JP3445992B2 - Hot-dip Al-Zn-based alloy plated steel sheet with excellent crack resistance and corrosion 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 containing 20 to 95% by weight of Al in a plating film and generally used after chemical conversion treatment or coating.

[0002]

2. Description of the Related Art 20 to 95% by weight of Al in a plating film
The hot-dip Al-Zn alloy plated steel sheet containing is
As shown in No. 6-7161, it shows excellent corrosion resistance as compared with ordinary hot-dip galvanized steel sheet, and thus its demand is 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 molding, roll molding, bending, etc., it is used in fields such as building materials and home appliances.

However, this hot-dip Al-Zn alloy plated steel sheet has a drawback that cracks are likely to occur in the worked portion when subjected to severe bending, and the cracks impair the appearance. Conventionally, in order to prevent the occurrence of cracks in such a processed portion, a method of reducing the amount of plating adhered (Japanese Patent Laid-Open No. 5-271895) or a method of heat-treating a product after plating (Japanese Patent Publication No. 61-61). -2874
No. 8) has been proposed.

[0004]

However, of these methods, the former method has a problem of reduced corrosion resistance, and the latter method has a problem of increased manufacturing cost due to an increased number of manufacturing steps. In addition, in order to prevent deterioration of corrosion resistance due to the reduction of the amount of plating, misch metal, Mg, Mn in the bath.
Although a method of adding such as (Japanese Patent Publication No. 64-10593) has been proposed, it is not preferable because it causes an increase in material cost.

Therefore, the object of the present invention is to solve the problems of the prior art and to have both excellent crack resistance and corrosion resistance, without increasing the number of manufacturing processes and adding a special element to the plating bath. Manufacturable molten Al-Zn
To obtain a system alloy plated steel sheet.

[0006]

As a result of intensive studies to solve the above-mentioned problems, the present inventors have found the following facts. (1) In the hot-dip Al-Zn alloy plated steel sheet containing 20 to 95% by weight of Al in the plating film, the crack generation state in the severely worked part changes as the coating amount is reduced. As a result, the opening width of the crack is reduced, and the appearance of the crack becomes almost unrecognizable.

(2) Further, 20 to 9 Al is contained in the plating film.
Regarding the corrosion resistance of the molten Al-Zn alloy plated steel sheet containing 5% by weight, the amount of coating adhered on one surface is 10 g /
If it is m ² or more, excellent corrosion resistance peculiar to this plating film will be exhibited in the non-processed part of the plated steel sheet, but spot-like red rust will occur in the part subjected to severe processing, and corrosion will start from this. Progresses.

Then, as a result of investigating and examining the mechanism of such corrosion, the following facts were found. (3) The coating film structure of a conventionally produced Al-Zn alloy-plated steel sheet with a coating deposition amount per one surface of about 75 g / m ² has an Al-based dendrite portion and an Al-dendrite portion as shown in FIG. The base is composed of an interdendrite portion in which a large amount of Zn is deposited on the base and an interface alloy layer formed at the interface with the steel sheet, and a Zn agglomerated portion is present in a part of the interdendrite portion. Such a plating film structure can be explained by an Al-Zn system equilibrium diagram. That is, the alloy system in the region containing 20 to 95% by weight of Al is composed of an Al phase having a wide range of solid solubility of Zn and a Zn phase containing almost no solid solution of Al. Since the solid solubility limit of Zn in Al changes as the temperature decreases, when dendrites having a high Al concentration are precipitated in the liquid phase, Zn is concentrated between the dendrites and finally solidifies at the lowest temperature as Zn agglomerates. In the dendrite part solidified first and in the interdendrite part solidified while rejecting Zn, Zn is finely precipitated by solid phase diffusion during cooling thereafter, and at room temperature, Zn particles form a small amount of Zn in the solid solution of Zn particles. It becomes a deposited structure. Then, when cracks are formed in the plating film by subjecting such a plating film to severe processing, first, the corrosion products of the agglomerated Zn cover the cracks with the corrosion product, and Al dissolves over time. Then, it changes to stable rust and the anticorrosion ability continues.

(4) On the other hand, when the plating adhesion amount is reduced in order to improve the crack resistance, the Zn agglomerated portion in the interdendrite portion as described above disappears,
As a result of the plating film structure changing to Al containing Zn in a supersaturated state and the sacrificial anticorrosive action described above being lost, dot-like red rust is likely to occur.

(5) Therefore, in order to exert the above-mentioned sacrificial anticorrosion effect by Zn and the rust stabilizing effect by Al in a well-balanced manner even when the amount of plating adhesion is reduced, the plating film structure is formed on the Al-based dendrite portion. It is necessary to have a structure composed of an interdendrite part which is Al-based and has a Zn agglomeration part at an appropriate ratio and an interface alloy layer. Specifically, there is a Zn agglomeration part in the interdendrite part. The area ratio of the plating film cross section is 1.
It is necessary that 0 to 30% be present. Also,
Such a plating film structure can be obtained by performing hot dip plating in a plating bath having a predetermined Al concentration according to the amount of plating adhered on the manufactured plated steel sheet.

The present invention has been made on the basis of such findings, and is characterized by having the following configuration. [1] A hot-dip Al—Zn alloy-plated steel sheet containing 20 to 95% by weight of Al in the plating film, wherein the amount of coating adhered to one surface of the steel plate is 10 to 45 g / m ² and the plating film is Al. It consists of a dendrite part in which Zn is finely precipitated, an interdendrite part in which Zn is precipitated in Al, and an interfacial alloy layer that exists at the interface with the steel sheet. Al-Zn excellent in crack resistance and corrosion resistance, characterized by being present in an area ratio of 1.0 to 30%
System alloy plated steel sheet.

[2] A galvanized steel sheet according to the above-mentioned [1], which has a chemical conversion treatment film on the surface of the plating film and is excellent in crack resistance and corrosion resistance. [3] A galvanized steel sheet according to the above [1], which has a coating film on the surface of the plating film and is excellent in crack resistance.

[4] In the plated steel sheet of the above [1], a molten Al- which is excellent in crack resistance and corrosion resistance, characterized by having a chemical conversion treatment film on the surface of the plating film and having a coating film on the upper layer thereof. Zn-based alloy plated steel sheet. [5] The molten Al-Zn system having excellent crack resistance and corrosion resistance, characterized in that the plated steel sheet according to the above [1] has a chemical conversion treatment film on the surface of the plating film and an organic resin film as an upper layer. Alloy plated steel sheet.

[0014]

DETAILED DESCRIPTION OF THE INVENTION The details of the present invention will be described below.
In the plated steel sheet of the present invention, the plating film contains Al of 20 to 95.
Contains 10% by weight, and the amount of plating adhered per side is 10
It is a 45 g / m ² hot-dip Al-Zn alloy plated steel sheet, and its plating film has a dendrite portion in which Zn is finely deposited on the Al base and an interdendrite portion in which Zn is deposited on the Al base, as shown in FIG. And an alloy layer having a steel sheet interface, and Zn agglomerates are present in the interdendrite portion in an area ratio of 1.0 to 30% in the cross section of the plating film.

Here, the Zn agglomeration part is defined as follows. That is, the Zn agglomerated part is a part that is finally solidified in the solidification process of the Al—Zn-based film, and is a part that can be observed as a white region in SEM observation as shown in FIG. FIG. 4 is an SEM enlarged photograph of the cross-sectional structure of the plated coating of the plated steel sheet of the present invention, in which the white portion surrounded by the white line is the Zn agglomerated portion in the interdendrite portion. Since the Zn agglomerated part is the final solidified part, most of it consists of Zn in which a small amount of Al is solid-solved.
Since there are often shrinkage cavities that appear as black dot-like portions in the white area (Zn agglomerated portion) in the photograph of FIG. 4 within the agglomerated portion, the composition of the Zn agglomerated portion is not 100% Zn. I won't. Therefore, in the present invention, the EDX or EPM is used.
A region where the Al intensity is 50% or less with respect to pure Al when analyzed by A is defined as a Zn agglomerated portion. The boundary of the Zn agglomerated portion defined in this way is in good agreement with the boundary of the Zn agglomerated portion that can be observed and confirmed by the SEM as shown in FIG. Moreover, the area ratio of the Zn agglomerated portion is the area ratio of the Zn agglomerated portion in an arbitrary cross section of the plating film, and is, for example, 50.
It can be determined by observing the cross section of the plating film at a magnification of about 00 times and measuring the area ratio of Zn agglomerated portions in the observation visual field.

When the hot dip Al—Zn system plated steel sheet is manufactured, the Al concentration in the plating bath and the coating adhesion amount are variously changed, and the area ratio of the Zn agglomerates present in the interdendrite portion of the plating film (plating film cross section The area ratio of Zn agglomerated parts in the interdendrite part is the processed part (0T bend).
The effect on the corrosion resistance was investigated. In this test,
After the manufactured plated steel sheet was subjected to 0T bending, a combined cycle test (300 cycles) and an atmospheric exposure test (inland residential area 6 months) were performed to evaluate red rust resistance. The test method and the red rust resistance evaluation method were the same as in the examples described later.

The results are shown in FIG. According to this, in terms of corrosion resistance, there is an optimum range for the area ratio of Zn agglomerates in the interdendrite portion, and the area ratio of Zn agglomerates is 1.0.
If it is less than%, red rust is likely to occur in a short period of time in an air-exposed environment, while if the area ratio of Zn agglomerates exceeds 30%, long-term stable red rust resistance is obtained in a corrosion-promoted environment. Absent. This is because the area ratio of Zn agglomerates is 1.
If it is less than 0%, the sacrificial anticorrosive action of Zn is insufficient, and if the area ratio of Zn agglomerates is more than 30%, the rust stabilizing action by Al is insufficient, so that the corrosion resistance (red rust resistance) is deteriorated. It is considered to be a thing.

Regarding the coating weight of the plated steel sheet, when the coating weight of one side of the steel sheet is 10 g / m ² , the corrosion resistance of the flat plate portion is poor. On the other hand, the upper limit of the coating weight per one side of the steel sheet is specified from the viewpoint of crack resistance. FIG. 2 shows the amount of coating adhered per one side of a molten Al-Zn alloy plated steel sheet containing 20 to 95% by weight of Al in the plating film and crack resistance (the crack opening in the processed part when the coated steel sheet is bent by 0T). It shows the relationship with the average width), and the opening width of cracks in the part subjected to severe processing decreases as the coating weight decreases, and the coating weight per side is 45 g / m ² or less. The average value of the crack opening width is less than 30 μm, and the appearance is such that almost no crack can be recognized.

The hot-dip Al—Zn alloy-plated steel sheet of the present invention is a hot-rolled steel sheet that is cast and hot-rolled by an ordinary method and then pickled and descaled, or a cold-rolled sheet obtained by further cold-rolling the steel sheet. A steel plate is charged into a continuous hot dip galvanizing machine, and Al is added to 20 ~
It is manufactured by applying a molten Al-Zn-based plating containing 95% by weight.

There is no particular limitation on the method of forming the Zn agglomerated portion in the interdendrite portion in the area ratio of 1.0 to 30%, but such a plating film structure is
For example, it can be obtained by performing hot dip plating in a plating bath having a predetermined Al concentration according to the amount of plated coating on the manufactured plated steel sheet. The Al concentration in the molten Al-Zn-based plating bath is changed to produce plated steel sheets with various coating adhesion amounts, and the Al concentration in the plating bath and the plating adhesion amount per one side of the product-plated steel sheet are generated in the interdendrite part. FIG. 5 shows the result of investigation on the influence of Zn on the area ratio of the agglomerated part.

According to FIG. 5, the Al concentration in the plating bath [A
L] is 0.25 CW + in relation to the coating weight [CW]
If it exceeds 40, the area ratio of the Zn agglomerated portion in the interdendrite portion becomes less than 1.0%, while the Al concentration [AL] in the plating bath is 0.25 CW + 34. When it is less than 5, the area ratio of Zn agglomerated portions in the interdendrite portion exceeds 30%. Therefore, the area ratio in the interdendrite part is 1.0 to
In order to generate a Zn agglomerated portion of 30%, the Al concentration [AL] (wt) that satisfies the following equation (1) according to the coating weight [CW] (g / m ² ) per surface of the product-plated steel sheet
%), It is sufficient to perform hot dipping in a plating bath. 0.25CW + 40 ≧ AL ≧ 0.25CW + 34.5 (1)

Further, when the coating film is kept in the solid-liquid coexistence temperature range for a long time during solidification, the separation of Al and Zn in the coating film is promoted. Therefore, the cooling conditions after plating are adjusted. Then, the area ratio of the Zn agglomerated portion may be controlled within the above range by promoting the separation of Al and Zn. In this case, for example, a method of installing a heat retaining device or a heating device directly above the plating bath of the continuous hot dip plating facility and appropriately adjusting the cooling rate of the plating film is adopted.

The hot dip Al-Zn alloy plated steel sheet of the present invention contains 20 to 95% by weight of Al in the plating film, and is a so-called hot-dip 55% Al-Zn alloy plated steel sheet. is there. In the plating film of this molten Al-Zn alloy plated steel sheet, usually, in addition to Al and Zn, Si: about 0.3 to 3.0% by weight (Si is a bath to suppress the growth of a brittle interfacial alloy layer). (Added in), and other than this, appropriate amounts of Fe, Ti, Sr,
One or more kinds of V, Cr, Mg, Mn, etc., and other unavoidable impurities may be contained. The molten Al of the present invention
The Zn-based alloy-plated steel sheet has excellent crack resistance regardless of the plate thickness, but from the viewpoint of corrosion resistance of the cut end, the plate thickness is 1.2 mm or less (more preferably 0.7 mm). preferable.

The hot-dip Al-Zn alloy-plated steel sheet of the present invention is subjected to chemical conversion treatment such as phosphate treatment or chromate treatment on its plated surface, or coated on the plated surface or the chemical conversion treatment film surface. You can Molten Al-Zn
The system alloy-plated steel sheet may be discolored to be black (so-called black discoloration phenomenon) if it is left outdoors for a long time and the surface of the steel sheet is wet due to dew condensation or rain. In order to prevent this, it is preferable to form a chromate film on the surface of the plating film. This chromate film is trivalent C
It contains r and hexavalent Cr, and has a Cr adhesion amount (adhesion amount in terms of metal chromium) of 3 to 80 mg / m ² , and more preferably 10
It is preferably 50 mg / m ² . Blackening can be effectively prevented by forming such a chromate film. If the amount of deposited Cr is less than 3 mg / m ² , the effect of preventing blackening cannot be sufficiently obtained, while the amount of deposited Cr is 80 mg / m ²
Even if it exceeds, the effect corresponding to the adhered amount cannot be obtained, and conversely Cr
Is easily dissolved, which is not preferable.

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

In the chromate film formed on the surface of the plating film, for example, water-dispersible organic resin, silica,
Anions such as mineral acids, fluorides and the like can be added. Of these, the addition of an organic resin can impart scratch resistance during processing and the like, and the addition of silica can improve the corrosion resistance. Further, by adding an anion or a fluoride, it is possible to suppress the coloration of the chromate film or adjust the reactivity with the plating film. However, these additives may reduce the blackening resistance depending on the type and the amount added, and therefore the type and the amount added need to be appropriately selected.

Usually, the chromate film is formed by applying the treatment liquid on the surface of the plating film by spraying, dipping, roll coater or the like, and drying at a plate temperature of about 60 to 250 ° C. At this time, a part of hexavalent Cr in the treatment liquid
Reacts on the surface of the plating to produce trivalent Cr. Therefore, even if a treatment liquid containing no trivalent Cr is used, the coating contains trivalent Cr. In addition, the upper layer of the chromate film has 0.
It is also possible to form an organic resin film having a film 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, a crack may occur in the coating film in a severely processed part, and such a crack also impairs the appearance as described above. One of the causes of such cracks is cracks in the base plating film, and if the hot-dip Al—Zn 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 ( The crack resistance) is also improved. Further, the corrosion resistance of the processed part is also markedly 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 degreasing treatment before coating and, if necessary, after 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, and 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 treatment film, but in order to further improve the workability and the white rust resistance, the undercoat paint (so-called primer) usually used for painted steel sheets is used. It is desirable that the coating composition (1) is applied and baked before coating, that is, the coating composition is composed of an undercoating film and an upper coating film on the upper layer thereof. As the resin for the undercoat paint, those having an epoxy resin, a polyester resin, a polyester resin modified with epoxy, an epoxy resin modified with polyester or the like as a main component are preferable from the viewpoint of workability and white rust resistance. Further, as the curing agent, one or more kinds 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 coating film. As the chromate compound, zinc chromate, strontium chromate, calcium chromate, barium chromate and the like are suitable, and the content thereof is 1 to 6 in terms of the solid content in the coating film.
It is suitable to be 0% by weight. The thickness of the undercoat coating is preferably about 5 to 20 μm in order to obtain the above effects.

As the paint for forming the top coating film, usual paints such as polyester resin paints, fluororesin paints, acrylic resin paints, vinyl chloride paints and silicone paints can be used. The thickness of the top coating film is preferably 5 to 40 μm from the viewpoint of workability and white rust resistance. If the thickness of the coating film is less than 5 μm, the weather resistance of the coating film is lowered (the ultraviolet transmittance is increased),
In addition, the ability to suppress white rust exposure of the coating film is also reduced, which is not preferable. On the other hand, if the thickness exceeds 40 μm, the coating workability is deteriorated, the appearance of the coating film is deteriorated, and the cost is increased, which is not preferable.

If necessary, additives such as color pigments, extender pigments and scratch preventives may be added to the undercoating film and the topcoating film. Examples of the color pigment include titanium oxide, carbon black, iron oxide, lead chromate, metal powder, calcined pigment, pearl pigment and the like. Examples of extender pigments include calcium carbonate, clay, talc,
Examples include antimony trioxide, barium sulfate, kaolin and the like. As the scratch-preventing agent, ceramic beads such as silica and alumina, glass beads, glass fibers, resin beads and fluorine beads are preferable from the viewpoint of workability.

Examples of the solvent used in the undercoat paint and the topcoat paint include toluene, xylene, ethyl acetate, butyl acetate, cellosolve-based solvent, methyl isobutyl ketone, methyl ethyl ketone, diisobutyl ketone,
Examples thereof include isophorone and cyclohexanone. Also,
As an additive, for example, an antifoaming agent, a pigment dispersant, an anti-dripping agent or the like can be added to the coating material.

There is no particular limitation on the coating method of the coating material, and conventionally-used coating methods such as roll coater method, curtain flow coater method, spray coating and brush coating can be applied. The roll coater method is the most common. When the roll coater method is used, the baking treatment after applying the paint is usually 20
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 melt curing of the resin component is insufficient, while if it exceeds 180 seconds, thermal deterioration including the undercoat paint component begins, and in any case the original performance of the paint will not be exhibited. Not preferable. There is also no particular limitation on the heating method of the baking treatment, and a hot air heating method, a high frequency heating method or the like can be applied.

[0036]

[Examples] [Example 1] Cold-rolled steel sheet (sheet thickness: 0.28) obtained by casting, hot rolling, pickling and cold rolling by a conventional method.
~ 1.8 mm) is charged into a continuous hot dip plating equipment, and plating is performed by using hot dip plating baths having various Al concentrations in the bath.
A molten Al-Zn alloy plated steel sheet was manufactured.

The workability (crack resistance) and corrosion resistance of the hot-dip Al-Zn alloy plated steel sheet thus obtained were evaluated by the following test methods. The area ratio of the Zn agglomerated portion in the interdendrite portion was determined by image analysis of an SEM image of the cross section of the plating film as shown in FIG. (1) Workability (crack resistance) A test piece was bent by 0T and the processed portion was visually observed to evaluate the occurrence of cracks by the following. ⊚: No cracks are visible to the naked eye ○: Slight cracks occur Δ: Small amount of clear cracks ×: Large amount of clear cracks

(2) White rust resistance A cut end surface of a 150 mm × 70 mm test piece is sealed,
Combined cycle test specified in JIS-K5621 (15
(0 cycle) and an outdoor atmospheric exposure test (3 months in the coastal area) were conducted, and the white rust occurrence area ratio on the surface of the test piece after the test was evaluated. The evaluation criteria are as follows. ◎: White rust occurrence area ratio 0% ◯: White rust occurrence area ratio 1% or more and less than 25% △: White rust occurrence area ratio 25% or more, less than 50% ×: White rust occurrence area ratio 50% or more

(3) Red rust resistance of processed part (acceleration test) The cut end face of a 150 mm × 70 mm test piece is sealed,
After bending this test piece to 0T, JIS-K5621
The combined cycle test (300 cycles) specified in Section 1 was carried out to evaluate the occurrence of red rust on the surface of the test piece after the test. The evaluation criteria are as follows. ◎: No discoloration or red rust occurred ○ +: Slightly discolored ○: Small amount of spot rust △: Spot rust ×: Red rust

(4) Red rust resistance of processed part (exposure test) A cut end face of a 150 mm × 70 mm test piece is sealed,
After bending the test piece to 0T, an outdoor atmospheric exposure test (inland residential area for 6 months) was performed to evaluate the red rust occurrence on the surface of the test piece after the test. The evaluation criteria are as follows. ◎: No discoloration or red rust occurred ○ +: Slightly discolored ○: Small amount of spot rust △: Spot rust ×: Red rust

(5) Red rust resistance of the cut end portion A test piece of 150 mm × 70 mm, which has 1
An air exposure test (inland residential area for 6 months) was performed on a test piece that did not seal only the sides, and the state of red rust generation at the cut end of the test piece after the test was evaluated. The evaluation criteria are as follows. ◎: No discoloration or red rust ○ +: Slight discoloration ○: Discoloration △: Spot rust ×: Red rust

The results of these tests are shown as the thickness of the plated steel sheet,
Tables 1 to 4 show the concentrations of Al and Si in the used plating bath, the amount of deposited plating, the area ratio of Zn agglomerated portions in the interdendrite portion, and the upper and lower limit values of the above formula (1). According to this, it is understood that the plated steel sheet satisfying the conditions of the present invention has both excellent corrosion resistance and crack resistance. Also,
The corrosion resistance of the cut end is better in a plated steel sheet having a plate thickness of 1.2 mm or less (particularly, a plate thickness of 0.7 mm or less) as compared with a plated steel sheet having a plate thickness of more than 1.2 mm.

[0043]

[Table 1]

[0044]

[Table 2]

[0045]

[Table 3]

[0046]

[Table 4]

[Example 2] A part of the hot dip Al-Zn alloy plated steel sheet of the present invention manufactured in Example 1 was coated with a chromate treatment (weight of hexavalent Cr / total Cr in chromic acid in the treatment liquid). Ratio: 0.5, liquid temperature: 50 ° C., coating method: spraying method) and immediately dried to form a chromate film (Cr adhesion amount: 20 mg / m ² ), chromate treatment molten Al-Zn alloy plating A steel plate was obtained. The blackening resistance of these chromate-treated molten Al-Zn alloy plated steel sheets was evaluated by the following test method.

(1) 0.5 k by overlaying the chromate treated surfaces of the blackening resistance test pieces
Stacked at a surface pressure of g / cm ² , 60 ℃, 98%
The change in the appearance surface after standing for 240 hours in a humid environment of RH or higher was visually evaluated according to the following evaluation criteria. 5: No change at all 4: 1 There is a slight change (blackening) in the area of 1 to 5%, there is a clear blackening in the area of 3: 1 to 5% 2: There is a clear blackening in the area of 6 to 25% 1 : There is clear black discoloration in an area of 26% or more. The results of these tests are shown in Tables 5 and 6, and in both cases, good blackening resistance is obtained.

[0049]

[Table 5]

[Example 3] Casting, hot rolling, and
Cold-rolled steel sheet obtained by pickling and cold rolling (sheet thickness 0.3
~ 1.8 mm) is charged into a continuous hot dip plating equipment, and plating is performed by using hot dip plating baths having various Al concentrations in the bath.
A molten Al-Zn alloy plated steel sheet was manufactured. These hot-dip Al-Zn alloy plated steel sheets are coated with chromate to form a chromate film having a Cr adhesion amount of 30 mg / m ² , and then an epoxy / melamine resin-based paint is used as an undercoat paint to give a dry film thickness of 5 μm. And then baked at about 200 ° C for 60 seconds, and then a polyester resin paint as the top coat paint is dried to a film thickness of 20 μm.
The coated steel sheet was baked at about 250 ° C. for 60 seconds and then water-cooled.

The workability (crack resistance) and red rust resistance of the cut edges of these coated steel sheets were evaluated by the following test methods. (1) Coating processability (crack resistance) The test piece was bent 180 ° in a room at 20 ° C, and the bent portion was observed with a magnifying glass of 30 times to minimize cracking. The number of plate scissors was evaluated. ◎: 0T ○: 1T △: 2T ×: 3T or more

(2) Red rust resistance of cut end A test piece of 150 mm x 70 mm was subjected to an outdoor air exposure test (inland residential area for 2 years), and after the test, red rust occurred at the cut end. Was evaluated. The evaluation criteria are as follows. ◎: No discoloration or red rust ○ +: Slight discoloration ○: Discoloration △: Spot rust ×: Red rust

These test results are shown as the thickness of the plated steel sheet,
Table 7 shows the Al and Si concentrations in the plating bath used, the amount of plating deposited, the area ratio of Zn agglomerated portions in the interdendrite portion, and the upper and lower limit values of the above formula (1). According to this, in the coated steel sheet of the present invention example, the crack resistance of the coating film is significantly improved as compared with the coated steel sheet of the comparative example. In addition, 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 of is better.

[0054]

[Table 6]

[0055]

As described above, the molten Al-Z of the present invention is used.
The n-based alloy plated steel sheet has excellent corrosion resistance and crack resistance.

[Brief description of drawings]

FIG. 1 is a graph showing the influence of the area ratio of Zn agglomerates in the interdendrite portion of the plating film on the corrosion resistance of the plated steel sheet in the hot-dip Al—Zn alloy-plated steel sheet.

FIG. 2 is a graph showing the relationship between the amount of coating adhered to one surface of a molten Al-Zn alloy plated steel sheet and crack resistance.

FIG. 3 is an explanatory view showing a plating film structure of the molten Al—Zn alloy plated steel sheet of the present invention.

FIG. 4 is an enlarged micrograph showing a cross-sectional structure of a plated coating of the molten Al—Zn alloy plated steel sheet of the present invention.

FIG. 5 is a graph showing the influence of the Al concentration in the plating bath and the amount of plating deposited on one surface of the product-plated steel sheet on the area ratio of Zn agglomerated portions in the interdendrite portion of the plating film.

FIG. 6 is an explanatory view showing a plating film structure of a conventional hot dip Al—Zn alloy plated steel sheet.

─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Yasuhide Yoshida Marunouchi 1-2-2 Marunouchi, Chiyoda-ku, Tokyo Japan Steel Pipe Co., Ltd. (72) Masaaki Yamashita 1-2-1 Marunouchi Marunouchi, Chiyoda-ku, Tokyo Nippon Steel Pipe Co., Ltd. (72) Inventor Jiromaru Kazou 1-2-2 Marunouchi, Chiyoda-ku, Tokyo Japan Steel Tube Co., Ltd. (72) Inventor Toshiyuki Okuma 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Japan Steel Tube Co., Ltd. (72) Inventor Nobuyuki Ishida 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Within Nippon Steel Tube Co., Ltd. (56) References JP-A-5-271895 (JP, A) JP-A-58-177450 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C23C 2/00-2/40

Claims (5)

(57) [Claims] 1. The plating film contains 20 to 95% by weight of Al.
It is a hot-dip Al-Zn alloy plated steel sheet containing, the coating amount per one side of the steel sheet is 10 to 45 g / m ² , and the plating film has a dendrite portion in which Zn is finely precipitated in Al, and It consists of an interdendrite part where Zn is deposited and an interfacial alloy layer that exists at the interface with the steel sheet, and the agglomerated part of Zn is present in the interdendrite part in an area ratio of 1.0 to 30% in the cross section of the plating film. Melt A with excellent crack resistance and corrosion resistance
1-Zn alloy plated steel sheet. 2. The hot dip Al—Zn alloy plated steel sheet having excellent crack resistance and corrosion resistance according to claim 1, wherein the surface of the plating film has a chemical conversion treatment film. 3. The hot dip Al—Zn alloy plated steel sheet having excellent crack resistance and corrosion resistance according to claim 1, wherein the surface of the plating film has a coating film. 4. The chemical conversion treatment film is provided on the surface of the plating film, and the coating film is provided as an upper layer thereof.
Al-Z excellent in crack resistance and corrosion resistance described in 1.
N-based alloy plated steel sheet. 5. The molten Al—Zn-based alloy excellent in crack resistance and corrosion resistance according to claim 1, characterized in that a chemical conversion treatment film is provided on the surface of the plating film, and an organic resin film is provided thereon. Plated steel sheet.