JPS5853437A - Welding painted steel plate and its manufacture - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a coated steel sheet that is slightly weldable by imparting electrical conductivity to a coating film using zinc powder, and a method for manufacturing the same.

In recent years, some automobiles use single-sided anti-corrosion steel plates that have been previously treated with anti-corrosion treatment on one side to prevent corrosion from the backside of the car body due to rock salt sprinkled on roads to prevent freezing in winter.

However, in the manufacturing process of automobiles, changing the manufacturing process when cold-rolled steel sheets and single-sided anti-corrosion steel sheets are used is extremely inconvenient due to equipment that employs mass production methods. For this reason, single-sided corrosion-resistant steel sheets are required to be handled in the same way as cold-rolled steel sheets, and conventional exterior painting can be applied to the side that is not treated with anti-corrosion treatment. Traditionally, steel manufacturers have supplied single-sided electrogalvanized steel sheets and single-sided hot-dip galvanized steel sheets to meet such performance requirements, but the former uses a lot of electricity. However, since the production efficiency is low, the crystal price is low, and the latter cannot be plated on one side with current hot-dip plating technology, so the plating layer on one side must be removed after plating, which is still expensive [ there was.

Therefore, coated steel plates with weldability and anti-corrosion properties are being considered as single-sided anti-corrosion steel plates that can be manufactured at low cost.

A layer containing chromium and an overcoat layer containing conductive metal powder (e.g. Japanese Patent Publication No. 47-6882, Japanese Patent Application Laid-open No. 49/1989)
-114540) and those in which the undercoat layer also contains chromium and conductive metal powder (for example, Japanese Patent Publication No. 54-117
No. 80, JP-A No. 48-296.41), etc. are known. However, in order to improve electrical conductivity, the undercoat layer of these steel plates is designed to contain no resin at all, or to contain as little resin as possible5, and the film thickness is also 1.5 mm for the purpose of emphasizing corrosion resistance. The rim, which is made to be as thick as ~2μ, has the disadvantage of poor workability.

On the other hand, as an undercoat layer that overcomes this drawback, one using polyacrylic acid and an acrylic emulsion polymer as a binder is known. This undercoat layer is JP-A-54-11
A stable aqueous solution for coating metal surfaces having the following composition described in No. 0145 was applied and dried.

Composition of a stable aqueous solution for coating metal surfaces (a) 10 parts by weight of chromium trioxide in which 40 to 50 ions have been reduced to a trivalent state (b) Phosphoric acid (1004Hs PO4) 3 to 4 parts by weight
c> 4 to 5 parts by weight of polyacrylic acid (d) Acrylic emulsion polymer solid content 17 to 20 parts by weight (e) 200 to 4000 parts by weight of water to make an aqueous solution This undercoat layer is applied when the material is cold-rolled steel sheet. , 1- on dry weight
It exhibits excellent adhesion and anti-corrosion properties when applied to the surface, but it does not contain conductive metal powder, and is used as a binder. Even if an overcoat layer containing powder was formed, there was a drawback that the electro-wax weldability was slightly inferior.

The present invention provides a weldable coated steel sheet and a method for manufacturing the same, which solve the electrical conductivity of the dry aqueous solution film described in JP-A-54-110145 by improving the surface of the base steel sheet.

The weldable coated steel sheet of the present invention consists of a steel sheet such as a hot-air steel sheet or a cold-rolled steel sheet, an undercoat layer obtained by applying the above-mentioned aqueous solution on the steel sheet and drying it, and a resin containing zinc powder formed on the undercoat layer. It consists of an overcoat layer, and the steel plate has a surface roughness.

In other words, the present inventors proposed a method for improving weldability defects due to poor conductivity of the undercoat layer.[If the surface of the steel plate is roughened to form unevenness, the convex portions will be higher than when the surface of the steel plate is smooth. We conducted an experiment based on the assumption that the distance between the coating layer surface and the welding machine tip would be smaller, improving conductivity, and it was confirmed that the desired effect was obtained. .

Figure 1 (a) and (b) schematically show the cross sections of a steel plate with a smooth surface and a steel plate with a rough surface, respectively, with an undercoat layer and a topcoat layer formed thereon. 1 is a steel plate, and 2 is a steel plate. Undercoat layer, 3ii
This is the top coat layer. As shown in the figure, when the coating amount (weight) of the topcoat layer is the same, in the case of a steel plate with a rough surface, the convex portion 4 physically approaches the surface of the topcoat layer 30. Therefore, when electrically welding, the distance between the steel plate 1 and the chip becomes smaller, and the undercoat layer 2
The poor current conductivity is improved.

Table 1 shows 1-light cold-rolled steel sheets of ordinary steel and cold-rolled steel sheets with roughened surfaces by shot blasting (each sheet thickness 0.8cm).
The aqueous solution was applied and dried to form an undercoat layer with a total chromium content (total amount of chromium contained) of 431 mm, and an epoxy resin top coat layer containing 85 mm of zinc powder on top of the base coat layer. This shows the electric weldability (spot weldability) of a single-sided coated steel sheet with a thickness of 15μ, and the cold-rolled steel sheet with a rougher surface is superior.

Table 1 (Note 1) Electric welding conditions (Note 2) Evaluation criteria for weldability Evaluation was made based on the incidence of welding failures with tensile shear strength of less than 350 Ayf.

0 21G or less Δ More than 21i and 35- or less However, the improvement in weldability due to surface roughness is when the surface roughness is 15μ.
It is saturated at the above value, and if it is too small, air will be drawn in when applying the top coat, so it is preferable to set it to 20μ or less.

Furthermore, weldability is determined by the relationship between the surface roughness and the thickness of the topcoat layer, so if the standard thickness of the topcoat layer is 10 to 25μ, the surface roughness is preferably 4 to 18μ.

On the other hand, the present inventors have newly discovered that the corrosion resistance of a painted steel plate can be further improved by roughening the surface of the steel plate.

The paint film adhesion (180 degree close bending (Ot) cellophane tape application and peeling) of the coated steel sheet with the dried residue of the above aqueous solution as the undercoat layer was determined when the topcoat layer was a zinc-containing epoxy resin (film thickness 15μ, zinc powder content 87%). % by weight), and when the steel plate is a prite cold-rolled steel plate, the smaller the coating amount, the better the coating amount, as shown in Fig. 2. However, as shown in FIG. 3, its anticorrosion properties (240 hour salt spray test according to JIS-Z 2371) decrease as the amount of coating decreases. Therefore, conventionally, the coating amount of the undercoat layer is determined by selecting a range that harmonizes coating film adhesion and corrosion resistance, and when the surface roughness of the steel plate is 3 μ or less, the total chromium content in the layer is 2 to 15-15 μm.
I was trying to make it so.

However, when the surface of the steel plate is roughened, since the coating liquid for the undercoat layer is an aqueous solution, it flows down after coating and accumulates in the recess 5 as shown in FIG. 1(b). As a result, the convex portion 4 and its sloped portion have an appropriate film thickness. Therefore, when the surface roughness is high, even if the coating amount of the undercoat layer is increased, the excess will flow down into the recesses 5, and the convex portions 4 and their slopes will have an appropriate coating amount.

However, since the amount of coating in the recesses 5 increases, the adhesion of the coating film decreases. However, the decrease in paint film adhesion in this area is compensated for by the increase in surface area due to the roughened surface, the anchor effect, etc., and the overall paint film adhesion improves. As the material that has flowed down accumulates in the recessed portion 5, the amount of coating in that portion increases, and the anticorrosion property improves accordingly.

Table 2 shows how much the amount of undercoat layer can be increased when the surface of the steel plate is made rough, and the relationship between paint film adhesion and corrosion resistance when increased. .

Table 2 (Note 1) The thickness of the steel plate is 0.8-. .

(Note 2) The coating amount of the undercoat layer is the total amount of chromium in the layer.

(Note 3) The top coating layer is a 15μ epoxy resin film containing 87% by weight of zinc powder.

(Note 4) Paint film adhesion was determined by a bending test in a normal condition □ according to the test method for colored galvanized iron plates of JIS-G 3312. For the bending test, the number of spaced plates on the inside of the bend was 0 (Ol), 1 (1t), and 2 (2t). The cellophane tape was peeled off rapidly and evaluated using the following 5-point method. .

(Note 5.) Corrosion resistance was determined by making a cross cut in advance with a knife on the coating film of the test piece, reaching the steel plate, and conducting a salt spray test based on JIS-Z 2371. The test was conducted at 240 hours, and the state of occurrence of red rust on the cross-cut portion was evaluated using the following 5-point method.

As shown in Table 2, the coating amount of the undercoat layer can be increased up to 50 mm. Also, the lower limit is 101! It becomes VI.

Figure 4 shows the results of investigating the distribution of chromium concentration contained in the undercoat layer on the surface of the steel plate using an X-ray microanalyzer. Although some of this is in charge, the top coat layer is responsible for most of it.

For this reason, the top coat layer contains zinc powder as an anticorrosive and conductive material, and is bound with a binder. Therefore, it is preferable to increase the amount of zinc powder in the top coat layer and minimize the amount of binder, but since the binder needs to be at least 4% by weight in the dry coating for film formation, the upper limit of zinc powder is 96%.
There are heavy molecules.

In addition, in order to impart good electrical conductivity to the top coat layer, at least 8
Required above 0 weight. In addition, zinc powder average weight 1
．． 5 to 6μ is appropriate. Although various resins can be used as the binder, epoxy resins with excellent adhesiveness are preferred. Furthermore, if the film thickness is less than 10 μm, the anticorrosion properties will be poor, and if it exceeds 50 μm, the weldability will not be improved even if the steel plate surface roughness is increased, so the film thickness is set to 10 to 50 μm.

The production of the coated steel plate of the present invention involves first roughening the surface of the steel plate and then
A surface roughness of 20 μm is imparted, and the surface is then cleaned by degreasing, pickling, water washing, etc. using conventional methods. In this case, as for surface polishing, shot butt polishing or chemical polishing provides better coating film adhesion and adhesion than polishing with paper or buffing.

After the surface is cleaned, a stable aqueous solution for coating metal surfaces is applied to form an undercoat layer, and the coating can be done by various known methods such as spraying, burping, and burping. And when coating, the amount of coating after drying is 10~50wV as the total amount of chromium? 5 Adjust the liquid concentration, squeezing pressure, etc.

After forming the undercoat layer in this manner, a resin containing 80 to 96 mos of zinc powder is applied thereon as a dry film. In this case, as the resin, it is preferable to use an epoxy resin as described above, but epoxy resins having a molecular weight of 10,000 to 100,000 are particularly advantageous because of their excellent processability and adhesion.

When an epoxy resin with a molecular weight of less than 10,000 is used as a binder and baked in a continuous coating line, a sufficient dry coating film cannot be obtained unless a curing agent and a curing catalyst are used together, making it unusable for practical use. Therefore, a curing catalyst or the like must be used in combination, but in that case, the coating becomes hard, resulting in poor processability, and unreacted curing agent remains in the coating, resulting in decreased corrosion resistance. For this reason, we focused on a method that does not use a hardening agent, and found a region where both workability and corrosion resistance are satisfied only within the molecular weight range of 10,000 to 100,000 (no hardening agent is used within this range). When the molecular weight is 10,000 or more, a complete coating film is formed, and when the molecular weight is 100,000 or more, there is no industrial solvent that can dissolve the resin, so it cannot be made into a paint. Therefore, the optimum range for the epoxy resin used in the present invention is 10,000 to 10,000, and within this range, the plate temperature is 200 to 26,000 without using a curing agent or curing accelerator.
Dry baking can be done in 10 to 60 seconds at 0C.

As the solvent, methyl ethyl ketone, cyclohexanone, isophoro, diaseptone alcohol, etc. may be used.

The coated steel sheet of the present invention can be made into a double-sided coated steel sheet depending on the application, and can also be used for automobile bodies, interiors of ship bodies,
It can also be used for furniture that comes into contact with water, home appliances, etc. that require electric welding during manufacturing.

As described above, the present invention is characterized in that the undercoat layer contains a resin.
This problem was solved by imparting surface roughness to the surface of the steel plate to solve the special situation that the weldability becomes worse and the adhesion of the coating becomes worse when the amount of coating is increased. It is easier and cheaper.

[Brief explanation of the drawing]

FIG. 1 is a schematic cross-sectional view of the weldable coated steel sheet of the present invention, (J
1) shows the case where the steel plate is a bright cold-rolled steel plate, and (b) shows the case where the steel plate is a cold-rolled steel plate whose surface has been roughened by shot blasting. Figure 2 shows the relationship between the coating amount of the undercoat layer and the film adhesion.
Moreover, FIG. 3 shows the relationship between the coating amount of the undercoat layer and the corrosion resistance. FIG. 4 shows the distribution state of chromium in the undercoat layer when it was investigated using an X-ray microanalyzer. l... Steel plate, 2... Undercoat layer, 3... Top coat layer, 4...
... Convex portion, 5... Concave portion Patent applicant Nisshin Steel Co., Ltd. Agent Mitsuru Shindo Figure 1 Figure 3 Figure 41! ]

Claims (1)

[Claims] (1) On the surface of a steel plate with a surface roughness of 4 to 20μ,
10 parts by weight of chromium trioxide reduced to trivalent state, 3-4 parts by weight of phosphoric acid (100% H1l PO4), 4-5 parts by weight of polyacrylic acid, 17-20 parts by weight of acrylic emulsion polymer solids. The undercoat layer obtained by applying and drying a stable aqueous solution for coating metal surfaces containing 200 to 4000 parts by weight of water to form an aqueous solution has a total chromium content of 10 to 5 parts by weight.
0i2, and a resin overcoat layer containing zinc powder is formed on this undercoat layer with a thickness of 10 to 50μ. A weldable coated steel sheet characterized by having a higher content of . <2) The weldable coated steel sheet according to claim 1, wherein the steel sheet has a surface roughness of 4 to 18 μm and a topcoat layer thickness of 10 to 25 μm. (3) The top coat layer contains zinc powder with an average particle size of 1.5 to 6μ.
0-96. The weldable coated steel sheet according to claim 1, characterized in that the weldable painted steel sheet is weight engaged. (4) The weldable coated steel sheet according to claim 1, wherein the resin of the top coat layer is an epoxy resin.
(5) On the surface of a steel plate with a surface roughness of 4 to 20μ,
10 parts by weight of chromium trioxide reduced to trivalent state, 3-4 parts by weight of phosphoric acid (100% H*PO4), 4-5 parts by weight of polyacrylic acid, 17-20 parts by weight of acrylic emulsion polymer solids. Parts by weight and 2 parts water for making an aqueous solution
0 to 50 liters of a stable aqueous solution for coating metal surfaces containing 0 to 4000 parts by weight, assuming that the total dry film is 4 ml.
If/s islands are applied to form an undercoat layer, and then zinc powder is dried to form a 5o-jc+6 weight epoxy resin-corrugated paint with a molecular weight of 1 to 100,000. A method for manufacturing a weldable coated steel sheet, which includes applying the coating to a film thickness of 10 to 50μ.