JPS6133067B2 - - Google Patents

Description

[Detailed description of the invention] The present invention has excellent corrosion resistance and does not cause "kinks" on the plating surface.
The present invention relates to a method for producing hot-dip galvanized steel sheets that are free from "hair" and have good gloss.

Hot-dip galvanized steel sheets have rapidly grown due to their excellent corrosion-resistant properties, and are used as corrosion-resistant materials in a wide range of fields such as building materials, home appliance materials, and automobile body materials.
It has reached 6 million tons, accounting for approximately 30% of cold-rolled steel sheets.

Zinc is an inexpensive and chemically active metal, and at the same time, the compound produced by the reaction is dense, so that a moderate corrosion rate can be obtained, making it a metal suitable for corrosion protection of steel materials. In general, the sacrificial anticorrosion ability of zinc against steel materials in a neutral environment is in an overly protective state, and even if the corrosion rate of zinc is further suppressed, sufficient sacrificial anticorrosion ability can be exhibited. For example, when measured in 3% saline, sacrificial corrosion protection against steel is effective even if the corrosion rate of pure zinc is suppressed to 1/20 to 1/50. Therefore, in a neutral environment, if the corrosion rate of zinc can be suppressed to 1/20 to 1/50 by some means, a life of 20 to 50 times longer than that of pure zinc with the same basis weight can be achieved, and the current It is possible to lower the basis weight to 1/20 to 1/50 to obtain the same performance.

The present invention focuses on this point and is a method for manufacturing a plated steel strip, which is as follows.

(1) A method for producing a hot-dip galvanized steel strip that includes plating at least one side in a zinc bath and controlling the amount of plating, using a bath consisting of 0.1 to 2.0% Mg, the balance zinc and unavoidable impurities. At least part of the time during which the plating metal adhering to the steel strip surface from the bath surface solidifies is surrounded by a seal box, and the oxygen concentration on the bath surface side including the wiping nozzle is reduced to 100%.
~1000ppm, and the oxygen concentration on the solidification zone side of the plating metal above it is not controlled if the plating weight is less than 50g/ ^m2 , or if the plating weight is 50g/m2 ^or more. A method for producing a plated steel strip, which comprises solidifying the unsolidified plated metal on the surface of the steel strip by controlling the concentration to 100 to 1000 ppm.

(2) The method for producing a plated steel strip according to item 1 above, characterized in that the bath surface side is a space of 1 m at most above the wiping nozzle from the plated bath surface.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The mode of carrying out the present invention will be described below in detail with reference to the drawings.

Figure 1 shows electrolytic zinc (99.97% purity) and its
The corrosion resistance of a steel plate hot-dipped using a Sendzimer pilot line using a zinc bath containing Mg as shown in Figure 1 was tested for 3 days in accordance with the salt spray test method limited to Japanese Industrial Standards (JIS) Z2371. This is a record of the corrosion loss measured using a straight line.
N ₁ indicates the level of corrosion rate of the electrolytic zinc bath without Mg addition, and curve A ₁ is the result of the electrolytic zinc bath with Mg addition. Figure 1 shows that the corrosion resistance of molten alloy plated steel sheets in an electrolytic zinc bath improves significantly depending on the amount of Mg added; with 0.5% Mg, the weight loss is 1/10, and with 1.0% Mg, the weight loss is 1/14th. The amount will be reduced. However, a large amount
The amount of Mg added has no meaning; 1.5% Mg gives only a slight improvement over 1.0%, and even 2.0% Mg makes almost no difference and the performance is saturated. Therefore, for the purpose of improving corrosion resistance, the effective amount of Mg added is 0.1 to 2.0%.

Incidentally, a hot-dip galvanized steel sheet composition characterized by containing 0.1 to 2.0% Mg as an alloying element and having an Al content of 0.01% or less was disclosed by the applicant on April 25, 1980, as a special patent. Although a patent application has been filed in Japanese Patent Application No. 55-55152, the present invention relates to a manufacturing method characterized by controlling the oxygen concentration of the atmosphere in the unsolidified area and solidified area of the plated metal.

Steel plates plated with a zinc bath containing Mg are extremely advantageous in improving corrosion resistance, but are also effective in terms of top coatability. In particular, with respect to phosphate treatment performed as a pre-painting treatment, a stable phosphate film can be formed under the same conditions as conventional galvanized steel sheets. Top coating is excellent for almost all types of coatings, such as electrodeposition coating (cationic, anionic) and baking coating used in the automobile industry, primer coating for color steel plates, baking coating for home appliances, etc. Demonstrates excellent performance.

As mentioned above, the addition of Mg to the zinc bath is very effective in improving corrosion resistance, but there are many problems in terms of manufacturing and quality. In terms of manufacturing, there are problems such as increased dross, uncontrollable basis weight, and poor appearance due to skin covering, etc. In terms of quality, problems such as unevenness and spangle-freeness occur. In other words, if we simply use the current manufacturing method with a bath containing Mg,
It cannot be manufactured, and in terms of quality, it can only be applied to extremely limited fields. In this regard, the inventors conducted many pilot line experiments and were able to complete a method for manufacturing galvanized steel sheets containing Mg.

The manufacturing method of the present invention will be explained in more detail below.

Mg has a strong affinity with oxygen and easily generates dross. In particular, in the current method of controlling the basis weight using gas jets (abbreviated as G, J, and C), there is a large amount of contact between the atmosphere and the molten metal, and depending on the conditions, a splash of molten metal may occur in the atmosphere. The increase in dross is so severe that it is virtually impossible to operate in a bath containing Mg. In order to prevent or reduce dross, the generation of dross can be prevented by reducing the amount of oxygen in the G, J, and C parts to zero, and there are several known examples regarding this. However, the inventor has experimentally found that when the oxygen concentration is lowered, metal vapor is generated and solidified inside the device, causing trouble. Figure 2 shows the experimental results of the relationship between the amount of steam and the oxygen concentration in the atmosphere with respect to dross. Figure 2 shows the oxygen concentration during sheet threading using the non-oxidizing furnace method molten galvanizing line (abbreviated as NOF-CGL) shown in Figure 3, with a line speed of 80 m/min in a molten zinc bath containing 0.5% Mg. and metal vapor content and dross generation by current atmospheric wiping.
It is expressed as a relative ratio as 1.0. Straight line I in Figure 2 is the amount of steam generated in a bath containing electrolytic zinc (99.99% purity) and 0.5% Mg. Also, the curved line is dross generated in the same bath as the straight line. The bath temperature was 450°C in all cases. In the case of a straight line, steam generation stops when the oxygen concentration exceeds 100 ppm.
The amount of dross generated is almost non-existent when the oxygen concentration is below 1000 ppm, and it rapidly occurs when the oxygen concentration exceeds 1000 ppm. In FIG. 3, 1 is a strip, 2 is a seal box for controlling oxygen concentration, and 7 is a GJC gas wiping nozzle, in which nitrogen was used as the gas. 3 is a pot pot, 4 is a plating bath heated to 450℃, 5 is a pot roll, and 6 is a snout.
Oxygen concentration was measured using a highly sensitive oxygen meter by sucking gas from 100 mm above and below the wiping nozzle.

As mentioned above, it was found that the area weight control operation was possible by controlling the oxygen concentration to an appropriate level. In addition, it has been found that the production of dross can be suppressed by measuring the oxygen concentration, so it is possible to operate even when the plate is passed at a higher speed.

That is, the current hot-dip galvanizing line has difficulty running the sheet at a speed of 150 m/min or more even in a normal galvanizing bath due to the increase in dross associated with the generation of splash. Oxygen concentration control makes it possible to increase the speed, and it is also possible to thread the sheet at high speed even in a bath containing Mg. On the other hand, under wiping conditions where the coating weight is controlled to be light, splashes are generated more often due to the proximity of the wiping nozzle and increased gas pressure, and a large amount of dross and steam are generated outside the appropriate oxygen concentration range. Therefore, problems similar to those caused by speeding up occur, and there is a limit to reducing the area weight. However, by controlling the oxygen concentration, the problem associated with splash can be solved, and at the same time, it is easy to control the weight to be thin. That is, when comparing atmospheric wiping and wiping with an oxygen concentration of 100 ppm under the same wiping conditions, the latter has a basis weight that is about 20% lower.

For the above reasons, oxygen concentration control has made it possible to control the area weight of Mg-containing alloy-plated steel sheets, including light weight areas, at high speed.

As described above, due to quality problems and manufacturing problems of galvanized steel sheets, it is necessary to separately control the oxygen concentration in the area up to the vicinity of the wiping nozzle and in the solidification process. The specific area near the wiping nozzle can be determined by considering the area where splash occurs.
Determined by strip speed, basis weight control, and bath composition in an area of 1000 mm or less above the nozzle. The oxygen concentration range in this region in the present invention is 50~
It is 1000ppm. (Figure 4, double hatched area) The oxygen concentration in the seal box must be controlled within an appropriate range for the appearance to be plated. Previously known examples of adding Mg are aimed at permanently lowering the oxygen concentration, even if they are qualitatively expressed as non-oxidizing or weakly oxidizing atmospheres, and there are no concrete examples of oxygen concentration control. No practical method has been proposed, and a plated steel sheet suitable for practical use has not been obtained. The influence of oxygen concentration on appearance is particularly problematic in the skinning phenomenon in the wiping nozzle section, that is, the area control section. In leather covering, the surface of the plating metal is oxidized by the surrounding oxygen and turns into a solid, while the inside is in a state of fluid molten metal, so the high pressure gas sprayed onto the plating surface creates a flow and wrinkles. A pattern occurs. Naturally, the ability to control the area weight also decreases. Figure 5 shows the area where skinning occurs in terms of the oxygen concentration in the wiping area and the Mg content in the bath (in the figure, x indicates full skinning, △ indicates slight skinning, and ○ indicates no skinning, respectively). ). For zinc, use an electrolytic zinc bath, bath temperature 450℃, line speed 80m/m.
The in-wiping gas pressure was 1.0 Kg/cm ² , the slit of the wiping nozzle was 0.5 mm, and the nozzle interval was 20 mm.
As is clear from FIG. 5, in a bath containing 2% Mg or less, skinning does not occur if the oxygen concentration is below 1000 ppm, and in a bath containing 0.5% Mg, it does not occur even at 3000 ppm. The relationship between oxygen concentration and plating appearance is also a problem in the solidification process of plating metal, so this point will be explained below.

The range of the solidification process in the present invention is located above the above-mentioned area weight control region, and is a part or all of the region until the plating metal solidifies.

FIG. 6 shows the area where appearance defects occur during the solidification process when plating is performed using a bath containing 0.5% and 1.0% Mg (the results are the same in both cases) in electrolytic zinc. The oxygen concentration control method will be described later. If the oxygen concentration during the coagulation process is high, a "hair" plating appearance will occur, and if it is low, the appearance will be a granular or hexagonal mottled pattern (referred to as "wavy"). “Hair” is a phenomenon unique to baths containing Mg. On the other hand, "waving" is a common phenomenon with zinc. 6th
The figure shows "hair" (□: no occurrence, ■: occurrence) and "kink" (○: no occurrence, ●: occurrence), and the areas of occurrence are indicated by diagonal lines in relation to the basis weight. From Figure 6, in the area weight range of 50g/m2 or ^more , the optimum oxygen concentration is 50 to 1000ppm.
There is no need to control the oxygen concentration at a low basis weight of less than g/m ² .

The seal box for controlling oxygen in the present invention will be described in detail below with reference to FIGS. 7 to 18. The symbols in these figures are as follows unless otherwise specified.

1: Strip 1 _S : Plated metal (solid) attached 1 _L : Plated metal (liquid) attached 2: Seal box 2a: Lower casing of seal box (wiping, bath surface) 2b: Upper casing of seal box (solidification process) 2c: Inner room of double structure seal box 2d: Inner room of gas curtain seal box 2e: Inner wall of gas curtain double structure seal box 3: Pot pot 4: Plating bath 5: Pot roll 6: Snout 7: Wiping nozzle 8: Seal Gas conduit 9: Inert gas supply port 10: Air supply port 11: Air cushion part (also serves as oxygen concentration control), 1
2: Circulation blower 13: Gas suction port 14: Circulation pipe 15: Seal wall M, M ₁ , M ₂ , M ₃ :
Gas suction port for measuring oxygen concentration M ₁ : Bottom (bath surface,
(corresponds to the wiping part) M ₂ : Middle part (corresponds to the solidification process) M ₃ : Upper part (corresponds to the solidification process) W ₁ ,
W ₂ : Opening of the seal box through which the strip passes. Figures 7, 8 and 9 show three positions of the seal box. In the figure, 1 indicates the strip, 1 _L (black) indicates the plated metal is in a molten state,
1 _S (white) is in a solid state. 2a is a seal box that controls the wiping nozzle and bath surface, 2b
is a seal box that controls the solidification process. In the case of solidification in the atmosphere, 2b can be omitted, resulting in FIG. 7. Figure 8 shows that the oxygen concentration is completely regulated until solidification.
FIG. 9 is an example of regulating part of the coagulation process.

Below, FIGS. 10 to 18 show an example of the structure of a seal box, an oxygen concentration control method, and the results thereof. In the present invention, methods other than those shown in the above figures can be applied as long as a limited oxygen concentration can be obtained.

The structure of the seal box (particularly the 2a part) must have a structure that can reduce the oxygen concentration to 10,000 ppm or less, and the oxygen concentration in the 2a part must be controlled by introducing a certain amount of oxygen from the outside. .
In boxes where the oxygen concentration in the 2a part cannot be kept below 1000 ppm, there will be large variations, causing increased dross and skin formation.

Fig. 10 shows a double seal box composed of double walls 2a and 2c, and the oxygen concentration in the box is controlled by inert gas such as nitrogen injected from the wiping nozzle and into the inner chamber of the double seal. An inert gas, such as air, containing oxygen whose flow rate is controlled at 10 is fed into the box from the bath surface and controlled. The internal pressure of the box is 5~5 in the water column.
Reaching 10mmH ₂ O, seal box top opening W ₁
Prevents air from entering. If the air volume is not allowed to flow through the box, the oxygen inside the box will be maintained below 10 ppm.

FIG. 11 shows a method of preventing air from entering through the opening _W1 of the seal box by using an inert gas curtain. Oxygen enters from 10, is mixed with nitrogen, and sent into the box where its concentration is controlled.

FIG. 12 shows a 2b box structure in which the opening extends along the strip, and because the opening is long, it is possible to prevent air from entering the 2a box.

Fig. 13 has a structure in which the wiping nozzle part 2a and the solidification process 2b are separated, and the wiping nozzle part 2a and the solidification process 2b are separated, and the wiping nozzle part 2a and the solidification process 2b are separated.
Determine the position of b. 2a and 2b can be matched. In this case, a nitrogen curtain structure is required to prevent air from entering each box. The oxygen concentration in the box is controlled at 9 and 10.

In FIG. 14, nitrogen curtains are created in the openings W ₁ and W ₂ in the seal box interior 2d having a double curtain structure. This seal box is used for pilot line testing of hot-dip galvanizing (line speed
Figure 15 shows the results of the test at a speed of 80 m/min and a strip width of 150 mm. The oxygen concentration measurement location (indicated by M in FIG. 14) is 50 mm above the wiping nozzle and 30 mm from the side wall of the seal box. The openings for W ₁ and W ₂ were made with an opening width of 20 mm. 26 m ³ /hr of nitrogen Q ₁ to the outer curtain, 16 m ³ /hr of nitrogen to the inner curtain Q ₂ , and a wiping nozzle (strip width 400
When the pressure is set to 0.5 Kg/cm ² (slit gap 0.3 mm, nozzle distance 30 mm), the oxygen concentration in the box is stabilized at 5 to 15 ppm (varies with the meandering of the strip). Air to _Q2 2.5/hr, 10/hr,
When mixing 17.5/hr and 25/hr, the respective oxygen concentrations will be 100 to 250, as shown in Figure 15.
It can be controlled to 100-500, 900-1000, 1500-2000ppm.

In FIG. 16, an air action pad (ACP) indicated by 11 is built into the seal box 2b. Since the vibration of the strip 1 can be suppressed by the ACP, the area of the opening W of the seal box can be reduced, and the ACP can exert a gas curtain effect. The gas is sucked from the middle seal box 13 by the hermetic blower 12 and then passed through the pipe 1.
4 to the ACP. Nitrogen into the box is supplied from a wiping nozzle. Further, the oxygen concentration inside the box is controlled by introducing air from the pipe 10 into the middle of the pipe 14. Of course, if necessary, nitrogen 9 and air 10 may be provided in 2a and 2b and mixed and supplied.

Figure 17 shows the results of a pilot line test (line speed 80 m/min, strip width 150 mm) of hot-dip galvanizing using the seal box shown in Figure 16. The ACP had a slit opening of 100 x 100 x 3 mm, and the flow rate to the ACP was adjusted to 4.4 m ³ /min by adjusting the blower valve. The distance between ACPs is 30
mm. The wiping nozzle (GJC) conditions were a nozzle slit of 0.5 mm, a width of 350 mm, and a nozzle distance of 20 mm. The amount of nitrogen gas ejected from GJC is 3.9m ³ /mi
n (pressure 1.0 Kg/cm ² ). In the case of this box structure, the gas amount ratio of ACP gas and GJC is ACP/
Control to GJC=1.4 or less/1. If the ACP exceeds 1.4, the pressure inside the box becomes negative and air tends to enter, making it necessary to separately introduce nitrogen into the box.

Under the above conditions, air was supplied from Fig. 10 in Fig. 16, and the oxygen concentration was measured at M ₁ , M ₂ , M ₃ in Fig. 16.
This is shown in curves M ₁ , M ₂ , and M ₃ in Figure 7. When no air is supplied, M ₁ , M ₂ , and M ₃ all stabilize around 10 ppm. When air is supplied at 20/min, M ₁ = 50,
M ₂ = 100, M ₃ = 500ppm, when air is supplied at 50/min
M ₁ = 100, M ₂ = 500, M ₃ = 2000ppm, air 100/
When supplying min. M ₁ = 500, M ₂ = 2000, M ₃ = 5000ppm
It was hot.

Figure 18 shows a structure in which the ACP 11 is separated from the seal box, and if necessary, the seal wall 15
can be provided. Oxygen in the seal box is mixed and supplied from 9 and 10, and ACP is supplied from 13→
The gas in the seal box is reused from step 12 to step 14, and the oxygen concentration is controlled by the air supply port 10 provided in the pipe 14. In this structure,
ACP also serves as 2b of the seal box.

Other seal boxes than those detailed above can be applied as long as the oxygen concentration can be controlled as defined in the present invention.

Furthermore, the present invention can be similarly applied to single-sided plating in addition to normal double-sided plating.

[Brief explanation of the drawing]

Figure 1 is a curve showing the corrosion loss of electrolytic zinc and galvanized steel sheets with 0 to 2% Mg added to it;
The figure shows a curve showing the amount of steam and dross generated in a bath containing 0.5% Mg added to electrolytic zinc. Figure 3 is an explanatory diagram of the molten galvanizing line using the non-oxidation furnace method shown in Figure 2.
Figure 4 is a diagram showing the relationship between oxygen concentration, amount of steam generated, amount of dross, and amount of skin tension. Figure 5 is a diagram showing the area where skin tension occurs in terms of oxygen concentration in the wiping area and Mg in the bath.
The figure shows "hair" and "curvature" based on basis weight and oxygen concentration.
Figures showing the outbreak area, Figures 7, 8, and 9 are diagrams showing the three positions of the seal box, Figures 10 to 1.
FIG. 8 is a diagram showing the structure of the seal box. 1: Strip, 1 _S : Plated metal (solid) attached, 1 _L : Plated metal (liquid) attached, 2: Seal box, 2a: Lower casing of seal box (wiping, bath surface), 2b: Upper part of seal box Casing (solidification process), 2c: Inner chamber of double structure seal box, 2d: Inner chamber of gas curtain seal box, 2e: Inner wall of gas curtain double structure seal box, 3: Pot pot, 4: Plating bath, 5: Pot roll, 6: Snout, 7: Wiping nozzle, 8: Seal gas conduit, 9: Inert gas supply port, 10: Air supply port, 11: Air action part, 12: Circulation blower, 13: Gas suction port, 14 : Circulation pipe, 15: Seal wall.

Claims (1)

[Scope of Claims] 1. A method for producing a hot-dip galvanized steel strip including a step of plating at least one side in a zinc bath and controlling the amount of plating, comprising 0.1 to 2.0% Mg, the balance zinc and inevitable impurities. Using a bath, at least part of the time during which the plated metal adhering to the steel strip surface solidifies from the bath surface is surrounded by a seal box, and the oxygen concentration on the bath surface side including the wiping nozzle is controlled to 100 to 1000 ppm. If the plating weight is less than 50g/ ^m2 , the oxygen concentration on the solidification zone side of the plating metal above it is not controlled;
A method for producing a plated steel strip, which comprises solidifying the unsolidified plated metal on the surface of the steel strip by controlling it to 100 to 1000 ppm when the concentration is 50 g/m ² or more. 2. Claim 1, in which the bath surface side is a space of 1 m at most above the wiping nozzle from the plating bath surface.
A method for producing a plated steel strip as described in Section 1.