High-chromium heat-resistant steel

Abstract

The present invention provides a high-chromium heat-resistant steel. The steel contains in mass %, C: 0.08% to 0.13%; Si: 0.15% to 0.45%; Mn: 0.1% to 1.0%; Ni: 0.01% to 0.5%; Cr: 10.0% to 11.5%; Mo: 0.3% to 0.6%; V: 0.10% to 0.25%; Nb: 0.01% to 0.06%; N: 0.015% to 0.07%; B: ≤0.005%, and Al: ≤0.04%. The balance consists of Fe and inevitable impurity elements. The steel shows a martensitic microstructure.

Claims

1. A high-chromium heat-resistant steel, consisting of, in mass %: C: 0.08% to 0.13%; Si: 0.15% to 0.45%; Mn: 0.1% to 1.0%; Ni: 0.01% to 0.5%; Cr: 10.0% to 11.5%; Mo: 0.3% to 0.6%; V: 0.15% to 0.25%; Nb: 0.01% to 0.06%; N: 0.015% to 0.07%, B: 0 to 0.005%; and Al: 0 to 0.04%; wherein the balance is Fe and inevitable impurity elements.

2. The high-chromium heat-resistant steel of claim 1, wherein B is in the range of 0.001% to 0.005% by mass.

3. The high-chromium heat-resistant steel of claim 1, wherein the mass % of the inevitable impurity elements is lower than 0.4%.

4. The high-chromium heat resistant steel of claim 1, consisting of, in mass %: C: 0.08% to 0.11%; Si: 0.15% to 0.35%; Mn: 0.40% to 0.60%; Ni: 0.01% to 0.2%; Cr: 10.45% to 11.0%; Mo: 0.45% to 0.55%; V: 0.15% to 0.25%; Nb: 0.035% to 0.06%; N: 0.040% to 0.070%; B: 0 to 0.005%; and Al: 0 to 0.04%; wherein the balance is Fe and inevitable impurity elements.

5. The high-chromium heat-resistant steel of claim 4, wherein B is in the range of 0.002% to 0.004%.

6. The high-chromium heat-resistant steel of claim 4, wherein Al: 0 to 0.025% by mass.

7. A steam contacting component made from the high-chromium heat-resistant steel of claim 1.

8. A pressure boiler comprising one or more steam contacting components made from the high-chromium heat-resistant steel of claim 1.

9. A thermal power plant comprising the steam contacting component of claim 7.

10. A thermal power plant comprising the pressure boiler of claim 8.

11. The steam contacting component of claim 7, wherein the steam contacting component is a tube.

12. The pressure boiler of claim 8, wherein the one or more steam contacting components is a boiler drum.

13. The pressure boiler of claim 8, wherein the one or more steam contacting components is a tube.

14. The high-chromium heat-resistant steel of claim 1, wherein the steel has a martensitic microstructure.

15. The high-chromium heat-resistant steel of claim 14, wherein the steel does not contain delta ferrite.

16. The high-chromium heat-resistant steel of claim 1, wherein the steel has a creep rupture time of at least 21,985 hours under a temperature of 650° C. and a stress of 70 MPa.

17. The high-chromium heat-resistant steel of claim 1, wherein the steel has a creep rupture time of at least 23,801 hours under a stress of 70 MPa at a temperature of 650° C.

18. The high-chromium heat-resistant steel of claim 1, wherein the steel has a creep rupture time of at least 25,451 hours under a stress of 70 MPa at a temperature of 650° C.

19. The high-chromium heat-resistant steel of claim 1, wherein the steel has a creep rupture time of between 21,985 and 25,451 hours under a stress of 70 MPa at a temperature of 650° C.

20. The high-chromium heat-resistant steel of claim 1, wherein the steel forms an average scale thickness of at most 33 μm under a steam oxidation temperature of 650° C. for 1000 hours.

21. The high-chromium heat-resistant steel of claim 1, wherein the steel forms an average scale thickness of at most 39 μm of under a steam oxidation temperature of 650° C. for 1000 hours.

22. The high-chromium heat-resistant steel of claim 1, wherein the steel forms an average scale thickness of at most 40 μm under a steam oxidation temperature of 650° C. for 1000 hours.

23. The high-chromium heat-resistant steel of claim 1, wherein the steel forms an average scale thickness of 33 to 40 μm under a steam oxidation temperature of 650° C. for 1000 hours.

24. The high-chromium heat-resistant steel of claim 1, wherein the steel has a creep rupture time of between 21,985 and 25,451 hours under a stress of 70 MPa at a temperature of 650° C., and wherein the steel forms an average scale thickness of 33 to 40 μm under a steam oxidation temperature of 650° C. for 1000 hours.

25. The high-chromium heat-resistant steel of claim 1, wherein Ni is in the range of 0.01% to 0.2% by mass.

26. The high-chromium heat-resistant steel of claim 1, wherein Mo is in the range of 0.45% to 0.6% by mass.

Description

DETAILED DESCRIPTION FOR CARRYING OUT THE INVENTION

(1) Reasons for limitations for the individual elements will be discussed below.

(2) C: 0.08% to 0.13%;

(3) C is an austenite forming element suppressing ferrite formation. Hence, an appropriate amount of C is determined with ferrite forming elements such as Cr, in order to obtain the tempered martensite structure. In addition, C precipitates as carbides of the MC type (M represents an alloying element (The same will applies hereinafter.)) and M.sub.23C.sub.6 type, which greatly affect the high temperature strength, and in particular, creep rupture strength. With C content of less than 0.08%, the amount of precipitation is insufficient for precipitation strengthening, and also the suppression of delta ferrite phase is imperfect. For this reason, the lower limit thereof is set to 0.08%. With the addition of more than 0.13% of C, weldability is impaired and toughness is decreased. Further, agglomerated coarsening of carbides is accelerated resulting in a decrease in the creep rupture strength on the high-temperature and long term side. For this reason, the range thereof is set to 0.08% to 0.13%, preferably within the range of 0.08% to 0.11% (mass percentage)

(4) Si: 0.15% to 0.45%;

(5) Si is added as a deoxidizing agent and for oxidation resistance. However, Si is a strong ferrite forming element and toughness is impaired by the ferrite phase. For this reason, the range thereof is set to 0.15% to 0.45% to balance the oxidation resistance and the tempered martensite structure; preferably within the range of 0.15% to 0.35% (mass percentage)

(6) Mn: 0.1% to 1.0%;

(7) Mn is added as a deoxidizing agent and a desulfurizing agent. In addition, it is also an austenite forming element suppressing the delta ferrite phase, but excessive addition thereof impairs the creep strength. For this reason, the range thereof is set to 0.1% to 1%; preferably within the range of 0.40% to 0.60% (mass percentage)

(8) Ni: 0.01% to 0.5%;

(9) Ni is a strong austenite forming element suppressing ferrite phase formation. However, excessive addition thereof impairs long-term creep rupture strength. For this reason, the range suggested is set from 0.01% to 0.5%, preferably within the range of 0.01% to 0.20% (mass percentage)

(10) Cr: 10.0% to 11.5%;

(11) Cr is an important element for securing steam oxidation resistance. Cr content of 10.0% or more is necessary from the viewpoint of steam oxidation resistance for high temperature steam. However, excessive addition of Cr as well as Si causes ferrite formation and also causes formation of brittle phases in long-term creep, thereby impairing the rupture strength. For this reason, the upper limit thereof is set to 11.5%, preferably within the range of 10.45% to 11% (mass percentage)

(12) Mo: 0.3% to 0.6%;

(13) Mo is a ferrite forming element while it increases the creep strength due to the effect of solid-solution hardening. However, excessive addition thereof results in the formation of delta ferrite and the precipitation of coarse intermetallic compounds not contributing to the creep rupture strength. For this reason, the range thereof is set to 0.3% to 0.6%, preferably within the range of 0.45% to 0.55% (mass percentage)

(14) V: 0.10% to 0.25%;

(15) V precipitates as fine carbonitrides and thereby improves both high temperature strength and creep rupture strength. With a content of less than 0.1%, the amount of precipitation is insufficient to increase the creep strength. In contrast, excessive addition thereof results in formation of bulky V (C, N) precipitates not contributing to the creep strength. For this reason, the range thereof is set to 0.1% to 0.25%, preferably within the range of 0.15% to 0.25% (mass percentage)

(16) Nb: 0.01% to 0.06%;

(17) Nb also precipitates as fine carbonitrides, and is an important element improving the creep rupture strength. A content of 0.01% or more is necessary to obtain this effect. However, similarly as V, excessive addition of Nb results in formation of bulky carbonitrides to reduce the creep rupture strength. Hence, the range thereof is set to 0.01% to 0.06%, preferably within the range of 0.035% to 0.06% (mass percentage)

(18) N: 0.015% to 0.07%,

(19) N precipitates as either nitrides or carbonitrides thereby to improve the creep rupture strength. It is also an austenite forming element to suppress delta ferrite phases. However, excessive addition thereof impairs toughness. For this reason, the range thereof is set to 0.015% to 0.070%, preferably within the range of 0.040% to 0.070% (mass percentage)

(20) Al: ≤0.04%; and

(21) Al can be used as a deoxidizing agent, but it impairs the long-term creep rupture strength with excessive addition. For this reason, when optionally used, the upper limit thereof is set to 0.04%, preferably less than 0.025% (mass percentage)

(22) B: 0.001% to 0.005%.

(23) B is an element strengthening the grain boundary and that has also the effect of the precipitation hardening as M.sub.23(C,B).sub.6, thus being effective for improving the creep rupture strength. However, excessive addition thereof impairs workability under high temperatures leading to a cause of cracking, and also impairs the creep rupture ductility. For this reason, when optionally used, the range thereof is set to 0.001% to 0.005%, preferably from 0.002% to 0.004% (mass percentage).

(24) P: ≤0.03%;

(25) P is an Inevitable impurity element contained in melting raw materials and not easily reduced in steel making process. It impairs toughness at room temperatures and high temperatures as well as hot workability. If present, the upper limit is set to 0.03%, preferably lees than 0.018% (mass percentage)

(26) S: ≤0.01%;

(27) S is also an inevitable impurity element and it impairs hot workability. It also can be a cause of cracks, scratches, or the like. If present, the upper limit is set to 0.01%, preferably lees than 0.005% (mass percentage)

(28) In the present invention, the manufacturing conditions are not specifically limited. The tempered martensite structure can be obtained by conventional normalizing treatment heated at temperatures in the range of 950 to 1150 degree centigrade followed by air cooling and tempering treatment heated at temperatures in the range of 700 to 800 degree centigrade.

EXAMPLES

(29) Steels according to the present invention (Nos. A to C) and comparative steels (Nos. D to F) having chemical compositions shown in Table 2 were melted using a vacuum induction melting furnace, cast into 50 kg or 70 kg ingot, and then hot-rolled into steel plates with a thickness of 12 mm to 15 mm. Then, the steel plates were heat treated by normalizing and then tempering. The normalizing temperature is in a range of 1050° C. to 1100° C., and the tempering temperature is in a range of 770° C. to 780° C. Obtained microstructure is a tempered martensite structure, not containing delta ferrite. Among comparative steels, Steel D has a component system of 9Cr-1Mo steels called Grade 91 steels, which are widely used at present. Steel D was used as a steel representing existing materials.

(30) TABLE-US-00002 TABLE 2 Division Steel C Si Mn P S Ni Cr Mo V Nb Al N B Inventive steel A 0.09 0.21 0.25 0.012 0.002 0.20 10.6 0.51 0.22 0.04 0.012 0.044 — Inventive steel B 0.12 0.42 0.75 0.009 0.003 0.15 10.3 0.55 0.18 0.05 0.008 0.028 — Inventive steel C 0.11 0.18 0.48 0.013 0.001 0.41 11.3 0.34 0.20 0.03 0.015 0.040 0.0025 Comparative D 0.10 0.32 0.47 0.011 0.003 0.20  8.5 0.98 0.25 0.07 0.013 0.045 — steel Grade91 Comparative E 0.13 0.29 0.53 0.015 0.004 0.17 12.2 0.48 0.21 0.03 0.007 0.048 — steel Comparative F 0.09 0.36 0.38 0.009 0.002 0.31  9.2 0.38 0.16 0.04 0.019 0.035 — steel
(mass %) The underlined figures indicate the value that is out of the range in the present invention.

(31) Test specimens were taken from the heat treated plates and were subjected to creep rupture testing and steam oxidation testing. Creep rupture testing was performed using a 6 mm diameter specimen under testing temperature of 650° C. and stresses of 110 MPa and 70 MPa. For steels of this type, testing requires tens of thousands hours to clarify superiority or inferiority at testing temperature of 600° C., which is an actual temperature for real thermal power plants. Therefore, the testing temperature was elevated to 650° C., and two stress conditions were applied with estimated rupture time periods of about 1,000 hours and about 10,000 hours. Since the difference in the rupture time among steels is assumed to be small on a short-term side testing of about 1,000 hours using a 110 MPa testing condition, 70 MPa testing condition was applied as long-term testing of about 10,000 hours to differentiate the rupture strength among steels.

(32) For steam oxidation testing, the temperature was set to 650° C., which is the same as that for the creep rupture testing. In the testing, an average thickness of scale formed on the surface of the specimen subjected to 1,000-hour steam oxidation testing was measured using an optical microscope. In this manner, the steam oxidation resistance was evaluated. The specimen is a small sample of 15 mm×20 mm×10 mm taken from the heat treated plate material.

(33) The results of the creep rupture testing and the steam oxidation testing are shown in Table 3.

(34) TABLE-US-00003 TABLE 3 Creep rupture time (h) Test Steam oxidation temperature 650° C. testing 650° C., Stress: Stress: 1000 h Average Division Steel 110 MPa 70 MPa scale thickness (μm) Inventive steel A 883 25,451 39 Inventive steel B 923 23,801 40 Inventive steel C 783 21,985 33 Comparative D 482 8,862 92 steel Comparative E 1,034 7,075 30 steel Comparative F 804 21,904 72 steel
Compared to the steel D equivalent to the existing Grade 91 steel, steels for the present invention demonstrate excellent high temperature properties. For example, the rupture time is three times or more in the long-term testing with the stress of 70 MPa and the average thickness of scale formed in steam oxidation is no more than half. Thus, significant improvements are shown in the creep rupture strength and the steam oxidation resistance.

(35) Comparative steel E having higher Cr content of 12.2% significantly improves the steam oxidation resistance, however it decreases the long-term creep rupture strength. Although the microstructure of Steel E is tempered martensite, not containing delta ferrite, the decreased creep rupture strength is considered owing to an increase in Cr content. Comparative steel F having equivalent Cr content to the existing Grade 91 steels cannot improve the steam oxidation properties with considerably thick scales compared with steels of the present invention.

INDUSTRIAL APPLICABILITY

(36) According to the present invention, it is possible to provide a high-chromium heat-resistant steel that enhances both the creep rupture strength and the steam oxidation resistance even not containing expensive elements such as W and Co and less containing Mo. Therefore the present invention provides excellent economical efficiency. The inventive steel can be advantageously used for steam contacting components, e.g. tubes for a pressure boiler and/or a boiler drum.