JPWO2019098233A1 - Duplex Stainless Steel and Duplex Stainless Steel Manufacturing Methods - Google Patents

Abstract

Provided is a duplex stainless steel in which the occurrence of pitting corrosion is suppressed. Duplex stainless steel according to the present disclosure has Cr: more than 27.00% 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, and the balance consists of Fe and impurities. It contains a chemical composition satisfying the formula (1), a ferrite phase of 35 to 65% by volume, and a microstructure composed of an austenite phase in the balance, and the area ratio of Cu precipitated in the ferrite phase is 0.5% or less. is there. Cr + 4.0 x Mo + 2.0 x W + 20 x N-5 x ln (Cu) ≧ 65.2 ... (1)

Description

The present invention relates to duplex stainless steel and methods for producing duplex stainless steel.

A two-phase stainless steel having a two-phase structure of a ferrite phase and an austenite phase is known to have excellent corrosion resistance. Duplex stainless steel is particularly excellent in corrosion resistance (hereinafter referred to as "pitting corrosion resistance") against pitting corrosion and / or crevice corrosion, which is a problem in aqueous solutions containing chloride. Therefore, duplex stainless steel is widely used in moist environments containing chlorides such as seawater. Duplex stainless steel is used, for example, in flow line pipes, umbilical tubes, heat exchangers and the like in wet environments containing chlorides.

In recent years, the corrosion conditions in the environment in which duplex stainless steel is used have become more and more severe. Therefore, duplex stainless steel is required to have even better pitting corrosion resistance. Various techniques have been proposed to further enhance the pitting corrosion resistance of duplex stainless steel.

International Publication No. 2013/191208 (Patent Document 1) is based on mass%, Ni: 3 to 8%, Cr: 20 to 35%, Mo: 0.01 to 4.0%, N: 0.05 to 0. Two phases containing .60% and further containing one or more selected from Re: 2.0% or less, Ga: 2.0% or less, and Ge: 2.0% or less. Disclose stainless steel. In Patent Document 1, by incorporating Re, Ga, or Ge in duplex stainless steel, the critical potential (pitting potential) at which pitting corrosion occurs is increased, and pitting corrosion resistance and crevice corrosion resistance are enhanced. There is.

International Publication No. 2010/082395 (Patent Document 2) is based on mass%, Cr: 20 to 35%, Ni: 3 to 10%, Mo: 0 to 6%, W: 0 to 6%, Cu: 0 to 0. A duplex stainless steel material containing 3% and N: 0.15-0.60% is hot-worked or further solidified and heat-treated to prepare a cold-working raw pipe, and then cold-rolled for 2 Disclose a method of manufacturing a duplex stainless steel pipe. The method for producing a duplex stainless steel pipe in Patent Document 2 is a process of Rd (= exp [{In (MYS) -In (14.5 × Cr + 48.3 × Mo + 20.)) At the cross-sectional reduction rate in the final cold rolling step. 7 × W + 6.9 × N)} /0.195]) is a method for producing a duplex stainless steel pipe that is cold-rolled within the range of 10 to 80% and has 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 duplex stainless steel tube that can be used in oil wells and gas wells, for example, can be obtained in a carbon dioxide corrosion environment and a stress corrosion environment, and has high strength as well as excellent corrosion resistance. ing.

Japanese Unexamined Patent Publication No. 2007-84837 (Patent Document 3) describes Cr: 20 to 30%, Ni: 1 to 11%, Cu: 0.05 to 3.0%, Nd: 0.005 to 0 in mass%. Two-phase stainless steels containing 1.5%, N: 0.1-0.5%, and Mo: 0.5-6 and / or both of W: 1-10 are disclosed. In Patent Document 3, hot workability of duplex stainless steel is enhanced by containing Nd.

Japanese Patent Application Laid-Open No. 2005-520934 (Patent Document 4) shows Cr: 21.0% to 38.0%, Ni: 3.0% to 12.0%, Mo: 1.5% to 6 in weight%. It contains 5.5%, W: 0 to 6.5%, N: 0.2% to 0.7%, Ba: 0.0001 to 0.6%, and has a duplex equivalent index PREW of 40 ≦ PREW ≦. A super duplex stainless steel satisfying 67 is disclosed. As a result, super two-phase stainless steel with excellent corrosion resistance, embrittlement resistance, castability and hot workability, in which the formation of metal-phases such as brittle sigma (σ) phase and chi (χ) phase is suppressed, is produced. It is described in Patent Document 4 that it can be obtained.

International Publication No. 2013/191208 International Publication No. 2010/082395 JP-A-2007-88437 Special Table 2005-520934

As described above, in recent years, duplex stainless steels having more excellent pitting corrosion resistance have 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.

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.

Duplex stainless steel according to the present disclosure has Cr: more than 27.00% 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 is composed of Fe and impurities, and has a chemical composition satisfying the formula (1), and 35 to 65% by volume of the ferrite phase and the balance are the austenite phase. Contains 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, the content (mass%) of each element is substituted for each element symbol in the formula (1).

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 preparatory step, a material having the above-mentioned chemical composition is prepared. In the hot working step, the material is hot worked at 850 ° C. or higher. In the cooling step, the hot-worked material is cooled at 5 ° C./sec or higher. In the solution heat treatment step, the cooled material is solution heat treated at 1070 ° C. or higher.

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-mentioned duplex stainless steel.

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.

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 is the main component of the passivation film on the surface of duplex stainless steel as an oxide. Passivation coatings interfere with the contact of corrosive factors with the surface of duplex stainless steel. As a result, duplex stainless steel with a passivation film formed on its surface has improved pitting corrosion resistance. Mo is contained in the passivation film and further enhances the pitting corrosion resistance of the passivation film.

On the other hand, among 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 corrosion occurs. The first step is the development of pitting corrosion (early stage). The next step is the progress of pitting corrosion (progress stage). Conventionally, Cu has been considered to have an effect of suppressing the progress of pitting corrosion. In particular, in an acidic solution, active sites with a high dissolution rate are formed on the surface of duplex stainless steel. Cu coats its active site and suppresses the melting of duplex stainless steel. As a result, Cu has been considered to suppress the progress of pitting corrosion of duplex stainless steel.

Based on the above mechanism, Cr, Mo and Cu have been considered to be effective elements for improving pitting corrosion resistance in duplex stainless steel. Therefore, conventionally, in 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 studies by the present inventors, the following findings that have not been known in the past have been obtained. Specifically, the present inventors have found that among Cr, Mo and Cu, Cu may rather reduce pitting corrosion resistance in the development of pitting corrosion (initial stage).

Table 1 is a table showing the chemical composition of the test pieces of test numbers 2 and 5 and the pitting potential, which is an index of pitting corrosion resistance, in Examples described later. The chemical composition of Table 1 is an excerpt of the chemical composition of steel types B and E corresponding to test numbers 2 and 5 from Table 3 described later, and is described in two stages. The chemical composition in Table 1 is described in% 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 later.

With reference to Table 1, the Cu content of the test piece of Test No. 2 was higher than the Cu content of the test piece of Test No. 5. Further, the Cr and Mo contents of the test piece of Test No. 2 were higher than the Cr and Mo contents of the test piece of Test No. 5. Therefore, based on the conventional knowledge, it can be expected that the test piece of Test No. 2 having a high content of Cr, Mo and Cu has better pitting corrosion resistance than the test piece of Test No. 5. However, the pitting potential, which is an index of pitting corrosion resistance of the test piece of Test No. 2, is 71 mV vs. It is SCE, and the pitting potential of the test piece of test number 5 is 346 mV vs. It was lower than SCE.

That is, from the conventional knowledge, the test piece of test number 2, which is expected to have better pitting corrosion resistance than the test piece of test number 5, has lower pitting corrosion resistance than the test piece of test number 5. Was there. Therefore, the present inventors focused on the microstructure of the test pieces of test numbers 2 and 5 and investigated them in more detail. As a result, it was clarified that the test piece of test number 2 had a higher area ratio of Cu precipitated in the ferrite phase (referred to as the Cu area ratio in the ferrite phase) than the test piece of test number 5.

Therefore, the present inventors further investigated and examined 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 composition of the test pieces of test numbers 3 and 6, the Cu area ratio in the ferrite phase, and the pitting potential, which is an index of pitting corrosion resistance, in Examples described later. The chemical composition of Table 2 is an excerpt of the chemical composition of steel type C corresponding to test numbers 3 and 6 from Table 3 described later, and is described in two stages. The chemical composition in Table 2 is described in% by mass, and the balance is Fe and impurities. The Cu area ratio in the ferrite phase in Table 2 is the Cu area ratio in the ferrite phase having 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 later.

With reference to Table 2, the chemical compositions of the test piece of test number 3 and the test piece of test number 6 were the same. On the other hand, the test piece of test number 6 had a Cu area ratio in the ferrite phase lower than that of the test piece of test number 3 in the ferrite phase. As a result, the pitting potential of the test piece of test number 6 was 204 mV vs. It is SCE, and the pitting potential of the test piece of test number 3 is -12 mV vs. It was higher than 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 reducing the precipitation of Cu in the ferrite phase.

As described above, it has been conventionally considered that increasing the content of Cr, Mo and Cu increases the pitting corrosion resistance. However, the present inventors have found for the first time that among Cr, Mo and Cu, Cu may rather reduce pitting corrosion resistance. The present inventors have further obtained a previously unknown finding that the pitting corrosion resistance can be improved by reducing the amount of Cu precipitated in the ferrite phase.

The detailed reason why the Cu precipitated in the ferrite phase reduces the pitting corrosion resistance of the duplex stainless steel has not been clarified. However, the present inventors think as follows. Cu precipitated in the ferrite phase may hinder the uniform formation of the passivation 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 due to the passive coating may be reduced. As a result, it is considered that pitting corrosion occurs on the surface of duplex stainless steel.

The duplex stainless steel according to the present embodiment completed based on the above findings has a mass of Cr: more than 27.00% to 29.00%, Mo: 2.50 to 3.50%, and Ni: 5.00. ~ 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 is composed of Fe and impurities, and the chemical composition satisfying the formula (1), and 35 to 65% by volume of the ferrite phase and the balance are the austenite phase. Contains 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, the content (mass%) of each element is substituted for each element symbol in the formula (1).

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.

Preferably, the chemical composition 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% in mass%. Contains one or more of the above.

In this case, the hot workability of the duplex stainless steel according to the present embodiment is enhanced.

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 preparatory step, a material having the above-mentioned chemical composition is prepared. In the hot working step, the material is hot worked at 850 ° C. or higher. In the cooling step, the hot-worked material is cooled at 5 ° C./sec or higher. In the solution heat treatment step, the cooled material is solution heat treated at 1070 ° C. or higher.

The duplex stainless steel produced by the production method 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 produced by the production method according to the present embodiment has excellent pitting corrosion resistance.

Hereinafter, duplex stainless steel according to this embodiment will be described in detail.

[Chemical composition]
The chemical composition of duplex stainless steel according to this embodiment contains the following elements. Unless otherwise specified,% with respect to the element means mass%.

[About essential elements]
The chemical composition of duplex stainless steel according to this embodiment indispensably contains the following elements.

Cr: Over 27.00% to 29.00%
Chromium (Cr) forms a passivation film on the surface of duplex stainless steel as an oxide. Passivation coatings interfere with the contact of corrosive factors with the surface of duplex stainless steel. As a result, the occurrence of pitting corrosion of duplex stainless steel is suppressed. Cr is also an element required to obtain the 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 duplex stainless steel deteriorates. Therefore, the Cr content is more than 27.00% to 29.00%. The preferred lower limit of the Cr content is 27.50%, more preferably 28.00%. The preferred upper limit of the Cr content is 28.50%.

Mo: 2.50 to 3.50%
Molybdenum (Mo) is contained in the passive film to further enhance the corrosion resistance of the passive film. As a result, the pitting corrosion resistance of 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 when assembling a steel pipe made of duplex stainless steel is lowered. Therefore, the Mo content is 2.50 to 3.50%. The preferred lower limit of the Mo content is 2.80%, more preferably 3.00%. The preferred upper limit of the Mo content is 3.30%.

Ni: 5.00 to 8.00%
Nickel (Ni) is an austenite stabilizing element, which 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 preferred lower limit of the Ni content is 5.50%, more preferably 6.00%. The preferred upper limit of the Ni content is 7.50%.

W: 4.00 to 6.00%
Tungsten (W) is contained in the passivation film like Mo, and further enhances the corrosion resistance of the passivation film. As a result, the occurrence of pitting corrosion of 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 is likely to precipitate and the toughness is lowered. Therefore, the W content is 4.00 to 6.00%. The preferable lower limit of the W content is 4.50%. The preferred upper limit of the W content is 5.50%.

Cu: 0.01 to less than 0.10% Copper (Cu) is an element effective in suppressing the progress (progress 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 lowers the pitting corrosion resistance at 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 is reduced. Therefore, the Cu content is less than 0.01-0.10%. The preferred upper limit of the Cu content is 0.07%, more preferably 0.05%.

N: Over 0.400% ~ 0.600%
Nitrogen (N) is an austenite stabilizing element, which 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 duplex stainless steel will decrease. 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%.

C: 0.030% or less Carbon (C) is inevitably contained. That is, the C content is more than 0%. C forms Cr carbides at the grain boundaries and increases the corrosion sensitivity at the grain boundaries. Therefore, the C content is 0.030% or less. The preferred upper limit of the C content is 0.025%, more preferably 0.020%. The C content is preferably as low as possible. However, an extreme reduction in C content significantly increases manufacturing costs. Therefore, when industrial production is taken into consideration, the preferable lower limit of the C content is 0.001%, and more preferably 0.005%.

Si: 1.00% or less Silicon (Si) deoxidizes steel. When Si is used as an antacid, the Si content is greater than 0%. On the other hand, if the Si content is too high, the hot workability of duplex stainless steel deteriorates. Therefore, the Si content is 1.00% or less. The preferred upper limit of the Si content is 0.80%, more preferably 0.70%. The lower limit of the Si content is not particularly limited, but is, for example, 0.20%.

Mn: 1.00% or less Manganese (Mn) deoxidizes steel. When Mn is used as a deoxidizer, the Mn content is more than 0%. On the other hand, if the Mn content is too high, the hot workability of duplex stainless steel deteriorates. Therefore, the Mn content is 1.00% or less. The preferred 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, for example, 0.20%.

sol. Al: 0.040% or less Aluminum (Al) deoxidizes steel. When Al is used as a deoxidizer, the Al content is more than 0%. On the other hand, if the Al content is too high, the hot workability of duplex stainless steel is reduced. Therefore, the Al content is 0.040% or less. The preferred 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, for example, 0.005%. In the present embodiment, the Al content refers to the acid-soluble Al (sol.Al) content.

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 increase excessively and the toughness and corrosion resistance of the duplex stainless steel may decrease. Therefore, the V content is 0.50% or less. The preferred upper limit of the V content is 0.40%, more preferably 0.30%. The lower limit of the V content is not particularly limited, but is, for example, 0.05%.

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 preferred upper limit of the O content is 0.007%, more preferably 0.005%. The O content is preferably as low as possible. However, an extreme reduction in O content significantly increases manufacturing costs. Therefore, when industrial production is taken into consideration, the preferable lower limit of the O content is 0.0001%, and more preferably 0.0005%.

P: 0.030% or less Phosphorus (P) is an impurity. That is, the P content is more than 0%. P reduces pitting corrosion resistance and toughness of duplex stainless steel. Therefore, the P content is 0.030% or less. The preferred 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, an extreme reduction in P content significantly increases manufacturing costs. Therefore, when industrial production is taken into consideration, the preferable lower limit of the P content is 0.001%, and more preferably 0.005%.

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 preferred upper limit of the S content is 0.010%, more preferably 0.005%, and even more preferably 0.003%. It is preferable that the S content is as low as possible. However, an extreme reduction in S content significantly increases manufacturing costs. Therefore, when industrial production is taken into consideration, the preferable lower limit of the S content is 0.0001%, and more preferably 0.0005%.

The balance of the chemical composition of the duplex stainless steel of this embodiment consists of Fe and impurities. Here, the impurities in the chemical composition are mixed from ore, scrap, manufacturing environment, etc. as a raw material when the two-phase stainless steel is industrially manufactured, and the two-phase stainless steel according to the present embodiment. It means what is allowed as long as it does not adversely affect the steel.

[About arbitrary elements]
The chemical composition of duplex stainless steel according to this embodiment may optionally contain the following elements.

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 contained, Ca enhances the hot workability of duplex stainless steel. If even 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, coarse oxides are generated and the hot workability of duplex stainless steel is lowered. Therefore, the Ca content is 0 to 0.0040%. The preferred lower limit of the Ca content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferable upper limit of the Ca content is 0.0030%.

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 duplex stainless steels, similar to Ca. If even a small amount of Mg is contained, this effect can be obtained to some extent. On the other hand, if the Mg content is too high, coarse oxides are generated and the hot workability of duplex stainless steel is lowered. 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 even more preferably 0.0010%. The upper limit of the Ca content is preferably 0.0030%.

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 duplex stainless steel, similar to Ca and Mg. If even a small amount of B is contained, this effect can be obtained to some extent. On the other hand, if the B content is too high, the toughness of the duplex stainless steel will decrease. 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 even more preferably 0.0010%. The upper limit of the Ca content is preferably 0.0030%.

[About equation (1)]
The chemical composition of duplex stainless steel according to this embodiment satisfies the content of each of the above elements and satisfies the following formula (1).
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).

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 duplex stainless steel is reduced. Therefore, F1 ≧ 65.2. The lower limit of F1 is preferably 68.0, more preferably 69.0, and even more preferably 70.0. The upper limit of F1 is not particularly limited, but is, for example, 90.0.

[About microstructure]
The microstructure of the two-phase stainless steel according to this embodiment is composed of ferrite and austenite. Specifically, the microstructure of the two-phase stainless steel according to the present embodiment consists of a ferrite phase of 35 to 65% by volume and the rest of which is an austenite phase. If the volume fraction of the ferrite phase (hereinafter, also referred to as the ferrite fraction) is less than 35%, the possibility of stress corrosion cracking increases depending on the usage 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 two-phase stainless steel of the present embodiment consists of a ferrite phase of 35 to 65% by volume and an austenite phase in the balance.

[Measurement method of ferrite fraction]
In the present embodiment, the ferrite fraction of the duplex stainless steel can be determined by the following method. First, a test piece for microstructure observation is collected from duplex stainless steel. If duplex stainless steel is a steel sheet, the cross section perpendicular to the width direction of the steel sheet (hereinafter referred to as the observation surface) is polished. If duplex stainless steel is a steel pipe, the cross section (observation surface) including the axial direction and the wall thickness direction of the steel pipe is polished. If the duplex stainless steel is a bar steel or wire rod, the cross section (observation surface) of the bar steel or wire rod including the axial direction is polished. Next, the observation surface after polishing is etched with a mixed solution of aqua regia and glycerin.

The etched observation surface is observed in 10 fields with an optical microscope. The visual field area is, for example, 2000 μm ² (magnification 500 times). In each observation field, ferrite and the other phases can be distinguished by contrast. Therefore, the ferrite in each observation is specified from the contrast. The area ratio of the specified ferrite is measured by a point calculation method based on JIS G0555 (2003). Assuming that the measured area fraction is equal to the volume fraction, this is defined as the ferrite fraction (volume%).

[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 duplex stainless steel is considered to suppress the progress of pitting corrosion of 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 less than 0.01 to 0.10% Cu, metallic Cu may precipitate in the ferrite phase. As described above, it has been clarified that Cu precipitated in the ferrite phase reduces the effect of suppressing the occurrence of pitting corrosion due to the passivation film. That is, the metallic Cu deposited in the ferrite phase lowers the pitting corrosion resistance of the duplex stainless steel.

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 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%.

[Measurement method of Cu area ratio in ferrite phase]
In the present specification, the Cu area ratio in the ferrite phase means the area ratio of Cu precipitated in the ferrite phase to the ferrite phase in the microstructure of the duplex stainless steel. In the present embodiment, the Cu area ratio in the ferrite phase can be measured by the following method. A thin film sample for observation with a transmission electron microscope (TEM: Transmission Electron Microscope) is prepared by the FIB-microsampling method. A focused ion beam processing device (MI4050, manufactured by Hitachi High-Tech Science Corporation) is used to prepare a thin film sample. A thin film sample for TEM observation is prepared from any part of duplex stainless steel. A Mo mesh is used to prepare the thin film sample, and a carbon depot film is used as the surface protective film.

A field emission transmission electron microscope (JEM-2100F manufactured by JEOL Ltd.) is used for TEM observation. TEM observation is performed with an observation magnification of 10000 times. The contrast is different from that of the ferrite phase and the austenite phase in the field of view. Therefore, the grain boundaries are specified based on the contrast. The phase of the region surrounded by each grain boundary is specified by X-ray diffraction (XRD: X-Ray Diffraction). Of the regions surrounded by each grain boundary, the area of the region specified as the ferrite phase is determined by image analysis.

Elemental analysis is performed on the observation field of view by an energy dispersive X-ray analysis method (EDS: Energy Dispersive X-ray Spectrum) to generate an elemental map. Furthermore, the precipitate can be identified from the contrast. Therefore, it can be specified by EDS that the precipitate specified based on the contrast in the ferrite phase specified by XRD is metallic Cu.

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.

The duplex stainless steel according to the present embodiment satisfies both the chemical composition including the formula (1) and the microstructure including the Cu area ratio in the ferrite phase described above. Therefore, the duplex stainless steel according to the present embodiment has excellent pitting corrosion resistance.

[Yield strength]
The yield strength of duplex stainless steel according to this 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.

[Measurement method of yield strength]
In the present specification, the yield strength means the 0.2% proof stress obtained by the method according to JIS Z2241 (2011).

[Shape of duplex stainless steel]
The shape of the duplex stainless steel according to this 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.

[Production method]
The duplex stainless steel of the present embodiment can be produced by, for example, the following method. The manufacturing method includes a preparatory step, a hot working step, a cooling step, and a solution heat treatment step.

[Preparation process]
In the preparatory step, a material having the above-mentioned chemical composition is prepared. The material may be a slab produced by a continuous casting method (including round CC) or a steel slab produced from the slab. Further, a steel piece produced by hot-working an ingot produced by the ingot method may be used.

[Hot working process]
The prepared material is placed in a heating furnace or a soaking furnace and heated to, for example, 1150 to 1300 ° C. Subsequently, the heated material is hot-processed. The hot working may be hot forging, hot extrusion using, for example, the Eugene-Sejurne method or the Erhard pushbench method, or hot rolling. The hot working may be carried out once or multiple times.

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 deposited 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 later. In this case, the pitting corrosion resistance of the duplex stainless steel is reduced. Therefore, the surface temperature of the steel material at the end of hot working is 850 ° C. or higher. When the hot working is carried out a plurality of times, the surface temperature of the steel material at the end of the final hot working is 850 ° C. or higher. As a result, it is possible to suppress the precipitation of Cu 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, for example, 1300 ° C. The time when the hot working is completed means within 3 seconds after the hot working is completed.

[Cooling process]
Subsequently, the hot-worked material is cooled at 5 ° C./sec or higher. 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 is deposited 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 later. In this case, the pitting corrosion resistance of the duplex stainless steel is reduced. Therefore, the cooling rate after hot working is 5 ° C./sec or more. Here, when the 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.

[Soluble heat treatment process]
Subsequently, the cooled material is subjected to solution heat treatment at 1070 ° C. or higher. Cu precipitated in the ferrite phase is solid-solved by solution heat treatment. The Cu area ratio in the ferrite phase can be reduced to 0.5% or less by performing solution heat treatment at 1070 ° C or higher on a material in which Cu precipitation in the ferrite phase is sufficiently suppressed at the end of hot working and after cooling. .. The upper limit of the solution heat treatment temperature is not particularly limited, but is, for example, 1150 ° C. 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.

Duplex stainless steel according to this embodiment can be produced by the above steps. In this embodiment, it is preferable not to perform cold working because the manufacturing cost increases.

The alloy having the chemical composition shown in Table 3 was melted in a vacuum melting furnace of 50 kg, the obtained ingot was heated at 1200 ° C., and hot forging and hot rolling were carried out to process it into a steel sheet 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 post-rolling cooling rate 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 test pieces having each test number.

[Ferrite fraction measurement test]
The ferrite fraction (volume%) was measured for each test piece of each test number by the above-mentioned method. The results are shown in Table 4. The rest of the microstructure of the test piece of each test number was the austenite phase.

[Cu area ratio measurement test in ferrite phase]
For the test piece of each test number, the Cu area ratio (%) in the ferrite phase was measured by the above method. The results are shown in Table 4.

[Pitting corrosion potential measurement test]
The pitting potential of the test piece of each test number after the 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 inside. Conditions other than the test temperature and NaCl concentration were performed according to JIS G0577 (2014). The measurement results of the pitting potential Vc _'100 of the specimen of each test number shown in Table 4.

[Tensile test]
For the test piece of each test number, 0.2% proof stress was determined by a method conforming to JIS Z2241 (2011). The results are shown in Table 4.

[Evaluation results]
With reference to Tables 3 and 4, the test pieces of Test Nos. 5 to 8 had an appropriate chemical composition and appropriate production conditions. Therefore, the test pieces of test numbers 5 to 8 are two-phase stainless steel having a ferrite fraction of 35 to 65% by volume and the balance being an austenite phase, and further, the Cu area ratio in the ferrite phase is 0.5%. It was below. As a result, the pitting potential (mV vs. SCE) of the steel sheets of test numbers 5 to 8 was 100 or more, showing excellent pitting corrosion resistance.

On the other hand, in the test piece of Test No. 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 No. 1 was 0.8%. As a result, the pitting potential (mV vs. SCE) of the test piece of Test No. 1 was -60, and did not show excellent pitting corrosion resistance.

The Cu content of the test piece of Test No. 2 was too high. Therefore, the Cu area ratio in the ferrite phase of the test piece of Test No. 2 was 0.6%. As a result, the pitting potential (mV vs. SCE) of the test piece of Test No. 2 was 71, and did not show excellent pitting corrosion resistance.

In the test piece of test number 3, the solution temperature was 1050 ° C, which was too low. Therefore, the Cu area ratio in the ferrite phase of the test piece of Test No. 3 was 0.7%. As a result, the pitting potential (mV vs. SCE) of the test piece of Test No. 3 was -12, and did not show excellent pitting corrosion resistance.

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 pitting potential (mV vs. SCE) of the test piece of Test No. 4 was 85, which did not show excellent pitting corrosion resistance.

The W content was too low in the test piece of test number 9. As a result, the pitting potential (mV vs. SCE) of the test piece of Test No. 9 was 70, which did not show excellent pitting corrosion resistance.

The Mo content was too low in the test piece of test number 10. 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 corrosion resistance.

In the test piece of test number 11, the Cr content was too low. As a result, the pitting potential (mV vs. SCE) of the test piece of Test No. 11 was 81, which did not show excellent pitting corrosion resistance.

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 No. 12 was 1.1%. As a result, the pitting potential (mV vs. SCE) of the test piece of Test No. 12 was −150, and did not show excellent pitting corrosion resistance.

The test piece of test number 13 had a cooling rate of 3 ° C./sec after the completion of hot rolling, which was too slow. Therefore, the Cu area ratio in the ferrite phase of the test piece of Test No. 13 was 1.6%. As a result, the pitting potential (mV vs. SCE) of the test piece of Test No. 13 was -71, which did not show excellent pitting corrosion resistance.

The embodiments of the present invention have been described above. However, the embodiments described above are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and the above-described embodiment can be appropriately modified and implemented without departing from the spirit of the present invention.

Claims (3)

By mass%
Cr: Over 27.00% 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: Over 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: 0-0.0040%,
Mg: 0 to 0.0040%,
B: 0 to 0.0040% and
The balance consists of Fe and impurities, and has a chemical composition that satisfies formula (1).
A ferrite phase of 35 to 65% by volume and the balance containing a microstructure composed of an austenite phase.
Duplex stainless steel in which 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, the content (mass%) of each element is substituted for each element symbol in the formula (1). The duplex stainless steel according to claim 1, wherein the chemical composition is mass%.
Ca: 0.0001 to 0.0040%,
Mg: 0.0001 to 0.0040% and
B: Duplex stainless steel containing one or more selected from the group consisting of 0.0001 to 0.0040%. By mass%
Cr: Over 27.00% 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: Over 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: 0-0.0040%,
Mg: 0 to 0.0040%,
B: 0 to 0.0040% and
The rest consists of Fe and impurities, and the step of preparing a material having a chemical composition satisfying the formula (1) and
The process of hot-working the material at 850 ° C or higher and
A step of cooling the material after hot working at 5 ° C./sec or higher, and
A method for producing duplex stainless steel, which comprises a step of solution heat treating the cooled material at 1070 ° C. or higher.
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).