Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/38—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for roll bodies

Definitions

• the present invention relates to low-alloy heat-resistant steels which show excellent performance as heat-resistant structural members, particularly as turbine rotor members, and relates to processes for producing the low-alloy heat-resistant steels.
• the present invention also relates to turbine rotors in which the low-alloy heat-resistant steels are used.
• a steel of a conventional type is insufficient in mechanical properties at high temperatures, particularly in terms of creep strength. Accordingly, need for developing a material which is durable in use at higher steam temperatures has been growing.
• a CrMoV steel is used after quenching the CrMoV steel heated to a temperature of about 950°C. A higher heating temperature before quenching results in a higher strength of the material because precipitation of a pro-eutectoid ferrite phase, which is soft, is inhibited, and dissolution of the strengthening elements in a solid solution is promoted.
• the preferred aim of the present invention is to provide a heat-resistant steel which can be quenched after heating to a higher temperature, has a toughness equivalent to or higher than that of a conventional CrMoV steel, and has excellent creep properties such as a high creep rupture property, according to a creep test on an unnotched test piece, and inhibition of creep embrittlement.
• Another preferred aim of the present invention is to provide a turbine rotor comprising this novel heat-resistant steel.
• the present inventors have diligently carried out research, and found that impurities greatly affect the properties of a steel at high temperatures, particularly the creep embrittlement resistance.
• the present inventors found that a low-alloy heat-resistant steel and a turbine rotor which can be quenched after heating to a high temperature of at least 1000°C, having a high toughness as explained below, and having excellent properties at high temperatures such as not being subject to creep embrittlement, can be obtained not only by mixing alloy components with predetermined proportions, but also by minimizing the amount of trace impurity elements which are harmful, such as phosphorus, sulfur, copper, aluminum, arsenic, tin, and antimony.
• the present inventors have thus achieved the present invention.
• test piece As a test piece, a round bar having a constant cross section is used.
• the measuring method is defined by JIS Z-2271 and JIS Z-2272.
• the measuring methods defined by the JIS standards are for creep tests on unnotched test pieces, and test pieces which are finished by smoothly shaving between gauge marks in the portion to be measured are used in these methods.
• a test piece having a notch between gauge marks is used in a creep test on a notched test piece.
• the cross section of the portion to be stretched and subject to measurement is set to be the same as the cross section of the part subject to the measurement in a creep test on an unnotched test piece, and the stress is determined.
• a tensile stress which is applied gradually elongates the distance between gauge marks, and narrows the portion between the gauge marks, which finally will rupture.
• notch softening Such a phenomenon is called “notch softening", which can be used as an index for expressing creep embrittlement. That is to say, by conducting creep rupture tests on an unnotched test piece and a notched test piece under the same conditions such as stress and temperature, and comparing the times elapsed before creep rupture, the level of creep embrittlement can be clearly demonstrated.
• the present inventors succeeded in developing a material which can be quenched after heating to a high temperature of 1000°C or higher, which is inhibited from producing precipitation of a pro-eutectoid ferrite phase, and which is not subject to creep embrittlement, by minimizing the amount of trace impurity elements which are harmful, such as phosphorus, sulfur, copper, aluminum, arsenic, tin, and antimony.
• an alloy according to the first aspect of the present invention is a low-alloy heat-resistant steel comprising:
• Tungsten is added to a conventional CrMoV steel with the intention of improving the creep properties. Furthermore, by limiting the permissible amounts of phosphorus, sulfur, copper, aluminum, arsenic, tin, and antimony impurities, which are harmful in causing creep embrittlement, to low levels, the creep embrittlement resistance is particularly improved.
• An alloy according to the second aspect of the present invention is a low-alloy heat-resistant steel comprising:
• cobalt is further added to the alloy of the first aspect of the invention with the intention of improving the toughness of the alloy. Furthermore, like the alloy of the first aspect, by limiting the permissible amounts of phosphorus, sulfur, copper, aluminum, arsenic, tin, and antimony impurities, which are harmful in causing creep embrittlement, to low levels, the creep embrittlement resistance is particularly improved.
• An alloy according to the third aspect of the present invention is a low-alloy heat-resistant steel comprising:
• a trace amount of at least one type of element selected from niobium, tantalum, nitrogen, and boron is further added to the alloy of the first aspect of the invention with the intention of further improving particularly the creep properties of an unnotched test piece. Furthermore, like the alloy of the first aspect, by limiting the permissible amounts of phosphorus, sulfur, copper, aluminum, arsenic, tin, and antimony impurities, which are harmful in causing creep embrittlement, to low levels, the creep embrittlement resistance is particularly improved.
• An alloy according to the fourth aspect of the present invention is a low-alloy heat-resistant steel comprising:
• cobalt and a trace amount of at least one type of element selected from niobium, tantalum, nitrogen, and boron are further added to the alloy of the first aspect of the invention with the intention of improving the toughness as well as further improving particularly the creep properties of an unnotched test piece. Furthermore, as in the case of alloy of the first aspect, the creep embrittlement resistance is intended to be improved.
• An alloy according to the fifth aspect of the present invention is a low-alloy heat-resistant steel comprising:
• the creep embrittlement resistance of this alloy is improved by limiting the permissible amounts of phosphorus, sulfur, copper, aluminum, arsenic, tin, and antimony impurities, which are harmful in causing creep embrittlement, to low levels compared to conventional CrMoV steels.
• An alloy according to the sixth aspect of the present invention is a low-alloy heat-resistant steel comprising:
• cobalt is added to the alloy of the fifth aspect of the invention so as to improve the toughness of the alloy. Furthermore, as in the alloy of the fifth aspect, the creep embrittlement resistance is improved.
• An alloy according to the seventh aspect of the present invention is a low-alloy heat-resistant steel comprising:
• a trace amount of at least one type of element selected from niobium, tantalum, nitrogen, and boron is further added to the alloy of the fifth aspect of the invention with the intention of further improving the creep properties of an unnotched test piece. Furthermore, as in the case of the alloy of the fifth aspect, the creep embrittlement resistance is improved.
• An alloy according to an eighth aspect of the present invention is a low-alloy heat-resistant steel comprising:
• cobalt and a trace amount of at least one type of element selected from niobium, tantalum, nitrogen, and boron are added to the alloy of the fifth aspect of the invention so as to improve toughness as well as to further improve the creep properties of an unnotched test piece. Furthermore, as in the case of the alloy of the fifth aspect, the creep embrittlement resistance is improved.
• An alloy according to the ninth aspect of the present invention is a low-alloy heat-resistant steel according to any one of the alloys of the first to eighth aspects, wherein the amount of pro-eutectoid ferrite phase is not larger than 10% by volume.
• the tenth aspect of the present invention is a process for producing a low-alloy heat-resistant steel, the process comprising the step of:
• the ingot By heating the ingot to 1000°C or higher, the amount of pro-eutectoid ferrite phase can be restricted, and the properties at high temperatures are improved due to thorough dissolution of reinforcing elements in the alloy matrix.
• the eleventh aspect of the present invention is a turbine rotor comprising a low-alloy heat-resistant steel as in any one of the first to ninth aspects.
• this turbine rotor is superior to a conventional turbine in creep embrittlement resistance, and is durable in use at high temperatures.
• the low-alloy heat-resistant steel according to the present invention can be easily manufactured, has a yield strength and toughness which are equivalent to or greater than those of a conventional CrMoV steel, and has excellent high-temperature properties.
• this steel can be used at a high temperature and is very useful because it allows construction of power plants of high energy efficiency.
• Carbon has the effect of increasing the material strength as well as ensuring the hardenability during the heat treatment. In addition, carbon forms a carbide and contribute to improvement of the creep rupture strength at high temperatures.
• the lower limit of the carbon content is 0.20% since a carbon content of less than 0.02% does not impart sufficient material strength to the alloy.
• an excessive carbon content deteriorates the toughness, and while the alloy is being used at a high temperature, carbon nitride aggregates to form coarse grains, which cause degradation in the creep rupture strength and creep embrittlement. Accordingly, the upper limit of the carbon content is 0.35%.
• a particularly preferred range within which both material strength and the toughness are imparted to the alloy is from 0.25 to 0.30%.
• Si is an element which is effective as a deoxidizer but embrittles the alloy matrix.
• an Si content of up to 0.35% is permissible.
• the silicon content can be minimized.
• the lower limit of the silicon content is 0.005%. Accordingly, the range of the silicon content is from 0.005 to 0.35%. A preferable range is from 0.01 to 0.30%.
• Manganese functions as a deoxidizer as well as having the effect of preventing hot cracks during forging. In addition, manganese has the effect of enhancing the hardenability during heat treatment. However, since too large a manganese content deteriorates the creep rupture strength, the upper limit of the manganese content is 1.0%. However, since limiting the manganese content to less than 0.05% requires careful selection of materials and excessive refining steps, and therefore brings about a higher cost, the lower limit of the manganese content is 0.05%. Accordingly, the range of the manganese content is from 0.05 to 1.0%, preferably from 0.1 to 0.8%.
• Nickel particularly has the effect of enhancing the toughness as well as enhancing the hardenability during the heat treatment and improving the tensile strength and the yield strength. If the nickel content is less than 0.05%, these effects are not discernible. On the other hand, a large amount of nickel added reduces the long-term creep rupture strength.
• the upper limit of the nickel content is 0.3%. Taking account of the balance between this harmful effect and the effect of enhancing the toughness, the range of the nickel content is from 0.05 to 0.3%, preferably from 0.08 to 0.20%.
• Chromium enhances the hardenability of the alloy during the heat treatment as well as contributing to improvement of the creep rupture strength by forming a carbide and/or a carbonitride, and improving the antioxidation effect by dissolving in the matrix of the alloy.
• chromium has the effect of strengthening the matrix itself and improving the creep rupture strength.
• a chromium content of less than 0.8% does not provide a sufficient effect, and a chromium content exceeding 2.5% has the adverse effect of reducing the creep rupture strength. Accordingly, the range of the chromium content is from 0.8 to 2.5%, preferably from 1.0 to 1.5%.
• Molybdenum enhances the hardenability of the alloy during the heat treatment as well as improving the creep rupture strength by dissolving in the matrix of the alloy or in a carbide and/or a carbonitride. If the molybdenum content is less than 0.1%, these effects are not sufficiently discernible. Addition of molybdenum exceeding 2.0% has the adverse effect of deteriorating the toughness and brings about a higher cost. Accordingly, the molybdenum content is from 0.1 to 1.5%, preferably 0.5 to 1.5%.
• Vanadium Vanadium enhances the hardenability of the alloy during the heat treatment as well as improving the creep rupture strength by forming a carbide and/or a carbonitride.
• a vanadium content of less than 0.05% does not provide a sufficient effect.
• a vanadium content exceeding 0.3% has the opposite effect of deteriorating the creep rupture strength. Accordingly, the vanadium content is from 0.05 to 0.3%, preferably from 0.15 to 0.25%.
• Tungsten dissolves in the matrix of the alloy or a carbide to improve the creep rupture strength. If the tungsten content is less than 0.1%, the above effect is not sufficient. If the tungsten content exceeds 2.5%, there is a possibility of segregation in the alloy, and a ferrite phase tends to emerge, which deteriorates the strength. Accordingly, the tungsten content is from 0.1 to 2.5%, preferably 1.0 to 2.4%.
• Phosphorus (P), Sulfur (S) Both phosphorus and sulfur are impurities transferred from materials for steel production, and are harmful impurities which noticeably deteriorate the toughness of the steel product by forming a phosphide or a sulfide therein.
• phosphorus and sulfur also adversely affect the high-temperature properties. Phosphorus tends to be segregated, and secondarily causes segregation of carbon which embrittles the steel product. It was also found that phosphorus greatly affects the embrittlement when a high load is applied at a high temperature over a long time.
• the upper limits of phosphorus and sulfur were sought such that the rupture time in a creep test on a notched test piece is 10,000 hours or longer. As a result, it has been determined that the upper limit of phosphorus is 0.012%, and the upper limit of sulfur is 0.005%. More preferably, phosphorus is 0.010% or less, and sulfur is 0.002% or less.
• Copper is diffused along crystal grain boundaries in the steel product, and embrittles the steel product. Copper particularly degrades high-temperature properties. In view of the results of creep tests on notched test pieces, it has been determined that the upper limit of the copper content is 0.10%. More preferably, the copper content is 0.04% or less.
• Aluminum is brought into steel mainly from deoxidizers during the steel production process, and forms an oxide-type inclusion in the steel product, which embrittles it. In view of the results of creep tests on notched test pieces, it has been determined that the upper limit of the aluminum content is 0.01%. More preferably, the copper content is 0.005% or less.
• Arsenic (As), Tin (Sn), Antimony (Sb) It is often the case that arsenic, tin, and antimony are brought into the steel from materials for steel production. They are precipitated along crystal grain boundaries, which deteriorates the toughness of the steel product. Arsenic, tin, and antimony are aggregated in crystal grain boundaries particularly at high temperatures, and accelerate the embrittlement. In view of the results of creep tests on notched test pieces, the upper limits of these impurities are 0.01% for arsenic, 0.01% for tin, and 0.003% for antimony. More preferably, the arsenic content is 0.007% or less, the tin content is 0.007% or less, and the antimony content is 0.0022% or less.
• Cobalt dissolves in the matrix of the alloy, and strengthens the matrix itself as well as inhibiting the precipitation of the ferrite phase. In addition, cobalt has an effect of improving the toughness, and thus is effective in keeping the balance between the hardness and the toughness. If the amount of cobalt added is less than 0.1%, the above effects are not discernible. If the amount of cobalt added exceeds 3.5%, precipitation of carbides is accelerated, which leads to deterioration of the creep properties. Accordingly, a permissible range of the cobalt content is from 0.1% to 3.5%, and more preferably from 0.5% to 2.5%.
• Niobium enhances the hardenability of the alloy as well as improving the creep rupture strength by forming a carbide and/or a carbonitride. In addition, niobium restricts the growth of crystal grains during heating at high temperatures, and contributes to homogenization of the alloy structure. If the amount of niobium added is less than 0.01%, the effects are not discernible. An amount of niobium added exceeding 0.15% will bring about noticeable deterioration of the toughness as well as causing formation of coarse grains of the carbide or the carbonitride of niobium during use of the alloy, which deteriorates long-term creep rupture strength. Accordingly, it has been determined that a permissible niobium content is from 0.01% to 0.15%, preferably 0.05 to 0.1%.
• Tantalum in a manner similar to niobium, enhances the hardenability of the alloy as well as improving the creep rupture strength by forming a carbide and/or a carbonitride. If the amount of tantalum added is less than 0.01%, the effects are not discernible. An amount of tantalum added exceeding 0.15% will bring about noticeable deterioration of the toughness as well as causing formation of coarse grains of the carbide or the carbonitride of niobium during use of the alloy, which deteriorates long-term creep rupture strength. Accordingly, it has been determined that a permissible tantalum content is from 0.01% to 0.15%, preferably 0.05 to 0.1%.
• the main components of the alloy according to fifth to eighth aspect of the invention correspond to main components of a conventional CrMoV steel, and do not include tungsten.
• the reasons for limiting the amounts of the other components are as described above.
• the alloy according to the fifth aspect of the invention is an alloy in which the contents of impurities in an alloy corresponding to a conventional CrMoV steel are limited to extremely low levels. It was discovered that by limiting the impurity contents to extremely low levels, a material which has a creep strength according to a test on an unnotched test piece equivalent to the creep strength of a conventional CrMoV steel and which is not subject to creep embrittlement can be obtained without having the toughness reduced. Accordingly, tungsten is not added to the alloy of the fifth aspect of the invention.
• the carbon content is from 0.20 to 0.40%
• the silicon content is from 0.005 to 0.440%
• the manganese content is from 0.05 to 1.0%
• the nickel content is from 0.05 to 0.6%
• the chromium content is from 0.8 to 1.5%
• the vanadium content is from 0.1 to 0.3%.
• the alloy according to the sixth aspect of the invention is an alloy in which cobalt is added to the alloy components of the fifth aspect of the invention to strengthen the matrix of the alloy and improve the toughness.
• the alloy according to the seventh aspect of the invention is an alloy in which at least one of the trace elements of niobium, tantalum, nitrogen, and boron is added to the alloy components of the fifth aspect of the invention with the intention of further improving the creep properties according to tests on an unnotched test piece.
• the alloy according to the eighth aspect of the invention is an alloy in which cobalt and at least one of niobium, tantalum, nitrogen, and boron are added to the alloy components of the fifth aspect of the invention in order to improve the toughness and with the intention of further improving the creep properties according to tests on an unnotched test piece.
• the low-alloy steel of the present invention is normally used after it is heated to a high temperature of 950°C or higher and quenched, and then tempered at a temperature of 580 to 680°C.
• a heating temperature of 1000°C or higher before quenching a conventional CrMoV steel causes so-called creep embrittlement, by which the material becomes brittle. Accordingly, the heating temperature before quenching a conventional CrMoV steel is from 950 to 970°C.
• the heating temperature before quenching a system of components in which amounts of strengthening elements such as tungsten and molybdenum are increased is within a range of 950 to 970°C, the strength of the material cannot be ensured since a soft ferrite phase is precipitated in large amounts, and yet the high-temperature creep strength does not reach a satisfactory level.
• the ferrite phase precipitated is in a small amount and is finely distributed, and the harmful effects are small.
• the proportion of the ferrite phase in the light microscopic structure can be determined using an image analyzing device which is commonly used.
• a base material is produced by a melting process so as to have a predetermined alloy composition.
• a method for reducing the trace impurities is not particularly limited, and various known refining methods including careful selection of raw materials can be employed.
• an alloy melt with a predetermined composition is cast by a known method to form a steel ingot, which is subjected to a predetermined forging/molding process to produce a material for the turbine rotor member.
• this material is quenched after heating to a temperature between 1000°C and 1100°C, preferably between 1030°C and 1070°C, and is tempered at a temperature between 600°C and 750°C, preferably 650°C and 700°C. If the heating temperature before quenching is lower than 1000°C, a great amount of soft ferrite phase is precipitated because dissolution of the strengthening elements is insufficient, and the strength is not increased. if the heating temperature before quenching exceeds 1100°C, coarse crystal grains are formed, which deteriorates the toughness. If the tempering temperature is lower than 600°C, the tempering is insufficient, and the high-temperature creep strength is deteriorated while the desired toughness cannot be obtained.
• tempering temperature exceeds 750°C, the tensile strength and the yield strength are deteriorated.
• the range of the tempering temperature is appropriately chosen such that the 0.2% yield strength will be about 63 ⁇ 2 kgf/mm 2 .
• any known means can be appropriately chosen taking account of the use and size of the material.
• test pieces were prepared by a melting process using a 50 kg vacuum high-frequency furnace, and forging at a heating temperature of 1200°C. Heat treatments of the test pieces used in the various evaluation tests were carried out by hardening the test pieces under conditions which simulated the central part of an oil-quenched rotor having a drum diameter of 1,200 mm, and then tempering them at a temperature which had been determined so as to give a 0.2% yield strength of about 63 ⁇ 2 kgf/mm 2 . However, some test pieces did not achieve this target yield strength.
• Example 1 chemical compositions of materials tested in Example 1 (Samples Nos. 1 to 6) and of comparative materials (Samples Nos. 7 to 14) are shown.
• the amounts of pro-eutectoid ferrite phase in each material quenched after heating to 950°C, 1000°C, and 1050°C were quantified using an image analyzing device, and the results are shown in Table 2.
• the 0.2% yield strength, the Charpy impact absorbed energy, and the creep rupture time at 600°C under 15 kgf/mm 2 for each material quenched after heating to 1050°C were measured for notched and unnotched test pieces, and the results are also shown in Table 2.
• Each of Samples Nos. 7 to 9 of Comparative Example A exhibited insufficient strength even though the trace impurity contents were reduced, mainly because the amounts of carbon, silicon, and manganese were inappropriate.
• Each of Samples Nos. 7 and 8 had a large amount of pro-eutectoid ferrite phase, having reduced hardenability, and exhibited an insufficient strength.
• Sample No. 9 exhibited an inferior toughness.
• Each of Samples Nos. 10 and 11 of Comparative Example B had an insufficient creep strength according to the test using an unnotched test piece even though the trace impurity contents were reduced, since the amounts of nickel, chromium, molybdenum, tungsten, vanadium, and the like were inappropriate. Samples Nos.
• Example 2 is based on the material of Sample No. 2 or 5 in Example 1, and furthermore in Example 2, cobalt or a trace element such as niobium, tantalum, nitrogen, and boron was added to the material of Sample No. 2 or 5, and trace impurities were restricted to low levels.
• 0.2% yield strength, the Charpy impact absorbed energy, and the creep rupture time at 600°C under 15 kgf/mm 2 of each material quenched after heating to 1050°C were measured for notched and unnotched test pieces, and the results are shown in Table 4.
• Example 2 From the results in Table 4, it is understood that the levels of the 0.2% yield strength and the Charpy impact absorbed energy of each material (Samples Nos. 15 to 21) of Example 2 are about the same as those of the materials in Example 1, but the creep properties were improved, and particularly the creep rupture time in the creep test using the unnotched test piece was prolonged greatly. In addition, it is understood that in Example 2 the amounts of pro-eutectoid ferrite phase were reduced in the test pieces which were quenched after heating to 1000°C, and creep embrittlement did not occur in the test pieces which were quenched after heating to 1050°C since the trace impurities were restricted to low levels.
• Example 5 the chemical compositions of the materials of the present invention tested in Example 3 (Samples Nos. 22 and 23) and of comparative materials (Samples Nos. 12 and 24) are shown.
• Example 3 the study of trace impurities in the material corresponding to a conventional CrMoV steel was carried out. The 0.2% yield strength, the Charpy impact absorbed energy, and the creep rupture time at 600°C under 15 kgf/mm 2 of each material quenched after heating to 950°C, 1000°C, or 1050°C were measured for notched and unnotched test pieces, and the results are shown in Table 6.
• Example 3 the materials of Example 3 (Samples Nos. 22 and 23) exhibited the strength and the toughness at the same level as those of the materials of the Comparative Examples (Samples Nos. 12 and 24) when the samples were quenched after heating to 1000°C.
• Example 3 the high-temperature creep properties were greatly improved and the creep properties of the unnotched test pieces were slightly improved, and the notched test pieces did not break in 12,000 hours in the creep rupture time tests while comparative materials with notches broke in 2,000 to 2,500 hours. It is understood that even in a conventional CrMoV steel, creep embrittlement can be prevented by restricting the amount of trace impurities to low levels.
• Example 4 is based on the alloy of Sample No. 23 in Example 3, which showed good results, and furthermore in Example 4, cobalt or a trace element such as niobium, tantalum, nitrogen, and boron is added to the material of Sample No. 23, and trace impurities are restricted to low levels.

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