JP2003328090A - Corrosion- and heat-resistant cast steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide heat-resistant cast steel which is corrosion-resistant as highly as that intergranular corrosion is prevented in such a strongly corrosive environment as to cause chloriation corrosion or sulfidation corrosion simultaneously with high-temperature oxidation. <P>SOLUTION: An alloy prevented from the decrease in thickness due to continuous intergranular corrosion and falling of crystal granules is prepared by substituting Nb or W carbide having a partially or continuously high corrosion resistance for a lowly corrosion-resistant Cr carbide precipitated continuously at the grain boundaries. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat-resistant cast steel exhibiting excellent corrosion resistance in a highly corrosive environment in which chlorination corrosion, sulfide corrosion and the like occur simultaneously with high temperature oxidation.

[0002]

2. Description of the Related Art Fe is a heat-resistant cast steel used at high temperatures.
-Cr-Ni-C heat-resistant cast steel (for example, JIS SCH
2 or SCH13), but when the present alloy is used in a severe corrosive environment in which high temperature oxidation and chlorination corrosion or sulfide corrosion simultaneously occur, corrosion causes material damage. Therefore, when these alloys are used in a high temperature corrosive environment, it is necessary to prolong the life of the alloy surface by cooling and to periodically perform maintenance to replace the device members.

Reducing the surface temperature of a material by cooling to prolong the life of the material is an effective means for reducing corrosion, but when these alloys are used in, for example, a combustion apparatus, excessive cooling is performed. Causes a decrease in combustion efficiency.

One of the applications of the above-mentioned cast steel is a grate of an incinerator, but in the incinerator, high temperature combustion is required from the viewpoint of suppressing the generation of dioxins, and the grate has a more severe corrosive environment. Tend to be used in. Here, if the grate is excessively cooled for the purpose of preventing corrosion of the grate, the combustion temperature is lowered, which is not preferable. However, since the current materials have poor corrosion resistance at high temperatures, it is necessary to periodically stop the furnace and perform maintenance in a short period of time. In recent years, most incinerators have a waste power generation facility, so continuous operation for a long period of time is required, and there is a problem that the current materials cannot handle.
For this reason, many studies have been conducted on the material.

In the prior art disclosed in JP-A-5-140700, the weight ratio of C: 0.05-0.25%, Si: 0.
3 to 2.0%, Mn: 0.2 to 1.0%, P: <0.0
5%, S. : <0.05%, Cr: 16 to 20%, N
b: 0.5 to 1.5%, B: 0.02 to 0.15, the composition of balance Fe is presented, and coarse Cr carbide harmful to thermal fatigue can be suppressed by adding an appropriate amount of Nb and B. It is said that excellent fatigue strength and heat resistance were obtained.

In the prior art disclosed in Japanese Unexamined Patent Publication No. 11-131192, the high temperature wear resistance was examined, and the weight ratio of C: 0.
3 to 0.7%, Si: 0.5 to 2.0%, Mo: 0.1
-2.0%, Cr: 20-30%, W: 0.5-4.5
%, Al: 0.01 to 0.05, Ca: 0.01 to 0.
05, the cast steel consisting of the balance Fe is presented and the conventional material (SCH
It is said that it has a high temperature strength exceeding that of 2), and in particular, excellent high temperature hardness and wear resistance were obtained in a temperature range of about 600 ° C. or higher.

In the prior art disclosed in Japanese Patent Laid-Open No. 11-140600, in order to improve the restraint thermal fatigue resistance and the oxidation resistance and to reduce the thickness, the weight ratio of C: 0.01 to 0.2%, S
i: 3.0-5.0%, Mn: 0.5-0.25%, C
r: 6.0 to 14.5%, Ni. 6.0-11.5%,
Mo. 0.05-2.0%, Nb: 0.05-2.5
%, W: 0.05 to 4.5, Cu: 0.5 to 2.0%,
He proposed a cast steel composed of the balance Fe, and claimed that the oxidation resistance and the castability improved the thickness.

Further, in the prior art disclosed in JP-A-11-131192, in order to improve high hardness and wear resistance in a high temperature range, C: 0.3 to 0.7% by weight and Si:
0.5-2.0%, Mo: 0.1-2.0%, Cr: 2
0.0-30.0%, W: 0.5-4.5%, Al:
0.01-0.05%, Ca: 0.01-0.05%,
He presented a heat-resistant cast steel consisting of the balance Fe and claimed that a hardness exceeding that of the conventional material (SCH2) was obtained in a high temperature range exceeding 500 ° C.

The above-mentioned prior art is mainly concerned with mechanical properties such as fatigue strength and high temperature wear resistance, and it is said that oxidation resistance is slightly improved with respect to high temperature corrosion. On the other hand, the present invention relates to a heat-resistant cast steel exhibiting improved corrosion resistance not only in high-temperature oxidation but also in a highly corrosive environment in which chlorination corrosion, sulfide corrosion, etc. occur simultaneously.

[0010]

In order to investigate the corrosion behavior under a high temperature corrosive environment, heat-resistant cast steel (SCH2) is treated with HC.
The corrosion test was carried out under the condition that the molten salt was present in the presence of 1 and SO2. A cross-sectional photograph after the corrosion test is shown in FIG. Cr carbide continuously precipitated at the grain boundaries is selectively corroded,
Further, near the surface, all grain boundaries were corroded and crystal grains were dropped. In an actual high-temperature device, thermal stress is further applied due to temperature fluctuations, cracks easily propagate from corroded crystal grain boundaries, and crystal grains are accelerated to fall off. Due to such intergranular corrosion, severe metal thinning occurs.

The present invention addresses the above-mentioned problems.
An object of the present invention is to provide an alloy having excellent corrosion resistance without excessively cooling the equipment members in a high temperature corrosive environment.

[0012]

[Means for Solving the Problems] By the above corrosion mechanism, it is possible to reduce the corrosion damage of cast steel in a high temperature corrosive environment by suppressing the intergranular corrosion resulting from the Cr carbide precipitated at the grain boundaries. Becomes By replacing Cr carbide with low corrosion resistance that is continuously precipitated at grain boundaries with another carbide with high corrosion resistance partially or continuously, reduction due to continuous intergranular corrosion and loss of crystal grains It is possible to prevent meat. An alloy was produced in which Nb or W was added as an element forming a carbide that replaces the Cr carbide.

2 is SCH2, FIG. 3 is SCH2 with 1% Nb added (hereinafter 1 Nb), and FIG. 4 is SCH2 with 3% Nb.
b is added (hereinafter 3 Nb), and FIG. 5 is a cross-sectional photograph of an as-cast material of an alloy in which 3% W is added to SCH2 (hereinafter 3 W). The carbide identified by X-ray diffraction is S
Cr carbide (Cr ₂₃ C ₆ ) in CH2, C in 1Nb
Nb carbide (Nb ₂ C) together with r carbide (Cr ₂₃ C ₆ ).
And NbC) and Nb carbide (Nb ₆ C ₅ ) in 3Nb
In the same manner, even at 3W, only W carbide (WC) was formed. From this, the carbide precipitated at the grain boundary in FIG.
In contrast to the carbide, Cr carbide was partially replaced by Nb carbide in 1Nb of FIG. 3Nb in FIG.
In, the Cr carbide was almost replaced with the Nb carbide. Similarly, in FIG. 5, Cr carbide was almost replaced by W carbide. When the precipitation of these Cr carbides was not continuous, intergranular corrosion that causes the drop of crystal grains did not occur, leading to a significant prolongation of life.

The present invention has solved the above-mentioned problems by the following means based on the finding that the corrosion resistance is improved by the continuous control of Cr carbide by the experimental research.

In mass%, C is 0.1-0.5%, Si is 0.5-5.0%, Mn is 2.0% or less, and Cr is 20-.
Corrosion resistant heat-resistant cast steel containing 35%, Ni 4% or less, Mo 0.4% or less, 0.1 to 5.0% of at least one of Nb and W, and the balance Fe except unavoidable impurities.

Corrosion-resistant heat-resistant cast steel according to the above (1), characterized in that it contains 0.1 to 5.0% by mass of Nb.

Corrosion-resistant heat-resistant cast steel according to the above (1), characterized in that it contains 0.1 to 5.0% by weight of W.

Corrosion-resistant heat-resistant cast steel according to the above (1), characterized in that intergranular corrosion is suppressed by precipitating at least one of Nb carbide and W carbide.

[0019] At least one of Nb carbide and W carbide is dispersed in a grain boundary to be precipitated, and the corrosion-resistant heat-resistant cast steel according to the above (1) is characterized.

As described above, the present invention improves corrosion resistance by adding a carbide-forming element to heat-resistant cast steel and suppressing the continuous precipitation of Cr carbide.

BEST MODE FOR CARRYING OUT THE INVENTION The proportion of each element added to the alloy of the present invention and the reason therefor will be described.

In conventional alloys, C forms Cr carbides and promotes intergranular corrosion, so it is desirable that the content of C is small from the viewpoint of corrosion resistance. However, the formation of carbides improves the high temperature strength, and
In order to improve castability, it is essential to add a certain amount. If the content is less than 0.1%, the desired effect cannot be obtained, and if it exceeds 0.5%, the formation of carbides becomes excessive and intergranular corrosion becomes severe.
I decided.

Si exerts a strong deoxidizing action on the molten metal and also has an action of improving castability.
%, The effect becomes remarkable. However, if added excessively, the toughness of the material is lowered, and if it exceeds 5%, the toughness is remarkably lowered. Therefore, the content is 0.5 to 5.
It was set to 0%.

Mn acts as a desulfurizing material as well as a deoxidizing agent, and further improves the castability, but addition of a large amount causes a decrease in high temperature oxidation resistance, so the effect of deterioration of corrosion resistance is not significant 2% or less Was defined as the content. The content is preferably 1% or less.

Cr is an essential element for improving high temperature corrosion resistance. Reacts with oxygen in the atmosphere to form a Cr ₂ O ₃ film that is protective against corrosion on the alloy surface and suppresses corrosion of the base material. In order to form a uniform Cr ₂ O ₃ film on the surface of the alloy, it is necessary to add more than 20%, but addition of more than 35% forms a σ phase with extremely poor toughness, which may deteriorate the toughness of the material. There is a nature. Therefore, the content is 2
It was limited to 0 to 35%.

When Ni is added in a large amount, the base is austenitized. The austenitic type has a slower diffusion rate in the alloy than the ferritic type and is inferior in the high temperature oxidation property of the base material.
In addition, N changes on the alloy surface due to the structural change of the base material due to high temperature corrosion.
i may be concentrated and the surface may become austenite,
The amount of addition of 4% or less which does not cause austenite formation on the surface of such an alloy was determined.

Mo is an additive element effective in improving high temperature strength, but it deteriorates high temperature oxidation resistance. Especially when high temperature strength is required, the upper limit is set to 0.4% in a range that does not deteriorate the high temperature oxidation resistance.

As described above, Nb improves the intergranular corrosion resistance by forming a solid solution in the alloy, forming carbides and precipitating at the grain boundaries, continuously cutting the Cr carbides and replacing them with the Cr carbides. In order to cut the continuous Cr carbide and improve the corrosion resistance, it is necessary to add 0.1% or more, but the effect becomes constant when the addition exceeds 5%. From an economical point of view, it is desirable to add a small amount of Nb,
The content was set to 0.1 to 5.0%. Preferably 0.2
~ 2.0%.

W, like Nb, also becomes a carbide and precipitates at grain boundaries, contributing to the improvement of intergranular corrosion resistance. The effect appears when 0.1% or more is added, and the effect becomes constant when the content exceeds 5%. Since it is desirable that the content of W is small from the economical point of view, the content is set to 0.1 to 5.0%.

[0029]

The present invention will be described in detail with reference to the following examples. However, the present invention is not limited to these examples.

In order to investigate the properties of the alloy of the present invention, test pieces of the alloy of the present invention having a different composition and various alloys used as comparative materials were prepared and subjected to a corrosion test. All the alloys used in the test were melted in the atmosphere using a normal high-frequency induction melting furnace to prepare ingots. Table 1 shows the composition of the alloy used in the test.

[0031]

[Table 1]

A test piece was cut out from the ingot, held at 700 ° C. for 24 hours under a condition simulating the grate environment of a refuse incinerator, and the amount of corrosion was measured by observing a cross section after the test. As shown in the results shown in Table 2, the alloys of the present invention to which Nb and W were added exhibited excellent corrosion resistance with respect to the comparative material.

[0033]

[Table 2]

EFFECTS OF THE INVENTION The alloy according to the present invention exhibits excellent intergranular corrosion resistance particularly in a high temperature corrosive environment where intergranular corrosion is a problem, because chloride corrosion and sulfidic corrosion occur simultaneously with high temperature oxidation. Therefore, by using the alloy of the present invention, it is possible to provide an alloy having a long life and high reliability without excessive cooling when cooling deteriorates the device performance. When these alloys are required for these characteristics, for example, in the grate of an incinerator, they can be used for a long period of time without requiring maintenance without lowering the combustion temperature by excessive cooling.

[0035]

[Brief description of drawings]

FIG. 1 is a cross-sectional morphology of SCH2 after a corrosion test.

FIG. 2 is a cross-sectional form of an as-cast material of SCH2.

FIG. 3 is a cross-sectional structure of an as-cast material of the developed alloy containing 1% Nb.

FIG. 4 is a cross-sectional structure of an as-cast material of the developed alloy containing 3% Nb.

FIG. 5 is a cross-sectional structure of the as-cast material of the developed alloy containing 3% W.

[Table 1] Chemical composition of cast steel.

[Table 2] High-temperature corrosion test results of cast steel.

Continued front page    (72) Inventor Hidenori Takahashi             11-chome, Kita-ku, Kita-ku, Sapporo, Hokkaido             Inside the Hokkaido Industrial Test Station (72) Inventor Yasuki Miyakoshi             11-chome, Kita-ku, Kita-ku, Sapporo, Hokkaido             Inside the Hokkaido Industrial Test Station (72) Inventor Shuichi Kamoda             11-chome, Kita-ku, Kita-ku, Sapporo, Hokkaido             Inside the Hokkaido Industrial Test Station (72) Inventor Gen Jinbo             11-1 Haneda Asahi-cho, Ota-ku, Tokyo Edo Co., Ltd.             In the original factory (72) Inventor Yoshinobu Urakami             11-1 Haneda Asahi-cho, Ota-ku, Tokyo Edo Co., Ltd.             In the original factory (72) Inventor Takehiro Oka             11-1 Haneda Asahi-cho, Ota-ku, Tokyo Edo Co., Ltd.             In the original factory (72) Inventor Hiroshi Yawa             Fujisawa City, Kanagawa Prefecture Fujisawa 4-2-1 Stock Association             Inside the Ebara Research Institute (72) Inventor Manabu Noguchi             Fujisawa City, Kanagawa Prefecture Fujisawa 4-2-1 Stock Association             Inside the Ebara Research Institute (72) Inventor Toshio Narita             Hokkaido, Sapporo, Kita-ku, Kita 13-jo Nishi 8-chome, Hokkaido             Inside the university F term (reference) 3K065 AA24 AB01

Claims (5)

[Claims] 1. C-0.1 to 0.5%, Si 0.5 to 5.0%, Mn 2.0% or less, and Cr 20 to 20% by mass.
Corrosion resistant heat-resistant cast steel containing 35%, Ni 4% or less, Mo 0.4% or less, 0.1 to 5.0% of at least one of Nb and W, and the balance Fe except unavoidable impurities. 2. Corrosion-resistant heat-resistant cast steel according to claim 1, characterized in that it contains 0.1 to 5.0% by mass of Nb. 3. Corrosion-resistant heat-resistant cast steel according to claim 1, characterized in that it contains 0.1 to 5.0% by weight of W. 4. Corrosion-resistant heat-resistant cast steel according to claim 1, wherein intergranular corrosion is suppressed by precipitating at least one of Nb carbide and W carbide. 5. At least one of Nb carbide and W carbide is dispersed in a grain boundary to be precipitated, whereby Cr is formed.
The corrosion-resistant heat-resistant cast steel according to claim 1, which suppresses intergranular corrosion due to selective corrosion of carbides.