JPH072997B2 - Zinc-based plated steel sheet with excellent corrosion resistance and paintability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention has excellent corrosion resistance and coating performance when exposed to a corrosive environment containing Cl ⁻ ions such as NaCl and CaCl ₂ and is excellent in automobile protection. The present invention relates to galvanized steel sheets used for rust steel sheets and building materials.

(Prior Art and Problems Thereof) So-called cold-rolled steel sheets, which are not provided with an anticorrosion coating layer, have been conventionally used for automobile steel sheets. This cold-rolled steel sheet is processed into various parts for automobiles by an automobile company, assembled, subjected to a phosphate treatment, and then painted. That is, cold-rolled steel sheets used for automobiles are protected from corrosion by a coating film. However, in recent years, the demand for improved durability of automobiles, especially due to corrosion, has increased,
Conventional painting alone cannot always meet this requirement. For example, in Canada, where salt is sprayed to prevent roads from freezing in the winter, 1985 targets "no pitting" and "no rusting for 5 years" as guidelines for car body corrosion in 1985. This guideline is known as the "Canada Code", and as a result, various measures are being taken as a goal for car body rust prevention.

At present, electrogalvanized steel sheets are widely used because they improve the corrosion resistance of cold-rolled steel sheets and the corrosion resistance after painting and can be mass-produced without impairing workability.

However, although the zinc layer itself of the galvanized steel sheet has excellent corrosion resistance in the severe corrosive environment containing Cl ⁻ as described above, zinc is much more anodized than the base steel (plating base plate), The sacrificial anticorrosion ability against iron has a high dissolution / corrosion rate, and the anticorrosion effect of zinc is lost in a relatively short period of time, making it difficult to obtain a long-term corrosion resistance life.

The easiest way to improve corrosion resistance is to increase the plating amount. However, an increase in the plating amount causes a marked decrease in productivity and an increase in cost in electroplating, which is not only economically undesirable, but also has the following problems in terms of workability and weldability.

That is, when a plated steel sheet is processed into an automobile part, particularly in the drawing process, the plating layer is peeled off or a part of it is scraped off (so-called powdering) and deposited on a press die,
There is a phenomenon that the product is flawed. If such powdering occurs, not only the productivity will be significantly reduced by the maintenance of the mold, but also the performance of the product will be adversely affected. Therefore, it is necessary to reduce the plating amount. On the other hand, resistance welding (spot welding) is mostly used for assembling various processed members, and importance is attached to good or bad weldability. The weldability is greatly affected by the plating amount, and if the plating amount increases to a certain extent or more, defects such as insufficient strength of the welded portion and poor appearance are likely to occur, and further, the life of the welding electrode is significantly reduced. Therefore, from the viewpoint of workability and weldability, it is desirable that the amount of plating is as low as possible. Furthermore, galvanized steel sheets for automobiles are finally coated, but since zinc is easily corroded by the coating film defects and the corrosive aqueous solution that permeates the coating film, coating film "blisters" (so-called blisters) are generated. However, there is a drawback that the coating film is generated and floats from the substrate and peels off.

Corrosion progresses from these coating film peeling portions, and the corrosion resistance life thereof is remarkably reduced.

Similarly, when it is used as a building material such as a roof or a wall material, long-term corrosion resistance life is required, and there is a strong demand for improvement of the corrosion resistance.

In these cases as well, they are often used after being coated, and like the above, the corrosive aqueous solution that has penetrated the coating film defects or coating film corrodes zinc and causes "blister" (so-called blister) on the coating film surface. However, there is a drawback that the coating film is peeled from the base material, corrosion from the coating film peeled portion progresses, and the corrosion resistance life is significantly deteriorated.

Such a problem is caused not only by the galvanized steel sheet, but also by the zinc-plated steel sheet (here, "zinc-based" means a zinc alloy with a eutectic structure containing zinc as a main component, or a polyamide-based or polyimide-based material for zinc, etc.). Of the organic matter, or those containing inorganic matter such as phosphorus, silica, and alumina), the corrosion rate of the plating layer itself in the corrosive environment is improved by alloying, but sacrificial anticorrosive action Since it depends on zinc in the alloy plating layer, there were often problems that corrosion resistance, coating film properties, etc. deteriorated.

(Means for Solving Problems) The present inventor has solved the problems of such a galvanized steel sheet or a zinc-based steel sheet, and applied coating such as corrosion resistance, end face corrosion resistance or coating adhesion, and post-coating corrosion resistance. Various studies were conducted for the purpose of obtaining a zinc-based plated steel sheet with excellent performance.

As a result, adjusting the raw steel composition of the zinc-based plated steel sheet,
Corrosion resistance and pitting resistance of the plated steel sheet are ensured by ensuring the anodic corrosion protection function of zinc, reducing the couple corrosion current to the plating base plate, and suppressing the dissolution rate of zinc at the plating defect portion or the end surface of the plated steel sheet by sacrificial corrosion of zinc. It was found that the corrosion resistance can be improved or the corrosion from the end face can be prevented. In addition, when a coating base treatment with excellent adhesion to the coating film is applied, zinc corrosion generation is suppressed at the coating film defective portion, the plating defective portion under the coating film, the end surface, etc. by suppressing the corrosion rate of zinc. It has been found that it is possible to improve the corrosion resistance after coating by preventing the coating film blistering (so-called blister) caused by the object, the peeling of the coating film, and the reduction of the coating peeling portion.

That is, a steel plate in which Cr is added to the plating original plate and Ti, N
By applying a zinc-plated layer or a zinc-based plated layer to a steel sheet to which one or more of b, V, and Zr are appropriately adjusted and added, the interaction between the original plating plate and the plated layer or the mutual effect is caused. It was found that the above-mentioned corrosion resistance and coating performance can be improved.

Further, it was found that the performance improving effect is further promoted by applying a Ni-based undercoating layer to the plated steel sheet having the above composition.

Therefore, the gist is (1)% by weight, C; 0.15% or less, acid-soluble Al; 0.005-0.10%, Cr; 1.5-20%
A zinc-based plated steel sheet having excellent corrosion resistance and coatability, which is obtained by applying a zinc-plated layer or a zinc-based plated layer to one or both sides of a steel sheet containing the.

(2)% by weight, C; 0.15% or less, acid-soluble Al; 0.005-0.10%, Cr; 1.5-20
%, One or more of Ti, Nb, V and Zr and 0.03 to 0.50% of steel plate containing a zinc plating layer or a zinc-based plating layer on one or both sides. Excellent corrosion resistance and paintability. Zinc plated steel sheet.

(3)% by weight, C; 0.15% or less, acid-soluble Al; 0.005-0.10%, Cr; 1.5-20
%, A zinc-based plated steel sheet excellent in corrosion resistance and coatability, which is obtained by applying a Ni-based undercoating layer and a zinc-plated layer or a zinc-based plated layer to one or both sides of the steel sheet.

(4) C: 0.15% or less by weight%, acid-soluble Al; 0.005-0.10%, Cr; 1.5-20
%, One or more of Ti, Nb, V, Zr and 0.03 to 0.50% of the steel sheet containing Ni-based undercoating layer and zinc-plated layer or zinc-plated layer on one or both sides We sometimes provide galvanized steel sheets with excellent corrosion resistance and paintability.

Hereinafter, the present invention will be described in detail.

A steel plate having the above-mentioned steel components manufactured through a normal steel plate manufacturing process is used as a plating original plate. In a corrosive environment where a zinc-based plated steel sheet is used, especially in a Cl ^- ion containing corrosive environment where the corrosion is remarkable, a steel sheet containing 1.5% or more of Cr, particularly C
A steel sheet containing 3% or more of r exhibits excellent corrosion resistance as compared with a steel sheet containing only Cr inevitable impurities.

At the same time, as shown in FIG. 1, in a Cl ⁻ ion-containing corrosive environment, when the Cr content is 1.5% or more, the couple corrosion current between the zinc plating layer and the plating original plate is remarkably reduced.

As a result, the corrosion current between the conventional galvanized layer and the original plating plate is extremely large, and the defective portion or the defective portion due to the flaw of the plated layer generated at the time of processing or the corrosive environment where the end surface contains Cl ^- ions. Although the galvanized steel sheet was inferior in corrosion resistance due to remarkable dissolution of the galvanized layer due to the sacrificial anticorrosive action, it can be solved by using the Cr-containing steel sheet as in the present invention.

In general, it is difficult to completely eliminate plating defects such as pinholes and non-plating, no matter how strictly the zinc-based plating layer is plated on the steel plate.

Therefore, as in the present invention, by significantly reducing the couple corrosion current between the zinc plating layer and the plating base plate, and using a steel plate containing Cr as an essential component of the plating base plate for anodic corrosion as a plating base plate, a plating defect portion or At the same time as the anodic corrosion of the end face part, the dissolution rate of the zinc-based plating layer in these parts is significantly suppressed, and the zinc-based plating layer is corroded,
Due to the corrosion resistance of the original plating plate itself even after disappearing, a zinc-based plated steel sheet having an extremely long corrosion resistance life can be obtained.

Further, this effect brings about an improvement in the adhesion of the coating film after aging and an improvement in the corrosion resistance after coating even when the coating film is used.

That is, in the defective portion reaching the base metal, the crack occurrence portion reaching the coating film surface due to processing, or the coating film defective portion, in order to suppress remarkable corrosion dissolution due to the sacrificial anticorrosive ability of the zinc-based plating layer, a corrosion product of zinc To prevent the formation of blisters on the coating film, to prevent the peeling of the coating film when exposed to a corrosive environment for a long time, to improve the paint adhesion in the defective part as described above, and to significantly improve the corrosion resistance after coating. The improvement effect can be obtained. Thus, Cr, which is the main component that obtains these effects,
The upper limit of the content is regulated to 20% or less.

When the Cr content exceeds 20%, the above effects are almost saturated, and the workability and weldability deteriorate. In particular, from the viewpoint of formability, in the composition of the transformation region of γ phase and α phase of Cr 11% or less, it is difficult for crystal grains to coarsen due to these transformations during steel sheet production, and it is subjected to severe forming process. In this case, a phenomenon called ridging which is extremely rough is less likely to occur, which is preferable. Therefore, the Cr content is set to 1.5 to 20% in view of the corrosion resistance against the corrosive atmosphere, workability, and weldability. It is preferably 3 to 11%.

As described above, the effect of Cr is the greatest from the viewpoint of corrosion resistance, but in the present invention, from the viewpoint of targeting a rustproof steel plate for automobiles or building materials, C and acid-soluble Al and other components are also
Its content is limited.

As the content of C increases, the amount of chromium carbide precipitated increases, which deteriorates the mechanical properties and corrosion resistance of the steel and, at the same time, hinders the uniform coverage of the galvanized layer. Therefore, C
The content is 0.15% or less, preferably 0.10% or less.

Further, in the present invention, when Ti, Nb or the like is added to further improve the workability and the corrosion resistance, the C content is 0.02%.
The following is preferable in terms of economy, workability, and uniform coating property of the plating layer due to precipitation of titanium carbide and the like.

Al is a small content of less than 0.005% acid-soluble Al (SolAl) remaining in steel, it is difficult to prevent the generation of bubbles due to oxidizing gas, and the surface defect occurrence rate of steel is remarkably high. High,
It becomes the starting point of deterioration of the corrosion resistance of steel materials. Excessive acid-soluble Al exceeding 0.10% causes Al-based oxides to be scattered on the steel surface, resulting in no plating, pinholes, etc. on the starting point of corrosion resistance deterioration or on the plated surface applied to this steel sheet. Occurs,
Impairs the soundness of the plating layer.

In addition, the present invention includes, in addition to the above steel components, Ti, Nb, Zr, and V containing 0.03 to 0.50% of one type or two or more types of Cr, which is contained by combining with C in the steel. Effectiveness is improved to further improve moldability and corrosion resistance.

When the content of steel components such as Ti is less than 0.03%, the effect of preventing the precipitation of chromium carbide and improving the formability and corrosion resistance is small, and when the content exceeds 0.50%, the effect becomes saturated. It is not economical, and the precipitation of these components causes the material to harden, which tends to deteriorate the moldability. The preferred content is 0.075-0.2
It is 0%.

In the steel plate itself composed of the above composition components,
Although it has excellent corrosion resistance, red rust occurs and corrosion resistance deteriorates remarkably in sea breeze areas or in places with severe corrosion such as snow melting salt for road freezing prevention sprayed. In addition, in applications where coating is applied to improve the corrosion resistance, it is difficult to uniformly apply a phosphate treatment as a coating base treatment for improving the adhesion of the coating, and the performance after coating is significantly deteriorated.

Therefore, it is insufficient in view of the corrosion resistance currently required for steel plates for automobiles or building materials.

Therefore, in order to impart more excellent corrosion resistance and coating performance to the plated steel sheet, a zinc-based plating layer is applied.

Thus, when used in the present invention, when a zinc-based plating layer is applied to a steel sheet containing Cr as an essential component, when compared with the conventional steel sheet itself, the couple corrosion current of the zinc plating layer and the plating original plate is significantly Decrease.

Therefore, the corrosion rate of the galvanized layer on the steel sheet due to sacrificial corrosion is significantly reduced, and the amount of corrosion of the plated layer when exposed to a corrosive environment is reduced.

Therefore, in comparison with the case of using a conventional steel plate containing Cr as an unavoidable impurity as a plating base plate, the zinc-based plated steel plate of the present invention maintains a sacrificial anticorrosion effect on the steel plate for a significantly long period of time, Corrosion rate is low and corrosion resistance is extremely excellent.

As a result of this, and even when painted,
The following advantages are obtained.

That is, even with respect to under-coating corrosion, the effect of reducing the dissolution rate of the zinc-based plating layer reduced the occurrence of coating blistering, which is thought to be due to the corrosion product of zinc, and was exposed to the corrosive environment for a long time. In this case, the paint adhesion is significantly improved.

In particular, when a coating film defect that reaches the base metal is generated, or on the end face and the like, coating film blistering is significantly reduced and coating film peeling is significantly reduced as compared with the conventional case. Therefore, the corrosion progressing from the coating film peeling portion is significantly reduced.

As described above, the zinc-based plated steel sheet of the present invention has its corrosion resistance life remarkably extended due to the increase of the corrosion resistance due to the decrease of the corrosion rate of the plating layer and the improvement of the corrosion resistance due to the improvement of the coating performance, together with the effect of complementing with the plating original plate. It

Next, in the present invention, the zinc plating layer and the zinc-based plating layer applied to the original plating plate are preferably the following plating layers.

That is, as the plating layer, a zinc plating layer having an alloy layer to the extent that plating separation does not occur during the forming process by the hot dipping method and a zinc plating layer by the electroplating method are applied. Further, the zinc-based plating layer is a mixture of 50% or more Zn with alloying elements, for example, Zn-Al, Zn-Al-Si, Zn
-Al-Sb, Zn-Al-Mg based alloy plated steel sheet and the like are included.

Next, the plating in the present invention is not particularly specified, and after the surface of the steel sheet is cleaned and activated, it is applied to one or both sides of the steel sheet by hot dipping, electroplating, vacuum deposition or the like. It The hot dipping method may be either a gas reduction method or a flux method. In the electroplating method, the zinc plating layer is formed using an electrolytic bath composition such as a (zinc sulfate-sodium sulfate) -based aqueous solution or a (zinc chloride-sodium chloride) -based aqueous solution which is usually used.

Although the thickness of the zinc-based plating layer is not particularly specified, a plating film layer having a thickness of 1.5 μm or more, preferably 3 μm or more is provided from the viewpoint of corrosion resistance and corrosion life by ensuring uniform coverage. To be Further, from the viewpoints of adhesion, forming workability, weldability, etc. of the coating layer, the coating layer is applied with a thickness of 25 μm or less, preferably 15 μm or less.

Further, in the present invention, in order to further improve the corrosion resistance and the coating film performance due to the combined effect of the steel plate having the above-described steel composition and the zinc-based plating layer, a Ni-based material is used as an intermediate layer between the plating base plate and the zinc-based plating layer. An undercoat layer is provided.

By providing this Ni-based undercoating layer, an effect of improving corrosion resistance can be obtained by reducing pinholes due to the superposition effect of the Zn or Zn-based alloy plating layer and the undercoating layer.

In addition, the metal or alloy forming the Ni-based undercoating layer has a relatively high diffusion rate with the zinc-based plating layer, and a Zn-Ni-based alloy layer is likely to be formed during paint baking, etc., reducing pinholes that reach the base metal. Further, the effect of improving corrosion resistance can be obtained.

Thus, in this Ni-based undercoat layer, Ni, Ni-Co alloy, Ni-
A P alloy, a Ni-Fe alloy, and a Ni diffusion treatment layer are provided in a thickness of 0.01 to 1 µ. This is because when the thickness is less than 0.01 μ, the uniform coating effect of the undercoating layer tends to be insufficient and the above-mentioned pinhole reduction effect tends to be unobtainable, and when the thickness exceeds 1 μ; This is because the above effect is saturated and the undercoat layer tends to crack during processing, which is not preferable.

Among these Ni-based undercoat layers, the method of providing a Ni diffusion coating layer is particularly excellent. This diffusion layer is provided with a base coating layer as described above and then a diffusion treatment layer is provided by utilizing the heating and annealing process of a cold rolled steel sheet, etc. Since it becomes an electrically noble plated base plate with a high concentration, the effect of improving the corrosion resistance of the base plate itself can be obtained, and at the same time, the couple corrosion current with the zinc-based plating layer is reduced, and plating defects and end face parts of the plating layer, etc. Corrosion rate due to sacrificial dissolution is reduced, and as a result, the corrosion resistance life of the plated steel sheet is extended, or the corrosion rate of the galvanized layer, etc. at the coating film defects and end faces that reach the base metal even after coating is reduced. In addition, the under-coating corrosion is remarkably suppressed, and the effect of improving the corrosion resistance after coating and the adhesiveness of the coating after aging can be further expected.

Next, in the present invention, the Ni-based undercoating layer to be applied to the plating original plate, the method for providing the zinc-based plating layer and the amount of adhesion thereof are not particularly limited, but about the method for providing the coating layer For example, the following method is used, and it is preferable to use the following amount in the attached amount.

That is, the Ni-based undercoating method is (1) nickel undercoating; electroplating method using nickel sulfate-nickel chloride-boric acid-based bath (2) nickel-cobalt alloy undercoating; nickel and cobalt ions of desired composition Electroplating method using nickel sulfate-cobalt sulfate-nickel chloride-cobalt chloride-boric acid-containing bath (3) Nickel-iron alloy undercoating; nickel sulfate containing the desired composition nickel and iron ions- Electroplating method using iron sulfate-boric acid system bath (4) Nickel-P alloy substrate treatment; nickel sulfate-nickel chloride-sodium hypophosphite-phosphoric acid system containing nickel and P ions of desired composition Electroplating using a bath is performed.

Further, in the case of providing a Ni-based diffusion treatment layer for the Ni-based undercoating method, the above Ni and Ni alloys are applied to the original plate by electroplating, or Ni ions or Ni ions and other alloying element ions are applied. The contained aqueous solution is applied to the surface of the original plating plate, and each is subjected to heat diffusion treatment.

For example, after the above Ni plating or nickel acetate-surfactant aqueous solution, or nickel acetate-ammonium phosphate-
After applying a surfactant aqueous solution with a roll coater and drying, heat diffusion treatment in a non-oxidizing atmosphere (for example, 650-910 ℃)
Heat treatment for 30 to 180 seconds) is performed.

When providing this diffusion coating layer, use either the as-cold rolled steel sheet (As Cold material) or the steel sheet that has been annealed after cold rolling (full finisher material) for degreasing, pickling, etc. After the surface cleaning and activation treatment, the Ni-based coating layer may be provided by the electroplating method or the aqueous solution coating method to perform the heat diffusion treatment. However, when cold-rolled steel sheet is
Providing a system-based coating layer and performing diffusion treatment simultaneously with annealing of the original plate is intended for short-time heat treatment because the process strain of the cold-rolled material further promotes mutual diffusion between the Ni-based coating layer and the steel sheet. A diffusion layer is formed, which is economically and industrially advantageous.

As described above, these Ni-based underground coating layers have a thickness of 0.
It is provided with a coating layer having a thickness of 01 to 1.0 μ.

As described above, the plated steel sheet composed of the steel sheet having the steel component of the present invention and the zinc-based plating layer, or the Ni-based underlayer and the zinc-based plating layer is the original plating plate when exposed to a corrosive environment. By the combined effect of the plating layer and the plating layer, the dissolution rate due to the sacrificial anticorrosive action of the plating layer at the plating defect, the processing flaw portion at the time of molding processing, or the end surface portion is reduced,
The corrosion resistance life of the plated layer is extended, and when used after being coated, the effect of improving the paint adhesion after aging and the corrosion resistance after coating is obtained by reducing the undercoat corrosion.

Thus, when the steel sheet of the present invention is used after being coated, a phosphate crystal coating or a chromate-treated coating layer is applied as a coating base treatment. In this case, when exposed in a corrosive environment for a long period of time, when the corrosive aqueous solution penetrates under the coating film, the coating undercoating layer is difficult to dissolve, and the adhesiveness between the coating film and the coating undercoat layer is extremely excellent. When the phosphate crystal film is applied, the excellent effect of improving the coating performance can be obtained.

That is, as described above, the zinc-based plating layer of the plated steel sheet of the present invention has a low dissolution rate in a plating defect portion, a flaw portion reaching the base iron, and the like, and thus a phosphate crystal insoluble in a corrosive aqueous solution. When the coating is applied as the coating base treatment, the following advantages are obtained as compared with the conventional galvanized steel sheet.

In zinc-based plated steel sheets, the phosphate crystal coating has a pore-sealing effect on pinholes and the like that are inevitably generated, but this is not always sufficient, and a corrosive aqueous solution that has penetrated into the defective portions of the phosphate crystal coating, etc. As a result, the dissolution rate of zinc due to the sacrificial anticorrosive action on the defective part of the plating layer is high, so the adhesion between the coating film and the phosphate crystal film is excellent in the conventional zinc-based plated steel sheet, but this corrosion portion of the plating layer The phenomenon that blistering of the coating film, peeling of the coating film, etc. often occurred.

However, in the steel sheet of the present invention, by suppressing the dissolution rate of zinc with respect to the plating layer defects, when a phosphate crystal coating film having excellent adhesion to the coating film is formed, The corrosion product has the effect of significantly reducing blistering and peeling of the coating from the interface between the phosphate coating and the plating layer.

Therefore, the adhesiveness of the coating film, the adhesiveness over time, and the corrosion resistance after coating are remarkably superior to those of the conventional zinc-based plated steel sheet. This advantage is that, even in a flawed portion or an end surface reaching the base steel, similarly, the dissolution of the plating layer is suppressed,
The same effects as described above are obtained, and extremely excellent effects are obtained for improving the adhesion of the coating film, improving the corrosion resistance after coating, and the like.

As described above, the present invention results from corrosion of the zinc plating layer due to excessive sacrificial anticorrosive action of the zinc plating layer by applying the phosphate crystal coating having excellent adhesion to the coating layer to the surface of the zinc plating layer. Since the peeling of the coating film and the deterioration of the corrosion resistance of the peeled portion of the coating film are suppressed, it is possible to provide a coated steel sheet having an extremely long corrosion resistance life.

(Example) Below, the Example of this invention is demonstrated with a comparative example.

Using a steel sheet having a steel composition in which the Cr content shown in Table 1 is mainly changed, a zinc-based plating layer or
A Ni-based undercoat layer and a zinc-based plating layer were provided.

That is, the galvanized layer by the electroplating method uses a cold rolled steel sheet (full finish material) in a (3% NaOH + 0.3% surfactant) type degreasing bath, and after degreasing and washing with water, a 10% H ₂ SO ₄ aqueous solution is used. Anodic pickling for 1 second at a current density of 20 A / dm ² at 50 ℃
Cathodic acid cleaning was performed for 1 second by electrolytic pickling and water cleaning to perform surface cleaning and activation treatment. Then, using a (350 g / zinc sulfate-80 g / sodium sulfate) -based electrolytic bath at 60 ° C., 4
A galvanized layer having a predetermined thickness was provided at a current density of 0 A / dm ² .

On the other hand, for the galvanized layer or the zinc-based plated layer by the hot dip plating method, the cold rolled As Cold material is used, and the hot dip galvanizing apparatus by the non-oxidizing furnace method is used.
A Zn-0.2% Al plating bath and a zinc alloy plating bath shown in Table 1 were used to form zinc-based plating layers each having a predetermined thickness.

Furthermore, in the case of providing a Ni-based undercoat layer, in the case of the undercoat layer by electroplating, the same method as the above electroplating method is used, and a Ni-based electrolytic bath composition of a predetermined thickness is used.
A Ni-based undercoat layer was applied to provide a zinc-based plating layer. In the case of Ni-based diffusion coating layer, As Cold material is used, and the surface is cleaned and activated by the same method as in the case of electroplating. At the same time, a diffusion layer was provided. After that, a zinc-based plating layer was applied. After that, untreated material or
Chromate treatment using a CrO ₃ —SiO ₂ system bath and full dip type phosphate treatment were carried out, and a prescribed coating amount was provided for each, and the following evaluation test was conducted.

As the performance evaluation test results show in Table 2, the steel sheet of the present invention has extremely excellent characteristics in corrosion resistance and coating performance as compared with the comparative steel sheet.

○ Evaluation test method (I) Corrosion resistance of unpainted material Corrosion resistance by salt water spray test By measuring the rate of red rust after 240 hours of salt water spray test, the corrosion resistance is evaluated from the state of red rust generated due to the plating defect. It was

◎ …… Red rust occurrence rate is less than 3% ○ …… 〃 3% or more and less than 10% △ …… 〃 10% or more and less than 30% × …… 〃 30% or more Corrosion resistance by cyclic corosion test 0.8mm thickness Using the evaluation material, (i) salt spray (5% NaCl 35 ° C x 4 hours) → (ii) dry (70 ° C humidity 60% 2 hours) → (iii) wet (49 ° C)
Humidity 98% 2 hours) → cooling (-20 ℃ x 2 hours) → (i)
The corrosion resistance was evaluated according to the following evaluation criteria by measuring the pitting corrosion depth after 50 cycles of the cyclic corosion test under the condition of salt spray ((i) to 1 cycle).

◎ …… Maximum perforation corrosion depth less than 0.3mm ○ …… 〃0.3mm or more-less than 0.45mm △ …… 〃0.45mm or more-less than 0.60mm × …… 〃0.60mm or more-perforation occurred Corrosion resistance by outdoor exposure test 0.8 5% NaCl water was sprayed to the evaluation material once per day using the evaluation material with mm plate thickness, and the outdoor exposure test was conducted for 2 years. After that, the pitting corrosion depth was measured and corrosion from the end surface of the evaluation material was performed. The situation was observed and the corrosion resistance was evaluated according to the following evaluation criteria.

◎ …… The maximum perforation depth is less than 0.25 mm, almost no corrosion from the end face ○ …… The maximum perforation corrosion depth is 0.25 mm or more and less than 0.40 mm, some corrosion from the end face occurs △ …… The maximum perforation corrosion depth From 0.40 mm or more to less than 0.60 mm, the corrosion from the end face part is possible × …… Maximum pitting corrosion depth 0.60 mm or more to partial pitting corrosion, and the initial shape of the end face due to corrosion from the end face part Almost none (II) Corrosion resistance of coating materials Coating performance after salt spray test A 20μ thick coating was applied by cation electrodeposition, and scratch marks reaching the base steel were added, and the coating film after 300 hours of salt spray test The blister generation state and the maximum perforation corrosion depth of the scratch part were measured, and the paint adhesion and the post-coating corrosion resistance after the lapse of time were evaluated for the coating film defect part.

The evaluation criteria were as follows.

◎ …… Maximum swell width on one side from scratch is 3.5mm or less and maximum pitting corrosion depth is 0.1mm or less ○ …… Maximum swell width on one side from scratch is 5mm or less and maximum pitting corrosion depth is 0.2mm or less △ …… Maximum swell width on one side from scratch of 7.5 mm or less and maximum puncture corrosion depth of 0.3 mm or less × ... Maximum swell width on one side from scratch of more than 7.5 mm or maximum pitting corrosion depth of more than 0.2 mm Cyclic corosion test Coating performance A cationic electrodeposition coating material (coating thickness 18μ) was used to add scratches that reach the base metal, and the coating film blistering width and scratches after 100 cycles of the cyclic corosion test under the conditions above The maximum perforation corrosion depth of the part was measured, and the coating film defects were evaluated, focusing on the paint adhesion after the passage of time and the corrosion resistance after coating. In addition, the evaluation criteria were based on the following methods.

◎ ... Maximum swell width on one side from scratch of 5mm or less and maximum pitting corrosion depth of 0.1mm or less ○ …… Maximum swell width of 7.0mm or less on one side from scratch and maximum pitting corrosion depth of 0.2mm or less △ ...... Maximum swell width of 8.5 mm or less on one side from the scratch part and maximum piercing corrosion depth of 0.3 mm or less × …… One side maximum swell width of 8.5 mm or more from the scratch part or maximum piercing corrosion depth of 0.3 mm or more Corrosion resistance by cyclic corosion test Using a 20 μm thick cationic electrodeposition material with a plate thickness of 0.8 mm, observation of the red rust occurrence state of the end face portion after 75 cycles under the cyclic corosion test conditions of the above item, The evaluation was carried out as one measure showing the corrosion state of the plating layer on the end surface and the coating film performance.

◎ …… Red rust occurrence rate is less than 20% ○ …… 〃 20% or more to less than 40% △ …… 〃 40% or more to less than 60% × …… 〃 60% or more Molding workability 0.8mm × 480 × 480mm blank size From this, with a wrinkle holding pressure of 20T, a 200 × 200 mm size, 110 mm deep drawing square tube drawing was performed, and the cracking status and surface “rusting” (riding) occurrence status were compared in relative terms, and the molding process was performed. The sex was evaluated.

◎ …… Excellent ○ …… Satisfactory good △ …… Satisfactory inferior × …… Very inferior

[Brief description of drawings]

FIG. 1 is a graph showing a couple corrosion current in a 5% NaCl aqueous solution when various types of steel plates having different Cr contents are plated with Zn.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Reference number within the agency FI Technical indication location C25D 5/26 F (72) Inventor Koji Umeno 1-1-1 Edemitsu, Hachimanto-ku, Kitakyushu, Fukuoka Inside Nippon Steel Co., Ltd. Yawata Works (72) Inventor Kenichi Asagawa 1-1-1 Edamitsu, Yawatahigashi-ku, Kitakyushu, Kitakyushu, Fukuoka (56) References Japanese Patent Laid-Open No. 57- 89494 (JP, A) JP-A-57-171692 (JP, A)

Claims (4)

[Claims] 1. By weight%, C: 0.15% or less, acid-soluble Al; 0.005-0.10%, Cr; 1.5-20%
A zinc-based plated steel sheet having excellent corrosion resistance and coatability, which is obtained by applying a zinc-plated layer or a zinc-based plated layer to one or both sides of a steel sheet containing the. 2. By weight%, C: 0.15% or less, acid-soluble Al; 0.005-0.10%, Cr; 1.5-20
%, One or more of Ti, Nb, V and Zr and 0.03 to 0.50% of steel plate containing a zinc plating layer or a zinc-based plating layer on one or both sides. Excellent corrosion resistance and paintability. Zinc plated steel sheet. 3. By weight%, C: 0.15% or less, acid-soluble Al; 0.005-0.10%, Cr; 1.5-20
%, A zinc-based plated steel sheet excellent in corrosion resistance and coatability, which is obtained by applying a Ni-based undercoating layer and a zinc-plated layer or a zinc-based plated layer to one or both sides of the steel sheet. 4. By weight%, C: 0.15% or less, acid-soluble Al; 0.005-0.10%, Cr; 1.5-20
%, One or more of Ti, Nb, V, Zr and 0.03 to 0.50% of the steel sheet containing Ni-based undercoating layer and zinc-plated layer or zinc-plated layer on one or both sides Galvanized steel sheet with excellent corrosion resistance and paintability.