JPH0120057B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a composite coated steel sheet with improved electric welding workability by forming a manganese-rich coating on a zinc-rich coating. In recent years, there has been an increasing demand for highly corrosion-resistant surface-treated steel sheets as materials for the undercarriage of automobile bodies, doors, etc.
However, in the case of automobile parts, assembly involves electric welding, so electric welding is possible, and the coating system is for cold-rolled steel sheets, so surface treatment is required on only one side. . 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 plated with multiple layers of 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 blemishes, 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 is true for hot-dip galvanized steel sheets;
−Fe-based and Zn-Al-based materials were used, but
Its 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 were still insufficient, and it did not have the anti-corrosion performance required in recent years for more than 10 years without perforation. Also, during press processing, the paint film exhibits a powdering phenomenon, and the powder is picked up, causing scratches and dents on the product.Also, during spot welding, the chips become zinc alloyed and the number of consecutive welds is about 2000. It was inferior to galvanized steel plate. In view of the recent strong demand from automobile manufacturers for surface-treated steel sheets with particularly excellent corrosion resistance, the present invention has been developed to provide surface-treated steel sheets with even better corrosion resistance than zinc-rich coated steel sheets.
Moreover, the present invention provides a composite coated steel sheet with improved spot weldability. The inventors of the present invention have conducted various studies to improve the corrosion resistance and welding workability of conventional zinc-rich coated steel sheets, and have found that if a manganese-rich coating is further formed on the zinc-rich coating, the corrosion resistance and welding performance will be improved. We have found that workability can be improved. The present inventors have developed a surface-treated steel sheet with excellent corrosion resistance and welding workability that has an average surface roughness Rz of 4 to 30μ.
(1) A chromate-based film having a total chromium content of 5 to 100 mg/m ² on the steel plate, starting from the steel plate side;
(2) Zinc powder in epoxy resin at a weight percent of 70~
A zinc-rich coating film with a film thickness of 2 to 20μ containing 90% and (3) manganese powder in epoxy resin at a weight%
They developed a composite coated steel sheet on which a manganese-rich coating with a thickness of 2 to 20 μm containing 50 to 90% manganese was formed. The present invention will be explained in detail below. The attached drawings schematically show a cross section of a composite coated steel plate of the present invention, where 1 is a steel plate and 2 is a chromate film formed on this steel plate. 3 is a zinc-rich coating film formed on this coating, in which zinc powder 4 is dispersed in an epoxy resin coating film. 5 is a manganese-rich coating film formed on the zinc-rich coating film 3, in which manganese powder 6 is dispersed in the epoxy resin coating film. In the present invention, these steel plates, coatings, and coatings are constructed as follows. First, regarding the steel plate 1, irregularities with an average surface roughness Rz of 4 to 30 μm are formed on the surface of the steel plate 1. In the case of the present invention, these irregularities have the general purpose of improving the adhesion of the coating film, and are also intended to increase the amount of film deposited in the recesses 7 when a coating-type processing solution is used to form the chromate-based film 2. The total amount of film deposited is increased, thereby improving corrosion resistance, and the distance to the welding machine tip is shortened by the convex portion 8 during electric welding, thereby improving weldability. If the average surface roughness Rz of the irregularities is less than 4μ, the adhesion of the coating will decrease, and powdering or peeling of the coating will likely occur during processing. Moreover, if it exceeds 30μ, the coating surface will become rough, reducing its anticorrosion properties and impairing its appearance. Therefore, in the case of the present invention, the average surface roughness Rz of the steel plate 1 is set to 4 to 30μ. Note that there are no particular limitations on the surface roughness profile, and one formed by a general forming method such as the Dull skin pass method or the shot blasting method is sufficient.
Further, as for the type of steel plate 1, ordinary steel plate is mainly used in the case of a member of an automobile body, but depending on the purpose, it is also possible to use steel plates of other steel types. The chromate-based film 2 covers the surface of the steel plate 1 with a passive film to inactivate it and at the same time improves the adhesion of the coating.
If it is less than mg/m ² , corrosion resistance will be poor, and coating film adhesion will not be stable, and good adhesion may not be obtained. If it exceeds 100 mg/m ² , the corrosion resistance will improve, but the adhesion of the coating will decrease, and it may peel off during press processing, making it impossible to obtain a product that is stable and durable for practical use. Therefore, in the present invention, the amount of the chromate-based film 2 is set to 5 to 100 mg/m ² in terms of the total amount of chromium in the film. The chromate-based film 2 may be formed by a known chromate treatment method, for example, a reactive chromate treatment, a no-rinse coating type chromate treatment, or an electrolytic chromate treatment. Zinc-rich coating film 3 contains zinc powder 4 in the dry coating film.
If the zinc powder content is less than 70% in an epoxy resin coating containing 70 to 90% by weight, the coating adhesion is excellent, but the electrical conductivity decreases and weldability rapidly become lower. On the other hand, if it exceeds 90%, the adhesion of the coating deteriorates, powdering tends to occur, and the interlayer adhesion with the upper manganese rich coating 5 becomes extremely poor. If the dry coating thickness is less than 2μ, the particle size of the zinc powder used is 5 to 10μ, making it difficult to distribute it uniformly and bind each powder to each other during coating. This results in a coating that is not uniform and has excellent adhesion. Also, if the coating thickness is thin like this, the absolute amount of zinc for the corrosion-resistant steel plate 1 will be insufficient,
Corrosion resistance decreases. On the other hand, if it exceeds 20μ, there will be no quality problem, but the cost will be high. The manganese rich coating film 5 is an epoxy resin coating film containing 50 to 90% by weight of manganese powder in the dry coating film, and exerts a galvanic effect on the underlying zinc rich coating film 3 to provide anticorrosion properties. improve,
It also prevents alloying of chips in welding machines and improves welding workability. 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 manganese-rich coating film 5
has sacrificial anticorrosive action and passive anticorrosive action, and improves corrosion protection from both of these aspects. Furthermore, the fact that manganese has a galvanic effect on zinc means that it also has the effect of suppressing the self-consuming excessive galvanic effect of zinc in conventional zinc-rich coated steel sheets and preventing zinc from leaching out more than necessary. Conceivable. On the other hand, manganese is difficult to alloy with the welding machine's tip (made of copper), so the number of consecutive spot welding points can be increased to 4,000 to 5,000 points. In the case of the present invention, if the manganese powder content in the coating film (dry coating film) is less than 50% by weight, the electrical conductivity will decrease, weldability will decrease, and the galvanic effect will also decrease in terms of corrosion protection. In addition, the production of oxides and hydroxides, which have anticorrosion properties, decreases, resulting in a decrease in anticorrosion properties.
On the other hand, if it exceeds 90%, powdering and delamination are likely to occur, causing processing problems. If the film thickness is less than 2μ in terms of dry film thickness, the absolute amount of zinc will be insufficient and long-term anticorrosion effect will not be obtained. Also, if the thickness exceeds 20μ, there will be no quality problem, but the cost will increase. The epoxy resin for the zinc-rich coating 3 and the manganese-rich coating 5 preferably has a molecular weight of 10,000 to 100,000 in terms of workability and quality. Further, these coating films can be easily formed by applying each coating material using a known coating method and then baking and drying at a temperature (board temperature) of 200 to 260°C. Next, the present invention will be explained with reference to examples. Example: A cold-rolled steel plate with a thickness of 0.8 mm is subjected to a dull skin pass to form irregularities on the surface, and then a 2-coat, 2-bake continuous painting line is used to degrease, chromate treatment, form a zinc-rich coating, and manganese-rich. A composite coated steel sheet was manufactured by sequentially forming a coating film. Table 1 shows the obtained steel plate, and Table 2 shows its quality performance. In addition, both the zinc-rich coating film and the manganese-rich coating film are baked at a board temperature of 260.
It was carried out for 60 seconds at a temperature of ℃.

【table】

[Table] The quality performance test shown in Table 2 was conducted 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. In the bending test, 180 degrees of close bending was performed using 0 sheets (0t) and 2 sheets (2t) at intervals on the inside of the bend, and then Sellotape was pasted on the coating on the processed area, and Sellotape was removed by rapidly peeling it off. , evaluated according to the following criteria.

[Table] (2) Corrosion resistance The test piece was cross-cut using a knife to reach the steel substrate and bent 1t at 180 degrees, and then subjected to a salt spray test for 1000 hours based on JIS Z 2371. The incidence of rust and red rust was evaluated according to the following criteria.

[Table] (3) Welding workability After continuous spot welding without dressing using Cu-Cr chips, the tensile shear strength of the welded specimens was measured, and the tensile shear strength was less than 450Kgf-14%. The point at which this occurred was defined as a possible welding point, and the number of welding points was investigated. Welding was performed at a pressure of 250 kg, a current of 7500 A, and 12 cycles.

[Table] As is clear from Table 2, the composite coated steel sheet of the present invention, which has a manganese-rich coating film on a zinc-rich coating film, is superior in corrosion resistance and welding workability to conventional zinc-rich coated steel plates.

[Brief explanation of drawings]

The attached drawing is a schematic cross-sectional view of the composite coated steel sheet of the present invention. 1... Steel plate, 2... Chromate film, 3...
Zinc-rich coating film, 4... Zinc powder, 5... Manganese-rich coating film, 6... Manganese powder, 7... Concave portion, 8... Convex portion.

Claims (1)

[Claims] 1. A chromate-based film having a total chromium content of 5-100 mg/m ² on a steel plate having an average surface roughness Rz of 4 to 30 μ, sequentially from the steel plate side (1) and (2) Zinc powder in epoxy resin at 70% by weight
(3) a manganese-rich coating film with a thickness of 2 to 20 μm containing 50 to 90% by weight of manganese powder in an epoxy resin; Composite coated steel plate with excellent welding workability.