CA1256054A - Process for preparation of highly anticorrosive surface-treated steel plate - Google Patents

Abstract

ABSTRACT

The present invention relates to a process for the preparation of a highly anticorrosive surface-treated steel plate. This process comprises the steps of subjecting a surface of a steel plate having a plating layer of the zinc or aluminum type deposited thereon to a chromate treatment to form a chromate film, treating the steel plate with an organic composite silica solution comprising an epoxy resin in an amount exceeding a certain level as an indispensable component and a curing agent optionally incorporated therein to form an organic composite silicate film on the chromate film, and heat-treating the steel plate at a specific temperature. As the zinc plating method, there may be adopted an electro-lytic plating, a melting or gas plating method. For depositing zinc alloys, the electrolytic or melting methods are ordinarily used.

Description

~Z5~V54 The present invention relates to a highly anticor-rosive surface-treated steel plate, especially a rust-proof steel plate suitable for a car body, and to a process for its preparation.
~ecently, the demand for steel plate excellent in corrosion resistance as a steel plate for a car body has been increasing, and there is observed a strong tendency to use a highly anticorrosive surface-treated steel plate instead of a cold-rolled steel plate heretofore used.
As this surface-treated steel plate, there can be mentioned a zinc-deposited steel plate. However, in the case of a steel plate of this type, in order to increase the corrosion resistance, it is necessary to increase the amount of zinc deposited and this increase results in degradation of the processability and weldability. In order to eliminate this defect, there has been proposed a steel having a deposit of an alloy of zinc and at least one element selected from Ni, Fe, Mn, Mo, Co, Al and Cr and a mLltilayer deposit on a steel plate.
These steel plates are advantageous as compared with zinc-deposited steel plate, in that the corrosion resistance can be improved without degradation of the weldability and process-abllity. However, when these steel plates are applied ~o a bag structure portion or bent portion (hemmed portion) of an inner plate of a car body, for which a high corrosion resistance is required, the corrosion resistance is still insufficient. As ' ~

~256~)~4 steel plate having a high corrosion resistance, there has been developed a rust-proof steel plate having a zinc-rich coating, as disclosed in Japanese Patent Publication No. 24230/70 (published Augus-t 13, 1970;
Applicant: Diamond Samlock Inc.) and No. 6882/72 (published February 28, 1972; Applicant: Diamond Samlock Inc.), and as a typical instance of the rust-proof steel plate, there is known ZINCHROMETAL (trade mark). However, even in the case of this rust-proof coated steel plate, peeling of the coating film sometimes occurs in the portion subjected to processing such as pressing, and this steel plate is still insufficient as a highly anticorrosive rust-proof coated steel plate satisfying requirements for the material of a car body.
As means for eliminating the foregoing defects, there has been proposed a composite coated steel plate comprising a steel plate deposited with a plating layer of the zinc type as a substrate and a double coating layer of a chromate film and an organic composite silicate film formed thereon and a process for the preparation of this composite coated steel plate in Japanese Laid-Open Patent Specifications No. 108292/82 (published July 6, 1982; Applicant: Nippon Kokan K.K.) and No.
224174/83 (published December 26, 1983; Applicant: Nippon Kokan K.K.). This composite coated steel plate is excellent as compared with conventional surface-treated steel plates in corrosion resistance and processability.
However, if this steel plate is used for a car body, the adhesion to cationic electro-deposition paint is poor and the corrosion resistance of the coating is not satisfactory.
The inner surfaces of many parts of a car body such as the door and the fender are ordinarily coated with a cationic ~' .

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electrdepoSitiOn paint, and a topcoat is applied onto the - cationic electrodepsoition coating on the outer or inner surfaces of the car body, for example, on the inner surface of a hood or the like. The adhesion to such a multiple coating sh should satisfy conditions severer than those required for the adhesion to tne single cationic electrodeposition coating, and these conditions cannot be satisfied by the conventional com-posite coated steel plate.
In connection with the outer surface of a car body having a composite coating of cationic electrodeposition coating/
middle coating/top coating J the coating finish (appearance) and the adhesion to the coating are considered more important than the corrosion resistance. For this reason; cold-rolled steel plates have ordinarily been used. Recently, however, it has been required to improve the corTosion resistance of the coating for the outer surface of the car body, and this tendency is conspicuouS in Canada and the United States. A steel plate having a zinc or zinc alloy plating is suitable for improvement of the corrosion resistance of the coating. However, the steel plate of this type is inferior to a cold-rolled steel plate in the coating finish ~cratering resistance: formation of pinhole-like defects on the cationic electrodeposition coating is called "cratering phenomenon", the cause of which is not clarifiedJ this cratering phenomenon occurs on a steel plate having deposited on it a zinc or a zinc alloy and this cratering phenomenon is observed also after middle coatlng-top coating 56~S4 and has a serious influence on the coating finish) and the adhesion to the coating [water-resistant adhesion: this pro-perty is evaluated at the test of the coating adhesion under wet conditions where a sample coa~ed even with a topcoat paint is immersed in pure water (deionized water) for a predetermined time, suitably about 5 to about 10 days, 100 cross cuts extending to the substrate ~steel~:and having sides of 1 to 2 mm are formed on the coating, an adhesive tape is applied to the cross cuts and peeled and the number of cross cuts peeled together witn the adhesive tape is counted; the cold-rolled steel plate is excellent in water-resistance adhesion and the steel plate plated with zinc or a zinc alloy is inferio~ .
Accordingly, practical application of the steel plate deposited with zinc or a zinc alloy is difficult.
As the rust-proof steel plate for a car body, there has been used a steel plate having one surface plated and a steel plate having a rust-proof coating formed on one surface (in each steel plate, the other surface is the surface of the cold-rolled steel plate). However, these steel plates fail to satisfy the above-mentioned requirement for corrosion resis-tance at the inner and outer surfaces of a car body. Accordingly, development of a highly anticorrosive rust-proof steel plate having basic properties required for the inner and outer sur-faces ~corrosion resistance, processability and spot weldability for the inner surface and coating finish property, coating adhesion, corrosion resistance of the coating, processability and spot weldability for the outer surface)has been desired.

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~25~ 4 .The present invention has been completed as the result of researches made having.regard to this background;
in particular the invention sGeks to provide a process for the preparation of a highly anticorrosive surface-treated steel plate which is excellent in corrosion resistance and process-ability and which has good adhesion to a coating, especially a cationic electrodeposition paint, and enjoys a high corrosion resistance of the coating.
The invention also seeks to provide a process for the preparation or a highly anticorrosive surface-treated steel Plate which has a good adhesion to a multiple coating of at least two coatings, which adhesion is to be higher than that to a cationic elec.rodeposition coating, while maintaining high corrosion resistance and processability.
Still further the invention seeks to provide a process for the preparation of a highly anticorrosive surface-treated stell plate which suitably satisfies the requirements of basic properties for the inner and outer surfaces of a car body.
According to the present invention, there is provided a process wnich comprises subjecting the sur~ace of a steel plate having a zinc type plating or aluminum type plating layer, to a chromate treatment to for~ a chromate film, treating the steel plate with an organic composite silicate s solution containing an epoxy resin at a concentration of at least 15% by weight based on the total solids to form an organic 3L256~S4 ,~

composite silicate film in an amount deposited of 0.5 to 4.0 g/m on the chromate film, and heat-treating the steel plate at a temperature of 100 to 300C.
The organic composite silicate referred to herein comprises an organic resin, a silica sol and a silane compound;
and more especially a colloidal silica, the organic resin and the silane compound; more particularly the composite silicate resin film consists essentially of a colloidal silica, an organic resin and a silane compound.
In accordance with another aspect of the invention there is provided a highly anticorrosive surface-treated steel plate comprising: a steel plate having a plating layer of zince or aluminum type, a chromate film, and an organic composite silicate film on said chromate film, said composite film containing an epoxy resin in an amount of at least 15%, by weight, based on thc weight of *he composite film, said plate with said film thereon having been heat treated at an elevated temperature.
In the case where a severer adhesion to the coating, that is, a higher adhesion to a multiple coating of at least two coating layers, is required, the treatment is carried out with an organic composite silicate solution having an epoxy resin concentration of at least 26%, by weight, based on the total solids and an SiO2/resin ratio of from 10/90 to 60/40 and the heat treatment is then carried out at 250 to 300C.

-` 1256VS4 A curing agent such as melamine may be added to the organic composite silicate solution. If the curing agent is used, the upper limit of the heat treatment temperature can be elevated to 350C and hence, the lower limit of the amount deposited of the organic composite silicate film can be lowered to 0.4 g/m2. Addition of the curing agent is es-pecially effective when a coated steel plate is prepared under the preparation conditions meeting the requirement of the adhesion to the coating, that is, under the conditions where the treatment is carried out with an organic composite silicate solution having an epoxy resin concentration of at least 26%, by weight, based on the total solids and an SiO2/resin ratio of from 10/90 to 60/40 and the heat treatment is then carried out at a temperature of 250 to 300 C.
.In the surface-treated steel plate prepared according to the above-mentioned process, a high corrosion resistance is obtained by chromate and organic composite silicate films densified by the heat treatment, and by the presence of an epoxy resin contained in an amount exceeding a certain level as an indis~ensable component in the organic composite silicate and also by the effect of the heat treatment, an excellent adhesion to a coating, especially a cationic electrodeposition coating, can be obtained.
Especially when the above-mentioned severer prepara-tion conditions are adopted, the above-mentioned effects are 1256~S4 enhanced and hence, the adhesion to a multiple coating of at least two coating layers is highly improved.
When a curing agent is added to the organic composite silicate solution, the heat decomposition temperature of the organic composite silicate film at lZ56~4 the heat treatment is shifted to a higher temperature side, and the heat treatment at a higher temperature becomes possible. If the heat treatment is carried out at a higher temperature, the crosslinking reaction of the organic composite silicate is promoted, with the result that the corrosion resistance and the adhesion to the coating are further imprcved.
When the treatment of the present invention is conducted only on one surface of a steel plate, and a plating layer of the zinc type or zinc alloy type (Zn-Fe, Zn-Ni, Zn-Mn, Zn-al or the like) is kept on the other surface, the basic properties required for the inner and outer surfaces of a car body are suffi-ciently manifested, and especially when a plating layer of the iron type or the iron-zinc alloy type having an iron content of at least 50% by weight is deposited on a plating layer of the zinc type on the other surface corresponding to the outer surface of a car body, there can be obtained a surface-treated steel plate in which the requirements of the basic pro-perties for the inner and outer surfaces of a car body are more suitably satisfied.
Figs. l-(A) through l-(C) are charts of an X-ray microanalyzer obtained when cationic electrodeposition coating-peeled portions are inspected.
Fig. 2 is a diagram illlustrating the relation between the temperature for heating the organic com-posite silicate and the ratio of the weight loss by heating.
The present invention will now be described in detail.
As pointed outhereinbefore, in case of a car body, the coating on the inner surfaces of relatively many parts such as a door and a fender is composed solely of a cationic electrodeposition coating layer, and on the outer surface of a car body and the inner surfaces of such members as a hood, a double or triple coating _ 9 _ ~Z56~5~

including another coating layer formed on the cationic electrodeposition coating layer is ordinarily formed.
The gist of the present invention resides in a process for the preparation of a surface-treated steel plate which is excellent in not only the corrosion resistance and processability but also the adhesion to the cationic electrodeposition coating. The treatment to be applied to a single layer of the cationic electro-deposition coating, however, is insufficientfor a multiple coating including a cationic electrodeposition coating layer, and severer requirements should be satisfied. The present invention provides a prepara-tion process which is conducted under such severe conditions as will produce an excellent adhesion to a multiple coating including at least two layers.
In accordance with the fundamental aspect of the present invention, there is provided a process for the preparation of a highly anticorrosive surface-treated steel plate, which comprises subjecting a plated sur-face of a steel plate having a plating layer of the zinc or aluminum type as a substrate to the follow-ing treatments:
(a) subjecting the steel plate to a chromate treatment to form a chromate film, (b) treating the steel plate with an organic com-posite silicate solution containing an epoxy resin in an amount of at least 15% by weight based on the total solids, in which a curing agent may be incorporated according to need, to form an organic composite sili-cate film in an amount deposited of 0.5 to 4.0 g/m2 or 0.4 to 4.0 g/m2 when the curing agent is incorpo-rated, on the chromate film, and (c) heat-treating the steel plate at a temperature of 100 to 300C or a temperature of 100 to 350C when the curing agent is incorporated in the organic com-posite silicate solution.
In the case where severer adhesion conditions 1256~S4 are required, that is, an excellent adhesion to a multiple coating of at least two coating layers is desired, a plated surface of a steel plate having a plating layer of the zinc or aluminum type as a sub-strate to the following treatments under restricted conditions:
(i) subjecting the steel plate to a chromate treatment to form a chromate film, (ii) treating the steel plate with an organic composite silicate solution containing an epoxy resin in an amount of at least 26% by weight based on the total solids and having an SiO2/resin ratio of from 10/90 to 60/40, in which a curing agent may be incor-porated according to need, to form an organic compo-site silicate film in an amount deposited of 0.5 to 4.0 g/m or 0.4 to 4.0 g/m when the curing agent is incorporated, on the chromate film, and (iii) heat-treating the steel plate at a temper-ature of 250 to 300C or 250 to 350Cwhen the curing agent is incorporated in the organic composite sili-cate.
As the steel plate having a plating layer of the zinc type as the substrate, there may be used a zinc-deposited steel plate, a zinc-iron alloy-deposited steel plate, a zinc-nickel alloy-deposited steel plate, a zinc-manganese alloy-deposited steel plate, a zinc-aluminum alloy-deposited steel plate and a zinc-cobalt-chromium alloy-deposited steel plate. Furthermore, at least one element selected from Ni, Fe, Mn, Mo, Co, Al and Cr may be incorporated into the above-mentioned metals to be deposited. Moreover, a composite depo-sited steel plate having at least two same or different plating layers selected from the above-mentioned plating layers may be used.
As the plating means for obtaining the steel having a plating layer of the zinc type, there may be ~256~5~

adopted an electrolytic plating method, a melting method and a gas phase plating method. The rust-proof steel plate intended in the present invention is mainly used for a car body and in this field, it is important that the properties of the cold-rolled steel plate to be platedshould not be impaired. In viel~ of this importance, it is preferred that the electric plating method in which no heat is generated be adopted.
In the present invention, the plating layer de-posited on the substrate steel plate has great influ-ences on the corrosion resistance of the steel plate.
Among the above-mentioned plating layers, the steel plate deposited with zinc alone is relatively poor in the corrosion resistance, and the steel plates de-posited with zinc alloys, especially Ni-Zn and Zn-Mn alloys, and these alloys containing incorporated there-in, at least one plating-element as described above are excellent in the corrosion resistance and attain-ment of a highly improved corrosion resistance can be expected by use of these deposited steel plates as the substrate.
These alloy deposition layers may be formed under conditions described below. Ni-Zn alloy deposition is ordinarily carried out acording to the electrolytic plating method, and the content of Ni in the alloy to be plated is ordinarily 1 to 90% by weight, preferably 5 to 30% by weight. Fe-Zn alloy deposition is ordinarily carried out according to the electrolytic plating or melting method, and the Fe content in the alloy to be plated is ordinarily 1 to 70% by weight and an Fe content of 5 to 35% by weight is preferred from the viewpoint of the corrosion resistance. Zn-Mn alloy deposition is carried out according to the electrolytic method, and the Mn content in the alloy to be deposited is ordinarily 20 to 90% by wcight and preferably 30 to 85% by weight. Zn-al alloy deposition is ordinarily carried out according to the melting method, and the Al content in the alloy to be plated is 2 to 60% by 12560S~

weight. In case of Zn-Co-Cr alloy deposition, the Co and Cr contents in the alloy to be plated are 0.01 to 15% by weight and 0.01 to 1~ by weight, respectively.
In the case where two or more of plating layers are deposited, the contents of the plating metals should preferably be controlled within the above-mentioned ranges in the respective layers.
In the above-mentioned deposited steel plate, it is preferred that the amount deposited of the plating layer be at least 1 g/m2 on one surface. If the amount deposited of the plating layer is smaller than 1 g/m2 on one surface, there is a risk of reduction of the corrosion resistance. Even if the amount deposited of the plating layer exceeds 300 g/m , no high improve-ment of the corrosion resistance can be expected but the cost is increased. When the above-mentioned electric plating method is adopted, it is preferred that the amount deposited of the plating layer be 5 to 60 g/m on one surface.
According to the present invention, the plated surface of the steel plateis subjected to a chromate treatment to form a chromate film on the plated sur-face. It is preferred that the amount deposited (dry amount) of chromium in the chromate film be 1 to 1000 mg/m2 (as metallic chromium). If the amount deposited of chromium exceeds 1000 mg/m2, the processability and weldability are degraded, and if the amount deposited of chromium is smaller than 1 mg/m2, the chromate film becomes uneven and no good results can be obtained. In order to obtain good processability and weldability and a high evenness of the film simultaneously, it is preferred that the amount deposited of chromium be 10 to 200 mg/m2, especially 20 to 90 mg/m2.
This chromate treatment for formation of the under-coating film may be carried ^u-~ acco~ding 'o any of known methods, such as the reaction type chromate treatment method, the coating type chromate treatment 1256~54 method and the electrolytic chromate treatment method.
As the reaction type chromate treatment solution, there may be used, for example, a solution formed by incorporating at least one mineral acid (selected from H2S04, H2P04, HF and silicofluoric acid) as the main reaction-promoting component into chromic acid or the like. A solution having a pH value adjusted to 0.5 to

2.0 and a Cr3 /Cr6 ratio of from l/lto 1/10 is ordi-narily used. If the deposited steel plate is immersed in or sprayed with this treatment solution for a pre-determined time, reaction is caused between the plated surface and the treatment solution to form a chromate film. The unreacted substance is then removed by water washing and the treated steel plate is dried to obtain a chromate film.
The coating type treatment solution comprises a partially reduced chromic acid solution as the main component and contains, incorporated therein, an organic resin such as a water-dispersible or water-soluble acrylic resin and/or silica particles (silica sol or fumed silica) having a particle size of from scores of A to several thousand ~ according to need.
In this case, it is preferred that the Cr3 /Cr6 ratio be from 1/1 to 1/3 and the pH value be l.S to 4.0, especially 2 to 3. The Cr3+/Cr6 ratio may be adjuste~
to a desired value by using an ordinary organic reduc-ing agent such as a saccharide or an alcohol or an ordinary inorganic reducing agent. Any of the roll coater method, the dipping method and the spraying method may be adopted for the coating type chromate treatment. In case of the coating type chromate treat-ment, the treated steel plate is dried after the chromate treatment and a chromate film is obtained without performing water washing. The reason why drying is carried out withoutperforming water washing is that since Cr6+ is not removed by ordinary water washing, the Cr3 /Cr6 ratio is stably maintained and the treatment is conducted with an organic composite ~2561:~54 silicate solution at the subsequent step to effect sealing.
In case of the elctrolytic chromate treatment, the plated surface is subjected to a cathodic electro-lytic treatment with a solution comprising chromic anhydride and at least one anion selected from sulfuric acid, phosphoric acid, a fluoride and a halogen oxy-acid, and the treated steel plate is washed with water and dried to obtain a chromate film.
When chromate films obtained according to the above-mentioned three chromate treatment methods are compared with each other, the reaction type chromate is obtained relatively easily because the reaction type chromate treatment is a known technique which is generally adopted in various fields, though it is difficult to treat only one surface. Since the coat-ing type chromate film contains hexavalent chromium in a larger amount than in the electrolytic chromate film, the coating type chromate is excellent in the corrosion resistance. In other words, it is preferred that hexavalent chromium be present in the chromate film. By the action of Cr6 , the crosslinking of the film is promoted at the organic composite silicate treatment of the subsequent step and the film is strengthened. Moreover, Cr6 has a repairing action and when the steel plate is impaired, Cr6 inhibits advance of corrosion from the impaired portion. When the coating type chromate film is subjected to the heat treatment described hereinafter, the film is densified and strengthened, and hence, there is attained a higher corrosion resistance than in case of the reaction type chromate film or electrolytic chromate film. The electrolytic chromate treatment is advantageous in that the amount deposited of the chromate film can easily be controlled. In view of the corrosion resistance; the coating type chromate film is most preferred. A steel ~LZSi6~54 plate having one surface treated is often used as the rust-proof steel plate for a car body. In view of this fact, the coating type chromate treatment and the electrolytic chromate treatment are preferred.
After the above-mentioned chromate treatment, the treatment is carried out with an organic composite silicate solution, whereby an organic composite sili-cate film is formed on the chromate film. As pointed out hereinbefore, the organic composite silicate com-prises an organic resin, a silica sol and a silane compound.
The intended rust-proof steel plate of the present invention is mainly used for a car body, and cationic electrodeposition coating is ordinarily performed on both the inner and outer surfaces of a car body. In view of this fact, according to the present invention, the chromate-treated steel plate is subjected to a treatment with an organic composite silicate solution having an epoxy resin concentration adjusted to at least 15~ by weight based on the total solids to obtain a film containing an epoxy resin in a specific amount.
In the coated steel plate obtained according to the present invention, that is, the steel plate com-prising a deposited steel plate, a chromate film and an organic composite silicate film, the adhesion to a cationic electrodeposition coating layer is determined by the properties of the organic composite silicate film as the topcoat. When we examined the peeled sur-faces at the adhesion test conducted after cationic electrodeposition coating, it was found that peeling of the cationic electrodeposition coating layer is due to the cohesive failure of the organic composite sili-cate and the fracture of the interface between the organic composite silicate and the cationic electro-deposition coating layer. Figs. l-(A~ through 1-(C) are charts of an X-ray microanalyzer (XMA). Fig. l-(A) is a chart obtained when a coating type chromate film ~L;2S;6~ 4 was formed on a starting deposited steel plate having N-Zn deposited in an amount of 30 g/m2 and Fig. l-(B) is a chart obtained when an organic composite silicate was coated on the above chromate film. When both the charts are compared, it is seen that although the peak of Si is very slight on the chromate surLace of Fig. 1-(A), the peak of Si is prominent in Fig. l-(B) obtained when the organic composite silicate was further coated. Fig. l-(C) is an XMA chart of the peeled sur-face obtained when chromate and organic composite sili-cate films were formed on the starting deposited steel plate and an adhesive tape was applied and peeled at the adhesion test. The pattern is substantially the same as the pattern of Fig. l-(B) (the organic compo-site silicate-coated surface). Accordingly, it is seen that peeling of the cationic electrodeposition coating layer is peeling on the interface between the cationic electrodeposition coating layer and the organic composite sili-cate. When this peeled surface was examined by an operation type electron microscope, it was found that cohesive failure of the organic composite silicate partially takes place. This tend-ency is observed irrespectively of the kind of the starting depos-ited steel plate and the kind of the chromate treatment conducted on the starting deposited steel plate.
When the relation between the organic composite silicate component and the adhesion to the cationic electrodeposition coating layer was examined based on the foregoing facts, it was found that if an epoxy resin is incorporated into the organic composite sili-cate in an amount of at least 15% by weight based on the total solids, a good adhesion can be obtained. It is considered that the adhesion to the cationic electro-deposition coating layer is improved for the following reasons. In the first place, since an epoxy resin is contained in a cationic electrodeposition pair.t and the epoxy resin is contained in the organic composite silicate in an amount exceeding a cer~ain level, a ~L~2S6~)S~

strong mutual action is obtained between the epoxy resin in the cationic electrodeposition paintand the epoxy resin in the organic composite silicate and hence, a high adhesion is obtained in the interface between the cationic electrodeposition coating layer and the organic composite silicate film. In the second place, since the epoxy resin is contained in the organic composite silicate in an amount exceeding a certain level, the organic composite silicate film per se is strengthened by the epoxy resin and occur-rence of the cohesive failure is effectively prevented.
This effect is appropriately obtained when the epoxy resin is incorporated in an amount of at least 15% by weight based on the total solids.
In the present invention, the organic composite silicate contains at least 15% by weight of an epoxy resin, and an epoxy resin composite silicate or a mixture of an epoxy resin composite silicate with at least one other organic composite silicate is prefer-ably used.
It is preferred that in the organic composite sili-cate, the SiO2/organic resin weight ratio be from 95/5 to 5/95, especially from 60/40 to 10/90. The reason why the weight ratio between SiO2 and the organic resin is thus controlled is that the SiO2 component and the organic resin component are effective for improving the corrosion resistance and the coating adhesion, respectively. Accordingly, this weight ratio is especially important when a high adhesion to a multiple coating layer of at least two layers, as described hereinafter, is obtained as well as a high corrosion resistance.
In the present invention, a curing agent such as melamine may be added to the above-mentioned organic composite silicate solution. When the curing agent is thus added to the organic composite silicate solution, the critical decomposition temperature of the organic ~L25~(3S~

composite silicate film by heating is shifted to a high temperature side, and a heat treatment at a higher temperature becomes possible. If heating is carried out at a higher temperature, the adhesion is further enhanced for the above-mentioned reason.
Addition of the curing agent is especially effect-ive when it is desired to obtain a good adhesion to a multiple coating including at least two layers.
Namely, if addition of the curing agent is combined with adoption of treatment conditions, described herein-after, for obtaining a good adhesion to a multiple coating layer, a particularly high effect can be attained. This feature will be described in detail hereinafter.
The amount deposited of the organic composite silicate film is adjusted to 0.5 to 4 0 g/m2 (on the dry base; the same will apply hereinafter),- if the curing agent is not added. If the amount deposited of the organic composite silicate film is smaller than 0.5 g/m2, no sufficient corrosion resistance can be obtained. If the amount deposited of the organic composite silicate exceeds 4.0 g/m , the spot weld-ability is degraded. If it is desired to obtain a good spot weldabili~ty assuredly, it is preferred that the amount deposited of the organic composite sili-cate be smaller than 3.0 g/m2.
If a curing agent such as melamine is added to the organic composite silicate, since the adhesion and corrosion resistance are improved by the addition of the curing agent, the lower limit of the amount de-posited of the organic composite silicate may be reduced to 0.4 g/m . As pointed out hereinbefore, reduction of the amount deposited of the organic com-posite silicate results in improvement of the spot weldability. Acordingly, the curing agent--incor~orated organic composite silicate film is excellent in that 1256~)54 predetermined adhesion and corrosion resistance can be obtained even if the amount deposited of the organic composite silicate is small. Therefore, when the curing agent is incorporated in the organic composite silicate, the amount deposited of the organic composite silicate film is adjusted to 0.4 to 4.0 g/m2.
The organic composite silicate comprises water-dispersible silica as an indispensable component, and the organic composite silicate is obtained by mixing water-dispersible silica with an organic polymeric resin in the presence of a silane compound and reacting them at a temperature in the range of from 10C to the boiling point, preferably from 50 to 90C. The water-dispersible silica is so-called silica sol or colloidal silica having a particle size of from scores of ~ to several thousand A. The silane compound acts as a reaction promoter when the silica is combined with the organic resin. A commercially available silane coupling agent may be used as the silane compound. For example, there can be used trialkoxysilanes such as vinyltriethoxysilane, vinyltris(~-methoxyethoxy) silane,~ -glycidoxypropyltrimethoxysilane,,~-methacryl-oxypropyltrimethoxysilane, N-~-(aminoethyl)-~-amino-propyltrimethoxysilane andQ-aminopropyltriethoxysilane.
As the water-soluble or water-dispersible organic polymeric resin, there can be mentioned, for example, polyvinyl alcohol, hydroxyethyl cellulose, a polyester, an alkyd resin, an epoxy resin and an acrylic co-polymer. As pointed out hereinbefore, in the present invention, the epoxy resin is an indispensable com-ponent. As the epoxy resin, there can be mentioned a fatty acid-modified epoxy resin, a polybasic acid-modified epoxy resin, an acrylic resin-modified epoxy resin, an alkyd resin-modified epoxy resin, a phenolic resin-modified epoxy resin, a polybutadiene resin-modified epoxy resin and an amine-modified epoxy resin.
An amine compound or ammonium compound may be added 1256~)S4 so as to render the foregoing organic resins water-soluble or water-dispersible.
As pointed out hereinbefore, the weight ratio of the water-dispersible silica to the water-soluble or water-dispersible organic resin in the organic com-posite silicate is in the range of from 5/95 to 95/5, preferably from 10/90 to 60/40. It is preferred that the silane compound be added in an amount of 0.5 to 15% by weight based on the total solids of the silica and organic resin.
One or more of organic composite silicates obtained according to the foregoing procedures may be used, as pointed out hereinbefore. An oxyacid of molybdenum, tungsten or vanadium, a salt thereof or an alkoxide chelate of titanium or zirconium may be added to the organic composite silicate. If at least one of such additives is added in an amount of up to 14% by weight, preferably 0.2 to 8% by weight, based on the total solids of SiO2 and the organic resin, the corrosion resistance can further be improved.
A roll coating method, a spray coating method or other coating method may optionally be adopted for coating the organic composite silicate solution.
After the coating operation, the coated steel plate is dried to form an organic composite silicate film.
In the present invention, after the above-mentioned treatment with the organic composite silicate solution, the heat treatment is carried out at a temperature of 100 to 300C or at a temperature of up to 350C as the upper limit when the curing agent is added to the organic composite silicate solution. The reason why the heat treatment temperature is thus limited is that if the heat treatment temperature is lower than 100C, no sufficient corrosion resistanoe Aor coating adhesion can be obtained and if the treatment temperature exceeds the upper limit of 300 or 350C, the organic composite silicate film is thermally decomposed and -20a-lZS6~54 the weight loss is caused. As pointed out herein-before, if the curing agent is added, the upper limit of the treatment temperature for prevention of the thermal decomposition is shifted to the high temper-ature side, and the heat treatment may be carried out at a temperature of about 350C at highest.
It is considered that the corrosion resistance is improved by the heat treatment for the following two reasons. In the first place, the chromate film as the undercoat is densified by the heat treatment and the corrosion resistance is improved. Namely, such reac-tions as reduction of Cr6 and dehydration are caused by the heat treatment, and a dense chromic chromate film is formed. In the case where silica and/or the organic resin is contained in the chromate film, cross-linking is caused between chromium and these components by the heat treatment, and hence, the chromate film is densified to increase the corrosion resistance. In the second place, the organic composite silicate per se is strengthened. Namely, dehydration condensation is caused in the organic composite silicate by the heat treatment to increase the crosslinking density. In contrast, if the organic composite silicate is dried and cured at normal temperature, the crosslinking density is not sufficient and the organic composite silicate film is readily swollen in a wetting atmosphere or-the organic composite silicate film is readily deteriorated by alkali degreasing because of a poor alkali resistance. In addition to the fore-going two main reasons, in the case where C6+ is present in the chromate film as the undercoat, this Cr6+ is reacted with a polar group in the organic composite silicate, such as - 20b -~Z56VS4 a hydroxyl group or carboxyl group, by the heat treatment and the crosslinking is further advanced to improve the corrosion resistance.
Fig. 2 i5 a graph showing the results obtained when the relation between 4he heating temperature and the ratio of the weight loss of the organic com-posite silicate by the above-mentioned dehydration condensation. As pointed out hereinbefore, curing of the organic composite silicate is caused by u~hydration condensation, and the reaction ratio is proportional to the ratio of the weight loss. Test conditions adopted for obtaining the results shown in Table 2 are as follows.
Organic composite silicate:
SiO2/organic resin weight ratio = 40/60 Acrylic resin/epoxy resin weight ratio - 50/50 Temperature-elevating rate:
20C/min Preparation of sample:
5 cc of an organic composite silicate solution was charged in a 200-m~
beaker and was dried in a desiccator for 3 days, and about 200 9 of the solution was sampled and used for the measurement.
In Fig. 2, curve (I) shows the results obtained when a curing agent was not added, and curve (II) shows the results obtained when melamine was added as the curing agent in an amount of 3 parts by weight per 100 parts by weight of the epoxy resin. From Fig. 2, it is seen that the weight loss begins at about 60C and becomes conspicuous at temperatures higher than 100C. The weight loss at temperatures lower than 100C is due mainly to evaporation of water, and substantial dehydration condensation is caused at temperatures higher than 100C. In the curve (I~ obtained when the curing agent was not added, the weight is drastically reduced if the tem-perature exceeds 300C and it is seen that thermal decomposition of the silicate is initiated. Therefore, it is understood that when the curing agent is not added, the temperature for heating the organic composite sili-cate should be 100 to 300C, preferably 200 to 300C. In the curve (II)~

obtained when melamine was added in an amount of 3 O by weight as the curinu ~z~v~ l agent, the time of initiation of the dehydration condensation reaction is substantially the same as in the curve (I) obtained when the curing agent was not added, but with elevation of the temperature, the weight loss by the dehydration condensation reaction is increased and made larger than in the curve (I), and this tendency is especially conspicuous at temperatures higher than Z30C. Furthermore, in the curve (II) obtained when the curing agent was added, the thermal decomposition temperature is shifted to the high temperature side and it is seen that heating may be performed at temperatu-res of up to 350C. From the curve (II) obtained when the curing agent was added, it is seen that thermal decomposition is caused at a temperature exceeding 350CJ and the~mal decomposition becomes conspicuous at about 360C. Thus, it will be understood that the temperature for heating the organic composite silicate in which the curing agent is incorporated is suitably 100 to 350 C. Incidentally, if the heating temperature is lower than 230C, the unreacted curing agent is left free and attainment of an adhesion corresponding to the weight loss can hardly be expected. Accordingly, in order to sufficiently enjoy the advahtages of the addition of the curing agent, it is preferred that the heating temperature be at least 230C.

In the above-mentioned heat treatment, it is preferred that after heating the steel plate at a predetermined temperature, this temperature be maintained within several seconds to several minutes. Long-time maintenance of this temperature is not preferred from the economical viewpoint and there is a risk of degradation of the properties.
The foregoing preparation conditions are ordinary conditions adopted in the present invention for attaining a good adhesion to a cationic electro-deposition coating layer. However, these conditions are sti~l insufficient for obtaining an excellent adhesion to a multiple electrodeposition coating including two or ~hree layers but more restricted preparation condi-~-ons are required.

More specifically, in order to attain a high adhesion to a multple , . , .`
., , .. , ,.~, ~2S~ 4 coating of at least two layers, the SiO2/or~anic resin weight ratio in the organic composite silicate should be in the range of from 10/90 to 60/40, preferably from 2û/8û to 50/50, the amount of the epoxy resin should be at least 26 O by weight, preferably at least 35 O by weight, base~ an the total solids in the organic composite silicate, and the temperature of the heat treatment conducted after the organic composite silicate solution treat-ment should be at least 250C.
If the epoxy resin concentration in the organic composite silicate is thus increased, a strong mutual action is manifested between this epoxy resin and the epoxy resin in the cationic electrodeposition coating and a high coating adhesion is obtained. Furthermore, if the epoxy resin content in the organic composite silicate is increased, the critical thermal decompo-sitiOn temperature for the resin is shifted to the high temperature side and the heat treatment can be carried out at a higher temperature. Accord-ingly, by this high temperature heat treatment, crosslinking of the orgaaiccomposite silicate is promoted, resulting in improvement of the adhesion.
More specifically, the cationic electrodeposition coating is characterized in that the coated interface becomes alkaline, and an organic composite silicate film is ordinarily poor in the resistance to an alkaline environ-ment and is readily softened and swollen by the alkaline characteristic ofthe interface formed by the cationic electrodeposition coating and this is a main cause of inhibition of attainment of a good adhesion. Furthermore, in the case of a multiple coating of at least two layers where a topcoat is applied to a cationic electrodeposition coating layer, since the coating film thickness on the steel plate is increased, the internal stress is increased and this incre æ of the internal stress is a large cause of reduction of the adhesion. In order to cope with these problems concerning the adhesion, according to the present invention, crosslinking is promoted to increase the crosslinking density and reduction of the adhesion to the coating layer due to sotening and swelling under an alkaline environment or to generation of the internal stress is controlled.
Furthermore, as pointed out hereinbefore, since the SiO2 component lZS6~54 and the organic resin component in the organic composite silicate are effective for improving the corrosion resistance and the adhesion to the coating, respect-ively, in order to attain a good adhesion to a multiple coating of two or three layers with a good corrosion resistance, to which attainment of a good adhesion is very difficult, control of the SiO2/organic resin weight ratio is indispensable. If this weight ratio exceeds 60/40, the corrosion resistance is increased but the adhesion becomes insufficient. In contrast, if this weight ratio is lower than 10/90, the corrosion resistance is degraded. Accordingly, the SiO2/organic resin weight ratio should be adjusted within the range of from 60/40 to 10/90, preferably from 50/50 to 20/80.
In the case where a multiple coating including at least two layers is intended, as pointed out herein-before, incorporation of a curing agent such as melamine into an organic composite silicate solution is recom-mended. As described hereinafter, if it is desired to attain a good adhesion to a multiple coating of at least two layers, the lower limit of the heat treatmenttemper-ature is elevated and the heat treatment is carried out at a higher temperature, and in this case, the incorporated curing agent exerts functions of promoting the curing of the organic cc~Tposite silicate fi]m and enhancing the a~esion of th2 0~ g~-.ic composite silicate film per se. Moreover, the curing agent exerts a function of shifting the critical thermal decomposition temper-ature of the organic composite silicate film to a high temperature side and it becomes possible to perform the hea~ treatment at a higher temperature where a high adhesion is attained.

As the curing agent to be incorporated, there can be mentioned a blocked isocyanate, urea, melamine and a phenol. Furthermore, a polyamide, an amino resin, an amine, an organic acid, an inorganic acid, an alcohol 1:256~54 a mercaptan and an acid anhydride may be used.
The curing agent is incorporated in an amount of 0.1 to 100 parts by weight, preferably 0.3 to 50 parts by weight, per 100 parts of the - 24a -~ ;256VS~
epoxy resin in the organic composite silicate. If the curing agent is incor-porated in too large an amount, the free curing agent not reacted with the epoxy resin inhibits the adhesion. Therefore, the upper limit of the amount incorporated of the curing agent is preferably set at 1ûO parts by weight per 100 parts by ~eight of the epoxy resin.
The lower limit of the temperature for the heat treatment of the organic composite silicate is set at 250 C and the heat treatment tempera-ture is adjusted within the range of from 250 to 300 C, and in the case where the curing agent is incorporated, the heat treatment is carried out at a temperature within the range of from 250 to 350C. When improvement of the adhesion to a multiple coating including at least two layers is intended, heating at a relatively low temperature not higher than 250C is insuffici-ent and attainment of a satisfactory adhesion cannot be expected.
The above-mentioned plating treatment,-chromate treatment and organic composite solution treatment may be conducted on both the surfaces or one surface of a steel plate. The steel plate prepared according to the present invention lncludes, for example, the following embodiments.
(1) One surface: plating layer/chromate fllm~organic composite silicate film Other surface: Fe surface (2) One surface: plating layer/chromate film/organic composite silicate film Other surface: plated surface

(3) Both Surfaces: plating layer/chromate film/organic composite silicate film The above-mentioned steel plate (2) has the basic properties required for the inner and outer surfaces of a car body. Namely, it is preferred that one surface of the steel plate to be formed into an inner surface of a car body be treated according to the present invention to form a plating layer, a chromate film and an or9aniC composite silicate filr,1 and that the other .~ surface of the steel plate to be formed into an outer surface of a car body S3S~
be plated with zinc or a zinc alloy to form a plating layer. It is especially preferred that the other surface of the steel plate be subjected to at least two plating treatments to fonm (a) a-zinc plating ilm in an amount deposited of 1 to 60 g/m on the lower side and (b) a plating layer film of iron or an iron-zinc alloy having an iron content of at least 50 O by weight as the topmost iayer. According to this preferred embodiment, there is obtained a steel plate having both the surfaces treated, in which one surface ( corresponding to the inner surface of a car body ) has a higher corrosion resistance than that of a rust-proof steel plate such as Zinchrometal and the other surface ( corresponding to the outer surface of a car body ) is comparable to a cold-rolled steel plate in the coating finish property and the adhesion to the coating and lS much superior to a cold-rolled steel plate in the corrosion resistance ( red rust resistance ).
- If a composite plating film lS formed on the surface of the steel plate to be formed into the outer surface of a car body by depositing a pl~rality of plating layers appropriately so that various properties required for the outer surface of a carbody can effectively be manifested, high cratering resistance, water-rsistant adhesion and corrosion resistance can simultaneously be attained very satisfactorily.
The zinc type plating film on the lower side is formed so as to improve the corrosion resistance ( red rust resistance and blister resistance ) and this plating film has a single-layer or multi-layer structure of a zinc pla-ting or a zinc-based alloy plating. The zinc-based alloy plating is especially excellent in the corrosion resistance, and an Fe-Zn alloy plating ( the Fe content is up to 40 0, preferably 5 to 35 O ), an Ni-Zn alloy plating ( the Ni content is 5 to 20 ' ) and an Mn-Zn alloy plating ( the Mn content is 30 to 85 O ) are especially preferred. The above-mentioned iron, nickel and manganese contents in the foregoing zinc alloys are determined~wi~ a view of obtaining corrosiorl resistance. If these ccntents aré outside the above-range and too high or too low, the corrosion resistance is degraded and no good results can be obtained. At least one of the above-rnentioned zinc and . .

zinc alloy layers is deposited in the form of a single layer or multiple layer coating.
The topmost plating film deposited on the above-mentioned plating film on the lower side is a plating film of iron or an iron-zinc alloy having an Fe content of at least 50 O by weight and this plating film is formed so as to improve the cratering resistance and water-resistant adhesion of the surface to be coated ( the outer surface ).
The machanism of occurrence of cratering and degradation of the water-re-sistant adhesion in a zinc- or zinc alloy-deposited steel plate has not 1~ been completely elucidated. However, we noted that these properties are good in a cold-rolled steel plate andcraterin~takes place at the cationic electro-deposition step, and when samples where the water-resistant adhesion was degraded were examined, it was found that peeling of the interface was due to fracture of the interface between the phosphate film and the cationic electrodeposition coating film or the cohesive failure of the phosphate film. While taking the foregoing facts into consideration, we selected the topmost plating film in the following manner.
In the first place, we presumed that the difference of the water-resis-tant adhesion and cratering resistance between a cold-rolled steel plate and a zinc- or zinc alloy-deposited steel plate would be due to the surface layer of the material and the composition and crystal structure of a phosphate film formed by reaction. The difference of the surface layer of the material is such that the Zn content in the zinc- or zinc alloy deposited steel plate is 100 to 80 O by weight while the Fe content should naturally be 100 O by weight in the cold-rolled steel. Both the steels are different in the formed phosphate film. For example, in a steel plate coated with a zinc type plat-ing layer having a zinc content of 80 to 100 O by weight, the pphosphate film is composed solely of hopeite [Zn3(P04)2 4H20] and has a needle crystal structure. On the other hand, in case of a cold-rolled steel plate, the phosp~ate film is composed of hopeite and phosphophyllite [Zn2Fe(P04)2-4H20]
and ~hen a dipping type phoaphate treatment which has b~en recently broadly adopb3d lZ56~)54 is employed, the phosphate will be composed mainly of phosphophyllite and have a columnar crystal structure.
We made research based on the foregoing facts with a view to obtaining a film having cratering resistance and water-~esistant adhesion characteris-tics comparable to those of the cold-rolled steel plate, and we found the following facts in connection with a plating layer of an iron-zinc alloy.
Namely, in connection with the plating surface characteristics, it was found that if the Fe content exceeds the range of from 20 to 40 O by weight, phosphophyllite is formed in the phosphate film and with increase of the Fe con~ent in the plating layer, the proportion of phosphophyllite is increased.
Furthermore, if the Fe content in the plating layer exceeds 50 O by weight, the ~-phase ( according to the X-ray diffractometry ) is observed, and the a-phase is increased with increase of the Fe content. It also was confirmed that the cratering resistance becomes substantially comparable to that of the cold-rolled steel plate if the Fe content in the Fe-Zn alloy plating layer is hish~r tll~n ~ y weig~lt, an~ .ha~ ~e water-resistant adhesion beccmes substantially ccnparable to that of the cold-rolled steel plate if the Fe content in the Fe-Zn alloy plating layer is higher than 40% by weight.

From the foregoing results, in the present invention, an iron plating film or an iron-zinc alloy plating film having an Fe content of at ieast 50 O
by weight is selected as the topmost plating layer.
The amounts deposited of the respective plating films will now be des-cribed. The amount deposited of the iron plating film or the iron-zinc alloy plating film hsving an Fe content of at least 50 O by weight as the topmost layer is selected within the range of 0.5 to 10 g~m , preferably 1 to 5 g/m2. If the amount deposited of this topmost layer is smaller than 0.5 g/m , the characteristics of the plating film o~ the lower side are mani-fested and the cratering resistance and water-resistant adhesion are degraded. If the amount deposited of the topmost plating layer exceeds 10 g/m2, the adhesion to the coating and the processability are degraded and no goo~ results can be obtained. The-amount deposited of the lower zinc or ~2~ 5~ .
zinc elloy plating layer is selected within the range of from 1 to 60 g/m2, preferably from 10 to 40 g/m . If the amount depoQited of the lower plating layer is smaller than 1 g/m , the corrosion resistance is degraded and if the amount deposited D~ ~he lower plating layer exceeds 60 g/m2, the proces-sability of the plating film i9 degraded and the manufacturing cost is in-creased. ~
The plating methods for obtaining the foregoing plating layers are not particularly critical.For example, electric, gas phase and meiting plating methods ~ay be adopted for deposition of a zinc or iron-zinc alloy plating layer, and electric and gas phase plating methods may be adopted for de?osition of a nickel-zinc or manganese-zinc alloy plating layer.
In the above-mentloned steel plate, a certain plating film is formed on each of both the surfaces. Namely, differ~nt kinds of plating films may be formed on both the surfaces or both the surfaces may be pl~ted accord ng to ditferent plating methods. However, ~he.lowermost plating layers of both the surfaces can be formed by the .inc type plating. When the process of the present invention is indust ially carried out on the steel plate of this type, there is preferably adopted a method in which the same zinc alloy plating (selected from Fe-Zn alloy plating, Co -Zn alloy plating and ~n-Zn alloy p!ating ! is effected on both the surfaces of the starting steel plate, a plating layer of iron or an iron-~inc alloy having an Fe content of at least~50 by weight is deposited on one surface to be formed into the outer surface of a car body and on the other surface to be formed into the inner surface of a car body, a chromate film is formed as a first layer and an organic com-posite silicate film is formed as the second layer, and finally, the steel plate is subjected to the above-mentioned heat treatment. As pointed out hereinbefore, optional plating methods may be adopted for obtaining this .. . . .
steel plate. If different plating layers are to be formed on the two surfaces or when the uppermost plating layer for the outersurface is being formed, the electric plating method is preferably adopted.
In the steel plate of this type, excellent coating appearance, high adhesion and high corrosion resistanc ofthe coated surface are obtained on one surface and high corrosion resistance of the uncoated surface and high adhesion to the cationic electrodeposition coating are obtained on the other surface.
The present invention will now be described in detail with reference to the following examples that by no means limit the scope of the invention.
Example 1 In this example, the treatment was conducted under such treatment conditions of the present invention that a good adhesion to the cationic electrodeposition coating could be obtained.
Namely, various surface-treated steel plates differing in the plating components and the amounts of the deposited films, as shown in Table 1, were prepared as steel plates corresponding to the inner surface of a car body according to the process of the present invention, and these-surface-treated steel plates were subjected to the adhesion and corrosion resistance tests.Ccmparative steel plates shown in Table 2 were prepared and similarly tested.
The plating components of the respective steel plates were as described below. With respect to each of the steel plates shown in Table 1 and the steel plates having a chromate film and an organic compo-site silicate film, shown in Table 2, the plated steel plate was alkali~degreased, washed with water and dried, and the steel plate was coated with a coating type chromate treatment solution by a roll coater or was dipped in an electrolytic chromate treatment solu-tion to form a chromate film. After drying, the steel plate was coated with an organic composite silicate treatment solution by a roll coater to form an organic composite silicate layer as the second layer. After drying, the steel plate was subjected to a heat treat-ment and was then air-cooled.
Ni-Zn alloy electric plating: Ni content of 12~o Fe-Zn alloy electric plating: Fe content of 25%

'.~

1256~5~

Mn-Zn alloy electric plating: Mn content of 60%
Zn-Al alloy electric plating: Al content of 5%
The coating type chromate treatment conditions, the electrolytic chromate treatment conditions and the organic composite silicate treatment solution were as described below.
Coating type chromate treatment conditions:
The Cr3 /Cr ratio was 2/3, and the pH value was adjusted to 2.5 by KOH. A chromate treatment solution having a solid content of 20 g/l was coated at normal temperature by a roll coater, followed by drying.
Electrolytic chromate treatment conditions:
The CrO3 concentration was 50 g/l and the H2SO4 concentration was 0.5 g/l, and the cathodic electroly-tic treatment was conducted at a current density of

4.9 A/dm2 in the chromate treatment solution maintained at a bath temperature of 50C for an electrolysis time of 2.0 seconds. The treated steel plate was washed with water and dried.
Organic composite silicate treatment solution:
An acrylic composite silicateand an epoxy composite silicate which had an organic resin/SiO2 weight ratio of 20/80, 40/60, 60/40 or 80/20 were synthesized according to procedures described below, and both the composite silicates were mixed at a ratio shown in Table 1 or 2 to obtain an organic composite silicate solution having a solid content of 20%.
(A) Synthesis of acrylic composite silicate:
A 1-liter 4-neck flask equipped with a thermometer, a stirrer, a cooler and dropping funnel was charged with 180 parts of isopropyl alcohol, and the inner atmosphere of the flask was substituted by nitrogen and the inner temperature of the flask was adjusted to about 85 C. A monomeric mixture comprising 140 parts of ethyl acrylate, 68 parts of methyl methacryl-ate, 15 parts of styrene, 15 parts of N-n-butoxymethyl-acrylamide, 38 parts of 2-hydroxyethyl acrylate and 24 ~256~54 parts of acrylic acid was dropped into the flask together with a catalyst consisting of 6 parts of 2,2'-azobis(2,4-dimethylbutyronitrile) over a period of about 2 hours. After the dropwise addition, reaction was further conducted at the same temperature for 5 hours to obtain a colorless transparent resin solution having a solid content of 63% and an acid value of 67. To SOO parts of the so-obtained acrylic co-- 31a -1:2S6~54 polymer resin solution was added 45 parts of 38 6 aqueous ammonia, and water was added and the mixture was stirred sufficiently to obtain an aqueous dispersion of the acrylic copolymer having a solid content of 20~ and a pH value of 9.5. The flask was charged with 300 parts of this aqueous dispersion and a predetermined amount of colloidal silica (sold under the trademark STOTEX N by Nissan Kagaku Kogyo K.K.) was added to the dispersion at room temperature with sufficient stirring. Then, 1 part of~'-meth-acryloxypropyltrimethoxysilane (sold under the trademark KBM 503 by Shinets~ Kagaku Kogyo K.K.j was dropped to the mixture with stirring. Then, the mixture was heated at 85C and maintained at this temperature for 2 hours to effect reaction and obtain a milky white water-dispersible acrylic composite silicate.
(B) Synthesis of epoxy composite silicate:
A flask was charged with 310 parts of a bisphenol A type epoxy resin having an epoxy equi-valent of 950 (sold under the trademark EPIKOTE 1004 by Shell Kagaku K.K.), 95 parts of linseed fatty acid, 95 parts of tung oil fatty acid and 15 parts of xylene, and the mixture was gradually heated under a nitrogen current until the temperature was elevated to 240 C. Then, the mixture was cooled, and when the temperature was lowered to 70C, 200 parts of ethyl-ene glycol monoethyl ether were added to -the reaction mixture to obtain an oil-modified epoxy resin solu-tion having a solid content of 70% and an acid valueof 54. An epoxy composite silicate was prepared from this oil-modified epoxy resin solu-tion in the same manner as described in (A) above.

A
~j 1256'054 Each of sample steel plates including comparative samples was coated with a rust-preventive oil (DIAMOND PA920 (trademark) supplied by Mitsubishi Sekiyu K.K.), allowed to stand still for one day and subjected to a phosphate treatment under a standard condition for BONDERITE 3004 (trademark; Nippon Parkerizing K.K.). Then, the corrosion resistance test was carried out according to the following procedures.
10Namely, the cycle test was conducted by performing the following operations as one cycle:
dipping in 5% NaCl, 40 C, 30 minutes (inclusive of ~wettling at 95% RH, 50 C, 60 minutes transfer L drying, 60C, 30 minutes time) 15At 250, 500 and 1000 cycles, the area where red rusting occurred was measured with respect to each sample. The adhesion test was carried out according to the following procedures.
Each sample that had been subjected to the phosphate treatment was subjected to electrodeposi-tion using a cationic electrodeposition paint (U-50 (trademark) supplied by Nippon Paint K.K.) to form a coating film having a thickness of 20~4, and the primary and secondary adhesion tests were carried out. At the primary adhesion test, 100 cross cuts were formed at intervals of l mm on the coated surface of each sample, and an adhesive tape was applied to the cross cuts and peeled. At the second-ary adhesion test, after the electrodeposition, each sample was dipped in warm water (pure water) main-tained at 40C for 120 hours, and within 30 minutes, cross cuts were formed at intervals of l mm in the same manner as described above and an adhesive tape was applied to the cross cuts and peeled.

The results of the above-mentioned corro-sion resistance and adhesion tests are shown in Tables 3 and 4. As is apparent from the results shown in these Tables, the samples of the present invention are excellent over the compara-tive samples in that the samples of the present invention have a high corrosion resistance and a good adhesion to the cationic electrodeposition coating in combination.
In Tables 2 and 4, compara-tive samples Nos. 1 and 2 were given to show the criticality of the proportion of the epoxy resin to the total solids in the organic composite silicate film, comparative samples Nos. 3 and 4 were given to show the criticality of the amount deposited of the organic composite silicate film, and comparative samples Nos. 5 and 6 were given to show the influences of the heat treatment. From the results obtained with respect to these compara-tive samples, it is seen that if the requirements specified in the present invention are not satisfied, obtained steel plates are insufficient in at least one of corrosion resistance, adhesion, spot weld-ability and processability (peeling at the pressing step) and are not suitable as rust-proof steel plates for a car body. It will also be understood that in the samples of the present invention, the larger is the proportion of the epoxy resin in the organic composite silicate, the more improved is the adhesion to the cationic electrodeposition coatings.
Example 2 In this example, the treatments were carried out under severer conditions so that a good adhesion to a multiple coating including at least two coating layers. Namely, a treatment was carried out with an organic composite silicate solution in which the content of the epoxy resin was at least 26~ by ~2S6~5~

weight based on the total solids and the SiO2/organic resin weight ratio was from 10/90 to 60/40, and a heat treatment was carried out at a temperature of at least 250C. Surface-treated steel plates having plating components and deposition amounts shown in Tables 5-a through 5-d were prepared as steel plates corresponding to the inner surface of a carbody, and the adhesions to two-layer and three-layer coatings and the corrosion resistance after the coating were tested. For comparison, steel plates shown in Tables 6-a and 6-b were prepared, and they were similarly tested. In Table 6-a, samples Nos. 1, 2,4, 5, 6, 8, 9, 11 and 13 are included within the scope of the present invention, but the content of the epoxy resin was lower than 26% by weight or the heat treatment temperature was lower than 250 C. Accordingly, in this example, these samples were designated as comparative samples The plating components, chromate treatment conditions and organic composite silicate treatment conditions were the same as in Example 1.
The adhesion test was carried out accord-ing to the following procedures.
In case of the two-layer coating, a sample that had been subjected to the phosphate treatment was subjected to electrodeposition with a cationic electrodeposition paint(U-50 supplied by Nippon Paint K.K.) to form a coating film having a thickness of 20 ~, and then, the sample was spray-coated with AMILAC No. 002 (trademark; Kansai Paint K.K.) in a thickness of 30 ~ . in case of the three-layer coating, after the electrodeposition coating, a sample was spray-coated with ORGA S89 and ORGA S50 (trademark; Nippon Paint K.K.) in thicknesses of2~
and 25~ -respectively, and the sample was spray-coated lZS6~S4 with AMILAC No. 805 White (supplied by Kansai Paint K.K.) in a thickness of 40 ~. Each sample was subjected to the primary adhesion test and secondary adhesion test. At the primary adhesion test, lO0 cross cuts were formed on the coated surface of each sample at intervals of l mm in case of the two-layer coating or 2 mm in case of the three-layer coating, and an adhesive tape was applied to the cross cuts and peeled. At the secondary adhesion test, after the coating operation, each sample was dipped in warm water (pure water) maintained at 40C for 120 hours, and within 30 minutes cross cuts were formed in the same manner as described above at intervals of l mm in case of the two-layer coating or 2 mm in case of the three-layer coating. An adhesive tape was applied to cross cuts and peeled.
The corrosion resistance was determined according to the cycle test conducted by performing the following operations as one cycle:
20 ~ dipping in 5% NaCl, 40 C, 30 minutes (inclusive of wettlng at 95% RH, 50 C, 60 minu-tes -transfer time) - durlng, 60C, 30 minutes At 500 and 1000 cycles, the degree of rusting was checked with respect to each sample.
The results of the adhesion test and corrosion reslstance test are shown in Tables 7a through 7-d and 8-a through 8-b. From these results, it will readily be understood that if there is adopted the preferred embodiment of the present invention where the epoxy resin content in the organic composite silicate is at least 26% by weight, the SiO2/organic resin weight ratio is adjusted to from lO/90 to 60/40 and the heat treatment is carried out at a higher temperature of at least 250, there ,.~, lZS~054 can be obtained a surEace-treated steel plate which is excellent in the corrosion resistance and the adhesion to a multiple coating includi.ng at least 2 coating layers over not only the conventional coated steel plates but also steel plates prepared according to other embodiments of the present inventin.
The terminology of "organic composite silicate film" referred to in the foregoing explanation of the invention is defined as --composite silicate resin film formed on the chromate film and consisting essentially of a colloidal silica, an organic resin and a silane compound--.

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Claims (33)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for the preparation of a highly anti-corrosive surface-treated steel plate, which comprises subjecting a plated surface of a steel plate having a plating layer of the zinc or aluminum type to the following treatments:
(a) subjecting the steel plate to a chromate treat-ment to form a chromate film, (b) treating the steel plate with an organic composite silicate solution containing an epoxy resin in an amount of at least 15%, by weight, based on the total solids to form an organic composite silicate film in an amount deposited of 0.5 to 4.0 g/m on the chromate film, and (c) heat-treating the steel plate at a temperature of 100 to 300 C.

2. A process according to claim 1, wherein said organic composite silicate solution comprises a colloidal silica, an organic resin and a silane compound.

3. A process according to claim 2, wherein the steel plate is treated with an organic composite silicate solution containing an epoxy resin in an amount of at least 26%, by weight, based on the total solids and having an SiO2/organic resin weight ratio of from 60/40 to 10/90, to form an organic composite silicate film on the chromate film and the steel plate is heat-treated at a temperature of 250 to 300°C.

4. A process according to claim 3, wherein said solution contains said epoxy resin in an amount of at least 35%, by weight, based on the total solids, and has a SiO2/ organic resin weight ratio of from 50/50 to 20/80.

5. A process according to claim 1 or 2, wherein the steel plate having a plating layer of the zinc type is a steel plate having a plating layer of Zn, an Ni-Zn alloy, an Fe-Zn alloy, a Zn-Mn alloy, a Zn-Al alloy or a Zn-Co-Cr alloy, a steel plate having a plating layer of at least one element selected from Ni, Fe, Mn, Co, Al and Cr, incorporated into one of the foregoing plating components, or a steel plate having at least two same or different plating layers selected from the foregoing plating layers.

6. A process according to claim 1 or 2, wherein the chromate treatment is carried out so that the chromate film is deposited in an amount of 1 to 1000 mg/m2, as chromium.

7. A process according to claim 1 or 2, wherein the chromate treatment is carried out so that the chromate film is deposited in an amount of 10 to 200 mg/m2, as chromium.

8. A process according to claim 1 or 2, wherein the chromate treatment is a coating type chromate treatment.

9. A process according to claim 1 or 2, wherein the chromate treatment is an electrolytic chromate treatment.

10. A process according to claim 1 or 2, wherein the chromate treatment is a reaction type chromate treatment.

11. A process according to claim 1 or 2, wherein the organic composite silicate is an epoxy composite silicate or a mixture of an epoxy composite silicate with at least one other organic composite silicate.

12. A process according to claim 1 or 2, wherein at least one additive selected from oxyacids of molybdenum, tungsten and vanadium, salts thereof and alkoxide chelates of titanium and zirconium is incorporated into the organic composite silicate in an amount of up to 14%, by weight, based on the total solids of the organic resin and SiO2.

13. A process according to claim 1 or 2, wherein at least one additive selected from oxyacids of molybdenum, tungsten and vanadium, salts thereof and alkoxide chelates of titanium and zirconium is incorporated into the organic com-posite silicate in an amount of 0.2 to 8%, by weight, based on the total solids of the organic resin and SiO2.

14. A process for the preparation of a highly anticor-rosive surface-treated steel plate, which comprises subjecting a plated surface of a steel plate having a plating layer of the zinc type or aluminum type to the following treatments:
(a) subjecting the steel plate to a chromate treatment to form a chromate film, (b) treating the steel plate with an organic com-posite silicate solution containing an epoxy resin in an amount of at least 15%, by weight, based on the total solids, in which a curing agent is incorporated, to form an organic composite silicate film in an amount of 0.4 to 4.0 g/m2 on the chromate film, and (c) heat-treating the steel plate at a temperature of 100 to 350°C.

15. A process according to claim 14, wherein said organic composite silicate solution comprises a colloidal silica, an organic resin and a silane compound.

16. A process according to claim 15, wherein the steel plate is treated with an organic composite silicate solution containing an epoxy resin in an amount of at least 26%, by weight, based on the total solids and having an SiO2/organic resin weight ratio of from 60/40 to 10/90, to form an organic composite silicate film on the chromate film and the steel plate is heat-treated at a temperature of 250 to 350 C.

17. A process according to claim 16, wherein said solution contains said epoxy resin in an amount of at least 35%, by weight, based on the total solids, and has a 5:02/organic resin weight ratio of from 50/50 to 20/80.

18. A process according to claim 15, wherein at least one member selected from melamine, a blocked isocyanate, urea, a phenol, a polyamide, an amino resin, an amine, an organic acid, an inorganic acid, an alcohol, a mercaptan and an acid anhydride is incorporated as the curing agent in the organic composite silicate solution in an amount of 0.1 to 100 parts by weight, per 100 parts by weight of the epoxy resin in the organic composite silicate.

19. A process according to claim 18, wherein said curing agent is incorporated in said solution in an amount of 0.3 to 50 parts by weight.

20. A process according to claim 14 or 15, wherein the steel plate having a plating layer of the zinc type is a steel plate having a plating layer of Zn, an Ni-Zn alloy, an Fe-Zn alloy, a Zn-Mn alloy, a Zn-Al alloy or a Zn-Co-Cr alloy, a steel plate having a plating layer of at least one element selected from Ni, Fe, Mn, Co, Al and Cr, incorporated into one of the foregoing plating components, or a steel plate having at least two same or different plating layers selected from the foregoing plating layers.

21. A process according to claim 14 or 15, wherein the chromate treatment is carried out so that the chromate film is deposited in an amount of 1 to 1000 mg/m2, as chromium.

22. A process according to claim 14 or 15, wherein the chromate treatment is a coating type chromate treatment.

23. A process according to claim 14 or 15, wherein the chromate treatment is an electrolytic chromate treatment.

24. A process according to claim 14 or 15, wherein the chromate treatment is a reaction type chromate treatment.

25. A process according to claim 14 or 15, wherein the organic composite silicate is an epoxy composite silicate or a mixture of an epoxy composite silicate with at least one other organic composite silicate.

26. A process according to claim 14 or 15, wherein at least one additive selected from oxyacids of molybdenum, tungsten and vanadium, salts thereof and alkoxide chelates of titanium and zirconium is incorporated into the organic com-posite silicate in an amount of up to 14%, by weight, based on the total solids of the organic resin and SiO2.

27. A process according to claim 14 or 15, wherein at least one additive selected from oxyacids of molybdenum, tungsten and vanadium, salts thereof and alkoxide chelates of titanium and zirconium is incorporated into the organic composite silicate in an amount of 0.2 to 8%, by weight, based on the total solids of the organic resin and SiO2.

28. A process according to claim 1 or 2, wherein one surface of a steel plate having both the surfaces deposited with a plating layer of the zinc type is subjected to the chromate treatment, the organic composite silicate solution treatment and the heat treatment.

29. A process according to claim 14 or 15, wherein one surface of a steel plate having both the surfaces deposited with a plating layer of the zinc type is subjected to the chromate treatment, the organic composite silicate solution treatment and the heat treatment.

30. A process according to claim 1 or 2, wherein one surface of a steel plate having both the surfaces deposited with a plating layer of the zinc type is plated with iron or an iron-zinc alloy having an iron content of at least 50%, by weight and the other surface is subjected to the chromate treatment, the organic composite silicate solution treatment and the heat treatment.

31. A process according to claim 14 or 15, wherein one surface of a steel plate having both the surfaces deposited with a plating layer of the zinc type is plated with iron or an iron-zinc alloy having an iron content of at least 50%, by weight, and the other surface is subjected to the chromate treatment, the organic composite silicate solution treatment and the heat treatment.

32. A highly anticorrosive surface-treated steel plate comprising:
a steel plate having a plating layer of zinc or aluminum type, a chromate film, and an organic composite silicate film on said chromate film, said composite film containing an epoxy resin in an amount of at least 15%, by weight, based on the weight of the composite film, said plate with said film thereon having been heat treated at a temperature of 100 to 300°C.

33. A steel plate according to claim 32, wherein said organic composite silicate film is derived from a colloidal silica, an epoxy resin and a silane compound.