JP2001316774A - Heat resistant ferritic stainless steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide heat resistant steel suitable for the exhaust gas pipe of a gas turbine in a class of the combustion temperature of 1,400 to 1,500 deg.C. SOLUTION: This heat resistant ferritic stainless steel excellent in high temperature strength, low temperature toughness and workability has a chemical composition containing, by mass, <=0.03% C, <=1.5% Si, <=1.5% Mn, <=0.6% Ni, 11 to 19% Cr, <=0.3% Nb, 0.1 to 0.5% V and 0.02 to 0.07% N and further containing, at need, one or more kinds selected from Cu, Mo, Ti, W and Zr in the range of <=3% in total, and the balance Fe with inevitable impurities, has a metallic structure in which the ratio of a martensitic phase is 0 to 30 vol.% and particularly suitable for the exhaust gas passage member of a gas turbine.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas path member such as an exhaust diffuser, an exhaust duct, a silencer, a denitration device, etc., provided on an outlet side of an industrial gas turbine such as a power plant or a small gas turbine for industrial use. The present invention relates to a heat-resistant ferritic stainless steel material suitable for a heat-resistant part.

[0002]

2. Description of the Related Art Power plants are required to have high thermal efficiency, excellent environmental characteristics and plant operability.
Construction is being promoted in various countries as a system in which a combined cycle power plant satisfies these requirements. recent years,
In order to further improve the power generation efficiency of the plant, a plan to raise the combustion temperature of the gas turbine from the conventional 1300 ° C class to the 1400 to 1500 ° C class is being promoted.

[0003] The gas temperature of the exhaust gas path members after the outlet side of the gas turbine, for example, the exhaust gas duct section and the exhaust silencer section, is at most 60 in a conventional 1300 ° C-class plant.
It was about 0 to 700 ° C. Among these members, SUH409 steel has been used for a relatively low temperature portion of 600 ° C. or lower. On the other hand, the portion having a high temperature, such as disclosed in JP-A-6-228715 and JP-A-6-323108,
Ferritic stainless steels based on r-Si-Nb have been used. However, in a high-temperature combustion type plant of 1400-1500 ° C class, the temperature of the exhaust gas path member is 650-800
It is expected to rise to about ° C. When that happens,
If the conventional materials and structures are used, various types of fractures such as thermal fatigue fracture, high-temperature high-cycle fatigue fracture, and creep fracture are likely to occur over a long period of use. In addition, when the structure is significantly changed by heating for a long time, it is considered that low-temperature toughness after use is reduced due to, for example, generation of brittle precipitates, and brittle fracture is likely to occur during operation.

[0004] As means for preventing these various destructions,
i) change the design of the exhaust gas path, or
i) It is conceivable to change the material to one having more excellent heat resistance and structural stability.

In the case of a design change such as i), a measure is basically taken to increase the thickness of the sheet and to use a reinforcing material in the stress concentration portion. And the load of welding work when assembling the duct section increases. Further, the increase in the thickness of the used material causes an increase in heat loss at the site, which may be a factor that hinders high-efficiency power generation.

On the other hand, as an improvement in the material aspect as in ii), it is conceivable to use a martensitic stainless steel or an austenitic stainless steel instead of the conventional low Cr ferritic stainless steel. However, martensitic stainless steel has high strength but has a drawback of poor workability. Further, even if workability is improved by tempering, use in a high temperature environment of 650 to 800 ° C. may cause decomposition into ferrite and carbide, and may further cause formation of an austenite phase. In the former case, only the same strength as that of ferritic stainless steel can be expected, and in the latter case, there is the danger that the expansion and contraction due to phase transformation and the expansion and contraction due to heat will overlap and the member will be locally deformed significantly. is there.
The same applies to the use of 2.25Cr-based steel, 9Cr-based steel, and 12Cr-based steel conventionally known as high-temperature, high-strength ferritic heat-resistant steel. In addition, austenitic stainless steel has a larger thermal expansion coefficient than ferritic stainless steel, so in power plants that repeatedly start and stop operation every day, there is a concern that thermal fatigue fracture may occur in stress-concentrated parts such as welds.
As a result, it is considered that the design of the exhaust gas path must be changed. Further, austenitic stainless steels are generally expensive, which leads to an increase in construction costs.

[0007]

As described above, at present, the construction of a 1400 to 1500 ° C-class combined power generation plant requires a change in the design of the exhaust gas path section from the conventional 1300 ° C-class, It is sufficiently considered that a design change based on making the thickness larger is necessary. At that time, in terms of materials, the selection of existing steel grades with the best properties such as heat resistance, workability, etc. as much as possible reduces the power generation efficiency, raises construction costs, and improves on-site workability. It is only about minimizing deterioration.

If it becomes possible to construct an exhaust gas path section of a 1400-1500 ° C. plant by using a material having the same thickness as that of a conventional exhaust gas path member of 1300 ° C. class, it would be necessary to increase the weight of the member and to increase welding. The increase in cost due to the increase in construction load can be reduced, and the obstacle to improving the power generation efficiency will be eliminated. However, there are many reliable materials that are exposed to high temperatures of 650 to 800 ° C and returned to room temperature when the engine is stopped, and can exhibit stable performance enough to withstand use under severe repetitive environments. Even though heat-resistant steel is being developed, it has not yet been identified.
That is, it can be said that there is still room for study on heat-resistant materials suitable for use in gas turbine exhaust gas path members of 1400-1500 ° C. combined power plants.

The same applies to other gas turbine applications, such as industrial small gas turbines, as well as power plants. That is, as the combustion temperature of the gas turbine rises, the characteristics required for the exhaust gas path member are becoming severe. As a result of studies by the inventors, in developing a heat-resistant material applicable to such gas turbine exhaust gas applications, in addition to having general properties such as heat resistance, workability, weldability, etc., in particular, high-temperature long It has been found that it is extremely important to improve the creep rupture strength when used for a long time and to sufficiently secure the toughness when the temperature is returned to room temperature after heating for a long time. Moreover, it is necessary to realize this with a ferritic steel type having a small coefficient of thermal expansion.

An object of the present invention is to provide a ferritic stainless steel material applicable to an exhaust gas path member of a gas turbine satisfying such a demand. More specifically, in comparison with the current material 14Cr-Si-Nb steel, 700
The purpose is to develop a ferritic stainless steel material that has a creep characteristic (rupture strength) of 1.5 ° C or more and a workability and low-temperature toughness after long-time heating are equal to or more than that.

[0011]

Means for Solving the Problems The inventors of the present invention used 14Cr-0.8-1.2Si-0.4-0.6Nb steel, which is considered to be suitable for an exhaust gas duct material of a 1300 ° C-class combined cycle power plant. The precipitation morphology after aging treatment in was investigated in detail. As a result, Nb-Si-added steel shows that Nb is in a solid solution state before aging and the high-temperature strength is improved by solid solution strengthening.In the early stage of aging, some Nb precipitates as an intermetallic compound and precipitates. It was clarified that the high-temperature strength was maintained by the strengthening, and that after the long-term aging, the strength increase due to the precipitation strengthening disappeared and the high-temperature strength decreased. Furthermore, the present inventors have investigated and studied the effects of alloying elements on the high-temperature strength of ferritic stainless steel after long-term aging, and as a result, by adding a predetermined amount of N and V, the Nb-Si-added steel Also, it was found that the high-temperature strength was hardly reduced due to the disappearance of the precipitation strengthening, and excellent high-temperature characteristics were maintained. In addition, it has been found that by strictly defining the upper limit of N, a steel material having excellent high-temperature strength and also having workability and toughness can be obtained. The present invention has been completed based on these findings.

[0012] That is, the above-mentioned object is, in mass%, C: 0.03
%, Si: 1.5% or less, Mn: 1.5% or less, Ni: 0.6% or less, Cr: 11 to 19%, Nb: 0.3% or less, V: 0.1 to 0.5%,
N: 0.02 to 0.07%, and if necessary, Cu, Mo,
A metal containing one or more of Ti, W, and Zr in a total range of 3% by mass or less, a balance having a chemical composition of Fe and unavoidable impurities, and a martensite phase of 0 to 30% by volume. This is achieved by a heat-resistant ferritic stainless steel material having a structure and excellent in high-temperature strength, low-temperature toughness, and workability, particularly a steel material for an exhaust gas path member of a gas turbine.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows the basic composition of 12 to 15Cr-0.05Nb-0.25V steel in order to understand the effect of N content on the high-temperature strength characteristics and toughness of ferritic stainless steel. The creep rupture stress at 700 ° C. × 1000 hours and the elongation at room temperature are shown when the temperature is changed. Here, the creep rupture stress is a value obtained by performing a creep rupture test at 700 ° C. under various stresses and obtaining a rupture strength for 1000 hours. The elongation at room temperature shows the elongation (%) when a tensile test was performed in the rolling direction at room temperature on a cold-rolled annealed sheet having a thickness of 2.0 mm.

As can be seen from FIG. 1, the creep rupture stress at 700 ° C. for 1000 hours sharply increases with an increase in the N content, and is about 25 N / mm ² at 0.02 mass% and 0.025 mass% for the same.
Is about 27 N / mm ² . The value of the 27N / mm ² is about 1.5 times the 14Cr-Si-Nb steel used in high-temperature portion of 1300 ° C. class power plant exhaust gas path of the current situation described above, the SUH409 steel is used in a low temperature portion Corresponding to more than 3 times the breaking stress.

On the other hand, the elongation at room temperature decreases as the N content increases. In particular, when the N content exceeds 0.07% by mass, the decrease becomes remarkable. This is considered to be due to the fact that, in addition to the increase in the N content, the steel becomes hard and the ductility deteriorates, and in this component system, the martensite content increases with the N content. In order to obtain sufficient workability as ferritic stainless steel, at least
The elongation of 30% or more is necessary. From this point, the N content is
It is limited to 0.07% by mass or less.

The toughness of the steel material of the present invention after aging (that is, after prolonged heating assuming actual use) will be described in detail in Examples, but even after aging at 600 to 900 ° C. for 1000 hours,
In the case of a plate thickness of 2.0 mm, a Charpy impact value of 100 J / cm ² or more was shown at 0 ° C., and it was confirmed that the steel had low-temperature toughness equivalent to or higher than that of 14Cr-Si-Nb steel.

According to our detailed investigation, N is equal to V
To form nitrides, effectively contributing to an increase in high-temperature strength. The effect is remarkable when the N content is 0.02% by mass or more. However, if N is added excessively, the steel becomes harder and the amount of martensitic phase increases, and the content of N increases by 0.07% by mass.
If it is contained in a large amount exceeding the above, ductility is reduced. Therefore, the N content needs to be in the range of 0.02 to 0.07% by mass. The lower limit of the N content is more preferably 0.025% by mass or more, and even more preferably an amount exceeding 0.03% by mass. Further, the upper limit of the N content is more preferably set to 0.06% by mass.

C is generally considered to be an effective element for high-temperature strength such as creep strength. However, on the other hand, as the content increases, the oxidation characteristics, workability and toughness decrease. Further, in the present invention, since the effect of improving the high-temperature strength due to solid-solution Nb is used as described later, if the amount of C is large, Nb is undesirably consumed for fixing the Nb. Therefore, in the present invention, the content of C is desirably low, and the upper limit is limited to 0.03% by mass. A more preferred upper limit of the C content is 0.02% by mass.

Si is a very effective element for improving high-temperature oxidation characteristics. However, when Si is added excessively, the hardness increases, and the workability and toughness decrease.
Limit to 1.5 mass% or less. A more preferable upper limit of the Si content is 1.0% by mass.

Mn has the effect of improving the high-temperature oxidation properties of ferritic stainless steel, especially the resistance to scale peeling, but excessive addition deteriorates workability and weldability. Further, since Mn is an austenite phase stabilizing element, excessive addition causes an increase in the amount of martensite phase formed, thereby deteriorating workability. For this reason, the Mn content is specified to be 1.5% by mass or less. A preferred Mn content range is 1.0% by mass or less.

Since Ni is an austenite phase stabilizing element, when Ni is excessively added to ferritic stainless steel,
Like Mn, it causes an increase in the martensite phase, and the workability deteriorates. Also, since the raw material price is high, excessive addition of Ni should be avoided. Therefore, in the present invention, the Ni content is specified to be 0.6% by mass or less. A more preferable range of the Ni content is 0.1.
5 mass% or less.

Cr is an element that stabilizes the ferrite phase and is indispensable for improving the oxidation resistance, which is regarded as important for high-temperature materials. From the viewpoint of oxidation resistance, the Cr content is preferably as large as possible, but if added excessively, it causes brittleness of the steel, and the workability is also deteriorated due to the increase in hardness. In the present invention, considering such characteristics of Cr, the Cr content is specified to be 11 to 19% by mass. A particularly preferred range of the Cr content is 12 to 15% by mass.
It is.

Nb has the effect of fixing C and N as carbonitrides. In addition, Nb in the solid solution state where C and N are fixed effectively acts to increase the high-temperature strength of the material.
However, since N is added in the present invention, if Nb is added excessively, a large amount of carbonitride is generated, and as a result, toughness is deteriorated. In addition, it leads to an increase in the production cost of steel materials. Therefore, excessive addition of Nb is not preferable, and the Nb content range is set to 0.3% by mass or less. The lower limit of the Nb content is more preferably 0.02% by mass, and the upper limit is more preferably less than 0.20% by mass.

V is an element necessary for improving the high-temperature strength in the present invention. In the temperature range of 600 to 800 ° C. assumed when using the steel material of the present invention, V is finely dispersed and precipitated mainly as nitride. Since this V nitride precipitates later and grows more slowly than in the case of Nb-Si added steel,
In particular, it is considered that the high-temperature strength on the long-time side is further improved. To achieve the object of the present invention from the viewpoint of high-temperature strength, V content of 0.1% by mass or more is required. On the other hand, since the workability and toughness decrease with an increase in the V content, the upper limit of the V content is limited to 0.5% by mass or less. A more preferred lower limit of the V content is 0.2% by mass, and a more preferred upper limit is 0.4% by mass.

Cu, Mo, Ti, W and Zr are effective elements for improving the high-temperature strength, and in order to sufficiently exert their effects, the larger the amount of addition, the better. These elements are
They may be used alone or in combination of two or more. On the other hand, adding too much will make the steel harder,
Also, the raw material cost increases. Therefore, Cu, Mo, Ti, W
Or when adding Zr, the total amount of these is 3% by mass.
Make sure that: More preferably, the total content range of Cu, Mo, Ti, W and Zr is 0.1 to 2% by mass.

It is preferable to reduce P, S, O, etc., which are general impurity elements, as much as possible. For example, P is 0.
It is preferable to reduce the content of S to 0.04% by mass or less and the content of O to 0.02% by mass or less. In order to ensure the above-mentioned workability and toughness at a higher level, the upper limits of these impurity elements are further increased. It may be strictly defined. Al, Y, REM, which are generally known as elements for improving heat resistance,
Ca, M known as elements that improve hot workability and toughness
Elements such as g, B, Co, etc. can also obtain their respective effects by appropriately adding them as needed.

It is known that the martensite phase is very effective in improving the high-temperature strength when the operating temperature is in the range of 650 ° C. or lower, and the high-temperature strength is improved by increasing the ratio of the martensite phase. Attempts have been made since ancient times. However, assuming a case where the steel material of the present invention is used by being exposed to a high temperature of 700 ° C. or more, the martensite phase is tempered and thus is not very effective in increasing the high temperature strength. Further, when the Ac ₁ point is low, the martensite phase is transformed into an austenite phase during use, so that the transformation strain and the thermal expansion coefficient of the steel material increase, which may cause deterioration of the thermal fatigue properties. Furthermore, when the martensite phase is large at room temperature, the ductility becomes significantly lower than that of a ferritic single-phase steel, resulting in poor workability. Therefore, in the present invention, the smaller the martensite phase, the better. As a result of the study by the inventors, in the present invention,
Before use (state after annealing), it was found that the amount of martensite in the steel material should be limited to 30% by volume or less. A state in which no martensite phase is present (0% by volume), such as a ferrite single phase, may be used. A more preferred amount of martensite is 0 to 20% by volume.

The martensite phase mentioned here includes a tempered martensite phase as well as a martensite phase as-quenched. The martensite phase in the steel material can be identified by observing the etched metal structure of the steel material cross section using, for example, an optical microscope. The ratio (volume%) of the martensite phase in the steel material can be determined by measuring the area ratio of the martensite phase in the above-described metallographic observation. For the measurement, for example, a method of image analysis using a computer can be used, and it is also possible to apply a method of determining the area percentage by visual measurement of ferrite and pearlite in JIS G 0552 (test method of ferrite crystal grain size of steel). .

The method for producing the steel material of the present invention is not particularly limited. However, in the case of a ferrite single phase structure, it is necessary to obtain the steel material of the present invention which exhibits excellent heat resistance, workability and toughness in a hot-rolled and annealed state. Is possible. When the steel contains a martensite phase, the ductility is lower than that of a ferritic single-phase steel, so that a tempering treatment may be performed if necessary.
When a steel sheet having a desired thickness cannot be produced only by hot rolling, cold rolling and annealing are repeated once or a plurality of times to obtain a steel sheet having heat resistance equivalent to that of a hot rolled annealed sheet. Furthermore, even if such a steel sheet is processed or welded (including forming a pipe, etc.) into a desired shape, the characteristics intended by the present invention can be secured, and the steel material of the present invention can be obtained.

[0030]

EXAMPLES Table 1 shows the chemical compositions of the test materials. In Table 1, N
o.1 to 14 are steel materials of the present invention, and No.15 to 22 are comparative steel materials. Among them, No. 21 is a SUH409 equivalent steel and No. 22 is a 14Cr-Si-Nb steel, which is a steel suitable for an exhaust gas duct material of a conventional LNG combined cycle power plant of 1100 to 1300 ° C class.

[0031]

[Table 1]

Each of the above steels was melted in a vacuum melting furnace and cast into a 30 kg ingot. After that, the steel ingot is forged into a round bar,
After annealing, it was subjected to a creep rupture test. In addition, a part of the steel ingot was forged into a plate, subjected to hot rolling, annealing, cold rolling, and finish annealing, and the resulting cold-rolled annealed plate having a thickness of 2.0 mm was observed for metallographic structure, oxidation test and room temperature. Was subjected to a tensile test. Further, after the cold-rolled annealed plate was heated at 700 ° C. for 1000 hours, it was subjected to a Charpy impact test.

The metallographic structure was observed by etching the cross section of the steel material with a mixed solution of hydrofluoric acid and nitric acid and then observing it with an optical microscope.
The area ratio of the martensite phase was measured according to the method for determining the area percentage by visual measurement of ferrite and pearlite in SG0052 (test method for ferrite crystal grain size of steel).
The measured values are shown in Table 1 as "amount of martensite (% by volume)".

The creep rupture test was performed at 700 ° C. in accordance with JIS Z 2272. The creep rupture curve was created so that the stress applied during the test was changed for each test and the longest rupture time was about 5000 hours, and the rupture strength of 1000 hours, that is, the rupture time was 1000 hours in the creep rupture curve The load stress at the time was determined. The high-temperature oxidation test was performed by continuous heating at 700 ° C. × 1000 hours in accordance with JIS Z 2281. After the test, the occurrence of abnormal oxidation, that is, the presence or absence of a thick oxide having a bump-like shape penetrating in the plate thickness direction was visually observed. Charpy impact test conforms to JIS Z 2242, 700 ° C x 100
The cold-rolled annealed plate that had been aged for 0 hours was processed into a 2.0 mm-thick sub-sized test piece, and a test was performed at 0 ° C. to determine the Charpy impact value. The Charpy impact test specimen was sampled so that the longitudinal direction was perpendicular to the rolling direction. In the tensile test at room temperature, a 2.0 mm-thick cold-rolled annealed plate was processed into a No. 13B test piece in accordance with JIS Z 2241, and the elongation at break after the tensile test was determined. In addition, the tensile test piece was sampled so that the longitudinal direction was the rolling direction. Table 2 shows the results.

[0035]

[Table 2]

The steel materials of Nos. 1 to 14 of the invention are all
The creep rupture strength at 700 ° C for 1000 hours is superior to the current materials No. 21 and 22, and the appearance after continuous heating at 700 ° C for 1000 hours (absence of abnormal oxidation), after aging at 700 ° C for 1000 hours The Charpy impact value at 0 ° C. has the same properties as the current material. Further, the room temperature elongation of the cold-rolled annealed plate is 30% or more, and it is considered that the processing into the exhaust gas path member of the gas turbine is sufficiently possible.

On the other hand, Nos. 15 and 2 having a small N content
1 and 22 are inferior in creep rupture characteristics. No.16, which has a high N content, has poor Charpy impact value at 0 ° C after aging at 700 ° C for 1000 hours and room temperature elongation of the cold-rolled annealed sheet, so it cannot be processed sufficiently or has insufficient toughness during use. May occur. No. 18 having a high Nb content and No. 20 having a high V content have low Charpy impact values after aging. No. 19 containing V but having a small amount thereof is inferior in creep rupture characteristics. Further, No. 17 having a chemical composition within the range specified in the present invention but having too much martensite phase has low room temperature elongation.

[0038]

As described above, despite the fact that many heat-resistant steels have been developed, it is possible to stably exhibit characteristics suitable for exhaust gas path members of a high-temperature combustion type gas turbine having a combustion temperature of 1400 to 1500 ° C. No steel grade was present. The reason is that such a member repeatedly undergoes a severe thermal cycle in which the material temperature reaches 650 to 800 ° C., is exposed to such a high temperature for a long time, and then is returned to room temperature. In other words, the material is required to have strict characteristics such as high creep rupture strength during high-temperature long-time heating and sufficient low-temperature toughness after long-time heating, and these characteristics are required to have a small coefficient of thermal expansion. The fact that it must be realized with ferritic steel has made material development difficult. The present invention has solved this problem and has achieved the development of a new steel type suitable for an exhaust gas path member of a high-temperature combustion type gas turbine.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between N content in Nb and V-containing ferritic stainless steel, creep rupture stress at 700 ° C. for 1000 hours, and elongation at room temperature of a cold-rolled annealed sheet.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) F02C 7/00 F02C 7/00 CB

Claims (3)

[Claims] 1. In mass%, C: 0.03% or less, Si: 1.5%
Mn: 1.5% or less, Ni: 0.6% or less, Cr: 11 to 19%,
Metal structure containing Nb: 0.3% or less, V: 0.1-0.5%, N: 0.02-0.07%, the balance being a chemical composition comprising Fe and unavoidable impurities, and a martensite phase of 0-30% by volume. A heat-resistant ferritic stainless steel with excellent high-temperature strength, low-temperature toughness and workability. 2. The steel material according to claim 1, further comprising one or more of Cu, Mo, Ti, W, and Zr in a total amount of 3% by mass or less. 3. The steel material according to claim 1, which is used for an exhaust gas path member of a gas turbine.