JP7739005B2 - Sintered alloy consisting of precipitation hardened stainless steel and carbide - Google Patents

Description

The present invention relates to a sintered alloy consisting of precipitation-hardened stainless steel and carbides, and more specifically to a sintered alloy consisting of precipitation-hardened stainless steel and carbides that has high hardness and toughness.

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

For example, when it comes to cutting tool materials, typical alloys include high-speed tool steel and WC-Co and TiC cermets, which use particles of high-hardness ceramics and a metal binder. However, cermets are generally extremely difficult to machine due to their high hardness. For this reason, several materials have been proposed that use Ti-based carbides or nitrides as dispersed particles in an age-hardenable matrix.

For example, a hard alloy has been proposed that has excellent cutting tool properties and can significantly improve machinability by (1) containing 1.8 to 2.2% C and 0.1≦C−Ceq≦0.4, (2) containing 6 to 10% W, 5 to 8% Mo in the substrate powder, with 18 to 22% W+2Mo, and 3 to 5% V, (3) controlling the particle size of MC carbides to an average particle size of 0.5 to 2.0 μm, (4) adding 3 to 7% of hard particles of TiN and TiCN, controlling the average particle size to 2.0 μm or less, and (5) restricting the total amount of M ₆ C + MC + TiN + TiCN, which has a significant effect on grindability, to 29 to 39% (see Patent Document 1). This is a hard alloy made by mixing and sintering high-speed tool steel powder produced by water atomization with TiN, TiCN, and TiC, and aims to obtain a material with excellent machinability by controlling the total amount of hard particles. However, its corrosion resistance is insufficient.

In addition, there has been a proposal to produce a sintered steel with hard particles dispersed in a wet mixture of hexane or xylene as a solvent, in which TiC is dispersed in an age-hardenable Fe-based matrix, thereby achieving high strength and wear resistance (see Patent Document 2).

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 achieves high hardness through age hardening (see Patent Document 3).

In addition, a carbide-dispersed material has been proposed in which Ti and Mo carbides are dispersed in a matrix of stainless steel composition (see Patent Document 4).

Japanese Patent Application Publication No. 9-287059 Publication No. 2000-273503 Japanese Patent Application Publication No. 6-207246 Japanese Patent Application Publication No. 11-92870

When using Ti carbides or nitrides as hard dispersoids, as in the proposed innovations described above, it was necessary to increase the amount of these dispersoids added to achieve high hardness. However, increasing the amount of hard dispersoids added tends to result in a decrease in toughness, and an increase in the amount of hard dispersoids added directly leads to increased costs. Since high toughness and low cost are always in a trade-off 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.

The inventors conducted extensive research to solve the above problems, and discovered that 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, it is possible to achieve high hardness and toughness with a small amount of added ceramic 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.

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,
This is a sintered alloy made of precipitation hardened stainless steel and carbide, characterized by Fe-based particles dispersed in an island shape.

The second means is a sintered alloy composed of precipitation-hardened stainless steel and carbide as described in the first means, characterized in that the diameter of the largest inscribed circle that can be drawn by the Fe-based particles in the microstructure is 12 μm or more.

The third means is a sintered alloy composed of precipitation-hardened stainless steel and carbide as described in either the first or second means, characterized in that the circularity of the Fe-based particles in the microstructure is 0.25 or more.

The fourth means is a sintered alloy made of precipitation-hardened 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.

The fifth means is a sintered alloy composed 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.

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 propagation of cracks when stress is applied is suppressed, and fracture does not easily occur, thereby ensuring toughness.
Furthermore, when the Fe-based particles have high circularity, stress concentration between the continuous phase of MC carbide 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.

1 is an image of a sintered alloy having a TiC matrix according to Example 1 of the present invention, taken with an optical microscope.

Before describing the embodiments of the present invention, the chemical components of the Fe-based sintered alloy of the present invention will be described. The manufacturing method of the sintered alloy of the present invention is not particularly limited, but it can generally be manufactured by mixing MC carbide components with Fe-based particles and then sintering them using a method such as HIP. 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 by predicting the reaction during sintering.

(C: 4.0-10.8%)
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%.

(Cr: 4.0-12.0%)
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 obtain 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%.

(Ni: 1.0-4.0%)
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 exceeds 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%.

(Mo: 1.0-4.0%)
Mo precipitates intermetallic compounds, contributing to precipitation strengthening, and its combined effect with Cr contributes to improved corrosion resistance. 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 generated, 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%.

(Al: 0.2-1.4%)
Al precipitates intermetallic compounds and contributes to precipitation strengthening. If Al is less than 0.2%, this effect is not fully achieved, and if Al is more than 1.4%, a large amount of intermetallic compounds is formed, making it difficult to obtain sufficient toughness. Therefore, the Al content 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%.

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

(Ti: 16.0-41.0%)
Ti is an essential element for forming carbides and 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%.

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

(Nb: 0-18.0%)
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%.

(Optional components: Si and/or Mn, 0.0 to 0.4% in total)
Since Si and Mn are deoxidizing agents and are effective elements for improving hardenability and hardness, 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%.

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 is 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.

The maximum inscribed circle that can be drawn on an Fe-based particle in a microstructure is the maximum inscribed circle that can be drawn on the ^largest Fe-based particle among the three fields of view when the cross-sectional microstructure of a sintered alloy is observed with an optical microscope and images of the Fe-based particles are taken in three fields of view, each of which is approximately 34,000 μm². The diameter of the largest inscribed circle that touches at least a portion of the particle is determined.

(The area-weighted average circularity of the Fe-based particles in the microstructure is 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 bending 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.

The roundness referred to in the present invention is calculated by 4π × area/contour ^length2 . A value of 1.0 indicates a perfect circle, and the closer to 0 the value, the longer and thinner or more complex the shape. Each Fe-based particle is observed in an optical microscope image, and an area-weighted average roundness value weighted by the measured particle area is calculated.

(MC carbide is the continuous phase, and Fe-based particles are dispersed in an island-like pattern)
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 areas (1) are MC carbides, which exist continuously as a matrix.

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 a matrix. A suitable Fe-based particle is, for example, a stainless steel composition, such as a precipitation-hardened stainless steel composition. Precipitation-hardened stainless steel hardens through aging, so it can be processed before aging the sintered alloy and then subjected to aging to achieve a final high hardness, making it suitable for use in the present invention.

In this way, relatively soft Fe-based particles are dispersed within a continuous phase of hard MC carbide particles, allowing for a reduction in the amount of ceramic particles added, making it possible to obtain a high-hardness, high-toughness Fe-based sintered alloy at low cost.

(Hardness after aging treatment is 60 HRC or more and bending strength after aging treatment is 1000 MPa 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.

Conventional sintered metals have a hardness of approximately 58 HRC when the amount of carbide is 30%. The sintered alloy of the present invention has a hardness of 60 HRC or higher after aging treatment when the amount of carbide is approximately 25%, making it less costly. More preferably, the hardness after aging treatment is 62 HRC or higher, and even more preferably, 65 HRC or higher.

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. Even more preferably, the flexural strength is 1370 MPa or more.

(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.

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, filled into a mild steel capsule, and sealed by vacuum degassing, followed by HIP to obtain a sintered body.
By mixing and sintering the spherical gas atomized powder with amorphous TiC and NbC powders in this way, a structure is obtained in which the MC carbides form a continuous phase and the Fe-based particles are dispersed in island-like form.

Specifically, materials with the chemical composition shown in Example Nos. 1 to 11 and Comparative Examples Nos. 1 to 3 in Table 1 (note that each chemical composition in Table 1 is the total amount of spherical gas-atomized powder and amorphous TiC and NbC powders) were subjected to HIP at a temperature of 1,350°C, a pressure of 147 MPa, and a holding time of 5 hours.

Note that Examples 1 to 4 are examples in which only TiC was added as the MC carbide. Examples 5 to 11 are examples in which TiC and NbC were added as the MC carbides. Comparative Examples 1 to 3 are comparative examples in which the C content exceeded 10.8%.

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, followed by holding at 480°C for 6 hours and aging treatment by furnace cooling.

The hardness of the aged test specimens was measured using the Rockwell hardness test method in accordance with Japanese Industrial Standard JIS Z 2245. Five measurements were taken for each specimen. The values in Table 1 are the average values of these measurements.

The flexural strength of the samples after aging treatment was also measured. Test pieces with a thickness (t): 1.8 mm × width (W): 1.8 mm × 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 while pressing down in the vertical direction was measured and calculated as the three-point bending strength 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).

(Regarding the diameter of the largest inscribed circle)
For each example and comparative example, the microstructure of the cross section 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 imaged as shown in Figure 1. When the diameter of the largest inscribed circle of the Fe-based particle is large, the propagation of cracks when stress is applied is suppressed, and fracture does not easily occur, ensuring toughness.

(Regarding roundness)
The circularity here can be calculated by 4π × area/contour ^length2 . A value of 1.0 indicates a perfect circle, and the closer to 0 the value, the longer and thinner or more complex the shape. The circularity was calculated as an area-weighted average by averaging the circularity of each Fe-based particle weighted by area after binarizing the projected image of the optical microscope.
When the Fe-based particles have high circularity, stress concentration between the continuous phase of MC carbide and the dispersed Fe-based particles can be suppressed, thereby improving the bending strength.

As shown in Fig. 1, an optical electron microscope image of the microstructure of Example 1 shows that 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.

All of Examples 1 to 11 are sintered alloys whose composition falls within the ranges specified by the present invention. Examples 1 to 4 contain 25 to 50 mass% TiC, while Examples 5 to 11 contain 25 to 40% TiC and 5 to 20% NbC, forming a continuous phase with Fe-based particles formed in an island-like pattern. In these Examples, the hardness after aging was 61.6 to 69.7 MPa, demonstrating high hardness. Furthermore, in the Examples, the flexural strength after aging was 1020 to 1630 MPa.

On the other hand, in Comparative Examples 1 to 3, the C concentration was higher than the range specified in the present invention, resulting in a large volume of the ceramic phase. Although hardness was achieved after aging, the toughness was below 1000 MPa in all cases.

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

Claims (5)

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,
A sintered alloy consisting of precipitation hardened stainless steel and carbide, characterized in that Fe-based particles are dispersed in an island-like pattern. A sintered alloy consisting of precipitation-hardened stainless steel and carbide as described in claim 1, characterized in that the diameter of the largest inscribed circle that can be drawn by Fe-based particles in the microstructure is 12 μm or more. A sintered alloy comprising precipitation-hardened stainless steel and carbide according to claim 1 or 2, characterized in that the circularity of the Fe-based particles in the microstructure is 0.25 or more. A sintered alloy comprising precipitation-hardened stainless steel and carbides according to any one of claims 1 to 3, characterized in that the hardness after aging is 60 HRC or more and the flexural strength after aging is 1000 MPa or more. A sintered alloy comprising precipitation-hardening stainless steel and carbide according to any one of claims 1 to 4, characterized in that the Fe-based particles are precipitation-hardening stainless steel.