Alloy steel powders, sintered bodies and method

Abstract: Alloy steel powders capable of obtaining high strength in a sintered state and having excellent compacting compressibility and methods of manufacturing a sintered body. The alloy steel powder comprises, by wt%, about 0.5 - 2% of Cr, not greater than about 0.08% of Mn, about 0.1 - 0.6% of Mo, about 0.05 - 0.5% of V, not greater than about 0.015 of S, not greater than about 0.2% of O, and the balance being Fe and incidental impurities. The alloy steel powder is compacted and sintered at a temperature of about 1100 - 1300°C and then cooled at a cooling rate no higher than about 1°C/s in a temperature range of from about 800°C to 400°C. The alloy steel powder can contain Nb and/or Ti and one or more of Co, W and B. Additionally, Ni powder and/or Cu powder may be adhered and dispersed onto the surface of the alloy steel powder.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a method of manufacturing sintered bodies. Description of the Related Art When iron parts requiring high strength are manufactured by conventional powder metallurgy, alloy steel powders are compacted with added strength-enhancing alloy element powders such as Ni, Cu, Mo, Cr and the like. Alternatively, this is done using alloy steel powders made by adding such strength-enhancing alloy elements to molten steel, sintering these alloy steel powders, then carburizing and nitriding and thereafter quenching and tempering the resulting alloy steel powders. Further repeating compacting and sintering of the alloy steel powders, after the first sintering, may be practiced to obtain high strength. It is inevitable, however, that the repetition of the heat treatment and compacting steps increases manufacturing cost. Further, repetition of heat treatment reduces dimensional accuracy of the resulting sintered body. For example, Cr-Mn alloy steel powders capable of obtaining high strength and exhibiting excellent hardenability are examples of sintered and heat-treated materials whose strength is improved by the addition of strengthening elements (such as Cr) with molten steel (Japanese Patent Publication No. 58(1983)-10962). However, Cr and Mn lower compressibility when powder particles are hardened and compacted, thus shortening the life of a mold. Additional drawbacks include cost increases caused by heat treatments such as quenching, tempering and the like in the manufacturing of steel powders and low dimensional accuracy from the repetition of heat treatments. Through extensive study, we have discovered remarkable steel powders which can achieve high strength and excellent compressibility after a single sintering operation (omitting the above-described heat treatment). The inventors have proposed Japanese Patent Application Laid-Open No. Hei 4(1992)-165002 and Japanese Patent Application Laid-Open No. Hei 5(1993)-287452 based on these discoveries. Japanese Patent Application Laid-Open No. Hei 4(1992)-165002 increases the strength of a sintered body by adding Nb and V to Cr alloy powders and utilizing a carbide and nitride precipitation mechanism such that the content of Mn is reduced. Since the powders contain only 0.005 - 0.08 wt% of V, however, the strengthening effect of the carbides and nitrides of V is lessened. Further, since a large amount of Mo (0.5 - 4.5 wt%) is used to improve the strength of the sintered body, coarse upper bainite is produced causing the strength of the resulting sintered body to be lower than that of a heat-treated body. Japanese Patent Application Laid-Open No. 5(1993)-287452 improves strength and fatigue strength by reducing the number of sites of fracture caused by oxide and the like. This is accomplished by further reducing the contents of Mn, P, S in conventional Cr alloy steel powders and limiting the cooling rate after sintering, thereby creating a fine pearlite structure in the sintered body. However, such alloy steel powders are sensitive to the cooling rate after sintering such that the strength of the sintered body is greatly dispersed depending upon the cooling rate. Thus, it is difficult for users to handle these alloy steel powders. SUMMARY OF THE INVENTION An object of this invention is to obtain high strength sintered bodies without heat treating and by sintering only once. Through zealous study, we have discovered remarkable alloy steel powders possessing excellent compressibility as well as sintered bodies made from the alloy steel powders that are substantially unaffected by the cooling rate after sintering. As a result, high strength can be stably obtained even when the sintered bodies are used in the sintered state. The invention is defined in claim 1. Preferred embodiments are disclosed in claims 2-11. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a graph showing the relationship between the cooling rate and the tensile strength of a sintered body after sintering; Fig. 2 is a graph showing the relationship between the sintering temperature and the tensile strength of a sintered body; and Fig. 3 is a graph showing the relationship between the cooling rate after sintering and the tensile strength of a sintered body. DETAILED DESCRIPTION OF THE INVENTION This invention will first be described by classifying the components of the alloy steel powders and the sintering conditions. (1) Components Cr increases strength through solution hardening. To obtain this effect, Cr must constitute not less than 0.5 wt%. However, if it constitutes more than 2 wt%, it decreases the compressibility of steel powders due to the solution hardening of Cr. Thus, Cr content is set to 0.5 - 2 wt%. A preferable lower Cr content limit is 0.6 wt% from the viewpoint of improving strength, and a preferable upper content limit is 1.2 wt% from the viewpoint of improving compressibility. Mo improves the strength of steel by solution hardening and precipitation hardening of Mo carbide, and the like. When Mo content is less than 0.1 wt%, its effect is small. Further, when Mo content exceeds 0.6 wt%, upper bainite is liable to be produced because Mo greatly delays pearlite transformation during cooling after sintering, thus lowering strength. Therefore, Mo content is set to 0.1 - 0.6 wt%. A preferable lower Mo content limit is 0.15 wt% from the viewpoint of increasing strength, and a preferable upper limit thereof is 0.4 wt% from the viewpoint of easily producing pearlite. V improves strength through the precipitation hardening of V carbide and nitride. When the V content is less than 0.005 wt%, however, the effect is small. Further, when the V content exceeds 0.5 wt%, strength is lowered from the increased size of the V carbide and nitride precipitates. Thus, the V content is set to 0.05 wt% - 0.5 wt%. In this range, grain sizes are reduced by a pining effect from the V carbides and nitrides so that the hardenability is lowered. Therefore, even if V is added in this range, a base structure of coarse upper bainite is not produced. V content is preferably 0.1 wt% - 0.4 wt%. As shown in Fig. 1, when the cooling rate after sintering exceeds 0.6°C/sec, steel powders of 1 wt% Cr and 0.3 wt% Mo (Japanese Patent Application Laid-Open No. Hei 4 (1994)-165002) which have no added V form an upper bainite structure having little strength. Fig. 1 also shows that such steel powders can be formed into a fine pearlite structure by the addition of 0.3 wt% V even if the cooling rate is 0.6°C/sec or higher, thus securing high strength sintered bodies. Mn improves the strength of a heat-treated material by improving its hardenability. However, when Mn content exceeds 0.08 wt%, oxide is produced on the surface of alloy steel powders such that compressibility is lowered and hardenability is increased beyond the required level. Hence, a coarse upper bainite structure is formed and strength is lowered. Mn content is preferably not greater than 0.06 wt% to improve compressibility. Mn content can be reduced by, for example, increasing the amount of oxygen to be blown into molten steel such that the slag exhibits a high degree of oxidation in the steel making process. S content is set to an amount not greater than 0.015 wt%. A consequence of the Mn content being only 0.08 wt% or less is a reduced production of MnS and an increased solid solution S. When S content exceeds about 0.015 wt%, the amount of solid solution S increases and strength at grain boundaries is lowered. Thus, S content is preferably not greater than 0.01 wt% to improve strength. Reducing O content is another feature of this invention. When O content exceeds 0.2 wt%, oxides are formed with Cr and V which reduce strength and compressibility. O content is preferably limited to not greater than 0.2 wt% and more preferably to not greater than 0.15 wt%. O content can be decreased by reducing pressure to about 10 -2 Torr. Although this invention is fundamentally arranged as described above, an enhanced effect can be obtained through the addition of the following components. Nb and Ti may be added because strength can be improved by the precipitation hardening of carbides and nitrides of Nb and/or Ti. When the content of Nb and Ti is each less than 0.01 wt%, their effect is small. Further, when the content of either of them exceeds 0.08 wt%, the carbide and nitride precipitates of Nb and/or Ti are coarsened, thus lowering strength. Therefore, the content for each of Nb and Ti is 0.01 - 0.08 wt%. Since both Nb and Ti produce carbide and nitride in this range, amounts of solid solution Nb and Ti are reduced and hardenability cannot be improved. Thus, even if Nb and/or Ti are added in this range, coarse upper bainite is not produced. A content for each of Nb and Ti is preferably 0.01 wt% - 0.04 wt% to improve strength. Co, W, B may be added because Co and W improve strength through solution hardening and B improves strength by strengthening grain boundaries. To obtain this effect, the content for each of Co and W is preferably not less than 0.1 wt%, and the content of B is preferably not less than 0.001 wt%. When Co and/or W are contained in an amount exceeding 1 wt%, and B is contained in an amount exceeding 0.01 wt%, compressibility of steel powders is lowered. Thus, it is preferable to contain Co and W each in the range of 0.1 - 1 wt%, and to contain B in the range of 0.001 - 0.01 wt%. Further, additions of Co, W and/or B in these ranges does not cause the production of coarse upper bainite. The content for each of Co and W is more preferably 0.3 wt% - 0.8 wt%, and the content of B is more preferably 0.003 wt% - 0.008 wt%. Ni and/or Cu may be added to increase strength. Diffusion bonding Ni or Cu powder does not reduce compressibility and is therefore the preferred method of adding these alloys. When alloys are added by diffusion bonding, a composite structure of fine pearlite and martensite is formed in the sintered body such that strength is improved. Additive amounts of these alloys are limited to Ni: 0.5 - 5 wt% and Cu: 0.5 - 3 wt%. When the amount added of each element is less than the respective lower limit amount, the strengthening effects are not observed. Further, when each element exceeds the respective upper limit amount, compressibility abruptly decreases. Concerning incidental impurities such as P, C, N, Si, Al and the like, it is preferable to limit P to an amount not greater than 0.015 wt%, C to an amount not greater than 0.02 wt%, N to an amount not greater than 0.004 wt%, Si to an amount not greater than 0.1 wt%, and Al to an amount not greater than 0.01 wt%. This is because that when P, C, N, Si, Al are present in amounts exceeding their upper limits, they greatly reduce compressibility. It is preferable to limit P to an amount not greater than 0.01 wt%, C to an amount not greater than 0.01 wt%, N to an amount not greater than 0.002 wt%, Si to an amount not greater than 0.05 wt%, and Al to an amount not greater than 0.005 wt%. (2) Sintering Conditions When the above alloy steel powders are sintered, graphite powder is added in the range of 0.3 - 1.2 wt% and about 1 wt% of zinc stearate powder is added as a lubricant, and compacted. Graphite powders are added in the amount of 0.3- 1.2 wt% because C improves steel strength when contained in sintered bodies in an amount not less than 0.3 wt%. When C is contained in an amount exceeding 1.2 wt%, however, cementite precipitates and lowers the strength and toughness of the sintered bodies. When the sintering temperature is less than 1100°C, sintering does not proceed well, whereas when the sintering temperature exceeds 1300°C, production costs increase. Thus, the sintering temperature is set to about 1100 - 1300°C. If the cooling rate exceeds about 1°C/s after sintering , a coarse bainite structure is produced which reduces strength. A fine pearlite structure can be obtained by setting the cooling rate to 1°C/s or less in the temperature range of from 800°C to 400°C so that the strength of the sintered bodies can be improved. The cooling rate is preferably set to 0.2 - 0.8°C/s. Examples The following examples, directed to specific forms of the invention, are merely illustrative and are not intended to limit the scope of the invention defined in the appended claims. Example 1 Alloy steel powders having chemical components shown in Table 1 were made through the processes of water atomization, vacuum reduction, and pulverization/classification. The resultant alloy steel powders were added and blended with 1 wt% of zinc stearate and compacted at 6 t/cm 2 and subjected to measurements of green density. Further, the alloy steel powders were blended with 0.8 wt% of graphite powders and 1 wt% of zinc stearate powders as a lubricant and then compacted to green compacts having a green density of 7.0 g/cm 3 . These green compacts were sintered in a N 2 -10% H 2 atmosphere at 1250°C for 60 minutes and thereafter cooled at a cooling rate of 0.4°C/s in a temperature range of from 800°C to 400°C. Tensile strengths of the resulting sintered bodies were measured. Table 1 shows the results of the tensile strength and green density measurements. When specimens Nos. 1, 2 and 3 are compared with specimens Nos. 21 and 22, it is observed that when the content of Cr exceeds 2%, compressibility decreases. When specimens Nos. 4, 5 and 6 are compared with specimens Nos. 24 and 25, it is observed that when the content of Mo is outside of the range of this invention, strength decreases. When specimens Nos. 7, 8 and 9 are compared with specimens Nos. 26 and 27, it is observed that when the content of V is outside of the range of this invention, strength decreases. When specimens Nos. 10 and 11 are compared with a specimen No. 29, it is observed that when the content of Mn exceeds 0.08%, green density and strength decrease. When specimens Nos. 12 and 13 are compared with a specimens No. 31, it is observed that when the content of P exceeds 0.015%, green density decreases. When specimens Nos. 14 and 15 are compared with a specimen No. 32, it is observed that when the content of S exceeds 0.015%, green density and strength decrease. When specimens Nos. 16 and 17 are compared with a specimen No. 33, it is observed that when the content of Nb exceeds 0.08%, strength decreases. When specimens Nos. 18 and 19 are compared with a specimen No. 34, it is observed that when the content of Ti exceeds 0.08%, strength decreases. Further, since contents of Cr and P of specimen No. 23 are outside of the ranges of this invention, the observed green density is very low. Specimen No. 28 shows a composition disclosed in Japanese Patent Application Laid-Open No. Hei 4(1994)-165002. Since the contents of Mo and V are outside of the ranges of this invention, the observed strength is very low. Specimen No. 30 shows a composition disclosed in Japanese Patent Publication No. Sho 58(1983)-10962. Since contents of Cr, Mn and Mo are outside of the ranges of this invention, the observed strength is very low. As is apparent from Table 1, utilizing the specified chemical components within the composition ranges of this invention enables the remarkable combination of high compressibility and high strength in the same sintered body. Example 2 Alloy steel powders having chemical components shown in Table 2 were made through the processes of water atomization, vacuum reduction, and pulverization/classification. The resultant alloy steel powders were added and blended with 1 wt% of zinc stearate as a lubricant, compacted at 6 t/cm 2 and subjected to a measurement of green density. Further, the alloy steel powders were blended with 0.9 wt% of graphite powders and 1 wt% of zinc stearate powder as a lubricant and then compacted to green compacts having a green density of 7.0 g/cm 3 . These green compacts were sintered in a N 2 -10% H 2 atmosphere at 1250°C for 60 minutes and thereafter cooled at a cooling rate of 0.4°C/s in a temperature range of from 800°C to 400°C. Tensile strengths of the resulting sintered bodies were measured. Table 2 shows the results of the tensile strength and green density measurements. It is apparent from Table 2 that when any one of the O, C, N, Si and Al quantities exceeds the upper limit of this invention, compressibility and strength decrease. Example 3 Alloy steel powders having chemical components shown in Table 3 were subjected to measurement of green density and tensile strength under the same conditions as those of Example 2. Table 3 shows the results of the measurements. Although strength of the alloy powder steels is increased by the addition of Co, W or B, it is apparent that if they are added in amounts exceeding the upper limits of the invention, compressibility and strength decrease. Example 4 Carbonyl nickel powders and copper powders were mixed with alloy steel powder No. 8 shown Table 1 in a predetermined ratio and annealed at 875°C for 60 minutes in hydrogen gas so that they were partially prealloyed onto the alloy steel powders, thus producing the alloy steel powders of the compositions shown Table 4. The resulting alloy steel powders were subjected to measurement of green density and tensile strength under the same conditions as those of Example 2 except that in this case the amount of graphite powder added was 0.6 wt%. Table 4 shows the results of the measurements. Although strength of the alloy powder steels is increased by the addition of Ni or Cu, it is apparent from Table 4 that if they are added in amounts exceeding the upper limits of the invention, strength and compressibility decrease. Example 5 Alloy steel powder No. 2 shown in Table 1 was added and mixed with 1 wt% graphite powder and 1 wt% zinc stearate and compacted to green compacts having densities of 7.0 g/cm 3 . These green compacts were sintered in a N 2 -75% H 2 atmosphere at temperatures ranging from 1000 - 1300°C for 30 minutes and then cooled at a cooling rate of 0.3°C/s. The tensile strengths of the resulting sintered bodies were measured, then the tensile strengths were plotted against the respective sintering temperatures to produce the graph in Fig. 2. It is observed in Fig. 2 that high strength is obtained at sintering temperatures not lower than about 1100°C. Example 6 The Alloy steel powder No. 8 shown in Table 1 was added and mixed with 0.9 wt% graphite powder and 1 wt% zinc stearate and compacted to green compacts having a density of 6.9 g/cm 3 . These green compacts were sintered in a N 2 -10% H 2 atmosphere at 1250°C for 60 minutes and then cooled at various cooling rates. The tensile strengths of the resulting sintered bodies were measured, then the tensile strengths were plotted against the respective cooling speeds to produce the graph in Fig. 3. It is observed in Fig. 3 that high strength is obtained at cooling rates not higher than about 1°C/s. The alloy steel powders of the invention and the method of manufacturing sintered bodies from the alloy steel powders of the invention enables the production of low cost iron sintered bodies having high strength and excellent compressibility during compacting without conducting post-sintering heat treatments. Additionally, special limits on the cooling rate after sintering are unnecessary, even if the sintered bodies are used in the sintered state. This enables the use of conventional sintering furnaces unequipped with cooling control devices. Moreover, quenching and tempering equipment are not required, further reducing production costs. Also, since compacting and sintering processes need not be repeated after the first sintering process, the invention conserves both manpower and wear on production equipment.

Claims: A method of manufacturing a sintered body having high strength from alloy steel powder comprising the steps of mixing alloy steel powder comprising, by wt%, 0.5- 2% of Cr, ≤0.08% of Mn, 0.1 - 0.6% of Mo, 0.05 - 0-5% of V, ≤0.015% of S, ≤0.2% of 0, optionally 0.01 to 0.08% of Nb; ≤0.08% of Ti, 0.1 to 1.0% of Co, ≤1.0% of W, 0.001 to 0.01% of B, with the balance being Fe and incidental impurities, the surface of the alloy steel powder optionally having dispersed thereon and adhered thereto one or more component powders selected from the group consisting of (a) 0.5 - 5% of Ni and (b) 0.5- 3% of Cu, with a lubricant and 0.3 - 1.2wt% of graphite powder, compacting the mixture, and sintering the compacted mixture, wherein the sintering is performed at a temperature of 1100 - 1300°C, and the sintered mixture is cooled at a cooling rate not higher than 1°C/s over a temperature range of from 800°C to 400°C, wherein the sintered body has a structure substantially composed of fine pearlite. A method according to claim 1, wherein the content of Cr is 0.6 - 1.2 wt%. A method according to claim 1 or 2, wherein the content of Mo is 0.15 - 0.4wt%. A method according to claim 1, 2 or 3 wherein the content of V is 0.1 - 0.4 wt%. A method according to any preceding claim, wherein the content of Mn is not greater than 0.06 wt%. A method according to any preceding claim wherein the content of Nb is 0.01 - 0.04 wt%. A method according to any preceding claim wherein the content of Ti is 0.01 - 0.04 wt%. A method according to any preceding claim wherein the content of Co is 0.3 - 0.8wt%. A method according to any preceding claim wherein the content of W is 0.3 - 0.8wt%. A method according to any preceding claim wherein the content of B is 0.003 - 0.008wt%. A method according to any preceding claim wherein at least one of the elements selected from the group consisting of (a) P in an amount not greater than 0.015%, (b) C in an amount not greater than 0.02%, (c) N in an amount not greater than 0.004%, (d) Si in an amount not greater than 0.1%, and (e) Al in an amount not greater than 0.01% is present as incidental impurity.

Inventors: UENOSONO SATOSHI, UNAMI SHIGERU, UENOSONO, SATOSHI, C/O IRON & STEEL RESEARCH LAB., UNAMI, SHIGERU, C/O IRON & STEEL RESEARCH LAB., Uenosono, Satoshi, c/o Iron &amp; Steel Research Lab., Unami, Shigeru, c/o Iron &amp; Steel Research Lab.
Applicants: KAWASAKI STEEL CO, KAWASAKI STEEL CORPORATION
Application year: 1995
Main classification: C22C33
IPC codes: C22C33, C22C38