CAST MATERIAL AND METHOD OF MANUFACTURING CAST MATERIAL

BACKGROUND OF THE INVENTION
Field of the Invention

The present invention relates to a cast material and a method of manufacturing a cast material.

Brief Description of the Related Art

Requirements for wear resistant materials used in various mechanical facilities and mechanical devices have come to be increasingly severe year by year; recently, wear resistant materials have been demanded to be not only high in wear resistance but also excellent in corrosion resistance, heat resistance and the like.

As such wear resistant materials, cermet materials, namely, composite materials between ceramics and metals have hitherto been investigated. As the methods of manufacturing such cermet materials, there has been known a method in which, for example, by a powder metallurgy method, powders to be raw materials are mixed with each other, and subjected to firing at a temperature equal to or lower than the melting points of the raw materials, in a state of being molded by mold pressing or the like.

When the powder metallurgy method is used, because the raw materials are not melted, excessive grain growth in raw materials can be suppressed, and the generation of shrinkage cavities or dendrite microstructures (columnar crystals) can be prevented. On the other hand, when the powder metallurgy method is used, because voids remain in the interior of the obtained cermet material, the density of the obtained cermet material sometimes comes to be insufficient.

In contrast, Patent Document 1 discloses a method for obtaining a cast material including Mo (molybdenum), Ni (nickel), B (boron) and the like, by using cast method.

PRIOR ART DOCUMENT

[Patent Document]

[Patent Document 1] WO 2012/063879

SUMMARY OF THE INVENTION
Problems to be Solved by Invention

However, a cast cermet material obtained by the cast method described in foregoing Patent Document 1 is improved in density, and additionally, dendrite microstructure tend to grow in the interior of the cast cermet material. Accordingly, the cast material obtained by the cast method described in Patent Document 1 is liable to be broken due to the grown dendrite microstructures to function as breakage origins. Therefore, it has been difficult to use the cast material obtained by the cast method described in Patent Document 1, in particular, in applications requiring bending strength.

An object of the present invention is to provide a cast material being excellent in corrosion resistance and wear resistance, and achieving a high hardness and a high bending strength.

Means for Solving Problems

The present inventors have perfected the present invention by discovering that the foregoing object can be achieved by controlling the average particle size of hard phase particles, the average value of the aspect ratios of the hard phase particles, the content of the hard phase particles having a major axis exceeding 5 μm, and the contact ratio between the hard phase particles so as to fall within specific ranges, in a cast material comprising the hard phase particles mainly composed of a boride or a carbide and a binder phase including an alloy mainly composed of Co and/or Ni.

That is, according to an aspect of the present invention, there is provided a cast material comprising hard phase particles mainly composed of a boride and/or a carbide and a binder phase including an alloy mainly composed of Co and/or Ni, wherein the average particle size of the hard phase particles is 3 μm or less, the average value of the aspect ratios of the hard phase particles is 2.3 or less, the content of the hard phase particles having a major axis exceeding 5 μm is 3 particles or less per 2,450 μm², and the contact ratio between the hard phase particles is 40% or less.

In the cast material according to the present invention, the hard phase particles are preferably particles of the boride and/or the carbide composed of at least one of Ni, Co, Cr, Mo, Mn, Cu, W, Fe and Si, and B and/or C.

In the cast material according to the present invention, the binder phase is preferably the alloy composed of at least one metal of Cr, Mo, Mn, Cu, W, Fe and Si, and Co and/or Ni.

In the cast material according to the present invention, the content of B in the cast material is preferably 1 to 6 wt %, and the content of C in the cast material is 0 to 2.5 wt %.

In the cast material according to the present invention, the hard phase particles are preferably composed of a composite boride represented by Mo₂NiB₂or Mo₂(Ni,Cr)B₂, and the binder phase is preferably composed of a Ni-based alloy.

Moreover, according to another aspect of the present invention, there is provided a method of manufacturing a cast material comprising the hard phase particles mainly composed of a boride and/or a carbide, and a binder phase including an alloy mainly composed of Co and/or Ni, wherein the production method includes a step of obtaining a fused mixture by dissolving the raw materials for forming the cast material in a state of being mixed with each other, and a step of cooling the fused mixture, the step of cooling the fused mixture including a process of continuously cooling the fused mixture, at a cooling rate of 100° C./min or more, in a temperature range from the cooling starting temperature to 400° C.

It is preferred in the manufacturing method according to the present invention to perform the cooling of the fused mixture by pouring the fused mixture into a mold set at room temperature to 1100° C.

Effect of Invention

According to the present invention, there can be provided a cast material excellent in corrosion resistance and wear resistance, and achieving a high hardness and a high bending strength.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram for illustrating the measurement method of the microstructure of the cast material according to the present invention.

FIG. 2A and FIG. 2B are photographs showing a backscattered electron image of a cross section of the cast material of Example 1 taken by using a scanning electron microscope (SEM).

FIG. 3A and FIG. 3B are photographs showing a backscattered electron image of a cross section of the cast material of Example 2 taken by using a scanning electron microscope (SEM).

FIG. 4A and FIG. 4B are photographs showing a backscattered electron image of a cross section of the cast material of Example 3 taken by using a scanning electron microscope (SEM).

FIG. 5A and FIG. 5B are photographs showing a backscattered electron image of a cross section of the cast material of Example 4 taken by using a scanning electron microscope (SEM).

FIG. 6A and FIG. 6B are photographs showing a backscattered electron image of a cross section of the cast material of Example 5 taken by using a scanning electron microscope (SEM).

FIG. 7A and FIG. 7B are photographs showing a backscattered electron image of a cross section of the cast material of Comparative Example 1 taken by using a scanning electron microscope (SEM).

FIG. 8A and FIG. 8B are photographs showing a backscattered electron image of a cross section of the cast material of Comparative Example 2 taken by using a scanning electron microscope (SEM).

FIG. 9A and FIG. 9B are photographs showing a backscattered electron image of a cross section of the cast material of Comparative Example 3 taken by using a scanning electron microscope (SEM).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the cast material according to the present invention will be described.

The cast material according to the present invention comprises hard phase particles mainly composed of a boride or a carbide and a binder phase including an alloy mainly composed of Co and/or Ni, wherein the average particle size of the hard phase particles is 3 μm or less, the average value of the aspect ratios of the hard phase particles is 2.3 or less, the content of the hard phase particles having a major axis exceeding 5 μm is 3 particles or less per 2,450 μm², and the contact ratio between the hard phase particles is 40% or less.

The hard phase particles constituting the cast material according to the present invention mainly comprise a boride and/or a carbide, and contributes to the hardness and the wear resistance of the cast material. In the cast material according to the present invention, the hard phase particles are present in a state of being dispersed in the matrix of the binder phase to be described later.

Examples of the boride and the carbide constituting the hard phase particles may include, but are not particularly limited to, precipitated particles comprising at least one of Ni, Co, Cr, Mo, Mn, Cu, W, Fe and Si, and B and/or C. The hard phase particles may be particles in which particles different from each other in composition are mixed.

Examples of the boride may include, but are not particularly limited to, MB-type, MB₂-type, M₂B-type, M₂B₅-type, and M₂M′B₂-type borides (M and M′ each represent at least one metal of Ni, Co, Cr, Mo, Mn, Cu, W, Fe and Si, and M′ represents a metal element different from M); specific examples of the boride may include borides such as CrB, MoB, Cr₂B, Mo₂B, Mo₂B₅, Mo₂FeB₂, Mo₂CrB₂, and Mo₂NiB₂.

Examples of the carbide may include, but are not limited to, M₂₃C₆-type, M₄C-type, M₃C₂-type, M₂C-type, and MC-type carbides (M represents at least one metal of Ni, Co, Cr, Mo, Mn, Cu, W, Fe and Si, and M may be substituted with a different metal element(s)); specific examples of the carbide may include carbides such as Cr₂₃C₆, Cr₃C₂, Cr₆C, Mo₂C, and CrC.

The content ratio of the above-described hard phase particles in the cast material according to the present invention is preferably 10 to 50 vol %, and further preferably 20 to 45 vol %. As a method for controlling the content ratio of the hard phase particles in the cast material, there can be used a method in which the content of B contained in the cast material or the content of C contained in the cast material is adjusted. By setting the content ratio of the hard phase particles within the above range, it is possible to make the cast material according to the present invention be highly balanced among the corrosion resistance, the wear resistance, and the mechanical strengths such as the hardness and the bending strength. Moreover, by setting the content ratio of the hard phase particles within the above range, it is possible to prevent the contact ratio between the hard phase particles from being too large, and the bending strength of the cast material from being decreased due to the aggregation of the hard phase particles. By setting the content ratio of the hard phase particles within the above range, the temperature required for melting the raw materials of the hard phase particles can be lowered, and thus the energy required for melting is suppressed to lead to an advantage in cost.

The binder phase constituting the cast material according to the present invention comprises an alloy containing Co and/or Ni as a main component(s), and is a phase for forming a matrix for binding the above-described hard phase particles. Specific examples of the alloy constituting the binder phase include Co-based alloys and/or Ni-based alloys containing at least one of Cr, Mo, Mn, Cu, W, Fe and Si. In the cast material according to the present invention, by allowing the binder phase to include an alloy mainly composed of Co and/or Ni, the corrosion resistance of the obtained cast material is improved as compared with the case where the binder phase includes an alloy mainly composed of a Fe-based alloy.

As the hard phase particles and the binder phase constituting the cast material according to the present invention, among the above-described constitutions, preferred is a constitution in which in particular, the hard phase particles comprise a composite boride represented by Mo₂NiB₂, and the binder phase comprises a Ni-based alloy.

In the cast material according to the present invention, the microstructures thereof, specifically, the average particle size of the hard phase particles, the average value of the aspect ratios of the hard phase particles, the content of the hard phase particles having a major axis exceeding 5 μm, and the contact ratio between the hard phase particles are controlled within the specific ranges to be described later. According to the present invention, by controlling these quantities within the specific ranges to be described later, the cast material can be excellent in corrosion resistance and wear resistance, and can be provided with a high hardness and a high bending strength.

In the cast material according to the present invention, the average particle size of the above-described hard phase particles is 3 μm or less, preferably 2.8 μm or less, and further preferably 2.5 μm or less. By setting the average particle size of the hard phase particles within the above range, the hardness and the bending strength of the obtained cast material can be made sufficient. When the average particle size of the hard phase particles exceeds 3 μm, there occurs a failure that the hard phase particles function as origins, and the bending strength of the cast material is remarkably lowered. The lower limit of the average particle size of the hard phase particles is not particularly limited, but is preferably 0.5 μm. In order to make the average particle size of the hard phase particles less than 0.5 μm, the cooling rate is required to be extremely large, such a large cooling rate is hardly achieved by usual water cooling or the like; an achievement of such a large cooling rate leads to a production cost increase.

The average particle size of the hard phase particles can be measured, for example, by calculating the equivalent circle diameters of the hard phase particles, and by calculating the average value of the calculated equivalent circle diameters. Specifically, first, by using a scanning electron microscope (SEM), a backscattered electron image of a cross section of the cast material is taken, and by using the obtained backscattered electron image, the average particle size of the hard phase particles can be calculated on the basis of the formula of Fullman (the following formula (1)).

d
_m=(4/π)×(N_L/N_S) (1)

In the above-described formula (1), d_mrepresents the average particle size of the hard phase particles; n represents the ratio of the circumference of a circle to its diameter; N_Lrepresents the number of the hard phase particles per unit length of an arbitrary straight-line segment hit by the arbitrary straight-line segment (when an arbitrary straight-line segment is drawn, brought into contact with or intersect with the arbitrary straight-line segment) on a cross sectional microstructure; specifically, N_Lis the value calculated by dividing, by the length L of the arbitrary straight-line segment, the number of the particles hit by the arbitrary straight-line segment having the length of L on a cross sectional microstructure; and N_Srepresents the number of the hard phase particles included in an arbitrary unit area, namely, the value obtained by dividing the number of the particles included in an arbitrary measurement area having a measurement region range S by the arbitrary measurement region range S. In this case, the length of the straight-line segment L can be a length intersecting a sufficient number of the hard phase particles for the measurement of the average particle size, and is preferably set to be 20 μm or more. In Examples to be described later, the length of the straight-line segment L is set to be 42 μm. The measurement region range S can be a range including a sufficient number of the hard phase particles for the measurement of the average particle size, and is preferably a range having a length of 20 μm or more and a width of 20 μm or more. In Examples to be described later, the length and the width of the measurement region range S are 57 μm and 43 μm, respectively (namely, the area is 2,450 μm²).

In the cast material according to the present invention, the average value of the aspect ratios of the hard phase particles, namely, the average value of the ratio (major axis/minor axis) of the major axis to the minor axis is 2.3 or less, preferably 2.2 or less and further preferably 2.1 or less. By setting the average value of the aspect ratios of the hard phase particles within the above range, the bending strength of the cast material can be remarkably improved. When the average value of the aspect ratios of the hard phase particles is too large due to, for example, the growth of the dendrite microstructures (columnar crystals), in the dendrite microstructure portion, the bending strength of the cast material is lowered and the cast material tends to be broken.

The average value of the aspect ratios of the hard phase particles can be determined according to JIS R1670, as follows. First, a cast material is cut, and the cut cross section is photographed by using a scanning electron microscope (SEM) to obtain a backscattered electron image. Next, from the obtained backscattered electron image, in the same manner as in the above-described measurement of the average particle size, a predetermined number of the hard phase particles are selected from the above-described measurement region range S (a range of a length of 20 μm or more and a width of 20 μm or more), and the length (major axis) of the longest portion and the length (minor axis) of the longest portion in the direction perpendicular to the major axis of each of the hard phase particles are measured. Then, from the measured major axis and minor axis, the ratio (major axis/minor axis) of the major axis to the minor axis can be determined as the aspect ratio of the hard phase particle. In the present invention, for a predetermined number (for example, 10 or more) of the hard phase particles, such aspect ratios are determined and the average value of the aspect ratios is calculated, and thus the average value of the aspect ratios of the hard phase particles can be determined.

In the cast material according to the present invention, the content of the hard phase particles having a major axis exceeding 5 μm is 3 particles or less, preferably 2 particles or less and further preferably 1 particle or less, per 2,450 μm². By setting the content of the hard phase particles having a major axis exceeding 5 μm within the above range in the cast material, the bending strength of the cast material as well as the control of the average value of the aspect ratios of the hard phase particles can be remarkably improved. The number of the hard phase particles having a major axis exceeding 5 μm can be determined by counting the number of the hard phase particles having a major axis exceeding 5 μm in the measurement region range S (the range having a length of 20 μm or more and a width of 20 μm or more) in a backscattering electron image taken with a SEM in an arbitrary cross section, in the same manner as in above-described the measurement of the aspect ratios of the hard phase particles. In the present invention, in Examples to be described later, the content of the hard phase particles having a major axis exceeding 5 μm on the basis of the number of particles in the measurement region range S of 2,450 μm²for performing actual measurement; however, actual measurements are not particularly limited to such an area range; when measurements are performed by using the backscattered electron images in different area ranges, the aforementioned content of the hard phase particles can be determined by a proportional calculation. For example, in the present invention, the content of the hard phase particles having a major axis exceeding 5 μm per an area range of 5,000 μm²can be controlled, and in this case, the content of the hard phase particles having a major axis exceeding 5 μm per 5,000 μm²is 6 particles or less, preferably 4 particles or less and more preferably 2 particles or less.

Moreover, in the cast material according to the present invention, the contact ratio (contiguity) between the hard phase particles is 40% or less, preferably 39% or less and further preferably 38% or less. The contact ratio between the hard phase particles is an index indicating the dispersibility of the hard phase particles; the lower the contact ratio, the more excellent in the dispersibility the hard phase particles, and accordingly, the improvement of the strength is possible. When the contact ratio between the hard phase particles is too high, the contact between the hard phase particles causes the generation of coarse aggregates, or the occurrence of grain growth due to the mutual bonding of the hard phase particles; thus, there occurs a failure that the grain growth generation portion functions as the origin to lower the bending strength of the cast material.

The contact ratio between the hard phase particles can be measured, for example, as follows. Specifically, first, by using a scanning electron microscope (SEM), the backscattered electron image of the surface of the cast material is photographed, a straight-line segment L for measurement having a predetermined length is arbitrarily drawn on the backscattered electron image, as shown in FIG. 1, in the same manner as in the above-described measurement of the average particle size, and the hard phase interface present on the straight-line segment L is observed. FIG. 1 is a diagram for illustrating the measurement method of the microstructure of the cast material according to the present invention. Specifically, the hard phase particle interfaces are observed, the interfaces on which the hard phase particles are brought into mutual contact is designated as the hard phase-hard phase interfaces I_HH, the interfaces on which the hard phase particles and the binder phase are brought into mutual contact are designated as the hard phase-binder phase interfaces I_HB, and the numbers of these interfaces are counted. Then, in the present invention, from the number N(I_HH) per unit length of L1 of the hard phase-hard phase interfaces I_HHand the number N(I_HB) per unit length of L1 of the hard phase-binder phase interfaces I_HB, the contact ratio Cont (in units of %) between the hard phase particles can be calculated on the basis of the following formula (2):

Cont=2N(I_HH)/[2N(I_HH)+N(I_HB)]×100 (2)

When the contact ratio between the hard phase particles is calculated according to the above-described method, it is preferred to calculate the contact ratio between the hard phase particles by performing the following set of operations six times and by averaging the results of the six measurements in total: in a set of operations, the straight-line segment L for measurement other than the above-described straight-line segment is drawn on the SEM photograph so as to pass through the route other than the above-described route, and the number of the hard phase-hard phase interfaces I_HHand the number of the hard phase-binder phase interfaces I_HBare counted in the same manner as described above.

In the present invention, the method allowing the following quantities to fall within the above ranges is not particularly limited, but the following method may be quoted: the aforementioned quantities are the average particle size of the hard phase particles, the average value of the aspect ratios of the hard phase particles, the content of the hard phase particles having a major axis exceeding 5 μm, and the contact ratio between the hard phase particles. Specifically, there is quoted a method in which when the cast material is manufactured, first, a fused mixture is obtained by melting the raw materials for forming the hard phase particles and the binder phase; and next when the obtained fused mixture is cooled, there is included a process of continuously cooling at a cooling rate of 100° C./min or more, in the temperature range from the cooling starting temperature to 400° C.

It is to be noted that the contact ratio between the hard phase particles can also be controlled by, for example, adjusting the composition of the cast material so as to fall within a specific range.

The composition of the cast material according to the present invention is not particularly limited, but preferably comprises, when the binder phase includes a Ni-based alloy mainly composed of Ni, 1 to 6 wt % of B, 0 to 2.5 wt % of C, 0 to 30 wt % of Co, 0 to 5 wt % of Si, 0 to 20 wt % of Cr, 5 to 40 wt % of Mo, 0 to 25 wt % of Fe, and the balance of Ni. Alternatively, when the binder phase includes a Co-based alloy mainly composed of Co, the composition of the cast material according to the present invention preferably comprises 1 to 6 wt % of B, 0 to 2.5 wt % of C, 0 to 5 wt % of Ni: 0 to 5 wt % of Si, 0 to 25 wt % of Cr, 5 to 40 wt % of Mo, 0 to 25 wt % of Fe, and the balance of Co.

B (boron) is an element for forming a boride to produce hard phase particles. By setting the content ratio of B within the above range, the content ratio of the hard phase particles in the cast material can be appropriate, and accordingly, the wear resistance of the cast material is improved. By setting the content ratio of B within the above range, the contact ratio between the hard phase particles can also be within the above range, and the hardness and the bending strength of the cast material can be improved. The content of B in the cast material is preferably 1 to 6 wt %, and more preferably 2 to 5 wt %, either in the cast material in which the binder phase is mainly composed of Ni or in the cast material in which the binder phase is mainly composed of Co.

By including C (carbon) in a large amount, it is possible to form a carbide to produce hard phase particles. The formation of the carbide improves the wear resistance. On the other hand, by setting the content ratio of C within the above range, the content ratio of the hard phase particles in the cast material is made appropriate, and accordingly, the contact ratio between the hard phase particles can be made to fall within the above range, and the hardness and the bending strength of the cast material can be improved. The content of C in the cast material is preferably 0.15 wt % to 2.5 wt %, and more preferably 0.2 to 1 wt %, either in the cast material in which the binder phase is mainly composed of Ni or in the cast material in which the binder phase is mainly composed of Co. When carbon is included as an inevitable impurity without forming any carbide, the content of C is preferably, for example, 0.06 wt % or less.

Ni (nickel) is, when a Ni-based alloy is used as the binder phase of the cast material, an element capable of forming the hard phase particles, and at the same time, an element capable of constituting the binder phase, and has a function to improve the corrosion resistance of the cast material. Ni has, when a Co-based alloy is used as the binder phase of the cast material, a function to improve the corrosion resistance of the cast material.

Co (cobalt) is, when a Ni-based alloy is used as the binder phase of the cast material, an element capable of forming the hard phase particles, and at the same time, an element capable of constituting the binder phase, and has a function to improve the corrosion resistance of the cast material.

Si (silicon) is an element capable of constituting the binder phase of the cast material, and has a function to lower the melting temperatures of the raw materials for forming the cast material. By controlling the content ratio of Si so as to be appropriate, the above-described melting temperature can be lowered, and additionally, the lowering of the bending strength of the cast material due to the increase of the contents of the silicides in the cast material can be suppressed.

Cr is an element capable of forming the hard phase particles, and at the same time, an element capable of constituting the binder phase, and has a function to improve the corrosion resistance, wear resistance, high temperature properties, hardness and bending strength of the cast material. By controlling the content ratio of Cr so as to be appropriate, the content ratio of the hard phase particles in the cast material is within the above range, and the bending strength of the cast material can be improved.

Mo (molybdenum) is an element capable of forming hard phase particles, and at the same time, an element capable of forming the binder phase, and has a function to improve the corrosion resistance of the cast material. In particular, a fraction of Mo is solid-dissolved in the binder phase, and accordingly has a function to improve the corrosion resistance of the cast material. By controlling the content ratio of Mo so as to be appropriate, the wear resistance and the corrosion resistance of the cast material can be improved.

Next, a method of manufacturing the cast material according to the present invention will be described.

First, a raw material powder for forming the cast material according to the present invention is prepared. The raw material powder can be prepared in such a way that the content ratios of the respective elements forming the cast material are the desired composition ratios. In the present invention, the hard phase particles mainly composed of a boride and/or a carbide may be preliminarily included in the raw material powder; or alternatively, the hard phase particles are not included in the raw material powder, but in the process of preparing the cast material by using the raw material powder, the hard phase particles mainly composed of a boride and/or a carbide as originating from boron and carbon contained in the raw material powder may be formed in the cast material.

Next, in order to pulverize, if necessary, the prepared raw material powder to a predetermined particle size, a binder, an organic solvent and the like are added to the raw material powder, and these are mixed and crushed by using a crusher such as a ball mill.

The binder is added for the purpose of improving the moldability during molding and preventing the oxidation of the powder. The binder is not particularly limited, and heretofore known binders can be used; examples of the binder may include paraffin. The addition amount of the binder is not particularly limited, but is preferably 3 to 6 parts by weight in relation to 100 parts by weight of the raw material powder. The organic solvent is not particularly limited, but low-boiling-point solvents such as acetone can be used. The pulverization-mixing time is not particularly limited; it may be recommended to select the conditions such that the average particle size of the hard phase particles formed in the obtained cast material is within the above range; the pulverization-mixing time is usually 15 to 30 hours.

Then, the above-described raw material powder is fused into a fused mixture, and subsequently, the removal of impurities such as gases and oxides is performed, if necessary. The fusing temperature in this case can be determined according to the raw materials used, and is preferably 1100 to 1300° C., and more preferably 1200 to 1250° C.

Successively, the thus obtained fused mixture is poured into a casting mold such as a mold according to the desired shape and then cooled for casting, and consequently the cast material can be obtained.

In the present invention, when the fused mixture is cooled, there is included a process of continuously cooling the fused mixture at a cooling rate of 100° C./min or more, in the temperature range from the cooling starting temperature to 400° C. In the present invention, the inclusion of the process of continuously cooling the fused mixture at a cooling rate of 100° C./min or more means that there can be adopted a mode in which the cooling rate is 100° C./min or more over a certain continuous period; there can be included a process of continuously cooling at a cooling rate of 100° C./min or more over a period time of preferably 1 minute or more and more preferably 5 minutes or more; there is not included, for example, a mode in which the cooling rate is instantaneously 100° C./min or more (such as a mode in which the cooling rate is 100° C./min or more, for example, only for 1 second or less). When the fused mixture is cooled, there can be included a process of continuously cooling the fused mixture at a cooling rate of 100° C./min or more in the temperature range from the cooling starting temperature to 400° C.; the cooling rate in this case is preferably 200° C./min or more and more preferably 400° C./min or more. By performing the cooling of the fused mixture under the above-described conditions, for the obtained cast material, the average particle size of the hard phase particles, the average value of the aspect ratios of the hard phase particles, the average value of the major axes of the hard phase particles, and the contact ratio between the hard phase particles can be controlled within the above ranges.

In the present invention, examples of the method for cooling the fused mixture under the above-described conditions may include, but are not limited to, a method in which the fused mixture is cooled by pouring the fused mixture into a mold preferably at room temperature to 1100° C., and more preferably at 300 to 1100° C. As room temperature, the temperatures of 1 to 30° C. may be quoted.

The cast method is not particularly limited, but it is preferred to use, for example, a mold cast method, a lost-wax cast method, a continuous cast method, a centrifugal cast method, from the viewpoint of being capable of forming a cast material having a complicated shape or from the viewpoint of being capable of a forming a thick-walled cast material.

In such a manner as described above, the cast material according to the present invention is manufactured.

The cast material according to the present invention comprises the hard phase particles mainly composed of a boride and/or a carbide, and the binder phase including an alloy mainly composed of Co and/or Ni, wherein the average particle size of the hard phase particles is controlled to be 3 μm or less, the average value of the aspect ratios of the hard phase particles is controlled to be 2.3 or less, the content of the hard phase particles having a major axis exceeding 5 μm is controlled to be 3 particles or less per 2,450 μm², and the contact ratio between the hard phase particles is controlled to be 40% or less. Therefore, the cast material according to the present invention is excellent in corrosion resistance and wear resistance, and achieves a high hardness and a high bending strength.

Since the cast material according to the present invention is excellent in corrosion resistance and wear resistance, and achieves a high hardness and a high bending strength, the cast material according to the present invention can be suitably used as wear resistant materials capable of achieving an excellent durability even in the environments in which high loads are applied, namely, in rolls, cylinders, bearings, industrial pump components and the like.

EXAMPLES

Hereinafter, the present invention will be more specifically described with reference to examples, but the present invention is not limited to these examples.

The definitions of the properties and the evaluation methods of the properties are as follows.

By using a scanning electron microscope (SEM), the backscattered electron image of the cross section of the cast material was taken, and according to the above-described methods, the average particle size of the hard phase particles, the average value of the aspect ratios of the hard phase particles, the number of the hard phase particles having a major axis exceeding 5 μm, and the contact ratio between the hard phase particles were measured. In the present examples, when the respective measurements were performed, the length of the straight-line segment L was set at 42 μm and the measurement region range S was set at 2,450 μm².

For the cast material, the measurement of the hardness (Rockwell C scale) was performed.

A test piece was obtained by cutting the cast material so as to be a size of 4 mm×7 mm×24 mm, and the bending strength (three-point bending test) of the obtained test piece was performed according to CIS 026.

Example 1

A mixed powder was obtained by dry mixing 20 wt % of a Mo₂NiB₂-type composite boride and, 80 wt % of a Ni-based self-fluxing alloy (composition: Cr: 10 wt %, B: 2 wt %, Si: 2.7 wt %, C: 0.4 wt %, Fe: 2 wt %, Ni: the balance). Next, the obtained mixed powder was placed in a crucible, and fused by using a vacuum casting machine (TCP-5250, manufactured by Tanabe Kenden Co., Ltd.) and by raising the temperature to 1200° C. in a high-frequency fusion furnace to obtain a fused mixture; the obtained fused mixture at 1200° C. was poured into a mold heated to 400° C., and thereafter air cooled to room temperature to obtain a cast material. In this case, the temperature of the fused mixture was measured after 2 minutes from the taking out of the fused mixture from the high-frequency fusion furnace and the temperature was found to be 400° C. In other words, the fused mixture was cooled from 1200° C. to 400° C. in 2 minutes after being taken out from the high-frequency fusion furnace; in this case, the cooling rate of the fused mixture was 400° C./min; from this results, it can be said that the fused mixture was continuously cooled at a cooling rate of approximately 400° C./min in the range from 1200° C. to 400° C.

Subsequently, for the obtained cast material, according to the above-described methods, the average particle size of the hard phase particles, the average value of the aspect ratios of the hard phase particles, the number of the hard phase particles having a major axis exceeding 5 μm, the contact ratio between the hard phase particles, the hardness, and the bending strength were measured. The backscattered electron images taken by a scanning electron microscope (SEM) for the respective measurements are shown in FIG. 2(A) and FIG. 2(B). Here, FIG. 2(B) is an enlarged image obtained by enlarging a portion of FIG. 2(A). In the backscattered electron images of FIG. 2(A) and FIG. 2(B), the white regions are made of a boride (hard phase particle), the black regions are made of carbides, and the rest gray region is made of a Ni-based alloy.

The results of the respective measurements in Example 1 were as follows: the average particle size of the hard phase particles was 2.2 μm, the average value of the aspect ratios of the hard phase particles was 2.0, the contact ratio between the hard phase particles was 37%, the hardness (HRC) was 54.8, and the bending strength was 1143 MPa. For the arbitrary 10 particles extracted from the hard phase particles used in the calculation of the aspect ratio, the measured values of the major axis as examples were 2.4 μm, 3.0 μm, 3.5 μm, 3.8 μm, 3.9 μm, 3.9 μm, 4.0 μm, 4.4 μm, 4.9 μm, and 5.1 μm; and the average value of the major axis as the average value of these was 3.89 μm. For all the hard phase particles present within measurement region range S (namely, the range of 2,450 μm²), the major axes were measured, and the number of the particles having a major axis exceeding 5 μm was 1. The measurement of the number of the particles having a major axis exceeding 5 μm was performed for five measurement ranges while the measurement range was being altered; in any one of the measurement ranges, the number of the particles having a major axis exceeding 5 μm was 0 to 1. Alternatively, when for all the hard phase particles present within a range of 5,000 μm², the major axes were measured, and the number of the particles having a major axis exceeding 5 μm was 2.

Example 2

A mixed powder was obtained by dry mixing 10 wt % of a Mo₂NiB₂-type composite boride and, 90 wt % of a Ni-based self-fluxing alloy (composition: B: 2.3 wt %, Si: 7.1 wt %, C: 0.06 wt % or less, Fe: 1.5 wt %, Ni: the balance). Next, an ingot was obtained by sintering in vacuum the obtained mixed powder by using a vacuum furnace under the conditions of 1160° C. and 30 minutes. Then, a fused mixture was obtained by raising the temperature of the ingot in the air to 1200° C. to fuse the ingot by using an air atmospheric furnace, and the obtained fused mixture at 1200° C. was poured into a mold at room temperature of 20° C. and subsequently air cooled to obtain a cast material. In this case, the temperature of the fused mixture was measured after approximately 1 minute from the taking out of the fused mixture from the air atmospheric furnace and the temperature was found to be 400 to 500° C. In other words, the fused mixture was cooled from 1200° C. to 400 to 500° C. in 1 minute after being taken out from the air atmospheric furnace; in this case, the cooling rate of the fused mixture was 700 to 800° C./min; from this result, it can be said that the fused mixture passed through a process of being continuously cooled at a cooling rate of approximately 700 to 800° C./min in the range from 1200° C. to 400° C.

Subsequently, for the obtained cast material, according to the above-described methods, the average particle size of the hard phase particles, the average value of the aspect ratios of the hard phase particles, the number of the hard phase particles having a major axis exceeding 5 μm, the contact ratio between the hard phase particles, the hardness, and the bending strength were measured. The backscattered electron images taken by a scanning electron microscope (SEM) for the respective measurements are shown in FIG. 3(A) and FIG. 3(B). Here, FIG. 3(B) is an enlarged image obtained by enlarging a portion of FIG. 3(A). In the backscattered electron images of FIG. 3(A) and FIG. 3(B), the white regions are made of a boride (hard phase particle), the black regions are made of impurities, and the rest gray region is made of a Ni-based alloy.

The results of the respective measurements in Example 2 were as follows: the average particle size of the hard phase particles was 2.8 μm, the average value of the aspect ratios of the hard phase particles was 1.5, the contact ratio between the hard phase particles was 14%, the hardness (HRC) was 64, and the bending strength was 1101 MPa. For the arbitrary 10 particles extracted from the hard phase particles used in the calculation of the aspect ratio, the measured values of the major axis as examples were 2.8 μm, 3.8 μm, 2.7 μm, 3.6 μm, 2.8 μm, 2.4 μm, 3.2 μm, 3.7 μm, 4.2 μm, and 2.9 μm; and the average value of the major axis as the average value of these was 3.20 μm. For all the hard phase particles present within measurement region range S (namely, the range of 2,450 μm²), the major axes were measured, and the number of the particles having a major axis exceeding 5 μm was 2. The measurement of the number of the particles having a major axis exceeding 5 μm was performed for five measurement ranges while the measurement range was being altered; in any one of the measurement ranges, the number of the particles having a major axis exceeding 5 μm was 0 to 2. Alternatively, when for all the hard phase particles present within a range of 5,000 μm², the major axes were measured, and the number of the particles having a major axis exceeding 5 μm was 4.

Example 3

A cast material was obtained in the same manner as in Example 2 except that there was used a mixed powder prepared by dry mixing 15 wt % of a Mo₂NiB₂-type composite boride and 85 wt % of a Ni-based self-fluxing alloy (composition: B: 2.3 wt %, Si: 7.1 wt %, C: 0.06 wt % or less, Fe: 1.5 wt %, Ni: the balance), and the respective measurements were performed in the same manner as in Example 2. The backscattered electron images taken by a scanning electron microscope (SEM) for the respective measurements are shown in FIG. 4(A) and FIG. 4(B). Here, FIG. 4(B) is an enlarged image obtained by enlarging a portion of FIG. 4(A). In the backscattered electron images of FIG. 4(A) and FIG. 4(B), the white regions are made of a boride (hard phase particle), the black regions are made of impurities, and the rest gray region is made of a Ni-based alloy. In the gray region, the elongate shape portion (in FIG. 4(B), the portion indicated by an arrow) is shown in deeper color due to the difference in crystalloid in the Ni-based alloy, than the other portion of the Ni-based alloy, is still the Ni-based alloy, and is considered not to constitute the hard phase particles.

The results of the respective measurements in Example 3 were as follows: the average particle size of the hard phase particles was 2.1 μm, the average value of the aspect ratios of the hard phase particles was 1.8, the contact ratio between the hard phase particles was 13%, the hardness (HRC) was 65, and the bending strength was 993 MPa. For the arbitrary 10 particles extracted from the hard phase particles used in the calculation of the aspect ratio, the measured values of the major axis as examples were 2.2 μm, 3.1 μM, 3.2 μm, 3.2 μM, 2.6 μm, 4.3 μm, 3.7 μm, 3.7 μm, 2.8 μm, and 3.2 μm; and the average value of the major axis as the average value of these was 3.20 μm. For all the hard phase particles present within measurement region range S (namely, the range of 2,450 μm²), the major axes were measured, and the number of the particles having a major axis exceeding 5 μm was 0. The measurement of the number of the particles having a major axis exceeding 5 μm was performed for five measurement ranges while the measurement range was being altered; in any one of the measurement ranges, the number of the particles having a major axis exceeding 5 μm was 0 to 1. Alternatively, when for all the hard phase particles present within a range of 5,000 μm², the major axes were measured, and the number of the particles having a major axis exceeding 5 μm was 2.

Example 4

A cast material was obtained in the same manner as in Example 2 except that there was used a mixed powder prepared by dry mixing 20 wt % of a Mo₂NiB₂-type composite boride and 80 wt % of a Ni-based self-fluxing alloy (composition: B: 2.3 wt %, Si: 7.1 wt %, C: 0.06 wt % or less, Fe: 1.5 wt %, Ni: the balance), and the respective measurements were performed in the same manner as in Example 2. The backscattered electron images taken by a scanning electron microscope (SEM) for the respective measurements are shown in FIG. 5(A) and FIG. 5(B). Here, FIG. 5(B) is an enlarged image obtained by enlarging a portion of FIG. 5(A). In the backscattered electron images of FIG. 5(A) and FIG. 5(B), the white regions are made of a boride (hard phase particle), the black regions are made of impurities, and the rest gray region is made of a Ni-based alloy. In the gray region, the elongate shape portion (in FIG. 5(B), the portion indicated by an arrow) is shown in deeper color due to the difference in crystalloid in the Ni-based alloy, than the other portion of the Ni-based alloy, is still the Ni-based alloy, and is considered not to constitute the hard phase particles.

The results of the respective measurements in Example 4 were as follows: the average particle size of the hard phase particles was 2.1 μm, the average value of the aspect ratios of the hard phase particles was 1.8, the contact ratio between the hard phase particles was 13%, the hardness (HRC) was 65, and the bending strength was 1198 MPa. For the arbitrary 10 particles extracted from the hard phase particles used in the calculation of the aspect ratio, the measured values of the major axis as examples were 3.2 μm, 4.0 μm, 3.4 μm, 3.2 μm, 3.2 μm, 3.7 μm, 3.2 μm, 3.0 μm, 3.2 μm, and 3.2 μm; and the average value of the major axis as the average value of these was 3.31 μm. For all the hard phase particles present within measurement region range S (namely, the range of 2,450 μm²), the major axes were measured, and the number of the particles having a major axis exceeding 5 μm was 2. The measurement of the number of the particles having a major axis exceeding 5 μm was performed for five measurement ranges while the measurement range was being altered; in any one of the measurement ranges, the number of the particles having a major axis exceeding 5 μm was 0 to 2. Alternatively, when for all the hard phase particles present within a range of 5,000 μm², the major axes were measured, and the number of the particles having a major axis exceeding 5 μm was 4.

Example 5

A cast material was obtained in the same manner as in Example 2 except that only a Ni-based self-fluxing alloy (composition: Cr: 10 wt %, B: 2 wt %, Si: 2.7 wt %, C: 0.4 wt %, Fe: 2 wt %, Ni: the balance) was used in place of a mixed powder, and the respective measurements were performed in the same manner as in Example 2. The backscattered electron images taken by a scanning electron microscope (SEM) for the respective measurements are shown in FIG. 6(A) and FIG. 6(B). Here, FIG. 6(B) is an enlarged image obtained by enlarging a portion of FIG. 6(A). In the backscattered electron images of FIG. 6(A) and FIG. 6(B), the black regions are made of a carbide (hard phase particle), and the rest gray region is made of a Ni-based alloy. It is to be noted that the carbide is probably formed by the reaction of the carbon in the Ni-based self-fluxing alloy with a metal element (Ni, Cr or Fe).

The results of the respective measurements in Example 5 were as follows: the average particle size of the hard phase particles was 1.1 μm, the average value of the aspect ratios of the hard phase particles was 2.4, the contact ratio between the hard phase particles was 18.7%, the hardness (HRC) was 44.7, and the bending strength was 1118 MPa. For the arbitrary 10 particles extracted from the hard phase particles used in the calculation of the aspect ratio, the measured values of the major axis as examples were 1.5 μm, 2 μm, 1.5 μm, 4.5 μm, 4 μm, 2 μm, 2 μm, 1.75 μm, 2 μm, and 2 μm; and the average value of the major axis as the average value of these was 2.4 μm. For all the hard phase particles present within measurement region range S (namely, the range of 2,450 μm²), the major axes were measured, and the number of the particles having a major axis exceeding 5 μm was 0. The measurement of the number of the particles having a major axis exceeding 5 μm was performed for five measurement ranges while the measurement range was being altered; in any one of the measurement ranges, the number of the particles having a major axis exceeding 5 μm was 0 to 1. Alternatively, when for all the hard phase particles present within a range of 5,000 μm², the major axes were measured, and the number of the particles having a major axis exceeding 5 μm was 1.

Comparative Example 1

A cast material was obtained as follows: a mixed powder obtained in the same manner as in Example 1 was placed in a crucible, the temperature of the mixed powder was raised to 1200° C. by using a vacuum heat treatment furnace (PVSGgr 20/20, manufactured by SHIMADZU CORPORATION) to fuse the mixed powder and thus a fused mixture was obtained, the fused mixture was slowly cooled to 400° C., over approximately 1 hour in a high-frequency fusion furnace, and thus a cast material was obtained. Next, for the obtained cast material, the respective measurements were performed in the same manner as in Example 1. The backscattered electron images taken by a scanning electron microscope (SEM) for the respective measurements are shown in FIG. 7(A) and FIG. 7(B). Here, FIG. 7(B) is an enlarged image obtained by enlarging a portion of FIG. 7(A). In the backscattered electron images of FIG. 7(A) and FIG. 7(B), the white regions are made of a boride (hard phase particle), the black regions are made of a carbide, and the rest gray region is made of a Ni-based alloy.

The results of the respective measurements in Comparative Example 1 were as follows: the average particle size of the hard phase particles was 3.5 μm, the average value of the aspect ratios of the hard phase particles was 2.4, the contact ratio between the hard phase particles was 43%, the hardness (HRC) was 49.4, and the bending strength was 664 MPa. For the arbitrary 10 particles extracted from the hard phase particles used in the calculation of the aspect ratio, the measured values of the major axis as examples were 3.4 μm, 3.6 μm, 4.1 μm, 4.2 μm, 4.7 μm, 5.1 μm, 5.1 μm, 5.1 μm, 5.6 μm, and 6.4 μm; and the average value of the major axis as the average value of these was 4.73 μm. For all the hard phase particles present within measurement region range S (namely, the range of 2,450 μm²), the major axes were measured, and the number of the particles having a major axis exceeding 5 μm was 5. Alternatively, when for all the hard phase particles present within a range of 5,000 μm², the major axes were measured, and the number of the particles having a major axis exceeding 5 μm was 10.

Comparative Example 2

A cast material was obtained as follows: a mixed powder obtained in the same manner as in Example 1 was placed in a crucible, the temperature of the mixed powder was raised to 1200° C. by using a vacuum heat treatment furnace (PVSGgr 20/20, manufactured by SHIMADZU CORPORATION) to fuse the mixed powder and thus a fused mixture was obtained, the fused mixture was slowly cooled to 800° C., over approximately 5 hours in a high-frequency fusion furnace, and thus a cast material was obtained. Next, for the obtained cast material, the respective measurements were performed in the same manner as in Example 1. The backscattered electron images taken by a scanning electron microscope (SEM) for the respective measurements are shown in FIG. 8(A) and FIG. 8(B). Here, FIG. 8(B) is a higher magnification electron image of the same test piece as for FIG. 8(A). In the backscattered electron images of FIG. 8(A) and FIG. 8(B), the white regions are made of a boride (hard phase particle), the black regions are made of a carbide, and the rest gray region is made of a Ni-based alloy.

The results of the respective measurements in Comparative Example 2 were as follows: the average particle size of the hard phase particles was 3.68 μm, the average value of the aspect ratios of the hard phase particles was 1.7, the contact ratio between the hard phase particles was 21%, the hardness (HRC) was 47.0, and the bending strength was 522 MPa. For the arbitrary 10 particles extracted from the hard phase particles used in the calculation of the aspect ratio, the measured values of the major axis as examples were 3.5 μm, 6 μm, 4 μm, 6 μm, 7.5 μm, 8.5 μm, 9.5 μm, 8.5 μm, 6.5 μm, and 5.5 μm; and the average value of the major axis as the average value of these was 6.6 μm. For all the hard phase particles present within measurement region range S (namely, the range of 2,450 μm²), the major axes were measured, and the number of the particles having a major axis exceeding 5 μm was 10. Alternatively, when for all the hard phase particles present within a range of 5,000 μm², the major axes were measured, and the number of the particles having a major axis exceeding 5 μm was 20.

Comparative Example 3

A cast material was obtained in the same manner as in Comparative Example 2 except that only a Ni-based self-fluxing alloy (composition: Cr: 10 wt %, B: 2 wt %, Si: 2.7 wt %, C: 0.4 wt %, Fe: 2 wt %, Ni: the balance) was used in place of a mixed powder, and the respective measurements were performed in the same manner as in Comparative Example 2 The backscattered electron images taken by a scanning electron microscope (SEM) for the respective measurements are shown in FIG. 9(A) and FIG. 9(B). Here, FIG. 9(B) is an enlarged image obtained by enlarging a portion of FIG. 9(A). In the backscattered electron images of FIG. 9(A) and FIG. 9(B), the black regions are made of a carbide (hard phase particle), and the rest gray region is made of a Ni-based alloy.

In Comparative Example 3, the hard phase particles were mutually bonded to form large agglomerates, and accordingly, the average particle size, the average value of the aspect ratios and the contact ratio of the hard phase particles were not able to be measured. In the cast material of Comparative Example 3, the hardness (HRC) was 38.0, and the bending strength was 519 MPa.

Form the measurement results of Examples 1 to 5, the following cast material was shown to be high in bending strength and hardness: a casting material having an average particle size of the hard phase particles of 3 μm or less, an average value of the aspect ratios of the hard phase particles of 2.3 or less, a content of the hard phase particles having a major axis exceeding 5 μm of 3 particles or less per 2,450 μm²and a contact ratio between the hard phase particles of 40% or less. In other words, from the results of Examples 1 to 5, the cast materials are shown to be cast materials provided with an excellent corrosion resistance and an excellent resistance, and additionally, a high bending strength and a high hardness, namely, the properties possessed by a cast material comprising hard phase particles mainly composed of a boride and/or a carbide, and a binder phase including an alloy mainly composed of Co and/or Ni.

In above-described Examples 2 to 5, the cast materials were obtained by pouring a fused mixture into a mold at room temperature and subsequently air-cooling the fused mixture; however, even in a case where a mold heated to 400° C. was used as a mold in the same manner as in Example 1, the fused mixture is regarded to pass through a process of continuously cooling the fused mixture at a cooling rate of 100° C./min or more, in the temperature range from the cooling starting temperature to 400° C.; accordingly, the obtained cast material is considered to be a cast material similarly having, in addition to an excellent corrosion resistance and an excellent wear resistance, properties being high in bending strength and hardness.

On the other hand, from the results of Comparative Examples 1 and 2, the following cast materials were shown to be low in bending strength: the cast materials having an average particle size of the hard phase particles exceeding 3 μm, an average value of the aspect ratios of the hard phase particles exceeding 2.3, a content of the hard phase particles having a major axis exceeding 5 μm is 3 particles per 2,450 μm², and a contact ratio between the hard phase particles exceeding 40% major axis.

From the measurement results of Comparative Example 3, a cast material in which the hard phase particles were mutually bonded to form large agglomerates was shown to be low both in bending strength and in hardness.

CAST MATERIAL AND METHOD OF MANUFACTURING CAST MATERIAL

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