Tool steel excellent in corrosion resistance

Abstract

To provide tool steel excellent in corrosion resistance, abrasion resistance, and toughness.SOLUTION: Tool steel contains C: 2.0 mass% or more and 3.0 mass% or less, Si: 0.1 mass% or more and 2.0 mass% or less, Mn: 0.1 mass% or more and 2.0 mass% or less, Cr: 15.0 mass% or more and 30.0 mass% or less, Mo: 2.0 mass% or less, W: 4.0 mass% or less, V: 3.0 mass% or more and 8.0 mass% or less, Nb: 3.0 mass% or less, Cu: 0.01 mass% or more and 0.15 mass% or less, N: 0 or more and 0.100 mass% or less, and P and / or S: 0 mass% or more and 0.100 mass% or less in total. The tool steel satisfies the mathematical expression below. The tool steel has a volume fraction Pγ of a retained austenite phase of 30% or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a tool steel suitable for applications requiring corrosion resistance, wear resistance and high toughness. [Background technology]

[0002] Tool steel is used for dies. Tool steel has excellent wear resistance and toughness. However, general tool steel has poor corrosion resistance. For applications requiring corrosion resistance, martensitic stainless steel is sometimes used instead of tool steel.

[0003] Japanese Patent Application Laid-Open No. 9-291346 discloses a tool steel containing C, Si, Mn, Cr, Mo, W, V, and Nb. This alloy has excellent wear resistance and toughness. This alloy also has excellent corrosion resistance. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 9-291346 Summary of the Invention [Problem to be solved by the invention]

[0005] When the tool steel disclosed in JP-A-9-291346 is used in an environment where a weak acid is present, corrosion occurs, and there is room for improvement in the corrosion resistance of this tool steel.

[0006] An object of the present invention is to provide a tool steel having excellent corrosion resistance, wear resistance and toughness. [Means for solving the problem]

[0007] The tool steel according to the present invention comprises: C: 2.0% by mass or more and 3.0% by mass or less, Si: 0.1% by mass or more and 2.0% by mass or less, Mn: 0.1% by mass or more and 2.0% by mass or less, Cr: 15.0 mass% or more and 30.0 mass% or less, Mo: 2.0% by mass or less, W: 4.0% by mass or less, V: 3.0% by mass or more and 8.0% by mass or less, Nb: 3.0% by mass or less, Cu: 0.01% by mass or more and 0.15% by mass or less, N: 0% by mass or more and 0.100% by mass or less, and P and / or S: 0 mass% or more and 0.100 or less in total The remainder is Fe and unavoidable impurities. This tool steel satisfies the following formulas (1), (2), and (3). Mo% + 0.5 * W% ≦ 2.0 (1) 3.0 ≦ V% + 0.5 * Nb% ≦ 8.0 (2) Pγ ≦ 30 (3) In these formulas, Mo% represents the mass content of Mo, W% represents the mass content of W, V% represents the mass content of V, Nb% represents the mass content of Nb, and Pγ represents the volume fraction (volume %) of the retained austenite phase after quenching and tempering.

[0008] Preferably, the tool steel satisfies the following formula (4). 20 ≦ Pγ / Cu% ≦ 1000 (4) In this formula, Cu% represents the mass content of Cu.

[0009] Preferably, the content of N is 0.005% by mass or more and 0.050% by mass or less. Preferably, the total content of P and S is 0.005% by mass or more and 0.050% by mass or less. [Effects of the Invention]

[0010] The tool steel according to the present invention is suitable for applications requiring corrosion resistance, wear resistance and high toughness. DETAILED DESCRIPTION OF THE INVENTION

[0011] The tool steel according to the present invention can be obtained by a melting method, a powder metallurgy method, or the like. Typically, the tool steel is obtained by sintering powder. In other words, the alloy is a sintered body. The powder is typically obtained by atomization. The tool steel is obtained through heat treatment. Typical heat treatments include quenching and tempering. The tool steel contains predetermined amounts of additive elements. Preferably, the balance is Fe and unavoidable impurities. The role of each element in the tool steel will be explained in detail below.

[0012] [Carbon (C)] C dissolves in the matrix upon quenching. C precipitates from the matrix upon tempering. Furthermore, C combines with other elements to form carbides. Therefore, C can contribute to the wear resistance and strength of tool steel. From these viewpoints, the C content is preferably 2.0% by mass or more, more preferably 2.1% by mass or more, and particularly preferably 2.2% by mass or more. Excess C leads to excessive precipitation of carbides, which impairs the toughness of the tool steel. Excess C also impairs the corrosion resistance of the tool steel. From the viewpoints of toughness and corrosion resistance, the C content is preferably 3.0% by mass or less, more preferably 2.8% by mass or less, and particularly preferably 2.6% by mass or less.

[0013] [Silicon (Si)] Si contributes to deoxidation during the steelmaking process. Si also contributes to solid-solution strengthening of tool steel. From these viewpoints, the Si content is preferably 0.1 mass% or more, more preferably 0.3 mass% or more, and particularly preferably 0.4 mass% or more. Excessive Si impairs the workability of tool steel. From the viewpoint of workability, the Si content is preferably 2.0 mass% or less, more preferably 1.5 mass% or less, and particularly preferably 1.0 mass% or less.

[0014] [Manganese (Mn)] Mn contributes to deoxidation during the steelmaking process. Mn also improves the heat treatment properties of tool steel. From these viewpoints, the Mn content is preferably 0.1 mass% or more, more preferably 0.2 mass% or more, and particularly preferably 0.3 mass% or more. Excess Mn inhibits the toughness of tool steel. From the viewpoint of toughness, the Mn content is preferably 2.0 mass% or less, more preferably 1.5 mass% or less, and particularly preferably 1.0 mass% or less.

[0015] Chromium Cr forms carbides, which contribute to the wear resistance of tool steel. Cr also contributes to the corrosion resistance of tool steel. From these viewpoints, the Cr content is preferably 15.0 mass% or more, more preferably 16.0 mass% or more, and particularly preferably 17.0 mass% or more. Excess Cr leads to excessive precipitation of carbides, which impairs the toughness of tool steel. From the viewpoint of toughness, the Cr content is preferably 30.0 mass% or less, more preferably 25.0 mass% or less, and particularly preferably 20.0 mass% or less.

[0016] [Molybdenum (Mo), Tungsten (W)] Mo and W form fine carbides M6C (M is Mo and / or W) in tool steel. These carbides contribute to the strength and wear resistance of tool steel. The effect of W on strength and wear resistance is about half that of Mo. Therefore, in the present invention, the equivalent E1 of the content of Mo and W is calculated by the following formula: E1 = Mo% + 0.5 * W% In this formula, Mo% represents the mass content of Mo, and W% represents the mass content of W. From the viewpoint of strength and wear resistance, the equivalent E1 is preferably 0.2 mass% or more, more preferably 0.3 mass% or more, and particularly preferably 0.4 mass% or more. Excess Mo and W cause excessive carbide precipitation and impair the toughness of the tool steel. From this viewpoint, the equivalent E1 is preferably 2.0 mass% or less. In other words, a preferred tool steel satisfies the following formula (1): Mo% + 0.5 * W% ≦ 2.0 (1) This equivalent E1 is more preferably 1.5 mass % or less. In other words, the tool steel more preferably satisfies the following formula: Mo% + 0.5 * W% ≦ 1.5 This equivalent E1 is particularly preferably 1.0 mass % or less. In other words, it is particularly preferable that the tool steel satisfy the following formula: Mo% + 0.5 * W% ≦ 1.0 From the viewpoint of satisfying the above mathematical formula (1), the Mo content is preferably 2.0 mass % or less, and the W content is preferably 4.0 mass % or less.

[0017] [Vanadium(V)] V suppresses the coarsening of crystal grains during quenching. Furthermore, V exists as fine carbides VC in tool steel. These carbides contribute to the high-temperature strength, softening resistance, and wear resistance of the tool steel. From these viewpoints, the V content is preferably 3.0% by mass or more, more preferably 3.5% by mass or more, and particularly preferably 4.0% by mass or more. Excess V leads to the precipitation of excessive carbides, which impairs the toughness of the tool steel. From the viewpoint of toughness, the V content is preferably 8.0% by mass or less, more preferably 7.0% by mass or less, and particularly preferably 6.0% by mass or less.

[0018] [Niobium (Nb)] Nb suppresses the coarsening of crystal grains during quenching. Furthermore, Nb exists as fine carbide NbC in tool steel. This carbide contributes to the high-temperature strength, softening resistance, and wear resistance of tool steel. Excessive Nb leads to the precipitation of excessive NbC carbide. In tool steel, NbC carbide tends to be coarser than VC carbide. This NbC carbide impairs the toughness of tool steel. From the viewpoint of toughness, the Nb content is preferably 3.0 mass% or less.

[0019] [V and Nb] As mentioned above, Nb and V suppress the coarsening of crystal grains during quenching. On the other hand, excessive V and excessive Nb impair the toughness of tool steel. The effect of Nb on suppressing coarsening and toughness is about half that of V. Therefore, in the present invention, the equivalent E2 of the V and Nb content is calculated by the following formula: E2 = V% + 0.5 * Nb% In this formula, V% represents the mass content of V, and Nb% represents the mass content of Nb. From the viewpoint of suppressing grain coarsening, the equivalent weight E2 is preferably 3.0 mass% or more. From the viewpoint of toughness, the equivalent weight E2 is preferably 8.0 mass% or less. In other words, a preferred tool steel satisfies the following formula (2): 3.0 ≦ V% + 0.5 * Nb% ≦ 8.0 (2) This equivalent E2 is more preferably 3.5% by mass or more, and particularly preferably 4.0% by mass or more. This equivalent E2 is more preferably 7.0% by mass or less, and particularly preferably 6.0% by mass or less.

[0020] Copper (Cu) In the tool steel according to the present invention, Cu is an extremely important additive element. According to the findings of the present inventors, Cu can contribute to the corrosion resistance of the tool steel in an environment where a weak acid such as phosphoric acid is present. From the viewpoint of corrosion resistance, the Cu content is preferably 0.01% by mass or more, more preferably 0.02% by mass or more, and particularly preferably 0.03% by mass or more. Cu is an austenite-stabilizing element. Excessive Cu in this tool steel leads to excessive retention of the austenite phase after quenching and tempering. Excessive retained austenite phase impairs the toughness of the tool steel. From the viewpoint of toughness, the Cu content is preferably 0.15% by mass or less, more preferably 0.10% by mass or less, and particularly preferably 0.07% by mass or less.

[0021] [Iron (Fe)] The main component of tool steel is Fe. Therefore, this alloy has excellent toughness. From the viewpoint of toughness, the Fe content is preferably 60 mass % or more, more preferably 65 mass % or more, and particularly preferably 70 mass % or more.

[0022] Nitrogen N causes coarsening of carbides and nitrides. Coarse carbides and nitrides impair the toughness of tool steel. From the viewpoint of toughness, the N content is preferably 0.100% by mass or less, and particularly preferably zero. The inclusion of N as an unavoidable impurity is acceptable. On the other hand, N contributes to corrosion resistance in an environment where weak acids are present through a synergistic effect with Cu. From the viewpoint of corrosion resistance, the N content is preferably 0.005% by mass or more, more preferably 0.010% by mass or more, and particularly preferably 0.015% by mass or more.

[0023] [Sulfur (S), Phosphorus (P)] S and P impair the strength of tool steel. From the viewpoint of strength, the total content of S and P is preferably 0.100% by mass or less, and particularly preferably zero. The inclusion of S and P as unavoidable impurities is permissible. On the other hand, S and P contribute to corrosion resistance in an environment where a weak acid is present through a synergistic effect with Cu. From the viewpoint of corrosion resistance, the total content of S and P is preferably 0.005% by mass or more, more preferably 0.010% by mass or more, and particularly preferably 0.015% by mass or more. The tool steel may contain only S, only P, or both S and P.

[0024] [Metal structure] The metal structure of this tool steel includes a matrix and numerous metal carbides dispersed in the matrix. The main element of the matrix is ​​Fe. In the matrix, other elements are dissolved in Fe as a solid solution. The metal carbides are compounds of C with Fe or other elements. Examples of metal carbides include Fe3C, MC3, MC, and MC. Here, M represents one or more elements selected from Cr, Mo, W, V, and Nb.

[0025] [Retained austenite phase] As mentioned above, tool steel is obtained through quenching and tempering. The structure of an alloy held at high temperatures during quenching is austenite. Cooling after quenching transforms much of the austenite into martensite. Some austenite remains even after cooling. Tempering transforms the retained austenite into martensite. This transformation causes secondary hardening. Since the tool steel according to the present invention contains Cu, an austenite-stabilizing element, austenite can remain even after tempering. Retained austenite generally contributes to toughness. In the present invention, excessive retained austenite actually impairs the toughness of the tool steel. From the viewpoint of toughness, it is preferable to select heat treatment conditions that can obtain an appropriate volume fraction of the retained austenite phase. The fraction Pγ of the retained austenite phase after tempering is preferably 30% by volume or less. In other words, it is preferable that the tool steel satisfy the following mathematical formula (3): Pγ ≦ 30 (3) This ratio Pγ is more preferably 27% by volume or less, and particularly preferably 25% by volume or less.

[0026] The fraction of retained austenite, Pγ, is measured by X-ray diffraction. A typical measurement device is the Rigaku X-ray stress measurement device PSPC-MSF-3M.

[0027] In the present invention, the ratio R1 is calculated by the following formula. R1 = Pγ / Cu% In this formula, Pγ represents the volume fraction (volume %) of the retained austenite phase after quenching and tempering, and Cu% represents the mass content of Cu. The ratio R1 is preferably 20 or more and 1000 or less. In other words, a preferred tool steel satisfies the following formula (4): 20 ≦ Pγ / Cu% ≦ 1000 (4) According to the findings of the present inventors, tool steels having a ratio R1 of 20 or greater and 1000 or less have an excellent balance between corrosion resistance and toughness. From this viewpoint, the ratio R1 is more preferably 50 or greater and 800 or less, and particularly preferably 100 or greater and 500 or less.

[0028] A desirable ratio R1 can be achieved by adjusting the alloy composition. A desirable ratio R1 can be achieved by appropriate heat treatment conditions. For example, a desirable ratio R1 can be achieved by adjusting the quenching temperature, quenching time, tempering temperature, tempering time, number of temperings, etc.

[0029] [Powder metallurgy method] The tool steel according to the present invention can be obtained by powder metallurgy. In powder metallurgy, metal powder is first produced by gas atomization, water atomization, disk atomization, pulverization, or other methods. This metal powder is then packed into a sealed container and solidified under high-temperature pressure to obtain a compact. A preferred pressing method is hot isostatic pressing. In hot isostatic pressing, the powder is pressed at high temperatures of several hundred to 2,000 degrees Celsius under an isotropic pressure of several tens to 200 MPa. Preferably, an inert gas, such as argon or helium, is used as the pressurizing medium. The use of an inert gas suppresses oxidation of the metal powder. The compact is then hot-worked. The tool steel is then obtained by heat treatment. A typical heat treatment is annealing-quenching-tempering. These heat treatments precipitate preferred metal carbides. These heat treatments can achieve a preferred ratio Pγ. [Example]

[0030] The effects of the present invention will be clarified below by examples, but the present invention should not be construed as being limited based on the descriptions of these examples.

[0031] [Example 1] The molten metal was atomized to obtain a metal powder. This metal powder was filled into a cylindrical steel can. The can was sealed and then vacuum-degassed. A compact was obtained by hot isostatic pressing under an argon gas atmosphere at a pressure of 200 MPa and a temperature of 950°C. This compact was then forged, rolled, hot extruded, and annealed to obtain a round bar with a diameter of 70 mm. Test specimens for wear tests, immersion tests, and impact tests were cut from the round bar. These test specimens were quenched at approximately 1150°C and tempered once for 3 hours. The tempering temperature was adjusted so that the tool steel would achieve a hardness of 62 HRC or more and 65 HRC or less (the tempering temperature range was 500-600°C). The composition of this tool steel is shown in Table 1 below. In addition to the elements listed in Table 1, this tool steel also contains unavoidable impurities.

[0032] [Examples 2-17 and Comparative Examples 1-8] Tool steels of Examples 2-17 and Comparative Examples 1-8 were obtained in the same manner as in Example 1, except that the compositions were as shown in Tables 1 and 2 below.

[0033] [Wear test] A test piece with a diameter of 30 mm and a thickness of 10 mm was set in a Nishihara abrasion tester. The amount of abrasion was measured under the following conditions in an environment containing tap water. Mating material: SUJ2 (diameter: 30 mm, thickness: 6 mm) Load: 882N Rotation speed: 860 rpm Lubrication: dripping sewer water (10cm 3 / min) The results are shown in Tables 3 and 4 below.

[0034] [Immersion test] A test piece measuring 20 mm in length, width and height was immersed in a phosphoric acid aqueous solution and the corrosion weight loss was measured under the following conditions. Phosphoric acid aqueous solution concentration: 0.033 mol / L (calculated hydrogen ion concentration: 0.1 mol / L) pH: 1.5 Temperature: 25℃ Time: 1 hour The results are shown in Tables 3 and 4 below.

[0035] [Impact test] A test piece measuring 10 mm in length, 10 mm in width, and 50 mm in length was prepared. This test piece had a notch. The notch size was "10R, 2 mmC." This test piece was subjected to a Charpy impact test in accordance with the provisions of "JIS Z 2242:2005," and the impact value was measured. The results are shown in Tables 3 and 4 below.

[0036] [comprehensive evaluation] Each tool steel was rated based on the following criteria: A: The alloy satisfies the following (1) to (3). (1) Wear amount is less than 30 mg (2) Corrosion weight loss is 1g / (m 2 Less than 1 hour (3) Impact value: 16 J / cm 2 Greater than B: The alloy satisfies the following (1) to (3). (1) Wear amount is less than 30 mg (2) Corrosion weight loss is 1g / (m 2 ·hr) or more 5g / (m 2 Less than 1 hour (3) Impact value: 16 J / cm 2 Greater than C: The alloy satisfies the following (1) to (3). (1) Wear amount is less than 30 mg (2) Corrosion loss: 5g / (m 2 ·hr) or more 10g / (m 2 Less than 1 hour (3) Impact value: 16 J / cm 2 Greater than D: The alloy satisfies the following (1) to (3). (1) Wear amount is less than 30 mg (2) Corrosion loss: 5g / (m 2 ·hr) or more 10g / (m 2 Less than 1 hour (3) Impact value: 14 J / cm2 Greater than F: The alloy does not satisfy at least one of the following (1) to (3): (1) The amount of wear is less than 30 mg. (2) Corrosion loss is 10g / (m 2 hr) (3) Impact value: 16 J / cm 2 Greater than The results are shown in Tables 3 and 4 below.

[0037] [Table 1]

[0038] [Table 2]

[0039] [Table 3]

[0040] [Table 4]

[0041] The units of evaluation items in Tables 3 and 4 are as follows: Friction amount: mg Corrosion weight loss: g / (m 2 hr) Impact value: J / cm 2

[0042] As shown in Tables 3 and 4, the tool steels of the examples were excellent in all evaluation items. From the above evaluation results, the superiority of the present invention is clear. [Industrial Applicability]

[0043] The tool steel according to the present invention can be used for a variety of purposes, such as molds, injection molding machines, die sets, punches, hand tools, machine tools, and blades.

Claims

1. (1) A process for obtaining a molded body by compressing metal powder in a high-temperature atmosphere to solidify it; (2) a step of subjecting the compact to hot working; and (3) A step of subjecting the compact to heat treatment A method for producing a tool steel, comprising: The tool steel is C: 2.0% by mass or more and 3.0% by mass or less, Si: 0.1% by mass or more and 2.0% by mass or less, Mn: 0.1% by mass or more and 2.0% by mass or less, Cr: 15.0% by mass or more and 30.0% by mass or less, Mo: 2.0% by mass or less, W: 4.0% by mass or less, V: 3.0% by mass or more and 8.0% by mass or less, Nb: 3.0% by mass or less, Cu: 0.01% by mass or more and 0.15% by mass or less, N: 0% by mass or more and 0.100% by mass or less, and P and / or S: 0 mass% or more and 0.100 mass% or less in total and the balance being Fe and unavoidable impurities, The method for producing a tool steel, wherein the tool steel satisfies the following formulas (1), (2), and (3): Mo% + 0.5 * W% ≦ 2.0 (1) 3.0 ≦ V% + 0.5 * Nb% ≦ 8.0 (2) Pγ≦30 (3) (In these formulas, Mo% represents the mass content of Mo, W% represents the mass content of W, V% represents the mass content of V, Nb% represents the mass content of Nb, and Pγ represents the volume fraction (volume %) of the retained austenite phase after quenching and tempering.)

2. The manufacturing method described in claim 1, wherein the tool steel satisfies the following formula (4): 20≦Pγ / Cu%≦1000 (4) (In this formula, Cu% represents the mass content of Cu.)

3. A manufacturing method described in claim 1 or 2, wherein the N content in the tool steel is 0.005 mass% or more and 0.050 mass% or less.

4. A manufacturing method described in any of claims 1 to 3, wherein the total content of P and S in the tool steel is 0.005 mass% or more and 0.050 mass% or less.

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