Cemented carbide and composite cemented carbide roll for rolling

Abstract

A cemented carbide comprising 55-90 parts by mass of WC particles and 10-45 parts by mass of a Fe-based binder phase; the binder phase having a composition comprising 0.5-10% by mass of Ni, 0.2-2% by mass of C, 0.5-5% by mass of Cr, 0.2-2.0% by mass of Si, and 0.1-5% by mass of W, the balance being Fe and inevitable impurities, and containing 0.05-2.0% by area of Fe—Si—O-based particles.

Claims

1. A cemented carbide comprising 55-90 parts by mass of WC particles and 10-45 parts by mass of a binder phase containing Fe in an amount of 50% by mass or more; said binder phase having a composition comprising 2.5-10% by mass of Ni, 0.2-2% by mass of C, 0.5-5% by mass of Cr, 0.2-2.0% by mass of Si, and 0.1-5% by mass of W, the balance being Fe and inevitable impurities, and containing 0.05-2.0% by area of Fe—Si—O-containing particles.

2. The cemented carbide according to claim 1, wherein said cemented carbide does not contain Fe—Si—O-containing particles having equivalent circle diameters of 3 μm or more, where the equivalent circle diameter of an Fe—Si—O-containing particle is a diameter of a circle having the same area as that of the Fe—Si—O-containing particle in a polished cross section of the cemented carbide.

3. The cemented carbide according to claim 2, wherein among said Fe—Si—O-containing particles, the ratio of particles having equivalent circle diameters of 0.1-3 μm is 0.05-2.0% by area in total.

4. The cemented carbide according to claim 1, wherein said cemented carbide contains no composite carbides having equivalent circle diameters of 5 μm or more, where the equivalent circle diameter of a composite carbide is a diameter of a circle having the same area as that of the composite carbide in the cemented carbide.

5. The cemented carbide according to claim 1, wherein said WC particles have a median diameter D50 of 0.5-10 μm.

6. The cemented carbide according to claim 1, wherein said binder phase further contains 0-5% by mass of Co, and 0-1% by mass of Mn.

7. The cemented carbide according to claim 1, wherein the total amount of bainite phases and/or martensite phases in said binder phase is 50% or more by area.

8. The cemented carbide according to claim 1, wherein said cemented carbide has compressive yield strength of 1200 MPa or more.

9. A composite cemented carbide roll for rolling, which comprises an outer layer made of the cemented carbide recited in claim 1, which is metallurgically bonded to an outer peripheral surface of a steel sleeve or shaft.

10. The cemented carbide according to claim 1, wherein said Fe—Si—O-containing particles comprise 10-30% by mass of Si, 10-40% by mass of 0, 0.3-5% by mass of Ni, 0-3% by mass of C, 0.3-3% by mass of Cr, and 1-10% by mass of W, the balance being Fe and inevitable impurities.

11. The cemented carbide according to claim 1, which comprises 55-85 parts by mass of WC particles and 15-45 parts by mass of the binder phase.

12. The cemented carbide according to claim 1, wherein the Ni content of said binder phase is 3-10% by mass.

13. The cemented carbide according to claim 1, wherein the Ni content of said binder phase is 4-10% by mass.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1(a) is a photograph of a secondary electron image of SEM showing a cross section structure of the cemented carbide of Sample 1 (within the present invention).

(2) FIG. 1(b) is a photograph of a backscattered electron image of SEM showing a cross section structure of the cemented carbide of Sample 1 (within the present invention) in the same field as in FIG. 1(a).

(3) FIG. 2(a) is a photograph of a secondary electron image of SEM showing a cross section structure of the cemented carbide of Sample 2 (Comparative Example).

(4) FIG. 2(b) is a photograph of a backscattered electron image of SEM showing a cross section structure of the cemented carbide of Sample 2 (Comparative Example) in the same field as in FIG. 2(a).

(5) FIG. 3 is a graph showing a stress-strain curve of Sample 2, which were obtained by a uniaxial compression test.

(6) FIG. 4 is a schematic view showing a test piece used in the uniaxial compression test.

(7) FIG. 5 is a schematic view showing a test machine for evaluating thermal shock by friction.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(8) The embodiments of the present invention will be explained in detail below. Explanations of one embodiment are applicable to other embodiments unless otherwise mentioned. The following explanations are not restrictive, but various modifications may be made within the scope of the present invention.

(9) [1] Cemented Carbide

(10) (A) Composition

(11) The cemented carbide of the present invention comprises 55-90 parts by mass of WC particles and 10-45 parts by mass of an Fe-based binder phase.

(12) (1) WC Particles

(13) The amount of WC particles in the cemented carbide of the present invention is 55-90 parts by mass. When WC particles are less than 55 parts by mass, the amount of hard WC particles is relatively small, providing the cemented carbide with too low Young's modulus. On the other hand, when WC particles exceed 90 parts by mass, the amount of the binder phase is relatively small, failing to provide the cemented carbide with enough strength. The lower limit of the amount of WC particles is preferably 60 parts by mass, and more preferably 65 parts by mass. Also, the upper limit of the amount of WC particles is preferably 85 parts by mass.

(14) The WC particles preferably have a median diameter D50 (corresponding to a particle size at a cumulative volume of 50%) of 0.5-10 μm. When the average particle size is less than 0.5 μm, there are increased boundaries between the WC particles and the binder phase, making it likely to generate composite carbides described below, thereby reducing the strength of the cemented carbide. On the other hand, when the average particle size exceeds 10 μm, the strength of the cemented carbide is lowered. The lower limit of the median diameter D50 of WC particles is preferably 1 μm, more preferably 2 μm, and most preferably 3 μm. Also, the upper limit of the median diameter D50 of WC particles is preferably 9 μm, more preferably 8 μm, and most preferably 7 μm.

(15) Because WC particles densely exist in a connected manner in the cemented carbide, it is difficult to determine the particle sizes of WC particles on the photomicrograph. Because the cemented carbide of the present invention is produced by sintering a green body at a temperature between (liquid phase generation-starting temperature) and (liquid phase generation-starting temperature+100° C.) in vacuum as described below, there is substantially no particle size difference between WC powder in the green body and WC particles in the cemented carbide. Accordingly, the particle sizes of WC particles dispersed in the cemented carbide are expressed by the particle sizes of WC powder in the green body.

(16) WC particles preferably have relatively uniform particle sizes. Accordingly, in a cumulative particle size distribution curve determined by a laser diffraction and scattering method, the WC particles have a preferable particle size distribution defined below. The lower limit of D10 (particle size at a cumulative volume of 10%) is preferably 0.3 μm, and more preferably 1 μm, and the upper limit of D10 is preferably 3 μm. Also, the lower limit of D90 (particle size at a cumulative volume of 90%) is preferably 3 μm, and more preferably 6 μm, and the upper limit of D90 is preferably 12 μm, and more preferably 8 μm. The median diameter D50 is as described above.

(17) (2) Binder Phase

(18) In the cemented carbide of the present invention, the binder phase has a composition comprising

(19) 0.5-10% by mass of Ni,

(20) 0.2-2% by mass of C,

(21) 0.5-5% by mass of Cr,

(22) 0.2-2.0% by mass of Si, and

(23) 0.1-5% by mass of W,

(24) the balance being Fe and inevitable impurities.

(25) (i) Indispensable Elements

(26) (a) Ni: 0.5-10% by Mass

(27) Ni is an element necessary for securing the hardenability of the binder phase. When Ni is less than 0.5% by mass, the binder phase has insufficient hardenability, likely lowering the material strength. On the other hand, when Ni exceeds 10% by mass, the binder phase is turned to have an austenite phase, failing to provide the cemented carbide with sufficient compressive yield strength. The lower limit of the Ni content is preferably 2.0% by mass, more preferably 2.5% by mass, further preferably 3% by mass, and most preferably 4% by mass. Also, the upper limit of the Ni content is preferably 8% by mass, and more preferably 7% by mass.

(28) (b) C: 0.2-2% by Mass

(29) C is an element necessary for securing the hardenability of the binder phase and suppressing the generation of coarse composite carbides. When C is less than 0.2% by mass, the binder phase has insufficient hardenability, and large amounts of composite carbides are generated, resulting in low material strength. On the other hand, when C exceeds 2% by mass, coarse composite carbides are generated, providing the cemented carbide with low strength. The lower limit of the C content is preferably 0.3% by mass, and more preferably 0.5% by mass, and the upper limit of the C content is preferably 1.5% by mass, more preferably 1.2% by mass, and most preferably 1.0% by mass.

(30) (c) Cr: 0.5-5% by Mass

(31) Cr is an element necessary for securing the hardenability of the binder phase. When Cr is less than 0.5% by mass, the binder phase has too low hardenability, failing to obtain sufficient compressive yield strength. On the other hand, when Cr exceeds 5% by mass, coarse composite carbides are generated, providing the cemented carbide with low strength. Cr is preferably 4% or less by mass, and more preferably 3% or less by mass.

(32) (d) Si: 0.2-2.0% by Mass

(33) Si is an element necessary for strengthening the binder phase. Less than 0.2% by mass of Si insufficiently strengthens the binder phase. On the other hand, when Si, a graphitization element, is more than 2.0% by mass, graphite is likely crystallized, providing the cemented carbide with low strength. The lower limit of the Si content is preferably 0.3% by mass, and more preferably 0.5% by mass. Also, the upper limit of the Si content is preferably 1.9% by mass.

(34) (e) W: 0.1-5% by Mass

(35) The W content in the binder phase is 0.1-5% by mass. When the W content in the binder phase exceeds 5% by mass, coarse composite carbides are generated, providing the cemented carbide with low strength. The lower limit of the W content is preferably 0.8% by mass, and more preferably 1.2% by mass. Also, the upper limit of the W content is preferably 4% by mass.

(36) (ii) Optional Elements

(37) (a) Co: 0-5% by Mass

(38) Co, which has a function of improving sinterability, is not indispensable in the cemented carbide of the present invention. Namely, the Co content is preferably substantially 0% by mass. However, 5% or less by mass of Co does not affect the structure and strength of the cemented carbide of the present invention. The upper limit of the Co content is more preferably 2% by mass, and most preferably 1% by mass.

(39) (b) Mn: 0-1% by Mass

(40) Mn, which has a function of improving hardenability, is not indispensable in the cemented carbide of the present invention. Namely, the Mn content is preferably substantially 0% by mass. However, 1% or less by mass of Mn does not affect the structure and strength of the cemented carbide of the present invention. The upper limit of the Mn content is more preferably 0.5% by mass, and most preferably 0.3% by mass.

(41) (iii) Inevitable Impurities

(42) The inevitable impurities include Mo, V, Nb, Ti, Al, Cu, N, O, etc. Among them, at least one selected from the group consisting of Mo, V and Nb is preferably 2% or less by mass in total. At least one selected from the group consisting of Mo, V and Nb is more preferably 1% or less by mass, and most preferably 0.5% or less by mass, in total. Also, at least one selected from the group consisting of Ti, Al, Cu, N and O is preferably 0.5% or less by mass alone and 1% or less by mass in total. Particularly, each of N and O is preferably less than 1000 ppm. The inevitable impurities within the above ranges do not substantially affect the structure and strength of the cemented carbide of the present invention.

(43) (B) Structure

(44) The cemented carbide of the present invention has a structure comprising WC particles, a binder phase, and Fe—Si—O-based particles.

(45) (1) Fe—Si—O-Based Particles

(46) The cemented carbide of the present invention has a structure containing 0.05-2.0% by area of Fe—Si—O-based particles. As shown in FIGS. 1(a) and 1(b), the Fe—Si—O-based particles are black portions (shown by the arrows) particularly in the backscattered electron image [FIG. 1(b)], when observed by SEM on a polished cross section of the cemented carbide. Incidentally, in FIG. 1(b), white portions are WC particles, and gray portions are binder phases. the EDX analysis (acceleration voltage: 5 kV, and beam diameter: 1 μm) of the SEM image has confirmed that the Fe—Si—O-based particles comprise 10-30% by mass of Si, 10-40% by mass of 0, 0.3-5% by mass of Ni, 0-3% by mass of C, 0.3-3% by mass of Cr, and 1-10% by mass of W, the balance being Fe and inevitable impurities composition. It is considered that the Fe—Si—O-based particles improve the sticking resistance of the cemented carbide. When the Fe—Si—O-based particles are less than 0.05% by area in total, they do not exhibit a sufficient effect of improving the sticking resistance. More than 2.0% by area of Fe—Si—O-based particles disadvantageously lower the material strength. The lower limit of the total amount of Fe—Si—O-based particles is preferably 0.1% by area, and more preferably 0.2% by area, and the upper limit of the total amount of Fe—Si—O-based particles is preferably 1.8% by area, more preferably 1.6% by area, further preferably 1.4% by area, and most preferably 1.2% by area.

(47) The Fe—Si—O-based particles preferably have equivalent circle diameters of 3 μm or less. When the equivalent circle diameters of the Fe—Si—O-based particles are more than 3 μm, the particles appear as a pattern on a polished surface of the cemented carbide. Accordingly, when such cemented carbide is used for tools such as rolling rolls, etc., the pattern is transferred to a rolled strip, deteriorating the quality of the rolled strips. The lower limit is not particularly restricted, but it is difficult to observe particles having equivalent circle diameters of 0.1 μm or less with high accuracy, and they do not have appreciable influence on the sticking resistance. Accordingly, the Fe—Si—O-based particles having equivalent circle diameters of 0.1-3 μm in the structure of the cemented carbide of the present invention is preferably 0.05-2.0% by area in total. Herein, the equivalent circle diameter of an Fe—Si—O-based particle is a diameter of a circle having the same area as that of the Fe—Si—O-based particle in a photomicrograph (magnification of 1000) of a polished cross section of the cemented carbide.

(48) (2) Composite Carbides

(49) The cemented carbide of the present invention preferably has a structure containing substantially no composite carbides having equivalent circle diameters of 5 μm or more. The composite carbides are those composed of W and metal elements, for example, (W, Fe, Cr).sub.23C.sub.6, (W, Fe, Cr).sub.3C, (W, Fe, Cr).sub.2C, (W, Fe, Cr).sub.7C.sub.3, (W, Fe, Cr).sub.6C, etc. Herein, the equivalent circle diameter of a composite carbide is a diameter of a circle having the same area as that of the composite carbide particle in a photomicrograph, like the above Fe—Si—O-based particle. The cemented carbide containing no composite carbides having equivalent circle diameters of 5 μm or more in the binder phase has bending strength of 1700 MPa or more. Herein, “containing substantially no composite carbides” means that composite carbides having equivalent circle diameters of 5 μm or more are not observed on the SEM photograph (magnification of 1000). Composite carbides having equivalent circle diameters of less than 5 μm may exist in an amount of less than about 5% by area when measured by EPMA, in the cemented carbide of the present invention.

(50) (3) Bainite Phases and/or Martensite Phases

(51) The binder phase in the cemented carbide of the present invention preferably has a structure containing 50% or more in total by area of bainite phases and/or martensite phases. The use of the term “bainite phases and/or martensite phases” is due to the fact that bainite phases and martensite phases have substantially the same function, and that it is difficult to distinguish them on the photomicrograph. With such structure, the cemented carbide of the present invention has high compressive yield strength and mechanical strength.

(52) Because the total amount of bainite phases and/or martensite phases in the binder phase is 50% or more by area, the cemented carbide of the present invention has compressive yield strength of 1200 MPa or more. The total amount of bainite phases and/or martensite phases is preferably 70% or more by area, more preferably 80% or more by area, and most preferably substantially 100% by area. Other phases than bainite phases and martensite phases are pearlite phases, austenite phases, etc.

(53) (4) Diffusion of Fe into WC Particles

(54) The EPMA analysis has revealed that in the sintered cemented carbide, WC particles contain 0.3-0.7% by mass of Fe.

(55) (C) Properties

(56) The cemented carbide of the present invention having the above composition and structure has compressive yield strength of 1200 MPa or more and bending strength of 1700 MPa or more. Accordingly, when a rolling composite cemented carbide roll having an outer layer made of the cemented carbide of the present invention is used for the cold rolling of metal (steel) strips, dents due to the compressive yield of the roll surface can be reduced, enabling the continuous high-quality rolling of metal strips with a long life span of the composite roll. Also, because the cemented carbide of the present invention contains 0.05-2.0% by area of Fe—Si—O-based particles, it has excellent sticking resistance. Accordingly, the rolling composite cemented carbide roll of the present invention is also suitable as a roll for hot-rolling metal strips.

(57) The compressive yield strength is yield stress determined by a uniaxial compression test of a test piece shown in FIG. 4 under an axial load. Namely, in a stress-strain curve determined by the uniaxial compression test as shown in FIG. 3, stress at a point at which the stress and the strain deviate from a straight linear relation is defined as the compressive yield strength.

(58) The cemented carbide of the present invention has compressive yield strength of more preferably 1500 MPa or more, and most preferably 1600 MPa or more, and bending strength of more preferably 2000 MPa or more, and most preferably 2300 MPa or more.

(59) The cemented carbide of the present invention further has Young's modulus of 385 GPa or more and Rockwell hardness of 80 HRA or more. The Young's modulus is preferably 400 GPa or more, and more preferably 450 GPa or more. Also, the Rockwell hardness is preferably 82 HRA or more.

(60) [2] Production Method of Cemented Carbide

(61) (A) Powder for Molding

(62) 55-90 parts by mass of WC powder, and 10-45 parts by mass of metal powder comprising 0.5-10% by mass of Ni, 0.3-2.2% by mass of C, 0.5-5% by mass of Cr, 0.2-2.5% by mass of Si, and 300-5000 ppm of O, the balance being Fe and inevitable impurities, are wet-mixed in a ball mill to prepare the powder for molding. Because O is adsorbed onto the metal powder or exists as surface oxides, its amount can be adjusted to 300-5000 ppm by reduction after mixing or molding. Because W is diffused from the WC powder to the binder phase during sintering, the metal powder may not contain W. Also, to prevent the generation of composite carbides, the amount of C in the metal powder should be 0.3-2.2% by mass, and is preferably 0.5-1.7% by mass, and more preferably 0.5-1.5% by mass.

(63) The metal powder for forming the binder phase may be a mixture of constituent element powders, or alloy powder containing all constituent elements. Carbon may be added in the form of powder such as graphite, carbon black, etc., or may be added to powder of each metal or alloy. Cr may be added in the form of its alloy with Si (for example, CrSi.sub.2).

(64) (1) Si: 0.2-2.5% by Mass

(65) Si is necessary to form Fe—Si—O-based particles, and to strengthen the binder phase as described above. Less than 0.2% by mass of Si does not form Fe—Si—O-based particles sufficiently, and has an insufficient effect of strengthening the binder phase. On the other hand, when Si is more than 2.5% by mass, a large amount of Fe—Si—O-based particles are formed, and graphite is likely crystallized, providing the cemented carbide with low strength. The lower limit of the Si content is preferably 0.3% by mass, and more preferably 0.5% by mass. Also, the upper limit of the Si content is preferably 2.4% by mass, and more preferably 2.3% by mass.

(66) (2) O: 300-5000 ppm

(67) O is necessary for forming Fe—Si—O-based particles with Si and Fe in the metal powder. When O is less than 300 ppm, 0.05% or more by area of Fe—Si—O-based particles cannot be formed, providing an insufficient effect of improving the sticking resistance. On the other hand, when O exceeds 5000 ppm, more than 2% by area of Fe—Si—O-based particles are formed, resulting in low strength. The lower limit of the O content is preferably 400 ppm, and more preferably 500 ppm, and the upper limit of the O content is preferably 4000 ppm, and more preferably 3000 ppm.

(68) (B) Molding

(69) After drying, the powder for molding is formed into a green body having a desired shape by a method such as die-pressing, cold-isostatic pressing (CIP), etc. Incidentally, the powder for molding may be charged into a HIP can for a HIP treatment without molding.

(70) (C) HIP Treatment

(71) The green body is charged into a steel HIP can, which is evacuated and sealed. This HIP can is placed in a HIP furnace, and subjected to a HIP treatment at 1240±40° C. and 100-140 MPa. When the powder for molding is subjected to a HIP treatment without molding, the powder for molding is charged into a steel HIP can, evacuated, and sealed to conduct the HIP treatment.

(72) (D) Cooling

(73) The HIPed body is cooled at an average rate of 60° C./hour or more between 900° C. and 600° C. When cooled at an average rate of less than 60° C./hour, the resultant binder phase in the cemented carbide contains a large percentage of pearlite phases, failing to have 50% or more in total by area of bainite phases and/or martensite phases, thereby providing the cemented carbide with low compressive yield strength. Cooling at an average rate of 60° C./hour or more may be conducted in the cooling process in a HIP furnace, or after cooling in a HIP furnace, the HIPed body may be heated again to 900° C. or higher in another HIP furnace, and then cooled.

(74) [3] Uses

(75) The cemented carbide of the present invention is preferably used for an outer layer metallurgically bonded to a tough steel sleeve or shaft of a composite roll. Because the outer layer of this rolling composite cemented carbide roll has high compressive yield strength, bending strength, Young's modulus, hardness and sticking resistance, it is particularly suitable for the cold rolling and hot rolling of metal (steel) strips. The rolling composite cemented carbide roll of the present invention is preferably used as a work roll in (a) a 6-roll stand comprising a pair of upper and lower work rolls for rolling a metal strip, a pair of upper and lower intermediate rolls for supporting the work rolls, and a pair of upper and lower backup rolls for supporting the intermediate rolls, or (b) a 4-roll stand comprising a pair of upper and lower work rolls for rolling a metal strip, and a pair of upper and lower backup rolls for supporting the work rolls. At least one stand described above is preferably arranged in a tandem mill comprising pluralities of stands.

(76) In addition, the cemented carbide of the present invention can also be widely used for wear-resistant tools, corrosion-resistant, wear-resistant parts, molding dies, etc., which have been made of conventional cemented carbides.

(77) The present invention will be explained in further detail by Examples below, without intention of restricting the present invention thereto.

Example 1

(78) 80 parts by mass of WC powder [purity: 99.9%, and D10: 4.3 μm, median diameter D50: 6.4 μm, and D90: 9.0 μm, which were measured by a laser diffraction particle size distribution meter (SALD-2200 available from Shimadzu Corporation)], and 20 parts by mass of a binder phase-forming powder having the composition shown in Table 1 were mixed to prepare mixture powders (Samples 1-3). Each binder phase-forming powder had a median diameter D50 of 1-10 μm, and contained trace amounts of inevitable impurities.

(79) Each of the mixture powders was wet-mixed for 20 hours in a ball mill, dried, and then subjected to a reduction reaction in a reducing atmosphere of a hydrogen-helium mixture gas at 750° C., such that the amount of oxygen in the metal powder was adjusted to 450 ppm in Sample 1, 2330 ppm in Sample 2, and 150 ppm in Sample 3.

(80) TABLE-US-00001 TABLE 1 Sample Composition of Binder Phase-Forming Powder (% by mass, ppm for O) No. Si Mn Ni Cr Mo V C Co O Fe.sup.(1) 1 0.80 — 5.02 1.21 — — 1.29 — 450 Bal. 2 0.81 — 5.01 1.20 — — 1.23 — 2330 Bal. 3* 0.80 — 5.02 1.21 — — 1.29 — 150 Bal. Note: * Comparative Example.

(81) (1) The balance includes in evitable impurities.

(82) Each of the powders was charged into a HIP can, which was evacuated and sealed, subjected to a HIP treatment at 1230° C. and 140 MPa for 2 hours, cooled at an average cooling rate of 100° C./hour between 900° C. and 600° C., and then annealed at 350° C., to produce a cemented carbide (outer diameter: 60 mm, and length: 40 mm) of Sample 1 (within the present invention), Sample 2 (within the present invention), and Sample 3 (Comparative Example). Each cemented carbide was evaluated by the following methods.

(83) (1) Compressive Yield Strength

(84) Each compression test piece shown in FIG. 3 was cut out of each cemented carbide, and a strain gauge was attached to a center portion of a surface thereof to obtain a stress-strain curve under an axial load. In the stress-strain curve, stress at a point at which the stress and the strain deviated from a straight linear relation was regarded as the compressive yield strength. The results are shown in Table 2.

(85) (2) Bending Strength

(86) A test piece of 4 mm×3 mm×40 mm cut out of each cemented carbide was measured with respect to bending strength under 4-point bending conditions with an interfulcrum distance of 30 mm. The results are shown in Table 2.

(87) (3) Young's modulus

(88) A test piece of 10 mm in width, 60 mm in length and 1.5 mm in thickness, which was cut out of each cemented carbide, was measured by a free-resonance intrinsic vibration method (JIS Z2280). The results are shown in Table 2.

(89) (4) Hardness

(90) The Rockwell hardness (A scale) of each cemented carbide was measured. The results are shown in Table 2.

(91) (5) Area Ratio of Sticking

(92) To evaluate the sticking resistance, a sticking test was conducted on the cemented carbide test pieces of Samples 1 and 2, using a test machine for evaluating thermal shock by friction shown in FIG. 5. In the test machine for evaluating thermal shock, a weight 12 is dropped onto a rack 11 to rotate a pinion 13, so that a member to be bitten is brought into strong contact with a test piece 14. The sticking was evaluated by an area ratio of sticking.

(93) TABLE-US-00002 TABLE 2 Compressive Bending Young’s Area Ratio of Sample Yield Strength Strength Modulus Hardness sticking No. (MPa) (MPa) (GPa) (HRA) (% by area) 1 1500 2297 460 84.2 25 2 1500 2297 450 83.8 23 3* 1700 2488 475 84.4 33 Note: *Comparative Example.

(94) (6) Observation of Structure

(95) Each sample was mirror-polished, and observed by SEM. FIGS. 1 and 2 are SEM photographs of the cemented carbides of Samples 1 and 3. FIGS. 1(a) and 2(a) are photographs showing secondary electron images, and FIGS. 1(b) and 2(b) are photographs showing backscattered electron images. In the photographs showing backscattered electron images, white granular portions are WC particles, gray portions are binder phases, and black spots are Fe—Si—O-based particles. The Fe—Si—O-based particles are clearly discernable particularly in the backscattered electron image of FIG. 1(b). The presence or absence of composite carbides, and the total area ratios of bainite phases and martensite phases in the binder phase were determined from these SEM photographs. The results are shown in Table 3. The Fe—Si—O-based particles were also observed as black particles in an optical microscopic observation of a polished surface, confirming one-to-one correspondence. Accordingly, the area ratio of Fe—Si—O-based particles was calculated from the areas of black particles in optical microscopic observation (magnification of 1000).

(96) Further, the compositions of Fe—Si—O-based particles and the binder phase in Sample 1 were measured by SEM-EDX (acceleration voltage: 5 kV, and beam diameter: 1 μm). C in the binder phase was point-analyzed by a field emission electron probe microanalyzer (FE-EPMA) with a beam diameter of 1 μm. The results are shown in Table 4.

(97) TABLE-US-00003 TABLE 3 Sample Fe-Si-O-Based Composite Bainite Phases and/or No. Particles .sup.(1) Carbides .sup.(2) Martensite Phases .sup.(3) 1 0.32% by area No 50% or more by area 2 0.85% by area No 50% or more by area 3*   0% by area No 50% or more by area Note: * Comparative Example.

(98) (1) The total area ratio (%) of Fe—Si—O-based particles having equivalent circle diameters of 0.1-3 μm.

(99) (2) The presence of absence of composite carbides having diameters of 5 μm or more in the binder phase.

(100) (3) The area ratio (%) of bainite phases and martensite phases in the binder phase.

(101) TABLE-US-00004 TABLE 4 Composition of Sample 1 W Si Ni Fe Cr C O Binder Phase 2.8 0.6 4.5 Bal. 0.9 0.8 — Fe—Si—O-based 5.4 17.8 2.2 Bal. 0.7 — 20.2 Particles Note: The amount of each element is shown by “% by mass.”

(102) Samples 1 and 2 were better than Sample 3, exhibiting lower area ratios of sticking than that of Sample 3.