CN111344426A

CN111344426A - Duplex stainless steel and method for producing duplex stainless steel

Info

Publication number: CN111344426A
Application number: CN201880073296.6A
Authority: CN
Inventors: 相良雅之; 富尾悠索; 小薄孝裕; 鹈川裕介
Original assignee: Nippon Steel and Sumitomo Metal Corp
Current assignee: Nippon Steel Corp
Priority date: 2017-11-15
Filing date: 2018-11-14
Publication date: 2020-06-26
Also published as: BR112020009434A2; CA3080706A1; EP3712289A4; KR20200080312A; US20200332378A1; JPWO2019098233A1; WO2019098233A1; EP3712289A1

Abstract

The disclosed duplex stainless steel has a chemical composition and a microstructure, wherein the chemical composition comprises, in mass%, more than 27.00% and not more than 29.00% of Cr, 2.50 to 3.50% of Mo, 5.00 to 8.00% of Ni, 4.00 to 6.00% of W, 0.01% and not more than 0.10% of Cu, more than 0.400% and not more than 0.600% of N, 0.030% or less of C, 1.00% or less of Si, 1.00% or less of Mn, 0.040% or less of sol.Al, 0.50% or less of V, 0.010% or less of O, 0.030% or less of P, 0.020% or less of S, and the balance Fe and impurities, and satisfies the formula (1), wherein the microstructure comprises, in terms of 35 to 65% by volume, the balance of ferrite and austenite, the balance of Cu, 0.350.7% or more, and the balance of Mo and the balance of Cu, the area of Cu, the balance being 0.355 + 365.3.7.26% or more, and the balance of Mo and the area of Cu being 0.365 + 3.3.3.3.3.3.3.3.3.3.3.3.3.3% or less.

Description

Duplex stainless steel and method for producing duplex stainless steel

Technical Field

The present application relates to duplex stainless steel and a method of manufacturing duplex stainless steel.

Background

Duplex stainless steels having a duplex structure of a ferrite phase and an austenite phase are known to have excellent corrosion resistance. Duplex stainless steel is excellent in corrosion resistance (hereinafter referred to as "pitting corrosion resistance") particularly against pitting corrosion and/or crevice corrosion which are problematic in chloride-containing aqueous solutions. Therefore, duplex stainless steel is widely used in a humid environment containing chlorides such as seawater. Duplex stainless steel is used, for example, for flow tubes, umbilical lines, and heat exchangers in humid environments containing chloride.

In recent years, the corrosion conditions under the use environment of duplex stainless steel have become more and more severe. Therefore, the duplex stainless steel is required to have more excellent pitting corrosion resistance. In order to further improve pitting corrosion resistance of duplex stainless steel, various techniques have been proposed.

International publication No. 2013/191208 (patent document 1) discloses a duplex stainless steel containing Ni: 3-8%, Cr: 20-35%, Mo: 0.01-4.0%, N: 0.05 to 0.60%, and further contains one or more compounds selected from the group consisting of Re: 2.0% or less, Ga: 2.0% or less, and Ge: 2.0% or less and 1 or more. In patent document 1, by adding Re, Ga, or Ge to duplex stainless steel, the critical potential at which pitting occurs (pitting potential) is increased, and pitting corrosion resistance and crevice corrosion resistance are improved.

International publication No. 2010/082395 (patent document 2) discloses a method for producing a duplex stainless steel pipe, which comprises a duplex stainless steel material containing, by mass%, 20 to 35% of Cr, 3 to 10% of Ni, 0 to 6% of Mo, 0 to 6% of W, 0 to 3% of Cu, and 0.15 to 0.60% of N, and is subjected to hot working or further solution heat treatment to produce a cold working blank, and then is subjected to cold rolling, wherein the method for producing a duplex stainless steel pipe of patent document 2 is characterized In that the cold rolling is performed within a range of 10 to 80% of a reduction ratio of a cross section In a final cold rolling step, and a stainless steel pipe having a minimum yield strength of 758.3 to 965.2MPa (In [ { In mys) -In (14.5 × Cr +48.3 × Mo +20.7 × W +6.9 × N) }/0.195]) is produced, and thereby, for example, the duplex stainless steel pipe can be used In an oil well, a high corrosion resistance environment, and also has a high carbon dioxide gas corrosion resistance.

Jp 2007 & 84837 a (patent document 3) discloses a method for producing a chromium-containing alloy containing Cr in mass%: 20-30%, Ni: 1-11%, Cu: 0.05 to 3.0%, Nd: 0.005-0.5%, N: 0.1 to 0.5% and Mo: 0.5-6 and W: 1 to 10 or both of them. In patent document 3, the hot workability of the duplex stainless steel is improved by including Nd.

Japanese patent application laid-open No. 2005-520934 (patent document 4) discloses a method for producing a chromium-containing alloy containing Cr in% by weight: 21.0% -38.0%, Ni: 3.0% -12.0%, Mo: 1.5% -6.5%, W: 0-6.5%, N: 0.2% -0.7%, Ba: 0.0001-0.6% of super duplex stainless steel satisfying the pitting corrosion resistance equivalent index PREW of 40-67 is disclosed. Patent document 4 describes: thus, a super duplex stainless steel is obtained which is excellent in corrosion resistance, embrittlement resistance, castability and hot workability, while suppressing the formation of brittle intermetallic phases such as sigma (σ) phase and happy (χ) phase.

Documents of the prior art

Patent document

Patent document 1: international publication No. 2013/191208

Patent document 2: international publication No. 2010/082395

Patent document 3: japanese patent laid-open publication No. 2007-84837

Patent document 4: japanese Kohyo publication No. 2005-520934

Disclosure of Invention

Problems to be solved by the invention

As described above, in recent years, duplex stainless steel having more excellent pitting corrosion resistance has been demanded. Therefore, a duplex stainless steel exhibiting excellent pitting corrosion resistance can 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 manufacturing the duplex stainless steel.

Means for solving the problems

Duplex stainless steels based on the present disclosure contain the following chemical composition and microstructure: the chemical composition is, in mass%, Cr: more than 27.00% and 29.00% or less, Mo: 2.50-3.50%, Ni: 5.00-8.00%, W: 4.00-6.00%, Cu: 0.01% or more and less than 0.10%, N: greater than 0.400% and 0.600% or less, 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: 0-0.0040%, B: 0-0.0040%, and the balance: fe and impurities, and satisfies formula (1); the microstructure consists of 35-65 vol% of a ferrite phase and the balance of an austenite phase. In the duplex stainless steel according to the present disclosure, 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)

Wherein the content (mass%) of each element is substituted into each element symbol in the formula (1).

The method for producing a duplex stainless steel according to the present disclosure includes the steps of: a preparation process; a hot working procedure; a cooling step; and a solution heat treatment step. In the preparation step, a billet having the above-described chemical composition is prepared. In the hot working step, the billet is hot worked at 850 ℃ or higher. In the cooling step, the hot worked billet is cooled at a temperature of 5 ℃/sec or more. In the solution heat treatment step, the cooled billet is subjected to solution heat treatment at 1070 ℃ or higher.

ADVANTAGEOUS EFFECTS OF INVENTION

The duplex stainless steel based on the present disclosure has excellent pitting corrosion resistance. The method for manufacturing a duplex stainless steel according to the present disclosure can manufacture the duplex stainless steel described above.

Detailed Description

The present inventors have conducted investigations and studies on a technique for improving pitting corrosion resistance of a duplex stainless steel. As a result, the following findings were obtained.

Cr, Mo, and Cu are known to be effective for improving pitting corrosion resistance of duplex stainless steel. Among Cr, Mo, and Cu, the mechanism by which Cr and Mo improve pitting corrosion resistance of duplex stainless steel is considered as follows. Cr becomes a main component of a passive film on the surface of the duplex stainless steel as an oxide. The passivating coating hinders contact of corrosion factors with the surface of the duplex stainless steel. As a result, the pitting corrosion resistance of the duplex stainless steel having the passive film formed on the surface thereof is improved. Mo is contained in the passivation film, and the pitting corrosion resistance of the passivation film is further improved.

On the other hand, the mechanism by which Cu improves pitting corrosion resistance of duplex stainless steel among Cr, Mo, and Cu is considered as follows. It is considered that until the pitting corrosion is generated, there are 2 processes as follows. The initial process is the occurrence of pitting (initial stage). The next process is the development of pitting corrosion (development stage). Conventionally, Cu is considered to have an effect of suppressing the progress of pitting corrosion. In particular, 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, inhibiting dissolution of the duplex stainless steel. From this, Cu is considered to inhibit the development of pitting corrosion of the duplex stainless steel.

Based on the above mechanism, Cr, Mo, and Cu are 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 actively contained for the purpose of improving pitting corrosion resistance. However, as a result of the studies by the present inventors, the following findings, which have not been known in the past, were obtained. Specifically, the present inventors found that: among Cr, Mo, and Cu, Cu sometimes deteriorates pitting corrosion resistance when pitting corrosion occurs (initial stage).

Table 1 shows the chemical compositions of test pieces of test nos. 2 and 5 and the pitting potentials as indicators of pitting corrosion resistance in examples described later. The chemical compositions in table 1 are summarized in table 3 below as 2 lines, excepting the chemical compositions of steel grades B and E corresponding to test numbers 2 and 5. The chemical composition of table 1 is described in mass%, with the balance being Fe and impurities. The pitting potentials in table 1 are indicated by extracting the pitting potentials of the corresponding test numbers from table 4 described later.

[ Table 1]

TABLE 1

Referring to table 1, the Cu content of the test piece of test No. 2 was higher than that 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 those of the test piece of test No. 5. Therefore, based on conventional findings, it is expected that the test piece of test No. 2 having high contents of Cr, Mo, and Cu has superior pitting corrosion resistance to the test piece of test No. 5. However, the pitting potential of the test piece of test No. 2 as an index of pitting resistance was 71mvvs.

That is, the test piece of test No. 2, which is expected from conventional findings to have superior pitting corrosion resistance compared to the test piece of test No. 5, is rather lower in pitting corrosion resistance than the test piece of test No. 5. Therefore, the present inventors paid attention to the microstructures of the test pieces of test numbers 2 and 5 and examined the microstructures in more detail. As a result, it was found that the area ratio of Cu precipitated in the ferrite phase (referred to as Cu area ratio in the ferrite phase) was higher in the test piece of test No. 2 than in the test piece of test No. 5.

Therefore, the present inventors have further investigated and studied the influence of Cu precipitated in a ferrite phase on the pitting corrosion resistance of the duplex stainless steel in detail. Table 2 shows the chemical compositions of test pieces of test Nos. 3 and 6 in examples described later; cu area fraction in ferrite phase; and a table of pitting potential as an index of pitting corrosion resistance. The chemical composition in table 2 is a chemical composition described in 2 lines extracted from table 3 below for the chemical composition of steel type C corresponding to test nos. 3 and 6. The chemical compositions in table 2 are described in mass%, with the balance being Fe and impurities. The Cu area ratio in the ferrite phase in table 2 is a Cu area ratio described by extracting the Cu area ratio in the ferrite phase of the corresponding test number from table 4 described later. The pitting potentials in table 2 are indicated by extracting the pitting potentials of the corresponding test numbers from table 4 described later.

[ Table 2]

TABLE 2

Referring to table 2, the chemical compositions of the test piece of test No. 3 and the test piece of test No. 6 were the same. On the other hand, the Cu area ratio in the ferrite phase of the test piece of test No. 6 was lower than that of the test piece of test No. 3. As a result, the pitting potential of the test piece of test No. 6 was 204mvvs. sce, which was higher than the pitting potential of the test piece of test No. 3, which was-12 mvvs. sce. That is, in the test piece of test No. 6, the precipitation of Cu in the ferrite phase was reduced, and as a result, the pitting corrosion resistance was superior to that of the test piece of test No. 3.

As described above, it has been conventionally considered that the pitting corrosion resistance is high when the contents of Cr, Mo and Cu are increased. However, the present inventors have found for the first time that Cu rather lowers pitting corrosion resistance among Cr, Mo and Cu. The present inventors have further found that the pitting corrosion resistance can be improved by reducing the amount of Cu deposited in the ferrite phase.

The detailed reason why Cu precipitated in the ferrite phase lowers the pitting corrosion resistance of the duplex stainless steel is not clear. However, the present inventors considered the following. Cu precipitated in the ferrite phase may inhibit the uniform formation of the passivation film. Therefore, when the amount of Cu precipitated in the ferrite phase is large, there is a possibility that the effect of suppressing the contact of the corrosion factor with the surface of the duplex stainless steel by the passivation film is reduced. As a result, it is considered that pitting corrosion occurs on the surface of the duplex stainless steel.

The duplex stainless steel obtained by the present embodiment based on the above findings contains the following chemical composition and microstructure: the chemical composition is, in mass%, Cr: more than 27.00% and 29.00% or less, Mo: 2.50-3.50%, Ni: 5.00-8.00%, W: 4.00-6.00%, Cu: 0.01% or more and less than 0.10%, N: greater than 0.400% and 0.600% or less, 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: 0-0.0040%, B: 0-0.0040%, and the balance: fe and impurities, and satisfies formula (1); the microstructure consists of 35-65 vol% of a ferrite phase and the balance of an austenite phase. The duplex stainless steel according to the present embodiment has an area ratio of Cu precipitated in a ferrite phase of 0.5% or less.

Cr+4.0×Mo+2.0×W+20×N-5×ln(Cu)≥65.2···(1)

The duplex stainless steel according to the present embodiment has the above chemical composition and the above microstructure, and further has an area ratio of Cu in a ferrite phase of 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 contains, in mass%, a chemical component selected from the group consisting of Ca: 0.0001-0.0040%, Mg: 0.0001-0.0040%, and B: 0.0001-0.0040% of 1 or more than 2 of the group.

In this case, the duplex stainless steel according to the present embodiment has high hot workability.

The method for producing a duplex stainless steel according to the present embodiment includes the steps of: a preparation process; a hot working procedure; a cooling step; and a solution heat treatment step. In the preparation step, a billet having the above-described chemical composition is prepared. In the hot working step, the billet is hot worked at 850 ℃ or higher. In the cooling step, the hot worked billet is cooled at a temperature of 5 ℃/sec or more. In the solution heat treatment step, the cooled billet is subjected to solution heat treatment at 1070 ℃ or higher.

The duplex stainless steel produced by the production method according to the present embodiment has the above chemical composition and the above microstructure, and further has an area ratio of Cu in the ferrite phase of 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, the duplex stainless steel according to the present embodiment will be described in detail.

[ chemical composition ]

The chemical composition of the duplex stainless steel according to the present embodiment contains the following elements. Unless otherwise specified, the% of the element represents mass%.

[ essential elements ]

The chemical composition of the duplex stainless steel according to the present embodiment must contain the following elements.

Cr: more than 27.00% and less than 29.00%

Chromium (Cr) forms a passive film on the surface of the duplex stainless steel in the form of an oxide. The passivating coating hinders contact of corrosion factors with the surface of the duplex stainless steel. As a result, the occurrence of pitting corrosion in the duplex stainless steel is suppressed. Cr is also an essential element for obtaining the ferrite structure of the duplex stainless steel. By obtaining a ferrite structure sufficiently, 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 is lowered. Therefore, the Cr content is more than 27.00% and 29.00% or less. The lower limit of the Cr content is preferably 27.50%, more preferably 28.00%. The preferable upper limit of the Cr content is 28.50%.

Mo：2.50～3.50％

Molybdenum (Mo) is contained in the passivation film, and the corrosion resistance of the passivation film is further improved. As a result, pitting corrosion resistance of the duplex stainless steel is improved. If the Mo content is too low, the effect cannot be obtained. On the other hand, if the Mo content is too high, workability in the case of assembling a steel pipe or the like made of duplex stainless steel is lowered. Therefore, the Mo content is 2.50 to 3.50%. The preferable lower limit of the Mo content is 2.80%, more preferably 3.00%. The preferable upper limit of the Mo content is 3.30%.

Ni：5.00～8.00％

Nickel (Ni) is an austenite stabilizing element and is an essential element for obtaining a dual phase structure of ferrite and austenite. If the Ni content is too low, the 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 preferred upper limit of the Ni content is 7.50%.

W：4.00～6.00％

Tungsten (W) is contained in the passivation film in the same manner as Mo, and the corrosion resistance of the passivation film is further improved. As a result, the occurrence of pitting corrosion in the duplex stainless steel is suppressed. If the W content is too low, the effect cannot be obtained. On the other hand, if the W content is too high, σ -compatibility is liable to precipitate, and the toughness is lowered. Therefore, the W content is 4.00 to 6.00%. A preferred lower limit of the W content is 4.50%. The preferable upper limit of the W content is 5.50%.

Cu: more than 0.01 percent and less than 0.10 percent

Copper (Cu) is an element effective for suppressing the development (development stage) of pitting corrosion. If the Cu content is too low, the effect cannot be obtained. On the other hand, among Cr, Mo, and Cu, Cu decreases pitting corrosion resistance when pitting corrosion occurs (initial stage). Therefore, the duplex stainless steel of the present embodiment has a reduced Cu content as compared with conventional duplex stainless steels. As a result, precipitation of Cu in the ferrite phase is suppressed, and occurrence of pitting corrosion of the duplex stainless steel is suppressed (initial stage). If the Cu content is too high, the Cu area ratio in the ferrite phase becomes too high. At this time, the pitting corrosion resistance of the duplex stainless steel is lowered. Therefore, the Cu content is 0.01% or more and less than 0.10%. The preferable upper limit of the Cu content is 0.07%, more preferably 0.05%.

N: more than 0.400% and less than 0.600%

Nitrogen (N) is an austenite stabilizing element and is an essential element for obtaining a ferrite-austenite duplex structure. N further improves pitting corrosion resistance of the 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 are reduced. Therefore, the N content is more than 0.400% and 0.600% or less. A preferred lower limit of the N content is 0.420%. The preferable upper limit of the N content is 0.500%.

C: less than 0.030%

Inevitably containing carbon (C). That is, the C content is more than 0%. C forms Cr carbides at grain boundaries, increasing the 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%. The C content is preferably as low as possible. However, the extreme reduction in C content greatly increases the manufacturing cost. Therefore, in view of industrial production, the preferable lower limit of the C content is 0.001%, more preferably 0.005%.

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 is lowered. Therefore, the Si content is 1.00% or less. The preferable upper limit of the Si content is 0.80%, more preferably 0.70%. The lower limit of the Si content is not particularly limited, and 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 the duplex stainless steel is lowered. 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, and is, for example, 0.20%.

Al: less than 0.040%

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 the duplex stainless steel is lowered. Therefore, the Al content is 0.040% or less. The preferable upper limit of the Al content is 0.030%, and more preferably 0.025%. The lower limit of the Al content is not particularly limited, and is, for example, 0.005%. In the present embodiment, the Al content refers to an acid-soluble Al (sol. Al) content.

V: less than 0.50%

Vanadium (V) is inevitably contained. That is, the V content is more than 0%. If the V content is too high, the ferrite phase excessively increases, 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%, more preferably 0.30%. The lower limit of the V content is not particularly limited, and is, for example, 0.05%.

O: 0.010% or less

Oxygen (O) is an impurity. I.e., an O content of greater than 0%. O decreases the hot workability of the 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 in the O content greatly increases the manufacturing cost. Therefore, in view of industrial production, the preferable lower limit of the O content is 0.0001%, more preferably 0.0005%.

P: less than 0.030%

Phosphorus (P) is an impurity. I.e. a P content of more than 0%. P decreases pitting corrosion resistance and toughness of the duplex stainless steel. Therefore, the P content is 0.030% or less. The upper limit of the P content is preferably 0.025%, more preferably 0.020%. The P content is preferably as low as possible. However, the extreme decrease in P content greatly increases the manufacturing cost. Therefore, in view of industrial production, the preferable lower limit of the P content is 0.001%, more preferably 0.005%.

S: 0.020% or less

Sulfur (S) is an impurity. I.e., the S content is greater than 0%. S reduces the hot workability of the duplex stainless steel. Therefore, the S content is 0.020% or less. The upper limit of the S content is preferably 0.010%, more preferably 0.005%, and still more preferably 0.003%. The S content is preferably as low as possible. However, the extreme reduction of the S content greatly increases the manufacturing cost. Therefore, in view of industrial production, 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 the present embodiment is Fe and impurities. Here, the impurities in the chemical composition mean substances mixed from ores, scraps, production environments, and the like as raw materials in the industrial production of the duplex stainless steel, and are allowable within a range not adversely affecting the duplex stainless steel according to the present embodiment.

[ for any element ]

The chemical composition of the duplex stainless steel according to the present embodiment may optionally contain the following elements.

Ca：0～0.0040％

Calcium (Ca) is an arbitrary element, and may or may not be contained. That is, the Ca content may be 0%. When contained, Ca improves hot workability of the duplex stainless steel. This effect can be obtained to some extent by only containing a small amount of Ca. On the other hand, if the Ca content is too high, coarse oxides are formed, and the hot workability of the duplex stainless steel is lowered. Therefore, the Ca content is 0 to 0.0040%. The lower limit of the Ca content is preferably 0.0001%, more preferably 0.0005%, and still more preferably 0.0010%. The preferable upper limit of the Ca content is 0.0030%.

Mg：0～0.0040％

Magnesium (Mg) is an arbitrary element, and may or may not be contained. That is, the Mg content may be 0%. When contained, Mg improves the hot workability of the duplex stainless steel as in Ca. This effect can be obtained to some extent by only containing a small amount of Mg. On the other hand, if the Mg content is too high, coarse oxides are formed, and the hot workability of the duplex stainless steel is lowered. Therefore, the Mg content is 0 to 0.0040%. The lower limit of the Mg content is preferably 0.0001%, more preferably 0.0005%, and still more preferably 0.0010%. The upper limit of the Ca content is preferably 0.0030%.

B：0～0.0040％

Boron (B) is an arbitrary element and may not be contained. That is, the B content may be 0%. When contained, B improves the hot workability of the duplex stainless steel in the same manner as Ca and Mg. This effect can be obtained to some extent by containing B 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 lower limit of the B content is preferably 0.0001%, more preferably 0.0005%, and still more preferably 0.0010%. The upper limit of the Ca content is preferably 0.0030%.

[ for formula (1) ]

The chemical composition of the duplex stainless steel according to the present embodiment satisfies the contents 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)

The definition is that F1 ═ Cr +4.0 × Mo +2.0 × W +20 × N-5 × ln (cu) and F1 are indices indicating pitting corrosion resistance, and if F1 is less than 65.2, the pitting corrosion resistance of the duplex stainless steel decreases, and therefore, the lower limit of F1 ≧ 65.2 and F1 is preferably 68.0, more preferably 69.0, still more preferably 70.0, and the upper limit of F1 is not particularly limited, for example, 90.0.

[ for the 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 includes 35 to 65 vol% of a ferrite phase and the balance of an austenite phase. If the volume fraction of the ferrite phase (hereinafter referred to as ferrite fraction) is less than 35%, stress corrosion cracking is highly likely to occur due to the use environment. On the other hand, when the volume fraction of the ferrite phase is more than 65%, the toughness of the duplex stainless steel may be lowered. Therefore, the microstructure of the duplex stainless steel according to the present embodiment is composed of 35 to 65 vol% of a ferrite phase and the balance of an austenite phase.

[ method for measuring 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 was collected from duplex stainless steel. When the duplex stainless steel is a steel sheet, a cross section (hereinafter referred to as an observation plane) perpendicular to the sheet width direction of the steel sheet is polished. When the duplex stainless steel is a steel pipe, a cross section (observation surface) including the axial direction and the wall 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 steel bar or the wire rod in the axial direction is polished. Next, the polished observation surface was etched using a mixed solution of aqua regia and glycerin.

The etched observation surface was observed in 10 fields with an optical microscope. The viewing area is, for example, 2000 μm²(magnification 500 times). In each observation field, ferrite can be distinguished from other phases by contrast. Therefore, ferrite in each observation was determined according to the contrast. The area ratio of the ferrite thus determined was measured by a point counting method based on JIS G0555 (2003). The measured area ratio was regarded as equivalent to the volume fraction, which was defined as ferrite fraction (vol%).

[ area ratio of Cu in ferrite phase ]

The area ratio of Cu precipitated in the ferrite phase in the duplex stainless steel according to the present embodiment is 0.5% or less. As described above, it is considered that Cu contained in the duplex stainless steel suppresses the development of pitting corrosion of the duplex stainless steel. Therefore, the duplex stainless steel according to the present embodiment contains 0.01% or more and less than 0.10% of Cu. On the other hand, in duplex stainless steel containing 0.01% or more and less than 0.10% of Cu, metallic Cu may be precipitated in the ferrite phase. As described above, it was found that Cu precipitated in the ferrite phase reduces the effect of suppressing the occurrence of pitting corrosion by the passivation film. That is, metallic Cu precipitated in the ferrite phase lowers pitting corrosion resistance of the duplex stainless steel.

Therefore, the duplex stainless steel according to the present embodiment has a Cu area ratio in the ferrite phase reduced to 0.5% or less. Therefore, occurrence of pitting corrosion of the duplex stainless steel is suppressed. The lower the Cu area ratio in the ferrite phase is, the more preferable. The upper limit of the Cu area ratio in the ferrite phase is preferably 0.3%, and more preferably 0.1%. The lower limit of the Cu area ratio in the ferrite phase is 0.0%.

[ method of measuring Cu area fraction in ferrite phase ]

In the present specification, the area fraction of Cu in the ferrite phase refers to the area fraction of Cu precipitated in the ferrite phase relative 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 by a Transmission Electron Microscope (TEM) was prepared by the FIB-microsampling method. A focused ion beam processing apparatus (manufactured by Hitachi High-Tech Science Corporation, MI4050) was used for the preparation of the thin film samples. Thin film samples for TEM observation were prepared from arbitrary portions of the duplex stainless steel. For the production of the thin film sample, a Mo mesh was used, and a carbon deposited film was used as a surface protective film.

In the TEM observation, a field emission type transmission electron microscope (JEM-2100F, manufactured by Nippon electronics Co., Ltd.) was used. TEM observation was performed with the observation magnification set to 10000 times. The contrast of the ferrite phase and the austenite phase in the field of view is different. Therefore, based on the contrast, the grain boundary is determined. The phase of the region surrounded by each grain boundary is determined by X-Ray Diffraction (XRD). The area of the region identified as the ferrite phase among the regions surrounded by the grain boundaries is determined by image analysis.

For the observation field, elemental analysis by Energy Dispersive X-ray Spectrometry (EDS) was performed to generate an elemental map. Further, the precipitates can be determined by contrast. Therefore, when the precipitates identified by contrast in the ferrite phase identified by XRD are metallic Cu, they can be identified by EDS.

The area of the Cu precipitated in the ferrite phase was determined by image analysis. The total area of the ferrite phase is divided by the total area of Cu precipitated in the ferrite phase. In this manner, the Cu area ratio (%) in the ferrite phase was measured.

The duplex stainless steel according to the present embodiment satisfies both the chemical composition including 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.

[ 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 750MPa or less, cold working can be omitted in the production process. In this case, the manufacturing cost can be reduced. Therefore, the yield strength is preferably 750MPa or less. More preferably, the yield strength is 720MPa or less. The lower limit of the yield strength is not particularly limited, and is, for example, 300 MPa.

[ method for measuring yield Strength ]

In the present specification, the yield strength represents 0.2% yield strength obtained by a method according to JIS Z2241 (2011).

[ shape of Duplex stainless Steel ]

The shape of the duplex stainless steel according to the present embodiment is not particularly limited. For duplex stainless steel, for example, steel pipe, steel plate, steel bar, or wire rod may be used.

[ production method ]

The duplex stainless steel of the present embodiment can be produced, for example, by the following method. The manufacturing method comprises the following steps: a preparation process; a hot working procedure; a cooling step; and a solution heat treatment step.

[ preparation Process ]

In the preparation step, a billet having the above-described chemical composition is prepared. The billet may be a cast slab produced by a continuous casting method (including round billet continuous casting) or a billet produced from a cast slab. Further, the billet may be produced by hot working an ingot produced by an ingot casting method.

[ Hot working Process ]

The prepared billet is loaded into a heating furnace or soaking furnace, for example, heated to 1150-1300 ℃. Next, the heated material is hot worked. The hot working may be hot forging, for example, hot extrusion using a glass lubricant high-speed extrusion method or an Edison tube method, or hot rolling. The heat treatment may be performed 1 time or more.

The heated billet is hot worked at a temperature of 850 ℃ or higher. More specifically, the surface temperature of the steel material at the time of completion of hot working is 850 ℃ or higher. When the surface temperature of the steel material at the time of completion of hot working is less than 850 ℃, 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 later. At this time, the pitting corrosion resistance of the duplex stainless steel is lowered. Therefore, the surface temperature of the steel material at the time of completion of hot working is 850 ℃ or higher. When hot working is performed a plurality of times, the surface temperature of the steel material at the end of the final hot working is 850 ℃ or higher. Therefore, Cu deposition in the ferrite phase can be suppressed at the time of termination of hot working. The upper limit of the surface temperature of the steel material at the time of completion of hot working is not particularly limited, and is, for example, 1300 ℃. The time of termination of the hot working means within 3 seconds after the termination of the hot working.

[ Cooling Process ]

Subsequently, the hot worked billet is cooled at 5 ℃/sec or more. In the vicinity of 850 ℃, 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 later. At this time, the pitting corrosion resistance of the duplex stainless steel is lowered. Therefore, the cooling rate after hot working is 5 ℃/sec or more. Here, when the hot working is performed a plurality of times, the hot working is performed after the final hot working. That is, in the present embodiment, the blank after the final hot working is cooled at 5 ℃/sec or more. The upper limit of the cooling rate is not particularly limited. Examples of cooling methods are air cooling, water cooling, oil cooling, and the like.

[ solution Heat treatment Process ]

Then, the cooled billet is subjected to solution heat treatment at 1070 ℃ or higher. The Cu precipitated in the ferrite phase is dissolved by the solution heat treatment. The billet in which the precipitation of Cu in the ferrite phase at the time of termination of hot working and after cooling is sufficiently suppressed can be subjected to solution heat treatment at 1070 ℃ or higher to make the Cu area ratio in the ferrite phase 0.5% or less. The upper limit of the solution heat treatment temperature is not particularly limited, and is, for example, 1150 ℃. 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.

Through the above steps, the duplex stainless steel according to the present embodiment can be manufactured. In the present embodiment, since the manufacturing cost is high, it is preferable not to perform cold working.

Examples

An alloy having a chemical composition shown in table 3 was melted in a vacuum melting furnace of 50kg, and the obtained ingot was heated at 1200 ℃. 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 sheets were subjected to solution treatment at the solution temperature (c) shown in table 4 to obtain test pieces of each test number.

[ Table 3]

[ Table 4]

TABLE 4

[ ferrite fraction measurement test ]

For each test piece of test No., the ferrite fraction (% by volume) was measured by the method described above. The results are shown in Table 4. The balance of the microstructure of the test piece of each test number was an austenite phase.

[ measurement test for Cu area ratio in ferrite phase ]

The Cu area ratio (%) in the ferrite phase was measured for each test piece of each test number by the method described above. The results are shown in Table 4.

[ pitting potential measuring test ]

For after solution treatmentThe pitting potential of the test piece of each test number was measured. First, a test piece was machined to obtain a test piece having a diameter of 15mm and a thickness of 2 mm. The obtained test piece was used to measure the pitting potential at 80 ℃ in a 25% nacaq. The conditions other than the test temperature and the NaCl concentration were performed based on JIS G0577 (2014). Table 4 shows pitting potentials Vc 'of the test pieces of the test numbers'₁₀₀The measurement result of (1).

[ tensile test ]

The 0.2% yield strength of each test piece was determined according to JIS Z2241 (2011). The results are shown in Table 4.

[ evaluation results ]

Referring to tables 3 and 4, the chemical compositions of the test pieces of test numbers 5 to 8 were appropriate, and the production conditions were appropriate. Therefore, the test pieces of test nos. 5 to 8 were duplex stainless steels having a ferrite fraction of 35 to 65% by volume and the balance being an austenite phase, and further had a Cu area ratio in the ferrite phase of 0.5% or less. As a result, the steel sheets of test nos. 5 to 8 exhibited pitting potential (mvvs. sce) of 100 or more and excellent pitting corrosion resistance.

On the other hand, in the test piece of test No. 1, the Cu content was excessively high. In the test piece of test No. 1, F1 was 59.8, and formula (1) was not satisfied. Therefore, the test piece of test No. 1 had a Cu area ratio in the ferrite phase of 0.8%. As a result, the test piece of test No. 1 had a pitting potential (mvvs. sce) of-60, and did not exhibit excellent pitting corrosion resistance.

In the test piece of test No. 2, the Cu content was excessively 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 test piece of test No. 2 had a pitting potential (mvvs. sce) of 71, and did not exhibit excellent pitting corrosion resistance.

In the test piece of test No. 3, the solid solution temperature was 1050 ℃ and 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 test piece of test No. 3 had a pitting potential (mvvs. sce) of-12, and did not exhibit excellent pitting corrosion resistance.

In the test piece of test No. 4, the contents of the respective elements were proper, but F1 was 65.1, and formula (1) was not satisfied. As a result, the test piece of test No. 4 had a pitting potential (mvvs. sce) of 85, and did not exhibit excellent pitting corrosion resistance.

In the test piece of test No. 9, the W content was too low. As a result, the test piece of test No. 9 had a pitting potential (mvvs. sce) of 70, and did not exhibit excellent pitting corrosion resistance.

In the test piece of test No. 10, the Mo content was too low. As a result, the test piece of test No. 10 had a pitting potential (mvvs. sce) of 76, and did not exhibit excellent pitting corrosion resistance.

In the test piece of test No. 11, the Cr content was too low. As a result, the test piece of test No. 11 had a pitting potential (mvvs. sce) of 81, and did not exhibit excellent pitting corrosion resistance.

In the test piece of test No. 12, the temperature at the termination of hot rolling was 840 ℃ and 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 test piece of test No. 12 had a pitting potential (mvvs. sce) of-150, and did not exhibit excellent pitting corrosion resistance.

The cooling rate of the test piece of test No. 13 after the completion of hot rolling was 3 ℃/sec, 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 test piece of test No. 13 had a pitting potential (mvvs. sce) of-71, and did not exhibit excellent pitting corrosion resistance.

The embodiments of the present application have been described above. However, the above embodiments are merely examples for carrying out the present application. Therefore, the present invention is not limited to the above embodiments, and the above embodiments may be modified as appropriate without departing from the scope of the present invention.

Claims

1. A duplex stainless steel comprising a chemical composition and a microstructure as follows:

the chemical composition is calculated by mass percent

Cr: more than 27.00% and not more than 29.00%,

Mo：2.50～3.50％、

Ni：5.00～8.00％、

W：4.00～6.00％、

Cu: more than 0.01% and less than 0.10%,

N: more than 0.400% and less than 0.600%,

C: less than 0.030%,

Si: less than 1.00 percent,

Mn: less than 1.00 percent,

Al: less than 0.040%,

V: less than 0.50 percent of,

O: less than 0.010%,

P: less than 0.030%,

S: less than 0.020%,

Ca：0～0.0040％、

Mg：0～0.0040％、

B: 0 to 0.0040%, and

and the balance: fe and impurities, and

satisfies formula (1);

the microstructure is composed of 35-65 vol% of ferrite phase and the balance of austenite phase,

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)

wherein the content of each element in mass% is substituted at each element symbol in the formula (1).

2. Duplex stainless steel according to claim 1, wherein the chemical composition contains in mass% is selected from the group consisting of

Ca：0.0001～0.0040％、

Mg: 0.0001 to 0.0040%, and

b: 0.0001-0.0040% of 1 or more than 2 of the group.

3. A method for producing a duplex stainless steel, comprising the steps of:

a step of preparing a billet having the following chemical composition:

in mass%)

Cr: more than 27.00% and not more than 29.00%,

Mo：2.50～3.50％、

Ni：5.00～8.00％、

W：4.00～6.00％、

Cu: more than 0.01% and less than 0.10%,

N: more than 0.400% and less than 0.600%,

C: less than 0.030%,

Si: less than 1.00 percent,

Mn: less than 1.00 percent,

Al: less than 0.040%,

V: less than 0.50 percent of,

O: less than 0.010%,

P: less than 0.030%,

S: less than 0.020%,

Ca：0～0.0040％、

Mg：0～0.0040％、

B: 0 to 0.0040%, and

and the balance: fe and impurities, and

satisfies formula (1);

a step of hot working the blank at 850 ℃ or higher;

cooling the hot worked billet at a temperature of 5 ℃/sec or more; and the number of the first and second groups,

a step of subjecting the cooled billet to solution heat treatment at 1070 ℃ or higher,

Cr+4.0×Mo+2.0×W+20×N-5×ln(Cu)≥65.2· · · (1)