JP7248893B2 - duplex stainless steel - Google Patents

Description

TECHNICAL FIELD The present invention relates to duplex stainless steel, and more particularly to duplex stainless steel having excellent pitting resistance.

Duplex stainless steel, which has a two-phase structure of ferrite phase and austenite phase, is known to have excellent corrosion resistance. Duplex stainless steel is particularly excellent in corrosion resistance to pitting corrosion and/or crevice corrosion (hereinafter referred to as "pitting corrosion resistance"), which is a problem in aqueous solutions containing chlorides. Duplex stainless steels are therefore widely used in wet environments containing chlorides such as seawater. Duplex stainless steels are used, for example, in flowline pipes, umbilical tubes and heat exchangers in wet environments containing chlorides.

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

International Publication No. 2013/191208 (Patent Document 1) is mass %, Ni: 3 to 8%, Cr: 20 to 35%, Mo: 0.01 to 4.0%, N: 0.05 to 0 .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 including Re, Ga, or Ge in the duplex stainless steel, the critical potential at which pitting corrosion occurs (pitting corrosion potential) is raised, and pitting corrosion resistance and crevice corrosion resistance are enhanced. there is

International Publication No. 2010/082395 (Patent Document 2) is mass %, Cr: 20 to 35%, Ni: 3 to 10%, Mo: 0 to 6%, W: 0 to 6%, Cu: 0 to Duplex stainless steel material containing 3% N: 0.15 to 0.60% is hot worked or further solution heat treated to prepare a mother tube for cold working, and then cold rolled to 2 A method of manufacturing a phase stainless steel pipe is disclosed. In the method for manufacturing a duplex stainless steel pipe of Patent Document 2, the workability Rd(=exp[{In(MYS)−In(14.5×Cr+48.3×Mo+20. 7×W+6.9×N)}/0.195]) is cold rolled within the range of 10 to 80%, and a duplex stainless steel pipe having a minimum yield strength of 758.3 to 965.2 MPa is produced. characterized by being As described in Patent Document 2, this makes it possible to obtain a duplex stainless steel pipe that can be used, for example, in oil wells and gas wells, and exhibits excellent corrosion resistance even in carbon dioxide gas corrosive environments and stress corrosive environments, as well as having high strength. ing.

WO2013/191208 WO2010/082395

As described above, duplex stainless steels are required to have even better pitting corrosion resistance. Therefore, a duplex stainless steel exhibiting even better pitting corrosion resistance may be obtained by means other than the techniques described in Patent Documents 1 and 2.

SUMMARY OF THE DISCLOSURE It is an object of the present disclosure to provide a duplex stainless steel with excellent pitting resistance.

The duplex stainless steel according to the present disclosure contains Cr: 27.0 to 29.0%, Mo: 2.50 to 4.00%, Ni: 5.00 to 8.00%, N: 0.00%, in mass %. Over 400% to 0.600%, Ag: 0.01 to 0.50%, C: 0.030% or less, Si: 1.00% or less, Mn: 2.00% or less, sol. Al: 0.040% or less, V: 0.50% or less, O: 0.010% or less, P: 0.030% or less, S: 0.0200% or less, W: 0 to 6.00%, Cu : 0 to less than 0.10%, and the balance has a chemical composition consisting of Fe and impurities.

Duplex stainless steels according to the present disclosure have excellent pitting corrosion resistance.

The present inventors have investigated and studied techniques for improving the pitting corrosion resistance of duplex stainless steel. As a result, the following findings were obtained.

Cr and Mo have been known as elements that improve the pitting corrosion resistance of duplex stainless steel. Specifically, the mechanism by which Cr and Mo enhance the pitting corrosion resistance of duplex stainless steel is considered as follows.

Cr, as an oxide, is the main component of the passivation film on the surface of duplex stainless steel. The passivation coating inhibits contact between corrosive agents and the surface of the duplex stainless steel. As a result, the duplex stainless steel with the passivation film formed on the surface has enhanced pitting corrosion resistance. Mo is contained in the passive coating to further enhance the pitting corrosion resistance of the passive coating.

Therefore, the inventors of the present invention, in mass %, Cr 27.0 to 29.0%, Mo 2.50 to 4.00%, Ni 5.00 to 8.00%, and N 0.00%. Focusing attention on duplex stainless steel containing more than 400% to 0.600%, various studies were made on techniques for improving pitting corrosion resistance.

Specifically, various kinds of duplex stainless steel containing other elements in addition to the chemical composition described above were produced, and their pitting corrosion resistance was investigated and studied in detail. As a result, the inventors of the present invention found that the duplex stainless steel having the chemical composition described above may have improved pitting corrosion resistance by containing silver (Ag), which has not received attention so far. rice field. That is, as a result of detailed studies by the present inventors, it has become clear that the pitting corrosion resistance of the duplex stainless steel is enhanced if 0.01% or more of Ag is contained in addition to the chemical composition described above.

The detailed mechanism by which 0.01% or more Ag content enhances pitting corrosion resistance in the duplex stainless steel having the chemical composition described above has not been elucidated. However, the inventors consider as follows. It is believed that the following two steps exist before pitting corrosion occurs. The first step is the initiation of pitting corrosion (initial stage). The next step is the progress of pitting corrosion (advancement stage).

In acidic solutions, active sites with high dissolution rates are formed on the surface of duplex stainless steel. Ag may form a solid solution in steel and coat its active sites. In this case, Ag dissolved in the steel can suppress dissolution of the duplex stainless steel. It is believed that Ag suppresses the progress of pitting corrosion in the duplex stainless steel.

It is proved by examples described later that the pitting corrosion resistance of the duplex stainless steel having the chemical composition described above is enhanced by containing 0.01% or more of Ag. That is, even if Ag increases the pitting corrosion resistance of the duplex stainless steel by a mechanism different from the above mechanism considered by the present inventors, Ag is effective for the pitting corrosion resistance of the duplex stainless steel. That has been proven.

Therefore, the duplex stainless steel according to this embodiment further contains 0.01 to 0.50% Ag in addition to the chemical composition described above. As a result, the duplex stainless steel according to this embodiment can obtain excellent pitting corrosion resistance.

The duplex stainless steel according to the present embodiment completed based on the above knowledge has, in mass%, Cr: 27.0 to 29.0%, Mo: 2.50 to 4.00%, Ni: 5.00 to 8 .00%, N: more than 0.400% to 0.600%, Ag: 0.01 to 0.50%, C: 0.030% or less, Si: 1.00% or less, Mn: 2.00% Below, sol. Al: 0.040% or less, V: 0.50% or less, O: 0.010% or less, P: 0.030% or less, S: 0.0200% or less, W: 0 to 6.00%, Cu : 0 to less than 0.10%, and the balance has a chemical composition consisting of Fe and impurities.

The duplex stainless steel according to this embodiment has the chemical composition described above. As a result, the duplex stainless steel according to this embodiment has excellent pitting corrosion resistance.

Preferably, the above chemical composition contains W: 0.10 to 6.00% by mass.

Preferably, the above chemical composition contains Cu: 0.01 to less than 0.10% by mass.

In this case, the pitting corrosion resistance of the duplex stainless steel according to this embodiment is further enhanced.

The duplex stainless steel according to this embodiment will be described in detail below.

[Chemical composition]
The chemical composition of the duplex stainless steel according to this embodiment contains the following elements. In addition, unless otherwise specified, % regarding an element means mass %.

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

Cr: 27.0-29.0%
Chromium (Cr) forms a passive film on the surface of duplex stainless steel as an oxide. The passivation coating inhibits contact between corrosive agents and 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 element necessary to obtain a ferritic structure of duplex stainless steel. Stable pitting corrosion resistance can be obtained by obtaining a sufficient ferrite structure. If the Cr content is too low, these effects cannot be obtained. On the other hand, if the Cr content is too high, the hot workability of the duplex stainless steel deteriorates. Therefore, the Cr content is 27.0-29.0%. The lower limit of the Cr content is preferably over 27.0%, more preferably 27.5%, still more preferably 28.0%. A preferred upper limit for the Cr content is 28.5%.

Mo: 2.50-4.00%
Molybdenum (Mo) is contained in the passive coating to further enhance the corrosion resistance of the passive coating. As a result, the pitting corrosion resistance of the duplex stainless steel is enhanced. If the Mo content is too low, this effect cannot be obtained. On the other hand, if the Mo content is too high, workability in assembling a steel pipe made of duplex stainless steel is lowered. Therefore, the Mo content is 2.50-4.00%. A preferred lower limit for the Mo content is 2.80%, more preferably 3.00%. A preferable upper limit of the Mo content is 3.80%, more preferably 3.60%, still more preferably 3.50%, still more preferably 3.30%.

Ni: 5.00-8.00%
Nickel (Ni) is an austenite stabilizing element, and is an element necessary for obtaining a two-phase structure of ferrite and austenite. If the Ni content is too low, these effects 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 5.00-8.00%. A preferable lower limit of the Ni content is 5.50%, more preferably 6.00%. A preferable upper limit of the Ni content is 7.50%.

N: more than 0.400% to 0.600%
Nitrogen (N) is an austenite stabilizing element, and is an element necessary for obtaining a ferrite-austenite two-phase structure. N also enhances the pitting corrosion resistance of duplex stainless steels. If the N content is too low, these effects cannot be obtained. On the other hand, if the N content is too high, the toughness and hot workability of the duplex stainless steel deteriorate. Therefore, the N content is more than 0.400% to 0.600%. A preferable lower limit of the N content is 0.420%. A preferable upper limit of the N content is 0.500%.

Ag: 0.01-0.50%
Silver (Ag) dissolves in the duplex stainless steel and covers the active sites formed on the surface of the duplex stainless steel in an acidic solution. As a result, the pitting corrosion resistance of the duplex stainless steel is enhanced. If the Ag content is too low, this effect will not be obtained. On the other hand, if the Ag content is too high, the hot workability of the duplex stainless steel deteriorates. Too high an Ag content also reduces the pitting resistance of the duplex stainless steel. Therefore, the Ag content is 0.01-0.50%. The preferred lower limit of the Ag content is 0.02%, more preferably 0.03%, still more preferably 0.04%, still more preferably 0.05%, still more preferably 0.07 %, more preferably 0.08%, more preferably 0.10%. A preferable upper limit of the Ag content is 0.45%, more preferably 0.40%, and still more preferably 0.35%.

C: 0.030% or less Carbon (C) is inevitably contained. That is, the C content is over 0%. C forms Cr carbides at the grain boundaries and increases corrosion susceptibility at the grain boundaries. Therefore, the C content is 0.030% or less. A preferable upper limit of the C content is 0.025%, more preferably 0.020%. The C content is preferably as low as possible. However, a drastic reduction of the C content greatly increases manufacturing costs. Therefore, considering industrial production, the lower limit of the C content is preferably 0.001%, more preferably 0.005%.

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

Mn: 2.00% or less Manganese (Mn) deoxidizes steel. When Mn is used as a deoxidizing agent, the Mn content is greater than 0%. On the other hand, if the Mn content is too high, the hot workability of the duplex stainless steel deteriorates. Therefore, the Mn content is 2.00% or less. A preferable upper limit of the Mn content is 1.75%, more preferably 1.50%, and still more preferably 1.25%. Although the lower limit of the Mn content is not particularly limited, it is, for example, 0.20%.

sol. Al: 0.040% or less Aluminum (Al) deoxidizes steel. When Al is used as a deoxidizing agent, the Al content is greater than 0%. On the other hand, if the Al content is too high, the hot workability of the duplex stainless steel deteriorates. Therefore, the Al content is 0.040% or less. A preferable upper limit of the Al content is 0.030%, more preferably 0.025%. Although the lower limit of the Al content is not particularly limited, it is, for example, 0.005%. In the present embodiment, the Al content refers to acid-soluble Al (sol. Al) content.

V: 0.50% or less Vanadium (V) is inevitably contained. That is, the V content is over 0%. If the V content is too high, the ferrite phase may excessively increase, resulting in deterioration of the toughness and corrosion resistance of the duplex stainless steel. Therefore, the V content is 0.50% or less. A preferable upper limit of the V content is 0.40%, more preferably 0.30%. Although the lower limit of the V content is not particularly limited, it is, for example, 0.05%.

O: 0.010% or less Oxygen (O) is an impurity. That is, the O content is over 0%. O lowers the hot workability of duplex stainless steel. Therefore, the O content is 0.010% or less. A preferable upper limit of the O content is 0.007%, more preferably 0.005%. It is preferable that the O content is as low as possible. However, drastic reduction of O content greatly increases manufacturing cost. Therefore, considering industrial production, the preferred lower limit of the O content is 0.0001%, more preferably 0.0005%.

P: 0.030% or less Phosphorus (P) is an impurity. That is, the P content is over 0%. P reduces the pitting corrosion resistance and toughness of duplex stainless steel. Therefore, the P content is 0.030% or less. A preferable upper limit of the P content is 0.025%, more preferably 0.020%. The lower the P content is, the better. However, drastic reduction of the P content greatly increases manufacturing costs. Therefore, considering industrial production, the preferable lower limit of the P content is 0.001%, more preferably 0.005%.

S: 0.0200% or less Sulfur (S) is an impurity. That is, the S content is over 0%. S reduces the hot workability of duplex stainless steel. Therefore, the S content is 0.0200% or less. A preferable upper limit of the S content is 0.0100%, more preferably 0.0050%, and still more preferably 0.0030%. It is preferable that the S content is as low as possible. However, drastic reduction of the S content greatly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the S content is 0.0001%, more preferably 0.0005%.

The remainder 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 those that are mixed from ore, scrap, or the manufacturing environment as raw materials when industrially producing the duplex stainless steel. It means a permissible range that does not adversely affect steel.

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

W: 0-6.00%
Tungsten (W) is an optional element and may not be contained. That is, the W content may be 0%. When included, W is included in the passive coating in the same manner as Mo and further enhances the corrosion resistance of the passive coating. As a result, the occurrence of pitting corrosion in duplex stainless steel is suppressed. This effect can be obtained to some extent if even a small amount of W is contained. On the other hand, if the W content is too high, the σ phase tends to precipitate and the toughness decreases. Therefore, the W content is 0-6.00%. A preferable lower limit of the W content is more than 0%, more preferably 0.10%, still more preferably 0.30%, still more preferably 0.80%, still more preferably 1.00% is. A preferable upper limit of the W content is 5.90%, more preferably 5.70%, and still more preferably 5.50%.

Cu: 0 to less than 0.10% Copper (Cu) is an optional element and may not be contained. That is, the Cu content may be 0%. When included, Cu dissolves in the duplex stainless steel and covers the active sites formed on the surface of the duplex stainless steel in acidic solutions. As a result, the pitting corrosion resistance of the duplex stainless steel is enhanced. This effect can be obtained to some extent if even a small amount of Cu is contained. On the other hand, if the Cu content is too high, metallic Cu may precipitate in the steel, resulting in a decrease in pitting corrosion resistance. Therefore, the Cu content is between 0 and less than 0.10%. The lower limit of the Cu content is preferably over 0%, more preferably 0.01%, still more preferably over 0.01%, still more preferably 0.02%, still more preferably 0.02%. 03%. A preferable upper limit of the Cu content is 0.09%, more preferably 0.08%, and still more preferably 0.06%.

[About the microstructure]
The microstructure of the duplex stainless steel according to this embodiment consists of ferrite and austenite. The volume fractions of ferrite and austenite are not particularly limited. However, if the volume fraction of the ferrite phase (hereinafter also referred to as the ferrite fraction) or the volume fraction of the austenite phase (hereinafter also referred to as the austenite fraction) is too low, the elements are not properly distributed, and the duplex stainless steel characteristics may not be obtained. Therefore, the ferrite fraction is, for example, less than 35-65%. In this case, the austenite fraction is, for example, greater than 35-65%.

[Measuring method of ferrite fraction]
In this embodiment, when obtaining the ferrite fraction of the duplex stainless steel, it can be obtained by the following method. Specimens for microstructural observation are obtained from duplex stainless steel. If the duplex stainless steel is a steel plate, a cross section (hereinafter referred to as an observation surface) perpendicular to the width direction of the steel plate is polished. If the duplex stainless steel is a steel pipe, the cross section (observation surface) including the axial direction and thickness direction of the steel pipe is polished. If the duplex stainless steel is a steel bar or wire rod, the cross section (observation surface) including the axial direction of the steel bar or wire rod is polished. Next, using a mixture of aqua regia and glycerin, the observation surface after polishing is etched.

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

[Shape of Duplex Stainless Steel]
The shape of the duplex stainless steel according to this embodiment is not particularly limited. 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 this embodiment can be produced, for example, by the following method. The manufacturing method includes a preparation step, a hot working step, and a solution heat treatment step.

[Preparation process]
In the preparation step, a material having the chemical composition described above is prepared. The raw material may be a slab (slab, bloom, and billet) produced by a continuous casting method, or a steel ingot (ingot) produced by an ingot casting method. If necessary, the cast slab or steel ingot may be bloomed to produce a steel slab (billet).

[Hot working process]
In the hot working process, the raw material prepared in the preparation process is hot worked to manufacture a steel material. The hot working may be hot forging, hot extrusion, or hot rolling. The method of hot working is not particularly limited, and a known method may be used.

When performing hot extrusion, for example, the Eugene Sejournet method or the Erhardt pushbench method may be performed. When performing hot rolling, for example, piercing rolling by the Mannesmann method may be performed. Note that the hot working may be performed only once, or may be performed multiple times. For example, the above-described hot extrusion may be performed after the above-described piercing-rolling is performed on the raw material.

[Solution heat treatment step]
In the solution heat treatment step, the steel material produced in the hot working step is subjected to solution heat treatment. The method of the solution heat treatment is not particularly limited, and a well-known method may be used. For example, a steel material is charged into a heat treatment furnace, soaked at a desired temperature, and then quenched.

The soaking temperature (solution heat treatment temperature) and the soaking time (solution heat treatment time) in the solution heat treatment step according to the present embodiment are not particularly limited, and may be carried out under known conditions. The solution heat treatment temperature is, for example, 980-1200.degree. The solution heat treatment time is, for example, 2 to 60 minutes. A rapid cooling method is, for example, water cooling.

It should be noted that, if necessary, cold working or pickling treatment may be performed on the steel material that has been subjected to the solution heat treatment. In this case, the cold working and the pickling treatment may be performed by a well-known method, and are not particularly limited.

Through the steps described above, the duplex stainless steel according to the present embodiment can be manufactured. The method for producing the duplex stainless steel described above is merely an example, and the duplex stainless steel may be produced by other methods.

An alloy having the chemical composition shown in Table 1 was melted in a 50 kg vacuum melting furnace, the resulting ingot was heated at 1200° C., hot forged and hot rolled, and worked into a steel plate with a thickness of 10 mm. . The obtained steel sheets of each test number were subjected to solution treatment at 1150° C. for 15 minutes. The microstructures of the steel sheets of each test number were all composed of ferrite and austenite. Further, the microstructure of the steel sheets of each test number had a ferrite fraction (ferrite volume fraction) within the range of 35 to less than 65%.

[pitting corrosion test]
A ferric chloride immersion corrosion test in accordance with ASTM G48 (2015) was performed as a pitting corrosion test on the steel plate of each test number on which the solution treatment was performed. Specifically, the steel plate of each test number was machined to obtain a pitting corrosion test piece having a width of 30 mm, a thickness of 3 mm, and a length of 50 mm. The weight of the sampled pitting corrosion test piece was measured. The pitting test piece was immersed in a 6% FeCl ₃ aqueous solution acidified with hydrochloric acid heated to 80±1° C. for 24 hours.

The weight of the pitting test piece after immersion for 24 hours was measured. Based on the amount of change in the weight of the pitting corrosion test piece before and after immersion, the weight loss due to corrosion of the pitting corrosion test piece of each test number was determined. The corrosion rate (g/m ² /hr) in the pitting corrosion test piece of each test number was determined from the weight loss obtained for each test number. Table 1 shows the corrosion rates of the pitting corrosion test pieces obtained for each test number. The corrosion rate "-" in Table 1 means that the corrosion rate was less than 0.01 g/m ² /hr.

[Evaluation results]
With reference to Table 1, the steel sheets of test numbers 1 to 7 had appropriate chemical compositions. As a result, the steel sheets of test numbers 1 to 7 had a corrosion rate of less than 0.01 g/m ² /hr, indicating excellent pitting corrosion resistance.

On the other hand, in the steel sheet of test number 8, the Ag content was too low. As a result, the corrosion rate was 0.01 g/m ² /hr or more, and excellent pitting corrosion resistance was not exhibited.

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

Claims (3)

in % by mass,
Cr: 27.0 to 29.0%,
Mo: 2.50-4.00%,
Ni: 5.00 to 8.00%,
N: more than 0.400% to 0.600%,
Ag: 0.01-0.50%,
C: 0.030% or less,
Si: 1.00% or less,
Mn: 2.00% or less,
sol. Al: 0.040% or less,
V: 0.50% or less,
O: 0.010% or less,
P: 0.030% or less,
S: 0.0200% or less,
W: 0 to 6.00%,
Cu: 0 to less than 0.10%, and
A duplex stainless steel having a chemical composition with the balance being Fe and impurities. 2. The duplex stainless steel of claim 1, wherein the chemical composition comprises:
Duplex stainless steel containing W: 0.10-6.00%. 3. The duplex stainless steel according to claim 1 or claim 2, wherein the chemical composition is
Cu: Duplex stainless steel containing less than 0.01-0.10%.