Martensitic stainless steel and martensitic stainless steel components for nitrogen enrichment treatment

Abstract

To provide a martensitic stainless steel for nitrogen-enriching treatment that has excellent hot workability and excellent cold workability, and a martensitic stainless steel member.SOLUTION: The present invention relates to a martensitic stainless steel for nitrogen-enriching treatment, comprising 0.10≤C≤0.30 mass%, Si≤0.20 mass%, 0.20≤Mn≤1.50 mass%, P≤0.05 mass%, S≤0.01 mass%, Cu≤0.3 mass%, 10.5≤Cr≤17.0 mass%, 0.50≤Ni≤3.00 mass%, 0.50≤Mo≤3.00 mass%, and 0.1≤Nb≤0.5 mass%, with the balance being Fe and inevitable impurities, satisfying 1.00≤[Ni]*[Mo]≤9.00 and [Nieq] / [Creq]≥1.00, and having a ferrite phase area ratio of 50% or more. A martensitic stainless steel member comprises a base part composed of the martensitic stainless steel and a nitrogen enriched layer formed on the surface of the base part.SELECTED DRAWING: None

Description

Technical Field

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

Background Art

[0002] Martensitic stainless steel refers to stainless steel having a martensite structure at room temperature. Martensitic stainless steel is manufactured by quenching and tempering from the austenite region. Martensitic stainless steel is used for knives, turbine blades, bearings, etc. because it is excellent in corrosion resistance, strength, wear resistance, etc.

[0003] It is known that when nitrogen is further dissolved in such martensitic stainless steel, the corrosion resistance, hardness, wear resistance, etc. are further improved. However, when a method of dissolving nitrogen in molten steel is used, blow holes may occur in the ingot. Therefore, after producing a base material made of low-nitrogen martensitic stainless steel, nitrogen is dissolved only in the surface layer portion of the base material, and a layer having a higher nitrogen concentration than the base material (hereinafter, also referred to as "nitrogen enrichment layer") is formed on the surface layer portion of the base material. A method has been proposed. When a nitrogen enrichment layer is formed on the surface layer portion of the low-nitrogen base material, the corrosion resistance and wear resistance of the base material can be improved without generating blow holes.

[0004] Regarding such a method for forming a nitrogen enrichment layer, various proposals have been made conventionally. For example, in Patent Document 1, (a) Quenching and tempering are performed on a steel wire made of martensitic stainless steel containing a predetermined amount of Mo, V, and Al, (b) The steel wire is further subjected to gas nitriding treatment A manufacturing method of martensitic stainless steel is disclosed. The document describes how this method can yield thick spring materials with excellent delayed fracture resistance and fatigue resistance.

[0005] Patent Document 2 contains: (a) Prepare a component (annealed material) made of martensitic stainless steel containing a predetermined amount of C, Si, Mn, and Cr, (b) Heat the component in nitrogen gas (99% purity) at atmospheric pressure, and then furnace cool it to below 600°C. (c) Furthermore, the components are subjected to quenching and tempering. A method for manufacturing martensitic stainless steel components is disclosed. The document describes how martensitic stainless steel components with excellent corrosion resistance and wear resistance can be obtained using this method.

[0006] Patent Document 3 describes a method for forming a nitrogen-enriched layer, but (a) Prepare a steel bar made of martensitic stainless steel containing a predetermined amount of C, Si, Mn, S, P, Ni, Cr, Mo, N, and Al. (b) Softening annealing is performed on the steel bar under specified conditions. (c) Cold working is performed on the softened annealed steel bar, (d) The cold-worked components are subjected to a hardening treatment to increase their hardness and become the final product. A method for manufacturing martensitic stainless steel is disclosed. The document states: (A) When carbonitrides are finely dispersed in steel after softening annealing, the fine carbonitrides pin the movement of dislocations and grain boundaries, which reduces cold workability, and (B) When softening annealing is performed at high temperatures, the amount of fine carbonitrides decreases, which improves cold workability. It is stated.

[0007] Applying a nitrogen-enrichment treatment (hereinafter also referred to as "nitrogen enrichment treatment") to the surface of a component made of low-nitrogen martensitic stainless steel can improve its wear resistance. However, if the treatment conditions are inappropriate, abnormal grain growth may occur during nitrogen enrichment treatment. Abnormal grain growth can cause a decrease in the fatigue properties of the component. To solve this problem, one could consider adding carbonitride-forming elements to induce the deposition of large amounts of carbonitrides in the steel, thereby suppressing abnormal grain growth. However, excessive carbonitride deposition can reduce hot workability and / or cold workability. Furthermore, excessive carbonitride deposition can also reduce the corrosion resistance of the component. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2021-143388 [Patent Document 2] Japanese Patent Publication No. 2019-167630 [Patent Document 3] Japanese Patent Publication No. 2020-050916 [Overview of the Initiative] [Problems that the invention aims to solve]

[0009] The problem that this invention aims to solve is to provide a martensitic stainless steel for nitrogen enrichment treatment that has excellent hot workability and cold workability, and a martensitic stainless steel component using the same. Furthermore, another problem that the present invention aims to solve is to provide a martensitic stainless steel for nitrogen enrichment treatment that can be used to manufacture components with excellent fatigue properties, and a martensitic stainless steel component using the same. Furthermore, another problem to be solved by the present invention is to provide a martensitic stainless steel for nitrogen enrichment treatment capable of manufacturing a member excellent in fatigue characteristics and corrosion resistance, and a martensitic stainless steel member using the same.

Means for Solving the Problems

[0010] In order to solve the above problems, the martensitic stainless steel for nitrogen enrichment treatment according to the present invention is 0.10 ≦ C ≦ 0.30 mass%, Si ≦ 0.20 mass%, 0.20 ≦ Mn ≦ 1.50 mass%, P ≦ 0.05 mass%, S ≦ 0.01 mass%, Cu ≦ 0.3 mass%, 10.5 ≦ Cr ≦ 17.0 mass%, 0.50 ≦ Ni ≦ 3.00 mass%, 0.50 ≦ Mo ≦ 3.00 mass%, and 0.1 ≦ Nb ≦ 0.5 mass% including, with the balance being composed of Fe and unavoidable impurities, satisfying the following formulas (1) and (2), The area ratio of the ferrite phase is 50% or more.

[0011] 1.00 ≦ [Ni] * [Mo] ≦ 9.00 …(1) [Nieq] / [Creq] ≧ 1.00 …(2) However, [X] is the content (mass%) of element X, [Nieq] = [Ni] + 30[C] + 0.5[Mn] + 8, [Creq] = [Cr] + [Mo] + 1.5[Si].

[0012] The martensitic stainless steel member according to the present invention is a base portion made of martensitic stainless steel, and a nitrogen enrichment layer formed on the surface of the base portion and The aforementioned martensitic stainless steel is 0.10 ≤ C ≤ 0.30 mass%, Si ≤ 0.20 mass%, 0.20 ≤ Mn ≤ 1.50 mass%, P ≤ 0.05 mass%, S ≤ 0.01 mass%, Cu ≤ 0.3 mass%, 10.5 ≤ Cr ≤ 17.0 mass%, 0.50 ≤ Ni ≤ 3.00 mass%, 0.50 ≤ Mo ≤ 3.00 mass%, and, 0.1 ≤ Nb ≤ 0.5 mass% It contains, with the remainder consisting of Fe and unavoidable impurities. The martensitic stainless steel satisfies the following equations (1) and (2): The base has a ferrite phase area ratio of 5% or less. The thickness of the nitrogen-enriched layer is 100 μm or more.

[0013] 1.00≦[Ni]*[Mo]≦9.00 …(1) [Nieq] / [Creq]≧1.00 …(2) however, [X] is the mass %) of element X. [Nieq]=[Ni]+30[C]+0.5[Mn]+8, [Creq] = [Cr] + [Mo] + 1.5 [Si]. [Effects of the Invention]

[0014] In martensitic stainless steel having a predetermined composition, optimizing the Ni and Mo content to satisfy formula (1) can suppress grain coarsening during nitrogen enrichment treatment. As a result, martensitic stainless steel components with excellent fatigue properties can be obtained. Furthermore, by optimizing the content of each element to satisfy equation (2), a martensitic stainless steel containing a relatively large amount of martensitic phase after quenching and tempering can be obtained. As a result, the fatigue properties of components using this martensitic stainless steel are improved.

[0015] Furthermore, optimizing the composition (especially the amount of Ni) of a martensitic stainless steel having a predetermined composition improves the cold workability of the martensitic stainless steel. Furthermore, if martensitic stainless steel contains excess Cu, P, and / or S, its hot workability decreases. Conversely, if the amounts of Cu, P, and S are kept below critical values, the hot workability of martensitic stainless steel improves. Furthermore, optimizing the nitrogen enrichment treatment method and / or treatment conditions suppresses the formation of excess nitrides within the nitrogen enrichment layer. As a result, a martensitic stainless steel component with excellent corrosion resistance can be obtained. [Modes for carrying out the invention]

[0016] One embodiment of the present invention will be described in detail below. [1. Martensitic stainless steel for nitrogen enrichment treatment] [1.1. Main constituent elements] The martensitic stainless steel for nitrogen enrichment 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 unavoidable impurities. The types of added elements, their component ranges, and the reasons for their limitations are as follows.

[0017] (1) 0.10 ≤ C ≤ 0.30 mass%: Carbon (C) is an element that increases the strength of martensitic stainless steel. To obtain a sufficient effect, the amount of C must be 0.10 mass% or more. Preferably, the amount of C is 0.15 mass% or more. On the other hand, if the amount of carbon is excessive, it can increase the amount of undissolved chromium carbides during quenching, which can reduce the corrosion resistance of the matrix. Therefore, the amount of carbon needs to be 0.30 mass% or less.

[0018] (2) Si ≤ 0.20 mass%: Si is an effective deoxidizing element. However, excessive Si content can impair ductility and toughness. Therefore, the Si content must be 0.20 mass% or less. Preferably, the Si content is 0.15 mass% or less.

[0019] (3) 0.20 ≤ Mn ≤ 1.50 mass%: Mn is a deoxidizing element. Furthermore, Mn is an element that increases the solid solubility limit of nitrogen during nitrogen enrichment treatment (especially solid-phase nitrogen absorption treatment). To obtain a sufficient effect, the amount of Mn must be 0.20 mass% or more. Preferably, the amount of Mn is 0.40 mass% or more. On the other hand, if the amount of Mn is excessive, corrosion resistance may be impaired. Therefore, the amount of Mn needs to be 1.50 mass% or less. Preferably, the amount of Mn is 1.20 mass% or less.

[0020] (4) P ≤ 0.05 mass %: P is an unavoidable impurity. Excessive P can lead to embrittlement of grain boundaries and reduced hot workability. Therefore, the P content should be 0.05% or less.

[0021] (5) S ≤ 0.01 mass %: S is an unavoidable impurity. Excessive S content can lead to the formation of sulfides, which may reduce hot workability and / or corrosion resistance. Therefore, the S content must be 0.01 mass% or less.

[0022] (6) Cu ≤ 0.3 mass %: Cu is an unavoidable impurity. If the amount of Cu is excessive, undissolved Cu may precipitate in the steel. Undissolved Cu melts during hot working, causing a decrease in hot workability. Therefore, the amount of Cu must be 0.3 mass% or less. Preferably, the amount of Cu is 0.2 mass% or less.

[0023] (7) 10.5 ≤ Cr ≤ 17.0 mass%: Cr is an element that improves corrosion resistance. To obtain a sufficient effect, the amount of Cr needs to be 10.5 mass% or more. Preferably, the amount of Cr is 12.0 mass% or more. On the other hand, if the amount of Cr is excessive, a large amount of ferrite phase may be formed after quenching and tempering, which can reduce the strength. Therefore, the amount of Cr needs to be 17.0 mass% or less. Preferably, the amount of Cr is 16.0 mass% or less.

[0024] (8) 0.50 ≤ Ni ≤ 3.00 mass%: Ni is an effective element for improving corrosion resistance. To obtain a sufficient effect, the amount of Ni must be 0.50 mass% or more. Preferably, the amount of Ni is 0.70 mass% or more. On the other hand, if the amount of Ni is excessive, the cold workability may decrease. Therefore, the amount of Ni needs to be 3.00 mass% or less. Preferably, the amount of Ni is 2.50 mass% or less.

[0025] (9) 0.50 ≤ Mo ≤ 3.00 mass%: Mo is an element that improves corrosion resistance. To obtain a sufficient effect, the amount of Mo needs to be 0.50 mass% or more. Preferably, the amount of Mo is 0.70 mass% or more. On the other hand, if the amount of Mo is excessive, a large amount of ferrite phase may be formed after quenching and tempering, which can reduce the strength. Therefore, the amount of Mo needs to be 3.00 mass% or less. Preferably, the amount of Mo is 2.50 mass% or less.

[0026] (10) 0.1 ≤ Nb ≤ 0.5 mass%: Nb is an element that forms carbonitrides, refines the crystal grains, and contributes to improving strength. Therefore, the Nb content must be 0.1 mass% or more. Preferably, the Nb content is 0.15 mass% or more. On the other hand, if the amount of Nb is excessive, coarse carbonitrides may form, impairing the grain refinement effect. Furthermore, carbonitrides can act as fracture initiation points, potentially reducing strength. Therefore, the Nb content needs to be 0.5 mass% or less.

[0027] Other elements that exhibit a grain refinement effect include Ti and V. Among these, Nb has the advantage of being less prone to the formation of coarse crystal carbides compared to Ti, and having higher carbonitride stability and less prone to precipitated particle growth compared to V. Therefore, Nb is an essential element in this invention.

[0028] [1.2. Inevitable Impurities] "Unavoidable impurities" refer to trace components that are introduced from raw materials and refractories during the manufacturing of martensitic stainless steel. Examples of unavoidable impurities include: (a) P less than or equal to 0.050 mass%, (b) S less than or equal to 0.010 mass%, (c) Al less than 0.05 mass%, (d) O less than or equal to 0.030 mass%, These are some examples.

[0029] [1.3. Sub-constituent elements] The martensitic stainless steel according to the present invention may further contain one or more elements in addition to the elements described above. The types of additive elements, their component ranges, and the reasons for their limitations are as follows.

[0030] (1) Ti ≤ 0.5 mass %: Adding Ti to martensitic stainless steel containing Nb allows Ti to play an auxiliary role to Nb, enhancing the grain refinement effect. Therefore, martensitic stainless steel may contain additional Ti. However, Ti is more prone to forming coarse carbides than Nb. Therefore, if the amount of Ti is excessive, the fine carbonitrides may decrease, and the crystal grains may become coarser. For this reason, the amount of Ti is preferably 0.5 mass% or less.

[0031] (2) V ≤ 1.0 mass %: V, like Ti, plays a supporting role to Nb and enhances the grain refinement effect. Therefore, martensitic stainless steel may contain even more V. However, V exhibits lower carbonitride stability at high temperatures compared to Nb, and precipitated particles tend to grow more easily. Therefore, if the amount of V is excessive, the fine carbonitrides may decrease, and the crystal grains may become coarser. Consequently, the amount of V is preferably 1.0 mass% or less.

[0032] [1.4. Ingredient Balance] The martensitic stainless steel according to the present invention satisfies the following formulas (1) and (2). 1.00≦[Ni]*[Mo]≦9.00 …(1) [Nieq] / [Creq]≧1.00 …(2) however, [X] is the mass %) of element X. [Nieq]=[Ni]+30[C]+0.5[Mn]+8, [Creq] = [Cr] + [Mo] + 1.5 [Si].

[0033] [1.4.1. Formula (1)] [Ni]*[Mo] correlates with the average grain size of martensitic stainless steel after nitrogen enrichment treatment. If [Ni]*[Mo] becomes too small, the grains may coarseen after nitrogen enrichment treatment. As a result, repeated stress acting on the nitrogen-enriched material may lead to intergranular fracture. Therefore, [Ni]*[Mo] must be 1.00 or higher. Preferably, [Ni]*[Mo] is 1.05 or higher, and more preferably 1.10 or higher.

[0034] On the other hand, if the [Ni]*[Mo] ratio becomes too high, the crystal grains may coarseen even before the nitrogen enrichment treatment, and fine crystal grains may not be obtained even after the nitrogen enrichment treatment. Therefore, the [Ni]*[Mo] ratio needs to be 9.00 or less. Preferably, the [Ni]*[Mo] ratio is 6.00 or less, and more preferably 4.00 or less.

[0035] [1.4.2. Formula (2)] [Nieq] is the Ni equivalent, which is an indicator of the austenite-stabilizing element. [Creq] is the Cr equivalent, which is an indicator of the ferrite-stabilizing element. If [Nieq] / [Creq] becomes too small, the amount of ferrite phase remaining in the quenched structure increases, which may reduce the fatigue strength. Therefore, [Nieq] / [Creq] must be 1.00 or higher. [Nieq] / [Creq] is preferably 1.02 or higher, and more preferably 1.05 or higher.

[0036] [1.5. Area ratio of the ferrite phase] The martensitic stainless steel according to the present invention has an optimized composition, resulting in a ferrite phase area ratio of 50% or more at 23°C after annealing (before nitrogen enrichment treatment). Optimizing the manufacturing conditions can increase the ferrite phase area ratio to 70% or more, or even 90% or more. Furthermore, optimizing the composition can sometimes result in a ferrite phase area ratio of 100%. The "ferrite phase area ratio" will be discussed later. The martensitic stainless steel according to the present invention has excellent cold workability because, in its annealed state, the area ratio of the ferrite phase is relatively large (i.e., the area ratio of the martensite phase is relatively small).

[0037] [1.6. Usage] The martensitic stainless steel according to the present invention is used to produce a base by annealing and cold working, and then to perform nitrogen enrichment treatment on the surface of the base. After nitrogen enrichment treatment, quenching and tempering are performed. When nitrogen enrichment treatment is performed, a layer with a higher nitrogen concentration than the core of the base (nitrogen enriched layer) is formed on the surface of the base. Further quenching and tempering harden the nitrogen enriched layer. As a result, a component with excellent fatigue properties is obtained.

[0038] [2. Martensitic stainless steel components] The martensitic stainless steel component according to the present invention has the following configuration. (1) The martensitic stainless steel member is A base made of martensitic stainless steel, The nitrogen-enriched layer formed on the surface of the base and It is equipped with. (2) The martensitic stainless steel is 0.10 ≤ C ≤ 0.30 mass%, Si ≤ 0.20 mass%, 0.20 ≤ Mn ≤ 1.50 mass%, P ≤ 0.05 mass%, S ≤ 0.01 mass%, Cu ≤ 0.3 mass%, 10.5 ≤ Cr ≤ 17.0 mass%, 0.50 ≤ Ni ≤ 3.00 mass%, 0.50 ≤ Mo ≤ 3.00 mass%, and, 0.1 ≤ Nb ≤ 0.5 mass% It contains, with the remainder consisting of Fe and unavoidable impurities. The following equations (1) and (2) are satisfied. (3) The base portion has a ferrite phase area ratio of 5% or less. (4) The thickness of the nitrogen-enriched layer is 100 μm or more.

[0039] 1.00≦[Ni]*[Mo]≦9.00 …(1) [Nieq] / [Creq]≧1.00 …(2) however, [X] is the mass %) of element X. [Nieq]=[Ni]+30[C]+0.5[Mn]+8, [Creq] = [Cr] + [Mo] + 1.5 [Si].

[0040] [2.1. Base] [2.1.1. Materials] The base is made of martensitic stainless steel according to the present invention. Details of the martensitic stainless steel are as described above, so a further explanation is omitted.

[0041] [2.1.2. Area ratio of the ferrite phase] "Ferrite phase area percentage (%)" refers to the ratio of the area of ​​the ferrite phase to the total cross-sectional area of ​​martensitic stainless steel at 23°C. In other words, "the area percentage of the ferrite phase (%)" means, (a) A martensitic stainless steel member after annealing, or a martensitic stainless steel member after nitrogen enrichment treatment + quenching and tempering, is cut perpendicular to the surface, and the cross-section is polished and etched. (b) Observe the region near the center of the cross-section (in the case of a nitrogen-enriched material, the region where the nitrogen concentration is at the level before nitrogen enrichment (core)) at room temperature using an optical microscope, and calculate the field of view area (S0) and the area of ​​the ferrite phase included in the field of view (S), (c) The value obtained by dividing S by S0 (= S × 100 / S0) It refers to. In this invention, S0 is set to 2 mm x 3 mm.

[0042] The martensitic stainless steel according to the present invention has an optimized composition, resulting in a relatively high area ratio of the ferrite phase after annealing. On the other hand, after nitrogen enrichment treatment + quenching and tempering, almost the entire base becomes the martensite phase, and the area ratio of the ferrite phase in the base becomes 5% or less. By optimizing the composition of the martensitic stainless steel, the area ratio of the ferrite phase in the base after nitrogen enrichment treatment + quenching and tempering becomes 0.5% or less.

[0043] [2.1.3. Average grain size] "Average grain size" refers to the average value of the grain size of the prior austenite crystal grains, which is obtained by measuring the grain size from optical microscope photographs using the sectioning method with straight test lines described in JIS G0551, and then converting it using Table A.1 in Annex A of JIS G0551.

[0044] The average grain size of the core of the base after nitrogen enrichment treatment affects the fatigue properties of the martensitic stainless steel component. Generally, the smaller the average grain size of the core of the base after nitrogen enrichment treatment, the better the fatigue properties of the martensitic stainless steel component. The martensitic stainless steel component according to the present invention has elements added that have the effect of refining the crystal grains, so grain growth during nitrogen enrichment treatment is suppressed. By optimizing the manufacturing conditions, the average grain size of the core of the base after nitrogen enrichment treatment becomes 50 μm or less. By further optimizing the manufacturing conditions, the average grain size becomes 30 μm or less.

[0045] [2.2. Nitrogen-enriched layer] [2.2.1. Definition] The "nitrogen-enriched layer" refers to a layer formed by applying nitrogen enrichment treatment to the base after annealing and cold working, where the nitrogen concentration is higher than that of the core of the base (the region where the nitrogen concentration is at the level before nitrogen enrichment treatment). More specifically, the "nitrogen-enriched layer" refers to a region where the amount of N is between 0.1 mass% and 1.0 mass%. Furthermore, if the amount of N in the nitrogen-enriched layer becomes excessive, (a) Wear resistance decreases because martensitic transformation does not proceed during quenching. (b) Corrosion resistance is reduced due to the precipitation of a large amount of nitride. Problems such as these can arise. Therefore, the amount of N in the nitrogen-enriched layer needs to be 1.0 masss% or less.

[0046] "Nitrogen enrichment treatment" refers to a process of solid-dissolving nitrogen into the surface layer of the base. Examples of nitrogen enrichment treatment methods include solid-phase nitrogen absorption, gas nitriding, soft nitriding, and ion nitriding. Among these, solid-phase nitrogen absorption is preferred as a nitrogen enrichment treatment method because it yields a material with excellent corrosion resistance. Solid-phase nitrogen absorption has the advantage of suppressing the formation of chromium carbonitrides and improving wear resistance without reducing the corrosion resistance of the outermost surface. Details of solid-phase nitrogen absorption will be described later.

[0047] [2.2.2. Thickness] "Thickness of the nitrogen-enriched layer" refers to the thickness of the region formed on the surface of the base by nitrogen enrichment treatment of the base, where the amount of N is between 0.1 mass% and 1.0 mass%. The thickness of the nitrogen-enriched layer affects the wear resistance of the component. If the nitrogen-enriched layer is too thin, the wear resistance will be insufficient. Therefore, the thickness of the nitrogen-enriched layer is preferably 100 μm or more. More preferably, the thickness is 150 μm or more, and even more preferably, 200 μm or more.

[0048] [3. Method for manufacturing martensitic stainless steel] The martensitic stainless steel according to the present invention is (a) The raw materials, which have been blended to have a predetermined composition, are melted and cast, (b) The obtained ingot is hot forged, (c) Annealing the hot-forged rough material. It can be manufactured by doing so.

[0049] [3.1. Melting and Casting Process] First, the raw materials, which have been blended to achieve a predetermined composition, are melted and cast. The method and conditions of melting and casting are not particularly limited, and the most suitable method and conditions can be selected according to the purpose.

[0050] [3.2. Hot Forging Process] Next, the resulting ingot is hot forged. Hot forging is performed to break down the cast structure and process the ingot into a rough material with a predetermined shape. The method and conditions for hot forging are not particularly limited, and the optimal conditions can be selected according to the purpose.

[0051] [3.3. Annealing Process] The martensitic stainless steel according to the present invention is too hard in its as-hot-forged state and has poor cold workability. This is because the martensitic phase is partially formed during the cooling process after hot forging. To improve cold workability, it is necessary to anneal the hot-forged rough material to increase the area ratio of the ferrite phase.

[0052] The annealing conditions are not particularly limited, as long as the required cold workability is obtained. Annealing, specifically, (a) Maintain at a temperature of 850°C to 950°C for 1 hour to 10 hours. (b) Slowly cool the temperature range from the annealing temperature to 650°C at an average cooling rate of 30°C / h or less. It is preferable to do so.

[0053] [4. Method for manufacturing martensitic stainless steel components] The martensitic stainless steel component according to the present invention is (a) Cold working is performed on the annealed rough material, (b) The cold-worked component is subjected to nitrogen enrichment treatment. (c) The nitrogen-enriched components are subjected to quenching and tempering. It can be manufactured by doing so.

[0054] [4.1. Cold working process] First, the annealed rough material is subjected to cold working. The method and conditions of the cold working are not particularly limited, as long as they enable the manufacture of a component having a predetermined shape.

[0055] [4.2. Nitrogen Enrichment Process] Next, the cold-worked component is subjected to nitrogen enrichment treatment. Methods for nitrogen enrichment treatment include solid-phase nitrogen absorption, gas nitriding, soft nitriding, and ion nitriding. Any of these methods may be used in this invention. To obtain a component with excellent corrosion resistance, solid-phase nitrogen absorption is preferred as the nitrogen enrichment treatment method.

[0056] Solid-phase nitrogen absorption treatment is performed by heating the material to a predetermined temperature in a predetermined nitrogen atmosphere. Examples of nitrogen atmospheres include nitrogen gas atmospheres.

[0057] The heating temperature affects the nitrogen concentration on the surface of the material. Therefore, it is preferable to select an optimal heating temperature according to the composition of the material. Generally, the lower the heating temperature, the higher the equilibrium nitrogen concentration on the surface of the material. However, if the heating temperature is too low, a large amount of nitride may precipitate, reducing corrosion resistance. Therefore, a heating temperature of 900°C or higher is preferable. On the other hand, if the heating temperature becomes too high, the equilibrium nitrogen concentration on the surface of the material may decrease, resulting in a decrease in hardness. Therefore, a heating temperature of 1200°C or lower is preferable.

[0058] The partial pressure of nitrogen affects the nitrogen concentration on the surface of the material. Therefore, it is preferable to select an optimal value for the partial pressure of nitrogen according to the composition of the material. Generally, the higher the partial pressure of nitrogen, the higher the equilibrium nitrogen concentration on the surface of the material. To obtain this effect, a partial pressure of nitrogen of 0.1 atm (0.01 MPa) or higher is preferable. On the other hand, if the nitrogen partial pressure becomes too high, the nitrogen concentration increases excessively, lowering the martensitic transformation initiation temperature. As a result, the austenite phase is more likely to remain after quenching, which can reduce hardness. Therefore, a nitrogen partial pressure of 3.0 atm (0.3 MPa) or less is preferable.

[0059] The nitrogen concentration on the surface of the component is determined by the heating temperature and the partial pressure of nitrogen. On the other hand, the processing time affects the thickness of the nitrogen absorption layer (nitrogen-enriched layer). Therefore, it is preferable to select the optimal processing time according to the required thickness of the nitrogen absorption layer. The processing time is usually around 60 to 600 minutes.

[0060] [4.3. Quenching and Tempering Process] Next, the nitrogen-enriched material is subjected to quenching and tempering. This causes the austenite phase to transform into martensitic material. The quenching conditions are not particularly limited, as long as they are conditions that enable martensitic transformation of both the surface and the core.

[0061] For example, if solid-phase nitrogen absorption treatment is used as the nitrogen enrichment treatment method, quenching may be performed by rapidly cooling the material from the treatment temperature. Alternatively, quenching may be performed by performing solid-phase nitrogen absorption treatment, cooling to near room temperature, reheating to the quenching temperature, and then rapidly cooling. Rapid cooling can be performed using gas cooling, water cooling, ice water cooling, oil cooling, etc. To achieve a martensitic structure across the entire core of the component, a faster cooling rate during quenching is preferable. Specifically, an average cooling rate of 200°C / min or higher from the quenching temperature to 500°C is desirable.

[0062] Furthermore, a nitrogen diffusion treatment may be performed to diffuse nitrogen into the interior of the material before quenching. Specifically, following solid-phase nitrogen absorption treatment, nitrogen is diffused into the interior of the material by holding it at a high temperature of about 900 to 1200°C in an inert atmosphere such as argon gas. Performing a nitrogen diffusion treatment thickens the nitrogen absorption layer, which allows for stable hardness of the material surface after quenching. Furthermore, a sub-zero treatment may be performed after quenching to maintain the material temperature below 0°C.

[0063] Next, after quenching (or sub-zero treatment), tempering is performed. The tempering conditions are not particularly limited, and the optimal conditions can be selected according to the purpose. Specifically, tempering is preferably performed by holding the material at a temperature of 150°C to 500°C for about 1 to 3 hours.

[0064] [5. Effect] In martensitic stainless steel having a predetermined composition, optimizing the Ni and Mo content to satisfy formula (1) can suppress grain coarsening during nitrogen enrichment treatment. As a result, martensitic stainless steel components with excellent fatigue properties can be obtained. Furthermore, by optimizing the content of each element to satisfy equation (2), a martensitic stainless steel containing a relatively large amount of martensitic phase after quenching and tempering can be obtained. As a result, the fatigue properties of components using this martensitic stainless steel are improved.

[0065] Furthermore, optimizing the composition (especially the amount of Ni) of a martensitic stainless steel having a predetermined composition improves the cold workability of the martensitic stainless steel. Furthermore, if martensitic stainless steel contains excess Cu, P, and / or S, its hot workability decreases. Conversely, if the amounts of Cu, P, and S are kept below critical values, the hot workability of martensitic stainless steel improves. Furthermore, optimizing the nitrogen enrichment treatment method and / or treatment conditions suppresses the formation of excess nitrides within the nitrogen enrichment layer. As a result, a martensitic stainless steel component with excellent corrosion resistance can be obtained. [Examples]

[0066] (Examples 1-14, Comparative Examples 1-16) [1. Sample Preparation] 50 kg of steel with the compositions shown in Tables 1 and 2 was melted in a vacuum induction furnace. The resulting ingot was hot forged to produce a 20 mm diameter bar. Next, the bar was heated to 900°C and then softened by annealing, which involved slow cooling to 650°C at 20°C / h. A φ15 mm × 8 mm test specimen was taken from this bar.

[0067] Next, each test specimen was subjected to solid-phase nitrogen absorption treatment and quenching. Specifically, first, the test specimen was placed in the treatment chamber, and the chamber was evacuated using a vacuum device. Then, nitrogen gas was introduced into the treatment chamber, and the pressure and temperature inside the chamber were maintained at predetermined values. By appropriately adjusting the nitrogen partial pressure between 0.1 and 3.0 atm (0.01 MPa to 0.30 MPa) and the processing temperature between 900 and 1200°C, a nitrogen absorption layer with an N content of 0.1 to 1.0 mass% in the range from at least 100 μm from the surface of the material was obtained. The heating time was adjusted so that the thickness of the nitrogen absorption layer after quenching was 100 μm or more.

[0068] Furthermore, after the solid-phase nitrogen absorption treatment was completed, the test specimens were quenched. Quenching was performed by gas cooling. Subsequently, sub-zero treatment at -80°C for 2 hours and tempering at 200°C were carried out.

[0069] [Table 1]

[0070] [Table 2]

[0071] [2. Test Method] [2.1. Ferrite area ratio of the quenched structure core] After solid-phase nitrogen absorption treatment and quenching / tempering, the test specimens were cut, and the area ratio of the ferrite phase in the core was calculated.

[0072] [2.2. Average crystal grain size after nitrogen absorption treatment] The specimens were cut after solid-phase nitrogen absorption treatment, and the average grain size of the core was calculated. The average grain size was measured using the line segment method. First, a photograph of the microstructure of the core of the nitrogen-treated specimen was taken using an optical microscope (magnification: 100x). Next, 10 different lines (5 vertical and 5 horizontal) were drawn on this photograph, and for each line, the length of the line (L) was divided by the number of grain boundaries (n) that intersected the line (=L / n) to calculate the value. Furthermore, the average grain size was calculated by averaging these values.

[0073] [2.3. Rotational Bending Fatigue Test] A rotational bending fatigue test was conducted according to the test method specified in JIS Z2274. The fracture surface was observed to confirm the fracture initiation point.

[0074] [2.4. 5% sulfuric acid immersion test] The corrosion loss was calculated for each sample after solid-phase nitrogen absorption treatment according to the following procedure. Specifically, the mass of the test specimen before the corrosion test was measured after solid-phase nitrogen absorption treatment. Next, the test specimen was immersed for 6 hours in a 5% sulfuric acid aqueous solution maintained at 30°C in a constant temperature bath. After that, corrosion products adhering to the test specimen were removed by ultrasonic cleaning, and the mass of the test specimen after the corrosion test was measured. The corrosion loss was calculated by determining the mass reduction of the test specimen and dividing it by the surface area and time.

[0075] [2.5. Hot workability] The bar stock was visually inspected for cracks after hot working. [2.6. Microstructure after softening annealing (cold workability)] The presence or absence of the ferrite phase was confirmed by optical microscopy observation of the rod material after softening and annealing. Furthermore, the area ratio of the ferrite phase in the core was calculated.

[0076] [3. Results] The results are shown in Tables 3 and 4. From Tables 3 and 4, the following can be seen. In Tables 3 and 4, regarding the "ferrite area ratio of the quenched structure core," "◎" indicates that the ferrite area ratio is 0.5% or less, "○" indicates that the ferrite area ratio is greater than 0.5% but 5% or less, and "×" indicates that the ferrite area ratio is greater than 5%. Regarding the "average crystal grain size after nitrogen absorption treatment," "◎" indicates that the average crystal grain size is 30 μm or less, "○" indicates that the average crystal grain size is greater than 30 μm but 50 μm or less, and "×" indicates that the average crystal grain size is greater than 50 μm.

[0077] Regarding the "rotational bending fatigue test," "×" indicates that fracture originating from the ferrite phase, intergranular fracture originating from the grain boundaries of crystal grains with an equivalent circular diameter of 100 μm or more, and / or intergranular fracture originating from coarse carbonitrides occurred, while "○" indicates that no such fracture occurred. Regarding the "5% sulfuric acid immersion test," "◎" indicates a corrosion loss of 1 g / (m²). 2 This indicates that the corrosion loss is less than or equal to 1 g / (m²), and "○" means the corrosion loss is 1 g / (m²). 2 ·hr) super 10g / (m 2 ·hr) indicates that it is less than or equal to 10g / (m), and "×" means that the corrosion loss is 10g / (m 2 ·hr) indicates that something is greater than or equal to hr.

[0078] Regarding "hot workability," "○" indicates that no cracks occurred during hot forging, and "×" indicates that cracks occurred during hot forging. Furthermore, regarding the "structure after softening and annealing," "◎" indicates a single ferrite phase (α) (the area ratio of the ferrite phase exceeds 90.0%), "○" indicates a mixed structure of ferrite and martensite phase (α') (the area ratio of the ferrite phase is between 50.0% and 90.0%), and "×" indicates that the area ratio of the martensite phase is 50.0% or more (the area ratio of the ferrite phase is less than 50.0%).

[0079] (1) In Comparative Example 1, the area ratio of the ferrite phase in the core of the quenched structure exceeded 5%. As a result, fracture originating from the ferrite phase occurred in the rotational bending fatigue test. This is thought to be because the [Nieq] / [Creq] ratio was less than 1.0, and a large amount of ferrite phase remained even after quenching. (2) Comparative Example 2 showed increased corrosion loss. This is thought to be because the amount of C was excessive, resulting in the formation of a large amount of undissolved Cr carbonitrides. (3) Comparative Example 3 showed increased corrosion loss. This is thought to be due to an excess of Mn.

[0080] (4) Comparative Example 4 showed reduced hot workability. This is thought to be due to an excessive amount of Cu. (5) In Comparative Example 5, the average grain size after nitrogen absorption treatment exceeded 50 μm. As a result, in the rotational bending fatigue test, intergranular fracture occurred, starting from the grain boundaries of coarse grains. This is thought to be because the [Ni]*[Mo] ratio was less than 1.0. (6) In Comparative Example 6, the microstructure after softening annealing was α' single phase. This is thought to be due to an excess of Ni.

[0081] (7) Comparative Example 7 had a corrosion loss of 10 g / (m 2 It exceeded (hr). This is thought to be due to the low amount of Cr. (8) In Comparative Example 8, the area ratio of the ferrite phase in the core of the quenched structure exceeded 5%. As a result, fracture originating from the ferrite phase occurred in the rotational bending fatigue test. This is thought to be due to an excessive amount of Cr. (9) Comparative Example 9 had an average grain size exceeding 50 μm after nitrogen absorption treatment. As a result, in the rotational bending fatigue test, intergranular fracture occurred, starting from the grain boundaries of coarse grains. In addition, Comparative Example 9 had a corrosion loss of 10 g / (m²). 2 The result exceeded (hr). This is thought to be because the amount of Mo is low and the [Ni]*[Mo] ratio is less than 1.0.

[0082] (10) In comparative example 10, the area ratio of the ferrite phase in the core of the quenched structure exceeded 5%. As a result, fracture originating from the ferrite phase occurred in the rotational bending fatigue test. This is thought to be due to an excessive amount of Mo and a [Nieq] / [Creq] ratio of less than 1. (11) Comparative Example 11 had an average grain size exceeding 50 μm after nitrogen absorption treatment. As a result, in the rotational bending fatigue test, intergranular fracture occurred, starting from the grain boundaries of coarse grains. This is thought to be due to the low Nb content, which caused abnormal grain growth during nitrogen absorption treatment. (12) Comparative Example 12 had an average grain size exceeding 50 μm after nitrogen absorption treatment. As a result, grain boundary fracture originating from coarse carbonitrides occurred in the rotational bending fatigue test. This is thought to be because the excessive amount of Nb led to the formation of coarse carbonitrides.

[0083] (13) Comparative Examples 13 and 14 had an average grain size exceeding 50 μm after nitrogen absorption treatment. As a result, in the rotational bending fatigue test, intergranular fracture occurred, starting from the grain boundaries of coarse grains. This is thought to be because the [Ni]*[Mo] ratio was less than 1.0. (14) In Comparative Example 15, the average grain size after nitrogen absorption treatment exceeded 50 μm. As a result, in the rotational bending fatigue test, grain boundary fracture originating from coarse carbonitrides occurred. This is thought to be because the amount of V was excessive, leading to the formation of coarse carbonitrides. (15) Comparative Example 16 had an average grain size exceeding 50 μm after nitrogen absorption treatment. As a result, grain boundary fracture originating from coarse carbonitrides occurred in the rotational bending fatigue test. This is thought to be because coarse carbonitrides were formed due to an excess of Ti.

[0084] (16) In all of Examples 1 to 14, the ferrite area ratio of the quenched core structure was 0.5% or less, and the average grain size after nitrogen absorption treatment was 50 μm or less. As a result, all of Examples 1 to 14 showed good rotational bending fatigue characteristics. In addition, all of Examples 1 to 14 had a corrosion loss of 10 g / (m²). 2 The working time was less than 1.5 hours, and no cracks occurred during hot working. (17) The microstructure after softening annealing in Examples 1-4 and 6-14 was a mixed α+α' structure. On the other hand, the microstructure after softening annealing in Example 5 was a single α phase.

[0085] [Table 3]

[0086] [Table 4]

[0087] Although embodiments of the present invention have been described in detail above, the present invention is not limited in any way to the above embodiments, and various modifications are possible without departing from the spirit of the present invention. [Industrial applicability]

[0088] The martensitic stainless steel for nitrogen enrichment treatment according to the present invention can be used in shafts, bearings, gears, pins, bolts, screws, turbine blades, valves, cutting tools, nozzles, and the like.

Claims

1. 0.10≦C≦0.30mass%, Si≦0.20mass%, 0.20≦Mn≦1.50 mass%, P≦0.05mass%, S≦0.01mass%, Cu≦0.3mass%, 10.5≦Cr≦17.0mass%, 0.50≦Ni≦3.00mass%, 0.50 ≤ Mo ≤ 3.00 mass%, and, 0.1≦Nb≦0.5mass% It contains, with the remainder consisting of Fe and unavoidable impurities. The following equations (1) and (2) are satisfied, It is used to induce martensitic transformation by subjecting it to softening annealing, nitrogen enrichment treatment, and quenching and tempering treatments. After the softening annealing treatment and before the nitrogen enrichment treatment, the area ratio of the ferrite phase at 23°C is 50% or more. Martensitic stainless steel for nitrogen enrichment treatment. 1.00≦[Ni]*[Mo]≦9.00…(1) [Nieq] / [Creq]≧1.00…(2) however, [X] is the mass %) content of element X. [Nieq]=[Ni]+30[C]+0.5[Mn]+8, [Creq]=[Cr”+[Mo]+1.5[Si].

2. Ti ≤ 0.5 mass%, and / or, V≦1.0mass% The martensitic stainless steel for nitrogen enrichment treatment according to claim 1, further comprising:

3. A base made of martensitic stainless steel, The nitrogen-enriched layer formed on the surface of the base and Equipped with, The aforementioned martensitic stainless steel is 0.10≦C≦0.30mass%, Si≦0.20mass%, 0.20≦Mn≦1.50 mass%, P≦0.05mass%, S≦0.01mass%, Cu≦0.3mass%, 10.5≦Cr≦17.0mass%, 0.50≦Ni≦3.00mass%, 0.50 ≤ Mo ≤ 3.00 mass%, and, 0.1≦Nb≦0.5mass% It contains, with the remainder consisting of Fe and unavoidable impurities. The martensitic stainless steel satisfies the following formulas (1) and (2): The aforementioned base has a ferrite phase area ratio of 5% or less. The thickness of the nitrogen-enriched layer is 100 μm or more. Martensitic stainless steel components. 1.00≦[Ni]*[Mo]≦9.00…(1) [Nieq] / [Creq]≧1.00…(2) however, [X] is the mass %) content of element X. [Nieq]=[Ni]+30[C]+0.5[Mn]+8, [Creq]=[Cr”+[Mo]+1.5[Si].

4. The aforementioned martensitic stainless steel is Ti ≤ 0.5 mass%, and / or, V≦1.0mass% The martensitic stainless steel member according to claim 3, further comprising:

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