JP2001081539A - Hot dip aluminum plated steel sheet excellent in high temperature corrosion resistance and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a steel sheet excellent in high temperature strength characteristics and corrosion resistance, by depositing an aluminum coating layer by a plating bath to which Si, Fe and Mg or further Ca is added respectively by specified weight % on the surface of a steel sheet having an intermetallic compound coating layer contg. Si, Fe, Cr and Mg respectively by specified weight % and contg Cr in a specified ratio. SOLUTION: The intermetallic compound coating layer contains 25 to 50% Fe, 3 to 18% Si, 0.5 to 5% Cr and <=1% Mg, and the balance substantial Al with inevitable impurity elements, and its thickness is preferably controlled to <=5 μm. Preferably, the steel sheet contains Cr by at least 3 to 30%. It is preferable that a coating layer having a composition containing 2 to 12 % Si, <=1% Fe and 0.1 to 15% Mg or 0.01 to 1.0% Ca as well, and the balance substantial Al which inevitable impurity elements, and in which the total content of Zn and Sn in the impurties is <=1% formed on the surface.

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 aluminum-plated layers (hereinafter referred to as plated layers) made of aluminum.
And a steel plate having a steel plate to be plated and a layer made of an intermetallic compound that is a reaction product of aluminum (hereinafter referred to as an alloy layer),
It is known that it 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. On the other hand, stainless steel sheets are also excellent in heat resistance and corrosion resistance, and are used for similar applications.

However, in recent years, for example, the demand for high durability of automobile exhaust system materials has increased, and the need for products with further improved heat resistance and corrosion resistance has been expanding. Therefore, there have been various inventions in which corrosion resistance and heat resistance are further improved by using stainless steel as an aluminum plating base sheet. The applicant of the present application also
-What used Mo added steel (Japanese Patent Publication No. 6-60380). On the other hand, there are many examples in which elements are added to an aluminum plating bath in order to improve corrosion resistance. For example, JP-B-63-23264 discloses an example of a steel sheet having a plating layer containing Si: 3% or less and Mn: 0.5 to 4%, and JP-B-6-11906 discloses a steel sheet.
A method for producing a plated steel sheet in which r: 0.01 to 2% is added to a plating bath is disclosed.

[0004]

However, the above-described invention has the following problems. For example,
Corrosion resistance is certainly improved by using Cr steel as the plating base sheet, but this is mainly due to the improvement in corrosion resistance due to the base sheet, and there is not much difference from ordinary aluminum plated steel sheet from the viewpoint of the corrosion resistance of the plating itself. Was. 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 hot-dip aluminum-coated steel sheet having high corrosion resistance and high heat resistance 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, M
Most of the g is concentrated in the plating layer and S in the plating bath.
combines with i to form Mg ₂ Si. This Mg ₂ Si is an intermetallic compound having a certain degree of solubility in an aqueous solution. When the plated steel sheet is exposed to a corrosive environment, Mg ₂ Si is dissolved and Mg 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, AlFeSiMg exists in the alloy layer. When an alloy layer containing a small amount of Mg is formed as described above, the alloy layer is formed of Mg or Fe having a small atomic radius.
It becomes a structure that has penetrated around the 1 atom. Fe at high temperature
Even if Al and Al attempt to interdiffuse, diffusion (movement) is suppressed because Mg exists around these atoms. Therefore, it took time until Fe diffused to the outermost surface of the plated steel sheet, and it was clarified that alloying was suppressed. Further, the Cr in the steel sheet is also concentrated in the alloy layer, whereby the potential of the alloy layer is increased, so that the corrosion resistance of the alloy layer is improved, and a higher temperature corrosion resistance than before can be obtained by the synergistic effect of these. 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 oxidizability of Mg.
Since the Mg oxide film 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. Thus, the above-mentioned plated steel sheet contains a specific amount of C, Ti, Mn, Cr, Al, N as a plating base plate, and after adding a predetermined amount of a specific amount of Mg, Ca, Fe, Si into a plating bath. It can be manufactured by limiting the sum of the impurities Sn and Zn to a specific amount. Hereinafter, the present invention will be specifically described.

According to the present invention, C:
0.02% or less, Mn: 0.1-1.5%, Si: 0.
1% or less, Ti: 0.1 to 0.5%, Cr: 3 to 30
%, N: 0.004% or less, Al: 0.01 to 0.08
%, Ni: 0.1 to 10%, Mo: 0.1 to 2
%, Cu: 0.1 to 1% of one or more kinds, and the balance is substantially Fe and an unavoidable impurity element.
%, Si: 3 to 18%, and Mg: 1% or less, the balance being substantially composed of Al and unavoidable impurity elements, and having an intermetallic compound coating layer having a thickness of 5 μm or less. Further, on the surface of the intermetallic compound layer, Si: 2 to 12% by weight, Fe: 1% or less, Mg: 0.1 to 1
0% or further Ca: 0.01 to 1.0%, with the balance substantially consisting of Al and unavoidable impurity elements, and the total content of Zn and Sn in the impurities is 1% or less. A hot-dip aluminized steel sheet having a coating layer and excellent in high-temperature high-strength properties and corrosion resistance is provided. As a method for producing such a hot-dip aluminized steel sheet, S
i: 3 to 12%, Fe: 0.5 to 2.5%, Mg: 0.
1 to 1.0%, or Ca: 0.01 to 1.
0% balance is substantially composed of Al, and Zn in impurities
And a method for plating in a plating bath in which the sum of Sn and Sn is 1% or less.

Hereinafter, the reasons for limitation of the present invention will be described.
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 small, and in the present invention, it is desirably 0.02% or less. Ti: Ti is an element that reacts with C, N in steel or O entering from 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%). It is desirable that the lower limit of the content be 0.1%. On the other hand, the upper limit is set to 0.
It is desirable to set it to 5%.

Mn: Mn is an element that hardens a steel sheet. If its content exceeds 1.5%, workability is impaired, and therefore the upper limit is preferably this amount. In addition, it is difficult to reduce Mn to 0.1% or less by a normal steel making method, and thus it is desirable to set this amount as the lower limit. Cr: Cr is a very important element that contributes to improvement in heat resistance and corrosion resistance. As described above, Al-Si-M
When g-based plating is performed, the corrosion resistance is dramatically improved by adding 3% or more, but if it exceeds 30%, the effect is saturated and the workability is reduced. Therefore, the lower limit was set to 3% and the upper limit was set to 30%.

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 the upper limit is preferably 2.0%. More preferably 0.0
8%. 0.08% to prevent non-plating
In the case of the above addition, it is desirable to apply a pre-plating of Ni, Cu, Cr or the like to the surface of the steel sheet. 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 a lower N is desirable. Therefore, 0.004% or less is desirable.

In the present invention, one or more of the following elements can be added. By coexisting with an element such as Ni: Cr, it is effective in suppressing local corrosion progress. However, 0.1%
% Is required. If the content exceeds 10%, the effect is saturated, so that the amount of addition is limited to the upper limit. Mo: Mo has a great effect on the occurrence of local corrosion and suppression of progress of steel. The effect is remarkable at 0.1% or more, but 2%
Above, it saturates.

Cu: The corrosion resistance is improved by coexisting Cu in a component system based on Cr and Mo. If it is 0.1% or less, the coexistence effect is not sufficient, and if it exceeds 1%, the corrosion resistance is saturated and the workability is deteriorated, which is not desirable. Nb: Nb has the same effect as Ti and is an element that contributes to heat resistance and corrosion resistance. Effective at 0.3% or more,
Since the effect is saturated at 0.5%, addition in this range is desirable.

Next, the composition of the plating layer and the bath composition of the production 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 about 12%.

Fe: Fe is eluted from the original plate for plating or the equipment in the 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 in the bath is set to 2.5% because corrosion resistance is hindered 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 in 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 3, the alloy layer becomes thicker due to an increase in the bath temperature, and the workability deteriorates. Therefore, the upper limit is set to 1.0%. 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 limited to 1% or less both in the plating layer and in the bath.

Next, the reasons for limiting the composition of the 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 presence of a very small amount improves the heat resistance. 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. Cr: the lower limit of 0.5% for the Cr content in steel in the range of the present invention;
The upper limit is 5%.

Further, the plated steel sheet of the present invention can be obtained by either the Sendzimer method or the pre-plating method. However, in the case of the pre-plating method, one of Ni, Cu, and Cr is used for 10 to 1000 m.
g / m ² . Pre-plating adhesion amount is 10mg /
If it is less than m ^2, it will dissolve in the plating bath and the wettability of the plating will be significantly reduced. In addition, 1000 mg /
If it is more than m ^2, it is uneconomical, so the lower limit of the amount of pre-plated coating is 10 mg / m ² and the upper limit is 1000 mg / m ² .
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, plating was performed by adding Si, Mg, and Ca as plating bath components. Further, pre-plating of Ni, Cu, and Cr was performed, and hot-dip aluminum plating was performed in a line of a type in which a steel sheet was heated in a non-oxidizing furnace. 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. The aluminum-plated steel sheet thus obtained was evaluated. The evaluation method is shown below. Tables 2, 3, and 4 summarize the manufacturing conditions and evaluation results.

[0023]

[Table 1]

[0024]

[Table 2]

[0025]

[Table 3]

[0026]

[Table 4]

(1) Method for analyzing composition of plating layer and alloy layer Plating layer: 50% 50 × 50% sample
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 ^TwoNi layer or alloy layer
The electrolysis was stopped when the potential of. Adhered to steel plate
Gently wipe the residue with absorbent cotton and collect the analysis solution together.
Was. Next, this analysis solution is filtered, and the undissolved residue is 10% hydrochloric acid.
Dissolved in water. Quantitative analysis of combined filtrate and lysate
By ICP (inductively coupled plasma) emission spectroscopy
Was. When the steel sheet is subjected to the chemical conversion treatment, C
Since there may be errors in r, Si, etc., lightly sand the surface.
It is good to peel off after polishing. 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 samples up to 70 × 150 mm in size
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 5 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.

[0029]

[Table 5]

(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 Spotted plating peeled 4 Thin plating peeled 5 All plating peeled

(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.

[0032]

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.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Teruaki Izaki 1-1 Niwahata-machi, Tobata-ku, Kitakyushu-shi, Fukuoka Prefecture Nippon Steel Corporation Yawata Works (72) Inventor Masao Kurosaki 20 Shintomi, Futtsu-shi, Chiba -1 Nippon Steel Corporation Technology Development Division (72) Inventor Hisaaki Sato 1-1 Hibata-cho, Tobata-ku, Kitakyushu-city, Fukuoka Prefecture F-term in Nippon Steel Corporation Yawata Works (reference) 4K027 AA02 AA23 AB05 AB08 AB26 AB32 AB48 AC15 AE03 AE21 AE23 AE27 4K044 AA02 AA03 AB02 BA01 BA02 BA06 BA10 BA19 BB03 BB04 BC02 BC11 CA04 CA11 CA15 CA18

Claims (5)

[Claims] 1. A steel sheet having a content of at least 3 to 30% by weight of Cr in a steel component and having an average composition of 25 to 50% by weight of Fe and 3 to 18% of Si by weight%. C
r: 0.5 to 5% in addition to Mg: 1% or less, the balance being substantially composed of Al and unavoidable impurity elements, and having an intermetallic compound coating layer having a thickness of 5 µm or less. , On the surface of the intermetallic compound coating layer,
i: 2 to 12%, Fe: 1% or less, Mg: 0.1 to 15
% Or further Ca: 0.01 to 1.0%, the balance being substantially composed of Al and unavoidable impurity elements, and having a total Zn and Sn content of 1% or less in the impurities. Hot-dip aluminized steel sheet with excellent corrosion resistance characterized by having layers. 2. The steel component has a content of at least 3 to 30% by weight of Cr and a content of Al of 0.0% by weight.
The hot-dip aluminized steel sheet having excellent high-temperature corrosion resistance according to claim 1, wherein a steel sheet of 1 to 2.0% is used. 3. C: 0.002% or less by weight in steel
Mn: 0.1-1.5%, Si: 0.1% or less, Ti:
0.1-0.5%, Cr: 4-25%, N: 0.004
% Or less, Al: 0.01 to 0.08%, and Ni:
0.1 to 10%, Mo: 0.1 to 2%, Cu: 0.1 to
A steel sheet containing one or more of 1% and Nb: 0.3 to 0.5%, the balance being substantially Fe and unavoidable impurity elements. 2. A hot-dip aluminized steel sheet having excellent high-temperature corrosion resistance according to 2. 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. Manufacturing method of excellent hot-dip aluminized steel sheet. 5. The steel sheet according to claim 1, wherein
One of Ni, Cu, and Cr is 10 to 1000 mg / m ^2.
After being attached, Si: 3 to 12%, Fe: 0.5 to 2.
5%, Mg: 0.1 to 10%, or further Ca:
Hot-dip aluminum plating in a plating bath of 0.01 to 1.0%, with the balance being substantially composed of Al and unavoidable impurity elements, and having a total Zn and Sn content of 1% or less in the impurities. Manufacturing method of hot-dip aluminized steel sheet with excellent high-temperature corrosion resistance.