JP5930635B2 - Austenitic heat resistant steel having excellent high temperature strength and method for producing the same - Google Patents

Description

  The present invention relates to an austenitic heat-resistant steel having high-temperature strength, which is a material of a steel pipe for a high-strength boiler used in coal-fired power generation or coal gasification combined power generation with ultra super critical pressure, and a method for producing the same.

  Coal-fired power generation systems are widely used as a major power source all over the world because of their high economic efficiency and safety. However, since it is a power generation method that emits most carbon dioxide, in recent years, there has been a strong demand for higher efficiency in power generation.

  As a conventional technique, a high-strength austenitic heat-resistant alloy having a high content of Mo, W, and Nb has been proposed (see, for example, Patent Document 1). This method for producing a high-strength austenitic heat-resistant alloy is a method for significantly increasing the creep rupture strength by the synergistic effect by the combined addition of Mo and W and the effects by the combined addition of Nb and Ti and the addition of B. In general, elemental segregation in a steel material lowers the high-temperature strength. Therefore, in order to reduce segregation as much as possible, homogenization heat treatment and hot working at a higher temperature are performed. In particular, hot working at a higher temperature is known as the most effective means for homogenization because a diffusion effect by recrystallization can be obtained simultaneously. However, in this method, due to the effect that a considerable amount of B is added to improve the strength, the overheating temperature of the steel material is lowered, making it difficult to process at higher temperatures, and the strengthening elements Mo, W, and Nb are sufficient. Therefore, there was room for further strength improvement.

  Furthermore, the steel material containing W and Nb can be welded by prescribing the heating temperature in hot extrusion, the cooling rate after processing, the solution heat treatment temperature, and the amount of Nb solid solution in the steel after solution heat treatment. A production method for obtaining an excellent austenitic heat-resistant steel has been proposed (for example, see Patent Document 2). This method uses a hot extrusion method in which high reduction processing is performed in a short time and cooling is performed immediately. For this reason, it cannot be said that the homogenization of the reinforcing elements W and Nb is sufficient, and there is room for improvement in strength.

  Furthermore, a manufacturing method has been proposed in which an austenitic heat-resistant steel pipe having excellent creep rupture strength is obtained by subjecting a steel pipe after solution heat treatment to a further small amount of cold working (see, for example, Patent Document 3). . However, in actual boiler production, it is necessary to cold-bend the steel pipe greatly, and then a solution heat treatment of the bent portion must be performed. Therefore, the strengthening by cold working is reset in the bent portion, and a significant decrease in strength is inevitable. As a result, it is difficult to apply the steel pipe by this method.

JP 63-183155 A Japanese Patent Laid-Open No. 11-21624 JP 2002-212634 A

  In order to increase the efficiency of a coal-fired power generation system, a heat-resistant steel having excellent creep rupture strength at 700 ° C. is currently required. Many of the high-strength materials that have been developed so far contain Nb, Mo, W, and the like. However, while Nb is an element that is easily segregated in the steel material, Mo and W are elements that diffuse slowly in the steel material, so that the strength may decrease due to segregation.

  Therefore, the problem to be solved by the present invention is to provide an austenitic heat resistant steel having excellent creep rupture strength at 700 ° C. and a method for producing the same.

  The means of the present invention for solving the above-mentioned problems are to optimize the chemical components of Mo, W and Nb-added austenitic heat-resisting steel, and to heat-hold and hot-work the steel at a higher temperature. It is to homogenize the chemical composition.

  Therefore, the first amount to be considered is the amount of B added in the alloy composition. In general, B is an element effective for improving the high temperature strength and hot work ductility of the austenitic heat resistant steel, so about 0.001% to 0.005% is added to the steel material. On the other hand, B generates iron chromium boride and lowers the overheating temperature of the steel material, so that hot working at a higher temperature cannot be performed, making it difficult to homogenize the steel material composition. Therefore, in order to achieve homogenization of the composition, it is important to keep the B content to a minimum. The second is a small amount of undissolved carbonitride during hot working. A small amount of undissolved carbide suppresses the coarsening of crystal grains during high-temperature heating, contributes to the refinement of crystal grains during hot working without deteriorating hot workability, and has the effect of homogenization by diffusion. Increase further. Therefore, in order to further increase the homogenization of Mo, W and Nb by recrystallization during hot working, it is important that a very small amount of undissolved Nb carbonitride is present during hot working. Thirdly, the steel ingot is heated at 1200 ° C. or higher and 1290 ° C. or lower for 1 hour or longer, thereby increasing the self-diffusion of Mo, W and Nb in the steel material and homogenizing. Fourthly, the steel ingot is subjected to forging or rolling at a higher temperature of 1200 ° C. or higher and a forging ratio of 3 or higher, thereby adding a diffusion effect by dynamic recrystallization to the self-diffusion effect, and Mo and W in the steel material. And homogenizing Nb dramatically.

That is, in the above means, the invention of claim 1 is mass%, C: 0.02 to 0.08%, Si: more than 0.3 to 0.8%, Mn: 0.6 to 2.0. %, P: ≦ 0.040%, S: ≦ 0.005%, Ni: more than 15 to 26%, Cr: 18 to 23%, W: 1.8 to 4.2%, Mo: <0.5 %, Nb: 0.2-0.5%, Al: 0.001-0.040%, N: 0.07-0.13%, B: <0.0010%, the balance Fe and inevitable An austenite having excellent high-temperature strength consisting of impurities, satisfying the following formulas (1), (2) and (3), and having a creep rupture strength of not less than 110 MPa at 700 ° C. and 100,000 hours This is a heat resistant steel.
0.05% ≦ Nb−0.031 (C + N) ^{(−0.744Nb−0.772)} ≦ 0.15% …… (1)
2.8% ≦ W + 2Mo ≦ 4.2% (2)
7.0% ≦ Ni + 27C + 23N + 0.2Mn + 0.3Cu−1.2 (Cr + Mo + 0.5W) −0.5Si−0.3Nb + 10% (3)

Invention of Claim 2 is the mass%, C: 0.02-0.08%, Si: more than 0.3-0.8%, Mn: 0.6-2.0%, P: <= 0.0. 040%, S: ≦ 0.005%, Ni: more than 15 to 26%, Cr: 18 to 23%, W: 1.8 to 4.2%, Mo: <0.5%, Nb: 0.2 -0.5%, Al: 0.001-0.040%, N: 0.07-0.13%, B: <0.0010%, and Cu: 2.0-3.2 % And Ca: 0.001 to 0.007%, either 1 type or 2 types, consisting of remaining Fe and inevitable impurities, satisfying the following formulas (1), (2) and (3) 700 ° C. An austenitic heat-resistant steel having excellent high-temperature strength consisting of a creep rupture strength at a time of 100,000 hours of 110 MPa or more .
0.05% ≦ Nb−0.031 (C + N) ^{(−0.744Nb−0.772)} ≦ 0.15% …… (1)
2.8% ≦ W + 2Mo ≦ 4.2% (2)
7.0% ≦ Ni + 27C + 23N + 0.2Mn + 0.3Cu−1.2 (Cr + Mo + 0.5W) −0.5Si−0.3Nb + 10% (3)

Invention of Claim 3 is the mass%, C: 0.02-0.08%, Si: more than 0.3-0.8%, Mn: 0.6-2.0%, P: <= 0.0. 040%, S: ≦ 0.005%, Ni: more than 15 to 26%, Cr: 18 to 23%, W: 1.8 to 4.2%, Mo: <0.5% , Nb: 0.2 -0.5%, Al: 0.001-0.040%, N: 0.07-0.13%, B: <0.0010% , comprising the remainder Fe and inevitable impurities, Billet formed by forging or rolling with a forging ratio of 3 or more after heating a steel ingot of austenitic heat-resistant steel satisfying formulas (1), (2) and (3) at a temperature of 1200 ° C. or more for 1 hour or more It is a manufacturing method of the steel material of the austenitic heat-resistant steel which has the outstanding high temperature strength consisting of manufacturing a steel material using.
0.05% ≦ Nb−0.031 (C + N) ^{(−0.744Nb−0.772)} ≦ 0.15% …… (1)
2.8% ≦ W + 2Mo ≦ 4.2% (2)
7.0% ≦ Ni + 27C + 23N + 0.2Mn + 0.3Cu−1.2 (Cr + Mo + 0.5W) −0.5Si−0.3Nb + 10% (3)

Invention of Claim 4 is the mass%, C: 0.02-0.08%, Si: more than 0.3-0.8%, Mn: 0.6-2.0%, P: <= 0.0. 040%, S: ≦ 0.005%, Ni: more than 15 to 26%, Cr: 18 to 23%, W: 1.8 to 4.2%, Mo: <0.5% , Nb: 0.2 -0.5%, Al: 0.001-0.040%, N: 0.07-0.13%, B: <0.0010% , and Cu: 2.0-3.2 % And Ca: 0.001 to 0.007%, or any one or two of them, consisting of the balance Fe and inevitable impurities, satisfying the following formulas (1), (2) and (3) After heating a steel ingot of austenitic heat-resistant steel at a temperature of 1200 ° C. or higher for 1 hour or longer, a steel material is manufactured using a billet formed by forging or rolling at a forging ratio of 3 or higher. A method for manufacturing the steel austenitic heat-resistant steel having excellent high temperature strength consisting.
0.05% ≦ Nb−0.031 (C + N) ^{(−0.744Nb−0.772)} ≦ 0.15% …… (1)
2.8% ≦ W + 2Mo ≦ 4.2% (2)
7.0% ≦ Ni + 27C + 23N + 0.2Mn + 0.3Cu−1.2 (Cr + Mo + 0.5W) −0.5Si−0.3Nb + 10% (3)

  The reasons for limiting the steel components and the reasons for limiting the formulas (1) to (3) will be described below. In addition,% is the mass%.

C: 0.02 to 0.08%
C is required to be 0.02% or more for improving the high temperature strength. However, if C exceeds 0.08%, a large amount of carbide precipitates at the grain boundaries, and the toughness deteriorates significantly after a long period of time at high temperatures. Therefore, C is set to 0.02 to 0.08%.

Si: more than 0.3 to 0.8%
Si is an element necessary for deoxidation, and therefore more than 0.3% is necessary. However, if Si exceeds 0.8%, precipitation of grain boundaries in the σ phase is promoted, and the toughness is remarkably deteriorated after a high temperature and a long time. Therefore, Si is more than 0.3 to 0.8%, preferably more than 0.3 to 0.6%.

Mn: 0.6 to 2.0%
Mn is an element necessary for deoxidation and needs to be 0.6% or more. However, if Mn exceeds 2.0%, excessive addition results in high cost. Therefore, Mn is set to 0.6 to 2.0%.

P: ≦ 0.040%
P is an element contained as an inevitable impurity. However, when P is contained more than 0.040%, the weldability is remarkably deteriorated. Therefore, P is set to 0.040% or less.

S: ≦ 0.005%
S is an element contained as an inevitable impurity. However, if S is contained in an amount of more than 0.005%, the hot workability is deteriorated and the generation of cracks during processing is promoted. Therefore, S is set to 0.005% or less.

Ni: more than 15 to 26%
Ni is an element necessary for stabilizing the austenite structure, and more than 15% is necessary. However, if Ni exceeds 26%, excessive addition results in high cost. Therefore, Ni is more than 15 to 26%.

Cr: 18-23%
Cr is an element necessary for ensuring excellent high-temperature corrosion resistance and steam oxidation resistance, and needs to be 18% or more. However, when Cr exceeds 23%, the grain boundary precipitation of the σ phase is promoted, and the toughness is remarkably deteriorated after a high temperature and a long time. Therefore, Cr is 18 to 23%, preferably 19 to 22%.

W: 1.8-4.2%
W is an element necessary for 1.8% or more for improving high-temperature strength. However, if W exceeds 4.2%, the Laves phase will precipitate at the grain boundaries, and the toughness will deteriorate significantly after a high temperature and a long time. Therefore, W is set to 1.8 to 4.2%.

Mo: <0.5%
Mo is an element necessary for improving high-temperature strength. However, when Mo is contained in an amount of 0.5% or more, a Laves phase is precipitated, and the toughness is remarkably deteriorated after a high temperature and a long time. Therefore, Mo is less than 0.5%, preferably 0.3% or less.

Nb: 0.2-0.5%
Nb is an element necessary for 0.2% or more for improving high-temperature strength. However, if Nb is contained in an amount of more than 0.5%, the hot workability deteriorates and cracks occur during processing. Therefore, Nb is set to 0.2 to 0.5%.

Al: 0.001 to 0.040%
Al is an element necessary for deoxidation, and for that purpose, 0.001% or more is necessary. However, if Al exceeds 0.040%, AlN is generated at the grain boundary, and the toughness is significantly deteriorated after a high temperature and a long time. Therefore, Al is made 0.001 to 0.040%.

N: 0.07 to 0.13%
N is an element necessary for 0.07% or more for improving the high-temperature strength. However, when N exceeds 0.13%, precipitation of the Laves phase is promoted, and the toughness is significantly deteriorated after a long period of time at a high temperature. Therefore, N is set to 0.07 to 0.13%.

B: <0.0010%
B is an element effective for improving the hot work ductility of a steel material. However, when B is added in an amount of 0.0010% or more, the overheating temperature of the steel material is lowered, hot working cannot be performed at a higher temperature, and the homogenization effect by dynamic recrystallization cannot be obtained. Therefore, B is less than 0.0010% .

0.05% ≦ Nb−0.031 (C + N) ^{(−0.744Nb−0.772)} ≦ 0.15% …… (1)
The formula (1) needs to be 0.05% or more in order to promote dynamic recrystallization during hot working of the material, homogenize the elemental composition in the material and improve the high temperature strength. However, if the formula (1) is more than 0.15%, the hot workability is deteriorated and cracks are generated during the processing. Therefore, the expression (1) is set to 0.05 to 0.15%.

2.8% ≦ W + 2Mo ≦ 4.2% (2)
The formula (2) needs to be 2.8% or more in order to ensure excellent high temperature strength of the material. However, when the formula (2) is more than 4.2%, a large amount of the Laves phase is precipitated at the grain boundary, and the toughness is remarkably deteriorated after a high temperature and a long time. Therefore, formula (2) is set to 2.8 to 4.2%.

7.0% ≦ Ni + 27C + 23N + 0.2Mn + 0.3Cu−1.2 (Cr + Mo + 0.5W) −0.5Si−0.3Nb + 10% (3)
The formula (3) is a conditional formula necessary for stabilizing the austenite structure and needs to be 7.0% or more in order to ensure an excellent high temperature strength. Therefore, Equation (3) is set to 7.0% or more.

  The present invention is an austenitic heat-resisting steel that exhibits excellent creep rupture strength at 700 ° C. by comprising the chemical components of the above means and satisfying the formulas (1), (2), and (3). . The present invention has an effect that can be applied to a steel pipe for a high-strength boiler used for ultra super critical pressure coal-fired power generation or coal gasification combined power generation.

  EMBODIMENT OF THE INVENTION The form for implementing this invention is demonstrated with reference to a table | surface sequentially below.

No. of the invention example which is an austenitic heat-resistant steel satisfying the chemical composition shown in Table 1 and the formulas (1), (2) and (3) shown in Table 2. 1-12 and Comparative Example No. About 13 to 21 steels were each melted in a steel ingot of 1000kg in a vacuum melting furnace. The steel ingot was heated at a high temperature of 1200 ° C. or higher for 1 hour or longer, and formed into a billet by hot forging or hot bill salt with a forging ratio of 3 or higher. The steel material consisting of this billet was subjected to a solution heat treatment at 1180 ° C. to 1220 ° C., and this material was subjected to a creep rupture test. Further, in order to confirm that the effect can be obtained even with a cold-worked steel material, the above-mentioned hot-forged forging material is cold-worked and further subjected to a solution heat treatment at 1180 ° C. to 1220 ° C. The specimen was subjected to a creep rupture test.
In addition, the formula (1) is 0.05% ≦ Nb−0.031 (C + N) ^(−0.744 Nb ^−0.772) ≦ 0.15%, and the formula (2) is 2.8% ≦ W + 2Mo ≦ 4.2. %, The formula (3) is 7.0% ≦ Ni + 27C + 23N + 0.2Mn + 0.3Cu−1.2 (Cr + Mo + 0.5W) −0.5Si−0.3Nb + 10%.

  The creep rupture test was conducted at 700 ° C., 750 and 800 ° C. using a test piece having a parallel part diameter of 6 mm and a gauge distance of 30 mm. From the test results, the creep rupture strength (estimated value) at 700 ° C. and 100,000 hours was determined using the Larson-Miller parameter. In Table 2, a material having an estimated value of 115 MPa or more is indicated with “◎”, a material with 110 MPa to less than 115 MPa is indicated with “◯”, a material with 105 MPa to less than 110 MPa is indicated with “Δ”, and a material with less than 105 MPa is indicated with “x”. Each was evaluated and shown.

  No. All examples of the present invention 1 to 12 are 1-No. 5, no. 7, no. 9, no. 11 and no. No. 12 does not perform cold working. 6, no. 8 and no. No. 10 is a product obtained by cold working. These have an estimated value of creep rupture strength at 700 ° C. and 100,000 hours of 110 MPa or more. 1, no. 2, no. 5, no. 7, no. 9, no. 11 and no. No. 12 is “◯” of 110 MPa or more and less than 115 MPa. 3, no. 4, no. 6, no. 8 and no. 10 was 115 MPa or more of “◎”. Therefore, it can be seen that these examples are austenitic steels having excellent creep rupture strength at 700 ° C.

  In contrast, no. Comparative examples 13 to 21 are No. 13 and no. B of 14 chemical components is out of the scope of the present invention. Further, as shown in Table 2, these Nos. In Comparative Examples 13 to 21, No. 16, no. 18-21 do not perform cold working, No. 15 and no. No. 17 is a product obtained by cold working. Furthermore, no. 16 and no. No. 21 has a heating temperature outside the range of 1200 ° C. or higher of the method of the present invention. 17 and no. No. 19 has a heating time outside the range of 1 hour or more of the method of the present invention. 18-No. No. 21 has a training ratio outside the range of 3 or more of the method of the present invention. 15 and no. 20 is out of the range of 0.05 to 0.15% in the value of equation (1). As a result, the comparative example No. 13 and no. No. 14, as described above, the addition amount of B is in the range of 0.0010% or more of the range of the present invention. The creep rupture strength could not be evaluated.

  As described above, the comparative example No. Since the value of formula (1) is not within the scope of claim 15, the evaluation of creep rupture strength was “Δ”, and the strength was 105 MPa or more and less than 110 MPa. Comparative Example No. No. 16 has a heating temperature of 1180 ° C. No. 21 had a heating temperature of 1190 ° C. and both were lower than 1200 ° C., so the creep rupture strength evaluation was “x” and the strength was less than 105 MPa. Comparative Example No. In No. 17, since the heating time was 0.5 hour and shorter than 1 hour, the creep rupture strength evaluation was “Δ”, and the strength was 105 MPa or more and less than 110 MPa. Comparative Example No. Since No. 18 had a forging ratio of 2.5 and smaller than 3, the creep rupture strength evaluation was “x”, and the strength was less than 105 MPa. Comparative Example No. 19 was shorter than 1 hour in 0.5 hours and the training ratio was 2 and smaller than 3. Therefore, the creep rupture strength evaluation was “x” and the strength was less than 105 MPa. Comparative Example No. No. 20 had a forging ratio of 2.5 and smaller than 3, and the value of the formula (1) was smaller than 0.05%. Therefore, the creep rupture strength evaluation was “x” and the strength was less than 105 MPa. Comparative Example No. No. 21 had a heating temperature of 1190 ° C., which was lower than 1200 ° C. and a forging ratio of less than 3, and therefore the creep rupture strength evaluation was “x” and the strength was less than 105 MPa.

Claims (4)

In mass%, C: 0.02 to 0.08%, Si: more than 0.3 to 0.8%, Mn: 0.6 to 2.0%, P: ≦ 0.040%, S: ≦ 0 0.005%, Ni: more than 15 to 26%, Cr: 18 to 23%, W: 1.8 to 4.2%, Mo: <0.5%, Nb: 0.2 to 0.5%, Al : 0.001 to 0.040%, N: 0.07 to 0.13%, B: <0.0010%, consisting of the balance Fe and inevitable impurities, the following formula (1), (2) An austenitic heat-resisting steel having excellent high-temperature strength, characterized by satisfying the formulas (3) and (3) and having a creep rupture strength of not less than 110 MPa at 700 ° C. and 100,000 hours .
0.05% ≦ Nb−0.031 (C + N) ^{(−0.744Nb−0.772)} ≦ 0.15% …… (1)
2.8% ≦ W + 2Mo ≦ 4.2% (2)
7.0% ≦ Ni + 27C + 23N + 0.2Mn + 0.3Cu−1.2 (Cr + Mo + 0.5W) −0.5Si−0.3Nb + 10% (3) In mass%, C: 0.02 to 0.08%, Si: more than 0.3 to 0.8%, Mn: 0.6 to 2.0%, P: ≦ 0.040%, S: ≦ 0 0.005%, Ni: more than 15 to 26%, Cr: 18 to 23%, W: 1.8 to 4.2%, Mo: <0.5%, Nb: 0.2 to 0.5%, Al : 0.001-0.040%, N: 0.07-0.13%, B: <0.0010%, Cu: 2.0-3.2% and Ca: 0.001 ~ 0.007% containing any one or two of them, consisting of the balance Fe and inevitable impurities, satisfying the following formulas (1), (2) and (3), 700 ° C, 100,000 hours An austenitic heat-resistant steel having excellent high-temperature strength, wherein the creep rupture strength at the time is 110 MPa or more .
0.05% ≦ Nb−0.031 (C + N) ^{(−0.744Nb−0.772)} ≦ 0.15% …… (1)
2.8% ≦ W + 2Mo ≦ 4.2% (2)
7.0% ≦ Ni + 27C + 23N + 0.2Mn + 0.3Cu−1.2 (Cr + Mo + 0.5W) −0.5Si−0.3Nb + 10% (3) In mass%, C: 0.02 to 0.08%, Si: more than 0.3 to 0.8%, Mn: 0.6 to 2.0%, P: ≦ 0.040%, S: ≦ 0 0.005%, Ni: more than 15 to 26%, Cr: 18 to 23%, W: 1.8 to 4.2%, Mo: <0.5% , Nb: 0.2 to 0.5%, Al : 0.001-0.040%, N: 0.07-0.13%, B: <0.0010% , comprising the balance Fe and inevitable impurities, the following formula (1), (2) A steel ingot is produced using a billet formed by forging or rolling at a forging ratio of 3 or more after heating a steel ingot of austenitic heat-resistant steel satisfying the formulas (3) and (3) for 1 hour or more at a temperature of 1200 ° C. or higher. A method for producing a steel material of an austenitic heat-resistant steel having excellent high-temperature strength.
0.05% ≦ Nb−0.031 (C + N) ^{(−0.744Nb−0.772)} ≦ 0.15% …… (1)
2.8% ≦ W + 2Mo ≦ 4.2% (2)
7.0% ≦ Ni + 27C + 23N + 0.2Mn + 0.3Cu−1.2 (Cr + Mo + 0.5W) −0.5Si−0.3Nb + 10% (3) In mass%, C: 0.02 to 0.08%, Si: more than 0.3 to 0.8%, Mn: 0.6 to 2.0%, P: ≦ 0.040%, S: ≦ 0 0.005%, Ni: more than 15 to 26%, Cr: 18 to 23%, W: 1.8 to 4.2%, Mo: <0.5% , Nb: 0.2 to 0.5%, Al : 0.001 to 0.040%, N: 0.07 to 0.13%, B: <0.0010% , Cu: 2.0 to 3.2% and Ca: 0.001 Steel ingot of austenitic heat-resisting steel that contains any one or two of ~ 0.007%, consists of remaining Fe and inevitable impurities, and satisfies the following formulas (1), (2), and (3) Is manufactured using a billet formed by forging or rolling at a forging ratio of 3 or more after being heated at a temperature of 1200 ° C. or more for 1 hour or more. Of austenitic heat-resisting steel having high-temperature strength.
0.05% ≦ Nb−0.031 (C + N) ^{(−0.744Nb−0.772)} ≦ 0.15% …… (1)
2.8% ≦ W + 2Mo ≦ 4.2% (2)
7.0% ≦ Ni + 27C + 23N + 0.2Mn + 0.3Cu−1.2 (Cr + Mo + 0.5W) −0.5Si−0.3Nb + 10% (3)