JPH0665717A - Surface treated steel sheet having excellent thermal discoloration resistance - Google Patents

Abstract

PURPOSE:To produce the steel sheet which is less discolored by oxidation in a high-temp. atmosphere, maintains metallic gloss for a long time and has excellent thermal discoloration resistance by forming a film mixture composed of MgO and Al2O3 on the surface of the steel sheet by a vacuum vapor deposition method, etc. CONSTITUTION:The thin film consisting of the compsn. mixture contg. MgO at 10 to 80wt.% average content and the balance Al2O3 is formed at 0.01 to 10mum thickness by the vacuum vapor deposition method, ion plating method, etc., on the surface of the heat resistant steel sheet, such as stainless steel sheet or carbon steel sheet. The thin film mixture composed of the MgO and the Al2O3 is formed of the compsn. provided with a concn. gradient in such a manner that the concn. of the Al2O3 increases toward the depth direction of the film mixture in order to additionally improve the thermal deformation resistance of the flat plate part of the steel sheet or with such a concn. gradient in such a manner that the concn. of the MgO increases toward the depth direction in order to improve the adhesion and thermal deformation resistance of the worked part.

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 heat resistance and discoloration resistance, and particularly used for covers of electric heaters, heat reflecting members, and kitchen equipment, which shows little discoloration due to oxidation in a high temperature atmosphere, and metal. The present invention relates to a plated steel sheet having excellent heat discoloration resistance that maintains luster for a long time.

[0002]

2. Description of the Related Art Conventionally, a heating device such as a cover of an electric heater or a heat reflecting member, a heating device such as an oil stove or a fan heater, or a member used at high temperature such as a kitchen device,
Bright annealed stainless steel plates with excellent surface gloss are often used. This is because Cr contained in stainless steel is concentrated on the surface in a high temperature atmosphere to form a protective oxide film, which prevents outward diffusion of metal ions from the inside, and promotes oxidation. This is because it is possible to provide excellent corrosion resistance, heat resistance, and heat reflection characteristics in order to suppress the above. However, since this oxide film does not contain pure Cr ₂ O ₃ but contains Fe or an impurity element in steel, discoloration during use is inevitable and blackening progresses. Further, inclusions in the steel existing near the surface serve as a starting point of oxidation and discoloration and cause point defects. In particular, in a combustion atmosphere, the oxygen partial pressure is low, and since a large amount of water vapor is included, it is difficult to form a dense protective oxide film at the initial stage of oxidation, so that oxidation proceeds rapidly.
Therefore, the stainless steel plate is used as an electric heater (cover,
When used in a heating device such as a heat reflecting plate) or a combustion device such as an oil stove or a kitchen device, not only the appearance is deteriorated, but also the emissivity is changed due to the discoloration of the surface and the combustion state and the heating effect are deteriorated. In addition, since the degree of oxidation and discoloration varies depending on the parts used, the appearance and emissivity also become non-uniform and the efficiency decreases. In addition, in the food industry, generation of such discoloration and generation of scale due to oxidation are regarded as a problem from the viewpoint of hygiene.

In order to reduce the above-mentioned discoloration at high temperature, conventionally, the Cr concentration in the stainless base material is increased or Si is added to form Cr in the initial stage of oxidation.
By reducing the defects in the oxide film (densification) and reducing the diffusion of Fe and impurity elements into the film, the protection of the oxide film is improved and the progress of oxidation is suppressed. It has also been attempted to add Al to the base material and form a protective oxide film of Al instead of Cr on the surface of the steel sheet to reduce the oxidation rate. In addition, Zr, Ti, Y, rare earth elements and the like are reported to improve the adhesion of the scale to the base steel sheet by internal oxidation.

By improving the alloy addition components as described above, the oxidation resistance of the stainless steel sheet is improved, and the weight change due to oxidation is reduced. In particular, stainless steel containing 4% or more of Al shows excellent oxidation resistance by forming a uniform protective oxide film of Al ₂ O _{3 at} the initial stage of oxidation. However, in any of the above methods, an oxide film of Cr or Al in steel is formed in the vicinity of the surface at the initial stage of oxidation, which causes Fe
It is a mechanism for suppressing the oxidation of base material components such as. Therefore, since a large amount of Fe and impurity elements in steel are contained in the oxide film, the oxide film of Cr or Al formed in the surface layer is not transparent and is fundamentally discolored.

On the other hand, Japanese Unexamined Patent Publication No. 63-76861 does not attempt to improve the oxidation resistance and the heat discoloration resistance by improving the alloy addition components as described above, but aluminum oxide and silicon oxide are formed on the surface of a stainless steel plate. It is disclosed that a transparent oxide film having excellent heat resistance, such as chromium oxide or chromium oxide, can be formed by a vapor phase plating method to prevent discoloration of a stainless steel sheet due to oxidation. In addition, the silicon oxide film has the highest thermal stability and protection effect among these oxide films,
It is known to have excellent heat resistance and oxidation resistance (SRJ Saunders and JRNicholis.Materials Scienc
e and Technology, August 1989 vol.5).

[0006]

However, although the oxide single-layer film material made of silicon oxide or the like has excellent oxidation resistance and heat resistance, thermal discoloration occurs due to long-term exposure in a high temperature atmosphere. It was confirmed to occur. In particular, since these materials have insufficient workability and adhesion to the processed portion, thermal discoloration and oxidation rapidly progress in the processed portion because the base is exposed due to peeling or cracking of the film.

Therefore, the object of the present invention is
Discoloration does not occur not only in the flat plate part but also in the processed part,
It is intended to provide a surface-treated steel sheet excellent in heat discoloration resistance that maintains a metallic luster for a long time.

[0008]

In order to achieve the above object, the present inventor coated various oxide films or mixed films of plural kinds of oxides on stainless steel plates, heat resistant steel plates and carbon steel plates. Materials were made and evaluation tests and studies were repeated.

As described above, when not only cracks are generated in the film at the processed portion but also the base is exposed by peeling,
Sudden oxidation and thermal discoloration occur from this exposed part. Therefore, we first evaluated the workability of the film and the adhesion of the processed part, and found that the magnesium oxide film was less likely to crack in the processed part than other oxide films, and had excellent adhesion. Since it was clarified, the investigation was focused on the film containing magnesium oxide. As a result, by coating a mixed film layer of a magnesium oxide film and an aluminum oxide film on a stainless steel plate or the like in an amount of 0.01 to 10 μm, thermal discoloration under a high temperature atmosphere is significantly suppressed not only on a flat plate but also after processing. I found that I could do it. Furthermore, when aluminum oxide has a concentration gradient that increases in the depth direction of the plating layer (the composition analysis diagram in the depth direction is shown in FIG. 1), the flat plate portion is extremely excellent in heat distortion resistance. Do you get it. The reason for this is considered to be that aluminum oxide has a small diffusion constant of the metal element and has an effect of suppressing diffusion into the plating layer in the base steel sheet. On the other hand, when magnesium oxide has a concentration gradient that increases in the depth direction of the plating layer (the composition analysis diagram in the depth direction is shown in FIG. 2), the adhesion of the processed part and the heat deformation resistance of the processed part Was found to be extremely good. The reason for this is that magnesium oxide has excellent adhesion to the processed part, as described above,
It is considered that the surface of the base steel sheet is uniformly covered and protected even after the processing. As a result, the present invention has been completed.

That is, according to the present invention, a steel plate such as a stainless steel plate, a heat-resistant steel plate, a carbon steel plate, etc.
It is a surface-treated steel sheet having a heat-resistant discoloration resistance of a processed part, which is provided with a mixed film layer containing magnesium oxide and aluminum oxide and is coated with a thickness of μm.

[0011]

[Function] Here, both a stainless steel plate and a carbon steel plate can be applied as the base steel plate, but when used in a severe environment, a stainless steel plate excellent in heat resistance and oxidation resistance is particularly suitable. Is. When carbon steel is coated with the above-mentioned film, high performance is required because oxidation and discoloration of the underlying carbon steel are likely to progress rapidly from the small bores and defects existing in the film. It is difficult to apply to the part.

The thickness of the film is limited for the following reason. If the thickness of the film is less than 0.01 μm, the permeation / diffusion of oxygen from the atmosphere cannot be suppressed, the oxidation of the underlayer rapidly progresses, and scale formation and falling off occur. The oxide film is transparent, but when it is coated on a stainless steel plate or a carbon steel plate, it is colored due to interference. It is necessary to make it a transparent film if you want to take advantage of the metallic color and luster of the base due to the needs of the application, but to obtain a transparent oxide film, it is necessary to make the film thickness 1.0 μm or more. There is. However, when a thick film having a thickness of more than 10 μm is provided, a crack is generated in the film due to a difference in coefficient of thermal expansion between the undercoat material and the film, and the undercoat is exposed. For the above reasons, the thickness of the film is 0.01 to 1
It was set to 0 μm.

The magnesium oxide in the mixed film layer preferably has a content (average concentration) of 10 to 80% by weight in order to prevent discoloration and maintain metallic luster. If the magnesium oxide content is too high or too low, discoloration may occur in both the flat plate material and the processed material, and the metallic luster may be lost.

The concentration of aluminum oxide or magnesium oxide in the mixed film layer may be uniform in the depth direction, but an excellent effect can be provided by having a concentration gradient. For example, in order to further improve the heat distortion resistance of the flat plate portion, the mixed film layer having a concentration gradient in which aluminum oxide becomes higher in the depth direction of the mixed film layer (see the composition analysis diagram of the depth direction in FIG. 1). (One example is shown). The reason for this is as described above. Further, in order to further improve the adhesion of the processed part and the heat distortion resistance of the processed part, the mixed film layer having a concentration gradient in which magnesium oxide becomes higher in the depth direction of the plating layer (the depth of which is shown in FIG. 2). Directional composition analysis diagram is shown). The reason for this is as described above. Whether to give priority to the concentration gradient of aluminum oxide or the concentration gradient of magnesium oxide is arbitrarily determined according to the application and the characteristics required for the application. When giving priority to the concentration gradient of aluminum oxide, the concentration gradient is preferably 10 to 80% / μm. When giving priority to the concentration gradient of magnesium oxide, the concentration gradient is preferably 5 to 80% / μm.

It is difficult to coat the above oxide film by a wet method, and it can be coated by a vapor phase plating method such as CVD, sputtering, vacuum deposition, ion plating and plasma spraying. Among them, plasma spraying is not suitable for coating a thin film as in the present invention, and CVD and sputtering are not suitable for large-area processing and mass production because the film forming rate is low. Therefore, it is most preferable to apply the vacuum deposition and the ion plating, which have excellent uniform coverage and can easily control the film thickness in a wide film thickness region.

As a method for forming a concentration gradient of aluminum oxide or magnesium oxide in the mixed film layer, for example,
There is a continuous coating method using a binary vapor deposition crucible shown in FIG. In FIG. 3, 1 is a steel plate such as a stainless steel strip, 2 is a binary vapor deposition crucible, 3 is an electron gun, and when a concentration gradient of magnesium oxide enrichment type is formed on the surface of the plated steel plate as shown in FIG. Aluminum oxide in the tank 2a of
Magnesium oxide is put into the second tank 2b. Conversely, FIG.
When an aluminum oxide-enriched concentration gradient is formed on the surface of a plated steel sheet, magnesium oxide is put in the first tank 2a and aluminum oxide is put in the second tank 2b. In the present invention, as a method for measuring the concentration of aluminum oxide or magnesium oxide and its concentration gradient, for example, a secondary ion mass spectrometry method in which a sample is irradiated with an ion beam and the ions generated by sputtering are subjected to mass spectrometry, and abnormal glow discharge There is a glow discharge mass spectrometry method in which secondary ions generated by sputtering are subjected to mass spectrometry.

[0017]

EXAMPLE An example in which a plated steel sheet according to the present invention is manufactured and an evaluation test is performed will be shown below. Table 1 shows commercially available stainless steel sheets (SUS304, SUS43) used as test materials.
The chemical composition (% by weight) of 0) is shown. All of them are BA-finished through a normal process.

A single layer film of silicon oxide, aluminum oxide, chromium oxide, magnesium oxide and a mixed film of aluminum oxide and magnesium oxide were coated on the above-mentioned test material by a vacuum deposition method. When providing a concentration gradient of aluminum oxide and magnesium oxide, the coating was performed by the method of FIG. Tables 2 to 4 show the film configurations and film thicknesses of the produced materials. In this example, the film concentration was measured by secondary ion mass spectrometry.

As an evaluation test, a flat plate material and a processed material (R8m
m, 90 ° bending) thermal cycle test (in an electric furnace,
The atmosphere was then evaluated, and the amount of oxidation increase, glossiness and appearance (transparency, color) were evaluated. Thermal cycle test is as follows: (1) Hold at 500 ° C for 7 hours → 1 cycle of cooling
And (2) after being kept at 700 ° C. for 7 hours and then being allowed to cool, the cycle was repeated 7 times for 2 conditions. The gloss level is 1
The surface of 0 mm square is irradiated with white light at an incident angle of 60 °, and the intensity of reflected light is represented by the ratio (%) to the incident light. In addition, the surface of the processed portion (R8 mm) is scanned with a scanning electron microscope (S
The adhesiveness of the processed portion was evaluated based on the state of crack generation and the state of peeling of the composite film layer by observing with EM). The results are shown in Table 2.
~ Shown in Table 4.

Stainless steel 30 used in the test as a comparative material
4 and 430 materials (Comparative Examples 39 and 40) were 500 ° C.,
By one heat cycle test, oxidation proceeded and the surface lost the metallic luster and discolored. On the other hand, an oxide film of silicon oxide, aluminum oxide, chromium oxide, etc.
Those coated with 1 μm or more (Comparative Examples 2-9, 11-14)
However, almost no weight increase due to oxidation was observed, but with these materials, the metallic luster was lost, discoloration occurred, and the glossiness also decreased. Particularly, in the case of the processed material, the oxidation rapidly progressed and discoloration occurred.

On the other hand, Examples 15 to 15 of the present invention coated with a mixed film layer of aluminum oxide and magnesium oxide
In the case of No. 32, both the flat plate material and the processed material maintained the metallic luster after the heat cycle test, and almost no discoloration was observed. In particular, when the thickness of the oxide film is 1.0 μm or more (Examples 15, 17, 19 to 24, 26 to 32), a transparent film is obtained, but since the transparent appearance is maintained even after the thermal cycle test, The bright finished stainless steel with good gloss was not impaired.

Furthermore, when the aluminum oxide has a concentration gradient that increases in the depth direction of the mixed film layer (Examples 21 to 24), the evaluation results of the flat plate material after the heat cycle test at 500 ° C. and 700 ° C. From this, it can be seen that the flat plate portion is particularly excellent in heat distortion resistance. When the magnesium oxide has a concentration gradient that increases in the depth direction of the mixed film layer (Examples 25 to 29), the appearance of the processed material was evaluated from the appearance evaluation result after the thermal cycle test at 700 ° C. It can be seen that the adhesiveness is excellent and thermal deformation does not easily occur in the processed part. The concentration gradients of Examples 21 to 29 were 2 in order, respectively.
It is 1, 30, 19, 21, 61, 19, 33, 10, 6.

Further, the average content of magnesium oxide is less than 10% by weight or more than 80% by weight (Example 3
0 to 32) and a single layer film of magnesium oxide (Comparative Example 38), it was found that the glossiness of the surface was slightly decreased by the heat cycle test. from this result,
It is understood that the average content of magnesium oxide is preferably 10 to 80% by weight.

Further, when the film thickness of the mixed film layer was less than 0.01 μ (Comparative Examples 34 and 36), oxidation and thermal deformation occurred.
It is presumed that this is because the mixed film layer did not completely cover the surface of the stainless steel plate of the base steel plate, so that the permeation and diffusion of oxygen from the atmosphere could not be suppressed, and the oxidation of the base steel plate proceeded rapidly. On the other hand, when it exceeds 10 μm (Comparative Example 3)
5, 37), as described above, due to the difference in the thermal expansion coefficient between the coating and the base steel sheet, cracks in the coating and peeling of the coating occur and the base steel sheet is exposed, so sufficient characteristics were not obtained. .

[0025]

As described above, the mixed film layer of magnesium oxide and aluminum oxide is formed on the surface of the steel sheet in an amount of 0.01 to 10 μm.
The material coated with m is excellent in heat discoloration resistance, and not only the flat plate material but also the processed material does not undergo thermal discoloration in a high temperature atmosphere and the metallic luster is maintained for a long time. In particular, this effect is remarkable when the average content of magnesium oxide is 10 to 80% by weight. Further, when the aluminum oxide has a concentration gradient that increases in the depth direction of the plating layer in the mixed film layer, the heat deformation resistance of the flat plate portion can be further improved. Further, when the magnesium oxide has a concentration gradient that increases in the depth direction of the plating layer in the mixed film layer, the adhesion of the processed portion and the heat deformation resistance of the processed portion are further improved. Therefore, the present invention is extremely useful as a cover for an electric heater, a heat reflection member, and an interior material for kitchen equipment.

[0026]

[Table 1]

[0027]

[Table 2]

[0028]

[Table 3]

[0029]

[Table 4]

[Brief description of drawings]

FIG. 1 is a composition analysis diagram of a plating layer in a depth direction according to an embodiment of the present invention.

FIG. 2 is a composition analysis diagram in the depth direction of a plating layer according to another embodiment of the present invention.

FIG. 3 is an explanatory view showing a method for forming a concentration gradient in a plating layer according to the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Steel plate, 2 ... Dual vapor deposition crucible, 2a ... 1st tank, 2b
... second tank, 3 ... electron gun.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tomoaki Hyodo 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Inventor Shuichi Nagaishi 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Inside the Steel Pipe Corporation (72) Inventor Hiroshi Kibe 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Inside the Nippon Steel Pipe Corporation

Claims (4)

[Claims] 1. A surface-treated steel sheet having excellent resistance to heat discoloration, comprising a steel sheet and a mixed film layer containing magnesium oxide and aluminum oxide, which is coated on the steel sheet to a thickness of 0.01 to 10 μm. 2. The surface treatment excellent in heat discoloration resistance according to claim 1, wherein the mixed film layer is composed of magnesium oxide, aluminum oxide and unavoidable impurities, and the average content of magnesium oxide is 10 to 80% by weight. steel sheet. 3. The surface treatment excellent in heat discoloration resistance according to claim 1, wherein the aluminum oxide of the mixed film layer has a concentration gradient that increases in the depth direction of the mixed film layer. steel sheet. 4. The surface treatment excellent in heat discoloration resistance according to claim 1, wherein the magnesium oxide of the mixed film layer has a concentration gradient that increases in the depth direction of the mixed film layer. steel sheet.