JP7518342B2 - Duplex Stainless Steel - Google Patents

Description

This disclosure relates to duplex stainless steel materials.

Oil wells and gas wells (hereinafter, oil wells and gas wells will be collectively referred to simply as "oil wells") may be in a corrosive environment containing corrosive gases. Here, corrosive gas means carbon dioxide gas and/or hydrogen sulfide gas. In other words, the steel materials used in oil wells are required to have excellent corrosion resistance in corrosive environments.

To date, a method for improving the corrosion resistance of steel materials has been known in which the chromium (Cr) content is increased and a passive film mainly composed of Cr oxide is formed on the surface of the steel material. Therefore, duplex stainless steel materials with an increased Cr content are sometimes used in environments where excellent corrosion resistance is required. On the other hand, duplex stainless steel materials having a two-phase structure of ferrite and austenite phases have excellent corrosion resistance against pitting corrosion and/or crevice corrosion (hereinafter referred to as "pitting corrosion resistance"), which are problems in aqueous solutions containing chlorides.

In recent years, there has been active development of deep wells below sea level. This has created a demand for duplex stainless steel materials with higher strength. In other words, there is a demand for duplex stainless steel materials that combine high strength with excellent pitting corrosion resistance.

JP 5-132741 A (Patent Document 1), JP 9-195003 A (Patent Document 2), JP 2014-043616 A (Patent Document 3), and JP 2016-003377 A (Patent Document 4) propose duplex stainless steels that have high strength and excellent corrosion resistance.

The duplex stainless steel disclosed in Patent Document 1 has a chemical composition, by weight percent, of C: 0.03% or less, Si: 1.0% or less, Mn: 1.5% or less, P: 0.040% or less, S: 0.008% or less, sol. Al: 0.040% or less, Ni: 5.0-9.0%, Cr: 23.0-27.0%, Mo: 2.0-4.0%, W: over 1.5% to 5.0%, N: 0.24-0.32%, with the balance being Fe and unavoidable impurities, and a PREW (= Cr + 3.3 (Mo + 0.5W) + 16N) of 40 or more. Patent Document 1 states that this duplex stainless steel exhibits excellent corrosion resistance and high strength.

The duplex stainless steel disclosed in Patent Document 2 contains, by weight, C: 0.12% or less, Si: 1% or less, Mn: 2% or less, Ni: 3-12%, Cr: 20-35%, Mo: 0.5-10%, W: over 3-8%, Co: 0.01-2%, Cu: 0.1-5%, N: 0.05-0.5%, with the balance being Fe and unavoidable impurities. Patent Document 2 states that this duplex stainless steel has excellent corrosion resistance without reducing strength.

The duplex stainless steel disclosed in Patent Document 3 contains, in mass percent, C: 0.03% or less, Si: 0.3% or less, Mn: 3.0% or less, P: 0.040% or less, S: 0.008% or less, Cu: 0.2-2.0%, Ni: 5.0-6.5%, Cr: 23.0-27.0%, Mo: 2.5-3.5%, W: 1.5-4.0%, N: 0.24-0.40%, and Al: 0. The chemical composition is such that the σ-phase susceptibility index X (=2.2Si+0.5Cu+2.0Ni+Cr+4.2Mo+0.2W) is 52.0 or less, the strength index Y (=Cr+1.5Mo+10N+3.5W) is 40.5 or more, and the pitting corrosion resistance index PREW (=Cr+3.3(Mo+0.5W)+16N) is 40 or more. The structure of the steel is such that when a straight line parallel to the thickness direction from the surface layer to a depth of 1 mm is drawn in a thickness direction cross section parallel to the rolling direction, the number of boundaries between the ferrite phase and the austenite phase that intersect the straight line is 160 or more. This duplex stainless steel can be strengthened without impairing corrosion resistance, and by combining it with high-processing cold working, it exhibits excellent hydrogen embrittlement resistance, as described in Patent Document 3.

The duplex stainless steel disclosed in Patent Document 4 contains, in mass %, C: 0.03% or less, Si: 0.2 to 1%, Mn: 0.5 to 2.0%, P: 0.040% or less, S: 0.010% or less, Sol. The chemical composition is Al: 0.040% or less, Ni: 4 to less than 6%, Cr: 20 to less than 25%, Mo: 2.0 to 4.0%, N: 0.1 to 0.35%, O: 0.003% or less, V: 0.05 to 1.5%, Ca: 0.0005 to 0.02%, B: 0.0005 to 0.02%, with the balance being Fe and impurities, the metal structure is composed of a two-phase structure of a ferrite phase and an austenite phase, there is no precipitation of a sigma phase, the ratio of the ferrite phase to the metal structure is 50% or less in terms of area ratio, and the number of oxides with a grain size of 30 μm or more present in a 300 ^mm2 field of view is 15 or less. This duplex stainless steel is described in Patent Document 4 as being excellent in strength, pitting corrosion resistance, and low-temperature toughness.

Japanese Patent Application Publication No. 5-132741 Japanese Patent Application Publication No. 9-195003 JP 2014-043616 A JP 2016-003377 A

As mentioned above, in recent years, there has been active development of deep wells below sea level. The water temperature in deep wells below sea level is low. That is, when used in deep wells below sea level, duplex stainless steel materials are required to have high strength, excellent pitting corrosion resistance, as well as excellent low-temperature toughness. Therefore, duplex stainless steel materials having a yield strength of 552 MPa or more, excellent low-temperature toughness, and excellent pitting corrosion resistance may be obtained by techniques other than those disclosed in the above Patent Documents 1 to 4.

The objective of this disclosure is to provide a duplex stainless steel material that has a yield strength of 552 MPa or more, excellent low-temperature toughness, and excellent pitting corrosion resistance.

The duplex stainless steel material according to the present disclosure comprises:
In mass percent,
C: 0.030% or less,
Si: 0.20-1.00%,
Mn: 0.50-7.00%,
P: 0.040% or less,
S: 0.020% or less,
Al: 0.100% or less,
Ni: 4.20-9.00%,
Cr: 20.00-30.00%,
Mo: 0.50-2.00%,
Cu: 1.50-4.00%,
N: 0.150-0.350%,
V: 0.01-1.50%,
Nb: 0 to 0.100%,
Ta: 0-0.100%,
Ti: 0 to 0.100%,
Zr: 0 to 0.100%,
Hf: 0-0.100%,
W: 0-0.200%,
Co: 0 to 0.500%,
Sn: 0-0.100%,
Sb: 0 to 0.1000%,
Ca: 0-0.020%,
Mg: 0 to 0.020%,
B: 0 to 0.020%,
Rare earth elements: 0 to 0.200%, and
A chemical composition satisfying formula (1), with the balance being Fe and impurities;
A microstructure having a volume fraction of 30.0 to 70.0% ferrite and the remainder being austenite,
The yield strength is 552 MPa or more,
The proportion of the austenite having a KAM value of 1 to 5° is 25.0% by area or more.
Cr+3.3(Mo+0.5W)+16N≧30.0 (1)
Here, the content (mass%) of the corresponding element is substituted for the element symbol in formula (1). When the corresponding element is not contained, "0" is substituted for the element symbol.

The duplex stainless steel material disclosed herein has a high yield strength of 552 MPa or more, excellent low-temperature toughness, and excellent pitting corrosion resistance.

FIG. 1 is a diagram showing the relationship between the proportion (area %) of austenite with a KAM value of 1 to 5° and the yield strength (MPa) of the steel material in this embodiment.

First, the inventors investigated duplex stainless steel materials having a yield strength of 552 MPa or more, excellent low-temperature toughness, and excellent pitting corrosion resistance from the viewpoint of chemical composition. As a result, the inventors found that the following components, by mass%, were found to be C: 0.030% or less, Si: 0.20-1.00%, Mn: 0.50-7.00%, P: 0.040% or less, S: 0.020% or less, Al: 0.100% or less, Ni: 4.20-9.00%, Cr: 20.00-30.00%, Mo: 0.50-2.00%, Cu: 1.50-4.00%, N: 0.150-0.350%, V: 0.01-1.50%, Nb: 0-0.100%, Ta: 0-0.100%, Ti: 0-0.100% It was believed that a duplex stainless steel material with a chemical composition of Zr: 0-0.100%, Hf: 0-0.100%, W: 0-0.200%, Co: 0-0.500%, Sn: 0-0.100%, Sb: 0-0.1000%, Ca: 0-0.020%, Mg: 0-0.020%, B: 0-0.020%, rare earth elements: 0-0.200%, and the balance being Fe and impurities, could potentially achieve a yield strength of 552 MPa or more, excellent low-temperature toughness, and excellent pitting corrosion resistance.

Here, the microstructure of the duplex stainless steel material having the above-mentioned chemical composition is composed of ferrite and austenite. Specifically, the microstructure of the duplex stainless steel material having the above-mentioned chemical composition is composed of ferrite with a volume fraction of 30.0 to 70.0%, and the remainder is composed of austenite. In this specification, "composed of ferrite and austenite" means that phases other than ferrite and austenite are negligibly small.

Next, the inventors investigated various methods for improving the pitting corrosion resistance of a duplex stainless steel material having the above-mentioned chemical composition and a microstructure consisting of 30.0 to 70.0% by volume of ferrite and the remainder being austenite, and as a result, the inventors discovered that the pitting corrosion resistance of a duplex stainless steel material can be improved if the chemical composition of the duplex stainless steel material further satisfies the following formula (1).
Cr+3.3(Mo+0.5W)+16N≧30.0 (1)
Here, the content (mass%) of the corresponding element is substituted for the element symbol in formula (1). When the corresponding element is not contained, "0" is substituted for the element symbol.

Fn1 is defined as Cr + 3.3 (Mo + 0.5W) + 16N. Fn1 is an index related to the pitting corrosion resistance of steel material. Increasing Fn1 can improve the pitting corrosion resistance of duplex stainless steel material. In other words, if Fn1 is too low, the pitting corrosion resistance of duplex stainless steel material decreases. Therefore, the duplex stainless steel material according to this embodiment satisfies the above-mentioned chemical composition and has Fn1 of 30.0 or more.

Next, the inventors investigated various methods for improving both the low-temperature toughness and yield strength while maintaining pitting corrosion resistance for a duplex stainless steel material that satisfies the above-mentioned chemical composition, has an Fn1 of 30.0 or more, and has a microstructure with a volume fraction of 30.0 to 70.0% ferrite and the remainder being austenite. As a result, the inventors obtained the following findings.

As described above, the microstructure of a duplex stainless steel material that satisfies the above-mentioned chemical composition and has an Fn1 of 30.0 or more consists of a volume fraction of 30.0 to 70.0% ferrite and the remainder austenite. Here, austenite has lower strength than ferrite. Therefore, the yield strength of duplex stainless steel material is easily reduced due to the properties of austenite. On the other hand, if the yield strength of duplex stainless steel material is increased too much, the low-temperature toughness of the duplex stainless steel material will decrease. Therefore, if the strength of austenite in duplex stainless steel material can be appropriately increased, it may be possible to achieve both the yield strength and low-temperature toughness of the duplex stainless steel material.

The inventors therefore investigated various methods for increasing the strength of austenite. As a result, they obtained the following findings. First, the inventors focused on strain hardening as a method for increasing the yield strength of steel. Strain hardening is a method for increasing the strength of steel by increasing the amount of plastic strain in the steel. Plastic strain in steel impedes the movement of dislocations in the microstructure of the steel, thereby increasing the strength. In other words, if the amount of plastic strain in austenite is increased, the strength of austenite increases, and it is possible to increase the yield strength of duplex stainless steel while maintaining low-temperature toughness.

The inventors therefore investigated various methods for quantitatively calculating the plastic strain in austenite in the microstructure of steel. After detailed investigation, the inventors focused on the crystal orientation in the microstructure of steel. By observing the crystal orientation, it may be possible to observe the microscopic strain in the austenite in the microstructure of steel. If it is possible to calculate how the microscopic strain is distributed in the austenite, it may be possible to quantitatively calculate the plastic strain in the austenite.

Specifically, the crystal orientation of a duplex stainless steel material that satisfies the above-mentioned chemical composition, has an Fn1 of 30.0 or more, and has a microstructure with a volume fraction of 30.0 to 70.0% ferrite and the remainder being austenite was investigated in detail. The crystal orientation was determined by an electron backscatter diffraction pattern (EBSD) described below. EBSD can also identify the crystal phase at the measurement point. In other words, EBSD can determine whether the phase at any point is austenite or not, and can determine the crystal orientation at any point.

Next, the inventors calculated the distribution of microscopic strain in austenite by calculating the deviation of the crystal orientation from the surroundings at any point. Specifically, the inventors calculated the KAM (Kernel Average Misorientation) value from the crystal orientation obtained for any point. The KAM value was defined as follows: The field of view was divided into regular hexagonal pixel units, and any one regular hexagonal pixel in the field of view was selected as the center pixel. The orientation difference between the selected center pixel and each of the six pixels arranged adjacent to the outside of the center pixel was calculated. The average of the obtained orientation differences was calculated, and the average was defined as the KAM value of the center pixel.

In other words, the KAM value defined as above is an index that indicates the deviation of the crystal orientation from the surroundings in the microstructure of the steel material. Specifically, at a measurement point where the KAM value is large, the difference in crystal orientation between the measurement point and the surroundings is large. In this case, the microscopic distortion of the steel material is locally large at that measurement point. On the other hand, at a measurement point where the KAM value is small, the difference in crystal orientation between the measurement point and the surroundings is small. In this case, the microscopic distortion of the steel material is reduced at that measurement point.

The inventors further investigated the use of the calculated KAM value to quantify the plastic strain in austenite. Specifically, the inventors calculated the KAM value obtained at any pixel when the pixel is austenite, without crossing the grain boundary, to create a map (KAM map) that represents the change in crystal orientation within the austenite grain. The KAM map makes it possible to visualize the distribution of microscopic strain in the austenite, which is part of the microstructure in the steel material.

Based on the created KAM map, the inventors plotted the relationship between the KAM value and the area showing the KAM value in a histogram for a duplex stainless steel material that satisfies the above-mentioned chemical composition, has an Fn1 of 30.0 or more, and has a microstructure consisting of ferrite with a volume fraction of 30.0 to 70.0% and the remainder being austenite. As a result, the inventors found that if the region in austenite where the KAM value is high is broad to a certain extent, the yield strength of the steel material can be increased while maintaining low-temperature toughness. Therefore, the inventors conducted further detailed research and investigation, and found that there is a correlation between the proportion of the 1 to 5° region in austenite and the yield strength. This point will be specifically explained using a diagram.

FIG. 1 is a diagram showing the relationship between the proportion (area %) of the KAM value of 1 to 5° in austenite and the yield strength (MPa) of the steel. FIG. 1 was obtained by the following method. Among the examples described later, for duplex stainless steel materials that satisfy the above-mentioned chemical composition, have a microstructure with Fn1 of 30.0 or more, a volume fraction of ferrite of 30.0 to 70.0% and the remainder consisting of austenite, and have excellent low-temperature toughness and excellent pitting corrosion resistance, FIG. 1 was created using the proportion (area %) of the KAM value of 1 to 5° in austenite and the yield strength (MPa) of the steel. The proportion (area %) of the region with a KAM value of 1 to 5° in austenite and the yield strength (MPa) of the steel were obtained by the method described later. In addition, excellent low-temperature toughness means that the absorbed energy obtained by a Charpy impact test at -10°C is 130 J/cm ² or more. In addition, the pitting corrosion resistance of the steel was evaluated by the method described later.

With reference to Figure 1, it has been revealed that in a duplex stainless steel material that satisfies the above-mentioned chemical composition, has an Fn1 of 30.0 or more, has a microstructure consisting of 30.0 to 70.0% volume fraction of ferrite and the remainder austenite, and has excellent low-temperature toughness and excellent pitting corrosion resistance, if the proportion of KAM values 1 to 5° in austenite is 25.0 area % or more, the yield strength of the steel material will be 552 MPa or more. On the other hand, in the above-mentioned duplex stainless steel material, if the proportion of KAM values 1 to 5° in austenite is less than 25.0 area %, the yield strength of the steel material will be less than 552 MPa.

Therefore, the duplex stainless steel material according to this embodiment satisfies the above-mentioned chemical composition, has Fn1 of 30.0 or more, has a microstructure with a volume fraction of 30.0 to 70.0% ferrite and the remainder being austenite, and further has a proportion of KAM values of 1 to 5° in austenite of 25.0 area % or more. As a result, the duplex stainless steel material according to this embodiment has a yield strength of 552 MPa or more, excellent low-temperature toughness, and excellent pitting corrosion resistance.

The gist of the duplex stainless steel material according to this embodiment, which was completed based on the above findings, is as follows:

[1]
In mass percent,
C: 0.030% or less,
Si: 0.20-1.00%,
Mn: 0.50-7.00%,
P: 0.040% or less,
S: 0.020% or less,
Al: 0.100% or less,
Ni: 4.20-9.00%,
Cr: 20.00-30.00%,
Mo: 0.50-2.00%,
Cu: 1.50-4.00%,
N: 0.150-0.350%,
V: 0.01-1.50%,
Nb: 0 to 0.100%,
Ta: 0-0.100%,
Ti: 0 to 0.100%,
Zr: 0 to 0.100%,
Hf: 0-0.100%,
W: 0-0.200%,
Co: 0-0.500%,
Sn: 0-0.100%,
Sb: 0 to 0.1000%,
Ca: 0-0.020%,
Mg: 0 to 0.020%,
B: 0 to 0.020%,
Rare earth elements: 0 to 0.200%, and
A chemical composition satisfying formula (1), with the balance being Fe and impurities;
A microstructure having a volume fraction of 30.0 to 70.0% ferrite and the remainder being austenite,
The yield strength is 552 MPa or more,
The proportion of the KAM value of 1 to 5° in the austenite is 25.0 area% or more.
Duplex stainless steel material.
Cr+3.3(Mo+0.5W)+16N≧30.0 (1)
Here, the content (mass%) of the corresponding element is substituted for the element symbol in formula (1). When the corresponding element is not contained, "0" is substituted for the element symbol.

[2]
The duplex stainless steel material according to [1],
The chemical composition is
Nb: 0.001-0.100%,
Ta: 0.001-0.100%,
Ti: 0.001 to 0.100%,
Zr: 0.001 to 0.100%,
Hf: 0.001 to 0.100%, and
W: Contains one or more elements selected from the group consisting of 0.001 to 0.200%;
Duplex stainless steel material.

[3]
The duplex stainless steel material according to [1] or [2],
The chemical composition is
Co: 0.001 to 0.500%,
Sn: 0.001 to 0.100%, and
Sb: Contains one or more elements selected from the group consisting of 0.0001 to 0.1000%;
Duplex stainless steel material.

[4]
[1] to [3], wherein the duplex stainless steel material is
The chemical composition is
Ca: 0.001-0.020%,
Mg: 0.001-0.020%,
B: 0.001 to 0.020%, and
Rare earth elements: containing one or more elements selected from the group consisting of 0.001 to 0.200%;
Duplex stainless steel material.

The duplex stainless steel material according to this embodiment is described in detail below.

[Chemical composition]
The chemical composition of the duplex stainless steel material according to the present embodiment contains the following elements: "%" for each element means mass % unless otherwise specified.

C: 0.030% or less Carbon (C) is inevitably contained. That is, the lower limit of 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, if the C content is too high, the pitting corrosion resistance of the steel material decreases even if the contents of other elements are within the range of this embodiment. Therefore, the C content is 0.030% or less. The preferred upper limit of the C content is 0.028%, more preferably 0.025%. The C content is preferably as low as possible. However, an extreme reduction in the C content significantly increases the manufacturing cost. Therefore, when considering industrial production, the preferred lower limit of the C content is 0.001%, more preferably 0.005%.

Si: 0.20-1.00%
Silicon (Si) deoxidizes steel. If the Si content is too low, the above effect cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment. If the Si content is too high, the low temperature toughness and hot workability of the steel material will decrease even if the contents of other elements are within the ranges of this embodiment. The lower limit of the Si content is preferably 0.25%, and more preferably 0.30%. The upper limit of the Si content is preferably 0.80%, and more preferably 0.60%.

Mn: 0.50-7.00%
Manganese (Mn) deoxidizes and desulfurizes the steel. Mn also improves the hot workability of the steel. If the Mn content is too low, the other element contents may be within the range of this embodiment. However, even if the Mn content is high, the above-mentioned effect cannot be sufficiently obtained. On the other hand, Mn segregates at grain boundaries together with impurities such as P and S. Therefore, if the Mn content is too high, the contents of other elements are not sufficient to achieve the effect of the present embodiment. Even if the Mn content is within this range, the pitting corrosion resistance of the steel material in a high-temperature environment decreases. Therefore, the Mn content is 0.50 to 7.00%. The preferable lower limit of the Mn content is 0.75%. The upper limit of the Mn content is preferably 6.50%, more preferably 6.20%.

P: 0.040% or less Phosphorus (P) is inevitably contained. That is, the lower limit of the P content is more than 0%. P segregates at grain boundaries. Therefore, if the P content is too high, the low-temperature toughness and pitting corrosion resistance of the steel material will decrease even if the contents of other elements are within the range of this embodiment. Therefore, the P content is 0.040% or less. The preferred upper limit of the P content is 0.035%, more preferably 0.030%. The P content is preferably as low as possible. However, an extreme reduction in the P content significantly increases the manufacturing cost. Therefore, in consideration of industrial production, the preferred lower limit of the P content is 0.001%, more preferably 0.003%.

S: 0.020% or less Sulfur (S) is inevitably contained. That is, the lower limit of the S content is more than 0%. S segregates at grain boundaries. Therefore, if the S content is too high, the low-temperature toughness and pitting corrosion resistance of the steel material will decrease even if the contents of other elements are within the range of this embodiment. Therefore, the S content is 0.020% or less. The preferred upper limit of the S content is 0.018%, more preferably 0.016%. The S content is preferably as low as possible. However, an extreme reduction in the S content significantly increases the manufacturing cost. Therefore, in consideration of industrial production, the preferred lower limit of the S content is 0.0001%, more preferably 0.0003%, even more preferably 0.001%, and even more preferably 0.002%.

Al: 0.100% or less Aluminum (Al) is inevitably contained. That is, the lower limit of the Al content is more than 0%. Al deoxidizes the steel. On the other hand, if the Al content is too high, even if the contents of other elements are within the range of this embodiment, coarse oxide-based inclusions are generated, and the low-temperature toughness of the steel material decreases. Therefore, the Al content is 0.100% or less. The preferred lower limit of the Al content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit of the Al content is 0.090%, and more preferably 0.085%. The Al content in this specification means the content of "acid-soluble Al", that is, sol. Al.

Ni: 4.20-9.00%
Nickel (Ni) stabilizes the austenite structure of steel. In other words, Ni is an element necessary to obtain a stable ferrite-austenite two-phase structure. Ni also enhances the pitting corrosion resistance of steel. If the Ni content is too low, the above-mentioned effects cannot be sufficiently obtained even if the contents of the other elements are within the range of this embodiment. Even within the range of the embodiment, the volume fraction of austenite becomes too high, and the yield strength of the steel material decreases. Therefore, the Ni content is 4.20 to 9.00%. is 4.25%, more preferably 4.30%, even more preferably 4.35%, even more preferably 4.40%, and even more preferably 4.50%. The upper limit of the amount is preferably 8.75%, more preferably 8.50%, even more preferably 8.25%, even more preferably 8.00%, and even more preferably 7.75%. be.

Cr:20.00~30.00%
Chromium (Cr) enhances the pitting corrosion resistance of steel. Specifically, Cr forms a passive film on the surface of steel as an oxide. As a result, the pitting corrosion resistance of steel is improved. Cr also enhances the The volume fraction of the ferrite structure is increased. By obtaining a sufficient ferrite structure, the pitting corrosion resistance of the steel material is stabilized. If the Cr content is too low, the other element contents are within the range of this embodiment, and On the other hand, if the Cr content is too high, the hot workability of the steel material is reduced even if the contents of other elements are within the ranges of this embodiment. The Cr content is 20.00 to 30.00%. The lower limit of the Cr content is preferably 20.50%, more preferably 21.00%, and further preferably 21.50%. The upper limit of the content is preferably 29.50%, more preferably 29.00%, and further preferably 28.00%.

Mo: 0.50-2.00%
Molybdenum (Mo) improves the pitting corrosion resistance of steel. Mo also dissolves in steel to increase the yield strength of steel. Mo also forms fine carbides in steel to increase the yield strength of steel. If the Mo content is too low, the above effects cannot be sufficiently obtained even if the contents of the other elements are within the range of this embodiment. Even if the content is within the range of this embodiment, the hot workability of the steel material is reduced. Therefore, the Mo content is 0.50 to 2.00%. The preferred lower limit of the Mo content is 0. The upper limit of the Mo content is preferably less than 2.00%, more preferably 1.85%, and more preferably 0.60%, more preferably 0.70%, and further preferably 0.80%. More preferably, it is 1.50%.

Cu: 1.50-4.00%
Copper (Cu) increases the yield strength of the steel. Cu also increases the pitting corrosion resistance of the steel in high temperature environments. If the Cu content is too low, the other element contents may be within the range of this embodiment. On the other hand, if the Cu content is too high, the hot workability of the steel material decreases even if the contents of other elements are within the ranges of this embodiment. , the Cu content is 1.50 to 4.00%. The lower limit of the Cu content is preferably 1.60%, more preferably 1.80%, and further preferably 1.90%. The upper limit of the Cu content is preferably 3.90%, more preferably 3.75%, and even more preferably 3.50%. %.

N: 0.150-0.350%
Nitrogen (N) stabilizes the austenite structure of steel. In other words, N is an element necessary to obtain a stable ferrite-austenite two-phase structure. N also increases the pitting corrosion resistance of steel. If the N content is too low, the above-mentioned effects cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment. Even if the N content is within the range of the embodiment, the low temperature toughness and hot workability of the steel material are reduced. Therefore, the N content is 0.150 to 0.350%. The preferable lower limit of the N content is 0.170 %, more preferably 0.180%, and further preferably 0.190%. A preferred upper limit of the N content is 0.340%, and more preferably 0.330%.

V: 0.01-1.50%
Vanadium (V) increases the yield strength of steel. If the V content is too low, the above effect cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment. If the amount is too high, even if the contents of other elements are within the range of this embodiment, the strength of the steel material becomes too high, and the low-temperature toughness and hot workability of the steel material decrease. The lower limit of the V content is preferably 0.02%, more preferably 0.03%, and further preferably 0.05%. The upper limit is 1.20%, and more preferably 1.00%.

The remainder of the chemical composition of the duplex stainless steel material according to this embodiment is composed of Fe and impurities. Here, impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore, scrap, or the manufacturing environment when industrially manufacturing duplex stainless steel material, and are acceptable to the extent that they do not adversely affect the duplex stainless steel material according to this embodiment.

[Optional element]
The chemical composition of the above-mentioned duplex stainless steel material may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Nb, Ta, Ti, Zr, Hf, and W. All of these elements are optional elements, and increase the strength of the steel material.

Nb: 0-0.100%
Niobium (Nb) is an optional element and may not be contained. That is, the Nb content may be 0%. When contained, Nb forms carbonitrides and increases the strength of the steel material. If even a small amount of Nb is contained, the above effect can be obtained to some extent. However, if the Nb content is too high, the strength of the steel material will be high even if the contents of other elements are within the range of this embodiment. If the Nb content is too high, the low temperature toughness of the steel material will decrease. Therefore, the Nb content is 0 to 0.100%. The lower limit of the Nb content is preferably more than 0%, more preferably 0.001%, and even more preferably The upper limit of the Nb content is preferably 0.080%, more preferably 0.070%. be.

Ta: 0~0.100%
Tantalum (Ta) is an optional element and may not be contained. That is, the Ta content may be 0%. When contained, Ta forms carbonitrides and increases the strength of the steel material. If even a small amount of Ta is contained, the above effect can be obtained to some extent. However, if the Ta content is too high, the strength of the steel material will be high even if the contents of other elements are within the range of this embodiment. If the Ta content is too high, the low temperature toughness of the steel material will decrease. Therefore, the Ta content is 0 to 0.100%. The lower limit of the Ta content is preferably more than 0%, more preferably 0.001%, and even more preferably The upper limit of the Ta content is preferably 0.080%, more preferably 0.070%. be.

Ti: 0~0.100%
Titanium (Ti) is an optional element and may not be contained. That is, the Ti content may be 0%. When contained, Ti forms carbonitrides and increases the strength of the steel material. If even a small amount of Ti is contained, the above effect can be obtained to some extent. However, if the Ti content is too high, the strength of the steel material will be high even if the contents of other elements are within the range of this embodiment. If the Ti content is too high, the low temperature toughness of the steel material will decrease. Therefore, the Ti content is 0 to 0.100%. The lower limit of the Ti content is preferably more than 0%, more preferably 0.001%, and even more preferably The upper limit of the Ti content is preferably 0.080%, more preferably 0.070%. be.

Zr: 0-0.100%
Zirconium (Zr) is an optional element and may not be contained. That is, the Zr content may be 0%. When contained, Zr forms carbonitrides and increases the strength of the steel material. The above effects can be obtained to some extent if even a small amount of Zr is contained. However, if the Zr content is too high, the strength of the steel material will be high even if the contents of other elements are within the range of this embodiment. If the Zr content is too high, the low-temperature toughness of the steel material will decrease. Therefore, the Zr content is 0 to 0.100%. The lower limit of the Zr content is preferably more than 0%, more preferably 0.001%, and even more preferably The upper limit of the Zr content is preferably 0.080%, more preferably 0.070%. be.

Hf: 0-0.100%
Hafnium (Hf) is an optional element and may not be contained. That is, the Hf content may be 0%. When contained, Hf forms carbonitrides and increases the strength of the steel material. The above effects can be obtained to some extent if even a small amount of Hf is contained. However, if the Hf content is too high, the strength of the steel material will be increased even if the contents of other elements are within the ranges of this embodiment. If the content is too high, the low-temperature toughness of the steel material will decrease. Therefore, the Hf content is 0 to 0.100%. The lower limit of the Hf content is preferably more than 0%, more preferably 0.001%, and even more preferably The upper limit of the Hf content is preferably 0.080%, more preferably 0.070%. be.

W: 0-0.200%
Tungsten (W) is an optional element and may not be contained. That is, the W content may be 0%. When W is contained, W forms carbonitrides and increases the strength of the steel material. The above effect can be obtained to some extent if even a small amount of W is contained. However, if the W content is too high, the strength of the steel material will be high even if the contents of other elements are within the range of this embodiment. If the content is too high, the low-temperature toughness of the steel material will decrease. Therefore, the W content is 0 to 0.200%. The lower limit of the W content is preferably more than 0%, more preferably 0.001%, and even more preferably The upper limit of the W content is preferably 0.002%, more preferably 0.003%, and even more preferably 0.005%. The upper limit of the W content is preferably 0.180%, and even more preferably 0.150%. be.

The chemical composition of the above-mentioned duplex stainless steel material may further contain one or more elements selected from the group consisting of Co, Sn, and Sb in place of a portion of Fe. All of these elements are optional and increase the corrosion resistance of the steel material.

Co: 0-0.500%
Cobalt (Co) is an optional element and may not be contained. That is, the Co content may be 0%. When Co is contained, it forms a coating on the surface of the steel material and improves the strength of the steel material. Co improves corrosion resistance. Co also improves the hardenability of steel and stabilizes the strength of steel. The above effects can be obtained to some extent if even a small amount of Co is contained. However, if the Co content is too high, Even if the contents of other elements are within the ranges of this embodiment, the manufacturing cost will be extremely high. Therefore, the Co content is 0 to 0.500%. The preferred lower limit of the Co content is more than 0%. The upper limit of the Co content is preferably 0.480%, more preferably 0.001%, further preferably 0.010%, and further preferably 0.020%. 0.460%, and more preferably 0.450%.

Sn: 0-0.100%
Tin (Sn) is an optional element and may not be contained. In other words, the Sn content may be 0%. When contained, Sn enhances the corrosion resistance of the steel material. Even if even a small amount of Sn is contained, However, if the Sn content is too high, even if the contents of other elements are within the ranges of this embodiment, liquation embrittlement cracking occurs at the grain boundaries, resulting in the deterioration of the steel material. The hot workability of the steel sheet is deteriorated. Therefore, the Sn content is 0 to 0.100%. The lower limit of the Sn content is preferably more than 0%, more preferably 0.001%, and even more preferably 0.001%. The upper limit of the Sn content is preferably 0.080%, and more preferably 0.070%.

Sb: 0-0.1000%
Antimony (Sb) is an optional element and may not be contained. In other words, the Sb content may be 0%. When contained, Sb enhances the corrosion resistance of the steel material. Even if even a small amount of Sb is contained, However, if the Sb content is too high, the ductility of the steel material at high temperatures decreases even if the contents of other elements are within the ranges of this embodiment, and the steel material The hot workability is deteriorated. Therefore, the Sb content is 0 to 0.1000%. The lower limit of the Sb content is preferably more than 0%, more preferably 0.0001%, and further preferably 0. The upper limit of the Sb content is preferably 0.0800%, and more preferably 0.0700%.

The chemical composition of the above-mentioned duplex stainless steel material may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Ca, Mg, B, and rare earth elements. All of these elements are optional elements, and improve the hot workability of the steel material.

Ca: 0-0.020%
Calcium (Ca) is an optional element and may not be contained. In other words, the Ca content may be 0%. When contained, Ca fixes S in the steel material as sulfides, The above effect can be obtained to some extent if even a small amount of Ca is contained. However, if the Ca content is too high, the contents of other elements will be within the range of this embodiment. Even if Ca is present in the steel material, the oxides in the steel material become coarse and the low-temperature toughness of the steel material decreases. Therefore, the Ca content is 0 to 0.020%. The preferable lower limit of the Ca content is more than 0%. , more preferably 0.001%, further preferably 0.002%, further preferably 0.003%, further preferably 0.005%. The preferred upper limit of the Ca content is 0. It is preferably 0.018%, and more preferably 0.015%.

Mg: 0-0.020%
Magnesium (Mg) is an optional element and may not be contained. That is, the Mg content may be 0%. When Mg is contained, it fixes S in the steel material as sulfides, Even if even a small amount of Mg is contained, the above effects can be obtained to some extent. However, if the Mg content is too high, the contents of other elements will be within the range of this embodiment. Even if the Mg content is less than 0.020%, the oxides in the steel material will become coarse and the low-temperature toughness of the steel material will decrease. Therefore, the Mg content is 0 to 0.020%. The preferable lower limit of the Mg content is more than 0%. , more preferably 0.001%, further preferably 0.002%, further preferably 0.003%, further preferably 0.005%. The preferred upper limit of the Mg content is 0. It is preferably 0.018%, and more preferably 0.015%.

B: 0-0.020%
Boron (B) is an optional element and may not be contained. That is, the B content may be 0%. When contained, B suppresses the segregation of S to grain boundaries in the steel material. Even if even a small amount of B is contained, the above effect can be obtained to some extent. However, if the B content is too high, the contents of other elements may be within the range of this embodiment. Even if B is present, boron nitride (BN) is formed, which reduces the low-temperature toughness of the steel material. Therefore, the B content is 0 to 0.020%. The preferred lower limit of the B content is more than 0%, The B content is more preferably 0.001%, further preferably 0.002%, further preferably 0.003%, and further preferably 0.005%. The preferred upper limit of the B content is 0.018%. %, and more preferably 0.015%.

Rare earth elements: 0-0.200%
Rare earth elements (REM) are optional elements and may not be contained. That is, the REM content may be 0%. When contained, REM fixes S in the steel material as sulfides. Even if even a small amount of REM is contained, the above effect can be obtained to some extent. However, if the REM content is too high, the contents of other elements will be within the range of this embodiment. Even if the REM content is within the range, the oxides in the steel material become coarse and the low-temperature toughness of the steel material decreases. Therefore, the REM content is 0 to 0.200%. The preferable lower limit of the REM content is more than 0%. The upper limit of the REM content is preferably 0.001%, more preferably 0.005%, still more preferably 0.010%, and still more preferably 0.020%. .180%, more preferably 0.160%.

In this specification, REM refers to one or more elements selected from the group consisting of scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. In addition, the REM content in this specification refers to the total content of these elements.

[Regarding formula (1)]
The chemical composition of the duplex stainless steel material according to this embodiment further satisfies the following formula (1).
Cr+3.3(Mo+0.5W)+16N≧30.0 (1)
Here, the content (mass%) of the corresponding element is substituted for the element symbol in formula (1). When the corresponding element is not contained, "0" is substituted for the element symbol.

Fn1 (=Cr + 3.3 (Mo + 0.5W) + 16N) is an index related to the pitting corrosion resistance of steel material. Increasing Fn1 can improve the pitting corrosion resistance of duplex stainless steel material. In other words, if Fn1 is too low, the pitting corrosion resistance of duplex stainless steel material decreases. Therefore, the duplex stainless steel material according to this embodiment satisfies the above-mentioned chemical composition and has Fn1 of 30.0 or more.

The preferred lower limit of Fn1 is 30.5, more preferably 31.0, and even more preferably 31.5. The higher Fn1 is, the better. However, in the duplex stainless steel material according to this embodiment having the above-mentioned chemical composition, the upper limit of Fn1 is substantially 42.5.

[Microstructure]
The microstructure of the duplex stainless steel material according to the present embodiment is composed of 30.0 to 70.0% by volume of ferrite and the remainder being austenite. In this specification, "composed of ferrite and austenite" means that phases other than ferrite and austenite are negligibly small. For example, in the chemical composition of the duplex stainless steel material according to the present embodiment, the volume fraction of precipitates and inclusions is negligibly small compared to the volume fractions of ferrite and austenite. In other words, the microstructure of the duplex stainless steel according to the present embodiment may contain minute amounts of precipitates, inclusions, etc. in addition to ferrite and austenite.

The microstructure of the duplex stainless steel material according to this embodiment further has a ferrite volume fraction of 30.0 to 70.0%. If the ferrite volume fraction is too low, the yield strength and/or pitting corrosion resistance of the steel material may decrease. On the other hand, if the ferrite volume fraction is too high, the low-temperature toughness and/or hot workability of the steel material may decrease. Therefore, in the microstructure of the duplex stainless steel material according to this embodiment, the ferrite volume fraction is 30.0 to 70.0%. The preferred lower limit of the ferrite volume fraction is 31.0%, more preferably 32.0%. The preferred upper limit of the ferrite volume fraction is 68.0%, more preferably 65.0%.

In this embodiment, the volume fraction of ferrite in the duplex stainless steel material can be determined by a method conforming to ASTM E562 (2011). A test piece for microstructure observation is prepared from the duplex stainless steel material according to this embodiment. When the steel material is a steel plate, a test piece having an observation surface 5 mm in the rolling direction and 5 mm in the plate thickness direction from the center of the plate thickness is prepared. When the steel material is a steel pipe, a test piece having an observation surface 5 mm in the pipe axial direction and 5 mm in the pipe radial direction from the center of the wall thickness is prepared. When the steel material is a steel bar, a test piece having an observation surface 5 mm in the axial direction and 5 mm in the radial direction from the center of the cross section perpendicular to the axial direction of the steel bar is prepared. Note that the size of the test piece is not particularly limited as long as the above observation surface can be obtained.

The observation surface of the prepared test piece is mirror-polished. The mirror-polished observation surface is electrolytically etched in a 7% potassium hydroxide etching solution to reveal the structure. The observation surface with the revealed structure is observed in 10 fields of view using an optical microscope. The field area is not particularly limited, but is, for example, 1.00 mm ² (magnification 100 times). In each field of view, ferrite is identified from the contrast. The area ratio of the identified ferrite is measured by a point counting method in accordance with ASTM E562 (2011). In this embodiment, the arithmetic average value of the obtained area ratio of ferrite in the 10 fields of view is defined as the volume ratio (%) of ferrite.

[Proportion of the region with KAM value of 1 to 5° in austenite]
In the duplex stainless steel material according to this embodiment, the proportion of KAM values of 1 to 5° in austenite is 25.0 area % or more. As described above, the KAM value is an index showing the deviation of the crystal orientation from the surroundings in the microstructure of a steel material. In austenite, at a measurement point where the KAM value is large, the difference in crystal orientation between the measurement point and the surroundings is large. On the other hand, in austenite, at a measurement point where the KAM value is small, the difference in crystal orientation between the measurement point and the surroundings is small.

Therefore, a KAM map created based on the KAM value in austenite can visualize the distribution of microscopic crystal strain in the austenite of the microstructure of the steel. If there is a large distribution of microscopic crystal strain in the austenite, there is a large amount of plastic strain in the austenite. Therefore, the width of the area in the KAM map where the KAM value is 1° or more is a quantitative indicator of the plastic strain in the austenite.

Therefore, the duplex stainless steel material according to this embodiment increases the proportion of the region with a KAM value of 1 to 5°, as visualized by the KAM map. Specifically, in the duplex stainless steel material according to this embodiment, the proportion of the austenite with a KAM value of 1 to 5° is set to 25.0 area % or more. As a result, the duplex stainless steel material according to this embodiment has increased austenite strength, and can increase the yield strength of the steel material.

The preferred lower limit of the proportion of the KAM value of 1 to 5° in the austenite of the duplex stainless steel material according to this embodiment is 25.5 area %, and more preferably 26.0 area %. The preferred upper limit of the proportion of the KAM value of 1 to 5° in the austenite of the duplex stainless steel material according to this embodiment is not particularly limited. However, as will be described in detail later, in the preferred manufacturing method of the duplex stainless steel material according to this embodiment, solution treatment is performed, and further, cold working is not performed.

When a duplex stainless steel material satisfying the above-mentioned chemical composition is subjected to solution treatment and then cold working, the plastic strain in the austenite increases excessively. In this case, the proportion of the KAM value of 1 to 5° in the austenite is 40.0 area% or more. As a result, the low-temperature toughness of the duplex stainless steel material decreases. Therefore, the preferred upper limit of the proportion of the KAM value of 1 to 5° is less than 40.0 area%. In other words, if the proportion of the KAM value of 1 to 5° is less than 40.0 area%, the duplex stainless steel material according to this embodiment can stably obtain excellent low-temperature toughness, provided that other regulations are satisfied. Therefore, in the duplex stainless steel material according to this embodiment, the proportion of the KAM value of 1 to 5° is preferably less than 25.0 to 40.0 area%. In this case, the duplex stainless steel material can stably obtain not only excellent pitting corrosion resistance, but also a yield strength of 552 MPa or more and excellent low-temperature toughness.

The percentage of the area in austenite where the KAM value is 1-5° is not the average value of the microscopic distortion of the crystal, as represented by dislocation density, but an index showing the distribution of the microscopic distortion of the crystal. For example, when comparing a case where dislocations are locally accumulated at high density with a case where dislocations are uniformly distributed, the percentage of the area where the KAM value is 1-5° will differ even if the dislocation density is about the same. Furthermore, microscopic distortion of the crystal is not caused only by dislocations. In other words, the percentage of the area in austenite where the KAM value is 1-5° and the dislocation density cannot be simply compared.

The KAM value in the austenite of the duplex stainless steel material according to this embodiment can be determined by the following method. A test piece for measuring the KAM value is prepared from the steel material according to this embodiment. When the steel material is a steel plate, a test piece having an observation surface 100 μm in the rolling direction and 100 μm in the plate thickness direction from the center of the plate thickness is prepared. When the steel material is a steel pipe, a test piece having an observation surface 100 μm in the tube axial direction and 100 μm in the tube radial direction from the center of the wall thickness is prepared. When the steel material is a steel bar, a test piece having an observation surface 100 μm in the axial direction and 100 μm in the radial direction from the center of the cross section perpendicular to the axial direction of the steel bar is prepared. Note that the size of the test piece is not particularly limited as long as the above observation surface can be obtained.

The above-mentioned observation surface is mirror-polished to finish the surface. EBSD measurements are performed on the surface-finished test piece in a field of view of 100 μm x 100 μm with a pitch of 0.3 μm. The acceleration voltage for the EBSD measurements is 20 kV. From the EBSD measurement results, the phase at the measurement point (austenite or not) is identified, and the KAM value at the measurement point is further determined.

The KAM value is defined as described above. Specifically, a 100 μm x 100 μm range is divided into regular hexagonal pixel units. One side of a pixel is, for example, 0.15 μm. An arbitrary regular hexagonal pixel is selected as the central pixel. The crystal orientation difference between the selected central pixel and each of the six pixels arranged adjacent to the outside of the central pixel is calculated. The average of the obtained crystal orientation differences is calculated, and the calculated average is defined as the KAM value of the central pixel. The KAM value is calculated using the same method for pixels identified as austenite within the 100 μm x 100 μm range.

After calculating the KAM value for each pixel in the austenite in the observed field of view, a KAM map is created showing the KAM value for each pixel. That is, the KAM map illustrates the KAM value in austenite. In the resulting KAM map, the KAM values of all pixels identified as austenite are tallied. Of the KAM values of all pixels identified as austenite, the proportion of the area with a KAM value of 1-5° is determined. The determined proportion is defined as the proportion (area %) of austenite with a KAM value of 1-5°.

The EBSD analysis program for determining the KAM value may be a well-known program. For example, OIM Data Collection/Analysis 6.2.0 by TSL Solutions Co., Ltd. may be used.

[Yield strength]
The duplex stainless steel material according to this embodiment has a yield strength of 552 MPa or more. The duplex stainless steel material according to this embodiment has the above-mentioned chemical composition, and further satisfies formula (1), has a microstructure consisting of 30.0 to 70.0% by volume of ferrite and the remainder being austenite, and the proportion of KAM values of 1 to 5° in the austenite is 25.0 area % or more. As a result, the duplex stainless steel material according to this embodiment has excellent low-temperature toughness and excellent pitting corrosion resistance, even if it has a yield strength of 552 MPa or more.

The preferred lower limit of the yield strength of the duplex stainless steel material according to this embodiment is 555 MPa, more preferably 560 MPa, and even more preferably 570 MPa. The upper limit of the yield strength of the duplex stainless steel material according to this embodiment is not particularly limited, but is, for example, 700 MPa.

The yield strength of the duplex stainless steel material according to this embodiment can be determined by the following method. Specifically, a tensile test is performed according to ASTM E8/E8M (2013). A round bar test piece is prepared from the steel material according to this embodiment. If the steel material is a steel plate, the round bar test piece is prepared from the center of the plate thickness. If the steel material is a steel pipe, the round bar test piece is prepared from the center of the wall thickness. If the steel material is a steel bar, the round bar test piece is prepared from the center of the cross section perpendicular to the axial direction of the steel bar. The size of the round bar test piece is, for example, a parallel part diameter of 6 mm and a parallel part length of 24 mm. The axial direction of the round bar test piece is parallel to the rolling direction of the steel material. A tensile test is performed using the round bar test piece at room temperature (25°C) in the air, and the obtained 0.2% offset yield strength is defined as the yield strength (MPa).

[Low temperature toughness]
The duplex stainless steel material according to this embodiment has the above-mentioned chemical composition, and further satisfies formula (1), has a microstructure consisting of 30.0 to 70.0% by volume of ferrite and the remainder being austenite, and the proportion of KAM values of 1 to 5° in the austenite is 25.0 area % or more. As a result, the duplex stainless steel material according to this embodiment exhibits excellent low-temperature toughness and excellent pitting corrosion resistance even when the yield strength is 552 MPa or more. In this embodiment, excellent low-temperature toughness is defined as follows.

The low temperature toughness of the duplex stainless steel material according to this embodiment can be evaluated by a Charpy impact test according to ASTM E23 (2018). A V-notch test piece is prepared from the steel material according to this embodiment. Specifically, the V-notch test piece is prepared according to API 5CRA (2010). A Charpy impact test according to ASTM E23 (2018) is performed on the V-notch test piece prepared according to API 5CRA (2010) to obtain the absorbed energy (J) at -10 ° C. The obtained absorbed energy (J) can be divided by the cross-sectional area (cm ² ) of the V-notch test piece to obtain the absorbed energy per unit area (J/cm ² ) at -10 ° C. The cross-sectional area of the V-notch test piece means the area of the cross section in the longitudinal direction of the V-notch test piece. Specifically, when a V-notch test specimen having a longitudinal cross section of a square of 10 mm x 10 mm is used, the absorbed energy per unit area (J/ ^cm2 ) can be calculated by dividing the determined absorbed energy (J) by the cross-sectional area of 1.0 ^cm2 .

In this embodiment, when the absorbed energy per unit area at −10° C. is 130 J/cm ² or more, it is determined that the material has excellent low-temperature toughness. In this specification, the absorbed energy per unit area at −10° C. is also simply referred to as “absorbed energy”.

[Pitting corrosion resistance]
The duplex stainless steel material according to this embodiment has the above-mentioned chemical composition, and further satisfies formula (1), has a microstructure consisting of 30.0 to 70.0% by volume of ferrite and the remainder being austenite, and the proportion of KAM values of 1 to 5° in the austenite is 25.0 area % or more. As a result, the duplex stainless steel material according to this embodiment has excellent low-temperature toughness and excellent pitting corrosion resistance even if the yield strength is 552 MPa or more. In this embodiment, excellent pitting corrosion resistance is defined as follows.

The pitting corrosion resistance of the duplex stainless steel material according to this embodiment can be evaluated by a corrosion test in accordance with ASTM G48 (2011) Method E. A test piece for the corrosion test is prepared from the steel material according to this embodiment. If the steel material is a steel plate, the test piece is prepared from the center of the plate thickness. If the steel material is a steel pipe, the test piece is prepared from the center of the wall thickness. If the steel material is a steel bar, the test piece is prepared from the center of a cross section perpendicular to the axial direction of the steel bar. The size of the test piece is, for example, 3 mm thick, 25 mm wide, and 50 mm long. The longitudinal direction of the test piece is parallel to the rolling direction of the steel material.

The test solution is 6% _FeCl3 + 1% HCl. The test piece is immersed in the test solution with a specific liquid volume of 5 mL/ ^cm2 or more. The test is started at 15°C, and the temperature of the test solution is increased by 5°C every 24 hours. The temperature at which pitting occurs on the test piece is defined as the critical pitting temperature (CPT). In this embodiment, if the obtained CPT is higher than 15°C, the duplex stainless steel material is determined to have excellent pitting corrosion resistance.

[Shape of duplex stainless steel material]
The shape of the duplex stainless steel material according to this embodiment is not particularly limited. The duplex stainless steel material according to this embodiment may be, for example, a steel pipe, a steel plate, a steel bar, or a wire rod. Preferably, the duplex stainless steel material according to this embodiment is a seamless steel pipe. When the duplex stainless steel material according to this embodiment is a seamless steel pipe, it has a yield strength of 552 MPa or more, excellent low-temperature toughness, and excellent pitting corrosion resistance, even if the wall thickness is 5 mm or more.

[Production method]
An example of a method for producing a duplex stainless steel material according to the present embodiment having the above-mentioned configuration will be described. Note that the method for producing a duplex stainless steel material according to the present embodiment is not limited to the method described below. The example of a method for producing a duplex stainless steel material according to the present embodiment includes a material preparation step, a hot working step, and a solution treatment step. Each manufacturing step will be described in detail below.

[Material preparation process]
In the material preparation step according to the present embodiment, a material having the above-mentioned chemical composition is prepared. The material may be prepared by manufacturing or by purchasing from a third party. In other words, the method of preparing the material is not particularly limited.

When manufacturing the raw material, for example, it is manufactured by the following method. Molten steel having the above-mentioned chemical composition is manufactured. The molten steel is used to manufacture a cast piece (slab, bloom, or billet) by a continuous casting method. The molten steel may be used to manufacture a steel ingot by an ingot casting method. If necessary, the slab, bloom, or ingot may be rolled to manufacture a billet. The raw material is manufactured by the above-mentioned steps.

[Hot processing process]
In the hot working step according to this embodiment, the raw material prepared in the preparation step is hot worked to produce an intermediate steel material. In this specification, the intermediate steel material refers to a plate-shaped steel material when the final product is a steel plate, a blank pipe when the final product is a steel pipe, a bar-shaped steel material when the final product is a steel bar, and a wire-shaped steel material when the final product is a wire rod. The hot working may be hot forging, hot extrusion, or hot rolling. The method of hot working is not particularly limited and may be a well-known method.

When the intermediate steel material is a blank pipe (seamless steel pipe), in the hot working process, for example, the Eugène-Séjournet method or the Erhardt push bench method (i.e., hot extrusion) may be performed, or piercing rolling by the Mannesmann method (i.e., hot rolling) may be performed. Note that hot working may be performed only once or multiple times. For example, the above-mentioned hot extrusion may be performed after the above-mentioned piercing rolling is performed on the material. For example, the above-mentioned piercing rolling may be further performed on the material, followed by elongation rolling. That is, in the hot working process, hot working is performed by a well-known method to manufacture an intermediate steel material of the desired shape.

[Solution treatment process]
In the solution treatment process according to the present embodiment, the intermediate steel material produced in the hot working process is subjected to solution treatment to produce a duplex stainless steel material. The solution treatment refers to a heat treatment that dissolves compounds in the intermediate steel material. Specifically, the solution treatment process includes a process of heat treating the intermediate steel material at a desired temperature (heat treatment process) and a process of quenching the heat-treated intermediate steel material (quenching process). Each process will be described in detail below.

[Heat treatment process]
In the heat treatment step according to the present embodiment, the intermediate steel material produced in the hot working step is subjected to heat treatment. Specifically, the intermediate steel material is preferably subjected to heat treatment at a heat treatment temperature of 950 to 1100°C for a heat treatment time of 5 to 180 minutes. In this specification, the heat treatment temperature means the temperature (°C) of a heat treatment furnace for carrying out a solution treatment. In this specification, the heat treatment time means the time from when the intermediate steel material is charged into a heat treatment furnace for carrying out a solution treatment to when it is removed.

If the heat treatment temperature in the heat treatment process is too low, precipitates may remain in the duplex stainless steel material after the solution treatment process. In this case, the pitting corrosion resistance of the duplex stainless steel material decreases. On the other hand, if the heat treatment temperature in the heat treatment process is too high, the volume fraction of ferrite may exceed 70.0%. In this case, the pitting corrosion resistance of the duplex stainless steel material decreases. Therefore, in the heat treatment process according to this embodiment, the heat treatment temperature is preferably 950 to 1100°C. A more preferable lower limit of the heat treatment temperature is 960°C. A more preferable upper limit of the heat treatment temperature is 1080°C.

If the heat treatment time in the heat treatment process is too short, precipitates may remain in the duplex stainless steel material after the solution treatment process. In this case, the pitting corrosion resistance of the duplex stainless steel material may decrease. On the other hand, if the heat treatment time in the heat treatment process is too long, the effect of solutionizing the precipitates becomes saturated. Therefore, in the heat treatment process according to this embodiment, the heat treatment time is preferably 5 to 180 minutes.

[Quenching process]
In the quenching step according to the present embodiment, the intermediate steel material heat-treated in the heat treatment step is quenched to produce a duplex stainless steel material. In the intermediate steel material having the above-mentioned chemical composition, most of the precipitates such as intermetallic compounds precipitate at 400°C or higher. That is, if the intermediate steel material at 950°C or higher is quenched to 400°C or lower, the intermediate steel material can be solution-treated. Therefore, from the viewpoint of productivity, in a normal solution treatment, the part of the cross section of the intermediate steel material that is cooled the slowest (for example, the central part of the plate thickness in the case of a steel plate, the central part of the wall thickness in the case of a steel pipe, and the central part of the cross section in the axial direction in the case of a steel bar) is cooled to about 300 to 400°C.

On the other hand, the duplex stainless steel material according to this embodiment has an increased amount of plastic strain in the austenite, achieving both yield strength and low-temperature toughness. As described above, the duplex stainless steel material according to this embodiment has a microstructure with a volume fraction of 30.0 to 70.0% ferrite and the remainder being austenite. Here, the ferrite and austenite have different thermal expansion coefficients. Specifically, the thermal expansion coefficient of austenite is greater than that of ferrite. Therefore, when cooled from a high temperature to a low temperature, the austenite contracts in volume to a greater extent than the ferrite. However, it is believed that austenite cannot contract completely due to constraints from the surrounding ferrite. Therefore, it is believed that austenite is subjected to more tensile stress during cooling than ferrite, and more plastic strain is introduced.

Therefore, in the manufacturing process according to this embodiment, rapid cooling in the solution treatment is used to increase the amount of plastic strain in the austenite. Specifically, in the rapid cooling process according to this embodiment, the cooling end temperature of the rapid cooling in the solution treatment process, which was previously cooled to about 300 to 400°C, is lowered to 50°C or less. As a result, in the rapid cooling process according to this embodiment, the cooling temperature range is widened, and the amount of plastic strain can be increased in the microstructure of the duplex stainless steel material, particularly in the austenite.

That is, as an example, in the quenching process according to this embodiment, if cooling is performed so as to satisfy the following conditions, the amount of plastic strain in the austenite of the duplex stainless steel material can be increased. As a result, the duplex stainless steel material according to this embodiment can achieve a proportion of the austenite with a KAM value of 1 to 5° of 25.0 area % or more.

Average cooling rate in the range of 900 to 400°C (solution cooling rate CR _900-400 ): 3.0°C/sec or more Average cooling rate in the range of 400 to 100°C (strain-increasing cooling rate CR _400-100 ): 1.5°C/sec or more End of cooling temperature: 50°C or less In this specification, the cooling rate in the range of 900 to 400°C is defined as the solution cooling rate CR _900-400 . In this specification, the cooling rate in the range of 400 to 100°C is defined as the strain-increasing cooling rate CR _400-100 . The end of cooling temperature, the solution cooling rate CR _900-400 and the strain-increasing cooling rate CR _400-100 are determined from the temperatures measured at the center of the intermediate steel in the rolling direction. Specifically, in this embodiment, a thermocouple is embedded in the center of the plate thickness when the steel material is a steel plate, in the center of the wall thickness when the steel material is a steel pipe, and in the center of the cross section perpendicular to the axial direction when the steel material is a steel bar, to measure the temperature. From the obtained temperature change, the solution cooling rate CR _900-400 and the strain increase cooling rate CR _400-100 can be determined.

If the solution cooling rate CR _900-400 is too slow, precipitates may precipitate during cooling. In this case, the solution treatment becomes insufficient, and the steel material does not obtain excellent pitting corrosion resistance. If the solution cooling rate CR _900-400 is too slow, the plastic strain introduced into the austenite by cooling may not be sufficiently increased. In this case, the proportion of the KAM value of 1 to 5° in the austenite becomes less than 25.0 area %. Therefore, in the quenching process according to this embodiment, it is preferable to set the solution cooling rate CR _900-400 to 3.0 ° C./sec or more. A more preferable lower limit of the solution cooling rate CR _900-400 is 5.0 ° C./sec, and even more preferably 10.0 ° C./sec. The upper limit of the solution cooling rate CR _900-400 is not particularly limited, but is, for example, 500.0 ° C./sec.

The strain-increasing cooling rate CR _400-100 has not been paid much attention to in the past. However, if the strain-increasing cooling rate CR _400-100 is too slow, the plastic strain introduced into the austenite by cooling may not be sufficiently increased. In this case, the proportion of the KAM value of 1 to 5° in the austenite is less than 25.0 area %. Therefore, in the quenching process according to the present embodiment, it is preferable to set the strain-increasing cooling rate CR _400-100 to 1.5°C/sec or more. A more preferable lower limit of the strain-increasing cooling rate CR _400-100 is 3.0°C/sec, and even more preferably 5.0°C/sec. The upper limit of the strain-increasing cooling rate CR _400-100 is not particularly limited, but is, for example, 250.0°C/sec.

If the cooling end temperature is too high, it may not be possible to sufficiently increase the plastic strain introduced into the austenite by cooling. In this case, the proportion of austenite with a KAM value of 1 to 5° will be less than 25.0 area %. Therefore, in the rapid cooling process according to this embodiment, it is preferable to set the cooling end temperature to 50°C or less. A more preferable upper limit of the cooling end temperature is 45°C, and even more preferably 40°C. There is no particular limit on the lower limit of the cooling end temperature, but if it is reduced too much, the manufacturing costs will increase significantly. Therefore, the lower limit of the cooling end temperature is, for example, 20°C.

In addition, the cooling method of the intermediate steel material in the quenching process is not particularly limited as long as the solution cooling rate CR _900-400 and the strain increase cooling rate CR _400-100 satisfy the above ranges. The cooling method may be, for example, accelerated cooling in which the intermediate steel material is immersed in a water tank and the water tank is stirred, or cooling by performing shower water cooling from one side of the intermediate steel material. That is, the cooling method can be selected depending on the shape, size, thickness, etc. of the intermediate steel material. In short, in the quenching process according to this embodiment, the cooling method is appropriately selected so that the cooling end temperature, the solution cooling rate CR _900-400 , and the strain increase cooling rate CR _400-100 satisfy the above ranges. As a result, in the duplex stainless steel material according to this embodiment, the proportion of the austenite with a KAM value of 1 to 5° is 25.0 area % or more.

If necessary, the duplex stainless steel material that has been subjected to solution treatment may be subjected to pickling treatment. In this case, the pickling treatment may be performed by a well-known method, and is not particularly limited. Furthermore, if cold working is performed on the duplex stainless steel material that has been subjected to solution treatment, the strength of the steel material becomes too high, and the toughness of the steel material decreases drastically. Therefore, it is preferable not to perform cold working on the duplex stainless steel material according to this embodiment.

The above steps allow the production of duplex stainless steel material according to this embodiment. Note that the above-mentioned method for producing duplex stainless steel material is just one example, and duplex stainless steel material may be produced by other methods. The present disclosure will be explained in more detail below with reference to examples.

Molten steel having the chemical composition shown in Table 1 was melted using a 50 kg vacuum melting furnace, and steel ingots were produced using the ingot casting method. Note that "-" in Table 1 means that the content of the corresponding element was at the impurity level. The chemical composition described in Table 1 and Fn1 calculated from the above definition are also shown in Table 1.

The obtained ingot was heated at the rolling temperature (℃) shown in Tables 2 and 3, and then hot rolling was performed to produce a raw pipe (seamless steel pipe). In this example, the rolling temperature (℃) was the temperature (℃) of the heating furnace used for heating. The "shape" column in Tables 2 and 3 was as follows. "A" means a seamless steel pipe shape with an outer diameter of 139.7 mm and a wall thickness of 9.17 mm. "B" means a seamless steel pipe shape with an outer diameter of 114.3 mm and a wall thickness of 7.4 mm. "C" means a seamless steel pipe shape with an outer diameter of 177.8 mm and a wall thickness of 10.4 mm. "D" means a seamless steel pipe shape with an outer diameter of 130.0 mm and a wall thickness of 17.8 mm. "E" means a seamless steel pipe shape with an outer diameter of 159.0 mm and a wall thickness of 22.1 mm. "F" means a seamless steel pipe shape with an outer diameter of 198.0 mm and a wall thickness of 23.9 mm. "G" means a seamless steel pipe shape with an outer diameter of 378.5 mm and a wall thickness of 34.0 mm.

The blank pipes of each test number manufactured by hot rolling were subjected to solution treatment under the conditions shown in Tables 2 and 3 to manufacture seamless steel pipes of each test number. Specifically, the blank pipes of each test number were subjected to heat treatment at the heat treatment temperature (°C) and heat treatment time (min) shown in Tables 2 and 3. In this example, the furnace temperature of the heat treatment furnace in which the solution treatment was performed was taken as the heat treatment temperature (°C). Furthermore, the time from when the blank pipes were loaded into the heat treatment furnace in which the solution treatment was performed until when they were removed was taken as the heat treatment time (min).

The mother pipe of each test number after the heat treatment was cooled to obtain a seamless steel pipe of each test number. The solution cooling rate CR _900-400 (°C/sec), strain increasing cooling rate CR _400-100 (°C/sec), and cooling end temperature (°C) were obtained by embedding a thermocouple in the center of the wall thickness of the mother pipe and measuring the temperature. In the solution treatment, the solution cooling rate CR _900-400 and strain increasing cooling rate CR _400-100 were controlled by changing the cooling method. The solution cooling rate for the mother pipe of each test number is "CR _900-400 (°C/sec)", the strain increasing cooling rate is "CR _400-100 (°C/sec)", and the cooling end temperature (°C) are shown in Tables 2 and 3, respectively. Through the above process, a seamless steel pipe of each test number was obtained. The shape of the mother pipe of each test number was the same as the shape of the seamless steel pipe of the corresponding test number.

[Evaluation test]
For the seamless steel pipes of each test number after the solution treatment, microstructural observation, a KAM value measurement test, a tensile test, a Charpy impact test, and a corrosion test were carried out.

[Microstructure Observation]
For the seamless steel pipe of each test number, microstructural observation was performed by the above-mentioned method based on ASTM E562 (2011), and the ferrite volume fraction (%) was obtained. First, a test specimen was prepared from the center of the wall thickness of the seamless steel pipe of each test number. The ferrite volume fraction was obtained by the above-mentioned method using the prepared test specimen. The obtained ferrite volume fraction (%) for each test number is shown in Tables 2 and 3.

[KAM value measurement test]
For each seamless steel pipe with each test number, the proportion of the region in austenite where the KAM value is 1 to 5° was determined. The proportion (area %) of the KAM value in austenite where the KAM value is 1 to 5° was determined using the method described above. In this example, the test specimen for the KAM value measurement test was prepared from the center of the wall thickness of the seamless steel pipe. Furthermore, the cross section perpendicular to the axial direction of the seamless steel pipe was used as the observation surface. The proportion (area %) of the KAM value in austenite where the KAM value is 1 to 5° for each test number is shown in Tables 2 and 3 as "KAM value 1 to 5° proportion (area %)".

[Tensile test]
A tensile test was performed on each seamless steel pipe of each test number by the above-mentioned method in accordance with ASTM E8/E8M (2013) to determine the yield strength (MPa). First, a round bar test piece was prepared from the center of the wall thickness of each seamless steel pipe of each test number. The axial direction of the round bar test piece was parallel to the axial direction of the seamless steel pipe. A tensile test was performed on the round bar test piece of each test number prepared in accordance with ASTM E8/E8M (2013). The 0.2% offset proof stress obtained in the tensile test was defined as the yield strength (MPa). The yield strength of each test number obtained is shown in Tables 2 and 3 as "YS (MPa)".

[Charpy impact test]
A Charpy impact test in accordance with ASTM E23 (2018) was performed on the seamless steel pipe of each test number to evaluate the low temperature toughness. First, a V-notch test piece for the Charpy impact test was prepared from the center of the wall thickness of the seamless steel pipe of each test number in accordance with API 5CRA (2010). A Charpy impact test was performed on the prepared V-notch test piece of each test number in accordance with ASTM E23 (2016).

Specifically, three test pieces of each test number prepared in accordance with API 5CRA (2010) were cooled to -10°C, and a Charpy impact test in accordance with ASTM E23 (2016) was performed to determine the absorbed energy (J). The absorbed energy thus determined was divided by the longitudinal cross-sectional area (cm ² ) of the V-notch test piece used to determine the absorbed energy (J/cm ² ) per unit area at -10°C. The longitudinal cross-sectional area (cm ² ) of the V-notch test piece was defined as described above. The absorbed energy (J/cm ² ) per unit area at -10°C for each test number thus obtained is shown in Tables 2 and 3 as "E(-10°C) (J/cm ² )".

[Corrosion test]
The seamless steel pipes of each test number were subjected to a corrosion test according to the above-mentioned method in accordance with ASTM G48 (2011) Method E to evaluate the pitting corrosion resistance. First, a test piece was prepared from the center of the wall thickness of the seamless steel pipe of each test number. The size of the test piece for the corrosion test was 3 mm thick, 25 mm wide, and 50 mm long, and the longitudinal direction of the test piece was parallel to the axial direction of the seamless steel pipe.

The prepared test pieces of each test number were immersed in a test solution (6% _FeCl3 + 1% HCl) with a specific liquid volume of 5 mL/cm2 or ^more and at 15°C. After the test pieces were immersed in the test solution, the temperature of the test solution was increased by 5°C every 24 hours, and the occurrence of pitting corrosion was confirmed by the naked eye. The temperature at which pitting corrosion occurred was taken as the CPT (°C). The obtained CPT (°C) for each test number is shown in Tables 2 and 3.

[Evaluation results]
With reference to Tables 1 to 3, the steel materials of test numbers 1 to 29 had appropriate chemical compositions, and Fn1 was 30.0 or more. Furthermore, the manufacturing method was the preferred manufacturing method described in the specification. As a result, the volume fraction of ferrite was 30.0 to 70.0%, and the proportion of austenite with a KAM value of 1 to 5° was 25.0 area % or more. As a result, the yield strength was 552 MPa or more, the CPT exceeded 15°C, and the absorbed energy per unit area at -10°C was 130 J/ ^cm2 or more. That is, the steel materials of test numbers 1 to 29 had a yield strength of 552 MPa or more, and the steel materials had excellent pitting corrosion resistance and excellent low-temperature toughness.

On the other hand, in the steel material of test number 30, the solution cooling rate CR _900-400 was too slow. Furthermore, the strain-increasing cooling rate CR _400-100 was too slow. As a result, the proportion of KAM values of 1 to ⁵ ° in austenite was less than 25.0 area %. As a result, the yield strength was less than 552 MPa, the CPT was 15°C, and the absorbed energy per unit area at -10°C was less than 130 J/cm2. That is, the steel material of test number 30 did not have a yield strength of 552 MPa or more, excellent pitting corrosion resistance, or excellent low-temperature toughness.

In the steels of test numbers 31 and 32, the strain increasing cooling rate CR _400-100 was too slow. As a result, the proportion of KAM values of 1 to 5° in austenite was less than 25.0 area %. As a result, the yield strength was less than 552 MPa. In other words, the steels of test numbers 31 and 32 did not have a yield strength of 552 MPa or more.

In the steels of test numbers 33 and 34, the cooling end temperature was too high. As a result, the proportion of austenite with KAM values of 1 to 5° was less than 25.0 area %. As a result, the yield strength was less than 552 MPa. In other words, the steels of test numbers 33 and 34 did not have a yield strength of 552 MPa or more.

In the steel material of test number 35, Fn1 was less than 30.0. As a result, the CPT was 15°C. In other words, the steel material of test number 35 did not have excellent pitting corrosion resistance.

In the steel material of test number 36, the Cr content was too low. As a result, the volume fraction of ferrite was less than 30.0%. As a result, the yield strength was less than 552 MPa. In other words, the steel material of test number 36 did not have a yield strength of 552 MPa or more.

In the steel material of test number 37, the Ni content was too low. As a result, the volume fraction of ferrite exceeded 70.0%. As a result, the absorbed energy per unit area at -10°C was less than 130 J/ ^cm2 . In other words, the steel material of test number 377 did not have excellent low-temperature toughness.

The above describes the embodiments of the present disclosure. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by modifying the above-described embodiments as appropriate within the scope of the spirit of the present disclosure.

Claims (4)

In mass percent,
C: 0.030% or less,
Si: 0.20-1.00%,
Mn: 0.50-7.00%,
P: 0.040% or less,
S: 0.020% or less,
Al: 0.100% or less,
Ni: 4.20-9.00%,
Cr: 20.00-30.00%,
Mo: 0.50-2.00%,
Cu: 1.50-4.00%,
N: 0.150-0.350%,
V: 0.01-1.50%,
Nb: 0 to 0.100%,
Ta: 0-0.100%,
Ti: 0 to 0.100%,
Zr: 0 to 0.100%,
Hf: 0-0.100%,
W: 0-0.200%,
Co: 0 to 0.500%,
Sn: 0-0.100%,
Sb: 0 to 0.1000%,
Ca: 0-0.020%,
Mg: 0 to 0.020%,
B: 0 to 0.020%,
Rare earth elements: 0 to 0.200%, and
A chemical composition satisfying formula (1), with the balance being Fe and impurities;
A microstructure having a volume fraction of 30.0 to 70.0% ferrite and the remainder being austenite,
The yield strength is 552 MPa or more,
The proportion of the KAM value of 1 to 5° in the austenite is 25.0 area% or more.
Duplex stainless steel material.
Cr+3.3(Mo+0.5W)+16N≧30.0 (1)
Here, the content (mass%) of the corresponding element is substituted for the element symbol in formula (1). When the corresponding element is not contained, "0" is substituted for the element symbol. 2. The duplex stainless steel material according to claim 1,
The chemical composition is
Nb: 0.001 to 0.100%,
Ta: 0.001 to 0.100%,
Ti: 0.001 to 0.100%,
Zr: 0.001 to 0.100%,
Hf: 0.001 to 0.100%, and
W: Contains one or more elements selected from the group consisting of 0.001 to 0.200%;
Duplex stainless steel material. The duplex stainless steel material according to claim 1 or 2,
The chemical composition is
Co: 0.001 to 0.500%,
Sn: 0.001 to 0.100%, and
Sb: Contains one or more elements selected from the group consisting of 0.0001 to 0.1000%;
Duplex stainless steel material. The duplex stainless steel material according to any one of claims 1 to 3,
The chemical composition is
Ca: 0.001-0.020%,
Mg: 0.001-0.020%,
B: 0.001 to 0.020%, and
Rare earth elements: containing one or more elements selected from the group consisting of 0.001 to 0.200%;
Duplex stainless steel material.

Images