JPH0867989A - Rustproof steel sheet for automobile excellent in joined part corrosion resistance, coating suitability and weldability and its production - Google Patents

Abstract

PURPOSE: To develop a rustproof steel sheet for the automobile excellent in joined part corrosion resistance, coating suitability, weldability, etc., by forming specified amts. of the phosphate crystal film and phosphate film on both sides of galvanized steel sheet. CONSTITUTION: The surface of a thin steel sheet such as mild-steel sheet and high-strength steel sheet is plated with Zn, Zn-Ni alloy, etc., then a phosphate crystal film is formed on one side of the substantially unoxidized steel sheet before winding and oiling at 0.05-0.5g/m<2> , and a phosphate film is formed on the other side at 0.5-3.0g/m<2> . A rustproof steel sheet for the automobile excellent in chemical convertibility, coating suitability, joined part corrosion resistance and weldability is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an anticorrosion steel sheet suitable for automobiles, which has excellent corrosion resistance, coating properties and weldability, and a method for producing the same.

[0002]

2. Description of the Related Art Various measures have been taken on automobile bodies in terms of coating and vehicle design in order to improve corrosion resistance. For steel sheets for automobile bodies, rust-preventing steel sheets are mainly used from the viewpoint of reliability for long-term rust prevention, and among the rust-preventing steel sheets, galvanized steel sheets or galvannealed steel sheets are most often used.

Corrosion of an actual vehicle is most noticeable in a steel plate joint portion that is necessarily present in the structure of the vehicle. This is because the electrodeposition coating does not go around in the case of the mating portion and the corrosive environment is harsh because the drying and wetting are repeated more frequently than in the normal portion. The conventional anticorrosive steel sheet for automobiles needs to have a large basis weight in order to secure the corrosion resistance in such a severe part.

By the way, the outline of the process of using a rust preventive steel sheet in an automobile maker consists of a press process, an assembly welding process, a chemical conversion treatment process, and a painting process. Problems such as peeling of plating and deterioration of spot weldability occur.

As described above, the conventional rust preventive steel sheet has a problem that it is necessary to overcome the inconvenience of handling because of the rust preventive property of only a specific portion such as a joint.

Further, even in the chemical conversion treatment step for forming a phosphate film, which is the coating base treatment, the conventional rust-proof steel sheet has the following problems. Usually, anti-corrosion steel sheets for automobiles manufactured by steel manufacturers are treated with anti-corrosion oil,
It will be delivered to the automobile manufacturer after a certain period of time. Despite the presence of anti-rust oil, some surface oxidation is unavoidable depending on the environment before delivery to the automobile manufacturer. Such an oxide film is mainly a zinc oxide and is easily removed in a strong alkaline degreasing step which is usually a pretreatment of the chemical conversion treatment step. However, a chemical conversion treatment failure may occur due to abnormal formation of an oxide after the plating treatment or defective degreasing at an automobile manufacturer. Examples of these include that the activation state of the steel sheet surface is non-uniform and causes uneven treatment, or that the chemical conversion treatment film is unhealthy and sufficient coating film adhesion cannot be obtained. As described above, the currently used rustproof steel sheets for automobiles still have many problems to be overcome.

[0007]

To solve these problems,
For example, JP-A-4-88196 discloses a technique of improving press formability and chemical conversion treatability by coating a zinc-based plating surface with 1 to 500 mg / m ² of P oxide and other oxides. Has been done. This technique is an effective technique for improving press formability, but the chemical conversion treatability is not always good. The feature of this technology lies in the amorphous phosphorous oxide, but since the phosphorous oxide is a film, when this is locally coated on the plating surface, the activity between the coated part and the uncoated part is different. Therefore, uneven chemical conversion treatment is likely to occur. Further, since the amount of P oxide film adhered is as small as 500 mg / m ² or less, almost no contribution to the corrosion resistance is recognized, and the basis weight is increased in order to overcome the corrosion resistance in a severe part such as a joint part. There is a need.

[0008] Further, JP publication 4-507436, electroplating both surfaces 0.7~1.6g / m ^2, the zinc phosphate layer 0.8~1.6g / m ² on both surfaces hot dipping A technique related to the zinc-based plated steel sheet has been disclosed.
This technique has a drawback that since the zinc phosphate layer is present, sparks are likely to occur on the electrode surface during spot welding during automobile assembly, resulting in poor continuous spotting properties. It is considered that this is because the zinc phosphate layer has a large adhered amount and the plating surface is coated with zinc phosphate having low electric conductivity, so that the contact resistance between the electrode tip and the steel sheet becomes high.

The present invention has been made in view of the above problems, and is suitable for automobiles without increasing the weight per unit area, and is a rust-preventing steel plate excellent in corrosion resistance, coating property and weldability of a mating part, and a method for producing the same. The purpose is to provide.

[0010]

SUMMARY OF THE INVENTION The present invention is to solve this problem. After applying zinc-based plating to both sides of a steel sheet,
Adhesion amount of 0.05 g / m ² or more 0.5 on one surface of the zinc-plated steel sheet with the surface not substantially oxidized
Disperse and present a phosphate crystal film of g / m ² or less,
Adhesion amount on the other surface exceeds 0.5 g / m ² and 3.0 g / m ²
It is an anticorrosive steel sheet for automobiles which is excellent in corrosion resistance, paintability, and weldability of a joint part, which is characterized by forming the following phosphate film, and a method for producing the same.

[0011]

In the present invention, first, zinc-based plating is applied to both sides of the steel sheet. The base steel sheet is not particularly specified, and any of ordinary steel sheets for automobiles such as mild steel sheet, high-tensile steel sheet, deep-drawn steel sheet, corrosion-resistant steel sheet, and steel sheet subjected to surface modification treatment can be applied.

Zinc-based plating applied to both surfaces of the steel sheet can be performed by a conventional method, and either hot dipping or electroplating can be performed. The type of plating film is also not particularly specified, zinc plating, or alloy plating of zinc-iron, zinc-nickel, zinc-chromium, zinc-manganese, etc.,
Zinc -SiO _2, none of the dispersion plating, such as zinc -Cr ₂ O ₃ has the effect of the present invention is obtained.

Next, in the present invention, a phosphate treatment is applied to the surface of the zinc-based plated steel sheet, and the amount of adhesion is 0.05 g /
^{2. A} phosphate crystal film consisting of m ² or more and 0.5 g / m ² or less is formed, and the adhered amount exceeds 0.5 g / m ² on the other surface.
A phosphate film of 0 g / m ² or less is formed. The phosphate film formed on the other surface may be crystalline, amorphous, or a mixture of both.

The reason why the phosphate crystal film is formed on one surface of the steel sheet is as follows. In automobiles, there is always a joint portion of steel plates due to its structure. However, in the case of the joint portion, the electrodeposition coating does not go around and the corrosive environment is harsh because it is more frequently dried and wet compared to the normal portion. When phosphate crystals are formed on the surface of the steel sheet in advance, this also effectively acts in the joint portion, the occurrence of the starting point of corrosion is suppressed, and the corrosion resistance is markedly improved as compared with the normal zinc-based plated steel sheet.

When the rustproof steel plate for automobiles is used as a vehicle body outer plate, one surface is always an outer surface. Since the outer surface is coated through the steps of chemical conversion treatment, electro-painting, intermediate coating, and top coating, if chemical conversion treatment unevenness occurs on the rust-preventive steel plate, this directly leads to uneven coating. Further, since the chemical conversion treatment effectively acts on the adhesion of the coating, the deterioration of the chemical conversion treatment also brings about the decrease of the coating adhesion. On the other hand, when phosphate crystals are applied to the surface of the steel sheet in advance, the surface is hard to be oxidized and the surface state is kept uniform. This effect is
This is remarkable in a plated steel sheet such as a Cr alloy plating whose surface is easily oxidized. The treatment applied to the surface must be a phosphate treatment. Phosphate treatment has little adverse effect on the chemical conversion treatment (phosphate treatment) line at automobile manufacturers. However, if other treatments such as chromate are used, the influence of elution of chromium is unavoidable.

At this time, the phosphate formed must be precipitated as crystals. When the adhesion amount is small, since the phosphate is dispersed in a dot shape if it is crystalline, the active sites without the phosphate can be uniformly left. As a result, the phosphate treatment is uniformly formed in the subsequent chemical conversion treatment process at the automobile manufacturer. As a suitable dispersion state of the existing crystals, it is preferable that the crystals are sufficiently dispersed on the plating surface and do not cover the plating surface, and more preferably, only 50% or less of the surface is dispersed.

Next, the deposition amount of phosphate crystals coating on one surface was 0.05 to 0.5 g / m ² formed, to 3.0 g / m exceeds 0.5 g / m ² on the opposite side ^The reason for forming ² will be described.

[0018] First, the surface to be mating portion has a phosphate crystal deposition amount 0.5 g / m ² greater than to 3.0 g / m ^2. If the adhesion amount is 0.5 g / m ² or less, the contribution to the corrosion resistance is small, so it is necessary to exceed 0.5 g / m ² . Further, if the amount of adhesion is increased, the weldability deteriorates, so the upper limit is 3 g / m ² .

Although depending on the type of zinc-based plating, considering the weldability, the amount of phosphate deposited is 0.5 g / m ²
The range over 2.4 g / m ² is particularly good. On the other hand, the weldability can be secured by applying it to the vehicle body part so that the surface on which the phosphate crystal is deposited in an amount of 0.05 to 0.5 g / m ² is on the electrode tip side. Further, by making the surface on which the phosphate crystal deposition amount is 0.05 to 0.5 g / m ^{2 the} outer surface side, stable chemical conversion treatability can be obtained,
Good coatability.

That is, when the plating surface in contact with the electrode tip is completely covered with the phosphate, the electrode is severely damaged and the continuous spotting property is extremely lowered. Therefore, when the relationship between the amount of phosphate crystals deposited and the weldability was changed on the contact surface with the electrode tip and on the opposite side, and was examined, if the amount deposited was 0.5 g / m ² or less, It was revealed that the influence on the weldability was small even if the amount of phosphate adhering on one side was 3 g / m ² at maximum. This is because if the amount of adhering phosphate on the side in contact with the electrode tip is 0.5 g / m ² or less, the crystals do not sufficiently cover the plating surface, so that the current during welding is not hindered. It is believed that there is.
In order to obtain such an effect, the amount of the phosphoric acid crystals attached should be such that they block the oxidation active points on the surface.
g / m ² is sufficient.

According to the present invention, with the above-mentioned constitution, it is possible to obtain a rust-preventive steel sheet for automobiles which is excellent in the corrosion resistance, the coatability and the weldability of the joint portion. This type of rustproof steel plate is subjected to a pressing process, an assembly welding process, a chemical conversion treatment (phosphoric acid treatment) process, and a painting process in an automobile manufacturer. Here, the phosphoric acid treatment is intended to improve the coating property, but the phosphoric acid treatment liquid hardly wraps around the joint portion, and the effect of improving the corrosion resistance cannot be expected. Further, if the plated steel sheet is surface-oxidized in the process up to the phosphoric acid treatment, the uniformity of the phosphoric acid treatment at this automobile manufacturer cannot be expected.
However, since the rust-preventing steel sheet according to the present invention has already been subjected to the above-mentioned treatment, all of these problems are solved.

Whether or not the phosphate film is crystalline is
It can be judged by X-ray analysis. Phosphate ordinary Zn-based plating is comprised of mainly _{Zn 3 (PO 4) 2 ·} 4H 2 O hopeite, when Fe is present in the plating film _{Zn 2 Fe (PO 4) 2} · 4H 2 O phosphophyllite etc. Is formed. If the presence of these crystals is confirmed, it can be said that phosphate crystals are present.

Next, a method for producing a phosphate film will be described. For the formation of the phosphate film, normal immersion type,
Either spray type can be applied. However, the spray type is desirable in order to change the amount of the phosphate film deposited on each surface. This is because it is possible to limit the adhered amount by changing the number of spray nozzles or the amount of spraying. When the immersion type chemical conversion treatment is used, the chemical conversion treatment is controlled by changing the chemical conversion pretreatment on each surface. Specifically, spraying is used as the normal surface adjustment process, and this is adjusted on each surface. Alternatively, a method of providing a surface modification treatment tank after plating and similarly controlling the surface activity of each surface by a spray treatment may be used.

It is necessary that the phosphate treatment step be continuously carried out after the plating production, without passing through steps such as winding and oiling. As a result, a uniform phosphate film can be obtained without causing surface oxidation. This is because in the case of a low deposition amount such as 0.05 g / m ^{2 to} 0.5 g / m ² , in order to stably obtain crystalline phosphate,
The reactivity of the plated surface must be high. For this reason, it is advantageous that the plating surface is not oxidized.

The phosphating solution contains normal zinc phosphate as a main component, and as the cation component, in addition to zinc, Ni ²⁺ , Mn ²⁺ , Ca ⁺ , Na ²⁺ , Fe ²⁺ , NH ₄
⁺ , H ⁺ , as anion components, PO ₄ ³⁻ , NO ₃ ⁻ ,
NO ₂ ⁻ , F ⁻ , SiF ₆ ²⁻ , ClO ₃ ⁻ , SO ₄ ^{2− and the} like may be contained. Further, the effect of the present invention can be obtained even if the organic acid is partially contained.

After production, the plated steel sheet can be subjected to surface activation treatment or inactivation treatment according to a conventional method before reaching the phosphate treatment step. This makes it possible to control the amount of the phosphate film deposited on each surface and the crystal morphology. In particular,
A phosphate film may be formed on the zinc-based plating layer without substantially interposing zinc oxide. In this case, immediately after the formation of the zinc-based composite plating (a simple cleaning process without oiling and degreasing-may occur during the removal of contamination of the plating solution components or during the plate passing process from the plating section to the phosphate processing section. Since phosphate-treated steel sheets can be provided to automobile manufacturers (to the extent that surface contamination is removed), it is possible to prevent oxidation over time, including the steel sheet distribution process, and it is excellent due to the synergistic effect of the plating layer and phosphate coating. Weldability shows excellent corrosion resistance, excellent corrosion resistance at the joint part, and paintability. In this case, prior to the phosphate treatment step, even if the surface activation treatment or deactivation treatment is not performed, the phosphate treatment step can be performed without passing through the steps such as oil coating and winding after plating production. It suffices to prevent oxidation of the plated surface. It should be noted that it is also possible to apply a commonly used surface tone processing.

[0027]

EXAMPLES Examples of the present invention will be described below. 1) Sample preparation The zinc-based plated steel sheet shown in Table 1 was subjected to a phosphate treatment with dipping type and spray type treatment liquids with different amounts of adhesion. The phosphoric acid treatment was performed immediately after the plating film formation. The amount of phosphate adhering was controlled by the immersion time and the spraying amount. However, when the immersion type phosphate treatment liquid is used, one side of the plated steel sheet is sprayed with the following treatment liquid to activate or deactivate it, resulting in a different adhesion from the opposite side. A quantity of phosphating was applied.

The amount of the obtained phosphate crystals adhered was calculated by immersing it in an ammonium dichromate solution to dissolve and remove the phosphate crystals, and measuring the weight method before and after the dissolution. Treatment liquid: Activation ~ 10 g / l sulfuric acid (98%), 10 g
/ L Hydrochloric acid (36%) inactivated to 10 g / l nitric acid (63%), 2) Corrosion resistance The corrosion resistance was evaluated by a sample assuming a steel sheet mating portion. Samples that assume a steel plate mating part are 1)
The above-mentioned 150 × 70 mm plate subjected to chemical conversion treatment in 1 and
Two 00 × 50 mm plates were spot-welded with their phosphate-treated surfaces facing each other, then subjected to chemical conversion treatment and electrodeposition coating. Next, the sample was subjected to a corrosion acceleration test, taken out after a predetermined cycle, disassembled, and the maximum corrosion depth on the plated surface was measured.

In all of the examples of the present invention, the amount of the phosphate treatment adhered exceeds 0.5 g / m ^{2 to} 3.0 g / m ^2.
The surface which became is evaluated as the inside of the seam. Corrosion acceleration test 50 ° C 85% RH → 50 ° C 30% RH → Indoor exposure → 5%
The NaCl water immersion was timed so that one cycle was 24 hours.

Evaluation method Corrosion resistance of mating part Maximum perforation depth after 120 cycles less than 0.1 mm ○ 0.1 or more and less than 0.2 mm △ 0.2 or more and less than 0.4 mm mm × 0.4 mm or more × × 3) Coating Chemical conversion treatment Assuming chemical conversion treatment by an automobile manufacturer, the sample prepared in 1) was first oiled and then subjected to normal immersion type phosphate treatment.

The evaluation in the examples, the surface adhesion amount of the phosphate crystals is ^{0.05g / m 2 ~0.5g / m 2} , was evaluated by the degree of chemical conversion unevenness. In the test, the number of test chances was 5 and the number of test pieces was 10 for each level, and the data of the sample with the worst performance was adopted.

Small unevenness ○ Large unevenness × Coating adhesion The coating adhesion was measured by applying the oil-coated sample prepared in 1) above.
The surface to be evaluated is set to 0.05 g / m ^{2 to} 0.5 g / m ² phosphate adhesion side, and chemical conversion treatment is further performed to simulate the manufacturing process in an automobile, and electrodeposition coating, intermediate coating, and top coating are performed. Was applied. The electrodeposition coating is 20 μm and the middle coating,
The total film thickness of the top coat was 90 μm for all samples.

The sample after coating was placed in pure water at 40 ° C. for 24 hours.
After holding it for 0 hour, immediately after taking it out, scratches are made on a 2 mm square × 100 squares with a cutter. Immediately thereafter, the tape was peeled off, and the number of peeled squares was evaluated.

In the test, the number of test chances was 5 and the number of test pieces was 10 for each level, and the data of the sample with the worst performance was adopted. No tape peeling ○ Less than 5 pieces △ More than 5 pieces × 4) Weldability Electrode: DR type, tip diameter 6 mm, electrode pressure: 200 kgf Initial pressure time: 36 cycles / 60 Hz, energization time: 10 cycles / 60 Hz Welding speed: 1 point / 1 second Welding current: A continuous dotability test was conducted under the condition of 9.5 kA.

The amount of phosphate attached is 0.05 to 0.5 g
/ M ² side is the electrode tip side, 0.5 g / m ² exceeds ~ 3.0
The g / m ² side was the inside. For evaluation, a new electrode is set and spot welding is continuously performed under the above conditions.
The diameter of the nugget formed at every 00 points was measured, and the point at which no nugget was formed was taken as the end point.

5000 points or more ○ ○ 3000 points or more and less than 5000 points ○ 1000 points or more and less than 3000 points △ Less than 1000 points or no electric current × Table shows the test results. Phosphate crystal adhesion amount is 0.05g
The / m ^{2 to} 0.5 g / m ² side was the front side and the 0.5 g / m ² excess to 3.0 g / m ² side was the back side. At this time, the evaluation target surface of each test is as follows.

Phosphate treatment Front side Paintability Front side Corrosion resistance (joint) Create a joint sample with the back side inside Weldability Electrode tip contact side Table 2 From these test results shown in Table 2 to Example, The anticorrosion steel sheet of the present invention shown in Fig. 2 has good corrosion resistance, paintability, and weldability at the joint, but in the conventional technique shown in Comparative Example, one or more of these characteristics must be the present invention. It turns out that it is inferior to the example.

[0038]

EFFECTS OF THE INVENTION By using the rustproof steel sheet for automobiles according to the present invention, stable chemical conversion treatment and paintability can be stably obtained regardless of the condition of the chemical conversion treatment line at the automobile manufacturer.
In addition, since the corrosion resistance and weldability of the mating portion are also good, it is possible to obtain the effect that the productivity of the automobile manufacturer can be improved.

[0039]

[Table 1]

[0040]

[Table 2]

[0041]

[Table 3]

Claims (2)

[Claims] 1. A exist zinc phosphate crystals film comprising on one surface adhesion amount 0.05 g / m ² or more 0.5 g / m ² or less of plated steel sheet are dispersed, the amount adhering to the other surface 0.
An anticorrosive steel sheet for automobiles having excellent corrosion resistance, paintability, and weldability at the joint part, in which a phosphate film of more than 5 g / m ² and 3.0 g / m ² or less is formed. 2. After applying zinc-based plating to both sides of the steel sheet,
Adhesion amount of 0.05 g / m ² or more 0.5 on one surface of the zinc-plated steel sheet with the surface not substantially oxidized
Disperse and present a phosphate crystal film of g / m ² or less,
Adhesion amount on the other surface exceeds 0.5 g / m ² and 3.0 g / m ²
A method for producing an anticorrosive steel sheet for automobiles, which is excellent in corrosion resistance, paintability, and weldability at the joint part, characterized by forming the following phosphate film.