JP2010070784A - HOT-DIP Al-Zn PLATED STEEL SHEET, AND METHOD FOR MANUFACTURING THE SAME - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hot-dip Al-Zn plated steel sheet which has superior press formability, and to provide a method for manufacturing the same. <P>SOLUTION: An Al content in the plated film is 20-95 mass%. A Si content in the plated film is 5 mass% or more with respect to the Al content. The plated film includes an upper layer and an alloy phase existing in the interface between the upper layer and a base steel sheet, and the upper layer contains 3 mass% or more non-dissolved Si with respect to the Al content in the plated film. Thus, by increasing the amount of the non-dissolved Si, the number of crack increases, the aperture width of each crack is decreased. Then, the aperture is difficult to become a starting point of the peeling of the plated film due to sliding, and as a result, the press formability is enhanced. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a molten Al—Zn-based plated steel sheet excellent in press workability, in particular, a molten Al—Zn-based plated steel sheet having an Al content of 20 to 95 mass% in a plating film and a method for producing the same.

  As shown in Patent Document 1, a hot-dip galvanized steel sheet containing 20 to 95 mass% Al in the plating film exhibits excellent corrosion resistance as compared with hot-dip galvanized steel sheet. Demand is growing mainly in the field of building materials such as roofs and walls.

  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 continuous hot dip plating equipment, first, the underlying steel plate is heated to a predetermined temperature in an annealing furnace maintained in a reducing atmosphere, and simultaneously with annealing, the rolling oil adhering to the steel plate surface is removed, and the oxide film is reduced and removed. . Next, the base steel sheet is immersed in a hot dipping bath containing a predetermined concentration of Al by passing through the inside of the snout whose lower end is immersed in the plating bath. And the steel plate immersed in the plating bath is pulled up above the plating bath via the sink roll, and by injecting pressurized gas toward the surface of the steel plate from the gas wiping nozzle arranged on the plating bath. The plating 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.

  At this time, in order to ensure the desired plating quality and material, the heat treatment conditions and atmospheric conditions of the annealing furnace in the continuous hot-dip plating equipment, the operating conditions such as the plating bath composition and the cooling rate after plating are within a predetermined management range. It is managed accurately.

  The plating film of the molten Al—Zn-based plated steel sheet produced as described above is composed of an alloy phase present at the interface with the base steel sheet and an upper layer present thereon. Furthermore, the upper layer mainly comprises Zn in a supersaturated state and Al is dendrite solidified and the remaining dendrite gap is formed, and the dendrite solidified part is laminated in the film thickness direction of the plating film. Such a characteristic coating structure makes the path of corrosion progress from the surface complicated, making it difficult for corrosion to reach the underlying steel sheet, and hot-dip galvanized steel sheets with the same coating thickness. It will show superior corrosion resistance.

Usually, about 3 mass% of Si is added to the plating bath, and the action of this Si suppresses the growth of the alloy phase at the interface of the molten Al—Zn-based plated steel sheet. The length is about 1 to 2 μm. If the plating film thickness is the same, the thinner the alloy phase, the thicker the upper layer that is effective for improving the corrosion resistance. Therefore, suppressing the growth of the alloy phase contributes to the improvement of the corrosion resistance. Further, since the alloy phase is harder than the upper layer and acts as a starting point of cracks during processing, the suppression of the growth of the alloy phase reduces the occurrence of cracks and also brings about the effect of improving the bending workability. And since the base steel plate is exposed and the crack part which generate | occur | produced is inferior to corrosion resistance, reducing generation | occurrence | production of a crack will also improve bending process part corrosion resistance.
Japanese Examined Sho 46-7161

As described above, the molten Al—Zn-based plated steel sheet exhibits excellent corrosion resistance. On the other hand, when processing such as bending is performed, cracks may occur in the plating film of the processed part depending on the degree of processing.
In addition, since the alloy phase is the starting point of the crack and the upper dendrite gap is the propagation path of the crack, the crack propagation is higher than that of the hot-dip galvanized steel sheet with the same plating thickness when bent to the same degree. The path is limited, and the number of occurrences of cracks decreases while each crack tends to open relatively large.
Therefore, depending on the degree of bending, there is a problem that the crack is visually recognized with the naked eye and 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.

Moreover, when trying to use the molten Al—Zn-based plated steel sheet in the automobile field, there are the following problems.
In recent years, as a part of global warming countermeasures, it has been required to reduce the body weight, improve fuel efficiency and reduce CO ₂ emissions, thereby reducing the weight by using high-strength steel sheets and molten Al- It is strongly desired to achieve both Zn-based plating. In the automobile field, complicated press work is performed in accordance with the shape of the vehicle body, and therefore, a machining mode in which sliding is added as well as simple bending deformation as in the building material field is used. However, high-strength steel sheets are prone to form defects such as springback for such processing, while hot-dip Al-Zn-based plated steel sheets are prone to peeling of the plating film starting from cracks that are greatly open during sliding. Tend to be.

  In view of such circumstances, an object of the present invention is to provide a molten Al—Zn-plated steel sheet having excellent press workability and a method for producing the same.

  The inventors of the present invention have made extensive studies to solve the above problems. As a result, it has been found that by controlling the plating film structure, excellent press workability that has never been obtained can be obtained.

The present invention has been made based on the above findings, and the gist thereof is as follows.
[1] A molten Al-Zn-based plated steel sheet having an Al content of 20 to 95 mass% in the plating film, wherein the Si content in the plating film is 5 mass% or more with respect to the Al content, The said plating film consists of an alloy phase which exists in the interface of an upper layer and a base steel plate, and contains 3 mass% or more of non-solid solution Si with respect to the said Al content in the said plating film in this upper layer.
[2] In the above [1], the alloy phase forms a layer having a thickness of 5 μm or less.
[3] In the above [1] or [2], the insoluble Si is a particle of 5 μm or less.
[4] When producing a molten Al-Zn-based plated steel sheet, a molten Al-Zn-based plating in which the Al content in the plating bath is 20 to 95 mass%, and the Si content is 5 mass% or more with respect to the Al content. It is a manufacturing method of a steel plate.
[5] In the above [4], after plating, the steel sheet is held so that the alloy phase has a thickness of 0.5 μm or more and 5 μm or less at a temperature not lower than the melting point of the plating bath and not higher than the temperature of the plating bath.

  In the present invention, regardless of whether or not alloying is performed, a steel plate obtained by plating Al—Zn on a steel plate by a plating method is generically referred to as a molten Al—Zn-based plated steel plate. That is, the hot-dip Zn-Al-plated steel sheet in the present invention includes both a hot-melt Al-Zn-plated steel sheet that has not been subjected to alloying treatment and an alloyed hot-melt Al-Zn-plated steel sheet that has undergone alloying treatment.

  According to the present invention, a molten Al-Zn-based plated steel sheet having excellent press workability can be obtained. And by applying the molten Al-Zn-based plated steel sheet of the present invention to a high-strength steel sheet, it is possible to achieve both weight reduction and excellent corrosion resistance in the automotive field.

  The plated steel sheet which is the subject of the present invention is a molten Al—Zn-based plated steel sheet containing 20 to 95 mass% of Al in the plating film. From the balance of performance (corrosion resistance, workability, etc.) and operation, the preferable range of the Al content in the plating film is 45 to 85 mass%.

  As described above, Si added to the plating bath suppresses alloy phase growth of the molten Al—Zn-based plated steel sheet after the plating treatment. However, surplus Si that has not contributed to the alloy phase growth is dissolved in the upper layer. Further, surplus Si exceeding the amount of Si solid solution in the upper layer crystallizes as non-solid solution Si in the upper layer and becomes a physical barrier that hinders the growth of the upper layer Al dendrite. Moreover, since non-solid Si does not mix with Al dendrite, it is dispersed in the dendrite gap. In the dendrite gap containing non-solid Si, the boundary portion of non-solid Si becomes easy to peel off, so that cracks propagate more easily than the dendrite gap without non-solid Si. Therefore, as the amount of non-solid Si increases, a large number of fine cracks are likely to occur during deformation.

  However, in the present invention, contrary to the above-described conventional technical idea, the Si content in the plating film is set to 5 mass% or more with respect to the Al content, and the non-solid Si is contained in the upper layer in the Al content in the plating film. 3 mass% or more. This is because, in the present invention, the basic technical idea is to actively use non-solid solution Si, which has been considered to be preferable because it is easy to generate fine cracks. That is, the present invention utilizes the crack fine dispersion effect, and the number of cracks increases, so that the opening width of each crack is reduced, making it difficult to start plating peeling due to sliding, and as a result, press working Improves.

  When the Si content in the plating film is less than 5 mass% with respect to the Al content, Si is consumed for the growth of the alloy phase, so the non-solid Si in the upper layer is reduced and the Al content in the plating film is reduced. Therefore, the non-solid solution Si in the upper layer does not become 3 mass% or more. Therefore, Si content in a plating film shall be 5 mass% or more with respect to Al content.

  Further, in the upper layer, non-solid Si is contained in an amount of 3 mass% or more with respect to the Al content in the plating film. This is because when the non-solid solution Si is less than 3 mass% with respect to the Al content in the plating film, the amount of the non-solid solution Si is small and the above-described crack fine dispersion effect cannot be sufficiently exhibited. Therefore, non-solid solution Si shall be 3 mass% or more with respect to Al content in a plating film.

  The alloy phase preferably forms a layer having a thickness of 5 μm or less. By setting the thickness of the alloy phase to 5 μm or less, the alloy phase that acts as the starting point of the crack is thinned, and the cracks that penetrate the alloy phase are finely dispersed. This is because the width is reduced. In addition, by forming the alloy phase into a layer, the occurrence of cracks from the protruding portion where the alloy phase is thick can be suppressed.

Furthermore, it is preferable that non-solution Si is a particle | grain of 5 micrometers or less.
By making insoluble Si into particles of 5 μm or less, the particles are reduced, the number is increased, and cracks are finely dispersed. When the thickness exceeds 5 μm, there is a portion where non-solid solution Si particles penetrate through the entire thickness of the upper layer, and there is a concern that the corrosion resistance of the portion is reduced.
As described above, non-solid Si is crystallized from the excess Si exceeding the amount of Si solid solution in the upper layer. For this reason, the size of particles made of Si alone, which is insoluble Si, can be controlled by appropriately controlling the solidification process of the plating film. Examples of the method for controlling the size of the non-solid solution Si particles include a method in which the upper layer of the plated steel sheet is temporarily held at a temperature at which the upper layer of the plated steel sheet is maintained in the melt, and then cooled immediately after the plating process. . That is, after the plating treatment, the steel sheet is held at a temperature not lower than the melting point of the plating bath and not higher than the temperature of the plating bath. At this time, the holding is terminated so that the thickness of the alloy phase is 0.5 μm to 5 μm or less. When the thickness of the alloy phase is maintained to exceed 5 μm, there is a concern that the non-solid solution Si particles may penetrate through the entire thickness of the upper layer as described above, leading to a decrease in corrosion resistance of the portion. , The holding effect is not obtained.
Usually, the plating film is solidified by cooling immediately after plating for the purpose of suppressing the growth of the alloy phase. Therefore, when the upper layer is in a molten state, a concentration gradient of Si occurs, and the surface of Si is closer to the surface than the vicinity of the alloy phase. The concentration increases, and large non-solid Si particles tend to be formed on the uppermost layer side of the upper layer after solidification. However, once the upper layer is maintained at a temperature at which it is maintained in the melt, the Si concentration in the upper layer becomes uniform, and after solidification, a large amount of non-solution-soluble Si particles are not formed on the outermost layer side of the upper layer. It is possible to reduce the solid solution Si particles.
However, the method of making non-solid solution Si particles 5 μm or less is not limited to the above, and the same effect can be obtained by any method as long as the non-solid solution Si particles in the upper layer are 5 μm or less. can get.

  In addition, the thickness of the alloy phase and the size of the particles of the non-solid solution Si in the present invention can be measured by polishing and observing the cross section of the plating film with a scanning electron microscope or the like. The maximum thickness of the alloy phase and the maximum diameter of the particles of insoluble Si obtained by observing and obtaining the cross section of the plating film of the sample taken from any place except for at least a continuous range of 10 mm (in any direction). Although there are several methods for polishing and etching the cross section, any method may be used as long as it is a method used for observing the cross section of the plating film.

Next, the manufacturing method of the fusion | melting Al-Zn type plated steel plate excellent in the press workability of this invention is demonstrated.
The hot-dip Al-Zn-based plated steel sheet of the present invention is manufactured with a continuous hot-dip plating equipment or the like, and the Al content in the plating bath is 20 to 95 mass%, and the Si content is 5 mass% or more with respect to the Al content. .
In addition, in order to control the size of non-solid-solution Si particles to 5 μm or less, after plating, the steel sheet is at least the melting point of the plating bath and below the temperature of the plating bath, and the thickness of the alloy phase is 0.5 μm or more. It is preferable to hold it so that it becomes 5 μm or less, and then cool it. As described above, the upper layer of the plating film is cooled after being kept at a temperature at which the upper layer is maintained in the melt, so that the Si concentration in the upper layer becomes uniform, and after solidification, a large amount of non-solid solution Si is formed on the surface of the upper layer. The particles of non-solid solution Si become 0.5 μm or more and 5 μm or less without generating particles.

  In addition to the Al, Zn, and Si described above, the plating bath of the plated steel sheet of the present invention may have some sort of Mg, Sr, V, Mn, Ni, Co, Cr, Ti, Sb, Ca, Mo, B, etc. Although an element may be added, it is applicable as long as the effect of the present invention is not impaired.

  Furthermore, the plated steel plate on which the predetermined plating film is obtained as described above can be made into a surface-treated steel sheet by having a chemical conversion treatment film on its surface. 1 or including a chromate treatment or a chromium-free chemical conversion treatment in which, for example, a chromate treatment solution or a chromium-free chemical conversion treatment solution is applied to the plating layer surface of the plated steel sheet and dried at 80 to 300 ° C. without washing with water. Two or more treatments are performed to form a chemical conversion treatment film. 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 fluorine resin coating film. Moreover, for example, an epoxy-modified polyester resin coating film in which a part of the resin is modified with another resin can be applied. Further, 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. Examples of the coating method include roll coater coating, curtain flow coating, and spray coating. After the paint is applied, 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.
As described above, the molten Al—Zn-based plated steel sheet having excellent press workability according to the present invention can be obtained.

Next, the present invention will be described in more detail with reference to examples.
Cold-rolled steel sheets produced in a conventional manner were passed through a continuous hot-dip plating facility and plated with the plating bath compositions shown in Tables 1 and 2 to produce molten Al-Zn plated steel sheets. The plating bath temperature was set to melting point + 30 ° C. or higher according to the bath composition, and the line speed was 150 m / min. In addition, using an alloying furnace installed directly above the plating bath, the upper layer is maintained at a temperature higher than the melting point (460 ° C. to 620 ° C.) of each bath so that the upper layer is maintained in a molten state up to the exit side of the alloying furnace. After holding for 2 seconds, it was cooled.

  For the molten Al-Zn-based plated steel sheet obtained as described above, the alloy phase thickness and the size of non-solid solution Si particles in the upper layer are measured as shown below, and the press workability is further evaluated. did.

  The alloy phase thickness and the size of insoluble Si particles in the upper layer were observed with a scanning electron microscope after the cross section was polished. A continuous 10 mm range parallel to the rolling direction of the steel sheet was photographed at 1500 times, and the maximum thickness of the alloy phase and the maximum diameter of the particles of non-solid Si were obtained.

A draw bead test was conducted as a method for evaluating press workability. In the draw bead test, a bead with a bead tip diameter of 0.5 mmR was pushed in by 10 mm and pulled out with a holding load of 4903 N (500 kgf). Further, as an evaluation of press workability, the appearance of plating peeling 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. No plating peeling 4: Cracks are observed by visual observation. No plating peeling 3: Partial plating peeling (peeling area <non-peeling area)
2: Partial plating peeling (peeling area> non-peeling area)
1: Full surface plating peeling The results obtained by the above are shown in Table 1, Table 2, and FIG.
Table 1 is a table showing examples of the present invention, and Table 2 is a table showing comparative examples. Moreover, FIG. 1 is a figure which shows the relationship between the ratio of the amount of non-solution Si with respect to Al contained in a plating film, and press workability.

  It can be seen from Table 1 and FIG. 1 that in the present invention example, the press workability is improved and a molten Al—Zn-based plated steel sheet having excellent press workability is obtained.

  In addition to corrosion resistance, it is also excellent in press workability, so it can be applied in a wide range of fields, mainly in the field of building materials and automobiles.

It is a figure which shows the relationship between the ratio of the amount of non-solution Si contained in a plating film, and press workability. (Example 1)

Claims (5)

  A molten Al-Zn-based plated steel sheet having an Al content in a plating film of 20 to 95 mass%, wherein the Si content in the plating film is 5 mass% or more with respect to the Al content, and the plating film Is composed of an alloy phase existing at the interface between the upper layer and the base steel sheet, and the upper layer contains 3 mass% or more of insoluble Si with respect to the Al content in the plating film. -Zn-based plated steel sheet.   2. The molten Al—Zn-based plated steel sheet according to claim 1, wherein the alloy phase forms a layer having a thickness of 5 μm or less.   The molten Al-Zn-based plated steel sheet according to claim 1 or 2, wherein the non-solid solution Si is particles having a size of 5 µm or less.   When manufacturing a molten Al-Zn-based plated steel sheet, the Al content in the plating bath is 20 to 95 mass%, and the Si content is 5 mass% or more with respect to the Al content. Manufacturing method of a galvanized steel sheet.   5. The molten Al according to claim 4, wherein after the plating treatment, the steel sheet is held so that the thickness of the alloy phase is not less than 0.5 μm and not more than 5 μm at a temperature not lower than the melting point of the plating bath and not higher than the temperature of the plating bath. A method for producing a Zn-based plated steel sheet.

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