JPS6133069B2 - - 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 a hot-dip galvanized steel sheet that does not generate "hair" and has good adhesion.

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. Generally, 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.

In recent years, the applications of galvanized steel sheets have expanded from the traditional field of building materials to home appliances, automobiles, and steel furniture, and they must have performance that matches the application. In other words, it has excellent corrosion resistance, and
Requirements include (1) good plating adhesion, (2) good appearance and no discoloration, and (3) good topcoat performance (including chemical conversion treatment).

The object of the present invention is to provide a galvanized steel sheet with quality that meets the above-mentioned requirements. In order to achieve this object, the present invention limits the composition of the zinc bath and maintains the unsolidified and solidified areas of the plating metal attached to the steel strip surface under specific conditions for treatment, as follows: be.

(1) In a method for producing a hot-dip galvanized steel strip, which includes a step of plating at least one side in a zinc bath and controlling the amount of plating, Mg0.1 to 2.0 is added to the zinc bath.
%, Al 0.1 to 0.5%, the balance being zinc and unavoidable impurities, and at least a portion of the plating metal adhering to the steel strip surface from the bath surface is surrounded by a seal box and wiped during solidification. Oxygen concentration on the bath surface side including the nozzle is 50 to 1000 ppm
At the same time, the oxygen concentration on the solidification zone side of the plating metal above it is not controlled when the plating weight is less than 50 g/ ^m2 , and when the plating weight is 50 g/m2 or ^more , A method for producing a hot-dip galvanized steel strip, characterized by solidifying unsolidified galvanized metal on the surface of the steel strip at a concentration of 100 to 100 ppm.

(2) The method for producing a hot-dip galvanized 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 plating 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 (purity 99.97%), electrolytic zinc and mixed zinc (Al 0.22%, other impurities).
The corrosion resistance of steel sheets hot-dipped using a Sendzimer pilot line using three types of baths containing Mg (including 0.1% Pb, 0.01% Od, and 0.02% Fe) as shown in Figure 1 was determined according to the Japanese Industrial Standards. (JIS) This is a record of the corrosion loss measured in a 3-day test based on the salt spray test method specified in Z2371, where the straight line M ₁ is Mg-free electrolytic zinc bath, and the curves N ₁ and A ₁ are electrolytic zinc bath. These are the results of baths in which Mg was added to zinc and mixed zinc as shown. From Figure 1, the amount of corrosion on hot-dip galvanized steel sheets
M ₁ is significantly improved by the addition of Mg as shown in curve N ₁ , and compared to the case without Mg addition, the weight loss is 1/10 with 0.5% Mg and 1/20 with 1.0% Mg. However, a large amount
Adding Mg is meaningless, and performance is saturated at 2.0% Mg. Therefore, for the purpose of improving corrosion resistance, the effective amount of Mg added is 0.1 to 2.0%.

In this case, the corrosion resistance of the blended zinc is lower than that of electrolytic zinc, and the corrosion loss is large, but as the amount of Mg added increases, the corrosion resistance improves (at about 1.0%, it becomes equal to electrolytic zinc and the performance is saturated). Although the addition of Al to electrolytic zinc does not improve corrosion resistance, it is preferable to add a small amount of Al together with Mg in order to improve plating adhesion.Figure 2 shows that Al is added to an electrolytic zinc bath containing 1.0% Mg. 0.05%, 0.1%, 0.15%, 0.2%, 0.3
%, 0.4%, and 0.5% were added, and the plating adhesion was evaluated using a ball impact test. The ball impact test was conducted by hand using a ball with a diameter of 25 mm. After the test, it was forcibly peeled off with an adhesive tape, and the peeling was expressed as an area percentage. As is clear from Figure 2
Adhesion improves with Al content of 0.1% or more. Also, Figure 3 is
Mg0.2% + Al 0.2% (○ mark), Mg0.5% + Al 0.2%
(△ mark), Mg 1.0% + Al 0.2% (□ mark), Mg 0.2% only (● mark), the wiping gas pressure of GJC was controlled to create plated steel sheets with different plating amounts.
The adhesion was evaluated using a ball impact test in the same manner as shown in FIG. The one containing 0.2% Al shows good plating adhesion over a wide range of area weight.
It cannot be said that the adhesion to plating is sufficient if Al is not added. The addition of Al is 0.1 for plating adhesion.
% or more, but if it is too large, intergranular corrosion will occur due to the combination with Sn or the influence of impurities such as Pb, so the upper limit should be set at 0.5%.

Incidentally, a hot-dip galvanized steel sheet composition characterized by containing 0.1 to less than 2.5% Mg as an alloying element and having an Al content of 0.1 to less than 3% was developed by the applicant in December 1978. Although a patent application has been filed in Japanese Patent Application No. 173435/1983 on the 28th, 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 plating metal.

Steel plates plated with a zinc bath containing Mg are extremely effective in improving corrosion resistance, but are also effective in terms of repaintability. 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 zinc baths is often effective for corrosion resistance, but there are many problems in terms of manufacturing and quality. In terms of manufacturing, the increase in dross,
There are problems such as uncontrollable basis weight and poor appearance due to skin covering, etc., and quality problems such as discoloration such as blackening, unevenness, spangle-free, etc. occur. That is, simply using the current production method with a bath containing Mg does not allow production, and in terms of quality, it can only be applied to extremely limited fields. In the future, the inventors conducted many pilot line experiments, and found that Mg and Al
We were able to complete a manufacturing method for galvanized steel sheets including

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, with the current method of controlling the basis weight using gas jets (abbreviated as GJC), the contact between the atmosphere and molten metal is large, and depending on the conditions, molten metal may splash into the atmosphere, resulting in an increase in dross. It is virtually impossible to operate in a bath containing intense Mg.
GJC to prevent or reduce dross
If the amount of oxygen in the area is reduced to zero, it will no longer be generated, and there are several known examples regarding this. However,
The present inventor has experimentally found that when the oxygen concentration is lowered, metal vapor is generated and solidified inside the device, causing trouble. FIG. 4 shows the experimental results of the relationship between the amount of steam and the oxygen concentration in the atmosphere with respect to dross. Figure 4 shows the non-oxidizing furnace type hot-dip galvanizing line (abbreviated as NOF-CGL) shown in Figure 5.
The oxygen concentration, amount of metal vapor, and dross generation during sheet threading in a molten zinc bath containing 0.5% Mg at a line speed of 80 m/min are shown as relative ratios, assuming the current atmospheric wiping as 1.0. The curve in Figure 4 shows the amount of steam generated in a bath containing 0.5% Mg in electrolytic zinc (99.99% purity), and the curve in the bath containing 0.2% Al and 0.5% Mg in zinc containing unavoidable impurities. It is. Also, the curve is the dross generated in the same bath as the curve. The bath temperature was 450° in all cases. In the case of the curve, the evolution of steam depends on the oxygen concentration
Generation stops at 100ppm or more. When containing Al in the curve, there is a critical point at an oxygen concentration of 50 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. 5, 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. It has also been found that the production of dross can be suppressed by controlling the oxygen concentration, making it possible to operate the steel plate 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, controlling the oxygen concentration can solve the problem associated with splashing, and at the same time has the characteristic that it is easy to control the coating weight to a light weight.In other words, if you compare atmospheric wiping and wiping with an oxygen concentration control of 100ppm under the same wiping conditions, the latter has a coating weight that is approximately 20% lower. Become.

For the above reasons, oxygen concentration control has made it possible to control the area weight of galvanized steel sheets containing Mg and Al, including light weights, 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 region up to the vicinity of the wiping noisle and in the solidification process. The specific area near the wiping nozzle can be determined by considering the area where splash occurs, and is determined by stripping speed, control of basis weight, and bath composition in an area of 1000 mm or less above the nozzle. The oxygen degree range in this region in the present invention is 50~
It is 1000ppm. (Figure 6, 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 though the non-oxidizing region is qualitatively expressed as a weakly oxidizing atmosphere, and it is difficult to control the oxygen concentration. No specific method has been disclosed, 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 area weight control ability also decreases. Figure 7 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, × indicates full skinning, △ indicates slight skinning, and ○ indicates no skinning, respectively). ). For zinc, use a mixed zinc bath, bath temperature 450℃, line speed 80m/m.
The wiping gas pressure was 1.0 Kg/cm ² , the wiping nozzle slit was 0.5 mm, and the nozzle interval was 20 mm. The same results as in FIG. 7 were also obtained when Al was added to Mg. As is clear from Figure 7, 2
In a bath with an Mg content of 0.5% or less, skinning will not occur if the oxygen concentration is below 1000 ppm, and in a bath with a 0.5% Mg content, skinning will 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 basis weight control, and is a part or all of the region until the plating metal solidifies.

Figure 8 shows Mg0.5 in mixed zinc (containing 0.22% Al).
This shows the area where poor appearance occurs during the solidification process when plating is performed using a bath containing 1.0% and 1.0% (the results are the same in both cases). 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. Figure 8 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. . In Figure 8, oxygen concentration
The optimal range is 100 to 1000 ppm, and there is no need to control the oxygen concentration at a low basis weight of less than 50 g/m ² (for example, 20 g/m ² ).

The seal box for controlling oxygen in the present invention will be described in detail below with reference to FIGS. 9 to 20. 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 9, 10 and 11 show three positions of the seal box. In the figure, 1 indicates a strip, 1 _L (black) indicates that the plated metal is in a molten state, and 1 _S (white) indicates that the plated metal 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 and the result is shown in FIG. FIG. 10 shows an example in which the oxygen concentration is completely regulated until solidification, and FIG. 11 is an example in which a part of the solidification process is regulated.

Below, FIGS. 12 to 20 show an example of the structure of the 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 (especially part 2a) must have a structure that can reduce the oxygen concentration to 1000 ppm or less, and the oxygen concentration in part 2a 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. 12 shows a double seal box composed of double walls 2a and 2c. 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 supplied to the box 10, the oxygen inside the box will be maintained at 10 ppm.

FIG. 13 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. 14 shows a 2b box structure in which the opening extends along the strip, and since the opening is long, it is possible to prevent air from entering the 2a box.

Fig. 15 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 by 9 and 10.

Fig. 16 shows a seal box having a double curtain structure, and nitrogen curtains are formed in the openings W ₁ and W ₂ in the inner chamber 2d. Figure 17 shows the results of a hot-dip galvanizing pilot line test (line speed 80 m/min, strip width 150 mm) using this seal box. The oxygen concentration measurement location (indicated by M in FIG. 16) is 50 mm above the wiping nozzle and 30 mm from the side wall of the seal box. W ₁ , W ₂
The opening was made with an opening width of 20 mm. Nitrogen amount Q ₁ to the outer curtain 26 m ³ /hr and to the inner curtain Q ₂ 16 m ³ /hr
Flow nitrogen through the wiping nozzle (strip width
400mm, slit gap 0.3mm, nozzle distance 30mm) and set the pressure to 0.5Kg/ ^cm2 , the oxygen concentration inside the box will be 5 to 15ppm (varies with the meandering of the strip).
stabilized at Air to _Q2 2.5/hr, 10/hr,
When mixed at 17.5/hr and 25/hr, the respective oxygen concentrations will be 100 to 250, as shown in Figure 17.
It can be controlled to 100-500, 900-1000, 1500-2000ppm.

In FIG. 18, 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 14.
is sent to the ACP through 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 19 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 18. 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 ² /min
(Pressure 1.0Kg/cm ² ). In the case of this box structure, the gas amount ratio of ACP 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 Figure 18 10, and the oxygen concentration was measured at M ₁ , M ₂ , M ₃ in Figure 16.
This is shown in curves M ₁ , M ₂ , and M ₃ in Figure 9. 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 20 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 seal box 2b.

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.

[Brief explanation of the drawing]

Figure 1 shows electrolytic zinc and the electrolytic zinc and Al 0.22%.
A curve showing the corrosion loss of a galvanized steel sheet prepared by adding 0 to 2% Mg to a zinc compound containing Mg 1.0%.
Figure 3 shows the plating adhesion of zinc plating containing Mg and Al by controlling the wiping gas pressure, and Figure 4 shows the adhesion of plating with 0.05 to 0.5% Al. A curve showing the amount of steam and dross generated in a bath containing 0.5% Mg, Figure 5 is an explanatory diagram of the hot-dip galvanizing line using the non-oxidizing furnace method in Figure 4, and Figure 6 is the oxygen concentration and amount of steam generated. , a diagram showing the relationship between the amount of dross and the amount of skin tension, Figure 7 is a diagram showing the area where skin tension occurs using the oxygen concentration of the wiping area and Mg in the bath, and Figure 8 is a diagram showing the relationship between the amount of dross and the amount of skin tension.
FIGS. 9, 10 and 11 are diagrams showing the area where "kink"occurs; FIGS. 9, 10, and 11 are diagrams showing three positions of the seal box; and FIGS. 12 to 20 are diagrams 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: Pottrol, 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, which includes a step of plating at least one side in a zinc bath and controlling the amount of plating, wherein 0.1 to 2.0% Mg and 2.0% Al are added to the zinc bath.
A bath consisting of 0.1 to 0.5% zinc and unavoidable impurities is used, and at least a portion of the bath surface during which the plated metal adhering to the steel strip solidifies is surrounded by a seal box and includes a wiping nozzle. In addition to controlling the oxygen concentration on the bath surface side to 50 to 1000 ppm,
The oxygen concentration on the solidification zone side of the plating metal above is not controlled when the plating weight is less than 50g/ ^m2 , and is controlled to 100 to 1000 ppm when the plating weight is 50g/m2 or ^more . A method for producing a hot-dip galvanized steel strip, which comprises solidifying unsolidified galvanized metal on the surface of the steel strip. 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 manufacturing a hot-dip galvanized steel strip as described in Section 1.