JPH04365851A - Steel sheet plated with al or al alloy by vapor deposition and having excellent corrosion resistance - Google Patents

Abstract

PURPOSE:To provide a low-cost steel sheet plated with Al or Al alloy by vapor deposition, having improved corrosion resistance without deteriorating workability and suitable for use as the material of the exhaust system of an automobile. CONSTITUTION:A steel sheet consisting of, by weight, < 0.05% C, 0.01-0.1% P, 2-6% Cr, 0.2-0.6% Cu, 0.2-0.6% Ni and the balance Fe with inevitable impurities is plated with Al or Al alloy by vapor deposition.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a vapor-deposited Al-plated steel sheet having excellent corrosion resistance and suitable as a material for an automobile exhaust system.

0002

2. Description of the Related Art Characteristics required for automobile exhaust system materials include corrosion resistance, heat resistance, and workability. The inside of a muffler, which is one of the above-mentioned automobile exhaust system materials, is a highly corrosive environment where exhaust gas condensation is generated.Especially after stricter exhaust gas regulations, catalytic converters were introduced and the exhaust gas temperature decreased.
The problem of corrosion inside mufflers is becoming more serious as condensed water stays there for longer periods of time.

[0003] Hot-dip Al-plated steel sheets have been widely used as the muffler material. However, melting A
l-Plated steel sheets have the problem that corrosion of the base steel sheet progresses from defects in the plating layer, and as mentioned above, the requirements for corrosion resistance have become stricter, so there is a demand for materials with better corrosion resistance. .

[0004] Corrosion resistance can be improved by using stainless steel containing 17 to 19% Cr, but it is expensive and has poor workability. Further, stainless steel containing 11 to 13% Cr progresses through pitting corrosion and has insufficient corrosion resistance.

[0005] Therefore, in the above-mentioned stainless steel and Cr-containing steel,
Some materials are also used in combination with hot-dip Al plating. However, if the Cr content in the steel is 5% or less, the hot-dip Al plating layer does not exhibit stable sacrificial corrosion protection, and the effect of improving corrosion resistance is insufficient. On the other hand, in the case of steel with a Cr content of 5% or more, a strong oxide film is formed during hot-dip Al plating and the wettability of the plating decreases, so pre-plating with Ni, Fe, etc. is recommended as a pre-plating treatment. This has led to decreased productivity and increased costs.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned circumstances, and has improved corrosion resistance without impairing workability compared to conventional Al-plated steel sheets.
Furthermore, the present invention aims to provide a vapor-deposited Al-based plated steel sheet that is inexpensive.

[0007]

[Means for Solving the Problems] The present invention that achieves the above objects includes C: 0.05%, P: 0.01 to 0.1%,
Cr: 2-6%, Cu: 0.2-0.6%, Ni:
0.2 to 0.6%, with the remainder being Fe and unavoidable impurities, and is coated with vapor-deposited Al plating or vapor-deposited Al alloy plating.

[0008]

[Operation] Although corrosion resistance is improved by adding a large amount of Cr, processability and economic efficiency are reduced.

[0009] Therefore, the present inventors suppressed the amount of Cr added to an extent that does not impair workability and added Cu, thereby improving corrosion resistance due to the synergistic effect of Cr and Cu. Furthermore, by adding Ni, we prevented the embrittlement during hot rolling caused by Cu, and by applying vapor-deposited Al-based plating, we obtained plated steel for automobile mufflers with excellent condensed water resistance, thereby completing the present invention. . The reasons for limiting the steel material composition will be explained below.

C<0.05% C is an element that deteriorates corrosion resistance, and also reduces the content of Cr, which is effective in improving corrosion resistance, by forming Cr carbide, so the smaller the content, the better.
It is necessary to keep it below 5%.

2%≦Cr≦6% When Cr is contained in an amount of 2% or more when it coexists with Cu, it reduces the dissolution rate of the base steel sheet from defects in the plating layer and improves the corrosion resistance.

[0012] In the case of corrosion of Al-plated steel sheets, the progress of corrosion is suppressed by covering the surface of plating defects such as pinholes with Al corrosion products, but the presence of Cr and Cu in the base steel sheet prevents the corrosion from progressing. Then, as the corrosion of the plating layer progresses, it is thought that a protective corrosion product consisting of Al-Cr-Cu hydroxide is formed, thereby exerting the effect of improving corrosion resistance.

On the other hand, if Cr is added in an amount exceeding 6%, processability and economic efficiency will be impaired. Therefore, the Cr content was set to 2% or more and 6% or less.

0.2%≦Cu≦0.6%, 0.2%≦Ni≦0
．． 6% Cu forms a dense corrosion product consisting of Fe-Cr-Cu-S in the exhaust gas condensed water containing S in the case where there is no vapor-deposited plating layer, and exhibits the effect of suppressing the progress of corrosion. When combined with vapor-deposited Al-based plating, Cu enters the Al-Cr hydroxide in the corrosion products of the plating layer, further improving corrosion resistance.

However, since the addition of Cu causes embrittlement during hot rolling, it is necessary to add Ni in an amount equivalent to the amount of Cu added. Adding a large amount of Cu requires the same amount of Ni, which significantly impairs economic efficiency, so the upper limit of the amount of Cu added is set at 0.6.
%.

0.01%≦P≦0.1 %P is Al, C
Since it coexists with r and Cu and increases corrosion resistance, it is added at least 0.01%, but too much will harden the material, so 0.1% or more is added.
The upper limit was %.

The present invention uses Al or an Al alloy as a plating material, which has superior corrosion resistance in an acidic environment containing concentrated exhaust gas condensed water compared to Zn-based plating materials. The above Al alloys include Al-Cr, Al-Ti, Al-M
n etc. are preferable from the viewpoint of improving corrosion resistance.

Further, the present invention employs a vacuum evaporation method as a plating method. Regarding the sacrificial corrosion protection ability, both the vacuum deposition method and the melt-dip method are at the same level, but since the corrosion resistance of the vapor-deposited plating layer itself is superior to that of the hot-dip plating layer, the corrosion resistance after vapor-deposited plating is improved compared to hot-dip plating. Furthermore, formation of an Fe-Al alloy layer and non-plating, which are problems in the case of hot-dip plating of Cr-containing steel, do not occur.

[0019] To obtain sufficient corrosion resistance, the coating weight should be 2.
It is preferably 0 g/m2 or more, more preferably 40 g/m2 or more.

Further, for the purpose of improving corrosion resistance, a chromate treatment may be performed after plating, and an organic coating treatment may also be performed.

[0021]

[Example] Using steel having the composition shown in Table 1, a vapor-deposited Al-plated steel plate or a vapor-deposited Al--Cr alloy plated steel plate was prepared. The following corrosion resistance evaluation test was conducted using the above-mentioned plated steel sheet in an environment simulating exhaust gas condensed water, and evaluation was performed based on the maximum perforation depth. The amount of plating deposited is 40g/m2 in each case,
Similar tests were also conducted on the comparative examples shown in Table 1.

1. Cycle test SO4 2-(10000ppm), Cl-(100
0ppm), CO3 2-(3000ppm), HC
The next cycle test was conducted using an aqueous solution containing O3 - (3000 ppm) as an ammonium salt.

[0023] The end face of a 400 mm x 100 mm test piece was coated, and the test liquid was evaporated and concentrated by immersing it in the above aqueous solution and heating for 4 hours, and then 1/5 of the above test piece was coated.
One cycle consisted of immersing the sample in a semi-sealed state, storing it at room temperature for 20 hours, and then exchanging the liquid. After 30 cycles, the perforation depth was measured.

2. Half immersion test SO4 2- (60000 ppm), Cl- (600
An aqueous solution containing NH4 + (25,500 ppm) was adjusted to pH 3.5, and 1/3 of the above test piece was prepared.
After soaking at 80°C for 336 hours, the perforation depth was measured.

The evaluation criteria for the above two types of tests are as follows, and the results are also listed in Table 1. ◎: Maximum perforation depth less than 50 μm ○: Maximum perforation depth 50 μm or more and less than 100 μm △: Maximum perforation depth 100 μm or more and less than 150 μm ×: Maximum perforation depth 150 μm or more

[0026]

[Table 1]

[0027]

[Effects of the Invention] Since the present invention is constructed as described above, it is possible to provide a vapor-deposited Al-based plated steel sheet that has improved corrosion resistance without impairing workability and is inexpensive.

Claims (1)

[Claims] [Claim 1] C: less than 0.05% (meaning of weight %, the same applies hereinafter) P: 0
．． 01 to 0.1% Cr: 2 to 6% Cu: 0.2 to 0.6% Ni: 0.2 to 0.6%, with the balance consisting of Fe and unavoidable impurities, vapor-deposited Al plating Alternatively, a vapor-deposited Al-based plated steel sheet with excellent corrosion resistance is formed by applying vapor-deposited Al alloy plating.