JP2020158816A - Austenitic stainless steel and method for producing the same - Google Patents

Abstract

To provide an austenitic stainless steel that is excellent in strength and hydrogen embrittlement resistance, and to provide a method for producing the same.SOLUTION: An austenitic stainless steel, containing, in mass%, Cr: 10.5 to 20.0%, Ni: 10.0% or more, N: 0.020 to 0.500%, and in which the value of Md30 is -180 to -20 (°C), the average crystal grain size is 10.0 μm or less, the average lattice constant difference, which is a difference between the average lattice constant of the austenite phase in the surface layer part and the average lattice constant of the austenite phase in the center part, is 0.003 Å or more, and, in the surface layer part, the amount of austenite phase is 98.0% or more.SELECTED DRAWING: None

Description

The present invention relates to austenitic stainless steel and a method for producing the same.

In recent years, attempts have been made to control the emission of greenhouse gases such as carbon dioxide from the viewpoint of reducing the environmental load. From this point of view, fuel cells are attracting attention as a clean energy source that does not emit greenhouse gases, and are expected to spread in a wide range of fields such as transportation means such as automobiles, household or commercial power sources, etc. ing.

In general, fuel cells often use hydrogen as a fuel. Hydrogen is a very small element and is known to penetrate into metallic materials and cause embrittlement. In order to realize the widespread use of fuel cells, the metal materials used for the fuel cells themselves, hydrogen storage tanks, pipes for supplying hydrogen, etc. are hydrogen embrittled even in an environment where hydrogen is present at a high concentration. Is required to be suppressed.

Under the above-mentioned environment, the use of austenitic stainless steel has been studied. For example, Patent Documents 1 and 2 disclose austenitic stainless steel having excellent hydrogen embrittlement resistance.

Japanese Unexamined Patent Publication No. 2009-13301 Japanese Unexamined Patent Publication No. 2014-114471

Hydrogen used as fuel for fuel cells is compressed at high pressure. For this reason, austenitic stainless steels and the like used in the above-mentioned hydrogen storage tanks and the like are required to have higher strength from the viewpoint of fuel transportation efficiency and the like. On the other hand, the higher the strength of a metal material such as austenitic stainless steel, the more likely it is that hydrogen embrittlement will occur. Therefore, the austenitic stainless steel used in the above-mentioned hydrogen gas usage environment has a problem that it is difficult to achieve both strength and hydrogen embrittlement resistance.

The austenitic stainless steels disclosed in Patent Documents 1 and 2 are considered to have room for improvement in strength and hydrogen embrittlement resistance.

Based on the above, it is an object of the present invention to solve the above problems and to provide an austenitic stainless steel having excellent strength and hydrogen embrittlement resistance and a method for producing the same.

The present invention has been made to solve the above problems, and the gist of the present invention is the following austenitic stainless steel and a method for producing the same.

(1) By mass%
Cr: 10.5 to 20.0%,
Ni: 10.0% or more,
N: 0.020 to 0.500%,
Contains,
The value of Md _{30 represented by} the following equation (i) is −180 to −20 (° C.).
The average crystal grain size is 10.0 μm or less,
When calculating the average lattice constant of the austenite phase using the following equation (ii) based on the diffraction peak of X-ray diffraction, the average lattice constant of the austenite phase in the surface layer and the average lattice constant of the austenite phase in the center The difference, the average lattice constant difference is 0.003 Å or more,
An austenitic stainless steel having an austenitic phase amount of 98.0% or more calculated by the following formula (iii) in the surface layer portion.
Md ₃₀ (° C.) = 497-462C-9.2Si-8.1Mn-13.7Cr-20Ni-18.7Mo ... (i)
d _Ave. = {Dγ ₍₁₁₁₎ × Iγ ₍₁₁₁₎ + dγ ₍₂₀₀₎ × Iγ ₍₂₀₀₎ + dγ ₍₂₂₀₎ × Iγ ₍₂₂₀₎ + dγ ₍₃₁₁₎ × Iγ ₍₃₁₁₎ } / {Iγ ₍₁₁₁₎ + Iγ ₍₂₀₀₎ + Iγ ₍₂₂₀₎ + Iγ ₍₃₁₁₎ } ・ ・ ・ (ii)
r = 100 × ΣIγ / ΣI all ... (iii)
However, each element symbol in the above formula (i) represents the content (mass%) of each element contained in the steel, and if it is not contained, it is set to zero, and each of the above formulas (ii) and (iii). The symbol is defined by:
d _Ave. : Average lattice constant of austenite phase dγ _(hkl) : Lattice constant (Å) calculated from the diffraction peak of the (hkl) plane of the austenite phase
Iγ _(hkl) : Integrated intensity (cps) of the diffraction peak of the (hkl) plane of the austenite phase.
r: Austenite phase amount ΣIγ: Sum of integrated intensities of diffraction peaks of all measured austenite phases (cps)
All ΣI: Sum of integrated intensities of all measured diffraction peaks (cps)

(2) The austenitic stainless steel according to (1) above, wherein the average lattice constant difference is 0.03 Å or more.

(3) The chemical composition is mass%
C: 0.1% or less,
Si: 1.0% or less,
Mn: 3.0% or less,
P: 0.045% or less,
S: 0.030% or less,
Cr: 10.5 to 20.0%,
Ni: 10.0% or more,
N: 0.020 to 0.500%,
Mo: 0-3.0%,
Nb: 0-0.5%,
Ti: 0-0.5%,
V: 0-0.5%,
Remaining: The austenitic stainless steel according to (1) or (2) above, which is Fe and impurities.

(4) A method for manufacturing austenitic stainless steel.
(A) The process of hot-working steel pieces to make them into processed materials,
(B) A step of cold-working the processed material at a temperature of 20 ° C. or lower and a cross-sectional reduction rate of 80% or more.
(C) A step of heat-treating the cold-worked processed material at a temperature in the range of 700 to 1000 ° C.
(D) A step of allowing the processed material to absorb nitrogen and setting the N content within the range according to claim 1.
The method for producing an austenitic stainless steel according to any one of (1) to (3) above.

(5) The method for producing an austenitic stainless steel according to (4) above, wherein the step (c) and the step (d) are simultaneously performed.

(6) After the step (b) and before the step (c),
(E) The process of bending and returning
The method for producing austenitic stainless steel according to (4) and (5) above.

(7) After the step (b) and before the step (e),
(F) Welding process
The method for producing an austenitic stainless steel according to (6) above.

(8) In the chemical composition of the steel piece, the N content is less than 0.020%, the content of each element other than N is in the range described in (3) above, and the value of Md ₃₀ is The method for producing an austenitic stainless steel according to any one of (4) to (7) above, which is in the range of −180 to −20 (° C.).

According to the present invention, an austenitic stainless steel having excellent strength and hydrogen embrittlement resistance can be obtained.

The present inventors have conducted various studies in order to obtain an austenitic stainless steel having excellent strength and hydrogen embrittlement resistance. As a result, the following findings (a) to (f) were obtained.

(A) Some austenitic stainless steels undergo transformation when processed to form a martensite phase. Hydrogen easily penetrates into the martensite phase, specifically the process-induced martensite, resulting in hydrogen embrittlement. Therefore, it is important to stabilize the austenite phase and suppress the transformation to the martensite phase described above. In order to stabilize the austenite phase, it is effective to contain a predetermined amount of Ni, which is an austenite stabilizing element.

(B) In order to obtain high strength, it is effective to make the crystal grains finer. However, in the austenite single-phase metal structure, there is a limit to the refinement of crystal grains when the conventional production method is used. Therefore, it is effective to refine the crystal grains by utilizing the transformation between the austenite phase and the martensite phase described above.

(C) Specifically, during production, a transformation from the austenite phase to the martensite phase is caused by strong processing or processing at a low temperature. Then, heat treatment is performed at a temperature higher than the temperature at which the austenite transformation occurs, and this time, a transformation from the martensite phase to the austenite phase (hereinafter, also referred to as “reverse transformation”) is generated. Strain is accumulated during the transformation from the austenite phase to the martensite phase, and the accumulated strain causes the crystal grains to become finer during the reverse transformation. As a result, high strength and a good balance between strength and elongation can be obtained, and austenite stability can be improved by grain refinement.

(D) However, if the metal structure is as it is, it will be deformed when it is processed again. Therefore, it is effective to absorb nitrogen from the surface and dissolve it in the austenitic stainless steel together with the heat treatment or separately from the heat treatment.

Austenitic stainless steel with finer crystal grains has an increased grain boundary density. Therefore, nitrogen is sufficiently absorbed even at a relatively low temperature. Nitrogen is a powerful austenite stabilizing element. Therefore, the solid solution of nitrogen stabilizes the austenite phase, and subsequent processing does not cause martensitic transformation.

As a result, hydrogen embrittlement resistance is improved. Nitrogen is an intrusive solid solution element, and it dissolves by invading between crystal lattices. Like nitrogen, hydrogen, which is a factor of hydrogen embrittlement, is an intrusive solid solution element and is considered to invade between the crystal lattices from the surface of steel. Here, since nitrogen has penetrated into the surface in advance and is dissolved in solid solution, hydrogen is less likely to penetrate. As a result, it is considered that hydrogen embrittlement resistance may be improved.

(E) In addition, nitrogen dissolved in austenitic stainless steel improves the balance between strength and elongation.

(F) Based on the above, the martensite phase is formed by processing, etc., but it is necessary to adjust the chemical composition of the austenitic stainless steel so that the austenitic phase can be stabilized by the subsequent absorption of nitrogen. is there.

The present invention has been made based on the above findings. Hereinafter, each requirement of the present invention will be described in detail.

1. 1. Austenitic stainless steel 1-1. Chemical composition The austenitic stainless steel according to the present invention contains Cr: 10.5 to 20.0%, Ni: 10.0% or more, and N: 0.020 to 0.500% in mass%. In addition to these elements, other specific chemical compositions will be described. In the following description, "%" for the content means "mass%".

Cr: 10.5 to 20.0%
Cr is an important element in stainless steel and is an effective element for obtaining corrosion resistance. It also promotes the absorption of nitrogen. Therefore, the Cr content is set to 10.5% or more. However, since Cr is a ferrite stabilizing element, if Cr is excessively contained, the austenite phase becomes unstable. It also increases the likelihood of forming compounds with C and / or N. As a result, corrosion resistance is reduced. Therefore, the Cr content is set to 20.0% or less. The Cr content is preferably 11.0% or more, and more preferably 12.0% or more. The Cr content is preferably 19.4% or less, and more preferably 19.0% or less.

Ni: 10.0% or more Ni is a strong austenite-stabilizing alloy element and has the effect of stabilizing the austenite phase and suppressing hydrogen embrittlement. Therefore, the Ni content is set to 10.0% or more. The Ni content is preferably more than 12.0%. However, since Ni is an expensive and rare alloying element, excessive content is not desirable.

Here, when 10.5% of Cr, which is an element constituting stainless steel, is contained and the value of Md ₃₀ , which will be described later, is −180 ° C., the upper limit of Ni is about 27%. However, it is also difficult to set the other elements to 0% in order to adjust the value of Md ₃₀ . Therefore, the Ni content is preferably 20.0% or less, more preferably 19.0% or less, and further preferably 16.0% or less.

N: 0.020 to 0.500%
N is a strong austenite stabilizing element and a solid solution strengthening element. Therefore, the N content is set to 0.020% or more. However, if N is excessively contained, a nitride is formed, which is not preferable from the viewpoint of strength and corrosion resistance. Therefore, the N content is set to 0.500% or less. The N content is preferably 0.050% or more, and more preferably 0.070% or more. The N content is preferably 0.460% or less, and more preferably 0.400% or less.

Here, the austenitic stainless steel according to the present invention absorbs nitrogen and dissolves it from the surface side. Therefore, in the austenitic stainless steel according to the present invention, it is considered that nitrogen is concentrated near the surface, and a difference in the solid solution amount of nitrogen occurs between the surface side and the central portion of the steel, which will be described later. , The N content on the surface is considered to be about 0.1% or more higher than that in the central part.

The above-mentioned N content is a value when the N content is averaged in the entire austenitic stainless steel after nitrogen absorption. For example, in the case of a steel sheet or the like, it is a value averaged over the total thickness of the steel sheet after nitrogen absorption. Therefore, it can be measured in the same way as other elements.

It is desirable to reduce nitrogen as much as possible in steel before it absorbs nitrogen. This is because inclusions containing nitrogen are formed during melting, which has an adverse effect and deteriorates hot workability. Therefore, before absorbing nitrogen, it may be contained as an impurity as long as the N content is at a level of less than 0.200%. Before absorbing nitrogen, the N content is preferably less than 0.130%. And it is desirable to increase by absorption of nitrogen.

The austenitic stainless steel according to the present invention may contain the following elements in the following ranges.

C: 0.1% or less C, together with N, is a strong austenite stabilizing alloy element and a solid solution strengthening element. However, if C is excessively contained, a large amount of carbides are precipitated in the heat treatment for the purpose of grain refinement and the subsequent heat treatment for the purpose of N absorption at a lower temperature. As a result, the required austenite stability and strength cannot be obtained. Therefore, the C content is preferably 0.1% or less, and more preferably 0.08% or less. On the other hand, in order to obtain the above effect, the C content is preferably 0.01% or more.

Si: 1.0% or less Si is an element used as a deoxidizer during melting. However, if Si is excessively contained, coarse inclusions are generated and the workability is deteriorated. Therefore, the Si content is preferably 1.0% or less. The Si content is more preferably 0.9% or less. In order to obtain the above effect, the Si content is preferably 0.05% or more.

Mn: 3.0% or less Mn is a relatively inexpensive and effective austenite stabilizing alloy element. However, if Mn is excessively contained, coarse inclusions are generated and the workability is deteriorated. Therefore, the Mn content is preferably 3.0% or less, and more preferably 2.6% or less. On the other hand, in order to obtain the stabilizing effect of the austenite phase, the Mn content is preferably 0.1% or more.

P: 0.045% or less P is an element contained as an impurity, but it is preferable to reduce it as much as possible because it deteriorates workability. Therefore, the P content is preferably 0.045% or less, and more preferably 0.030% or less. On the other hand, excessive reduction of P increases the manufacturing cost. Therefore, the P content is preferably 0.005% or more.

S: 0.030% or less S is an element contained as an impurity, but it is preferable to reduce it as much as possible because it forms sulfide-based inclusions and lowers corrosion resistance. Therefore, the S content is preferably 0.030% or less, and more preferably 0.020% or less. On the other hand, excessive reduction of S increases the manufacturing cost. Therefore, the S content is preferably 0.001% or more.

Mo: 0-3.0%
Mo is an element effective for improving the corrosion resistance of stainless steel. Mo is an extremely expensive alloying element. If Mo is contained in an excessive amount, coarse inclusions are generated and the workability is deteriorated. Therefore, the Mo content is preferably 3.0% or less, and more preferably 2.6% or less. On the other hand, in order to obtain the above effect, the Mo content is preferably 0.1% or more.

The austenitic stainless steel according to the present invention may contain one or more selected from Nb, Ti, and V, if necessary, in order to refine the crystal grains.

Nb: 0-0.5%
Ti: 0-0.5%
V: 0-0.5%
Nb, Ti and V are elements that bind to C and N to form a compound that suppresses the growth of crystal grains by a pinning effect. Therefore, it may be contained as needed. However, if any of the elements exceeds 0.5%, a coarse compound may be formed and the formation of the austenite phase may become unstable. Further, the processability is deteriorated, and a coarse compound containing these elements may be the starting point of fracture. Therefore, it is preferable that Nb: 0.5% or less, Ti: 0.5% or less, and V: 0.5% or less. It is more preferable that Nb: 0.4% or less, Ti: 0.4% or less, and V: 0.4% or less. On the other hand, in order to obtain the above effects, it is preferable that Nb: 0.01% or more, Ti: 0.01% or more, and V: 0.01% or more.

In the chemical composition of the present invention, the balance is Fe and impurities. Here, the "impurity" is a component mixed with raw materials such as ore and scrap, and various factors in the manufacturing process when steel is industrially manufactured, and is allowed as long as it does not adversely affect the present invention. Means something.

About Md ₃₀ Md ₃₀ represented by the following formula (i) is an index showing the stability of the austenite phase. That is, Md ₃₀ is an index showing the susceptibility to process-induced martensitic transformation, and the higher the value of Md ₃₀ , the more likely it is to transform into process-induced martensite.

Md ₃₀ (° C.) = 497-462C-9.2Si-8.1Mn-13.7Cr-20Ni-18.7Mo ... (i)
However, each element symbol in the above formula (i) represents the content (mass%) of each element contained in the steel, and if it is not contained, it is set to zero.

Here, in the austenitic stainless steel according to the present invention, the value of Md _{30 represented by} the above formula (i) is −180 to −20 (° C.). When the value of Md ₃₀ is less than −180 ° C., the transformation from the austenite phase to the martensite phase does not occur, and therefore the reverse transformation does not occur. As described above, in the austenitic stainless steel according to the present invention, the crystal grains are refined by utilizing the above transformation.

The martensite phase formed by transformation acts extremely effectively as a base structure for miniaturization. Therefore, the crystal grains cannot be sufficiently refined unless the transformation to process-induced martensite occurs. As a result, the strength cannot be improved. Therefore, the value of Md ₃₀ is preferably −180 (° C.) or higher, and preferably −160 (° C.) or higher. However, when the value of Md ₃₀ is more than -20 (° C.), the transformation from the austenite phase to the martensite phase easily occurs, and as will be described later, even if nitrogen is absorbed and dissolved, the austenite phase The organization cannot be maintained. Therefore, the value of Md ₃₀ is preferably −20 (° C.) or less, and preferably −30 (° C.) or less.

1-2. Average crystal grain size In the austenitic stainless steel according to the present invention, the average crystal grain size is set to 10.0 μm or less in order to obtain desired strength and elongation. The average crystal grain size is preferably 8.0 μm or less, more preferably 5.0 μm or less. The lower limit of the average crystal grain size is not particularly determined, but is usually considered to be 0.1 μm or more. By setting the average crystal grain size within the above range, the grain boundary density increases, and the absorption and solid solution of nitrogen, which will be described later, become easy.

The average crystal grain size is measured by the following procedure. Specifically, a test piece is collected so that the vertical cross section of the steel in the rolling direction is the observation surface. The test piece is embedded in a resin or the like, polished, and the observation surface is corroded with a corrosive liquid that corrodes grain boundaries, specifically, an acid mixed aqueous solution or the like. Then, the crystal grain size is measured by observing the structure of the observation surface using an optical microscope or a scanning electron microscope (SEM).

The observation magnification shall be the same value of 400 times or more, and the total observation field of view shall be 12 fields of view, and the measurement shall be performed by the intercept method. The observation visual fields are 4 visual fields at the center of the plate thickness, 2 visual fields each at the 1/4 position of the plate thickness from both surface sides (4 visual fields in total), and 2 visual fields in the vicinity of both surfaces (4 visual fields in total), for a total of 12 The field of view. In observing these structures, the average crystal grain size is calculated from the average value of the obtained crystal grain sizes.

1-3. Average lattice constant difference of austenitic phase In the austenitic stainless steel according to the present invention, nitrogen is absorbed from the surface of the steel and dissolved in solid solution in order to stabilize the austenitic phase, as will be described later. Nitrogen is called an invading solid solution element and enters between the crystal lattices of austenite and dissolves in a solid solution. For this reason, the solid solution of nitrogen distorts the crystal lattice and increases the distance between the lattices.

Nitrogen penetrates from the surface of the steel, dissolves in a solid solution, diffuses toward the central portion in order, and after absorbing nitrogen, the N content of the surface side is higher than that of the central portion. That is, the interstitial distance on the surface, that is, the average lattice constant is larger, and the average lattice constant becomes smaller toward the center.

Therefore, in the austenitic stainless steel according to the present invention, the difference between the average lattice regularity of the austenite phase in the surface layer portion and the average lattice regularity of the austenite phase in the central portion (hereinafter, simply referred to as “average lattice regularity difference”) is defined as It should be 0.003 Å or more. If the difference between the average lattice constants is less than 0.003 Å, nitrogen absorption is insufficient and the austenite phase cannot be stabilized. As a result, transformation to the martensite phase is likely to occur, and the hydrogen embrittlement resistance is deteriorated. Further, since the solid solution of nitrogen is not sufficient, the effect of improving the strength cannot be obtained.

Therefore, the average lattice constant difference is preferably 0.003 Å or more, and preferably 0.005 Å or more. Further, in order to completely suppress the martensitic transformation of the steel surface when the Ni content is 12.0% or more, the average lattice constant difference is more preferably 0.03 Å or more.

The "surface layer portion" here means a depth at which X-rays can penetrate from the surface when Cu is used as an X-ray source and X-ray diffraction described later is performed. Specifically, from the measurement surface. It means a region up to about 10 μm in the vertical depth direction.

By the way, the change in the lattice constant depending on the solid solution amount N of nitrogen can be obtained from the following empirical formula (1).
d (Å) = 3.5946 + 0.0348 × N ・ ・ ・ (1)
However, each symbol in the above equation (1) is defined by the following.
N: Nitrogen solid solution d (Å): Lattice constant

With reference to the above equation (1), when the average lattice constant difference is 0.003 Å or more, the N content differs by about 0.1%. That is, it is considered that the amount of N solid solution on the surface is about 0.1% or more higher than that in the central portion.

The average lattice constant difference can be measured by the following procedure. Specifically, a sample is cut out from the surface of the steel and the (thickness) center in parallel with the surface. For example, in the case of a steel plate, it is the surface of the steel plate, and the central portion corresponds to the central portion of the plate thickness. The obtained sample is measured using an X-ray diffractometer. In X-ray diffraction, a general 2θ / θ method is used. An X-ray diffraction profile is obtained by using CuKα1 (wavelength: 1.54056 nm) as an X-ray source and setting a voltage: 40 kV, a current: 40 mA, and 2θ = 20 to 120 °.

Based on the obtained diffraction peak of the diffraction profile at the surface and the center, the average lattice constant of the austenite phase at the surface and the center is calculated by using the following equation (ii). Then, the average lattice constant of the austenite phase in the central portion is subtracted from the average lattice constant of the austenite phase on the obtained surface, and the average lattice constant difference is calculated.
d _Ave. = {Dγ ₍₁₁₁₎ × Iγ ₍₁₁₁₎ + dγ ₍₂₀₀₎ × Iγ ₍₂₀₀₎ + dγ ₍₂₂₀₎ × Iγ ₍₂₂₀₎ + dγ ₍₃₁₁₎ × Iγ ₍₃₁₁₎ } / {Iγ ₍₁₁₁₎ + Iγ ₍₂₀₀₎ + Iγ ₍₂₂₀₎ + Iγ ₍₃₁₁₎ } ・ ・ ・ (ii)

However, each symbol in the above equation (ii) is defined by the following.
d _Ave. : Average lattice constant of austenite phase dγ _(hkl) : Lattice constant (Å) calculated from the diffraction peak of the (hkl) plane of the austenite phase
Iγ _(hkl) : Integrated intensity (cps) of the diffraction peak of the (hkl) plane of the austenite phase.

In addition, dγ _(hkl) can be calculated based on the following equation (2).
dγ _(hkl) = 1 / (2sinθ) × nλ ・ ・ ・ (2)
However, each symbol in the above equation (2) is defined by the following.
n: Surface index (calculated by (h ² + k ² + l ² ) ^1/2 )
θ: Peak angle (angle at the center of the half-value width, which is the part that becomes the value of 1/2 of the maximum intensity of each peak)
λ: X-ray wavelength

1-4. Austenitic phase amount In the austenitic stainless steel according to the present invention, the amount of austenite phase on the surface layer is specified. The austenite phase amount is calculated by the following formula (iii), and in the austenitic stainless steel according to the present invention, the austenite phase amount is 98.0% or more in the surface layer portion. This is to suppress the formation of the martensite phase, which is prone to hydrogen embrittlement, so that the steel surface, which is the starting point of hydrogen embrittlement, is covered with the austenite phase.

Here, for example, when the amount of the austenite phase is 98.0% or more, it is considered that the surface that is easily hydrogen embrittled is sufficiently covered with the austenite phase, and the desired hydrogen embrittlement resistance property can be obtained.

In the present invention, even when the martensite phase is partially formed locally, if 98.0% or more is the austenitic phase, it is collectively referred to as austenitic stainless steel.

r = 100 × ΣIγ / ΣI all ... (iii)
However, each symbol in the above equation (iii) is defined by the following.
r: Austenite phase amount ΣIγ: Sum of integrated intensities of peaks of all measured austenite phases (cps)
All ΣI: Sum of integrated intensities of all measured peaks (cps)

In addition, also in the diffraction intensity integrated intensity ratio, the "surface layer portion" means a depth at which X-rays can penetrate from the surface when Cu is used as an X-ray source and X-ray diffraction described later is performed. Means a region from the measurement surface to about 10 μm in the vertical depth direction.

Based on the above, the amount of austenite phase obtained by X-ray diffraction and calculated by the following formula (iii) is 98.0% or more, preferably 100.0%.

The integrated intensity ratio of diffraction is measured by the following procedure. Specifically, a sample is taken from the surface of the steel. The obtained sample is measured using an X-ray diffractometer. In X-ray diffraction, a general 2θ / θ method is used. An X-ray diffraction profile is obtained by using CuKα1 (wavelength: 0.154056 nm) as an X-ray source and setting a voltage: 40 kV, a current: 40 mA, and 2θ = 20 to 120 °. From the obtained profile, all peaks are identified and substituted into the above equation (iii) to calculate the austenite phase amount.

2. 2. Target Characteristics In the austenitic stainless steel according to the present invention, a tensile strength of 800 MPa or more is evaluated as a good strength. Further, in the austenitic stainless steel according to the present invention, the tensile properties are measured after holding (hydrogen charging) at 200 ° C. for 100 hours in hydrogen gas at 60 MPa, and hydrogen embrittlement is evaluated based on the width before holding. Specifically, when the plate width reduction rate, which will be described later, is 85% or more, it is evaluated as having good hydrogen embrittlement resistance. Regarding the balance between strength and elongation, a case where the product of tensile strength and elongation is 40,000 (MPa ·%) or more is evaluated as a good characteristic.

3. 3. Steel material The austenitic stainless steel according to the present invention is not particularly limited in terms of the type of steel material. For example, thin plates, thick plates, shaped steels, steel bars, wire rods, steel pipes, etc. can be considered as an example.

4. Manufacturing Method The manufacturing method of the austenitic stainless steel sheet according to the present invention will be described below. The austenitic stainless steel sheet according to the present invention can be stably obtained by the method described below.

4-1. Casting and hot rolling steps The N content is less than 0.200%, the content of each element except N is in the above range, and the value of Md ₃₀ is −180 to −20 (° C.) described above. ) Is preferably produced. Subsequently, the produced steel piece is hot-processed to obtain a processed material. During hot working or after hot working, annealing and pickling may be carried out as appropriate, if necessary.

4-2. Cold processing The obtained processed material is cold processed. Cold working is performed at a temperature of room temperature or lower, that is, a temperature of 20 ° C. or lower, and under a condition that the cross-sectional reduction rate is 80% or more.

In cold working, the cross-sectional reduction rate is set to 80% or more and the working temperature is set to room temperature or lower by promoting the transformation from the austenite phase to the martensite phase and forming a martensite single-phase structure after cold working. This is preferable. Both the increase in the cross-sectional reduction rate and the decrease in the processing temperature promote the processing-induced martensitic transformation, and there is a possibility of single-phase structure formation only by adjusting each factor, but by utilizing both factors, single-phase structure formation Becomes easier.

Therefore, the cross-sectional reduction rate of cold working is preferably 80% or more, more preferably 85% or more. The processing temperature is preferably 20 ° C. or lower, more preferably 15 ° C. or lower. Furthermore, it is most preferable to immerse the steel sheet in liquid nitrogen at a lower temperature and perform strong processing at the liquid nitrogen temperature (-196 ° C.). After cold working, annealing may be performed as appropriate.

4-3. Bending and bending back processing The processed material that has undergone the above cold processing may be subjected to bending and bending back processing without changing its shape and dimensions. Bending and bending back processing can be performed, for example, by winding austenitic stainless steel around a roll or the like. Such bending and bending back processing can introduce a large strain to the surface and is effective even when the austenite phase is stable, and by repeating the process, the transformation to the martensite phase is sufficiently generated. be able to.

As a result, the crystal grains on the surface can be further refined. After the bending / bending back processing, a heat treatment at 700 to 1000 ° C., which will be described later, is performed to reverse transform the work-induced martensite formed by the bending / bending back processing into austenite again. This bending / bending back processing and the heat treatment described later may be repeated a plurality of times. Further, it is preferable that the bending / bending back processing is performed at the same temperature as the above-mentioned cold processing.

The bending / bending back processing is effective when welding is performed to form a product shape. In welding, the crystal grains of the welded portion are usually coarsened by heat input. Therefore, after welding, the welded portion is subjected to bending and bending back processing without changing the shape, so that the coarsened crystal grains can be transformed again to obtain a fine austenite phase.

4-4. Heat treatment Subsequently, it is preferable to perform a heat treatment for holding the processed material at a temperature in the range of 700 to 1000 ° C. This is because austenite transformation does not occur when the holding temperature in the heat treatment is less than 700 ° C. Therefore, the holding temperature in the heat treatment is preferably 700 ° C. or higher, preferably 750 ° C. or higher. However, when the heat treatment temperature exceeds 1000 ° C., the crystal grains become coarser than 10 μm, and it is not possible to obtain a strength of 800 MPa or more. Therefore, the heat treatment temperature is preferably 1000 ° C. or lower, preferably 980 ° C. or lower.

By heat-treating the processed material at a temperature in the range of 700 to 1000 ° C., it is possible to form a metal structure of an austenite phase of fine crystal grains using a fine martensite structure as a base. As a result, the hydrogen embrittlement resistance can be improved by improving the balance between the strength and elongation of the austenitic stainless steel and stabilizing the austenitic phase. The average crystal grain size of the steel during the heat treatment is preferably 0.50 μm or more.

4-5. Nitrogen absorption It is preferable to absorb nitrogen from the surface of the processed material during the heat treatment, that is, at the same time as the heat treatment or after the heat treatment. The temperature at which nitrogen is absorbed is preferably 400 ° C. or higher, more preferably 420 ° C. or higher. The upper limit is preferably a temperature equal to or lower than the above-mentioned heat treatment temperature due to the problem of grain size. The hydrogen, hydrogen sulfide, and a reducing gas for the purpose of removing the oxide film on stainless steel such as hydrogen fluoride, N _2, preferably in the condition which is a mixed atmosphere of nitrogen source to become gas that ammonia.

By absorbing nitrogen on the surface in this way, the austenite phase reverse-transformed by the heat treatment can be stabilized and the hydrogen embrittlement resistance can be improved. In addition, hydrogen embrittlement occurs when hydrogen invades from the surface and accumulates between crystal lattices. Here, nitrogen is an intrusive solid solution element, and it is considered that the presence of nitrogen on the surface of austenitic stainless steel makes it difficult for hydrogen to invade and improves hydrogen embrittlement resistance.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.

Steel pieces having the chemical compositions shown in Table 1 were produced.

The obtained steel pieces were heated at 1200 ° C. and held, then hot-rolled, then heated at 1150 ° C. and annealed for 30 minutes. Subsequently, cold rolling (cold working) and annealing at 1100 ° C. for 30 minutes were repeated once or multiple times on the obtained hot-rolled sheet, and then the final cold rolling resulted in a sheet thickness of 0.4 mm. It was made into a steel plate. In addition, No. In No. 19, after cold rolling, welding was performed, and bending and bending back processing was performed. Specifically, after cutting into strips to a predetermined size, both ends were electron beam welded, and then the welded portion was bent and bent back at least 100 times and the heat treatment described later was repeated 10 times while being immersed in liquid nitrogen. .. In addition, No. In No. 20, after cold rolling, only bending and bending back processing was performed.

In the bending and bending back processing, bending and bending back was performed by winding around a plurality of rolls having a diameter of 10 mm. The rolling rate (working rate) in the final cold rolling (cold working), that is, the total cold rolling rate and the working temperature are as shown in Table 2.

Subsequently, heat treatment was performed at the temperatures shown in Table 2. The heat treatment was carried out in a mixed gas atmosphere of 75% hydrogen (H ₂ ) + 25% nitrogen (N ₂ ) with a holding time of 1 minute. Then, it cooled to 20 degreeC.

Nitrogen absorption was carried out under two types of conditions at the temperature and atmosphere shown in Table 2. Nitrogen absorption at a holding temperature of 700 ° C or higher is carried out with a mixed gas of 49% hydrogen (H ₂ ) + 50% nitrogen (N ₂ ) + 1% hydrogen sulfide (H ₂ S) until it reaches 700 ° C, and from 700 ° C. After heating to the treatment temperature, holding and cooling to room temperature were carried out with a mixed gas of 50% hydrogen (H ₂ ) + 50% nitrogen (N ₂ ). The holding time is 1 minute. In Table 2, the above atmosphere is described as “H ₂ + N ₂ + H ₂ S”. It takes about 1 minute to reach 700 ° C. during heating.

In this example, nitrogen absorption at a relatively low temperature of less than 700 ° C., which will be described later, is also possible. Therefore, for convenience, heat treatment and nitrogen absorption were performed separately. However, when nitrogen absorption at 700 ° C. or higher is performed, heat treatment and nitrogen absorption can be performed in one step at the same time, which is rational. That is, only nitrogen absorption at 700 ° C. or higher may be carried out. No. In 33, No. The result of performing only nitrogen absorption at 900 ° C. using the same material as No. 2 is shown.

On the other hand, in the example in which nitrogen absorption was carried out by heating at a low temperature of 700 ° C. or lower, the nitrogen absorption was maintained at the same temperature for 4 hours. At this time, the atmosphere is a mixed gas of 98% ammonia (NH ₃ ) + 2% hydrogen sulfide (H ₂ S) during heating, keeping at 700 ° C or lower and cooling to room temperature with only 100% ammonia (NH ₃ ). did. This is indicated by "NH ₃ + H ₂ S" in Table 2. The time to reach the heating temperature was about 30 minutes.

(Average crystal grain size)
The average crystal grain size of the obtained steel sheet was measured by the following procedure. A test piece was collected so that the vertical cross section of the steel sheet in the rolling direction was the observation surface. The test piece was embedded in a resin or the like, polished, and the observation surface was corroded with a corrosive liquid that corrodes grain boundaries, specifically, an acid mixed aqueous solution or the like. Then, the crystal grain size was measured by observing the structure of the observation surface using an optical microscope or a scanning electron microscope (SEM). The observation magnification was the same value of 400 times or more, the observation field of view was 12 fields of view, and the measurement was performed by the intercept method. The observation visual fields are 4 visual fields at the center of the plate thickness, 2 visual fields each at the 1/4 position of the plate thickness (4 visual fields in total), and 2 visual fields each near both surfaces (4 visual fields in total), for a total of 12 visual fields. did. The average crystal grain size was calculated from those average values.

(Average lattice constant difference and austenite phase amount)
The average lattice constant difference was calculated by the following procedure. Specifically, a sample of a measurement surface parallel to the steel plate surface was cut out from the steel plate surface and the center of the plate thickness. The obtained sample was measured using an X-ray diffractometer. The 2θ / θ method was used for X-ray diffraction. An X-ray diffraction profile was obtained using CuKα1 (wavelength: 1.54056 nm) as the X-ray source, with a voltage of 40 kV, a current of 40 mA, and 2θ = 20 to 120 ° C. As the measuring device, a SmartLab manufactured by Rigaku Co., Ltd. was used.

Based on the diffraction peaks of the diffraction profile on the surface and the center, the average lattice constants of the austenite phase on the surface and the center were calculated using the above equation (ii). The average lattice constant difference of the austenite phase in the central part was subtracted from the average lattice constant of the austenite phase on the obtained surface to calculate the average lattice constant difference.

The amount of austenite phase was measured by the following procedure. A sample was taken from the surface of the steel plate. The obtained sample was measured using an X-ray diffractometer. The 2θ / θ method was used for X-ray diffraction. An X-ray diffraction profile was obtained by using CuKα1 (wavelength: 1.54056 nm) as an X-ray source and setting a voltage of 40 kV and a current of 40 mA, 2θ = 20 to 120 °. From the obtained profile, all peaks were identified and substituted into the above-mentioned equation (iii) to calculate the austenite phase amount. As a measuring device, a SmartLab manufactured by Rigaku Co., Ltd. was used.

(Tensile strength, elongation)
The tensile properties and elongation were measured by the following procedure. Specifically, JIS No. 13B (JIS G 0567: 2012) test pieces were sampled by cutting in parallel with the rolling direction, and tensile strength and elongation were measured using an Instron type tensile tester. The measurement was carried out at room temperature.

(Hydrogen embrittlement resistance)
Hydrogen embrittlement resistance was measured by the following procedure. Specifically, the tensile characteristics were examined after holding at 200 ° C. for 100 hours (hydrogen charge) in hydrogen gas at 60 MPa, and hydrogen embrittlement was determined by the plate width reduction rate. The judgment is the amount of width reduction after holding with respect to the amount of plate width reduction before holding (hereinafter, referred to as "plate width reduction rate"), that is, (width reduction amount after holding / plate width reduction amount before holding × 100). ). When the plate width reduction rate was less than 85%, it was indicated by x and judged to be embrittlement. On the other hand, a plate width reduction rate of less than 95% (85% or more) was indicated by ◯, and 95% or more was indicated by ⊚, and it was judged that the embrittlement was small or absent.

Test No. 1 to 22 are examples of the present invention satisfying the provisions of the present invention. Therefore, the product of strength and elongation exceeded 40,000, the plate width reduction rate was 85% or more, and a good balance between strength and elongation and hydrogen embrittlement resistance were obtained.

Among the above examples, Test No. At 4, 5, 11, 14, 15, 17, 19, 20, 21 and 22, the average lattice constant difference was 0.03 Å or more, so the plate width reduction rate was 95% or more, and even better hydrogen embrittlement resistance. It showed embrittlement. Further, as shown in Table 2, No. 1 in which rolling was carried out at a liquid nitrogen temperature. No. 5 and 15 and No. 5 and No. In No. 19, the crystal grains became finer, the product of strength and elongation became good, and this characteristic was improved.

Test No. 23 to 33 are comparative examples that do not satisfy the provisions of the present invention. No. 1 having a chemical composition satisfying the provisions of the present invention but was not produced under the production conditions of the present invention. In 23 to 26, at least one of the average crystal grain size, the average lattice constant difference, and the austenite phase amount is out of the specified range of the present invention, and at least one of the tensile strength, elongation, balance and hydrogen embrittlement resistance thereof. One has dropped.

No. In No. 23, since the rolling ratio was low as shown in Table 2, the crystal grains became coarse after that, and the austenite phase amount was also insufficient at 86%. Similarly, No. In 24 as well, as shown in Table 2, since the processing temperature was high, the crystal grains became coarse after that, and the amount of austenite phase was also insufficient.

In addition, No. At No. 25, although the heat treatment temperature was low and the high strength of more than 1000 MPa was maintained, the reverse transformation to the austenite phase did not sufficiently occur and fine crystal grains could not be obtained, so that the balance with elongation was the lowest. The proportion of the austenite phase was also low, and hydrogen embrittlement occurred. No. In No. 26, since gases such as H ₂ , H ₂ S, and HF that remove the oxide film of stainless steel were not used during nitrogen absorption, nitrogen absorption was insufficient, and the average lattice constant difference was specified in the present invention. It was out of range.

Therefore, the austenite phase was not sufficiently stabilized, the martensite phase remained, and hydrogen embrittlement occurred. Further, the average crystal grain size exceeded 10.0 μm, and the product of strength and elongation decreased.

No. whose chemical composition does not satisfy the provisions of the present invention. In 27 to 33, one or more of the strength and elongation, the balance between them, and the plate width reduction rate were deteriorated. No. In 27 to 31, it is considered that hydrogen embrittlement occurred because the austenite single-phase structure in which the crystal grains were refined was not adjusted and the martensite phase remained. No. In 32 and 33, the content of Nb, Ti, or V was excessive, so that a coarse compound was produced, and the balance between strength and elongation was lowered.

Claims (8)

By mass%
Cr: 10.5 to 20.0%,
Ni: 10.0% or more,
N: 0.020 to 0.500%,
Contains,
The value of Md _{30 represented by} the following equation (i) is −180 to −20 (° C.).
The average crystal grain size is 10.0 μm or less,
When calculating the average lattice constant of the austenite phase using the following equation (ii) based on the diffraction peak of X-ray diffraction, the average lattice constant of the austenite phase in the surface layer and the average lattice constant of the austenite phase in the center The difference, the average lattice constant difference is 0.003 Å or more,
An austenitic stainless steel having an austenitic phase amount of 98.0% or more calculated by the following formula (iii) in the surface layer portion.
Md ₃₀ (° C.) = 497-462C-9.2Si-8.1Mn-13.7Cr-20Ni-18.7Mo ... (i)
d _Ave. = {Dγ ₍₁₁₁₎ × Iγ ₍₁₁₁₎ + dγ ₍₂₀₀₎ × Iγ ₍₂₀₀₎ + dγ ₍₂₂₀₎ × Iγ ₍₂₂₀₎ + dγ ₍₃₁₁₎ × Iγ ₍₃₁₁₎ } / {Iγ ₍₁₁₁₎ + Iγ ₍₂₀₀₎ + Iγ ₍₂₂₀₎ + Iγ ₍₃₁₁₎ } ・ ・ ・ (ii)
r = 100 × ΣIγ / ΣI all ... (iii)
However, each element symbol in the above formula (i) represents the content (mass%) of each element contained in the steel, and if it is not contained, it is set to zero, and each of the above formulas (ii) and (iii). The symbol is defined by:
d _Ave. : Average lattice constant of austenite phase dγ _(hkl) : Lattice constant (Å) calculated from the diffraction peak of the (hkl) plane of the austenite phase
Iγ _(hkl) : Integrated intensity (cps) of the diffraction peak of the (hkl) plane of the austenite phase.
r: Austenite phase amount ΣIγ: Sum of integrated intensities of diffraction peaks of all measured austenite phases (cps)
All ΣI: Sum of integrated intensities of all measured diffraction peaks (cps) The austenitic stainless steel according to claim 1, wherein the average lattice constant difference is 0.03 Å or more. The chemical composition is mass%,
C: 0.1% or less,
Si: 1.0% or less,
Mn: 3.0% or less,
P: 0.045% or less,
S: 0.030% or less,
Cr: 10.5 to 20.0%,
Ni: 10.0% or more,
N: 0.020 to 0.500%,
Mo: 0-3.0%,
Nb: 0-0.5%,
Ti: 0-0.5%,
V: 0-0.5%,
The balance: the austenitic stainless steel according to claim 1 or 2, which is Fe and impurities. A method for manufacturing austenitic stainless steel
(A) The process of hot-working steel pieces to make them into processed materials,
(B) A step of cold-working the processed material at a temperature of 20 ° C. or lower and a cross-sectional reduction rate of 80% or more.
(C) A step of heat-treating the cold-worked processed material at a temperature in the range of 700 to 1000 ° C.
(D) A step of allowing the processed material to absorb nitrogen and setting the N content within the range according to claim 1.
The method for producing an austenitic stainless steel according to any one of claims 1 to 3. The method for producing an austenitic stainless steel according to claim 4, wherein the step (c) and the step (d) are performed at the same time. After the step (b) and before the step (c),
(E) The process of bending and returning
The method for producing austenitic stainless steel according to claims 4 and 5, further comprising. After the step (b) and before the step (e),
(F) Welding process
The method for producing an austenitic stainless steel according to claim 6, further comprising. In the chemical composition of the steel piece, the N content is less than 0.020%, the content of each element other than N is in the range according to claim 3, and the value of Md ₃₀ is −180 to −. The method for producing an austenitic stainless steel according to any one of claims 4 to 7, which is in the range of 20 (° C.).