JPH0360919B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing a single-sided highly corrosion-resistant coated steel sheet for automobiles. In recent years, there has been an increasing demand for steel sheets with excellent corrosion resistance as steel sheets for automobile bodies, and for this reason, there is a strong tendency to use surface-treated steel sheets with high corrosion resistance in place of conventionally used cold-rolled steel sheets. The first example of such surface-treated steel sheets is galvanized steel sheets, but with this type of steel sheet, it is necessary to increase the amount of zinc deposited in order to improve corrosion resistance, and along with this, workability and weldability There is a problem of deterioration. In order to improve these problems, zinc alloy plated steel sheets and multi-layer plated steel sheets are being researched and developed to which one or more elements such as Ni, Fe, Mn, Mo, Co, Al, Cr, etc. are added. Compared to the above-mentioned galvanized steel sheet, the steel sheet can have improved corrosion resistance without deteriorating weldability and workability. However, when a steel plate is applied to the bag structure or bent part (hemming part) of an automobile body plate, a high degree of corrosion resistance is required on the surface, and zinc alloy plated steel plates and multilayer plated steel plates as mentioned above are required. Steel plates have a problem in that their corrosion resistance is not quite sufficient. As a steel plate with a high degree of corrosion resistance, special
Research and development has been carried out on anti-corrosion steel sheets coated with zinc-rich coatings, such as the one seen in No. 6882, and the representative one is known as Zinchrome Metal.
However, even with this anti-corrosion coated steel sheet, peeling of the coating may occur in the processed parts such as press forming, and it is still sufficient as a corrosion-resistant and anti-rust coated steel sheet that meets the requirements for automobile body materials, etc. It's hard to say. In addition, a poorly soluble oxide film called temper color is formed on the non-rust-preventing painted surface (steel plate surface) of zinc chrome metal when the zinc-rich coating is baked at a high temperature of approximately 270°C. The reaction of the phosphate treatment carried out on the house side is poor, and an uneven phosphate film is formed, resulting in a deterioration of quality performance after painting. In particular, Si and Mn are included in the steel components.
In the case of a high-strength steel sheet containing more of Si, Mn, etc. than a normal mild steel sheet, a selective oxidation film is formed due to Si, Mn, etc., and this tendency becomes noticeable. For this reason, for example, JP-A No. 56-152768 proposes a method in which a specific elemental compound is applied to a non-corrosive painted surface before the paint baking treatment to prevent temper color. Adoption of this method not only complicates the manufacturing process, but also causes problems such as the specific elements applied actually inhibiting chemical conversion treatment properties, so it cannot be said to be a complete measure against temper color. The non-corrosion coated surface of steel plates is used on the exterior of automobile bodies, and a high level of surface appearance and paint quality is required after 3-coat, 3-bake painting.In this sense, the above-mentioned temper color measures are essential. It can be said that it is a thing. The present invention has been researched and developed in view of the shortcomings of the prior art, and provides good chemical conversion treatment and paintability without producing temper color on the uncoated surface, as well as excellent corrosion resistance on one side. It is an object of the present invention to provide a method capable of manufacturing a single-sided coated steel sheet for automobiles having high corrosion resistance. For this reason, the present invention applies zinc alloy plating to one side of a steel sheet to form a zinc alloy plating film with an adhesion amount of 1 g/m ² or more, and then applies chromate treatment to only the plated surface to reduce the adhesion amount. 1000
A chromate film of 0.5 to 4.0 g/m ² is formed on the chromate film by treatment with an organic composite silicate solution ^, and then a chromate film of 100 to 250 ℃
The steel plate is heat-treated at a temperature of 250°C, which makes it possible to obtain a film structure with excellent corrosion resistance on one side.In addition, since the heat treatment is carried out at a temperature below 250°C, the tempering of the uncoated surface of the steel plate due to high-temperature heating is reduced. It is possible to appropriately prevent color generation and obtain good chemical conversion treatment properties and paintability. The applicant had previously filed a patent application No. 55-182112 (Japanese Unexamined Patent Publication No. 57-182) for a composite coated steel sheet in which a two-layer film of a chromate film and an organic composite silicate film was formed on the surface of an alloy-plated steel sheet using zinc as the base metal. 108292), and this composite coated steel sheet has excellent corrosion resistance. The present invention focuses on this composite coating, and by applying it to one side of a steel plate and combining it with appropriate heat treatment, higher corrosion resistance can be obtained on the coated side, and excellent chemical conversion treatment properties can be obtained on the uncoated side. This makes it possible to manufacture steel sheets that are paintable. In the present invention, only one side of the steel plate is subjected to (1) zinc alloy plating, (2) coating-type chromate treatment, (3) further treatment with an organic composite silicate solution, and (4) subsequent heating at 100 to 250°C. heat-treated to obtain a finished product. In the present invention, zinc alloy plating is first applied to only one side of a steel plate (cold rolled steel plate). In this way, the reason why we use alloy plating that uses zinc as a base metal is that this type of plating is less prone to corrosion than normal zinc plating, and is also less susceptible to corrosion due to the chromate film and organic composite silicate film that are formed on it. This is because high corrosion resistance can be exhibited due to the synergistic effect of. The coating weight of this zinc alloy plating is 1g/m ²
If it is less than this, the corrosion resistance will deteriorate. Furthermore, even if the amount of plating exceeds 300 g/m ² , no significant improvement in corrosion resistance can be expected, and on the contrary, the cost will increase. The amount of plating deposited is usually 1 to 60g/
m ² , preferably in the range of 5 to 40 g/m ² . This zinc alloy plating includes zinc-iron alloy plating, zinc-nickel alloy plating, zinc-manganese alloy plating, zinc-aluminum alloy plating, and zinc-aluminum alloy plating.
Cobalt-chromium alloy plating, furthermore, these optional plating components include Ni, Fe, Mn, Mo, Co,
One or more types of elements such as Al and Cr may be added, and furthermore, it may be a composite plating in which two or more of the same or different types of platings as described above are applied. For example, two or more layers of Fe-Zn alloy plating with different Fe contents can be applied. As for the plating method for these zinc-based alloy platings, any practicable method such as electrolytic method, melting method, vapor phase method, etc. can be adopted, but in general, electroplating is advantageous due to the ease of single-sided plating. . In the present invention, the above plated surface is subjected to chromate treatment to form a chromate film. This chromate film has a chromium deposit (dry)
It needs to be 1000mg/m2 or ^less ; if it exceeds 1000mg/ ^m2 , the chromate film itself will peel off, which will cause the film to peel off during pressing, resulting in poor workability and poor weldability. . However, if the amount of chromium deposited is less than 1 mg/m ² , the film will be non-uniform, which is not preferable. The preferred amount of chromate film is 10
~200mg/m ² (the above is converted into metallic chromium). There are three types of methods for forming such a chromate film: a coating type, an electrolytic type, and a reaction type. In the present invention, it is necessary to form a chromate film by a coating type chromate treatment. Reactive chromate treatment is difficult to treat on one side of a steel plate, and therefore is not applicable to the present invention. The coating-type chromate treatment liquid has a partially cyclic chromic acid solution as its main component, and if necessary, it is coated with an organic resin such as water-dispersible or water-soluble acrylic resin and/or tens to thousands of Å. It contains silica particles (silica sol, fumed silica). In this case, the ratio of Cr ³⁺ /Cr ⁶⁺ is preferably 1/1 to 1/3, and the pH is preferably 1.5 to 4.0 (more preferably 2 to 3). The ratio of Cr ³⁺ /Cr ⁶⁺ is adjusted to a predetermined ratio using a general organic reducing agent (eg, sugar, alcohol, etc.) or an inorganic reducing agent. Further, as the coating type chromate treatment, any method such as a roll coater method, a dipping method, a spray method, etc. may be used, but the roll coater method is advantageous since it is necessary to perform the chromate treatment on only one side.
In coating type chromate treatment, a film is obtained by drying without washing with water after the chromate treatment. The reason for drying without washing with water is that Cr ⁶⁺ is removed by normal washing with water, so the Cr ³⁺ /Cr ⁶⁺ ratio remains stable and the organic composite silicate is used in the next process. This is for sealing by treatment with a solution. On the other hand, in electrolytic chromate treatment, cathodic electrolysis is performed in a bath containing chromic anhydride and one or more anions such as sulfuric acid, fluoride, or halogen acids, and a film is formed by washing with water and drying. urge Comparing the chromate films produced by the above two treatment methods, the coated chromate film contains more hexavalent chromium in the film than the electrolytic chromate film, so it has superior corrosion resistance. When heat-treated at a relatively high temperature, the film becomes dense and strong, resulting in better corrosion resistance. In the present invention, it is important to perform chromate treatment on only one side and avoid adhering chromium to the other side. If even a small amount of chromium adheres to this other surface, the reactivity of the chemical conversion treatment will be poor and the quality of the coating will deteriorate. Following the above chromate treatment, the plated surface is similarly treated with an organic composite silicate solution on one side, and an organic composite silicate film is formed on the chromate film. The coating weight (dry) of this organic composite silicate needs to be selected in the range of 0.5g/m ² to 4.0g/m ² , more preferably
A range of 1.0 g/m ² to 3.0 g/m ² is suitable. If the amount of adhesion is less than 0.5g/ ^m2 , sufficient corrosion resistance cannot be obtained.
Moreover, if it exceeds 4.0 g/m ² , spot weldability deteriorates, which is not preferable. The organic composite silicate has water-dispersible silica as an essential component, and a water-soluble or water-dispersible organic polymer resin is mixed therein in the presence of a silane compound to produce a product at a temperature of 10°C or higher and below the boiling point, preferably 50 to 90°C. It can be obtained by reacting at a temperature range of . The water-dispersible silica is so-called silica sol or colloidal silica and has a particle diameter of several tens of angstroms to several thousand angstroms.
As water-soluble or water-dispersible organic polymer resins,
Examples include polyvinyl alcohol, hydroxyethyl cellulose, polyester, alkyd, epoxy, and acrylic copolymers, but any resin may be used as long as it reacts with silica. However, when attaching importance to the adhesion of a cationic electrodeposition paint (based on epoxy resin), which is an undercoat for automobiles, epoxy resin is preferable, and it is preferable to contain at least 30 parts per 100 parts of organic resin. Further, the silane compound is used as a reaction accelerator when silica and organic resin are combined. As this silane compound, a commercially available silane coupling agent may be used.
For example, vinyltriethoxysilane, vinyltris(β-methoxyethoxy)silane, vinyltris(β-methoxyethoxy)silane, γ-glycidoxypropyltrimethoxysilane, γ-methacryloxypropyltrimethoxysilane, N=β
Examples include trialkoxysilane compounds such as (aminoethyl)-γ-aminopropyltrimethoxysilane and γ-aminopropyltriethoxysilane. The mixing ratio of water-dispersible silica and water-soluble or water-dispersible organic resin in the organic composite silicate is 5:95 to 95:5, preferably 5:95 to 95:5 in terms of solid weight percentage.
The hours will be 10:90-60:40. The addition ratio of the silane compound is 0.5 to 15 wt% based on the total solid weight of silica and organic resin. The organic composite silicates obtained as described above may be used alone or in combination of two or more. Furthermore, molybdenum, tungsten, or vanadium oxygen acids or salts thereof, or titanium or zirconium alkoxide oxylate compounds may be added. Corrosion resistance can be improved by adding one or more of these additives to 14 wt% or less, preferably 0.2 to 8 wt%, based on the total solid content of the silica sol and organic resin. Further, if a curing agent such as melamine is added to the organic composite silicate solution, a greater effect can be obtained. After forming the chromate film and the organic composite silicate film as described above, in the present invention, heat treatment is performed at a surface plate temperature of 100 to 250°C. In the present invention, the corrosion resistance of the coated surface is further improved by heating in such a temperature range, and since the effect of improving corrosion resistance by such heating can be achieved in a temperature range of 250°C or less, the occurrence of temper color on the uncoated surface is prevented. can be appropriately prevented. If the heating temperature exceeds 250℃,
Corrosion resistance may deteriorate due to cracks in the chromate film or a decrease in soluble Cr ⁶⁺ , which has a passivating effect.
is the upper limit of the heating temperature. Heat treatment must be performed at a temperature of 100°C or higher from the viewpoint of corrosion resistance. The heating method may be any method such as hot air drying, infrared heating, induction heater, etc. as long as a predetermined temperature is obtained. Further, the heating holding time is about several seconds to several minutes, and holding for a long time is not only economically disadvantageous but also undesirable because there is a risk that the film performance will deteriorate. The purpose of this heat treatment is first to thermally cure the film by the combination of silica sol/silica sol, silica sol/organic polymer, and organic polymer/organic polymer in the organic composite silicate component. For example, silica sol has the property of forming a dry film as water evaporates even at room temperature, but when heated, the silanol groups of silica sol

[Formula] loxane bond

During the formation of [Formula], it is estimated that a more dense film is formed due to the condensation reaction between the silanol group and the hydroxyl group of the organic resin component. The second purpose of heat treatment is to reduce Cr ⁶⁺ on the surface layer of the chromate film.
reacts with polar groups such as hydroxyl groups and carboxyl groups in the organic composite silicate film to strengthen the bond between the two layers. Furthermore, the third purpose of the heat treatment is to improve the corrosion resistance by densifying the underlying chromate film. As described above, the steel sheet manufactured by the method of the present invention has a zinc alloy plating as a base plate to prevent corrosion of the material itself as much as possible, and a coated chromate film is formed on it as a base layer. By improving corrosion resistance and further sealing with an organic composite silicate film as a top layer, it is possible to minimize the elution of Cr ⁶⁺ , which has a passivating effect in corrosive environments, and exhibits excellent corrosion resistance. can also be
The organic composite silicate film itself provides high corrosion resistance due to its excellent barrier effect and paint adhesion effect due to its organic components, and as a whole provides excellent unpainted corrosion resistance and moderate paint adhesion. On the other hand, the non-coated surface (steel plate surface) is coated with chromate, which is easy to treat on one side, so there is almost no chromium adhesion, and since the heat treatment temperature is less than 250℃, there is no temper color and no phosphorescence. It is possible to obtain steel sheet surface properties with good acid treatment properties and paintability. Next, examples of the present invention will be described. Example 1 Corrosion resistance tests were conducted on the steel sheets of the present invention for use in automobile bodies, with different plating components, coating amounts, and heat treatment temperatures as shown in Table 1. Similar tests were also conducted on each steel plate shown in Table 2 as comparative materials. The plating components of each steel plate are as follows.
Regarding each steel plate shown in the table and each steel plate with a chromate film and an organic composite silicate film in Table 2, the plated steel plate is degreased with alkaline, then washed with water.
After drying, apply a coating type chromate treatment liquid using a roll coater or immerse it in an electrolytic chromate treatment bath to form an electrode chromate film by cathodic electrolysis.After drying, apply an organic composite silicate treatment liquid as a second layer using a roll coater. Coated. Furthermore, after drying, it was heat-treated, and after being left for 10 days, a corrosion resistance test was conducted. Ni-Zn alloy electroplating...Ni content 12% Fe-Zn alloy electroplating...Fe content 25% Mn-Zn alloy electroplating...Mn content 60% Paint type chromate treatment liquid, electrolytic chromate conditions The details of the organic composite silicate treatment solution and corrosion resistance test are as follows. Γ coating type chromate treatment conditions Cr ³⁺ /Cr ⁶⁺ = 2/3, PH = 2.5 (PH adjustment with KOH),
A chromate treatment solution with a solid content of 20 g was applied at room temperature using a roll coater and then dried. Γ electrolytic chromate treatment conditions CrO ₃ : 50g/, H ₂ SO ₄ : 0.5g/, bath temperature
A current density of 0.1 to 20 A/dm ² using a 50°C bath.
Cathode electrolysis treatment with an electrolysis time of 0.5 to 30 seconds, washing with water,
A film was obtained by drying. [Conditions] Current density Electrolysis time Chromium deposition amount 0.1A/dm ² 2.0sec 0.5mg/m ² 4.9 〃 0.5 〃 10.0 〃 4.9A/dm ² 2.0sec 40 mg/m ² 9.6 〃 4.0 〃 150 〃 20.0 〃 30.0〃 1500 〃 In some comparative examples, reactive chromate treatment was performed, but the treatment was performed using CrO ₃ :15g/,
Cr ⁶⁺ /Cr ³⁺ : 3, Zn ²⁺ : 3g/, H ₂ SO ₄ :
With a reactive chromate treatment solution containing 9g/component,
The treatment was carried out at 50°C for 5 seconds and with a spray of 1 atm. Γ Organic composite silicate treatment liquid Acrylic composite silicate and epoxy composite silicate (organic resin: silica sol = 60:40, respectively) synthesized as follows were mixed at a ratio of 70:30 to prepare an organic composite silicate treatment liquid (PH :95,
Solid content 20%) A Synthesis of acrylic composite silicate Put 180 parts of isopropyl alcohol into a four-necked flask equipped with a thermometer, stirrer, condenser, and dropping funnel, and after purging with nitrogen, lower the temperature inside the flask to about 85%. ℃, 140 parts of ethyl acrylate, 68 parts of methyl methacrylate,
A monomer mixture consisting of 15 parts of styrene, 15 parts of N-n-butoxymethylacrylamide, 38 parts of 2-hydroxyethyl acrylate, and 24 parts of acrylic acid was mixed with 6 parts of 2,2'-azobis(2,4-dimethylclero-tolyl). It takes about 2 hours to drop the mixture together with a catalyst consisting of 1.0 parts. After the dropwise addition, the reaction was continued for another 5 hours at the same temperature, and the solid content decreased to 63
%, and a colorless and transparent resin solution with an acid value of 67 was obtained. For 500 parts of this acrylic copolymer resin solution
Mix 45 parts of 38% ammonia water, add water and stir thoroughly to reduce solid content to 20% and pH
An aqueous dispersion of acrylic copolymer No. 9.5 was obtained.
Pour 300 parts of this aqueous dispersion into a flask,
Add a predetermined amount of colloidal silica (manufactured by Nissan Chemical Co., Ltd., trade name "Stotex N") while stirring thoroughly at room temperature, and then add γ-methacryloxypropyltrimethoxysilane (manufactured by Shin-Etsu Chemical Co., Ltd., 1 part of the product (trade name: "KBM503") was added dropwise and mixed under stirring, and then heated to 85°C and kept at the same temperature for 2 hours to react, to obtain a milky white, water-dispersible acrylic composite silicate. B Synthesis of epoxy composite silicate Bisphenol A with epoxy equivalent weight 950
310 parts of type epoxy resin (manufactured by Ciel Kagaku Co., Ltd., trade name ``Epicote 10004''), 95 parts of linseed oil fatty acid, 95 parts of tung oil fatty acid, and 15 parts of xylene were placed in a flask, and heated gradually while passing nitrogen through the flask. After raising to 70°C, cool
When the temperature has dropped to ℃, add 200 parts of ethylene glycol monoethyl ether to make the solid content 70%.
An oil-modified epoxy resin solution with oxidation level 54 was obtained. An epoxy composite silicate was obtained from this oil-modified epoxy resin solution in the same manner as in the case of A above. Γ Corrosion resistance test → ｜ ｜ ｜ ｜ ｜ ｜ ｜ -5%NaCl immersion 40℃ 30 minutes ↓ 95%RH humidity 50℃ 60 minutes ↓ Drying 60℃ 30 minutes (including transition time) Cycle test with the above as one cycle The red rust occurrence area of each sample material was evaluated at 250, 500, and 1000 cycles. The results of the corrosion resistance test are shown in the right column of each table. As is clear from these tables, it can be seen that the one-sided film structure of the material of the present invention has superior corrosion resistance compared to conventional zinc metal or alloy plating.
Furthermore, as is clear from the comparative materials in Table 2, within the range that deviates from the conditions of the present invention, there are problems in one or more of corrosion resistance, spot weldability, and formability (film peeling during pressing, etc.). It can be said that it is unsuitable as a rust-proof steel plate for automobile bodies.

【table】

[Table] * Dacromet (300mg/m ² of chromium deposited) is applied as the first layer on the cold-rolled steel plate.
After heating to 170℃, apply Zinchromet (film thickness 2μ) as a second layer and bake at 270℃. Corrosion resistance evaluation criteria ◎… Red rust occurrence area 0-2% 〇… 〃 3-10% △… 〃 11-50% ×… 〃 51-100% Example 2 This example shows the surface texture of the non-coated surface (steel plate surface) This study investigated the occurrence of temper color on the non-coated surface and the chemical conversion treatment of test materials manufactured by applying chromate treatment and organic composite silicate treatment to the plated surface of various single-sided zinc alloy plated steel sheets, and then heat treating them. I looked into gender. The results are shown in Table 3 together with comparative materials. The investigation of chemical conversion properties regarding phosphate treatment was conducted in the following manner. Test material → Degreasing (Nippon Parkerizing Co., Ltd. "FC4357" 28g/, 55
～60℃, 120sec dip)→Water washing→Surface conditioning (“RL4031” manufactured by Nippon Parkerizing Co., Ltd. 1g/, 10sec
spray) → Phosphate treatment (“Bt 3004” manufactured by Nippon Parkerizing Co., Ltd. TA22 points, FAI 1 point, AC1.5-2.0, 48-52℃, 120sec dip) → Water washing → Pure water washing → Drying → Scanning of crystal appearance Observe with an electron microscope (X1000-2000x). As is clear from Table 3, it can be seen that the uncoated surface of the steel sheet obtained by the method of the present invention exhibits a surface quality that is the same as that of a normal cold-rolled steel sheet. On the other hand, other steel sheets, including zinc chrome metal, which are heat-treated at temperatures exceeding the upper limit of the heating temperature of the present invention, have problems such as generation of temper color and deterioration of phosphate treatability.

【table】

[Table] 〓 ×… coarse 〓

Claims (1)

[Claims] 1 Zinc alloy plating is applied to one side of the steel plate and the adhesion amount is 1.
After forming a zinc alloy plating film with a coating weight of 1,000 mg/m ² or more, a coating-type chromate treatment is applied only to the plating surface to form a chromate film with a coating amount of 1000 mg/m ² , and then an organic composite silicate solution is applied. The amount of adhesion on the chromate film is
A method for manufacturing a single-sided highly corrosion-resistant coated steel sheet for automobiles, which comprises forming an organic composite silicate film of 0.5 to 4.0 g/m ² and then heat-treating the steel sheet at 100 to 250°C.