JP2012036491A - HOT-DIP Al-Zn PLATED STEEL SHEET - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot-dip Al-Zn plated steel sheet excellent in corrosion resistance of a joint part.SOLUTION: There is provided the hot-dip Al-Zn plated steel sheet that has Al-Zn plating film having Al content of 20-95 mass% and a coating layer having the thickness of 0.1-5 μm, which contains aluminum oxide and/or aluminum hydroxide on the surface of the steel sheet in sequence.

Description

  The present invention relates to a molten Al—Zn-based plated steel sheet having a plating film mainly composed of Al and Zn, and more particularly to a molten Al—Zn-based plated steel sheet having excellent corrosion resistance at a joint portion formed when the steel sheets are overlapped.

  Since it has been disclosed in Patent Document 1 that a steel sheet having a plating film containing 25 to 70 mass% of Al and the balance being made of Zn has superior corrosion resistance as compared to conventional Zn-plated steel sheets, Al of the plating film has been disclosed. The demand for hot-dip Al-Zn-coated steel sheets with a content of about 55 mass% is growing mainly in the field of building materials.

  This hot-dip Al-Zn plated steel sheet is a hot-rolled steel sheet that has been descaled by pickling, or a cold-rolled steel sheet that has been cold-rolled as a base steel sheet, and uses a continuous hot-dip plating facility as follows. Manufactured. That is, the base steel sheet is first heated to a predetermined temperature in an annealing furnace adjusted to a reducing atmosphere, and at the same time as the annealing, dirt and oxide films such as rolling oil adhering to the steel sheet surface are removed, and then via a snout. After being immersed in a plating bath containing a predetermined concentration of Al and Zn and subjected to plating treatment, the coating amount of the coating is adjusted by a gas wiping nozzle outside the plating bath to obtain a molten Al-Zn plated steel sheet. Become.

  The coating film of the molten Al-Zn-plated steel sheet produced in this way is supersaturated with the Fe-Al alloy phase formed at the interface with the underlying steel sheet and mainly Zn formed on this alloy phase. It is comprised with the dendrite solidification structure | tissue phase which consists of contained Al. Although voids are observed in the dendritic solidified structure phase, the progress of corrosion is hindered by the characteristic dendritic morphology, and it is said that the corrosion resistance is superior to that of hot-dip Zn-plated steel sheets. Also, since the alloy phase is brittle, it is easy to break by processing and causes a decrease in corrosion resistance, so it is common to suppress the growth of the alloy phase by adding about 3 mass% Si to the Al content in the plating bath. is there. If the growth of the alloy phase is suppressed and the thickness of the alloy phase is reduced, the dendrite solidified structure phase contributing to the corrosion resistance can be increased, which is advantageous for improving the corrosion resistance.

Recently, studies have been made to apply this molten Al-Zn plated steel sheet having excellent corrosion resistance not only to the field of building materials but also to steel sheets for automobiles. For example, Patent Document 2 proposes a molten Al—Zn-based plated steel sheet having an oxide layer made of zinc oxide and aluminum oxide having a thickness of 10 mm or more on the surface of the plating film in order to improve adhesive bondability. Further, in Patent Document 3, in order to improve the elongation characteristics, a molten Al-Zn-based plated steel sheet on which a plating film having a surface roughness Ra of 0.20 μm or more is heated to a temperature below 300 ° C. offline. There has been proposed a method for producing a molten Al-Zn-based plated steel sheet having an Al content of 20 to 95 mass% which is wound and kept at a tension of 3.0 kg / mm ² or less.

Japanese Patent Publication No.46-7161 Japanese Patent Laid-Open No. 6-10156 Japanese Patent No.3533177

  However, when the molten Al—Zn-plated steel sheet described in Patent Documents 2 and 3 is used, there is a problem that excellent corrosion resistance cannot always be obtained at a portion where the steel sheets are overlapped by spot welding or the like, that is, a so-called mating portion.

  An object of the present invention is to provide a molten Al—Zn-based plated steel sheet having excellent corrosion resistance at the mating portion.

  As a result of diligent investigations to achieve the above object, the present inventors have provided a coating layer having a thickness of 0.1 μm or more containing aluminum oxide and / or aluminum hydroxide on the surface of the plated film of the molten Al—Zn-based plated steel sheet. Has been found effective.

  The present invention has been made on the basis of such knowledge, and on the steel sheet surface, an Al-Zn-based plating film having an Al content of 20 to 95 mass% and a thickness containing aluminum oxide and / or aluminum hydroxide in this order. A molten Al—Zn-based plated steel sheet having a thickness of 0.1 to 5 μm is provided.

  In the molten Al—Zn-based plated steel sheet of the present invention, the thickness of the coating layer is preferably 0.1 to 2 μm.

  According to the present invention, it has become possible to produce a molten Al-Zn-based plated steel sheet having excellent corrosion resistance at the mating portion. The molten Al—Zn-based plated steel sheet of the present invention is excellent in chemical conversion treatment and is suitable for automobile members.

It is a figure which shows typically the test piece for evaluating the corrosion resistance of a mating part. It is a figure which shows the corrosion resistance test conditions of 1 cycle.

  The molten Al-Zn-based plated steel sheet of the present invention is a hot-rolled steel sheet after descaling, or an Al-Zn-based plated film formed on the surface of a cold-rolled steel sheet that has been cold-rolled. Another feature is that a coating layer containing aluminum hydroxide and having a thickness of 0.1 to 5 μm is provided. The details will be described below.

1) About the Al-Zn plating film From the viewpoint of corrosion resistance and workability of the non-matching part, the Al content in the Al-Zn plating film is 20 to 95 mass%, preferably 45 to 85 mass%, and the remaining Zn and unavoidable Impurities.

  In order to form an Al-Zn plating film having such a composition on the surface of a steel sheet, as described above, using a continuous hot-dip plating facility, the Al content is 20 to 95 mass%, and the balance is substantially composed of Zn. This is possible by immersing the steel sheet in a Zn plating bath. In the plating bath, it is preferable to add 0.5 to 10 mass% Si with respect to the Al content in order to suppress the growth of the alloy phase formed at the interface with the steel plate. If it is 0.5 mass% or more, the growth of the alloy phase can be suppressed, and the plating adhesion during the molding process is sufficient. On the other hand, if it is 10 mass% or less, since a massive precipitate does not precipitate in the plating film, the workability is not hindered and the melting point of the bath does not increase, which is preferable. In addition, elements such as Mo, Ca, Mn, Mg, Ti, Sr, V, Ni, Co, Cr, Sb, and B can be added to the plating bath as long as the effects of the present invention are not impaired.

The adhesion amount of the Al—Zn-based plating film per one side of the steel sheet is preferably 15 to 100 g / m ² . If it is this range, it will become a plated steel plate excellent in the corrosion resistance and workability of a non-matching part.

2) Coating layer containing aluminum oxide and / or aluminum hydroxide The present inventors have investigated the cause of the poor corrosion resistance of the mating part of the molten Al-Zn-based plated steel sheet. Therefore, it was clarified that the corrosion progresses along the voids existing in the dendritic solidified phase of the Al-Zn plating film. Therefore, if the gap is sealed by covering the Al—Zn plating film with a coating layer containing chemically stable aluminum oxide and / or aluminum hydroxide, the progress of corrosion can be prevented and the corrosion resistance can be improved. The content of aluminum oxide and / or aluminum hydroxide in the coating layer may be an amount that can seal the voids in the Al—Zn-based plating film. In order to effectively seal the voids in the Al—Zn plating film, it is preferable that almost 100% of the coating layer is made of aluminum oxide and / or aluminum hydroxide. However, if the thickness of the coating layer is less than 0.1 μm, such an effect cannot be obtained sufficiently. In order to further stabilize the chemical conversion property, the thickness of the coating layer is more preferably 0.1 to 2 μm.

  In addition, a coating layer of aluminum oxide and / or aluminum hydroxide can be easily formed on the Al—Zn-based plating film as described below. That is, using a continuous hot dipping system, a sodium hydroxide aqueous solution is sprayed onto a molten Al-Zn plated steel sheet on which an Al-Zn plating film is formed, or a molten Al-Zn plated steel sheet is placed in a sodium hydroxide aqueous solution. Just dipping. In the former case, specifically, a 5 to 25 mass% sodium hydroxide aqueous solution maintained at a temperature of 10 to 45 ° C was sprayed uniformly on the entire surface of the molten Al-Zn-based plated steel sheet, and then the relative temperature was 20 ° C or higher. What is necessary is just to dry, after hold | maintaining for 10 minutes or more in the moisture state of 50-98% of humidity. In the latter case, specifically, after dipping in a 5 to 25 mass% sodium hydroxide aqueous solution maintained at a temperature of 10 to 45 ° C. for at least 2 seconds, wet at 20 ° C. or more and a relative humidity of 50 to 98%. After being kept in the state for 10 minutes or more, it may be dried. Since Al is an amphoteric metal, not only an alkaline solution such as an aqueous sodium hydroxide solution but also an acid solution such as nitric acid can be sprayed or immersed in the acid solution. In any case, it may be performed under the condition that the thickness of the coating layer containing aluminum oxide and / or aluminum hydroxide is 0.1 to 5 μm.

  The formation of the aluminum oxide and / or aluminum hydroxide coating layer may be performed in a continuous hot dip plating facility, but the effect can be sufficiently exerted even immediately before use as a product.

  The presence of aluminum oxide or aluminum hydroxide in the coating layer can be confirmed by an X-ray diffraction method or the like. Further, the thickness of the coating layer can be determined by observation of the cross section of the plating film with a scanning electron microscope.

  A chemical conversion treatment film can be further formed on the molten Al—Zn plated steel sheet of the present invention. As the chemical conversion coating, for example, a chromate coating or a chromium-free chemical conversion coating formed by applying a chromate processing solution or a chromium-free chemical conversion processing solution and drying it at a steel plate temperature of 80 to 300 ° C. without washing with water. Can be mentioned. These chemical conversion treatment films may be a single layer or multiple layers, and in the case of multiple layers, a plurality of chemical conversion treatments may be performed sequentially. In addition, on the Al-Zn plating film or the chemical conversion treatment film, a coating film containing an organic resin, for example, a polyester resin coating film, an epoxy resin coating film, an acrylic resin coating film, a urethane resin coating film A fluorine-based resin coating film can be formed. For example, an epoxy-modified polyester resin coating film in which a part of the resin is modified with another resin can also be applied. Further, a curing agent, a curing catalyst, a pigment, an additive and the like can be added to the resin as necessary. Such a coating film containing an organic resin can be formed by applying a paint containing the organic resin with a roll coater, curtain flow, spray, or the like, and then drying by means of hot air drying, infrared heating, induction heating or the like.

Using the continuous hot-dip plating equipment, changing the Al content in the plating bath, the Al content shown in Table 1 on a cold-rolled steel plate with a line thickness of 150 mm / min and a sheet thickness of 0.8 mm, the balance Zn and unavoidable An Al—Zn plating film made of impurities was formed on both sides. At this time, 3 mass% Si was added to the Al content in the plating bath (Si content in the plating bath: 0.8 to 2.7 mass%). The plating amount per one side of the steel plate was 35 to 45 g / m ² . Samples Nos. 1, 3, 5, 7, 14, 16, and 18 were used as plated. Furthermore, after immersing some plated steel sheets in a 5 mass% sodium hydroxide aqueous solution maintained at a temperature of 20 ° C. for 10 seconds and then holding them in a wet state at 20 ° C. and a relative humidity of 60% for 60 minutes, After drying, a coating layer made of aluminum oxide and / or aluminum hydroxide having the thickness shown in Table 1 was formed on the Al-Zn-based plating film, and sample Nos. 2, 4, 6, 11, 15, 17, 19 was produced. Also, change the sodium hydroxide concentration and immersion time in the aqueous sodium hydroxide solution, or change the temperature, relative humidity, and retention time in the subsequent wet state, and then air dry, then use the Al-Zn system. A coating layer made of aluminum oxide and / or aluminum hydroxide having the thickness shown in Table 1 was formed on the plating film, and sample Nos. 8 to 10, 12, and 13 were produced. For the thickness of the coating layer, the cross section of the plating film was observed with a scanning electron microscope of 1000 times, and the thickness of the coating layer at each of three visual fields was measured for an arbitrary 10 visual fields, and an average value of a total of 30 positions was obtained.

And about the produced sample, the corrosion resistance test was done with the following method and the corrosion resistance of the joint part was evaluated.
Mating portions of the corrosion resistance test: As shown in FIG. 1, the plate thickness 0.8 mm, width 70 mm, length 150 mm, coating weight per steel sheet one side to the alloyed hot-dip Zn plated steel sheet 45 g / m ^2, taken from each sample Spot welded 40mm wide and 110mm long test pieces were spot welded at 3 points to form a joint, and then started from the wetting process under the 1-cycle condition (SAE J 2334) shown in Fig. 2, and a 120-cycle corrosion resistance test was conducted. went. After the corrosion resistance test, disassemble the mating part to remove rust and plating film on the test piece, divide the corroded part into 10 sections with a unit section of 20 mm x 15 mm size, and use a micrometer to Calculate the corrosion depth as the plate thickness difference for the uncorroded healthy part, apply the Gumbel distribution to the calculated maximum corrosion depth data of each section, perform extreme value statistical analysis, and determine the mode of the maximum corrosion depth When the mode value of the maximum corrosion depth is 0.5 μm or less, the corrosion resistance of the joint is excellent.

In addition, after forming the mating part as shown in Fig. 1, chemical conversion treatment is performed on one side of the test piece so that the amount of zinc phosphate deposited is 2.0 to 3.0 g / m ² and electrodeposition coating of 20 ± 1 μm is applied. Then, the corrosion resistance of the mating portion was evaluated by the above method, and the adhesion state of the electrodeposition coating of the non-matching portion was visually inspected.

  The results are shown in Table 1. A sample in which an aluminum oxide and / or aluminum hydroxide coating layer having a thickness of 0.1 to 5.0 μm, which is an example of the present invention, has the maximum corrosion depth regardless of the presence or absence of coating treatment including chemical conversion treatment and electrodeposition coating. The mode value is 0.5 μm or less, which indicates that the corrosion resistance of the mating portion is excellent. Moreover, there is no problem in the adhesion state of the electrodeposition coating of the non-matching portion, and it can be said that the sample of the present invention is excellent in chemical conversion treatment.

Claims (2)

  The steel sheet has, in order, an Al-Zn plating film having an Al content of 20 to 95 mass% and a coating layer having a thickness of 0.1 to 5 μm containing aluminum oxide and / or aluminum hydroxide. Molten Al-Zn plated steel sheet.   2. The molten Al—Zn-based plated steel sheet according to claim 1, wherein the coating layer has a thickness of 0.1 to 2 μm.

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