JPS61163242A - Steel having superior corrosion resistance to oxidation at high temperature as well as to wet corrosion by exhaust gas - Google Patents

Abstract

PURPOSE:To obtain the steel having superior corrosion resistance to oxidation at high temp. as well as to the wet corrosion by exhaust gas, by incorporating specific amounts of C, Si, Mn, and Cr to steel. CONSTITUTION:The steel consisting of, by weight, <0.06% C, 0.3-1.5% Si, <0.5% Mn, 1.5-5% Cr, and the balance Fe with inevitable impurities is refined; Cr is preferably 1.5-3%, more preferably 1.5-2%. This steel is cast and subjected to hot forging, hot rolling, cold rolling, and annealing to produce the steel sheet.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a highly corrosion-resistant steel alloy suitable as a material constituting an exhaust gas member. In particular, the steel alloy of the present invention exhibits excellent corrosion resistance against both alkaline substances and acidic substances (both alkaline and acidic aqueous solutions) and has excellent high-temperature oxidation resistance. The present invention relates to an economical corrosion-resistant steel alloy that is suitable as a material for exhaust gas systems that are subject to wet corrosion due to condensation and high-temperature oxidation.

[Conventional technology]

In machinery and equipment that involve the combustion of fossil fuels, especially petroleum-based fuels, such as internal combustion engines, boilers, and heating appliances, the materials of the parts that come into contact with combustion exhaust gas must have high-temperature oxidation resistance. Aluminized steel sheet is a material used in this field.

This aluminum-plated steel sheet has the advantage of being cheaper than heat-resistant steel or stainless steel that contains a large amount of one alloy element, and has a certain degree of high-temperature oxidation resistance. Often used for road construction members. More specifically, the automobile exhaust gas line is usually the front tube from the side closest to the engine.

It consists of parts such as an exhaust gas purification device (converter using a catalyst), central tube, muffler, tail tube, etc., which are connected in sequence, and aluminum-plated steel sheets are used for some or all of these parts. There are many examples where it is used.

As for materials other than aluminum plated steel sheets.

Special Publication No. 54-23327, Special Publication No. 54-3557
Publication No. 1.

JP-A-58-224148, JP-A-59-179
There is a steel proposed in Publication No. 758 and the like.

[Problem that the invention seeks to solve]

For materials used in fields that require high-temperature oxidation resistance as described above, in addition to high-temperature oxidation resistance, corrosion resistance against alkaline and acidic substances may also be required. . In this case, conventional aluminum-plated steel sheets may not be able to meet the requirements.

For example, if we take an automobile exhaust gas pipe as an example, the exhaust gas that has passed through the converter passes through the central tube.

Exhaust gas is discharged from the car body through the muffler 3-tail tube, etc., but since the pipe is not sufficiently warmed, especially when the engine is started, moisture in the exhaust gas condenses inside the pipe, causing dew condensation water. (drain) accumulates (for example, in the muffler) or adheres to the pipe wall. This condensed water eventually evaporates as the temperature of the exhaust gas pipe increases, but during this time the nature of the condensed water changes from alkaline to acidic. This is because the degree to which various components in exhaust gas dissolve into condensed water differs depending on the temperature, and in the process of evaporation of condensed water, components that have already been dissolved decompose or scatter, but the degree of dissolution varies depending on the temperature. The reason for this is thought to be that there are differences between each. Therefore, in addition to high-temperature oxidation resistance when the temperature eventually rises, corrosion resistance against alkali and acid is also required. In conventional aluminum-plated steel sheets, the aluminum plating layer did not have sufficient corrosion resistance against alkaline condensed water, and the base steel sheet did not have sufficient corrosion resistance against acidic condensed water.

And for wet corrosion involving water, there are connections between dissimilar metals with different standard electrode potentials.

It is a well-known phenomenon that a corrosion current flows between two metals through water, making one metal more susceptible to corrosion.
In automobile exhaust gas pipes that partially use aluminum-plated steel plates, there are many joints between the aluminum-coated steel plates and other metals (special steel and stainless steel), so the standard electrode potential with the aluminum steel plate is In contrast, a phenomenon occurs in which the aluminum-plated steel sheet side becomes more susceptible to corrosion.

The present invention is designed to protect against corrosive environments that undergo wet corrosion and high-temperature oxidation by repeatedly coming into contact with such alkaline substances and acidic substances (alkaline aqueous solutions and acidic aqueous solutions) and being exposed to high-temperature combustion exhaust gas. This is an attempt to solve the problem that conventional materials (particularly aluminum-plated steel sheets) did not have sufficient service life. This kind of problem is particularly common in automobile exhaust gas pipes, especially in mufflers where a large amount of condensed water accumulates during the early stages of engine startup. This is a problem that is more or less common in many industries, and it is also a problem that accompanies chemical plants and the like.

In addition to aluminum-plated tM steel sheets, there is also a steel that is said to be suitable for use in 8 automobile exhaust gas paths:
Publication No. 7, Japanese Patent Publication No. 54-35571, Japanese Patent Application Publication No. 1983
-224148 publication and JP-A-59-179758
However, only the steels described in JP-A-58-224148 and JP-A-59-179758 have been studied for wet corrosion due to dew condensation.

However, the steels described in JP-A No. 58-224148 and JP-A No. 59-179758 both contain a large amount of Cr and also contain expensive elements such as Ti, Nb, and Zr to prevent condensation water. It cannot be manufactured at low cost because it has high resistance to wet corrosion. Special Public Service 1977
No. 23327 and Japanese Patent Publication No. 54-35571 do not consider moisture corrosion, and the former contains a large amount of Cr, while the latter contains Al as an essential element and has a relatively large amount of Cr. In both cases, the processability is further improved by adding Ti and Zr, so there is a problem that they cannot be manufactured at low cost.

[Means for solving problems]

The present invention is a steel alloy for solving the above-mentioned problems, as described in the claims, 2% by weight,
C up to 0.06%, 0.30-1.50% Si
, Mn up to 0.50%, 1.50-5.
00%, preferably 1.50 to 3.00%, more preferably 1.50 to 2.00% Cr, the balance being Fe and unavoidable impurities. Steel with excellent corrosion resistance against wet corrosion due to exhaust gas and high temperature oxidation. It provides:

When the steel of the present invention is used as a material constituting automobile exhaust gas pipes that are exposed to the special +G eating environment as mentioned above, 1. For example, when applied to muffler materials, it can be Shows better corrosion resistance (service life) than steel plate. Since the steel of the present invention is not an alloy steel with a high content of expensive alloying elements, it is economically very advantageous in that it can be manufactured at low cost.

〔Example〕

Preferred embodiments and characteristics of the steel alloy of the present invention will be specifically explained below based on test results.

Table 1 lists 3 types of steel whose chemical composition values (weight%) are shown.
After melting and casting in a 0 kg high-frequency vacuum melting furnace, hot forging, hot rolling, cold rolling, and annealing were performed under exactly the same conditions.

A steel plate with a thickness of 1.0 mm was manufactured. Each steel plate was then subjected to three types of extremely long-term corrosion resistance tests under the following test conditions.

Test specimen: 60 mm diameter from a 1.0 mm thick plate
A disk was cut out, and the central part of this disk was made to protrude into a hemispherical shape using an Erichsen tester (diameter: 25 m).
A test piece was prepared by forming a hemispherical depression with a maximum depth of 4.0 mm at the center.

[Test I]

Since the exhaust gas condensation water at the initial stage of engine startup is an alkaline aqueous solution, this test is a corrosion resistance test using the typical composition of this alkaline condensation water as it is.

After immersing the test piece in the following alkaline test solution A for 3 minutes at room temperature, it was taken out from the solution with test solution A filled in the hemispherical depression (referred to as the liquid reservoir) of the test piece. One cycle consists of holding the sample in a warm air atmosphere at ℃ for 17 minutes and completely evaporating the liquid in the liquid reservoir to dryness during this period. This cycle is then repeated 1080 times.

After completing this 1080 cycle corrosion test.

Corrosion reduction amount (gr) + plate thickness reduction amount (mm) of each test piece
Measure and visually observe the presence or absence of pitting corrosion.

Test liquid A This liquid contains the following ionic components, which are actually contained in the condensed water that accumulates in the muffler of automobile exhaust gas, in the following amounts, and the pH value is kept constant at 8.8. ,
pHtJl was adjusted with ammonium salt. The addition of activated carbon simulates the fact that a large amount of unburned carbon is deposited inside the muffler.

GO322000ppm 11co3 2000 ppm 502 500 ppm Cl-50ppm HCHo 12 ppm Activated carbon 10g/j! [Test ■] This test investigated the corrosion resistance after the condensed water of automobile exhaust gas changed from alkaline to acidic in the process of evaporation, and was a corrosion resistance test for the following test liquid B (acidic liquid).

After immersing the above-mentioned test piece in the following acidic test solution B for 3 seconds at room temperature, take it out from the solution with test solution B filled in the reservoir of the test piece, then dry it at 120 ° C. for 15 minutes, One cycle consists of holding the material in a furnace held at 500° C. for 10 minutes to perform high-temperature oxidation, and then allowing it to cool to room temperature. This cycle is then repeated 41 times. After completing this 41-cycle corrosion test, the amount of corrosion reduction (gr) of each test piece was measured and the surface condition was observed.

Test Solution B This solution is an acidic aqueous solution obtained by concentrating automobile exhaust gas condensation water equivalent to Test Solution A at 60° C. to 1/4 of its volume. PR was adjusted with ammonium salt so that the pH value was constant at 4.0. The liquid composition is as follows.

SOi 20000 ppm Cl-600ppm 111”0=i2200 ppln Nll: ; 8000 ppm Activated carbon 10g/2 These test results are summarized in Table 2.

Table 1 Table 2 From these test results, 1 Steel No. of the present invention, 1 to No.
All types 3 are good in a special and harsh corrosive environment (test ■) in which the cycle of wet corrosion due to condensed water from automobile exhaust gas, high temperature oxidation, and further wet corrosion is repeated over and over again. In test I (alkaline aqueous solution), aluminum plated steel plate (No. 11
), it can be seen that the performance is better than that of

With reference to these test results, an outline of the reasons why the content of each component of the steel of the present invention is limited as described in the claims will be explained below.

C is 2 In steel containing Cr such as the alloy of the present invention,
Cr forms a carbide with Cr and dissolves in the matrix.
As a result, the effect of improving corrosion resistance due to Cr is reduced.

Since the presence of the formed carbide itself promotes corrosion, the smaller the amount, the better. However, in the case of the present invention, where one of the objectives is to provide a material with a low carbon content, ultra-low carbonization goes against this objective when the cost required for this purpose is taken into account.

As seen in the results in Table 2, in the alloy of the present invention whose corrosion resistance is improved by Cr and Si, C is 0.
．． Even if it is allowed up to 0.06%, sufficient corrosion resistance can be maintained.

The upper limit of the 0 content is 0.06%, preferably 0.05
%.

Si is a characteristic element in the steel of the present invention.

For example, comparison 6MNo, 4. 5. 6. 7 and 9
When the Si content is less than 0.3%, even if Cr is included, Ni, Cu, P
Even if elements thought to have an effective effect on corrosion resistance are included, sufficient corrosion resistance cannot be obtained.

On the other hand, when Si is contained at 0.3% or more, corrosion resistance properties such as Ni, Cu, and P are not actively added, and even if the Cr content is 5.00% or less, , Even if the CrM content is as low as 3.00% or less (actually 2.00% or less) as in positive 1 and 2, Ni,
Cu.

It is possible to obtain sufficient corrosion resistance than those containing P and those containing 5% or more of Cr (No. 7), and the corrosion resistance against exhaust gas condensation water and 500%
The objective of high temperature oxidation resistance at temperatures around °C is fully achieved. One of the characteristics of the steel of the present invention is that it has been found that the effect of Si is noticeable at low Cr contents. To obtain the synergistic effect of Si on moisture corrosion resistance and high temperature oxidation resistance due to this low Cr content, + Si
The content should be at least 0.30%. . However, if there is a large amount of Si, especially if the content exceeds 1.50%, the material will become extremely brittle, making cold workability difficult and making it difficult to form the complex shapes required for the parts that make up the automobile exhaust gas path. It may be difficult to process, so
The upper limit of the Si content is 1.50%.

From the viewpoint of corrosion resistance, it is better to have a low Mn content, but since it is an element necessary for deoxidation reactions and sulfide formation in the steel manufacturing process, it is not possible to omit Mn. Cr,
In the alloy of the present invention, whose corrosion resistance is comprehensively improved by Si + Cu + Ni, P, etc., + Mn
Since sufficient corrosion resistance is achieved even when Mn is contained up to 0.50%, the upper limit of Mn is set at 1.50%.

Cr is disclosed in Japanese Patent Publication No. 54-23327, Japanese Patent Application Laid-open No. 58-
As recommended in Japanese Patent Application Laid-open No. 224148 and Japanese Patent Application Laid-Open No. 179758/1987, it was necessary to contain the content in an amount exceeding 5% for aqueous corrosion and high temperature oxidation. In this case, even if the Cr content is 5% or less (3,00% or less, in fact, 2.00% or less), if an appropriate amount of Si is added, it will be sufficient to protect against dew condensation water and high-temperature oxidation in the exhaust gas. The above test revealed that it exhibited a high resistance. For example, comparative steel No. 9 is C
Although it contains 5.56% of r, unlike the steel of the present invention, it does not properly contain St, so pinting occurred in test ①, and corrosion weight loss occurred in tests I to ① compared to the steel of the present invention. It is more common than steel. However, comparison 11
Even if Si is added within the range specified by the present invention, as seen in No. 8 and No. 010, if the Cr content is lower than 1.5%, sufficient test results cannot be obtained. Therefore, to achieve the object of the present invention, the Cr content must be at least 1.5%.

In the invention steel N11l~3, the Cr content is 5
% or less, and as in NO, 1 to 2, C
Sufficient corrosion resistance results can be obtained even when the r amount is 3.00% or less, in fact, 2.00% or less. In this way, the corrosion resistance objective of the present invention was achieved at low Cr content.
Compared to the steel with a high Cr content as seen in the above-mentioned publication, the amount of expensive Cr added can be reduced, and in this respect it is economically very advantageous.

The steel of the present invention can be manufactured according to conventional steel manufacturing methods. Impurity elements are unavoidably included in ordinary steel manufacturing processes.

For example, S, P, N, O, Aβ, etc. exhibit sufficient corrosion resistance even if they are contained as impurities in the range where they are unavoidably contained. This means that a special degassing process is not necessarily required, which means that it can be produced economically.

Furthermore, according to the test results in Table 2, the invention steel No. 1 shows better results than the aluminum-plated steel sheet (No. 11). In other words, in a corrosive environment with an alkaline aqueous solution caused by exhaust gas condensation water, it is better to use a type I aluminum plated steel plate containing Si to improve heat resistance (Floor 11 is this type I aluminum plated steel plate). It also shows excellent resistance.

Therefore, it is possible to provide an alternative material to conventional aluminum-plated steel sheets for automobile exhaust gas path members, especially for muffler members downstream of converters where corrosion caused by condensed water is a problem. Since this product has succeeded in developing this corrosion resistance by adding a certain amount of alloying elements, it has lower corrosion resistance than aluminum-plated steel sheets, chromium steel proposed in the above-mentioned publications, and even special corrosion-resistant steels such as stainless steel. It is also very advantageous in terms of speed.

Claims (3)

[Claims] (1) C up to 0.06% in weight %, 0.30-1
．． 50% Si, up to 0.50% Mn, 1.50-5
．． A steel with excellent corrosion resistance against wet corrosion caused by exhaust gas and high-temperature oxidation, consisting of 0.00% Cr and the balance Fe and unavoidable impurities. (2) A steel excellent in corrosion resistance against wet corrosion due to exhaust gas and high temperature oxidation according to claim 1, wherein Cr is 1.50 to 3.00%. (3) A steel excellent in corrosion resistance against wet corrosion due to exhaust gas and high temperature oxidation according to claim 1, wherein Cr is 1.50 to 2.00%.