JP6696495B2 - Method for manufacturing hot dip galvanized steel sheet - Google Patents

Description

  The present invention relates to a method for manufacturing a galvanized steel sheet.

  In recent years, with increasing awareness of environmental issues, regulations on carbon dioxide emissions from automobiles have become strict. In addition, the safety of the vehicle body is required to be higher than ever before, such as the regulations on collision safety of automobiles being tightened. Therefore, in order to achieve both weight reduction and strength improvement, automobile manufacturers are promoting the expansion of application of hot-dip galvanized high-strength steel sheets to vehicle bodies.

  The hot-dip galvanized steel sheet is manufactured by the following method. The coil after cold rolling is passed through a continuous galvanizing line (CGL), and first, the oil content on the surface of the base material is burned and removed in a preheating furnace. After that, heating is performed in an oxidizing atmosphere or a reducing atmosphere to recrystallize the steel sheet. Further, in an oxidizing atmosphere or a reducing atmosphere, the steel sheet is cooled to a temperature suitable for plating and immersed in molten zinc.

  In order to increase the tensile strength of steel sheets, addition of solid solution strengthening elements such as Si, Mn, P and Al is often performed. In particular, Si has the advantages that the addition cost is lower than that of other elements and that the strength can be increased without impairing the ductility of steel. Therefore, Si-containing steel is promising as a high-tensile steel plate. However, adding a large amount of Si to steel causes the following problems.

  The high-strength steel sheet is annealed in a reducing atmosphere in a temperature range of 600 to 900 ° C. Since Si is an easily oxidizable element as compared with Fe, Si concentrates on the surface of the steel sheet at this time. As a result, Si oxide is formed on the surface of the steel sheet, and this Si oxide significantly deteriorates the wettability with zinc and causes non-plating.

  Furthermore, if Si is concentrated on the surface, even if zinc plating adheres, a significant delay in alloying occurs in the alloying process after hot dip galvanizing. As a result, productivity deteriorates.

For such problems, the steel sheet is heated in a heating zone with an open flame burner to form an oxide film on the steel sheet surface, and then reduced iron is formed on the steel sheet surface by reduction annealing to improve wettability with zinc. A method of doing so is disclosed in Patent Document 1. Patent Document 2 discloses a method of maintaining a uniform oxide film thickness by using an open flame burner to regulate the concentration of oxidizing gas (O ₂ , CO ₂ , H ₂ O) in the atmosphere of the heating zone. ing.

  Further, Patent Document 3 discloses a technique for annealing a steel sheet using superheated steam.

  Further, Patent Document 4 discloses a technique of injecting superheated steam onto the surface of a steel sheet in a part of an annealing furnace in order to secure the internal oxidation amount of Si-added steel.

JP-A-4-202630 JP-A-6-306561 JP, 9-241734, A JP, 2014-122390, A

  In Patent Document 1, an oxide film is formed on the surface of the steel sheet by using an open flame burner in the heating zone. It is known that the flame temperature is affected by the air ratio, and the flame temperature changes due to air ratio fluctuations that are unavoidable in control, and the steel plate temperature changes due to the influence. As a result, the temperature of the steel sheet changes in the longitudinal direction of the coil, and the amount of oxidation is not constant, which causes problems such as non-plating and excessive oxide film picked up by the roll in the furnace.

In Patent Document 2, in order to directly control the atmosphere in the furnace, a gas is introduced into the furnace separately from the combustion gas of the direct-burning burner to try to control the atmosphere. In this method, the temperature unevenness occurs in the steel sheet due to the arrangement of the open flame burners. Therefore, as in Patent Document 1, the final oxide film is not uniform in the coil, resulting in non-plating and pickup. Further, since there are three kinds of oxidizing gases (O ₂ , CO ₂ , H ₂ O), in order to control the oxygen potential in the furnace, it is necessary to manage the gas concentrations of the three kinds, which is complicated. It is necessary to build a sophisticated control system. In addition to this, since there is a gas flow from the downstream side to the upstream side in the heating zone, the combustion gas of the direct-burning burner and the gas introduced separately are not sufficiently mixed, and the atmosphere in the furnace is not uniform. Plating and pickup will occur.

In Patent Document 3, the steam pressure (0.5 to 5 kg / cm ² ), the steam temperature (150 to 500 ° C.), and the heating time (5 to 60 minutes) when the steel sheet is overheated with superheated steam are specified. When a steel sheet is heated by this method, when the heat capacity of the steel sheet changes, the temperature of the steel sheet on the exit side of the heating furnace changes greatly, so the oxide film formed on the steel plate surface becomes too small or too large, and part of the oxide film peels from the steel plate. Then there is the problem of picking up on the roll.

  In Patent Document 4, there is a problem that the controllability of the steel sheet temperature is poor in the heating zone using superheated steam, and the oxide film formed on the steel sheet surface may be picked up on the soaking zone roll surface.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide a method for manufacturing a hot-dip galvanized steel sheet having excellent plating quality without using an open flame burner.

  The present inventors can easily control the steel strip temperature on the heating strip side without using an open flame burner, and even when the line speed or the strip thickness of the steel sheet changes, the steel strip temperature on the heating strip side. The present inventors have earnestly studied a heating method of a steel sheet capable of controlling the temperature within a certain range. As a result, in the heating zone of the all-radiant tube type continuous hot dip galvanizing equipment, the steel sheet is heated by superheated steam, the zones of the heating zone are divided, and the superheated steam temperature of each zone is controlled, It was found that a steel plate with good plating quality can be manufactured by controlling the steel plate temperature on the side.

The present invention is based on the above findings, and its features are as follows.
[1] Using an all-radiant tube type continuous hot-dip galvanizing facility 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 In the method of manufacturing a hot-dip galvanized steel sheet, the heating zone is divided into a plurality of zones in the longitudinal direction of the steel sheet, and the heat transfer coefficient of the heating zone is 100 to 400 W / (m ^2). -Introducing superheated steam to be K), and controlling the superheated steam temperature in each zone of the heating zone so that the temperature of the latter half of the zones in the latter half of the zones of the heating zone becomes lower than that in the first half of the zones. A method for producing a hot-dip galvanized steel sheet, wherein the steel sheet temperature T _f on the heating zone exit side is set to 400 to 700 ° C.
[2] The method for producing a hot-dip galvanized steel sheet according to [1], wherein when the number of zones in the heating zone is n (n is an integer), the following formulas (1) and (2) are satisfied.

However,
n: the number of zones in the heating zone. However, from the upstream side to the downstream side of the heating zone, the first, second, third, ... Nth zones are set.
T _f : Steel plate temperature (° C.) on the heating zone exit side
G _i : Superheated steam temperature (° C.) in the i-th heating zone (i = 1, 2, 3, ... N)
α: heat transfer coefficient of superheated steam (W / (m ² · K))
And
[3] The method for producing a hot-dip galvanized steel sheet according to [1] or [2], wherein the steel sheet heated to a temperature exceeding 100 ° C. is conveyed to the heating zone.
[4] The hot dip galvanizing according to any one of [1] to [3], characterized in that the steel sheet is heated in a reducing atmosphere on the downstream side of the heating zone and before being transported to the soaking zone. Steel plate manufacturing method.

  According to the present invention, an excellent hot-dip galvanized steel sheet having a beautiful surface appearance without unplating or pick-up flaws can be obtained. INDUSTRIAL APPLICABILITY The present invention is particularly effective when a high Si-added steel sheet that is difficult to hot dip galvanize is used as a base material and is useful as a method for improving the plating quality in the production of a high Si-added hot-dip galvanized steel sheet.

FIG. 1 is a schematic view of a heating zone and a soaking zone according to an embodiment of the present invention. FIG. 2 is a schematic diagram showing the arrangement of superheated steam spray nozzles.

  An embodiment of the present invention will be specifically described below with reference to FIG.

  FIG. 1 is a schematic diagram of a heating zone 1 and a soaking zone 2 in an all-radiant tube type continuous hot dip galvanizing facility 100 according to an embodiment of the present invention. A cooling zone, a hot dip galvanizing device, an alloying treatment device, etc. are arranged downstream of the soaking zone 2 (not shown). The soaking zone 2, the cooling zone, the hot dip galvanizing device, the alloying treatment device and the like are not particularly limited and may be those usually employed.

  The steel sheet S is heat-treated in the continuous hot-dip galvanizing facility 100. The heating zone 1 is divided into a plurality of zones. In addition, in FIG. 1, although it is divided into the first zone 1Z and the second zone 2Z, it is not limited to this embodiment.

  The superheated steam generator 3 is connected to the first zone 1Z and the second zone 2Z of the heating zone 1. A pipe 4 is connected to an inlet side of the superheated steam generator 3, and water is introduced into the superheated steam generator 3 through the pipe 4. The introduced water is heated to a predetermined temperature and becomes superheated steam. After that, the superheated steam is transported by the superheated steam pipe 5 and introduced into the first zone 1Z and the second zone 2Z of the heating zone 1. The pipe heating device 6 heats the superheated steam pipe 5 so that the temperature of the superheated steam does not decrease during transportation, and the temperature of the superheated steam is kept constant. As a result, the temperature of the superheated steam introduced into the heating zone 1 becomes constant, and the steel sheet S can be heated without causing temperature unevenness.

  A recovery pipe 7 for recovering the superheated steam is connected to the downstream side of the first zone 1Z and the second zone 2Z of the heating zone 1, and the recovery pipe 7 transfers the superheated steam to the superheated steam generator 3. It is a mechanism to circulate.

  A reduction zone 8 may be installed on the downstream side of the heating zone 1. A HNx gas pipe 9 that is a reducing gas is connected to the reduction zone 8, and the HNx gas is introduced into the reduction zone 8. Further, in order to secure an exhaust system for HNx gas, an exhaust pipe 10 for HNx gas is provided on the downstream side of the reduction zone 8. Note that a gas other than HNx gas may be used as long as it is a reducing gas, for example, CO gas may be used.

A preheating zone 11 may be installed in front of the heating zone 1 on the entry side. A N ₂ supply pipe 12 is connected to the preheating zone 11, and N ₂ is introduced into the preheating zone 11.

  Further, as shown in FIG. 1, multiple reflection type thermometers 13 are installed on the downstream side of the second zone 2Z and the downstream side of the reduction zone 8, respectively. Thereby, the temperature of the steel plate on the outlet side of the heating zone 1 is monitored. A multiple reflection type thermometer 13 may be provided on the exit side of the preheating zone 11 (the entrance side of the heating zone 1). This is for grasping the temperature of the steel sheet after being heated and conveyed to the heating zone 1 when the steel sheet is heated by the preheating zone 11.

  A roll 14 is arranged between the heating zone 1 and the preheating zone 11. In the present invention, the roll 14 is installed between the heating zone 1 and the preheating zone 11 to suppress the inflow of air into the heating zone 1. Further, a ceramic roll 15 may be arranged between the heating zone 1 and the soaking zone 2 in order to suppress the gas flow from the soaking zone 2 to the heating zone 1.

When performing pre-oxidation in a direct fire burner furnace (DFF) or a non-oxidizing furnace (NOF), the amount of oxidation is affected by the gas concentrations of three kinds of O ₂ , CO ₂ , and H ₂ O. Even if manufacturing is performed under the same operating conditions, the atmosphere inside the furnace changes due to fluctuations in fuel gas components and seasonal dew point fluctuations in the atmosphere.

  Therefore, as a result of intensive studies by the present inventors, in the hot dip galvanizing equipment of the all radiant tube method, if heating is performed by only superheated steam, the atmosphere of the heating zone 1 is always constant, so that the temperature of the steel sheet on the outlet side of the heating zone We thought that the amount of oxidation could be controlled by. Further, in the method using the direct flame burner, the flame temperature changes due to the fluctuation of the air ratio that is unavoidably caused by the control, and finally the steel plate temperature on the heating zone side fluctuates. When heating with superheated steam, a multiple reflection thermometer 13 is installed in front of the superheated steam pipe 5 to control the superheated steam temperature, and if necessary, the pipe heating device 6 is used to heat the steam. To control. As a result, the temperature of the superheated steam introduced into the heating zone 1 can be made constant, and the controllability of the steel sheet temperature on the heating zone exit side is higher than in the case of using an open flame burner.

  In the present invention, the heating zone 1 is composed of a plurality of zones (first zone 1Z, second zone 2Z, third zone 3Z ... Nth zone in the longitudinal direction of the steel sheet. It is divided into. When the heating zone 1 is divided into a plurality of zones, the combustion load can be controlled for each zone when the line speed or the sheet thickness changes, so that the steel sheet temperature on the heating zone exit side can be precisely controlled.

In order to obtain good plating properties, it is necessary to secure an optimum amount of oxidation. When heating a steel sheet with a single superheated steam temperature, when the plate thickness or line speed changes and the heat capacity of the superheated steam changes, the steel plate temperature on the outlet side of the heating zone 1 changes greatly, and the amount of oxidation is not constant. , Non-plating and pickup occurs. As a result of earnest studies by the present inventors, if the heat transfer coefficient of superheated steam is set to 100 to 400 W / (m ² · K), even if the line speed or the plate thickness is changed, the heating zone 1 is output. It was found that the steel plate temperature T _f on the side is within the range of 400 to 700 ° C., and the amount of oxidation required for ensuring the plating property can be obtained. This is because if the heat transfer coefficient is less than 100 W / (m ² · K), the furnace length of the heating zone 1 must be set to be extremely long, which requires a huge construction cost. On the other hand, if it exceeds 400 W / (m ² · K), the flow rate of superheated steam increases, the steel sheet is affected by the vibration, and the shape deteriorates. Further, in the present invention, by controlling the steel plate temperature _Tf on the outlet side of the heating zone 1 to 400 to 700 ° C, it is possible to achieve the amount of oxidation necessary for ensuring the plating property. When the temperature of the steel sheet on the outlet side of the heating zone 1 is less than 400 ° C, it is impossible to obtain the amount of oxidation necessary for ensuring the plating property. On the other hand, when the temperature exceeds 700 ° C, pickup occurs.

For superheated steam having a heat transfer coefficient of 100 to 400 W / (m ² · K), it may be adjusted by estimating the discharge flow velocity from the pressure of the superheated steam and the nozzle diameter and calculating the heat transfer coefficient. ..

  In zone division, superheated steam is introduced so that the superheated steam temperature in the latter half of the heating zone is lower than that in the first half. That is, the superheated steam temperature of the i heating zone is lower than the superheated steam temperature of the (i-1) th heating zone (i = 1, 2, 3, ... N). This is to prevent the steel sheet temperature on the heating zone exit side from deviating from the target temperature by rapidly heating the steel sheet in the initial zone and decreasing the heating rate in the subsequent zone.

  Further, when the number of zones of the heating zone 1 is n (n is an integer), it is preferable to set the superheated steam temperature of each zone of the heating zone 1 so as to satisfy the following formulas (1) and (2).

However,
n: the number of zones in the heating zone. However, from the upstream side to the downstream side of the heating zone, the first, second, third, ... Nth zones are set.
T _f : Steel plate temperature (° C.) on the heating zone exit side
G _i : Superheated steam temperature (° C.) in the i-th heating zone (i = 1, 2, 3, ... N)
α: heat transfer coefficient of superheated steam (W / (m ² · K))
And

  By satisfying the above formulas (1) and (2), the steel plate temperature on the exit side of the heating zone 1 can be controlled more precisely, and the amount of oxidation necessary for ensuring the plating property can be obtained.

  When the steel sheet S is heated with superheated steam in the heating zone 1, the steel sheet temperature rises rapidly to 100 ° C., all the moisture condensed on the steel sheet surface evaporates, and then the steel sheet temperature rises again. Therefore, when the steel plate S at room temperature is put in the superheated steam atmosphere, uneven spots of appearance may occur due to uneven evaporation of water. In order to avoid this, it is preferable to heat the steel sheet S in the preheating zone 11 to a temperature higher than 100 ° C. and convey the steel sheet S heated to a temperature higher than 100 ° C. to the heating zone 1. The heating mode of the steel sheet is not limited, but induction heating that can uniformly and rapidly heat the steel sheet is preferable.

  Further, in order to prevent the oxide film on the surface of the steel sheet from being picked up by the roll in the soaking zone 2, the steel sheet may be heated in a reducing atmosphere after being heated in the heating zone 1 and before being transported to the soaking zone 2. It is preferable to heat the steel sheet in the reduction zone 8 which is the reducing atmosphere shown in FIG. That is, it is preferable to heat the steel sheet S in the reducing atmosphere on the downstream side of the heating zone 1 and before being conveyed to the soaking zone 2. A hydrogen atmosphere is preferable as the reducing atmosphere. Further, the furnace length in the reducing atmosphere is preferably 1 m or more and 5 m or less. This is because if it is less than 1 m, the reduction becomes insufficient, while if it is 5 m or more, there is no difference in the effect. Even when the steel sheet is reduced, pickup occurs when the steel sheet temperature (steel sheet temperature on the exit side of the reduction zone 8) exceeds 800 ° C. Therefore, the upper limit of the steel sheet temperature was set to 800 ° C.

  The superheated steam may be sprayed onto the steel plate S using, for example, a nozzle (superheated steam spray nozzle). In order to efficiently heat the steel plate S, it is preferable to arrange the spray holes of the superheated steam spraying nozzles perpendicular to the steel plate S and spray them. Further, the superheated steam spray nozzles are preferably arranged in multiple stages in the traveling direction of the steel plate S. Further, the superheated steam spray nozzles are preferably arranged in a zigzag manner in the traveling direction of the steel plate S, that is, in the steel plate longitudinal direction. For example, as shown in FIG. It is preferable to arrange the steam pipes 5 in a zigzag pattern on the front and back of the steel plate in the width direction. Both are for heating the steel sheet S with high efficiency without temperature unevenness.

It is preferable to use a ceramic roll as the roll 14. This is to prevent the pickup of oxides from the surface of the steel sheet to the roll. The material of the thermal spray material of the ceramic roll is preferably Al ₂ O ₃ , Cr ₂ O ₃ , ZrO ₂ or a material obtained by sintering two or more selected from these. Further, in order to suppress gas inflow from the reduction furnace into the heating zone, it is preferable to dispose the ceramic roll 15 between the heating zone and the soaking zone.

  For the superheated steam generator 3, it is preferable to use an induction heating method capable of generating steam with high efficiency. With the induction heating method, it is possible to efficiently generate superheated steam. The water introduced into the superheated steam generator 3 from the pipe 4 may be liquid or gas (steam). Further, for the superheated steam pipe 5 that transports the superheated steam, SUS steel having corrosion resistance and heat resistance is preferable.

  The steel sheet targeted by the present invention is preferably a high Si steel, and specifically, the Si content is preferably 0.3 mass% or more.

  Si is contained as a deoxidizer, as a solid solution strengthening element for achieving high strength, or as an element for improving magnetic properties. In particular, Si is an element that has a relatively small effect of increasing the strength but has a relatively small deterioration of mechanical properties such as workability, and thus can be preferably used. However, when the content is less than 0.3% by mass, the thickening to the surface layer of the steel sheet during annealing is small and it is not necessary to apply the present invention. Therefore, the Si content is preferably 0.3% by mass or more. When the Si content exceeds 3.0% by mass, the diffusion rate of Si into the surface layer cannot be suppressed only by the oxide film formed by this method, and the ratio of the steel sheet in which the surface layer is concentrated increases. Therefore, the upper limit is preferably 3.0 mass or less. The more preferable range of Si is 0.8 to 1.5 mass%.

  In addition, elements other than Si can be contained in the range contained in a normal cold rolled steel sheet. For example, C, Mn, Al, P, and S have almost no effect on the oxide adhesion to the furnace roll that is to be solved by the present invention, so they are component ranges required in terms of mechanical strength characteristics, manufacturability, and the like. C: 0.05 to 0.25 mass%, Mn: 0.5 to 3.0 mass%, Al: 0.01 to 3.00 mass%, P: 0.001 to 0.10 mass%, S: It can be contained in the range of 0.200 mass% or less.

  In the all radiant tube type CGL, as shown in FIG. 1, a heating zone 1 with superheated steam was installed on the inlet side of the CGL, and a test was performed to evaluate the plating property by changing the line speed and the plate thickness. As a comparison, the same test was performed on CGL having DFF in the heating zone. It should be noted that although there are two zones of the heating zone in FIG. 1, in the present embodiment, a device in which the zones of the heating zone are divided into four is used, and the burning rate and the air ratio can be individually set in each zone. The reducing zone was used in the final zone, and the oxidizing atmosphere was used in the other zones.

  Two types of steel plates (steel types A and B) were prepared for the test. The Si content of steel type A is 0.5%, and the Si content of steel type B is 1.5%. The width of the steel plate S was 1 m. The furnace length of the heating zone 1 was 35 m.

  Other manufacturing conditions are shown in Table 1. The annealing temperature was 830 ° C., 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. After hot-dip galvanizing, alloying treatment was performed at an alloying temperature of 530 ° C. In Table 1, No. No. 1 and 2 steel plates, No. Steel plates Nos. 3 and 4, No. Steel sheets Nos. 5 and 6, No. No. 7 and No. 8 steel plates. Since it is assumed that the steel sheets 12 and 13 are each continuously flown into the heating zone, the two steel sheets were collectively evaluated as one example (for example, No. 1 and 2 are one invention example). In addition, No. 1 in Table 1 In No. 7, a 5 m reduction zone 8 for heating the steel sheet in a hydrogen atmosphere is provided on the downstream side of the heating zone 1, and HNx gas with a hydrogen concentration of 10% is introduced so that the steel sheet temperature becomes 600 ° C. at the section exit side. Heating was performed under a reducing atmosphere. In addition, # 1 to # 4 in Table 1 are heating steam temperatures (° C.) in each zone (first zone to fourth zone). In addition, No. 1 in Table 1. The notation of # 1 to # 4 in 12 and 13 is the furnace temperature (° C.) of each zone.

The preheating zone 11 provided in front of the heating zone 1 had a furnace length of 2 m, and the steel plate S was heated in the preheating zone 11 as needed. Nitrogen gas was introduced into the preheating zone 11 through the N ₂ supply pipe 12. Nitrogen gas was introduced so that the oxygen concentration in the preheating zone 11 would be 0.5% or less. In the preheating zone 11, the steel plate S was heated to 110 ° C. using an induction heating device. The steel plate temperature of the preheating zone 11 was determined by measuring the steel sheet temperature with the multiple reflection type thermometer 13 installed on the outlet side of the preheating zone 11.

  A total of superheated steam having a mass flow rate of 740 kg / h was introduced into the heating zone 1. The superheated steam was sprayed by arranging the superheated steam pipe 5 equipped with a superheated steam spray nozzle in the width direction of the front and back of the steel sheet. The superheated steam piping 5 was arranged from the upper part to the lower part of the heating zone 1 at a pitch of 0.3 m in the longitudinal direction of the steel plate, and further sprayed using the superheated steam spray nozzles arranged in a staggered manner on the front and back of the steel plate.

  As the superheated steam generator 3, a heating device using an induction heating system was used. Water was supplied to the superheated steam generator 3 through the pipe 4. Further, SUS316L steel was used for the superheated steam pipe 5 for transporting the superheated steam. As the pipe heating device 6, a heating device using an induction heating method was used.

  The steel plate temperature on the outlet side of the heating zone 1 was measured by the multiple reflection type thermometer 13 on the outlet side of the heating zone 1. The temperature profile in the plate width direction was acquired every 5 seconds, and the steel plate temperature was obtained by dividing all the obtained temperatures by the number of measurement points.

  The dew point of soaking zone 2 was controlled in the range of -40 to -10 ° C.

The plating appearance and plating adhesion of the obtained plated steel sheet were evaluated as follows.
(1) Plating Appearance The plating appearance was evaluated as follows based on the presence or absence of non-plating and alloy unevenness. 1 and 2 are passed.
1 Non-plating, non-oxidation and uneven alloying 2 Non-plating, non-oxidizing and slight alloying unevenness 3 Non-plating and / or uneven alloying 4 Peroxidation and / or uneven alloying (2) Adhesion of plating A unit when a Sellotape (registered trademark) with a tape width of 24 mm and a unit length of 1 m is attached to an alloyed hot-dip galvanized steel sheet (GA) and the tape surface is bent back by 90 ° and then bent back. For the amount of peeling per length, the Zn count number was measured by fluorescent X-rays and ranked according to the following criteria. Ranks 1 and 2 are passed.
Less than 10-500 (good)
2,500 or more and less than -1000 3 1000 or more and less than 2,000 4 2000 or more and less than 3,000 Manufacturing conditions and results are shown in Table 1.

  No. 1, which is an example of the present invention. For Nos. 1, 2, 5, and 6, the plate temperature was within the target range even if the line speed or the plate thickness was changed, and good appearance and plating adhesion were obtained. The target temperatures are different because the steel types are different. On the other hand, No. 1 in which the steel sheet was heated with a single superheated steam temperature. Nos. 3, 4, 7, and 8 were out of the target temperature range, non-plating and pickup occurred, and the appearance and adhesion were deteriorated. In addition, the temperature of the superheated steam is outside the range shown in claim 1, No. In Nos. 7 and 8, the temperature was out of the target temperature range, non-plating occurred, and the appearance deteriorated. Before the steel sheet was introduced into the heating zone 1, the steel sheet was heated to 100 ° C. or higher in the preheating zone 11 No. In No. 9, No. 9 The appearance was improved as compared with 1, 2, 5, and 6. It is presumed that this is because the condensation in the heating zone 1 was avoided and the waterdrop-shaped defects disappeared. No. 1 in which the steel sheet was reduced in a hydrogen atmosphere on the downstream side of the heating zone 1. No. 10 is No. Plating adhesion was improved as compared with 1, 2, 5, 6, and 9. It is considered that this is because the oxide film was sufficiently reduced. The superheated steam temperatures of # 1 to # 4 in the invention examples of Table 1 all satisfied the expressions (1) and (2). In addition, No. 1 in Table 1 12 and 13 are the results of the test conducted in CGL including DFF as a heating zone. Due to the change of the plate thickness, the temperature on the heating zone exit side deviated from the target temperature, and the pickup due to the peroxidation occurred.

1 heating zone 2 soaking zone 3 superheated steam generator 4 piping 5 superheated steam piping 6 piping heating device 7 recovery piping 8 reduction zone 9 HNx gas piping 10 HNx gas exhaust piping 11 preheating zone 12 N ₂ supply piping 13 multiple reflection type Thermometer 14 rolls 15 Ceramic rolls 100 Continuous hot dip galvanizing equipment S Steel plate 1Z to 2Z Heating zone 1 zone

Claims (4)

A heating zone, a soaking zone, and an annealing furnace in which a cooling zone and a cooling zone are arranged in this order, and using an all-radiant tube system continuous hot dip galvanizing equipment having a hot dip galvanizing device adjacent to the cooling zone In the method for producing a galvanized steel sheet for producing a galvanized steel sheet,
The heating zone is divided into a plurality of zones in the longitudinal direction of the steel plate,
Superheated steam having a heat transfer coefficient of 100 to 400 W / (m ² · K) is introduced into the heating zone, and in the plurality of zones of the heating zone, the superheated steam temperature of the zone on the downstream side of the zone on the upstream side is Control the superheated steam temperature of each zone of the heating zone to be low,
The method for producing a hot-dip galvanized steel sheet, wherein the steel sheet temperature _Tf on the heating zone exit side is set to 400 to 700 ° C. When the number of zones of the said heating zone is n (n is an integer), the following formulas (1) and (2) are satisfy | filled, The manufacturing method of the hot dip galvanized steel sheet of Claim 1 characterized by the above-mentioned.

However,
n: the number of zones in the heating zone. However, from the upstream side to the downstream side of the heating zone, the first, second, third, ... Nth zones are set.
T _f : Steel plate temperature (° C.) on the heating zone exit side
G _i : Superheated steam temperature (° C.) in the i-th heating zone (i = 1, 2, 3, ... N)
α: heat transfer coefficient of superheated steam (W / (m ² · K))
And   The method for producing a hot-dip galvanized steel sheet according to claim 1, wherein the steel sheet heated to a temperature exceeding 100 ° C. is conveyed to the heating zone.   The method for manufacturing a hot-dip galvanized steel sheet according to claim 1, wherein the steel sheet is heated in a reducing atmosphere on the downstream side of the heating zone and before being transported to the soaking zone.

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