JP2000239820A - Hot-dip aluminized steel sheet excellent in corrosion resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a hot-dip aluminized steel sheet having drastically excellent corrosion resistance in comparison with the conventional steel sheet and hardly generating rust of the base steel on the edge surface. SOLUTION: On the surface of the steel sheet, an intermetallic compound coated layer containing Al, Fe, Si and having <=5 μm thickness, is applied, and further, on the surface of this intermediate compound coated layer, a coated layer composed of by weight, 2-13% Si, >3% to 15% Mg and the balance substantially Al, is applied. Furthermore, it is desirable that the ratio of Mg/Si in the coated layer is <=1.7.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-dip aluminized steel sheet having excellent corrosion resistance for use in exhaust system members of automobiles, gasoline tank materials, metal building materials such as roof walls, and household heating appliances such as toasters and stoves. .

[0002]

2. Description of the Related Art Hot-dip aluminized steel sheets have been used in steel sheets for automobile parts, building materials, home electric parts and the like as described above because of their high corrosion resistance, heat resistance and beautiful appearance. In order to respond to the recent demand for improvement of corrosion resistance of automobile exhaust system members, a number of steel plates having a high corrosion resistance have been developed by applying a Cr-containing steel plate or a stainless steel to an aluminum plating to provide a high corrosion resistance (see Japanese Patent Application Laid-Open No. Sho 61). 231152, JP-A-3-277761, etc.). When Cr-containing steel or stainless steel is used as the base plate, the corrosion resistance naturally increases, but the production cost increases and the workability tends to deteriorate. Accordingly, various studies have been made on adding a corrosion resistance improving element to the plating bath. In Japanese Patent Application Laid-Open Nos. 2-88754 and 7-20091, Cr, M
The addition of n and the like are disclosed.

On the other hand, recently, Pb of an automobile fuel tank has been
Freedom is being studied, and the use of aluminum-plated steel sheets for this purpose is also advancing. The problem at this time is a high balance between corrosion resistance, workability, and weldability. In general, in a surface-treated steel sheet, as the amount of plating increases, the corrosion resistance naturally increases, but the workability and weldability tend to decrease. The weldability at this time means the reactivity with the electrode,
Aluminum easily reacts with copper as an electrode material, and therefore, when the amount of adhesion increases, the aluminum easily reacts with the electrode, resulting in a reduction in the life of the electrode. Therefore, many inventions have been made in order to achieve both of these characteristics (Japanese Patent Application Laid-Open No. 10-46358), but it is difficult to say that corrosion resistance, weldability, and workability can be completely compatible. Aluminum plating is particularly excellent in corrosion resistance especially in a dry-wet environment, but tends to be easily dissolved in a constantly wet environment. In a salt damage environment, aluminum plating dissolves preferentially by sacrificial corrosion of the steel sheet, but in an environment that is constantly wet, its speed is large and the plating may melt in a short period of time.

[0004]

Therefore, in the present invention, by adding Mg which contributes to corrosion resistance to the aluminum plating layer, the corrosion resistance of the plating layer is improved remarkably compared with the prior art, whereby a low adhesion amount is sufficient. An object of the present invention is to provide a galvanized steel sheet that can exhibit corrosion resistance and has improved workability and weldability.

[0005]

Means for Solving the Problems The present inventors have conducted detailed studies on the behavior of added elements, particularly Si and Mg, in a plating bath and completed the present invention. Hereinafter, the present invention will be described in detail. In JP-A-56-127762, the present applicants have already disclosed a method for producing an aluminum-plated steel sheet containing Si and Mg. We have:
The plating structure when the amounts of Si and Mg were changed and the corrosion resistance at that time were examined in more detail, and the following findings were obtained. That is, A
As is clear from the l-Si-Mg ternary phase diagram, this system forms an intermetallic compound called Mg ₂ Si. This weight ratio is Mg / Si = 1.73. When a line is drawn with this composition from the Al corner, the phase diagram is divided into two parts above and below the line. Above this line, Al—Mg ₂ Si—Al ₈ Mg
Ternary eutectic of No. ₅ ; below, Al-Mg ₂ Si-S
It becomes ternary eutectic of i. On the other hand, the line of Mg / Si = 1.73 is the binary eutectic of Al-Mg ₂ Si.

In the present invention, the finding that the corrosion resistance is remarkably improved by adding Mg to the aluminum plating layer was obtained, but this was greatly related to Mg / Si, and the ratio was 1.7.
3 before and after above, the effect of almost Mg added was not observed in ternary Akiraiki of words Al-Mg ₂ Si binary eutectic or _{Al-Mg 2 Si-Al 8} Mg 5,. At this time, the microstructure of the plating layer also changed substantially according to the phase diagram, indicating that the microstructure of the plating layer greatly affected the corrosion resistance. The reason why the metal structure affects the corrosion resistance is not yet known in detail, but it is considered that the presence of Mg ₂ Si contributes to the corrosion resistance.

According to the present invention, Mg is added and Mg is added.
By properly controlling the ratio of / Si, it is possible to obtain a galvanized steel sheet with dramatically improved corrosion resistance. Due to its excellent corrosion resistance, the amount of plating adhesion can be reduced, or the post-treatment coating can be simplified. Processing, weldability,
Alternatively, it has an effect of improving workability. The effect of Mg is remarkable in a salt damage environment, particularly in a wet environment, but also has a sufficient effect in an air environment, a sulfuric acid environment, and the like. Furthermore,
By adding Mg, an end face anticorrosion effect in an atmospheric environment, which has been conventionally difficult, can be obtained.

The gist of the present invention is as follows. (1) An intermetallic compound coating layer containing Al, Fe, and Si and having a thickness of 5 μm or less is provided on the surface of the steel sheet.
A hot-dip galvanized steel sheet having excellent corrosion resistance, characterized by having a coating layer of 2 to 13%, Mg: more than 3% to 15%, and the balance substantially consisting of Al. (2) The galvanized steel sheet having excellent corrosion resistance according to (1), wherein Mg / Si is 1.70 or less. (3) The hot-dip aluminized steel sheet having excellent corrosion resistance according to the above (1) or (2), wherein the thickness of the intermetallic compound layer is 3 μm or less.

(4) The melt having excellent corrosion resistance as described in (1) to (3) above, wherein the surface of the Al-based coating layer has a post-treatment film containing 10 to 1000 mg / m ^{2 in} terms of silica. Aluminum plated steel sheet. (5) The hot-dip aluminized steel sheet according to (1) to (4), wherein the outermost layer on at least one surface of the steel sheet is an organic resin layer. (6) The hot-dip aluminized steel sheet having excellent corrosion resistance according to (1) to (5), wherein the total coating amount of the aluminum coating layer and the intermetallic compound is 20 to 60 g / m ² per one side. .

Next, the reasons for limiting the numerical values of the present invention will be described. First, the reasons for limiting the aluminum-based coating layer (hereinafter abbreviated as a plating layer) and the intermetallic compound layer (hereinafter abbreviated as an alloy layer) will be described. Si: In hot-dip aluminum plating, Si is usually added to suppress the growth of the alloy layer. For this reason, Si is contained in the aluminum coating layer or the alloy layer. If the amount of Si is too small, the effect of suppressing the growth of the alloy layer described above is lost, while if the amount is too large, coarse Si primary crystals are formed and the corrosion resistance is impaired. Therefore, the preferred amount of addition is 2 to
13%.

Mg: In the present invention, Mg is added for improving corrosion resistance. When Mg is added in excess of 3%, the corrosion resistance is improved, and when it is 5 to 6%, the maximum effect is obtained. Although Mg is an element that is very easily oxidized, even if such an amount is added to the aluminum plating bath, generation of dross does not particularly increase. However, as the amount of Mg added increases, the viscosity of the bath gradually increases, and the operability deteriorates.
Also, its effect on corrosion resistance tends to saturate.
The upper limit of g is set to 15%. In the present invention, M
g / Si is limited to 1.70 or less. This is because, as described above, a ternary eutectic plating structure of Al—Mg ₂ Si—Si is exhibited in this region, and at this time, corrosion resistance is significantly improved.

[0012] In addition to these elements, the plating layer may contain Fe as an unavoidable impurity.
% Or less, and the effect on the plating structure is relatively small. Further, it is also possible to add trace elements such as Cr, Mn and Ti to the plating layer. Also, low-melting metals such as Sn, Zn, and Bi generally have an adverse effect on corrosion resistance.
These are at the impurity level.

[0013] The thickness of the alloy layer greatly affects the workability and corrosion resistance in applications where the amount of plating is small and processing is severe, such as in an automobile fuel tank. First, the alloy layer is very brittle because of an intermetallic compound, and when the thickness increases, the alloy layer is broken inside and causes plating peeling. For this reason, in order to ensure good workability, the thickness needs to be 5 μm or less. Furthermore, a large alloy layer thickness means that the plating layer is relatively thin, which greatly impairs corrosion resistance. When the thickness of the alloy layer is 3 μm or less, workability,
Deterioration of corrosion resistance is reduced. The thinner the alloy layer, the better, but it is practically difficult to reduce the thickness to 1 μm or less.
The method for reducing the alloy layer is not particularly limited, but can be controlled by controlling the temperature of the sheet entering the plating bath, the bath temperature, the line speed, and the time until solidification. The alloy layer contains Fe and Al. Since Mg is hardly contained in the alloy layer, the alloy layer is basically composed of Fe, Al, and Si.
It may contain trace amounts of Mg and the like.

Next, the steel composition of the base material will be described. There is no particular limitation on the steel composition, and any steel type has an effect of improving corrosion resistance. As steel types, in addition to IF steel, Al-k steel, Cr-containing steel, stainless steel, and high tensile steel to which Ti, Nb, B, etc. are added, Ti-added steel for heat resistance, and free-N addition for alloying suppression effect It is also possible to use steel or the like. Al-k or stainless steel for construction materials, Ti-IF or Ti-added steel for exhaust systems, Al-k or free-N-added steel for home appliances, fuel tanks It is preferable to use a B-added IF steel.

Aluminum-plated steel sheets are usually made of
It is subjected to post-treatment and used for use. In the present invention,
To silica-based treatment or organic resin coating treatment
Thus, it is possible to further improve the performance. For example,
Silica-based treatment includes chromic acid-silica-based chrome
Treatment, phosphoric acid-added white chromate-based treatment,
Car resin treatment, silica-phosphate treatment, etc. are possible.
The amount of these particles is 10 to 1000 mg in terms of silica.
/ M^Two And The purpose of these treatments is paint adhesion,
Contact resistance, corrosion resistance, etc.
10 mg / m minimum ^Two Is necessary, but the effect is saturated
And the lower the uniformity, the lower the upper limit.
1000mg / m^Two And Further to the surface of this process
You may coat with a resin.

As the organic resin, there are a clear treatment utilizing the appearance of aluminum, a color treatment containing a pigment, and a treatment for improving weldability. The resin is also an acrylic resin, a polyethylene resin, a polyester resin, a melamine resin. General-purpose treatments such as epoxy-based, epoxy-based, urethane-based, and fluorine-based are all possible. The film thickness is not particularly limited, and a normal process of about 0.5 to 20 μm can be performed.
Inhibitors that do not use chromium have recently been studied as post-treatments, but these treatments are naturally applicable. In addition to this, post-plating post-treatments such as zero spangle treatment, annealing, and temper rolling may be applied, but these are not particularly limited and can be applied.

Finally, the reason for limiting the amount of plating adhesion will be described. As described at the outset, the present invention is a material having both high weldability and high corrosion resistance, and both properties are greatly affected by the amount of adhesion. In the present invention, it is desirable that the total coating amount of the aluminum coating layer and the intermetallic compound layer (hereinafter, referred to as plating adhesion amount) is 20 to 60 g / m ² per one surface. The lower limit of the amount that can be produced by the normal hot-dip plating process is 20 g / m
It is about ² , which is the lower limit. When the amount of adhesion increases, the weldability decreases, so the upper limit is 60 g / m ^2.
And

[0018]

Next, the present invention will be described in more detail with reference to Examples. (Example 1) A hot-rolled steel sheet (sheet thickness 0.8 mm) having a steel composition as shown in Table 1 and having undergone ordinary hot-rolling and cold-rolling steps was subjected to hot-dip aluminum plating. For hot-dip aluminum plating, use a non-oxidizing furnace-reducing furnace type line, adjust the amount of plating by gas wiping after plating, then cool,
Zero spangle processing was performed. Some materials were treated with silica, and some of them were painted. In this case, the composition of the plating bath was based on Al-2% Fe,
g: 0.2 to 20%, and Si: 1 to 15%. At this time, Fe in the bath is inevitable supplied from plating equipment and strip in the bath. The bath temperature was a value obtained by adding 60 ° C. to the melting point read from the ternary phase diagram. The plating appearance was good with no plating or the like. The thickness of the alloy layer was reduced with consideration of the temperature of the invading plate, the cooling rate after plating, and the like. Table 2 shows the manufacturing conditions at this time. The plating adhesion amount was uniform on both sides, and the indication was the adhesion amount on both sides.

The performance of the hot-dip aluminized steel sheet thus manufactured was evaluated by the method shown below. Two types of chromate are used: α: chromic acid-silica type, β: resin chromate (resin / chromium = 8, using acrylic resin in dry weight ratio), and the coating is silica-based epoxy type. The resin was 1 μm on both sides. The chromate, the amount of coating film, and the film thickness are indicated per one side.

[0020]

[Table 1]

[0021]

[Table 2]

[0022] (1) plating layer, the alloy layer composition, thickness analytical methods plating layer: 3% NaOH + 1% AlCl 3 · 6H 2 O
Only the plating layer was peeled off by constant current electrolytic peeling. M
Since g was insoluble in the alkaline solution, after electrolytic stripping, it was further treated with 20% nitric acid and mixed with an electrolytic stripping solution to obtain a plating layer composition analysis solution. The analysis of each element was performed by ICP. When analyzing after chromate treatment, it is necessary to slightly polish the surface to reduce the influence of Cr in the chromate. Alloy layer thickness: The alloy layer thickness was measured from a 400 times cross-sectional microscopic photograph.

(2) Evaluation of Corrosion Resistance Corrosion Resistance to Salt Damage JIS Z 23 for a sample having a size of 70 × 150 mm
The salt spray test according to No. 71 was performed for 30 days, and the corrosion products were peeled off and the corrosion weight loss was measured. The indication of the corrosion weight loss is a value for one side of the plating. [Evaluation Criteria] ◎: corrosion weight loss 5 g / m ² or less ○: less than corrosion loss 10 g / m ² △: Corrosion weight loss 10 to 25 g / m ² ×: corrosion weight loss 25 g / m ² greater

Corrosion resistance after painting A melamine-based black paint 20
μm and baked at 140 ° C. for 20 minutes. Thereafter, a cross cut was made and subjected to a salt spray test. The appearance after 60 days was visually observed. [Evaluation Criteria] ：: No red rust generated Δ: No red rust generated except for cross cut △: Red rust generation rate 5% or less ×: Red rust generation rate 5% or more

Corrosion resistance to fuel Corrosion resistance to gasoline was evaluated. The method is as follows, using the above-mentioned hydraulic forming tester, flange width 20 mm, diameter 50 m
m, a flat-bottomed cylindrical drawing with a depth of 25 mm
The test solution was charged and covered with glass via a ring made of silicone rubber. The corrosion state after this test was visually determined. [Test conditions] Test solution: Gasoline + distilled water 10% + formic acid 200 ppm Test period: Left at 40 ° C for 3 months [Evaluation criteria] 〇: Less than 0.1% of red rust △: 0.1 to 5% of red rust or white Rusted ×: Red rust generation is more than 5% or white rust is noticeable

Corrosion resistance to exhaust system condensed water A sample having a size of 25 × 100 mm was subjected to a test in accordance with the JASO M 611-92B method specified by the Society of Automotive Engineers of Japan. The test period was four cycles. After the test, the corrosion products were peeled off and the corrosion depth was measured. [Evaluation criteria] 〇: Corrosion depth less than 0.05 mm △: Corrosion depth 0.05-0.2 mm ×: Corrosion depth more than 0.2 mm

(2) Weldability Spot welding was performed under the following welding conditions, and the number of continuous hits until the nugget diameter fell below 4 ッ ト t (t: plate thickness) was evaluated. [Welding conditions] Welding current: 10 kA Pressure: 220 kg Welding time: 12 cycles Electrode diameter: 6 mm Electrode shape: Dome type, tip 6φ-40R [Evaluation criteria] 〇: Continuous hitting point exceeds 700 △: Continuous hitting point 400 to 700 points X: less than 400 continuous hit points

(3) Workability Using a hydraulic forming tester, cup molding was performed at a drawing ratio of 2.25 using a cylindrical punch having a diameter of 50 mm. The test was performed by applying oil, and the wrinkle suppressing force was 500 kg. The evaluation of workability was based on the following index. [Evaluation criteria] 〇: No abnormality △: Cracking of plating ×: Peeling of plating

[0029]

[Table 3]

As shown in Table 3, Comparative Example No. When the addition amount of Mg is small as in No. 21, the corrosion resistance is poor. In Comparative Example No. If the amount of Si is too low as in 22, the alloy layer grows too much, causing plating peeling during processing and poor corrosion resistance. In the examples of the present invention as well, When Mg / Si exceeds 1.70 as in Nos. 18, 19 and 20, the corrosion resistance tends to be slightly inferior. Even if the amount of Mg is too large as in 4, the contribution to corrosion resistance is small, and Mg is preferably about 4 to 9% from the viewpoint of corrosion resistance. No. When the amount of Si is low as in Nos. 5 and 8, the alloy layer easily grows, and is slightly inferior in workability. No. If the amount of plating adhered is too large as in 10, the weldability tends to decrease.
When the amount of Mg, the amount of Si, and the ratio of Mg / Si are all within appropriate ranges, extremely excellent corrosion resistance, weldability, and workability are exhibited.

Example 2 The composition of the plating bath and the operating conditions were fixed, and the influence of the original plating was investigated. The plating bath composition was Al-9% Si-6% Mg-2% Fe, the coating weight was 60 g / m ^{2 on} both sides, and the post-treatment was a treatment not containing silica-phosphoric acid chromium. 70
mg / m ² . The evaluation items and methods are basically the same as those in Example 1, but the draw ratio at the time of workability evaluation was 2.1 because only a stainless steel material was used. At this time,
The composition of the plating layer was approximately 8.5% Si:
5.7%, Fe: 0.5%, Mg / Si: 0.67, alloy layer thickness: 1.8 to 2.8 μm were obtained.
Table 4 shows the steel types used, and Table 5 summarizes the evaluation results at that time. Excellent characteristics were obtained regardless of the type of steel used.

[0032]

[Table 4]

[0033]

[Table 5]

[0034]

According to the present invention, there is provided a hot-dip aluminized steel sheet having a significantly superior corrosion resistance as compared with the prior art.
The application of aluminum-plated steel sheet has been started from conventional automobile exhaust system members, heat appliances, roof walls, etc., and recently it has also started to be applied to automobile fuel tanks. It is expected to be maintenance-free, and will greatly contribute to work.

[Brief description of the drawings]

FIG. 1 is a liquidus surface of an Al—Si—Mg ternary phase diagram;
The line connecting Al-F is Al-Mg _TwoSi pseudo-binary eutectic
is there. Above this line, Al-Al₈Mg_Five-Mg_Two
Si ternary eutectic (B), lower part: Al-Si-Mg_TwoS
It has a ternary eutectic (D) structure of i.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Teruaki Izaki 1-1, Hibata-cho, Tobata-ku, Kitakyushu-shi, Fukuoka Prefecture Nippon Steel Corporation Yawata Works (72) Inventor Masao Kurosaki 20 Shintomi, Futtsu-shi, Chiba -1 Inside the Technology Development Division of Nippon Steel Corporation (72) Inventor Hisaaki Sato 1-1 Hibata-cho, Tobata-ku, Kitakyushu-shi, Fukuoka F-term in the Nippon Steel Corporation Yawata Works 4K027 AA02 AA22 AB05 AB26 AB48 AC82 AE03 AE23 4K044 AA02 AB02 BA10 BA14 BA19 BA21 BB03 BB04 BC02 CA11 CA16 CA53 CA62

Claims (6)

[Claims] 1. A steel sheet having an intermetallic compound coating layer containing Al, Fe, Si and having a thickness of 5 μm or less on the surface of a steel sheet, wherein the surface of the intermetallic compound coating layer has a weight percentage of
A hot-dip aluminized steel sheet having excellent corrosion resistance, characterized by having a coating layer of Si: 2 to 13%, Mg: more than 3% to 15%, and the balance substantially consisting of Al. 2. The galvanized steel sheet having excellent corrosion resistance according to claim 1, wherein Mg / Si is 1.70 or less. 3. The hot-dip aluminized steel sheet having excellent corrosion resistance according to claim 1, wherein the thickness of the intermetallic compound layer is 3 μm or less. 4. The surface of the Al-based coating layer is coated with 1
The hot-dip aluminized steel sheet having excellent corrosion resistance according to claim 1, further comprising a post-treatment film containing 0 to 1000 mg / m ² . 5. The hot-dip aluminized steel sheet having excellent corrosion resistance according to claim 1, wherein at least one outermost layer of the steel sheet is an organic resin layer. 6. The hot-dip galvanized steel sheet having excellent corrosion resistance according to claim 1, wherein the total coating amount of the aluminum coating layer and the intermetallic compound layer is 20 to 60 g / m ² per one side. .