JPH0120058B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a zinc-rich weldable coated steel sheet that has improved corrosion resistance, powdering resistance, and welding workability by adding manganese powder or an anticorrosion pigment together with the manganese powder. In recent years, highly anticorrosion-resistant surface-treated steel sheets have been required as materials for parts exposed to extremely corrosive environments, such as the underside of automobile bodies, doors, and these parts. However, in the case of automobile parts, assembly generally involves electric welding, so electric welding is possible for surface-treated steel sheets, and anti-corrosion treatment is required only on the back side of the parts, and the surface should be painted with cold-rolled steel sheet exterior paint. Therefore, one side is required to be a cold-rolled steel plate. Conventionally, galvanized steel sheets such as zinc-based electroplated steel sheets or hot-dip galvanized steel sheets, or painted steel sheets such as zinc-rich coated steel sheets have been used as such surface-treated steel sheets, but each has advantages and disadvantages, and is suitable for automobiles. It has not yet reached the point where it satisfies the manufacturer's requirements. For example, in the case of zinc-based electroplated steel sheets, conventional Zn
- Steel plates plated with Ni-based or Zn-Fe-based alloys or multi-layer plated with Zn and Ni layers have been used, but these steel plates suffer from peeling of the plated layer and powdering during press working. It has excellent workability and weldability, with very little occurrence of spot welding, and the number of consecutive spots possible during spot welding is up to 5,000 without dressing the chip. However, in the case of electroplated steel sheets, the formation of pinholes is generally unavoidable, so there is the problem of rust starting from the pinholes, and even with electroplating, blistering occurs easily in salt spray tests. Corrosion resistance is inferior to zinc-rich coated steel sheets. Furthermore, the adhesion of the electrodeposited coating film is not as good as that of zinc-rich coated steel sheets. The same applies to coated galvanized steel sheets;
−Fe-based and Zn-Al-based materials were used, but
These advantages and disadvantages were similar to those of the zinc-based electroplated steel sheet. However, the biggest drawback of this steel plate is that a reliable single-sided plating method has not yet been developed. For this reason, in the past, single-sided plated products were manufactured by mechanically removing the plated layer on one side after plating, resulting in extremely high manufacturing costs. On the other hand, in the case of zinc-rich coated steel sheets, such as zinc-rich coated steel sheets such as Zinchrome Metal (trade name) and coated steel sheets in which an epoxy-based zinc-rich paint is applied on top of a chromium-containing highly corrosion-resistant undercoat layer, single-sided coated products can be easily produced using existing strip coating lines. It exhibits superior corrosion resistance than zinc-based electroplated steel sheets and hot-dip galvanized steel sheets. However, its anti-corrosion properties are still insufficient, and the significant galvanic action on the steel substrate causes sub-film corrosion at the interface with the steel substrate, which appears as blisters, or causes powdering during press forming. The powder was picked up, causing scratches and dents on the product, and furthermore, during spot welding, the tip (made of copper) and zinc powder became alloyed, and the zinc powder was picked up on the tip, resulting in a poor number of continuous points. . In view of the recent demand for surface-treated steel sheets with corrosion resistance of 10 years or more until perforation, the present invention aims to improve the corrosion resistance and resistance of zinc-rich coated steel sheets, which have particularly excellent corrosion resistance among the surface-treated steel sheets. The present invention provides a weldable coated steel sheet with improved powdering properties and welding workability. The inventors of the present invention have conducted various studies to improve the above-mentioned drawbacks of conventional zinc-rich coated steel sheets. As a result, they have added manganese powder to the zinc-rich coat, or a rust-preventive pigment together with it, to create a zinc-manganese-rich coat. I found out that I can improve it by doing this. The present invention was made based on this knowledge,
The gist is that a zinc-manganese rich coating film containing 60% by weight or more of zinc powder and manganese powder is formed on a steel plate with a roughened surface via a chromate-based film. It is made of painted steel plate. The present invention will be explained in detail below. FIGS. 1 and 2 schematically show a cross section of a coated steel plate according to the present invention, where 1 is a steel plate and 2 is a chromate film formed on this steel plate 1. FIG. Reference numeral 3 denotes a zinc-manganese rich coating film formed on this coating 2, in which zinc powder 4 and manganese powder 5 are dispersed in the resin coating film. These steel plate 1, chromate-based coating 2, and zinc-manganese rich coating 3 are constructed as follows. First, regarding the steel plate 1, a normal cold-rolled steel plate with a roughened surface is usually used as the steel plate 1. Here, the surface of the steel sheet 1 is roughened not only for the general purpose of improving the adhesion of the zinc-manganese rich coating 3, but also for the purpose of roughening the surface when a coating-type treatment liquid is used to form a chromate-based film. By increasing the amount of chromate-based film deposited on rough concave areas, the overall amount of film deposited is increased, improving corrosion resistance, and shortening the distance between the welding machine tip and convex parts during electric welding. This is to improve conductivity in this way. The surface roughness of this steel plate 1 is the average surface roughness Rz of 4
~20μ is appropriate. If it is less than 4μ, the adhesion of the zinc-manganese rich coating 3 will decrease, and powdering and peeling will easily occur during processing.
This is because if it exceeds 20μ, roughness will occur on the coating surface, reducing corrosion resistance and appearance. Such surface roughening may be formed by conventional methods such as dull skin bathing and shot blasting. The chromate-based film 2 may be formed by a known chromate treatment method such as a reactive chromate treatment (for example, chromate alone or an etching agent added thereto), a no-rinse coating type chromate treatment, or an electrolytic chromate treatment. However, after various studies, it is preferable to use a coating type chromate treatment liquid having the composition shown below in terms of corrosion protection and coating adhesion. Treatment liquid 1 (a) 10 parts by weight of chromium trioxide, 40-50% reduced to trivalent state (b) 3-4 parts by weight of phosphoric acid (100% H ₃ PO ₄ ) (c) Polyacrylic acid 4 -5 parts by weight (d) Acrylic emulsion polymer solid content 17-20 parts by weight (e) 200-4000 parts by weight of water for making an aqueous solution Treatment liquid 2 In the composition of the treatment liquid 1, (a) trioxide A treatment liquid in which all or part of the hexavalent chromium is reduced to a trivalent state, and the ratio of the amount of hexavalent chromium to the amount of trivalent chromium is 0 to 2.3. In the case of the present invention, the amount of deposited chromate film 2 is controlled by the total amount of chromium contained in the film, and the total amount of chromium is controlled to be 10 to 50 mg/m ² . this is
If it is less than 10 mg/m ² , corrosion resistance will be poor and coating film adhesion will not be stable, making it impossible to always obtain good adhesion. On the other hand, if it exceeds 50 mg/m ² , corrosion resistance improves, but the adhesion of the coating decreases and it becomes easy to peel off during press working. The zinc-manganese rich coating film 3 contains manganese powder 5 together with zinc powder 4 to improve corrosion resistance, powdering resistance during processing, and pick-up during electric welding. Since manganese is electrochemically less base than zinc, it has a galvanic effect on zinc and protects it from leaching out. Furthermore, when manganese is placed in a corrosive environment, it produces manganese oxides and hydroxides that have anti-corrosion properties, and these corrosion products suppress the dissolution of manganese itself. Therefore, the zinc-manganese rich coating film 3 has a sacrificial anti-corrosion effect and a passive anti-corrosion effect, and improves the anti-corrosion property from both of these aspects. Furthermore, in addition to these anticorrosion effects, manganese also has the effect of alleviating the excessive self-consuming galvanic effect of zinc on iron through its own galvanic effect, and suppressing the elution of zinc more than necessary. Furthermore, since the manganese powder 5 has a higher hardness than zinc, the powdering property during processing is improved compared to the case of using zinc powder alone. Furthermore, manganese has a higher melting point than zinc, so
It is difficult to alloy with chips during welding, and pick-up is suppressed. In the case of the present invention, this zinc-manganese rich coating film 3
The content ratio of zinc powder 4 and manganese powder 5 is set to be zinc powder/manganese powder = 2/98 to 98/2. This means that 4 zinc powder is 2 of the total of both powders.
If the amount of manganese powder 5 is less than 2% by weight, the galvanic effect of zinc will not be exhibited and the anticorrosion property will decrease.On the other hand, if the amount of manganese powder 5 is less than 2% by weight,
This is because the above-mentioned effects due to this will be reduced. If the total content of these in the coating film is less than 60% by weight, the electrical conductivity will be poor and the corrosion resistance will also be reduced.
Make it 60% by weight or more. However, if the content exceeds 91% by weight, the adhesion of the coating film will decrease and powdering will occur more easily, so the upper limit is preferably 91% by weight or less. The best resin for forming a coating film is an epoxy resin, especially one with a molecular weight of 1 to 1.
One with a linear structure of 100,000 is preferable. If the film thickness is too thin, the content of anti-corrosive metal powder will be reduced and the anti-corrosion properties will be lowered, and if the film is too thick, workability will be reduced, so a thickness of 5 to 50 μm is appropriate. In the case of the present invention, the drawbacks of conventional zinc-rich coated steel sheets are improved by including zinc powder 4 and manganese powder 5 in the coating film.
It is also possible to add anti-corrosion pigment 6 to further improve the anti-corrosion properties (Fig. 2). In the case of coating films of general paints, the anticorrosive properties of the coating film are also improved by adding anticorrosive pigments, but the anticorrosive effect is due to the anticorrosive effect of the anticorrosive pigment itself. However, in the case of the present invention, in addition to such a corrosion-preventing effect, it can be expected to suppress excessive elution of zinc and elute only the amount of zinc necessary for corrosion protection of the steel substrate. The amount of the rust-preventing pigment added here is suitably 0.2 to 5% by weight, and if it is less than 0.2% by weight, the effect of the addition will be small.
On the other hand, if the amount exceeds 5% by weight, chromium may be eluted if the anticorrosive pigment is a chromate. The painted steel sheet of the present invention can be manufactured using a steel sheet as a raw material in the same manner as a colored galvanized iron sheet. In this case, if an epoxy resin is used as the resin for the zinc-magnetic coating, the baking temperature is 200 to 260.
It may be carried out at ℃ for 10 to 80 seconds. Example: A cold-rolled steel plate with a thickness of 0.8 mm is subjected to a dull skin pass to form a surface roughness of Rz 20μ, then degreased to clean the surface, and then a coating-type chromate treatment is applied to the surface by roll coating to reduce the total chromium content. A chromate film of 40 mg/m ² was formed. Thereafter, a zinc-manganese rich coating film (dry coating thickness of 20 μm in each case) was formed on this film by adding zinc powder and manganese powder or an anticorrosion pigment to these powders. Table 1, 2
The table shows the relationship between the composition and performance of these coatings.
The chromate film and the zinc-manganese rich film were formed under the following conditions. (1) Chromate film formation conditions: 10 parts by weight of chromic acid trioxide, 3 parts by weight of phosphoric acid,
5 parts by weight of polyacrylic acid, 18 parts by weight of acrylic emulsion polymer solids, 2000 parts by weight of water,
A coating type chromate treatment solution consisting of Cr ⁺⁶ /Cr ⁺³ = 1.4 is applied using the roll coating method. (2) Zinc-manganese rich coating film formation conditions 250℃ (plate temperature) after coating by roll coating method
Bake for 60 seconds.

【table】

[Table] The coating film performance was also investigated in the following manner. (1) Paint film adhesion A bending test was conducted in accordance with the JIS G 3312 test method for colored galvanized iron plates. The bending test was performed using 0 sheets (0t), 1 sheet (1t), and 2 sheets (2t) at the bending inner interval.
After performing a 180 degree close bending process, cellophane tape was applied to the processed part coating film, and cellotape peeling was performed by rapidly peeling it off, and evaluation was made according to the following criteria.

[Table] (2) Powdering resistance After cutting the test specimen into blanks with a diameter of 360 mm, they were coated with anti-corrosion lubricant (Oil Coat Z2, manufactured by Idemitsu Kosan) and punched with a punch diameter of 200 mm using a 300-ton hydraulic press.
Punch R13mm, die R4mm, drawing height 65mm,
A cylindrical deep drawing test was performed with the coated surface facing outward under the conditions of a 22-ton presser. After the test, the powder adhering to the die mold was scraped off with abrasive paper, the amount was visually evaluated, and the next step was performed. It was evaluated based on the criteria.

[Table] (3) Corrosion resistance Test specimens were cross-cut with a knife and bent 180 degrees at 4t.
The products subjected to the deep drawing test for powdering resistance were tested for 1000 hours and 240 hours, respectively, based on JIS Z 2371, and evaluated according to the following criteria.

【table】

[Table] (4) Weldability After performing spot welding under the following welding conditions, a tensile test was conducted to investigate the incidence of tensile shear strength of less than 350 Kgf and the number of consecutive welding points under constant welding conditions. The welding conditions for tensile shear strength were the following three conditions, and the welding conditions for the number of continuous welding points were a 4.5φ-CF electrode, a pressure of 250 kg, a current application time (cycle) of 12,
The current was 7500A. In the latter case, the tensile shear strength is determined every 100 points, and the strength is
The life of the electrode was defined as the point in time when the value became lower than the allowable lower limit shown in RWMA.

【table】

[Table] From Table 1, when manganese powder is included together with zinc powder, corrosion resistance is improved compared to zinc powder alone, and even with a small content, the same or better weldability can be obtained, and powdering is also reduced. I understand that. Furthermore, when comparing Tables 1 and 2, it can be seen that the addition of anticorrosion pigments further improves the anticorrosion properties.

[Brief explanation of drawings]

FIGS. 1 and 2 are schematic cross-sectional views of the coated steel plate of the present invention. 1... Steel plate, 2... Chromate film, 3...
Zinc-manganese rich coating film, 4...Zinc powder, 5
...Manganese powder, 6...Anti-rust pigment.

Claims (1)

[Claims] 1. A total of 60% by weight of zinc powder and manganese powder on a steel plate with a roughened surface via a chromate-based film.
A weldable coated steel sheet, characterized in that a zinc-manganese rich coating film containing the above is formed. 2 The ratio of zinc powder/manganese powder is 2/98 to 98/
2, the total content of both powders in the coating film is 60 to 91% by weight.
The weldable coated steel sheet according to claim 1, characterized in that: 3. The weldable coated steel sheet according to claim 1, characterized in that a rust preventive pigment is added to the zinc-manganese rich coating film.