Patent Application
20240209468
This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2022-206477 filed on Dec. 23, 2022, the contents of which are incorporated herein by reference.
The present invention relates to a martensitic stainless steel material for a hydrogen gas environment and a manufacturing method therefor, and more particularly to a martensitic stainless steel material for a hydrogen gas environment having excellent strength and hydrogen embrittlement resistance and a manufacturing method therefor.
As one of methods for achieving carbon neutrality, use of hydrogen is exemplified. For example, in the case of a fuel cell vehicle (FCV), which is a typical example of the use of hydrogen, since high-pressure hydrogen gas is used, a problem of hydrogen embrittlement of a steel material exposed to a hydrogen gas environment arises. As the steel material exposed to such a hydrogen gas environment, austenitic stainless steels excellent in hydrogen embrittlement resistance have been used in the related art. The austenitic stainless steels have both the hydrogen embrittlement resistance and corrosion resistance. However, it is difficult to achieve weight reduction because strength of the austenitic stainless steels is lower than that of martensitic stainless steels and the like. In addition, since expensive elements such as Ni are used, the cost is high.
In order to solve this problem, various proposals have been made in the related art.
For example. Patent Literature 1 discloses a substrate for hydrogen equipment including
Patent Literature 1 discloses that (A) when molten aluminum plating is applied on the surface of the substrate by using an Al—Si aluminum alloy, the three-layered structure film can be formed on the surface of the substrate, and (B) both the three-layered structure film and a two-layered structure film of an aluminum-based intermetallic compound layer and an alumina layer have a function of preventing hydrogen penetration, but the three-layered structure film is superior to the two-layered structure film in this function.
Patent Literature 2 discloses a martensitic stainless steel pipe containing predetermined amounts of C, Si, Mn, P. S, Cr, Al, N, and B, with the balance being Fe and inevitable impurities, although the martensitic stainless steel pipe is not intended to improve a hydrogen embrittlement resistance property.
Patent Literature 2 discloses that, when components are optimized, a martensitic stainless steel pipe with high strength exceeding 650 MPa in yield stress and excellent low-temperature toughness can be obtained.
Further, Patent Literature 3 discloses a stainless steel wire rod for a high-strength bolt, containing predetermined amounts of C, Si, Mn, Ni, Cr, N, and a carbide-forming element (Nb, Ti, V. W, Ta, and Zr), with the balance being Fe and inevitable impurities, although the stainless steel wire rod is not intended to improve a hydrogen embrittlement resistance property.
Patent Literature 3 discloses that when components are optimized, a stainless steel wire rod for a high-strength bolt having a tensile strength of 400 N/mm2 to 700 N/mm2 can be obtained.
As described above, although the austenitic stainless steels are excellent in the hydrogen embrittlement resistance, it is difficult to enhance the strength thereof. Therefore, the austenitic stainless steels are not suitable for a structural member requiring weight reduction and size reduction, for example, a vehicle component. In addition, since a large amount of expensive elements such as Ni and Mo are used, the cost is high.
On the other hand, since the martensitic stainless steels are high in corrosion resistance and have high strength as compared to the austenitic stainless steels, the weight reduction and size reduction of the component can be achieved. However, since the martensitic stainless steels are inferior in hydrogen embrittlement resistance, it is necessary to apply a special plating treatment to a surface thereof in order to use the martensitic stainless steels for a component exposed to a hydrogen gas environment (see Patent Literature 1). In addition to the cost of the plating treatment, in the case where even a small fraction of defects, scratches, or the like is present on the surface of the steel material due to the plating treatment, there is a concern of causing hydrogen embrittlement, and there is a problem in terms of safety assurance.
An object of the present invention is to provide a martensitic stainless steel material for a hydrogen gas environment excellent in strength and hydrogen embrittlement resistance, and a manufacturing method therefor.
In order to achieve the above object, a martensitic stainless steel material for a hydrogen gas environment according to the present invention,
has a composition consisting of:
having:
D
H2(0.7)
/D
air≥0.8 (1)
here, Dair represents a displacement at a time point when stress shows a local maximum in a stress-displacement curve obtained by performing a tensile test under a condition of a strain rate of 5×10−5/s in the atmosphere at normal temperature (25° C.), and
DH2(0.7) represents a displacement at a time point when stress shows a local maximum or a maximum value in a stress-displacement curve obtained by performing a tensile test under a condition of a strain rate of 5; 10/s in hydrogen gas of 0.7 MPa at normal temperature (25° C.).
One aspect of a manufacturing method for a martensitic stainless steel material for a hydrogen gas environment according to the present invention, contains:
Another aspect of a manufacturing method for a martensitic stainless steel material for a hydrogen gas environment according to the present invention, contains:
When a martensitic stainless steel material having a predetermined composition is subjected to a spheroidizing annealing treatment or a treatment corresponding thereto, the hardness of the martensitic stainless steel material can be reduced to a degree of hardness at which cold working is enabled.
Next, when the material is subjected to the cold working as necessary, and then the material is subjected to quenching (and sub-zero treatment as necessary), a metal structure including a martensite structure can be obtained. In this case, when quenching conditions are optimized, the crystal grain size number increases (a crystal grain size decreases), and a part of the spheroidized precipitates (mainly carbonitride) remains.
In the case where tempering is performed from this state, when tempering conditions are optimized, at the same time as toughness of the martensite phase is appropriately recovered (in other words, tensile strength is appropriately reduced), new precipitates grow in a branched shape (or a mesh-like shape) along an interface of the martensite substructure starting from the remaining fine precipitates.
Fine crystal grains, an appropriate tensile strength, and precipitates grown in a branched shape all have an effect of preventing spreading of cracks. Therefore, when such a steel material is employed to a member exposed to a hydrogen gas environment, brittle fracture caused by hydrogen can be prevented.
Hereinafter, an embodiment of the present invention will be described in detail.
The martensitic stainless steel material for a hydrogen gas environment (hereinafter, also referred to as a “martensitic stainless steel material” or simply a “steel material”) according to the present invention contains the following elements, with the balance being Fe and inevitable impurities. Types of additive elements, component ranges thereof, and reasons for limitation thereof are as follows.
C is an austenite stabilization element and is an element required for martensitic transformation at the time of quenching. In addition, C is an interstitial element and contributes to an improvement in strength. In addition. C bonds to Cr, Mo, V, Nb, and the like to be described later to form a precipitate (mainly carbonitride), and improves tempering hardness. Further, C contributes to formation of a branched carbonitride and improves hydrogen embrittlement resistance. In order to achieve such effects, the amount of C needs to be 0.03 mass % or more. The amount of C is preferably 0.05 mass % or more, and more preferably 0.10 mass % or more.
On the other hand, in the case where the amount of C is excessive, the amount of the retained austenite at the time of quenching may be increased, and the hydrogen embrittlement resistance may deteriorate. Therefore, the amount of C needs to be 1.20 mass % or less. The amount of C is preferably 0.80 mass % or less, and more preferably 0.50 mass % or less.
Si is a deoxidizing element and is effective for preventing an oxide that causes a reduction in toughness and ductility. Therefore, the martensitic stainless steel material may contain Si. In order to prevent the reduction in toughness and ductility, the amount of Si is preferably 0.005 mass % or more.
On the other hand, in the case where the amount of Si is excessive, deterioration of hydrogen embrittlement resistance or reduction of hot workability may be caused. Therefore, the amount of Si needs to be 1.00 mass % or less. The amount of Si is preferably 0.50 mass % or less, and more preferably 0.30 mass % or less.
In the case where Mn is further added in the case where the martensitic stainless steel material contains S, MnS is formed, and hot workability and machinability are improved. Therefore, the martensitic stainless steel material may contain Mn. In order to improve hot workability and machinability, the amount of Mn is preferably 0.005 mass % or more.
On the other hand, in the case where the amount of Mn is excessive, the amount of the retained austenite at the time of quenching is increased, and the hydrogen embrittlement resistance may deteriorate. Therefore, the amount of Mn needs to be 1.50 mass % or less. The amount of Mn is preferably 1.25 mass % or less, and more preferably 1.00 mass % or less.
P is an element that deteriorates hot workability, grain boundary strength, toughness and ductility, or hydrogen embrittlement resistance, and thus is preferably reduced. Therefore, the amount of P needs to be 0.060 mass % or more. The amount of P is preferably 0.050 mass % or less, and more preferably 0.040 mass % or less.
When the amount of P is reduced more than necessary, the cost increases. An optimum amount of P is preferably selected in consideration of these points.
S has an effect of promoting formation of MnS and enhancing machinability. Therefore, the martensitic stainless steel material may contain S.
On the other hand, in the case where the amount of S is excessive, corrosion resistance, toughness and ductility, and hot workability may be reduced. Therefore, the amount of S needs to be 0.250 mass % or less. The amount of S is preferably 0.200 mass % or less, and more preferably 0.150 mass % or less.
When the amount of S is reduced more than necessary, the cost increases. An optimum amount of S is preferably selected in consideration of these points.
In the case where the amount of Cu is excessive, the amount of retained austenite at the time of quenching increases, and the hydrogen embrittlement resistance may deteriorate. Therefore, the amount of Cu needs to be 0.50 mass % or less.
Cr forms a precipitate (mainly carbonitride) and contributes to an improvement in strength. In addition, Cr is an element effective in improving corrosion resistance. Further. Cr contributes to the formation of the branched carbonitride and improves hydrogen embrittlement resistance. In order to achieve such effects, the amount of Cr needs to be 8.0 mass % or more. The amount of Cr is preferably 10.0 mass % or more, and more preferably 11.5 mass % or more.
On the other hand, in the case where the amount of Cr is excessive, the amount of the retained austenite at the time of quenching is increased, and the hydrogen embrittlement resistance may deteriorate. Therefore, the amount of Cr needs to be 22.0 mass % or less. The amount of Cr is preferably 19.0 mass % or less, and more preferably 18.0 mass % or less.
In the case where the amount of Ni is excessive, the amount of the retained austenite at the time of quenching is increased, and the hydrogen embrittlement resistance may deteriorate. Therefore, the amount of Ni needs to be 1.00 mass % or less. The amount of Ni is preferably 0.60 mass % or less.
N is an interstitial element and contributes to an improvement in strength. In addition, N bonds to Cr, Mo, V, Nb, and the like to form a precipitate (mainly carbonitride), and improves tempering hardness. Further. N contributes to the formation of the branched carbonitride and improves hydrogen embrittlement resistance. Therefore, the martensitic stainless steel material may contain N.
On the other hand, in the case where the amount of N is excessive, the amount of the retained austenite at the time of quenching is increased, and the hydrogen embrittlement resistance may deteriorate. In addition, in the case where the amount of N is excessive, generation of a nitrogen blow may be induced. Therefore, the amount of N needs to be 0.40 mass % or less. The amount of N is preferably 0.30 mass % or less, and more preferably 0.20 mass % or less.
The martensitic stainless steel material according to the present invention may further contain one or two or more of the following elements in addition to the above-described main constituent elements. Types of additive elements, component ranges thereof, and reasons for limitation thereof are as follows.
Mo improves corrosion resistance and also improves strength as a solid solution-strengthening element. In addition, Mo bonds to C and N at the time of tempering to form a precipitate (mainly carbonitride), and also contributes to an improvement in hardness. Therefore, the martensitic stainless steel material may contain Mo. The amount of Mo is preferably 0.01 mass % or more, and more preferably 0.05 mass % or more.
On the other hand, in the case where the amount of Mo is excessive, the amount of the retained austenite at the time of quenching is increased, and the hydrogen embrittlement resistance may deteriorate. Therefore, the amount of Mo is preferably 3.00 mass % or less. The amount of Mo is more preferably 2.00 mass % or less, and still more preferably 1.00 mass % or less.
V bonds to C and N at the time of tempering to form a precipitate (mainly carbonitride), and contributes to an improvement in hardness. Therefore, the martensitic stainless steel material may contain V in addition to Mo or instead of Mo. In order to improve hardness, the amount of V is preferably 0.01 mass % or more. The amount of V is more preferably 0.05 mass % or more.
On the other hand, in the case where the amount of V is excessive, toughness and ductility may be significantly reduced due to a formation of coarse precipitates. Therefore, the amount of V is preferably 1.50 mass % or less. The amount of V is more preferably 1.0 mass % or less, and still more preferably 0.6 mass % or less.
Nb bonds to C and N at the time of tempering to form a precipitate (mainly carbonitride), and contributes to an improvement in hardness. Therefore, the martensitic stainless steel material may contain Nb in addition to or instead of Mo and/or V. In order to achieve such an effect, the amount of Nb is preferably 0.01 mass % or more. The amount of Nb is more preferably 0.05 mass % or more.
On the other hand, in the case where the amount of Nb is excessive, toughness and ductility may be significantly reduced due to a formation of coarse precipitates. Therefore, the amount of Nb is preferably 1.00 mass % or less. The amount of Nb is more preferably 0.60 mass % or less.
Pb has an effect of enhancing machinability. Therefore, the martensitic stainless steel material may contain Pb in addition to or instead of one or more elements selected from the group consisting of Mo. V, and Nb. In order to obtain high machinability, the amount of Pb is preferably 0.05 mass % or more.
On the other hand, in the case where the amount of Pb is excessive, hot workability may be significantly reduced. Therefore, the amount of Pb is preferably 0.30 mass % or less.
B has an effect of contributing to an improvement in toughness and ductility or improving hot workability. Therefore, the martensitic stainless steel material may contain B in addition to or instead of one or more elements selected from the group consisting of Mo. V. Nb. and Pb.
On the other hand, in the case where the amount of B is excessive, hot workability may be rather reduced. Therefore, the amount of B is preferably 0.0500 mass % or less. The amount of B is more preferably 0.0400 mass % or less, and still more preferably 0.0300 mass % or less.
[A. Content of Precipitate after Tempering]
The martensitic stainless steel material according to the present invention can be obtained by subjecting a steel material containing a predetermined component to a spheroidizing annealing or a treatment corresponding thereto, quenching (including sub-zero treatment), and tempering, under predetermined conditions. In this case, in the case where the composition and heat treatment conditions of the martensitic stainless steel material are optimized, precipitates are contained in the structure after the tempering. The precipitates mainly include carbonitrides.
The carbonitride in the present invention includes not only a compound containing both carbon and nitrogen but also a carbide and a nitride. Examples of the carbide include M23C6, M6C, M2C, and MC. Examples of the nitride include M2N. Here, M is one or more of Cr, Mo. V, Nb, and Fe. Examples of the carbonitride include a carbonitride in which a part of carbon of the above-mentioned carbide is replaced by nitrogen.
Here, the “content (mass %) of precipitate” refers to a value obtained by (a) sampling a test material having a predetermined size from a steel material, (b) anodic-dissolving a part of the test material at a current density of 25 mA/cm2 for approximately 2 hours to 5 hours by a constant current electrolysis using a methanol solution containing 10 vol % of acetylacetone and 1 mass % of tetramethylammonium chloride, as an electrolyte solution, (c) suction-filtering a solution obtained by the anodic dissolution by using a microfilter with a pore size of 0.1 μm, and (d) dividing the mass of the electrolytic extraction residue deposited on the microfilter by a difference in mass of the test material before and after the electrolytic extraction.
Accordingly, the “content (mass %) of precipitate” may include, in addition to the precipitate precipitated at the time of tempering, inclusions such as oxides and sulfides present at the time of molten steel and carbides crystallized during solidification.
In this case, the amount of the electrolytic extraction residue hardly varies depending on the size and shape of the test material. Therefore, the size and shape of the test material are not particularly limited, and an optimum size and shape can be selected according to a purpose thereof. For example, a 10 mm-cubic body may be used as the test material.
The precipitate precipitated in the martensitic stainless steel material after the tempering have (a) a function of trapping and detoxifying hydrogen, (b) a function of strengthening an interface such as a boundary surface of a prior austenite grain boundary and a martensite substructure (lath, block, and packet), and (c) a function as resistance against spreading of cracks. Therefore, as the content of the precipitate increases, the hydrogen embrittlement resistance is improved.
In order to obtain an excellent hydrogen embrittlement resistance, the content of the precipitate contained in the steel material after tempering needs to be 1.50 mass % or more. The content of the precipitate is preferably 2.00 mass % or more, and more preferably 2.50 mass % or more.
The shape of the precipitate is not particularly limited. In order to improve the hydrogen embrittlement resistance, the martensitic stainless steel material preferably contains a branched carbonitride as the precipitate.
Here, the “branched carbonitride” refers to a carbonitride having a structure in which elongated rod-shaped precipitates intersect in a branched shape to each other. Each of the rod-shaped carbonitrides constituting the branched carbonitride preferably has an aspect ratio of 5 or more. The shape of the carbonitride can be determined by observing the electrolytic extraction residue with a scanning electron microscope (SEM).
The “aspect ratio” of the branched carbonitride refers to a value obtained by dividing the length (L) in a long axis direction of the longest rod-shaped carbonitride among the rod-shaped carbonitrides constituting the branched carbonitride by the length (D) in a short axis direction of the rod-shaped carbonitrides.
When a spheroidizing annealing treatment is performed before quenching the steel material, or a slow-cooling treatment is performed after hot-working the steel material, spherical precipitates (mainly carbonitride) can be precipitated in a matrix. Next, when the steel material from which the spherical precipitates are precipitated is subjected to a quenching under appropriate conditions, the spherical precipitates do not completely disappear, and fine precipitates remain in an as quenched state (in an as quenched and sub-zero treated state in the case where a sub-zero treatment is performed). Further, when the steel material in which the fine precipitates remain is subjected to a tempering, new precipitates are precipitated in a branched shape along the interface of the martensite substructure starting from the fine precipitates. Since the branched carbonitride has an effect of strengthening the interface of the martensite substructure, the spreading of cracks is prevented, and the hydrogen embrittlement resistance is improved.
The “crystal grain size number of prior austenite grain” refers to a value measured in accordance with JIS G 0551:2020.
In the case where the crystal grain size number of the steel material after tempering is small (=grain size is large), the resistance against cracks caused by hydrogen and spreading thereof becomes small. As a result, the hydrogen embrittlement resistance deteriorates, resulting in an unsuitable result in a slow strain rate tensile (SSRT) test. In order to obtain an excellent hydrogen embrittlement resistance, the crystal grain size number of the prior austenite grain needs to be 2.0 or more. The crystal grain size number is preferably 3.0 or more, more preferably 4.0 or more.
The martensitic stainless steel material according to the present invention can be obtained by subjecting a steel material containing a predetermined component to a spheroidizing annealing, quenching (including sub-zero treatment), and tempering, under appropriate conditions. Therefore, the martensitic stainless steel material has a metal structure including a martensite structure.
The “tensile strength” refers to a value obtained by (a) performing a tensile test (slow strain rate tensile test) under a condition of a strain rate of 5×10−5/s in the atmosphere at normal temperature (25° C.), and (b) dividing the maximum value of stress in the slow strain rate tensile test by an area of a parallel portion of a tensile test piece.
In the case where a component used in a hydrogen gas environment is manufactured by using the martensitic stainless steel material according to the present invention, in order to reduce the weight of the component, the tensile strength of the martensitic stainless steel material is preferably as high as possible. However, in the case where the tensile strength becomes too high, toughness is reduced, and a brittle fracture easily occurs. In order to prevent the brittle fracture, the tensile strength needs to be 1,800 MPa or less. The tensile strength is preferably 1,500 MPa or less, and more preferably 1,200 MPa or less.
However, in the case where the tensile strength becomes too low, it may be difficult to reduce the weight of the component. Therefore, the tensile strength is preferably 540 MPa or more. The tensile strength is more preferably 600 MPa or more, and still more preferably 700 MPa or more.
[1.3.5. DH2/Dair]
The martensitic stainless steel material needs to satisfy the following formula (1).
D
H2(0.7)
/D
air≥0.8 (1)
here,
Dair represents a displacement at a time point when stress shows a local maximum in a stress-displacement curve obtained by performing a tensile test under a condition of a strain rate of 5×10−5/s in the atmosphere at normal temperature (25° C.), and
DH2(0.7) represents a displacement at a time point when stress shows a local maximum or a maximum value in a stress-displacement curve obtained by performing a tensile test under a condition of a strain rate of 5×10−5/s in hydrogen gas of 0.7 MPa at normal temperature (25° C.).
In addition, the martensitic stainless steel material preferably satisfies the following formula (2) in addition to the formula (1):
D
H2(90)
/D
air≥0.8 (2)
here,
Dair represents a displacement at a time point when stress shows a local maximum in the stress-displacement curve obtained by performing the tensile test under a condition of a strain rate of 5×10−5/s in the atmosphere at normal temperature (25° C.), and
DH2(90) represents a displacement at a time point when stress shows a local maximum or a maximum value in a stress-displacement curve obtained by performing a tensile test under a condition of a strain rate of 5×10−5/s in hydrogen gas of 90.0 MPa at normal temperature (25° C.).
In the case where the tensile test is performed in a hydrogen atmosphere, when the test piece is broken before the stress reaches a local maximum or when whether the stress shows a local maximum cannot be clearly determined, a “displacement at the time when the stress shows the maximum value” is regarded as the “displacement at the time when the stress shows a local maximum”, and values of the formulae (1) and (2) are calculated based thereon.
DH2(0.7)/Dair and DH2(90)/Dair (hereinafter, also collectively referred to as “DH2/Dair”) represent indices of the hydrogen embrittlement resistance. DH2/Dair being large indicates that the deformation amount under a hydrogen atmosphere is large, that is, the material is excellent in the hydrogen embrittlement resistance. The martensitic stainless steel material according to the present invention is excellent in the hydrogen embrittlement resistance since the composition and structure thereof are optimized. Specifically, by optimizing the composition and structure. DH2(0.7)/Dair becomes 0.8 or more. In the case where the composition and structure are further optimized, DH2(0.7)/Dair becomes 0.9 or more.
In the case where the composition and structure are further optimized, DH2(90)/Dair becomes 0.8 or more or 0.9 or more in addition to DH2(0.7)/Dair satisfying the above-described conditions.
The martensitic stainless steel material according to the present invention can be used for various members used in a hydrogen gas environment. Examples of such members include (a) a valve, pipe, and pressure reducing valve for high-pressure hydrogen gas used in a hydrogen station or an FCV, and (b) a member for a compressor used to increase a pressure of hydrogen gas.
A manufacturing method for a martensitic stainless steel material for a hydrogen gas environment, according to a first embodiment of the present invention includes:
First, the material A1 having the same composition as that of the martensitic stainless steel material for hydrogen gas environment according to the present invention is subjected to a hot-working and/or a heat treatment to obtain the material A2 containing a spheroidized precipitate (step A).
There are various methods for spheroidizing the precipitate, and a method of heating the material to a temperature equal to or higher than an Af point (a temperature at which reverse transformation from martensite to austenite is completed) and slowly cooling the material (so-called “spheroidizing annealing”) is preferable. The reason for heating the material to a temperature equal to or higher than the Af point is to transform a structure before the spheroidizing annealing into an austenite phase.
In the case where a spheroidizing annealing temperature T3 is too low, the transformation to the austenite phase becomes insufficient, and the structure before the spheroidizing annealing (for example, a martensite structure) remains, so that spheroidizing of the precipitate is not promoted. In the case of the present component system, the spheroidizing annealing temperature T3 is preferably 800° C. or more. The spheroidizing annealing temperature T3 is more preferably 820° C. or more.
As a heating time of the spheroidizing annealing, an optimum time is selected according to the heating temperature. The spheroidizing annealing time is usually 1 hour to 10 hours.
After being held at a predetermined temperature for a predetermined time, the material is slowly cooled. In the case where a cooling rate of the slow-cooling is too high, precipitation and growth of the precipitate become insufficient. On the other hand, slow-cooling more than necessary has no practical benefit. The cooling rate is preferably 30° C./h or less. The temperature range within which the slow-cooling is performed may be a range within which precipitation and growth of the precipitate occur. Normally, the final temperature of the slow-cooling is about 550° C. to 650° C.
The amount of the spheroidized precipitate affects the precipitation and growth of the precipitate in the subsequent tempering step, and as a result, affects the hydrogen embrittlement resistance.
However, for the generation of such a spheroidized precipitate, it is not always necessary to adopt the above-described step, and for example, a hot-working at a temperature at which the precipitate is precipitated, a hot-direct annealing after a hot-working, and the like are also effective.
Next, the material A2 is subjected to a quenching or the quenching and a sub-zero treatment (step C1). As a result, the material Cl having a retained austenite content of 10.0 vol % or less is obtained.
The quenching is performed by heating the material A2 that has been subjected to the spheroidizing treatment, to a quenching temperature, followed by rapidly cooling. Generally, in the case where the quenching temperature T4 is too low, an austenite single phase state cannot be obtained. Therefore, martensitic transformation does not occur at the time of the rapid cooling, and sufficient hardness cannot be obtained. Therefore, the quenching temperature T4 needs to be equal to or higher than the Af point.
On the other hand, in the case where the quenching temperature T4 becomes too high, (a) coarsening of crystal grains occurs, (b) precipitates present before the quenching are completely dissolved, and (c) the amount of retained austenite after the quenching becomes excessive, so that the hydrogen embrittlement resistance may deteriorate.
Therefore, the quenching temperature T4 is preferably 1,200° C. or less.
As a holding time at the quenching temperature T4, an optimum time is selected according to the quenching temperature T4. In general, as the quenching temperature T4 increases, an austenite single phase structure can be obtained in a short time. The holding time at the quenching temperature T4 depends on a dimension of the material, and is usually about 10 minutes to 2 hours.
A rapid cooling method is not particularly limited, and an optimum method is selected according to a purpose thereof. Specific examples of the rapid cooling method include oil cooling and blast cooling.
The sub-zero treatment is performed by further cooling the quenched material to a temperature of 0° C. or lower. The step C1 may be a step of performing only the quenching, or may be a step of performing both the quenching and the sub-zero treatment. In the case where a large amount of austenite remains with only the quenching, the sub-zero treatment is preferably performed. The sub-zero treatment is usually performed by holding the member at −30° C. to −196° C.
In the case where the amount of retained austenite is excessive in the as quenched state or in the as quenched and sub-zero treated state, the retained austenite undergoes the martensitic transformation at the time of tempering to generate new martensite. The new martensite causes deterioration of the hydrogen embrittlement resistance. In contrast, when the retained austenite is reduced by the sub-zero treatment, the deterioration of the hydrogen embrittlement resistance due to the new martensite can be prevented.
In order to prevent the deterioration of the hydrogen embrittlement resistance, the amount of the retained austenite needs to be 10.0 vol % or less in the as quenched state or in the as quenched and sub-zero treated state. The amount of the retained austenite is preferably 8.0 vol % or less, and more preferably 3.0 vol % or less.
[2.2.4. Content of Precipitate after Quenching or after Sub-Zero Treatment]
The fine precipitates contained in the steel material after the quenching or after the sub-zero treatment promote the generation of the branched carbonitride at the time of tempering and improve the hydrogen embrittlement resistance. It is considered that the precipitates contained in the steel material after the quenching or after the sub-zero treatment are derived from the spherical precipitates that have generated in the spheroidizing treatment and remained without being completely dissolved at the time of quenching. Therefore, even in the case where an appropriate amount of precipitates is contained at the end of the spheroidizing treatment, the precipitates disappear when the quenching temperature is too high. Therefore, it is necessary to pay attention to settings of the quenching conditions.
In order to grow the branched carbonitride in the tempering step, the content of the precipitates after the quenching or after the sub-zero treatment is preferably 0.01 mass % or more. The content is more preferably 0.03 mass % or more, and still more preferably 0.05 mass % or more.
In addition, it is considered that for the growth of the branched carbonitride, a tempering at 400° C. or higher is preferable.
Next, the material C1 is subjected to a tempering (step D1). As a result, the austenitic stainless steel material according to the present invention is obtained.
The tempering is performed to appropriately reduce the strength of the martensite structure generated by the quenching or the sub-zero treatment and recover the toughness. In addition, since precipitation of precipitates effective in improving the hydrogen embrittlement resistance occurs during the tempering, the tempering step is an important step.
In the case where the amount of the retained austenite in a stage before the tempering step is excessive, new martensite is generated at the time of tempering, and the hydrogen embrittlement resistance deteriorates. In addition, in the case where the amount of precipitate is too small in the stage before the tempering step, new precipitates are not appropriately precipitated and grown at the time of tempering, and the branched carbonitride is hardly developed. Therefore, the hydrogen embrittlement resistance deteriorates. This is considered to be because fine precipitates present in the steel in the stage before the tempering step serve as nuclei for the precipitation and growth of the new precipitates at the time of tempering.
In the case where the tempering temperature T5 is too low, the strength cannot be reduced. In addition, precipitation of new precipitates and growth into a branched carbonitride become insufficient, and the hydrogen embrittlement resistance deteriorates.
On the other hand, in the case where the tempering temperature T5 becomes too high and becomes equal to or higher than an As point (a temperature at which reverse transformation from martensite to austenite starts), reverse transformation from the martensite structure to austenite occurs. As a result, new martensite is generated in a cooling process, and conversely, the strength may be increased.
Therefore, as the tempering temperature T5, it is preferable to select an optimum temperature according to the composition, required properties, and the like of the martensitic stainless steel material. The tempering temperature T5 is usually about 400° C. to 780° C.
As a tempering time, an optimum time is selected according to the tempering temperature T5. In general, as the tempering temperature T5 increases, the strength can be reduced in a short time. The tempering time depends on the dimension of the material, and is usually about 30 minutes to 4 hours.
[3. Manufacturing Method for Martensitic Stainless Steel Material for Hydrogen Gas Environment (2)]
A manufacturing method for a martensitic stainless steel material for a hydrogen gas environment according to a second embodiment of the present invention includes:
In the present embodiment, the material A2 that has been subjected to the spheroidizing treatment is subjected to a cold-working, and the material B after the cold-working may be subjected to the quenching, the quenching and the sub-zero treatment, and the tempering. This point is different from that in the first embodiment.
The cold-working of the material A2 is performed as necessary. A cold-working method of the material A2 is not particularly limited, and an optimal method can be selected according to a purpose thereof.
The other points are the same as those in the first embodiment, and descriptions thereof are omitted.
When a martensitic stainless steel material having a predetermined composition is subjected to a spheroidizing annealing treatment or a treatment corresponding thereto, the hardness of the martensitic stainless steel material can be reduced to a degree of hardness at which cold working is enabled.
Next, when the material is subjected to the cold working as necessary, and then the material is subjected to quenching (and sub-zero treatment as necessary), a metal structure including a martensite structure can be obtained. In this case, when quenching conditions are optimized, the crystal grain size number increases (a crystal grain size decreases), and a part of the spheroidized precipitates (mainly carbonitride) remains.
In the case where tempering is performed from this state, when tempering conditions are optimized, at the same time as toughness of the martensite phase is appropriately recovered (in other words, tensile strength is appropriately reduced), new precipitates grow in a branched shape (or a mesh-like shape) along an interface of the martensite substructure starting from the remaining fine precipitates.
Fine crystal grains, an appropriate tensile strength, and precipitates grown in a branched shape all have an effect of preventing spreading of cracks. Therefore, in the case where such a steel material is employed to a member exposed to a hydrogen gas environment, the brittle fracture caused by hydrogen can be prevented.
In a vacuum induction furnace, a 50 kg of steel having a composition shown in Table 1 was melted and cast into an ingot. Then, a hot-forging, hot-rolling, and machining were performed on the ingot to manufacture a steel bar having a diameter of 30 mm.
Next, the steel bar was subjected to a spheroidizing annealing, quenching, sub-zero treatment, and tempering.
The spheroidizing annealing was performed by heating the steel bar at 850° C. for 2 hours, followed by slowly cooling to 600° C. at a cooling rate of 5° C./h.
The quenching was performed by holding the steel bar at 850° C. to 1,200° C. for 30 minutes, followed by oil cooling. The sub-zero treatment was performed by further holding the quenched steel bar at −80° C. for 3 hours.
Further, the tempering was performed by holding the steel bar after the quenching or after the sub-zero treatment at 180° C. to 700° C. for 2 hours, followed by water cooling or air cooling.
The steel bar after the quenching or after the sub-zero treatment was subjected to an X-ray diffraction measurement using a Mo tube. Next, the amount of the retained austenite (vol %) was calculated by using a “5-peak method”. The “5-peak method” refers to a method of calculating the amount of the retained austenite (vol %) by using peak intensities of (200) and (211) of a ferrite phase and peak intensities of (200), (220), and (311) of an austenite phase appearing in an X-ray profile.
The crystal grain size of a prior austenite grain of the steel bar after the tempering was measured. The crystal grain size was measured in accordance with JIS G 0551:2020.
Electrolytic extraction was performed on the steel bar after the tempering. Further, the amount of precipitates was calculated based on the mass of the electrolytic extraction residue and the mass difference of the test piece between before and after the electrolytic extraction.
The electrolytic extraction residue was subjected to microscopic observation to evaluate the shape of the precipitates.
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A tensile test piece was sampled from the steel bar after the tempering and subjected to a slow strain rate tensile (SSRT) test. A round bar test piece having a parallel portion diameter of 6 mm was used as the tensile test piece. The strain rate thereof was 5×10−5/s. The test temperature thereof was normal temperature (25° C.). The test atmosphere thereof was the atmosphere, hydrogen gas of 0.7 MPa, or hydrogen gas of 90 MPa Based on the obtained stress-displacement diagram, presence or absence of a local maximum was evaluated, and DH2(0.7)/Dair and DH2(90)/Dair were calculated.
A tensile test piece was sampled from the steel bar after the tempering and subjected to the slow strain rate tensile test. A round bar test piece having a parallel portion diameter of 6 mm was used as the tensile test piece. The strain rate thereof was 5×10−5/s. The test temperature thereof was normal temperature (25° C.). The test atmosphere thereof was the atmosphere. The tensile strength was calculated by dividing the maximum value of the stress by an area of the parallel portion.
Results are shown in Table 2. From Table 2, the following can be understood.
Regarding the shape of precipitate, “A” indicates that branched (mesh-like) precipitates were formed, and “B” indicates that branched (mesh-like) precipitates were not formed.
Regarding SSRT, “A” represents DH2/Dair≥0.9, “B” represents 0.8≤DH2/Dair<0.9, and “C” represents DH2/Dair<0.8.
Regarding the tensile strength, “A” represents that the tensile strength is 540 MPa or more and 1,200 MPa or less, “B” represents that the tensile strength is more than 1,200 MPa and 1,500 MPa or less, “C” represents that the tensile strength is more than 1,500 MPa and 1,800 MPa or less, and “D” represents that the tensile strength is more than 1,800 MPa.
(1) Comparative Example 1 is a material corresponding to SUS410. In addition, Comparative Example 2 is a material corresponding to SUS420J2. Both of Comparative Examples 1 and 2 had a poor hydrogen embrittlement resistance. This is considered to be because quenching conditions and tempering conditions were inappropriate, and thus the amount of precipitates in the as quenched state and the amount of precipitates after the tempering were both small, and no branched carbonitride was formed.
(2) Comparative Example 3 is a material corresponding to SUS420J. Comparative Example 3 had a poor hydrogen embrittlement resistance. This is considered to be because tempering conditions were inappropriate, and thus the amount of precipitates in the as quenched state and the amount of precipitates after the tempering were both small, and no branched carbonitride was formed.
(3) Comparative Example 4 is a material corresponding to SUS420J. Comparative Example 4 had a poor hydrogen embrittlement resistance.
This is considered to be because quenching conditions were inappropriate, the crystal grain size number became small (crystal grains became coarse), and because no branched carbonitride was formed.
(4) Comparative Example 5 is a material corresponding to SUS420J2. Comparative Example 5 had a poor hydrogen embrittlement resistance. This is considered to be because since spheroidizing annealing was not performed, the amount of precipitates in the as quenched state was reduced, and thus no branched carbonitride was formed.
(5) Comparative Example 6 is a material corresponding to SUS440C. Comparative Example 6 had a poor hydrogen embrittlement resistance.
This is considered to be because tempering conditions were inappropriate, and thus no branched carbonitride was formed, and because the tensile strength was too high.
(6) Comparative Example 7 is a material corresponding to SUS440C. Comparative Example 7 had a poor hydrogen embrittlement resistance.
This is considered to be because no sub-zero treatment was performed, and thus the amount of the retained austenite became excessive. The retained austenite in the as quenched state undergoes martensitic transformation at the time of tempering to become new martensite. It is considered that the resulting new martensite deteriorates the hydrogen embrittlement resistance.
(8) Comparative Example 8 had a poor hydrogen embrittlement resistance. This is considered to be because the amount of Si was excessive.
(9) Comparative Example 9 had a poor hydrogen embrittlement resistance. This is considered to be because the amount of P was excessive.
(10) All of Examples 1 to 19 were excellent in the hydrogen embrittlement resistance. This is considered to be due to fine crystal grains, appropriate tensile strength, and an appropriate amount of the branched carbonitride.
(11) Example 3 exhibited excellent hydrogen embrittlement resistance in the hydrogen gas of 0.7 MPa.
However, in the hydrogen gas of 90 MPa, the hydrogen embrittlement resistance deteriorated. This is considered to be because the tempering temperature was too low and the amount of precipitates was slightly small.
(12) Example 9 exhibited excellent hydrogen embrittlement resistance in the hydrogen gas of 0.7 MPa.
However, in the hydrogen gas of 90 MPa, the hydrogen embrittlement resistance deteriorated. This is considered to be because the amount of Si was relatively large and the tensile strength was slightly high.
(13) Example 10 exhibited excellent hydrogen embrittlement resistance in the hydrogen gas of 0.7 MPa. However, in the hydrogen gas of 90 MPa, the hydrogen embrittlement resistance deteriorated. This is considered to be because the amount of Si was relatively large.
(14) Example 11 exhibited excellent hydrogen embrittlement resistance in the hydrogen gas of 0.7 MPa. However, in the hydrogen gas of 90 MPa, the hydrogen embrittlement resistance deteriorated. This is considered to be because the amount of P was relatively large.
(15) Example 18 exhibited excellent hydrogen embrittlement resistance in the hydrogen gas of 0.7 MPa. However, in the hydrogen gas of 90 MPa, the hydrogen embrittlement resistance deteriorated. This is considered to be because the tensile strength was high.
The martensitic stainless steel material for a hydrogen gas environment according to the present invention can be used as a structural member used in a high-pressure hydrogen gas device.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2022-206477 | Dec 2022 | JP | national |
