Technical field
[0001]
The present disclosure relates to steel materials, and more particularly to austenitic stainless steel materials and welded joints using the austenitic stainless steel materials.
Background technology
[0002]
High temperature strength is required for steel materials used in chemical plant equipment such as petroleum refining plants and petrochemical plants. Austenitic stainless steel is used as the steel for these chemical plant equipment applications.
[0003]
Chemical plant equipment includes multiple equipment. Each device of the chemical plant equipment is, for example, an atmospheric distillation device, a vacuum distillation device, a direct desulfurization device, a catalytic reforming device, and the like. These devices include heating furnace tubes, reaction towers, tanks, heat exchangers, piping and the like. These devices are welded structures formed by welding steel materials.
[0004]
The average temperature of each device during operation is different. Hereinafter, the average temperature during operation is referred to as "average operating temperature". For example, the vacuum distillation apparatus operates at 400-450 ° C. The direct desulfurization equipment is operated at 400-450 ° C. The catalytic reformer operates at 420-700 ° C. Therefore, in the case of steel materials used for heating furnace pipes, reaction towers, tanks, heat exchangers, pipes, etc. of these devices, they are maintained at an average operating temperature of about 400 to 700 ° C. for a long time during the operation of the devices. There is. Some of the equipment in the chemical plant equipment operates at a temperature of over 700 ° C.
[0005]
Furthermore, when constructing a new chemical plant equipment or repairing a chemical plant equipment, the steel materials used for the equipment in the chemical plant equipment are the planned construction site of the chemical plant or the site where the chemical plant is located. Will be welded at. In recent welding work, in order to reduce the number of welding passes, large heat input welding with a large amount of heat input is often adopted.
[0006]
By the way, when an austenitic stainless steel material is welded, it is known that sensitization due to Cr carbide occurs in the weld heat-affected zone (hereinafter, also referred to as HAZ (Heat Affected Zone)). When sensitization occurs, solid solution Cr is deficient at the grain boundaries. Such a region where the solid solution Cr is deficient is referred to as a “Cr deficient region”. The Cr-deficient region near the grain boundaries causes intergranular corrosion and stress corrosion cracking.
[0007]
Stabilized austenitic stainless steel has been developed for the purpose of suppressing the sensitization of austenitic stainless steel in HAZ. The stabilized austenitic stainless steel material contains Nb or Ti. The affinity with C is higher in Nb and Ti than in Cr. Therefore, in the stabilized austenitic stainless steel material, Nb carbide and Ti carbide are generated by Nb and Ti, and the formation of Cr carbide is suppressed. This suppresses the formation of Cr-deficient regions near the grain boundaries. As a result, the stabilized austenitic stainless steel material can suppress the sensitization of HAZ.
[0008]
However, with stabilized austenitic stainless steel, there is a possibility that knife line attack will occur when high heat input welding is performed. Knife line attack means the following phenomenon. When high heat input welding is performed, the temperature of the portion near the weld metal (the portion corresponding to HAZ) of the stabilized austenitic stainless steel material rises to near the melting point. Specifically, the temperature of the vicinity of the weld metal described above rises to about 1200 ° C. At this time, the Nb carbide and Ti carbide that fixed C in the steel material are melted. In the solidification stage (cooling stage) of the weld metal, Nb and Ti try to combine with C again. However, the cooling rate of the vicinity portion in the solidification stage is high. Therefore, in the solidification step, the temperature in the vicinity thereof drops to 800 to 500 ° C., which is the temperature range for forming Cr carbides, while Nb and Ti cannot be completely bonded to C. In this case, Nb and Ti cannot be bonded to C, Cr is bonded to C, and Cr carbide is generated. As a result, sharp cracks occur in the portion of HAZ near the boundary with the weld metal. This phenomenon is called knife line attack. Knife line attack is a type of sensitization. Therefore, it is desired that the occurrence of sensitization can be suppressed even when high heat input welding is performed.
[0009]
Further, among the above-mentioned chemical plant equipment, in the case of steel materials used in equipment having an average operating temperature of 400 to 700 ° C., it is preferable that sensitization can be suppressed even during the operating period of the equipment. In the previous research, for steel materials for equipment applications with an average operating temperature of about 400 to 700 ° C, the temperature range of 550 ° C was maintained for 1000 hours, and the presence or absence of sensitization was examined. However, considering the operating period of the chemical plant, holding at 550 ° C for 1000 hours is too short. Therefore, it is preferable that the sensitization of the steel material can be suppressed even after holding at 550 ° C for 10,000 hours, which is much longer than 1000 hours at 550 ° C.
[0010]
Patent Document 1 proposes an austenitic stainless steel having excellent embrittlement and cracking resistance of HAZ when used at a high temperature for a long time. The austenitic stainless steel disclosed in Patent Document 1 has a mass% of C: less than 0.04%, Si: 1.5% or less, Mn: 2% or less, Cr: 15 to 25%, Ni: 6 to. 30%, N: 0.02 to 0.35%, sol. Al: 0.03% or less, Nb: 0.5% or less, Ti: 0.4% or less, V: 0.4% or less, Ta: 0.2% or less, Hf: 0.2% The following and Zr: Contains one or more of 0.2% or less, the balance consists of Fe and impurities, and P, S, Sn, As, Zn, Pb and Sb in the impurities are P, respectively. : 0.04% or less, S: 0.03% or less, Sn: 0.1% or less, As: 0.01% or less, Zn: 0.01% or less, Pb: 0.01% or less and Sb: 0 The values ​​of F1 and F2, which are 0.01% or less and are represented by the following equations (1) and (2), are F1 ≤ 0.075 and 0.05 ≤ F2 ≤ 1.7-9, respectively. Satisfy F1.
F1 = S + {(P + Sn) / 2} + {(As + Zn + Pb + Sb) / 5} (1)
F2 = Nb + Ta + Zr + Hf + 2Ti + (V / 10) Equation (2)
Prior art literature
Patent documents
[0011]
Patent Document 1: International Publication No. 2009/044802
Outline of the invention
Problems to be solved by the invention
[0012]
The austenitic stainless steel proposed in Patent Document 1 enhances the embrittlement cracking resistance of HAZ when used at high temperature for a long time. However, Patent Document 1 does not assume large heat input welding. Therefore, Patent Document 1 does not study the sensitization resistance property after long-term use at an average operating temperature of 400 to 700 ° C. after high heat input welding.
[0013]
An object of the present disclosure is to provide an austenitic stainless steel material having excellent sensitization resistance properties even after long-term use at an average operating temperature of 400 to 700 ° C. after high heat input welding.
Means to solve problems
[0014]
The austenitic stainless steel material according to this disclosure is
Chemical composition by mass%
C: 0.020% or less,
Si: 1.50% or less,
Mn: 2.00% or less,
P: 0.045% or less,
S: 0.0300% or less,
Cr: 15.00 to 25.00%,
Ni: 9.00 to 20.00%,
N: 0.05-0.15%,
Nb: 0.1-0.8%,
Mo: 0.10-4.50%,
W: 0.01-1.00%,
Ti: 0 to 0.50%,
Ta: 0 to 0.50%,
V: 0 to 1.00%,
Zr: 0 to 0.10%,
Hf: 0 to 0.10%,
Cu: 0 to 2.00%,
Co: 0 to 1.00%,
Sol. Al: 0 to 0.030%,
B: 0-0.0100%,
Ca: 0-0.0200%,
Mg: 0-0.0200%,
Rare earth elements: 0 to 0.100%,
Sn: 0 to 0.010%,
As: 0 to 0.010%,
Zn: 0 to 0.010%,
Pb: 0 to 0.010%,
Sb: 0 to 0.010% and
The balance consists of Fe and impurities,
Satisfy formula (1)
The Nb content in the residue obtained by the extraction residue method is 0.050 to 0.267% by mass, and the Cr content in the residue is 0.125% or less by mass.
21.9Mo + 5.9W-5.0 ≧ 0 (1)
Here, the content (mass%) of the corresponding element in the chemical composition is substituted for each element symbol in the formula (1).
The invention's effect
[0015]
The austenitic stainless steel material of the present disclosure has excellent sensitization resistance even after being used for a long time at an average operating temperature of 400 to 700 ° C. after high heat input welding.
A brief description of the drawing
[0016]
FIG. 1 is a plan view showing an example of a welded joint according to the present embodiment.
FIG. 2 is a cross-sectional view of the welded joint of FIG. 1 cut in the width direction of the weld metal.
FIG. 3 is a cross-sectional view of the welded joint of FIG. 1 cut in the weld metal extending direction.
FIG. 4 is a cross-sectional view of a welded joint cut in a weld metal extending direction, which is different from FIG.
FIG. 5 is a view showing a cross section in a direction perpendicular to the extending direction of the weld metal in the welded joint of the present embodiment.
FIG. 6 is a side view of a large heat input welded joint simulated test piece produced in the example.
Embodiment for carrying out the invention
[0017]
The present inventors have studied an austenitic stainless steel material having excellent sharpening resistance even after long-term use at an average operating temperature of 400 to 700 ° C. after large heat welding.
[0018]
The present inventors first examined the chemical composition of steel materials. In order to enhance the sensitization resistance property, it is effective to suppress the formation of Cr-deficient regions at the grain boundaries. In order to suppress the formation of Cr-deficient regions at the grain boundaries, it is effective to suppress the formation of Cr carbides in the steel material. In order to suppress the formation of Cr carbides, it is effective to reduce the C content in the chemical composition of the steel material. Further, in order to suppress C in the steel material from being bonded to Cr, it is effective to contain Nb in the steel material and bond C in the steel material to Nb. Therefore, the present inventors first examined the chemical composition of the steel material in order to enhance the sharpening resistance property of the steel material. As a result, the chemical composition is C: 0.020% or less, Si: 1.50% or less, Mn: 2.00% or less, P: 0.045% or less, S: 0.0300% or less, Cr: 15 .00 to 25.00%, Ni: 9.00 to 20.00%, N: 0.05 to 0.15%, Nb: 0.1 to 0.8%, Ti: 0 to 0.50%, Ta: 0 to 0.50%, V: 0 to 1.00%, Zr: 0 to 0.10%, Hf: 0 to 0.10%, Cu: 0 to 2.00%, Co: 0-1 .00%, sol. Al: 0 to 0.030%, B: 0 to 0.0100%, Ca: 0 to 0.0200%, Mg: 0 to 0.0200%, rare earth elements: 0 to 0.100%, Sn: 0 to 0.010%, As: 0 to 0.010%, Zn: 0 to 0.010%, Pb: 0 to 0.010%, Sb: 0 to 0.010%, and the balance consists of Fe and impurities. It was thought that the formation of Cr carbide could be suppressed if the austenite-based stainless steel material was used.
[0019]
By the way, as mentioned above, large heat input welding may be performed on austenitic stainless steel materials at the time of new construction or repair of chemical plant equipment. When high heat input welding is performed, the temperature of the portion near the weld metal (the part corresponding to HAZ) of the steel material exceeds 1200 ° C. due to the welding heat at the time of high heat input welding. Therefore, even if a large amount of Cr carbide is not present in the steel material before the large heat input welding, Cr carbide may be generated in the steel material after the large heat input welding. In this case, the austenitic stainless steel material may become sensitized when the chemical plant equipment is operated and maintained at an average operating temperature of 400 to 700 ° C. for a long time.
[0020]
Therefore, the present inventors further investigated a means capable of suppressing the occurrence of sensitization even when the austenitic stainless steel material is heat-welded and then held at an average operating temperature of 400 to 700 ° C. for a long time. bottom. As a result, the present inventors The following findings were obtained.
[0021]
In the chemical composition of the above-mentioned austenitic stainless steel material, Mo: 0.10 to 4.50% and W: 0.01 to 1.00% are contained as essential elements in place of a part of Fe. The Cr carbide generated in the steel material during the steel material manufacturing process and high heat input welding is M 23C 6 type carbide. Mo and W enter the Cr site (M site) of the Cr carbide of M 23C 6 type by substituting with Cr, and lower the free energy of the Cr carbide. Further, the diffusion rate of Mo and the diffusion rate of W are slower than the diffusion rate of Cr. Therefore, the growth rate of Cr carbide in which Mo and / or W are replaced with Cr and enter the M site is significantly slowed down. By the above mechanism, the present inventors considered that the inclusion of Mo and W suppresses the formation and growth of Cr carbides during the production of steel materials and the time of high heat input welding.
[0022]
However, as a result of the study by the present inventors, even if the steel material contains the above-mentioned contents of Mo and W, sensitization is achieved when the steel material is maintained at an average operating temperature of 400 to 700 ° C. for a long time after high heat input welding. In some cases, it could not be sufficiently suppressed. Therefore, the present inventors further studied. As a result, if the Mo content (mass%) and W content (mass%) in the steel material satisfy the formula (1), it is maintained at an average operating temperature of 400 to 700 ° C. for a long time after high heat input welding. Even so, it was found that the sensitization resistance property was enhanced.
21.9Mo + 5.9W-5.0 ≧ 0 (1)
Here, the content (mass%) of the corresponding element in the chemical composition is substituted for each element symbol in the formula (1).
[0023]
The present inventors further, in an austenitic stainless steel material in which the content of each element in the chemical composition is within the above range and satisfies the formula (1), the average operating temperature of 400 to 700 ° C. after high heat input welding. We investigated a means that can further enhance the sensitization resistance even if it is held for a long time.
[0024]
Here, the present inventors paid attention to the precipitate in the steel material. The proportion of CrNb nitride in the precipitate in the austenitic stainless steel material having the above-mentioned chemical composition is increased. That is, the proportion of CrNb nitride in the precipitate is increased. The CrNb nitride is a fine precipitate (nitride) containing Cr and Nb. CrNb nitride increases the grain boundary area of ​​the steel material. If the grain boundary area is increased, the sharpening resistance property is enhanced even when the crystal grain boundary area is maintained at an average operating temperature of 400 to 700 ° C. for a long time after high heat input welding.
[0025]
CrNb nitride is very fine. Therefore, it is difficult to quantitatively measure the number density of CrNb nitrides with a scanning electron microscope or the like by the current measurement technique. However, if the extraction residue method is carried out on the steel material and the chemical composition of the residue obtained by the extraction residue method is quantified, the precipitate in the steel material can be predicted. As a result of the studies by the present inventors, it was obtained by carrying out the extraction residue method on an austenitic stainless steel material in which the content of each element in the chemical composition is within the above range and which satisfies the formula (1). When the Nb content in the residue is 0.050 to 0.267% by mass and the Cr content in the residue is 0.125% or less by mass, in the precipitate in the steel material. The proportion of CrNb nitride becomes sufficiently high. As a result, it was found that excellent sharpening resistance characteristics can be obtained even when the product is held at an average operating temperature of 400 to 700 ° C. for a long time after high heat input welding.
[0026]
The austenitic stainless steel material of the present embodiment completed based on the above findings has the following constitution.
[0027]
[1]
Austenitic stainless steel,
Chemical composition by mass%
C: 0.020% or less,
Si: 1.50% or less,
Mn: 2.00% or less,
P: 0.045% or less,
S: 0.0300% or less,
Cr: 15.00 to 25.00%,
Ni: 9.00 to 20.00%,
N: 0.05-0.15%,
Nb: 0.1-0.8%,
Mo: 0.10-4.50%,
W: 0.01-1.00%,
Ti: 0 to 0.50%,
Ta: 0 to 0.50%,
V: 0 to 1.00%,
Zr: 0 to 0.10%,
Hf: 0 to 0.10%,
Cu: 0 to 2.00%,
Co: 0 to 1.00%,
Sol. Al: 0 to 0.030%,
B: 0-0.0100%,
Ca: 0-0.0200%,
Mg: 0-0.0200%,
Rare earth elements: 0 to 0.100%,
Sn: 0 to 0.010%,
As: 0 to 0.010%,
Zn: 0 to 0.010%,
Pb: 0 to 0.010%,
Sb: 0 to 0.010% and
The balance consists of Fe and impurities,
Satisfy formula (1)
The Nb content in the residue obtained by the extraction residue method is 0.050 to 0.267% by mass, and the Cr content in the residue is 0.125% or less by mass.
Austenitic stainless steel material.
21.9Mo + 5.9W-5.0 ≧ 0 (1)
Here, the content (mass%) of the corresponding element in the chemical composition is substituted for each element symbol in the formula (1).
[0028]
Here, the "Nb content in the residue" is the ratio (mass) of the mass of the Nb content in the residue to the mass of the austenitic stainless steel material (the mass of the austenitic stainless steel material electrolyzed by the extraction residue method). %) Means. The "Cr content in the residue" is the ratio (mass%) of the mass of the Cr content in the residue to the mass of the austenitic stainless steel material (the mass of the austenitic stainless steel material electrolyzed by the extraction residue method). means.
[0029]
The austenitic stainless steel material of the present embodiment has excellent sensitization resistance even after being used for a long time at an average operating temperature of 400 to 700 ° C. after large heat welding.
[0030]
[2]
The austenitic stainless steel material described in [1].
The chemical composition is
Mo: 2.50-4.50%, and
Co: 0.01-1.00%,
Containing, further satisfying equations (2) and (3),
The Nb content in the residue obtained by the extraction residue method is 0.065 to 0.245% by mass, and the Cr content in the residue is 0.104% or less in mass%. ,
Austenitic stainless steel material.
2 ≦ 73W + 5Co ≦ 60 (2)
0.20 ≤ Nb + 0.1 W ≤ 0.58 (3)
[0031]
The austenitic stainless steel material of the above [2] further has excellent polythionic acid SCC resistance, excellent liquefaction cracking resistance, and excellent naphthenic acid corrosion resistance.
[0032]
[3]
The austenitic stainless steel material according to [1] or [2].
The chemical composition contains at least one element or two or more elements belonging to any of the groups 1 to 5.
Austenitic stainless steel material.
Group 1:
Ti: 0.01-0.50%,
Ta: 0.01-0.50%,
V: 0.01-1.00%,
Zr: 0.01-0.10% and
Hf: 0.01-0.10%,
Group 2:
Cu: 0.01-2.00% and
Co: 0.01-1.00%,
Group 3:
Sol. Al: 0.001 to 0.030%,
Group 4:
B: 0.0001 to 0.0100%,
Group 5:
Ca: 0.0001-0.0200%,
Mg: 0.0001 to 0.0200% and
Rare earth element: 0.001 to 0.100%.
[0033]
[4]
It is a welded joint
With the pair of austenitic stainless steel materials described in [2] or [3],
It is equipped with a weld metal placed between the pair of austenitic stainless steel materials.
Of the cross section of the austenitic stainless steel material perpendicular to the extending direction of the weld metal, the average crystal grain size in the range of 200 μm in the width direction of the weld metal from the fusion line in the weld heat affected zone is the average crystal grain size. When defined as R1 and the average crystal grain size of the portion other than the weld heat affected zone is defined as the average crystal grain size R2,
The average crystal grain size R1 and the average crystal grain size R2 satisfy the formula (4).
Welded joint.
R1 / R2 ≤ 4.8 (4)
[0034]
Hereinafter, the austenitic stainless steel material and the welded joint of the present embodiment will be described in detail. Unless otherwise specified, "%" for an element means mass%.
[0035]
[Chemical composition]
The chemical composition of the austenitic stainless steel material of this embodiment contains the following elements.
[0036]
C: 0.020% or less
Carbon (C) is inevitably contained. That is, the C content is more than 0%. C produces M 23C 6 type Cr carbides at the grain boundaries. When the C content exceeds 0.020%, even if the content of other elements is within the range of the present embodiment, Cr carbides are excessively generated and the sharpening resistance property of the steel material is significantly deteriorated. Therefore, the C content is 0.020% or less. The upper limit of the C content is preferably 0.018%, more preferably 0.016%, still more preferably 0.014%, still more preferably 0.012%. The C content is preferably as low as possible. However, excessive reduction of the C content increases the manufacturing cost. Therefore, in terms of industrial production, the lower limit of the C content is preferably 0.001%, more preferably 0.002%.
[0037]
Si: 1.50% or less
Silicon (Si) is inevitably contained. That is, the Si content is more than 0%. Si deoxidizes steel in the steelmaking process. If even a small amount of Si is contained, the above effect can be obtained to some extent. However, if the Si content exceeds 1.50%, the weld crack sensitivity is significantly increased even if the content of other elements is within the range of the present embodiment. Furthermore, since Si is a ferrite stabilizing element, the stability of austenite is reduced. In this case, a sigma phase (σ phase) is formed in the steel material during long-term use at an average operating temperature of 400 to 700 ° C. The sigma phase reduces the toughness and ductility of the steel during use at an average operating temperature of 400-700 ° C. Therefore, the Si content is 1.50% or less. The lower limit of the Si content is preferably 0.01%, more preferably 0.05%, still more preferably 0.10%, still more preferably 0.15%, still more preferably 0.20. %. The preferred upper limit of the Si content is 1.40%, more preferably 1.20%, still more preferably 1.00%, still more preferably 0.80%, still more preferably 0.70. %, More preferably 0.60%, still more preferably 0.50%.
[0038]
Mn: 2.00% or less
Manganese (Mn) is inevitably contained. That is, the Mn content is more than 0%. Mn combines with S in the steel material to form MnS and enhances the hot workability of the steel material. Mn further deoxidizes the welded portion of the steel during welding. If even a small amount of Mn is contained, the above effect can be obtained to some extent. However, if the Mn content exceeds 2.00%, even if the content of other elements is within the range of this embodiment, the sigma phase in the steel material when used at an average operating temperature of 400 to 700 ° C. (Σ phase) is easily generated. The sigma phase reduces the toughness and ductility of the steel during use at an average operating temperature of 400-700 ° C. Therefore, the Mn content is 2.00% or less. The preferred lower limit of the Mn content is 0.01%, more preferably 0.10%, still more preferably 0.50%, still more preferably 1.00%, still more preferably 1.20. %, More preferably 1.30%. The preferred upper limit of the Mn content is 1.80%, more preferably 1.60%, still more preferably 1.55%.
[0039]
P: 0.045% or less
Phosphorus (P) is an impurity that is inevitably contained. That is, the P content is more than 0%. P segregates at the grain boundaries of the steel material during large heat input welding. As a result, the sensitization resistance of the steel material deteriorates. P is also steel at the time of welding.Increases the weld crack sensitivity of the material. When the P content exceeds 0.045%, even if the content of other elements is within the range of the present embodiment, the sharpening resistance property of the steel material is lowered and the welding crack sensitivity is increased. Therefore, the P content is 0.045% or less. The preferred upper limit of the P content is 0.040%, more preferably 0.035%, still more preferably 0.030%. It is preferable that the P content is as low as possible. However, excessive reduction of P content raises the manufacturing cost of steel materials. Therefore, considering normal industrial production, the preferred lower limit of the P content is 0.001%, more preferably 0.002%.
[0040]
S: 0.0300% or less
Sulfur (S) is an impurity that is inevitably contained. That is, the S content is more than 0%. S segregates at grain boundaries during use of steel materials in a high temperature environment. As a result, the sensitization resistance of the steel material deteriorates. S further enhances the weld crack sensitivity of the steel material during welding. When the S content exceeds 0.0300%, even if the content of other elements is within the range of the present embodiment, the sharpening resistance property of the steel material is lowered and the welding crack sensitivity is increased. Therefore, the S content is 0.0300% or less. The preferred upper limit of the S content is 0.0200%, more preferably 0.0150%, still more preferably 0.0100%, still more preferably 0.0060%, still more preferably 0.0050. %, More preferably 0.0040%, still more preferably 0.0030%. It is preferable that the S content is as low as possible. However, excessive reduction of S content raises the manufacturing cost of steel materials. Therefore, considering normal industrial production, the preferred lower limit of the S content is 0.0001%, more preferably 0.0002%.
[0041]
Cr: 15.00 to 25.00%
Chromium (Cr) enhances the oxidation resistance and corrosion resistance of steel materials when used at an average operating temperature of 400 to 700 ° C. If the Cr content is less than 15.00%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Cr content exceeds 25.00%, the stability of austenite in the steel material at an average operating temperature of 400 to 700 ° C. decreases even if the content of other elements is within the range of this embodiment. do. In this case, the creep strength of the steel material decreases. Therefore, the Cr content is 15.00 to 25.00%. The lower limit of the Cr content is preferably 15.50%, more preferably 16.00%, still more preferably 16.20%, still more preferably 16.40%. The preferred upper limit of the Cr content is 24.00%, more preferably 23.00%, still more preferably 22.00%, still more preferably 21.00%, still more preferably 20.00%. %, More preferably 19.00%.
[0042]
Ni: 9.00 to 20.00%
Nickel (Ni) stabilizes austenite and increases the creep strength of steel materials at an average operating temperature of 400 to 700 ° C. If the Ni content is less than 9.00%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Ni content exceeds 20.00%, the above effect is saturated and the manufacturing cost is further increased. Therefore, the Ni content is 9.00 to 20.00%. The preferred lower limit of the Ni content is 9.50%, more preferably 9.80%, still more preferably 10.00%. The preferred upper limit of the Ni content is 18.00%, more preferably 16.00%, still more preferably 15.00%, still more preferably 14.50%, still more preferably 14.00%. %, More preferably 13.50%.
[0043]
N: 0.05 to 0.15%
Nitrogen (N) dissolves in the matrix (matrix) to stabilize austenite. N further produces CrNb nitride in the steel. CrNb nitride increases the total area of ​​grain boundaries. Therefore, even when the product is operated for a long time at an average operating temperature of 400 to 700 ° C., the formation of Cr carbide can be suppressed. As a result, the sharpening resistance characteristics of the steel material are enhanced. If the N content is less than 0.05%, the above effect cannot be sufficiently obtained. On the other hand, if the N content exceeds 0.15%, Cr nitride (Cr 2N) is formed at the grain boundaries. In this case, the amount of solid solution Cr in the steel material is reduced, and as a result, the sharpening resistance property of the steel material is lowered. Therefore, the N content is 0.05 to 0.15%. The preferred lower limit of the N content is 0.06%, more preferably 0.07%. The preferred upper limit of the N content is 0.14%, more preferably 0.12%, still more preferably 0.10%, still more preferably 0.09%.
[0044]
Nb: 0.1-0.8%
Niobium (Nb), together with N, produces CrNb nitride in austenite crystal grains, increasing the total area of ​​grain boundaries. Therefore, even when the product is operated for a long time at an average operating temperature of 400 to 700 ° C., the formation of Cr carbide can be suppressed. As a result, the sharpening resistance characteristics of the steel material are enhanced. Nb further combines with C to form MX-type Nb carbides. By generating Nb carbide and fixing C, the amount of solid solution C in the steel material is reduced. As a result, during the use of the steel material at an average operating temperature of 400 to 700 ° C., the formation of Cr carbides at the grain boundaries is suppressed, and the sharpening resistance property of the steel material is enhanced. The Nb carbide further enhances the creep strength of the steel material at an average operating temperature of 400-700 ° C. by precipitation strengthening. If the Nb content is less than 0.1%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Nb content exceeds 0.8%, CrNb nitride and Nb carbide are excessively produced even if the content of other elements is within the range of this embodiment. In this case, the strength in the crystal grains becomes excessively high, and the difference in strength between the inside of the crystal grains and the grain boundaries becomes large. Therefore, stress concentration occurs at the grain interface, and welding cracks and embrittlement cracks are likely to occur. Therefore, the Nb content is 0.1 to 0.8%. The preferred lower limit of the Nb content is 0.2%, more preferably 0.3%. The preferred upper limit of the Nb content is 0.7%, more preferably 0.6%, still more preferably 0.5%, still more preferably 0.4%.
[0045]
Mo: 0.10-4.50%
Molybdenum (Mo) suppresses the formation and growth of M 23C 6 type Cr carbides at the grain boundaries during the use of steel materials at an average operating temperature of 400 to 700 ° C. Mo, as a solid solution strengthening element, further enhances the creep strength of the steel material at an average operating temperature of 400 to 700 ° C. If the Mo content is less than 0.10%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Mo content exceeds 4.50%, the formation of intermetallic compounds such as the LAVES phase is promoted in the crystal grains even if the content of other elements is within the range of the present embodiment. In this case, the strength in the crystal grains becomes excessively high, and the difference in strength between the inside of the crystal grains and the grain boundaries becomes large. Therefore, stress concentration occurs at the grain interface, and welding cracks and embrittlement cracks are likely to occur. Therefore, the Mo content is 0.10 to 4.50%.
[0046]
In the chemical composition of the steel material, when the content of elements other than Mo is within the range of this embodiment, if the Mo content is 2.50% or more, the average operating temperature of 400 to 700 ° C. is further increased. In the steel material used in the above, polythionic acid SCC resistance and naphthenic acid corrosion resistance can be enhanced. Therefore, if sufficient polythionic acid SCC resistance and sufficient naphthenic acid corrosion resistance are required for steel materials used at an average operating temperature of 400 to 700 ° C., the Mo content is 2.50 to 4.50%. be.
[0047]
When an austenitic stainless steel material is used for applications in which resistance to polythionic acid SCC and resistance to naphthenic acid corrosion is not particularly required, the preferable lower limit of the Mo content is 0.15%, more preferably 0.20%, and further. It is preferably 0.25%, more preferably 0.27%, still more preferably 0.30%.
[0048]
When an austenitic stainless steel material is used for applications in which resistance to polythionic acid SCC and resistance to naphthenic acid corrosion is not particularly required, the upper limit of the Mo content is preferably less than 2.50%, more preferably 2.45%. It is more preferably 2.20%, still more preferably 2.00%, still more preferably 1.70%, still more preferably 1.50%, still more preferably 1.30%. It is more preferably 1.00%, further preferably 0.90%, still more preferably 0.80%, still more preferably 0.70%, still more preferably 0.60%, and more preferably 0.60%. More preferably, it is 0.50%.
[0049]
When austenitic stainless steel is used for applications requiring SCC resistance to polythionic acid and corrosion resistance to naphthenic acid, the preferable lower limit of the Mo content is 2.50% as described above, and more preferably 2.70%. Yes, more preferably 2.90%, still more preferably 3.00%, still more preferably 3.05%, still more preferably 3.10%. When austenitic stainless steel is used for applications requiring SCC resistance to polythionic acid and corrosion resistance to naphthenic acid, the preferred upper limit of the Mo content is 4.30%, more preferably 4.20%, and further. It is preferably 4.15%, more preferably 4.05%, still more preferably 3.95%.
[0050]
W: 0.01-1.00%
Tungsten (W), like Mo, suppresses the formation and growth of M 23C 6 type Cr carbides at grain boundaries during the use of steel materials at average operating temperatures of 400-700 ° C. W, as a solid solution strengthening element, further enhances the creep strength of the steel material at an average operating temperature of 400 to 700 ° C. If the W content is less than 0.01%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the W content exceeds 1.00%, the formation of intermetallic compounds such as the LAVES phase is promoted in the crystal grains even if the content of other elements is within the range of the present embodiment. In this case, the strength in the crystal grains becomes excessively high, and the difference in strength between the inside of the crystal grains and the grain boundaries becomes large. Therefore, stress concentration occurs at the grain interface, and welding cracks and embrittlement cracks are likely to occur. Therefore, the W content is 0.01 to 1.00%. The preferred lower limit of the W content is 0.02%, more preferably 0.04%, still more preferably 0.06%, still more preferably 0.08%, still more preferably 0.10. %. The preferred upper limit of the W content is 0.80%, more preferably 0.60%, still more preferably 0.40%, still more preferably 0.35%, still more preferably 0.30. %.
[0051]
The balance of the chemical composition of the austenitic stainless steel material according to this embodiment is composed of Fe and impurities. Here, the impurities are mixed from ore, scrap, or the manufacturing environment as a raw material when the austenitic stainless steel material is industrially manufactured, and have an adverse effect on the austenitic stainless steel material of the present embodiment. It means what is allowed within the range that does not give.
[0052]
Among the impurities, the contents of Sn, As, Zn, Pb and Sb are as follows.
Sn: 0 to 0.010%
As: 0 to 0.010%
Zn: 0 to 0.010%
Pb: 0 to 0.010%
Sb: 0 to 0.010%
Tin (Sn), arsenic (As), zinc (Zn), lead (Pb) and antimony (Sb) are all impurities. The Sn content may be 0%. Similarly, the As content may be 0%. The Zn content may be 0%. Pb content is 0% May be. The Sb content may be 0%. When contained, all of these elements segregate at the grain boundaries to lower the melting point of the grain boundaries or reduce the binding force of the grain boundaries. When the Sn content exceeds 0.010%, the hot workability and weldability of the steel material deteriorate even if the content of other elements is within the range of this embodiment. Similarly, when the As content exceeds 0.010%, the hot workability and weldability of the steel material are lowered even if the content of other elements is within the range of the present embodiment. When the Zn content exceeds 0.010%, the hot workability and weldability of the steel material deteriorate even if the content of other elements is within the range of this embodiment. When the Pb content exceeds 0.010%, the hot workability and weldability of the steel material deteriorate even if the content of other elements is within the range of this embodiment. When the Sb content exceeds 0.010%, the hot workability and weldability of the steel material deteriorate even if the content of other elements is within the range of this embodiment. Therefore, the Sn content is 0 to 0.010%. The As content is 0 to 0.010%. The Zn content is 0 to 0.010%. The Pb content is 0 to 0.010%. The Sb content is 0 to 0.010%. The lower limit of the Sn content may be more than 0% or 0.001%. The lower limit of the As content may be more than 0% or 0.001%. The lower limit of the Zn content may be more than 0% or 0.001%. The lower limit of the Pb content may be more than 0% or 0.001%. The lower limit of the Sb content may be more than 0% or 0.001%.
[0053]
[About arbitrary elements]
[Group 1 arbitrary element]
The chemical composition of the austenitic stainless steel material according to the present embodiment may further contain one element or two or more elements selected from the group consisting of Ti, Ta, V, Zr and Hf instead of a part of Fe. .. All of these elements combine with C to form carbides. Therefore, the solid solution C is reduced, and the sharpening resistance property of the steel material is enhanced.
[0054]
Ti: 0 to 0.50%
Titanium (Ti) is an optional element and does not have to be contained. That is, the Ti content may be 0%. When contained, Ti combines with C in the steel to form carbides. As a result, the formation of Cr carbides is suppressed, and the sharpening resistance characteristics of the steel material are enhanced. If even a small amount of Ti is contained, the above effect can be obtained to some extent. However, if the Ti content exceeds 0.50%, carbides will be excessively deposited in the crystal grains even if the content of other elements is within the range of this embodiment. In this case, the strength in the crystal grains becomes excessively high, and the difference in strength between the inside of the crystal grains and the grain boundaries becomes large. Therefore, stress concentration occurs at the grain interface, and welding cracks and embrittlement cracks are likely to occur. Therefore, the Ti content is 0 to 0.50%. The lower limit of the Ti content is more than 0%, more preferably 0.01%, still more preferably 0.02%. The preferred upper limit of the Ti content is 0.45%, more preferably 0.40%, still more preferably 0.35%, still more preferably 0.30%.
[0055]
Ta: 0 to 0.50%
Tantalum (Ta) is an optional element and does not have to be contained. That is, the Ta content may be 0%. When contained, Ta combines with C to form carbides. As a result, the formation of Cr carbides is suppressed, and the sharpening resistance characteristics of the steel material are enhanced. If even a small amount of Ta is contained, the above effect can be obtained to some extent. However, if the Ta content exceeds 0.50%, carbides will be excessively deposited in the crystal grains even if the content of other elements is within the range of the present embodiment. In this case, the strength in the crystal grains becomes excessively high, and the difference in strength between the inside of the crystal grains and the grain boundaries becomes large. Therefore, stress concentration occurs at the grain interface, and welding cracks and embrittlement cracks are likely to occur. Therefore, the Ta content is 0 to 0.50%. The lower limit of the Ta content is more than 0%, more preferably 0.01%, still more preferably 0.02%. The preferred upper limit of the Ta content is 0.45%, more preferably 0.40%, still more preferably 0.35%, still more preferably 0.30%.
[0056]
V: 0 to 1.00%
Vanadium (V) is an optional element and does not have to be contained. That is, the V content may be 0%. When contained, V combines with C to form carbides. As a result, the formation of Cr carbides is suppressed, and the sharpening resistance characteristics of the steel material are enhanced. If V is contained even in a small amount, the above effect can be obtained to some extent. However, if the V content exceeds 1.00%, carbides will be excessively deposited in the crystal grains even if the content of other elements is within the range of the present embodiment. In this case, the strength in the crystal grains becomes excessively high, and the difference in strength between the inside of the crystal grains and the grain boundaries becomes large. Therefore, stress concentration occurs at the grain interface, and welding cracks and embrittlement cracks are likely to occur. Therefore, the V content is 0 to 1.00%. The preferable lower limit of the V content is more than 0%, more preferably 0.01%, still more preferably 0.02%, still more preferably 0.04%, still more preferably 0.06. %. The preferred upper limit of the V content is 0.80%, more preferably 0.70%, still more preferably 0.50%, still more preferably 0.40%, still more preferably 0.35. %, More preferably 0.30%.
[0057]
Zr: 0 to 0.10%
Zirconium (Zr) is an optional element and may not be contained. That is, the Zr content may be 0%. When contained, Zr combines with C to form carbides. As a result, the formation of Cr carbides is suppressed, and the sharpening resistance characteristics of the steel material are enhanced. If even a small amount of Zr is contained, the above effect can be obtained to some extent. However, if the Zr content exceeds 0.10%, carbides will be excessively deposited in the crystal grains even if the content of other elements is within the range of the present embodiment. In this case, the strength in the crystal grains becomes excessively high, and the difference in strength between the inside of the crystal grains and the grain boundaries becomes large. Therefore, stress concentration occurs at the grain interface, and welding cracks and embrittlement cracks are likely to occur. Therefore, the Zr content is 0 to 0.10%. The preferred lower limit of the Zr content is more than 0%, more preferably 0.01%, still more preferably 0.02%. The preferred upper limit of the Zr content is 0.09%, more preferably 0.08%, still more preferably 0.07%, still more preferably 0.06%.
[0058]
Hf: 0 to 0.10%
Hafnium (Hf) is an optional element and does not have to be contained. That is, the Hf content may be 0%. When contained, Hf combines with C to form carbides. As a result, the formation of Cr carbides is suppressed, and the sharpening resistance characteristics of the steel material are enhanced. If even a small amount of Hf is contained, the above effect can be obtained to some extent. However, if the Hf content exceeds 0.10%, carbides will be excessively deposited in the crystal grains even if the content of other elements is within the range of the present embodiment. In this case, the strength in the crystal grains becomes excessively high, and the difference in strength between the inside of the crystal grains and the grain boundaries becomes large. Therefore, stress concentration occurs at the grain interface, and welding cracks and embrittlement cracks are likely to occur. Therefore, the Hf content is 0 to 0.10%. The lower limit of the Hf content is more than 0%, more preferably 0.01%, still more preferably 0.02%. The preferred upper limit of the Hf content is 0.09%, more preferably 0.08%, still more preferably 0.07%, still more preferably 0.06.
[0059]
[Group 2 arbitrary element]
The chemical composition of the austenitic stainless steel material according to the present embodiment may further contain one or more elements selected from the group consisting of Cu and Co, instead of a part of Fe. All of these elements increase the creep strength of steel at average operating temperatures of 400-700 ° C.
[0060]
Cu: 0 to 2.00%
Copper (Cu) is an optional element and does not have to be contained. That is, Cu may be 0%. When contained, Cu precipitates as a Cu phase in the grains during use of the steel material at an average operating temperature of 400 to 700 ° C., and the precipitation strengthening enhances the creep strength of the steel material. If even a small amount of Cu is contained, the above effect can be obtained to some extent. However, if the Cu content exceeds 2.00%, the Cu phase is excessively precipitated even if the content of other elements is within the range of this embodiment. In this case, the embrittlement crack sensitivity in HAZ after welding is increased. Therefore, the Cu content is 0 to 2.00%. The lower limit of the Cu content is more than 0%, more preferably 0.01%, still more preferably 0.03%, still more preferably 0.05%, still more preferably 0.10%. Is. The preferred upper limit of the Cu content is 1.50%, more preferably 1.00%, still more preferably 0.80%, still more preferably 0.60%.
[0061]
Co: 0 to 1.00%
Cobalt (Co) is an optional element and does not have to be contained. That is, the Co content may be 0%. When contained, Co stabilizes austenite and increases the creep strength of the steel at an average operating temperature of 400-700 ° C. Co further enhances the polythionic acid SCC resistance of the steel material, similar to W. If even a small amount of Co is contained, the above effect can be obtained to some extent. However, if the Co content exceeds 1.00%, the raw material cost increases even if the content of other elements is within the range of the present embodiment. Therefore, the Co content is 0 to 1.00%. The lower limit of the Co content is more than 0%, more preferably 0.01%, still more preferably 0.05%, still more preferably 0.10%, still more preferably 0.20%. Is. The preferred upper limit of the Co content is 0.90%, more preferably 0.80%, still more preferably 0.70%, still more preferably 0.60%.
[0062]
[Group 3 arbitrary element]
The chemical composition of the austenitic stainless steel material according to the present embodiment may further contain Al instead of a part of Fe. Al deoxidizes the steel in the steelmaking process.
[0063]
Sol. Al: 0 to 0.030%
Aluminum (Al) is an optional element and does not have to be contained. That is, the Al content may be 0%. When contained, Al deoxidizes the steel in the steelmaking process. If Al is contained even in a small amount, the above effect can be obtained to some extent. However, sol. If the Al content exceeds 0.030%, the workability and ductility of the steel material will decrease even if the content of other elements is within the range of this embodiment. Therefore, sol. The Al content is 0 to 0.030%. sol. The lower limit of the Al content is more than 0%, more preferably 0.001%, still more preferably 0.005%, still more preferably 0.010%. sol. The preferred upper limit of the Al content is 0.029%, more preferably 0.028%, still more preferably 0.025%. In this embodiment, sol. The Al content means the content of acid-soluble Al (sol.Al).
[0064]
[Group 4 arbitrary element]
The chemical composition of the austenitic stainless steel material according to the present embodiment may further contain B instead of a part of Fe. B segregates at the grain boundaries and strengthens the grain boundaries.
[0065]
B: 0 to 0.0100%
Boron (B) is an optional element and does not have to be contained. That is, the B content may be 0%. When contained, B segregates at the grain boundaries during use of the steel material at an average operating temperature of 400-700 ° C., increasing the grain boundary strength. If B is contained even in a small amount, the above effect can be obtained to some extent. However, if the B content exceeds 0.0100%, the formation of Cr carbides at the grain boundaries is promoted even if the content of other elements is within the range of the present embodiment. Therefore, the sharpening resistance property of the steel material is deteriorated. If the B content exceeds 0.0100%In addition, the melting point of the grain boundaries is lowered, and liquefaction cracking occurs at the grain boundaries of HAZ during welding. Therefore, the B content is 0 to 0.0100%. The lower limit of the B content is more than 0%, more preferably 0.0001%, still more preferably 0.0005%, still more preferably 0.0010%. The preferred upper limit of the B content is 0.0050%, more preferably 0.0040%, still more preferably 0.0030%, still more preferably 0.0020%.
[0066]
[Group 5 arbitrary element]
The chemical composition of the austenitic stainless steel material according to the present embodiment may further contain one element or two or more elements selected from the group consisting of Ca, Mg and rare earth elements (REM) instead of a part of Fe. .. All of these elements enhance the hot workability of steel materials.
[0067]
Ca: 0-0.0200%
Calcium (Ca) is an optional element and does not have to be contained. That is, the Ca content may be 0%. When contained, Ca fixes O (oxygen) and S (sulfur) as inclusions and enhances the hot workability of the steel material. Ca further immobilizes S and suppresses the grain boundary segregation of S. This reduces embrittlement cracking of HAZ during welding. If even a small amount of Ca is contained, the above effect can be obtained to some extent. However, if the Ca content exceeds 0.0200%, the cleanliness of the steel material is lowered, and the hot workability of the steel material is rather lowered. Therefore, the Ca content is 0 to 0.0200%. The lower limit of the Ca content is more than 0%, more preferably 0.0001%, still more preferably 0.0002%, still more preferably 0.0005%. The preferred upper limit of the Ca content is 0.0150%, more preferably 0.0100%, still more preferably 0.0080%, still more preferably 0.0050%, still more preferably 0.0040. %.
[0068]
Mg: 0-0.0200%
Magnesium (Mg) is an optional element and does not have to be contained. That is, the Mg content may be 0%. When contained, Mg fixes O (oxygen) and S (sulfur) as inclusions and enhances the hot workability of the steel material. Mg further fixes S and suppresses the grain boundary segregation of S. This reduces the embrittlement cracking of HAZ during welding. If even a small amount of Mg is contained, the above effect can be obtained to some extent. However, if the Mg content exceeds 0.0200%, the cleanliness of the steel material is lowered, and the hot workability of the steel material is rather lowered. Therefore, the Mg content is 0 to 0.0200%. The lower limit of the Mg content is more than 0%, more preferably 0.0001%, still more preferably 0.0002%, still more preferably 0.0005%. The preferred upper limit of the Mg content is 0.0150%, more preferably 0.0100%, still more preferably 0.0080%, still more preferably 0.0050%, still more preferably 0.0040. %.
[0069]
Rare earth element: 0 to 0.100%
Rare earth element (REM) is an optional element and does not have to be contained. That is, the REM content may be 0%. When contained, REM immobilizes O (oxygen) and S (sulfur) as inclusions and enhances the hot workability and creep ductility of the base metal. However, if the REM content is too high, the hot workability and creep ductility of the base metal will decrease. Therefore, the REM content is 0 to 0.100%. The preferred lower limit of the REM content is more than 0%, more preferably 0.001%, still more preferably 0.002%. The preferred upper limit of the REM content is 0.080%, more preferably 0.060%.
[0070]
The REM in the present specification contains at least one element or two or more elements of Sc, Y, and a lanthanoid (La of atomic number 57 to Lu of 71), and the REM content is the total content of these elements. Means quantity.
[0071]
[About formula (1)]
The chemical composition of the austenitic stainless steel material of the present embodiment further satisfies the formula (1).
21.9Mo + 5.9W-5.0 ≧ 0 (1)
Here, the content (mass%) of the corresponding element in the chemical composition is substituted for each element symbol in the formula (1).
[0072]
Defined as F1 = 21.9Mo + 5.9W-5.0. F1 is an index of the amount of M 23C 6 type Cr carbide produced in the steel material. Both Mo and W replace Cr at the M site of Cr carbide to reduce the free energy of Cr carbide. Therefore, Mo and W suppress the formation of Cr carbides. Further, the diffusion rates of Mo and W are slower than the diffusion rates of Cr. Therefore, the growth rate of the Cr carbide in which Cr at the M site is replaced with Mo or W becomes slow.
[0073]
If F1 is 0 or more, Mo and W in an amount capable of suppressing the formation of Cr carbides are sufficiently contained. Therefore, it is possible to sufficiently suppress the formation and growth of Cr carbides during welding and when steel materials are used at an average operating temperature of 400 to 700 ° C. As a result, even when the steel material is operated for a long time at the above-mentioned average operating temperature after performing high heat input welding to the steel material, excellent sharpening resistance characteristics can be obtained. The preferable lower limit of F1 is 0.1, more preferably 0.2, still more preferably 0.5, still more preferably 1.0, still more preferably 1.5, still more preferably. It is 2.0. The upper limit of F1 is not particularly limited, but is 99.45 when the maximum content of Mo and the maximum content of W in the chemical composition are taken into consideration. F1 is a value obtained by rounding off the second decimal place of the obtained numerical value (that is, F1 is the first decimal place).
[0074]
[Chemical composition analysis method for austenitic stainless steel materials]
The chemical composition of the austenitic stainless steel material of this embodiment can be obtained by a well-known component analysis method. Specifically, when the austenitic stainless steel material is a steel pipe, a drill is used to drill at the center position of the wall thickness to generate chips, and the chips are collected. When the austenitic stainless steel material is a steel plate, a drill is used to drill at the center of the plate width and the center of the plate thickness to generate chips, and the chips are collected. When the austenitic stainless steel material is steel bar, drilling is performed at the R / 2 position using a drill to generate chips, and the chips are collected. Here, the R / 2 position means the central position of the radius R in the cross section perpendicular to the longitudinal direction of the steel bar.
[0075]
Dissolve the collected chips in acid to obtain a solution. ICP-OES (Inductively Coupled Plasma Optical Precision Spectroscopy) is carried out on the solution to perform elemental analysis of the chemical composition. The C content and S content are determined by a well-known high-frequency combustion method. Specifically, the above solution is burned by high-frequency heating in an oxygen stream, and the generated carbon dioxide and sulfur dioxide are detected to determine the C content and the S content. By the above analysis method, the chemical composition of austenitic stainless steel can be obtained.
[0076]
[About deposits (residues) in austenitic stainless steel materials]
In the austenitic stainless steel material of the present embodiment, the Nb content in the residue obtained by the extraction residue method is 0.050 to 0.267% by mass, and the Cr content in the residue is mass. % Is 0.125% or less.
[0077]
Here, the "Nb content in the residue" is the ratio (mass) of the mass of the Nb content in the residue to the mass of the austenitic stainless steel material (the mass of the austenitic stainless steel material electrolyzed by the extraction residue method). %) Means. The "Cr content in the residue" is the ratio (mass%) of the mass of the Cr content in the residue to the mass of the austenitic stainless steel material (the mass of the austenitic stainless steel material electrolyzed by the extraction residue method). means.
[0078]
When the Nb content in the residue extracted by the extraction residue method is 0.050 to 0.267% by mass and the Cr content in the residue is 0.125% or less by mass, austenitic stainless steel. The proportion of CrNb nitride in the precipitates in the austenitic stainless steel increases. That is, the Nb content in the residue extracted by the extraction residue method is 0.050 to 0.267% in mass%, and the Cr content in the residue is 0.125% or less in mass%. In this case, it means that the amount of precipitates other than CrNb nitride (Cr carbide, Cr 2N, other carbides, nitrides, carbonitrides, etc.) is sufficiently smaller than the amount of CrNb nitride.
[0079]
When the Nb content in the residue is less than 0.050%, it means that CrNb nitride is not sufficiently precipitated in the steel material. In this case, when the steel material after high heat input welding is held at an average operating temperature of 400 to 700 ° C. for a long time, sufficient sharpening resistance characteristics cannot be obtained.
[0080] [0080]
On the other hand, when the Nb content in the residue exceeds 0.267% and / or when the Cr content in the residue exceeds 0.125%, the steel material before use at an average operating temperature of 400 to 700 ° C. It means that a large number or coarse CrNb nitrides or other precipitates have already been formed in the grain boundaries. Therefore, when the steel material after high heat input welding is held at an average operating temperature of 400 to 700 ° C. for a long time, a Cr-deficient region is excessively generated, and as a result, sufficient sharpening resistance characteristics cannot be obtained.
[0081]
In the residue extracted by the extraction residue method, the lower limit of the Nb content is preferably 0.052%, more preferably 0.054%, still more preferably 0.055%. The preferred upper limit of the Nb content in the residue is 0.265%, more preferably 0.263%, still more preferably 0.260%, still more preferably 0.250%, still more preferably 0. .240%.
[0082]
The preferred upper limit of the Cr content in the residue obtained by the extraction residue method is 0.120%, more preferably 0.110%, still more preferably 0.100%, still more preferably 0.090. %, More preferably 0.080%. The lower limit of the Cr content is not particularly limited. The lower limit of the Cr content is preferably 0.001%, more preferably 0.003%, still more preferably 0.005%.
[0083]
[Measurement method of chemical composition in residue]
The Nb content and Cr content in the residue can be measured by the following method. Collect test pieces from austenitic stainless steel. The cross section perpendicular to the longitudinal direction of the test piece may be circular or rectangular. When the austenitic stainless steel material is a steel pipe, the test piece is collected so that the center of the cross section perpendicular to the longitudinal direction of the test piece is the center position of the wall thickness of the steel pipe and the longitudinal direction of the test piece is the longitudinal direction of the steel pipe. .. When the austenitic stainless steel material is a steel plate, the center of the cross section perpendicular to the longitudinal direction of the test piece is the center position of the plate width and the center position of the plate thickness of the steel plate, and the longitudinal direction of the test piece is the longitudinal direction of the steel plate. Collect a test piece. When the austenitic stainless steel material is steel bar, the test piece is collected so that the center of the cross section perpendicular to the longitudinal direction of the test piece is the R / 2 position of the steel bar and the longitudinal direction of the test piece is the longitudinal direction of the steel bar. ..
[0084]
The surface of the collected test piece is polished by about 50 μm by preliminary electrolytic polishing to obtain a new surface. The electropolished test piece is electrolyzed (mainly electrolyzed) with an electrolytic solution (10% acetylacetone + 1% tetraammonium + methanol). The electrolytic solution after the main electrolysis is passed through a 0.2 μm filter to capture the residue. The obtained residue is acid-decomposed, and the mass of Nb in the residue and the mass of Cr in the residue are determined by ICP (inductively coupled plasma) emission analysis. Further, the mass of the electrolyzed base material (austenitic stainless steel material) is obtained. Specifically, the mass of the test piece before the main electrolysis and the mass of the test piece after the main electrolysis are measured. Then, from the mass of the test piece before the main electrolysis, the test after the main electrolysis The value obtained by subtracting the mass of the piece is defined as the amount of the base material electrolyzed.
[0085]
The mass of Nb in the residue is divided by the amount of the base material electrolyzed to obtain the Nb content (% by mass) in the residue. That is, the Nb content (mass%) in the residue is determined based on the following formula (i). Further, the Cr mass in the residue is divided by the amount of the base material subjected to the main electrolysis to obtain the Cr content (mass%) in the residue. That is, the Cr content (% by mass) in the residue is obtained based on the following formula (ii).
Nb content in the residue = Nb mass in the residue / base material amount x 100 (i)
Cr content in the residue = Cr mass in the residue / Mother material amount x 100 (ii)
[0086]
As described above, the austenitic stainless steel material of the present embodiment has the content of each element in the chemical composition within the above range and satisfies the formula (1). Further, the Nb content in the residue obtained by the extraction residue method is 0.050 to 0.267% by mass, and the Cr content in the residue is 0.125% or less by mass. Therefore, the austenitic stainless steel material of the present embodiment has excellent sharpening resistance even after being used for a long time at an average operating time of 400 to 700 ° C. after large heat welding.
[0087]
Here, having excellent sensitization resistance characteristics even after long-term use at an average operating time of 400 to 700 ° C. after high heat input welding means the following matters.
[0088]
Collect square test pieces from austenitic stainless steel. When the austenitic stainless steel material is a steel pipe, the square test piece is placed so that the center of the cross section perpendicular to the longitudinal direction of the test piece is the center position of the wall thickness of the steel pipe and the longitudinal direction of the test piece is the longitudinal direction of the steel pipe. Collect. When the austenitic stainless steel material is a steel plate, the center of the cross section perpendicular to the longitudinal direction of the test piece is the center position of the plate width and the center position of the plate thickness of the steel plate, and the longitudinal direction of the test piece is the longitudinal direction of the steel plate. Collect a horny test piece. When the austenitic stainless steel material is steel bar, the square test piece is placed so that the center of the cross section perpendicular to the longitudinal direction of the test piece is the R / 2 position of the steel bar and the longitudinal direction of the test piece is the longitudinal direction of the steel bar. Collect.
[0089]
The length of the square test piece is not particularly limited, but is, for example, 100 mm. The cross section (cross section) perpendicular to the longitudinal direction of the square test piece is not particularly limited, but is, for example, a rectangle of 10 mm × 10 mm.
[0090]
Using a high frequency thermal cycle device, give the following thermal history to the angular test piece. Specifically, the central portion having a predetermined width (for example, 10 mm) at the central position in the longitudinal direction of the angular test piece is heated in the atmosphere from room temperature to 1350 to 1400 ° C. at 70 to 100 ° C./sec. It is held at a further raised temperature for 1 to 60 seconds. Then, the angular test piece is cooled to room temperature at a cooling rate of 20 ° C./sec. By applying the above heat history to the angular test piece, a large heat input welded joint simulated test piece is produced.
[0091]
Using a large heat input welded joint simulation test piece, carry out the following long-term sensitization treatment. The large heat-entry welded joint simulation test piece is charged into the heat treatment furnace. In the heat treatment furnace, the large heat input welded joint simulated test piece is held in the air at atmospheric pressure at 550 ° C. for 10,000 hours (sensitization treatment). After 10000 hours have passed, the large heat input welded joint simulated test piece is extracted from the heat treatment furnace and allowed to cool.
[0092]
The following Strauss test and reactivation rate measurement test will be carried out on the large heat input welded joint simulated test piece that has been sensitized for a long time.
[0093]
[Strauss test (sulfuric acid / copper sulfate corrosion test)]
The Strauss test compliant with ASTM A262-15 PRACTICE E will be conducted as follows. From the large heat input welded joint simulated test piece that has been sensitized for a long time, the plate-shaped test piece is collected so that the central part is at the center position in the longitudinal direction of the plate-shaped test piece. The size of the plate-shaped test piece is not particularly limited. The size of the plate-shaped test piece is, for example, 2 mm in thickness, 10 mm in width, and 80 mm in length. The plate-shaped test piece is immersed in a copper sulfate test solution containing 16% sulfuric acid and boiled for 15 hours. Then, the plate-shaped test piece is taken out from the copper sulfate test solution. A bending test is performed on the removed plate-shaped test piece. In the bending test, the plate-shaped test piece is bent 180 ° in the atmosphere around the center position in the longitudinal direction of the large heat input welded joint simulated test piece. Cut the bent part of the bent test piece. Observe the cut surface with a 20x optical microscope. If cracks are observed, determine the length of the cracks. If no cracks are observed, or if cracks are observed but the length of the cracks is 100 μm or less, it is judged that the sensitization resistance is excellent.
[0094]
[Reactivation rate measurement test]
An electrochemical reactivation rate measurement test (Electrochemical Reaction test) conforming to ASTM G108-94 will be carried out using a large heat input welded joint simulated test piece that has been sensitized for a long time. Specifically, the plate-shaped test piece is collected from the central portion (the portion to which the large heat input is applied) of the large heat input welded joint simulated test piece that has been sensitized for a long time. In the collected plate-shaped test piece, the area other than the surface portion of the evaluation area of ​​100 mm 2 is masked. Using the masked plate-shaped test piece as an electrode, it is immersed in a 0.5 mol sulfuric acid + 0.01 mol potassium thiocyanate solution having a temperature of 30 ° C. and a capacity of 200 cm 3. Next, the plate-shaped test piece is scanned in the noble direction from the natural potential to 300 mV with a linear polarization at a polarization rate of 100 mV / min. Immediately after reaching 300 mV based on the saturated sweet electrode, scanning is performed in the base direction to the original natural potential. Measure the current that flows when the voltage is applied in the noble direction (outward route). Then, the current flowing when the voltage is applied in the base direction (return path) is measured. Based on the obtained current value, the reactivation rate (%) is defined as follows.
Reactivation rate = (maximum anode current on the return path / maximum anode current on the outward path) x 100
[0095]
The lower the reactivation rate, the lower the degree of sensitization (DOS) and the higher the sensitization resistance. When the reactivation rate is 10% or less, it is judged that the sensitization resistance property is excellent.
[0096]
No cracks are observed in the ASTM A262-15 PRACTICE E compliant Strauss test in the simulated test piece of the large heat input welded joint that has been sensitized for a long time, or even if cracks are observed, the crack length is 100 μm or less. Yes, and if the reactivation rate obtained in the ASTM G108-94 compliant electrochemical reactivation rate measurement test is 10% or less, the average operating time is 400 to 700 ° C after high heat welding. It is judged to have excellent sensitization resistance even after long-term use.
[0097]
[About steel materials with improved polythionic acid SCC resistance and naphthenic acid corrosion resistance]
With the recent decline in gasoline prices, the proportion of low-priced, low-grade crude oil containing naphthenic acid is increasing in chemical plant equipment. Therefore, excellent naphthenic acid corrosion resistance may be required for steel materials used in chemical plant equipment. Further, in steel materials used for heating furnace pipes of atmospheric distillation devices and vacuum distillation devices, sulfur is contained in crude oil in order to suppress the adhesion of a large amount of coke generated in the distillation step. Sulfur contained in crude oil suppresses the adhesion of cork. However, sulfur contained in crude oil tends to cause polythionic acid stress corrosion cracking (hereinafter, also referred to as polythionic acid SCC) in steel materials. Therefore, steel materials used in chemical plant equipment may also be required to have excellent polythionic acid SCC resistance.
[0098]
When sufficient polythionic acid SCC resistance and sufficient naphthenic acid corrosion resistance are obtained, the austenitic stainless steel material of the present embodiment further satisfies the following requirements.
(I) The Mo content is 2.50 to 4.50%, and the Co content is 0.01 to 1.00%.
(II) The chemical composition of the steel material satisfies the formulas (2) and (3).
2 ≦ 73W + 5Co ≦ 60 (2)
0.20 ≤ Nb + 0.1 W ≤ 0.58 (3)
(III) The Nb content in the residue obtained by the extraction residue method is 0.065 to 0.245% by mass, and the Cr content in the residue is 0.104% or less by mass.
Hereinafter, (I) to (III) will be described.
[0099]
[About (I)]
In the chemical composition of the austenitic stainless steel material of the present embodiment, when the Mo content is 2.50% or more, as described above, the content of other elements is within the range of the present embodiment and the formula (1). ) Is satisfied, and excellent naphthenic acid corrosion resistance can be obtained. Furthermore, W and Co enhance polythionic acid SCC resistance. Therefore, for the purpose of obtaining sufficient polythionic acid SCC resistance and sufficient naphthenic acid corrosion resistance, the Mo content in the austenitic stainless steel material is 2.50 to 4.50%, and Co. The content is 0.01-1.00%.
[0100]
[About (II)]
The chemical composition of the steel material further satisfies the formulas (2) and (3).
2 ≦ 73W + 5Co ≦ 60 (2)
0.20 ≤ Nb + 0.1 W ≤ 0.58 (3)
Here, the content (mass%) of the corresponding element in the chemical composition is substituted for each element symbol in the formula (2) and the formula (3).
Hereinafter, equations (2) and (3) will be described.
[0101]
[About formula (2)]
Defined as F2 = 73W + 5Co. F2 is an index related to polythionic acid SCC resistance and liquefaction crack resistance during large heat input welding. If F2 is less than 2, the total content of W and Co in the chemical composition of the austenitic stainless steel is not sufficient. In this case, the polythionic acid SCC resistance of the steel material is lowered. On the other hand, when F2 exceeds 60, W and Co promote the formation of intermetallic compounds such as LAVES phase when the Mo content is 2.50% or more. In this case, the intermetallic compound is excessively produced. Therefore, the strength in the crystal grains becomes excessively high, and the difference in strength between the inside of the crystal grains and the grain boundaries becomes large. Therefore, stress concentration occurs at the grain interface. As a result, the liquefaction-resistant cracking property is lowered at the time of high heat input welding.
[0102]
In the chemical composition of the austenitic stainless steel material of the present embodiment, the Mo content is 2.50 to 4.50%, the Co content is 0.01 to 1.00%, and F2 is 2. If it is to 60, sufficient polythionic acid SCC resistance can be obtained on the premise that the content of other elements is within the range of the present embodiment, and the occurrence of liquefied cracks can be suppressed during large heat input welding. .. The preferred lower limit of F2 is 3, more preferably 4, and even more preferably 5. The preferred upper limit of F2 is 58, more preferably 55, still more preferably 53, still more preferably 50. F2 is a value obtained by rounding off the first decimal place of the obtained numerical value.
[0103]
[About formula (3)]
Defined as F3 = Nb + 0.1W. F3 means the amount of effective Nb. Both Nb and W combine with C to form carbides and reduce the amount of solid solution C in the steel material. This suppresses the formation of Cr carbides in the steel material and enhances the polythionic acid SCC resistance of the steel material. However, when the N content in the steel material is 0.05 to 0.15%, if the total content of Nb and W is too high, Nb precipitates typified by the Laves phase are excessively generated. In this case, liquefaction cracking may occur in HAZ during large heat input welding, and the liquefaction cracking resistance may decrease.
[0104]
If F3 is less than 0.20, the formation of Cr carbides cannot be sufficiently suppressed, and the polythionic acid SCC resistance of the steel material is lowered. On the other hand, if F3 exceeds 0.58, Nb precipitates typified by the Laves phase are excessively generated, and liquefaction cracking in HAZ may occur at the time of high heat input welding. Further, when the above (I) and (II) are satisfied in the austenitic stainless steel material of the present embodiment, that is, when F3 is 0.20 to 0.58%, excellent polythionic acid SCC resistance can be obtained. In addition, it is possible to suppress liquefaction cracking in HAZ during high heat input welding.
[0105]
The preferred lower limit for F3 is 0.22 It is more preferably 0.24, still more preferably 0.26. The preferred upper limit of F3 is 0.56, more preferably 0.54, still more preferably 0.50, still more preferably 0.48, still more preferably 0.45. F3 is a value obtained by rounding off the third decimal place of the obtained numerical value.
[0106]
[About (III)]
When the austenitic stainless steel material of the present embodiment satisfies (I) and (II), the Nb content in the residue obtained by the extraction residue method is 0.065 to 0.245% by mass. When the Cr content in the residue is 0.104% or less in mass%, excellent polythionic acid SCC resistance can be obtained.
[0107]
If the Nb content in the residue extracted by the extraction residue method is 0.065 to 0.245% in mass% and the Cr content in the residue is 0.104% or less in mass%, austenitic acid is used. Since the proportion of CrNb nitride in the precipitates in the austenitic stainless steel is sufficiently large and the grain boundary area is sufficiently increased, excellent polythionic acid SCC resistance can be obtained.
[0108]
When the Nb content in the residue is less than 0.065%, it means that CrNb nitride is not sufficiently precipitated in the steel material to the extent that sufficient polythionic acid SCC resistance can be obtained. In this case, when the steel material after high heat input welding is held at an average operating temperature of 400 to 700 ° C. for a long time, sufficient polythionic acid SCC resistance cannot be obtained.
[0109]
On the other hand, when the Nb content in the residue exceeds 0.245% and / or when the Cr content in the residue exceeds 0.104%, the steel material before use at an average operating temperature of 400 to 700 ° C. It means that a large number or coarse CrNb nitrides are already formed at the grain boundaries to the extent that the polythionic acid SCC resistance is deteriorated. Therefore, when the steel material after high heat input welding is held at an average operating temperature of 400 to 700 ° C. for a long time, sufficient polythionic acid SCC resistance cannot be obtained.
[0110]
In the austenitic stainless steel material of the present embodiment, the Nb content in the residue satisfying (I) and (II) and obtained by the extraction residue method is 0.065 to 0.245% in mass%. When the Cr content in the residue is 0.104% or less in mass%, excellent polythionic acid SCC resistance and naphthenitic acid corrosion resistance can be obtained.
[0111]
The preferable lower limit of the Nb content in the residue extracted by the extraction residue method is 0.070%, more preferably 0.075%, still more preferably 0.085%, still more preferably 0.090. %. The preferred upper limit of the Nb content in the residue is 0.240%, more preferably 0.235%, still more preferably 0.230%.
[0112]
The upper limit of the Cr content in the residue extracted by the extraction residue method is 0.100%, more preferably 0.095%, still more preferably 0.090%, and the lower limit of the Cr content is Not particularly limited. The lower limit of the Cr content is preferably 0.001%, more preferably 0.003%, still more preferably 0.005%.
[0113]
As described above, the content of each element in the chemical composition of the austenitic stainless steel material of the present embodiment is within the range of the present embodiment, the formula (1) is satisfied, and the above-mentioned (I) to (III) are satisfied. ) Is satisfied, excellent naphthenic acid corrosion resistance, excellent polythionic acid SCC resistance, and excellent liquefaction cracking resistance can be obtained. Here, excellent naphthenic acid corrosion resistance, excellent polythionic acid SCC resistance, and excellent liquefaction cracking resistance mean the following matters.
[0114]
[Resistant to naphthenic acid corrosion]
Collect test pieces from austenitic stainless steel. When the austenitic stainless steel material is a steel pipe, the test piece is collected so that the center of the cross section perpendicular to the longitudinal direction of the test piece is the center position of the wall thickness of the steel pipe and the longitudinal direction of the test piece is the longitudinal direction of the steel pipe. .. When the austenitic stainless steel material is a steel plate, the center of the cross section perpendicular to the longitudinal direction of the test piece is the center position of the plate width and the center position of the plate thickness of the steel plate, and the longitudinal direction of the test piece is the longitudinal direction of the steel plate. Collect a test piece. When the austenitic stainless steel material is steel bar, the test piece is collected so that the center of the cross section perpendicular to the longitudinal direction of the test piece is the R / 2 position of the steel bar and the longitudinal direction of the test piece is the longitudinal direction of the steel bar. .. The size of the test piece is not particularly limited. The size of the test piece is, for example, 2 mm in thickness, 10 mm in width, and 30 mm in length. The collected test piece is immersed in a 100% cyclohexanecarboxylic acid solution at 200 ° C. for 720 hours under normal pressure. After soaking for 720 hours, the test piece is ultrasonically cleaned with acetone for 3 minutes.
[0115]
The difference between the mass of the test piece before the test and the mass of the test piece after ultrasonic cleaning is calculated as the corrosion weight loss. Further, the corrosion rate (mm / year) is obtained from the surface area, specific gravity, and test time of the test piece. When the corrosion rate is 0.01 mm / year or less, it is judged that the naphthenic acid corrosion resistance is excellent.
[0116]
[Polythionic acid SCC resistance]
Prepare a large heat input welded joint simulation test piece similar to the above-mentioned evaluation test for sharpening resistance characteristics. The above-mentioned long-term sensitization treatment is carried out on the large heat input welded joint simulated test piece. From the large heat input welded joint simulated test piece after long-term sensitization treatment, the plate-shaped test piece is collected so that the central part is at the center position in the longitudinal direction of the plate-shaped test piece. The size of the plate-shaped test piece is not particularly limited. The size of the plate-shaped test piece is, for example, 2 mm in thickness, 10 mm in width, and 75 mm in length. Using the collected plate-shaped test pieces, a polythionic acid SCC resistance evaluation test is carried out by the following method. The plate-shaped test piece is bent around a punch having an inner radius of 5 mm to form a U-bend shape. The U-bend type test piece is immersed in a 1% K 2S 4O 6 solution adjusted to PH = 2 with sulfuric acid at room temperature for 100 hours. In the test piece after immersion, the bent portion of the bent test piece is cut in a direction perpendicular to the longitudinal direction, and the cut surface is observed with a 20x optical microscope. If cracks are observed, the depth of cracks on the cut surface is determined. When no crack is observed, or when crack is observed but the crack depth is less than 20 μm, it is judged that the polythionic acid SCC resistance is excellent.
[0117]
[Liquid resistance crack resistance evaluation test]
At the center position in the longitudinal direction of the large heat input welded joint simulated test piece, cut in the direction perpendicular to the longitudinal direction. The cut surface is used as the observation surface. The observation surface is etched with mixed acid. Arbitrary three fields of view (each field of view is 250 μm × 250 μm) of the etched observation surface is observed with a 400 × optical microscope. In the three observed visual fields, the presence or absence of a partial melting mark at the grain boundary is determined.
[0118]
Of the partial melting marks generated at the grain boundaries, no partial melting marks with a length of 25 μm or more were observed, or partial melting marks with a length of 25 μm or more were observed on the cut surface of the three visual fields. However, if no partial melting mark having a length of 50 μm or more is observed, it is judged that the liquefaction resistance and cracking resistance are high.
[0119]
[Shape of austenitic stainless steel material of this embodiment]
The shape of the austenitic stainless steel material of this embodiment is not particularly limited. The austenitic stainless steel material of the present embodiment may be a steel pipe, a steel plate, or a steel bar. The austenitic stainless steel material of the present embodiment may be a forged product or a cast product.
[0120]
[About the use of the austenitic stainless steel material of this embodiment]
The austenitic stainless steel material of the present embodiment is suitable for equipment applications used at an average operating temperature of 400 to 700 ° C. The austenitic stainless steel material of the present embodiment is particularly suitable for equipment applications that are used for a long period of time at an average operating temperature of 400 to 700 ° C. after high heat input welding is performed. 400 to 700 ° C is the average operating temperature, and even if the operating temperature temporarily exceeds 700 ° C, if the average operating temperature is 400 to 700 ° C, the austenitic stainless steel material of the present embodiment can be used. Suitable for use. The maximum temperature reached for these devices may be 750 ° C. Such equipment is, for example, equipment for chemical plant equipment represented by petroleum refining and petrochemicals. These devices include, for example, a heating furnace pipe, a tank, a pipe, and the like. Further, the austenitic stainless steel material of the present embodiment may be used for chemical plant equipment having an average operating temperature of less than 400 ° C.
[0121]
When the steel material of the present embodiment satisfies the above (I) to (III), that is, it contains Mo: 2.50 to 4.50% and Co: 0.01 to 1.00% in the chemical composition. Further, the formula (2) and the formula (3) are satisfied, and the Nb content in the residue obtained by the extraction residue method is 0.065 to 0.245% by mass in mass%, and the residue is contained. When the Cr content is 0.104% or less in mass%, it is suitable for chemical plant equipment applications where polythionic acid SCC resistance and naphthenic acid corrosion resistance are required.
[0122]
The austenitic stainless steel material of the present embodiment can naturally be used for equipment other than chemical plant equipment. The equipment other than the chemical plant equipment is, for example, a thermal power generation boiler equipment (for example, a boiler tube, etc.), which is expected to be used at an average operating temperature of about 400 to 700 ° C. like the chemical plant equipment.
[0123]
[About the welded joint of this embodiment]
FIG. 1 is a plan view showing an example of a welded joint of the present embodiment. With reference to FIG. 1, the welded joint 1 according to the present embodiment includes a pair of austenitic stainless steel materials 100 and a weld metal 200. The weld metal 200 is arranged between a pair of austenitic stainless steel materials 100. The weld metal 200 is formed between a pair of austenitic stainless steel materials 100, and is connected to the pair of austenitic stainless steel materials 100. In the following description, the austenitic stainless steel material 100 is also referred to as a "base material" 100.
[0124]
The ends of the pair of base materials 100 are, for example, grooved. The weld metal 200 is formed by mating the ends of a pair of base materials 100 having grooved ends, and then performing single-layer welding or multi-layer welding. For example, the welding methods include Tig welding (Gas Tungsten Arc Welding: GTAW), shielded metal arc welding (SMAW), flux-welded wire arc welding (Flux Code Arc Welding: FCAW), and gas metal arc welding (GasArc). Welding: GMAW), Submerged Arc Welding (SAW).
[0125]
In FIG. 1, the direction in which the weld metal 200 extends is defined as the weld metal extension direction L. The direction perpendicular to the weld metal extension direction L is defined as the weld metal width direction W. The direction perpendicular to the weld metal extension direction L and the weld metal width direction W is defined as the weld metal thickness direction T. FIG. 2 is a cross-sectional view of the welded joint 1 of FIG. 1 cut in the weld metal width direction W. As shown in FIGS. 1 and 2, the weld metal 200 is formed (arranged) between a pair of base materials 100.
[0126]
FIG. 3 is a cross-sectional view of the welded joint 1 of FIG. 1 cut in the weld metal extending direction L, and FIG. 4 is a cross-sectional view of the welded joint 1 cut in the weld metal extending direction L, which is different from FIG. be. As shown in FIG. 3, the base material 100 may be a steel plate. Further, as shown in FIG. 4, the cross section of the base material 100 perpendicular to the longitudinal direction may be a circular pipe (that is, a steel pipe). Although not shown, the base metal 100 may be steel bar.
[0127]
[About the base material 100]
Each of the pair of base materials 100 is the austenitic stainless steel material of the present embodiment having the above-mentioned excellent polythionic acid SCC resistance and excellent naphthenic acid corrosion resistance. That is, the base material 100 has a chemical composition of% by mass, C: 0.020% or less, Si: 1.50% or less, Mn: 2.00% or less, P: 0.045% or less, S: 0. .0300% or less, Cr: 15.00 to 25.00%, Ni: 9.02 to 20.00%, N: 0. 05 to 0.15%, Nb: 0.1 to 0.8%, Mo: 2.50 to 4.50%, W: 0.01 to 1.00%, Ti: 0 to 0.50%, Ta : 0 to 0.50%, V: 0 to 1.00%, Zr: 0 to 0.10%, Hf: 0 to 0.10%, Cu: 0 to 2.00%, Co: 0.01 to 1.00%, sol. Al: 0 to 0.030%, B: 0 to 0.0100%, Ca: 0 to 0.0200%, Mg: 0 to 0.0200%, rare earth elements: 0 to 0.100%, Sn: 0 to 0.010%, As: 0 to 0.010%, Zn: 0 to 0.010%, Pb: 0 to 0.010%, Sb: 0 to 0.010%, and the balance consists of Fe and impurities. , The Nb content in the residue obtained by the extraction residue method satisfying the formulas (1) to (3) is 0.065 to 0.245% in mass%, and the Cr content is 0 in mass%. It is 104% or less.
[0128]
[About weld metal 200]
The chemical composition of the weld metal 200 is not particularly limited. The weld metal 200 may be formed using a well-known welding material. Well-known welding materials are, for example, compliant with AWS A5.9, standard names: ERNiCrCoMo-1, ERNiCrMo-3, NiCrCoMo-1, 22Cr-12Co-1Al-9Mo-Ni, NiCrMo-3, 22Cr-8Mo- 3.5Nb-Ni or the like.
[0129]
[Preferable range of average crystal grain size R1 in weld heat affected zone (HAZ) and average crystal grain size R2 in parts other than HAZ]
FIG. 5 is a diagram showing a cross section of the welded joint 1 of the present embodiment in the direction perpendicular to the weld metal extending direction L. With reference to FIG. 5, in the cross section of the welded joint 1 in the direction perpendicular to the weld metal extending direction L, the base metal (austenitic stainless steel) 100 has a weld heat affected zone (HAZ) 101 and a portion other than the HAZ 101. Includes 102. The HAZ 101 is a region of the base metal 100 adjacent to the molten wire 200E of the weld metal 200 and is affected by heat during welding. On the other hand, the portion of the base material 100 other than the HAZ 101 is referred to as a normal portion 102. Of the base metal 100, the normal portion 102 is a portion that is not substantially affected by heat during welding.
[0130]
With reference to FIG. 5, in the cross section of the base metal 100 in the direction perpendicular to the weld metal extending direction L, a range of 200 μm in the HAZ101 from the molten wire 200E to the weld metal width direction W (broken line in FIG. 5). The area hatched in) is defined as the range Dref. The range Dref is part of HAZ101. The average crystal grain size in the range Dref is defined as the average crystal grain size R1 (μm). Further, the average crystal grain size of the portion other than HAZ101 (that is, the normal portion 102) in the cross section of the base metal 100 is defined as the average crystal grain size R2 (μm). At this time, preferably, the average crystal grain size R1 and the average crystal grain size R2 satisfy the formula (4).
R1 / R2 ≤ 4.8 (4)
[0131]
Here, the average crystal grain size R1 is measured by the following method. From the welded joint 1, a test piece including a cross section in the direction perpendicular to the weld metal extending direction L is collected. The cross section in the direction perpendicular to the weld metal extending direction L is used as the observation surface. The observation surface is mirror-polished. After mirror polishing, etching is performed with a 10% oxalic acid solution. Of the etched observation surfaces, any three fields of view within the range Dref are observed with a 200x optical microscope to generate a photographic image. Each field of view is 100 μm × 100 μm. In each field of view, the crystal particle size number is obtained by the cutting method in accordance with JIS G0551 (2013). The arithmetic mean value of the obtained three crystal particle size numbers is obtained and defined as the average crystal particle size number. The average crystal grain size R1 (μm) is obtained from the obtained average crystal grain size number.
[0132]
Similarly, the average crystal grain size R2 is measured by the following method. With reference to FIG. 5, a test piece including a cross section in the direction perpendicular to the weld metal extending direction L is collected from the normal portion 102 of the base metal 100 of the welded joint 1. The cross section in the direction perpendicular to the weld metal extending direction L is used as the observation surface. The observation surface is mirror-polished. After mirror polishing, etching is performed with a 10% oxalic acid solution. Of the etched observation surfaces, any three fields of view are observed with a 200x optical microscope to generate a photographic image. Each field of view is 100 μm × 100 μm. In each field of view, the crystal particle size number is obtained by the cutting method in accordance with JIS G0551 (2013). The arithmetic mean value of the obtained three crystal particle size numbers is obtained and defined as the average crystal particle size number. The average crystal grain size R2 (μm) is obtained from the obtained average crystal grain size number.
[0133]
In the welded joint 1 of the present embodiment, the base material 100 is the austenitic stainless steel material of the present embodiment described above, and the average crystal grain size R1 at HAZ101 near the molten wire 200E and the average crystal at the normal portion 102. If the particle size R2 satisfies the formula (4), the welded joint 1 of the present embodiment has further excellent polythionate SCC resistance and further excellent liquefaction crack resistance even after high heat input welding. ..
[0134]
[Manufacturing method of austenitic stainless steel material of this embodiment]
Hereinafter, the method for manufacturing the austenitic stainless steel material of the present embodiment will be described. The method for producing an austenitic stainless steel material described below is merely an example of the method for producing an austenitic stainless steel material according to the present embodiment. Therefore, the austenitic stainless steel material having the above-mentioned structure may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferable example of the manufacturing method of the austenitic stainless steel material of the present embodiment.
[0135]
The method for producing an austenitic stainless steel material of the present embodiment includes the following steps.
1. Process of preparing materials (preparation process)
2. Process of manufacturing intermediate steel by performing hot working on the material (hot working process)
3. If necessary, a process of pickling the intermediate steel material after the hot working process and then performing cold working (cold working process).
4. A step of precipitating CrNb nitride on the intermediate steel material after the hot working step or the cold working step (CrNb nitride formation treatment step).
Hereinafter, each process will be described.
[0136]
[1. Preparation process]
In the preparation process, a material having the above-mentioned chemical composition is prepared. The material may be supplied by a third party or may be manufactured. The material may be ingot, slab, bloom, billet. When manufacturing a material, the material is manufactured by the following method. A molten steel having the above-mentioned chemical composition is produced. The ingot is manufactured by the ingot method using the manufactured molten steel. The molten steel produced may be used to produce slabs, blooms and billets by continuous casting. Billets may be manufactured by hot working the manufactured ingots, slabs and blooms. For example, the ingot may be hot forged to produce a cylindrical billet, and this billet may be used as a material. In this case, the temperature of the material immediately before the start of hot forging is not particularly limited, but is, for example, 1000 to 1300 ° C. The cooling method of the material after hot forging is not particularly limited.
[0137]
[2. Hot working process]
In the hot working process, hot working is performed on the material prepared in the preparatory process to manufacture an intermediate steel material. The intermediate steel material may be, for example, a steel pipe, a steel plate, or a steel bar.
[0138]
If the intermediate steel material is a steel pipe, the following processing is performed in the hot processing process. First, prepare a cylindrical material. By machining, a through hole is formed along the central axis of the cylindrical material. An intermediate steel material (steel pipe) is manufactured by performing hot extrusion represented by the Eugene Sejurne method on a cylindrical material having through holes. The temperature of the material immediately before hot extrusion is not particularly limited. The temperature of the material immediately before hot extrusion is, for example, 1000 to 1300 ° C. Instead of the hot extrusion method, a hot punching pipe manufacturing method may be carried out.
[0139]
Instead of hot extrusion, a steel pipe may be manufactured by performing perforation rolling by the Mannesmann method. In this case, the round billet is drilled and rolled by a drilling machine. In the case of drilling and rolling, the drilling ratio is not particularly limited, but is, for example, 1.0 to 4.0. The perforated round billet is further hot-rolled with a mandrel mill, reducer, sizing mill or the like to form a raw pipe. The cumulative surface reduction rate in the hot working process is not particularly limited, but is, for example, 20 to 80%. When the steel pipe is manufactured by hot working, the steel pipe temperature (finishing temperature) immediately after the hot working is completed is not particularly limited, but is preferably 900 ° C. or higher.
[0140]
When the intermediate steel material is a steel plate, the hot working process uses, for example, one or a plurality of rolling mills equipped with a pair of work rolls. A steel plate is manufactured by hot rolling a material such as a slab using a rolling mill. The material is heated before hot rolling, and hot rolling is performed on the heated material. The temperature of the material immediately before hot rolling is, for example, 1000 to 1300 ° C. When the steel sheet is manufactured by hot working, the temperature of the steel sheet (finishing temperature) immediately after the hot working is completed is not particularly limited, but is preferably 900 ° C. or higher.
[0141]
When the intermediate steel material is bar steel, the hot working process includes, for example, a rough rolling process and a finish rolling process. In the rough rolling process, the material is hot-processed to produce billets. For the rough rolling step, for example, a lump rolling mill is used. Billets are manufactured by performing slab rolling on the material with a slab rolling mill. When a continuous rolling mill is installed downstream of the lump rolling mill, hot rolling is further performed on the billet after lump rolling using the continuous rolling mill to produce a smaller billet. You may. In a continuous rolling mill, for example, a horizontal stand having a pair of horizontal rolls and a vertical stand having a pair of vertical rolls are alternately arranged in a row. The material temperature immediately before the rough rolling process is not particularly limited, but is, for example, 1000 to 1300 ° C. In the finish rolling process, the billet is first heated. The billets after heating are hot-rolled using a continuous rolling mill to produce steel bars. The heating temperature in the heating furnace in the finish rolling step is not particularly limited, but is, for example, 1000 to 1300 ° C. When the steel bar is manufactured by hot working, the temperature of the steel bar (finishing temperature) immediately after the hot working is completed is not particularly limited, but is preferably 900 ° C. or higher.
[0142]
[3. Cold processing process]
The cold processing process will be carried out as needed. That is, the cold working process does not have to be carried out. When this is carried out, the intermediate steel material that has been hot-worked is pickled and then cold-worked. When the intermediate steel material is a steel pipe or steel bar, the cold working is, for example, cold drawing or cold rolling. When the intermediate steel material is a steel plate, the cold working is, for example, cold rolling. By carrying out the cold working step, strain is applied to the intermediate steel material before the CrNb nitride formation treatment step. As a result, recrystallization and sizing can be performed during the CrNb nitride formation treatment step. The surface reduction rate in the cold working process is not particularly limited, but is, for example, 10 to 90%.
[0143]
[4. CrNb nitride formation process]
In the CrNb nitride formation treatment step, the CrNb nitride formation treatment is carried out on the intermediate steel material after the hot working step or the cold working step. As a result, an appropriate amount of CrNb nitride is precipitated while suppressing the formation of other precipitates (Cr carbide, Cr 2N, other carbides, nitrides, carbonitrides, etc.). As a result, the Nb content in the residue obtained by the extraction residue method from the produced austenitic stainless steel material can be set to 0.050 to 0.267% by mass, and the Cr content in the residue can be set to 0.050 to 0.267%. Can be 0.125% or less in terms of mass%.
[0144]
The CrNb nitride formation process is carried out by the following method. An intermediate steel material is charged into a heat treatment furnace in which the atmosphere in the furnace is an atmospheric atmosphere. The atmospheric atmosphere here means an atmosphere containing 78% or more by volume of nitrogen, which is a gas constituting the atmosphere, and 20% or more by volume of oxygen.
[0145]
[Conditions for CrNb nitride formation processing]
In the CrNb nitride formation process, the following three conditions (Article 1) The condition, the second condition, the third condition) are satisfied.
[0146]
[First condition: heat treatment temperature T in CrNb nitride formation treatment]
In the CrNb nitride formation process, the heat treatment temperature T (° C.) is maintained in the following temperature range in the furnace in an atmospheric atmosphere.
1000 ≤ T ≤ T max
Here, Tmax (° C.) is as follows according to the Mo content.
<1> When the Mo content is 0.10 to 1.00%
T max = T x-100 (Mo + W) + 200C-80Nb
<2> When the Mo content is more than 1.00% and less than 2.50%
T max = T x-50 (Mo + W) + 200C-80Nb
<3> When the Mo content is 2.50 to 4.50%
T max = T x-20 (Mo + W) + 200C-80Nb
Here, T x = 1300.
[0147]
If the heat treatment temperature T is less than 1000 ° C., the precipitates such as Cr carbides deposited in the steel material in the hot working process do not sufficiently dissolve. In this case, the ratio of Nb carbide and Cr carbide in the precipitate is remarkably high in the austenitic stainless steel material in which the element content in the chemical composition is within the range of the present embodiment and the formula (1) is satisfied. Therefore, the proportion of CrNb nitride becomes significantly low. Therefore, the Nb content in the residue exceeds 0.267% by mass and / or the Cr content in the residue exceeds 0.125% by mass.
[0148]
Further, the element content in the chemical composition of the steel material is within the range of this embodiment, the Mo content is 2.50 to 4.50%, and Co: 0.01 to 1.00%. When the formulas (1) to (3) are satisfied, if the heat treatment temperature T is less than 1000 ° C., the Nb content in the residue exceeds 0.245% by mass and / or is contained in the residue. Cr content exceeds 0.104% by mass.
[0149]
On the other hand, if the heat treatment temperature T exceeds T max, not only the Nb carbides and Cr carbides generated in the steel material in the hot working process are solid-dissolved, but also the precipitation of CrNb nitrides in the CrNb nitride formation process is insufficient. do. Therefore, the element content in the chemical composition is within the range of the present embodiment, and the proportion of CrNb nitride present in the austenitic stainless steel material satisfying the formula (1) is remarkably reduced. As a result, the Nb content in the residue is less than 0.050% by mass.
[0150]
Further, the element content in the chemical composition of the steel material is within the range of this embodiment, the Mo content is 2.50 to 4.50%, and Co: 0.01 to 1.00%. When the formulas (1) to (3) are satisfied, if the heat treatment temperature T exceeds T max, the Nb content in the residue is less than 0.065% by mass.
[0151]
When the heat treatment temperature T is 1000 ° C. or higher and T max or lower, the Cr carbides produced in the hot working step can be sufficiently dissolved, the excessive formation of Nb carbides can be suppressed, and an appropriate amount can be obtained. CrNb nitride can be produced. As a result, in the austenitic stainless steel material in which the element content in the chemical composition is within the range of the present embodiment and the formula (1) is satisfied, the Nb content in the residue is 0.050 to 0. It is 267% and the Cr content is 0.125% or less. Therefore, the austenitic stainless steel material has improved sensitization resistance.
[0152]
Further, the element content in the chemical composition of the steel material is within the range of this embodiment, the Mo content is 2.50 to 4.50%, and Co: 0.01 to 1.00%. When the formulas (1) to (3) are satisfied, the Nb content in the residue is 0.065 to 0.245% by mass, and the Cr content is 0.104% or less. Therefore, the polythionic acid SCC resistance of the austenitic stainless steel material is enhanced.
[0153]
In T max, the preferred T x is 1290, more preferably 1280.
[0154]
[Second condition]
Further, in the CrNb nitride formation treatment, the heat treatment temperature T (° C.) and the holding time t (minutes) at the heat treatment temperature T satisfy the following conditions.
(A) When the Mo content in the chemical composition is 0.10 to 1.00%
F1 ≦ f2 and f2 ≦ f3
Here, f1 to f3 are defined as follows.
F1 = 760
F2 = T × Log10 (20Nb + 0.1Cr + 10Mo + t / 60)
F3 = 1680
(B) When the Mo content in the chemical composition is more than 1.00% to less than 2.50%
F1 ≦ f2 and f2 ≦ f3
Here, f1 to f3 are defined as follows.
F1 = 1200
F2 = T × Log10 (20Nb + 0.1Cr + 10Mo + t / 60)
F3 = 1900
(C) When the Mo content in the chemical composition is 2.50 to 4.50%
F1 ≦ f2 and f2 ≦ f3
Here, f1 to f3 are defined as follows.
F1 = 1520
F2 = T × Log10 (20Nb + 0.1Cr + 10Mo + t / 60)
F3 = 2050
The heat treatment temperature T (° C.) is substituted for T in f2, and the holding time t (minutes) is substituted for t. The content (mass%) of the corresponding element is substituted for each element symbol in f2.
[0155]
F2 is a parameter of the heat treatment temperature T and the holding time t required to generate an appropriate amount of CrNb nitride in the steel material in which the content of each element in the chemical composition is within the range of the present embodiment. Hereinafter, f2 is referred to as "CrNb nitride generation parameter". Cr and Nb in the chemical composition are elements constituting CrNb nitride. Furthermore, Mo is an element that affects the formation of CrNb nitrides and induces the formation of the LAVES phase.
[0156]
A different value is applied to f1 based on the Mo content in the chemical composition of the steel material. Specifically, when the Mo content in the chemical composition of the steel material is 0.10 to 1.00%, f1 = 760. When the Mo content in the chemical composition of the steel material is more than 1.00% to less than 2.50%, f1 = 1200. When the Mo content in the chemical composition of the steel material is 2.50 to 4.50%, f1 = 1520.
[0157]
Similar to f1, different values ​​are applied to f3 based on the Mo content in the chemical composition of the steel material. Specifically, when the Mo content in the chemical composition of the steel material is 0.10 to 1.00%, f3 = 1680. When the Mo content in the chemical composition of the steel material is more than 1.00% to less than 2.50%, f3 = 1900. When the Mo content in the chemical composition of the steel material is 2.50 to 4.50%, f3 = 2050.
[0158]
If f2 is less than f1, the CrNb nitride generation parameter is too low. In this case, the element content in the chemical composition is within the range of the present embodiment, and the ratio of Nb carbide and Cr carbide in the precipitate is high in the austenitic stainless steel material satisfying the formula (1). The proportion of CrNb nitride is significantly reduced. Therefore, the Nb content in the residue exceeds 0.267% by mass and / or the Cr content in the residue exceeds 0.125% by mass.
[0159]
Further, the element content in the chemical composition of the steel material is within the range of this embodiment, the Mo content is 2.50 to 4.50%, and Co: 0.01 to 1.00%. When formulas (1) to (3) are satisfied, if f2 is less than f1, the Nb content in the residue exceeds 0.245% by mass, and / or Cr content in the residue. The amount exceeds 0.104%.
[0160]
If f2 exceeds f3, the CrNb nitride generation parameter is too high. In this case, the precipitation of CrNb nitride is insufficient. Therefore, the proportion of CrNb nitride present in the austenitic stainless steel material is significantly reduced. As a result, the element content in the chemical composition is within the range of the present embodiment, and the Nb content in the residue is less than 0.050% by mass in the austenitic stainless steel material satisfying the formula (1). ..
[0161]
Further, the element content in the chemical composition of the steel material is within the range of this embodiment, the Mo content is 2.50 to 4.50%, and Co: 0.01 to 1.00%. When the formulas (1) to (3) are satisfied, if f2 exceeds f3, the Nb content in the residue is less than 0.065% by mass.
[0162]
If f2 is f1 or more and f2 is f3 or less, the CrNb nitride generation parameter is within an appropriate range. In this case, an appropriate amount of CrNb nitride is deposited. Therefore, in the austenitic stainless steel material in which the element content in the chemical composition is within the range of the present embodiment and the formula (1) is satisfied, the Nb content in the residue is 0.050 to 0. It is 267%, and the Cr content in the residue is 0.125% or less in mass%. As a result, the austenitic stainless steel material has excellent sensitization resistance.
[0163]
Further, the element content in the chemical composition of the steel material is within the range of this embodiment, the Mo content is 2.50 to 4.50%, and Co: 0.01 to 1.00%. When the formulas (1) to (3) are satisfied, if f2 is f1 or more and f2 is f3 or less, the Nb content in the residue of the austenitic stainless steel material is 0. It is 065 to 0.245%, and the Cr content in the residue is 0.104% or less in mass%. As a result, the austenitic stainless steel material has excellent polythionic acid SCC resistance.
[0164]
[Third condition]
The CrNb nitride formation treatment is further held at a heat treatment temperature of T ° C. for a holding time of t, and then cooled. At this time, the average cooling rate CR in a temperature range of at least 800 to 500 ° C. is cooled at 15 ° C./sec or higher. When the average cooling rate CR is less than 15 ° C./sec, CrNb nitride is also deposited at the grain boundaries in the steel material while cooling in the temperature range of 800 to 500 ° C., and further, M 23C 6 type. Cr carbides are also generated at the grain boundaries. Therefore, the element content in the chemical composition is within the range of the present embodiment, and the Nb content in the residue exceeds 0.267% by mass in the austenitic stainless steel material satisfying the formula (1). And / or the Cr content exceeds 0.125%. In this case, the austenitic stainless steel material has a reduced sensitization resistance.
[0165]
Further, the element content in the chemical composition of the steel material is within the range of this embodiment, the Mo content is 2.50 to 4.50%, and Co: 0.01 to 1.00%. When the formulas (1) to (3) are satisfied, if the average cooling rate CR is less than 15 ° C./sec, the Nb content in the residue exceeds 0.245% by mass. And / or the Cr content exceeds 0.104%. In this case, the polythionic acid SCC resistance of the austenitic stainless steel material is lowered.
[0166]
When the average cooling rate CR is 15 ° C./sec or more, it is possible to suppress excessive formation of Cr carbide in the steel material while cooling in the temperature range of 800 to 500 ° C. Therefore, on the premise that the first condition and the second condition are satisfied, in the austenitic stainless steel material in which the element content in the chemical composition is within the range of the present embodiment and the formula (1) is satisfied, The Nb content in the residue is 0.050 to 0.267% by mass, and the Cr content in the residue is 0.125% or less by mass. Therefore, the austenitic stainless steel material can be enhanced in sharpening resistance.
[0167]
Further, the element content in the chemical composition of the steel material is within the range of this embodiment, the Mo content is 2.50 to 4.50%, and Co: 0.01 to 1.00%. When the formulas (1) to (3) are satisfied, if the average cooling rate CR is 15 ° C./sec or more, the austenitic stainless steel material is premised on the condition that the first condition and the second condition are satisfied. The Nb content in the residue is 0.065 to 0.245% by mass, and the Cr content in the residue is 0.104% or less. Therefore, the polythionic acid SCC resistance of the austenitic stainless steel material is enhanced.
[0168]
By the above steps, the austenitic stainless steel material of the present embodiment can be manufactured. The above-mentioned manufacturing method is the method for manufacturing an austenitic stainless steel material of the present embodiment.This is just one example. Therefore, the method for producing the austenitic stainless steel material of the present embodiment is not limited to the above-mentioned production method. The content of each element in the chemical composition of the steel material is within the range of this embodiment, the formula (1) is satisfied, the Nb content in the residue is 0.050 to 0.267% by mass, and The austenitic stainless steel material of the present embodiment is not limited to the above-mentioned production method as long as the Cr content in the residue is 0.125% or less in mass%.
[0169]
As described above, in the austenitic stainless steel material of the present embodiment, each element in the chemical composition is within the range of the present embodiment and satisfies the formula (1). Further, the Nb content in the residue is 0.050 to 0.267% by mass, and the Cr content in the residue is 0.125% or less by mass. Therefore, the austenitic stainless steel material of the present embodiment has excellent sensitization resistance.
[0170]
Further, when the austenitic stainless steel material of the present embodiment further satisfies the above (I) to (III), that is, in the chemical composition, Mo: 2.50 to 4.50% and Co: 0.01 to 1 The Nb content in the residue obtained by the extraction residue method, which contains .00% and further satisfies the formulas (2) and (3), is 0.065 to 0.245% in mass%. When the Cr content in the residue is 0.104% or less in mass%, the austenitic stainless steel material of the present embodiment has sufficient polythionic acid SCC resistance and naphthenic acid corrosion resistance.
Example 1
[0171]
Hereinafter, the effect of the austenitic stainless steel material of the present embodiment will be specifically described with reference to Examples. The conditions in the following examples are one condition example adopted for confirming the feasibility and effect of the austenitic stainless steel material of the present embodiment. Therefore, the austenitic stainless steel material of the present embodiment is not limited to this one condition example.
[0172]
[Manufacturing of austenitic stainless steel]
A material (ingot) having the chemical composition shown in Table 1 was manufactured.
[0173]
[table 1]

[0174]
"0" and blanks in Table 1 indicate that the corresponding element content was below the detection limit. If it was below the detection limit, it was considered that the element was not contained. For example, the Mo content of test number B1 means that it was "0" when the third decimal place was rounded off. Further, the W content of the test number B1 means that it was "0" when the third decimal place was rounded off. In the "Arbitrary element, etc." column in Table 1, the contained arbitrary element or impurity element and its content (mass%) are described. For example, test number A3 indicates that Ti was contained in 0.02%, V was contained in 0.04%, and B was contained in 0.0014%. Regarding the impurity elements Sn, As, Zn, Pb, and Sb, the Sn content is 0 to 0.010% and the As content is 0 to 0.010% in any of the test numbers. The Zn content was 0 to 0.010%, the Pb content was 0 to 0.010%, and the Sb content was 0 to 0.010%.
[0175]
Using molten steel, an ingot having the chemical composition shown in Table 1 and having an outer diameter of 120 mm and a diameter of 30 kg was manufactured. Hot forging was performed on the ingot to obtain a material with a thickness of 30 mm. The temperature of the ingot before hot forging was 1250 ° C. Further, hot rolling was carried out on the material to produce an intermediate steel material (steel plate) having a thickness of 15 mm. The material temperature immediately before hot working (hot rolling) was 1250 ° C. The finishing temperature of the intermediate steel material after hot rolling was 900 ° C. or higher.
[0176]
CrNb nitride formation treatment was carried out on the intermediate steel material after hot rolling. In the CrNb nitride formation process, the T max of each test number was as shown in Table 2. The heat treatment temperatures T of test numbers A1 to A18 and B1 to B6, B9, B10, and B13 to B17 were all 1000 ° C. or higher and T max or lower. On the other hand, the heat treatment temperature T of test number B8 was less than 1000 ° C. Further, the heat treatment temperatures T of test numbers B7, B11 and B12 exceeded T max.
[0177]
Further, the CrNb nitride generation parameters f2, f1 and f3 of each test number are as shown in Table 2. In the "f1 ≦ f2" column in Table 2, "T" indicates that f1 ≦ f2. "F" indicates that f1> f2. In the "f2 ≦ f3" column in Table 2, "T" indicates that f2 ≦ f3. "F" indicates that f2> f3.
[0178]
Further, the average cooling rate CR from 800 to 500 ° C. in the CrNb nitride formation treatment of test numbers A1 to A18, B1 to B5, B7 to B14, B16 and B17 was 15 ° C./sec or more. On the other hand, the average cooling rate CR of test numbers B6 and B15 from 800 to 500 ° C. was 5 ° C./sec. Through the above steps, an austenitic stainless steel material was produced.
[0179]
[Evaluation test]
The following evaluation test was conducted on the austenitic stainless steel materials manufactured by the above manufacturing process.
[0180]
[Preparation of large heat input welded joint simulation test piece]
Using the manufactured austenitic stainless steel material, a large heat input welded joint simulation test piece simulating high heat input welding was produced by the following method.
[0181]
Square test pieces including the center position of the plate width and the center position of the plate thickness of the austenitic stainless steel material of each test number were collected. The longitudinal direction of the angular test piece was parallel to the longitudinal direction of the austenitic stainless steel material. The length of the square test piece was 100 mm. The cross section (cross section) perpendicular to the longitudinal direction of the square test piece was a rectangle of 10 mm × 10 mm. The center position of the cross section of the square test piece almost coincided with the center position of the plate width and the center position of the plate thickness of the austenitic stainless steel material.
[0182]
The following thermal history was given to the angular test piece using a high frequency thermal cycle device. Specifically, with reference to FIG. 6, the central portion 60 having a width of 10 mm at the center position in the longitudinal direction of the angular test piece (that is, a width of 5 mm to the left and right from the center position in the longitudinal direction) is set in the atmosphere. The temperature was raised from room temperature to 1400 ° C at 70 ° C / sec. It was further held at 1400 ° C. for 10 seconds. Then, the angular test piece was cooled to room temperature at a cooling rate of 20 ° C./sec. By applying the above heat history to the angular test piece, a large heat input welded joint simulated test piece was produced.
[0183]
[Long-term sensitization treatment]
Using a large heat input welded joint simulation test piece, the following long-term sensitization treatment was carried out. A simulated test piece of a large heat input welded joint was charged into a heat treatment furnace. In the heat treatment furnace, the large heat input welded joint simulated test piece was held in the air at atmospheric pressure at 550 ° C. for 10,000 hours (sensitization treatment). After 10000 hours had passed, the large heat input welded joint simulated test piece was extracted from the heat treatment furnace and allowed to cool.
[0184]
The following Strauss test and reactivation rate measurement test were carried out on the large heat input welded joint simulated test piece that had been sensitized for a long time.
[0185]
[Strauss test (sulfuric acid / copper sulfate corrosion test)]
The Strauss test compliant with ASTM A262-15 PRACTICE E was conducted as follows. A plate-shaped test piece having a thickness of 2 mm, a width of 10 mm, and a length of 80 mm was collected from a large heat input welded joint simulated test piece that had been sensitized for a long time so that the central portion 60 was located at the center position in the longitudinal direction. The plate-shaped test piece was immersed in a copper sulfate test solution containing 16% sulfuric acid and boiled for 15 hours. Then, the plate-shaped test piece was taken out from the copper sulfate test solution. A bending test was performed on the plate-shaped test piece taken out. In the bending test, the plate-shaped test piece was bent 180 ° in the atmosphere around the center position in the longitudinal direction of the large heat input welded joint simulated test piece. The bent portion of the bent test piece was cut. The cut surface was observed with a 20x optical microscope. If cracks were observed, the length of the cracks was determined. If no cracks were observed, or if cracks were observed but the length of the cracks was 100 μm or less, the Strauss test was judged to be acceptable (“E” (Excellent) in Table 2). On the other hand, when cracks exceeding 100 μm were observed, the Strauss test was judged to be unsuccessful (“B” (Bad) in Table 2).
[0186]
[Reactivation rate measurement test]
An electrochemical reactivation rate measurement test (Electrochemical Reaction test) based on ASTM G108-94 was carried out using a large heat input welded joint simulated test piece that had been sensitized for a long time. Specifically, a plate-shaped test piece was collected from the central portion 60 (the portion to which the large heat input was applied) of the large heat input welded joint simulated test piece that had been subjected to the sensitization treatment for a long time. In the collected plate-shaped test piece, the area other than the surface portion of the evaluation area of ​​100 mm 2 was masked. Using the masked plate-shaped test piece as an electrode, it was immersed in a 0.5 mol sulfuric acid + 0.01 mol potassium thiocyanate solution having a temperature of 30 ° C. and a capacity of 200 cm 3. Next, the plate-shaped test piece was scanned in the noble direction from the natural potential to 300 mV with a linear polarization at a polarization rate of 100 mV / min. Immediately after reaching 300 mV based on the saturated sweet electrode, scanning was performed in the base direction to the original natural potential. The current flowing when the voltage was applied in the noble direction (outward route) was measured. Then, the current flowing when the voltage was applied in the base direction (return path) was measured. Based on the obtained current value, the reactivation rate (%) was defined as follows.
Reactivation rate = (maximum anode current on the return path / maximum anode current on the outward path) x 100
[0187]
The lower the reactivation rate, the lower the degree of sensitization (DOS) and the higher the sensitization resistance. In this example, when the reactivation rate is 10% or less, it is judged to be acceptable. ("E" (Excellent) in Table 2). On the other hand, when the reactivation rate exceeds 10%, it is judged to be unacceptable (“B” (Bad) in Table 2).
[0188]
If the Strauss test is passed and the reactivation rate is 10% or less in the large heat input welded joint simulated test piece that has been subjected to the above-mentioned long-term sensitization treatment, the austenitic stainless steel material has excellent resistance. It was judged to have sensitization characteristics.
[0189]
[Test results]
Table 2 shows the test results.
[0190]
[Table 2]

[0191]
With reference to Tables 1 and 2, in test numbers A1 to A18, the content of each element in the chemical composition was appropriate, and F1 satisfied the formula (1). Further, the Nb content in the residue was 0.050 to 0.267% by mass, and the Cr content in the residue was 0.125% or less. Furthermore, in the Strauss test, no cracks exceeding 100 μm were confirmed. Further, in the reactivation rate measurement test, the reactivation rate was 10% or less. Therefore, the austenitic stainless steel materials of test numbers A1 to A18 showed excellent sensitization resistance even when the sensitization treatment was performed at 550 ° C. for 10,000 hours after the large heat input welding.
[0192]
On the other hand, in the test numbers B1 to B3, the Mo content and / or the W content was low. Therefore, in the Strauss test, cracks exceeding 100 μm were confirmed. Furthermore, in the reactivation rate measurement test, the reactivation rate exceeded 10%. That is, when the sensitization treatment was carried out at 550 ° C. for 10,000 hours after the large heat input welding, the sensitization resistance property was low.
[0193]
In test number B4, F1 did not satisfy equation (1). Therefore, in the Strauss test, cracks exceeding 100 μm were confirmed. Furthermore, in the reactivation rate measurement test, the reactivation rate exceeded 10%. That is, when the sensitization treatment was carried out at 550 ° C. for 10,000 hours after the large heat input welding, the sensitization resistance property was low.
[0194]
In test number B5, the C content was high. Therefore, in the Strauss test, cracks exceeding 100 μm were confirmed. Furthermore, in the reactivation rate measurement test, the reactivation rate exceeded 10%. That is, when the sensitization treatment was carried out at 550 ° C. for 10,000 hours after the large heat input welding, the sensitization resistance was low.
[0195]
In test number B6, F1 did not satisfy equation (1). Further, in the CrNb nitride treatment process The average cooling rate CR at 800 to 500 ° C. was less than 15 ° C./sec. Therefore, the Cr content in the residue was too high. Therefore, in the Strauss test, cracks exceeding 100 μm were confirmed. Furthermore, in the reactivation rate measurement test, the reactivation rate exceeded 10%. That is, when the sensitization treatment was carried out at 550 ° C. for 10,000 hours after the large heat input welding, the sensitization resistance was low.
[0196]
In test number B7, the heat treatment temperature T was higher than T max in the CrNb nitride formation treatment. Therefore, the Nb content in the residue was too low. Therefore, in the Strauss test, cracks exceeding 100 μm were confirmed. Furthermore, in the reactivation rate measurement test, the reactivation rate exceeded 10%. That is, when the sensitization treatment was carried out at 550 ° C. for 10,000 hours after the large heat input welding, the sensitization resistance was low.
[0197]
In test number B8, the heat treatment temperature T was less than 1000 ° C. in the CrNb nitride formation treatment. Therefore, the Nb content in the residue and the Cr content in the residue were too high. Therefore, cracks exceeding 100 μm were confirmed in the Strauss test, and the reactivation rate exceeded 10% in the reactivation rate measurement test. That is, when the sensitization treatment was carried out at 550 ° C. for 10,000 hours after the large heat input welding, the sensitization resistance was low.
[0198]
In test number B9, although the chemical composition was appropriate and the formula (1) was satisfied, the CrNb nitride formation parameter f2 was less than f1 in the CrNb nitriding treatment step. Therefore, the Cr content in the residue was too high. As a result, cracks exceeding 100 μm were confirmed in the Strauss test. Furthermore, in the reactivation rate measurement test, the reactivation rate exceeded 10%. That is, when the sensitization treatment was carried out at 550 ° C. for 10,000 hours after the large heat input welding, the sensitization resistance was low.
[0199]
In test number B10, the chemical composition was appropriate and the formula (1) was satisfied, but the CrNb nitride formation parameter f2 exceeded f3 in the CrNb nitriding treatment step. Therefore, the Nb content in the residue was too low. As a result, cracks exceeding 100 μm were confirmed in the Strauss test. Furthermore, in the reactivation rate measurement test, the reactivation rate exceeded 10%. That is, when the sensitization treatment was carried out at 550 ° C. for 10,000 hours after the large heat input welding, the sensitization resistance was low.
[0200]
In test number B11, the heat treatment temperature T was higher than T max in the CrNb nitride formation treatment. Therefore, the Nb content in the residue was too low. Therefore, in the Strauss test, cracks exceeding 100 μm were confirmed. Furthermore, in the reactivation rate measurement test, the reactivation rate exceeded 10%. That is, when the sensitization treatment was carried out at 550 ° C. for 10,000 hours after the large heat input welding, the sensitization resistance was low.
[0201]
In test number B12, the heat treatment temperature T was higher than T max in the CrNb nitride formation treatment. Therefore, the Nb content in the residue was too low. Therefore, in the Strauss test, cracks exceeding 100 μm were confirmed. Furthermore, in the reactivation rate measurement test, the reactivation rate exceeded 10%. That is, when the sensitization treatment was carried out at 550 ° C. for 10,000 hours after the large heat input welding, the sensitization resistance was low.
[0202]
In test number B13, although the chemical composition was appropriate and the formula (1) was satisfied, the CrNb nitride formation parameter f2 was less than f1 in the CrNb nitriding treatment step. Therefore, the Cr content in the residue was too high. As a result, cracks exceeding 100 μm were confirmed in the Strauss test. Furthermore, in the reactivation rate measurement test, the reactivation rate exceeded 10%. That is, when the sensitization treatment was carried out at 550 ° C. for 10,000 hours after the large heat input welding, the sensitization resistance was low.
[0203]
In test number B14, the chemical composition was appropriate and the formula (1) was satisfied, but the CrNb nitride formation parameter f2 exceeded f3 in the CrNb nitriding treatment step. Therefore, the Nb content in the residue was too low. As a result, cracks exceeding 100 μm were confirmed in the Strauss test. Furthermore, in the reactivation rate measurement test, the reactivation rate exceeded 10%. That is, when the sensitization treatment was carried out at 550 ° C. for 10,000 hours after the large heat input welding, the sensitization resistance was low.
[0204]
In test number B15, the chemical composition was appropriate and the formula (1) was satisfied, but the average cooling rate CR at 800 to 500 ° C. was less than 15 ° C./sec in the CrNb nitriding treatment step. Therefore, the Cr content in the residue was too high. As a result, cracks exceeding 100 μm were confirmed in the Strauss test. Furthermore, in the reactivation rate measurement test, the reactivation rate exceeded 10%. That is, when the sensitization treatment was carried out at 550 ° C. for 10,000 hours after the large heat input welding, the sensitization resistance was low.
[0205]
In test number B16, although the chemical composition was appropriate and the formula (1) was satisfied, the CrNb nitride formation parameter f2 was less than f1 in the CrNb nitriding treatment step. Therefore, the Cr content in the residue was too high. As a result, cracks exceeding 100 μm were confirmed in the Strauss test. Furthermore, in the reactivation rate measurement test, the reactivation rate exceeded 10%. That is, when the sensitization treatment was carried out at 550 ° C. for 10,000 hours after the large heat input welding, the sensitization resistance was low.
[0206]
In test number B17, the chemical composition was appropriate and the formula (1) was satisfied, but the CrNb nitride formation parameter f2 exceeded f3 in the CrNb nitriding treatment step. Therefore, the Nb content in the residue was too low. As a result, cracks exceeding 100 μm were confirmed in the Strauss test. Furthermore, in the reactivation rate measurement test, the reactivation rate exceeded 10%. That is, when the sensitization treatment was carried out at 550 ° C. for 10,000 hours after the large heat input welding, the sensitization resistance was low.
Example 2
[0207]
[Manufacturing of austenitic stainless steel]
A material (ingot) having the chemical composition shown in Table 3 was manufactured.
[0208]
[Table 3]

[0209]
Blanks in Table 3 indicate that the corresponding element content was below the detection limit. If it was below the detection limit, it was considered that the element was not contained. In the "Arbitrary element, etc." column in Table 3, the contained arbitrary element or impurity element and its content (mass%) are described. For example, test number A3 indicates that Ti was contained in 0.08%, V was contained in 0.16%, and the impurity Sn was contained in 0.005%. Regarding the impurity elements Sn, As, Zn, Pb, and Sb, the Sn content is 0 to 0.010%, the As content is 0 to 0.010%, and Zn in any of the test numbers. The content was 0 to 0.010%, the Pb content was 0 to 0.010%, and the Sb content was 0 to 0.010%.
[0210]
Using molten steel, an ingot having the chemical composition shown in Table 3 and having an outer diameter of 120 mm and a diameter of 30 kg was manufactured. Hot forging was performed on the ingot to obtain a material with a thickness of 30 mm. The temperature of the ingot before hot forging was 1150 ° C. Further, hot rolling was carried out on the material to produce a steel material (steel plate) having a thickness of 15 mm. The material temperature before hot working (hot rolling) was 1150 ° C. The finishing temperature of the steel material after hot rolling was 900 ° C. or higher.
[0211]
CrNb nitride formation treatment was carried out on the steel material after hot rolling. The T max of each test number in the CrNb nitride formation process is as shown in Table 4. In test numbers A1 to A13, B1 to B8, and B11 to 14, the heat treatment temperature T was 1000 ° C. or higher and T max or lower. On the other hand, in test number B9, the heat treatment temperature T exceeded T max. Further, in the test number B10, the heat treatment temperature T was less than 1000 ° C.
[0212]
Further, the CrNb nitride generation parameters f2, f1 and f3 of each test number are as shown in Table 4. In the "f1 ≦ f2" column in Table 4, "T" indicates that f1 ≦ f2. "F" indicates that f1> f2. In the "f2 ≦ f3" column in Table 4, "T" indicates that f2 ≦ f3. "F" indicates that f2> f3.
[0213]
Further, the average cooling rate CR from 800 to 500 ° C. in the CrNb nitride formation treatment of test numbers A1 to A13, B1 to B10, and B12 to B14 was 15 ° C./sec or more. On the other hand, the average cooling rate CR of test number B11 from 800 to 500 ° C. was less than 15 ° C./sec. Through the above steps, an austenitic stainless steel material was produced.
[0214]
[Naphthenic acid corrosion resistance evaluation test]
A test piece having a thickness of 2 mm, a width of 10 mm, and a length of 30 mm was collected from the center position of the width and the center of the plate thickness of the austenitic stainless steel material of each test number. The longitudinal direction of the test piece was parallel to the longitudinal direction (rolling direction) of the steel material. The collected test piece was immersed in a 100% cyclohexanecarboxylic acid solution at 200 ° C. for 720 hours under normal pressure. After soaking for 720 hours, the test piece was ultrasonically washed with acetone for 3 minutes.
[0215]
The difference between the mass of the test piece before the test and the mass of the test piece after ultrasonic cleaning was calculated as the corrosion weight loss. Furthermore, the corrosion rate (mm / year) was determined from the surface area, specific gravity, and test time of the test piece. When the corrosion rate was 0.01 mm / year or less, it was judged to be excellent in naphthenic acid corrosiveness (indicated as "E" in the "naphthenic acid corrosiveness" column in Table 4). On the other hand, when the corrosion rate exceeded 0.01 mm / year, it was judged that the naphthenic acid corrosiveness was low (indicated as "B" in the "naphthenic acid corrosiveness" column in Table 4).
[0216]
[Preparation of large heat input welded joint simulation test piece]
Using the manufactured austenitic stainless steel material, a large heat input welded joint simulation test piece simulating a welded joint manufactured by high heat input welding was produced by the following method.
[0217]
Square test pieces including the center position of the plate width and the center position of the plate thickness of the austenitic stainless steel material of each test number were collected. The longitudinal direction of the angular test piece was parallel to the longitudinal direction of the austenitic stainless steel material. The length of the square test piece was 100 mm. The cross section (cross section) perpendicular to the longitudinal direction of the square test piece was a rectangle of 10 mm × 10 mm. The center position of the cross section of the square test piece almost coincided with the center position of the plate width and the center position of the plate thickness of the austenitic stainless steel material.
[0218]
The following thermal history was given to the angular test piece using a high frequency thermal cycle device. Specifically, with reference to FIG. 6, the 10 mm wide portion 60 at the center position in the longitudinal direction of the square test piece was heated from room temperature to 1350 ° C. at 100 ° C./sec in the atmosphere. Further, it was held at 1350 ° C. for 1 to 60 seconds. Then, the angular test piece was cooled to room temperature at a cooling rate of 20 ° C./sec. By applying the above heat history to the angular test piece, a large heat input welded joint simulated test piece 50 was produced.
[0219]
[Average crystal grain size R1 and R2 measurement test]
The average crystal grain sizes R1 and R2 were measured by the following method using a large heat input welded joint simulated test piece 50. The region 60 of the 10 mm wide portion at the center position in the length direction of the large heat-affected zone simulated test piece 50 corresponds to the HAZ range Dr (reproduced HAZ structure) of the welded joint. Therefore, the region 60 was recognized as the HAZ range Dref (reproduced HAZ structure) 60. A sample was taken with the surface of the range Dref 60 as the observation surface. The observation surface was mirror-polished. Then, in accordance with JIS G0551 (2013), the crystal particle size numbers in any three fields of view were obtained by a cutting method. Each field of view was 100 μm × 100 μm. The arithmetic mean value of the obtained three crystal particle size numbers was obtained and defined as the average crystal particle size number. The average crystal grain size R1 (μm) was obtained from the obtained average crystal grain size number.
[0220]
Similarly, the position 25 mm from the end portion in the longitudinal direction of the large heat input welded joint simulated test piece 50 was certified as the normal portion 70. In the normal part 70, the average crystal grain size R2 was measured by the following method. Large heat input welded joint simulation test A sample was taken with the surface of the normal portion 70 of the test piece 50 as the observation surface. The observation surface was mirror-polished. Then, in accordance with JIS G0551 (2013), the crystal particle size numbers in any three fields of view were obtained by a cutting method. Each field of view was 100 μm × 100 μm. The arithmetic mean value of the obtained three crystal particle size numbers was obtained and defined as the average crystal particle size number. The average crystal grain size R2 (μm) was obtained from the obtained average crystal grain size number.
[0221]
R1 / R2 was obtained by using the average crystal grain size R1 in the obtained range Dref 60 and the average crystal grain size R2 in the normal part 70. The obtained R1 / R2 is shown in the "R1 / R2" column of Table 4. Further, "T" in the "Equation (4)" column of Table 4 means that R1 / R2 is 4.8 or less and satisfies the equation (4). On the other hand, "F" in the "Equation (4)" column means that R1 / R2 exceeded 4.8 and did not satisfy the equation (4).
[0222]
[Polythionic acid SCC resistance evaluation test]
The following long-term sensitization treatment test was carried out using a large heat input welded joint simulation test piece. A simulated test piece of a large heat input welded joint was charged into a heat treatment furnace. In the heat treatment furnace, the large heat-injection welded joint simulated test piece was held in the air at atmospheric pressure at 550 ° C. for 10,000 hours (sensitization treatment). After 10000 hours had passed, the large heat input welded joint simulated test piece was extracted from the heat treatment furnace and allowed to cool.
[0223]
From the large heat input welded joint simulated test piece after long-term sensitization treatment, a plate-shaped test piece with a thickness of 2 mm, a width of 10 mm, and a length of 75 mm was collected so that the range Dref60 was located at the center position in the longitudinal direction. Using the collected plate-shaped test pieces, a polythionic acid SCC resistance evaluation test was carried out by the following method. The plate-shaped test piece was bent around a punch having an inner radius of 5 mm to form a U-bend shape. The U-bend type test piece was immersed in a 1% K 2S 4O 6 solution adjusted to PH = 2 with sulfuric acid at room temperature for 100 hours. In the test piece after immersion, the bent portion of the bent test piece was cut in a direction perpendicular to the longitudinal direction, and the cut surface was observed with a 20x optical microscope. When cracks were observed, the depth of cracks on the cut surface was determined. When no crack was observed, it was judged that the polythionic acid SCC resistance was extremely excellent (indicated as "E" (Excellent) in the "PTASCC resistance" column in Table 4). Although cracks were observed on the cut surface, when the crack depth was less than 20 μm, it was judged to be excellent in polythionic acid SCC resistance (denoted as “G” (Good) in the “PTASSC resistance” column in Table 4). ). When cracks were observed on the cut surface and the crack depth was 20 μm or more, it was judged that the polythionic acid SCC resistance was low (indicated as “B” (bad) in the “PTASSC resistance” column in Table 4). ..
[0224]
[Liquid resistance crack resistance evaluation test]
The large heat input welded joint simulated test piece 50 was cut in the direction perpendicular to the longitudinal direction at the center position in the longitudinal direction (that is, within the range of the range Dref60). The cut surface was used as the observation surface. The observation surface was etched with mixed acid. Arbitrary three fields of view (each field of view is 250 μm × 250 μm) of the etched observation surface was observed with a 400 × optical microscope. In the three observed visual fields, the presence or absence of partial melting marks at the grain boundaries was determined.
[0225]
If no partial melting marks with a length of 25 μm or more were observed on the cut surface of the three visual fields, it was judged that the liquid resistance cracking resistance was very high (Table 4). Indicated by "E" (Excellent) in the "Liquid resistance cracking resistance" column inside). Of the partial melting marks generated at the grain boundaries, if partial melting marks with a length of 25 μm or more are observed but no partial melting marks with a length of 50 μm or more are observed, it is judged that the liquefaction resistance is high. (Indicated by "G" (Good) in the "Liquid resistance cracking resistance" column in Table 4). When even one partial melting mark having a length of 50 μm or more was observed, it was judged that the liquefaction cracking resistance was low (indicated by “B” (Bad) in the “liquidation cracking resistance” column in Table 4).
[0226]
[Test results]
Table 4 shows the test results.
[Table 4]

[0227]
With reference to Tables 3 and 4, in test numbers A1 to A13, the content of each element in the chemical composition was appropriate, and the formulas (1) to (3) were satisfied. Further, the Nb content in the residue was 0.065 to 0.245% by mass, and the Cr content was 0.104% or less by mass. Therefore, it is excellent in polythionic acid SCC resistance and naphthenic acid corrosion resistance. Furthermore, it has excellent liquid resistance and crack resistance.
[0228]
Further, in the test numbers A1 to A12, the average crystal grain size R1 of the range Drf in the large heat input welded joint simulated test piece and the average crystal grain size R2 of the normal portion satisfied the formula (4). Therefore, the polythionic acid SCC resistance was extremely high, and the liquefaction cracking corrosion resistance was extremely high.
[0229]
On the other hand, in the test number B1, although the content of each element in the chemical composition was appropriate, F2 exceeded the upper limit of the formula (2) and F3 exceeded the upper limit of the formula (3). As a result, the liquid resistance and cracking resistance were low.
[0230]
In test number B2, the Mo content was low. Further, F3 was less than the lower limit of the formula (3). Therefore, the polythionic acid SCC resistance and the naphthenic acid corrosion resistance were low.
[0231]
In test number B3, the Mo content was low. Therefore, the naphthenic acid corrosion resistance was low.
[0232]
In test number B4, F3 was less than the lower limit of equation (3). Therefore, the polythionic acid SCC resistance was low.
[0233]
In test number B5, F2 was less than the lower limit of equation (2). Therefore, the polythionic acid SCC resistance was low.
[0234]
In test number B6, F2 exceeded the upper limit of equation (2). As a result, the liquid resistance and cracking resistance were low.
[0235]
In test number B7, F3 was less than the lower limit of equation (3). As a result, the polythionic acid SCC resistance was low.
[0236]
In test number B8, F3 exceeded the upper limit of equation (3). As a result, the liquid resistance and cracking resistance were low.
[0237]
In test number B9, the heat treatment temperature T of the CrNb nitride formation treatment exceeded T max. Therefore, the Nb content in the residue was too low. As a result, the polythionic acid SCC resistance was low.
[0238]
In test number B10, the heat treatment temperature T of the CrNb nitride formation treatment was too low. Therefore, the Cr content in the residue was high. As a result, the liquid resistance and cracking resistance were low.
[0239]
In test number B11, the average cooling rate CR was too slow in the CrNb nitride formation process. Therefore, the Nb content in the residue was high, and the Cr content in the residue was high. As a result, the polythionic acid SCC resistance was low.
[0240]
In test number B12, the chemical composition was appropriate and the formulas (1) to (3) were satisfied, but the CrNb nitride formation parameter f2 was less than f1 in the CrNb nitriding treatment step. Therefore, the Cr content in the residue was too high. As a result, the polythionic acid SCC resistance was low.
[0241]
In test number B13, the chemical composition was appropriate and the formulas (1) to (3) were satisfied, but the CrNb nitride formation parameter f2 exceeded f3 in the CrNb nitriding treatment step. Therefore, the Nb content in the residue was too low. As a result, the polythionic acid SCC resistance was low.
[0242]
In test number B14, the Mo content was low. Therefore, the naphthenic acid corrosion resistance was low.
[0243]
The embodiment of the present invention has been described above. However, the embodiments described above are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and the above-mentioned embodiment can be appropriately modified and carried out within a range not deviating from the gist thereof.
Code description
[0244]
1 Welded joint
100 Austenitic stainless steel (base material)
101 Welding heat-affected zone (HAZ)
102 Normal part
200 Welded metal
200E melting wire
The scope of the claims
[Claim 1]
Austenitic stainless steel,
Chemical composition by mass%
C: 0.020% or less,
Si: 1.50% or less,
Mn: 2.00% or less,
P: 0.045% or less,
S: 0.0300% or less,
Cr: 15.00 to 25.00%,
Ni: 9.00 to 20.00%,
N: 0.05-0.15%,
Nb: 0.1-0.8%,
Mo: 0.10-4.50%,
W: 0.01-1.00%,
Ti: 0 to 0.50%,
Ta: 0 to 0.50%,
V: 0 to 1.00%,
Zr: 0 to 0.10%,
Hf: 0 to 0.10%,
Cu: 0 to 2.00%,
Co: 0 to 1.00%,
Sol. Al: 0 to 0.030%,
B: 0-0.0100%,
Ca: 0-0.0200%,
Mg: 0-0.0200%,
Rare earth elements: 0 to 0.100%,
Sn: 0 to 0.010%,
As: 0 to 0.010%,
Zn: 0 to 0.010%,
Pb: 0 to 0.010%,
Sb: 0 to 0.010% and
The balance consists of Fe and impurities,
Satisfy formula (1)
The Nb content in the residue obtained by the extraction residue method is 0.050 to 0.267% by mass, and the Cr content in the residue is 0.125% or less by mass.
Austenitic stainless steel material.
21.9Mo + 5.9W-5.0 ≧ 0 (1)
Here, the content (mass%) of the corresponding element in the chemical composition is substituted for each element symbol in the formula (1).
[Claim 2]
The austenitic stainless steel material according to claim 1.
The chemical composition is
Mo: 2.50-4.50%, and
Co: 0.01-1.00%,
Containing, further satisfying equations (2) and (3),
The Nb content in the residue obtained by the extraction residue method is 0.065 to 0.245% by mass, and the Cr content in the residue is 0.104% or less in mass%. be,
Austenitic stainless steel material.
2 ≦ 73W + 5Co ≦ 60 (2)
0.20 ≤ Nb + 0.1 W ≤ 0.58 (3)
[Claim 3]
The austenitic stainless steel material according to claim 1 or 2.
The chemical composition contains at least one element or two or more elements belonging to any of the groups 1 to 5.
Austenitic stainless steel material.
Group 1:
Ti: 0.01-0.50%,
Ta: 0.01-0.50%,
V: 0.01-1.00%,
Zr: 0.01-0.10% and
Hf: 0.01-0.10%,
Group 2:
Cu: 0.01-2.00% and
Co: 0.01-1.00%,
Group 3:
Sol. Al: 0.001 to 0.030%,
Group 4:
B: 0.0001 to 0.0100%,
Group 5:
Ca: 0.0001-0.0200%,
Mg: 0.0001 to 0.0200% and
Rare earth element: 0.001 to 0.100%.
[Claim 4]
It is a welded joint
The pair of austenitic stainless steel materials according to claim 2 or 3,
It is equipped with a weld metal placed between the pair of austenitic stainless steel materials.
Of the cross section of the austenitic stainless steel material perpendicular to the extending direction of the weld metal, the average crystal grain size in the range of 200 μm in the width direction of the weld metal from the fusion line in the weld heat affected zone is the average crystal grain size. When defined as R1 and the average crystal grain size of the portion other than the weld heat affected zone is defined as the average crystal grain size R2,
The average crystal grain size R1 and the average crystal grain size R2 satisfy the formula (4).
Welded joint.
R1 / R2 ≤ 4.8 (4)