JPH11302813A - Hot dip aluminum zinc based alloy steel sheet excellent in workability and white rust corrosion resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the workability and the white rust corrosion resistance of a plated steel sheet by controlling the ratio of Al concentration between a dendrite part and a dendrite gap part in a plated film of a hot dip Al-Zn based alloy plated steel sheet to less than a specific value. SOLUTION: The ratio d/i of Al concentration between the dendrite part (d) and the dendrite gap part (i) in the plated film of the hot dip Al-Zn based allay plated steel, sheet containing 20-95 wt.% Al is controlled to less than 1.5. The coating weight of the plated film is controlled to <=45 g/m<2> per one surface, a chemical conversion treated film is formed on the surface of the plated film and further, an organic resin coating film is preferably formed on the upper surface. The chemical conversion treated film is a chromated film formed by applying a chromating solution containing chromic acid having a ratio (hexavalent Cr)/(total Cr) of 0.1-3.0 by weight and the coating weight of the chromated film is preferably 3-80 mg/m<2> expressed in terms of metal chromium. The plated steel sheet is obtained by adjusting the cooling speed of the plated film to control the solidification of the plated film or the like.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a method for producing a plating film, comprising:
The present invention relates to a hot-dip Al-Zn-based alloy-plated steel sheet containing 20 to 95% by weight of l and generally used after being subjected to a chemical conversion treatment, painting, or the like.

[0002]

2. Description of the Related Art Al is 20 to 95% by weight in a plating film.
The hot-dip Al-Zn alloy-plated steel sheet contained is
As shown in No. 6-7161, since it shows superior corrosion resistance as compared with a hot-dip galvanized steel sheet, its demand has been increasing in recent years mainly in the field of building materials. This hot-dip Al-Zn-based alloy-plated steel sheet is hot-rolled, and then hot-rolled steel sheet pickled and descaled, or a cold-rolled steel sheet obtained by further cold-rolling the hot-rolled steel sheet as a base sheet, and is used in a continuous hot-dip plating facility. It is manufactured by the following process.

[0003] A steel sheet as a plating base sheet first enters an annealing furnace maintained in a reducing atmosphere, is heated to a predetermined temperature, and at the same time as the annealing, the rolling oil and the like adhering to the steel sheet surface are removed and the oxide film is reduced and removed. After that, the lower end passes through the snout immersed in the plating bath and is immersed in the hot-dip galvanizing bath containing a predetermined amount of Al. The steel sheet subjected to predetermined plating in this plating bath is pulled up above the plating bath via a sink roll, and after the amount of plating is adjusted by gas wiping by a wiping nozzle arranged on the plating bath, It becomes a hot-dip Al-Zn-based alloy-plated steel sheet on which a predetermined plating film is formed by cooling by a cooling device.

[0004] In continuous hot-dip plating equipment for producing hot-dip Al-Zn alloy-plated steel sheets, operating conditions such as heat treatment conditions in an annealing furnace, atmospheric conditions, plating bath composition, and cooling rate after plating are determined by the desired plating conditions. In order to ensure the quality and material, it is precisely controlled within a predetermined management range. The thus-produced hot-dip Al-Zn alloy-plated steel sheet has an alloy phase having a thickness of about 1 to 2 μm at the interface between the coating film and the base steel sheet. And the remaining dendrite gap. The dendrite portion and the dendrite gap portion have different compositions. Usually, the dendrite portion has a higher Al content than the dendrite gap portion, and the Al content between the dendrite portion [d] and the dendrite space portion [i] in the plating film. The concentration ratio d / i is 1.5 or more.

[0005]

By the way, a hot-dip Al-Zn alloy-plated steel sheet containing 20 to 95% by weight of Al in a plating film may be subjected to processing such as bending, depending on the degree of processing. Cracks occur in the plating film. In this hot-dip Al-Zn-based alloy-plated steel sheet, the alloy phase having a thickness of about 1 to 2 μm at the interface between the plating film and the base steel sheet becomes the starting point of cracks, and the dendrite gap of the plating film tends to be a crack propagation path. Therefore, cracks tend to open relatively large as compared to a case where hot-dip galvanized steel sheets having the same plating film thickness are processed to the same extent. For this reason, depending on the degree of processing, there is a problem that cracks are generated that can be visually recognized by the naked eye, and the appearance is impaired.

A hot-dip Al—Zn alloy plated steel sheet containing 20 to 95% by weight of Al causes white rust due to the sacrificial corrosion protection of Zn contained in the plating film. This is also undesirable in appearance. Therefore, an object of the present invention is to solve such problems of the prior art, and to form a molten Al-
It is to obtain a Zn-based alloy-plated steel sheet.

[0007]

The present inventors have conducted intensive studies to obtain a hot-dip Al-Zn-based alloy coated steel sheet having excellent workability and white rust resistance. %, The Al concentration ratio d / i between the dendrite part [d] and the dendrite gap part [i] in the plating film of the hot-dip Al-Zn-based alloy-plated steel sheet is less than 1.5, so that the workability and the durability are improved. It has been found that the white rust property is dramatically improved.

[0008] The present invention has been made based on such knowledge, and has the following features. [1] In a hot-dip Al—Zn alloy plated steel sheet containing 20 to 95% by weight of Al in a plating film, an Al concentration ratio d / i between a dendrite part [d] and a dendrite gap part [i] in the plating film. Is less than 1.5, a hot-dip Al-Zn-based alloy-coated steel sheet having excellent workability and white rust resistance. [2] In the hot-dip Al-Zn-based alloy-coated steel sheet according to the above [1], a coating amount of a plating film containing 20 to 95% by weight of Al is 45 g / m ² or less per side. Al-Zn alloy plated steel sheet with excellent resistance and white rust resistance.

[3] The hot-dip Al—Zn-based alloy-coated steel sheet according to the above [1] or [2], which has a chemical conversion treatment film on the surface of the plating film and is excellent in workability and white rust resistance. Hot-dip Al-Zn alloy plated steel sheet. [4] The molten Al-Zn alloy-plated steel sheet according to [1] or [2], characterized by having a coating film on the surface of the plating film, and having excellent workability and white rust resistance. Alloy plated steel sheet.

[5] The hot-dip Al-Zn-based alloy-coated steel sheet according to [1] or [2], characterized in that it has a chemical conversion coating on the surface of the plating film and has a coating film thereon. Al-Zn alloy plated steel sheet with excellent resistance and white rust resistance. [6] The hot-dip Al-Zn-based alloy-plated steel sheet according to [1] or [2], further comprising a chemical conversion treatment film on a surface of the plating film,
A hot-dip Al-Zn-based alloy-plated steel sheet having excellent workability and white rust resistance characterized by having an organic resin film as an upper layer.

[7] In any one of the above-mentioned [3], [5], and [6], the chemical conversion coating film has a hexavalent Cr / total Cr weight ratio of 0.3. A chromate film formed by applying and drying a chromate treatment solution containing chromic acid of about 1.0 to 1.0, wherein the chromate film has an adhesion amount of metal chromium equivalent of 3 to 80 mg / m ^2. Al- with excellent workability and white rust resistance
Zn-based alloy plated steel sheet.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the present invention will be described below.
FIG. 1 shows a cross-sectional structure of a plating film of a conventionally manufactured hot-dip Al-Zn alloy-plated steel sheet (Al content: 20 to 95% by weight), and FIG. 2 shows hot-dip Al-Zn-based alloy plating of the present invention. The cross-sectional structure of the plating film of a steel plate (Al content: 20 to 95% by weight) is schematically shown.

In the conventionally manufactured hot-dip Al—Zn alloy plated steel sheet, the dendrite portion has a higher Al concentration (Al content) than the dendrite gap portion, and the dendrite portion [d] and the dendrite gap portion [i ] Is 1.5 or more, the difference in strength between the dendrite portion and the dendrite gap portion is remarkable. For this reason, as shown in FIG. 1, when the cracks generated during the processing propagate from the alloy phase into the plating film, they tend to preferentially propagate through the dendrite gap, and stress concentrates on the cracks. As a result, the cracks appear to be large in appearance.

On the other hand, as in the hot-dip Al—Zn alloy-coated steel sheet of the present invention, the Al concentration ratio d / d between the dendrite portion [d] and the dendrite gap portion [i] in the plating film.
If i is less than 1.5, the difference in strength between the dendrite portion and the dendrite gap becomes small, so that the tendency of the crack to propagate preferentially through the dendrite gap as shown in FIG. 2 is lost. This makes cracks easily propagate. For this reason, the number of cracks increases, the opening width of each crack becomes small, and it becomes difficult to visually recognize the crack.

[0015] Further, the Al concentration ratio d / i between the dendrite part [d] and the dendrite gap part [i] in the plating film of the hot-dip Al-Zn-based alloy-coated steel sheet containing 20 to 95% by weight of Al is 1. When the Al concentration ratio is less than 5,
JIS-K5
It has been found that the generation of white rust in an accelerated test such as a combined cycle test specified in 621 and an outdoor exposure test in the air is delayed, and the white rust resistance is improved. For the above reasons, in the hot-dip Al-Zn-based alloy-plated steel sheet of the present invention, the Al concentration ratio d / i between the dendrite part [d] and the dendrite gap part [i] in the plating film is set to less than 1.5.

The Al concentration ratio d / i between the dendrite part [d] and the dendrite gap part [i] in the plating film is determined by scanning the cross section of the plating film of the hot-dip Al—Zn alloy plated steel sheet with a scanning electron microscope or EPMA. It can be measured by observation. Note that the vicinity of both edges of the steel sheet, particularly the 100 mm width portions at both ends in the width direction of the steel sheet, is a unique part when viewed from the entire steel sheet, and the sheet temperature and the amount of plating applied during manufacturing are unstable. The specified Al concentration ratio d / i is an Al concentration ratio measured at a portion excluding both end portions in the plate width direction.

The Al concentration ratio d / i between the dendrite part [d] and the dendrite gap part [i] in the plating film depends on the coating weight, the plating bath composition and the plating bath temperature, the thickness of the base steel sheet, It changes due to the effects of cooling conditions, etc.
In order to make the Al concentration ratio d / i between the dendrite part [d] and the dendrite gap part [i] in the plating film less than 1.5, it is advantageous that the coating amount is small, so that the plating amount is one side. It is preferably 45 g / m ² or less.

The hot-dip Al-Zn alloy-coated steel sheet of the present invention contains 20 to 95% by weight of Al in a plating film, and is represented by a so-called hot-dip 55% Al-Zn alloy-coated steel sheet. is there. Usually, the plating film of the hot-dip Al-Zn-based alloy-plated steel sheet contains about 0.3 to 3.0% by weight of Si in addition to Al and Zn, and an appropriate amount of Fe, Ti, Sr, V, Cr, M
g, Mn, etc., and other unavoidable impurities. The hot-dip Al-Zn-based alloy-plated steel sheet of the present invention is cast in a conventional manner, hot-rolled, then pickled and descaled hot-rolled steel sheet, or a cold-rolled steel sheet obtained by further cold-rolling the same. Into a hot-dip galvanizing machine,
It is manufactured by performing hot-dip Al-Zn-based plating containing 0 to 95% by weight.

In producing a hot-dip Al—Zn alloy-plated steel sheet, in order to make the Al concentration ratio d / i between the dendrite part [d] and the dendrite gap part [i] in the plating film less than 1.5. Can take various methods, for example,
There is a method in which a heat retaining device or a heating device is installed immediately above a plating bath of a continuous hot-dip plating facility, a cooling rate of a plating film is appropriately adjusted, and solidification of the plating film is controlled. That is, when the time during which the coating is maintained in the solid-liquid coexisting temperature range when the plating coating solidifies is shortened, the separation of Al and Zn in the coating is suppressed, and the dendrite part [d] and the dendrite gap [i] Is apt to be small, and therefore A
1 The concentration ratio d / i tends to be small. Further, when the solidification rate from the solid-liquid coexistence temperature range is increased, the substance diffusion is suppressed, and the A concentration ratio d / i tends to decrease.

Further, the Al concentration ratio d / d between the dendrite part [d] and the dendrite gap part [i] in the plating film.
Since i changes depending on the plating bath composition, the plating bath temperature, the bath penetration plate temperature and the like, the Al concentration ratio d / i can be controlled by appropriately adjusting these. Further, as described above, the Al concentration ratio d / i between the dendrite portion [d] and the dendrite gap portion [i] in the plating film is set to 1.
In order to make it less than 5, the coating weight per one side should be 45 g /
Although it is advantageous to set it to m ² or less, it is not always possible to make the Al concentration ratio d / i less than 1.5 alone.
The hot-dip Al-Zn-based alloy plated steel sheet of the present invention has excellent workability and white rust resistance regardless of the sheet thickness, but from the viewpoint of corrosion resistance at the cut end, the sheet thickness is less than 1.2 mm. It is preferable to do so.

Normally, the hot-dip Al-Zn alloy-coated steel sheet of the present invention is subjected to a chemical conversion treatment such as a phosphate treatment or a chromate treatment on the plated surface, or a coating is applied to the plated surface or the chemical conversion treated film surface. And may be a product. When a hot-dip Al-Zn-based alloy-plated steel sheet is left outdoors, for example, for a long period of time while the surface of the steel sheet is wet due to condensation or rain, the surface may be discolored black (a so-called black discoloration phenomenon). In order to prevent this, it is preferable to form a chromate film on the surface of the plating film. This chromate film contains trivalent Cr and hexavalent Cr, and the amount of deposited Cr (the amount of deposited metal chromium) is preferably ³ to 80 mg / m ² , more preferably 10 to 50 mg / m ² . Blackening can be effectively prevented by forming such a chromate film. If the amount of Cr attached is less than 3 mg / m ² , the effect of preventing blackening cannot be sufficiently obtained. On the other hand, if the amount of Cr exceeds 80 mg / m ² , an effect commensurate with the amount of attached Cr cannot be obtained. It is not preferable because it becomes easy.

The chromate film is formed by applying a chromate treatment solution containing chromic acid to the surface of the plating film and drying the chromate treatment solution. Chromic acid contained in the chromate treatment solution is hexavalent Cr / total Cr. Weight ratio 0.3 ~
It is preferably 1.0, and when the weight ratio of hexavalent Cr / total Cr is less than 0.3, blackening resistance may be reduced. This is considered to be because the passivation effect of the chromate film on the plating film surface is reduced. From the above viewpoints, the weight ratio of hexavalent Cr in chromic acid / total Cr is preferably in the range of 0.4 to 1.0, particularly preferably 0.5 to 1.0. Before performing the chromate treatment, it is also possible to wash the plating surface with hot water, water, or an alkaline solution.

In the chromate film formed on the plating film surface, for example, water-dispersible organic resin, silica,
Anions such as mineral acids, fluorides and the like can be added. Among these, it is possible to impart scratch resistance during processing or the like by adding an organic resin, and it is possible to improve corrosion resistance by adding silica. Further, by adding an anion or a fluoride, the coloring of the chromate film can be suppressed or the reactivity with the plating film can be adjusted. However, these additives may lower the blackening resistance depending on the type and amount of the additive, and thus the type and the amount of the additive must be appropriately selected.

Usually, the chromate film is formed by applying a treating solution to the surface of the plating film by spraying, dipping, roll coater or the like, and drying the coating solution at a plate temperature of about 60 to 250 ° C. At this time, some hexavalent Cr in the processing solution
Reacts on the plating surface to produce trivalent Cr, so that even if a treatment solution containing no trivalent Cr is used, the coating contains trivalent Cr. In addition, the upper layer of the chromate film is 0.1.
It is also possible to form an organic resin film having a thickness of about 1 to 5 μm.

The hot-dip Al-Zn alloy plated steel sheet of the present invention can also be used as a base steel sheet for coating materials. When processing a coating material, cracks may occur in the coating film in severely processed parts, and such cracks also impair the appearance as described above. One of the causes of such cracks is a crack in the base plating film. If a hot-dip Al-Zn-based alloy plated steel sheet having improved workability according to the present invention is used as the base steel sheet, the workability of the coated steel sheet itself ( Crack resistance) is also improved. In addition, the corrosion resistance of the processed part is significantly improved by applying the coating.

When the hot-dip Al-Zn alloy-plated steel sheet of the present invention is used as a coated steel sheet, it is usually subjected to a degreasing treatment before coating and, if necessary, to a further pickling.
It is preferable to perform a chemical conversion treatment such as a chromate treatment or a phosphate treatment. The chromate treatment is as described above. In particular, the workability (crack resistance) can be improved by adding an aqueous resin to the chromate film.

The paint can be applied directly on the chemical conversion coating. However, in order to further improve the workability and the white rust resistance, an undercoat paint (a so-called primer) usually used for coated steel sheets is used. ) Is applied and baked, and then applied, that is, it is preferable to form a coating film composed of an undercoating film and an overcoating film on the undercoating film. The resin for the undercoat paint is preferably a resin mainly composed of an epoxy resin, a polyester resin, a polyester resin modified with epoxy, an epoxy resin modified with polyester, etc. from the viewpoint of processability and white rust resistance. Further, as the curing agent, one or more of melamine, isocyanate and the like can be used.

Further, when a high degree of white rust resistance is required, it is preferable to add a chromate compound as a rust preventive pigment to the undercoating film. As the chromate compound, zinc chromate, strontium chromate, calcium chromate, barium chromate and the like are preferable, and the content thereof is 1 to 6 in terms of the solid content in the coating film.
Suitably, it is 0% by weight. The thickness of the undercoating film is preferably about 5 to 20 μm in order to obtain the above-described effects.

As the paint for forming the top coat, there can be used ordinary paints such as polyester resin paint, fluororesin paint, acrylic resin paint, PVC paint, silicone paint and the like. The thickness of the top coat is preferably from 5 to 40 μm from the viewpoints of workability and white rust resistance. When the coating thickness is less than 5 μm, the weather resistance of the coating decreases (UV transmittance increases),
In addition, the ability of the coating film to suppress white rust exposure is reduced, which is not preferable. On the other hand, if it exceeds 40 μm, the coating workability and the appearance of the coating film will be reduced, and the cost will increase, which is not preferable.

The undercoating film and the overcoating film may contain additives such as coloring pigments, extender pigments, and anti-scratch agents, if necessary. Examples of the coloring pigment include titanium oxide, carbon black, iron oxide, lead chromate, metal powder, calcined pigment, and pearl pigment. As extender pigments, for example, calcium carbonate, clay, talc,
Examples include antimony trioxide, barium sulfate, and kaolin. As the scratch preventing agent, ceramic beads such as silica and alumina, glass beads, glass fibers, resin beads, fluorine beads and the like are preferable from the viewpoint of processability.

Examples of the solvent used for the undercoat or topcoat include toluene, xylene, ethyl acetate, butyl acetate, cellosolve solvents, methyl isobutyl ketone, methyl ethyl ketone, diisobutyl ketone, and the like.
Isophorone, cyclohexanone and the like. Also,
For example, an antifoaming agent, a pigment dispersant, an anti-sagging agent and the like can be added to the paint.

The coating method of the paint is not particularly limited, and a coating method such as a roll coater method, a curtain flow coater method, a spray coating method, a brush coating method, etc., which are conventionally generally used, can be applied. The roll coater method is most common. When the roll coater method is used, the baking treatment after the coating is applied is usually 20 times.
It is performed by heating for ~ 180 seconds to reach a plate temperature of 150 ° C or higher. If the baking time is less than 20 seconds, the resin component is insufficiently melt-cured. On the other hand, if it exceeds 180 seconds, thermal degradation including the undercoat paint component starts, and the original performance of the paint cannot be exhibited in any case. Not preferred. There is no particular limitation on the heating method of the baking treatment, and a method such as a hot air heating method or a high frequency heating method can be applied.

[0033]

[Example 1] A cold-rolled steel sheet obtained by casting, hot rolling, pickling and cold rolling by a conventional method was charged into a continuous hot-dip plating facility, and 55 wt% Al-1. 5wt% S
i-Molten Al by plating bath consisting essentially of zinc
-Zn plating was performed to produce a hot-dip 55% Al-Zn-based alloy-plated steel sheet. In the production of this hot-dip Al-Zn alloy plated steel sheet, the dendrite in the plating film is changed by changing the plating bath temperature and the penetration plate temperature and the capability of a heat retaining device installed immediately above the plating bath of the continuous hot-dip plating equipment. Part [d]
The Al concentration ratio d / i between the and the dendrite gap [i] was adjusted.

The cross section of the plating film of the hot-dip Al—Zn alloy plated steel sheet thus obtained was observed by EPMA.
The Al concentration ratio d / i between the dendrite part [d] and the dendrite gap part [i] in the plating film was measured. In this measurement, a sample was arbitrarily sampled from a range excluding a portion having a width of 100 mm at both ends in the width direction of the hot-dip Al-Zn alloy-plated steel sheet, and the Al concentration ratio d / i of the plating film cross section was measured by EPMA every 100 μm. The maximum value measured at 10 points was taken as the value of the Al concentration ratio d / i. In the measurement of the Al concentration ratio d / i by EPMA, EPM manufactured by JEOL Ltd.
A Using JXA-8600MX, acceleration voltage 15 kV,
Observation was performed under the conditions of an irradiation current of 3 × 1/10 ⁷ A and a beam diameter of 1 μmφ, and a point analysis was performed at an acquisition time of 10 sec. Also, the workability of hot-dip Al-Zn alloy plated steel sheet,
The white rust resistance and the corrosion resistance of the cut end were evaluated by the following test methods.

(1) Workability The test piece was subjected to 0T bending, and the processed portion was
The opening width of the crack was measured by observing with a double magnifier, and the workability was evaluated based on the maximum value of the opening width of the crack. The evaluation criteria are as follows. ◎: crack opening width of less than 20 μm ：: crack opening width of 20 μm or more and less than 50 μm Δ: crack opening width of 50 μm or more and less than 100 μm ×: crack opening width of 100 μm or more

(2) White rust resistance A cut end face of a test piece of 150 mm × 70 mm was sealed, and a combined cycle test specified in JIS-K5621 and an outdoor air exposure test (coastal area and inland residential area) were performed. After the elapse of a certain time, the evaluation was made based on the area ratio of white rust on the test piece surface. The evaluation criteria are as follows. ：: White rust generation area rate 0% ○: White rust generation area rate 1% or more and less than 25% △: White rust generation area rate 25% or more, 50% or less ×: White rust generation area rate more than 50%

(3) Corrosion resistance of cut end (red rust resistance) Using the same sample as in the evaluation test of white rust resistance, an air exposure test in an inland residential area was conducted on a test piece that did not seal only one side of the cut end. Was performed, and the state of occurrence of red rust at the cut end of the test piece after three months was evaluated. The evaluation criteria are as follows. ◎: No occurrence of red rust was observed even when observed with a 20-power loupe. ：: Red rust generation was observed by observation with a 20 × magnifier, but no red rust was observed by visual observation. Δ: Discontinuous spot-like red rust is observed by visual observation.

The test results are shown in Tables 1 to 3 together with the thickness of the plated steel sheet and the coating weight. According to this, the plated steel sheet of the present invention in which the Al concentration ratio d / i between the dendrite part [d] and the dendrite gap part [i] in the plating film is less than 1.5, has the same Al concentration ratio d / i. Is 1.5
Workability and white rust resistance are significantly improved as compared with the plated steel sheets of the comparative examples described above. In addition, the corrosion resistance of the cut end is 1.2 mm thicker than a plated steel plate having a thickness of 1.2 mm or more.
A plated steel sheet with less than is better.

[0039]

[Table 1]

[0040]

[Table 2]

[0041]

[Table 3]

Example 2 Molten Al produced in Example 1
-Coating type chromate treatment on a part of Zn-based alloy plated steel sheet (weight ratio of hexavalent Cr / total Cr in chromic acid of the treatment liquid:
0.5, liquid temperature: 50 ° C., coating method: spraying method), and immediately dried to form a chromate film to obtain a chromate-treated hot-dip Al—Zn alloy-plated steel sheet. The blackening resistance of these chromate-treated hot-dip Al-Zn alloy-plated steel sheets was evaluated by the following test method.

(1) Blackening resistance
g / cm ^{2 at} a surface pressure of 60 ° C, 98%
Changes in the external surface after being left in a humid environment of RH or higher for 240 hours were visually evaluated according to the following evaluation criteria. 5: No change at all 4: A slight change (blackening) at an area of 1 to 5% 3: A clear blackening at an area of 1 to 5% 2: A clear blackening at an area of 6 to 25% 1 : Clear blackening was observed in an area of 26% or more. Table 4 shows the test results. In each case, the inventive examples show good blackening resistance.

[0044]

[Table 4]

Example 3 Molten Al produced in Example 1
-Apply a coating-type chromate treatment to a Zn-based alloy-plated steel sheet to form a chromate film having a Cr adhesion amount of 30 mg / m ² , and then apply an epoxy-melamine resin-based paint as an undercoat so that the dry coating thickness becomes 5 μm. After application, baking was performed at about 200 ° C. for 60 seconds, and a polyester resin paint was further applied as a top coating so that the dry coating film thickness became 20 μm, and then baked at about 250 ° C. for 60 seconds, followed by water cooling to obtain a coated steel sheet. .

The workability, white rust resistance and corrosion resistance of the cut end of these coated steel sheets were evaluated by the following test methods. (1) Film workability (crack resistance) The test piece was bent at 180 ° in a room at 20 ° C, and the bent portion was observed with a 30-fold loupe to minimize cracks. Was evaluated by the number of plate scissors. ：: 0T ○: 1T △: 2T ×: 3T or more

(2) White rust resistance A cut end face of a test piece of 150 mm × 70 mm was sealed,
The combined cycle test specified in JIS-K5621 and the outdoor air exposure test (coastal area and inland residential area) were conducted.
The evaluation was made based on the area ratio of white rust occurrence on the surface of the test piece after a certain time. The evaluation criteria are as follows. ：: White rust generation area rate 0% ○: White rust generation area rate 1% or more and less than 20% △: White rust generation area rate 20% or more, 50% or less ×: White rust generation area rate more than 50%

(3) Corrosion resistance of cut end (red rust resistance) Using a sample similar to that for the evaluation test of white rust resistance, an air exposure test in an inland residential area was performed on a test piece that did not seal only one side of the cut end. After 5 years, the occurrence of red rust at the cut end of the test piece was evaluated. The evaluation criteria are as follows. ◎: No occurrence of red rust was observed even when observed with a 20-power loupe. ：: Red rust generation was observed by observation with a 20 × magnifier, but no red rust was observed by visual observation. Δ: Discontinuous spot-like red rust is observed by visual observation.

Tables 5 to 7 show the test results. According to this, the coated steel sheet using the plated steel sheet of the present invention as a base has significantly improved coating film workability and white rust resistance as compared with the coated steel sheet using the plated steel sheet of the comparative example as the base. Also,
The corrosion resistance of the cut end is better in a plated steel sheet having a thickness of less than 1.2 mm than in a plated steel sheet having a thickness of 1.2 mm or more.

[0050]

[Table 5]

[0051]

[Table 6]

[0052]

[Table 7]

[0053]

As described above, the molten Al-Z of the present invention
The n-based alloy-plated steel sheet has much better workability and white rust resistance than the conventional hot-dip Al-Zn-based alloy-plated steel sheet.

[Brief description of the drawings]

FIG. 1 is a schematic view showing a cross-sectional structure of a plating film of a conventionally manufactured hot-dip Al—Zn alloy-plated steel sheet (Al content: 20 to 95% by weight) and a mode of propagation of cracks in a film thickness direction.

FIG. 2 is a hot-dip Al—Zn-based alloy plated steel sheet (A
1 content: 20 to 95% by weight) schematically showing the cross-sectional structure of a plating film and the mode of crack propagation in the film thickness direction.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Nobuyuki Ishida 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Inside Nihon Kokan Co., Ltd. (72) Inventor Kazumi Jiromaru 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Inside Nippon Kokan Co., Ltd. (72) Inventor Toshiyuki Okuma 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd.

Claims (7)

[Claims] 1. A plating film containing 20 to 95% by weight of Al.
Workability, characterized in that, in the hot-dip Al-Zn alloy-plated steel sheet to be contained, the Al concentration ratio d / i between the dendrite part [d] and the dendrite gap part [i] in the plating film is less than 1.5. Al-Zn with excellent white rust resistance
Alloy plated steel sheet. 2. The processability and white rust resistance according to claim 1, wherein the coating weight of the plating film containing 20 to 95% by weight of Al is 45 g / m ² or less per one surface. Excellent hot-dip Al-Zn alloy plated steel sheet. 3. A hot-dip Al-Zn alloy plated steel sheet having excellent workability and white rust resistance according to claim 1, further comprising a chemical conversion coating on the surface of the plating film. 4. The hot-dip Al-Zn alloy plated steel sheet having excellent workability and white rust resistance according to claim 1 or 2, having a coating film on the surface of the plating film. 5. A coating film having a chemical conversion treatment film on a surface of the plating film, and a coating film on an upper layer thereof.
Or molten Al- having excellent workability and white rust resistance described in 2 above
Zn-based alloy plated steel sheet. 6. The molten Al having excellent workability and white rust resistance according to claim 1 or 2, wherein a chemical conversion treatment film is provided on the surface of the plating film, and an organic resin film is provided thereon. -Zn alloy plated steel sheet. 7. The chemical conversion coating film is a chromate coating film formed by applying and drying a chromate treatment solution containing chromic acid having a weight ratio of hexavalent Cr / total Cr of 0.3 to 1.0, and processability and耐白rust excellent in molten Al-Zn-based alloy according to claim 3, 5 or 6 attached amount of metal chromium conversion of the chromate film is characterized in that it is a 3~80mg / m ² Plated steel sheet.