JPH059689A - Apparatus for producing galvanized steel sheet minimized in surface defects - Google Patents

Abstract

PURPOSE:To reduce the capacity of a plating cell and to take out suspended particles before forming dross by sticking these particles onto the surface of a steel sheet so as to ameliorate surface defects by regulating the distance between the bottom of the plating cell and the rear surface of a sink roll and the distance between a pass line and the inside wall of the plating cell. CONSTITUTION:This apparatus for producing the galvanized steel sheet is formed of the plating cell 10, the sink roll 24, a snout 20 provided upstream of this, a support roll 25, a gas wiping device 26 provided downstream of this, etc., and a sealing means 28, etc. The pass line from the snout 20 to the gas piping device 26 and a plating bath surface 16 are exposed to an inert gaseous and/or reducing gaseous atmosphere by this means 28, etc. The distance C between the bottom of the plating cell 10 and the rear surface of the roll 24 is specified to <=200mm and the distance A between the pass line before the steel sheet 12 comes into contact with the roll 24 and the inside wall 14 of the plating cell to <=200mm. Further, the distance D between the pass line changed in direction by the roll 24 and the above-mentioned inside wall 14 is specified to <=700mm.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for producing a hot-dip galvanized steel sheet with few surface defects and a method for supplying a plating raw material to the apparatus. More specifically, the present invention relates to an apparatus for producing a hot-dip galvanized steel sheet with a small number of surface defects and a method for supplying a plating raw material, which can realize a variety of small lots, energy saving, resource saving, and low noise.

[0002]

2. Description of the Related Art Today, the specifications required for thin steel plates (hereinafter referred to as "steel plates") are becoming strict, and the corrosion resistance is becoming severe especially for building materials, home appliances and automobiles, and further improvement is required. Therefore, plated steel sheets have become common. The hottest galvanized steel sheet currently used is hot-dip galvanized steel sheet. Usually, this is used by further coating, and in addition to corrosion resistance, smoothness of the finished surface after coating, adhesion to the coated surface, etc. are required, so the surface quality of the plated surface is important.

In the case of hot-dip galvanized steel sheet, defects caused by dross floating in the plating bath include a so-called press pit, which causes a non-uniform surface during press to impair the sharpness, and a non-uniform surface part locally. There is something called a dross butt that forms a battery and deteriorates corrosion resistance.On the other hand, surface defects due to surface oxidation of the plated surface include surface defects caused by the oxide being stuck on the surface of the plated steel sheet or the appearance due to surface oxidation. It can be exacerbated. Hereinafter, in the present specification, these are collectively referred to as surface defects, but they are used by being distinguished from dross defects and surface oxidation defects as necessary.

Several attempts have been proposed so far to prevent the surface defects caused by the dross. The method proposed by Japanese Examined Patent Publication No. 57-58434 is a method of separating the plating tank and the Al reaction tank by partition walls and performing high Al operation and stirring the bath to make the bottom dross into top dross and remove the bottom dross. .

(1) According to this method, the dross reacts with Al and tends to float during high Al operation, while the dross reacts with Al during low Al operation, that is, during manufacturing of galvannealed steel sheet (alloyed steel sheet). The light dross generated as a result of this does not form, but the heavy bottom dross formed by reacting with Zn accumulates but cannot be removed. (2) Since the plating tank and the Al reaction tank are separated, the entire equipment has a large capacity, and an increase in equipment cost cannot be avoided. (3) When stirring in a reaction tank to remove dross, oxygen in the air is entrained and oxidation of the Zn bath is promoted, which can be said to be a problem in terms of Zn yield and quality control.

On the other hand, in order to prevent surface oxidation defects, it is necessary to prevent surface oxidation of the plated steel sheet. However, since the conventional apparatus is a large-scale apparatus, sealing with an inert gas is required to prevent oxidation. However, a large-scale device is required, and an increase in equipment cost is unavoidable, and the effect due to that is not sufficient.

[0007]

Particularly in recent years, mainly for automobiles, the demand for alloyed steel sheets has been increasing more and more, and the need for higher quality is also increasing. Against this background, there is a demand for establishment of a manufacturing technique for hot-dip galvanized steel sheets that does not have surface defects due to dross even in low Al operation.

Therefore, the object of the present invention is to prevent dross defects due to the adhesion of dross on the surface of a steel sheet even in a low Al operation, indentation surface scratches due to oxide adhesion, and deterioration of appearance due to surface oxidation. An object of the present invention is to provide an apparatus for producing a hot-dip galvanized steel sheet having no surface defects and a method for supplying a plating raw material to the apparatus.

[0009]

Means for Solving the Problems The present inventors have made various studies to achieve the above-mentioned object, and have obtained the following findings,
The present invention has been completed. (1) If the dross becomes coarse and adheres to the steel plate, it becomes a dross defect when it is pressed after being alloyed.However, even if it adheres to the steel plate, if it is in a fine particle state, it will diffuse and disappear during the heat treatment performed in the alloying process. Or, it disappears in the temper rolling performed in the finishing process and becomes harmless.

(2) So-called dross floats as fine particles in the plating bath at the initial stage of formation, but when the system has a small volume, these fine particles are constantly and sufficiently stirred and easily adhere to the steel plate, Before growing and settling, it is discharged outside the system in a harmless state.

(3) The capacity of the plating tank is reduced to 50% or less, preferably 30% or less of the conventional plating tank, the dead space for the bath flow is eliminated, and the amount of the plating bath is reduced, so that the steel plate can be passed through. The stirring force in the system due to the movement of the plate and the rotation of the sink roll increases, and the sedimentation of dross can be suppressed. For example, when a typical plating tank with a capacity of around 200 tons is operated at 30,000 T / month, the amount of molten zinc contained in the plating tank is equivalent to the consumption of about half a month, but the plating tank is about 30 tons. If so, the molten zinc contained is replaced in 2 days, and the dross growth time, that is, the residence time in the bath is shortened to 1/7, and eventually the sufficient time required for growth is lost, and the fine particles can be always maintained.

(4) Although the dross defects can be prevented by the above means, the generation of surface oxide defects can also be prevented by performing the above means in an inert gas or non-oxidizing atmosphere.

(5) As a means for supplying the inert gas or the non-oxidizing gas, when an air diffuser is provided in the bath and the gas is bubbled and supplied, the agitation effect of the above-mentioned suspended matter is further increased, and the suspended matter is suspended. Emissions outside the system are promoted.

(6) By suppressing the amount of bubbling at this time, it is possible to keep the plating bath temperature substantially constant even if the temperature fluctuation is large with a small amount of plating bath. That is, the plating bath is usually kept at around 460 ° C, but the temperature of the steel sheet invading the plating bath is usually 460-500 ° C, and the heat supply tends to be over in consideration of the heat radiation. As the cooling means, room temperature inert gas may be blown into the bath to control the bath temperature in addition to the atmosphere adjustment and the stirring effect described above.

(7) When the capacity of the plating layer is further reduced,
Although it is necessary to constantly replenish the plating component, the liquid level can be accurately controlled by controlling the amount of ingot immersed in the bath.

Here, the gist of the present invention is, in a hot-dip galvanized steel sheet manufacturing apparatus provided with a sink roll, a support roll and a plating tank, the distance between the bottom of the plating tank and the bottom surface of the sink roll is 200 mm. Hereinafter, the distance between the steel sheet passing line and the inner wall of the plating tank until the steel sheet comes into contact with the sink roll is 200 mm or less, and the steel sheet passing line after the direction is changed by the sink roll and the inner wall of the plating tank are The distance is 700 mm or less, which is an apparatus for producing hot-dip galvanized steel sheet with few surface defects.

In the present invention, the array is limited by numerical values in order to specifically specify the capacity reduction of the plating tank.
In short, this means that the inner wall surface of the plating is arranged so as to follow the traveling line of the traveling steel plate, that is, the locus of the wide surface as close as possible.

According to a preferred embodiment of the present invention, a snout is provided upstream of the sink roll and a gas wiping device is provided downstream of the support roll, and the strip line and the plating bath surface from the snout to the gas wiping device are not provided. A sealing means for establishing an atmosphere of active gas and / or reducing gas may be provided on the upper part of the plating tank.

By providing the above-mentioned sealing means, the surface oxidation of the galvanized steel sheet is effectively prevented,
More preferably, by supplying an inert gas and / or a reducing gas into the plating bath by bubbling, the stirring force in the system is increased and the dross winding is promoted, whereby the dross defects can be more effectively prevented.

Further, an air diffuser for bubbling the inert gas and / or the reducing gas may be provided in the plating tank. Further, from another aspect, the present invention controls the plating bath surface height by partially immersing a lump of a plating raw material in a plating bath and adjusting the immersion depth of the plating raw material. And a method of supplying a plating raw material to a hot-dip galvanized steel sheet manufacturing apparatus.

[0021]

Next, the present invention will be described more specifically with reference to the accompanying drawings. FIG. 1 is a schematic explanatory view of a galvanizing apparatus which is an apparatus for producing a hot-dip galvanized steel sheet according to the present invention. In the figure, a plating tank body 10 is, for example, 1700 × 800 × 3.
About 500 mm and conventional type (Example: 4000 × 1800 × 3500 mm)
Compared with, the capacity is about 1/5. The hatched area in FIG. 1 is an unnecessary space in the conventional plating tank. The distance between the traveling steel plate 12 and the inner wall 14 of the plating tank, the height of the liquid surface 16 and the arrangement of the sink rolls are defined as follows.

A: Distance between the wide surface of the steel plate and the inner wall of the plating tank until the steel plate comes into contact with the sink roll (mm) B: Distance between the plating bath surface and the top surface of the sink roll (mm) C: Distance between bottom of plating tank and bottom surface of sink roll (mm) D: Distance between wide surface of steel sheet after change of direction by sink roll and inner wall of plating tank (mm) When hot dip galvanizing steel sheet The steel sheet 12 guided into the bath through the snout 20 travels along the inner wall 14 of the plating tank provided in parallel along the running line of the steel sheet 12 while being immersed and immersed in the plating bath, and the sink. After orbiting the roll 24, it is pulled upward through a pair of support rolls 25.

When the plating bath is discharged, the basis weight is adjusted. In the illustrated example, the gas wiping nozzle 26 is provided, but this may be a conventional one, and since the whole plating tank is configured compactly, it is provided in the gas sealing region of the upper space of the plating tank 10. Can be provided. Alternatively, as a modification, it may be provided above the plating tank and gas-sealed as a whole.

The plating bath surface is a sealing plate provided on the plating bath
The sealing means 28 is cut off from the outside to keep the inside in a reducing gas or inert gas atmosphere (eg, nitrogen gas atmosphere), and the gas wiping nozzle 26 is arranged in this gas sealing area. May be done. The inert gas atmosphere in this region prevents the bath surface and the plated surface of the steel sheet from being oxidized. When an inert gas such as nitrogen gas or a reducing gas is supplied as the wiping gas from the gas wiping nozzle 26, the gas introduced into the gas seal region may be replaced with that.

In the figure, reference numeral 30 is a ceramic air diffuser for a reducing gas or an inert gas (eg, nitrogen gas), and the reducing gas or the inert gas bubbled in the bath is the bath. It is effective to cool and stir the mixture to promote the adhesion of the suspended matter to the steel plate and to make the atmosphere in the seal plate 28 a reducing gas or inert gas atmosphere. The ingot 36 is for replenishing the plating bath, that is, for maintaining the plating surface in a steady state.If the plating surface fluctuates for some reason during operation, raise or lower the ingot 36 to reduce the immersion amount. It has a function of adjusting and always maintaining the upper surface of the plating bath at a constant height.

According to the present invention, the distance between the wide surface of the steel sheet and the inner wall of the plating tank is 200 mm or less, preferably 100 mm from the time when the steel sheet enters the sink roll to the sink roll.
Within The distance between the steel plate vertically turned by the sink roll and the inner wall of the plating tank is 700 mm or less, generally 100 to 700 mm, preferably 200 mm or less. Also, the distance between the bottom of the sink roll and the bottom of the plating tank is within 200 mm,
It is preferably within 100 mm.

According to a preferred embodiment of the present invention, as shown in FIG. 1, in the plating bath 10 of the continuous hot dip galvanizing equipment, the snout 20 and the inner wall 14 of the plating bath are integrated to form a plating bath. Can be made more compact. Therefore, the steel plate 12 penetrating from the snout 20 moves along the surface of the snout extension surface and keeps the distance (within 100 mm) from the minimum wall. 10 top and bottom sink rolls
Keep within 0 mm. Due to their proximity, stirring in the system can be sufficiently maintained.

The shorter the distance from the inner wall on the side of the steel sheet which has been vertically turned, the more preferable it is for improving the stirring force. However, for supplying the plating raw material, the distance is kept within 100 to 700 mm. Can be balanced. In this way, by reducing the capacity of the plating tank, the inert gas atmosphere is sealed in a small space, and the basis weight is controlled by the gas wiping nozzle 26 in the sealed area, oxidation of the plated surface can be completely prevented. Generation of surface oxidation defects can be effectively prevented.

The pair of support rolls 25, 25 provided on the upstream side of the gas wiping nozzle 26 have a function of stabilizing paths and a warp correction function as a snap roll. By the way, as a method of supplying a plating raw material to the above-described apparatus for manufacturing a hot-dip galvanized steel sheet, the liquid surface height may be controlled to be constant by the immersion amount of the ingot. However, the form of the raw material includes a plate shape, a wire material, and powder addition in addition to the ingot. A large ingot is preferable because the liquid level height is changed by changing the immersion amount.

As a method of adjusting the components of the plating bath, Al = 0.
It is preferable to use plating raw materials of 05 to 0.20%, Al≈0%, and Al = 1 to 100%. According to the present invention, by reducing the capacity of the plating tank, there is no dead space (unnecessary space) at the rear part of the snout, there is no stagnation part of the liquid, and the stirring force is greatly improved. Therefore, the fine particles that generate dross are taken out of the tank by adhering to the surface of the plated steel sheet before they grow sufficiently and are not deposited in the tank. In addition, the amount of hot-dip zinc contained is small, for example, 30 to 40 tons, and not only the composition of the bath can be changed promptly, but also the amount of energy for maintaining the hot-dip galvanizing bath is small. , The following benefits are obtained.

(1) It is possible to modify using an existing large capacity plating tank. (2) The contact area between the equipment and the bath in the bath is reduced, and the elution of Fe into the plating bath can be reduced. (3) The immersed ingot has the function of controlling the liquid level and at the same time has the function of the baffle plate, so that the stirring effect can be further enhanced. (4) Due to the above space-saving, the amount of molten zinc held in the plating tank for about half a month is reduced to about two days, and it is possible to swiftly respond to a large variety of small lots. (5) By sealing the atmosphere, it is possible to prevent surface oxide defects, and it is possible to manufacture a hot-dip galvanized steel sheet with extremely good surface quality without both dross defects and surface oxide defects. Next, the present invention will be described more specifically by way of examples.

[0032]

EXAMPLE A small-capacity plating tank 10 (about 30 tons of Zn capacity) shown in FIG. 1 was prototyped, and the unit weight was controlled by blowing air with a gas wiping nozzle 26. Further, gas wiping was performed with N ₂ , and N ₂ gas wiping was also performed with hot dip galvanizing under conditions in which an atmosphere was sealed with an inert gas and air oxidation was not performed.

N ₂ gas was used as the inert gas and was blown from the bottom of the plating tank through the ceramic diffuser 30. As the plating raw material, Al concentration (trace amount, 0.1%, 1%, 10
%) Zinc ingots (weight of about 1 ton) or small piece ingots (about 20 kg), plate-shaped (eg cathode Zn), wire Al, lump, powder Zn, Al, Zn
The -Al mixture was used in appropriate combination. The level of the bath surface was controlled by the degree of immersion of an ingot of about 1 ton, and the liquid level meter and the amount of indentation were linked. Under the plating conditions as described above, the relationship between the distance between the inner wall of the plating tank and the steel sheet and the surface defect was changed after changing the distances A, C, and D.
The number of dross defects during stable operation after 24 hours (number / 100 m ² ) was evaluated.

(1) The distance between the steel plate coming from the snout and the inner wall of the plating was changed to find the relationship with the amount of dross defects. As distance, A = 30, 50, 100, 200, 500 mm 5
It compared by the level. However, the distance from the liquid surface on the upper surface of the sink roll and the distance from the bottom of the plating tank on the lower surface B = 600
mm, distance C = 100 mm, and distance D = 700 mm between the steel plate and the inner wall of the plating tank after changing the direction. (2) Distance from bottom of sink roll to bottom of plating tank C = 3
Comparison was made at five levels of 0, 50, 100, 200, and 500 mm. However, A = 100 mm, B = 600 mm, and D = 700 mm. (3) The distance between the steel plate vertically turned by the sink roll and the inner wall of the plating tank is D = 100, 300, 500, 700.
, 1000 and 5 levels were compared. However, A = 100
mm, B = 600 mm, and C = 100 mm.

The operation is carried out with an effective Al concentration of 0.08 to 0.10% by weight, a snout material temperature of 480 ° C., a bath temperature of 460 ° C., and a line speed of 100.
Evaluation was performed using m / min, plate thickness 1 mm, plate width 1800 mm, and ultra-low carbon material. The number of dross defects was measured by pressing with a press tester and visually counting the number of generated spots. These results are shown in FIGS.
Is shown in the graph.

FIG. 2 is a graph showing the relationship between the number of dross defects generated and the distance from A. As shown in FIG. 2, A is 20
If it is within 0 mm, preferably within 100 mm, the number of dross defects is significantly reduced to 25 or less. FIG. 3 is a graph showing the relationship between the number of dross defects generated and the distance from C. As shown in FIG. 3, C also has a dross defect number of 20 or less, which is significantly reduced within 200 mm, preferably within 100 mm. FIG. 4 is a graph showing the relationship between the number of dross defects generated and the distance between D and D. As shown in FIG. 4, D is within 700 mm, preferably 20
If it is within 0 mm, the number of dross defects will be 10 or less.

FIG. 5 is a graph showing the effect of blowing an inert gas into the bath, in which a zinc oxide defect is caused by blowing an inert gas into the bath from an air diffusing tube for bubbling or by wiping the inert gas. The number of occurrences can be zero. As in the case of dross defects, the number of flaws due to zinc oxide on the surface was measured by visual inspection. FIG. 6 is a graph showing the effect of blowing an inert gas on the generation of dross defects. The number of dross defects generated decreased more greatly as the amount of inert gas blown from the diffuser increased.

Next, when the controllability of the bath surface level control by the ingot was examined under the conditions A, C and D selected above, it was found that an accuracy of ± 20 mm or less could be obtained as shown in FIG. The bath temperature was controlled by cooling with N ₂ gas injection as shown in Fig. 8 against the target value of 460 ° C.
As shown in Example 1, an accuracy of ± 5 ° C or less was obtained.

Example 2 Using the apparatus shown in FIG. 1, line speed 100 m / min, A = 10
With 0, B = 100, C = 600, D = 700 (mm), N ₂ gas injection [bubble] (120 l / min) from an inert gas diffuser, and N ₂ gas wiping 50 Nm ³ / min When hot-dip galvanizing was performed and then alloying treatment was performed, an alloyed steel sheet having a total basis weight of 100 g / m ^{2 on} both sides, which had very good appearance without dross defects and surface oxidation defects, was obtained.

[0040]

According to the present invention, surface defects and deterioration of appearance due to dross defects and surface oxidation defects can be greatly improved. In addition, the small-capacity plating tank can smoothly respond to requests for many kinds of small lots, and at the same time, it has great effect of saving resources and energy. As described above, the present invention is an invention that meets the needs of an era in which the conventional idea of hot-dip galvanizing is revolutionarily changed.

[Brief description of drawings]

FIG. 1 is a schematic explanatory view of an apparatus for manufacturing a hot-dip galvanized steel sheet according to the present invention.

FIG. 2 is a graph showing the results of the examples of the present invention.

FIG. 3 is a graph showing the results of the examples of the present invention.

FIG. 4 is a graph showing the results of the examples of the present invention.

FIG. 5 is a graph showing the results of the examples of the present invention.

FIG. 6 is a graph showing the results of the examples of the present invention.

FIG. 7 is a graph showing the results of the examples of the present invention.

FIG. 8 is a graph showing the results of the examples of the present invention.

[Explanation of symbols]

10: Plating tank 12: Steel plate 14: plating tank inner wall 16: Liquid level 20: Snout 24: Syncroll 25: Support roll 26: Gas wiping nozzle

Claims (4)

[Claims] 1. A hot-dip galvanized steel sheet manufacturing apparatus equipped with a sink roll, a support roll and a plating tank, wherein the distance between the bottom of the plating tank and the bottom surface of the sink roll is 200 mm or less, and the steel sheet contacts the sink roll. The distance between the steel sheet passing line and the inner wall of the plating tank is 200 mm or less, and the distance between the steel sheet passing line after the direction is changed by the sink roll and the inner wall of the plating tank is 700 mm or less. Equipment for producing hot-dip galvanized steel sheets with few surface defects. 2. A snout upstream of the sink roll,
A hot-dip galvanized steel sheet manufacturing apparatus having a gas wiping device downstream of the support roll, the seal comprising a strip line from the snout to the gas wiping device and a plating bath surface with an inert gas and / or reducing gas atmosphere. The apparatus according to claim 1, wherein the means is provided in the upper part of the plating tank. 3. The apparatus according to claim 1, wherein an air diffuser for bubbling the inert gas and / or the reducing gas is provided in the plating tank. 4. A plating bath surface height is controlled by partially immersing a lump of a plating raw material in a plating bath and adjusting a dipping depth of the plating raw material. 4. A method of supplying a plating raw material to a hot-dip galvanized steel sheet manufacturing apparatus according to any one of 1 to 3.