FERRITIC HEAT-RESISTANT STEEL

Abstract

A ferritic heat-resistant steel includes, by mass: from 0.06 to 0.11% of C; from 0.15 to 0.35% of Si; from 0.35 to 0.65% of Mn; from 0 to 0.02% of P; from 0 to 0.003% of S; from 0.005 to 0.25% of Ni; from 0.005 to 0.25% of Cu; from 2.7 to 3.3% of Co; from 8.3 to 9.7% of Cr; from 2.5 to 3.5% of W; from 0.15 to 0.25% of V; from 0.03 to 0.08% of Nb; from 0.002 to 0.04% of Ta; from 0.01 to 0.06% of Nd; from 0.006 to 0.016% of B; from 0.005 to 0.015% of N; from 0 to 0.02% of Al; and from 0 to 0.02% of O, with a balance consisting of Fe and impurities, and an amount of W, which is analyzed as an electrolytically extracted residue, [% W].sub.ER, satisfying: −10×[% B]+0.26≤[% W].sub.ER≤10×[% B]+0.54.

Claims

1. A ferritic heat-resistant steel comprising, by mass: from 0.06% to 0.11% of C; from 0.15% to 0.35% of Si; from 0.35% to 0.65% of Mn; 0.020% or less of P; 0.0030% or less of S; from 0.005% to 0.250% of Ni; from 0.005% to 0.250% of Cu; from 2.7% to 3.3% of Co; from 8.3% to 9.7% of Cr; from 2.5% to 3.5% of W; from 0.15% to 0.25% of V; from 0.030% to 0.080% of Nb; from 0.002% to 0.040% of Ta; from 0.010% to 0.060% of Nd; from 0.006% to 0.016% of B; from 0.005% to 0.015% of N; 0.020% or less of Al; and 0.020% or less of O, with a balance consisting of Fe and impurities, wherein an amount of W, which is analyzed as an electrolytically extracted residue, satisfies the following Formula (1):
−10×[% B]+0.26≤[% W].sub.ER≤10×[% B]+0.54  Formula (1) wherein, in Formula (1), [% W].sub.ER represents an amount of W, which is analyzed as an electrolytically extracted residue, in % by mass, and [% B] represents a content of B, in % by mass, that is contained in the ferritic heat-resistant steel.

2. The ferritic heat-resistant steel according to claim 1, wherein: by mass, a total content of Ta and Nb is from 0.040% to 0.110%, and a ratio Ta/Nb of a content of Ta to a content of Nb is from 0.10 to 0.70.

3. The ferritic heat-resistant steel according to claim 1, further comprising, by mass, at least one selected from the group consisting of: 0.50% or less of Mo; 0.20% or less of Ti; 0.015% or less of Ca; 0.015% or less of Mg; and 0.005% or less of Sn.

4. The ferritic heat-resistant steel according to claim 1, wherein a tensile strength at room temperature, as defined in JIS Z2241: 2011, is 620 MPa or more, and a full-size Charpy absorbed energy at 20° C., as defined in JIS Z2242: 2005, is 27 J or more.

5. The ferritic heat-resistant steel according to claim 2, further comprising, by mass, at least one selected from the group consisting of: 0.50% or less of Mo; 0.20% or less of Ti; 0.015% or less of Ca; 0.015% or less of Mg; and 0.005% or less of Sn.

6. The ferritic heat-resistant steel according to claim 2, wherein a tensile strength at room temperature, as defined in JIS Z2241: 2011, is 620 MPa or more, and a full-size Charpy absorbed energy at 20° C., as defined in JIS Z2242: 2005, is 27 J or more.

7. The ferritic heat-resistant steel according to claim 3, wherein a tensile strength at room temperature, as defined in JIS Z2241: 2011, is 620 MPa or more, and a full-size Charpy absorbed energy at 20° C., as defined in JIS Z2242: 2005, is 27 J or more.

8. The ferritic heat-resistant steel according to claim 4, wherein a tensile strength at room temperature, as defined in JIS Z2241: 2011, is 620 MPa or more, and a full-size Charpy absorbed energy at 20° C., as defined in JIS Z2242: 2005, is 27 J or more.

9. The ferritic heat-resistant steel according to claim 5, wherein a tensile strength at room temperature, as defined in JIS Z2241: 2011, is 620 MPa or more, and a full-size Charpy absorbed energy at 20° C., as defined in JIS Z2242: 2005, is 27 J or more.

Description

MODE FOR CARRYING OUT THE INVENTION

[0046] In the present disclosure, the reasons for limiting the composition of the ferritic heat-resistant steel are as follows.

[0047] In the following explanation, the description “%” used to describe the content of each element means the content thereof in “% by mass”. Further, in the present disclosure, a numerical range that is expressed by using “to” means a range including the numerical values described before and after “to” as a lower limit value and an upper limit value, unless otherwise defined.

[0048] In numerical ranges that are described in a stepwise manner in the present disclosure, the upper limit value or the lower limit value in one numerical range that is described in a stepwise manner may be replaced with the upper limit value or the lower limit value of another numerical range that is described in a stepwise manner, or may be replaced with values shown in the examples.

[0049] The definition of the term “step” in the present disclosure includes not only an independent step, but also a step which is not clearly distinguishable from another step, as long as the intended purpose of the step is achieved.

[0050] C: from 0.06% to 0.11%

[0051] C is effective for obtaining a martensite structure, in addition to forming fine carbide or carbonitride and contributing to an improvement in tensile strength and creep strength. In the range of content of W and B in the present disclosure, it is necessary for C to be contained at 0.06% or more in order to sufficiently obtain these effects. However, since including an excessive amount of C instead causes a decrease in creep strength and toughness, the content of C is set to 0.11% or less. The lower limit of the content of C is preferably 0.07%, and the upper limit thereof is preferably 0.10%. The lower limit of the content of C is more preferably 0.08%, and the upper limit thereof is more preferably 0.09%.

[0052] Si: from 0.15% to 0.35%

[0053] Si is contained as a deoxidizing agent, but is also an element that is effective for improving steam oxidation resistance (that is, resistance to oxidation by water vapor). It is necessary for Si to be contained at 0.15% or more in order to sufficiently obtain this effect. However, including an excessive amount of Si causes a decrease in ductility. Therefore, the content of Si is set to 0.35% or less. The lower limit of the content of Si is preferably 0.18%, and the upper limit thereof is preferably 0.32%. The lower limit of the content of Si is more preferably 0.20%, and the upper limit thereof is more preferably 0.30%.

[0054] Mn: from 0.35% to 0.65%

[0055] Mn is contained as a deoxidizing agent, as is the case with Si, but is also effective for obtaining a martensite structure. It is necessary for Mi to be contained at 0.35% or more in order to sufficiently obtain this effect. However, since including an excessive amount of Mn causes creep embrittlement, the content of Mn is set to 0.65% or less. The lower limit of the content of Mn is preferably 0.38%, and the upper limit thereof is preferably 0.62%. The lower limit of the content of Mn is more preferably 0.40%, and the upper limit thereof is more preferably 0.60%.

[0056] P: 0.020% or less

[0057] Including an excessive amount of P causes a decrease in creep ductility. Therefore, it is necessary for the content of P to be 0.020% or less. The content of P is preferably 0.018% or less, and more preferably 0.015% or less. It should be noted that the lower the content of P, the better. However, an extreme reduction in the content of P causes an extreme increase in material costs. Therefore, the lower limit of the content of P is preferably 0.0005%, and more preferably 0.001%.

[0058] S: 0.0030% or less

[0059] As is the case with P, including an excessive amount of S causes a decrease in creep ductility. Therefore, it is necessary for the content of S to be 0.0030% or less. The content of S is preferably 0.0025% or less, and more preferably 0.0020% or less. It should be noted that the lower the content of S, the better. However, an extreme reduction in the content of S causes an extreme increase in production costs. Therefore, the lower limit of the content of S is desirably 0.0002%, and more preferably 0.0004%.

[0060] Ni: from 0.005% to 0.250%

[0061] Ni is an element that is effective for obtaining a martensite structure. It is necessary for Ni to be contained at 0.005% or more in order to obtain this effect. However, in the present disclosure in which the content of Co is in the following range, including Ni in excess of 0.250%, results in saturation of the effect, and causes an increase in costs due to Ni being an expensive element. Therefore, the upper limit of the content of Ni is set to 0.250%. The lower limit of the content of Ni is preferably 0.020%, and the upper limit thereof is preferably 0.200%. The lower limit of the content of Ni is more preferably 0.050%, and the upper limit thereof is more preferably 0.150%.

[0062] Cu: from 0.005% to 0.250%

[0063] Cu is an element that is effective for obtaining a martensite structure, as is the case with Ni. It is necessary for Cu to be contained at 0.005% or more in order to obtain the effect. However, in the present disclosure in which the content of Co is in the following range, the upper limit of the content of Cu is set to 0.250% since including Cu in excess of 0.250% results in saturation of the effect. The preferable lower limit of the content of Cu is 0.020%, and the preferable upper limit thereof is 0.200%. The lower limit of the content of Cu is more preferably 0.050%, and the upper limit thereof is more preferably 0.150%.

[0064] Co: from 2.7% to 3.3%

[0065] Co is an element that is effective for obtaining a martensite structure and ensuring creep strength. It is necessary for Co to be contained at 2.7% or more in order to sufficiently obtain this effect. However, Co is an extremely expensive element, and including an excessive amount of Co causes an increase in material costs and instead causes a decrease in creep strength and creep ductility. Therefore, the content of Co is set to 3.3% or less. The lower limit of the content of Co is preferably 2.8%, and the upper limit thereof is preferably 3.2%. The lower limit of the content of Co is more preferably 2.9%, and the upper limit thereof is more preferably 3.1%.

[0066] Cr: from 8.3% to 9.7%

[0067] Cr is an element that is effective for ensuring steam oxidation resistance at a high temperature and corrosion resistance. Further, Cr precipitates as carbide, and also contributes to an improvement in creep strength. It is necessary for Cr to be contained at 8.3% or more in order to sufficiently obtain these effects. However, including an excessive amount of Cr causes a decrease in stability of the carbide, and instead decreases creep strength. Therefore, the content of Cr is set to 9.7% or less. The lower limit of the content of Cr is preferably 8.5%, and the upper limit thereof is preferably 9.5%. The lower limit of the content of Cr is more preferably 8.7%, and the upper limit thereof is more preferably 9.3%.

[0068] W: from 2.5% to 3.5%

[0069] W dissolves as a solid solution in the matrix or precipitates as M.sub.23C.sub.6-type carbide, and contributes to ensuring tensile strength, in addition to precipitating as an intermetallic compound and contributing to ensuring creep strength at a high temperature during use for a long period of time. It is necessary for W to be contained at 2.5% or more in order to obtain the effect. However, including an excessive amount of W results in saturation of the effect of increasing creep strength, and causes an increase in material costs since W is an expensive element. Therefore, the content of W is set to 3.5% or less. The lower limit of the content of W is preferably 2.6%, and the upper limit thereof is preferably 3.3%. The lower limit of the content of W is more preferably 2.8%, and the upper limit thereof is more preferably 3.1%.

[0070] It should be noted that the content of W as used herein means the total amount of W contained in the ferritic heat-resistant steel. In other words, the content of W means the total of the amount of W dissolved as a solid solution in the matrix and the amount of W present as a precipitate.

[0071] Further, in the present disclosure, in addition to satisfying the above-described range of the content of W, it is necessary for the amount of W that is present as a precipitate, that is, the amount of W analyzed as the electrolytically extracted residue, to satisfy the relationship with the amount of B, as will be described later.

[0072] V: from 0.15% to 0.25%

[0073] V precipitates within the grains as fine carbonitride, and contributes to an improvement in creep strength. It is necessary for V to be contained at 0.15% or more in order to sufficiently obtain this effect. However, if the content of V is excessive, the carbonitride precipitates coarsely and in a large amount, and instead causes a decrease in creep strength and creep ductility. Therefore, the content of V is set to 0.25% or less. The lower limit of the content of V is preferably 0.16%, and the upper limit thereof is preferably 0.24%. The lower limit of the content of V is more preferably 0.18%, and the upper limit thereof is more preferably 0.22%.

[0074] Nb: from 0.030% to 0.080%

[0075] Nb precipitates within the grains as fine carbonitride, and contributes to an improvement in creep strength. It is necessary for Nb to be contained at 0.030% or more in order to sufficiently obtain the effect. However, if the content of Nb is excessive, the carbonitride precipitates coarsely and in a large amount, and instead causes a decrease in creep strength and creep ductility. Therefore, the content of Nb is set to 0.080% or less. The lower limit of the content of Nb is preferably 0.035%, and the upper limit thereof is preferably 0.075%. The lower limit of the content of Nb is more preferably 0.040%, and the upper limit thereof is more preferably 0.070%.

[0076] Ta: from 0.002% to 0.040%

[0077] As is the case with Nb, Ta precipitates within the grains as fine carbonitride, and contributes to an improvement in creep strength. It is necessary for Ta to be contained at 0.002% or more in order to obtain the effect. However, if the content of Ta is excessive, the carbonitride precipitates coarsely and in a large amount, and instead causes a decrease in creep strength and creep ductility. Therefore, the content of Ta is set to 0.040% or less. The lower limit of the content of Ta is preferably 0.003%, and the upper limit thereof is preferably 0.035%. The lower limit of the content of Ta is more preferably 0.004%, and the upper limit thereof is more preferably 0.030%.

[0078] Total content of Nb and Ta: from 0.040% to 0.110%

[0079] Nb and Ta precipitate within the grains as fine carbonitride, and contribute to an improvement in creep strength. In order to sufficiently obtain this effect, it is preferable that a total of 0.040% or more of Nb and Ta is contained. However, if the content of Nb and Ta is excessive, Nb and Ta carbonitrides precipitate coarsely and in a large amount, and instead causes a decrease in creep strength and creep ductility. Therefore, it is preferable to set the upper limit of the total content of Nb and Ta to 0.110%. The lower limit of the total content of Nb and Ta is more preferably 0.050%, and the upper limit thereof is more preferably 0.100%. The lower limit of the total content is more preferably 0.060%, and the upper limit thereof is more preferably 0.090%.

[0080] Ratio of Ta/Nb of content of Ta to content of Nb: from 0.10 to 0.70

[0081] As described above, Nb and Ta precipitate within the grains as fine carbonitride, and contribute to an improvement in creep strength. However, if the content ratio Ta/Nb of the content of Ta and Nb is small, there are cases in which the growth of the carbonitride becomes faster during use for a long period of time, the precipitation strengthening effect thereof disappears at an early stage, and stable creep strength cannot be sufficiently obtained when used for a long period of time. On the other hand, if the content ratio Ta/Nb of the content of Ta and Nb is large, there are cases in which the start of precipitation of the carbonitride is delayed at the initial stage of use, and sufficient high-temperature strength cannot be obtained. Therefore, it is preferable to set the content ratio Ta/Nb of the content of Ta and Nb to from 0.10 to 0.70. A more preferable range of the content ratio Ta/Nb of the content of Ta and Nb is from 0.12 to 0.68, and an even more preferable range thereof is 0.15 to 0.65.

[0082] Nd: from 0.010% to 0.060%

[0083] Nd binds to S and P, removes the adverse effects thereof, and improves creep ductility. It is necessary for Nd to be contained at 0.010% or more in order to sufficiently obtain this effect. However, if an excessive amount of Nd is included, Nd binds with oxygen to cause a decrease in cleanliness and cause a deterioration in hot workability. Therefore, the content of Nd is set to 0.060% or less. The lower limit of the content of Nd is preferably 0.015%, and the upper limit thereof is preferably 0.055%. The lower limit of the content of Nd is more preferably 0.020%, and the upper limit thereof is more preferably 0.050%.

[0084] B: from 0.006% to 0.016%

[0085] B is effective for obtaining a martensite structure. In addition, B has the effect of dissolving in M.sub.23C.sub.6-type carbide, to allow the carbide to finely disperse, and thereby contributes to ensuring tensile strength and toughness. Further, B also contributes to an improvement in creep strength. It is necessary for B to be contained at 0.006% or more in order to obtain the effects. However, if an excessive amount of B is included, B mixes into a weld metal during welding, resulting in an increased solidification crack susceptibility; therefore, the upper limit of the content of B is set to 0.016%. The lower limit of the content of B is preferably 0.007%, and the upper limit thereof is preferably 0.014%. The lower limit of the content of B is more preferably 0.008%, and the upper limit thereof is preferably 0.012%.

[0086] It should be noted that, in the present disclosure, it is necessary for the amount of W present as a precipitate, that is, the amount of W analyzed as the electrolytically extracted residue, to satisfy a predetermined relationship with the content of B, as will be described later.

[0087] N: from 0.005% to 0.015%

[0088] N binds to Nb and Ta during use at a high temperature and precipitates within the grains as fine carbonitride, and therefore contributes to an improvement in creep strength. It is necessary for N to be contained at 0.005% or more in order to obtain this effect. However, including an excessive amount of N leads to coarsening of the carbonitride, and instead causes a decrease in creep ductility; therefore, the content of N is set to 0.015% or less. The lower limit of the content of N is preferably 0.006%, and the upper limit thereof is preferably 0.014%. The lower limit of the content of N is more preferably 0.008%, and the upper limit thereof is more preferably 0.012%.

[0089] Al: 0.020% or less

[0090] Although Al is contained as a deoxidizing agent, including a large amount of Al significantly impairs cleanliness, resulting in deterioration in workability. Further, including a large amount of Al is not preferable from the viewpoint of creep strength. Therefore, the content of Al is set to 0.020% or less. The content of Al is preferably 0.018% or less, and more preferably 0.015% or less. It should be noted that it is not necessary to set a lower limit of the content of Al. However, since an extreme decrease in the content of Al causes an increase in production costs, the content of Al is preferably set to 0.001% or more.

[0091] O: 0.020% or less

[0092] O is present as an impurity, and causes a decrease in workability in a case of being contained in a large amount. Therefore, the content of O is set to 0.020% or less. The content of O is preferably 0.015% or less, and more preferably 0.010% or less. It should be noted that it is not necessary to set a lower limit of the content of O, in particular. However, since an extreme decrease in the content of O causes an increase in production costs, the content of O is preferably set to 0.001% or more.

[0093] Balance: Consists of Fe and Impurities

[0094] The ferritic heat-resistant steel according to the present disclosure includes each of the above-mentioned elements, and a balance consists of Fe and impurities.

[0095] It should be noted that “impurities” are so-called unavoidably mixed-in components that are mixed in due to various factors in the production process, including raw materials such as ore or scrap, when steel materials are industrially produced, and refers to components that are not intentionally added.

[0096] Further, the ferritic 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 reasons for the limitation will be described below.

[0097] Group Mo: 0.50% or less [0098] Ti: 0.20% or less [0099] Ca: 0.015% or less [0100] Mg: 0.015% or less [0101] Sn: 0.005% or less

[0102] Mo: 0.50% or less

[0103] Mo may be included since, as is the case with W, Mo dissolves as a solid solution in the matrix, and contributes to ensuring creep strength at a high temperature. However, including an excessive amount of Mo results in saturation of the effect, and causes an increase in material costs since Mo is an expensive element; therefore the content of Mo is set to 0.50% or less. The upper limit of the content of Mo is preferably 0.40%, and more preferably 0.20% or less. A lower limit in a case in which Mo is contained is preferably 0.01%, and the lower limit thereof is more preferably 0.02%.

[0104] Ti: 0.20% or less

[0105] Ti may be contained as necessary since, as is the case with Nb and Ta, Ti precipitates within the grains as fine carbonitride during use at a high temperature, and contributes to an improvement in creep strength. However, if the content of Ti is excessive, the carbonitride precipitates coarsely and in a large amount, and causes a decrease in creep strength and creep ductility; therefore the content of Ti is set to 0.20% or less. The upper limit of the content of Ti is preferably 0.15%, and more preferably 0.10% or less. A lower limit in a case in which Ti is contained is preferably 0.01%, and the lower limit thereof is more preferably 0.02%.

[0106] Ca: 0.015% or less

[0107] Ca may be contained as necessary since Ca has the effect of improving hot workability in the production process. However, including an excessive amount of Ca causes Ca to bind to oxygen and significantly decreases cleanliness, and instead results in deterioration in hot workability; therefore, the content of Ca is set to 0.015% or less. The content of Ca is preferably 0.012% or less, and more preferably 0.010% or less. A lower limit in a case in which Ca is contained is preferably 0.0005%, and the lower limit thereof is more preferably 0.001%.

[0108] Mg: 0.015% or less

[0109] As is the case with Ca, Mg may be contained as necessary, since Mg has the effect of improving the hot workability in the production process. However, including an excessive amount of Mg causes Mg to bind to oxygen and significantly decreases cleanliness, and instead results in deterioration in hot workability; therefore, the content of Mg is set to 0.015% or less. The content of Mg is preferably 0.012% or less, and more preferably 0.010% or less. A lower limit in a case in which Mg is contained is preferably 0.0005%, and the lower limit thereof is more preferably 0.001%.

[0110] Sn: 0.005% or less

[0111] Sn may be contained as necessary, since Sn is concentrated under the scales on the surface of the steel and has the effect of improving corrosion resistance. However, since including an excessive amount of Sn causes a decrease in toughness, the content of Sn is set to 0.005% or less. The content of Sn is preferably 0.004% or less, and more preferably 0.003% or less. A lower limit in a case in which Sn is contained is preferably 0.0005%, and the lower limit thereof is more preferably 0.0010%.

[0112] Amount of W Analyzed as Electrolytically Extracted Residue ([% W].sub.ER):

−10×[% B]+0.26≤[% W].sub.ER≤10×[% B]+0.54

[0113] W contained in the ferritic heat-resistant steel precipitates in the form contained in the M.sub.23C.sub.6-type carbide during the tempering heat treatment in the production process. When precipitated finely, this carbide contributes to ensuring tensile strength. However, on the other hand, if this carbide is precipitated excessively, toughness decreases. The amount of this carbide can be estimated as the amount of W analyzed as the electrolytically extracted residue.

[0114] When the amount of the M.sub.23C.sub.6-type carbide which contains W is small, a strengthening effect due to precipitation of the carbide is small, and in addition to not being able to obtain the required tensile strength, recovery and softening of the structure do not proceed and toughness also decreases. On the other hand, if this carbide is excessively and coarsely precipitated, the carbide does not contribute to the enhancement of tensile strength and the like, and the amount of W that is dissolved as a solid solution in the matrix of the steel is reduced, and a solid solution strengthening effect is also reduced. As a result, the required tensile strength cannot be obtained, and the carbide becomes the initiation of fracture, and toughness also decreases.

[0115] Further, B contained in the steel has the effect of allowing the M.sub.23C.sub.6-type carbide to precipitate finely, without affecting the amount of precipitation of the above-described carbide. Therefore, it is easy to obtain the strengthening effect due to precipitation of the carbide even with a small amount of precipitation, and at the same time, B is capable of reducing the size of precipitates (that is, making the precipitates fine), to prevent the disappearance of the strengthening effect due to precipitation and prevent a decrease in toughness due to the carbide being the initiation of fracture. Therefore, in order to obtain the required tensile strength and toughness, it is necessary for the lower limit and the upper limit of the amount of W present as the precipitation, namely, the amount of W analyzed as electrolytically extracted residue ([% W].sub.ER), to be set in a relationship so as to satisfy the following Formula (1), depending on the content of B in the steel.

−10×[% B]+0.26≤[% W].sub.ER≤10×[% B]+0.54  Formula (1)

[0116] In Formula (1), [% W].sub.ER represents the amount (% by mass) of W analyzed as electrolytically extracted residue, and [% B] represents the content (% by mass) of B in the ferritic heat-resistant steel.

[0117] The amount of W analyzed as the electrolytically extracted residue can be adjusted according to the amounts of W and C contained in the steel, in addition to the conditions of the tempering heat treatment and the like. Specifically, the higher the amount of W and the higher the amount of C which are contained in the steel, the larger the amount of W analyzed as the electrolytically extracted 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 electrolytically extracted residue. Further, in the cooling after the tempering treatment, the lower the cooling rate, the larger the amount of W analyzed as the electrolytically extracted residue.

[0118] The amount of W analyzed as the electrolytically extracted residue is measured as follows.

[0119] A test material of a predetermined size is collected from the ferritic heat-resistant steel. Anodic dissolution of the test material is carried out at a current density of 20 mA/cm.sup.2, by galvanostatic electrolysis method using a 10% by volume acetylacetone-1% by mass tetramethyl ammonium chloride-methanol solution, as an electrolytic solution, and carbide is extracted as a residue. After acid decomposition of the extracted residue, ICP (Inductively Coupled Plasma) optical emission spectrometry is carried out to measure the mass of W in the residue. The mass of W in the residue is divided by the amount of dissolution of the test material, to determine the amount of W present as carbide. That is, the thus determined amount of W is the amount of W analyzed as the electrolytically extracted residue.

[0120] Characteristics of the Ferritic Heat-Resistant Steel

[0121] (1) Tensile Strength

[0122] The ferritic heat-resistant steel according to the present disclosure preferably has a tensile strength at room temperature of 620 MPa or more, and more preferably 670 MPa or more.

[0123] The tensile strength is measured at room temperature (from 10° C. to 35° C.) in accordance with JIS Z2241: 2011 using a No. 14A round-bar test specimen in which a parallel portion diameter is 8 mm and a parallel portion length is 55 mm.

[0124] (2) Full-Size Charpy Absorbed Energy

[0125] The ferritic heat-resistant steel according to the present disclosure preferably has a full-size Charpy absorbed energy at 20° C. of 27 J or more.

[0126] The full-size Charpy absorbed energy is measured at 20° C. in accordance with JIS Z2242: 2005 and using a full-size Charpy impact test specimen with a 2 mm V-notch.

[0127] (3) Creep Performance

[0128] A creep rupture test is carried out on the ferritic heat-resistant steel according to the present disclosure under the conditions of a temperature of 650° C. and a pressure of 157 MPa, in which a target rupture time of the base metal is 1,000 hours, and it is preferable that the rupture time exceeds the target rupture time.

[0129] The creep rupture test is carried out using a round-bar creep test specimen in accordance with JIS Z2271: 2010.

[0130] Next, a method for producing the ferritic heat-resistant steel according to the present disclosure will be described with reference to an example.

[0131] Forming Step

[0132] In the production of the ferritic heat-resistant steel according to the present disclosure, first, a raw material is formed into a final shape of the ferritic heat-resistant steel. The forming step includes all the processes involving the deformation of the steel material into a final shape, such as casting, forging, rolling, and the like.

[0133] One example of the forming step may be, for example, a step in which an ingot that is obtained by melting and casting the raw material is subjected to hot forging and hot rolling, or alternatively, subjected to hot forging, hot rolling and cold working, and is formed into a final shape of the ferritic heat-resistant steel.

[0134] Normalizing Heat Treatment Step

[0135] After the forming step, for example, a normalizing heat treatment may be carried out. For example, it is preferable that the normalizing heat treatment is carried out under the conditions of a temperature of from 1,130° C. to 1,170° C., and a period of time from 0.1 to 1.5 hours.

[0136] Tempering Heat Treatment Step

[0137] Further, after the normalizing heat treatment step, for example, a tempering heat treatment may be carried out. For example, it is preferable that the tempering heat treatment is carried out under the conditions of a temperature of from 770° C. to 790° C., and a period of time from 1 to 5 hours.

[0138] Applications

[0139] The ferritic heat-resistant steel according to the present disclosure is used, for example, in equipment used at a high temperature, such as a boiler for power generation.

[0140] Examples of equipment used at a high temperature include: boiler pipes for use in coal-fired power generation plants, oil-fired power generation plants, waste incineration power plants, biomass power generation plants, and the like; and cracking tubes for use in petrochemical plants.

[0141] Here, the expression “used at a high temperature” in the present disclosure includes, for example, an embodiment in which the steel is used in an environment of from 350° C. to 700° C. (further, from 400° C. to 650° C.).

EXAMPLES

[0142] Hereinafter, the present disclosure will be described in more specific detail with reference to Examples. However, the present disclosure is not limited to these Examples.

Example 1

[0143] Ingots which were prepared by melting and casting raw materials A to I, each having a chemical composition shown in Table 1-1 and Table 1-2, in a laboratory, were subjected to hot forging and hot rolling, in this order, and formed at a thickness of 15 mm. The resulting raw materials were processed into plate materials each having a length of 150 mm and a width of 150 mm. In Table 1-1 and Table 1-2, the unit of each component except for “Ratio Ta/Nb” is % by mass, and the balance is Fe and impurities. In addition, the underlined values in the tables below indicate values that are outside the scope of the present disclosure.

TABLE-US-00001 TABLE 1-1 Symbol C Si Mn P S Ni Cu Co Cr W V Nb Ta A 0.11 0.25 0.62 0.018 0.0012 0.011 0.018 3.0 9.6 3.4 0.16 0.034 0.006 B 0.10 0.35 0.38 0.015 0.0020 0.150 0.010 2.8 9.0 2.9 0.18 0.045 0.012 C 0.06 0.31 0.40 0.016 0.0008 0.020 0.050 3.1 8.8 2.5 0.20 0.039 0.004 D 0.07 0.20 0.52 0.014 0.0025 0.051 0.022 3.3 8.7 3.0 0.21 0.050 0.023 E 0.08 0.18 0.38 0.020 0.0010 0.085 0.142 3.2 9.3 3.2 0.20 0.052 0.016 F 0.06 0.22 0.35 0.018 0.0012 0.015 0.032 2.9 8.4 2.1 0.16 0.033 0.008 G 0.13 0.15 0.55 0.016 0.0022 0.225 0.189 3.1 9.7 3.7 0.15 0.033 0.008 H 0.04 0.32 0.45 0.015 0.0018 0.023 0.015 2.8 8.3 2.3 0.16 0.034 0.010 I 0.11 0.31 0.39 0.017 0.0015 0.240 0.020 3.0 9.6 3.9 0.15 0.032 0.009

TABLE-US-00002 TABLE 1-2 Symbol Nd B N Al O Others Ta + Nb Ratio Ta/Nb A 0.012 0.009 0.012 0.008 0.006 Mo: 0.03 0.040 0.18 B 0.023 0.010 0.008 0.010 0.009 0.057 0.27 C 0.049 0.009 0.010 0.009 0.008 Ca: 0.002 0.043 0.10 D 0.021 0.013 0.011 0.010 0.010 0.073 0.46 E 0.032 0.010 0.009 0.011 0.009 Mg: 0.001, Ti: 0.03 0.068 0.31 F 0.025 0.007 0.012 0.013 0.012 0.041 0.24 G 0.033 0.012 0.009 0.012 0.012 0.041 0.24 H 0.026 0.006 0.015 0.012 0.010 Ca: 0.002 0.044 0.29 I 0.022 0.009 0.009 0.010 0.008 0.041 0.28

[0144] The thus obtained plate materials were subjected to normalizing and tempering heat treatments as shown in Table 2, to prepare test materials.

[0145] Measurement of Amount of W Analyzed as Electrolytically Extracted Residue

[0146] From each of the resulting test materials, a test specimen having a size of 8 mm square and a length of 40 mm was collected, and the amount of W analyzed as the electrolytically extracted residue was measured by the method described in the above described-embodiment, namely, by galvanostatic electrolysis method. Specifically, anodic dissolution of the test material was carried out at a current density of 20 mA/cm.sup.2, by galvanostatic electrolysis method using a 10% by volume acetylacetone-1% by mass tetramethyl ammonium chloride-methanol solution, as an electrolytic solution, and carbides were extracted as a residue. After the acid decomposition of the extracted residue, ICP (high frequency inductively coupled plasma) emission spectrometry was carried out to measure the mass of W in the residue. The mass of W in the residue was divided by the amount of dissolution of the test material, to determine the amount of W present as carbide, for each test material.

TABLE-US-00003 TABLE 2 Normalizing Heat Treatment Tempering Heat Treatment Symbol of Raw Material Amount of B Temperature Time Temperature Time Steel Plate Used % by mass (° C.) (hours) (° C.) (hours) A1 A 0.009 1150 0.5 780 1 A2 A 0.009 1150 0.5 780 2 A3 A 0.009 1150 0.5 780 3 A4 A 0.009 1150 0.5 760 3 A5 A 0.009 1150 0.5 790 3 A6 A 0.009 1150 0.5 780 4 A7 A 0.009 1150 0.5 780 5 A8 A 0.009 1150 0.5 780 0.1 A9 A 0.009 1150 0.5 780 0.5 A10 A 0.009 1150 0.5 780 7 A11 A 0.009 1150 0.5 780 10 B1 B 0.010 1150 0.5 780 1 B2 B 0.010 1150 0.5 780 2 B3 B 0.010 1150 0.5 780 3 B4 B 0.010 1150 0.5 760 3 B5 B 0.010 1150 0.5 790 3 B6 B 0.010 1150 0.5 780 4 B7 B 0.010 1150 0.5 780 5 B8 B 0.010 1150 0.5 780 0.1 B9 B 0.010 1150 0.5 780 0.5 B10 B 0.010 1150 0.5 780 7 B11 B 0.010 1150 0.5 780 10 C1 C 0.009 1150 0.5 780 1 C2 C 0.009 1150 0.5 780 2 C3 C 0.009 1150 0.5 780 3 C4 C 0.009 1150 0.5 780 4 C5 C 0.009 1150 0.5 780 5 D1 D 0.013 1150 0.5 780 1 E1 E 0.010 1150 0.5 780 1 F1 F 0.007 1150 0.5 780 1 G1 G 0.012 1150 0.5 780 1 H1 H 0.006 1150 0.5 780 1 I1 I 0.009 1150 0.5 780 1 Amount of W Analyzed as Electrolytically Left Side of Right Side of Symbol of Extracted Residue Formula (1) −10 × Formula (1) Steel Plate (% by mass) [% B] + 0.26 10 × [% B] + 0.54 A1 0.42 0.17 0.63 A2 0.44 0.17 0.63 A3 0.47 0.17 0.63 A4 0.42 0.17 0.63 A5 0.59 0.17 0.63 A6 0.49 0.17 0.63 A7 0.51 0.17 0.63 A8 0.16 0.17 0.63 A9 0.20 0.17 0.63 A10 0.60 0.17 0.63 A11 0.64 0.17 0.63 B1 0.38 0.16 0.64 B2 0.39 0.16 0.64 B3 0.41 0.16 0.64 B4 0.35 0.16 0.64 B5 0.42 0.16 0.64 B6 0.44 0.16 0.64 B7 0.46 0.16 0.64 B8 0.15 0.16 0.64 B9 0.19 0.16 0.64 B10 0.59 0.16 0.64 B11 0.65 0.16 0.64 C1 0.19 0.17 0.63 C2 0.25 0.17 0.63 C3 0.26 0.17 0.63 C4 0.31 0.17 0.63 C5 0.40 0.17 0.63 D1 0.34 0.13 0.67 E1 0.40 0.16 0.64 F1 0.18 0.19 0.61 G1 0.67 0.14 0.66 H1 0.10 0.20 0.60 I1 0.65 0.17 0.63

[0147] Tensile Test/Tensile Strength

[0148] Further, a No. 14A round-bar test specimen in which the parallel portion diameter was 8 mm and the length of the parallel portion length is 55 mm, and which was defined in JIS Z2241: 2011, was collected from each test material, and subjected to a tensile test at room temperature (from 10° C. to 35° C.) in accordance with JIS Z2241:2011. The test materials having a tensile strength of 620 MPa or more, which is the tensile strength required for a base metal, were evaluated as “Pass”, and among these, those having a tensile strength of 670 MPa or more were evaluated as “Pass/excellent”, and the test materials other than those were evaluated as “Pass/acceptable”. On the other hand, test materials having a tensile strength below 620 MPa were evaluated as “Fail”.

[0149] Charpy Impact Test/Toughness

[0150] Three pieces of V-notch full-size Charpy impact test specimens provided with a 2 mm notch were collected from the central portion in a plate thickness direction of each test material, and subjected to a Charpy impact test. The Charpy impact test was carried out in accordance with JIS Z2242: 2005. The test was carried out at 20° C., and the test materials in each of which the mean value of the absorbed energy of the three pieces of test specimens was 27 J or more were evaluated as “Pass”. Among these, the test materials in each of which the three pieces of test specimens each had an absorbed energy of 27 J or more were evaluated as “Pass/excellent”, and the test materials other than those were evaluated as “Pass/acceptable”. On the other hand, the test materials in each of which the mean value of the absorbed energy of the three pieces of test specimens was below 27 J were evaluated as “Fail”.

[0151] Creep Rupture Test

[0152] In addition, from each of the test materials which passed the tensile test and the Charpy impact test, a round-bar creep test specimen was collected, and subjected to a creep rupture test. As the evaluation of creep strength when used for a long period of time, the creep rupture test was carried out under the conditions of a temperature of 650° C. and a pressure of 157 MPa, in which a target rupture time of the base metal was 1,000 hours. The creep rupture test was carried out in accordance with JIS Z2271: 2010. The test materials with a rupture time exceeding the target rupture time were evaluated as “Pass”, and those with a rupture time below the target rupture time were evaluated as “Fail”.

TABLE-US-00004 TABLE 3 Symbol of Steel (Tensile Strength) (Impact Strength) Plate Tensile test Charpy Impact Test Creep Rupture Test A1 Pass/Excellent Pass/Excellent Pass A2 Pass/Excellent Pass/Excellent Pass A3 Pass/Excellent Pass/Excellent Pass A4 Pass/Excellent Pass/Excellent Pass A5 Pass/Excellent Pass/Excellent Pass A6 Pass/Excellent Pass/Excellent Pass A7 Pass/Excellent Pass/Excellent Pass A8 Fail Fail Not performed A9 Pass/Excellent Pass/Excellent Pass A10 Pass/Acceptable Pass/Acceptable Pass A11 Fail Fail Not performed B1 Pass/Excellent Pass/Excellent Pass B2 Pass/Excellent Pass/Excellent Pass B3 Pass/Excellent Pass/Excellent Pass B4 Pass/Excellent Pass/Excellent Pass B5 Pass/Excellent Pass/Excellent Pass B6 Pass/Excellent Pass/Excellent Pass B7 Pass/Excellent Pass/Excellent Pass B8 Fail Fail Not performed B9 Pass/Excellent Pass/Excellent Pass B10 Pass/Excellent Pass/Acceptable Pass B11 Fail Fail Not performed C1 Pass/Excellent Pass/Excellent Pass C2 Pass/Excellent Pass/Excellent Pass C3 Pass/Excellent Pass/Excellent Pass C4 Pass/Excellent Pass/Excellent Pass C5 Pass/Excellent Pass/Excellent Pass D1 Pass/Excellent Pass/Excellent Pass E1 Pass/Excellent Pass/Excellent Pass F1 Fail Fail Not performed G1 Fail Fail Not performed H1 Fail Fail Not performed I1 Fail Fail Not performed

[0153] From Table 3, it can be seen that excellent tensile strength and high toughness can be stably obtained from the steels satisfying the requirements defined in the present disclosure. In addition, it can be seen that these steels also have a high creep strength when used for a long period of time.

[0154] In contrast, in each of the test materials of symbols A8, B8, F1, and H1, the amount of W analyzed as the electrolytically extracted residue was below the range defined by Formula (1), that is, the amount of precipitation of carbide was not sufficient, and thus the target tensile strength and toughness were not satisfied.

[0155] In each of the test materials of symbols A11, B11, G1, and I1, the amount of W analyzed as the electrolytically extracted residue exceeded the range defined by Formula (1), that is, the carbide precipitated excessively and coarsely, and thus the target tensile strength and toughness were not satisfied.

Example 2

[0156] Ingots which were prepared by melting and casting raw materials J to 0, each having a chemical composition shown in Table 4-1 and Table 4-2, in a laboratory, were subjected to hot forging and hot rolling, in this order, and formed to a thickness of 15 mm. The resulting raw materials were processed into plate materials each having a length of 150 mm and a width of 150 mm. In Table 4-1 and Table 4-2, the unit of each component except for “Ratio Ta/Nb” is % by mass, and the balance is Fe and impurities.

TABLE-US-00005 TABLE 4-1 Symbol C Si Mn P S Ni Cu Co Cr W V Nb Ta J 0.09 0.23 0.54 0.017 0.0010 0.034 0.025 2.9 9.4 3.0 0.22 0.035 0.007 K 0.10 0.28 0.60 0.015 0.0015 0.040 0.018 3.0 9.2 3.2 0.19 0.068 0.015 L 0.07 0.25 0.55 0.016 0.0012 0.028 0.020 2.7 9.0 2.7 0.16 0.032 0.007 M 0.06 0.26 0.61 0.017 0.0011 0.030 0.020 2.8 9.1 2.6 0.17 0.077 0.035 N 0.07 0.25 0.58 0.018 0.0009 0.035 0.024 2.9 9.3 2.6 0.18 0.044 0.004 O 0.07 0.25 0.63 0.017 0.0015 0.027 0.026 2.7 9.2 2.7 0.16 0.036 0.026

TABLE-US-00006 TABLE 4-2 Symbol Nd B N Al O Others Ta + Nb Ratio Ta/Nb J 0.018 0.010 0.010 0.009 0.008 0.042 0.20 K 0.020 0.009 0.009 0.008 0.008 Sn: 0.004 0.083 0.22 L 0.032 0.007 0.012 0.010 0.009 0.039 0.22 M 0.025 0.007 0.014 0.008 0.008 0.112 0.45 N 0.028 0.006 0.013 0.012 0.010 0.048 0.09 O 0.021 0.006 0.013 0.012 0.002 0.062 0.72

[0157] The thus obtained plate materials were subjected to a normalizing heat treatment of heating at 1,150° C. for 0.5 hours and cooling, and a tempering heat treatment of heating at 780° C. for 1 hour and cooling, to prepare test materials.

[0158] Measurement of Amount of W Analyzed as Electrolytically Extracted Residue

[0159] Tensile Test/Tensile Strength

[0160] Charpy Impact Test/Toughness

[0161] Each of the thus obtained test materials was subjected to measurement of the amount of W analyzed as the electrolytically extracted residue, the tensile test and the Charpy impact test, as described above.

[0162] Creep Rupture Test

[0163] In addition, from each of the test materials which passed the tensile test and the Charpy impact test, a round-bar creep test specimen was collected, and subjected to a creep rupture test. Then, as an evaluation of the high temperature strength at the initial stage of use, the creep rupture test was carried out under the conditions of 650° C. and a pressure of 206 MPa in which a target rupture time of the base metal was 50 hours, for each of the three pieces of test specimens, and as an evaluation of creep strength when used for a long period of time, the creep rupture test was carried out under the conditions of a temperature of 650° C. and a pressure of 157 MPa, in which a target rupture time of the base metal was 1,000 hours, for each of the three pieces of test specimens. It should be noted that the creep rupture test was carried out in accordance with JIS Z2271: 2010.

[0164] Cases in which a rupture time of all three pieces of test specimens exceeded the target rupture time were evaluated as “Pass (excellent)”, cases in which a rupture time of two of the three pieces of test specimens exceeded the target rupture time and a rupture time of the remaining one piece of test specimen was below the target rupture time but 90% or more of the target rupture time were evaluated as “Pass (acceptable)”, and anything else was evaluated as “Fail”.

TABLE-US-00007 TABLE 5 Normalizing Heat Treatment Tempering Heat Treatment Symbol of Raw Material Amount of B Temperature Time Temperature Time Steel Plate Used % by mass (° C.) (hours) (° C.) (hours) J1 J 0.010 1150 0.5 780 1 K1 K 0.009 1150 0.5 780 1 L1 L 0.007 1150 0.5 780 1 M1 M 0.007 1150 0.5 780 1 N1 N 0.006 1150 0.5 780 1 O1 O 0.006 1150 0.5 780 1 Amount of W Analyzed as Electrolytically Left Side of Right Side of Symbol of Extracted Residue Formula (1) −10 × Formula (1) Steel Plate (% by mass) [% B] + 0.26 10 × [% B] + 0.54 J1 0.39 0.16 0.64 K1 0.41 0.17 0.63 L1 0.23 0.19 0.61 M1 0.21 0.19 0.61 N1 0.22 0.20 0.60 O1 0.24 0.20 0.60

TABLE-US-00008 TABLE 6 Symbol of (Tensile Strength) (Impact Strength) Creep Rupture Test Steel Plate Tensile test Charpy Impact Test 650° C. × 206 MPa 650° C. × 157 MPa J1 Pass/Excellent Pass/Excellent Pass/Excellent Pass/Excellent K1 Pass/Excellent Pass/Excellent Pass/Excellent Pass/Excellent L1 Pass/Excellent Pass/Excellent Pass/Acceptable Pass/Acceptable M1 Pass/Excellent Pass/Excellent Pass/Excellent Pass/Acceptable N1 Pass/Excellent Pass/Excellent Pass/Excellent Pass/Acceptable O1 Pass/Excellent Pass/Excellent Pass/Acceptable Pass/Excellent

[0165] From Table 5 and Table 6, it can be seen that excellent tensile strength and high toughness can be stably obtained due to steels J to 0 satisfying the requirements defined in the present invention. In addition, it can be seen that in a case in which Ta and Nb satisfy a predetermined range, it is easy to stably obtain a high creep strength at an initial stage of use and when used for a long period of time.

[0166] In this manner, it can be understood that it is possible to obtain a ferritic heat-resistant steel which stably provides an excellent tensile strength and toughness, and which also provides a high creep strength when used for a long period of time, only in a case in which the requirements of the present disclosure are satisfied.

INDUSTRIAL APPLICABILITY

[0167] According to the present disclosure, it is possible to provide a ferritic heat-resistant steel which contains W and B in large amounts, and which stably provides an excellent tensile strength and toughness.

[0168] The disclosures of Japanese Patent Application No. 2019-075661, which was filed on Apr. 11, 2019, and Japanese Patent Application No. 2019-051751 and Japanese Patent Application No. 2019-051752, which were filed on Mar. 19, 2019, are incorporated herein by reference in their entirety. All documents, patent applications, and technical standards described herein are incorporated by reference herein to the same extent as if the individual documents, patent applications, and technical standards were specifically and individually described.