JP6908062B2 - Manufacturing method of hot-dip galvanized steel sheet - Google Patents

Description

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

In recent years, due to growing awareness of environmental issues, regulations on carbon dioxide emissions from automobiles have become stricter. In addition, the safety of the vehicle body is required more than before, such as the tightening of regulations on the collision safety of automobiles. Therefore, in order to achieve both weight reduction and strength improvement, automobile manufacturers are promoting the expansion of the 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 hot-dip galvanizing line (CGL), and first, the oil content on the surface of the base metal is burned and removed in the preheating furnace. Then, the steel sheet is recrystallized by heating in an oxidizing atmosphere or a reducing atmosphere. Further, the steel sheet is cooled to a temperature suitable for plating in an oxidizing atmosphere or a reducing atmosphere, and immersed in hot-dip zinc.

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

The high-strength steel sheet is annealed in a temperature range of 600 to 900 ° C. in a reducing atmosphere. Since Si is an easily oxidizing element as compared with Fe, Si is concentrated on the surface of the steel sheet at this time. As a result, a 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.

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.

To solve this problem, the steel sheet is heated in a heating zone with a direct flame burner to form an oxide film on the surface of the steel sheet, and then reduced iron is formed on the surface of the steel sheet by reduction annealing to improve the wettability with zinc. The method of doing this is well known. Therefore, it is very important to keep the oxide film thickness of the heating zone constant, and various methods have been published to make the oxide film thickness constant. In addition, if the oxide film formed in the heating zone is not sufficiently reduced, plating defects will occur. Therefore, a method for keeping the capacity of the reduction zone uniform has been disclosed.

For example, Patent Document 1 discloses a method of equalizing the oxide film thickness by dividing the furnace into a plurality of zones and providing a zone in which the direct flame burner is not burned in the direct flame heating type non-oxidizing furnace.

In Patent Document 2, in a direct-fire heating type non-oxidizing furnace, a method of _{defining the concentration of oxidizing gas (O 2} , CO ₂ , H ₂ O) in the atmosphere of the heating zone to keep the oxide film thickness uniform is described. It is disclosed.

In Patent Document 3, in a direct-fire heating type non-oxidizing furnace, the heating zone is divided into a pre-tropical zone, a non-oxidizing zone, an oxidizing zone, and a reducing zone, and the air ratio of the burner in the oxidizing zone adjacent to the reducing zone is set high. This stabilizes the oxidizing atmosphere and stabilizes the non-oxidizing atmosphere by burning the exhaust gas containing residual oxygen flowing in from the oxidation zone by setting the air ratio of the burner in the oxidation zone adjacent to the non-oxidizing zone low. As a result, a method for forming a uniform oxide film on the surface of the steel plate is disclosed.

According to Patent Document 4, in a direct-fire heating type non-oxidizing furnace, gas is exhausted from between the direct-fire zone and the reduction zone to suppress the inflow of the direct-fire zone gas into the reduction zone, thereby reducing the tropics. A method for preventing a decrease in capacity and sufficiently reducing the oxide film is disclosed.

Japanese Unexamined Patent Publication No. 2009-19253 Japanese Unexamined Patent Publication No. 6-306561 Japanese Unexamined Patent Publication No. 2013-142174 JP-A-2007-146242

In Patent Document 1, it is described that oxidation proceeds slowly in a zone where an open flame burner is not burned, but since each zone is connected, combustion gas flows in from other zones and the atmosphere is unpleasant. It tends to be stable. Therefore, the amount of oxidation tends to be non-uniform, and non-plating tends to occur.

Further, 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 open flame burner to try to control the atmosphere. Since there are three types of oxidizing gas (O ₂ , CO ₂ , H ₂ O), in order to control the oxygen potential in the furnace, it is necessary to control the concentration of the three types of gas, which is a complicated control. It is necessary to build a system. In addition to this, in the heating zone, there is a gas flow from the downstream to the upstream, so the combustion gas of the direct flame burner and the gas introduced separately are not sufficiently mixed, and the atmosphere inside the furnace is not uniform, which is not possible. Plating occurs.

Also in Patent Document 3, since the reducing gas flows from the tropics in the latter stage of the heating zone, the atmosphere of the heating zone cannot be kept constant only by adjusting the air ratio of the burner locally. Therefore, non-plating is likely to occur.

In the method of Patent Document 4, since the gas in the direct flame oxidation zone is mixed with the gas in the direct flame reduction zone in the direct flame heating zone, the oxygen concentration, carbon dioxide concentration, and water vapor concentration that affect the oxidation amount of the steel sheet are non-uniform. As a result, the amount of oxidation cannot be kept constant in the width direction of the steel sheet, and non-plating occurs.

The present invention has been made in view of such circumstances, and an object of the present invention is to provide a method for producing a hot-dip galvanized steel sheet having a beautiful surface appearance without non-plating.

In order to obtain good plating properties, it is necessary to secure the optimum amount of oxidation. In a continuous hot-dip galvanizing facility equipped with a heating zone having a direct-fired heating furnace (DFF), the present inventors can easily control the atmosphere inside the furnace and form a uniform oxide film on the surface of a steel sheet. Diligently examined. As a result, it was clarified that a uniform oxide film is formed on the surface of the steel sheet by controlling the oxygen concentration in the oxidation region of DFF (DFF oxidation zone) in the heating zone within an appropriate range.

The present invention is based on the above findings, and the gist thereof is as follows.
[1] In a method for manufacturing a hot-dip galvanized steel sheet using a continuous hot-dip galvanizing facility equipped with an annealing furnace in which a heating zone including a direct-fired heating furnace (DFF) and a soaking furnace are adjacent to each other.
The direct-fired heating furnace has a DFF oxidation zone operated at an air ratio of 1 or more and a DFF reduction zone operated at an air ratio of less than 1.
A method for producing a hot-dip galvanized steel sheet, which comprises exhausting furnace gas from between the DFF oxide zone and the DFF reduction zone to keep the furnace pressure of the DFF oxidation zone higher than that of the DFF reduction zone.
[2] The method for producing a hot-dip galvanized steel sheet according to [1], wherein the hot-dip galvanized steel sheet is operated at an oxygen concentration of 0.7% or more in the DFF oxide band.
[3] In the method for producing a hot-dip galvanized steel sheet according to [1] or [2], the hot-dip galvanized steel sheet is characterized in that the exhausted furnace gas is burned and then returned to the pre-tropical zone in the preheating zone. Manufacturing method.
[4] The method for producing a hot-dip galvanized steel sheet according to any one of [1] to [3], wherein the hot-dip galvanized steel sheet is alloyed after the hot-dip galvanized treatment.

According to the present invention, an excellent hot-dip galvanized steel sheet having a beautiful surface appearance without non-plating can be obtained. The present invention is particularly effective when a high-Si-added steel sheet, which is difficult to perform hot-dip galvanizing, 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 continuous hot-dip galvanizing facility having a DFF according to an embodiment of the present invention. FIG. 2 is a schematic view of an experimental device for an oxidation experiment carried out using an open flame burner. FIG. 3 is a graph showing the experimental results of an oxidation experiment carried out using an open flame burner. FIG. 4 is a schematic view showing a pipe provided on the DFF oxidation band entry side.

Embodiments of the present invention will be specifically described with reference to FIGS. 1 to 4.

FIG. 1 is a schematic view of a continuous hot-dip galvanizing facility 100 having a DFF according to an embodiment of the present invention.

In FIG. 1, the continuous hot-dip galvanizing facility 100 has an annealing furnace in which the heating zone 1 and the soaking tropics 2 are adjacent to each other in this order. A cooling zone, a hot-dip galvanizing device, an alloying treatment device, and the like are arranged downstream of the soothing tropics 2 (not shown). The soaking tropics 2, the cooling zone, the hot-dip galvanizing device, the alloying treatment device, and the like are not particularly limited, and those usually adopted may be used.

The steel sheet S is heat-treated in the continuous hot-dip galvanizing facility 100. As shown in FIG. 1, a pre-tropical 1-1 is arranged in front of the heating zone 1, and the heating zone 1 has a direct-fired heating furnace (DFF) 1-2. The direct flame heating furnace (DFF) 1-2 has a DFF oxidation zone 3 and a DFF reduction zone 4.

The DFF oxidation zone 3 is operated at an air ratio of 1 or more, and the DFF reduction zone 4 is operated at an air ratio of less than 1 because the surface concentration of Si and Mn is suppressed by the preoxidation of DFF.

A pipe 5 for discharging the gas to the outside of the furnace is connected between the DFF oxidation zone 3 and the DFF reduction zone 4. Further, in order to monitor the furnace pressures of the DFF oxidation zone 3 and the DFF reduction zone 4, furnace pressure gauges 6 are installed in the DFF oxidation zone 3 and the DFF reduction zone 4, respectively.

Further, in order to monitor the oxygen concentrations in the DFF oxidation zone 3 and the DFF reduction zone 4, oxygen concentration meters 7 are installed in each of the DFF oxidation zone 3 and the DFF reduction zone 4. Further, in order to monitor the outlet temperature of the direct flame heating furnace (DFF) 1-2, a multiple reflection thermometer 8 is installed on the outlet side of the DFF reduction band 4.

On the downstream side of the pipe 5, a blower 9 for discharging the furnace gas from between the DFF oxidation zone 3 and the DFF reduction zone 4 and a flow meter 10 for monitoring the flow rate of the furnace gas are connected in this order. The flow meter 10 is connected to the afterburning chamber 11 on the downstream side. Further, in the after-burning chamber 11, an air introduction pipe 12 for introducing air into the after-burning chamber 11, an oxygen concentration meter 13 for monitoring the oxygen concentration in the after-burning chamber 11, and a combustion gas in the after-burning chamber 11 Is connected to each of the pipes 14 for returning the air to the pre-tropical 1-1. Oxygen may be injected into the pipe 14 instead of air. Further, the pipe 5 and the pipe 14 are covered with the heat insulating material 15 for the purpose of suppressing the temperature drop of the gas.

In order to obtain good plating properties, it is necessary to secure the optimum amount of oxidation. The experiments leading to the present invention will be described below.

An oxidation experiment was carried out using a high-strength steel plate containing 1.5% of Si as a test material using an open flame burner. FIG. 2 shows a schematic diagram of the experimental device. As shown in FIG. 2, the experimental apparatus is arranged in the order of the preparation chamber, the heating chamber, and the cooling chamber, and each is partitioned by a partition. The experimental equipment is designed so that the steel sheet can be continuously heated and cooled. Further, in order to suppress oxidation outside the heating chamber, the atmospheres of the preparation chamber and the cooling chamber are replaced by _{N 2 gas.} Nitrogen is constantly sprayed into the gaps that are inevitably created to move the steel sheet, suppressing the inflow of air into the heating chamber and suppressing the intrusion of air into the heating chamber when the burner is not burning. .. The steel plate is installed on a movable transport table, moves from the preparation chamber to the heating chamber, is heated, and then is cooled by _{N 2 in the cooling chamber.}

The present inventors conducted an oxidation experiment in which the steel plate was heated to a predetermined temperature (500 ° C., 600 ° C. or 700 ° C.) and cooled by the experimental apparatus shown in FIG. The relationship between the amount of oxidation and the amount of oxidation was investigated. In the oxidation experiment, C: 0.12 mass%, Si: 1.4 mass%, Mn: 1.9 mass%, P: 0.01 mass% were contained, and the balance Fe and unavoidable. A steel plate having a component composition composed of impurities was used. The amount of oxidation was measured by a fluorescent X-ray analyzer. The measurement result of the amount of oxidation is shown in FIG.

From FIG. 3, the higher the temperature of the steel sheet, the larger the amount of oxidation. In addition, at any steel sheet temperature, the amount of oxidation increases once in the range of 0.2 to 0.5% of oxygen concentration, then decreases, and the amount of oxidation is almost the same when the oxygen concentration is 0.7% or more. It was a constant value.

From this, it was found that it is important to keep the oxygen concentration in the DFF oxide zone at a predetermined concentration or higher in order to keep the oxide film of the high Si-added steel uniform over the entire surface of the steel sheet.

As a method of keeping the oxygen concentration of the DFF oxidation zone 3 above a predetermined concentration, it is conceivable to adjust the air ratio of the burner. However, as described above, the reducing gas from the DFF reduction zone 4 and the solitary tropics 2 flows into the DFF oxidation zone 3 in the direction opposite to the traveling direction of the steel sheet S. Therefore, it is difficult to control the gas concentration in the vicinity of the steel sheet S, which is considered to be the most important for the oxidation of the steel sheet S.

Therefore, as a means for adjusting the atmosphere in the vicinity of the steel plate S of the DFF oxide zone 3, attention was paid to gas discharge from between the DFF oxide zone 3 and the DFF reduction zone 4.

Therefore, as a result of further studies by the present inventors, in order to keep the oxygen concentration in the DFF oxidation zone 3 above a predetermined concentration, in the present invention, the gas in the DFF reduction zone 3 is suppressed from flowing into the DFF oxidation zone 4. Therefore, it has been found that the furnace gas is exhausted from between the DFF oxidation zone and the DFF reduction zone, and the furnace pressure in the DFF oxidation zone is kept higher than the furnace pressure in the DFF reduction zone. As a method of keeping the furnace pressure of the DFF oxidation zone higher than the furnace pressure of the DFF reduction zone, for example, a method of making the cross-sectional area of the DFF oxidation zone 3 smaller than the cross-sectional area of the DFF reduction zone 3 with respect to the steel plate advancing direction, or a method of reducing the cross-sectional area of the DFF reduction zone Examples thereof include a method of suppressing the gas flow rate of the DFF reduction band 4 within a range that does not impair the reduction ability of the DFF reduction band 4, and a method of exhausting the gas of the DFF reduction band 4.

The hydrogen gas in the nitrogen-hydrogen mixed gas input to the somatic tropical zone 2 and the hydrogen and carbon monoxide in the exhaust gas discharged from the burner in the DFF reduction zone 4 are the oxygen discharged from the burner in the DFF oxidation zone 3. It reacts as shown in the following formulas (1) and (2).
H ₂ + 0.5O ₂ → H ₂ O ... Equation (1)
CO + 0.5O ₂ → CO ₂ ... Equation (2)
Normally, as shown in the above formulas (1) and (2), oxygen is consumed by hydrogen and carbon monoxide. Therefore, the oxygen concentration in the DFF oxidation zone 3 is lower than the oxygen concentration calculated from the C gas composition, the flow rate, and the air flow rate.

Therefore, based on the formulas (1) and (2), the mass balance of gas in the tropics 2, the DFF oxidation zone 3 and the DFF reduction zone 4 was examined. As a result, from the following equations (3) and (4), the lower limit QL (Nm ³ / h) of the flow rate of the furnace gas discharged from between the DFF oxidation zone 3 and the DFF reduction zone 4, and the DFF oxidation zone 3 It has been found that ^{the upper limit QH (Nm 3} / h) of the flow rate of the furnace gas discharged from the DFF reduction zone 4 is determined respectively.
QL = λ {1.4f (i + j + k) + (0.6g + 1.4) h + (2e + 2d) f + (1.4-2b) c} / {0.7 (i + j + k + 1) + 0.3g + e + d} ... Equation (3) )
QH = λ {3f (i + j + k) + (-g + 1.4) h + (2e + 2d) f + (3-2b) c} / {1.5 (i + j + k + 1) -0.5g + e + d} ... Equation (4)
In the above equations (3) and (4),
b: Oxygen concentration of exhaust gas calculated from the air ratio of the final zone of the DFF oxidation zone and the C gas composition (-)
c: Total exhaust gas flow rate in the final zone of the DFF oxidation zone (Nm ³ / h)
d: Carbon monoxide concentration of exhaust gas calculated from the air ratio of the DFF reduction band and the C gas composition (-)
e: Hydrogen concentration of exhaust gas calculated from the air ratio of the DFF reduction band and the C gas composition (-)
f: Total exhaust gas flow rate in the DFF reduction zone (Nm ³ / h)
g: Hydrogen concentration of hydrogen-nitrogen mixed gas input to the tropics (-)
h: Total flow rate of hydrogen-nitrogen mixed gas input to the tropics (Nm ³ / h)
i: Water vapor concentration of exhaust gas calculated from the air ratio of the DFF reduction band and the C gas composition (-)
j: Nitrogen concentration of exhaust gas calculated from the air ratio of the DFF reduction band and the C gas composition (-)
k: Carbon dioxide concentration of exhaust gas calculated from the air ratio of the DFF reduction band and the C gas composition (-)
λ: Coefficient determined by the production line (-)
Is.

The coefficients b, d, e, i, j, and k are values obtained by reacting the unburned component of C gas with oxygen in the air in a closed system. For example, assuming that the C gas component is given as shown in Table 1, various coefficients b, d, e, i, j, and k with respect to the air ratio can be obtained as follows.

Assuming that the C gas component is each concentration (%) in Table 1, various coefficients are calculated on the assumption that hydrogen and oxygen and carbon monoxide and oxygen are completely burned in the oxidation zone (b, d, e). .. In the reduction zone (i, j, k), the ratio of carbon monoxide and hydrogen not reacting with oxygen changes according to the air ratio (for example, when the air ratio is 0.9, 10% of CO7.32% does not react. ), And can be obtained as follows.
b = -10.125R ² + 37.624R-27.503
d = 1.6131R ² -4.7638R + 3.1509
e ⁼ 12.234R 2 -36.13R + 23.898
i = 26.585R ² -40.288R + 36.427
j = -93.56R ² + 178.53R-15.197
k ⁼ 47.421R 2 -80.048R + 40.28
In addition, R is an air ratio.

The coefficient calculation methods of equations (3) and (4) from the C gas composition are shown below. As a representative, how to obtain the coefficient b will be described. First, the amount of air (= theoretical amount of air) required to completely burn ^{1 m 3} of the fuel is calculated from the C gas composition in Table 1. The theoretical amount of air can be obtained from the following equation (5) using the concentrations (%) of carbon monoxide, hydrogen, methane, ethylene, and oxygen.
100 (0.5CO + 0.5H ₂ + 2CH ₄ + 3C ₂ H _4- O ₂ ) /0.21 ... Equation (5)
Substituting each concentration in Table 1 into equation (5), 100 (0.5 × 7.48 + 0.5 × 56.73 + 2 × 24.76 + 3 × 2.4-0.2) /0.21=4.22 next, when to complete combustion of C gas 1 m ^3, the air becomes a 4.22M ³ required.

For example, when the air ratio is 1.1, the oxygen concentration is obtained as follows. Since the air ratio is 1.1, the amount of air to be introduced is 4.22 × 1.1 = 4.64 m ³ . Here, assuming that the oxygen concentration of the air is 21%, the nitrogen concentration is 79%, and the C gas fuel is 1 m ³ , the volumes occupied by the C gas and air components before the combustion reaction are shown in Table 2.

Based on the chemical reaction formulas of the formulas (1) and (2), the volume of each component of the exhaust gas is calculated, and then the concentration is calculated, which is shown in Table 3.

From the above, the oxygen concentration with an air ratio of 1.1 was determined to be 1.64%. This operation is performed for a typical air ratio, various gas concentrations are plotted against the air ratio, and the gas concentration is obtained by approximating with a quadratic polynomial. That is, b is a quadratic polynomial of oxygen concentration, and b, d, e, i, j, and k correspond to quadratic polynomials of various gas concentrations.

From the above, in the present invention, by exhausting the furnace gas, the furnace pressure of the DFF oxidation zone 3 can be kept higher than the furnace pressure of the DFF reduction zone 4, and the DFF derived by the above formulas (3) and (4) can be maintained. It is preferable to control ^{the lower limit QL (Nm 3} / h) of the flow rate discharged from between the oxidation zone 3 and the DFF reduction zone 4 and the upper limit QH (Nm ^{3 / h) of the flow rate.}

If the flow rate of the discharged furnace gas is less than the lower limit QL, favorable atmospheric conditions cannot be maintained, and the plating property may deteriorate. If the flow rate of the discharged furnace gas exceeds the upper limit QH, the DFF exit plate temperature drops significantly, and the plating property may be extremely deteriorated.

In the present invention, it is desirable to keep the furnace pressure in the DFF oxidation zone higher than the furnace pressure in the DFF reduction zone by 13.5 Pa or more (differential pressure is 13.5 Pa or more).

In the present invention, it is preferable to operate the DFF oxidation zone 3 with an oxygen concentration of 0.7% or more. On the other hand, even if an excessive amount of oxygen is added, the combustion efficiency is deteriorated, and considering the variation in the amount of oxidation, the oxygen concentration is more preferably 1.0% or more. Further, from the viewpoint of combustion efficiency, the upper limit is preferably 1.5%.

Further, in order to control the oxygen concentration of the DFF oxidation zone 3 to an oxygen concentration of 0.7% or more, as shown in FIG. 4, a separate pipe 18 is provided on the DFF oxidation zone entry side to exhaust the combustion exhaust gas on the DFF oxidation zone entry side. Exhaust gas may be exhausted by the blower 16 and combustion exhaust gas may be blown by the nozzle 17 on the DFF oxidation band out side. When the combustion exhaust gas is blown by the nozzle 17, a flow meter 19 may be provided to control the amount of the blown exhaust gas.

The furnace gas discharged from between the DFF oxidation zone 3 and the DFF reduction zone 4 reacts with oxygen (or oxygen only) in the air in the afterburning chamber 11 and burns to generate reaction heat. In the present invention, this combustion gas may be refluxed to Pretropical 1-1. By refluxing the combustion gas, the heating capacity of Pretropical 1-1 is increased, and high-efficiency heating of the steel sheet becomes possible. Further, the combustion exhaust gas of the DFF oxide zone 3 flows to the pre-tropical 1-1 and is used for preheating the steel sheet.

The steel sheet S may be annealed in a reducing atmosphere in a temperature range of 600 to 900 ° C. in the solitary tropics 2 following the DFF oxidation zone 3 and the DFF reduction zone 4. When the steel sheet S is heated above 700 ° C. in the DFF oxidation band 3 and the DFF reduction band 4, the amount of oxidation of the steel sheet becomes excessive, and a part of the oxide is picked up by the roll. Therefore, it is preferable to heat the steel sheet to 700 ° C. or lower in the DFF oxidation zone 3 and the DFF reduction zone 4.

The annealed steel sheet S is cooled in a cooling zone and then hot-dip galvanized by a hot-dip galvanizing apparatus. It is preferable to use a galvanized bath having a bath temperature of 440 to 550 ° C. and an Al concentration of 0.10 to 0.20% in the bath for producing the hot-dip galvanized steel sheet.

If the bath temperature is less than 440 ° C., Zn solidification may occur in a place where the temperature variation in the bath is large. If the temperature exceeds 550 ° C., the bath evaporates violently and the operating cost and vaporized Zn adhere to the inside of the furnace, which causes a problem in operation. Furthermore, since alloying progresses during plating, it tends to become overalloyed.

When the Al concentration in the plating bath is less than 0.10%, a large amount of ζ phase is generated and the powdering property is deteriorated, and when it exceeds 0.20%, Fe—Zn alloying does not proceed.

Next, an alloying treatment is performed. The alloying treatment is optimally performed when the heating temperature of the steel sheet exceeds 460 ° C and is lower than 570 ° C. At 460 ° C. or lower, the alloying progress is slow, and at 570 ° C. or higher, a hard and brittle Zn—Fe alloy layer formed at the ground iron interface is excessively formed due to overalloying, and the plating adhesion deteriorates. Further, since the retained austenite phase is decomposed, the balance between strength and ductility may be deteriorated.

^{The plating adhesion amount is preferably 20 g / m 2} or more (adhesion amount per one side) from the viewpoint of corrosion resistance and plating adhesion amount control. However, if the amount of adhesion is large, the adhesion may decrease, so 120 g / m ² or less (the amount of adhesion per side) is preferable.

The steel sheet 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 deoxidizing agent or as a solid solution strengthening element for increasing the strength. In particular, Si is an element that has a large effect of increasing the strength but has a relatively small deterioration in mechanical properties such as workability, and therefore can be preferably used. However, if the content is less than 0.3% by mass, the concentration on 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. If the Si content exceeds 3.0% by mass, the oxide film formed by this method alone cannot suppress the diffusion of Si into the surface layer, and the proportion of steel sheets that thicken the surface layer increases. Therefore, the upper limit is preferably 3.0% by mass or less. A more preferable range of Si is 0.8 to 1.5% by mass.

Elements other than Si can be contained within the range contained in ordinary cold-rolled steel sheets. For example, C, Mn, Al, P and S have almost no effect on the adhesion of oxides to the in-core rolls to be solved by the present invention, and thus are in the component range required from the mechanical strength characteristics, manufacturability and the like. 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 be contained in the range of 0.200% by mass or less. The balance is Fe and unavoidable impurities.

As shown in FIG. 1, in a CGL having DFF in the heating zone 1, a test was conducted in which the oxygen concentration of the DFF oxidation zone 3 was changed to evaluate the plating property. The oxygen concentration in the DFF oxidation zone 3 was controlled by exhausting gas from between the DFF oxidation zone 3 and the DFF reduction zone 4. Further, the steel plate temperature was measured on the exit side of the DFF reduction band 4 and used as the DFF exit side steel plate temperature. The temperature of the DFF output steel plate was controlled to 680 ± 20 ° C. Further, the respective furnace pressures were measured using the furnace pressure gauges 6 installed on the exit side of the DFF oxidation zone 3 and the outlet side of the DFF reduction zone 4.

The chemical composition of the steel sheet used in the test is shown in Table 4 (the balance is Fe and unavoidable impurities). The width of the steel plate was 1 m. Further, λ = 0.7.

As for the heating zone, the three zones (# 1 to # 3) on the steel plate S entry side were set to the DFF oxide zone 3 (air ratio 1 or more), and the final zone (# 4) was set to the DFF reduction zone 4 (air ratio less than 1). .. Direct fire burners were arranged in each zone so as to face the steel plate, and the number of burners was 72 in each zone (36 on one side). In addition, an oxygen concentration meter 7 was placed in each zone so that the oxygen concentration could be monitored.

In some experiments (condition 7), the gas discharged from the pipe 5 is introduced into the after-burning chamber 11, the air is further flowed into the after-burning chamber 11 from the air introduction pipe 12, and the combustion exhaust gas is pre-tropical 1-1. Refluxed to.

The manufacturing conditions are shown in Table 5. The annealing temperature was adjusted to 830 ° C., the plating bath temperature was adjusted to 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 galvanizing, the alloying treatment was performed at an alloying temperature of 530 ° C.

The plated appearance of the obtained plated steel sheet was evaluated as follows.
(1) Plating appearance The plating appearance was visually evaluated based on the presence or absence of non-plating.

Under conditions 1, 3, 5, and 7, which are examples of the present invention, the oxygen concentration in the DFF oxidation zone (# 1 to # 3) in the heating zone is controlled to 0.7 or more, and a steel sheet having an excellent plating appearance can be produced. Is possible. Non-plating occurs under conditions 2, 4 and 6 in which the furnace gas is not exhausted. Under condition 7 in which the gas discharged from the pipe 5 was burned in the after-burning chamber 11, the plate passing speed was improved and the steel plate could be produced with high efficiency. Further, in Example 8 in which the furnace pressure difference did not satisfy the optimum range, non-plating occurred.

100 Hot-dip galvanizing equipment 1 Heating zone 1-1 Pre-tropical 1-2 Direct-fired heating furnace (DFF)
2 Normal tropical 3 DFF oxidation zone 4 DFF reduction zone 5 piping 6 furnace pressure gauge 7 oxygen concentration meter 8 multiple reflection type thermometer 9 blower 10 flow meter 11 afterburning room 12 air introduction piping 13 oxygen concentration meter 14 piping 15 insulation material 16 blower 17 Nozzle 18 Piping 19 Flowmeter S Steel plate

Claims (4)

In a method for manufacturing a hot-dip galvanized steel sheet using a continuous hot-dip galvanizing facility equipped with an annealing furnace in which a heating zone including a direct-fired heating furnace (DFF) and a soaking furnace are adjacent to each other.
The direct-fired heating furnace has a DFF oxidation zone operated at an air ratio of 1 or more and a DFF reduction zone operated at an air ratio of less than 1.
A method for producing a hot-dip galvanized steel sheet, which comprises exhausting furnace gas from between the DFF oxide zone and the DFF reduction zone to keep the furnace pressure of the DFF oxidation zone higher than that of the DFF reduction zone. The method for producing a hot-dip galvanized steel sheet according to claim 1, wherein the hot-dip galvanized steel sheet is operated at an oxygen concentration of 0.7% or more in the DFF oxide band. The method for producing a hot-dip galvanized steel sheet according to claim 1 or 2, wherein the exhausted furnace gas is burned and then returned to the pre-tropical zone in front of the heating zone. The method for producing a hot-dip galvanized steel sheet according to any one of claims 1 to 3, wherein the hot-dip galvanized steel sheet is alloyed after the hot-dip galvanized treatment.

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