Sintered alloy consisting of precipitation hardened stainless steel and carbide

Abstract

To provide an Fe-based sintered alloy that has high strength and high toughness and can be produced at low cost.SOLUTION: The present invention discloses a sintered alloy composed of precipitation-hardening stainless steel and carbide, the sintered alloy including, as essential components, in mass%, C: 4.0-10.8%, Cr: 4.0-12.0%, Ni: 1.0-4.0%, Mo: 1.0-4.0%, Al: 0.2-1.4%, Cu: 0.2-1.4%, Ti: 16.0-41.0%, Co: 2.0-10.0%, and Nb: 0-18.0%, and further including, as an optional component, Si and / or Mn of 0.4% or less in total, with the balance being Fe and inevitable impurities. In MC carbide as a continuous phase, Fe-based particles are dispersed like islands.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a sintered alloy made of precipitation hardened stainless steel and carbide, and more particularly to a sintered alloy made of precipitation hardened stainless steel and carbide, which has high hardness and high toughness. [Background technology]

[0002] With the recent trend toward lower costs for metal materials used in molds and cutting tools, there is a growing desire to use low-cost materials even in more severe environments, which has led to a demand for materials that combine high hardness and high toughness at even lower costs.

[0003] For example, in the case of cutting tool materials, typical alloys include high-speed tool steels and WC-Co and TiC cermets, which are made of high-hardness ceramic particles and a metal binder. However, cermets are generally extremely difficult to machine due to their high hardness. Therefore, several materials have been proposed that use Ti-based carbides or nitrides as dispersed particles in an age-hardenable matrix.

[0004] For example, a hard alloy has been proposed that has excellent cutting tool properties and significantly improved machinability by: (1) containing 1.8–2.2% C with 0.1≦C−Ceq≦0.4; (2) containing 6–10% W, 5–8% Mo, or 18–22% W+2Mo in the substrate powder; and (3–5% V); (3) controlling the particle size of MC carbides to an average particle size of 0.5–2.0 μm; (4) adding 3–7% TiN and TiCN hard particles with an average particle size of 2.0 μm or less; and (5) limiting the total amount of M6C+MC+TiN+TiCN, which significantly affects grindability, to 29–39% (see Patent Document 1). This hard alloy is made by blending and sintering high-speed tool steel powder produced by water atomization with TiN, TiCN, and TiC, and the control of the total amount of hard particles is intended to produce a material with excellent machinability. However, the corrosion resistance is not sufficient.

[0005] In addition, there has been a proposal to obtain high strength and wear resistance by dispersing TiC in an Fe-based matrix with age hardening properties in a hard particle-dispersed sintered steel obtained by wet mixing hexane or xylene as a solvent (see Patent Document 2).

[0006] In addition, a carbide-dispersed maraging steel has been proposed in which Ti and Mo carbides are dispersed in the matrix of the maraging steel, which is easy to machine and can obtain high hardness by age hardening (see Patent Document 3).

[0007] Also, a carbide dispersed material has been proposed in which carbides of Ti and Mo are dispersed in a matrix of a stainless steel composition (see Patent Document 4). [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 9-287059 [Patent Document 2] Publication No. 2000-273503 [Patent Document 3] Japanese Patent Application Publication No. 6-207246 [Patent Document 4] Japanese Patent Application Publication No. 11-92870 Summary of the Invention [Problem to be solved by the invention]

[0009] When Ti carbides or nitrides are used as hard dispersoids, as in the proposed inventions described above, it is necessary to increase the amount of these dispersoids to achieve high hardness. However, increasing the amount of hard dispersoids tends to reduce toughness, and increasing the amount of hard dispersoids directly leads to increased costs. Since high toughness and low cost are always in a trade-off relationship when it comes to achieving high hardness, achieving both is difficult and not easy. Therefore, the present invention aims to provide a material that is high in hardness, high in toughness, and low in cost. [Means for solving the problem]

[0010] To solve the above problems, the inventors conducted extensive research and discovered that high hardness and toughness can be achieved with a small amount of added ceramic particles by using a hard phase as a continuous phase (matrix), which is the opposite of conventional material structures, and dispersing a relatively soft phase as particles. In particular, they discovered that an Fe-based sintered alloy in which TiC and MC carbides containing Ti, Nb, Cr, Mo, Co, and Fe are used as a continuous phase and precipitation-hardened stainless steel is dispersed as Fe-based particles in an island-like pattern can achieve high hardness and toughness at low cost.

[0011] That is, a first means for solving the problems of the present invention is a steel sheet containing, as essential components, in mass %, C: 4.0 to 10.8%, Cr: 4.0 to 12.0%, Ni: 1.0 to 4.0%, Mo: 1.0 to 4.0%, Al: 0.2 to 1.4%, Cu: 0.2 to 1.4%, Ti: 16.0 to 41.0%, Co: 2.0 to 10.0%, and Nb: 0 to 18.0%, Furthermore, as optional components, one or two selected from Si and Mn are contained in a total amount of 0.4% or less. the balance being Fe and unavoidable impurities; MC carbide is the continuous phase, Fe-based particles are dispersed in an island pattern It is a sintered alloy consisting of precipitation hardened stainless steel and carbide, characterized by the above.

[0012] The second means is a sintered alloy comprising precipitation hardening stainless steel and carbide according to the first means, characterized in that the diameter of the maximum inscribed circle that can be drawn in the Fe-based particles in the microstructure is 12 μm or more.

[0013] The third means is a sintered alloy comprising precipitation hardened stainless steel and carbide according to either the first or second means, characterized in that the circularity of the Fe-based particles in the microstructure is 0.25 or more.

[0014] The fourth means is a sintered alloy consisting of precipitation hardening stainless steel and carbide according to any one of the first to fifth means, characterized in that the hardness after aging is 60 HRC or more and the flexural strength after aging is 1000 MPa or more.

[0015] The fifth means is a sintered alloy made of precipitation hardening stainless steel and carbide according to any one of the first to fourth means, characterized in that the Fe-based particles are precipitation hardening stainless steel. [Effects of the Invention]

[0016] According to the present invention, relatively soft Fe-based particles are dispersed in a continuous phase of hard particle MC carbide, and the amount of ceramic particles added can be reduced, making it possible to obtain a high-hardness and high-toughness Fe-based sintered alloy at low cost. Furthermore, since the diameter of the maximum inscribed circle of the Fe-based particles is large, the progression of cracks when stress is applied is suppressed, fracture does not easily occur, and toughness can be ensured. Furthermore, when the Fe-based particles have high circularity, stress concentration between the continuous phase of MC carbides and the dispersed Fe-based particles can be suppressed, thereby improving the bending strength. Furthermore, the sintered alloy of the present invention can have a hardness of 60 HRC or more and a flexural strength of 1000 MPa or more by undergoing an appropriate aging treatment. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is an image of a sintered alloy containing TiC as a matrix according to Example 1 of the present invention, taken with an optical microscope. DETAILED DESCRIPTION OF THE INVENTION

[0018] Prior to describing the embodiments of the present invention, the chemical composition of the Fe-based sintered alloy of the present invention will be described. The method for producing the sintered alloy of the present invention is not particularly limited, but it can generally be produced by mixing MC carbide components with Fe-based particles and then sintering them by HIP or other methods. The MC carbide may be the same as the carbide that is ultimately dispersed in the steel, or it may be added as carbide or metal particles, with the reaction during sintering predicted.

[0019] (C: 4.0 to 10.8%) C is an essential element for forming carbides. The amount of C in the present invention is mainly determined by the amounts of TiC and NbC. If the C content is less than 4.0%, the sintered alloy will not have sufficient hardness. On the other hand, if the C content exceeds 10.8%, the volume of the ceramic phase will increase and sufficient toughness will not be obtained. Therefore, the C content is set to 4.0 to 10.8%. Preferably, C is 4.5 to 10.0%, and more preferably, C is 5.0 to 9.0%.

[0020] (Cr: 4.0 to 12.0%) Cr is an element essential for martensitic transformation and also contributes to improving corrosion resistance. If the Cr content is less than 4.0%, this effect cannot be fully achieved. If the Cr content exceeds 12.0%, the amount of retained austenite increases, making it difficult to achieve sufficient hardness. Therefore, the Cr content is set to 4.0 to 12.0%. Preferably, the Cr content is 5.0 to 11.0%, and more preferably, the Cr content is 4.7 to 8.0%.

[0021] (Ni: 1.0 to 4.0%) Ni is an element essential for martensitic transformation and contributes to precipitation strengthening and improved corrosion resistance. If Ni is less than 1.0%, this effect cannot be fully achieved. If Ni is more than 4.0%, the amount of retained austenite increases, making it difficult to achieve sufficient hardness. Therefore, Ni is set to 1.0 to 4.0%. Preferably, Ni is 1.5 to 3.5%, and more preferably, Ni is 2.0 to 3.0%.

[0022] (Mo: 1.0-4.0%) Mo contributes to precipitation strengthening by precipitating intermetallic compounds, and also contributes to improving corrosion resistance through its combined effect with Cr. If Mo is less than 1.0%, this effect cannot be fully achieved. If Mo exceeds 4.0%, a large amount of intermetallic compounds is formed, making it difficult to achieve sufficient toughness. Therefore, Mo is set to 1.0 to 4.0%. Preferably, Mo is 1.5 to 3.5%, and more preferably, Mo is 2.0 to 3.0%.

[0023] (Al: 0.2 to 1.4%) Al precipitates intermetallic compounds and contributes to precipitation strengthening. If Al is less than 0.2%, this effect is not sufficiently obtained, and if Al is more than 1.4%, a large amount of intermetallic compounds is formed, making it difficult to obtain sufficient toughness. Therefore, Al is set to 0.2 to 1.4%. Preferably, Al is 0.3 to 1.3%, and more preferably, Al is 0.7 to 0.8%.

[0024] (Cu: 0.2 to 1.4%) Cu contributes to precipitation strengthening by precipitating intermetallic compounds, and also contributes to improving corrosion resistance through its combined effect with Cr. If Cu content is less than 0.2%, this effect cannot be fully achieved. If Cu content exceeds 1.4%, the amount of retained austenite increases, making it difficult to achieve sufficient hardness. Therefore, Cu content is set to 0.2 to 1.4%. Preferably, Cu content is 0.3 to 1.3%, and more preferably, Cu content is 0.4 to 1.2%.

[0025] (Ti: 16.0 to 41.0%) Ti is an essential element for forming carbides, and also contributes to precipitation strengthening by precipitating intermetallic compounds. If Ti is less than 16.0%, sufficient hardness cannot be obtained. If Ti exceeds 41.0%, the volume of the ceramic phase increases and sufficient toughness cannot be obtained. Therefore, Ti is set to 16.0 to 41.0%. Preferably, Ti is 22.0 to 33.0%, and more preferably, Ti is 24.0 to 33.0%.

[0026] (Co: 2.0 to 10.0%) Co promotes the precipitation reaction during age hardening. If Co is less than 2.0%, sufficient hardness cannot be obtained. If Co exceeds 10.0%, martensite becomes embrittled and sufficient toughness cannot be obtained. Therefore, Co is set to 2.0 to 10.0%, preferably 3.0 to 9.0%, and more preferably 4.0 to 9.0%.

[0027] (Nb: 0 to 18.0%) Nb is an element that forms carbides. Nb forms a complete solid solution with TiC and improves the toughness of the ceramic phase. However, if Nb exceeds 18.0%, the volume of the ceramic phase increases and sufficient toughness cannot be obtained. Therefore, the upper limit of Nb, if added, is set to 18.0%. Preferably, Nb is 0 to 15.0%, and more preferably, Nb is 0 to 9.0%.

[0028] (Optional components: Si and / or Mn, 0.0-0.4% in total) Si and Mn are deoxidizing agents and are effective elements for improving hardenability and hardness, so they may be added optionally to the components of the present invention. However, if the total amount of Si and Mn exceeds 0.4%, toughness tends to decrease. Therefore, when either or both of them are added to the Fe-based alloy of the present invention, the total amount of Si and Mn should be limited to 0.4% or less. Preferably, the total amount of Si and Mn should be 0.1 to 0.4%.

[0029] Next, the reasons for defining the microstructure of the Fe-based sintered alloy will be explained. (The diameter of the largest inscribed circle that can be drawn on the Fe-based particles in the microstructure must be 12 μm or more.) If the diameter of the maximum inscribed circle of the Fe-based particles is less than 12 μm, crack propagation under stress cannot be sufficiently suppressed, leading to easy fracture and insufficient toughness. Therefore, the diameter of the maximum inscribed circle that can be drawn by the Fe-based particles in the microstructure is set to 12 μm or more. Preferably, the diameter of the maximum inscribed circle is 14 μm or more, and more preferably, the diameter of the maximum inscribed circle is 16 μm or more.

[0030] The maximum inscribed circle that can be drawn in an Fe-based particle in a microstructure is approximately 34,000 μm per field of view when observing the microstructure of the cross section of a sintered alloy with an optical microscope. 2 When an Fe-based particle is imaged in three fields of view, the diameter of the largest inscribed circle that can be drawn on the largest Fe-based particle in the three fields of view is calculated.

[0031] (The area-weighted average circularity of Fe-based particles in the microstructure must be 0.25 or more.) By making the circularity of the dispersed Fe-based particles 0.25 or more, stress concentration between the ceramic phase and the dispersed particles that occurs when stress is applied can be suppressed, thereby improving the flexural strength. Therefore, the circularity of the Fe-based particles in the microstructure is made 0.25 or more. The circularity is preferably 0.3 or more, and more preferably 0.35 or more.

[0032] The roundness in the present invention is defined as 4π×area / contour length 2 A value of 1.0 indicates a perfect circle, and the closer to 0 the particle is, the longer and thinner it becomes, or the more complex the shape becomes. Each Fe-based particle is observed in an optical microscope image, and the area-weighted average of the circularity is calculated, weighted by the measured particle area.

[0033] (MC carbide is the continuous phase, with Fe-based particles dispersed in island-like formations) MC carbides are MC type carbides formed in steel by carbide-forming elements M (for example, Ti, Nb, Cr, Mo, Co, and Fe). In the Fe-based sintered alloy of the present invention, MC carbides such as TiC constitute a continuous phase (matrix) as hard dispersed particles. Figure 1 is an optical microscope image of the sintered alloy of Example 1 of the present invention. The dark gray area (1) is the MC carbide, which exists continuously as a matrix.

[0034] On the other hand, the slightly whitish circular areas (2) scattered like islands in the optical microscope image in Figure 1 appear to be Fe-based particles dispersed in the matrix. For example, a stainless steel composition, such as a precipitation-hardened stainless steel composition, is suitable for these Fe-based particles. Precipitation-hardened stainless steel hardens through aging treatment, and therefore can be suitably applied to the present invention because it can be processed before aging treatment and then subjected to aging treatment to achieve a final high hardness.

[0035] In this way, relatively soft Fe-based particles are dispersed in a continuous phase of hard MC carbide particles, and the amount of ceramic particles added can be reduced, making it possible to obtain a high-hardness, high-toughness Fe-based sintered alloy at low cost.

[0036] (Hardness after aging must be 60HRC or more and flexural strength after aging must be 1000MPa or more) The Fe-based sintered alloy of the present invention can be processed before aging treatment, but if aging treatment is properly performed, the hardness after aging treatment will be 60 HRC or more and the bending strength will be 1000 MPa or more, so that both high hardness and high toughness will be achieved.

[0037] In conventional sintered metals, the hardness was about 58 HRC when the amount of carbide was 30%. In the sintered alloy of the present invention, the hardness after aging is 60 HRC or more when the amount of carbide is about 25%, which makes it less costly. More preferably, the hardness after aging is 62 HRC or more, and even more preferably, 65 HRC or more.

[0038] The sintered alloy of the present invention has a flexural strength of 1000 MPa or more after aging treatment. The toughness of the sintered alloy of the present invention improves as the flexural strength increases. The flexural strength is more preferably 1250 MPa or more. The flexural strength is even more preferably 1370 MPa or more.

[0039] (Example) The sintered alloy of the present invention can be obtained, for example, by the following procedure: Note that the present invention is not limited to the following description, and the present invention should not be construed as being limited by the examples.

[0040] First, the powder used in the sintered alloy of the present invention can be obtained by gas atomization in an Ar or nitrogen atmosphere, and has a spherical shape. The obtained powder is classified to a predetermined particle size, for example, 25 μm or less. The powders were mixed by adding a predetermined amount of gas atomized powder to amorphous TiC powder of 2 to 3 μm and NbC powder of 1 to 3 μm, and dry mixing was carried out in a pot mill for 10 minutes. The mixed powder is then removed from the mill and filled into a mild steel capsule, which is then sealed by vacuum degassing, and a sintered body is obtained by the HIP method. By mixing and sintering spherical gas atomized powder with amorphous TiC and NbC powders in this way, a structure is obtained in which MC carbides form a continuous phase and Fe-based particles are dispersed in island-like shapes.

[0041] Specifically, for materials having the chemical composition shown in Examples Nos. 1 to 11 and Comparative Examples Nos. 1 to 3 in Table 1 (note that each chemical component in Table 1 is the total amount of spherical gas atomized powder and amorphous TiC and NbC powder), HIP was performed under the conditions of temperature: 1350°C, pressure: 147 MPa, and holding time: 5 hours.

[0042] In Examples 1 to 4, only TiC was added as the MC carbide. In Examples 5 to 11, TiC and NbC were added as the MC carbides. In Comparative Examples 1 to 3, the C content exceeded 10.8%.

[0043] [Table 1]

[0044] The sintered body obtained by the HIP method was held at 1050°C for 3 hours, then air-cooled and subjected to solution heat treatment, and then held at 480°C for 6 hours and subjected to aging treatment by furnace cooling.

[0045] The hardness of the aged test pieces was measured using the Rockwell hardness test method in accordance with Japanese Industrial Standard JIS Z 2245. Each measurement was performed five times. The values ​​in Table 1 are the average values ​​of these measurements.

[0046] The flexural strength of the samples after aging treatment was also measured. Test pieces with a thickness (t): 1.8 mm x width (W): 1.8 mm x length (L): 20 mm were prepared and evaluated by a three-point bending test. The measurement was performed twice. The values ​​in Table 1 are the average values. The three-point bending test was carried out with a distance between supports of 10 mm, and the load (N) applied was measured in the vertical direction, and the three-point bending strength was calculated based on the following formula. Three-point bending strength (MPa) = (3 x load (N) x distance between supports (mm)) / (2 x width of test piece (mm) x thickness of test piece (mm)) 2 ). This three-point bending strength was expressed as flexural strength (MPa).

[0047] (The diameter of the largest inscribed circle) For each example and comparative example, the cross-sectional microstructure of the sintered alloy was observed with an optical microscope, and the diameter of the largest inscribed circle that could be drawn within the Fe-based particle, i.e., the diameter of the largest inscribed circle that touches at least a part of the particle, was determined for the Fe-based particle images shown in Figure 1. When the diameter of the largest inscribed circle of the Fe-based particle is large, crack propagation under stress is suppressed, fracture does not easily occur, and toughness can be ensured.

[0048] (Regarding roundness) Here, the circularity is 4π x area / contour length 2 A value of 1.0 indicates a perfect circle, and the closer to 0 the particle is, the more elongated or complex the shape becomes. Circularity was calculated as the area-weighted average by averaging the circularity of each Fe-based particle weighted by area after binarizing the projected image from the optical microscope. High circularity of the Fe-based particles can suppress stress concentration between the continuous phase of MC carbides and the dispersed Fe-based particles, thereby improving the bending strength.

[0049] As shown in the optical electron microscope image of the microstructure of Example 1 in Figure 1, TiC, which is the MC carbide (1), is observed as a dark gray continuous phase in the matrix. On the other hand, the Fe-based particles (2) are slightly lighter gray and are confirmed to be dispersed in the matrix like islands. Since Example 1 has high hardness and high toughness, it can be said that the present invention is a low-cost sintered alloy that has the desired hardness and toughness while having a low amount of carbide, compared to the conventional case in which TiC is dispersed in a matrix of Fe-based particles.

[0050] All of Examples 1 to 11 are sintered alloys whose composition is within the range of the present invention. Examples 1 to 4 contain 25 to 50 mass% of TiC, and Examples 5 to 11 contain 25 to 40% of TiC and 5 to 20% of NbC, forming a continuous phase with Fe-based particles formed in an island shape. In these Examples, the hardness after aging was 61.6 to 69.7 MPa, which was high hardness. In addition, in the Examples, the flexural strength after aging was 1020 to 1630 MPa.

[0051] On the other hand, in Comparative Examples 1 to 3, the C concentration was higher than the range specified in the present invention, so the volume of the ceramic phase was large and although hardness was obtained after aging treatment, the toughness was 1000 MPa or less in all cases. [Explanation of symbols]

[0052] 1. Continuous phase MC carbide 2. Island-shaped Fe-based particles

Claims

1. In mass%, C: 4.0 to 10.1%, Cr: 4.0 to 12.0%, Ni: 1.0 to 4.0%, Mo: 1.0 to 4.0%, Al: 0.2 to 1.4%, Cu: 0.2 to 1.4%, Ti: 16.0 to 41.0%, Co: 2.0 to 10.0%, Nb: 0 to 18.0%, Furthermore, as optional components, one or two selected from Si and Mn are contained in a total amount of 0.4% or less, the balance being Fe and unavoidable impurities; MC carbide is the continuous phase, Fe-based particles are dispersed in an island-like pattern A sintered alloy consisting of precipitation hardened stainless steel and carbide, characterized by:

2. 2. A sintered alloy comprising precipitation hardened stainless steel and carbide according to claim 1, characterized in that the diameter of the largest inscribed circle that can be drawn in the Fe-based particles in the microstructure is 12 μm or more.

3. 3. A sintered alloy comprising precipitation hardened stainless steel and carbide according to claim 1, wherein the Fe-based particles have a circularity of 0.25 or more in the microstructure.

4. 4. A sintered alloy comprising precipitation hardening stainless steel and carbide according to claim 1, characterized in that the hardness after aging treatment is 60 HRC or more and the flexural strength after aging treatment is 1000 MPa or more.

5. 5. A sintered alloy comprising precipitation hardening stainless steel and carbide according to claim 1, wherein the Fe-based particles are precipitation hardening stainless steel.

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