JP2000290764A - Hot-dip aluminized steel sheet with excellent resistance to blackening by heating and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a hot-dip aluminized steel sheet excellent in heat blackening resistance, which is mainly used for heat appliances, exhaust system materials and the like. The present invention provides a stable
It became possible to obtain an aluminum-plated steel sheet having a silver-white appearance up to 0 ° C. SOLUTION: The composition is C: 0.01% or less by weight ratio, S
i: 0.10% or less, N: 0.0005 to 0.010
%, Al: 0.01% or less, total of 0.01% or less of Ti, Nb, V, B, steel sheet composed of balance of Fe and unavoidable impurities, Si: 3 to 12% by weight in composition, Mg: 0.
A hot-dip aluminized steel sheet having excellent resistance to blackening by heating, characterized in that the hot-dip aluminized steel sheet is formed by performing a hot-dip aluminum plating in a plating bath consisting of 5 to 15%, the balance being Al and unavoidable impurities, comprising a plating layer, an intermetallic compound and a steel sheet Production method.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the production of a hot-dip galvanized steel sheet having excellent resistance to blackening by heating, in which the surface of the plating layer does not turn black even when it is held at a high temperature and maintains high heat reflection. It is about the law.

[0002]

2. Description of the Related Art A so-called type 1 hot-dip aluminized steel sheet in which a steel sheet is plated with an Al-Si alloy retains silver white even at high temperatures because of the addition of Si to the plating layer, and exhibits heat reflection. Excellent characteristics. For this reason, it has been conventionally used for various heat-resistant applications. However, this type 1 hot-dip aluminized steel sheet is also 400-450 ° C.
Exposure to the above high temperature causes interdiffusion of Al and Fe,
It is known that the aluminum plating layer changes to an Fe-Al-Si intermetallic compound layer (hereinafter, this phenomenon is referred to as alloying) and turns black, loses luster, and significantly impairs heat reflectivity.

[0003] The alloying of aluminum plating involves the so
It is known that the l-N content is deeply involved, and in a steel sheet containing sol-N in a certain amount or more, the formation of an AlN layer at the interface between the alloy layer and the steel sheet to suppress the alloying reaction may be, for example, iron. And steel 70 (1984) S475. It is also known that the AlN layer grows by annealing the steel sheet containing sol-N after plating, and the blackening resistance temperature further increases.

[0004] Based on such knowledge, various studies have been made on techniques for suppressing blackening caused by alloying. For example, the present applicants disclosed in Japanese Patent Publication No. 2-61544 that the amount of sol-N was 0.0003 to 0.00.
Disclosed is an aluminum-plated steel sheet which is limited to 20% and Al to 0.005% or less, and subjected to annealing under specific conditions after hot-dip aluminum plating to impart blackening resistance to heating. Further, in Japanese Patent Publication No. 5-26864, N: 0.0005 to
An example of a galvanized steel sheet containing 0.0060%, Al: 0.005 to 0.020%, and annealed under specific conditions after plating is disclosed.

[0005] In these disclosed examples, it is true that Fe-A
It is possible to suppress the alloying reaction of
The silver-white appearance can be maintained even after exposure to a high temperature such as ５570 ° C. However, its performance is somewhat unstable,
Alloying may occur at a temperature of about 530 ° C. depending on the steel composition and operating conditions. On the other hand, when heated at about 500 ° C., alloying did not occur, but only the surface sometimes exhibited a black color as if alloyed.

[0006]

SUMMARY OF THE INVENTION According to the present invention, the reason why the quality instability occurs as described above is clarified, and it is possible to stably solve the problem.
An object of the present invention is to provide a production method for achieving a blackening resistance temperature of 0 ° C. or higher. First, alloying occurs at about 530 ° C., which is largely affected by the amount of Mg in the plating bath and the total amount of Ti, Nb, V, and B in steel. In particular, they have found that the presence of 3% or more of Mg in the plating bath contributes to the stability of blackening resistance due to heating.

Although the reason is still unclear, when Mg enters the plating bath, Mg is taken into the alloy layer and Fe-
Fe-Al-Si- with Al-Si intermetallic compound layer
An Mg intermetallic compound layer is formed. This Fe-Al-S
It is presumed that the presence of the i-Mg intermetallic compound layer affects the susceptibility of the intermetallic compound layer to change to monoclinic, and becomes a barrier layer when exposed to a high-temperature environment, thereby suppressing secondary alloy layer growth. ing. Elements such as Ti, Nb, V, and B are all elements that form compounds with N, and the more these elements are added, the more the amount of sol-N is reduced. The critical amount is that the total amount of these elements is 0.005%. Therefore, in order to obtain stable performance, the total amount of Ti, Nb, V, and B needs to be 0.005% or less.

Next, a description will be given of a blackening phenomenon, which is not alloying, which occurs when the hot-dip aluminum plating is maintained at a high temperature.
For example, Japanese Patent Publication No. 2-61544 or Japanese Patent Publication No.
Even in a hot-dip aluminized steel sheet manufactured by a manufacturing method as disclosed in Japanese Patent No. -26864, a phenomenon of blackening after heating at 500 ° C. or more was found as if alloying had occurred. The color tone at this time is very similar to the blackening caused by the alloying. However, when observed from the cross section, no alloying has occurred, and the color tone is slightly light at a temperature of 500 ° C. or less. For these reasons, this blackening phenomenon is a phenomenon that depends only on the surface of the aluminum plating layer. That is, in order to obtain a hot-dip aluminized steel sheet having excellent resistance to blackening after heating, it is necessary to leave sol-N in the steel in a stable manner as described above, but this is not necessarily a sufficient condition. It is necessary to take measures to prevent blackening.

When the blackened hot-dip aluminized steel sheet was examined by GDS (glow discharge spectroscopy), a strong Cr peak appeared on the surface layer. When the substance was examined by thin film X-ray diffraction, CrO (OH) was identified. Therefore, it can be concluded that this surface blackening is due to the chromate treatment. Normally, a hot-dip aluminized steel sheet is subjected to a chromate treatment mainly containing chromic acid for primary rust prevention. It is considered that when the chromate is heated to 500 ° C. or more, the chromate is transformed into a black film.

The present invention has been completed based on the above findings. That is, Ti, Nb,
V, B, etc. are limited to form a component system that stably retains sol-N, and this is subjected to hot-dip aluminum plating containing Mg to some extent to form an Fe-Al-Si-Mg alloy in the alloy layer. Easily change to monoclinic, avoiding chromate treatment after hot-dip aluminum plating or treating it with a certain amount or less to prevent both blackening due to alloying and blackening due to surface, The object is to obtain a hot-dip aluminized steel sheet having excellent resistance to blackening after heating. That is, the present invention prevents blackening due to interdiffusion of elements by forming an intermetallic compound having a monoclinic crystal structure between a plating layer and a steel sheet, or by forming an AlN layer. is there.

The gist of the invention is as follows: (1) The composition is C: 0.01% or less by weight, Si:
0.10% or less, N: 0.0005 to 0.010%, A
l: 0.01% or less, total 0.0 of Ti, Nb, V and B
1% or less, a composition of steel: 3 to 12% by weight and Mg: 0.5 to
A hot-dip aluminized steel sheet having excellent resistance to blackening by heating, characterized in that hot-dip aluminizing is carried out in a plating bath comprising 15%, the balance being Al and inevitable impurities, and an intermetallic compound is present between the plated layer and the steel sheet.

(2) The hot-dip aluminized steel sheet according to (1), wherein the intermetallic compound present between the plating layer and the steel sheet has a monoclinic crystal structure. (3) The hot-dip aluminized steel sheet according to (1) or (2), wherein an AlN layer is present at an interface between the intermetallic compound and the steel sheet. (4) After plating, the adhesion amount per side is 15m in Cr conversion
A hot-dip aluminized steel sheet having excellent resistance to blackening after heating, characterized by being subjected to a chromate treatment of g / m ² or less.

(5) Composition: C: 0.01% or less, Si: 0.10% or less, N: 0.0005 to 0.0
10%, Al: 0.01% or less, total of 0.01% or less of Ti, Nb, V, and B, the composition of a steel sheet composed of the balance of Fe and unavoidable impurities by weight: Si: 3 to 12%, Mg :
Hot-dip aluminum plating in a plating bath consisting of 0.5 to 15%, with the balance being Al and inevitable impurities, and then or after further chromate treatment, at a temperature of 300 to 500 ° C. for 2 hours
This is a method for producing a hot-dip aluminized steel sheet having excellent resistance to blackening by heating, characterized by performing an annealing treatment for up to 20 hours.

Next, the reasons for limitation of the present invention will be described.
First, the components in steel will be described. C: At the same level of sol-N, the lower the C content, the more the alloying between the Al-Si plating layer and the ground iron is suppressed. On the other hand, since the component system of the present invention necessarily contains sol-N, the processability is slightly inferior. Therefore, a low C is preferable in terms of both suppression of alloying and workability. In the present invention, the content is limited to 0.01% or less. However, from the above point, the content is desirably 0.005% or less.

Si: Si reacts with oxygen in the steel making stage to form silicon and silicate, and removes oxygen in the molten steel. In the steel strip manufacturing process, there is a possibility of reacting with sol-O in steel. Also, Si reacts with N in steel to form Si.
₃ to generate N _4, SiN, reducing the sol-N.
Furthermore, when the amount of Si increases, it is concentrated as an oxide on the surface at the time of heating during the hot-dip plating process, causing non-plating. In any case, a lower element is a desirable element, and 0.10%
Or less, preferably limited to 0.05% or less.

N: In order to ensure gloss retention after aluminum plating, it is necessary to leave sol-N on the steel sheet. For this purpose, 0.0005% or more of N is required. On the other hand, the steel sheet hardens due to the increase of sol-N,
Strength and tensile strength are greatly improved, and elongation is reduced. In addition, press formability also deteriorates. Therefore, the upper limit of the amount of N is set to 0.
010%.

Al: Al is usually used as a deoxidizing agent for molten steel in a steelmaking process. However, the remaining Al reacts with sol-N in the steel strip manufacturing process to become AlN.
In that case, the gloss retention after aluminum plating is deteriorated, so that the lower the Al content, the better. However, the degree of the solubility product of AlN may remain, and the upper limit is set to 0.0.
Limited to 1%.

Ti, Nb, V, B: These elements all form a compound with N. Therefore, in order to secure sol-N, a smaller amount is preferable. In particular, Ti is an element having a small solubility product with N and easily precipitated as TiN, and thus needs to be kept low. In order to stably secure sol-N, the total amount of these elements is limited to 0.010% or less. Desirably, the content of Ti is set to 0.001% or less.

Next, the reason for limiting Si in the hot-dip aluminum plating bath will be described. In aluminum plating, an Al-Fe intermetallic compound layer (hereinafter referred to as an alloy layer) is thick and tends to grow, and the grown alloy layer causes plating peeling during processing. Usually, Si is added to suppress the growth of the alloy layer. For the purpose of reducing the alloy layer, the amount of Si is 3
% Or more is necessary. On the other hand, the effect saturates at about 10%, and further addition lowers the corrosion resistance and workability. Therefore, the upper limit and the lower limit of the amount of Si are limited to 12% and 3%.

Next, the reason for limiting Mg in the plating bath will be described. In the present invention, 0.5% or more is contained in the plating bath. The reason for this is that Fe-Al
Due to the barrier effect of the -Si-Mg intermetallic compound, the effect increases as the amount of Mg increases. However, when the amount of Mg is 10% or more, plating becomes hard and workability deteriorates, so the upper limit is set to 15%. Al, Si, M in bath
As an unavoidable element other than g, about 2% of Fe normally eluted from a plated steel sheet or plating equipment is contained, but this is not particularly limited.

Next, the reason for limiting the amount of adhesion in the chromate treatment after plating will be described. As described above, in general, the surface of a hot-dip aluminized steel sheet is treated with a chemical mainly composed of chromic acid called a chromate treatment in order to prevent the generation of initial rust. The normal amount of adhesion is 2 in terms of Cr per side.
It is around 0 g / m ² . In the present invention, the amount of adhesion is limited to 15 mg / m ² or less in order to bring out blackening when the chromate film is heated. If this amount is 50
Heating to 0 ° C or higher does not cause blackening and has little effect on the rust-preventive performance of the treatment.
₍₃₎ The treatment may be carried out in a single bath, or may be performed by adding SiO ₂ thereto, or may be performed by adding phosphoric acid, a resin component or the like as necessary. Alternatively, the chromate treatment may not be performed if the initial rust prevention performance is inferior but the color tone is emphasized.

According to the present invention, a stable blackening resistance at 530 ° C. is exhibited by specifying the components of the plating bath and the components in the steel. However, annealing after plating is even higher 57
Blackening resistance at 0 ° C. can be imparted. The reason for limiting the annealing conditions in this case will be described. The purpose of this annealing treatment is to react Al in the plating layer with sol-N in the steel to generate AlN at the interface between the alloy layer and the iron base. This A
In order to generate 1N, 300 ° C. or higher is required,
At 0 ° C. or higher, the reaction of the plating layer Al—base iron occurs preferentially, which is inappropriate. Also, as for the time, a processing time of 2 hours or more is necessary for the generation of AlN, and if it is too long, it is disadvantageous in terms of cost.

[0023]

The present invention will be described in more detail with reference to examples.
You. 200 kg of steel having the composition shown in Table 1
Smelted as a steel ingot, which is rough-rolled to a thickness of 25 mm.
It was rolled to a thickness of 4 mm by up-rolling to obtain a hot-rolled sheet. Cool this
Cold rolled at an elongation of 80% to produce a 0.8mm thick thin steel coil
Done. Components other than the elements shown in Table 1 were Mn: 0.3
0%, P: 0.006%, S: 0.008%. Ma
A in Table 1 is the basic component, and the parts not described in the table are
It is the basic component. NOF-RF melting of this coil
Changing the amount of Si in the bath on the plating line
And the amount of adhesion is about 120 on both sides by gas wiping.
g / m ^Two Was adjusted to Some samples were then 30 g /
1, CrO_Three+10 g / l SiO _TwoDrop the solution on the steel plate
And adjust the amount of chromate with a roll squeezer and dry with hot air.
Thereafter, a sample was prepared by subjecting it to temper rolling of 1%. At this time
The operating conditions were to adjust the air-fuel ratio of the non-oxidizing furnace to 0.9,
Furnace plate temperature 650 ° C, reduction furnace plate temperature 800 ° C, bath temperature 660
° C. Some of the prepared samples were further box-annealed.
, Using different annealing conditions to perform annealing.
1% temper rolling was performed. Roll at the time of temper rolling
Was used. The amount of chromate adhesion is
The plating surface is peeled off with 20% NaOH.
The amount was measured, and the amount of adhesion per one side was calculated. Measurement is burning
Although performed before and after dulling, almost the same values were obtained.

[0024]

[Table 1]

The sample thus produced was evaluated by the following method. (1) Heat-resistant blackening Sample (50 × 100) was placed in a box annealing furnace at 530,
Each annealing was performed at 570 ° C. for 200 hours. After annealing, the presence or absence of alloying was determined by visual judgment and cross-sectional structure observation, and the L * value was measured by a color computer.

(2) Workability After the press oil was applied, drawing was performed with a blank diameter of 100 mm and a punch diameter of 50 mm (drawing ratio 2.0) to determine whether or not drawing was possible. Further, after degreasing, taping was performed on the side surface after squeezing, and the plating peeling state was observed to evaluate the workability of the plating layer. Table 2 summarizes the details of the as-plated samples without annealing and the evaluation results. Table 3 summarizes the sample specifications and the evaluation results when the annealing conditions were changed.

[0027]

[Table 2]

If the contents of C, Si, N in the steel component and Mg in the plating bath are too large, the workability of the steel sheet is impaired and the steel sheet cannot withstand press working. Al, Ti, Nb,
If the sum of N and B is too large, or if N is too small, the performance of suppressing alloying during heating is insufficient and the surface turns black. If the amount of chromate is too large, alloying is suppressed, but the chromate on the surface is altered and blackened. On the other hand, if the amounts of Si and Mg in the plating bath are insufficient, the alloy layer grows too much during plating, and the plating adhesion is insufficient. On the other hand, if the amount of Mg is too large, the workability deteriorates. When all of these conditions are appropriate, a steel sheet having excellent blackening resistance after heating, workability of the base material, and workability of plating can be obtained.

[0029]

[Table 3]

In order to further improve the blackening resistance by annealing, the blackening resistance in Table 3 was evaluated at 570 ° C. which was higher than that in Table 2.
It is done in. If there is too much C, Si, N in the steel component,
It impairs the workability of the steel sheet and cannot withstand press working.
If the total amount of Al, Ti, Nb, N, and B in the steel is too large, or if the N content is too small, the ability to suppress alloying during heating is insufficient and the surface turns black. If the amount of chromate is too large, alloying is suppressed, but the chromate on the surface is altered and blackened. On the other hand, Si, M
If the amount of g is insufficient or the annealing temperature is too high, the alloy layer will grow too much during plating, and the plating adhesion will be insufficient. Further, when the amount of Mg in the plating bath is large, the workability becomes insufficient. When the annealing temperature is low or the annealing time is short, the effect of annealing is insufficient and the blackening resistance temperature does not reach 570 ° C. When all of these conditions are appropriate, a steel sheet having excellent blackening resistance after heating, workability of the base material, and workability of plating can be obtained.

[0031]

As described above, the present invention provides a method for producing a hot-dip aluminized steel sheet excellent in heat blackening resistance, which is mainly used for heat appliances, exhaust system materials and the like. Further, according to the present invention, it has become possible to obtain an aluminum-plated steel sheet which maintains a silver-white appearance stably up to 570 ° C. Conventionally, expensive stainless steel sheets have been used for such applications, but inexpensive hot-dip aluminized steel sheets can be used, which greatly contributes to the industry.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification code FI Theme coat ゛ (Reference) C23C 2/28 C23C 2/28 2/40 2/40 28/00 28/00 C (72) Inventor Izaki Teruaki 1-1 Nichihata-cho, Tobata-ku, Kitakyushu-shi, Fukuoka Nippon Steel Corporation Yawata Works (72) Inventor Masao Kurosaki 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation Technology Development Division ( 72) Inventor Hisaaki Sato 1-1 Futaba-cho, Tobata-ku, Kitakyushu-shi, Fukuoka F-term in the Nippon Steel Corporation Yawata Works (reference) 4K027 AA02 AA23 AB01 AB08 AB14 AB32 AB48 AC72 AC82 AE03 AE21 4K037 EA01 EA02 EA04 EA15 EA18 EA19 EA23 EA25 EA27 EA31 EA32 FM02 GA02 GA05 4K044 AA02 AB02 BA10 BA15 BA18 BA19 BB03 BB04 BC02 BC09 BC11 CA11 CA16 CA53 CA62

Claims (5)

[Claims] 1. The composition is, by weight, C: 0.01% or less, Si: 0.10% or less, N: 0.0005 to 0.010%, Al: 0.01% or less, Ti, Nb, A steel sheet consisting of V and B in total of 0.01% or less, the balance being Fe and unavoidable impurities has a composition in weight ratio of S
i: 3 to 12%, Mg: 0.5 to 15%, the balance being hot-dip aluminum plating in a plating bath consisting of Al and unavoidable impurities, and having an intermetallic compound between the plating layer and the steel sheet. Hot-dip aluminized steel sheet with excellent heat blackening resistance. 2. The crystal structure of an intermetallic compound present between a plating layer and a steel sheet is monoclinic.
Hot-dip aluminized steel sheet with excellent resistance to heat blackening described. 3. The hot-dip aluminized steel sheet according to claim 1, wherein an AlN layer is present at an interface between the intermetallic compound and the steel sheet. 4. A hot-dip aluminized steel sheet excellent in resistance to blackening after heating after heating, wherein after plating, a chromate treatment is performed so that the amount of adhesion per one side is 15 mg / m ² or less in terms of Cr. 5. The composition is, by weight, C: 0.01% or less, Si: 0.10% or less, N: 0.0005 to 0.010%, Al: 0.01% or less, Ti, Nb, The composition of the steel sheet composed of V and B in a weight ratio of 0.01% or less and the balance of Fe and unavoidable impurities is represented by S
i: 3 to 12%, Mg: 0.5 to 15%, balance aluminum and molten aluminum plating in a plating bath composed of unavoidable impurities, and then or after further chromate treatment at a temperature of 30
A method for producing a hot-dip aluminized steel sheet having excellent resistance to blackening by heating, characterized by performing an annealing treatment at 0 to 500 ° C. for 2 to 20 hours.