JP6955322B2 - Austenitic heat-resistant steel with excellent workability, high-temperature strength and toughness after aging - Google Patents

Description

The present invention relates to ultra-supercritical coal-fired power generation, that is, steel for high-strength boilers used for advanced ultra-supercritical pressure-fired power generation and integrated coal gasification combined cycle, and austenite having excellent workability, high-temperature strength, and toughness after aging. Regarding heat-resistant steel.

In recent years, reduction of carbon dioxide emissions has been required as a measure against global warming. By the way, coal-fired power generation systems are widely adopted all over the world as a main power source because of their high economic efficiency and stability. However, since it is a power generation method that emits the largest amount of carbon dioxide, further improvement in power generation efficiency is required.

Therefore, the applicant examined the components with the Fe group composition in consideration of cost, and had good high-temperature creep strength and toughness after aging (the impact value after aging for a long period of time is referred to as "toughness after aging"). Austenite-based heat-resistant steel has been developed (see, for example, Patent Document 1). However, this steel is a high N material that requires 0.07% or more of N as a component element, and needs to be improved in terms of hot workability and toughness after aging.

On the other hand, it is a high-temperature austenitic stainless steel with excellent workability after long-term use, as a steel pipe used as a boiler superheater tube or reheater tube, a reaction furnace tube for the chemical industry, and a heat-resistant pressure-resistant member. Austenitic stainless steels for high temperatures, which are suitable as materials for steel sheets, steel bars, forged steels, etc. and have excellent workability after long-term use, have been developed (see, for example, Patent Document 2). This steel material is similar to the above steel material, but this steel material is also a high N material, and also needs to be improved in terms of hot workability and toughness after aging.

Further, austenitic heat-resistant steel has been proposed as having excellent creep strength and toughness in a high temperature environment of 700 ° C. or higher (see, for example, Patent Document 3). However, the proposed steel also needs to be improved in terms of high temperature creep strength.

Japanese Patent No. 5661001 Japanese Patent No. 4946758 Japanese Unexamined Patent Publication No. 2015-183261

In the field of thermal power generation, in order to increase the power generation efficiency more than before, a heat-resistant material having a creep strength of 100 MPa or more at 700 ° C. and 100,000 hours is required. So far, materials that achieve this goal have been found, but the current candidate materials are Ni-based alloys with a Ni content of 40% by mass or more, so the alloy cost is high and the economy is poor. Therefore, from these viewpoints, Fe-based alloys are also being studied. In the study of this Fe-based alloy, it is possible to reduce the amount of Ni and reduce the alloy cost, but since the reduction of Ni promotes the precipitation of the σ phase, which is the embrittlement phase, this alloy is kept at a high temperature. Is significantly embrittled. Further, a material that requires high-temperature strength naturally has a high deformation resistance at a high temperature, and therefore has a problem of poor hot workability.

Therefore, the problem to be solved by the present invention is that, in addition to the target creep strength, it has thermal stability while suppressing the alloy cost, has good hot workability, and is excellent even after high temperature holding. It is an object of the present invention to provide an austenite-based heat-resistant steel having toughness (hereinafter referred to as "toughness after aging").

In the means of claim 1 of the present application, the means for solving the above problems are, in terms of mass%, C: 0.01 to 0.10%, Si: 0.25 to 1.00%, Mn: 1. 00 to 2.50%, Ni: 9.5 to 32.5%, Cr: 13.0 to 25.0%, Mo: 0.01 to 2.00%, Al: 0.05% or less, Nb: It contains 0.10 to 0.80, W: 5.00 to 9.00%, N: 0.005 to 0.015%, B: 0.001 to 0.005%, and Ti: 0. It contains one or more of Ti, V, and Ta which are 500% or less, V: 0.20% or less, and Ta: 1.000% or less, and as unavoidable impurities, P: 0.040% or less, S. : 0.010% or less, Cu: 0.10% or less, the balance consists of Fe and unavoidable impurities other than the above, and formula 1: {([Mo] +0.5 [W]) / 5} + {( 15 [C] +13 [N]) / (3.8 [Ti] +1.9 [Nb] +3.5 [V] +1.1 [Ta])} = 1.5 to 4.0, and Equation 2: Of the fine carbides, intermetallic compounds, and monometal phases that satisfy both the equations ([Cr] +3 [Mo] -15.8) / [Ni] ≤0.25 and have high thermal stability. The ratio of precipitation of one or more types at 700 ° C is 5% or more, the drawing value by hot working is 70% or more, the high temperature breaking strength is 100 MPa or more, and the shearing impact value after aging is 30 J / cm. It is an austenite-based heat-resistant steel having excellent workability, high-temperature strength, and toughness after aging, which is characterized by having ^{2 or more.}
However, the [element symbol] of the above formulas 1 and 2 is a numerical value in 100% of each element in the above chemical composition.

In the means of claim 2, in terms of mass%, C: 0.01 to 0.10%, Si: 0.25 to 1.00%, Mn: 1.00 to 2.50%, Ni: 9.5 to 5 32.5%, Cr: 13.0 to 25.0%, Mo: 0.01 to 2.00%, Al: 0.05% or less, Nb: 0.10 to 0.80%, W: 5. It contains 00 to 9.00%, N: 0.005 to 0.015%, B: 0.001 to 0.005%, and further, Ti: 0.500% or less, V: 0.20% or less, Ta: Contains one or more of Ti, V, and Ta which is 1.000% or less, and further, Ca: 0.0001 to 0.0200%, Mg: 0.0001 to 0.0200%, REM: It contains one or more of Ca, Mg, and REM, which are 0.0001 to 0.0200% below, and as unavoidable impurities, P: 0.040% or less, S: 0.010% or less, Cu: 0. It contains 10% or less, and consists of the balance Fe and unavoidable impurities other than the above. Formula 1: {([Mo] +0.5 [W]) / 5} + {(15 [C] + 13 [N]) / (3.8 [Ti] +1.9 [Nb] +3.5 [V] +1.1 [Ta])} = 1.5 to 4.0, and Equation 2: ([Cr] +3 [Mo] -15 .8) / [Ni] ≤ 0.25, the area when one or more of the fine carbides, intermetallic compounds, and monometal phases with high thermal stability are at 700 ° C. Processing characterized by a rate precipitation rate of 5% or more, a drawing value by hot working of 70% or more, a high temperature breaking strength of 100 MPa or more, and a charpy impact value after aging of 30 J / cm ² or more. It is an austenite-based heat-resistant steel with excellent properties, high-temperature strength, and toughness after aging.
However, the [element symbol] of the above formulas 1 and 2 is a numerical value in 100% of each element in the above chemical composition.

By using the means of claim 1 and the means of claim 2 of the present application, good hot workability can be obtained while suppressing the alloy cost, and further, the σ phase is precipitated by limiting the value of the formula 1. It is possible to obtain a precipitate with high thermal stability by achieving excellent high-temperature creep rupture strength while suppressing the precipitation of Excellent effects such as being able to be secured can be obtained.

Prior to the description of the embodiment for carrying out the invention of the present application, the reason for limiting the chemical composition of the austenitic heat-resistant steel excellent in processability and high-temperature strength in the means of the above-mentioned claim of the present invention, P, which is an unavoidable impurity, The reasons for limiting the chemical components of S and Cu, and the reasons for limiting the formulas 1 and 2 will be described. In addition,% in these is mass%.

C: 0.01 to 0.10%
C is an element necessary for improving high-temperature creep strength by solid solution strengthening and formation of fine carbonitride. For this purpose, it is necessary to add 0.01% or more of C. However, when C exceeds 0.10%, the formation of coarse carbides is promoted, and the high temperature creep strength and the toughness after aging deteriorate. Therefore, C is set to 0.01 to 0.10%.

Si: 0.25 to 1.00%, preferably Si: 0.25 to 0.95%
Si is an element required for deoxidation during refining. For this reason, it is necessary to add 0.20% or more of Si. However, even if Si is contained in an amount of more than 1.00%, the above effect is saturated, and further, the formation of the σ phase in the steel is promoted, and the toughness after aging is deteriorated. Therefore, Si is set to 0.25 to 1.00%, and Si is preferably set to 0.25 to 0.95%.

Mn: 1.00 to 2.50%, preferably Mn: 1.00 to 2.45%
Mn is an element required for deoxidation during refining and further for stabilizing austenite in steel. Therefore, it is necessary to add 1.00% or more of Mn. However, even if Mn is contained in an amount of more than 2.50%, it is only added excessively and the cost is increased. Therefore, Mn is set to 1.00 to 2.50%, preferably Mn: 1.00 to 2.45%.

Ni: 9.5 to 32.5%
Ni is an element that stabilizes the austenite structure. Therefore, Ni needs to contain 9.5% or more. However, since Ni is an expensive element, if it contains more than 32.5% of Ni, the cost becomes high. Therefore, Ni is set to 9.5 to 32.5%.

Cr: 13.0 to 25.0%
Cr is an element that improves high-temperature corrosion resistance and water vapor oxidation resistance. Therefore, Cr needs to be contained in an amount of 13.0% or more. However, even if Cr is contained in an amount of more than 24.0%, the effect of improving the high temperature corrosion resistance and the water vapor oxidation resistance is saturated, and the formation of the σ phase is promoted to deteriorate the toughness after aging. Therefore, Cr is set to 13.0 to 24.0%.

Mo: 0.01-2.00%
Mo is an element that improves high-temperature creep strength by strengthening solid solution and forming fine precipitates. Therefore, it is necessary to add 0.01% or more of Mo. However, even if Mo is added in a large amount exceeding 2.00%, the effects of strengthening the solid solution and improving the high-temperature creep strength are saturated, and the formation of the σ phase is promoted, so that the toughness after aging deteriorates. , Increase the cost. Therefore, Mo is set to 0.01 to 2.00%.

Al: ≤0.05%
Al is an element added for deoxidation during refining. However, if Al is added in excess of 0.05%, the toughness after aging is deteriorated due to the formation of AlN. Therefore, Al is set to 0.05% or less.

Nb: 0.10 to 0.80%
Nb is an element that improves high-temperature creep characteristics by forming fine precipitates. For that purpose, it is necessary to add 0.10% or more of Nb. However, even if Nb is added in a large amount exceeding 0.80%, the effect of improving the high temperature creep property is saturated, the formation of the σ phase is promoted, and the toughness after aging deteriorates. Therefore, Nb is set to 0.10 to 0.80%.

W: 5.00 to 9.00%
W is an element that improves high-temperature creep strength by strengthening solid solution and forming fine precipitates. For that purpose, it is necessary to add 5.00% or more of W. However, even if W is contained in an amount of more than 9.00%, the effect of improving the high temperature creep characteristic is saturated, and the formation of the σ phase is promoted, the toughness after aging deteriorates, and the cost increases. Therefore, W is set to 5.00 to 9.00%.

N: 0.005 to 0.015%
N is an element that improves high-temperature creep strength by solid solution strengthening and formation of fine carbonitride. For that purpose, it is necessary to add 0.005% or more of N. However, even if N is contained in an amount of more than 0.015%, the effect of improving the high temperature creep strength is saturated, and the hot workability and the toughness after aging deteriorate due to the excessive formation of nitrides. Therefore, N is set to 0.005 to 0.015%.

B: 0.001 to 0.005%
B is an element that improves high-temperature creep strength and hot workability by strengthening grain boundaries. Therefore, it is necessary to add 0.001% or more of B. However, even if B is contained in an amount of more than 0.005%, the effect of improving the high temperature creep strength and the hot workability is saturated, and the hot workability is deteriorated by excessive addition. Therefore, B is set to 0.001 to 0.005%.

Ti: 0.500% or less, preferably Ti: 0.480% or less When Ti is contained in excess of 0.500%, it promotes the formation of coarse carbonitride in the steel, resulting in an excess of intermetallic compounds. Produces to reduce high temperature creep strength and deteriorate toughness after aging. Therefore, Ti is 0.500% or less, preferably 0.480% or less.

V: 0.20% or less When V is contained in excess of 0.20%, it promotes the formation of coarse carbonitride in steel, excessively produces intermetallic compounds, and lowers high-temperature creep strength. However, the toughness after aging deteriorates. Therefore, V is set to 0.20% or less.

Ta: 1.000% or less When Ta is contained in excess of 1.000%, it promotes the formation of coarse carbon nitride in the steel, excessively produces intermetallic compounds, and lowers the high temperature creep strength. However, the toughness after aging deteriorates. Therefore, Ta is set to 1.000% or less.

In the invention of the means of claim 1, one or more of the above Ti, V, and Ta shall be contained within the above range.

In the means of claim 2, one or more of Ca: 0.0200% or less, Mg: 0.0200% or less, and REM: 0.0200% or less In the invention of the means of claim 2, Ca, Mg, REM. Is required to improve hot workability by fixing S in steel by any one or more of these elements. For that purpose, it is necessary to add one or more of each of these elements in an amount of 0.0200% or less.

P: 0.040% or less P is an element contained as an unavoidable impurity in the invention of the means according to the present application. By the way, if P is contained in an amount of more than 0.040%, the hot workability of steel is deteriorated. Therefore, P is set to 0.040% or less.

S: 0.010% or less, preferably S: 0.008% or less S is an element as an unavoidable impurity in the invention of the means according to the present application. By the way, if S is contained in an amount of more than 0.010%, the hot workability of steel is deteriorated. Therefore, S is 0.010% or less, preferably S: 0.008% or less.

Cu: 0.10%
Cu is an element as an unavoidable impurity in the invention of the means according to the present application. By the way, if Cu is contained in an amount of more than 0.10%, the hot workability of steel is deteriorated. Therefore, Cu is set to 0.10% or less.

Equation 1 = {([Mo] +0.5 [W]) / 5} + {(15 [C] +13 [N]) / (3.8 [Ti] +1.9 [Nb] +3.5 [V] When +1.1 [Ta])}, equations 1: 1.5 to 4.0
When the value of the formula 1 is less than 1.5, the amount of solid solution strengthening and the amount of thermally stable precipitates produced are reduced, and the high temperature creep strength is deteriorated. On the other hand, when the value of Equation 1 exceeds 4.0, the formation of the σ phase is promoted and the toughness after aging deteriorates. Therefore, Equation 1 is set to 1.5 to 4.0. The [element symbol] described in Equation 1 is a numerical value in 100% of each element in the above chemical composition.

When equation 2 = ([Cr] +3 [Mo] -15.8) / [Ni], equation 2: 0.25 or less When the value of equation 2 exceeds 0.25, the generation of σ phase is promoted. , Deteriorates toughness after aging. Therefore, Equation 2 is set to 0.25 or less. The [element symbol] in the formula 2 is a numerical value in 100% of each element in the above chemical composition.

Percentage of area ratio of one or more precipitates of fine carbon nitride, intermetal compound, and monometal phase with high thermal stability at 700 ° C: 5% or more The reason why the ratio of the area ratio precipitated at 700 ° C. is 5% or more is that if it is 5% or more, the precipitates are in a dispersed state.

Here, a mode for carrying out the invention of the present application will be described below including examples and comparative examples. First, No. 1 of the examples shown in Table 1. Nos. 1 to 16 and Comparative Examples shown in Table 2 100 kg each of the steels of the chemical components of the test materials 17 to 38 were melted in a vacuum induction melting (VIM) furnace to obtain 100 kg of steel ingots. These ingots were then heated to 1200-1300 ° C. for homogenization. This homogenized ingot was indexed and a google test was carried out. On the other hand, the cooled and homogenized ingot is heated to 1150 to 1200 ° C. and forged into a steel bar having a diameter of 20 mm. Then, a test piece for evaluating the material properties of hot workability, creep breaking strength, and toughness after aging was prepared.

The precipitate area ratios in the examples shown in Table 1 and the comparative examples shown in Table 2 are obtained by performing a heat treatment for holding at 700 ° C. for 3000 hours after a solution treatment using the above-mentioned steel bar having a diameter of 20 mm. It was identified by observing the precipitate with a transmission electron microscope. For this purpose, the area ratios of the carbonitrides, intermetallic compounds, and monometallic phases precipitated in the grains, excluding _{the M 23} C ₆ type carbides at the grain boundaries with low thermal stability, ^{were measured in a 100 μm 2 field.} bottom. Each area ratio is shown in Tables 1 and 2 as "precipitate area ratio (%)". The value satisfying the range of the precipitate area ratio of the present invention is 5% or more.

In the evaluation of hot workability in Tables 1 and 2, the ingot homogenized above was indexed into a gleeble test piece of steel bar having a diameter of 8 mm, and rapidly heated in the gleeble test to draw a drawing at 1100 ° C. As an evaluation of hot workability, in the "Hot workability" column of Tables 1 and 2, a drawing value of 70% or more is indicated by "○", and a drawing value of less than 70% is indicated by "x". It was written as.

In the evaluation of the high-temperature creep rupture strength in Tables 1 and 2, the steel bars with a diameter of 20 mm forged above were subjected to solidification heat treatment and processed to a parallel portion diameter of 6 mm and a scoring distance of 30 mm to obtain a test piece. Fracture tests were performed at 700 ° C, 750 ° C, and 800 ° C for up to 10,000 hours using test pieces, and the test results were organized by the parameters of the Larson-Miller method, and 100,000 at 700 ° C. The estimated high temperature creep rupture strength at the time point was determined. As an evaluation of the estimated high temperature creep rupture strength, the estimated value of 100 MPa or more is indicated by "○" in the "creep rupture strength" column of Tables 1 and 2, and the estimated value of less than 100 MPa is shown in Tables 1 and 2. Indicated as "x" in the "creep rupture strength" column.

The evaluation of toughness after aging in Tables 1 and 2 is performed by aging at 700 ° C., 750 ° C., and 800 ° C. for a maximum of 10,000 hours after solution heat treatment using the steel bars with a diameter of 20 mm forged above. After the subsequent treatment, it was processed into an impact test piece having a width of 10 mm and a 2 mm V notch. Using this test piece, a Charpy impact test was carried out at room temperature. The results were arranged according to the parameters of Orr-Sherby-Dorn, and the estimated Charpy impact value at 700 ° C. for 100,000 hours was obtained. An estimated Charpy impact value of 30 J / cm ² or more is indicated as "○" in the "Aging toughness" column of Tables 1 and 2 as an evaluation of toughness after aging, and the estimated Charpy impact value is less than ^{30 J / cm 2.} Is indicated by "x" in the column of "aging toughness" in Tables 1 and 2.

The values of Equation 1 in Tables 1 and 2 are No. 1 of Examples. 1 to 16 are all in the range of 1.5 to 4.0 of the value specified in the claim, but No. 1 of the comparative example. In 25, the value of Equation 1 is larger than 4.0, which is the upper limit of the value specified in claim in 4.3, and in No. 36 of the comparative example, the value of Equation 1 is 1.0, which is specified in claim. It is less than the lower limit of 1.5. Further, the values of Equation 2 in Tables 1 and 2 are No. 1 of Examples. In 1 to 16, all of them are 0.25 or less, which is the upper limit of the value specified in the claim. In 17, the value of Equation 2 is 0.28, which is larger than the upper limit of 0.25 of the value specified in the claim, and No. In 34, the value of Equation 2 is 0.51, which is larger than the upper limit of 0.25, which is also defined in the claims.

In Tables 1 and 2,% of the precipitation area ratio is No. 1 of Example. In 1 to 16, all of them are 5% or more of the value specified in the claim, but No. 1 of the comparative example. At 24, the precipitation area ratio is 3%, which is smaller than the lower limit of 5% of the value specified in the claims.

Further, the hot workability, creep rupture strength, and aging toughness, which are the material property evaluations in Tables 1 and 2, are described in No. 1 of Examples. In all of 1 to 16, the aperture value is 70% or more, the high temperature breaking strength is 100 MPa or more, and the Charpy impact value after aging is 30 J / cm ² or more, which are defined by the claims by "○". Example No. 17 to 38 are any of the drawing value, the high temperature rupture strength value, and the Charpy impact value after aging, wherein any of the hot workability, creep rupture strength, and aging toughness is defined by "x". Is not satisfied.

Claims (2)

By mass%, C: 0.01 to 0.10%, Si: 0.25 to 1.00%, Mn: 1.00 to 2.50%, Ni: 9.5 to 32.5%, Cr: 13.0 to 25.0%, Mo: 0.01 to 2.00%, Al: 0.05% or less, Nb: 0.10 to 0.80, W: 5.00 to 9.00%, N : 0.005 to 0.015%, B: 0.001 to 0.005%, Ti: 0.500% or less, V: 0.20% or less, Ta: 1.000% or less. It contains one or more of certain Ti, V, and Ta, and further contains P: 0.040% or less, S: 0.010% or less, Cu: 0.10% or less as unavoidable impurities, and the balance Fe. And unavoidable impurities other than the above, formula 1: {([Mo] +0.5 [W]) / 5} + {(15 [C] + 13 [N]) / (3.8 [Ti] +1.9 [Nb] +3.5 [V] +1.1 [Ta])} = 1.5 to 4.0, and Equation 2: ([Cr] +3 [Mo] -15.8) / [Ni] ≤ 0. One or more of fine carbon nitrides , intermetallic compounds, and monometal phases that satisfy both equations 25 and have high thermal stability are precipitated at an area ratio when held at 700 ° C. for 3000 hours. The ratio is 5% or more, the drawing value by hot working at 1100 ° C is 70% or more, the estimated high temperature creep breaking strength at 700 ° C for 100,000 hours is 100 MPa or more, and after aging at 700 ° C for 100,000 hours. An austenitic heat-resistant steel having excellent workability, high-temperature strength, and toughness after aging, characterized in that the estimated value of the Charpy impact value at room temperature is 30 J / cm ^{2 or more.}
However, the [element symbol] of the above formulas 1 and 2 is a numerical value in 100% of each element in the above chemical composition. By mass%, C: 0.01 to 0.10%, Si: 0.25 to 1.00%, Mn: 1.00 to 2.50%, Ni: 9.5 to 32.5%, Cr: 13.0 to 25.0%, Mo: 0.01 to 2.00%, Al: 0.05% or less, Nb: 0.10 to 0.80%, W: 5.00 to 9.00%, N: 0.005 to 0.015%, B: 0.001 to 0.005%, Ti: 0.500% or less, V: 0.20% or less, Ta: 1.000% or less It contains one or more of Ti, V, and Ta, and further, Ca: 0.0001 to 0.0200%, Mg: 0.0001 to 0.0200%, REM: 0.0001 to 0.0200. It contains one or more of Ca, Mg, and REM which are%, and further contains P: 0.040% or less, S: 0.010% or less, and Cu: 0.10% or less as unavoidable impurities. It consists of the balance Fe and unavoidable impurities other than the above, and formula 1: {([Mo] +0.5 [W]) / 5} + {(15 [C] +13 [N]) / (3.8 [Ti] + 1 .9 [Nb] +3.5 [V] +1.1 [Ta])} = 1.5 to 4.0, and Equation 2: ([Cr] +3 [Mo] -15.8) / [Ni] Area ratio when one or more of fine carbon nitrides , intermetallic compounds, and monometal phases that satisfy both equations of ≤0.25 and have high thermal stability are held at 700 ° C. for 3000 hours. The rate of precipitation is 5% or more, the drawing value by hot working at 1100 ° C is 70% or more, the estimated high temperature creep breaking strength at 700 ° C for 100,000 hours is 100 MPa or more, and 100,000 hours at 700 ° C. An austenitic heat-resistant steel having excellent workability, high-temperature strength, and toughness after aging, characterized in that the estimated value of the Charpy impact value at room temperature after aging is 30 J / cm ^{2 or more.}
However, the [element symbol] of the above formulas 1 and 2 is a numerical value in 100% of each element in the above chemical composition.