Specification
Title of invention: Austenitic stainless steel
Technical field
[0001]
　The present invention relates to austenitic stainless steel.
Background technology
[0002]
　TP316H regulated by American Society of Mechanical Engineers (ASME) SA213 and SA213M contains Mo and has excellent corrosion resistance at high temperature, so it is widely used as a material for heat transfer tubes and heat exchangers in thermal power plants and petrochemical plants. Has been done.
[0003]
　For example, Patent Document 1 proposes an austenitic stainless steel containing Mo and Ce in the same manner as TP316H to improve high-temperature corrosion resistance. Further, Patent Document 2 proposes an austenitic stainless steel containing Nb, Ta and Ti to further enhance high temperature strength.
[0004]
　By the way, as disclosed in Non-Patent Documents 1 and 2, when TP316H containing Mo is used as a thick structural member at high temperature, it is widely known that creep damage due to σ phase precipitation occurs. There is. For example, Non-Patent Document 2 proposes increasing the Ni balance and decreasing the Nv-Nc value in order to suppress σ-phase precipitation.
Prior art documents
Patent literature
[0005]
Patent Document 1: JP-A-57-2869 JP
Patent Document 2: JP-A-61-23749
Non-patent literature
[0006]
Non-Patent Document 1: T.W. C. MCGOUGH et al.: Welding Journal, January (1985), page 29,
Non-Patent Document 2: John F DeLong et al.: Thermal power generation, Vol. 35 No. 11, (1984), p. 1249.
Summary of the invention
Problems to be Solved by the Invention
[0007]
　However, if the stability of the austenite phase is increased by the measures described in Non-Patent Document 2, cracking easily occurs in the weld heat affected zone. In particular, it became clear that cracks in the weld heat affected zone may not be able to be prevented in the case of weld joint shapes with strong restraints such as when used as a thick welded structure in an actual large plant. .. Therefore, it is required to suppress cracks generated during welding work and realize excellent weldability.
[0008]
　On the other hand, even if excellent weldability is achieved, the creep strength of the welded structure may be poor. Therefore, it is required to realize stable creep strength as a structure in addition to weldability.
[0009]
　It is an object of the present invention to provide an austenitic stainless steel capable of achieving both excellent weldability when performing welding and stable creep strength as a structure.
Means for solving the problem
[0010]
　The present invention has been made to solve the above problems, and has as its gist the following austenitic stainless steels.
[0011]
　(1) Chemical composition in mass %,
　C: 0.04 to 0.12%,
　Si: 0.25 to 0.55%,
　Mn: 0.7 to 2.0%,
　P: 0.035% Below,
　S: 0.0015% or less,
　Cu: 0.02 to 0.80%,
　Co: 0.02 to 0.80%,
　Ni: 10.0 to 14.0%,
　Cr: 15.5 to 17 0.5%,
　Mo: 1.5 to 2.5%,
　N: 0.01 to 0.10%,
　Al: 0.030% or less,
　O: 0.020% or less,
　Sn: 0 to 0.01% , 　Sb:
　0 ~ 0.01%, 　As: 0 ~ 0.01%, Bi: 0 ~ 　0.01%, V: 0 ~ 0.10%, Nb: 0 ~ 　0.10%, Ti: 0 ~ 0.10%, 　W:0 to 0.50%, 　B:0 to 0.005%, 　Ca:0 to 0.010%, 　Mg:0 to 0.010%,

　REM: 0 to 0.10%,
　balance: Fe and impurities, and 　austenitic stainless steel
　satisfying the following formulas (i) and (ii)
.
　　18.0≦Cr+Mo+1.5×Si≦20.0 (i)
　　14.5≦Ni+30×(C+N)+0.5×(Mn+Cu+Co)≦19.5 (ii)
　However, in the above formula The element symbol represents the content (mass %) of each element contained in the steel.
[0012]
　(2)
　The austenitic stainless steel according to (1) above , wherein the chemical composition contains, in mass%, one or more selected from Sn, Sb, As and Bi in total of more than 0% and 0.01% or less. steel.
[0013]
　(3) The chemical composition is% by mass,
　V: 0.01 to 0.10%,
　Nb: 0.01 to 0.10%,
　Ti: 0.01 to 0.10%,
　W: 0.01 ~ 0.50%,
　B: 0.0002 to 0.005%,
　Ca: 0.0005 to 0.010%,
　Mg: 0.0005 to 0.010%, and
　REM: 0.0005 to 0.10. %, 　the austenitic stainless steel according to (1) or (2) above
　, containing one or more selected from the
group consisting of:
Effect of the invention
[0014]
　ADVANTAGE OF THE INVENTION According to this invention, the austenitic stainless steel which can be compatible with the outstanding weldability at the time of welding construction, and the stable creep strength as a structure can be obtained.
Brief description of the drawings
[0015]
FIG. 1 is a schematic cross-sectional view showing the shape of a test material subjected to groove processing in an example.
MODE FOR CARRYING OUT THE INVENTION
[0016]
　The present inventors conducted a detailed investigation in order to achieve both excellent weldability when performing welding and stable creep strength as a structure. As a result, the following findings have been obtained.
[0017]
　As a result of investigating cracks generated in the welded joint using a thick austenitic stainless steel, (a) cracks occurred at a position adjacent to the melting boundary and at a position slightly away from the melting boundary, and (b) the former Melting traces are observed at the grain boundaries, and it is likely to occur in a component system in which the stability of the austenite phase is high. I found it easy.
[0018]
　From this, the former is so-called liquefaction cracking, and the stability of the austenite phase is increased, so that P and S are easily segregated at the grain boundaries during the heat cycle during welding, and the melting point near the grain boundaries decreases, It was considered to be a crack that melted and opened due to thermal stress. Further, the latter is so-called ductility-decreasing crack, and S segregated at the grain boundary during the heat cycle during welding lowers the bond strength of the grain boundary, and the thermal stress exceeds the bond strength and is a crack formed by opening. it was thought.
[0019]
　As a result of repeated studies, in a thick austenitic stainless steel having the composition targeted by the present invention, in order to stably prevent cracking of the weld heat affected zone, Cr+Mo+1.5×Si was added to 18. It has been found that it is necessary to set it to 0 or more, set Ni+30×(C+N)+0.5×(Mn+Cu+Co) to 19.5 or less, and limit the S content to 0.0015% or less. In addition, it has been found that it is necessary to contain Cu and Co in a predetermined amount or more in order to sufficiently obtain the effect of reducing the weld crack susceptibility.
[0020]
　By the way, although cracks during welding could be prevented by these measures, Cr+Mo+1.5×Si exceeded 20.0, or Ni+30×(C+N)+0.5×(Mn+Cu+Co) became less than 14.5. In this case, on the contrary, it became clear that the austenite phase became unstable, the σ phase was generated during use at high temperature, and the creep strength was significantly reduced.
[0021]
　Further, S has an effect of increasing the penetration depth at the time of welding, and particularly improving the weldability at the time of initial layer welding, while having an adverse effect on weld cracking. From the viewpoint of weld cracking, it was found that when the S content was controlled to 0.0015% or less, a sufficient penetration depth could not be obtained. In order to solve this, simply increasing the heat input to the welding, but the increase in the heat input increases the susceptibility to hot cracking during welding.
[0022]
　Therefore, in order to obtain this effect sufficiently, it was also found that it is effective to contain at least one selected from Sn, Sb, As and Bi in a predetermined range. It is thought that this is because these elements affect the convection of the molten pool during welding, and by evaporating from the surface of the molten pool and contributing to the formation of current-carrying paths, promote melting in the depth direction. Was given.
[0023]
　The present invention has been made based on the above findings. Hereinafter, each requirement of the present invention will be described in detail.
[0024]
　(A) Chemical composition The
　reasons for limiting each element are as follows. In addition, in the following description, "%" regarding the content means "mass %".
[0025]
　C: 0.04 to 0.12%
　C stabilizes the austenite phase and also combines with Cr to form fine carbides to improve the creep strength during high temperature use. However, when C is excessively contained, a large amount of carbide is precipitated, which causes sensitization of the welded portion. Therefore, the C content is 0.04 to 0.12%. The C content is preferably 0.05% or more, more preferably 0.06% or more. The C content is preferably 0.11% or less, more preferably 0.10% or less.
[0026]
　Si: 0.25 to 0.55%
　Si is an element that has a deoxidizing effect and is necessary for ensuring corrosion resistance and oxidation resistance at high temperatures. However, when Si is contained excessively, the stability of the austenite phase is lowered, and the creep strength is lowered. Therefore, the Si content is set to 0.25 to 0.55%. The Si content is preferably 0.28% or more, more preferably 0.30% or more. Further, the Si content is preferably 0.45% or less, and more preferably 0.40% or less.
[0027]
　Mn: 0.7 to 2.0%
　Mn is an element having a deoxidizing action, like Si. It also stabilizes the austenite phase and contributes to the improvement of creep strength. However, when the Mn content becomes excessive, the creep ductility decreases. Therefore, the Mn content is 0.7 to 2.0%. The Mn content is preferably 0.8% or more, more preferably 0.9% or more. Further, the Mn content is preferably 1.9% or less, more preferably 1.8% or less.
[0028]
　P: 0.035% or less
　P is contained as an impurity and is an element that segregates at the crystal grain boundaries of the heat-affected zone during welding to enhance liquefaction cracking susceptibility. Furthermore, creep ductility is also reduced. Therefore, the upper limit of P content is set to 0.035% or less. The P content is preferably 0.032% or less, more preferably 0.030% or less. The P content is preferably reduced as much as possible, that is, the P content may be 0%, but the extreme reduction causes an increase in steelmaking cost. Therefore, the P content is preferably 0.0005% or more, more preferably 0.0008% or more.
[0029]
　S: 0.0015% or less
　S is contained in the alloy as an impurity like P, and segregates at the grain boundaries of the heat-affected zone during welding to enhance liquefaction cracking susceptibility and ductility reduction cracking. Therefore, the upper limit of S content is set to 0.0015% or less. The S content is preferably 0.0012% or less, and more preferably 0.0010% or less. The S content is preferably reduced as much as possible, that is, the S content may be 0%, but on the other hand, it is an element effective for increasing the penetration depth during welding. Therefore, the S content is preferably 0.0001% or more, and more preferably 0.0002% or more.
[0030]
　Cu: 0.02 to 0.80%
　Cu enhances the stability of the austenite phase and contributes to the improvement of creep strength. Further, as compared with Ni and Mn, the effects on segregation energies of P and S are small, and grain boundary segregation is reduced, and welding crack susceptibility is expected to be reduced. However, when Cu is contained excessively, hot workability is deteriorated. Therefore, the Cu content is 0.02 to 0.80%. The Cu content is preferably 0.03% or more, more preferably 0.04% or more. Further, the Cu content is preferably 0.60% or less, and more preferably 0.40% or less.
[0031]
　Co: 0.02 to 0.80%
　Like Cu, Co is an element that enhances the stability of the austenite phase and contributes to the improvement of creep strength. Further, as compared with Ni and Mn, the effects on segregation energies of P and S are small, and grain boundary segregation is reduced, and welding crack susceptibility is expected to be reduced. However, since Co is an expensive element, excessive inclusion causes an increase in cost. Therefore, the Co content is 0.02 to 0.80%. The Co content is preferably 0.03% or more, more preferably 0.04% or more. Further, the Co content is preferably 0.75% or less, and more preferably 0.70% or less.
[0032]
　Ni: 10.0 to 14.0%
　Ni is an essential element for ensuring the stability of the austenite phase during long-term use. However, Ni is an expensive element, and the inclusion of a large amount causes an increase in cost. Therefore, the Ni content is set to 10.0 to 14.0%. The Ni content is preferably 10.2% or more, more preferably 10.5% or more. Further, the Ni content is preferably 13.8% or less, more preferably 13.5% or less.
[0033]
　Cr: 15.5 to 17.5%
　Cr is an essential element for ensuring the oxidation resistance and corrosion resistance at high temperatures. Also, it contributes to secure the creep strength by forming fine carbide. However, a large content lowers the stability of the austenite phase and, conversely, impairs the creep strength. Therefore, the Cr content is set to 15.5 to 17.5%. The Cr content is preferably 15.8% or more, more preferably 16.0% or more. Further, the Cr content is preferably 17.2% or less, and more preferably 17.0% or less.
[0034]
　Mo: 1.5 to 2.5%
　Mo is an element that forms a solid solution in the matrix and contributes to the improvement of creep strength and tensile strength at high temperatures. In addition, it is also effective in improving corrosion resistance. However, if it is contained excessively, the stability of the austenite phase is lowered and the creep strength is impaired. Furthermore, since Mo is an expensive element, excessive inclusion causes an increase in cost. Therefore, the Mo content is set to 1.5 to 2.5%. The Mo content is preferably 1.7% or more, more preferably 1.8% or more. In addition, the Mo content is preferably 2.4% or less, more preferably 2.2% or less.
[0035]
　N: 0.01 to 0.10%
　N stabilizes the austenite phase, and forms a solid solution or precipitates as a nitride to contribute to the improvement of high temperature strength. However, if it is contained excessively, ductility is lowered. Therefore, the N content is set to 0.01 to 0.10%. The N content is preferably 0.02% or more, more preferably 0.03% or more. The N content is preferably 0.09% or less, more preferably 0.08% or less.
[0036]
　Al: 0.030% or less
　Al is added as a deoxidizing agent. However, if a large amount of Al is contained, the cleanliness of steel deteriorates and the hot workability deteriorates. Therefore, the Al content is 0.030% or less. The Al content is preferably 0.025% or less, more preferably 0.020% or less. It is not necessary to set a lower limit for the Al content, that is, the Al content may be 0%, but an extreme reduction leads to an increase in steelmaking cost. Therefore, the Al content is preferably 0.0005% or more, more preferably 0.001% or more.
[0037]
　O: 0.020% or less
　O (oxygen) is included as an impurity. If the content is excessive, the hot workability is deteriorated and the toughness and ductility are deteriorated. Therefore, the O content is 0.020% or less. The O content is preferably 0.018% or less, more preferably 0.015% or less. It is not necessary to set a lower limit for the O content, that is, the O content may be 0%, but an extreme reduction causes an increase in steelmaking cost. Therefore, the O content is preferably 0.0005% or more, more preferably 0.0008% or more.
[0038]
　As mentioned above, Cr, Mo and Si affect the stability of the austenite phase. Therefore, it is necessary that not only the content of each element falls within the above range, but also the following formula (i) is satisfied. When the median value of the formula (i) exceeds 20.0, the stability of the austenite phase is reduced, and a brittle σ phase is generated during use at high temperature to lower the creep strength. On the other hand, if it is less than 18.0, the stability of the austenite phase is enhanced, but hot cracking during welding is likely to occur. The value on the left side of the formula (i) is preferably 18.2, and more preferably 18.5. On the other hand, the value on the right side of the expression (i) is preferably 19.8, and more preferably 19.5.
　　18.0≦Cr+Mo+1.5×Si≦20.0 (i)
　However, the element symbols in the above formula represent the content (% by mass) of each element contained in the steel.
[0039]
　Further, Ni, C, N, Mn, Cu and Co affect the stability of the austenite phase. Therefore, not only the content of each element falls within the above range, but also the following formula (ii) needs to be satisfied. When the median value of the formula (ii) is less than 14.5, the stability of the austenite phase is not sufficient, and a brittle σ phase is generated during use at high temperature to lower the creep strength. On the other hand, if it exceeds 19.5, the austenite phase becomes excessively stable, and hot cracking during welding tends to occur. The value on the left side of the equation (ii) is preferably 14.8, and more preferably 15.0. On the other hand, the value on the right side of the equation (ii) is preferably 19.2, and more preferably 19.0.
　　14.5≦Ni+30×(C+N)+0.5×(Mn+Cu+Co)≦19.5 (ii)
　However, the element symbol in the above formula is the content (mass %) of each element contained in the steel. Represents.
[0040]
　In the chemical composition of the steel of the present invention, in addition to the above-mentioned elements, one or more selected from Sn, Sb, As and Bi may be contained in the range shown below. The reason will be described.
[0041]
　Sn: 0 to 0.01%
　Sb: 0 to 0.01%
　As: 0 to 0.01%
　Bi: 0 to 0.01%
　Sn, Sb, As and Bi affect the convection of the molten pool during welding. To accelerate the heat transfer in the vertical direction of the molten pool, or to evaporate from the molten pool surface to form a current-carrying path to increase the concentration of the arc, thereby increasing the penetration depth. Therefore, one or more selected from these elements may be contained if necessary. However, an excessive content increases crack susceptibility in the heat-affected zone during welding, so the content of any element is set to 0.01% or less. The content of each element is preferably 0.008% or less, and more preferably 0.006% or less.
[0042]
　In order to obtain the above effect, the content of one or more selected from the above elements is preferably more than 0%, more preferably 0.0005% or more, and 0.0008% or more. Is more preferable, and 0.001% or more is even more preferable. When two or more elements selected from these elements are contained in a complex manner, the total content thereof is preferably 0.01% or less, more preferably 0.008% or less, It is more preferably 0.006% or less.
[0043]
　In the chemical composition of the steel of the present invention, in addition to the above elements, one or more selected from V, Nb, Ti, W, B, Ca, Mg and REM may be contained in the range shown below. Good. The reasons for limiting each element will be described.
[0044]
　V: 0 to 0.10%
　V combines with C and/or N to form fine carbides, nitrides or carbonitrides, and contributes to creep strength. Therefore, V may be contained if necessary. .. However, if it is contained in excess, a large amount of carbonitride precipitates, leading to a decrease in creep ductility. Therefore, the V content is 0.10% or less. The V content is preferably 0.09% or less, more preferably 0.08% or less. When it is desired to obtain the above effects, the V content is preferably 0.01% or more, more preferably 0.02% or more.
[0045]
　Nb: 0 to 0.10%
　Similar to V, Nb is combined with C and/or N and precipitates as fine carbides, nitrides or carbonitrides in the grains, and creep strength and tensile strength at high temperature. Since it is an element that contributes to the improvement of the above, it may be contained if necessary. However, if it is contained in excess, a large amount of carbonitride precipitates, leading to a decrease in creep ductility. Therefore, the Nb content is 0.10% or less. The Nb content is preferably 0.08% or less, more preferably 0.06% or less. In order to obtain the above effect, the Nb content is preferably 0.01% or more, more preferably 0.02% or more.
[0046]
　Ti: 0 to 0.10%
　Similar to V and Nb, Ti combines with C and/or N to form fine carbide, nitride or carbonitride, which contributes to creep strength. May be included. However, if it is contained excessively, a large amount of carbonitrides are deposited, which causes a decrease in creep ductility. Therefore, the Ti content is 0.10% or less. The Ti content is preferably 0.08% or less, more preferably 0.06% or less. In order to obtain the above effects, the Ti content is preferably 0.01% or more, more preferably 0.02% or more.
[0047]
　W: 0 to 0.50%
　W is an element which, like Mo, forms a solid solution in the matrix and contributes to the improvement of creep strength and tensile strength at high temperatures, so W may be contained if necessary. However, if it is contained excessively, the stability of the austenite phase is lowered, and the creep strength is rather lowered. Therefore, the W content is 0.50% or less. The W content is preferably 0.40% or less, more preferably 0.30% or less. In order to obtain the above effect, the W content is preferably 0.01% or more, more preferably 0.02% or more.
[0048]
　B: 0 to 0.005%
　B finely disperses the grain boundary carbides to improve the creep strength and segregate at the grain boundaries to strengthen the grain boundaries and reduce the ductility reduction cracking susceptibility of the weld heat affected zone. Since this also has a certain effect, it may be contained if necessary. However, if it is contained excessively, the susceptibility to liquefaction cracking is increased. Therefore, the B content is 0.005% or less. The B content is preferably 0.004% or less, more preferably 0.003% or less, and further preferably 0.002% or less. When it is desired to obtain the above effects, the B content is preferably 0.0002% or more, more preferably 0.0005% or more.
[0049]
　Ca: 0 to 0.010% Since
　Ca has an effect of improving hot workability during production, it may be contained if necessary. However, if it is contained in excess, it combines with oxygen and significantly deteriorates the cleanability, and rather deteriorates the hot workability. Therefore, the Ca content is 0.010% or less. The Ca content is preferably 0.008% or less, more preferably 0.005% or less. When it is desired to obtain the above effects, the Ca content is preferably 0.0005% or more, more preferably 0.001% or more.
[0050]
　Mg: 0 to 0.010%
　Similar to Ca, Mg has an effect of improving hot workability at the time of production, and may be contained if necessary. However, if it is contained in excess, it combines with oxygen, significantly deteriorating the detergency and conversely degrading the hot workability. Therefore, the Mg content is 0.010% or less. The Mg content is preferably 0.008% or less, more preferably 0.005% or less. In order to obtain the above effect, the Mg content is preferably 0.0005% or more, more preferably 0.001% or more.
[0051]
　REM: 0 to 0.10%
　Similar to Ca and Mg, REM has an effect of improving hot workability during production, and thus may be contained if necessary. However, if it is contained in excess, it combines with oxygen, significantly deteriorating the detergency and conversely degrading the hot workability. Therefore, the REM content is 0.10% or less. The REM content is preferably 0.08% or less, more preferably 0.06% or less. In order to obtain the above effects, the REM content is preferably 0.0005% or more, more preferably 0.001% or more.
[0052]
　Here, REM refers to a total of 17 elements of Sc, Y and lanthanoid, and the content of the REM means the total content of these elements.
[0053]
　In the chemical composition of the steel of the present invention, the balance is Fe and impurities. Here, the "impurity" is a component that is mixed in by raw materials such as ore and scrap when manufacturing steel industrially, and various factors of the manufacturing process, and is allowed within a range that does not adversely affect the present invention. Means something.
[0054]
　(B) Manufacturing Method
　The method for manufacturing the austenitic stainless steel according to the present invention is not particularly limited, but for example, steel having the above chemical composition is subjected to hot forging, hot rolling, heat treatment and It can be manufactured by sequentially performing machining.
[0055]
　Hereinafter, the present invention will be described more specifically by way of examples, but the present invention is not limited to these examples.
Example
[0056]
　A test material having a plate thickness of 15 mm, a width of 50 mm and a length of 100 mm was produced from an ingot obtained by melting and casting steel having the chemical composition shown in Table 1 by hot forging, hot rolling, heat treatment and machining. Using the obtained test material, various performance evaluation tests shown below were performed.
[0057]
[table 1]

[0058]
　
　A groove in the shape shown in FIG. 1 was applied to the end portion in the longitudinal direction of the test material. After that, two test materials having a groove formed therein were butted, and butt welding was performed by TIG welding without using a filler material. Two weld joints were produced for each test material with a heat input of 8 kJ/cm. Of the obtained welded joints, those in which the back bead was formed over the entire length of the welding line were considered to have good weldability and were designated as “passed”. Among them, those having a back bead width of 2 mm or more over the entire length were judged to be “good”, and those having a part less than 2 mm were judged to be “good”. In addition, in some of the two welded joints, if there was a portion where the back bead was not formed, it was determined as "fail".
[0059]
　
　After that, the above-mentioned welded joint, in which only the first layer was welded, was restrained and welded on the commercially available steel plate for four rounds. The commercially available steel plate was a steel plate specified in JIS G 3160 (2008) of SM400B and had a thickness of 30 mm, a width of 200 mm, and a length of 200 mm. In addition, the above-mentioned constraint welding was performed using a covered arc welding rod ENi6625 specified in JIS Z 3224 (2010).
[0060]
　Then, laminated welding was performed by TIG welding in the groove. The above-mentioned lamination welding was performed using a filler material corresponding to SNi6625 specified in JIS Z 3334 (2011). Heat input was 10 to 15 kJ/cm, and two welded joints were produced for each test material. Then, one of the welded joints manufactured from each test material was sampled from five test pieces. The cross-section of the collected test piece was mirror-polished and then corroded, and observed by an optical microscope to examine the presence or absence of cracks in the heat affected zone. Then, in all of the five test pieces, a welded joint having no crack was judged as "pass", and a welded joint in which crack was observed was judged as "fail".
[0061]
　
　Furthermore, from the remaining one welded joint made from the test material that passed the evaluation of weld cracking resistance, the round bar creep rupture so that the weld metal is in the center of the parallel portion. A test piece was sampled, and a creep rupture test was performed under the conditions of 650° C. and 167 MPa at which the target rupture time of the base material was about 1000 hours. Then, the material that broke in the base material and the rupture time was 90% or more of the target rupture time of the base material was defined as “pass”.
[0062]
　The results are summarized in Table 2.
[0063]
[Table 2]

[0064]
　As can be seen from Table 2, Test Nos. using Steels A to F satisfying the requirements of the present invention. In Nos. 1 to 6, the workability and weld cracking resistance required when producing a welded joint were obtained, and the creep strength was excellent. In addition, the test No. 4 and test No. As can be seen by comparing 5 and 6, when S was reduced, the welding workability was improved by incorporating one or more selected from Sn, S, As and Bi.
[0065]
　On the other hand, since the S content of Steel G, which is a comparative example, is out of the regulation, the test No. In No. 7, cracks considered to be ductility-decreasing cracks occurred in the weld heat affected zone. Further, Steel H was below the lower limit of the formula (i) and above the upper limit of the formula (ii), so the test No. In No. 8, the stability of the austenite phase was excessively increased, the segregation of S and P due to the welding heat cycle was promoted, and cracks considered to be liquefaction cracks were generated in the weld heat affected zone.
[0066]
　Steel I was below the lower limit of the formula (ii) and Steel J was above the upper limit of the formula (i). Therefore, the stability of the austenite phase was insufficient. In Nos. 9 and 10, the σ phase was generated in the high temperature creep test, and the required creep strength was not obtained. Further, since Steel K was below the lower limit of the formula (i) and Steel L was above the upper limit of the formula (ii), the test Nos. In Nos. 11 and 12, the stability of the austenite phase was excessively increased, the segregation of S and P due to the welding heat cycle was promoted, and cracks considered to be liquefaction cracks were generated in the weld heat affected zone.
[0067]
　Furthermore, since the steels M, N and O do not contain one or both of Cu and Co, the test No. In Nos. 13 to 15, the grain boundary segregation reducing effect of P and S was not obtained, and cracks considered to be liquefaction cracks occurred in the weld heat affected zone.
[0068]
　As described above, it is understood that the necessary weldability, weld crack resistance, and excellent creep strength can be obtained only when the requirements of the present invention are satisfied.
Industrial availability
[0069]
　ADVANTAGE OF THE INVENTION According to this invention, the austenitic stainless steel which can be compatible with the outstanding weldability at the time of welding construction, and the stable creep strength as a structure can be obtained.
The scope of the claims
[Claim 1]
　The chemical composition is% by mass,
　C: 0.04 to 0.12%,
　Si: 0.25 to 0.55%,
　Mn: 0.7 to 2.0%,
　P: 0.035% or less,
　S : 0.0015% or less,
　Cu: 0.02 to 0.80%,
　Co: 0.02 to 0.80%,
　Ni: 10.0 to 14.0%,
　Cr: 15.5 to 17.5% ,
　Mo:
　1.5 ~ 2.5%, N: 0.01
　~ 0.10%, Al: 0.030% or
　less, O: 0.020% or
　less, Sn: 0 ~
　0.01%, Sb: 0 to 0.01%,
　As: 0 to 0.01%,
　Bi: 0 to 0.01%,
　V: 0 to 0.10%,
　Nb: 0 to 0.10%,
　Ti: 0 to 0.10. %,
　W: 0 to 0.50%,
　B: 0 to 0.005%,
　Ca: 0 to 0.010%,
　Mg: 0 to 0.010%,
　REM: 0 to 0.10%,
　balance: Fe and impurities, and 　austenitic stainless steel
　satisfying the following formulas (i) and (ii)
.
　　18.0≦Cr+Mo+1.5×Si≦20.0 (i)
　　14.5≦Ni+30×(C+N)+0.5×(Mn+Cu+Co)≦19.5 (ii)
　However, in the above formula The element symbol represents the content (mass %) of each element contained in the steel.
[Claim 2]

　The austenitic stainless steel according to claim 1, 　wherein the chemical composition contains, in mass%, one or more selected from Sn, Sb, As, and Bi in total of more than 0% and 0.01% or less .
[Claim 3]
　The chemical composition is% by mass
　: V: 0.01-0.10%,
　Nb: 0.01-0.10%,
　Ti: 0.01-0.10%,
　W: 0.01-0.
　% 50,
　B: 0.0002 ~
　0.005%, Ca: 0.0005 ~ 0.010%, Mg: 0.0005 ~ 0.010%,
　and, REM: 0.0005 ~ 0.10%,
　from
　The austenitic stainless steel according to claim 1 or 2, containing at least one selected .