Corrosion-resistant tool steel

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

[0002] Tool steel is used in molds. Tool steel has excellent wear resistance and toughness. However, general tool steel has poor corrosion resistance. In applications where corrosion resistance is required, martensitic stainless steel is sometimes used instead of tool steel.

[0003] Japanese Patent Publication 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. Furthermore, this alloy also has excellent corrosion resistance. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 9-291346 [Overview of the project] [Problems that the invention aims to solve]

[0005] The tool steel disclosed in Japanese Patent Publication No. 9-291346 corrodes when used in an environment where weak acids are present. There is room for improvement in the corrosion resistance of this tool steel.

[0006] The objective of the present invention is to provide tool steel with excellent corrosion resistance, wear resistance, and toughness. [Means for solving the problem]

[0007] The tool steel according to the present invention 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 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 mass% or more and 0.15 mass% or less, N: 0% by mass or more and 0.100% by mass or less, and P and / or S: Total of 0% by mass or more and 0.100% or less It contains [a certain substance]. 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 copper.

[0009] Preferably, the N content is 0.005% by mass or more and 0.050% by mass or less. Preferably, the total P and S content 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. [Modes for carrying out the invention]

[0011] The tool steel according to the present invention can be obtained by a melting method, powder metallurgy method, etc. Typically, this tool steel is obtained by sintering of powder. In other words, this alloy is a sintered body. The powder is typically obtained by atomization. This tool steel is obtained through heat treatment. Typical heat treatments are quenching and tempering. This tool steel contains a predetermined amount of additive elements. Preferably, the balance is Fe and unavoidable impurities. Hereinafter, the roles of each element in this tool steel will be described in detail.

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

[0013] [Silicon (Si)] Si contributes to deoxidation in the steelmaking process. Si further contributes to the solid solution strengthening of the tool steel. From these viewpoints, the content of Si is preferably 0.1 mass% or more, more preferably 0.3 mass% or more, and particularly preferably 0.4 mass% or more. Excessive Si inhibits the workability of the tool steel. From the viewpoint of workability, the content of Si 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 in the steelmaking process. Furthermore, Mn enhances the heat treatment properties of tool steel. 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 inhibits the toughness of tool steel. From the viewpoint of toughness, the Mn content is preferably 2.0% by mass or less, more preferably 1.5% by mass or less, and particularly preferably 1.0% by mass or less.

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

[0016] [Molybdenum (Mo), Tungsten (W)] Mo and W form fine carbides M6C (where M is Mo and / or W) in tool steel. These carbides contribute to the strength and wear resistance of the tool steel. The effect of W on strength and wear resistance is about half that of Mo. Therefore, in this invention, the equivalent amounts E1 of the Mo and W content are calculated using 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 lead to excessive carbide precipitation, which inhibits 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% by mass or less. In other words, more preferably, the tool steel satisfies the following formula. Mo% + 0.5 * W% ≤ 1.5 This equivalent weight E1 is particularly preferably 1.0 mass% or less. In other words, the tool steel is particularly preferably satisfied with the following formula. Mo% + 0.5 * W% ≤ 1.0 From the viewpoint of satisfying the above formula (1), the Mo content is preferably 2.0% by mass or less, and the W content is preferably 4.0% by mass or less.

[0017] [Vanadium (V)] V suppresses grain coarsening during quenching. Furthermore, V exists in tool steel as fine carbides VC. These carbides contribute to the high-temperature strength, softening resistance, and wear resistance of 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. Excessive V leads to excessive carbide precipitation, which inhibits the toughness of 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 grain coarsening during quenching. Furthermore, Nb exists in tool steel as fine carbides NbC. These carbides contribute to the high-temperature strength, softening resistance, and wear resistance of tool steel. Excessive Nb leads to the precipitation of excessive NbC carbides. In tool steel, NbC carbides tend to be coarser than VC carbides. These NbC carbides impair the toughness of tool steel. From the viewpoint of toughness, an Nb content of 3.0% by mass or less is preferable.

[0019] [V and Nb] As mentioned above, Nb and V suppress grain coarsening during quenching. On the other hand, excess V and excess Nb impair the toughness of tool steel. The effect of Nb on grain coarsening suppression and toughness is about half that of V. Therefore, in this invention, the equivalent amounts E2 of V and Nb content are 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, an equivalent E2 of 3.0 mass% or more is preferable. From the viewpoint of toughness, an equivalent E2 of 8.0 mass% or less is preferable. In other words, a preferred tool steel satisfies the following formula (2). 3.0 ≦ V% + 0.5 * Nb% ≦ 8.0 (2) This equivalent amount E2 is more preferably 3.5% by mass or more, and particularly preferably 4.0% by mass or more. This equivalent amount 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 inventors' findings, Cu can contribute to the corrosion resistance of the tool steel in environments where weak acids such as phosphoric acid are 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 austenitically stable element. Excess Cu in this tool steel leads to the excess retention of the austenite phase after quenching and tempering. Excess retained austenite phase inhibits 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% by mass or more, more preferably 65% ​​by mass or more, and particularly preferably 70% by mass or more.

[0022] [Nitrogen (N)] Nitrogen (N) leads to the 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. However, the presence of N as an unavoidable impurity is acceptable. On the other hand, N, in synergy with Cu, contributes to corrosion resistance in environments with weak acids. 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 mass% or less, and particularly preferably zero. However, the inclusion of S and P as unavoidable impurities is acceptable. On the other hand, S and P, in synergy with Cu, contribute to corrosion resistance in environments where weak acids are present. From the viewpoint of corrosion resistance, the total content of S and P is preferably 0.005 mass% or more, more preferably 0.010 mass% or more, and particularly preferably 0.015 mass% or more. Tool steel may contain only S, only P, or both S and P.

[0024] [Metal structure] The microstructure of this tool steel consists of a matrix and numerous metal carbides dispersed within it. The dominant element of the matrix is ​​Fe. Other elements are dissolved in the matrix. Metal carbides are compounds of C and Fe or other elements. Examples of metal carbides include Fe3C, M7C3, M6C, and MC, where M represents one or more elements selected from Cr, Mo, W, V, and Nb.

[0025] [Residual austenite phase] As mentioned above, tool steel is obtained through quenching and tempering. In alloys held at high temperatures during quenching, the microstructure is austenite. Upon cooling after quenching, most of the austenite transforms into martensite. Some austenite remains after cooling. This retained austenite transforms into martensite through tempering. This transformation causes secondary hardening. Since the tool steel according to the present invention contains Cu, an austenite-stable element, austenite may remain after tempering. Retained austenite generally contributes to toughness. In the present invention, excess retained austenite actually hinders 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 percentage Pγ of the retained austenite phase after tempering is preferably 30 volume% or less. In other words, it is preferable that the tool steel satisfies the following formula (3). Pγ ≤ 30 (3) This Pγ ratio is more preferably 27% by volume or less, and particularly preferably 25% by volume or less.

[0026] The fraction Pγ of the retained austenite phase is measured by X-ray diffraction. A typical instrument for this measurement is the Rigaku Corporation's X-ray stress analyzer "PSPC-MSF-3M".

[0027] In this invention, the ratio R1 is calculated using 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 between 20 and 1000. 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 steel with a ratio R1 of 20 to 1000 exhibits an excellent balance between corrosion resistance and toughness. From this viewpoint, a ratio R1 of 50 to 800 is more preferable, and a ratio of 100 to 500 is particularly preferable.

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

[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, disc atomization, pulverization, etc. This metal powder is filled into a sealed container and solidified under pressure in a high-temperature atmosphere to obtain a molded body. A preferred pressurization method is hot isotropic pressurization. In hot isotropic pressurization, the powder is pressurized at a high temperature of several hundred to 2000 degrees Celsius under an isotropic pressure of several tens of MPa to 200 MPa. Preferably, an inert gas such as argon gas or helium gas is used as the pressurizing medium. The use of an inert gas suppresses oxidation of the metal powder. This molded body is subjected to hot working. Furthermore, this molded body is subjected to heat treatment to obtain tool steel. A typical heat treatment is "annealing-quenching-tempering". These heat treatments precipitate desirable metal carbides. These heat treatments can achieve a desirable ratio Pγ. [Examples]

[0030] The effects of the present invention will be demonstrated below by the examples, but the present invention should not be interpreted restrictively based on the description in these examples.

[0031] [Example 1] A metal powder was obtained by atomizing molten metal. This metal powder was filled into a cylindrical steel can. The steel can was sealed and then vacuum-degassed. A molded body was obtained by hot isotropic pressurization in an argon gas atmosphere at a pressure of 200 MPa and a temperature of 950°C. A round bar with a diameter of 70 mm was obtained by forging, rolling, hot extrusion, and annealing of this molded body. Test specimens for wear testing, immersion testing, and impact testing were cut from this round bar. These test specimens were hardened to approximately 1150°C and then tempered once for 3 hours. The tempering temperature was adjusted so that a hardness of 62 HRC to 65 HRC could be achieved in the tool steel (the tempering temperature range was 500-600°C). The composition of this tool steel is shown in Table 1 below. This tool steel contains unavoidable impurities in addition to the elements shown in Table 1.

[0032] [Examples 2-17 and Comparative Examples 1-8] Tool steels for Example 2-17 and Comparative Example 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] [Abrasion test] A test specimen with a diameter of 30 mm and a thickness of 10 mm was placed in a Nishihara-type abrasion tester. The amount of abrasion was measured under the following conditions in an environment containing tap water. Countering material: SUJ2 (Diameter: 30mm, Thickness: 6mm) Load: 882N Rotation speed: 860 rpm Lubrication: dripping sewer water (10cm 3 / min) These results are shown in Tables 3 and 4 below.

[0034] [Immersion test] Test specimens with dimensions of 20 mm in length, width, and height were immersed in a phosphoric acid aqueous solution, and the corrosion loss was measured. The conditions were as follows: Concentration of phosphoric acid solution: 0.033 mol / L (Calculated hydrogen ion concentration: 0.1 mol / L) pH: 1.5 Temperature: 25°C Time: 1 hour This result is shown in Tables 3 and 4 below.

[0035] [Impact Test] A test piece with a length of 50 mm, a width of 10 mm, and a thickness of 10 mm was prepared. This test piece has a notch. The size of the notch is "10R, 2 mm C". A Charpy impact test was performed on this test piece in accordance with the provisions of "JIS Z 2242:2005", and the impact value was measured. This result is shown in Tables 3 and 4 below.

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

[0037] [Table 1]

[0038] [Table 2]

[0039] [Table 3]

[0040] [Table 4]

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

[0042] As shown in Tables 3 and 4, the tool steels of each embodiment perform well in all evaluation categories. Based on these evaluation results, the superiority of the present invention is clear. [Industrial applicability]

[0043] The tool steel according to the present invention can be used in a variety of applications, such as molds, injection molding machines, dies, punches, hand tools, machine tools, and cutting tools.

Claims

1. 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: Total of 0% by mass or more and 0.100% by mass or less It contains [a certain substance], with the remainder being Fe and unavoidable impurities. Tool steel that 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 tool steel according to claim 1, satisfying the following formula (4). 20≦Pγ / Cu%≦1000 (4) (In this formula, Cu% represents the mass content of Cu.)

3. The tool steel according to claim 1 or 2, wherein the N content is 0.005% by mass or more and 0.050% by mass or less.

4. The tool steel according to any one of claims 1 to 3, wherein the total content of P and S is 0.005% by mass or more and 0.050% by mass or less.

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