JP2005290418A - HOT-DIP Al-Zn ALLOY PLATED STEEL SHEET SUPERIOR IN PRESS FORMABILITY, AND MANUFACTURING METHOD THEREFOR - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the press formability of a hot-dip Al-Zn alloy plated steel sheet of which the plated film contains 20 to 80 mass% Al. <P>SOLUTION: The hot-dip Al-Zn alloy plated steel sheet superior in press formability has a plated film with such a structure that the amount of an elementary Si crystallizing in dendrite spaces is controlled. Specifically, the plated film except an alloy phase region formed between the substrate steel sheet and the plated film contains 2-9 mass% elemental Si. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a molten Al—Zn-based plated steel sheet having an Al content in a plating film of 20 to 80 mass% and a method for producing the same.

Since the hot-dip Al-Zn-based plated steel sheet containing 20 to 80 mass% of Al in the plating film exhibits excellent corrosion resistance as compared with the hot-dip galvanized steel sheet, demand has recently increased mainly in the building material field (for example, , See Patent Document 1).
This hot-dip Al-Zn-based plated steel sheet is manufactured as follows using a hot-rolled steel sheet pickled and descaled or a cold-rolled steel sheet obtained by further cold rolling the base steel sheet in a continuous hot-dip plating facility. Is done.

  In the continuous hot dip plating facility, the base steel sheet was heated to a predetermined temperature in an annealing furnace maintained in a reducing atmosphere, and simultaneously with the annealing, the rolling oil adhered to the steel sheet surface was removed, and the oxide film was reduced and removed. Thereafter, the lower end passes through the inside of the snout immersed in the plating bath and is immersed in a hot dip galvanizing bath containing a predetermined concentration of Al. The steel plate immersed in the plating bath is pulled up above the plating bath via a sink roll, and then plated by injecting pressurized gas toward the steel plate surface from a gas wiping nozzle disposed on the plating bath. The adhesion amount is adjusted and then cooled by a cooling device to obtain a molten Al—Zn-based plated steel sheet on which a predetermined plating film is formed.

Operating conditions such as annealing furnace heat treatment conditions and atmosphere conditions, plating bath composition and cooling rate after plating in a continuous hot-dip plating facility are accurately controlled within a predetermined control range to ensure the desired plating quality and material. The
The plating film of the plated steel sheet produced as described above is mainly composed of a part where Al containing Zn is supersaturated and dendrite solidified and a part of the remaining dendrite gap, and the dendrite is in the film thickness direction of the plating film. Laminated. Due to such a characteristic film structure, the molten Al—Zn-based plated steel sheet exhibits excellent corrosion resistance.

  In addition, about 3 mass% Si is usually added to the plating bath with respect to Al, but the action of Si suppresses the growth of the alloy phase at the plating film / underlying steel plate interface in the molten Al—Zn-based steel plate. The alloy phase thickness is about 1-2 μm. The thinner the alloy phase, the more part of the characteristic film structure that exhibits excellent corrosion resistance. Therefore, the suppression of the growth of the alloy phase contributes to the improvement of the corrosion resistance. In addition, since the alloy phase is harder than the plating film and acts as a starting point of cracks during processing, suppressing the growth of the alloy phase reduces the occurrence of cracks and brings about an effect of improving workability. Moreover, since the base steel plate is exposed in the crack portion and the corrosion resistance is poor, reducing the occurrence of cracks also improves the corrosion resistance of the processed portion.

Usually, the plating bath contains unavoidable impurities, Fe eluting from steel plates and equipment in the plating bath, Si for alloy phase suppression, but some other elements may be added in addition to these alloys. In the phase and plating film, these elements exist in the form of an alloy or a simple substance.
In addition, the hot-dip Al-Zn-based plated steel sheet is rarely used as it is in hot-dip plating when it is put to practical use, and is usually a surface-treated steel sheet that has been subjected to chemical conversion treatment or coating on the surface of the plated steel sheet, Used as a painted steel plate.
Japanese Patent Publication No.46-7161

  When the molten Al—Zn-based plated steel sheet is subjected to processing such as bending, cracks occur in the plating film of the processed part depending on the degree of processing. In this plated steel sheet, an alloy phase having a thickness of about 1 to 2 μm existing at the plating film / underlying steel sheet interface serves as a crack starting point, and a dendrite gap portion of the plating film serves as a crack propagation path. For this reason, even when the same degree of processing is performed, the propagation path of cracks is limited compared to hot-dip galvanized steel sheets with the same plating film thickness, and the number of cracks generated is small, but each crack is relatively large in size. Tend to. For this reason, the crack is visually recognized with the naked eye depending on the degree of processing, and there is a problem that the appearance is impaired. Moreover, since the base steel plate is exposed at the crack portion, there is also a problem that the corrosion resistance is remarkably lowered as compared with the portion without the crack.

  Furthermore, as described above, the hot-dip Al-Zn-based plated steel sheet exhibits excellent corrosion resistance compared to the hot-dip galvanized steel sheet with the same plating film thickness, but it is not only deformed but also subjected to sliding as well as deformation. In the mode, the plating film tends to peel more easily as the opening of the crack becomes larger and the interval between adjacent cracks becomes longer.

  Accordingly, an object of the present invention is to provide a plated steel sheet having an unprecedented excellent press workability in a molten Al-Zn-based plated steel sheet having an Al content in a plated film of 20 to 80 mass%.

  In order to solve the above-mentioned problems, the present inventors diligently studied a method for improving the press workability of the molten Al—Zn-based plated steel sheet. As a result, it is possible to obtain an unprecedented excellent press workability by adopting a plating film structure in which the amount of elemental Si crystallized in the dendrite gap is controlled, and the amount of Si contained in the plating bath and the plating bath It has been found that such a plated film structure can be obtained by optimizing the temperature.

The present invention has been made based on such findings, and the features thereof are as follows.
[1] A molten Al—Zn-based plated steel sheet having an Al content of 20 to 80 mass% in the plated film, and Si alone is 2 to 2 in the plated film excluding the alloy phase portion of the base steel sheet and the plated film interface. A molten Al-Zn-based plated steel sheet excellent in press workability, characterized by containing 9 mass%.
[2] A molten Al-Zn-based plated steel sheet having an Al content of 20 to 80 mass% in the plated film, and Si is contained in the plated film including the alloy phase portion at the interface between the base steel sheet and the plated film (1) A hot-dip Al—Zn-based plated steel sheet excellent in press workability, characterized by containing the formula and the formula (2).
0.02 × T-14.5 ≦ S1 ≦ 0.02 × T-7.5… (1)
770 ≦ T ≦ 1000 (2)
T: Plating bath temperature (K) during molten Al-Zn plating
S1: Si content in the plating film (mass%)

[3] A surface-treated steel sheet excellent in press workability, comprising a chemical conversion coating on the surface of the molten Al—Zn-plated steel sheet of [1] or [2].
[4] The above-mentioned [1] or [2] has a chemical conversion treatment film on the surface of the molten Al-Zn-based plated steel sheet, and further has a single-layer or multiple-layer coating on the upper layer, Painted steel sheet with excellent press workability.
[5] In producing the hot-dip Al-Zn-based plated steel sheet according to [1] or [2] above in a continuous hot-dip plating facility,
A method for producing a molten Al-Zn-based plated steel sheet excellent in press workability, comprising plating a steel sheet in a molten Al-Zn plating bath having a plating bath temperature satisfying the following formula (3) below 1000K .
2.5 × A + 720 ≦ T ≦ 50 × S2 + 725 (3)
T: Plating bath temperature (K)
A: Al content in the plating bath (mass%)
S2: Si content in the plating bath (mass%)

[6] A method for producing a surface-treated steel sheet having excellent press workability, wherein a chemical conversion treatment film is formed on the surface of the molten Al—Zn-based plated steel sheet obtained by the production method of [5].
[7] A chemical conversion film is formed on the surface of the molten Al—Zn-plated steel sheet obtained by the production method of [5] above, and a single-layer film or a multi-layer film is formed thereon. The manufacturing method of the coated steel plate excellent in press workability.

  According to the present invention, a molten Al—Zn-based plated steel sheet having excellent press workability can be obtained.

The plated steel sheet of the present invention is a molten Al-Zn-based plated steel sheet containing 20 to 80 mass% of Al in the plated film, but from the balance of performance (corrosion resistance, workability, etc.) and operational surface, A more preferable range of the amount of Al is 45 to 65 mass%.
As described above, the molten Al-Zn-based plated steel sheet has a crack propagation path limited to the dendrite gap of the plating film, so the number of cracks is small and each crack is relatively large. Tend to. In the present invention, by utilizing such a relationship between the plating film structure and cracks, a crack propagation path is actively introduced into the plating film, thereby generating a large amount of fine cracks, that is, the number of cracks. The aim is to reduce the opening width of each crack by increasing it, and as described below, the generation form of such cracks is plating in which the amount of simple Si crystallized in the dendrite gap is controlled. This can be realized by adopting a film structure. Moreover, this film structure can be obtained by controlling the amount of Si contained in the plating bath and the temperature of the plating bath within an appropriate range.

  As described above, in a plating bath for producing a molten Al—Zn-based plated steel sheet, Si of about 3 mass% is usually added to Al. Due to the function of Si, molten Al—Zn-based plating is performed. In the steel plate, alloy phase growth at the plating film / underlying steel plate interface is suppressed, and the alloy phase thickness is about 1 to 2 μm. Excess Si, which did not contribute to the suppression of alloy phase formation, crystallizes as a simple substance in the plating film when it exceeds the amount of Si solid solution in the plating film, and becomes a physical barrier that hinders the growth of Al dendrite in the plating film. . Further, since crystallized Si does not mix with Al dendrite, it is dispersed in the dendrite gap. In the dendrite gap including crystallized Si, the boundary between the crystallized Si becomes easy to peel off, so that the crack propagates more easily than the dendrite gap without crystallized Si. For this reason, the more crystallized Si, the more likely to generate many fine cracks during deformation. Even in the conventional molten Al-Zn-based plated steel sheet, there was a case where the surplus Si that did not contribute to the suppression of the formation of the alloy phase was crystallized very slightly as a simple substance in the plating film, but the amount is 1 mass. It was a very small amount of less than%.

  If the amount of crystallized Si is too small, the number of crack propagation paths will not increase sufficiently. Conversely, if the number of crystallized Si is too large, the number of crack propagation paths will increase and countless cracks will occur. There is a concern that the plating film becomes unevenly distributed and becomes locally brittle. Therefore, the amount of crystallized Si (Si simple substance) in the plating film excluding the alloy phase portion at the interface between the base steel plate and the plating film is 2 to 9 mass%.

Moreover, in order to make the content of Si simple substance in the plating film excluding the alloy phase part of the base steel plate and the plating film interface as described above to be 2 to 9 mass%, the plating including the alloy phase part of the base steel plate and the plating film interface is performed. The Si content in the film is preferably set to satisfy the following formulas (1) and (2).
0.02 × T-14.5 ≦ S1 ≦ 0.02 × T-7.5… (1)
770 ≦ T ≦ 1000 (2)
T: Plating bath temperature (K) during molten Al-Zn plating
S1: Si content in the plating film (mass%)
Si consumed in the alloy phase increases as the bath temperature rises, but by adding Si in the plating film including the alloy phase part of the base steel sheet and the plating film interface under the above conditions, the alloy phase Since surplus Si that has not contributed to the suppression of the formation of crystallization is sufficiently crystallized, the amount of single Si in the dendrite gap portion of the plating film is increased, and the crack propagation path is sufficiently increased.

  In addition to Al, Zn, and Si described above, for example, Mg, Sr, V, Mn, Ni, Co, Cr, Ti, Sb, Ca, Mo, and B are used in the molten Al—Zn plating bath. One or more elements may be added, but any element may be added as long as it does not prevent the introduction of a crack propagation path, which is a function and effect peculiar to the plated steel sheet of the present invention.

The molten Al—Zn plated steel sheet of the present invention having the plating film structure as described above is plated with a molten Al—Zn plating bath having a plating bath temperature of 1000 K or less and satisfying the following expression (3). Can be obtained.
2.5 × A + 720 ≦ T ≦ 50 × S2 + 725 (3)
T: Plating bath temperature (K)
A: Al content in the plating bath (mass%)
S2: Si content in the plating bath (mass%)
As the Al content in the plating bath increases, the melting point of the bath increases, so the plating bath temperature must be increased. On the other hand, the amount of Si consumed in the alloy phase varies depending on the bath temperature. Therefore, in order to ensure sufficient single Si, it is desirable to manufacture under the above conditions.

  The molten Al—Zn plated steel sheet of the present invention can be made a surface-treated steel sheet by forming a chemical conversion coating on the surface thereof. Although there is no restriction | limiting in particular in the kind of this chemical conversion treatment film, For example, the chromate process liquid is apply | coated to the plating film surface, and 1 or 2 or more processes including the chromate process of drying at 80-300 degreeC are performed without washing with water. A chemical conversion treatment film can be formed. The chemical conversion treatment film may be formed of a multilayer film, and in this case, a plurality of treatments are sequentially performed.

  Furthermore, a single-layer or multi-layer coating film can be formed on the surface of the surface-treated steel sheet to obtain a coated steel sheet. Examples of the coating film include a polyester resin coating film, an epoxy resin coating film, an acrylic resin coating film, a urethane resin coating film, and a fluororesin coating film. Moreover, for example, an epoxy resin-modified polyester resin-based coating film in which a part of the resin is modified with another resin can be applied. Furthermore, a curing agent, a curing catalyst, a pigment, an additive, and the like can be added to the resin as necessary.

A coating method for forming a coating film on the surface of the surface-treated steel sheet is not particularly defined, but examples of the coating method include roll coater coating, curtain flow coating, and spray coating. After coating the paint, it is generally heated and dried by hot air drying, infrared heating, induction heating or the like to form a coating film.
However, the manufacturing method of the said surface-treated steel plate and a coated steel plate is an example, and is not limited to this.

A cold-rolled steel plate (plate thickness: 1.0 mm) manufactured by a conventional method was passed through a continuous hot-dip plating facility, and hot-dip plating was performed using a hot-dip Al-Zn plating bath having the composition shown in Tables 1 to 4. The line speed was 120 m / min. Next, in-line chemical conversion treatment was performed on the hot-dip Al—Zn-based plated steel sheet obtained as described above in a continuous hot-dip plating facility. The chemical conversion treatment was carried out by applying a liquid obtained by mixing acrylic emulsion resin and chromic acid at a resin solid content: Cr = 100: 1 (mass ratio) to the surface of the hot-dip plated steel sheet so that the Cr adhesion amount was 40 mg / m ^2, and 120 Dried at ℃.

The surface-treated steel sheet thus obtained was evaluated for press workability by a draw bead test. The draw bead test conditions were as follows: the bead tip diameter was 0.5 R, and the drawing was performed with the press load being 200 kg, while being pushed in by 4 mm. For the sample after the draw bead test, tape peeling was repeated until no new deposits adhered to the tape, and then the appearance was observed and evaluated according to the following criteria. If this evaluation criterion is 4 points or more, it is judged that the press workability is good.
5: No cracks are observed by visual observation.
4: Cracks are observed by visual observation.
3: Partially peeled (peeled area <unpeeled area).
2: Partially peeled (peeled area ≧ unpeeled area).
1: Full surface peeling

  FIG. 1 shows the relationship between the Si content contained in the plating film excluding the alloy phase portion at the interface between the base steel plate and the plating film and the press workability in this example. From these results, it can be seen that by satisfying the conditions of the present invention, the press workability is improved, and a surface-treated steel sheet having excellent press workability is obtained.

A graph showing the relationship between the amount of elemental Si contained in the plating film excluding the alloy phase part at the base steel plate / plating film interface and press workability

Claims (7)

  It is a molten Al-Zn-based plated steel sheet having an Al content of 20-80 mass% in the plating film, and contains 2-9 mass% of Si alone in the plating film excluding the alloy phase portion of the base steel sheet and the plating film interface. A molten Al—Zn-based plated steel sheet excellent in press workability. A molten Al-Zn-based plated steel sheet having an Al content of 20 to 80 mass% in the plating film, wherein Si is contained in the plating film including the alloy phase portion of the base steel sheet and the plating film interface in the following formula (1) and ( 2) A molten Al-Zn-based plated steel sheet excellent in press workability, characterized by containing the compound under conditions satisfying the formula.
0.02 × T-14.5 ≦ S1 ≦ 0.02 × T-7.5… (1)
770 ≦ T ≦ 1000 (2)
T: Plating bath temperature (K) during molten Al-Zn plating
S1: Si content in the plating film (mass%)   A surface-treated steel sheet excellent in press workability, comprising a chemical conversion coating on the surface of the molten Al-Zn-based plated steel sheet according to claim 1 or 2.   It has a chemical conversion treatment film on the surface of the molten Al-Zn-plated steel sheet according to claim 1 or 2, and further has a single-layer or multi-layer coating film on the upper layer, for press workability. Excellent coated steel sheet. In producing the hot-dip Al-Zn-based plated steel sheet according to claim 1 or 2, in a continuous hot-dip plating facility,
A method for producing a molten Al-Zn-based plated steel sheet excellent in press workability, comprising plating a steel sheet in a molten Al-Zn plating bath having a plating bath temperature satisfying the following formula (3) below 1000K .
2.5 × A + 720 ≦ T ≦ 50 × S2 + 725 (3)
T: Plating bath temperature (K)
A: Al content in the plating bath (mass%)
S2: Si content in the plating bath (mass%)   A method for producing a surface-treated steel sheet excellent in press workability, comprising forming a chemical conversion coating on the surface of a molten Al-Zn-based plated steel sheet obtained by the production method according to claim 5.   6. A press characterized by forming a chemical conversion coating on the surface of the molten Al—Zn-based plated steel sheet obtained by the production method of claim 5 and further forming a single-layer or multi-layer coating on the upper layer. A method for producing coated steel sheets with excellent workability.

Images