Specification
Title of invention: Duplex stainless steel and method for producing duplex stainless steel
Technical field
[0001]
　The present invention relates to a duplex stainless steel and a method for producing a duplex stainless steel.
Background technology
[0002]
　It is known that a duplex stainless steel having a two-phase structure of a ferrite phase and an austenite phase has excellent corrosion resistance. Duplex stainless steel is particularly excellent in corrosion resistance to pitting corrosion and/or crevice corrosion (hereinafter referred to as “pitting corrosion resistance”) which is a problem in an aqueous solution containing chloride. Therefore, duplex stainless steel is widely used in a humid environment containing chloride such as seawater. In a chloride-containing wet environment, duplex stainless steels are used, for example, in flow line pipes, umbilical tubes and heat exchangers.
[0003]
　In recent years, the corrosion conditions in the use environment of duplex stainless steel have become more and more severe. Therefore, the duplex stainless steel is required to have further excellent pitting corrosion resistance. Various techniques have been proposed to further improve the pitting corrosion resistance of duplex stainless steel.
[0004]
　International Publication No. 2013/191208 (Patent Document 1) has a mass% of Ni: 3 to 8%, Cr: 20 to 35%, Mo: 0.01 to 4.0%, N: 0.05 to 0. 2 phases containing 0.60% of Re: 2.0% or less, Re: 2.0% or less of Ga, and 2.0% or less of Ge: 2.0% or less. Disclosed is stainless steel. In Patent Document 1, by containing Re, Ga, or Ge in a duplex stainless steel, the critical potential (pitting corrosion potential) at which pitting corrosion occurs is increased, and pitting corrosion resistance and crevice corrosion resistance are increased. There is.
[0005]
　In WO 2010/082395 (Patent Document 2), Cr: 20 to 35%, Ni: 3 to 10%, Mo: 0 to 6%, W: 0 to 6%, Cu: 0 to A duplex stainless steel material containing 3% and N: 0.15 to 0.60% is hot-worked or further subjected to solution heat treatment to prepare a cold-working raw pipe, and then cold-rolled. A method of making a duplex stainless steel tube is disclosed. The method for manufacturing the duplex stainless steel pipe of Patent Document 2 is the workability Rd(=exp[{In(MYS)-In(14.5×Cr+48.3×Mo+20. 7×W+6.9×N)}/0.195]) is cold-rolled within the range of 10 to 80% to produce a duplex stainless steel pipe having a minimum yield strength of 758.3 to 965.2 MPa. It is characterized by being. It is described in Patent Document 2 that a two-phase stainless steel pipe that can be used in, for example, an oil well or a gas well and that has excellent strength while exhibiting excellent corrosion resistance even in a carbon dioxide gas corrosive environment or a stress corrosive environment can be obtained. ing.
[0006]
　Japanese Patent Laid-Open No. 2007-84837 (Patent Document 3) discloses that, in terms of mass%, Cr: 20 to 30%, Ni: 1 to 11%, Cu: 0.05 to 3.0%, Nd: 0.005 to 0. Disclosed is a duplex stainless steel containing 0.5%, N: 0.1 to 0.5%, and one or both of Mo: 0.5 to 6 and W: 1 to 10. In Patent Document 3, the hot workability of the duplex stainless steel is improved by containing Nd.
[0007]
　Japanese Patent Publication No. 2005-520934 (Patent Document 4) discloses that, by weight, Cr: 21.0% to 38.0%, Ni: 3.0% to 12.0%, Mo: 1.5% to 6%. 0.5%, W:0 to 6.5%, N: 0.2% to 0.7%, Ba: 0.0001 to 0.6%, and the pitting corrosion resistance equivalent index PREW is 40≦PREW≦. A super duplex stainless steel satisfying 67 is disclosed. As a result, a super duplex stainless steel having excellent corrosion resistance, embrittlement resistance, castability and hot workability, in which formation of brittle sigma (σ) phase, chi (χ) phase and other intermetallic phases is suppressed, is obtained. It is described in Patent Document 4 that it is obtained.
Prior art documents
Patent literature
[0008]
Patent Document 1: International Publication No. 2013/191208
Patent Document 2: International Publication No. 2010/082395
Patent Document 3: Japanese Patent Laid-Open No. 2007-84837
Patent Document 4: Japanese Patent Publication No. 2005-520934
Summary of the invention
Problems to be Solved by the Invention
[0009]
　As described above, in recent years, duplex stainless steel having further excellent pitting corrosion resistance has been demanded. Therefore, duplex stainless steel exhibiting excellent pitting corrosion resistance may be obtained by means other than the techniques described in Patent Documents 1 to 4.
[0010]
　An object of the present disclosure is to provide a duplex stainless steel having excellent pitting corrosion resistance and a method for producing the duplex stainless steel.
Means for solving the problem
[0011]
　The duplex stainless steel according to the present disclosure has, in mass %, Cr: more than 27.0% to 29.00%, Mo: 2.50 to 3.50%, Ni: 5.00 to 8.00%, W: 4.0 to 6.00%, Cu: 0.01 to less than 0.10%, N: more than 0.400% to 0.600%, C: 0.030% or less, Si: 1.00% or less, Mn: 1.00% or less, sol. Al: 0.040% or less, V: 0.50% or less, O: 0.010% or less, P: 0.030% or less, S: 0.020% or less, Ca: 0 to 0.0040%, Mg : 0 to 0.0040%, B: 0 to 0.0040%, and the balance consisting of Fe and impurities, a chemical composition satisfying the formula (1), and a ferrite phase of 35 to 65% by volume and the balance of an austenite phase. And a microstructure consisting of The duplex stainless steel according to the present disclosure has an area ratio of Cu precipitated in the ferrite phase of 0.5% or less.
　Cr+4.0×Mo+2.0×W+20×N-5×ln(Cu)≧65.2 (1)
　Here, each element symbol in the formula (1) represents the content (mass) of each element. %) is substituted.
[0012]
　The method for producing duplex stainless steel according to the present disclosure includes a preparation step, a hot working step, a cooling step, and a solution heat treatment step. In the preparing step, a material having the above chemical composition is prepared. In the hot working step, the material is hot worked at 850°C or higher. In the cooling step, the material after hot working is cooled at 5°C/sec or more. In the solution heat treatment step, the cooled material is solution heat treated at 1070° C. or higher.
Effect of the invention
[0013]
　Duplex stainless steel according to the present disclosure has excellent pitting corrosion resistance. The method for producing duplex stainless steel according to the present disclosure can produce the above-described duplex stainless steel.
MODE FOR CARRYING OUT THE INVENTION
[0014]
　The present inventors have investigated and investigated a method for improving the pitting corrosion resistance of duplex stainless steel. As a result, the following findings were obtained.
[0015]
　Cr, Mo and Cu are known to be effective in improving the pitting corrosion resistance of duplex stainless steel. Among Cr, Mo and Cu, the mechanism by which Cr and Mo enhance the pitting corrosion resistance of duplex stainless steel is considered as follows. Cr, as an oxide, becomes the main component of the passive film on the surface of the duplex stainless steel. The passivation coating interferes with the contact of corrosion factors with the surface of duplex stainless steel. As a result, the duplex stainless steel having the passivation film formed on the surface has improved pitting corrosion resistance. Mo is contained in the passivation film and further enhances the pitting corrosion resistance of the passivation film.
[0016]
　On the other hand, of Cr, Mo and Cu, the mechanism by which Cu enhances the pitting corrosion resistance of duplex stainless steel is considered as follows. It is believed that there are two steps before pitting occurs. The first step is the occurrence of pitting corrosion (early stage). The next step is the progress of pitting (progress stage). Conventionally, Cu has been considered to have an effect of suppressing the progress of pitting corrosion. Particularly, in an acidic solution, active sites having a high dissolution rate are formed on the surface of the duplex stainless steel. Cu covers the active sites and suppresses the melting of the duplex stainless steel. Therefore, Cu has been considered to suppress the progress of pitting corrosion of duplex stainless steel.
[0017]
　Due to the above mechanism, Cr, Mo and Cu have been considered to be effective elements for improving the pitting corrosion resistance in the duplex stainless steel. Therefore, in the conventional duplex stainless steel, Cr, Mo and Cu have been positively contained for the purpose of enhancing pitting corrosion resistance. However, as a result of the study by the present inventors, the following findings, which have not been known so far, were obtained. Specifically, the present inventors have found that among Cr, Mo and Cu, Cu may rather reduce the pitting corrosion resistance in the occurrence of pitting corrosion (initial stage).
[0018]
　Table 1 is a table showing the chemical compositions of the test pieces of Test Nos. 2 and 5 and the pitting corrosion potential, which is an index of pitting corrosion resistance, in Examples described later. The chemical composition of Table 1 is the chemical composition of the steel types B and E corresponding to the test numbers 2 and 5 extracted from Table 3 described later and described in two stages. The chemical composition of Table 1 is described by mass %, and the balance is Fe and impurities. The pitting potentials in Table 1 are the pitting potentials of the corresponding test numbers extracted from Table 4 described below.
[0019]
[table 1]

[0020]
　With reference to Table 1, the Cu content of the test piece of test number 2 was higher than the Cu content of the test piece of test number 5. Furthermore, the Cr and Mo contents of the test piece of test number 2 were higher than the Cr and Mo contents of the test piece of test number 5. Therefore, based on the conventional knowledge, it can be expected that the test piece of test number 2 having a high content of Cr, Mo and Cu has better pitting corrosion resistance than the test piece of test number 5. However, the pitting potential, which is an index of the pitting corrosion resistance of the test piece of Test No. 2, is 71 mV vs. SCE, the pitting potential of the test piece of Test No. 5 was 346 mV vs. It was lower than SCE.
[0021]
　That is, from the conventional knowledge, the test piece of test number 2 which is expected to have superior pitting corrosion resistance to the test piece of test number 5 has a lower pitting corrosion resistance than the test piece of test number 5. Was there. Therefore, the present inventors paid attention to the microstructures of the test pieces of test numbers 2 and 5 and investigated them in more detail. As a result, it was revealed that the test piece of Test No. 2 had a higher area ratio of Cu precipitated in the ferrite phase (called Cu area ratio in the ferrite phase) than the test piece of Test No. 5.
[0022]
　Therefore, the present inventors have further investigated and studied in detail the effect of Cu precipitated in the ferrite phase on the pitting corrosion resistance of the duplex stainless steel. Table 2 is a table showing the chemical compositions of the test pieces of Test Nos. 3 and 6, the Cu area ratio in the ferrite phase, and the pitting corrosion potential, which is an index of pitting corrosion resistance, in Examples described later. The chemical composition of Table 2 is the chemical composition of steel type C corresponding to test numbers 3 and 6 extracted from Table 3 described later, and is described in two stages. The chemical composition of Table 2 is described by mass %, and the balance is Fe and impurities. The Cu area ratio in the ferrite phase of Table 2 is a description of the Cu area ratio in the ferrite phase of the corresponding test number, which is extracted from Table 4 described later. The pitting potentials in Table 2 are the pitting potentials of the corresponding test numbers extracted from Table 4 described below.
[0023]
[Table 2]

[0024]
　With reference to Table 2, the chemical composition of the test piece of test number 3 and the test piece of test number 6 were the same. On the other hand, in the test piece of test number 6, the Cu area ratio in the ferrite phase was lower than the Cu area ratio in the ferrite phase of the test piece of test number 3. As a result, the pitting potential of the test piece of Test No. 6 was 204 mV vs. SCE, the pitting potential of the test piece of Test No. 3-12 mV vs. It was high compared to SCE. That is, the test piece of test number 6 had better pitting corrosion resistance than the test piece of test number 3 as a result of the reduction of the precipitation of Cu in the ferrite phase.
[0025]
　As described above, it has been conventionally considered that pitting corrosion resistance is enhanced by increasing the contents of Cr, Mo and Cu. However, among the Cr, Mo, and Cu, the present inventors have for the first time found that Cu may rather reduce the pitting corrosion resistance. The present inventors have further found that pitting corrosion resistance can be improved by reducing the precipitation amount of Cu in the ferrite phase, which has not been known at all.
[0026]
　The detailed reason why Cu precipitated in the ferrite phase deteriorates the pitting corrosion resistance of the duplex stainless steel has not been clarified. However, the present inventors consider as follows. Cu precipitated in the ferrite phase may hinder the uniform formation of the passive film. Therefore, when the amount of Cu precipitated in the ferrite phase is large, the effect of suppressing the contact between the corrosion factor and the surface of the duplex stainless steel by the passive film may be reduced. As a result, it is considered that pitting corrosion occurs on the surface of the duplex stainless steel.
[0027]
　The duplex stainless steel according to the present embodiment completed based on the above findings is, in mass %, Cr: more than 27.0% to 29.00%, Mo: 2.50 to 3.50%, Ni: 5.00. ~ 8.00%, W: 4.00 ~ 6.00%, Cu: 0.01 ~ 0.10%, N: over 0.400% ~ 0.600%, C: 0.030% or less, Si: 1.00% or less, Mn: 1.00% or less, sol. Al: 0.040% or less, V: 0.50% or less, O: 0.010% or less, P: 0.030% or less, S: 0.020% or less, Ca: 0 to 0.0040%, Mg : 0 to 0.0040%, B: 0 to 0.0040%, and the balance consisting of Fe and impurities, a chemical composition satisfying the formula (1), and a ferrite phase of 35 to 65% by volume and the balance of an austenite phase. And a microstructure consisting of In the duplex stainless steel according to the present embodiment, the area ratio of Cu precipitated in the ferrite phase is 0.5% or less.
　Cr+4.0×Mo+2.0×W+20×N-5×ln(Cu)≧65.2 (1)
　Here, each element symbol in the formula (1) represents the content (mass) of each element. %) is substituted.
[0028]
　The duplex stainless steel according to the present embodiment has the above-mentioned chemical composition and the above-mentioned microstructure, and further, the area ratio of Cu in the ferrite phase is 0.5% or less. As a result, the duplex stainless steel according to the present embodiment has excellent pitting corrosion resistance.
[0029]
　Preferably, the chemical composition is mass% and is selected from the group consisting of Ca: 0.0001 to 0.0040%, Mg: 0.0001 to 0.0040%, and B: 0.0001 to 0.0040%. 1 type or 2 or more types are included.
[0030]
　In this case, the hot workability of the duplex stainless steel according to this embodiment is enhanced.
[0031]
　The method for producing duplex stainless steel according to the present embodiment includes a preparation step, a hot working step, a cooling step, and a solution heat treatment step. In the preparing step, a material having the above chemical composition is prepared. In the hot working step, the material is hot worked at 850°C or higher. In the cooling step, the material after hot working is cooled at 5°C/sec or more. In the solution heat treatment step, the cooled material is solution heat treated at 1070° C. or higher.
[0032]
　The duplex stainless steel manufactured by the manufacturing method according to the present embodiment has the above-described chemical composition and the above-described microstructure, and further, the area ratio of Cu in the ferrite phase is 0.5% or less. As a result, the duplex stainless steel manufactured by the manufacturing method according to the present embodiment has excellent pitting corrosion resistance.
[0033]
　Hereinafter, the duplex stainless steel according to the present embodiment will be described in detail.
[0034]
　[Chemical composition]
　The chemical composition of the duplex stainless steel according to the present embodiment contains the following elements. In addition, unless otherwise specified,% regarding an element means mass %.
[0035]
　[About essential elements]
　The chemical composition of the duplex stainless steel according to the present embodiment essentially contains the following elements.
[0036]
　Cr: over 27.0% to 29.00%
　Chromium (Cr) forms an passive film on the surface of the duplex stainless steel as an oxide. The passivation coating interferes with the contact of corrosion factors with the surface of duplex stainless steel. As a result, the occurrence of pitting corrosion of the duplex stainless steel is suppressed. Cr is an element necessary for obtaining a ferrite structure of duplex stainless steel. By obtaining a sufficient ferrite structure, stable pitting corrosion resistance can be obtained. If the Cr content is too low, these effects cannot be obtained. On the other hand, if the Cr content is too high, the hot workability of the duplex stainless steel deteriorates. Therefore, the Cr content is more than 27.00% and 29.00%. The preferable lower limit of the Cr content is 27.50%, more preferably 28.00%. The preferable upper limit of the Cr content is 28.50%.
[0037]
　Mo: 2.50 to 3.50%
　Molybdenum (Mo) is contained in the passivation film to further enhance the corrosion resistance of the passivation film. As a result, the pitting corrosion resistance of the duplex stainless steel is enhanced. If the Mo content is too low, this effect cannot be obtained. On the other hand, if the Mo content is too high, the workability in assembling a steel pipe made of duplex stainless steel is deteriorated. Therefore, the Mo content is 2.50 to 3.50%. The preferable lower limit of the Mo content is 2.80%, and more preferably 3.00%. The preferable upper limit of the Mo content is 3.30%.
[0038]
　Ni: 5.00 to 8.00%
　Nickel (Ni) is an austenite stabilizing element and is an element necessary for obtaining a two-phase structure of ferrite/austenite. If the Ni content is too low, this effect cannot be obtained. On the other hand, if the Ni content is too high, the balance between the ferrite phase and the austenite phase cannot be obtained. In this case, duplex stainless steel cannot be stably obtained. Therefore, the Ni content is 5.00 to 8.00%. The preferable lower limit of the Ni content is 5.50%, more preferably 6.00%. The preferable upper limit of the Ni content is 7.50%.
[0039]
　W: 4.00 to 6.00%
　Tungsten (W) is contained in the passivation film similarly to Mo, and further enhances the corrosion resistance of the passivation film. As a result, the occurrence of pitting corrosion of the duplex stainless steel is suppressed. If the W content is too low, this effect cannot be obtained. On the other hand, if the W content is too high, the σ phase tends to precipitate and the toughness decreases. Therefore, the W content is 4.00 to 6.00%. The preferable lower limit of the W content is 4.50%. The preferable upper limit of the W content is 5.50%.
[0040]
　Cu: 0.01 to less than 0.10%
　Copper (Cu) is an element effective in suppressing the progress (progressing stage) of pitting corrosion. If the Cu content is too low, this effect cannot be obtained. On the other hand, among Cr, Mo and Cu, Cu reduces the pitting corrosion resistance in the occurrence of pitting corrosion (initial stage). Therefore, the duplex stainless steel of the present embodiment has a lower Cu content than the conventional duplex stainless steel. As a result, the precipitation of Cu in the ferrite phase is suppressed, and the occurrence of pitting corrosion (initial stage) of the duplex stainless steel is suppressed. If the Cu content is too high, the Cu area ratio in the ferrite phase becomes too high. In this case, the pitting corrosion resistance of the duplex stainless steel decreases. Therefore, the Cu content is 0.01 to less than 0.10%. The preferable upper limit of the Cu content is 0.07%, more preferably 0.05%.
[0041]
　N: over 0.400% to 0.600%
　Nitrogen (N) is an austenite stabilizing element and is an element necessary for obtaining a two-phase structure of ferrite/austenite. N further enhances the pitting corrosion resistance of duplex stainless steel. If the N content is too low, these effects cannot be obtained. On the other hand, if the N content is too high, the toughness and hot workability of the duplex stainless steel deteriorate. Therefore, the N content is more than 0.400% to 0.600%. The preferable lower limit of the N content is 0.420%. The preferable upper limit of the N content is 0.500%.
[0042]
　C: 0.030% or less
　Carbon (C) is inevitably contained. That is, the C content is more than 0%. C forms Cr carbides at grain boundaries and increases corrosion susceptibility at grain boundaries. Therefore, the C content is 0.030% or less. The preferable upper limit of the C content is 0.025%, more preferably 0.020%. It is preferable that the C content is as low as possible. However, the extreme reduction of the C content significantly increases the manufacturing cost. Therefore, when industrial production is taken into consideration, the lower limit of the C content is preferably 0.001%, more preferably 0.005%.
[0043]
　Si: 1.00% or less
　Silicon (Si) deoxidizes steel. When Si is used as a deoxidizer, the Si content is more than 0%. On the other hand, if the Si content is too high, the hot workability of the duplex stainless steel deteriorates. Therefore, the Si content is 1.00% or less. The upper limit of the Si content is preferably 0.80%, more preferably 0.70%. The lower limit of the Si content is not particularly limited, but is 0.20%, for example.
[0044]
　Mn: 1.00% or less
　Manganese (Mn) deoxidizes steel. When Mn is used as the deoxidizer, the Mn content is more than 0%. On the other hand, if the Mn content is too high, the hot workability of the duplex stainless steel deteriorates. Therefore, the Mn content is 1.00% or less. The preferable upper limit of the Mn content is 0.80%, more preferably 0.70%. The lower limit of the Mn content is not particularly limited, but is 0.20%, for example.
[0045]
　sol. Al: 0.040% or less
　Aluminum (Al) deoxidizes steel. When Al is used as the deoxidizer, the Al content is more than 0%. On the other hand, if the Al content is too high, the hot workability of the duplex stainless steel deteriorates. Therefore, the Al content is 0.040% or less. The preferable upper limit of the Al content is 0.030%, more preferably 0.025%. The lower limit of the Al content is not particularly limited, but is 0.005%, for example. In the present embodiment, the Al content refers to the acid-soluble Al (sol.Al) content.
[0046]
　V: 0.50% or less
　Vanadium (V) is inevitably contained. That is, the V content is more than 0%. If the V content is too high, the ferrite phase may excessively increase, and the toughness and corrosion resistance of the duplex stainless steel may decrease. Therefore, the V content is 0.50% or less. The preferable upper limit of the V content is 0.40%, and more preferably 0.30%. The lower limit of the V content is not particularly limited, but is, for example, 0.05%.
[0047]
　O: 0.010% or less
　Oxygen (O) is an impurity. That is, the O content is more than 0%. O reduces the hot workability of duplex stainless steel. Therefore, the O content is 0.010% or less. The preferable upper limit of the O content is 0.007%, and more preferably 0.005%. The O content is preferably as low as possible. However, the extreme reduction of the O content significantly increases the manufacturing cost. Therefore, when industrial production is taken into consideration, the preferable lower limit of the O content is 0.0001%, more preferably 0.0005%.
[0048]
　P: 0.030% or less
　Phosphorus (P) is an impurity. That is, the P content is more than 0%. P reduces the pitting corrosion resistance and toughness of duplex stainless steel. Therefore, the P content is 0.030% or less. The preferable upper limit of the P content is 0.025%, more preferably 0.020%. It is preferable that the P content is as low as possible. However, the extreme reduction of the P content significantly increases the manufacturing cost. Therefore, when industrial production is taken into consideration, the preferable lower limit of the P content is 0.001%, and more preferably 0.005%.
[0049]
　S: 0.020% or less
　Sulfur (S) is an impurity. That is, the S content is more than 0%. S reduces the hot workability of duplex stainless steel. Therefore, the S content is 0.020% or less. The preferable upper limit of the S content is 0.010%, more preferably 0.005%, and further preferably 0.003%. It is preferable that the S content is as low as possible. However, the extreme reduction of the S content significantly increases the manufacturing cost. Therefore, when industrial production is taken into consideration, the preferable lower limit of the S content is 0.0001%, more preferably 0.0005%.
[0050]
　The balance of the chemical composition of the duplex stainless steel of this embodiment is Fe and impurities. Here, the impurities in the chemical composition are those that are mixed from ore as a raw material, scrap, or the manufacturing environment when the duplex stainless steel is industrially manufactured, and the duplex stainless steel according to the present embodiment. It means that it is acceptable as long as it does not adversely affect steel.
[0051]
　[About Arbitrary Elements]
　The chemical composition of the duplex stainless steel according to the present embodiment may optionally contain the following elements.
[0052]
　Ca: 0 to 0.0040%
　Calcium (Ca) is an optional element and may not be contained. That is, the Ca content may be 0%. When included, Ca enhances the hot workability of duplex stainless steel. If a small amount of Ca is contained, this effect can be obtained to some extent. On the other hand, if the Ca content is too high, a coarse oxide is generated, and the hot workability of the duplex stainless steel deteriorates. Therefore, the Ca content is 0 to 0.0040%. The preferable lower limit of the Ca content is 0.0001%, more preferably 0.0005%, and further preferably 0.0010%. The preferable upper limit of the Ca content is 0.0030%.
[0053]
　Mg: 0 to 0.0040%
　Magnesium (Mg) is an optional element and may not be contained. That is, the Mg content may be 0%. When contained, Mg enhances the hot workability of the duplex stainless steel similarly to Ca. This effect can be obtained to some extent if a small amount of Mg is contained. On the other hand, if the Mg content is too high, a coarse oxide is generated, and the hot workability of the duplex stainless steel deteriorates. Therefore, the Mg content is 0 to 0.0040%. The preferable lower limit of the Mg content is 0.0001%, more preferably 0.0005%, and further preferably 0.0010%. The upper limit of the Ca content is preferably 0.0030%.
[0054]
　B: 0 to 0.0040%
　Boron (B) is an optional element and may not be contained. That is, the B content may be 0%. When contained, B enhances the hot workability of the duplex stainless steel similarly to Ca and Mg. This effect can be obtained to some extent if B is contained in a small amount. On the other hand, if the B content is too high, the toughness of the duplex stainless steel decreases. Therefore, the B content is 0 to 0.0040%. The preferable lower limit of the B content is 0.0001%, more preferably 0.0005%, and further preferably 0.0010%. The upper limit of the Ca content is preferably 0.0030%.
[0055]
　[Regarding Formula (1)]
　The chemical composition of the duplex stainless steel according to the present embodiment satisfies the content of each element described above and also satisfies the following formula (1).
　Cr+4.0×Mo+2.0×W+20×N-5×ln(Cu)≧65.2 (1)
　Here, each element symbol in the formula (1) represents the content (mass) of each element. %) is substituted.
[0056]
　It is defined as F1=Cr+4.0×Mo+2.0×W+20×N-5×ln(Cu). F1 is an index showing pitting corrosion resistance. If F1 is less than 65.2, the pitting corrosion resistance of the duplex stainless steel decreases. Therefore, F1≧65.2. The lower limit of F1 is preferably 68.0, more preferably 69.0, and further preferably 70.0. The upper limit of F1 is not particularly limited, but is 90.0, for example.
[0057]
　[Microstructure]
　The microstructure of the duplex stainless steel according to the present embodiment is composed of ferrite and austenite. Specifically, the microstructure of the duplex stainless steel according to the present embodiment is composed of 35 to 65% by volume of ferrite phase and the balance of austenite phase. If the volume ratio of the ferrite phase (hereinafter, also referred to as ferrite fraction) is less than 35%, the possibility of stress corrosion cracking increases depending on the use environment. On the other hand, when the volume fraction of the ferrite phase exceeds 65%, the toughness of the duplex stainless steel is likely to decrease. Therefore, the microstructure of the duplex stainless steel of this embodiment is composed of 35 to 65% by volume of the ferrite phase and the balance of the austenite phase.
[0058]
　[Measurement Method of Ferrite Fraction] In the
　present embodiment, the ferrite fraction of the duplex stainless steel can be obtained by the following method. First, a test piece for microstructure observation is taken from a duplex stainless steel. If the duplex stainless steel is a steel plate, a cross section (hereinafter referred to as an observation surface) perpendicular to the plate width direction of the steel plate is polished. If the duplex stainless steel is a steel pipe, a cross section (observation surface) including the axial direction and the thickness direction of the steel pipe is polished. When the duplex stainless steel is a steel bar or a wire rod, a cross section (observation surface) including the axial direction of the steel bar or the wire rod is polished. Next, the observation surface after polishing is etched using a mixed solution of aqua regia and glycerin.
[0059]
　10 fields of view of the etched observation surface are observed with an optical microscope. The visual field area is, for example, 2000 μm 2 (magnification: 500 times). In each visual field, ferrite and other phases can be distinguished from the contrast. Therefore, the ferrite in each observation is specified from the contrast. The area ratio of the specified ferrite is measured by the point calculation method based on JIS G0555 (2003). The measured area ratio is defined as the ferrite fraction (volume %), assuming that it is equal to the volume fraction.
[0060]
　[Regarding Cu Area Ratio in Ferrite Phase]
　The area ratio of Cu precipitated in the ferrite phase of the duplex stainless steel according to the present embodiment is 0.5% or less. As described above, Cu contained in the duplex stainless steel is considered to suppress the progress of pitting corrosion of the duplex stainless steel. Therefore, the duplex stainless steel according to the present embodiment contains Cu in an amount of 0.01 to less than 0.10%. On the other hand, in duplex stainless steel containing Cu in an amount of 0.01 to less than 0.10%, metallic Cu may be precipitated in the ferrite phase. As described above, it has been revealed that Cu precipitated in the ferrite phase reduces the effect of suppressing the occurrence of pitting corrosion due to the passive film. That is, the metallic Cu precipitated in the ferrite phase reduces the pitting corrosion resistance of the duplex stainless steel.
[0061]
　Therefore, the duplex stainless steel according to the present embodiment reduces the Cu area ratio in the ferrite phase to 0.5% or less. Therefore, the occurrence of pitting corrosion of the duplex stainless steel is suppressed. The lower the Cu area ratio in the ferrite phase, the more preferable. The upper limit of the Cu area ratio in the ferrite phase is preferably 0.3%, more preferably 0.1%. The lower limit of the Cu area ratio in the ferrite phase is 0.0%.
[0062]
　[Measurement Method of Cu Area Ratio in Ferrite Phase] In the
　present specification, the Cu area ratio in the ferrite phase means the area of ​​Cu precipitated in the ferrite phase with respect to the ferrite phase in the microstructure of the duplex stainless steel. Means rate. In this embodiment, the Cu area ratio in the ferrite phase can be measured by the following method. A thin film sample for transmission electron microscope (TEM) observation is prepared by the FIB-micro sampling method. A focused ion beam processing device (MI4050, manufactured by Hitachi High-Tech Science Co., Ltd.) is used for manufacturing a thin film sample. A thin film sample for TEM observation is prepared from an arbitrary portion of duplex stainless steel. For the production of the thin film sample, a Mo mesh and a carbon deposition film as the surface protective film are used.
[0063]
　A field emission transmission electron microscope (JEM-2100F manufactured by JEOL Ltd.) is used for TEM observation. The observation magnification is 10,000 times and TEM observation is performed. The contrast is different from the ferrite phase and the austenite phase in the visual field. Therefore, the crystal grain boundary is specified based on the contrast. The phase in the region surrounded by each crystal grain boundary is specified by an X-ray diffraction method (XRD: X-Ray Diffraction). The area of ​​the region specified as the ferrite phase among the regions surrounded by the crystal grain boundaries is obtained by image analysis.
[0064]
　Elemental analysis is performed on the observation visual field by an energy dispersive X-ray analysis method (EDS: Energy Dispersive X-ray Spectrometry) to generate an element map. Furthermore, the precipitate can be identified from the contrast. Therefore, it can be specified by EDS that the precipitate specified in the ferrite phase specified by XRD based on the contrast is metallic Cu.
[0065]
　The area of ​​Cu precipitated in the specified ferrite phase is determined by image analysis. The total area of ​​Cu precipitated in the ferrite phase is divided by the total area of ​​the ferrite phase. In this way, the Cu area ratio (%) in the ferrite phase is measured.
[0066]
　The duplex stainless steel according to the present embodiment satisfies both the above-described chemical composition including the formula (1) and the microstructure including the Cu area ratio in the ferrite phase. Therefore, the duplex stainless steel according to the present embodiment has excellent pitting corrosion resistance.
[0067]
　[Yield Strength]
　The yield strength of the duplex stainless steel according to the present embodiment is not particularly limited. However, if the yield strength is 750 MPa or less, cold working can be omitted in the manufacturing process. In this case, the manufacturing cost can be reduced. Therefore, the yield strength is preferably 750 MPa or less. More preferably, the yield strength is 720 MPa or less. The lower limit of the yield strength is not particularly limited, but is, for example, 300 MPa.
[0068]
　[Measurement Method of Yield Strength] In the
　present specification, the yield strength means 0.2% proof stress obtained by a method based on JIS Z2241 (2011).
[0069]
　[Shape of Duplex Stainless Steel]
　The shape of the duplex stainless steel according to the present embodiment is not particularly limited. The duplex stainless steel may be, for example, a steel pipe, a steel plate, a steel bar, or a wire rod.
[0070]
　[Manufacturing Method]
　The duplex stainless steel of the present embodiment can be manufactured, for example, by the following method. The manufacturing method includes a preparation step, a hot working step, a cooling step, and a solution heat treatment step.
[0071]
　[Preparation Step] In the
　preparation step, a material having the above chemical composition is prepared. The material may be a slab manufactured by a continuous casting method (including round CC) or a steel slab manufactured from the slab. Further, a steel slab produced by hot working an ingot produced by the ingot making method may be used.
[0072]
　[Hot working step] The
　prepared material is charged into a heating furnace or a soaking furnace and heated to, for example, 1150 to 1300°C. Then, the heated material is hot worked. The hot working may be hot forging, hot extrusion using the Eugene-Sejournet method or the Erhard pushbench method, or hot rolling. The hot working may be performed once or plural times.
[0073]
　The heated material is hot worked at 850°C or higher. More specifically, the surface temperature of the steel material at the end of hot working is 850°C or higher. When the surface temperature of the steel material at the end of hot working is less than 850°C, a large amount of Cu is precipitated in the ferrite phase. As a result, the Cu area ratio in the ferrite phase may not be sufficiently reduced even by the solution treatment described below. In this case, the pitting corrosion resistance of the duplex stainless steel decreases. Therefore, the surface temperature of the steel material at the end of hot working is 850°C or higher. When carrying out hot working a plurality of times, the surface temperature of the steel material at the end of the final hot working is 850°C or higher. This makes it possible to prevent Cu from precipitating in the ferrite phase at the end of hot working. The upper limit of the surface temperature of the steel material at the end of hot working is not particularly limited, but is 1300° C., for example. The end of hot working means within 3 seconds after the end of hot working.
[0074]
　[Cooling Step]
　Subsequently, the material after hot working is cooled at 5° C./sec or more. At around 850° C., Cu begins to precipitate in the ferrite phase. Therefore, if the cooling rate after hot working is too slow, a large amount of Cu precipitates in the ferrite phase. As a result, the Cu area ratio in the ferrite phase may not be sufficiently reduced even by the solution treatment described below. In this case, the pitting corrosion resistance of the duplex stainless steel decreases. Therefore, the cooling rate after hot working is 5° C./sec or more. Here, when hot working is performed a plurality of times, the term “after hot working” means after the final hot working. That is, in the present embodiment, the material after the final hot working is cooled at 5° C./or higher. The upper limit of the cooling rate is not particularly limited. The cooling method is, for example, air cooling, water cooling, oil cooling or the like.
[0075]
　[Solution heat treatment step]
　Subsequently, the cooled material is subjected to solution heat treatment at 1070°C or higher. The solution heat treatment heat-dissolves Cu precipitated in the ferrite phase. The Cu area ratio in the ferrite phase can be set to 0.5% or less by subjecting the material in which the precipitation of Cu in the ferrite phase is sufficiently suppressed at the end of hot working and after cooling to a solution heat treatment at 1070° C. or higher. .. The upper limit of the solution heat treatment temperature is not particularly limited, but is 1150° C., for example. The treatment time of the solution heat treatment is not particularly limited. The treatment time of the solution heat treatment is, for example, 1 to 30 minutes.
[0076]
　Through the above steps, the duplex stainless steel according to the present embodiment can be manufactured. In addition, in this embodiment, it is preferable not to perform cold working because the manufacturing cost increases.
Example
[0077]
　Alloys having the chemical compositions shown in Table 3 were melted in a 50 kg vacuum melting furnace, the obtained ingot was heated at 1200° C., hot forged and hot rolled to carry out processing into a steel plate having a thickness of 10 mm. .. The temperature at the end of rolling shown in Table 4 is the surface temperature of the steel sheet at the end of hot rolling. The cooling rate after rolling shown in Table 4 is the cooling rate after hot rolling. Further, the steel sheet was solution-treated at the solution temperature (° C.) shown in Table 4 to obtain a test piece of each test number.
[0078]
[Table 3]

[0079]
[Table 4]

[0080]
　[Ferrite Fraction Measurement Test] The
　ferrite fraction (volume %) was measured on the test piece of each test number by the method described above. The results are shown in Table 4. The rest of the microstructure of the test piece of each test number was an austenite phase.
[0081]
　[Cu Area Ratio Measurement Test
　in Ferrite Phase] The Cu area ratio (%) in the ferrite phase was measured by the above-described method for the test piece of each test number. The results are shown in Table 4.
[0082]
　[Piting potential measurement test] The pitting potential
　of the test piece of each test number after solution treatment was measured. First, the test piece was machined into a test piece having a diameter of 15 mm and a thickness of 2 mm. Using the obtained test piece, 80° C., 25% NaClaq. The pitting potential was measured therein. The conditions other than the test temperature and the NaCl concentration were according to JIS G0577 (2014). Table 4 shows the measurement results of the pitting potential Vc′ 100 of the test pieces of each test number .
[0083]
　[Tensile test] The
　0.2% proof stress was determined for the test piece of each test number by the method according to JIS Z2241 (2011). The results are shown in Table 4.
[0084]
　[Evaluation Results] With
　reference to Tables 3 and 4, the test pieces of Test Nos. 5 to 8 had appropriate chemical compositions and production conditions. Therefore, the test pieces of test Nos. 5 to 8 are duplex stainless steels having a ferrite fraction of 35 to 65% by volume and the balance consisting of an austenite phase. Further, the Cu area ratio in the ferrite phase is 0.5%. It was below. As a result, the pitting corrosion potentials (mV vs. SCE) of the steel sheets of Test Nos. 5 to 8 were 100 or more, showing excellent pitting corrosion resistance.
[0085]
　On the other hand, in the test piece of test number 1, the Cu content was too high. Further, in the test piece of test number 1, F1 was 59.8, which did not satisfy the formula (1). Therefore, the Cu area ratio in the ferrite phase of the test piece of test number 1 was 0.8%. As a result, the test piece of Test No. 1 had a pitting potential (mVvs.SCE) of -60, and did not show excellent pitting resistance.
[0086]
　In the test piece of test number 2, the Cu content was too high. Therefore, the Cu area ratio in the ferrite phase of the test piece of test number 2 was 0.6%. As a result, the pitting potential (mVvs.SCE) of the test piece of Test No. 2 was 71, which did not show excellent pitting resistance.
[0087]
　The test piece of test number 3 had a solution temperature of 1050° C., which was too low. Therefore, the Cu area ratio in the ferrite phase of the test piece of test number 3 was 0.7%. As a result, the pitting corrosion potential (mVvs.SCE) of the test piece of Test No. 3 was -12, which did not show excellent pitting corrosion resistance.
[0088]
　In the test piece of test number 4, although the content of each element was appropriate, F1 was 65.1, which did not satisfy the formula (1). As a result, the test piece of Test No. 4 had a pitting potential (mVvs.SCE) of 85, which did not show excellent pitting resistance.
[0089]
　In the test piece of test number 9, the W content was too low. As a result, the pitting potential (mVvs.SCE) of the test piece of Test No. 9 was 70, which did not show excellent pitting resistance.
[0090]
　In the test piece of test number 10, the Mo content was too low. As a result, the pitting potential (mV vs. SCE) of the test piece of Test No. 10 was 76, which did not show excellent pitting resistance.
[0091]
　In the test piece of test number 11, the Cr content was too low. As a result, the pitting corrosion potential (mV vs. SCE) of the test piece of Test No. 11 was 81, which did not show excellent pitting corrosion resistance.
[0092]
　In the test piece of test number 12, the temperature at the end of hot rolling was 840°C, which was too low. Therefore, the Cu area ratio in the ferrite phase of the test piece of test number 12 was 1.1%. As a result, the test piece of Test No. 12 had a pitting corrosion potential (mV vs. SCE) of −150, and did not show excellent pitting corrosion resistance.
[0093]
　The test piece of Test No. 13 had a cooling rate of 3° C./sec after completion of hot rolling and was too slow. Therefore, the Cu area ratio in the ferrite phase of the test piece of test number 13 was 1.6%. As a result, the pitting corrosion potential (mVvs.SCE) of the test piece of Test No. 13 was -71, which did not show excellent pitting corrosion resistance.
[0094]
　The embodiments of the present invention have been described above. However, the embodiments described above are merely examples for implementing the present invention. Therefore, the present invention is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments without departing from the spirit thereof.
The scope of the claims
[Claim 1]
　In mass%,
　Cr: over 27.0% to 29.00%,
　Mo: 2.50 to 3.50%,
　Ni: 5.00 to 8.00%,
　W: 4.00 to 6.00%,
　Cu: 0.01 to less than 0.10%,
　N: more than 0.400% to 0.600%,
　C: 0.030% or less,
　Si: 1.00% or less,
　Mn: 1.00% or less,
　sol ．Al: 0.040% or less,
　V: 0.50% or less,
　O: 0.010% or less,
　P: 0.030% or less,
　S: 0.020% or less,
　Ca: 0 to 0.0040%,
　Mg : 0 to 0.0040%,
　B: 0 to 0.0040%, and the
　balance consisting of Fe and impurities, a chemical composition satisfying the formula (1),
　and a ferrite phase of 35 to 65% by volume and the balance of an austenite phase. And a microstructure consisting
　of, the area ratio of Cu precipitated in the ferrite phase being 0.5% or less.
　Cr+4.0×Mo+2.0×W+20×N-5×ln(Cu)≧65.2 (1)
　Here, the content (mass %) of each element is substituted for each element symbol in the formula (1).
[Claim 2]
　The
　duplex stainless steel according to claim 1, wherein the chemical composition is% by mass, Ca: 0.0001 to 0.0040%,
　Mg: 0.0001 to 0.0040%, and
　B: 0. Duplex stainless steel containing one or more selected from the group consisting of 0001 to 0.0040%.
[Claim 3]
　In mass%,
　Cr: over 27.0% to 29.00%,
　Mo: 2.50 to 3.50%,
　Ni: 5.00 to 8.00%,
　W: 4.00 to 6.00%,
　Cu: 0.01 to less than 0.10%,
　N: more than 0.400% to 0.600%,
　C: 0.030% or less,
　Si: 1.00% or less,
　Mn: 1.00% or less,
　sol ．Al: 0.040% or less,
　V: 0.50% or less,
　O: 0.010% or less,
　P: 0.030% or less,
　S: 0.020% or less,
　Ca: 0 to 0.0040%,
　Mg :0 to 0.0040%,
　B: 0 to 0.0040%, and the
　balance consisting of Fe and impurities, the step of preparing a raw material having a chemical composition satisfying the formula (1), and
　said raw material at 850° C. The method
　comprises the steps of hot working as described above, cooling the material after hot working at 5° C./sec or more, and
　subjecting the cooled material to solution heat treatment at 1070° C. or more. Method for producing duplex stainless steel.
　Cr+4.0×Mo+2.0×W+20×N-5×ln(Cu)≧65.2 (1)
　Here, each element symbol in the formula (1) represents the content (mass) of each element. %) is substituted.