Manufacturing method for rolled metal strip and rolling facility

Abstract

In a manufacturing method for a rolled metal strip, the method including rolling, by a rough rolling mill 70, a cast piece cast by a continuous casting machine (CC) 12, the CC 12 is configured to be capable of selecting an endless mode in which a cast piece cast into a first thickness X is rolled by the rough rolling mill 70 without being cut and a batch mode in which the cast piece cast into a second thickness Y thicker than the first thickness X is rolled by the rough rolling mill 70 after being cut. This provides a manufacturing method for a rolled metal strip and a rolling facility that can improve the productivity of the rolled metal strip in a continuous casting facility that uses both the endless mode and the batch mode compared with conventional techniques.

Description

MANUFACTURING METHOD FOR ROLLED METAL STRIP AND ROLLING FACILITYThe present invention relates to a manufacturing method for a rolled metal strip and a rolling facility.As one example of solutions for continuous manufacturing of a hot rolled metal strip or metal plate, there is described a plant for endless manufacturing and batch manufacturing of a metal strip and a metal plate of hot rolled steel with a thickness of 0.6 mm to 50 mm or a thickness half the maximum thickness of a cast slab, in Patent Document 1. The plant according to Patent Document 1 includes a continuous casting machine (CC) using liquid core reduction to fabricate a thin slab, subsequently an induction heater with the interposition of a first shear, and a rolling mill. Subsequently, the plant includes also a second shear and a run out table having a cooling device and a pusher / piler for a metal plate. Further, the plant includes also a third shear and a plurality of coilers. Furthermore, the plant includes a minimum reduction rolling mill stand disposed between the CC and the first shear. The minimum reduction rolling mill stand is designed to execute rolling reduction in which the reduction rate is approximately 10% and 20% or lower in any case.[PTL 1] JP-2022-107666-AFor example, there is a technique described in the above-described Patent Document 1 as a conventional technique that allows both an endless mode in which a slab cast by a CC is rolled without being cut and a batch mode in which a slab is rolled after being cut.In recent years, it has been required that rolled metal strips with various steel glades and thicknesses can be produced in an apparatus that executes a process from casting to rolling in an integrated manner, and it has become necessary to enhance the productivity by selecting appropriate casting velocity, slab thickness, and rolling mode that do not lead to a breakout, according to the steel grade.However, in the technique described in the above-described Patent Document 1, although a slab having a minimum thickness of 80 mm and having a thickness of, for example, 100 mm is rolled to fabricate a metal strip with a thickness of 0.6 to 12 mm or a metal plate with a thickness of 12 to 50 mm, all modes are modes to roll a slab that is comparatively thinly cast and has the same thickness, and a slab with an appropriate thickness according to the steel grade is not employed.In particular, regarding a steel grade for which it is difficult to raise the casting velocity, like peritectic steel, there is a need to not only select the batch mode but also consider a temperature drop of a slab that occurs because of the casting velocity being low. Thus, further improvement is required.The present invention provides a manufacturing method for a rolled metal strip and a rolling facility that can improve the productivity of the rolled metal strip in a continuous casting facility that uses both an endless mode and a batch mode, compared with conventional techniques.Means for Solving the ProblemThe present invention includes a plurality of means for solving the above-described problem. To cite one example thereof, in a manufacturing method for a rolled metal strip, the method including rolling, by a rough rolling mill, a cast piece cast by a continuous casting machine, the continuous casting machine is configured to be capable of selecting an endless mode in which the cast piece cast into a first thickness X is rolled by the rough rolling mill without being cut and a batch mode in which the cast piece cast into a second thickness Y thicker than the first thickness X is rolled by the rough rolling mill after being cut.Advantages of the InventionAccording to the present invention, the productivity of the rolled metal strip in a continuous casting facility that uses both the endless mode and the batch mode can be improved compared with conventional techniques. Problems, configurations, and effects other than the above-described ones will be made apparent by the following description of embodiments.FIG. 1 is a diagram illustrating the outline of the configuration of a rolling facility of a first embodiment.FIG. 2 is a diagram illustrating the relationship between the slab thickness and the casting velocity.FIG. 3 is a diagram illustrating the relationship between the slab thickness and the throughput.FIG. 4 is a diagram illustrating the relationship between the slab thickness and the ratio of the throughput of a mode, where a thin slab is produced, to the throughput of a mode where a thick slab is produced.FIG. 5 is a diagram illustrating the outline of a state in endless rolling in the rolling facility of the first embodiment.FIG. 6 is a diagram illustrating the outline of another configuration of the rolling facility of the first embodiment.FIG. 7 is a diagram illustrating the outline of a state in endless rolling in the state of FIG. 6.FIG. 8 is a diagram illustrating one example of the amount of energy consumption when a slab is heated up by an IH slab heater in execution of rolling in a batch mode in the rolling facility of the first embodiment.FIG. 9 is a diagram obtained by rearranging the relationship between the slab thickness and the amount of energy consumption with use of data on the casting velocity being 1.0 m / min in FIG. 8.FIG. 10 is a diagram illustrating the outline of the configuration of a rolling facility of a second embodiment.FIG. 11 is a diagram illustrating the outline of the configuration of a rolling facility of a third embodiment.FIG. 12 is a diagram illustrating the outline of another configuration of the rolling facility of the third embodiment.FIG. 13 is a diagram illustrating the outline of the configuration of a rolling facility of a fourth embodiment.Modes for Carrying Out the InventionEmbodiments of a manufacturing method for a rolled metal strip and a rolling facility according to the present invention will be described below with use of the drawings. In the drawings used in the present specification, the same or corresponding constituent elements are given the same or similar numerals and repeated description is omitted regarding these constituent elements in some cases.<First Embodiment>A first embodiment of the manufacturing method for a rolled metal strip and the rolling facility according to the present invention will be described with use of FIGs. 1 to 9.First, the overall configuration of the rolling facility will be described with use of FIG. 1. FIG. 1 is a diagram illustrating the outline of the rolling facility of the present invention.A rolling facility 1 in FIG. 1 includes a ladle turret 10, a continuous casting machine (CC) 12, a torch cutting machine 14, a first temperature raising device 501, a second temperature raising device 502, a first slab transportation device 40, a second slab transportation device 42, a third slab transportation device 44, a fourth slab transportation device 46, a storage yard 50 for rejected slab, a scale breaker 60, an IH slab heater 503, a rough rolling mill 70, a coil box 520, a shear 510, an IH bar heater 504, a finishing rolling mill 75, a run out table 80, a dividing shear 530, a down coiler 85, a controller 90, and so forth.The CC 12 has a first strand 401 and a second strand 402. In the present embodiment, the same line as the line on which the first temperature raising device 501 is disposed is defined as the first strand 401, and the line side on which the second temperature raising device 502 is disposed is defined as the second strand 402. A slab of the second strand 402 is transported to the first strand 401 by the first slab transportation device 40, and thereafter is transported in the direction toward the rough rolling mill 70 and so forth.The CC 12 of the present embodiment is configured to be capable of selecting an endless mode, where a cast piece cast into a first thickness X is rolled by the rough rolling mill 70 without being cut, and a batch mode where a cast piece cast into a second thickness Y thicker than the first thickness X is rolled by the rough rolling mill 70 after being cut.When the machine length is the same, the CC 12 executes production in which the casting velocity is lowered to a larger extent from the condition under which solidification is completed in the CC 12, as the slab thickness is larger. Furthermore, because the casting velocity is low when the slab thickness is large, the large slab thickness is not suitable for endless rolling but is excellent in production of a slab for peritectic steel or an outer plate of an automobile.In contrast, with the first thickness X smaller than the second thickness Y, high velocity casting is possible and the throughput can be made higher than that with the second thickness Y even when the machine length of the CC 12 is the same. Thus, the first thickness X is suitable for the endless rolling.Moreover, the present embodiment is configured to be capable of, when the batch mode is selected, further selecting at least one from among: a first mode in which a cast piece cut by the torch cutting machine 14 is rolled by the rough rolling mill 70 after the temperature thereof is raised by the first temperature raising device 501 or the second temperature raising device 502; a second mode in which a cast piece cut by the torch cutting machine 14 is transported to the storage yard 50 for rejected slab by the second slab transportation device 42 and the fourth slab transportation device 46 without raising the temperature thereof by the first temperature raising device 501 or the second temperature raising device 502; and a third mode in which a cast piece stored in the storage yard 50 for rejected slab is rolled by the rough rolling mill 70 after the temperature thereof is raised by the first temperature raising device 501 or the second temperature raising device 502.The first mode and the second mode are selected and executed for each cast piece during execution of the batch mode, whereas the third mode can be executed when the first mode or the second mode of the batch mode is executed.Although being omitted for convenience of illustration, a ladle is mounted on the ladle turret 10, and molten steel is poured from this ladle into a tundish. The tundish has two exits and molten steel is poured from this exit into each of molds of both the first strand 401 and the second strand 402.Because the CC 12 has the two strands, cast pieces are cast on two rows and are split into slabs with a desired length by the torch cutting machine 14 existing on the exit side of the CC 12.When switching is executed between the endless mode in which a cast piece cast into the first thickness X is rolled by the rough rolling mill 70 without being cut and the batch mode in which a cast piece cast into the second thickness Y thicker than the first thickness X is rolled by the rough rolling mill 70 after being cut, replacement of a mold in the CC 12 is necessary. However, when the tundish exists above the mold at this time, the replacement of the mold is impossible. Therefore, in replacement of the mold, the tundish is shifted from the molten steel pouring position to a standby position existing in the width direction of the slab. That is, the molten steel pouring from the tundish is stopped and the continuous casting is interrupted.When it is impossible to transport a cast slab to the rough rolling mill 70 because of trouble in the rolling facility or a reason in terms of the quality of the slab, the slab is transported to the storage yard 50 for rejected slab as a rejected slab by using the second slab transportation device 42, the fourth slab transportation device 46, the first slab transportation device 40, and the third slab transportation device 44 to be described later. In the storage yard 50 for rejected slab, the rejected slab is stored and repair and inspection of the rejected slab are executed when necessary.The first temperature raising device 501 is disposed on the line between the first strand 401 in the CC 12 and the rough rolling mill 70, and has a fourth soaking device 36, a first IH slab heater 20, a first soaking device 30, a thermometer 201 in the first temperature raising device, and so forth. This first temperature raising device 501 is disposed on the same line as the first strand 401 of the CC 12.Desired heating-up of a slab transported from the CC 12 in the direction toward the rough rolling mill 70 is executed by at least the first IH slab heater 20, and thereafter the slab is transported to the first soaking device 30 or the fourth soaking device 36. In these first soaking device 30 and fourth soaking device 36, homogenization of temperature distribution in a section of the slab and adjustment of the rolling timing are executed while heat dissipation from the slab is suppressed.The first soaking device 30 is a device that is disposed on the exit side of the slab of the first IH slab heater 20 and employs heat retained by the slab as a heat source. The fourth soaking device 36 is a device that is disposed on the entry side of the slab of the first IH slab heater 20 and employs heat retained by the slab as a heat source.The second temperature raising device 502 is disposed between the second strand 402 different from the first strand 401 in the CC 12 and the rough rolling mill 70, and has a third soaking device 34, a second IH slab heater 22, a second soaking device 32, a thermometer 203 in the second temperature raising device, and so forth.The second soaking device 32 is a device that is disposed on the exit side of the slab on the line of the second IH slab heater 22 and employs heat retained by the slab as a heat source. The third soaking device 34 is a device that is disposed on the entry side of the slab on the line of the second IH slab heater 22 and employs heat retained by the slab as a heat source.This second temperature raising device 502 is disposed on the same line as the second strand 402 and is disposed in order to execute heating-up in the second IH slab heater 22 or soaking in the second soaking device 32, the third soaking device 34, or the like when the temperature of a slab that has flown from the second strand 402 is raised or when a rise in the temperature of a slab is insufficient with the first IH slab heater 20 of the first temperature raising device 501.Furthermore, the fourth soaking device 36 and the third soaking device 34 act to suppress a temperature drop of slabs in the case in which insertion of the slabs into the first IH slab heater 20 and the second IH slab heater 22 is executed at a low velocity when the slabs supplied from the first strand 401 and the second strand 402, respectively, are heated by the first IH slab heater 20 and the second IH slab heater 22. The insertion velocity in the case of temperature difference compensation heating-up to be described later is low compared with the slab transportation velocity. Thus, the fourth soaking device 36 and the third soaking device 34 effectively act for suppression of a temperature drop on that occasion.Although the configuration in which the first temperature raising device 501 and the second temperature raising device 502 each have two soaking devices is illustrated, both may have either one or have three or more soaking devices. Moreover, also regarding the IH slab heater, one or more heaters may be disposed.The first IH slab heater 20, the second IH slab heater 22, the first soaking device 30, the second soaking device 32, the third soaking device 34, and the fourth soaking device 36 are equipped with table rollers (omitted for convenience of illustration), and a slab can be moved back and forth in the traveling direction.The first IH slab heater 20 and the second IH slab heater 22 can be configured to intend equalization of the temperature in a section while causing reciprocation (oscillation) in which a slab is moved back and forth in the traveling direction to intend homogenization of a temperature rise in the longitudinal direction of the slab.The above-described first soaking device 30, second soaking device 32, third soaking device 34, and fourth soaking device 36 can employ a configuration in which the escape of heat from a slab disposed inside the soaking device is suppressed and equalization of the temperature inside the slab is promoted, by surrounding the slab by a panel formed of a reflective plate and a heat insulator. It is also possible to employ a structure obtained by combining a heat insulator like a fire-resistant object and an iron shell outside the heat insulator without disposing the reflective plate. Furthermore, disposing heating means inside the soaking device to raise the temperature in the soaking device by using the heating means at the start of operation is also effective for shortening of the time for the temperature rise. Gas heating-up may be used for the heating means inside the soaking device. However, using an electrical heater can suppress direct CO2emission.These first soaking device 30, second soaking device 32, third soaking device 34, and fourth soaking device 36 can also be configured to intend equalization of the temperature in a section while causing reciprocation (oscillation) in which a slab is moved back and forth in the traveling direction to intend homogenization of a temperature rise in the longitudinal direction of the slab.The first slab transportation device 40 is configured to allow movement of a slab between the downstream side of the first temperature raising device 501 and the downstream side of the second temperature raising device 502. The first slab transportation device 40 is configured to also transport a slab from the line on which the second temperature raising device 502 is disposed toward the line on which the first temperature raising device 501 is disposed, and transport a slab from the line on which the first temperature raising device 501 is disposed toward the line on which the second temperature raising device 502 is disposed.The second slab transportation device 42 is configured to allow movement of a slab between the upstream side of the first temperature raising device 501 and the upstream side of the second temperature raising device 502.The third slab transportation device 44 is configured to allow movement of a slab between the storage yard 50 for rejected slab in which a slab is stored and the downstream side of the second temperature raising device 502. The fourth slab transportation device 46 is configured to allow movement of a slab between the storage yard 50 for rejected slab and the upstream side of the second temperature raising device 502. It is possible to make a configuration including only either one of the third slab transportation device 44 and the fourth slab transportation device 46.The thermometer 201 in the first temperature raising device is a thermometer that is disposed in the first temperature raising device 501 and measures the temperature of a slab in the first temperature raising device 501. For example, a configuration in which the surface temperature of a slab in the first IH slab heater 20 is measured is employed.The thermometer 203 in the second temperature raising device is a thermometer that is disposed in the second temperature raising device 502 and measures the temperature of a slab in the second temperature raising device 502. For example, a configuration in which the surface temperature of a slab in the second IH slab heater 22 is measured is employed.A thermometer 205 in the storage yard is a thermometer that is disposed in the storage yard 50 for rejected slab and measures the surface temperature of a slab in the storage yard 50 for rejected slab.The controller 90 predicts the average temperature of a slab and the temperature of the center of the slab on the basis of the measurement result of the temperature of the slab by any one or more of the thermometers in the above-described thermometer 201 in the first temperature raising device, thermometer 203 in the second temperature raising device, and thermometer 205 in the storage yard, and decides the transportation destination of the slab on the basis of the measured surface temperature and the predicted inside temperature.Moreover, the controller 90 decides the transportation velocity of a slab in addition to the transportation direction in each device in the temperature raising device regarding the slab, and controls the velocity.After heating-up by the first temperature raising device 501 or the second temperature raising device 502 ends, in the rolling facility 1, first, because oxide scales are generated on the surface of the slab due to the high temperature heating-up prior to rolling, the scales on the surface are removed by the scale breaker 60 that jets high pressure water to the surface of the slab to remove the scales before the rolling.The IH slab heater 503 is disposed on the entry side of the rough rolling mill 70, and the configuration thereof is substantially the same as that of the first IH slab heater 20 and the second IH slab heater 22. As a difference therebetween, that the IH slab heater 503 is often configured to be capable of being retracted and inserted from and into the transportation line of the cast piece is cited. There is no particular limit on the configuration for allowing retraction and insertion from and into the transportation line of the cast piece, and various publicly known configurations can be employed.The rough rolling mill 70 is composed of three rolling mill stands and is a facility capable of unidirectional rolling. When the rough rolling mill 70 executes unidirectional rolling of a slab with the second thickness Y, which is thick, a warp does not exist at the end of the slab before rough rolling, and thus there is no fear of interference with the IH slab heater 503. Therefore, the IH slab heater 503 is not retracted during the passing of the slab in some cases.When the rough rolling mill 70 executes reverse rolling, a warp often occurs at the end of the rolled metal strip due to the rough rolling, and thus a fear of interference is caused in passing through the IH slab heater 503. Therefore, the IH slab heater 503 is retracted in some cases. The rough rolling mill 70 executes rough rolling, and unidirectional rolling is executed by the finishing rolling mill 75 of a plurality of rolling mill stands. Then, strip cooling is executed by the run out table 80, and the rolled metal strip is coiled up by the down coiler 85.The IH bar heater 504 is disposed on the exit side of the rough rolling mill 70 and on the entry side of the finishing rolling mill 75. This IH bar heater 504 is also configured to be capable of being retracted and inserted from and into the transportation line of the cast piece. In FIG. 1, the state in which the IH bar heater 504 is retracted from the rolling line is illustrated.The configuration in which the IH slab heater 503 can be retracted and inserted from and into the transportation line of the cast piece in some cases, and in which the IH bar heater 504 is configured to be capable of being retracted and inserted from and into the transportation line of the cast piece has been illustrated. However, a configuration in which either one is configured to be capable of retracting and inserting may be employed. Alternatively, it is also possible to employ a configuration in which neither moves nor is retracted from the transportation line of the cast piece.When the exit side velocity of the rough rolling mill 70 does not correspond with the entry side velocity of the finishing rolling mill 75, the bar is coiled up by the coil box 520, and then the bar is uncoiled and finishing rolling is executed.For example, the coil box 520 is used in batch rolling. When rough rolling is executed by the rough rolling mill 70 with three rolling mill stands, the velocity on the exit side of the last rolling mill stand of the rough rolling mill 70 often exceeds 300 m / min.Executing the rough rolling at a high velocity can suppress the temperature drop during the rolling. However, this velocity is high velocity as the entry side velocity of the finishing rolling mill 75 and it is often difficult to continuously execute the rough rolling and the finishing rolling. In this case, by coiling-up the bar once by the coil box 520 and subsequently uncoiling the bar, the finishing rolling can be executed with the entry side velocity of the finishing rolling mill 75 set to a desired value.When the bar is not coiled up by the coil box 520, the velocity on the exit side of the last rolling mill stand of the rough rolling mill 70 with a plurality of rolling mill stands is set to become the velocity on the entry side of the finishing rolling mill 75. In this case, the bar velocity on the exit side of the rough rolling mill 70 becomes low, and therefore the temperature drop during the rough rolling and during the transportation until the finishing rolling becomes large. Thus, raising the temperature by the IH bar heater 504 is often selected. In this case, the IH bar heater 504 is inserted into the rolling line and the bar is heated up to a desired temperature by the IH bar heater 504 to execute the finishing rolling. As above, when the batch mode is selected and the rough rolling and the finishing rolling are continuously executed without coiling-up the bar by the coil box 520, it is desirable to raise the temperature of the cast piece by the IH bar heater 504 disposed on the exit side of the rough rolling mill 70 for ensuring the rolling temperature.An upward warp or downward warp often occurs at the end of the bar after the rough rolling by the plurality of rolling mill stands. For this case, the shear 510 is disposed on the entry side of the IH bar heater 504 and the warp part at the bar end is split. Alternatively, a leveler is disposed on the entry side of the IH bar heater 504 and the warp at the bar end is corrected.When the exit side velocity of the rough rolling mill 70 and the entry side velocity of the finishing rolling mill 75 can be equalized, the state is made in which the rough rolling mill 70 and the finishing rolling mill 75 are simultaneously executing rolling.Subsequently, description will be made with use of FIGs. 2 to 4 about the definition of the thickness of the slab in the present invention, a preferable relationship between the first thickness X in the endless mode and the second thickness Y in the batch mode, and so forth.First, the definition of the first thickness X will be described.In endless rolling, a temperature drop in the rough rolling occurs after a slab before the rough rolling is heated up, and a temperature drop in the finishing rolling occurs after a bar before the finishing rolling is heated up.In the finishing rolling, the finishing rolling mill 75 executes desired rolling by executing rolling in one direction. However, normally heating means is not disposed in the finishing rolling mill 75. Therefore, after heating-up before the finishing rolling, the strip temperature lowers in the process of descaling before the finishing rolling and the rolling at each rolling mill stand.According to an actual example of the endless rolling, the CC produced a slab with a slab thickness of 90 mm at a casting velocity of 5 m / min, and the slab went through the rough rolling. Thereafter, the strip temperature was heated up to a high temperature by the IH bar heater on the entry side of the finishing rolling mill 75. Then, rolling was executed to the thickness of 1.0 mm by the finishing rolling mill 75 with five rolling mill stands, and a desired strip temperature on the exit side of the finishing rolling mill 75 was obtained.When the throughput at this time is defined as strip thickness (mm) × casting velocity (m / min), the throughput is 90 × 5 = 450 (mm × m / min). If the throughput is equal to or higher than 450 (mm × m / min), the endless rolling is possible. If the throughput is lower than 450 (mm × m / min), the strip temperature becomes equal to or lower than the desired strip temperature, and there is a possibility of the occurrence of a problem in terms of quality.Here, the throughput means the weight that passes per unit time, and therefore, normally, is obtained by multiplying the thickness, the width, the specific gravity, and the casting velocity regarding the passing material. However, comparison of the throughput is allowed by comparison of the thickness × the casting velocity because the width and the specific gravity are the same. Thus, the thickness × the casting velocity is defined as the throughput in the present specification.Subsequently, the preferable relationship between the second thickness Y and the first thickness X will be described.First, when the distance from the molten metal level (meniscus) in the mold to the exit side of the CC 12, which is the machine length of the CC 12, is defined as Lm, and the distance from the molten metal level of the mold to the position at which solidification is completed is defined as a solidification length Ls, if the position of the solidification completion exists on the exit side relative to the position at Lm, bulging attributed to the ferrostatic pressure occurs and it is impossible to continue the casting. Thus, there is a need to set at least Ls ≤ Lm.In general, the following expression is used as an expression to obtain the solidification thickness (Hs / 2).Hs / 2 = K × (Ls / Vc)^0.5Here, Hs is the slab thickness (mm). K is a solidification coefficient (mm / min^0.5). Ls is the solidification length (m). Vc is the casting velocity (m / min).Therefore, when the second thickness Y is defined as Hs1 and the casting velocity is defined as Vc1, the expression becomes (Hs1) / 2 = K × (Ls / Vc1)^0.5. When the first thickness X is defined as Hs2 and the casting velocity is defined as Vc2, the expression is represented as (Hs2) / 2 = K × (Ls / Vc2)^0.5.K is a value that varies depending on the condition of the continuous casting. However, in the following, in both the batch rolling mode and the endless rolling mode, the maximum casting velocity in each rolling mode is employed as the subject of consideration. Thus, it is assumed that there is no large difference in the value of K in the case of this comparatively high velocity between the batch rolling mode and the endless rolling mode.Furthermore, the maximum casting velocity with the slab thickness Hs is obtained when Ls = Lm. Thus, assuming that the value of K × Ls^0.5 is the same with both the second thickness Y and the first thickness X on the basis of this premise, the relationship among the second thickness Y, the first thickness X, and the casting velocity, Hs1 / Hs2 = (Vc2 / Vc1)^0.5, holds.FIG. 2 illustrates the relationship between the slab thickness and the casting velocity. In FIG. 2, circle marks and a solid line are obtained by plotting, as the ordinate, the casting velocity Vc2 that yields the same solidification length Ls when the slab thickness is changed to Hs2 with the same CC in the case in which the second thickness Y is set to Hs1 = 200 mm and the casting velocity thereof is set to Vc1 = 2.0 m / min. The casting velocity exceeding this leads to Lm < Ls, which means failure in the completion of the casting.Moreover, in FIG. 2, triangle marks and a dashed line are obtained by plotting, as the ordinate, the casting velocity Vc2 that yields the same solidification length Ls when the slab thickness is changed to Hs2 with the same CC in the case in which the second thickness Y is set to Hs1 = 230 mm and the casting velocity thereof is set to Vc1 = 1.74 m / min.As illustrated in FIG. 2, for example, at the circle mark, the casting velocity Vc with which solidification is completed within the machine length Lm is 9.88 m / min when the slab thickness is 90 mm.On the other hand, the risk of leading to a breakout is high when the casting velocity Vc is too high. In actual operation, approximately 5.0 to 6.0 m / min is the limit of the casting velocity although depending on the steel grade. Here, the maximum of the casting velocity is set to 5.0 m / min. At the places where the casting velocity exceeds 5 m / min as the value of the circle mark or the triangle mark, the casting velocity is corrected to 5.0 m / min.FIG. 3 illustrates the relationship between the slab thickness and the throughput.In FIG. 3, circle marks and a solid line are obtained by plotting, as the ordinate, the throughput Hs × Vc obtained by using the casting velocity Vc2 that yields the same solidification length Ls when the slab thickness is changed to Hs2 with the same CC in the case in which the second thickness Y is set to Hs1 = 200 mm and the casting velocity thereof is set to Vc1 = 2.0 m / min.Furthermore, in FIG. 3, triangle marks and a dashed line are obtained by plotting, as the ordinate, the throughput Hs × Vc obtained by using the casting velocity Vc2 that yields the same solidification length Ls when the slab thickness is changed to Hs2 with the same CC in the case in which the second thickness Y is set to Hs1 = 230 mm and the casting velocity thereof is set to Vc1 = 1.74 m / min.Moreover, in FIG. 3, the solid line and the dashed line are corrected with the maximum of the casting velocity regarded as 5.0 m / min. For example, the throughput when the slab thickness is 90 mm is 450 (mm × m / min) (= 90 mm × 5 m / min).A range of diagonal lines in FIG. 3 is a range in which the throughput is equal to or higher than 450 (mm × m / min). It is easy to execute the endless rolling within this range. For example, when the second thickness Y is set to 200 mm, the endless rolling is allowed to be easily executed by setting the first thickness X to (90 to 178 (= the second thickness Y × 0.89)) mm.Furthermore, when the second thickness Y is set to 230 mm, the endless rolling is allowed to be easily executed by setting the first thickness X to (90 to 205 (= the second thickness Y × 0.89)) mm.From the above, the endless rolling is allowed to be easily executed by causing the relationship between the first thickness X and the second thickness Y to satisfy 90 ≤ X ≤ 0.89Y (unit is mm).Moreover, when the second thickness Y is 200 mm, the throughput reaches 620 when the first thickness X is 130 mm (= the second thickness Y × 0.65). This is equivalent to 1.38 times (= 620 / 450) the condition 450, under which the finishing rolling mill 75 can keep the rolling temperature with five rolling mill stands. Furthermore, the maximum of the throughput is a value of 575 to 620 when the first thickness X is 120 mm (the second thickness Y × 0.60) to 140 mm (the second thickness Y × 0.70).When the throughput is 575, because of 575 / 450 = 1.28, the passing time becomes 1 / 1.28 when the distance between an F1 stand and an F5 stand is set to 20 m in the case in which the finishing rolling mill 75 is composed of five rolling mill stands. Thus, passing through the finishing rolling mill 75 is executed in a shorter time, and therefore temperature lowering can be suppressed, which is preferable in terms of keeping the rolling temperature.Moreover, when the distance between an F1 stand and an F6 stand is 25 m (< 20 m × 1.28 = 25.6 m) in the case in which the finishing rolling mill 75 is composed of six rolling mill stands, the passing time when 1.0 mm is obtained by a six-stand finishing rolling mill with a throughput 575 becomes shorter than the passing time when 1.0 mm is obtained by a five-stand finishing rolling mill with a throughput 450. From this, it becomes possible to easily produce 1.0 mm while keeping the rolling temperature by the six-stand finishing rolling mill.From the above, it can be said that the relationship between the first thickness X and the second thickness Y satisfying 0.60Y ≤ X ≤ 0.70Y (unit is mm) is a more preferable relationship between the second thickness Y and the first thickness X in production of an extremely thin metal strip.FIG. 4 illustrates the relationship between the slab thickness and the ratio of the throughput of the endless rolling mode to the throughput of the batch rolling mode.In FIG. 4, □ is the throughput (mm × m / min) when production is executed with two strands in the batch rolling mode with the second thickness Y being 200 mm and the casting velocity set to 2.0 m / min. This is regarded as 1.0.Production in the endless rolling mode is executed with one strand. Therefore, the throughput becomes 0.58 to 0.77 times that of the batch rolling mode when the first thickness X is set to (90 to the second thickness Y × 0.89) mm.Furthermore, as illustrated in FIG. 4, when the first thickness X is 130 mm (= the second thickness Y × 0.65), the production amount of the endless rolling mode becomes the maximum and becomes the production amount that is 0.77 times that of the batch rolling mode.Moreover, when the first thickness X is 120 mm (the second thickness Y × 0.60) to 140 mm (the second thickness Y × 0.70), the production amount in the endless rolling becomes a production amount that is (0.71 to 0.77) times that of the batch rolling mode.As above, by setting the first thickness X to “(the second thickness Y × 0.60) to (the second thickness Y × 0.70),” the maximum production amount of the endless rolling mode can be made equal to or higher than 70% of the maximum production amount of the batch rolling mode. Here, the maximum production amount means a production amount when casting is executed in the CC at the casting velocity that yields Ls = Lm. Alternatively, when this casting velocity exceeds the upper limit of the casting velocity at and under which casting without the occurrence of a breakout is possible, the maximum production amount means a production amount when casting is executed in the CC at the casting velocity of the upper limit at and under which casting without the occurrence of a breakout is possible.For example, when the production amount of the batch rolling mode is 100% and the production amount of the endless rolling mode is 58%, the casting time of molten steel of one ladle in the endless rolling mode is 86 min if the casting time of molten steel of one ladle in the batch rolling mode is assumed to be 50 min. Thus, the lowering of the molten steel temperature in the ladle becomes large in casting in the endless rolling mode. Because change in the casting condition from the start of molten steel pouring from the ladle to the end of the molten steel pouring becomes large, it often becomes impossible to execute casting of the whole amount of the ladle depending on the case.If the maximum production amount of the endless rolling mode can be made equal to or higher than 70% of the maximum production amount of the batch rolling mode, the casting time of molten steel of one ladle in the endless rolling mode becomes 71 min if the casting time of molten steel of one ladle in the batch rolling mode is assumed to be 50 min. Thus, the condition under which casting of the whole amount of the ladle is possible is widened.In the above, description has been made with an assumption of approximately 170 to 250 mm as the second thickness Y. However, for example, the case in which the second thickness Y is set to 150 mm and the first thickness X is set to 90 to 134 (second thickness Y × 0.89) is also included in the present invention.If the second thickness Y exceeds 250 mm to become thicker, the size of the CC becomes larger and Lm becomes longer. In this case, when the first thickness X is thin, a problem that a temperature drop in the CC becomes large and the energy loss becomes high is caused. Furthermore, when the casting velocity at the time of the second thickness Y is lowered and Lm is shortened, the energy loss of a slab in which the first thickness X is thin lowers, whereas a problem that the productivity of casting of the second thickness Y lowers is caused. Thus, although the second thickness Y is not limited to this range, it is desirable to set the upper limit of the second thickness Y to a thickness of approximately 250 mm, which is used for a conventional thick slab.FIG. 5 illustrates the time of endless rolling in FIG. 1 and illustrates the state in which the IH bar heater 504 is inserted into the rolling line.In the endless rolling, the first strand 401 of the CC 12 is used and the second strand 402 does not execute production. Furthermore, because of the endless rolling, a slab produced by the first strand 401 is transported in the direction toward the rough rolling mill 70 at the casting velocity without being cut in the middle by the torch cutting machine 14.When the slab passes through the first temperature raising device 501, the temperature is raised by using the first IH slab heater 20 and the first soaking device 30, the fourth soaking device 36, and so forth. After the heating-up by the first temperature raising device 501, scales on the surface are removed by the scale breaker 60, and thereafter the slab is heated up to a desired temperature by the IH slab heater 503.Rough rolling is executed as unidirectional rolling, and the rolled metal strip is transported to the finishing rolling mill 75. In the middle of the transportation, heating-up to a desired temperature can be executed by the IH bar heater 504. Moreover, after rolling by the finishing rolling mill 75, the rolled metal strip is divided by the dividing shear 530 installed in front of the down coiler 85 and is coiled up by the down coiler 85 for each coil.Here, although not illustrated for convenience of illustration, a descaling device is disposed between the IH slab heater 503 and the rough rolling mill 70, and scales on the surface of the slab are removed before the rough rolling. Furthermore, a descaling device is disposed between the IH bar heater 504 and the finishing rolling mill 75, and scales on the surface are removed before the finishing rolling.As above, when the endless mode is selected, the temperature of the cast piece can be raised at at least one place among the exit side of the CC 12 (first IH slab heater 20 of the first temperature raising device 501), the entry side of the rough rolling mill 70, 70A, or 70B (IH slab heater 503), and the entry side of the finishing rolling mill 75 (IH bar heater 504) disposed on the exit side of the rough rolling mill 70, 70A, or 70B. By employing the IH slab heater in this manner, the heating device can be made into a short facility, and the distances between the CC 12 and the rough rolling mill 70, 70A, or 70B and between the rough rolling mill 70, 70A, or 70B and the finishing rolling mill 75 can be shortened. This can suppress heat dissipation of the metal strip that is traveling, and thus enables reduction in the energy loss.FIG. 6 is a diagram illustrating a rolling facility with another configuration different from the rolling facility illustrated in FIG. 1, and illustrates the state in which the IH slab heater 503 and the IH bar heater 504 are retracted from the rolling line.In a rolling facility 1A illustrated in FIG. 6, a table 710 on the entry side of the rough rolling mill is disposed on the downstream side of the scale breaker 60 and on the upstream side of the IH slab heater 503. At least a steel grade for which it is impossible to set the casting velocity to a high velocity (peritectic steel or the like) is subjected to a temperature rise by the first temperature raising device 501 or the second temperature raising device 502 in the middle of transportation with the second thickness Y, and thereafter is transported to a rough rolling mill 70A.As illustrated in FIG. 6, the rough rolling mill 70A is composed of two rolling mill stands and is a facility capable of both reverse rolling and unidirectional rolling. The reverse rolling at the rough rolling mill 70A is executed at a comparatively high velocity. Therefore, the amount of temperature drop is small, and heating-up by the IH slab heater 503 disposed on the entry side of the rough rolling mill 70A is not required.However, an upward warp or downward warp often occurs at the end of the rolled metal strip when the rough rolling mill 70A executes the reverse rolling for the second thickness Y. Thus, it is desirable to retract the IH slab heater 503 from the rolling line in order to avoid breakage by the end of the rolled metal strip.The IH bar heater 504 is disposed on the entry side of the finishing rolling mill 75.When the exit side velocity of the third pass of the rough rolling mill 70A does not correspond with the entry side velocity of the finishing rolling mill 75, a bar is coiled up by the coil box 520 and then the bar is uncoiled, and finishing rolling is executed.When the exit side velocity of the third pass of the rough rolling mill 70A and the entry side velocity of the finishing rolling mill 75 can be equalized, the state in which rolling is being simultaneously executed in the third pass of the rough rolling mill 70A and the finishing rolling mill 75 is made.FIG. 7 illustrates the time of endless rolling in FIG. 6 and illustrates the state in which the IH slab heater 503 and the IH bar heater 504 are inserted into the rolling line.In the endless rolling, similarly to the rolling facility 1 illustrated in FIG. 1, only the first strand 401 of the CC 12 is used and production is not executed on the second strand 402. Furthermore, because of the endless rolling, a slab produced by the first strand 401 is transported in the direction toward the rough rolling mill 70A at the casting velocity without being cut in the middle by the torch cutting machine 14.When the slab passes through the first temperature raising device 501, the temperature is raised by using the first IH slab heater 20, the first soaking device 30, the fourth soaking device 36, and so forth. Thereafter, scales on the surface are removed by the scale breaker 60, and then the slab is heated up to a desired temperature by the IH slab heater 503.Rough rolling is executed as unidirectional rolling, and the rolled metal strip is transported to the finishing rolling mill 75. In the middle, heating-up to a desired temperature is executed by the IH bar heater 504. After rolling by the finishing rolling mill 75, the rolled metal strip is divided by the dividing shear 530 installed in front of the down coiler 85, and is coiled up by the down coiler 85 for each coil.Because the slab passes in a continuous state, oscillation of the slab is not executed in the first IH slab heater 20, the first soaking device 30, and the fourth soaking device 36.Although being omitted for convenience of illustration, a descaling device is disposed between the IH slab heater 503 and the rough rolling mill 70A, and scales on the surface of the slab are removed before the rough rolling. Furthermore, although not illustrated, a descaling device is disposed between the IH bar heater 504 and the finishing rolling mill 75, and scales on the surface are removed before the finishing rolling.FIG. 8 illustrates the amount of energy consumption when a slab is heated up by the first IH slab heater 20 or the second IH slab heater 22 in execution of rolling in the batch mode in the rolling facility 1 of FIG. 1, and is a simulation result with a condition under which the average temperature of the slab after the heating-up becomes approximately 1200°C.The size of each slab was set as follows as the condition of the simulation. Regarding t150, the slab thickness is 150 mm, the slab length is 18 m, and the weight per unit width is 21.2 kg / mm. Regarding t225, the slab thickness is 225 mm, the slab length is 12 m, and the weight per unit width is 21.2 kg / mm. Regarding t235, the slab thickness is 235 mm, the slab length is 12 m, and the weight per unit width is 22.1 kg / mm. The weight per unit width is set to almost the same value.As illustrated in FIG. 8, when the slab thickness is 150 mm, the amount of energy consumption becomes large compared with the slab thicknesses 225 mm and 235 mm. That is, when the casting velocity is low, both temperature lowering in the CC and temperature lowering during transportation in the case of the slab thickness 150 mm become large compared with the slab thicknesses 225 mm and 235 mm. Thus, it turns out that the slab thickness 150 mm requires more energy for the temperature rise by the first IH slab heater 20 or the second IH slab heater 22.FIG. 9 is obtained by rearrangement with use of the data when the casting velocity is 1.0 m / min in FIG. 8. The abscissa indicates the slab thickness and the ordinate indicates the amount of energy consumption of the IH slab heater similarly to FIG. 8.In particular, regarding a steel grade for which it is difficult to raise the casting velocity, like peritectic steel, there is a need to not only select the batch mode but also consider a temperature drop of the slab that occurs because the casting velocity is low.In contrast, by executing casting and rolling for the second thickness Y in the batch mode at a velocity lower than that for the first thickness X, it becomes possible to suppress the amount of heating-up by the first IH slab heater 20 or the second IH slab heater 22, and execute direct rolling with low energy loss also for the steel grade for which it is impossible to raise the casting velocity.Moreover, by executing casting and rolling for the first thickness X at a comparatively high velocity in the endless mode, keeping the rolling temperature in the finishing rolling mill 75 is enabled.Here, in the case of the endless rolling, a slab is transported without being cut at the casting velocity of the CC 12. The casting velocity of the CC 12 is at most approximately 5 m / min. In contrast, the transportation velocity when a slab is cut and transported is approximately 30 to 100 m / min. Thus, the temperature drop of the slab can be suppressed when the slab is cut and transported, whereas the temperature drop of the slab becomes large in the case of the endless rolling. Thus, it is desirable to dispose the IH slab heater 503 and execute heating-up.When the CC 12 produces a slab with the second thickness Y and the slab is transported while being cut to execute rough rolling, the slab is transported at a higher velocity than the casting velocity and the rough rolling is executed at a high velocity. Thus, a bar at a high temperature can be obtained. Therefore, heating-up previous to the rough rolling mill is not necessarily required.When the rough rolling of the slab is executed, an upward warp or downward warp occurs at the end. In the case of reverse rolling by the rough rolling mill with a plurality of rolling mill stands, the breakage of the IH slab heater 503 can be avoided in the reverse rough rolling by the plurality of rolling mill stands, by retracting the IH slab heater 503 to the outside of the rolling line.It is possible to heat up the slab by using the first temperature raising device 501 or the second temperature raising device 502 in the middle of transportation of the slab. However, on the exit side thereof, a certain level of distance exists to the rough rolling mill 70 with the plurality of rolling mill stands. Thus, the temperature drop until the rough rolling is large because the transportation velocity is low.Thus, in the case of the endless rolling, it is desirable to insert the IH slab heater 503 into the rolling line and heat up the slab. This enables the rough rolling after heating-up of the slab to the rolling temperature. In the case of executing the endless rolling of the slab, an upward warp or downward warp after the rough rolling does not exist at the slab end. Therefore, the IH slab heater 503 is not broken, and thus can be used as it is.In the endless rolling, the bar resulting from the rough rolling is transported to the finishing rolling mill 75 without being cut.Assuming that the first thickness X is 120 mm and the casting velocity is 5 m / min in the endless rolling, the exit side velocity of the rough rolling mill with the plurality of rolling mill stands is 20 m / min when a bar with a thickness of 30 mm is obtained after the rolling by the rough rolling mill with the plurality of rolling mill stands.Here, when the rough rolling mill is composed of two rolling mill stands, by setting the reduction amounts of the respective rolling mill stands to 50 mm and 40 mm, which are considerably high reduction, a strip thickness of 120 − (50 + 40) = 30 mm can be obtained.In contrast, when the rough rolling mill is composed of three rolling mill stands, the reduction amount of each rolling mill stand to obtain the strip thickness of 30 mm becomes comparatively small. The reduction amounts of the respective rolling mill stands can be set to 40 mm, 30 mm, and 20 mm, for example, (120 − (40 + 30 + 20) = 30). Moreover, it is also possible to make the strip thickness on the exit side of the rough rolling mill smaller than 30 mm.When the bar travels at 20 m / min between the rough rolling mill 70 and the finishing rolling mill 75, a large temperature drop occurs if the distance between the rough rolling mill 70 and the finishing rolling mill 75 is long. Thus, the distance between the rough rolling mill 70 and the finishing rolling mill 75 is set short. This can intend reduction in the energy loss.However, even when the distance between the rough rolling mill 70 and the finishing rolling mill 75 can be set as short as approximately 50 m, 2.5 min (= 50 m / 20 m / min) is required until the start of the finishing rolling. A temperature drop occurs at approximately 1°C / s in the bar with the thickness of 30 mm. Therefore, after the rough rolling, a temperature drop by 150°C occurs during transportation for 2.5 min (150 sec) to the finishing rolling. Furthermore, also in the rough rolling mill with the plurality of rolling mill stands, a temperature drop occurs also when rolling of one pass is executed.Thus, the bar can be heated up to the finishing rolling temperature by the IH bar heater 504 on the exit side of the rough rolling mill 70. A temperature drop due to descaling occurs before the finishing rolling. Therefore, the heating condition is set for the IH bar heater 504 also in consideration of the temperature drop other than that in the transportation. For example, heating-up to approximately 1050°C to 1150°C is selected.Subsequently, description will be made about the significance of enabling the rough rolling mill 70A to execute reverse rolling by a plurality of rows of rolling mill stand. Here, the second thickness Y is set to 200 mm.When rolling of one pass is executed in the case of two rolling mill stands as in the rough rolling mill 70A, the strip thickness after the rough rolling is 200 − 50 × 2 = 100 when it is assumed that one time of rolling allows reduction by 50 mm. Thus, it is impossible to execute the finishing rolling in this state.In contrast, when reverse rolling is executed by two rolling mill stands of the rough rolling mill, rolling is executed six times if rolling of total three passes is executed, for example. Thus, the strip thickness on the entry side of the finishing rolling mill 75 can be turned to a desired thickness (for example, 28 to 40 mm) for the finishing rolling mill 75 by the rolling.Furthermore, when three rolling mill stands are employed as in the rough rolling mill 70 in FIG. 1, unidirectional rough rolling provides total three times of rolling in one pass. Therefore, the strip thickness on the entry side of the finishing rolling mill 75 becomes somewhat thinner. Assuming that reduction by 50 mm, 50 mm, and 40 mm is executed by the respective rolling mill stands, approximately 200 − (50 + 50 + 40) = 60 mm can be obtained as the strip thickness on the exit side of the rough rolling. However, this results in a condition in which the thickness is large as the bar thickness on the entry side of the finishing rolling mill 75, and the finishing rolling is subject to restrictions.In contrast, when reverse rolling is executed in the case of three rolling mill stands as in the rough rolling mill 70, total nine times of rolling is executed in three passes. Therefore, reduction to the desired strip thickness on the entry side of the finishing rolling mill 75 is sufficiently possible. However, with the three passes, the reduction can be executed even by two rolling mill stands of the rough rolling mill. Thus, in the case of executing the reverse rolling by the rough rolling mill, two rolling mill stands as in the rough rolling mill 70A are preferable in terms of intending to reduce the facility cost.When an extremely thin strip of approximately 1.0 mm is rolled in the finishing rolling, the possibility of the occurrence of a rolling trouble becomes high in the case of rolling the slab one by one. In particular, troubles such as bending at the time of threading and strip pinching at the time of tailout are likely to occur. Therefore, the endless rolling in which unidirectional rolling is executed until the exit side of the finishing rolling mill 75 without cutting the slab and the rolled metal strip is cut before coiling-up to make a coil is preferably employed.Moreover, when there is a need to make the strip thickness on the entry side of the finishing rolling mill 75 thinner in the endless rolling, three rolling mill stands as in the rough rolling mill 70 are selected in some cases because it is often impossible to make the strip thickness on the entry side of the finishing rolling mill 75 thin in unidirectional rolling by two rolling mill stands as in the rough rolling mill 70A.Furthermore, in the case of executing the endless rolling, three rolling mill stands as in the rough rolling mill 70 allow the strip thickness on the entry side of the rough rolling (= the slab thickness on the exit side of the CC 12) to be larger. Thus, three rolling mill stands are selected in some cases.Next, effects of the present embodiment will be described.In the manufacturing method for a rolled metal strip in which a cast piece cast by the CC 12 of the above-described first embodiment of the present invention is rolled by the rough rolling mill 70 or 70A, the CC 12 is configured to be capable of selecting the endless mode in which a cast piece cast into the first thickness X is rolled by the rough rolling mill 70 or 70A without being cut and the batch mode in which a cast piece cast into the second thickness Y thicker than the first thickness X is rolled by the rough rolling mill 70 or 70A after being cut.In a conventional mass production, a plurality of blast furnaces and converters are disposed on the upstream side. A large amount of slab is produced by a CC in a steelmaking plant and the large amount of slab is heated up by a reheating furnace and is collectively rolled in a rolling plant.In recent years, CO2emission has been a large issue. There has been a measure to suppress the CO2emission by manufacturing molten steel by an electric furnace with use of scrap and reduced iron, and executing continuous casting and rolling, and there has been an increasing need for it.Furthermore, reduction in the energy loss exists as a continuous issue. For the reduction in the energy loss, the amount of reheating can be suppressed by supplying a slab to the rolling facility while the temperature drop of the slab is small. This leads to large reduction in the energy loss. Therefore, direct rolling in which a slab is directly sent from casting and hot rolling is executed is effective.In the case of disposing electric furnaces on the upstream side, production at 3.0 to 3.5 Mton / y by two electric furnaces is one target although depending on the capability of the electric furnaces.Moreover, a facility that meets the needs of the market and can produce conventional steel grades is required. For example, peritectic steel is a steel grade frequently used in the market. In continuous casting of peritectic steel, it is impossible to raise the casting velocity in terms of suppression of a breakout. Thus, production at a low casting velocity from the second thickness Y is preferable. In this case, the throughput with the second thickness Y in the continuous casting (passing weight per unit time) is low compared with the throughput of rolling. Thus, batch rolling is executed.Moreover, there is also a high need for production of a new product by a new facility. For example, there is also a need for production of an extremely thin strip of approximately 1.0 mm by a finishing rolling mill and omission of cold rolling. This leads also to reduction in the production cost and production of a new steel grade. The production of such an extremely thin strip is enabled by rolling in an endless state from continuous casting to rolling.Furthermore, in conventional techniques, a slab with a thickness similar to that at the time of endless rolling is used in batch rolling. Therefore, the temperature drop of the slab on the entry side of the batch rolling is large, and thus the temperature is raised by an IH slab heater or the like.Moreover, in the above-described Patent Document 1, the strip length is long, and therefore the following problems exist. The connecting furnace is a facility that is long in the slab traveling direction and is wide in the width direction of the slab. Thus, the connecting furnace is a large-size facility. In addition, in endless rolling, the large connecting furnace needs to be kept at a high temperature state and the energy loss becomes high.In contrast, when the slab is made thick in the batch rolling compared with that in the endless rolling as in the present invention, the temperature drop is small and the amount of reheating is also small. Thus, the productivity can be improved compared with the conventional techniques.Furthermore, it becomes possible to execute the endless rolling with the first thickness X on the line on which the batch rolling from the second thickness Y is executed. Therefore, reduction in the energy loss can be intended by direct rolling. In addition, it becomes possible to produce many steel grades by one production facility. Accordingly, an effect that it is possible to meet the needs of the market and provide new steel grades such as an extremely thin strip to the market can also be provided. Moreover, restrictions on the produced steel grade can be reduced.For example, in the present invention, in the endless rolling, a slab is produced by one strand of the CC 12 and the casting velocity is made high in order to keep the rolling temperature. Therefore, an extremely thin metal strip of approximately 1.0 mm can be produced by the endless rolling. Furthermore, peritectic steel or the like cannot be produced at a high casting velocity. Thus, a slab with the second thickness Y is produced with the casting velocity lowered and the slab can be rolled in batches while being cut on the exit side of the CC. Moreover, when the production amount of the second thickness Y is insufficient, the insufficiency of the production amount can be supplemented by employing a two strands CC as the CC. As above, the present invention enables production in a wide range from rolling of a steel grade cast at a low velocity to rolling of an extremely thin metal strip of approximately 1.0 mm.Furthermore, easy and sure execution of the endless rolling is enabled by causing the relationship between the first thickness X and the second thickness Y to satisfy 90 ≤ X ≤ 0.89Y (unit is mm) or causing the relationship between the first thickness X and the second thickness Y to satisfy 0.60Y ≤ X ≤ 0.70Y (unit is mm).Moreover, the configuration is made in such a manner that it is possible to, when the batch mode is selected, further select at least one from among: the first mode in which a cut cast piece is rolled by the rough rolling mill 70 or 70A after the temperature thereof is raised; the second mode in which a cut cast piece is transported to the storage yard 50 for rejected slab; and the third mode in which a cast piece stored in the storage yard 50 for rejected slab is rolled by the rough rolling mill 70 or 70A after the temperature thereof is raised. This makes it possible to deal with rolling under various conditions.Furthermore, when the endless mode is selected, the temperature of a cast piece is raised at at least one place among the exit side of the CC 12, the entry side of the rough rolling mill 70 or 70A, and a place that is on the exit side of the rough rolling mill 70 or 70A and on the entry side of the finishing rolling mill 75. Due to this, a roller hearth furnace of a gas heating system or a walking beam furnace does not need to be employed as the device for transportation in the rolling direction. Thus, a furnace such as a connecting furnace becomes unnecessary. In particular, during the endless rolling, the rolling velocity is determined depending on the casting velocity at the CC 12, and the rolling velocity is comparatively low, and thus the temperature drop of the slab becomes large. Therefore, the rolled metal strip with desired characteristics and thickness can be surely obtained due to the capability of heating-up before the rough rolling and before the finishing rolling.Moreover, rolling is executed in one direction by the rough rolling mill 70 or 70A, and reverse rolling is executed by the rough rolling mill 70 or 70A when the batch mode is selected. Due to this, it becomes possible to deal with rolling under various conditions. In addition, rolling with a configuration in which scale enlargement of the rolling facility beyond necessity is avoided can be implemented.Furthermore, the temperature raising device that is disposed at at least one of a place on the entry side of the rough rolling mill 70 or 70A and a place that is on the exit side of the rough rolling mill 70 or 70A and on the entry side of the finishing rolling mill 75, and raises the temperature of a cast piece is configured to be capable of being retracted and inserted from and into the transportation line of the cast piece. This makes it possible to easily deal with both the case in which heating-up is required before the finishing rolling in the endless rolling in which the bar temperature has lowered after the rough rolling and the case in which the temperature raising device is retracted from the rolling line during the batch rolling while avoiding the risk of the breakage of the temperature raising device.In particular, the IH slab heater 503 that is disposed on the entry side of the rough rolling mill 70 or 70A and raises the temperature of a cast piece is configured to be capable of being retracted from the transportation line of the cast piece. Due to this, when an upward warp or downward warp is made at the end of the strip because the strip thickness is large during the rough rolling, the breakage of the IH slab heater 503 can be surely prevented against the fear of the breakage attributed to the warp part.Moreover, when the batch mode is selected, the rough rolling and the finishing rolling can be continuously executed by raising the temperature of a cast piece by the temperature raising device disposed on the entry side of the rough rolling mill 70 or 70A.<Second Embodiment>A manufacturing method for a rolled metal strip and a rolling facility according to a second embodiment of the present invention will be described with use of FIG. 10. FIG. 10 is a diagram illustrating the outline of the configuration of the rolling facility of the second embodiment.In a rolling facility 1B of the present embodiment illustrated in FIG. 10, the second slab transportation device 42 and the fourth slab transportation device 46 are omitted and the first temperature raising device 501 and the second temperature raising device 502 are disposed closer to the exit side of the CC 12. Due to this, a distance L1 from the mold of the CC 12 to a rough rolling mill 70B is shortened. That is, due to the shortening of the distance L1, the amount of temperature drop of the slab can be more suppressed and the amount of heating-up at the first IH slab heater 20, the second IH slab heater 22, and the IH slab heater 503 can be suppressed to be smaller.Furthermore, the first IH slab heater 20 in the first temperature raising device 501 and the second IH slab heater 22 in the second temperature raising device 502 are shifted in the traveling direction of the slab. This can avoid interference between the first IH slab heater 20 and the second IH slab heater 22. In addition, maintenance places for the first IH slab heater 20 and the second IH slab heater 22 are ensured.Moreover, the rolling facility 1B includes a pendulum shear 16 instead of the torch cutting machine 14. The pendulum shear 16 cuts a cast piece while traveling synchronously with the cast piece, and can complete the cutting in a shorter time than the torch cutting machine 14.Here, the rough rolling mill 70B is composed of four rolling mill stands and the finishing rolling mill 75 is composed of five rolling mill stands.Although illustration is omitted, a descaling device is disposed between the IH slab heater 503 and the rough rolling mill 70B, and scales on the surface of the slab are removed before the rough rolling. Furthermore, a descaling device is disposed between the IH bar heater 504 and the finishing rolling mill 75, and scales on the surface of the slab are removed before the finishing rolling.The other configuration and operation are substantially the same configuration and operation as the manufacturing method for a rolled metal strip and the rolling facility according to the above-described first embodiment, and details thereof are omitted.Also in the manufacturing method for a rolled metal strip and the rolling facility according to the second embodiment of the present invention, almost the same effects as the manufacturing method for a rolled metal strip and the rolling facility according to the above-described first embodiment are obtained.<Third Embodiment>A manufacturing method for a rolled metal strip and a rolling facility according to a third embodiment of the present invention will be described with use of FIGs. 11 and 12. FIG. 11 is a diagram illustrating the outline of the configuration of the rolling facility of the third embodiment. FIG. 12 is a diagram illustrating the outline of another configuration of the rolling facility of the third embodiment.A rolling facility 1C of the present embodiment illustrated in FIG. 11 is obtained by omitting the second temperature raising device 502 from the rolling facility 1 illustrated in FIG. 1.In the rolling facility 1 illustrated in FIG. 1, heating-up is executed by the first IH slab heater 20 in the first temperature raising device 501 in the case of executing batch rolling of a slab with the second thickness Y on the first strand 401, and heating-up is executed by the second IH slab heater 22 in the second temperature raising device 502 in the case of executing batch rolling of a slab with the second thickness Y on the second strand 402. Then, the slab is supplied to the rough rolling mill 70. In this case, the IH slab heater 503 is in an unused state. The IH slab heater 503 is used mainly for heating up a slab with the first thickness X in endless rolling.In contrast, in the rolling facility 1C illustrated in FIG. 11, in the case of executing the batch rolling, both a slab of the first strand 401 and a slab of the second strand 402 are heated up by both the first IH slab heater 20 and the IH slab heater 503.The slabs that pass through the first IH slab heater 20 are the slabs of both the first strand 401 and the second strand 402. Therefore, the amount of passing per unit time increases by a factor of two. Thus, the heating time by the first IH slab heater 20 becomes half per one slab. However, the insufficiency of the heating-up can be supplemented by the IH slab heater 503.Due to making the configuration like the rolling facility 1C of FIG. 11, both the first IH slab heater 20 and the IH slab heater 503 are used in both the endless rolling of the slab produced by only the first strand 401 and the batch rolling of the slabs produced by both the first strand 401 and the second strand 402. Therefore, the second temperature raising device 502 becomes unnecessary. Thus, the facility cost can be reduced relative to the rolling facility 1 illustrated in FIG. 1.A rolling facility 1D illustrated in FIG. 12 is obtained by disposing IH slab heaters 801 and 802 between the CC 12 and the torch cutting machine 14 in the rolling facility 1C illustrated in FIG. 11. In particular, when the casting velocity is low, the temperature drop of the surface temperature of the slab, particularly the temperature drop of a width end corner part, is large in the CC 12 and on the exit side of the CC 12. Therefore, when insufficiency exists in the first IH slab heater 20 and the IH slab heater 503, the insufficiency can be supplemented.The cast piece travels at the casting velocity on the entry side of the torch cutting machine 14. However, the velocity is low, and thus the capacity of the IH slab heaters 801 and 802 does not need to be made excessively high. In particular, the inside remains at a high temperature compared with the surface during the casting. Therefore, the IH, which heats up the surface, is effective.Furthermore, depending on the steel grade, AlN (aluminum nitride) often precipitates at a slab edge part when the temperature lowers. The precipitation of AlN at this stage is not preferable in a deep drawing steel strip that requires box annealing treatment. Thus, there is a case in which the temperature needs to be kept equal to or higher than approximately 900°C, which is the temperature at which AlN precipitation starts, in the process of the temperature drop after the continuous casting. In this case, the IH slab heaters 801 and 802 effectively act.Here, the IH slab heaters 801 and 802 can include either or both of width end part heating and whole area heating.The other configuration and operation are substantially the same configuration and operation as the manufacturing method for a rolled metal strip and the rolling facility according to the above-described first embodiment, and details thereof are omitted.Also in the manufacturing method for a rolled metal strip and the rolling facility according to the third embodiment of the present invention, almost the same effects as the manufacturing method for a rolled metal strip and the rolling facility according to the above-described first embodiment are obtained.<Fourth Embodiment>A manufacturing method for a rolled metal strip and a rolling facility according to a fourth embodiment of the present invention will be described with use of FIG. 13. FIG. 13 is a diagram illustrating the outline of the configuration of the rolling facility of the fourth embodiment.A rolling facility 1E of the present embodiment illustrated in FIG. 13 is obtained by omitting the IH slab heater 503 from the rolling facility 1D illustrated in FIG. 12. Moreover, the first slab transportation device 40 and the third slab transportation device 44 are omitted and a continuous casting machine exit-side slab transportation device 48 is added. Due to this, the distance L1 from the mold of the CC 12 to the rough rolling mill 70 is shortened.Since the distance L1 becomes shorter, the temperature drop of the slab during transportation in endless rolling becomes smaller. Thus, the amount of temperature rise by heating-up before rough rolling for the endless rolling can be suppressed, and the slab heating-up is enabled by only the first IH slab heater 20. On that occasion, somewhat enhancing the heating-up capability of the first IH slab heater 20 is selected in some cases. Furthermore, by giving consideration to increase the slab temperature to be high by adjusting the casting condition such as the casting velocity, and to extend the slab supply time interval by increasing the slab length, batch rolling of the slabs produced from both the first strand 401 and the second strand 402 is enabled without enhancing the heating-up capability of the first IH slab heater 20, in some cases.The IH slab heaters 801 and 802 allow suppression of the temperature drop during transportation of the slab, and also have an effect of reducing the burden of heating-up of the first IH slab heater 20. Here, when heating-up is possible with only the first IH slab heater 20, it is also possible to omit the IH slab heaters 801 and 802.Moreover, the present embodiment can be made into one configured to be capable of, when the batch mode is selected, further selecting at least one from among: a first mode in which a cast piece cut by the torch cutting machine 14 is rolled by the rough rolling mill 70 after the temperature thereof is raised by the first temperature raising device 501; a second mode in which a cast piece cut by the torch cutting machine 14 is transported to the storage yard 50 for rejected slab by the continuous casting machine exit-side slab transportation device 48 or the second slab transportation device 42 and the fourth slab transportation device 46 without raising the temperature of the cast piece by the first temperature raising device 501; and a third mode in which a cast piece stored in the storage yard 50 for rejected slab is transported to the rolling line by the fourth slab transportation device 46 and the second slab transportation device 42 or the continuous casting machine exit-side slab transportation device 48 and is rolled by the rough rolling mill 70 after the temperature of the cast piece is raised by the first temperature raising device 501.Note that the first mode and the second mode are selected and executed for each cast piece during execution of the batch mode, whereas the third mode can be executed when the first mode or the second mode of the batch mode is executed.In the third mode, in order to allow the slab transported to the rolling line to be rolled by the rough rolling mill 70 after the temperature thereof is raised by the first temperature raising device 501, a necessary device is appropriately disposed, although not illustrated, such that heating means for the slab is disposed in the storage yard 50 for rejected slab or disposed between the storage yard 50 for rejected slab and the rolling line.Here, it is also possible to omit either the continuous casting machine exit-side slab transportation device 48 or the second slab transportation device 42 and the fourth slab transportation device 46, and execute the first mode, the second mode, and the third mode by using the other one. This can make the distance L1 shorter. Therefore, in particular, the amount of temperature drop of the slab in the endless mode can be made small and the amount of temperature rise by the first temperature raising device can be suppressed. Note that, for example, when the second mode and the third mode are simultaneously executed, omitting the either one is allowed by appropriately disposing a required facility such that a standby position for the slab is provided, although not illustrated, in order to avoid the interference between the slabs during transportation of the slabs.The other configuration and operation are substantially the same configuration and operation as the manufacturing method for a rolled metal strip and the rolling facility according to the above-described first embodiment, and details thereof are omitted.Also in the manufacturing method for a rolled metal strip and the rolling facility according to the forth embodiment of the present invention, almost the same effects as the manufacturing method for a rolled metal strip and the rolling facility according to the above-described first embodiment are obtained.<Others>The present invention is not limited to the above-described embodiments and various modification examples are included therein. The above-described embodiments are described in detail in order to explain the present invention in an easy-to-understand manner and are not necessarily limited to that including all configurations described. In the above description, the case where the cast piece is a slab has been explained, but cases where the cast piece is a billet, bloom, and beam blank, etc. are also included in the present invention.Furthermore, it is also possible to replace part of a configuration of a certain embodiment by a configuration of another embodiment. Moreover, it is also possible to add a configuration of another embodiment to a configuration of a certain embodiment. In addition, it is also possible to execute addition, deletion, or substitution of another configuration regarding part of a configuration of each embodiment.Description of Reference Characters1, 1A, 1B, 1C, 1D, 1E: Rolling facility10: Ladle turret12: Continuous casting machine (CC)14: Torch cutting machine16: Pendulum shear20: First IH slab heater22: Second IH slab heater30: First soaking device32: Second soaking device34: Third soaking device36: Fourth soaking device40: First slab transportation device42: Second slab transportation device44: Third slab transportation device46: Fourth slab transportation device48: Continuous casting machine exit-side slab transportation device50: Storage yard for rejected slab60: Scale breaker70, 70A, 70B: Rough rolling mill75: Finishing rolling mill80: Run out table85: Down coiler90: Controller201: Thermometer in the first temperature raising device203: Thermometer in the second temperature raising device205: Thermometer in the storage yard401: First strand402: second strand501: first temperature raising device502: Second temperature raising device503: IH slab heater504: IH bar heater510: Shear520: Coil box530: Dividing shear710: Table on the entry side of the rough rolling mill801, 802: IH slab heater

Claims

1. A manufacturing method for a rolled metal strip, comprising: rolling, by a rough rolling mill, a cast piece cast by a continuous casting machine, wherein the continuous casting machine is configured to be capable of selecting an endless mode in which the cast piece cast into a first thickness X is rolled by the rough rolling mill without being cut, and a batch mode in which the cast piece cast into a second thickness Y thicker than the first thickness X is rolled by the rough rolling mill after being cut.

2. The manufacturing method for a rolled metal strip according to claim 1, wherein a relationship between the first thickness X and the second thickness Y satisfies 90 ≤ X ≤ 0.89Y (unit is mm).

3. The manufacturing method for a rolled metal strip according to claim 1, wherein a relationship between the first thickness X and the second thickness Y satisfies 0.60Y ≤ X ≤ 0.70Y (unit is mm).

4. The manufacturing method for a rolled metal strip according to any one of claims 1 to 3, wherein a configuration is made in such a manner that it is possible to, when the batch mode is selected, further select at least one of a first mode in which the cast piece that has been cut is rolled by the rough rolling mill after temperature of the cast piece is raised, a second mode in which the cast piece that has been cut is transported to a storage yard, and a third mode in which the cast piece stored in the storage yard is rolled by the rough rolling mill after temperature of the cast piece is raised.

5. The manufacturing method for a rolled metal strip according to any one of claims 1 to 3, wherein, when the endless mode is selected, temperature of the cast piece is raised at at least one place among an exit side of the continuous casting machine, an entry side of the rough rolling mill, and a place that is on an exit side of the rough rolling mill and on an entry side of a finishing rolling mill.

6. The manufacturing method for a rolled metal strip according to any one of claims 1 to 3, wherein rolling is executed in one direction by a plurality of the rough rolling mills.

7. The manufacturing method for a rolled metal strip according to any one of claims 1 to 3, wherein reverse rolling is executed by a plurality of the rough rolling mills when the batch mode is selected.

8. The manufacturing method for a rolled metal strip according to any one of claims 1 to 3, wherein a temperature raising device that is disposed at at least one of a place on an entry side of the rough rolling mill and a place that is on an exit side of the rough rolling mill and on an entry side of a finishing rolling mill, the temperature raising device raising temperature of the cast piece, is configured to be capable of being retracted and inserted from and into a transportation line of the cast piece.

9. The manufacturing method for a rolled metal strip according to claim 7, wherein a temperature raising device that is disposed on an entry side of the rough rolling mill and raises temperature of the cast piece is configured to be capable of being retracted from a transportation line of the cast piece.

10. The manufacturing method for a rolled metal strip according to any one of claims 1 to 3, wherein temperature of the cast piece is raised by a temperature raising device disposed on an entry side of the rough rolling mill when the batch mode is selected.

11. A rolling facility including a continuous casting machine and a rough rolling mill that rolls a cast piece cast by the continuous casting machine, wherein the continuous casting machine is configured to be capable of selecting an endless mode in which the cast piece cast into a first thickness X is rolled by the rough rolling mill without being cut, and a batch mode in which the cast piece cast into a second thickness Y thicker than the first thickness X is rolled by the rough rolling mill after being cut.

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