JPH049478A - Corrosion resistant chromating plated steel product excellent in adhesive property of coating and production thereof - Google Patents

Abstract

PURPOSE:To develop a chromating plated steel product which is uniform in appearance and excellent in corrosion resistance and adhesive property of coating by applying the surface of the Zn-based plated steel product with chromating liquid contg. stringlike silica sol and drying it. CONSTITUTION:The surface of a steel product such as a steel plate, a bar steel and a steel pipe which are galvanized, hot galvanized, electroplated or hot dipped with Zn alloy is applied with chromating liquid described hereunder and dried. This chromating liquid contains chromium oxide wherein the ratio of Cr<3+> to Cr<6+> is previously reduced to 3/7-5/5 at 5-50g/l expressed in terms of CrO3 and stringlike silica gel having (2-100) ratio of length to diameter at 5-250g/l expressed in terms of SiO2 or furthermore spherical silica gel wherein the ratio of stringlike silica gel to the tame plus spherical silica gel is at least 0.1. The chromating Zn-based plated steel product is obtained which has composition of 10-100mg/m<2> as Cr and (0.5-5.0) expressed in terms of SiO2/CrO3 as the ratio of silica to chromium oxide.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a corrosion-resistant chromate-treated plated steel material that has a uniform appearance and excellent corrosion resistance and topcoat adhesion (hereinafter referred to as paint adhesion), and a method for producing the same. It is.

[Conventional technology]

In recent years, in the home appliance industry, which is the final manufacturer of plated steel sheets,
Conventionally, plated steel sheets were processed into the shape of products such as refrigerators and washing machines, and then painted after assembly. However, by using coated steel sheets (hereinafter referred to as pre-painted steel sheets), this process can be easily completed by the final manufacturer. Efforts are being made to reduce costs by omitting the painting process.

On the other hand, with the improved performance of chromate-treated plated steel sheets, chromate-treated plated steel sheets are increasingly being used as base plated steel sheets for pre-coated steel sheets. Such pre-painted steel sheets cannot be assembled using spot welding, which was conventionally used to join steel sheets together when assembled into products such as refrigerators, so the method of assembling the steel sheets must be performed by caulking the steel sheets together. This caulking process involves bending the steel plates together, so it is a very severe process called IT bending (bending 180 degrees with one steel plate sandwiched between them). Therefore, chromate-treated plated steel sheets used for pre-coated steel sheets are required to have high paint adhesion.

For this purpose, the inventor previously developed a coating-type chromate solution containing reduced chromic acid, silica sol, phosphoric acid, and resin as main components (Japanese Patent Laid-Open No. 63-270480).

[Problem to be solved by the invention]

Conventional techniques related to the present invention include a method using a chromate solution composed of hexavalent or trivalent chromium and an acrylic emulsion (Japanese Unexamined Patent Publication No. 145785/1985), and a method using a chromium compound containing a large amount of trivalent chromium, although the purpose is different. A method described in JP-A-136685 using a chromate solution consisting of silica, phosphoric acid, fluoride and acrylic acid polymer is known.

Unfortunately, however, these conventional techniques are not capable of achieving the object of the present invention. Paint adhesion is improved by adding a water-soluble resin such as acrylic acid to a chromate solution containing chromic acid and silica sol. However, the drawback is that the appearance of chromate changes over time. In addition, as a countermeasure for these improvements, the problem of aging deterioration can be improved by using an emulsion with a resin with a large molecular weight. However, problems such as the emulsion resin deteriorating over time in the chromate solution cannot be used in the high-speed production lines of steel manufacturers, which continuously process large quantities.

As a result of intensive research into a method for improving the paint adhesion of chromate-treated steel materials, the inventor of the present invention discovered that the shape of the silica sol in the chromate treatment solution was changed from the conventional particle shape to a string shape with a thickness and length. Based on this knowledge, they discovered that paint adhesion could be improved without using organic polymers such as water-soluble resins or emulsion resins, which cause deterioration of the appearance of chromate-treated steel materials over time and deterioration of chromate treatment solutions over time. Based on these findings, the present invention has been completed.

[Means for solving problems]

The present invention provides a chromate-treated plated steel material with high quality, which has the following coating composition and can be obtained by the method shown below.

(Adhesion of a chromate film obtained by applying a chromate treatment solution consisting mainly of a chromium compound and a string-like silica sol with a length/diameter ratio of 2 to 10 O to the surface of the 11-plated steel material and drying it. The amount is 10 as Cr
~1100n/m, and the weight ratio of silica/chromium compound is 5i02/Crux conversion ratio of 0.5 to 5.0
A corrosion-resistant chromate-treated plated steel material with excellent paint adhesion.

(2) String-like silica sol and spherical silica sol, which mainly consist of a chromium compound and silica sol and have a length/diameter ratio of 2 to 100, are applied to the surface of the plated steel material. The amount of chromate coating obtained by applying and drying a chromate treatment solution having a ratio of 0.1 or more (decaspheric silica sol) is 10 to 100 Cr/d,
and the weight ratio of silica/chromium compound is 5I02/Cr
A corrosion-resistant chromate-treated plated steel material with excellent paint adhesion, characterized by an Os conversion ratio of 0.5 to 5.0.

(3) CrO is applied to the surface of the plated steel by applying a chromic acid compound that has been reduced in advance to a Cr''/Cr''' ratio of 3/7 to 515.

Converted to 5-50g/l, length/diameter ratio 2-100
The string-like silica sol is converted to Sin, 5 to 250 g/l.
A method for manufacturing a corrosion-resistant chromate-treated plated steel material with excellent paint adhesion, which comprises applying a chromate solution containing it and then drying it.

(4) Apply a chromic acid compound that has been reduced to a Cr3+/Cr'+ ratio of 3/7 to 515 on the surface of the plated steel. ~10
The weight ratio of string-like silica sol to spherical silica sol is 0.1 or more as string-like silica sol/(string-like silica sol/ten-spherical silica sol).

A method for producing a corrosion-resistant chromate-treated plated steel material with excellent paint adhesion, which comprises applying a chromate solution containing 5 to 250 g/M in terms of conversion and then drying it.

The galvanized steel materials targeted by the present invention include electrically and hot-dip galvanized steel sheets, electrically and hot-dip galvanized steel sheets, steel bars, steel pipes, and thick plates.

Alloys include ferrous metals and zinc, aluminum and zinc alloys,
Further, multilayer zinc plating, thermal spray plating, and vapor deposition plating are also included in the present invention.

[For production]

The silica sol conventionally used in chromate solutions is spherical particles. However, with spherical silica, paint adhesion decreases as the amount of adhesion increases, but by using the string-like silica sol described in the present invention, it is greatly improved and the adhesion of paint is increased with a high amount of adhesion. A chromate film with excellent properties can be obtained. Such string-like silica sol is produced by passing silicate such as sodium silicate or potassium silicate through a cation exchange resin to remove cations, and adding Ca ions and Mg ions in the process of turning the silica into a sol.
By setting H in the range of 3 to 10, the shape of the silica sol can be changed into a string shape, and the silica sol can be manufactured.

The string-like silica sol produced in this way has a string-like (columnar, branch-like) shape. Paint adhesion can be improved by adding silica sol having such a shape to a chromate solution and applying it to plated steel.

The effect of adding this string-like silica sol to a chromate solution to improve paint adhesion of chromate-treated plated steel will be explained below.

The chromate film is a very brittle film, and as the amount of chromate deposited increases, paint adhesion decreases.

In order to improve this property, a commonly used method is to add an organic polymer or the like to the chromate solution and use it as a binder to improve paint adhesion. However, as mentioned above, organic polymers deteriorate over time and are difficult to withstand. In this respect, conventional inorganic substances such as spherical silica sol are stable substances in chromate solution or chromate film, but unfortunately they have the disadvantage that they have little effect as a binder and have little effect on improving paint adhesion. Ta.

The string-like silica sol of the present invention satisfies both of these characteristics, such as aging deterioration and binder effect in the chromate film.

In other words, by changing the conventional spherical shape of silica sol to a string-like shape, the deterioration over time, which is a characteristic of silica sol, does not occur, and in the drying process after applying chromate, the string-like silica sol becomes entangled with each other in the chromate film, and the binder They discovered that by demonstrating this effect, the brittleness, which is a drawback of chromate films, can be improved and paint adhesion can be improved.

Needless to say, the length/diameter ratio of the string-like silica sol is very important for the binder effect and influences the paint adhesion improvement effect. The length-to-diameter ratio of the string-like silica sol of the present invention is in the range of 2 to 100. If it is less than 2, the effect of improving paint adhesion will be insufficient, and if it exceeds +00, stability with chromate solution will deteriorate and there will be a problem that silica will precipitate.

Moreover, the most preferable range is 2-50.

Hereinafter, the mechanism of action of the chromate film of the present invention will be described in detail.

The amount of chromate attached is C "converted from 10 to 100 ■/11
(is within the scope of the present invention. If it is less than 10 mg/n, the corrosion resistance will be insufficient, and if it exceeds 100 mg/n, weldability will decrease and the appearance will be more likely to be uneven. Adhesion within the scope of the present invention) A chromate film with excellent performance such as corrosion resistance and uneven appearance can be obtained with the amount of
It is 60mg/m.

Silica is SiO□/C with total chromium converted to chromic acid.
The range of the present invention is 0.5 to 5.0 in terms of weight ratio of 0.03.

Silica is necessary to ensure the uniformity, corrosion resistance, and paint adhesion of the chromate film. 5io2. / Croa
If the weight ratio is less than 0.5, it is difficult to obtain the performance;
, exceeds 0, weldability deteriorates.

Regarding the shape of the silica, as mentioned above, it is assumed that string-like silica sol with a length/diameter ratio of 2 to 100 is used, but in some cases, it is also possible to mix and use conventional spherical silica sol. It is. However, the mixing ratio must be 0.1 or more in weight ratio of string-like silica sol/total silica sol (string-like + spherical silica sol), and if it is less than 0.1, it is difficult to ensure paint adhesion.

A known organic resin is further added on the chromate film of the present invention.
．． By coating with a thickness of 1 to 3.0 μm, plated steel materials with even higher performance can be obtained, and these steel materials are also included in the present invention.

The manufacturing method will be described in detail below.

The present invention uses a chromate solution containing chromic acid and silica sol as main components. Chromic acid is used as a reducing agent in advance, and
A chromic acid (reduced chromic acid) solution whose /Cr'+ ratio is reduced to a range of 3/7 to 515 is used.

In this case, in order to minimize the amount of undecomposed reducing agent, the reducing agent is added to the acidic, high chromic acid concentration solution and adjusted over time.

Further, a mixed concentrated solution of phosphoric acid and chromic acid may be used for reducing chromic acid.

Reducing agents are polysaccharides such as starch, fructose, and sucrose.

Alternatively, alcohols such as methanol and ethanol, organic acids such as oxalic acid and formic acid, or inorganic and organic compounds such as phenols and hydrogen peroxide can be used.

The Cr3"lcr" ratio affects the paint adhesion of the chromate film. It also affects the appearance color tone and corrosion resistance.

If the ratio of CS"/Cr"+ is less than 3/7, there will be a large amount of soluble chromium and it will be difficult to obtain sufficient paint adhesion. Also 5
If it exceeds 15, the corrosion resistance, especially the corrosion resistance after processing, will be insufficient.

The concentration of chromium compound is 5 to 50 g in terms of chromic anhydride.
Used at a concentration of /l. If it is less than 5g/1, the thickness of the chromate solution applied will be thick and the appearance will be uneven during the drying process.
If it exceeds 0 g/l, the stability of the bath will be poor and long-term storage will be difficult. Further, a desirable range is 10 to 30 g/l.

Silica sol has 5iOz/CrO for chromium compounds.
, at a ratio of 0.5 to 5.0, and at a concentration of 5 to 100 g/l in terms of SiO□.

If the amount is less than 5 g/1, the effect of improving the wettability of the liquid to be applied is small, making it difficult to obtain uniformity. Moreover, if it exceeds 100 g/l, the stability of the bath is poor and long-term storage is difficult. The silica sol is preferably a sol in which amorphous silica is uniformly dispersed. The shape of the silica is string-like (columnar, branch-like) with a diameter of 5 to 1100 nm, and the length/diameter ratio of the silica sol is in the range of 2 to +00. In some cases, it is also possible to use a mixture of spherical silica sol.

However, the mixing ratio must be 0.1 or more in weight ratio of string-like silica sol/total silica sol (string-like + spherical silica sol), and if it is less than 0.1, it is difficult to ensure paint adhesion.

As the chromium compound, a partially reduced aqueous solution of chromic acid anhydride or dichromic acid compound is used.

The silica sol used is one produced from sodium silicate as described above. Other tower components include metal ions (C
a2++ Mg"+ Zn"+ Ni", Na' +
K'NH'+, co! + etc.), anions (F-9 fluorine complex ions.

organic acids).

After contacting the above-mentioned chromate solution to the surface of the plated steel material by spraying or dipping, a gas knife,
Adjust the amount of adhesion using roll squeezing, etc.

Alternatively, the chromate solution is directly applied using a roll coater 1 or electrostatic coating.

Immediately after coating, preferably within 10 seconds, it is desirable to heat the coating to a final temperature of 50 to 150° C. using a means such as hot air or a heating furnace. If the temperature is less than 50°C, the effectiveness of the film tends to be insufficient, and if it exceeds 150°C, cracks will occur in the chromate film. The acidity of the chromate solution is determined to have a pH of 0.5 to 5, preferably 2 to 5, in terms of reaction with the plating surface and bath stability.
It is 3.

〔Example〕

Examples will be described below.

The conditions in the examples are as follows: (1) Treatment method A cold rolled steel strip (0.8n+m x 914mm) was plated by a known method, and then a chromate solution was sprayed and applied, and the amount of chromium deposited was adjusted using a gas knife. Thereafter, it was immediately passed through a direct fire furnace for 3 seconds to heat it. The plate temperature reached by heating was measured at the exit side of the furnace.

(2) Analysis of film For typical examples, measurements were made using fluorescent X-rays for which a calibration curve was prepared using a chemical analysis method. Total chromium, silica, and phosphoric acid were almost equal to the liquid composition, and the bath composition was directly converted to the film composition.

(3) Measurement of shape of silica sol The shape (diameter, length) of silica sol was measured using a transmission electron microscope, magnified 100,000 times, and its diameter and length were measured.
The length/diameter ratio was measured.

(4) Evaluation method a) Paint adhesion For baking, a commercially available hardening melamine alkyd resin paint was applied to a dry film thickness of approximately 25 μm using a bar coater, and then baked at 125°C for 20 minutes to conduct a paint adhesion test. . As a paint adhesion test, after performing an IT bending test, the film was peeled off with an adhesive tape, and the appearance was visually evaluated. Rating: 1 (100%) Peeling...5 (30%) Peeling...
-10 (No peeling) b) Corrosion resistance The time (hrs) at which 5% of rust occurred was measured by a salt spray test according to JIS-Z-2371.

C) Uniformity The unevenness of the chromate appearance was determined with the naked eye.

Rating = 1 (very noticeable), 2 (unevenness observed)
, 3 (Almost no unevenness observed) Example 1 After electrogalvanizing a steel sheet to a thickness of 3 μm, treatment was performed using the chromate solution shown in Table 1. cr3+/Cr'+
A chromic acid solution reduced with starch was used for preparation.

No. 1 is a comparative example in which the ratio of Cr''/Cr'+ is 3/7 and all spherical silica sol is used, but the paint adhesion is low at 2 points.

In addition, Nα2 is N(L 1's Cr''/Cr6+ ratio is 4/
However, the paint adhesion was 4 points, and the paint adhesion was still insufficient.

Nα3 has a lower chromate deposition amount than Nα2 at 20 μ/bo, and has good paint adhesion at 8 points, but poor corrosion resistance.

On the other hand, the examples of the present invention, Nos. 4 to Nα24, have good overall performance and are well balanced.

Example 2 A steel plate was hot-dip galvanized to a thickness of 25 μm, and Nα8 in Table 1 was applied.
The sample was treated with chromate as shown in the figure below. Paint adhesion is 8
A good score of 1 point was obtained, and the appearance uniformity was also 2 points.
It exhibited excellent corrosion resistance of 120 hours or more.

Example 3 A steel sheet was hot-dip galvanized to a thickness of 8 μm and heated and alloyed in a direct-fired furnace to produce an alloyed hot-dip galvanized steel sheet with an iron content of 8% in the plating layer. It was subjected to chromate treatment and used as a sample.

Paint adhesion was 10 points, indicating very excellent performance.
Corrosion resistance was good for 72 hours, and uniformity was good at 3 points.

〔Effect of the invention〕

The present invention has great advantages because a plated steel sheet having a corrosion-resistant chromate film with excellent surface properties can be obtained by a simple coating-type treatment method.

In particular, it has a wide range of uses, including as a base steel plate for painting due to its improved paint adhesion. The applicable galvanized steel materials are also coating type.

Due to the waterless washing manufacturing process, thick plates are produced in addition to thin steel plates.

Wide range of applications including wire rods, shaped steel, steel pipes, etc.

Conventionally, these improvement effects by adding resin have been recognized, but there have been problems such as bath stability with chromic acid and silica sol systems and performance deterioration over time. The present invention solves these problems for the first time, and provides a steel material that can be manufactured with stable quality and has excellent performance.

Agent Patent Attorney Masaaki Aki Sawa 1 other person

Claims (4)

[Claims] (1) A chromate coating is obtained by applying a chromate treatment solution consisting mainly of a chromium compound and string-like silica sol with a length/diameter ratio of 2 to 100 on the surface of plated steel and drying it. The amount of adhesion is 10 as Cr
~100mg/m^2, and the weight ratio of silica/chromium compound is 0.5~ in terms of SiO_2/CrO_3 ratio
Corrosion-resistant chromate-treated plated steel material with excellent paint adhesion, characterized by a corrosion resistance of 5.0. (2) String-like silica sol and spherical silica sol, which mainly consist of a chromium compound and silica sol and have a length/diameter ratio of 2 to 100, are applied to the surface of the plated steel material. The amount of chromate coating obtained by applying and drying a chromate treatment solution with a ratio of spherical silica sol + spherical silica sol of 0.1 or more is 10 to 100 mg/m^2 as Cr, and the silica/chromium compound is The weight ratio of is SiO_2/C
A corrosion-resistant chromate-treated plated steel material with excellent paint adhesion, characterized by an rO_3 conversion ratio of 0.5 to 5.0. (3) Cr^3^+/Cr^ on the surface of the plated steel material in advance
6^+ A chromic acid compound reduced to a ratio of 3/7 to 5/5 is 5 to 50 g/l in terms of CrO_3, and a string-like silica sol with a length/diameter ratio of 2 to 100 is 5 to 5 in terms of SiO_2.
A method for producing a corrosion-resistant chromate-treated plated steel material with excellent paint adhesion, which comprises applying a chromate solution containing 250 g/l and then drying it. (4) C^3^+/Cr^6 on the surface of the plated steel material in advance
^+ Chromate compound reduced to a ratio of 3/7 to 5/5 is C
5 to 50 g/l in terms of rO_3, length/diameter ratio is 2
A chromate solution containing 5 to 250 g/l of silica sol of 0.1 or more in terms of SiO_2 was applied with a weight ratio of string-like silica sol of ~100 and spherical silica sol as string-like silica sol/(string-like silica sol + spherical silica sol). rear,
A method for producing corrosion-resistant chromate-treated plated steel with excellent paint adhesion, which is characterized by drying.