WO2021015283A1 - オーステナイト系ステンレス鋼材及び溶接継手 - Google Patents
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Definitions
- the present disclosure relates to steel materials, and more particularly to austenitic stainless steel materials and welded joints using the austenitic stainless steel materials.
- Austenitic stainless steel is used as the steel for these chemical plant equipment applications.
- 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 reformer, 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.
- the average temperature during operation of each device is different.
- the average temperature during operation is referred to as "average operating temperature”.
- a vacuum distillation apparatus is operated at 400-450 ° C.
- the direct desulfurization equipment is operated at 400-450 ° C.
- the catalytic reformer operates at 420-700 ° C. Therefore, the steel materials used for the heating furnace pipes, reaction towers, tanks, heat exchangers, pipes, etc. of these devices are maintained at an average operating temperature of about 400 to 700 ° C. for a long time during the operation of the devices.
- some of the equipment of chemical plant equipment operates at a temperature of more than 700 ° C.
- 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.
- 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. As a result, the formation of Cr-deficient regions near the grain boundaries is suppressed. As a result, the stabilized austenitic stainless steel material can suppress the sensitization of HAZ.
- Knife line attack means the following phenomenon.
- 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.
- the temperature of the portion near the weld metal described above rises to about 1200 ° C.
- the Nb carbide and Ti carbide that fixed C in the steel material are melted.
- the solidification stage (cooling stage) of the weld metal Nb and Ti try to combine with C again.
- the cooling rate of the vicinity portion in the solidification stage is high.
- the temperature of the vicinity portion is lowered to 800 to 500 ° C., which is the temperature range for forming Cr carbides, while Nb and Ti cannot be completely bonded to C.
- Nb and Ti cannot be bonded to C
- Cr is bonded to C
- Cr carbide is formed.
- 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.
- 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%
- P, S, Sn, As, Zn, Pb and Sb in the impurities are P, respectively.
- 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)
- Patent Document 1 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.
- 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 large heat input welding.
- 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 large heat welding.
- the austenitic stainless steel material according to the present disclosure is Chemical composition is C: 0.020% or less in mass%, 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: 0.1-0.8%, Mo: 0.10 to 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: 0-2.00%, Co: 0 to 1.00%, sol.
- the rest consists of Fe and impurities Satisfy equation (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)
- the content (mass%) of the corresponding element in the chemical composition is substituted for each element symbol in the formula (1).
- 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 large heat welding.
- FIG. 1 is a plan view showing an example of a welded joint of 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 the welded joint cut in the weld metal extending direction, which is different from FIG.
- FIG. 5 is a view showing a cross section in a direction perpendicular to the welding metal extending direction in the welded joint of the present embodiment.
- FIG. 6 is a side view of the large heat input welded joint simulated test piece produced in the examples.
- the present inventors have studied an austenitic stainless steel material having excellent sensitization resistance characteristics even after long-term use at an average operating temperature of 400 to 700 ° C. after large heat welding.
- 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 the bond of C in the steel material with Cr, it is effective to contain Nb in the steel material and bond C in the steel material with 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.
- 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%
- Zn 0 to 0.010%
- Pb 0 to 0.010%
- Sb 0 to 0.010%
- 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.
- large heat input welding may be performed on austenitic stainless steel materials at the time of new construction or repair of chemical plant equipment.
- the temperature of the portion of the steel material in the vicinity of the weld metal exceeds 1200 ° C. due to the welding heat during the large heat input welding. Therefore, even if a large amount of Cr carbides are not present in the steel material before the large heat input welding, Cr carbides may be generated in the steel material after the large heat input welding.
- the austenitic stainless steel material may become sensitized when the chemical plant equipment is operated and held at an average operating temperature of 400 to 700 ° C. for a long time.
- 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. did. As a result, the present inventors obtained the following findings.
- 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 carbides formed in the steel materials during the manufacturing process of the steel materials and during the high heat input welding are M 23 C 6 type carbides. Mo and W enter the Cr site (M site) of the Cr carbide of M 23 C 6 type by replacing 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 enter the M site in place of Cr becomes significantly slower. Based on 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 high heat input welding.
- 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.
- 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.
- the Nb content in the residue is 0.050 to 0.267% in mass% and the Cr content in the residue is 0.125% or less in 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.
- the austenitic stainless steel material of the present embodiment completed based on the above knowledge has the following constitution.
- Austenitic stainless steel Chemical composition is C: 0.020% or less in mass%, 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: 0.1-0.8%, Mo: 0.10 to 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: 0-2.00%, Co: 0 to 1.00%, sol.
- 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.
- 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.
- the chemical composition is Mo: 2.50 to 4.50%, and Co: 0.01-1.00%, , And further satisfy the formulas (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%.
- 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.
- the chemical composition contains at least one element or two or more elements belonging to any of the first to fifth groups.
- Group 1 Ti: 0.01-0.50%, Ta: 0.01-0.50%, V: 0.01-1.00%, Zr: 0.01 to 0.10%, and Hf: 0.01 to 0.10%
- Group 2 Cu: 0.01 to 2.00% and Co: 0.01-1.00%
- Group 3 sol.
- Al 0.001 to 0.030%
- Group 5 Ca: 0.0001-0.0200%
- Mg 0.0001 to 0.0200%
- Rare earth element 0.001 to 0.100%.
- the chemical composition of the austenitic stainless steel material of the present embodiment contains the following elements.
- C 0.020% or less Carbon (C) is inevitably contained. That is, the C content is more than 0%. C produces M 23 C 6 type Cr carbides at the grain boundaries. If 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 remarkably lowered. Therefore, the C content is 0.020% or less.
- the preferable upper limit of the C content is 0.018%, more preferably 0.016%, further preferably 0.014%, still more preferably 0.012%.
- the C content is preferably as low as possible. However, excessive reduction of C content increases manufacturing costs. Therefore, in terms of industrial production, the preferable lower limit of the C content is 0.001%, and more preferably 0.002%.
- 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 cracking sensitivity is remarkably increased even if the other element content 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 ( ⁇ phase) is formed in the steel material during long-term use at an average operating temperature of 400 to 700 ° C.
- ⁇ phase sigma phase
- the ⁇ phase reduces the toughness and ductility of the steel material 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%.
- 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, which enhances the hot workability of the steel material. Mn further deoxidizes the welded portion of the steel material 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. ( ⁇ phase) is easily generated.
- ⁇ phase the sigma phase in the steel material when used at an average operating temperature of 400 to 700 ° C.
- the Mn content is 2.00% or less.
- the preferable lower limit of the Mn content is 0.01%, more preferably 0.10%, further 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%.
- 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 sharpening resistance property of the steel material is lowered. P further increases the weld crack sensitivity of the steel material during welding. 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 weld cracking 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 preferable lower limit of the P content is 0.001%, more preferably 0.002%.
- 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 sharpening resistance property of the steel material is lowered. S further increases 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%, even more preferably 0.0100%, even 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%.
- Chromium (Cr) enhances the oxidation resistance and corrosion resistance of the steel material when the steel material is 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. To 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%, further preferably 22.00%, still more preferably 21.00%, still more preferably 20.00. %, More preferably 19.00%.
- Ni 9.00 to 20.00%
- Nickel (Ni) stabilizes austenite and increases the creep strength of steel materials at an average operating temperature of 400-700 ° 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%.
- 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 property of the steel material is 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 2 N) is formed at the grain boundaries.
- 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%.
- Nb 0.1-0.8% Niobium (Nb), together with N, produces CrNb nitride in the 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 property of the steel material is 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.
- 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 the present embodiment.
- the Nb content is 0.1-0.8%.
- 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%.
- Mo 0.10 to 4.50%
- Molybdenum (Mo) suppresses the formation and growth of M 23 C 6 type Cr carbides at grain boundaries during the use of steel materials at an average operating temperature of 400-700 ° 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.
- the strength in the crystal grains becomes excessively high, and the strength difference 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%.
- the Mo content is 2.50% or more, the average operating temperature of 400 to 700 ° C.
- 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%. is there.
- 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%.
- the upper limit of Mo content is preferably less than 2.50%, more preferably 2.45%. It is even more preferably 2.20%, even more preferably 2.00%, even more preferably 1.70%, even more preferably 1.50%, still more preferably 1.30%. It is even more preferably 1.00%, even more preferably 0.90%, even more preferably 0.80%, even more preferably 0.70%, still more preferably 0.60%, and more preferably 0.60%. More preferably, it is 0.50%.
- 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%.
- the preferable upper limit of the Mo content is 4.30%, more preferably 4.20%, and further. It is preferably 4.15%, more preferably 4.05%, and even more preferably 3.95%.
- W 0.01 to 1.00% Tungsten (W), like Mo, suppresses the formation and growth of M 23 C 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.
- the W content is 0.01 to 1.00%.
- the preferable lower limit of the W content is 0.02%, more preferably 0.04%, further preferably 0.06%, still more preferably 0.08%, still more preferably 0.10. %.
- the preferable upper limit of the W content is 0.80%, more preferably 0.60%, further preferably 0.40%, further preferably 0.35%, still more preferably 0.30. %.
- the rest of the chemical composition of the austenitic stainless steel material according to this embodiment consists of Fe and impurities.
- the impurities are those mixed from ore, scrap, or the manufacturing environment as a raw material when the austenitic stainless steel material is industrially manufactured, and adversely affect the austenitic stainless steel material of the present embodiment. Means what is allowed within the range that does not give.
- 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%.
- the As content may be 0%.
- the Zn content may be 0%.
- the Pb content may be 0%.
- 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.
- 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.
- 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.
- the Zn content exceeds 0.010%
- the hot workability and weldability of the steel material are lowered even if the other element content is within the range of this embodiment.
- the Pb 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 this embodiment.
- 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%.
- 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.
- Titanium (Ti) is an optional element and may not 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 carbide is suppressed, and the sharpening resistance property of the steel material is 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 precipitated 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 strength difference between the inside of the crystal grains and the grain boundaries becomes large.
- the Ti content is 0 to 0.50%.
- the preferred 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%.
- Tantalum (Ta) is an optional element and may not 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 carbide is suppressed, and the sharpening resistance property of the steel material is 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 precipitated 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 strength difference between the inside of the crystal grains and the grain boundaries becomes large.
- the Ta content is 0 to 0.50%.
- the preferred 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%.
- V 0 to 1.00%
- Vanadium (V) is an optional element and may not 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 carbide is suppressed, and the sharpening resistance property of the steel material is enhanced. If even a small amount of V is contained, the above effect can be obtained to some extent. However, if the V content exceeds 1.00%, carbides will be excessively precipitated 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 strength difference between the inside of the crystal grains and the grain boundaries becomes large.
- the V content is 0 to 1.00%.
- the preferable lower limit of the V content is more than 0%, more preferably 0.01%, further 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%.
- Zr Zirconium
- Zr Zirconium
- the Zr content may be 0%.
- Zr combines with C to form carbides.
- the formation of Cr carbide is suppressed, and the sharpening resistance property of the steel material is enhanced.
- the above effect can be obtained to some extent.
- the Zr content exceeds 0.10%, carbides will be excessively precipitated 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 strength difference between the inside of the crystal grains and the grain boundaries becomes large.
- 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%.
- Hf 0 to 0.10%
- Hafnium (Hf) is an optional element and may not 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 carbide is suppressed, and the sharpening resistance property of the steel material is 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 precipitated 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 strength difference between the inside of the crystal grains and the grain boundaries becomes large.
- the Hf content is 0 to 0.10%.
- the preferred 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.
- 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.
- Cu 0 to 2.00% Copper (Cu) is an optional element and may not 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 creep strength of the steel material is increased by precipitation strengthening. 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 cracking sensitivity in HAZ after welding increases. 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 preferable upper limit of the Cu content is 1.50%, more preferably 1.00%, still more preferably 0.80%, still more preferably 0.60%.
- Co is an optional element and may not 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, like W, further enhances the polythionic acid SCC resistance of steel materials. 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%.
- 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.
- sol. Al 0 to 0.030%
- Aluminum (Al) is an optional element and may not be contained. That is, the Al content may be 0%. When contained, Al deoxidizes the steel in the steelmaking process. If even a small amount of Al is contained, 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).
- 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 to strengthen the grain boundaries.
- B 0 to 0.0100% Boron (B) is an optional element and may not 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 to 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 lowered.
- the B content is 0 to 0.0100%.
- the preferable 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%.
- 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.
- REM rare earth elements
- Ca 0-0.0200% Calcium (Ca) is an optional element and may not 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 fixes S and suppresses 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 preferable lower limit of the Ca content is more than 0%, more preferably 0.0001%, further preferably 0.0002%, still more preferably 0.0005%.
- the preferred upper limit of the Ca content is 0.0150%, more preferably 0.0100%, even more preferably 0.0080%, even more preferably 0.0050%, still more preferably 0.0040. %.
- Mg 0 to 0.0200%
- Magnesium (Mg) is an optional element and may not 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 grain boundary segregation of S. This reduces 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 preferable 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%, even more preferably 0.0080%, even more preferably 0.0050%, still more preferably 0.0040. %.
- Rare earth element 0 to 0.100%
- Rare earth elements are optional elements and may not be contained. That is, the REM content may be 0%. When contained, REM immobilizes O (oxygen) and S (sulfur) as inclusions to enhance 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 preferable 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%.
- the REM in the present specification contains at least one element or two or more elements of Sc, Y, and a lanthanoid (Atomic number 57 La to 71 Lu), and the REM content is the total content of these elements. Means quantity.
- F1 21.9Mo + 5.9W-5.0.
- F1 is an index of the amount of M 23 C 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. Furthermore, 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.
- F1 When F1 is 0 or more, Mo and W in an amount capable of suppressing the formation of Cr carbide 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, excellent sharpening resistance can be obtained even when the steel material is operated for a long time at the above-mentioned average operating temperature after the large heat input welding is performed on the steel material.
- the preferred lower limit of F1 is 0.1, more preferably 0.2, even more preferably 0.5, even more preferably 1.0, even more preferably 1.5, even more preferably. It is 2.0.
- the upper limit of F1 is not particularly limited, but is 99.45 in consideration of the maximum content of Mo and the maximum content of W in the chemical composition.
- 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).
- the chemical composition of the austenitic stainless steel material of the present embodiment can be determined 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.
- 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.
- ICP-OES Inductively Coupled Plasma Optical Mission Spectrum
- 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. From the above analysis method, the chemical composition of austenitic stainless steel can be determined.
- 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.
- 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.
- 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.
- CrNb nitride accounts for a large proportion of the precipitates in the austenitic stainless steel material. 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%. If, precipitates other than the CrNb nitride (Cr carbides, Cr 2 N, other carbides, nitrides, and carbonitrides, etc.) the amount of means that sufficiently small for the amount of CrNb nitride ..
- the Nb content in the residue is less than 0.050%, it means that CrNb nitride is not sufficiently precipitated in the steel material.
- 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.
- 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%, even more preferably 0.260%, even more preferably 0.250%, still more preferably 0. .240%.
- the preferable upper limit of the Cr content in the residue obtained by the extraction residue method is 0.120%, more preferably 0.110%, further preferably 0.100%, still more preferably 0.090. %, More preferably 0.080%.
- the lower limit of the Cr content is not particularly limited.
- the preferred lower limit of the Cr content is 0.001%, more preferably 0.003%, still more preferably 0.005%.
- the Nb content and Cr content in the residue can be measured by the following method.
- Specimens are collected from austenitic stainless steel.
- the cross section perpendicular to the longitudinal direction of the test piece may be circular or rectangular.
- 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. ..
- 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 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 sampled 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 surface of the collected test piece is polished by about 50 ⁇ m 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 ⁇ m 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 spectrometry. Further, the mass of the electrolyzed base material (austenitic stainless steel material) is determined.
- the mass of the test piece before the main electrolysis and the mass of the test piece after the main electrolysis are measured. Then, the value obtained by subtracting the mass of the test piece after the main electrolysis from the mass of the test piece before the main electrolysis is defined as the amount of the base material subjected to the main electrolysis.
- 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 sensitization resistance characteristics even after being used for a long time with an average operating time of 400 to 700 ° C. after large heat welding.
- Collect a square test piece 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 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.
- 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.
- 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 angular test piece is not particularly limited, but is, for example, a rectangle of 10 mm ⁇ 10 mm.
- 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 from room temperature to 1350 to 1400 ° C. at 70 to 100 ° C./sec in the atmosphere. Hold 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.
- the large heat input welded joint simulation test piece is charged into 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.
- the Strauss test in accordance 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.
- 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 the cracks are observed but the length of the cracks is 100 ⁇ m or less, it is judged that the sharpening resistance is excellent.
- 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.
- scanning is performed in the base direction to the original natural potential.
- the current that flows when the voltage is applied in the base direction (return path) is measured.
- polythionic acid stress corrosion cracking (hereinafter, also referred to as polythionic acid SCC) is likely to occur in the steel material. Therefore, steel materials used in chemical plant equipment may also be required to have excellent polythionic acid SCC resistance.
- the austenitic stainless steel material of the present embodiment further satisfies the following requirements.
- the Mo content is 2.50 to 4.50%, and the Co content is 0.01 to 1.00%.
- the chemical composition of the steel material satisfies the formulas (2) and (3). 2 ⁇ 73W + 5Co ⁇ 60 (2) 0.20 ⁇ Nb + 0.1W ⁇ 0.58 (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%.
- 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 material 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 the 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 strength difference 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 cracking resistance is lowered during large heat input welding.
- the Mo content is 2.50 to 4.50%
- the Co content is 0.01 to 1.00%
- F2 is 2. If it is ⁇ 60, on the premise that the content of other elements is within the range of this embodiment, sufficient polythionic acid SCC resistance can be obtained, and the occurrence of liquefaction cracking 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, even more preferably 53, still more preferably 50.
- F2 is a value obtained by rounding off the first decimal place of the obtained numerical value.
- 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.
- Nb precipitates typified by the Laves phase are excessively generated. In this case, liquefaction cracking in HAZ may occur at the time of large heat input welding, and the liquefaction cracking resistance may decrease.
- the preferable lower limit of F3 is 0.22, more preferably 0.24, and even more preferably 0.26.
- the preferred upper limit of F3 is 0.56, more preferably 0.54, even 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.
- the Nb content in the residue extracted 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 acid is used. Since CrNb nitride accounts for a sufficiently large proportion of the precipitates in the austenitic stainless steel material and the grain boundary area is sufficiently increased, excellent polythionic acid SCC resistance can be obtained.
- 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.
- the steel material after the large 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.
- the Nb content in the residue obtained by the extraction residue method is 0.065 to 0.245% by mass.
- 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.
- the preferable lower limit of the Nb content in the residue extracted by the extraction residue method is 0.070%, more preferably 0.075%, further 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%.
- 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 There is no particular limitation.
- the preferred lower limit of the Cr content is 0.001%, more preferably 0.003%, still more preferably 0.005%.
- 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.
- excellent naphthenic acid corrosion resistance, excellent polythionic acid SCC resistance, and excellent liquefaction cracking resistance mean the following items.
- [Naphthenic acid corrosion resistance] Collect test pieces from austenitic stainless steel.
- 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. ..
- 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 of the plate thickness of the steel plate
- the longitudinal direction of the test piece is the longitudinal direction of the steel plate.
- the test piece is sampled 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.
- 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 determined 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.
- a large heat input welded joint simulation test piece similar to the above-mentioned evaluation test for sensitization resistance is produced.
- the above-mentioned long-term sensitization treatment is carried out on the large heat input welded joint simulated test piece.
- 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.
- the polythionic acid SCC resistance evaluation test is carried out by the following method using the collected plate-shaped test piece.
- the plate-shaped test piece is bent around a punch having an inner radius of 5 mm to form a U-bend shape.
- 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, determine the crack depth on the cut surface. If no cracks are observed, or if cracks are observed but the crack depth is less than 20 ⁇ m, it is judged that the polythionic acid SCC resistance is excellent.
- the shape of the austenitic stainless steel material of the present 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.
- 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 by these devices may be 750 ° C.
- Such equipment is, for example, equipment for chemical plant equipment represented by petroleum refining and petrochemistry. These devices include, for example, heating furnace pipes, tanks, pipes 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.
- the steel material of the present embodiment satisfies the above (I) to (III), that is, in the chemical composition, Mo: 2.50 to 4.50% and Co: 0.01 to 1.00% are contained. Further, the formulas (2) and (3) are satisfied, and the Nb content in the residue obtained by the extraction residue method is 0.065 to 0.245% in mass%, and the residue is contained.
- the Cr content of 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.
- austenitic stainless steel material of the present embodiment can naturally be used for equipment other than chemical plant equipment.
- Equipment other than chemical plant equipment is, for example, thermal power generation boiler equipment (for example, boiler tube, etc.), which is expected to be used at an average operating temperature of about 400 to 700 ° C. like chemical plant equipment.
- FIG. 1 is a plan view showing an example of a welded joint of the present embodiment.
- 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.
- the austenitic stainless steel material 100 is also referred to as a "base material" 100.
- the ends of the pair of base materials 100 are grooved, for example.
- the weld metal 200 is formed by associating the ends of a pair of base materials 100 having grooved ends with each other and then performing single-layer welding or multi-layer welding.
- the welding methods include, for example, TIG welding (Gas Tungsten Arc Welding: GTAW), coated arc welding (Shelded Metal Arc Welding: SMAW), flux-welded wire arc welding (Flux Code Arc Welding: FCAW), and gas metal arc welding (GasArc). Welding (GMAW) and submerged arc welding (Submerged Arc Welding: SAW).
- 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.
- FIG. 3 is a cross-sectional view of the welded joint 1 of FIG. 1 cut in the weld metal extending direction L
- 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. is there.
- the base material 100 may be a steel plate.
- the cross section of the base material 100 perpendicular to the longitudinal direction may be a circular pipe (that is, a steel pipe).
- the base metal 100 may be a steel bar.
- 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 mass%, C: 0.020% or less, Si: 1.50% or less, Mn: 2.00% or less, P: 0.045% or less, S: 0.
- 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.
- 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, based on AWS A5.9, standard names: ER NiCrCoMo-1, ER NiCrMo-3, NiCrCoMo-1, 22Cr-12Co-1Al-9Mo-Ni, NiCrMo-3, 22Cr-8Mo- It is 3.5Nb-Ni or the like.
- FIG. 5 is a view showing a cross section of the welded joint 1 of the present embodiment in a direction perpendicular to the weld metal extending direction L.
- the base metal (austenitic stainless steel material) 100 has a weld heat affected zone (HAZ) 101 and a portion other than the HAZ 101.
- HAZ101 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.
- the portion of the base material 100 other than the HAZ 101 is referred to as a normal portion 102.
- the normal portion 102 is a portion that is substantially unaffected by heat during welding.
- a range of 200 ⁇ m in the HAZ101 from the fusion line 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 ( ⁇ m).
- the average crystal grain size of the portion other than HAZ101 (that is, the normal portion 102) of the cross section of the base metal 100 is defined as the average crystal grain size R2 ( ⁇ m).
- the average crystal grain size R1 and the average crystal grain size R2 satisfy the formula (4).
- 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. Mirror polish the observation surface. 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 ⁇ m ⁇ 100 ⁇ m. In each field of view, the crystal particle size number is obtained by the cutting method in accordance with JIS G 0551 (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 ( ⁇ m) is obtained from the obtained average crystal grain size number.
- the average crystal grain size R2 is measured by the following method.
- a test piece including a cross section in a direction perpendicular to the welding 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 crystal particle size number is obtained by the cutting method in accordance with JIS G 0551 (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 particle size R2 ( ⁇ m) is obtained from the obtained average crystal particle size number.
- the base material 100 is the austenitic stainless steel material of the present embodiment described above, and the average grain size R1 at HAZ101 near the fusion line 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 polythionic acid SCC resistance and further excellent liquefaction cracking resistance even after high heat input welding. ..
- 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 methods 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.
- the method for producing an austenitic stainless steel material of the present embodiment includes the following steps. 1. 1. Process of preparing materials (preparation process) 2. 2. Process of manufacturing intermediate steel by performing hot working on the material (hot working process) 3. 3. If necessary, a step of performing a pickling treatment on the intermediate steel material after the hot working step and then performing a cold working (cold working step). 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).
- steps will be described.
- 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.
- 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.
- Slabs, blooms, and billets may be produced by a continuous casting method using the produced molten steel.
- the billets may be produced by hot working the produced ingots, slabs and blooms.
- the ingot may be hot forged to produce a cylindrical billet, and this billet may be used as a material.
- 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.
- the intermediate steel material may be, for example, a steel pipe, a steel plate, or a steel bar.
- the intermediate steel material is a steel pipe
- the following processing is performed in the hot processing process.
- An intermediate steel material (steel pipe) is manufactured by performing hot extrusion represented by the Eugene Sejurne method on a cylindrical material in which through holes are formed.
- 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.
- a hot punching pipe manufacturing method may be carried out.
- a steel pipe may be manufactured by performing perforation rolling by the Mannesmann method.
- the round billet is drilled and rolled by a drilling machine.
- 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%.
- the steel pipe temperature (finishing temperature) immediately after the hot working is completed is not particularly limited, but is preferably 900 ° C. or higher.
- the hot working process uses, for example, one or more 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.
- 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.
- the hot working process includes, for example, a rough rolling process and a finish rolling process.
- the material is hot-processed to produce billets.
- a bulk rolling mill is used for the rough rolling process. Billets are manufactured by performing slab rolling on the material with a slab rolling mill.
- a continuous rolling mill is installed downstream of the ingot rolling mill, hot rolling is further performed on the billet after the ingot rolling using the continuous rolling mill to produce a smaller billet. You may.
- a continuous rolling mill for example, horizontal stands having a pair of horizontal rolls and vertical stands having a pair of vertical rolls are alternately arranged in a row.
- the material temperature immediately before the rough rolling step is not particularly limited, but is, for example, 1000 to 1300 ° C.
- 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.
- the steel bar temperature (finishing temperature) immediately after the hot working is completed is not particularly limited, but is preferably 900 ° C. or higher.
- the cold working process is carried out as needed. That is, the cold working process does not have to be carried out.
- the intermediate steel material after hot working is pickled and then cold worked.
- the cold working is, for example, cold drawing or cold rolling.
- the intermediate steel material is a steel plate
- the cold working is, for example, cold rolling.
- strain is applied to the intermediate steel material before the CrNb nitride formation treatment step.
- 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%.
- 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.
- other precipitates Cr carbides, Cr 2 N, other carbides, nitrides, and carbonitrides, etc.
- Cr 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 0.125% or less in mass%.
- 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 inside the furnace is an atmospheric atmosphere.
- the atmospheric atmosphere referred to 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.
- T max T x- 100 (Mo + W) + 200C-80Nb ⁇ 2>
- T max T x -50 (Mo + W) + 200C-80Nb ⁇ 3>
- T max T x -20 (Mo + W) + 200C-80Nb
- T x 1300.
- the heat treatment temperature T is less than 1000 ° C.
- the precipitates such as Cr carbides precipitated in the steel material in the hot working process do not sufficiently dissolve.
- 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.
- 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%.
- the heat treatment temperature T is less than 1000 ° C.
- the Nb content in the residue exceeds 0.245% by mass and / or in the residue.
- Cr content exceeds 0.104% by mass.
- 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-solved, but also the precipitation of CrNb nitrides in the CrNb nitride formation process is insufficient. To 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.
- 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%.
- the heat treatment temperature T exceeds T max , the Nb content in the residue is less than 0.065% by mass.
- 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.
- 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.
- 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%.
- 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.
- the preferred T x is 1290, more preferably 1280.
- the heat treatment temperature T (° C.) and the holding time t (minutes) at the heat treatment temperature T satisfy the following conditions.
- f1 to f3 are defined as follows.
- F2 is a parameter of the heat treatment temperature T and the holding time t required to produce 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.
- f2 is referred to as "CrNb nitride formation parameter”.
- Cr and Nb in the chemical composition are elements constituting CrNb nitride.
- Mo is an element that affects the formation of CrNb nitrides and induces the formation of the LAVES phase.
- the CrNb nitride formation parameter is too low.
- the ratio of Nb carbide and Cr carbide in the precipitate is 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.
- 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.
- 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%.
- the Nb content in the residue exceeds 0.245% by mass, and / or Cr content in the residue. The amount exceeds 0.104%.
- the CrNb nitride formation 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). ..
- 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%.
- the formulas (1) to (3) are satisfied, if f2 exceeds f3, the Nb content in the residue is less than 0.065% by mass.
- the CrNb nitride formation parameter is within an appropriate range. In this case, an appropriate amount of CrNb nitride is precipitated. 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.
- 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%.
- 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.
- 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.
- the average cooling rate CR in a temperature range where the steel material temperature is at least 800 to 500 ° C. is cooled at 15 ° C./sec or more. If the average cooling rate CR is lower than 15 ° C. / sec, while cooling the temperature range of 800 ⁇ 500 ° C., CrNb nitrides in the steel material is precipitated in the grain boundary, and further, M 23 C 6 type Cr carbides are also generated at the grain boundaries.
- 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.
- 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%.
- 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.
- the average cooling rate CR is 15 ° C./sec or more, it is possible to suppress excessive formation of Cr carbides 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, the element content in the chemical composition is within the range of the present embodiment, and the austenitic stainless steel material satisfying the formula (1) is used.
- 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.
- 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%.
- the average cooling rate CR is 15 ° C./sec or more
- the austenitic stainless steel material is premised on satisfying the first condition and the second condition.
- 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.
- the austenitic stainless steel material of the present embodiment can be manufactured.
- the above-mentioned manufacturing method is an example of the manufacturing method of the austenitic stainless steel material of the present embodiment. 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%.
- each element in the chemical composition is within the range of the present embodiment and satisfies the formula (1).
- the Nb content in the residue is 0.050 to 0.267% by mass
- 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.
- 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%.
- 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.
- 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.
- test number A3 indicates that Ti was contained in 0.02%, V was contained in 0.04%, and B was contained in 0.0014%.
- 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%
- the Sb content was 0 to 0.010%.
- 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 produced.
- Hot forging was carried out on the ingot to obtain a material having a thickness of 30 mm.
- the temperature of the ingot before hot forging was 1250 ° C.
- the material was hot-rolled 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 materials after hot rolling was 900 ° C. or higher.
- the intermediate steel material after hot rolling was subjected to CrNb nitride formation treatment.
- 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.
- the heat treatment temperature T of test number B8 was less than 1000 ° C.
- the heat treatment temperatures T of test numbers B7, B11 and B12 exceeded T max .
- the CrNb nitride formation parameters f2, f1 and f3 of each test number are as shown in Table 2.
- Table 2 the CrNb nitride formation parameters f2, f1 and f3 of each test number are as shown in Table 2.
- T indicates that f1 ⁇ f2.
- F indicates that f1> f2.
- T indicates that f2 ⁇ f3.
- F indicates that f2> f3.
- 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.
- the average cooling rate CR of test numbers B6 and B15 from 800 to 500 ° C. was 5 ° C./sec.
- 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 angular 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.
- the following thermal history was given to the angular test piece using a high frequency thermal cycle device. Specifically, with reference to FIG. 6, a 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 thermal history to the angular test piece, a large heat input welded joint simulated test piece was produced.
- a Strauss test compliant with ASTM A262-15 PRACTICE E was conducted as follows. From the large heat input welded joint simulated test piece that had been sensitized for a long time, a plate-shaped test piece having a thickness of 2 mm, a width of 10 mm, and a length of 80 mm was collected 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.
- 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 crack was observed, or if the crack was observed but the length of the crack was 100 ⁇ m or less, the Strauss test was judged to be acceptable (“E” (Excellent) in Table 2). On the other hand, when cracks exceeding 100 ⁇ m were observed, the Strauss test was judged to be unacceptable (“B” (Bad) in Table 2).
- 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.
- scanning was performed in the base direction to the original natural potential.
- the current that flowed when the voltage was applied in the noble direction (outward route) was measured.
- the current flowing when the voltage was applied in the base direction (return path) was measured.
- the austenitic stainless steel material has excellent resistance. It was judged to have sensitization characteristics.
- 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 ⁇ m 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 exhibited excellent sensitization resistance even when the sensitization treatment was performed at 550 ° C. for 10,000 hours after the large heat input welding.
- test numbers B1 to B3 the Mo content and / or W content was low. Therefore, in the Strauss test, cracks exceeding 100 ⁇ m 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.
- test number B4 F1 did not satisfy equation (1). Therefore, in the Strauss test, cracks exceeding 100 ⁇ m 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.
- test number B5 the C content was high. Therefore, in the Strauss test, cracks exceeding 100 ⁇ m 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.
- 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 ⁇ m 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.
- 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 ⁇ m 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 performed at 550 ° C. for 10,000 hours after the large heat input welding, the sensitization resistance property was low.
- 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, in the Strauss test, cracks exceeding 100 ⁇ m 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.
- test number B10 Although the chemical composition was appropriate and the formula (1) was satisfied, 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, in the Strauss test, cracks exceeding 100 ⁇ m 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.
- 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 ⁇ m 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.
- 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 ⁇ m 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.
- test number B14 Although the chemical composition was appropriate and the formula (1) was satisfied, 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, in the Strauss test, cracks exceeding 100 ⁇ m were confirmed. Furthermore, in the reactivation rate measurement test, the reactivation rate exceeded 10%. That is, when the sensitization treatment was performed at 550 ° C. for 10,000 hours after the large heat input welding, the sensitization resistance property was low.
- test number B15 although the chemical composition was appropriate and the formula (1) was satisfied, 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, in the Strauss test, cracks exceeding 100 ⁇ m 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.
- 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, in the Strauss test, cracks exceeding 100 ⁇ m 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.
- test number B17 Although the chemical composition was appropriate and the formula (1) was satisfied, 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, in the Strauss test, cracks exceeding 100 ⁇ m were confirmed. Furthermore, in the reactivation rate measurement test, the reactivation rate exceeded 10%. That is, when the sensitization treatment was performed at 550 ° C. for 10,000 hours after the large heat input welding, the sensitization resistance property was low.
- 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.
- the contained arbitrary element or impurity element and its content (mass%) are described.
- test number A3 indicates that Ti was contained in 0.08%, V was contained in 0.16%, and Sn, which is an impurity, was contained in 0.005%.
- 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 is obtained 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%.
- 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 produced.
- Hot forging was carried out on the ingot to obtain a material having a thickness of 30 mm.
- the temperature of the ingot before hot forging was 1150 ° C.
- the material was hot-rolled 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.
- a 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 treatment was as shown in Table 4.
- the heat treatment temperature T was 1000 ° C. or higher and T max or lower.
- the heat treatment temperature T exceeded T max .
- the heat treatment temperature T was less than 1000 ° C.
- the CrNb nitride formation parameters f2, f1 and f3 of each test number are as shown in Table 4.
- 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.
- 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.
- the average cooling rate CR of test number B11 from 800 to 500 ° C. was less than 15 ° C./sec.
- test piece having a thickness of 2 mm, a width of 10 mm, and a length of 30 mm was collected from the austenitic stainless steel material of each test number at the center width position and the center plate thickness position.
- 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 immersion for 720 hours, the test piece was ultrasonically cleaned with acetone for 3 minutes.
- 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 that the naphthenic acid corrosiveness was excellent (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).
- 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 angular 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.
- 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 angular 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.
- the average crystal grain sizes R1 and R2 were measured by the following methods using the 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 identified as the HAZ range Dr (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 G 0551 (2013), the crystal particle size numbers in any of the three fields of view were determined by the cutting method.
- Each field of view was 100 ⁇ m ⁇ 100 ⁇ m.
- 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 particle size R1 ( ⁇ m) was determined from the obtained average crystal particle size number.
- the position 25 mm from the end in the longitudinal direction of the large heat input welded joint simulated test piece 50 was certified as the normal part 70.
- the average crystal grain size R2 was measured by the following method. A sample was taken with the surface of the normal portion 70 of the large heat input welded joint simulated test piece 50 as the observation surface. The observation surface was mirror-polished. Then, in accordance with JIS G 0551 (2013), the crystal particle size numbers in any of the three fields of view were determined by the cutting method. Each field of view was 100 ⁇ m ⁇ 100 ⁇ m. 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 particle size R2 ( ⁇ m) was determined from the obtained average crystal particle size number.
- R1 / R2 was determined using the average crystal grain size R1 in the obtained range Dref60 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.
- T means that R1 / R2 is 4.8 or less and satisfies the equation (4).
- F in the “Equation (4)” column means that R1 / R2 exceeded 4.8 and did not satisfy the equation (4).
- a plate-shaped test piece having 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.
- the polythionic acid SCC resistance evaluation test was carried out by the following method using the collected plate-shaped test pieces.
- the plate-shaped test piece was bent around a punch having an inner radius of 5 mm to form a U-bend shape.
- the average crystal grain size R1 of the range Dref 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.
- 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.
- test number B4 F3 was less than the lower limit of equation (3). Therefore, the polythionic acid SCC resistance was low.
- test number B5 F2 was less than the lower limit of equation (2). Therefore, the polythionic acid SCC resistance was low.
- test number B6 F2 exceeded the upper limit of equation (2). As a result, the liquid resistance and cracking resistance were low.
- test number B7 F3 was less than the lower limit of equation (3). As a result, the polythionic acid SCC resistance was low.
- test number B8 F3 exceeded the upper limit of equation (3). As a result, the liquid resistance and cracking resistance were low.
- 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.
- 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.
- 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.
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Abstract
Description
F1=S+{(P+Sn)/2}+{(As+Zn+Pb+Sb)/5} (1)式
F2=Nb+Ta+Zr+Hf+2Ti+(V/10) (2)式
化学組成が、質量%で
C:0.020%以下、
Si:1.50%以下、
Mn:2.00%以下、
P:0.045%以下、
S:0.0300%以下、
Cr:15.00~25.00%、
Ni:9.00~20.00%、
N:0.05~0.15%、
Nb:0.1~0.8%、
Mo:0.10~4.50%、
W:0.01~1.00%、
Ti:0~0.50%、
Ta:0~0.50%、
V:0~1.00%、
Zr:0~0.10%、
Hf:0~0.10%、
Cu:0~2.00%、
Co:0~1.00%、
sol.Al:0~0.030%、
B:0~0.0100%、
Ca:0~0.0200%、
Mg:0~0.0200%、
希土類元素:0~0.100%、
Sn:0~0.010%、
As:0~0.010%、
Zn:0~0.010%、
Pb:0~0.010%、
Sb:0~0.010%、及び、
残部がFe及び不純物からなり、
式(1)を満たし、
抽出残渣法により得られた残渣中のNb含有量が質量%で0.050~0.267%であり、かつ、前記残渣中のCr含有量が質量%で0.125%以下である。
21.9Mo+5.9W-5.0≧0 (1)
ここで、式(1)中の各元素記号には、前記化学組成中の対応する元素の含有量(質量%)が代入される。
21.9Mo+5.9W-5.0≧0 (1)
ここで、式(1)中の各元素記号には、化学組成中の対応する元素の含有量(質量%)が代入される。
オーステナイト系ステンレス鋼材であって、
化学組成が、質量%で
C:0.020%以下、
Si:1.50%以下、
Mn:2.00%以下、
P:0.045%以下、
S:0.0300%以下、
Cr:15.00~25.00%、
Ni:9.00~20.00%、
N:0.05~0.15%、
Nb:0.1~0.8%、
Mo:0.10~4.50%、
W:0.01~1.00%、
Ti:0~0.50%、
Ta:0~0.50%、
V:0~1.00%、
Zr:0~0.10%、
Hf:0~0.10%、
Cu:0~2.00%、
Co:0~1.00%、
sol.Al:0~0.030%、
B:0~0.0100%、
Ca:0~0.0200%、
Mg:0~0.0200%、
希土類元素:0~0.100%、
Sn:0~0.010%、
As:0~0.010%、
Zn:0~0.010%、
Pb:0~0.010%、
Sb:0~0.010%、及び、
残部がFe及び不純物からなり、
式(1)を満たし、
抽出残渣法により得られた残渣中のNb含有量が質量%で0.050~0.267%であり、かつ、前記残渣中のCr含有量が質量%で0.125%以下である、
オーステナイト系ステンレス鋼材。
21.9Mo+5.9W-5.0≧0 (1)
ここで、式(1)中の各元素記号には、前記化学組成中の対応する元素の含有量(質量%)が代入される。
[1]に記載のオーステナイト系ステンレス鋼材であって、
前記化学組成は、
Mo:2.50~4.50%、及び、
Co:0.01~1.00%、
を含有し、さらに、式(2)及び式(3)を満たし、
前記抽出残渣法により得られた前記残渣中のNb含有量は質量%で0.065~0.245%であり、かつ、前記残渣中のCr含有量が質量%で0.104%以下である、
オーステナイト系ステンレス鋼材。
2≦73W+5Co≦60 (2)
0.20≦Nb+0.1W≦0.58 (3)
[1]又は[2]に記載のオーステナイト系ステンレス鋼材であって、
前記化学組成は、第1群~第5群のいずれかの群に属する少なくとも1元素又は2元素以上を含有する、
オーステナイト系ステンレス鋼材。
第1群:
Ti:0.01~0.50%、
Ta:0.01~0.50%、
V:0.01~1.00%、
Zr:0.01~0.10%、及び、
Hf:0.01~0.10%、
第2群:
Cu:0.01~2.00%、及び、
Co:0.01~1.00%、
第3群:
sol.Al:0.001~0.030%、
第4群:
B:0.0001~0.0100%、
第5群:
Ca:0.0001~0.0200%、
Mg:0.0001~0.0200%、及び、
希土類元素:0.001~0.100%。
溶接継手であって、
[2]又は[3]に記載の一対のオーステナイト系ステンレス鋼材と、
前記一対のオーステナイト系ステンレス鋼材の間に配置された溶接金属とを備え、
前記溶接金属の延在方向と垂直な前記オーステナイト系ステンレス鋼材の断面のうち、溶接熱影響部内であって溶融線から前記溶接金属の幅方向に200μmの範囲における平均結晶粒径を平均結晶粒径R1と定義し、前記溶接熱影響部以外の部分の平均結晶粒径を平均結晶粒径R2と定義したとき、
前記平均結晶粒径R1と前記平均結晶粒径R2とは式(4)を満たす、
溶接継手。
R1/R2≦4.8 (4)
本実施形態のオーステナイト系ステンレス鋼材の化学組成は、次の元素を含有する。
炭素(C)は不可避に含有される。つまり、C含有量は0%超である。Cは、粒界にM23C6型のCr炭化物を生成する。C含有量が0.020%を超えれば、他の元素含有量が本実施形態の範囲内であっても、Cr炭化物が過剰に生成して鋼材の耐鋭敏化特性が顕著に低下する。したがって、C含有量は0.020%以下である。C含有量の好ましい上限は0.018%であり、さらに好ましくは0.016%であり、さらに好ましくは0.014%であり、さらに好ましくは0.012%である。C含有量はなるべく低い方が好ましい。しかしながら、C含有量の過剰な低減は製造コストを高くする。したがって、工業生産上、C含有量の好ましい下限は0.001%であり、さらに好ましくは0.002%である。
シリコン(Si)は不可避に含有される。つまり、Si含有量は0%超である。Siは、製鋼工程において、鋼を脱酸する。Siが少しでも含有されれば、上記効果がある程度得られる。しかしながら、Si含有量が1.50%を超えれば、他の元素含有量が本実施形態の範囲内であっても、溶接割れ感受性が顕著に高まる。さらに、Siはフェライト安定化元素であるため、オーステナイトの安定性が低下する。この場合、400~700℃の平均操業温度での長時間使用時において、鋼材中にシグマ相(σ相)が生成する。σ相は、400~700℃の平均操業温度での使用時における鋼材の靱性及び延性を低下する。したがって、Si含有量は1.50%以下である。Si含有量の好ましい下限は0.01%であり、さらに好ましくは0.05%であり、さらに好ましくは0.10%であり、さらに好ましくは0.15%であり、さらに好ましくは0.20%である。Si含有量の好ましい上限は1.40%であり、さらに好ましくは1.20%であり、さらに好ましくは1.00%であり、さらに好ましくは0.80%であり、さらに好ましくは0.70%であり、さらに好ましくは0.60%であり、さらに好ましくは0.50%である。
マンガン(Mn)は不可避に含有される。つまり、Mn含有量は0%超である。Mnは、鋼材中のSと結合してMnSを形成し、鋼材の熱間加工性を高める。Mnはさらに、溶接時において鋼材の溶接部を脱酸する。Mnが少しでも含有されれば、上記効果がある程度得られる。しかしながら、Mn含有量が2.00%を超えれば、他の元素含有量が本実施形態の範囲内であっても、400~700℃の平均操業温度での使用時において、鋼材中にシグマ相(σ相)が生成しやすくなる。σ相は、400~700℃の平均操業温度での使用時における鋼材の靱性及び延性を低下する。したがって、Mn含有量は2.00%以下である。Mn含有量の好ましい下限は0.01%であり、さらに好ましくは0.10%であり、さらに好ましくは0.50%であり、さらに好ましくは1.00%であり、さらに好ましくは1.20%であり、さらに好ましくは1.30%である。Mn含有量の好ましい上限は1.80%であり、さらに好ましくは1.60%であり、さらに好ましくは1.55%である。
燐(P)は不可避に含有される不純物である。つまり、P含有量は0%超である。Pは、大入熱溶接時において、鋼材の粒界に偏析する。その結果、鋼材の耐鋭敏化特性が低下する。Pはさらに、溶接時において、鋼材の溶接割れ感受性を高める。P含有量が0.045%を超えれば、他の元素含有量が本実施形態の範囲内であっても、鋼材の耐鋭敏化特性が低下し、溶接割れ感受性が高まる。したがって、P含有量は0.045%以下である。P含有量の好ましい上限は0.040%であり、さらに好ましくは0.035%であり、さらに好ましくは0.030%である。P含有量はなるべく低い方が好ましい。しかしながら、P含有量の過剰な低減は、鋼材の製造コストを引き上げる。したがって、通常の工業生産を考慮すれば、P含有量の好ましい下限は0.001%であり、さらに好ましくは0.002%である。
硫黄(S)は不可避に含有される不純物である。つまり、S含有量は0%超である。Sは、高温環境下での鋼材使用中において、粒界に偏析する。その結果、鋼材の耐鋭敏化特性が低下する。Sはさらに、溶接時において、鋼材の溶接割れ感受性を高める。S含有量が0.0300%を超えれば、他の元素含有量が本実施形態の範囲内であっても、鋼材の耐鋭敏化特性が低下し、溶接割れ感受性が高まる。したがって、S含有量は0.0300%以下である。S含有量の好ましい上限は0.0200%であり、さらに好ましくは0.0150%であり、さらに好ましくは0.0100%であり、さらに好ましくは0.0060%であり、さらに好ましくは0.0050%であり、さらに好ましくは0.0040%であり、さらに好ましくは0.0030%である。S含有量はなるべく低い方が好ましい。しかしながら、S含有量の過剰な低減は、鋼材の製造コストを引き上げる。したがって、通常の工業生産を考慮すれば、S含有量の好ましい下限は0.0001%であり、さらに好ましくは0.0002%である。
クロム(Cr)は、400~700℃の平均操業温度での鋼材使用時において、鋼材の耐酸化性及び耐食性を高める。Cr含有量が15.00%未満であれば、他の元素含有量が本実施形態の範囲内であっても、上記効果が十分に得られない。一方、Cr含有量が25.00%を超えれば、他の元素含有量が本実施形態の範囲内であっても、400~700℃の平均操業温度での鋼材中のオーステナイトの安定性が低下する。この場合、鋼材のクリープ強度が低下する。したがって、Cr含有量は15.00~25.00%である。Cr含有量の好ましい下限は15.50%であり、さらに好ましくは16.00%であり、さらに好ましくは16.20%であり、さらに好ましくは16.40%である。Cr含有量の好ましい上限は24.00%であり、さらに好ましくは23.00%であり、さらに好ましくは22.00%であり、さらに好ましくは21.00%であり、さらに好ましくは20.00%であり、さらに好ましくは、19.00%である。
ニッケル(Ni)はオーステナイトを安定化して、400~700℃の平均操業温度での鋼材のクリープ強度を高める。Ni含有量が9.00%未満であれば、他の元素含有量が本実施形態の範囲内であっても、上記効果が十分に得られない。一方、Ni含有量が20.00%を超えれば、上記効果が飽和し、さらに、製造コストが高くなる。したがって、Ni含有量は9.00~20.00%である。Ni含有量の好ましい下限は、9.50%であり、さらに好ましくは9.80%であり、さらに好ましくは10.00%である。Ni含有量の好ましい上限は18.00%であり、さらに好ましくは16.00%であり、さらに好ましくは15.00%であり、さらに好ましくは14.50%であり、さらに好ましくは14.00%であり、さらに好ましくは13.50%である。
窒素(N)はマトリクス(母相)に固溶してオーステナイトを安定化する。Nはさらに、鋼材中にCrNb窒化物を生成する。CrNb窒化物は、結晶粒界の総面積を増大する。そのため、400~700℃の平均操業温度で長時間操業した場合であっても、Cr炭化物の生成を抑えることができる。その結果、鋼材の耐鋭敏化特性が高まる。N含有量が0.05%未満であれば、上記効果が十分に得られない。一方、N含有量が0.15%を超えれば、結晶粒界にCr窒化物(Cr2N)が生成する。この場合、鋼材中の固溶Cr量が低減してしまい、その結果、鋼材の耐鋭敏化特性が低下する。したがって、N含有量は0.05~0.15%である。N含有量の好ましい下限は0.06%であり、さらに好ましくは0.07%である。N含有量の好ましい上限は0.14%であり、さらに好ましくは0.12%であり、さらに好ましくは0.10%であり、さらに好ましくは0.09%である。
ニオブ(Nb)は、Nとともに、オーステナイト結晶粒内にCrNb窒化物を生成し、結晶粒界の総面積を増大する。そのため、400~700℃の平均操業温度で長時間操業した場合であっても、Cr炭化物の生成を抑えることができる。その結果、鋼材の耐鋭敏化特性が高まる。Nbはさらに、Cと結合してMX型のNb炭化物を生成する。Nb炭化物を生成してCを固定することにより、鋼材中の固溶C量が低減する。これにより、400~700℃の平均操業温度での鋼材の使用中において、粒界でのCr炭化物の生成が抑制され、鋼材の耐鋭敏化特性が高まる。Nb炭化物はさらに、析出強化により、400~700℃の平均操業温度での鋼材のクリープ強度を高める。Nb含有量が0.1%未満であれば、他の元素含有量が本実施形態の範囲内であっても、上記効果が十分に得られない。一方、Nb含有量が0.8%を超えれば、他の元素含有量が本実施形態の範囲内であっても、CrNb窒化物及びNb炭化物が過剰に生成する。この場合、結晶粒内の強度が過剰に高くなり、結晶粒内と結晶粒界との強度差が大きくなる。そのため、粒界面で応力集中が発生し、溶接割れや脆化割れが発生しやすくなる。したがって、Nb含有量は0.1~0.8%である。Nb含有量の好ましい下限は0.2%であり、さらに好ましくは0.3%である。Nb含有量の好ましい上限は0.7%であり、さらに好ましくは0.6%であり、さらに好ましくは0.5%であり、さらに好ましくは0.4%である。
モリブデン(Mo)は、400~700℃の平均操業温度での鋼材の使用中において、粒界でM23C6型のCr炭化物が生成及び成長するのを抑制する。Moはさらに、固溶強化元素として、400~700℃の平均操業温度での鋼材のクリープ強度を高める。Mo含有量が0.10%未満であれば、他の元素含有量が本実施形態の範囲内であっても、上記効果が十分に得られない。一方、Mo含有量が4.50%を超えれば、他の元素含有量が本実施形態の範囲内であっても、結晶粒内において、LAVES相等の金属間化合物の生成を促進する。この場合、結晶粒内の強度が過剰に高くなり、結晶粒内と結晶粒界との強度差が大きくなる。そのため、粒界面で応力集中が発生し、溶接割れや脆化割れが発生しやすくなる。したがって、Mo含有量は0.10~4.50%である。
タングステン(W)は、Moと同様に、400~700℃の平均操業温度での鋼材の使用中において、粒界でのM23C6型のCr炭化物が生成及び成長するのを抑制する。Wはさらに、固溶強化元素として、400~700℃の平均操業温度での鋼材のクリープ強度を高める。W含有量が0.01%未満であれば、他の元素含有量が本実施形態の範囲内であっても、上記効果が十分に得られない。一方、W含有量が1.00%を超えれば、他の元素含有量が本実施形態の範囲内であっても、結晶粒内において、LAVES相等の金属間化合物の生成を促進する。この場合、結晶粒内の強度が過剰に高くなり、結晶粒内と結晶粒界との強度差が大きくなる。そのため、粒界面で応力集中が発生し、溶接割れや脆化割れが発生しやすくなる。したがって、W含有量は0.01~1.00%である。W含有量の好ましい下限は0.02%であり、さらに好ましくは0.04%であり、さらに好ましくは0.06%であり、さらに好ましくは0.08%であり、さらに好ましくは0.10%である。W含有量の好ましい上限は0.80%であり、さらに好ましくは0.60%であり、さらに好ましくは0.40%であり、さらに好ましくは0.35%であり、さらに好ましくは0.30%である。
Sn:0~0.010%
As:0~0.010%
Zn:0~0.010%
Pb:0~0.010%
Sb:0~0.010%
すず(Sn)、ヒ素(As)、亜鉛(Zn)、鉛(Pb)及びアンチモン(Sb)はいずれも、不純物である。Sn含有量は0%であってもよい。同様に、As含有量は0%であってもよい。Zn含有量は0%であってもよい。Pb含有量は0%であってもよい。Sb含有量は0%であってもよい。含有される場合、これらの元素はいずれも、粒界に偏析して粒界の融点を下げたり、粒界の結合力を低下したりする。Sn含有量が0.010%を超える場合、他の元素含有量が本実施形態の範囲内であっても、鋼材の熱間加工性及び溶接性が低下する。同様に、As含有量が0.010%を超える場合、他の元素含有量が本実施形態の範囲内であっても、鋼材の熱間加工性及び溶接性が低下する。Zn含有量が0.010%を超える場合、他の元素含有量が本実施形態の範囲内であっても、鋼材の熱間加工性及び溶接性が低下する。Pb含有量が0.010%を超える場合、他の元素含有量が本実施形態の範囲内であっても、鋼材の熱間加工性及び溶接性が低下する。Sb含有量が0.010%を超える場合、他の元素含有量が本実施形態の範囲内であっても、鋼材の熱間加工性及び溶接性が低下する。したがって、Sn含有量は0~0.010%である。As含有量は0~0.010%である。Zn含有量は0~0.010%である。Pb含有量は0~0.010%である。Sb含有量は0~0.010%である。Sn含有量の下限は0%超であってもよいし、0.001%であってもよい。As含有量の下限は0%超であってもよいし、0.001%であってもよい。Zn含有量の下限は0%超であってもよいし、0.001%であってもよい。Pb含有量の下限は0%超であってもよいし、0.001%であってもよい。Sb含有量の下限は0%超であってもよいし、0.001%であってもよい。
[第1群任意元素]
本実施形態によるオーステナイト系ステンレス鋼材の化学組成はさらに、Feの一部に代えて、Ti、Ta、V、Zr及びHfからなる群から選択される1元素又は2元素以上を含有してもよい。これらの元素はいずれも、Cと結合して炭化物を生成する。そのため、固溶Cを低減して、鋼材の耐鋭敏化特性が高まる。
チタン(Ti)は任意元素であり、含有されなくてもよい。つまり、Ti含有量は0%であってもよい。含有される場合、Tiは、鋼材中のCと結合して炭化物を生成する。これにより、Cr炭化物の生成が抑制され、鋼材の耐鋭敏化特性が高まる。Tiが少しでも含有されれば、上記効果がある程度得られる。しかしながら、Ti含有量が0.50%を超えれば、他の元素含有量が本実施形態の範囲内であっても、炭化物が結晶粒内に過剰に析出する。この場合、結晶粒内の強度が過剰に高くなり、結晶粒内と結晶粒界との強度差が大きくなる。そのため、粒界面で応力集中が発生し、溶接割れや脆化割れが発生しやすくなる。したがって、Ti含有量は0~0.50%である。Ti含有量の好ましい下限は0%超であり、さらに好ましくは0.01%であり、さらに好ましくは0.02%である。Ti含有量の好ましい上限は0.45%であり、さらに好ましくは0.40%であり、さらに好ましくは0.35%であり、さらに好ましくは0.30%である。
タンタル(Ta)は任意元素であり、含有されなくてもよい。つまり、Ta含有量は0%であってもよい。含有される場合、Taは、Cと結合して炭化物を生成する。これにより、Cr炭化物の生成が抑制され、鋼材の耐鋭敏化特性が高まる。Taが少しでも含有されれば、上記効果がある程度得られる。しかしながら、Ta含有量が0.50%を超えれば、他の元素含有量が本実施形態の範囲内であっても、炭化物が結晶粒内に過剰に析出する。この場合、結晶粒内の強度が過剰に高くなり、結晶粒内と結晶粒界との強度差が大きくなる。そのため、粒界面で応力集中が発生し、溶接割れや脆化割れが発生しやすくなる。したがって、Ta含有量は0~0.50%である。Ta含有量の好ましい下限は0%超であり、さらに好ましくは0.01%であり、さらに好ましくは0.02%である。Ta含有量の好ましい上限は0.45%であり、さらに好ましくは0.40%であり、さらに好ましくは0.35%であり、さらに好ましくは0.30%である。
バナジウム(V)は任意元素であり、含有されなくてもよい。つまり、V含有量は0%であってもよい。含有される場合、Vは、Cと結合して炭化物を生成する。これにより、Cr炭化物の生成が抑制され、鋼材の耐鋭敏化特性が高まる。Vが少しでも含有されれば、上記効果がある程度得られる。しかしながら、V含有量が1.00%を超えれば、他の元素含有量が本実施形態の範囲内であっても、炭化物が結晶粒内に過剰に析出する。この場合、結晶粒内の強度が過剰に高くなり、結晶粒内と結晶粒界との強度差が大きくなる。そのため、粒界面で応力集中が発生し、溶接割れや脆化割れが発生しやすくなる。したがって、V含有量は0~1.00%である。V含有量の好ましい下限は0%超であり、さらに好ましくは0.01%であり、さらに好ましくは0.02%であり、さらに好ましくは、0.04%であり、さらに好ましくは0.06%である。V含有量の好ましい上限は0.80%であり、さらに好ましくは0.70%であり、さらに好ましくは0.50%であり、さらに好ましくは0.40%であり、さらに好ましくは0.35%であり、さらに好ましくは0.30%である。
ジルコニウム(Zr)は任意元素であり、含有されなくてもよい。つまり、Zr含有量は0%であってもよい。含有される場合、Zrは、Cと結合して炭化物を生成する。これにより、Cr炭化物の生成が抑制され、鋼材の耐鋭敏化特性が高まる。Zrが少しでも含有されれば、上記効果がある程度得られる。しかしながら、Zr含有量が0.10%を超えれば、他の元素含有量が本実施形態の範囲内であっても、炭化物が結晶粒内に過剰に析出する。この場合、結晶粒内の強度が過剰に高くなり、結晶粒内と結晶粒界との強度差が大きくなる。そのため、粒界面で応力集中が発生し、溶接割れや脆化割れが発生しやすくなる。したがって、Zr含有量は0~0.10%である。Zr含有量の好ましい下限は0%超であり、さらに好ましくは0.01%であり、さらに好ましくは0.02%である。Zr含有量の好ましい上限は0.09%であり、さらに好ましくは0.08%であり、さらに好ましくは0.07%であり、さらに好ましくは0.06%である。
ハフニウム(Hf)は任意元素であり、含有されなくてもよい。つまり、Hf含有量は0%であってもよい。含有される場合、Hfは、Cと結合して炭化物を生成する。これにより、Cr炭化物の生成が抑制され、鋼材の耐鋭敏化特性が高まる。Hfが少しでも含有されれば、上記効果がある程度得られる。しかしながら、Hf含有量が0.10%を超えれば、他の元素含有量が本実施形態の範囲内であっても、炭化物が結晶粒内に過剰に析出する。この場合、結晶粒内の強度が過剰に高くなり、結晶粒内と結晶粒界との強度差が大きくなる。そのため、粒界面で応力集中が発生し、溶接割れや脆化割れが発生しやすくなる。したがって、Hf含有量は0~0.10%である。Hf含有量の好ましい下限は0%超であり、さらに好ましくは0.01%であり、さらに好ましくは0.02%である。Hf含有量の好ましい上限は0.09%であり、さらに好ましくは0.08%であり、さらに好ましくは0.07%であり、さらに好ましくは0.06である。
本実施形態によるオーステナイト系ステンレス鋼材の化学組成はさらに、Feの一部に代えて、Cu及びCoからなる群から選択される1元素以上を含有してもよい。これらの元素はいずれも、400~700℃の平均操業温度での鋼材のクリープ強度を高める。
銅(Cu)は任意元素であり、含有されなくてもよい。つまり、Cuは0%であってもよい。含有される場合、Cuは400~700℃の平均操業温度での鋼材の使用中において、粒内にCu相として析出して、析出強化により鋼材のクリープ強度を高める。Cuが少しでも含有されれば、上記効果がある程度得られる。しかしながら、Cu含有量が2.00%を超えれば、他の元素含有量が本実施形態の範囲内であっても、Cu相が過剰に析出する。この場合、溶接後のHAZでの脆化割れ感受性が高まる。したがって、Cu含有量は0~2.00%である。Cu含有量の好ましい下限は0%超であり、さらに好ましくは0.01%であり、さらに好ましくは0.03%であり、さらに好ましくは0.05%であり、さらに好ましくは0.10%である。Cu含有量の好ましい上限は1.50%であり、さらに好ましくは1.00%であり、さらに好ましくは0.80%であり、さらに好ましくは0.60%である。
コバルト(Co)は任意元素であり、含有されなくてもよい。つまり、Co含有量は0%であってもよい。含有される場合、Coはオーステナイトを安定化して、400~700℃の平均操業温度での鋼材のクリープ強度を高める。Coはさらに、Wと同様に、鋼材の耐ポリチオン酸SCC性を高める。Coが少しでも含有されれば、上記効果がある程度得られる。しかしながら、Co含有量が1.00%を超えれば、他の元素含有量が本実施形態の範囲内であっても、原料コストが高まる。したがって、Co含有量は0~1.00%である。Co含有量の好ましい下限は0%超であり、さらに好ましくは0.01%であり、さらに好ましくは0.05%であり、さらに好ましくは0.10%であり、さらに好ましくは0.20%である。Co含有量の好ましい上限は0.90%であり、さらに好ましくは0.80%であり、さらに好ましくは0.70%であり、さらに好ましくは0.60%である。
本実施形態によるオーステナイト系ステンレス鋼材の化学組成はさらに、Feの一部に代えて、Alを含有してもよい。Alは製鋼工程において、鋼を脱酸する。
アルミニウム(Al)は任意元素であり、含有されなくてもよい。つまり、Al含有量は0%であってもよい。含有される場合、Alは製鋼工程において、鋼を脱酸する。Alが少しでも含有されれば、上記効果がある程度得られる。しかしながら、sol.Al含有量が0.030%を超えれば、他の元素含有量が本実施形態の範囲内であっても、鋼材の加工性及び延性が低下する。したがって、sol.Al含有量は0~0.030%である。sol.Al含有量の好ましい下限は0%超であり、さらに好ましくは0.001%であり、さらに好ましくは0.005%であり、さらに好ましくは0.010%である。sol.Al含有量の好ましい上限は0.029%であり、さらに好ましくは0.028%であり、さらに好ましくは0.025%である。本実施形態においてsol.Al含有量は、酸可溶Al(sol.Al)の含有量を意味する。
本実施形態によるオーステナイト系ステンレス鋼材の化学組成はさらに、Feの一部に代えて、Bを含有してもよい。Bは、粒界に偏析して粒界を強化する。
ボロン(B)は、任意元素であり、含有されなくてもよい。つまり、B含有量は0%であってもよい。含有される場合、Bは、400~700℃の平均操業温度での鋼材の使用中において、粒界に偏析し、粒界強度を高める。Bが少しでも含有されれば、上記効果がある程度得られる。しかしながら、B含有量が0.0100%を超えれば、他の元素含有量が本実施形態の範囲内であっても、粒界でのCr炭化物の生成を促進する。そのため、鋼材の耐鋭敏化特性が低下する。B含有量が0.0100%を超えればさらに、粒界の融点が低下して、溶接時において、HAZの粒界で液化割れが生じる。したがって、B含有量は0~0.0100%である。B含有量の好ましい下限は0%超であり、さらに好ましくは0.0001%であり、さらに好ましくは0.0005%であり、さらに好ましくは0.0010%である。B含有量の好ましい上限は0.0050%であり、さらに好ましくは0.0040%であり、さらに好ましくは0.0030%であり、さらに好ましくは0.0020%である。
本実施形態によるオーステナイト系ステンレス鋼材の化学組成はさらに、Feの一部に代えて、Ca、Mg及び希土類元素(REM)からなる群から選択される1元素又は2元素以上を含有してもよい。これらの元素はいずれも、鋼材の熱間加工性を高める。
カルシウム(Ca)は任意元素であり、含有されなくてもよい。つまり、Ca含有量は0%であってもよい。含有される場合、Caは、O(酸素)及びS(硫黄)を介在物として固定し、鋼材の熱間加工性を高める。Caはさらに、Sを固定して、Sの粒界偏析を抑制する。これにより、溶接時のHAZの脆化割れが低減する。Caが少しでも含有されれば、上記効果がある程度得られる。しかしながら、Ca含有量が0.0200%を超えれば、鋼材の清浄性が低下し、鋼材の熱間加工性がかえって低下する。したがって、Ca含有量は0~0.0200%である。Ca含有量の好ましい下限は0%超であり、さらに好ましくは0.0001%であり、さらに好ましくは0.0002%であり、さらに好ましくは0.0005%である。Ca含有量の好ましい上限は0.0150%であり、さらに好ましくは0.0100%であり、さらに好ましくは0.0080%であり、さらに好ましくは0.0050%であり、さらに好ましくは0.0040%である。
マグネシウム(Mg)は任意元素であり、含有されなくてもよい。つまり、Mg含有量は0%であってもよい。含有される場合、Mgは、O(酸素)及びS(硫黄)を介在物として固定し、鋼材の熱間加工性を高める。Mgはさらに、Sを固定して、Sの粒界偏析を抑制する。これにより、溶接時のHAZの脆化割れを低減する。Mgが少しでも含有されれば、上記効果がある程度得られる。しかしながら、Mg含有量が0.0200%を超えれば、鋼材の清浄性が低下し、鋼材の熱間加工性がかえって低下する。したがって、Mg含有量は0~0.0200%である。Mg含有量の好ましい下限は0%超であり、さらに好ましくは0.0001%であり、さらに好ましくは0.0002%であり、さらに好ましくは0.0005%である。Mg含有量の好ましい上限は0.0150%であり、さらに好ましくは0.0100%であり、さらに好ましくは0.0080%であり、さらに好ましくは0.0050%であり、さらに好ましくは0.0040%である。
希土類元素(REM)は任意元素であり、含有されなくてもよい。つまり、REM含有量は0%であってもよい。含有される場合、REMは、O(酸素)及びS(硫黄)を介在物として固定し、母材の熱間加工性及びクリープ延性を高める。しかしながら、REM含有量が高すぎれば、母材の熱間加工性及びクリープ延性が低下する。したがって、REM含有量は0~0.100%である。REM含有量の好ましい下限は0%超であり、さらに好ましくは0.001%であり、さらに好ましくは0.002%である。REM含有量の好ましい上限は0.080%であり、さらに好ましくは0.060%である。
本実施形態のオーステナイト系ステンレス鋼材の化学組成はさらに、式(1)を満たす。
21.9Mo+5.9W-5.0≧0 (1)
ここで、式(1)中の各元素記号には、化学組成中の対応する元素の含有量(質量%)が代入される。
本実施形態のオーステナイト系ステンレス鋼材の化学組成は、周知の成分分析法により求めることができる。具体的には、オーステナイト系ステンレス鋼材が鋼管である場合、ドリルを用いて、肉厚中央位置にて穿孔加工して切粉を生成し、その切粉を採取する。オーステナイト系ステンレス鋼材が鋼板である場合、ドリルを用いて、板幅中央位置かつ板厚中央位置にて穿孔加工して切粉を生成し、その切粉を採取する。オーステナイト系ステンレス鋼材が棒鋼である場合、ドリルを用いてR/2位置にて穿孔加工して切粉を生成し、その切粉を採取する。ここで、R/2位置とは、棒鋼の長手方向に垂直な断面における、半径Rの中央位置を意味する。
本実施形態のオーステナイト系ステンレス鋼材では、さらに、抽出残渣法により得られた残渣中のNb含有量が質量%で0.050~0.267%であり、かつ、残渣中のCr含有量が質量%で0.125%以下である。
残渣中のNb含有量及びCr含有量は次の方法で測定できる。オーステナイト系ステンレス鋼材から、試験片を採取する。試験片の長手方向に垂直な断面は、円形であっても矩形であってもよい。オーステナイト系ステンレス鋼材が鋼管である場合、試験片の長手方向に垂直な断面の中心が鋼管の肉厚中央位置となり、試験片の長手方向が鋼管の長手方向となるように、試験片を採取する。オーステナイト系ステンレス鋼材が鋼板である場合、試験片の長手方向に垂直な断面の中心が鋼板の板幅中央位置かつ板厚中央位置となり、試験片の長手方向が鋼板の長手方向となるように、試験片を採取する。オーステナイト系ステンレス鋼材が棒鋼である場合、試験片の長手方向に垂直な断面の中心が棒鋼のR/2位置となり、試験片の長手方向が棒鋼の長手方向となるように、試験片を採取する。
残渣中のNb含有量=残渣中のNb質量/母材質量×100 (i)
残渣中のCr含有量=残渣中のCr質量/母材質量×100 (ii)
ASTM A262-15 PRACTICE Eに準拠したストラウス試験を次のとおり実施する。長時間鋭敏化処理を実施した大入熱溶接継手模擬試験片から、中央部分が板状試験片の長手方向の中央位置にくるように、板状試験片を採取する。板状試験片のサイズは特に限定されない。板状試験片のサイズは例えば、厚さ2mm、幅10mm、長さ80mmである。板状試験片を、16%硫酸を含有する硫酸銅試験液中に浸漬して、15時間沸騰する。その後、板状試験片を硫酸銅試験液から取り出す。取り出した板状試験片に対して、曲げ試験を実施する。曲げ試験では、大気中において、大入熱溶接継手模擬試験片の長手方向中央位置を中心として、板状試験片を180°曲げる。曲げた試験片の曲げ部を切断する。切断面を20倍の光学顕微鏡で観察する。割れが観察された場合、割れの長さを求める。割れが観察されなかった場合、又は、割れが観察されても、割れの長さが100μm以下である場合、耐鋭敏化特性に優れると判断する。
長時間鋭敏化処理を実施した大入熱溶接継手模擬試験片を用いて、ASTM G108-94に準拠した電気化学的再活性化率測定試験(Electrochemical Reactivation test)を実施する。具体的には、長時間鋭敏化処理を実施した大入熱溶接継手模擬試験片の中央部分(大入熱が加えられた部分)から板状試験片を採取する。採取した板状試験片内において、評価面積100mm2の表面部分以外の領域をマスキングする。マスキングされた板状試験片を電極として、温度30℃、容量200cm3の0.5mol硫酸+0.01molチオシアン酸カリウム溶液に浸漬する。次に、板状試験片に対して、分極速度100mV/分の直線分極で、自然電位から300mVまで貴方向に走査する。飽和甘こう電極基準で300mVに到達後、直ちに元の自然電位まで卑方向に走査する。貴方向(往路)への電圧印加時に流れた電流を測定する。そして、卑方向(復路)への電圧印加時に流れた電流を測定する。得られた電流値に基づいて、再活性化率(%)を次のとおり定義する。
再活性化率=(復路の最大アノード電流/往路の最大アノード電流)×100
最近のガソリン価格の低下に伴い、化学プラント設備では、ナフテン酸を含有する低価格の低品位原油の使用比率が高まっている。したがって、化学プラント設備に利用される鋼材では、優れた耐ナフテン酸腐食性が求められる場合がある。また、常圧蒸留装置や減圧蒸留装置の加熱炉管等に利用される鋼材では、蒸留工程で発生する多量のコークの付着を抑制するために、原油中に硫黄が含有される。原油に含有される硫黄により、コークの付着は抑制される。しかしながら、原油に含有される硫黄により、鋼材にポリチオン酸応力腐食割れ(以下、ポリチオン酸SCCともいう)が発生しやすくなる。したがって、化学プラント設備に使用される鋼材では、優れた耐ポリチオン酸SCC性も要求される場合がある。
(I)Mo含有量が2.50~4.50%であり、かつ、Co含有量が0.01~1.00%である。
(II)鋼材の化学組成が、式(2)及び式(3)を満たす。
2≦73W+5Co≦60 (2)
0.20≦Nb+0.1W≦0.58 (3)
(III)抽出残渣法により得られた残渣中のNb含有量が質量%で0.065~0.245%であり、残渣中のCr含有量が質量%で0.104%以下である。
以下、(I)~(III)について説明する。
本実施形態のオーステナイト系ステンレス鋼材の化学組成において、Mo含有量が2.50%以上である場合、上述のとおり、他の元素含有量が本実施形態の範囲内であり、かつ、式(1)を満たすことを前提として、優れた耐ナフテン酸腐食性が得られる。さらに、W及びCoは、耐ポリチオン酸SCC性を高める。したがって、十分な耐ポリチオン酸SCC性及び十分な耐ナフテン酸腐食性を得ることを目的とした場合、オーステナイト系ステンレス鋼材において、Mo含有量は2.50~4.50%であり、かつ、Co含有量は0.01~1.00%である。
鋼材の化学組成はさらに、式(2)及び式(3)を満たす。
2≦73W+5Co≦60 (2)
0.20≦Nb+0.1W≦0.58 (3)
ここで、式(2)及び式(3)中の各元素記号には、化学組成中の対応する元素の含有量(質量%)が代入される。
以下、式(2)及び式(3)について説明する。
F2=73W+5Coと定義する。F2は耐ポリチオン酸SCC性と、大入熱溶接時の耐液化割れ性に関する指標である。F2が2未満であれば、オーステナイト系ステンレス鋼材の化学組成中のW及びCoの総含有量が十分ではない。この場合、鋼材の耐ポリチオン酸SCC性が低下する。一方、F2が60を超えれば、Mo含有量が2.50%以上である場合に、W及びCoがLAVES相等の金属間化合物の生成を促進する。この場合、金属間化合物が過剰に生成する。そのため、結晶粒内の強度が過剰に高くなり、結晶粒内と結晶粒界との強度差が大きくなる。そのため、粒界面で応力集中が発生する。その結果、大入熱溶接時において、耐液化割れ性が低下する。
F3=Nb+0.1Wと定義する。F3は有効Nb量を意味する。Nb及びWはいずれも、Cと結合して炭化物を生成し、鋼材中の固溶C量を低減する。これにより、鋼材中にCr炭化物が生成するのを抑制し、鋼材の耐ポリチオン酸SCC性を高める。しかしながら、鋼材中のN含有量が0.05~0.15%である場合、Nb及びWの総含有量が高すぎれば、Laves相に代表されるNb析出物が過剰に生成してしまう。この場合、大入熱溶接時において、HAZでの液化割れが発生し、耐液化割れ性が低下する場合がある。
本実施形態のオーステナイト系ステンレス鋼材が(I)及び(II)を満たす場合、さらに、抽出残渣法により得られた残渣中のNb含有量が質量%で0.065~0.245%であり、残渣中のCr含有量が質量%で0.104%以下であれば、優れた耐ポリチオン酸SCC性が得られる。
オーステナイト系ステンレス鋼材から試験片を採取する。オーステナイト系ステンレス鋼材が鋼管である場合、試験片の長手方向に垂直な断面の中心が鋼管の肉厚中央位置となり、試験片の長手方向が鋼管の長手方向となるように、試験片を採取する。オーステナイト系ステンレス鋼材が鋼板である場合、試験片の長手方向に垂直な断面の中心が鋼板の板幅中央位置かつ板厚中央位置となり、試験片の長手方向が鋼板の長手方向となるように、試験片を採取する。オーステナイト系ステンレス鋼材が棒鋼である場合、試験片の長手方向に垂直な断面の中心が棒鋼のR/2位置となり、試験片の長手方向が棒鋼の長手方向となるように、試験片を採取する。試験片のサイズは特に限定されない。試験片のサイズは例えば、厚さ2mm、幅10mm、長さ30mmである。採取した試験片を、常圧下において、200℃の100%シクロヘキサンカルボン酸溶液に720時間浸漬する。720時間浸漬後、アセトンを用いて試験片を3分間超音波洗浄する。
上述の耐鋭敏化特性の評価試験と同様の大入熱溶接継手模擬試験片を作製する。大入熱溶接継手模擬試験片に対して、上述の長時間鋭敏化処理を実施する。長時間鋭敏化処理後の大入熱溶接継手模擬試験片から、中央部分が板状試験片の長手方向の中央位置にくるように、板状試験片を採取する。板状試験片のサイズは特に限定されない。板状試験片のサイズは例えば、厚さ2mm、幅10mm、長さ75mmである。採取した板状試験片を用いて、耐ポリチオン酸SCC性評価試験を次の方法で実施する。板状試験片を、内側半径5mmのポンチ周りに曲げてUベンド形とする。Uベンド形の試験片を、硫酸を用いてPH=2に調整した1%K2S4O6溶液中に常温で100時間浸漬する。浸漬後の試験片において、曲げた試験片の曲げ部を、長手方向に垂直な方向に切断し、切断面を20倍の光学顕微鏡で観察する。割れが観察された場合、切断面における割れの深さを求める。割れが観察されなかった場合、又は、割れが観察されるが、割れの深さが20μm未満である場合、耐ポリチオン酸SCC性に優れると判断する。
大入熱溶接継手模擬試験片の長手方向の中央位置で、長手方向に垂直な方向に切断する。切断面を観察面とする。観察面を混酸でエッチングする。エッチングされた観察面の任意の3視野(各視野は250μm×250μm)を、400倍の光学顕微鏡で観察する。観察された3視野において、粒界での部分溶融痕の有無を判断する。
本実施形態のオーステナイト系ステンレス鋼材の形状は特に限定されない。本実施形態のオーステナイト系ステンレス鋼材は、鋼管であってもよいし、鋼板であってもよいし、棒鋼であってもよい。本実施形態のオーステナイト系ステンレス鋼材は、鍛造品であってもよいし、鋳造品であってもよい。
本実施形態のオーステナイト系ステンレス鋼材は、400~700℃の平均操業温度で使用される装置用途に適する。本実施形態のオーステナイト系ステンレス鋼材は特に、大入熱溶接が実施された後、400~700℃の平均操業温度で長期間使用される装置用途に適する。400~700℃は平均の操業温度であり、一時的に操業温度が700℃を超える場合があっても、平均の操業温度が400~700℃であれば、本実施形態のオーステナイト系ステンレス鋼材の使用に適する。これらの装置の最高到達温度は750℃であってもよい。このような装置はたとえば、石油精製や石油化学に代表される化学プラント設備の装置である。これらの装置はたとえば、加熱炉管、槽、配管等を備える。また、本実施形態のオーステナイト系ステンレス鋼材を、平均操業温度が400℃未満の化学プラント設備に利用してもよい。
図1は、本実施形態の溶接継手の一例を示す平面図である。図1を参照して、本実施形態による溶接継手1は、一対のオーステナイト系ステンレス鋼材100と、溶接金属200とを備える。溶接金属200は、一対のオーステナイト系ステンレス鋼材100の間に配置されている。溶接金属200は、一対のオーステナイト系ステンレス鋼材100の間に形成されており、一対のオーステナイト系ステンレス鋼材100とつながっている。以降の説明では、オーステナイト系ステンレス鋼材100を「母材」100ともいう。
一対の母材100の各々は、上述の優れた耐ポリチオン酸SCC性及び優れた耐ナフテン酸腐食性を有する本実施形態のオーステナイト系ステンレス鋼材である。つまり、母材100は、化学組成が、質量%で、C:0.020%以下、Si:1.50%以下、Mn:2.00%以下、P:0.045%以下、S:0.0300%以下、Cr:15.00~25.00%、Ni:9.00~20.00%、N:0.05~0.15%、Nb:0.1~0.8%、Mo:2.50~4.50%、W:0.01~1.00%、Ti:0~0.50%、Ta:0~0.50%、V:0~1.00%、Zr:0~0.10%、Hf:0~0.10%、Cu:0~2.00%、Co:0.01~1.00%、sol.Al:0~0.030%、B:0~0.0100%、Ca:0~0.0200%、Mg:0~0.0200%、希土類元素:0~0.100%、Sn:0~0.010%、As:0~0.010%、Zn:0~0.010%、Pb:0~0.010%、Sb:0~0.010%、及び、残部がFe及び不純物からなり、式(1)~式(3)を満たし、抽出残渣法により得られた残渣中のNb含有量が質量%で0.065~0.245%であり、Cr含有量が質量%で0.104%以下である。
溶接金属200の化学組成は、特に限定されない。溶接金属200は、周知の溶接材料を使用して形成すればよい。周知の溶接材料はたとえば、AWS A5.9に準拠した、規格名:ER NiCrCoMo-1、ER NiCrMo-3、NiCrCoMo-1、22Cr-12Co-1Al-9Mo-Ni、NiCrMo-3、22Cr-8Mo-3.5Nb-Ni等である。
図5は、本実施形態の溶接継手1において、溶接金属延在方向Lに垂直な方向の断面を示す図である。図5を参照して、溶接継手1の溶接金属延在方向Lに垂直な方向の断面では、母材(オーステナイト系ステンレス鋼材)100は、溶接熱影響部(HAZ)101と、HAZ101以外の部分102とを含む。HAZ101は、母材100のうち、溶接金属200の溶融線200Eと隣接した領域であって、溶接時の熱影響を受けている部分である。一方、母材100のうち、HAZ101以外の部分を、通常部102と称する。母材100のうち、通常部102は、溶接時の熱影響を実質的に受けていない部分である。
R1/R2≦4.8 (4)
以下、本実施形態のオーステナイト系ステンレス鋼材の製造方法を説明する。以降に説明するオーステナイト系ステンレス鋼材の製造方法は、本実施形態のオーステナイト系ステンレス鋼材の製造方法のあくまでも一例である。したがって、上述の構成を有するオーステナイト系ステンレス鋼材は、以降に説明する製造方法以外の他の製造方法により製造されてもよい。しかしながら、以降に説明する製造方法は、本実施形態のオーステナイト系ステンレス鋼材の製造方法の好ましい一例である。
1.素材を準備する工程(準備工程)
2.素材に対して熱間加工を実施して中間鋼材を製造する工程(熱間加工工程)
3.必要に応じて、熱間加工工程後の中間鋼材に対して酸洗処理を実施した後冷間加工を実施する工程(冷間加工工程)
4.熱間加工工程後又は冷間加工工程後の中間鋼材に対して、CrNb窒化物を析出させる工程(CrNb窒化物生成処理工程)
以下、各工程について説明する。
準備工程では、上述の化学組成を有する素材を準備する。素材は第三者から供給されてもよいし、製造してもよい。素材はインゴットであってもよいし、スラブ、ブルーム、ビレットであってもよい。素材を製造する場合、次の方法により、素材を製造する。上述の化学組成を有する溶鋼を製造する。製造された溶鋼を用いて、造塊法によりインゴットを製造する。製造された溶鋼を用いて、連続鋳造法によりスラブ、ブルーム、ビレットを製造してもよい。製造されたインゴット、スラブ、ブルームに対して熱間加工を実施して、ビレットを製造してもよい。たとえば、インゴットに対して熱間鍛造を実施して、円柱状のビレットを製造し、このビレットを素材としてもよい。この場合、熱間鍛造開始直前の素材の温度は特に限定されないが、たとえば、1000~1300℃である。熱間鍛造後の素材の冷却方法は特に限定されない。
熱間加工工程では、準備工程において準備された素材に対して熱間加工を実施して、中間鋼材を製造する。中間鋼材はたとえば鋼管であってもよいし、鋼板であってもよいし、棒鋼であってもよい。
冷間加工工程は必要に応じて実施する。つまり、冷間加工工程は実施しなくてもよい。実施する場合、熱間加工後の中間鋼材に対して、酸洗処理を実施した後、冷間加工を実施する。中間鋼材が鋼管又は棒鋼である場合、冷間加工はたとえば、冷間抽伸又は冷間圧延である。中間鋼材が鋼板である場合、冷間加工はたとえば、冷間圧延である。冷間加工工程を実施することにより、CrNb窒化物生成処理工程前に、中間鋼材に歪を付与する。これにより、CrNb窒化物生成処理工程時において再結晶の発現及び整粒化を行うことができる。冷間加工工程における減面率は特に限定されないが、たとえば、10~90%である。
CrNb窒化物生成処理工程では、熱間加工工程後又は冷間加工工程後の中間鋼材に対して、CrNb窒化物生成処理を実施する。これにより、他の析出物(Cr炭化物、Cr2N、その他の炭化物、窒化物、及び、炭窒化物等)の生成を抑えつつ、CrNb窒化物を適量析出させる。その結果、製造されたオーステナイト系ステンレス鋼材から抽出残渣法により得られた残渣中のNb含有量を質量%で0.050~0.267%とすることができ、かつ、残渣中のCr含有量を質量%で0.125%以下とすることができる。
CrNb窒化物生成処理では、次の3つの条件(第1の条件、第2の条件、第3の条件)を満たす。
CrNb窒化物生成処理では、大気雰囲気の炉内において、熱処理温度T(℃)を次の温度範囲に保持する。
1000≦T≦Tmax
ここで、Tmax(℃)は、Mo含有量に応じて、次のとおりである。
<1>Mo含有量が0.10~1.00%である場合
Tmax=Tx-100(Mo+W)+200C-80Nb
<2>Mo含有量が1.00%超~2.50%未満である場合
Tmax=Tx-50(Mo+W)+200C-80Nb
<3>Mo含有量が2.50~4.50%である場合
Tmax=Tx-20(Mo+W)+200C-80Nb
ここで、Tx=1300である。
CrNb窒化物生成処理はさらに、熱処理温度T(℃)、及び、熱処理温度Tでの保持時間t(分)とが、次の条件を満たす。
(A)化学組成中のMo含有量が0.10~1.00%である場合
f1≦f2、かつ、f2≦f3
ここで、f1~f3は次のとおり定義される。
f1=760
f2=T×Log10(20Nb+0.1Cr+10Mo+t/60)
f3=1680
(B)化学組成中のMo含有量が1.00%超~2.50%未満である場合
f1≦f2、かつ、f2≦f3
ここで、f1~f3は次のとおり定義される。
f1=1200
f2=T×Log10(20Nb+0.1Cr+10Mo+t/60)
f3=1900
(C)化学組成中のMo含有量が2.50~4.50%である場合
f1≦f2、かつ、f2≦f3
ここで、f1~f3は次のとおり定義される。
f1=1520
f2=T×Log10(20Nb+0.1Cr+10Mo+t/60)
f3=2050
f2中のTには、熱処理温度T(℃)が代入され、tには保持時間t(分)が代入される。f2中の各元素記号には、対応する元素の含有量(質量%)が代入される。
CrNb窒化物生成処理はさらに、熱処理温度T℃で保持時間t分保持した後、冷却する。このとき、少なくとも鋼材温度が800~500℃の温度域での平均冷却速度CRを15℃/秒以上で冷却する。平均冷却速度CRが15℃/秒未満である場合、800~500℃の温度範囲を冷却している間に、鋼材中にCrNb窒化物が粒界にも析出し、さらに、M23C6型のCr炭化物も粒界に生成してしまう。そのため、化学組成中の元素含有量が本実施形態の範囲内であり、かつ、式(1)を満たすオーステナイト系ステンレス鋼材中において、残渣中のNb含有量が0.267質量%を超える。及び/又は、Cr含有量が0.125%を超える。この場合、オーステナイト系ステンレス鋼材の耐鋭敏化特性が低下する。
表1の化学組成を有する素材(インゴット)を製造した。
以上の製造工程により製造されたオーステナイト系ステンレス鋼材に対して、次の評価試験を実施した。
製造されたオーステナイト系ステンレス鋼材を用いて、次の方法により、大入熱溶接を模擬した大入熱溶接継手模擬試験片を作製した。
大入熱溶接継手模擬試験片を用いて、次に示す長時間鋭敏化処理を実施した。大入熱溶接継手模擬試験片を熱処理炉に装入した。熱処理炉において、大入熱溶接継手模擬試験片を大気中、大気圧にて、550℃で10000時間保持した(鋭敏化処理)。10000時間経過後の大入熱溶接継手模擬試験片を熱処理炉から抽出して、放冷した。
ASTM A262-15 PRACTICE Eに準拠したストラウス試験を次のとおり実施した。長時間鋭敏化処理を実施した大入熱溶接継手模擬試験片から、中央部分60が長手方向の中央位置にくるように厚さ2mm、幅10mm、長さ80mmの板状試験片を採取した。板状試験片を、16%硫酸を含有する硫酸銅試験液中に浸漬して、15時間沸騰した。その後、板状試験片を硫酸銅試験液から取り出した。取り出した板状試験片に対して、曲げ試験を実施した。曲げ試験では、大気中において、大入熱溶接継手模擬試験片の長手方向中央位置を中心として、板状試験片を180°曲げた。曲げた試験片の曲げ部を切断した。切断面を20倍の光学顕微鏡で観察した。割れが観察された場合、割れの長さを求めた。割れが観察されなかった場合、又は、割れが観察されても、割れの長さが100μm以下である場合、ストラウス試験を合格と判断した(表2中で「E」(Excellent))。一方、100μmを超える割れが観察された場合、ストラウス試験を不合格と判断した(表2中で「B」(Bad))。
長時間鋭敏化処理を実施した大入熱溶接継手模擬試験片を用いて、ASTM G108-94に準拠した電気化学的再活性化率測定試験(Electrochemical Reactivation test)を実施した。具体的には、長時間鋭敏化処理を実施した大入熱溶接継手模擬試験片の中央部分60(大入熱が加えられた部分)から板状試験片を採取した。採取した板状試験片内において、評価面積100mm2の表面部分以外の領域をマスキングした。マスキングされた板状試験片を電極として、温度30℃、容量200cm3の0.5mol硫酸+0.01molチオシアン酸カリウム溶液に浸漬した。次に、板状試験片に対して、分極速度100mV/分の直線分極で、自然電位から300mVまで貴方向に走査した。飽和甘こう電極基準で300mVに到達後、直ちに元の自然電位まで卑方向に走査した。貴方向(往路)への電圧印加時に流れた電流を測定した。そして、卑方向(復路)への電圧印加時に流れた電流を測定した。得られた電流値に基づいて、再活性化率(%)を次のとおり定義した。
再活性化率=(復路の最大アノード電流/往路の最大アノード電流)×100
表2に試験結果を示す。
表3の化学組成を有する素材(インゴット)を製造した。
各試験番号のオーステナイト系ステンレス鋼材の幅中央位置かつ板厚中央位置から、厚さ2mm、幅10mm、長さ30mmの試験片を採取した。試験片の長手方向は、鋼材の長手方向(圧延方向)に平行であった。採取した試験片を、常圧下において、200℃の100%シクロヘキサンカルボン酸溶液に720時間浸漬した。720時間浸漬後、アセトンを用いて試験片を3分間超音波洗浄した。
製造されたオーステナイト系ステンレス鋼材を用いて、次の方法により、大入熱溶接で製造された溶接継手を模擬した大入熱溶接継手模擬試験片を作製した。
大入熱溶接継手模擬試験片50を用いて、平均結晶粒径R1及びR2を次の方法で測定した。大入熱溶接継手模擬試験片50の長さ方向の中央位置の10mm幅部分の領域60は、溶接継手のHAZの範囲Dref(再現HAZ組織)に相当する。そこで、領域60をHAZの範囲Dref(再現HAZ組織)60と認定した。範囲Dref60の表面を観察面とするサンプルを採取した。観察面を鏡面研磨した。その後、JIS G 0551(2013)に準拠して、任意の3視野における結晶粒度番号を切断法により求めた。各視野は100μm×100μmであった。求めた3つの結晶粒度番号の算術平均値を求め、平均結晶粒度番号と定義した。得られた平均結晶粒度番号から平均結晶粒径R1(μm)を求めた。
大入熱溶接継手模擬試験片を用いて、次に示す長時間鋭敏化処理試験を実施した。大入熱溶接継手模擬試験片を熱処理炉に装入した。熱処理炉において、大入熱溶接継手模擬試験片を大気中、大気圧にて、550℃で10000時間保持(鋭敏化処理)した。10000時間経過後の大入熱溶接継手模擬試験片を熱処理炉から抽出して、放冷した。
大入熱溶接継手模擬試験片50の長手方向の中央位置(つまり、範囲Dref60の範囲内)で、長手方向に垂直な方向に切断した。切断面を観察面とした。観察面を混酸でエッチングした。エッチングされた観察面の任意の3視野(各視野は250μm×250μm)を、400倍の光学顕微鏡で観察した。観察された3視野において、粒界での部分溶融痕の有無を判断した。
100 オーステナイト系ステンレス鋼材(母材)
101 溶接熱影響部(HAZ)
102 通常部
200 溶接金属
200E 溶融線
Claims (4)
- オーステナイト系ステンレス鋼材であって、
化学組成が、質量%で
C:0.020%以下、
Si:1.50%以下、
Mn:2.00%以下、
P:0.045%以下、
S:0.0300%以下、
Cr:15.00~25.00%、
Ni:9.00~20.00%、
N:0.05~0.15%、
Nb:0.1~0.8%、
Mo:0.10~4.50%、
W:0.01~1.00%、
Ti:0~0.50%、
Ta:0~0.50%、
V:0~1.00%、
Zr:0~0.10%、
Hf:0~0.10%、
Cu:0~2.00%、
Co:0~1.00%、
sol.Al:0~0.030%、
B:0~0.0100%、
Ca:0~0.0200%、
Mg:0~0.0200%、
希土類元素:0~0.100%、
Sn:0~0.010%、
As:0~0.010%、
Zn:0~0.010%、
Pb:0~0.010%、
Sb:0~0.010%、及び、
残部がFe及び不純物からなり、
式(1)を満たし、
抽出残渣法により得られた残渣中のNb含有量が質量%で0.050~0.267%であり、かつ、前記残渣中のCr含有量が質量%で0.125%以下である、
オーステナイト系ステンレス鋼材。
21.9Mo+5.9W-5.0≧0 (1)
ここで、式(1)中の各元素記号には、前記化学組成中の対応する元素の含有量(質量%)が代入される。 - 請求項1に記載のオーステナイト系ステンレス鋼材であって、
前記化学組成は、
Mo:2.50~4.50%、及び、
Co:0.01~1.00%、
を含有し、さらに、式(2)及び式(3)を満たし、
前記抽出残渣法により得られた前記残渣中のNb含有量は質量%で0.065~0.245%であり、かつ、前記残渣中のCr含有量が質量%で、0.104%以下である、
オーステナイト系ステンレス鋼材。
2≦73W+5Co≦60 (2)
0.20≦Nb+0.1W≦0.58 (3) - 請求項1又は請求項2に記載のオーステナイト系ステンレス鋼材であって、
前記化学組成は、第1群~第5群のいずれかの群に属する少なくとも1元素又は2元素以上を含有する、
オーステナイト系ステンレス鋼材。
第1群:
Ti:0.01~0.50%、
Ta:0.01~0.50%、
V:0.01~1.00%、
Zr:0.01~0.10%、及び、
Hf:0.01~0.10%、
第2群:
Cu:0.01~2.00%、及び、
Co:0.01~1.00%、
第3群:
sol.Al:0.001~0.030%、
第4群:
B:0.0001~0.0100%、
第5群:
Ca:0.0001~0.0200%、
Mg:0.0001~0.0200%、及び、
希土類元素:0.001~0.100%。 - 溶接継手であって、
請求項2又は請求項3に記載の一対のオーステナイト系ステンレス鋼材と、
前記一対のオーステナイト系ステンレス鋼材の間に配置された溶接金属とを備え、
前記溶接金属の延在方向と垂直な前記オーステナイト系ステンレス鋼材の断面のうち、溶接熱影響部内であって溶融線から前記溶接金属の幅方向に200μmの範囲における平均結晶粒径を平均結晶粒径R1と定義し、前記溶接熱影響部以外の部分の平均結晶粒径を平均結晶粒径R2と定義したとき、
前記平均結晶粒径R1と前記平均結晶粒径R2とは式(4)を満たす、
溶接継手。
R1/R2≦4.8 (4)
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