JPS6340666B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a zinc-rich coated steel sheet with excellent weldability, workability, and corrosion resistance. Currently, roads in North America and Canada are sprayed with rock salt to prevent ice from freezing in order to prevent motor vehicle slip-up accidents during the winter. However, from the perspective of corrosion, automobiles are exposed to a harsh corrosive environment due to salt water, and in particular, anti-corrosion measures are needed for parts such as the underside of the car body, which have a bag structure and are exposed to a harsh corrosive environment where salt water tends to accumulate. In addition to improving car body structures and painting systems, automobile manufacturers are using highly corrosion-resistant surface-treated steel sheets. Such surface-treated steel sheets for automobiles include, in addition to zinc-plated steel sheets, painted steel sheets coated with zinc-rich paints as organic coated steel sheets. In general, the manufacturing process for automobile bodies consists of the steps of press forming, assembling, and painting the above-mentioned painted steel sheets.The performance of automobile steel sheets is determined by their workability in terms of press workability and spot weldability during car body assembly. , is required along with corrosion resistance. The zinc-rich coated steel sheets currently used as automotive steel sheets contain more than 80% zinc powder and have a coating thickness of 12 to 10%.
It has a 17μm zinc-rich coating layer. Regarding this kind of zinc-rich coated steel plate,
The content of zinc powder in the coating film and the coating thickness greatly affect the performance, weldability, workability, and corrosion resistance required of automotive steel sheets. Although the zinc-rich coated steel sheets currently in use are designed with a good balance of weldability, workability, and corrosion resistance, they cannot be said to have sufficient performance as steel sheets for automobiles. In other words, in such coated steel sheets, the zinc powder content in the coating film is high (80% or more), and the coating film is thick at about 15 μm, so the electrochemical corrosion prevention effect of zinc and the barrier of the coating film are As a result, the corrosion resistance of the flat plate part is good, but due to the high content of zinc powder in the coating film, the elongation of the coating film is poor.
Furthermore, since the film is thick, the coating is likely to peel off in processed areas, and the anticorrosive effect of the zinc-rich coating is lost. Regarding workability, the coating film tends to peel off during press molding and powdering phenomenon occurs easily, which poses a problem in terms of press workability. Regarding weldability, the high content of zinc in the coating film increases the conductivity of the coating film, but due to the thick coating, the electrode wears out due to an increase in electrical resistance on the electrode surface due to organic matter adhesion. It is fast and has insufficient continuous spot welding properties. Therefore, an object of the present invention is to eliminate the above-mentioned drawbacks of conventionally coated steel plates and to provide a zinc-rich coated steel plate that has excellent weldability, workability, and corrosion resistance. The present inventors coated a steel plate with a chromate treatment film layer mainly composed of zinc powder and chromic acid and zinc powder.
The present invention was achieved as a result of intensive research on zinc-plated steel sheets as material steel sheets, chromate treatment liquids, and additives to zinc-rich paints for coated steel sheets having a zinc-rich coating layer containing ~80% by weight. In other words, in order to obtain a zinc-rich steel sheet with high corrosion resistance, we investigated a zinc-based galvanized steel sheet with sacrificial corrosion resistance as a material steel sheet, and found that in a severe corrosive environment, when a normal galvanized steel sheet is used, the underlying zinc It was found that the elution rate of zinc in the plating layer was extremely fast, which caused blisters to form on the coating surface, causing the coating to peel off, resulting in a decrease in anticorrosion performance. However, even among alloyed zinc platings in which the elution rate of zinc is properly controlled,
By using Zn-Ni alloy plated steel plate,
A zinc-rich coated steel sheet with extremely excellent corrosion resistance was obtained. The Zn-Ni alloy plated steel sheet used in the present invention preferably has a Ni content of 5 to 20 wt%. Ni content is 5wt
If it is less than 20wt%, the elution rate of zinc is fast and blisters are likely to occur, and if it exceeds 20wt%, the sacrificial anticorrosion effect of zinc will be reduced. On the other hand, the amount of plating deposited is preferably 5 to 30 g/m ² . If it is less than 5 g/m ^{2 ,} there is no sufficient anticorrosive effect, and if it exceeds 30 g/m ² , it is disadvantageous from an economic standpoint. In the present invention, such a Zn--Ni alloy plated steel plate is used as a material steel plate, which is subjected to chromate treatment and further coated with a zinc-rich paint. The chromate treatment liquid used in the present invention mainly contains zinc powder and chromic acid, and also contains small amounts of additives such as a dispersant and a reducing agent, and is a type of coating-type chromate treatment liquid. This is applied with a roll or the like, and then heated and dried for about 90 seconds at a plate temperature of about 170°C to form a chromate film layer on the steel plate. The content of zinc powder in the chromate treatment solution is preferably 3 to 5 in weight ratio (Zn/CrO ₃ ) to chromic acid. The reason for this is that if Zn/CrO ₃ is less than 3, the conductivity of the film will be poor and weldability will be poor.
This is because if Zn/CrO ₃ exceeds 5, corrosion resistance due to zinc and chromic acid will be poor. In such chromate treatment, the coating has electrical conductivity due to the inclusion of zinc powder, and the controlled self-sacrificing protection of the zinc particles and the passivation of the base surface with chromic acid improve weldability and corrosion resistance. It works extremely effectively. In addition, in order to obtain even higher corrosion resistance, it was found that it is effective to add a chromium-based soluble rust preventive pigment (abbreviated as A) to the chromate treatment solution. This chromium-based rust-preventing pigment uses zinc potassium chromate ( _{K 2 O.}
4CrO ₃ .4ZnO .3H ₂ O (hereinafter abbreviated as ZPC) or strontium chromate (SrCrO ₄ ) is preferable. That is, it is thought that the addition of a chromium-based anticorrosive pigment increases the effect of passivation of the base surface by chromic acid in the chromate coating, and is effective in improving corrosion resistance. Further, the amount of the rust preventive pigment added is preferably 0.11 or less in weight ratio (A/Zn) to the zinc powder in the chromate treatment solution. This is because if this exceeds 0.11, blisters are likely to occur on the painted surface and weldability will be significantly reduced. The amount of chromate treatment film deposited as described above is preferably 100 to 500 mg/m ² in terms of chromium amount. When the amount of chromium deposited is less than 100 mg/m ² , sufficient corrosion resistance cannot be obtained, and when it exceeds 500 mg/m ² , workability is significantly reduced. The zinc-rich paint used in the present invention to form the zinc-rich coating layer on the chromate film is based on a general zinc-rich paint based on an organic resin containing a high content of zinc powder of 70 to 85 wt%. As the organic resin, epoxy resin is preferred, but other resins may also be used. In steel sheets using such zinc-rich paints, the contact between the zinc particles in the paint film is maintained, the paint film itself is conductive, and the electrochemical corrosion prevention effect works effectively, resulting in improvements in weldability and corrosion resistance. It is advantageous from
However, from the viewpoint of processability, there was a problem in that the coating film did not spread well. Therefore, in the present invention, by maintaining the blending amount of zinc powder in the coating film at 70 to 85 wt%, adding a lubricity imparting agent to improve workability, and reducing the coating film thickness, A coated steel plate with high workability was obtained. As a wettability imparting agent, as a result of investigation, it was found that molybdenum disulfide is effective. In addition, its content is 0.2 to 0.8wt relative to the nonvolatile content of the paint.
% is preferable. If it is less than 0.2wt%, the processability will not improve, and if it exceeds 0.8wt%, no further improvement in processability can be expected, and if it is added to the paint, the viscosity will increase and the paintability will deteriorate. The coating thickness is 3
~10 μm is preferred. If it is less than 3 μm, it will be difficult to obtain a uniform coating and corrosion resistance will be poor, and if it exceeds 10 μm, not only workability but also weldability will deteriorate. However, when the film thickness of the zinc-rich coating film is reduced in this way, the barrier effect of the coating film becomes smaller and the corrosion resistance decreases. In order to obtain a highly corrosion-resistant steel sheet, which is the object of the present invention, it is necessary to prevent a decrease in corrosion resistance due to thinning of the coating film. Therefore, in the present invention, a chromium-based soluble rust preventive pigment is added to the coating film. As a chromium-based soluble rust preventive pigment, the water-soluble component at room temperature is 0.40 to 1.20 as _CrO3 .
Zinc potassium chromate (K ₂ O・4CrO ₃・4ZnO・
3H ₂ O, abbreviated as ZPC) or strontium chromate (SrCrO ₄ ) were effective. If the water-soluble component is less than 0.40g/ as _CrO3 , the effect of passivating the base steel plate by soluble chromium cannot be expected, and if it exceeds 1.20g/, blisters will occur on the coating surface and the desired corrosion resistance will not be achieved. is not obtained. The content of the antirust pigment in the coating film is preferably 2 to 6 wt% based on the nonvolatile content of the coating. If it is less than 2wt%, no improvement in corrosion resistance can be expected, and if it exceeds 6wt%, blisters are likely to occur in the coating film and weldability is also reduced. On the other hand, weldability is improved by reducing the thickness of the zinc-rich coating, but the content of chromium-based rust-preventing pigment (abbreviated as A) and molybdenum disulfide (abbreviated as B) in the zinc-rich coating It has been found that weldability is dramatically improved when the content ratio is in the range of 0.05≦B/A≦0.25 when the content is in the range of 2≦A≦6wt% and 0.2≦B≦0.8wt%, respectively ( (See Figure 1). Zinc-rich coatings containing chromium-based anticorrosive pigments or molybdenum disulfide alone do not improve weldability, and it is only by using these in combination that weldability can be significantly improved. Although the details of the reason for this are unknown, when observing the surface of the welding electrode, it was found that the amount of organic matter derived from the resin in the zinc-rich coating film attached to the electrode was extremely low on the steel sheet of the present invention. It is thought that this suppresses the wear and tear of the electrodes due to an increase in resistance. That is, the adhesion of the resin in the zinc-rich coating film to the electrode during welding is suppressed by the appropriate content ratio of the chromium-based anticorrosive pigment and molybdenum disulfide in the zinc-rich coating film. As mentioned above, on a Zn-Ni alloy plated steel sheet, a chromate treatment film containing zinc powder and chromic acid as the main components, or a chromate treatment film with a chromium rust prevention pigment added, is coated with a chromium rust prevention pigment. By forming a zinc-rich coating containing molybdenum disulfide and molybdenum disulfide, it is possible to obtain a zinc-rich coated steel sheet that has dramatically improved performance in all aspects of weldability, workability, and corrosion resistance compared to conventional zinc-rich coated steel sheets. The present invention will be specifically described below with reference to Examples and Comparative Examples. Examples 1 to 10, Comparative Examples 1 to 10 (1) Preparation of test pieces A chromate treatment solution having the composition shown below was applied onto various 0.8 mm steel plates using a roll coater.
It was dried by heating at 170°C for 90 seconds. Immediately after cooling, a zinc-rich paint based on epoxy resin and containing the additives shown in Table 1 was applied using a roll coater, and baked at 270°C for 90 seconds to prepare a sample. Chromate treatment solution composition Chromic acid 42g Zinc powder 160g ZPC or strontium chromate 0-24g Succinimide 6.0g Xanthan gum 1.5g Water 1 (2) Performance test The performance of the above sample was evaluated by conducting the following tests. (2-1) Weldability test A continuous spot welding test was performed under the following conditions: 100 spot welds were made on two stacked plates, then 1 spot welded on two 30 x 100 mm plates,
Evaluation was made by the number of dots until a tensile shear strength of 400 kg or more was achieved. Welding surface Paint film - cold rolled surface pressure 200Kg Current 8.5KA Current application time 10 cycle electrode R40 (radius type), Cr-Cu (2-2) Workability test Blank diameter 69mmφ, die diameter 33mmφ 25mm
Cup drawing was performed to the desired height, and the processed portion was repeatedly peeled off with cellophane tape three times, and workability was evaluated based on the amount of film peeled off as shown below. Amount of peeling (mg) = (Weight of blank) - (Weight of cup after peeling) Workability evaluation 〇... Amount of film peeled off 5 mg or less △... 〃 5 to 20 mg ×... 〃 20 mg or more (2-3) Corrosion resistance test Composite of samples Corrosion test method (salt spray for 4 hours,
2 hours at 60℃ dry, 2 hours at 50℃ humidity, 95% or more
(1 cycle is 1 cycle, 100 cycles in total)
The corrosion resistance was evaluated according to the state of red rust occurrence as follows. 〇… Red rust occurrence area rate 0% △… 〃 1~5% ×… 〃 5% or more As is clear from the performance test results shown in Table 2, the coated steel sheet of the present invention has a higher resistance to welding than conventional zinc-rich steel sheets. It has far superior properties in terms of hardness, processability, and corrosion resistance. In addition, in the present invention, the Zn-Ni alloy plated steel plate used in the present invention,
The object of the present invention can be achieved for the first time by combining a chromate treatment containing zinc powder or a chromium-based rust-preventive pigment and a coating treatment of a zinc-rich paint containing a chromium-based rust-preventive pigment and molybdenum disulfide. . In particular, the content ratio of chromium-based rust-preventive pigments and molybdenum disulfide in the zinc-rich coating has a large effect on the performance of painted steel sheets, with the content of chromium-based rust-preventive pigments affecting corrosion resistance and the content of molybdenum disulfide affecting workability. ,
It was found that the content ratio of molybdenum disulfide to the chromium-based anticorrosive pigment affects weldability.
Zinc-rich coated steel sheets with different content ratios of chromium-based rust-preventing pigment and molybdenum disulfide in the zinc-rich coating were prepared and their performance was evaluated. The results are shown in Figure 1. It can be seen that weldability, workability, and corrosion resistance are all excellent as long as the content ratio of the chromium-based anticorrosion pigment and molybdenum disulfide in the zinc-rich coating is within the range of the present invention.

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【table】 [Brief explanation of the drawing]

FIG. 1 is a graph showing the appropriate content ratio of zinc potassium chromate and molybdenum disulfide in the zinc-rich coating.

Claims (1)

[Claims] 1 Ni content is 5 to 20 wt%, plating amount is 5
Zn-Ni alloy plating layer with ~30g/ ^m2 , zinc powder and chromic acid as main components, and their weight ratio
A chromate coating layer with a chromium adhesion amount of 100 to 500 mg/m ² obtained by treatment with a chromate treatment solution containing 3 to 5 Zn/CrO ₃ , a chromium-based soluble rust preventive pigment (abbreviated as A), and disulfide. Contains molybdenum (abbreviated as B),
The content ratio is 2≦A≦6 relative to the nonvolatile content of the paint.
(wt%), 0.2≦B≦0.8 (wt%), 0.05≦B/A≦
0.25 and a zinc-rich coating layer having a thickness of 3-10 μm obtained by applying and drying a zinc-rich paint having a zinc powder content of 70-85 wt% based on the non-volatile content of the paint. Painted steel sheet with excellent strength, workability, and corrosion resistance. 2 Ni content is 5 to 20wt%, plating amount is 5
The main components are a Zn-Ni alloy plated layer of ~30g/ ^m2 , zinc powder, chromium-based soluble rust preventive pigment (abbreviated as A) and chromic acid, and the weight ratio is
A chromate coating layer with a chromium deposition amount of 100 to 500 mg/m ² obtained by treatment with a chromate treatment solution having a Zn/CrO ₃ ratio of 3 to 5 and a weight ratio A/Zn of 0.11 or less, and a chromium-based soluble rust preventive. Contains a pigment (abbreviated as A) and molybdenum disulfide (abbreviated as B),
The content ratio is 2≦A≦6 relative to the nonvolatile content of the paint.
(wt%), 0.2≦B≦0.8 (wt%), 0.05≦B/A≦
0.25 and a zinc-rich coating layer having a thickness of 3-10 μm obtained by applying and drying a zinc-rich paint having a zinc powder content of 70-85 wt% based on the non-volatile content of the paint. Painted steel sheet with excellent strength, workability, and corrosion resistance.