KR102026708B1 - Continuous hot-dip galvanizing apparatus and method of producing hot-dip galvanized steel sheet - Google Patents

Abstract

The present invention provides a continuous hot dip galvanizing apparatus capable of suppressing the occurrence of roll pick-up of cracks that may occur due to condensation in a humidification gas pipe and the like, and obtaining a good plating appearance. The continuous hot dip galvanizing apparatus of the present invention has a heating zone, a cracking zone, an annealing furnace in which cooling zones are arranged in this order, and a hot dip galvanization facility adjacent to the cooling zone. The gas supplied to the cracks 12 is a mixed gas obtained by mixing a gas humidified by the humidifier 50 with a gas not humidified by the humidifier, and a dry gas that is not humidified by the humidifier. to be. The continuous hot dip galvanizing apparatus of the present invention has a drainage apparatus 80 for draining the circulating water from the humidifying apparatus 50 when the mixed gas is not supplied to the cracks 12.

Description

CONTINUOUS HOT-DIP GALVANIZING APPARATUS AND METHOD OF PRODUCING HOT-DIP GALVANIZED STEEL SHEET}

The present invention provides a continuous hot dip galvanizing apparatus having an annealing furnace in which a heating zone, a soaking zone and a cooling zone are arranged in this order, and a hot dip galvanizing apparatus adjacent to the cooling zone, and melting using the same. A method for producing a galvanized steel sheet.

In recent years, in the fields of automobiles, home appliances, building materials, and the like, the demand for high tensile steel sheets (high tensile steel sheets), which contributes to weight reduction of structures, has increased. As a high-tensile steel, for example, a steel sheet having good hole expansion formability by containing Si in the steel, or a steel sheet which is easy to form residual γ and has good ductility by containing Si or Al can be produced. Know.

However, when the alloyed hot dip galvanized steel sheet is produced using a high tensile strength steel sheet containing a large amount of Si (particularly 0.2% by mass or more) as a base material, there are the following problems. The alloyed hot-dip galvanized steel sheet is subjected to hot-dip galvanizing treatment on the steel sheet after heat-annealing the steel sheet of the base material at a temperature of about 600 to 900 ° C in a reducing atmosphere or a non-oxidizing atmosphere. It is produced by heat alloying zinc plating.

Here, Si in steel is a oxidizable element, is selectively oxidized in a reducing atmosphere or a non-oxidizing atmosphere which is generally used, and is concentrated on the surface of the steel sheet to form an oxide. This oxide lowers the wettability with molten zinc at the time of a plating process, and produces fire plating. Therefore, with increasing Si concentration in steel, wettability falls rapidly and many unplatings generate | occur | produce. Moreover, even if it does not lead to unplating, there exists a problem that plating adhesion is inferior. Moreover, when Si in steel is selectively oxidized and it concentrates on the surface of a steel plate, there exists also a problem that a remarkable alloying delay occurs in the alloying process after hot dip galvanizing, and it inhibits productivity significantly.

For this problem, for example, Patent Document 1 discloses an annealing furnace having a heating stage upstream, a downstream stage of a heating stage, a heating stage, and a cooling stage and a continuous annealing hot dip plating method using a hot dip bath. Indirect heating or heating of the steel sheet in the region where the steel sheet temperature is at least 300 ° C is used, and the atmosphere in each furnace is set to an atmosphere of 1 to 10% by volume of hydrogen and the balance of nitrogen and inevitable impurities. The temperature reached to the steel sheet being heated at the front of the heating table is set to 550 ° C. or more and 750 ° C. or less, and the dew point is less than −25 ° C., and the dew point of the heating table subsequent to the heating table and the dew point of the heating table are −30 ° C. or more. By annealing under the condition of 0 ° C or less and the dew point of the cooling stand below -25 ° C, the internal oxidation of Si is suppressed to suppress the concentration of Si on the surface of the steel sheet. The base material is described. It is also described to humidify and introduce a mixed gas of nitrogen and hydrogen into the rear stage of the heating table and / or the heating table.

International Publication No. 2007/043273

When manufacturing a high tensile strength steel plate, in order to raise the dew point in a crack zone, in addition to reducing or non-oxidizing dry gas, humidification gas is thrown in as a crack as described in patent document 1. On the other hand, when manufacturing the steel plate of normal strength (henceforth a "normal steel plate"), a humidifying gas is not thrown in and only a reducing or non-oxidizing dry gas is thrown in as a crack. Therefore, when manufacturing a high tension steel plate and a normal steel plate continuously, for example, it is necessary to operate, switching the use / nonuse of a humidification gas.

The present inventors have recognized the following problems that occur when operating while switching the use / nonuse of such humidifying gas. That is, even when the humidification gas is not used, even if the gas of the humidification system is simply stopped, water from the humidifier diffuses into condensation in the piping of the humidification system, or excess humidified gas remains. Then, when the humidification system is switched from non-use to use, condensed water or excess humidified gas in the pipe is blown into the crack, damaging the hearth roll in the crack and pick-up. ), Or a water drop pattern occurs in the steel sheet. Due to this, unplating may occur in the subsequent hot dip galvanizing process, and plating appearance may be damaged.

Therefore, in view of the above problem, the present invention provides continuous hot dip galvanizing which can suppress the occurrence of roll pick-up of cracks that may occur due to condensation or the like in the humidifying gas piping and obtain a good plating appearance. It is an object to provide an apparatus and a method for producing a hot dip galvanized steel sheet.

MEANS TO SOLVE THE PROBLEM In order to solve the said subject, the present inventors do not let dew condensate generate | occur | produce in the piping for humidification gas, or do not stay in the humidified gas excessively when the humidification gas is not used (while supplying the humidification gas to a crack is stopped). The present invention was completed, and the present invention was completed.

The summary structure of this invention is as follows.

(1) A continuous hot dip galvanizing apparatus having an annealing furnace in which a heating stand, a crack stand, a cooling stand is arranged in this order, and a hot dip galvanizing facility adjacent to the cooling stand,

A first pipe through which the reducing or non-oxidizing dry gas passes,

A gas distribution device connected to the first pipe and configured to distribute dry gas passing through the first pipe;

2nd piping, 3rd piping, and 4th piping which branched from the said gas distribution apparatus, and which the dry gas distribute | distributed to the said gas distribution apparatus passes,

A humidifying device 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 a humidifying gas humidified by the humidifier passes;

A gas mixing device connected to the third pipe and the fifth pipe, for mixing a dry gas having passed through the third pipe and the humidifying gas having 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 formed in the crack zone for supplying the mixed gas passing through the sixth pipe into the crack zone;

Dry gas supply holes formed in the cracks for supplying the dry gas passing through the fourth pipe into the cracks

With

The humidifier has a module including a water vapor permeable membrane, and while the dry gas passing through the second pipe passes through one of the spaces between the water vapor permeable membranes in the module, the constant temperature is circulated in the other space. Configured to humidify the dry gas by circulating water using a water bath,

And a drainage device for draining water from the other space of the module when the mixed gas is not supplied to the crack zone.

(2) The continuous hot dip galvanizing apparatus according to (1), having an alloying installation adjacent to the hot dip galvanizing facility.

(3) As a manufacturing method of a hot-dip galvanized steel plate using the continuous hot-dip galvanizing apparatus as described in said (1),

A step of conveying a steel strip in the order of the heating table, the cracking zone, and the cooling zone in the interior of the annealing furnace, and annealing the steel strip;

Process of performing hot dip galvanizing on the steel strip discharged from the said cooling stand using the said hot dip galvanizing installation

With

In a first operation state in which the mixed gas and the dry gas are supplied to the crack zone, water is circulated using the circulating constant temperature water tank,

In the second operation state in which only the dry gas is supplied to the crack zone and the mixed gas is not supplied, distribution of the dry gas to the second pipe is stopped, and the drainage device is used to 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 constant temperature water bath.

(4) When switching from the second operation state to the first operation state, after the circulation of water using the circulation constant temperature water tank is resumed, the distribution of the dry gas to the second piping is resumed (3). The manufacturing method of the hot dip galvanized steel plate of description).

(5) The manufacturing method of the hot-dip galvanized steel sheet as described in said (3) or (4) which controls the dew point in the said crack zone to -20 degreeC or more and 0 degrees C or less in the said 1st operation state.

(6) Hot-dip galvanizing as described in any one of said (3)-(5) which further has the process of carrying out the heat alloying of the zinc plating performed on the said steel strip using the said alloying equipment of said (2). Method of manufacturing steel sheet.

According to the continuous hot dip galvanizing apparatus and the hot dip galvanized steel sheet production method of the present invention, it is possible to suppress the occurrence of roll pickup of cracks that may occur due to condensation or the like in the piping for humidifying gas, and obtain a good plating appearance.

1 is a schematic view showing the configuration of a continuous hot dip galvanizing apparatus 100 according to an embodiment of the present invention.
FIG. 2 is a schematic diagram showing a supply system of a mixed gas and a dry gas to the crack zone 12 in FIG. 1.
3 is an enlarged schematic view of the humidifying device 50 and the drainage device 80 in FIG. 2.

(Form to carry out invention)

The structure of the continuous hot dip galvanizing apparatus 100 which concerns on one Embodiment of this invention is demonstrated with reference to FIG. The continuous hot dip galvanizing apparatus 100 includes an annealing furnace 20 in which the heating table 10, the cracking table 12, and the cooling tables 14 and 16 are arranged in this order, and the molten metal adjacent to the cooling table 16. The hot dip galvanizing bath 22 as a galvanizing installation, and the alloying installation 24 adjacent to this hot dip galvanizing bath 22 are provided. In the present embodiment, the heating table 10 includes a first heating table 10A (a heating table front end) and a second heating table 10B (a heating table rear end). The cooling stand includes a first cooling stand 14 (quench zone) and a second cooling stand 16 (slow cooling zone). In the snout 18 connected to the 2nd cooling zone 16, the tip is immersed in the 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 introduce | transduced into 10 A of 1st heating tables from the steel strip introduction port below the 1st heating table 10A. At each stage 10, 12, 14, 16, one or more hearth rolls are arranged at the top and the bottom. When the steel strip P is folded back 180 degrees starting from a hearth roll, the steel strip P is conveyed in the up-down direction several times inside the predetermined | prescribed stand of the annealing furnace 20, and forms several path | pass. In FIG. 1, an example of 10 passes in the crack zone 12, 2 passes in the first cooling zone 14, and 2 passes in the second cooling zone 16 is illustrated, but the number of passes is not limited thereto. It can set suitably according to conditions. Moreover, in some hearth rolls, the steel strip P is orientated at right angles without returning, and the steel strip P is moved as follows. In this way, the steel strip P is conveyed in the order of the heating table 10, the crack table 12, and the cooling tables 14 and 16 in the inside of the annealing furnace 20, and annealing is performed with respect to the steel strip P. I can do it.

In the annealing furnace 20, adjacent zones communicate with each other via a communication section connecting the upper or lower portions of the respective zones. In the present embodiment, the first heating table 10A and the second heating table 10B communicate with each other via a throat (tightening portion) connecting the upper portions of the respective tables. The 2nd heating stand 10B and the crack stand 12 communicate with each other through the throat which connects the lower parts of each stand. The cracks 12 and the 1st cooling stand 14 communicate with each other through the throat which connects the lower parts of each stand. The 1st cooling stand 14 and the 2nd cooling stand 16 communicate with each other through the throat which connects the lower parts of each stand. Although the height of each throat may be set suitably, it is preferable that the height of each throat is as low as possible from a viewpoint of improving the independence of each atmosphere. The gas in the annealing furnace 20 flows downstream from the furnace upstream, and is discharged from the steel strip introduction port below the first heating table 10A.

(Heating stand)

In the present embodiment, the second heating table 10B is a direct heating furnace DFF. DFF can use a well-known thing. Although not shown in FIG. 1, a plurality of burners are disposed to face the steel strip P on the inner wall of the direct heating furnace in the second heating table 10B. It is preferable that the plurality of burners are divided into a plurality of groups, and the fuel rate and the air ratio can be independently controlled for each group. The combustion exhaust gas of the 2nd heating table 10B is supplied inside 10 A of 1st heating tables, 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 of the burner at the maximum combustion load. The burn rate is 100% when the burner is burned at the maximum combustion load. The burner cannot obtain a stable combustion state when the combustion load is low. Therefore, it is preferable to make combustion rate into 30% or more normally.

The air ratio is a value obtained by dividing the amount of air introduced into the actual burner by the amount of air required to completely burn the fuel gas. In the present embodiment, the burner for heating of the second heating table 10B is divided into four groups # 1 to # 4, and the three groups # 1 to # 3 on the upstream side of the steel plate moving direction are burners for oxidation. The final zone # 4 was a burner for reduction, and the air ratios of the burner for oxidation and the burner for reduction were individually controllable. In the burner for oxidation, it is preferable to make air ratio 0.95 or more and 1.5 or less. In the reduction burner, it is preferable to make air ratio into 0.5 or more and less than 0.95. In addition, it is preferable that the temperature inside the 2nd heating table 10B shall be 800-1200 degreeC.

(Crack stand)

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

The crack zone 12 is supplied with a reducing gas or a non-oxidizing gas. As the reducing gas, a H ₂ -N ₂ mixed gas is usually used. For example, a gas having a composition composed of H ₂ : 1-20% by volume and the balance of N ₂ and unavoidable impurities (dew point: around -60 ° C) Can be mentioned. Further, as the non-oxidizing gas, a gas having a composition consisting of N ₂ and inevitable impurities may be mentioned (dew point about -60 ℃).

In the present embodiment, the reducing gas or the non-oxidizing gas supplied to the crack 12 is two types of mixed gas and dry gas. Here, a "dry gas" is a reducing point or a 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, "mixed gas" is obtained by mixing the gas humidified by the humidifier and the gas not humidified by the humidifier at a predetermined mixing ratio such that the dew point is -20 to 10 ° C.

With reference to FIG. 2, the supply system of the mixed gas and dry gas to the crack 12 is demonstrated. The supply system connects the first pipe 31, the second pipe 32, the third pipe 33, the fourth pipe 34, the fifth pipe 35, and the sixth pipe 36 from the upstream side. It further has the gas distribution apparatus 40, the humidification apparatus 50, the gas mixing apparatus 60, and the drainage apparatus 80.

The dry gas supplied from the gas supply source which is not shown in figure passes through the 1st piping 31.

The gas distribution device 40 is connected to the 1st piping 31, and the following 2nd piping 32, 3rd piping 33, and 4th piping which carry out the dry gas which passed the inside of the 1st piping 31 are as follows. The three systems in (34) may be arbitrarily distributed at a variable rate. The 2nd piping 32, the 3rd piping 33, and the 4th piping 34 branch off from the gas distribution apparatus 40, and the dry gas distribute | distributed to the gas distribution apparatus 40 passes. That is, a part of the dry gas which passed the inside of the 1st piping 31 passes to the humidifier 50 through the 2nd piping 32, and the other part passes through the 3rd piping, and the gas mixing apparatus 60 is carried out. ), The remainder passes through the fourth pipe 34 and is supplied into the crack stage 12 as it is. The gas distribution device 40 interrupts the distribution to the second pipe 32 and the third pipe 33 when the mixed gas, which will be described later, is not used.

First, the supply of dry gas is demonstrated. The dry gas which passed the 4th piping 34 is supplied into the crack 12 through the dry gas supply ports 72A, 72B, 72C, 72D formed in the crack 12. As shown in FIG. The position and number of the dry gas supply port are not particularly limited, and may be appropriately determined in consideration of various conditions. However, it is preferable to arrange | position a plurality of dry gas supply ports in the same height position, and it is preferable to be arrange | positioned evenly in the strip moving direction.

Next, the supply of the mixed gas will be described. The humidifier 50 is connected to the 2nd piping 32, and the dry gas which passed through the 2nd piping 32 is introduce | transduced. The 5th piping 35 extends from the humidifier 50 and the humidifying gas humidified by the humidifier 50 passes.

The gas mixing device 60 is connected to the third pipe 33 and the fifth pipe 35, and mixes the dry gas passing through the third pipe and the humidifying gas passing through the fifth pipe at a predetermined and variable ratio. Thus, a mixed gas of a desired dew point is prepared. The sixth pipe 36 is a pipe for the mixed gas, extends from the gas mixing device 60, and the mixed gas from the gas mixing device 60 passes therethrough. The mixed gas having passed through the sixth pipe 36 is supplied into the crack zone 12 through the mixed gas supply port formed in the crack zone 12. In this embodiment, the mixed gas is supplied to two systems of the mixed gas supply ports 70A, 70B, and 70C and the 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, it is preferable that a plurality of mixed gas supply ports are arranged at two or more different height positions, as in the present embodiment, and are evenly arranged in the moving direction of the strip. The dew point of the mixed gas can be measured by the mixed gas dew point 74 formed in the sixth pipe.

Next, with reference to FIG. 3, the structure of the humidification apparatus 50 and the drainage apparatus 80 which is the characteristic structure of this invention is demonstrated. The humidifier 50 has a tubular module 52 and a circulating constant temperature water tank 54. The 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. Although only two are shown in Fig. 3, about 50 to 500 hollow fiber membranes are arranged substantially in parallel. . While the dry gas passing through the inner side 53A of the water vapor permeable membrane in the module 52 passes through the second pipe 32, the outer side 53B of the water vapor permeable membrane is circulated at a predetermined temperature by using a circulating constant temperature water tank 54. Circulate adjusted pure water. 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 through the flow paths 55A and 55B.

A fluorine-based or polyimide-based hollow fiber membrane is a kind of ion exchange membrane having affinity with water molecules. When the difference in moisture concentration occurs inside and outside the hollow fiber membrane, a force is generated to equalize the difference in concentration, and the moisture moves through the membrane toward the lower moisture concentration as a driving force. Therefore, the drying gas is humidified in the process of passing through the inner side 53A of the water vapor permeable membrane in the module 52 to become a humidifying gas. The dry gas temperature changes depending on the season and the daily temperature change. However, in the present embodiment, since the heat exchange can also be performed by sufficiently taking the contact area between the gas and the water through the water vapor permeable membrane 51, the dry gas temperature Even if the gas is higher than or lower than the circulating water temperature, the dry gas is a gas humidified to the same dew point as the set water temperature, so that high-precision dew point control becomes possible. The dew point of a humidifying gas can be arbitrarily controlled in the range of 5-50 degreeC. If the dew point of the humidifying gas is higher than the piping temperature, condensation may occur in the piping, and condensed water may invade the furnace directly. Therefore, the piping for the humidifying gas is heated and heated above the dew point of the humidifying gas and above the external air temperature. It is kept warm.

In addition, the structure in the module 52 is not limited to FIG. 3, For example, a vapor permeable membrane may be a fluorine-type or polyimide-type flat membrane. In that case, while the dry gas which passed the inside of the 2nd piping 32 passed through the one space which interposed the vapor permeable membrane in a module, the other space will circulate water using the circulation constant temperature water tank 54, , Humidify the dry gas.

The continuous hot dip galvanizing apparatus 100 of the present embodiment has a drainage device 80 for draining water from the space of the outer side 53B of the vapor permeable membrane of the module when the mixed gas is not supplied to the crack. It is characteristic. An example of the drainage apparatus 80 is shown in FIG. The drainage device 80 includes a first shutoff valve 82, a second flow path 84, a second shutoff valve 86, and a drainage tank 88. The 1st shut-off valve 82 is provided in the flow path 55B which the water which moves to the circulation constant temperature water tank 54 from the outer side 53B of the water vapor permeable membrane in a module passes. The second flow path 84 branches at 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 thereof is located above the drain tank 88. . The second shut-off valve 86 is provided in the second flow path 84. The drain tank 88 accommodates water discharged from the second flow path 82.

When generating a humidification gas, the 1st shut-off valve 82 is expanded, the 2nd shut-off valve 86 is fully closed, and the water vapor permeation | transmission in a module is used using the constant temperature circulation tank 54. As shown in FIG. The water is circulated to the outer side 53B of the membrane. When no humidification gas is generated, the circulation of water is stopped, the second shut-off valve 86 is opened, and the first shut-off valve 82 is fully closed, so that the space of the outer 53B of the water vapor permeable membrane of the module is removed. Water is drained to the drain 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 drain tank 88, it is preferable to install a suction device or the like on the drain tank side to drain the water in the humidifier.

For example, at the time of manufacture of a high tensile strength steel plate, the mixed gas containing a humidification gas is supplied to the crack 12 in addition to a dry gas. In this invention, this state is called "1st operating state." On the other hand, for example, in the case of manufacturing a normal steel sheet, only dry gas is supplied to the crack zone 12, and mixed gas is not supplied. In this invention, this state is called "2nd operation state."

In the second operation state, when the humidification gas is not needed, 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 inside 53A of the water vapor permeable membrane in the module. Do it. However, if left for a long period of time while circulating the water using the circulating constant temperature water tank 54, the inside (2nd piping 32 and the fifth piping 35) before and after the module 52, and further downstream The sixth pipe 36 I condensation. Even if the piping is heated and moisturized, the excess humidified gas is retained because the moisture in the piping is always saturated. In addition, even if the circulation of water is stopped, the same problem occurs even if the water is left in a state in which the space on the outer side 53B of the module's water vapor permeable membrane is filled for a long time.

Therefore, in this embodiment, switching of a 1st operating state / a 2nd operating state is performed as follows. First, in a 1st operating state, the 1st shut-off valve 82 is developed and the 2nd shut-off valve 86 is fully closed, the water is circulated using the circulating constant temperature water tank 54, and a humidification gas is produced | generated. . In the second operation state, the distribution of the dry gas to the second pipe 32 is stopped, and the circulation of the water using the circulating constant temperature water tank 54 is further stopped, and then the module is discharged using the drainage device 80. Water is drained from the space on the outer side 53B of the water vapor permeable membrane. Specifically, the second shutoff valve 86 is deployed, and the first shutoff valve 82 is fully closed. That is, in the second operating state, the space of the outer side 53B of the water vapor permeable membrane is in a state without water, and the water circulation using the circulating constant temperature water tank 54 is not performed. In addition, temperature adjustment of the circulating constant temperature water tank 54 may be continued.

Thereby, inside the piping before and after the module 52 (the 2nd piping 32 and the 5th piping 35) between the 2nd operation states, and inside the 6th piping 36 downstream, Excessive humidified gas may or may not stay. Therefore, when switching from the second operation state to the first operation state next, condensed water and excessively humidified gas do not enter the crack zone 12, and thus the roll pick-up of the crack zone 12 is performed. It can suppress generation | occurrence | production, and as a result, favorable 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 the steel sheet to the production of the high tensile steel sheet), the circulation of the water using the circulation constant temperature water tank 54 is resumed. Distribution of dry gas to 2 piping 32 is resumed.

In the first operating state and the second operating state, the gas flow rate Qrd of the dry gas supplied to the cracks 12 through the fourth pipe 34 is a gas flow meter (shown in the fourth pipe 34). is measured by a not) is not particularly limited, but is about 0~600 (Nm ³ / hr). As a result, the pressure in the crack zone 12 is properly maintained (higher than that of the weaving zone), and the excessive pressure is not caused.

In the first operating state, the gas flow rate Qrw of the mixed gas supplied to the crack 12 through the sixth pipe 36 is controlled by a gas flow meter (not shown) provided in the sixth pipe 36. measurement is not particularly limited, and about 100~500 (Nm ³ / hr). As a result, the pressure in the crack zone 12 is properly maintained (higher than that of the weaving zone), and the excessive pressure is not caused.

Moreover, in the 1st operating state, it is preferable to always control the dew point in the crack 12 to -20 degreeC or more and 0 degrees C or less. The dew point system is higher than the height direction 1/2 of the crack stage in at least one place (the dew point measurement position 75A) in the vicinity of the lower hearth roll 73B (lowest point of the cracking stand) and below the upper hearth roll 73A. At least one position (dew point measurement position 75B) is provided at the position (upper part of the cracker). When the dew point in the crack zone 12 is controlled to be -20 ° C or higher, an appropriate alloying temperature is obtained at the time of subsequent alloying treatment, and desired mechanical properties can be obtained. On the other hand, in the crack zone 12, when the dew point reaches + 10 ° C or more, the steel bar iron begins to oxidize, so the upper limit of the dew point is minimized for reasons of minimizing the uniformity of the dew point distribution and the dew point fluctuation range in the crack zone 12. It is preferable to manage at 0 degreeC.

When the mixing ratio of the gas in the gas mixing device 30 is adjusted, the mixed gas of arbitrary dew point can be supplied into the crack 12. When the dew point in the crack zone 12 is below the target range, a mixed gas having a high dew point is supplied. When the dew point in the crack zone 12 exceeds the target range, a mixed gas having a low dew point can be supplied. In this way, in the first operating state, the dew point in the crack zone 12 can always be controlled to be -20 ° C or more and 0 ° C or less.

(Cooling stand)

In the present embodiment, the steel strips P are cooled in the cooling tables 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.

The reducing gas or the non-oxidizing gas is also supplied to the cooling zones 14 and 16, but only the drying gas is supplied here. Although supply of the dry gas to the cooling stand 14 and 16 is not specifically limited, It is preferable to supply from two or more height directions and two or more length openings so that it may be thrown in evenly into a cooling stand. 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 200 to 1000 (Nm ³ / hr) It is enough. As a result, the pressure in the crack zone 12 is properly maintained (higher than that of the weaving zone), and the excessive pressure is not caused.

(Molten Zinc Plating Bath)

The hot dip galvanizing can be performed to the steel strip P discharged | emitted from the 2nd cooling stand 16 using the hot dip galvanizing bath 22. FIG. The hot dip galvanizing may be performed in accordance with a conventional method.

(Alloying equipment)

By using the alloying facility 24, the zinc plating applied to the steel strip P can be heat-alloyed. The alloying treatment may be performed in accordance with the regular method. According to this embodiment, since alloying temperature does not become high temperature, the tensile strength of the manufactured galvanized steel plate does not fall. However, in this invention, the alloying installation 24 and the alloying process by it are not essential. This is because the effect of suppressing the occurrence of roll pick-up of the cracking band which may be caused due to condensation of the piping for humidifying gas and obtaining a good plating appearance can be obtained even when no alloying treatment is performed.

Example

(Experimental conditions)

Using the continuous hot dip galvanizing apparatus shown in FIGS. 1-3, the steel strip of the component composition shown in Table 1 was annealed under the various annealing conditions shown in Table 2, and the hot dip galvanization and alloying process were performed after that. Steel grade A is a normal steel, and steel grade B is a high tensile strength steel, and both the comparative examples and the invention examples performed annealing, hot dip galvanization, and alloying process continuously in the mail order shown in Table 2.

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

The crack was set to RT furnace having a volume Vr of 700 m 3. The average temperature Tr inside the crack was set as shown in Table 2. As the drying gas, gas (dew point: -50 ℃) having a composition comprising the balance of N ₂ and unavoidable impurities as H ₂ of 15% by volume it was used. A part of this dry gas was humidified with the humidifier which has ten hollow fiber membrane type humidification modules, and the mixed gas was prepared. Each module was made to flow up to 500 L / min of dry gas and up to 10 L / min of circulating water. The circulating constant temperature water tank is common in each module, and can supply pure water of 100 L / min in total. The dry gas supply port and the mixed gas supply port were disposed at the position shown in FIG. 2. Moreover, the drainage apparatus shown in FIG. 3 was also provided.

In both the comparative example and the invention example, in the mail order of the steel grade A, it was made into the 2nd operation state, and in the mail order of the steel grade B, it was set as the 1st operation state, and gas was supplied to the cracking zone. The dry gas flow rate Qrd, the mixed gas flow rate Qrw, and the mixed gas dew point of Table 2 are stable values in each board | plate.

In the comparative example, although the supply of the dry gas to the 2nd piping was stopped in the 2nd operation state in steel plate A in the board | plate, the circulation of water using the circulation constant temperature water tank was continued. In the invention example, the distribution of the dry gas to the 2nd piping in the 2nd operation state in the board | substrate is stopped, and also the circulation of the water using a circulation constant temperature water tank is stopped, and the drainage apparatus is used of the module. Water was drained from the space outside the water vapor permeable membrane.

The dry gas (dew point: -50 degreeC) was supplied to the 1st cooling stand and the 2nd cooling stand at the flow volume shown in Table 2 from the lowest part of each stand.

Plating bath temperature was 460 degreeC, 0.130% of Al concentration in a plating bath, and the adhesion amount was adjusted to 45 g / m <2> per side by gas wiping. In addition, the line speed was 80-100 mpm. In addition, after performing hot dip galvanizing, the alloying process was performed in the induction heating type alloying furnace so that the film alloying degree (Fe content rate) may be 10 to 13%. The alloying temperature at that time is shown in Table 2.

(Assessment Methods)

The evaluation of the appearance of the plating is performed by inspection by an optical surface defect meter (detecting φ0.5 or more of non-plating defects or peroxidation defects) and visualized alloying nonuniformity, and all items passed the pass. When there was a problem, △, even if one failed, it was made x. The results are shown in Table 2.

In addition, the tensile strength of the alloyed hot dip galvanized steel sheet produced under various conditions was measured. The steel grade A of the ordinary steel was 270 MPa or more, and the steel grade B of the high tensile steel was 980 MPa or more. The results are shown in Table 2.

(Evaluation results)

In No. 1 of the comparative example, since the mixed gas was supplied to raise the crack zone dew point in the sheet of steel grade B, the alloying temperature did not have to be increased excessively, and tensile strength was not a problem. However, when the humidification gas is supplied to the second plate, moisture condensation in the pipe is introduced into the cracking zone, which causes a high dew point locally in the vicinity of the hearth roll, resulting in roll pick-up, and rolls on the steel strip surface. Scratches due to pickup have occurred. Therefore, the plating appearance was damaged to the 2nd to 4th mail order. On the other hand, in No. 2 of the example of this invention, dew condensation did not generate | occur | produce in piping and switching of humidification gas was possible. As a result, all evaluation items passed.

(Industrial availability)

According to the continuous hot dip galvanizing apparatus and the hot dip galvanized steel sheet production method of the present invention, it is possible to suppress the occurrence of roll pickup of cracks that may occur due to condensation or the like in the piping for humidifying gas, and obtain a good plating appearance.

100: continuous hot dip galvanizing device
10: heating stand
10A: 1st heating table (shear)
10B: 2nd heating table (rear end, direct type heating furnace)
12: cracking band
14: 1st cooling stand (quench cooling stand)
16: 2nd cooling stand (defroster)
18: snout
20 annealing furnace
22: hot dip galvanizing bath
24: alloying equipment
31: the first pipe
32: second pipe
33: third pipe
34: fourth pipe
35: fifth pipe
36: sixth piping
40: gas distribution device
50: humidifier
51: water vapor transmission membrane
52: module
53A: Inner side of the water vapor permeable membrane
53B: Outside of water vapor permeable membrane (space on the other side)
54: circulating constant temperature water tank
55A, 55B: Euro
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 part hearth roll
73B: lower part hearth roll
74: dew point meter for mixed gas
75A, 75B: Dew point measurement position
80: drainage
82: first shutoff valve
84: the second euro
86: second shutoff valve
88: drainage tank
P: steel strip

Claims (7)

A continuous hot dip galvanizing apparatus having an annealing furnace in which a heating stage, a crack stage, a cooling stage is arranged in this order, and a hot dip galvanizing facility adjacent to the cooling stage,
A first pipe through which a reducing or non-oxidizing dry gas passes,
A gas distribution device connected to the first pipe and configured to distribute dry gas passing through the first pipe;
2nd piping, 3rd piping, and 4th piping which branched from the said gas distribution apparatus, and which the dry gas distribute | distributed to the said gas distribution apparatus passes,
A humidifying device 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 a humidifying gas humidified by the humidifier passes;
A gas mixing device connected to the third pipe and the fifth pipe, for mixing a dry gas having passed through the third pipe and the humidifying gas having 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 formed in the crack zone for supplying the mixed gas passing through the sixth pipe into the crack zone;
Dry gas supply holes formed in the cracks for supplying the dry gas passing through the fourth pipe into the cracks.
With
The humidifier has a module including a water vapor permeable membrane, and while the dry gas passing through the second pipe passes through one of the spaces between the water vapor permeable membranes in the module, the constant temperature is circulated in the other space. Configured to humidify the dry gas by circulating water using a water bath,
And a drainage device for draining water from the other space of the module when the mixed gas is not supplied to the crack zone. The method of claim 1,
And a continuous hot dip galvanizing apparatus having an alloying facility adjacent to said hot dip galvanizing facility. As a method for producing a hot dip galvanized steel sheet using the continuous hot dip galvanizing apparatus according to claim 1,
Conveying a steel strip in the inside of the annealing furnace in the order of the heating table, the cracking table, and the cooling table, and performing annealing on the steel sheet;
Process of performing hot dip galvanizing on the steel strip discharged from the said cooling stand using the said hot dip galvanizing installation
With
In a first operation state in which the mixed gas and the dry gas are supplied to the crack zone, water is circulated using the circulating constant temperature water tank,
In the second operation state in which only the dry gas is supplied to the crack zone and the mixed gas is not supplied, distribution of the dry gas to the second pipe is stopped, and the drainage device is used to 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 constant temperature water bath. The method of claim 3,
Production of the hot-dip galvanized steel sheet which resumes distribution of the dry gas to the second pipe after restarting the circulation of water using the circulating constant temperature water bath when switching from the second operation state to the first operation state. Way. The method according to claim 3 or 4,
The manufacturing method of the hot-dip galvanized steel sheet which controls the dew point in the said crack zone to -20 degreeC or more and 0 degrees C or less in the said 1st operation state. The method according to claim 3 or 4,
The manufacturing method of the hot-dip galvanized steel sheet which further has the process of carrying out the heat alloying of the zinc plating performed on the said steel strip using the said alloying installation of Claim 2. The method of claim 5,
The manufacturing method of the hot-dip galvanized steel sheet which further has the process of carrying out the heat alloying of the zinc plating performed on the said steel strip using the said alloying installation of Claim 2.

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