JPH01312082A - Production of organic coated steel sheet having superior corrosion resistance and adhesion to coating film - Google Patents

Abstract

PURPOSE:To improve the fixing rate of Cr and adhesion to a coating film as well as to obtain high corrosion resistance, weldability and workability by specifying a chromating soln. when a Zn alloy plated steel sheet is successively subjected to chromating and org. coating. CONSTITUTION:At least one side of a steel sheet is plated with a Zn alloy and chromated with a chromating soln. contg. a reducing agent, a silane coupling agent and one or more among acid residue, resin and silica. An org. coating film is then formed on the resulting chromate film. By this method, an org. coated steel sheet having a high fixing rate of Cr and superior adhesion to a coating film as well as high corrosion resistance, weldability and workability is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to the production of an organic coated steel sheet with a good chromium fixation rate, excellent corrosion resistance and coating adhesion for use in automobiles.

<Prior Art> In recent years, increasing the corrosion resistance of automobile steel sheets has attracted attention as a social demand, and various rust-proof steel sheets have been proposed to meet this challenge, and are gradually becoming established.

Some of these antirust steel sheets have been surface-treated by hot-dip Zn plating, hot-dip Zn alloy plating, electrolytic Zn plating, electrolytic Zn-based alloy plating, organic film-based zinc-rich coating, and the like.

Furthermore, in addition to these, composite coated steel sheets have also been developed in which organic coatings are applied to plated steel sheets. This composite type coated steel sheet is currently known as an excellent rust-proof steel sheet.

In general, the manufacturing process for automobile bodies consists of press forming of the above-mentioned painted steel sheets, assembly, and painting.The performance of automobile steel sheets requires workability in terms of press workability and spot weldability in assembly, as well as corrosion resistance. be done.

As a product that meets these requirements, (a) coated steel sheet with excellent weldability, workability, and corrosion resistance using a sink paint containing a predetermined amount of chromium-based soluble rust-preventing pigment and molybdenum disulfide (Japanese Patent Laid-Open No. 60-10553);
No. 5) (b) On a Zn alloy plated steel plate having bake hardenability,
A composite type coated steel sheet having an organic coating treatment and having heat hardening properties (Japanese Patent Application No. 1983-013229) has been proposed.

<Problems to be Solved by the Invention> Composite coated steel sheets are excellent rust-proof steel sheets, but chromate is often used as a base treatment for organic coatings to improve corrosion resistance.

If normal chromate is used as a base treatment, chromium will be eluted during the alkaline degreasing and chemical conversion treatment during the coating process, which will interfere with waste liquid treatment. cause

In addition, when the base treatment is normally done using chromate,
In a high humidity environment (warm pure water, immersion at 50°C for 10 days),
The so-called water-resistant secondary adhesion deteriorates due to the chromium-[--organic film being brushed at the interface.

The purpose of the present invention is to solve the above-mentioned problems of the prior art, and by applying an organic coating treatment, it not only provides high corrosion resistance, weldability, and workability, but also provides high corrosion resistance, weldability, and workability during alkaline degreasing and/or chemical conversion treatment in the automobile painting process. It is an object of the present invention to provide a method for manufacturing an organic coated steel sheet that suppresses chromium elution and improves secondary water-resistant adhesion.

<Means for Solving the Problems> The present inventors performed alkaline degreasing on automotive steel sheets in which chromate treatment and organic N treatment are applied to Zn-based alloy plated steel sheets, thereby eluting chromium and removing the chromium after degreasing. We have confirmed that the amount of chromate deposits on steel sheets can be significantly reduced in some cases.

Also, after applying alkaline degreasing, chemical conversion treatment, electrodeposition coating, intermediate coating, and top coating on an organic coated steel sheet, what should I do? ! Pure water (so℃)
After soaking for 240 hours, cut the base with a knife.
After removing the tape, I confirmed that it was peeling off at the chromate resin interface.

Therefore, in order to improve these, addition of reducing agent, addition of acid, addition of resin, addition of silica to the chromate solution,
As a result of investigating the addition of a coupling agent, etc., the present invention was achieved.

<Specific configuration of the invention> In other words, the present invention provides Zn on at least one surface of a steel plate.
After chromate plating is performed, a reducing agent and Provided is a method for producing an organic coated steel sheet with excellent corrosion resistance and coating adhesion, which is characterized by adding a silane coupling agent and further adding one or more selected from acid radicals, resins, and silica. .

Since the organic coated steel sheet improved by the present invention has a good chromium fixation rate, chromium elution can be suppressed during alkaline degreasing and/or chemical conversion treatment in the automobile painting process. By adding a silane coupling agent to Zara,
The bonding strength between the chromate layer and the resin increases, and the water resistant secondary adhesion improves.

The present invention will be explained in more detail below.

In the present invention, Z is formed on at least one surface of the square steel plate.
Apply n-based alloy plating.

Zn-based alloy plating includes Zn-Ni alloy plating (preferably Ni content 5 to 13).
wt%), Zn-Fe alloy plating (preferably Fe content 8-25w)
t%), Zn COA fl 203 Cr 203 alloy plating (preferably Co content 1-5 wt%), Zn-A
J2 alloy plating (preferably An content 1 to 15 wt%)
, Zn-Ni/Fe-P two-layer alloy plating (preferably P content in Fe-P 0.0003-5 w
t%), Zn-Fe/Fe-P two-layer alloy plating, etc. can be used. Since these Zn-based alloy platings have corrosion resistance several times higher than conventional pure Zn platings, they further improve the object of the present invention. Also, the coating weight is preferably 10 to 40 g/m2. 10
If it is less than 40 g/m2, corrosion resistance is insufficient;
If it exceeds m2, there will be no further significant improvement in corrosion resistance.
This is because it is not economical.

The reason why the content of each element in the Zn-based alloy is limited as described above is as follows.

The N1 content of the Zn-Ni alloy is preferably 5 to 13 wt%. If it is less than 5wt%, corrosion resistance will be insufficient, and 13w
This is because if it exceeds t%, the plating layer will become too hard. Z
The Fe content of the n-Fe alloy is preferably 8 to 25 wt%. If it is less than 8 wt%, corrosion resistance will be insufficient, and if it exceeds 25 wt%, red rust will easily occur.

Zn Co AJZ203 C of Cr2o3 alloy
The o content is preferably 1 to 5 wt%. This is because if it is less than 1 wt%, corrosion resistance will be insufficient, and if it exceeds 5 wt%, it will be economically disadvantageous.

The AI content of the Zn-Al alloy is preferably 1 to 15 wt%. This is because if it is less than 1 wt%, corrosion resistance will be insufficient, and if it exceeds 15 wt%, sacrificial corrosion protection will be lost.

Zn-Ni/Fe-P alloy and Zn-
The P content in Fe-P of the Fe/Fe-P alloy is preferably 0.0003 to 5 wt%. 0.0003
If it is less than 5 wt%, the chemical conversion treatment will be poor, and if it exceeds 5 wt%, the current efficiency during plating will be poor and it will be uneconomical.

In order to improve the adhesion of the below-mentioned organic film on the Zn-based alloy plating and further improve its corrosion resistance,
Perform chromate treatment.

This chromate treatment is 10mg/metal chromium.
A coating amount of m' or more is required, and for this purpose, coating type chromate or electrolytic chromate is effective in controlling the coating amount. If it is less than 10 mg/m2, not only the corrosion resistance will be insufficient, but also the adhesion to the organic film will be poor.

On the chromate-treated film, an organic film treatment is applied to improve rust prevention as described above.As the resin for this organic film, one or more of the following ■~■ may be used alone or in combination. use

■Water-dispersible resin; Acrylic, polyethylene, epoxy, alkyd, etc. and modified resins of these.

■Solvent-based resins: Epoxy 1, polyester, etc., and modified resins of these.

■Ultraviolet or electron beam curable resin Acrylic, epoxy, polyurethane, etc.

One or more of these (1) to (4) are coated on the chromate-treated film to a thickness of 0.5 to 3 tones. Film thickness is 05
If it is less than μs, corrosion resistance will be insufficient, and if it exceeds 3 units, weldability will deteriorate.

Note that the corrosion resistance is further improved by adding up to 60 wt% of silica sol to the organic resin film. 60wt
If it exceeds %, the viscosity of the treatment liquid will increase, making it easier to gel.

Furthermore, the present inventors conducted the following tests to investigate the chromium fixation rate during alkaline degreasing and/or chemical conversion treatment in the automobile painting process. As a material steel plate, a zinc-based plating (Zn-Ni) treatment (area weight 10 to 40 g/m2) with sacrificial anti-corrosion properties is applied to an ultra-low carbon steel plate, and the surface is coated with hexavalent chromium/trivalent chromium (c). r6
A chromate solution having a ratio of 80/20 to 20/80 (+/c r3 + ) was applied and baked, and a water-based or solvent-based resin (epoxy resin) was applied thereon and baked.

However, the ratio of Cr"/Cr3" was determined by redox titration method. The maximum plate temperature reached was room temperature to 150°C for chromate baking, and room temperature to 150°C for resin baking.

In the case of using chromate solution alone (without additives in the present invention), after applying the chromate solution and baking, resin is applied and baked.The higher the baking temperature, the higher the chromium fixation rate as it is reduced to hexavalent chromium or trivalent chromium. will improve. For example 150℃
When baked with the above conditions, the chromium fixation rate during alkaline degreasing is 8.
It was more than 0%, which was a level that caused no problems in the automobile painting process.

However, in this case, yield strain occurs due to the material, which causes problems such as the need to remove the strain, increase in yield stress, and deterioration of pressability.

Therefore, in the present invention, as mentioned above, the maximum temperature for baking is limited to room temperature to 150°C, and the chromium fixation rate is improved.

By the method of the present invention, a chromate solution containing methanol as a reducing agent and phosphoric acid as an acid radical is applied and baked at room temperature to 150°C, followed by resin coating and baking. A stock solution that achieves a chromium fixation rate of 80% or more as described below. Cr”/Cr
The range of the ratio of 3''' is as shown in Figure 1. Here, methanol was used as the reducing agent and phosphoric acid was used as the additive, but other reducing agents or additives such as acid groups other than phosphoric acid, resins, etc. The same applies to the case where Onihi silica is used.

As shown in Figure 1, in order to achieve a good chromium fixation rate of 80% or more at a maximum seizure temperature of 150°C to room temperature, the present inventors have found that reducing agents and silane are added to the chromate treatment solution. It has been found that it is effective to add one or more selected from the group consisting of acid radicals, resins and silica in addition to the coupling agent.

First, representative reducing agents suitable for the present invention include methanol, hydrogen peroxide, ethylene glycol, succinic acid, and succinimide. The addition amount is preferably such that RAH is 0.2 to 1.9 relative to CrO2. However, RAH represents the number of Durham atoms of hydrogen in the reducing agent. If this ratio is less than 0.2, even if the following additives are added, the chromium fixation rate will be insufficient, and it will be 1.9
This is because the processing solution will gel if it exceeds this temperature.

As a silane coupling agent, by adding a silane coupling agent represented by general formula (a) to the chromate solution, the water-resistant secondary adhesion is dramatically improved.

(a) X, 1-n-5i (OR), n=0 to 4
n - Integer It is known that such coupling agents have two types of functional groups with different lipophilic and hydrophilic properties in the same molecule, thereby bridging organic resins and inorganic materials and strengthening mutual bonds. .

Therefore, by adding the above-mentioned coupling agent to the chromate solution, the bond between the chromate film and the organic resin is strengthened, and the water-resistant secondary adhesion and corrosion resistance are improved.

Next, the acid radical, resin, and silica to be further added will be explained.

(1) Acid radical As the acid radical, phosphoric acid, chloric acid, etc. are preferable, and the amount added is A-x/CrO, (A-' is the acid radical) at a weight ratio of 0.
05 to 0.3. The reason is that the above ratio is 0.05
This is because the chromium fixation rate will be less than 80% in the future, and if it exceeds 03, the corrosion resistance will deteriorate.

(2) The resin to be added to the resin chromate treatment solution is an acrylic resin with an internal acid value of 250 or more, or an acrylic resin with acrylic acid monomer and/or methacrylic acid monomer added to stabilize it. is suitable.

The amount added is such that the weight ratio of resin/CrO3 is 0.1 to 2.
A range of 0 is good. This is because when this ratio is less than 01, the chromium fixation rate is less than 80%, and when it exceeds 20, the adhesion to the substrate becomes poor.

(3) Silica It is preferable to use colloidal silica as the silica. The amount added is 0.3 in weight ratio of 5i02/Cry3.
~3.0 is preferred. This is because if it is less than 03, the chromium fixation rate will be less than 80%, and if it exceeds 3.0, the adhesion to the substrate will be poor.

When the above-mentioned additives are added to the chromate treatment solution, the chromium fixation rate in the automobile painting process, particularly during alkaline degreasing and chemical conversion treatment, is greatly improved. In the following examples, an immersion alkaline degreaser commonly used in automotive painting processes was used.

FIG. 2 shows the relationship between the amount added and the chromium fixation rate when methanol is used as the reducing agent.

According to this, it is difficult to achieve a chromium fixation rate of 80% or more by adding methanol or reducing agent alone, but if the RA H/CrO3 value is between 0.2 and 1.9, the Cr"/Cr" The value is 80/20 to 20/80, and according to Figure 1, the above (1), (
It can be seen that by adding one or more of the additives 2) and (3), the chromium fixation rate becomes 80% or more.

Figure 3 shows methanol (RAM/Cry3=
1.0) and the relationship between the amount added and the chromium fixation rate when phosphoric acid is added as an acid radical. When the amount of phosphoric acid added is more than o in terms of P O4'-/CrO3,
Even when the ratio of Cr''/Cr'+ is 80/20,
The chromium fixation rate was 100%, which was good.

Cr6+/Cr5 (f is 20/80 Te+fP0
It can be seen that even when 43-/CrO3 is 0.01, the chromium fixation rate is 100%. The amount of phosphoric acid added is P
When O<3-/CrO3 ratio is C'0.3 or more,
Due to the non-volatile nature of phosphoric acid, baking the resin coating later deteriorates the resin and impairs corrosion resistance.

Methanol (RAH/C) was added as a reducing agent to the chromate solution.
FIG. 4 shows the relationship between the amount of resin added and the chromium fixation rate when a resin was added (rO3=1.0) and a resin was added. As the resin, acidic acrylic resin was used.

The amount of resin added was calculated as resin solid content/CrO3. Resin solid content/Cr03=01~2
Within the range of 0.0, it was effective in improving the chromium fixation rate.

It can be seen that when it exceeds 20.0, the adhesion to the substrate deteriorates, and the workability and weldability also deteriorate.

Methanol (RAH/
FIG. 5 shows the relationship between the amount of silica added and the chromium fixation rate when CrO3=1,0) and silica were added. The silica used is ultrafine anhydrous silica. Silica content was expressed as SiO2/CrO3.
When S i 02 /Cry was 0.3 or more, the fixation rate was good and was 80% or more. When the silica content was 3.0 or more, adhesion to the substrate deteriorated and weldability decreased.

As mentioned above, in order to increase the chromate fixation rate in the automobile painting process (during alkaline degreasing), it is necessary to add a reducing agent to the chromate, and in addition to that, (1) introduce acid groups into the chromate, (2) resin (3) Addition of silica was effective. These are thought to be due to the following effects.

First, regarding the addition of a reducing agent to chromate, when methanol, hydrogen peroxide, ethylene glycol, etc. are added to chromate, chromic acid is reduced and Cr670 is added.
The ratio of r3+ decreases and Cr6+, which is easily eluted, decreases, thereby improving the chromium fixation rate.

Next, the acid radicals, resins and silica added in addition to the reducing agent will be described.

(1) Introduction of acid radicals into chromate Analysis of samples after applying and baking chromate solutions with and without phosphoric acid radicals in chromate revealed that in addition to trivalent chromium and hexavalent chromium, 3 A peak that appears to be a hydrate or hydroxide of valent chromium is observed.

There is no significant difference in the proportion of hexavalent chromium in the chromate film between when no acid radicals are introduced and when they are introduced, and the fixation rate of chromate alone does not depend on the introduction of acid radicals. However, when a resin is applied thereon and baked, the hydrate or hydroxide of trivalent chromium is strongly bound to the resin, and as a result, it is thought that the elution of hexavalent chromium is suppressed. It was also confirmed that hexavalent chromium itself reacts with the resin and is reduced. Both of these effects reduce the elution of hexavalent chromium.

(2) Addition of resin When an acidic type resin such as acrylic resin that is stable in the chromate solution is added to the chromate solution, Cr6'
Although the ratio of ``/Cr 3+ does not change, seizure is later suppressed either by bonding with the resin or by elution of raw hexavalent chromium.

(3) Addition of silica In the case of chromate coating alone (without resin treatment), when silica was added, the chromium fixation rate decreased as the amount added increased.

When the Chrome-1 film to which silica was added was analyzed, it was found that there were more OH groups than the film to which silica was not added. on the other hand,
When the distribution of Si in the chromate film was examined, it was found that Sl was unevenly distributed in the surface layer.

From these facts, silica forms a strong film on the surface layer of chromate, and chromate itself is a chromium ion hydrate or hydroxide. Apply chromate O to this second layer and bake it.
The H group and the resin form a strong bond, and as a resin-treated steel sheet, chromium elution is suppressed.

Furthermore, in order to suppress the peeling from the chromate and organic resin interface after immersion in warm pure water, we investigated the addition of a coupling agent and found that the molecular structure shown below has good compatibility and liquid stability in the chromate solution. It was found that the bonding strength between the chromate film and the organic film increased, improving the adhesion of the paint film.

Among the above-mentioned coupling agents, those having the following structures, which are X.vinyl group, mercapto group, methyl group, epoxy group, chromium group and R.methoxy group, are particularly preferred.

However, a coupling agent containing an amino group as X is not used here because it reacts with the chromate solution and increases the viscosity.

(2) H5CH2CH2CH25l(OCH3)3(
3) Cll35i (QC) I3), 1(5) C
u-CII□ ClI2 ClI2 51 (ocl
13) 3(6) ClI2 -CI+Si (OC
H3) The amount of the coupling agent added in the 3-chromate treatment solution is 5 to 50 wt% based on the amount of chromium in terms of Cry in terms of 5i02. If the amount is less than 5 wt%, the improvement in coating adhesion is insufficient, and the water resistant secondary adhesion is not improved. If it exceeds 50 wt%, the compatibility between the chromate and the coupling agent will deteriorate, and the chromate will not be applied uniformly.

<Example> The present invention will be specifically described below based on Examples.

Ultra-low carbon steel sheets were subjected to Zn-based alloy plating using chromate IA treatment as element A, and then subjected to organic coating treatment, and these were used as test steel sheets. The treatment steps are shown in Table 1.

Table 2 shows the results of the chromium fixation rate, water resistant secondary adhesion, corrosion resistance, weldability, and workability of each test steel plate.

The evaluation of each test was conducted according to the following method.

[Test method and its evaluation]

(1) Corrosion resistance (1-a) SST A cross cut was made on the organic film formed on each test steel plate, and a 5% NaCj2 solution was sprayed onto these at 35°C, and the time until red rust appeared was measured.

(1-b) CCT Each test steel plate was sprayed with 5% NaCj2 for 4 hours at 35℃ under the following conditions ↓ Dry for 2 hours at 60℃ ↓ 1 cycle of leaving at 50℃ and 95% RH (humidity) for 2 hours A cycle corrosion test was carried out as
The number of cycles until red rust appeared was counted.

(2) Weldable electrode type R (40R), pressing force 170 kg, welding time 10
Two of each test steel plate was stacked alternately under cycle conditions, and the number of continuous welding points was determined.

(3) Workability Each test steel plate (90mmφ) has a diameter of 50mmφ and a depth of 25m.
The sample was subjected to cylindrical drawing (BHF 1tor), and the processed portion was peeled off with cellophane tape, and the amount of film peeled off (mg/circumference) was measured.

(4) Chromium fixation rate The chromium fixation rate was determined by measuring the number of chromium counts before and after degreasing using a conventional immersion type alkaline degreaser using fluorescent X-rays. As shown in Table 3, the treatment method of the present invention provides high corrosion resistance, weldability, workability, and bake hardenability as well as high corrosion resistance chromium fixation rate.
-1 Curing type organic coated steel sheet is obtained.

(5) Water resistant secondary adhesion Organic coated steel plate was subjected to alkaline degreasing and phosphate treatment → electrodeposition coating → top coating, and after being immersed in pure water at 50°C for 240 hours,
Immediately it was taken out, a 2 mm square cut was made with a cutter knife, the tape was peeled off, and the peeled area ratio (%) was evaluated.

○: No peeling △, peeling ×・Full surface peeling The coupling agents shown in Table 1 are as follows.

(2) H5CH2C112C4125j (OCR,)
.

(3) CHs 5i(QC:)I3)s(5) 1
-C1h CH2CH25t(OCRs)s(6) C
H2-CH5i (OCH3) 3(7) NH4CO
3CHa N)ICl3 CH2CH25i(OCH3
)3 Table 2 (Part 1) Table 2 (Part 2) <Effects of the Invention> As detailed above, according to the present invention, high corrosion resistance is achieved by performing a prescribed chromate treatment and organic coating treatment. , an organic coated steel sheet with good weldability, workability, chromium fixation rate, and excellent coating film adhesion can be obtained.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between baking temperature, Cr''/C, r3+, and chromium fixation rate. Figure 2 is a graph showing the effect of methanol addition to the chromate treatment solution on the chromium fixation rate. Figure 3 is a graph showing the relationship between the amount of phosphoric acid added to the chromate treatment solution and the chromium fixation rate. Figure 4 is a graph showing the relationship between the amount of resin added to the chromate treatment solution and the chromium fixation rate. Figure 5 is a graph showing the relationship between the amount of silica added to the chromate treatment solution and the chromium fixation rate.

Claims (1)

[Claims] (1) After applying Zn-based alloy plating to at least one surface of the steel sheet, chromate treatment is performed, and then an organic coating is formed to produce an organic coated steel sheet with excellent coating film adhesion and corrosion resistance. An organic product with excellent corrosion resistance and coating adhesion, characterized by adding a reducing agent and a silane coupling agent to the chromate treatment solution in chromate treatment, and further adding one or more selected from acid radicals, resins, and silica. Method for manufacturing coated steel plate.