JP3149687B2 - Stainless steel with excellent oxidation resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat-resistant stainless steel which constitutes a non-pressure-resistant member of a boiler for thermal power generation, such as a non-pressure-resistant and non-cooling member, which is required to have repeated oxidation resistance.

[0002]

2. Description of the Related Art In recent years, with the progress of high-temperature plant technology, a new type of combined cycle power plant, namely, a coal gasification combined plant, PFBC (Pressurized Fluidized Bed Combusio) has been developed.
n, that is, a pressurized fluidized bed combustion) power generation system, a topping cycle and the like have been proposed, and a test plant has been operated. In these new-type plants, unlike conventional boilers, the range of non-cooled components exposed to high temperatures besides steam heating tubes has been greatly expanded, and non-cooled components that can be used at up to about 900 ° C are used. There is also a new need for heat-resistant steel.

Hitherto, as a high-temperature member, steel types mainly focused on high-temperature strength have been actively developed, and 18Cr-8Ni-based stainless steel has been widely used as a tube material. The mainstream is JIS SUS304H (18Cr-10Ni-C), and there is also JIS SUS321H with Ti added to it. Basically, 18-8 series is considered to have long-term structural stability. It was possible.

[0004]

However, in general, the parts such as the non-cooling part in a new type plant have a high working temperature and a steel type that can be used even when designed as a non-pressure-resistant member stored in a pressure vessel. I can hardly find it. 18
If the solution is to be solved with -8 series heat-resistant steel, see JP-A-53-63.
It is also conceivable to use a Cr-Ni stainless steel containing Al and Si, which are elements for improving high-temperature oxidation resistance disclosed in, for example, Japanese Patent Publication No. 210-210. However, such steel grades are required as structural members in a plant requiring a certain structural strength, such as measures against repeated oxidation characteristics and maintenance of long-term phase stability.
In this respect, consideration is not sufficient.

[0005] Here, the term "repetitive oxidizing property" refers to a phenomenon in which when a material is repeatedly subjected to a thermal cycle by starting and stopping a power plant, high-temperature oxidation at 800 to 900 ° C promotes a reduction in thickness of a steel material. The conventional Cr-Ni stainless steel containing Al and Si has a problem that the repetitive oxidation resistance is poor. In addition, the expensive 25Cr-20Ni-based austenitic stainless heat-resistant steel represented by JIS SUS310S, which is currently being used for this purpose, easily precipitates a sigma phase when used at 800 to 900 ° C for 200 to 700 hours. However, it is feared that cracking may occur when hammering is performed during periodic inspections. Therefore, at present, in the high-temperature material technology, it is strongly desired to establish an alloy design guideline that simultaneously satisfies the two points of such resistance to repeated oxidation and long-term phase stability.

[0006]

Means for Solving the Problems The present inventors consider that it is essential to consider repetitive oxidation resistance when the material life in actual plant operation is determined by high-temperature oxidation. Various alloys were trial-produced starting from 18Cr-8Ni-based stainless steel, which is excellent in quality, and performance evaluation was repeated. As a result, unlike the case of simple heating, the elemental composition of the surface scale layer became important for repeated oxidation resistance. Was obtained. That is, it has been found that Si, which is often used as an inexpensive element for improving oxidation resistance, is rather harmful, while increasing the content of Al or increasing Cr is effective.

Further, as a result of a detailed investigation of the state of the surface scale layer, this scale is an oxide such as Cr, Al, or Si. When all these elements are simultaneously contained, Cr is present on the environmental side. It was found that the material side, that is, the mother phase side was an oxide enriched in Al or Si. Therefore, it is presumed that the difference in the effect of these elements on the repetitive oxidation resistance is caused by the composition of the scale, and particularly due to the difference in thermal expansion between the scale and the matrix.

For example, the average expansion coefficient of SiO ₂ from room temperature to 1000 ° C. is as low as 0.5 × 10 ⁻⁶ K ⁻¹ , but the average expansion coefficient is Al ₂ O _3.
Has an average expansion coefficient of 8 × 10 ⁻⁶ K ^−1, which is relatively close to the value of 20 × 10 ⁻⁶ K ⁻¹ of ordinary stainless steel. Therefore, even if a difference occurs in the thermal stress between the mother phase and the interface at the time of temperature change, the Al ₂ O ₃
A mechanism is conceivable in which the film is hardly peeled off and falls off, so that the property of protecting the inner material is maintained.

On the other hand, with respect to the phase stability when subjected to long-time heating, various means for mainly suppressing precipitation of a brittle second phase such as a sigma phase were examined. Generally, regarding the delta ferrite suppression or the amount of delta ferrite in austenitic stainless steel, for example, de Long
Cr equivalent, Ni as proposed in tal Progress
It has been found that the equivalent concept can be used. However, the above equation could not be diverted as it is for the prediction of the amount of sigma phase precipitation, but it was experimentally determined that the critical point of sigma phase precipitation is when both sides of the following equation (3) are equal. Newly found. σ = 3.6 (1.5Si + 3Al + Cr)-(0.5Mn + Ni + 30C + 30N) = 70- (3)

The present invention has been made based on the above-mentioned viewpoints, and is excellent in economic efficiency and superior to long-term tissue stability.
800-900 ° C based on Cr-8Ni stainless steel
An object of the present invention is to provide a heat-resistant steel excellent in high-temperature oxidation resistance and repetitive oxidation resistance in steel. (1) The invention of claim 1 is a stainless steel having the following component composition and excellent in repeated oxidation resistance (component composition is wt.
%). (A) As main components, C: 0.12% or less, Si: 0.31 to 1.0% Mn: 2.0% or less, P: 0.04% or less S: 0.03% or less, Al: 0.33 to 3.0% , N: 0.2% or less, Ni : 10 to 33%, Cr: 14% to 28%, the balance being Fe and unavoidable impurities. (B) The component composition satisfies the following formulas (1) to (3). R = Cr + 2 (Al + Si) ≧ 20.2-(1) α = (1.5Si + 3Al + Cr)-(0.5Mn + Ni + 30C + 30N) <9- (2) σ = 3.6 (1.5Si + 3Al + Cr)-(0.5Mn + Ni + 30C + 30N) <70- (3)

(2) The invention of claim 2 provides the component composition of the invention of claim 1 with a repetitive oxidation resistance characteristic in which at least one of Ce, Y, and La contains 0.07% or less in total. Excellent stainless steel.

[0012]

The reasons for containing the elements contained in the stainless steel according to the present invention and the reasons for limiting the content range will be described below. C is an element which gives the high-temperature strength of the parent phase of the steel of the present invention and is effective for phase stability. However, if it is contained in excess of 0.12%, coarse carbides precipitate in the form of traversing the crystal grains. Its content is 0.
12% or less.

Since Si is an element effective for deoxidation, 1.0%
The following may be included, but if the content exceeds 1.0%, the mother phase side of the oxide film on the surface becomes Si-rich, which is harmful to repeated oxidation characteristics. Therefore, the content is set to 1.0% or less.

Mn is an element effective for phase stability.
The content may be 0% or less, but if the content exceeds 2.0%, it is harmful to high-temperature oxidation resistance. Therefore, the content is set to 2.0% or less.

P is an element that segregates at the grain boundaries and impairs the ductility during rolling. The smaller the content, the better. Therefore, in order to prevent cracking due to a decrease in ductility during rolling, the content is set to 0.04% or less.

S, like P, is an element that segregates at grain boundaries and impairs ductility during rolling. The smaller the content, the better. Therefore, in order to prevent cracks due to a decrease in ductility during rolling, the content is set to 0.03% or less.

N, like C, imparts the high-temperature strength of the parent phase of the steel of the present invention and is an element effective for phase stability. Therefore, N may contain up to 0.20% or less. A substance is formed and is harmful to the toughness, so the content is set to 0.20% or less.

Ni is an essential element for obtaining a stable austenite structure. The content is required to be 10% or more in view of the relationship between other contained elements, particularly, Cr and Al. On the other hand, if the Ni content exceeds 33%, the effect of stabilizing austenite decreases, and even if the Ni content is increased, the content of elements that improve high-temperature oxidation resistance, such as Cr, cannot be greatly increased. The content was set to 10% or more and 33% or less.

[0019] Cr is important as a basic element that provides oxidation resistance at high temperatures. If the content is less than 14%, sufficient oxidation resistance cannot be obtained at 800 ° C. to 900 ° C. even if Al which is effective for high-temperature oxidation resistance is contained. Meanwhile, 28
If the content exceeds 0.1%, a large amount of expensive Ni is required to maintain the stability of the austenite phase, which impairs economic efficiency. In addition, the contribution to the improvement of high-temperature oxidation resistance is reduced, so that the content is 14%. % And 28% or less.

Al alone, in an oxidizing environment, Al ₂ O ₃
A very dense oxide film is formed, and in the presence of Cr oxide, it is contained therein as a composite oxide to increase the denseness of the oxide. In such a case, the element is not only extremely high in surface protection but also hardly peels off even under repeated thermal stress, and gives excellent resistance to repeated high-temperature oxidation.
However, if the content of Al exceeds 3.0%, it becomes difficult to maintain the phase stability. From this finding, the content of Al is 3.
0% or less.

Rare earth elements such as Ce, Y and La are Al ₂ O
_{(3) Since} it dissolves in the oxide film and increases its general resistance to high-temperature oxidation, it may contain one or more of these. If the content exceeds 0.07%, hot ductility is impaired, so the total content is set to 0.07% or less.

High-temperature oxidation resistance: The main factor that determines high-temperature oxidation resistance is the total content of Cr, Al, and Si. The high temperature oxidation resistance is determined by the content of these elements R = Cr +
2 (Al + Si), and 800
It has been found that the oxidation resistance at temperatures between 900C and 900C is not sufficient when R is less than 20.2. That is, it is necessary to contain Cr, Al, and Si satisfying R = Cr + 2 (Al + Si) ≧ 20.2 (1).

Hot workability and austenite phase stability: The main factors determining phase stability are Cr equivalents and Ni equivalents. Cr equivalent = 1.5 Si + 3Al + Cr, Ni equivalent = 0.5 Mn + Ni + 30C + 30N. When the following formula (2) is satisfied using these, there is no austenitic single phase structure after rolling, and an austenitic single phase structure, and a steel sheet side edge We found that there was almost no crack in the part.

On the other hand, when these formulas did not hold, significant cracking occurred at the steel sheet side end, and the presence of a ferrite phase after rolling was recognized. Cracking at the steel sheet side end is not economically desirable because it lowers the yield. The ferrite phase that exists after rolling is 800 ° C to 900 ° C
Changes to a brittle σ phase during use in, and significantly reduces toughness. Therefore, it is necessary that the component system satisfy α = (1.5Si + 3Al + Cr)-(0.5Mn + Ni + 30C + 30N) <9- (2).

Even when the ferrite phase does not exist and the austenitic single phase structure is obtained after rolling, a brittle σ phase may be formed by long-time use at 800 ° C. to 900 ° C. The formation of the σ phase can also be expressed by the above-described relationship between the Cr equivalent and the Ni equivalent. When the following equation (3) constituted using these holds, it is found that the σ phase is not generated even after long-time use. Was. That is, good toughness is maintained even after long-term use. Therefore, it is necessary to make the component system satisfying σ = 3.6 (1.5 Si + 3 Al + Cr) − (0.5 Mn + Ni + 30 C + 30 N) <70 (3)

This steel is manufactured by adding a predetermined amount of a predetermined component to a molten steel in the form of a simple substance or a master alloy.

[0027]

Next, an embodiment of the present invention will be described. Table 1
Table 4 shows the chemical compositions of the steels studied. In the table, N
o.1 to No. 21 are invention steel grades that do not contain Ce, Y, and La and satisfy the component range of the present invention.
No. 45 is a comparative steel grade. In the table, No. 22
No. 29 to No. 29 are invention steel grades containing Ce, Y and La and satisfying the component range of the present invention, and No. 46 to No. 57
Is the comparative steel. In the table, (1), (2),
The values of R, α, and σ, respectively defined by equation (3), are also shown.

[0028]

[0029]

[0030]

[Table 3]

[0031]

[Table 4]

The results of repeated oxidation tests at 900 ° C. for these steels are shown in Tables 5 and 6. The repeated oxidation test uses a corrosion test piece of 20 mm x 30 mm x 5 mm,
It was heated in an annular furnace with a mullite tube as the inner tube. After soaking for 96 hours in air humidified and adjusted to a dew point of 30 ° C with a humidity control system, move the side wall to a water-cooled cooling room to cool it to 200 ° C or less, and repeat this for 10 cycles with this as one cycle. went. The actual measured values of the heating rate and the cooling rate were about 130 ° C./min and 150 ° C./min, respectively. The repetitive oxidation resistance was determined by removing weight of the test piece after the test with a nylon brush, measuring the weight, and evaluating the weight loss by oxidation with respect to the weight before the test. The number of repetitions (n number) is 4 for each steel type,
The average value was evaluated. Variations between individual specimens were generally within 20-40%.

[0033]

[Table 5]

[0034]

[Table 6]

As can be seen from the table, the repetitive oxidation resistance is as follows:
The invention steels of the present invention not containing the rare earth elements No. 1 to No. 21 and the inventions containing the rare earth elements No. 22 to No. 29 were all used as comparative steel No. 30 18Cr-8Ni stainless steel (SU
The corrosion loss was less than 1/2 that of S304H), indicating good characteristics. This is because the oxide film is stabilized by increasing the amount of Cr, and the denseness of the oxide film is increased due to the inclusion of Al and rare earth elements in the oxide film, and the diffusion rate of oxygen is reduced to improve the internal protection. It is thought to be.

On the other hand, Comparative Steel No. 32, No. 50,
In No. 55, although the content of Si is high and shows relatively small corrosion weight loss at the beginning of the oxidation test,
It is characterized in that the oxide film starts to be peeled off from the third to the third cycle and shows a large corrosion weight loss in the long term.
In Comparative Steel No. 33, the high-temperature oxidation resistance was reduced by containing a large amount of Mn, and as a result, a large corrosion weight loss was exhibited. No. 41, No. 48 and No. 54 show that the value of Cr + 2 (Al + Si) did not reach 20.2, and Comparative Steel No. 37 and No. 57 showed that Cr + 2 (A
Although the value of (l + Si) has reached 20.2, sufficient high-temperature oxidation resistance has not been obtained due to a shortage of Cr.

Next, regarding the hot workability during rolling, the results of workability evaluation based on the presence or absence of edge cracks in the rolled sheet material are shown in Tables 5 and 6. In the rolling step of heating the steel of the present invention at 1200 ° C. and setting the finishing temperature to 900 ° C., no edge cracks occurred and good rolling results were obtained. On the other hand, P
Steel No. 34, No. 35, N
In Comparative Steel No. 39 containing a large amount of, and Comparative Steel No. 47 containing an excessive amount of rare earth elements, remarkable edge cracks occurred on the plate end faces. Further, a comparative steel having an α value of 9 or more in equation (2)
No. 40, No. 42, No. 43, No. 52 and No. 53 also had edge cracks on the plate end faces.

Lastly, tissue stability is important as a property that is problematic when used at high temperatures. Therefore JIS 4
Table 5 and Table 6 also show the measured values of the Charpy absorbed energy at 0 ° C using a No. Charpy test piece for aging at 850 ° C for 1000 hours. The invention steel has a good absorption energy at 0 ° C. of 30 J or more, and does not show a decrease in toughness due to the formation of carbonitrides, intermetallic compounds, and the like.

On the other hand, in the comparative steel No. 31, deterioration of toughness due to excessive precipitation of carbide was observed. Comparative steel No. 3
No. 6 is a comparative steel No. 38 and a comparative steel
In No. 49, since the Al content and the Cr content were excessive, respectively, a σ phase was precipitated and a decrease in toughness was recognized. Comparative steel No. 40, N
o. 43, No. 44, No. 45, No. 46, No. 50, N
o. 51, No. 53, and No. 58 show that the value of σ in equation (3) is 7
Since it was 0 or more, the σ value was precipitated by the heat treatment for a long time, and deterioration of toughness was recognized. Comparative steel No. 56 is (3)
Although the σ value of the equation is less than 70, the effect of Ni is not sufficient because the Ni content is a value obtained by including a large amount of more than 33%, and the stability of the austenite phase can be maintained. No, the toughness has deteriorated.

As is clear from the above Examples and Comparative Examples, according to the composition of the invention steel, the repetitive oxidation resistance at high temperatures can be improved, and the structure stability at the time of use at high temperatures is excellent, so that the toughness of the steels is high. A heat-resistant stainless steel having no deterioration and excellent hot workability during rolling production can be obtained.

[0041]

According to the present invention, a heat-resistant stainless steel capable of withstanding the operating conditions characteristic of a new type power plant such as a non-pressure-resistant and non-cooled portion at 800 ° C. to 900 ° C. is produced with a good yield. be able to. In addition, since it has remarkably superior oxidation resistance to 18Cr-8Ni stainless steel, which is a conventional general-purpose stainless steel, it withstands repeated startup and shutdown of the plant in a short cycle, and is suitable for long-term use. This can help in producing a reliable member that exhibits tissue stability.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-49-102511 (JP, A) JP-A-59-182956 (JP, A) JP-A-62-202056 (JP, A) JP-A-62-202056 164852 (JP, A) JP-A-2-25545 (JP, A) JP-A-62-164854 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00-38 / 60

Claims (2)

(57) [Claims] 1. A stainless steel having the following component composition and excellent in repeated oxidation resistance (the component composition is wt%). (A) C: 0.12% or less, Si: 0.31 to 1.0% Mn: 2.0% or less, P: 0.04% or less S: 0.03% or less, Al: 0.33 to 3.0% , N: 0.2% or less, Ni : 10 to 33%, Cr: 14% to 28%, the balance being Fe and unavoidable impurities. (B) The component composition satisfies the following formulas (1) to (3). R = Cr + 2 (Al + Si) ≧ 20.2-(1) α = (1.5Si + 3Al + Cr)-(0.5Mn + Ni + 30C + 30N) <9- (2) σ = 3.6 (1.5Si + 3Al + Cr)-(0.5Mn + Ni + 30C + 30N) <70- (3) 2. A stainless steel having the following component composition and excellent in repeated oxidation resistance (component composition is wt%). (A) C: 0.12% or less, Si: 0.31 to 1.0% Mn: 2.0% or less, P: 0.04% or less S: 0.03% or less, Al: 0.33 to 3.0% , N: 0.2% or less, Ni : 10 to 33%, Cr: 14% to 28%, and at least one of Ce, Y and La as a total content of 0.07
%, And the balance consists of Fe and inevitable impurities. (B) The component composition satisfies the following formulas (1) to (3). R = Cr + 2 (Al + Si) ≧ 20.2-(1) α = (1.5Si + 3Al + Cr)-(0.5Mn + Ni + 30C + 30N) <9- (2) σ = 3.6 (1.5Si + 3Al + Cr)-(0.5Mn + Ni + 30C + 30N) <70- (3)