JPH06264172A - High chromium ferritic heat resisting alloy steel - Google Patents

Abstract

PURPOSE:To produce a high chromium ferritic heat resisting alloy steel excellent in high temp. creep strength, oxidation resistance, erosion-corrosion resistance, etc., by incorporating specific percentages of C, Si, Mn, Cr, Al, B, Mo, W, Ta, and Nb into Fe. CONSTITUTION:A heat resisting alloy steel having a composition consisting of, by weight, <=0.15% C, <=2% Si, <=2% Mn, 65-80% Cr, 0.5-1.5% Al, <=1% B, <=5% (in total, 10 the case of >=2 elements) of one or >=2 elements selected from the group consisting of Mo, W, Ta, and Nb, and the balance essentially Fe is prepared. By this method, the high chromium ferritic heat resisting alloy steel having high melting point, high thermal conductivity, and low coefficient of thermal expansion can be obtained.

Description

Detailed Description of the Invention

[0001]

The present invention relates to a radiant tube, a hearth constituent member of a heating furnace, etc., which is excellent in high temperature creep strength, oxidation resistance, etc., has a high melting point, and has a high thermal conductivity and a low thermal expansion coefficient. A heat-resistant alloy steel having

[0002]

2. Description of the Related Art As a material for a radiant tube or a hearth of a steel material heating furnace, 20Ni-25Cr-Fe has been conventionally used.
(ASTM standard HK40, JIS G5122 SCH
22), 35Ni-25Cr-Fe (HP45, SCH
24) or austenitic high Ni-high Cr alloy steel such as 50Ni-30Cr-13W-Fe.

[0003]

A radiant tube used under severe thermal conditions is apt to be deformed by creep or thermal stress during its use, or cracked due to the deformation. This is particularly noticeable on the burner side, and oxidation / melting damage of the tube wall due to direct contact with the burner flame also has a large effect on the tube life. Therefore, the radiant tube made of conventional heat-resistant alloy steel has a heating temperature of about 10
It is used by limiting the temperature to 00 ° C or lower. Moreover, its service life is short. The present invention, high temperature creep strength, oxidation resistance,
Excellent erosion resistance, etc., 100 as a radiant tube etc.
It is intended to provide a heat-resistant alloy steel that can be used in a high temperature range exceeding 0 ° C.

[0004]

The high chromium ferritic heat resistant alloy steel of the present invention comprises C: 0.15% or less and Si: 2%.
Below, Mn: 2% or less, Cr: 65 to 80%, Al:
0.5-1.5%, B: 1% or less, Mo, W, Ta, N
One or more elements selected from the group b): 5% or less (in the case of two or more elements, the total amount), the balance being a chemical composition consisting essentially of Fe.

[0005]

The heat-resistant alloy steel of the invention having the above-described composition has a high creep strength even in a high temperature range of about 1000 ° C. or higher, and has little deformation such as creep elongation and drawing. Further, since it has a high thermal conductivity, the occurrence of temperature unevenness and thermal stress due to it is small, and since it has a low coefficient of thermal expansion, thermal deformation due to local heating or temperature change is small.
Further, it has a high oxidation resistance and a high melting point, so that it is also excellent in melting resistance. Therefore, even when the temperature exceeds 1000 ° C, such as the burner-side tubing of the radiant tube, and even in a severe environment where local heating and temperature change occur, deformation due to creep, thermal stress, etc. is alleviated and alleviated.・ Suppressing and preventing melting damage.

The reasons for limiting the components of the heat resistant alloy steel of the present invention are as follows. Cr: 65-80% Cr enhances the creep strength of the alloy. Further, the alloy matrix is made to be a ferrite phase, and high thermal conductivity and low thermal expansion coefficient are imparted. In a high temperature environment of about 1000 ° C. or higher, at least 65% is necessary to maintain high creep strength and secure a ferrite matrix. However, as the amount of Cr increases, it becomes difficult to melt the alloy in the atmosphere, and in particular, the increase in the amount of nitrogen (N) stored in the melting process causes deterioration of mechanical properties such as the toughness of the alloy, and Since it will be difficult to secure it, the upper limit is 80%.

Al: 0.5-1.5% Al forms an oxide film on the alloy surface in the high temperature range. The oxide film is B in the high temperature use environment of the alloy,
It prevents the oxidation and consumption of elements such as Mo, W, Ta and Nb and the change of alloy composition. To obtain this effect, at least 0.
Requires 5%. However, if added in a large amount, the fluidity of the molten alloy becomes poor, casting becomes difficult, and the amount of non-metallic inclusions increases to cause casting defects (slag bite), so 1.5% is made the upper limit. .

B: 1% or less B lowers the melting point of the alloy steel of the present invention having a high Cr content to a temperature (about 1580 ° C. or less) that does not hinder the casting operation, and easily secures sound casting quality. To be added. However, if added in a large amount, casting cracks are likely to occur, and it is difficult to secure improved high-temperature characteristics because the melting point is excessively lowered (about 1450 ° C. or less). Therefore, the upper limit is 1%. . It is preferably 0.1 to 1%.

Mo, W, Ta, Nb: 5% or less Each element of Mo, W, Ta, Nb has the effect of increasing the high temperature creep strength of the alloy. In particular, Mo, combined with B, has a remarkable effect on long creep life and reduction of creep elongation and drawing. Nb also has the effect of improving the weld crack susceptibility. The addition amount of these elements is sufficient up to 5%, and if the amount is increased beyond that, casting cracks and work cracks are likely to occur, so this is the upper limit. 2
When using multiple elements in combination, the total amount should be 5%.
Below.

C: 0.15% or less When the content of C is large, the tendency of casting cracking of the alloy increases. Further, the precipitation effect of forming carbides such as Cr, W, and Ta to increase the creep strength is a phenomenon in the temperature range of about 1000 ° C. or lower, and does not exist in the high temperature range of about 1100 ° C. or higher. Therefore, the upper limit of the amount of C is set to 0.15%.

Si: 2% or less Si is a deoxidizing agent in the alloy melting process, and also improves the fluidity of the molten alloy and is effective in improving the castability. However, a large amount of addition causes work cracking of the alloy. 2% because it causes the
Below.

Mn: 2% or less Mn is a desulfurizing element in the alloy melting step and has the effect of improving the fluidity of the alloy and increasing the castability, but the addition amount for that is sufficient up to 2%. Yes, no further addition required.

The heat-resistant alloy steel of the present invention can be produced by applying the atmospheric melting method in a high frequency melting furnace or the like. Also,
For a tubular body such as a radiant tube, centrifugal force casting can be applied to cast a tubular body having an arbitrary tube size. The temperature of the cast material is approximately 1300 to 14 as a heat treatment.
After heating and holding at 00 ° C. for an appropriate time, heat treatment for rapid cooling (forced air cooling, water cooling, etc.) is performed. By this heat treatment, the cast material is homogenized by eliminating the microscopic component segregation, and a relatively coarse crystal structure is formed. Forming this coarse-grained crystal structure is preferable for effectively exhibiting the high creep strength of the alloy steel of the present invention.

[0014]

【Example】

[1] Specimen material A molten alloy steel melted by a high-frequency melting furnace (atmosphere melting) was subjected to centrifugal force casting, and the resulting cast pipe material was heated to a temperature of 1350.
After heating and holding at 0 ° C., a heat treatment of forced air cooling was performed. This is designated as test material A. Test pieces were cut out from the test material A, and various tests were performed to obtain the results shown in Table 1. As a comparative example,
Commercially available 20Ni-25Cr-Fe heat-resistant alloy steel (AS
The same test was performed for TM standard HK40 material). This comparative material is designated as B. The measurement results are also shown in the table.

Chemical composition of test material A (wt% ) C: 0.11, Si: 1.1, Mn: 1.2. Cr: 7
0.5. Al: 1.0, B: 0.80, Mo: 2.1,
Fe: Bal. Chemical composition of test material B (wt% ) C: 0.42, Si: 1.3, Mn: 1.1. Cr: 2
6.8. Ni: 20.2, Mo: 0.45. Fe: Ba
l.

[2] Tests of various characteristics (1) High temperature creep rupture test According to JIS Z2272. However, test temperature: 1
250 ° C., load: 0.5 Kg / mm ² . (2) High Temperature Oxidation Test A test piece is held in a heating furnace (atmosphere atmosphere) set at a temperature of 1250 ° C. for 100 hours to measure the surface oxidation loss.

[0017]

[Table 1] Specimen A (Invention example) Specimen B (Conventional example) Creep property Break time (Hr) 15 0.5 Break elongation (%) 7 12 Break reduction (%) 9 76 Oxidation resistance Oxidation weight loss (Mm / year) 1.8 9.2 Melting point (liquid phase) (° C) 1540 1400 Thermal expansion coefficient (~ 1250 ° C) (× 10 ^-6 / ° C) 16 21 Thermal conductivity (1250 ° C) (Kcal / mhk ) 41 26

The heat-resistant alloy of the present invention has higher creep strength and higher creep deformation resistance than the conventional representative heat-resistant alloy steels. Moreover, since it has both a high thermal conductivity and a low thermal expansion coefficient, thermal deformation due to uneven heat distribution and temperature change is small. Further, it has a high oxidation resistance and also has a high melting point, so that it is also excellent in melting resistance.

[0019]

The heat-resistant alloy steel of the present invention is excellent in high-temperature creep strength, oxidation resistance, and melt damage resistance, and has high thermal conductivity and low thermal expansion coefficient.
Even when used as a burner tube material, creep deformation in high temperature environment is unlikely to occur, deformation due to thermal stress is small even with local heating of the burner flame, and resistance to deterioration damage due to melting and oxidation is high. Is also excellent,
These improved material properties provide benefits such as improved service life, reduced maintenance, and improved furnace operating efficiency. Further, the use of the heat-resistant alloy steel of the present invention is not limited to the above-mentioned examples, and is also useful as a material for a skid pipe, a hearth roll or the like which is a hearth constituent member of a steel material heat treatment furnace.

Claims (1)

[Claims] 1. C: 0.15% or less, Si: 2% or less,
Mn: 2% or less, Cr: 65-80%, Al: 0.5-
1.5%, B: 1% or less, one or more elements selected from the group of Mo, W, Ta, Nb: 5% or less (in the case of 2 or more, the total amount), the balance substantially High chromium ferritic heat resistant alloy steel made of Fe.