KR20130137705A - Heat-resistant austenitic stainless steel having excellent cyclic oxidation resistance - Google Patents

Abstract

Heat-resistant austenitic stainless steel of the present invention is C: 0.05 to 0.2%, Si: 0.1 to 1%, Mn: 0.1 to 2.5%, Cu: 1 to 4%, Ni: 7 to 12%, Cr: 16 to 20%, Nb: 0.1 to 0.6%, Zr: 0.05 to 0.4%, Ce: 0.005 to 0.1%, Ti: 0.1 to 0.6%, B: 0.0005 to 0.005%, N: 0.001 to 0.15%, S: 0.005% or less (Which does not contain 0%) and P: 0.05% or less (does not contain 0%), respectively, and the remainder is made of iron and unavoidable impurities.

Description

Heat-Resistant Austenitic Stainless Steels with Excellent Repetitive Oxidation Properties {HEAT-RESISTANT AUSTENITIC STAINLESS STEEL HAVING EXCELLENT CYCLIC OXIDATION RESISTANCE}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to heat resistant austenitic stainless steels suitably used as heat transfer tube materials such as boilers, and more particularly to heat resistant austenitic stainless steels having excellent repeated oxidation resistance.

In recent years, in order to suppress the emission of carbon dioxide, which is a warming gas, high efficiency of thermal power generation by coal has been progressing. In order to improve this power generation efficiency, an increase in the steam temperature and the pressure of the boiler is effective, and as the heat transfer tube material of the boiler, one having excellent high temperature strength and oxidation resistance is applied. In addition, austenitic stainless steel is generally used as a material having such excellent characteristics.

The oxidation resistance required for the heat transfer tube material includes repeated oxidation resistance. Since the boiler starts and stops repeatedly, the oxide formed on the surface of the steel pipe (heat pipe) is placed in a cyclic oxidizing environment that receives alternating high and low temperature environments. Under such circumstances, there is a problem that the oxide is peeled off due to the difference in thermal expansion with the material base material, and further the oxidation progresses due to scale peeling and the lack of strength due to the reduction in the thickness of the steel pipe occurs. Even in such an environment, the above-described phenomenon is unlikely to occur (in the present invention, this is referred to as "repeated oxidation resistance").

As a heat resistant material having excellent oxidation resistance in a broad sense including properties other than repeated oxidation resistance, 25Cr-20Ni austenitic stainless steel (SUS310S) is known, but this stainless steel contains a large amount of expensive Ni. Therefore, there is a problem that the cost is high. For this reason, as a heat transfer tube material of a boiler, it is an important requirement to keep Ni content low and to make 18Cr-8Ni austenitic stainless steel (SUS304) which has high temperature strength and corrosion resistance as a basic component.

As a component close to 18Cr-8Ni austenitic stainless steel, a component system of SUS321 to which Ti is added is known, and fire SUS321J2HTB is known as a boiler stainless steel having a thermal power generation standard that conforms to the component system of SUS321. As a technique for improving oxidation resistance in the broad sense, REM (rare earth elements) containing (1) surface treatment such as shot peening or mechanical polishing, and (2) Al, Si, Ce, and La, which are components that improve corrosion resistance ), (3) crystal grain refinement, and the like, and related technologies such as Patent Documents 1 and 2 have been proposed as related to austenitic stainless steels using Ti compounds as precipitation strengthening mechanisms.

Patent Document 1 of these techniques discloses that oxidation resistance is improved by adding Al, which contributes to improvement of corrosion resistance, and promoting formation of a Cr ₂ O ₃ layer by surface polishing. In addition, as an alternative means of obtaining the same effect as the surface polishing treatment, it is possible to increase the total amount of Al and Si to 4% or more, and to further improve oxidation resistance by adding REM or Ca such as Ce, Y or La. It is described.

However, by adding Al and Si or forming a Cr ₂ O ₃ layer, the effect of retarding the growth rate of the oxide formed on the surface of the steel pipe can be expected. However, the formation of the oxide itself can not be completely prevented. Good exertion of repeated oxidation cannot be expected. In addition, in steel materials to which Al is added, there is also a problem that surface damage is likely to occur at the time of pipe making.

Patent Document 2 discloses adding Ce, La, and Hf in order to improve the oxidation resistance, but similarly to the above technique, it is expected that the resistance to repeated oxidation is low, and the improvement of the resistance to repeated oxidation is recognized. It is not made by.

As a technique for improving the resistance to repeated oxidation, a technique similar to that of Patent Document 3 is also proposed. However, in this technique, since Al and Si are contained a lot, there is a problem of causing surface damage of the steel pipe or embrittlement after a long heat treatment. In addition, in this technique, it is described that adding REM such as La, Ce, etc. to Y, exhibits the effect of improving the adhesion of the scale, but does not have sufficient characteristics and recognizes the improvement of the repeated oxidation characteristics. It is not made by.

On the other hand, as a technique of improving the oxidation resistance of the austenitic stainless steel for boilers, the technique similar to patent document 4 is also proposed. This technique is a component system of "fire SUS304J1HTB" which uses Nb and N for precipitation strengthening and solid solution strengthening. Also in this technique, about 0.002 to 0.05% of Ti is added for the purpose of forming oxide inclusions, but in steels using the precipitation of Ti compounds such as fire SUS321J2HTB as a reinforcing mechanism, Ti is about 0.1 to 0.25%. If it is not added, it is anticipated that high temperature strength cannot be ensured. In addition, since this technique was not made in recognition of the improvement of the resistance to repeated oxidation, it is expected that the resistance to repeated oxidation is low.

In the technique of patent document 5, oxidation resistance is improved by addition of REM and particle injection pinning. However, there is another problem that pinning causes a high cost due to an increase in the manufacturing process, and it is not made in recognition of the improvement of the resistance to repeated oxidation, so that the resistance to repeated oxidation is expected to be low.

Japanese Patent Application Publication No. 2004-43903 Japanese Patent Application Laid-open No. Hei 9-165655 Japanese Patent Application Laid-open No. Hei 8-337850 Japanese Patent Application Publication No. 2003-268503 Japanese Patent Application Laid-open No. Hei 6-322489

The present invention has been made under such circumstances, and its object is that the content of Ni and Cr has a chemical composition equivalent to that of 18Cr-8Ni austenitic stainless steel, and does not depend on the addition of Al or Si or the surface treatment, and it is repeated oxidation. The present invention provides a heat-resistant austenitic stainless steel having low peeling of oxides in the environment and excellent in repeated oxidation characteristics in which thickness reduction is unlikely to occur.

The heat-resistant austenitic stainless steel of this invention which solved the said subject is C: 0.05-0.2% (mean of mass%. Hereinafter, it is the same with respect to a chemical component composition), Si: 0.1-1%, Mn: 0.1- 2.5%, Cu: 1-4%, Ni: 7-12%, Cr: 16-20%, Nb: 0.1-0.6%, Zr: 0.05-0.4%, Ce: 0.005-0.1%, Ti: 0.1-0.6 %, B: 0.0005 to 0.005%, N: 0.001 to 0.15%, S: 0.005% or less (not including 0%) and P: 0.05% or less (not including 0%), respectively, It is characterized by consisting of iron and unavoidable impurities.

The heat resistant austenitic stainless steel of the present invention is also useful, if necessary, further containing Mo: 3% or less (not including 0%) and / or W: 5% or less (not including 0%). By containing these components, the high temperature strength is further improved.

The heat-resistant austenitic stainless steel of this invention further contains Ca: 0.005% or less (0%) and / or Mg: 0.005% or less (0%) as needed, The yield can be improved while the toughness can be improved.

By adjusting the chemical composition as described above, a heat-resistant austenitic stainless steel having improved repeat oxidation resistance is obtained, but also higher repeat oxidation by setting the crystal grain size of the metal structure to 6 or more and less than 12 in the ASTM grain size number. A characteristic can be obtained and it can be exhibited stably.

In the heat-resistant austenitic stainless steel of the present invention, even in a repeated oxidation environment, since the progress of oxidation due to the peeling of the scale and the decrease in the thickness of the steel material accompanying it hardly occur, the steam temperature is used as a heat transfer tube for coal-fired power generation. It is possible to improve the power generation efficiency by increasing the temperature of heat, and to reduce the maintenance cost by extending the life of the heat pipe compared to the existing material. Moreover, since there is little peeling of a scale, when used as a heat exchanger, the scale scattering inside can be suppressed and damage to a turbine can also be reduced.

MEANS TO SOLVE THE PROBLEM The present inventors examined from various angles, in order to implement | achieve an austenitic stainless steel which improved repetitive oxidation resistance, maintaining the required high temperature strength. As a result, it was found that when a predetermined amount of Zr and Ce is contained in a stainless steel having a chemical composition of Ni and Cr equivalent to that of 18Cr-8Ni austenitic stainless steel, very excellent repeated oxidation resistance can be exhibited. The present invention has been completed.

The heat-resistant austenitic stainless steel of the present invention is characterized by containing Zr and Ce in a predetermined amount with respect to the chemical composition of Ni and Cr equivalent to that of 18Cr-8Ni austenitic stainless steel. The reason for setting the range of Ce content is as follows.

Zr and Ce express an effect which suppresses peeling of an oxide by these synergistic effects. In order to exhibit such an effect, it is necessary to contain 0.05% or more about Zr. However, when the Zr content becomes excessive, coarse inclusions are formed to deteriorate the surface properties and toughness of the steel (or steel pipe), so the upper limit needs to be 0.4% or less. Moreover, about Ce, it is necessary to contain 0.005% or more in order to exhibit the effect. However, when the Ce content is excessively more than 0.1%, economic cost increases.

Since addition of Zr and Ce brings high cost of steel materials, what is necessary is just to set appropriate content by the balance of the effect by containing and high cost. From such a viewpoint, the minimum with preferable Zr content is 0.10% or more (more preferably 0.15% or more), and a preferable upper limit is 0.3% or less (more preferably 0.25% or less). Moreover, the minimum with preferable Ce content is 0.01% or more (more preferably 0.015% or more), and a preferable upper limit is 0.05% or less (more preferably 0.03% or less).

In addition, although the raw material of Ce may add pure Ce, it is also possible to add necessary Ce pure components using the master alloy containing Ce and the misch metal containing Ce, and La, Nd contained in Misch metal. Even if Pr and the like were contained in the steel as impurities at a lower concentration than Ce, respectively, there is no problem, and it is possible to simplify the handling during the dissolution operation by using a master alloy or misch metal, compared to pure Ce, which is easily oxidized.

Moreover, although the patent document 1, 3, 5 is disclosed in patent document 1, 3, 5 by adding REM containing Y, La, and Ce, the adhesiveness of oxide improves, but these indications assume that REM is single addition alone. The synergistic effect of adding Ce together with Zr is not disclosed at all.

Moreover, although the said patent document 2 also discloses that Zr and Ce can be used together, it also discloses that it is not an essential component in this technique, and all are added as needed including non-addition, especially Zr is grain boundary strengthening and creep. It is contained less than the range prescribed | regulated by this invention in anticipation of the ductility improvement.

The heat-resistant austenitic stainless steel of the present invention has a chemical composition of Ni and Cr equivalent to that of 18Cr-8Ni austenitic stainless steel, but the chemical composition of each element other than Zr and Ce (C, Si, Mn). , Cu, Ni, Cr, Nb, Ti, B, N, S, P) also needs to be adjusted appropriately. The action of these components and the reason for setting the range are as follows.

[C: 0.05-0.2%]

C is an element having a function of forming carbide in a high temperature use environment and improving the high temperature strength and creep strength required as a heat transfer tube, and it is necessary to contain C at least 0.05% in order to secure the amount of precipitation of the carbide serving as a reinforcing mechanism. have. However, when C content becomes excess and exceeds 0.2%, it will become coarse carbide exceeding the solid solution limit, and further strengthening will not be obtained. The minimum with preferable C content is 0.07% or more (more preferably 0.09% or more), and a preferable upper limit is 0.18% or less (more preferably 0.15% or less).

[Si: 0.1 to 1%]

Si is an element having a deoxidation action in molten steel. Moreover, even if it contains a trace amount, it acts effectively for the improvement of oxidation resistance. In order to exhibit these effects, Si content needs to be 0.1% or more. However, when Si content becomes excess and exceeds 1%, it will cause formation of an sigma phase, and will cause brittleness ((sigma) embrittlement) of steel materials. The minimum with preferable Si content is 0.2% or more (more preferably 0.3% or more), and a preferable upper limit is 0.9% or less (more preferably 0.8% or less).

[Mn: 0.1 to 2.5%]

Mn, like Si, is an element having a deoxidation action in molten steel and has a function of stabilizing austenite. In order to exhibit these effects, Mn content needs to be 0.1% or more. However, when Mn content becomes excess and exceeds 2.5%, hot workability will be inhibited. The minimum with preferable Mn content is 0.2% or more (more preferably 0.3% or more), and a preferable upper limit is 2.0% or less (more preferably 1.8% or less).

[Cu: 1 to 4%]

Cu is an element which forms a matching precipitate (precipitate with a base material and an atomic arrangement continuous) in steel, and remarkably improves high temperature creep strength, and is one of the major reinforcing mechanisms in stainless steel. In order to exhibit this effect, Cu content needs to be 1% or more. However, even if Cu content becomes excess and exceeds 4%, the effect will be saturated. The minimum with preferable Cu content is 2.0% or more (more preferably 2.5% or more), and a preferable upper limit is 3.7% or less (more preferably 3.5% or less).

[Ni: 7 to 12%]

Ni has an effect of stabilizing austenite, and in order to maintain the austenite phase, it is necessary to contain Ni 7% or more. However, when Ni content becomes excess and exceeds 12%, cost will increase. The minimum with preferable Ni content is 7.5% or more (more preferably 8.0% or more), and a preferable upper limit is 11.5% or less (more preferably 11.0% or less).

[Cr: 16-20%]

Cr is an essential element in order to express corrosion resistance as stainless steel. In order to exhibit such an effect, it is necessary to contain Cr 16% or more. However, when Cr content becomes excess and exceeds 20%, the ferrite phase which causes the fall of high temperature strength will increase. The minimum with preferable Cr content is 16.5% or more (more preferably 17.0% or more), and a preferable upper limit is 19.5% or less (more preferably 19.0% or less).

[Nb: 0.1 to 0.6%]

Nb is an element effective for improving the high temperature strength by depositing carbonitride (carbide, nitride or carbonitride), and furthermore, the precipitate suppresses coarsening of crystal grains and promotes diffusion of Cr, thereby additionally improving corrosion resistance. It works. In order to secure the required amount of precipitation, it is necessary to contain Nb 0.1% or more. However, when Nb content exceeds 0.6% and becomes excessive, a precipitate will coarsen and will lead to fall of toughness. The minimum with preferable Nb content is 0.12% or more (more preferably 0.15% or more), and a preferable upper limit is 0.5% or less (more preferably 0.3% or less).

[Ti: 0.1 to 0.6%]

Ti also has the same effect as Nb, but by adding it in combination with Nb and Zr, the precipitate is further stabilized and is effective for maintaining long-term high temperature strength. In order to exhibit such an effect effectively, Ti content needs to be 0.1% or more. However, when Ti content becomes excess, since precipitate precipitates coarsening similarly to the case of Nb, and will fall toughness, it is necessary to be 0.6% or less. The minimum with preferable Ti content is 0.12% or more (more preferably 0.15% or more), and a preferable upper limit is 0.5% or less (more preferably 0.3% or less).

[B: 0.0005 to 0.005%]

B has a function of promoting the formation of M ₂₃ C ₆ type carbide (M is a carbide forming element), which is one of the main reinforcing mechanisms, by solid solution in steel. In order to exhibit these effects effectively, the B content needs to be 0.0005% or more. However, when the B content is excessive, the hot workability and the weldability are deteriorated, so it is necessary to be 0.005% or less. The minimum with preferable B content is 0.001% or more (more preferably 0.0012% or more), and a preferable upper limit is 0.004% or less (more preferably 0.003% or less).

[N: 0.001 to 0.15%]

N has an effect of improving the high temperature strength by solid solution strengthening by solid solution in steel, and is an element effective in improving the high temperature strength by forming nitride with Cr or Nb under a long time high temperature load. In order to exhibit these effects effectively, the N content needs to be 0.001% or more. However, when N content becomes excess and exceeds 0.15%, coarse Ti nitride and Nb nitride will be formed and toughness will deteriorate. The minimum with preferable N content is 0.002% or more (more preferably 0.003% or more), and a preferable upper limit is 0.10% or less (more preferably 0.08% or less, More preferably, 0.02% or less).

[S: 0.005% or less (not including 0%)]

Although S is an unavoidable impurity, it is necessary to be 0.005% or less because its content deteriorates hot workability. In addition, since S impairs the effect of adding Ce by fixing Ce as a sulfide, it is preferable to suppress it to 0.002% or less (more preferably 0.001% or less).

[P: 0.05% or less (not including 0%)]

Although P is an unavoidable impurity, it is necessary to make it 0.05% or less because its content deteriorates weldability. Preferably it is suppressed to 0.04% or less (more preferably 0.03% or less).

The contained elements defined in the present invention are as described above, the remainder being iron and unavoidable impurities, and in addition to La, Nd, Pr, etc., which are contained at a lower concentration than Ce when the Ce raw material is added to the misch metal, raw materials, materials, manufacturing facilities, etc. Depending on the situation, the incorporation of the imported element may be allowed. However, low-melting-point impurity metals such as Sn, Pb, Sb, As, and Zn derived from the scrap raw material lower the strength of the grain boundary during hot working or when used in a high temperature environment. In order to improve a crack, it is desirable to suppress it at low concentration. Moreover, the steel material of this invention may contain Mo, W, Ca, Mg, etc. as needed, and the characteristic of a steel material improves further according to the kind of element to contain.

[Mo: 3% or less (not including 0%) and / or W: 5% or less (without 0%)]

Mo and W have the effect of improving the high temperature strength by solid solution strengthening, and the content of Mo and W can be further increased by containing as necessary. However, when the Mo content becomes excessive, hot workability is inhibited, so it is preferable to set it as 3% or less. More preferably, it is 2.5% or less (more preferably 2.0% or less). In addition, when the W content is excessive, coarse intermetallic compounds are formed to cause high temperature ductility to be lowered. Therefore, the W content is preferably 5% or less. More preferably, it is 4.5% or less (more preferably 4.0% or less). Moreover, the minimum with preferable for exerting such an effect effectively is 0.1% or more (more preferably 0.5% or more) in Mo, and is 0.1% or more (more preferably 1.0% or more) in W. However, by containing these elements, the above-described effect is exerted, but at the same time, the cost is increased, so the content may be set in accordance with the required reinforcement amount and the allowable cost.

[Ca: 0.005% or less (not including 0) and / or Mg: 0.005% or less (not including 0)]

Since Ca and Mg act as desulfurization and deoxidation elements, the formation of Ce sulfide and Ce oxide can be suppressed to improve the yield of Ce and to suppress the decrease in toughness due to inclusion formation. The minimum with preferable minimum for exerting such an effect effectively is 0.0002% or more, More preferably, it is 0.0005% or more. However, when these contents become excess, since the operational restrictions, such as bumping of molten steel generate | occur | produce during a melt | dissolution operation, were made, all the upper limits were made 0.005% or less. More preferably, they are all 0.002% or less.

The heat-resistant austenitic stainless steel of the present invention can improve the repeated oxidation resistance by containing a predetermined amount of Zr and Ce, but in order to further improve the characteristics, it is effective to control the grain size of the metal structure. . From this point of view, it is preferable that the crystal grain size of the metal structure of the heat-resistant austenitic stainless steel be a microstructure of 6 or more and less than 12 in ASTM (American Society for Testing and Materials) particle size number. The particle size number (crystal particle size number) is determined by ASTM and means a particle size number calculated by a planimetric method.

If the crystal grain size of the metal structure is less than 6 as the ASTM particle size number, the effect of improving the repeated oxidation resistance by containing Zr and Ce is obtained, but the improvement effect cannot be sufficiently increased. This particle size number is more preferably 7 or more, and still more preferably 9 or more. On the other hand, in the steel-making process by hot / cold processing and heat processing, since an extremely fine grain structure cannot be produced substantially, it is preferable to set the upper limit of crystal grain size to less than 12. When manufacturing cost and productivity are considered, it is more preferable that it is 10 or less.

The crystal grain size range as described above is obtained by adjusting the addition amount of the component contributing to the pinning of the grain boundary, and the conditions of hot and cold working and heat treatment such as drawing and extrusion in the tube making process. The optimum conditions vary depending on these three factors. However, in order to refine the crystal grain size, it is necessary to increase the addition amount of the precipitated element, increase the workability, and lower the heat treatment temperature. Cold and hot working are aimed at aligning the grain structure in the heat treatment after processing by introducing thickness adjustment and deformation, and are usually performed at a cross-sectional reduction rate of 30% or more. In addition, the heat processing aims at removing distortion, and is implemented in the temperature range of about 1000 degreeC or more and less than 1300 degreeC. For example, when the reduction ratio of the cross section is about 35%, the prescribed particle size range can be obtained by setting the heat treatment temperature to 1250 ° C or lower, preferably 1225 ° C or lower, particularly preferably 1150 ° C or lower. ㆍ It is not limited to this condition depending on the balance of heat processing.

By using heat-resistant austenitic stainless steel as described above, a heat exchanger tube for a boiler is formed, thereby exhibiting excellent characteristics under a repeated oxidation environment.

Hereinafter, the present invention will be described more specifically by way of examples. This invention is not restrict | limited by the following example, Of course, it is also possible to carry out by changing suitably in the range which may be suitable for the meaning mentioned later, and they are all included in the technical scope of this invention.

Example

Example 1

The various steel materials which consist of the chemical composition shown in following Table 1 are melt | dissolved, 20 kg ingot melt | dissolved in the vacuum melting furnace (VIF) is hot-forged-processed by the dimension of width: 120 mm x thickness: 20 mm, and heat-processed at 1250 degreeC. After implementation, it was processed to thickness: 13 mm by cold rolling. Thereafter, heat treatment was performed at 1150 ° C. for 5 minutes again to form a base material. The steel material of 20 mm x 30 mm x 2 mm was cut out from this base material by machining, the surface of the steel material was smoothed and mirror-hardened by the grinding | polishing using the emery paper, and the buffing | polishing using the diamond abrasive grain, and the test piece was produced.

In addition, among the steels shown in Table 1, Test Nos. 1 to 10 are steels (steel of the present invention) that satisfy the requirements specified in the present invention, and Test Nos. 11 to 16 are steels that deviate from the requirements specified in the present invention. Comparative steel), and tests No. 14, 15, and 16 are the existing steels "Fire SUS304J1HTB equivalent steel", "SUS304L equivalent steel", and "SUS310S equivalent steel", respectively. In addition, Test Nos. 7 and 8 are steel materials in which Ce is added as a misch metal, and La, Pr, Nd, etc. are contained as impurities. Test No. 9 and 10 are steel materials which added Mg and Ca, respectively.

Said "fire SUS304J1HTB equivalency steel" (test No. 14) belongs to 18Cr-8Ni austenitic stainless steel, and is a steel grade which has been used as a boiler heat exchanger tube (for example, "MATERIA Japan" 46 Vol. 2, 2007, P99-101). In addition, SUS310S equivalent steel (test No. 16) belongs to 25Cr-20Ni austenitic stainless steel and is expensive because it contains more Ni than 18Cr-8Ni austenitic stainless steel. Corrosion resistance is better than 18Cr-8Ni austenitic stainless steel.

In order to evaluate the thickness reduction amount, the repeated oxidation test was done using the various test pieces obtained above. In this repeated oxidation test, the sample was taken out from the atmosphere furnace of 1100 degreeC by the cycle of 25 minutes of furnace heating and 5 minutes of air cooling, and heating and cooling were repeated to 20 cycles. After the repeated oxidation test, the weight change of the test piece was measured by electronic balance, and the thickness reduction amount (mg · cm ⁻² ) of the steel was calculated. In addition, the surface roughness of the test piece after the repeated oxidation test was observed by visual observation.

The measurement results (thickness reduction amount, surface roughness) are shown in Table 2 below.

From this result, it can be considered as follows. Steels satisfying the chemical component composition specified in the present invention (steel of the present invention: Test Nos. 1 to 10) are conventional steels (test Nos. 14 and 15) or comparative steels deviating from the chemical component composition specified in the present invention ( It turns out that the amount of thickness reduction is small compared with Test No.11-13), scale peeling hardly arises by the composite addition of Zr and Ce, and thickness reduction amount can be suppressed.

Further, it can be seen that generation and peeling of scale did not occur even when the roughness of the scale surface was smooth in the steel of the present invention. The steel of the present invention exhibits properties equivalent to that of 25Cr-20Ni conventional steel SUS310S equivalent steel (test No. 16), which is considered to be excellent in corrosion resistance due to its high Ni content. Nevertheless, it can be seen that the repeated oxidation resistance can be improved to be equivalent to that of 25Cr-20Ni austenitic stainless steel.

[Example 2]

For the inventive steels of Test Nos. 1 to 6 shown in Tables 1 and 2 and the comparative steel of Test No. 14, the heat treatment temperature was changed to a temperature range of 1125 to 1275 ° C after cold working at a section reduction rate of 35%, respectively. A sample having a crystal grain size number of 4.5 to 10.0 was made of steel. The repeated oxidation test is carried out with a temperature cycle of 25 minutes of heating in the furnace and 5 minutes of atmospheric cooling, and the sample is taken out from the atmosphere furnace at 1100 ° C., and the mass reduction amount (thickness reduction amount: mg Cm- ² ) was obtained.

Regarding the number of cycles, the reduction in thickness was greatly improved in a part of the steel to which Zr and Ce were added, and the heating and cooling were repeated up to 40 cycles because the thickness reduction after 20 cycles was an error depending on the particle size. Calculation of the crystal grain size was carried out at 3 o'clock per steel class.

The said measurement result (thickness reduction amount) is shown in following Table 3 with crystal grain size.

From this result, it can be considered as follows. A sample having a crystal grain size number of 6 or more is an invention example satisfying the provisions of the present invention up to the crystal grain size in addition to the chemical composition, and a sample of less than 6 satisfies the chemical composition but does not satisfy the crystal grain size. Underlining). As shown in the results of the comparative steel of Test No. 14, in the steel that deviates from the chemical composition of the present invention, the amount of decrease in thickness hardly changes even when the crystal grain size changes, but in the inventive steels of Test Nos. 1 to 6, the crystal grain size It is understood that the larger the number is, the smaller the thickness reduction tends to be. In addition, since all invention steels having different crystal grain sizes can reduce the amount of thickness reduction than the existing steel of Test No. 14, the addition of Zr and Ce itself improves the resistance to repeated oxidation and the chemical composition Even within the range defined by the invention, it can be seen that the finer the grain size, the better the characteristics.

In view of the particle size dependence of the steels of the present invention Nos. 1 to 6, although there is a characteristic difference as an absolute value due to the content of Zr and Ce in each steel grade, when the grain size number is 6 or more compared to less than 6 in any steel grade, It turns out that it becomes a high repetitive oxidation characteristic and especially the remarkable improvement effect is acquired in 7 or more and 9 or more particle sizes. In other words, by using a steel material satisfying the composition range of the present invention, it is possible to improve the repeated oxidation resistance, but it can be seen that by adjusting the crystal grain size, the effect can be further enhanced, and excellent repeated oxidation characteristics can be stably obtained. .

Although this invention was detailed also demonstrated with reference to the specific embodiment, it is clear for those skilled in the art that various changes and correction can be added without deviating from the mind and range of this invention.

This application is a Japanese patent application (Japanese Patent Application No. 2011-106588) filed on May 11, 2011, a Japanese Patent Application (Japanese Patent Application No. 2011-203604) filed on September 16, 2011, It is based on the Japanese patent application (Japanese Patent Application No. 2012-048357) for which it applied on March 5, 2012, The content is taken in here as a reference.

The heat-resistant austenitic stainless steel of this invention is used suitably as heat exchanger tube materials, such as a boiler.

Claims (3)

C: 0.05 to 0.2% (meaning of mass%, which is the same with respect to chemical component composition), Si: 0.1 to 1%, Mn: 0.1 to 2.5%, Cu: 1 to 4%, and Ni: 7 to 12% , Cr: 16-20%, Nb: 0.1-0.6%, Zr: 0.05-0.4%, Ce: 0.005-0.1%, Ti: 0.1-0.6%, B: 0.0005-0.005%, N: 0.001-0.15%, S: 0.005% or less (does not contain 0%) and P: 0.05% or less (does not contain 0%), respectively, and the residual amount consists of iron and inevitable impurities, Excellent heat-resistant austenitic stainless steel. The method of claim 1,
A heat-resistant austenitic stainless steel excellent in repeated oxidation resistance, further containing at least one of the following elements.
Mo: 3% or less (does not contain 0%)
W: 5% or less (does not include 0%)
Ca: 0.005% or less (does not contain 0%)
Mg: 0.005% or less (does not contain 0%) 3. The method according to claim 1 or 2,
Heat-resistant austenitic stainless steel having excellent repeated oxidation resistance, wherein the grain size of the metal structure is 6 or more and less than 12 in ASTM particle size number.