Facility having a continuous annealing furnace and a galvanization bath and method for continuously manufacturing hot-dip galvanized steel sheet

Abstract

A facility for continuously manufacturing a galvanized steel sheet includes a continuous annealing furnace, a snout, and a galvanization bath. The furnace is divided into a heating zone, a soaking zone, and a cooling zone. The bath is directly connected to the furnace through the snout. The facility has a dewpoint meter and a spray port and a suction port for a furnace gas that are provided in at least one of the zones, gas cyclic paths which connect the spray port and the suction port to a refiner, and a dewpoint meter and a humidification device that are provided in the snout. The gas cyclic paths are separately formed for the respective connected zones. The refiner functions so that a measured value of the dewpoint meter is equal to a target dewpoint, and the humidification device functions so that a measured value of the dewpoint meter in the snout is equal to a target dewpoint for the snout.

Claims

1. A facility for continuously manufacturing a galvanized steel sheet, comprising: a furnace, said furnace is a continuous annealing furnace divided into three zones including a heating zone that heats a steel strip in form of a strip-shaped steel sheet to be passed through the furnace, a soaking zone that soaks the steel strip which has been heated in the heating zone, and a cooling zone that cools the steel strip which has been soaked in the soaking zone, which are arranged in this order heating zone, soaking zone, cooling zone from an upstream side of a transport path; a galvanization bath; a snout that directly connects the furnace to the galvanization bath therethrough, the snout being a closed space through which the steel strip is directly fed into the galvanization bath from the furnace; a first dewpoint meter, a suction port, and a spray port for a gas within the furnace are provided in each of the heating zone and the soaking zone of the furnace; a refiner, which is a moisture removing device provided outside the furnace, a plurality of gas cyclic paths, including a gas cyclic path that connects the suction port and the spray port of the heating zone to the refiner and a gas cyclic path that connects the suction port and the spray port of the soaking zone to the refiner, the plurality of gas cyclic paths being separate from one another along their entire respective lengths and a second dewpoint meter and a humidification device, which are provided in the snout, the humidification device which humidifies an inside of the snout, wherein: the refiner functions so that, for each of the gas cyclic paths, a first measured value of the first dewpoint meter in the heating zone is equal to a first target dewpoint in the heating zone and a first measured value of the first dewpoint meter in the soaking zone is equal to a first target dewpoint in the soaking zone; and the humidification device functions so that a second measured value of the second dewpoint meter in the snout is equal to a second target dewpoint for the snout.

2. The facility for continuously manufacturing a galvanized steel sheet according to claim 1, further comprising a snout suction port and a snout spray port for a gas within the snout that are provided in the snout, and a snout gas cyclic path formed between the refiner and the snout by connecting the ports thereof to the refiner, wherein the refiner functions along with the humidification device so that the second measured value of the further dewpoint meter in the snout is equal to the second target dewpoint for the snout.

3. The facility for continuously manufacturing a galvanized steel sheet according to claim 1, wherein the second target dewpoint for the snout is 35 C. or higher.

4. A facility for continuously manufacturing a galvanized steel sheet, comprising: a furnace, said furnace is a continuous annealing furnace divided into three zones including a heating zone that heats a steel strip in form of a strip-shaped steel sheet to be passed through the furnace, a soaking zone that soaks the steel strip which has been heated in the heating zone, and a cooling zone that cools the steel strip which has been soaked in the soaking zone, which are arranged in this order heating zone, soaking zone, cooling zone from an upstream side of a transport path; a galvanization bath; a snout that directly connects the furnace to the galvanization bath therethrough, the snout being a closed space through which the steel strip is directly fed into the galvanization bath from the furnace; a first dewpoint meter, a suction port, and a spray port for a gas within the furnace are provided in each of the heating zone and the soaking zone of the furnace; a refiner, which is a moisture removing device provided outside the furnace, a plurality of gas cyclic paths, including a gas cyclic path that connects the suction port and the spray port of the heating zone to the refiner, a gas cyclic path that connects the suction port and the spray port of the soaking zone to the refiner, and a gas cyclic path that connects the suction port and the spray port of the cooling zone to the refiner, the plurality of gas cyclic paths being separate from one another along their entire respective lengths; and a second dewpoint meter and a humidification device, which are provided in the snout, the humidification device which humidifies an inside of the snout, wherein: the refiner functions so that, for each of the plurality of gas cyclic paths, a first measured value of the first dewpoint meter in the heating zone, is equal to a first target dewpoint in the heating zone, a first measured value of the first dewpoint meter in the soaking zone is equal to a first target dewpoint in the soaking zone, and a first measured value of the first dewpoint meter in the cooling zone is equal to a first target dewpoint in the cooling zone; and the humidification device functions so that a second measured value of the second dewpoint meter in the snout is equal to a second target dewpoint for the snout.

5. The facility for continuously manufacturing a galvanized steel sheet according to claim 4, further comprising a snout suction port and a snout spray port for a gas within the snout that are provided in the snout, and a snout gas cyclic path formed between the refiner and the snout by connecting the ports thereof to the refiner, wherein the refiner functions along with the humidification device so that the second measured value of the further dewpoint meter in the snout is equal to the second target dewpoint for the snout.

6. The facility for continuously manufacturing a galvanized steel sheet according to claim 4, wherein the second target dewpoint for the snout is 35 C. or higher.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic diagram illustrating an embodiment of the present invention.

(2) FIG. 2 is a schematic diagram illustrating the relationship between the surface oxidation amounts of zinc coatability-deteriorating elements and the dewpoint of a furnace atmospheric gas.

(3) FIG. 3 is a schematic diagram illustrating an embodiment of the present invention, which is different from the one illustrated in FIG. 1.

(4) FIG. 4 is a schematic diagram illustrating an alternative emboli ent of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

(5) As illustrated in, for example, FIG. 1, embodiments of the present invention are premised on a facility including a continuous annealing furnace, a snout 4, and a galvanization bath 5. The furnace is divided into three zones, i.e., a heating zone 1 for heating a steel strip S, which is a strip-shaped steel sheet that is passed through the furnace, a soaking zone 2 for soaking the heated steel strip, and a cooling zone 3 for cooling the soaked steel strip, which are arranged in this order from the upstream side of a transport path. The bath is directly connected to the furnace through the snout, which is a closed space through which the steel strip is directly fed into the galvanization bath from the furnace. There are seal rolls 9 provided at pivotal points between the cooling zone 3 and the snout 4 in order to prevent the atmospheric gases of the different treatment sections from mixing with each other, and there are seal rolls 10 provided at the entrance of the heating zone 1 in order to prevent outer air from entering the furnace. There is also a case where a heater is provided in a part on the downstream side of the cooling zone 3 in order to perform an over-aging treatment. Such a premise is within a range of a well-known technique.

(6) Under such a premise, aspects of the present invention include a facility including: (i) a dewpoint meter 6 and a suction port 7 and a spray port 8 of the furnace gas provided in at least one of the three zones (two zones which are the heating zone 1 and the soaking zone 2 in the present example); (ii) a refiner 11, which is a moisture removing device provided outside the furnace; (iii) gas cyclic paths 12 and 13, which connect the suction ports and the spray ports to the refiner 11, separately formed for the respective connected zones; (iv) and a dewpoint meter 6 and a humidification device 14 for humidifying the inside of the snout 4 provided in the snout 4 (as indicated by arrow in FIG. 1). In the facility, the refiner 11 functions so that, for each of the gas cyclic paths, a measured value of the dewpoint meter in the connected zone is equal to a target dewpoint, and the humidification device 14 functions so that a measured value of the dewpoint meter 6 in the snout 4 is equal to a target dewpoint for the snout 4.

(7) Here, a pair of a suction port 7 and a spray port 8 is provided in each of the heating zone 1 and the soaking zone 2 in FIG. 1. FIG. 4 illustrates an alternative embodiment in which a suction port and a spray port are provided in each of the three zones of the furnace, including the cooling zone 3. Because it is easier to control the dewpoint when a plural pairs is provided, however, two or more pairs of suction ports 7 and spray ports 8 may be provided in each zone, and the number of pairs is appropriately determined in order to achieve the target dewpoint.

(8) The gas cyclic paths 12 and 13, which are connected to different zones, are independent of each other and do not join together in the refiner 11. The refiner 11 functions, for each of the gas cyclic paths, to remove moisture in the gas in the gas cyclic path in order to control the dewpoint measured by the dewpoint meter of the zone to which the gas cyclic path is connected to be equal to a target dewpoint. On the other hand, the humidification device 14 functions so that humidification is performed by feeding moisture in the snout in order to control a dewpoint measured by the dewpoint meter in the snout to be equal to a target dewpoint, which is higher than that of the zone in the furnace.

(9) As described above, some of the gas from the zones of the furnace is suctioned and is sprayed back to the same zones after removing moisture, and humidification in the snout is performing using a humidification device. Accordingly, it is possible to stably control the dewpoint in the furnace to be low, i.e., 50 C. or lower, and it is also possible to separately control the dewpoint in the furnace and the dewpoint in the snout. Thereby it is possible to stably achieve high quality and high zinc coatability for a galvanized steel sheet.

(10) The dewpoint meter 6, the suction port 7, and the spray port 8 may be provided in at least one of the three zones of the furnace. However, it is preferable to provided these devices in the soaking zone 2. The soaking zone 2 is a zone in which surface concentration of Si and Mn likely to occur due to higher furnace temperature than in the other zones. Accordingly, it is suitable to control the dewpoint to be low by preferentially placing the dewpoint meter, the suction port, and the spray port in this zone in order to stably achieve high zinc coatability. Here, it is needless to say that it is most preferable to provide these devices in all of the three zones.

(11) In the case of the example illustrated in FIG. 3, in addition to the example illustrated in FIG. 1, the facility further includes a suction port 7 and a drawing port 8 for the gas in the snout provided in the snout 4, and a gas cyclic path 15 between the refiner 11 and the snout formed by connecting these ports to the refiner 11. In the facility, the refiner 11 also functions along with the humidification device 14 so that a dewpoint measured by the dewpoint meter in the snout is equal to a target dewpoint for the snout. Using this facility, it is possible to control the dewpoint in the snout with increased accuracy. Accordingly, it is possible to prevent the attachment of vaporized zinc in the snout more effectively, which results in high galvanized product quality achieved with significantly increased stability.

(12) In a galvanization operation using the facility according to aspects of the present invention, it is preferable that, as described above, the target dewpoint in the furnace is set to be 50 C. or lower in order to suppress surface concentration of Si and Mn. Using the facility according to aspects of the present invention achieves such control for a low dewpoint, and it becomes possible to effectively prevent surface concentration of Si and Mn and stably achieve high zinc coatability. Here, because there is a significant increase in cost in order to lower the dewpoint to lower than 80 C., it is preferable that the dewpoint is 80 C. or higher. On the other hand, it is possible to control the dewpoint in the snout independently of the dewpoint in the furnace by using the facility according to aspects of the present invention. It is preferable that the target dewpoint in the snout is 35 C. or higher in order to effectively prevent vaporized zinc from attaching to a steel strip in the snout. However, it is preferable that the target dewpoint is 10 C. or lower because, in the case where the dewpoint is excessively high, a zinc oxide film is formed on the bath surface, which is disadvantageous in that the film attaches to a steel strip.

(13) Here, among examples of the refiner, refiners having strong dehumidification capability such as desiccant-type ones which continuously perform dehumidification using, for example, calcium oxide, zeolite, silica gel, calcium chloride, or the like and compressor-type ones, which use, for example, substitute chlorofluorocarbon, are preferably used.

EXAMPLES OF THE PRESENT INVENTION

(14) An EXAMPLE in which aspects of the present invention was applied to a continuous line for manufacturing a galvanized steel sheet in the embodiment similar to the example illustrated in FIG. 3 will be described. In this line, conventionally, a dewpoint in the furnace was controlled using a method in which furnace gas was suctioned from a cooling zone, moisture was removed using a refiner provided outside of the furnace, and then the gas was returned to a heating zone. However, the lower limit of achievable dewpoint in the furnace was about 40 C., and the dewpoint was not controlled in the snout. Therefore, in the operation of the galvanization of a high-strength steel sheet to which Si and Mn are added, the control was not satisfactorily effective for suppressing bare spots and poor surface quality. In contrast, in the case of EXAMPLE, it was possible to stably control the dewpoint in the soaking zone to be 50 C. or lower and 80 C. or higher, and it was possible to stably control the dewpoint in the snout to be 35 C. or higher and 10 C. or lower. As a result, the occurrence frequency of bare spots was significantly decreased to 10, and the occurrence frequency of poor surface quality was significantly decreased to 20 when the conventional occurrence frequency of bare spots was defined as 100, which indicates a significant effect of aspects of the present invention.

REFERENCE SIGNS LIST

(15) 1 heating zone 2 soaking zone 3 cooling zone 4 snout 5 galvanization bath 6 dewpoint meter 7 suction port 8 spray port 9, 10 seal roll 11 refiner (moisture removing device) 12, 13, 15 gas cyclic path 14 humidification device S steel strip