JP2852718B2 - Hot-dip aluminized steel sheet with excellent corrosion resistance - Google Patents

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 which is hardly perforated by corrosion even when exposed to a severe corrosive environment such as a combustion exhaust system of an automobile or the like, and has extremely excellent corrosion resistance. is there.

[0002]

2. Description of the Related Art Hot-dip aluminized steel sheet has corrosion resistance,
Because of its excellent high-temperature oxidation resistance, heat resistance, and the like, it is used in many fields such as exhaust system materials for automobiles, household heat appliances, and industrial furnace materials. Generally, aluminum-killed steel, Ti-added ultra-low C steel, or the like is used for the steel sheet to be plated (hereinafter referred to as a plating base sheet). In a field where corrosion resistance is particularly required, for example, Japanese Patent Publication No. 2-50979 discloses "Cr. 0.1-5%
Hot-dip aluminized steel sheet with improved high-temperature corrosion resistance "and JP-B-2-61542" Cr
A hot-dip aluminum-plated steel sheet having 12% added to improve corrosion resistance in a corrosive liquid "has been developed.

[0003] The idea of improving the corrosion resistance is to use Cr steel or Cr-based stainless steel as the base plate of the hot-dip aluminum-plated steel plate to improve the corrosion resistance of the base plate and to reduce the Cr in the steel diffused into the aluminum plating layer. The effect also improves the corrosion resistance of the plating layer, and the synergistic effect significantly improves the overall corrosion resistance.

[0004] Such a Cr-containing hot-dip galvanized steel sheet having excellent corrosion resistance is mainly used for an exhaust gas muffler of an engine of an automobile or a motorcycle or the like and an exhaust pipe thereof (hereinafter, collectively referred to as a muffler). . However, after the above muffler is actually mounted on a car or the like, its outer surface is exposed to various anti-freezing salts scattered on the road surface, and its inner surface is exposed to high temperature and high temperature due to condensation and corrosive liquid generated from engine exhaust gas. There is a problem that the service life is short because of exposure under wet conditions.

[0005] That is, in the outer surface of Cl various salts were sprinkled on the road surface generated lysed by ice and snow ^- progresses corroded by corrosive components such as ions composed mainly of. On the inner surface, the high-temperature exhaust gas generated by the engine reaches the muffler via the exhaust manifold and the center pipe, but when the engine is started, the condensed water in which the moisture in the exhaust gas condenses on the inner surface of the muffler because the temperature of the muffler is low. Stay. CO ₃ ^2- , NH
_It contains corrosive components such as ₄ ⁺ , SO ₄ ²⁻ , Cl ⁻ ions, and organic acids, and causes corrosion. Further, due to changes in fuels used in recent years (for example, change from gasoline to fuel containing alcohol), use of catalytic converters accompanying emission control, and the like, the conditions tend to be more severe.

[0006] As described above, there is a demand for maintenance-free automobile exhaust system materials, which are exposed to severe corrosive environments on both the inner and outer surfaces, from the viewpoint of comprehensive rust prevention. There is a demand for an exhaust system material having the following.

[0007]

SUMMARY OF THE INVENTION The present invention has been made in view of these circumstances, and it is an object of the present invention to provide a hot-dip aluminized steel sheet having excellent corrosion resistance under severe corrosive environments such as combustion exhaust systems of automobiles and the like. It is intended for.

[0008]

SUMMARY OF THE INVENTION In view of the above-mentioned problems,
In order to further improve the corrosion resistance, the Cr
Although it is conceivable to increase the amount of addition, it is difficult to apply hot-dip aluminum plating to a plating original plate containing a large amount of Cr, because it deteriorates plating performance. This is considered to be because Cr, which is an element that is easily oxidized, is selectively oxidized in a non-oxidizing furnace or a reducing furnace to form an oxide film on the steel sheet surface that is difficult to react with Al. Further, Cr steel and stainless steel-based materials are expensive materials due to high alloy costs such as Cr, pickling and cold rolling are difficult, and productivity is low. Increases with increasing. For this reason, there is a limit to the method for increasing the amount of Cr in steel.

In view of such circumstances, the present inventors have studied measures to improve the corrosion resistance of a hot-dip aluminized steel sheet using a Cr steel material in an environment where automobile exhaust system parts are exposed. As a result of disassembling a muffler using a base steel as a plating base plate and observing the corrosion behavior on the inner and outer surfaces of the muffler made of a galvanized steel sheet in detail, it was found that the corrosion behavior was as follows.

An aluminum-plated steel sheet is usually made of a base plate and a plating layer made of Al-Si (hereinafter referred to as a plating layer),
And an Al-Si-Fe intermetallic compound (hereinafter referred to as an alloy layer) generated at the interface. In an exhaust system environment, corrosion of a hot-dip aluminized steel sheet made of steel containing almost no Cr starts from Al along the eutectic Si on the surface of the coating layer, and the progress of corrosion to the surroundings and the formation of corrosion products Deposition continues. Then, when the corrosion reaches the alloy layer, the corrosion rate sharply decreases. This is because the alloy layer is hardly corroded, but this alloy layer is made of an intermetallic compound and is hard and brittle. I do. Further, since the alloy layer has almost no sacrificial anticorrosion ability with respect to the original plate, corrosive components penetrate the cracks and reach the original plate, and the corrosion of the original plate proceeds from that portion.

On the other hand, when the plating base plate is made of a Cr-containing material, the plating base plate becomes electrochemically noble with respect to the plating layer and the alloy layer, and even if the alloy layer has cracks, Since the layer and the alloy layer sacrifice corrosion of the original plate, corrosion of the original plate is slight. Further, since the original plating itself has improved corrosion resistance, corrosion hardly progresses.

[0012] Such a form of corrosion was also confirmed in a follow-up test in a laboratory, and can be said to be a general behavior of a Cr-containing hot-dip aluminum-coated steel sheet in an exhaust system environment. Therefore, the present inventors have conducted various studies on factors affecting corrosion resistance in taking the above-mentioned corrosion form, and found that increasing the thickness of the alloy layer is extremely effective in improving corrosion resistance. is there. That is, since the Cr-containing steel has a slower reaction rate with Al than the ordinary steel, the growth of the alloy layer is slower than that of the ordinary steel. Under ordinary operating conditions, the alloy layer is thinner than the ordinary steel. Therefore, when the thickness of this alloy layer is increased, the corrosion rate of the alloy layer becomes extremely low, and the alloy layer of the Cr-containing steel-plated aluminum-plated steel sheet is liable to be sacrificed to the original plate. Is greatly improved. However, since the alloy layer has a very hard and brittle property as described above, if the alloy layer is too thick, it breaks inside the alloy layer when it is processed, and easily causes plating peeling. Therefore, it is necessary to control the thickness of the alloy layer within a certain range.

The present invention is based on the above findings,
The main points are as follows: C: 0.1% or less, Si:
0.3% or less, Cr: 5 to 23%, or Mo: 0.3 to 1.5%, the balance being Fe and unavoidable impurities on one or both sides of the steel plate to be plated, Si: 5 to 12%, with the balance being substantially Al
A hot-dip aluminum-plated steel sheet having excellent corrosion resistance characterized by having an aluminum-plated layer made of Al and having an Al-Si-Fe alloy layer having a thickness of 2.5 to 8 µm at the interface.

Hereinafter, the present invention will be described in detail.

[0015]

According to the present invention, molten steel produced in a normal melting furnace such as a converter or an electric furnace is made into a slab by ingot casting, slabging or continuous casting, and then hot-rolled and pickled. A cold-rolled sheet is manufactured in a general manufacturing process of a cold-rolled thin steel sheet or a Cr-based stainless steel sheet. Next, the cold-rolled steel strip is subjected to hot-dip aluminum plating by the Sendzimer method or the non-oxidizing furnace method. At this time, since the Cr-containing steel is a hard-to-plate material as described above, Fe, N
i, Fe-Ni or the like may be pre-plated, or a method for controlling the air-fuel ratio of the non-oxidizing furnace and the temperature of the inner plate of the non-oxidizing furnace as disclosed in JP-A-2-163357. Is also good. The plating base plate is preheated while oxidizing and burning surface dirt and rolling oil in an oxidation furnace or a non-oxidation furnace, and then introduced into a reduction furnace, where the surface oxide film is reduced and activated and recrystallized. Is promoted. Further, the steel strip is adjusted to a temperature suitable for plating and then immersed in an Al-Si-based hot-dip aluminum plating bath to adjust the plating thickness and to be wound up. The original plate for plating of the present invention, in addition to using the cold-rolled sheet as described above as it is, after cold rolling, degreasing,
A so-called full finish material or the like subjected to annealing and surface grinding may be used.

Next, the reasons for limitation of the present invention will be described. The thickness of the steel component and the alloy layer in the present invention are determined from the effects on various properties of the steel sheet after hot-dip aluminum plating. (1) Reasons for limiting plating base sheet C increases the content, not only deteriorating workability, but also Cr carbide that has reacted with Cr (for example, Cr ₂₃ C ₆ ).
Is a harmful element that lowers the corrosion resistance, such as by forming chromium at the grain boundaries and causing sensitization of the Cr-containing steel. The lower the content of the harmful element, the more preferable it is. The upper limit was set to 0.1%.

Si is not only an element that forms a stable oxide on the steel surface and inhibits the plating adhesion of the steel sheet, but also an element that hardens the steel sheet and deteriorates the workability like C, Again, the lower the content, the better,
The upper limit is set to 0.3%. Cr is an essential element for improving the corrosion resistance of an aluminum-plated steel sheet. In order for the plating layer and the alloy layer to have a sacrificial corrosion-preventing ability with respect to the base plate and exhibit sufficient corrosion resistance, 5% or more is required. is necessary. As the Cr content increases, the corrosion resistance also improves, but the effect is saturated at 23%.
Since r is an element that inhibits the plating adhesion like Si, the excessive content is harmful, and the upper limit is set to 23%.

Mo is an element capable of improving the corrosion resistance of the steel sheet, especially the pitting corrosion resistance, and is added as necessary. The effect is exhibited especially when coexisting with 10% or more of Cr. For this purpose, it is necessary to add Mo of 0.3% or more. However, the effect is saturated when the content is increased, so the upper limit is set to 1.5%. In addition, Mn,
P, S, Ti and the like are elements that deteriorate workability,
The smaller the better, the better.

The plated steel sheet manufactured with the above composition is subjected to aluminum plating in a molten aluminum plating bath containing Si. (2) Reasons for Limiting Aluminum Plating Layer In a hot-dip aluminum-plated steel sheet, a hard alloy layer generated at the interface between the plated steel sheet and the plating layer controls plating adhesion. In order to suppress excessive growth of this alloy layer and improve the heat resistance of the aluminum-plated steel sheet, Si is added to the aluminum plating bath. Therefore, although the thickness of the alloy layer, which is the gist of the present invention, can be controlled to some extent by the amount of added Si, if the Si content is too low, heat resistance cannot be obtained, so the Si content needs to be 5% or more. Si
If the content is too high, coarse Si precipitates in the plating layer and impairs corrosion resistance and plating adhesion.
2% or less. In addition, in the plating layer of the normally Cr-containing hot-dip galvanized steel sheet, as inevitable impurities,
Fe: around 0.5%, Cr: 0.1% other than Al and Si
Contains before and after.

(3) Reasons for Limiting the Thickness of the Alloy Layer The thickness of the alloy layer formed between the plating layer and the original plating plate, that is, at the interface, is an important item of the present invention as described above. The purpose of the present invention is to improve the corrosion resistance by increasing the thickness of an alloy layer having high corrosion resistance and having a sacrificial anticorrosion ability with respect to an original plate for plating. The thickness of the alloy layer is 2.
If it is less than 5 μm, the corrosion resistance is not sufficient, and if it is more than 8 μm, the plating tends to peel off inside the alloy layer or at the interface between the plating layer and the alloy layer when subjected to severe processing. Therefore, the thickness of the alloy layer is set to 2.5 to 8 μm. In the present invention, the method for forming the alloy layer is not particularly limited, and may be any of a method of performing annealing after plating, a method of increasing a plating bath temperature and a entering plate temperature, and a method of increasing the immersion time in a plating bath. Depending on the method.

[0021]

Next, the present invention will be described in detail with reference to examples. [Example 1] Table 1 shows the chemical composition and thickness of the steel of the present invention and the comparative steel. The steel of the present invention is a cold-rolled sheet which has been subjected to component adjustment in a normal converter and subjected to hot rolling, pickling and cold rolling. These plating plates are treated in a bath with a non-oxidizing furnace method.
Hot-dip aluminum plating was performed in a plating bath having an i concentration of 9% and a bath temperature of 660 ° C., and the basis weight was 12 by gas wiping.
After adjusting to 0 g / m ² , alumina powder was sprayed with a zero spangle device, temper rolling was performed at an elongation of 1%, and then wound around a coil. Thereafter, in a bell-type batch annealing furnace, the atmosphere gas composition contained 5% H ₂ , and the balance was substantially N ₂ ,
Annealing was performed while changing the temperature and time. Table 2 summarizes the manufacturing conditions at this time.

Various properties were evaluated for the materials thus manufactured, and the results are shown in Table 3. The evaluation method at this time is as follows. (1) Plating property After exfoliating only the plating layer of the plated steel sheet with 20% NaOH to expose the alloy layer, the occurrence of non-plating is visually determined,
The evaluation was performed in four steps according to the following criteria.

◎: 3 / dm ² or less Δ: 11 to 11
15 pieces / dm ² ○: 4 to 10 pieces / dm ² ×: 16 pieces / dm ² or more (2) Alloy layer thickness The cross section of the manufactured steel sheet is polished, etched with nitric hydrofluoric acid, and magnified 1000 times with an optical microscope. And the thickness of the alloy layer was calculated by averaging five points.

(3) Plating Adhesion The following two types of processing tests were carried out to observe the state of peeling of the plating in the processed portion, and a relative evaluation of the plating adhesion was performed. Reverse bend hiding: A plated steel sheet is subjected to impact bending into a V-shape, the V-shape is stretched, and then the same portion is impact-bent into an inverted V-shape, and the V-shape is further stretched.

Cup squeezing: squeezing with a blank mold 50 mm, die shoulder diameter 2 mm, drawing depth 10 mm, punch diameter 33 mm. (4) Corrosion resistance The product of the present invention has a diameter of 2 for corrosion of the exhaust pipe inner surface.
It was bent into a 5.4 mm pipe shape, and the following evaluation tests were performed. Corrosion resistance was relatively evaluated from the measurement results such as the occurrence of red rust and a decrease in the thickness of the rust portion.

Evaluation for the inner surface of the exhaust system The test liquid composition was 1.5 g / l NH ₄ (SO ₄ ) ₂ -1.5
g / l NH ₄ O ₃ -1.5 g / l NH ₄ Cl, 90
The test was repeated 100 cycles, in which the sample was half-submerged in the solution heated to ° C., and the cycle was set to 24 hr and left at room temperature for 24 hr as one cycle. The gas phase part and the liquid phase part were evaluated.

Evaluation on the outer surface of the exhaust system: 5 minutes of immersion in 0.5% salt water and 10 minutes of drying at 80 ° C. were repeated, and heated once a week at 400 ° C. for 120 minutes.
As a cycle, a 10-cycle test was repeated.

[0028]

[Table 1]

[0029]

[Table 2]

[0030]

[Table 3]

Example 2 Plating was carried out by using a plating original sheet of C shown in Table 1 by an oxidation-free furnace method. At that time, the line speed was lowered to extend the immersion time in the bath, or the bath temperature was raised to adjust the thickness of the alloy layer. Therefore, no annealing was performed after plating. Table 4 shows the experimental conditions and the performance at that time. The other conditions and the performance evaluation method are the same as in the first embodiment.

[0032]

[Table 4]

[0033]

According to the present invention, it is possible to manufacture a hot-dip aluminum-coated steel sheet having excellent corrosion resistance in a severe environment such as an automobile muffler, which greatly contributes industrially.

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-5-311379 (JP, A) JP-A-5-295513 (JP, A) JP-A 5-112859 (JP, A) JP-A-62-162 161944 (JP, A) JP-A-58-224159 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C23C 2/00-2/40

Claims (2)

(57) [Claims] 1. One or both surfaces of a steel sheet to be plated containing C: 0.1% or less, Si: 0.3% or less, Cr: 5 to 23%, and the balance being Fe and inevitable impurities.
An aluminum plating layer containing 5% to 12% of Si and substantially the remainder of Al is provided, and an Al—Si—Fe alloy layer having a thickness of 2.5 to 8 μm is provided at the interface. Hot-dip aluminized steel sheet with excellent corrosion resistance. 2. C: 0.1% or less, Si: 0.3% or less, Cr: 5 to 23%, Mo: 0.3 to 1.5%, the balance being Fe and inevitable impurities. An aluminum plating layer containing 5% to 12% of Si and the remainder substantially made of Al is applied to one or both surfaces of the steel sheet to be plated, and the interface is formed with an Al—Si— layer having a thickness of 2.5 to 8 μm.
A hot-dip aluminized steel sheet having excellent corrosion resistance, characterized by having an Fe alloy layer.