High toughness heat-resistant steel, turbine rotor and method of producing the same

Abstract: A high toughness heat-resistant steel, a turbine rotor formed of this steel and a method of producing the turbine rotor are described. The heat-resistant steel has a composition consisting essentially of: 0.05 to 0.30wt% C, 0 to 0.20wt% Si, 0 to 1.0wt% Mn, 8.0 to 14.0wt% Cr, 0.5 to 3.0wt% Mo, 0.10 to 0.50wt% V, 1.5 to 5.0wt% Ni, 0.01 to 0.50wt% Nb, 0.01 to 0.08wt% N, 0.001 to 0.020wt% B, balance Fe and unavoidable impurities. The steel has excellent characteristics for not only a tensile strength and toughness at a relatively low temperature condition of a steam turbine such as high/low pressure combined type one but also a creep rupture strength at a high temperature condition of this turbine.

Description: BACKGROUND OF THE INVENTION The present invention relates to a high toughness heat-resistant steel, a turbine rotor and a method of producing the same, and more particularly, to improvements in material of the high toughness heat-resistant steel used for high/low pressure combined type turbine rotor and the like which are especially suitable for a power plant aiming at a large volume and high efficiency. In generally, in a steam turbine in which a plurality of turbine rotors are mechanically coupled together, materials for the rotors are selected in accordance with steam conditions used from high pressure side to low pressure side. For example, CrMoV steel (ASTM-A470 (class8)) or 12Cr steel (Japanese Patent Application Publication No.60-54385) is used as a material for turbine rotor used at the side of high temperature (550 to 600°C) and high pressure, and NiCrMoV steel (ASTM-A471 (classes 2 to 7)) including 2.5% or more of Ni is used as a material for turbine rotor used at the side of low temperature (400°C or lower) and high pressure. In a recent power plant having a tendency to achieve a large volume and high efficiency, a so-called high/low pressure combined type turbine rotor in which a high pressure side portion and a low pressure side portion are integrally formed of the same material has attracted attention, in view of miniaturization of the steam turbine and simplification of the structure. However, since the conventional steel for the above-described turbine rotor is not a material intended to be used under the condition which covers all of the requirements from high pressure side to low pressure side, if such a conventional steel is used to form the high/low pressure combined type turbine rotor, the following problems are conceived to be arisen: 1): In the case of CrMoV steel, although it is excellent in creep rupture strength in a high temperature region about 550°C, a tensile strength and toughness are not always satisfactory in a low temperature region, and a ductile fracture, brittle fracture or the like are anticipated. Therefore, as a countermeasure therefor, it is necessary to reduce stress acting on the lower pressure portion of the turbine rotor. As a result, a size of blade mounted at a low pressure stage, especially at the final stage is restrained. From the view of this point, it is difficult to increase the volume of a power plant. Further, with respect to a high temperature creep rupture strength also, the CrMoV steel does not always satisfy the condition of high temperature (about 600°C) and high pressure of steam at entrance of turbine that is required for enhancing the efficiency of the recent power plant. 2) In the case of 12Cr steel, this steel superior to the CrMoV steel in high temperature creep rupture strength, and thus can satisfy the above-described condition for the steam at entrance of turbine. However, since this steel does not have enough toughness, a countermeasure is also required as in the case of the CrMoV steel, and a size of blade that can be mounted at the low pressure stage is limited. 3) In the case of NiCrMoV steel, although this steel has excellent tensile strength and toughness at low temperature region, its creep rupture strength is not always satisfactory, and since a strength of this steel used at the high pressure side is not sufficient, it is necessary to limit a degree of high temperature of the steam at entrance of turbine, and it is difficult to enhance the efficiency of the power plant. As described above, when a high/low pressure combined type turbine rotor is formed using the conventional steel, there is a problem that a great restriction can not be avoided when effort is made for increasing a volume and enhancing the efficiency in a steam turbine in which a long low pressure final stage blade is mounted. EP 0 639 691 A1 discloses a heat-resistant steel comprising 0.05 - 0.3 wt-% C, 1.0 wt-% or less Si, 1.0 wt-% or less Mn, 8.0 - 13.0 wt-% Cr, 1.5 wt-% or less Mo, 0.1 - 0.5 wt-% V, 2.0 wt-% or less Ni, 0.03 - 0.25 wt-% Nb, 0.025 - 0.10 wt-% N, 0 to 0.05 wt-% B, 0.5 - 5.0 wt-% W, the balance being Fe and unavoidable impurities. EP 0 333 129 A2 discloses a heat-resistant steel comprising 0.05 - 0.2 wt-% C, not more than 0.5 wt-% Si, not more than 1.5 wt-% Mn, 8 to 13 wt-% Cr, 1.5 - 3.0 wt-% Mo, 0.05 - 0.3 % V, not more than 3 wt-% Ni, 0.02 - 0.2 wt-% Nb, 0.02 - 0.1 wt-% N, not more than 0.01 wt-% B, not more than 1 wt-% W, the balance being Fe and unavoidable impurities. EP 0 867 523 11, which is prior art under Art 54(3) EPC, discloses a heat-resistant steel comprising 1.0 to 5.0 wt-% Co. SUMMARY OF THE INVENTION The present invention has been accomplished in view of the conventional problems, and it is an object of the invention to provide a heat-resistant steel having excellent characteristics for both the tensile strength and toughness at a relatively low temperature region and a creep rupture strength at a high temperature region. Further, it is another object of the invention to provide a turbine rotor such as high/low pressure combined type turbine rotor suitable for a power plant requiring a large volume and high efficiency. The above objects are achieved by a high toughness heat-resistant steel according to claim 1, a turbine rotor according to claim 9, and by the method according to claim 10. The reason for limiting the ranges of contents of compositions of each of the elements in the high toughness heat-resistant steel of the present invention will be described below. Here, it should be noted that the sign of % showing composition (content) of each the elements means % by weight, unless there is a description to the contrary. C is bonded to elements such as Cr, Nb and V to form carbohydrate and contributes to strengthening precipitation, and is indispensable element for enhancing the hardening properties or for suppressing the generation of 6 ferrite. Here, if an amount of C added is less than 0.05%, a desired creep rupture strength can not be obtained, and if the amount of C added exceeds 0.30%, this facilitates to coarsen carbohydrate, and the creep rupture strength over long time period is lowered. Therefore, the C content should be in the range of 0.12 to 0.16%. Si is a necessary element as a deoxidizer at the time of melting. However, if a large amount of Si is added, a portion thereof remains in the steel as an oxide to lower the toughness and therefore, Si content is set in a range of 0.05 to 0.13%. Mn is a necessary element as a deoxidizer or desulfurizing agent at the time of melting. However, if a large amount of Mn is added, the creep rupture strength of the steel is lowered and therefore, the Mn content should be in the range of 0.09 to 0.23%. Cr is a necessary element as a component element of M23C6-type precipitation which enhances antioxidation properties and anticorrosive, and contributes to strengthen the solid solution and precipitation. However, if an amount of Cr added is less than 8.0%, its effect is small, and if the amount of Cr added exceeds 14.0%, δ ferrite which is harmful for the toughness and the creep rupture strength is prone to be generated. Therefore, the Cr content should be in the range of 10.15 to 11.67 %. Mo is a necessary element as a component element as a solid solution strengthen element and carbohydrate. However, if an amount of Mo added is less than 0.5%, such effects are small, and if the amount of Mo added exceeds 3.0%, the toughness is largely lowered, and δ ferrite is prone to be generated. Therefore, the Mo content should be in the range of 0.10 to 1.40% Here, W (which will be described later) which exhibits substantially the same function as that of Mo is added. If, in this case, an amount of Mo added is less than 0.1%, its effects as a solid solution strengthening element and a carbohydrate element are small, and if the amount of Mo added exceeds 1.40%, the toughness is largely lowered, and δ ferrite is prone to be generated. V is an element contributing to strengthen the solid solution and to form V-carbohydrate. If an amount of V is equal to or greater than 0.10%, the fine precipitation is precipitated in the creep mainly on martensite lath boundary to suppress the recovery. However, if the amount of V exceeds 0.50%, δ ferrite is prone to be generated. Further, if the amount of V is less than 0.10%, solid solution amount and precipitation amount are small and the above-mentioned effects can not be obtained. Therefore, the V content should be in the range of 0.18 to 0.26%. Ni is an element which largely enhances the hardening properties and toughness, and suppresses the precipitation of δ ferrite. However, if an amount of Ni added is less than 1.5%, such effects are small, and if the amount of Ni added exceeds 5.0%, a creep resistance is lowered. Therefore, the Ni content should be in a range of 2.31 to 2.71%. Nb is an element which forms fine carbon-nitride of Nb(C, N) by bonding to C and N, and contributes to strengthen the precipitation dispersion. However, if an amount of Nb added is less than 0.01%, precipitation density is low and the above-mentioned effects can not be obtained, and if the amount of Nb added exceeds 0.50%, a coarse Nb (C, N) which has not yet been solidified is prone to be created, and ductile and toughness are lowered. Therefore, the Nb content should be in a range of 0.05 to 0.09 %. N is an element which forms nitride or carbon-nitride and contributes to strengthen the precipitation dispersion, and which remains in base phase to also contribute to strengthen the solid solution. However, if an amount of N added is less than 0.01%, such effects can not be obtained, and if the amount of N added exceeds 0.08%, this facilitates to coarsen nitride or carbon-nitride and the creep resistance is lowered, and ductile and toughness are lowered also. Therefore, the N content should be in a range of 0.021 to 0.026%. B is an element which facilitates the precipitation of precipitation on crystal grain boundary with a small amount of B added, and enhances stability of carbon-nitride at high temperature for a long time. However, if an amount of B added is less than 0.001%, such effects can not be obtained, and if the amount of B added exceeds 0.020%, toughness is largely lowered and hot-working properties are deteriorated. Therefore, the B content should be in a range of 0.006% to 0.010%. W is an element which contributes as solid solution reinforcing element and as a carbide, and also contributes to formation of intermetallic compound comprising Fe, Cr, and W and the like. Therefore, W is added such that a more excellent creep rupture strength results. However, if the amount of W added is less than 0.3%, such effect can little be obtained, and if the amount of W added exceeds 5.0%, δ ferrite is prone to be created, and the toughness and heat fragile characteristics are remarkably lowered. Therefore, the W content should be in a range of 1.17 to 3.99%. Co is an element which contributes to strengthen the solid solution and suppresses δ ferrite from being creased. However, Co is added in comparative examples only. When each of the above-described elements and Fe are added, it is desirable to reduce, to the utmost, the amount of impurities which may be mixed attendantly. A turbine rotor according to the present invention is characterized in that it is formed of high toughness heat-resistant steel according to the invention. A method of producing a turbine rotor according to the present invention comprises the steps of: preparing a steel material having the chemical composition according to the present invention; forming a turbine rotor blank using the material; subjecting the turbine rotor blank to a hardening under the condition of heating temperature of 950°C to 1,120°C, and then; subjecting the turbine rotor blank to a tempering at least once under the condition of heating temperature of 550°C to 740°C. Preferably, the condition of heating temperature in the hardening step is set in a range of 1,030°C (inclusive) to 1,120°C (inclusive) for a high pressure portion or an intermediate pressure portion of the turbine rotor blank, and is set in a range of 950°C (inclusive) to 1,030°C (inclusive) for a low pressure portion of the turbine rotor blank. Preferably, the condition of heating temperature in the tempering step is set in a range of 550°C (inclusive) to 630°C (inclusive) for a high pressure portion or an intermediate pressure portion of the turbine rotor blank, and is set in a range of 630°C (inclusive) to 740°C (inclusive) for a low pressure portion of the turbine rotor blank. Reasons for defining the thermal treatment conditions of the present invention will be described below. Hardening treatment is a necessary thermal treatment for providing a turbine rotor blank with an excellent strength. However, if a heating temperature is less than 950°C, austenitization is no sufficient and the hardening can not be performed, and if the heating temperature exceeds 1,120°C, austenitic crystal grain is excessively coarsened, and ductile is lowered and therefore, the heating temperature is set in a range of 950°C to 1,120°C. Here, since the creep rupture strength is especially important for the portion of the rotor blank corresponding to its high pressure or intermediate pressure portion, it is desirable that each of the precipitations is sufficiently formed into solid solution by hardening at a high heating temperature in a range of 1,030°C to 1,120°C and then, it is again precipitated finely by tempering. Further, since a tensile strength and toughness are especially important for a portion of the rotor blank corresponding to its low pressure portion, it is desirably to finely pulverize the crystal grains by hardening at a low heating temperature in a range of 950°C to 1,030°C. Tempering treatment is a thermal treatment which is necessary to be carried out once or more so as to adjust to provide the turbine rotor blank with a desired strength. However, if a heating temperature of the tempering is less than 550°C, a sufficient tempering effect can not be obtained and thus an excellent toughness can not be obtained, and if the heating temperature exceeds 740°C, a desired strength can not be obtained. Therefore, the heating temperature is set in a range of 550°C to 740°C. Here, since the creep rupture strength is especially important for the portions of the rotor blank corresponding to its high pressure portion and intermediate pressure portion, it is desirable that a tempering treatment at a high heating temperature in a range of 630°C to 740°C is carried out at least once, and a precipitation which has been formed into solid solution by hardening is again precipitated sufficiently. Further, since a tensile strength and toughness are especially important for a portion of the rotor blank corresponding to its low pressure portion, it is desirably to carry out the tempering treatment at least once at a low heating temperature in a range of 550°C to 630°C, thereby satisfying both a desired tensile strength and an excellent toughness. As a process for forming the turbine rotor blank, it is preferable to use a process in which a steel ingot for the turbine rotor blank is produced using electroslag remelting. In a large-sized blank, typified by a steam turbine rotor, when a steel ingot is solidified, segregation of added element or ununiformity in solidified composite are prone to be generated. Especially, when various elements are added aiming at enhancement in material characteristics, a tendency of segregation is increased at center portion of the steel ingot, and the ductile or toughness at the center portion of the rotor blank tends to be lowered. Therefore, if the electroslag remelting is used as a producing method of the steel ingot for forming the turbine rotor blank, more homogeneous and cleaner steel ingot can be obtained. As other measures, a vacuum carbon deoxidization and the like may be used. According to the present invention, as described above, it is possible to provide a high toughness heat-resistant steel having a high creep rupture strength even under a high temperature steam condition, and having high tensile strength and toughness even under a relatively low temperature steam condition. Therefore, if a turbine rotor, especially a high/low pressure combined type turbine rotor is formed using this high toughness heat-resistant steel, there is a merit that the turbine rotor can be used in a high temperature steam environment and a low pressure final long stage can be mounted, and it is possible to construct a power plant having a large volume and high efficiency using a high/low pressure combined type turbine rotor which was not realized before. DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of a high toughness heat-resistant steel, a turbine rotor and a method for producing the same will be described below, wherein the first embodiment is an embodiment of the present invention. FIRST EMBODIMENT Examples 1 to 44 As examples 31, 32, 34, and 36 to 39 of the present invention, sample materials were prepared having chemical compositions (sample materials M31, M32, M34, M36 to M39) as shown in Table 1. Furthermore, examples 1 to 30, 33, 35, and 40 to 44 were prepared as comparative examples having chemical compositions as shown in Table 1, which are not covered by the claims. Here, the sample materials M1 to M30 do not include W and Co, the materials M31 to M40 include W, and the materials M41 to M44 include W and Co. 50kg of each of the sample materials of the examples 1 to 44 was melted using a vacuum high frequency induction electric furnace, and after casting, it was heated to 1,200°C, press-forged and stretched to prepare a round rod having a diameter of 60mm. Thereafter, the round rod was subjected to the thermal treatment condition HM1 shown in Table 2, i.e., a hardening at 1,030°C and then, a tempering once at 630°C once. A test piece was cut out from each of the round rod sample materials obtained in this manner, tensile test, Charpy impact test and creep fracture test were conducted. Here, the tensile test is for finding out a tensile strength, a yield strength, an elongation, a reduction of area and the like for evaluating that the tensile strength is excellent as the tensile strength and the yield strength are greater, and the ductility is excellent as the elongation and the reduction of area are greater. The Charpy impact test is for finding out impact value, FATT and the like of the sample materials for evaluating that the toughness is excellent as the impact value is greater or the FATT value is smaller. Generally, the impact value is a temperature variable value showing unfrangibility, i.e., toughness when an impact force is applied to the sample material at room temperature (20°C). FATT means a ductile-brittle transition temperature obtained by fracture ratio of the impact test piece, i.e., a temperature at which an area ratio of the ductile fracture measured at high temperature region having greater impact value and a brittle fracture measured at low temperature region having smaller impact value becomes 50% - 50% in intermediate temperature region in which both the ductile fracture and the brittle fracture mixedly exist. The creep rupture test is for finding out the creep rupture strength and the like of the sample material. The creep rupture strength is a characteristic corresponding to creep rupture time, and such strength increases as the rupture time is longer. Here, if results of creep rupture tests (test temperature, test stress and fracture time) obtained from a plurality of test pieces are sorted out using Larson-Miller parameter, it is possible to find out a creep rupture strength (such as 105 hours rupture strength) at an arbitrary temperature (such as 580°C). Table 3 shows measurement results of the above described material tests for tensile strength, 0.02% yield strength, an elongation, a reduction of area, FATT and 100,000 (=10 5 ) hours rupture strength. For comparison, the same material tests were conducted with respect to conventional steels which were actually used for turbine rotors. As the conventional steels, there were prepared three kinds of samples, typified by conditions of chemical compositions (sample materials No.S1 to S3) shown in Table 4, i.e., CrMoV steel (ASTM-A470) for high temperature turbine rotor material ("conventional example 1", hereafter), NiCrMoV steel (ASTM-A471) for low temperature turbine rotor material ("conventional example 2", hereafter), and 12Cr steel (Japanese Patent Application Publication No.60-54385) for high temperature turbine rotor material ("conventional example 3", hereafter). The three kinds of conventional steels shown in Table 4 were processed using the thermal conditions HS1 to HS3 shown in Table 2 to prepare samples, and the same material tests as those described above were conducted for the samples. The test results are shown in Table 5 below. Comparing to the characteristics of the three kinds of conventional steels, it was-confirmed that the conventional example 1 was inferior in tensile strength and toughness, the conventional example 2 was most excellent in toughness, and the conventional example 3 was most excellent in tensile strength and creep rupture strength. Characteristics of the steels of the present invention were compared to those of the conventional steels and analyzed. As a result, it was confirmed that any of the examples 1 to 44, wherein examples 31, 32, 34 and 36 to 39 are examples according to the present invention were superior to the conventional examples 1 to 3 with respect to the values of tensile strength and 0.02% yield strength, and that the steels of the present invention were superior to the three kinds of conventional steels in tensile strength and creep rupture strength. Further, with respect to elongation and reduction of area, it was confirmed that the examples 1 to 44 showed substantially the same values as those of the conventional examples 1 to 3, and had sufficient ductile properties. With respect to FATT, any of the examples 1 to 44 showed the same or lower values as comparing to the conventional example 2 which was most excellent in toughness among all of the three conventional steels. With respect to creep rupture strength, it was confirmed that any of the examples 1 to 44 were superior to the conventional example 1, and some of the examples showed substantially the same level as the conventional example 3 which was most excellent in creep rupture strength among all of the three conventional steels, and that the steels of the present invention had extremely excellent creep rupture strength. From the above, it was confirmed that the steels according to examples 31, 32, 34 and 36 to 39 were superior in tensile strength and toughness to the conventional steels used for steam turbine rotor, and had the creep rupture strength substantially equal to or close to that of the 12Cr steel which was most excellent among all of the three conventional steels, and that the steels of the present invention were high toughness heat-resistant steel of excellent new characteristics having tensile strength, toughness and creep rupture strength. Comparative examples 1 to 20 As further comparative steels, comparative examples 1 to 20 were prepared using conditions (sample materials S4 to S23) of chemical compositions in which any one of the various elements shown in Table 4 exceeded upper or lower limit of the range of the present invention, and using the above-described thermal treatment condition HM1, and the same tests as described above were performed. As a result, as shown in Table 5, it was confirmed that said further comparative steels were inferior to the steels of the prevent invention in all of the tensile strength, toughness and creep rupture strength, and that the comparative examples 1 to 5, 7, 10, 11, 13 to 15, 17 and 19 were inferior in creep rupture strength, the comparative examples 6, 8, 9, 12, 14, 16, 18 and 20 were inferior in toughness, and the comparative examples 1 and 13 were inferior in tensile strength. It was also confirmed that another comparative example including Co showed the same results, i.e., was also inferior in all of the tensile strength, toughness and creep rupture strength. SECOND EMBODIMENT (not covered by the claims) In the second embodiment, an influence under thermal treatment condition was specifically observed by experiments in regard to a producing method of turbine rotors and the like using a high toughness heat-resistant steel. Example 45 In the example 45, the same test as described above was carried out for the sample material M1 which did not include W or Co and is not covered by the claims using the thermal treatment condition HM1. As a result, it was confirmed as shown in Table 6 that the sample material M1 was excellent in all of the tensile strength, toughness and creep rupture strength. Therefore, according to the example 45, it is possible to provide a high toughness heat-resistant steel having preferable characteristics as a blank for, e.g., high/low pressure combined type turbine rotors, more particularly, to provide a high toughness heat-resistant steel having extremely excellent tensile strength and toughness for a low pressure portion, and extremely excellent creep rupture strength for high a pressure portion. Example 46 In the example 46, the thermal treatment condition HM2 was used that was different from HM1 only in that a step for conducting a second tempering at 475°C was added. As a result, it was confirmed as shown in Table 6 that 0.02% yield strength was largely increased, and FATT and creep rupture strength were little varied, as compared to the example 45 using HM1. Therefore, according to the example 46, the tensile strength can further be enhanced by conducting the second tempering, and if the example is used for producing, e.g., rotor blanks, such effects can be exhibited more effectively. Example 47 In the example 47, the thermal treatment condition HM3 was used that was the same as the condition HM1 except that a hardening temperature was set at 1,000°C. As a result, it was confirmed as shown in Table 6 that although creep rupture strength tended to be lowered, tensile strength and 0.02% yield strength were little varied, and FATT was largely lowered, as compared to the example 45 using HM1. Therefore, according to the example 47, it is possible to obtain a high toughness heat-resistant steel having characteristics suitable for, e.g., a low pressure portion and the like of a high/low pressure combined type turbine rotor, i.e., a superior toughness, by conducting a hardening at a low heating temperature in a range of 950°C to 1,030°C. Example 48 In the example 48, the thermal treatment condition HM4 was used that was the same as the condition HM1 except that a hardening temperature was set at 1,070°C. As a result, it was confirmed as shown in Table 6 that although FATT is increased, tensile strength and 0.02% yield strength were little varied, and creep rupture strength was increased, as compared to the example 45 using HM1. Therefore, according to the example 48, it is possible to obtain a high toughness heat-resistant steel having characteristics suitable for, e.g., a high or intermediate pressure portion and the like of a high/low pressure combined type turbine rotor, i.e., a superior creeping fracture strength, by conducting a hardening at a high heating temperature in a range of 1,030°C to 1,120°C. Example 49 In the example 49, the thermal treatment condition HM5 was used that was the same as the condition HM1 except that a tempering temperature was set at 600°C. As a result, it was confirmed as shown in Table 6 that creeping fracture strength was slightly lowered, FATT was slightly increased, and tensile strength and 0.02% yield strength were largely increased, as compared to the example 45 using HM1. Therefore, according to the example 49, it is possible to obtain a high toughness heat-resistant steel having characteristics suitable for, e.g., a low pressure portion and the like of a high/low pressure combined type turbine rotor, i.e., a superior tensile strength, by conducting a tempering at a low heating temperature in a range of 550°C to 630°C. Example 50 In the example 50, the thermal treatment condition HM6 was used that was the same as the condition HM1 except that a tempering temperature was set at 680°C. As a result, it was confirmed as shown in Table 6 that 0.02% yield strength was lowered, FATT was slightly lowered, creep rupture strength was increased, as compared to the example 45 using HM1. Therefore, according to the example 50, it is possible to obtain a high toughness heat-resistant steel having characteristics suitable for, e.g., a high or intermediate pressure portion and the like of a high/low pressure combined type turbine rotor, i.e., a superior creeping fracture strength, by conducting a tempering at a high heating temperature in a range of 630°C to 740°C. Example 51 In the example 51, the thermal treatment condition HM7 was used that was the same as the condition HM1 except that a hardening temperature was set at 1,000°C and a tempering temperature was set at 600°C. As a result, it was confirmed as shown in Table 6 that although creep rupture strength was lowered, FATT was largely lowered, and 0.02% yield strength was largely increased, as compared to the example 45 using HM1. Therefore, according to the example 51, it is possible to obtain a high toughness heat-resistant steel having characteristics suitable for, e.g., a low pressure portion and the like of a high/low pressure combined type turbine rotor, i.e., a superior tensile strength and toughness, by conducting a hardening at a low temperature in a range of 950°C to 1,030°C, and a tempering at a low heating temperature in a range of 550°C to 630°C. Example 52 In the example 52, the thermal treatment condition HM8 was used that was the same as the condition HM1 except that a hardening temperature was set at 1,070°C and a tempering temperature was set at 680°C. As a result, it was confirmed as shown in Table 6 that although tensile strength and 0.02% yield strength were lowered and FATT was increased, creep rupture strength was largely increased, as compared to the example 45 using HM1. Therefore, according to the example 52, it is possible to obtain a high toughness heat-resistant steel having characteristics suitable for, e.g., a low pressure portion and the like of a high/low pressure combined type turbine rotor, i.e., a further superior creeping fracture strength, by conducting a hardening at a high temperature in a range of 1,030°C to 1,120°C, and a tempering at a high heating temperature in a range of 630°C to 740°C. Example 53 In the example 53, the thermal treatment condition HM9 was used that was the same as the condition HM7 except that a step for conducting a second tempering at 475°C was added. As a result, it was confirmed as shown in Table 6 that 0.02% yield strength was largely increased, and FATT and creep rupture strength were little varied, as compared to example 51 using HM7. Therefore, according to the example 53, it is possible to obtain a high toughness heat-resistant steel having characteristics suitable for, e.g., a low pressure portion and the like of a high/low pressure combined type turbine rotor, i.e., a further superior tensile strength and toughness, by conducting a hardening at a low temperature in a range of 950°C to 1,030°C, a tempering at a low heating temperature in a range of 550°C to 630°C, and a second tempering. Example 54 In the example 54, the thermal treatment condition HM10 was used that was the same as the condition HM8 except that a step for conducting a second tempering at 475°C was added. As a result, it was confirmed as shown in Table 6 that 0.02% yield strength was increased, and FATT and creep rupture strength were little varied, as compared to example 52 using HM8. Therefore, according to the example 54, if a hardening is conducted at a high temperature in a range of 1,030°C to 1,120°C and a tempering is conducted at a low heating temperature in a range of 630°C to 740°C, it is possible to obtain a high toughness heat-resistant steel maintaining characteristics suitable for, e.g., a high pressure portion of a high/low pressure combined type turbine rotor, i.e., a further superior creep rupture strength, even if a second tempering is conducted. Example 55 In the example 55, the thermal treatment condition HS4 was used that was the same as the condition HM1 except that a hardening temperature was set at 930°C. As a result, it was confirmed as shown in Table 6 that all of the tensile strength, toughness and creep rupture strength were low, as compared to the example 45 using HM1. Example 56 In the example 56, the thermal treatment condition HS5 was used that was the same as the condition HM1 except that a hardening temperature was set at 1,140°C. As a result, it was confirmed as shown in Table 6 that especially toughness and ductile properties were low, as compared to the example 45 using HM1. Example 57 In the example 57, the thermal treatment condition HS6 was used that was the same as the condition HM1 except that a tempering temperature was set at 530°C. As a result, it was confirmed as shown in Table 6 that especially toughness and ductile properties were low, as compared to the example 45 using HM1. Example 58 In the example 58, the thermal treatment condition HS7 was used that was the same as the condition HM1 except that a tempering temperature was set at 760°C. As a result, it was confirmed as shown in Table 6 that especially tensile strength and creep rupture strength were low, as compared to the example 45 using HM1. Examples 59 to 72 In the examples 59 to 72, the conditions HM1 to HM10 and HS4 to HS7 having different thermal conditions as described above were respectively applied to sample materials M31 including W. As a result, substantially the same results as those of the sample materials M1 were obtained as shown in Table 6. Examples 73 to 86 In the examples 73 to 86, the conditions HM1 to HM10 and HS4 to HS7 having different thermal conditions as described above were respectively applied to sample materials M41 including W and Co which is not covered by the claims. As a result, substantially the same results as those of the sample materials M1 were obtained as shown in Table 6. THIRD EMBODIMENT (not covered by the claims) This embodiment was carried out by changing a producing method of steel ingot which constitutes a turbine rotor blank. Example 87 In the example 87, a chemical composition as shown in Table 7 was used to prepare a sample material E1, which is not covered by the claims. The sample material was melted in an electrical furnace and then, was casted in electrode mole of electroslag remelting to produce a steel ingot. The steel ingot was used as consumable electrode to produce a steel ingot using electroslag remelting. The resultant steel ingot was heated to 1,200°C and press-forged to provide a model (1,000mm×800mm) of a portion corresponding to a rotor. The model was subjected to thermal treatments, i.e., a hardening at 1,030°C and then, a tempering at a heating temperature of 630°C. Test pieces were cut out from a surface layer portion and center portion of the sample material obtained in the above described manner, and tensile test, Charpy impact test and creep fracture test were conducted with respect the test pieces at room temperature, thereby providing a tensile strength, 0.02% yield strength, elongation, reduction of area, FATT and fracture strength for 105 hours at 580°C. As a result, it was confirmed that the surface layer portion and the center portion showed substantially the same values of the tensile strength, 0.02% yield strength, elongation, reduction of area, FATT and creep rupture strength, as shown in Table 8. Therefore, according this example, a more uniform rotor blank having little difference in the tensile strength, ductile properties, toughness and creep rupture strength between the surface layer portion and the center portion, by producing a steel ingot using electroslag remelting for forming a turbine rotor blank made of high toughness heat-resistant steel. Example 88 In the example 88, a chemical composition including W and Co as shown in Table 7 was used as a sample material E2 which is not covered by the claims . According to this example 88, it was confirmed that the same results as those described above could be obtained, and especially its effect was exhibited remarkably when a large amount of alloy element was added. Example 89 In the example 89, a sample material was prepared (sample material V1 (which is not covered by the claims)) which was substantially the same as the sample material E1 used in the example 87 as shown in Table 7. The sample material was melted in an electrical furnace and then, was formed into a steel ingot using vacuum carbon deoxidization, and was heated to 1,200°C and press-forged to provide a model (1,000mm(×800mm) of a portion corresponding to a rotor. The model was subjected to the same thermal treatments as those described above, and the same tests as those described above were carried out on the resultant sample material. As a result, as shown in Table 8, it was confirmed that although the surface layer portion and the center portion showed substantially the same values of the tensile strength, 0.02% yield strength, and creep rupture strength, the center portion had lower elongation and reduction of area, and FATT had an upward tendency at the center portion. Example 90 In the example 90, a sample material was prepared (sample material V2 (which is not covered by the claims)) which was substantially the same as the sample material E2 used in the example 88 as shown in Table 7 except that the same as the example 89. According to this example 90, it was confirmed that the same results as those described above could be obtained, and especially its effect was exhibited remarkably when a large amount of alloy element was added. Various modifications and alterations to the above-described preferred embodiment will be apparent to those skilled in the art. Accordingly, this description of the invention should be considered exemplary and not as limiting the scope of the invention as set forth in the following claims.

Claims: A high toughness heat-resistant steel, consisting of 0.12 - 0.16 wt-% C, 0.05 - 0.13 wt-% Si, 0.09 - 0.23 wt-% Mn, 10.15 - 11.67 wt-% Cr, 0.10 - 1.40 wt-% Mo, 0.18 - 0.26 wt-% V, 2.31 - 2.71 wt-% Ni, 0.05 - 0.09 wt-% Nb, 0.021 - 0.026 wt-% N, 0.006 - 0.010 wt-% B, 1.17 - 3.99 wt-% W, the balance being Fe and unavoidable impurities. A high toughness heat-resistant steel according to claim 1, consisting of 0.13 wt-% C, 0.05 wt-% Si, 0.09 wt-% Mn, 11.63 wt-% Cr, 0.68 wt-% Mo, 0.21 wt-% V, 2.58 wt-% Ni, 0.06 wt-% Nb, 0.021 wt-% N, 0.006 wt-% B, 1.81 wt-% W, the balance being Fe and unavoidable impurities. A high toughness heat-resistant steel according to claim 1, consisting of 0.14 wt-% C, 0.08 wt-% Si, 0.17 wt-% Mn, 10.88 wt-% Cr, 1.06 wt-% Mo, 0.20 wt-% V, 2.43 wt-% Ni, 0.09 wt-% Nb, 0.026 wt-% N, 0.008 wt-% B, 1.17 wt-% W, the balance being Fe and unavoidable impurities. A high toughness heat-resistant steel according to claim 1, consisting of 0.14 wt-% C, 0.10 wt-% Si, 0.13 wt-% Mn, 11.67 wt-% Cr, 0.56 wt-% Mo, 0.18 wt-% V, 2.51 wt-% Ni, 0.07 wt-% Nb, 0.022 wt-% N, 0.007 wt-% B, 2.84 wt-% W, the balance being Fe and unavoidable impurities. A high toughness heat-resistant steel according to claim 1, consisting of 0.14 wt-% C, 0.08 wt-% Si, 0.14 wt-% Mn, 11.45 wt-% Cr, 0.70 wt-% Mo, 0.22 wt-% V, 2.49 wt-% Ni, 0.09 wt-% Nb, 0.025 wt-% N, 0.007 wt-% B, 3.99 wt-% W, the balance being Fe and unavoidable impurities. A high toughness heat-resistant steel according to claim 1, consisting of 0.12 wt-% C, 0.13 wt-% Si, 0.22 wt-% Mn, 10.15 wt-% Cr, 0.30 wt-% Mo, 0.26 wt-% V, 2.31 wt-% Ni, 0.08 wt-% Nb, 0.025 wt-% N, 0.007 wt-% B, 2.04 wt-% W, the balance being Fe and unavoidable impurities. A high toughness heat-resistant steel according to claim 1, consisting of 0.13 wt-% C, 0.08 wt-% Si, 0.23 wt-% Mn, 10.78 wt-% Cr, 1.40 wt-% Mo, 0.21 wt-% V, 2.60 wt-% Ni, 0.08 wt-% Nb, 0.023 wt-% N, 0.010 wt-% B, 1.36 wt-% W, the balance being Fe and unavoidable impurities. A high toughness heat-resistant steel according to claim 1, consisting of 0.16 wt-% C, 0.12 wt-% Si, 0.13 wt-% Mn, 11.43 wt-% Cr, 0.10 wt-% Mo, 0.22 wt-% V, 2.71 wt-% Ni, 0.05 wt-% Nb, 0.022 wt-% N, 0.007 wt-% B, 2.31 wt-% W, the balance being Fe and unavoidable impurities. A turbine rotor formed of a high toughness heat-resistant steel according to any one of claims 1 to 8. A method of producing a turbine rotor, comprising the steps of: preparing a steel material having a chemical composition as the steel of any one of claims 1 to 8; forming a steel material into a blank body of the turbine rotor; performing a hardening on the blank body; and subsequently performing at least one tempering on the hardened blank body, thereby the tempered blank body providing the turbine rotor having high toughness. The method according to claim 10, wherein said hardening is performed at a temperature in the range of 950°C to 1120°C, said temperature being performed at a temperature in the range of 550°C to 740°C. The method according to claim 10 or 11, wherein said turbine rotor comprises a high pressure portion, an intermediate pressure portion, and a low pressure portion, said hardening being performed at a temperature in the range of 1,030°C to 1,120°C for the high or intermediate pressure portion and at a temperature in the range of 950°C to 1,030°C for the low pressure portion. The method according to any of claims 10 to 12, wherein the tempering is performed at a temperature in the range of 550°C to 630°C for the high or the intermediate pressure portion and at a temperature in the range of 630°C to 740°C for the low pressure portion. The method according to any of claims 10 to 13, wherein the steel material is a steel ingot formed by using electroslag remelting.

Inventors: ISHII RYUICHI, TSUDA YOICHI, YAMADA MASAYUKI, ISHII, RYUICHI, TSUDA, YOICHI, YAMADA, MASAYUKI
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1998
Main classification: C22C38
IPC codes: C22C38, C22B9, C21D6, C21D9, F01D5