TIN-CONTAINING IRON POWDER AND PROCESS FOR ITS PRODUCTION

Abstract: Tin-containing iron powder comprising powder particles mainly composed of iron having formed on the surface thereof a tin-rich portion wherein at least part of the tin is in the form of a compound with iron and the content of total tin is 1 to 20 wt%. This tin-containing iron powder is produced by mixing a powdered tin compound capable of being decomposed by heat to produce metallic tin with a powder mainly composed of iron, and heating the mixture to 450 to 700°C in a reductive or non-oxidative atmosphere. This tin-containing iron powder can be sintered to yield a product having a high density and excellent magnetic properties.

Description: This invention relates to the use of a powder mixture for the production of, soft magnetic parts such as cores in electric apparatus. Electrical sheets, silicon steel sheets and the like have heretofore been widely used as soft magnetic parts such as cores in electric apparatus such as electric motors as is well known in the art. Recently, sintered magnetic materials formed by compacting and sintering iron powder have progressively replaced the electrical sheets and silicon steel sheets. These sintered materials have some advantages characteristic of powder metallurgy including an increased percent yield based on the stock, a low processing cost, and an increased degree of freedom in shape, but have the disadvantage that their magnetic characteristics are imperatively inferior to those of electrical sheets and silicon steel sheets due to residual voids in the sintered materials. To overcome the drawbacks of sintered iron base materials as mentioned above, attempts have been made to add a variety of additives. Among such additives, tin (Sn) forms a liquid phase at a relatively low temperature. If tin is added, a liquid phase is created during sintering and tin forms a solid solution with iron to allow α-phase iron to develop during sintering, resulting in an increased sinter density, reduced influence of voids, and promoted growth of α-phase crystals, and hence, the possibility of achieving excellent magnetic characteristics. If high density sinters are made by adding tin, then it is expectable to apply them to sintered mechanical parts requiring wear resistance and high strength. Known among processes for adding tin to sintered iron-base material is a process comprising mixing a tin powder with an iron powder, compacting the mixture and sintering it, as disclosed in 7P-A 48-102008. In this process, however, since tin is melted during sintering to penetrate between iron particulates to spread the interstices between them and depleted voids are left where tin particulates have occupied before melting, the sinter density is not sufficiently increased in practice, failing to provide satisfactory magnetic characteristics. In order to overcome such problems, it may be contemplated to use a powder iron alloy which has previously been alloyed. However, alloying with tin makes iron base powder harder to considerably deteriorate its compressibility to provide a reduced compact density although the development of tin-depleted voids is prevented. It is thus eventually difficult to provide a high sinter density. It may also be contemplated that if very finely divided tin is used as the metallic tin powder to be added to and blended with an iron powder, then depleted voids of a substantial size are not left even after melting of tin during sintering so that uniform sintering may take place to yield a coherent sinter. However, atomizing and triturating techniques normally employed in the preparation of tin powder are difficult to effectively produce such very finely divided tin. GB-A-549 449 describes a ferromagnetic poor material of an alloy of iron and tin, the tin content lying in the range of 2 to 20 % by weight of the alloy. The ferromagnetic powder can be obtained by reducing tin oxide and iron in a hydrogen atmosphere at a temperature of at least 1100°F (593°C), preferably 1400 to 1500°F (760 to 816°C), but for high frequency cores, however, an annealing operation is subsequently conducted. The annealing operation of the alloy is carried out at temperatures between 1550°F (about 845°C) and 400°F, the annealing process being started at 1550°C and the temperature being reduced over a period of at least 4 hours to 400°F. In embodiments for producing the powder for high frequency cores, production conditions are observed which include temperatures of at least 1500°F (816°C). Therefore the object of the present invention is to provide the use of a Sn-containing iron base sintering powder stock which may be converted by compacting and sintering into Sn-containing iron base sinters having high sinter density and high wear resistance by which soft magnetic parts such as cores in electric apparatus having improved magnetic properties and a low iron loss can be produced. This object is solved according to the invention by the use of a powder mixture comprising a tin-containing iron base powder, obtained by heat treatment at a temperature of 450 to 700° C, consisting of iron base particles each having tin-rich portions at the surface in which at least a part of the tin forms a compound with iron, the total tin content being in the range of 1 to 20 % by weight of the iron base powder, and a phosphor containing powder, the phosphor content of the powder mixture being such that the phosphor content of the sinter is in the range of 0,1 to 2 % by weight, for the production of soft magnetic parts such as cores in electric apparatus by compacting and sintering said powder mixture. It has been found that in preparing a sintering iron-tin base material, coherent sinters having improved magnetic characteristics can be produced using as a tin-providing sintering powder stock, a composite powder comprising iron particulates each having tin-rich portions formed on the surface in which at least a part of the tin forms a compound with iron. It has also been found that the above-mentioned composite powder is easily prepared by mining ground iron with a powder tin compound which is thermally decomposable into Sn, for example, tin oxides, and reducing the resultant mixture. It should be noted that the desired effect is achievable by controlling the content of tin in the composite powder to 1 to 20% by weight. Accordingly, the tin-containing powder which is used for the powder mixture according to the present invention is a tin-containing iron base powder having improved sintering property comprising iron base particulates each having tin-rich portions at the surface in which at least a part of the tin forms a compound with iron, wherein the total content of tin is in the range of 1 to 20% by weight of the powder. The composite powder as defined above is mixed with a phorphorus-containing powder as a phorphorus source, for example, iron-phosphorus alloy powder, red phosphorus powder or the like, or the composite powder may be mixed with a phosphorus-containing powder and an iron powder before the resultant mixture is compacted and sintered. It is possible to incorporate a predetermined amount of a lubricant before compacting. The powder mixture used according to the present invention is produced by mixing an iron base powder with at least one powder selected from the group consisting of tin oxide, tin chloride, tin oxalate, tin nitrate, tin sulfate, and tin sulfide powders in an amount of 1 to 20% by weight calculated as tin, and effecting heat treatment at a temperature of 450 to 700°C in a non-oxidizing or reducing atmosphere and by mixing this base powder mixture with a phosphorus containing powder wherein the phosphorus content of the powder mixture being such that the phosphorus content of the sinter is in the range of 0.1 to 2% by weight. Fig. 1 is the phase diagram of Fe-Sn system; Fig. 2 is a scanning electron photomicrograph of Fe-Sn composite particulate surface; Fig. 3 is a schematic view showing a portion of Fe-Sn composite particulate surface; Figs. 4A to 4C are photographs by an X-ray micro-analyzer of Fe-Sn composite particulate surface, Fig. 4A being a secondary electron image, Fig. 4B being an X-ray image of Sn character, and Fig. 4C being an X-ray image of Fe character; Fig. 5 is the phase diagram of Fe-P system; The used powder mixture according to the present invention is produced as follows: There is provided as a sintering powder stock for producing tin-containing iron base sinters, a composite iron base powder comprising iron base particulates each having tin-rich portions at the surface in which at least a part of the tin forms a compound with iron, wherein the total content of tin is in the range of 1 to 20% by weight of the powder. This composite powder is subject to sintering in admixture with iron powder and phosphorus-containing powder (for example, iron-phosphorus alloy powder). When the composite powder is used in sintering, tin which is finely distributed, rapidly diffuses into the interior of powder particulates (composite powder particulates themselves and iron powder particulates mixed therewith) even if tin is melted during sintering. Not only the behavior of tin to spread the interstices between particulates and the behavior or leaving large voids are precluded, but also alloying occurs fast and uniformly to facilitate the development of α-phase to promote sintering so that sinters having a high density and hence, improved mechanical and magnetic properties may be obtained. Moreover, the addition of tin allows crystal particles to grow larger, resulting in further improved magnetic properties. With tin contents of the composite powder of less than 1% by weight, the Fe-Sn phase diagram shown in Fig. 1 suggests that even when the composite powder alone is sintered, the development of α-phase does not occur at usual sintering temperatures in the range of 950 to 1300°C and thus the promotion of sintering is not fully accomplished. On the other hand, it is difficult to incorporate more than 20% by weight of tin into iron base powder particulates as tin-rich portions at the surface. In this case, tin agglomerates during sintering to give rise to behavior as occurring when a coarse tin powder is admixed, failing to effectively increase the density of sinters. The above-mentioned composite powder may desirably be prepared by mixing a powder having a major proportion of iron (to be referred to as iron-base matrix powder, hereinafter) with a tin compound powder having a particle size equal to or smaller than that of the iron-base matrix powder particulates, and heating the mixture at a temperature range of from 450 to 700°C in a reducing or non-oxidizing atmosphere to decompose the tin compound. The tin compounds used herein may be any desired one as long as it is decomposed by heating to generate tin, and special mention may be made of one or mixtures of more than one selected from tin oxide (SnO or SnO₂), tin hydroxide (Sn(OH)₂ or SnO₂.nH₂o), tin chloride (SnCl₂ or SnCl₄ with or without water of crystallization), tin oxalate (C₂O₄Sn) tin nitrate (Sn(NO₃)₂ or Sn(NO₃)₄ with or without water of crystallization), tin sulfate (SnSO₄), and tin sulfide (SnS or SnS₂). All these tin compounds are markedly more brittle than metallic tin so that they may be readily comminuted. Such a comminuted tin compound may be mixed with an iron-base matrix powder and heated in a reducing or non-oxidizing atmosphere to produce a composite powder consisting essentially of iron-base matrix powder particulates having tin-rich portions uniformly formed or distributed at the surface thereof. The iron-base matrix powder used herein is basically a powder having a major proportion of iron and desirably, substantially free of Sn. Examples of the iron-base matrix powder include atomized pure iron powders having an iron content of at least 99.0% and containing as impurity elements not more than 0.02% of carbon (C), not more than 0.10% of silicon (Si), not more than 0.15% or not more than 0.35% of manganese (Mn), not more than 0.020% of phosphorus (P), and not more than 0.020% of sulfur (S); reduced iron powders having an iron content of at least 98.5% by weight and containing as impurities not more than 0.05% of carbon, not more than 0.15% of silicon, not more than 0.40% of manganese, not more than 0.020% of phosphorus, and not more than 0.020% of sulfur; low alloy steel powders containing as an alloying constituent at least one selected from 1.3 to 1.6% of nickel (Ni), 0.2 to 0.6% of molybdenum (Mo), 0.4 to 0.7% of copper (Cu), and 0.9 to 1.2% of chromium (Cr) and the balance being substantially iron and concomitant impurities; and the like. At heat treating temperatures of lower than 450°C reduction does not fully proceed to leave the hard tin compound which would cause molds to be worn during compacting amd prevent the compact density from being fully increased, resulting in sinters of a relatively low density. On the other hand, at heat treating temperatures of higher than 700°C tin is extremely diffused and alloyed into the iron-base matrix powder to make the powder harder, resulting in a reduction in compact density, and hence, sinter density. For these reasons, the heat treating temperature is limited to the range of from 450 to 700°C. Now, it will be further described in detail how the tin-containing composite iron base powder is produced. At the outset, mixing an iron-base matrix powder with a tin compound results in a mixture in which the iron value is in macroscopic admixture with the tin value. In this case, the finer the tin compound powder, the more intimate the mixture becomes. However, because of further comminutation in the subsequent steps, relatively coase tin compound powder may be used as long as its particle size is equal to or smaller than that of the iron-base matrix powder. The thus obtained mixture is heated to a temperature of at least 450°C in reducing or non-oxidizing atmosphere to decompose the tin compound to yield metallic tin. Since the heating temperature is enough higher than the melting point of tin, the tin generated is instantaneously converted into molten tin which exhibits good wetting properties to the iron-base matrix powder and partially or entirely covers the surface of matrix powder particulates. A part of the molten tin reacts with iron to form an iron-tin compound in solid state at the surface of iron-base matrix powder particulates, forming Sn-rich portions on the powder particulate surface. Fig. 2 is a scanning electron photomicrograph of the surface of iron-tin composite powder particulates produced in this way, and Fig. 3 is a schematic view showing a portion of the particulate surface. In Fig. 3, reference numeral 1 designates ridges and recesses on the particulate surface, and minute precipitates 2 in the form of very fine white spots on the particulate surface consist essentially of iron-tin compounds. This is attested by the photographs by an X-ray microanalyzer and X-ray identification analysis of iron-tin composite powder particulates as shown in Figs. 4A to 4C. More specifically, Fig. 4A is a secondary electron image, Fig. 4B is the corresponding X-ray image of Sn character, and Fig. 4C is the corresponding X-ray image of Fe character. The X-ray analysis shows that the fine product on the particulate surface consists predominantly of iron-tin compounds (FeSn or Fe₃Sn₂, or FeSn₂ or the like) and metallic tin is locally identified. In the above-described process, the lower the heating temperature and the shorter the heating time, the reaction of tin with iron terminates in more incomplete state and depending on the extent of the reaction, metallic tin may sometimes remain on the surface of iron-base matrix powder particulates. Alternatively, the residual tin which is left without covering the surface of iron base matrix powder particulates (or iron-tin compounds resulting from the reaction of this residual tin with iron) may sometimes take the form of grains attached to the surface of iron-base matrix powder particulates. For improved powder quality, it is preferred to form or distribute tin-rich portions on the iron-base matrix powder particulate surface as uniformly as possible and for the segregated tin to be of iron-tin compounds. However, if the heating temperature is increased above 700°C merely for these purposes, then the tin segregated on the iron-base matrix powder particulate surface is readily diffused into the particulate interior and alloyed, resulting in hardened powder particulates. It is thus rather unavoidable that metallic tin partially remains. Depending on the particular element contained in the iron-base matrix powder, the tin rich portions can contain a third element other than iron and tin. The iron-tin composite powder is substantially improved in sinterability for the following two reasons. First, the fine distribution of tin on the surface of iron-base matrix powder particulates, even if the tin should be metallic tin, prevents large tin-depleted voids from being left during sintering, resulting in more coherent sinters. Secondly, at least a part of the tin value is present in the form of an iron-tin compound having a higher melting point so that the diffusion of tin into iron proceeds to some extent until the development of a liquid phase during sintering, precluding the phenomenon that large tin-depleted voids are left as a result of instant melting of tin-rich portions. The ultimately obtained sinters may desirably have a tin content in the range of 1 to 10% by weight. Tin contents of sinters of less than 1% by weight presuppose tin contents of the starting composite powder of less than 1% by weight, which is too low to achieve promoted sintering as described above. If the tin content of sinters exceeds 10% by weight, as seen from the Fe-Sn phase diagram shown in Fig. 1, a non-magnetic intermetallic compound phase (FeSn) precipitates upon colling after sintering, resulting in sinters with poor magnetic properties. The phosphorus content of sinters is in the range of from 0.1 to 2% by weight. If the phosphorus content is less than 0.1% by weight, as understood from the Fe-P system phase diagram shown in Fig. 5, α-phase does not develop at usual sintering temperatures of 950 to 1300°C, failing to obtain the effect of promoted sintering due to the addition of phosphorus. Of course, the co-existence of tin changes the quantity of phosphorus required to develop α-phase. With sufficiently small quantities of phosphorus to prevent the development of α-phase in iron-phosphorus system, it is estimated that the additive effect is also very slight for an iron-tin-phosphorus system. On the other hand, the addition of a powder to be a phosphorus source detracts from the compressibility of a mixed powder as is well known and extremely reduces compact density particularly at phosphorus contents in excess of 2% by weight, resulting in sinters with reduced sinter density and increased dimensional change before and after sintering, which leads to deteriorated dimensional accuracy of sinter. The Example of the present invention are presented below together with a comparative example. Example An atomized iron having a particle size of -80 mesh (- 177 µm) was mixed with an SnO powder having a particle size of -325 mesh (- 44 µm)in an amount of 4% by weight calculated as tin, and heated at 600°C for one hour in a stream of decomposed ammonia gas, preparing an iron-tin composite powder. To this iron-tin composite powder was added an iron-phosphorus alloy powder of -325 mesh (- 44 µm) (phosphorus content 16% by weight) as a phosphorus source in such an amount as to give a phosphorus content of 0.6% by weight based on the powder mixture. Further, 1% by weight of zinc stearate was added thereto as a lubricant. The mixture was then compacted. Compacting was carried out in the presence of 1% of zinc stearate and under a compression pressure of 7 t/cm² and sintering was carried out at 1150°C for 60 minutes in decomposed ammonia gas. The density and various magnetic properties of the sinter are also shown in Table 1. Comparative Example 1 According to the process described in JP-A-48-102 008, an atomized iron having a particle size of -80 mesh (- 177 µm) was mixed with 4 % by weight of a tin powder of -250 mesh (- 57,5 µm), 1 % by weight of zinc stearate was added as an additive, and the resulting powder was compacted and sintered to produce an iron-tin sinter. Compacting was carried out under a compression pressure of 7 t/cm² and sintering was carried out at 1150°C for 60 minutes in decomposed ammonia gas. The density and various magnetic properties of the sinter are also shown in Table 1. Comparative Example 2 An atomized iron having a particle size of -80 mesh (- 177 µm) was mixed with a tin powder of -250 mesh (- 57,5 µm) and an iron-phosphrus alloy powder of -325 mesh (- 44 µm) phosphorus content 16 % by weight) in such amounts as to give a tin content of 4 % by weight and a phosphorus content of 0,6 % by weight based on the powder mixture. Further 1 % by weight of zinc stearate was added thereto as a lubricant. The mixture was then compacted and sintered to produce an iron-tin-phosphorus sinter. Compacting was carried out under a compression pressure of 7 t/cm² and sintering was carried out at 1200°C for 60 minutes in decomposed ammonia gas. The density and various magnetic properties of the sinter are also shown in Table 1. As seen from Table 1, the sintered product made from the powder mixture used according to the present invention have a higher sinter density than the sintered products of the same compositions obtained in Comparative Examples 1 and 2 according to the prior art, and hence exhibit improved magnetic properties including high magnetic flux density, low coercive force, high permeability, and low iron loss. INDUSTRIAL APPLICABILITY As obvious from the foregoing description, the iron-tin composite powder according to the present invention has the outstanding benefit of making possible the practical manufacturing of tin-containing iron base sintered products or iron-tin sintered products having a high sinter density and particularly, improved magnetic properties. Then the composite powders according to the present invention are best suited as sintering stock materials intended for the production of soft magnetic parts useful as cores used in electric apparatus such as motors, mechanical parts requiring high strength and high wear resistance, and the like.

Claims: The use of a powder mixture comprising a tin-containing iron base powder, obtained by heat treatment at a temperature of 450 to 700° C, consisting of iron base particles each having tin-rich portions at the surface in which at least a part of the tin forms a compound with iron, the total tin content being in the range of 1 to 20 % by weight of the iron base powder, and a phosphor containing powder, the phosphor content of the powder mixture being such that the phosphor content of the sinter is in the range of 0,1 to 2 % by weight, for the production of soft magnetic parts such as cores in electric apparatus by compacting and sintering said powder mixture.

Inventors: TAKAJO, SHIGEAKI
Applicants: KAWASAKI STEEL CORPORATION
Application year: 1984
Main classification: B22F1
IPC codes: B22F1, H01F1, C22C33