JP6684620B2 - High-strength austenitic stainless steel excellent in hydrogen embrittlement resistance, its manufacturing method, and hydrogen equipment used in high-pressure hydrogen gas and liquid hydrogen environment - Google Patents

Description

The present invention relates to a high-strength austenitic stainless steel having excellent hydrogen embrittlement resistance and a method for producing the same, and hydrogen equipment used in a high-pressure hydrogen gas and liquid hydrogen environment, and particularly under a high-pressure hydrogen gas and liquid hydrogen environment. TECHNICAL FIELD The present invention relates to an austenitic stainless steel having high strength and excellent in hydrogen embrittlement resistance and a method for producing the same.
The present application claims priority based on Japanese Patent Application No. 2015-64951 filed in Japan on March 26, 2015, and the content thereof is incorporated herein.

In recent years, from the viewpoint of preventing global warming, technological development has been advanced in which hydrogen is used as a transportation / storage medium of energy in order to suppress the emission of greenhouse gases (CO ₂ , NO _x , SO _x ). Therefore, the development of metallic materials used in hydrogen storage and transportation equipment is expected.

  Conventionally, hydrogen gas up to a pressure of about 40 MPa is filled and stored as a high-pressure gas in a thick (thick) Cr-Mo steel cylinder. Further, as a pipe material or a high-pressure hydrogen gas tank liner for a fuel cell vehicle, JIS standard SUS316-based austenitic stainless steel (hereinafter referred to as "SUS316 steel") is used. The SUS316 steel has hydrogen embrittlement resistance under the environment of high-pressure hydrogen gas, for example, carbon steel including the above Cr-Mo steel, and JIS standard SUS304-based austenitic stainless steel (hereinafter referred to as "SUS304 steel"). ) Is better than

In recent years, prior to the general sale of fuel cell vehicles, public trial manufacture and verification tests of hydrogen stations have been underway. For example, a hydrogen station that can store a large amount of hydrogen as liquid hydrogen and is capable of pressurizing the liquid hydrogen and supplying it as high-pressure hydrogen gas of 70 MPa or more is in the demonstration stage. Further, in a hydrogen station, a technique called precooling for precooling hydrogen filling a tank of a fuel cell vehicle to a low temperature of about -40 ° C has been put into practical use.
From these, it is assumed that the metal material used for the storage container for liquid hydrogen, the hydrogen gas pipe, and the like that accompanies the dispenser of the hydrogen station is exposed to the high-pressure and low-temperature hydrogen gas of 70 MPa.

  As a metal material that does not become hydrogen embrittlement under a more severe hydrogen embrittlement environment, SUS316 steel and SUS316L steel containing about 13% of Ni can be mentioned, but it is recommended to use these two steel types at a 70 MPa class hydrogen station in Japan. It is recognized according to the example standards set by the Gas Safety Association.

  On the other hand, in order to popularize and autonomously develop a hydrogen energy society centered on fuel cell vehicles in the future, cost reduction of fuel cell vehicles and hydrogen stations is essential. That is, for a metal material used in a hydrogen embrittlement environment, higher strength is required in order to reduce the amount of steel used by downsizing and thinning various devices. In particular, for high pressure hydrogen piping applications, a tensile strength of about 650 MPa is required for the metal material used.

  However, the SUS316-based austenitic stainless steel described in the above-mentioned exemplary standards is expensive because it contains a large amount of rare metals Ni and Mo. However, even if the SUS316-based austenitic stainless steel is subjected to the solution treatment, such tensile strength is not satisfied, so that the SUS316-based austenitic stainless steel is cold-worked to be used to reinforce the strength.

  As a steel material having increased tensile strength of SUS316 steel and SUS316L steel, JIS standard steel types SUS316N and SUS316LN utilizing solid solution strengthening of N are known. However, for example, as reported in Non-Patent Document 1, SUS316N and SUS316LN have low ductility in high-pressure hydrogen gas at low temperature.

  The stainless steel disclosed in Patent Document 1 (Japanese Patent Laid-Open No. 2002-173742) controls the metal structure into a two-phase structure of an austenite phase and a martensite phase by thermomechanical treatment while keeping the Ni content at 4 to 12%. ing. As a result, very hard stainless steel having a Vickers hardness of about 500 is achieved.

  The stainless steel disclosed in Patent Document 2 (Japanese Unexamined Patent Publication No. 2009-13001) has improved hydrogen embrittlement resistance by utilizing carbonitrides of Ti and Nb having a size of 1 μm or more. On the other hand, since the addition of Mo is omitted, it is excellent in economic efficiency.

JP 2002-173742 A Japanese Patent Laid-Open No. 2009-13001 JP, 2014-47409, A JP, 2014-1422, A

"Effect of Temperature on Hydrogen Environment Embrittlement of SUS316 Series Stainless Steel at Low Temperature", Journal of Japan Institute of Metals, Vol. 67, No. 9, p456-459.

However, the stainless steel described in Patent Document 1 contains a martensite phase that is easily embrittled by hydrogen, and thus is difficult to apply in a hydrogen environment.
Further, the stainless steel described in Patent Document 2 is about the same in strength as SUS316 steel, and further improvement in strength is desired.

As described above, at present, a high-strength austenitic stainless steel having hydrogen embrittlement resistance in a high-pressure hydrogen gas environment at a low temperature of over 40 MPa has not yet appeared.
The present invention has been made in view of the above circumstances, and an object thereof is to provide a high-strength austenitic stainless steel having excellent hydrogen embrittlement resistance that can be suitably used in a high-pressure hydrogen gas environment at a low temperature and higher than 40 MPa. And

A stainless steel for high-pressure hydrogen, which is aimed at increasing the strength by precipitation strengthening, is disclosed in, for example, Patent Document 3 (JP-A-2014-47409).
The stainless steel described in Patent Document 3 utilizes an η phase intermetallic compound. However, it is necessary to add 20% or more of Ni, which causes an increase in alloy cost.
Therefore, the present inventors have focused on Cr-based carbonitrides as precipitates obtained by utilizing the main elements.

  On the other hand, generally, the various properties of stainless steel deteriorate due to the influence of Cr-based carbonitrides. For example, as disclosed in Patent Document 4 (Japanese Unexamined Patent Publication No. 2014-1422), when a Cr-based carbonitride precipitates, the interface between the Cr-based carbonitride and the mother phase serves as a starting point of fracture, resulting in moldability. Cause decline.

  Furthermore, the effect of Cr-based carbonitrides on the hydrogen gas embrittlement resistance of stainless steel is no exception. According to Non-Patent Document 1, when a Cr-based carbonitride is precipitated in the metal structure, a Cr-deficient layer having a significantly reduced Cr concentration is formed around the precipitate. Since the stability of the austenite phase decreases near the Cr-deficient layer, the work-induced martensite phase is preferentially generated at the time of deformation, resulting in a decrease in ductility in high-pressure hydrogen gas. The Cr deficient layer can be eliminated by performing additional heat treatment to diffuse Cr, but this increases the manufacturing cost.

  Here, the inventors have found that the alloy composition of the austenitic stainless steel composed of Cr, Ni, Mo, which are the main elements, and trace elements, the metal structure, the average size of the precipitates, and the high pressure hydrogen gas environment. An intensive study was conducted on the relationship between hydrogen embrittlement resistance and strength characteristics. As a result, the following new findings (a) to (f) were obtained.

(A) In the test piece showing hydrogen embrittlement, cracks are formed around the Cr-based carbonitride or the Ni / Fe / Cr / Mo / Si intermetallic compound. The ductility decreases due to the connection and propagation of cracks generated around these precipitates.
(B) However, the mass% to 1.0%, further precipitation generation and progression significantly inhibited the size of a crack generated by hydrogen embrittlement by controlling under 100nm or less of, and as a result, resistance to hydrogen Brittleness characteristics are improved.
(C) In Rukoto such deposits amount to control 0.001% to 1.0% in mass% of Gyosu further control the average size of the precipitates 100nm or less, Ya Cr-based carbonitride Ni, Fe, Cr, Mo, and Si intermetallic compounds or precipitates such as Ti, Nb, and V-based carbonitrides also effectively act to increase the strength of austenitic stainless steel. Further, by utilizing the solid solution strengthening of N while making the precipitation strengthening act compositely, it is possible to obtain a tensile strength of about 650 MPa which is equal to or higher than that of the cold-worked material of SUS316 steel.
(D) The size of precipitates is strongly influenced by heat treatment conditions. The precipitation nose of Cr-based carbonitrides and Ni / Fe / Cr / Mo / Si intermetallic compounds is about 800 ° C, and if kept at a temperature higher than this, precipitation will occur in a short time, but coarsening will occur rapidly. proceed. Therefore, it is difficult to control the average size of the precipitates to 100 nm or less. If the steel material is held at 800 ° C or lower, coarsening of precipitates can be suppressed, but it takes time to start precipitation.
(E) When cooling after the final heat treatment, by controlling the average cooling rate up to 750 ° C. to be less than 2.0 ° C./s, the size of precipitates that achieve both high strength and improved hydrogen embrittlement resistance of stainless steel. -The mass% can be secured.
(F) Further, Ti, Nb, and V-based carbonitrides are precipitated by adding a small amount of Ti, Nb, and V that easily form carbonitrides, individually or in combination, or Cu is precipitated by addition of Cu. By doing so, the strength can be further increased without impairing the hydrogen embrittlement resistance.

  The present invention has been made based on the new findings (a) to (f), and the summary thereof is as follows.

(1)% by mass, C: 0.2% or less, Si: 0.2 to 1.5%, Mn: 0.5 to 2.5%, P: 0.06% or less, S: 0.008 % Or less, Ni: 10.0 to 20.0%, Cr: 16.0 to 25.0%, Mo: 3.5% or less, Cu: 3.5% or less, N: 0.04 to 0.50. %, O: 0.015% or less, the balance being Fe and unavoidable impurities,
Furthermore, the amount of precipitates is not more than 1.0% less than 0.001% by mass%, is not less 100nm or less average size of more precipitate, the water tensile strength, characterized in Rukoto exceed 650MPa containing High-strength austenitic stainless steel with excellent brittleness characteristics.

(2) Furthermore, in mass%, selected from Al: 0.3% or less, Mg: 0.01% or less, Ca: 0.01% or less, REM: 0.10% or less, B: 0.008% or less. The high-strength austenitic stainless steel excellent in hydrogen embrittlement resistance as set forth in (1), characterized in that it contains one or more of the following.

(3) It is characterized by containing, in mass%, one or more selected from Ti: 0.5% or less, Nb: 0.5% or less, and V: 0.5% or less (1). Alternatively, a high-strength austenitic stainless steel excellent in hydrogen embrittlement resistance according to (2).

(4) The high-strength austenitic stainless steel excellent in hydrogen embrittlement resistance according to any one of (1) to (3), which is used in a high-pressure hydrogen gas and liquid hydrogen environment.

(5) After the step of hot working a steel slab having the component composition according to any one of (1) to (3), the step of final heat treatment at 1000 ° C to 1200 ° C, and the step of the final heat treatment. And a step of cooling, wherein in the step of cooling, the average cooling rate up to 750 ° C. is controlled to be less than 2.0 ° C./s (1) to (3). The method for producing a high-strength austenitic stainless steel excellent in hydrogen embrittlement resistance according to 1.

(6) An apparatus for hydrogen used in a high-pressure hydrogen gas and liquid hydrogen environment, which uses the high-strength austenitic stainless steel excellent in hydrogen embrittlement resistance according to any one of (1) to (4).

  According to one aspect of the present invention, it is possible to provide an austenitic stainless steel that is suitably used in a high-pressure hydrogen gas and liquid hydrogen environment, has high strength, and is excellent in hydrogen embrittlement resistance, and a method for producing the same.

Hereinafter, the austenitic stainless steel according to the present embodiment and the method for manufacturing the same will be described in detail.
First, the component composition of the austenitic stainless steel of this embodiment will be described. In addition, in the following description, the "%" display of the content of each element means "mass%".

  The austenitic stainless steel according to the present embodiment is, in mass%, C: 0.2% or less, Si: 0.2 to 1.5%, Mn: 0.5 to 2.5%, P: 0.06. % Or less, S: 0.008% or less, Ni: 10.0 to 20.0%, Cr: 16.0 to 25.0%, Mo: 3.5% or less, Cu: 3.5% or less, N : 0.01 to 0.50% and O: 0.015% or less. Furthermore, the average size of the deposits is 100 nm or less, and the amount of the deposits is 0.001% or more and 1.0% or less by mass%.

<C: 0.2% or less>
C is an element effective for stabilizing the austenite phase and contributes to the improvement of hydrogen embrittlement resistance. Further, solid solution strengthening and precipitation strengthening by Cr-based carbide contribute to increase in strength. In order to obtain these effects, the C content is preferably 0.01% or more. On the other hand, addition of an excessive amount of C leads to excessive precipitation of Cr-based carbides, leading to a reduction in hydrogen embrittlement resistance. Therefore, it is necessary to set the upper limit of the C content to 0.2%. The more preferable upper limit of the C content is 0.15%.

<Si: 0.2 to 1.5%>
Si is an element effective for stabilizing the austenite phase. In order to improve the hydrogen embrittlement resistance by stabilizing the austenite phase, the Si content needs to be 0.2% or more. The Si content is preferably 0.4% or more. On the other hand, the addition of an excessive amount of Si promotes the formation of intermetallic compounds such as sigma phase, resulting in deterioration of hot workability and toughness. Therefore, it is necessary to set the upper limit of the Si content to 1.5%. The Si content is more preferably 1.1% or less.

<Mn: 0.5 to 2.5%>
Mn is an element effective in stabilizing the austenite phase. The stabilization of the austenite phase suppresses the formation of the work-induced martensite phase, which improves the hydrogen embrittlement resistance. Therefore, the Mn content needs to be 0.5% or more. The Mn content is preferably 0.8% or more. On the other hand, the addition of an excessive amount of Mn promotes the formation of coarse MnS inclusions, lowers the ductility of the austenite phase, and also has the effect of promoting the formation of nitrides, so the upper limit is 2.5%. There is a need to. The Mn content is more preferably 2.0% or less.

<P: 0.06% or less>
P is contained as an impurity in the austenitic stainless steel of this embodiment. Since P is an element that reduces hot workability, it is preferable to reduce the P content as much as possible. Specifically, the P content is preferably limited to 0.06% or less, more preferably 0.05% or less. However, since the extreme reduction of the P content leads to an increase in the steelmaking cost, the P content is preferably 0.008% or more.

<S: 0.008% or less>
S is an element that segregates at the austenite grain boundaries during hot working and weakens the bond strength of the grain boundaries to induce cracking during hot working. Therefore, it is necessary to limit the upper limit of the S content to 0.008%. The preferable upper limit of the S content is 0.005%. Since it is preferable to reduce the S content as much as possible, no particular lower limit is set, but the extreme reduction leads to an increase in steelmaking cost. Therefore, the S content is preferably 0.0001% or more.

<Ni: 10.0 to 20.0%>
Ni is an element having a large effect of improving the hydrogen embrittlement resistance of austenitic stainless steel. It also promotes the formation of Ni, Fe, Cr, Mo, and Si intermetallic compounds and contributes to the enhancement of strength. In order to sufficiently obtain these effects, the Ni content needs to be 10.0% or more. Since these effects are further improved by eliminating the segregation of the components, the Ni content is preferably 11.5% or more. On the other hand, addition of an excessive amount of Ni causes an increase in material cost, so the upper limit of the Ni content is set to 20.0%. The Ni content is preferably 14.0% or less.

<Cr: 16.0 to 25.0%>
Cr is an essential element for obtaining the corrosion resistance required for stainless steel. In addition, Cr is an element that also contributes to the strength increase of austenitic stainless steel. In order to sufficiently obtain this effect, the Cr content needs to be 16.0% or more. The Cr content is preferably 16.5% or more. On the other hand, the addition of an excessive amount of Cr causes excessive precipitation of Cr-based carbonitrides, which deteriorates the hydrogen embrittlement resistance. Therefore, it is necessary to set the upper limit of the Cr content to 25.0%. The Cr content is preferably 22.5% or less.

<Mo: 3.5% or less>
Mo is an element that contributes to the increase in strength and the corrosion resistance of austenitic stainless steel. However, the addition of Mo causes an increase in alloy cost. Therefore, the Mo content is set to 3.5% or less. On the other hand, Mo is an element that is inevitably mixed from the scrap material. Excessive reduction of the Mo content causes restrictions on the melting raw material, leading to an increase in manufacturing cost. Therefore, the lower limit of the Mo content is preferably set to 0.05% in order to achieve both the above effects and manufacturability.

<Cu: 3.5% or less>
Cu is an element effective in stabilizing the austenite phase. The Cu content is preferably 0.15% or more in order to improve the hydrogen embrittlement resistance by stabilizing the austenite phase. On the other hand, Cu also contributes to the strength increase due to Cu precipitation strengthening, but the addition of an excessive amount of Cu leads to a decrease in the strength of the austenite phase and impairs the hot workability, so the upper limit of the Cu content is set to 3. It should be 5%. The Cu content is more preferably 3.0% or less.

<N: 0.01 to 0.50%>
N is an element effective in stabilizing the austenite phase and improving the corrosion resistance. Further, solid solution strengthening and precipitation strengthening of Cr-based nitride contribute to increase in strength. In order to obtain these effects, the N content is 0.01% or more. The N content is preferably 0.04% or more. On the other hand, addition of an excessive amount of N promotes excessive formation of Cr-based nitrides, and deteriorates hydrogen embrittlement resistance, corrosion resistance, and toughness of the austenite phase. Therefore, it is necessary to set the upper limit of the N content to 0.50%. The N content is more preferably 0.35% or less.

<O: 0.015% or less>
O reduces the hot workability and toughness of the austenite phase by forming an oxide in steel. Therefore, it is necessary to limit the upper limit of the O (oxygen) content to 0.015% or less. The O content is preferably 0.010% or less. Although it is preferable to reduce the O (oxygen) content as much as possible, extreme reduction leads to an increase in steelmaking cost. Therefore, the O (oxygen) content is preferably 0.001% or more.

  The austenitic stainless steel according to the present embodiment is composed of Fe and inevitable impurities other than the above-mentioned elements, but may contain an optionally added element described later.

<Al: 0.3% or less, Mg, Ca: 0.01% or less, REM: 0.10% or less, B: 0.008% or less>
Al, Mg, Ca, REM and B are elements effective in improving deoxidation, hot workability and corrosion resistance. If necessary, one or more elements selected from these may be added. However, addition of an excessive amount of these elements causes a significant increase in manufacturing cost. Therefore, it is necessary to set the upper limits of the contents of these elements to Al: 0.3% or less, Mg, Ca: 0.01% or less, REM: 0.10% or less, and B: 0.008% or less. The lower limits of the contents of these elements do not have to be set in particular, but in order to obtain a sufficient deoxidizing effect, Al: 0.01%, Mg, Ca: 0.0002%, REM: 0.001%, B: 0. It is preferably set to 0.0002%. Here, REM (rare earth element) is a general term for two elements, scandium (Sc) and yttrium (Y), and 15 elements (lanthanoids) from lanthanum (La) to lutetium (Lu) according to a general definition. A single element may be added, or two or more elements may be added. The content of REM is the total amount of these elements.

<Ti, Nb, V: 0.50% or less>
Ti, Nb, and V are solid elements in steel or are precipitated as carbonitrides, and are effective elements for increasing strength. If desired, one or more elements selected from these may be added. In this case, the content of each of Ti, Nb, and V is preferably 0.01% or more. However, if the content of each of Ti, Nb, and V exceeds 0.50%, the formation of Cr-based carbonitrides is suppressed, and the effect of precipitation strengthening by Cr-based carbonitrides cannot be sufficiently obtained. . Therefore, the upper limit of the content of each of Ti, Nb, and V needs to be 0.50% or less. The preferable upper limit of the content of each of Ti, Nb, and V is 0.40%.

  Elements other than the elements described above can be contained in a range that does not impair the effects of the present embodiment.

"Reason for limitation of deposits"
Next, the size and amount of precipitates in steel will be described.
In the test piece showing hydrogen embrittlement, a crack is formed around the Cr-based carbonitride or the Ni / Fe / Cr / Mo / Si intermetallic compound. This is because the Cr-deficient layer formed around each precipitate locally lowers the hydrogen gas embrittlement resistance around each precipitate. Ductility decreases due to the connection and propagation of cracks generated from the periphery of the precipitate.

  However, by controlling the average size of the precipitates to 100 nm or less and controlling the amount of the precipitates generated to be 1.0% or less by mass%, the generation and propagation of cracks generated by hydrogen gas embrittlement can be suppressed. Remarkably suppressed. As a result, the hydrogen gas embrittlement resistance is improved.

  Furthermore, by increasing the strength by precipitation strengthening of precipitates and by causing the solid solution strengthening of N to act in a composite manner, it is possible to obtain a tensile strength of about 650 MPa, which is equal to or higher than that of the cold-worked material of SUS316 steel. . In order to enjoy the effect, the lower limit value of the amount of precipitate produced is 0.001% or more. The lower limit of the amount of precipitate produced is preferably 0.005% or more.

The average size of the precipitates and the amount of the produced precipitates can be controlled by controlling the average cooling rate after the final heat treatment described later. However, the slower the average cooling rate, the coarser the precipitates become. Therefore, it becomes possible to confirm the presence of precipitates with a transmission microscope (TEM). The preferable average size of the precipitate is 70 nm or less.
On the other hand, when the average cooling rate is high (close to the upper limit), the precipitates are very fine. Therefore, the lower limit of the average size of the precipitates is not particularly set, but it is preferably 5 nm or more.

The amount of carbonitride and intermetallic compound (precipitate) formed can be measured by, for example, an electrolytic extraction residue method.
If an excessive amount of precipitate is generated, the connection and propagation of cracks generated from the periphery of the precipitate as an origin is promoted. Therefore, the amount of precipitate generated must be 1.0% by mass or less. . Preferably, the amount of the generated precipitates is 0.90% or less by mass%. On the other hand, when the cooling rate is fast (close to the upper limit), the precipitates are extremely fine, and therefore the lower limit of the average size of the precipitates is not particularly set. However, in order to obtain the strength increasing effect by the Cr-based carbonitride and the Ni, Fe, Cr, Mo, Si intermetallic compound, it is preferably 0.02% or more by mass%.

Moreover, the average size of the precipitates is measured, for example, by the following method. The precipitate is observed by TEM, the precipitate is identified by EDX, and the precipitate is specified. Next, the major axis and minor axis of one precipitate are measured by a TEM photograph. Then, the average value of the major axis and the minor axis ((major axis + minor axis) / 2) is determined and used as the size of the precipitate. Similarly, the sizes of a plurality of precipitates are obtained. It is possible to calculate an average value of the sizes of a plurality of precipitates and use the average value as the average size of the precipitates in the stainless steel. In this embodiment, a circumscribed rectangle is drawn so that the area of one precipitate is minimized. The long side of this circumscribed rectangle is the major axis of the precipitate, and the short side of the circumscribed rectangle is the minor axis of the precipitate.
Further, the "precipitate" in the present invention means all the precipitates deposited in the steel, and in addition to Cr-based carbonitrides, Ni.Fe.Cr.Mo.Si intermetallic compounds, Ti, Nb, V-based carbonitrides and precipitated Cu are also included.

"Production method"
Next, an example of a method for manufacturing austenitic stainless steel according to this embodiment will be described.
In order to manufacture the austenitic stainless steel of this embodiment, first, the stainless steel having the above-described composition is melted to manufacture a steel slab such as a slab. Next, the steel slab is heated to a predetermined temperature and hot working such as hot rolling is performed (hot working step).
The austenitic stainless steel of this embodiment is not limited to a steel plate. Therefore, the billet is not limited to the slab, and can be achieved by selecting a billet (billet, bloom, etc.) having a preferable shape with respect to the shape of the target product (rod, tube, etc.). Needless to say.

Hereinafter, the conditions of the final heat treatment after the hot working will be described in detail.
If the temperature of the final heat treatment after hot working is too high, the strength of the steel material may decrease due to excessive grain growth, or a grinding step may be added due to abnormal oxidation, which may lead to an increase in production cost. Therefore, the upper limit of the final heat treatment temperature is 1200 ° C. On the other hand, if the temperature of the final heat treatment is too low, the deformed structure during hot working remains and the ductility of the steel product decreases, so the lower limit is made 1000 ° C. A preferable temperature range for the final heat treatment is 1050 ° C to 1180 ° C.
The holding time of the heat treatment in the above temperature range is 1 second to 1 hour. If the holding time is shorter than this, the work structure remains in the steel, leading to a decrease in ductility. The lower limit of the preferable holding time is 30 seconds. If the holding time of the heat treatment is too long, the strength may be reduced due to excessive grain growth, or a grinding step may be added due to abnormal oxidation, which may lead to an increase in production cost. The preferable upper limit holding time is 40 minutes.

  The precipitation nose temperature of the Cr-based carbonitride and the Ni, Fe, Cr, Mo, Si intermetallic compound is about 800 ° C. If the steel material is held at a temperature higher than this, coarsening of the precipitates proceeds rapidly, so it is difficult to control the average size of the precipitates to 100 nm or less. On the other hand, if the steel material is held at 800 ° C. or lower, coarsening of precipitates can be suppressed, but it takes time to start precipitation. Therefore, this leads to an increase in manufacturing cost.

  However, after the final heat treatment at 1000 ° C. to 1200 ° C., by controlling the average cooling rate up to 750 ° C. to be less than 2.0 ° C./s, precipitation that achieves both high strength and improved hydrogen embrittlement resistance of stainless steel can be achieved. It is possible to secure the average size and production amount of the product.

  From the above, in the cooling step after the final heat treatment, it is necessary to control the average cooling rate up to 750 ° C to be less than 2.0 ° C / s. When the average cooling rate is higher than 2.0 ° C./s, the time required for deposits to be deposited cannot be secured, so that the strength of the steel product cannot be increased. On the other hand, if the cooling rate is excessively slow, the average size of the precipitates may become larger than 100 nm, and it may not be possible to ensure good hydrogen embrittlement resistance of the steel product. Therefore, the lower limit of the preferable average cooling rate is 0.3 ° C./s or more. A more preferable lower limit is 0.4 ° C / s or higher.

  After the hot working and the final heat treatment, pickling and cold working may be performed as necessary.

Further, the austenitic stainless steel according to the present embodiment is not limited to the above-described manufacturing method, and any manufacturing method may be adopted as long as it is a method capable of controlling the average size and amount of precipitates within the above range. .
Further, even if the average size and amount of precipitates are controlled within the above range by heat treatment in the manufacturing process of hydrogen equipment using austenitic stainless steel satisfying the components of the present invention, or by heat treatment to hydrogen equipment. Good.

Examples of the present invention will be described below, but the present invention is not limited to the conditions used in the following examples.
The underlines in the table indicate those outside the range of the present embodiment.

  A stainless steel test material having the composition shown in Table 1 was melted to produce a slab having a thickness of 120 mm. Then, the slab was heated at 1200 ° C., and hot forging and hot rolling were performed to produce a hot rolled sheet having a thickness of 20 mm. Then, the hot rolled sheet was subjected to final heat treatment and cooling under the conditions shown in Table 2 to obtain a hot rolled annealed sheet. The holding time in the final heat treatment was within the range of 3 minutes to 20 minutes. In Table 2, "heat treatment temperature (° C)" indicates the temperature of the final heat treatment, and "cooling rate (° C / s)" indicates the average cooling rate up to 750 ° C.

Table 2 shows the average size of the precipitates and the amount of the precipitates of each test material.
A sample was prepared from the obtained hot rolled annealed plate by the extraction replica method, and then the precipitate was observed by TEM. The size of one precipitate was defined as the average value of the major axis and the minor axis ((major axis + minor axis) / 2). The size of 30 precipitates was measured, and the average value of the sizes of 30 precipitates was defined as the average size of the precipitates in the test material.
Similarly, the amount of the deposit was measured by collecting an analytical sample from the test material and performing the electrolytic extraction residue method. The mesh size of the filter for filtering the residue was 0.2 μm.

Next, the hot rolled annealed plate of each test material was evaluated for hydrogen gas embrittlement resistance by the method described below.
A round bar tensile test piece having a parallel portion with an outer diameter of 3 mm and a length of 20 mm was sampled from the longitudinal direction of the 20 mm thick hot rolled annealed plate and the center of the plate thickness. Using this round bar tensile test piece, (1) tensile test in air and (2) tensile test in high-pressure hydrogen gas were performed.

The tensile test in the atmosphere (1) was carried out under the conditions of test temperature: 25 ° C. and −40 ° C., strain rate: 5 × 10 ⁻⁵ / s. Those having a tensile strength of more than 650 MPa obtained by a tensile test at 25 ° C were evaluated as acceptable.
The tensile test in (2) in high-pressure hydrogen gas was carried out under the conditions of test temperature: -40 ° C, test environment: 70 MPa hydrogen gas, strain rate: 5 x 10 ^-5 / s. The test piece No. For A3, A4, and A6, the same tensile test was performed in a test environment: 103 MPa hydrogen gas.
Then, the value of “(throttle in high-pressure hydrogen gas / throttle in air) × 100 (%)” at -40 ° C. (relative throttle) was calculated. The co-test material having this value of 80% or more was evaluated as having an acceptable hydrogen embrittlement resistance in high-pressure hydrogen gas. Particularly, those having a tensile strength at 25 ° C. of more than 650 MPa and a reduction of 80% or more and less than 85% were evaluated as “◯”, and those of 85% or more were evaluated as “⊚”.
The results are shown in Tables 3 and 4.

The test pieces A1a, A1c, and A2 to A18 are test materials (invention examples) subjected to final heat treatment and cooling under preferable conditions.
These relative drawing values were 80% or more, but the tensile strength in air at 25 ° C. was 650 MPa or more. In particular, Ni, Cu, which has a great effect on the improvement of hydrogen embrittlement resistance, and A1a, A2 to A6, and A8 to A17 in which the average cooling rate is the preferable range of the present embodiment, the relative aperture value is 85% or more, The result was that the hydrogen embrittlement resistance was very excellent.
Further, the tensile test was performed on the test pieces A3, A4, and A6 even in 103 MPa hydrogen, but the relative drawing was 90% or more, which was higher than the target value of 80%.

  The cooling rate after the final heat treatment of the test piece A1b does not satisfy the range of the present invention. As a result, during the cooling after the final heat treatment, no precipitate was deposited in the sample material, and the effect of precipitation strengthening could not be obtained, so the tensile strength at room temperature and in the atmosphere was less than 650 MPa.

  The test piece B1 has a Ni content below the range of the present invention. As a result, the hydrogen embrittlement resistance was insufficient and the relative aperture value was 59%.

  The Cu content of the test piece B2 exceeds the range of the present invention. As a result, the strength of the austenite phase decreased, and the tensile strength in air at 25 ° C fell below the target value of 650 MPa.

  The Si content of the test piece B3 exceeds the range of the present invention. As a result, the hydrogen embrittlement resistance was insufficient and the relative aperture value was 68.8%.

  In the test piece B4, the Cr amount exceeds the range of the present invention. As a result, as a result of depositing an amount of precipitates exceeding the range of the present invention, hydrogen gas embrittlement susceptibility was increased, hydrogen embrittlement resistance was insufficient, and the relative drawing value was 61.5%.

  The test piece B5 has a Mn content exceeding the range of the present invention. As a result, the hydrogen embrittlement resistance was insufficient and the relative aperture value was 71.3%.

  The test piece B6 has a Cr content below the range of the present invention. As a result, the hydrogen embrittlement resistance was insufficient due to the decreased austenite phase stability, and the relative drawing value was 77.5%.

  The N content of the test piece B7 is below the range of the present invention. As a result, the strength of the austenite phase decreased, and the tensile strength in air at 25 ° C fell below the target value of 650 MPa.

  The austenitic stainless steel of the present invention has extremely excellent hydrogen embrittlement resistance and tensile strength of more than 650 MPa in hydrogen gas at a pressure of more than 40 MPa. Therefore, the austenitic stainless steel of the present invention is a high-pressure hydrogen gas tank for storing hydrogen gas having a pressure exceeding 40 MPa, a high-pressure hydrogen gas tank liner, a high-pressure hydrogen gas heat exchanger, a high-pressure hydrogen gas and liquid hydrogen pipe. It is applicable as a material.

Claims (6)

% By mass, C: 0.2% or less, Si: 0.2 to 1.5%, Mn: 0.5 to 2.5%, P: 0.06% or less, S: 0.008% or less, Ni: 10.0 to 20.0%, Cr: 16.0 to 25.0%, Mo: 3.5% or less, Cu: 3.5% or less, N: 0.04 to 0.50%, O : 0.015% or less, the balance being Fe and unavoidable impurities,
The amount of precipitates is not more than 1.0% less than 0.001% by mass%, is not less 100nm or less average size of more precipitate, resistance to hydrogen embrittlement tensile strength, characterized in Rukoto exceed 650MPa High-strength austenitic stainless steel with excellent properties.   In mass%, one kind selected from Al: 0.3% or less, Mg: 0.01% or less, Ca: 0.01% or less, REM: 0.10% or less, B: 0.008% or less, or The high-strength austenitic stainless steel excellent in hydrogen embrittlement resistance according to claim 1, comprising two or more kinds.   % Or less in mass%, Ti: 0.5% or less, Nb: 0.5% or less, V: 0.5% or less, and 1 or more types selected, It is characterized by the above-mentioned. A high-strength austenitic stainless steel excellent in hydrogen embrittlement resistance as described in 2.   The high-strength austenitic stainless steel excellent in hydrogen embrittlement resistance according to any one of claims 1 to 3, which is used in a high-pressure hydrogen gas and liquid hydrogen environment. A step of hot working a steel slab having the component composition according to any one of claims 1 to 3, a step of final heat treatment at 1000 ° C to 1200 ° C, and a step of cooling after the step of the final heat treatment. In the step of cooling, the average cooling rate up to 750 ° C. is controlled to be less than 2.0 ° C./s. 4. The hydrogen embrittlement resistance according to claim 1. A method for producing a high-strength austenitic stainless steel having excellent properties.   An apparatus for hydrogen used in a high-pressure hydrogen gas and liquid hydrogen environment, which uses the high-strength austenitic stainless steel excellent in hydrogen embrittlement resistance according to any one of claims 1 to 4.