JP2001131725A - Hot dip aluminized steel sheet excellent in heat resistance and corrosion resistance and its producing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot dip aluminized steel sheet excellent in the heat resistance and the corrosion resistance used for mainly base stock for gas exhausting system in a motor car, building material, home electric appliances, various heating devices, etc., and the producing method thereof. SOLUTION: On the steel sheet surface where the content of Ti, Mn, etc. in the components of the steel, is regulated, an intermetallic compound coated layer containing minute content of Mg is produced, and thereby, higher heat resistance than that of the conventional steel sheet is displayed, and further, on the surface of this intermetallic compound coated layer, an aluminum coated layer adding Si, Fe, Mg or further, Ca is provided to obtain the hot dip aluminized steel sheet excellent in the heat resistance and the corrosion resistance.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-dip galvanized steel sheet excellent in heat resistance and corrosion resistance mainly used for automobile exhaust system materials, building materials, home appliances, various heat appliances and the like, and a method for producing the same.

[0002]

2. Description of the Related Art Hot-dip aluminized steel sheets are mainly made of aluminum (hereinafter referred to as "plated layers").
It is known that a steel sheet having a steel sheet to be plated and a layer made of an intermetallic compound which is a reaction product of aluminum (hereinafter, referred to as an alloy layer) has excellent heat resistance and corrosion resistance. Taking advantage of these features, it is widely used in automobile exhaust system materials, home appliances mainly including heat appliances, and building materials such as roofs and walls. Stainless steel sheets are similarly excellent in heat resistance and corrosion resistance, but hot-dip aluminized steel sheets are less expensive than stainless steel sheets, and their use is expanding in recent years. However, there is a growing need for products with further improved heat resistance and corrosion resistance, and there have been many inventions in which various elements are added to aluminum plated original sheets. For example, the present applicants have disclosed a steel sheet using Ti-added steel as a plating base plate in order to increase heat resistance (Japanese Patent Publication No. 5-21978).

[0003] On the other hand, there are many examples in which elements are added to an aluminum plating bath to improve corrosion resistance. For example, Japanese Patent Publication No. 63-23264 discloses that Si: 3% or less, M
n: 0.5 to 4%, an example of a steel sheet having a plating layer is disclosed in Japanese Patent Publication No. 6-11906.
There is a method for producing a plated steel sheet in which 01 to 2% is added to a plating bath.

[0004]

However, the above-described invention has the following problems. For example,
Although the effect of improving heat resistance by Ti is effective for plating defects such as non-plating, it is not different from ordinary aluminum-plated steel sheets from the viewpoint of heat resistance of plating itself. On the other hand, even in the invention in which an element is added to a plating bath, the amount of the element added to the plating bath needs to be limited to a certain amount in order not to raise the temperature of the plating bath, and the corrosion resistance is sufficiently high. There was a problem that it was difficult to add the amount of addition.

[0005]

SUMMARY OF THE INVENTION The present invention provides a high heat-resistant and high corrosion-resistant hot-dip aluminum-coated steel sheet which overcomes the above-mentioned drawbacks. The present inventors have repeated various experiments on the properties of the plating layer and the alloy layer which affect various properties of the hot-dip aluminized steel sheet, and have obtained the following knowledge. In other words, when Mg is added to the plating bath,
Almost all of them are concentrated in the plating layer and Si in the plating bath
And form Mg ₂ Si. This Mg ₂ Si is an intermetallic compound having a certain solubility in an aqueous solution. When this plated steel sheet is exposed to a corrosive environment, Mg ₂ Si dissolves and
g ions are released. The released Mg ions form an oxide film on the plating layer and on the exposed base iron to suppress the reduction reaction of oxygen, thereby suppressing the dissolution of the plating metal and the base iron (so-called cathodic protection).

[0006] Further, a small amount of Mg is contained in the alloy layer (1% or less when Mg is 10% or less in the plating bath). At this time, the alloy layer contains AlFeSi and AlFeSi.
It exists in the form of SiMg. When an alloy layer containing a small amount of Mg is formed as described above, the reason is unknown at present, but Fe and Al are liable to interdiffusion at a high temperature. Therefore, it has been clarified that the oxidation of Fe is suppressed. Further, since Ti and Al in the steel sheet at high temperatures easily interdiffuse due to Ti in the steel sheet, abnormal oxidation from plating defects is also suppressed. It has been found that by these synergistic effects, heat resistance and corrosion resistance higher than before can be obtained. Also,
Both Sn and Zn in the plating bath are elements that significantly impair the corrosion resistance of the aluminized steel sheet. Therefore, the sum of these impurity elements must be limited to a certain amount or less.

Further, when producing a plated steel sheet in a bath containing Mg, if wiping with nitrogen gas is used to control the basis weight, wrinkles are generated on the surface, resulting in poor appearance. This is due to the high oxidizing property of Mg.
Since the oxide film of g is porous, it grows rapidly in an air environment. It is considered that wrinkles are generated due to the surface flow during wiping when the oxide film is thick as described above. Therefore, in order to suppress the generation of wrinkles, that is, to suppress the growth of the oxide film, it is necessary to provide an N ₂ seal box or the like and control the oxygen concentration from the bath surface to the vicinity of the wiping nozzle to 0.1% or less. It is. In addition, when it is difficult to install such equipment, Mg, which has higher oxidizing property than Mg and forms a dense oxide film, is also added to the bath to suppress the growth of the Mg oxide film and obtain a good appearance. can get.

[0008] The effect of improving the heat resistance due to the inclusion of Mg in the alloy layer and the effect of improving the corrosion resistance due to Mg ₂ Si in the plating layer are attributable to the plating itself. However, by using Ti-added steel as a plating base plate for abnormal plating baths, abnormal oxidation from plating defects can be suppressed, and extremely excellent heat resistance can be obtained overall. That is, when a Ti-added steel is used as a plating base plate and a Mg-added bath is used as a plating bath, a hot-dip aluminized steel sheet is obtained in which not only the heat resistance of the plating layer itself is improved, but also the heat resistance of the unplated portion is obtained. It becomes possible. Thus, the above-mentioned plated steel sheet has a specific amount of C, Ti, Mn, Cr,
Al, N, and a specific amount of Mg, Ca,
After adding predetermined amounts of Fe and Si, impurities Sn and Zn are added.
Can be produced by limiting the sum of Hereinafter, the present invention will be described specifically.

According to the present invention, the components in the steel are expressed by weight%,
C: 0.02% or less, Mn: 0.6 to 2.0%, Ti:
0.1-0.5%, Cr: 1% or less, N: 0.004%
Hereinafter, in addition to Al: 0.01 to 0.08%,
i: 1.5% or less, P: 0.1% or less, B: 0.000
The steel sheet contains 3% or less of one or more kinds, and the balance is substantially Fe and an unavoidable impurity element.
In addition to 8%, Mg contains 1% or less, and the balance substantially consists of Al and unavoidable impurity elements, and has a thickness of 5 μm.
m or less, and the surface of the intermetallic compound coating layer further includes Si:
%, Fe: 1% or less, Mg: 0.1 to 15%, or Ca: 0.01 to 1.0%, and the balance substantially consists of Al and unavoidable impurity elements, and Zn in the impurities. A hot-dip aluminized steel sheet having excellent heat resistance and corrosion resistance characterized by having a coating layer having a total Sn content of 1% or less is provided. As a method for producing such a hot-dip aluminized steel sheet, Si: 3 to 12%, Fe:
0.5 to 2.5%, high Mg: 0.1 to 10%, or Ca: 0.01 to 1.0%, the balance is substantially made of Al, and Zn and Sn in impurities are contained. A manufacturing method for plating in a plating bath having a sum of 1% or less is provided.

The reasons for limiting the present invention will be described below. First, the composition of the plating base sheet will be described. C: In general, as the C content increases, the amount of carbides precipitated at grain boundaries increases.
Promotes intergranular corrosion of steel. In this sense, it is desirable that C is as small as possible, and is limited to 0.02% or less in the present invention. Ti: Ti is an element that reacts with C, N in the steel or O entering from the outside to improve the heat resistance of the aluminum-plated steel sheet. For this effect, the amount of Ti needs to be about 20 times the total amount of C and N, and it is necessary to correspond to an industrially reducible value of C and N (C + N: 0.003 to 0.004%). The lower limit was set to 0.01% as the content. More preferably, it is 0.1%. On the other hand, the upper limit is set to 0.5% because the effect of Ti on heat resistance is saturated.

Mn: Mn is an element that hardens a steel sheet. If its content exceeds 2.0%, workability is impaired, which is not desirable. Mn is 0.1 in a normal steelmaking method.
Making it less than 1% leads to cost increase, so 0.1%
The above is desirable. More preferably, it is 0.6 to 2.0%. Cr: Cr is also an element that contributes to improvement in heat resistance. However, the effect is not as great as that of Ti. On the other hand, if the Cr content increases, the workability of the steel sheet is impaired, so the upper limit is desirably set to 1%.

Al: Al is added to adjust oxygen in steel in the smelting process of molten steel. However, if it is too much, it inhibits aluminum plating and causes non-plating, and the workability of steel sheet. Lower limit of 0.01
%, And an upper limit of 0.08% is desirable. N: N is an element that inhibits the workability of steel and is an element that increases the required amount of Ti by combining with Ti, so that 0.004% or less is desirable. Si: Si is an element that impairs the plating property during hot-dip plating when its addition amount is large, so it is desirable to suppress it to 1.5% or less. P: is an element that hardens the steel sheet, and its content is 0.1.
If it exceeds 1%, the workability is impaired, so that the upper limit is desirably set to this amount. B: B is desirably added in an amount of 0.003% or less in order to secure weld airtightness after welding, secondary workability, and the like.

Next, the composition and production of the coating layer (plating layer)
The bath composition of the method will be described. Si: In addition to aluminum plating layer for hot-dip aluminum-coated steel sheet
And a very hard and brittle alloy layer is formed,
Inhibits adhesion. Usually, to reduce this effect,
About 10% of Si is added to the bath to reduce the thickness of the alloy layer.
Control. In the present invention, the same purpose is simultaneously achieved.
Mg which is an intermetallic compound with Mg _TwoCrystallize Si
Si is added for the purpose. For these purposes
The Si content in the plating bath must be at least 3%.
The amount of Si in the plating layer is 2% or more. On the other hand, added Si
Excessive formation of coarse primary crystal Si in the plating layer causes corrosion resistance
The upper limit is set to 12% to adversely affect the properties. At this time
The amount of Si in the plating layer is also 12%.

Fe: Fe is eluted from an original plating plate or equipment in a bath, and is not particularly actively added in the present invention. Usually, about 0.3 to 0.8% is contained in the plating layer. Fe is preferable to be small because it adversely affects the corrosion resistance, and the upper limit in the plating layer is set to 1%. Originally, the smaller the better, the better. However, as described above, it is difficult to completely remove the element by inevitably mixed elements. In addition, it is an unavoidable element even in the bath, and it is almost impossible to remove it. If the pressure is forcibly reduced, the equipment in the bath tends to be melted, so the lower limit in the bath is set to 0.5%. The upper limit value in the bath is set to 25% because corrosion resistance is impaired or appearance stains due to dross appear.

Mg: This element is particularly important in the present invention. An element that is present as Mg ₂ Si in the plating layer and has a remarkable effect on corrosion resistance, heat resistance, etc. by containing a trace amount in the alloy layer. is necessary. When plating is performed in this plating bath, the plating layer contains about 0.1%, so this concentration is defined as the lower limit of the plating layer. On the other hand, the more Mg is added, the more the corrosion resistance is improved.
%, The workability deteriorates, so the upper limit of Mg in the plating layer is set to 15%. In the plating bath, Mg is 10%.
%, The plating bath temperature rises, the alloy layer becomes thick, and the workability deteriorates remarkably. Therefore, the lower limit in the plating bath is set to 0.1%, and the upper limit is set to 10%.

Ca: This element is effective for suppressing the occurrence of poor appearance when Mg is added as described above. This effect is effective from 0.01%. However, 1.0%
If the addition exceeds the limit, the alloy layer becomes thicker due to an increase in the bath temperature and the workability deteriorates.
And Zn, Sn: These are all elements that greatly inhibit the corrosion resistance of Al and accelerate the generation of white rust. Therefore, the sum of these elements is reduced to 1% in the plating layer and in the bath.
Limited to the following.

Next, the reasons for limiting the composition of the intermetallic compound coating layer (alloy layer) will be described. Si: As described above, 3 to 12% of Si is added to the plating bath for the purpose of suppressing the growth of the alloy layer. At this time, the Si concentration in the alloy layer is 3 to 18%. Therefore, S in the alloy layer
i is limited within this range. Fe: The alloy layer is mainly generated by the reaction between Al in the plating bath and Fe in the original plate. At this time, the Fe concentration in the alloy layer becomes 25 to 50%. Therefore, Fe in the alloy layer is limited to this range.

Mg: Almost no Mg is contained in the alloy layer, but the heat resistance is improved by the presence of a very small amount of Mg. When 0.1 to 10% is added to the plating bath for improving corrosion resistance, the content is 1% or less in the alloy layer. Therefore, Mg in the alloy layer
Is within this range. Regarding the thickness of the alloy layer, if the thickness is too large, the adhesion of the plating will be impaired. There is no particular lower limit for the alloy layer, which is preferably thinner to inhibit plating adhesion. Under normal operating conditions, the thickness of the alloy layer is 2-3 μm. The product of the present invention can be subjected to a zero spangle process in order to make the appearance beautiful. The use of a liquid containing an inhibitor such as chromic acid as a main component for the purpose of initial rust prevention does not impair the gist of the present invention.

Next, the present invention will be described in more detail with reference to examples.

[Example] Sheet thickness 0.8 mm after normal hot rolling and cold rolling processes
Using steels of several types of steel components as shown in Table 1 as plating base plates, hot-dip aluminum plating was performed in a non-oxidizing furnace-reducing furnace type line. After plating, the coating weight was adjusted to about 120 g / m ² on both sides by a gas wiping method, and the film was cooled and wound up. At this time, Si, Mg, Fe, C
a was added to perform plating, and plating with a good appearance was possible. The aluminum-plated steel sheet thus obtained was evaluated. The evaluation method is shown below. Tables 2 and 3 summarize the manufacturing conditions and evaluation results.

[0020]

[Table 1]

[0021]

[Table 2]

[0022]

[Table 3]

(1) Method of analyzing composition of plating layer and alloy layer Plating layer: Both sides of a sample of dimensions 50 × 50 were 5% NaO
Current density 10 mA / cm in H solution (% by weight)^TwoOpposite
Was electrolytically peeled off as stainless steel. Potential rises suddenly
At the end, the current density is reduced by half in order,
1mA / cm ^TwoTo show the potential of the alloy layer
By the way, electrolysis was stopped. Clean the residue adhering to the steel sheet with absorbent cotton
And the analysis solution was collected together. Then this analysis
The solution was filtered, and the undissolved residue was dissolved in 10% hydrochloric acid.
The combined quantitative analysis of the filtrate and the lysate was performed by ICP (induction).
Combined plasma) emission spectroscopy. In addition, steel sheet
When the pretreatment is performed, there is an error in Cr, Si, etc.
It is recommended that the surface be lightly polished with paper and then peeled off.
No. Alloy layer: After the above electrolytic stripping, strip the alloy layer with chemical soda
Separate to obtain an alloy layer composition analysis solution and perform quantitative analysis of each element
Was.

(2) Corrosion resistance test: The following three tests were performed. Salt spray test JISZ2371 for a sample of size 70 × 150 mm
Was carried out for 30 days, and the weight loss due to corrosion was measured. Automotive Exhaust System Simulated Condensation Water Immersion Test A sample having a size of 70 × 150 mm was immersed in the solution shown in Table 4 for 30 minutes and dried at 70 ° C. for 30 minutes. This cycle is 1000
The cycle was carried out and the corrosion weight loss after the test was measured.

[0025]

[Table 4]

(3) Plating adhesion: The following test was conducted. Cup drawing test Blank diameter: 50 mm Drawing depth: 10 mm Die shoulder radius: 2 mm Punch diameter: 33 mm Drawing was performed under the above conditions, and the state of peeling of the plating on the side surface was observed and rated. The rating criteria are shown below. Rating criteria 1 No abnormalities 2 Cracks occurred in plating layer 3 Spot-like plating peeled 4 Foil-like plating peeling 5 Overall plating peeling

(4) Heat Resistance Test A sample having a size of 100 × 100 mm was kept at 800 ° C. for 48 hours, and the cooling process was defined as one cycle. This cycle was repeated five times, and the oxidation increase after the test was measured. (5) Press formability A mold having a diameter of 80 mm was formed to a depth of 40 mm, and the formability was evaluated by the occurrence of cracks. (6) Weldability I groove butt TIG welding was performed, and an Erichsen test was performed on the welded portion to evaluate ductility. Welding conditions: current 95A, voltage 11V, welding speed 300m
m / min Arc length 1.5 m (7) High-temperature strength A high-temperature tensile test was performed at a test temperature of 600 ° C. to evaluate tensile strength and elongation.

[0028]

The aluminum-coated steel sheet obtained according to the present invention has extremely excellent heat resistance and high-temperature strength. Further, even in a severe environment such as salt spray or a vehicle exhaust system condensed water immersion environment, the plate thickness phenomenon due to corrosion is small. Therefore, this material is considered to be extremely promising as an automotive exhaust system material that can replace stainless steel.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C22C 38/38 C22C 38/38 (72) Inventor Teruaki Izaki 1-1 Tobata-cho, Tobata-ku, Kitakyushu-shi, Fukuoka New 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 Tobata-ku, Tobata-ku, Kitakyushu-shi, Fukuoka Town 1-1 Shin-Nippon Steel Corporation Yawata Works F-term (reference) 4K027 AA05 AA23 AB05 AB08 AB28 AB48 AC54

Claims (4)

[Claims] 1. A steel sheet having a content of at least 0.01 to 0.5% by weight of Ti in a steel component in a content of 0.01 to 0.5%, and having an average composition of 25 to 50% by weight of Fe, : 3 ~
18%, Mg: 1% or less, the balance being substantially Al
And an intermetallic compound coating layer comprising an unavoidable impurity element and having a thickness of 5 μm or less, and further, on the surface of the intermetallic compound coating layer, Si: 2 to 12% by weight%,
Fe: 1% or less, Mg: 0.1 to 15%, or Ca: 0.01 to 1.0%, and the balance substantially consists of Al and inevitable impurity elements.
A hot-dip galvanized steel sheet having excellent heat resistance and corrosion resistance, comprising a coating layer having a total Sn content of 1% or less. 2. A steel sheet in which at least the content of Ti in the steel components is 0.1 to 0.5% by weight and the content of Mn is 0.1 to 2.0% by weight. 2. A hot-dip aluminized steel sheet having excellent heat resistance and corrosion resistance according to claim 1. 3. The steel composition contains, by weight, C: 0.02% or less, Mn: 0.6 to 2.0%, and Ti: 0.1 to 0.5.
%, Cr: 1% or less, N: 0.004% or less, Al:
In addition to 0.01 to 0.08%, one or more of Si: 1.5% or less, P: 0.1% or less, B: 0.003% or less, and the balance is substantially 3. A hot-dip aluminized steel sheet having excellent heat resistance and corrosion resistance according to claim 1, wherein the steel sheet is Fe and an unavoidable impurity element. 4. The steel sheet according to claim 1, wherein
i: 3 to 12%, Fe: 0.5 to 2.5%, Mg: 0.
1 to 10%, or further Ca: 0.01 to 1.0
%, With the balance substantially consisting of Al and unavoidable impurity elements, and hot-dip aluminum plating in a plating bath in which the total content of Zn and Sn in the impurities is 1% or less. Of hot-dip aluminized steel sheet with excellent corrosion resistance.