JP2001247940A - Heat resistant alloy excellent in carburizing resistance and metal dusting resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the carburizing resistance and metal dusting resistance of a tubing, such as a reheater tube for a direct reduction iron making furnace and a cracking tube for ethylene production. SOLUTION: The alloy has a composition consisting of, by weight, 0.05-0.6% C, <=2.5% Si, 1.5-6% Mn, 23-46% Cr, 23-50% Ni, at least one kind selected from the group consisting of <=6% W, <=2% Nb and <=2% Mo, and the balance essentially Fe. If necessary, 0.005-0.25% Ce, 0.01-0.5% Al, 0.01-0.4% Ti and 0.01-0.3% Zr can be incorporated.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat-resistant alloy suitable as a tube material such as a reheater tube for a direct reduction steelmaking furnace and a cracking tube for ethylene production.

[0002]

2. Description of the Related Art In a direct reduction iron making process, metallic iron is produced by reducing iron ore (iron oxide) using hydrogen. The exhaust gas after reduction contains hydrogen, carbon monoxide, water vapor and the like, and is reused through a reheater tube heated to a temperature of about 450 to 750 ° C. In the ethylene production process, pyrolysis is performed by passing a liquid or gaseous hydrocarbon through a cracking tube heated to a temperature of about 1000 to 1200 ° C. As the reheater tube and the cracking tube, Fe—Cr—Ni heat-resistant alloys containing a large amount of Cr and Ni are widely used.

[0003]

In the case of a reheater tube for direct reduction steelmaking, there is a problem that if the tube is continuously used, a part of the tube is reduced in thickness. This thinning is a phenomenon called metal dusting, which is considered to be caused by carburizing embrittlement on the inner surface of the tube, and easily occurs in a temperature range of about 450 to 750 ° C. That is, when the reducing gas containing hydrogen and carbon monoxide is heated, CO + H ₂ → C + H ₂ O, 2
A reaction of CO → CO ₂ + C occurs, and carbon is deposited and adheres to the inner surface of the tube. The carbon penetrates into the tube and causes carburization. In the carburized part, a Cr carbide is formed, so that the amount of Cr in the matrix is reduced. As a result, the material is embrittled in an environment containing hydrogen, and the metal collapses like dust. If the thickness of the tube is significantly reduced, the tube must be replaced. Also, in the case of cracking tubes for ethylene production, carbon inevitably precipitates from the raw material hydrocarbons and adheres to the inner surface of the tubes. Therefore, if carburization at a high temperature proceeds, deterioration of the tube material occurs. therefore,
Materials of this type are required to have improved resistance to carburization and metal dusting.

[0004]

SUMMARY OF THE INVENTION The present invention provides, in weight percent,
C: 0.05-0.6%, Si: 2.5% or less, Mn: 1.0%
5-6%, Cr: 23-46%, Ni: 23-50%,
And W: 6% or less, Nb: 2% or less, and Mo: 2%
Including at least one selected from the group consisting of:
The present invention provides a heat-resistant alloy consisting essentially of Fe and having excellent carburization resistance and metal dusting resistance. The heat-resistant alloy of the present invention may contain Ce: 0.005 to 0.25, if necessary.
%, Al: 0.01 to 0.5%, Ti: 0.01 to 0.4
%, Zr: 0.01 to 0.3% as necessary.

[0005]

[Explanation of Reasons for Restricting Components] C: 0.05-0.6% C forms NbC or Cr-W-based carbide at the crystal grain boundaries at the time of casting and solidification, strengthens the grain boundaries and increases the creep rupture strength. The improvement in creep rupture strength is recognized from 0.05%, but if it exceeds 0.6%, the room-temperature tensile ductility decreases, so the content is made 0.05 to 0.6%.

[0006] Si: 2.5% or less Si has a deoxidizing effect, increases the fluidity of the molten metal,
It has the effect of improving carburization resistance. In a carburizing corrosion environment up to about 1200 ° C, an internal oxide film SiO ₂
And has an effect as a barrier for C intrusion. The effect increases as the content increases, but if it exceeds 2.5%, the creep rupture strength is significantly reduced. Therefore, the upper limit is 2.5
%. In addition, 1.3-2.0% is preferable.

Mn: 1.5 to 6% Mn has an effect of fixing S and improves weldability. In addition, there is an effect that a Mn oxide which forms a solid solution in the matrix and has high adhesion is formed on the metal surface to improve carburization resistance and metal dusting resistance. Therefore, 1.5%
It is contained above. On the other hand, if it exceeds 6%, the creep rupture strength decreases, so the upper limit is set to 6%. Note that 3.
5-5% is preferable, and 4-5% is more preferable.

[0008] Cr: 23 to 46% Cr contributes to the improvement of oxidation resistance. At an operating temperature up to about 1200 ° C., 23% or more is required to ensure oxidation resistance in the presence of Ni. However, if it exceeds 46% in the balance with Ni, the drop in the room temperature tensile stretching becomes remarkable, so the upper limit is 46%.

Ni: 23 to 50% Ni stabilizes the austenite phase and is effective for securing oxidation resistance and high-temperature strength. Considering the use of high temperatures up to about 1200 ° C., 23% or more is required. However, if the content exceeds 50%, a corresponding improvement in high-temperature strength cannot be expected, so the upper limit is 50%.

[0010] At least one of W, Nb and Mo is contained. W: 6% or less W forms a solid solution with the matrix and forms a Cr-W-based carbide to contribute to improvement in creep rupture strength. The effect increases as the content increases, but if it exceeds 6%, the tensile elongation at room temperature is significantly reduced, so the upper limit is 6%.

Nb: 2% or less Nb contributes to improvement in creep rupture strength. However, 2
%, The tensile elongation at room temperature is lowered.
%.

Mo: 2% or less Mo contributes to improvement in creep rupture strength and improves high temperature tensile strength. However, if it exceeds 2%, the tensile elongation at room temperature is reduced, so the upper limit is set to 2%.

Ce: 0.005 to 0.25% Ce improves carburization resistance and metal dusting resistance in a reducing atmosphere of CO—H ₂ —H ₂ O. For this reason, it is preferable to contain 0.005% or more as needed. However, if the content exceeds 0.25%, oxides are formed during casting and the cleanliness of the steel is greatly reduced, so the upper limit is 0.25%.

Al: 0.01 to 0.5% Al has an effect of improving the carburization resistance, so that 0.01% or more is contained as necessary. However, the content is 0.5
%, The strength is reduced. Therefore, the content is 0.1.
It is preferably from 0.01 to 0.5%.

Ti: 0.01 to 0.4% Ti contributes to the improvement of creep rupture strength, so it is contained in 0.01% or more as necessary. However, when the content is 0.1.
If it exceeds 4%, the room-temperature tensile ductility is reduced, so the upper limit is made 0.4%.

Zr: 0.01-0.3% Since Zr contributes to the improvement of creep rupture strength, it is contained at 0.01% or more as necessary. However, when the content is 0.1.
If it exceeds 3%, the room-temperature tensile ductility decreases, so the upper limit is made 0.3%.

The heat-resistant alloy of the present invention contains the above-mentioned component elements, and the balance substantially consists of Fe. It should be noted that impurities that are inevitably mixed during the melting of the alloy may be present as long as they are within the range normally allowed for this kind of alloy. For example, for P and S, P: 0.2% or less,
S: Content of 0.2% or less is allowable.

[0018]

[Example] Since the temperature range in which the reheater tube of the direct reduction steel is subjected to metal dusting is a relatively low temperature range of about 450 to 750 ° C, a carburizing test was conducted at 600 ° C, and the carburizing resistance and the metal resistance were evaluated. The dusting property was examined. The heating temperature of the cracking tube for ethylene production is about 10
Because of the high temperature range of 00 to 1150 ° C, 1000 ° C
A carburizing test was conducted to examine the carburizing resistance. Further, a high temperature tensile creep rupture test was performed to examine the creep rupture strength.

Carburizing test at 600 ° C. Test materials having the alloy chemical components shown in Table 1 were melted, and test pieces having a width of 20 mm × length 30 mm × thickness 5 mm were cut out from the obtained test materials. The test piece was formed with a through-hole having a diameter of 5 mm at a position 7.5 mm from the edge in the length direction and 10 mm from the edge in the width direction, and the surface was finished smoothly using # 400 emery paper. In Table 1, No. 1 to No. 12 are invention examples, and No. 13 to No. 16 are comparative examples having a small amount of Mn. CO flowing through the reheater tube of direct reduction steelmaking
As a gas simulating a rich CO—H ₂ —H ₂ O gas, a mixed gas of CO: 0.06 L / min and H ₂ : 0.02 L / min was passed through a humidifier at 25 ° C. It was supplied into the experimental furnace. Each specimen was weighed and then suspended in a laboratory furnace at a temperature of 600 ° C. by passing a rod having a diameter of 4 mm into the hole. After a lapse of 900 hours, the test piece was taken out of the experimental furnace, and the adhered substance on the surface was rubbed cleanly with a bristle brush, subjected to ultrasonic cleaning, and weighed again. The measurement result was converted into a weight change per 1 cm ² of the test piece, and the weight change amount is shown in Table 2.

[0020]

[Table 1]

[0021]

[Table 2]

Referring to Table 2, the comparative examples No. 15 and No. 15
No. 16 shows that the weight increase is large and carburization is progressing as compared with No. 1 to No. 12 of the invention examples. Comparative example
No. 13 and No. 14 have reduced weight. This is because metal collapse occurred due to the metal dusting phenomenon. That is, since the content of Mn is small, carburization proceeds and the amount of generated Cr carbide increases, and the content of Cr becomes 2%.
This is considered to be because the concentration of Cr in the matrix was reduced due to the small amount of 4 to 25%, and the material was embrittled. Note that No. 15 and No. 16 described above are No. 13 and N
Since the content of Cr is higher than that of o.14, even if carburization occurs, the decrease in the Cr concentration in the matrix has not yet led to embrittlement of the material. Thus, it can be seen that the Mn oxide has strong adhesion on the metal surface, and is very effective in carburization resistance and metal dusting resistance. Among the invention examples, the reason why the weight increase of No. 1 and No. 2 is slightly increased is that the content of Cr is small. Therefore, in order to improve carburization resistance, the content of Cr should be about 3%.
Desirably, it exceeds 0%, and more preferably, about 35% or more.

1000 ° C. carburizing test As a gas simulating CH ₄ —H ₂ —H ₂ O gas flowing through a cracking tube of a cracking furnace for ethylene production, CH ₄ : 0.06 liter / min, H ₂ : 0. After passing a mixed gas of 03 L / min through a humidifier at 34 ° C., it was supplied into the experimental furnace. Each specimen was weighed in the same manner as in the metal dusting test, and then suspended in a laboratory furnace at a temperature of 1000 ° C. through a rod having a diameter of 4 mm in the hole. After a lapse of 125 hours, remove the test specimen from the experimental furnace,
The attached matter on the surface was rubbed cleanly with a bristle brush, and after ultrasonic cleaning, the weight was measured again. The amount of change in weight is also shown in Table 2.

Referring to Table 2, the weight increase of No. 13 to No. 16 was larger than that of No. 1 to No. 12, indicating that the carburization resistance was inferior. This is because the content of Mn is small.

High temperature creep rupture test A test piece having a diameter of 6 mm and a length between gauge points of 30 mm was cut out from a test material having the chemical composition of the alloy shown in Table 1.
Tensile creep rupture test under the condition of stress 34.3N (JIS
Z 2272), and the rupture time was measured. The measurement results are also shown in Table 2. The invention example has the high temperature strength required for a reheater tube of a direct reduction steelmaking furnace and a cracking tube of a cracking furnace for ethylene production.

[0026]

The heat-resistant alloy of the present invention is excellent in carburization resistance and metal dusting resistance, and thus is suitable as a reheater tube of a direct reduction iron making furnace. Further, since it has excellent carburizing properties at high temperatures, it is also suitable as a cracking tube for a cracking furnace for ethylene production.

Claims (3)

[Claims] (1) C: 0.05 to 0.6% by weight, S
i: 2.5% or less, Mn: 1.5 to 6%, Cr: 23 to 4
6%, Ni: 23 to 50%, and W: 6% or less, N
b: A heat-resistant alloy containing at least one selected from the group consisting of 2% or less and Mo: 2% or less, and the balance being substantially Fe, which is excellent in carburization resistance and metal dusting resistance. 2. The heat-resistant alloy according to claim 1, which contains 0.005 to 0.25% of Ce and / or 0.01 to 0.5% of Al. 3. Ti: 0.01 to 0.4% and / or Z
The heat-resistant alloy according to claim 1 or 2, wherein r: 0.01 to 0.3% is contained.