JP2008248385A - Ferritic heat resistant steel material and heat-resistant structure excellent in the creep property of weld heat-affected zone - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a ferritic heat resistant steel material capable of suppressing the occurrence of a type IV type damage in a weld heat affected zone. <P>SOLUTION: The ferritic heat resistant steel material having the excellent creep characteristic of the weld heat affected zone contains, by mass%, 0.01 to 0.20% C, 0.02 to 0.50% Si, 0.05 to 1.0% Mn, ≤0.02% P, ≤0.01% S, 0.4 to 12.0% Cr, 0.001 to 0.05% Al, 0.001 to 0.07% N, is limited to ≤0.01% O, consists of the balance Fe and inevitable impurities, and is 0.5 to 80 in a weld heat-affected zone hardenability index HDL defined by formula. The low temperature transformation texture of 1×10<SP>12</SP>pieces/m<SP>2</SP>or more (in the case of 0.4 to 3.0% Cr) or 1×10<SP>13</SP>pieces/m<SP>2</SP>or more (in the case of 3.0 to 12.0% Cr) in dislocation density or 1×10<SP>12</SP>pieces/m<SP>2</SP>(in the case of 3.0 or over to 12.0% Cr) in dislocation density is created in the weld heat affected zone of the steel material heated to an AC<SB>1</SB>transformation point to the AC<SB>1</SB>transformation point +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 that make up the equipment, but such damage generally occurs at the welds of the steel sheets.

  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, no Type IV type damage prevention technology that has been put into practical use has 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 rate of decrease. 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 a certain carbide or carbonitride has an effect in terms of suppressing a decrease in strength 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.

  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 for heat-treating a welded part together with a base material after welding again, for example, a technique for solving strength reduction in a weld heat-affected zone by quenching and tempering. ing.

  This technology is a technology that returns the structure of the heat affected zone to the same structure as the base material structure, and is not a technique that prevents the occurrence of Type IV damage. In addition, when the component device or the constituent unit is larger than a certain size, it is difficult to heat treat the entire welded structure at the same time.

  Further, since the weld metal strength is usually designed by welding, that is, as cast, by tempering, the above-described method, that is, quenching and tempering the entire structure including the weld metal, the high temperature strength of the weld metal. It is difficult to ensure the strength of the welded joint before the occurrence of Type IV damage.

  In order to heat-treat the entire welded structure, it is necessary to use a large furnace. However, in the case of a large furnace, the equipment cost is high and the energy cost to be used is also increased. In order to apply this technology to industrial mass production, further technological development is required.

  However, it is actually impossible to heat treat the entire welded structure, 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. In this method, heating is performed to the Ac ₃ transformation point or higher before welding, 3% of retained austenite is introduced, and the growth coalescence prevents fine graining, but cementite or M ₂₃ C ₆ type carbide is used. It cannot be applied to an alloy system that does not generate retained austenite.

  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 an industrial special solution 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 reproduction material with reduced grain size", CAMP-ISIJ, Vol. 19 (2006), 1180

  The present invention uses a ferritic heat-resistant steel material, and when a heat-resistant welded structure constituting a thermal power plant or a petrochemical plant is constructed, a local strength decrease phenomenon in a weld heat affected zone inevitably generated in a welded portion. It is an object to prevent the destruction of the heat-resistant welded structure from the weld heat-affected zone by suppressing the occurrence of Type IV damage caused by the welding.

  And this invention solves the said subject, and does not impair the safety | security of a heat-resistant welded structure, even if design tolerance is made small in the design of the heat-resistant welded structure which comprises a power generation blunt or a petrochemical plant, or The purpose is to increase the design margin by utilizing the conventional design standards, increase the operating conditions, especially the pressure conditions, increase the energy conversion efficiency, and realize 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 inventors have obtained the following knowledge.

  (I) The main cause of the strength decrease in the welded part of 9% Cr steel is the decrease in the dislocation density in the fine grain region formed at the outer edge of the welded heat affected part (site close to the base steel) in the welded heat affected part. It is.

(Ii) In order to suppress the occurrence of Type IV damage in the weld zone, during cooling after being affected by the welding heat, even in the fine grain region heated just above the Ac ₃ point, some carbides Even if it remains undissolved and the amount of dissolved C necessary for low-temperature transformation structure formation decreases, martensite and / or bainite with high dislocation density is formed, and the structure of the weld heat-affected zone and the base steel structure are uniform. It is important to ensure consistency and consistency.

  Furthermore, in addition to the said knowledge, this inventor came to obtain the following knowledge.

  (Iii) In order to form martensite and / or bainite having a high dislocation density, a weld heat-affected zone hardenability index HDI is defined, and if this HDI is regulated within a required range, crystals are formed in the weld-heat-affected zone. Even if the particle size changes significantly, the occurrence of Type IV damage can be suppressed.

  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%, Al: 0.001 to 0.05%, N: 0.001 to 0.07%, O: 0.01% or less A ferritic heat resistant steel material having a weld heat-affected zone hardenability index HDI of 0.5 to 80 defined by the following formula (1):
Ac ₁ to weld heat affected zone of the steel is heated to a transformation point to Ac ₁ transformation point + 300 ° C., the dislocation density of 1 × 10 ¹² pieces / m ² or more (Cr: For 0.4 to 3.0%), Alternatively, a ferrite system having excellent creep characteristics in the heat affected zone of welding, wherein a low temperature transformation structure of 1 × 10 ¹³ pieces / m ² or more (in the case of Cr: more than 3.0 to 12.0%) is generated. Heat resistant steel.
HDI = √ [% C] √ [% N] (1 + 0.5 [% Si]) (1 + 3 [% Mn]) (1 + 2 [% Cr]) (1 + 3 [% Mo]) (1 + 0.8 [% W] ) (1 + 0.3 [% Cu]) (1 + 0.5 [% Ni]) (1 + 2.5 [% Nb]) (1 + 1.5 [% V]) (1 + 0.5 [% Al]) (1 + 0.3 [% Ti]) (1 + 0.3 [% Zr]) (1 + 0.2 [% Re]) (1 + 25 [% B]) (1 + 0.5 [% Co]) (1)

  (2) The ferritic heat resistant steel material further contains B: 0.0003 to 0.005% by mass%, and is excellent in the creep characteristics of the weld heat affected zone according to (1) above Ferritic heat resistant steel.

  (3) The ferritic heat resistant steel material is further in mass%, Mo: 0.05 to 2.0%, W: 0.05 to 3.0%, Re: 0.05 to 2.0% The ferritic heat resistant steel material excellent in the creep characteristics of the weld heat affected zone according to (1) or (2), characterized by containing one or more of the above.

  (4) The ferritic heat-resisting steel material is further in% by mass of Ni: 0.01 to 0.5%, Co: 0.01 to 3.0%, and Cu: 0.01 to 1.5%. 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 (3), characterized by containing one or more of the above.

  (5) The ferritic heat resistant steel material is further in mass%, Ti: 0.005 to 0.20%, Zr: 0.002 to 0.10%, Nb: 0.005 to 0.50%, V : Ferrite excellent in creep characteristics of weld heat affected zone according to any one of (1) to (4), characterized by containing one or more of 0.01 to 1.0% Heat resistant steel.

  (6) The ferritic heat resistant steel material is further in mass%, Ca: 0.0003 to 0.005%, Mg: 0.0003 to 0.01%, La: 0.005 to 0.05%, Ce. : 0.005 to 0.10%, Y: 0.005 to 0.10%, Ba: 0.0003 to 0.005%, or one or more of the above (1) The ferritic heat resistant steel material excellent in the creep characteristics of the weld heat affected zone according to any one of (1) to (5).

(7) in the weld heat affected zone of the steel is heated to the Ac ₁ transformation point to Ac ₁ transformation point + 300 ° C., in advance, 100 [mu] m or more low-temperature transformation structure with a grain size of old austenite is an average equivalent-sphere diameter is formed The ferritic heat resistant steel material having excellent creep characteristics of the weld heat affected zone according to any one of the above (1) to (6).

  (8) The ferritic heat resistant steel material having excellent creep characteristics of the weld heat affected zone according to any one of (1) to (7), wherein the low temperature transformation structure is bainite and / or martensite. .

  (9) The weld heat affected zone hardenability index HDI is 3.0 to 65, which is excellent in the creep characteristics of the weld heat affected zone according to any one of (1) to (8). Ferritic heat resistant steel.

  (10) 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 (9). Excellent heat resistant structure.

  According to the present invention, Type IV type damage does not occur in the heat-affected zone of the welded portion of the ferritic heat-resistant steel material. Therefore, in the design of the heat-resistant welded structure (heat-resistant structure) constituting the high-temperature and high-pressure plant equipment, the high temperature The strength can be designed as 0.67 times the normal creep rupture strength (normal safety factor). 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: 0.05-1.0%, P: 0.02% or less, S: 0.01% or less, Cr: 0.4-12.0%, Al: 0.001-0.05%, N: 0.001-0.07%, O: 0.01% It is a ferritic heat resistant steel which is limited to the following, consists of the remaining Fe and inevitable impurities, and has a weld heat affected zone hardenability index HDI defined by the following formula (1) of 0.5 to 80.

  HDI = √ [% C] √ [% N] (1 + 0.5 [% Si]) (1 + 3 [% Mn]) (1 + 2 [% Cr]) (1 + 3 [% Mo]) (1 + 0.8 [% W] ) (1 + 0.3 [% Cu]) (1 + 0.5 [% Ni]) (1 + 2.5 [% Nb]) (1 + 1.5 [% V]) (1 + 0.5 [% Al]) (1 + 0.3 [% Ti]) (1 + 0.3 [% Zr]) (1 + 0.2 [% Re]) (1 + 25 [% B]) (1 + 0.5 [% Co]) (1)

  Here, the expression (1) of the weld heat affected zone hardenability index HDI is generally used when evaluating the hardenability in the weld heat affected zone of a low alloy steel having a carbon content of 0.2% or less. This is an optimization of the formula for the ideal critical diameter DI.

That is, it is assumed that the hardenability factor f _X of the main alloy element X can be expressed by a linear expression: f _X = 1 + α _X [% X], and the effect of welding heat on a low alloy steel having a carbon content of 0.2% or less. Based on the result of observation of the tissue of the part with an optical microscope, α _X was determined and determined by regression analysis.

  The reason why the hardenability factor expression is assumed to be the linear equation is that the hardenability factor of each main alloy element in a well-known figure of hardenability factor obtained by Grossmann et al. , By the linear expression. However, since it has been found that the amount of carbon and the amount of nitrogen are proportional to the 1/2 power, they are incorporated as the 1/2 power in the equation (1).

  Ferritic heat-resistant steel is mostly tempered and used, and as a result, it has been found that the effective crystal grain size is approximately constant at 10 to 15 μm. Is not incorporated into the HDI formula.

  Therefore, the weld heat affected zone hardenability of the target product can be expressed by the index HDI. A satisfactory HDI threshold was determined from the relationship between the HDI and the characteristics of the steel material, and applied.

  First, the reason for limiting the ranges of chemical components and HDI values 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 contributes to the improvement of the creep rupture strength. This improvement effect becomes clear when 0.01% or more is added, but if added over 0.20%, the carbides are significantly coarsened, and the creep rupture strength may be impaired. 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 hardened structure and at the same time to obtain a required level of steam oxidation resistance, 1.0 to 9.0% is preferable. In order to further increase the creep strength, 3.0-7.0% is more preferable.

  N: N, like C, contributes to improving the hardenability of the steel material, and has the effect of increasing the HDI value in the steel material of the present invention. Furthermore, carbonitrides and the like are formed to contribute to the improvement of the creep strength of the steel material. When the addition is less than 0.001%, the effect does not become obvious. On the other hand, when the addition exceeds 0.07%, coarse nitrides or carbonitrides are formed, and the toughness of the steel material is reduced, or creep. Since the strength at break may be reduced, the addition range is set to 0.001 to 0.07%.

  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.

  In addition to the above elements, the steel material of the present invention may inevitably contain Al and O in a normal range that does not impair the characteristics of the steel of the present invention and the properties of the welded portion.

  When low Cr steel is used in a relatively low temperature region of 500 ° C. or lower, Al is sometimes added as a deoxidizing element with emphasis on toughness. In this case, it is possible to add Al up to 0.05%.

  On the other hand, although it is difficult in terms of steelmaking technology, it is usually preferable to suppress Al to 0.005% or less from the viewpoint of creep rupture strength. However, due to steelmaking technology limitations, the lower limit of Al is set to 0. 0.001%.

  O is a component premised on the addition of Cr, and is basically not added, but may be added in order to prevent wear of the refractory within a range of 0.01% or less of the amount of impurities. In this case, O produces | generates a micro oxide, exists in steel materials, and exhibits the effect which suppresses that a crystal grain grows abnormally at the time of a solution treatment.

  The steel of the present invention may contain other elements other than those described above for further improving the creep characteristics or for improving other characteristics.

  The steel of the present invention may contain 0.0003 to 0.005% B in order to improve the hardenability of the steel.

In order to increase the high temperature strength by utilizing the precipitation strengthening mechanism by precipitation of intermetallic compounds or precipitation of M ₂ C, and MC type carbide or carbonitride, the steel of the present invention has a Mo: 0.05-2. You may contain 1 type, or 2 or more types in 0%, W: 0.05-3.0%, Re: 0.05-2.0%.

  In addition, the steel material of the present invention has a large effect on the creep strength as compared with the case where a large amount of the above-mentioned ferrite stabilizing element is added, and as a result, the phase stability is lowered and δ ferrite is generated and the creep strength is impaired. , Ni: 0.01 to 0.5%, Co: 0.01 to 3.0%, Cu: 0.01 to 1.5%, within a range not significantly reducing the transformation point, one or more It may contain.

  Furthermore, in order to form the intragranular precipitation type carbide | carbonized_material or carbonitride required for this invention steel materials to support a high temperature creep characteristic over a long time, Ti: 0.005-0.20%, Zr: 0.002- You may contain 1 type, or 2 or more types in 0.10%, Nb: 0.005-0.50%, V: 0.01-1.0%.

  In addition, the steel of the present invention is a coarse sulfide, specifically, to prevent MnS from coarsely precipitating in the segregation part, and to fix S that segregates at the grain boundary and decreases the creep rupture strength. Ca: 0.0003 to 0.005%, Mg: 0.0003 to 0.01%, La: 0.005 to 0.05%, Ce: 0.005 to 0.10%, Y: 0.005 You may contain 1 type, or 2 or more types in 0.10% and Ba: 0.0003-0.005%.

  In the steel of the present invention, in addition to the composition of each element, the weld heat affected zone hardenability index HDI defined by the above formula (1) is limited to 0.5-80. As described below, the limitation of HDI is the most important in suppressing the occurrence of Type IV damage in the weld heat affected zone.

If the weld heat affected zone hardenability index HDI is within the required range, even if the average old γ grain size is reduced to about 5 μm at the outer edge of the weld heat affected zone, that is, the Ac ₁ transformation is caused by welding heat. Even if there is a fine grain structure formed by exposure to a temperature in the range of Ac ₁ transformation point + 300 ° C. above the above point, the internal dislocation density remains high, which has been a problem in the past. The “dislocation density drop in the region” can be directly prevented.

  If it is possible to prevent the occurrence of Type IV damage by preventing “decrease in dislocation density in the fine-grained structure region” in the weld heat-affected zone, the fact that HDI is limited to the required range is This is an extremely important requirement for the invention steel.

  HDI is an index for evaluating the hardenability of steel, in particular, the hardenability of the weld heat affected zone. Since this greatly affects the introduction of dislocations, an index for comprehensively evaluating the hardenability of individual elements is newly defined by the above formula (1). In the steel of the present invention, the value is 0.5-80. Regulated. This is the first feature of the present invention.

  The reason why the HDI is regulated to 0.5 to 80 is as follows.

  In the steel of the present invention, if the HDI is less than 0.5, the hardenability of the weld heat affected zone remains low, and the portion once heated depends on the composition and cooling rate of the weld heat affected zone. When transforming again, the ability to transform at low temperatures is lacking. As a result, the structure of the weld heat-affected zone is unlikely to be bainite or martensite, and part of it becomes a ferrite structure, so the high-temperature creep strength of the weld heat-affected zone is significantly reduced, and the typical Type IV type Damage will occur.

  Therefore, the weld heat affected zone hardenability index HDI has a minimum required value of 0.5 as a lower limit. In addition, in order to ensure reliably the ability to carry out low temperature transformation again in a welding heat affected zone, 3.0 or more are preferable and HDI or more is more preferable.

  On the other hand, when the steel material of the present invention has a strong hardenability exceeding HDI80, the strength of the weld heat-affected zone becomes extremely high, and as a result, hot cracking due to welding residual stress may occur, and tempering may occur. In such a case, temper embrittlement becomes remarkable, and in some cases, temper cracking may occur. Therefore, the upper limit of HDI is 80, but 65 or less is preferable in order to eliminate the possibility of the occurrence of cracks.

  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 present steel material of the above chemical composition and the above HDI, even if a low temperature transformation structure of fine grains is formed after cooling due to the influence of welding heat input of 1 kJ / mm or more, the low temperature transformation structure of the above dislocation density is generated. Generates and suppresses the occurrence of Type IV damage in the heat affected zone due to the dislocation behavior.

This point is the second feature of the steel of the present invention. However, the lower limit of the dislocation density differs at a boundary of 3% Cr (in the case of Cr: 0.4 to 3.0%: 1 × 10 ¹² pieces / m ² or more, Cr: more than 3.0% to 12.0%: 1 × 10 ¹³ pieces / m ² or more) The reason is as follows.

  The form of the alloy phase diagram changes depending on the amount of Cr. When the Cr content is small, a γ loop type phase diagram having a form close to that of an iron-carbon phase diagram is formed, and when the Cr content is large, a typical γ loop type phase diagram is formed. Therefore, the degree of influence of the third element also changes according to the Cr amount and the Cr amount. As a result, the transformation point changes, and naturally the hardenability changes.

  The change in hardenability changes the low temperature transformation structure itself, and when the Cr content is small, it becomes a bainite-based structure, and when the Cr content is large, it becomes a martensite-based structure. Therefore, the dislocation density of the dislocations to be introduced must be changed depending on the low temperature transformation structure, and naturally, it should be changed according to the amount of Cr.

That is, in the case of Cr: 0.4 to 3.0%, a low temperature transformation structure having a dislocation density of 1 × 10 ¹² pieces / m ² or more is formed, and Cr: more than 3.0% to 12.0% In this case, if a low temperature transformation structure having a dislocation density of 1 × 10 ¹³ pieces / m ² or more is formed, the dislocation density in the weld heat affected zone is not lowered as compared with the dislocation density of the base steel material.

In this specification, the dislocation density is displayed in "pieces / m ^2", even if you do not display the units, units of the dislocation density is "number / m ^2".

  The low temperature transformation structure is bainite and / or martensite, and the weld heat affected zone is etched with a corrosive liquid (nitral, picric acid, nitric acid, aqua regia, etc.) and observed with an optical microscope. Generation can be confirmed.

At the present invention steels, the welding heat affected zone of the steel is heated to Ac ₁ transformation point to Ac ₁ transformation point + 300 ° C., in advance, the 100μm or more low-temperature transformation structure with a grain size of old austenite is an average equivalent-sphere diameter formed If so, it is possible to further suppress the refinement of the low-temperature transformation structure after welding and more stably prevent the occurrence of Type IV damage in the heat affected zone.

  The prior austenite grain size was confirmed by observing the low temperature transformation structure with an optical microscope at least 10 locations at a magnification of 100 times.

  Next, the production of the steel of the present invention and the confirmation of the welded portion characteristics will be described.

  The steel of the chemical composition specified by the steel material of the present invention is melted by using an ordinary 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 slab is hot-rolled into a steel plate, and further, the steel plate is processed and molded into a steel pipe, or the slab is forged into a forged member, ie, the required tempering heat treatment, A structure containing substantially 80% or more of tempered bainite and / or tempered martensite is formed by performing quenching-tempering treatment or tempering-tempering treatment.

  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. In addition, the HDI defined by the above formula (1) is It is regulated to 0.5-80.

Therefore, in the present invention steel, even if re-heated sites Ac ₁ ~Ac ₁ + 300 ℃ in welding heat, also reduced the old γ grain size is to a minimum 5μm about an average value measured by light microscopy Even in the welded part, the structure of the part undergoes low-temperature transformation during cooling and becomes bainite (Cr: 0.4 to 3%) or martensite (Cr: more than 3 to 12%). Does not occur.

  The present inventor confirmed this by etching the cross-sectional structure of the above site with a picric acid saturated alcohol solution and observing the structure at a magnification of 100 times using an optical microscope.

  The presence or absence of Type IV type damage is difficult to determine by observation of the structure with an optical microscope when the old γ particle size is small. At the same time, the observation of the structure is performed at a magnification of 10,000 times using a transmission electron microscope. It was confirmed that the structure was healthy bainite or martensite even after heat treatment after welding.

Then, using the X-ray diffraction peak height determination method, the dislocation density in the above part was measured. In bainite (Cr: 0.4 to 3%) and martensite (Cr: more than 3 to 12%), respectively. It was confirmed that it was 1 × 10 ¹² or more and 1 × 10 ¹³ or more.

  Here, as a method for determining the height of the X-ray diffraction peak used for measuring the dislocation density, specifically, a method for determining and evaluating from the half-value width of the X-ray diffraction peak (see Reference Document 1 below) is used. It was.

The specimen material was cut into 10 mm × 10 mm × 2 mm, the main surface was mirror-polished, and then the mirror-polished surface was cut by 50 μm or more by chemical polishing or electrolytic polishing. This sample was placed in an X-ray diffractometer, and Cr—K _α characteristic X-ray or Cu—K _α characteristic X-ray was incident on the main polishing surface, and α-Fe (110 ), (211), and (220) plane diffraction lines were measured.

The Cr-K _α characteristic X-ray and the Cu-K _α characteristic X-ray are respectively composed of adjacent K _α1 line and K _α2 line. For this reason, the half-width of the K _α1 line diffraction peak was evaluated by subtracting the adjacent K _α2 line diffraction peak height from the diffraction peak of each crystal plane by the method of Rachinger (see Reference 2 below). .

  Since the half width of the diffraction peak is proportional to the average strain ε in the crystal, ε was determined from the half width by the Williamson-Hall method (see Reference 3 below).

Furthermore, from this ε, the equation (10) described in Reference Document 1 is: ρ = 14.4ε ² / b ²
Was used to determine the dislocation density ρ (pieces / m ² ). b is the magnitude of the Burgers vector (= 0.248 × 10 ⁻⁹ m).

Reference 1: Koichi Nakajima et al., “Method for evaluating dislocation density using X-ray diffraction” (Materials and Processes, Japan Iron and Steel Institute, Vol. 17 (2004), No. 3), p. 396-399
Reference 2: Guinier, A., translated by Kazutake Takara et al., “Theory and Actual Revision 3rd Edition of X-ray Crystallography” (Rigaku Denki, (1967), p. 406)
Reference 3: GKWilliamson and WHHall, “Acta Metall., 1 (1953)”, p. twenty two

  In this way, it was confirmed that the structure of the weld heat-affected zone was a structure in which Type IV damage was unlikely to occur within the range of the chemical composition of the steel material of the present invention.

  Subsequently, from the welded portion including the above structure, a creep rupture test piece having a diameter of 6 mmφ, a distance between scores and a parallel portion of 30 mm was taken along a direction perpendicular to the weld line, and a creep rupture test was performed. A temperature-accelerated creep test is performed in a temperature range from the assumed temperature of the steel material to a temperature higher by up to 100 ° C. Based on the test results, the creep rupture strength estimated for 100,000 hours is calculated using Larson- Obtained by Miller-Parameter method.

  The steel of the present invention prescribes the chemical composition and the HDI value, and regulates the dislocation density of the weld heat affected zone to suppress the occurrence of Type IV damage. The hardenability changes depending on the particle size.

  In particular, in the welding heat-affected zone where the prior austenite particle size (old γ particle size) continuously changes greatly, the influence of the old γ particle size is significant, and the old γ particle size is originally several tens of μm. Or whether the particle size exceeds 100 μm affects the old γ particle size of the weld heat-affected zone generated by subsequent welding.

  The reason for this is that when the welding heat input is relatively large, the time that the weld heat affected zone is exposed to the welding heat results in a time margin for grain growth in the γ grains after the α → γ transformation. It will be provided.

  Therefore, in the steel material of the present invention, it is particularly preferable that the old γ grain size of the previous structure affected by welding heat is 100 μm or more in order to surely suppress the occurrence of Type IV damage.

  If the old γ grain size of the base steel material is 100 μm or more, even if it is a steel material in a region where the HDI value is low, it is possible to demonstrate the same type IV damage suppression effect as a steel material in a region where the HDI value is high. The inventor found out the result of earnest research. This point is also one of the features of the present invention.

  In confirming the Type IV type damage suppressing effect of the steel of the present invention, the presence or absence of Type IV type damage was determined by the ratio of the creep rupture strength of the welded portion to the creep rupture strength of the base steel material.

  In the prior art, the creep rupture strength estimated for 100,000 hours of a weld that causes Type IV type damage is about 0.5 to 0.6 with respect to the creep rupture strength of the base steel, and exceeds 0.7. There is hardly anything.

  Therefore, when evaluating the creep rupture strength of the welded portion of the steel of the present invention, the threshold value of the creep rupture strength ratio in consideration of the weld state and weld metal strength, joint shape and soundness, which are disturbance factors, It was set to “0.8” exceeding “0.7”.

  Here, FIG. 2 shows the creep rupture strength ratio of the base steel, the weld of the steel of the present invention, and the weld of the conventional steel obtained based on the creep rupture strength estimated for 100,000 hours. FIG. 2 shows the creep rupture strength ratio of the welded portion of 2.25% Cr-1% Mo steel material (the steel material of the present invention of Cr 3.0% or less) and 9% Cr-1% Mo steel material (Cr over 3.0%). The creep rupture strength ratio of the present invention steel).

2. In the case of 2.25% Cr-1% Mo steel (the steel of the present invention of Cr 3.0% or less), the dislocation density at the weld heat affected zone heated from Ac ₁ transformation point to Ac ₁ transformation point + 300 ° C. is the creep test. Previously, it was 3.2 × 10 ¹² pieces / m ² , and in the case of 9% Cr-1% Mo steel material (the steel material of the present invention exceeding Cr 3.0%), it was 7.8 × 10 ¹⁴ pieces / m ^2. It was.

In the case of the conventional steel material, the dislocation density in the above part is only 9.8 × 10 ⁹ pieces / m ² before the creep test, and there is a significant difference in the dislocation density between the steel material of the present invention and the conventional steel material. Is recognized.

3, after the dislocation density (after welding heat treatment in the weld heat affected zone which is heated to Ac ₁ transformation point to Ac ₁ transformation point + 300 ° C., prior to the creep test was measured by the X-ray diffraction peak height determination method ) And the creep rupture strength ratio comparing the estimated creep rupture strength of the base steel and the welded portion for 100,000 hours are shown for various steel materials. In FIG. 3, the steel material is divided into a steel material of Cr 3.0% or less (black circle) and a steel material of Cr 3.0% or more (white circle).

From FIG. 3, in the steel material of Cr 3.0% or less, unless the dislocation density 1 × 10 ¹² is maintained, the creep rupture strength ratio does not exceed the threshold value 0.80 set in the present invention, and the Cr content exceeds 3%. It is understood that the creep rupture strength ratio does not exceed the threshold value 0.80 unless the dislocation density of 1 × 10 ¹³ is maintained in the steel material.

Furthermore, FIG. 4 shows the old γ grain size before welding in the welded portion of 9% Cr steel in which Type IV type damage did not occur, and the Ac ₁ transformation point to Ac ₁ transformation point +300 after welding and before the creep test. The relationship with the dislocation density of the welding heat affected zone heated to ° C is shown. FIG. 4 clearly shows that when the old γ grain size before welding is 100 μm or more, the dislocation density in the weld heat affected zone after welding is increased.

  The heat-resistant structure of the present invention is obtained by welding the steel of the present invention by a normal welding method, but the steel of the present invention is a ferritic heat-resistant steel having excellent creep characteristics in the weld heat affected zone. The body itself is a heat resistant structure excellent in the creep characteristics of the weld heat affected zone.

  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)
The steels of the present invention (invention examples) and comparative steels (comparative examples) having the chemical components and HDI values shown in Table 1 were produced by the above-described production methods, and the properties and structure of the welded parts were examined by the above-described test methods for these steel materials. investigated. The results are shown in Table 2.

In the comparative example No. 31, although the chemical component satisfies the chemical component of the steel of the present invention, the HDI value is lower than 0.5 (the lower limit of the steel of the present invention), and the Ac ₁ transformation point to Ac ₁ transformation point + 300 ° C. The dislocation density of the heated weld heat affected zone does not reach 1 × 10 ¹² (the lower limit in the steel of the present invention of Cr 3% or less), and type IV type damage occurs in the weld heat affected zone, and the base steel and weld zone This is an example in which the creep rupture strength ratio estimated for 100,000 hours (hereinafter simply referred to as “creep rupture strength ratio”) is significantly reduced.

  In Comparative Example No. 32, the amount of Mn was excessive (see Table 1), the HDI value was too high, and weld cracking occurred and broke from the boundary between the weld metal and the base steel. This is an example of a significant decrease.

  The No. 33 comparative example has an excessive Cr amount, the No. 34 comparative example has an excessive Mo amount, and the No. 35 comparative example has an excessive W amount (see Table 1). Similarly, this is an example in which temper cracking occurred near the boundary between the weld metal and the base metal, and the creep rupture strength ratio was significantly reduced.

In the comparative example No. 36, although the chemical component satisfies the chemical component of the steel of the present invention, the HDI value is higher than 80 (the upper limit of the steel of the present invention), and it is heated from Ac ₁ transformation point to Ac ₁ transformation point + 300 ° C. The dislocation density at the weld heat affected zone is sufficient and exceeds 1 × 10 ¹³ pieces / m ² (the lower limit of the steel of the present invention), but temper cracks occur near the boundary between the weld metal and the base steel, and the creep rupture strength This is an example in which the ratio is significantly reduced.

  Compared with the comparative example, in the inventive example, a creep rupture strength ratio exceeding the threshold value “0.8” set exceeding the conventional value is obtained.

  As described above, according to the present invention, the Type IV type damage does not occur in the heat-affected zone of the welded portion of the ferritic heat-resistant steel material. In designing, the high-temperature strength can be designed as 0.67 times the normal creep rupture strength (normal safety factor). 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 | produced in the welding heat affected zone of ferritic heat resistant steel. It is a figure which shows the creep rupture strength of the welded part of this invention steel material, and the conventional steel material estimated for 100,000 hours by ratio with respect to the creep rupture strength of a base steel material. It is a figure which shows the relationship between the dislocation density of the welding heat affected zone of this invention steel material, and the creep rupture strength ratio of 100,000 hours estimation of a welding part. It is a figure which shows the relationship between the old (gamma) particle size (average value) of the steel materials before welding, and the dislocation density of the welding heat affected zone of the welding part with which an HDI value satisfy | fills the range of this invention.

Claims (10)

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 %: Cr: 0.4-12.0%, Al: 0.001-0.05%, N: 0.001-0.07%, O: limited to 0.01% or less, The weld heat affected zone hardenability index HDI, which consists of the remaining Fe and inevitable impurities and is defined by the following formula (1), is a ferritic heat resistant steel material of 0.5 to 80,
Ac ₁ to weld heat affected zone of the steel is heated to a transformation point to Ac ₁ transformation point + 300 ° C., the dislocation density of 1 × 10 ¹² pieces / m ² or more (Cr: For 0.4 to 3.0%), Alternatively, a ferrite system having excellent creep characteristics in the heat affected zone of welding, wherein a low temperature transformation structure of 1 × 10 ¹³ pieces / m ² or more (in the case of Cr: more than 3.0 to 12.0%) is generated. Heat resistant steel.
HDI = √ [% C] √ [% N] (1 + 0.5 [% Si]) (1 + 3 [% Mn]) (1 + 2 [% Cr]) (1 + 3 [% Mo]) (1 + 0.8 [% W] ) (1 + 0.3 [% Cu]) (1 + 0.5 [% Ni]) (1 + 2.5 [% Nb]) (1 + 1.5 [% V]) (1 + 0.5 [% Al]) (1 + 0.3 [% Ti]) (1 + 0.3 [% Zr]) (1 + 0.2 [% Re]) (1 + 25 [% B]) (1 + 0.5 [% Co]) (1)   The ferritic heat resistant steel material having excellent creep characteristics of the weld heat affected zone according to claim 1, wherein the ferritic heat resistant steel material further contains B: 0.0003 to 0.005% by mass. Steel material.   The ferritic heat-resisting steel material is further, in mass%, Mo: 0.05 to 2.0%, W: 0.05 to 3.0%, Re: 0.05 to 2.0%, or The ferritic heat resistant steel material excellent in creep characteristics of the weld heat affected zone according to claim 1 or 2, wherein the ferritic heat resistant steel material contains two or more kinds.   The ferritic heat-resisting steel material is further, in mass%, Ni: 0.01 to 0.5%, Co: 0.01 to 3.0%, Cu: 0.01 to 1.5%, The ferritic heat resistant steel material excellent in creep characteristics of the weld heat affected zone according to any one of claims 1 to 3, comprising two or more kinds.   Further, the ferritic heat resistant steel material is, in mass%, Ti: 0.005 to 0.20%, Zr: 0.002 to 0.10%, Nb: 0.005 to 0.50%, V: 0.00. The ferritic heat resistant steel material having excellent creep characteristics of the weld heat affected zone according to any one of claims 1 to 4, wherein the ferritic heat resistant steel material contains at least one of 01 to 1.0%.   Further, the ferritic heat resistant steel material is, in mass%, Ca: 0.0003 to 0.005%, Mg: 0.0003 to 0.01%, La: 0.005 to 0.05%, Ce: 0.00. It contains one or more of 005 to 0.10%, Y: 0.005 to 0.10%, and Ba: 0.0003 to 0.005%. A ferritic heat resistant steel material excellent in creep characteristics of the weld heat affected zone according to any one of the items. The welding heat affected zone of the steel is heated to the Ac ₁ transformation point to Ac ₁ transformation point + 300 ° C., in advance, that the particle size of prior austenite is 100μm or more low-temperature transformation structure at an average equivalent-sphere diameter is formed The ferritic heat resistant steel material excellent in creep characteristics of the weld heat affected zone according to any one of claims 1 to 6.   The ferritic heat resistant steel material excellent in creep characteristics of the weld heat affected zone according to any one of claims 1 to 7, wherein the low temperature transformation structure is bainite and / or martensite.   The ferritic heat resistant steel material having excellent creep characteristics of the weld heat affected zone according to any one of claims 1 to 8, wherein the weld heat affected zone hardenability index HDI is 3.0 to 65. .   A heat-resistant structure excellent in creep characteristics of a weld heat-affected zone, manufactured by welding the ferritic heat-resistant steel material excellent in creep characteristics of the weld heat-affected zone according to any one of claims 1 to 9. body.