[0001]The present disclosure relates to ferritic heat resistant steels.
Background technology
[0002]
　Ferritic heat-resistant steel is not only cheaper than austenitic heat-resistant steel and Ni-based heat-resistant steel, but also has the advantage of being a heat-resistant steel used at high temperatures because it has a small coefficient of thermal expansion. Widely used for equipment used at high temperatures.
[0003]
　In recent years, in coal-fired power generation, high temperature and high pressure steam conditions have been promoted in order to improve thermal efficiency, and in the future, operations under ultra-supercritical pressure conditions of 650 ° C and 350 atm are planned. In order to cope with such severe steam conditions, many ferrite-based heat-resistant steels having increased creep strength by positively utilizing W and B have been proposed.
[0004]
　For example, Patent Document 1 describes in terms of mass%, C: 0.001 to 0.15%, Cr: 8 to 13%, V: 0.2 to 0.5%, Nb: 0.002% to 0. It contains 2%, W: 2-5%, N: 0.001-0.03%, B: 0.0001-0.01%, the metal structure consists of tempered martensite base, and inside the martensite lath. A high-Cr ferritic heat-resistant steel material having excellent high-temperature and long-term creep strength, in which a total of 0.4 pieces / μm 3 or more of M 23 C 6 having a particle size of 0.6 μm or less and an intermetallic compound are deposited, is disclosed. ..
[0005]
　Patent Document 2 describes, in terms of mass%, C: 0.05 to 0.15%, Cr: 8 to 15%, V: 0.05 to 0.5%, Nb: 0.002 to 0.18%, W: 0.1 to 5%, B: 0.0001 to 0.02%, N: 0.0005 to 0.1%, and Nd content determined from S, P, Ca and Mg contents. Disclosed are high Cr ferritic heat resistant steels that are excellent in creep strength and creep ductility at high temperatures.
[0006]
　In Patent Document 3, in mass%, C: 0.01 to 0.13%, Cr: 8.0 to 12.0%, W: 1.0 to 4.0%, Co: 1.0 to 5 .0%, V: 0.1 to 0.5%, Nb: 0.01 to 0.10%, B: 0.002 to 0.02%, N: 0.005 to 0.020%, Nd: Among the MX precipitates containing 0.005 to 0.050% and existing in the crystal grains, those having a particle size of 20 nm or more but an average inter-particle distance λ of 20 nm or more and 100 nm or less, creep strength at high temperature. High Cr ferrite ferritic heat resistant steels with excellent properties are disclosed.
[0007]
　Further, Patent Document 4 contains B: 0.003 to 0.03% in weight%, other alloying elements C: 0.03 to 0.15%, Cr: 8.0 to 13.0%. , Mo + W / 2: 0.1 to 2.0%, V: 0.05 to 0.5%, N: 0.06% or less, Nb: 0.01 to 0.2%, (Ta + Ti + Hf + Zr): 0. A welded joint having excellent creep strength, which is made of tempered martensite-based heat-resistant steel containing any one or more of 01 to 0.2%, is disclosed.
[0008]
　In addition, Patent Document 5 describes in terms of mass%, C: 0.01 to 0.18%, Cr: 8 to 14%, V: 0.05 to 1.8%, Mo: 0.01 to 2. By containing 5%, W: 0.02 to 5% and N: 0.001 to 0.1%, and setting the solid solubility amount Vs% of V in the matrix to Vs> 0.01 / (C + N). Manufacture of high Cr ferrite heat resistant steel that achieves both long-term creep strength and room temperature toughness, and high Cr ferrite heat resistant steel that is subjected to normalizing and tempering determined by the C and V contents to obtain it. The method is disclosed.
[0009]
　Further, in Patent Document 6, in terms of mass%, C: 0.05 to 0.12%, Cr: 8.0 to less than 12%, V: 0.15 to 0.25%, Nb: 0.03 to Contains 1 or 2 of 0.08%, N: 0.005 to 0.07%, Mo: 0.1 to 1.1% and W: 1.5 to 3.5%, and is processed. A method for producing a high Cr ferritic heat-resistant steel material that defines conditions in the process is disclosed.
[0010]
　Patent Document 7 contains C: less than 0.05% by mass, N: 0.055% by mass or less, Si: more than 0.05% by mass, 0.50% by mass or less, and further, Mn: 2. 20% by mass or less, Ni: 1.00% by mass or less, Cr: 10.50% by mass or less, Mo: 1.20% by mass or less, V: 0.45% by mass or less, Nb: 0.080% by mass or less, Welding containing one or more selected from the group of W: 2.0% by mass or less, Co: 3.0% by mass or less, and B: 0.005% by mass or less, the balance being Fe and unavoidable impurities. Wire and CaF 2 : 2 to 30% by mass, CaO: 2 to 20% by mass, MgO: 20 to 40% by mass, Al 2O 3 : 5 to 25% by mass, total of Si and SiO 2 : 5 to 25% by mass High Cr system containing % (SiO 2 equivalent) and used in combination with a welding flux regulated to BaO: 25% by mass or less, ZrO 2 : 10% by mass or less, and TiO 2 : less than 5% by mass. A single submerged arc welding method of CSEF (Creep Strength-Enhanced Ferritic) is disclosed.
[0011]
　Patent Document 8 describes in terms of mass%, C: 0.06 to 0.10%, Si: 0.1 to 0.4%, Mn: 0.3 to 0.7%, P: 0.01% or less. , S: 0.003% or less, Co: 2.6 to 3.4%, Ni: 0.01 to 1.10%, Cr: 8.5 to 9.5%, W: 2.5 to 3. 5%, Mo: less than 0.01%, Nb: 0.02 to 0.08%, V: 0.1 to 0.3%, Ta: 0.02 to 0.08%, B: 0.007 to 0.015%, N: 0.005 to 0.020%, Al: 0.03% or less, O: 0.02% or less, Cu: 0-1%, Ti: 0 to 0.3%, Ca: Ferrite containing 0 to 0.05%, Mg: 0 to 0.05%, and rare earth elements: 0 to 0.1%, the balance consisting of Fe and impurities, and having a chemical composition satisfying the formula (1). Welding materials for heat-resistant steel are disclosed.
0.5 ≦ Cr + 6Si + 1.5W + 11V + 5Nb + 10B-40C-30N-4Ni-2Co-2Mn ≦ 10.0 (1)
Here, each element symbol in the formula (1) has the content (mass%) of the corresponding element. Substituted.
Prior art literature
Patent documents
[0012]
　　Patent Document 1: Japanese Patent Application Laid-Open No. 2002-241903
　　Patent Document 2: Japanese Patent Application Laid-Open No. 2002-363709
　　Patent Document 3: Japanese Patent Application Laid-Open No.
　　2016-216815 Patent Document 4: Japanese Patent Application Laid-Open No. 2004-300532
　　Patent Document 5: Japanese Patent Application Laid -Open No. Japanese Patent Application Laid-Open No. 2001-192781 Patent
　　Document 6: Japanese Patent Application Laid-Open No. 2009-2933063
　　Patent Document 7: Japanese Patent Application Laid-Open No. 2016-22501
　　Patent Document 8: International Publication No. 2017/104815
Outline of the invention
Problems to be solved by the invention
[0013]
　By the way, ferritic heat-resistant steel is used not only at high temperature but also at high temperature (for example, during operation of the boiler in ferritic heat-resistant steel used for power generation boiler). In order to ensure the soundness of the structure before (for example, the assembly process until the start of operation of the boiler), it is required to have mechanical performance, that is, sufficient tensile characteristics and impact characteristics (toughness). Although the above-mentioned ferritic heat-resistant steel has excellent creep strength, it has been found that these mechanical performances may not be stably obtained. In particular, when the ferrite heat-resistant steel contains a large amount of W and 0.006% or more of B for the purpose of obtaining higher creep strength, sufficient tensile strength and toughness may not be obtained. ..
[0014]
　The present disclosure has been made in view of the above situation, and an object of the present invention is to provide a ferritic heat-resistant steel containing a large amount of W and B and having high tensile strength and toughness.
Means to solve problems
[0015]
　In order to solve the above-mentioned problems, the present inventors have conducted a detailed investigation on ferritic heat-resistant steels containing 2.5% to 3.5% W and 0.006% to 0.016% B. gone. As a result, the following findings were clarified.
[0016]
　As a result of comparative investigation of steels having differences in tensile strength and toughness, the steels in which sufficient performance in the tensile strength and toughness was obtained were M 23 C6 type carbides containing W at the grain boundaries and in the grains. It was finely and densely dispersed. On the other hand, in steels containing a small amount of M 23 C6 type carbides containing W , or conversely, steels in which a large amount of carbides are deposited, or steels in which the precipitates are coarse and sparsely deposited, The performance was unfavorable.
[0017]
　From this, it was inferred that the mechanical properties such as tensile strength and toughness were not stable as described in (1) and (2) below. (1) W in the steel is either solid - dissolved in the steel or finely dispersed and precipitated as M 23 C6 type carbide
containing W in the tempering heat treatment during the production of ferritic heat-resistant steel, which contributes to the tensile strength. However, when the precipitation amount of M 23 C6 type carbide containing W is small and sparsely precipitated, the strengthening effect due to the precipitation is not sufficient, and the required tensile strength cannot be obtained. On the contrary, if the M 23 C6 type carbide containing W is coarsely deposited, it does not contribute to the strengthening of the tensile strength, and the strengthening effect of W by solid solution to steel is also reduced, so that the required tensile strength is obtained. I can't get it.
[0018]
(2) Further, W is precipitated as M 23 C 6 type carbide in the tempering heat treatment, so that the structure is restored and softened. However, when the precipitation amount of M 23 C6 type carbide containing W is small, the effect of advancing the recovery and softening is small, so that sufficient toughness cannot be obtained. On the contrary, when the M 23 C6 type carbide containing W is coarsely deposited, the starting point of fracture increases, so that the required toughness cannot be obtained.
[0019]
　Therefore, as a result of repeated studies, by controlling the amount of W analyzed as the electrolytic extraction residue within a predetermined range according to the amount of B contained in the steel, stable performance in terms of tensile strength and toughness can be obtained. It was found that it was possible.
[0020]
　The reason for this is considered to be the following (3) and (4). (3) As described above, W precipitates as M 23 C 6
type carbide in the tempering heat treatment and contributes to the improvement of tensile strength. B has the effect of substituting C for the M 23 C6 type carbide and dissolving it in the carbide, and finely and densely dispersing the carbide without affecting the amount of precipitation. Therefore, the strengthening effect due to the precipitation of carbides can be obtained, and the required tensile strength can be easily obtained even with a small amount of precipitates of carbides. On the contrary, even when a large amount of carbide is deposited, the size of the carbide becomes small and the strengthening effect due to the precipitation of the carbide is maintained, so that sufficient tensile strength can be easily obtained.
[0021]
(4) In addition, B has the effect of reducing the size of the precipitate without affecting the amount of the carbide precipitated. As a result, the tissue is recovered and softened, and the starting point of destruction is less likely to be obtained, so that the required toughness can be easily obtained.
[0022]
　Furthermore, the present inventors conducted a detailed investigation on the effects of Ta and Nb on the high-temperature strength, that is, the creep strength of the above-mentioned ferritic heat-resistant steel containing W and B. As a result, the findings of (5) and (6) described below were clarified.
[0023]
(5) As a result of comparing the steels having a difference in creep strength at the initial stage of use at high temperature, the steel having excellent strength has a large amount of fine carbonitride containing Ta and Nb, and is further contained in the carbonitride. The proportion of Ta was low.
[0024]
(6) As a result of comparing the steels having different creep strengths when used for a long time, the fine carbonitrides containing Ta and Nb were finely and densely formed in the steels having excellent strength. Were present. Furthermore, the proportion of Nb contained in the carbonitride was small.
[0025]
　From the above results, the mechanism of action of Ta and Nb on the high temperature strength (particularly the high temperature strength at the initial stage of use and the high temperature strength at the time of long-term use) was inferred as described in (7) and (8) below.
(7) Ta and Nb are finely deposited as carbonitride in the heat treatment process during steel production and in the initial stage of use, which contributes to strength, but Ta, which has a slow diffusion rate, is more abundant in steel than Nb. If it is contained, the start of precipitation is delayed and a sufficient amount of precipitation cannot be obtained. As a result, the high temperature strength at the initial stage of use decreases.
[0026]
(8) On the other hand, the carbonitride containing these Ta and Nb contributes to the creep strength during long-term use, but when Nb having a high diffusion rate is contained in a large amount in the steel with respect to Ta, the carbonitride Growth becomes faster and becomes coarser at an early stage. As a result, it was speculated that the strengthening effect of these precipitates disappeared at an early stage, so that the creep strength at high temperature during long-term use decreased.
[0027]
　Therefore, as a result of repeated studies, the following findings (9) were obtained.
(9) By controlling the ratio of Ta and Nb contained in steel within an appropriate range, it is possible to secure the amount of fine carbonitride at the initial stage of use and delay the coarsening of carbonitride during long-term use. It is possible to obtain more stable high-temperature strength at the initial stage of use at high temperature and during long-term use.
[0028]
　The present disclosure has been completed based on the above findings, and the gist thereof is the ferrite heat-resistant steel shown below.
[0029]
<1>
　By mass%,
C: 0.06% to 0.11%,
Si: 0.15% to 0.35%,
Mn: 0.35% to 0.65%,
P: 0.020% or less,
S: 0.0030% or less,
Ni: 0.005% to 0.250%,
Cu: 0.005% to 0.250%,
Co: 2.7% to 3.3%,
Cr: 8.3% ~ 9.7%,
W: 2.5% ~ 3.5%,
V: 0.15% ~ 0.25%,
Nb: 0.030% ~ 0.080%,
Ta: 0.002% ~ 0 .040%,
Nd: 0.010% to 0.060%,
B: 0.006% to 0.016%,
N: 0.005% to 0.015%,
Al: 0.020% or less, and
O : Ferrite-based heat-resistant steel
containing 0.020% or less , the
　balance consisting of Fe and impurities,
　and the amount of W analyzed as an electrolytic extraction residue satisfies the following formula (1).
　-10 x [% B] +0.26≤ [% W] ER≦ 10 × [% B] +0.54 (1) In the formula
　(1), [% W] ER represents the amount (mass%) of W analyzed as the electrolytic extraction residue, and [% B] is. It represents the content (mass%) of B in the ferritic heat-resistant steel.
<2> 　The total content of the Ta and the Nb is 0.040% to 0.110% in
　mass%, 　and the ratio Ta / Nb of the Ta content to the Nb content is 0.10 to 0.10%. The ferritic heat-resistant steel according to <1>, which is 0.70. <3> 　The ferrite heat resistant steel according to <1> or <2>, wherein the ferritic heat resistant steel contains at least one element selected from the following group in mass% instead of a part of the Fe. steel. 　Group Mo: 0.50% or less 　　　Ti: 0.20% or less 　　　Ca: 0.015% or less 　　　Mg: 0.015% or less 　　　Sn: 0.005% or less <4> 　JIS Z2241: 2011 at room temperature The ferritic heat-resistant steel according to any one of <1> to <3>, which has a tensile strength of 620 MPa or more and a full-size charmy absorption energy of 27 J or more specified in JIS Z2242: 2005 at 20 ° C.

The invention's effect
[0030]
　According to the present disclosure, it is possible to stably obtain excellent tensile strength and toughness in a ferritic heat-resistant steel containing a large amount of W and B.
Embodiment for carrying out the invention
[0031]
　In the present disclosure, the reasons for limiting the composition of the ferrite heat-resistant steel are as follows.
　In the following description, the "%" indication of the content of each element means "mass%". Further, in the present disclosure, the numerical range represented by using "-" means a range including the numerical values ​​before and after "-" as the lower limit value and the upper limit value unless otherwise specified.
　In the numerical range described stepwise in the present disclosure, the upper or lower limit of a stepwise numerical range may be replaced with the upper limit or lower limit of another numerical range described in a stepwise manner. , May be replaced with the values ​​shown in the examples.
　In the present disclosure, the term "process" is included in this term not only as an independent process but also as long as the intended purpose of the process is achieved even if it cannot be clearly distinguished from other processes.
[0032]
C: 0.06% to 0.11%
　C is effective for obtaining a martensite structure and produces fine carbides or carbonitrides, which contributes to the improvement of tensile strength and creep strength. Within the range of W and B contents in the present disclosure, it is necessary to contain 0.06% or more of C in order to sufficiently obtain these effects. However, if the content of C is excessive, the creep strength and toughness are deteriorated. Therefore, the content of C is set to 0.11% or less. The preferred lower limit of the C content is 0.07%, and the preferred upper limit is 0.10%. A more preferable lower limit is 0.08%, and a further preferable upper limit is 0.09%.
[0033]
Si: 0.15% to 0.35%
　Si is contained as a deoxidizing agent, but is an element effective for water vapor oxidation resistance (that is, resistance to oxidation by water vapor). In order to obtain the effect sufficiently, it is necessary to contain 0.15% or more of Si. However, excessive content of Si causes a decrease in ductility. Therefore, the Si content is set to 0.35% or less. The preferred lower limit of the Si content is 0.18%, and the preferred upper limit is 0.32%. A more preferable lower limit is 0.20%, and a further preferable upper limit is 0.30%.
[0034]
Mn: 0.35% to 0.65%
　Mn is contained as a deoxidizing agent like Si, but is also effective in obtaining a martensite structure. In order to obtain the effect sufficiently, it is necessary to contain Mn of 0.35% or more. However, if Mn is excessively contained, creep embrittlement is caused, so the Mn content is set to 0.65% or less. The preferred lower limit of the Mn content is 0.38%, and the preferred upper limit is 0.62%. A more preferable lower limit is 0.40%, and a further preferable upper limit is 0.60%.
[0035]
P: 0.020% or less
　P, if contained in excess, reduces creep ductility. Therefore, the content of P needs to be 0.020% or less. The content of P is preferably 0.018% or less, and more preferably 0.015% or less. The smaller the P content, the better. However, an extreme reduction in P content will significantly increase material costs. Therefore, the preferable lower limit of the content of P is 0.0005%, and the more preferable lower limit is 0.001%.
[0036]
S: 0.0030% or less
　S, like P, reduces creep ductility when contained in excess. Therefore, the content of S needs to be 0.0030% or less. The S content is preferably 0.0025% or less, and more preferably 0.0020% or less. The smaller the S content, the better. However, the extreme reduction of the S content extremely increases the manufacturing cost. Therefore, the desirable lower limit of the S content is 0.0002%, and the more preferable lower limit is 0.0004%.
[0037]
Ni: 0.005% to 0.250%
　Ni is an element effective for obtaining a martensite structure. In order to obtain the effect, it is necessary to contain 0.005% or more of Ni. However, in the present disclosure in which the Co content is in the following range, even if Ni is contained in excess of 0.250%, the effect is saturated and the cost is increased because it is an expensive element. Therefore, the Ni content is limited to 0.250%. The preferred lower limit of the Ni content is 0.020%, and the preferred upper limit is 0.200%. A more preferable lower limit is 0.050%, and a further preferable upper limit is 0.150%.
[0038]
Cu: 0.005% to 0.250%
　Cu, like Ni, is an effective element for obtaining a martensite structure. In order to obtain the effect, it is necessary to contain 0.005% or more of Cu. However, in the present disclosure in which the Co content is in the following range, even if the Cu content exceeds 0.250%, the effect is saturated, so the Cu content is limited to 0.250%. .. The preferred lower limit of the Cu content is 0.020%, and the preferred upper limit is 0.200%. A more preferable lower limit is 0.050%, and a further preferable upper limit is 0.150%.
[0039]
Co: 2.7% to 3.3%
　Co is an element effective for obtaining a martensite structure and ensuring creep strength. In order to obtain the effect sufficiently, it is necessary to contain 2.7% or more of Co. However, since it is a very expensive element, if it contains an excessive amount of Co, the material cost is increased and the creep strength and creep ductility are rather lowered. Therefore, the Co content is 3.3% or less. The preferred lower limit of the Co content is 2.8% and the preferred upper limit is 3.2%. A more preferable lower limit is 2.9%, and a further preferable upper limit is 3.1%.
[0040]
Cr: 8.3% to 9.7%
　Cr is an element effective for ensuring water vapor oxidation resistance and corrosion resistance at high temperatures. It also precipitates as carbide and contributes to the improvement of creep strength. In order to obtain these effects sufficiently, it is necessary to contain Cr in an amount of 8.3% or more. However, if Cr is excessively contained, the stability of the carbide is lowered and the creep strength is lowered. Therefore, the Cr content is set to 9.7% or less. The preferred lower limit of the Cr content is 8.5%, and the preferred upper limit is 9.5%. A more preferable lower limit is 8.7%, and a further preferable upper limit is 9.3%.
[0041]
W: 2.5% to 3.5%
　W dissolves in the matrix or precipitates as M23 C6 type carbide, which contributes to ensuring the tensile strength and between the metals during long-term use . It precipitates as a compound and contributes to ensuring creep strength at high temperatures. In order to obtain this effect sufficiently, it is necessary to contain W in an amount of 2.5% or more. However, even if W is excessively contained, the effect of improving creep strength is saturated, and it is an expensive element, which increases the material cost. Therefore, the W content is set to 3.5% or less. The preferred lower limit of the W content is 2.6% and the preferred upper limit is 3.3%. A more preferable lower limit is 2.8%, and a further preferable upper limit is 3.1%.
[0042]
　The W content here means the total amount of W contained in the ferritic heat-resistant steel. That is, it means the total of the amount of W solidly dissolved in the matrix and the amount of W existing as a precipitate.
　Further, in the present disclosure, in addition to satisfying the above range of W content, the amount of W existing as a precipitate, that is, the amount of W analyzed as an electrolytic extraction residue is B as described later. It is necessary to satisfy the relationship with the quantity of.
[0043]
V: 0.15% to 0.25%
　V precipitates in the grains as fine carbonitrides and contributes to the improvement of creep strength. In order to obtain the effect sufficiently, it is necessary to contain V in an amount of 0.15% or more. However, if the V content is excessive, a large amount and coarse precipitation will occur, which will lead to a decrease in creep strength and creep ductility. Therefore, the V content is set to 0.25% or less. The preferred lower limit of the V content is 0.16%, and the preferred upper limit is 0.24%. A more preferable lower limit is 0.18%, and a further preferable upper limit is 0.22%.
[0044]
Nb: 0.030% to 0.080%
　Nb precipitates in the grains as fine carbonitride and contributes to the improvement of creep strength. In order to obtain the effect sufficiently, it is necessary to contain Nb in an amount of 0.030% or more. However, if Nb is excessively contained, it precipitates in a large amount and coarsely, which in turn causes a decrease in creep strength and creep ductility. Therefore, the content of Nb is set to 0.080% or less. The preferred lower limit of the Nb content is 0.035%, and the preferred upper limit is 0.075%. A more preferable lower limit is 0.040%, and a further preferable upper limit is 0.070%.
[0045]
Ta: 0.002% to 0.040%
　Ta, like Nb, precipitates in the grains as fine carbonitrides and contributes to the improvement of creep strength. In order to obtain the effect, it is necessary to contain Ta in 0.002% or more. However, if Ta is excessively contained, it precipitates in a large amount and coarsely, which in turn causes a decrease in creep strength and creep ductility. Therefore, the Ta content is 0.040% or less. The preferred lower limit of the Ta content is 0.003%, and the preferred upper limit is 0.035%. A more preferable lower limit is 0.004%, and a further preferable upper limit is 0.030%.
[0046]
Total content of Nb and Ta: 0.040% to 0.110%
　Nb and Ta are precipitated in the grains as fine carbonitrides and contribute to the improvement of creep strength. In order to sufficiently obtain the effect, it is preferable to contain Nb and Ta in a total of 0.040% or more. However, if Nb and Ta are excessively contained, a large amount and coarse precipitation will occur, which will lead to a decrease in creep strength and creep ductility. Therefore, it is preferable that the total content of Nb and Ta is up to 0.110%. A more preferable lower limit of the total content is 0.050%, and a more preferable upper limit is 0.100%. A more preferable lower limit is 0.060%, and a further preferable upper limit is 0.090%.
[0047]
Ratio of Ta content to Nb content Ta / Nb: 0.10 to 0.70
　As described above, Nb and Ta precipitate in the grains as fine carbonitrides and contribute to the improvement of creep strength. However, when the ratio of the contents of Ta and Nb, Ta / Nb, is small, the carbonitride grows faster during long-term use, its precipitation strengthening effect disappears early, and stable creep strength during long-term use. May not be obtained sufficiently. On the other hand, when the ratio of the contents of Ta and Nb, Ta / Nb, is large, the start of precipitation of the carbonitride may be delayed at the initial stage of use, and sufficient high-temperature strength may not be obtained. Therefore, it is preferable that the ratio Ta / Nb of the contents of Ta and Nb is 0.10 to 0.70. The more preferable range of the Ta / Nb content ratio Ta / Nb is 0.12 to 0.68, and the more preferable range is 0.15 to 0.65.
[0048]
Nd: 0.010% to 0.060%
　Nd binds to S and P to remove their adverse effects and improve creep ductility. In order to obtain this effect sufficiently, it is necessary to contain 0.010% or more of Nd. However, if Nd is excessively contained, it binds to oxygen, which lowers the cleanliness and deteriorates the hot workability. Therefore, the content of Nd is set to 0.060% or less. The preferred lower limit of the Nd content is 0.015%, and the preferred upper limit is 0.055%. A more preferable lower limit is 0.020%, and a preferred upper limit is 0.050%.
[0049]
B: 0.006% to 0.016%
　B is effective for obtaining martensite structure. In addition, it has the effect of being dissolved in M 23 C 6 type carbide and finely dispersed, which contributes to ensuring tensile strength and toughness. Furthermore, it also contributes to the improvement of creep strength. In order to obtain the effect, it is necessary to contain B in an amount of 0.006% or more. However, if B is excessively contained, it flows into the weld metal during welding and the susceptibility to solidification cracking is increased. Therefore, the upper limit of the B content is set to 0.016%. The preferred lower limit of the B content is 0.007%, and the preferred upper limit is 0.014%. A more preferred lower limit is 0.008% and a preferred upper limit is 0.012%.
　In the present disclosure, as described later, it is necessary that the amount of W existing as a precipitate, that is, the amount of W analyzed as an electrolytic extraction residue, satisfies a predetermined relationship according to the content of B. ..
[0050]
N: 0.005% to 0.015%
　N binds to Nb and Ta during use at a high temperature and finely precipitates in the grain as fine carbonitride, which contributes to the improvement of creep strength. In order to obtain this effect, it is necessary to contain 0.005% or more of N. However, if N is excessively contained, the carbonitride is coarsened and the creep ductility is lowered. Therefore, the N content is set to 0.015% or less. The preferred lower limit of the N content is 0.006%, and the preferred upper limit is 0.014%. A more preferred lower limit is 0.008% and a preferred upper limit is 0.012%.
[0051]
Al: 0.020% or less
　Al is contained as a deoxidizing agent, but if a large amount of Al is contained, the cleanliness is significantly impaired and the processability is deteriorated. Further, it is not preferable to contain a large amount of Al from the viewpoint of creep strength. Therefore, the Al content is 0.020% or less. It is preferably 0.018% or less, and more preferably 0.015% or less. It is not necessary to set a lower limit of the Al content. However, since the extreme reduction of Al increases the manufacturing cost, the Al content is preferably 0.001% or more.
[0052]
O: 0.020% or less
　O is present as an impurity, but if it is contained in a large amount, the processability is deteriorated. Therefore, the content of O is 0.020% or less. It is preferably 0.015% or less, and more preferably 0.010% or less. It is not necessary to set a lower limit of the O content. However, since the extreme reduction of O increases the manufacturing cost, the content of O is preferably 0.001% or more.
[0053]
Residue:
　The ferrite-based heat-resistant steel according to the present disclosure, which is composed of Fe and impurities, contains each of the above-mentioned elements, and the balance is composed of Fe and impurities.
　In addition, "impurities" are so-called unavoidable components that are mixed by various factors in the manufacturing process, including raw materials such as ore or scrap, when steel materials are industrially manufactured, and are intentionally mixed. Refers to ingredients that are not added to.
[0054]
　Further, the ferrite heat-resistant steel according to the present disclosure may contain at least one element belonging to the following group instead of a part of the Fe in the balance. The reason for the limitation is described below.
[0055]
Group Mo: 0.50% or less
　　Ti: 0.20% or less
　　Ca: 0.015% or less
　　Mg: 0.015% or less
　　Sn: 0.005% or less
[0056]
Mo: 0.50% or less
　Mo may be contained in the same manner as W because it dissolves in the matrix and contributes to ensuring the creep strength at high temperature. However, even if Mo is contained in an excessive amount, the effect is saturated and the element is expensive, which increases the material cost. Therefore, the Mo content is set to 0.50% or less. The preferred upper limit is 0.40%, more preferably 0.20% or less. When Mo is contained, the preferable lower limit is 0.01%, and the more preferable lower limit is 0.02%.
[0057]
Ti: 0.20% or less
　Ti, like Nb and Ta, precipitates in the grains as fine carbonitrides during use at high temperatures and contributes to the improvement of creep strength, so it is contained as necessary. May be good. However, if the Ti content is excessive, it precipitates in a large amount and coarsely, which causes a decrease in creep strength and creep ductility. Therefore, the Ti content is set to 0.20% or less. The preferred upper limit is 0.15%, more preferably 0.10% or less. When Ti is contained, the preferable lower limit is 0.01%, and the more preferable lower limit is 0.02%.
[0058]
Ca: 0.015% or less
　Ca has an effect of improving hot workability at the time of production, and may be contained as needed. However, the excess content of Ca combines with oxygen, which significantly reduces the cleanliness and rather deteriorates the hot workability. Therefore, the Ca content is set to 0.015% or less. It is preferably 0.012% or less, and more preferably 0.010% or less. When Ca is contained, the preferable lower limit is 0.0005%, and the more preferable lower limit is 0.001%.
[0059]
Mg: 0.015% or less
　Mg may be contained as necessary because it has an effect of improving hot workability at the time of production, like Ca. However, the excess content of Mg combines with oxygen and significantly lowers the cleanliness, which in turn deteriorates the hot workability. Therefore, the content of Mg is set to 0.015% or less. It is preferably 0.012% or less, and more preferably 0.010% or less. When Mg is contained, the preferable lower limit is 0.0005%, and the more preferable lower limit is 0.001%.
[0060]
Sn: 0.005% or less
　Sn may be contained as necessary because it is concentrated under the scale on the surface of the steel and is effective in improving corrosion resistance. However, since the excessive content of Sn causes a decrease in toughness, the Sn content is set to 0.005% or less. It is preferably 0.004% or less, and more preferably 0.003% or less. When Sn is contained, the preferable lower limit is 0.0005%, and the more preferable lower limit is 0.0010%.
[0061]
Amount of W analyzed as electrolytic extraction residue ([% W] ER ):
　-10 × [% B] +0.26 ≦ [% W] ER ≦ 10 × [% B] +0.54 Contained
　in ferrite heat-resistant steel W is precipitated in the form contained in M 23 C 6 type carbide in the tempering heat treatment at the time of production . When this carbide is finely deposited, it contributes to ensuring the tensile strength. However, on the other hand, if this carbide is excessively deposited, the toughness is lowered. The amount of this carbide can be estimated as the amount of W analyzed as the electrolytic extraction residue. When the amount of M 23 C6 type carbide
　containing W is small, the strengthening effect due to the precipitation of carbide is small, the required tensile strength cannot be obtained, the structure recovery and softening do not proceed, and the toughness also decreases. On the other hand, if this carbide is excessively and coarsely deposited, it does not contribute to the strengthening of tensile strength and the like, and the amount of W that is solid-solved in the steel matrix is ​​reduced, so that the strengthening effect by the solid solution is also reduced. As a result, the required tensile strength cannot be obtained, and it becomes a starting point of fracture and the toughness is lowered. 　Further, B contained in the steel does not affect the precipitation amount of the above-mentioned carbides, and M 23 C 6
It has the effect of finely precipitating the type carbide. Therefore, even if the amount of precipitation is small, the strengthening effect due to the precipitation of carbides can be easily obtained, and the precipitates are made smaller (that is, finer) to suppress the disappearance of the strengthening effect due to precipitation, and toughness due to being a starting point of fracture. Can be suppressed. Therefore, in order to obtain the required tensile strength and toughness, the amount of W present as a precipitate, that is, the amount of W analyzed as an electrolytic extraction residue [% W] , the lower and upper limits of ER are set to B in the steel. Depending on the content, it is necessary to satisfy the following equation (1).
　-10 x [% B] +0.26 ≤ [% W] ER ≤ 10 x [% B] +0.54 (1) In equation
　(1), [% W] ER is analyzed as an electrolytic extraction residue. The amount of W (% by mass) is represented, and [% B] represents the content (% by mass) of B in the ferritic heat-resistant steel.
[0062]
　The amount of W analyzed as the electrolytic extraction residue can be adjusted depending on the amount of W and C contained in the steel, the conditions of the tempering heat treatment, and the like. Specifically, the higher the amount of W and the amount of C contained in the steel, the larger the amount of W analyzed as the electrolytic extraction residue. Further, in the tempering heat treatment applied to the steel of the present disclosure, the higher the temperature and the longer the time, the larger the amount of W analyzed as the electrolytic extraction residue. Further, in the cooling after the tempering treatment, the smaller the cooling rate, the larger the amount of W analyzed as the electrolytic extraction residue.
[0063]
　The amount of W analyzed as the electrolytic extraction residue is measured as follows.
　A test material of a predetermined size is collected from ferritic heat-resistant steel. The test material is anodically dissolved at a current density of 20 mA / cm 2 by a constant current electrolysis method using a 10% by volume acetylacetone-1 mass% tetramethylammonium chloride methanol solution as an electrolytic solution, and the carbide is extracted as a residue. After acid decomposition of the extracted residue, ICP (radio frequency inductively coupled plasma) emission analysis is performed to measure the mass of W in the residue. The mass of W in the residue is divided by the amount of the test material dissolved to determine the amount of W present as a carbide. That is, the amount of W is the amount of W analyzed as the electrolytic extraction residue.
[0064]
Characteristics of Ferritic Heat-Resistant Steel
　(1) Tensile Strength
　The ferrite-based heat-resistant steel according to the present disclosure preferably has a tensile strength of 620 MPa or more at room temperature, more preferably 670 MPa or more.
　The tensile strength is measured at room temperature (10 ° C. to 35 ° C.) in accordance with JIS Z2241: 2011 using a No. 14A round bar test piece having a parallel portion diameter of 8 mm and a parallel portion length of 55 mm.
[0065]
　(2) Full-size Charpy Absorption Energy
　The ferrite-based heat-resistant steel according to the present disclosure preferably has a full-size Charpy absorption energy of 27 J or more at 20 ° C.
　Full size Charpy absorption energy is measured at 20 ° C. using a 2 mm V notch full size Charpy impact test piece in accordance with JIS Z2242: 2005.
[0066]
　(3) Creep performance
　The ferritic heat-resistant steel according to the present disclosure is preferably subjected to a creep rupture test under the condition of 650 ° C. × 157 MPa, which has a target rupture time of 1000 hours for the base metal, and the rupture time preferably exceeds the target rupture time. ..
　The creep rupture test is performed in accordance with JIS Z2271: 2010 using a round bar creep test piece.
[0067]
　Next, a method for producing the ferritic heat-resistant steel according to the present disclosure will be described with an example.
[0068]
Forming process
　In the production of ferritic heat-resistant steel according to the present disclosure, the material is first formed into the final shape of the ferrite-based heat-resistant steel. The forming step includes all steps involving deformation to obtain the final shape, and includes, for example, steps such as casting, forging, and rolling.
　As a forming process, for example, an ingot in which a material is melted and cast is formed by hot forging and hot rolling, or by hot forging, hot rolling, and cold working. , The process of making the final shape of ferritic heat-resistant steel can be mentioned.
[0069]
-Normalizing heat treatment step
　After the molding step, for example, normalizing heat treatment may be performed. For example, it is preferable to perform a normalizing heat treatment at 1130 ° C to 1170 ° C for 0.1 hour to 1.5 hours.
[0070]
-Tempering heat treatment step
　Further, after the normalizing heat treatment step, for example, a tempering heat treatment may be performed. For example, it is preferable to perform tempering heat treatment at 770 ° C. to 790 ° C. for 1 hour to 5 hours.
[0071]

　-Applications The ferritic heat-resistant steel according to the present disclosure is used for equipment used at high temperatures, such as boilers for power generation .
　Examples of equipment used at high temperatures include boiler pipes for coal-fired power plants, petroleum-fired power plants, waste incineration power plants, biomass power plants, etc.; decomposition pipes in petrochemical plants; and the like.
　Here, the "use at high temperature" in the present disclosure includes, for example, an embodiment of use in an environment of 350 ° C. or higher and 700 ° C. or lower (further, 400 ° C. or higher and 650 ° C. or lower).
Example
[0072]
　Hereinafter, the present disclosure will be described in more detail by way of examples. The present disclosure is not limited to these examples.
[0073]

　Hot forging and hot rolling are performed on an ingot in which the materials A to I having the chemical compositions shown in Table 1-1 and Table 1-2 are melted and cast in a laboratory. This was done in order and molded to a thickness of 15 mm. From this material, a plate material having a length of 150 mm and a width of 150 mm was processed. In Table 1-1 and Table 1-2, the unit of each component except “ratio Ta / Nb” is mass%, and the balance is Fe and impurities. In addition, the underlined values ​​in the table below indicate that they are outside the scope of this disclosure.
[0074]
[Table 1-1]

[0075]
[Table 1-2]

[0076]
　This plate material was subjected to normalizing and tempering heat treatment as shown in Table 2 to prepare a test material.
[0077]
[Measurement of the amount of W analyzed as an electrolytic extraction residue]
　A test piece of 8 mm square and 40 mm in length was collected from the obtained test material, and was subjected to the method described in the above embodiment, that is, the constant current electrolysis method. The amount of W analyzed as the electrolytic extraction residue was measured. Specifically, the test material was anodically dissolved at a current density of 20 mA / cm 2 by a constant current electrolysis method using a 10% by volume acetylacetone-1 mass% tetramethylammonium chloride methanol solution as an electrolytic solution, and the carbide was extracted as a residue. did. After acid decomposition of the extracted residue, ICP (radio frequency inductively coupled plasma) emission analysis was performed to measure the mass of W in the residue. The mass of W in the residue was divided by the amount of the test material dissolved to determine the amount of W present as a carbide.
[0078]
[Table 2]

[0079]
[Tensile test / Tensile strength]
　Furthermore, a 14A round bar test piece shown in JIS Z2241: 2011 with a parallel portion diameter of 8 mm and a parallel portion length of 55 mm was collected from the test material and conformed to JIS Z2241: 2011. Then, it was subjected to a tensile test at room temperature (10 ° C. to 35 ° C.). Then, those having a tensile strength of 620 MPa or more, which is the tensile strength required for the base material, are regarded as "pass", those having a tensile strength of 670 MPa or more are regarded as "pass / excellent", and the others are "pass / acceptable". On the other hand, those below 620 MPa were regarded as "failed".
[0080] [0080]
[Charpy impact test / toughness]
　In addition, three 2 mm V notch full-size Charpy impact test pieces with notches processed were collected from the central portion of the test material in the plate thickness direction and subjected to the Charpy impact test. The Charpy impact test was performed in accordance with JIS Z2242: 2005. The test was carried out at 20 ° C., and those with an average value of absorbed energy of 3 test pieces of 27 J or more were regarded as "pass", and among them, the individual values ​​of absorbed energy of all 3 test pieces were 27 J or more. Those that are "pass / excellent" and those that are not are "pass / acceptable", while those whose average absorbed energy of the three test pieces is less than 27J are "fail".
[0081]
[Creep rupture test]
　In addition, a round bar creep test piece was collected from a test material that passed the tensile test and the Charpy impact test, and a creep rupture test was performed. Then, as an evaluation during long-term use, a creep rupture test was conducted under the condition that the target rupture time of the base metal was 1000 hours at 650 ° C. × 157 MPa. The creep rupture test was performed in accordance with JIS Z2271: 2010. Then, those having a breaking time exceeding the target breaking time were regarded as "pass", and those having a breaking time shorter than that were regarded as "fail".
[0082]
[Table 3]

[0083]
　From Table 3, it can be seen that steels satisfying the conditions specified in the present disclosure can stably obtain excellent tensile strength and high toughness. In addition, it can be seen that it also has high creep strength during long-term use.
[0084]
　On the other hand, in the substitutes A8, B8, F1 and H1, the amount of W analyzed as the electrolytic extraction residue was less than the range specified in the formula (1), that is, the amount of carbide precipitated was not sufficient. The target tensile strength and toughness were not satisfied.
[0085]
　The substitutes A11, B11, G1 and I1 are targeted because the amount of W analyzed as the electrolytic extraction residue exceeds the range specified in the formula (1), that is, carbides are excessively and coarsely precipitated. Not satisfied with tensile strength and toughness.
[0086]

　Hot forging and hot rolling are performed on an ingot in which the materials J to O having the chemical compositions shown in Tables 4-1 and 4-2 are melted and cast in a laboratory. This was done in order and molded to a thickness of 15 mm. From this material, a plate material having a length of 150 mm and a width of 150 mm was processed. In Tables 4-1 and 4-2, the unit of each component except "ratio Ta / Nb" is mass%, and the balance is Fe and impurities.
[0087]
[Table 4-1]

[0088]
[Table 4-2]

[0089]
　This plate material was subjected to normalizing by heating at 1150 ° C. for 0.5 hours and then cooling, and by heat treatment of tempering by heating at 780 ° C. for 1 hour and cooling to obtain a test material.
[0090]
[Measurement of the amount of W analyzed as the electrolytic extraction residue]
[Tensile test / Tensile strength]
[Charpy impact test / Toughness]
　The amount of W analyzed as the electrolytic extraction residue with respect to the obtained test material. Measurements, tensile tests and Charpy impact tests were performed.
[0091]
[Creep rupture test]
　In addition, a round bar creep test piece was collected from a test material that passed the tensile test and the Charpy impact test, and a creep rupture test was performed. Then, as an evaluation of the high temperature strength at the initial stage of use, the target breaking time of the base metal is 50 hours each under the condition of 650 ° C. × 206 MPa, and as an evaluation during long-term use, the target breaking time of the base metal is determined. A creep rupture test was carried out for each of three under the condition of 650 ° C. × 157 MPa for 1000 hours. The creep rupture test was performed in accordance with JIS Z2271: 2010.
　Then, those having a breaking time exceeding the target breaking time are regarded as "pass (excellent)", two of the three are exceeding the target breaking time, and the remaining one is shorter than the target breaking time, but the breaking is performed. Those whose time is 90% or more of the target breaking time are "passed (acceptable)", and those other than that are "failed".
[0092]
[Table 5]

[0093]
[Table 6]

[0094]
　From Tables 5 and 6, it can be seen that the steels J to O satisfy the conditions specified in the present invention, and thus stable and excellent tensile strength and high toughness can be obtained. In addition, when Ta and Nb satisfy a predetermined range, it can be seen that high creep strength can be stably obtained at the initial stage of use and during long-term use.
[0095]
　As described above, it can be seen that a ferritic heat-resistant steel having stable and excellent tensile strength and toughness and also having high creep strength during long-term use can be obtained only when the requirements of the present disclosure are satisfied.
Industrial applicability
[0096]
　According to the present disclosure, it is possible to provide a ferritic heat-resistant steel which contains a large amount of W and B and has stable and excellent tensile strength and toughness.
[0097]
　The disclosures of Japanese Patent Application 2019-075661 filed on April 11, 2019 and Japanese Patent Application 2019-051751 and Japanese Patent Application 2019-051752 filed on March 19, 2019 are herein by reference in their entirety. Incorporated into the book. All documents, patent applications, and technical standards described herein are referenced herein to the same extent as if individual documents, patent applications, and technical standards were specifically and individually described. Is taken in by.

WE CLAIMS

[Claim 1]By mass%
C: 0.06% to 0.11%,
Si: 0.15% to 0.35%,
Mn: 0.35% to 0.65%,
P: 0.020% or less,
S: 0 .0030% or less,
Ni: 0.005% to 0.250%,
Cu: 0.005% to 0.250%,
Co: 2.7% to 3.3%,
Cr: 8.3% to 9. 7%,
W: 2.5% to 3.5%,
V: 0.15% to 0.25%,
Nb: 0.030% to 0.080%,
Ta: 0.002% to 0.040% ,
Nd: 0.010% to 0.060%,
B: 0.006% to 0.016%,
N: 0.005% to 0.015%,
Al: 0.020% or less, and
O: 0. A ferrite heat-resistant steel
containing 020% or less , the
　balance consisting of Fe and impurities,
　and the amount of W analyzed as an electrolytic extraction residue satisfying the following formula (1).
　-10 x [% B] +0.26≤ [% W] ER≦ 10 × [% B] +0.54 (1) In the formula
　(1), [% W] ER represents the amount (mass%) of W analyzed as the electrolytic extraction residue, and [% B] is the ferrite. It represents the content (mass%) of B in the ferritic stainless steel.
[Claim 2]
　In terms of mass%,
　the total content of the Ta and the Nb is 0.040% to 0.110%,
　and the ratio Ta / Nb of the Ta content to the Nb content is 0.10 to 0.70. The ferritic heat-resistant steel according to claim 1.
[Claim 3]
　The ferrite-based heat-resistant steel according to claim 1 or 2, which contains at least one element selected from the following group in mass% instead of a part of the Fe.
　Group Mo: 0.50% or less
　　　Ti: 0.20% or less
　　　Ca: 0.015% or less
　　　Mg: 0.015% or less
　　　Sn: 0.005% or less
[Claim 4]
　Any of claims 1 to 3 in which the tensile strength specified in JIS Z2241: 2011 at room temperature is 620 MPa or more and the full-size ferritic absorption energy specified in JIS Z2242: 2005 at 20 ° C. is 27 J or more. The ferritic heat-resistant steel according to item 1.