JP2008266786A - Heat-resistant ferritic steel material superior in creep characteristics at weld heat-affected zone, and heat-resistant structure - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a heat-resistant structure which inhibits a type-IV damage from occurring at a weld heat-affected zone of a heat-resistant ferritic steel material. <P>SOLUTION: The heat-resistant ferritic steel material includes, by mass%, 0.01-0.20% C, 0.02-0.50% Si, 0.05-1.0% Mn, 0.02% or less P, 0.01% or less S, 0.4-12.0% Cr, 0.002-0.15% N, one or more elements of 0.01-0.20% Ti, 0.003-0.20% Zr, 0.01-0.50% Nb, 0.01-0.50% V, and 0.01-0.15% Ta, and the balance Fe with unavoidable impurities, and has a Cr equivalent value of 0.4 to 20 at the weld heat-affected zone; and has such superior creep characteristics at the weld heat-affected zone that a low temperature transformation structure having former γ grains with average particle sizes of 10 μm or larger is formed (z) in the weld heat-affected zone of the steel material to be heated to the Ac<SB>1</SB>transformation temperature to (the Ac<SB>1</SB>transformation temperature +300°C). <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a heat-resistant welded structure used at a high pressure of 450 ° C. or higher, particularly a thermal power plant for energy conversion and a ferritic heat-resistant steel material constituting a petrochemical plant for energy purification. The present invention relates to a ferritic heat resistant steel material having excellent creep characteristics of a heat affected zone of weld (hereinafter sometimes referred to as “HAZ”).

  With the background of recent depletion of energy resources and a review of mass consumption, steel structures used at high temperature and high pressure, especially in the operation of pressure equipment, are required to efficiently convert energy to protect the global environment. Yes. In the future, it is expected to develop technologies for realizing low-emission large-scale power generation such as nuclear power generation and fast breeder reactors, light water reactors, and fusion reactors.

  Further, in oil, coal, or natural gas thermal power generation that has been operating in the past, it is important to develop a technique for efficiently acquiring electric energy from the viewpoint of preventing global warming.

  In addition, since exhaust gas discharged from vehicles for transportation contains substances that adversely affect the global environment, crude oil is desulfurized in order to clean the fuel itself and reduce the emissions of these substances. So-called deep immersion desulfurization technology, which is performed at higher temperature and pressure, has attracted attention.

  In order to improve the operation rate in such power plants and chemical plants, or to improve the purification rate, the usage environment of the equipment constituting the plant tends to increase in temperature and pressure, and at the same time, energy demand As the demand for construction of electric power plants and chemical plants advances on a global scale, the development of technology that can stably operate electric power plants and chemical plants even at high temperatures and high pressures is required.

  Currently, thermal power generation covers the majority of electrical energy, and in the situation where chemical plants are operating in the high temperature range of 450-500 ° C, accidents in the equipment that makes up these systems are from the perspective of energy supply. It is fatal, and even with the system shut down for several days, the impact on society and economic loss are immeasurable.

  Such large-scale accidents that cause inoperability are often caused by damage to the steel sheets constituting the equipment, but the damage generally occurs at the welded portion of the steel material.

  When the metallographic structure of the weld is collected and observed with an optical microscope, it is possible to identify the part that is heated beyond the transformation point of the steel material and the structure can change to become the starting point of damage. Destruction caused by a local decrease in creep strength occurring at a portion closest to the base material) is a serious problem in terms of plant equipment safety.

  The above destruction (damage) is a phenomenon (refer to Fig. 1) generally known as Type IV type damage (or Type IV type destruction) according to the classification according to the location of its occurrence. There are few (see Non-Patent Documents 1 and 2), and common recognition for elucidating the generation mechanism has not been established yet. Therefore, at present, Type IV type damage prevention technology that has been put into practical use has not been developed.

  In the design of plant equipment, the standards and regulations only provide guidelines for determining the high temperature allowable stress when there are welds, and by adding voluntary safety margins for equipment and plant manufacturers, The current situation is to prevent large-scale accidents.

  For this reason, the design is designed to ensure safety excessively, resulting in an increase in the weight of the plant equipment and an increase in manufacturing cost. Furthermore, there are concerns about an increase in the cost of energy to be supplied due to an increase in plant processes, an increase in operating costs. An increase in energy costs hinders stable supply.

  In addition, even if a design that ensures excessive safety is performed, there remains concern about the strength of the weld, and it is not expected to reduce the accident rate of the plant. It becomes a big factor to prevent. In addition, even if the composition design is performed to increase the strength of the steel material, the design strength of the plant is determined by the strength of the welded portion, so the improvement in the function of the steel material itself is meaningless.

  Thus, eliminating Type IV damage in the weld heat affected zone is extremely important for the construction of plants that perform high-temperature and high-pressure energy conversion. That is, if the occurrence of Type IV damage in the weld heat affected zone can be prevented, the high-temperature and high-pressure plant equipment will stably and sufficiently exert its function, and greatly contribute to the stable supply of low-cost energy.

  By the way, the strength reduction of the welded portion (welded joint) is generally said to be 30 to 50% in 100,000 hours. Eliminating this decrease in strength is equivalent to increasing the strength of the high-temperature plant equipment by 30 to 50%, as estimated from the decrease rate. This increase in strength is comparable to an increase in operating temperature of 50 to 80 ° C. in terms of plant operating conditions.

  For example, in the case of a thermal power plant, this increase in operating temperature improves the energy conversion efficiency by 5%. As a result, the thermal power plant becomes a high-efficiency energy conversion plant comparable to nuclear power generation.

  Under the above background, the development of technology for suppressing the strength reduction in the welded portion of the heat-resistant steel material for high-temperature and high-pressure plants has been vigorously performed, and many results have been reported so far. As one of the representative techniques, a precipitation strengthening factor that supports the creep characteristics of the weld heat-affected zone, for example, techniques for stabilizing carbides, carbonitrides, and oxides (Patent Documents 1 to 5, reference).

  Precipitates present in the weld heat-affected zone are migration obstacles that hinder the movement of dislocations contained in the martensite structure and bainite structure, so they may be reheated to a temperature above the transformation point and decompose and dissolve. Stabilizing certain carbides and carbonitrides is effective in suppressing the strength reduction at the weld (see Patent Documents 1 to 5).

  In addition, since the oxide does not decompose and dissolve even in the reheat temperature range, if the oxide is dispersed instead of carbonitride and precipitation strengthening is attempted, a decrease in strength at the welded portion can be suppressed (Patent Document 1). ~ 5).

  However, the effect of stabilizing the precipitates in the heat affected zone is large when the precipitates are deposited very finely and with high density, but usually in the bainite structure and martensite structure where the dislocation density is high, the fixed dislocations. Due to the high density, the stabilization of the precipitate may not be the main strengthening factor. In particular, it is difficult to finely disperse oxides, and special manufacturing methods such as mechanical milling are indispensable, which is not a general method.

  Also, if a large amount of carbide or carbonitride is deposited and does not decompose and dissolve, when it is cooled again, the carbon concentration and nitrogen concentration in the base material will decrease, adversely affecting the structure formation of the weld heat affected zone In some cases, depending on the welding method, the strength reduction of the weld heat affected zone may not be significantly improved.

  Patent Document 6 discloses a technique in which a welded portion is heat-treated again with a base material after welding, for example, a technique for solving strength reduction in a weld heat-affected zone by quenching-tempering or normalizing-tempering. Is disclosed.

  This technology is a technology that returns the structure of the heat affected zone to the same structure as the base material structure. For the above reasons, this technology was not established as an industrial technology to prevent the occurrence of Type IV damage. Compared to the present invention, the effect is less likely to appear. That is, when the component equipment and the constituent units are larger than a certain size, it is difficult to heat treat the entire welded structure at the same time.

  Furthermore, in the above method, the weld metal strength is designed by welding as it is, that is, as cast, and tempering. Therefore, it becomes difficult to ensure the high temperature strength of the weld metal by the entire quenching and tempering, and the Type IV type. Prior to the occurrence of damage, the strength of the welded joint becomes difficult to design.

  In order to heat treat the entire welded structure at a high temperature necessary for quenching or tempering, it is necessary to use a large furnace. In the case of a large furnace, the equipment cost is high, and the energy cost to be used is also high. Therefore, in order to apply the technique disclosed in Patent Document 6 to industrial mass production, further technological development is required.

  However, it is actually impossible to heat treat the entire welded structure at the high temperatures required for quenching or tempering, and this heat treatment cannot completely suppress Type IV damage in the heat affected zone. .

On the other hand, Non-Patent Document 3 reports a method for suppressing the refinement of the weld heat-affected zone structure and improving the creep characteristics. This method is to heat up to Ac ₃ transformation point or higher before welding, introduce 3% residual γ, and prevent grain refinement by the growth coalescence, but does not produce cementite and residual γ. It is not applicable to the system.

  Furthermore, according to the above method, after welding, residual austenite occurs in the base metal, and the deformation progresses gradually during creep, resulting in a large heat stress in the piping and heat exchanger system. As a fatal situation can not be avoided.

That is, Non-Patent Document 3 proposes only a special solution that is not industrial, and does not disclose a technique for stably suppressing Type IV damage. Rather, Non-Patent Document 3 suggests that the addition of 90 ppm of B can stably suppress Type IV damage.
JP 2002-332547 A Japanese Patent Laid-Open No. 2001-192761 JP-A-11-256269 Japanese Patent Application Laid-Open No. 07-242935 Japanese Patent Application Laid-Open No. 06-0665689 JP 2001-003120 A "Identification and generation mechanism of Type IV creep damage observed in heat affected zone of high Cr ferritic heat resistant steel", Iron and Steel, Vol.90 (2006) No.10, pp31-39 "Study on the structure controlling factor of type IV creep damage of high Cr ferritic advanced heat resistant steel", Iron and Steel, Vol.90 (2006) No.10, pp40-48 "Creep characteristics of P92HAZ reproducible material with reduced grain size", CAMP-ISIJ, Vol.19 (2006), 1180

  The present invention uses a ferritic heat-resistant steel material, and when constructing a heat-resistant welding structure constituting a thermal power plant or a petrochemical plant, a local strength lowering phenomenon in a weld heat affected zone inevitably generated in a welded portion. It is an object of the present invention to completely prevent the occurrence of Type IV damage due to the occurrence of structural damage and the introduction of stable nitrides to prevent the destruction of the heat-resistant welded structure from the weld heat affected zone.

  And, the present invention solves the above-mentioned problems, and in designing a heat-resistant welded structure that constitutes a power generation blunt or a petrochemical plant, does not impair the safety of the heat-resistant welded structure even if the design margin is reduced. Alternatively, it is an object of the present invention to make a design margin high by utilizing a conventional design standard, and to increase the operating conditions, in particular, the pressure condition to increase the energy conversion efficiency, thereby realizing the construction of a low emission type plant.

  The inventor has experimentally confirmed that Type IV damage occurs in 9% Cr steel according to the generation mechanism described in Non-Patent Documents 1 and 2.

  As a result, the present inventor found that (i) the main cause of the strength decrease in the welded portion of 9% Cr steel was formed at the outer edge of the welded heat affected zone (site close to the base steel) in the welded heat affected zone. This is a decrease in the dislocation density in the fine-grained region. (Ii) In order to suppress the occurrence of Type IV damage in the weld zone, the change in the carbon concentration in the base steel during cooling after being affected by the welding heat In addition, it is important to ensure that the weld heat-affected zone structure and the base steel structure are uniform and consistent, and (iii) rarely formed multiple heat Even in the affected fine grain part (described later), when the base steel material structure contains a stable MX type nitride, if the MX type nitride is finely left in the low temperature transformation structure of the welded portion, the welding condition It is known that Type IV type damage can be prevented stably regardless of fluctuations I came to see it.

  This invention was made | formed based on the said knowledge, and the summary is as follows.

(1) By mass%, C: 0.01 to 0.20%, Si: 0.02 to 0.50%, Mn: 0.05 to 1.0%, P: 0.02% or less, S: 0.01% or less, Cr: 0.4 to 12.0%, N: 0.002 to 0.15%, and Ti: 0.005 to 0.20%, Zr: 0.003 to 0.20 %, Nb: 0.01 to 0.50%, V: 0.01 to 0.50%, Ta: 0.01 to 0.15%, one or more, and the balance Fe and A ferritic heat resistant steel material consisting of inevitable impurities and having an HCreq (Cr equivalent of the weld heat affected zone) defined by the following formula (1) of 0.4 to 20,
(Z) Ac ₁ in weld heat affected zone of the transformation point to Ac ₁ steel is heated to a transformation point + 300 ° C., welding heat an average particle size of prior austenite grains and generating the 10μm or more low-temperature transformation structure Ferritic heat-resistant steel with excellent creep properties in the affected area.
HCreq = [% Cr] +6 [% Si] +11 [% V] +4 [% Mo] +1.5 [% W] +5 [% Nb] +12 [% Al] + [% Re] +2 [% Zr] +5 [% Ti] +2 [% Ta] +15 [% B] -18 [% C] -12 [% N] -4 [% Ni] -2 [% Mn]-[% Cu] -2 [% Co] ... (1)

  (2) The low-temperature transformation structure is bainite and / or martensite containing one or more MX type nitrides of Ti, Zr, Nb, V, and Ta (1) The ferritic heat resistant steel material having excellent creep characteristics of the weld heat affected zone as described in (1).

(3) By mass%, C: 0.01 to 0.20%, Si: 0.02 to 0.50%, Mn: 0.05 to 1.0%, P: 0.02% or less, S: 0.01% or less, Cr: 0.4 to 12.0%, N: 0.002 to 0.15%, and Ti: 0.005 to 0.20%, Zr: 0.003 to 0.20 %, Nb: 0.01 to 0.50%, V: 0.01 to 0.50%, Ta: 0.01 to 0.15%, one or more, and the balance Fe and A ferritic heat resistant steel material consisting of inevitable impurities and having an HCreq (Cr equivalent of the weld heat affected zone) defined by the following formula (1) of 0.4 to 20,
(X) Ac ₁ transformation point to Ac ₁ transformation point + 300 ° C. to tissue before the welding heat affected zone of the steel to be heated, (x1) low-temperature transformation structure of bainite and / or martensite, and, (x2) low-temperature transformation Having the same crystal orientation as the previous austenite, including residual austenite having a sphere equivalent average particle size of 10 nm or more present at the martensite lath boundary, or at the block grain boundary of bainite and / or martensite,
(Z) Any _{one of} Ti, Zr, Nb, V, and Ta having an average grain size of prior austenite grains of 10 μm or more in a welding heat-affected region of a steel material heated to Ac ₁ transformation point to Ac ₁ transformation point + 300 ° C. A ferritic heat resistant steel material excellent in creep characteristics of a weld heat affected zone, characterized in that a low temperature transformation structure of bainite and / or martensite containing one or more MX type nitrides is formed.
HCreq = [% Cr] +6 [% Si] +11 [% V] +4 [% Mo] +1.5 [% W] +5 [% Nb] +12 [% Al] + [% Re] +2 [% Zr] +5 [% Ti] +2 [% Ta] +15 [% B] -18 [% C] -12 [% N] -4 [% Ni] -2 [% Mn]-[% Cu] -2 [% Co] ... (1)

(4) In the lath of martensite or in the block grains of bainite and / or martensite, one or more MX types of Ti, Zr, Nb, V, Ta are included in the low temperature transformation structure. The welding heat-affected zone as described in (3) above, comprising 2 nitrides / μm ² or more of MX type nitrides, which are nitrides and can be confirmed by observation at 50,000 times using a transmission electron microscope Ferritic heat-resistant steel with excellent creep characteristics.

  (5) Ferrite based on excellent creep characteristics of weld heat affected zone according to (3) or (4), wherein the retained austenite is contained in a volume ratio of 0.5% or more and 5% or less. Heat resistant steel.

  (6) Ferritic heat resistance excellent in the creep characteristics of the weld heat affected zone according to (3) or (4), wherein the retained austenite is contained in a volume ratio of 0.5% or more and 3% or less. Steel material.

  (7) Ferritic heat resistant steel material excellent in creep characteristics of weld heat affected zone according to any one of (3) to (6), wherein 30% or more of the retained austenite has the same crystal orientation .

(8) By mass%, C: 0.01 to 0.20%, Si: 0.02 to 0.50%, Mn: 0.05 to 1.0%, P: 0.02% or less, S: 0.01% or less, Cr: 0.4 to 12.0%, N: 0.002 to 0.15%, and Ti: 0.005 to 0.20%, Zr: 0.003 to 0.20 %, Nb: 0.01 to 0.50%, V: 0.01 to 0.50%, Ta: 0.01 to 0.15%, one or more, and the balance being Fe And HCreq (Cr equivalent of welding heat-affected zone) defined by the following formula (1) is 0.4 to 20 and consists of inevitable impurities,
(Y) Ac ₁ transformation point to Ac ₁ transformation point + 300 ° C. to tissue before the welding heat affected zone of the steel to be heated, (y1) low-temperature transformation structure of bainite and / or martensite, and, (y2) low-temperature transformation It has a specific crystal orientation that matches the crystal orientation of the previous austenite with the crystal orientation of the low-temperature transformation structure, and the sphere equivalent average particle size present at the martensite lath boundary or at the block grain boundary of martensite and / or bainite Including matched cementite of 10 nm or more,
(Z) Any _{one of} Ti, Zr, Nb, V, and Ta having an average grain size of prior austenite grains of 10 μm or more in a welding heat-affected region of a steel material heated to Ac ₁ transformation point to Ac ₁ transformation point + 300 ° C. A ferritic heat resistant steel material excellent in creep characteristics of a weld heat affected zone, characterized in that a low temperature transformation structure of bainite and / or martensite containing one or more MX type nitrides is formed.
HCreq = [% Cr] +6 [% Si] +11 [% V] +4 [% Mo] +1.5 [% W] +5 [% Nb] +12 [% Al] + [% Re] +2 [% Zr] +5 [% Ti] +2 [% Ta] +15 [% B] -18 [% C] -12 [% N] -4 [% Ni] -2 [% Mn]-[% Cu] -2 [% Co] ... (1)

(9) In the lath of martensite or in the block grains of bainite and / or martensite, one or more MX types of Ti, Zr, Nb, V, Ta are included in the low temperature transformation structure. The welding heat-affected zone as described in (8) above, comprising 2 nitrides / μm ² or more of MX type nitrides that can be confirmed by 50,000-times observation using a transmission electron microscope Ferritic heat-resistant steel with excellent creep characteristics.

  (10) The ferrite system having excellent creep characteristics of the weld heat affected zone according to (8) or (9), wherein the matched cementite is contained in a volume ratio of 0.5% or more and 5% or less. Heat resistant steel.

  (11) The ferrite system having excellent creep characteristics of the weld heat affected zone according to (8) or (9), wherein the matched cementite is contained in a volume ratio of 0.5% or more and 3% or less. Heat resistant steel.

  (12) The ferritic heat resistant steel material having excellent creep characteristics of the weld heat affected zone according to (8) to (11), wherein 30% or more of the matched cementite has the same crystal orientation.

  (13) The ferritic heat resistant steel material is further in mass%, W: 0.01 to 3.0%, Mo: 0.01 to 3.0%, Re: 0.01 to 0.5%, Ni : 0.01 to 2.0%, Co: 0.01 to 5.0%, Cu: 0.01 to 2.0%, B: 0.0003 to 0.0050%, one or two of them The ferritic heat resistant steel material excellent in the creep characteristics of the weld heat affected zone according to any one of the above (1) to (12), characterized by containing the above.

  (14) The ferritic heat resistant steel material is further in mass%, Y: 0.005 to 0.05%, Ce: 0.005 to 0.5%, Mg: 0.0003 to 0.005%, Ba : 0.0003 to 0.005%, Ca: 0.0003 to 0.005%, and La: 0.005 to 0.05%, containing one or more of the above ( 1) A ferritic heat-resistant steel material excellent in creep characteristics of a weld heat-affected zone according to any one of (13).

  (15) The creep characteristics of the weld heat-affected zone characterized by being manufactured by welding the ferritic heat resistant steel material having excellent creep characteristics of the weld heat-affected zone according to any one of (1) to (14). Excellent heat resistant structure.

(16) The entire heat-resistant structure is subjected to a heat treatment for 1 minute or more at an Ac ₁ transformation point or less to reduce residual austenite or matched cementite to less than 0.5% by volume ratio. The heat-resistant structure excellent in creep characteristics of the weld heat-affected zone as described in (15).

  According to the present invention, in the heat-affected zone of the welded portion of ferritic heat-resistant steel, the strength is enhanced by utilizing a strengthening mechanism based on the fine dispersion of MX type nitride, and the concern that Type IV type damage may occur is completely eliminated. Therefore, when designing heat-resistant welded structures (heat-resistant structures) that constitute high-temperature and high-pressure plant equipment, the high-temperature strength can be designed to be 0.67 times the creep rupture strength (normal safety factor). it can. As a result, it is possible to prevent an accident at the starting point of the weld heat affected zone that has occurred in the past.

  FIG. 1 shows a cross-section of Type IV damage occurring in a weld heat affected zone of a ferritic heat resistant steel material. The ferritic heat resistant steel material of the present invention (the steel material of the present invention) is shown in FIG. Since no type IV damage occurs, the creep characteristics of the weld heat affected zone are remarkably excellent.

  This invention steel material is the mass%, C: 0.01-0.20%, Si: 0.02-0.50%, Mn: 0.05-1.00%, P: 0.02% or less, S: 0.01% or less, Cr: 0.4 to 12.0%, N: 0.002 to 0.15%, and Ti: 0.005 to 0.20%, Zr: 0.003 to 0 .20%, Nb: 0.01 to 0.50%, V: 0.01 to 0.50%, Ta: 0.01 to 0.15%, containing one or more, the balance It is a ferritic heat resistant steel material composed of Fe and unavoidable impurities and having an HCreq (Cr equivalent of the weld heat affected zone) defined by the following formula (1) of 0.4 to 20.

  HCreq = [% Cr] +6 [% Si] +11 [% V] +4 [% Mo] +1.5 [% W] +5 [% Nb] +12 [% Al] + [% Re] +2 [% Zr] +5 [% Ti] +2 [% Ta] +15 [% B] -18 [% C] -12 [% N] -4 [% Ni] -2 [% Mn]-[% Cu] -2 [% Co] ... (1)

  First, the reason for limiting the chemical components and HCreq as described above will be described. In addition,% means the mass%.

  C: C contributes to the improvement of the hardenability of the ferritic heat resistant steel material, and at the same time forms carbides and carbonitrides and contributes to the improvement of the creep rupture strength of the base steel material. This improvement effect becomes clear with addition of 0.01% or more, but if added over 0.20%, the coarsening of the carbide or carbonitride is remarkably increased, and on the contrary, the creep rupture strength may be impaired. The upper limit is 0.20%. In consideration of workability and structure stability, 0.05 to 0.12% is preferable.

  Si: Si is added as a deoxidizer in the steelmaking process, but is an element that contributes to improving the strength of the steel and the resistance to steam oxidation at high temperatures. The effect becomes remarkable with addition of 0.02% or more, but if added over 0.50%, oxide clusters are formed and toughness is lowered, so the upper limit is made 0.50%. In order to stably achieve both steam oxidation and toughness, 0.1 to 0.35% is preferable.

  Mn: Since Mn is an element that contributes to the improvement of the strength and toughness of steel, 0.05% or more is added. On the other hand, if added over 1.0%, the creep rupture strength decreases, so the upper limit is made 1.0%. For the purpose of improving the long-term creep rupture strength, 0.1 to 0.5% is preferable.

  Cr: Cr is an element that remarkably improves hardenability. In heat-resistant steel, it is an element that simultaneously improves high-temperature steam oxidation, so 0.4% or more is added. On the other hand, if added over 12.0%, the amount of precipitation of δ ferrite increases, and the creep rupture strength and toughness are remarkably reduced, so the upper limit is made 12.0%.

  Industrially, in order to obtain a uniform quenched structure and at the same time to obtain a required level of steam oxidation resistance, 1.0 to 9.0% is preferable, but in order to further increase the creep strength, 3 0.0 to 7.0% is more preferable.

  P, S: Since P and S are unavoidable impurity elements, it is preferable that P and S be less, P being 0.02% or less, and S being 0.01% or less.

  N: Since it is an element that forms MX-type nitride and contributes to the improvement of the creep rupture strength of the base steel material and the welded portion, 0.002% or more is added to ensure the above improvement effect. On the other hand, if added over 0.15%, the MX type nitride is coarsened, the creep rupture strength of the base steel material is not improved, and the MX type nitride remaining in the low temperature transformation structure of the weld is also coarse. Therefore, the upper limit is made 0.15%.

  Ti, Zr, Nb, V, and Ta: Ti, Zr, Nb, V, and Ta are essential elements for forming the MX type nitride and increasing the creep rupture strength of the base steel and the welded portion. In order to ensure the effect of improving the strength, one or more kinds are appropriately selected and added. Ti is 0.005% or more, Zr is 0.003% or more, and Nb is 0.01% or more. V must be added at 0.01% or more, and Ta must be added at 0.01% or more.

  On the other hand, excessive addition leads to excessive formation of MX-type nitride, aggregation and coarsening, and does not improve the creep rupture strength of the base steel, and MX-type nitride remaining in the low temperature transformation structure of the welded portion Since it coarsens and does not contribute to the improvement of the creep rupture strength of the welded portion, Ti is 0.20% or less, Zr is 0.20% or less, Nb is 0.50% or less, and V is 0.50. % And Ta are added in the range of 0.15% or less.

W, Mo, Re: W, Mo, Re is a precipitation strengthening mechanism by precipitation of intermetallic compounds, or precipitation of M ₂ C, and precipitation of MC type carbide or carbonitride. Add to increase the hardenability of the base steel. In order to obtain this improvement effect, it is necessary to add 0.01% or more of each of W, Mo, and Re.

  On the other hand, when W is added in excess of 3.0%, Mo is added in excess of 3.0%, or Re is added in excess of 0.5%, the respective precipitates are coarse. On the contrary, the high temperature creep strength decreases, so W is added to 3.0% or less, Mo is added to 3.0% or less, and Re is added within a range of 0.5% or less.

  Ni, Co, Cu: Ni, Co, and Cu are all γ-phase stabilizing elements, so are added as appropriate. When a large amount of a ferrite stabilizing element is added, phase stability is lowered, δ ferrite is generated, and creep strength may be lowered. In order to avoid this strength reduction, one or more kinds are appropriately added within a range that does not affect the creep rupture strength.

  In order to stabilize the γ phase, it is necessary to add Ni, Co, or Cu in an amount of 0.01% or more. However, if excessively added, the high temperature creep strength of the steel material is impaired, so Ni is 2.0% or less. Co is added in the range of 5.0% or less, and Cu is added in the range of 2.0% or less.

  B: B enhances the hardenability of the base steel and stabilizes the residual γ phase or the matched θ phase, which is important in the Type IV type damage avoidance heat treatment (this heat treatment will be described later). The gamma phase that coalesces or nucleates facilitates the reproduction of the γ grains of the pre-structure, making it more reliable to avoid the occurrence of Type IV type damage.

  If the addition is less than 0.0003%, the above effect cannot be obtained, so 0.0003% or more is added. However, if the addition exceeds 0.0050%, weldability, in particular, hot cracking, or after welding is added. Since reheat embrittlement may be caused, the upper limit is made 0.0050%.

  In addition to the above elements, the steel of the present invention may inevitably contain other elements, Al, and O within the normal range that does not impair the characteristics of the steel of the present invention and the characteristics of the weld. In addition, Al: less than 0.02% and O: less than 0.01% are preferable.

  Further, the steel of the present invention may contain one or more of Y, Ce, Mg, Ba, Ca, and La as long as the properties of the steel of the present invention and the properties of the weld are not impaired. Good.

  Preferably, in mass%, Y: 0.005 to 0.05%, Ce: 0.005 to 0.5%, Mg: 0.0003 to 0.005%, Ba: 0.0003 to 0.005% , Ca: 0.0003 to 0.005% and La: 0.005 to 0.05%.

  These elements act as sulfide form control elements, play a role of preventing the formation of coarse MnS and increasing toughness.

  In the steel material of the present invention, in addition to the composition of each element, HCreq (Cr equivalent of the weld heat affected zone) defined by the above formula (1) is limited to 0.4 to 20. This limitation is an important condition for obtaining a high-strength ferritic heat-resistant steel material, as described below, and effectively enables the technology to avoid Type IV damage that occurs in the structure of the heat affected zone. This is an important condition for application.

  HCreq (Cr equivalent of weld heat affected zone) is the total of ferrite forming ability of elements other than Cr converted into the ferrite forming ability of Cr. Ferrite formation when affected by welding heat in steel welded joints It is an index indicating performance. HCreq is calculated including elements mixed as impurities even for chemical components not intentionally added.

  For example, Al is an element that tends to be mixed as an impurity. Even if it is mixed as an impurity, HCreq is calculated including the mixed amount as long as the mixed amount is clear.

  The Cr equivalent formula (basic formula) related to ordinary steel is shown in the Schaeffler phase diagram. The present inventor, however, controls each element in order to control the low temperature transformation structure in the weld heat affected zone to a required structure. Based on the phase stability of various heat-resistant steel materials, experimentally confirmed the ferrite-forming ability of the heat-resisting steel, and converted to the ferrite-forming ability of Cr, and integrated it to more accurately determine the ferrite-forming ability of the heat-affected zone of the heat-resistant steel. As an index to be evaluated, the above formula (1) is defined on the premise of the above basic formula.

  Therefore, the above formula (1) is a Cr equivalent formula unique to the steel of the present invention, which is different from the Cr equivalent formula described in a general technical book, in terms of coefficient and target phase stability. Thus, it is also a feature of the present invention that a Cr equivalent formula unique to the steel of the present invention is set.

  And in this invention steel material, HCreq is defined using said Formula (1), The value is limited to 0.4-20, The reason is as follows.

  When the HCreq is low in the steel material of the present invention, it is difficult to make the structure into high-strength bainite or martensite in a normal usage form, that is, in the form of a steel pipe or steel plate used for high-temperature pressure equipment. This also means that the previous tissue for preventing Type IV damage cannot be acquired stably, so the lower limit of HCreq is set to 0.4.

  On the other hand, if HCreq exceeds 20, as a characteristic of Cr-containing steel, ferrite may not be produced to obtain bainite or martensite, so the upper limit of HCreq is set to 20.

  HCreq is preferably 1.0 to 15.0, and more preferably 5.0 to 12.0 from the viewpoint of creep strength.

  Usually, ferritic heat-resistant steel is welded with a heat input of 1 kJ / mm or more, but a fine low-temperature transformation structure is generated in the weld heat affected zone due to the influence of the heat input of 1 kJ / mm or more.

  In the steel material of the present invention having the above chemical composition and the above HCreq, a Type IV type damage avoiding heat treatment (this heat treatment will be described later) is carried out before welding even under the influence of welding heat input of 1 kJ / mm or more. In this case, after cooling, a low-temperature transformation structure having an average particle size of old austenite grains (hereinafter sometimes referred to as “old γ grains”) of 10 μm or more is generated. This point is also a feature of the steel material of the present invention.

  The low-temperature transformation structure is bainite and / or martensite, and the traces of the former γ grains are obtained by etching the heat affected zone of the weld with a caustic solution (nitral, picric acid, nitric acid, aqua regia, etc.) and observing with an optical microscope. This can be confirmed.

  The inventor measured the particle diameter of 50 or more old γ grains with an optical microscope, and determined the average value as “average particle diameter of old γ grains” defined in the steel of the present invention. However, some of the newly formed austenite grains (hereinafter sometimes referred to as “new γ grains”) produced by the α → γ transformation are not included in the “average grain size of the old γ grains” for the following reasons.

  The average crystal grain size means the old γ grain size in the low-temperature transformation structure, but when the old γ grain is cut in the vicinity of the ridgeline where the two faces are joined during cross-sectional observation, the actual crystal grain size is not necessarily measured. I can't.

  The α → γ transformation in the weld heat-affected zone of the steel of the present invention occurs exclusively from the inside of the crystal grains, and therefore has the same crystal orientation and is retained austenite (hereinafter sometimes referred to as “residual γ phase”) or matched. When γ grains regenerated from cementite (hereinafter sometimes referred to as “matched θ phase” will be described later) grow and coalesce, new γ grains having no trace of old γ grains are formed from old γ grain boundaries. In some cases, the maximum particle size is 10 μm.

  The new γ grain is formed adjacent to the old γ grain boundary using the old γ grain as a nucleation site, and toward the inside of the γ grain without disappearing the old γ grain boundary. Since it grows, it is continuously generated on one side or both sides of the old γ grain boundary, but the exclusive volume (area) is small, and the characteristics of the weld heat affected zone are not particularly affected.

  New γ grains that have a small area and do not affect the characteristics of the heat affected zone cannot be handled in the same row as the old γ grains that affect the characteristics of the heat affected zone. Invented steel materials, excluding new γ grains, the diameter of γ grains that are not adjacent to the old γ grains are represented by the old γ grain diameter, represented by the visual diameter on the optical microscope, and the “average grain diameter of the old γ grains” Was calculated.

  In addition, although it may be a phenomenon even when the above-mentioned adjacent fine new γ grains are not generated, since it is rare, in the steel material of the present invention, in any case, residual γ phase or matched θ phase in the grains The “average grain size of old γ grains” is defined as the average grain size of γ grains that reproduces the austenite crystal orientation of the previous structure by coalescence and growth.

  In the steel material of the present invention, the average grain size of austenite crystal grains before low temperature transformation must be 10 μm or more in order to suppress the refinement of the low temperature transformation structure formed at the outer edge of the weld heat affected zone. Even if crystal grains having a particle diameter of 10 μm or less are present, if the average particle diameter is 10 μm, it is possible to suppress the refinement of the low-temperature transformation structure.

  However, if the average grain size is less than 10 μm, there are inevitably many austenite crystal grains with a grain size of 10 μm or less. Become.

  The average grain size of the austenite crystal grains before the low temperature transformation is set to 10 μm or more, and in order to suppress the refinement of the low temperature transformation structure, at least the portion including the groove before welding (the influence of the welding heat input of 1 kJ / mm or more) Heat treatment (hereinafter referred to as “Type IV damage avoidance heat treatment”) at a maximum heating temperature of 1000 to 1400 ° C., a holding time of 1 to 60000 seconds, and a cooling rate of 0.1 to 50 ° C./second. In the subsequent welding, it is preferable to select a welding condition in which the entire region of the welding heat-affected zone is heated at a temperature rising rate of 10 ° C./second or more.

  When the type IV type damage avoidance heat treatment is applied to the part including the groove before welding (the part affected by the welding heat input of 1 kJ / mm or more), the structure of the part is in view of the uniformity of the structure, Focusing on the more preferable residual γ-phase after welding, (x1) contains the low-temperature transformation structure of bainite and / or martensite, and (x2) has the same crystal orientation as austenite before low-temperature transformation, and martensite The structure contains residual austenite having a sphere equivalent average particle size of 10 nm or more present at the lath boundary or at the block grain boundary of bainite and / or martensite.

  Note that the austenite structure before low-temperature transformation can be observed with an electron diffraction image of an electron microscope of the dislocation substructure, so that the orientation of the crystals constituting the structure can be known by analyzing the Kikuchi line, etc. it can.

  In recent years, analysis technology has improved, and this analysis can easily measure the area ratio in the cross section by a crystal orientation mapping technology called EBSP (Electron Back Scattering Pattern analysis). And the volume ratio can also be easily obtained from the measured area ratio.

  As described above, when the structure is formed before welding, formation of a low-temperature transformation structure of bainite and / or martensite having an average grain size of prior austenite grains of 10 μm or more in the weld heat affected zone after welding. Will be promoted.

  Also in this case, the retained austenite necessary for avoiding Type IV damage is preferably 0.5% or more and 5% or less, and more preferably 0.5% or more and 3% or less by volume. Further, it is preferable from the viewpoint of avoiding Type IV damage that 30% or more of retained austenite has the same crystal orientation. The crystal orientation distribution of retained austenite can be measured by the EBSP method or the electron diffraction method using a transmission electron microscope.

In addition, when Type IV type damage avoidance heat treatment is applied to at least a part including a groove before welding (a part affected by welding heat input of 1 kJ / mm or more), the structure of the part becomes structurally consistent. In view of the presence of cementite, (y1) includes the low temperature transformation structure of bainite and / or martensite, and (y2) matches the crystal orientation of the austenite before the low temperature transformation and the crystal orientation of the low temperature transformation structure. The normal vector of the “specific crystal orientation”, that is, the main orientation <111> of untransformed retained austenite and the main orientation <100> of orthorhombic Fe ₃ C is within 15 ° in consideration of translational symmetry. And a certain azimuth relationship (a certain azimuth relationship with the tempered bainite and martensite, Pitsch-Schrader relationship, or Pitsch relationship), and the martensite lath boundary or martensa It becomes a structure including “matched cementite having a sphere equivalent average particle diameter of 10 nm or more” present at the block grain boundary of theite and / or bainite.

  The present inventor has found that cementite having the above orientation relationship plays an important role in suppressing the refinement of the weld heat affected zone structure. In the present invention, such cementite is referred to as “matched cementite” (matched θ phase or matched θ). This “matched cementite” plays the same role as the “residual austenite” from the viewpoint of effect. The crystal orientation of cementite can be measured with an electron microscope.

  However, when the formation of the weld changes in various ways, that is, when the welding conditions change and the temperature history acting on the weld heat affected zone changes in various ways, the Type IV type damage avoidance heat treatment is completely It is not always possible to suppress the occurrence of Type IV damage.

  Although rare, there are sites in the structure of the weld that are repeatedly subjected to heat treatment corresponding to the fine grain region a plurality of times. The multiple heat-affected fine-grained part formed by repeatedly receiving the heat treatment has a very small proportion of the weld heat-affected zone, but in heat-resistant steel whose toughness has decreased after long-term use, minute cracks are large. Since this may lead to damage, the formation of multiple heat-affected fine-grained parts must be suppressed as much as possible to eliminate the possibility of cracking.

  In the steel of the present invention, in consideration of this point, even if the Type IV type damage avoidance heat treatment is carried out immediately before welding, the outer edge structure of the weld heat affected zone is extremely finely divided, and the avoidance heat treatment is performed. Although the degree is small compared to the case of not carrying out, even when a part where the dislocation density is reduced is partially formed, the high temperature creep strength is maintained, so that the precipitation form does not easily change in multiple thermal cycles. Nitride is dispersed and precipitated. This is the greatest feature of the present invention.

  The Type IV type damage avoidance heat treatment may be usually performed in accordance with heat treatment conditions employed for heat treatment applied to high-strength heat-resistant steel, that is, quenching-tempering or normalizing-tempering.

  As described above, the “stable nitride” is one or more MX type nitrides of Ti, Zr, Nb, V, Ta, and the base steel is heated immediately above the transformation point. However, if it is heating for a short time, it is a nitride that is difficult to decompose and dissolve, so even after welding, it remains in the heat-affected zone without changing its form and does not lose its precipitation strengthening ability.

  Furthermore, in the steel material of the present invention, if one or more of Ti, Zr, Nb, V, Ta and N and N are contained within a specified range, the MX type nitride is always stable. Since it is dispersed and precipitated, a large amount of solute N at the time of transformation is not lost, and even in the multiple heat-affected fine-grained part, the hardenability does not change, even if the part is present, Presence does not contribute to the occurrence of Type IV damage.

  As described above, the present invention is a welded part having multiple heat-affected fine-grained parts subjected to a complex thermal history such as multiple thermal cycles by using both pre-weld heat treatment and stable nitride dispersion precipitation. Even if it exists, it is a technique which can actually form a high-strength weld part in which the creep rupture strength does not decrease.

  Next, production of the steel material of the present invention and production of a heat resistant structure using the steel material of the present invention will be described.

  The steel having the composition defined by the steel material of the present invention is melted by using a conventional blast furnace-converter-continuous cast iron steel integrated process, or an electric furnace steelmaking method, a direct reduction steelmaking method, etc. Is cast by an ingot casting method or a continuous casting method to obtain a slab having a predetermined size and shape.

  The steel slab is subjected to hot rolling to form a steel plate, and further, the steel plate is processed and formed into a steel pipe, or the steel slab is made into a seamless steel pipe by hot rolling or hot extrusion, or the above The slab is forged into a forged member, and the required tempering heat treatment, that is, quenching-tempering treatment, or normalizing-tempering treatment, is performed, and tempered bainite and / or tempered martensite is substantially 80% or more. Form a tissue containing.

  In the steel material of the present invention, tempered bainite and / or tempered martensite are referred to as bainite and / or martensite, and the volume ratio (% by volume) of bainite / martensite can be obtained by observation with an optical microscope.

  The steel material subjected to the tempering heat treatment originally has good high-temperature creep characteristics and toughness, and further has workability, and is suitable for plant construction. However, tempering causes decomposition or alteration of residual austenite or consistent cementite. In some cases, the suppression of the refinement of the weld heat affected zone may not be achieved.

  Therefore, in order to avoid the above situation, it is necessary to heat the steel plate or the entire steel material or the portion including the groove again before welding to generate residual austenite or matched cementite again.

  The knowledge of the present invention is that the tempering treatment is omitted or not intentionally performed within the range where no problems occur due to processing, deformation, etc., after quenching or normalizing the steel material and before the subsequent welding process. It is also effective in a method of welding and constructing a plant or a structural part. In fact, when effective in avoiding Type IV damage, the construction method can be said to be substantially the same as the present invention.

  These phases (residual γ phase or matched θ phase) are affected by welding heat during welding and reproduce the old γ grains. In addition, the α → γ transformation slightly occurs from the old γ grain boundary, which is the nucleation point of new crystal grains, due to the influence of welding heat, but the above phase acts to hinder the growth of the new crystal grains.

  Due to the reproducibility of the structure, in the weld heat affected zone of the steel of the present invention, the average grain size of the old γ grains does not become less than 10 μm at the site reheated near the temperature just above the transformation point. As a result, in the outer edge portion of the weld heat affected zone, the structure is prevented from being refined and hardenability is not lowered.

  In addition, stable nitrides are dispersed and precipitated by adjusting the chemical components defined in the scope of the present invention, and by obtaining stable precipitation strengthening, even welds that have undergone complex multiple thermal cycles, Type IV damage, which has been a problem in the past, does not occur in the weld heat affected zone.

  In order to avoid delayed fracture or embrittlement, a post-weld heat treatment (hereinafter sometimes referred to as “PWHT”) may be further applied to a weld having the above structure.

  This PWHT improves both the strength and toughness of the welded part and the structure in the vicinity of the weld, but the residual austenite or coherent cementite intentionally introduced into the steel material or the pre-weld part including the groove is not removed after welding. However, if it remains in the steel material or in the above part, for example, during the use of pressure equipment at a high temperature, due to temperature and load stress, it transforms over time into martensite, ferrite and Cr carbide, etc. Or it may change and a big volume change may arise as the whole steel materials or the whole apparatus.

  This volume change gives stress to various parts of the pipes in addition to thermal stress, particularly to the pipes carrying the high-temperature steam, which causes damage to the pressure equipment.

Therefore, in the steel material of the present invention, the retained austenite or matched cementite is limited to 0.5% or less by volume, and even if the pressure device manufactured by welding the steel material of the present invention is exposed to high temperature, it is unique to the device. It is necessary to have a structure that gives only a small stress compared to the thermal stress generated by thermal expansion of the material. In practice, the welded member or welded structure may be subjected to tempering heat treatment under the condition that the retained austenite or the matched θ phase decomposes and deteriorates below the Ac ₁ transformation point after welding.

Welded structures manufactured by welding the steel of the present invention, such as pressure devices and plants, are heat resistant structures with excellent creep characteristics in the weld heat affected zone because Type IV type damage does not occur in the weld heat affected zone. However, the entire welded structure is subjected to a residual phase reduction heat treatment that is maintained for 1 minute or longer at an Ac ₁ transformation point or less, and residual austenite or matched cementite is 0.5% by volume throughout the welded structure. It is more preferable to reduce to less than% and suppress the generation of stress due to the transformation or alteration.

Even with the above heat treatment, the stable MX type nitride, which is a feature of the steel material of the present invention, is difficult to decompose and dissolve, so the strengthening mechanism by the nitride is not affected at all. The residual phase reducing heat treatment is preferably a heat treatment that is held at 400 ° C. or higher and below the Ac ₁ transformation point for 10 minutes or longer.

  Next, examples of the present invention will be described. The conditions of the examples are one example of conditions adopted for confirming the feasibility and effects of the present invention, and the present invention is limited to this one example of conditions. Is not to be done. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

Example 1
A heat resistant steel material having the chemical composition shown in Table 1 was subjected to Type IV avoidance heat treatment, and then the steel material was welded with a welding material having the chemical composition shown in Table 1 to constitute a pressure device. After the pressure device is subjected to a heat treatment for reducing the residual γ phase for 400 minutes or more and less than the Ac ₁ transformation point for 10 minutes or more, the test is performed on the device component and the welded part of the device from 4 to 6 mm in parallel part diameter and 30 mm in parallel part length. Pieces were collected.

  And using the said test piece, the creep test which adds the stress (20-100 MPa) assumed by the use temperature (500-650 degreeC) of a pressure apparatus as a constant load is implemented, an apparatus structural member and this member welding The creep rupture life of the part was investigated.

  For comparison, heat resistant steel materials with the chemical components shown in Table 1 are not subjected to Type IV avoidance heat treatment, but welded with welding materials of the chemical components shown in Table 1 to form a pressure device. The creep rupture life of the member and the welded part of the member was investigated.

  FIG. 2 shows the creep rupture life of the welded part of the present invention (welded part obtained by TIG welding of 9% Cr ferritic heat-resistant steel A using welding material D at a heat input of 2 kJ / mm), that is, , 650 ° C., estimated creep rupture strength (MPa) at 100,000 hours. Furthermore, FIG. 2 also shows the estimated creep rupture strength (MPa) at 650 ° C. for 100,000 hours of the base steel material before welding and not subjected to Type IV damage avoidance heat treatment.

  The estimated creep rupture strength at 650 ° C. and 100,000 hours for the conventional weld is only about half of the estimated creep rupture strength at 650 ° C. and 100,000 hours for the base steel material. The estimated creep rupture strength is equal to the estimated creep rupture strength of the base steel at 650 ° C. and 100,000 hours.

Next, FIG. 3 shows a weld heat-affected zone (Ac ₁ transformation point to Ac ₁ transformation point + 300 ° C.) in a weld zone formed by welding 9% Cr ferritic heat-resistant steel material B using a common metal welding material C. In the base steel material part in the welded portion reheated to the original, the part that originally becomes a fine grain region (hereinafter sometimes referred to as “fine grain region equivalent heat affected zone” or “FG-HAZ”)). The relationship between the average crystal grain size and the amount of residual γ before welding measured by the X-ray diffraction peak method, and also the above average crystal grain size, electron microscope observation, and acid dissolution constant potential electrolytic extraction residue of the base material The relationship with the fixed amount of aligned cementite (alignment θ) before welding is shown.

  The average crystal grain size means the old γ grain size in the low temperature transformation structure, and does not include the grain size of the new γ grain as described above.

  From FIG. 3, in order to suppress the occurrence of Type IV damage, in order to increase the average crystal grain size in FG-HAZ (fine grain region equivalent heat-affected zone) to 10 μm or more, the retained austenite or matched cementite is 0.5% or more. I understand that it must exist.

If consistent cementite or retained austenite remains or precipitates in excess of 5%, it may not be completely decomposed or dissolved when heat-treated to heat the equipment or the entire plant to a temperature below the Ac ₁ transformation point. In addition, when the plant is operated at a high temperature, pipes and the like are remarkably deformed or broken, so the amount of matched cementite or retained austenite is preferably 5% or less in volume ratio.

  Note that, in pressure equipment or plants, curved pipes provided after long straight pipes are severely limited in deformation of the pipes, and in particular, may be limited to 3% at high temperatures, so matched cementite or residual austenite The amount is preferably 3% or less in terms of volume ratio.

  The relationship between the amount of retained austenite and the deformation rate of piping is already empirically known in Europe. The present inventor conducted a virtual test using a small test piece, and investigated the influence on the deformation due to the intermediate transformation or alteration. The result is shown in FIG.

  From FIG. 4, at 650 ° C., in order for the deformation to fall below the deformation limit of 0.5% (a large deformation reaching 50 cm with a straight pipe of 100 m), residual austenite remains in the base steel. It can be seen that if it is left unusable (pipe breaks due to the deformation), the amount needs to be reduced to 0.5% or less by volume ratio.

  The present inventor has also experimentally confirmed that it is necessary to reduce the volume ratio to 0.5% or less in the case of matched cementite.

  The deformation of the piping is presumed to be due to the precipitation or alteration of carbide accompanying volume change, but in the steel of the present invention, it avoids Type IV damage that occurs in the weld zone of ferritic heat-resistant steel. At the same time, it is possible to prevent deformation of the pressure equipment or the plant constituent members inevitably caused by the avoidance technique.

Precipitation density of “stable MX type nitride” (M: one or more of Nb, V, Ti, Ta, Zr), which is an important feature in the steel of the present invention, and double or triple fine FIG. 5 shows the creep rupture strength of the weld zone subjected to repeated heat treatment within the grain region equivalent cycle, that is, within the range of Ac ₃ + 300 ° C. from the Ac ₃ transformation point, in comparison with the creep rupture strength of the base steel.

From FIG. 5, it is understood that the creep rupture strength ratio is 0.8 or more when the precipitation density of MX type nitride is 2 pieces / m ² or more. When the precipitation density is less than 2 pieces / m ² , the concentration of one or more of Nb, Ti, V, Zr, and Ta deviates from the range defined by the steel material of the present invention. When the precipitation density is 2 pieces / m ² or more, the effect of the steel of the present invention is remarkable.

  As shown in FIG. 6, when MX type nitride does not exist (refer to the left half in the figure), even if the structure is controlled by performing Type IV damage avoidance heat treatment before welding, Repeatedly, when subjected to the thermal cycle corresponding to the heat affected zone corresponding to the fine-grained area, although not necessarily frequent, Type IV type damage occurred in some welds, and the creep rupture strength ratio was 0.8 (Fig. Middle, dotted line, reference) may be significantly below.

  However, similarly, as shown in FIG. 6, when MX type nitride is introduced (refer to the right half in the figure), if the MX type nitride defined in the present invention is finely dispersed, creep rupture occurs. The strength ratio greatly exceeds 0.8 (see the dotted line in the figure), and it can be seen that Type IV damage can be avoided stably at the weld.

(Example 2)
The steel of the present invention having the chemical composition shown in Table 2 and the comparative steel having the chemical composition shown in Table 3 were heat-treated and welded under the conditions shown in Table 4 to confirm the effects of the steel of the present invention. The results are also shown in Table 4.

  In addition, the effect of this invention steel material was shown by the strength ratio with 100,000 hours estimated creep rupture strength of a base steel material using 100,000 hours estimated creep rupture strength.

  The creep characteristics of the welded part can not be evaluated only by the presence or absence of Type IV type damage, considering that it changes even with weld metal and subsequent stress relief annealing conditions, with a strength ratio of 0.8 as the threshold, The effect of the steel material of the present invention was expressed as 0.8 or more.

  In the welded part according to the prior art (conventional welded part), the strength ratio is around 0.5 (see Table 4, strength ratio of the comparative example). This means that there is almost no decrease in strength due to the occurrence of type IV damage.

  The estimated creep rupture strength includes the strength by a temperature accelerated creep test up to a temperature 100 ° C. higher than that at the use temperature of the ferritic heat resistant steel, for example, 450 to 600 ° C. The creep rupture data exceeding this is a value obtained by estimating the rupture strength by treating the temperature and time equivalently using the LMP method.

  Although the estimated calculation value is an extrapolation value, if it is a high-order polynomial function of degree 3 or higher, it can be calculated with high accuracy, so the actually required 100,000 hour creep test itself is not performed. .

Table 4 shows the results of the comparative example for comparison. Since the transition element for producing the stable MX type nitride is not added to the steel material in Comparative Example 31 (see Table 3), the welded portion is between Ac ₁ transformation point to Ac ₁ transformation point + 300 ° C. When heated multiple times, the average crystal grain size of the weld heat affected zone (in this example, the double thermal cycle site) becomes small, the hardenability decreases, and the estimated creep rupture strength of the weld zone is the base steel. The ratio is 0.78, which is slightly less than 0.8 and is an example of a decrease. The same applies to Comparative Example 32.

  Comparative Example 33 is a steel material that has generated stable MX type nitride, but since it has not been subjected to Type IV type damage avoidance heat treatment, the occurrence of Type IV type damage cannot be suppressed, and the creep rupture strength ratio is significantly reduced. This is an example.

  Since the comparative example 34 did not carry out the disappearance treatment of retained austenite or consistent cementite in the plant equipment or the entire plant after welding, a large amount of distortion occurred in the ferritic heat-resistant steel material, particularly in this case, the piping system. In this example, the welded portion was damaged and the strength ratio was significantly reduced.

  In Comparative Example 35, the steel material was subjected to Type IV type damage avoidance heat treatment, but under the conditions, the maximum heating temperature was low, and therefore, retained austenite or matched cementite could be sufficiently present in the structure before welding. This is an example in which the occurrence of Type IV damage could not be avoided.

  In Comparative Example 36, the steel material was subjected to Type IV type damage avoidance heat treatment, but under that condition, the cooling rate was too slow, so that the integrity of the structure disappeared due to decomposition of residual austenite or growth of matched cementite. This is an example in which residual austenite or matched cementite could not be sufficiently present in the structure before welding, and occurrence of Type IV damage could not be avoided.

  Comparative Example 37 satisfies all the treatments and treatment conditions specified in the present invention, but the temperature rise rate during welding is too slow due to the relationship between welding heat input and plate thickness, and the effect of the steel material of the present invention is obtained. As a result, a fine grain region was generated in the weld heat affected zone, and the occurrence of Type IV damage could not be avoided.

  In Comparative Example 38, HCreq is too high and becomes a ferritic single phase steel, the strength of the steel material itself is remarkably lowered, and further, the effect by the Type IV type damage avoiding heat treatment is not expressed, and eventually Type IV type damage occurs. This is an example in which the weld strength is significantly reduced with respect to the strength of the base steel.

  As described above, according to the present invention, in the heat-affected zone of the weld zone of the ferritic heat-resistant steel material, the strength is increased by utilizing the strengthening mechanism by the fine dispersion of MX type nitride, and Type IV type damage occurs. Since concerns can be completely wiped out, the high-temperature strength is 0.67 times the normal creep rupture strength (normal safety factor) in the design of heat-resistant welded structures (heat-resistant structures) that make up high-temperature and high-pressure plant equipment. Can be designed as As a result, it is possible to prevent an accident at the starting point of the weld that has occurred in the past. Therefore, the present invention has great applicability in the plant construction industry.

It is a figure which shows the Type IV type damage which generate | occur | produces in the welding heat affected zone of a ferritic heat-resistant steel material. It is the figure which compares the estimated creep rupture strength of 650 degreeC and 100,000 hours of the welding part (this invention welding part) of this invention steel material with the said estimated creep rupture strength of the welding part (conventional welding part) of a conventional heat-resistant steel material. is there. It is a figure which shows the relationship between the retained austenite (residual (gamma)) or matching cementite (matching (theta)) before welding, and the average prior gamma particle size of a welding heat-affected zone outer edge part (fine-grain area equivalent heat-affected zone). It is a figure which shows the relationship between the length change rate of the straight piping manufactured with 9% Cr ferritic heat-resistant steel materials, and the amount (volume%) of a retained austenite (residual (gamma)). The relationship between the ratio of the creep rupture strength at 650 ° C. and 100,000 hours estimated for the welded part and base steel subjected to multiple thermal cycles and the precipitation density (pieces / μm ² ) of MX-type nitrides with a sphere equivalent diameter of 200 nm or less FIG. It is a figure which shows that 650 degreeC and a 100,000-hour estimated creep rupture strength ratio of a multiple heat cycle influence part site | part (heat influence part outer edge) are stabilized by introduction of MX type nitride.

Claims (16)

In mass%, C: 0.01 to 0.20%, Si: 0.02 to 0.50%, Mn: 0.05 to 1.0%, P: 0.02% or less, S: 0.01 % Or less, Cr: 0.4 to 12.0%, N: 0.002 to 0.15%, and Ti: 0.005 to 0.20%, Zr: 0.003 to 0.20%, Nb : 0.01 to 0.50%, V: 0.01 to 0.50%, Ta: 0.01 to 0.15% of any one type or two or more types, the balance Fe and inevitable impurities And a ferritic heat resistant steel material having an HCreq (Cr equivalent of the weld heat affected zone) defined by the following formula (1) of 0.4 to 20,
(Z) Ac ₁ in weld heat affected zone of the transformation point to Ac ₁ steel is heated to a transformation point + 300 ° C., welding heat an average particle size of prior austenite grains and generating the 10μm or more low-temperature transformation structure Ferritic heat-resistant steel with excellent creep properties in the affected area.
HCreq = [% Cr] +6 [% Si] +11 [% V] +4 [% Mo] +1.5 [% W] +5 [% Nb] +12 [% Al] + [% Re] +2 [% Zr] +5 [% Ti] +2 [% Ta] +15 [% B] -18 [% C] -12 [% N] -4 [% Ni] -2 [% Mn]-[% Cu] -2 [% Co] ... (1)   The low temperature transformation structure is bainite and / or martensite containing one or more of MX type nitrides of Ti, Zr, Nb, V, and Ta. Ferritic heat-resistant steel with excellent creep characteristics in weld heat affected zone. In mass%, C: 0.01 to 0.20%, Si: 0.02 to 0.50%, Mn: 0.05 to 1.0%, P: 0.02% or less, S: 0.01 % Or less, Cr: 0.4 to 12.0%, N: 0.002 to 0.15%, and Ti: 0.005 to 0.20%, Zr: 0.003 to 0.20%, Nb : 0.01 to 0.50%, V: 0.01 to 0.50%, Ta: 0.01 to 0.15% of any one type or two or more types, the balance Fe and inevitable impurities And a ferritic heat resistant steel material having an HCreq (Cr equivalent of the weld heat affected zone) defined by the following formula (1) of 0.4 to 20,
(X) Ac ₁ transformation point to Ac ₁ transformation point + 300 ° C. to tissue before the welding heat affected zone of the steel to be heated, (x1) low-temperature transformation structure of bainite and / or martensite, and, (x2) low-temperature transformation Having the same crystal orientation as the previous austenite, including residual austenite having a sphere equivalent average particle size of 10 nm or more present at the martensite lath boundary, or at the block grain boundary of bainite and / or martensite,
(Z) Any _{one of} Ti, Zr, Nb, V, and Ta having an average grain size of prior austenite grains of 10 μm or more in a welding heat-affected region of a steel material heated to Ac ₁ transformation point to Ac ₁ transformation point + 300 ° C. A ferritic heat resistant steel material excellent in creep characteristics of a weld heat affected zone, characterized in that a low temperature transformation structure of bainite and / or martensite containing one or more MX type nitrides is formed.
HCreq = [% Cr] +6 [% Si] +11 [% V] +4 [% Mo] +1.5 [% W] +5 [% Nb] +12 [% Al] + [% Re] +2 [% Zr] +5 [% Ti] +2 [% Ta] +15 [% B] -18 [% C] -12 [% N] -4 [% Ni] -2 [% Mn]-[% Cu] -2 [% Co] ... (1) The low-temperature transformation structure is one or more MX type nitrides of Ti, Zr, Nb, V, Ta in the martensite lath or in the bainite and / or martensite block grains. 4. The creep characteristics of the weld heat-affected zone according to claim 3, further comprising ² MX / μm ² or more of MX type nitride that can be confirmed by 50,000-times observation using a transmission electron microscope. Excellent ferritic heat resistant steel.   5. The ferritic heat resistant steel material having excellent creep characteristics of the weld heat affected zone according to claim 3, wherein the retained austenite is contained in a volume ratio of 0.5% or more and 5% or less.   The ferritic heat resistant steel material having excellent creep characteristics of the weld heat affected zone according to claim 3 or 4, wherein the retained austenite is contained in a volume ratio of 0.5% or more and 3% or less.   The ferritic heat resistant steel material excellent in creep characteristics of the weld heat affected zone according to any one of claims 3 to 6, wherein 30% or more of the retained austenite has the same crystal orientation. In mass%, C: 0.01 to 0.20%, Si: 0.02 to 0.50%, Mn: 0.05 to 1.0%, P: 0.02% or less, S: 0.01 % Or less, Cr: 0.4 to 12.0%, N: 0.002 to 0.15%, and Ti: 0.005 to 0.20%, Zr: 0.003 to 0.20%, Nb : 0.01 to 0.50%, V: 0.01 to 0.50%, Ta: 0.01 to 0.15% of any one type or two or more types, the balance Fe and inevitable impurities And a ferritic heat resistant steel material having an HCreq (Cr equivalent of the weld heat affected zone) defined by the following formula (1) of 0.4 to 20,
(Y) Ac ₁ transformation point to Ac ₁ transformation point + 300 ° C. to tissue before the welding heat affected zone of the steel to be heated, (y1) low-temperature transformation structure of bainite and / or martensite, and, (y2) low-temperature transformation It has a specific crystal orientation that matches the crystal orientation of the previous austenite with the crystal orientation of the low-temperature transformation structure, and the sphere equivalent average particle size present at the martensite lath boundary or at the block grain boundary of martensite and / or bainite includes more matching cementite 10 nm, in (z) Ac ₁ in weld heat affected zone of the steel is heated to a transformation point to Ac ₁ transformation point + 300 ° C., an average particle size of prior austenite grains is 10μm or more, Ti, Zr, A welding heat-affected zone characterized in that a low-temperature transformation structure of bainite and / or martensite containing one or more MX type nitrides of Nb, V, and Ta is formed. Excellent ferritic heat resistant steel in creep characteristics.
HCreq = [% Cr] +6 [% Si] +11 [% V] +4 [% Mo] +1.5 [% W] +5 [% Nb] +12 [% Al] + [% Re] +2 [% Zr] +5 [% Ti] +2 [% Ta] +15 [% B] -18 [% C] -12 [% N] -4 [% Ni] -2 [% Mn]-[% Cu] -2 [% Co] ... (1) The low-temperature transformation structure is one or more MX type nitrides of Ti, Zr, Nb, V, and Ta in the martensite lath or in the block grains of bainite and / or martensite. 9. The creep characteristics of the heat-affected zone according to claim 8, characterized in that it contains ² MX / μm ² or more of MX type nitride that can be confirmed by 50,000-times observation using a transmission electron microscope. Excellent ferritic heat resistant steel.   The ferritic heat resistant steel material having excellent creep characteristics of the weld heat affected zone according to claim 8 or 9, wherein the matched cementite is contained in a volume ratio of 0.5% or more and 5% or less.   The ferritic heat resistant steel material having excellent creep characteristics of the weld heat affected zone according to claim 8 or 9, wherein the matched cementite is contained in a volume ratio of 0.5% or more and 3% or less.   The ferritic heat resistant steel material excellent in creep characteristics of the weld heat affected zone according to claim 8, wherein 30% or more of the matched cementite has the same crystal orientation.   Further, the ferritic heat-resistant steel material is, in mass%, W: 0.01 to 3.0%, Mo: 0.01 to 3.0%, Re: 0.01 to 0.5%, Ni: 0.00. Contains one or more of 01-2.0%, Co: 0.01-5.0%, Cu: 0.01-2.0%, B: 0.0003-0.0050% The ferritic heat resistant steel material excellent in creep characteristics of the weld heat affected zone according to any one of claims 1 to 12.   Further, the ferritic heat resistant steel material is, in mass%, Y: 0.005 to 0.05%, Ce: 0.005 to 0.5%, Mg: 0.0003 to 0.005%, Ba: 0.00. It contains any one or more of 0003 to 0.005%, Ca: 0.0003 to 0.005%, La: 0.005 to 0.05%. The ferritic heat resistant steel material excellent in the creep characteristics of the weld heat affected zone according to any one of the above.   The heat-resistant structure excellent in the creep characteristic of the weld heat affected zone characterized by being manufactured by welding the ferritic heat resistant steel material excellent in the creep property of the weld heat affected zone according to any one of claims 1 to 14. body. 16. The heat resistant structure is subjected to a heat treatment for 1 minute or more at an Ac ₁ transformation point or less to reduce residual austenite or matched cementite to less than 0.5% by volume. A heat-resistant structure excellent in creep characteristics of the weld heat-affected zone as described in 1.