Title of invention: Austenite-based stainless steel material
Technical field
[0001]
This disclosure relates to austenite-based stainless steel materials.
Background technology
[0002]
In recent years, the development of fuel cell vehicles that run on hydrogen as fuel and the practical research of hydrogen stations that supply hydrogen to fuel cell vehicles are underway. Stainless steel is one of the candidate materials used for these applications. However, in a high-pressure hydrogen gas environment, even stainless steel materials may cause brittleness (hydrogen brittleness) due to hydrogen gas. According to the standards for compressed hydrogen containers for automobiles stipulated in the High Pressure Gas Safety Law, the use of SUS316L as a stainless steel material with excellent hydrogen brittle resistance is permitted.
[0003]
However, considering the need for weight reduction of fuel cell vehicles, compactness of hydrogen stations, and high-pressure operation of hydrogen stations, stainless steel materials used for containers, joints, and pipes have excellent hydrogen brittle resistance in a hydrogen gas environment and are already available. It is desired to have a high strength of SUS316L or more.
[0004]
International Publication No. 2016/068009 (Patent Document 1) proposes austenite stainless steel having excellent hydrogen brittle resistance and high strength.
[0005]
The austenite stainless steel disclosed in Patent Document 1 has a chemical composition of% by mass, C: 0.10% or less, Si: 1.0% or less, Mn: 3.0% or more and less than 7.0%, Cr. : 15 to 30%, Ni: 12.0% or more and less than 17.0%, Al: 0.10% or less, N: 0.10 to 0.50%, P: 0.050% or less, S: 0. At least one of 050% or less, V: 0.01 to 1.0% and Nb: 0.01 to 0.50%, Mo: 0 to 3.0%, W: 0 to 6.0%, Ti: 0 ~ 0.5%, Zr: 0 ~ 0.5%, Hf: 0 ~ 0.3%, Ta: 0 ~ 0.6%, B: 0 ~ 0.020%, Cu: 0 ~ 5.0% , Co: 0 to 10.0%, Mg: 0 to 0.0050%, Ca: 0 to 0.0050%, La: 0 to 0.20%, Ce: 0 to 0.20%, Y: 0 to 0.40%, Sm: 0 to 0.40%, Pr: 0 to 0.40%, Nd: 0 to 0.50%, balance: Fe and impurities, the ratio of the minor axis to the major axis of the austenite crystal grains. Is larger than 0.1, the grain size number of the austenite crystal grains is 8.0 or more, and the tensile strength is 1000 MPa or more.
Prior art literature
Patent documents
[0006]
Patent Document 1: International Publication No. 2016/068009
Outline of the invention
Problems to be solved by the invention
[0007]
In the austenite stainless steel disclosed in Patent Document 1, the hydrogen brittle resistance is enhanced by setting the Ni content to 12.0% or more. Further, by finely precipitating the carbon nitride, the deformation of the crystal grains is suppressed by the pinning effect, and the crystal grains are made finer. As a result, high tensile strength can be obtained.
[0008]
However, Patent Document 1 contains a large amount of carbides such as V and Nb and alloying elements that form carbon nitride in order to utilize the pinning effect. Therefore, the manufacturing cost is high. There may be an austenite-based stainless steel material having excellent hydrogen brittleness resistance and high strength by means other than the means disclosed in Patent Document 1.
[0009]
An object of the present disclosure is to provide an austenite-based stainless steel material having high tensile strength and excellent hydrogen brittleness resistance.
Means to solve the problem
[0010]
The austenite-based stainless steel material according to this disclosure is
The chemical composition is mass%,
C: 0.100% or less,
Si: 1.00% or less,
Mn: 5.00% or less,
Cr: 15.00 to 22.00%,
Ni: 10.00-21.00%,
Mo: 1.20-4.50%,
P: 0.050% or less,
S: 0.050% or less,
Al: 0.100% or less,
N: 0.100% or less,
Cu: 0 to 0.70%, and
The rest consists of Fe and impurities,
The austenite crystal grain size number conforming to ASTM E112 is 5.0 to less than 8.0,
In the cross section perpendicular to the longitudinal direction of the austenite-based stainless steel material, the number density of precipitates having a dislocation cell structure ratio of less than 50 to 80% and a major axis of 1.0 μm or more is 5.0 / 0.2 mm. It is 2 or less.
Effect of the invention
[0011]
The austenite-based stainless steel material according to the present disclosure has high tensile strength and excellent hydrogen brittle resistance.
A brief description of the drawing
[0012]
[Fig. 1] Fig. 1 is an example of a bright-field image (TEM image) of an observation field in which a rearrangement cell structure is formed, which is obtained by observation with a transmission electron microscope in an austenite-based stainless steel material having a chemical composition of the present embodiment. It is a figure which shows.
FIG. 2 is a diagram showing an example of a TEM image in which a rearrangement cell structure is not formed in an austenite-based stainless steel material having a chemical composition of the present embodiment.
FIG. 3 is a diagram showing an example of a TEM image in which a rearrangement cell structure is not formed in an austenite-based stainless steel material having a chemical composition of the present embodiment, which is different from FIG.
FIG. 4 is an image obtained by binarizing the bright field image of FIG. 1 with the median value of the histogram of pixel values ​​as a threshold value.
FIG. 5 is a diagram extracted by drawing the extension of a low-density shift region (shift cell) having an area of ​​0.20 μm 2 or more based on the binarized image of FIG.
FIG. 6 is a schematic diagram for explaining a sampling position when the austenite-based stainless steel material of the present embodiment is a steel pipe.
FIG. 7 is a schematic diagram for explaining a sampling position when the austenite-based stainless steel material of the present embodiment is steel bar.
FIG. 8 is a schematic diagram for explaining a sample sampling position when the austenite-based stainless steel material of the present embodiment is a steel plate.
FIG. 9 is a diagram showing a backscattered electron image of a microstructure containing precipitates in austenite-based stainless steel material.
Embodiment for carrying out the invention
[0013]
The present inventors have studied an austenite-based stainless steel material having high tensile strength and excellent hydrogen brittle resistance. The inclusion of Cr, Ni and Mo is extremely effective in enhancing the hydrogen brittle resistance. Therefore, the present inventors have investigated the chemical composition of an austenite-based stainless steel material having excellent hydrogen brittleness resistance. As a result, the chemical composition is C: 0.100% or less, Si: 1.00% or less, Mn: 5.00% or less, Cr: 15.00 to 22.00%, Ni: 10. 00 to 21.00%, Mo: 1.20 to 4.50%, P: 0.050% or less, S: 0.050% or less, Al: 0.100% or less, N: 0.100% or less, It was considered that sufficient hydrogen brittleness resistance could be obtained if Cu: 0 to 0.70% and the balance was an austenite-based stainless steel material composed of Fe and impurities.
[0014]
Therefore, the present inventors further investigated the strength of the austenite-based stainless steel material having the above chemical composition. As described in Patent Document 1, it is considered that the strength is increased by producing fine precipitates such as V precipitates and Nb precipitates and making the crystal grains finer by the pinning effect of the fine precipitates. However, these precipitates can be the starting point for hydrogen cracking when cold working is performed.
[0015]
Therefore, the present inventors did not adopt a method of increasing the strength by the pinning effect of the precipitate, but dared to study a method of increasing the strength by a method different from the pinning effect of the precipitate. As a result, the present inventors have found for the first time that in the austenite-based stainless steel material having the above-mentioned chemical composition, high strength can be obtained by forming a rearranged cell structure instead of utilizing the pinning effect of the precipitate. ..
[0016]
FIG. 1 shows a visual field (4.2 μm × 4) in which a rearranged cell structure was formed, which was obtained by observing the structure of an austenite-based stainless steel material having the above-mentioned chemical composition using a transmission electron microscope (TEM). It is a figure which shows the bright-field image (hereinafter, referred to as a TEM image) of .2 μm). 2 and 3 are views showing an example of a TEM image in which a rearrangement cell structure is not formed in an austenite-based stainless steel material having the above-mentioned chemical composition. FIG. 1 corresponds to the test number 1 of the examples described later. FIG. 2 corresponds to test number 16. FIG. 3 corresponds to test number 12.
[0017]
FIGS. 1 to 3 are all austenite-based stainless steel materials having the above-mentioned chemical composition. In FIG. 2, short shifts 105 are sparsely present, but shifts 105 do not form cells. Further, in FIG. 3, a large number of shifts 105 are present, but the shifts 105 do not form cells.
[0018]
On the other hand, in the TEM image shown in FIG. 1, the state of the rearrangement is different from that in FIGS. 2 and 3. Specifically, in FIG. 1, a cell wall region 101 having a high dislocation density (a region having low brightness (black) in a TEM image) and a low density region surrounded by the cell wall region 101 and having a low dislocation density. There is a shift region 102 (a region with high brightness in the TEM image). In FIG. 1, the cell wall region 101 is formed in a mesh pattern. The low-density shift region 102 is surrounded by the cell wall region 101. In the present specification, the structure in which the mesh-like cell wall region 101 and the low-density shift region 102 are present is referred to as a “shift cell structure”. More specifically, as will be described later, the cell wall region 101 and the low-density dislocation region 102 exist in the field of view of 4.2 μm × 4.2 μm in the cross section perpendicular to the longitudinal direction of the austenite-based stainless steel material. In addition, when there are nine or more low-density dislocation regions 102 having an area of ​​20 μm 2 or more, the visual field is recognized as a visual field in which a “dislocation cell structure” is formed.
[0019]
In the austenite-based stainless steel material having the above-mentioned chemical composition, the present inventors set the austenite crystal grains to 5.0 or more with a crystal grain size number conforming to ASTM E112, and formed a rearranged cell structure to form a precipitate. It was found that high strength can be obtained without using the pinning effect of. More specifically, it was found that when the dislocation cell structure ratio defined by the following method is 50% or more, excellent hydrogen brittle resistance and high tensile strength can be obtained.
[0020]
Here, the dislocation cell structure ratio is defined by the following method.
[0021]
Select any 30 visual fields with a size of 4.2 μm × 4.2 μm in the cross section perpendicular to the longitudinal direction of the austenite stainless steel material. A bright field image (TEM image) by a transmission electron microscope (TEM) is generated in each selected field. In the generated TEM image, a cell wall region 101 having a high dislocation density and a low density dislocation region 102 that is a region surrounded by the cell wall region 101 and has a low dislocation density are specified. In each visual field, among the plurality of specified low-density dislocation regions 102, a visual field in which nine or more low-density dislocation regions 102 having an area of ​​0.20 μm 2 or more are present is a visual field in which a dislocation cell structure is formed. Recognize that there is. The ratio of the number of fields in which the shift cell structure is formed to all the fields (30 fields) is defined as the shift cell structure ratio (%).
[0022]
More specifically, the dislocation cell structure ratio is specified by the following method. Three samples are taken in a cross section perpendicular to the longitudinal direction of austenite stainless steel material. The surface to be inspected for each sample shall be a cross section perpendicular to the longitudinal direction of the austenite-based stainless steel material. Wet polishing is performed until the thickness of each sample reaches 30 μm. After wet polishing, a mixed solution of perchloric acid (10 vol.%) And ethanol (90 vol.%) Is used to perform electrolytic polishing on the sample to prepare a thin film sample. Structure observation using TEM is performed on the surface to be inspected of each thin film sample. Specifically, TEM observation is performed in any 10 fields on the surface to be inspected of each sample. The size of each field is a rectangle of 4.2 μm × 4.2 μm. The acceleration voltage during TEM observation is 200 kV. Observing crystal grains that can be observed by the incident electron beam of <110> Let's be an elephant. A bright field image (TEM image) is acquired in each field.
[0023]
Using the bright field image (TEM image) of each field, it is determined by the following method whether or not each field has a dislocation cell structure. In the following description, a method for determining the dislocated cell structure will be described using the bright field image (TEM image) shown in FIG. 1 as an example. In a bright field image (TEM image), a histogram showing the frequency of pixel values ​​(0 to 255) is generated, and the median value of the histogram is obtained. The number of pixels of the bright field image in each field is not particularly limited, but is, for example, 100,000 pixels or more and 150,000 pixels or less. The bright field image is binarized with the median value as the threshold value. FIG. 4 is an image obtained by binarizing the bright field image of FIG. 1 with the median value of the histogram of the pixel values ​​as a threshold value. In the binarized image, the black region is the region where the shift density is high. Therefore, the black region is recognized as the cell wall region 101. On the other hand, the white region is a region where the dislocation density is low. Therefore, the white closed region surrounded by the cell wall region 101 is defined as the low density shift region 102.
[0024]
The extension of the white closed region (low density shift region 102) is defined, and the area of ​​each low density shift region 102 is obtained. Then, the low-density shift region 102 having an area of ​​0.20 μm 2 or more is recognized as a “shift cell”.
[0025]
FIG. 5 is a diagram extracted by drawing the extension of the low density shift region 102 (shift cell) having an area of ​​0.20 μm 2 or more based on the binarized image of FIG. In FIG. 5, when the extensions of the low-density shift regions 102 are in contact with each other, the areas of the low-density shift regions 102 are calculated as one low-density shift region 102. In the case of the visual field of FIG. 1, there are 11 low-density shift regions 102.
[0026]
When the number of low-density shift regions 102 is determined by the same method for FIGS. 2 and 3 by the above-mentioned method, the number of low-density shift regions 102 in FIG. 2 is two, and that is shown in FIG. The number of low-density shift regions 102 is four.
[0027]
By the above analysis method, the number of shift cells (low density shift region 102 having an area of ​​0.20 μm 2 or more) in each visual field (4.2 μm × 4.2 μm) is determined. Then, when there are 9 or more shift cells in each visual field, the visual field is recognized as the visual field in which the shift cell structure is formed. In each field, if there are three or more straight lines that intersect both of the two opposite sides (opposite sides) of the field (4.2 μm × 4.2 μm rectangular bright field image), the field has a planner structure. It is recognized as, and it is not recognized as a translocation cell structure. Of the 30 visual fields observed, the number of visual fields in which the rearranged cell structure is formed is determined. Then, the dislocation cell structure ratio (%) is defined by the following equation.
Dislocation cell structure ratio = number of visual fields in which dislocation cell structure is formed / total number of visual fields x 100
[0028]
Calculation of the median value of the histogram of the pixel values ​​of the above-mentioned photographic image (bright-field image), binarization processing of the photographic image, identification of the extension of the low-density shift region 102, and the area of ​​the low-density shift region 102. For all calculations, well-known image processing software may be used. A well-known image processing software is, for example, ImageJ (trade name). It is well known to those skilled in the art that the same analysis can be performed by image processing software other than ImageJ.
[0029]
If it has the above chemical composition and the dislocation cell structure ratio based on the above definition is 50% or more, high strength can be obtained in the austenite-based stainless steel material. The reason is not clear, but the following reasons can be considered. Of the dislocation cell structures, in the cell wall region 101, which is a high-density dislocation region, dislocations are densely entangled with each other. Therefore, the dislocations constituting the cell wall region 101 are difficult to move and are fixed. As a result, it is considered that the strength of the austenite-based stainless steel material is increased.
[0030]
The content of each element in the chemical composition of the austenite-based stainless steel material is within the above range, the austenite crystal grain size number according to ASTM E112 is 5.0 or more, and the dislocation cell structure ratio is 50% or more. However, if a large amount of coarse precipitates are present in the steel material, hydrogen is stored at the interface between the coarse precipitates and the matrix (austenite), and the hydrogen brittle resistance is lowered. Therefore, the present inventors have an austenite crystal grain size number of 5.0 or more according to ASTM E112, a rearrangement cell structure ratio of 50% or more, and the content of each element in the chemical composition is within the above range. In a certain austenite-based stainless steel material, the relationship between the coarse precipitate and the hydrogen brittle resistance was investigated and investigated. As a result, the content of each element in the chemical composition is within the above range, the austenite crystal grain size number conforming to ASTM E112 is 5.0 or more, and the dislocation cell structure ratio is 50% or more. It was found that when the number density of precipitates having a major axis of 1.0 μm or more is 5.0 / 0.2 mm 2 or less, excellent hydrogen brittle resistance and high tensile strength can be obtained.
[0031]
The austenite-based stainless steel material of the present embodiment completed based on the above findings has the following constitution.
[0032]
[1]
Austenite stainless steel material
The chemical composition is mass%,
C: 0.100% or less,
Si: 1.00% or less,
Mn: 5.00% or less,
Cr: 15.00 to 22.00%,
Ni: 10.00-21.00%,
Mo: 1.20-4.50%,
P: 0.050% or less,
S: 0.050% or less,
Al: 0.100% or less,
N: 0.100% or less,
Cu: 0 to 0.70%, and
The rest consists of Fe and impurities,
The austenite crystal grain size number conforming to ASTM E112 is 5.0 to less than 8.0,
In the cross section perpendicular to the longitudinal direction of the austenite-based stainless steel material, the number density of precipitates having a dislocation cell structure ratio of less than 50 to 80% and a major axis of 1.0 μm or more is 5.0 / 0.2 mm. 2 or less
Austenite stainless steel material.
[0033]
[2]
The austenite-based stainless steel material described in [1].
The austenite crystal grain size number is 5.8 or more.
Austenite stainless steel material.
[0034]
[3]
The austenite-based stainless steel material according to [1] or [2].
The dislocation cell structure ratio is 55% or more.
Austenite stainless steel material.
[0035]
[4]
The austenite-based stainless steel material according to any one of [1] to [3].
The number density of precipitates having a major axis of 1.0 μm or more is 4.5 / 0.2 mm 2 or less.
Austenite stainless steel material.
[0036]
[5]
The austenite-based stainless steel material according to any one of [1] to [4].
The chemical composition is
Contains Cu: 0.01-0.70%,
Austenite stainless steel material.
[0037]
Hereinafter, the austenite-based stainless steel material of the present embodiment will be described in detail. Unless otherwise specified, "%" for an element means% by mass.
[0038]
[Chemical composition]
The chemical composition of the austenite-based stainless steel material of the present embodiment contains the following elements.
[0039]
C: 0.100% or less
Carbon (C) is an unavoidable impurity. That is, the C content is more than 0%. C produces carbides at the austenite crystal grain boundaries and reduces the hydrogen brittleness resistance of the steel material. If the C content exceeds 0.100%, the hydrogen brittle resistance of the steel material is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the C content is 0.100% or less. The preferred upper limit of the C content is 0.080%, more preferably 0.070%, still more preferably 0.060%, still more preferably 0.040%, still more preferably 0.035. %, More preferably 0.030%, still more preferably 0.025%. The C content is preferably as low as possible. However, if the C content is excessively reduced, the manufacturing cost will increase. Therefore, considering normal industrial production, the lower limit of the C content is preferably 0.001%, more preferably 0.002%, still more preferably 0.005%, still more preferably 0. It is 010%, more preferably 0.015%.
[0040]
Si: 1.00% or less
Silicon (Si) is inevitably contained. That is, the Si content is more than 0%. Si deoxidizes steel. However, if the Si content is too high, Si binds to Ni, Cr and the like to promote the formation of a sigma (σ) phase. When the Si content exceeds 1.00%, the hot workability and toughness of the steel material are lowered due to the formation of the σ phase even if the content of other elements is within the range of the present embodiment. Therefore, the Si content is 1.00% or less. The preferred upper limit of the Si content is 0.90%, more preferably 0.70%, still more preferably 0.60%, still more preferably 0.50%. If the Si content is excessively reduced, the manufacturing cost will increase. Therefore, considering normal industrial production, the preferred lower limit of the Si content is 0.01%, more preferably 0.02%. The preferable lower limit of the Si content for more effectively enhancing the deoxidizing action of the steel is 0.10%, more preferably 0.20%.
[0041]
Mn: 5.00% or less
Manganese (Mn) is inevitably contained. That is, the Mn content is more than 0%. Mn stabilizes austenite. However, if the Mn content is too high, the formation of δ-ferrite is promoted. If the Mn content exceeds 5.00%, δ ferrite is generated and the hydrogen brittle resistance of the steel material is lowered even if the content of other elements is within the range of the present embodiment. Therefore, the Mn content is 5.00% or less. The preferred lower limit of the Mn content is 0.30%, more preferably 0.50%, still more preferably 1.00%, still more preferably 1.50%, still more preferably 1.60%. %. The preferred upper limit of the Mn content is 4.80%, more preferably 4.30%, still more preferably 3.80%, still more preferably 3.30%, still more preferably 2.95%. %.
[0042]
Cr: 15.00 to 22.00%
Chromium (Cr) enhances the hydrogen brittleness resistance of steel materials. Cr further promotes the formation of rearranged cell structures. If the Cr content is less than 15.00%, these effects cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Cr content exceeds 22.00%, coarse charcoal such as M 23C 6 is produced even if the content of other elements is within the range of this embodiment. In this case, the hydrogen brittle resistance of the steel material is reduced. Therefore, the Cr content is 15.00 to 22.00%. The lower limit of the Cr content is preferably 15.50%, more preferably 16.00%, still more preferably 16.50%, still more preferably 17.00%. The preferred upper limit of the Cr content is 21.50%, more preferably 21.00%, still more preferably 20.50%, still more preferably 20.00%, still more preferably 19.50. %, More preferably 19.00%, still more preferably 18.50%.
[0043]
Ni: 10.00-21.00%
Nickel (Ni) stabilizes austenite and suppresses the formation of work-induced martensite. Therefore, the hydrogen brittle resistance of the steel material is increased. If the Ni content is less than 10.00%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Ni content exceeds 21.00%, the above effects are saturated and the production cost increases even if the content of other elements is within the range of the present embodiment. Therefore, the Ni content is 10.00 to 21.00%. The preferred lower limit of the Ni content is 10.50%, more preferably 11.00%, still more preferably 11... It is 50%, more preferably 12.00%, still more preferably 12.50%. The preferred upper limit of the Ni content is 17.50%, more preferably 17.00%, still more preferably 16.50%, still more preferably 16.00%, still more preferably 15.50%. %, More preferably 15.00%, still more preferably 14.50%.
[0044]
Mo: 1.20-4.50%
Molybdenum (Mo) enhances the hydrogen brittleness and strength of steel materials. Mo further refines the crystal grains and facilitates the formation of a rearranged cell structure. If the Mo content is less than 1.20%, this effect cannot be obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Mo content exceeds 4.50%, even if the content of other elements is within the range of the present embodiment, the effect is saturated and the production cost is only increased. Therefore, the Mo content is 1.20 to 4.50%. The lower limit of the Mo content is preferably 1.30%, more preferably 1.40%, still more preferably 1.60%. The preferred upper limit of the Mo content is 3.50%, more preferably 3.20%, still more preferably 3.00%.
[0045]
P: 0.050% or less
Phosphorus (P) is an impurity that is inevitably contained. That is, the P content is more than 0%. If the P content exceeds 0.050%, the hot workability and toughness of the steel material will decrease even if the content of other elements is within the range of this embodiment. Therefore, the P content is 0.050% or less. The preferred upper limit of the P content is 0.045%, more preferably 0.040%, still more preferably 0.035%, still more preferably 0.030%, still more preferably 0.025. %. It is preferable that the P content is as low as possible. However, excessive reduction of P content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the P content is 0.001%, more preferably 0.005%.
[0046]
S: 0.050% or less
Sulfur (S) is an impurity that is inevitably contained. That is, the S content is more than 0%. When the S content exceeds 0.050%, the hot workability and toughness of the steel material are lowered even if the content of other elements is within the range of the present embodiment. Therefore, the S content is 0.050% or less. The preferred upper limit of the S content is 0.030%, more preferably 0.025%. It is preferable that the S content is as low as possible. However, excessive reduction of S content increases manufacturing costs. Therefore, considering normal industrial production, the preferable lower limit of the S content is 0.001%.
[0047]
Al: 0.100% or less
Aluminum (Al) is inevitably contained. That is, the Al content is more than 0%. Al deoxidizes the steel. If Al is contained even in a small amount, this effect can be obtained to some extent. However, if the Al content exceeds 0.100%, oxides and intermetal compounds are likely to be formed in the steel material even if the content of other elements is within the range of the present embodiment, and the toughness of the steel material becomes high. descend. Therefore, the Al content is 0.100% or less. The preferable lower limit of the Al content for more effectively deoxidizing the steel material is 0.001%, and more preferably 0.002%. The preferred upper limit of the Al content is 0.050%, more preferably 0.040%, still more preferably 0.030%. In the present specification, the Al content is sol. It means the content of Al (acid-soluble Al).
[0048]
N: 0.100% or less
Nitrogen (N) is inevitably contained. That is, the N content is more than 0%. N increases the strength of the steel material. If N is contained even in a small amount, the above effect can be obtained to some extent. However, if the N content exceeds 0.100%, coarse nitrides are likely to be produced even if the content of other elements is within the range of the present embodiment. Therefore, the N content is 0.100% or less. The lower limit of the N content is preferably 0.001%, more preferably 0.005%, still more preferably 0.010%. The preferred upper limit of the N content is 0.090%, more preferably 0.080%, still more preferably 0.070%.
[0049]
The balance of the chemical composition of the austenite-based stainless steel material according to the present embodiment consists of Fe and impurities. Here, the impurities are those mixed from ore, scrap, or the manufacturing environment as a raw material when the austenite-based stainless steel material of the present embodiment is industrially manufactured, and the austenite of the present embodiment. It means that it is permissible as long as it does not adversely affect the stainless steel material.
[0050]
[About arbitrary elements]
The chemical composition of the austenite-based stainless steel material according to the present embodiment may further contain Cu instead of a part of Fe.
Cu: 0 to 0.70%
Copper (Cu) is an optional element and does not have to be contained. That is, the Cu content may be 0%. When contained, Cu enhances the corrosion resistance of the steel material. If even a small amount of Cu is contained, the above effect can be obtained to some extent. However, if the Cu content exceeds 0.70%, the hot workability of the steel material deteriorates even if the content of other elements is within the range of the present embodiment. Therefore, the Cu content is 0 to 0.70%. The lower limit of the Cu content is preferably 0.01%, more preferably 0.05%, still more preferably 0.10%, still more preferably 0.15%, still more preferably 0.20. %. The preferred upper limit of the Cu content is 0.65%, more preferably 0.60%, still more preferably 0.55%, still more preferably 0.50%.
[0051]
[Austenite crystal grain size number]
In the austenite-based stainless steel material of the present embodiment, the austenite crystal grain size number conforming to ASTM E112 is 5.0 to less than 8.0. Here, ASTM is an abbreviation for American Society for Testing and Material.
[0052]
If the austenite crystal grain size number is less than 5.0, it is difficult to form the rearranged cell structure described later. If the rearranged cell structure is not formed, the strength of the austenite-based stainless steel material having the above-mentioned chemical composition is low.
[0053]
When the austenite crystal grain size number is 5.0 or more, a rearranged cell structure is formed in the austenite-based stainless steel material having the above-mentioned chemical composition. Specifically, when the austenite crystal grain size number is 5.0 or more, the crystal grains become fine. Therefore, the rearrangements formed in the crystal grains are short. Since short dislocations are easy to move, they are likely to be entangled with each other, and as a result, a dislocation cell structure is likely to be formed.
[0054]
In the steel material having the above-mentioned chemical composition, if the austenite crystal grain size number is 5.0 or more and the dislocation cell structure ratio is 50% or more in the microstructure, not only excellent hydrogen brittle resistance can be obtained. High strength can be obtained due to the finer grain size and the synergistic effect of the rearranged cell structure. The lower limit of the preferable crystal grain size number is 5.5, more preferably 5.8, still more preferably 5.9, still more preferably 6.0, still more preferably 6.1.
[0055]
The upper limit of the austenite crystal grain size number is not particularly limited. However, when the austenite-based stainless steel material is produced by the production method described later, the austenite crystal grain size number is less than 8.0. Therefore, in the present embodiment, the upper limit of the crystal grain size number of the austenite-based stainless steel material is less than 8.0. The upper limit of the crystal grain size number of the austenite-based stainless steel material is preferably 7.9, more preferably 7.8, still more preferably 7.5, still more preferably 7.0.
[0056]
The austenite crystal grain size number is obtained by the following method. Cut austenite-based stainless steel material vertically in the longitudinal direction. When the austenite-based stainless steel material is a steel pipe, as shown in FIG. 6, the wall thickness is defined as t (mm) in the cross section perpendicular to the longitudinal direction of the austenite-based stainless steel material. The t / 2 position (that is, the center position of the wall thickness) in the wall thickness direction from the outer surface is defined as the sampling position P1. The t / 4 position in the wall thickness direction from the outer surface is defined as the sampling position P2. The t / 4 position in the wall thickness direction from the inner surface is defined as the sampling position P3. The sample collected from the collection position P1 is referred to as sample P1. The sample collected from the collection position P2 is referred to as sample P2. The sample collected from the collection position P3 is referred to as sample P3. The test surface of each sample P1 to P3 has a cross section perpendicular to the longitudinal direction of the austenite-based stainless steel material. The sample P1 is collected so that the center position of the surface to be inspected substantially corresponds to the t / 2 position. The sample P2 is collected so that the center position of the surface to be inspected substantially corresponds to the t / 4 position. The sample P3 is collected so that the center position of the test surface corresponds to approximately the t / 4 position.
[0057]
When the austenite-based stainless steel material is steel bar, the radius is defined as R (mm) in the cross section perpendicular to the longitudinal direction of the austenite-based stainless steel material, as shown in FIG. The R position in the radial direction from the surface, that is, the center position of the cross section perpendicular to the longitudinal direction of the austenite stainless steel material is defined as the sampling position P1. In the diameter including the center position of the cross section, the R / 2 position in the radial direction from the surface of one end of the diameter is defined as the sampling position P2. The R / 2 position in the radial direction from the surface of the other end of the diameter is defined as the sampling position P3. Samples P1 to P3 are collected from the collection positions P1 to P3. The test surface of each sample P1 to P3 has a cross section perpendicular to the longitudinal direction of the austenite-based stainless steel material. Sample P1 is taken so that the center position of the surface to be inspected corresponds to the center position of the cross section perpendicular to the longitudinal direction of the steel bar. The sample P2 is collected so that the center position of the test surface substantially corresponds to the R / 2 position. The sample P3 is collected so that the center position of the test surface substantially corresponds to the R / 2 position.
[0058]
When the austenite-based stainless steel material is a steel plate, the plate thickness is defined as t (mm) in the cross section perpendicular to the longitudinal direction of the austenite-based stainless steel material, as shown in FIG. The t / 2 position in the plate thickness direction from the upper surface is defined as the sampling position P1. The t / 4 position in the plate thickness direction from the upper surface is defined as the sampling position P2. The t / 4 position in the plate thickness direction from the lower surface is defined as the sampling position P3. Samples P1 to P3 are collected from the collection positions P1 to P3. The test surface of each sample P1 to P3 has a cross section perpendicular to the longitudinal direction of the austenite-based stainless steel material. The sample P1 is collected so that the center position of the surface to be inspected substantially corresponds to the t / 2 position. The sample P2 is collected so that the center position of the surface to be inspected substantially corresponds to the t / 4 position. The sample P3 is collected so that the center position of the test surface corresponds to approximately the t / 4 position.
[0059]
The surface to be inspected of each sample P1 to P3 is mirror-polished. The mirror-polished surface to be inspected is corroded with a mixed acid (a solution mixed with hydrochloric acid: nitric acid = 1: 1) to reveal austenite crystal grain boundaries. The texture of each sample P1 to P3 to be inspected is observed using an optical microscope. The magnification of the optical microscope for tissue observation is set to 100 times. Arbitrary three visual fields are selected on the test surface of each sample P1 to P3. The size of each field is 1000 μm × 1000 μm. In each field, the austenite crystal grain size number is measured according to ASTM E112. The arithmetic average value of the austenite crystal grain size numbers obtained in nine visual fields (three visual fields in each sample P1 to P3) is defined as the austenite crystal grain size number of the austenite-based stainless steel material.
[0060]
[Transition cell structure]
The austenite-based stainless steel material of the present embodiment further has a dislocation cell structure ratio of less than 50 to 80% in a cross section perpendicular to the longitudinal direction of the austenite-based stainless steel material. Here, the shift cell set The weaving ratio is defined by the following method.
[0061]
[Definition of dislocation cell structure ratio]
In the cross section perpendicular to the longitudinal direction of the austenite-based stainless steel material of the present embodiment, any 30 visual fields having a size of 4.2 μm × 4.2 μm are selected. A TEM image (bright field image) is generated in each selected field. In the generated TEM image, the cell wall region 101 having a high dislocation density and the low density dislocation region 102 having a low dislocation density are specified. In each visual field, among the plurality of specified low-density dislocation regions 102, a visual field in which nine or more low-density dislocation regions 102 of 0.20 μm 2 or more are present is recognized as a visual field in which a dislocation cell structure is formed. The ratio of the number of fields in which the shift cell structure is formed to all the fields (30 fields) is defined as the shift cell structure ratio (%).
[0062]
More specifically, the dislocation cell structure ratio is specified by the following method.
[0063]
[Measurement method of dislocation cell structure ratio]
Samples P1 to P3 for observing the dislocation cell structure are collected from the above-mentioned collection positions P1 to P3 in a cross section perpendicular to the longitudinal direction of the austenite-based stainless steel material. The test surface of each sample P1 to P3 has a cross section perpendicular to the longitudinal direction of the austenite-based stainless steel material. Wet polishing is performed until the thickness of the samples P1 to P3 reaches 30 μm. After wet polishing, electrolytic polishing is performed on each sample P1 to P3 using a mixed solution of perchloric acid (10 vol.%) And ethanol (90 vol.%) To obtain thin film samples P1 to P3. The texture of each of the thin film samples P1 to P3 to be inspected is observed using a transmission electron microscope (TEM). Specifically, TEM observation is performed in any 10 fields of the test surface of each sample. The size of each field is a rectangle of 4.2 μm × 4.2 μm. The acceleration voltage during TEM observation is 200 kV. Crystal grains that can be observed by the incident electron beam of <110> are targeted for observation. A bright field image is generated in each field.
[0064]
Using the bright field image of each field, determine whether each field has a dislocated cell structure by the following method. In each bright field image, a histogram showing the frequency of pixel values ​​(0 to 255) is generated, and the median value of the histogram is obtained. The number of pixels of the bright field image in each field is not particularly limited, but is, for example, 100,000 pixels or more and 150,000 pixels or less. The bright field image is binarized with the median value as the threshold value. In FIG. 4, which is an example of a binarized image, the black region is a region having a high shift density. Therefore, the black region is recognized as the cell wall region 101. On the other hand, the white region is a region where the dislocation density is low. Therefore, the white closed region surrounded by the cell wall region 101 is defined as the low density shift region 102. The extension of the white closed region (low density shift region 102) is defined, and the area of ​​each low density shift region 102 is obtained. Then, the low-density shift region 102 having an area of ​​0.20 μm 2 or more is recognized as a “shift cell”.
[0065]
The number of shift cells (low density shift region 102 having an area of ​​0.20 μm 2 or more) in each visual field (4.2 μm × 4.2 μm) is determined. Then, when there are 9 or more shift cells in each visual field, the visual field is recognized as the visual field in which the shift cell structure is formed. In each field, if there are three or more straight lines that intersect both of the two opposite sides (opposite sides) of the field (4.2 μm × 4.2 μm rectangular bright field image), the field has a planner structure. It is recognized as, and it is not recognized as a translocation cell structure. Of the 30 visual fields observed, the number of visual fields in which the rearranged cell structure is formed is determined. Then, the dislocation cell structure ratio (%) is defined by the following equation.
Dislocation cell structure ratio = number of visual fields in which dislocation cell structure is formed / total number of visual fields x 100
[0066]
In the austenite-based stainless steel material according to the present embodiment, the dislocation cell structure ratio determined by the above definition is 50% or more. Therefore, the austenite-based stainless steel material according to the present embodiment not only has excellent hydrogen brittleness resistance, but also has high strength. In the cell wall region 101, the dislocations are densely entangled with each other. Therefore, the dislocations constituting the dislocation cell structure are difficult to move. As a result, it is considered that the strength of the austenite-based stainless steel material is increased.
[0067]
The upper limit of the dislocation cell structure ratio is not particularly limited, and it is preferable that the dislocation cell structure ratio is high. However, when the dislocation cell structure ratio is less than 50 to 80%, excellent hydrogen brittle resistance and sufficiently high strength can be obtained. The preferred lower limit of the dislocation cell structure ratio is 53%, more preferably 55%, still more preferably 56%, still more preferably 57%, still more preferably 58%, still more preferably 59%. It is more preferably 60%. The upper limit of the dislocation cell structure ratio may be 79%, 78%, 77%, 75%, or 72%. However, it may be 70% or 68%.
[0068]
[About the number density of coarse precipitates in steel materials]
Further, in the austenite-based stainless steel material of the present embodiment, the number density of precipitates having a major axis of 1.0 μm or more is 5.0 pieces / 0.2 mm 2 or less in a cross section perpendicular to the longitudinal direction of the austenite-based stainless steel material. ..
[0069]
In the austenite-based stainless steel material having the above-mentioned chemical composition, a precipitate having a major axis of 1.0 μm or more is defined as a “coarse precipitate”. The precipitate is a carbide, a nitride, a carbon nitride or the like, and is, for example, an M 23C 6 type carbide. The coarse precipitate easily adsorbs hydrogen at the interface with the matrix (austenite). Therefore, if the number of coarse precipitates is large, the hydrogen brittle resistance of the austenite-based stainless steel material is lowered. Precipitates having a major axis of less than 1.0 μm are less likely to adsorb hydrogen than coarse precipitates. Therefore, the effect on hydrogen brittle resistance is extremely small as compared with the coarse precipitate. Therefore, in this embodiment, attention is paid to the coarse precipitate.
[0070]
If the number of coarse precipitates is more than 5.0 / 0.2 mm 2, the content of each element in the chemical composition of the austenite-based stainless steel material is within the range of this embodiment, and the austenite crystal grain size conforming to ASTM E112. Even if the number is less than 6.0 to 8.0 and the dislocation cell structure ratio is less than 50 to 80%, sufficient hydrogen brittle resistance cannot be obtained. When the number of coarse precipitates is 5.0 / 0.2 mm 2 or less, the content of each element in the chemical composition of the austenite-based stainless steel material is within the range of this embodiment, and the austenite crystal grain size according to ASTM E112. Excellent hydrogen brittle resistance is obtained on the premise that the number is less than 5.0 to 8.0 and the dislocation cell structure ratio is less than 50 to 80%.
[0071]
[Method of measuring the number density of coarse precipitates]
The number density of coarse precipitates can be measured by the following method. A sample for measuring the number and density of coarse precipitates is collected from the above-mentioned sample collection positions P1 to P3. Hereinafter, the sample collected from the collection position P1 is referred to as a sample P1. The sample collected from the collection position P2 is referred to as sample P2. The sample collected from the collection position P3 is referred to as sample P3.
[0072]
The surface to be inspected for each sample P1 to P3 has a cross section perpendicular to the longitudinal direction of the austenite-based stainless steel material. The surface to be inspected is mirror-polished. The mirror-polished samples P1 to P3 are corroded with a mixed acid (a solution mixed with hydrochloric acid: nitric acid = 1: 1) to reveal austenite crystal grain boundaries and precipitates. The surface to be inspected after etching is observed in one field with a backscattered electron image using a scanning electron microscope (SEM). The field size is 400 μm × 500 μm. Precipitates in the field can be identified by contrast. FIG. 9 is an example of a backscattered electron image. With reference to FIG. 9, the black region 500 in the visual field is a precipitate.
[0073]
Measure the long axis of the precipitate. Specifically, the longest straight line connecting any two points at the interface between the precipitate and the matrix (austenite) is defined as the long axis (μm). Among the precipitates, those having a major axis of 1.0 μm or more are specified as “coarse precipitates”. The number of specified coarse precipitates is determined. Based on the number of obtained coarse precipitates and the visual field area (0.2 mm 2), the number density of coarse precipitates (pieces / 0.2 mm 2) in each sample P1 to P3 is determined. Then, the arithmetic average value of the three number densities is defined as the number densities of coarse precipitates (pieces / 0.2 mm 2).
[0074]
As described above, in the austenite-based stainless steel material of the present embodiment, each element in the chemical composition is within the above range, the austenite crystal grain size number according to ASTM E112 is less than 5.0 to 8.0, and the rearrangement. The cell structure ratio is less than 50 to 80%, and the number density of precipitates having a major axis of 1.0 μm or more is 5.0 pieces / 0.2 mm 2 or less. Therefore, in the austenite-based stainless steel material of the present embodiment, not only excellent hydrogen brittleness resistance but also high tensile strength can be obtained. The preferred upper limit of the number density of precipitates having a major axis of 1.0 μm or more is 4.7 / 0.2 mm 2, more preferably 4.3 / 0.2 mm 2, and even more preferably 4.0. Pieces / 0.2 mm 2, more preferably 3.7 pieces / 0.2 mm 2, still more preferably 3.3 pieces / 0.2 mm 2, and even more preferably 3.0 pieces / 0.2 mm. It is 2, more preferably 2.7 pieces / 0.2 mm 2.
[0075]
[Shape of austenite-based stainless steel material of this embodiment]
The shape of the austenite-based stainless steel material of the present embodiment is not particularly limited. The austenite-based stainless steel material of the present embodiment may be a steel pipe. The austenite-based stainless steel material of the present embodiment may be steel bar. The austenite-based stainless steel material of the present embodiment may be a steel plate. The austenite-based stainless steel material of the present embodiment may have a shape other than steel pipes, steel bars, and steel plates.
[0076]
[Use of austenite-based stainless steel material of this embodiment]
The austenite-based stainless steel material of the present embodiment can be widely applied to applications that require hydrogen brittleness resistance and high strength. The austenite-based stainless steel material of the present embodiment can be particularly used as a member for high-pressure hydrogen gas environmental use. The high-pressure hydrogen gas environmental application is, for example, a member used for a high-pressure hydrogen container mounted on a fuel cell vehicle, a member used for a high-pressure hydrogen container installed at a hydrogen station that supplies hydrogen to a fuel cell vehicle, and the like. be. However, the austenite-based stainless steel material of the present embodiment is not limited to high-pressure hydrogen gas environmental use. As described above, the austenite-based stainless steel material of the present embodiment can be widely applied to applications that require hydrogen brittleness resistance and high strength.
[0077]
[Production method]
Hereinafter, the method for manufacturing the austenite-based stainless steel material of the present embodiment will be described. The method for producing an austenite-based stainless steel material described below is an example of the method for producing an austenite-based stainless steel material according to the present embodiment. Therefore, the austenite-based stainless steel material having the above-mentioned structure may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferable example of the manufacturing method of the austenite-based stainless steel material of the present embodiment.
[0078]
An example of the method for producing an austenite-based stainless steel material of the present embodiment includes a preparation step, a heat treatment step, and a cold working step. Each process will be described in detail.
[0079]
[Preparation process]
In the preparation step, an intermediate steel material having the above-mentioned chemical composition is prepared. As the intermediate steel material having the above-mentioned chemical composition, those purchased from a third party may be used. Further, the manufactured product may be used. When manufacturing an intermediate steel material, for example, it is manufactured by the following method.
[0080] [0080]
The molten steel having the above-mentioned chemical composition is manufactured by a well-known method. Casting materials are manufactured using the manufactured molten steel by a well-known casting method. For example, an ingot is manufactured by the bulking method. Shards (slabs, bubs) by continuous casting method Rooms, billets, etc.) may be manufactured. Slabs, blooms and billets may be manufactured by performing hot working such as slab rolling and hot forging on the ingot. The material is manufactured by the above process.
[0081]
Perform hot working on the prepared material (hot working process). Hot working is, for example, hot forging, hot extrusion, hot rolling and the like. Hot forging is, for example, forging and stretching. Hot rolling is performed, for example, by performing tandem rolling using a tandem rolling machine including a plurality of rolling stands arranged in a row (each rolling stand has a pair of work rolls), and performing multiple pass rolling. Alternatively, reverse rolling may be carried out by a reverse rolling machine or the like having a pair of work rolls, and the rolling of the pass may be carried out a plurality of times. Hot extrusion is, for example, hot extrusion by the Eugene-Sejurne method. An intermediate steel material may be manufactured by the above manufacturing process. The preferred heating temperature T0 before hot working is 950 to 1100 ° C. The preferable holding time t0 at the heating temperature T0 is 20 minutes to 150 minutes (2.5 hours). If the heating temperature exceeds 1100 ° C., the crystal grains will be coarsened. As a result, even if the heat treatment step and the cold working step are carried out, the austenite crystal grain size number based on ASTM E112 tends to be less than 5.0.
[0082]
The preferable surface reduction rate in hot working is 50% or more. Here, the reduction rate (%) is defined by the following equation.
Surface reduction rate = (1-cross-sectional area perpendicular to the longitudinal direction of the intermediate steel material after hot working / cross-sectional area perpendicular to the longitudinal direction of the material before hot working) x 100
[0083]
The preferable lower limit of the reduction rate is 55%, and more preferably 60%. The upper limit of the reduction rate is not particularly limited. Considering the equipment load, the preferable upper limit of the surface reduction rate is, for example, 90%.
[0084]
[Heat treatment process]
In the heat treatment step, heat treatment is performed on the intermediate steel material having the above-mentioned chemical composition. Specifically, the holding time is t1 at the heat treatment temperature T1 (° C.). Then, after the holding time has elapsed, the intermediate steel material is rapidly cooled. Quenching is, for example, water cooling or oil cooling. The cooling rate is, for example, 100 ° C./sec or higher. The conditions for the heat treatment temperature T1 (° C.) and the holding time t1 (minutes) are as follows.
Heat treatment temperature T1: 950-1200 (° C)
Holding time at heat treatment temperature T1 t1: 5 ~ (1400-T1) / 5 (minutes)
[0085]
[About heat treatment temperature T1]
If the heat treatment temperature T1 is less than 950 ° C., the precipitate in the intermediate steel material does not sufficiently dissolve and remains in the steel material. In this case, the number density of coarse precipitates exceeds 5.0 / 0.2 mm 2. On the other hand, if the heat treatment temperature T1 exceeds 1200 ° C., the austenite crystal grains become coarse, and the austenite crystal grain size number of the produced austenite-based stainless steel material becomes less than 5.0. Therefore, the heat treatment temperature T1 is 950 to 1200 ° C. The preferred lower limit of the heat treatment temperature T1 is 980 ° C, more preferably 1050 ° C, still more preferably 1100 ° C. The preferred upper limit of the heat treatment temperature T1 is 1180 ° C.
[0086]
[About holding time t1]
F1 = (1400-T1) / 5. The heat treatment temperature T1 is substituted for "T1" in F1. When the holding time t1 is less than 5 minutes, the precipitate in the intermediate steel material does not sufficiently dissolve and remains in the steel material. In this case, the number density of coarse precipitates exceeds 5.0 / 0.2 mm 2. On the other hand, if the retention time t1 exceeds (1400-T1) / 5 minutes, the dislocation cell structure ratio becomes less than 50%. Therefore, the holding time t1 at the heat treatment temperature T1 is 5 to (1400-T1) / 5 minutes. The preferred lower limit of the holding time t1 is 10 minutes, more preferably 15 minutes. The preferred upper limit of the holding time t1 is F1-5 (minutes), and more preferably F1-10 (minutes).
[0087]
As described above, the intermediate steel material is rapidly cooled after being held at the heat treatment temperature T1 for the holding time t1. This suppresses the precipitation of alloy elements solidly dissolved by heat treatment during cooling. Quench cooling is, for example, water cooling or oil cooling. As a water cooling method, the steel material may be immersed in a water tank for cooling, or the steel material may be rapidly cooled by shower water cooling or mist cooling.
[0088]
When the steel material is manufactured by hot working, the heat treatment step may be carried out on the steel material immediately after the hot working is completed. For example, the steel material temperature (finishing temperature) immediately after the completion of hot working may be set to 950 to 1200 ° C., maintained for t1 hour, and then rapidly cooled. In this case, the same effect as the heat treatment using the above-mentioned heat treatment furnace can be obtained. When the steel material immediately after the completion of hot working is rapidly cooled, the heat treatment temperature T1 in the heat treatment step corresponds to the temperature (° C.) of the intermediate steel material immediately after hot working.
[0089]
[Cold processing process]
In the cold working process, cold working is performed on the intermediate steel material after the heat treatment process. Cold working is, for example, cold drawing, cold forging, cold rolling and the like. For example, if the steel material is a steel pipe or steel bar, cold drawing is performed. If the steel material is a steel plate, cold rolling is performed.
[0090]
The cross-sectional reduction rate RR in the cold working process shall be 15.0% or more. The cross-sectional reduction rate RR (%) in the cold working process is defined by the following equation.
Cross-section reduction rate RR = (1- (cross-sectional area of ​​intermediate steel material after completion of cold working in cold working process / cross-sectional area of ​​intermediate steel material before cold working process)) x 100
Here, the cross-sectional area of ​​the intermediate steel material means the area (mm 2) of the cross section perpendicular to the longitudinal direction (axial direction) of the intermediate steel material.
[0091]
When the cross-sectional reduction rate RR in the cold working process is less than 15.0%, the dislocation cell structure rate is less than 50%. Therefore, sufficiently high strength cannot be obtained. Therefore, the cross-sectional reduction rate RR in the cold working step is 15.0% or more. The preferred lower limit of the cross-sectional reduction rate RR is 18.0%, more preferably 19.0%, still more preferably 20.0%.
[0092]
The upper limit of the cross-sectional reduction rate RR is not particularly limited. However, if the cross-sectional reduction rate exceeds 80.0%, the effect of improving the strength is saturated. Therefore, the preferable upper limit of the cross-sectional reduction rate RR is 80.0%. A more preferable upper limit of the cross-sectional reduction rate RR is 75.0%, and even more preferably 70.0%. The processing direction in the cold processing step (cold drawing or cold rolling) is one direction. For example, when cold rolling is performed from a plurality of directions, the cell wall region 101 formed by performing cold rolling in one direction collapses due to cold rolling in the other direction. As a result, the rearranged cell structure is not sufficiently formed. Therefore, in the present embodiment, the cold working direction is one direction.
[0093]
Through the above manufacturing process, the austenite crystal grain size number having the above-mentioned chemical composition and conforming to ASTM E112 is 5.0 to less than 8.0, the dislocation cell structure ratio is less than 50 to 80%, and the major axis. It is possible to produce an austenite-based stainless steel material in which the number density of precipitates having a diameter of 1.0 μm or more is 5.0 pieces / 0.2 mm 2 or less.
[0094]
The above-mentioned manufacturing method is an example of the method for manufacturing the austenite-based stainless steel material of the present embodiment. Therefore, the austenite-based stainless steel material of the present embodiment has the above-mentioned chemical composition, has an austenite crystal grain size number of 5.0 to less than 8.0 according to ASTM E112, and has a dislocation cell structure ratio of 50 to 80%. If the number of precipitates having a major axis of 1.0 μm or more is less than 5.0 pieces / 0.2 mm 2 or less, it may be produced by another production method. The above-mentioned manufacturing method is a suitable example for manufacturing the austenite-based stainless steel material of the present embodiment.
Example
[0095]
The effect of the austenite-based stainless steel material of the present embodiment will be described more specifically by way of examples. The conditions in the following examples are one condition example adopted for confirming the feasibility and effect of the austenite-based stainless steel material of the present embodiment. Therefore, the austenite-based stainless steel material of the present embodiment is not limited to this one condition example.
[0096]
180 kg of austenite-based stainless steel having the chemical composition shown in Table 1 was melted in a vacuum to produce an ingot.
[0097]
[table 1]

[0098]
Hot forging and hot rolling were carried out on the ingot to manufacture a steel plate (intermediate steel material) with a width of 200 mm and a thickness of 20 mm. In any of the test numbers (see Table 2), the heating temperature T0 (° C.) during hot forging and the holding time t0 (minutes) at the heating temperature T0 (° C.) are as shown in Table 2. rice field. The surface reduction rate during hot forging was 65%. A heat treatment step was carried out on the manufactured intermediate steel materials of each test number. The heat treatment temperature T1 in the heat treatment step and the holding time t1 (minutes) at the heat treatment temperature T1 (° C.) are as shown in Table 2. After the holding time had elapsed, the steel plate was water-cooled immediately after being extracted from the heat treatment furnace. The cooling rate was, for example, 100 ° C./sec or higher.
[0099]
[Table 2]

[0100]
A cold working process was carried out on the intermediate steel material after the heat treatment process. Cold rolling was carried out as a cold working step. The cross-sectional reduction rate RR in the cold working step is as shown in Table 2. In test number 16, the cold working step was not carried out. Therefore, the cross-sectional reduction rate RR in the cold working step of test number 16 was 0%. The rolling direction of cold rolling was one direction. Through the above manufacturing process, an austenite-based stainless steel material (steel plate) was manufactured.
[0101]
[Evaluation test]
[Crystal particle size number measurement test]
As shown in FIG. 8, the plate thickness was defined as t (mm) in the cross section perpendicular to the longitudinal direction of the austenite-based stainless steel material. The t / 2 position in the plate thickness direction from the upper surface was defined as the sampling position P1. The t / 4 position in the plate thickness direction from the upper surface was defined as the sampling position P2. The t / 4 position in the plate thickness direction from the lower surface was defined as the sampling position P3. Samples P1 to P3 were collected from the collection positions P1 to P3. The test surface of each sample P1 to P3 has a cross section perpendicular to the longitudinal direction of the austenite-based stainless steel material. Sample P1 was taken so that the center position of the test surface substantially corresponds to the t / 2 position. Sample P2 was taken so that the center position of the test surface substantially corresponds to the t / 4 position. Sample P3 was taken so that the center position of the test surface substantially corresponds to the t / 4 position.
[0102]
The surface to be inspected for each sample P1 to P3 was mirror-polished. The mirror-polished surface to be inspected was corroded with a mixed acid (a solution mixed with hydrochloric acid: nitric acid = 1: 1) to reveal austenite crystal grain boundaries. The structure of each sample P1 to P3 to be inspected was observed using an optical microscope. The magnification of the optical microscope for tissue observation was set to 100 times. Arbitrary three visual fields were selected on the test surface of each sample P1 to P3. The size of each field was 1000 μm × 1000 μm. In each field, the austenite crystal grain size number was measured according to ASTM E112. The arithmetic average value of the austenite crystal grain size numbers obtained in nine visual fields (three visual fields in each sample P1 to P3) was defined as the austenite crystal grain size number. The obtained austenite crystal grain size numbers are shown in Table 2.
[0103]
[Dislocation cell structure ratio calculation test]
As shown in FIG. 8, the sampling position P1 is t / 2 in the plate thickness direction from the upper surface and t in the plate thickness direction from the upper surface, where t (mm) is the plate thickness in the cross section perpendicular to the longitudinal direction of the austenite stainless steel material. Samples P1 to P3 for observing the dislocation cell structure were collected from the collection position P2 at the / 4 position and the collection position P3 at the t / 4 position in the plate thickness direction from the lower surface. The test surface of each sample P1 to P3 has a cross section perpendicular to the longitudinal direction of the austenite-based stainless steel material. Wet polishing was performed until the thickness of the sample reached 30 μm. After wet polishing, electropolishing was performed on each sample P1 to P3 using a mixed solution of perchloric acid (10 vol.%) And ethanol (90 vol.%) to prepare thin film samples P1 to P3. .. Structure observation using TEM was performed on the test surface of each thin film sample P1 to P3. Specifically, among the test surfaces of each thin film sample P1 to P3, any 10 visual fields (10 visual fields in the thin film sample P1, thin film).TEM observation was performed with 10 fields of sample P2 and 10 fields of thin film sample P3). The size of each field was 4.2 μm × 4.2 μm. The acceleration voltage during TEM observation was 200 kV. Crystal grains observable by the incident electron beam of <110> were used as observation targets. A bright field image was generated in each field.
[0104]
Using the bright field image of each field, it was determined by the following method whether or not each field had a dislocated cell structure. In the obtained bright field image, a histogram showing the frequency of pixel values ​​(0 to 255) was generated, and the median value of the histogram was obtained. The number of pixels of the bright field image of each field was 117306 pixels. The bright field image was binarized with the median as the threshold. In the binarized image, the low density shift region 102, which is a white region, was identified. The extension of the low-density shift region 102 was defined, and the area of ​​each low-density shift region 102 was determined. Then, the low-density shift region 102 having an area of ​​0.20 μm 2 or more was designated as a “shift cell”. The number of shift cells (low density shift region 102 having an area of ​​0.20 μm 2 or more) in each field (4.2 μm × 4.2 μm) was determined. Then, when there are 9 or more shift cells in each visual field, the visual field is recognized as the visual field in which the shift cell structure is formed. Of the 30 visual fields observed, the number of visual fields in which the rearranged cell structure was formed was determined. Then, the dislocation cell structure ratio (%) was defined by the following equation.
Dislocation cell structure ratio = number of visual fields in which dislocation cell structure is formed / total number of visual fields x 100
Table 2 shows the obtained dislocation cell structure ratio.
[0105]
[Measurement test of the number density of coarse precipitates]
The number density of coarse precipitates was measured by the following method. Samples for measuring the number and density of coarse precipitates were collected from the above-mentioned sample collection positions P1 to P3.
[0106]
The test surface of each sample P1 to P3 had a cross section perpendicular to the longitudinal direction of the austenite-based stainless steel material. The surface to be inspected was mirror-polished. The mirror-polished samples P1 to P3 were corroded with a mixed acid (a solution mixed with hydrochloric acid: nitric acid = 1: 1) to reveal austenite crystal grain boundaries and precipitates. The surface to be inspected after etching was observed in one field with a backscattered electron image using SEM. The field size was 400 μm × 500 μm. The long axis of the precipitate in the field was measured. Specifically, the longest straight line connecting any two points at the interface between the precipitate and the matrix (austenite) is defined as the long axis (μm). Among the precipitates, those having a major axis of 1.0 μm or more were identified as “coarse precipitates”. The number of identified coarse precipitates was determined. Based on the number of obtained coarse precipitates and the visual field area (0.2 mm 2), the number density of coarse precipitates (pieces / 0.2 mm 2) in each sample P1 to P3 was determined. Then, the arithmetic average value of the three number densities was defined as the number densities of coarse precipitates (pieces / 0.2 mm 2). The number densities of the obtained coarse precipitates are shown in Table 2.
[0107]
[Low strain rate tensile test]
A low strain rate tensile test (Slow Straight Rate Test: SSRT) was carried out on the steel plate of each test number. Specifically, a plurality of round bar tensile test pieces were produced from the center position of the plate thickness of the steel plate. The diameter of the parallel portion of the round bar tensile test piece was 3.0 mm, and the parallel portion was parallel to the longitudinal direction (corresponding to the rolling direction) of the steel plate. The central axis of the parallel portion almost coincided with the center position of the plate thickness of the steel plate. The surface of the parallel portion of the round bar tensile test piece was polished in the order of # 150, # 400, and # 600 Emily paper, and then degreased with acetone. Using the obtained round bar tensile test piece, a tensile test was carried out in a room temperature atmosphere at a strain rate of 3.0 × 10-5 / sec, and a breaking drawing (break elongation, unit:%) and Tensile strength (MPa) was obtained. The obtained tensile strength is shown in Table 2.
[0108]
Furthermore, using another round bar tensile test piece, a tensile test was carried out in hydrogen gas at 90 MPa at a strain rate of 3.0 × 10-5 / sec, and a breaking drawing (break elongation, unit:%) was performed. Obtained. The relative breaking throttle (%) of each test number was determined using the following equation.
Relative break throttle = break throttle in hydrogen gas at 90 MPa / break throttle in room temperature atmosphere x 100
[0109]
If the obtained relative rupture drawing was 90.0% or more, it was judged to be excellent in hydrogen brittleness resistance (“○” in the “Relative rupture drawing evaluation” column in Table 2). On the other hand, if the obtained relative fracture drawing was less than 90.0%, it was judged that the hydrogen brittleness resistance was low (“x” in the “Relative breaking drawing evaluation” column in Table 2).
[0110]
[Test results]
Table 2 shows the test results. The chemical compositions of test numbers 1 to 8 and 17 were appropriate, and the production method was also appropriate. Therefore, in the austenite-based stainless steel material, the austenite crystal grain size number based on ASTM E112 was 5.0 to less than 8.0. Further, the dislocation cell structure ratio was less than 50 to 80% in each case. Further, the number density of coarse precipitates was 5.0 / 0.2 mm 2 or less in each case. As a result, in Test Nos. 1 to 8, the tensile strength was 800 MPa or more, and high tensile strength was obtained. Further, the relative break drawing was 90.0% or more, showing excellent hydrogen brittleness resistance.
[0111]
On the other hand, in test number 9, the Cr content was too low. Therefore, the relative break drawing was less than 90.0%, and the hydrogen brittle resistance was low.
[0112]
In test number 10, the Cr content was too high. Therefore, the number density of the coarse precipitates exceeded 5.0 / 0.2 mm 2. As a result, the relative break drawing was less than 90.0%, and the hydrogen brittle resistance was low. It is probable that Cr carbide was excessively generated and became the starting point of hydrogen cracking.
[0113]
In test number 11, the Mo content was too low. Therefore, the dislocation cell structure ratio was less than 50%. As a result, the tensile strength was less than 800 MPa. Further, the relative break drawing was less than 90.0%, and the hydrogen brittle resistance was low.
[0114]
In test number 12, although the chemical composition was appropriate, the heat treatment temperature T1 in the heat treatment step was too high. Therefore, the austenite crystal grain size number was as low as less than 5.0. Furthermore, the dislocation cell structure ratio was less than 50%. As a result, the tensile strength TS was less than 800 MPa.
[0115]
In test number 13, although the chemical composition was appropriate, the heat treatment temperature T1 in the heat treatment step was too low. Therefore, the number density of the coarse precipitates exceeded 5.0 / 0.2 mm 2. As a result, the relative break drawing was less than 90.0%, and the hydrogen brittle resistance was low.
[0116]
In test number 14, although the chemical composition was appropriate, the retention time t1 in the heat treatment step exceeded F1. Therefore, the dislocation cell structure ratio was less than 50%. Therefore, the tensile strength was less than 800 MPa.
[0117]
In test number 15, the cross-sectional reduction rate RR in the cold working process was too low. Further, in test number 16, the cold working step was not carried out. Therefore, in test numbers 15 and 16, the dislocation cell structure ratio was less than 50%. Therefore, the tensile strength was less than 800 MPa.
[0118]
The embodiment of the present invention has been described above. However, the above-described embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and the above-mentioned embodiment can be appropriately modified and carried out within a range not deviating from the gist thereof.
Description of the sign
[0119]
101 cell wall area
102 Low density dislocation region
The scope of the claims
[Claim 1]
Austenite stainless steel material
The chemical composition is mass%,
C: 0.100% or less,
Si: 1.00% or less,
Mn: 5.00% or less,
Cr: 15.00 to 22.00%,
Ni: 10.00-21.00%,
Mo: 1.20-4.50%,
P: 0.050% or less,
S: 0.050% or less,
Al: 0.100% or less,
N: 0.100% or less,
Cu: 0 to 0.70%, and
The rest consists of Fe and impurities,
The austenite crystal grain size number conforming to ASTM E112 is 5.0 to less than 8.0,
In the cross section perpendicular to the longitudinal direction of the austenite-based stainless steel material, the number density of precipitates having a dislocation cell structure ratio of less than 50 to 80% and a major axis of 1.0 μm or more is 5.0 / 0.2 mm. 2 or less
Austenite stainless steel material.
[Claim 2]
The austenite-based stainless steel material according to claim 1,
The austenite crystal grain size number is 5.8 or more.
Austenite stainless steel material.
[Claim 3]
The austenite-based stainless steel material according to claim 1 or claim 2.
The dislocation cell structure ratio is 55% or more.
Austenite stainless steel material.
[Claim 4]
The austenite-based stainless steel material according to any one of claims 1 to 3.
The number density of precipitates having a major axis of 1.0 μm or more is 4.5 / 0.2 mm 2 or less.
Austenite stainless steel material.
[Claim 5]
The austenite-based stainless steel material according to any one of claims 1 to 4.
The chemical composition is
Contains Cu: 0.01-0.70%,
Austenite stainless steel material.