JP3745730B2 - Method for producing hot-dip galvanized steel sheet having a beautiful surface appearance - Google Patents

Description

【００３０】
【発明の効果】
以上述べたように、本発明により浮遊ドロスの付着が抑制され、美麗な表面外観を有する溶融亜鉛めっき鋼板を得ることができると共に、そのめっき品質の向上と製品歩留りの向上を図ることが出来る優れた効果を奏するものである。
【図面の簡単な説明】
【図１】本発明に係るめっき浴面でのメタル随伴流の状態を示す図である。
【図２】鋼帯との境界層近傍でのメタル流の状況を示す図である。
【図３】本発明に係るめっき浴面でのメタル流層を発生させる装置の概略図である。
【図４】本発明に係るめっき浴面でのメタル流層を発生させる装置の上断面図である。
【図５】一般的な連続溶融金属めっき装置の概略図である。
【符号の説明】
１ 鋼板
２ 溶融めっき槽
３ スナウト
４ ターンダンロール
５ めっき浴
６ シンクロール
７ ガスワイピングノズル
８ めっき浴面
９ 速度勾配
１０ 境界層
１１ メタル吐出口
１２ サポートロール
１３ 主流[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a hot-dip galvanized steel sheet having a beautiful surface appearance.
[0002]
[Prior art]
Conventionally, when continuous molten metal plating is performed, generally, as shown in FIG. 5, a steel plate 1 annealed in a reducing atmosphere is transferred from a snout 3 to a hot dipping bath 2 via a turn-roller roll 4. A pair of the molten zinc that is guided and immersed in the hot dipping bath 5, then changed in direction by the sink roll 6 and pulled up in the vertical direction, and the excess of the hot zinc adhering to the surface of the steel plate 1 is provided on the bath. 5 is a plating method in which a desired plating amount is applied to the surface of the steel sheet 1 by adjusting the coating weight per unit area by the gas wiping nozzle 7, and a typical facility is a schematic diagram of a continuous molten metal plating apparatus shown in FIG. 5. It is. Reference numeral 8 denotes a plating bath surface.
[0003]
On the other hand, in continuous hot dip galvanizing, in addition to using zinc in the plating bath, the interface between zinc and the base steel is made hard so that the growth of the alloy layer of Fe-Zn is suppressed and the plating adhesion is improved. In some cases, Al is added. That is, by forming an Al-enriched layer at the interface between the plating layer and the base steel, the Fe—Zn alloy layer at the interface between zinc and the base steel is moderately suppressed, and peeling of the plating layer is prevented. . In such continuous hot dip galvanizing, dross generated by chemical reaction with molten oxygen and oxygen added to the atmosphere, molten zinc, and even steel sheets inevitably occurs in the molten zinc bath. There is a problem of doing. In particular, when switching from the manufacturing process of the galvannealed steel sheet to the manufacturing process of the hot dip galvanized steel sheet, there is a great problem in generating floating dross.
[0004]
The dross generated in the plating bath during the operation in the above-mentioned continuous hot dip galvanizing line adheres to the steel plate when the steel plate is pulled up from the plating bath, which causes deterioration of the appearance and quality of the steel plate in terms of quality. It was. Therefore, as a method of removing this dross and preventing the appearance and quality deterioration of the steel sheet, for example, as disclosed in Patent Document 1, a U-shaped weir surrounding the strip is installed in the molten zinc pot, Pour clean molten zinc pumped from inside the molten zinc pot into the place delimited by this weir to create a one-way flow in the molten zinc inside the weir and eliminate dross from the vicinity of the strip to prevent adhesion to the product A method for preventing dross from adhering to a strip in a hot dip galvanizing facility is known.
[0005]
Further, as disclosed in Patent Document 2, in the method for producing a Zn-Al-Mg hot-dip plated steel sheet, the steel strip fed out of the heating furnace is immersed in the bath by immersing the tip of the snout in the bath. Surface appearance that keeps the inside of the snout in a non-oxidizing atmosphere and prevents floating of objects other than metal from drifting to the steel strip passing through the bath in the snout. The manufacturing method of the hot-dip Zn-Al-Mg type hot-dip plated steel sheet is known.
[0006]
In Patent Document 3, a steel strip rising vertically from the plating bath is sandwiched, and the plating bath surface and the steel strip in the bath are penetrated between the magnetic poles of a pair of moving magnetic field generating coils provided immediately above the plating bath. A magnetic field having a U-shaped trajectory is generated, and this magnetic field moves in one of the width directions of the steel strip to generate an electromagnetic force in the same direction as the moving direction. This electromagnetic force penetrates the magnetic field. A method is known in which the plating metal in the plating bath of the part is convected in the width direction of the steel strip, thereby removing the dross floating on the plating bath surface from the periphery of the steel strip and preventing the dross from adhering to the steel strip. ing.
[0007]
In addition, Patent Document 4 discloses a hot water surface in the vicinity of a rising portion of a steel strip while running the steel strip along a predetermined transfer line with a moving magnetic field generating coil facing both front and back surfaces of the steel strip pulled up from the hot dipping bath. In addition, an electromagnetic force is generated by a moving magnetic field from the widthwise center of the steel strip to both ends in the width direction, and the molten metal is caused to flow from the center in the width direction to both ends in the width direction by the electromagnetic force. A method for removing the top dross from the plating bath is known.
[0008]
Further, Patent Document 5 discloses a molten metal bath top dross removal method for removing generated top dross using a propulsive force due to electromagnetic induction in a molten metal bath of a molten metal plating apparatus. A method for hot metal plating of a steel strip characterized by filtering precipitates contained in the molten metal in a plating bath and then spraying the filtered molten metal over the entire width of the steel strip immersed in the molten metal Etc. are known.
[0009]
[Cited document]
(1) Patent Document 1 (Japanese Patent Laid-Open No. 6-32298)
(2) Patent Document 2 (Japanese Patent Laid-Open No. 2000-64015)
(3) Patent Document 3 (Japanese Patent Laid-Open No. 11-36046)
(4) Patent Document 4 (Japanese Patent Laid-Open No. 10-53850)
(5) Patent Document 5 (Japanese Patent Laid-Open No. 54-33234)
(6) Patent Document 6 (Japanese Patent Laid-Open No. 64-28354)
[0010]
[Problems to be solved by the invention]
However, the dross in the case of the above-mentioned patent document 1 is intended to float near the zinc surface in the molten zinc pot, and also takes a method in which molten zinc is poured into the weir and a method in which the velocity gradient flows greatly for the bath surface. Is adopted. Furthermore, the discharge speed from the pump is 2 to 5 m / s (120 to 300 m / min). Therefore, the present invention is not intended for the dross floating on the bath surface from the support roll intended by the present invention. Also, a flow rate different from the velocity gradient of the present invention is given, and a stirring flow is generated due to the high flow rate. As a result, there is a drawback that buoyant dross adheres to the steel plate.
[0011]
Moreover, in the case of patent document 2, in order to suppress floating substances other than a metal drifting to the steel plate which has passed the bath in a snout in Zn-Al-Mg system hot dipping, in a snout The flow rate of forced horizontal flow is 0.015 to 0.5 m / sec in the width direction of the steel sheet. The maximum flow rate is as low as 3 m / min, and a sufficient effect is obtained to completely prevent floating dross from adhering. I can't. Patent Documents 3 to 5 are all hot-dip plating baths in which molten metal near the molten metal surface flows from the center in the width direction to both ends in the width direction by electromagnetic force provided directly on the plating bath or on the surface of the plating bath. This is a top dross removal method. Furthermore, in the case of patent document 6, the complicated process called a filtration process is required.
[0012]
[Means for Solving the Problems]
In order to solve the problems as described above, the inventors have intensively developed, and as a result, the steel plate is within a depth of 100 mm from the plating bath surface where the plating bath exits, and within a range of 100 mm from the steel plate surface, Furthermore, the metal is discharged at an angle of 5 to 20 ° with respect to the steel plate, a predetermined flow of the metal is created, and the dross that floats in the plating bath is divided from the steel plate by the metal shearing force generated by this metal flow to the steel plate. Provided is a method for producing a hot-dip galvanized steel sheet having a beautiful surface appearance that prevents adhesion.
[0013]
The gist of the invention is that
(1) In a continuous hot dip galvanizing line in which a steel plate annealed in a reducing atmosphere is introduced into a hot dip galvanizing bath through a snout and plated, the steel plate is Al: 0.08 to 0.50 mass%, Fe: 0.10 mass. %, Less than 100 mm from the plating bath surface on the steel sheet exit side of the hot dip galvanizing bath, and when the steel sheet is pulled up from the hot dip galvanizing bath, the steel plate In a plating bath within a range of 100 mm from the surface, a metal flow having a metal flow rate of 30 to 100 m / min is generated at an angle of 5 to 20 ° parallel to the steel plate width direction and follows the steel plate. A method for producing a hot dip galvanized steel sheet having a beautiful surface appearance, characterized by preventing adhesion of floating dross in a plating bath.
[0014]
( 2 ) In the method of said (1) description, the manufacturing method of the hot dip galvanized steel plate which has the beautiful surface appearance characterized by making the relationship between a steel plate speed (SV) and a metal flow velocity (MV) into the following range.
(1) In the case of 30 ≦ SV <140, 30 ≦ MV ≦ 100
(2) If 140 ≦ SV <180, 30 + 0.5 (SV−140) <MV ≦ 100
(3) In the case of 180 ≦ SV ≦ 200, 50 ≦ MV ≦ 100
( 3 ) A method for producing a hot-dip galvanized steel sheet having a beautiful surface appearance, wherein the metal flow rate is 30 to 70 m / min as described in (1) or (2) above.
( 4 ) A method for producing a hot-dip galvanized steel sheet having a beautiful surface appearance, wherein the metal flow layer thickness described in (1) to (3) is 30 to 95 mm.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail.
As described above, in continuous hot dip galvanization, dross generated by chemical reaction with molten oxygen and oxygen added to the atmosphere, molten zinc, and steel sheets is unavoidable in the molten zinc bath. Occurs. This dross can be roughly classified into solid dross or sludge dross caused by the inside of the snout and fine dross caused by the Fe-Zn, Fe-Al alloy, or top dross that is dust dross and oxidized dross.
[0016]
The generation of the dross is greatly influenced by the composition of the molten zinc bath. When the Al concentration in the bath is increased, the top dross having a relatively low specific gravity (Fe ₂ Al ₅ : specific gravity about 4, Fe ₂ ZnAl ₄ : specific gravity about) 5, ZnO, Al ₂ O ₃ ), and when the Al concentration is low, bottom dross (FeZn ₇ : specific gravity 7.3) having a relatively high specific gravity is generated. Moreover, there exists a floating dross (Fe ₂ ZnAl ₄ + FeZn _x : specific gravity 5 to 7.3) belonging to the middle. The present invention is particularly effective as a countermeasure against dross that floats in the plating bath and floats with little buoyancy of 5 mm or less.
[0017]
In the present invention, the reason for the hot dip galvanizing bath composed of Al: 0.08 to 0.50 mass%, Fe: 0.10 mass% or less, and the balance Zn is the case of hot dip galvanizing production and hot galvanizing. In consideration of the case of plating, in the case of alloying plating, the Al concentration is low, whereas in the case of the hot dip galvanizing bath composition, the Al concentration is higher than that of alloying. From the relationship of switching between these two during continuous operation using the same plating tank, including the Al concentration for producing both, the lower limit is 0.08% by mass considering the powdering properties of alloying plating. did. In consideration of the maximum value and adhesiveness of the hot dip galvanizing bath composition, the upper limit value is 0.50% by mass.
[0018]
As a countermeasure against dross generated from the plating bath composition, in particular, not the top dross and the bottom dross as described above, but the floating dross belonging to the middle is targeted. The size of the floating dross at that time is 5 mmφ, or the long diameter is 5 mm or less. In addition, since the size of one piece of 0.5 mmφ or less has not reached the surface appearance after plating, it was excluded from the scope of the present invention. The reason why Fe is 0.10% by mass or less is that Fe is inevitably introduced as an impurity from the steel plate during continuous hot dip galvanizing operation, and it is preferably as low as possible. However, if it exceeds 0.10 mass%, the plating adhesion deteriorates, so the upper limit was made 0.10 mass%. Moreover, it is preferable that the plating bath temperature at that time is normally operated in the range of 430-500 degreeC.
[0019]
FIG. 1 is a diagram showing a state of metal accompanying flow on the plating bath surface according to the present invention. FIG. 1A is a cross-sectional view, and FIG. 1B is an upper cross-sectional view. As shown in this figure, the metal accompanying flow to the plating bath surface 8 on the steel sheet exit side of the hot dip galvanizing bath produces a boundary layer 10 with the steel sheet 1 as a velocity gradient 9. If this boundary layer thickness is δ, this boundary layer thickness δ = √η / (ρUL) × L can be estimated. However, L = 300 mm, U = 100 mpm = 10000/60 [cm / s], η = 3.93 × 10 ⁻² [P], and ρ = 7 [g / cm ³ ]. From the calculation of the boundary layer thickness, the boundary layer thickness δ = 0.3 mm can be calculated.
[0020]
Further, the movement of the particles in the boundary layer has a velocity gradient in the fluid, and when the velocity of the particles is smaller than the velocity of the fluid, the force that moves the particles to the larger fluid velocity works. The particle speed up = (81.2 / 12π) × d / v ^0.5 (up−up) √du / dy due to the Suffman force, but when up is the particle flying speed and the density is lower than the liquid, u <up Become. Therefore, up is negative, and the particles (the density of the dross is about 5 g / cm ³ ) are separated from the steel plate. Therefore, the particles (≦ 0.3 mm) in the boundary layer receive an outward force. However, there is a high possibility that particles having a size exceeding the boundary layer thickness of 0.3 mm will approach the steel plate before approaching the inertia force and receiving the outward force. Therefore, since the target dross according to the present invention is a floating dross of 0.5 to 5 mmφ, it has been found that the boundary layer flow around the steel plate may be ignored at this size. From this fact, it is a feature of the present invention that the particles flow in the lateral direction before the particles (dross) near the steel plate approach.
[0021]
FIG. 2 is a diagram showing the state of the metal flow in the vicinity of the boundary layer 10 with the steel strip. FIG. 2 (a) shows a conventional metal flow situation, and FIG. 2 (b) shows a metal flow situation according to the present invention. From the above, in the technique disclosed in Patent Document 1 which is a conventional technique, a large particle (top dross) can flow in the lateral direction because the metal flow velocity is extremely large, but the small particles adhere to the steel plate. It will be. That is, as shown in FIG. 2A, the lateral flow is turbulent, and the particles that are engulfed in the turbulent vortex approach and adhere to the steel plate 1 near the steel plate.
[0022]
On the other hand, since the flow of the present invention is supplied from the discharge port of the metal pump at an optimal position in the lateral direction and with an optimal metal flow velocity, as shown in FIG. ) Is separated from the steel plate 1, and the particles near the steel plate are attracted to the main flow 13 side by the resistance force u> up due to the above-described Suffman force principle. That is, the position (point A) of the metal discharge port from the steel plate 1 and the metal pump (not shown) needs to be separated from the steel plate 1 by about 5 mm (L ₀ ). When the point A is separated by about 5 mm or more, a gap is formed between the steel plate and the cross current, and particles enter the molten zinc together from the outside. On the other hand, [B point] is preferably located within 100 mm (L ₃ ). This is because when the diameter exceeds 100 mm, the velocity gradient 9 in the lateral direction is not reached, and the effect of attracting particles to the mainstream side is reduced.
[0023]
FIG. 3 is a schematic view of an apparatus for generating a metal flow layer on the plating bath surface according to the present invention. As shown in this figure, a metal discharge port 11 is installed at positions L ₁ and L ₂ from the plating bath surface 8 between the plating bath surface 8 and the support roll 12 on the steel sheet exit side from the hot dip galvanizing bath. The discharge port 11 is used to generate a metal flow having a metal flow rate of 30-100 m / min. That is, the position of L ₂ is arranged at a depth within 100 mm from the plating bath surface 8, and even if a metal flow is generated at a deep position exceeding 100 mm, the shearing force of the metal acting on the steel plate is small and the floating dross The effect of separating the steel plate from the steel plate and preventing it from adhering to the steel plate is small, and the effect is large immediately before leaving the bath, so the maximum depth was set to 100 mm. In addition, the metal flow layer thickness used in the specification is a length of L ₂ -L ₁ . Further, it is desirable that L ₁ is lowered by about 5 mm or more from the surface of the zinc bath. The reason for this is that when the metal flow layer comes out on the surface, the surface may be stirred and rolled up.
[0024]
In addition, the metal speed of 30 to 100 m / min was obtained when the plating speed was 30 to 100 mm / min. However, if it is less than 30 m / min, the shearing force of the metal cannot be obtained and the floating dross is separated from the steel plate, and the effect of preventing the floating dross from adhering to the steel plate is small, and a beautiful appearance cannot be obtained. Moreover, when it exceeds 100 m / min, the vortex | eddy by turbulent flow peeling will arise easily in the edge vicinity of the pipe | tube of a discharge outlet, the frequency which a dross will move to the steel plate side will go up, and a dross will adhere to a steel plate easily. Since it becomes dross adhesion to a steel plate, the upper limit was made into 100 m / min. Preferably, the speed is 30 to 70 m / min. The metal speed referred to here is the calculated value obtained by dividing the flow rate (m ³ / min) of the metal pump by the cross-sectional area (m ² ) of the discharge port, that is, the average flow velocity of the metal coming out of the discharge port. is there.
[0025]
FIG. 4 is a top sectional view of an apparatus for generating a metal flow layer on the plating bath surface according to the present invention. As shown in this figure, the metal discharge port 11 is located at a position of L ₃ from the steel plate 1 and the metal flow is generated with respect to the steel plate 1 at an angle θ of the metal discharge port 11 of 5 to 20 °. This prevents floating dross from adhering in the plating bath following the steel plate. In order to prevent floating dross from adhering, an angle is required to create an efficient flow around the steel plate. In other words, according to the Suffman force principle described above, in order to apply a force to pull the dross away from the steel plate, the direction of the flow and the velocity gradient around the steel plate become clearer and acts more strongly when the angle is applied. . However, when the angle exceeds 20 °, the effect decreases conversely, so the upper limit is set to 20 °.
[0026]
Next, the metal flow rate (MV) is related to the steel plate speed (SV), and it is necessary to increase the metal flow rate as the steel plate speed increases. The relationship is preferably performed within the following range.
(1) In the case of 30 ≦ SV <140, 30 ≦ MV ≦ 100
(2) If 140 ≦ SV <180, 30 + 0.5 (SV−140) <MV ≦ 100
(3) In the case of 180 ≦ SV ≦ 200, 50 ≦ MV ≦ 100
That is, it is desirable to control the metal flow rate in each case when the steel plate speed is 30 ≦ SV <140, 140 ≦ SV <180, and 180 ≦ SV ≦ 200.
[0027]
The metal flow is generated in the above-described range, and the metal flow layer thickness is 30 to 95 mm. If it is less than 30 mm, sufficient metal shearing force cannot be obtained and a beautiful appearance cannot be obtained. Moreover, since the shear force is saturated in the metal flow in the layer exceeding 95 mm, and forming a metal layer of more than that is expensive in terms of equipment and increases the cost, the upper limit is set to 95 mm. As a device for generating a metal flow with respect to the steel plate, a metal pump is generally used. However, the present invention is not limited to this, and an electromagnetic linear motor or the like can also be used. .
[0028]
【Example】
Hereinafter, the present invention will be specifically described with reference to examples. A low carbon steel plate having a thickness of 0.7 mm and a width of 1200 mm was used as a raw material, and immersed in a continuous hot dip galvanizing line at a plating bath composition and a steel plate speed shown in Table 1. Table 1 shows the conditions of the metal flow velocity, the metal pump position, and the metal discharge port angle θ at that time. The result was evaluated by plating appearance. As shown in Table 1, no. 1 to 12 are examples of the present invention. 13 to 20 are comparative examples. No. Since the metal speeds of Nos. 13 to 15 were slow, the surface appearance was poor. No. Since the metal speed of No. 16 was high, the surface appearance was poor. No. In Nos. 17 to 20, the metal discharge port is located away from the steel plate. No. 17 is located deep in the plating bath. Since 20 is located deep from the plating bath surface, it can be seen that the surface appearance is poor. On the other hand, the present invention No. Any of 1 to 12 was able to obtain a hot-dip galvanized steel sheet having an excellent surface appearance.
[0029]
[Table 1]

[0030]
【The invention's effect】
As described above, according to the present invention, it is possible to obtain a hot dip galvanized steel sheet having a beautiful surface appearance, which can suppress the adhesion of floating dross, and can improve the plating quality and the product yield. It is effective.
[Brief description of the drawings]
FIG. 1 is a diagram showing a state of metal accompanying flow on a plating bath surface according to the present invention.
FIG. 2 is a view showing a state of a metal flow in the vicinity of a boundary layer with a steel strip.
FIG. 3 is a schematic view of an apparatus for generating a metal flow layer on a plating bath surface according to the present invention.
FIG. 4 is a top sectional view of an apparatus for generating a metal flow layer on the plating bath surface according to the present invention.
FIG. 5 is a schematic view of a general continuous molten metal plating apparatus.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Steel plate 2 Hot dipping bath 3 Snout 4 Turn Dunn roll 5 Plating bath 6 Sink roll 7 Gas wiping nozzle 8 Plating bath surface 9 Speed gradient 10 Boundary layer 11 Metal discharge port 12 Support roll 13 Mainstream

Claims (4)

In a continuous hot dip galvanizing line in which a steel plate annealed in a reducing atmosphere is led into a hot dip galvanizing bath through a snout, the steel plate is made of Al: 0.08 to 0.50 mass%, Fe: 0.10 mass% or less, After being immersed in the hot dip galvanizing bath composed of the remaining Zn, when being pulled up from the hot dip galvanizing bath, the depth is within 100 mm from the plating bath surface on the steel sheet exit side of the hot dip galvanizing bath, and 100 mm from the steel plate surface. A plating bath that follows a steel plate by generating a metal flow having a metal flow rate of 30 to 100 m / min at an angle of 5 to 20 ° with respect to the steel plate in a plating bath within the range of A method for producing a hot-dip galvanized steel sheet having a beautiful surface appearance, characterized by preventing adhesion of floating dross inside. The method according to claim 1, wherein the relationship between the steel plate speed (SV) and the metal flow rate (MV) is in the following range.
(1) In the case of 30 ≦ SV <140, 30 ≦ MV ≦ 100
(2) When 140 ≦ SV <180, 30 + 0.5 (SV−140) <MV ≦ 100
(3) When 180 ≦ SV ≦ 200, 50 ≦ MV ≦ 100 A method for producing a hot-dip galvanized steel sheet having a beautiful surface appearance, wherein the metal flow rate is 30 to 70 m / min according to claim 1 or 2. A method for producing a hot-dip galvanized steel sheet having a beautiful surface appearance, wherein the metal flow layer thickness according to claim 1 is 30 to 95 mm .

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