JPH01212799A - Production of electroplated steel sheet - Google Patents

Abstract

PURPOSE:To produce the subject Z electroplated steel sheet having excellent corrosion resistance and workability by incorporating a specified amt. of condensed phosphate into the Zn and Zn alloy plating soln. contg. soluble metal salts for forming metal oxides, and uniformly and compositely coprecipitating the fine particles of the metal oxides. CONSTITUTION:When a steel sheet, etc., are plated with Zn or a Zn alloy such as Zn-Ni, 0.1-30g/l of the water soluble metal salts of Mo, Ti, V, Cr, Mn, W, etc., are incorporated into the Zn or Zn alloy plating soln., and further 0.05-1.0wt.% of the condensed phosphates such as Na4P2O7 and Na5P3O10 are added to the soln. When electroplating is conducted by using such a plating soln., the fine particles of the oxides of Mo, Ti, Cr, V, Mn, and W are uniformly dispersed and coprecipitated in the plating film of Zn or a Zn alloy without being flocculated, the plating soln. is not deteriorated with the lapse of time, and a steel sheet excellent in corrosion resistance and workability and having a plating film of coprecipitated fine particles of metal oxides is obtained.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to a method for producing zinc and zinc-based alloy electroplated steel sheets with excellent corrosion resistance in which fine metal oxide particles are compositely eutectoided in the plating layer. It is.

(Prior art) In recent years, surface-treated steel sheets for automobiles have been required to have high corrosion resistance that can withstand harsh corrosive environments, especially in consideration of salt spraying on roads in cold regions. .

However, conventional electrogalvanized steel sheets have insufficient corrosion resistance even if the coating weight is increased. Against this background, in recent years, various zinc-based alloy electroplated steel sheets such as Zn-Ni and Zn-Fe, which have superior corrosion resistance than electrogalvanized steel sheets, have been developed, and their practical use is progressing. There is. Furthermore, recently, various so-called dispersion electroplated steel sheets in which fine particles such as metal oxides are compositely eutectoided in the plating film have been developed, and many patents have been filed for these.

Regarding dispersion electroplated steel sheets, for example, Japanese Patent Publication No. 61-47919 discloses an electrogalvanized steel sheet in which alumina is compositely eutectoid;
The publication contains 1 of silica, titanium oxide, and zirconium oxide.
Electrogalvanized steel sheets with composite eutectoids of more than
Japanese Patent Publication No. 56-49999 discloses an electrogalvanized steel sheet in which Stow is compositely eutectoid.

It is certainly possible to improve corrosion resistance by applying a dispersion electroplating method to electrogalvanized steel sheets or zinc-based alloy electroplated steel sheets. However, when these plated steel sheets subjected to dispersion electroplating are press-formed, there is a drawback that a so-called powdering phenomenon occurs in which the plating film peels off in the form of powder. In the case of dispersion electroplating, some of the oxides eutectoided in the plating film are present at the interface between the plating film and the steel sheet, reducing the adhesion of the plating film and causing powdering.

This powdering phenomenon becomes a big problem because it affects the quality of surface-treated steel sheets for automobiles. In other words, surface-treated steel sheets for automobiles always involve press forming, but poor powdering resistance not only reduces workability and post-processing corrosion resistance, but also causes peeled plating film pieces to adhere to the press mold. This can also cause scratches on the plating film. Due to such problems, it is no longer possible to take measures such as increasing the amount of plating deposited to improve corrosion resistance.

As described above, conventional dispersion electroplating has problems in terms of quality as well as difficulties in manufacturing as described below.

The production of surface-treated steel sheets for automobiles mainly involves electroplating, which involves unwinding a cold-rolled steel sheet wound into a coil, continuously introducing it into a plating bath, and plating it by continuously passing a large current through it. equipment is used. For this purpose, the amount of plating solution used is extremely large, and the equipment used is large-scale. In conventional dispersion electroplating, the oxide to be dispersed and eutectoided into the film is directly suspended in the plating bath. In large-scale electroplating equipment that uses electroplating, it is extremely difficult to maintain the dispersed particles uniformly in the plating bath and to bring the dispersed particles close to the steel plate surface at a constant rate to uniformly eutectoid. Therefore, it is difficult to say that the resulting plated steel sheet has excellent uniformity in quality.

In order to solve these problems, the present inventors aimed at dispersion electroplating by directly suspending the oxide to be eutectoid in the plating film in the plating bath, targeting Zn-Ni alloy electroplating. We have previously filed a patent application for an improved manufacturing method that is different from the method of manufacturing (Japanese Patent Application Laid-Open Nos. 62-50497 and 62-67197).

The invention of the earlier application is a method in which metal ions of the target oxide are added to a Zn-Ni alloy electroplating bath in the form of a soluble salt, and the oxide is generated from the ionic state by an electrode reaction during plating. According to this method, compared to the method of dispersion electroplating by directly suspending the oxide in a plating bath, the oxide to be dispersed and eutectoid exists in an ionic state until just before it approaches the surface of the plated steel sheet. Therefore, it is easily maintained uniformly in the plating bath, there is little coagulation of the dispersed particles, and the oxide is uniformly incorporated into the plating film in a very fine form. Therefore, the obtained Zn-Ni alloy electroplated steel sheet has excellent quality without deterioration in workability.

(Problem to be Solved by the Invention) However, it is not possible to apply the invention of the earlier application to zinc or zinc-based alloy plating and to manufacture zinc or zinc-based alloy electroplated steel sheets by continuously applying electricity on an industrial scale. It has been newly discovered that there is a problem in that the properties of the plating bath and the plated steel sheets manufactured using the plating bath deteriorate over time.

In other words, some of the oxides generated by dispersion electroplating that are not incorporated into the film aggregate, and the aggregated oxides become suspended, which is the same state as in conventional dispersion electroplating, and processing of the plated steel sheet is difficult. It causes a decline in sexuality. Not only that, but the oxide generated by the electrode reaction using the aggregated oxide as a core is consumed by the growth of the aggregated oxide, which essentially reduces the amount of effective oxide that is eutectoided into the plating film. This may even lead to a decrease in corrosion resistance.

The object of the present invention is to provide a zinc and zinc-based alloy electroplating bath in which metals are added in the form of soluble salts to generate metal oxides to be compositely eutectoided into a film, and the oxides are composited using electrode reactions. To continuously produce zinc and zinc-based alloy electroplated steel sheets of excellent quality, with no deterioration in workability and corrosion resistance over time even when dispersion electroplating is carried out on an industrial scale for long periods of time. The goal is to provide a method that can be used.

(Means for Solving the Problems) The dispersion electroplating method of the prior invention in which oxides are generated and eutectoided by an electrode reaction is an excellent method that can produce stable and homogeneous dispersion electroplated steel sheets. However, as mentioned above, there is a problem in that continuous plating for a long time causes oxides to aggregate and deteriorate the quality of the plated steel sheet over time. Therefore, the present inventors believe that in order to prevent oxide agglomeration, other substances effective in preventing agglomeration may be further added to the zinc and zinc-based alloy electroplating baths to which soluble salts have been added. Based on the idea that oxides are effective, we conducted intensive research into such substances, and as a result, we discovered that condensed phosphates have the most effective effect of preventing oxide aggregation, leading to the present invention. .

Here, the gist of the present invention is that ``0.05 to 1.0 w is added to the plating bath to which a soluble salt of a metal is added to generate a metal oxide to be compositely eutectoided and dispersed in the plating film.
``A method for producing an electroplated steel sheet'' characterized in that electroplating is carried out using a zinc and zinc-based alloy electroplating bath to which t% of condensed phosphate is added.

In carrying out the present invention, the condensed phosphates include:
Na4PtO+ , Na5P301o, Na1PaO+
It is desirable to use one or more of the condensed phosphates having the chemical formulas s and Na4PJ+s.

A condensed phosphate having the above chemical formula will stably exist in the bath without forming a poorly soluble salt with zinc, and a long lasting surfactant effect can be obtained.

Furthermore, in the present invention, the type of metal to be compositely eutectoided as a metal oxide in the plating film is not particularly limited. For example, any metal can be used as long as it is eutectoidally dispersed in the grain coating. , Ti, V, Cr, Mn, W
etc.

Then, one or more of these metals is used in the form of a soluble salt, for example, the metal for producing an oxide is Mo.
In the case of Na! As Mo0a, (NH4) Jon4, if the same <Ti, as Tiz(SOa)i,
Similarly, in the case of v, VC/! z, VCj! As z,
Similarly, in the case of Cr, the same (Mn
In the case of Mn(, l!, MnCj! 2 H48z
As O, in the case of W, WClz and WCla are 0.1 to 30% in zinc and zinc-based alloy electroplating baths.
If added at 1 g/g, the effect of improving corrosion resistance can be obtained. Furthermore, there is no significant difference in the effects depending on the type of these metals.

(Function) Hereinafter, the method for manufacturing highly corrosion-resistant zinc and zinc-based alloy electroplated steel sheets of the present invention will be explained in detail along with its functions and effects.

In the above-mentioned dispersion electroplating that utilizes the electrode reaction of metal salts, the mechanism by which a metal changes from an ionic state to an oxide is as follows, taking titanium as an example. That is, the ions added in the form of Ti3' to the plating bath exhibit a reaction as shown in equation (1) and are anodized.

? +"+HzOe--TIO"+2H"...11
) On the other hand, on the surface of the steel plate, which is the cathode, hydrogen is generated along with metal precipitation due to electricity. For this reason, near the cathode interface (steel plate surface), all increases due to the consumption of H, and Ti generated by the reaction shown in equation (1) near the steel plate surface
Due to the shift in pH equilibrium, the e'' ion becomes an oxide through the reaction of the following equation (2).

Tie"10J→Ti0z+H" ... (2) The oxide produced by the reaction shown in equations (11 and (2) above) adsorbs H (hereinafter referred to as protons) on its surface, and its electrical charge increases. It exists stably in the plating bath due to the repulsive force of the electrostatic force, but it is inevitable that some of it will aggregate in order to obtain sufficient static electricity. Therefore, to prevent this aggregation, it is necessary to use substances other than protons. One possible method is to make the substance adsorb onto an oxide and give the oxide an electric charge based on the charge of the substance, thereby increasing its repulsive force. There are several known additives that provide charges other than protons, but there are many different types, and there are many different types of particles. The effects vary depending on the environment, etc., and it is extremely difficult to select the optimal additive. For example, examples of additives include those disclosed in JP-A-62-87242, JP-A-59-193300, etc., but their use is limited to applications other than plating baths or even in plating baths. under low current, weak acid conditions. If such additives are used in an industrial-scale plating environment under special conditions of low pH, high temperature, and heavy current, the additives will decompose due to electrolytic reactions, not only reducing their effectiveness but also causing precipitation. Characteristics of plating film
There are times when I am a student.

Therefore, the present inventors conducted extensive research into additives that can prevent the agglomeration of oxides and do not require consideration of the above considerations, and as a result, condensed phosphates were found to be optimal. We have also discovered that if a small amount of this condensed phosphate is added, agglomeration of oxides can be prevented even when electricity is continuously applied for a long time.

By adding a small amount of this condensed phosphate to a zinc and zinc-based alloy electroplating bath containing a soluble metal salt to generate oxides, agglomeration of oxides is prevented, resulting in stable and excellent quality. The exact reason why zinc and zinc-based alloy electroplated steel sheets can be obtained is not clear, but it is thought to be because condensed phosphates have the following effects. Department,
Anions adsorb to the condensed phosphates and provide a negative charge to the oxides, thus preventing oxide aggregation. In addition, it is believed that it is possible to produce products with stable quality by preventing partially aggregated oxides from approaching the cathode, which is the surface of the steel sheet, and from eutectoid deposition.

Such effects are obtained when the concentration of condensed phosphate is 0.05
It can be seen from wt% or more. However, if the amount is less than this, agglomeration of oxides cannot be prevented, leading to deterioration of workability and corrosion resistance over time.

This effect also occurs when the concentration of condensed phosphate is 1. Qwt%
becomes saturated. Therefore, increasing the concentration any further would be meaningless and would result in an economic disadvantage.

For these reasons, in the present invention, the amount of condensed phosphate added is 0.05 wt% to 1. It is specified as Oi+t%.

The present invention will be further explained below with reference to Examples.

(Example) A plating solution having the plating bath composition, pH, and bath temperature shown in (1) to (3) below was prepared in a plating tank of 12 beakers, and (
Zinc and zinc-based alloy electroplating was carried out according to the energization conditions shown in 4), aiming for a coating weight of 208/m" and a plating area of 1 dm" per sample. Evaluation samples with a content of 11.5 to 12.5 wt% were successively produced.

Conditions (1) Plating bath composition (a) (Zn plating) ZnSO, -7H,O: 400g/INazSO
a: 100g/e (b) (Zn-N
i alloy plating] Zn5O,-78,O: 100-240g/lN
15Oa・6H,O: 260-300g/INa
, SO,: 100g/ 1 (c) (Zn-Fe alloy plating) ZnSOa ・7) IJ: 250g/ 42F
eSO4・7HzO: 250g/ INazSO4
: Add soluble salts and condensed phosphates of the types shown in Table 1 based on 80g/1-.

(2) pH: 1.0 to 3.0 (3) Bath temperature: 50 to 60°C (4) Current conditions: 60 to 100 A/d+w” The obtained evaluation samples were evaluated for deterioration over time in workability and corrosion resistance. The results and the amounts of soluble salts and condensed phosphates added are also shown in Table 1.

In Table 1, evaluation 1 is an evaluation sample that was plated immediately after the plating bath was prepared, evaluation 2 is an evaluation sample that was plated after continuous energization of the equivalent of 5 evaluation samples, and evaluation 3 is an evaluation sample that was plated after continuous energization of the equivalent of 5 evaluation samples.
means the characteristics for the evaluation sample plated after continuous energization for the same <100 sheets, and the evaluation 4 means the characteristics for the evaluation sample plated after the same <200 sheets energized.

Corrosion resistance was evaluated by conducting a salt spray test using 5% salt water and determining the time until red rust appeared.

For workability, a 90mmφ disc blank was taken from the evaluation sample, deep drawn into a cylindrical shape with a diameter of 501m + *φ and an opening depth of 28, and the plating film on the side wall surface was peeled off with adhesive tape. A test was conducted, and the amount of peeling was visually inspected and evaluated on a five-point scale. Rating: 5: No peeling at all, 4:
: Tape area with peeling pieces attached to the total area of the tape is less than 10%, 3: Same <10% or more and less than 30%, 2: Same <30% or more and less than 50%, and the entire surface of the tape is It was ranked as having peeling pieces attached.

As is clear from Table 1, Comparative Examples Nα1, Nα7, and Nα9, which are ordinary zinc and zinc-based alloy electroplating without the addition of soluble salts and condensed phosphates, have no deterioration in corrosion resistance and workability over time; The quality is poor. Comparative examples Nα2, Nα3, Nα8 and Nα in which condensed phosphate is not added or the amount added is less than the range specified in the present invention
In No. 10, the effect of improving corrosion resistance and workability due to dispersion plating is seen after the plating bath is prepared, but the effect has already been lost after energizing five evaluation samples. It is clear that in Comparative Example No. 4, in which the amount of condensed phosphate added is larger than the range specified by the present invention, there is no deterioration over time, but the effect is saturated. In addition, Comparative Example No. 5 and Comparative Example Nα6, which are conventional dispersion plating in which no condensed phosphate is added and a special grade powder reagent is used as the oxide and directly suspended in the plating bath, show little deterioration over time. Its quality is poor, especially its workability.

In contrast to these comparative examples, all of the present invention examples Nα1 to No. 16 have stable quality over a long period of time without deterioration of corrosion resistance and workability over time. In other words, the inventive example achieved long-term quality stability that was 20 times or more greater than that of the comparative example.

(Effects of the Invention) As explained above, according to the method of the present invention, even if dispersion electroplating is performed with continuous current application, oxide agglomeration does not occur, so there is no deterioration of the plating bath over time.Therefore, corrosion resistance and workability are improved. It is possible to continuously produce zinc and zinc-based alloy coated steel sheets of excellent quality and stability over a long period of time.

Claims (1)

[Claims] Zinc in which 0.05 to 1.0 wt% of condensed phosphate is further added to a plating bath to which a soluble salt of a metal is added to generate a metal oxide to be compositely eutectoided and dispersed in a plating film. and a method for producing an electroplated steel sheet, comprising performing electroplating using a zinc-based alloy electroplating bath.