Powder high-speed steel

Abstract

To provide a powder high-speed steel that achieves a superior balance of wear resistance and toughness.SOLUTION: The powder high-speed steel contains: C at 1.00 mass% or more and 1.50 mass% or less; Si at 0.60 mass% or more and 1.20 mass% or less; Mn at 0.60 mass% or more and 1.50 mass% or less; Cr at 2.0 mass% or more and 4.0 mass% or less; Mo at 0.5 mass% or more and 4.0 mass% or less; V at 0.5 mass% or more and 3.0 mass% or less; and Co at 2.0 mass% or more and 6.0 mass% or less, the balance being Fe and unavoidable impurities. A metal structure of the powder high-speed steel includes a plurality of crystal grains and grain boundaries. Each crystal grain contains a matrix and a plurality of carbides dispersed in the matrix. In the crystal grains, an area ratio P1 of carbides having a circle-equivalent diameter of 5 μm or more is 10% or less. An area ratio P2 of carbides present at the grain boundaries is 15% or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This specification discloses a powder metallurgically obtained high speed steel. [Background technology]

[0002] Cutting tools made of high-speed steel are widely used. High-speed steel is also called "high-speed tool steel." High-speed steel obtained by powder metallurgy is called "powdered high-speed steel." Powdered high-speed steel is also called "powdered high-speed steel" or "sintered high-speed steel."

[0003] Cutting tools rub against the workpiece. Therefore, cutting tools need to be wear-resistant. Japanese Patent Laid-Open Publication No. 2-175846 discloses a powder high-speed steel intended to improve wear resistance. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2-175846 Summary of the Invention [Problem to be solved by the invention]

[0005] A cutting tool that is less likely to break during use is desirable. From the viewpoint of suppressing breakage, powder high-speed steel with excellent toughness is desired. Powder high-speed steel with excellent toughness is also desired for applications other than cutting. The toughness of conventional powder high-speed steel is insufficient.

[0006] The applicant's intention is to provide a powder high-speed steel having an excellent balance between wear resistance and toughness. [Means for solving the problem]

[0007] The powder high-speed steel disclosed in this specification is C: 1.00% by mass or more and 1.50% by mass or less, Si: 0.60 mass% or more and 1.20 mass% or less, Mn: 0.60 mass% or more and 1.50 mass% or less, Cr: 2.0% by mass or more and 4.0% by mass or less, Mo: 0.5% by mass or more and 4.0% by mass or less, V: 0.5% by mass or more and 3.0% by mass or less, and Co: 2.0% by mass or more and 6.0% by mass or less, The remainder is Fe and unavoidable impurities. The metal structure of this powder high-speed steel includes a plurality of crystal grains and grain boundaries. Each crystal grain includes a matrix and a plurality of carbides dispersed in the matrix. The area fraction P1 of carbides having an equivalent circle diameter of 5 μm or more in the crystal grains is 10% or less. The area fraction P2 of carbides present at the grain boundaries is 15% or less. [Effects of the Invention]

[0008] This powder high-speed steel has an excellent balance of wear resistance and toughness, and is suitable for applications where large frictional forces and large stresses are generated. DETAILED DESCRIPTION OF THE INVENTION

[0009] The high-speed steel according to this embodiment is obtained by sintering powder. In other words, this high-speed steel is a sintered body. The powder is typically obtained by atomization. This powder high-speed steel is further obtained by heat treatment. Typical heat treatments include solution treatment, annealing, quenching, and tempering.

[0010] [composition] This powder high-speed steel is C: 1.00% by mass or more and 1.50% by mass or less, Si: 0.60 mass% or more and 1.20 mass% or less, Mn: 0.60 mass% or more and 1.50 mass% or less, Cr: 2.0% by mass or more and 4.0% by mass or less, Mo: 0.5% by mass or more and 4.0% by mass or less, V: 0.5% by mass or more and 3.0% by mass or less, and Co: 2.0% by mass or more and 6.0% by mass or less, Preferably, the balance is Fe and unavoidable impurities.

[0011] [Metal structure] The metal structure of powder high-speed steel is polycrystalline. This metal structure includes multiple crystal grains and grain boundaries. Each crystal grain includes a matrix and multiple carbides dispersed in this matrix. The base of the matrix is ​​Fe. In the matrix, other elements are dissolved in Fe as solid solutions. The carbides are compounds of C and other elements.

[0012] [element] The elements contained in the powder high speed steel will be described in detail below.

[0013] [Carbon (C)] Carbon combines with other elements to form carbides. These carbides can contribute to the wear resistance of powder high-speed steel. Typical examples of carbides include chromium carbide, vanadium carbide, and molybdenum carbide. Carbon may combine with two or more elements. Carbon also dissolves in Fe, the matrix base. This dissolution can contribute to the high hardness, wear resistance, and fatigue life of powder high-speed steel. From these viewpoints, the C content is preferably 1.00% by mass or more, and particularly preferably 1.05% by mass or more. Excessive C leads to the formation of excessive carbides, which impairs the toughness of powder high-speed steel. From the viewpoint of toughness, the C content is preferably 1.50% by mass or less, more preferably 1.35% by mass or less, and particularly preferably 1.20% by mass or less.

[0014] [Silicon (Si)] Si contributes to deoxidation during the steelmaking process. Si also contributes to the hardenability of steel. Powder high-speed steel containing sufficient Si has excellent wear resistance. From these viewpoints, the Si content is preferably 0.60 mass% or more, more preferably 0.65 mass% or more, and particularly preferably 0.70 mass% or more. Excessive Si inhibits the toughness and workability of powder high-speed steel. From the viewpoints of toughness and workability, the Si content is preferably 1.20 mass% or less, more preferably 1.15 mass% or less, and particularly preferably 1.10 mass% or less.

[0015] [Manganese (Mn)] Mn contributes to deoxidation during the steelmaking process. Mn also contributes to the strength and toughness of powder high-speed steel. From these viewpoints, the Mn content is preferably 0.60% by mass or more, more preferably 0.70% by mass or more, and particularly preferably 0.80% by mass or more. Excess Mn impairs the toughness of powder high-speed steel. Mn can combine with S as an impurity to form MnS. This MnS promotes cracking of powder high-speed steel during use. From the viewpoints of toughness and suppression of cracking, the Mn content is preferably 1.50% by mass or less, more preferably 1.40% by mass or less, and particularly preferably 1.30% by mass or less.

[0016] Chromium Cr combines with C to form carbides. These carbides can contribute to the wear resistance of powder high-speed steel. Cr also contributes to the hardenability of the steel. Powder high-speed steel containing sufficient Cr has high hardness. From these viewpoints, the Cr content is preferably 2.0 mass% or more, more preferably 2.1 mass% or more, and particularly preferably 2.2 mass% or more. Excessive Cr leads to the precipitation of excessive carbides. Excessive carbides impair the toughness of powder high-speed steel. From the viewpoint of toughness, the Cr content is preferably 4.0 mass% or less, more preferably 3.8 mass% or less, and particularly preferably 3.5 mass% or less.

[0017] [Molybdenum (Mo)] Mo combines with C to form carbides. These carbides can contribute to the strength and wear resistance of powder high-speed steel. Mo also contributes to the toughness of powder high-speed steel. From these viewpoints, the Mo content is preferably 0.5% by mass or more, more preferably 0.6% by mass or more, and particularly preferably 0.7% by mass or more. Excess Mo leads to the precipitation of excessive carbides. Excessive carbides impair the toughness of powder high-speed steel. From the viewpoint of toughness, the Mo content is preferably 4.0% by mass or less, more preferably 3.7% by mass or less, and particularly preferably 3.5% by mass or less.

[0018] [Vanadium(V)] In this powder high-speed steel, V is an extremely important element. V can combine with C during heating for quenching to form fine carbides. These carbides pin the grain boundaries during heating. This pinning allows for a metallographic structure with fine grains. Powder high-speed steel having this metallographic structure has excellent toughness. From the viewpoint of fine grains, the V content is preferably 0.5% by mass or more, more preferably 0.7% by mass or more, and particularly preferably 1.0% by mass or more. Excess V leads to the precipitation of excessive carbides. Excessive carbides impair the hot workability and toughness of the powder high-speed steel. From the viewpoint of hot workability and toughness, the V content is preferably 3.0% by mass or less, more preferably 2.7% by mass or less, and particularly preferably 2.5% by mass or less.

[0019] [Cobalt (Co)] Co forms a substitutional solid solution with Fe, which is the base of the matrix. This solid solution contributes to the hardenability of the steel. Co also contributes to the strength of the powder high-speed steel. From these viewpoints, the Co content is preferably 2.0 mass% or more, more preferably 2.3 mass% or more, and particularly preferably 2.5 mass% or more. From the viewpoint of low cost, the Co content is preferably 6.0 mass% or less, more preferably 5.5 mass% or less, and particularly preferably 5.0 mass% or less.

[0020] [Iron (Fe)] The main component of the powder high-speed steel is Fe. This powder high-speed steel has excellent toughness. From the viewpoint of toughness, the Fe content is preferably 70 mass % or more, more preferably 80 mass % or more, and particularly preferably 85 mass % or more.

[0021] [impurities] Powdered high-speed steel may contain unavoidable impurities. A typical example of such an impurity is oxygen (O). O generates oxides, which can impair the toughness of the powdered high-speed steel. From the viewpoint of suppressing toughness, the O content is preferably 0.10% by mass or less, more preferably 0.05% by mass or less, and particularly preferably 0.01% by mass or less. Another impurity is nitrogen (N). N causes coarsening of carbides. N also generates nitrides. These carbides and nitrides impair the toughness of the powdered high-speed steel. From the viewpoint of toughness, the N content is preferably 0.10% by mass or less, more preferably 0.05% by mass or less, and particularly preferably 0.01% by mass or less.

[0022] [Area ratio P1] As described above, the crystal grains contain a matrix and a plurality of carbides dispersed in the matrix. The area fraction P1 of carbides having an equivalent circle diameter of 5 μm or more in the crystal grains is preferably 10% or less. Carbides can impair the toughness of the powder high-speed steel. A powder high-speed steel having an area fraction P1 of 10% or less has excellent toughness. When a tool made of this powder high-speed steel is used for cutting, the tool is less likely to break. From the viewpoint of preventing breakage, the area fraction P1 is more preferably 8% or less, and particularly preferably 5% or less. The area fraction P1 may be 0%. A powder high-speed steel having an area fraction P1 of 0% does not contain carbides having an equivalent circle diameter of 5 μm or more. Carbides having an equivalent circle diameter of less than 5 μm may be present among these carbides.

[0023] Coarse carbides significantly impair the toughness of powder high-speed steel. The inventors focused on carbides with an equivalent circle diameter of 5 μm or more and calculated the area ratio P1. Carbides whose circle having the same area as the carbide has a diameter of 5 μm or more correspond to "carbides whose equivalent circle diameter is 5 μm or more."

[0024] A test piece for measuring the area ratio P1 is obtained from powder high-speed steel after tempering, which will be described in detail later. The test piece is cut, and the exposed cut surface is polished. The cut surface is etched with Vilela's solution. The cut surface is photographed, and an image of the inside of the crystal grain is obtained. The image is analyzed to calculate the area ratio P1. The area ratio P1 is calculated using the following formula. P1 = (S1 / S2) 100 In this formula, S1 represents the total area of ​​multiple carbides with an equivalent circle diameter of 5 μm or more contained within the field of view, and S2 represents the area of ​​this field of view. This field of view is selected from randomly selected crystal grains, avoiding grain boundaries.

[0025] Carbides present in crystal grains and having an equivalent circle diameter of less than 5 μm have little adverse effect on toughness. These carbides can contribute to the wear resistance of powder high-speed steel. From these viewpoints, the area ratio of carbides having an equivalent circle diameter of less than 5 μm in the crystal grains is preferably 3% or more.

[0026] [Carbide area ratio P2] As mentioned above, the metal structure of this powder high-speed steel has grain boundaries. The area fraction P2 of carbides present at the grain boundaries is preferably 15% or less. Carbides can impair the toughness of the powder high-speed steel. Powder high-speed steel with an area fraction P2 of 15% or less has excellent toughness. When cutting is performed with a tool made of this powder high-speed steel, the tool is less likely to break. From the viewpoint of preventing breakage, this area fraction P2 is more preferably 12% or less, and particularly preferably 10% or less. The smaller the area fraction P2, the better.

[0027] The area ratio P2 is measured from powder high-speed steel after tempering, which will be described in detail later. The test piece is cut, and the exposed cut surface is polished. The cut surface is then etched with Vilela's solution. The cut surface is photographed, and an image including the crystal grains and grain boundaries is obtained. The image is analyzed to calculate the area ratio P2. The area ratio P2 is calculated using the following formula. P2 = (S3 / S4) 100 In this formula, S3 represents the total area of ​​multiple carbides (amorphous) contained within the field of view and present at the grain boundaries, and S4 represents the area of ​​this field of view. This field of view is randomly determined. This area S4 is the sum of the area of ​​the crystal grains and the area of ​​the grain boundaries contained in this field of view.

[0028] [Hardness] The hardness of powder high-speed steel correlates with its wear resistance. From the viewpoint of wear resistance, the hardness (HRC) of powder high-speed steel is preferably 50 or more, more preferably 55 or more, and particularly preferably 58 or more. From the viewpoint of toughness of powder high-speed steel, the hardness (HRC) is preferably 70 or less, more preferably 67 or less, and particularly preferably 65 or less.

[0029] [Manufacturing method] The high speed steel according to the present invention can be obtained through powder metallurgy and heat treatment, typically annealing-solution-quenching-tempering.

[0030] [Powder metallurgy method] In powder metallurgy, metal powder is first produced by gas atomization, water atomization, disk atomization, pulverization, or the like. This metal powder is pressed in a high-temperature atmosphere to solidify, resulting in a green body (sintering). A preferred pressing method is hot isostatic pressing (HIP). In hot isostatic pressing, powder is pressed with isostatic pressure at high temperature. Preferably, an inert gas such as argon gas or helium gas is used as the pressurizing medium. This green body is subjected to hot working as needed. This green body is then annealed as needed.

[0031] [Solution treatment] Carbides are present in the metal structure of the compact after sintering or hot working. In the solution treatment, the compact is held in a high-temperature environment. By holding in a high-temperature environment, C and other metal atoms contained in the carbides dissolve in Fe. This holding reduces the amount of carbides. As mentioned above, this alloy contains V. V carbides are less likely to decompose at high temperatures than Fe carbides and Cr carbides. For example, by holding at a temperature of 1250°C or less, carbides other than V carbide (VC) dissolve in Fe, and VC may remain. The compact is then cooled. This solution treatment results in a metal structure in which fine VC is sufficiently dispersed.

[0032] [Quenching] During quenching, the metal structure is held at a temperature higher than the A3 line. This holding results in an austenite structure. Furthermore, this holding results in the precipitation of fine V carbides. As mentioned above, there is sufficient V dissolved in the metal structure after solution treatment. Therefore, quenching results in the precipitation of sufficient V carbides. These carbides pin the grain boundaries. This pinning suppresses the growth of grains. The compact is then rapidly cooled. This rapid cooling results in a metal structure in which C is supersaturated in solid solution. This structure is martensite. Due to the effect of pinning, the martensite is fine.

[0033] [Tempering] During tempering, the compact is held at a temperature lower than the A1 line and slowly cooled. This tempering process causes the precipitation of fine carbides. These carbides contribute to the wear resistance of powder high-speed steel. Because the carbides are fine, they do not impair the toughness of powder high-speed steel.

[0034] [Heat treatment conditions] An example of preferable heat treatment conditions from the viewpoint of achieving the preferable area ratios P1 and P2 is shown below. Solution treatment Hold at 1150℃ for 1.5 hours Hold at 600℃ for 1.5 hours Slow cooling Quenching Hold at 1100℃ for 2 hours oil cooled Tempering Hold at 200℃ for 2 hours air-cooled

[0035] [Cutting tools] This specification is also directed to cutting tools such as drills and taps. The material of these cutting tools is the powder high-speed steel described above. That is, this powder high-speed steel is C: 1.00% by mass or more and 1.50% by mass or less, Si: 0.60 mass% or more and 1.20 mass% or less, Mn: 0.60 mass% or more and 1.50 mass% or less, Cr: 2.0% by mass or more and 4.0% by mass or less, Mo: 0.5% by mass or more and 4.0% by mass or less, V: 0.5% by mass or more and 3.0% by mass or less, and Co: 2.0% by mass or more and 6.0% by mass or less, The remainder is Fe and unavoidable impurities. The metal structure of this powder high-speed steel includes a plurality of crystal grains and grain boundaries. Each crystal grain includes a matrix and a plurality of carbides dispersed in the matrix. The area ratio P1 of carbides having an equivalent circle diameter of 5 μm or more in the crystal grains is 10% or less. The area ratio P2 of carbides present at grain boundaries is 15% or less. This cutting tool has excellent wear resistance and toughness. This cutting tool is suitable for machining difficult-to-cut materials. [Example]

[0036] The effects of the powder high-speed steel according to the examples will be clarified below, but the scope of the present disclosure should not be construed as being limited based on the description of these examples.

[0037] [Example 1] The raw materials were placed in a refractory crucible. The raw materials were melted in an argon gas atmosphere using a dielectric method to obtain a molten metal. The molten metal was dropped from a nozzle at the bottom of the crucible and atomized with nitrogen gas to obtain a powder. The powder was classified to adjust the particle size to 500 μm or less. The powder was filled into a cylindrical steel can. The steel can was vacuum degassed and then sealed. A compact was obtained by hot isostatic pressing under an argon gas atmosphere at a pressure of 100 MPa and a temperature of 1100°C. The compact was then forged, rolled, hot extruded, and annealed to obtain a round bar with a diameter of 50 mm. The round bar was then subjected to a solution treatment. In this solution treatment, the round bar was first held at 1150°C for 1.5 hours, then held at 600°C for 1.5 hours, and then slowly cooled. This round bar was quenched at 1100°C and then tempered at 200°C for 2 hours to obtain powder high-speed steel according to Example 1. The composition of this powder high-speed steel is shown in Table 1 below. In addition to the elements shown in Table 1, this powder high-speed steel contains Fe and unavoidable impurities.

[0038] [Examples 2-14 and Comparative Examples 1-16] Powder high-speed steels of Examples 2-14 and Comparative Examples 1-16 were obtained in the same manner as in Example 1, except that the compositions were as shown in Table 1 below and the quenching and tempering conditions were changed.

[0039] [Wear resistance] Test pieces were cut out from the tempered powder high-speed steel. The Rockwell hardness (HRC) of the test pieces was measured at five points. The average values ​​of the five measurement results are shown in Table 2 below. Powder high-speed steel with a higher average value has better wear resistance.

[0040] [Toughness] Test pieces were cut out from the tempered powder high-speed steel, and the sizes of the test pieces were as follows: Test piece: JIS No. 3 10mm x 10mm x 50mm Notch: 10RC The test pieces were subjected to a Charpy impact test in accordance with the provisions of "JIS Z 2242:2005" to measure the impact value. The results are shown in Table 2 below. Powdered high-speed steel with a high impact value has excellent toughness.

[0041] [Table 1]

[0042] [Table 2]

[0043] As shown in Table 2, the powder high-speed steel of each example has an excellent balance between wear resistance and toughness. The above evaluation results clearly demonstrate the superiority of this powder high-speed steel. [Industrial Applicability]

[0044] This powder high speed steel can be used in a variety of applications, such as cutting tools, molds, injection molding machines, die sets, punches, hand tools, machine tools, and blades.

Claims

[Claim 1] C: 1.00% by mass or more and 1.50% by mass or less, Si: 0.60% by mass or more and 1.20% by mass or less, Mn: 0.60% by mass or more and 1.50% by mass or less, Cr: 2.0% by mass or more and 4.0% by mass or less, Mo: 0.5% by mass or more and 4.0% by mass or less, V: 0.5% by mass or more and 3.0% by mass or less, and Co: 2.0% by mass or more and 6.0% by mass or less, It contains The balance is Fe and unavoidable impurities, The metal structure includes a plurality of crystal grains and grain boundaries, Each crystal grain includes a matrix and a plurality of carbides dispersed in the matrix, The area ratio P1 of carbides having an equivalent circle diameter of 5 μm or more in the crystal grains is 10% or less, The powder high-speed steel has an area ratio P2 of carbides present at the grain boundaries of 15% or less.

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