Metastable austenitic stainless steel, and method for manufacturing metastable austenitic stainless steel

Abstract

Metastable austenitic stainless steel consists of the following chemical components by mass: 0.05%≦C≦0.15%, 0.05%≦N≦0.15%, 16%≦Cr≦18%, 0.1%≦Mn≦2%, 4%≦Ni≦6%, 2.5%≦Mo≦3.5%, 0.05%≦Si≦1.0%, with the remainder being Fe and unavoidable impurities. Metastable austenitic stainless steel has a metallic structure in which the austenite phase is the main phase, and a work-induced martensite phase exists, which is a work-induced transformation from the austenite phase. Metastable austenitic stainless steel has a volume ratio of 1 volume% or more and 15 volume% or less of the work-induced martensite phase, and a volume ratio of 85 volume% or more and 99 volume% or less of the austenite phase. When observing the microstructure of the work-induced martensite phase of metastable austenitic stainless steel using TEM, the total number of carbide particles, nitride particles, and carbonitride particles with a short side length or diameter of 5 nm or more is 78,000 nm. 2 Includes 5 or more items within its field of view.

Description

Technical Field

[0001] Embodiments of the present invention relate to a metastable austenitic stainless steel capable of improving the fatigue durability in a welded portion and its vicinity generated by welding when welded in a subsequent process, and a method for manufacturing the same.

Background Art

[0002] Conventionally, various stainless steels have been used as materials for products according to the functions and applications of the products (see Patent Documents 1 to 3). For example, when manufacturing bellows in a form where stainless steel plates are stacked and welded, the stacked stainless steel plates are joined by TIG welding or laser welding. Bellows are components used in manufacturing equipment for semiconductors and perovskite solar cells, etc., and repeatedly expand and contract while maintaining the vacuum level of the movable part of the semiconductor manufacturing equipment. Further, for example, a diaphragm is a member that punches a stainless steel plate into a circular shape and joins the outer periphery of the circle to a flange or the like by TIG welding or laser welding to adjust or sense the pressure or flow rate of a liquid or gas. [[ID=~]]

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0004] In bellows, stress is concentratedly generated at the welded part as the bellows expands and contracts. Therefore, when the expansion and contraction are repeated, the bellows may break from the welded part or its vicinity. In that case, there is a risk that the degree of vacuum of the movable part in the semiconductor manufacturing apparatus cannot be maintained. For example, when the internal degree of vacuum in the semiconductor manufacturing apparatus decreases, it takes several hours to several days or more to recover. Therefore, improvement in the fatigue durability of bellows, specifically, the fatigue durability of the welded part and its vicinity in the welded stainless steel, is required. Also in diaphragms, stress tends to concentrate on the welded part and its vicinity during use, and there is a risk of breakage or the like occurring in these parts. Similarly to bellows, improvement in the fatigue durability of the welded part and its vicinity is required.

[0005] In order to solve such problems, the present invention provides a metastable austenitic stainless steel capable of improving the fatigue durability of the welded part and its vicinity generated by the welding process when welded in a post-process, and a method for manufacturing the same.

Means for Solving the Problems

[0006] The metastable austenitic stainless steel according to one embodiment contains, in terms of mass % of chemical components, 0.05[%] ≤ C ≤ 0.15[%], 0.05[%] ≤ N ≤ 0.15[%], 16[%] ≤ Cr ≤ 18[%], 0.1[%] ≤ Mn ≤ 2[%], 4[%] ≤ Ni ≤ 6[%], 2.5[%] ≤ Mo ≤ 3.5[%], 0.05[%] ≤ Si ≤ 1.0[%], and the balance consists of Fe and inevitable impurities. Such metastable austenitic stainless steel has an austenite phase as the main phase and a metallographic structure in which a strain-induced martensite phase transformation-induced from the austenite phase exists. Such metastable austenitic stainless steel has the strain-induced martensite phase in a volume ratio of 1% by volume or more and 15% by volume or less, and the austenite phase in a volume ratio of 85% by volume or more and 99% by volume or less, respectively. When the strain-induced martensite phase is observed using a transmission electron microscope, such metastable austenitic stainless steel contains 5 or more in total of the number of carbide particles, nitride particles, and carbonitride particles having a short side length or diameter of 5 [nm] or more in a field of view of 78000 [nm 2 and includes 5 or more.

[0007] When the austenite phase or the grain boundary between the austenite phases is observed using the transmission electron microscope, such metastable austenitic stainless steel contains, in total, 5 or more of the number of carbide particles, nitride particles, and carbonitride particles having a short side length or diameter of 5 [nm] or more in a field of view of 78000 [nm 2 and includes 5 or more.

[0008] When the 0.2% proof stress is σ 0.2 [MPa], the maximum tensile strength is σ UTS [MPa], the nominal strain at the 0.2 % proof stress is ε 0.2 [[ID=,20]] [%], and the nominal strain at the maximum tensile strength is ε UTS [%], when the first determination value JV1 is defined by the following formula (1), JV1 = log(σ<, 0.2 ×(1 + ε 0.2 / 100) / (σ UTS ×(1 + ε UTS / 100))) / log(log(1+ε 0.2 / 100) / log(1+ε UTS / 100)) …(1) JV1≦0.3 Satisfying the relationship, The true stress at the 0.2% yield strength is σt 0.2 [MPa], true stress at 5% true strain is σt5[MPa], true strain at 0.2% proof stress is εt 0.2 [%], if we set εt5, which represents a true strain of 5%, to 5[%], and define the second judgment value JV2 by the following equation (2), JV2=(σt5-σt 0.2 ) / (εt5-εt 0.2 ) × 100 …(2) where εt = 5 JV2≦5000 To satisfy the relationship.

[0009] The aforementioned metastable austenitic stainless steel may contain V in a mass percentage of 0.001% or more and 0.5% or less of the chemical composition.

[0010] Furthermore, when the grain size number of the aforementioned metastable austenitic stainless steel is measured by evaluation based on a microscopic test method for grain size, the average grain size obtained by converting the grain size number to the grain size of the crystals is less than the thickness of the plate.

[0011] A method for producing metastable austenitic stainless steel according to one embodiment includes the step of preparing a stainless steel material whose chemical composition, in mass%, is 0.05[%]≦C≦0.15[%], 0.05[%]≦N≦0.15[%], 16[%]≦Cr≦18[%], 0.1[%]≦Mn≦2[%], 4[%]≦Ni≦6[%], 2.5[%]≦Mo≦3.5[%], 0.05[%]≦Si≦1.0[%], with the remainder being Fe and unavoidable impurities. This method for producing metastable austenitic stainless steel includes the step of processing the stainless steel material in a processing rate of 0.01% or more and 7% or less, which is the difference between the cross-sectional area before processing and the cross-sectional area after processing divided by the cross-sectional area before processing, thereby forming a processing-induced martensite phase from the austenite phase. The method for producing such metastable austenitic stainless steel is defined as follows, where the processing temperature is T[K], the holding time is t[hour], the coefficient CV is 20, and the third determination value TP is defined by the following equation (3): TP = T × (CV + log t) …(3) 6800 ≤ TP ≤ 17800 The system includes a step of performing an annealing treatment to satisfy the following relationship. The method for producing such metastable austenitic stainless steel involves producing a stainless steel material having a volume ratio of 1% or more and 15% or less of the work-induced martensite phase, and 85% or more and 99% or less of the austenite phase, and when the structure of the work-induced martensite phase is observed using a transmission electron microscope, the total number of carbide particles, nitride particles, and carbonitride particles with a short side length or diameter of 5 [nm] or more is 78,000 [nm]. 2 The microstructure is defined as having five or more elements within the field of view of [ ].

[0012] The method for producing the metastable austenitic stainless steel involves observing the structure of the austenite phase or the grain boundaries between austenite phases using the transmission electron microscope, and determining that the total number of carbide particles, nitride particles, and carbonitride particles with a short side length or diameter of 5 nm or more is 78,000 nm. 2 The microstructure is defined as having five or more elements within the field of view of [ ].

[0013] The method for producing the metastable austenitic stainless steel involves giving the stainless steel material a metal structure having a volume ratio of 1% or more and 15% or less of the work-induced martensite phase, and a volume ratio of 85% or more and 99% or less of the austenite phase, and a 0.2% proof tolerance of σ 0.2 [MPa], maximum tensile strength σ UTS [MPa], 0.2 % The nominal strain at load-bearing capacity is ε 0.2 [%], nominal strain at maximum tensile strength is ε UTS If we set it as [%] and define the first judgment value JV1 by the following equation (1), JV1=log(σ 0.2 ×(1+ε 0.2 / 100) / (σ UTS ×(1+ε UTS / 100))) / log(log(1+ε 0.2 / 100) / log(1+ε UTS / 100)) …(1) JV1≦0.3 The relationship satisfies, and The true stress at the 0.2% yield strength is σt 0.2 [MPa], true stress at 5% true strain is σt5[MPa], true strain at 0.2% proof stress is εt 0.2 [%], if we set εt5, which represents a true strain of 5%, to 5[%], and define the second judgment value JV2 by the following equation (2), JV2=(σt5-σt 0.2 ) / (εt5-εt 0.2 ) × 100 …(2) where εt = 5 JV2≦5000 The mechanical properties satisfy the following relationship.

[0014] In the method for producing the metastable austenitic stainless steel, the stainless steel material may contain V in a mass percentage of 0.001% or more and 0.5% or less of the chemical composition.

[0015] The method for producing the aforementioned metastable austenitic stainless steel involves determining the particle size number of the stainless steel material by evaluation based on a microscopic test method for grain size, and then determining the average particle size when converted from the particle size number to the grain size, such that the average grain size is less than the thickness of the plate. [Effects of the Invention]

[0016] According to the metastable austenitic stainless steel and its manufacturing method according to the present invention, when welding is performed in a subsequent process, the fatigue durability of the welded portion and its vicinity can be improved. [Brief explanation of the drawing]

[0017] [Figure 1] Figure 1 shows the results of determining whether various samples of metastable austenitic stainless steel belong to the metastable austenitic stainless steel according to this embodiment, based on the first determination value for each sample. [Figure 2] Figure 2 shows the results of determining whether various samples of metastable austenitic stainless steel belong to the metastable austenitic stainless steel according to this embodiment, based on the second determination value for each sample. [Figure 3] Figure 3 shows the final determination result of whether each sample corresponds to the metastable austenitic stainless steel according to this embodiment, based on the determination results of the first and second determination values. [Figure 4] Figure 4 shows the values ​​of the low-temperature annealing treatment temperature (annealing temperature) and holding time (10 [sec]) for calculating the third determination value for a sample of metastable austenitic stainless steel, as well as the value of the third determination value based on these values. [Figure 5] Figure 5 shows the values ​​for the low-temperature annealing treatment temperature (annealing temperature) and holding time (60 [sec]) for calculating the third determination value for a sample of metastable austenitic stainless steel, as well as the value of the third determination value based on these values. [Figure 6]Figure 6 shows the values ​​for the low-temperature annealing treatment temperature (annealing temperature) and holding time (72 hours) for calculating the third determination value for a sample of metastable austenitic stainless steel, as well as the value of the third determination value based on these values. [Figure 7] Figure 7 schematically shows the carbonitride precipitation state (before welding) in an example of metastable austenitic stainless steel according to a comparative example of this embodiment. [Figure 8] Figure 8 schematically shows the carbonitride deposition state near the weld (state after welding) after welding an example of metastable austenitic stainless steel relating to a comparative example of this embodiment shown in Figure 7. [Figure 9] Figure 9 schematically shows the carbonitride precipitation state (state before welding) in an example of metastable austenitic stainless steel according to this embodiment (this example). [Figure 10] Figure 10 schematically shows the state of carbonitride deposition near the weld area (state after welding) resulting from the welding process, as shown in Figure 9. [Figure 11] Figure 11 shows the results of fatigue endurance tests conducted on this case and the comparative example. [Figure 12] Figure 12 shows the relationship between the value of the third judgment value and the fatigue durability of welded metastable austenitic stainless steel. [Modes for carrying out the invention]

[0018] The following describes metastable austenitic stainless steels according to embodiments of the present invention.

[0019] (Composition of chemical components) The metastable austenitic stainless steel according to this embodiment comprises the following chemical components in the following mass ratios (mass%). That is, the metastable austenitic stainless steel has C at 0.05% or more and 0.15% or less (0.05% ≤ C ≤ 0.15%), N at 0.05% or more and 0.15% or less (0.05% ≤ N ≤ 0.15%), Cr at 16% or more and 18% or less (16% ≤ Cr ≤ 18%), and Mn at 0. It consists of 1% or more and 2% or less (0.1% ≤ Mn ≤ 2%), Ni of 4% or more and 6% or less (4% ≤ Ni ≤ 6%), Mo of 2.5% or more and 3.5% or less (2.5% ≤ Mo ≤ 3.5%), and Si of 0.05% or more and 1.0% or less (0.05% ≤ Si ≤ 1.0%). Furthermore, metastable austenitic stainless steel consists of Fe and unavoidable impurities as the remainder of the chemical components other than those listed above.

[0020] However, metastable austenitic stainless steel may also contain V in addition to each of the above chemical components. In this case, the metastable austenitic stainless steel only needs to contain V in a mass ratio (mass%) of 0.001% or more and 0.5% or less (0.001% ≤ V ≤ 0.5%).

[0021] Carbon (C) plays a crucial role in the mechanical stability of austenite and the strength of martensite, which is formed from austenite through work-induced transformation. For this reason, metastable austenitic stainless steel contains 0.05% or more C. On the other hand, if metastable austenitic stainless steel contains an excess of C, the amount of carbide precipitation increases during low-temperature annealing, which will affect the corrosion resistance and fatigue resistance of the stainless steel. Therefore, the C content is kept below 0.15%.

[0022] Nitrogen (N) plays an important role in improving the mechanical stability of austenite and the strength and corrosion resistance of martensite, which is formed by work-induced transformation from austenite. For this reason, metastable austenitic stainless steel contains 0.05% or more N. On the other hand, if metastable austenitic stainless steel contains an excess of N, the amount of nitride precipitated during low-temperature annealing, as described later, increases, which adversely affects the corrosion resistance, fatigue resistance, and toughness of the stainless steel. Therefore, the N content is kept below 0.15%.

[0023] Chromium (Cr) is an important element for achieving corrosion resistance in stainless steel. For this reason, metastable austenitic stainless steel contains 16% or more Cr. On the other hand, if metastable austenitic stainless steel contains an excess of Cr, it promotes the formation of the δ-ferrite phase during annealing and welding, which will be discussed later, and adversely affects the mechanical properties and corrosion resistance of the stainless steel. Therefore, the Cr content is kept below 18%.

[0024] Manganese (Mn) is included in stainless steel for its austenite-stabilizing effect. For this reason, metastable austenitic stainless steel contains 0.1% or more Mn. On the other hand, if metastable austenitic stainless steel contains an excess of Mn, it promotes the formation of MnS, which reduces the corrosion resistance of the stainless steel. Therefore, the Mn content is kept below 2%.

[0025] Nickel (Ni) is included for its effect as an austenite-stabilizing element. For this reason, metastable austenitic stainless steel contains 4% or more Ni. On the other hand, if metastable austenitic stainless steel contains an excessive amount of Ni, the amount of work-induced martensitic transformation relative to the processing rate (cold rolling rate) during cold rolling, as described later, will be suppressed, resulting in failure to obtain the required strength and adverse effects such as increased alloy costs. Therefore, the Ni content of such stainless steel is kept at 6% or less.

[0026] Mo (molybdenum) is included in metastable austenitic stainless steel with the expectation that it will improve the corrosion resistance. For this reason, metastable austenitic stainless steel contains 2.5% or more Mo. On the other hand, if metastable austenitic stainless steel contains an excessive amount of Mo, it promotes the formation of the δ-ferrite phase, which adversely affects the mechanical properties of the stainless steel and increases the cost of the alloy. Therefore, the Mo content is kept below 3.5%.

[0027] Vanadium (V) is included in metastable austenitic stainless steel with the expectation of improving its weldability. For this reason, metastable austenitic stainless steel contains 0.001% or more V. On the other hand, if metastable austenitic stainless steel contains an excessive amount of V, it can adversely affect the fatigue durability of the stainless steel after welding, for example, when the stainless steel is welded. Therefore, the V content is kept below 0.5%.

[0028] Unavoidable impurities may be present in a reasonable range as long as they do not significantly affect the performance of metastable austenitic stainless steel. Examples of unavoidable impurities include Cu (copper), Al (aluminum), P (phosphorus), S (sulfur), and Sn (tin). Of these, P and S can cause grain boundary embrittlement when metastable austenitic stainless steel is welded, by segregating at the grain boundaries during welding. Therefore, it is desirable to limit the P content to 0.04% or less and the S content to 0.03% or less.

[0029] (Metal structure) Metastable austenitic stainless steel is characterized by a microstructure in which the austenite phase is the main phase, with a small amount of work-induced martensite phase present, which is formed by work-induced transformation from the austenite phase. Metastable austenitic stainless steel contains a work-induced martensite phase in a volume ratio of 1% to 15%. Furthermore, metastable austenitic stainless steel contains an austenite phase in a volume ratio of 85% to 99%.

[0030] By setting the volume ratio of the austenite phase to the work-induced martensite phase in this manner, the precipitation of carbides and nitrides into the martensite phase during the low-temperature annealing process described later can be promoted, and the precipitation of carbides and nitrides when metastable austenitic stainless steel is welded can be suppressed. For example, if a microstructure with a low volume ratio of the austenite phase (i.e., a high volume ratio of the work-induced martensite phase) is achieved, a high cold rolling rate is required, which may impair the mechanical isotropy required for bellows, diaphragms, and other components manufactured by welding metastable austenitic stainless steel.

[0031] Furthermore, when the grain size number of metastable austenitic stainless steel is measured by evaluation based on a microscopic test method for grain size, the average grain size of the metastable austenitic stainless steel, when converted from the grain size number to the grain size, is less than the plate thickness. As a result, when bellows or diaphragms manufactured by welding metastable austenitic stainless steel are plastically deformed, surface roughness (orange peel) and a decrease in fatigue durability due to the growth of grain size that occurs during welding can be suppressed.

[0032] Furthermore, when the martensite phase of metastable austenitic stainless steel, which is transformed from the austenite phase by processing, is observed using a transmission electron microscope (TEM), the field of view contains carbide particles, nitride particles, and carbonitride particles. In this embodiment, the total number of carbide particles, nitride particles, and carbonitride particles with a short side length or diameter of 5 nm or more in the metastable austenitic stainless steel is 78,000 nm. 2 The field of view contains five or more of these particles. However, metastable austenitic stainless steel is even better if the total number of such particles in the field of view is preferably 10 or more, and more preferably 15 or more. In the following description, unless otherwise specified, carbides and nitrides are referred to collectively as carbonitrides.

[0033] Furthermore, when the austenite phase or grain boundaries between austenite phases of metastable austenitic stainless steel are observed using a transmission electron microscope (TEM), carbonitride particles are present in the field of view. In this embodiment, the total number of carbonitride particles (number of carbide particles, number of nitride particles, and carbonitride particles) with a short side length or diameter of 5 [nm] or more is 78,000 [nm]. 2 The field of view contains five or more of these particles. However, metastable austenitic stainless steel is even better if the total number of such particles in the field of view is preferably 10 or more, and more preferably 15 or more.

[0034] (mechanical properties) Metastable austenitic stainless steel has a 0.2% yield strength of σ 0.2 If [MPa] is used, σ 0.2 The pressure is 500 MPa or greater and 1000 MPa or less (500 MPa ≤ σ). 0.2 (≤1000 [MPa]). Furthermore, metastable austenitic stainless steel has a ductility of 9 [%] or more and 45 [%] or less (9 [%] ≤ EL ≤ 45 [%]) when ductility is defined as EL.

[0035] Furthermore, metastable austenitic stainless steel has the following mechanical properties. In metastable austenitic stainless steel, the 0.2% yield strength is σ 0.2 [MPa], maximum tensile strength σ UTS [MPa], 0.2 % The nominal strain at load-bearing capacity is ε 0.2 [%], nominal strain at maximum tensile strength is ε UTS Let it be in [%]. Then, the determination value (first determination value) for determining the mechanical properties here will be JV1, and JV1 will be defined as shown in the following equation (1). JV1=log(σ 0.2 ×(1+ε 0.2 / 100) / (σ UTS ×(1+ε UTS / 100))) / log(log(1+ε 0.2 / 100) / log(1+ε UTS / 100)) …(1)

[0036] In the metastable austenitic stainless steel according to this embodiment, the first determination value JV1 is 0.3 or less (JV1 ≤ 0.3). JV1 is the work hardening index (n value) of metastable austenitic stainless steel, and in this embodiment, it is the 0.2% yield strength (σ) of the metastable austenitic stainless steel. 0.2 [MPa]) and maximum tensile strength (σ UTS This is the work hardening index calculated based on [MPa]).

[0037] Furthermore, in metastable austenitic stainless steel, the true stress of the 0.2% yield strength is σt 0.2 [MPa], true stress at 5% true strain is σt5[MPa], true strain at 0.2% proof stress is εt 0.2 [%], and εt5, which represents a true strain of 5%, is set to 5[%]. Then, the judgment value (second judgment value) for determining the mechanical properties here is JV2, and JV2 is defined as shown in the following equation (2). JV2=(σt5-σt 0.2 ) / (εt5-εt 0.2 ) × 100 …(2) where εt = 5

[0038] In the metastable austenitic stainless steel according to this embodiment, the second determination value JV2 is 5000 or less (JV2 ≤ 5000). JV2 is the work hardening rate of metastable austenitic stainless steel, and in this embodiment, it is the true stress (σt) of the 0.2% yield strength in metastable austenitic stainless steel. 0.2 This is the stress increase rate calculated based on the true stress (σt5[MPa]) at a true strain of 5% (εt5).

[0039] In other words, the mechanical properties of metastable austenitic stainless steel are defined by the work hardening index (n value) and the work hardening rate. According to equations (1) and (2), metastable austenitic stainless steel has mechanical properties such that the work hardening index (n value), JV1, is 0.3 or less, and the work hardening rate, JV2, is 5000 or less.

[0040] (Manufacturing method) A general process for producing the metastable austenitic stainless steel according to this embodiment, having the chemical composition, metal structure, and mechanical properties described above, will now be explained. Such metastable austenitic stainless steel is produced through five processes, the first, second, third, fourth, and fifth processes, which are described below.

[0041] The first step is to prepare a stainless steel material consisting of the following chemical components by mass percentage: 0.05[%]≦C≦0.15[%], 0.05[%]≦N≦0.15[%], 16[%]≦Cr≦18[%], 0.1[%]≦Mn≦2[%], 4[%]≦Ni≦6[%], 2.5[%]≦Mo≦3.5[%], 0.05[%]≦Si≦1.0[%], with the remainder being Fe and unavoidable impurities. Such stainless steel material may be obtained by conventional means.

[0042] However, in addition to each of the above chemical components, the stainless steel material may also contain V in a mass ratio (mass%) of 0.001% or more and 0.5% or less (0.001% ≤ V ≤ 0.5%).

[0043] The second step is to cold-roll the stainless steel material having the chemical composition obtained in the first step. The cold rolling here is performed in a step earlier than the cold rolling performed in the fifth step described later (cold rolling with a higher cold rolling rate than the low-rate cold rolling in the fourth step described later). The cold rolling speed is not particularly limited, but for example, it should be between 50 [m / sec] and 6000 [m / sec]. It is known that the amount of work-induced martensitic transformation relative to the processing rate of cold rolling (hereinafter referred to as the cold rolling rate) is less as the processing temperature increases and more as the processing temperature decreases. For this reason, temperature control of the stainless steel material during cold rolling is important in controlling the amount of work-induced martensitic transformation. The processing temperature is preferably in the range of 10 [°C] or higher and 150 [°C] or lower, more preferably in the range of 25 [°C] or higher and 100 [°C] or lower. Within this temperature range, cold rolling is performed while maintaining the temperature as constant as possible, for example, 30°C ± 15°C. The reduction load in cold rolling can be arbitrarily determined in relation to the cold rolling rate and processing temperature, for example, according to the performance and capacity of the rolling mill. Under these rolling conditions, the stainless steel material is cold-rolled until it reaches a predetermined cold rolling rate, for example, 5% or more and 70% or less.

[0044] The third step is to solution anneal the stainless steel material that was cold-rolled in the second step. The solution annealing temperature is in the range of 950°C or higher and 1200°C or lower. The solution annealing time (holding time) is in the range of 1 minute or higher and 5 minutes or lower. The stainless steel material is processed at the above temperature and holding time, and then rapidly cooled. If the solution annealing temperature is below 950°C, the strain in the stainless steel material obtained by cold rolling in the second step will not be removed, or the processing-induced martensite will not undergo reverse transformation to austenite, and a sufficient annealing effect will not be obtained. Consequently, carbonitrides will precipitate, reducing the amount of interstitial elements in the austenite. On the other hand, if the solution annealing temperature exceeds 1200°C, it can lead to the formation of δ-ferrite, which is concentrated with Cr and Mo in addition to austenite, and coarsening of the grain size of the austenite. Therefore, this may worsen the final mechanical properties and corrosion resistance. The atmosphere used in solution annealing is an inert gas, such as hydrogen, argon, nitrogen, or a mixture thereof, to prevent oxidation and decarburization of the material surface. Through solution annealing, the stainless steel material develops a metallic structure with austenite as the dominant phase.

[0045] The second step (cold rolling) and the third step (solution annealing) described above may be omitted or repeated multiple times as appropriate, depending on the composition of the stainless steel material and the thickness of the product.

[0046] The fourth step is to cold-roll the stainless steel material that has been processed to a predetermined cold-rolling ratio in the first three steps. The cold-rolling here is a later step than the cold-rolling performed in the second step described above (hereinafter referred to as low-rate cold-rolling). The basic processing conditions are the same as those for the cold-rolling in the second step.

[0047] The cold rolling ratio for low-rate cold rolling is in the range of 0.01% or more and 7% or less. However, the cold rolling ratio for low-rate cold rolling is preferably in the range of 0.1% or more and 7% or less, and even more preferably in the range of 1% or more and 5% or less. The cold rolling ratio is the value obtained by dividing the difference between the cross-sectional area of ​​the stainless steel material before processing and the cross-sectional area after processing by the cross-sectional area before processing (processing rate). The higher the cold rolling ratio, the higher the proportion of martensite that is transformed from austenite by processing (process-induced martensite). If the cold rolling ratio is less than 0.1%, the amount of processing-induced martensite transformation will be small, and carbonitrides will not precipitate properly on the stainless steel material in the fifth step (low-temperature annealing) described later. On the other hand, if the cold rolling ratio exceeds 7%, the amount of processing-induced martensite transformation will be large. In this case, the strength of the finished metastable austenitic stainless steel will be high, and there is a risk that it may exceed the appropriate strength and mechanical isotropy required for processed products welded from the metastable austenitic stainless steel, such as bellows and diaphragms.

[0048] In the fourth step (low-rate cold rolling), the stainless steel material is given a metallic structure in which the austenite phase is the main phase and a small amount of work-induced martensite phase, which is a work-induced transformation from the austenite phase, is present. The amount of work-induced martensite transformation varies depending on the chemical composition of the stainless steel material obtained in the first step. For example, the work-induced martensite phase is 1 volume% or more and 15 volume% or less. In contrast, the main phase, the austenite phase, is 85 volume% or more and 99 volume% or less. That is, the metastable austenitic stainless steel according to this embodiment has the austenite phase and the work-induced martensite phase, each in a volume ratio within the above ranges.

[0049] The fourth step (low-rate cold rolling), as described above, is a process performed under light reduction, where the cold rolling rate is kept within the range of 0.01% (almost no processing) to a maximum of 7%. Therefore, the processing method for stainless steel materials in this step is not limited to cold rolling. For example, instead of or in addition to cold rolling, the stainless steel material may be processed using a tension leveler to introduce strain or improve the shape of the stainless steel material.

[0050] After the fourth step (low-rate cold rolling) described above, the process moves to the fifth step. However, a predetermined intermediate step may be inserted between the fourth step and the fifth step. For example, press working to form bellows, diaphragms, and other components by welding the metastable austenitic stainless steel according to this embodiment may be performed as an intermediate step.

[0051] The fifth step is to perform low-temperature annealing on the stainless steel material that was cold-rolled in the fourth step. Low-temperature annealing is performed under the following conditions. Under these conditions, the processing temperature for low-temperature annealing is T [K], the holding time is t [hour], and the coefficient CV is 20. The processing temperature T [K] is a value converted using relative temperature (Celsius) [°C] + 273.15. The determination value (third determination value) for determining the conditions of low-temperature annealing is TP, and TP is defined as shown in the following equation (3). TP = T × (CV + log t) …(3)

[0052] In this embodiment, the third determination value TP is 6800 or greater and 17800 or less (6800 ≤ TP ≤ 17800). The third determination value TP is a parameter known as the tempering parameter. Therefore, in the fifth step, annealing (low-temperature annealing) is performed so as to satisfy the relationship that the third determination value TP is 6800 or greater and 17800 or less.

[0053] The processing temperature for low-temperature annealing is in the range of 100°C or higher and 700°C or lower. The processing time (holding time) for low-temperature annealing is in the range of 10 seconds or higher and 259,200 seconds (72 hours) or lower. The holding time depends on the performance of the furnace used for low-temperature annealing, for example, so it can be set to any value within the above range such that the third judgment value TP is 6,800 or higher and 17,800 or lower (6,800 ≤ TP ≤ 17,800), depending on the performance. The atmosphere for low-temperature annealing is an inert gas, such as argon or nitrogen. However, if surface oxidation of the stainless steel material during low-temperature annealing is not a problem, the atmosphere for low-temperature annealing may be air. In addition to low-temperature annealing, pickling, mechanical polishing, or chemical polishing may be performed to remove surface oxidation.

[0054] In the fifth step (low-temperature annealing), the stainless steel material maintains a metallic structure in which the austenite phase is the main phase and a small amount of work-induced martensite phase, which is a work-induced transformation from the austenite phase, is present. However, fine carbonitrides are precipitated within the work-induced martensite phase and at the grain boundaries between the work-induced martensite phase and the austenite phase, resulting in a metastable austenitic stainless steel. As a result, when manufacturing processed products, such as bellows, that are welded from metastable austenitic stainless steel, even if exposed to a predetermined temperature during welding that would cause carbonitride precipitation and growth of the precipitated carbonitrides, the precipitation of coarse carbonitrides in and near the weld (hereinafter collectively referred to as "near the weld") of the metastable austenitic stainless steel can be suppressed. Such predetermined temperatures are, for example, 600°C or higher and 950°C or lower.

[0055] The method for producing metastable austenitic stainless steel according to this embodiment, which involves the five steps described above—the first, second, third, fourth, and fifth steps—is primarily characterized by performing low-temperature annealing on a stainless steel material that has been lightly reduced after solution annealing. In other words, according to this manufacturing method, a portion of the solution-treated austenite phase is transformed into work-induced martensite containing supersaturated interstitial elements by cold rolling. Then, carbonitrides are appropriately precipitated on the stainless steel material during the subsequent low-temperature annealing.

[0056] Therefore, compared to stainless steel manufactured without cold rolling and low-temperature annealing after solution annealing, for example, the stainless steel according to this embodiment suppresses the amount (size and number) of carbonitrides precipitated at the boundary between the heat-affected zone (HAZ) near the weld and the molten zone when bellows are manufactured by welding. As a result, the number of fatigue fracture initiations near the weld is reduced, improving the fatigue durability of bellows and the like.

[0057] Next, we will explain the effects and benefits of the metastable austenitic stainless steel according to the embodiment described above.

[0058] Figure 1 shows the results of determining whether various samples of metastable austenitic stainless steel correspond to the metastable austenitic stainless steel according to this embodiment, based on the first determination value JV1 for each sample. Figure 2 shows the results of determining whether various samples of metastable austenitic stainless steel correspond to the metastable austenitic stainless steel according to this embodiment, based on the second determination value JV2 for each sample. Figure 3 shows the final determination result of whether each sample corresponds to the metastable austenitic stainless steel according to this embodiment, based on the determination results of the first determination value JV1 and the second determination value JV2. Note that in Figures 1, 2, and 3, samples with the same sample number are metastable austenitic stainless steels that have the same chemical composition, metal structure, mechanical properties, and manufacturing method.

[0059] Figure 1 shows the 0.2% yield strength (σ) for calculating the first judgment value JV1 using the above formula (1) for 19 types of metastable austenitic stainless steel samples with sample numbers S101 to S119. 0.2 [MPa]), maximum tensile strength (σ UTS [MPa]), 0.2 % Nominal strain at load-bearing capacity (ε 0.2 [%]), at maximum tensile strength (nominal strain ε UTS The values ​​of each of [%]) and the value of the first judgment value JV1 based on these values ​​are shown.

[0060] As shown in Figure 1, the sample with sample No. S101 has a first judgment value JV1 (n value) of 0.32, which is greater than 0.3 (JV1 > 0.3). Therefore, the first judgment value JV1 (n value) for the sample with sample No. S101 does not fall within the desired range (judgment result is NG). In contrast, the first judgment value JV1 (n value) for all samples with sample No. S102 to S119 is 0.3 or less (JV1 ≤ 0.3). Therefore, the first judgment value JV1 for the samples with sample No. S102 to S119 falls within the desired range (judgment result is OK).

[0061] Figure 2 shows the true stress (σt) of the 0.2% yield strength for calculating the second judgment value JV2 using the above-mentioned formula (2) for 19 types of metastable austenitic stainless steel samples, with sample numbers S101 to S119. 0.2 [MPa]), true stress at 5% true strain (σt5[MPa]), true strain at 0.2% proof stress (εt 0.2 The values ​​for true strain (εt5=5[%]) and true strain (εt5=5[%]) are shown, along with the second judgment value JV2 based on these values.

[0062] As shown in Figure 2, for samples No. S102, S108, and S114, the first judgment value JV1 (n value) is all greater than 5000 (JV2 > 5000). Therefore, for samples No. S102, S108, and S114, the second judgment value JV2 is not within the desired range (judgment result is NG). In contrast, for samples No. S101, S103 to S107, S109 to S113, and S115 to S119, the second judgment value JV2 is all 5000 or less (JV2 ≤ 5000). Therefore, for samples No. S101, S103 to S107, S109 to S113, and S115 to S119, the second judgment value JV2 is within the desired range (judgment result is OK).

[0063] Figure 3 shows the final determination results for whether each sample from sample No. S101 to S119 corresponds to the metastable austenitic stainless steel according to this embodiment, based on the determination results using the first determination value JV1 shown in Figure 1 and the determination results using the second determination value JV2 shown in Figure 2.

[0064] As shown in Figures 1 to 3, sample No. S101 has a second judgment value JV2 within the desired range (judgment result OK), but the first judgment value JV1 is not within the desired range (judgment result NG), therefore it does not qualify as a metastable austenitic stainless steel according to this embodiment (final judgment result NG). Similarly, samples No. S102, S108, and S114 have a first judgment value JV1 within the desired range (judgment result OK), but the second judgment value JV2 is not within the desired range (judgment result NG), therefore none of them qualify as metastable austenitic stainless steel according to this embodiment (final judgment result NG).

[0065] In contrast, for samples S103 to S107, S109 to S113, and S115 to S119, both the first judgment value JV1 and the second judgment value JV2 are within the desired range (all judgment results are OK). Therefore, all of these samples correspond to metastable austenitic stainless steel according to this embodiment (final judgment result is OK).

[0066] Figures 4 to 6 show the third determination value TP for various samples of metastable austenitic stainless steel, and the results of determining whether or not each sample corresponds to metastable austenitic stainless steel according to this embodiment based on the third determination value TP.

[0067] Figure 4 shows the processing temperature (annealing temperature) (T[K]) and holding time (t=10[sec]=10 / 3600[hour]) of the fifth step (low-temperature annealing) used to calculate the third judgment value TP using the above-described equation (3), for 15 types of metastable austenitic stainless steel samples numbered S201 to S215, and the values ​​of the third judgment value TP based on these values. Note that the processing temperature T[K] in equation (3) is the annealing temperature [°C] shown in Figure 4 plus 273.15. The coefficient CV is 20 for all samples.

[0068] As shown in Figure 4, the samples with sample numbers S201 and S202 have a third judgment value TP of less than 6800 (TP < 6800). Also, the sample with sample number S215 has a third judgment value TP exceeding 17800 (TP > 17800). Therefore, the third judgment value TP of the samples with sample numbers S201, S202, and S215 does not fall within the desired range. For this reason, these samples (S201, S202, and S215) do not qualify as metastable austenitic stainless steel according to this embodiment (judgment result is NG).

[0069] In contrast, for samples S203 to S214, the third judgment value TP is always 6800 or greater and 17800 or less (6800 ≤ TP ≤ 17800). Therefore, for samples S203 to S214, the third judgment value TP is within the desired range. For this reason, these samples (S203 to S214) correspond to metastable austenitic stainless steel according to this embodiment (judgment result is OK).

[0070] Figure 5 shows the annealing temperature [°C] and holding time (t=60 [sec]=60 / 3600 [hour]) for the fifth step (low-temperature annealing) used to calculate the third judgment value TP using the above-described equation (3), for 15 types of metastable austenitic stainless steel samples numbered S301 to S315, and the values ​​of the third judgment value TP based on these values. Note that the processing temperature T [K] in equation (3) shown in Figure 5 is the annealing temperature [°C] plus 273.15. The coefficient CV is 20 for all samples.

[0071] As shown in Figure 5, the samples with sample numbers S301 and S302 have a third judgment value TP of less than 6800 (TP < 6800). Also, the sample with sample number S315 has a third judgment value TP exceeding 17800 (TP > 17800). Therefore, the third judgment value TP of the samples with sample numbers S301, S302, and S315 does not fall within the desired range. For this reason, these samples (S301, S302, and S315) do not qualify as metastable austenitic stainless steel according to this embodiment (judgment result is NG).

[0072] In contrast, for samples No. S303 to S314, the third judgment value TP is always 6800 or greater and 17800 or less (6800 ≤ TP ≤ 17800). Therefore, for samples No. S203 to S214, the third judgment value TP is within the desired range. For this reason, these samples (S203 to S214) correspond to the metastable austenitic stainless steel according to this embodiment (judgment result is OK).

[0073] Figure 6 shows the annealing temperature [°C] and holding time (t=72 [hours]) for the fifth step (low-temperature annealing) used to calculate the third judgment value TP using the above-described equation (3), for 15 types of metastable austenitic stainless steel samples numbered S401 to S415, and the values ​​of the third judgment value TP based on these values. Note that the processing temperature T [K] in equation (3) shown in Figure 6 is the annealing temperature [°C] plus 273.15. The coefficient CV value is 20 for all samples.

[0074] As shown in Figure 6, sample No. S401 has a third judgment value TP of less than 6800 (TP < 6800). Also, samples No. S411 to S415 all have a third judgment value TP exceeding 17800 (TP > 17800). Therefore, the third judgment value TP of samples No. S401 and S411 to S415 does not fall within the desired range. For this reason, these samples (S401, S411 to S415) do not qualify as metastable austenitic stainless steel according to this embodiment (judgment result is NG).

[0075] In contrast, for samples No. S402 to S410, the third judgment value TP is always 6800 or greater and 17800 or less (6800 ≤ TP ≤ 17800). Therefore, for samples No. S402 to S410, the third judgment value TP is within the desired range. For this reason, these samples (S402 to S410) correspond to the metastable austenitic stainless steel according to this embodiment (judgment result is OK).

[0076] Figures 7 to 10 schematically show the carbonitride precipitation state in metastable austenitic stainless steel. Figure 7 schematically shows the carbonitride precipitation state (before welding) in an example of metastable austenitic stainless steel according to a comparative example of this embodiment. Figure 8 schematically shows the carbonitride precipitation state (after welding) near the weld in the example of metastable austenitic stainless steel according to a comparative example of this embodiment shown in Figure 7, caused by welding. Figure 9 schematically shows the carbonitride precipitation state (before welding) in an example of metastable austenitic stainless steel according to this embodiment (hereinafter referred to as "this example"). Figure 10 schematically shows the carbonitride precipitation state (after welding) near the weld in the example shown in Figure 9, caused by welding.

[0077] In this example, for instance, when observing the structure of the processing-induced transformation martensite phase using a transmission electron microscope (TEM), the total number of carbonitride particles with a short side length or diameter of 5 nm or more was found to be 78,000 nm. 2 This is a metastable austenitic stainless steel containing five or more, preferably ten or more, and even more preferably fifteen or more, elements within its field of view.

[0078] A comparative example is a metastable austenitic stainless steel that does not fall under the above example, for example, in which the processing temperature of the third step (solution annealing) is higher and the processing temperature (annealing temperature) of the fifth step (low-temperature annealing) is lower (or low-temperature annealing is omitted).

[0079] In Figures 7 to 10, γ represents the austenite phase grains, α' represents the processing-induced martensite phase grains, and δ represents the ferrite phase grains. Also, in Figures 7 to 10, the black circles represent precipitated carbonitrides, and the larger the size or the greater the number of circles, the greater the amount of precipitated carbonitrides.

[0080] As shown in Figures 7 and 9, in both the comparative example and the present example before welding, carbonitrides precipitate in the metastable austenitic stainless steel by performing low-temperature annealing in the fifth step. These carbonitrides precipitate mainly in the work-induced martensite phase, which is a work-induced transformation from the austenite phase, and precipitation at the grain boundaries (boundaries between crystal grains) between the austenite phase and the work-induced martensite phase is suppressed. Carbonitrides precipitate more readily as the processing temperature (annealing temperature) of the low-temperature annealing increases. For this reason, the present example (Figure 9) shows a larger amount of carbonitrides precipitated in the work-induced martensite phase compared to the comparative example (Figure 7) (the black circles shown are larger and more numerous).

[0081] As shown in Figure 8, in the metastable austenitic stainless steel obtained by welding the comparative example shown in Figure 7, many carbonitrides are precipitated at the grain boundaries between the austenite phase and the work-induced martensite phase, and at the grain boundaries between austenite phases (the black circles shown are large and numerous). In contrast, as shown in Figure 10, in the metastable austenitic stainless steel obtained by welding the example shown in Figure 9, only a small amount of carbonitrides are precipitated at the grain boundaries between austenite phases (the black circles shown are small and few in number).

[0082] Thus, compared to the comparative example, this example can suppress the precipitation of carbonitrides at the grain boundaries between the austenite phase and the processing-induced martensite phase, as well as at the grain boundaries between austenite phases, after the welding process.

[0083] For example, when observing the structure of the processing-induced transformation martensite phase using a transmission electron microscope (TEM) in the state before welding shown in Figure 9, the total number of carbonitride particles with a short side length or diameter of 5 [nm] or more was 78,000 [nm]. 2 By using metastable austenitic stainless steel containing 5 or more of these particles within the field of view, the deposition of carbonitrides near the weld caused by welding can be suppressed after welding. When the structure of the work-induced transformation martensite phase of the metastable austenitic stainless steel is observed under the above conditions, the total number of carbonitride particles with a short side length or diameter of 5 [nm] or more is 78,000 [nm]. 2 The field of view of the ] preferably contains 10 or more, and more preferably 15 or more. In addition to having such a metallic structure, when the austenite phase or grain boundaries between austenite phases are observed under the above conditions in the metastable austenitic stainless steel before welding, the total number of carbonitride particles with a short side length or diameter of 5 [nm] or more is 78,000 [nm]. 2 The field of view may include five or more, preferably ten or more, and even more preferably fifteen or more.

[0084] In Figures 7 to 10, the shaded areas in the work-induced martensite phase indicate defects that have formed in the work-induced martensite phase due to processes such as cold rolling. These defective areas are weaker than the surrounding areas. Therefore, as shown in each figure, carbonitrides precipitate in the defective areas of the work-induced martensite phase. When carbonitrides precipitate in the defective areas, the amount of carbon dissolved in the work-induced martensite phase decreases. Furthermore, the more carbonitrides precipitate (in terms of size and number), the greater the amount of carbon consumed from the work-induced martensite phase. Since carbon plays an important role in the strength of the work-induced martensite phase, the more carbon consumed by carbonitrides, the more likely the strength of the work-induced martensite phase is to decrease.

[0085] Therefore, according to this example, after welding, the amount of carbonitrides resulting from the welding process does not precipitate at the grain boundaries between the austenite phase and the work-induced martensite phase, or between austenite phases, as is the case with the metastable austenitic stainless steel after welding shown in Figure 8. In other words, according to this example, the precipitation of a large amount of carbonitrides resulting from the welding process at the grain boundaries after welding can be suppressed. The grain boundaries between the austenite phase and the work-induced martensite phase, and between austenite phases, correspond to fragile areas, similar to the defective areas mentioned above. For this reason, if carbonitrides precipitate at such grain boundaries, the fatigue durability of the metastable austenitic stainless steel after welding decreases, for example, at the boundary between the heat-affected zone (HAZ) and the molten area, near the weld.

[0086] In this example, the precipitation of carbonitrides caused by welding at the grain boundaries between the austenite phase and the work-induced martensite phase, as well as at the grain boundaries between austenite phases, can be suppressed after welding. Therefore, the fatigue durability of metastable austenitic stainless steel near the weld, for example, at the boundary between the heat-affected zone (HAZ) and the molten area, can be improved after welding. In other words, with the metastable austenitic stainless steel according to this example, when welding is performed in a subsequent process, the fatigue durability near the weld created by the welding can be improved.

[0087] For example, let's consider the fatigue durability of a bellows manufactured by welding metastable austenitic stainless steel, as in this case. A bellows is made by punching out ring shapes from thin sheets of metastable austenitic stainless steel, for example, about 0.1 mm to 0.2 mm thick, stacking multiple such ring-shaped sheets, and alternately welding the outer and inner circumferences of the overlapping sheets together to form a bellows-like shape. As such a bellows expands and contracts, the greatest stress is applied to the outer and inner circumferences, which are the welded parts.

[0088] Therefore, when bellows are manufactured by welding thin sheets of metastable austenitic stainless steel as in this example, the fatigue durability of the welded parts of the bellows, i.e., the parts that are subjected to the greatest stress during expansion and contraction, can be improved. As a result, the lifespan of the bellows can be extended, preventing, for example, the semiconductor manufacturing equipment from being shut down for an extended period due to bellows fracture. This also makes it possible to extend the lifespan of the semiconductor manufacturing equipment as a whole.

[0089] Figure 11 shows the results of fatigue endurance tests (simplified accelerated tests) performed on metastable austenitic stainless steel welded together, as described in the present example and comparative example. The fatigue endurance test involves placing a test piece of welded metastable austenitic stainless steel on a pulley and repeatedly applying a predetermined stress, measuring the number of repetitions until the test piece fractures. In Figure 11, the horizontal axis represents the number of repetitions of the endurance test until the test piece fractures, and the vertical axis represents the value of the stress applied to the test piece. According to this fatigue endurance test, when the same stress is applied, a larger number of repetitions, i.e., the number of times until fracture occurs, means that the fatigue endurance of the test piece, i.e., the welded metastable austenitic stainless steel, is higher and the fatigue life is longer. Figure 11 shows the results of the simplified accelerated test described above, and for example, in bellows used in semiconductor manufacturing equipment that repeatedly expands and contracts, it is expected that a life improvement effect of more than 10 times can be obtained depending on the usage conditions.

[0090] In Figure 11, the open white circles indicate the test results of the metastable austenitic stainless steel welded to the present example, and the solid line is a trajectory that schematically shows the fatigue durability of the metastable austenitic stainless steel welded to the present example based on the test results. In Figure 11, the open white squares indicate the test results of the metastable austenitic stainless steel welded to the comparative example, and the dashed line is a trajectory that schematically shows the fatigue durability of the metastable austenitic stainless steel welded to the comparative example based on the test results.

[0091] As shown in Figure 11, the metastable austenitic stainless steel welded to the comparative example fractured after approximately 10,000 cycles when the stress was around 720 to 740 MPa, and after approximately 70,000 cycles when the stress was around 660 MPa. In contrast, the metastable austenitic stainless steel welded to the present example fractured after approximately 20,000 cycles when the stress was around 840 MPa, and after approximately 60,000 cycles when the stress was around 720 MPa. On the other hand, the metastable austenitic stainless steel welded to the present example did not fracture even after more than 10,000,000 cycles when the stress was around 660 MPa. Thus, according to the fatigue endurance test results shown in Figure 11, the welded metastable austenitic stainless steel in the present example has higher fatigue endurance compared to the comparative example.

[0092] Figure 12 shows the relationship between the third judgment value TP and the fatigue durability of the welded metastable austenitic stainless steel. In Figure 12, the horizontal axis shows the value of the third judgment value TP, and the vertical axis shows the fatigue durability value of the metastable austenitic stainless steel welded using the example shown in Figure 11. The fatigue durability value on the vertical axis is the fatigue durability value of the metastable austenitic stainless steel welded using the example shown in Figure 11, with the average fatigue durability of the metastable austenitic stainless steel welded using the comparative example shown in Figure 11 set to 1, and indicates the improvement rate in fatigue durability compared to the comparative example.

[0093] Therefore, if the improvement rate is greater than 1, it means that the fatigue durability of the metastable austenitic stainless steel after welding has improved, and if it is less than 1, it means that the fatigue durability of the metastable austenitic stainless steel after welding has decreased. Fatigue durability is calculated based on the results of the fatigue durability test shown in Figure 11, that is, when a test piece of welded metastable austenitic stainless steel is subjected to a predetermined stress and repeatedly pulled by a pulley, the number of cycles until the test piece breaks.

[0094] In Figure 12, the circles, diamonds, and squares represent the differences in processing time (holding time) for low-temperature annealing (corresponding to the fifth step) performed on stainless steel material before welding. The circles represent 10 seconds, the diamonds represent 60 seconds, and the squares represent 72 hours. When the symbols are the same, the processing time (holding time) is the same for the circles, diamonds, and squares. Therefore, the higher the processing temperature (annealing temperature) for low-temperature annealing, the larger the third judgment value TP becomes (see equation (3)).

[0095] Furthermore, in Figure 12, the black circles, diamonds, and squares each represent the fatigue endurance values ​​when the third judgment value TP is 6800 or greater and 17800 or less (6800 ≤ TP ≤ 17800). In contrast, in Figure 12, the white circles, diamonds, and squares each represent the fatigue endurance values ​​when the third judgment value TP is less than 6800 (TP < 6800) or greater than 17800 (TP > 17800).

[0096] As shown in Figure 12, when the third judgment value TP is less than 6800 (TP < 6800) or greater than 17800 (TP > 17800), regardless of whether the low-temperature annealing treatment time (holding time) is 10 [sec], 60 [sec], or 72 [hours], the fatigue durability of the metastable austenitic stainless steel welded in this example is at most about 1. In other words, in this case, the fatigue durability of the metastable austenitic stainless steel in this example after welding is lower than that of the metastable austenitic stainless steel in the comparative example after welding.

[0097] In particular, if the third judgment value TP is greater than 17800 (TP > 17800), the fatigue durability of the metastable austenitic stainless steel in this case after welding will be less than 1, regardless of whether the low-temperature annealing treatment time (holding time) is 10 [sec], 60 [sec], or 72 [hours]. This is because when the third judgment value TP is greater than 17800 (TP > 17800), it is more likely to lead to coarsening of the carbonitrides precipitated by low-temperature annealing, which worsens the fatigue durability of the metastable austenitic stainless steel before welding.

[0098] In contrast, as shown in Figure 12, when the third judgment value TP is 6800 or greater and 17800 or less (6800 ≤ TP ≤ 17800), the fatigue durability of the metastable austenitic stainless steel welded in this example is greater than 1, regardless of whether the low-temperature annealing treatment time (holding time) is 10 [sec], 60 [sec], or 72 [hours]. In other words, in this case, the metastable austenitic stainless steel in this example after welding exhibits improved fatigue durability compared to the metastable austenitic stainless steel in the comparative example after welding.

[0099] Therefore, in the explanation of Figures 4, 5, and 6 described above, the materials that correspond to the metastable austenitic stainless steel according to this embodiment (those for which the third judgment value TP is judged as OK) can have higher fatigue durability after welding than the comparative examples.

[0100] For example, when a bellows is manufactured by welding a thin sheet of metastable austenitic stainless steel according to this embodiment, the fatigue durability of the welded joint in the bellows can be improved. As a result, the lifespan of the bellows can be extended, preventing, for example, the semiconductor manufacturing equipment from being shut down for an extended period due to bellows fracture.

[0101] Although embodiments of the present invention have been described above, these embodiments are presented as examples and are not intended to limit the scope of the invention. Such novel embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their variations are included in the scope and spirit of the invention, as well as in the claims of the invention and its equivalents.

Claims

1. In terms of mass percentage of chemical components, 0.05[%] ≤ C ≤ 0.15[%], 0.05[%] ≤ N ≤ 0.15[%], 16[%] ≤ Cr ≤ 18[%], 0.1[%] ≤ Mn ≤ 2[%], 4[%] ≤ Ni ≤ 6[%], 2.5[%] ≤ Mo ≤ 3.5[%], 0.05[%] ≤ Si ≤ 1.0[%], with the remainder being Fe and unavoidable impurities. The microstructure consists of an austenite phase as the main phase, with a work-induced martensite phase present, which is formed by a work-induced transformation from the austenite phase. The processing-induced martensite phase is present in a volume ratio of 1% or more and 15% or less, and the austenite phase is present in a volume ratio of 85% or more and 99% or less, When the microstructure of the processing-induced martensite phase was observed using a transmission electron microscope, the total number of carbide particles, nitride particles, and carbonitride particles with a short side length or diameter of 5 nm or more was found to be 78,000 nm. ２ The field of view of ] is characterized by containing five or more of them. Metastable austenitic stainless steel.

2. When the structure of the austenite phase or the grain boundaries between the austenite phases was observed using the transmission electron microscope, the total number of carbide particles, nitride particles, and carbonitride particles with a short side length or diameter of 5 nm or more was found to be 78,000 nm. ２ The field of view of ] is characterized by containing five or more of them. The metastable austenitic stainless steel according to claim 1.

3. 0.2% proof stress σ ０．２ [MPa], maximum tensile strength σ ＵＴＳ [MPa], nominal strain at 0.2% proof stress is ε ０．２ [%], nominal strain at maximum tensile strength is ε ＵＴＳ Let [%] be used, and if the first judgment value, JV1, is defined by the following formula (1), JV1=log(s) ０．２ ×(1+e) ０．２ / 100) / (s ＵＴＳ ×(1+e) ＵＴＳ / 100))) / / oo(ooo(1+5 ０．２ / 100) / L / / 1+M ＵＴＳ / 100)) …(1) JV1 ≤ 0.3 Satisfying the relationship, The true stress at the 0.2% yield strength is σt ０．２ [MPa], true stress at true strain of 5% is σt ５ [MPa], true strain at 0.2% yield strength is εt ０．２ [%], εt representing true strain of 5% ５ If we set this to 5 [%] and define the second judgment value, JV2, by the following formula (2), JV2 = (σt ５ -σt ０．２ ) / (εt ５ -εt ０．２ ) × 100 … (2) where εt = 5 JV2 ≤ 5000 It is characterized by satisfying the following relationship The metastable austenitic stainless steel according to claim 1.

4. This product is characterized by containing V in a mass percentage of 0.001% or more and 0.5% or less of the chemical components. Metastable austenitic stainless steel according to any one of claims 1 to 3.

5. The characteristic feature is that, when the particle size number is measured by evaluation based on a microscopic test method for crystal grain size, the average particle size obtained by converting the particle size number to the crystal grain size is less than the plate thickness. The metastable austenitic stainless steel according to claim 4.

6. A process to prepare a stainless steel material consisting of the following chemical components in mass percent: 0.05[%]≦C≦0.15[%], 0.05[%]≦N≦0.15[%], 16[%]≦Cr≦18[%], 0.1[%]≦Mn≦2[%], 4[%]≦Ni≦6[%], 2.5[%]≦Mo≦3.5[%], 0.05[%]≦Si≦1.0[%], with the remainder being Fe and unavoidable impurities. The process involves processing the stainless steel material in a manner where the processing rate, which is the difference between the cross-sectional area before processing and the cross-sectional area after processing divided by the cross-sectional area before processing, is in the range of 0.01% or more and 7% or less, thereby forming a processing-induced martensite phase from the austenite phase. When the processing temperature is T [K], the holding time is t [hour], the coefficient CV is 20, and the third judgment value TP is defined by the following equation (3), TP=T×(CV+logt)…(3) 6800 ≤ TP ≤ 17800 The process includes an annealing treatment to satisfy the following relationship, The aforementioned stainless steel material, The processing-induced martensite phase is present in a volume ratio of 1% or more and 15% or less, and the austenite phase is present in a volume ratio of 85% or more and 99% or less. When the structure of the processing-induced martensite phase is observed using a transmission electron microscope, the total number of carbide particles, nitride particles, and carbonitride particles with a short side length or diameter of 5 [nm] or more is 78,000 [nm]. ２ The characteristic feature is that the metal structure contains five or more elements within the field of view of the ]. A method for manufacturing metastable austenitic stainless steel.

7. The aforementioned stainless steel material, When the structure of the austenite phase or the grain boundaries between the austenite phases was observed using the transmission electron microscope, the total number of carbide particles, nitride particles, and carbonitride particles with a short side length or diameter of 5 nm or more was found to be 78,000 nm. ２ The characteristic feature is that the metal structure contains five or more elements within the field of view of the ]. A method for producing metastable austenitic stainless steel according to claim 6.

8. The aforementioned stainless steel material, The microstructure is such that the processing-induced martensite phase is present in a volume ratio of 1% or more and 15% or less, and the austenite phase is present in a volume ratio of 85% or more and 99% or less, 0.2% proof stress σ ０．２ [MPa], maximum tensile strength σ ＵＴＳ [MPa], nominal strain at 0.2% proof stress is ε ０．２ [%], nominal strain at maximum tensile strength is ε ＵＴＳ Let [%] be used, and if the first judgment value, JV1, is defined by the following formula (1), JV1=log(s) ０．２ ×(1+e) ０．２ / 100) / (s ＵＴＳ ×(1+e) ＵＴＳ / 100))) / / oo(ooo(1+5 ０．２ / 100) / L / / 1+M ＵＴＳ / 100)) …(1) JV1 ≤ 0.3 The relationship satisfies, and The true stress at the 0.2% yield strength is σt ０．２ [MPa], true stress at true strain of 5% is σt ５ [MPa], true strain at 0.2% yield strength is εt ０．２ [%], εt representing true strain of 5% ５ If we set this to 5 [%] and define the second judgment value, JV2, by the following formula (2), JV2 = (σt ５ -σt ０．２ ) / (εt ５ -εt ０．２ ) × 100 … (2) where εt = 5 JV2 ≤ 5000 It is characterized by having mechanical properties that satisfy the following relationship. A method for producing metastable austenitic stainless steel according to claim 6.

9. The aforementioned stainless steel material is characterized by containing V in a mass percentage of 0.001% or more and 0.5% or less of the chemical component. A method for producing metastable austenitic stainless steel according to any one of claims 6 to 8.

10. The aforementioned stainless steel material, The microstructure is characterized by having a particle size number measured by an evaluation based on a microscopic test method for crystal grain size, where the average particle size obtained by converting the particle size number to the crystal grain size is less than the thickness of the plate. A method for producing metastable austenitic stainless steel according to claim 9.

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