High-chromium containing ferrite based heat resistant steel

Abstract: A high-Cr containing ferrite heat resistant steel having not only an excellent long-term creep strength at a high temperature exceeding 650 °C, but also an improved oxidation resistance, which is based on ferritic phase and contains 13 % by weight or more of chromium, and containing precipitates of intermetallic compounds.

Description: TECHNICAL FIELD OF THE INVENTION The present invention relates to a process for producing a heat resistant high-Cr containing steel based on a ferrite phase, and a steel obtainable by such a process. In further detail, the invention according to the present application relates to a high-Cr ferrite heat resistant steel having not only an excellent long-term creep strength at a high temperature exceeding 650 °C, but also an improved oxidation resistance. BACKGROUND OF THE INVENTION Conventionally, the creep strength of a ferrite based, heat resistant steel has been improved heretofore by converting the ferritic texture into a tempered martensitic texture having a higher creep strength. However, a tempered martensitic texture is unstable at high temperatures because it undergoes textural change and becomes heterogeneous. This decreases the creep strength. Furthermore, dislocations present in the martensite accelerates the long term creep deformation. Thus, the texture is changed influenced by the heat applied at welding as to impair the creep strength at the welded portion Although Cr (chromium) is known as an element effective for improving the oxidation resistance of a steel, the incorporation of Cr at a higher concentration of 12 % by weight or more results in the generation of a δ-ferrite phase which decreases the creep strength and the toughness. Accordingly, austenite stabilizing agents such as Ni, Cu and Co, have been added to the ferritic heat resistant steel known heretofore in order to suppress the generation of δ-ferrite phase. However, the addition of Ni or Cu lowers the transformation temperatures of austenite and ferrite. To achieve long term stability of the high-temperature strength, it is advantageous to set the tempering temperature higher after the normalization; however, the addition of Ni or Cu results in a lower tempering temperature because it thus lowers the transformation temperature of austenite and ferrite. Accordingly, it is practically unfeasible to add Cr at a quantity exceeding a concentration of 12 % by weight. JP-A-9118961 discloses a ferritic stainless steel composed of, by weight, ≤0.003 % C, ≤0.005 % N, 0.05 to 2.0 % Si, 0.1 to 2.0 % Mn, 10 to 22 % Cr, (3 x 93/12 x C + 93/14 x N) % to 1% Nb and the balance Fe with inevitable impurities. The steel is subjected to final annealing and is allowed to contain the formed Nb precipitates as Fe 3 Nb 3 C or Fe 2 Nb. JP-A-3006354 discloses an alloy composed of, by weight, 0.002 to 0.1 % C, 0.5 to 8.0 % of one or more kinds of ≤ 7.0 % Ni, ≤5.0 % Cu and ≤4.0 % Co, 0.3 to 4.0 % of one or both of ≤4.0 % Al and Ti, 5.0 to 25 % Cr, 0.1 to 12.0 % of one or both of ≤10.0 % Mo and ≤3.0 % Si, 0.0005 to 0.003 % O and 0.0002 to 0.03 % N and optionally prescribed amounts of Mn, W, V, Nb, Ta, Zr, Hf, B etc. and the balance Fe with inevitable impurities. US-A-5 772956 relates to a martensitic heat resistant steel, which does not form an intermetallic compound having a composition substantially of Cr 40 Mo 20 Co 20 W 10 C 2 -Fe at a temperature of 600 °C or more. This document further describes in vol. 2, lines 32-49 (see also Table 2) the precipitation of an intermetallic compound in a Cr steel, with Co, W and Mo being added in combination, under actual service conditions. DISCLOSURE OF THE INVENTION The invention according to the present application has been made in the light of the aforementioned circumstances, and an object thereof is to provide a high-Cr ferrite heat resistant steel having excellent long-term creep strength at a high temperature exceeding 650 °C, and yet having an improved oxidation resistance. As described above, a conventional ferritic heat resistant steel based on the tempered martensitic texture suffers an abrupt drop in creep strength because it undergoes a heterogeneous textural change in the vicinity of the grain boundaries when subjected to higher temperatures over 650 °C for a long duration of time because of the unstable texture. Accordingly, the inventors of the present invension extensively studied a means for achieving textural stability at higher temperatures. As a result, it has been found that the ferritic heat resistant steel having a greatly improved long term creep strength at high temperatures can be obtained by realizing a texture based on a ferritic phase and precipitating therein an intermetallic compound of a Laves phase or a µ phase. The present invention has been accomplished based on these findings. More specifically, in accordance with the first aspect of the invention of the present application, there is provided a process for producing a heat resistant high-chromium containing steel based on a ferritic phase according to claim 1. In a preferred aspect of the process, said steel contains a precipitate of an intermetallic compound and said process further comprises heating said steel at a temperature at or above 650 °C. Furthermore, according to a second aspect of the invention of the present application, there is provided a heat resistant high-chromium containing steel obtainable by the process of the invention. Preferably, the steel contains a precipitate of an intermetallic compound. More preferably, the intermetallic compound is at least one type of precipitate selected from the group consisting of a Laves phase or a µ phase. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows the stress vs. time to breakage curve at 650°C of text specimens according to Reference Examples 1 to 9, and Comparative Examples 1 to 3; Fig. 2 shows the stress vs. time to breakage curve at 650°C of test specimens according to Reference Examples 10 to 16; Fig. 3 shows the creep rate vs. time curve obtained as a result of creep tests performed at 650 °C and 70 MPa on test specimens according to Reference Examples 1 and 2; Fig. 4 is a micrograph obtained under transmission electron microscopy showing the texture of the test specimen just after annealing according to Reference Example 2; Fig. 5 is a micrograph obtained under transmission electron microscopy showing the texture of the test specimen according to Reference Example 2, obtained 100 hours after performing the creep test; Fig. 6 is a micrograph obtained under transmission electron microscopy showing the texture of the test specimen according to Reference Example 2, obtained 1,000 hours after performing the creep test; Fig. 7 shows the creep rate vs. time curve obtained as a result of creep tests performed at 650 °C and 100 MPa on test specimens according to Reference Examples 2 to 9; Fig. 8 is a graph showing the creep rate vs. time curve obtained as a result of creep tests performed at 650 °C and 70 MPa on test specimens according to Reference Examples 10 to 12; Fig. 9 is a micrograph obtained under transmission electron microscopy showing the texture of the test specimen just after annealing according to Reference Example 12; Fig. 10 is a micrograph obtained under transmission electron microscopy showing the texture of the test specimen according to Reference Example 12, obtained 100 hours after performing the creep test; Fig. 11 is the X-ray diffractogram of an electrolytically extracted residue obtained from the test specimen subjected to creep test at 650°C and 70 MPa and stopped after 1,000 hours; Fig. 12 shows the creep rate vs. time curve obtained as a result of creep tests performed at 650 °C and 100 MPa on test specimens according to Reference Examples 12 to 16; Fig. 13 shows the creep rate vs. time curve obtained as a result of creep tests performed at 700 °C and 70 MPa on test specimens according to Reference Examples 1 to 3, and Reference Example 8; and Fig. 14 shows the creep rate vs. time curve obtained as a result of creep tests performed at.700 °C and 70 MPa on test specimens according to Reference Examples 10 to 12 and 14. As described above, the high-Cr ferrite heat resistant steel according to the invention of the present application contains 13 to 30 % by weight of chromium and is based on ferritic phase, and at the same time, contains precipitates of intermetallic compounds. As the intermetallic compounds, there can be specifically mentioned at least one type of phase selected from the group consisting of a Laves phase (Fe 2 W, Fe 2 Mo) or a µ phase. The intermetallic compounds above precipitation harden the ferritic phase. Furthermore, because the basic phase constituting the high-Cr ferrite heat resistant steel is ferrite and not the tempered martensite that is unstable at high temperatures, the high-Cr ferrite heat resistant steel according to the invention of the present application realizes an excellent creep strength for a long duration of time. Because a ferritic matrix phase equivalent to that of the mother material is obtained by performing heat treatment after welding, the strength can be maintained without being impaired by the thermal influence at the welded portion. In the high-Cr ferrite heat resistant steel according to the invention of the present application, the basic ferritic phase preferably accounts for 70 % by volume or more. Furthermore, because the high-Cr ferrite heat resistant steel according to the invention of the present application contains Cr at a high quantity of 13 to 30 % by weight, it exhibits excellent resistances against oxidation and water vapor oxidation as compared with a conventional ferritic heat resistant steel. Although the incorporation of Cr at a high quantity may lower the toughness, the toughness of the high-Cr ferrite heat resistant steel according to the invention of the present application is maintained favorably because the intermetallic compounds form a uniform subgrain as to suppress the growth of basic ferritic phase into coarse crystals. The heat resistant high-chromium containing steel based on a ferritic phase consists of the following chemical composition (weight %): Cr 13 ~ 30 Mo 0.5 ~ 8.0 W 1.0 ~ 8.0 Co 1.0 ~ 10.0 C 0.50 or less N 0.20 or less B 0.01 or less Nb 0.01 ~ 2.0 Fe residue and may contain incidental impurities. The present application also provides a process for producing the heat resistant high-chromium containing steel based on a ferritic phase as mentioned above. Said process comprises hot working the bulky steel derived from a melt of raw materials, annealing the hot worked steel at a temperature of 1000 °C or more and cooling in a furnace. The present invention is described in further detail by reference to specific examples. REFERENCE EXAMPLES not according to the invention 1 - 16 and COMPARATIVE EXAMPLES 1 TO 3: Test specimens each having the chemical composition shown in Table 1 were prepared. Each of the test specimens was prepared by first producing an ingot 10 kg in weight in a vacuum high frequency melting furnace, hot forging the resulting ingot into a cylindrical rod about 13 mm in diameter, and annealing by holding at 1,200 °C for a duration of 30 minutes and cooling in the furnace. The test specimens were subjected to creep tests at 600 °C, 650 °C, and 700 °C, as well as to the measurement of hardness and observation under a transmission electron microscope. Chemical Composition (% by weight) Alloy No. C Cr Mo W V Nb Cu Co N B Ref. Ex. 1 1501 0.10 15.0 0.5 1.8 0.20 0.05 - - 0.07 0.003 Ref. Ex. 2 1502 0.10 15.0 1.0 3.0 0.20 0.05 - - 0.07 0.003 Ref. Ex. 3 1503 0.10 15.0 1.0 3.0 0.40 0.10 - - 0.09 0.003 Ref. Ex. 4 1504 0.10 15.0 1.0 6.0 0.20 0.05 - - 0.07 0.003 Ref. Ex. 5 1505 0.10 15.0 1.0 6.0 0.40 0.10 - - 0.08 0.003 Ref. Ex. 6 1506 0.10 15.0 1.0 3.0 0.20 0.06 - 3.0 0.08 0.003 Ref. Ex. 7 1507 0.10 15.0 1.0 3.0 0.40 0.10 - 3.0 0.08 0.003 Ref. Ex. 8 1509 0.10 15.0 1.0 6.0 0.40 0.10 - 3.0 0.08 0.003 Ref. Ex. 9 1508 0.10 15.0 1.0 6.0 0.20 0.05 - 3.0 0.07 0.003 Ref. Ex. 10 2001 0.10 20.0 0.5 1.8 0.20 0.05 - - 0.07 0.003 Ref. Ex. 11 2002 0.10 20.0 1.0 3.0 0.20 0.05 - - 0.07 0.003 Ref. Ex. 12 2003 0.10 20.0 1.0 3.0 0.20 0.05 - 5.0 0.07 0.003 Ref. Ex. 13 2004 0.10 20.0 1.0 3.0 0.40 0.10 - 5.0 0.06 0.002 Ref. Ex. 14 2005 0.10 20.0 1.0 6.0 0.20 0.05 - 5.0 0.07 0.003 Ref. Ex. 15 2006 0.10 20.0 1.0 6.0 0.40 0.10 - 5.0 0.09 0.003 Ref. Ex. 16 2007 0.10 20.0 1.0 9.0 0.40 0.10 - 5.0 0.07 0.002 Comp.1 ASME T91 0.10 9.0 1.0 - 0.20 0.05 - - 0.05 - Comp.2 ASME T92 0.10 9.0 0.5 1.8 0.20 0.05 - - 0.06 0.003 Comp.3 ASME T122 0.10 11.0 0.5 2.0 0.20 0.05 1.0 - 0.05 0.003 The texture of each of the test specimens obtained in Reference Examples 1 to 16 after annealing was found to be a ferrite containing carbides, but the precipitation density of the carbides was low. For the test specimens of Examples 6 to 9, and 12 to 16 each containing Co, martensite was found to account for about 5 to 6 % by volume. After the annealing, the test specimens of Reference Examples 1 to 5, and 10 to 11 were found to yield a hardness Hv in the range of from 160 to 180, and those of Reference Examples 6 to 9 and 12 to 16 yielded a high hardness Hv in the range of from 230 to 250. Figs. 1 and 2 show the stress vs. time to breakage curves at 650 °C. The curve shows that the test specimens (ferritic steel) for Reference Examples 1 to 9 and 10 to 16 yield higher stability in creep strength for a long duration of time as compared with the test specimens of Comparative Examples 1 to 3 (martensitic steel), and SUS 304 of the conventional type. On the other hand, the test specimens of Comparative Examples 1 to 3, and SUS 304 show considerable drop in long term creep strength. Fig. 3 shows the creep rate vs. time curve obtained as a result of creep tests performed at 650 °C and 70 MPa on test specimens according to Reference Examples 1 and 2. The test specimens of Reference Examples 1 and 2 both. contain 15 % by weight or Cr, and the test specimen of Reference Example 2 contains the intermetallic compound elements Mo and W at a higher amount as compared with that of Reference Example 1. It can be seen that the creep rate is lower and that the time to creep rupture is about 10 times as long as that of the Reference Example 1. Thus, it can be understood that the creep strength of the test specimen of Reference Example 2 is higher than that of the test specimen of Reference Example 1. Figs. 4 to 6 each show the textures of the test specimen according to Reference Example 2, obtained just after the annealing, after 100 hours of the creep test, and after 1,000 hours of the creep test. The figures show a uniform texture, and the black spots observed in the figure represent the intermetallic compound. It can be seen that the intermetallic compound precipitates in a larger amount during the creep test. From the results above, it can be understood that the creep strength is improved by the precipitation of the intermetallic compound which reinforces the ferritic phase, and that the precipitation hardening of the intermetallic compound is further accelerated by increasing the addition of Mo and W. Fig. 7 shows the creep rate VS. time curve obtained as a result of creep tests performed at 650°C and 100 MPa on test specimens according to Reference Examples 2 to 9. The test specimens of Reference Examples 2 to 9 each contain 15 % by weight of Cr, and the test specimens of Reference Examples 4 to 5, and 8 to 9 contain the intermetallic compound elements W at a higher amount as compared with that of Reference Examples 2 to 3, and 6 to 7. The test specimens of Reference Examples 6 to 9 each contain 3 % by weight of Co. Based on the higher amount of intermetallic compound element W, it can be understood that the creep strength of the test specimens of Reference Examples 4 and 5 are higher than that of the test specimens of Reference Examples 2 and 3. Additionally, based on the element Co, it can be understood that the creep strength of the test specimens of Reference Examples 6 and 7 are higher than.that of the test specimens of Reference Examples 2 and 3, and that the creep strength of the test specimens of Reference Examples 8 and 9 are higher than that of the test specimens of Reference Examples 4 and 5. Fig. 8 shows the creep rate vs. time curve obtained as a result of creep tests performed at 650 °C and 70 MPa on test specimens according to Reference Examples 10 to 12 The test specimens according to Reference Examples 10 to 12 contain Cr at a higher amount as compared with those according to Reference Examples 1 to 9. Similar to the case of Reference Examples 1 and 2, the results obtained in the creep test for the test specimens of Reference Examples 10 and 11 show that the precipitation hardening attributed to the intermetallic compound increases with increasing amount of addition of Mo and W. The test specimen according to Reference Example 12 is obtained by adding Co to the test specimen of Reference Example 11. By comparing the result of Reference Example 12 with that of Reference Example 11, it can be understood that the amount of intermetallic compound precipitate increases with the addition of Co, and that the creep strength is thereby improved. Figs. 9 and 10 each show the texture of the test specimen of Reference Example 12, each obtained just after annealing and 100 hours after the creep test. Referring to Fig. 9 and 10, the intermetallic compounds can be seen as black spots, and it can be understood that the intermetallic compound precipitates at a large amount. Fig. 11 shows an X-ray diffractogram of an electrolytically extracted residue obtained from the test specimen subjected to creep test at 650 °C and 70 MPa and by stopping the test after 1,000 hours. The formation of an intermetallic compound, i.e., the Laves phase, is confirmed. Fig. 12 shows the creep rate vs. time curve obtained as a result of creep tests performed at 650 °C and 100 MPa on test specimens according to Reference Examples 12 to 16. By comparing Reference Examples 12 and 13, it can be understood that the creep strength is decreased by the addition of an excess amount of the elements V and Nb. However, by comparing Reference Example 13 and Reference Examples 15 to 16, it can be understood that addition of the element W can increase the creep strength. Fig. 13 shows the creep rate vs. time curve obtained as a result of creep tests performed at 700 °C and 70 MPa on test specimens according to Reference Examples 1 to 3, and 8. It can be seen therefrom that the creep strength of the test specimen increases in the order of Reference Example 1,Reference Example 2, Reference Example 3, and Reference Example 8. The test specimens of Reference Examples 1 to 3, and 8 all contain 15 % by weight of Cr, and the test specimen of Reference Example 2 contains the intermetallic compound elements Mo and W at a higher amount as compared with that of Reference Example 1. The test specimen of Reference Example 3 contains the intermetallic compound element W at a higher amount as compared with the case of Reference Example 2. Furthermore, the test specimen of Reference Exemple 8 is obtained by adding Co, an element which increases the amount of precipitated intermetallic compound, to the test specimen of Reference Example 3. From the above facts, it can be understood that the amount of precipitated intermetallic compound increases in the order of Reference Example 1, Reference Example 2, Reference Example 3, and Reference Example 8, and resulted in an increase in creep strength. Fig. 14 shows the creep rate vs. time curve obtained as a result of creep tests performed at 700 °C and 70 MPa on test specimens according to Reference Examples 10 to 12 and 14. It can be seen therefrom that the creep strength of the test specimen increases in the order of Reference Example 10, Reference Example 11, Reference Example 12, and Reference Example 14. The test specimens of Reference Examples 10 to 12 and 14 all contain 20 % by weight of Cr, and the test specimen of Reference Example 11 contains the intermetallic compound elements Mo and W at a higher amount as compared with that of Reference Example 10. The test specimen of Reference Example 12 is obtained by adding Co, an element which increases the amount of precipitated intermetallic compound, to the test specimen of Reference Example 11. The test specimen of Reference Example 14 contains the intermetallic compound element W at a higher amount as compared with the case of Reference Example 12. It can be understood from the above facts that the amount of precipitated intermetallic compound increases in the order of Reference Example 10, Reference Example 11, Reference Example 12, and Reference Example 14, and that this resulted in an increase in creep strength in this order. As described above, the invention according to the present application provides a high-Cr ferrite heat resistant steel having not only an excellent long-term creep strength at a high temperature exceeding 650 °C, but also an improved oxidation resistance. By taking into consideration the distinguished properties, the high-Cr ferrite heat resistant steel of the present invention is suitable as a material of apparatuses for use under high temperature and high pressure, such as boilers, nuclear power plant installations, chemical industry apparatuses, etc., and the use thereof is believed to bring about an improvement in energy efficiency of power plants, an improvement in reaction efficiency of chemical industry apparatuses, etc.

Claims: A process for producing a heat resistant high-chromium containing steel based on a ferritic phase, said steel having the following chemical composition (weight %) Cr 13 - 30 Mo 0.5 - 8.0 W 1.0 - 8.0 Co 1.0 - 10.0 C 0.50 or less N 0.20 or less B 0.01 or less Nb 0.01 - 2.0 the balance being Fe and any incidental impurities, said process comprising hot working bulky steel derived from a melt of the raw materials, annealing said steel at a temperature of 1000°C or more and cooling in a furnace. A process for producing a heat resistant high-chromium containing steel as claimed in claim 1, wherein said steel contains a precipitate of an intermetallic compound, said process further comprising heating said steel at a temperature at or above 650°C. A heat resistant high-chromium containing steel obtainable by the process of claim 1. A heat resistant high-chromium containing steel obtainable by the process of claim 2. The heat resistant high-chromium containing steel as claimed in claim 4, wherein the intermetallic compound is at least one type of precipitate selected from the group consisting of a Laves phase or a µ phase. A method of improving the high temperature creep strength of a heat resistant high-chromium steel according to claim 3 comprising heating said steel at or above 650°C to precipitate an intermetallic compound.

Inventors: KIMURA KAZUHIRO, KIMURA, KAZUHIRO, Kimura, Kazuhiro c/o Nat. Research Ins.for Metals
Applicants: JP NAT RES INST FOR METALS, JAPAN AS REPRESENTED BY DIRECTOR GENERAL OF NATIONAL RESEARCH INSTITUTE FOR METALS
Application year: 2000
Main classification: C22C38
IPC codes: C21D6, C22C38