JP2000109982A - Surface treated steel sheet excellent in high temperature thermal deformation resistance and electrical conductivity - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the steel sheet small in the change of the color tone before and after heating treatment, and capable of securing electrical conductivity and withstanding bare use by allowing a chromate film to contain a specified amt. of Cr, a silica sol having a specified particle size and a water soluble emulsion resin and specifying the weight ratios between the silica sol and the Cr, between the water soluble emulsion resin and the Cr and between the sexivalent chromium and the whole chromium in the film. SOLUTION: The chromate film contains Cr of 20 to 120 mg/m2, a silica sol having 5 to 50 nm particle size and a water soluble emulsion resin, the weight ratio between the silica sol and the Cr lies in the range of SiO/Cr=2 to 5, the weight ratio of the water soluble emulsion resin to the Cr is within the range of 0.2 to 2.0, and the weight ratio of Cr6+ to total Cr is within the range of 0.2 to 0.8. It is suitable that the chromate film moreover contains organic or inorganic lubricating particles, and the weight ratio of the organic or inorganic lubricating particles to the resin is within the range of 0.01 to 0.2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface-treated steel sheet having excellent resistance to high-temperature thermal discoloration and electrical conductivity, particularly, for example, when performing heat processing such as shrink fitting when attaching an explosion-proof band of a cathode ray tube. The present invention relates to a surface-treated steel sheet which does not change its color tone even under a heating environment such as when used for a gas stove, and which can be lightly worked or welded.

[0002]

2. Description of the Related Art In recent years, the use of electroplated steel and hot-dip steel for home appliances and OA equipment has been increasing as a measure against rust and corrosion. More recently, with the improvement of chromate treatment technology, there has been an increasing tendency to reduce costs by using a chromate-treated steel material as it is and omitting an overcoating step such as painting, mainly in these industrial fields.

[0003] In particular, in the field of audio and the like, a printing style called silk printing, in which the underlying steel sheet can be viewed as it is, is preferred. Therefore, among the chromate-treated steel materials, those having a bright color are preferred. Chromate treated steel sheets have been developed to meet the demands.

[0004] Japanese Patent Application Laid-Open No. 2-104673 discloses a chromate-treated galvanized steel material having a bright color tone.
This is essentially an invention relating to electropure zinc plating, and does not make the color tone bright by providing a chromate film, but makes the color tone bright by adjusting the orientation of the plating surface. On the other hand, the chromate film may be discolored by heating, and the color tone may be significantly changed. Further, from the viewpoint of the plane orientation of the plating, in the case of a pure galvanized steel sheet, the color tone changes greatly when heating is applied so as to greatly exceed the melting point of 419 ° C. of zinc. As described above, in the prior art, no consideration was given to the stability of the color tone change after heating.

Japanese Unexamined Patent Publication (Kokai) No. 63-307281 discloses a surface-treated steel sheet having improved corrosion resistance, fingerprint resistance, and paint adhesion, but also in this case, the color tone change before and after heating is a problem. Not in.

Japanese Unexamined Patent Publication (Kokai) No. 63-105976 discloses a plated steel sheet having improved corrosion resistance. However, since the coating composition is such that a resin is coated on an upper layer, the heat discoloration resistance further deteriorates. .

Japanese Unexamined Patent Publication No. Hei 6-93461 discloses a bare chromate treated steel sheet having improved corrosion resistance and lubricity, but does not disclose any improvement in heat discoloration resistance.

Japanese Patent Application Laid-Open No. 7-278837 discloses, for example, 500
A surface-treated steel sheet having improved high-temperature discoloration resistance when heated at 40 ° C. for 40 seconds is disclosed.

Japanese Patent Application Laid-Open No. 8-333688 discloses a surface-treated steel sheet having improved lubricity and conductivity.

[0010]

As described above, various types of surface-treated steel sheets having a chromate film have been proposed even in the prior art. However, for example, in
Even in the case of a surface-treated steel sheet excellent in high-temperature discoloration resistance disclosed in Japanese Patent No. 278837, its high-temperature discoloration resistance is that after a heat treatment of 500 ° C. × 40 seconds. It has been found that resistance to high-temperature discoloration under even more severe conditions may be required. In addition, at the same time, it is also required to improve conductivity in order to secure sufficient weldability. The surface-treated steel sheet disclosed in JP-A-8-333688 contains a large amount of lubricating particles for the purpose of lubricating properties. It has been found that the properties may be impaired and discoloration may be adversely affected.

Here, an object of the present invention is to provide a surface-treated steel sheet which has little change in color tone before and after heat treatment, can secure conductivity even after heat treatment, and can withstand naked use.

Specifically, for example, at 550 ° C. in a surface-treated steel material used by performing a chromate treatment on a plated steel sheet.
Provide surface-treated steel sheets that have improved resistance to high-temperature discoloration when heated for 60 seconds and have electrical conductivity important for welding and other applications, and can be used in high-temperature corrosive environments such as explosion-proof bands for CRTs and gas stoves. It is to be.

[0013]

Means for Solving the Problems As a result of intensive studies conducted by the present inventor to achieve the object, for example, when a chromate film is provided on a Zn-Ni electric alloy plated steel sheet, silica sol and resin are contained in the chromate film. By blending and defining the respective proportions to the amount of Cr, it was found that the high-temperature discoloration resistance at high temperatures was significantly improved and stabilized, and the present invention was completed.

Accordingly, the gist of the present invention is a steel sheet and a surface-treated steel sheet having a chromate film provided on the surface of the steel sheet, wherein the chromate film has a thickness of 20 mm.
It contains Cr / mg / m ² or more and 120 mg / m ² or less, a silica sol having a particle diameter of 5 to 50 nm, and a water-soluble emulsion resin, and the silica sol and Cr have a weight ratio of SiO ₂ / Cr = 2 to 2. 5, the water-soluble emulsion resin and Cr are in a weight ratio of resin / Cr = 0.2 to 2.0, and the amount of hexavalent chromium and the total amount of chromium in the chromate film are Cr ⁶⁺ / All Cr = 0.
It is a surface-treated steel sheet excellent in high-temperature thermal discoloration resistance and conductivity, characterized by having a ratio of 2 to 0.8.

In an improved embodiment of the present invention, the chromate film further contains organic or inorganic lubricating particles, and the organic or inorganic lubricating particles and the resin are in a weight ratio of lubricating particles / resin =
You may comprise so that it may be 0.01-0.2.

In one embodiment of the present invention, the steel sheet is a Zn-Ni-based electroplated steel sheet containing 9 to 15 wt% of Ni in a plating film and having a single-side plating adhesion amount of 3 to 40 g / m ^2. It is. In addition, the plating film is Ni, Cr, Co, Mo, F
e, Pb and the like may be contained in a small amount.

In another embodiment of the present invention, the steel sheet is a hot-dip galvanized steel sheet or a hot-dip Al-Zn alloy-plated steel sheet with 4 to 95% Al and the balance being Zn. At this time, the plating film may contain a small amount of Si, Mg, Pb, Sb, La, Ce, or the like.

[0018]

Next, embodiments of the present invention will be specifically described. The present invention comprises a steel sheet and a chromate film provided on the surface thereof. Since the steel sheet is required to have high-temperature discoloration resistance as an application of the surface-treated steel sheet according to the present invention, a chromate film is easily provided and a required resistance is provided. There is no particular limitation as long as high-temperature discoloration can be exhibited, but a cold-rolled steel sheet is generally used, and in many cases, it is used as a plated steel sheet.

The preferred steel sheet has a coating weight of 3 to 40.
g / m ² and an electric Zn—Ni content of 9 to 15 wt% Ni
Alloy plated steel sheet or the adhesion amount is 30 to 150 g / m ² of the molten Zn,
It is a plated steel sheet or a hot-dip Al-Zn alloy plated steel sheet with 4 to 95% Al and the balance Zn.

Further, the chromate treatment itself is already known, and in the present invention, except that the composition of the chromate film is adjusted as follows, the chromate film is formed on the surface of the steel sheet by such a known operation. May be provided.

SiO ₂ / Cr = 2-5 Water-soluble emulsion resin / Cr = 0.2-2.0 Cr ⁶⁺ / Total Cr = 0.2-0.8 If desired, organic or inorganic lubricating particles / resin = 0.01-0.
2 However, Cr: 20 to 120 mg / m ² , silica sol particle size: 5 to 5
50 nm For example, a Zn-Ni-based electroplated steel sheet has been used in a larger amount than before because of its good corrosion resistance. However, in the case of use without treatment after plating, red rust does not occur at an early stage, but white rust of zinc is inevitable. As a preventive measure, chromate treatment is performed.

Therefore, from such a viewpoint, there are many products in which a chromate chromate film is applied on a plated steel sheet. In addition, although a chromate-treated steel sheet containing silica exists in a pure galvanized steel sheet for the purpose of improving corrosion resistance, when it is heated, the plating film itself is in a molten state due to its low melting point, and the base material Fe
And the sacrificial corrosion resistance may be significantly reduced.

When a conventional chromate chromate is subjected to heat treatment, the chromate film is significantly discolored and the anticorrosion function is impaired, so that naked use is impossible.

Further, when a large amount of resin is added to the chromate film or the resin is coated on the chromate film,
When exposed to a high temperature of 0 ° C. or more, the discoloration is accelerated by the burning of the resin component, and the corrosion resistance also deteriorates.

However, when a chromate film containing a small amount of silica and a small amount of a resin is used on a plating film having a high melting point, for example, an electric Zn-Ni plating or a hot-dip Al-Zn plated steel sheet, deterioration of corrosion resistance can be suppressed and high temperature environment can be suppressed. It is possible to obtain a steel sheet having excellent heat discoloration resistance underneath.

Therefore, in the present invention, a plating film having a high melting point, for example, a chromate film having excellent high-temperature corrosion resistance and conductivity, which is based on electric Zn-Ni plating or hot-dip Al-Zn plated steel plate, has been developed. In the present invention, discoloration in a high-temperature environment causes interference color by crystallization in the chromate film, oxidation, reduction of Cr ⁶⁺ and oxidation of the base steel sheet. The appropriate composition of the chromate film can be determined by a method that greatly suppresses it. That is, in the chromate film, Cr ⁶⁺ / total Cr, silica / total Cr and resin /
It specifies all Cr.

(Cr ⁶⁺ / total Cr = 0.2 to 0.8) Cr ⁶⁺ / total Cr is 0.8
When the temperature exceeds the threshold value, the initial yellowness is large, and the change in color tone due to the reduction reaction of Cr ⁶⁺ → Cr ³⁺ occurring in the heat treatment process becomes very conspicuous. However, if Cr ⁶⁺ / total Cr is less than 0.2, the self-healing function of Cr ⁶⁺ can hardly be expected,
Corrosion resistance will be significantly degraded. For that, Cr ⁶⁺
/ Total Cr = 0.2-0.8, more preferably Cr ⁶⁺ / total Cr = 0.
4 to 0.6.

The effect of adding (silica / total Cr = 2 to 5) silica sol is to disperse in the chromate film to stabilize the function as a barrier and corrosion products and suppress the progress of corrosion. The addition of silica sol in the present invention,
It may be carried out by adding a predetermined amount to the chromate solution before coating the steel sheet.

When the silica particle size is smaller than 5 nm, a sufficient effect of suppressing color tone cannot be obtained, and the silica tends to be aggregated, which is a cause of impairing liquid stability. If the silica particle size is larger than 50 nm, sufficient corrosion resistance cannot be obtained. Therefore, the particle size of the silica is 5 to 50 nm, more preferably 10 to 30 nm.

If the ratio of silica / total Cr is less than 2, sufficient effects on heat discoloration resistance and corrosion resistance cannot be obtained.
If it is larger than 5, there is a problem in cost increase and stability of the chromate solution. Therefore, silica / total Cr =
It is 2-5, more preferably 2.5-4.5.

The effect of improving the heat resistance by adding (resin / total Cr = 0.2 to 2.0) resin is to prevent interference color from being generated by coating the resin with a thin film. However, if the ratio of resin / total Cr is less than 0.2, the effect of preventing interference color cannot be obtained sufficiently. If it exceeds 2.0, the resin film thickness becomes too large, and the discoloration of the resin itself during heating becomes remarkable. Although the color is suppressed, the discoloration itself becomes large, and the conductivity is remarkably reduced, so that the weldability is deteriorated. Therefore, resin / total Cr = 0.
It is 2 to 2.0, more preferably 0.5 to 1.0.

Here, as the resin used in the present invention, acrylic styrene type, polyester type, acrylic type,
Urethane-based resins are exemplified and used as an aqueous emulsion.

(Lubricating particles / resin = 0.01-0.2) For processing applications, it is more advantageous to disperse organic or inorganic lubricating particles in a chromate film for appearance and cracking after processing. Therefore, lubricating particles may coexist in the resin for applications requiring processing. However, in order to satisfy the heat discoloration resistance and the electrical conductivity, which are the objects of the present invention, it is necessary that lubricating particles / resin = 0.01 to 0.2. More preferably, it is 0.05 to 0.1. If the ratio of the lubricating particles / resin is less than 0.01, a sufficient lubricating effect cannot be obtained. If the ratio exceeds 0.2, the lubricating component is melted upon heating, thereby deteriorating heat resistance discoloration and conductivity.

Here, inorganic lubricating particles include graphite molybdenum sulfide, boron nitride, calcium stearate particles, and the like, while organic lubricating particles include natural wax, polyolefin wax, and the like.
Examples include fluorocarbon particles. These may be used alone or in combination of two or more as necessary.

(Reason for Limiting Plating Film) As a phenomenon caused by heat-resistant discoloration, a decrease in lightness in a high temperature range can be cited.
When the brightness is lowered, the original brightness is more noticeable when the brightness is higher, and is less noticeable when the brightness is lower. Therefore, in the present invention, in the case of a Zn-Ni-based electroplated steel sheet, the synergistic effect combined with the above-described improvement by the chromate film composition is used by limiting the Ni content in the plating film to 9 to 15%. .

Generally, when Ni is contained in Zn plating,
The color tone of the plating film tends to be dark. Therefore, a fixed ratio of Ni is always contained. In addition, the stability of production can be ensured by adding Ni. Furthermore, since the dissolution rate of the plating film in a corrosive environment decreases, high corrosion resistance can be expected. Melting point of plating film is 419 ℃ of pure Zn plating
Steel sheet Fe under high temperature environment to rise to above 800 ℃
Diffusion into the plating film is delayed, the stability of the color tone is secured, and the workability after that does not deteriorate.

On the other hand, the same is true for hot-dip plating. When Al% in Zn increases, the melting point of plating increases and oxidation resistance also improves. Therefore, Al% in the plating film is 4% to 95%,
Desirably, it is 15% to 93%. Next, the operation and effect of the present invention will be described more specifically with reference to examples.

[0038]

EXAMPLES (Example 1) A Zn-Ni electroplated steel sheet having a composition as shown in Table 1 was applied to a SPCC-grade cold-rolled steel sheet by a conventional means at a coating weight of 20 g / m ² , In the same manner, 50 mg of a chromate film having the composition shown in Table 3 was used.
/ m ² only.

The prepared test material was subjected to a heat treatment at 550 ° C. for 60 seconds, and the color tone before and after heating was calculated using the following equation (1). Here, L: lightness, a: saturation, b: saturation,
Using a Cr-300 color difference meter manufactured by Minolta Co., Ltd., it was determined by Hunter display. Color difference (ΔE) = (ΔL ² + Δa ² + Δb ² ) ^(1/2) (1) Discoloration was evaluated as follows.

[0040] The results are shown in Tables 4-1 and -2.

(Example 2) A hot-dip Al-Zn plated steel sheet having a composition changed as shown in Table 2 was applied to an SPCC grade cold-rolled steel sheet at a coating weight of 80 g / m ² , and then a chromate film having a composition shown in Table 3 Was applied at 50 mg / m ² .

The prepared test material was evaluated for discoloration in the same manner as in Example 1. The results are shown in Tables 5-1 and -2. In these results, the chromate film in Table 3
Nos. 5, 11, and 18 do not contain a resin and correspond to the contents disclosed in JP-A-7-278837.

Next, the conductivity was tested in the following manner for the plating Nos. 7 and 16, and the chromates No. 12 and 19, which are representative examples obtained in Examples 1 and 2, and all were rated 1
Met.

[0044]

[0045]

[Table 1]

[0046]

[Table 2]

[0047]

[Table 3]

[0048]

[Table 4-1]

[0049]

[Table 4-2]

[0050]

[Table 5-1]

[0051]

[Table 5-2]

[0052]

As described above, according to the present invention, it is possible to obtain a surface-treated steel sheet having a chromate film having high-temperature discoloration resistance and excellent conductivity, which has not received much attention until now. In addition to greatly contributing to the future expansion of bare-plated chromate-treated steel sheets, the characteristics of explosion-proof bands and combustion appliances already used today are greatly improved, and their practical significance is great.

 ────────────────────────────────────────────────── ─── Continued on the front page F term (reference) 4K026 AA02 AA07 AA09 AA12 AA13 AA22 BA06 BB01 BB04 BB10 CA16 CA19 CA20 CA39 CA41 DA16

Claims (2)

[Claims] 1. A Zn—Ni-based electroplated steel sheet containing 9 to 15 wt% of Ni in a plating film and having a single-side plating adhesion amount of 3 to 40 g / m ² , or a hot-dip zinc-coated steel sheet or Al 4 to 95%.
A hot-dip Al-Zn alloy-plated steel sheet of the remaining Zn, and a surface-treated steel sheet having a chromate film provided on the surface of the steel sheet,
The chromate coating, 20 mg / m ² or more 120 mg / m ² or less of
Cr, a silica sol having a particle size of 5 to 50 nm, and a water-soluble emulsion resin, wherein the silica sol and Cr are in a weight ratio.
In the range of SiO ₂ / Cr = 2 to 5, wherein the water-soluble emulsion resin and Cr is in the range of resin /Cr=0.2 to 2.0 by weight ratio, and hexavalent chromium content and the total chromium in the chromate film A surface-treated steel sheet excellent in high-temperature thermal discoloration resistance and conductivity, characterized in that the amount is Cr ⁶⁺ / total Cr = 0.2 to 0.8. 2. The chromate film further contains organic or inorganic lubricating particles, wherein the weight ratio of the organic or inorganic lubricating particles to the resin is 0.01 to 0.2. 2. The surface-treated steel sheet according to 1.