MARTENSITIC STAINLESS STEEL FOR NITROGEN-ENRICHING TREATMENT AND MARTENSITIC STAINLESS STEEL MEMBER

Abstract

 The present invention relates to a martensitic stainless steel for a nitrogen-enriching treatment, consisting of: 0.10 mass% ≤ C ≤ 0.30 mass%; Si ≤ 0.20 mass%; 0.20 mass% ≤ Mn ≤ 1.50 mass%; P ≤ 0.05 mass%; S ≤ 0.01 mass%; Cu ≤ 0.3 mass%; 10.5 mass% ≤ Cr ≤ 17.0 mass%; 0.50 mass% ≤ Ni ≤ 3.00 mass%; 0.50 mass% ≤ Mo ≤ 3.00 mass%; 0.1 mass% ≤ Nb ≤ 0.5 mass%; Ti ≤ 0.5 mass%; and V ≤ 1.0 mass%, with the remainder being Fe and inevitable impurities, having an area ratio of a ferrite phase of 50% or more, and satisfying the following formulae (1) and (2), 1.00 ≤ [Ni]*[Mo] ≤ 9.00 (1), [Nieq]/[Creq] ≥ 1.00 (2), in the formulae, [X] represents a content (mass%) of an element X, [Nieq] = [Ni] + 30[C] + 0.5[Mn] + 8, and [Creq] = [Cr] + [Mo] + 1.5[Si].

Description

TECHNICAL FIELD

[0001] The present invention relates to a martensitic stainless steel for a nitrogen-enriching treatment and a martensitic stainless steel member, and more particularly, to martensitic stainless steel for a nitrogen-enriching treatment that has excellent hot workability, excellent cold workability, and excellent fatigue characteristics, and a martensitic stainless steel member using the same.

BACKGROUND ART

[0002] The martensitic stainless steel refers to a stainless steel having a martensite structure at normal temperature. The martensitic stainless steel is manufactured by performing quenching from an austenite region and tempering. The martensitic stainless steel is excellent in corrosion resistance, strength, wear resistance, and the like, and thus is used for bladed objects, turbine blades, bearings, and the like.

[0003] It is known that corrosion resistance, hardness, wear resistance, and the like are further improved in the case where nitrogen is further solid-dissolved in such a martensitic stainless steel. However, in the case where a method of solid-dissolving nitrogen in molten steel is used, a blowhole may be generated in an ingot. Therefore, a method has been proposed in which after preparing a base material including low-nitrogen martensitic stainless steel, nitrogen is solid-dissolved only in a surface layer portion of the base material, and a layer having a higher nitrogen concentration than the base material (hereinafter, also referred to as a "nitrogen-enriched layer") is formed on the surface layer portion of the base material. In the case where the nitrogen-enriched layer is formed on the surface layer portion of the low-nitrogen base material, corrosion resistance and wear resistance of the base material can be improved without generating blowholes.

[0004] In the related art, various proposals have been made in relation to such a method for forming a nitrogen-enriched layer.

[0005] For example, Patent Literature 1 discloses a method for manufacturing martensitic stainless steel in which (a) a steel wire, which includes a martensitic stainless steel containing predetermined amounts of Mo, V, and Al, is subjected to quenching and tempering, and (b) the steel wire is further subjected to a gas nitriding treatment.

[0006] Patent Literature 1 discloses that a thick spring material having excellent delayed fracture resistance and excellent fatigue resistance can be obtained by such a method.

[0007] Patent Literature 2 discloses a method for manufacturing a martensitic stainless steel member in which (a) a member (annealed material), which includes a martensitic stainless steel containing predetermined amounts of C, Si, Mn, and Cr, is prepared, (b) the member is heated in a nitrogen gas (purity: 99%) at atmospheric pressure, and then furnace cooled to 600°C or lower, and (c) the member is further subjected to quenching and tempering.

[0008] Patent Literature 2 discloses that a martensitic stainless steel member having excellent corrosion resistance and excellent wear resistance can be obtained by such a method.

[0009] Patent Literature 3 discloses a method for manufacturing martensitic stainless steel, which is not a method for forming a nitrogen-enriched layer, but a method in which (a) a steel bar, which includes a martensitic stainless steel containing predetermined amounts of C, Si, Mn, S, P, Ni, Cr, Mo, N, and Al, is prepared, (b) the steel bar is subjected to soft annealing under predetermined conditions, (c) the soft-annealed steel bar is subjected to cold working, and (d) the cold-worked member is subjected to a quenching treatment to increase the hardness, thereby obtaining a final product.

[0010] Patent Literature 3 discloses that (A) in the case where carbonitrides are finely dispersed in the steel obtained after soft annealing, fine carbonitrides perform pinning on a dislocation or a crystal grain boundary movement, and thus the cold workability is reduced, and (B) in the case where the soft annealing is performed at a high temperature, the fine carbonitride is reduced, and thus the cold workability is improved.

[0011] In the case where a treatment (hereinafter, also referred to as "nitrogen-enriching treatment") for forming a nitrogen-enriched layer is performed on a surface of a member including low-nitrogen martensitic stainless steel, wear resistance can be improved. However, in the case where treatment conditions are inappropriate, abnormal grain growth may occur during the nitrogen-enriching treatment. The abnormal grain growth causes deterioration in fatigue characteristics of the member.

[0012] In order to solve this problem, it is also considered that a carbonitride-forming element is added to have a large amount of carbonitride precipitated in the steel such that the carbonitride prevents abnormal grain growth. However, in the case where a precipitation amount of the carbonitride is excessive, hot workability and/or cold workability may be reduced. Further, in the case where the precipitation amount of the carbonitride is excessive, corrosion resistance of the member may also be reduced.

Patent Literature 1: JP2021-143388A

Patent Literature 2: JP2019-167630A

Patent Literature 3: JP2020-050916A

SUMMARY OF INVENTION

[0013] An object of the present invention is to provide a martensitic stainless steel for a nitrogen-enriching treatment that has excellent hot workability and excellent cold workability, and a martensitic stainless steel member using the same.

[0014] Another object of the present invention is to provide a martensitic stainless steel for a nitrogen-enriching treatment that is capable of manufacturing a member having excellent fatigue characteristics, and a martensitic stainless steel member using the same.

[0015] A further object of the present invention is to provide a martensitic stainless steel for a nitrogen-enriching treatment that is capable of manufacturing a member having excellent fatigue characteristics and excellent corrosion resistance, and a martensitic stainless steel member using the same.

[0016] In order to solve the above-mentioned problems, the martensitic stainless steel for a nitrogen-enriching treatment according to the present invention consists of:
0.10 mass% ≤ C ≤ 0.30 mass%, Si ≤ 0.20 mass%, 0.20 mass% ≤ Mn ≤ 1.50 mass%, P ≤ 0.05 mass%, S ≤ 0.01 mass%, Cu ≤ 0.3 mass%, 10.5 mass% ≤ Cr ≤ 17.0 mass%, 0.50 mass% ≤ Ni ≤ 3.00 mass%, 0.50 mass% ≤ Mo ≤ 3.00 mass%, and 0.1 mass% ≤ Nb ≤ 0.5 mass%, Ti ≤ 0.5 mass%, and V ≤ 1.0 mass%, with the remainder being Fe and inevitable impurities, satisfies the following formulae (1) and (2), and has an area ratio of a ferrite phase of 50% or more.

[0017] In the formulae (1) and (2), [X] represents a content (mass%) of an element X, [Nieq] = [Ni] + 30[C] + 0.5 [Mn] + 8, and [Creq] = [Cr] + [Mo] + 1.5[Si].

[0018] The martensitic stainless steel member according to the present invention includes: a base portion including a martensitic stainless steel; and a nitrogen-enriched layer formed on a surface of the base portion, in which the martensitic stainless steel member consists of 0.10 mass% ≤ C ≤ 0.30 mass%, Si ≤ 0.20 mass%, 0.20 mass% ≤ Mn ≤ 1.50 mass%, P ≤ 0.05 mass%, S ≤ 0.01 mass%, Cu ≤ 0.3 mass%, 10.5 mass% ≤ Cr ≤ 17.0 mass%, 0.50 mass% ≤ Ni ≤ 3.00 mass%, 0.50 mass% ≤ Mo ≤ 3.00 mass%, and 0.1 mass% ≤ Nb ≤ 0.5 mass%, Ti ≤ 0.5 mass%, and V ≤ 1.0 mass%, with the remainder being Fe and inevitable impurities, in which the martensitic stainless steel satisfies the following formulae (1) and (2), the base portion has an area ratio of a ferrite phase of 5% or less, and the nitrogen-enriched layer has a thickness of 100 µm or more.

[0019] In the formulae (1) and (2), [X] represents a content (mass%) of an element X,

[0020] In a martensitic stainless steel having a predetermined composition, in the case where an amount of Ni and an amount of Mo are optimized so as to satisfy the formula (1), coarsening of a crystal grain during the nitrogen-enriching treatment can be prevented. As a result, a martensitic stainless steel member having excellent fatigue characteristics is obtained.

[0021] In addition, in the case where a content of each element is optimized so as to satisfy the formula (2), a martensitic stainless steel, which includes a relatively large amount of martensite phases, is obtained in a state after quenching and tempering. As a result, fatigue characteristics of a member using the martensitic stainless steel are improved.

[0022] In the martensitic stainless steel having a predetermined composition, in the case where the composition (in particular, the amount of Ni) is optimized, cold workability of the martensitic stainless steel is improved.

[0023] In addition, in the case where the martensitic stainless steel contains excessive Cu, P, and/or S, hot workability is reduced. In contrast, in the case where an amount of Cu, an amount of P, and an amount of S are set to critical values or less, the hot workability of the martensitic stainless steel is improved.

[0024] Further, in the case where the method and/or the treatment conditions of the nitrogen-enriching treatment are optimized, generation of excessive nitrides in the nitrogen-enriched layer is prevented. As a result, a martensitic stainless steel member having excellent corrosion resistance is obtained.

DESCRIPTION OF EMBODIMENTS

[0025] Hereinafter, an embodiment of the present invention will be described in detail.

[1. Martensitic Stainless Steel for Nitrogen-enriching Treatment]

[1.1. Main Constituent Elements]

[0026] Martensitic stainless steel for a nitrogen-enriching treatment according to the present invention (hereinafter, also simply referred to as "martensitic stainless steel") contains the following elements, with the remainder being Fe and inevitable impurities. Types of additive elements, component ranges thereof, and reasons for limitation thereof are as follows.

[0027] C is an element having an effect of increasing the strength of the martensitic stainless steel. In order to obtain a sufficient effect, an amount of C needs to be 0.10 mass% or more. The amount of C is preferably 0.15 mass% or more.

[0028] On the other hand, in the case where the amount of C is excessive, an amount of non-solid-dissolved Cr carbide may be increased at the time of quenching, and corrosion resistance of a matrix may be reduced. Therefore, the amount of C needs to be 0.30 mass% or less.

[0029] Si is an effective element as a deoxidizing element. However, in the case where an amount of Si is excessive, ductility and toughness may be impaired. Therefore, the amount of Si needs to be 0.20 mass% or less. The amount of Si is preferably 0.15 mass% or less.

[0030] Mn is a deoxidizing element. Mn is also an element that increases a solid solubility limit of nitrogen at the time of a nitrogen-enriching treatment (in particular, a solid-phase nitrogen absorption treatment). In order to obtain a sufficient effect, an amount of Mn needs to be 0.20 mass% or more. The amount of Mn is preferably 0.40 mass% or more.

[0031] On the other hand, in the case where the amount of Mn is excessive, corrosion resistance may be impaired. Therefore, the amount of Mn needs to be 1.50 mass% or less. The amount of Mn is preferably 1.20 mass% or less.

[0032] P is an inevitable impurity. In the case where an amount of P is excessive, a grain boundary may become brittle, and hot workability may be reduced. Therefore, the amount of P needs to be 0.05% or less.

[0033] S is an inevitable impurity. In the case where an amount of S is excessive, a sulfide may be formed, and hot workability and/or corrosion resistance may be reduced. Therefore, the amount of S needs to be 0.01 mass% or less.

[0034] Cu is an inevitable impurity. In the case where an amount of Cu is excessive, non-solid-dissolved Cu in the steel may be precipitated. The non-solid-dissolved Cu is melted during hot working and causes a decrease in hot workability. Therefore, the amount of Cu needs to be 0.3 mass% or less. The amount of Cu is preferably 0.2 mass% or less.

[0035] Cr is an element having an effect of improving corrosion resistance. In order to obtain a sufficient effect, an amount of Cr needs to be 10.5 mass% or more. The amount of Cr is preferably 12.0 mass% or more.

[0036] On the other hand, in the case where the amount of Cr is excessive, a large amount of a ferrite phase may be generated after the quenching and tempering, and the strength may be reduced. Therefore, the amount of Cr needs to be 17.0 mass% or less. The amount of Cr is preferably 16.0 mass% or less.

[0037] Ni is an element effective in improving corrosion resistance. In order to obtain a sufficient effect, an amount of Ni needs to be 0.50 mass% or more. The amount of Ni is preferably 0.70 mass% or more.

[0038] On the other hand, in the case where the amount of Ni is excessive, cold workability may be reduced. Therefore, the amount of Ni needs to be 3.00 mass% or less. The amount of Ni is preferably 2.50 mass% or less.

[0039] Mo is an element having an effect of improving corrosion resistance. In order to obtain a sufficient effect, an amount of Mo needs to be 0.50 mass% or more. The amount of Mo is preferably 0.70 mass% or more.

[0040] On the other hand, in the case where the amount of Mo is excessive, a large amount of a ferrite phase may be generated after the quenching and tempering, and the strength may be reduced. Therefore, the amount of Mo needs to be 3.00 mass% or less. The amount of Mo is preferably 2.50 mass% or less.

[0041] Nb is an element that forms a carbonitride to refine the crystal grain and contributes to improvement in strength. Therefore, an amount of Nb needs to be 0.1 mass% or more. The amount of Nb is preferably 0.15 mass% or more.

[0042] On the other hand, in the case where the amount of Nb is excessive, a coarse carbonitride may be formed, and a crystal-grain-refining effect may be impaired. In addition, the carbonitride may become a starting point of fracture, and thus the strength may be reduced. Therefore, the amount of Nb needs to be 0.5 mass% or less.

[0043] Examples of similar elements having the crystal-grain-refining effect include Ti and V Among these, Nb is advantageous in that a coarse crystallized carbide is less likely to be formed as compared with Ti, and stability of the carbonitride is higher and a precipitated particle is less likely to grow as compared with V. Therefore, Nb is an essential element in the present invention.

[1.2. Inevitable Impurities]

[0044] The term "inevitable impurities" refers to minor components mixed from raw materials and refractory materials when the martensitic stainless steel is prepared. Examples of the inevitable impurities include (a) 0.050 mass% or less of P, (b) 0.010 mass% or less of S, (c) 0.3 mass% or less of Cu (d) 0.05 mass% or less of Al, and (e) 0.030 mass% or less of O.

[1.3. Sub-constituent Element]

[0045] The martensitic stainless steel according to the present invention may further contain one or two or more of the following elements in addition to the above-described elements. Types of additive elements, component ranges thereof, and reasons for limitation thereof are as follows.

[0046] In the case where Ti is further added to the martensitic stainless steel containing Nb, Ti plays an auxiliary role for Nb and has an effect of enhancing the crystal-grain-refining effect. Therefore, the martensitic stainless steel may further contain Ti.

[0047] However, Ti is more likely to form the coarse crystallized carbide as compared with Nb. Therefore, in the case where an amount of Ti is excessive, the fine carbonitride may be reduced, and the crystal grain may be coarsened. Therefore, the amount of Ti is preferably 0.5 mass% or less.

[0048] Similar to Ti, V plays an auxiliary role for Nb and has an effect of enhancing the crystal-grain-refining effect. Therefore, the martensitic stainless steel may further contain V

[0049] However, V causes a reduction in stability of carbonitride at a high temperature and facilitates growth of the precipitated particle as compared with Nb. Therefore, in the case where an amount of V is excessive, the fine carbonitride may be reduced, and the crystal grain may be coarsened. Therefore, the amount of V is preferably 1.0 mass% or less.

[1.4. Component Balance]

[0050] The martensitic stainless steel according to the present invention satisfies the following formulae (1) and (2),

and

[0051] In the formulae (1) and (2), [X] represents a content (mass%) of an element X,

[1.4.1. Formula (1)]

[0052] [Ni]*[Mo] correlates with an average crystal grain size of the martensitic stainless steel after the nitrogen-enriching treatment. In the case where [Ni]^∗[Mo] is too small, the crystal grain after the nitrogen-enriching treatment may be coarsened. As a result, in the case where repeated stress acts on the member after the nitrogen-enriching treatment, grain boundary fracture may progress. Therefore, [Ni]^∗[Mo] needs to be 1.00 or more. [Ni]^∗[Mo] is preferably 1.05 or more, and more preferably 1.10 or more.

[0053] On the other hand, in the case where [Ni]*[Mo] is too large, the crystal grain is coarsened before the nitrogen-enriching treatment, and even after the nitrogen-enriching treatment is performed, a fine crystal grain may not be obtained. Therefore, [Ni]*[Mo] needs to be 9.00 or less. [Ni]*[Mo] is preferably 6.00 or less, and more preferably 4.00 or less.

[1.4.2. Formula (2)]

[0054] [Nieq] represents a Ni equivalent serving as an index of an austenite stabilizing element. [Creq] represents a Cr equivalent serving as an index of a ferrite stabilizing element. In the case where [Nieq]/[Creq] is too small, a residual amount of a ferrite phase in a quenched structure may be increased, and fatigue strength may be reduced. Therefore, [Nieq]/[Creq] needs to be 1.00 or more. [Nieq]/[Creq] is preferably 1.02 or more, and more preferably 1.05 or more.

[1.5. Area Ratio of Ferrite Phase]

[0055] Since the components of the martensitic stainless steel according to the present invention are optimized, an area ratio of the ferrite phase at 23°C after annealing (before nitrogen-enriching treatment) is 50% or more. In the case where manufacturing conditions are optimized, the area ratio of the ferrite phase may be 70% or more or 90% or more. In addition, in the case where the components are optimized, the area ratio of the ferrite phase may be 100%. The "area ratio of ferrite phase" will be described later.

[0056] The martensitic stainless steel according to the present invention has a relatively large area ratio of the ferrite phase (that is, a relatively small area ratio of a martensite phase) in a state after annealing, and thus has excellent cold workability.

[1.6. Application]

[0057] The martensitic stainless steel according to the present invention is used for performing a nitrogen-enriching treatment on a surface of a base portion after the base portion is prepared by annealing and cold working. After the nitrogen-enriching treatment, the quenching and tempering are further performed. When the nitrogen-enriching treatment is performed, a layer (nitrogen-enriched layer), which has a higher nitrogen concentration than a core portion of the base portion, is formed in the surface layer portion of the base portion. Further, when the quenching and tempering are performed, the nitrogen-enriched layer is hardened. As a result, a member having excellent fatigue characteristics is obtained.

[2. Martensitic Stainless Steel Member]

[0058] The martensitic stainless steel member according to the present invention has the following configuration.

(1) The martensitic stainless steel member includes a base portion including martensitic stainless steel and a nitrogen-enriched layer formed on a surface of the base portion.

(2) The martensitic stainless steel consists of 0.10 mass% ≤ C ≤ 0.30 mass%, Si ≤ 0.20 mass%, 0.20 mass% ≤ Mn ≤ 1.50 mass%, P ≤ 0.05 mass%, S ≤ 0.01 mass%, Cu ≤ 0.3 mass%, 10.5 mass% ≤ Cr ≤ 17.0 mass%, 0.50 mass% ≤ Ni ≤ 3.00 mass%, 0.50 mass% ≤ Mo ≤ 3.00 mass%, and 0.1 mass% ≤ Nb ≤ 0.5 mass%, Ti ≤ 0.5 mass%; and V ≤ 1.0 mass%, with the remainder being Fe and inevitable impurities, and satisfies the following formulae (1) and (2).

(3) The base portion has an area ratio of a ferrite phase of 5% or less.

(4) A thickness of the nitrogen-enriched layer is 100 µm or more.

[0059] In the formulae (1) and (2), [X] represents a content (mass%) of an element X,

[2.1. Base Portion]

[2.1.1. Material]

[0060] The base portion includes the martensitic stainless steel according to the present invention. Since details of the martensitic stainless steel are as described above, the description thereof is omitted.

[2.1.2. Area Ratio of Ferrite Phase]

[0061] The "area ratio of a ferrite phase (%)" refers to a ratio of an area of a ferrite phase to an area of a cross section of the martensitic stainless steel at 23°C.

[0062] In other words, the "area ratio of a ferrite phase (%)" refers to a value (= S × 100/S₀) obtained by (a) cutting the martensitic stainless steel after the annealing or the martensitic stainless steel member after the nitrogen-enriching treatment and the quenching and tempering in a direction perpendicular to the surface, followed by polishing and etching the cross section, (b) observing a region near the center of the cross section (in the member subjected to the nitrogen-enriching treatment, region (core portion) having a nitrogen concentration at a level before the nitrogen-enriching treatment) at room temperature with an optical microscope, followed by calculating a viewing area (S₀) and an area (S) of the ferrite phase included in a field of view, and (c) dividing S by S₀.

[0063] In the present invention, S₀ is 2 mm × 3 mm.

[0064] Since the composition of the martensitic stainless steel according to the present invention is optimized, the area ratio of the ferrite phase is relatively high in a state after the annealing. On the other hand, after the nitrogen-enriching treatment and the quenching and tempering, almost an entire surface of the base portion becomes a martensite phase, and the area ratio of the ferrite phase of the base portion becomes 5% or less. In the case where the composition of the martensitic stainless steel is optimized, the area ratio of the ferrite phase in the base portion after the nitrogen-enriching treatment and the quenching and tempering is 0.5% or less.

[2.1.3. Average Crystal Grain Size]

[0065] The "average crystal grain size" refers to an average value of grain sizes of prior austenite crystal grains, which is a value obtained by measuring a crystal grain size based on an optical microscopic photograph with a cutting method using a linear test line described in JIS G0551:2020, and performing conversion based on the crystal grain size using Table A.1 of Annex A in JIS G0551:2020.

[0066] The average crystal grain size of the core portion of the base portion after the nitrogen-enriching treatment affects the fatigue characteristics of the martensitic stainless steel member. In general, the smaller the average crystal grain size of the core portion of the base portion after the nitrogen-enriching treatment, the better the fatigue characteristics of the martensitic stainless steel member. In the martensitic stainless steel member according to the present invention, since an element having an effect of refining the crystal grain is added, grain growth during the nitrogen-enriching treatment is prevented. In the case where the manufacturing conditions are optimized, the average crystal grain size of the core portion of the base portion after the nitrogen-enriching treatment may be 50 µm or less. In the case where the manufacturing conditions are further optimized, the average crystal grain size may be 30 µm or less.

[2.2. Nitrogen-enriched Layer]

[2.2.1. Definition]

[0067] The "nitrogen-enriched layer" refers to a layer formed by subjecting the base portion after the annealing and the cold working to the nitrogen-enriching treatment, which is a region (region having a nitrogen concentration at a level before the nitrogen-enriching treatment) having a higher nitrogen concentration than the core portion of the base portion. More specifically, the "nitrogen-enriched layer" refers to a region in which an amount of N is 0.1 mass% or more and 1.0 mass% or less.

[0068] In the case where the amount of N in the nitrogen-enriched layer is excessive, problems may occur such as that (a) wear resistance is reduced because martensitic transformation does not proceed during the quenching, and (b) corrosion resistance is reduced because a large amount of nitride is precipitated. Therefore, the amount of N in the nitrogen-enriched layer needs to be 1.0 mass% or less.

[0069] The "nitrogen-enriching treatment" refers to a treatment of solid-dissolving nitrogen in a surface layer portion of the base portion. Examples of a nitrogen-enriching treatment method include a solid-phase nitrogen absorption method, a gas nitriding method, a soft nitriding method, and an ion nitriding method. Among these, the solid-phase nitrogen absorption method is preferred as the nitrogen-enriching treatment method because a member having excellent corrosion resistance can be obtained. The solid-phase nitrogen absorption method is advantageous in that precipitation of chromium carbonitride can be prevented and the wear resistance can be improved without reducing corrosion resistance of the outermost surface. Details of the solid-phase nitrogen absorption method will be described later.

[2.2.2. Thickness]

[0070] The "thickness of the nitrogen-enriched layer" refers to a thickness of a region that is formed on the surface of the base portion by subjecting the base portion to the nitrogen-enriching treatment and has an amount of N of 0.1 mass% or more and 1.0 mass% or less.

[0071] The thickness of the nitrogen-enriched layer affects wear resistance of the member. In the case where the thickness of the nitrogen-enriched layer is too thin, the wear resistance becomes insufficient. Therefore, the thickness of the nitrogen-enriched layer is 100 µm or more. The thickness of the nitrogen-enriched layer is preferably 150 µm or more, and more preferably 200 µm or more.

[3. Method for Manufacturing Martensitic Stainless Steel]

[0072] The martensitic stainless steel according to the present invention can be manufactured by (a) melting and casting raw materials blended so as to have a predetermined composition, (b) performing hot forging on the obtained ingot, and (c) performing annealing on the hot-forged roughly shaped material.

[3.1. Melting and Casting Process]

[0073] First, the raw materials blended so as to have a predetermined composition are melted and cast. Methods and conditions of the melting and casting are not particularly limited, and an optimum method and optimum conditions can be selected in accordance with a purpose.

[3.2. Hot Forging Process]

[0074] Next, the obtained ingot is hot-forged. The hot forging is performed in order to destroy a casting structure and process the ingot into a roughly shaped material having a predetermined shape. Methods and conditions of the hot forging are not particularly limited, and optimum conditions can be selected in accordance with a purpose.

[3.3. Annealing Process]

[0075] The martensitic stainless steel according to the present invention has an excessively high hardness in a state as hot-forged, and is poor in cold workability. This is because a martensite phase is partially formed in a cooling step after hot forging. In order to improve the cold workability, it is necessary to increase the area ratio of the ferrite phase by performing the annealing on the hot-forged roughly shaped material.

[0076] Annealing conditions are not particularly limited as long as necessary cold workability is obtained.

[0077] Specifically, the annealing is preferably performed by (a) holding the roughly shaped material at a temperature of 850°C or more and 950°C or less for 1 hour or more and 10 hours or less, and (b) gradually cooling the roughly shaped material at an average cooling rate of 30°C/h or less in a temperature range from an annealing temperature to 650°C.

[4. Method for Manufacturing Martensitic Stainless Steel Member]

[0078] The martensitic stainless steel member according to the present invention can be manufactured by (a) performing cold working on the annealed roughly shaped material, (b) performing the nitrogen-enriching treatment on the cold-worked member, and (c) performing the quenching and tempering on the nitrogen-enriched member.

[4.1. Cold Working Process]

[0079] First, the cold working is performed on the annealed roughly shaped material. Methods and conditions of the cold working are not particularly limited as long as a member having a predetermined shape can be manufactured.

[4.2. Nitrogen-enriching Treatment Process]

[0080] Next, the nitrogen-enriching treatment is performed on the cold-worked member. Examples of a nitrogen-enriching treatment method include a solid-phase nitrogen absorption method, a gas nitriding method, a soft nitriding method, and an ion nitriding method. In the present invention, any of the methods may be used. In order to obtain a member having excellent corrosion resistance, the solid-phase nitrogen absorption method is preferable as the nitrogen-enriching treatment method.

[0081] The solid-phase nitrogen absorption treatment is performed by heating the member to a predetermined temperature in a predetermined nitrogen atmosphere. Examples of the nitrogen atmosphere include a nitrogen gas atmosphere.

[0082] A heating temperature affects the nitrogen concentration on a member surface. Therefore, it is preferable to select an optimum temperature as the heating temperature in accordance with the composition of the member. In general, the lower the heating temperature, the higher an equilibrium nitrogen concentration on the member surface. However, in the case where the heating temperature is too low, a large amount of nitride may be precipitated and the corrosion resistance may be reduced. Therefore, the heating temperature is preferably 900°C or higher.

[0083] On the other hand, in the case where the heating temperature is too high, the equilibrium nitrogen concentration on the member surface may be reduced and the hardness may be reduced. Therefore, the heating temperature is preferably 1,200°C or less.

[0084] A nitrogen partial pressure affects the nitrogen concentration on the member surface. Therefore, it is preferable to select an optimum value as the nitrogen partial pressure in accordance with the composition of the member. In general, the higher the nitrogen partial pressure, the higher the equilibrium nitrogen concentration on the member surface. In order to obtain such an effect, the nitrogen partial pressure is preferably 0.1 atm (0.01 MPa) or more.

[0085] On the other hand, in the case where the nitrogen partial pressure is too high, the nitrogen concentration is excessively increased and a martensite-transformation-starting temperature is lowered. As a result, an austenite phase tends to remain after the quenching, and the hardness may be reduced. Therefore, the nitrogen partial pressure is preferably 3.0 atm (0.3 MPa) or less.

[0086] The nitrogen concentration on the member surface is determined by the heating temperature and the nitrogen partial pressure. On the other hand, a processing time affects a thickness of a nitrogen absorption layer (nitrogen-enriched layer). Therefore, it is preferable to select an optimum time as the processing time in accordance with a required thickness of the nitrogen absorption layer. The processing time is usually about 60 minutes to 600 minutes.

[4.3. Quenching and Tempering Process]

[0087] Next, the quenching and tempering are performed on the nitrogen-enriched member. Thus, the austenite phase undergoes martensitic transformation. Quenching conditions are not particularly limited as long as the surface layer portion and the core portion of the nitrogen-enriched member can be transformed into martensite.

[0088] For example, in the case where the solid-phase nitrogen absorption treatment is used as the nitrogen-enriching treatment method, quenching may be performed by rapidly cooling the member from a treatment temperature. Alternatively, the quenching may be performed by performing the solid-phase nitrogen absorption treatment, cooling the member to around room temperature, then reheating the member to a quenching temperature, and rapidly cooling the member. For rapid cooling, gas cooling, water cooling, ice water cooling, oil cooling, or the like may be used.

[0089] In order to transform an entire surface of the core portion of the member into a martensite structure, a cooling rate at the time of quenching is preferably as high as possible. Specifically, an average cooling rate from the quenching temperature to 500°C is preferably 200°C/min or more.

[0090] Before performing the quenching, a nitrogen diffusion treatment for diffusing nitrogen into an interior of the member may be performed. Specifically, following the solid-phase nitrogen absorption treatment, nitrogen is diffused into the interior of the member by holding the member at a high temperature of about 900°C to 1,200°C in an inert atmosphere such as argon gas. In the case where the nitrogen diffusion treatment is performed, the nitrogen absorption layer becomes thick, so that a hardness of the member surface after the quenching can be stably obtained.

[0091] Further, a sub-zero treatment in which a material is held at 0°C or lower after the quenching may be performed.

[0092] Next, the tempering is performed after the quenching (or after the sub-zero treatment). Tempering conditions are not particularly limited, and an optimal condition can be selected in accordance with a purpose. Specifically, it is preferable to perform the tempering by holding the member at a temperature of 150°C to 500°C for about 1 hour to 3 hours.

[5. Effect]

[0093] In a martensitic stainless steel having a predetermined composition, in the case where an amount of Ni and an amount of Mo are optimized so as to satisfy the formula (1), coarsening of a crystal grain during the nitrogen-enriching treatment can be prevented. As a result, a martensitic stainless steel member having excellent fatigue characteristics is obtained.

[0094] In addition, in the case where a content of each element is optimized so as to satisfy the formula (2), a martensitic stainless steel including a relatively large amount of martensite phases is obtained in a state after a quenching and tempering. As a result, fatigue characteristics of a member using the martensitic stainless steel are improved.

[0095] In the martensitic stainless steel having a predetermined composition, in the case where the composition (in particular, the amount of Ni) is optimized, cold workability of the martensitic stainless steel is improved.

[0096] In addition, in the case where the martensitic stainless steel contains excessive Cu, P, and/or S, hot workability is reduced. In contrast, in the case where an amount of Cu, an amount of P, and an amount of S are set to critical values or less, the hot workability of the martensitic stainless steel is improved.

[0097] Further, in the case where the method and/or the treatment conditions of the nitrogen-enriching treatment are optimized, generation of excessive nitrides in the nitrogen-enriched layer is prevented. As a result, a martensitic stainless steel member having excellent corrosion resistance is obtained.

EXAMPLES

(Examples 1 to 14 and Comparative Examples 1 to 16)

[1. Preparation of Sample]

[0098] In a vacuum induction furnace, 50 kg of steels each having compositions shown in Tables 1 and 2 were melted. Each of the obtained ingots was subjected to hot forging to thereby manufacture each bar material having a diameter of 20 mm. Next, each of the bar material was heated to 900°C and then subjected to soft annealing in which the bar material was gradually cooled to 650°C at 20°C/h. Test pieces each having a diameter of 15 mm and a length of 8 mm were obtained from each of the bar materials.

[0099] Next, each test piece was subjected to solid-phase nitrogen absorption treatment and quenching. That is, first, the test piece was placed in a treatment chamber, and the treatment chamber was evacuated by a depressurizing unit. Next, nitrogen gas was introduced into the treatment chamber, and a pressure and a temperature in the treatment chamber were maintained at predetermined values.

[0100] A nitrogen partial pressure was appropriately adjusted between 0.1 atm to 3.0 atm (0.01 MPa to 0.30 MPa), and a treatment temperature was appropriately adjusted between 900°C to 1,200°C, thereby obtaining a nitrogen absorption layer in which an amount of N in a range of at least 100 µm from a member surface was 0.1 mass% to 1.0 mass%. A heating time was adjusted such that a thickness of the nitrogen absorption layer after quenching was 100 µm or more.

[0101] Further, after the solid-phase nitrogen absorption treatment was completed, the test piece was subjected to quenching. The quenching was performed by gas cooling. Thereafter, a sub-zero treatment at -80°C for 2 hours and tempering at 200°C were performed.

[2. Test Method]

[2.1. Ferrite Area Ratio of Core Portion of Quenched Structure]

[0102] The test piece after the solid-phase nitrogen absorption treatment and the quenching and tempering was cut, and an area ratio of a ferrite phase in the core portion was calculated.

[2.2. Average Crystal Grain Size after Nitrogen Absorption Treatment]

[0103] The test piece after the solid-phase nitrogen absorption treatment was cut, and an average crystal grain size of the core portion was calculated. The average crystal grain size was measured by a line intercept method. First, a photograph of a structure of the core portion of the test piece after the nitrogen absorption treatment was captured with an optical microscope (magnification: 100 times). Next, a total of ten different straight lines of five vertical lines and five horizontal lines were drawn on the captured photograph, and for each of the straight lines, a value (= L/n) obtained by dividing a length (L) of the straight line by the number (n) of crystal grain boundaries intersecting the straight line was calculated. Further, the average crystal grain size was calculated by calculating an average value thereof.

[2.3. Rotating Bending Fatigue Test]

[0104] A rotating bending fatigue test was performed by a test method specified in JIS Z2274:1978. A fracture surface was observed, and the starting point of a fracture was confirmed.

[2.4. 5% Sulfuric Acid Immersion Test]

[0105] A corrosion loss was calculated for each sample after the solid-phase nitrogen absorption treatment according to the following procedure. That is, a mass before a corrosion test of the test piece after the solid-phase nitrogen absorption treatment was measured. Next, the test piece was immersed in a 5% sulfuric acid aqueous solution maintained at 30°C in a thermostatic bath for 6 hours. Thereafter, corrosion products adhered to the test piece were removed by ultrasonic cleaning, and a mass of the test piece after the corrosion test was measured. An amount of mass loss of the test piece was calculated and divided by a surface area and a time to thereby calculate the corrosion loss.

[2.5. Hot Workability]

[0106] Presence or absence of a crack in the bar material after hot working was visually confirmed.

[2.6. Structure after Soft Annealing (Cold Workability)]

[0107] Presence or absence of a ferrite phase in the bar material after soft annealing was confirmed by optical microscopic observation. Further, the area ratio of the ferrite phase in the core portion was calculated.

[3. Results]

[0108] Results are shown in Tables 3 and 4. From Tables 3 and 4, the followings can be understood.

[0109] In Tables 3 and 4, regarding "ferrite area ratio of core portion of quenched structure", "AA" indicates that the ferrite area ratio is 0.5% or less, "A" indicates that the ferrite area ratio is more than 0.5% and 5% or less, and "C" indicates that the ferrite area ratio is more than 5%.

[0110] Regarding "average crystal grain size after nitrogen absorption treatment", "AA" indicates that the average crystal grain size is 30 µm or less, "A" indicates that the average crystal grain size is more than 30 µm and 50 µm or less, and "C" indicates that the average crystal grain size is more than 50 µm.

[0111] Regarding "rotating bending fatigue test", "C" indicates occurrences of fracture starting from the ferrite phase, grain boundary fracture starting from a grain boundary of crystal grains having an equivalent circle diameter of 100 µm or more, and/or grain boundary fracture starting from a coarse carbonitride, and "A" indicates that such fracture did not occur.

[0112] Regarding "5% sulfuric acid immersion test", "AA" indicates that the corrosion loss is 1 g/(m²·hr) or less, "A" indicates that the corrosion loss is more than 1 g/(m²·hr) and 10 g/(m²·hr) or less, and "C" indicates that the corrosion loss is more than 10 g/(m²·hr).

[0113] Regarding "hot workability", "A" indicates that no crack occurred during hot forging, and "C" indicates that the crack occurred during the hot forging.

[0114] Further, regarding "structure after soft annealing", "AA" represents a single phase of the ferrite phase (α) (area ratio of the ferrite phase being more than 90.0%), "A" represents a mixed structure of the ferrite phase and the martensite phase (α') (area ratio of the ferrite phase being 50.0% or more and 90.0% or less), and "C" represents that the area ratio of the martensite phase is 50.0% or more (area ratio of the ferrite phase being less than 50.0%).

(1) In Comparative Example 1, the area ratio of the ferrite phase in the core portion of the quenched structure exceeded 5%. As a result, in the rotating bending fatigue test, fracture starting from the ferrite phase occurred. The reason is considered to be that [Nieq]/[Creq] was less than 1.0 and a large amount of ferrite phase remained even after quenching.

(2) In Comparative Example 2, the corrosion loss increased. The reason is considered to be that the amount of C was excessive and a large amount of non-solid-dissolved Cr carbonitride was generated.

(3) In Comparative Example 3, the corrosion loss increased. The reason is considered to be that the amount of Mn was excessive.

(4) In Comparative Example 4, the hot workability is reduced. The reason is considered to be that the amount of Cu was excessive.

(5) In Comparative Example 5, the average crystal grain size after the nitrogen absorption treatment exceeded 50 µm. As a result, in the rotating bending fatigue test, grain boundary fracture starting from a grain boundary of the coarse crystal grain occurred. The reason is considered to be that [Ni]*[Mo] was less than 1.0.

(6) In Comparative Example 6, the structure after soft annealing was a single phase of α'. The reason is considered to be that the amount of Ni was excessive.

(7) In Comparative Example 7, the corrosion loss exceeded 10 g/(m²·hr). The reason is considered to be that the amount of Cr was small.

(8) In Comparative Example 8, the area ratio of the ferrite phase in the core portion of the quenched structure exceeded 5%. As a result, in the rotating bending fatigue test, fracture starting from the ferrite phase occurred. The reason is considered to be that the amount of Cr was excessive.

(9) In Comparative Example 9, the average crystal grain size after the nitrogen absorption treatment exceeded 50 µm. As a result, in the rotating bending fatigue test, grain boundary fracture starting from a grain boundary of the coarse crystal grain occurred. In Comparative Example 9, the corrosion loss exceeded 10 g/(m²·hr). The reason is considered to be that the amount of Mo was small and [Ni]*[Mo] was less than 1.0.

(10) In Comparative Example 10, the area ratio of the ferrite phase in the core portion of the quenched structure exceeded 5%. As a result, in the rotating bending fatigue test, fracture starting from the ferrite phase occurred. The reason is considered to be that the amount of Mo was excessive and [Nieq]/[Creq] was less than 1.

(11) In Comparative Example 11, the average crystal grain size after the nitrogen absorption treatment exceeded 50 µm. As a result, in the rotating bending fatigue test, grain boundary fracture starting from a grain boundary of the coarse crystal grain occurred. The reason is considered to be that the amount of Nb was small and abnormal grain growth occurred during the nitrogen absorption treatment.

(12) In Comparative Example 12, the average crystal grain size after the nitrogen absorption treatment exceeded 50 µm. As a result, in the rotating bending fatigue test, grain boundary fracture starting from a coarse carbonitride occurred. The reason is considered to be that a coarse carbonitride was formed due to an excessive amount of Nb.

(13) In Comparative Examples 13 and 14, the average crystal grain size after the nitrogen absorption treatment exceeded 50 µm. As a result, in the rotating bending fatigue test, grain boundary fracture starting from a grain boundary of the coarse crystal grain occurred. The reason is considered to be that [Ni]*[Mo] was less than 1.0.

(14) In Comparative Example 15, the average crystal grain size after the nitrogen absorption treatment exceeded 50 µm. As a result, in the rotating bending fatigue test, grain boundary fracture starting from a grain boundary of the coarse crystal grain occurred. The reason is considered to be that no fine crystal grain was obtained because Nb was not included.

(15) In Comparative Example 16, the average crystal grain size after the nitrogen absorption treatment exceeded 50 µm. As a result, in the rotating bending fatigue test, grain boundary fracture starting from a grain boundary of the coarse crystal grain occurred. The reason is considered to be that no fine crystal grain was obtained because Nb was not included.

(16) In any of Examples 1 to 14, the ferrite area ratio of the core portion of the quenched structure was 0.5% or less, and the average crystal grain size after the nitrogen absorption treatment was 50 µm or less. As a result, Examples 1 to 14 all exhibited good rotating bending fatigue characteristics. In addition, in any of Examples 1 to 14, the corrosion loss was 10 g/(m²·hr) or less, and the crack did not occur during hot working.

(17) Structures after soft annealing in Examples 1 to 4 and 6 to 14 were mixed structures of α+α'. On the other hand, the structure after soft annealing in Example 5 was a single phase of α.

[0115] Although the embodiment of the present invention has been described in detail, the present invention is not limited to the above embodiment, and various modifications can be made without departing from the gist of the present invention.

[0116] The present application is based on Japanese Patent Application No. 2022-110838 filed on July 8, 2020, and the contents thereof are incorporated herein by reference.

INDUSTRIAL APPLICABILITY

[0117] The martensitic stainless steel for a nitrogen-enriching treatment according to the present invention can be used for shafts, bearings, gears, pins, bolts, screws, turbine blades, valves, bladed objects, nozzles, and the like.

Claims

1. A martensitic stainless steel for a nitrogen-enriching treatment, consisting of

0.10 mass% ≤ C ≤ 0.30 mass%;

Si ≤ 0.20 mass%;

0.20 mass% ≤ Mn ≤ 1.50 mass%;

P ≤ 0.05 mass%;

S ≤ 0.01 mass%;

Cu ≤ 0.3 mass%;

10.5 mass% ≤ Cr ≤ 17.0 mass%;

0.50 mass% ≤ Ni ≤ 3.00 mass%;

0.50 mass% ≤ Mo ≤ 3.00 mass%;

0.1 mass% ≤ Nb ≤ 0.5 mass%;

Ti ≤ 0.5 mass%; and

V ≤ 1.0 mass%,

with the remainder being Fe and inevitable impurities comprising 0.05 mass% or less of Al, and 0.030 mass% or less of O,

having an area ratio of a ferrite phase of 50% or more, and satisfying the following formulae (1) and (2),



in the formulae,
[X] represents a content (mass%) of an element X,

and

2. A martensitic stainless steel member, comprising:

a base portion comprising a martensitic stainless steel; and

a nitrogen-enriched layer formed on a surface of the base portion, wherein

the martensitic stainless steel consists of

0.10 mass% ≤ C ≤ 0.30 mass%;

Si ≤ 0.20 mass%;

0.20 mass% ≤ Mn ≤ 1.50 mass%;

P ≤ 0.05 mass%;

S ≤ 0.01 mass%;

Cu ≤ 0.3 mass%;

10.5 mass% ≤ Cr ≤ 17.0 mass%;

0.50 mass% ≤ Ni ≤ 3.00 mass%;

0.50 mass% ≤ Mo ≤ 3.00 mass%;

0.1 mass% ≤ Nb ≤ 0.5 mass%;

Ti ≤ 0.5 mass%; and

V ≤ 1.0 mass%,

with the remainder being Fe and inevitable impurities 0.05 mass% or less of Al, and 0.030 mass% or less of O,

the martensitic stainless steel satisfies the following formulae (1) and (2),



in the formulae,
[X] represents a content (mass%) of an element X,

and

the base portion has an area ratio of a ferrite phase of 5% or less, and

the nitrogen-enriched layer has a thickness of 100 µm or more.

3. The martensitic stainless steel member according to claim 2, wherein the base portion comprises a core portion, and
the core portion of the base portion has an average crystal grain size of 50 µm or less.

4. The martensitic stainless steel member according to claim 2 or 3, wherein the nitrogen-enriched layer has an amount of N of 0.1 mass% or more and 1.0 mass% or less.

5. The martensitic stainless steel member according to one of claims 2 to 4, wherein the martensitic stainless steel member is a shaft, bearing, gear, pin, bolt, screw, turbine blade, valve, bladed object, or nozzle.

6. Use of the martensitic stainless steel according to claim 1, for manufacturing the martensitic stainless steel member according to one of claims 2 to 5.

7. The use of claim 6, comprising a nitrogen-enriching treatment.

8. The use of claim 7, wherein the nitrogen-enriching treatment is performed on a cold-worked steel member.

9. The use of claim 7 or 8, wherein the nitrogen-enriching treatment is selected from the group consisting of solid-phase nitrogen absorption method, a gas nitriding method, a soft nitriding method, and an ion nitriding method.

10. The use of claim 9, wherein the nitrogen-enriching treatment is a solid-phase nitrogen absorption method.