JPH0633296A - Method for galvanizing on steel sheet - Google Patents

Abstract

PURPOSE:To obtain a galvanized steel sheet excellent in uniformity and adhesion by subjecting a steel sheet immersed in high temp. molten salt or subjected to anodic treatment to cathodic treatment in a molten bath contg. zinc salt to remove the concentrated layer of hard-to-plate components on the surface of the steel sheet. CONSTITUTION:The steel sheet is immersed in a molten salt (such as sodium chloride and lithium chloride) of 200 to 600 deg.C for 1 to 60sec or is subjected to anodic electrolytic treatment. Then, the concentrated layer of hard-to-plate components on the surface of the steel sheet are removed. Next, cathodic electrolytic treatment is executed in the molten salt bath contg. zinc salt to obtain the galvanized steel sheet excellent in uniform plating properties and plating adhesion.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing zinc and zinc-iron alloy plated steel sheets used for automobiles, home appliances, building materials, etc., and particularly to Si, Mn, P, Ti, Nb, Al, Cr, Cu, Ni,
The present invention relates to a zinc-based plating method which contains one or more of Mo, V, B and the like, and can be advantageously used for a hard-to-plate high-strength steel sheet in which these steel components are concentrated on the surface.

[0002]

2. Description of the Related Art Zinc and zinc-iron alloy plated steel sheets have been widely used for automobiles, home appliances, building materials and the like since they have been produced by a hot dip zinc dipping method or an electroplating method. By the way, in recent years, weight reduction has been promoted mainly for automobiles for the purpose of energy saving, and along with this, the use of high-strength steel sheets which are thinner and have higher strength than conventional ones is expanding. High-strength steel sheets have Si, Mn, P, and
Although it contains a large amount of Ti, Nb, Al, Cr, Cu, Ni, Mo, V, B, etc., these elements tend to be concentrated on the surface of the steel sheet, and these elements and their oxides concentrated on the surface are Inhibits platability. This tendency is due to Si, Ti, Ni, Cu, Mo, C
For r and V, 0.1 wt% or more, for Mn, 0.5 wt% or more, and for P, Al and Nb, 0.05 wt% or more
%, And B becomes remarkable when 0.001 wt% or more is contained.

That is, in the molten zinc dipping method, the wettability of the surface concentrated layer and the molten zinc is poor and non-plating frequently occurs, and in electroplating, the adhesion between the surface concentrated layer and the electrodeposited layer is poor and plating peeling occurs. Therefore, in order to perform uniform zinc plating and zinc-iron alloy plating on a high-strength steel plate, it is necessary to remove the surface concentrated layer by mechanical polishing or special pickling before plating. Japanese Unexamined Patent Publication No. 3-207845 discloses a method for producing a hot-dip galvanized steel sheet by grinding and removing a surface concentrated layer of a plating original sheet (steel sheet) with a brush roll. However, such a method has poor productivity and is difficult to carry out industrially. In addition, electroplating has a drawback that hydrogen enters the steel during plating and causes hydrogen embrittlement.

[0004]

SUMMARY OF THE INVENTION It is an object of the present invention to solve these problems and to provide a zinc-based plating method which is excellent in productivity, uniform plating property and plating adhesion. is there.

[0005]

In order to achieve the above object, the present invention provides a steel sheet in a molten salt bath at 200 to 600 ° C.
Dipping for 60 seconds or anodic electrolysis followed by zinc salt 2
Method for zinc-based plating of steel sheet for cathodic electrolysis in a molten salt bath at 00 to 600 ° C, or soaking the steel sheet in a molten salt bath at 200 to 600 ° C containing zinc salt for 1 to 60 seconds or anodic electrolysis, and then cathodic This is a zinc-based plating method for a steel sheet to be electrolyzed.

[0006]

The inventors of the present invention have studied various methods for removing the steel sheet surface concentrated layer of the hard-to-plate component, and have arrived at a method using a molten salt. The molten salt is usually a high temperature liquid of 100 ° C. or higher, and the non-oxidizing or weakly oxidizing molten salt is highly corrosive, so that the thickened layer on the surface can be removed only by immersing the steel sheet for a short time. In addition, the surface concentrated layer can be removed more effectively by anodic electrolysis. Furthermore, the molten salt is very convenient because it can be used as a Zn plating bath as long as it has excellent removability of the surface concentrated layer and contains Zn ions. That is, when a molten salt containing Zn ions is used, the steel sheet is immersed or anodized, and then subjected to cathodic electrolysis in the molten salt as it is to obtain a plating having excellent uniform plating property and plating adhesion.

In the case of Zn plating with a molten salt, if the plating temperature is low or the plating time is short, a Zn-based plating layer is obtained, and if the plating temperature is high or the plating time is long, the plated Zn is plated. Is alloyed with a steel sheet to obtain a Zn-Fe alloy plating layer, which is convenient because it is possible to separately produce zinc plating and zinc-iron alloy plating simply by controlling the plating temperature and time, and the zinc-based plating in the present invention is It is meant to include both such zinc plating and zinc alloy plating.

The molten salt used is preferably a non-oxidizing or weakly oxidizing one having a strong corrosive property, and the salts constituting the molten salt include sodium chloride, potassium chloride, lithium chloride,
Chlorides such as aluminum chloride, zinc chloride and magnesium chloride, hydroxides such as sodium hydroxide, potassium hydroxide, lithium hydroxide and barium hydroxide, sodium sulfate,
Sulfates such as potassium sulfate and lithium sulfate are suitable,
Since carbonates such as sodium carbonate, potassium carbonate and lithium carbonate and nitrates such as sodium nitrate, potassium nitrate and lithium nitrate have strong oxidizing properties, it is desirable not to use much. The molten salt may be composed of one or more of these chlorides, hydroxides, sulfates, carbonates, nitrates and the like.

As the molten salt, one for removing the surface concentrated layer and one for performing the Zn plating may be provided in different baths, or the removal of the surface concentrated layer and the Zn plating may be performed in the same bath. When Zn plating is performed, the Zn salt content is 10 mol% or more, preferably 30 m
ol% or more, more preferably 50mol% or more, and 10mool of one or more of chlorides, hydroxides and sulfates
% Or more, preferably 30 mol% or more, it is desirable to use a molten salt. If only to remove the surface-enriched layer, use one or more of chloride, hydroxide, and sulfate.
It is desirable to use a molten salt containing 10 mol% or more, preferably 30 mol% or more.

The temperature of the molten salt is in the range of 200 to 600 ° C. If the temperature is below 200 ° C, the dissolution and removal rate of the surface concentrated layer is slow and it takes a long time, resulting in poor productivity. The material will deteriorate due to heat. The surface concentrated layer can be dissolved and removed by immersing it in the molten salt for 1 to 60 seconds. If the immersion time is less than 1 second, the removal of the surface concentrated layer is insufficient, and if it exceeds 60 seconds, the removal of the surface concentrated layer is sufficient, but the melting loss of the steel sheet itself is large and the productivity is poor.

Although the surface concentrated layer can be removed only by dipping, it can be removed more efficiently by anodic electrolytic treatment. Anodic electrolysis is performed with a steel plate as the anode, 1 to 200 A / dm ²
The current may be applied at the current density of, and the electrolysis time may be determined according to the amount of the surface concentrated layer of the steel sheet. Zn plating is performed by passing a current through a steel plate as a cathode, and the current density is 20 ~
500 A / dm ² is desirable. The electrolysis time may be determined according to the required plating amount. Current density of Zn plating is 20 A / dm
^If it is less than ² , it takes a long time to secure the plating amount, resulting in poor productivity. If it exceeds 500 A / dm ² , a huge current is required to flow the current, which not only costs but also the electrical resistance of the steel sheet itself. It is not preferable because the bath temperature rises due to heat.

[0012]

EXAMPLES Examples and comparative examples of the present invention will be described below. A steel plate containing 1.0 wt% of Si, 0.3 wt% of Mn and 0.1 wt% of P is subjected to normal degreasing and pickling, and then heated to the plating temperature, and zinc and zinc are applied using the molten salt baths A to D below. −
An iron alloy plated steel sheet was produced under the conditions shown in Table 1.

[Molten salt bath A] ZnCl ₂ 60 mol% NaCl 20 mol% KCl 20 mol% [molten salt bath B] ZnCl ₂ 60 mol% NaCl 20 mol% KaOH 20 mol% [molten salt bath C] LiCl ₂ 58 mol% KCl 42 mol% [molten Salt bath D] NaOH 90 mol% Na ₂ CO ₃ 10 mol% Table 1 also shows the evaluations regarding the uniform plating property and the plating adhesion property. The evaluation of each plating property was performed as follows. (Evaluation of uniform plating property) The plating surface was visually observed to evaluate the uniformity of plating.

Rating 3-Uniformly plated Rating 2-Partially unplated Rating 1-Half or more unplated (plating adhesion evaluation) Bending a plated steel sheet 180 degrees to evaluate plating adhesion did.

Rating 3-No peeling Rating 2-Peeling slightly Rating 1-Large peeling

[0016]

[Table 1]

As is clear from Table 1, the production method of the present invention makes it possible to produce zinc and zinc-iron alloy plated steel sheets which are excellent in uniform plating property and plating adhesion.

[0018]

As described above, according to the present invention, the difficult-to-plate high-strength steel sheet is immersed in a molten salt bath containing zinc salt at 200 to 600 ° C. for 1 to 60 seconds, or subjected to anodic electrolysis and then cathodic electrolysis. It is possible to manufacture a zinc and zinc-iron alloy plated steel sheet excellent in uniform plating property and plating adhesion, and its industrial value is very large.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Kiyoji Nakajima 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Corporation Technical Research Headquarters (72) Inventor Yoichi Toyama 1 Kawasaki-cho, Chuo-ku, Chiba Address Kawasaki Iron & Steel Co., Ltd. Technical Research Headquarters (72) Inventor Nobuyuki Morito 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Kawasaki Iron & Steel Co., Ltd.

Claims (5)

[Claims] 1. A steel sheet in a molten salt bath at 200 to 600 ° C.
A zinc-based plating method for a steel sheet, which comprises immersing for 60 seconds and then subjecting it to cathodic electrolysis in a molten salt bath containing zinc salt. 2. A zinc-based plating method for a steel sheet, which comprises subjecting the steel sheet to an anodic electrolytic treatment in a molten salt bath at 200 to 600 ° C., and then performing a cathodic electrolytic treatment in a molten salt bath containing a zinc salt. 3. A zinc-based plating method for a steel sheet, which comprises immersing the steel sheet in a molten salt bath containing a zinc salt at 200 to 600 ° C. for 1 to 60 seconds, and then subjecting it to cathodic electrolysis. 4. A zinc-based plating method for a steel sheet, which comprises subjecting the steel sheet to an anodic electrolytic treatment in a molten salt bath containing a zinc salt at 200 to 600 ° C., and then performing a cathodic electrolytic treatment. 5. The zinc-based plating method for a steel sheet according to claim 1, 2, 3 or 4, wherein the steel sheet is a high-strength steel sheet having a hard-to-plate component concentrated on its surface.