EP3305439B1

EP3305439B1 - Mixed powder for iron-based powder metallurgy, method for producing same, and sintered body produced using same

Info

Publication number: EP3305439B1
Application number: EP16799745.1A
Authority: EP
Inventors: Nobuaki Akagi
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 2015-05-27
Filing date: 2016-04-27
Publication date: 2021-05-26
Anticipated expiration: 2036-04-27
Also published as: JP6480266B2; KR102113996B1; US20180104739A1; KR20180008730A; JP2016222945A; WO2016190037A1; CN107614159A; EP3305439A1; EP3305439A4

Description

The present invention relates to a mixed powder for iron-based powder metallurgy and a sintered body prepared by using the same, and more particularly to a mixed powder for iron-based powder metallurgy containing calcium sulfate anhydrite II at a specific ratio and a sintered body prepared by using the same.
Powder metallurgy is widely used as a method for industrial production of various kinds of mechanical parts. A procedure for the iron-based powder metallurgy is such that, first, a mixed powder is prepared by mixing an iron-based powder with a powder for alloy such as a copper (Cu) powder or a nickel (Ni) powder, a graphite powder, and a lubricant. Next, this mixed powder is put into a mold to perform press-molding, and the resultant is sintered to prepare a sintered body. Finally, this sintered body is subjected to cutting such as drilling process or turning on a lathe, so as to be prepared into a mechanical part having a desired shape.
An ideal for powder metallurgy is such that the sintered body is processed to be made usable as a mechanical part without performing cutting on the sintered body. However, the aforesaid sintering may generate non-uniform contraction of the raw material powder. In recent years, the dimension precision required in the mechanical parts is increasing, and the shapes of the parts are becoming more complex. For this reason, it is becoming essential to perform cutting on the sintered body. From such a technical background, machinability is imparted to the sintered body so that the sintered body can be smoothly processed.
There is a technique of adding a manganese sulfide (MnS) powder to the mixed powder as means for imparting the machinability. Addition of the manganese sulfide powder is effective for cutting at a comparatively low speed, such as drilling. However, addition of a manganese sulfide powder is not necessarily effective for cutting at a high speed that is performed in recent years, and raises problems such as generation of contamination on the sintered body and decrease in the mechanical strength.
Patent Literature 1 (Japanese Examined Patent Application Publication No. S52-16684 ) discloses a method of imparting machinability other than the aforesaid addition of manganese sulfide. Patent Literature 1 discloses a sintered steel in which 0.1 to 1.0% of calcium sulfide (CaS), 0.1 to 2% of carbon (C), and 0.5 to 5.0% of copper (Cu) are incorporated into an iron-based raw material powder obtained by allowing a needed amount of carbon and copper to be contained in an iron powder.
Incorporation of calcium sulfide into an iron-based raw material powder disclosed in Patent Literature 1 raises problems such as considerable decrease in the strength of the mechanical parts and unstable product quality caused by change with lapse of time of the mixed powder. Further, when the sintered steel disclosed in Patent Literature 1 is processed with use of a cutting tool, the chips are hardly fragmented finely. From this, the sintered steel disclosed in Patent Literature 1 can hardly be said to be excellent to a degree such as to satisfy the current demand for the chip controllability. Patent Literature 2 discloses an iron-based powder for powder metallurgy. Non-Patent Literature 1 discloses effects of metal sulfate on dimensional change
The present invention has been made in view of the aforementioned problems, and an object thereof is to provide a mixed powder for iron-based powder metallurgy capable of preparing a sintered body having a stable product quality and performance.

Patent Literature 1: Japanese Examined Patent Application Publication No. S52-16684 Patent Literature 2: JP 2014/080683 A
Non-Patent Literature 1: Adam et al., Advances in Powder Metallurgy & Particulate Materials, 1997, vol. 2, pp. 107 to 113

A mixed powder for iron-based powder metallurgy of the present invention comprises an iron-based powder at a weight ratio of 60 wt% or more, where wt% is relative to the total weight of the constituent components of the mixed powder for iron-based powder metallurgy other than the lubricants, a powder containing calcium sulfate anhydrite II such that a weight ratio of CaS after sintering is 0.01 wt% or more to 0.1 wt% or less, and one or more ternary oxides selected from the group consisting of Ca-Al-Si oxides and Ca-Mg-Si oxides, wherein the weight ratio of calcium sulfate anhydrite II in the powder containing calcium sulfate anhydrite is at least 70 wt%.
A method for producing a mixed powder for iron-based powder metallurgy of the present invention comprises:

preparing a powder containing calcium sulfate anhydrite II by heating a powder containing dihydrate gypsum or hemihydrate gypsum at a temperature of 300°C or higher to 900°C or lower; and
mixing the powder containing calcium sulfate anhydrite II with an iron-based powder and one or more ternary oxides selected from the group consisting of Ca-Al-Si oxides and Ca-Mg-Si oxides, wherein the mixed powder comprises an iron-based powder at a weight ratio of 60 wt% or more, where wt% is relative to the total weight of the constituent components of the mixed powder for iron-based powder metallurgy other than the lubricants, and the weight ratio of calcium sulfate anhydrite II in the powder containing calcium sulfate anhydrite II is at least 70 wt%.

FIG. 1 is an image showing one example of an outer appearance of chips having a good chip controllability.
FIG. 2 is an image showing one example of an outer appearance of chips without having a good chip controllability.
FIG. 3 is an observation image of a worn part of a tool rake face after a sintered body prepared in Example 26 was turned on a lathe with a cermet tip.
FIG. 4 is an observation image of a worn part of a tool rake face after a sintered body prepared in Example 30 was turned on a lathe with a cermet tip.
FIG. 5 is an observation image of a worn part of a tool rake face after a sintered body prepared in Example 32 was turned on a lathe with a cermet tip.
FIG. 6 is an observation image of a worn part of a tool rake face after a sintered body prepared in Example 33 was turned on a lathe with a cermet tip.
FIG. 7 is an observation image of a worn part of a tool rake face after a sintered body prepared in Example 34 was turned on a lathe with a cermet tip.
FIG. 8 is an observation image of a worn part of a tool rake face after a sintered body prepared in Reference Example 1 was turned on a lathe with a cermet tip.

In order to achieve the aforementioned object, the present inventor has made investigations on why the sintered body disclosed in Patent Literature 1 undergoes decrease in the product quality and performance with lapse of time. Then, the present inventors have found out that, when the sintered body contains calcium sulfide and hemihydrate gypsum (hereafter, these two components will be referred to as "CaS components"), the product quality and performance of the sintered body decreases. In other words, the present inventors have found out that, when the CaS components absorb moisture in ambient air, the CaS components are changed into calcium sulfate dihydrate (CaSO₄·2H₂O), or the CaS components are aggregated by a hardening reaction to form coarse grains of 63 µm or greater. It has been made clear that this lets the CaS components be non-uniformly dispersed in the mixed powder or in the sintered body to decrease the machinability of the sintered body, or lets the moisture adsorbed onto the CaS components be dilated during the sintering to decrease the strength of the sintered body.
The present inventor has completed the present invention shown below by further making eager studies on the crystal structure of calcium sulfate having a low moisture absorptivity based on the above findings.
According to the present invention, there can be provided a mixed powder for iron-based powder metallurgy capable of preparing a sintered body having a stable product quality and performance According to the invention, ternary oxides are added to enhance machinability of the sintered body.
Hereafter, a mixed powder for iron-based powder metallurgy according to the present invention and a method for producing the same will be specifically described.

A mixed powder for iron-based powder metallurgy of the present invention is a mixed powder obtained by mixing an iron-based powder with a powder containing calcium sulfate anhydrite II (which may hereafter be referred to also as "II type CaSO₄ powder"). Various kinds of additives such as binary oxides, powders for alloy, graphite powders, lubricants, and binders may be appropriately added into this mixed powder. In addition to these, the mixed powder may contain a slight amount of inevitable impurities during the process of producing the mixed powder for iron-based powder metallurgy. The mixed powder for iron-based powder metallurgy of the present invention may be put into a mold or the like to be molded and thereafter sintered to give a sintered body. The sintered body thus prepared may be subjected to cutting process, so as to be made usable in various kinds of mechanical parts. The use and the production method of this sintered body will be described later.

<Iron-based powder>

The iron-based powder is a main constituent component constituting the mixed powder for iron-based powder metallurgy, and is contained at a weight ratio of 60 wt% or more relative to the total amount of the mixed powder for iron-based powder metallurgy. Here, wt% of the iron-based powder as used herein refers to the occupied ratio relative to the total weight of the constituent components of the mixed powder for iron-based powder metallurgy other than the lubricants. Hereafter, it is assumed that, when wt% of each component is defined, the definition refers to the occupied weight ratio relative to the total weight of the constituent components of the mixed powder for iron-based powder metallurgy other than the lubricants.
The above iron-based powder usable in the present invention may be, for example, a pure iron powder such as an atomized iron powder or a reduced iron powder, a partially diffused alloyed steel powder, a completely alloyed steel powder, a hybrid steel powder obtained by partially diffusing alloy components into a completely alloyed steel powder, or the like. A volume-average particle size of the iron-based powder is preferably 50 µm or more, more preferably 70 µm or more. When the volume-average particle size of the iron-based powder is 50 µm or more, the handling property is excellent. Further, the volume-average particle size of the iron-based powder is preferably 200 µm or less, more preferably 100 µm or less. When the volume-average particle size of the iron-based powder is 200 µm or less, a precision shape can be readily molded, and also a sufficient strength can be obtained.

<II-type CaSO₄ powder>

The mixed powder for iron-based powder metallurgy of the present invention is characterized by comprising a powder containing calcium sulfate anhydrite II (II type CaSO₄ powder). The present invention overturns a conventional technical common sense (for example, of Patent Literature 1) that mere addition of a component that becomes calcium sulfide (CaS) after sintering can enhance the machinability of the sintered body. In other words, dihydrate gypsum (CaSO₄·2H₂O), calcium sulfate anhydrite III (III type CaSO₄), hemihydrate gypsum (CaSO₄·1/2H₂O), and the like may in some cases absorb moisture with lapse of time, thereby decreasing the machinability of the sintered body. In contrast, calcium sulfate anhydrite II has low moisture absorptivity and does not absorb moisture in ambient air, so that the mass of calcium sulfate anhydrite II does not increase even when the calcium sulfate anhydrite II is stored for a certain period of time in a state of being contained in the mixed powder for iron-based powder metallurgy. Moreover, calcium sulfate anhydrite II can enhance the machinability of the sintered body by being changed into CaS after sintering. For this reason, a mixed powder for iron-based powder metallurgy containing II type CaSO₄ powder can enhance various performances of the sintered body stably as compared with dihydrate gypsum (CaSO₄·2H₂O), calcium sulfate anhydrite III (III type CaSO₄), and hemihydrate gypsum (CaSO₄·1/2H₂O).
The II type CaSO₄ powder contains calcium sulfate anhydrite II as a major component; however, the II type CaSO₄ powder may contain dihydrate gypsum (CaSO₄·2H₂O), calcium sulfate anhydrite III (III type CaSO₄), hemihydrate gypsum (CaSO₄·1/2H₂O), and the like. The more the ratio occupied by calcium sulfate anhydrite II in the II type CaSO₄ powder is, the more preferable it is. The weight ratio of calcium sulfate anhydrite II is 70 wt% or more, preferably 80 wt% or more, and it is particularly preferable that the II type CaSO₄ powder is made of calcium sulfate anhydrite II alone. Further, the surface of the II type CaSO₄ powder may be covered with a lubricant or a binder described later.
The mixed powder for iron-based powder metallurgy contains a II type CaSO₄ powder such that a weight ratio of CaS after sintering is 0.01 wt% or more to 0.1 wt% or less. The II type CaSO₄ powder is preferably such that a weight ratio of CaS after sintering is 0.02 wt% or more, more preferably such that a weight ratio of CaS after sintering is 0.03 wt% or more. A sintered body containing CaS at such a weight ratio is excellent particularly in machinability. The II type CaSO₄ powder is contained more preferably so that a weight ratio of CaS after sintering is 0.09 wt% or less, still more preferably so that a weight ratio of CaS after sintering is 0.08 wt% or less. Incorporation of CaS at such a weight ratio can enhance the strength of the sintered body.
The term "weight ratio of CaS after sintering" refers to the weight ratio occupied by CaS in the sintered body obtained by sintering the mixed powder for iron-based powder metallurgy. The weight ratio of CaS contained in the sintered body after sintering can be adjusted by the weight ratio of II type CaSO₄ powder contained before the sintering.
The weight ratio of CaS contained in the sintered body is calculated by collecting a sample piece through processing the sintered body with a drill or the like and converting the weight of Ca, which is obtained by performing quantitative analysis of the weight of Ca contained in the sample piece, into the weight of CaS. Such conversion is carried out by dividing with the atomic weight of Ca (40.078) and multiplying with the molecular weight of CaS (72.143). Little amount of Ca disappears by reacting during the sintering, so that the weight of Ca does not change between before and after the sintering, and Ca and S are bonded at a ratio of 1 : 1.
The volume-average particle size of the II type CaSO₄ powder is preferably 0.1 µm or more, more preferably 0.5 µm or more, and still more preferably 1 µm or more. Further, the volume-average particle size of the II type CaSO₄ powder is preferably 60 µm or less, more preferably 30 µm or less, and still more preferably 20 µm or less. A II type CaSO₄ powder having such a volume-average particle size can be obtained, for example, by heating hemihydrate gypsum to 350°C or higher and 900°C or lower, holding the heated hemihydrate gypsum for 1 hour or more to 10 hours or less, and crushing and classifying the resultant. According as the volume-average particle size of the II type CaSO₄ powder is smaller, machinability of the sintered body can be improved even if the amount of addition of the II type CaSO₄ powder is reduced to be smaller. The above volume-average particle size is a value of the particle size D₅₀ at an accumulated value of 50% in the particle size distribution obtained by using a laser diffraction particle size distribution measurement device (Microtrac "MODEL9320-X100" manufactured by Nikkiso Co., Ltd.).
When the volume-average particle size of the II type CaSO₄ powder is R (µm) and the weight ratio of CaS contained in the sintered body after the sintering is W (wt%), it is preferable that a lower limit of R^1/3/W is 15 or more, more preferably 20 or more, and still more preferably 25 or more. Further, an upper limit of R^1/3/W is preferably 400 or less, more preferably 340 or less, and still more preferably 270 or less. Such a definition is based on an experience of the present inventors that the relationship between the volume-average particle size, which is proportional to the cubic root of the volume ratio, and the weight ratio is correlated to various properties of the sintered body. When such a numerical value range is satisfied, a sintered body that is good in all of radial crushing strength, machinability, and chip controllability can be obtained.

Ternary oxides are added in order to improve the machinability when the sintered body is used for a long period of time in a cutting process. Addition of the ternary oxides in combination with addition of the II type CaSO₄ powder can considerably enhance the machinability of the sintered body. The ternary oxide means a composite oxide of three types of elements. Specifically, the ternary oxide is a Ca-Al-Si oxide, or a Ca-Mg-Si oxide. The Ca-Al-Si oxide may be, for example, 2CaO·Al₂O₃·SiO₂ or the like. The Ca-Mg-Si oxide may be, for example, 2CaO·MgO·2SiO₂ or the like. Among these, it is preferable to add 2CaO·Al₂O₃·SiO₂. The aforementioned 2CaO·Al₂O₃·SiO₂ reacts with TiO₂ contained in the cutting tool or in the coating formed on the cutting tool to form a protection coating film on the surface of the cutting tool, whereby the wear resistance of the cutting tool can be considerably increased.
A shape of the ternary oxide is not particularly limited; however, the ternary oxide preferably has a spherical shape or a crushed spherical shape, that is, a shape that is round as a whole.
A lower limit of the volume-average particle size of the ternary oxide is preferably 0.1 µm or more, more preferably 0.5 µm or more, and still more preferably 1 µm or more. There is a tendency such that, according as the volume-average particle size is smaller, the machinability of the sintered body can be improved by a smaller amount of addition. Further, an upper limit of the volume-average particle size of the ternary oxide is preferably 15 µm or less, more preferably 10 µm or less, and still more preferably 9 µm or less. When the volume-average particle size is too large, it is difficult to improve the machinability of the sintered body. The volume-average particle size of the ternary oxide is a value obtained by a measurement method similar to the above-described method for measuring the volume-average particle size of the II type CaSO₄ powder.
A lower limit of the content of the ternary oxide is preferably 0.01 wt% or more, more preferably 0.03 wt% or more, and still more preferably 0.05 wt% or more. Further, an upper limit of the content of the ternary oxide is preferably 0.25 wt% or less, more preferably 0.2 wt% or less, and still more preferably 0.15 wt% or less. When the ternary oxide is contained at such a weight ratio, a sintered body can be obtained that has an excellent machinability even in a cutting process of a long period of time while suppressing the costs. Use of the ternary oxides in combination with II type CaSO₄ powder can improve the machinability in a cutting process of a long period of time even when the amount of addition of the ternary oxide is small.
The weight ratio of the ternary oxides and CaS after the sintering is preferably 1 : 9 to 9 : 1, more preferably 3 : 7 to 9 : 1, and still more preferably 4 : 6 to 7 : 3. When the two components are contained at such a weight ratio, the machinability of the sintered body can be considerably improved.

Binary oxides may be added in order to improve the machinability at an initial stage of cutting when the sintered body is used in a cutting process. The binary oxide means a composite oxide of two types of elements. Specifically, the binary oxide is preferably a composite oxide of two types of elements selected from the group consisting of Ca, Mg, Al, Si, Co, Ni, Ti, Mn, Fe, and Zn, and is more preferably a Ca-Al oxide, a Ca-Si oxide, or the like. The Ca-Al oxide may be, for example, CaO·Al₂O₃, 12CaO·7Al₂O₃, or the like. The Ca-Si oxide may be, for example, 2CaO·SiO₂ or the like.
The shape and the volume-average particle size of the binary oxide as well as the method of measurement and the weight ratio thereof are preferably similar to those of the ternary oxide described above.

The mixed powder for iron-based powder metallurgy of the present invention preferably contains both of binary oxides and ternary oxides in a sum weight of 0.02 wt% or more to 0.3 wt% or less. The sum weight of the binary oxides and ternary oxides is preferably 0.05 wt% or more, more preferably 0.1 wt% or more. In view of costs, the weight ratio of the binary oxides and ternary oxides is preferably as small as possible. Further, the sum weight of the binary oxides and ternary oxides is preferably 0.25 wt% or less, more preferably 0.2 wt% or less. When the sum weight of the oxides is 0.25 wt% or less, the radial crushing strength of the sintered body can be sufficiently ensured.
The weight ratio of the binary oxides and CaS after the sintering is preferably 1 : 9 to 9 : 1, more preferably 3 : 6 to 9 : 1, and still more preferably 4 : 6 to 7 : 3. When the two components are contained at such a weight ratio, a sintered body having an excellent machinability at an initial stage of cutting can be prepared.

A powder for alloy is added for the purpose of promoting bonding between the iron-based powders and enhancing the strength of the sintered body after the sintering. Such a powder for alloy is contained preferably at a ratio of 0.1 wt% or more to 10 wt% or less relative to the whole of the mixed powder for iron-based powder metallurgy. When the ratio is 0.1 wt% or more, the strength of the sintered body can be enhanced. Further, when the ratio is 10 wt% or less, the dimension precision of the sintered body at the time of sintering can be ensured.
The powder for alloy may be, for example, a non-ferrous metal power such as copper (Cu) powder, nickel (Ni) powder, Mo powder, Cr powder, V powder, Si powder, or Mn powder, a copper suboxide powder, or the like. These may be used either alone as one kind or in combination of two or more kinds.

A lubricant is added into the mixed powder for iron-based powder metallurgy so that the molded body obtained by compressing the mixed powder for iron-based powder metallurgy in a mold can be readily taken out from the mold. In other words, when a lubricant is added into the mixed powder for iron-based powder metallurgy, the withdrawing pressure at the time of taking the molded body out from the mold can be reduced, so that cracking of the molded body and damage of the mold can be prevented. The lubricant may be added into the mixed powder for iron-based powder metallurgy or may be applied on the surface of the mold. When the lubricant is added into the mixed powder for iron-based powder metallurgy, the lubricant is contained preferably at a ratio of 0.01 mass% or more to 1.5 mass% or less, more preferably at a ratio of 0.1 mass% or more to 1.2 mass% or less, and still more preferably at a ratio of 0.2 mass% or more to 1.0 mass% or less, relative to the weight of the mixed powder for iron-based powder metallurgy. When the content of the lubricant is 0.01 mass% or more, the effect of reducing the withdrawing pressure of the mold can be readily obtained. When the content of the lubricant is 1.5 mass% or less, a sintered body having a high density can be readily obtained, and a sintered body having a high strength can be obtained.
The lubricant that can be put to use may be one or more selected from the group consisting of metal soap (lithium stearate, calcium stearate, zinc stearate, or the like), stearamide, fatty acid amide, amide wax, hydrocarbon-based wax, and cross-linked alkyl (meth)acrylate resin. Among these, it is preferable to use an amide-based lubricant from the viewpoint of having a good performance of allowing the powder for alloy, graphite powder, or the like to adhere onto the iron-based powder surface and being capable of readily reducing the segregation of the iron-based mixed powder.

A binder is added for the purpose of allowing the powder for alloy, graphite powder, or the like to adhere onto the iron-based powder surface. The binder that is put to use may be a butene-based polymer, a methacrylate-based polymer, or the like. As the butene-based polymer, it is preferable to use a 1-butene homopolymer made of butene alone or a copolymer of butene and alkene. The alkene is preferably a lower alkene, and is preferably ethylene or propylene. As the methacrylate-based polymer, it is possible to use one or more selected from the group consisting of methyl methacrylate, ethyl methacrylate, butyl methacrylate, cyclohexyl methacrylate, ethylhexyl methacrylate, lauryl methacrylate, methyl acrylate, and ethyl acrylate.
The binder is contained preferably at a ratio of 0.01 mass% or more to 0.5 mass% or less, more preferably at a ratio of 0.05 mass% or more to 0.4 mass% or less, and still more preferably at a ratio of 0.1 mass% or more to 0.3 mass% or less, relative to the weight of the mixed powder for iron-based powder metallurgy.

In preparing the mixed powder for iron-based powder metallurgy of the present invention, a II type CaSO₄ powder contained in the mixed powder for iron-based powder metallurgy is prepared first. The II type CaSO₄ powder is preferably obtained by heating a hemihydrate gypsum or dihydrate gypsum having a volume-average particle size of 0.1 µm or more to 60 µm or less, to a temperature of 300°C or higher to 900°C or lower. The volume-average particle size of the hemihydrate gypsum or dihydrate gypsum that is put to use is preferably equivalent to or slightly smaller than the volume-average particle size of the II type CaSO₄ powder in consideration of aggregation at the time of heating. A lower limit of the heating temperature is 300°C or higher, preferably 350°C or higher, more preferably 400°C or higher. Further, an upper limit of the heating temperature is 900°C or lower, preferably 800°C or lower, more preferably 700°C or lower, and still more preferably 500°C or lower. When the heating temperature is 900°C or lower, it is possible to obtain a II type CaSO₄ powder having a particle size of 100 µm or less, which is general as a powder to be mixed into the iron-based powder. In particular, when the heating temperature is 700°C or lower, aggregation of the hemihydrate gypsum or dihydrate gypsum is less likely to occur, so that the II type CaSO₄ powder can be obtained while maintaining the volume-average particle size of the hemihydrate gypsum or dihydrate gypsum. When the heating temperature is high, a strong and firm aggregation occurs, so that it is preferable to perform a grinding step. When the heating temperature is 300°C or higher, moisture of the hemihydrate gypsum or dihydrate gypsum can be dehydrated to form the II type CaSO₄ powder. When the heating temperature is low, it is not preferable because calcium sulfate anhydrite III may be formed instead of calcium sulfate anhydrite II.
The heating time is preferably such that the time for dehydrating the hemihydrate gypsum or dihydrate gypsum into the II type calcium sulfate can be ensured, and is preferably one hour or more to eight hours or less. The higher the heating temperature is, the shorter the heating time can be made. When the heating time is short, part of the hemihydrate gypsum may remain as it is without being changed to II type calcium sulfate, or may change into calcium sulfate anhydrite III. For this reason, the heating time is preferably two hours or more, more preferably three hours or more.
The mixed powder for iron-based powder metallurgy of the present invention can be prepared by mixing the iron-based powder with the II type CaSO₄ powder prepared in the above with use of, for example, a mechanical agitation mixer. In addition to these powders, one or more ternary oxides selected from the group consisting of Ca-Al-Si oxides and Ca-Mg-Si oxides are added. Moreover, various kinds of additives such as a powder for alloy, a graphite powder, a lubricant, a binary oxide, and a binder may be suitably added. The mechanical agitation mixer may be, for example, a high-speed mixer, a Nauta Mixer, a V-type mixer, a double-cone blender, or the like. The order of mixing these powders is not particularly limited. The mixing temperature is not particularly limited; however, the mixing temperature is preferably 150°C or lower in view of suppressing oxidation of the iron-based powder in the mixing step.

After the mixed powder for iron-based powder metallurgy prepared in the above is put into a mold, a pressure of 300 MPa or higher to 1200 MPa or lower is applied to produce a pressed-powder molded body. The molding temperature during this time is preferably 25°C or higher to 150°C or lower.
The pressed-powder molded body prepared in the above is sintered by an ordinary sintering method to obtain a sintered body. The sintering conditions may be a non-oxidizing atmosphere or a reducing atmosphere. The above pressed-powder molded body is preferably sintered at a temperature of 1000°C or higher to 1300°C or lower for 5 minutes or more to 60 minutes or less in an atmosphere such as a nitrogen atmosphere, a mixed atmosphere of nitrogen and hydrogen, or a hydrocarbon atmosphere.

The sintered body thus prepared can be used as a mechanical part of an automobile, an agricultural instrument, a power tool, a home electrical appliance, or the like by being processed with various kinds of tools such as a cutting tool in accordance with the needs. The cutting tool for processing the sintered body may be, for example, a drill, an end mill, a cutting tool for milling, a cutting tool for turning on a lathe, a reamer, a tap, or the like.
According to the mixed powder for iron-based powder metallurgy of the above-described embodiment, a sintered body having a stable product quality and performance can be prepared. The calcium sulfate anhydrite II contained in the mixed powder for iron-based powder metallurgy of the above embodiment has low moisture absorptivity and does not absorb moisture in ambient air, so that the mass of a powder containing calcium sulfate anhydrite II does not increase even when the powder is stored for a certain period of time in ambient air. For this reason, various performances of the sintered body can be stably enhanced by using a powder containing calcium sulfate anhydrite II (II type CaSO₄ powder) instead of using calcium sulfide and hemihydrate gypsum, as a component that is turned into CaS by sintering.
In the above embodiment, since the II type CaSO₄ powder has a volume-average particle size of 0.1 µm or more to 60 µm or less, the machinability of the sintered body can be enhanced.
When the volume-average particle size of the II type CaSO₄ powder is R µm and the weight ratio of CaS contained in the sintered body after the sintering is W wt%, it is satisfied that R^1/3/W is 15 or more to 400 or less, so that a sintered body that is good in all of radial crushing strength, machinability, and chip controllability can be obtained.
The mixed powder for iron-based powder metallurgy of the above-described embodiment further contains one or more ternary oxides selected from the group consisting of Ca-Al-Si oxides and Ca-Mg-Si oxides, so that the machinability in a cutting process for a long period of time can be improved.
In the mixed powder for iron-based powder metallurgy of the above-described embodiment, the weight ratio of the ternary oxides and CaS after the sintering is 3 : 7 to 9 : 1, so that the machinability in a cutting process for a long period of time can be improved.

Examples

Hereafter, the present invention will be described in further detail by way of Examples; however, the present invention is not limited to these.

(Reference Example 1)

First, a commercially available powder of hemihydrate gypsum was classified with a sieve into -63/+45 µm (volume-average particle size of 54 µm). The classified hemihydrate gypsum was heated at 350°C for five hours in an ambient air heating furnace to obtain an calcium sulfate anhydrite II powder (II type CaSO₄ powder). This II type CaSO₄ powder was classified with a sieve into -63/+45 µm (volume-average particle size of 54 µm). The yield of the obtained II type CaSO₄ powder was 100%. This yield is a value of percentage relative to the weight of the II type CaSO₄ powder after the heating and represents the weight obtained by subtracting the weight of the II type CaSO₄ powder that was removed by the classification, from the weight of the II type CaSO₄ powder after the heating.
Next, a pure iron powder (trade name: ATOMEL 300M (manufactured by Kobe Steel, Ltd.)) was mixed with 2 wt% of copper powder (trade name: CuATW-250 (manufactured by Fukuda Metal Foil & Powder Co., Ltd.)), 0.8 wt% of graphite powder (trade name: CPB (manufactured by Nippon Graphite Industries Co., Ltd.)), 0.75 wt% of an amide-based lubricant (ACRAWAX C (manufactured by Lonza Ltd.)), and the II type CaSO₄ powder prepared in the above, so as to prepare a mixed powder for iron-based powder metallurgy. The graphite powder was added at an amount such that the amount of carbon after the sintering would be 0.75 wt%. The II type CaSO₄ powder was added at an amount such that the weight of CaS after the sintering would be 0.5 wt%.
With use of the above mixed powder for iron-based powder metallurgy, two types of sintered bodies were prepared. One was a sintered body prepared by using the mixed powder for iron-based powder metallurgy which was in a state immediately after the preparation (which will be hereafter referred to as "sintered body immediately after"), and the other one was a sintered body prepared by using the mixed powder for iron-based powder metallurgy that had been stored in ambient air for ten days after the preparation (which will be hereafter referred to as "sintered body after 10 days").
A procedure of producing the sintered body immediately after is as follows. First, the mixed powder for iron-based powder metallurgy which was in a state of immediately after the preparation was put into a mold, and a test piece was molded so as to have a ring shape with an outer diameter of 64 mm, an inner diameter of 24 mm, and a thickness of 20 mm and to have a molding density of 7.00 g/cm³. Next, this test piece having a ring shape was sintered at 1130°C for 30 minutes in a 10 vol% H₂-N₂ atmosphere, so as to prepare a sintered body. On the other hand, the sintered body after 10 days was prepared in the same manner as the sintered body immediately after except that the mixed powder for iron-based powder metallurgy was left to stand in ambient air for ten days after the preparation, and thereafter put into a mold.

(Reference Examples 2 to 8)

In each of Reference Examples 2 to 8, a sintered body was prepared in the same manner as in Reference Example 1 except that the temperature of heating the hemihydrate gypsum powder was changed as shown in the section of "heating treatment temperature" in Table 1.

(Comparative Examples 1 to 3)

In each of Comparative Examples 2 to 3, a sintered body was prepared in the same manner as in Reference Example 1 except that the calcium sulfate anhydrite II was changed to a material shown in the section of "CaS component" in Table 1. In Comparative Example 1, a sintered body was prepared in the same manner as in Reference Example 1 except that the calcium sulfate anhydrite II was not added.

[Table 1]

Experiment number	Reference Example 1	Reference Example 2	Reference Example 3	Reference Example 4	Reference Example 5	Reference Example 6	Reference Example 7	Reference Example 8	Comparative Example 1	Comparative Example 2	Comparative Example 3
Heating temperature (°C)	350	400	500	600	700	800	900	300	-	-	-
CaS component	II type CaSO₄							II, III type CaSO₄	None	CaS	Hemihydrate gypsum
Molded body density (g/cm³)	7.00/7.00	7.00/7.00	7.00/7.00	7.00/7.00	7.00/7.00	7.00/7.00	7.00/7.00	7.00/7.00	7.00/7.00	7.00/7.00	7.00/7.00
Sintered body density (g/cm³)	6.82/6.80	6.81/6.81	6.82/6.82	6.82/6.81	6.82/6.82	6.82/6.82	6.82/6.81	6.81/6.77	6.94/6.94	6.83/6.78	6.80/6.76
Radial crushing strength (MPa)	779/783	776/778	785/779	787/788	790/792	789/791	792/788	780/730	915/920	781/714	775/708
Tool wear amount (µm)	45/46	47/41	51/53	52/54	67/61	71/69	76/72	46/64	253/248	57/144	44/160
Yield (%)	100	99	99	97	85	73	65	100	-	-	-

In Table 1, the evaluation results of molded body density, sintered body density, radial crushing strength, and tool wear amount were given in a form of "sintered body immediately after / sintered body after 10 days". Such notation means that the value on the left side of the slash mark is the evaluation result of the sintered body immediately after, whereas the value on the right side of the slash mark is the evaluation result of the sintered body after 10 days.
The molded body density and the sintered body density of the sintered body immediately after and the sintered body after 10 days of each Reference Example and each Comparative Example were values as determined by making measurements in accordance with Japan Powder Metallurgy Association Standard (JPMA M 01). Further, the radial crushing strength was a value as determined by making measurements on each sintered body of each Reference Example and each Comparative Example in accordance with JIS Z 2507-2000. The higher the radial crushing strength is, the less likely the sintered body is broken, so that the sintered body has a higher strength.
The sintered body prepared in each Reference Example and each Comparative Example was turned on a lathe for 1150 m by using a cermet tip (ISO type number: SNGN120408 non-breaker) under conditions with a circumferential speed of 160 m/min, a cutting rate of 0.5 mm/pass, and a feed rate of 0.1 mm/rev, and with a dry type. The tool wear amount (µm) of the cutting tool after the sintered body was turned on the lathe was measured with a tool microscope. The results thereof are shown in the section of "tool wear amount" in Table 1. The smaller the value of the tool wear amount is, the more excellent the machinability of the sintered body is.
From the results of each Reference Example and each Comparative Example shown in Table 1, it has been found out that various properties (sintered body density, radial crushing strength, and tool wear amount) of the sintered body immediately after and the sintered body after 10 days are almost equivalent when the II type CaSO₄ powder is contained as the CaS component as in each Reference Example. On the other hand, in Comparative Examples 2 and 3, CaS single or hemihydrate gypsum was contained as the CaS component, so that various properties of the sintered body after 10 days were considerably deteriorated as compared with those of the sintered body immediately after.
The reason why the product quality and performance of the sintered body after 10 days were considerably deteriorated in Comparative Examples 2 and 3 seems to be that the CaS or hemihydrate gypsum in the mixed powder for iron-based powder metallurgy absorbed moisture during the period of time in which the mixed powder for iron-based powder metallurgy was left to stand for ten days. In other words, this seems to be due to the fact that, in Comparative Examples 2 and 3, the CaS single or hemihydrate gypsum in the mixed powder for iron-based powder metallurgy absorbed moisture during the storage in ambient air for 10 days, so that the density of the sintered body decreased, or the radial crushing strength decreased. In Comparative Example 1, the CaS component was not contained, so that the tool wear amount was considerably high both in the sintered body immediately after and in the sintered body after 10 days, and the machinability of the sintered body was considerably low.
Also, the degree of deterioration in various performances of the sintered body after 10 days of Reference Example 8 from the sintered body immediately after of Reference Example 8 is greater than that of the Reference Examples 1 to 7. This seems to be due to the fact that, because the temperature of heating the hemihydrate gypsum of Reference Example 8 was lower than that of Reference Examples 1 to 7, part of the hemihydrate gypsum was changed into III type calcium sulfate or remained, as it was, as the hemihydrate gypsum instead of being changed into II type calcium sulfate, these components exhibited the moisture absorptivity. However, the stability of various performances of the sintered body obtained in Reference Example 8 is outstandingly excellent as compared with those of Comparative Examples 1 to 3. For this reason, it has been made clear that the effect of enhancing the stability of the sintered body can be obtained even if the whole of the hemihydrate gypsum is not turned into II type calcium sulfate, as shown in Reference Example 8.
When attention is paid to the "yield" of the Reference Examples 1 to 7 of Table 1, there is a tendency such that, the higher the temperature of heating the hemihydrate gypsum is, the lower the yield is. This seems to be because, according as the heating temperature is raised, the II type calcium sulfate is aggregated to form a large granular substance, and this large granular substance is removed by classification. Accordingly, it has been made clear that, in order to obtain a power made of II type calcium sulfate at a high yield, the temperature of heating the hemihydrate gypsum is preferably set to be 350°C or higher to 600°C or lower.
From the results shown in Table 1, it has been made clear that, when a II type CaSO₄ powder is contained as the CaS component, various properties (sintered body density, radial crushing strength, and tool wear amount) of the sintered body immediately after and the sintered body after 10 days are almost equivalent, and the product quality and performance of the sintered body is stable, thereby showing the effect of the present invention.

(Reference Examples 9 to 29)

A sintered body was prepared in the same manner as in Reference Example 1 except that the volume-average particle size of the II type CaSO₄ powder and the weight ratio of CaS after the sintering were changed as shown in the sections of "volume-average particle size" and "CaS weight ratio" in Table 2, and each evaluation item was evaluated by a method similar to that of Reference Example 1. The results are shown in Table 2. Adjustment of the volume-average particle size of the II type CaSO₄ powder used in each Example was made by performing various grinding and classifying treatments on the heated II type CaSO₄ powder.
Here, in Reference Examples 9 to 29 as well, two types of sintered bodies, that is, a sintered body immediately after and a sintered body after 10 days, were prepared in the same manner as in Reference Example 1, and the properties of each of the two types were evaluated; however, the two measurement values were the same as each other or of a slight difference such that the difference could be ignored in all of the evaluation items, so that only one measurement value is shown in Table 2. From the results shown in Table 2, it has been made clear that the sintered body prepared by using the mixed powder for iron-based powder metallurgy of Reference Examples 9 to 29 has a stable product quality and performance, thereby showing the effect of the present invention.
The "chip controllability" in Table 2 is a result obtained by evaluating the outer appearance of the chips, which are generated by turning the sintered body on the lathe with use of the cermet tip, in accordance with the following evaluation criterion.

(Evaluation criterion of chip controllability)

very good: The number of spring-like windings (number of curls) is one or less (for example, FIG. 1).
good: The number of curls is within a range from one to three.
poor: The number of curls exceeds three (for example, FIG. 2).

As shown in FIG. 1, when the chips are finely fragmented, the frequency of cleaning the chip hopper of the cutting machine can be suppressed to be low. On the other hand, when the chips are extended long in a coil form as shown in FIG. 2, the chips are entangled with each other in a complex manner within the chip hopper, so that the labor of cleaning may become cumbersome, or the frequency of cleaning the chip hopper may increase, thereby leading to lower production efficiency. When this occurs, automatic operation for a long period of time can not be carried out even if the tool wear amount can be reduced. This does not lead to power saving or increase in efficiency.
From the results shown in Table 2, it has been made clear that a sintered body being excellent in all of radial crushing strength, tool wear amount, and chip controllability can be prepared when R^1/3/W is 20 or more to 340 or less. On the other hand, it has been confirmed that, when R^1/3/W is less than 20, the chip controllability tends to decrease, whereas when R^1/3/W exceeds 340, the radial crushing strength tends to be high, and the tool wear amount tends to increase considerably.

(Examples 30 to 34 and Reference Examples 35 and 36)

In Examples 30 to 34, a sintered body was prepared in the same manner as in Reference Example 26 except that a part of the II type CaSO₄ powder was changed to 2CaO·Al2O₃·SiO₂ or 2CaO·MgO·2SiO₂, as shown in Table 3. In Reference Examples 35 and 36, a sintered body was prepared in the same manner as in Reference Example 26 except that the whole amount of the II type CaSO₄ powder was changed to 2CaO·Al₂O₃·SiO₂ or 2CaO·MgO·2SiO₂. With respect to the 2CaO·Al₂O₃·SiO₂ or 2CaO·MgO·2SiO₂, those having a volume-average particle size of 2 µm were used. Furthermore, with respect to the II type CaSO₄ powder, one having a volume-average particle size of 18.4 µm was used.

Each evaluation item was evaluated by a method similar to that of Reference Example 26 on the sintered body of each Example and each Comparative Example prepared in this manner. The results are shown in Table 3. In Examples 30 to 34 as well, two types of sintered bodies, that is, a sintered body immediately after and a sintered body after 10 days, were prepared, and the properties of each of the two types were evaluated; however, the two measurement values were the same as each other or of a slight difference such that the difference could be ignored in all of the evaluation items, so that only one measurement value is shown in Table 3. Therefore, it has been made clear that the sintered body prepared by using the mixed powder for iron-based powder metallurgy of Examples 30 to 34 has a stable product quality and performance, thereby showing the effect of the present invention.

[Table 3]

Experiment number	Examples						Reference Examples
Experiment number	26*	30	31	32	33	34	35	36
CaS weight ratio (II type CaSO₄)	0.1	0.05	0.05	0.01	0.03	0.07	-	-
2CaO·Al₂O₃·SiO₂	-	0.05	-	0.09	0.07	0.03	0.1
2CaO·MgO·2SiO₂		-	0.05	-	-	-	-	0.1
Radial crushing strength (MPa)	801	846	835	871	866	844	907	889
Tool wear amount (µm)	51	38	48	47	43.2	46.4	58.5	69.6
Chip controllability	very good	very good	very good	very good	very good	very good	very good	very good

* indicates Reference Example

From the results shown in Table 3, it has been made clear that the tool wear amount can be further reduced by replacing a part of the II type CaSO₄ powder with ternary oxides. In particular, as shown by the results of Examples 32 to 34, it has been made clear that the tool wear amount can be considerably reduced when the weight ratio of the ternary oxides and CaS after the sintering is 3 : 7 to 9 : 1.
The reason why the tool wear amount can be reduced in such a manner seems to be that, by combined use of the II type CaSO₄ powder and the ternary oxides, interaction of the two occurs.
The reason why it is considered so is that the mode of wear of the tool rake face and the component of the worn part were different between the case in which the II type CaSO₄ powder and the ternary oxides were used in combination and the case in which the ternary oxides were used alone. FIGS. 3 to 8 show observation images of a worn part of a tool rake face after the sintered bodies prepared in Reference Example 26 and Examples 30, 32 to 34, and Reference Example 35, respectively, were turned on a lathe with a cermet tip. The observation images were obtained with an optical microscope. Referring to FIGS. 4 to 7, in the case in which the II type CaSO₄ powder and the ternary oxides were used in combination (Examples 30, and 32 to 34), the adhesion of iron was reduced, and no groove-like wear was observed. In contrast, in the case in which only the ternary oxides were added without addition of the II type CaSO₄ powder (Reference Example 35), a groove-like wear was formed, and adhesion of iron was observed, as shown in FIG. 8. Further, in the worn part of Examples 30, and 32 to 34, the ternary oxide components were detected over the whole worn surface. In contrast, in the worn part of Reference Example 35, the ternary oxides were detected only in a part of the half-moon-shaped worn part. Here, in the case in which only the II type CaSO₄ powder was added without addition of the ternary oxides (Reference Example 26), the area of the half-moon-shaped worn part of the tool rake face was smaller (i.e., received with a smaller area of the tool) than that of Examples 30, and 32 to 34, and partial adhesion of iron (Fe) was large. When such an iron adherent substance adheres to and drops off from the tool repeatedly, the wear of the cutting tool is liable to proceed, or the surface of the workpiece material may become non-smooth.
From the above results, it has been made clear that the machinability of the sintered body is furthermore excellent when the II type CaSO₄ powder and the ternary oxides are used in combination as in Examples 30 to 34.

Claims

A mixed powder for iron-based powder metallurgy, which comprises an iron-based powder at a weight ratio of 60 wt% or more, where wt% is relative to the total weight of the constituent components of the mixed powder for iron-based powder metallurgy other than the lubricants, a powder containing calcium sulfate anhydrite II such that a weight ratio of CaS after sintering is 0.01 wt% or more to 0.1 wt% or less, and one or more ternary oxides selected from the group consisting of Ca-Al-Si oxides and Ca-Mg-Si oxides, wherein the weight ratio of calcium sulfate anhydrite II in the powder containing calcium sulfate anhydrite is at least 70 wt%.
The mixed powder for iron-based powder metallurgy according to claim 1, wherein the powder containing calcium sulfate anhydrite II has a volume-average particle size of 0.1 µm or more to 60 µm or less, wherein the volume-average particle size is a value of the particle size D₅₀ at an accumulated value of 50% in the particle size distribution obtained by using a laser diffraction particle size distribution measurement device.
The mixed powder for iron-based powder metallurgy according to claim 1 or 2, wherein a weight ratio of the ternary oxides and CaS after the sintering is 3 : 7 to 9 : 1.
The mixed powder for iron-based powder metallurgy according to claim 1, wherein, when the powder containing calcium sulfate anhydrite II has a volume-average particle size of R µm and the weight ratio of CaS contained in a sintered body after the sintering is W wt%, it is satisfied that R^1/3/W is 15 or more to 400 or less.
The mixed powder for iron-based powder metallurgy according to claim 1, wherein the powder containing calcium sulfate anhydrite II is covered with a lubricant or a binder.
A method for producing a mixed powder for iron-based powder metallurgy, comprising:
preparing a powder containing calcium sulfate anhydrite II by heating a powder containing dihydrate gypsum or hemihydrate gypsum at a temperature of 300°C or higher to 900°C or lower; and

mixing the powder containing calcium sulfate anhydrite II with an iron-based powder and one or more ternary oxides selected from the group consisting of Ca-Al-Si oxides and Ca-Mg-Si oxides, wherein

the mixed powder comprises an iron-based powder at a weight ratio of 60 wt% or more where wt% is relative to the total weight of the constituent components of the mixed powder for iron-based powder metallurgy other than the lubricants, and the weight ratio of calcium sulfate anhydrite II in the powder containing calcium sulfate anhydrite II is at least 70 wt%.