WO2024038645A1 - Fe-Cr-Ni ALLOY HAVING EXCELLENT WORKABILITY AND HIGH-TEMPERATURE STRENGTH - Google Patents

Abstract

[Problem] To provide an Fe-Cr-Ni alloy having excellent workability and high-temperature characteristics. [Solution] An Fe-Cr-Ni alloy comprising, in mass%, C: 0.03-0.08%, Si: 0.10-0.50%, Mn: 0.20-1.20%, P: 0.001-0.040%, S: 0.0001-0.0030%, Ni: 35.5-45.5%, Cr: 23.5-26.0%, Mo: 0.30-1.50%, Cu: 0.01-0.30%, Al: 0.010-0.150%, Ti: 0.10% or less, B: 0.0005-0.0050%, Co: 0.02-0.30%, Nb: 0.45-0.60%, and N: 0.15-0.30%, the remainder being Fe and unavoidable impurities, the 0.2% yield strength being 400 MPa or less and the elongation being 40.0% or more in a normal temperature tensile test and the 0.2% yield strength being 140 MPa or more and the elongation being 50.0% or more in a high-temperature tensile test at 1100°F.

Description

Fe-Cr-Ni alloy with excellent workability and high-temperature strength

The present invention relates to a Fe--Cr--Ni alloy that is used as a structural material for reaction towers that requires excellent high-temperature strength. Specifically, it relates to a technology that achieves both excellent yield strength and elongation at room temperature and high temperature by optimizing the components during alloy production.

In recent years, clean energy through solar power generation has been attracting attention. In particular, they are attracting more and more attention year by year, as they do not emit carbon dioxide and help prevent global warming. When manufacturing solar power generation equipment, polysilicon refined from crude silicon is often used as a raw material for power generation elements. Inside the reaction tower for refining this polysilicon, the pressure of the reaction gas is at a high pressure of 100 MPa or more, and the temperature is as high as 1100° F. or more, resulting in a very harsh environment. The material used for the reaction tower must not only have excellent processability for molding and corrosion resistance, but also must have excellent high-temperature strength. If the high-temperature strength is insufficient, the useful life of the reaction tower will be shortened, and productivity will be greatly reduced. In order to satisfy these characteristics, a suitable Fe--Cr--Ni alloy is required.

However, as the high-temperature strength increases, the room-temperature strength also increases, which poses the problem of making material processing difficult when manufacturing a reaction tower. Furthermore, since the reaction gas is a corrosive gas containing chlorine, excellent corrosion resistance is also required, and there is a need for a Fe--Cr--Ni alloy that solves these problems and has excellent workability and high-temperature strength.

As a prior art that focuses on high-temperature strength, Patent Document 1 discloses a ferritic stainless steel that has excellent high-temperature oxidation resistance, high-temperature creep strength, and high-temperature tensile strength, but there is no specified value regarding high-temperature elongation in the claim. Moreover, the yield strength value of the high-temperature tensile strength described in Examples is also low.

In addition, Patent Document 2 discloses a Ni-based alloy with excellent structure stability and high-temperature strength and a method for manufacturing Ni-based alloy materials, but Ni-based alloys require the addition of expensive elements such as Mo and W. Yes, the cost will increase.

Furthermore, Patent Document 3 proposes a Fe-Cr-Ni alloy that achieves high creep rupture properties by controlling the components and precipitates or crystallized substances during alloy manufacturing, but the room temperature strength, crystal grain size, There is no description of high-temperature 0.2% yield strength or high-temperature elongation, and this is not an alloy that takes workability into consideration.

JP2016-079437A Japanese Patent Application Publication No. 2008-297579 Patent No. 6675846

The present invention has been made in view of the above-mentioned problems with the prior art, and its purpose is to provide a Fe-Cr-Ni alloy having excellent workability and high-temperature properties.

In order to solve the above problems, the present inventors have conducted extensive research. First, raw materials such as electrolytic iron, Cr, Mo, and Ni were weighed and melted in a high frequency induction furnace. The crucible is magnesia, and the amount of melting is 20 kg. At this time, the purpose was to vary the concentration of each element such as Ni and use it for measurement. After melting, it was poured into a mold, then forged to a thickness of 20 mm, and annealed at 1300° C. for 20 to 60 minutes.

For the room temperature tensile test, the annealed plate was prepared as a round bar test piece in accordance with ASTM E8, and the tensile test specified in ASTM E8 was conducted to evaluate the 0.2% yield strength and elongation.

For the high temperature tensile test, a round bar test piece was prepared from the annealed plate in accordance with ASTM E8, and this was subjected to a high temperature tensile test at 1100°F specified in ASTM E21 to evaluate the 0.2% yield strength and elongation. did.

To measure the grain size, a test piece of 5 mm x 20 mm x 15 mm was cut out from the annealed plate, and the surface of the cross section perpendicular to the forging direction was mirror-polished, and a microscopic test specified by ASTM E112 was performed using an optical microscope. Then, the average crystal grain size was measured.

To measure the number of carbonitrides of NbN, NbC, TiN, and TiC and to measure the distribution of precipitates of NbN, NbC, TiN, TiC, and M23C6, a test piece of 5 mm x 10 mm x 10 mm was measured from the 1/4 plate thickness position of the annealed sheet. It was cut out, and the surface of the cross section perpendicular to the forging and elongation direction was mirror-polished, and observation and composition analysis were performed using SEM-EDS.

For the hardness test, a 20 mm x 50 mm x 50 mm test piece was cut out from the surface of the annealed plate, the surface was polished to No. 320, and a Brinell hardness test was performed using a cemented carbide ball specified by ASTM E10 to determine the hardness. It was measured.

In this way, by determining the appropriate alloy component range, we were able to determine an alloy that ensures appropriate workability and high-temperature strength.

The Fe-Cr-Ni alloy of the present invention has a mass percentage of C: 0.03 to 0.08%, Si: 0.10 to 0.50%, Mn: 0.20 to 1.20%, and P: 0. .001 to 0.040%, S: 0.0001 to 0.0030%, Ni: 35.5 to 45.5%, Cr: 23.5 to 26.0%, Mo: 0.30 to 1.50 %, Cu: 0.01-0.30%, Al: 0.010-0.150%, Ti: 0.10% or less, B: 0.0005-0.0050%, Co: 0.02-0 .30%, Nb: 0.45 to 0.60%, and N: 0.15 to 0.30%, with the remainder consisting of Fe and unavoidable impurities, and passed the room temperature tensile test specified by ASTM E8. The 0.2% proof stress value is 400 MPa or less, the elongation is 40.0% or more, and the 0.2% proof stress value is 140 MPa or more and the elongation is 140 MPa or more in the high temperature tensile test at 1100°F specified in ASTM E21. 50.0% or more, one or more types of NbN, NbC, TiN, and TiC with an average grain size of -3 to 6 in the microscope test specified in ASTM E112 and a major axis of 0.5 to 5 μm. The total of these must contain an average of 20 to 180 pieces/mm ² in any cross section, and if it contains one or more of NbN, NbC, TiN, and TiC with a major axis of more than 5 μm, the total of these can be arbitrary. It is an Fe-Cr-Ni alloy characterized by an average number of 5.0 pieces/mm ² or less in the cross section.

The present invention is characterized in that the mass % of each component is adjusted to 0.120≦Nb×N+Nb×C+Ti×N+Ti×C≦0.150.

In the present invention, the Fe-Cr-Ni alloy has NbN, NbC, TiN with a major axis of 0.5 to 5 μm, and the mass percentage of each component is adjusted to 0.0020%≦B+P≦0.0400%. It is characterized in that the total percentage of precipitates of one or more of TiC and M23C6 precipitated at grain boundaries in any cross section is 10 to 30% in number.

The present invention is characterized by a hardness test value of 130 to 210 using a cemented carbide ball having a Brinell hardness specified by ASTM E10.

1 is an electron micrograph showing typical NbC precipitates observed in precipitate counting according to the present invention. It is an electron micrograph which shows the grain boundary precipitate and the intragranular precipitate of Invention Example 17 in an Example. Blue dots are precipitates precipitated at grain boundaries, and red dots are precipitates precipitated within grains.

In order to improve the workability and high-temperature properties of Fe-Cr-Ni alloys, the present invention aims to improve the workability and high-temperature properties of Fe-Cr-Ni alloys. .20-1.20%, P: 0.001-0.040%, S: 0.0001-0.0030%, Ni: 35.5-45.5%, Cr: 23.5-26.0 %, Mo: 0.30-1.50%, Cu: 0.01-0.30%, Al: 0.01-0.15%, Ti: 0.10% or less, B: 0.0005-0 .0050%, Co: 0.02-0.30%, Nb: 0.45-0.60% and N: 0.15-0.30%, with the balance consisting of Fe and inevitable impurities. -Cr-Ni alloy. The reason for specifying this range will be explained.

C: 0.03-0.08%
C is an element necessary to ensure 0.2% yield strength of the alloy at room temperature and high temperature, and if it is less than 0.03%, the required strength cannot be obtained. On the other hand, when it exceeds 0.08%, the number of NbC and TiC increases, and the room temperature 0.2% yield strength becomes too high. Therefore, the C content was determined to be 0.03 to 0.08%. Preferably it is 0.035 to 0.075%, more preferably 0.04 to 0.07%.

Si: 0.10~0.50%
Si is added as a deoxidizing agent and is an element necessary to ensure oxidation resistance at high temperatures, and if it is less than 0.10%, a sufficient effect cannot be obtained. On the other hand, if it exceeds 0.50%, the stability of the austenite phase decreases and the necessary high temperature 0.2% yield strength cannot be obtained, so it is set at 0.10 to 0.50%. Preferably it is 0.12 to 0.45%, more preferably 0.13 to 0.40%.

Mn: 0.20-1.20%
Mn is an element added as a deoxidizing agent and stabilizes the austenite phase, and if it is less than 0.20%, a sufficient effect cannot be obtained. On the other hand, if it exceeds 1.20%, the stability of the austenite phase decreases and the necessary high temperature 0.2% proof stress cannot be obtained, so it is set at 0.20 to 1.20%. Preferably it is 0.35 to 1.10%, more preferably 0.50 to 1.00%.

P:0.001-0.040%
P is an important element in the present invention and is an impurity element that is inevitably mixed into steel. If the content is large, it segregates at grain boundaries and reduces high temperature elongation. Therefore, it is necessary to strictly limit the upper limit. In the present invention, P is limited to 0.040% or less. A preferable upper limit of the content is 0.030%, and a more preferable upper limit is 0.020%. The lower limit of the P content is preferably as close to 0% as possible, but 0.001% is listed based on Invention Example 1.

S: 0.0001-0.0030%
S is an important element in the present invention, forms sulfide, and reduces toughness. Therefore, the S content should be as low as possible, and the upper limit is preferably 0.0030%. Preferably it is 0.0015% or less, more preferably 0.0010%. However, S is an element that improves weldability because even a small amount of S greatly increases the fluidity of the melt during melting. Although S is not particularly limited, it is preferably contained in an amount of 0.0001% or more in order to obtain good weldability. Note that S is adjusted within the range of the present invention by desulfurizing it by adding Al and Si.

Ni: 35.5-45.5%
Ni is an important element for stabilizing the austenite phase and maintaining high temperature 0.2% yield strength. If it is less than 35.5%, the effect cannot be obtained, and if it exceeds 45.5%, the raw material cost will increase. Therefore, it was set at 35.5% to 45.5%. Preferably it is 36.0 to 44.0%, more preferably 37.0 to 40.0%.

Cr:23.5-26.0%
Cr is an element that improves corrosion resistance, forms a dense oxide film at high temperatures, and has the effect of maintaining the high temperature 0.2% yield strength of the base material. If it is less than 23.5%, the effect cannot be obtained. On the other hand, if it exceeds 26.0%, the stability of the austenite phase decreases and the necessary high temperature 0.2% yield strength cannot be obtained. Therefore, it was set at 23.5 to 26.0%. Preferably it is 24.0 to 25.5%, more preferably 24.5 to 25.4%.

Mo: 0.30~1.50%
Mo is an element that improves corrosion resistance and has the effect of strengthening grain boundaries, and is an element necessary to ensure high-temperature 0.2% yield strength. If it is less than 0.30%, the effect cannot be obtained. On the other hand, if it exceeds 1.50%, the σ phase will precipitate and the high temperature elongation will decrease. Therefore, it was set at 0.30 to 1.50%. Preferably it is 0.35 to 1.30%, more preferably 0.40 to 1.00%.

Cu: 0.01~0.30%
Cu is an element that stabilizes the austenite phase and improves room temperature elongation, and is an essential element for the alloy of the present invention. In order to obtain this effect, it is necessary to contain 0.01% or more. On the other hand, as the Cu content increases, the high temperature 0.2% yield strength decreases. Therefore, the upper limit of the Cu content is set to 0.30%. It is preferably 0.02 to 0.25%, more preferably 0.03 to 0.20%.

Al: 0.010-0.150%
Al is a useful element for making the oxide film after rolling more dense. From this point of view, addition of 0.010% is necessary. Furthermore, it is an effective element as a deoxidizing agent. If it is less than 0.010%, sufficient effects cannot be obtained, oxide inclusions increase, and sufficient high temperature 0.2% yield strength cannot be obtained. Moreover, if it exceeds 0.150%, it combines with N to form Al nitride, resulting in a decrease in high temperature 0.2% yield strength. Therefore, it was set at 0.010 to 0.150%. Preferably it is 0.015 to 0.130%, more preferably 0.020 to 0.100%.

Ti: 0.10% or less Ti is useful because it forms nitrides and carbides to ensure high-temperature 0.2% yield strength, so it may be added. Addition of more than 0.10% increases the number of nitrides or carbides, which goes beyond the scope of the present invention. Therefore, addition should be limited to 0.10% or less. Preferably it is 0.07% or less, more preferably 0.05% or less.

B: 0.0005-0.0050%
B is an element that segregates at grain boundaries, has the effect of increasing grain boundary strength at high temperatures, and is an important element for ensuring high temperature 0.2% yield strength. If it is less than 0.0005%, the effect will not be obtained, and if it exceeds 0.0050%, high temperature elongation will decrease. Therefore, it was set at 0.0005 to 0.0050%. Preferably it is 0.0010 to 0.0045%, more preferably 0.0015 to 0.0040%.

Co:0.02~0.30%
Co is a useful element that improves only the high temperature 0.2% yield strength without changing the strength at room temperature, and is an essential element for the alloy of the present invention. If it is less than 0.02%, the effect will not be obtained, and if it is added in excess of 0.30%, high temperature elongation will decrease. Moreover, raw material cost increases. Therefore, it was set at 0.02 to 0.30%. Preferably it is 0.03 to 0.28%, more preferably 0.04 to 0.25%.

Nb: 0.45-0.60%
Nb is useful because it forms nitrides and carbides and ensures high-temperature 0.2% yield strength. If it is less than 0.45%, the effect cannot be obtained. However, if it is added in excess of 0.60%, the number of nitrides or carbides increases and the high temperature 0.2% yield strength decreases. Therefore, it was set at 0.45 to 0.60%. Preferably it is 0.47 to 0.58%, more preferably 0.50 to 0.56%.

N: 0.15-0.30%
N is an element that improves corrosion resistance and is an element useful for forming nitrides with Nb and Ti. If it is less than 0.15%, a sufficient effect cannot be obtained, and the required room temperature 0.2% proof stress and high temperature 0.2% proof stress cannot be obtained. On the other hand, if it exceeds 0.30%, the number of nitrides or carbides increases, which exceeds the scope of the present invention and the room temperature 0.2% yield strength becomes too high. Therefore, it was set at 0.15 to 0.30%. Preferably it is 0.17 to 0.28%, more preferably 0.20 to 0.25%.

By satisfying the above component range, the 0.2% proof stress value in the room temperature tensile test specified by ASTM E8 can be satisfied to 400 MPa or less. If it exceeds 400 MPa, workability is poor and it is difficult to manufacture a reaction tower. Preferably it is 390 MPa or less, more preferably 380 MPa or less.

Furthermore, by satisfying the above component ranges, the elongation value in the normal temperature tensile test specified by ASTM E8 can be satisfied to be 40.0% or more. If it is less than 40.0%, processability is poor and it is not suitable for manufacturing a reaction tower. Preferably it is 42.0% or more, more preferably 43.0% or more.

Furthermore, by satisfying the above component range, the 0.2% proof stress in the high temperature tensile test at 1100°F specified by ASTM E21 can be satisfied to be 140 MPa or more. If it is less than 140 MPa, cracks will occur in the above-mentioned high temperature environment, making it unsuitable for use as an alloy. Preferably it is 150 MPa or more, more preferably 160 MPa or more.

Furthermore, by satisfying the above component ranges, the elongation in the high temperature tensile test specified by ASTM E21 can be 50.0% or more. If it is less than 50.0%, cracks will occur in the above-mentioned high temperature environment, making the alloy unsuitable for use. Preferably it is 55.0% or more, more preferably 60.0% or more.

Furthermore, by satisfying the above-mentioned components, the average crystal grain size in the microscope test specified by ASTM E112 can be satisfied from -3 to 6. If it is less than -3, the crystal grains will be coarse and the high temperature 0.2% yield strength will decrease. If it exceeds 6, the 0.2% proof stress at room temperature will be high and the workability will be poor, making it unsuitable for manufacturing a reaction tower. It is preferably -2 to 5, more preferably -1 to 4.

In addition, by satisfying the above component range, one or more types of NbN, NbC, TiN, and TiC with a major axis of 0.5 to 5 μm can be added at an average of 20 to 180 pieces/mm ² in any cross section. Must contain, and if it contains one or more of NbN, NbC, TiN, and TiC with a major axis of more than 5 μm, the total number of these in any cross section is 5.0 pieces/mm2 ^or less on average. It becomes like this. One or more types of NbN, NbC, TiN, and TiC with a major axis of 0.5 to 5 μm are combined to an average of 20 pieces/mm in any cross section.If it is less than ² , the crystal grains will become coarse during annealing and the high temperature .2% yield strength cannot be satisfied, and the total of one or more types of NbN, NbC, TiN, and TiC with a major axis of 0.5 to 5 μm exceeds 180 pieces/mm ² on average in any cross section. When containing one or more types of NbN, NbC, TiN, and TiC with a major axis of more than 5 μm, if the total number of these exceeds an average of 5.0 pieces/ ^mm2 in any cross section, carbonitrides become the starting point of voids. High temperature elongation is lower. Preferably, it always contains one or more precipitates of NbN, NbC, TiN, and TiC with an average size of 30 to 170 particles of 0.5 to 5 μm, and one type of NbN, NbC, TiN, and TiC with a major axis of more than 5 μm. Or two or more types of precipitates with an average size of 3.0 pieces/mm ² or less, more preferably 0.5-5 μm, with an average of 40-160 pieces of one or more types of NbN, NbC, TiN, and TiC. The number of precipitates of one or more of NbN, NbC, TiN, and TiC with a major axis of more than 5 μm is 1.0 pieces/mm ² or less on average.

0.120≦Nb×N+Nb×C+Ti×N+Ti×C≦0.150...(Formula 1)
Equation 1 shows the range of the amount of carbonitride precipitates. If it is less than 0.120, precipitation of carbonitrides is small, crystal grains become coarse during annealing, and good high temperature 0.2% yield strength cannot be obtained. When it exceeds 0.150, carbonitrides become the starting point of voids and high temperature elongation becomes low. It is preferably 0.125 to 0.145, more preferably 0.130 to 0.140.

0.0020%≦B+P≦0.0400%...(Formula 2)
Equation 2 shows a range of favorable addition amounts of elements that segregate at grain boundaries. If it is less than 0.0020%, the amount of grain boundary segregation will be insufficient, the grain boundaries will not be strengthened, and a good high temperature 0.2% yield strength will not be obtained. If it exceeds 0.0400%, the amount of grain boundary segregation becomes excessive, causing the grain boundaries to become brittle and resulting in low high-temperature elongation. Preferably it is 0.0025% to 0.0300%, more preferably 0.0030% to 0.0200%.

Further, in the Fe-Cr-Ni alloy, one or more of NbN, NbC, TiN, TiC, and M23C6 having a major axis of 0.5 to 5 μm is precipitated at the grain boundaries in an arbitrary cross section. It is characterized in that the proportion of precipitates is 10 to 30%. If it is less than 10%, high temperature 0.2% proof stress cannot be obtained, and if it exceeds 30%, high temperature elongation cannot be satisfied. Preferably it is 12 to 28%, more preferably 15 to 25%.

It is also characterized by a hardness test value of 130 to 210 using a cemented carbide ball with a Brinell hardness specified by ASTM E10. If it is less than 130, high temperature 0.2% proof stress cannot be obtained, and if it exceeds 210, workability is poor and it is not suitable for manufacturing a reaction tower. Preferably it is 140-200, more preferably 150-190.

Raw materials such as iron scrap, ferrochrome, ferronickel, stainless steel scrap, etc. adjusted to a predetermined ratio are melted in an electric furnace and subjected to secondary refining in an AOD (Argon Oxygen Decarburization) furnace or a VOD (Vacuum Oxygen Decarburization) furnace. and table After adjusting the main component composition shown in 1, continuous casting was performed to obtain a steel billet (slab). The composition of C and S shown in the table was determined using a simultaneous carbon and sulfur analyzer (combustion in an oxygen stream - infrared absorption method), and the composition of N was determined using a simultaneous analyzer of oxygen and nitrogen (inert gas - impulse heating). The compositions other than the above are the values analyzed using fluorescent X-ray analysis. Next, the above slab was hot rolled and annealed at 1180° C. to 1300° C. to obtain a hot rolled annealed steel plate having a thickness of 20 mm.

For the evaluation of room temperature tensile strength, a round bar test piece conforming to ASTM E8 was prepared so that the longitudinal end was located in the direction perpendicular to the rolling direction of the above-mentioned hot rolled annealed steel sheet, and this test piece was used as specified in ASTM E8. A tensile test was conducted to measure 0.2% yield strength and elongation. ◎: 0.2% proof stress is 380 MPa or less and elongation is 43.0% or more; ○: 0.2% proof stress is 390 MPa or less and elongation is 42.0% or more. Those with a 0.2% yield strength of 400 MPa or less and an elongation of 40.0% or more were evaluated as Δ, and those with a 0.2% yield strength of 400 MPa or less and an elongation of 40.0% or more were evaluated as ×, as shown in Table 2.

For evaluation of high-temperature tensile strength, round bar test pieces were prepared in accordance with ASTM E8M so that their longitudinal sides were located in the direction perpendicular to the rolling direction of the hot-rolled annealed steel sheet. This test piece was subjected to a high-temperature tensile test at 1100°F as specified by ASTM E21, and the 0.2% proof stress, tensile strength, and elongation were measured. ◎: 0.2% proof stress is 160 MPa or more and elongation is 60.0% or more; ○: 0.2% proof stress is 150 MPa or more and elongation is 55.0% or more. Those with a 0.2% yield strength of 140 MPa or more and an elongation of 50.0% or more were evaluated as Δ, and those with a 0.2% yield strength of 140 MPa or more and an elongation of 50.0% or more were evaluated as ×, and the results are shown in Table 2.

The average crystal grain size was evaluated by cutting the hot-rolled annealed sheet, mirror polishing it, and electrolytically etching it with oxalic acid at 0.16 mA/mm ² . This test piece was subjected to a microscope test specified in ASTM E112 using a comparative method. Those that satisfied -1 to 4 were evaluated as ◎, those that satisfied -2 to 5 as ○, those that satisfied -3 to 6 as △, and those that did not satisfy -3 to 6 as ×, and are shown in Table 2.

Carbonitrides of NbN, NbC, TiN, and TiC were evaluated by cutting the hot-rolled annealed steel sheet, mirror-polishing it, and performing observation and composition analysis using SEM-EDS. 100 photographs were taken at a magnification of 10,000 times and the average number of photographs was counted. Furthermore, the chemical composition was determined by EDS. At this magnification, carbonitrides up to 0.5 μm were visible, so 0.5 μm is set as the lower limit in the present invention. The average number of carbonitrides with a major axis of 0.5 to 5 μm is 40 to 160 pieces/ ^mm2 , and the average number of carbonitrides with a major axis of more than 5 μm is 0 to 1.0 pieces/ ^mm2 . The average number of carbonitrides with a diameter of 0.5 to 5 μm is 30 to 170 pieces/ ^mm2 , and the average number of carbonitrides with a major axis of more than 5 μm is more than 1.0 to 3.0 pieces/ ^mm2. ○, the average number of carbonitrides with a major axis of 0.5 to 5 μm is 20 to 180 pieces/mm2, ^and the average number of carbonitrides with a major axis of more than 5 μm is more than 3.0 to 5.0 pieces/ ^mm2. △, the average number of carbonitrides with a major axis of 0.5 to 5 μm is 20 to 180 pieces/ ^mm2 , or the average number of carbonitrides with a major axis of more than 5 μm is 5.0 pieces/mm Those exceeding ² were evaluated as × and shown in Table 2.

The evaluation of formula 1 is ◎ if it satisfies 0.130 to 0.140, ○ if it satisfies 0.125 to 0.145, △ if it satisfies 0.120 to 0.150, and △ if it satisfies 0.120 to 0.145. Those that did not satisfy 150 were evaluated as × and shown in Table 3.

The evaluation of formula 2 is: ◎ if it satisfies 0.0030% to 0.0200%, ○ if it satisfies 0.0025% to 0.0300%, △ if it satisfies 0.0020% to 0.0400%. Those that did not satisfy 0.0020% to 0.0400% were evaluated as × and shown in Table 3.

For evaluation of the precipitate distribution of NbN, NbC, TiN, TiC, and M23C6, the hot rolled annealed steel sheet was cut, mirror polished, and observed and compositionally analyzed using SEM-EDS. 100 photographs were taken at a magnification of 1000 times and the average distribution was measured. Furthermore, the chemical composition was determined by EDS. ◎ if the ratio of precipitates precipitated at grain boundaries is 15 to 25%, ○ if it is 12 to 28%, △ if it is 10 to 30%, × if it is not 10 to 30%. The results are shown in Table 3.

The hardness was evaluated by cutting out the hot-rolled annealed plate, polishing the surface to No. 320, and performing a Brinell hardness tester using cemented carbide balls specified in ASTM E10. Those whose hardness satisfies 150 to 190 are evaluated as ◎, those whose hardness satisfies 140 to 200 are evaluated as ○, those whose hardness satisfies 130 to 210 are evaluated as △, and those whose hardness does not satisfy 130 to 210 are evaluated as ×, and are shown in Table 3.

As a comprehensive evaluation, if the components according to claim 1, room temperature 0.2% proof stress, room temperature elongation, high temperature 0.2% proof stress, high temperature elongation, average grain size, and number of precipitates are evaluated as ◎, ○, or △. If there is ○ or one or more ×, it is evaluated as × and shown in Table 3.

Since all of the invention examples satisfied the scope of the present invention, the room temperature strength, high temperature strength, crystal grain size, and number of carbonitrides were satisfied.

In sample number 18, S exceeded the upper limit, so a large amount of sulfide was formed, and the high temperature 0.2% yield strength and high temperature elongation did not satisfy the specified values.

In sample No. 19, B and P exceeded the upper limit, so the grain boundaries became brittle, and elongation at room temperature, 0.2% proof stress at high temperature, and elongation at high temperature did not satisfy the specified values. Furthermore, the values of Equations 1 and 2 exceeded the upper limits, and the grain boundary precipitation ratio of precipitates did not satisfy the lower limit.

In sample number 20, N and Nb exceeded the upper limit, so a large amount of nitride precipitated, and the 0.2% yield strength at room temperature, high temperature elongation, number of precipitates, and crystal grain size did not satisfy the specified values. Moreover, the value of Formula 1 exceeded the upper limit.

Sample No. 21 had C and Ti exceeding the upper limit, so many carbides were precipitated, and the high temperature elongation, number of precipitates, and crystal grain size did not satisfy the specified values. Moreover, the value of Formula 1 exceeded the upper limit.

In sample No. 22, the Co content did not meet the lower limit, so the high temperature strength was not enhanced, and the high temperature 0.2% proof stress and high temperature elongation did not satisfy the specified values.

Sample No. 23 had less nitrides because Nb and N did not meet the lower limits, and the lower limits of high temperature 0.2% yield strength, number of precipitates, grain size, and hardness test did not satisfy the specified values. Moreover, the value of Formula 1 did not satisfy the lower limit.

Sample No. 24 had less carbide because C did not meet the lower limit, and the high temperature 0.2% yield strength did not satisfy the specified value. Moreover, the value of Formula 1 did not satisfy the lower limit. Cu also exceeded the upper limit and did not satisfy the lower limit of the hardness test.

In sample number 25, C exceeded the upper limit and Cu did not meet the lower limit, so the room temperature strength was high, and the room temperature 0.2% proof stress and room temperature elongation did not satisfy the specified values. Since B also did not satisfy the lower limit, the high temperature 0.2% yield strength did not satisfy the specified value. Moreover, the value of Formula 2 did not satisfy the lower limit, and the hardness test value also exceeded the upper limit.

In sample number 26, since B did not meet the lower limit, the grain boundaries were not strengthened and the high temperature 0.2% yield strength did not satisfy the specified value. Furthermore, the value of Equation 2 did not satisfy the upper limit, and the grain boundary precipitation ratio of precipitates was low.

In sample number 27, C and Nb did not meet the lower limits, so there were few carbides, and the high temperature 0.2% proof stress, number of precipitates, and crystal grain size did not satisfy the specified values. Moreover, the value of Formula 1 did not satisfy the lower limit.

In sample No. 28, Co exceeded the upper limit, so the high temperature elongation did not satisfy the specified value.

Claims (8)

Mass%: C: 0.03-0.08%, Si: 0.10-0.50%, Mn: 0.20-1.20%, P: 0.001-0.040%, S: 0 .0001 to 0.0030%, Ni: 35.5 to 45.5%, Cr: 23.5 to 26.0%, Mo: 0.30 to 1.50%, Cu: 0.01 to 0.30 %, Al: 0.010-0.150%, Ti: 0.10% or less, B: 0.0005-0.0050%, Co: 0.02-0.30%, Nb: 0.45-0 .60% and N: 0.15 to 0.30%, the remainder consists of Fe and unavoidable impurities, and the 0.2% proof stress value in the room temperature tensile test specified by ASTM E8 is 400 MPa or less, The elongation satisfies 40.0% or more, the 0.2% proof stress value in the high temperature tensile test at 1100°F specified by ASTM E21 is 140 MPa or more, the elongation satisfies 50.0% or more, and the value is specified by ASTM E112. One or more types of NbN, NbC, TiN, and TiC with an average crystal grain size of −3 to 6 and a major axis of 0.5 to 5 μm in a total of 20 to 180 pieces/ ^mm2 , and in cases where one or more types of NbN, NbC, TiN, and TiC with a major axis of more than 5 μm are included, the total average of these is 5.0 pieces/ ^mm2 in any cross section. A Fe-Cr-Ni alloy characterized by the following: The Fe-Cr-Ni alloy according to claim 1, wherein the mass % of each component is adjusted to 0.120≦Nb×N+Nb×C+Ti×N+Ti×C≦0.150. The mass percentage of each component is adjusted to 0.0020%≦B+P≦0.0400%, and the total of one or more types of NbN, NbC, TiN, TiC, and M23C6 with a major axis of 0.5 to 5 μm is used. The Fe-Cr-Ni alloy according to claim 1, characterized in that the proportion of precipitates precipitated at grain boundaries in any cross section is 10 to 30% in terms of number ratio. The mass percentage of each component is adjusted to 0.0020%≦B+P≦0.0400%, and the total of one or more types of NbN, NbC, TiN, TiC, and M23C6 with a major axis of 0.5 to 5 μm is used. The Fe-Cr-Ni alloy according to claim 2, wherein the proportion of precipitates precipitated at grain boundaries in any cross section is 10 to 30% in terms of number ratio. The Fe-Cr-Ni alloy according to claim 1, wherein the Fe-Cr-Ni alloy has a hardness test value of 130 to 210 using a cemented carbide ball having a Brinell hardness specified by ASTM E10. The Fe-Cr-Ni alloy according to claim 2, wherein the Fe-Cr-Ni alloy has a hardness test value of 130 to 210 using a cemented carbide ball having a Brinell hardness specified by ASTM E10. The Fe-Cr-Ni alloy according to claim 3, wherein the Fe-Cr-Ni alloy has a hardness value of 130 to 210 in a hardness test using a cemented carbide ball having a Brinell hardness specified by ASTM E10. The Fe-Cr-Ni alloy according to claim 4, characterized in that the hardness test value of a cemented carbide ball having a Brinell hardness specified by ASTM E10 is 130 to 210.

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