Patent Publication Number: US-2021162499-A1

Title: Method for manufacturing sintered member

Description:
TECHNICAL FIELD 
     The present disclosure relates to a method for manufacturing a sintered member. This application claims priority to Japanese Patent Application No. 2017-105088 filed May 26, 2017, which is incorporated herein by reference in its entirety. 
     BACKGROUND ART 
     The method for manufacturing a sintered body in PTL 1 is known as a method for manufacturing a sintered member used for, for example, automobile components and general machinery components. This method for manufacturing a sintered body includes a step of producing a compact by compacting metal powder, a step of calcinating the compact, a step of machining the calcined body, and a step of firing the machined calcined body after the machining step. In the step of producing a compact, the pressure during compaction is 100 MPa to 1500 MPa. 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Patent Application Laid-Open No. 2007-77468 
     SUMMARY OF INVENTION 
     A method for manufacturing a sintered member according to the present disclosure includes: 
     a step of preparing a raw material powder containing an iron-based powder; 
     a step of forming a green compact having a relative density of 97% or more and having a solid cylindrical shape or hollow cylindrical shape by compacting the raw material powder; and 
     a step of sintering the green compact. 
     The raw material powder contains at least one of a mixed powder containing pure iron powder and Ni powder and an iron alloy powder containing Ni as an additive element. 
     The total amount of the Ni powder and Ni serving as the additive element in the raw material powder is 1 mass % or more. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a graph showing the relationship between compacting pressure and green density in Test Example 1. 
         FIG. 2  is a graph showing the relationship between green density and tensile strength in Test Example 1. 
     
    
    
     DESCRIPTION OF EMBODIMENT OF PRESENT DISCLOSURE 
     First, features of an embodiment of the present disclosure will be listed and described. 
     (1) A method for manufacturing a sintered member according to an aspect of the present disclosure includes:
         a step of preparing a raw material powder containing an iron-based powder;       

     a step of forming a green compact having a relative density of 97% or more and having a solid cylindrical shape or hollow cylindrical shape by compacting the raw material powder; and 
     a step of sintering the green compact, 
     wherein the raw material powder contains at least one of a mixed powder containing pure iron powder and Ni powder and an iron alloy powder containing Ni as an additive element, and 
     a total amount of the Ni powder and Ni serving as the additive element in the raw material powder is 1 mass % or more. 
     According to the above features, a sintered member having high density and high strength can be manufactured. Since the green compact has a relative density of 97% or more and shrinks during sintering, the relative density of the sintered member is larger than the relative density of the green compact. The sintered member has high relative density and thus has high strength. 
     (2) The method for manufacturing a sintered member according to one aspect further includes a step of cutting the green compact between the forming step and the sintering step. 
     According to the above features, a sintered member having high density and complicated shape can be manufactured easily. Even when the green compact contains 1 mass % of Ni, which degrades cutting workability, in the form of powder or contains 1 mass % or more of Ni in the form of alloy additive element, the green compact is softer and less viscous than a sintered member or wrought material having the same composition. It is thus easy to cut the green compact. In addition, the green compact has very high relative density and relatively high strength although it is softer than a sintered member or wrought material. The green compact is thus unlikely to undergo chipping or cracking during cutting. 
     &lt;&lt;Details of Embodiment of Present Disclosure&gt;&gt; 
     The details of the embodiment of the present disclosure will be described below. 
     [Method for Manufacturing Sintered Member] 
     A method for manufacturing a sintered member according to an embodiment includes a step (raw material preparation step) of preparing a raw material powder, a step (forming step) of forming a green compact by compacting the raw material powder, and a step (sintering step) of sintering the green compact. One of characteristics of the method for manufacturing a sintered member is that a particular raw material powder is prepared in the raw material preparation step and a green compact satisfying a particular relative density is produced in the forming step. Each step will be described below in detail. 
     [Raw Material Preparation Step] 
     The raw material preparation step involves preparing a raw material powder containing an iron-based powder having iron-based particles. The term iron-based refers to pure iron or an iron alloy containing iron as a main component. The raw material powder has any one of a mixed powder containing Ni in the form of powder, an iron alloy powder containing Ni as an additive element, and a composite powder containing both the mixed powder and the iron alloy powder. 
     (Mixed Powder) 
     The mixed powder contains pure iron powder and Ni powder. The amount of the pure iron powder is, for example, 90 mass % or more, or 95 mass % or more relative to 100 mass % of the raw material powder. The amount of the Ni powder is, for example, 1 mass % or more relative to 100 mass % of the raw material powder. When the amount of the Ni powder is 1 mass % or more, the hardenability is improved so that the sintered member has improved mechanical characteristics. The amount of the Ni powder may be, for example, 2 mass % or more and 10 mass % or less. The mixed powder is sintered in the later sintering step to form an iron-based alloy. 
     The mixed powder may further contain alloying element powder that forms an iron-based alloy during sintering in the later sintering step. The alloying element is, for example, at least one selected from Cu, Sn, Cr, Mo, Mn, and C. The alloying element improves the mechanical characteristics of the sintered member. The total amount of powders of Cu, Sn, Cr, Mn, and Mo selected from the above alloying elements is, for example, more than 0 mass % and 5.0 mass % or less, or 0.1 mass % or more and 2.0 mass % or less relative to 100 mass % of the raw material powder. The amount of C powder is, for example, more than 0 mass % and 2.0 mass % or less, or 0.1 mass % or more and 1.0 mass % or less relative to 100 mass % of the raw material powder. 
     (Iron Alloy Powder) 
     The iron alloy powder contains iron as a main component and has iron alloy particles containing Ni as an alloying element. 
     The amount of iron is, for example, 90 mass % or more, or 95 mass % or less relative 100 mass % of the iron alloy. The amount of Ni is, for example, 1 mass % or more, or 2 mass % or more and 10 mass % or less relative to 100 mass % of the iron alloy. 
     The iron alloy may further contain at least one additive element selected from Cu, Sn, Cr, Mo, Mn, and C. The iron alloy may contain inevitable impurities. Specific examples of the iron alloy include Fe—Ni—Mo alloy, Fe—Ni—Mo—C alloy, Fe—Ni—C alloy, Fe—Ni—Mo—Cr alloy, Fe—Ni—Mo—Mn alloy, Fe—Ni—Cr alloy, Fe—Ni—Cu alloy, Fe—Cu—Ni—Mo alloy, and Fe—Ni—Mo—Cu—C alloy. The total amount of Cu, Sn, Cr, Mn, and Mo in the iron alloy is, for example, more than 0 mass % and 5.0 mass % or less, or 0.1 mass % or more and 2.0 mass % or less. The amount of C in the iron alloy is, for example, more than 0 mass % and 2.0 mass % or less, or 0.1 mass % or more and 1.0 mass % or less. Carbon C may be contained in the form of powder in the raw material powder instead of being contained as an alloying element in the iron alloy. In other words, the raw material powder may contain C powder in addition to the iron alloy powder. 
     (Composite Powder) 
     The composite powder contains both the mixed powder and the iron alloy powder. Specifically, the composite powder contains pure iron powder, Ni powder, and iron alloy powder having iron alloy particles containing iron as a main component and Ni as an alloying element. The total amount of the pure iron powder and iron contained in the iron alloy in the raw material powder is, for example, 90 mass % or more, or 95 mass % or more relative to 100 mass % of the entire raw material powder. The total amount of the Ni powder and Ni contained as an alloying element in the raw material powder is, for example, 1 mass % or more, or 2 mass % or more and 10 mass % or less relative to 100 mass % of the entire raw material powder. 
     The iron-based powder may be, for example, water atomized powder, reduced powder, gas atomized powder, or carbonyl powder. The iron-based powder has an average particle size of, for example, 20 μm or more and 200 μm or less. When the iron-based powder has an average particle size in this range, it is easy to handle and compact the iron-based powder. In particular, it is easy to ensure the fluidity of the iron-based powder when the iron-based powder has an average particle size of 20 μm or more. It is easy to form a sintered body having a dense structure when the iron-based powder has an average particle size of 200 μm or less. The iron-based powder has an average particle size of, for example, 50 μm or more and 150 μm or less. The average particle size of the iron-based powder refers to a particle size (D50) at 50% cumulative volume in the volume particle size distribution determined with a laser diffractometry particle size distribution measuring apparatus. 
     The raw material powder may contain at least one of a lubricant and an organic binder. However, the amount of lubricant and organic binder is preferably as small as possible. The total amount of lubricant and organic binder is, for example, 0.1 mass % or less. In the case where the raw material powder contains at least one of a lubricant and an organic binder in this range, the metal powder accounts for a large proportion of the compact, which makes it easy to form a dense green compact. In the case where the raw material powder is free of lubricant or organic binder, it is not necessary to degrease the green compact in a later step. 
     [Forming Step] 
     The forming step involves compacting the raw material powder into a green compact having a relative density of 97% or more. The relative density is preferably 98% or more and more preferably 99% or more. The shape of the green compact is, for example, a shape in conformity with the final shape of the sintered member, a shape suitable for cutting in a later step, specifically, a solid cylindrical or hollow cylindrical shape. The green compact is produced by using, for example, a suitable forming device (mold) that allows the raw material powder to form into the above-described shape. Specifically, a mold that enables uniaxial pressing so as to perform press-forming in the axial direction of the solid cylinder or the hollow cylinder is preferably used. Uniaxial pressing may use a mold including a die having openings on its top and bottom and a pair of punches to be fitted into the openings on the top and bottom. The cavity in the die of the mold is filled with the raw material powder, and the raw material powder in the cavity is compressed with the upper and lower punches to produce a green compact. 
     The compacting pressure (surface pressure) is, for example, 1560 MPa or more. At a high compacting pressure, a green compact having high relative density can be produced. The compacting pressure is preferably 1660 MPa or more or 1760 MPa or more, or more preferably 1860 MPa or 1960 MPa or more. There is no upper limit of the compacting pressure. 
     The forming step is preferably performed by a mold (external) lubricating method in which a lubricant is applied to the inner circumferential surfaces of the mold (the inner circumferential surface of the die and the press surfaces of the punches). It is thus easy to prevent galling of the raw material powder on the mold. Examples of the lubricant include higher fatty acids, metallic soaps, fatty acid amides, and higher fatty acid amides. Examples of metallic soaps include zinc stearate and lithium stearate. Examples of fatty acid amides include stearamide, lauramide, and palmitamide. Examples of higher fatty acid amides include ethylene bis stearamide. 
     The relative density of the green compact to be produced is preferably 98% or more and more preferably 99% or more. The relative density of the green compact is obtained from “{(green density of green compact)/(true density of green compact)}×100”. The green density of the green compact is obtained by immersing the green compact in an oil and calculating the green density in accordance with “oil-impregnated density×{(mass of green compact before oil impregnation)/(mass of green compact after oil impregnation)}”. The oil-impregnated density is a value obtained by dividing the mass of the green compact after oil impregnation by the volume of the green compact after oil impregnation. 
     The relative density of the green compact can be obtained on the basis of image analysis on the cross section of the green compact with commercially available image analysis software. First, the images of the cross sections of the green compact are obtained from 10 or more fields of view. The cross sections may be freely defined, and 10 or more cross sections are taken from fields of view (one field of view per cross section or two or more fields of view per cross section). The size of each field of view is 500 μm×600 μm. The image in each field of view is binarized to obtain the area ratio of metal in each field of view, and the area ratio is taken as a relative density in each field of view. The mean relative density of all fields of view is obtained and taken as the relative density of the green compact. 
     [Sintering Step] 
     The sintering step involves sintering the green compact. The sintering provides a sintered member in which metal powder particles are in contact with and bonded to each other. The sintered member has a relative density of more than 97%. Since the green compact has a relative density of 97% or more and shrinks during sintering, the relative density of the sintered member after sintering is larger than the relative density of the green compact. Although the amount of shrinkage of the green compact is very small during sintering because of the green compact having very high density, the relative density of the sintered member is larger than relative density of the green compact although. 
     The sintering conditions can be appropriately selected according to the composition of the raw material powder. The sintering temperature is, for example, 1100° C. or more and 1400° C. or less, or 1200° C. or more and 1300° C. or less. The sintering time is, for example, 15 minutes or more and 150 minutes or less, or 20 minutes or more and 60 minutes or less. The sintering conditions can be known conditions. 
     [Other Steps] 
     In addition to the raw material preparation step, the forming step, and the sintering step, the method for manufacturing a sintered member may include at least one of a step (compact processing step) of cutting the compact, a step (heat treatment step) of carburizing, quenching, and annealing the sintered member, and a step (finishing step) of finishing the sintered member. 
     (Compact Processing Step) 
     The compact processing step involves cutting the green compact between the forming step and the sintering step. In cutting, the green compact is processed into a predetermined shape by using a cutting tool. Since the green compact before sintering is cut, it is easy to manufacture a sintered member having high density and complicated shape. Even when the green compact contains 1 mass % or more of Ni, which degrades cutting workability, in the form of powder or contains 1 mass % or more of Ni in the form of alloy additive element, the green compact is softer and less viscous than a sintered member or wrought material having the same composition. It is thus easy to cut the green compact. Moreover, the green compact is softer than a sintered member or wrought material, but has very high relative density and relatively high strength. The green compact is thus unlikely to undergo chipping or cracking during cutting. The green compact is formed by simply compressing the raw material powder, and the metal powder particles in the green compact are mechanically attached to each other. The sintered member is formed by diffusion-bonding metal powder particles to each other through sintering, and the metal powder particles are strongly bonded to each other. Assuming that the wrought material has the same size as the green compact, the wrought material is an integrally formed object much larger than metal particles that constitute the green compact. 
     In particular, the processing speed for producing the green compact by using the mixed powder can be higher than the processing speed for producing the green compact by using the alloy powder. For example, in dry-cutting a green compact formed of the alloy powder and a green compact formed of the mixed powder by using a hob made of powder high-speed steel, the maximum peripheral speed (cutting speed) of the tool is 350 m/min for cutting the green compact formed of the alloy powder and 450 m/min for cutting the green compact formed of the mixed powder. The maximum peripheral speed of the tool is 150 m/min for cutting the wrought material and 150 m/min for cutting the sintered member. 
     Examples of cutting include milling and turning. Milling includes drilling. Examples of the cutting tool include drills and reamers for drilling; milling cutters and end mills for milling; and tool bits and edge replaceable cutting tips for turning. In addition, for example, a hob, a broach, or a pinion cutter may be used, or a machining center that can automatically perform various types of processing may be used. 
     Before cutting, a volatile solution or plastic solution in which an organic binder is dissolved may be applied to the surface of the green compact by coating or immersion. In this case, it is easy to suppress cracking or chipping in the surface layer of the green compact during cutting. Examples of the organic binder include polyethylene, polypropylene, polymethyl methacrylate, polystyrene, polyvinyl chloride, polyvinylidene chloride, polyamide, polyester, polyether, polyvinyl alcohol, vinyl acetate, paraffin, and various waxes. 
     Cutting may be performed with the compression stress applied to the green compact to offset the tensile stress acting on the green compact. In this case, it is easy to suppress cracking or chipping of the green compact. For example, in the case where a processing hole is formed in the green compact by using a broach, a high tensile stress acts near the exit of the processing hole when the broach is inserted into the green compact. In this case, plural green compacts may be stacked in a multilayer manner. For example, a dummy green compact or plate may be placed under the lowest green compact. In the plural green compacts stacked in a multilayer manner, the lower surfaces of the upper green compacts are pressed against the upper surfaces of the lower green compacts, and compression stress acts on the lower surfaces. When the green compacts stacked in a multilayer manner undergo broaching from above, it is possible to effectively prevent cracking or chipping near the exits of processing holes formed in the lower surfaces of the green compacts. In the case where processing grooves are formed in the green compacts by using a milling cutter, a high tensile stress acts near the exits of the processing grooves. In this case, for example, plural green compacts are arranged in the direction in which the milling cutter moves, and compression stress is applied to parts for forming the exits of the processing grooves. 
     (Heat Treatment Step) 
     The heat treatment step involves carburizing, quenching, and annealing the sintered member. This step improves the mechanical characteristics, particularly hardness and toughness, of the sintered member. 
     (Finishing Step) 
     The finishing step involves reducing the surface roughness of the sintered member and adjusting the dimensions of the sintered member to designed dimensions. For example, the surface of the sintered member is polished. 
     [Applications] 
     A sintered member having high density and high strength can be manufactured in the method for manufacturing a sintered member according to the embodiment. It is also easy to manufacture a sintered member having high density and complicated shape in this method. The method for manufacturing a sintered member according to the embodiment can be preferably applied to the manufacture of various ordinary structural components (sintered components, such as mechanical components, including sprockets, rotors, gears, rings, flanges, pulleys, and bearings). 
     [Sintered Member] 
     The sintered member contains metal particles bonded to each other and has a relative density of more than 97%. This sintered member can be manufactured by using the method for manufacturing a sintered member (the raw material preparation step, the forming step, and the sintering step). Since the compact has a relative density of 97% or more, the sintered member obtained after the subsequent sintering step also has a relative density of more than 97%. The method for measuring this relative density is the same as the method for measuring the relative density of the green compact. The density of this sintered member in the range of 1 mm from its surface does not substantially change. In other words, the density is substantially uniform. This is because the sintered member does not undergo rolling. The metallographic structure of the sintered member has no streamline pattern formed by stretching metal particles. This is because the sintered member does not undergo forging. 
     Test Example 1 
     The sintered member was produced, and the relative density and strength of the sintered member were evaluated. 
     [Sample No. 1-1 to No. 1-3, and No. 1-101 to No. 1-103] 
     The sintered members of Sample No. 1-1 to No. 1-3 and No. 1-101 to No. 1-103 were produced through the raw material preparation step, the forming step, and the sintering step as in the method for manufacturing a sintered member. 
     [Raw Material Preparation Step] 
     Carbon powder and iron alloy powder produced by water atomization were prepared for raw material powder. The iron alloy powder has a composition of 2 mass % Ni-0.5 mass % Mo with the balance being Fe and inevitable impurities and has an average particle size (D50) of 70 μm. The carbon powder has an average particle size (D50) of 5 μm. The amount of the carbon powder is 0.3 mass % relative to 100 mass % of the raw material powder, and the balance is iron alloy powder. The raw material powder is free of lubricant or organic binder. 
     [Forming Step] 
     The raw material powder was compacted into a green compact having a solid cylindrical shape (outer diameter: 75 mm, height: 20 mm). A mold that enables uniaxial pressing was used for production of the green compact. This mold includes a die and upper and lower punches. The die has openings on its top and bottom and a circular insertion hole for forming the outer circumferential surface of the green compact having a solid cylindrical shape. The upper and lower punches each have a circular press surface for forming the opposite end surfaces of the green compact having a solid cylindrical shape. The inner circumferential surface of the die was coated with a solution of myristic acid in alcohol, which was a lubricant. 
     The compacting pressure is as shown in Table 1. The compacting pressure in Table 1 is a value obtained by converting “8 ton/cm 2  to 20 ton/cm 2 ” into “MPa” and rounding off decimals. 
     The relative density of the produced green compact was measured. The relative density of the green compact was obtained from “{(green density of green compact)/(true density of green compact)}×100”. The green density of the green compact was obtained by immersing the green compact in an oil and calculating the green density in accordance with “oil-impregnated density×((mass of green compact before oil impregnation)/(mass of green compact after oil impregnation)).” The oil-impregnated density is a value obtained by dividing the mass of the green compact after oil impregnation by the volume of the green compact after oil impregnation. The true density of the green compact (raw material powder) is about 7.8 g/cm 3 . The green density and relative density of the green compact of each sample are shown in Table 1. The relationship between the compacting pressure (MPa) and the green density (g/cm 3 ) of the green compact of each sample is shown in  FIG. 1 . The horizontal axis of the graph shown in  FIG. 1  represents compacting pressure (MPa), and the vertical axis represents green density (g/cm 3 ). In  FIG. 1 , the results of Sample No. 1-1 to No. 1-3 are plotted with black circles, and the results of Sample No. 1-101 to No. 1-103 are plotted with black rhombuses. 
     [Sintering Step] 
     The green compact was sintered to produce the sintered member. The sintering conditions were as follows: the sintering temperature was 1150° C.; the sintering time was 60 minutes; and the sintering atmosphere was a nitrogen atmosphere. 
     [Sample No. 1-111 to No. 1-117] 
     The sintered members of Sample No. 1-111 to No. 1-117 were produced in the same manner as for Sample No. 1-1 except that the mold was not coated with a lubricant, and the raw material powder further contained a lubricant. The lubricant was ethylene bis stearamide, and the amount of the lubricant in the raw material powder was 0.6 mass %. The relative density of the green compacts of Sample No. 1-111 to No. 1-117 was measured in the same manner as for Sample No. 1-1. The green density and relative density of the green compact of each sample are shown in Table 1. The relationship between the compacting pressure (MPa) and the green density (g/cm 3 ) of the green compact of each sample is shown in  FIG. 1 . The results of Sample No. 1-111 to No. 1-117 are plotted with white squares. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Forming Step 
                 Green Compact 
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 Compacting 
                 Green 
                 Relative 
               
               
                   
                 Sample 
                 Pressure 
                 Density 
                 Density 
               
               
                   
                 No. 
                 (MPa) 
                 (g/cm 3 ) 
                 (%) 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 1-1 
                 1569 
                 7.63 
                 97.82 
               
               
                   
                 1-2 
                 1765 
                 7.68 
                 98.46 
               
               
                   
                 1-3 
                 1961 
                 7.70 
                 98.72 
               
               
                   
                 1-101 
                 980 
                 7.28 
                 93.33 
               
               
                   
                 1-102 
                 1176 
                 7.46 
                 95.64 
               
               
                   
                 1-103 
                 1372 
                 7.56 
                 96.92 
               
               
                   
                 1-111 
                 784 
                 7.04 
                 90.26 
               
               
                   
                 1-112 
                 980 
                 7.21 
                 92.44 
               
               
                   
                 1-113 
                 1176 
                 7.29 
                 93.46 
               
               
                   
                 1-114 
                 1372 
                 7.34 
                 94.10 
               
               
                   
                 1-115 
                 1569 
                 7.35 
                 94.23 
               
               
                   
                 1-116 
                 1765 
                 7.36 
                 94.36 
               
               
                   
                 1-117 
                 1961 
                 7.36 
                 94.36 
               
               
                   
                   
               
            
           
         
       
     
     Table 1 and  FIG. 1  show that the green compacts of Sample No. 1-1 to No. 1-3 each have a relative density of 97% (green density≈7.57 g/cm 3 ) or more. This result suggests that the sintered members of Sample No. 1-1 to No. 1-3 each have a relative density of more than 97% (green density≈7.57 g/cm 3 ). 
     However, it is found that the green compacts of Sample No. 1-101 to No. 1-103 and No. 1-111 to No. 1-117 each have a relative density of less than 97%. This result suggests that the sintered members of Sample No. 1-101 to No. 1-103 and No. 1-111 to No. 1-117 each have a relative density of less than 97%. 
     [Evaluation of Strength] 
     The strength of the sintered members was evaluated by measuring tensile strength in tension testing. The sintered members of Sample Nos. 2-1, 2-2, and 2-101 to 2-103 having a solid cylindrical shape were produced in the manner as for Sample No. 1-1. The compacting pressure was controlled such that the green density (relative density) of each sample was a value shown in Table 2. The sintered member having a solid cylindrical shape was processed into a predetermined shape and subjected to carburizing and quenching to produce a test piece for measurement of tensile strength. The test piece has a plate shape including a narrow part and wide parts formed at the opposite ends of the narrow part. The test piece has a thickness of 4 mm and a length of 72 mm. The narrow part includes a central portion and shoulder portions each having arc-shaped side surfaces formed from the central portion to the wide part. The central portion has a length of 32 mm, a width of 5.7 mm at its center, and a width of 5.96 mm at its opposite ends. The side surfaces of the shoulder portions have a radius R of 25 mm. The wide parts have a width of 8.7 mm. 
     This test piece was subjected to tensile testing by using a general-purpose tensile testing machine. 
     The results of tensile strength (MPa) are shown in Table 2. The tensile strength (MPa) shown in Table 2 is a mean tensile strength taken from evaluation number n=5. The relationship between green density (g/cm 3 ) and tensile strength (MPa) is shown in  FIG. 2 . The horizontal axis of the graph shown in  FIG. 2  represents green density, and the vertical axis represents tensile strength. In  FIG. 2 , the mean tensile strength of each of Sample No. 2-1 and No. 2-2 is plotted with black circles, the mean tensile strength of each of Sample No. 2-101 to No. 2-103 is plotted with black rhombuses, and the maximum tensile strength and the minimum tensile strength of these samples are indicated by error bars. 
     
       
         
           
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 Test Piece 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 Green 
                 Relative 
                 Tensile 
               
               
                   
                 Sample 
                 Density 
                 Density 
                 Strength 
               
               
                   
                 No. 
                 (g/cm 3 ) 
                 (%) 
                 (MPa) 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 2-1 
                 7.6 
                 97.44 
                 1803.8 
               
               
                   
                 2-2 
                 7.7 
                 98.72 
                 1813.6 
               
               
                   
                 2-101 
                 7.1 
                 91.03 
                 1214.6 
               
               
                   
                 2-102 
                 7.4 
                 94.87 
                 1500.4 
               
               
                   
                 2-103 
                 7.5 
                 96.15 
                 1675.4 
               
               
                   
                   
               
            
           
         
       
     
     Table 2 and  FIG. 2  show that the tensile strength of the test pieces (sintered members) of Sample No. 2-1 and No. 2-2 is 1700 MPa or more, or 1750 MPa or more, or 1800 MPa or more. However, it is found that the tensile strength of the test pieces (sintered members) of Sample No. 2-101 to No. 2-103 is less than 1700 MPa. It is thus revealed that sintering a green compact having a relative density of 97% or more produces a sintered member having high density and high strength. 
     The tensile strength of a common chromium molybdenum steel (SCM415), which is used for structural components that experience very high load, such as transmission gears for automobiles, was measured in the same manner as for Sample No. 1-1 and the like. The tensile strength was 1372 MPa. In other words, it is found that the tensile strength of the sintered members of Sample No. 2-1 and No. 2-2 was very high. 
     It should be understood that the embodiment disclosed herein is illustrative in any respect and non-restrictive from any viewpoint. The scope of the present invention is defined by the claims, rather than the above description, and is intended to include all modifications within the meaning and range of equivalency of the claims.