TWI597385B - Continuous molten metal plating method and continuous molten metal plating equipment - Google Patents

Description

Continuous molten metal plating method and continuous molten metal plating equipment

The present invention relates to a continuous molten metal plating method and a continuous molten metal plating apparatus for continuously manufacturing, for example, a molten zinc plated steel sheet.

In the continuous molten zinc plating production line of the steel strip, usually, the steel strip of the cleaned surface is continuously annealed in the annealing furnace, and after cooling to a predetermined temperature, it is introduced into the molten zinc bath, and the steel strip is Molten zinc plating is performed. Typically, the annealing and cooling steps in the annealing furnace are carried out in a reducing environment. Moreover, in order to prevent the steel strip from coming out to the molten zinc plating bath during the annealing furnace, the steel strip passage path is isolated from the atmosphere, so that the steel strip can pass through the reducing environment, and the annealing furnace is formed with A path of a rectangular cross section called a snout is provided between the plating tanks of the molten zinc bath. A sink roll is provided in the molten zinc bath, and the steel strip entering the molten zinc bath rises in the vertical direction by the sinking roller shifting the traveling direction. The steel strip lifted from the molten zinc bath is adjusted to a prescribed plating thickness by a gas wiping nozzle, and then cooled to guide to a subsequent step.

The furnace nose is connected to the cooling zone of the annealing furnace (the exit side of the steel strip), so that the interior thereof is usually a reducing environment. Therefore, it is difficult to form an oxide film on the molten zinc bath surface in the furnace nose, and only a thin oxide film is formed. So formed on the molten zinc bath surface in the nose of the furnace The oxide film is not strong, and when the steel strip enters the molten zinc bath, the molten zinc is exposed to the bath surface by vibration or the like, and the zinc is evaporated into the furnace nose. At this time, the molten zinc is evaporated until reaching a saturated vapor pressure at an ambient temperature inside the furnace nose.

The zinc vapor reacts with a small amount of oxygen present in the reducing ambient gas to form an oxide. Further, even when the zinc vapor is not oxidized, when the vapor pressure of the zinc vapor reaches a saturated vapor pressure or higher, a part of the zinc vapor changes to a liquid phase or a solid phase zinc. In particular, since the furnace nose contains only a thin heat-resistant material, the temperature of the inner wall surface of the furnace nose is easily affected by the outside air, and the temperature below the saturation temperature of the vapor pressure of the zinc vapor is lower than the temperature below the temperature. The zinc vapor turns into zinc powder and adheres to the inner surface of the nose.

When an oxide or an adherend (so-called ash) as described above adheres to a steel strip, quality defects such as an unplated portion may occur. In the present specification, as described above, the quality defect such as the unplated portion caused by the ash generated by the zinc vapor in the furnace nose is referred to as "defect due to ash".

As a technique for suppressing defects caused by ash, there are the following techniques. Patent Document 1 describes a technique in which a heater is used to heat a furnace nose, and then the outside of the heater is insulated by a heat insulating material to set an ambient temperature and an inner wall in the furnace nose. The temperature difference between the temperature and the plating bath temperature is set to 150 ° C or less to prevent ash from adhering to the inner wall of the furnace nose. Patent Document 2 describes a technique in which a suction blower is provided in a plating bath, and a suction port is connected to a suction side of the suction fan at a position higher than a bath surface in the furnace nose. The suction tube discharges the zinc vapor in the furnace nose out of the system. Patent Document 3 describes that The technique of suppressing the generation of smoke (zinc vapor) by using an atmosphere in the furnace nose as a non-oxidizing gas for the steel sheet and making the molten zinc an oxidizing gas.

[Prior Art Literature] [Patent Literature]

[Patent Document 1] Japanese Patent Laid-Open No. Hei 8-176773

[Patent Document 2] Japanese Patent Laid-Open No. Hei 8-302453

[Patent Document 3] Japanese Patent Laid-Open No. Hei 6-330271

According to the technique of Patent Document 1, by heating the furnace nose, crystallization of zinc vapor on the inner wall of the furnace nose, that is, generation of ash can be suppressed to some extent. However, since the generation of zinc vapor from the molten zinc bath surface cannot be prevented, it is unavoidable that ash is generated in an unheated place, and the potential danger of ash adhering to the steel strip cannot be excluded.

According to the technique of Patent Document 2, since the zinc vapor in the furnace nose cannot be reliably discharged, the zinc vapor which is not discharged adheres to the inner wall of the furnace nose to generate ash, so that the effect of preventing defects due to ash is insufficient. Further, the discharge of the zinc vapor adversely affects the evaporation of the molten zinc, so that there is no effect.

In the technique of Patent Document 3, there is a relatively fast convection of gas in the furnace nose. Therefore, a large amount of oxidizing gas to be supplied is not released on the bath surface and is released outside the system. Therefore, if a very large amount of gas is not supplied, an appropriate oxide film cannot be formed, and it is difficult to prevent evaporation of molten zinc.

As described above, in the techniques of Patent Documents 1 to 3, the effect of suppressing defects caused by ash is not sufficient. Further, according to the investigation by the present inventors, it has been clarified that when the oxide film is too thick, when the steel strip enters the molten zinc bath, the oxide film adheres to the surface of the steel strip, which also causes quality defects such as unplated portions. the reason. In the present specification, as described above, a quality defect such as an unplated portion due to an oxide film on the molten zinc bath surface in the furnace nose is referred to as "a defect caused by an oxide film".

Further, the techniques described in Patent Documents 1 to 3 also have the following problems. That is, the suitable oxidizing power of the environment in the furnace nose (especially in the vicinity of the bath surface) varies depending on the chemical composition of the steel strip, the annealing conditions in the annealing step, and the composition of the molten metal bath. Therefore, when switching the operating conditions, the oxidizing power of the environment inside the furnace nose also needs to be switched quickly. However, in the techniques of Patent Documents 1 to 3, there is a problem that the oxidizing power of the environment in the furnace nose cannot be stably and rapidly changed. In particular, in Patent Document 3, since there is a large natural convection in the furnace nose, the oxidizing power of the environment in the furnace nose cannot be stably and rapidly changed.

These problems are not limited to molten zinc plating, but are also applicable to all molten metal plating.

Accordingly, the present invention has been made in view of the above problems, and an object thereof is to provide an unplated which is caused by a metal vapor generated in a furnace nose and an unplated film caused by an oxide film on a molten metal bath surface in a furnace nose. A continuous molten metal plating method and a continuous molten metal plating apparatus which can stably and rapidly change the oxidizing power of the environment in the furnace nose.

In order to solve the above problem, the inventors of the present invention have conducted the following findings.

(A) In order to suppress evaporation of molten zinc (production of zinc vapor) and suppress defects caused by ash, it is necessary to form an oxide film having a certain fixed thickness or more on the bath surface. On the other hand, in order to suppress defects caused by the oxide film, it is necessary to suppress the oxide film to a certain fixed thickness or less. That is, in order to suppress both defects caused by ash and defects caused by an oxide film, it is necessary to form an oxide film of an optimum thickness.

(B) In order to form an oxide film having an optimum thickness as described above, it is necessary to supply an oxidizing gas to the inside of the furnace after suppressing the convection of the environment in the furnace nose, thereby strictly managing the vicinity of the molten zinc bath surface in the furnace nose. The dew point of the environment. Therefore, it is preferable to supply the minimum oxidizing gas to the furnace nose in a state where the heat convection of the environment in the furnace nose has been suppressed. The reason for this is that the oxidizing gas supplied to the vicinity of the bath surface can be substantially directly retained in the vicinity of the bath surface.

(C) As a result, it is also possible to obtain an effect of stably and rapidly changing the oxidizing power of the environment in the furnace nose. Therefore, when the operating conditions are switched, the oxidizing power of the environment in the furnace nose can be quickly switched in accordance with the changed operating conditions.

The present invention has been completed based on the above findings, and the gist thereof is as follows.

(1) A continuous molten metal plating method comprising the steps of: continuously annealing a steel strip in an annealing furnace; and continuously supplying the annealed steel strip to a molten metal bath and forming a molten metal bath a plating tank for performing metal plating on the steel strip; the continuous molten gold The coating method is characterized in that: when the steel strip passes through the space defined by the furnace nose from the annealing furnace to the molten metal bath, an oxidizing gas is supplied into the furnace nose, and the furnace nose is The temperature of the inner wall surface is set to (plating bath temperature - 150 ° C) or more, and the ambient temperature of the upper portion in the furnace nose is set to (plating bath temperature - 100 ° C) or more, and the furnace nose is placed at the The steel strip exit side of the annealing furnace is provided so that the end portion is immersed in the molten metal bath.

(2) The continuous molten metal plating method according to (1), wherein the oxidizing gas is nitrogen gas containing water vapor or a mixed gas of nitrogen and hydrogen containing water vapor.

(3) The continuous molten metal plating method according to (1) above, wherein the oxidizing power of the oxidizing gas is changed in accordance with an operating condition.

(4) The continuous molten metal plating method according to (2) above, wherein the amount of water vapor in the oxidizing gas is changed in accordance with an operating condition.

(5) The continuous molten metal plating method according to (2), further comprising the step of: inspecting a dew point in the furnace nose and performing a metal under the operating condition for each operating condition a relationship between the amount of defects caused by the unplated steel strip being plated, determining a target dew point within the furnace nose under the operating conditions; and based on a target dew point determined for each of the operating conditions, The amount of water vapor in the oxidizing gas is determined.

(6) The continuous molten metal plating method according to (5) above, wherein When the operating condition is switched, the amount of water vapor in the oxidizing gas is changed based on the target dew point corresponding to the changed operating condition.

(7) The continuous molten metal plating method according to any one of (3), wherein the operating condition is a composition of the steel strip, and annealing in the annealing step At least one of a condition and a composition of the molten metal bath.

(8) The continuous molten metal plating method according to any one of (3) to (6), wherein the operating condition is a composition of the steel strip.

(9) The continuous molten metal plating method according to any one of (1), wherein the oxidizing property is supplied from both end portions of the furnace nose in the width direction of the steel strip. gas.

(10) A continuous molten metal plating apparatus, comprising: an annealing furnace for continuously annealing a steel strip; a plating tank for containing molten metal to form a molten metal bath; and a furnace nose disposed in the annealing furnace The steel strip exit side is provided so that the end portion is immersed in the molten metal bath, and the space through which the steel strip continuously supplied from the annealing furnace to the molten metal bath passes is divided; the heating body is set An outer wall of the furnace nose and an upper portion of the furnace nose; a gas supply mechanism coupled to the furnace nose; and a control unit that controls the heating body and the gas supply mechanism to supply an oxidizing gas Into the furnace nose, and the temperature of the inner wall surface of the furnace nose is set to (plating bath temperature -150 ° C) or more, and the ambient temperature of the upper portion of the furnace nose Set to (plating bath temperature -100 ° C) or more.

According to the continuous molten metal plating method and the continuous molten metal plating apparatus of the present invention, it is possible to simultaneously suppress the unplating caused by the metal vapor generated in the furnace nose and the oxide film caused by the molten metal bath surface in the furnace nose. Unplated, in addition, the oxidizing power of the environment in the furnace nose can be changed stably and rapidly.

10‧‧‧ Annealing furnace

12‧‧‧ plating tank

12A‧‧‧ molten zinc bath

14‧‧‧Hose

14A‧‧‧End of the nose

16, 17‧‧‧ heater

18‧‧‧Insulation materials

20‧‧‧ gas supply mechanism

22A, 22B, 22C, 22D, 22E‧‧‧ piping

24‧‧‧ valve

26‧‧‧Bottom bending roll

28‧‧‧ sunk roll

30‧‧‧Support roller

32A, 32B‧‧‧ dew point measuring hole

100‧‧‧Continuous molten zinc plating equipment

P‧‧‧ steel strip

Fig. 1 is a schematic view of a continuous molten zinc plating apparatus 100 according to an embodiment of the present invention.

Fig. 2 is a view showing only a half of the inside of the furnace nose 14 in Fig. 1 from the center in the width direction of the steel strip P.

Figure 3 is an enlarged schematic view of the furnace nose 14 of Figure 1.

Fig. 4 is a graph showing the relationship between the oxidizing power and the defect rate in the bath surface environment.

Fig. 5(A) is a graph showing the relationship between the oxidizing power of the bath surface environment and the defect rate for the Si high content steel and the Si low content steel, and Fig. 5(B) is for the high Al content bath and the Al low content bath. A graph showing the relationship between the oxidizing power of the bath surface environment and the thickness of the oxide film.

6(A) and 6(B) are graphs showing the relationship between the dew point and the defect rate in the furnace nose of the steel type A and the steel type B, respectively.

Fig. 7 is a graph showing the fluctuation of the dew point in the furnace nose in Inventive Example 1 to Invention Example 3, Comparative Example 1, and Comparative Example 2.

Hereinafter, a continuous molten zinc plating apparatus 100 according to an embodiment of the present invention and a continuous molten zinc plating method using the continuous molten zinc plating apparatus 100 will be described.

Referring to FIG. 1, a continuous molten zinc plating apparatus 100 includes an annealing furnace 10, a plating tank 12, and a furnace nose 14.

The annealing furnace 10 is a device for continuously annealing the steel strip P passing through the inside thereof, and is arranged in parallel in the order of the heating belt, the soaking zone, and the cooling zone. Only the cooling belt is illustrated in FIG. As the annealing furnace, a furnace having a known configuration or an arbitrary configuration can be used. A reducing gas or a non-oxidizing gas is usually supplied to the inside of the annealing furnace. As the reducing gas, a mixed gas of H ₂ -N ₂ is usually used, and for example, a gas having a composition of H ₂ of 1% by volume to 20% by volume and the balance containing N ₂ and unavoidable impurities (dew point) :-60 ° C or so). Further, examples of the non-oxidizing gas include a gas having a composition containing N ₂ and unavoidable impurities (dew point: about -60 ° C). The annealed steel strip P is cooled to about 470 ° C to 500 ° C by a cooling belt.

In the plating tank 12, molten zinc is accommodated to form a molten zinc bath 12A. The furnace nose 14 is provided on the steel strip outlet side of the annealing furnace 10. In the present embodiment, it is provided in connection with the cooling belt. The end portion 14A of the furnace nose is provided so as to be immersed in the molten zinc bath 12A. The furnace nose 14 is a member that divides a space through which the steel strip P continuously supplied from the annealing furnace 10 to the molten zinc bath 12A passes. In the upper portion of the furnace nose 14, a lower bending roller that turns the traveling direction of the steel strip P from the horizontal direction to the obliquely downward direction is disposed. Down roll) 26. The portion in which the space through which the steel strip P passes after the lower bending roll 26 is divided is formed into a rectangular shape in a cross-sectional view perpendicular to the traveling direction of the steel strip P.

The steel strip P passes through the inside of the furnace nose 14 and continuously enters into the molten zinc bath 12A. A sinking roller 28 and a support roll 30 are provided in the molten zinc bath 12A, and the steel strip P which has entered the molten zinc bath 12A is changed to the upward direction by the sinking roller 28, and is supported by the backup roller. 30 guides and continuously comes out of the molten zinc bath 12A. The steel strip P is subjected to molten zinc plating in the manner as described above.

Referring to Fig. 2, the continuous molten zinc plating apparatus 100 includes a gas supply mechanism 20 coupled to the furnace nose 14. The gas supply mechanism 20 includes a first pipe 22A through which hydrogen gas passes, a second pipe 22B through which nitrogen gas passes, a third pipe 22C through which steam as an oxidizing gas passes, and a valve for adjusting a flow rate attached to the pipes. (valve) 24, the fourth pipe 22D through which the mixed gas obtained by mixing the gas supplied from the pipes, and the fifth pipe 22E connected to the fourth pipe 22D and having the tip end located inside the furnace nose 14. The first pipe 22A and the third pipe 22C are connected to the second pipe 22B, and by adjusting the valve 24, hydrogen gas, nitrogen gas, and water vapor can be mixed at an arbitrary flow rate ratio.

The oxidizing gas is not particularly limited as long as it contains a gas such as steam, oxygen, or carbon dioxide. However, it is preferable to use a gas containing water vapor from the viewpoint that the oxidizing power is not high, it is easy to manage, the cost is low, and the oxidizing power can be easily measured by a dew point meter.

Referring to Fig. 3, a heater 16 as a heating body is disposed on the outer wall of the furnace nose 14, and the heater 16 is covered by a heat insulating material 18. Furthermore, heater 16 is in addition to The entire surface of the outer wall is covered except for the front end portion of the furnace nose 14 (near the bath surface). Further, a heater 17 as a heating body is also disposed in the upper portion of the furnace nose. As described below, the upper portion of the furnace nose has a large influence on the generation of heat convection. Therefore, by providing the heater 17, the ambient temperature of the upper portion of the furnace nose can be surely increased.

In the present embodiment, it is important to control the heater 16, the heater 17, and the gas supply mechanism 20 by a control unit (not shown) to supply an oxidizing gas into the furnace nose 14 and to open the inner wall surface of the furnace nose 14. The temperature is controlled to (plating bath temperature - 150 ° C) or more, and the ambient temperature of the upper portion in the furnace nose 14 is managed to (plating bath temperature - 100 ° C) or more. Hereinafter, the technical significance will be described in detail.

As described above, in the environment inside the furnace nose, there is an optimum value regarding its oxidizing property. Fig. 4 is a view showing the concept thereof. When the oxidizing property is low, an oxide film is not formed on the bath surface, or even if an oxide film is formed, it is difficult to cause defects due to the oxide film, but zinc evaporation is actively generated, so that defects caused by ash increase. Big. On the other hand, when the oxidizing property is high, the thick oxide film serves as a protective film and hardly causes evaporation of zinc, so that defects caused by ash are hard to occur, but defects caused by the oxide film are generated in a large amount.

Therefore, it is necessary to strictly control the oxidizing power of the environment near the bath surface where zinc is evaporated and oxidized at an optimum level (central portion in Fig. 4). The present inventors have found that, for example, when the gas containing water vapor is supplied into the furnace nose to control the oxidizing power of the environment near the bath surface, the dew point of the environment near the bath surface is strictly controlled at a predetermined point. (Target dew point) The range of ±4 °C or so can control both defects caused by ash and defects caused by oxide film to a low level. Again, about the purpose Dew point, as long as the operating conditions other than the target dew point are determined, can be determined by the following method.

Here, it is the convection of the environment in the furnace nose that makes it difficult to manage the dew point near the bath surface. As the convection in the furnace nose, the accompanying flow by the movement of the steel strip, the heat convection accompanying the temperature difference in the furnace nose, and the pressure flow due to the pressure difference in the furnace nose can be mainly cited. However, under normal furnace conditions, the effects caused by thermal convection are dominant. For example, in the case of a steel strip temperature of 500 ° C and a plating bath temperature of 450 ° C, a temperature difference of 400 ° C or more is present inside the furnace nose and outside the furnace nose. Further, in general, the upper portion of the furnace nose is connected to the cooling belt, and therefore the ambient temperature in the upper portion of the furnace nose is often in the range of 200 ° C to 300 ° C. At this time, the wind speed generated by the heat convection is about 4 m/s to 5 m/s, which is quite large compared with the typical value of the accompanying flow of the steel strip, that is, 1 m/s.

In the above-mentioned situation, even if a gas which promotes oxidation of the bath surface, for example, a gas containing water vapor, does not remain on the bath surface, it is desired to form an oxide film having a suitable thickness for suppressing defects caused by ash. Need to invest a lot of water vapor. In addition to this, in order to suppress defects caused by the oxide film, it is advantageous that the oxide film is as thin as possible, and as a result, it is necessary to minimize the concentration distribution of the oxidizing gas in the vicinity of the bath surface. However, under the condition of large heat convection, the concentration distribution of the oxidizing gas near the bath surface is increased (that is, the concentration becomes uneven in the plane), so the dew point management near the bath surface is extremely difficult.

Based on the above findings, the inventors have concluded that in order to strictly control the dew point near the bath surface, it is most effective to suppress the evaporation of zinc itself by suppressing defects caused by ash and defects caused by the oxide film. Therefore, the best is to suppress After the heat convection in the furnace nose, a minimum amount of oxidizing gas is supplied to the furnace nose.

Therefore, the inventors intend to reduce the temperature difference in the furnace nose, which causes the heat convection as described above. The steel strip is the highest inside the furnace nose, but usually the steel strip is only about 10 ° C above the bath temperature. Therefore, in the present invention, the temperature reference is set as the plating bath temperature. Further, since the heat convection and the accompanying flow of the steel strip are opposite directions, the convection in the furnace nose can be greatly suppressed as long as the magnitude of the heat convection can be made twice or less the size of the accompanying flow of the steel strip.

As a result of various investigations, it has been found that if the temperature of the inner wall surface of the furnace nose is set to (plating bath temperature -150 ° C) or more, the convection of the environment in the furnace nose can be suppressed until the temperature state of the flow influence is ignored. . However, the ambient temperature in the upper portion of the furnace nose has a greater influence on the heat convection, so it is necessary to set it to (plating bath temperature - 100 ° C) or more. The reason for this is that the density flow further increases when the dense gas is present at a high position. (The airflow due to density is proportional to Δρgh. h is the difference in height position, and if the high density is at a high position, the flow rate is increased.)

Further, the ambient temperature in the upper portion of the furnace nose is preferably set to (plating bath temperature + 100 ° C) or less. The reason for this is that the higher the upper ambient temperature, the more stable the convection in the furnace nose (the low-density material is stable in the upper state), but the stabilizing effect is achieved when it exceeds (plating bath temperature + 100 ° C). limit. Further, the temperature of the inner wall surface of the furnace nose is preferably set to (plating bath temperature + 0 ° C) or less. When the temperature of the inner wall surface is higher than the plating bath temperature, an upward flow occurs in the vicinity of the side wall in the furnace nose, and a downward flow is generated in the central portion under the influence thereof. The gas stream is phased with the gas stream generated by the accompanying flow of the steel strip In the same direction, it will cause a large flow in the nose. Therefore, it can be said that the temperature of the inner wall surface does not necessarily exceed the temperature of the plating bath, but the possibility of increasing the flow is high.

In the present invention, the "upper portion in the furnace nose" is defined as an area in the furnace nose within 1 m from the surface of the lower bending roller. In Fig. 3, "the upper portion in the furnace nose" is within a range of 1 m from the surface of the lower bending roller 26 in the furnace nose 14.

As described above, by supplying the oxidizing gas into the furnace nose while the temperature of the inner wall surface of the furnace nose and the ambient temperature of the upper portion in the furnace nose are managed, the oxidation to the vicinity of the bath surface can be advanced in advance. Most of the gas is retained on the bath surface, so that the generation of zinc vapor can be suppressed with a smaller amount of gas. Further, since the gas component supplied into the furnace nose is substantially directly present in the vicinity of the bath surface, environmental control is facilitated, and fluctuations in the dew point of the environment in the vicinity of the bath surface can be suppressed. As a result, defects caused by the oxide film can also be suppressed. In this way, it is desirable to maintain the oxidation state of the bath surface in the furnace nose, so that both defects caused by ash and defects caused by the oxide film can be almost eliminated. Further, it is also possible to obtain an effect of stably and rapidly changing the oxidizing power of the environment in the furnace nose. Therefore, when the operating conditions are switched, the oxidizing power of the environment in the furnace nose can be quickly switched in accordance with the changed operating conditions.

The oxidizing gas supplied into the furnace nose is preferably nitrogen gas containing water vapor or nitrogen-hydrogen mixed gas containing water vapor, and the dew point can be appropriately set according to the composition of the plating bath, the type of steel to be produced, and other operating conditions. However, it is generally good in the range of -20 ° C ~ -35 ° C. Further, the supply amount of the oxidizing gas is affected by various operating conditions, but in the case of the same conditions except for the temperature of the inner wall surface of the furnace nose and the ambient temperature of the upper portion in the furnace nose, The same dew point can be achieved with a supply of about 1/4 compared to the external conditions. Therefore, the supply amount of the oxidizing gas can be set to a minimum necessary amount for forming an appropriate oxide film.

As shown in Fig. 2, the oxidizing gas is preferably supplied into the furnace nose 14 from both end portions of the furnace nose in the width direction of the steel strip. The reason why the fifth pipe 22E including the gas inlet port is provided on the side surface of the furnace nose 14 is that the temperature in the vicinity of the side surface in the furnace nose is often low. Therefore, the downstream flow is usually reduced in the vicinity of the side surface, so that oxidation can be performed. The gas reaches the bath surface efficiently. The height from the bath surface of the gas inlet can be set to about 100 mm to 3000 mm. If it is less than 100 mm, the possibility that the gas directly reaches the bath surface is high, and as a result, the concentration distribution of the oxidizing gas in the vicinity of the bath surface is increased. On the other hand, when it exceeds 3000 mm, the distance from the bath surface is large, and the gas concentration is lowered. As a result, a large amount of gas is required.

Here, the suitable oxidizing power of the environment in the vicinity of the bath surface in the furnace nose fluctuates due to operating conditions such as the chemical composition of the steel strip, the annealing conditions in the annealing step, and the components of the molten zinc bath. That is, the two curves shown in FIG. 4 can be shifted to the left and right depending on the operating conditions. Hereinafter, the case will be described with reference to FIGS. 5(A) and 5(B) as an example.

First, as described above, the defects caused by the ash and the defects caused by the oxide film are all related to the thickness of the oxide film formed on the bath surface. Specifically, the defect caused by the ash is related to the amount of ash generation and the adhesion rate thereof, and the defect caused by the oxide film depends on the amount of the oxide film and the adhesion rate thereof.

Fig. 5(A) shows an example of the influence of the chemical composition of the steel strip on the suitable oxidizing power in the environment in the vicinity of the bath surface in the furnace nose. When the steel strip contains a lot of Si, When a so-called oxidizable element such as Mn or Al is formed, a large amount of surface concentrate is formed on the surface of the steel strip immediately before entering the plating bath. When the plating is performed in the surface-concentrated state as described above, the oxide film is likely to adhere to the steel strip, that is, the adhesion rate of the oxide film is increased, so that defects due to the oxide film are likely to occur. On the other hand, the amount of ash generated hardly depends on the surface concentration state of the steel strip, and thus the composition of the steel strip has little effect on defects caused by ash.

Further, the surface concentration state of the steel strip is also different depending on the annealing conditions such as the annealing temperature or the dew point in the furnace. Therefore, the annealing conditions also affect the easiness of the defects caused by the oxide film, but the defects caused by the ash are roughly no effect.

Fig. 5(B) shows an example of the influence of the composition of the molten zinc bath on the suitable oxidizing power of the environment in the vicinity of the bath surface in the furnace nose. As shown in FIG. 5(B), the higher the Al concentration in the bath, the easier it is to form an oxide film on the bath surface. Therefore, the more the Al high-content bath, the more difficult it is to cause defects caused by ash, and the more easily the defects caused by the oxide film occur. That is, the two curves of FIG. 4 are shifted to the left.

Therefore, it is preferred to change the oxidizing power of the oxidizing gas in accordance with the operating conditions. In other words, when the oxidizing gas contains water vapor, the appropriate dew point of the environment near the bath surface, that is, the target dew point differs depending on the operating conditions. Therefore, the amount of water vapor in the oxidizing gas may be changed in accordance with the operating conditions. Further, the amount of water vapor in the oxidizing gas is usually 100 ppm or more.

At this time, the operating conditions can be determined by examining in advance the relationship between the dew point in the furnace nose and the defect rate caused by the ash and the defect rate of the defect caused by the oxide film (ie, the information of FIG. 4) for each operating condition. The target dew point inside the furnace nose. and, The amount of water vapor in the oxidizing gas can be determined based on the target dew point determined for each operating condition. When the operating conditions are switched, the amount of water vapor in the oxidizing gas may be changed based on the target dew point corresponding to the changed operating conditions.

Here, the relationship between the dew point in the furnace nose as shown in FIG. 4 and the defect rate caused by the ash and the defect rate caused by the oxide film can be grasped in advance by the dew point in the furnace nose in the past operation. The tendency of the defect rate of each defect is obtained. The presence or absence of each defect can be determined by visual inspection. The size of the defect that can be discerned by visual observation is about 100 μm or more. Further, the defect mixing rate per length of 0.5 m is defined as "defect rate". The defect rate of 1% is equivalent to 1 / 50m.

Furthermore, the dew point in the furnace nose must be the dew point directly above the bath surface (near the bath surface). When the portion where the dew point is actually measured is not directly above the bath surface, the following treatment is performed. First, according to the present invention, as long as the heat convection in the furnace nose has disappeared, since there is almost no dew point distribution in the furnace nose, the actual dew point can be directly used. However, when there is thermal convection in the furnace nose, the measured dew point is corrected to the dew point near the bath surface. The correction can be made using the dew point distribution predicted by the flow analysis. For example, in the flow analysis, the dew point at a height of 500 mm from the bath surface is -35 ° C, and when the dew point near the bath surface is -30 ° C, the difference between the two is +5 ° C, and the difference in moisture ratio is 150 ppm. Therefore, the dew point obtained by adding 150 ppm of the dew point value at the measured height of 500 mm can be used as the bath surface dew point.

As an operating condition that affects the oxidizing power (the target dew point of the environment in the vicinity of the bath surface when the oxidizing gas contains water vapor) in the vicinity of the bath surface in the furnace nose, the steel type (the composition of the steel strip) Composition), annealing in the annealing step Conditions and composition of the molten zinc bath. Therefore, it is preferable to obtain the information of FIG. 4 in advance in consideration of at least one of the conditions. For example, in a specific continuous molten zinc plating apparatus, in the case where the annealing conditions and the composition of the molten zinc bath are known to be changed, the information of FIG. 4 is investigated in advance as long as the predetermined steel grade is introduced to the apparatus. Decide on the target dew point. Further, when the steel grade is switched, the amount of water vapor in the oxidizing gas may be changed so as to reach the target dew point corresponding to the steel grade after the change.

The present invention is not limited to the above embodiment, and the same applies to the case where the steel strip is continuously subjected to molten metal plating.

[Examples]

<Example 1>

The steel strip (hereinafter referred to as steel type A) is introduced into a molten zinc bath at a sheet speed of 60 mpm to 100 mpm by using a continuous molten zinc plating apparatus as shown in FIGS. 1 to 3 to produce a molten zinc-plated steel sheet. In the belt, the composition of the component is C: 0.001%, Si: 0.01%, Mn: 0.1%, P: 0.003%, S: 0.005%, Al: 0.03% in terms of mass percentage (%), and the remainder contains Fe and is not The impurities to be avoided have a plate thickness of 0.6 mm to 1.2 mm, a plate width of 900 mm to 1250 mm, and a tensile strength of 270 MPa. As shown in Fig. 2, the fifth pipe including the gas inlet port was provided on the side surface of the furnace nose, and the height from the bath surface of the gas inlet port was set to 500 mm. Based on past operational data, the relationship between the dew point in the furnace nose and the defect rate caused by ash and the defect rate caused by the oxide film was investigated in advance. The results are shown in Fig. 6(A). Based on Fig. 6(A), the target dew point in the furnace nose was determined to be -30 °C. And know that as long as the dew point in the nose By controlling the range of about -30 ° C ± 4 ° C, both the defects caused by ash and the defects caused by the oxide film can be suppressed to a low level.

When the steel strip passes through the furnace nose, a nitrogen-hydrogen mixed gas containing water vapor is supplied in Test No. 1 to Test No. 5 (in Table 1, "supply of water vapor, yes" is expressed. In the test example No. 6 and the test example No. 7, a nitrogen-hydrogen mixed gas containing no water vapor was supplied from the gas inlet port (in Table 1, "supply of water vapor, no"). The dew point of the input gas in Test Example No. 1 to Test No. 5 was measured by a dew point meter provided in the dew point measurement hole 32A in the fifth pipe in Fig. 2, and is shown in Table 1.

The temperature of the inner wall surface of the furnace nose when the steel strip passed through the furnace nose and the ambient temperature of the upper portion in the furnace nose were managed as the temperatures shown in Table 1. In Test Example No. 6, heating was not performed by a heater provided on the outer wall of the furnace nose and the upper portion in the furnace nose.

Each of the test examples was a dew point measuring device provided in the dew point measuring hole 32B at the center of the back surface of the furnace nose at a height of 500 mm as shown in Fig. 2, and the dew point of the environment in the furnace nose was measured over time. Further, in each of Test No. 1 to Test No. 7, the flow rate of the input gas was changed based on the difference between the measured dew point and the target dew point (-30 ° C) so that the measured dew point was close to the target dew point. The control is performed by a general proportional integral derivative (PID) control logic. Table 2 shows the histograms of the measured dew points in each of Test No. 1 to Test No. 7. In Test Example No. 1 to Test No. 5, the ratio of the volume of water vapor to the total volume of the input gas in the test is referred to as "water content", and is shown in Table 1, and the total amount of gas in the test. Inject traffic to set number 5 to 1 The manner in which the index is expressed is shown in Table 1.

Furthermore, if the dew point to be managed is considered to be the dew point directly above the bath surface, the position of the dew point meter should be near the lower bath surface, but according to the present invention, there is almost no dew point distribution in the furnace nose. Even if the dew point is measured at a position of 500 mm in height, the dew point near the bath surface can be grasped with high precision. Further, in the case of the comparative example in which zinc vapor is generated, there is a risk that zinc vapor adheres to the sensor portion of the dew point meter at a position lower by about 100 mm from the bath surface, so that the dew point meter cannot be provided in the lower portion of the furnace nose. many. Further, in the example of the present invention, since water vapor is used for the oxidizing gas, the gas measuring device is used as a dew point meter. However, when an oxidizing gas other than steam is used, it is of course necessary to provide a measuring device for detecting the gas. .

(evaluation of defect rate)

The defect rate of each of the defects caused by ash and the defects caused by the oxide film was evaluated by the following method. The presence or absence of each defect is determined by visual inspection. The size of the defect that can be discerned by visual observation is about 100 μm or more. Further, the defect mixing ratio per 0.5 m length is defined as "defect rate", which is shown in Table 1. The defect rate of 1% is equivalent to 1 / 50m.

(evaluation result)

The evaluation results will be described with reference to Tables 1 and 2. No. 1 (Invention Example) is an example in which no temperature difference is applied to the bath temperature, the wall surface temperature, and the upper temperature, and there is almost no dew point fluctuation. As a result, defects caused by ash and defects caused by the oxide film hardly occur. No. 2 (invention example) is an example in which the wall surface temperature is low, and No. 3 (invention example) is an example in which the ambient temperature in the upper portion of the furnace nose is low, but the environment inside the furnace nose can be The dew point suppression is within the management range (-30 °C ± 4 °C), and the defect rate can be maintained at a low level. Further, in the numbers 1 to 3, the gas input flow rate can be made sufficiently lower than the number 5.

On the other hand, the number 4 (comparative example) is an example in which the wall surface temperature is out of the range of the present invention, and the number 5 (comparative example) is an example in which the ambient temperature of the upper portion of the furnace nose is out of the range of the present invention, so that the dew point of the environment inside the furnace nose cannot be obtained. Inhibition in the management range (-30 ° C ± 4 ° C). As a result, a large amount of defects caused by ash or defects caused by an oxide film are generated in a large amount. No. 6 (Comparative Example) is an example in which no steam is supplied and heating is not performed by a heater. At this time, the dew point becomes low before and after -40 ° C, so that defects caused by the oxide film are not generated, but a large number of defects caused by ash are generated. In No. 7 (comparative example), since the temperature difference was not given, the dew point was stable, but it was low before and after -40 ° C, so that a large number of defects due to ash still occurred.

<Example 2>

In addition to the steel strip of the steel type A, the composition of the component is C: 0.12%, Si: 1.0%, Mn: 1.7%, P: 0.006%, S: 0.006%, Al: 0.03%, and the remainder is used. In the same manner as in the first embodiment, except for a steel strip having a thickness of 0.6 mm to 1.2 mm, a sheet width of 900 mm to 1250 mm, and a tensile strength of 780 MPa (hereinafter referred to as steel type B) containing Fe and unavoidable impurities. The relationship between the dew point in the furnace nose and the defect rate caused by the ash and the defect rate caused by the oxide film. The results are shown in Fig. 6(B).

6(A) and FIG. 6(B), it is understood that both the steel type A and the steel type B have a dew point which can sufficiently suppress both defects caused by ash and defects caused by the oxide film, but the optimum of the steel type B is obtained. The value is lower than the target dew point, and the defects of both The range of dew points that are sufficiently reduced is also narrower. From this, it is understood that, for example, when switching from the steel type A to the steel type B, it is necessary to change the environmental dew point with high precision in a short time.

<Example 3>

When the bath temperature, the wall surface temperature, and the upper temperature were set to No. 1 to No. 5 (Inventive Example 1 to Inventive Example 3 and Comparative Example 1 and Comparative Example 2) described in Table 1, the nitrogen-hydrogen mixed gas containing water vapor was used. The speed of the dew point switching was investigated. As shown in Fig. 7, the input dew point was switched from -35 ° C to -20 ° C at a time point of 50 minutes.

In the first invention, the bath temperature, the wall surface temperature, and the upper temperature were all set to 450 ° C, so that heat convection was hardly generated. Therefore, the measurement of the change in the dew point shows an behavior that is substantially the same as the fluctuation of the dew point of the input gas. Therefore, the dew point in the furnace nose can be directly controlled by the dew point of the supplied gas, so that it is very advantageous in quality management. In the second and third inventions, the follow-up delay can be seen in the dew point after the switching, but the dew point can be changed after about 30 minutes, and the dew point can be changed. Therefore, the quality control is also sufficient.

On the other hand, in Comparative Example 1 and Comparative Example 2, the dew point in the furnace nose was changed while switching the input dew point, and the temperature continued to rise slowly, and it was difficult to say that it was stabilized even after one hour. In such a state, it is difficult to cope with, for example, a change in the target dew point when the steel type A is switched to the steel type B.

[Industrial availability]

The continuous molten metal plating method and the continuous molten metal plating apparatus according to the present invention can simultaneously suppress unplating caused by metal vapor generated in the furnace nose and caused by an oxide film on the molten metal bath surface in the furnace nose. Not plated.

10‧‧‧ Annealing furnace

12‧‧‧ plating tank

12A‧‧‧ molten zinc bath

14‧‧‧Hose

14A‧‧‧End of the nose

26‧‧‧Bottom bending roll

28‧‧‧ sunk roll

30‧‧‧Support roller

100‧‧‧Continuous molten zinc plating equipment

P‧‧‧ steel strip

Claims (8)

A continuous molten metal plating method comprising the steps of: continuously annealing a steel strip in an annealing furnace; and continuously feeding the annealed steel strip to a plating tank containing a molten metal bath and forming a molten metal bath Metal plating is applied to the steel strip; the continuous molten metal plating method is characterized in that: when the steel strip passes from the annealing furnace to the molten metal bath through the space defined by the furnace nose, An oxidizing gas is supplied to the inside of the furnace, and the temperature of the inner wall surface of the furnace nose is set to (plating bath temperature - 150 ° C) or more (plating bath temperature + 0 ° C) or less, and the furnace nose is The upper ambient temperature is set to (plating bath temperature -100 ° C) or more, (plating bath temperature + 100 ° C) or less, and the furnace nose is disposed on the steel strip outlet side of the annealing furnace, and ends It is provided by immersing in the molten metal bath. The continuous molten metal plating method according to claim 1, wherein the oxidizing gas is nitrogen gas containing water vapor or a mixed gas of nitrogen and hydrogen containing water vapor. The continuous molten metal plating method according to claim 1, wherein at least one of a composition of the steel strip, an annealing condition in the annealing step, and a composition of the molten metal bath is The oxidizing power of the oxidizing gas is changed. The continuous molten metal plating method according to claim 2, wherein at least one of a composition of the steel strip, an annealing condition in the annealing step, and a composition of the molten metal bath is Changing the oxidizing gas The amount of water vapor. The continuous molten metal plating method according to claim 2, further comprising the steps of: a composition of each of the steel strips, an annealing condition in the annealing step, and a composition of the molten metal bath a combination of pre-investigating the relationship between the dew point in the furnace nose and the amount of defects caused by the unplating of the steel strip in which the metal plating is combined, and determining the inside of the furnace in the combination The target dew point; and the amount of water vapor in the oxidizing gas is determined based on the target dew point determined for each of the combinations. The continuous molten metal plating method according to claim 5, wherein at least one of a composition of the steel strip, an annealing condition in the annealing step, and a composition of the molten metal bath is switched The amount of water vapor in the oxidizing gas is changed based on the target dew point corresponding to the changed combination. The continuous molten metal plating method according to any one of claims 1 to 6, wherein the oxidizing gas is supplied from both end portions of the furnace nose in the width direction of the steel strip. A continuous molten metal plating apparatus, comprising: an annealing furnace for continuously annealing a steel strip; a plating tank for containing molten metal to form a molten metal bath; a furnace nose, a steel strip disposed in the annealing furnace a outlet side, wherein the end portion is immersed in the molten metal bath, and a space through which the steel strip continuously supplied from the annealing furnace to the molten metal bath passes is divided; a heating body provided on an outer wall of the furnace nose and an upper portion in the furnace nose; a gas supply mechanism coupled to the furnace nose; and a control unit that controls the heating body and the gas supply mechanism An oxidizing gas is supplied into the furnace nose, and a temperature of an inner wall surface of the furnace nose is set to (plating bath temperature - 150 ° C) or more (plating bath temperature + 0 ° C) or less, and the The ambient temperature in the upper portion of the furnace nose is set to (plating bath temperature - 100 ° C) or more (plating bath temperature + 100 ° C) or less.