JPH0349980B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to a steel material for a heat exchanger used for recovering heat from high-temperature waste gas or heating reaction gas. Conventional technology and problems In recent years, due to the rise in energy prices, melting furnaces for steel, non-ferrous metals, glass, etc., heat treatment furnaces, garbage incinerators,
In equipment that generates a large amount of heat, such as high-temperature processing equipment for industrial waste, it is possible to use the heat of waste gas to preheat combustion air and fuel gas in a heat exchanger to improve the thermal efficiency of the furnace. Usually done. Heat exchangers are also used in various plants to heat reaction gas, but in order to reduce fuel costs, low-cost heat sources such as heavy oil with a high sulfur content and waste oil have recently been used as heat sources. things are preferred. In the case of such heat exchangers, various types of high-temperature corrosion and corrosion caused by single substances or compounds of S, Cl, C, P, Na, K, etc. contained in fuel and processed materials are the materials. accelerates damage. For this reason, conventional heat exchangers are
Cyclomaru (Cr-Si-Al steel), a ferritic heat-resistant stainless steel known as ferritic stainless steel, was often used. Although Cyclomal has relatively good high-temperature corrosion resistance, the essential drawback of ferritic stainless steel is that it has low high-temperature strength, and as the material temperature increases, deformation becomes severe due to insufficient strength of the component, so the material temperature is 850℃. It was necessary to regulate the temperature as follows, and for this reason, it was not possible to raise the temperature of the heated gas to 700°C or higher. Therefore, if the temperature of the heat source gas was too high, it was diluted with air to lower the temperature. On the other hand, austenitic heat-resistant stainless steels and alloys with high high temperature strength such as SUS310S and Incoloy 800
These materials are also used as materials for heat exchangers, but since these materials have insufficient high-temperature corrosion resistance, in this case, the above-mentioned temperature regulation has been necessary to prevent high-temperature corrosion. In other words, there is no existing steel material that has both sufficient high-temperature corrosion resistance and high-temperature strength as a steel material for heat exchangers. The present inventors have previously proposed an austenitic stainless steel with excellent oxidation resistance in Japanese Patent Publication Nos. 55-43498 and 1987-11302. These steel materials exhibit excellent oxidation resistance in high-temperature oxidizing atmospheres, and also exhibit high high-temperature strength as austenitic steels. When the amount of oxygen in the atmosphere is insufficient, carburization and nitridation tend to occur, which deteriorates the high-temperature corrosion resistance of the steel and often causes problems when used in heat exchangers. Structure and Function of the Invention In view of the above-mentioned circumstances, the present inventors further investigated materials suitable as materials for heat exchangers, and as a result, they discovered that the following composition is extremely excellent as a material for heat exchangers. That is, the first aspect of the present invention includes, in weight percent, C: 0.2% or less, Si: 2% or less, Mn: 3% or less, Cr: 9 to 25%, Ni: 12 to 45%, Al: more than 5% to 8%. This steel material for heat exchangers is characterized by containing one or more kinds of Ca, Y, and rare earth elements in an amount of 0.1% or less each, and having a metal structure mainly in an austenite phase. The second aspect of the present invention, in addition to the first one, includes 5% or less Mo, 5% or less W, and 10% or less.
Contains 15% or less of one or more types of Co in total. Further, the third and fourth materials of the present invention each contain 5% or less of Cu in addition to the first and second materials. The present invention will be explained in detail below. First, the reason for limiting the components as described above in the present invention is as follows: C is necessary to stabilize the austenite phase in austenitic stainless steel and ensure strength at room temperature and high temperature. The upper limit was set at 0.2% because it deteriorates machinability, workability and toughness at room temperature. Si is useful for improving oxidation resistance in ordinary heat-resistant steels, but in the steel of the present invention, oxidation resistance is maintained by forming an Al ₂ O ₃ film, so addition of Si is not particularly necessary, and only 2.0% of Si is added. If Si exceeds this amount, the SiO ₂ layer will prevent the formation of the Al ₂ O ₃ film, so the upper limit was set at 2.0%. Mn is an austenite forming element, but 3.0%
If added in excess of this amount, the oxidation resistance deteriorates and it becomes too hard, so the upper limit was set at 3.0%. Cr is an essential element for stable formation of the Al ₂ O ₃ film and ensuring adhesion, and for this purpose, it is necessary to add 9% or more. However, adding more than 25% deteriorates the toughness of the material and also promotes embrittlement during use, so the upper limit was set at 25%. Ni is a basic element necessary to make the metal structure into an austenite phase, and even if the amount of Cr or Al added is the lower limit of the present invention's ingredients, it is necessary to add at least 12%. However, if it exceeds 45%, the hot workability deteriorates and it becomes expensive, so the composition range is reduced to 12 to 45%.
%. Next, in the steel material of the present invention, Al content of more than 5% to 8%
The biggest feature is that it contains In other words, with austenitic stainless steels such as SUS310S and Incoloy 800, a film mainly composed of Cr ₂ O ₃ is formed on the steel surface at high temperatures, but by using such high Al-containing steel materials, A film consisting mainly of Al ₂ O ₃ is formed at high temperatures. A film mainly composed of Cr ₂ O ₃ contains a considerable amount of iron oxide and does not have sufficient high-temperature corrosion resistance against corrosive substances that cause problems in heat exchangers, but as mentioned above, a film mainly composed of Al ₂ O ₃ In addition to the effects of one or more of Ca, Y, and rare earth elements that are contained in small amounts in steel materials, its high-temperature corrosion resistance is extremely good, and when used in a heat exchanger, it is highly resistant to various types of corrosive substances that fly in with the heat source gas. Not easily attacked by substances. In this case, the surface of the steel material of the present invention is coated with Al ₂ O ₃ which has excellent high temperature corrosion resistance as described above.
In order to form a film, it is necessary to add more than 5% of Al, so the lower limit was set to more than 5%. However, if added in excess of 8%, the toughness and ductility of the steel will deteriorate significantly, so the upper limit was set at 8%. Ca, Y, and rare earth elements are all effective in improving hot workability and stability of the Al ₂ O ₃ film by adding one or more of them to the steel of the present invention. If the element is added in an amount exceeding 0.1%, the hot workability will be adversely affected, so the upper limit was set at 0.1%. Furthermore, the rare earth elements (REM) referred to herein refer to lanthanide elements with atomic numbers 57 to 71. The above are the specific components of the steel material of the present invention, but in the present invention, in order to further improve high temperature strength and creep strength, one or more of Mo, W, and Co are added, and furthermore, the dew point corrosion resistance is improved. Expect it
Cu can be added. First, Mo, W, and Co are all effective in improving high-temperature strength and creep strength, but they are also expensive.
5% each for Mo and W, 10% for Co
%, and even if the total amount exceeds 15%, the effect will not increase significantly, so the upper limit was set as above. Cu is added to improve dew point corrosion resistance when the temperature of the heat exchanger drops, but if added in excess of 5.0%, hot workability and high temperature toughness will rapidly deteriorate, so the upper limit has been set at 5.0%. %. In addition, one of the important aspects of the steel material of the present invention is that its metal structure is mainly an austenite phase. That is, the reason why the structure is mainly composed of austenite phase is that it has higher high-temperature strength than Cyclomal, a ferritic stainless steel that has been widely used in the past, and is less likely to deform at high temperatures. In other words, the yield strength of cyclomal at 1000°C is about 1 Kg/mm ² , whereas the steel of the present invention has a low C content because it mainly consists of austenite phase, which means that the high temperature strength is relatively low. However, the yield strength is 5Kg/mm ² or more, which is equivalent to the yield strength of Cyclomaru at 700℃. When the steel of the present invention having such composition and structure is used in a heat exchanger, there are temperature constraints due to high-temperature corrosion when using austenitic stainless steel such as SUS310S, and high-temperature limitations when using Cyclomal. As a result of both the temperature constraints from deformation being relaxed, the temperature of the heat transfer surface of the heat exchanger can be lowered than that of the conventional one.
The temperature can be increased by 100 to 300°C, making it possible to provide a more efficient heat exchanger. As mentioned above, the metal structure of the steel of the present invention is mainly an austenite phase, but a delta ferrite phase containing a precipitated β phase (NiAl) may be included within the range permitted by the design stress for a heat exchanger member. It can contain up to 40%. However, if particular emphasis is placed on workability at room temperature and high-temperature strength, it is desirable to set the upper limit of the delta ferrite phase to 10%. Next, the effects of the present invention will be explained in more detail with reference to Examples. Example 1 Steel containing each component (wt%) of A to S shown in Table 1 was
A 25Kg steel ingot was produced by vacuum melting, and a portion of the steel ingot was forged, hot rolled, and annealed to create a 7mm thick hot rolled medium plate. A test piece measuring 5 ^t × 30 ^w × 500 ^l was cut out from this medium plate by machining, and a hole of 8〓 was drilled in one end.
It was hung in the flue of a glass melting furnace for 27 days, and the elongation due to its own weight and the Na ₂ O and K ₂ O that came from the glass raw materials, as well as Na ₂ SO ₄ that was produced when these reacted with S in the fuel,
The high temperature corrosion resistance due to adhered molten salts such as K ₂ SO ₄ was investigated. The gas temperature at the place where the test piece is hung is 900 to 1000.
The average temperature is 960°C and the oxygen concentration is 4-6%.
These results are shown in Table 2, and the creep elongation is expressed as the elongation length in the longitudinal direction of the test sample as a percentage of the length of the test piece of 500 mm before the test. Weight loss due to high-temperature corrosion was calculated by lightly sandblasting the test piece after the test to remove scale, and dividing the weight change of the test piece before and after the test by the surface area of the test piece before the test. However, the cyclomal of B is 30 as a whole.
It was not possible to collect any data because it had elongated by more than 30% and had abnormally elongated around the hanging hole, broken and fallen off. On the other hand, SUS310S and Incoloy 800 are
As for creep elongation, it was expected to show a small elongation of around 1%, but the loss due to high-temperature corrosion due to the salt coming from the glass raw material was large, and the plate thickness was lost by 2 to 3 mm. On the other hand, all of the steel materials of the present invention had creep elongations equal to or lower than SUS310S, and the amount of high-temperature corrosion due to glass raw materials was much smaller than that of SUS310S. This shows that the steel material of the present invention can be used as a steel material for heat exchangers even in such harsh environments, even when the temperature of the heat transfer surface is 900 to 1000°C.

【table】

【table】

[Table] Effects of the Invention As detailed above, the alloy of the present invention is excellent in both high-temperature strength and high-temperature corrosion resistance, and as shown in the examples above, the present invention is suitable for manufacturing heat exchangers with extremely high heat exchange efficiency. This greatly contributes to improving the energy saving rate of high-temperature furnaces.

Claims (1)

[Claims] 1 C: 0.2% by weight or less Si: 2% by weight or less Mn: 3% by weight or less Cr: 9 to 25% by weight Ni: 12 to 45% by weight Al: More than 5% to 8% by weight Ca, Y, one or more rare earth elements:
A steel material for heat exchangers, containing 0.1% by weight or less of each element, and having a metal structure mainly in an austenite phase. 2 C: 0.2% by weight or less Si: 2% by weight or less Mn: 3% by weight or less Cr: 9 to 25% by weight Ni: 12 to 45% by weight Al: More than 5% to 8% by weight Ca, Y, 1 of rare earth elements Species or two or more species:
0.1% by weight or less of each element, 5% by weight or less of Mo, 5% by weight or less of W, and
One or more types of Co, 10% by weight or less: 15 in total
% by weight or less, and the metal structure is mainly an austenite phase. 3 C: 0.2% by weight or less Si: 2% by weight or less Mn: 3% by weight or less Cr: 9 to 25% by weight Ni: 12 to 45% by weight Al: More than 5% to 8% by weight Ca, Y, 1 of rare earth elements Species or two or more species:
A steel material for a heat exchanger, containing 0.1% by weight or less of each element and 5% by weight or less of Cu, and having a metal structure mainly in an austenite phase. 4 C: 0.2% by weight or less Si: 2% by weight or less Mn: 3% by weight or less Cr: 9 to 25% by weight Ni: 12 to 45% by weight Al: More than 5% to 8% by weight Ca, Y, 1 of rare earth elements Species or two or more species:
0.1% by weight or less, 5% by weight or less of Mo, 5% by weight or less of W, and
10% by weight or less of one or more types of Co: total
A steel material for a heat exchanger, containing 15% by weight or less of Cu: 5% by weight or less, and having a metal structure mainly in an austenite phase.