JP2016180137A - Continuous molten zinc plating apparatus, and manufacturing method for molten zinc plated steel plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a continuous molten zinc plating apparatus enabled to acquire a satisfactory plated appearance by suppressing the occurrence of the roll pickup of a homogeneous heat zone which could occur due to a condensation or the like in a piping for humidification gases.SOLUTION: A continuous molten zinc plating apparatus of the invention comprises: an annealing furnace having a heating zone, a homogeneous heat zone and a cooling zone juxtaposed in the recited order; and a molten zinc plating facility adjacent to said cooling band. The gas to be fed to a homogeneous heat zone 12 is a mixed gas obtained by mixing a gas humidified by a humidifier 50 and a gas unhumidified by the humidifier, and a dry gas unhumidified by the humidifier. The continuous molten zinc plastic apparatus of the invention comprises a draining device 80 for draining the circulation water from the humidifier 50 when the mixed gas is not fed to the homogeneous heat zone 12.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a continuous galvanizing apparatus having an annealing furnace in which a heating zone, a soaking zone and a cooling zone are juxtaposed in this order, and a hot dip galvanizing equipment adjacent to the cooling zone, and a hot dip galvanizing using the same. The present invention relates to a method of manufacturing a steel plate.

  In recent years, in the fields of automobiles, home appliances, building materials, etc., there is an increasing demand for high-tensile steel plates (high-tensile steel plates) that contribute to weight reduction of structures. As a high-tensile steel material, for example, it has been found that a steel plate with good hole expansibility by containing Si in the steel, and a steel plate with good ductility can be produced by easily containing residual γ by containing Si or Al. Yes.

  However, when an alloyed hot-dip galvanized steel sheet is produced using a high-tensile steel sheet containing a large amount of Si (particularly 0.2% by mass or more) as a base material, there are the following problems. An alloyed hot-dip galvanized steel sheet is obtained by subjecting a base steel sheet to heat annealing at a temperature of about 600 to 900 ° C. in a reducing atmosphere or a non-oxidizing atmosphere, and then subjecting the steel sheet to hot-dip galvanization and further heating the galvanizing. Manufactured by alloying.

  Here, Si in steel is an easily oxidizable element, and is selectively oxidized in a generally used reducing atmosphere or non-oxidizing atmosphere to concentrate on the surface of the steel sheet to form an oxide. This oxide reduces wettability with molten zinc during the plating process and causes non-plating. Therefore, as the Si concentration in the steel increases, the wettability decreases sharply and non-plating occurs frequently. In addition, even when non-plating does not occur, there is a problem that the plating adhesion is poor. Further, when Si in the steel is selectively oxidized and concentrated on the surface of the steel sheet, there is a problem that a remarkable alloying delay occurs in the alloying process after hot dip galvanizing, and the productivity is remarkably hindered.

  For such a problem, for example, in Patent Document 1, in the continuous annealing hot dipping method using an annealing furnace and a hot dipping bath having a heating zone first stage, a heating zone latter stage, a heat retention zone, and a cooling zone in order, the steel plate temperature The heating or heat retention of the steel sheet in the region of at least 300 ° C. or more is indirect heating, the atmosphere in the furnace of each zone is 1 to 10% by volume of hydrogen, the remainder is an atmosphere consisting of nitrogen and inevitable impurities, The temperature reached by the steel sheet during heating is set to 550 ° C. or higher and 750 ° C. or lower, and the dew point is set to less than −25 ° C., and the dew point after the heating zone and the retentive zone is set to −30 ° C. or higher and 0 ° C. or lower. A technique is described in which Si is internally oxidized by annealing under the condition that the dew point of the band is less than −25 ° C., and Si is concentrated on the surface of the steel sheet. In addition, it is also described that a mixed gas of nitrogen and hydrogen is introduced after humidification into the latter stage of the heating zone and / or the tropical zone.

International Publication No. 2007/043273

  When producing a high-tensile steel sheet, in order to raise the dew point in the soaking zone, in addition to the reducing or non-oxidizing dry gas, a humidified gas is introduced into the soaking zone as described in Patent Document 1. . On the other hand, when manufacturing a normal strength steel plate (hereinafter referred to as “normal steel plate”), the humidified gas is not input, and only the reducing or non-oxidizing dry gas is input into the soaking zone. Therefore, for example, when manufacturing a high-strength steel plate and a normal steel plate continuously, it is necessary to operate while switching between use / non-use of the humidified gas.

  The present inventors have recognized the following problems that occur when operating while switching between use / nonuse of such humidified gas. That is, when the humidifying gas is not used, even if the humidifying system gas is simply stopped, water from the humidifying device diffuses in the piping of the humidifying system and condensation occurs, or excessively humidified gas is retained. To do. Then, when the humidification system is switched from non-use to use, the condensed water and excessively humidified gas in the piping blows into the soaking zone, damages the hearth roll in the soaking zone, and picks up, The problem that a water droplet pattern is attached to a steel plate occurs. Due to this, non-plating may occur in the subsequent hot dip galvanizing process, and the plating appearance may be impaired.

  Therefore, in view of the above problems, the present invention suppresses generation of a soaking zone roll pickup that may be caused by dew condensation or the like in a humidified gas pipe, and can obtain a good plating appearance. And it aims at providing the manufacturing method of a hot-dip galvanized steel plate.

  In order to solve the above-mentioned problems, the present inventors have formed dew condensation in the humidifying gas pipe when the humidifying gas is not used (while the humidifying gas supply to the soaking zone is stopped) or excessively humidified. The present inventors have intensively studied to prevent the generated gas from staying, and found that the object can be achieved by the following configuration, thereby completing the present invention.

The gist of the present invention is as follows.
(1) A continuous hot dip galvanizing apparatus having an annealing furnace in which a heating zone, a soaking zone, and a cooling zone are juxtaposed in this order, and a hot dip galvanizing facility adjacent to the cooling zone,
A first pipe through which a reducing or non-oxidizing drying gas passes;
A gas distributor connected to the first pipe for distributing the dry gas that has passed through the first pipe;
A second pipe, a third pipe, and a fourth pipe branched from the gas distributor and through which the dry gas distributed to the gas distributor passes;
A humidifier connected to the second pipe and into which the dry gas that has passed through the second pipe is introduced;
A fifth pipe extending from the humidifier and through which humidified gas humidified by the humidifier passes;
A gas mixing device that is connected to the third pipe and the fifth pipe and mixes the dry gas that has passed through the third pipe and the humidified gas that has passed through the fifth pipe to produce a mixed gas;
A sixth pipe extending from the gas mixing device and through which the mixed gas passes;
A mixed gas supply port provided in the soaking zone for supplying the mixed gas that has passed through the sixth pipe into the soaking zone;
A dry gas supply port provided in the soaking zone for supplying dry gas that has passed through the fourth pipe into the soaking zone;
Have
The humidifier has a module including a water vapor permeable membrane, and a dry gas passing through the second pipe passes through one space between the water vapor permeable membranes in the module, while passing through the other space. Is configured to humidify the dry gas by circulating water using a circulating constant temperature water bath,
The continuous galvanizing apparatus further comprising a drainage device for draining water from the other space of the module when the mixed gas is not supplied to the soaking zone.

  (2) The continuous hot dip galvanizing apparatus according to the above (1) having an alloying equipment adjacent to the hot dip galvanizing equipment.

(3) A method for producing a hot dip galvanized steel sheet using the continuous hot dip galvanizing apparatus described in (1) above,
Conveying the steel strip in the annealing furnace in the order of the heating zone, the soaking zone, and the cooling zone, and annealing the steel strip; and
Using the hot dip galvanizing equipment, applying hot dip galvanizing to the steel strip discharged from the cooling zone;
Have
In the first operation state in which the mixed gas and the dry gas are supplied to the soaking zone, the water is circulated using the circulation thermostatic bath,
In the second operation state in which only the dry gas is supplied to the soaking zone and the mixed gas is not supplied, the distribution of the dry gas to the second pipe is stopped and the module is used to stop the distribution of the dry gas. A method for producing a hot-dip galvanized steel sheet, wherein water is drained from the other space and water is not circulated using the circulating thermostatic bath.

  (4) When switching from the second operation state to the first operation state, after resuming the circulation of water using the circulation thermostatic water tank, the distribution of the dry gas to the second pipe is resumed. The manufacturing method of the hot dip galvanized steel sheet as described in said (3).

  (5) The manufacturing method of the hot dip galvanized steel sheet according to (3) or (4), wherein the dew point in the soaking zone is controlled to -20 ° C or higher and 0 ° C or lower in the first operating state.

  (6) The method according to any one of (3) to (5), further including a step of heat-alloying the galvanization applied to the steel strip using the alloying equipment according to (2). The manufacturing method of the hot-dip galvanized steel sheet as described in 2.

  According to the continuous hot dip galvanizing apparatus and the method for producing a hot dip galvanized steel sheet of the present invention, it is possible to suppress the occurrence of a soaking zone roll pickup that may be caused by condensation in the humidified gas pipe, and to provide a good plating appearance. It is possible to obtain.

It is a schematic diagram which shows the structure of the continuous hot dip galvanizing apparatus 100 by one Embodiment of this invention. 2 is a schematic diagram showing a mixed gas and dry gas supply system to the soaking zone 12 in FIG. 1. FIG. 3 is an enlarged schematic diagram of a humidifier 50 and a drain device 80 in FIG. 2.

  A configuration of a continuous hot dip galvanizing apparatus 100 according to an embodiment of the present invention will be described with reference to FIG. The continuous hot dip galvanizing apparatus 100 includes an annealing furnace 20 in which a heating zone 10, a soaking zone 12, and cooling zones 14 and 16 are arranged in this order, and a hot dip galvanizing bath 22 as a hot dip galvanizing facility adjacent to the cooling zone 16. And a hot dip galvanizing bath 22 and an adjacent alloying equipment 24. In the present embodiment, the heating zone 10 includes a first heating zone 10A (a heating zone upstream) and a second heating zone 10B (a heating zone downstream). The cooling zone includes a first cooling zone 14 (quenching zone) and a second cooling zone 16 (cooling zone). The tip of the snout 18 connected to the second cooling zone 16 is immersed in a hot dip galvanizing bath 22, and the annealing furnace 20 and the hot dip galvanizing bath 22 are connected. Another embodiment of the present invention is a method for producing a hot dip galvanized steel sheet using the continuous hot dip galvanizing apparatus 100.

  The steel strip P is introduced into the first heating zone 10A from the steel strip inlet at the bottom of the first heating zone 10A. In each of the bands 10, 12, 14, and 16, one or more hearth rolls are disposed at the upper and lower portions. When the steel strip P is folded back 180 degrees starting from the hearth roll, the steel strip P is conveyed a plurality of times in the vertical direction inside a predetermined strip of the annealing furnace 20 to form a plurality of passes. In FIG. 1, an example of 10 passes in the soaking zone 12, 2 passes in the first cooling zone 14, and 2 passes in the second cooling zone 16 is shown. However, the number of passes is not limited to this, and it depends on the processing conditions. It can be set as appropriate. Further, in some hearth rolls, the steel strip P is changed to a right angle without turning back, and the steel strip P is moved to the next strip. In this way, the steel strip P can be transported in the annealing furnace 20 in the order of the heating zone 10, the soaking zone 12, and the cooling zones 14 and 16, and the steel strip P can be annealed.

  In the annealing furnace 20, the adjacent bands communicate with each other via a communication portion that connects the upper parts or the lower parts of the respective bands. In the present embodiment, the first heating zone 10 </ b> A and the second heating zone 10 </ b> B communicate with each other via a throat (throttle portion) that connects the upper portions of the respective zones. The second heating zone 10B and the soaking zone 12 communicate with each other via a throat that connects the lower portions of each zone. The soaking zone 12 and the first cooling zone 14 communicate with each other via a throat connecting the lower portions of the respective zones. The 1st cooling zone 14 and the 2nd cooling zone 16 are connected via the throat which connects the lower parts of each zone. The height of each throat may be set as appropriate, but it is preferable that the height of each throat is as low as possible from the viewpoint of increasing the independence of the atmosphere of each band. The gas in the annealing furnace 20 flows from the downstream to the upstream of the furnace and is discharged from the steel strip inlet at the bottom of the first heating zone 10A.

(Heating zone)
In the present embodiment, the second heating zone 10B is a direct-fired heating furnace (DFF). A well-known DFF can be used. Although not shown in FIG. 1, a plurality of burners are arranged in a distributed manner facing the steel strip P on the inner wall of the direct-fired heating furnace in the second heating zone 10B. The plurality of burners are preferably divided into a plurality of groups, and the fuel ratio and the air ratio can be independently controlled for each group. The combustion exhaust gas from the second heating zone 10B is supplied into the first heating zone 10A, and the steel strip P is preheated by the heat.

  The combustion rate is a value obtained by dividing the amount of fuel gas actually introduced into the burner by the amount of fuel gas in the burner at the maximum combustion load. When the burner is burned at the maximum combustion load, the burning rate is 100%. The burner cannot obtain a stable combustion state when the combustion load becomes low. Therefore, it is preferable that the combustion rate is usually 30% or more.

  The air ratio is a value obtained by dividing the amount of air introduced into the actual burner by the amount of air necessary for complete combustion of the fuel gas. In the present embodiment, the heating burner of the second heating zone 10B is divided into four groups (# 1 to # 4), and the three groups (# 1 to # 3) on the upstream side in the steel plate moving direction are oxidation burners, The final zone (# 4) is a reduction burner, and the air ratio of the oxidation burner and the reduction burner can be individually controlled. In the oxidation burner, the air ratio is preferably 0.95 or more and 1.5 or less. In the reduction burner, the air ratio is preferably 0.5 or more and less than 0.95. Moreover, it is preferable that the temperature inside the 2nd heating zone 10B shall be 800-1200 degreeC.

(Soaking)
In this embodiment, in the soaking zone 12, the steel strip P can be indirectly heated using a radiant tube (RT) (not shown) as a heating means. The average temperature Tr (° C.) inside the soaking zone 12 is measured by inserting a thermocouple into the soaking zone, but is preferably 700 to 900 ° C.

The soaking zone 12 is supplied with reducing gas or non-oxidizing gas. As the reducing gas, a mixed gas of H ₂ —N ₂ is usually used, for example, H ₂ : 1 to 20% by volume, and the balance is composed of N ₂ and inevitable impurities (dew point: about −60 ° C.) Is mentioned. Examples of the non-oxidizing gas include a gas (dew point: about −60 ° C.) having a composition composed of N ₂ and inevitable impurities.

  In the present embodiment, the reducing gas or non-oxidizing gas supplied to the soaking zone 12 is in two forms: a mixed gas and a dry gas. Here, the “dry gas” is the reducing gas or non-oxidizing gas having a dew point of about −60 ° C. to −50 ° C., and is not humidified by a humidifier. On the other hand, the “mixed gas” is obtained by mixing a gas humidified by a humidifier and a gas not humidified by a humidifier at a predetermined mixing ratio so that the dew point is −20 to 10 ° C. Is.

  With reference to FIG. 2, the supply system of the mixed gas and dry gas to the soaking zone 12 will be described. The supply system includes a first pipe 31, a second pipe 32, a third pipe 33, a fourth pipe 34, a fifth pipe 35, and a sixth pipe 36 from the upstream side, and further includes a gas distribution device 40 and a humidifier. 50, a gas mixing device 60, and a drainage device 80.

  The first pipe 31 passes dry gas supplied from a gas supply source (not shown).

  The gas distribution device 40 is connected to the first pipe 31, and the dry gas that has passed through the first pipe 31 is arbitrarily and variable in the following three systems: the second pipe 32, the third pipe 33, and the fourth pipe 34. Distribute with. The second piping 32, the third piping 33, and the fourth piping 34 branch from the gas distribution device 40, and the dry gas distributed to the gas distribution device 40 passes therethrough. That is, a part of the dry gas that has passed through the first pipe 31 is sent to the humidifier 50 through the second pipe 32, and the other part is sent to the gas mixer 60 through the third pipe. The remainder is supplied into the soaking zone 12 through the fourth pipe 34 as it is. The gas distribution device 40 blocks distribution to the second pipe 32 and the third pipe 33 when the mixed gas described later is not used.

  First, supply of dry gas will be described. The dry gas that has passed through the fourth pipe 34 is supplied into the soaking zone 12 via the drying gas supply ports 72A, 72B, 72C, 72D provided in the soaking zone 12. The position and number of the drying gas supply ports are not particularly limited, and may be appropriately determined in consideration of various conditions. However, it is preferable that a plurality of the drying gas supply ports be arranged at the same height position, and it is preferable that the drying gas supply ports are arranged uniformly in the steel strip traveling direction.

  Next, the supply of the mixed gas will be described. The humidifier 50 is connected to the second pipe 32 and the dry gas that has passed through the second pipe 32 is introduced. The fifth pipe 35 extends from the humidifier 50, and the humidified gas humidified by the humidifier 50 passes therethrough.

  The gas mixing device 60 is connected to the third pipe 33 and the fifth pipe 35, and mixes the dry gas that has passed through the third pipe and the humidified gas that has passed through the fifth pipe at a predetermined and variable ratio to obtain a desired value. Prepare a dew point gas mixture. The sixth pipe 36 is a mixed gas pipe, which extends from the gas mixing device 60 and through which the mixed gas discharged from the gas mixing device 60 passes. The mixed gas that has passed through the sixth pipe 36 is supplied into the soaking tropics 12 through a mixed gas supply port provided in the soaking tropics 12. In the present embodiment, the mixed gas is supplied by two systems of mixed gas supply ports 70A, 70B, and 70C and mixed gas supply ports 71A, 71B, and 71C. The position and number of the mixed gas supply ports are not particularly limited, and may be appropriately determined in consideration of various conditions. However, a plurality of mixed gas supply ports are preferably arranged at two or more different height positions as in the present embodiment, and are preferably arranged evenly in the steel strip traveling direction. The dew point of the mixed gas can be measured by a mixed gas dew point meter 74 provided in the sixth pipe.

  Next, with reference to FIG. 3, the structure of the humidification apparatus 50 and the discharge apparatus 80 which is the characteristic structure of this invention is demonstrated. The humidifier 50 has a cylindrical module 52 and a circulating constant temperature water tank 54. A water vapor permeable membrane 51 is disposed in the module 52. In the present embodiment, the water vapor permeable membrane 51 is a fluorine-based or polyimide-based hollow fiber membrane, and although only two are illustrated in FIG. 3, about 50 to 500 hollow fiber membranes are arranged substantially in parallel. Pure water that has been adjusted to a predetermined temperature using a circulating constant temperature water tank 54 on the outer side 53B of the water vapor permeable membrane while the dry gas that has passed through the second pipe 32 passes through the inner side 53A of the water vapor permeable membrane in the module 52. Circulate. That is, the outer side 53B of the water vapor permeable membrane in the module is connected to the circulating constant temperature water tank 54 via the flow paths 55A and 55B.

  A fluorine-based or polyimide-based hollow fiber membrane is a kind of ion exchange membrane having an affinity for water molecules. When a difference in moisture concentration occurs between the inside and outside of the hollow fiber membrane, a force is generated to make the concentration difference uniform, and the moisture permeates through the membrane toward a lower moisture concentration using the force as a driving force. Therefore, the dry gas is humidified in the process of passing through the inner side 53 </ b> A of the water vapor permeable membrane in the module 52 and becomes a humidified gas. The dry gas temperature changes according to the season and daily temperature change, but in this embodiment, heat exchange can also be performed by taking a sufficient contact area between the gas and water through the water vapor permeable membrane 51. Regardless of whether the temperature is higher or lower than the circulating water temperature, the dry gas is humidified to the same dew point as the set water temperature, and highly accurate dew point control is possible. The dew point of the humidified gas can be arbitrarily controlled in the range of 5 to 50 ° C. If the dew point of the humidified gas is higher than the piping temperature, condensation may occur in the piping, and the condensed water may directly enter the furnace.Therefore, the humidifying gas piping should be above the humidifying gas dew point and above the ambient temperature. It is heated and insulated.

  In addition, the structure in the module 52 is not limited to FIG. 3, For example, a water vapor permeable film | membrane may be a fluorine-type or a polyimide-type flat film. In this case, the dry gas that has passed through the second pipe 32 passes through one space that separates the water vapor permeable membrane in the module, and water is circulated in the other space using the circulating thermostatic water tank 54. Humidify the dry gas.

  The continuous hot dip galvanizing apparatus 100 of this embodiment is characterized in that it has a drainage device 80 for draining water from the space 53B outside the water vapor permeable membrane of the module when the mixed gas is not supplied in the soaking zone. . An example of the drainage device 80 is shown in FIG. The drainage device 80 includes a first cutoff valve 82, a second flow path 84, a second cutoff valve 86, and a drainage tank 88. The 1st cutoff valve 82 is provided in the flow path 55B through which the water which moves to the circulation thermostatic water tank 54 from the outer side 53B of the water-vapor-permeable membrane in a module passes. The second flow path 84 branches from a portion upstream of the first shutoff valve 82 of the flow path 55B (near the outer side 53B of the water vapor permeable membrane), and the tip is located above the drainage tank 88. The second shutoff valve 86 is provided in the second flow path 84. The drainage tank 88 stores the water discharged from the second flow path 82.

  When the humidified gas is generated, the first shut-off valve 82 is fully opened, the second shut-off valve B is fully closed, and water is circulated to the outer side 53B of the water vapor permeable membrane in the module using the constant temperature circulating water tank 54. When the humidified gas is not generated, water circulation is stopped, the second shut-off valve 86 is fully opened, and the first shut-off valve 82 is fully closed, so that the water in the space 53B outside the water vapor permeable membrane of the module is drained. Drained into tank 88. When the height of the module 52 cannot be increased by 200 mm or more with respect to the upper end of the drainage tank 88, it is preferable to provide a suction device or the like on the drainage tank side to drain the water in the humidifier.

  For example, when manufacturing a high-strength steel sheet, a mixed gas containing a humidified gas is supplied to the soaking zone 12 in addition to the dry gas. In the present invention, this state is referred to as a “first operation state”. On the other hand, for example, when manufacturing a normal steel plate, only the dry gas is supplied to the soaking zone 12 and the mixed gas is not supplied. In the present invention, this state is referred to as a “second operation state”.

  When the humidified gas is not necessary in the second operation state, the distribution of the dry gas to the second pipe 32 and the humidifier 50 is stopped so that the dry gas does not flow to the inner side 53A of the water vapor permeable membrane in the module. You can do it. However, if the water circulation using the circulation thermostatic bath 54 is continued for a long period of time, the pipes before and after the module 52 (the second pipe 32 and the fifth pipe 35) and the sixth pipe 36 further downstream are arranged. Condensation will occur. Even if the pipe is heated and kept warm, since the moisture is always saturated in the pipe, excessively humidified gas stays there. Even if the circulation of water is stopped, the same problem occurs even if the space of the outer side 53B of the module's water vapor permeable membrane is filled with water and left for a long time.

  Therefore, in the present embodiment, switching between the first operation state and the second operation state is performed as follows. First, in the first operation state, water is circulated using the circulation thermostatic water tank 54 with the first shut-off valve 82 fully opened and the second shut-off valve B is fully closed to generate humidified gas. In the second operation state, the distribution of the dry gas to the second pipe 32 is stopped, and further, the circulation of water using the circulating constant temperature water tank 54 is stopped, and then the water vapor permeable membrane of the module is discharged using the discharge device 80. Water is drained from the space of the outer side 53B. Specifically, the second cutoff valve 86 is fully opened and the first cutoff valve 82 is fully closed. That is, in the second operation state, there is no water in the outer space 53B of the water vapor permeable membrane, and water is not circulated using the circulation thermostatic water tank 54. The temperature adjustment of the circulating constant temperature water tank 54 may be continued.

  As a result, during the second operating state, condensation in the pipes before and after the module 52 (the second pipe 32 and the fifth pipe 35) and further inside the sixth pipe 36 are dewed or excessively humidified. Does not stay. Therefore, when switching from the second operation state to the first operation state next, condensed water and excessively humidified gas are not mixed into the soaking zone 12, and the occurrence of roll pickup in the soaking zone 12 is suppressed. As a result, a good plating appearance can be obtained.

  When switching from the second operation state to the first operation state (for example, when switching from the production of a normal steel plate to the production of a high-tensile steel plate), after resuming the circulation of water using the circulation thermostatic water tank 54, the second pipe Restart the distribution of dry gas to 32.

In the first operation state and the second operation state, the gas flow rate Qrd of the dry gas supplied to the soaking zone 12 through the fourth pipe 34 is determined by a gas flow meter (not shown) provided in the fourth pipe 34. Although it is measured and is not particularly limited, it is set to about 0 to 600 (Nm ³ / hr). As a result, the furnace pressure in the soaking zone 12 is maintained appropriately (higher than the direct fire zone), and an excessive amount does not become the furnace pressure.

In the first operation state, the gas flow rate Qrw of the mixed gas supplied to the soaking zone 12 through the sixth pipe 36 is measured by a gas flow meter (not shown) provided in the sixth pipe 36 and is particularly limited. Although not, it is set to about 100 to 500 (Nm ³ / hr). As a result, the furnace pressure in the soaking zone 12 is appropriately maintained (higher than the direct flame zone), and the furnace pressure does not become excessive.

  In the first operating state, it is preferable to always control the dew point in the soaking zone 12 to -20 ° C or higher and 0 ° C or lower. At least one dew point meter (dew point measurement position 75A) in the vicinity of the lower hearth roll 73B (the lowest part of the soaking tropics) and a position below the upper hearth roll 73A and higher than 1/2 of the soaking zone height direction ( At least one place (dew point measurement position 75B) is installed in the upper part of the soaking zone. When the dew point in the soaking zone 12 is controlled to -20 ° C or higher, an appropriate alloying temperature is obtained during the subsequent alloying treatment, and desired mechanical properties can be obtained. On the other hand, in the soaking zone 12, when the dew point becomes + 10 ° C. or higher, the steel strip iron starts to oxidize. Therefore, the upper limit of the dew point is because the uniformity of the dew point distribution in the soaking zone 12 and the dew point fluctuation range are minimized. It is preferable to manage at 0 ° C.

  By adjusting the gas mixing ratio in the gas mixing device 30, a mixed gas having an arbitrary dew point can be supplied into the soaking zone 12. If the dew point in the soaking zone 12 is below the target range, supply a mixed gas with a high dew point. If the dew point in the soaking zone 12 is above the target range, supply a mixed gas with a low dew point. Can do. In this way, in the first operating state, the dew point in the soaking zone 12 can always be controlled to -20 ° C or higher and 0 ° C or lower.

(Cooling zone)
In the present embodiment, the steel strip P is cooled in the cooling zones 14 and 16. The steel strip P is cooled to about 480 to 530 ° C. in the first cooling zone 14 and is cooled to about 470 to 500 ° C. in the second cooling zone 16.

Although the reducing gas or non-oxidizing gas is also supplied to the cooling zones 14 and 16, only the dry gas is supplied here. The supply of the drying gas to the cooling zones 14 and 16 is not particularly limited, but it is preferable to supply the drying gas from two or more inlets in the height direction and two or more inlets in the longitudinal direction so as to be uniformly introduced into the cooling zone. . The total gas flow rate Qcd of the dry gas supplied to the cooling zones 14 and 16 is measured by a gas flow meter (not shown) provided in the pipe, and is not particularly limited, but is about 200 to 1000 (Nm ³ / hr). And As a result, the furnace pressure in the soaking zone 12 is maintained appropriately (higher than the direct fire zone), and an excessive amount does not become the furnace pressure.

(Hot galvanizing bath)
Using the hot dip galvanizing bath 22, hot dip galvanization can be performed on the steel strip P discharged from the second cooling zone 16. Hot dip galvanization may be performed according to a conventional method.

(Alloying equipment)
Using the alloying equipment 24, the galvanization applied to the steel strip P can be heated and alloyed. The alloying process may be performed according to a conventional method. According to this embodiment, since the alloying temperature does not become high, the tensile strength of the manufactured alloyed hot-dip galvanized steel sheet does not decrease. However, in the present invention, the alloying equipment 24 and the alloying treatment using it are not essential. This is because the effect of suppressing the generation of a soaking zone roll pickup that may be caused by dew condensation on the humidified gas pipe and obtaining a good plating appearance can be obtained even when the alloying treatment is not performed. .

(Experimental conditions)
Using the continuous hot dip galvanizing apparatus shown in FIGS. 1 to 3, the steel strip having the composition shown in Table 1 was annealed under various annealing conditions shown in Table 2, and then hot dip galvanized and alloyed. Steel type A was plain steel and steel type B was high-strength steel. In both the comparative example and the inventive example, annealing, hot dip galvanizing, and alloying treatment were performed continuously in the order of sheet passing shown in Table 2.

  The second heating zone was DFF. The heating burner is divided into four groups (# 1 to # 4), and the three groups (# 1 to # 3) on the upstream side in the steel plate moving direction are oxidation burners, and the final zone (# 4) is a reduction burner. The air ratio of the oxidation burner and the reduction burner was set to the values shown in Table 2. In addition, the length of the steel plate conveyance direction of each group is 4 m.

The soaking zone was an RT furnace with a volume Vr of 700 m ³ . The average temperature Tr in the soaking zone was set as shown in Table 2. As the drying gas, a gas (dew point: −50 ° C.) having a composition composed of 15% by volume of H ₂ and the balance of N ₂ and inevitable impurities was used. A part of this dry gas was humidified by a humidifier having 10 hollow fiber membrane humidification modules to prepare a mixed gas. Each module was supplied with a maximum of 500 L / min of dry gas and a maximum of 10 L / min of circulating water. A circulating water bath is common to each module and can supply a total of 100 L / min of pure water. The dry gas supply port and the mixed gas supply port were arranged at the positions shown in FIG. Moreover, the drainage apparatus shown in FIG. 3 was also installed.

  In both the comparative example and the inventive example, the gas was supplied to the soaking zone with the second operation state during the passing of the steel type A and the first during the passing of the steel type B. The dry gas flow rate Qrd, the mixed gas flow rate Qrw, and the mixed gas dew point in Table 2 are stable values in the respective plates.

  In the comparative example, in the second operation state in which the steel type A was passing through, the supply of the dry gas to the second pipe was stopped, but the water circulation using the circulation thermostatic water tank was continued. In the invention example, in the second operation state in which the steel type A is passing through, the distribution of the dry gas to the second pipe is stopped, and further, the water circulation using the circulation thermostatic water tank is also stopped, and then the discharge device is used. Water was drained from the space outside the water vapor permeable membrane of the module.

  The dry gas (dew point: −50 ° C.) was supplied to the first cooling zone and the second cooling zone from the bottom of each zone at the flow rates shown in Table 2.

The plating bath temperature was 460 ° C., the Al concentration in the plating bath was 0.130%, and the adhesion amount was adjusted to 45 g / m ² per side by gas wiping. The line speed was 80-100 mpm. In addition, after hot dip galvanization, alloying treatment was performed in an induction heating type alloying furnace so that the degree of film alloying (Fe content) was within 10 to 13%. The alloying temperature at that time is shown in Table 2.

(Evaluation method)
Plating appearance is evaluated by optical surface defect meter inspection (detection of non-plating defects and peroxide defects of φ0.5 or more) and visual judgment of alloying unevenness. △ when there was an alloying unevenness, and × if there was any failure. The results are shown in Table 2.

  Moreover, the tensile strength of the galvannealed steel plate manufactured on various conditions was measured. The grade A of ordinary steel passed 270 MPa or higher, and the grade B of high-tensile steel passed 980 MPa or higher. The results are shown in Table 2.

(Evaluation results)
In No. 1 of the comparative example, since the mixed gas was supplied to the passing plate of the steel type B to raise the soaking zone, the alloying temperature did not need to be raised excessively and the tensile strength was not a problem. However, when the supply of humidified gas was started with the second plate, water condensed in the piping was thrown into the soaking zone, resulting in a local high dew point near the hearth roll and roll pick up. In addition, wrinkles due to roll pickup occurred on the surface of the steel strip. Therefore, the plating appearance was impaired for all the second through fourth plates. On the other hand, in No. 2 of the example of the present invention, no condensation occurred in the piping, and the humidified gas could be switched. As a result, all evaluation items passed.

  According to the continuous hot dip galvanizing apparatus and the method for producing a hot dip galvanized steel sheet of the present invention, it is possible to suppress the occurrence of a soaking zone roll pickup that may be caused by condensation in the humidified gas pipe, and to provide a good plating appearance. It is possible to obtain.

100 Continuous hot dip galvanizing equipment 10 Heating zone 10A First heating zone (previous stage)
10B Second heating zone (later, direct-fired heating furnace)
12 Soaking zone 14 First cooling zone (quenching zone)
16 Second cooling zone (cooling zone)
18 Snout 20 Annealing furnace 22 Hot dip galvanizing bath 24 Alloying equipment 31 1st piping 32 2nd piping 33 3rd piping 34 4th piping 35 5th piping 36 6th piping 40 Gas distribution device 50 Humidifier 51 Water vapor permeable membrane 52 Module 53A Inside the water vapor permeable membrane (one space)
53B Outside the water vapor permeable membrane (the other space)
54 Circulating constant temperature water tank 55A, 55B Flow path 60 Gas mixing device 70A, 70B, 70C Mixed gas supply port 71A, 71B, 71C Mixed gas supply port 72A, 72B, 72C, 72D Dry gas supply port 73A Upper hearth roll 73B Lower hearth roll 74 Dew point meter for mixed gas 75A, 75B Dew point measurement position 80 Drain device 82 First shut-off valve 84 Second flow path 86 Second shut-off valve 88 Drain tank P Steel strip

Claims (6)

A continuous hot dip galvanizing apparatus having an annealing furnace in which a heating zone, a soaking zone, and a cooling zone are juxtaposed in this order, and a hot dip galvanizing facility adjacent to the cooling zone,
A first pipe through which a reducing or non-oxidizing drying gas passes;
A gas distributor connected to the first pipe for distributing the dry gas that has passed through the first pipe;
A second pipe, a third pipe, and a fourth pipe branched from the gas distributor and through which the dry gas distributed to the gas distributor passes;
A humidifier connected to the second pipe and into which the dry gas that has passed through the second pipe is introduced;
A fifth pipe extending from the humidifier and through which humidified gas humidified by the humidifier passes;
A gas mixing device that is connected to the third pipe and the fifth pipe and mixes the dry gas that has passed through the third pipe and the humidified gas that has passed through the fifth pipe to produce a mixed gas;
A sixth pipe extending from the gas mixing device and through which the mixed gas passes;
A mixed gas supply port provided in the soaking zone for supplying the mixed gas that has passed through the sixth pipe into the soaking zone;
A dry gas supply port provided in the soaking zone for supplying dry gas that has passed through the fourth pipe into the soaking zone;
Have
The humidifier has a module including a water vapor permeable membrane, and a dry gas passing through the second pipe passes through one space between the water vapor permeable membranes in the module, while passing through the other space. Is configured to humidify the dry gas by circulating water using a circulating constant temperature water bath,
The continuous galvanizing apparatus further comprising a drainage device for draining water from the other space of the module when the mixed gas is not supplied to the soaking zone.   The continuous hot dip galvanizing apparatus according to claim 1, further comprising an alloying equipment adjacent to the hot dip galvanizing equipment. A method for producing a hot dip galvanized steel sheet using the continuous hot dip galvanizing apparatus according to claim 1,
Conveying the steel strip in the annealing furnace in the order of the heating zone, the soaking zone, and the cooling zone, and annealing the steel strip; and
Using the hot dip galvanizing equipment, applying hot dip galvanizing to the steel strip discharged from the cooling zone;
Have
In the first operation state in which the mixed gas and the dry gas are supplied to the soaking zone, the water is circulated using the circulation thermostatic bath,
In the second operation state in which only the dry gas is supplied to the soaking zone and the mixed gas is not supplied, the distribution of the dry gas to the second pipe is stopped and the module is used to stop the distribution of the dry gas. A method for producing a hot-dip galvanized steel sheet, wherein water is drained from the other space and water is not circulated using the circulating thermostatic bath.   4. When switching from the second operation state to the first operation state, the circulation of water using the circulation thermostatic bath is resumed, and then the distribution of the dry gas to the second pipe is resumed. The manufacturing method of the hot-dip galvanized steel sheet as described in 2.   The manufacturing method of the hot dip galvanized steel sheet according to claim 3 or 4 which controls the dew point in said soaking zone to -20 ° C or more and 0 ° C or less in said 1st operation state.   The hot-dip galvanized steel sheet according to any one of claims 3 to 5, further comprising a step of heat-alloying the galvanization applied to the steel strip using the alloying equipment according to claim 2. Manufacturing method.

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