JPH0280593A - Production of zn alloy electroplated steel sheet having superior chemical treatability - Google Patents

Abstract

PURPOSE:To improve the chemical treatability of the plated surface of a steel sheet by plating the steel sheet with a Zn alloy in a plating soln., further plating the plated surface by a specified coating weight in a plating soln. having the same compsn. at a specified current density and immersing the steel sheet in the plating soln. without supplying electric current. CONSTITUTION:A steel sheet is electroplated with a nearly prescribed coating weight of a Zn alloy in a Zn alloy electroplating soln. The plated surface is further electroplated with >=0.1g/m<2> Zn alloy in a plating soln. having the same compsn. at 1-10A/dm<2> current density and the steel sheet is immersed in the Zn alloy plating soln. without supplying electric current. The chemical treatability of the plated surface of the steel sheet can be improved.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to a method for manufacturing a Zn-based alloy electroplated steel sheet,
In particular, it relates to improving the chemical conversion treatment properties of plated surfaces.

<Conventional technology> Zn-based electroplated steel sheets are used in automobiles because they can achieve high corrosion resistance without deteriorating the material quality.

Applications are expanding to fields where cold-rolled steel sheets have traditionally been used, such as home appliances and building materials.

In recent years, Zn-based alloy electroplated steel sheets have been developed for the purpose of improving corrosion resistance, and have been applied as rust-preventive plated steel sheets for automobile bodies, and their production has increased significantly.

By the way, phosphate crystals generated on Zn-based alloy electroplated steel sheets during phosphate chemical conversion treatment are called hopeite (
Zns(POa)t H4111O), this hope
i Le is phosphophy generated on cold rolled steel sheet
llite (Zn, Fe(PO4)*・4H!) Since the adhesion of the paint is inferior to that of 01, it is mainly used on the inside of the car body, and there are problems when using it on the outside of the car body.

Furthermore, Zn-based alloy electroplated steel sheets tend to form a non-uniform passive film on the surface of the plating, and in the subsequent chemical conversion treatment with phosphate, the film formation tends to be uneven (and this unevenness It also affects the coating that is applied later, resulting in uneven appearance after painting, which can significantly impede the product value in terms of appearance.In particular, Zn-based alloy electroplated steel sheets are being adopted for the exterior of automobiles to improve the durability of automobiles. can be,
The above-mentioned unevenness also poses a problem as well as the adhesion of the paint.

To address these problems, conventionally,
The two-layer plating method disclosed in Japanese Patent Publication No. 8 and Japanese Patent Publication No. 60-57518 is used. This technology improves the chemical conversion treatability and paintability of Zn-based alloy plating by applying Fe lynch plating on top of Zn-based plating, and sharing the roles of corrosion resistance in the lower layer and chemical conversion treatability in the upper layer. It is carried out.

However, the above technology has the following drawbacks.

(1) Upper layer plating increases costs.

(2) Since the basis weight of the upper layer plating is small, the upper layer plating can be controlled easily.

(3) Since a harmful metal is plated on a base metal, as the amount of upper layer plating increases, the amount of melting of the lower layer metal increases, resulting in poor corrosion resistance.

(4) Because different types of plating solutions are used, existing lines require equipment modification.

(5) Since different types of plating solutions are used, quality deterioration occurs due to contamination. Also, contamination control is costly.

<Problems to be Solved by the Invention> An object of the present invention is to provide a method for manufacturing a Zn-based alloy electroplated steel sheet that eliminates the above problems and has excellent chemical conversion treatment properties.

Means for Solving the Problems> The present inventors have conducted extensive research and investigation into the cause of the uneven chemical conversion treatment.

For example, when Zn-Ni alloy electroplating has uneven chemical formation, a large number of films of zinc hydroxide, zinc oxide, etc. are formed unevenly on the plating surface layer, whereas when there is no uneven chemical formation, It was found that the amount produced was small and uniform. In other words, the cause of uneven chemical conversion is that there are many films of zinc hydroxide, etc. present on the plating surface layer (and they are formed unevenly), which causes differences in the chemical conversion reaction and causes uneven chemical conversion. It became clear.

Therefore, the present inventors proposed that the surface layer of Zn-based alloy electroplating be coated with zinc hydroxide, etc., without making any changes to the existing Zn-based alloy electroplating equipment (and assuming the use of existing plating solutions). Various methods were investigated to suppress and uniformize film formation.

As a result, the present invention has been achieved. In applying Zn-based alloy electroplating to a steel plate, first, Zn-based alloy electroplating is applied to a substantially predetermined basis weight, and then the Zn-based alloy is further applied to the plated surface. Plating solution with the same bath composition as alloy electroplating, current density: 1 to IOA/dd, adhesion 1:
0.1g/n? This is a method for producing a Zn-based alloy electroplated W4 sheet with excellent chemical conversion treatment properties, which is characterized by applying the above-mentioned Zn-based alloy electroplating and then immersing the plate in the Zn-based alloy plating solution without applying electricity.

<For production> The present invention will be explained in detail below.

The Zn-based alloy electroplated steel sheet in the present invention broadly includes Zn-based alloy-plated steel sheets mainly containing Zn, such as Zn Ni-based, ZnFe-based, Zn-Mn-based, Zn-based alloy electroplated steel sheets, etc.
Alloy plating such as n-M system, and laminated plating including these, Soa Rani Co, Cr, Sn, S
b, VFe, T1. P, TI, As,
Pb, 81. In, Cdt, Ir, etc. are included unavoidably mixed or intentionally added, and sulfuric acid bath,
It is plated in a chloride bath or a mixed bath.

Normally, in order to industrially manufacture Zn-based alloy electroplated steel sheets, after performing pre-treatments such as electrolytic degreasing, water washing, pickling, and water washing, an electrolytic solution of a predetermined composition is used, and several types or ten tanks are used. An electroplated steel sheet with a predetermined alloy content and a predetermined coating weight is produced by sequential cathodic treatment in a plating bath consisting of:

In the present invention, after electroplating of a Zn-based alloy with a predetermined alloy content and approximately a predetermined deposition amount is completed, for example, a plating bath is further followed and immediately applied with the same plating solution as in the previous stage at a current density of 1 to 1. Adhesion amount is 0.1g/IOA/dj
After performing the above Zn-based alloy electroplating, it is further electrolessly immersed in the same or similar type of plating solution as in the previous step. After that, the process is the same as normal, but
Immediately without leaving the electrolyte on the steel plate to be plated,
The product is washed thoroughly and dried.

In the subsequent Zn-based alloy electroplating of the present invention, the current density is 1 to IOA/di, the deposition amount is Q, and Ig/rr is 1 or more.

The reason for this is that the current density is lower than that during normal plating, and the IO
By setting it below A/dd, the supply of metal ions to the cathode is delayed. Furthermore, the increase in ρ11 at the cathode interface is suppressed, and the formation of a barrier layer of zinc hydroxide is slowed down. Due to these synergistic effects, the formation of films such as zinc hydroxide on the plating surface layer is reduced and made uniform, thereby making the activity level of the plating surface uniform and improving the chemical conversion treatment properties.

Here, the current density is less than IA/di or the deposition amount is 0.1
If the current density is less than g/nf, the effect of improving chemical conversion treatment properties is small, while if the current density exceeds IOA/-, the effect disappears.

Further, the upper limit of the amount of plating is not particularly limited, but it is preferably Ig/rrf or less from the viewpoint of economic efficiency in line operation and the amount of plating commensurate with the etching layer during chemical conversion treatment.

Next, in the present invention, by further adding an electroless immersion treatment, the film of zinc hydroxide, etc. on the surface layer of the plating, which is the cause of the deterioration of the chemical conversion treatability mentioned above, is further completely dissolved and removed, further improving the chemical conversion treatability. It is something to do.

The pl+ of this electroless immersion liquid is preferably 0.5 or more and 2.5 or less.The reason is that if the pH is less than 0.5, the plating layer will melt too much and the plating area weight will decrease, which is economically disadvantageous. be. Moreover, if pi exceeds 2.5, the dissolving power decreases, requiring a longer soaking time, which reduces production capacity, which is not a good idea.

The immersion time is preferably 1 second or more and 20 seconds or less. Increasing the immersion time beyond 20 seconds is not a good idea because it reduces productivity, and on the contrary, it causes a decrease in chemical conversion treatment properties.If the immersion time is less than 1 second, there is little effect of improving chemical conversion treatment properties.

Further, the present invention naturally covers single-sided plating or double-sided plating.

Next, the present invention will be explained based on examples.

<Example> A cold-rolled 11 piece with a thickness of 0.70 and equivalent to 5 pcc was used for plating.The plating conditions were as follows.

Zn-Ni bath composition pn: 1.5 Bath temperature: 60°C 1t flow density: 50A/dj Plating time 715sec Plating weight: 20 g/m2<N+ content 12%
) Immediately after plating, using a plating bath having the same composition as above, cathodic treatment and electroless treatment were performed as shown in Table 1, followed by washing with water and drying.

In addition, the pH of the bath used for immersion! Fl adjustment was performed with sulfuric acid and sodium hydroxide.

The chemical conversion treatment was performed using a phosphate treatment liquid Granozin SD2000 (dip method) manufactured by Nippon Paint.

Chemical conversion treatment properties were evaluated by visual inspection of the appearance, and scanning electron microscopy observation at 10 locations at random for each sample, and evaluation was made based on uniformity, crystal density, etc. The evaluation criteria for the chemical conversion film at this time are as follows.

Chemical conversion film appearance evaluation: 5...The appearance is uniform and without unevenness, and the crystals are uniform and dense in all 10 locations.

4... Appearance is uniform and not uneven, but there are two crystals that are slightly uneven 3... Appearance is uniform and not uneven, but there are 4 crystals that are slightly uneven 2... Appearance is uneven in some areas The crystals are non-uniform in all 10 locations, and 5 of them are a mixture of coarse and fine grains.

l...The appearance is uneven over the entire surface, and the crystals are a mixture of coarse and fine grains in all 10 locations.

Hot water secondary adhesion test: After chemical conversion treatment, 20 g of cationic electrodeposition coating (Power Top U-30, manufactured by Nippon Paint), 30-9 intermediate coat, 40 Q top coat (both amino acid paint, 1 manufactured by Nippon Paint), 1 total of 901! After applying 3 coats of 50°C and immersing it in pure water at a temperature of 50°C for 240 hours,
For each location, use a knife to make 100 2m square scratches in the shape of a substrate, reaching the substrate, and remove the cellophane tape for 2 minutes.
The remaining rate (%) of the coating film was determined. Evaluation of water resistant secondary adhesion is as follows.

5...The average value of the coating film remaining rate at 10 locations is 100%4
... 90-99%3.
・・70～89%2・・
・50-69%1...
The results for each case of less than 50% are shown in Table 1.

In addition, as a comparative example, a steel plate plated in the same manner as in the example before the subsequent treatment was subjected to the cathodic treatment and/or electroless treatment shown in Comparative Examples 1-10 in Table 1, and a steel sheet plated with no subsequent treatment of Comparative Example 11. The chemical conversion treatment property and water resistant secondary adhesion of the untreated product were evaluated in the same manner.

All of the Examples of the present invention exhibited superior chemical conversion treatment properties and water-resistant secondary adhesion compared to the Comparative Examples.

<Effects of the Invention> As detailed above, according to the present invention, chemical conversion treatment can be improved without making any changes to existing Zn-based alloy electroplating equipment and by using a plating solution for several times. can be improved.

Claims (1)

[Claims] When applying Zn-based alloy electroplating to a steel plate, first apply Zn-based alloy electroplating with approximately a predetermined basis weight, and then apply a plating solution on the plated surface with the same bath composition as that of the Zn-based alloy electroplating at a current density of 1. -10 A/dm^2, coating amount: 0.1 g/m^2 or more of Zn-based alloy electroplating, and then further non-current immersion in the Zn-based alloy plating solution. A method for manufacturing a Zn-based alloy electroplated steel sheet with excellent properties.