JP2010138465A - Heat resistant steel having excellent creep strength, and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a new heat resistant steel having excellent high temperature properties, durability or the like for a long time, having no remarkable reduction in creep strength even if being used at 650°C for a long time, and suitable to the material for a turbine. <P>SOLUTION: The heat resistant steel comprises, by mass, 0.06 to 0.13% C, 8.5 to 9.8% Cr, 0.10 to 0.25% V, 0.03 to 0.08% Nb, 0.2 to 5.0% W, 1.0 to 5.0% Co, 0.002 to 0.015% B, 0.015 to 0.025% N and 0.010 to 2.0% Ru, if required, comprises 0.01 to 1.5% Re or/and 0 to 1.5% Mo or/and 0 to 0.2% Si, and, if required, further comprises one or two selected from 0 to 0.2% Mn and 0 to 0.25% Ni. Its long time creep strength is improved, and, by applying the steel to a turbine rotor or a turbine member, the increase of steam temperature is made possible, and it contributes to the improvement of power generating efficiency. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a heat-resistant steel that requires long-term creep strength at high temperatures, and is used in high-temperature equipment such as a turbine rotor shaft for power generation, a turbine disk, a turbine blade, bolts, piping for boilers, and pressure vessels for chemical plants. It is suitable.

Thermal power generation systems tend to raise the steam temperature of steam turbines in order to further increase power generation efficiency, and as a result, the high temperature characteristics required for turbine materials are becoming more severe. Many heat-resistant steels have been proposed as materials that can be used for this purpose. Among them, it is known that the developed heat-resistant steel proposed in Patent Document 1 and Patent Document 2 is relatively excellent in high-temperature strength.
Moreover, on the long time side in the vicinity of 600 ° C., there is a problem that the creep strength is extremely lowered due to deterioration of the metal structure, and various heat resistant steels have been proposed in order to suppress the long time creep strength from being lowered. . The developed heat-resistant steel proposed in Patent Document 3 is characterized in that it suppresses the decrease in creep strength mainly by defining the Cr content. Further, the heat resistant steels proposed in Patent Document 4, Patent Document 5, Patent Document 6 and the like are characterized by having not only creep strength but also excellent toughness, steam oxidation resistance, and high temperature oxidation resistance.
JP-A-4-147948 JP-A-8-3697 JP 2002-256396 A JP-A-11-217655 JP 2000-248341 A Japanese Patent Laid-Open No. 2000-19820

  However, when high Cr ferritic heat resistant steel is used at 650 ° C. for a long time, the Cr content is greatly affected by the Cr content, but the degree of creep is generally significantly reduced. Therefore, the current upper limit temperature is limited to about 620 ° C. at which no significant decrease in creep strength is observed. Therefore, it is desired to develop a turbine material that optimizes the Cr content and does not cause a significant decrease in creep strength even when used for a long time at 650 ° C.

  The present invention has been made against the background of the above circumstances, and is expected to have excellent high-temperature characteristics, durability, etc. over a long period of time by suppressing a significant decrease in high-temperature creep strength associated with long-term use at around 650 ° C. An object of the present invention is to provide a new heat resistant steel.

  In order to solve the above-mentioned problems, in the present invention, in a ferritic heat resistant steel having a tempered martensite structure, the Cr content that most affects the creep strength over a long period of time is optimized, and then a fine precipitate containing Ru is contained. By precipitating the phase within the lath and at the boundaries of the lath, block, packet, etc., a heat resistant steel having a high high temperature long time creep strength without causing a significant decrease in creep strength on the long time side is provided.

  In order to solve the above problems, the heat-resisting steel of the first invention is, in mass%, carbon (C): 0.06-0.13%, chromium (Cr): 8.5-9.8%, vanadium ( V) 0.10 to 0.25%, niobium (Nb): 0.03 to 0.08%, tungsten (W): 0.2 to 5.0%, cobalt (Co): 1.0 to 5. 0%, boron (B): 0.002-0.015%, nitrogen (N): 0.015-0.025%, ruthenium (Ru): 0.01-2.0%, the balance being iron It consists of (Fe) and inevitable impurities.

  The heat-resisting steel of the second invention is in mass%, carbon (C): 0.06-0.13%, chromium (Cr): 8.5-9.8%, vanadium (V) 0.10-0. .25%, niobium (Nb): 0.03-0.08%, tungsten (W): 0.2-5.0%, cobalt (Co): 1.0-5.0%, boron (B) : 0.002-0.015%, Nitrogen (N): 0.015-0.025%, Ruthenium (Ru): 0.01-2.0%, Rhenium (Re): 0.01-1.5 %, The balance being iron (Fe) and inevitable impurities.

  The heat-resisting steel according to a third aspect of the present invention is the heat-resisting steel according to the first or second aspect of the present invention. It is characterized by that.

  The heat-resisting steel according to a fourth aspect of the present invention is the heat-resistant steel according to any one of the first to third aspects, further comprising mass (%), silicon (Si): 0 to 0.2%, with the balance being iron (Fe) and inevitable impurities It is characterized by comprising.

  The heat-resisting steel according to a fifth aspect of the present invention is the heat-resisting steel according to any one of the first to fourth aspects of the present invention, further comprising, by mass, One or two types are included, and the balance consists of iron (Fe) and inevitable impurities.

  The heat resistant steel of the sixth invention has the heat resistant steel composition described in any one of the first to fifth inventions, and in terms of the component content, {6 [% Mo] +16 [% W] +38 [ % Ru] +14 [% Re] −59 [% Cr]} × 0.1 + 49 The long-time creep strengthening parameter ([%] indicates element mass%) is 0 or more. .

  The heat-resistant steel manufacturing method of the seventh invention is a method of hot forging a steel ingot having the heat-resistant steel composition of any one of the first to sixth inventions, followed by heating to 1050 to 1150 ° C. and quenching. The tempering process is performed at least once at 500 ° C. to 740 ° C., and the CrRuW phase is precipitated by the tempering process.

  The method for producing a heat-resistant steel according to an eighth aspect of the present invention comprises hot forging a steel ingot having the heat-resistant steel composition according to any one of the first to sixth aspects, and then heating to 1060 to 1120 ° C. to quench the steel ingot. The first tempering treatment is performed at 500 ° C. to 620 ° C., the second tempering treatment is performed at 660 ° C. to 740 ° C., and the CrRuW phase is precipitated by these tempering treatments.

  As described above, according to the heat resistant steel of the present invention, carbon (C): 0.06 to 0.13%, chromium (Cr): 8.5 to 9.8%, vanadium (V) 0.10. -0.25%, niobium (Nb): 0.03-0.08%, tungsten (W): 0.2-5.0%, cobalt (Co): 1.0-5.0%, boron ( B): 0.002-0.015%, Nitrogen (N): 0.015-0.025%, Ruthenium (Ru): 0.010-2.0%, optionally rhenium (Re): 0.01-1.5% or / and molybdenum (Mo): 0-1.5% or / and silicon (Si): 0-0.2%, further optionally manganese (Mn): 0 0.2%, nickel (Ni): 0 to 0.25% 1 or 2 types, the balance is iron (Fe) and inevitable Since from the object, a long time is improved creep strength, by applying the material used for the turbine rotor and turbine member, enables high temperature steam temperature, which contributes to the power generation efficiency. Moreover, it can provide as a material excellent in the high temperature characteristic and durability also for uses other than a turbine member.

  In the above component range, the long-time creep strengthening parameter represented by {6 [% Mo] +16 [% W] +38 [% Ru] +14 [% Re] −59 [% Cr]} × 0.1 + 49 is set to 0. By doing so, a high creep strength can be maintained for a longer time.

-Action, reason for limitation ---
Below, an effect | action of the component element of this invention heat-resistant steel and its limitation reason are demonstrated. In addition, all content of each component is shown by the mass%.

C: 0.06-0.13%
C is an element for improving hardenability, and is an essential element for transforming into a martensite structure, and combines with Fe, Cr, Mo, W, V, Nb, etc. in the alloy to form carbides and carbonitrides. To increase room temperature and high temperature strength. When the amount of C is small, delta ferrite and pro-eutectoid ferrite are precipitated, reducing strength and toughness, and reducing the amount of carbides precipitated, resulting in coarse intermetallic (Fe, Cr) ₂ (Mo, W) type Laves phase. Since the crystallization is promoted, the long-term creep strength also decreases. Therefore, a C content of at least 0.06% is required. On the other hand, if the content exceeds 0.13%, the coarsening of the carbide is promoted and the creep strength decreases, so the content is limited to 0.06 to 0.13%. Preferably, the lower limit of C is 0.08%, more preferably 0.09 to 0.12%.

Cr: 8.5 to 9.8%
In the present invention, Cr is one of the most important elements together with Ru and Re described later. The present inventors have clarified the phenomenon of the remarkable decrease in long-term creep strength observed at 650 ° C. and the mechanism thereof, and further conducted research on measures for suppressing the decrease in long-term creep strength. As a result of the research, as an important factor to suppress the decrease in long-term creep strength, a long-term creep strengthening parameter value, which will be described later in detail, has been proposed, and it is clear that the parameter value is 0 or more as a desirable form. I have to.
Cr has the largest absolute value of the coefficient of the elements constituting the accelerated creep suppression parameter equation, and Ru has the second largest value. By severely limiting the amount of Cr added, it is possible to suppress a decrease in long-term creep strength, which is a feature of the steel of the present invention, and to maintain a high creep strength for a long time. Generally in 8-12% Cr ferritic heat-resistant steels, conventionally, as Cr% increases, room temperature tensile strength and creep at high stress and short time (around 100-2000 hours) at a temperature of 600 ° C or higher. Since the strength is increased, the idea is that the higher Cr side is preferable in the range where delta ferrite is not generated. However, based on the results of detailed long-term creep tests at around 650 ° C. this time, when the Cr content exceeds 9.8%, the microstructure of the martensitic steel necessary for maintaining the creep strength is the high temperature of the creep test conditions. It was remarkably changed by the stress, and it was recognized from the microstructure observation that the martensite microstructure was recovered and formed into equiaxed subgrains. In addition, the finely precipitated Laves phase disappeared, a marked progress of agglomeration and coarsening of the precipitate was observed, and the dislocation density was also significantly reduced.
Cr is an essential element for transforming into a martensite structure. Further, Cr forms M ₂₃ C ₆ carbide and intermetallic compound (Fe, Cr) ₂ (Mo, W) type Laves phase, and precipitates these at the boundaries and grain boundaries of laths, blocks, packets, etc. Since it is an element that improves the creep strength, it needs to be at least 8.5%. The Cr content particularly affects the behavior of V carbonitride and Nb carbonitride. With Z phase as Cr amount is large is precipitated early, fine Nb carbonitrides and M _{2 X} carbonitride to disappear during or creep aging, long-term creep strength decreases significantly. On the other hand, when the amount of Cr is low, these fine carbonitrides exist stably for a long time during aging and creep, and fine V carbonitrides precipitate, reducing the reduction in long-term creep strength. be able to. Furthermore, when the Cr content is less than 8.5%, the room temperature and high temperature strength are low, and when it exceeds 9.8%, the M ₂₃ C ₆ carbide, the Laves phase and the CrRuW phase described later are promoted to increase the long-term creep strength. When the content exceeds 11%, delta ferrite precipitates and lowers the room temperature and high temperature strength and toughness. Therefore, the Cr content is limited to 8.5% to 9.8%. For the same reason as described above, the upper limit is desirably less than 9.5%. Further, in order to improve the creep strength for a long time, 8.7 to 9.3% is desirable. Even when Ru is added, the effect of Cr is greater, so the content is preferably less than 9.5%.

V: 0.10 to 0.25%
V is effective for improving the creep strength by forming fine carbides and carbonitrides together with Nb, Cr, Mo, W, Ru and the like, and requires at least 0.10%. On the other hand, if it exceeds 0.25%, the carbon is excessively fixed, the amount of precipitation of carbides increases, and the high temperature strength is lowered, so it is limited to 0.10 to 0.25%. More preferably, the lower limit is 0.14% and the upper limit is 0.22%.

Nb: 0.03 to 0.08%
Nb is an element that forms fine carbides and carbonitrides, improves creep strength, promotes refinement of crystal grains and improves low temperature toughness, and needs to be at least 0.03%. However, if the content exceeds 0.08%, coarse carbides and carbonitrides precipitate and lower the ductility, so 0.03 to 0.08%, more preferably the lower limit is 0.04%, The upper limit is limited to 0.07%.

W: 0.2-5.0%
W is an element that improves the strength and creep strength by solid-solution strengthening in the alloy and strengthening the matrix, or suppressing the agglomeration and coarsening of carbides, so it needs to be at least 0.2%. W forms a Laves phase together with Fe, Cr, Mo, Re, Ru, and the like, and deposits the Laves phase in the form of a lump on a boundary such as martensite to improve strength and creep strength. Moreover, addition of 2.5% or more of W improves the creep strength by precipitating in a fine plate shape in the lath. On the other hand, if W is added in excess of 5.0%, delta ferrite is formed, or excessive increase in strength or aggregation of Laves phase is promoted to reduce ductility, so 0.2% Limited to ~ 5.0%. For the same reason, the lower limit is preferably limited to 1.0% and the upper limit is limited to 4.0%. More preferably, the lower limit is limited to 3.5%.

Co: 1.0-5.0%
Co suppresses the formation of delta ferrite, has a solid solution strengthening effect, and improves the strength and creep strength by increasing the amount of precipitation of carbides and the Laves phase. In order to prevent the formation of delta ferrite and to effectively improve the creep strength, it is necessary to contain 1.0% or more. On the other hand, if the content exceeds 5.0%, ductility and high-temperature creep strength are reduced, and further, the cost is increased and the coarsening of carbides and Laves phases is promoted to reduce the creep strength. Limited to 5.0%. For the same reason, the lower limit is preferably limited to 2.0% and the upper limit is limited to 4.0%.

B: 0.002 to 0.015%
B is an element that has an effect of suppressing aggregation and coarsening of the prior austenite grain boundaries, martensite packets, martensite blocks, and precipitated carbides, precipitated carbonitrides, and precipitated Laves phases in the martensite lath over a high temperature for a long time. In addition, since B makes Nb and V carbonitrides stably present up to a high temperature, the quenching temperature can be increased, and the strength and the creep strength can also be increased. In order to obtain these effects, a minimum of 0.002% is necessary. On the other hand, when B exceeds 0.015%, BN (boron nitride) is bonded to nitrogen and precipitates, and the toughness is reduced, or the creep strength is reduced by reducing the effective amount of B. The content is limited to 0.002 to 0.015%. For the same reason, it is desirable that the lower limit is 0.005% and the upper limit is 0.010%.

N: 0.015-0.025%
N dissolves in the matrix, and forms carbonitride with Nb, V, W, Mo, Ru, etc., and improves strength and creep strength. N is also an element that suppresses the formation of delta ferrite, and a content of at least 0.015% is necessary to exert these effects. On the other hand, when N is added excessively, creep ductility and toughness are reduced due to the formation of BN (boron nitride), and the B content and N content effective for creep strengthening are reduced. Therefore, the content is limited to 0.015 to 0.025%. More preferably, it is limited to 0.015 to 0.020%.

Ru: 0.01 to 2.0%
Ru is the most important element in this patent application. Ru is known as an element that enhances phase stability and creep strength in nickel-base superalloys. The effect of adding this to ferritic steel has been confirmed by conducting a long-term creep test and a creep strain measurement test through experimental research.
Ru improves the creep strength by solid solution strengthening of the matrix. Moreover, since there exists an effect which improves toughness, at least 0.01% or more is required and 0.1% or more is desirable. Even if about 1% of Ru is added, the transformation point of the high Cr heat-resisting steel is not significantly changed, so that the stability of the tempered martensite structure is not lowered. Ru together with Cr, W, Mo, V, Re, etc. forms a fine precipitate phase (CrRuW phase described later) that is stable for a long time. This precipitated phase precipitates in the martensite lath and at the boundaries of laths, blocks, packets, etc., thereby improving the long-term creep strength. Ru forms an intermetallic compound (Fe, Cr, Ru, Re) ₂ (Mo, W) type Laves phase and M ₂₃ C ₆ carbide together with Fe, Cr, Mo, W, Re, etc., and provides long-term creep strength. Improve. Such dispersion of the finely precipitated phase can be realized by adopting a production method described later together with adjustment of the components. On the other hand, when Ru is added excessively, the coarsening of these precipitates is promoted and the creep strength is lowered. Since Ru is also an expensive metal, the upper limit is limited to 2.0%. Further, not only the addition of 1% or more of Ru alone but also the combined addition of Ru and Re can effectively improve the creep strength, so the upper limit is desirably 1.5%. More desirably, the lower limit is 0.15% and the upper limit is 1.0% of the Ru content.

Re: 0.01 to 1.5%
Re is one of the important elements together with Cr described above in the present invention. Re contributes significantly to solid solution strengthening with the addition of a very small amount (0.01% or more), has the effect of improving the high-temperature creep strength by increasing the high-temperature long-term structural stability of the matrix, and at the same time toughness Also has the effect of improving. Furthermore, Re is also included in the Laves phase, and suppresses the disappearance of the fine Laves phase and the coarsening of the Laves phase. Since these suppress the remarkable fall of long-term creep strength in the vicinity of 650 ° C., they are contained as desired. In order to fully exhibit this effect, the content of 0.1% or more is desirable. On the other hand, Re is an expensive metal, and if it is contained excessively, the workability is lowered, and even if Re is added excessively, the effect of increasing the creep strength is small, so its content is 0-1.5%. did. Preferably, the upper limit of Re is 1.0%, more preferably the lower limit is 0.15% and the upper limit is 0.6%.

Mo: 0 to 1.5%
Mo suppresses the agglomeration and coarsening of carbides, and solid dissolves in the alloy to strengthen the matrix. Furthermore, Mo is an element that works effectively to improve the high-temperature strength and the high-temperature creep strength by finely dispersing and precipitating as a Laves phase in the matrix, and is contained if desired. On the other hand, when Mo is excessively contained, delta ferrite is likely to be generated and cohesive coarsening of the Laves phase is promoted, and the creep strength is lowered. In order to sufficiently exhibit this effect, it is further desirable that the upper limit of Mo be 1.2%, more preferably 0.5%.

Si: 0 to 0.2%
Si is an element that is usually used as a deoxidizing material and improves steam oxidation characteristics, and is contained as desired. On the other hand, when Si is excessively added, segregation inside the steel ingot and susceptibility to temper embrittlement increase. Moreover, since the coarsening of a carbide | carbonized_material is promoted and high temperature creep strength falls, the content was limited to 0.2% or less. In order to fully exhibit this effect, the upper limit of Si is desirably 0.10%, and more preferably 0.02%.

Mn: 0 to 0.2%
Mn is an inexpensive austenite stabilizing element and contributes to the improvement of toughness. However, when Mn is added in excess of 0.2%, the coarsening of carbides and Laves phases is promoted, high-temperature creep strength is lowered, and temper embrittlement susceptibility is also increased. Therefore, the Mn content is limited to 0.2% or less. For the same reason, 0.1% or less is desirable.

Ni: 0 to 0.25%
Ni is an austenite stabilizing element like Mn, and suppresses the formation of delta ferrite and improves toughness, so Ni is included as desired. When Ni is added excessively, the coarsening and coarsening of carbides and Laves phases are promoted, and the high temperature creep strength is lowered. Therefore, the allowable content of Ni is limited to 0.25% or less. In addition, Preferably it limits to 0.15% or less.

Cu: 0.1 to 1.3%
Cu is an austenite stabilizing element like Mn and Ni, and contributes to the improvement of toughness, so is contained as desired. However, if the content is less than 0.1%, the above effect is not sufficient. On the other hand, if the content exceeds 1.3%, the high temperature creep strength is lowered and the hot workability is lowered. Therefore, the content is limited to 0.1 to 1.3%. Among these ranges, it is desirable that the lower limit is 0.3% and the upper limit is 0.8%.

Creep Strengthening Parameters When the steel of the present invention is subjected to a creep test near 650 ° C., the start of accelerated creep moves to the long time side in the creep strain-time curve, thereby suppressing a significant decrease in long-term creep strength. It is characterized by being able to do it. The resulting rupture time in the low-stress creep test greatly depends on the components of the material, and the above-described calculation formula calculated based on the content of each component can be used as the index. When this calculated value is less than 0, the coarsening of the carbide and the Laves phase is promoted and accelerated creep starts early, so that the fracture time is shortened. When this calculated value is 0 or more, coarsening of carbides and Laves phases can be suppressed, and the start of accelerated creep is delayed by fine dispersion of stable precipitated phases containing Cr, Ru, W, V, etc. Time can be increased.
Creep strengthening parameter formula {6 [% Mo] +16 [% W] +38 [% Ru] +14 [% Re] −59 [% Cr]} × 0.1 + 49

  The heat-resistant steel manufacturing method shown in the sixth to seventh inventions is a manufacturing method suitable for remarkably improving the high-temperature creep strength using the heat-resistant steel of the first to fifth inventions. Therefore, a fine CrRuW phase can be dispersed and precipitated in the martensite structure, and a remarkably good high temperature creep strength can be obtained. The reasons for limiting the method for producing this heat-resistant steel will be described below.

  The heat-resistant steel of the present invention can be melted in accordance with a conventional method so as to obtain the above components, and the melting method is not particularly limited. The obtained heat-resistant steel is subjected to a heat treatment under a processing condition such as forging or a desired condition.

Quenching heating temperature: 1050-1150 ° C
The steel according to the present invention improves the high-temperature creep strength by solid-dissolving the precipitated carbonitride and CrRuW phase by quenching heating and then uniformly finely dispersing and precipitating the carbonitride and CrRuW phase by subsequent tempering. In this heat-resistant steel, the solid solution temperature of precipitated carbides and carbonitrides shifts to the high temperature side due to the inclusion of B, so that the solid solution of precipitates is insufficient at a quenching heating temperature of less than 1050 ° C. and good high temperature creep strength is obtained. On the other hand, when the temperature exceeds 1150 ° C., the crystal grains become coarse and the toughness decreases, and the creep ductility also decreases. Therefore, it is limited to quenching heating in the temperature range of 1050 to 1150 ° C. Furthermore, quenching heating in a temperature range of 1060 to 1120 ° C. is desirable. The cooling at the time of quenching may be performed at a cooling rate equal to or higher than air cooling, and an appropriate cooling rate and cooling medium can be selected.

Tempering temperature: 500 ° C to 740 ° C
In tempering, the retained austenite generated during the quenching is decomposed into a tempered martensite single phase structure, and carbide, carbonitride, CrRuW phase, and Laves phase are uniformly finely dispersed and precipitated in the matrix, and the desired dislocation is recovered. Obtains room temperature and high temperature strength and toughness, and improves high temperature creep strength. Tempering may be performed once, but is preferably performed twice or more. In order to decompose the retained austenite in the first tempering, it is necessary to heat to a temperature equal to or higher than the _Ms temperature. When the tempering temperature is less than 500 ° C., the retained austenite is not sufficiently decomposed. On the other hand, when the temperature exceeds 620 ° C., precipitation of carbide, carbonitride, CrRuW phase and Laves phase is preferentially performed in the martensitic structure. Since it progresses, the precipitation of carbide, carbonitride, CrRuW phase and Laves phase in the retained austenite part becomes non-uniform, and the high temperature creep strength decreases. For this reason, it is desirable that the first tempering temperature be in the range of 500 ° C to 620 ° C. Furthermore, good ductility and toughness are obtained by the second tempering, and the precipitates are further stabilized to ensure high temperature and long-term creep strength. For this purpose, it is desirable to perform tempering at a temperature of 660 ° C. or higher. On the other hand, if tempering is performed at a temperature exceeding 740 ° C., the desired room temperature strength and high temperature strength cannot be obtained. It is desirable to set 660 to 740 ° C. If desired, preliminary heat treatment such as normalization can be performed between forging and quenching.

  Examples of the present invention will be described below in comparison with comparative examples. As test materials used in the examples, alloys having the compositions (remainder Fe and inevitable impurities) shown in Table 1 and Table 2 (invention steel, comparative steel) were prepared. These alloys were melted in a vacuum induction melting furnace as a 50 kg test steel ingot, forged, and then subjected to a predetermined heat treatment. In the heat treatment, after quenching treatment corresponding to oil cooling from 1070 ° C., the first tempering was performed at 570 ° C., and the second tempering was further performed at 680 ° C. to obtain each specimen.

The specimens obtained as described above were subjected to a creep test and a creep rupture test at a test temperature of 650 ° C. to evaluate the creep strength.
The creep curves at 650 ° C. and 137 MPa for the inventive steel 1 and the comparative steel 2 shown in Table 1 are shown in FIG. The creep curve of Invention Steel 1 shows that the start of accelerated creep shifts to a longer time side than that of Comparative Steel 2, and therefore the rupture time is greatly increased. This tendency is more prominent as the stress is lower, and the creep strength on the long-time side of the developed steel is also improved.

Invention Steel No. In order to investigate the effect of improving the creep strength of No. 1, the deposited phase was observed using a transmission electron microscope (TEM). FIG. 2 shows a photograph of precipitates observed with an extracted replica of the tempered material. Various precipitates (M ₂₃ C ₆ carbide, Laves phase, carbonitride) were observed. Further, as indicated by arrows in FIG. 2, it was confirmed that finer precipitated phases were arranged in a portion considered to be a martensitic lath boundary. The spectra obtained by these energy dispersive X-ray analyzes (EDX) are shown in FIG. From this, it was found that this was a precipitated phase (hereinafter referred to as CrRuW phase) mainly composed of Cr and having a composition of (Cr + Ru) :( W + V) ≈8: 2. Further, this CrRuW phase that was finely dispersed was also observed at a position considered to be in the martensite lath. In this way, by suppressing the movement of the martensite lath boundary by precipitating a fine precipitation phase containing Ru in the lath and the lath boundary, the structure recovery is suppressed, and in particular, the creep strength on the long time side is improved. Can do.
As described above, in the parameters proposed in the present invention, control of Cr, W, Ru, which are elements that form carbonitride with Ru, and Mo and Re having similar properties are particularly important. Therefore, a new parameter was proposed to define the steel grade that could delay the start of accelerated creep and maintain high creep strength for a long time. FIG. 4 shows the relationship between the rupture time and this parameter in the creep rupture test at 650 ° C. and 118 MPa for the inventive steel and the comparative steel shown in Table 2. By controlling this parameter to 0 or more, a high creep strength can be obtained.

It is a creep distortion-time curve in 650 degreeC-137MPa of the invention steel 1 and the comparative steel 2 in the Example of this invention. Invention Steel No. It is a transmission electron micrograph by the extraction replica which shows distribution of CrRuW phase about the tempering processing material of 1. FIG. Invention Steel No. 1 is an X-ray profile obtained by EDX analysis of a CrRuW phase for one tempered material. Cu is a copper mesh that supports the replica film and is independent of the composition of the precipitated phase. It is a graph which shows the relationship between a parameter and the creep rupture time in 650 degreeC-118 MPa about the test material in the Example of this invention.

Claims (8)

  In mass%, carbon (C): 0.06 to 0.13%, chromium (Cr): 8.5 to 9.8%, vanadium (V) 0.10 to 0.25%, niobium (Nb): 0.03-0.08%, tungsten (W): 0.2-5.0%, cobalt (Co): 1.0-5.0%, boron (B): 0.002-0.015% , Nitrogen (N): 0.015 to 0.025%, ruthenium (Ru): 0.01 to 2.0%, the balance being iron (Fe) and inevitable impurities, high Cr Ferritic heat resistant steel.   In mass%, carbon (C): 0.06 to 0.13%, chromium (Cr): 8.5 to 9.8%, vanadium (V) 0.10 to 0.25%, niobium (Nb): 0.03-0.08%, tungsten (W): 0.2-5.0%, cobalt (Co): 1.0-5.0%, boron (B): 0.002-0.015% , Nitrogen (N): 0.015 to 0.025%, ruthenium (Ru): 0.01 to 2.0%, rhenium (Re): 0.01 to 1.5%, the balance being iron (Fe ) And unavoidable impurities. High Cr ferritic heat resistant steel.   The high Cr content according to claim 1 or 2, further comprising, by mass%, molybdenum (Mo): 0 to 1.5% as a component, the balance being iron (Fe) and inevitable impurities. Ferritic heat resistant steel.   The content component further includes, in mass%, silicon (Si): 0 to 0.2%, and the balance is composed of iron (Fe) and inevitable impurities. High Cr ferritic heat resistant steel.   As a contained component, it further contains, by mass%, one or two of manganese (Mn): 0 to 0.2%, nickel (Ni): 0 to 0.25%, the balance being iron (Fe) and inevitable The high Cr ferritic heat resistant steel according to claim 1, comprising impurities.   In the relationship of the component content, the long-time creep strengthening parameter represented by {6 [% Mo] +16 [% W] +38 [% Ru] +14 [% Re] −59 [% Cr]} × 0.1 + 49 ([[ %] Indicates mass% of the element) is 0 or more, and the high Cr ferritic heat resistant steel according to claim 1, wherein:   A steel ingot having the composition according to any one of claims 1 to 6 is hot forged, then heated to 1050 to 1150 ° C and quenched, and then tempered at least once at 500 ° C to 740 ° C. A method for producing a ferritic heat resistant steel, characterized in that a CrRuW phase is precipitated by tempering.   A steel ingot having the composition according to any one of claims 1 to 6 is hot forged, then heated to 1060 to 1120 ° C and quenched, and then subjected to a first tempering process at 500 to 620 ° C. And a second tempering treatment at 660 ° C. to 740 ° C., and a CrRuW phase is precipitated by the tempering treatment.