Martensitic stainless steel

Abstract

To provide a metal product which has high hardness and is excellent in toughness and corrosion resistance.SOLUTION: A martensitic stainless steel contains 0.10 mass% or more and 0.17 mass% or less of C, 0.50 mass% or less of Si, 1.0 mass% or less of Mn, 0.040 mass% or less of P, 0.030 mass% or less of S, 0.60 mass% or less of Ni, 11.3 mass% or more and 13.0 mass% or less of Cr, 0.25 mass% or less of Mo, 0.40 mass% or less of Cu, 0.050 mass% or less of Al, 0.20 mass% or less of V, 0.20 mass% or less of W, 0.20 mass% or less of Nb and 0.050 mass% or less of N. The total percentage content of V, W and Nb is 0.20 mass% or less. The quenching temperature is Qt(°C) calculated by the following expression or higher. The quenching temperature is 200°C or higher and 300°C or lower. Expression: Qt=930+2.3Cr%+1667(N%-0.01)+125(V%+W%+Nb%).SELECTED DRAWING: None

Description

Technical Field

[0001] This specification discloses martensitic stainless steel. This specification further discloses a method for manufacturing a metal product whose material is martensitic stainless steel.

Background Art

[0002] Since martensitic stainless steel is excellent in strength, it is used in various applications. Various proposals have been made for the improvement of martensitic stainless steel. Japanese Patent Application Laid-Open No. 2015-137381 discloses stainless steel in which the contents of C, Mn, and S are adjusted. Japanese Patent Application Laid-Open No. 8-67950 discloses martensitic stainless steel in which quenching treatment in quenching is performed in a state where fine carbides are dispersed in a matrix. Japanese Patent Application Laid-Open No. 2001-158941 discloses martensitic stainless steel in which the contents of C, Mn, Cr, and N are adjusted.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0004] The stainless steel disclosed in Japanese Patent Application Laid-Open No. 2015-137381 is excellent in corrosion resistance, but has low hardness and poor toughness. The stainless steel disclosed in Japanese Patent Application Laid-Open No. 8-67950 is excellent in toughness, but has low hardness. The stainless steel disclosed in Japanese Patent Application Laid-Open No. 2001-158941 is poor in toughness.

[0005] The applicant's intention is to provide a martensitic stainless steel that can yield products with high hardness, toughness, and corrosion resistance. [Means for solving the problem]

[0006] Preferred martensitic stainless steels are, C: 0.10% by mass or more and 0.17% by mass or less Si: 0.50% by mass or less Mn: 1.0% by mass or less P: 0.040% by mass or less S: 0.030% by mass or less Ni: 0.60% by mass or less Cr: 11.3 mass% or more and 13.0 mass% or less Mo: 0.25% by mass or less Cu: 0.40% by mass or less Al: 0.050% by mass or less V: 0.20% by mass or less W: 0.20% by mass or less Nb: 0.20% by mass or less and N: 0.050 mass or less It contains [components]. The remainder consists of Fe and unavoidable impurities. The total content of V, W, and Nb in this stainless steel is 0.20% by mass or less. The quenching temperature of this stainless steel is Qt (°C) or higher, calculated by the following formula. The tempering temperature of this stainless steel is between 200°C and 300°C. The hardness of this stainless steel after tempering is 40 HRC or higher. Qt = 930 + 2.3Cr%+ 1667(N% - 0.01) + 125(V% + W% + Nb%) In this formula, Cr% represents the mass content of Cr, N% represents the mass content of N, V% represents the mass content of V, W% represents the mass content of W, and Nb% represents the mass content of Nb.

[0007] A preferred method for manufacturing metal products is: (A) The material is, C: 0.10% by mass or more and 0.17% by mass or less Si: 0.50 mass% or less Mn: 1.0 mass% or less P: 0.040 mass% or less S: 0.030 mass% or less Ni: 0.60 mass% or less Cr: 11.3 mass% or more and 13.0 mass% or less Mo: 0.25 mass% or less Cu: 0.40 mass% or less Al: 0.050 mass% or less V: 0.20 mass% or less W: 0.20 mass% or less Nb: 0.20 mass% or less and N: 0.050 mass or less containing the balance being Fe and inevitable impurities preparing an intermediate product which is a martensitic stainless steel in which the total content ratio of V, W and Nb is 0.20 mass% or less (B) subjecting the above intermediate product to quenching at a quenching temperature of Qt (°C) or higher calculated by the following formula and (C) subjecting the above intermediate product to tempering at a tempering temperature of 200°C or higher and 300°C or lower including Qt = 930 + 2.3Cr% + 1667(N% - 0.01) + 125(V% + W% + Nb%) In this formula, Cr% represents the mass content ratio of Cr, N% represents the mass content ratio of N, V% represents the mass content ratio of V, W% represents the mass content ratio of W, and Nb% represents the mass content ratio of Nb.

[0008] Preferably, the quenching temperature in the above step (B) is 1150°C or lower.

[0009] Preferably, this manufacturing method includes, between the above step (A) and step (B), (D) holding the above intermediate product at a temperature of 800°C or higher and 900°C or lower and (E) A process of cooling the above intermediate product to 600°C at a rate of 30°C / hr or less. This also includes: [Effects of the Invention]

[0010] From this martensitic stainless steel, metal products with high hardness, toughness, and corrosion resistance can be obtained. [Modes for carrying out the invention]

[0011] The main component of the martensitic stainless steel according to this embodiment is Fe. This stainless steel contains C and Cr. This stainless steel may contain other elements. By quenching this stainless steel, a martensitic structure can be obtained. By further tempering this steel, a tough metallic structure can be obtained.

[0012] [composition] This martensitic stainless steel is C: 0.10% by mass or more and 0.17% by mass or less Si: 0.50% by mass or less Mn: 1.0% by mass or less P: 0.040% by mass or less S: 0.030% by mass or less Ni: 0.60% by mass or less Cr: 11.3 mass% or more and 13.0 mass% or less Mo: 0.25% by mass or less Cu: 0.40% by mass or less Al: 0.050% by mass or less V: 0.20% by mass or less W: 0.20% by mass or less Nb: 0.20% by mass or less and N: 0.050 mass or less It contains [element name]. The remainder consists of Fe and unavoidable impurities. Details of each element are described below.

[0013] [Carbon (C)] Carbon (C) dissolves in Fe during quenching. Steel with dissolved C has high hardness and high strength. C precipitates as fine carbides during tempering. These fine carbides contribute to the strength of the steel. From these viewpoints, a C content of 0.10 mass% or more is preferable, 0.11 mass% or more is more preferable, and 0.12 mass% or more is particularly preferable. If the C content is excessive, some of the C will not dissolve in Fe and will form carbides. These carbides impair the toughness and corrosion resistance of the steel. From the viewpoint of toughness and corrosion resistance, a C content of 0.17 mass% or less is preferable, 0.16 mass% or less is more preferable, and 0.15 mass% or less is particularly preferable.

[0014] [Silicon (Si)] Si contributes to deoxidation in the steelmaking process. Si also contributes to hardenability. From these viewpoints, the Si content is preferably 0.10% by mass or more, more preferably 0.15% by mass or more, and particularly preferably 0.20% by mass or more. Excess Si causes ferrite formation during quenching, impairing the toughness of the steel. Furthermore, Si impairs the corrosion resistance of the steel. From the viewpoint of toughness and corrosion resistance, the Si content is preferably 0.50% by mass or less, more preferably 0.45% by mass or less, and particularly preferably 0.40% by mass or less. Si is not an essential element for this martensitic stainless steel. In other words, the Si content may be below the detection limit.

[0015] [Manganese (Mn)] Mn contributes to deoxidation in the steelmaking process. Mn contributes to austenite formation during heating before quenching. From these viewpoints, the Mn content is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, and particularly preferably 0.3% by mass or more. Excess Mn impairs the corrosion resistance and oxidation resistance of the steel. From the viewpoint of corrosion resistance and oxidation resistance, the Mn content is preferably 1.0% by mass or less, more preferably 0.8% by mass or less, and particularly preferably 0.7% by mass or less. Mn is not an essential element for this martensitic stainless steel. In other words, the Mn content may be below the detection limit.

[0016] [Lin(P)] P is an impurity. P inhibits the ductility, toughness, and hot workability of steel. The P content is preferably 0.040% by mass or less, more preferably 0.035% by mass or less, and particularly preferably 0.033% by mass or less. The P content may be below the detection limit.

[0017] [Sulfur (S)] S is an impurity. S inhibits the ductility, toughness, and hot workability of steel. The S content is preferably 0.030% by mass or less, more preferably 0.025% by mass or less, and particularly preferably 0.022% by mass or less. The S content may be below the detection limit.

[0018] [Nickel (Ni)] Steel containing nickel exhibits excellent toughness after tempering. From this viewpoint, a nickel content of 0.1% by mass or more is preferred, and 0.2% by mass or more is particularly preferred. Excess nickel leads to high costs for martensitic stainless steel. From a cost viewpoint, a nickel content of 0.6% by mass or less is preferred, and 0.5% by mass or less is particularly preferred. Nickel is not an essential element for this martensitic stainless steel. In other words, the nickel content may be below the detection limit.

[0019] [Chromium (Cr)] Cr forms a passive film on the surface of steel. This passive film suppresses corrosion of the steel. From the viewpoint of corrosion resistance, the Cr content is preferably 11.3% by mass or more, more preferably 11.4% by mass or more, and particularly preferably 11.5% by mass or more. Excess Cr generates ferrite during quenching, impairing the toughness and corrosion resistance of the steel. From the viewpoint of toughness and corrosion resistance, the Cr content is preferably 13.0% by mass or less, more preferably 12.6% by mass or less, and particularly preferably 12.0% by mass or less.

[0020] [Molybdenum (Mo)] In steel containing Mo, the passive film of Cr is dense. Mo can contribute to the corrosion resistance of steel. From the viewpoint of corrosion resistance, the Mo content is preferably 0.05 mass% or more, more preferably 0.07 mass% or more, and particularly preferably 0.10 mass% or more. Mo is expensive. Furthermore, excess Mo causes ferrite formation during quenching, impairing the toughness and corrosion resistance of the steel. From the viewpoint of cost, toughness, and corrosion resistance, the Mo content is preferably 0.25 mass% or less, more preferably 0.23 mass% or less, and particularly preferably 0.20 mass% or less. Mo is not an essential element for this martensitic stainless steel. In other words, the Mo content may be below the detection limit.

[0021] [Copper (Cu)] Steel containing Cu exhibits excellent corrosion resistance. From the viewpoint of corrosion resistance, a Cu content of 0.1% by mass or more is preferable. Steel containing excess Cu has poor hot workability. From the viewpoint of hot workability, a Cu content of 0.4% by mass or less is preferable, 0.3% by mass or less is more preferable, and 0.2% by mass or less is particularly preferable. Cu is not an essential element for this martensitic stainless steel. In other words, the Cu content may be below the detection limit.

[0022] [Aluminum (Al)] Al contributes to deoxidation in the steelmaking process. From the viewpoint of deoxidation, the Al content is preferably 0.001% by mass or more, more preferably 0.005% by mass or more, and particularly preferably 0.010% by mass or more. Al is interposed in the steel as an oxide. This oxide inhibits the toughness of the steel. Furthermore, excess Al causes ferrite formation during quenching, impairing the toughness and corrosion resistance of the steel. From the viewpoint of toughness and corrosion resistance, the Al content is preferably 0.050% by mass or less, more preferably 0.040% by mass or less, and particularly preferably 0.030% by mass or less. Al is not an essential element for this martensitic stainless steel. In other words, the Al content may be below the detection limit.

[0023] [Vanadium (V)] V is an impurity. V generates fine carbonitrides, which inhibit the toughness of the steel. From the viewpoint of toughness, the V content is preferably 0.20 mass% or less, more preferably 0.15 mass% or less, and particularly preferably 0.10 mass% or less. The V content may be below the detection limit.

[0024] [Tungsten (W)] W is an impurity. W generates fine carbonitrides, which inhibit the toughness of the steel. From the viewpoint of toughness, the W content is preferably 0.20 mass% or less, more preferably 0.15 mass% or less, and particularly preferably 0.10 mass% or less. The W content may be below the detection limit.

[0025] [Niobium (Nb)] Nb is an impurity. Nb forms fine carbonitrides, which inhibit the toughness of the steel. From the viewpoint of toughness, the Nb content is preferably 0.20% by mass or less, more preferably 0.15% by mass or less, and particularly preferably 0.10% by mass or less. The Nb content may be below the detection limit.

[0026] [V, W, and Nb] In this embodiment, the ratio Pc is calculated using the following formula. Pc = V% + W% + Nb% In this formula, V% represents the mass content of V, W% represents the mass content of W, and Nb% represents the mass content of Nb. The ratio Pc is the total content of V, W, and Nb. As mentioned above, V, W, and Nb inhibit the toughness of steel. From the viewpoint of toughness, the ratio Pc is preferably 0.20 mass% or less, more preferably 0.15 mass% or less, and particularly preferably 0.10 mass% or less.

[0027] [Nitrogen (N)] N dissolves in Fe during quenching. Steel with dissolved N is hard and strong. N precipitates as fine carbonitrides during tempering. These fine carbonitrides contribute to the strength of the steel. From these viewpoints, the N content is preferably 0.010% by mass or more, more preferably 0.015% by mass or more, and particularly preferably 0.020% by mass or more. Excess N inhibits the toughness of the steel due to excess solid solution. Excess N generates nitrides, which inhibit the toughness of the steel. Excess N further generates excess carbonitrides, which inhibit the corrosion resistance of the steel. From the viewpoint of toughness and corrosion resistance, the N content is preferably 0.050% by mass or less, more preferably 0.045% by mass or less, and particularly preferably 0.040% by mass or less.

[0028] [Manufacturing method] An example of the manufacturing method according to this embodiment will be described below. First, an intermediate product is obtained from molten steel having a predetermined composition through casting, plastic deformation, etc. Preferably, this intermediate product is annealed. This intermediate product is then hardened. This intermediate product is then tempered to obtain a metal product. Each step will be described in detail below.

[0029] [Antangling] In annealing, the intermediate material is held in a high-temperature environment. This temperature (annealing temperature) is preferably between 800°C and 900°C. The holding time is preferably 2 hours or more. After this holding, the intermediate material is slowly cooled. The cooling rate until the temperature of the intermediate material reaches 600°C is preferably 30°C / hour or less. This annealing process yields a homogeneous metal structure.

[0030] [Hardening] During quenching, the intermediate material is kept in a high-temperature environment. This temperature (quenching temperature) is above Qt (°C), which is calculated using the following formula. Qt = 930 + 2.3Cr%+ 1667(N% - 0.01) + 125(V% + W% + Nb%) In this formula, Cr% represents the mass content of Cr, N% represents the mass content of N, V% represents the mass content of V, W% represents the mass content of W, and Nb% represents the mass content of Nb. The holding time is preferably 10 minutes or more. This holding causes the intermediate material to develop an austenite structure. After this holding, the intermediate material is rapidly cooled. Examples of rapid cooling methods include water cooling and oil cooling. This rapid cooling causes a martensitic structure to form in the intermediate material.

[0031] To obtain metal products with high hardness and excellent toughness, it is important to quench them at an appropriate temperature according to their composition. The inventors of this invention have found that Qt, calculated by the above formula, correlates with the appropriate quenching temperature.

[0032] Quenching at a temperature of Qt or higher allows the carbides to sufficiently solid dissolve in the Fe matrix. Metal products obtained through this process have excellent toughness. From this viewpoint, the difference between the quenching temperature and Qt is preferably 5°C or higher, more preferably 10°C or higher, and particularly preferably 14°C or higher. The quenching temperature is preferably 1150°C or lower. Quenching at a temperature of 1150°C or lower can yield a metal structure with non-coarse grains. Products having this metal structure have excellent toughness. From this viewpoint, the quenching temperature is more preferably 1130°C or lower, and particularly preferably 1080°C or lower.

[0033] [Tempering] During tempering, the intermediate product is held in a high-temperature environment. This temperature (tempering temperature) is preferably between 200°C and 300°C. Metal products obtained through tempering at a temperature of 200°C or higher exhibit excellent toughness. From this viewpoint, a tempering temperature of 220°C or higher is more preferable, and 240°C or higher is particularly preferable. Metal products obtained through tempering at a temperature of 300°C or lower exhibit excellent hardness and corrosion resistance. From these viewpoints, a tempering temperature of 290°C or higher is more preferable, and 280°C or higher is particularly preferable. The intermediate product is preferably held at the tempering temperature for 1 hour or more. This intermediate product is then slowly cooled. Typically, the intermediate product is air-cooled. This process can yield a product with a stable metal structure.

[0034] [Hardness] The Rockwell hardness of the metal product is preferably 40 HRC or higher, more preferably 41 HRC or higher, and particularly preferably 42 HRC or higher. A Rockwell hardness of 45 HRC or lower is preferable. [Examples]

[0035] The effects of the martensitic stainless steel described in the following examples will be clarified, but the scope disclosed herein should not be interpreted as limiting based on the description of these examples.

[0036] [Example 1] A steel ingot having the composition shown in Table 1 below was melted in a vacuum induction melting furnace. The mass of this steel ingot was 100 kg. This steel ingot was hot forged to obtain a cylindrical bar with a diameter of 15 mm. This bar was annealed. The annealing temperature was 870°C. The annealing time was 2 hours. This bar was hardened. The hardening temperature was 1030°C. During this hardening, the bar was oil-cooled. This bar was tempered. The tempering temperature was 260°C. During this tempering, the bar was held at the tempering temperature for 1 hour. During this tempering, the bar was air-cooled.

[0037] [Examples 2-16 and Comparative Examples 1-15] A bar was obtained in the same manner as in Example 1, except that the composition was as shown in Tables 1 and 2 below, and the heat treatment conditions were as shown in Tables 3 and 4 below.

[0038] [Hardness] The hardness of the tempered bars at room temperature was measured using a Rockwell hardness tester. The results are shown in Tables 3 and 4 below.

[0039] [Resilience] Test specimens were cut from the bar and subjected to a Charpy impact test in accordance with the provisions of "JIS Z 2242:2005" to measure the impact value. The conditions were as follows: Test specimen: JIS-3, Length: 10mm, Width: 10mm, Length: 50mm Notch: 2mm V-notch Temperature: normal temperature These impact values ​​were graded according to the following criteria. Steel with a hardness of less than 40 HRC was not used for toughness evaluation. A: Impact value 45 J / cm 2 That's all. B: Impact value 40 J / cm 2 More than 45J / cm 2 It is less than. C: Impact value 40 J / cm 2 It is less than. -: Not evaluated These results are shown in Tables 3 and 4 below.

[0040] [Corrosion resistance] Test specimens were cut from the bar and subjected to a salt spray test. The conditions were as follows: Test specimen diameter: 12 mm Test specimen length: 21 mm Salt concentration: 50 ppm Saltwater temperature: 35℃ Spray duration: 16 hours Surface area of ​​the test specimen (area: 725 mm²) 2 The steel samples were visually inspected, and the number of spotted rust spots was counted. This number was graded according to the following criteria. Steel samples with a toughness rating of "C" were not used for corrosion resistance evaluation. A: The number is less than 15 B: Number of 15 or more -: Not evaluated These results are shown in Tables 3 and 4 below.

[0041] [Table 1]

[0042] [Table 2]

[0043] [Table 3]

[0044] [Table 4]

[0045] As shown in Tables 3 and 4, the martensitic stainless steels of each example performed well in all evaluation categories. The superiority of this stainless steel is evident from these evaluation results. [Industrial applicability]

[0046] The aforementioned martensitic stainless steel can be used as a material for various metal products. This martensitic stainless steel is particularly suitable for structural and mechanical components.

Claims

1. (A) The material is, C: 0.10% by mass or more and 0.17% by mass or less Si: 0.50% by mass or less Mn: 1.0% by mass or less P: 0.040% by mass or less S: 0.030% by mass or less Ni: 0.60% by mass or less Cr: 11.3% by mass or more and 13.0% by mass or less Mo: 0.25% by mass or less Cu: 0.40% by mass or less Al: 0.050% by mass or less V: 0.20% by mass or less W: 0.20% by mass or less Nb: 0.20% by mass or less and N: 0.050 mass or less It contains, The remainder consists of Fe and unavoidable impurities. A process for preparing an intermediate product which is a martensitic stainless steel in which the total content of V, W, and Nb is 0.20% by mass or less. (D) A step of holding the above intermediate product at a temperature of 800°C or higher and 900°C or lower. and (E) A step of cooling the above intermediate product to 600°C at a rate of 30°C / hr or less. (B) A step of quenching the above intermediate product to a quenching temperature of Qt (°C) or higher, calculated by the following formula: and (C) A process of tempering the above intermediate product, wherein the tempering temperature is between 200°C and 300°C. A method for manufacturing metal products, comprising the following elements in this order. Qt = 930 + 2.3Cr% + 1667(N%- 0.01) + 125(V% + W% + Nb%) (In this formula, Cr% represents the mass content of Cr, N% represents the mass content of N, V% represents the mass content of V, W% represents the mass content of W, and Nb% represents the mass content of Nb.)

2. The manufacturing method according to claim 1, wherein the quenching temperature in step (B) above is 1150°C or lower.

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