Sintered component, gear for starter motor, and production process thereof

Abstract

A sintered component that exhibits: a hardness distribution in which the hardness varies continuously from a surface to an inner portion, with the hardness increasing from the surface to the inner region and decreasing from the region of maximum hardness to the inner region; a Vickers hardness value of 550 or more and 730 or less at the surface; the maximum hardness being present in a region between 150 and 300 µm from the surface; the maximum hardness having a Vickers hardness value of 600 or more, and a carbonized and hardened layer being formed in a surface layer, and a decarbonized layer having a lower carbon concentration than the carbonized and hardened layer being formed outside the carbonized and hardened layer, the decarbonized layer forming the surface.

Description

Technical field

[0001] The present invention relates to a sintered component, a gear for starter motors and a production method thereof, and relates in particular to a technique for obtaining a high-strength equivalent to a component obtained from a raw block material by forging. State of the art

[0002] Powder metallurgy is a technique in which a raw material consisting of a metallic powder is compacted to obtain a predetermined shape and size and then sintered by heating to a temperature that does not cause melting, whereby the powder particles bond strongly together and this has the advantage that a "near-net" shape can be formed, the technique is suitable for mass production, special materials can be produced that cannot be made from raw block material, or similar, and the technique can be applied to machine parts for automobiles and machine parts for various industries.

[0003] Sintered components generally have lower strength than raw block materials because gaps form between the powder particles when a raw powder is compacted, and these gaps remain as pores after sintering. For example, speed reduction starters are widely used as engine starting devices for automobiles. The speed reduction starter comprises a speed reduction device with an internal gearbox and a planetary gear set for reducing the engine's speed. It includes a gear connected to a drive shaft of the speed reduction device and coupled to a ring gear of the engine, thereby starting the engine.In the starter, sintered parts are used as internal reduction gear components, such as internal gears and planetary gears; however, the application of sintered parts to the gear has been delayed because sintered parts do not possess the necessary strength for the gear, which could be subjected to a load up to six times higher compared to an internal reduction gear component.

[0004] This problem was addressed by strengthening the matrix through the addition of a large quantity of alloying elements to a sintered component; that is, by producing steel of a higher quality than that made from raw ingot materials. However, there is a limit to the strength of the matrix through alloying, and the cost of raw materials increases with the price of alloying elements.

[0005] On the other hand, strengthening the matrix of the sintered component by reducing the number of pores or by sealing them was investigated. Liquid-phase sintering is a technique in which an element is added that generates a liquid phase during sintering, causing pores to be filled and disappear with the liquid phase; however, it is difficult to ensure dimensional accuracy, thus requiring post-sintering machining and reducing the advantage of powder metallurgy, which can produce a near-net shaped body.A powder forging process in which a raw material powder is heated and forged in a heated mold set, or a sinter forging process in which a raw material formed by compaction and sintering is hot forged by powder metallurgy, requires heating devices to heat the raw material powder and the mold set or sintered compact, which could increase production costs.

[0006] Under such circumstances, sintering and cold forging processes were investigated, in which a raw material shaped by compaction and sintering using powder metallurgy is cold forged, thereby obtaining a higher density (Patent Publications 1 and 2). A component produced by the sintering and cold forging process is often subjected to carbonization and quenching to obtain hardness and strength in the same way as mechanical parts manufactured from raw ingot materials. That is, if the hardness of a mechanical part is increased uniformly, its load-bearing capacity is reduced.For this reason, carbonization and quenching are widely used as a method to improve load-bearing capacity while keeping the hardness of the internal portion low to a certain degree, as well as to improve wear resistance and fatigue strength with respect to repeated surface pressure by hardening only the surface that rubs against a counter-component and is repeatedly subjected to surface pressure.

[0007] During carbonization and quenching, a heat-treated object is exposed to a carbonized gas atmosphere with a Cp (carbon potential, corresponding to a carbon concentration [mass%] in the atmospheric gas) higher than the carbon content of the matrix at a temperature approximately 100°C above the austenitic transformation temperature. This causes carbon to diffuse from the gas atmosphere into the heat-treated object, forming the required carbonized layer. The heat-treated object is then immersed in an oil or similar solvent and rapidly cooled and quenched, forming a hard matrix such as martensite or bainite. Finally, the heat-treated object is tempered at a temperature of approximately 200°C for an appropriate period to improve its strength.In such a general hardening process, the thickness of the carbonized and hardened layer with a Vickers hardness value of 550 or more can be achieved by extending the quenching time. In this case, the hardness distribution is such that the hardness of the outermost layer is highest and decreases gradually towards the inner layer. Patent publication 1 is the Japanese unexamined patent application with publication number 2003-253372. Patent publication 2 is the Japanese unexamined patent application with publication number 2001-513143. DESCRIPTION OF THE INVENTION PROBLEMS SOLVED BY THE INVENTION

[0008] Although the maximum density in sintering and cold forging approaches the true density with increasing forging pressure, the proportion of remaining pores is approximately 1 to 2% by volume at the pressures used in industrial production. While the volume of pores remaining on the surface is reduced by the pressure, they exist there with the same frequency as in the interior. The depth of the surface pores reaches approximately 70 μm. On the other hand, the surface roughness of a sintered body is limited to 1 to 2 μm, with the pores having sharp edges. If a sintered body that has been cold forged and has sharp-edged pores on its surface is quenched, its notch sensitivity increases, even though the surface layer is hardened.As a result, sharp-edged pores remaining on the surface of the cold-forged sintered body act as a notch, and stress is concentrated at the notch, reducing flexural strength and impact toughness.

[0009] To prevent such disadvantages, the surface of a material is machined before quenching and pores remaining on the surface layer are removed, or alternatively, the surface layer is subjected to plastic flow and remaining pores are filled and disappear, thereby rendering the pores harmless, which allows mechanical properties corresponding to the density of the forged body to be obtained.

[0010] However, production costs increase if a machining step or a plastic processing step is added after cold forging.

[0011] For this reason, it is an objective of the present invention to provide a sintered body and a production method thereof in which the detrimental effect of pores remaining on a surface of the sintered body can be avoided without removing pores by a machining step and a plastic processing step, thereby achieving a strength equivalent to raw block materials, and the sintered body is applicable to gears for starters. MEANS TO SOLVE THE PROBLEMS

[0012] The inventors conducted intensive research to increase the strength of a sintered body that is forged after sintering, and they discovered a process in which the effect equivalent to eliminating residual pores through machining can be easily achieved through the carbonization and quenching conditions of a forged body. That is, the inventors found that stress concentration at remaining pores is strongly influenced by the hardness of a surface, and that by reducing the surface hardness to a required level, stress concentration can be avoided. Therefore, even if pores remain on the surface, the effects can be prevented.The inventors also found that, although the hardness distribution is such that the hardness is greatest at the surface and gradually decreases towards an inner proportion in a conventional carbonization and quenching process, the hardness at the outermost surface can be reduced by adjusting the carbon potential to one atmosphere after the carbonization step.

[0013] The present invention is based on the knowledge described above and provides a sintered component having: a hardness distribution in which hardness varies continuously from a surface to an internal portion; a Vickers hardness value (hereinafter referred to as Hv) of 730 or less at the surface; wherein the maximum hardness of the portion exists in a region between 150 and 300 μm from the surface; wherein the maximum hardness has an Hv of 600 or more.

[0014] The present invention provides a production method for a sintered component, the method comprising: a mixing step for mixing a raw material powder; a compaction step for compacting the raw material powder and obtaining a compact; a sintering step for sintering the compact and obtaining a sintered body; a forging step for forging the sintered body and obtaining a forged body; a carbonization step for heating the forged body in a gas atmosphere in which the carbon concentration is 0.7 to 1.2 wt% at a temperature of 850 to 950°C; a decarbonization step for heating the forged body in a carbonized gas atmosphere in which the carbon concentration is 0.3 to 0.6 wt% at a temperature of 800 to 950°C after the carbonization step; and a quenching step for rapidly cooling the forged body after the decarbonization step.and a tempering step to heat the forging to a temperature of 150 to 280°C and cooling the forging to ambient temperature after the quenching step. Various methods could be used for rapid cooling in the quenching step, such as immersing a heated forging in water or oil, or blowing air or mist onto the forging, etc. The method of immersing a forging in oil is preferable. The forging step is preferably carried out by cold forging.

[0015] According to the sintered component of the present invention, since the surface hardness is reduced, detrimental effects caused by remaining pores on a surface after forging are eliminated, thereby achieving a strength equivalent to that of raw block materials and enabling the component to be used on gears for starters. In the production process for a sintered component, since the surface hardness can be reduced, a strength equivalent to that of raw block materials can be obtained without an additional machining or plastic deformation step, which is advantageous for industry. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Fig. Figure 1 shows a perspective view illustrating an embodiment of a gear produced in an embodiment of the present invention.

[0017] Fig. 2A and Fig. Figure 2B shows drawings that explain a size fraction for obtaining a hardness distribution in the embodiment of the present invention.

[0018] Fig. 3A and Fig. Figure 3B shows drawings illustrating a hardness distribution in a conventionally sintered gear that has undergone cold forging, carbonizing, and quenching.

[0019] Fig. 4A and Fig. Figure 4B shows drawings illustrating a hardness distribution in a sintered gear of the present invention which has been subjected to cold forging, carbonizing and quenching.

[0020] Fig. Figure 5 shows a diagram illustrating the variation in hardness of a proportion of a surface when a carbon concentration in an atmosphere is varied in a decarbonization step in an example.

[0021] Fig. Figure 6 shows a diagram illustrating the variation in hardness in a proportion of the surface when a heating temperature is varied in the decarbonization step of the example.

[0022] Fig. Figure 7 shows a diagram illustrating the variation in hardness in a proportion of a surface when a carbon concentration in an atmosphere is varied in a carbonization step of the example.

[0023] Fig. Figure 8 shows a diagram illustrating the variation in hardness of a proportion of a surface when a heating temperature is varied in the carbonation step of the example. BEST METHOD FOR IMPLEMENTING THE INVENTION

[0024] In the following, a cold-forged sintered component of an embodiment of the present invention is described by way of an example of its application to a spur gear G, which is in Fig. As shown in point 1, it is explained. Fig. The spur gear G shown is manufactured such that multiple numerical fractions 11 , which protrude radially outwards, at equal intervals on an outer circumferential portion of a spur gear body 10 be formed and a fastening hole 12 on the central portion of the spur gear body 10 is formed. The spur gear G is treated so that a compacted blank, obtained by compacting a raw material powder, is sintered, the blank is cold forged and under conventional conditions is both carbonized and quenched, as well as tempered. Fig. 2B shows a cross-section of the spur gear G along a flat surface A perpendicular to the mounting hole. 12 of the spur gear G, which is in Fig. 2A is shown.

[0025] In conventional carbonizing and quenching, a cold-forged sintered component undergoes a carbonization step in which the sintered component is heated and held at a temperature of 850 to 950°C in a carbonizing gas atmosphere with a carbon concentration in the atmosphere of 0.7 to 1.2% by mass, and is then quenched by a rapid cooling step in which the sintered component is rapidly cooled by immersion in oil. In the conventionally cold-forged sintered gear obtained by such a process, carbon from the atmospheric gas is supplied to an austenitic surface and diffuses into an internal portion, thus affecting the hardness distribution of the tooth portion that is in Fig. 2B as the B share is shown, in Fig. 3A and Fig. Figure 3B shows that the hardness distribution of the tooth portion from the surface S to the inner portion O in the cold-forged sintered gear is as shown in Figure 3B. Fig. Figure 3B shows that the hardness varies continuously from the surface S to the inner portion O, with the carbonated hardening layer (SC), which has a hardness greater than that of the inner portion, forming in a region C from the surface S to a depth (distance from S) C. Within the carbonated hardening layer, the surface S has a carbon content equal to the carbon concentration in the atmospheric gas and exhibits the highest hardness. The hardness distribution within the SC shows that the hardness gradually decreases as the proportion of carbon in the distribution decreases from the surface S to the depth C in the direction of the inner portion.

[0026] On the other hand, when a cold-forged sintered component of the embodiment of the present invention is applied to the spur gear G, an example of a hardness distribution of the tooth portion, which is known as the B portion, is obtained. Fig. 2B is shown in Fig. 4A and Fig. 4B shown in the diagram of a surface hardness distribution. Fig. Figure 4B shows the solid line as an example of a hardness distribution of a cold-forged sintered component of the present invention, and the dashed line as an example of a hardness distribution as described in Figure 4B. Fig. Figure 3B shows a conventionally cold-forged sintered component. In the cold-forged sintered component of the present invention (solid line), a softened layer (SD) is formed in a region from the surface S of the tooth portion to a depth (distance from the surface) D, and a carbonized hardened layer (DC) is formed within the softened layer. The hardness distribution from the softened layer (SD) through the carbonized hardened layer (DC) to the inner portion O varies continuously from the surface to the inner portion, with the hardness of the carbonized hardened layer being at its maximum at a depth (distance from the surface) H and the surface hardness being less than the hardness at depth (distance from the surface) H.In the cold-forged sintered component of the present invention, the notch sensitivity of the remaining pores, which act as notches, is reduced, and stress concentration at the remaining pores is inhibited, since the softened layer (SD) is provided in the surface layer. It should be noted that the softened layer is formed by adjusting the amount of carbonization at the surface and the internal proportion as mentioned above, without causing any detrimental effects such as an abnormal oxidized layer, etc.

[0027] The surface hardness S of the cold-forged sintered component is not greater than Hv 730 to reduce notch sensitivity. There is no lower limit to the surface hardness of the cold-forged sintered component. This lower limit varies depending on the material of the cold-forged sintered component and could be the hardness of a material that does not contain carbon (a material that has not been heat-treated). If the surface hardness S is too low, the abrasion resistance is reduced, and abrasion occurs easily when the component is used, for example, as gears in contact with and rubbing against an opposing component. For this reason, the surface hardness S of the cold-forged sintered component is preferably Hv 500 or more, and even more preferably Hv 600 or more, when the component is used as in the examples above.

[0028] The softened layer forms not only on the surface of the cold-forged sintered component, but also on the surface of the remaining pores that are open to the surface. This eliminates the need to form the softened layer beyond the depth of the remaining pores. However, if the depth D of the softened layer is greater than the depth of the remaining pores, the reduced notch sensitivity effect is reliably maintained. In this case, the depth of the remaining pores at the surface reaches approximately 70 μm, making the depth of the softened layer preferably 100 μm or more. It should be noted that the depth D of the softened layer is the depth of the region below the surface where the hardness Hv is 730 or less.

[0029] In a case where surface pressure is repeatedly applied to a component, such as a gear, the stress (Hertz stress) could reach its maximum in a region 150 to 300 μm below the surface. Therefore, to improve fatigue strength with respect to repeatedly applied surface pressure, the hardness is maximized in the region 150 to 300 μm below the surface, and the hardness is Hv 600 or higher.

[0030] The cold-forged sintered component with the aforementioned hardness distribution can be easily obtained by incorporating a decarbonization step between the carbonization and quenching stages. In the carbonization stage, the cold-forged sintered component, treated as a heat-treated object, is heated to a temperature higher than its austenitic transformation temperature and exposed to a carbonizing gas atmosphere with a carbon concentration of 0.7 to 1.2 wt%, thus carbonizing it. The surface of the cold-forged sintered component then has a carbon content corresponding to the carbon concentration (0.7 to 1.2 wt%) of the atmospheric gas.When the carbon concentration of the atmospheric gas is reduced and maintained for a sufficient period, carbon concentrated in the surface layer disperses and penetrates the interior, while carbon tightly dissolved in a matrix is ​​released from the surface into the atmospheric gas. This process decarbonizes the surface layer, forming a hardened layer and increasing the depth of the hardened layer. Consequently, the notch hardness is reduced, the problem of stress concentration at the notch is resolved, and the product strength can be maintained in accordance with the density of the cold-forged sintered component.

[0031] If the atmospheric carbon concentration is greater than 0.6% during the decarbonization step, the amount of carbon removed from the surface is insufficient, making it difficult to reduce the surface hardness to Hv 730 or less. Conversely, if the atmospheric carbon concentration is less than 0.3% during the decarbonization step, the amount of carbon removed from the surface is too high, potentially resulting in a hardness of less than Hv 600 in the region 150 to 300 μm below the surface. Therefore, the atmospheric carbon concentration during the decarbonization step is between 0.3 and 0.6% by mass.

[0032] Carbon diffusion and decarbonization are promoted at high temperatures. Therefore, if the temperature is below 800°C, decarbonization is not promoted, resulting in an extremely long time required to achieve the desired hardness in the cold-forged sintered component, or even preventing it from being achieved at all. Conversely, if the temperature is above 950°C, decarbonization is excessively promoted, and the hardness of the internal components is slightly reduced. For this reason, the decarbonization step is carried out at a temperature between 800 and 950°C.

[0033] The carbonization step, which precedes the decarbonization step, could be carried out in a conventional manner, assuming an atmospheric carbon concentration of 0.7 to 1.2% by mass, as mentioned above. This means that if the atmospheric carbon concentration is less than 0.7% by mass, the amount of carbonization is insufficient, resulting in no improvement in wear resistance or strength. Conversely, if the atmospheric carbon concentration is greater than 1.2% by mass, the amount of carbonization is too high, leading to the easy deposition of brittle iron carbide in the matrix.

[0034] If the heating temperature in the carbonization step is less than 850°C, the diffusion of carbon from the atmospheric gas into the cold-forged sintered component is insufficient. If the heating temperature in the carbonization step is more than 950°C, the diffusion of carbon from the atmospheric gas into the cold-forged sintered component is too high, and carbon diffuses into the interior of the cold-forged sintered component, thus impairing its strength. For this reason, the carbonization step is carried out at a temperature of 850 to 950°C.

[0035] After the carbon distribution of the cold-forged sintered component has been adjusted as described above, the component is immersed in an oil or similar medium and rapidly cooled, resulting in quenching. During this process, supersaturated carbon in the high-carbon portion is forced to dissolve, increasing the hardness. In the decarbonized layer on the surface, the carbon content is lower, leading to lower hardness. This is why the cold-forged sintered component exhibits the hardness distribution described above.

[0036] In the cold-forged sintered component that has undergone quenching, excessive tensile stress accumulates in a high-carbon fraction, resulting in a hard and brittle metallic structure. Therefore, as is conventional practice, the cold-forged sintered component is subjected to a tempering step by reheating it to a temperature of 150–280°C and then cooling it back to ambient temperature. By performing this tempering step after quenching, the supersaturated carbon from martensite, which is transformed during the quenching process, is deposited as ε-carbide (Fe₂C), forming low-carbon martensite. This process removes the tensile stress that developed during quenching without reducing the hardness of the cold-forged sintered component.In this case, if the heating temperature during tempering is less than 150°C, the removal of tensile stress is insufficient. If the heating temperature during tempering is more than 280°C, the low-carbon martensite will readily decompose into ferrite and two-dimensional iron carbide, thus reducing the hardness.

[0037] The decarbonization step could be carried out such that the heating temperature of a heat-treated object is maintained during the carbonization step, and only the carbon concentration of the atmospheric gas is adjusted. The decarbonization step is then performed after the carbonization step, followed by the quenching step. The heating temperature in the carbonization step is preferably, for example, 900 to 950°C, thereby promoting carbonization of the heat-treated object. The heating temperature in the decarbonization step is preferably 800 to 870°C, thereby allowing for precise control of the decarbonization.By performing the carbonization and decarbonization steps, both the carbon distribution of the cold-forged sintered component can be easily brought into a required state, and the hardness distribution of the cold-forged sintered component can also be easily brought into a required state after the quenching treatment. In this case, since the quenching is carried out at a temperature of 800 to 870°C, the amount of tensile stress that accumulates in the heat-treated object can be reduced compared to the case where quenching is carried out at a temperature of 900 to 950°C, thereby limiting dimensional variation caused by tensile stress to a minimum.

[0038] The example above illustrates how a heat-treated object is heated in a heat treatment furnace while the carbon concentration in the atmospheric gas and the heating temperature are varied during the heat treatment process. If it is difficult to alter the carbon concentration in the atmospheric gas and the heating temperature in a heat treatment furnace, or if these changes are not desirable for a production line, the cold-forged sintered component is cooled once after the carbonization step, and the sintered body could then be reheated and decarbonized. In this case, the quenching treatment can be performed after the carbonization step because the heat-treated object is heated to a temperature higher than the austenitic transformation temperature in the subsequent decarbonization step.In this case, the quenching treatment is performed again after the decarbonization step. If the carbonization and decarbonization steps are performed in a single furnace, the carbonization step can be carried out with other, general-purpose sintered components that are not subject to decarbonization. In this case, only the cold-forged sintered component could be subjected to decarbonization, and processing can be discontinued according to the circumstances of a production line.

[0039] Ferrous sintered materials, used for a wide variety of machine structural components, can be used as raw materials for sintered bodies. For example, SMF2 type (iron-copper type), SMF3 type (iron-carbon type), SMF4 type (iron-copper-carbon type), SMF5 type (iron-nickel-copper-carbon type), SMF6 type (iron-copper-carbon type), SMF7 type (iron-nickel type), and SMF8 type (iron-nickel-carbon type), which are regulated by the Japanese industrial standard Z2550, and 4100 type (iron-chromium-manganese type) and 4600 type (iron-nickel-molybdenum type), which are regulated by the American Iron and Steel Institute (AISI) standard.

[0040] In such iron-containing sintered materials, the carbon content is preferably 0.6 wt% or less to ensure easy formability and compactability during forging. In cases where a carbon content greater than 0.6 wt% is required in a product, heat treatment in a carbonized atmosphere after forging is preferably performed to compensate for the carbon deficiency. Alternatively, annealing prior to forging could be carried out to render the matrix of the sintered material easily plastically deformable.

[0041] Suitable raw material powders in the present invention could include iron powder, simple powders of various alloying elements, raw materials mixed with graphite powder, iron-based alloy powders alloyed with various alloying elements, raw material powders mixed with iron-based alloy powder, simple powders of various alloying elements and graphite powder, etc. When a 4600-type steel regulated by the American Institute of Iron and Steel (AISI) standard is used as a sintered material, this results in a raw material powder mixed with an iron alloy powder consisting, for example, in mass percent, of 0.4 to 1.0% Ni, 0.2 to 1.0% Mo, 0.1 to 0.5% Mn, the balance of Fe and unavoidable impurities, and 0.2 to 0.6% graphite powder.When the 4100 type, which is regulated by the American Institute of Iron and Steel (AISI) standard, is used as a sintered material, it results in a raw material powder mixed with an iron alloy powder consisting of, for example, in mass% 0.4 to 1.0% Cr, 0.2 to 1.0% Mo, 0.1 to 0.8% Mn, the balance of Fe and unavoidable impurities, and 0.2 to 0.6% of a graphite powder.

[0042] The compaction step is a step to compress the raw material powder and obtain a compact, and is equivalent to a forging step in normal compaction processing. That is, a mold cavity of a mold used in normal compaction processing is filled with raw material, which is compressed by upper and lower punches, and the compressed compact is extracted from the mold cavity, resulting in a compact with a smaller radial diameter and greater thickness than that of the spur gear G, which is in Fig. The density of the pellet is 7.0 Mg / m³, as shown in Figure 1. 3 or more.

[0043] The sintering step can be carried out under normal conditions for conventional powder metallurgy processes. When a sintered body is oxidized as a raw material in the sintering step, it hardens and is not easily plastically deformable. For this reason, the sintering atmosphere is preferably a normal non-oxidizing atmosphere such as nitrogen gas, a gas mixture of nitrogen and hydrogen, or a vacuum atmosphere. The sintering temperature could be approximately 1000 to 1250°C.

[0044] The forging step is a step in which the sintered body, as a raw material obtained through the sintering step, is preferably cold-forged and machined so that a cold-forged shape with a die hole that has the same shape as the spur gear G that is in Fig. As shown in Figure 1, the cold forging process involves coating a lubricant, such as zinc stearate or similar, onto the surface of the die or the sintered body. The sintered body is then inserted into the die of the cold forging mold and compressed by a ram from the vertical direction at a pressure of 1500 to 2500 MPa. The forged body is then extracted from the die, resulting in a cold-forged sintered component. The compression ratio (compressed thickness / original thickness) could be between 8.1 and 9.3%. The density of the cold-forged sintered component could be 7.7 Mg / m³. 3 or more, and the density ratio could be 97.8% or more. In the present invention, the forging is preferably cold forging, which does not require heating the sintered body, but hot forging and warm forging could also be carried out.

[0045] The cold-forged sintered component, a heat-treated object obtained through the mixing step of the aforementioned raw material powder, the compaction step, the sintering step, and the forging step, is subjected to both carbonization and decarbonization steps, as well as a tempering step, resulting in a cold-forged sintered component with the hardness distribution described above. In this production process, any remaining pores in the cold-forged sintered component are not removed by any machining or plastic deformation step.In this process, since pores are prevented from functioning as notches in a state where pores remain on the surface of the cold-forged sintered component, a machining step and a plastic processing step after forging are not necessary, and cold-forged sintered components with the same high strength as raw block materials can be produced at low cost.

[0046] The production process described above is an example where the carbon concentration in the gas atmosphere during the decarbonization step is lower than the carbon concentration in the gas atmosphere during the carbonization step, resulting in the formation of a softened layer on the surface of the forged sintered component. It should be noted that an atmospheric gas containing a high amount of oxygen could be used in the decarbonization step, thereby reducing and decarbonizing the carbon that is tightly dissolved in a region extending from the surface of the cold-forged sintered component to its interior, and forming a softened layer. In this case, a gas with a low dew point, such as atmospheric air, or a vapor atmosphere can be used instead of the atmospheric gas with a high oxygen content. Examples

[0047] The present invention will be explained in detail below with reference to concrete examples.

[0048] An iron-based alloy powder (average diameter of 70 μm) consisting of 0.5 wt% Ni, 0.5 wt% Mo, 0.2 wt% Mn, the balance of Fe and unavoidable impurities, was mixed with 0.3 wt% of a graphite powder and 0.8 wt% zinc stearate, the total content of the iron-based alloy powder and the graphite powder of 100 wt%, and a raw material powder was obtained.

[0049] A predetermined weight of the raw material powder was weighed and placed in a mold, which was then compacted at a pressure of 700 MPa. The density of the fresh pellet was 7.0 Mg / m³. 3 and the density ratio was 90%.

[0050] The freshly pressed pellets were loaded into a sintering furnace containing an atmosphere of 5% by volume of H₂ and 95% by volume of N₂ and held at a temperature of 1120°C for 20 minutes, after which they were removed from the furnace. The density of the sintered pellets was 7.0 Mg / m³. 3 and the density ratio was 90%.

[0051] The sintered body was loaded into a forging mold. The clearance between the sintered body, the mold, and a core rod was 0.1 mm. The pressure was 1800 MPa, and the compression ratio (compressed thickness / original thickness) was 10%. The density of the gear obtained by forging was 7.7 Mg / m³. 3 and the density ratio was 97.8%.

[0052] The gears obtained by forging were placed in a furnace with a carbonizing gas atmosphere and subjected to a carbonization step for 130 minutes at the temperatures and carbon concentrations in the atmosphere shown in Table 1. They were then subjected to a decarbonization step for 90 minutes at the temperatures and carbon concentrations in the atmosphere shown in Table 1, and then immersed in an oil, which rapidly cooled and quenched them. The carbon concentration in the atmosphere was adjusted by varying the amount of carbonizing gas (propane) added to the nitrogen gas used as a carrier gas. The gears were then subjected to a tempering step at a temperature of 180°C for 90 minutes, yielding samples 01 to 06 and 09 to 22.As a conventional example, a gear was subjected to the carbonization step under the conditions shown in Table 1, and to the quenching and tempering steps under the conditions described above, without the decarbonization step, resulting in Sample 07. As another conventional example, a gear of the same size as the gear above was machined from a raw ingot of steel and subjected to the carbonization step under the conditions shown in Table 1, and to the quenching and tempering steps under the conditions described above, without the carbonization step, resulting in Sample 08. The term "Cp value" refers to the amount of carbon in the atmosphere during the carbonization and decarbonization steps.

[0053] The hardness of portions of the surface of the inner portion of the tooth portion in a cross-section along a planar surface A perpendicular to a mounting hole of the obtained samples 01 to 22 was measured using a Vickers hardness tester (load of 100 g). The results are shown in Table 1 and Fig. 5 to Fig. Figure 8 shows the hardness measured at a proportion of 0 μm from the surface. This is the hardness measured at the surface.

[0054] A load was applied to span three tooth segments, and the load at which the tooth segments broke was measured. The result is shown in Table 1. Since a breaking load of 3.4 tf or more might be required if the gear is used as a spur gear in a starter motor, it was decided that this value would be a criterion for suitability. Table 1 Effect of carbon concentration in atmospheric gas during the decarbonization step

[0055] The effect of carbon concentration (Cp) in a decarbonization step was determined based on the results of samples 01 to 07 from Table 1 and Fig. 5 examined.

[0056] In sample 07, a conventional example not subjected to the decarbonization step, the surface hardness (distance from the surface = 0) was highest at Hv 820, exhibiting a hardness distribution in which the hardness decreased from the surface to the interior. The overall breaking load of sample 07 with this hardness distribution was low at 2.9 tf.

[0057] On the other hand, in samples 01 to 06, which underwent the decarbonization step, the surface hardness (distance from surface = 0) was lower than the surface hardness of sample 07, and the surface hardness was as low as the carbon concentration (Cp value) of the atmospheric gas during the decarbonization step. In samples 02 to 05, where the carbon concentration of the atmospheric gas during the decarbonization step was 0.3 to 0.6 wt%, the surface hardness was Hv 730 or less, and the span failure load was high at 3.8 to 4.2 tf. The span failure load was not lower than that of sample 08, which was produced from a raw ingot. This could be due to the reduction in notch sensitivity of the pores remaining on the surface by the reduction in surface hardness during the decarbonization step. As in Fig. As shown in Figure 5, the hardness in samples 02 to 05 was highest in the region of 150 to 300 μm below the surface, and the maximum hardness was Hv 600 or more. Therefore, if these samples are used in a tooth portion of a gear subjected to repeated surface pressure, they may possess sufficient fatigue strength against repeated application of surface pressure.

[0058] In sample 01, where the carbon concentration in the atmospheric gas during the decarbonization step was less than 0.3 wt%, surface decarbonization was extreme, reducing the maximum hardness to Hv 550 and resulting in a low span failure strength of 3.2 tf. This could be due to the reduced surface hardness, where the effect of extreme carbon decarbonization to improve resistance was large compared to the effect of reducing surface hardness and notch sensitivity of pores, which did not sufficiently improve the span failure strength.

[0059] In sample 06, where the carbon concentration of the atmospheric gas during the decarbonization step exceeded 0.6 wt%, surface carbonization was weak and the surface hardness was Hv 780. Consequently, the span failure load was 3.1 tf, and the improvement in span failure load through decarbonization was weak. This may have resulted in a weak reduction in the notch sensitivity of remaining surface pores during decarbonization.

[0060] Therefore, it can be confirmed that the strength of the cold-forged sintered component can be improved by achieving a surface hardness of Hv 730 or higher and by reducing the notch sensitivity of any remaining surface pores. It has been confirmed that the cold-forged sintered component achieved the aforementioned surface hardness by adding a decarbonization step and adjusting the carbon concentration of the atmospheric gas during this step to between 0.3 and 0.6 wt%. Effect of heating temperature in the decarbonization step

[0061] The effect of the heating temperature in the decarbonization step was determined based on the results of samples 04 and 09 to 13 from Table 1 and Fig. 6 examined.

[0062] In sample 09, where the heating temperature in the decarbonization step was less than 800°C, surface decarbonization was weak and the surface hardness was Hv 770, resulting in a span failure load of 3.2 tf and a weak effect of decarbonization on improving the span failure load. This could be due to the weak effect of decarbonization on reducing the notch sensitivity of pores remaining on the surface.

[0063] On the other hand, in sample 10, where the heating temperature in the decarbonization step was 800°C, decarbonization from the sample's surface was promoted, reducing the surface hardness to Hv 700 and significantly improving the span failure load to 3.8 tf. In samples 04 and 11 to 13, where the heating temperature in the decarbonization step was 860°C or higher, decarbonization was promoted, resulting in surface hardness Hv 550 to 600 and high span failure loads of 4.0 to 4.2 tf, equivalent to the span failure load of sample 08, which was machined from a raw ingot.

[0064] As in Fig. As shown in Figure 6, in samples where the heating temperatures in the decarbonization step reached up to 860°C, the surface hardness decreased rapidly in proportion to the increase in heating temperature. When the heating temperature exceeded 860°C, the rate of decrease in surface hardness was slow. This is because, although decarbonization was promoted by increasing the heating temperature, the atmospheric Cp value was 0.5%, meaning that the carbon content of the sample surface was not less than 0.5% by mass, with the remainder being approximately 0.5% by mass.

[0065] However, in sample 13, where the heating temperature was greater than 950°C, the maximum hardness at Hv 580 was low. If the sample is used in applications involving repeated surface pressure, the fatigue strength against repeated surface pressure may be reduced.

[0066] On the other hand, in samples 04 and 10 to 12, where the heating temperatures were 800 to 950°C, the hardness was maximum in the region of 150 to 300 μm below the surface, and the maximum hardness was Hv 600 or more. Therefore, if the samples are used in applications involving repeated surface pressure, sufficient fatigue strength against repeated surface pressure could be obtained.

[0067] Therefore, it was confirmed that by setting the heating temperature in the decarbonization step to 800 to 950°C, the surface hardness Hv 730 or less, the hardness is at its maximum in the region of 150 to 300 μm below the surface, and the maximum hardness is Hv 600 or more. Effect of carbon concentration in the carbonization step

[0068] The effect of the carbon concentration (Cp value) in the carbonization step was determined based on the results of samples 04 and 14 to 18 from Table 1 and Fig. 7 examined.

[0069] In sample 14, where the carbon concentration of the atmospheric gas in the carbonization step was 0.6 mass%, the maximum hardness of the region from 150 to 300 μm below the surface was low at Hv 570, and the overspanning breaking load was low at 3.2 tf, as the amount of carbonization was weak.

[0070] On the other hand, in patterns 04 and 15 to 18, where the carbon concentration of the atmospheric gas in the carbonization step was 0.7 mass% or more, as in Fig. As shown in Figure 7, the amount of carbonization increased as the carbon concentration in the atmospheric gas increased, and the ratio of surface to internal carbon content showed a tendency toward increased hardness. In samples 04 and 15 to 17, where the atmospheric gas carbon concentration was 0.7 to 1.2 wt%, the maximum hardness of the region 150 to 300 μm below the surface was high at Hv 640 to 870, and the span failure load was high at 3.8 to 4.2 tf, which was equivalent to the span failure load of sample 08, which is machined from raw ingot steel.

[0071] However, in sample 18, where the carbon concentration of the atmospheric gas was more than 1.2 mass%, the amount of carbonization was very high and the decarbonization in the subsequent decarbonization step was insufficient, resulting in a surface hardness of Hv 780, which meant that the notch sensitivity of pores remaining on the surface could not be sufficiently reduced and the overspan fracture load was low at 3.3 tf.

[0072] Therefore, it was confirmed that a high over-span breaking load equivalent to that of a raw block steel could be obtained by adjusting the carbon concentration of the atmospheric gas in the carbonization step in the range of 0.7 to 1.2 mass%. Effect of the heating temperature in the carbonation step

[0073] The effect of the heating temperature in the carbonization step was determined based on the results of samples 04 and 19 to 23 in Table 1 and Fig. 8 examined.

[0074] In sample 19, where the heating temperature in the carbonization step was less than 850°C, the amount of carbonization was weak, the maximum hardness in the region of 150 to 300 μm below the surface was low at Hv 590, and the spanning breaking load was low at 3.2 tf.

[0075] On the other hand, in samples 04 and 20 to 23, where the heating temperature in the carbonization step was 850°C or more, as in Fig. As shown in Figure 8, the amount of carbonization increased when the heating temperature was increased, and the ratio of surface to internal hardness showed a tendency towards an increase in hardness. In the samples, the surface hardness Hv ranged from 580 to 620, and the overstress breaking load was high at 3.9 to 5.1 tf, equivalent to or greater than the overstress breaking load of Sample 08, which is machined from raw ingot steel.

[0076] However, in sample 23, where the heating temperature in the carbonization step was greater than 950°C, carbonization progressed in the inner portion, resulting in high hardness and potentially reduced load-bearing capacity. Therefore, this sample might not be suitable for applications where surcharges are applied.

[0077] Therefore, it was confirmed that a high overburden breaking load equivalent to or greater than that of raw ingot steel could be achieved by adjusting the heating temperature in the carbonization step to between 850 and 950°C. Since the samples in this range were not carbonized in their inner portion, the load-bearing capacity could be high.

[0078] The present invention can provide a resistance equivalent to that of a raw block steel for sintered components and is applicable to sintered components, such as gears and pinions, on which large stresses from counter-components are applied, and is applicable to gears for starters.

Claims

[1] Sintered component that exhibits: a hardness distribution in which hardness varies continuously from a surface to an inner portion; a Vickers hardness value of 730 or less at the surface; where the maximum hardness of the component exists in a region between 150 and 300 μm from the surface; and where the maximum hardness has a Vickers hardness value of 600 or more. [2] Sintered component according to claim 1, wherein a carbonized and hardened layer is formed in a surface layer and a softened layer is formed outside the carbonized and hardened layer, the softened layer forming the surface. [3] Sintered component according to claim 1 or 2, wherein the sintered component has a density ratio of 97.8% or more. [4] Gear made from the sintered component according to one of claims 1 to 3. [5] Production process for a sintered component, wherein the process comprises: a mixing step for mixing a raw material powder; a compaction step to compress the raw material powder and obtain a pressed pellet; a sintering step to sinter the compact and obtain a sintered body; a forging step to forge the sintered body and obtain a forged body; a carbonization step to heat the forged body in a gas atmosphere in which the carbon concentration is 0.7 to 1.2 wt% at a temperature of 850 to 950°C; a decarbonization step to heat the forged body in a carbonized gas atmosphere in which the carbon concentration is 0.3 to 0.6 wt% at a temperature of 800 to 950°C, after the carbonization step; a quenching step for rapid cooling of the forging after the decarbonization step; and a tempering step to heat the forging body to a temperature of 150 to 280°C and cooling the forging body to normal temperature after the quenching step. [6] Production method for a sintered component according to claim 5, wherein the decarbonization step is carried out in sequence after the carbonization step during the heating of the forging body and the release of the atmospheric gas. [7] Production method for a sintered component according to claim 5, wherein the quenching is carried out by immersing the forging in an oil directly after the carbonization step and the decarbonization step is carried out by reheating the forging. [8] Production method for a sintered component according to claim 5, wherein the forged body has a density ratio of 97.8% or more after the forging step. [9] Production method for a gear for a starter motor, which is produced by the production method for a sintered component according to any one of claims 5 to 8.