JPWO2009107475A1 - Austenitic stainless steel and its hydrogen removal method - Google Patents

Abstract

Focusing on diffusible hydrogen and non-diffusible hydrogen which cause hydrogen embrittlement of austenitic stainless steel, an austenitic stainless steel from which this has been removed, and a method for removing the hydrogen are provided. In order to remove diffusible hydrogen and non-diffusible hydrogen that cause hydrogen embrittlement of the austenitic stainless steel, the austenitic stainless steel is heated at a heating temperature of 200 ° C. or higher and 1100 ° C. or lower while being held in an air atmosphere. And perform aging treatment. Therefore, hydrogen (H) contained in the austenitic stainless steel is removed to 0.001 mass% (1 mass ppm) or less.

Description

  The present invention relates to an austenitic stainless steel with reduced hydrogen embrittlement and a method for removing the hydrogen. In particular, the present invention relates to an austenitic stainless steel in which the effect of hydrogen present in the austenitic stainless steel on the progress of fatigue cracks generated in the austenitic stainless steel is reduced, and a method for removing the hydrogen.

  The use of hydrogen as the next-generation energy is attracting attention from the viewpoint of global environmental problems, and research and development for that purpose is being actively conducted. In particular, the development of a fuel cell vehicle using hydrogen as a fuel and the development of a stationary fuel cell are attracting attention as important issues. The use of stainless steel as a material for high-pressure hydrogen tanks, various parts, piping, and the like in this fuel cell system has been studied (see, for example, Patent Document 1).

Table 1 illustrates typical austenitic stainless steel components. The first column of Table 1 is the names of stainless steel and heat-resistant steel defined by Japanese Industrial Standard (hereinafter referred to as JIS (Japan Industrial Standard) standard). The last list in Table 1 shows the Vickers hardness of stainless steel (hereinafter referred to as HV (Vickers hardness)). The other columns are chemical components contained in the stainless steel, and the unit of the component is expressed by mass%. In the case of hydrogen (H), the content is expressed in mass ppm.

  It is known that hydrogen penetrates into a metal material and reduces the static strength and fatigue strength of the material (for example, Non-Patent Documents 1 and 2). Various methods for removing this hydrogen and predicting the influence of hydrogen have been proposed. For example, in Patent Document 2, austenitic stainless steel is subjected to heat treatment after being plated at a temperature of 270 to 400 ° C. for 10 minutes or more to remove hydrogen to prevent hydrogen embrittlement. Patent Document 3 discloses a method for predicting and determining the degree of hydrogen embrittlement of austenitic stainless steel by chemical components.

Patent Documents 4 and 5 disclose dehydrogenated austenitic stainless steel wires. Patent Document 4 discloses a high-strength austenitic stainless steel wire in which hydrogen is reduced to 1.5 ppm or less by dehydrogenation treatment (at 300 ° C. for 24 hours) (Table 2 of Patent Document 4 and paragraph [0015]). , [0016].) Even when this stainless steel wire was subjected to a tensile test at a tensile strength of 1900 N / mm ² or more and up to about 2200 N / mm ² , no longitudinal wire cracking was observed, and the amount of work-induced martensite after wire drawing was 30 to 30 75%.

Patent Document 5 discloses a high-strength austenitic stainless steel wire in which hydrogen is reduced to 1.5 ppm by dehydrogenation treatment (low temperature aging at 400 ° C. for 30 minutes) (Table 2 and paragraph of Patent Document 5). [0042].) The stainless steel wire, even if a tensile test at a tensile strength of up to 2000N / mm ² or more 2800N / mm ^2, not to destroy, deformation-induced martensite amount after drawing is 25 to 70%. However, the inventions of Patent Documents 4 and 5 are inventions of austenitic stainless steel wires, and there is no description of fatigue tests with a slow repetition rate as will be described later.

  Non-Patent Document 1 discloses the fatigue test results of austenitic stainless steel compliant with SUS304, SUS316, and SUS316L. This fatigue test was conducted by comparing these austenitic stainless steels with the respective austenitic stainless steels charged with hydrogen. The fatigue crack growth rate of hydrogen-charged SUS304 and SUS316 is faster than that in the case of no charge. In the case of SUS316L, there is no clear difference.

  Furthermore, after pre-straining the test piece, the fatigue test results of JIS standard SUS304 and SUS316L austenitic stainless steel in which minute holes of about 100 μm are formed are announced. In SUS304 charged with hydrogen, the fatigue crack growth rate is accelerated 10 times as compared with the case of uncharged. In the case of SUS316L, the fatigue crack growth rate is accelerated twice.

  However, even a metastable austenitic stainless steel may undergo work-induced martensitic transformation due to cold working or cyclic stress. Austenitic stainless steels such as JIS SUS316L, etc. are generally recognized by those skilled in the industry, including the inside of academic societies, which is a group of researchers, and that the fatigue crack growth rate has almost no influence of hydrogen. It was a common sense. By showing the result of overturning this common sense, the above result was obtained by applying a repetitive load at a low frequency of 5 Hz or less, which is very significant.

In other words, in austenitic stainless steel such as SUS316L, it was confirmed that the fatigue crack growth rate was accelerated by a low frequency cyclic load. On the other hand, Non-Patent Document 2 states that “(3) The martensite phase generated by transformation in austenitic stainless steel becomes a passage for hydrogen diffusing in the material and increases the diffusion coefficient of hydrogen (see page 130). ) ”.
Japanese Patent Application Laid-Open No. 2004-339569 Japanese Patent Laid-Open No. 10-199380 JP-A-2005-9955 JP-A-10-121208 JP 2005-298932 A Toshihiko Kanazaki, Chihiro Amagasaki, Yoji Tsuji, Saburo Matsuoka, Takayoshi Murakami “Effect of Hydrogen on Fatigue Crack Propagation in Prestrained Austenitic Stainless Steel” Department of Mechanical Engineering, Japan 05-9] Proceedings of M & M 2005 Materials Mechanics Symposium ('05 .11.4-6, Fukuoka City), P86, P595-596 Toshihiko Kanazaki, Chihiro Amagasaki, Yoji Tsuji, Saburo Matsuoka, Takayoshi Murakami “The Effect of Hydrogen on the Fatigue Crack Growth Properties of Stainless Steel and the Martensitic Transformation” The Japan Society of Mechanical Engineers, Vol. 72, No. 723 (2006-2006) 11), P123-130 (original reception May 1, 2006)

  However, in austenitic stainless steel, it has been fully analyzed how diffusible hydrogen charged from the outside and non-diffusible hydrogen inherent in the crystal are related to the above-mentioned fatigue crack growth rate. There is no current situation. Furthermore, the relationship between the amount of martensite transformation in the material, the effect of acceleration of the hydrogen diffusion rate, and how diffusible and non-diffusible hydrogen affect the fatigue crack growth rate is fully elucidated. Not.

  Furthermore, when stainless steel is used in equipment and devices related to the use of hydrogen fuel, it is affected by various environments depending on the use environment. For example, when stainless steel is used for a high-pressure hydrogen container or piping for a fuel cell vehicle, loading and releasing are repeated in a relatively slow cycle due to filling and consumption of hydrogen gas in the high-pressure hydrogen container. However, this slow cycle is not assumed in past fatigue tests. That is, it was considered that the fatigue test with a load having a long cycle time could be replaced by a fatigue test with a high repetition rate.

  Also, low frequency repetitive loads are generated due to temperature changes caused by the outside air temperature. For example, the cyclic load due to changes in the outside air temperature may be due to compression and expansion of the stainless steel itself due to temperature difference between day and night, and thermal stress due to compression and expansion of the parts connected to the stainless steel parts. As for the frequency, for example, the temperature difference between day and night is from several degrees to 10 ° C. or more, and becomes one cycle in 24 hours, which is one day. In the fuel cell vehicle-related equipment, the high-pressure hydrogen tank, the fuel supply equipment for the fuel cell, etc. have a period of one day as described above, and the hydrogen filling time is long. In addition, there are a temperature difference of several degrees Celsius to several tens of degrees Celsius and a period of subseconds to several hours depending on the environment in which the fuel cell vehicle runs.

The present invention has been made based on the technical background as described above, and achieves the following objects.
An object of the present invention is to provide an austenitic stainless steel for reducing the influence of hydrogen on the fatigue crack growth rate of austenitic stainless steel, and a method for removing the hydrogen.
Another object of the present invention is to provide an austenitic stainless steel from which both are removed by focusing on diffusible hydrogen and non-diffusible hydrogen that cause hydrogen embrittlement of austenitic stainless steel, and a method for removing the hydrogen. It is in.

Still another object of the present invention is austenitic stainless steel, which is present in austenitic stainless steel and has been removed by paying attention to diffusible hydrogen and non-diffusible hydrogen which are problematic in repeated loads with a long cycle time. It is to provide steel and a method for removing hydrogen.
Still another object of the present invention is to provide an austenitic stainless steel for removing diffusible hydrogen and non-diffusible hydrogen present in the austenitic stainless steel in a manufacturing process of austenitic stainless steel, and a method for removing the hydrogen. There is.

Still another object of the present invention is a process for producing austenitic stainless steel, particularly austenitic stainless steel for removing diffusible hydrogen and non-diffusible hydrogen present in austenitic stainless steel in a heat treatment environment in the atmosphere. It is to provide steel and a method for removing hydrogen.
Still another object of the present invention is to provide an austenitic stainless steel capable of suppressing the growth rate of fatigue cracks at a low frequency cyclic load, and a method for removing hydrogen therefrom.

[Definition of technical terms]
The present invention uses the following technical terms in the meanings defined below. “Hydrogen charge” means that hydrogen enters the material. As a method of hydrogen charging, a method of exposing a material to a high-pressure hydrogen chamber, a method of applying a cathodic charge, and a method of immersing in a chemical solution or the like are used. "Fatigue crack growth" means that the crack grows due to repeated loads from defects or cracks that occur in the material during the manufacturing process, or holes or through holes that are artificially introduced into the material. .

  The fatigue crack growth rate means the speed of fatigue crack growth. The austenitic stainless steel is a Cr—Ni-based steel material, which is a stainless steel having an austenitic phase in which Cr and Ni are added to Fe to increase corrosion resistance against a corrosive environment or the like. Examples of this stainless steel are shown in Table 1. The austenite phase is an iron phase in a temperature range of 911 to 1392 ° C. in iron (Fe) having a purity of 100%, and has a face-centered cubic lattice structure (hereinafter referred to as an FCC (Face Centered Cubic Lattice) structure). Have.

  FIG. 11A shows a face-centered cubic lattice. By adding an alloy element such as Cr or Ni to Fe, an austenite phase can exist even at room temperature. The martensite phase is a structure obtained by rapidly cooling steel from a high-temperature stable austenite phase, and has a body-centered cubic lattice structure (hereinafter referred to as a BCC (Body Centered Cubic Lattice) structure). FIG. 11B illustrates a body-centered cubic lattice. Further, a martensite phase may be produced by applying cold working such as stress to stainless steel in the austenite phase state at room temperature.

  The transformation from the austenite phase of the FCC structure to the martensite phase of the BCC structure by cold working is referred to as processing-induced martensitic transformation. Diffusible hydrogen refers to hydrogen present in a material that exits the material over time at room temperature. This diffusible hydrogen causes hydrogen embrittlement of the material. Non-diffusible hydrogen refers to hydrogen that exists in a material and cannot escape from the material with time even at temperatures from room temperature to about 200 ° C.

In order to achieve the above object, the present invention employs the following means.
The inventors of the present invention have found that non-diffusible hydrogen in austenitic stainless steel is related to fatigue crack growth. We have invented an austenitic stainless steel utilizing this and a method for removing hydrogen. The present invention relates to an austenitic stainless steel having an austenitic phase whose crystal structure is a face-centered cubic lattice structure, and a method for removing hydrogen therefrom.

  The austenitic stainless steel of the present invention is obtained by heat-treating an austenitic stainless steel having an austenitic phase whose crystal structure is a face-centered cubic lattice structure at a heating temperature of 200 ° C. or higher and 1100 ° C. or lower in an air atmosphere. Removes diffusible hydrogen and non-diffusible hydrogen, which cause hydrogen embrittlement of stainless steel, and removes hydrogen (H) contained in austenitic stainless steel to 0.0001 mass% (1.0 mass ppm) or less It is characterized by that.

  The method for removing hydrogen from an austenitic stainless steel according to the present invention comprises heating an austenitic stainless steel having an austenitic phase having a face-centered cubic lattice structure in an air atmosphere at a heating temperature of 200 ° C. or higher and 1100 ° C. or lower. Thus, the amount of diffusible hydrogen and non-diffusible hydrogen in the austenitic stainless steel is removed so as to be 0.0001 mass% (1.0 mass ppm) or less.

  The diffusible hydrogen and non-diffusible hydrogen that are removed by the hydrogen removal method of the austenitic stainless steel of the present invention are diffused through a work-induced martensite phase caused by a low frequency cyclic load, and are subjected to stress concentration. By concentrating on the steel, the growth rate of the fatigue crack is increased, which causes hydrogen embrittlement of the austenitic stainless steel.

  The following description is common to the austenitic stainless steel of the present invention and its hydrogen removal method. The austenitic stainless steel of the present invention is obtained by removing diffusible hydrogen and non-diffusible hydrogen in the austenitic stainless steel, and the hydrogen (H) in the austenitic stainless steel is 0.00007. What was made into the mass% (0.7 mass ppm) or less is preferable. More preferably, the austenitic stainless steel of the present invention is obtained by removing diffusible hydrogen and non-diffusible hydrogen in the austenitic stainless steel, and hydrogen (H) in the austenitic stainless steel is: What was 0.00002 mass% (0.2 mass ppm) or less is good.

  The above diffusible hydrogen and non-diffusible hydrogen are removed by heat-treating austenitic stainless steel at a heating temperature of 200 ° C. or higher in an air atmosphere. The upper limit of this heating temperature is 1100 ° C., and in particular, it is preferably a temperature lower than the sensitization temperature which is the temperature at which chromium (Cr) carbide of austenitic stainless steel is precipitated by heating. The heat treatment time is 2 hours to 500 hours.

  Furthermore, the removal of diffusible hydrogen and non-diffusible hydrogen from the austenitic stainless steel of the present invention may be carried out for removal, but it is preferably not performed specially. In the manufacturing process, heat treatment is performed for a predetermined time to remove hydrogen, and hydrogen (H) is reduced to 0.00007 mass% (0.7 mass ppm) or less. Therefore, the manufacturing process of austenitic stainless steel can be simplified without providing a special process for removing diffusible hydrogen and non-diffusible hydrogen. The temperature at this time is preferably 200 ° C. or higher and lower than the melting point temperature of stainless steel.

  Although the heating time in this manufacturing process is changed depending on the capacity of the material, it is practical that it is 2 hours or more and tens of hours or less. This manufacturing process is more preferably in an inert gas flow atmosphere. The manufacturing process of austenitic stainless steel here is a concept including the steps of solution treatment and aging treatment used when manufacturing stainless steel.

  The temperature of the heat treatment is most preferably 920 ° C. or higher in the case of the solution treatment. Furthermore, the temperature of the heat treatment is most preferably 700 ° C. or higher in the case of aging treatment. Furthermore, the stainless steel of the present invention is not limited to those in Table 1 described above, but is austenitic stainless steel or austenitic heat resistant steel.

  When the aging treatment is performed on the austenitic stainless steel of the present invention in the atmosphere, it is considered that the heating temperature range is 200 to 1100 ° C. The supporting evidence is as follows. As can be seen from FIGS. 14A and 14B of other experimental example 3 described later, hydrogen is released at a temperature of 200 ° C. or higher. This indicates that the stainless steel needs to be heated to at least 200 ° C., that is, the lower limit of the heating temperature. The upper limit of the heating temperature is a temperature below the melting point of stainless steel.

  Usually, in conventional steelmaking methods, it is common for austenitic stainless steel to contain more than 1 ppm of hydrogen during the steelmaking process. The amount of hydrogen depends on the size of the material at the time of shipment. FIG. 15 is a graph showing the relationship between the amount of hydrogen and the material size of the materials shown in Table 1. This graph is a result of measurement of materials purchased by the inventors of the present invention from material manufacturers. The horizontal axis in the figure represents the size of the material, which is the minimum value of the material dimensions at the time of shipment.

  For example, a round bar having a diameter of 20 mm × a length of 1 m means 20 mm of the diameter, and a plate having a width of 200 mm × a plate thickness of 10 mm × a length of 1 m means a plate thickness of 10 mm. Further, in the case of a round bar having a diameter of 20 mm and a length of 10 mm, it means the length of 10 mm. The heating time described above is a time required for hydrogen to diffuse out of the sample. This time depends on the size of the material. The heating time can be estimated by calculation from the diffusion coefficient of hydrogen in the material at the heating temperature. When the sample size is reduced, the heat treatment takes a short time of several hours.

  On the other hand, when the sample size increases, a considerably long period of two weeks or more is required. When the heating time is longer than the time required for hydrogen to diffuse out of the sample, it is considered that the amount of hydrogen does not change even if the heating time is increased. Therefore, the effective range of the aging treatment time depends on the heating temperature and the sample size, but 500 hours or less is considered realistic. However, it is not limited to this time. As can be seen from the results shown in FIG. 15, when a large material is used, for example, as a structural material in a power plant or the like, more hydrogen is contained because of the large material size.

  Such a material is subjected to heat treatment in consideration of removing hydrogen that causes hydrogen fragility, as determined by the inventors of the present invention. As a result, by heating hydrogen in a large material to 1 ppm or less, the effect of improving crack propagation characteristics can be obtained, and the safety of buildings, machines, devices, and plants using the material can be improved. This is considered to be further improved. Needless to say, the effect is small and small stainless steel.

  According to the present invention, the following effects can be obtained. In the present invention, austenitic stainless steel is heat-treated in an air atmosphere at a temperature of 200 ° C. or higher to remove non-diffusible hydrogen and diffusible hydrogen in the austenitic stainless steel, and is resistant to fatigue crack growth. It has become possible to provide stainless steel.

FIG. 1 is a diagram illustrating an outline of a fatigue test piece, FIG. 1 (a) is a diagram illustrating the shape of a fatigue test piece, and FIG. 1 (b) is a shape of an artificial microhole formed in the fatigue test piece. FIG. FIG. 2 shows an outline of the test part of the fatigue test piece, the shape of the introduced artificial microhole, and the fatigue crack that is generated and propagates from the artificial microhole. FIG. 3 is a schematic view of a procedure for introducing prestrain into a fatigue test piece. FIG. 4 is a photograph of a fatigue crack generated from an artificial microhole after a fatigue test. FIG. 5 is a graph showing the result of inspecting the austenite phase and the martensite phase by X-rays on the surface of the test part before the fatigue test and the fatigue fracture surface after the fatigue test. FIG. 5 (b) shows the measurement result of SUS316, and FIG. 5 (c) shows the measurement result of SUS316L. FIG. 6 is a graph showing the relationship between the crack length and the number of repetitions in a fatigue test. FIG. 6 (a) is for SUS304, FIG. 6 (b) is for SUS316, and FIG. 6 (c) is for SUS316L. . FIG. 7 is a fatigue crack photograph of SUS304, SUS316, and SUS316L observed by the replica method. FIG. 8 is a graph illustrating the results of a fatigue test of SUS316L. FIG. 9 is a graph illustrating the results of a fatigue test of SUS316L. FIG. 10 is a conceptual diagram showing elements in which diffusible hydrogen and non-diffusible hydrogen move through martensitic transformation. FIG. 11 is a conceptual diagram showing the lattice of the crystal structure of the austenite phase and the martensite phase, FIG. 11 (a) is a face-centered cubic lattice structure (FCC) of the austenite phase, and FIG. 11 (b) is the martensite phase. It is a conceptual diagram of a body centered cubic lattice structure (BCC). FIG. 12 is a graph showing the results of other experimental example 1. FIG. 13 is a graph showing the results of other experimental example 2. FIG. 14 is a graph showing the results of other experimental example 3. FIG. 14A shows the case of SUS304, and FIG. 14B shows the case of SUS316L. FIG. 15 is a graph showing the relationship between the amount of hydrogen and the material size.

  Hereinafter, the embodiment of the present invention will be described by changing to experimental examples. First, how hydrogen affects the growth rate of fatigue cracks generated in austenitic stainless steel will be described. Austenitic stainless steels such as SUS304, SUS316, and SUS316L shown in Table 1 contain 1 to 4.7 ppm by mass of non-diffusible hydrogen even after the usual heat treatment (solution treatment). Traditionally, it was considered by those skilled in the art that this non-diffusible hydrogen had no effect on hydrogen embrittlement.

  However, the fatigue test described below has revealed that non-diffusible hydrogen has an effect on hydrogen embrittlement. Hydrogen embrittlement due to non-diffusible hydrogen has been confirmed particularly in the case of a low-frequency fatigue test speed of about 0.0015 Hz (repetition time of one cycle is about 11 minutes). The inventors of the present invention conducted the following experiment to observe how non-diffusible hydrogen affects the fatigue crack growth rate. An example of the experiment is shown.

[Test pieces]
The materials used were austenitic stainless steels SUS304, SUS316, and SUSU316L (A) (hereinafter simply referred to as SUSU316L) shown in Table 1. As SUS304, SUS316, and SUSU316L, those subjected to solution treatment were used. FIG. 1A illustrates the shape of a fatigue test piece. The surface of the test piece was polished to # 2000 with emery paper and then finished by buffing.

  In order to easily observe the fatigue crack growth, an artificial microhole having a diameter of 100 μm and a depth of 100 μm as shown in FIG. 1B is formed at the center in the length direction of the fatigue test piece and at the tip in the radial direction. It was opened with a drill with an angle of 120 degrees. An artificial microhole was introduced in the center of the test part of the test piece. The test part is a cylindrical part at the center of the test piece, and the length of the cylinder is about 20 mm. The upper surface and the bottom surface of this cylinder are parallel and perpendicular to the axis of the specimen in the length direction. FIG. 2 shows the outline of the test section and the shape of the introduced artificial microhole. In the case of a fatigue test piece charged with hydrogen, immediately after the completion of hydrogen charge, buffing was performed again to open an artificial microhole.

[X-ray diffraction]
For austenitic stainless steel, the amount of martensite in the test part of the fatigue test piece was measured by X-ray diffraction. X-ray diffraction was performed using a micro X-ray stress measurement apparatus (PSPC-RSF / KM) manufactured by Rigaku Corporation (Akishima City, Tokyo, Japan). Quantitative analysis was performed by using CrKα rays and calculating from the integrated intensity ratio of diffraction peaks of the austenite phase {220} plane and martensite phase {211} plane. The amount of martensite before the fatigue test contained in the test part was about 3% for SUS304, SUS316, and SUS316L.

  In the case of the test section charged with hydrogen, the same was about 3%. The amount of martensite was measured at two locations before introducing artificial microholes. The first area of this measurement is a circular area having a diameter of 1 mm centered on the position where the artificial microhole is to be introduced. The second area of this measurement is a circle area having a diameter of 1 mm centered on a position obtained by rotating the axis in the length direction of the test piece 180 degrees from the position where the artificial microhole is to be introduced. That is, the second measurement region is located on the opposite side of the cylinder from the first measurement region on the cylinder.

[Hydrogen charging method]
Hydrogen charging was performed by a cathodic charging method. The conditions for hydrogen charging are a sulfuric acid aqueous solution with pH = 3.5, a platinum anode, and a current density i = 27 A / m ² . When the solution temperature was 50 ° C. (323 K), hydrogen charging was performed for 672 hours (4 weeks), and when the temperature was 80 ° C. (353 K), hydrogen charging was performed for 336 hours (2 weeks). The aqueous sulfuric acid solution was changed every week in order to reduce the change in sulfuric acid concentration due to evaporation.

[Pre-strained material]
In order to investigate the relationship between the acceleration of fatigue crack growth rate due to hydrogen and the amount of martensite transformation, pre-strain was applied to SUS304 and SUS316L to prepare martensitic transformed specimens. FIG. 3 shows a schematic diagram of the pre-strain introduction procedure. Pre-strain was introduced in ethanol at -70 ° C. to promote martensitic transformation. After the introduction of this pre-strain, the test piece was processed into a shape as shown in FIG. For SUS304, a pre-strain of plastic strain (true strain) ε _p = 0.28 and for SUS316L, a plastic strain ε _p = 0.35 was applied.

  When the Vickers hardness (measurement load 9.8 N) after applying pre-strain was measured, SUS304 was HV = 426 (10-point average) and SUS316 was HV = 351 (10-point average). The variation was within ± 4%. After polishing the test piece, the amount of martensite in the test section after introduction of prestrain was measured by X-ray diffraction. The amount of martensite was 65 to 69% by volume in SUS304, and 26 to 28% in volume by SUS316L. The amount of martensite was measured at two locations before introducing artificial microholes. This measurement area is a circle having a diameter of 1 mm centered on a position where the artificial microhole is to be introduced and a position obtained by rotating the longitudinal axis of the test piece 180 degrees from the position where the artificial microhole is to be introduced. It is an area.

[Fatigue test method]
The fatigue test was performed using a hydraulic servo tension / compression fatigue tester servo pulser EHF-ED30KN manufactured by Shimadzu Corporation (Nakakyo-ku, Kyoto, Japan) with a repetition rate of 0.0015 to 5 Hz and a stress ratio R = -1. It was. The repetition rate was adjusted so that the surface temperature of the test part did not exceed 60 ° C. during the fatigue test. The fatigue crack was observed by the replica method and the length of the fatigue crack was measured.

  Observation of fatigue cracks by the replica method is as follows. An acetylcellulose film having a thickness of about 0.034 mm (hereinafter referred to as a replica film) was immersed in a methyl acetate solution for a while and then attached to a place to be observed. After applying the replica film, wait for 2 to 3 minutes. When the replica film dries, the replica film is collected. Fatigue cracks in the test part were observed by depositing gold on the collected replica film and observing it with a metal microscope.

Therefore, the target fatigue crack location could be observed without directly observing the specimen. In the case of a hydrogen charge material, immediately after the end of the fatigue test, a sample having a diameter of 7 mm and a thickness of 0.8 mm was cut out from the test part, placed in a vacuum chamber, and heated at a constant temperature increase rate. The pressure in the vacuum chamber was 1 × 10 ^{−7 to} 3 × 10 ⁻⁷ Pa before heating the sample. The temperature was raised to 800 ° C. at a temperature raising rate of 0.5 ° C./s.

  When the sample in the vacuum chamber was heated, hydrogen was desorbed from the sample, and the amount of desorbed hydrogen was measured with a quadrupole mass spectrometry temperature programmed desorption analyzer (hereinafter referred to as TDS). The TDS used for the measurement is a temperature programmed desorption analyzer (hereinafter referred to as TDS) EMD-WA1000S / H manufactured by Electronic Science Co., Ltd. (Musashino City, Tokyo, Japan). The accuracy of measurement by TDS was 0.01 mass ppm.

[Measured characteristics]
FIG. 4 is a photograph of a fatigue crack generated from an artificial microhole introduced into hydrogen-uncharged SUS304 after a fatigue test. From the photograph, fatigue cracks generated from artificial microholes can be confirmed. It can be seen that this fatigue crack originates from both sides of the artificial microhole and propagates almost symmetrically.

  FIG. 5 shows the results of inspecting the austenite phase and martensite phase with X-rays on the surface of the test part before the fatigue test and on the fatigue fracture surface after the fatigue test. The dotted line in FIG. 5 shows the result of measuring the surface of the test part before the fatigue test. The solid line shows the result of measuring the fatigue fracture surface after the fatigue test. FIG. 5A shows the measurement result of SUS304, and it can be seen from this measurement that the austenite phase decreased after the fatigue test and the martensite phase increased before the fatigue test.

  FIG. 5B shows the measurement result of SUS316. From this measurement, it can be seen that the austenite phase is slightly decreased after the fatigue test and the martensite phase is increased before the fatigue test. FIG.5 (c) is a measurement result of SUS316L, and it turns out from this measurement that the martensite phase is increasing after the fatigue test than before the fatigue test. In the case of SUS316L, the change of the austenite phase is not so much seen.

  FIG. 6 is a graph showing the relationship between the crack length and the number of repetitions in the fatigue test. 6A shows the case of SUS304, FIG. 6B shows the case of SUS316, and FIG. 6C shows the case of SUS316L. Each material SUS304, SUS316, and SUS316L shows the measurement results of those charged with hydrogen and those not charged with hydrogen. The repetition rate is 1.2 Hz for SUS304 and SUS316, and 5 Hz for SUS316L.

  From this graph, SUS304 and SUS316 that have been charged with hydrogen have a faster crack growth rate than those that are not charged with hydrogen. For example, the number of repetitions N until the crack length 2a reaches 400 μm is smaller when hydrogen is charged than when hydrogen is not charged. In this case, the fatigue crack growth rate is about twice as fast as when hydrogen is charged. On the other hand, in the case of SUS316L, the fatigue crack growth rate is slightly higher when hydrogen-charged than when not hydrogen-charged, but no clear difference is observed.

  FIG. 7 is a fatigue crack photograph of SUS304, SUS316, and SUS316L observed by the replica method. Since the fatigue crack propagates almost symmetrically as shown in the photograph of FIG. 4, only half of the photograph is shown in FIG. From this photograph, it can be observed that the fatigue crack of the hydrogen-charged material progresses linearly compared to the uncharged material. It can be seen that in the uncharged material, the slip band occurs over a wide region, whereas in the hydrogen charged material, the slip band is localized near the fatigue crack.

  FIG. 8 is a graph illustrating the results of a fatigue test of SUS316L. This FIG. 8 shows two materials with hydrogen of 0.4 mass ppm and 2.6 mass ppm when not charged, and a material obtained by hydrogen charging 2.6 mass ppm of material to 3.9 mass ppm. The fatigue test results are shown. The repetition rate is 1.5 Hz until the length of the fatigue crack reaches 200 μm. When the fatigue crack length was 200 μm, the repetition rate was changed from 1.5 Hz to 0.0015 Hz. Fatigue cracks have propagated for materials with 2.6 ppm by mass of hydrogen and 3.9 ppm by mass of hydrogen.

  However, in the case of a material having 0.4 ppm by mass of hydrogen, fatigue cracks have not progressed much. FIG. 9 is a graph illustrating the results of a fatigue test of SUS316L. In FIG. 9, two materials with hydrogen of 0.4 mass ppm and 2.6 mass ppm when uncharged and 2.6 mass ppm of material are charged with hydrogen to 3.9 mass ppm, and 5. The fatigue test result of the material made into 1 mass ppm is shown. There are two repetition rates, 1.5 Hz and 0.0015 Hz.

  This graph shows that fatigue cracks have progressed in the case of 2.6 mass ppm of hydrogen and the material charged with hydrogen to 3.9 mass ppm and 5.1 mass ppm. . It can be seen that when the repetition rate is as low as 0.0015 Hz, the fatigue crack growth rate is faster than at 1.5 Hz. However, it is understood that when the hydrogen content is 0.4 mass ppm, the fatigue crack growth rate is slower than when the repetition rate is 0.0015 Hz or 1.5 Hz. This indicates that fatigue cracks do not progress much when hydrogen in the material is 0.4 mass ppm or less.

  FIG. 10 is a conceptual diagram showing how diffusible hydrogen and non-diffusible hydrogen move through the transformed martensite phase. In the figure, the tip of the fatigue crack undergoes martensitic transformation, and diffusible hydrogen and non-diffusible hydrogen migrate through this martensite phase. That is, the martensite phase having a high hydrogen diffusion rate moves as a passage and gathers at the tip of the fatigue crack. This is a phenomenon related to the diffusion and movement time of hydrogen. The diffusion rate of hydrogen in the austenite phase (FCC) is four orders of magnitude slower than the diffusion rate in the martensite phase (BCC). The periphery of the fatigue crack undergoes martensitic transformation, and hydrogen around the martensite phase diffuses and collects at the tip of the fatigue crack.

[Involvement of non-diffusible hydrogen]
Therefore, the above experiment showed that not only diffusible hydrogen but also non-diffusible hydrogen, which has not been noticed in the past, is involved. This is a new finding regarding hydrogen embrittlement for stainless steel. This is influenced by the martensitic transformation at the tip of the fatigue crack (transformation from FCC to BCC).

[Relationship between fatigue test speed and fatigue crack growth speed]
Furthermore, as shown in FIG. 9 of the above experiment, it can be seen that if the austenitic stainless steel such as SUS316L decreases the fatigue test speed, the fatigue crack growth speed increases. Similarly, as shown in FIG. 6, a hydrogen charge material such as a test piece charged with diffusible hydrogen has a higher fatigue crack growth rate than an uncharged material. In the case of a material having hydrogen of 0.4 mass ppm or less, there is not much progress of fatigue cracks as shown in FIGS. Thus, the effect of slowing down the fatigue test speed is a phenomenon related to the diffusion and movement time of hydrogen (in the FCC, the 4-digit diffusion speed is slower than in the BCC).

Hereinafter, the alloy component contained in the austenitic stainless steel of the present invention, the content thereof, the production method defined in the production method of the present invention, and the like will be described.
[Austenitic stainless steel]
Austenitic stainless steel is also called Cr-Ni stainless steel, which is obtained by adding Cr and Ni to Fe. The main components of austenitic stainless steel are Fe (iron), Cr (chromium), and Ni (nickel). In addition, there are various additives shown in Table 2 below.

The following Table 2 shows a preferable example of the austenitic stainless steel of the present invention, and the embodiment of the present invention is not limited to this example.

[Composition of austenitic stainless steel]
Cr is added to Fe in order to improve corrosion resistance. Ni is added to Fe in combination with Cr in order to increase corrosion resistance. Ni and Mn (manganese) are elements for ensuring non-magnetism after cold rolling. In order to maintain non-magnetism after cold rolling, it is necessary to contain 10.0% by mass or more of Ni. Furthermore, it is necessary to adjust the amount of Ni according to the contents of Si (silicon) and Mn so that the work-induced martensite phase is not generated by 1 vol% or more. Mn also has the effect of increasing the solid solubility of N (nitrogen).

  C (carbon) is a strong element for forming austenite. Furthermore, C is an element effective for improving the strength of stainless steel. When C is added excessively, coarse Cr-based carbides precipitate during the recrystallization process, causing intergranular corrosion resistance and deterioration of fatigue characteristics. Si is added for the purpose of deoxidation and solid solution strengthening. If the Si content is high, the formation of a martensite phase is promoted during cold working, and therefore a small amount is preferably added. N brings about solid solution hardening.

  Mo (molybdenum) is added for the purpose of improving corrosion resistance. Furthermore, it also has an effect of finely dispersing carbonitride by aging treatment. Ti (titanium) is an element effective for precipitation hardening, and is added to increase the strength by aging treatment. B (boron) is an alloy component effective in preventing the occurrence of edge cracks in the hot-rolled steel strip caused by the difference in deformation resistance between the δ ferrite phase and the austenite phase in the hot working temperature range. Al (aluminum) is an element added for the purpose of deoxidation during steelmaking, and acts effectively on precipitation hardening in the same manner as Ti.

The embodiment of the present invention can be used by adding elements such as Nb and Cu as required in addition to the elements described in Table 2 above. Nb (niobium) can be an alternative element for Ti.
[About austenitic phase]
The austenitic stainless steel desirably has an austenitic phase of almost 100% of the total volume. It is desirable that there is no martensite phase in the austenitic stainless steel.

[Other properties]
The average crystal grain size is preferably about 50 μm or less. The average crystal grain size is about 50 μm in the current material, and an average crystal grain size smaller than that is desirable.
[Hydrogen removal treatment by heating]
Hereinafter, hydrogen removal treatment by heating austenitic stainless steel will be described. The inventors of the present invention have determined that non-diffusible hydrogen is involved in fatigue crack growth, and against this background, non-diffusible hydrogen and diffusible hydrogen in austenitic stainless steel are heat treated as follows. To remove.

  The removal of diffusible hydrogen and non-diffusible hydrogen is performed by heat-treating austenitic stainless steel at a heating temperature of 200 ° C. or higher. The heat treatment is performed in a vacuum atmosphere. The vacuum atmosphere is an environment of 0.2 Pa or less. Moreover, as for heat processing, the time which keeps austenitic stainless steel in a vacuum atmosphere and heating temperature is 460 hours or less. The heating temperature is lower than the sensitization temperature at which chromium (Cr) carbide of austenitic stainless steel is precipitated by heating.

  For example, in the case of the austenitic stainless steel shown in Tables 1 and 2, the upper limit of the heating temperature is 500 ° C. Therefore, diffusivity exists in austenitic stainless steel, diffuses through work-induced martensite phase due to cyclic loading, and collects in the stress-concentrated crack, causing hydrogen embrittlement of austenitic stainless steel Hydrogen and non-diffusible hydrogen can be removed.

  By such heat treatment, diffusible hydrogen that causes hydrogen embrittlement of austenitic stainless steel and non-diffusible hydrogen are removed from austenitic stainless steel, and hydrogen (H) contained in austenitic stainless steel is removed. 0.0001 mass% (1.0 mass ppm) or less. After this heat treatment, the amount of hydrogen (H) contained in the austenitic stainless steel is desirably 0.00007 mass% (0.7 mass ppm) or less. Further, the amount of hydrogen (H) contained in the austenitic stainless steel after this heat treatment is desirably 0.00002 mass% (0.2 mass ppm) or less, and 0.000007 mass% (0.07 More preferably, it is less than ppm by mass.

  As described above, the amount of hydrogen contained in the austenitic stainless steel is less than the conventional amount, and an excellent austenitic stainless steel that does not accelerate fatigue crack growth even under repeated loading with a long cycle time is provided. it can.

[Other Experimental Example 1]
A heat treatment experiment was conducted using a test piece made of SUS316. The test piece is a round bar having a diameter of 7 mm. The TDS measurement was performed by cutting into a disk shape having a diameter of 7 mm and a thickness of 0.8 mm. In the experiment, a test piece was put in a temperature of 800 ° C. and heat-treated for 20 minutes. The experimental atmosphere at this time was air, a vacuum atmosphere (about 0.006 Pa), and an Ar gas atmosphere. The Ar gas was heated while being supplied. The heating rate when heated to 700 ° C. was 0.5 ° C. per second. The hydrogen released during heating up to 700 ° C. was measured.

  The measurement was performed with a temperature programmed desorption analyzer EMD-WA1000S / H manufactured by Electronic Science Co., Ltd. (location: Musashino City, Tokyo, Japan). The measurement results are shown in FIG. In the graph, the horizontal axis indicates the measured temperature, and the vertical axis indicates the hydrogen release intensity. The hydrogen concentration of the test piece that was not heat-treated was 1.5 mass ppm. When this was heat-processed in air | atmosphere, the hydrogen concentration of the test piece became 0.7 mass ppm. When the heat treatment was performed in a vacuum atmosphere, the hydrogen concentration of the test piece was 0.4 mass ppm. In the case of Ar gas flow, it decreased to 0.4 mass ppm by the heat treatment.

[Other experimental example 2]
A heat treatment experiment was conducted using a test piece made of SUH660. The test piece is a round bar having a diameter of 7 mm. The TDS measurement was performed by cutting into a disk shape having a diameter of 7 mm and a thickness of 0.8 mm. In the experiment, a test piece was put in a temperature of 720 ° C. and heat-treated for 16 hours to perform an aging treatment. The experimental atmosphere at this time was a vacuum atmosphere (about 0.006 Pa). The hydrogen concentration of the test piece before aging treatment was 1.3 ppm. After the aging treatment, the hydrogen concentration of the test piece became 0.6 ppm.

  As described above, the stainless steel was subjected to aging treatment or the like in the production process, and hydrogen contained therein could be removed. The heating rate when heated to 700 ° C. was 0.33 ° C. per second. The hydrogen released during heating up to 600 ° C. was measured. The measurement was performed with a temperature programmed desorption analyzer (EMD-WA1000S / H) manufactured by Electronic Science Co., Ltd. (Musashino City, Tokyo, Japan). The measurement results are shown in FIG. In the graph, the horizontal axis indicates the measured temperature, and the vertical axis indicates the hydrogen release intensity.

[Other Experimental Example 3]
A heat treatment experiment was performed using test pieces made of SUS304 and SUS316L. The test piece is a disk-shaped sample having a diameter of 7 mm and a thickness of 0.4 mm. The experimental atmosphere during the heat treatment was an atmospheric atmosphere. This atmospheric atmosphere was about 0.1013 MPa. In the experiment, an aging treatment was performed by placing a test piece in an air atmosphere at temperatures of 300 ° C. and 450 ° C. and performing heat treatment for 2 hours.

  The TDS measurement was performed in a disk shape with a diameter of 7 mm and a thickness of 0.4 mm after the heat treatment. TDS measurement was performed with a temperature programmed desorption analyzer (EMD-WA1000S / H) manufactured by Electronic Science Co., Ltd. (Musashino City, Tokyo, Japan). The result of this measurement is shown in FIG. In the graph, the horizontal axis indicates the measurement temperature, and the vertical axis indicates the hydrogen emission intensity. At the time of TDS measurement, the heating rate when heated to 700 ° C. was 0.5 ° C. per second. During the TDS measurement, hydrogen released during heating up to 600 ° C. was measured.

  FIG. 14A shows the measurement result of a test piece made of SUS304. As shown in the figure, the hydrogen concentration of this test piece was 2.3 ppm before the aging treatment. After the aging treatment, the hydrogen concentration of the test piece was 0.19 ppm when heat-treated at a temperature of 300 ° C. and 0.07 ppm when heat-treated at a temperature of 450 ° C.

  FIG. 14B shows the measurement result of a test piece made of SUS316L. Prior to the aging treatment, the hydrogen concentration of this test piece was 2.6 ppm. After the aging treatment, the hydrogen concentration of the test piece was 0.07 ppm when heat-treated at a temperature of 300 ° C. and 0.03 ppm when heat-treated at a temperature of 450 ° C.

  As described above, the stainless steel was subjected to an aging treatment or the like in an air atmosphere to remove hydrogen contained therein.

  The present invention is preferably used in a field that utilizes high-pressure hydrogen as well as corrosion resistance. It is especially useful for metal gaskets, various valves for automobiles, springs, steel belts, blade materials, fuel cells, valves used in the vicinity of fuel cell systems, spring materials, etc., where hydrogen intrusion may cause hydrogen embrittlement and delayed fracture. . Furthermore, it may be used for buildings, machines, devices, and plants.

Claims (12)

An austenitic stainless steel having an austenitic phase whose crystal structure is a face-centered cubic lattice structure,
The austenitic stainless steel is heat-treated in an air atmosphere at a heating temperature of 200 ° C. or higher and 1100 ° C. or lower to remove diffusible hydrogen and non-diffusible hydrogen that cause hydrogen embrittlement of the austenitic stainless steel. An austenitic stainless steel, wherein hydrogen (H) contained in the austenitic stainless steel is removed to 0.0001 mass% (1.0 mass ppm) or less. In the austenitic stainless steel according to claim 1,
The austenitic stainless steel, wherein the diffusible hydrogen and the non-diffusible hydrogen are removed to reduce the hydrogen (H) to 0.00002 mass% (0.2 mass ppm) or less. In the austenitic stainless steel according to claim 2,
The austenitic stainless steel, wherein the diffusible hydrogen and the non-diffusible hydrogen are removed to reduce the hydrogen (H) to 0.00007 mass% (0.7 mass ppm) or less. In the austenitic stainless steel according to claim 3,
The austenitic stainless steel, wherein the diffusible hydrogen and the non-diffusible hydrogen are removed to reduce the hydrogen (H) to 0.000007 mass% (0.07 mass ppm) or less. In the austenitic stainless steel according to claim 1, selected from claims 1 to 4,
The austenitic stainless steel is heated at a heating temperature of 200 ° C. to 1100 ° C. for 2 hours to 500 hours to remove the diffusible hydrogen and the non-diffusible hydrogen. A feature of austenitic stainless steel. In the heat treatment method for removing hydrogen present in the austenitic stainless steel by heat-treating the austenitic stainless steel having an austenitic phase having a face-centered cubic lattice structure in the crystal structure,
The austenitic stainless steel is heated in an air atmosphere at a heating temperature of 200 ° C. or higher and 1100 ° C. or lower, and the amount of diffusible hydrogen and non-diffusible hydrogen in the austenitic stainless steel is 0.0001 mass. % (1.0 mass ppm) or less. A method for removing hydrogen from an austenitic stainless steel. In the method for removing hydrogen of austenitic stainless steel according to claim 6,
The diffusion that exists in the austenitic stainless steel, diffuses through a work-induced martensite phase due to repeated loading, and collects in a cracked portion that undergoes stress concentration, causing the hydrogen embrittlement of the austenitic stainless steel A method for removing hydrogen from austenitic stainless steel, characterized by removing volatile hydrogen and the non-diffusible hydrogen. In the method for removing hydrogen of austenitic stainless steel according to claim 6,
Holding the austenitic stainless steel for 2 hours or more and 500 hours or less at a temperature lower than a sensitization temperature at which chromium (Cr) carbide of the austenitic stainless steel is precipitated by heating;
A method for removing hydrogen from an austenitic stainless steel, comprising removing the diffusible hydrogen and non-diffusible hydrogen that cause hydrogen embrittlement of the austenitic stainless steel. In the method for removing hydrogen of austenitic stainless steel according to claim 7 or 8,
The hydrogen (H) contained in the austenitic stainless steel is 0.00007 mass% (0.7 mass ppm) or less. A method for removing hydrogen from an austenitic stainless steel. In the method for removing hydrogen of austenitic stainless steel according to claim 9,
The hydrogen (H) contained in the austenitic stainless steel is 0.00002 mass% (0.2 mass ppm) or less. A method for removing hydrogen from an austenitic stainless steel. In the method for removing hydrogen of austenitic stainless steel according to claim 10,
The hydrogen (H) contained in the austenitic stainless steel is 0.000007 mass% (0.07 mass ppm) or less. A method for removing hydrogen from an austenitic stainless steel. In the method for removing hydrogen of austenitic stainless steel according to claim 6,
The diffusible hydrogen and the non-diffusible hydrogen diffuse through the work-induced martensite phase due to low-frequency cyclic loading, and gather at the stress-concentrated crack, thereby increasing the fatigue crack growth rate. A method for removing hydrogen from an austenitic stainless steel, characterized in that it causes hydrogen embrittlement of the austenitic stainless steel.

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