JP2014043633A - Continuous hot dip galvanization method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a continuous hot dip galvanizing method which may suppress generation of quality defects attributed to molten zinc vapor generated in a snout, and furthermore may prevent generation of non-plated parts, and to provide an apparatus for the hot dip galvanizing.SOLUTION: In a continuous hot dip galvanizing method in which steel strips are continuously heat-treated in a continuous heat treatment furnace followed by introducing the strips into a plating tank holding molten zinc to perform hot dip galvanizing, atmosphere gas within a snout installed between the continuous heat treatment furnace and the plating tank is introduced to outside the snout, the amount of zinc vapor in the atmosphere gas is measured, and then the dew-point of the atmosphere gas is controlled such that the measured amount of zinc vapor is equal to a predetermined amount of zinc vapor in the atmosphere gas.

Description

  The present invention relates to a continuous hot dip galvanizing method for preventing the occurrence of quality defects due to hot dip zinc vapor generated in a snout.

  In the continuous hot dip galvanizing production line of steel strip, the steel strip whose surface has been cleaned is usually annealed continuously in a continuous heat treatment furnace, cooled to a predetermined temperature, and then introduced into the continuous hot dip galvanizing apparatus shown in FIG. Hot-dip galvanization is performed by passing the inside of the plating tank 2 filled with zinc. Normally, annealing and cooling processes in a continuous heat treatment furnace are performed in a reducing atmosphere, and the steel strip passageway between the steel strip S leaving the continuous heat treatment furnace and reaching the plating tank 2 is cut off from the atmosphere. In order to allow the steel strip S to pass through in a reducing atmosphere, a rectangular cross-section passage called a snout 1 is provided between the continuous heat treatment furnace and the plating tank 2. A sink roll 4 is installed in the plating tank, and the steel strip S is moved in the vertical direction by changing the traveling direction by the sink roll 4. The steel strip S pulled up from the plating tank 2 is adjusted to a predetermined plating thickness by the gas wiping nozzle 6 and then cooled and guided to a subsequent process.

  In this continuous hot dip galvanizing apparatus, since the inside of the snout 1 is a reducing atmosphere, an oxide film is hardly formed on the plating bath surface in the snout 1, and only a thin oxide film is formed. As described above, since the oxide film formed on the plating bath surface in the snout is not strong, when the steel strip S enters the plating bath 3, the molten zinc is exposed to the bath surface due to vibration or the like, from which the snout is formed. In 1, molten zinc evaporates. In this case, the molten zinc evaporates into the reducing atmosphere gas up to its saturated vapor pressure.

  The evaporated molten zinc vapor reacts with a small amount of oxygen in the reducing atmosphere gas to form an oxide (usually a solid). Even when the evaporated molten zinc is not oxidized, when the vapor pressure of the molten zinc becomes equal to or higher than the saturated vapor pressure, a part of the evaporated molten zinc changes into a liquid phase or solid phase zinc. In particular, since the snout 1 is only composed of a thin heat-resistant material, the temperature of the inner wall of the snout 1 tends to be lower than the saturation temperature in the vapor pressure of the molten zinc evaporated under the influence of outside air, and becomes lower than that temperature. Vapor turns into zinc powder and adheres to the inner surface of snout 1.

  When such oxides and deposits (so-called ash) directly adhere to the cleaned steel strip S, quality defects such as uneven plating and non-plated portions occur. Further, when the oxide falls on the plating bath surface in the snout 1, the melting temperature of the oxide is higher than the temperature of the molten zinc bath, so that it does not redissolve in the molten zinc bath. Furthermore, when the deposit falls on the plating bath surface in the snout 1, it is dissolved again when the deposit is the same zinc as the molten zinc, but in many cases, the deposit is contaminated with impurities. Often the deposits do not redissolve in the molten zinc bath. Therefore, the oxides and deposits that do not re-dissolve float on the bath surface in the snout, travel in the snout and flow in the molten zinc bath accompanying the steel strip S entering the plating bath 3, and the steel strip It moves to the S side and adheres to the surface of the steel strip S. In this case as well, oxides and deposits that do not re-dissolve act as a factor that inhibits the plating of the steel strip S, and as a result, the plating thickness becomes thin or non-plating, resulting in quality defects. .

  Many methods have been proposed in the past for solving the occurrence of quality defects (hereinafter sometimes referred to as ash quality defects) caused by molten zinc vapor generated in the snout. For example, Patent Document 1 discloses a method of reducing zinc vapor by suspending a ceramic ball on a snout bath surface. In Patent Document 2, the inner wall of the snout is heated with a heater, and the outside of the heater is further insulated with a heat insulating material, and the temperature difference between the bath temperature and the snout portion is set to 150 ° C. or less to prevent ash adhesion to the inner wall. A method is disclosed. Further, in Patent Documents 3 and 4, the metal fume contained in the furnace gas is discharged from the snout outside the furnace and recovered by ashing, and then the furnace gas from which the metal fume has been removed is provided with a diffusion tube from the snout. A method for dissipating into the outside air is disclosed. In Patent Document 5, a suction blower is installed in the plating bath, and a suction pipe having a suction port at a position higher than the bath surface in the snout is connected to the suction side of the suction blower to remove zinc vapor in the snout outside the system. A method of discharging is disclosed. Patent Document 6 discloses a technique for reducing the amount of zinc vapor in the snout by managing the oxygen concentration and dew point of the atmosphere in the snout to form a zinc oxide film on the zinc bath surface. Patent Document 7 discloses the relationship between the sum of the concentrations of Si and Al in the base steel sheet and the dew point of the atmosphere in the snout.

JP-A-7-62512 JP-A-8-176773 Japanese Patent Application Laid-Open No. 11-100550 JP 2003-328098 A JP-A-8-302453 JP-A-7-150320 JP 2006-111893 A

However, in the method of floating ceramic balls on the snout bath surface of Patent Document 1, it is not considered at all to prevent quality defects caused by the ash adhering to the snout wall falling directly on the surface of the steel strip, In addition, there is a concern about the occurrence of defects due to ceramic balls mixed in the bath.
The method of Patent Document 2 is not realistic because a large-scale heater and a heat insulating material are necessary to obtain a sufficient effect, and the risk of equipment damage due to thermal stress is high.
Moreover, in the method of providing the exhaust port of patent document 3, 4, since the ash produced | generated from a plating bath surface is certainly discharged | emitted from an exhaust port, a fixed effect is recognized, but in order to completely prevent an ash quality defect Requires a large discharge flow rate, and many operational problems such as an increase in cost for gas discharge and securing of furnace pressure occur, which are difficult to realize.
Moreover, since the method of patent document 5 cannot discharge | emit the zinc vapor | steam in a snout reliably, the zinc vapor | steam which was not discharged adheres to a snout wall, and the effect which prevents the quality defect resulting from the zinc vapor | steam in a snout is insufficient. It is.
In the methods of Patent Documents 6 and 7, there is a fluctuation range in the relationship between the atmospheric dew point in the snout and the amount of zinc vapor in the snout that is the cause of the ash quality defect. There is a problem that it is difficult to suppress quality defects.

  The present invention has been made in view of the above circumstances, and is a continuous hot dip galvanizing method and apparatus capable of suppressing the occurrence of quality defects due to hot zinc vapor generated in a snout and further preventing the occurrence of non-plating. The purpose is to provide.

The gist of the present invention is as follows.
[1] In a continuous hot dip galvanizing method in which a steel strip is continuously heat treated in a continuous heat treatment furnace and then introduced into a plating tank holding molten zinc to perform hot dip galvanization, it is provided between the continuous heat treatment furnace and the plating tank. Introducing the generated atmospheric gas in the snout outside the snout, measuring the amount of zinc vapor in the atmospheric gas, and so that the measured amount of zinc vapor is the amount of zinc vapor in the preset atmospheric gas A continuous hot dip galvanizing method characterized by controlling a dew point.

  According to the present invention, the proper amount of zinc vapor in the snout atmosphere can be maintained by directly measuring the amount of zinc vapor in the snout atmosphere and controlling the dew point. As a result, it is possible to suppress the occurrence of quality defects due to the molten zinc vapor generated in the snout, and to prevent the occurrence of non-plating, and to produce a hot-dip galvanized steel sheet having a good appearance.

It is a graph which shows the relationship between a snout dew point and zinc vapor quantity (Zn vapor collection amount). It is explanatory drawing which shows the conventional continuous hot dip galvanizing apparatus.

The inventors changed the dew point in the atmosphere inside the snout and measured the amount of zinc vapor and the non-plating occurrence rate in the atmosphere inside the snout. First, the atmosphere gas in the snout was led out of the snout, and the amount of zinc vapor in the atmosphere gas in the snout was measured. Specifically, a gas at a constant flow rate was sucked from the inside of the snout by the suction pump, and the atmospheric gas in the snout was passed through the filter. The filter passing gas amount and the zinc vapor amount to determine the zinc vapor per hour in filter area ^{1m 2 (g / m 2 ·} Hr). And the target value of the zinc vapor | steam amount in the atmosphere in a permitted snout was set to 13-17 g / m < ² > * Hr, and the dew point was controlled.

FIG. 1 shows the relationship between the dew point in the snout atmosphere, the amount of zinc vapor in the snout atmosphere, and the oxide film thickness (ZnO). In the snout, an equilibrium reaction of H ₂ O + Zn = H ₂ + ZnO occurs on the surface of the zinc bath. When the dew point in the snout is high, the formation of ZnO is promoted and the amount of zinc vapor is suppressed. On the other hand, when the dew point is low, the formation of ZnO becomes insufficient, and the amount of zinc vapor increases. The inventors of the present invention have the same relationship between the dew point of the atmosphere in the snout, the amount of zinc vapor in the atmosphere in the snout, and the oxide film thickness in a normal continuous hot dip galvanizing facility. The position was fluctuated in the dew point axis direction according to the manufacturing conditions such as the snout structure of the hot dip galvanizing equipment and the zinc bath temperature, and it was found that the position was not constant. Therefore, the absolute value of the dew point cannot control the zinc evaporation amount within a preferable range.

  Therefore, the present invention is characterized in that the amount of zinc vapor is actually measured to control the amount of zinc vapor within a preferable range. By controlling the amount of zinc vapor, the dew point can be controlled. That is, it can be said that the technique of raising the dew point to lower the amount of zinc vapor in the atmosphere inside the snout and lowering the dew point to raise the amount of zinc vapor is effective from the results of FIG. Therefore, the dew point can be controlled by appropriately setting the amount of zinc vapor in the atmosphere inside the snout.

  In the present invention, the amount of zinc vapor contained in the snout atmosphere gas is the concentration of zinc contained in the atmosphere gas. As described above, as a method for measuring the amount of zinc vapor contained in the snout atmosphere gas, a gas having a constant flow rate may be sucked from the inside of the snout by the suction pump. When sucking with a suction pump, the gas inside the snout is filtered by passing through the filter, and the filtered gas is preset. The amount of zinc vapor in the atmosphere gas in the snout varies depending on the product.

The steel strip was hot dip galvanized using the apparatus shown in FIG. The steel strip width is 1.5 m and the average steel strip processing speed is 120 mpm.
While changing the dew point of the atmosphere inside the snout, the amount of zinc vapor in the atmosphere inside the snout was measured by sucking the atmosphere gas inside the snout out of the snout and filtering it with a fixed amount filter. The measurement of the amount of zinc vapor was obtained by analyzing the dissolved zinc dissolved in acid. Specifically, it was dissolved in hydrochloric acid and quantitatively analyzed by ICP. At the same time, the amount of ash quality defects and the rate of non-plating were measured. Based on merchandise shipment criteria, the upper and lower limits of the amount of zinc vapor in the atmosphere in the snout that gives acceptable ash quality defect generation rates and non-plating generation rates were set to 13-17 (g / m 2 · Hr).

The rate of occurrence of ash quality defects and the rate of non-plating were calculated as the rate of occurrence of defects relative to the charged amount. In addition, the dew point of the atmosphere in the snout is about the dew point that is obtained by introducing N ₂ gas through a humidifier into the snout when the dew point is raised, and when the dew point is lowered and the moisture is removed and dried. It was controlled by introducing N ₂ gas at 50 ° C.

  About 1000 tons of hot-dip galvanized steel sheet was produced while controlling the dew point so that the amount of zinc vapor in the snout atmosphere was maintained within the upper and lower limits set above. In the case of the present invention in which the amount of zinc vapor was controlled, the occurrence rate of ash quality defects was 0.10%, and the non-plating occurrence rate was 0.12%. In the case of the conventional technology in which the dew point in the snout is controlled within a certain standard without measuring the amount of zinc in the snout atmosphere, the ash quality defect occurrence rate is 0.43% and the non-plating occurrence rate is 0.33%. It became. Therefore, according to the present invention, both the occurrence rate of ash quality defects and the non-plating occurrence rate are greatly reduced.

DESCRIPTION OF SYMBOLS 1 Snout 20 Snout upper wall part 30 Snout lower wall part 2 Plating tank 3 Plating bath 4 Sink roll 5 Support roll 6 Gas wiping nozzle 7 Touch roll S Steel strip

Claims (1)

  In a continuous hot dip galvanizing method in which a steel strip is continuously heat-treated in a continuous heat treatment furnace and then introduced into a plating tank holding molten zinc to perform hot dip galvanization, a snout provided between the continuous heat treatment furnace and the plating tank The internal atmospheric gas is led out of the snout, the amount of zinc vapor in the atmospheric gas is measured, and the dew point of the atmospheric gas is controlled so that the measured amount of zinc vapor becomes the preset amount of zinc vapor in the atmospheric gas A continuous hot dip galvanizing method.

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