Hard alloy

Abstract

A cemented carbide containing a total of 8.0-28.0% by mass of Co and Ni, (mass of Co) / (mass of Ni) being 1.0-4.0, and containing 0.5-2.5% by mass of Cr, 4.2-5.7% by mass of C, more than 0.0% by mass but not more than 1.0% by mass of Zr, the remainder comprising W and unavoidable impurities, {(mass% of Zr) / [(mass% of W) + (mass% of Zr)]} * 100 being 0.4-1.5, the cemented carbide having a binder phase, a main hard phase, and an auxiliary hard phase, co and Ni are the main components of the binder phase, W is the main component of the main hard phase as a carbide, a composite carbide of Zr and W is included in the sub-hard phase, the Zr content in the main hard phase accounts for 0.5-1.0 mass% of all the components of the main hard phase, and the W content in the sub-hard phase accounts for 25.0 mass% or less of all the components of the sub-hard phase.

Description

Technical Field

[0001] This invention relates to a cemented carbide used in a tool for inducing plastic deformation of a workpiece at high temperatures. This application claims priority to Japanese Patent Application No. 2023-145161, filed September 7, 2023. All descriptions in that Japanese patent application are incorporated herein by reference. Background Technology

[0002] For cemented carbide used in tools that apply plastic deformation to workpieces at high temperatures, the following approach is proposed: namely, to improve its performance by setting the average particle size of WC to a specific range, and / or by adding a composite compound of ZrC, Zr and Hf.

[0003] For example, Patent Document 1 describes a cemented carbide in which the bonding phase includes Co or Co and Cr3C2, the average particle size of WC particles is 0.5 to 6 μm, and the content of ZrC or (Zr,Hf)C accounts for 2.0 to 20% by mass of the total amount of the bonding phase. This cemented carbide has excellent strength and oxidation resistance at high temperatures.

[0004] Patent Document 1: Japanese Patent Publication No. 2003-113437 Summary of the Invention

[0005] The present invention was made in view of the above circumstances and solutions, and its object is to obtain a high-durability cemented carbide for use in tools that apply plastic deformation to workpieces at high temperatures (e.g., the hot working temperature of steel).

[0006] Regarding the hard alloys involved in the embodiments of the present invention.

[0007] It contains a total of 8.0 to 28.0% by mass of Co and Ni, with the ratio of (Co by mass%) to (Ni by mass%) being 1.0 to 4.0.

[0008] It also contains 0.5–2.5% by mass of Cr, 4.2–5.7% by mass of C, and more than 0.0% by mass but less than 1.0% by mass of Zr.

[0009] The remaining portion consists of W and unavoidable impurities.

[0010] {(mass% of Zr) / [(mass% of W)+(mass% of Zr)]}×100 is 0.4~1.5,

[0011] The cemented carbide has a bonding phase, a main hard phase, and a secondary hard phase.

[0012] Co and Ni are the main components of the bonded phase.

[0013] W is the main component of the main hard phase in the form of carbides.

[0014] The composite carbides of Zr and W are included in the secondary hard phase.

[0015] In the main hard phase, the Zr content accounts for 0.5 to 1.0% by mass of all components in the main hard phase.

[0016] In the secondary hard phase, the W content accounts for less than 25.0% by mass of all components of the secondary hard phase.

[0017] In addition, the cemented carbide may satisfy one or more of the following (1) and (2).

[0018] (1) In the main hard phase, (number of Zr atoms) / [(number of Zr atoms) + (number of W atoms)] × 100 is 1.5 to 2.5.

[0019] (2) The proportion of grain boundaries on the grain boundaries of the main components constituting the combined phase, where elements different from Co and Ni are segregated, is less than 30%.

[0020] When the cemented carbide described in the foregoing embodiments is used as a tool material for applying plastic deformation to workpieces at high temperatures, its durability is improved. Attached Figure Description

[0021] Figure 1 This is a schematic cross-sectional view of an apparatus used to simulate the thermal load test of hot rolling.

[0022] Figure 2 It means in Figure 1 A schematic diagram of the heating zone (heating width) of the hollow cylindrical sintered body simulating a hot rolling roll in the device. Detailed Implementation

[0023] The inventors have conducted in-depth research on methods for improving the durability of cemented carbide, wherein the cemented carbide is a cemented carbide used in tools that apply plastic deformation to workpieces at high temperatures, especially cemented carbide used in hot rolling rolls.

[0024] As a result, the physical properties of cemented carbide only provide strength reduction and oxidation inhibition at high temperatures, which limits the improvement of durability. In order to overcome this limitation, it is recognized that it is necessary to impart physical properties that inhibit the generation of cracks caused by thermal fatigue, especially inhibiting the development of mesh-like cracks (cracks that propagate in multiple directions on the surface of hot rolls) that are generated on the surface when used as hot rolls.

[0025] Furthermore, further research revealed that adding a specified amount of Zr to cemented carbide can suppress the progression of mesh-like cracks.

[0026] The cemented carbide involved in the embodiments of the present invention will now be described.

[0027] Furthermore, in this specification and claims, when "L to M" (where L and M are both numerical values) is used to represent a numerical range, it has the same meaning as "above L and below M," and the range includes an upper limit (M) and a lower limit (L). When only the unit of the upper limit is described, the units of the upper limit (M) and the lower limit (L) are the same. Additionally, the secondary hard phase in this embodiment is sometimes also referred to as the γ phase.

[0028] 1. Composition

[0029] The composition is explained.

[0030] (1) Co, Ni

[0031] The preferred composition includes a total of 8.0 to 28.0% by mass of Co and Ni.

[0032] If the combined content of Co and Ni is less than 8.0% by mass, the toughness of the cemented carbide will decrease; on the other hand, if it exceeds 28.0% by mass, the cemented carbide will not have the specified hardness.

[0033] Here, by adding Ni in addition to Co, the oxidation resistance at high temperatures is improved, making it easier for the bound phase to maintain the fcc crystal structure and suppressing the phase transition from the fcc crystal structure to other crystal structures, thereby preventing distortion caused by the phase transition. To suppress this distortion, the ratio of (mass% of Co) to (mass% of Ni) is preferably 1.0 to 4.0, more preferably 2.0 to 3.0.

[0034] (2) Cr

[0035] Preferably, it includes 0.5 to 2.5% by mass of Cr.

[0036] Cr is dissolved in the binding phase, inhibiting the growth of W carbides included in the main hard phase and refining the W carbides. As a result, the cemented carbide is endowed with oxidation resistance, toughness, and strength.

[0037] If the Cr content is below 0.5% by mass, the oxidation resistance of the cemented carbide will decrease; if it exceeds 2.5% by mass, it will exceed the solid solution limit in the bonding phase, leading to the formation of agglomerates, which will reduce the toughness of the cemented carbide. The preferred Cr content is 0.8% to 2.0% by mass.

[0038] (3) C

[0039] C is present to form carbides and is mainly included in the primary hard phase and the secondary hard phase. Its content is preferably 4.2 to 5.7% by mass. If it is within this range, a sufficient amount of carbides can be formed in the primary hard phase and the secondary hard phase.

[0040] (4) Zr

[0041] Zr is added to ensure uniform dispersion of the binder phase and to suppress the progression of mesh-like cracks. Its content is preferably more than 0.0% by mass and less than 1.0% by mass. More preferably, the Zr content is 0.1 to 0.9% by mass.

[0042] In addition, in the overall alloy, it is preferable that {(mass% of Zr) / [(mass% of W)+(mass% of Zr)]}×100 satisfies 0.4 to 1.5.

[0043] If this mass ratio is achieved, the propagation of mesh-like cracks at high temperatures is suppressed. Furthermore, the reason for setting this mass ratio is that if it is below 0.4, the bonding phase cannot be uniformly dispersed; on the other hand, if it exceeds 1.5, crack formation due to the aggregation of the secondary hard phase will increase.

[0044] The mass ratio is more preferably 0.5 to 1.3, and even more preferably 0.7 to 1.2.

[0045] (5) W

[0046] W is the main component of the primary hard phase, and the content of W carbides (almost entirely WC, but may also include other W carbides with compositions not limited to stoichiometry) accounts for more than 50 atomic percent of all components constituting the primary hard phase. There are also preferred values ​​for the W content in the secondary hard phase, which will be described later.

[0047] (6) Ti, V, Nb, Ta and Mo

[0048] While these Ti, V, Nb, Ta, and Mo are preferably absent, they may be present as unavoidable impurities as described later.

[0049] (7) Segregation of elements at the grain boundaries between the main components of the bonded phase

[0050] The proportion of grain boundaries at the grain boundaries of the main components constituting the bonding phase, namely Co, Ni, and Co-Ni solid solutions, where there are concentration peaks of elements different from Co and Ni (such as Cr or W, which cause segregation), is preferably 30% or less. This is because if it exceeds 30%, the precipitates on the grain boundaries will reduce the grain boundary strength and easily become crack propagation paths. While ideally the lower limit of this proportion is 0%, in an example following the manufacturing method described later, the lower limit is approximately 10%.

[0051] (8) Unavoidable impurities

[0052] Impurities that inevitably (unintentionally) are introduced during the manufacturing process may include the main hard phase, the secondary hard phase, and the bonding phase. Examples of unavoidable impurities include Fe, N, O, and Hf. Furthermore, when the total amount of cemented carbide is set at 100% by mass, the total amount of unavoidable impurities is preferably 0.3% by mass or less.

[0053] 2. Organization

[0054] Provide an explanation of the organization.

[0055] (1) Combining phase

[0056] The crystal structure of the bonded phase is either fcc or hcp.

[0057] The main components of the bound phase are Co and Ni. That is, the combined content of Co and Ni accounts for more than 50 atomic percent of the elements contained in the bound phase.

[0058] In addition to Co and Ni, the bound phase may also include Cr, W and C as the main components of the hard phase, and unavoidable impurities. When these components are included in the bound phase, they are presumed to be in a solid-dissolved state within the bound phase. Furthermore, Zr is substantially absent in the bound phase (its amount is below the detection limit).

[0059] (2) Secondary hard phase

[0060] The crystal structure of the secondary hard phase is an fcc structure.

[0061] The main component of the secondary hard phase is Zr carbides (stoichiometric composition not limited). Here, the main component of the secondary hard phase means that Zr carbides account for more than 50 atomic percent of all elements included in the secondary hard phase. In addition to Zr carbides, the secondary hard phase may also include Cr, W and C included in the main hard phase, Co and Ni included in the bonding phase, and unavoidable impurities.

[0062] The W content in all components contained in the secondary hard phase is preferably less than 25.0% by mass (the lower limit is about 2.0% by mass in one example of the manufacturing method described later, or it can be 0.0% by mass) in order to suppress the reaction between Zr and W and thus more effectively suppress the progression of mesh cracks.

[0063] Furthermore, the number of W atoms included in the secondary hard phase, calculated as the percentage of the sum of the number of Zr atoms included in the secondary hard phase and the number of W atoms, i.e., <W> / (<W>+<Zr>)×100 (where <W> and <Zr> are the number of W atoms and the number of Zr atoms, respectively), is more preferably 6.0 to 12.0. If this range is satisfied, the propagation of mesh-like cracks is more effectively suppressed.

[0064] While there is no particular limitation on the average particle size of the primary particles in the secondary hard phase, it is more preferably 0.5 μm or more and less than 1.5 μm. If it is less than 0.5 μm, the surface area of ​​the secondary hard phase increases, and ZrO2 may precipitate during sintering. On the other hand, if it reaches 1.5 μm or more, the secondary hard phase becomes too large, easily forming binder pools that act as aggregates of the bonding phase, leading to an increase in the distance between the primary hard phases, which may sometimes reduce the strength. The average particle size of the primary particles in the secondary hard phase is more preferably 0.6 to 1.3 μm. Furthermore, the definition of the average particle size of the primary particles in the secondary hard phase will be described later.

[0065] (3) Main hard phase

[0066] The crystal structure of the main hard phase is an hcp structure.

[0067] The main hard phase is primarily composed of W carbides (W carbides account for more than 50 atomic percent of all elements constituting the main hard phase). The main hard phase may include components of the bonding phase, Cr, and unavoidable impurities unintentionally included during the manufacturing process. The Zr content in the main hard phase is preferably 0.5 to 1.0% by mass of all components included in the main hard phase. This is because, at this content range, Zr removes oxides present at the interface between the main hard phase and the bonding phase, thereby improving the durability of tools used at high temperatures. The Zr content in the main hard phase is more preferably 0.6 to 0.9% by mass.

[0068] Furthermore, the content (number of atoms) of Zr included in the main hard phase, calculated as the percentage of the sum of the number of Zr atoms and the number of W atoms included in the main hard phase, i.e., <Zr> / (<Zr>+<W>)×100 (where <Zr> and <W> are the number of Zr atoms and the number of W atoms, respectively), is more preferably 1.5 to 2.5. If this range is met, the propagation of mesh-like cracks is more effectively suppressed.

[0069] While there are no particular limitations on the average particle size of the primary particles in the main hard phase, it is preferably 2.0 to 9.0 μm. If the particle size is less than 2.0 μm, the toughness of the cemented carbide will decrease; conversely, if the particle size exceeds 9.0 μm, the hardness of the cemented carbide will decrease. More preferably, the average particle size of the primary particles in the main hard phase is 3.0 μm to 8.0 μm, and even more preferably, 4.0 μm to 6.0 μm. The definition of the average particle size of the primary particles in the main hard phase will be described later.

[0070] (4) The ratio of the average particle size of primary particles in the main hard phase and the secondary hard phase

[0071] The ratio of the average particle size of the primary particles in the main hard phase to that in the secondary hard phase, i.e., (average particle size of the primary particles in the main hard phase) / (average particle size of the primary particles in the secondary hard phase), is more preferably 4.0 to 5.0. If the value of the ratio of the average particle size of the primary particles is within this range, the mesh-like cracks generated by the main hard phase and the secondary hard phase during hot rolling become smaller, which can prevent thermal stress concentration in specific cracks and further suppress the generation of deep cracks.

[0072] 3. Methods for determining the average particle size of primary particles

[0073] The particle size of primary particles in the secondary hard phase and the primary hard phase refers to the circle equivalent diameter, that is, the diameter (D50) of a circle having an area equal to that of the secondary hard phase and the primary hard phase, and is determined as follows.

[0074] (1) In order not to interfere with the backscattered electron diffraction (EBSD) measurement, any surface or cross section of the cemented carbide is machined to make it smooth by removing the fine bumps. Multiple fields of view are set on the machined surface, for example, each field of view is 190 μm (longitudinal) × 242 μm (horizontal). The observation is performed using a scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS) and an EBSD at an accelerating voltage of 15 kV and a measurement point interval of 0.1 μm. At the same time, the EBSD pattern and EDS data are read.

[0075] The number of observed phases can be the same or different in the identification of the binding phase, the secondary hard phase, and the primary hard phase. The number is preferably more than 300 (preferably 300 to 1000), and the aforementioned observation field is increased until it reaches more than 300.

[0076] In addition, surface or cross-section processing can be performed using devices such as focused ion beam (FIB) devices.

[0077] (2) Next, the average value of the EBSD measurement results and EDS count values ​​is taken to derive the composition of each phase.

[0078] (3) Here, each phase is identified according to the aforementioned definitions (crystal structure and composition). That is, the phase identified from the EBSD pattern as having an hcp structure and being mainly composed of W carbides is designated as the main hard phase. Next, from all measurement points identified as fcc or hcp phases, the average value of the detected Co and Ni EDS counts is calculated, and the measurement points of the fcc phase identified as having EDS counts of Co and Ni higher than the average value are designated as the binding phase, and the remaining part of the fcc phase is designated as the secondary hard phase.

[0079] (4) When adjacent measurement points are judged to be the same phase, if the orientation difference obtained from each other's measurement points is more than 5 degrees, then the middle between two of these measurement points is set as the phase interface.

[0080] (5) Regarding the average particle size of each phase, calculate the area of ​​more than 300 (preferably 300 to 1000) phases, and calculate the diameter of the circle with the same area.

[0081] (6) In the above (3), for each phase identified as the main hard phase and the secondary hard phase, EDS determination was performed again to confirm that W carbides account for more than 50 atomic% in the phase identified as the main hard phase and that the combined Co and Ni account for more than 50 atomic% in the phase identified as the bonding phase.

[0082] (7) Based on the results of (5) and (6) above, the particle sizes of the main hard phase and the secondary hard phase are respectively taken on the horizontal axis. A graph is made on the vertical axis to show the cumulative number ratio of the main hard phase and the secondary hard phase corresponding to each particle size on the horizontal axis ([(cumulative number of main hard phases up to each particle size) / (total number of main hard phases)] × 100, [(cumulative number of secondary hard phases up to each particle size) / (total number of secondary hard phases)] × 100). The particle size when the cumulative number ratio reaches 50% is set as the average particle size (D50).

[0083] 4. Determine the content of each component in the overall cemented carbide.

[0084] The content of W, Co, Ni, Zr, Cr, and C in the overall cemented carbide can be determined based on the composition of each phase (the proportion of each component in each phase) and the area ratio of each phase, but it can also be determined using electron beam microscopy (EPMA). In EPMA-based measurements, the mirror-finished surface of the sample placed in the EPMA apparatus (a surface of cemented carbide that has been machined to a smooth surface by removing fine irregularities so as not to interfere with EPMA measurements) is irradiated with electron beams. The composition and its amount are determined by the average value of the measurement results obtained from characteristic X-ray measurements obtained from an observation field of 512 μm (longitudinal) × 512 μm (horizontal).

[0085] 5. The proportion of interfaces containing elements other than Co at the interfaces between the bonding phases.

[0086] The proportion of interfaces at which elements different from Co (such as Cr and W) segregate at the interfaces of the two phases is determined as follows.

[0087] For example, multiple fields of view (preferably five or more) of size 190 μm (vertical) × 242 μm (horizontal) are set. Within each field of view, locations that may be binding phase interfaces are inferred based on dark-field scanning electron microscopy (BF-STEM) images and high-angle scattering annular dark-field scanning transmission microscopy (HAADF-STEM) images, and EDS line analysis is performed based on these inferences. This EDS line analysis determines whether segregation occurs at the interface due to the presence or absence of concentration peaks for elements other than Co.

[0088] Furthermore, (number of interfaces with segregation) / (cumulative number of interfaces that are the subject of measurement) × 100 is set as the ratio of the number of interfaces on which elements other than Co are segregated at the interfaces between the bound phases.

[0089] Here, the aforementioned field of view can be either the field of view that defines the aforementioned binding phase, or a newly defined field of view.

[0090] 6. Manufacturing method

[0091] The cemented carbide involved in this embodiment can be manufactured, for example, in the following manner.

[0092] WC powder, Co powder, Ni powder, Cr3C2 powder and ZrC powder, which are used as raw material powders, are mixed in a specified ratio, pulverized and mixed using a grinder, and then vacuum dried to obtain the compounded raw materials.

[0093] Then, in order to form a molded body of a specified shape, the molded body is pressed and molded repeatedly under a vacuum atmosphere, and the temperature is repeatedly raised and held so that each part of the molded body can be heated evenly until it reaches 1300°C to 1400°C, which is the sintering temperature, and is held at the sintering temperature for 30 to 90 minutes.

[0094] After sintering, the molded body undergoes slow cooling for 1.5 to 5 hours until it reaches a specified temperature between 900 and 1100°C. This specified temperature and slow cooling time depend on the shape of the molded body (the reason for the different slow cooling times is that the slow cooling is carried out simultaneously with electrical heating, resulting in a non-constant slow cooling rate). After reaching the target slow cooling temperature, Ar gas is injected for rapid cooling to room temperature.

[0095] Next, the temperature is raised again to 1250-1350℃, and HIP treatment is carried out in an Ar gas atmosphere. Then, while being heated by electricity, it is slowly cooled to 600-400℃ for 5.5-8.5 hours.

[0096] The above description includes the following features.

[0097] (Note 1)

[0098] A cemented carbide, characterized in that,

[0099] It contains a total of 8.0 to 28.0% by mass of Co and Ni, with a (mass of Co) / (mass of Ni) ratio of 1.0 to 4.0.

[0100] It also contains 0.5–2.5% by mass of Cr, 4.2–5.7% by mass of C, and more than 0.0% by mass but less than 1.0% by mass of Zr.

[0101] The remaining portion consists of W and unavoidable impurities.

[0102] {(mass% of Zr) / [(mass% of W)+(mass% of Zr)]}×100 is 0.4~1.5,

[0103] The cemented carbide has a bonding phase, a main hard phase, and a secondary hard phase.

[0104] Co and Ni are the main components of the bonded phase.

[0105] W is a carbide-based component of the main hard phase.

[0106] The composite carbides of Zr and W are included in the secondary hard phase.

[0107] In the main hard phase, the Zr content accounts for 0.5 to 1.0% by mass of all components in the main hard phase.

[0108] In the secondary hard phase, the W content accounts for less than 25.0% by mass of all components of the secondary hard phase.

[0109] (Note 2)

[0110] According to the cemented carbide described in Appendix 1, the characteristic is that, in the main hard phase, (number of Zr atoms) / [(number of Zr atoms) + (number of W atoms)] × 100 is 1.5 to 2.5.

[0111] (Note 3)

[0112] According to Appendix 1 or 2, the cemented carbide is characterized in that the proportion of grain boundaries at the grain boundaries of the main components constituting the bonding phase, where elements different from Co and Ni are segregated, is less than 30%.

[0113] (Note 4)

[0114] According to any one of Appendices 1 to 3, the cemented carbide is characterized in that the ratio of (average particle size of primary particles of the main hard phase) to (average particle size of primary particles of the secondary hard phase) is 4.0 to 5.0.

[0115] (Note 5)

[0116] The cemented carbide according to any one of Appendices 1 to 4 is characterized in that, in the secondary hard phase, (number of W atoms) / [(number of W atoms) + (number of Zr atoms)] × 100 is 6.0 to 12.0.

[0117] (Note 6)

[0118] The cemented carbide according to any one of Appendices 1 to 5 is characterized in that the average grain size of the secondary hard phase is 0.5 μm or more and less than 1.5 μm.

[0119] (Note 7)

[0120] The cemented carbide according to any one of Appendices 1 to 6 is characterized in that the average grain size of the main hard phase is 2.0 to 9.0 μm.

[0121] Example

[0122] The cemented carbide of the present invention is specifically described using a sintered body in the shape of a hollow cylinder made by simulating hot rolling rolls as an example, but the present invention is not limited to this example.

[0123] 1. Manufacturing of the Examples and Comparative Examples

[0124] The sintered bodies of hot-rolled rolls simulating the composition of cemented carbide shown in Table 3 (Examples 1-14 and Comparative Examples 1'-9') were manufactured as follows.

[0125] (1) Raw material powder and compounding process

[0126] First, as raw material powders for the sintered body of cemented carbide, WC powder (FSSS diameter: 6.2 μm), Co powder (FSSS diameter: 1.5 μm), Ni powder (average particle size D50: 0.8 μm), Cr3C2 powder (FSSS diameter: 3.3 μm) and ZrC powder (FSSS diameter: 2.4 μm) were prepared.

[0127] Here, the average particle size D of the Ni powder is the median diameter obtained by taking the results measured by a laser particle size distribution measuring device in the form of a volume ratio.

[0128] Mix these powders according to the proportions shown in Table 1, and then grind and mix them using a grinder.

[0129] (2) Sintering process

[0130] The mixed powder was dried and pressed into a hollow cylindrical shape with an outer diameter of 50 mm, an inner diameter of 18 mm, and a height of 37 mm under a pressure of 100 MPa, resulting in a molded body. Then, it was repeatedly heated and held at a vacuum of less than 1 Pa to ensure that all parts of the molded body could be considered to be uniformly (at the same rate) heated to 1250 °C, and sintered under the sintering conditions shown in Table 2. After sintering, it was electrically cooled (1.5–1.6 °C / min) for 5 hours until the target temperature of 900 °C was reached, and then rapidly cooled to room temperature using Ar gas. Then, it was heated again to 1290 °C and subjected to HIP treatment at 88.3 MPa under an Ar gas atmosphere for 7 hours while being electrically heated and then slowly cooled to 500 °C. Finally, the sintered body was cut into a hollow cylindrical shape with an outer diameter of 40 mm, an inner diameter of 16 mm, and a height of 30 mm.

[0131] Furthermore, the reason why the cooling rate of electro-cooling has a range is that it is difficult to keep the electro-cooling rate constant.

[0132] [Table 1]

[0133]

[0134] [Table 2]

[0135]

[0136] 2. Evaluation of the Examples and Comparative Examples

[0137] (1) Composition and average particle size

[0138] The content ratio of each component and the average particle size of the main hard phase and the secondary hard phase were determined using the aforementioned method. The results are shown in Table 3. Furthermore, for unavoidable impurities, their content was confirmed to be less than 0.3% by mass in all examples and comparative examples. Additionally, the Zr content in the bound phase was confirmed to be less than the detection limit.

[0139] Additionally, in Table 3, "-" indicates that the value is less than the detection limit. Since the secondary hard phase was not present in several comparative examples, the "average particle size of the secondary hard phase (D50: μm)" was recorded as "not measurable", and the "(primary average particle size of the main hard phase) / (primary average particle size of the secondary hard phase)" was recorded as "not calculable".

[0140] [Table 3]

[0141]

[0142] (2) The proportion of grain boundaries containing elements other than Co and Ni at the grain boundaries of the main components constituting the bounding phase.

[0143] For Example 1 and Comparative Example 1', the proportion of grain boundaries containing elements other than Co and Ni at the grain boundaries between Co grains was determined using the aforementioned method (5 fields of view). The EDS line analysis accompanying the STEM was performed at 738,000x magnification, with a spot diameter of 132 pm, a spot spacing of 132 pm, and a line length of 50 nm, measuring 400 points. The results are shown in Table 4. The segregation rate mentioned in Table 4 refers to the proportion of grain boundaries at the grain boundaries between the main components constituting the bounding phase where concentration peaks of elements other than Co and Ni are present (a concentration peak is considered to exist when the atomic percentage of the element at the location of the concentration peak is 1.5 times or more than the atomic percentage of the element at other locations) (segregation has occurred).

[0144] As can be seen from the table, compared with Comparative Example 1', Example 1 has a smaller proportion of grain boundaries containing elements other than Co and Ni at the grain boundaries between the main components of the bounding phase.

[0145] [Table 4]

[0146]

[0147] (3) Heat load test

[0148] A sintered body in the shape of a hollow cylinder with an outer diameter of 40 mm, an inner diameter of 16 mm, and a height (the roll width when considered as a roll) of 30 mm, simulating a hot rolling roll, has the composition of Example 1 and Comparative Example 1'. A hot rolling heat load test simulating repeated heating and cooling was conducted to evaluate the presence of mesh-like cracks.

[0149] The apparatus used for the thermal load test is as shown in Int. Journal of Refractory Metals and Hard Materials 60 (2016), pp. 118-124. (Refer to...) Figure 1 To illustrate, the hollow cylindrical sintered carbide body 1 is rotated at 4 rpm in the direction of the arrow. Approximately one-quarter of the rotation 2 is heated to 600°C using a high-frequency coil, followed by water cooling in the water jet zone 3. This heating and cooling cycle is repeated 3000 times. The heating width 5 is only 10 mm at the center (see reference). Figure 2 Cooling water is sprayed in a manner that does not flow into the high-frequency coil side and is recovered by water tank 4.

[0150] The heat load applied to this heating and cooling cycle is the heat load that simulates the heat cycle of the rolls during actual hot rolling.

[0151] That is, during hot rolling, the part of the roll that comes into contact with the rolled material is heated by the heat from the rolled material and becomes high temperature, while the two ends that come into contact with the rolled material are not heated but cooled by cooling water.

[0152] The reason for setting the heating temperature to 600℃ is that if it exceeds 600℃, WC will be oxidized and cannot be used as a roll. Therefore, it is speculated that the roll temperature during hot rolling will not exceed 600℃.

[0153] After the heat load test is completed, the hollow cylindrical cemented carbide sintered body is radially cut in a manner that includes the heated center (the center part in the height direction of the hollow cylinder), and the length of the cracks generated on its cross section is determined by the following sequence.

[0154] 1) Enlarge the two ends of the outer periphery of the heating area (the outer periphery corresponding to the central angle of the hollow cylinder of 1.8 degrees) by 200 times and place it on the diagonal of the quadrilateral observation area with a vertical length of about 450μm and a horizontal width of about 630μm.

[0155] 2) Starting from one end of the outer periphery and moving towards the other end, peripheral points are set on the outer periphery at 100μm intervals, parallel to the diagonal.

[0156] 3) Connect the outer perimeter points with a least-squares fit to form a straight line, and regard this straight line as the outer perimeter line used to determine the crack length.

[0157] 4) Draw a straight line parallel to the outer circumference line that passes through the deepest part of the crack (the point closest to the center of the hollow cylinder in the circumferential direction), and measure the distance between this straight line and the outer circumference line. This distance is considered the crack length. In addition, cracks with a length of 200 μm or more are observed at 100x magnification, and cracks with a length of less than 10 μm are not considered cracks.

[0158] Here, although the observation magnification is set to 100x, cracks with a length of 200μm or more sometimes extend beyond the field of view, making it sometimes impossible to measure their full length. Therefore, the 100x observation magnification is only one example, and the magnification is adjusted according to the crack length for measurement.

[0159] Furthermore, cracks with a length of less than 10µm were excluded from the observation because their thickness was thinner than the oxide layer formed on the roll surface and they were presumed to be caused by spalling rather than thermal cycling.

[0160] Table 4 shows the measurement results. The average crack length in Table 4 refers to the average value of the measured crack lengths.

[0161] (4) Fracture toughness value

[0162] Fracture toughness values ​​were determined using the Single Edge V-Notch Beam method (SEVNB method).

[0163] Having the same composition as in Examples 1 and 2, and under the same manufacturing conditions, a cuboid sintered body was fabricated, and a 3×4×40mm rectangular test piece was made from this sintered body. A V-shaped notch with a depth of 1.5mm, a right angle of less than 0.01mm, and a front radius of 15μm was machined at the center along the length direction, and a bending strength test was conducted. During the test, the test piece was positioned with the V-shaped notch located at the center of the pin on the two-point side of a three-point bend, under atmospheric conditions at 1000°C, with a distance of 16mm between the fulcrums, and a load speed of 0.5mm / min for the force sensor.

[0164] The results are shown in Table 5.

[0165] [Table 5]

[0166]

[0167] In Table 5, “Corresponding Comparative Example” indicates the number of the comparative example that has the same content of alloy components other than Zr and only does not contain Zr.

[0168] As can be seen from the columns "Ratio of Average Crack Depth to Corresponding Comparative Example" and "Ratio of Fracture Toughness to Corresponding Comparative Example," the embodiments, due to the presence of an appropriate amount of Zr, exhibit a smaller average crack depth and a higher fracture toughness value. Furthermore, in the column "Ratio of Average Crack Depth to Corresponding Comparative Example," Comparative Example 3' and Comparative Example 5' contain 0.2% by mass and 1.2% by mass of Zr, respectively, representing their ratios to the average crack depths of their corresponding Comparative Examples 2' and 4'. From the "Ratio of Average Crack Depth to Corresponding Comparative Example" for Comparative Examples 3' and 5', it can be seen that if the Zr content is not appropriate, the average crack depth becomes deeper.

[0169] (4) Re-grinding amount test

[0170] Having the same composition as Example 2 and Comparative Example 2', a roll with an outer diameter of 283.5 mm, an inner diameter of 160 mm, and a length of 70 mm was manufactured under the same manufacturing conditions as Example 2 and Comparative Example 2' and provided for rolling under the following hot rolling processing conditions.

[0171] Mouth shape: elliptical (width 33.33mm, depth 5.5mm, with one mouth diameter R of 28mm and shoulder diameter r of 3mm)

[0172] Rolled material: A material composed of SCM435 and SUM24L combined in a manner that makes up the total mass of the rolled material.

[0173] Rolling temperature: 500℃

[0174] Total rolled stock mass: 4400 tons

[0175] The amount of re-grinding is shown in Table 6.

[0176] Re-grinding amount refers to the amount of surface roughness and internal cracks caused by using the roller, and the amount of grinding required for reuse of the roller. Therefore, a higher average re-grinding amount indicates deeper surface roughness and internal cracks caused by roller use.

[0177] [Table 6]

[0178]

[0179] As can be seen from Table 6, since the amount of regrinding in Example 2 is less than that in Comparative Example 2', more regrinding can be performed until the discarded diameter, resulting in a longer service life when used as a roller.

[0180] The embodiments disclosed above are merely exemplary in all respects and are not restrictive. The scope of the invention is defined by the claims, not by the foregoing embodiments, and is intended to include all modifications within the meaning and scope of the claims.

[0181] Explanation of reference numerals in the attached figures

[0182] 1. A hollow cylindrical sintered body simulating a hot rolling roll

[0183] 2. Heating area using high-frequency coils

[0184] 3 Cooling water spray area

[0185] 4 sinks

[0186] 5 heating width

Claims

1. A cemented carbide, characterized in that, It contains a total of 8.0 to 28.0% by mass of Co and Ni, with a (mass of Co) / (mass of Ni) ratio of 1.0 to 4.

0. It also contains 0.5–2.5% by mass of Cr, 4.2–5.7% by mass of C, and more than 0.0% by mass but less than 1.0% by mass of Zr. The remaining portion consists of W and unavoidable impurities. {(mass% of Zr) / [(mass% of W)+(mass% of Zr)]}×100 is 0.4~1.5, The cemented carbide has a bonding phase, a main hard phase, and a secondary hard phase. Co and Ni are the main components of the bonded phase. W is the main component of the main hard phase in the form of carbides. The composite carbides of Zr and W are included in the secondary hard phase. In the main hard phase, the Zr content accounts for 0.5 to 1.0% by mass of all components in the main hard phase. In the secondary hard phase, the W content accounts for less than 25.0% by mass of all components of the secondary hard phase.

2. The cemented carbide according to claim 1, characterized in that, In the main hard phase, (number of Zr atoms) / [(number of Zr atoms) + (number of W atoms)] × 100 is 1.5 to 2.

5.

3. The cemented carbide according to claim 1 or 2, characterized in that, At the grain boundaries between the main components of the bounding phase, the proportion of grain boundaries in which elements different from Co and Ni segregate is more than 10% and less than 30%.

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