JP3324504B2 - Hot-dip Al-Zn alloy coated steel sheet with excellent crack resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-dip Al-Zn alloy-plated steel sheet which contains 20 to 95% by weight of Al in a plating film and is generally used after being subjected to chemical conversion treatment or painting.

[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 JP-A No. 6-7161, since the steel sheet shows superior corrosion resistance as compared with a normal hot-dip galvanized steel sheet, its demand has been increasing in recent years. Generally, this molten Al-Zn
System alloy plated steel sheet is subjected to chemical conversion treatment or painting,
Processed by press forming, roll forming, bending, etc., it is used in the fields of building materials and home appliances.

[0003] However, this hot-dip Al-Zn alloy-plated steel sheet has a drawback that when it is subjected to severe bending, cracks are liable to occur in the processed portion, and the cracks impair the appearance. Conventionally, in order to prevent the occurrence of cracks in the processed portion, a method of reducing the amount of plating (JP-A-5-271895) and a temperature range of 93 ° C. to 427 ° C. And then slowly cooling to at least 205 ° C. to obtain a plating film without age hardening (Japanese Patent Publication No. 61-2).
No. 8748) has been proposed.

[0004]

However, even with these methods, the improvement in crack resistance when severe bending is performed is not sufficient, and in addition, the corrosion resistance decreases in the former and the manufacturing process increases in the latter. This leads to a problem of an increase in manufacturing cost. In addition, misch metal, Mg,
Method of adding Mn or the like (Japanese Patent Publication No. 64-10593)
However, even if the amount of plating is reduced as described above, the crack resistance is not sufficiently improved,
The above method only increases the material cost.

[0005] Accordingly, an object of the present invention is to solve the problems of the prior art, to provide excellent crack resistance, and to be able to manufacture without increasing the number of manufacturing steps or adding a special element to a plating bath. An object is to obtain a hot-dip Al-Zn-based alloy-plated steel sheet.

[0006]

Means for Solving the Problems The present inventors have made intensive studies to solve the above-mentioned problems, and as a result, have found the following facts. (1) The plating film of the hot-dip Al-Zn-based alloy-coated steel sheet containing 20 to 95% by weight of Al mainly has a dendrite portion,
An interdendrite portion (dendrite gap portion) and an interfacial alloy layer (intermetallic compound layer) formed at the interface with the steel sheet. When such a steel sheet is subjected to severe bending (for example, 0T bending), an interfacial alloy layer is formed. , Cracks propagate in the dendrite / interdendrite interface, causing cracks in the plating film.

(2) The ease with which the cracks propagate and the form thereof greatly depend on the hardness difference between the dendrite portion and the interdendrite portion.
Since the hardness difference between the dendrite portion and the interdendrite portion is large in the system-alloy-plated steel sheet, the opening width of each crack increases, and the formation of such a crack having a large opening width results in impairing the appearance of the plating film. .
This is because, when the difference in hardness between the dendrite portion and the interdendrite portion is large, the propagation path of cracks generated from the interface alloy layer is concentrated at a limited location at the dendrite / interdendrite interface. This is because the opening width of each crack introduced to follow the deformation of the steel plate due to the working becomes large.

On the other hand, when the hardness difference between the dendrite portion and the interdendrite portion is small, a large number of cracks enter the plating film at random because the propagation path of the cracks is not limited as described above. The opening width of the crack is small and only small enough to be visually invisible. Therefore, if the difference in hardness between the dendrite portion and the interdendrite portion is reduced, the opening width of each crack can be reduced, and deterioration of the plating film appearance due to the crack can be suppressed.

(3) Further, in the conventional hot-dip Al-Zn alloy-plated steel sheet, the interdendrite portion itself is also very hard due to precipitation hardening after solidification, so that the propagation of the crack is easily promoted. Therefore, the propagation of cracks starting from the interface alloy layer can be suppressed by using a plating film having a low hardness of the interdendrite portion itself.

The present invention has been made based on such findings, and has the following features. [1] A hot-dip Al-Zn-based alloy-coated steel sheet containing 20 to 95% by weight of Al in a plating film, wherein a hardness ratio d / of a dendrite part [d] and an interdendritic part [i] in the plating film is d /
A hot-dip Al-Zn alloy plated steel sheet having excellent crack resistance, wherein i is 0.40 or more. [2] The coated steel sheet according to [1], wherein the interdendrite portion has a micro-Vickers hardness Hv of 500 or less, and is characterized by having excellent crack resistance.
n-based alloy plated steel sheet.

[3] The coated steel sheet according to [1] or [2], wherein the coating weight of the plating film containing 20 to 95% by weight of Al is 10 to 45 g / m ² per side. Hot-dip Al-Zn alloy plated steel sheet with excellent crack resistance. [4] A hot-dip Al-Zn-based alloy coated steel sheet having excellent crack resistance, wherein the coated steel sheet according to the above [1], [2] or [3] has a chemical conversion coating on the surface of the coating.

[5] The plated steel sheet of the above [1], [2] or [3], characterized by having a coating film on the surface of the plating film, characterized by having excellent crack resistance and hot-dip Al-Zn alloy plating. steel sheet. [6] The plated steel sheet of the above [1], [2] or [3], which has a chemical conversion treatment film on the surface of the plating film and has a coating film on an upper layer thereof, and has excellent crack resistance. Melt A
l-Zn alloy plated steel sheet.

[7] The crack-resistant steel sheet according to the above [1], [2] or [3], characterized in that the steel sheet has a chemical conversion coating on the surface of the coating and an organic resin film on the coating. Hot-dip Al-Zn-based alloy plated steel sheet. [8] In the plated steel sheet of the above [4], [6] or [7], the chemical conversion coating has a hexavalent Cr / total Cr weight ratio of 0.3 to 0.3.
A chromate film formed by applying and drying a chromate treatment solution containing chromic acid of 1.0, and having an adhesion amount of the chromate film of 3 to 80 mg in terms of metal chromium.
/ M ² , a hot-dip Al-Zn alloy plated steel sheet having excellent crack resistance.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the present invention will be described below.
FIG. 1 schematically shows a cross-sectional structure of a plated film of a hot-dip Al—Zn-based alloy plated steel sheet (Al content in the plated film: 20 to 95% by weight). The plated film is mainly composed of a dendrite portion and an interdendrite. (Dendrite gap) and an interface alloy layer (intermetallic compound layer) formed at the interface with the steel sheet. FIG. 2 shows a conventional molten Al-Z.
FIG. 3 schematically shows a crack generation mechanism in a plating film in an n-based alloy-plated steel sheet. When a severe bending process (for example, 0T bending) is performed on a plated steel sheet, cracks occur in an interface alloy layer, and the cracks are formed by plating. Propagation in the film causes cracks in the plating film.

As described above, in the conventional hot-dip Al-Zn alloy-plated steel sheet, the hardness difference between the dendrite portion and the interdendrite portion is large, and the hardness ratio d / i between the dendrite portion [d] and the interdendrite portion [i] is large. Since it is less than 0.40, the propagation path of cracks generated in the interface alloy layer is limited to the dendrite / interdendrite interface, and as a result, individual cracks introduced to follow the deformation of the steel sheet due to the bending process The opening width of the crack becomes large, and as a result, the appearance of the plating film surface is impaired.

Therefore, in the hot-dip Al—Zn alloy plated steel sheet of the present invention, the hardness ratio d / i between the dendrite part [d] and the interdendritic part [i] of the plating film is set to 0.40.
Above, more preferably 0.50 or more. By reducing the difference in hardness between the dendrite portion and the interdendrite portion in this way, the crack propagation path is not limited to the dendrite / interdendrite interface unlike the conventional plated steel sheet, and a large number of Since the cracks enter randomly, the opening width of each crack can be reduced to such a degree that it cannot be visually recognized in appearance.
FIG. 3 shows the relationship between the hardness ratio d / i of the dendrite part [d] and the interdendrite part [i] and the crack improvement rate. Here, the crack improvement rate refers to the improvement rate of the crack opening width with respect to the normal manufactured material defined in the description of the examples. According to FIG. 3, the hardness ratio d / i is 0.
It can be seen that the crack improvement rate is 30% at 40 or more, and the crack improvement rate of 50% or more can be stably obtained when the hardness ratio d / i is 0.50 or more.

In the conventional hot-dip Al-Zn alloy-plated steel sheet, the interdendrite itself becomes extremely hard with a micro Vickers hardness Hv of more than 500 due to precipitation hardening after solidification. Propagation is easily promoted. For this reason, in the present invention, it is preferable that the micro Vickers hardness Hv of the interdendrite portion be 500 or less, whereby the propagation of cracks can be effectively suppressed.

The hardness ratio d / i between the dendrite portion [d] and the interdendrite portion [i] of the plating film varies depending on the composition of the plating bath, the amount of plating applied, and the cooling conditions after plating. In order to obtain a plating film having / i of 0.40 or more, it is advantageous that the coating weight is small, and therefore, the coating weight is preferably 45 g / m ² or less per side. However, the coating weight is 10 g /
If it is less than m ² , the corrosion resistance of the plating film will not be sufficient, so the lower limit of the plating adhesion amount is preferably 10 g / m ² .

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 a so-called hot-dip 55% Al-Zn alloy-plated steel sheet. is there. In the plating film of this hot-dip Al-Zn-based alloy-plated steel sheet, usually, in addition to Al and Zn, Si: about 0.3 to 3.0% by weight (Si is used to suppress the growth of a brittle interface alloy layer. ), And an appropriate amount of Fe, Ti, Sr,
One or more of V, Cr, Mg, Mn, etc., and other unavoidable impurities may be contained.

The hot-rolled Al—Zn-based alloy-coated steel sheet of the present invention is cast and hot-rolled in a conventional manner, then pickled and descaled, and then cold-rolled. A steel sheet is charged into a continuous hot-dip coating apparatus, and Al
It is manufactured by applying hot-dip Al-Zn-based plating containing 95% by weight.

When producing a hot-dip Al—Zn alloy-plated steel sheet, a plating film having a hardness ratio d / i between the dendrite portion [d] and the interdendrite portion [i] of at least 0.40, preferably at least 0.50. Various methods can be used to obtain the coating. One example is a method of appropriately adjusting the solidification of the plating film by installing a heat retaining device immediately above the plating bath and performing a heat treatment for the plating film. There is. In this method, after adjusting the amount of plating, a heat treatment is performed for about 1 to 120 seconds in a solid-liquid coexistence temperature range determined by the plating composition (solid-liquid coexistence temperature range of the plating metal shown in FIG. 5), and the plating composition The heat retention temperature and the heat retention time are appropriately adjusted within the above range according to the plating conditions and the plating treatment conditions. By keeping the plating film in the solid-liquid coexisting temperature range during solidification by such a treatment, the aggregation and precipitation of zinc inside the interdendrites are promoted, so that the dendrite portion [d] and the interdendrite portion The hardness difference becomes smaller.

FIG. 4 shows a case where a hot-dip 55% Al—Zn alloy-plated steel sheet is manufactured and then heat-treated by the above-described method after the steel sheet is plated under specific plating conditions (heat retention time: 3 hours).
0 seconds), the heat treatment temperature is set at the dendrite part [d].
And the hardness ratio d / i of the interdendrite part [i] and the crack resistance (average value of the crack opening width in the processed part when the plated steel sheet is bent by 0T). In the case of an Al—Zn alloy plated steel sheet, the hardness ratio d / i is maintained by keeping heat for about 30 seconds in a temperature range of 500 to 580 ° C. which is a solid-liquid coexisting temperature range:
0.40 or more can be achieved, which indicates that good crack resistance can be obtained.

When the heat treatment is carried out at a temperature exceeding the solid-liquid coexistence temperature range, not only the above-mentioned effects cannot be obtained, but also
This is not preferable because the interface alloy layer grows thick. Although the hot-dip Al-Zn-based alloy-plated steel sheet of the present invention has excellent crack resistance irrespective of the sheet thickness, from the viewpoint of corrosion resistance at the cut end, the sheet thickness is 1.2 mm or less (more preferably 0.7 mm or less). ) Is preferred.

The hot-dip Al—Zn alloy plated steel sheet of the present invention may be subjected to a chemical conversion treatment such as a phosphate treatment or a chromate treatment on the plated surface, or may be coated on the plated surface or the chemical conversion treated film surface. Can be. Molten Al-Zn
When a steel plate is left outdoors, for example, for a long time in a state where the surface of the steel plate is wet due to condensation or rain, the surface of the steel plate may turn 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 has a Cr adhesion amount (an adhesion amount in terms of metal chromium) of 3 to 80 mg / m ² , more preferably 10 to 50 mg / m ² .
mg / m ² is preferred. 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. The chromate 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 treatment liquid to the surface of the plating film by spraying, dipping, a roll coater or the like, and drying it 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 above-mentioned chemical conversion treatment film. However, in order to further improve the workability and the white rust resistance, an undercoat paint (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 in the undercoat 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.

There are no particular restrictions on the method of coating the paint, and conventional coating methods such as a roll coater method, a curtain flow coater method, spray coating, and brushing 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 the baking time 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.

[0036]

[Example 1] A cold rolled steel sheet (sheet thickness 0.28) obtained by casting, hot rolling, pickling and cold rolling by a conventional method.
~ 1.8mm) was charged into a continuous hot-dip plating facility, and plated with a hot-dip bath (plating bath composition: 55 wt% Al-1.4 wt% Si- balance substantially Zn).
An l-Zn alloy plated steel sheet was manufactured. This molten Al-
In the production of Zn-based alloy-plated steel sheets, heat treatment for 500 to 550 ° C. for 1 to 30 seconds is performed on some of the coated steel sheets using an induction heating type heat retaining device (heating furnace) installed immediately above the plating bath. The hardness ratio d / i of the dendrite part [d] and the interdendrite part [i] was adjusted.

With respect to the hot-dip Al—Zn alloy plated steel sheet thus obtained, the hardness ratio d / i between the dendrite portion [d] and the interdendrite portion [i] of the plating film was obtained.
Was measured by the following method, and the crack resistance (the crack opening width on the plating film surface and the crack improvement rate) and the corrosion resistance of the cut end were evaluated by the following test methods. (1) Hardness measurement The average value of the measured values obtained by measuring 10 points with a measuring load of 1 g using an ultra-micro Vickers hardness tester manufactured by Matsuzawa Seiki Co., Ltd. was defined as the measured hardness. In addition, whether each hardness measurement location was a dendrite portion or an interdendrite portion was determined by observing a cross section of the plating film with a SEM.

(2) Crack resistance The 0T bent part of the test piece was observed with a 20 × optical microscope.
A photograph was taken to measure the crack opening width, and the average value was defined as the crack opening width. In addition, the crack opening width Dc of the normal production material (a steel plate having the same thickness and plated at the same plating adhesion amount, and then cooled at a cooling rate of 20 ° C./sec to solidify the plating film). The improvement rate of the crack opening width D of each test piece with respect to the crack opening width D measured by observing and photographing the 0T bent portion in the same manner as described above was [(Dc-D) / Dc] × 100. And this was taken as the crack improvement rate.

(3) Corrosion resistance (red rust resistance) of the cut end A 150 mm × 70 mm test piece was subjected to an atmospheric exposure test outdoors, and the occurrence of red rust at the cut end of the test piece after 6 months was evaluated. . The evaluation criteria are as follows. ◎: No discoloration and red rust generation ○ +: Slight discoloration generation ○: Discoloration generation △: Spot rust generation ×: Red rust generation

The results of these tests were used to determine the thickness of the plated steel sheet,
Tables 1 and 2 show the plating baths used and the coating weights used. According to this, the hardness ratio d / i between the dendrite part [d] and the interdendrite part [i] of the plating film is shown.
Is 0.40 or more, the crack resistance is significantly improved as compared with the plated steel sheet of the comparative example having the same hardness ratio d / i of less than 0.40. In addition, the corrosion resistance itself of the plating film does not change, but the corrosion resistance of the cut end is 1.2 mm or less (particularly 0.7 mm or less) as compared with a plated steel sheet having a thickness of more than 1.2 mm. The plated steel sheet is better.

[0041]

[Table 1]

[0042]

[Table 2]

Example 2 A part of the hot-dip Al—Zn alloy-plated steel sheet of the present invention manufactured in Example 1 was subjected to coating type chromate treatment (weight of hexavalent Cr / total Cr in chromic acid of the treatment liquid). (Ratio: 0.5, liquid temperature: 50 ° C., coating method: spray method), and immediately dried to form a chromate film, thereby obtaining 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) Resistance to blackening 0.5 k
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 black discoloration was observed in an area of 26% or more. The results of these tests are shown in Tables 3 and 4. In each case, good black discoloration resistance was obtained.

[0045]

[Table 3]

[0046]

[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 crack resistance and corrosion resistance of the cut end of these coated steel sheets were evaluated by the following test methods. (1) Crack resistance of the coating film The test piece was bent at 180 ° in a room at 20 ° C, and the bent portion was observed with a loupe of 30 times. The number was evaluated. ：: 0T ○: 1T △: 2T ×: 3T or more

(2) Corrosion resistance (red rust resistance) of cut end portion A 150 mm × 70 mm test piece was subjected to an atmospheric exposure test outdoors to evaluate the state of red rust occurrence at the cut end portion of the test piece two years later. . The evaluation criteria are as follows. ◎: No discoloration and red rust generation ○ +: Slight discoloration generation ○: Discoloration generation △: Spot rust generation ×: Red rust generation

Tables 5 and 6 show the test results. According to this, the coated steel sheet based on the plated steel sheet of the present invention has significantly improved crack resistance of the coating film as compared with the coated steel sheet based on the plated steel sheet of the comparative example. Although the corrosion resistance itself after coating is not changed, the corrosion resistance of the cut end is 1.2 mm or less (especially 0.7 mm or less) as compared with a plated steel sheet having a thickness of more than 1.2 mm. The plated steel sheet is better.

[0051]

[Table 5]

[0052]

[Table 6]

[0053]

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

[Brief description of the drawings]

FIG. 1 is an explanatory view schematically showing a cross-sectional structure of a plating film of a hot-dip Al—Zn alloy-plated steel sheet (Al content in a plating film: 20 to 95% by weight).

FIG. 2 is a schematic view showing a cross-sectional structure of a plating film of a conventionally manufactured hot-dip Al—Zn-based alloy-coated steel sheet (Al content in the plating film: 20 to 95% by weight) and a mode of crack propagation in the film thickness direction. Explanatory diagram

FIG. 3 is a graph showing the effect of the hardness ratio d / i between the dendrite part [d] and the interdendrite part [i] on crack resistance.

FIG. 4 is a graph showing the relationship between the hardness ratio d / i of the dendrite part [d] and the interdendritic part [i] in the heat treatment after the plating in the production of a 55% Al—Zn alloy plated steel sheet. Graph showing the effect on cracking properties

FIG. 5 is a Zn-Al equilibrium diagram of Al-Zn-based alloy plating

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Yasuhide Yoshida 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (72) Inventor Masaaki Yamashita 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Inside (72) Inventor Jiromaru Kazumi 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (72) Inventor Toshiyuki Okuma 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (72) Inventor Nobuyuki Ishida 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Inside Nippon Kokan Co., Ltd. (56) References JP-A-8-224829 (JP, A) JP-A-56-87654 (JP, A) JP-A-5-271895 (JP, A) JP-A-8-100248 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C23C 2/00-2/40

Claims (8)

(57) [Claims] 1. A plating film containing 20 to 95% by weight of Al.
A hot-dip Al-Zn-based alloy-coated steel sheet, wherein the hardness ratio d / i between the dendrite part [d] and the interdendrite part [i] in the plating film is 0.40 or more. Hot-dip Al-Zn-based alloy plated steel sheet. 2. The molten Al— excellent in crack resistance according to claim 1, wherein the micro Vickers hardness Hv of the interdendrite portion is 500 or less.
Zn-based alloy plated steel sheet. 3. The coating weight of a plating film containing 20 to 95% by weight of Al is 10 to 45 g / m ² per side.
The hot-dip Al-Zn-based alloy-plated steel sheet according to claim 1, wherein the steel sheet has excellent crack resistance. 4. The hot-dip Al-Zn alloy plated steel sheet having excellent crack resistance according to claim 1, further comprising a chemical conversion coating on the surface of the plating film. 5. The hot-dip Al-Zn-based alloy plated steel sheet having excellent crack resistance according to claim 1, wherein the steel sheet has a coating film on the surface of the plating film. 6. The molten Al having excellent crack resistance according to claim 1, further comprising a chemical conversion treatment film on the surface of the plating film, and a coating film thereon.
-Zn alloy plated steel sheet. 7. The molten Al—Zn excellent in crack resistance according to claim 1, wherein the plating film has a chemical conversion treatment film on a surface thereof and an organic resin film on an upper layer thereof. Alloy plated steel sheet. 8. 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 8. The molten Al having excellent crack resistance according to claim 4, wherein the chromate film has an adhesion amount of metal chromium equivalent of 3 to 80 mg / m < ² >.
-Zn alloy plated steel sheet.