Abstract:
A method and a device for producing a metal strip by continuous casting and rolling, in which a thin slab is initially cast into a casting machine, and is subsequently rolled in at least one rolling train using primary heat from the casting cycle. The cast thin slab is passed between the casting machine and the at least one rolling train and at least one holding oven as well as at least one induction oven. The holding oven and the induction oven are activated or deactivated according to a selected mode of operation, that is, a first mode of operation for the continuous production of the metal strip and a second mode of operation for the discontinuous production of the metal strip.

Description:
BACKGROUND OF THE INVENTION 
     The invention concerns a method for producing metal strip by direct strand reduction, in which a thin slab is first cast in a casting machine and then rolled in at least one rolling train with utilization of the primary heat of the casting process. The invention also concerns an installation for producing metal strip by direct strand reduction. 
     Installations of this type are known as thin slab-thin strip direct strand reduction installations and are referred to as CSP installations. 
     Continuous rolling from the casting heat has long been known but has not yet found commercial success. The rigid connection of the continuous casting installation and the rolling train and temperature control through the whole installation have proven difficult to control. 
     EP 0 286 862 A1 and EP 0 771 596 B1 disclose methods and installations for continuous rolling from the casting heat. In these cases, the casting process and the rolling process are directly coupled. The continuous strip is cut with a shear shortly before the coiler. 
     EP 0 415 987 B2 and EP 0 889 762 B1 disclose similar methods for the continuous production of strip steel with the coupling of casting and rolling installations. To overcome the temperature problems at the relatively low conveyance speed, inductive heating units are provided upstream of and within the rolling train. 
     An alternative technology to this is the rolling of single slabs and single strips. In the discontinuous rolling of strip, the casting and rolling are disconnected from each other. The casting speed is usually very low, and the rolling speed is at a high level and independent of the casting speed, such that the temperature for the last deformation is above the minimum temperature. Installations of this type, which are also referred to as CSP installations, are described, for example, in EP 0 266 564 B1, in which a high reduction is carried out in the thin slab installation. 
     EP 0 666 122 A1 describes a similar thin slab installation, in which discontinuous strips are rolled with the use of inductive heating between the first finishing stands. 
     The advantages of discontinuous rolling are that the casting and rolling speed can be adjusted independently of each other. In thin strip rolling, higher rolling speeds can be adjusted, e.g., flexibly, even when the casting installation operates at a lower speed or the speed is just then being adjusted there. 
     Both methods, i.e., on the one hand, continuous casting and rolling and, on the other hand, discontinuous casting and rolling, are difficult to combine due to the circumstances described above. 
     SUMMARY OF THE INVENTION 
     Therefore, the objective of the invention is to remedy this situation and to create a combined casting and rolling method and an associated installation, with which both continuous and discontinuous operation are possible. Accordingly, the goal is to combine the advantages of both methods in a new installation design. 
     The solution to the objective of the invention with respect to a method is characterized by the fact that the cast thin slabs pass through both at least one holding furnace and at least one induction furnace between the casting machine and the one or more rolling trains, where the holding furnace and the induction furnace are activated or deactivated and controlled by open-loop or closed-loop control as a function of a selected mode of operation, namely, a first mode of operation of continuous production of the metal strip and a second mode of operation of discontinuous production of the metal strip. The order of the two furnaces, i.e., the holding furnace and the induction furnace, can be chosen as desired. 
     The rolled metal strip is also preferably heated in at least one additional induction furnace downstream of a first rolling train in the direction of strip conveyance, and the one or more additional induction furnaces are activated or deactivated and controlled by open-loop or closed-loop control as a function of the selected mode of operation. 
     The selection of the mode of operation can be made as a function of the final thickness of the metal strip to be produced or as a function of the casting speed of the metal strip. It has also been found to be effective if it is provided that the mode of operation is selected as a function of the product of the thickness to be produced and the speed of the metal strip or thin slab. 
     The mode of operation can also be selected as a function of the material to be processed. This can also be related to the given allowable runout temperature of the strip from the rolling mill. 
     As an example, the continuous mode of operation can be selected if the product of the cast thickness and the casting speed is greater than 70 mm×6.5 m/min=455,000 mm 2 /min. Naturally, this value can also fall within a different general range, depending on the material; a value between 300,000 mm 2 /min and 600,000 mm 2 /min is preferably used as the criterion for the “switching point” from one mode of operation to the other. 
     An alternative criterion can be that the continuous mode of operation is selected for final thicknesses of the metal strip of less than 2 mm. 
     When the mode of operation of discontinuous production of the metal strip is selected, the thin slab is preferably held batchwise in the holding furnace at a desired temperature before being conveyed into the rolling train. 
     When the mode of operation of continuous production of the metal strip is selected, the thin slab can be brought to a desired temperature in the holding furnace and then heated by the induction furnace to the desired rolling temperature immediately before the rolling operation in the rolling train. In an especially preferred embodiment, it can be provided that the amount of heat introduced into the thin slab by the induction furnace depends on the casting speed. 
     The continuous mode of operation or discontinuous rolling can be set as a function of the casting speed, so that in each case the required final rolling temperature can be attained. 
     In order to achieve optimum energy input during the production of the metal strip, a refinement of the invention provides that the loss of heat from the heated metal strip and/or from the thin slab to the environment is hindered by thermal insulating means. These means do not have to be used constantly. Therefore, provision can be made to move at least some of the thermal insulating means into or out of the vicinity of the metal strip as a function of the desired mode of operation of the direct strand reduction installation. 
     One advantageous refinement provides that the metal strip is descaled in the rolling train in an upstream section of the rolling train with respect to the direction of strip conveyance and is then heated in a section of the rolling train that is farther downstream with respect to the direction of strip conveyance. Of course, this does not exclude the provision of additional descaling units. 
     The descaling of the metal strip and/or the thin slab by means of a descaling unit and the heating of the metal strip and/or the thin slab by means of an induction furnace preferably take place between two rolling stands. In this regard, the heating can follow the descaling in the direction of strip conveyance or vice versa. 
     The installation for producing a metal strip by direct strand reduction, which comprises a casting machine, in which a thin slab is first cast, and at least one rolling train, which is located downstream of the casting machine and in which the thin slab is rolled with utilization of the primary heat of the casting process, is characterized, in accordance with the invention, by the fact that at least one holding furnace and at least one induction furnace is installed between the casting machine and the one or more rolling trains. 
     As will be described in detail later, suitable control of the two furnaces, i.e., the holding furnace and the induction furnace, allows both efficient continuous operation and efficient discontinuous operation of the installation. For this purpose, control means are preferably present with which the holding furnace and/or the induction furnace is activated or deactivated and controlled by open-loop or closed-loop control as a function of the selected mode of operation, namely, a first mode of operation of continuous production of the metal strip and a second mode of operation of discontinuous production of the metal strip. 
     In the direction of conveyance of the thin slab and the metal strip, a holding furnace can be installed first and then an induction furnace. In addition, a roughing train and a finishing train can be provided, and another induction furnace is installed between the roughing train and the finishing train. Furthermore, at least one additional induction furnace can be installed between two rolling stands of the roughing train and/or the finishing train. 
     It is advantageous to install a strip shear downstream of the first induction furnace and upstream of the finishing train in the direction of conveyance of the thin slab or metal strip. In addition, as is already established practice, a thin slab shear can be installed downstream of the casting machine and upstream of the holding furnace in the direction of conveyance. A strip shear can be installed downstream of the finishing train in the direction of conveyance. 
     In accordance with another refinement of the invention, thermal insulating means for hindering the loss of heat from the heated metal strip and/or from the heated thin slab to the environment are present, which are arranged in the vicinity of the metal strip at least some of the time. In this regard, means are preferably present for moving at least some of the thermal insulating means into or out of the vicinity of the metal strip. 
     However, most of the thermal insulating means are generally installed in a stationary way. 
     Furthermore, it can be provided that at least one descaling unit is present, which is installed in an upstream section of the rolling train in the direction of strip conveyance. 
     In an especially preferred embodiment of the invention, a holding furnace, an induction furnace and a soaking furnace are installed in this order upstream of the rolling train in the direction of conveyance of the thin slab or metal strip. 
     The proposed method is supported by the use of efficient inductive furnaces, which today can be constructed in a relatively space-saving way, and by a suitable plant configuration that allows continuous operation or, optionally, discontinuous rolling. 
     The advantages of the continuous technology, i.e., continuous operation of the proposed installation, in conjunction with CSP technology consist in the following features: 
     A short overall length of the installation is realized, which results in low capital expenditures. 
     Energy savings are possible due to the consistent direct rolling without intermediate cooling and subsequent reheating. 
     In addition, a lower deformation strength is obtained due to the lower rolling speed. 
     The possibility is created of producing products that are difficult to roll and, e.g., very thin (ultrathin) strips (strip thickness of about 0.8 mm) in large production amounts. 
     Special materials (high-strength materials) can be processed. 
     A combination of wide and thin strips can be processed. 
     The rolling of strip tail ends and thus roll damage can be avoided or reduced. 
     The breakdown rate of the installation can be reduced, and strips with height defects* can be avoided. 
     Specific embodiments of the invention are illustrated in the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  shows schematically a direct strand reduction installation in accordance with a first embodiment of the invention. 
         FIG. 2  shows an alternative embodiment of the direct strand reduction installation of the invention in the same view as  FIG. 1 . 
         FIG. 3  shows another alternative embodiment of a direct strand reduction installation of the invention in the same view as  FIG. 1 . 
         FIG. 4  shows schematically the area between the casting machine and the rolling train with a shear and means for providing thermal insulation. 
         FIG. 5  shows schematically the section of the finishing train with two rolling stands, between which a descaling unit and induction furnace are arranged. 
         FIG. 6  shows another alternative embodiment of a direct strand reduction installation of the invention in the same view as  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a direct strand reduction installation, in which a metal strip  1  is produced by first casting a thin slab  3  in a casting machine  2  of a type that is already well known and then conveying it to a rolling train  4 ,  5 , which in the present case consists of a roughing train  4  and a finishing train  5 . 
     In order to allow both continuous operation and discontinuous operation in accordance with the above discussion, both a holding furnace  6  and an induction furnace  7  are provided upstream of the rolling train  4 ,  5 . The two furnaces  6 ,  7  are operated by a suitable control system (not shown) in such a way that the correct strip temperatures are present for the two modes of operation. The open-loop or closed-loop control systems required for this are sufficiently well known from the prior art. 
     The holding furnace  6  installed downstream of the casting machine  2  can be a conventionally gas-fired furnace. The order in which the holding furnace  6  and induction furnace  7  are arranged can be as desired. 
     In the embodiment illustrated in  FIG. 1 , the roughing train  4  has two rolling stands  10 , and the finishing train  5  has five rolling stands  11 . The drawing also shows that another induction furnace  8  is positioned between the roughing train  4  and the finishing train  5  in order to heat the strip, after it has been roughed in the roughing train  4 , to the optimum temperature before finish rolling is carried out in the finishing train  5 . In addition, in the embodiment according to  FIG. 1 , induction furnaces  9  are installed between some of the rolling stands  11  of the finishing train  5  in order to continue maintaining optimum temperature control of the strip. 
     A strip shear  13  is installed between the casting machine  2  and the holding furnace  6 , and another strip shear  14  is positioned downstream of the finishing train  5 . A novel feature is that an additional strip shear  12  is positioned downstream of the first induction furnace  7  and upstream of the finishing train  5 . 
     The shear  13  is used to cut the thin slabs  3  during the discontinuous operation, and the shear  14  is used to cut the strips during the continuous rolling operation. 
     The shear  12  is used to crop the leading end of the strip or the trailing end of the strip during startup or discharge in continuous operation or in discontinuous operation to guarantee reliable conveyance through the downstream active inductive furnaces. 
     The installation is additionally equipped with elements that are already well known in themselves. These include descaling units  15 , which are positioned in locations that are favorable from a process-engineering standpoint. In addition, a cooling line  16  is located downstream of the finishing train  5 . Similarly, coilers  17  are installed at the end of the installation. 
       FIG. 2  shows a plant design that includes a roughing train  4  with three rolling stands  10  and a finishing train  5  with four rolling stands  11 . Otherwise, the solution shown in  FIG. 2  is the same as that of  FIG. 1 . 
       FIG. 3  shows an installation with a compact finishing train, i.e., here there is no roughing train  4  in accordance with the solutions of  FIGS. 1 and 2 . In the present case, the compact finishing train  5  has seven rolling stands  11 , which finish roll the metal strip  1  following the induction furnace  7 . Additional inductive heating units  9  are provided between the finishing stands. 
     The use of the proposed types of installations makes possible a coupled, fully continuous strand reduction process (continuous rolling) and, optionally, decoupled, discontinuous charging of individual slabs (batch rolling). 
     The furnace  6 —preferably realized as a roller hearth furnace—serves as a holding furnace during the discontinuous operation, and it is advantageous for it to be constructed short, so that there is room in it for a thin slab  3 . In this way, cooling of the thin slab during conveyance at casting speed is prevented. With the inductive furnace  7 , the thin slab  3  is reheated during continuous operation or discontinuous operation. In this regard, the heat input can be individually adjusted as a function of the casting speed, so that when the thin slab  3  leaves the inductive furnace  7 , a constant temperature at the desired level is obtained. Another advantage of the inductive furnace  7  compared to a gas-fired furnace is the short overall length with a suitably high heating capacity. 
       FIG. 4  shows schematically the area between the casting machine  2  and the rolling train or the holding furnace  6 , with a shear  13 . Especially during continuous operation, in which rolling is carried out at the low casting speed, it is important to minimize heat losses. To this end, in this embodiment, the roller table is equipped with thermal insulating means  18 ,  19  between the casting machine  2  and the furnace  6  in the vicinity of the shear  13  (and upstream and downstream of the induction heater). In the present case, these means are designed as thermal insulation boards, which are positioned between the rollers of the roller table and above the rolls of the roller table. The thermal insulating means  18  are stationary. 
     It is not customary to position thermal insulating means in the area in which sequences of movements occur (e.g., in the vicinity of the shear  13 ), since a cropping cut is carried out at regular intervals of time. During continuous operation, on the other hand, the shears are inactive for long periods of time, so that provision is made in the present embodiment also to insulate the area of the shear closely alongside and below the slab  3  or strip  1  in order to have a positive influence on the energy balance. In other words, the roller table thermal insulation covering is normally active. Only if it is intended that a cut be made (especially at the start of casting or during batch rolling), are the thermal insulating means  19  moved out, especially swung out, from the insulation region into a holding position by moving means  20  (indicated in  FIG. 4  only in a highly schematic way by double arrows). 
     Temperature loss can be prevented by the thermal insulation explained above. 
     Since the rolling process occurs relatively slowly in the continuous operation, it makes sense to carry out a descaling of the surface of the slab  3  or strip  1  between the forward rolling stands and then to heat the strip. This has a positive effect on the surface quality. An embodiment of this type with respect to the installation of the invention is shown in  FIG. 5 , which shows the area between two rolling stands  11  of the finishing train  5 , where a descaling unit  15  is positioned first in the direction of conveyance F of the strip  1  or slab  3 . A looper  22  and a retaining roller  23  keep the strip  1  under tension. The strip  1  enters an induction furnace  9  and then the following rolling stand  11  via a transfer table  24  and a lateral guide  25 . The order of the rolling stands, furnaces and descaling units can also be combined in any other desired way. 
     As explained above, provision can be made for a holding furnace and an induction furnace to be arranged in succession, and, of course, the order can be selected as desired, namely, the induction heater can also be positioned upstream of the holding furnace. 
     It is also possible to install a soaking furnace  21  downstream of the first furnace in the form of a holding furnace  6  and of an induction furnace  7  that follows the holding furnace  6  in the direction of conveyance F, as is shown in  FIG. 6 . 
     This is advantageous especially when an especially high temperature is produced at the entrance to the finishing train, which may be necessary, e.g., for grain-oriented silicon steel. Here the first furnace  6  is a heating furnace, which is assisted by the induction furnace  7 . For the purpose of homogenizing the temperature distribution over the width and thickness of the strip, the soaking furnace  21  is advantageous. This furnace configuration is preferred for the process explained here, but it can also be used in a conventional CSP plant, i.e., in batch operation. 
     In continuous rolling, the level of the casting speed determines the temperature variation through the entire installation. Depending on the casting speed, a computer model dynamically controls the heating power of the inductive furnaces upstream and within the rolling train in such a way that the delivery temperature of the rolling train reaches the target temperature. 
     If the casting speed falls below a certain preset threshold value (in the event of problems in the casting installation, in the case of materials that are difficult to cast, during the startup process, etc.), the operation is automatically switched from continuous mode to discontinuous mode. 
     This means that the thin slab  3  is cut with the shear  13 , and the rolling speed is increased in such a way that the desired final rolling temperature is reached. In this regard, the slab segments and strip segments within the train  4 ,  5  are monitored, and the conveyance speed or rolling speed and the inductive heating power of the inductive furnaces over the length of the strip are dynamically adapted as a function of the temperature distribution. 
     If the casting process has stabilized again, and the casting speed rises above the preset minimum value, then the operation is automatically switched back from discontinuous mode to continuous mode. 
     During the continuous rolling, the inductive furnaces  8  are usually positioned within the finishing train  5 , and during the discontinuous operation or during the startup process at the leading end of the strip, they are in a safe holding position well above or well to the side of the strip. 
     The ability to switch or adjust to continuous operation or discontinuous operation results in a high degree of flexibility, which represents an increase in process reliability. This is especially true when a production plant is being started up. 
     The continuous mode of operation is not generally used during processing; the batch operation is used primarily when there are casting speed problems or during the startup operation. 
     For the purpose of energy optimization, it can be provided that mainly relatively thin strip or strip that is difficult to produce is rolled in the continuous mode of operation, while strip with a thickness greater than a critical thickness is rolled in the batch operation at a high speed and thus a low heating energy requirement. The correct combination of type of production optimizes the energy balance of the CSP continuous/batch installation for the entire product mix. 
     LIST OF REFERENCE SYMBOLS 
     
         
           1  metal strip 
           2  casting machine 
           3  thin slab 
           4 , 5  rolling train 
           4  roughing train 
           5  finishing train 
           6  holding furnace (roller hearth furnace) 
           7  induction furnace 
           8  induction furnace 
           9  induction furnace 
           10  rolling stand of the roughing train 
           11  rolling stand of the finishing train 
           12  strip shear 
           13  strip shear 
           14  strip shear 
           15  descaling unit 
           16  cooling line 
           17  coiler 
           18  thermal insulating means 
           19  thermal insulating means 
           20  moving means 
           21  soaking furnace 
           22  looper 
           23  retaining roller 
           24  transfer table 
           25  lateral guide 
         F direction of strip conveyance