it is preferable that the diameter of the cylindrical roll be large enough that the ratio of the thickness of the steel strip 60 to the cylindrical roll diameter is larger than the critical strain on the surface of the steel strip 60. Assuming, for example, that the critical strain on the surface of the steel strip 60 is 0.002, the minimum diameter of the cylindrical roll must be 150 mm for a thickness of 0.3 mm. It is desirable that the cylindrical roll be positioned so that it comes in contact with the steel strip 60 at a distance which is less than twice the width of the steel strip 60 from the point at which steel strip 60 starts to wind around or separate from the hearth roll. This is the range wherein the compressive stress of the steel strip 60 in the lateral direction becomes conspicuous due to uneven tension. In a continuous annealing apparatus in which cylindrical rolls 70 to 84 are provided near each hearth roll 50, transverse movement of the steel strip 60 is prevented and thus the meandering correction function may be spoiled by the frictional force between the cylindrical rolls 70 to 84 and the steel strip 60. In a conventional type of continuous annealing apparatus, there is no factor preventing transverse movement of the steel strip 60 between the hearth rolls. Therefore meandering of the steel strip 60 is corrected quickly by the meandering correction function provided by the hearth roll being convex. In this respect, the angles of contact between the cylindrical rolls 70 to 84 and the steel strip 60 must be minimized as far as possible so that the area of contact between the cylindrical rolls 70 to 84 and the steel strip 60, and the pressure normal to the contact surfaces, can be minimized and the transverse movement of the steel strip 60 can be facilitated. Thus the effect of correcting wrinkles produced in the steel strip 60 can be secured without reducing the meandering correction effect. This effect is easily obtainable by keeping the angles of contact of the steel strip 60 with the cylindrical rolls 70 to 84 to within 20 degrees at the roll center, and thus the effect of correcting wrinkles can be obtained without much loss of meandering correcting function. Smoothing the surface of the cylindrical rolls by minimizing their surface roughness Ra, and roughening the surface of the hearth rolls, is effective in preventing meandering. For example, when Ra=l pm for the cylindrical rolls, and Ra=4 to 5 pm for the hearth rolls. The embodiment shown in Fig. 13 comprises positioning a pair of cylindrical rolls 78, 80 and another pair 82, 84 against both front and rear surfaces of the steel strip 60 so that each pair sandwiches the steel strip 60. Each pair of cylindrical rolls 78, 80 or 82, 84 is positioned so that the gap between them is 1.0 to 1.2 times the thickness of the steel strip 60, and these act to correct deformations in the steel strip 60 within that gap. These cylindrical rolls are superior in that they provide no obstruction to the meandering correction function of the hearth roll. In the embodiment shown in Fig. 13, there is no problem concerning the occurrence of transversal creases in the steel strip 60, therefore no restrictions need be placed on the diameters of the cylindrical roll. For example, so long as the rigidity of the cylindrical rolls is within a permissible range, the diameter of the cylindrical rolls may be less than 150 mm. To prevent scratches occurring on the surfaces of the steel strip 60 as a result of the provision of the cylindrical rolls used in this invention, the rotational speed of the cylindrical rolls is made to coincide with the running speed of the steel strip 60. VVVF control (Variable Voltage Variable frequency control), for example, can be employed as a method of controlling the rotational speed of the cylindrical rolls. An illustrative example will be used to describe this invention further. ILLUSTRATIVE EXAMPLE Experiments were carried out by placing the cylindrical rolls as applied in this invention, in the manners shown in Figs. 9, 10, 11 and 13, at positions close to the three upper hearth rolls on the outlet side of the heating zone 30 of the vertical continuous annealing furnace, and similarly, near the three upper hearth rolls on the inlet side of the soaking zone 40, as shown in Fig. 1. The hearth rolls comprised a central cylindrical section of 460 mm and a conical section at each end tapering at tanG=0.0009 (0 being the angle of taper). All the cylindrical rolls used in this invention had an outer diameter of 300 mm. An extra-low carbon steel (C=0.003%) 0.7 mm thick and 1,280 mm wide was subjected to heat treatment at a rate of 200 m/min. The results are shown in Table 1, Fig. 18 and Fig. 19. Table 1 Meandering Example No. Characteristics 1 Comparative example A (conventional) 2 Fig. 9 (Embodiment) B 3 Fig. 10 (Embodiment) C 4 Fig. 11 (Embodiment) B 5 Fig. 13 (Embodiment) A Notes: Meandering characteristics A : Almost same as hitherto B : Almost no problem C : Meandering correction somewhat slow, but no problem in operation Fig. 18 and Fig. 19 indicate the relationship between buckling, if any, occurring in the steel strip and the mean tensile stress for each example in Table 1; Fig. 18 for the side of the steel strip advancing toward the hearth roll, and Fig. 19 for the side receding from the hearth roll. With Example 1 (conventional annealing furnace), buckling was produced on the side advancing toward the hearth roll when the mean tensile stress in the steel strip was 0.4 kg/mm2. In the embodiments of the continuous annealing apparatus provided with the cylindrical rolls used in this invention, the tensile stress at which buckling started to occur was great in each of Examples 2 to 5. With Examples 2 and 4, in which the cylindrical rolls were provided on the side of the steel strip advancing toward the hearth roll, buckling was not produced even at tensile stresses 1.6 to 1.7 times higher than hitherto. With Example 3, in which the cylindrical rolls 70, 72 were provided on both the advancing and receding sides, buckling was not produced even when tensile stresses more than twice as high as hitherto acted on both the advancing and receding sides. As seen from the meandering characteristics given for Example 3 in Table 1, the meandering control function was somewhat worse than that of the conventional system, however, this presents no problem in practice. With Example 5, superior performance was obseved in the control of both buckling and meandering.

Claims: CLAIMS 1. A continuous annealing apparatus comprising: a plurality of convex hearth rolls positioned horizontally in the upper and lower parts of a high temperature zone of a vertical continuous annealing furnace, guiding a steel strip up and down vertically as it passes through the furnace for heating or soaking, and one or more cylindrical rolls provided at positions close to at least one of said hearth rolls, being parallel to the hearth roll and being in contact with the adjacent part of the steel strip, whereby said cylindrical rolls may correct wrinkles produced in the steel strip by said convex hearth rolls. 2. The continuous annealing apparatus as defined in Claim 1, wherein said cylindrical rolls are respectively disposed in contact with the same surface of the steel strip which is in contact with the hearth roll on the advancing side relative to said hearth roll. 3. The continuous annealing apparatus as defined in Claim 1, wherein said cylindrical rolls are respect ively disposed in contact with the same surface of the steel strip which is in contact with the hearth roll both on the advancing and receding sides relative to said hearth roll. 4. The continuous annealing apparatus as defined in Claim 1, wherein said cylindrical rolls are respectively disposed in contact with the opposite surface of the steel strip which is not in contact with the hearth roll on the advancing side relative to said hearth roll. 5. The continuous annealing apparatus as defined in Claim 1, wherein said two cylindrical rolls are respectively disposed in contact with the opposite surface of the steel strip which is not in contact with the hearth roll both on the advancing and receding sides relative to said hearth roll. 6. The continuous annealing apparatus as defined in Claim 1, wherein said cylindrical rolls comprise pairs of rolls sandwiching said steel strip from both sides, provided at positions in contact with both surfaces of said steel strip on both the advancing and receding sides of said steel strip relative to said hearth roll.

Inventors: ABE, HIDEO C/O RESEARCH LABORATORIES, HIRA, TAKAAKI C/O RESEARCH LABORATORIES, SASAKI, TORU C/O RESEARCH LABORATORIES, SHIMOYAMA, YUJI C/O CHIBA WORKS, TAHARA, KOUICHI C/O HANSHIN WORKS, YANAGISHIMA, FUMIYA C/O CHIBA WORKS
Applicants: KAWASAKI STEEL CORPORATION
Application year: 1983
Main classification: C21D9
IPC codes: C21D9