JP2023145223A - Austenitic stainless steel, steel plate, and steel pipe and method for producing the same - Google Patents

Abstract

To provide an austenitic stainless steel with a reduced content of expensive Ni and improved fatigue resistance in a hydrogen environment.SOLUTION: An austenitic stainless steel has a predetermined chemical composition. The f value is more than 29.5 and less than 32.5. In a cross section parallel with a process direction and a thickness direction, when inclusions with a long side of 5 μm or more are termed "coarse inclusions", the number of coarse inclusions is 10 or less per 0.05 mm2, and the number of coarse inclusions that include 30 mol% or more of CaO constitutes 50% or more of the total number of coarse inclusions.SELECTED DRAWING: None

Description

The present invention relates to an austenitic stainless steel material, a steel plate, a steel pipe, and a method for manufacturing the same.

In recent years, hydrogen energy has attracted attention as a clean energy that does not emit greenhouse gases such as carbon dioxide. In order to utilize hydrogen energy, it is necessary to establish hydrogen-related technologies for producing, storing, and transporting hydrogen.

On the other hand, there are various problems in establishing hydrogen-related technology. One of them is the problem of so-called hydrogen embrittlement, in which strength and ductility decrease in a hydrogen environment. Austenitic stainless steels are examples of materials used in hydrogen environments. For example, Patent Documents 1 and 2 disclose austenitic stainless steels with improved hydrogen embrittlement resistance.

Further, in a hydrogen environment, the members used continue to be subjected to gas pressure above a certain level, that is, stress. When subjected to repeated stress, even if the stress is lower than the strength level that would normally cause rupture, rupture due to fatigue fracture may occur. Therefore, in anticipation of use in parts, Patent Documents 3 and 4 disclose austenitic stainless steels that have improved fatigue resistance in a hydrogen environment in addition to hydrogen embrittlement resistance.

JP2019-143228A JP 2021-109998 Publication International Publication No. 2016/68009 Japanese Patent Application Publication No. 2019-194357

Here, in the techniques disclosed in Patent Documents 1 and 2, the fatigue resistance characteristics of austenitic stainless steel in a hydrogen environment are not studied. Therefore, the austenitic stainless steel disclosed in the above document has room for further improvement in fatigue resistance in a hydrogen environment. Furthermore, in the techniques disclosed in Patent Documents 3 and 4, the fatigue resistance of austenitic stainless steel in a hydrogen environment has been studied, but since it contains a large amount of expensive Ni, it is difficult to reduce the alloy cost. There is still room for further improvement.

Based on the above, an object of the present invention is to solve the above problems and provide an austenitic stainless steel material that improves fatigue resistance in a hydrogen environment while reducing expensive Ni.

The present invention has been made to solve the above problems, and its gist is the following austenitic stainless steel material, steel plate, steel pipe, and manufacturing method thereof.

(1) The chemical composition is in mass%,
C: 0.1% or less,
Si: 2.0% or less,
Mn: 6.0 to 12.0%,
P: 0.030% or less,
S: 0.003% or less,
Cr: 13.0-18.0%,
Ni: 5.0 to 9.0%,
N: 0.15-0.25%,
Al: 0.005-0.080%,
Ca: 0.0005-0.01%,
B: 0.0001-0.01%,
Ga: 0.0010-0.020%,
Cu: less than 1.0%,
Mo: less than 2.0%,
Nb: 0 to 0.5%,
Ti: 0 to 0.5%,
V: 0-0.50%,
W: 0-0.50%,
Zr: 0 to 0.50%,
Co: 0 to 0.50%,
Mg: 0 to 0.005%,
Hf: 0-0.10%,
REM: 0-0.1%,
The remainder: Fe and impurities,
The f value calculated by the following formula (i) is more than 29.5 and less than 32.5,
In the cross section parallel to the processing direction and thickness direction,
When inclusions with long sides of 5 μm or more are coarse inclusions, the number of coarse inclusions is 10 or less per 0.05 mm ² ,
An austenitic stainless steel material, wherein the ratio of the number of coarse inclusions containing 30 mol% or more of CaO to the number of coarse inclusions is 50% or more.
f value=Ni+0.72Cr+0.88Mo+1.11Mn-0.27Si+0.53Cu+12.93C+7.55N-1.1Nb-2.2Ti...(i)
However, each element symbol in the above formula (i) represents the content (mass %) of each element contained in the steel, and is zero if it is not contained.

(2) In the cross section,
The negative segregation degree of Ni is 0.80 or more,
The austenitic stainless steel material according to (1) above, wherein the degree of negative segregation of Mn is 0.80 or more.

(3) The austenitic stainless steel material according to (1) or (2) above, which is a steel plate.

(4) The austenitic stainless steel material according to (1) or (2) above, which is a steel pipe.

(5) A manufacturing method for manufacturing the austenitic stainless steel material according to (1) or (2) above, comprising:
A cold working step of cold working a hot worked material having the chemical composition described in (1) above to obtain a cold worked material;
A method for producing an austenitic stainless steel material, comprising a bright annealing step of bright annealing at a temperature range of 950 to 1150°C with a dew point of -45°C or less.

(6) A manufacturing method for manufacturing the austenitic stainless steel material according to (3) above, comprising:
The method for producing an austenitic stainless steel material according to (5) above, wherein cold rolling is performed in the cold working step.

(7) A manufacturing method for manufacturing the austenitic stainless steel material according to (4) above, comprising:
The method for manufacturing an austenitic stainless steel material according to (5) above, wherein drawing is performed in the cold working step.

(8) A method for producing the austenitic stainless steel material according to (4) above, comprising:
The method for producing an austenitic stainless steel material according to (7) above, wherein the drawing process is performed twice or more.

According to the present invention, it is possible to obtain an austenitic stainless steel material with improved fatigue resistance in a hydrogen environment while reducing the amount of expensive Ni.

FIG. 1 is a diagram showing observation positions of inclusions and sample sampling positions in the case of a steel plate. FIG. 2 is a diagram showing the observation position of inclusions and the sample collection position in the case of a steel pipe.

The present inventors have studied an austenitic stainless steel material that has improved fatigue resistance in a hydrogen environment (hereinafter also simply referred to as "fatigue resistance") while reducing expensive Ni. The following findings (a) to (c) were obtained.

(a) When repeated stress is applied and fracture occurs due to fatigue, inclusions often become the starting point of the fracture. Therefore, it is desirable to reduce the number of inclusions formed. Therefore, it is effective to contain small amounts of Ga and Ca to suppress the formation of inclusions.

(b) Furthermore, if the size of the inclusion is large or the inclusion itself is hard, the inclusion is more likely to become a starting point for destruction. Therefore, it is effective to suppress the formation of coarse inclusions and to actively form soft CaO as inclusions. Here, in normal manufacturing processes, inclusions such as hard Al ₂ O ₃ and MgO are likely to be formed. The present inventors have also clarified that bright annealing performed under predetermined conditions is effective in reducing hard Al ₂ O ₃ and MgO by reducing it, while allowing CaO, which is difficult to reduce, to remain. I made it.

(c) Furthermore, even if the segregation of Ni and Mn is large, it becomes a starting point for fracture, so it is desirable to set the degree of negative segregation of these elements to 0.80 or more to eliminate segregation as much as possible. In order to make the degree of negative segregation of Ni and Mn 0.80 or more, it is desirable to perform cold working multiple times and then perform bright annealing in the final step.

One embodiment of the present invention has been made based on the above findings. Each requirement of this embodiment will be explained in detail below.

1. Chemical composition The reasons for limiting each element are as follows. In addition, in the following description, "%" regarding content means "mass %".

C: 0.1% or less C is an element effective in stabilizing the austenite phase and improving hydrogen embrittlement resistance. However, when C is contained excessively, Cr-based carbides are excessively precipitated, resulting in a decrease in fatigue resistance. Therefore, the C content is set to 0.1% or less. The C content is preferably 0.08% or less, more preferably 0.07% or less. On the other hand, in order to obtain the above effects, the C content is preferably 0.01% or more.

Si: 2.0% or less Si is an effective element for deoxidation and also contributes to improving hydrogen embrittlement resistance. However, when Si is contained excessively, the formation of intermetallic compounds such as hard inclusions SiO ₂ and sigma phase is promoted, resulting in a decrease in fatigue resistance. Therefore, the Si content is set to 2.0% or less. The Si content is preferably 1.0% or less. On the other hand, in order to obtain the above effects, the Si content is preferably 0.5% or more.

Mn: 6.0-12.0%
Mn is an element effective in stabilizing the austenite phase and contributes to improving hydrogen embrittlement resistance and fatigue resistance. Furthermore, since the solid solubility limit of N is increased, it indirectly contributes to saving expensive Ni. Therefore, the Mn content is set to 6.0% or more. The Mn content is preferably 8.0% or more, more preferably 9.0% or more. However, when Mn is contained excessively, it promotes the formation of an ε phase that is highly susceptible to hydrogen embrittlement, and on the contrary, the hydrogen embrittlement resistance decreases. In addition, it promotes the formation of inclusions, resulting in a decrease in fatigue resistance. Therefore, the Mn content is 12.0% or less, preferably 10% or less.

P: 0.030% or less P is an element contained in steel as an impurity, but it forms phosphides and deteriorates fatigue resistance. Therefore, the P content is set to 0.030% or less. The P content is preferably 0.025% or less, more preferably 0.015% or less. On the other hand, if P is reduced excessively, manufacturing costs will increase. Therefore, the P content is preferably 0.005% or more.

S: 0.003% or less S is an element contained in steel as an impurity, and reduces corrosion resistance, especially weather resistance. Therefore, the S content is set to 0.003% or less. The S content is preferably 0.002% or less, more preferably 0.001% or less. However, reducing S excessively increases manufacturing costs. Therefore, the S content is preferably 0.0001% or more.

Cr: 13.0-18.0%
Cr is an element contained in a certain amount in stainless steel, and has the effect of improving corrosion resistance, particularly weather resistance. Therefore, the Cr content is set to 13.0% or more. The Cr content is preferably 15.0% or more. However, Cr is a ferrite-forming element. Therefore, when Cr is contained excessively, the austenite phase becomes unstable and the hydrogen embrittlement resistance decreases. Furthermore, it promotes the formation of hard MnCr ₂ O ₄ inclusions, reducing fatigue resistance. Therefore, the Cr content is set to 18.0% or less. The Cr content is preferably 17.0% or less, more preferably 16.0% or less.

Ni: 5.0-9.0%
Ni is an element necessary to ensure hydrogen embrittlement resistance and fatigue resistance. Therefore, the Ni content is set to 5.0% or more. The Ni content is preferably 7.0% or more. However, when Ni is contained excessively, the alloy cost increases. Therefore, the Ni content is set to 9.0% or less. The Ni content is preferably 8.5% or less, more preferably 8.0% or less.

N: 0.15-0.25%
N is an element effective in improving hydrogen embrittlement resistance. Therefore, the N content is set to 0.15% or more. However, excessive N content promotes the formation of AlN and reduces fatigue resistance. Furthermore, internal defects such as blowholes may occur during welding, and the manufacturability of welded steel pipes also decreases. Therefore, the N content is set to 0.25% or less. The N content is preferably 0.22% or less, more preferably 0.20% or less.

Al: 0.005-0.080%
Al is an effective deoxidizing element and has the effect of improving fatigue resistance. Therefore, the Al content is set to 0.005% or more. The Al content is preferably 0.010% or more. However, when Al is contained excessively, hard Al ₂ O ₃ inclusions and AlN are likely to be formed, resulting in a decrease in fatigue resistance. Therefore, the Al content is set to 0.080% or less. The Al content is preferably 0.060% or less.

Ca: 0.0005-0.01%
Ca has a deoxidizing effect, and by reducing the O content, suppresses the formation of inclusions and improves fatigue resistance. It also has the effect of promoting the formation of CaO, which is effective in improving fatigue resistance. Therefore, the Ca content is set to 0.0005% or more. The Ca content is preferably 0.0020% or more, and preferably 0.0025% or more. However, when Ca is contained excessively, coarse CaO inclusions are excessively formed, and the fatigue resistance properties are deteriorated. Therefore, the Ca content is set to 0.01% or less. The Ca content is preferably 0.0050% or less.

B: 0.0001-0.01%
B has the effect of suppressing the propagation of fracture at the grain boundaries and improving fatigue resistance by strengthening the grain boundaries. Therefore, the B content is set to 0.0001% or more. The B content is preferably 0.0003% or more, and preferably 0.0005% or more. However, even if B is contained in excess, its effect is not only saturated, but also the grain boundary precipitation of boron compounds (BN, BC, Cr ₂ B) is promoted, so that the fatigue resistance properties are deteriorated. Therefore, the B content is set to 0.01% or less. The B content is preferably 0.0050% or less.

Note that the total content of Ca and B is preferably 0.005% or less. This is because if the total content of Ca and B exceeds 0.005%, Ca and B will form a compound at the grain boundaries, making it difficult to obtain a sufficient effect of improving fatigue resistance properties. .

Ga: 0.0010-0.020%
Ga is an important element in the steel material of this embodiment because it forms oxides and indirectly contributes to the formation of CaO, which is effective in improving fatigue resistance. Therefore, the Ga content is set to 0.0010% or more. The Ga content is preferably 0.0050% or more, more preferably 0.0090% or more. However, when Ga is contained excessively, manufacturability is reduced. Therefore, the Ga content is set to 0.020% or less. The Ga content is preferably 0.0110% or less.

Cu: less than 1.0% Cu is an element that is mixed in from raw materials such as scrap, and is an element that is effective in stabilizing the austenite phase and improving hydrogen embrittlement resistance. On the other hand, Cu is a low melting point element, segregates at grain boundaries, promotes high-temperature cracking due to P and S, and makes cracking more likely to occur. Therefore, the Cu content is set to less than 1.0%. The Cu content is preferably 0.5% or less. However, excessively reducing the Cu content leads to restrictions on melting raw materials and increases manufacturing costs. Therefore, the Cu content is preferably 0.01% or more.

Mo: less than 2.0% Mo is an element that is mixed in from raw materials such as scrap, but has the effect of improving strength and corrosion resistance. On the other hand, when it is contained in excess, it promotes the formation of δ ferrite phase and reduces hydrogen embrittlement resistance. Therefore, the Mo content is set to less than 2.0%. The Mo content is preferably 0.5% or less. On the other hand, excessively reducing the Mo content results in restrictions on melting raw materials and increases manufacturing costs. Therefore, the Mo content is preferably 0.01% or more.

In addition to the above elements, one or more selected from Nb, Ti, V, W, Zr, Co, Mg, Hf, and REM may be contained within the range shown below. The reasons for limiting each element will be explained.

Nb: 0-0.5%
Nb has the effect of forming carbonitrides, refining crystal grains, and strengthening grain boundaries. Therefore, it may be included if necessary. However, when Nb is contained excessively, carbonitrides are formed and fatigue resistance is deteriorated. Therefore, the Nb content is set to 0.5% or less. The Nb content is preferably 0.3% or less. On the other hand, in order to obtain the above effects, the Nb content is preferably 0.01% or more.

Ti: 0-0.5%
Ti has the effect of forming carbonitrides, refining crystal grains, and strengthening grain boundaries. As a result, Ti suppresses the occurrence of cracks during welding. Therefore, it may be included if necessary. However, when Ti is contained excessively, carbonitrides are formed and the fatigue resistance is deteriorated. Therefore, the Ti content is set to 0.5% or less. The Ti content is preferably 0.3% or less. On the other hand, in order to obtain the above effects, the Ti content is preferably 0.01% or more.

V: 0-0.50%
V precipitates as a solid solution or carbonitride in steel, and has the effect of improving strength. Therefore, it may be included if necessary. However, when an excessive amount of V is contained, carbonitrides are formed and the fatigue resistance is deteriorated. Therefore, the V content is set to 0.50% or less. The V content is preferably 0.30% or less. On the other hand, in order to obtain the above effects, the V content is preferably 0.01% or more.

W: 0-0.50%
W has the effect of improving strength and corrosion resistance. Therefore, it may be included if necessary. However, when W is contained excessively, manufacturing costs increase. Therefore, the W content is set to 0.50% or less. The W content is preferably 0.30% or less. On the other hand, in order to obtain the above effects, the W content is preferably 0.001% or more.

Zr: 0-0.50%
Zr has a deoxidizing effect and has an effect of improving weldability. Therefore, it may be included if necessary. However, when Zr is contained excessively, carbonitrides are excessively formed, resulting in a decrease in fatigue resistance. Therefore, the Zr content is set to 0.50% or less. The Zr content is preferably 0.30% or less. On the other hand, in order to obtain the above effects, the Zr content is preferably 0.01% or more.

Co: 0-0.50%
Co has the effect of improving corrosion resistance and stabilizing the austenite phase. Therefore, it may be included if necessary. However, when Co is contained excessively, manufacturing costs increase. Therefore, the Co content is set to 0.50% or less. The Co content is preferably 0.30% or less. On the other hand, in order to obtain the above effects, the Co content is preferably 0.01% or more.

Mg: 0-0.005%
Mg is an effective element for deoxidation and has the effect of improving the weldability of steel pipes. Therefore, it may be included if necessary. However, when Mg is contained excessively, MgO inclusions are excessively formed, resulting in a decrease in fatigue resistance. Therefore, the Mg content is set to 0.005% or less. From the viewpoint of effectiveness and manufacturability, the Mg content is preferably 0.002% or less. On the other hand, in order to obtain the above effects, the Mg content is preferably 0.0001% or more.

Hf: 0-0.10%
Hf has a deoxidizing effect and an effect of improving weldability. Therefore, it may be included if necessary. However, when Hf is contained excessively, inclusions are excessively formed and the fatigue resistance is deteriorated. Therefore, the Hf content is set to 0.10% or less. The Hf content is preferably 0.05% or less. On the other hand, in order to obtain the above effects, the Hf content is preferably 0.01% or more.

REM: 0~0.1%
REM has a deoxidizing effect and an effect of improving weldability. It also has the effect of improving corrosion resistance. Therefore, it may be included if necessary. However, when REM is contained excessively, not only its effect is saturated, but also inclusions are excessively formed, resulting in a decrease in fatigue resistance. Therefore, the REM content is set to 0.1% or less. The REM content is preferably 0.05% or less. On the other hand, in order to obtain the above effects, the REM content is preferably 0.01% or more.

REM refers to a total of 17 elements including Sc, Y, and lanthanoids, and the above REM content refers to the total content of these elements. REM is sometimes added industrially in the form of misch metal.

In the chemical composition of the steel sheet according to the present invention, the remainder is Fe and impurities. Here, "impurities" are components that are mixed in during the industrial production of austenitic stainless steel materials due to raw materials such as ore and scrap, and various factors in the manufacturing process, within a range that does not adversely affect the present invention. means what is permissible.

f value:
In the austenitic stainless steel material according to the present embodiment, the f value shown below is limited to a predetermined range as an index representing the stability of the austenite phase. Specifically, the f value calculated by the following formula (i) is set to be more than 29.5 and less than 32.5.

f value=Ni+0.72Cr+0.88Mo+1.11Mn-0.27Si+0.53Cu+12.93C+7.55N-1.1Nb-2.2Ti...(i)
However, each element symbol in the above formula (i) represents the content (mass %) of each element contained in the steel, and is zero if it is not contained.

Here, if the f value is 29.5 or less, the stability of the austenite phase is low, and the hydrogen embrittlement resistance and fatigue resistance are reduced. Therefore, the f value is set to exceed 29.5. The f value is preferably 30.0 or more. However, if the f value is 32.5 or more, the fatigue resistance properties will deteriorate due to high alloying. Also, raw material costs and manufacturing costs increase. Therefore, the f value is set to less than 32.5. From the viewpoints of manufacturability, weldability, and economic efficiency, the f value is preferably 31.5 or less.

2. Coarse inclusions 2-1. Number of Coarse Inclusions In the austenitic stainless steel material of this embodiment, in order to improve fatigue resistance, it is necessary to reduce the number of coarse inclusions. In other words, in a cross section parallel to the processing direction and the thickness direction (hereinafter also referred to as "L cross section"), when inclusions with long sides of 5 μm or more are considered coarse inclusions, the number of coarse inclusions is 0.05 mm. No more than 10 pieces per ² . Note that the processing direction is synonymous with the rolling direction in the case of a steel plate, and the drawing direction in the case of a steel pipe. Further, the above-mentioned thickness direction means the plate thickness direction in the case of a steel plate, and the wall thickness direction in the case of a steel pipe.

When the number of coarse inclusions exceeds 10 per 0.05 mm ² , the fatigue resistance in a hydrogen environment deteriorates. Therefore, the number of the coarse inclusions is 10 or less, preferably 7 or less per 0.05 mm ² . The lower limit of the number of coarse inclusions is not particularly determined, but it is preferably reduced as much as possible, and most preferably zero.

Here, a method for measuring the number of coarse inclusions per 0.05 mm ² will be explained. A sample for observation is taken from the L cross section of the steel material. From the viewpoint of observing the average metal structure, for example, in the case of a steel plate, as shown in FIG. 1, an L cross section near the center of the sheet width is selected, and in the selected L cross section, the surface layer, 3t/4, t/2, t/4, and 5 fields of view (black circles in FIG. 1) of the back and front layers are included, and the sample is collected so that observation can be made in these fields of view.

In the case of steel pipes, as shown in Figure 2, an L cross section near the circumferential center of the weld is selected, and in the wall thickness direction of the selected L cross section, the surface layer, 3t/4, t/2, t/4, and back Five fields of view (black circles in Figure 2) of the surface layer are included, and the sample is collected so that observations can be made in these fields.

The collected sample is polished using a corrosive solution so that it can be clearly observed. The sample whose observation surface is an L cross section is observed using a SEM (scanning electron microscope). As for the setting conditions of the SEM during observation, it is preferable that the accelerating voltage be 15 kV, for example. The observation field is 100 μm x 100 μm, and by observing this area in 5 fields, the total observation area is adjusted to 0.05 mm ² . Then, inclusions whose long sides are 5 μm or more are recognized as coarse inclusions, and the number of coarse inclusions is counted in the total observation area. Note that the long side refers to the longest line when connecting two points on the outer periphery of the inclusion.

2-2. Composition of Coarse Inclusions In the austenitic stainless steel material of this embodiment, it is necessary to form coarse inclusions containing a large amount of soft CaO. This is because inclusions such as hard Al ₂ O ₃ and MgO, which are usually contained in large quantities in inclusions, tend to become starting points for fracture and reduce fatigue resistance.

Therefore, in the L cross-section, the ratio of the number of coarse inclusions containing 30 mol% or more of CaO (hereinafter simply referred to as "CaO-based coarse inclusions") to the number of coarse inclusions is 50% or more. do. If the ratio of the number of CaO-based coarse inclusions is less than 50%, the inclusions become hard and the fatigue resistance cannot be sufficiently improved.

Therefore, the ratio of the number of CaO-based coarse inclusions to the number of coarse inclusions is 50% or more, preferably 60% or more, and more preferably 70% or more. Although there is no particular upper limit to the ratio of the number of CaO-based coarse inclusions, it is usually 90% or less. Further, hereinafter, the ratio of the number of CaO-based coarse inclusions to the number of coarse inclusions is simply referred to as the number ratio.

Here, a method for measuring the number ratio will be explained. For the inclusions recognized as coarse inclusions by the method described above, point analysis is performed at three points in the inclusions using EDX attached to the SEM. If the average value of the three point analysis points is the value of the mol% ratio in terms of oxide, and the CaO content is 30 mol% or more, it is recognized as a CaO-based coarse inclusion. The ratio (%) of the number of inclusions recognized as CaO-based coarse inclusions to all the observed coarse inclusions is calculated. Note that the inclusions are oxides, and one inclusion is one or more of MgO, Al ₂ O ₃ , MnO, Cr ₂ O ₃ , and CaO, or a composite oxide thereof, and other oxides, such as , ZrO ₂ etc. are not considered.

3. Negative Segregation Degree When the segregation of Ni and Mn is large, the segregation location tends to become the starting point of fracture. As a result, fatigue resistance deteriorates. Therefore, in the austenitic stainless steel material of this embodiment, it is preferable that the degree of negative segregation, which is an index of the degree of segregation of Ni and Mn, is in the following range. Specifically, in the L cross section, the negative segregation degree of Ni is preferably 0.80 or more, and the negative segregation degree of Mn is preferably 0.80 or more. In the L cross section, if the negative segregation degree of Ni is less than 0.80 or the negative segregation degree of Mn is less than 0.80, coarse inclusions remain in the steel material, and the fatigue resistance Characteristics cannot be improved sufficiently.

For this reason, in the L cross section, it is preferable that the negative segregation degree of Ni be 0.80 or more and the negative segregation degree of Mn be 0.80 or more. Note that here, negative segregation refers to an element being distributed at a lower concentration than the average concentration, and negative segregation degree refers to the degree thereof. For example, the negative segregation degree of Ni is calculated as (Ni concentration in the Ni negative segregation area)/(average Ni content of the steel material), and the negative segregation degree of Mn is calculated as (Mn concentration in the Mn negative segregation area)/(Ni concentration in the steel material average Mn content).

A method for measuring the degree of negative segregation will be explained. Regarding the negative segregation degree, the negative segregation degree of Ni and Mn is measured in an observation field of 250 μm×250 μm. Using EPMA analysis under the conditions of beam diameter 1 μm, step size 0.5 μm, acceleration voltage 15 kV, and irradiation current 2.0 × 10 ^-9 A, and when the average value of the Ni or Mn concentration of all analysis points is set to 1, The concentration ratio of Ni and Mn in the negative segregation area is defined as the degree of negative segregation. Samples for analysis are collected at the same location as when observing coarse inclusions.

4. Type of Steel Material In the austenitic stainless steel material of this embodiment, the type of steel material is not particularly limited. As an example, steel plates, steel pipes, etc. can be considered. Preferred applications include high-pressure hydrogen gas piping and the like. Note that a steel pipe has a welded portion consisting of a welded metal and a welded heat affected zone, and it is sufficient that this welded portion satisfies the above requirements.

In the steel pipe of this embodiment, the grain size at the center of the thickness of the welded portion is preferably 5.0 or more, more preferably 6.0 or more. This is because if the crystal grain size is less than 5.0, the segregation of Ni and Mn may not be sufficiently eliminated. Note that the crystal grain size is preferably determined by a cutting method in accordance with JIS G 0551:2013.

5. Manufacturing Method A preferred method for manufacturing the austenitic stainless steel material according to this embodiment will be described. The austenitic stainless steel material according to this embodiment can be stably manufactured, for example, by the following manufacturing method.

5-1. Manufacturing method of steel plate 5-1-1. Casting Steel having the above chemical composition is melted and cast. Hereinafter, the case where the obtained slab after casting is made into a slab or the like and then into a steel plate, and the case where the steel plate is manufactured into a steel pipe will be taken as an example and explained.

5-1-2. Hot Rolling Process The obtained slab is hot rolled to produce a hot rolled steel plate. The heating temperature during hot rolling is not particularly limited, but is often in the range of, for example, 1150 to 1250°C. In addition, other hot rolling conditions are not particularly limited, but for example, the hot rolling temperature should be in the range of 800 to 1200°C, the total rolling reduction should be more than 90%, and the coiling temperature should be 900°C or less. It is often adjusted to It may be adjusted as appropriate, taking into account the composition of the slab and other conditions. Note that hot working includes hot rolling. In addition, hot-processed materials include hot-rolled steel sheets.

5-1-3. Hot-rolled sheet annealing The hot-rolled steel sheet obtained in the above-mentioned hot rolling process may be subjected to hot-rolled sheet annealing, if necessary. The temperature for annealing the hot rolled sheet is not particularly limited, but is often in the range of 1000 to 1200°C, for example. Further, the annealing time for hot-rolled sheet annealing is similarly not particularly limited, but is often set in the range of 1 to 10 minutes, for example, from a manufacturing standpoint. Further, after annealing the hot rolled sheet, pickling may be performed to remove scale. In addition, in the case of materials other than steel plates, for example, the above-mentioned hot-processed material may be annealed.

5-1-4. Cold Rolling Step Next, the hot rolled steel sheet (if necessary, the hot rolled steel sheet has been annealed) is cold rolled to obtain a cold rolled steel sheet. The rolling reduction during cold rolling is not particularly limited. It is only necessary to crush and reduce coarse inclusions, which is often in the range of 30 to 60%, for example. Here, the cold rolling may be performed twice or more, with one rolling at the above rolling reduction ratio. By performing cold rolling two or more times, coarse inclusions can be finely pulverized and the number of coarse inclusions can be further reduced. Furthermore, segregation of Ni and Mn can be reduced and homogenization of these elements can be promoted. That is, the degree of negative segregation of Ni and Mn can be set to 0.80 or more, and the fatigue resistance can be improved.

When performing cold rolling two or more times, intermediate annealing may be performed between cold rolling. Further, the intermediate annealing performed between cold rolling may be normal annealing performed in an oxidizing atmosphere such as a combustion gas atmosphere, or bright annealing performed in a non-oxidizing atmosphere described below. If normal annealing is performed, pickling is recommended after that. The annealing temperature and annealing time of the intermediate annealing are not particularly limited, but for example, the annealing temperature is often in the range of 950 to 1150° C., and the annealing time is often 1 min or more and less than 1 hour. When bright annealing is performed, it is desirable to perform it in the atmosphere and dew point described below. After performing intermediate annealing, it is cooled to room temperature and cold rolled again. Note that cold working includes cold rolling. Further, cold-worked materials include cold-rolled steel sheets.

5-1-5. Bright Annealing Next, bright annealing is performed on the cold rolled steel sheet that has undergone the cold rolling process. Bright annealing is an annealing method that is usually performed on materials where aesthetics are important, so it is not performed for the purpose of improving fatigue resistance. However, in the steel material of this embodiment, by bright annealing, Control things.

Bright annealing is an annealing method performed in a non-oxidizing atmosphere using hydrogen gas or the like. Therefore, during annealing, hard inclusions Al ₂ O ₃ and MgO and composite inclusions in which these are combined are reduced, while CaO, which is relatively soft and effective in improving fatigue properties, is reduced. remains. As a result, the abundance ratio of CaO in the coarse inclusions becomes high, and the number ratio of CaO-based coarse inclusions can be made 50% or more.

In the bright annealing step, hydrogen or a mixed gas of hydrogen and nitrogen is used to create a reducing atmosphere, and the dew point during bright annealing is -45° C. or lower. If the dew point is higher than -45°C, the above-mentioned inclusions will not be sufficiently reduced, making it impossible to increase the abundance ratio of CaO in the coarse inclusions. For this reason, the dew point is set to -45°C or lower, preferably -50°C or lower.

Further, the annealing temperature in the bright annealing step is in the range of 950 to 1150°C. If the annealing temperature in the bright annealing step is less than 950° C., the annealing will not be performed sufficiently, the desired metal structure will not be obtained, and the hydrogen embrittlement resistance will decrease. Furthermore, the segregation of Ni and Mn is not sufficiently eliminated, and the fatigue resistance is also deteriorated. Therefore, the annealing temperature in the bright annealing step is 950°C or higher, preferably 980°C or higher, and more preferably 1000°C or higher.

On the other hand, when the annealing temperature in the bright annealing step exceeds 1150° C., crystal grains become coarse and hydrogen embrittlement resistance and hot cracking resistance tend to occur. Therefore, the annealing temperature in the bright annealing step is preferably 1150°C or lower, preferably 1120°C or lower. Further, the annealing time in bright annealing is not particularly limited. Although it may be adjusted as necessary, the annealing time in bright annealing is usually in the range of 1 min or more and less than 1 h.

By performing bright annealing, there is no need to perform pickling afterwards, so when manufacturing a steel plate, bright annealing is adjusted so that it is performed in the final process. After bright annealing, cooling may be performed to form an austenitic stainless steel plate.

5-2. Manufacturing method of steel pipe 5-2-1. Forming Process Next, the method for manufacturing the steel pipe will be explained. It is preferable to manufacture a steel pipe using the austenitic stainless steel plate obtained through the step 5-1 described above. The above steel plate is formed into a tubular shape. The forming method is not particularly limited, but so-called roll forming, in which the material is bent into a tubular shape using rolls having various curvatures, is common. Note that the outer diameter of the steel pipe is not particularly limited, but is generally in the range of, for example, 1/8 to 1/2 inch, ie, approximately 2 to 14 mm, in accordance with the ASTM A269 standard.

5-2-2. Welding Step Next, it is preferable to weld the ends in the width direction of the steel plate formed into the shape of a pipe to form a steel pipe. The welding method is not particularly limited, but may be, for example, high frequency electric resistance welding (also referred to as "ERW"), inert gas arc welding (also referred to as "TIG welding"), or laser welding. Other welding conditions may be adjusted as appropriate. Note that when the pipe is made by welding, welding burn occurs on the pipe, so it is preferable to perform pickling to remove the welding burn.

5-2-3. Annealing process before drawing Next, the welded pipe (hereinafter also referred to as "welded pipe") after the welding process is preferably annealed in a temperature range of 950 to 1150°C as necessary. If the annealing temperature of the welded pipe is less than 950°C, residual processing strain and segregation cannot be sufficiently eliminated. For this reason, the annealing temperature of the welded pipe is preferably 950°C or higher. On the other hand, if the annealing temperature of the welded pipe is higher than 1150° C., the yield will decrease and the crystal grains will not have a regular grain structure. For this reason, the annealing temperature of the welded pipe is preferably 1150°C or less.

In addition, in the annealing treatment of the welded pipe, annealing may be carried out in the normal atmosphere in an oxidizing atmosphere, but the above-mentioned bright annealing may also be carried out. The conditions for bright annealing are the same as those for the steel sheet described above. After the annealing, the tube is cooled at an appropriate cooling rate to form an austenitic stainless steel tube. Further, the annealing time at this time is not particularly limited, but is usually in the range of 1 min or more and less than 1 h, for example. Further, when the pre-drawing annealing step is performed in a combustion gas or in an oxidizing atmosphere, it is preferable to perform pickling after the annealing step in order to remove welding burn along with the oxide film generated during the annealing.

5-2-4. Drawing process Subsequently, when annealing is performed after the welding process or before drawing as necessary, cold drawing is performed after annealing. Note that the area reduction rate of one drawing process may be adjusted as appropriate.

Here, this drawing process is preferably performed two or more times. This is because by performing the drawing process two or more times, the number of coarse inclusions is reduced, and the negative segregation of Ni and Mn is also eliminated, resulting in a negative segregation degree of 0.80 or more. As a result, fatigue resistance is improved. Note that, if necessary, heat treatment may be performed between the drawing processes. The heat treatment may be performed under the same conditions as the annealing process of the welded pipe described above.

5-2-5. Bright annealing process It is preferable to perform bright annealing on the steel pipe that has undergone the drawing process. In addition, when bright annealing is performed before the drawing process, it is not necessarily necessary to perform bright annealing after the drawing process. Bright annealing may be performed at least once before and after drawing, but it is preferably performed at least twice. Here, various conditions for bright annealing may be the conditions described in 5-1-5. That is, the dew point may be set to -45°C or lower, and the annealing temperature may be set to 950 to 1150°C. The same applies to the annealing time. In bright annealing of steel pipes, as in the case of steel plates, hard Al ₂ O ₃ and MgO can be reduced and the proportion of soft CaO in inclusions can be increased. Thereafter, appropriate cooling may be performed to form an austenitic stainless steel pipe.

EXAMPLES Hereinafter, the austenitic stainless steel plate and steel pipe according to the present invention will be explained in more detail with reference to Examples, but the present invention is not limited to these Examples.

Steel having the chemical composition shown in Table 1 was melted to obtain a slab. The obtained slab was heated at 1230°C, hot rolled in a temperature range of 900°C or higher, and coiled at 850°C to obtain a 5.0 mm thick hot rolled steel plate. Subsequently, the obtained hot rolled steel sheet was annealed at 1100° C. for 3 minutes, and then pickled and descaled.

Subsequently, the annealed hot rolled steel sheet was cold rolled. Cold rolling was performed under the conditions shown in Table 2. In addition, for some examples, cold rolling was performed multiple times, and annealing was performed in between. Annealing was performed at a temperature of 1050 to 1120°C for 30 seconds to 1 minute. Regarding the annealing between cold rolling, some parts were bright annealed, but others were annealed in a combustion gas atmosphere. Note that the number of annealing operations in Table 2 indicates the number of bright annealing operations performed in all steps. After final annealing, it was cooled to obtain an austenitic stainless steel plate. In the case of bright annealing, hydrogen gas was used, the dew point was set to -45°C or less, and annealing was performed in the above-mentioned temperature range.

The state of inclusions, the degree of negative segregation of Ni and Mn, and the fatigue resistance of the obtained austenitic stainless steel plate were investigated.

(State of inclusions)
The state of inclusions in the steel sheet was evaluated by examining the number of coarse inclusions per 0.05 mm ² and the number ratio of CaO-based coarse inclusions. The observation sample was collected as described above, and adjusted so that the length of the L cross section of the sample in the longitudinal direction was 20 mm. The collected sample was polished using a corrosive solution or the like so that the L cross section was the observation surface. This sample was observed using a SEM (scanning electron microscope). The SEM setting conditions during observation were an accelerating voltage and 15k. The observation field of view was 100 μm x 100 μm, and this area was observed for 5 views at the above-mentioned 5 positions in the plate thickness direction, so that the total observation area was adjusted to 0.05 mm ² . Then, inclusions whose major axis was 5 μm or more were recognized as coarse inclusions, and their number was counted. Note that other matters are as described above.

Next, a method for measuring the number ratio of CaO-based coarse inclusions will be explained. After identifying coarse inclusions in the above-mentioned 0.05 mm ² area and counting the number, the composition of the coarse inclusions observed within the above-mentioned area was analyzed using EDX attached to the SEM. In the analysis, point analysis was performed at three points in the inclusion, and if the average value of the three points analyzed was the value of the mol% ratio converted to oxide, and CaO was 30 mol% or more, it was determined that CaO-based coarse inclusions were present. recognized as a thing. The ratio of the number of inclusions recognized as CaO-based coarse inclusions to all the coarse inclusions observed was calculated. Note that the inclusions are oxides, and one inclusion is one or more of MgO, Al ₂ O ₃ , MnO, Cr ₂ O ₃ , and CaO, or a composite oxide thereof, and other oxides, such as , ZrO ₂ etc. were not considered.

(Negative segregation degree)
The negative segregation degrees of Ni and Mn were measured using the following procedure. Specifically, the negative segregation degree of Ni and Mn is measured in an observation field of 250 μm×250 μm. Using EPMA analysis under the conditions of beam diameter 1 μm, step size 0.5 μm, acceleration voltage 15 kV, and irradiation current 2.0 × 10 ^-9 A, and when the average value of the Ni or Mn concentration of all analysis points is set to 1, The concentration ratio of Ni and Mn in the negative segregation area was defined as the degree of negative segregation. Note that the sample for analysis was collected at the same position as when observing coarse inclusions, and was collected so that the length of the L cross section of the sample in the longitudinal direction was 20 mm. The analysis was performed on the L cross section.

(Fatigue resistance properties)
The fatigue resistance properties of the steel plate were evaluated using a plane bending fatigue test. The test piece was held at 300° C. for 200 hours in hydrogen gas at 45 MPa to charge hydrogen into the steel material. The stress ratio was −1 and the frequency was 10 Hz. The fatigue limit is the maximum test stress that does not break after 1 × 10^7 cycles, and if the fatigue limit of the hydrogen-charged material is 0 to 20 MPa different from the hydrogen-uncharged material, ◎, and if it is over 20 MPa and within 30 MPa. It was marked as ○, and it was marked as × when it exceeded 30 MPa. Table 2 is shown below.

No. Samples Nos. 1 to 17 satisfied the requirements of this embodiment and exhibited good fatigue resistance. On the other hand, No. 1 that does not satisfy the requirements of this embodiment. Nos. 18 to 25 had poor fatigue resistance.

The cold-rolled steel sheets having the chemical composition of Example 1 were welded and formed into the shape of steel pipes, and then drawn the number of times shown in Table 3. In addition, the specific chemical composition of each steel plate is shown by the symbol in Table 3. In addition, in the case where the drawing process was performed multiple times, annealing was performed between the drawing processes. The annealing temperature was in the range of 1000 to 1100°C. Thereafter, it was cooled to obtain various types of austenitic stainless steel pipes. Note that the annealing between the drawing processes was carried out by ordinary combustion gas atmosphere annealing or bright annealing. Thereafter, bright annealing was performed in the final annealing, and the tube was cooled to obtain an austenitic stainless steel tube. Note that the number of times of bright annealing in Table 3 indicates the number of times of bright annealing performed between each drawing process and after the drawing process. In the case of bright annealing, hydrogen gas was used, the dew point was set to -45°C or less, and annealing was performed in the above-mentioned temperature range.

The obtained austenitic stainless steel pipe was examined for grain size, degree of negative segregation of Ni and Mn, state of inclusions, and fatigue resistance.

(crystal grain size)
The grain size of the steel pipe was measured using the following procedure. The weld was observed in a cross section perpendicular to the drawing direction of the steel pipe, and the grain size at the center of the wall thickness at the center of the weld was measured. In addition, the measurement method was carried out by a cutting method in accordance with JIS G 0551:2013, and in the observation during grain size measurement, five viewing fields of 200 to 1000 times magnification were measured, and the crystal grain size was calculated.

(State of inclusions)
As in the case of steel plates, the state of inclusions in steel pipes was also investigated. Regarding the measurement method, samples were collected and observed as described above. Note that other observation conditions were the same as in Example 1.

(Negative segregation degree)
As described above, the degree of negative segregation was measured using EPMA in the same way as when observing the state of inclusions. Samples for analysis were taken at the same location as when observing coarse inclusions. Note that other observation conditions were the same as in Example 1.

(Fatigue resistance properties)
The fatigue resistance properties of steel pipes were measured using the following procedure. A cycle test in which the inside of the steel pipe is filled with hydrogen gas and the gas pressure is set to a lower limit of 10 MPa (+0 to 3 MPa) and an upper limit of 70 MPa (+0 to 3 MPa) or a lower limit of 40 MPa (+0 to 3 MPa) and an upper limit of 100 MPa (+0 to 5 MPa). I did it. The pressure was increased and decreased in 20 to 30 seconds per cycle, and 10,000 cycles were performed. If there was no cracking under the condition of an upper limit of 100 MPa, the mark was ◎, if there was no cracking under the upper limit of 70 MPa, the mark was ○, and if there was a crack, the mark was x. The results are shown in Table 3 below.

No. 1 that satisfies the requirements of this embodiment. Nos. 1 to 17 showed good fatigue resistance. On the other hand, No. 1 that does not satisfy the requirements of this embodiment. Nos. 18 to 25 had poor fatigue resistance.

Claims (8)

The chemical composition is in mass%,
C: 0.1% or less,
Si: 2.0% or less,
Mn: 6.0 to 12.0%,
P: 0.030% or less,
S: 0.003% or less,
Cr: 13.0-18.0%,
Ni: 5.0 to 9.0%,
N: 0.15-0.25%,
Al: 0.005-0.080%,
Ca: 0.0005-0.01%,
B: 0.0001-0.01%,
Ga: 0.0010-0.020%,
Cu: less than 1.0%,
Mo: less than 2.0%,
Nb: 0 to 0.5%,
Ti: 0 to 0.5%,
V: 0-0.50%,
W: 0-0.50%,
Zr: 0 to 0.50%,
Co: 0 to 0.50%,
Mg: 0 to 0.005%,
Hf: 0-0.10%,
REM: 0-0.1%,
The remainder: Fe and impurities,
The f value calculated by the following formula (i) is more than 29.5 and less than 32.5,
In the cross section parallel to the processing direction and thickness direction,
When inclusions with long sides of 5 μm or more are coarse inclusions, the number of coarse inclusions is 10 or less per 0.05 mm ² ,
An austenitic stainless steel material, wherein the ratio of the number of coarse inclusions containing 30 mol% or more of CaO to the number of coarse inclusions is 50% or more.
f value=Ni+0.72Cr+0.88Mo+1.11Mn-0.27Si+0.53Cu+12.93C+7.55N-1.1Nb-2.2Ti...(i)
However, each element symbol in the above formula (i) represents the content (mass %) of each element contained in the steel, and is zero if it is not contained. In the cross section,
The negative segregation degree of Ni is 0.80 or more,
The austenitic stainless steel material according to claim 1, wherein the degree of negative segregation of Mn is 0.80 or more. The austenitic stainless steel material according to claim 1 or 2, which is a steel plate. The austenitic stainless steel material according to claim 1 or 2, which is a steel pipe. A manufacturing method for manufacturing the austenitic stainless steel material according to claim 1 or 2, comprising:
A cold working step of cold working a hot worked material having the chemical composition according to claim 1 to obtain a cold worked material;
A method for producing an austenitic stainless steel material, comprising a bright annealing step of bright annealing at a temperature range of 950 to 1150°C with a dew point of -45°C or lower. A manufacturing method for manufacturing the austenitic stainless steel material according to claim 3,
The method for manufacturing an austenitic stainless steel material according to claim 5, wherein cold rolling is performed in the cold working step. A manufacturing method for manufacturing the austenitic stainless steel material according to claim 4, comprising:
The method for manufacturing an austenitic stainless steel material according to claim 5, wherein drawing is performed in the cold working step. A method for manufacturing the austenitic stainless steel material according to claim 4,
The method for manufacturing an austenitic stainless steel material according to claim 7, wherein the drawing process is performed two or more times.