JPH06272008A - Production of galvanized steel sheet and alloyed galvannealed steel sheet excellent in uniform plating property and plating adhesion - Google Patents

Abstract

PURPOSE:To produce a galvanized steel sheet excellent in uniform plating properties and plating adhesion with high productivity by immersing an annealed steel sheet in a hot-dip salt bath of a specified temp. for specified time and thereafter galvanizing it. CONSTITUTION:A steel sheet subjected to continuous annealing is immersed in a hot-dip salt bath of >=420 deg.C for 10 to 60sec or is subjected to anodic electrolysis in the same hot-dip salt. As this hot-dip salt, the hot-dip salt of chloride is suitable from the viewpoints of treatability and cost, and particularly, the one contg. ZnCl2 by 40mol% is preferable. By this immersing treatment or anodic electrolysis treatment, concentrated layers having low wettability with hot-dip zinc on the surface of the steel sheet can be removed at high efficiency. After the hot-dip salt treatment, the steel sheet 15 galvanized, and, according to necessity, it is successively heated, and iron is allayed into the plated layer.

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 hot dip galvanized and alloyed hot dip galvanized 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 method for producing hot dip galvanized and galvannealed steel sheets suitable for hard-to-dip steel sheets containing one or more of Mo, V, B and the like.

[0002]

2. Description of the Related Art Galvanized steel sheets and galvannealed steel sheets used in automobiles, home appliances, building materials, etc. have been made by hot dip galvanizing for a long time by immersing the steel sheets in hot-dip zinc. 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 ever before is expanding. High-strength steel sheets are made of Si, Mn, P, T in order to obtain high strength and workability.
It contains a large amount of i, Nb, Al, Cr, Cu, Ni, Mo, V, B, etc., but these elements tend to concentrate on the surface of the steel sheet during annealing, and these elements concentrated on the surface and their The oxide hinders the plating property. That is, in the hot dip galvanizing method, the wettability between the surface concentrated layer and the hot dip zinc is poor and non-plating frequently occurs, and the adhesion between the surface concentrated layer and the plating is poor, resulting in stripping of the plating. Therefore, in order to perform high-strength steel sheet with uniform zinc plating and alloying zinc plating, the surface concentrated layer must be removed 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, it is difficult to carry out industrially.

Further, Japanese Patent Laid-Open No. 52-108334 discloses a method of treating the surface of a ferrous metal component with a molten salt before hot dip galvanizing, which involves annealing a steel strip and immersing it in a zinc bath. This is called a flux method of a different system from the so-called Sendzimir method, and its purpose is mainly to raise the temperature of ferrous metal parts and is a technology different from the present invention.

[0004]

The object of the present invention is to solve these problems, and to provide a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet which are excellent in productivity, uniform plating property and plating adhesion. Is to provide a method for manufacturing the same.

[0005]

Means for Solving the Problems The present invention is characterized in that a continuously annealed steel sheet is immersed in a molten salt bath at 420 ° C. or higher for 10 to 60 seconds, and then hot-dip galvanizing is applied. It is a method for producing a galvanized steel sheet having excellent plating adhesion, and in the present invention, the continuously annealed steel sheet may be subjected to anodic electrolysis in a molten salt bath at 420 ° C or higher. It is desirable that the content of zinc chloride be 40 mol% or more. In the present invention, after further hot dip galvanizing,
It is possible to produce an alloyed hot-dip galvanized steel sheet excellent in uniform plating property and plating adhesion even if the steel plate is continuously heated to alloy iron in the plating layer.

[0006]

The present inventors perform hot dip galvanizing on steel sheets with excellent uniform plating property and plating adhesion without mechanical polishing or special pickling for removing the concentrated layer on the steel plate surface. As a result of various studies on the method, it was found that the method of treating the steel sheet with the molten salt before the hot dip galvanizing is the best.

The molten salt is usually a high temperature liquid of 100 ° C. or higher, and the non-oxidizing or weakly acidic 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. it can. Further, if the steel sheet is used as an anode for electrolysis, the surface concentrated layer can be removed more effectively.
The molten salt used is preferably non-oxidizing or weakly oxidizing, which has strong corrosive properties, and chloride molten salt, carbonate molten salt, sulfate molten salt, and hydroxide molten salt are suitable. Molten salt is suitable in terms of handleability and cost. The nitrate molten salt is not preferable because it has a strong oxidizing property and easily forms an oxide having poor wettability with molten zinc on the surface of the steel sheet.

The temperature of the molten salt is limited to 420 ° C. or higher.
Since the treatment of the molten salt is carried out immediately before plating, the temperature of the molten salt bath must be higher than the melting point of zinc, 420 ° C. Therefore, the molten salt used should have a melting point of 420 ° C or less, more preferably 3 ° C.
It is below 70 ° C. Also, the molten salt used must be one that is difficult to evaporate from the viewpoint of workability, and the use of ammonium chloride or the like must be avoided. Further, the higher the temperature of the molten salt, the more easily the surface concentrated layer can be dissolved and removed in a short time, but a large amount of energy is required for keeping the temperature and the material of the steel sheet is deteriorated by heat.

The surface concentrated layer can be dissolved and removed by immersing it in the molten salt for 10 to 60 seconds. If the immersion time is less than 10 sec, the removal of the surface-enriched layer is insufficient, and if it exceeds 60 sec, the removal of the surface-enriched layer is sufficient, but the melting loss of the steel sheet itself is large and the productivity is poor. The surface concentrated layer can be removed only by the immersion treatment, but can be removed more efficiently by the anodic electrolytic treatment. Anodic electrolysis is performed with a steel plate as the anode, 1 to 200 A /
A current may be passed at a current density of dm ² , and the electrolysis time may be determined according to the amount of the surface concentrated layer of the steel sheet.

The molten salt used is preferably a chloride molten salt, but zinc chloride is suitable among the chlorides. Zinc chloride has a sufficiently low melting point of 283 ° C and a sufficiently high boiling point of 732 ° C. It is also highly corrosive and has the effect of improving the wettability between the steel sheet and molten zinc. The molten salt may be only zinc chloride, but it is preferable to use it in a mixture with other chlorides or carbonates, sulfates, hydroxides, etc. in order to increase the corrosiveness and the electric conductivity in electrolysis. When zinc chloride is mixed with other chlorides or carbonates, sulfates, hydroxides, etc., the zinc chloride content is 40 mol% or more. If it is less than 40 mol%, the melting point of the molten salt becomes high, which is not preferable.

Pure zinc plating is generally used as the hot-dip galvanizing method, but zinc-aluminum alloy plating, zinc-chromium alloy plating, or one in which one or more other elements are further added may be used. Further, after hot-dip plating, the steel plate may be heated to form a diffusion alloy of iron in the plating layer.

The effects of the present invention will be described below with reference to examples.

[0013]

Example: In% by weight, C is 0.004%, Si is 0.8%, Mn
Of cold-rolled steel sheet containing 0.3% of P, 0.1% of P, 0.03% of Ti, and 0.001% of B was degreased and pickled normally, then annealed in a non-oxidizing atmosphere and immediately treated with the molten salt shown in Table 1. Then, hot-dip galvanizing bath was passed to perform hot-dip galvanizing. A part of the alloy was hot dip galvanized and then alloyed at 470 ° C.

The uniform galvanizing properties and plating adhesion of the produced hot dip galvanized steel sheets were evaluated by the following methods. (Evaluation of Plating Adhesion) The plated steel sheet was bent 180 degrees and the adhesion of the plating was evaluated by tape peeling.

[0015] Table 2 shows the molten salt treatment status and the evaluation of plating characteristics.

As is apparent from Table 2, it is understood that the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet excellent in uniform plating property and plating adhesion can be manufactured by the manufacturing method of the present invention.

[0017]

[Table 1]

[0018]

[Table 2]

[0019]

INDUSTRIAL APPLICABILITY As described above, according to the present invention, the continuously annealed steel sheet is immersed in a molten salt bath at 420 ° C. or higher for 10 to 60 seconds or subjected to an anodic electrolytic treatment. An excellent hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet can be produced, and its industrial value is very large.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shuji Gomi 1 Kawasaki-cho, Chuo-ku, Chiba, Chiba Prefecture Technical Research Division, Kawasaki Steel Co., Ltd. (72) Inventor Kazuo Mochizuki, 1 Kawasaki-cho, Chuo-ku, Chiba-shi Kawasaki Steel Corporation Technical Research Division

Claims (4)

[Claims] 1. A molten zinc excellent in uniform plating property and plating adhesion, which comprises immersing a continuously annealed steel sheet in a molten salt bath at 420 ° C. or higher for 10 to 60 seconds and then performing hot dip galvanizing. Method for producing base-plated steel sheet 2. A hot-dip galvanized steel sheet excellent in uniform plating property and plating adhesion, which comprises subjecting a continuously annealed steel plate to anodic electrolysis in a molten salt bath at 420 ° C. or higher, and then hot-dip galvanizing. Manufacturing method 3. The method for producing a hot dip galvanized steel sheet excellent in uniform plating property and plating adhesion according to claim 1, wherein the molten salt bath contains zinc chloride in an amount of 40 mol% or more. 4. A uniform galvanizing property and a plating adhesion property, which is characterized in that after the hot dip galvanizing treatment according to claim 1, 2 or 3, the steel sheet is subsequently heated to alloy iron in the plating layer. Of excellent alloyed hot-dip galvanized steel sheet