JP2019019344A - Manufacturing method of hot-dip galvanized steel plate - Google Patents

Abstract

To provide a manufacturing method of a hot-dip galvanized steel plate having a beautiful surface appearance without a non-plating area.SOLUTION: A manufacturing method of a hot-dip galvanized steel plate for manufacturing the hot-dip galvanized steel plate uses 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 facility adjacent to the cooling zone. In the heating zone, superheated steam at 450 - 1200°C is introduced, and a steel plate temperature is raised to 700°C or less under an atmosphere consisting of 50 vol% or more of HO, a rest of N, and an inevitable impurity.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for producing a hot dip galvanized steel sheet.

  In recent years, carbon dioxide emission regulations for automobiles have become stricter due to increasing awareness of environmental issues. In addition, the safety of the vehicle body is required more than before, for example, regulations on the collision safety of automobiles are strengthened. Therefore, in order to achieve both weight reduction and strength improvement, automobile manufacturers are promoting the expansion of application of hot-dip galvanized high-tensile steel sheets to the vehicle body.

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

  Addition of solid solution strengthening elements such as Si, Mn, P, and Al is often performed to increase the tensile strength of the steel sheet. In particular, Si has the advantage that the addition cost is low compared to other elements and that the strength can be increased without impairing the ductility of the steel. Therefore, Si-containing steel is promising as a high-tensile steel plate. However, when Si is added in a large amount to steel, the following problems occur.

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

  Further, when 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, an oxide film is formed on the steel sheet surface, and then reduced iron is formed on the steel sheet surface by reduction annealing to improve the wettability with zinc. A technique to do this is disclosed in Patent Document 1. Patent Document 2 discloses a technique for 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. In patent document 3, a heating zone is divided into two for the purpose of forming a uniform oxide film on a steel plate surface with high efficiency using an open flame burner, and it adheres to a steel plate with the heating zone of the preceding paragraph. A technique is disclosed in which after removing the rolling oil, an oxide film is formed on the steel sheet surface in a subsequent heating zone where the steel sheet temperature is high.

  In recent years, the construction of CGLs equipped with an all-radiant tube type heating furnace has been increasing for reasons such as the ability to manufacture high-quality plated steel sheets at low cost due to the ease of operation and the difficulty of pickup. However, unlike DFF (direct flame type) and NOF type (non-oxidation type), all radiant tube type heating furnaces do not have an oxidation step immediately before annealing, so plating of steel sheets containing oxidizable elements such as Si and Mn It is disadvantageous in terms of securing sex.

Patent Document 4 describes an all radiant tube type continuous hot dip galvanizing apparatus for the purpose of preventing the occurrence of non-plating, in order to promote the internal oxidation of Si, in order to promote H ₂ O content in the heating zone and the soaking zone. A technique for improving the plating property by controlling the ratio of the pressure and the H ₂ partial pressure is disclosed.

  Further, Patent Document 5 discloses a technique for annealing a steel sheet using superheated steam.

  Furthermore, Patent Document 6 discloses a technique for injecting superheated steam onto a steel sheet surface in a part of an annealing furnace for the purpose of securing the internal oxidation amount of Si-added steel.

JP-A-4-202630 JP-A-6-306561 JP 2006-283109 A JP 2009-68041 A JP-A-9-241734 JP 2014-122390 A

In patent document 1, the atmosphere of a heating zone is managed by the air ratio. In this method, the concentration of oxidizing gas (O ₂ , CO ₂ , H ₂ O) in the heating zone changes in response to fluctuations in the air ratio that inevitably occur in the control, and concentration unevenness occurs in the heating zone. Arise. Therefore, the final oxide film is not uniform in the coil. Therefore, there is a problem that Si or Mn is concentrated on the surface in a portion where the oxide film is thin and unplating occurs, and an excessive oxide film is picked up by a roll in the furnace in a portion where the oxide film is thick.

In Patent Document 2, in order to directly control the atmosphere in the furnace, an attempt is made to control the atmosphere by introducing a gas into the furnace separately from the combustion gas of the direct fire burner. In this method, temperature unevenness occurs in the steel sheet due to the arrangement of the direct fire burner. Therefore, as in Patent Document 1, the final oxide film is not uniform in the coil, resulting in non-plating and pickup. Furthermore, 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, the concentration of the three kinds of gases must be managed, which is complicated. Need to build a simple control system. In addition, since there is a gas flow from the downstream to the upstream in the heating zone, the combustion gas of the direct fire burner and the separately introduced gas are not sufficiently mixed, and the furnace atmosphere is not uniform. Plating and pick-up occur.

  Patent Document 3 proposes a method in which the rolling oil adhering to the steel sheet surface is removed by a non-oxidizing furnace, and then an oxide film is formed by a subsequent oxidizing furnace. In this method, an open flame burner is used in a non-oxidation furnace and an oxidation furnace. For this reason, the oxidizing gas concentration in the furnace does not become constant due to the component variation and flow rate variation of air and fuel gas C gas. Therefore, there is a problem that non-plating and pickup occur.

In Patent Document 4, in claim 1, the ratio of H ₂ O and H ₂ partial pressure in the upper part of the soaking zone (H ₂ O / H ₂ ) U, and the ratio of H ₂ O and H ₂ partial pressure in the lower part of the soaking zone ( H ₂ O / H ₂ ) L is defined as follows.
1 ≧ (H ₂ O / H ₂ ) U ≧ 10 ^{− (0.5Si−3.25)} (1)
1 ≧ (H ₂ O / H ₂ ) L ≧ 10 ^{− (0.5Si−3.25)} (2)
Here, Si of Formula (1), (2) is Si addition amount in steel, and is Si = 0.3-2.5 mass%. Since the volume fraction is proportional to the partial pressure, the equations (1) and (2) can be read as equations defining the soaking zone H ₂ O and H ₂ volume fractions. In the formulas (1) and (2), when the H ₂ O volume fraction is the highest, H ₂ O = H ₂ , so the highest H ₂ O concentration in the soaking zone is 50%. Therefore, the H ₂ O concentration in the furnace does not exceed 50%. At such H ₂ O concentration, the internal oxides of Si and Mn are not sufficiently formed, and Si and Mn are concentrated on the surface, resulting in non-plating.

In patent document 5, the steam pressure (0.5-5 kg / cm < ² >), steam temperature (150-500 degreeC), and heating time (5-60 minutes) at the time of superheating a steel plate with superheated steam are prescribed | regulated. When the steel sheet is heated by this method, the oxide film formed on the steel sheet surface becomes excessive with respect to the oxide film necessary for suppressing the surface concentration of Si and Mn, and a part of the oxide film peels from the steel sheet, There is a problem of picking up on a roll.

In patent document 6, the atmospheric gas composition of the preheating apparatus which sprays superheated steam is not prescribed | regulated. Further, in the case of Patent Document 6, for example, when the line speed changes, the amount of outside air entering the preheating device changes with the accompanying flow of the steel plate, and as a result, the H ₂ O concentration in the preheating device changes. The oxide film does not have a constant thickness, resulting in non-plating and pickup.

  This invention is made | formed in view of this situation, Comprising: It aims at providing the manufacturing method of the hot dip galvanized steel plate which has the beautiful surface appearance without non-plating.

In order to obtain good plating properties, it is necessary to secure an optimal oxidation amount. In the all-radiant tube type continuous hot dip galvanizing facility, the present inventors have a method in which the atmosphere inside the furnace can be easily controlled and a uniform oxide film is formed on the surface of the steel sheet without using a direct fire burner. We conducted an intensive study. As a result, superheated steam at a predetermined temperature is introduced into the heating zone, and the high gas added steel is oxidized with the main gas component of the heating zone as H ₂ O, thereby forming a uniform oxide film on the steel plate surface. Was revealed.

The present invention is based on the above findings, and the features thereof 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 for producing a hot-dip galvanized steel sheet, 450 to 1200 ° C. superheated steam is introduced into the heating zone, and an atmosphere comprising 50 vol% or more of H ₂ O, the balance N ₂ and unavoidable impurities Among them, a method for producing a hot-dip galvanized steel sheet, wherein the steel sheet temperature is heated to 700 ° C. or lower.
[2] The method for producing a hot-dip galvanized steel sheet according to [1], wherein the steel sheet temperature and the H ₂ O concentration in the heating zone satisfy the following formula (1).

However, in Formula (1), f (x) = 9.8 × 10 ⁻⁴ x: adjustment coefficient (x: H ₂ O concentration in heating zone [vol%])
R = 8.314: Gas constant [J / mol · K]
Q: Activation energy of oxidation reaction [J / mol]
T: Steel plate temperature [K]
τ: Steel sheet heating time in the heating zone [sec]
It is.
[3] The method for producing a hot-dip galvanized steel sheet according to [1] or [2], wherein the superheated steam is introduced into the heating zone by a nozzle arranged in a staggered manner in the longitudinal direction of the steel sheet.

  According to the present invention, an excellent hot dip galvanized steel sheet having a beautiful surface appearance without unplating can be obtained. The present invention is particularly effective when a high Si-added steel sheet that is difficult to be hot-dip galvanized 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 diagram 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.

  The embodiment of the present invention will be specifically described with reference to FIGS.

  FIG. 1 is a schematic view 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. In addition, a cooling zone, a hot dip galvanizing apparatus, an alloying processing apparatus, etc. are arrange | positioned downstream of the soaking zone 2 (not shown). The soaking zone 2, the cooling zone, the hot dip galvanizing apparatus, the alloying processing apparatus and the like are not particularly limited and may be those normally employed.

  The steel sheet S is subjected to heat treatment in the continuous hot dip galvanizing equipment 100. A superheated steam generator 3 is connected to the heating zone 1. A pipe 4 is connected to the inlet side of the superheated steam generator 3, and water is supplied to the superheated steam generator 3 through the pipe 4. The charged water is heated to a predetermined temperature and becomes superheated steam. Thereafter, the superheated steam is transported by the superheated steam pipe 5 and put into the heating zone 1. The superheated steam pipe 5 is heated by the pipe heating device 6 so that the temperature of the superheated steam is kept constant so that the temperature of the superheated steam does not decrease during transportation.

Further, an exhaust pipe 7 for exhausting the superheated steam is connected to the downstream side of the heating zone 1 so that the superheated steam circulates from the exhaust pipe 7 to the superheated steam generator 3. Further, the dew point meters 8 are arranged uniformly along the longitudinal direction of the heating zone 1 so that it can be confirmed that the atmosphere (H ₂ O concentration) in the heating zone 1 is uniform. Further, a roll 9 is arranged between the heating zone 1 and the soaking zone 2. If this is a normally used heating zone, the soaking zone furnace pressure is set higher than the heating zone furnace pressure, and soaking zone gas flows into the heating zone and is exhausted. In this invention, by installing the roll 9 between the heating zone and the soaking zone, the soaking zone gas is prevented from flowing into the heating zone. Further, in order to secure an HNx gas exhaust system, an HNx gas exhaust pipe 10 is provided below the heating zone.

In order to obtain good plating properties, it is necessary to secure an optimal oxidation amount, and in the present invention, it is necessary to adjust the H ₂ O concentration in the heating zone. The temperature of the superheated steam thrown into the heating zone 1 is 450 to 1200 ° C., and the steel sheet is heated in an atmosphere composed of 50% or more of H ₂ O, the balance N ₂ and unavoidable impurities. The amount of oxidation is determined from the H ₂ O concentration in the heating zone, the temperature of the steel plate, and the time that the steel plate stays in the heating zone. If the superheated steam temperature is lower than 450 ° C., it is necessary to make the steel plate stay in the heating zone for a long time, which increases the scale of the equipment, which is not desirable from the viewpoint of construction cost. On the other hand, if the superheated steam temperature exceeds 1200 ° C., a great amount of heat is required to generate superheated steam, resulting in a huge amount of equipment costs. Therefore, the temperature of superheated steam shall be 450-1200 degreeC.

The atmosphere of the heating zone 1 consists of 50 vol% or more of H ₂ O, the balance N ₂ and unavoidable impurities. By introducing superheated steam at a predetermined temperature into the heating zone 1 and oxidizing the high Si-added steel with the main gas component of the heating zone 1 being H ₂ O, a uniform oxide film is formed on the steel plate surface. As a result, a hot dip galvanized steel sheet having a beautiful surface appearance free of unplating can be obtained. When the H ₂ O concentration is less than 50 vol%, the internal oxides of Si and Mn are not sufficiently formed, and Si and Mn are concentrated on the surface, resulting in non-plating. The balance consists of N ₂ and inevitable impurities, and N ₂ is supplied so that the H ₂ O concentration is 50 vol% or more. In order to make the heating zone have a predetermined H ₂ O concentration, for example, by attaching an N ₂ pipe (not shown) in the middle of the pipe that transports the superheated steam from the superheated steam generator to the heating zone, A mixed gas of superheated steam and N ₂ may be introduced into the heating zone.

The method for controlling the H ₂ O concentration in the heating zone 1 is not particularly limited. For example, it is divided into three in the longitudinal direction, the dew point of the exhaust gas is measured with the dew point meter 8, and the H ₂ O concentration is determined from the value measured with each dew point meter 8, and the H ₂ O concentration is 50 vol% or more. Thus, the flow rate of superheated steam introduced into the heating zone 1 may be appropriately adjusted. In the case of a direct fire burner, the gas concentration of the combustion gas varies due to the change in the flow rate of the C gas that is the fuel gas and the change in the input air amount due to the variation in the air ratio. On the other hand, in the present invention, since the introduced gas is mainly superheated steam, that is, H ₂ O, the concentration control in the furnace is easier compared to the direct fire burner.

  The dew point measurement method of the dew point meter 8 is not particularly limited.

  The steel sheet S is annealed in a temperature range of 600 to 900 ° C. in a reducing atmosphere following the heating zone 1. When the steel sheet S is heated above 700 ° C. in the heating zone 1, the amount of oxidation of the steel sheet becomes excessive, and part of the oxide is picked up by the roll. Therefore, in the heating zone 1, the steel plate temperature is heated to 700 ° C. or lower.

In order to obtain good plating properties, it is necessary to secure an optimum amount of oxidation. In actual operation, the temperature of the heated steel strip and the H ₂ O concentration in the heated zone depending on the steel composition, steel plate size, and line speed. Need to be adjusted. As a result of intensive studies by the present inventors, it has been found that the amount of oxidation necessary for plating high-Si steel as in the present invention is 0.1 to 0.6 g / m ² , so that the amount of oxidation falls within this range. Thus, it is preferable to operate the heating zone. As a result of further investigation, it was found that the optimum amount of Fe oxidation can be predicted by controlling the steel sheet temperature and the H ₂ O concentration in the heating zone so as to satisfy the following formula (1).

However, in Formula (1), f (x) = 9.8 × 10 ⁻⁴ x: adjustment coefficient (x: H ₂ O concentration in heating zone [vol%])
R = 8.314: Gas constant [J / mol · K]
Q: Activation energy of oxidation reaction [J / mol]
T: Steel plate temperature [K]
τ: Steel sheet heating time in the heating zone [sec]
It is.

By keeping the expression (1) in the range of 0.1 to 0.6, it is possible to secure 0.1 to 0.6 g / m ² which is an oxidation amount necessary for plating of high Si steel.

  Note that the activation energy Q is determined for each steel type, and can be appropriately determined by an oxidation experiment.

  What is necessary is just to spray superheated steam with respect to the steel plate S, for example using a nozzle (superheated steam spray nozzle). In order to efficiently heat the steel sheet S, it is desirable to spray the superheated steam spray nozzle with the injection holes arranged vertically with respect to the steel sheet S. The superheated steam spray nozzles are preferably arranged in multiple stages with respect to the traveling direction of the steel sheet S. Further, the superheated steam spray nozzle is preferably arranged in a staggered manner with respect to the traveling direction of the steel sheet S, that is, the longitudinal direction of the steel sheet. For example, as shown in FIG. It is preferable that the steam pipes 5 are staggered in the width direction on the front and back of the steel plate. In either case, the steel sheet S is heated with high efficiency without temperature unevenness.

As the roll 9 installed between the heating zone 1 and the soaking zone 2, a ceramic roll is preferably used. This is to prevent oxide pick-up from the steel sheet surface to the roll. As a material for the thermal spray material of the ceramic roll, Al ₂ O ₃ , Cr ₂ O ₃ , ZrO ₂ or a material obtained by sintering two or more selected from these is preferable. Furthermore, in order to suppress gas inflow from the reducing furnace to the heating zone, it is preferable to arrange a seal roll between the heating zone and the soaking zone.

  The superheated steam generator 3 preferably uses an induction heating method capable of generating steam with high efficiency. If it is an induction heating system, it is possible to produce | generate superheated steam efficiently. Moreover, the water thrown into the superheated steam generator 3 may be either liquid or gas (water vapor). Moreover, about the superheated steam piping 5 which transports superheated steam, SUS steel which has corrosion resistance and also has heat resistance is preferable.

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

  Si is contained as a deoxidizer, as a solid solution strengthening element for increasing the strength, or as an element for improving magnetic properties. In particular, Si is an element that has a relatively small deterioration in mechanical properties such as workability, although the effect of increasing the strength is large, and thus can be preferably used. However, when the content is less than 0.3% by mass, the concentration on the steel sheet surface layer 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 oxide film formed by this method alone cannot suppress the diffusion of Si to the surface layer, and the ratio of the steel sheet that becomes thickened is increased. Therefore, the upper limit is preferably set to 3.0 mass or less. A more preferable range of Si is 0.8 to 1.5% by mass.

  In addition, elements other than Si can be contained in a range included in a normal cold-rolled steel sheet. For example, C, Mn, Al, P and S are component ranges required from mechanical strength characteristics and manufacturability because they hardly affect the oxide adhesion to the in-furnace roll that the present invention is to solve. C: 0.05 to 0.25% by mass, Mn: 0.5 to 3.0% by mass, Al: 0.01 to 3.00% by mass, P: 0.001 to 0.10% by mass, S: It can contain in 0.200 mass% or less.

In the all radiant tube type CGL, as shown in FIG. 1, the heating zone 1 by superheated steam was installed on the inlet side of the CGL, and the test was performed to evaluate the plating property by changing the steel plate temperature and the H ₂ O concentration. . In addition, the steel plate temperature of the heating zone 1 was made into the steel plate temperature of the heating zone 1 by measuring the steel plate temperature in the exit side of the heating zone 1.

  Table 1 shows the chemical composition of the steel sheet used in the test. The width of the steel plate was 1 m and the plate thickness was 1 mm. Moreover, the furnace length of the heating zone 1 was 35 m, and the line speed was 100 mpm.

The heating zone 1 was sprayed with a gas in which superheated water vapor and N ₂ were mixed so as to have a predetermined H ₂ O concentration. In total, heating zone 1 was charged with superheated steam having a mass flow rate of 740 kg / h. In addition, an N ₂ pipe is attached in the middle of the pipe that transports the superheated steam from the superheated steam generator to the heating zone so that the heating zone has a predetermined H ₂ O concentration, and a mixed gas of superheated steam and N ₂ is added. Introduced into the tropics.

As the dew point meter 8, a capacitance type dew point meter was used. A dew point meter 8 was installed at each of the upper, middle and lower portions of the heating zone 1, and the dew point at each position was measured to obtain the H ₂ O concentration.

  The superheated steam was sprayed by arranging the superheated steam pipe 5 to which the superheated steam spray nozzle was attached in the width direction of the steel sheet. The superheated steam pipe 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 sheet (lattice arrangement). Further, some steel plates were sprayed using superheated steam spray nozzles arranged in a staggered manner on the front and back of the steel plates as shown in FIG.

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

A ceramic roll was disposed on the roll 9 between the heating zone 1 and the soaking zone 2. The material of the ceramic roll was ZrO _2. The soaking zone dew point was controlled in the range of -40 to -10 ° C.

The manufacturing conditions are shown in Table 2. 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. Further, after hot dip galvanization, alloying was performed at an alloying temperature of 530 ° C.

About the obtained plated steel plate, the plating appearance and plating adhesion were evaluated as follows.
(1) Plating appearance The plating appearance was evaluated based on the presence or absence of non-plating and unevenness of the alloy as follows. 1 and 2 are acceptable.
1 No plating, no over-oxidation and uneven alloying 2 No plating, no over-oxidation and slight alloying unevenness 3 No plating or / and alloying unevenness 4 Peroxidation or / and uneven alloying (2) Plating adhesion Unit when cellotape (registered trademark) with a tape width of 24 mm and a unit length of 1 m is applied to an alloyed hot-dip galvanized steel sheet (GA) and the tape surface is bent and bent back by 90 ° The amount of peel per length was measured by fluorescent X-rays and the Zn count was measured and ranked according to the following criteria. Ranks 1 and 2 are acceptable.
Less than 10-500 (good)
2 500 or more but less than −1000 3 1000 or more but less than −2000 4 2000 or more but less than −3000 Table 2 shows production conditions and results.

In conditions 1 to 3, which are examples of the present invention, by controlling the H ₂ O concentration in the heating zone and the steel plate temperature within appropriate ranges, it is possible to produce a steel plate excellent in plating appearance and plating adhesion.

DESCRIPTION OF SYMBOLS 1 Heating zone 2 Soaking zone 3 Superheated steam generator 4 Piping 5 Superheated steam piping 6 Piping heating device 7 Exhaust piping 8 Dew point meter 9 Roll 10 HNx exhaust piping 100 Continuous hot dip galvanizing equipment S Steel plate

Claims (3)

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 for producing a hot-dip galvanized steel sheet for producing a plated steel sheet, 450 to 1200 ° C. superheated steam is introduced into the heating zone, and in an atmosphere composed of 50 vol% or more of H ₂ O and the balance N ₂ and unavoidable impurities, A method for producing a hot dip galvanized steel sheet, wherein the steel sheet temperature is heated to 700 ° C or lower. The method for producing a hot-dip galvanized steel sheet according to claim 1, wherein the steel sheet temperature and the H ₂ O concentration in the heating zone satisfy the following formula (1).

However, in Formula (1), f (x) = 9.8 × 10 ⁻⁴ x: adjustment coefficient (x: H ₂ O concentration in heating zone [vol%])
R = 8.314: Gas constant [J / mol · K]
Q: Activation energy of oxidation reaction [J / mol]
T: Steel plate temperature [K]
τ: Steel sheet heating time in the heating zone [sec]
It is.   The method for producing a hot-dip galvanized steel sheet according to claim 1 or 2, wherein the superheated steam is introduced into the heating zone by a nozzle arranged in a staggered manner in the longitudinal direction of the steel sheet.

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