JPH09276789A - Weldable organic composite plated steel sheet excellent in workability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an org. composite plated steel sheet having good end face corrosion resistance and weldability and workability in pressing. SOLUTION: A rust-inhibiting layer contg. a chromium compd. is formed on both sides of a steel having a plating layer consisting of. a zinc-nickel alloy, a zinc-iron alloy or a zinc-chromium alloy. Further, a coating film consisting of 5-40vol.% rustinhibiting pigment, 1-40vol.% mixture of an iron compd. as a conductive material and a globular metallic grain and 35-94 vol.% org. resin and having 0.5-20μm thickness is formed thereon.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel sheet having high press formability, capable of being joined by resistance welding, and having high corrosion resistance.

[0002]

2. Description of the Related Art Conventionally, in the manufacture of automobile members and the like, after cold-rolled steel sheets or plated steel sheets are joined by welding, painting is performed by a plurality of steps. As an example of automobile parts,
The coating process includes chemical conversion treatment, electrodeposition coating, one or more intermediate coatings,
The chemical conversion treatment, the electrodeposition coating, and the intermediate coating are required for rust prevention of the vehicle body. The reason for joining the steel sheets prior to painting is that the electrical conductivity of the steel sheet surface is lost after painting and joining and assembling of the members by the resistance welding method becomes impossible.

[0003]

SUMMARY OF THE INVENTION In the prior art,
When painting after assembling steel plate bonded products, at the manufacturing site,
A coating / processing device of a size that can process the entire assembled member must be provided for each process, which is a large burden on the manufacturing cost. Further, it is also a cost burden that it is necessary to treat the volatile solvent and the waste liquid discharged in a large number of steps for each step.

[0004]

According to the present invention, an organic resin coating film is formed on the surface of a steel material, and a rust preventive pigment and conductive pigment particles are mixed in the coating film to improve weldability and corrosion resistance. It is what makes me. By using the steel material, molding,
It is possible to simplify the coating process after joining. That is, the organic coating film on the surface of the steel sheet contains a conductive material, which enables bonding by resistance welding, and the corrosion resistance of the steel sheet is improved by containing a rust preventive pigment. For aesthetic purposes, all you have to do is apply a top coat. Therefore, at a site where products are manufactured using steel sheets, manufacturing costs can be reduced by minimizing equipment and emissions by omitting steps.

[0005]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, a zinc-based alloy electroplating such as a zinc-nickel alloy, a zinc-iron alloy, or a zinc-chromium alloy is formed on the surface of a steel sheet as a plating layer. Since the melting point of the plated metal is higher than that of pure zinc plating, they are less likely to alloy with the welding tip and wear of the tip is suppressed to improve welding workability. Also,
There is an advantage that the corrosion resistance of the plating layer is high, and the occurrence of red rust due to the corrosion of the steel sheet is prevented by the sacrificial anticorrosion effect, while white rust, which is the rust of the plating layer, is less likely to occur.

The chromium compound-containing anticorrosion treatment layer improves the corrosion resistance particularly at the cut end surface of the steel sheet by the passivation of the surface of the metal by the chromium-containing component. This layer may be formed by any known method such as electrolytic chromate treatment, roll coating, and spray coating. The organic coating film increases corrosion resistance of the steel sheet by the action of the rust preventive pigment in the coating film,
It has the necessary conductivity to realize resistance welding. The thickness of the coating film is preferably 0.5 μm to 20 μm on each side. If the film thickness is less than 0.5 μm, the corrosion resistance is poor, and the film thickness is 20 μm.
When it exceeds m, the electric resistance of the organic coating layer increases and the weldability deteriorates. When the weldability of the steel sheet is important, the film thickness of the coating film is preferably 0.5 to 5 μm, and when the corrosion resistance is important, the film thickness of the coating film is preferably in the range of 5 to 20 μm.

The rust preventive pigments, strontium chromate, calcium chromate, zinc chromate, barium chromate, ammonium chromate, and ammonium dichromate, which are rust preventive pigments in the coating film, are all chromate salts. Similar to the chromium compound-containing anticorrosion treatment layer, the chromium-containing component passivates the surface of the metal, thereby improving the corrosion resistance particularly at the cut end surface of the steel sheet. If the content in the coating film is less than 5% by volume, the rust preventive effect is small, and if it exceeds 40% by volume, the coating film cohesive force decreases and the adhesion to the surface of the steel sheet deteriorates. In addition, it obstructs the energization path and reduces weldability. When the weldability is important, the content of the rust preventive pigment is preferably 5 to 20% by volume, and when the corrosion resistance is important, the content is preferably 10 to 40% by volume.

The spherical metal particles and the iron compound particles, which are the conductive materials in the coating film, come into contact with each other in the coating film during spot welding to form a welding current path. Specifically, when the steel plate is pressed by the welding electrode during spot welding, the conductive material partially destroys the coating film and makes contact with each other, thus ensuring an energization path from the welding electrode to the plated surface. Will be.

Iron phosphide, silicon / iron alloy, manganese /
Particles of iron alloys and particles of iron compounds such as cobalt / iron alloys are highly distorted in shape and thus have a large effect of breaking the coating film. However, on the other hand, during press forming of steel sheets,
It has a drawback that the coating film is easily gazed in the press mold. In the present invention, mold galling is improved by using spherical metal particles together as the conductive material. That is, the iron compound having a distorted shape is likely to be a starting point of galling because the sharp tip comes into contact with the mold in a very narrow area, but the spherical metal particles are
The contact with the press die is smooth and effective in preventing galling.

As the material of the spherical metal particles, stainless steel, nickel and the like can be used. Only the spherical metal particles have a low effect of ensuring an electric conduction path during welding. Therefore, it is important for the conductive material to use the metal particles in combination with the iron compound. The weight ratio of metal particles / iron compound in the coating film is preferably 1/9 to 9/1. If this ratio is less than 1/9, the effect of preventing mold galling during press molding is small,
If it exceeds 9/1, the weldability deteriorates. Also, metal particles /
When the content of the mixture of iron compounds in the coating film is less than 1% by volume, the effect of improving the weldability is small, and when it exceeds 40% by volume, the cohesive force of the coating film is reduced and thus the adhesion to the surface of the steel sheet is reduced. .

When the steel sheet is subjected to particularly severe forming processing, at least one of a polyolefin compound and a carboxylic acid ester compound should be contained in the coating film as an organic lubricant in the coating film. Thus, it is possible to reduce the frictional resistance value of the coating film and reduce the galling during molding. However, when the blending amount is large, the cohesive force of the coating film is reduced and the adhesion between the coating film and the steel sheet surface is reduced. Therefore, when the adhesion is important, the blending amount is preferably 10% by volume or less.

The organic coating may be either a solvent type or a water-soluble type, for example, epoxy resin, alkyd resin, oil-free polyester resin, acrylic resin, acrylic ethylene resin, melamine resin, urethane resin, phenol resin, polycarbonate resin, etc. One kind or a mixture of two or more kinds can be used. If necessary, various additives such as a rust preventive pigment, a curing agent, a coloring pigment, or a lubricant that further improves press workability may be added. The coating method is
Any known method such as roll coating, spray coating, or curtain flow coating may be used.

[0013]

Next, examples of the present invention will be described. An epoxy resin was used as the binder resin in Table 1. The plating applied to the steel sheet is electrogalvanized or Zn-Ni,
It is zinc-based alloy electroplating such as Zn-Fe and Zn-Cr. The chromium compound-containing rustproofing layer and the coating layer were formed by bar coating. In addition, among the following tests, the corrosion resistance test was carried out by spray-coating a shear-cut organic composite steel sheet with a urethane coating for automobile repairs to obtain a dry film thickness of 5
The one with 0 μm was used. In other tests, the organic composite steel sheets having the configurations shown in Tables 2 and 3 were used as they were.

[0014]

[Table 1]

[0015]

[Table 2]

[0016]

[Table 3]

(1) Corrosion resistance test 100 cycles of the following cycle corrosion test were carried out,
The generation area ratio of white rust and red rust on the cut end surface of the test piece having a plate thickness of 0.7 mm was investigated. Salt spray: 1 hour, Drying: 60 ° C, 3.5 hours,
Wetness: 50 ° C., humidity 90%, 3.5 hours Table 2 and Table 3 show the composition of test samples and the test results.
From Tables 2 and 3, it can be seen that the examples of the present invention have high end face corrosion resistance. In Example 1 in which the content of the rust preventive pigment in the coating film was small, the end surface rust area ratio was 50% or more, and the corrosion resistance was low. In Example 28, since Zn plating was used as the plating, white rust often occurred on the end faces. In Example 31, the end face corrosion resistance is low because the coating film was too thin.

(2) Weldability test The continuous weldability of the test plate by spot welding was investigated.
As a test method, first, an appropriate welding current range was obtained, and thereafter, a limit continuous welding point number was obtained. The proper welding range was obtained by the following procedure. Original plate: 0.7 mm thick plain steel is given a surface treatment and used in pairs. Electrode: Obara Co., Ltd. T-16D (material code DHO
M) is used. Electrode pressure: 200 kgf Welding pattern: Pressing / energizing pattern in {} below is set to one cycle of spot welding. {Starting pressing → (0.5 seconds) → Current value of predetermined current value Applying (0.2 seconds) → releasing the applied pressure} Appropriate welding current range: The welding current value is changed by 0.5 KA in accordance with the welding pattern, and the minimum current value that can secure a nugget diameter of 4 mm or more is the lower limit current value, the test. The minimum current value that causes strong welding between the plate and the electrode is defined as the upper limit current value. The proper welding current range is between the lower limit current value and the upper limit current value.

The limit number of continuous welding spots is the upper limit of the number of continuous welding spots that can secure the necessary nugget diameter, and was determined by the following procedure. Original plate: 0.7 mm thick ordinary steel is given a surface treatment and used in pairs. Electrode: Obara Co., Ltd. T-16D (material code DHO
M) is used. Pressure between electrodes: 200 kgf Welding pattern: The following pressurization / energization pattern in {} is one cycle of spot welding. {Start of pressurization → (0.
5 seconds) → Apply current value of predetermined current value (0.2 seconds) →
Pressurized pressure release} Welding current value: Intermediate value of proper welding current range obtained earlier =
(Lower limit current value + Upper limit current value) / 2 Limit continuous welding dot: Continuous welding of a set of two test pieces under the conditions of. The RBI speed is 1 point / 3 seconds. The maximum number of continuous dots that does not make the diameter of the nugget formed between test pieces less than 4 mm is the limit number of continuous dots.

The structures of the test samples and the test results are shown in Tables 2 and 3. With respect to weldability, all of the examples of the present invention showed excellent weldability with a continuous welding spot number of 500 or more. In Example 9, the continuous welding spot did not reach 500 because the amount of the conductive component in the coating film was insufficient. In Example 6, an excessive amount of anticorrosive pigment in the coating film hindered the current flow in the coating film. Example 19
Since only spherical stainless particles were used as the conductive component, the weldability deteriorated. In Example 28, the plating layer is pure Zn
Since it has a lower melting point than Zn-Ni alloys and the like, it is easily alloyed with the welding electrode, and electrode wear is severe, resulting in low weldability. Example 35 has an excessive coating thickness.

(3) Formability test In order to check the galling and peeling of the coating film during press working,
The following test was carried out. A steel sheet is sandwiched between dies that imitate a bead of press molding, and the steel sheet is pulled out at a constant speed while applying a load to examine the damage of the coating film.・ Sample withdrawal width: 30mm ・ Mold: φ4mm cylinder on one side, flat plate on the other side ・ Pressing load: 500kg ・ Pulling speed: 200mm / min ・ Oiling oil: None ・ Evaluation of coating damage: presence or absence of galling and peeling

The structures of the test samples and the test results are shown in Tables 2 and 3. With respect to moldability, all the examples of the present invention showed excellent moldability with no galling with the mold.
In Example 23, the amount of resin was small and the adhesion was poor, and therefore the coating film peeled. In addition, Example 16 contained no spherical stainless particles, and galling occurred between the mold and the mold. Example 20 caused galling due to the non-spherical shape of the stainless particles.

[0023]

As described above, the zinc-based alloy electroplating is used as the plating layer component of the organic composite plated steel sheet, the proportion of the rust preventive pigment and the conductive material in the organic coating film is specified, and
By using a mixture of an iron compound and spherical metal particles as the conductive material, it was possible to obtain an organic composite plated steel sheet having excellent corrosion resistance, weldability, and press formability.

Claims (3)

[Claims] 1. A rust preventive treatment layer containing a chromium compound is formed on both sides of a steel material having a plating layer made of an alloy of zinc and nickel, an alloy of zinc and iron, or an alloy of zinc and chromium. On top of that, 5 to 40% by volume of chromium-based rust preventive pigment, 1 to 40% by volume, based on the total solid content of the paint.
Of the electrically conductive material, and a coating film containing an organic resin consisting of a thermosetting resin of 35 to 94% by volume and a cross-linking material as an essential component with a thickness of 0.5 to 20 μm, corrosion resistance, Organic composite plated steel sheet with excellent weldability and press formability. 2. An organic composite plated steel sheet having excellent corrosion resistance, weldability and press formability, wherein the conductive material according to claim 1 is a mixture of spherical metal particles and iron compound particles. 3. An organic composite plated steel sheet having excellent corrosion resistance, weldability and press formability, wherein the spherical metal particles according to claim 2 are spherical stainless particles having an average particle size of 1 to 30 μm.