Patent Application
20240110266
The present application is based on, and claims priority from JP Application Serial Number 2022-158122, filed Sep. 30, 2022, the disclosure of which is hereby incorporated by reference herein in its entirety.
The present disclosure relates to a sintering metal powder and a sintered metal body.
JP-A-9-31505 discloses a timepiece external component containing a sintered body obtained by mixing and kneading an organic binder with an alloy powder of ferritic stainless steel, molding the mixture by an injection molding method, subjecting the resultant to a debindering treatment and then sintering the resultant. The ferritic stainless steel contains substantially no Ni. Therefore, the timepiece external component disclosed in JP-A-9-31505 can prevent metal allergy. In addition, the ferritic stainless steel is a magnetic body. Therefore, the timepiece external component disclosed in JP-A-9-31505 also contributes to improving antimagnetic performance of a movement.
On the other hand, the ferritic stainless steel is characterized by a relatively low crystal structure filling rate, resulting in a high diffusion rate and good densification. Therefore, when a molded body made of a ferritic stainless steel powder is sintered, a time difference between the inside and a surface layer of the molded body until the molded body is sintered tends to be large. As a result, the surface layer whose temperature easily rises during sintering is sintered first, and pores in the surface layer are blocked. Then, the remaining gas is confined inside, and finally, pores are generated inside a sintered body.
In this case, a thickness of the surface layer in which the pores are blocked, that is, a dense layer is fairly thin. Therefore, when a surface of the sintered body is polished, the dense layer is easily removed, and the pores generated therein are exposed on the surface. As a result, sufficient mirror finish performance may not be obtained on the polished surface even though the polishing is performed.
A sintering metal powder according to an application example of the present disclosure is
A sintered metal body according to an application example of the present disclosure contains:
Hereinafter, a sintering metal powder and a sintered metal body according to the present disclosure will be described in detail with reference to the accompanying drawings.
First, a sintering metal powder according to an embodiment will be described.
In a powder metallurgy technique, a composition containing a sintering metal powder and a binder is molded into a desired shape, and then subjected to a debindering treatment and a sintering treatment, whereby a sintered metal body having a desired shape can be obtained. According to such a powder metallurgy technique, as compared to other techniques, a sintered body having a complicated and fine shape can be produced in a near-net shape, that is, a shape close to a final shape.
As the sintering metal powder according to the embodiment, a powder in which C and Nb are added as necessary, with contents thereof adjusted as will be described later, to a composition of ferritic stainless steel is used.
Examples of the composition of ferritic stainless steel include chemical components defined in the JIS standards. In the JIS standards, a steel type of ferritic stainless steel is represented by a symbol. Examples of the steel type include SUS405, SUS410L, SUS429, SUS430, SUS430F, SUS430LX, SUS430J1L, SUS443J1, SUS434, SUS436J1L, SUS436L, SUS444, SUS445J1, SUS445J2, SUSXM27, SUS447J1, SUH409, and SUH409L.
The ferritic stainless steel originally has a high diffusion rate during sintering. Therefore, in a molded body containing a ferritic stainless steel powder, a difference in sintering progress tends to occur between the surface and the inside. In this case, sintering is completed earlier on the surface than the inside, and a gas generated due to the sintering remains inside. As a result, a dense layer in which sintering is completed (the dense layer) is formed extremely thin in a surface layer, whereas the inside becomes a region having a low density due to the remaining gas. In this case, the dense layer is extremely thin, and the dense layer is lost when the surface is polished.
In this regard, the sintering rate can be slowed down by using the sintering metal powder according to the embodiment. Accordingly, the difference in the sintering progress between the surface and the inside is reduced. As a result, it is possible to inhibit the gas from remaining inside and to thicken the dense layer formed on the surface. A sintered metal body having such a thick dense layer on a surface thereof is less likely to lose the dense layer even when the surface is polished, and thus a polished surface having favorable mirror finish performance can be obtained. In addition, the dense layer has excellent corrosion resistance. Therefore, the sintered metal body having the thick dense layer has favorable corrosion resistance. Meanwhile, the obtained sintered metal body can also maintain characteristics specific to ferritic stainless steel, such as being substantially nickel-free.
Hereinafter, the sintering metal powder according to the embodiment will be described in detail.
As described above, in a composition of the sintering metal powder according to the embodiment, Nb is added to the composition of ferritic stainless steel, and C is added as necessary. Therefore, a content of each element in the sintering metal powder, except for C, Nb, and impurities, may conform to a standard of ferritic stainless steel and is preferably as follows.
In addition, a remainder other than the above elements is Fe and impurities.
In the above composition, C (carbon) reduces a substance that inhibits sintering, for example, an oxide such as a silicon oxide or a chromium oxide. Examples of a reduction reaction of the oxide include a reaction represented by the following reaction formulas.
SiO2 (s)+C(s)→SiO (g)+CO (g)
Cr2O3 (s)+3C (s)→2Cr (s)+3CO (g)
In the above formulas, (s) represents a solid, and (g) represents a gas. In this example, the silicon oxide SiO2 reacts with carbon C, changes into a substance that is easily vaporized, and is removed from a molded body. In addition, the chromium oxide is reduced to metal chromium. As a result, it is possible to reduce the oxide that is likely to inhibit sintering from the molded body, and thus it is possible to increase a density of a sintered body.
On the other hand, in the reduction reaction, a gas is generated as a by-product. The gas may remain inside the sintered body. In the sintering metal powder according to the embodiment, the remaining of the gas can be inhibited.
Specifically, by using C (carbon) and Nb (niobium) in combination, NbC (niobium carbide) is precipitated on a particle surface of the sintering metal powder during sintering. Such NbC can slow down the sintering rate when the sintering metal powder is sintered, and inhibits rapid sintering progress on a surface of the molded body (debindered body). Accordingly, it is possible to reduce a difference in the sintering progress between the surface and the inside of the molded body, inhibit the gas from remaining inside and to thicken the dense layer formed on the surface.
The content of C is 0.05 mass % or more and 1.00 mass % or less, preferably 0.08 mass % or more and 0.50 mass % or less, and more preferably 0.13 mass % or more and 0.30 mass % or less. When the content of C is less than the lower limit value, an amount of C is insufficient with respect to an amount of Nb, and the reduction reaction described above may be less likely to occur, or precipitation of NbC may be reduced. On the other hand, when the content of C exceeds the upper limit value, the amount of C is excessive with respect to the amount of Nb, and thus a sintering reaction is hindered and a sintering density decreases. In addition, in the produced sintered metal body, a precipitate that reduces mirror finish performance and corrosion resistance may easily be generated.
The content of Nb is 0.05 mass % or more and 1.50 mass % or less, preferably 0.10 mass % or more and 1.20 mass % or less, and more preferably 0.15 mass % or more and 0.70 mass % or less. When the content of Nb is less than the lower limit value, the amount of Nb is insufficient with respect to the amount of C, and thus precipitation of NbC is reduced. On the other hand, when the content of Nb exceeds the upper limit value, the amount of Nb is excessive with respect to the amount of C, and thus the sintering reaction may be hindered and the sintering density may decrease. In addition, in the produced sintered metal body, a precipitate that reduces the mirror finish performance and the corrosion resistance may easily be generated.
C/Nb is preferably 0.10 or more and 1.80 or less, more preferably 0.20 or more and 1.20 or less, and still more preferably 0.30 or more and 1.00 or less, in which C/Nb is a ratio of the content of C to the content of Nb. Accordingly, a balance between the content of C and the content of Nb can be optimized. As a result, surplus and shortage of C and Nb are less likely to occur, an appropriate amount of NbC can be precipitated, the sintering rate can be slowed down, and the decrease in the sintering density can be inhibited. Accordingly, the dense layer formed on the surface of the sintered metal body can be thickened.
When C/Nb is less than the lower limit value, the amount of C may be insufficient with respect to the amount of Nb, or the amount of Nb may be excessive with respect to the amount of C. On the other hand, when C/Nb exceeds the upper limit value, the amount of C may be excessive with respect to the amount of Nb, or the amount of Nb may be insufficient with respect to the amount of C.
C+Nb is preferably 0.20 mass % or more and 1.50 mass % or less, more preferably 0.25 mass % or more and 1.20 mass % or less, and still more preferably 0.30 mass % or more and 0.80 mass % or less, in which C+Nb is a sum of the content of C and the content of Nb. Accordingly, NbC can be appropriately precipitated, the sintering rate can be slowed down, and the decrease in the sintering density can be inhibited. As a result, it is possible to obtain the sintering metal powder from which the sintered metal body having a high density as a whole and having a sufficiently thick dense layer can be produced.
The content of Si (silicon) is 1.00 mass % or less as described above, preferably 0.20 mass % or more and 0.80 mass % or less, and more preferably 0.30 mass % or more and 0.50 mass % or less. Accordingly, sintering performance of the sintering metal powder can be further improved.
The content of Cr (chromium) is 12.0 mass % or more and 30.0 mass % or less as described above, preferably 15.0 mass % or more and 25.0 mass % or less, and more preferably 18.0 mass % or more and 23.0 mass % or less. Accordingly, the corrosion resistance and heat resistance of the sintered metal body can be improved.
The sintering metal powder may contain at least one of Mo, Ni, Al, Ti, Cu, Zr, and N as necessary.
A content of Mo (molybdenum) is preferably 3.00 mass % or less, more preferably 0.70 mass % or more and 2.80 mass % or less, and still more preferably 1.80 mass % or more and 2.60 mass % or less.
A content of each of Ni (nickel), Al (aluminum), Ti (titanium), Cu (copper), Zr (zirconium), and N (nitrogen) is preferably 1.00 mass % or less, and more preferably 0.10 mass % or more and 0.80 mass % or less.
Fe (iron) and impurities occupy a remainder other than the components described above.
Among these, Fe is a main component of the sintering metal powder and has the highest content. The content of Fe is preferably 60 mass % or more, and more preferably 70 mass % or more.
A concentration of the impurities is preferably 0.10 mass % or less, and more preferably 0.05 mass % or less for each element. In addition, a total concentration of the impurities is preferably 1.00 mass % or less. Within this range, an element that is inevitably mixed or an element that is intentionally added can be regarded as an impurity since an effect of the sintering metal powder is not affected.
The impurities may contain O (oxygen). In view of final reduction, a concentration of oxygen may be higher than the concentration of the above impurities. Specifically, the concentration of oxygen is preferably 0.70 mass % or less, and more preferably 0.50 mass % or less. At this level, even if O is contained, the sintered metal body having intended characteristics can finally be produced.
The composition of the sintering metal powder has been described in detail above, and the composition is identified by the following analysis method.
Examples of the analysis method include iron and steel-atomic absorption spectrometric method defined in JIS G 1257:2000, iron and steel-ICP atomic emission spectrometric method defined in JIS G 1258:2007, iron and steel-method for spark discharge atomic emission spectrometric analysis defined in JIS G 1253:2002, iron and steel-method for X-ray fluorescence spectrometric analysis defined in JIS G 1256:1997, and gravimetric, titration, absorption spectrophotometric methods defined in JIS G 1211 to G 1237.
Specifically, examples include a solid-state emission spectrometer manufactured by SPECTRO Corporation, particularly a spark discharge emission spectrometer, model: SPECTROLAB, type: LAVMB08A, and an ICP apparatus CIROS 120 manufactured by Rigaku Corporation.
In particular, in order to identify C (carbon) and S (sulfur), an oxygen flow combustion (high-frequency induction heating furnace combustion)-infrared absorption method defined in JIS G 1211:2011 is also used. Specifically, examples include a carbon/sulfur elemental analyzer CS-200 manufactured by LECO Corporation.
Further, in particular, in order to identify N (nitrogen) and O (oxygen), iron and steel-methods for determination of nitrogen content defined in JIS G 1228:1997 and general rules for determination of oxygen in metallic materials defined in JIS Z 2613:2006 are also used. Specifically, examples include an oxygen/nitrogen elemental analyzer, TC-300/EF-300 manufactured by LECO Corporation.
An average particle diameter of the sintering metal powder is not particularly limited, and is preferably 0.5 μm or more and 30.0 μm or less, more preferably 0.5 μm or more and 15.0 μm or less, and still more preferably 1.0 μm or more and 10.0 μm or less. Accordingly, the dense layer formed in the sintered metal body can be further densified. As a result, when the produced sintered metal body is polished, particularly favorable mirror finish performance can be obtained on a polished surface.
When the average particle diameter of the sintering metal powder is less than the lower limit value, the powder is likely to aggregate and a filling property deteriorates, and thus the sintering density may decrease. On the other hand, when the average particle diameter of the sintering metal powder exceeds the upper limit value, the filling property at the time of molding deteriorates, and thus the sintering density may decrease.
The average particle diameter refers to a particle diameter D50 where a cumulative frequency is 50% from a small-diameter side in a cumulative particle size distribution on a volume basis of the sintering metal powder obtained using a laser diffraction type particle size distribution measuring apparatus.
With respect to the sintering metal powder, in the cumulative particle size distribution described above, (D90−D10)/D50 is preferably approximately 1.0 or more and 2.5 or less, and more preferably approximately 1.2 or more and 2.3 or less, in which D10 is a particle diameter when a cumulative frequency is 10% from the small-diameter side and D90 is a particle diameter when a cumulative value from the small-diameter side is 90%. (D90-D10)/D50 is an index indicating a degree of spread of the particle size distribution, and when this index is within the above range, the filling property of the sintering metal powder is particularly favorable. As a result, the sintered metal body having a high density can be produced.
The method for producing the sintered metal body shown in
In the composition preparation step S102, a molding composition containing a sintering metal powder and an organic binder is obtained.
The sintering metal powder is preferably produced by an atomization method, and more preferably produced by a water atomization method or a rotary water flow atomization method. The atomization method is a method of producing a metal powder by causing a molten metal to collide with a liquid or a gas injected at a high speed to pulverize and cool the molten metal. When the sintering metal powder is produced by the atomization method, a fine powder can be efficiently produced.
The sintering metal powder may be subjected to various pretreatments such as a heat treatment, a plasma treatment, an ozone treatment, and a reduction treatment.
As the organic binder, a resin that can be decomposed in a short time in a debindering treatment and a sintering treatment is used. Examples of the resin include polyolefins such as polyethylene, polypropylene, and ethylene-vinyl acetate copolymers, acrylic resins such as polymethyl methacrylate and polybutyl methacrylate, styrene resins such as polystyrene, polyesters such as polyvinyl chloride, polyvinylidene chloride, polyamide, polyethylene terephthalate, and polybutylene terephthalate, polyethers, polyvinyl alcohols, polyvinyl pyrrolidone, copolymers thereof, various waxes, paraffins, higher fatty acids, higher alcohols, higher fatty acid esters, and higher fatty acid amides, and one or more of these can be used in a mixture.
Examples of a form of the molding composition include a kneaded material and a granulated powder.
A mixing ratio of the organic binder is preferably approximately 0.2 mass % or more and 20.0 mass % or less of the molding composition, and more preferably approximately 0.5 mass % or more and 15.0 mass % or less.
In addition to the above components, various additives such as a plasticizer, a lubricant, an antioxidant, a debindering accelerator, and a surfactant may be added to the composition.
In the molding step S104, the molding composition is molded into a desired shape. Accordingly, a molded body is obtained.
Examples of a molding method include an injection molding method, a compression molding method, an extrusion molding method, and an additive manufacturing method. Among these, examples of the additive manufacturing method include a material extrusion deposition method and a binder jetting method.
In the debindering step S106, a debindering treatment is performed on the molded body to obtain a debindered body.
Examples of the debindering treatment include a method of heating the molded body to decompose the organic binder, and a method of exposing the molded body to a gas that decomposes the organic binder. All or a part of the organic binder in the molded body is removed by the debindering treatment.
When the method of heating the molded body is used, a heating condition of the molded body slightly varies depending on a composition and a blending amount of the organic binder, a temperature is preferably 100° C.. or higher and 750° C.. or lower, a time is preferably 0.1 hours or longer and 20 hours or shorter, the temperature is more preferably 150° C.. or higher and 600° C.. or lower, and the time is more preferably 0.5 hours or longer and 15 hours or shorter.
An atmosphere when heating the molded body is not particularly limited, and examples thereof include an inert atmosphere such as nitrogen or argon, an oxidizing atmosphere such as air, and a depressurized atmosphere obtained by depressurizing such an atmosphere.
As the method of exposing the molded body to a gas that decomposes the organic binder, for example, an acid debindering method is used. The acid debindering method is a method in which the molded body is heated in an acid-containing atmosphere to perform debindering using a catalytic action of an acid. According to the acid debindering method, since the organic binder can be decomposed in a short time even at a low temperature, the debindering treatment can be efficiently performed even for a molded body having a large volume.
The acid-containing atmosphere refers to an atmosphere containing an acid capable of decomposing the organic binder. Examples of the acid include nitric acid, oxalic acid, and ozone, and one or two or more of these can be used in combination. In addition, a mixed gas obtained by mixing these acids and another gas may be used. An example of the mixed gas is fuming nitric acid. A pressure of the atmosphere may be an atmospheric pressure, a reduced pressure, or an increased pressure.
A heating condition of the molded body in the acid-containing atmosphere is of a lower temperature or a shorter time than the heating condition described above. Therefore, an amount of heat applied to the molded body can be reduced, and oxidation of the sintering metal powder can be easily inhibited.
In the sintering step S108, the debindered body is subjected to a sintering treatment to obtain a sintered metal body.
A sintering temperature varies depending on a composition ratio, a particle diameter, and the like of the sintering metal powder, and is, for example, approximately 980° C.. or higher and 1330° C.. or lower. In addition, the temperature is preferably approximately 1050° C.. or higher and 1260° C.. or lower.
A sintering time is 0.2 hours or longer and 7 hours or shorter, and preferably approximately 1 hour or longer and 6 hours or shorter.
Examples of an atmosphere in the sintering treatment include a reducing atmosphere such as hydrogen, an inert atmosphere such as nitrogen or argon, and a depressurized atmosphere obtained by depressurizing such an atmosphere. A pressure of the depressurized atmosphere is not particularly limited as long as the pressure is less than an ambient pressure (100 kPa), is preferably 10 kPa or less, and more preferably 1 kPa or less. Accordingly, a gas remaining in the debindered body can be discharged particularly efficiently, and a density of the finally obtained sintered metal body can be increased.
The obtained sintered metal body may be subjected to a post-treatment such as annealing as necessary.
Next, the sintered metal body according to the embodiment will be described.
The sintered metal body according to the embodiment contains a composition of ferritic stainless steel, C having a content of 0.02 mass % or more and 1.00 mass % or less, Nb having a content of 0.05 mass % or more and 1.50 mass % or less, and impurities. The composition of ferritic stainless steel, the content of C, the content of Nb, and impurities are basically the same as those of the sintering metal powder.
The content of C in the sintered metal body may be lower than the content of C in the sintering metal powder due to the reduction reaction of C described above. Therefore, the content of C in the sintered metal body is 0.02 mass % or more and 1.00 mass % or less, preferably 0.05 mass % or more and 0.50 mass % or less, and more preferably 0.08 mass % or more and 0.30 mass % or less. When the content of C is less than the lower limit value, a sufficiently thick dense layer is not formed, and thus mirror finish performance and corrosion resistance of a polished surface of the sintered metal body deteriorate. On the other hand, when the content of C exceeds the upper limit value, a sintering density may decrease or a precipitate may easily be generated, and thus the mirror finish performance and the corrosion resistance deteriorate.
In addition, the content of Nb in the sintered metal body is 0.05 mass % or more and 1.50 mass % or less, preferably 0.10 mass % or more and 1.20 mass % or less, and more preferably 0.15 mass % or more and 0.70 mass % or less. When the content of Nb is less than the lower limit value, an amount of Nb is insufficient with respect to an amount of C, and thus the dense layer is thin, so that the mirror finish performance and the corrosion resistance of the polished surface of the sintered metal body deteriorate. On the other hand, when the content of Nb exceeds the upper limit value, the amount of Nb is excessive with respect to the amount of C, the sintering density may decrease or a precipitate may easily be generated, so that the mirror finish performance and the corrosion resistance deteriorate.
C/Nb is preferably 0.10 or more and 1.80 or less, more preferably 0.20 or more and 1.20 or less, and still more preferably 0.30 or more and 1.00 or less, in which C/Nb is a ratio of the content of C to the content of Nb. Accordingly, a balance between the content of C and the content of Nb can be optimized. As a result, the sintered metal body contains an appropriate amount of NbC, has a thicker dense layer, and has a high sintering density.
C+Nb is preferably 0.20 mass % or more and 1.50 mass % or less, more preferably 0.25 mass % or more and 1.20 mass % or less, and still more preferably 0.30 mass % or more and 0.80 mass % or less, in which C+Nb is a sum of the content of C and the content of Nb. Accordingly, NbC can be appropriately precipitated, and the sintered metal body having a high density as a whole and having a sufficiently thick dense layer can be obtained.
The sintered metal body 1 shown in
In the sintered metal body 1, since a thickness t of the dense layer CL is sufficiently large, the dense layer CL is hardly lost even when the surface SF is polished. When the dense layer CL is polished, the polished surface is a surface with a small amount of foreign matters 10, and is a surface having favorable mirror finish performance.
The thickness t of the dense layer CL is preferably 150 μm or more, more preferably 200 μm or more, and still more preferably 300 μm or more. The sintered metal body 1 having the dense layer CL which has such a thickness t has a low probability of losing the whole dense layer CL due to polishing. In addition, such a sintered metal body 1 is unlikely to lose the dense layer CL even when subjected to polishing a plurality of times. Therefore, the sintered metal body 1 is preferably used for, for example, various types of metal products requiring an aesthetic appearance, and various types of metal products expected to be repaired by polishing once or a plurality of times.
As described above, the dense layer CL is a region containing a small number of foreign matters 10, that is, a region mainly having a high-density portion 11, and a small number of foreign matters 10 may be contained. The foreign matters 10 allowed in the dense layer CL satisfy a condition that an area ratio in the dense layer CL is 0.50% or less and an average diameter is 2.5 μm or less. If the foreign matters 10 satisfy this condition, even when the foreign matters 10 are contained in the dense layer CL, the mirror finish performance is hardly affected. The area ratio and the average diameter of the foreign matters 10 are measured and calculated by the following procedure.
First, a cross-section of the sintered metal body 1 is observed with an electron microscope, and a range M of, for example, 100 μm×100 μm in contact with the surface SF on an observation image is selected. Next, binarization image processing is performed on the range M, and the high-density portion 11 and the foreign matters 10 are identified by using the fact that a concentration differs due to differences in density and composition. Next, the area ratio and the average diameter of the foreign matters 10 are calculated. The area ratio is a ratio of a total area of the foreign matters 10 reflected in the range M to an area of the range M. In addition, the average diameter is a value obtained by averaging 10 measured values obtained by randomly selecting 10 foreign matters 10 reflected in the range M and measuring diameters thereof. When the number of foreign matters 10 reflected in the range M is less than 10, the average diameter is a value obtained by averaging measured values for all the foreign matters 10.
Whether both the area ratio and the average diameter of the foreign matters 10 calculated in this manner satisfy the above condition is examined while changing a length of one side of the square range M. A maximum length when the condition is satisfied is the thickness t. Then, an average thickness of the dense layer CL is obtained by measuring the thickness t at 10 or more positions and calculating an average value.
The area ratio of the foreign matters 10 is preferably 0.30% or less, and more preferably 0.20% or less. In addition, the average diameter of the foreign matters 10 is preferably 2.0 μm or less, and more preferably 1.0 μm or less.
A relative density of the sintered metal body 1 is preferably 98.5% or more, and more preferably 99.0% or more. When the relative density of the sintered metal body 1 is within the above range, mechanical properties of the sintered metal body 1, and the mirror finish performance and the corrosion resistance of the polished surface are particularly favorable.
The corrosion resistance of the sintered metal body 1 can be evaluated according to method B among methods of pitting potential measurement for stainless steel defined in JIS G 0577:2014. Method B is a pitting potential measurement method implemented by an electrokinetic potential method in a 3.5 mass % sodium chloride aqueous solution. Then, a pitting potential at which a current density of the sintered metal body 1 is 100 μA/cm2 is measured by method B. That is, a potential at which the current density is 100 μA/cm2 is set, for convenience, as a potential at which corrosion starts to progress, that is, a pitting potential. In addition, the pitting potential is set to a reference value of a saturated calomel electrode (SCE). The sodium chloride aqueous solution has a pH of 7 and a temperature of 30° C.. In addition, a potential sweep rate is 20 mV/min.
The pitting potential of the sintered metal body 1 measured by such a method is preferably 200 mV or more, and more preferably 300 mV or more. When the pitting potential is within the above range, corrosion of the sintered metal body 1 is sufficiently inhibited, and particularly favorable corrosion resistance is obtained.
An upper limit value of the pitting potential may not be particularly set, and is preferably 1500 mV from the viewpoint of reducing individual differences.
The sintered metal body 1 as described above is used as a material constituting all or a part of, for example, a transportation equipment component such as an automobile component, a bicycle component, a railway vehicle component, a ship component, an aircraft component or a space transporter component, an electronic device component such as a personal computer component, a mobile phone terminal component, a tablet terminal component or a wearable terminal component, a component for electrical equipment such as a refrigerator, a washing machine or an air conditioner, a component for a machine such as a machine tool or a semiconductor manufacturing apparatus, a component for a plant such as a nuclear power plant, a thermal power plant, a hydroelectric power plant, an oil refinery or a chemical combination, a timepiece component, metalware, a decoration such as a jewelry decoration or an eyeglass frame, or a medical instrument such as a medical scalpel or forceps.
As described above, the sintering metal powder according to the embodiment is a sintering metal powder to be used in sintering, containing: a composition of ferritic stainless steel; C having a content of 0.05 mass % or more and 1.00 mass % or less; Nb having a content of 0.05 mass % or more and 1.50 mass % or less; and impurities.
According to such a configuration, a diffusion rate can be slowed down, and a difference in progress of sintering between the surface and the inside of a molded body can be reduced. Accordingly, it is possible to inhibit a gas from remaining inside and to thicken a dense layer formed on a surface of a sintered metal body. As a result, the dense layer is hardly lost even when the surface is polished. Therefore, it is possible to obtain a sintering metal powder from which a sintered metal body having favorable mirror finish performance on a polished surface can be produced. Meanwhile, the obtained sintered metal body can also maintain characteristics specific to ferritic stainless steel, such as being substantially nickel-free.
In the sintering metal powder, C/Nb is preferably 0.10 or more and 1.80 or less, in which C/Nb is a ratio of the content of C to the content of Nb. Accordingly, a balance between the content of C and the content of Nb can be optimized. As a result, surplus and shortage of C and Nb are less likely to occur, an appropriate amount of NbC can be precipitated, the sintering rate can be slowed down, and a decrease in a sintering density can be inhibited. Accordingly, the dense layer formed on the surface of the sintered metal body can be thickened.
In the sintering metal powder, C+Nb is preferably 0.20 mass % or more and 1.50 mass % or less, in which C+Nb is a sum of the content of C and the content of Nb. Accordingly, NbC can be appropriately precipitated, the sintering rate can be slowed down, and the decrease in the sintering density can be inhibited. As a result, it is possible to obtain a sintering metal powder from which a sintered metal body having a high density as a whole and having a sufficiently thick dense layer can be produced.
In the sintering metal powder, a content of Si is preferably 0.20 mass % or more and 0.80 mass % or less, and a content of Cr is preferably 12.0 mass % or more and 30.0 mass % or less.
Such a sintering metal powder has favorable sintering performance. In addition, a sintered metal body having excellent corrosion resistance and heat resistance can be obtained.
The sintered metal body 1 according to the embodiment contains a composition of ferritic stainless steel, C having a content of 0.02 mass % or more and 1.00 mass % or less, Nb having a content of 0.05 mass % or more and 1.50 mass % or less, and impurities.
According to such a configuration, the sintered metal body 1 having the thick dense layer CL is obtained. According to such a sintered metal body 1, since there is a low probability that the whole dense layer CL is lost even when a surface is polished, favorable mirror finish performance is obtained on the polished surface.
In the sintered metal body 1, C/Nb is preferably 0.40 or more and 1.80 or less, in which C/Nb is a ratio of the content of C to the content of Nb.
Accordingly, a balance between the content of C and the content of Nb can be optimized. As a result, the sintered metal body 1 contains an appropriate amount of NbC, has the thicker dense layer CL, and has a high sintering density.
The sintered metal body 1 preferably has the dense layer CL which has an average thickness of 150 μm or more on the surface.
The sintered metal body 1 having the dense layer CL which has such an average thickness has a low probability of losing the whole dense layer CL due to polishing. In addition, such a sintered metal body 1 is unlikely to lose the dense layer CL even when subjected to polishing a plurality of times. Therefore, the sintered metal body 1 is preferably used for, for example, various types of metal products requiring an aesthetic appearance, and various types of metal products expected to be repaired by polishing once or a plurality of times.
In the sintered metal body 1, a pitting potential is preferably 200 mV or more. The pitting potential is a potential at which a current density measured according to method B among methods of pitting potential measurement for stainless steel defined in JIS G 0577:2014 is 100 μA/cm2.
By satisfying such a pitting potential, corrosion of the sintered metal body 1 is sufficiently inhibited, and particularly favorable corrosion resistance is obtained.
Although the sintering metal powder and the sintered metal body according to the present disclosure have been described above based on preferred embodiments, the present disclosure is not limited thereto. For example, the sintered metal body may be a sintered body produced by using a metal powder different from the above-described sintering metal powder.
Next, Examples of the present disclosure will be described.
First, a kneaded material (composition) containing a sintering metal powder produced by a water atomization method and a binder was prepared. As the sintering metal powder, a powder having a composition shown in Table 1 and an average particle diameter of 8.0 μm was used. In addition, a mixture of polypropylene and wax was used as the binder. A mixing ratio of the binder in the kneaded material was 10 mass %.
Next, the kneaded material was molded by an injection molding machine to obtain a molded body. A shape of the molded body was a rectangular parallelepiped body having a length of 15 mm, a width of 15 mm, and a height of 3 mm. Next, the molded body was subjected to a debindering treatment to obtain a debindered body. The debindering treatment was a treatment of heating the molded body at 450° C.. for 2 hours in a nitrogen atmosphere.
Next, the debindered body was subjected to a sintering treatment to obtain a sintered metal body. The sintering treatment was a treatment of heating the debindered body at 1250° C.. for 3 hours in an argon atmosphere.
Sintered metal bodies were obtained in the same manner as in Sample No. 1 except that the composition of the sintering metal powder was changed as shown in Table 1.
In Table 1, those corresponding to the present disclosure are indicated by “Example”, and those not corresponding to the present disclosure are indicated by “Comparative Example”.
In each of Examples and Comparative Examples, the composition of the obtained sintered metal body was the same as that of the sintering metal powder except for the content of C and the content of O. In addition, in each sintered metal body in Examples, the content of C was lower than that of the sintering metal powder by about 0.10 mass % to 0.30 mass %. In addition, in each sintered metal body in Examples, the content of 0 was within a range of 0.10 mass % to 0.50 mass %.
Each sintered metal body obtained in Examples and Comparative Examples was cut. Then, a cut surface was polished, and the polished surface was observed with an electron microscope.
Next, a thickness of a dense layer was calculated based on an observation image. Calculation results are shown in Table 2.
On the other hand,
For each sintered metal body obtained in Examples and Comparative Examples, a relative density was calculated according to a method defined in JIS Z 2501:2000. Calculation results are shown in Table 2.
A surface of each sintered metal body obtained in Examples and Comparative Examples was subjected to a buffing and polishing treatment. In the buffing and polishing treatment, polishing was performed such that a range of a thickness of 50 μm from the surface is removed. Next, the polished surface was visually observed. Mirror finish performance of the polished surface was evaluated in view of the following evaluation criteria. Evaluation results are shown in Table 2.
For each sintered metal body obtained in Examples and Comparative Examples, a pitting potential was measured according to method B among methods of pitting potential measurement for stainless steel defined in JIS G 0577:2014. The measured pitting potential was evaluated according to the following evaluation criteria. Evaluation results are shown in Table 2.
In Table 2, those corresponding to the present disclosure are indicated by “Example”, and those not corresponding to the present disclosure are indicated by “Comparative Example”.
As shown in Table 2, it was observed that each sintered metal body in Examples had a sufficiently thick dense layer and a sufficiently high relative density as compared to each sintered metal body in Comparative Examples. In addition, each sintered metal body in Examples had better mirror finish performance and corrosion resistance than each sintered metal body in Comparative Examples.
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| Number | Date | Country | Kind |
|---|---|---|---|
| 2022-158122 | Sep 2022 | JP | national |
