JPH0230395B2 - YOSETSUSEITOSOKOHAN - Google Patents

Description

[Detailed description of the invention]

The present invention relates to improving the corrosion resistance of weldable coated steel sheets in which electrical conductivity is imparted to the coating film using metallic zinc powder. In recent years, one-sided anti-corrosion steel plates, which have undergone anti-corrosion treatment on one side in advance, have been used in the underbody of some automobiles to prevent corrosion from the back side. This single-sided corrosion-resistant steel sheet is required to have weldability and paintability on the uncorrosion-treated surface, just like the conventionally used cold-rolled steel sheet. Galvanized steel sheets such as galvanized steel sheets were used, but they had the drawback of being expensive due to low production efficiency and a large number of steps. For this reason, in recent years, single-sided coated steel plates with weldability and anti-corrosion properties have been studied as single-sided anti-corrosion steel plates that can be manufactured at low cost. The present inventors applied 10 parts by weight of chromium trioxide, 40 to 50% of which has been reduced to a trivalent state, and phosphoric acid (100% H ₃ PO ₄ ) 3 to 4 parts by weight, polyacrylic acid 4 to 5 parts by weight, acrylic emulsion polymer solid content 17 to 20 parts by weight, and water for making an aqueous solution.
The undercoat layer coated with a stable aqueous solution for metal surface coating containing 200 to 4000 parts by weight and dried has a total chromium content of 10
~50mg/ ^m2 , and on top of this undercoat layer, a resin topcoat layer containing zinc powder is formed at ~50mg/m2.
The present invention provides a weldable coated steel sheet, characterized in that the undercoat layer has a surface roughness of 50μ and the coating amount of the undercoat layer is larger in the concave portions than in the convex portions of the surface roughness. This steel plate has surface roughness, so
The convex parts of the surface roughness are close to the welding chip, and the undercoat layer is thin, so electric weldability is good, and the amount of undercoat applied is large in the concave parts of the surface roughness, so it has excellent corrosion resistance. Furthermore, since the top coat layer contains a resin as a binder, it has various characteristics such as good processability. However, in recent years, there has been a demand for materials with even better anti-corrosion properties in automobiles and the like due to longer service life. For this reason, methods such as increasing the thickness of the topcoat layer, which fundamentally contributes to corrosion resistance, and increasing the amount of metallic zinc powder in the topcoat layer have been considered, but none of these methods There were problems in implementation as it harmed the character. The present invention does not cause such deterioration in coating processability,
Furthermore, the present invention provides a weldable coated steel sheet with improved corrosion resistance compared to the conventional one. The present inventors conducted various studies in order to develop a weldable coated steel sheet with improved corrosion resistance while maintaining paint film processability, and as a result, succeeded by mixing an anticorrosive pigment into the top coat layer. FIG. 1 schematically shows a cross section of a weldable coated steel plate of the present invention, where 1 is a steel plate with surface roughness, 2 is a chromate-based undercoat layer formed on the surface of the steel plate 1, and 3 is an overcoat layer formed on this undercoat layer 2. The structures and compositions of these will be explained in detail below. First, regarding the steel plate 1, the surface of the steel plate 1 is rough, and its surface roughness (average roughness Rz measured by a surface roughness meter) is 4 to 20 μ. This surface roughness was created to shorten the distance between the tip of the welding machine that contacts the top coat layer 3 and the steel plate 1 during electric welding, and to improve weldability. Electric weldability is improved. However, if it is made too large, the thickness of the product coating film, including the undercoat layer 2, will become uneven, stress will be concentrated in specific areas during processing, and coating cracks will occur in those areas, as well as surface roughness. The coating thickness on the convex portions 4 becomes extremely thin and corrosion resistance deteriorates, so it is necessary to keep the coating thickness to 20μ or less. Furthermore, if the surface roughness is too small, no improvement in electric weldability can be expected, so it is necessary to set it to 4μ or more. The surface roughness of the steel plate 1 is most preferably formed by a chemical etching method (for example, etching with an aqueous ferric chloride solution) or a shotplast method, as it is uniform and dense. However, the latter method also has problems such as environmental pollution and difficulty in recovering the grid. Therefore, in order to industrially form a uniform and dense surface roughness, it is preferable to carry out skin pass rolling using dull rolls. The undercoat layer 2 formed on the surface of the steel plate 1 is 40 to 50%
10 parts by weight of chromium trioxide reduced to trivalent state, 3 to 4 parts by weight of phosphoric acid (100% H ₃ PO ₄ ), 4 to 5 parts by weight of polyacrylic acid, 17 to 17 parts by weight of acrylic emulsion polymer solids. A stable aqueous solution for coating metal surfaces containing 20 parts by weight and 200 to 4000 parts by weight of water to form an aqueous solution is coated and dried, and its composition is the residue of water evaporated from the aqueous solution during drying. When applying an aqueous solution to this undercoat layer 2, the aqueous solution flows down into the recesses 5 in the surface roughness of the steel plate, so the amount of application is greater in the recesses 5, and therefore the amount of application over the entire surface is greater than when the surface is smooth. , corrosion resistance improves. The coating amount of the undercoat layer 2 is controlled by the total amount of chromium, which is the amount of all chromium contained in the layer, and in order to exhibit corrosion resistance, the total amount of chromium must be 10 mg/m ² or more. However, if it exceeds 50 mg/m ² , although the corrosion resistance improves, the adhesion of the top coat layer 3 decreases, so the upper limit needs to be 50 mg/m ² or less. When forming this undercoat layer 2, since trivalent chromium in the aqueous solution greatly contributes to anticorrosion properties, an aqueous solution in which 40 to 50% of chromium trioxide has been reduced to trivalent chromium is used to form the undercoat layer 2. Apply. Top coat layer 3 on undercoat layer 2 is metal zinc powder 80.0 ~
91.0 parts by weight, anti-rust pigment 0.2-5.0 parts by weight, synthetic resin
A coating film consisting of 4.0 to 19.8 parts by weight is formed to a thickness of 10 to 50 μm, and the anti-corrosion properties are significantly improved compared to conventional coatings that only contain 80 to 96 parts by weight of metallic zinc powder due to the addition of anti-rust pigments. However, the processability is maintained as before. Metallic zinc powder is added to the paint film in the same manner as before to give it anti-corrosion properties and electrical conductivity.
Requires at least part by weight. The mixture of anti-rust pigments is a feature of the present invention.
As the anti-rust pigment, strontium chromate, zinc chromate, calcium leadate, etc. are used alone or in combination. In order to improve the corrosion resistance compared to conventional metal zinc powder by using these anticorrosion pigments, it is necessary to mix them in an amount of 0.2 parts by weight or more based on the entire coating film. However, if more than 5.0 parts by weight is mixed, the elution of hexavalent chromium from the coating surface becomes significant, and the anticorrosion effect becomes saturated. Therefore, the amount of the anticorrosion pigment mixed is 0.2 to 5.0 parts by weight based on the entire coating film. The synthetic resin is a binder for the metal zinc powder and antirust pigment, and the smaller the amount, the better in order to improve the electrical conductivity of the coating film. However, if the amount is reduced, the workability of the coating film decreases, making it unable to withstand even simple processing. Therefore, 4 parts by weight of the entire coating film is required as the synthetic resin. Therefore, the total amount of the metal zinc powder and the anti-rust pigment is 96 parts by weight at most, and since the maximum amount of the anti-rust pigment mixed is 5.0 parts by weight, the upper limit of the amount of the metal zinc powder is 91 parts by weight. On the other hand, the upper limit for synthetic resin is 19.8% lower than the lower limit for metal zinc powder and anti-rust pigment.
Weight part. Various synthetic resins can be used as the binder, but epoxy resins with excellent adhesion and processability are used, especially under the normal baking conditions of continuous coating lines without using curing agents or curing catalysts. It is preferable to use a linear molecular structure with a molecular weight of 10,000 to 100,000, which allows a coating film with the desired performance to be obtained at a plate temperature of 200 to 260°C and a baking time of 10 to 80 seconds. This is because when the molecular weight is less than 10,000, it is necessary to add a curing agent or curing catalyst to obtain a normal coating film under the above baking conditions, and addition of these hardens the coating film and reduces workability. This is because the unreacted residual curing agent also reduces corrosion resistance. On the other hand, if the molecular weight exceeds 100,000, it becomes industrially difficult to dissolve the resin, and a large amount of solvent is required to adjust the viscosity to an appropriate level, resulting in high costs and difficulty in adjusting the coating thickness. Top coat layer 3 is usually coated with base coat layer 2 by roll coating method.
paint over. For this purpose, metallic zinc powder, anticorrosive pigment and synthetic resin having the above composition are dissolved in 50 to 200 parts by weight of a solvent. As a solvent, the molecular weight is 1 to 10.
In the case of ten thousand epoxy resins, cyclohexanone, ethyl cellosolve acetate, diacetone alcohol, isophorone, etc. are used from the viewpoint of resin dissolving power and roll coating workability, and aliphatic hydrocarbons and others are used to improve workability and baking drying properties. Aromatic hydrocarbons, esters, ketones, ethers, etc. may be used in combination. If the coating amount of the top coat layer 3 is less than 10μ, corrosion resistance will be poor, and if it exceeds 50μ, electric weldability will not be improved even if the steel plate surface roughness is increased, so the coating amount is set to 10 to 50μ. In the present invention, the anti-corrosion property is improved by mixing the anti-rust pigment into the top coat layer 3. The anti-corrosive pigment suppresses the dissolution of the metal zinc powder, which conventionally dissolves at temperatures, and prevents the occurrence of red rust on the steel plate. As a result, the corrosion prevention efficiency per unit weight of zinc increases and the corrosion prevention property is considered to be maintained. It is also considered that this is because the antirust effect of the antirust pigment itself appears synergistically in parallel with this. Example 1 Cold rolled steel plate with average surface roughness Rz of 20μ (plate thickness 0.8mm)
A stable aqueous solution for coating metal surfaces having the following composition was applied by roll coating to form an undercoat layer having a total chromium content of 30 mg/m ² . Stable aqueous solution composition for metal surface coating Chromium trioxide (trivalent chromium 43%) 10 parts by weight Phosphoric acid 3.4 parts by weight Polyacrylic acid 4.4 parts by weight Acrylic emulsion polymer solid content 18.7 parts by weight Water 2100 parts by weight Then this undercoat layer An epoxy resin zinc-rich paint containing only metal zinc powder or an epoxy resin zinc-rich paint containing metal zinc powder mixed with anti-corrosion pigment is applied on the surface by roll coating.
Baking was performed at 235°C (plate temperature) for 60 seconds to form an overcoat layer with a thickness of 15μ. Table 1 shows the relationship between the top coating layer components and performance of the weldable coated steel sheets obtained, and shows that even though the product of the present invention contains anti-corrosion pigments, workability is maintained in the same way as before. Corrosion resistance has improved.

【table】

[Table] Example 2 An undercoat layer was formed on the surface of a cold rolled steel sheet in the same manner as in Example 1 using the same cold rolled steel sheet and a stable aqueous solution for coating metal surfaces, and then metallic zinc powder or zinc powder with an average particle size of 4μ was applied. The molecular weight of this mixed with anti-corrosion pigment is 10,005.
A 15 μm thick epoxy resin paint was applied to form a top coat layer. The top coat layer was applied using a roll coating method, and baking was performed at 235°C for 60 seconds. Table 2 shows the results of investigating the performance of the weldable coated steel sheet manufactured in this manner in the same manner as in Example 1, together with the composition of the top coat layer. Moreover, the corrosion resistance is improved. Here, the corrosion-resistant 4t bent part is made by bending the test piece by 4t in advance, and bending it by JIS Z 2371.
Based on this, a salt spray test was conducted for 240 hours to evaluate the occurrence of red rust on the 4t bent section. In addition, from Table 2, the metal zinc powder needs to be at least 80% by weight for good weldability, preferably at least 85% by weight, but if it exceeds 91% by weight, an anti-corrosive pigment should be added. As a result, workability deteriorates. Therefore, the metal zinc powder is preferably 85 to 90% by weight. Further, when the anticorrosive pigment is mixed in an amount of 1% by weight or more, the anticorrosive property is significantly improved, so it is preferable to mix the anticorrosive pigment in an amount of 1% by weight or more.

[Table] As described above, the present invention utilizes the anticorrosion effect of the anticorrosive pigment itself by mixing the anticorrosive pigment, and the effect of suppressing the excessive elution of the metallic zinc powder. It improves the corrosion resistance of a steel plate coated with paint and does not impair workability, so it can be used in applications that require even higher corrosion resistance than conventional coatings.

[Brief explanation of drawings]

FIG. 1 is a schematic cross-sectional view of a weldable coated steel plate of the present invention. 1... Steel plate, 2... Undercoat layer, 3... Top coat layer, 4
... Convex portion, 5... Concave portion.

Claims (1)

[Claims] 1. On the surface of a steel plate with an average surface roughness Rz of 4 to 20μ,
Chromium trioxide with ~50% reduced to trivalent state
10 parts by weight, 3 to 4 parts by weight of phosphoric acid (100% H ₃ PO ₄ ), 4 to 5 parts by weight of polyacrylic acid, 17 to 20 parts by weight of acrylic emulsion polymer solids, and 200 to 200 parts by weight of water to make an aqueous solution. A stable aqueous solution for metal surface coating containing 4000 parts by weight is applied and dried to form an undercoat layer with a total chromium content of 10 to 50 mg/ ^m2 ,
Furthermore, metal zinc powder 80.0~91.0 is applied on top of this undercoat layer.
Parts by weight, rust preventive pigment 0.2 to 5.0 parts by weight, synthetic resin 4.0 to 5.0 parts by weight
A weldable coated steel sheet, characterized in that a top coat layer consisting of 19.8 parts by weight is formed in a thickness of 10 to 50μ.