JP2000104153A - Zinc-aluminum alloy plated steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the corrosion resistance of a bent portion when subjected to a bending process, to provide a bent portion of a zinc-aluminum alloy-plated steel plate, and a zinc-aluminum alloy-plated steel plate having a surface coated. Provided is a zinc-aluminum alloy-plated steel sheet capable of improving corrosion resistance of a bent part and a cut edge part. SOLUTION: 25 to 75% by weight of aluminum;
An alloy plating layer containing 1.0% to 5.0% by weight of magnesium and 0.5% by weight or more of silicon with respect to the aluminum content, and the remainder essentially consisting of zinc is formed. Elution of zinc from the alloy plating layer in a corrosive atmosphere can be suppressed. Corrosion resistance of the cut edge portion and the bent portion of the zinc-aluminum alloy-plated steel sheet can both be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a zinc-aluminum alloy-plated steel sheet, which has good corrosion resistance in a plated steel sheet and a bent portion of a coated steel sheet whose surface has been coated, and further has a coated surface. The present invention relates to a zinc-aluminum alloy-plated steel sheet having good corrosion resistance at a cutting edge of a plated steel sheet.

[0002]

2. Description of the Related Art It has been widely known to improve the corrosion resistance of a steel sheet by subjecting the surface of the steel sheet to galvanization, and a large number of galvanized steel sheets are produced today. However, zinc plating may have insufficient corrosion resistance for many applications, and zinc-aluminum alloy plating has recently been proposed as a method for further improving the corrosion resistance of steel sheets as compared with zinc.

[0003] Such zinc-aluminum alloy plating is provided as Japanese Patent No. 617971. The patent discloses that an alloy plating consisting of 25 to 75% by weight of aluminum, silicon having an aluminum content of 0.5% or more, and the balance consisting essentially of zinc is provided. A zinc-aluminum alloy plating layer which is excellent, has good adhesion to a steel sheet, and has a beautiful appearance can be obtained. Thus, the steel sheet plated with zinc-aluminum alloy exhibited excellent corrosion resistance in each step as compared with the conventional steel sheet plated with zinc alloy.

[0004]

However, on the other hand, when the zinc-aluminum-plated steel sheet manufactured as described above is cut, sufficient corrosion resistance cannot be obtained at the cut edge. . This is because the corrosion of the steel plate portion exposed on the cut end surface of the steel plate is prevented by the sacrificial corrosion protection effect of zinc, and the zinc component disappears from the zinc segregated portion in the zinc-aluminum alloy plating layer and the corrosion resistance is reduced. It is. For this reason, when zinc is lost from the plating layer, when coating is further applied on the metal alloy plating layer of the steel sheet or when a plastic film is laminated, the coating is applied to the plating layer near the cut edge. Bleeding of the coating film (edge creep) due to generation of corrosive products between the film layer and the film layer occurred.

In order to improve the corrosion resistance in the vicinity of the cutting edge with the painted zinc-aluminum alloy plating layer, Japanese Patent No. 1330504 discloses that in a zinc-aluminum alloy plating layer, magnesium is contained in the zinc-aluminum alloy plating layer in an amount of 0.01 to 0.01%.
Although a zinc-aluminum alloy plated steel sheet containing 1.0% was provided, the corrosion resistance of the part subjected to processing such as bending was compared with that of the zinc-aluminum alloy plated layer not containing magnesium. Was not enough.

The present invention has been made in view of the above points, and has improved corrosion resistance of a bent portion when subjected to a bending process, and further has a cut edge portion when a coated plated steel plate is cut. It is an object of the present invention to provide a zinc-aluminum alloy plated steel sheet having improved corrosion resistance.

[0007]

The zinc-aluminum alloy-coated steel sheet according to claim 1 of the present invention contains 25 to 75% by weight of aluminum, 1.0 to 5.0% of magnesium, and contains aluminum. It is characterized by containing 0.5% by weight or more of silicon with respect to the amount and forming an alloy plating layer consisting essentially of zinc on the balance.

[0008]

Embodiments of the present invention will be described below.

[0009] The zinc-aluminum alloy-plated steel sheet of the present invention has, on the surface of the steel sheet, 25 to 75% by weight of aluminum, 1.0 to 5.0% of magnesium, and 0.5 to the aluminum content. % Or more of silicon, with the balance being an alloy plating layer consisting essentially of zinc.

In applying the above alloy plating to a steel sheet, a steel sheet serving as a base material is placed in a molten metal bath containing zinc, aluminum, magnesium, and silicon in the same mixing ratio as the composition of a desired plating layer. Known means such as immersion can be used. The conditions for forming the plating layer at this time are not particularly limited.
After immersing the steel sheet in a 50 ° C. molten metal bath for 1 to 10 seconds, the steel sheet is cooled at a cooling rate of 20 to 40 ° C./second, whereby an alloy-plated steel sheet can be obtained.

[0011] Here, in a molten metal bath containing magnesium, top dross formed by oxidization of magnesium is liable to be generated. Therefore, zinc-aluminum having an alloy plating layer formed using such a molten metal bath is used. In the alloy-plated steel sheet, the appearance of the alloy-plated layer may be deteriorated, or fine cracks may be generated in the alloy-plated layer, leading to a reduction in corrosion resistance. Therefore, in order to suppress the generation of top dross, the upper surface of the molten metal tank to which the molten metal is supplied is covered with a lid, and an inert gas such as nitrogen gas is supplied into the molten metal tank to supply the molten metal to the molten metal tank. To prevent the molten metal from being combined with oxygen, for example, by setting the inside to an inert atmosphere, or by floating a heat-resistant material such as glass wool in the molten metal bath to block the contact between the molten metal and air. Is preferred.

Here, the reason why the ratio of aluminum in the alloy plating layer is 25 to 75% by weight is to prevent the alloy plating layer from being cracked or flaked when the alloy plated steel sheet is formed. For this purpose, it is more preferable to set the mixing ratio of aluminum to 40 to 75% by weight.

When the alloy plating layer contains 0.5% by weight or more of silicon with respect to the aluminum content to form the alloy plating layer on the steel sheet, the interface between the steel sheet surface and the alloy plating is brittle. The formation of the iron-containing alloy layer can be suppressed, and the adhesion between the steel sheet surface and the alloy plating layer can be improved. Here, the upper limit of the silicon content is not limited, but if the upper limit is particularly set, it is set to 10% by weight or less based on the aluminum content, and 10% by weight.
In addition, the effect of suppressing the formation of a brittle iron-containing layer exceeding 200 may be saturated and the workability of the plating layer may be reduced.

In the composition of the alloy plating layer according to the present invention, the balance excluding aluminum, magnesium and silicon is essentially zinc. Here, “essentially” means that the zinc-aluminum alloy plated steel sheet of the present invention is used. Corrosion resistance, other substances that do not impair the properties such as adhesion to the coating film, such as impurities or impurities that are conventionally allowed in a molten metal bath for alloy plating and other conscious additives are present It shows good things, for example, lead contained in each raw material component,
Trace impurities such as iron, copper, cadmium and tin, iron, copper,
Impurities such as lead, which are inevitably mixed in the manufacturing process,
Components other than the essential components of the present invention, such as additives such as manganese, tin, nickel, molybdenum, tungsten, cobalt, chromium, titanium, cadmium, antimony, lanthanum, and selenium, may be included. The mixing amount of these impurities and additives is not particularly limited, but may be 1% by weight or less based on the entire alloy plating layer.
It is acceptable. It is also preferable to reduce the amount of copper, cadmium, tin, antimony and lead mixed as much as possible.

[0015] Further, by setting the ratio of magnesium in the alloy plating layer to 1.0 to 5.0% by weight, when the alloy plating layer is formed on the surface of the steel sheet, the alloy plating layer may be removed from the alloy plating layer in a corrosive atmosphere. Elution of zinc can be suppressed. The reason why the elution of zinc from the alloy plating layer is suppressed is that the ratio of magnesium in the alloy plating layer is 1.0%.
It is considered that when it is set to ~ 5.0% by weight, strongly reducing magnesium precipitates in the zinc segregated portion of the surface layer of the alloy plating layer, and the zinc segregated portion has a tendency to passivate as a whole. . And, as described above, when the elution of zinc from the alloy plating layer in a corrosive atmosphere is suppressed, it is possible to improve the corrosion resistance of the alloy plated steel sheet, and not only the flat part of the alloy plated steel sheet, The corrosion resistance of the cut edge portion when the cutting process is performed and the bent portion when the bending process is performed are improved, and particularly the corrosion resistance of the bent portion can be significantly improved. The surface of the zinc-aluminum alloy-plated steel sheet of the present invention is coated with a paint such as a polyester resin, an acrylic resin, a fluororesin, a vinyl chloride resin, a urethane resin, or an epoxy resin, by roll coating, spray coating, or curtain coating. When a coating film is formed by flow coating, dip coating, or film lamination when laminating a plastic film such as an acrylic resin film, the flat part is cut and processed in a corrosive atmosphere. The corrosion resistance of the cut edge portion in the case where the bending process is performed and the bent portion in the case where the bending process is performed can be improved, and in particular, the corrosion resistance of the bent portion can be significantly improved.

If the magnesium content exceeds 1.0% by weight, the hardness of the alloy plating layer increases, and when the steel sheet on which the alloy plating layer is formed is subjected to a forming process such as a bending process, the alloy plating layer is hardened. Although cracks may be generated in the layer, the effect of improving corrosion resistance by suppressing the elution of zinc exceeds the reduction in corrosion resistance due to the occurrence of cracks, and can improve corrosion resistance as a whole. is there. Therefore, in the zinc-aluminum alloy-plated steel sheet of the present invention, it is a matter of course that after the alloy plating, the corrosion resistance at the portion not subjected to any forming process or at the cut edge portion when the cutting process is performed can be improved. In addition, it is possible to improve the corrosion resistance in a portion subjected to a forming process such as a bending process. Here, when the proportion of magnesium in the alloy plating is less than 1.0% by weight, the corrosion resistance at the cut edge is improved as compared with the case of normal zinc-aluminum alloy plating not containing magnesium, but bending is performed. On the contrary, the corrosion resistance of the portion subjected to the treatment decreases, and when the content exceeds 5.0% by weight, the material cost increases even though the effect of improving the corrosion resistance is saturated. The hardness is too high to easily crack, and a large crack may occur, which is not preferable.

The amount of the alloy plating layer formed on the surface of the steel sheet is not particularly limited, but if the amount is small, a sufficient thickness for imparting corrosion resistance to the alloy plating layer cannot be obtained. Also, if the amount of adhesion is too large, cracks that occur when subjected to bending processing may be large, or the alloy plating may be easily peeled off from the steel sheet surface, in which case the corrosion resistance tends to decrease, In order to obtain sufficient corrosion resistance and adhesion of the alloy plating layer, preferably, the total of both sides of the steel sheet is 40 to 250 g / m2.
² .

[0018]

The present invention will be described below in detail with reference to examples. (Examples 1 to 14, Comparative Examples 1 to 4) A mild steel sheet having a thickness of 0.5 mm was placed in a molten metal bath at a temperature of 600 ° C. having a composition shown in Table 1.
After immersion for 2 seconds, it was cooled at a cooling rate of 30 ° C./second, and alloy plating was applied to both the front and back surfaces of the mild steel sheet. Here, the adhesion amount of the plating layer on the steel sheet surface is shown in Table 1 for both front and back surfaces.
It was made to become what is shown in. In the composition of the plating layer in the table, aluminum, magnesium, and zinc are shown by the ratio to the entire molten metal bath used for forming the plating layer, and silicon is shown by the ratio to the aluminum in the molten metal bath. It is.

The alloy-plated steel sheet obtained as described above was cut into a size of 100 mm × 50 mm, and the following evaluation test was performed.

(EPMA Analysis of Alloy Plating Layer Surface) Example 3 (Mg content: 3.0% by weight), Example 1 (M
g content: 1.0% by weight), Comparative Example 2 (Mg content:
0.5 wt%) and Comparative Example 1 (Mg content: 0 wt%), the surface of the alloy plating layer was photographed by SEM.
EPMA analysis (electron probem
The distribution of each component of zinc, aluminum, silicon, magnesium, and iron which is an impurity on the surface of the alloy plating layer was measured by performing (analysis). The results are shown in FIG. 1 for Example 3, FIG. 2 for Example 1, FIG. 3 for Comparative Example 2, and FIG. 4 for Comparative Example 1. Here, in each drawing,
(A) shows an SEM image, and (b) to (f) show EPMA analysis results showing distributions of aluminum, zinc, magnesium, silicon, and iron, respectively.

As shown in FIGS. 1 to 4, it was recognized that the surface of the alloy plating layer had a tendency to separate into a zinc segregated portion and an aluminum segregated portion. When the magnesium content in the alloy plating layer is 0% by weight, magnesium is naturally not deposited on the zinc segregated portion on the surface of the alloy plating layer, and when the magnesium content is 0.5% by weight, Almost no precipitation of magnesium was observed in the zinc segregated part.
On the other hand, the magnesium content is 1.0% by weight;
As the content became 0% by weight, the precipitation of magnesium on the zinc segregated portion increased.

(Adhesion test) In each of Examples and Comparative Examples, JIS G 3321 was applied to a steel plate subjected to alloy plating.
A bending process is performed by a T-bend method in accordance with the method described above, an adhesive tape is applied to the bent portion, and then the film is peeled off at a stretch, and the peeling state of the alloy plating layer at this time is observed.
The evaluation was performed in nine steps from (good workability) to 8T (poor workability). Table 1 shows the evaluation results.

(Crack resistance test) The alloy-coated steel sheets of the examples and comparative examples were subjected to 4T bending, and the presence or absence of cracks in the bent portions of the plated layer was evaluated according to the following evaluation criteria. It is shown in FIG. ◎: No cracks are visually observed. ：: Although slight cracks are visually observed, they are slight. Δ: Fine cracks are visually observed, and large cracks are also observed therein. X: A large crack was visually observed, and the plating layer was peeled in some places. (Evaluation of corrosion resistance) In the same manner as in the case of the above-described crack resistance evaluation, a 4T bending process was performed on an alloy-plated steel sheet, and two flaws reaching a steel plate body of 45 mm in length were formed in a cross with a cutter knife. Was exposed in the open air for one year in a coastal area, and the corroded portions (rust generation) were observed and evaluated according to the following evaluation criteria. The results are shown in Table 1. In this sample, chrome (manufactured by Okite Co., product number “Ochem Coat F1”) was used for primary rust prevention on the plating surface.
Was deposited at 15 to 40 mg / m ² .・ Cut edge ◎: No rust at all ○: Rust generated at a width of less than 1 mm from the end △: Rust generated at a width of 1 mm or more and less than 5 mm from the end ×: Rust at a width of 5 mm or more from the end Occurrence ・ 4T bent part ◎: No rust is generated at all ○: Rust is generated at an area of less than 20% of the 4T bent part △: Rust is generated at an area of 20% or more and less than 50% of the 4T bent part ×: Of the 4T bent part Rust is generated in an area of 50% or more. ・ Planar part ◎: No rust is generated at all. ○: Rust is generated in an area of less than 20% of the flat part. Rust is generated in an area of 50% or more of the flat part. Flawed part ◎: No rust is generated. ○: Rust is generated in an area of less than 20% of the flawed part. Δ: 20% or more and less than 50% of the flawed part. Rust generated in area ×: Rust generated in 50% or more of flawed area (corrosion resistance of painted alloy plated steel sheet) The steel sheet which has been subjected to sex) alloy plating, chromate-based coating surface treatment (Nippon Parkerizing Co., Ltd., the amount of chromium adhesion is deposited dry so as to 30~50mg / m ² by the number "1300AN"),
Epoxy base coat (Nippon Paint Co., Ltd., product number "P
-152S ", and a 20 µm coating film is baked with a polyester overcoat (trade name" Nippe Supercoat 300HQ ", manufactured by Nippon Paint Co., Ltd.), and the coated alloy plated steel sheet is coated. I got This coated alloy-plated steel sheet was subjected to 4T bending, and two flaws having a length of 45 mm were formed in a cross shape with a cutter knife.
The corroded portions (blisters) were observed and evaluated according to the following evaluation criteria. The results are shown in Table 1.・ Cutting edge ◎: No blister at all ○: Blister width from the end is less than 1 mm △: Blister width from the end is 1 mm or more and less than 5 mm ×: Blister width from the end is 5 mm or more ・ 4T bent part ◎: No blister at all ○: Blister generated in area less than 20% of 4T bent part △: Blister generated in area of 20% or more and less than 50% of 4T bent part ×: Blister in area of 50% or more of 4T bent part Occurrence ・ Planar portion ◎: No blister at all ○: Blister generation in an area of less than 20% of the flat portion △: Blister generation in an area of 20% or more and less than 50% of the flat portion ×: In an area of 50% or more of the flat portion Blister generation ・ Scratched part ：: No blister at all ○: Blister generation in area less than 20% of flawed part △: 20% or more and less than 50% of flawed part Blistering × the product: based on the material cost at the time of forming the alloy plating layer in blistering at least 50% of the area of the flaw-in portion (manufacturing cost) each of Examples and Comparative Examples, ◎,
△, △, and × were evaluated on four levels.

[0024]

[Table 1] As is evident from Table 1, in Comparative Example 4, which is a composition containing only 0.01% by weight of aluminum in zinc,
In both the case where the coating was not applied and the case where the coating was applied, the corrosion resistance of the bent portion, the flat portion, and the scratched portion was poor. Further, in Comparative Example 2 containing no magnesium, the corrosion resistance of the cut edge portion and the flawed portion when the coating was not applied, and the coating film edge and the flawed portion when the coating was performed were poor, and the content of magnesium was low. In Comparative Example 1 with a small amount, the corrosion resistance of the bent portion and the flawed portion in both the case where the coating was not applied and the case where the coating was applied was poor. In Comparative Example 3 in which the content of magnesium in the alloy plating layer was large, the corrosion resistance was improved, but the crack resistance was extremely poor and the production cost was high. On the other hand, in Examples 1 to 14, the cut end, the bent part, the flat part, the flawed part when the coating was not applied, and the coating end, the bent part when the coating was applied,
In all the corrosion resistance evaluations of the flat part and the flawed part, good results were obtained, and particularly the improvement of the corrosion resistance in the bent part was remarkable. Good results were also obtained with respect to alloy plating adhesion, and no extreme decrease was observed in crack resistance.

As is apparent from Examples 1 to 5,
It was confirmed that the corrosion resistance was improved as the content of magnesium was increased.

Further, as is apparent from Example 4 and Examples 7 to 11, when alloy plating having the same composition was formed, the amount of the deposited alloy plating layer was reduced in Examples 4, 8 to 1.
When both surfaces are in the range of 40 to 250 g / m ² as in 0, the adhesion of the alloy plating layer is particularly excellent, and the corrosion resistance is improved.

[0027]

As described above, the zinc-aluminum alloy-plated steel sheet according to the first aspect of the present invention has a
5 to 75% by weight of aluminum, 1.0 to 5.0% of magnesium and 0.5 to aluminum content
Weight percent or more of silicon, and the balance is formed with an alloy plating layer consisting essentially of zinc, which can suppress the elution of zinc from the alloy plating layer in a corrosive atmosphere. It is possible to improve both the corrosion resistance of the cut edge portion and the bent portion of the zinc-aluminum alloy plated steel sheet.

[Brief description of the drawings]

1A and 1B are SEM images and EPMA analysis results of Example 3, wherein FIG. 1A is an SEM image, and FIGS. 1B to 1F are EPMA analyzes showing distributions of aluminum, zinc, magnesium, silicon, and iron, respectively. It shows the results.

FIGS. 2A and 2B are SEM images and EPMA analysis results of Example 1. FIG. 2A is an SEM image, and FIGS. 2B to 2F are EPMA analyzes showing distributions of aluminum, zinc, magnesium, silicon, and iron, respectively. It shows the results.

3A and 3B are SEM images and EPMA analysis results of Comparative Example 2, wherein FIG. 3A is an SEM image, and FIGS. 3B to 3F are EPMA analyzes showing distributions of aluminum, zinc, magnesium, silicon, and iron, respectively. It shows the results.

FIG. 4 shows SEM images and EPMA analysis results of Comparative Example 1, wherein (a) is an SEM image, and (b) to (f) are EPMA analyzes showing distributions of aluminum, zinc, magnesium, silicon, and iron, respectively. It shows the results.

Claims (1)

[Claims] 1. 25% to 75% by weight of aluminum;
An alloy plating layer containing 1.0 to 5.0% by weight of magnesium and 0.5% by weight or more of silicon with respect to the aluminum content, and the balance being essentially zinc. A zinc-aluminum alloy plated steel sheet characterized by the following.