Ferritic heat-resistant steel

Abstract

Provided is ferritic heat-resistant steel having a chemical composition that is, in terms of mass%, 0.07-0.14% C, 0.15-0.35% Si, 0.30-0.55% Mn, not more than 0.0250% P, not more than 0.0030% S, 0.15-0.35% Ni, 8.0-9.7% Cr, 0.20-0.60% Mo, 1.50-2.30% W, 0.16-0.25% V, 0.020-0.120% Nb, 0.0010-0.0050% B, 0.010-0.080% N, not more than 0.020% Al, and not more than 0.020% O, the remainder being Fe and impurities, wherein the V content VER in precipitate obtained by extraction residue analysis satisfies [-0.2×C+0.060≤VER≤-0.1×C+0.160].

Description

Technical Field

[0001] The present invention relates to ferritic heat-resistant steel.

Background Art

[0002] Ferritic heat-resistant steel is not only less expensive than austenitic heat-resistant steel and Ni-based heat-resistant steel, but also has the advantage as a high-temperature steel of having a small coefficient of thermal expansion, and thus is widely used in equipment used at high temperatures, such as thermal power generation boilers.

[0003] In recent years, in coal-fired power generation, the steam conditions have been advanced to higher temperature and higher pressure in order to improve the thermal efficiency, and in the future, operation under ultra-supercritical pressure conditions of 650°C and 350 atmospheres is planned. Also, in the waste heat recovery boiler of gas turbine combined cycle power generation, the steam temperature is being increased.

[0004] In order to cope with such severe steam conditions, many ferritic heat-resistant steels with improved creep strength have been proposed.

[0005] For example, Patent Document 1 discloses a high-strength ferritic heat-resistant steel containing, by mass%, C: 0.03 to 0.12%, Cr: 8.0 to 13.0%, W: 1.8 to 3.0%, V: 0.05 to 0.30%, and Nb: 0.02 to 0.12%.

[0006] Patent Document 2 discloses a high-Cr ferritic heat-resistant steel excellent in high-temperature strength and toughness containing, by mass%, one or two of C: 0.02 to 0.15%, Cr: 8 to 16%, V: 0.1 to 0.3%, Nb: 0.01 to 0.2%, B: 0.001 to 0.02%, Mo: 0.01 to 1.2%, and W: 0.8 to 3.5%.

[0007] Patent Document 3 discloses a high-strength ferritic heat-resistant steel containing, by mass%, C: 0.01 to 0.30%, Cr: 8.00 to 13.00%, W: 0.10 to 5.00%, Mo: 0.01 to 3.00%, V: 0.002 to 0.800%, and Nb: 0.002 to 0.500%.

[0008] Patent Document 4 discloses a ferritic heat-resistant steel having excellent high-temperature strength and containing, by mass%, C: 0.01 to 0.08%, Cr: 8.00 to 13.00%, W: more than 1.50% and at most 3.50%, Mo: at most 0.50%, V: 0.10 to 0.30%, and Nb: 0.01 to 0.15%.

[0009] Patent Document 5 discloses a high-strength heat-resistant cast steel containing, by mass%, C: 0.06 to 0.16%, Cr: 8 to 12%, W: 1.9 to 3.0%, Mo: at most 0.7%, V: 0.05 to 0.3%, and a total amount of one or more of Nb, Ta, and Zr: 0.01 to 0.15%.

Prior Art Documents

Patent Documents

[0010]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Summary of the Invention

Problems to be Solved by the Invention

[0011] Although these ferritic heat-resistant steels certainly have excellent service performance, it has been found that when used at high temperatures, the creep strength varies, specifically, the time until fracture occurs under a certain stress may be shortened. Therefore, it is strongly desired that excellent high-temperature strength can be stably obtained.

[0012] The present invention has been made in view of the above situation, and an object thereof is to provide a ferritic heat-resistant steel having stable and excellent creep strength when used at high temperatures.

Means for Solving the Problem

[0013] The present invention features the following ferritic heat-resistant steel.

[0014] (1) The chemical composition is, by mass%, C: 0.07 to 0.14%, Si: 0.15 to 0.35%, Mn: 0.30 to 0.55%, P: 0.0250% or less, S: 0.0030% or less, Ni: 0.15 to 0.35%, Cr: 8.0 to 9.7%, Mo: 0.20 to 0.60%, W: 1.50 to 2.30%, V: 0.16 to 0.25%, Nb: 0.020 to 0.120%, B: 0.0010 to 0.0050%, N: 0.010 to 0.080%, Al: 0.020% or less, O: 0.020% or less, The balance: Fe and impurities, satisfying the following formula (i), ferritic heat-resistant steel. -0.2 × C + 0.060 ≤ V ER ≤ -0.1 × C + 0.160 ··· (i) However, the meanings of the symbols in formula (i) are as follows. C: C content in the steel (mass%) V ER : V content in the precipitate obtained by extraction residue analysis (mass%)

[0015] (2) The above-mentioned (1) ferritic heat-resistant steel, wherein part of Fe is replaced by, by mass%, Sn: 0.0200% or less, and As: 0.0200% or less, containing one or more selected therefrom. The ferritic heat-resistant steel according to the above (1).

[0016] (3) The chemical composition contains, in mass %, P: 0.0030 to 0.0250%, and satisfies the following formula (ii), The ferritic heat-resistant steel according to (2) above. 0.0005 ≦ Sn + As ≦ -2×P + 0.0600 ···(ii) However, the element symbols in the above formula represent the content (mass %) of each element.

[0017] (4) The chemical composition contains, in mass %, instead of a part of Fe, Nd: 0.0005 to 0.0400%, The ferritic heat-resistant steel according to any one of (1) to (3) above.

[0018] (5) The chemical composition contains, in mass %, instead of a part of Fe, Cu: 0.50% or less, Co: 0.50% or less, Ti: 0.20% or less, Ta: 0.20% or less, Ca: 0.0150% or less, Mg: 0.0150% or less, and REM: 0.0400% or less, contains one or more selected from The ferritic heat-resistant steel according to any one of (1) to (4) above.

Advantages of the Invention

[0019] According to the present invention, in the ferritic heat-resistant steel, excellent creep strength can be stably obtained during use at high temperatures.

Modes for Carrying Out the Invention

[0020] In order to achieve the above object, the present inventors conducted a detailed study on the relationship between creep strength and microstructure of ferritic heat resistant steels containing 0.07 to 0.14% C and 0.16 to 0.25% V. As a result, the following findings were obtained.

[0021] (a) Carbides and carbonitrides containing V were precipitated in the ferritic heat-resistant steel before the creep test. Furthermore, new carbides and carbonitrides containing V were precipitated during the high-temperature test, and their size was finer than that of the precipitates that had already precipitated before the test.

[0022] (b) Steel that had a large amount of precipitates containing V before the test exhibited good creep strength in a high-stress creep test (which fractured in a short time, corresponding to early use). On the other hand, steel that had excessive precipitates containing V before the test exhibited a short fracture time in a low-stress creep test (which fractured in a long time, corresponding to long-term use), and the precipitates quickly coarsened during use.

[0023] (c) In addition, when the C content is high, good creep strength is easily obtained in the creep test at high stress, but in the creep test at low stress, the coarsening of V-containing precipitates occurs quickly and the rupture time varies widely.

[0024] From the above results, the reasons for the variation in creep strength are inferred as follows.

[0025] (d) The precipitation strengthening effect of V carbides and carbonitrides in steel before use contributes to improving creep strength in the early stages of use. Therefore, when the amount of precipitates is large, good creep strength can be obtained in the early stages of use.

[0026] (e) On the one hand, during use, the precipitates containing V further precipitate finely within the grains, contributing to the maintenance of creep strength during long-term use. However, if excessive coarse precipitates containing V are precipitated before use, due to the interfacial energy difference caused by the difference in the size of the precipitates, the fine precipitates dissolve, promoting the growth of the surrounding coarse precipitates. As a result, a decrease in creep strength occurs during long-term use.

[0027] (f) And when the C content is high, since the dislocation density of the steel matrix before use is high, the creep strength at the initial stage of use is high. However, since the intragranular diffusion of alloying elements is fast, the growth of the surrounding precipitates associated with the solution of the precipitates containing V is accelerated, and the creep strength decreases.

[0028] Therefore, as a result of repeated studies, it has been found that by appropriately controlling the amount of V precipitates present in the steel before use (as manufactured), that is, the V content in the precipitates obtained by extraction residue analysis, in accordance with the C content in the steel, stable and good creep strength can be obtained at the initial stage of use and during long-term use.

[0029] Furthermore, the present inventors have confirmed that in a ferritic heat-resistant steel in which stable creep strength can be obtained by controlling the V content analyzed by extraction residue in accordance with the C content in the above steel, by containing either one or both of Sn and As, the corrosion resistance is improved.

[0030] However, when Sn and / or As are contained for the purpose of enhancing corrosion resistance, for example, when used in applications where heating to high temperatures and cooling to room temperature are repeated, such as the piping of a waste heat recovery boiler that repeats shutdown and operation according to the required power generation amount, it has been found that sufficient toughness may not be stably obtained.

[0031] To solve this problem, the present inventors conducted a detailed investigation on a ferritic heat-resistant steel containing a total of 0.0005% or more of Sn and As. As a result, the findings described below became clear.

[0032] (g) As a result of investigating the toughness of steel that had been repeatedly heated to high temperatures and cooled to room temperature, it was found that the toughness decreased as the contents of Sn and As increased. Furthermore, the greater the amount of P contained in the steel, the more significant the degree of this decrease was.

[0033] (h) As a result of observing the fracture surface after the impact test, regions fractured at the prior austenite grain boundaries were found to be mixed on the fracture surface, and the proportion of these increased as the amounts of Sn and As increased. Also, Sn and As were detected from the fracture surface fractured at the prior austenite grain boundaries. Furthermore, in addition to these elements, P was also detected simultaneously.

[0034] From the above results, it was speculated that the reason for the decrease in toughness was as follows. That is, Sn, As, and P contained in the steel segregate at the prior austenite grain boundaries during the process of repeatedly heating to high temperatures and cooling. Since all of these elements reduce the grain boundary bonding force, as a result, it was speculated that a decrease in toughness occurs.

[0035] Therefore, as a result of various studies, it was also found that by strictly controlling the upper limit of the total content of Sn and As contained in the steel according to the amount of P, and by reducing the grain boundary segregation of these elements during the repeated heating and cooling process during use, excellent toughness can be obtained.

[0036] The present invention has been made based on the above findings. Hereinafter, each requirement of the present invention will be described in detail.

[0037] (A) Chemical composition The reasons for limiting each element are as follows. In the following description, "%" regarding the content means "mass %".

[0038] C: 0.07 to 0.14% C is effective for obtaining a martensitic structure, generates fine carbides or carbonitrides, and contributes to creep strength. To obtain this effect, it is necessary to contain 0.07% or more. However, if it is contained in excess, it accelerates the growth of precipitates and rather causes a decrease in creep strength. Therefore, the C content is set to 0.07 - 0.14%. The C content is preferably 0.08% or more, more preferably 0.09% or more. Also, the C content is preferably 0.13% or less, more preferably 0.12% or less.

[0039] Si: 0.15 - 0.35% Si is contained as a deoxidizer and is an element effective for steam oxidation resistance characteristics. To obtain this effect, it is necessary to contain 0.15% or more. However, if it is contained in excess, it causes a decrease in ductility. Therefore, the Si content is set to 0.15 - 0.35%. The Si content is preferably 0.18% or more, more preferably 0.20% or more. Also, the Si content is preferably 0.32% or less, more preferably 0.30% or less.

[0040] Mn: 0.30 - 0.55% Mn, like Si, is contained as a deoxidizer and also has an effect on obtaining a martensitic structure. To obtain this effect, it is necessary to contain 0.30% or more. However, if it is contained in excess, it causes creep embrittlement. Therefore, the Mn content is set to 0.30 - 0.55%. The Mn content is preferably 0.33% or more, more preferably 0.35% or more. Also, the Mn content is preferably 0.52% or less, more preferably 0.50% or less.

[0041] P: 0.0250% or less When P is contained in excess, it reduces creep ductility and impairs toughness. Therefore, the P content needs to be 0.0250% or less. The P content is preferably 0.0200% or less, and more preferably 0.0180% or less. Although there is no particular need to set a lower limit for the P content, extreme reduction will extremely increase the material cost and contribute to the improvement of tensile strength and creep strength to some extent. Therefore, the P content is preferably 0.0030% or more, and more preferably 0.0050% or more.

[0042] S: 0.0030% or less When S is contained in excess, it causes a decrease in creep ductility. Therefore, the S content needs to be 0.0030% or less. The S content is preferably 0.0020% or less, and more preferably 0.0015% or less. Although the lower the S content, the better, extreme reduction will extremely increase the material cost. Therefore, the S content is preferably 0.0001% or more, and more preferably 0.0002% or more.

[0043] Ni: 0.15 - 0.35% Ni is an element effective for obtaining a martensite structure and ensuring creep strength. To obtain this effect, it is necessary to contain 0.15% or more. However, when contained in excess, the effect saturates and, since it is an expensive element, it increases the material cost. Therefore, the Ni content is set to 0.15 - 0.35%. The Ni content is preferably 0.18% or more, and more preferably 0.20% or more. Also, the Ni content is preferably 0.32% or less, and more preferably 0.30% or less.

[0044] Cr: 8.0 - 9.7% Cr is an element effective for ensuring resistance to steam oxidation and corrosion resistance at high temperatures. It also precipitates as carbides and contributes to the improvement of creep strength. To obtain this effect, it is necessary to contain 8.0% or more. However, if it is contained in excess, the stability of the carbides is reduced and the creep strength is rather decreased. Therefore, the Cr content is set to 8.0 - 9.7%. The Cr content is preferably 8.3% or more, more preferably 8.5% or more. Also, the Cr content is preferably 9.5% or less, more preferably 9.3% or less.

[0045] Mo: 0.20 - 0.60% Mo dissolves in the matrix and contributes to ensuring creep strength at high temperatures. To obtain this effect, it is necessary to contain 0.20% or more. However, even if it is contained in excess, the effect of improving creep strength saturates and, since it is an expensive element, the material cost is increased. Therefore, the Mo content is set to 0.20 - 0.60%. The Mo content is preferably 0.25% or more, more preferably 0.30% or more. Also, the Mo content is preferably 0.55% or less, more preferably 0.50% or less.

[0046] W: 1.50 - 2.30% W dissolves in the matrix or precipitates as intermetallic compounds during long - term use and contributes to ensuring creep strength at high temperatures. To obtain this effect, it is necessary to contain 1.50% or more. However, even if it is contained in excess, the effect of improving creep strength saturates and, since it is an expensive element, the material cost is increased. Therefore, the W content is set to 1.50 - 2.30%. The W content is preferably 1.60% or more, more preferably 1.70% or more. Also, the W content is preferably 2.20% or less, more preferably 2.10% or less.

[0047] V: 0.16 - 0.25% V precipitates in the grains as fine carbides or carbonitrides, contributing to the improvement of creep strength. To obtain this effect, it is necessary to contain 0.16% or more. However, if the content is excessive, it accelerates the growth of the precipitates and instead causes a decrease in creep strength. Therefore, the V content is set to 0.16 - 0.25%. The V content is preferably 0.17% or more, more preferably 0.18% or more. Also, the V content is preferably 0.24% or less, more preferably 0.23% or less.

[0048] Note that the V content here means the total amount of V contained in the ferritic heat-resistant steel. That is, it means the sum of the V content dissolved in the matrix and the V content present as precipitates. And in the present invention, in addition to the total amount of V satisfying the above V content range, the V content present as precipitates, that is, the V content analyzed as the electrolytic extraction residue, needs to satisfy the relationship with the C content described later.

[0049] Nb: 0.020 - 0.120% Nb precipitates in the grains as fine carbides or carbonitrides, contributing to the improvement of creep strength. To obtain this effect, it is necessary to contain 0.020% or more. However, if it is contained in excess, it precipitates in a large amount and coarsely, instead causing a decrease in creep strength and creep ductility. Therefore, the Nb content is set to 0.020 - 0.120%. The Nb content is preferably 0.030% or more, more preferably 0.050% or more. Also, the Nb content is preferably 0.100% or less, more preferably 0.080% or less.

[0050] B: 0.0010 - 0.0050% B is effective for obtaining a martensitic structure, dissolves in carbides, is finely dispersed, and contributes to the improvement of creep strength. To obtain this effect, it is necessary to contain 0.0010% or more. However, if it is contained in excess, it will flow into the weld metal during welding and increase the solidification cracking susceptibility. Therefore, the content of B is set to 0.0010 - 0.0050%. The B content is preferably 0.0015% or more, more preferably 0.0020% or more. Also, the B content is preferably 0.0045% or less, more preferably 0.0040% or less.

[0051] N: 0.010 - 0.080% N precipitates as fine carbonitrides during use at high temperatures and contributes to the improvement of creep strength. To obtain this effect, it is necessary to contain 0.010% or more. However, if it is contained in excess, it will rather cause a decrease in creep ductility. Therefore, the content of N is set to 0.010 - 0.080%. The N content is preferably 0.020% or more, more preferably 0.030% or more. Also, the N content is preferably 0.070% or less, more preferably 0.060% or less.

[0052] Al: 0.020% or less Al is contained as a deoxidizer, but if it is contained in a large amount, it will significantly harm the cleanliness and deteriorate the workability. Also, it is not preferable from the viewpoint of creep strength. Therefore, the content of Al is set to 0.020% or less. The Al content is preferably 0.018% or less, more preferably 0.015% or less. Note that there is no particular need to set a lower limit for the Al content, but extreme reduction will increase the manufacturing cost. Therefore, the Al content is preferably 0.001% or more, more preferably 0.002% or more.

[0053] O: 0.020% or less O exists as an impurity, but when contained in a large amount, it reduces the workability. Therefore, the content of O should be 0.020% or less. The O content is preferably 0.015% or less, and more preferably 0.010% or less. Note that there is no particular need to set a lower limit for the O content, but extreme reduction increases the manufacturing cost. Therefore, the O content is preferably 0.001% or more, and more preferably 0.002% or more.

[0054] A part of V contained in the steel exists in the steel as carbide or carbonitride in the as-manufactured (before use) state, and contributes to the creep strength in the initial stage of use due to its precipitation strengthening effect. Furthermore, the precipitates containing V also precipitate finely during use and contribute to ensuring the creep strength over a long time.

[0055] When the amount of precipitates containing V is small, the precipitation strengthening effect cannot be obtained in the initial stage of use, and sufficient creep strength cannot be obtained. On the other hand, when the amount of precipitates containing V is large in the state before use at high temperature, although the creep strength in the initial stage of use is improved, if it becomes excessive, the creep strength during long-term use decreases. This is due to the interfacial energy difference caused by the difference in the size between the precipitates existing in the state before use and the fine precipitates precipitating during use, which promotes the growth of the precipitates and causes the precipitation strengthening ability to disappear early.

[0056] In particular, when the C content is high, since the dislocation density of the matrix is high in the state before use at high temperature, the creep strength in the initial stage of use is high, but the grain boundary diffusion of alloying elements becomes fast, the growth of precipitates containing V is accelerated, the creep strength during long-term use decreases, and the variation in creep strength is likely to occur.

[0057] Therefore, it is necessary to control the amount of precipitates containing V in the state before use according to the C content. This amount of precipitates can be estimated as the V content in the precipitates obtained by extraction residue analysis. Specifically, in order to stably obtain good creep strength, it is necessary to satisfy the following formula (i). -0.2×C + 0.060 ≤ V ER ≤ -0.1×C + 0.160 ···(i) However, the meanings of the symbols in formula (i) are as follows. C: C content (mass %) in steel V ER : V content (mass %) in the precipitate obtained by extraction residue analysis

[0058] Note that the V content in the precipitate obtained by extraction residue analysis can be adjusted by the V content contained in the ferritic heat-resistant steel and heat treatment during manufacturing.

[0059] In order to further improve the creep strength during the initial use and long-term use, V ER is preferably [-0.2×C + 0.065] or more, and preferably [-0.2×C + 0.075] or more. Also, V ER is preferably [-0.1×C + 0.150] or less, and more preferably [-0.1×C + 0.140] or less.

[0060] Also, the V content in the precipitate obtained by extraction residue analysis is measured as follows. A test piece of a predetermined size is sampled from the ferritic heat-resistant steel. By using a constant current electrolysis method with a 10 vol% acetylacetone - 1 mass% tetramethylammonium chloride methanol solution as the electrolyte, the test piece is anodically dissolved at a current density of 20 mA / cm 2 to extract carbides and carbonitrides as residues. After acid-decomposing the extracted residues, ICP (inductively coupled plasma) emission analysis is performed to measure the mass of V in the residues. The mass of V in the residues is divided by the dissolution amount of the test piece to obtain the V content present as carbides and carbonitrides. That is, this V content is the V content in the precipitate obtained by extraction residue analysis.

[0061] The ferritic heat-resistant steel according to the present invention contains the above-mentioned respective elements, and the balance consists of Fe and impurities. Note that "impurities" refers to those mixed in due to various factors in the manufacturing process starting from raw materials such as ores or scraps when industrially manufacturing steel materials.

[0062] Furthermore, the ferrite heat-resistant steel according to the present invention may contain Sn and / or As in place of a part of Fe. The reasons for the limitation are described below.

[0063] Sn: 0.0200% or less Sn concentrates under the scale on the surface of the steel and is effective in improving corrosion resistance, especially in an environment where heating and cooling are repeated like in an exhaust heat recovery boiler and dew condensation water easily adheres to the surface. To obtain this effect, Sn may be contained as necessary. However, when contained in excess, it causes a decrease in toughness before use. Furthermore, when used at high temperatures, it decreases creep ductility. Therefore, when contained, the content of Sn should be 0.0200% or less.

[0064] Furthermore, in order to suppress grain boundary segregation during the heating and cooling processes during use and suppress a decrease in toughness, it is preferable to satisfy the following formula (ii) in relation to As and P. In addition, when it is desired to obtain the above effects, the Sn content is preferably 0.0005% or more, more preferably 0.0008% or more, and even more preferably 0.0010% or more. Also, the Sn content is preferably 0.0150% or less, and more preferably 0.0100% or less.

[0065] As: 0.0200% or less Similar to Sn, As concentrates under the scale on the surface of the steel and is effective in improving corrosion resistance, especially in an environment where heating and cooling are repeated like in an exhaust heat recovery boiler and dew condensation water easily adheres to the surface. To obtain this effect, As may be contained as necessary. However, when contained in excess, it not only causes a decrease in toughness before use but also segregates at the grain boundaries during the heating and cooling processes during use, leading to a further decrease in toughness. Therefore, when contained, the content of As should be 0.0200% or less.

[0066] Furthermore, in order to suppress grain boundary segregation during the heating and cooling processes during use and to suppress a decrease in toughness, it is preferable to satisfy the following formula (ii) in relation to Sn and P. In addition, when it is desired to obtain the above effects, the As content is preferably 0.0005% or more, more preferably 0.0008% or more, and even more preferably 0.0010% or more. Also, the As content is preferably 0.0150% or less, more preferably 0.0100% or less.

[0067] As described above, in the ferritic heat-resistant steel according to the present invention, when Sn and As are contained, these elements segregate at the prior austenite grain boundaries during the process of repeatedly heating to high temperatures and cooling to room temperature, reducing the grain boundary bonding force and causing a decrease in toughness. Therefore, while Sn and / or As are contained in a total amount of a predetermined amount or more, it is preferable to strictly control the total content according to the content of P that similarly segregates at the grain boundaries and causes embrittlement. Specifically, it is preferable to satisfy the following formula (ii). 0.0005 ≦ Sn + As ≦ -2 × P + 0.0600 ···(ii) However, the element symbols in the above formula represent the content (% by mass) of each element.

[0068] When it is desired to obtain excellent corrosion resistance, the value of Sn + As is preferably 0.0010 or more, more preferably 0.0020 or more. On the other hand, when it is desired to more surely suppress a decrease in toughness, the value of Sn + As is preferably [-2 × P + 0.0550] or less. Also, when it is desired to obtain the effect of further enhancing corrosion resistance without impairing creep ductility, it is preferable to contain both Sn and As in an amount of 0.0005% or more.

[0069] Furthermore, the ferritic heat-resistant steel according to the present invention may contain Nd in place of a part of Fe. The reasons for the limitation are described below.

[0070] Nd: 0.0005 to 0.0400% Nd may be contained as necessary to combine with S or P to remove its adverse effects and improve creep ductility. However, if contained in excess, it combines with oxygen, reducing cleanliness and deteriorating hot workability. Therefore, when contained, the Nd content is set to 0.0005 - 0.0400%. The Nd content is preferably 0.0010% or more, more preferably 0.0030% or more. Also, the Nd content is preferably 0.0350% or less, more preferably 0.0300% or less.

[0071] In addition, the ferritic heat-resistant steel according to the present invention may contain one or more elements selected from Cu, Co, Ti, Ta, Ca, Mg, and REM in place of a part of Fe. The reasons for the limitation are described below.

[0072] Cu: 0.50% or less Cu, like Ni, is effective for the formation of a martensite structure and may be contained as necessary. However, if contained in excess, it reduces creep ductility. Therefore, the Cu content is set to 0.50% or less. The Cu content is preferably 0.40% or less, more preferably 0.30% or less. When contained, the Cu content is preferably 0.01% or more, more preferably 0.02% or more.

[0073] Co: 0.50% or less Co, like Ni and Cu, is effective for the formation of a martensite structure and may be contained as necessary. However, if contained in excess, its effect saturates and it is an expensive element, leading to an increase in cost. Therefore, the Co content is set to 0.50% or less. The Co content is preferably 0.40% or less, more preferably 0.30% or less. When contained, the Co content is preferably 0.01% or more, more preferably 0.02% or more.

[0074] Ti: 0.20% or less Ti may be contained as needed because it precipitates as fine carbides or carbonitrides during use at high temperatures and contributes to the improvement of creep strength. However, if it is contained in excess, it will precipitate in large amounts and coarsely, leading to a decrease in creep strength and creep ductility. Therefore, the Ti content should be 0.20% or less. The Ti content is preferably 0.15% or less, and more preferably 0.10% or less. When contained, the Ti content is preferably 0.01% or more, and more preferably 0.02% or more.

[0075] Ta: 0.20% or less Ta may be contained as needed because it precipitates as fine carbides and carbonitrides during use at high temperatures and contributes to the improvement of creep strength. However, if it is contained in excess, it will precipitate in large amounts and coarsely, leading to a decrease in creep strength and creep ductility. Therefore, the Ta content should be 0.20% or less. The Ta content is preferably 0.15% or less, and more preferably 0.10% or less. When contained, the Ta content is preferably 0.01% or more, and more preferably 0.02% or more.

[0076] Ca: 0.0150% or less Ca may be contained as needed to improve the hot workability during manufacturing. However, if it is contained in excess, it will combine with oxygen, significantly reducing the cleanliness and rather deteriorating the hot workability. Therefore, the Ca content should be 0.0150% or less. The Ca content is preferably 0.0120% or less, and more preferably 0.0100% or less. When contained, the Ca content is preferably 0.0005% or more, and more preferably 0.0010% or more.

[0077] Mg: 0.0150% or less Mg, like Ca, may be contained as necessary to improve hot workability during manufacturing. However, if contained in excess, it binds with oxygen, significantly reducing cleanliness and, conversely, deteriorating hot workability. Therefore, the Mg content should be 0.0150% or less. The Mg content is preferably 0.0120% or less, and more preferably 0.0100% or less. When containing Mg, the Mg content is preferably 0.0005% or more, and more preferably 0.0010% or more.

[0078] REM: 0.0400% or less REM (rare earth elements), like Ca and Mg, may be contained as necessary to improve hot workability during manufacturing. However, if contained in excess, it binds with oxygen, significantly reducing cleanliness and, conversely, reducing hot workability. Therefore, the REM content should be 0.0400% or less. The REM content is preferably 0.0350% or less, and more preferably 0.0300% or less. When containing REM, the REM content is preferably 0.0005% or more, and more preferably 0.0010% or more.

[0079] "REM" is a general term for a total of 17 elements including Sc, Y, and lanthanoids, and the REM content refers to the total content of one or more of these REM elements. Also, REM is generally contained in mischmetal. Therefore, for example, mischmetal may be added to the alloy so that the REM content is within the above range. Since Nd is also one of the REMs, when containing REM other than Nd in addition to Nd, the total amount should be within the above range.

[0080] (B) Manufacturing method A method for manufacturing the ferritic heat-resistant steel according to the present invention will be described.

[0081] Forming process: First, the material is formed into the final shape of the ferritic heat-resistant steel. The forming process includes all processes involving deformation to obtain the final shape, such as processes like casting, forging, rolling, etc.

[0082] As the forming process, for example, as an example, for an ingot obtained by melting and casting the material, it is formed by hot forging, or by hot forging and hot rolling, or by hot forging, hot rolling, and cold working to obtain the final shape.

[0083] Normalizing heat treatment process: In order to obtain the ferritic heat-resistant steel according to the present invention, after the forming process, normalizing heat treatment needs to be performed under the conditions of 1040 to 1100 °C for 0.08 to 1.00 hours. If the normalizing temperature is too low or the normalizing time is too short, the precipitates containing V generated in the forming process do not dissolve sufficiently into the matrix, and a large amount of undissolved coarse precipitates remain. In addition, in the subsequent tempering heat treatment, the precipitation amount of fine carbides containing V decreases, so the creep strength at the initial stage of use decreases. Also, during long-term use, due to the interfacial energy difference between the coarse precipitates that did not dissolve in the normalizing heat treatment and the fine precipitates that precipitate during use, the growth of the precipitates is accelerated, and the creep strength is likely to vary. Moreover, if the normalizing temperature is too high or the normalizing time is too long, the grain size of the prior austenite becomes large and the toughness decreases.

[0084] V ER In order to make it [-0.1×C + 0.150] or less, it is preferable that the normalizing temperature exceeds 1050 °C. In order to make V ER [-0.1×C + 0.140] or less, it is preferable that the normalizing temperature exceeds 1070 °C. Also, the normalizing time is preferably 0.10 to 0.75 hours.

[0085] Tempering heat treatment process: The ferrite heat-resistant steel according to the present invention needs to be subjected to tempering heat treatment at 740 to 800 °C for 0.5 to 5.0 hours after the normalizing heat treatment process. If the tempering temperature is too low or the tempering time is too short, the precipitation amount of the precipitate containing V is small, so sufficient precipitation strengthening ability cannot be obtained, the creep strength at the initial stage of use decreases, and the dislocations of the substrate introduced in the normalizing heat treatment process do not recover sufficiently, so the grain boundary diffusion of alloying elements becomes faster, the growth of the precipitate containing V during long-term use is accelerated, and the creep strength is likely to vary.

[0086] On the other hand, if the tempering temperature is too high or the tempering time is too long, although there are many precipitates containing V to be precipitated, they become large, so the precipitation strengthening ability decreases, the creep strength at the initial stage of use decreases significantly, and during use, due to the interfacial energy difference caused by the size difference between the coarse precipitates containing V and the fine precipitates precipitated during use, the growth of the precipitates is accelerated, and the creep strength is likely to vary. Furthermore, partial austenite transformation and the generation of martensite with a high dislocation density also contribute to the variation of the creep strength.

[0087] V ER In order to make it [-0.2×C + 0.065] or more, or [-0.2×C + 0.075] or more, the tempering heat treatment conditions are preferably 750 °C or more and 1.0 hour or more. On the other hand, in order to make V ER [-0.1×C + 0.150] or less, or [-0.1×C + 0.140] or less, the tempering heat treatment conditions are preferably 790 °C or less and 4.0 hours or less.

[0088] Hereinafter, the present invention will be described more specifically by way of examples. It should be noted that the present invention is not limited to these examples.

Examples

[0089] For the ingots cast by melting materials A to L having the chemical compositions shown in Table 1 in the laboratory, hot forging and hot rolling were performed in sequence, and the ingots were formed into a thickness of 15 mm. From this material, plates with a length of 150 mm and a width of 150 mm were processed.

[0090]

Table 1

[0091] The obtained plates were annealed and stress-relieved under the conditions shown in Table 2 to obtain test specimens.

[0092]

Table 2

[0093] [V content in the precipitate obtained by extraction residue analysis] From the obtained test specimens, test pieces with a size of 10 mm square and a length of 40 mm were taken, and the V content in the precipitate obtained by extraction residue analysis was measured by the above method, that is, the constant current electrolysis method. Specifically, by the constant current electrolysis method using a 10 vol% acetylacetone - 1 mass% tetramethylammonium chloride methanol solution as the electrolyte, the test specimen was anodically dissolved at a current density of 20 mA / cm 2 to extract carbides and carbonitrides as residues. After acid-decomposing the extracted residues, ICP emission analysis was performed to measure the mass of V in the residues. The mass of V in the residues was divided by the dissolved amount of the test specimen to obtain the V content present as carbides or carbonitrides. The above test was performed three times for each test specimen, and the average value was taken as the amount of V present as a precipitate in the test specimen, that is, V ER was used.

[0094] [Creep rupture test] From the obtained test materials, round bar creep test pieces were sampled and a creep rupture test was conducted. As an evaluation at the initial stage of use, a creep rupture test was carried out under the conditions of 600°C × 186 MPa where the target rupture time of the base material is 500 hours. As an evaluation during long-term use, a creep rupture test was conducted under the conditions of 600°C × 157 MPa where the target rupture time of the base material is 3000 hours. The creep rupture test was carried out in accordance with JIS Z 2271:2010. Those with a rupture time exceeding the target rupture time were designated as "A", those with a rupture time less than or equal to the target rupture time but not less than 90% of the target rupture time were designated as "B", and those less than 90% were designated as "F".

[0095] From Table 2, it can be seen that the steel satisfying the conditions defined in the present invention has the target creep strength under both high stress and low stress conditions, that is, it has good creep strength at the initial stage of use and during long-term use.

[0096] On the other hand, for Test No. A6, since the V content present as precipitates in the steel, that is, V ER was below the predetermined range, sufficient precipitation strengthening effect could not be obtained at the initial stage of use, and the target creep performance was not satisfied under high stress conditions.

[0097] For Test Nos. A5, A13, A14, A16, C5 and E5, since V ER exceeded the predetermined range, there were many precipitates containing coarse V in the as-manufactured steel, so coarsening of the precipitates during use was promoted, and a stable precipitation strengthening effect could not be obtained over a long time, and the target creep performance was not satisfied under low stress conditions.

[0098] For Test Nos. J1 and K1, since the V content and C content were below the ranges of the present invention respectively, precipitates containing V did not precipitate sufficiently from the initial stage of use to during use, and the target creep performance was not satisfied under either low stress or high stress conditions.

[0099] Furthermore, in Test Nos. I1 and L1, since the V content and the C content exceeded the scope of the present invention, respectively, coarsening of the precipitates during use was promoted, and a stable precipitation strengthening effect could not be obtained over a long period of time, and the target creep performance under low stress conditions was not satisfied.

[0100] From Table 2, it can be seen that the steel satisfying the conditions defined in the present invention can stably obtain good creep strength at the initial stage of use and during long-term use.

Examples

[0101] For the ingots cast by melting the materials B and D having the chemical compositions shown in Table 1 and the materials M to Z having the chemical compositions shown in Table 3 in the laboratory, hot forging and hot rolling were sequentially performed to form them into a thickness of 15 mm. From this material, plates having a length of 150 mm and a width of 150 mm were processed.

[0102] The obtained plates were subjected to a heat treatment of annealing at 1075 °C for 0.5 hour and tempering at 780 °C for 1 hour to obtain test specimens.

[0103]

Table 3

[0104] [V content in the precipitates obtained by extraction residue analysis] For the obtained test specimens after annealing and tempering, the mass of V in the residue was measured by the above method.

[0105] [Charpy impact test / toughness] The above-mentioned test specimens after annealing and tempering, and test specimens obtained by repeating the heating and cooling cycle of "room temperature → 600 °C × 108 hours → room temperature" 5 times for these test specimens were prepared. From the central part in the plate thickness direction, 3 pieces of 2 mm V-notch full-size Charpy impact test pieces with notches processed were collected and subjected to the Charpy impact test. The Charpy impact test was performed in accordance with JIS Z 2242 (2005).

[0106] The tests were conducted at 20°C. Among those in which the average value of the absorbed energy of three test pieces was 54 J or more, those in which the absorbed energy values of all three test pieces were 54 J or more were designated as "A", and the others were designated as "B". On the other hand, those in which the average value of the absorbed energy of three test pieces was less than 54 J were designated as "F".

[0107] [Creep rupture test] In addition, round bar creep test pieces were collected from the test materials with the results of the Charpy impact test being A or B, and creep rupture tests were conducted. And as an evaluation for long-term use, creep rupture tests were conducted under the conditions of 600°C × 167 MPa where the target rupture time of the base material was 1000 hours. Also, for some test materials, creep rupture tests were conducted under the conditions of 600°C × 186 MPa where the target rupture time of the base material was 500 hours, and under the conditions of 600°C × 157 MPa where the target rupture time of the base material was 3000 hours, as an evaluation for long-term use. The creep rupture tests were carried out in accordance with JIS Z 2271 (2010). And those with a rupture time exceeding the target rupture time were designated as "A", and those below it were designated as "F".

[0108]

Table 4

[0109] From Table 4, for steel grade U and steel grade V, since the contents of Sn and As respectively exceeded the scope of the present invention, they did not satisfy the target toughness in any of the cases after annealing and tempering, and after repeated heating and cooling.

[0110] In contrast, for the steel satisfying the conditions defined in the present invention, good toughness was obtained after annealing and tempering. In addition, for the steel satisfying the above formula (ii), in addition to after annealing and tempering, good toughness was also obtained in the case of repeated heating and cooling, and it was found that good creep strength was stably obtained at the initial stage of use and during long-term use.

[0111] Steel grade W and steel grade X did not meet the target toughness after repeated heating and cooling because the relational expression between the total content of Sn and As and P did not satisfy the provisions of the present invention.

[0112] Thus, when Sn and As are contained for the purpose of enhancing corrosion resistance, it has been found that by satisfying the above formula (ii), excellent toughness can be obtained even after repeated heating and cooling, and good creep strength can be stably obtained at the initial stage of use and during long-term use.

Industrial Applicability

[0113] According to the present invention, in a ferritic heat-resistant steel, excellent creep strength can be stably obtained during use at high temperatures. Therefore, the ferritic heat-resistant steel of the present invention can be suitably used for equipment used at high temperatures, such as power generation boilers.

Claims

1. The chemical composition is by mass%, C: 0.07 to 0.14%, Si: 0.15 to 0.35%, Mn: 0.30 to 0.55%, P: 0.0030 to 0.0250%, S: 0.0030% or less, Ni: 0.15 to 0.35%, Cr: 8.0 to 9.7%, Mo: 0.20 to 0.60%, W: 1.50 to 2.30%, V: 0.16 to 0.25%, Nb: 0.020 to 0.120%, B: 0.0010 to 0.0050%, N: 0.010 to 0.080%, Al: 0.020% or less, O: 0.020% or less, and Sn: 0.0200% or less, and As: 0.0200% or less, contains one or more selected from the balance: Fe and impurities, satisfying the following formulas (i) and (ii), ferritic heat-resistant steel. -0.2×C + 0.060 ≤ V ＥＲ ≤ -0.1×C + 0.160...(i) 0.0005 ≦ Sn + As ≦ -2×P + 0.0600... (ii) However, the meanings of the symbols in formula (i) are as follows, and the element symbols in the above formula (ii) represent the content (mass%) of each element. C: C content in the steel (mass%) V ＥＲ : V content (mass%) in the precipitate obtained by extract residue analysis

2. The chemical composition contains, in place of a part of Fe, by mass%, Nd: 0.0005 to 0.0400%, the ferritic heat-resistant steel according to Claim 1.

3. The chemical composition contains, in place of a part of Fe, by mass%, Cu: 0.50% or less, Co: 0.50% or less, Ti: 0.20% or less, Ta: 0.20% or less, Ca: 0.0150% or less, Mg: 0.0150% or less, and REM: 0.0400% or less, contains one or more selected from the ferritic heat-resistant steel according to Claim 1 or Claim 2.

Images