WO2023189563A1

WO2023189563A1 - Martensite stainless steel for high-pressure hydrogen component, high-pressure hydrogen component using same, and method for producing same

Info

Publication number: WO2023189563A1
Application number: PCT/JP2023/009928
Authority: WO
Inventors: 和正窪田; 義典渡邊; 航花井; 遼川上
Original assignee: 愛知製鋼株式会社
Priority date: 2022-03-30
Filing date: 2023-03-14
Publication date: 2023-10-05
Also published as: WO2023188441A1

Abstract

This martensite stainless steel for high-pressure hydrogen component has a chemical configuration comprising, in terms of mass%, 0.01-0.05% of C; 0.70% or less of Is; 1.10% or less of Mn; 0.04% or less of P; 0.030% or less of S; 4.00-5.70% of Ni; and 17.00-20.50% of Cr, optionally comprising 0.0005-0.0030% of Ca, and also optionally comprising 0.0005-0.0050% of B, the balance being Fe and unavoidable impurities. The chemical configuration satisfies formula 1.　Formula 1: 3.0 > Ni – 0 > Ni – 0.8Cr + 11.36 > 0 (where the symbols of elements in formula 1 represent the value of the content ratio (%) of the respective elements.)

Description

Martensitic stainless steel for high-pressure hydrogen parts, high-pressure hydrogen parts using the same, and manufacturing method thereof

The present invention relates to a martensitic stainless steel for high-pressure hydrogen parts, a high-pressure hydrogen part using the same, and a method for manufacturing the same.

Parts that come into contact with high-pressure hydrogen gas in fuel cell vehicles and hydrogen stations (hereinafter referred to as "high-pressure hydrogen parts") are made of stainless steel materials with a mainly austenitic structure (austenitic stainless steel) such as SUS316, SUS316L, and SUS305. steel) is commonly used. These steel materials have the advantage of being easy to design equipment because there is no or only a slight decrease in strength in a high-pressure hydrogen environment. In addition, since they have excellent corrosion resistance, parts made of these steel materials do not need to be painted, and periodic inspections when using the parts are easy.

However, since these austenitic stainless steels have a high content of rare metals such as Ni and Mo, there is a problem that they are higher in cost compared to martensitic stainless steels that have a lower content of Ni and Mo. . More specifically, SUS316 and SUS316L contain 10% or more Ni and 2% or more Mo in terms of mass %, which poses a problem of high cost. Although SUS305 does not substantially contain Mo (its content as an impurity is allowed), it contains Ni in an amount of 10.5% or more by mass, which also poses the problem of high cost.

In addition, these austenitic stainless steels have a low 0.2% yield strength of about 270 MPa and a low tensile strength of about 580 MPa in the solution heat treated state, so high-pressure hydrogen parts such as pipe joints and valves have thick walls, making them large and heavy. There's a problem. To solve this problem, cold working may be performed after solution heat treatment. However, in this case, since the strain distribution within the cross section of the steel material after cold working is complex, there is a problem in that, for example, guaranteeing the strength of high-pressure hydrogen parts requires a lot of data and estimation using CAE calculations. Another problem is that the degree of freedom in shape obtained through machining is insufficient.

In this way, austenitic stainless steels such as SUS316 and SUS316L contain a large amount of rare metals and are expensive, and their strength is low in the heat-treated state, making it necessary to thicken the parts. It is difficult to design and manufacture. In order to solve this problem, as described in Patent Document 1, a proposal has been made to use low-alloy steel having a chemical composition in which the added amounts of Ni and Mo are relatively small for high-pressure hydrogen parts.

The above-mentioned low-alloy steel has a tensile strength of 1000 MPa or less without any strength loss in a high-pressure hydrogen environment, has a low content of rare metals such as Mo, and is resource-saving, and has a tensile strength of 500 MPa or more in the heat-treated state. .2% proof stress and tensile strength of 700 MPa or more can be obtained. Therefore, these low alloy steels are superior to austenitic stainless steels such as SUS316L in terms of strength, and are also superior in terms of freedom of part shape and ease of part manufacturing.

However, these low-alloy steels have a low Cr content and are easily corroded, so when applied to high-pressure hydrogen parts, they must be used in large-sized parts such as pressure accumulators, where wall thickness inspection using ultrasonic flaw detection is easy. Application limited to parts. Additionally, depending on the handling during manufacturing, there is a risk of releasing foreign substances such as rust into the gas flow path, and the coating makes it difficult to inspect the appearance for cracks and the like.

Additionally, as a plan to achieve both corrosion resistance and strength, it is conceivable to use general-purpose martensitic stainless steel such as SUS420J2 and SUS410 for high-pressure hydrogen parts. As a result of intensive research by the inventors, it was found that general-purpose martensitic stainless steel can maintain static strength in a high-pressure hydrogen environment by adjusting the tempering hardness, but its corrosion resistance is insufficient. It was concluded that it is difficult to use this material with the steel surface exposed, and in addition, its toughness at low temperatures is extremely low, making it difficult to apply it to high-pressure hydrogen parts that are exposed to low temperatures.

Japanese Patent Application Publication No. 2018-188716

In view of this background, the present invention aims to overcome the technical problems in reducing the cost of high-pressure hydrogen parts, and has almost no strength loss in a high-pressure hydrogen environment, excellent low-temperature toughness, and excellent corrosion resistance. The present invention aims to provide a martensitic stainless steel for high-pressure hydrogen parts that can save resources compared to conventional stainless steel materials for high-pressure hydrogen parts, a high-pressure hydrogen part using the same, and a method for manufacturing the same.

In one embodiment of the present invention, in mass %,
C: 0.01-0.05%,
Si: 0.70% or less,
Mn: 1.10% or less,
P: 0.040% or less,
S: 0.030% or less,
Ni: 4.00-5.70%,
Contains Cr: 17.00 to 20.50%,
As an optional element, Ca: 0.0005 to 0.0030%,
Contains B: 0.0005 to 0.0050% as an optional element,
The present invention is a martensitic stainless steel for high-pressure hydrogen components that satisfies the following formula 1 and has a chemical composition in which the balance is Fe and unavoidable impurities.
Formula 1: 3.0>Ni-0.8Cr+11.36>0,
(However, the element symbols in Formula 1 mean the content (%) value of each element.)

Another aspect of the present invention resides in a high-pressure hydrogen component made of the martensitic stainless steel for high-pressure hydrogen components.

Yet another aspect of the present invention is a method for manufacturing high-pressure hydrogen parts using the martensitic stainless steel for high-pressure hydrogen parts, comprising:
A part made of a steel material having the above chemical composition is held at a temperature of 900° C. to 1020° C. and then subjected to quenching heat treatment to be rapidly cooled,
Thereafter, the method of manufacturing a high-pressure hydrogen component includes performing a tempering heat treatment in which the component is held at a temperature of 640° C. to 770° C. and then rapidly cooled.

The martensitic stainless steel for high-pressure hydrogen parts and high-pressure hydrogen parts have a Ni content lower than that of conventional austenitic stainless steel to save resources, and are adjusted to satisfy formula 1. Because it has a specific chemical composition, if manufactured under appropriate conditions, there will be almost no decrease in strength even in a high-pressure hydrogen environment, and it will have excellent low-temperature toughness and corrosion resistance.

FIG. 7 is a diagram showing the results of a rotating bending fatigue test in a high-pressure hydrogen environment in Experimental Example 2. The figure which shows the relationship between test temperature (degreeC) and aperture (%) as a Greeble test result in Experimental example 5.

First, the reasons for limiting the chemical composition of the martensitic stainless steel for high-pressure hydrogen parts (hereinafter referred to as "stainless steel of the present application" as appropriate) will be explained.

C: 0.01-0.05%,
C (carbon) combines with Cr during tempering heat treatment and precipitates as carbides, thereby reducing the Cr concentration dissolved in steel and deteriorating corrosion resistance, and has the effect of lowering the mobile dislocation density of fresh martensite. Therefore, it is desirable that the C concentration is low. In addition, C has the effect of increasing the hardness of the martensitic structure during quenching, and cracks are likely to occur in the as-quenched state. From this point of view as well, it is desirable that the C concentration be low, since the handling of the steel material during forging becomes complicated and causes an increase in cost. Therefore, the content of C is limited to 0.05% or less. In addition, from the viewpoint of corrosion resistance, the C content is preferably 0.04% or less.

On the other hand, if the C concentration is too low and the chemical composition shifts to the austenite stable side (martensite phase rich side), many crystals with low mobile dislocation density will exist in the steel, and after rolling. After heat treatment, the steel material tends to bend due to room temperature creep deformation due to its own weight, which increases the effort of straightening and makes it difficult to produce the steel material. Therefore, the lower limit of the C content is set to 0.01%.

Si: 0.70% or less,
Si (silicon) is a necessary component for performing reduction refining in melting stainless steel. However, Si is a ferrite stabilizing element, and if added in excess, many ferrite crystals that are highly susceptible to hydrogen embrittlement will be generated in the metal structure, so the upper limit is set to 0.70%. Preferably, the Si content is 0.60% or less. Although the lower limit of the Si content is not particularly determined, it is usually 0.10% or more because it is unavoidable that it will be mixed in from the slag components used during refining.

Mn: 1.10% or less,
Mn (manganese) is a necessary component for refining together with Si in melting stainless steel. Further, when melting from scrap, it is an element that must be contained, and in this case, it is usually contained in an amount of 0.30% or more. However, if added in excess, MnS tends to form which reduces corrosion resistance, so the upper limit of the Mn content is set at 1.10%. The Mn content is preferably 1.00% or less, more preferably 0.80% or less.

P: 0.040% or less,
P (phosphorus) is an element that is inevitably mixed in in the refining process of stainless steel, which uses scrap collected from the city as a raw material. If excessive P is contained, cracks are likely to occur during solidification, so the upper limit of the P content is set at 0.040%. Preferably, the upper limit of the P content is 0.035%.

S: 0.030% or less,
S (sulfur) combines with Mn in steel to form MnS, thereby deteriorating corrosion resistance and hot workability, so the upper limit is set to 0.030%. Further, since S segregates at the grain boundary between austenite and ferrite during hot working and impedes hot workability, the upper limit is preferably 0.010%. More preferably, it is 0.005% or less. Note that the lower limit of the S content is not particularly determined, but it is usually 0.0005% or more because it is difficult to increase the reduction refining time because it is a low C steel type.

Ni: 4.00-5.70%,
Ni (nickel) is a strong austenite stabilizing element, has a high effect of suppressing the formation of ferrite crystals, and is an important element that improves corrosion resistance.In order to obtain these effects, it should be contained at 4.00% or more. . Preferably, Ni is contained in an amount of 4.60% or more. On the other hand, Ni is an expensive element, and if it is contained in a large amount, the Mf point will be below room temperature and a large amount of retained austenite will be produced, resulting in disadvantages such as a decrease in proof strength and a decrease in corrosion resistance due to micro-segregation of components. Additionally, retained austenite may produce deformation-induced martensite during plastic deformation at low temperatures, which is detrimental to maintaining strength in a high-pressure hydrogen environment. Therefore, the upper limit of the Ni content is set to 5.70%. Preferably, the upper limit of the Ni content is 5.10%.

Cr: 17.00-20.50%,
Cr (chromium) is an important element contributing to the corrosion resistance of stainless steel, and in order to obtain corrosion resistance as good as Type 439 (UNS S43035) stainless steel, the lower limit is set to 17.00%. Preferably, the lower limit of the Cr content is 17.50%. On the other hand, when the Cr concentration is high, the amount of ferrite increases, making it difficult to maintain tensile strength in a low-temperature, high-pressure hydrogen environment, so the upper limit is set at 20.50%. Preferably, the upper limit of the Cr content is 19.50%.

As an optional element, Ca: 0.0005 to 0.0030%,
By adding a small amount of Ca (calcium), it combines with S in steel to form CaS, thereby reducing the amount of S dissolved in the steel material and improving hot workability. In the case of the stainless steel of the present application, the addition of Ca suppresses the segregation of S to the grain boundaries by reducing the amount of solid solution S, especially during hot working at temperatures exceeding 1150°C, and has the effect of improving the ductility of the steel material during hot working. Obtained significantly. Note that Ca is not necessarily an element when hot working is performed at 1150° C. or lower.

If you want to obtain the effect of improving hot workability by adding Ca, it is preferable to contain it in an amount of 0.0005% or more. On the other hand, since excessive addition of Ca causes an increase in oxide inclusions in the steel, the upper limit is preferably 0.0030%. When adding Ca, it is preferable to add it simultaneously with B, which also has the effect of suppressing S segregation to grain boundaries.

As an optional element, B: 0.0005 to 0.0050%,
Adding a small amount of B (boron) has the effect of suppressing S segregation to grain boundaries and improving hot workability. In the case of the stainless steel of the present invention, the addition of B suppresses the segregation of S to the grain boundaries by reducing the amount of solid solution S, especially during hot working at temperatures exceeding 1150°C, and has the effect of improving the ductility of the steel material during hot working. Obtained significantly. Note that B is not necessarily a necessary element when hot working is performed at 1150° C. or lower.

As mentioned above, the addition of B is not necessarily essential, but if you want to obtain the effect of improving hot workability by adding B, it is necessary to contain at least 0.0005% or more, preferably 0.0020%. It is better to contain more than that. On the other hand, excessive addition of B causes an increase in unintended inclusions such as boride, so the upper limit is preferably 0.0050%, preferably 0.0040%. When adding B, it is preferable to add it simultaneously with Ca, which also has the effect of suppressing S segregation to grain boundaries.

Formula 1: 3.0>Ni-0.8Cr+11.36>0,
(However, the element symbols in Formula 1 mean the content (%) value of each element.)

Equation 1 defines the amount of ferrite structure to be within a range in which an appropriate metal structure can be obtained, with the martensitic structure as the main phase in the quenching and tempering heat treatment state, while limiting the amount of ferrite structure.

Although Equation 1 is simple, by setting the lower limit to the component range exceeding 0, the metal structure is controlled so that the amount of ferrite structure does not become excessive (for example, 40% or less in terms of area ratio). It becomes possible to make the mechanical properties (tensile strength, 0.2% proof stress) in a low temperature, high pressure hydrogen environment of 90 MPa almost equivalent to that in the atmosphere.

When plastic deformation occurs in iron with a BCC crystal structure in a hydrogen-containing state such as in a high-pressure hydrogen environment, the vacancy concentration within the crystal increases as dislocations move due to plastic deformation. This is widely known as the hole theory. Judging from the fact that the vacancy concentration in steel increases due to plastic deformation in a hydrogen environment, if the vacancy increases due to the influence of hydrogen during plastic deformation, the diffusion of iron atoms will become easier, and the crystal structure will increase. It is thought that this promotes sliding deformation, generates microcracks before tolerance slip occurs, and locally concentrates stress, leading to a decrease in the ductility of the steel material in a hydrogen environment.

Therefore, in order to increase elongation and maintain tensile strength equivalent to that in the atmosphere in hydrogen-containing conditions such as in a high-pressure hydrogen environment in iron with a BCC crystal structure, many dislocations must not move along the same slip plane. Therefore, it is necessary to ensure that as many slip surfaces as possible are active. Considering this, tempered martensitic crystals have more random crystal orientation per volume than ferrite crystals, and have more boundaries such as grain boundaries in an amorphous state, which reduces the decline in elongation in hydrogen. It is advantageous to do so. Therefore, it is better to have as little ferrite as possible. On the other hand, in the martensitic stainless steel for high-pressure hydrogen parts of the present application, the amount of ferrite tends to increase by setting the Cr content to at least 17.00% from the viewpoint of corrosion resistance, so Formula 1 is actively introduced. It was decided to limit the amount of ferrite by setting the lower limit to more than 0.

Furthermore, it is naturally conceivable that the amount of ferrite can be set to an appropriate amount by narrowly controlling the amounts of Cr and Ni added without using Formula 1, but the range of components becomes too narrow and manufacturing becomes difficult. Furthermore, in refining stainless steel, Ni, which is an expensive additive element, cannot be removed. Furthermore, in the reduction refining stage, where the amount of Cr, which is a relatively inexpensive additive element, is determined by controlling the composition of the steel material, there is no other way to lower the concentration other than diluting it. Therefore, in refining steel materials, it is useful to use Equation 1 also from the viewpoint of refining steel materials, where it is necessary to control the amount of Cr so that it does not become too high while looking at the actual value of expensive Ni.

Furthermore, residual γ (austenite) may induce deformation-induced martensite during use and cause a decrease in ductility, so it is preferable to suppress it as much as possible. Regarding the suppression of this residual γ, by setting an upper limit for various elements including Ni and setting the upper limit of the above formula 1 to less than 3.0, it is possible to suppress the structure containing ferrite (for example, % or more).

Next, the stainless steel of the present invention preferably has an absorbed energy of 100 J or more as determined by a V-notch Charpy test at a low temperature of -60°C. High-pressure hydrogen components are exposed to low temperatures not only due to the environmental temperature but also due to the effects of pre-cooling and temperature reduction due to expansion of hydrogen gas when a large amount of high-pressure hydrogen gas is consumed in a short period of time. Therefore, it is unfavorable for safety that the low-temperature toughness of the steel material is low. The KHKS0220 (2020) standard for ultra-high pressure gas equipment states that the toughness that high-pressure hydrogen equipment should satisfy is as follows: 3 pieces in the Charpy impact test of a V-notch with a thickness of 10 mm and a V-notch depth of 2 mm at the minimum design metal temperature. Test pieces were tested, and each test piece had an absorbed energy of 21 J or more, and the average of the three pieces had an absorbed energy of 27 J or more. It goes without saying that this requirement must be met, but in order to ensure the same level as conventional austenitic stainless steel, we decided to make an absorbed energy of 100 J or more an essential characteristic. Thereby, the reliability of the high-pressure hydrogen component can be further improved.

Next, the stainless steel of the present invention preferably has mechanical properties such as a tensile strength of 700 MPa or more, a 0.2% proof stress of 500 MPa or more, and a hardness of 230 HV to 300 HV at room temperature and in the atmosphere.

High-pressure hydrogen parts made of the stainless steel of this application are tested by applying gas pressure exceeding the design pressure during pressure tests, but plastic deformation occurs in the parts during this test, and the original function of the parts is lost before use. It is necessary to prevent damage, and high yield strength is desired. In addition, high tensile strength is preferred for lightweight design of parts. For these reasons, the tensile strength at room temperature in the atmosphere is preferably 700 MPa or more, and the 0.2% proof stress is preferably 500 MPa or more. On the other hand, the upper limits of tensile strength and 0.2% proof stress do not need to be specifically defined, but are preferably limited to 1000 MPa or less because if the tensile strength is too high, machining of the part becomes difficult. In this case, the 0.2% yield strength is also limited to a value lower than that value.

Hardness is measured using a Vickers hardness meter. The hardness at room temperature and in the atmosphere should be 230HV or higher to prevent scratches caused by cutting chips when machining the sealing surface of high-pressure hydrogen parts.On the other hand, if it is too hard, it will be difficult to machine small holes, which are common in high-pressure hydrogen parts. It is preferable to set it to 300HV or less.

Next, it is preferable that the mechanical properties of the stainless steel of the present invention do not deteriorate in a high-pressure hydrogen environment. In other words, if the 0.2% yield strength and tensile strength in a high-pressure hydrogen environment are lower than the 0.2% yield strength and tensile strength in the atmosphere, the strength of steel in the atmosphere cannot be utilized in the high-pressure hydrogen environment. This makes mechanical design and quality assurance of parts extremely difficult. Furthermore, it is generally known that the influence of high-pressure hydrogen on the mechanical properties of steel materials becomes particularly noticeable at low temperatures. Therefore, it is preferable that the tensile strength and 0.2% proof stress in a high-pressure hydrogen environment at low temperatures do not decrease significantly compared to in the air.

Specifically, the tensile strength A1 and 0.2% yield strength A2 in a low-temperature high-pressure hydrogen environment with a temperature of -45°C and a pressure of 90 MPa, and the tensile strength in a low-temperature nitrogen environment with a temperature of -45°C and a pressure of 0.1 MPa. In comparison with B1 and 0.2% proof stress B2, it is preferable that both A1/B1 and A2/B2 are 0.95 or more. By satisfying this ratio, the stainless steel of the present invention is extremely unaffected by high-pressure hydrogen, and even in a high-pressure hydrogen environment, it is possible to ensure mechanical properties that are almost equivalent to those under atmospheric pressure.

Note that the characteristics under a low-temperature, high-pressure hydrogen environment are representative of the characteristics under a low-temperature, high-pressure hydrogen environment with a temperature of -45° C. and a pressure of 90 MPa. In addition, a tensile test was conducted under the same conditions using a test piece having the same shape as above in a low-temperature nitrogen environment with a temperature of -45° C. and a pressure of 0.1 MPa, and used for comparison. The conditions of this low-temperature nitrogen environment assume a low-temperature atmosphere and exclude the influence of ice formation due to atmospheric moisture.

Next, the stainless steel of the present invention preferably has a pitting potential of 185 mV (vs SCE) or higher. The pitting potential is measured by a method based on JIS G0577 (2014, method for measuring pitting potential of stainless steel), as described below. In order to ensure excellent corrosion resistance, the corrosion resistance is preferably equal to or higher than that of Type 439 stainless steel, which has sufficient corrosion resistance in general environments, and the pitting corrosion potential of 185 mV (vs SCE) or higher is set as the preferable range.

Next, it is preferable that the stainless steel of the present application has a metal structure consisting essentially of martensite and ferrite, and the martensite includes a mixture of tempered martensite and fresh martensite. Here, fresh martensite refers to as-quenched martensite, and means a martensite structure newly generated by tempering heat treatment. Further, a metal structure consisting essentially of martensite and ferrite means that the area ratio of the combined phase of martensite (including tempered martensite and fresh martensite) and ferrite is 95% or more.

In order to ensure the low-temperature toughness of stainless steel, it is possible to increase the mobile dislocation density. In order to achieve this, it is effective to suppress the amount of C added in the chemical composition as much as possible and to use a martensitic structure containing an extremely low concentration of C. Further, it is preferable to use a component that slightly produces fresh martensite even when subjected to tempering heat treatment.

In addition, since retained austenite may induce deformation-induced martensite during use, it is preferable to avoid the formation of retained austenite as much as possible. Here, in a martensitic stainless steel containing 17% or more of Cr, such as the stainless steel of the present application, it is difficult to suppress retained austenite in a state of a metal structure with a high phase ratio of martensite. In order to suppress retained austenite, it is preferable to have a structure containing ferrite. That is, it is most preferable that the stainless steel of the present application contains an extremely low concentration of C, contains 17% or more of Cr, and has a metallographic state of "tempered martensite + ferrite + fresh martensite".

The area ratio of the ferrite is preferably in the range of 5% to 40%. When the area ratio of ferrite is less than 5%, there is a problem that the 0.2% yield strength decreases, while when it exceeds 40%, there is a problem that it is difficult to maintain strength in a low temperature and high pressure hydrogen environment. .

Next, the high-pressure hydrogen parts made of the stainless steel of the present invention can be applied to parts for any purpose that are used in contact with high-pressure hydrogen. Specific parts include, for example, pipe joints, valves, pressure accumulators, safety valves, receptacles, filling nozzles, and boosters.

In the method of manufacturing these high-pressure hydrogen parts, at least a part made of a steel material (stainless steel of the present invention) having the above chemical composition is subjected to a quenching heat treatment in which the part is held at a temperature of 900 ° C. to 1020 ° C. and then rapidly cooled, and then, A tempering heat treatment is performed in which the part is held at a temperature of 640° C. to 770° C. and then rapidly cooled.

The decrease in strength associated with the decrease in ductility in a high-pressure hydrogen environment is particularly problematic at low temperatures. It is well known that deformation-induced martensite is the cause of reduced ductility and strength in SUS304 and the like, which are mainly composed of FCC crystals. On the other hand, in the stainless steel of the present invention, which is mainly composed of BCC crystals and has almost no residual austenite, and even if it exists, it is less than 5% and can be ignored, the cause of the decrease in ductility in hydrogen with temperature decrease is This is thought to be due to the fact that lattice vibration decreases as the temperature decreases, making it difficult for dislocation motion to switch between slip planes to occur, and dislocation movement, that is, atomic diffusion, to concentrate on a specific slip plane.

In order to use BCC crystal steel in a high-pressure hydrogen environment, it is necessary to reduce the dislocation density before plastic deformation while maintaining the required strength, or to concentrate the movement of dislocations (diffusion of substitutional atoms) on specific slip planes. The important thing is not to let it happen. It is desirable to actively utilize crystal interfaces where the atomic arrangement is in an amorphous state and satisfies these requirements at the same time. From such a point of view, it is considered desirable to have a martensitic structure as the main phase, which has a feature of having more interfaces such as grain boundaries per volume than a ferrite structure.

The amount of ferrite mixed into the structure reduces the strength in a high-pressure hydrogen environment. Therefore, for steel materials defined by the above-mentioned formula 1 having the above-mentioned composition range, the temperature of the quenching heat treatment is preferably set at 900°C to 1020°C. If the temperature of the quenching heat treatment is less than 900°C, it is likely to be affected by changes in the austenite/ferrite phase ratio during cooling, and there is a risk that the hardness distribution in the cross section of the part will become uneven. If the temperature exceeds .degree. C., the amount of ferrite becomes excessive, which may cause a problem that the strength in high-pressure hydrogen decreases. Therefore, the temperature of the quenching heat treatment is preferably 950 to 1010°C.

Furthermore, water cooling, oil cooling, etc. can be employed as a method for rapidly cooling the parts after holding them in the quenching holding temperature range.

Next, after performing the above-mentioned quenching heat treatment, a tempering heat treatment is performed in which the part is held at a temperature of 640° C. to 770° C. and then rapidly cooled. By carrying out this tempering heat treatment, desired strength and elongation in a low-temperature, high-pressure hydrogen environment and targeted hardness in a room-temperature atmosphere can be obtained.

If the holding temperature in tempering heat treatment is less than 640°C, there is a risk that the hardness will become too hard due to insufficient tempering, and if it exceeds 770°C, fresh martensite will be produced excessively and the hardness will increase. There is a possibility that a problem may occur in which the material becomes too hard. Therefore, the holding temperature in the tempering heat treatment after the quenching heat treatment is preferably 730 to 760°C. Moreover, the rapid cooling in this case can be performed, for example, by water cooling.

Next, after performing the tempering heat treatment, it is preferable to further perform a re-tempering heat treatment in which the part is held at a temperature of 580° C. to 650° C. and then rapidly cooled. This has the effect of reducing the strength difference between the hard tissue and the soft tissue and increasing the yield strength by 0.2%. On the other hand, if the holding temperature of this second tempering heat treatment is less than 580°C, there is a risk that excessive precipitates will be formed and the hardness will become too high, and if it exceeds 650°C, There is a risk that an excessive amount of fresh martensite will be produced and the hardness will become too hard. Therefore, the holding temperature in the tempering heat treatment after the quenching heat treatment is preferably 580 to 620°C. Moreover, the rapid cooling in this case can also be performed by, for example, water cooling.

Examples regarding the stainless steel of the present application will be described.

(Experiment example 1)
In this example, as shown in Table 1, samples were prepared from 16 types of steel materials (Examples 1 to 10, Comparative Examples 1 to 3, and Conventional Examples 1 to 3), and various evaluations were performed.

<Preparation of steel material (sample)>
Except for Examples 1 and 4, each steel material was melted using 30 kg VIM, and was made into a φ25 round bar by hot forging and stretching. The steel material of Example 4 was melted using 2t VIM, and was made into a round bar of φ32 by hot rolling and forging. The steel material of Example 1 was melted using 50tEF-AOD, continuously cast, and then hot rolled into a round bar of φ40. All steel materials were produced by subjecting each round bar to heat treatment under the heat treatment conditions (heat treatment 1 to 3) shown in Table 2.

Heat treatment 1 is a condition for quenching heat treatment, heat treatment 2 is a condition for tempering heat treatment after quenching heat treatment, and heat treatment 3 is a condition for re-tempering heat treatment performed after tempering heat treatment. The steel materials of Examples 1 to 4, 7 to 9, Comparative Examples 1, 2, and 3, and Conventional Example 1 were produced by performing all of the quenching heat treatment, tempering heat treatment, and re-tempering heat treatment. The steel materials of Conventional Examples 2 and 3 were produced by performing only the quenching heat treatment and not performing the subsequent tempering heat treatment. Examples 5, 6, and 10 were produced by performing quenching heat treatment and tempering heat treatment without performing subsequent re-tempering heat treatment. Various evaluations were performed on the obtained steel material.

<Microstructure observation>
Using a sample cut from each round bar-shaped steel material so that the cross section perpendicular to the length direction is exposed as the observation surface, the observation surface is polished and then etched with Murakami reagent, and the observation surface is examined using an optical microscope. The area ratio of ferrite was confirmed by analyzing the image obtained by observing. A case where the area ratio of ferrite was 5% to 40% was evaluated as a pass, and a case outside this range was evaluated as a fail.

<Hardness measurement>
A sample was cut out from each round bar-shaped steel material so that the cross section perpendicular to the length direction served as the measurement surface, and the hardness was measured on the measurement surface using a Vickers hardness meter. A case where the hardness was between 230HV and 300HV was evaluated as a pass, and a case outside this range was evaluated as a failure.

<Tensile test in air>
The tensile strength and 0.2% proof stress at room temperature and in the atmosphere were determined by measuring JIS No. 14A test pieces with a parallel part diameter of φ8 mm in accordance with JIS Z2241 (2011, Metallic Materials Tensile Test Method) from each steel material in the longitudinal direction of the steel material. It was determined by taking a sample so that the distance was in the longitudinal direction of the test piece and performing a tensile test using it. Regarding the tensile strength at room temperature in the atmosphere, a case of 700 MPa or more was evaluated as a pass, and a case of less than 700 MPa was evaluated as a fail. Further, regarding the 0.2% yield strength in the air at room temperature, a case of 500 MPa or more was evaluated as a pass, and a case of less than 500 MPa was evaluated as a fail.

<Tensile test under low temperature high pressure hydrogen environment and low temperature nitrogen environment>
For the tensile test in a low-temperature, high-pressure hydrogen environment, a tensile test piece with a parallel part diameter of φ6 mm and a parallel part length of 30 mm was taken from each steel material so that the length direction of the steel material was the length direction of the test piece, and this was used. . Then, a tensile test was conducted at a slow stroke speed of 0.0015 mm/sec in an apparatus capable of realizing a low-temperature, high-pressure hydrogen environment with a temperature of -45° C. and a pressure of 90 MPa. Such a tensile test at a low strain rate is called SSRT (Slow Strain Rate Testing).

In addition, the tensile test in a low-temperature nitrogen environment was conducted in the same manner as in the low-temperature, high-pressure hydrogen environment described above, except that the atmospheric conditions were changed to a low-temperature nitrogen environment with a temperature of -45°C and a pressure of 0.1 MPa. .

In comparing the tensile strength A1 and 0.2% proof stress A2 in a low-temperature high-pressure hydrogen environment with the tensile strength B1 and 0.2% proof stress B2 in a low-temperature nitrogen environment, A1/B1 and A2/B2 are When both were 0.95 or more, it was evaluated as a pass, and when it was less than 0.95, it was evaluated as a fail. A1/B1 is called relative tensile strength (RTS), and A2/B2 is called relative yield strength (RYS). In all cases where RTS was 0.95 or higher, RYS was 0.95 or higher in all cases, so the value of A1/B1 was entered in the column of "Low temperature SSRT RTS" in Table 3, which will be described later.

<Low temperature Charpy test>
In the low-temperature Charpy test, a V-notch test piece with a thickness of 10 mm and a V-notch depth of 2 mm as specified in JIS Z2242 (2018, Charpy impact test method for metal materials) was taken from each steel material, and using this - The test was carried out in an air atmosphere at 60°C. When the absorbed energy obtained by this V-notch Charpy test was 100 J or more, it was evaluated as a pass, and when it was less than 100 J, it was evaluated as a fail.

<Pitting potential measurement>
The measurement of pitting corrosion potential is based on JIS G0577 (2014, pitting corrosion potential measurement method for stainless steel) using samples taken from each steel material so that the surface parallel to the longitudinal direction of the steel material is the test surface. It was measured using the following method. Specifically, the potential (V'c100) corresponding to a current density of 0.1 mA/cm ² was measured and evaluated as the pitting corrosion potential from the anode polarization curve in a 1 mol/L sodium chloride aqueous solution (30° C.). A 30° C. saturated KCl silver-silver chloride electrode was used as the reference electrode, and the value converted to the saturated Kanko electrode reference (vs SCE) by subtracting 45 mV from the potential value was used as the measured value of the pitting corrosion potential.

The measured value of 185 mV vs SCE for Type 439 stainless steel, which is recognized to have sufficient corrosion resistance in a general environment, was used as the standard, and anything higher than that was evaluated as a pass, and anything less than that was evaluated as a fail.

The above evaluation results are shown in Table 3.

As shown in Tables 1 to 3, Examples 1 to 10 have different melting methods and casting methods, but all have appropriate chemical compositions, satisfy Formula 1, and meet all evaluation criteria. It was found that it has excellent characteristics that pass the test, and that it is possible to overcome technical issues in applying it to high-pressure hydrogen parts and reducing their costs. It was also found that it is not significantly affected by the scale or type of manufacturing process.

From a comparison between Examples 1 to 4 and Examples 5 and 6, it was found that in cases where tempering heat treatment was performed twice after quenching heat treatment (Examples 1 to 4), tempering heat treatment was performed only once after quenching heat treatment. It can be seen that the 0.2% yield strength can be further improved than in the case where the coating is applied (Examples 5 and 6).

Although Examples 7, 8, and 10 have a relatively large amount of ferrite phase due to the influence of the components and heat treatment conditions, it can be seen that even in this case, they have sufficiently excellent characteristics.

Although Example 9 has a relatively small amount of ferrite phase due to the influence of the components and heat treatment conditions, it can be seen that even in this case, it has sufficiently excellent characteristics.

Comparative Example 1 has a high Ni content, does not satisfy Equation 1, and has an excessive martensitic structure, has a low ferrite area ratio, has a low yield strength in the atmosphere, and has low low temperature toughness. Corrosion resistance also decreased. Although the Cr content satisfied the conditions, it is thought that because the C content was high, the Cr content dissolved in solid solution decreased with carbide precipitation, resulting in a decrease in corrosion resistance.

Comparative Example 2 has a chemical composition far below the lower limit of formula 1, so the ferrite area ratio becomes too high, the low-temperature toughness decreases, and the tensile strength decreases in low-temperature high-pressure hydrogen, making it difficult to perform low-temperature SSRT. RTS failed.

Comparative Example 3 had a chemical component composition that greatly exceeded the upper limit of Formula 1, so the ferrite area ratio became too low and the low-temperature toughness decreased.

Conventional Example 1 is an example of martensitic stainless steel of SUS420J2 quenched and tempered material, which is excellent in resource saving, has high strength, and has almost no decrease in strength in hydrogen. However, the result was that the corrosion resistance was insufficient.

Conventional Example 2 is an example of ferritic stainless steel of SUS430J1L annealed material, which is excellent in resource saving and has good corrosion resistance. However, the strength was low, and the strength decreased significantly in a low-temperature hydrogen environment, and the low-temperature toughness and low-temperature SSRT RTS were rejected.

Conventional Example 3 is an austenitic stainless steel of SUS305 solution heat treated material, which is an example of a currently used steel used as high-pressure hydrogen parts. There is no. However, it is found that it contains a lot of Ni and is poor in resource-saving properties, and its strength (tensile strength, proof stress) and hardness in the atmosphere are low in the heat-treated state in the first place.

(Experiment example 2)
From the results of Experimental Example 1, it was confirmed that the stainless steel of the present invention according to the example has excellent mechanical strength in a high-pressure hydrogen environment at low temperatures. In Experimental Example 2, the results of a fatigue characteristic confirmation test under a high-pressure hydrogen environment using the stainless steel of the present invention of Example 4 will be shown.

The fatigue property confirmation test was conducted using a rotating bending fatigue tester (for details, see Japanese Patent Application No. 2021-45278) that uses a magnetic drive that can perform rotating bending tests in a test container filled with high-pressure hydrogen. went. The test conditions were such that rotational bending was applied at a frequency of 16.5 Hz in a high-pressure hydrogen environment of 88 MPa at room temperature in order to easily confirm the influence of the high-pressure hydrogen environment. Note that the test was also conducted in the atmosphere under the same conditions, and the results in the atmosphere and in 88 MPa high-pressure hydrogen were compared.

The test results are shown in Figure 1. In this figure, the number of stress repetitions (times) is plotted on the horizontal axis, and the stress amplitude (MPa) is plotted on the vertical axis. To explain the results around 10 ⁷ cycles in more detail, at a stress amplitude of 500 MPa, in the atmosphere it broke after 9.45 million cycles, but in 88 MPa high pressure hydrogen, it did not break and the durable state was maintained, so there is no breakage in the figure. The meaning is shown with an arrow. At a stress amplitude of 480 MPa, the test was conducted twice each in the air and in high-pressure hydrogen at 88 MPa, and the durability was maintained without any breakage. Indicated. In addition, with a stress amplitude of 460 MPa, tests were conducted once each in the air and in high-pressure hydrogen at 88 MPa, but both tests were not broken and the durability was maintained. I showed you the meaning.

As shown in FIG. 1, the fatigue strength of the stainless steel of the present application did not decrease even in a high-pressure hydrogen environment compared to in the atmosphere. From this, it was confirmed that the stainless steel of the present invention has excellent high-pressure hydrogen properties that are not affected by hydrogen, also in terms of fatigue properties.

(Experiment example 3)
Next, in this example, in order to confirm the importance of heat treatment conditions in the manufacturing process when the chemical composition is appropriate, three types of steel materials were used as shown in Table 4, and multiple different heat treatment conditions were applied. It was applied, manufactured, and its properties were evaluated. Steel material 1, steel material 2, and steel material 3 shown in Table 4 are steel materials having the same chemical composition as Example 2, Example 6, and Example 7 in Experimental Example 1, respectively. As shown in Table 5, the heat treatment conditions were those shown in the columns of Manufacturing Methods 1 to 7. Various evaluations were performed in the same manner as in Experimental Example 1. The results are shown in Table 6. Note that the results of Manufacturing Method 1, Manufacturing Method 2, and Manufacturing Method 3 refer to the results of Example 2, Example 6, and Example 7, respectively.

From the comparison between manufacturing method 1 and manufacturing method 4 using steel material 1, even if the chemical composition is within the same appropriate range, if the tempering temperature is too low, the martensite will not be tempered enough and the low-temperature toughness will decrease. Moreover, it was found that the tensile strength decreased in low-temperature, high-pressure hydrogen, and the RTS of low-temperature SSRT was rejected.

From the comparison between manufacturing method 2 and manufacturing method 5 using steel material 2, even if the chemical composition is within the same appropriate range, if the quenching temperature is too high, the ferrite area ratio becomes too high and the low temperature toughness decreases. Moreover, it was found that the tensile strength decreased in low-temperature, high-pressure hydrogen, causing the RTS of low-temperature SSRT to fail.

From the comparison between manufacturing method 3 and manufacturing method 6 using steel material 3, even if the chemical composition is within the same appropriate range, if the quenching temperature is too low, the ferrite area ratio becomes too high and the low temperature toughness decreases. Moreover, it was found that the tensile strength decreased in low-temperature, high-pressure hydrogen, causing the RTS of low-temperature SSRT to fail.

From the comparison between manufacturing method 2 and manufacturing method 7 using steel material 2, even if the chemical composition is within the same appropriate range, if the tempering temperature is too high, too much fresh martensite will be newly generated by the tempering heat treatment. It was found that the tensile strength and hardness increase, the low-temperature toughness decreases, and the tensile strength decreases in low-temperature, high-pressure hydrogen, causing the RTS of low-temperature SSRT to fail.

(Experiment example 4)
In this example, as shown in Table 7, the steel material of Example 11 was prepared in which trace amounts of Ca and B were added as optional elements. The steel material of Example 11 was manufactured under the same manufacturing conditions as Example 1 in Experimental Example 1, including heat treatment conditions (Heat Treatments 1 to 3), and various test pieces were prepared and evaluated in the same manner as in Experimental Example 1. Ta. The results are shown in Table 8.

As can be seen from Table 8, even when trace amounts of Ca and B were added as optional elements, there was almost no decrease in strength in a high-pressure hydrogen environment, and it was confirmed that the steel had excellent low-temperature toughness and excellent corrosion resistance.

(Experiment example 5)
Next, in this example, in order to confirm the effect of improving the ductility of the steel material of Example 11 by adding trace amounts of Ca and B during hot working, a Greeble test was conducted. The steel materials of Example 1 and Example 2 described above to which Ca and B were not added were also subjected to the same test and compared. Example 1 has an S content of 0.001% and does not contain Ca and B, and Example 2 has an S content of 0.012% and does not contain Ca and B. Example 11 has an S content of 0.001% and contains trace amounts of Ca and B.

The steel materials of Examples 1, 2, and 11 were machined, and cylindrical tensile test pieces with a diameter of 10 mm were obtained from the steel materials and used in the test. In the test, resistance heating was performed by passing electricity through the tensile test piece, heating it from room temperature to the test temperature in 100 seconds, holding it at the test temperature for 60 seconds, and then pulling it at a stroke speed of 50 mm/second until it broke. . After the test, the reduction of area of the test piece was measured, and it was evaluated that the larger the reduction of area, the higher the ductility at the tested temperature. The test temperature was set at 50°C intervals in the range of 900°C to 1300°C.

The measurement results of the aperture are shown in Table 9 and in FIG. 2. In this figure, the horizontal axis shows the test temperature (° C.) and the vertical axis shows the aperture (%), and the measurement results of Examples 1, 2, and 11 are plotted.

From Table 9 and FIG. 2, it can be seen from the comparison between Example 1 and Example 2 that the lower the S content, the larger the reduction of area, and that reducing the S content is advantageous for improving hot workability. . In addition, it can be seen that the addition of trace amounts of Ca and B increases the reduction of area at temperatures exceeding 1150°C, further improving hot ductility.

The deformation resistance (tensile strength) of the stainless steel of the present application in the temperature range from 1150°C to 1300°C is about 2/3 that of general steel materials such as SUS304. From this, for example, when hot rolling a long and slender steel material such as a steel billet as a rolling material, when heating the steel material in a rolling heating furnace, it is important to set the heating temperature too high. There is a risk that the steel material will deform and bend due to its own weight, making it impossible to transport it inside the furnace. Therefore, in order to avoid such problems, it is preferable that the heating temperature during hot rolling is 1150° C. or lower. If 1150°C or less is selected as the heating temperature for hot rolling, it is possible to ensure good hot workability by lowering the S content, and it is possible to actively add trace amounts of Ca and B. It can be said that there is no need to add it.

On the other hand, when hot forging is performed using a steel material such as a round bar cut into short lengths as a forging material, the length of the steel material is short, and there is no fear that it will deform and bend under its own weight during heating. Therefore, when it is desired to form parts with complex shapes with high precision, the heating temperature during hot forging is increased to a temperature exceeding 1150°C to lower the deformation resistance during hot forging, and the forging mold is filled with steel. It is desirable to obtain a comfortable condition. In such a case, if trace amounts of Ca and B are added to the steel material, particularly good hot ductility can be obtained at temperatures exceeding 1150°C and up to 1300°C. Then, it becomes easy to obtain a hot forged product that is free from cracks, including not only the product portion obtained by hot forging but also the so-called burr portion around the product portion.

As described above, when it is desired to increase the heating temperature for hot forging to a temperature exceeding 1150°C to improve hot forging formability, adding trace amounts of Ca and B to the steel material is very effective. Note that this improvement in hot forging formability has the effect of suppressing cracking in the burr portion as described above, but preventing cracking in the burr portion also leads to rationalization in the subsequent burr removal process. That is, in the burr removal step, if there is no crack in the burr portion, it is possible to separate the entire burr portion from the product in a unified state, and the separated burr portion can be easily handled. Therefore, it becomes possible to transport the burr portion using a robot or the like, and there is also an advantage that productivity can be improved.

Claims

In mass%,
C: 0.01-0.05%,
Si: 0.70% or less,
Mn: 1.10% or less,
P: 0.040% or less,
S: 0.030% or less,
Ni: 4.00-5.70%,
Contains Cr: 17.00 to 20.50%,
As an optional element, Ca: 0.0005 to 0.0030%,
Contains B: 0.0005 to 0.0050% as an optional element,
A martensitic stainless steel for high-pressure hydrogen parts, which satisfies the following formula 1 and has a chemical composition with the balance consisting of Fe and inevitable impurities.
Formula 1: 3.0>Ni-0.8Cr+11.36>0,
(However, the element symbols in Formula 1 mean the content (%) value of each element.)
The absorbed energy obtained by a V-notch Charpy test with a thickness of 10 mm and a V-notch depth of 2 mm as described in JIS Z2242 (2018, Charpy impact test method for metal materials) under a low temperature condition of -60 ° C. is 100 J or more,
At room temperature in the atmosphere, it has mechanical properties with a tensile strength of 700 MPa or more, a 0.2% proof stress of 500 MPa or more, and a hardness of 230 HV to 300 HV,
In a tensile test in which a tensile test piece with a parallel part diameter φ6 mm and a parallel part length 30 mm was pulled at a stroke speed of 0.0015 mm/sec, the tensile strength A1 and 0.2% in a low temperature high pressure hydrogen environment at a temperature of -45°C and a pressure of 90 MPa. In comparing yield strength A2 with tensile strength B1 and 0.2% yield strength B2 in a low-temperature nitrogen environment at a temperature of -45°C and a pressure of 0.1 MPa, both A1/B1 and A2/B2 are 0.95 or more. The martensitic stainless steel for high-pressure hydrogen parts according to claim 1.
The martensitic stainless steel for high-pressure hydrogen parts according to claim 1 or 2, which has a pitting potential of 185 mV (vs SCE) or higher.
It has a metal structure substantially consisting of martensite and ferrite, and the martensite includes a mixture of tempered martensite and fresh martensite, and the area ratio of the ferrite is in the range of 5% to 40%. The martensitic stainless steel for high-pressure hydrogen parts according to any one of claims 1 to 3, which is located in.
A high-pressure hydrogen component made of the martensitic stainless steel for high-pressure hydrogen components according to any one of claims 1 to 4.
A method for manufacturing high-pressure hydrogen parts using the martensitic stainless steel for high-pressure hydrogen parts according to any one of claims 1 to 4, comprising:
A part made of a steel material having the above chemical composition is held at a temperature of 900° C. to 1020° C. and then subjected to quenching heat treatment to be rapidly cooled,
A method for producing a high-pressure hydrogen component, wherein the component is then subjected to tempering heat treatment in which the component is held at a temperature of 640° C. to 770° C. and then rapidly cooled.
7. The method for manufacturing a high-pressure hydrogen component according to claim 6, wherein after the tempering heat treatment, a re-tempering heat treatment is further performed in which the component is held at a temperature of 580° C. to 650° C. and then rapidly cooled.