A manufacturing method of a cage and the cage manufactured by the method

Abstract

The present invention relates to a method for manufacturing a cage for a constant velocity joint and a cage for a constant velocity joint manufactured thereby. More specifically, the invention relates to a method for manufacturing a cage for a constant velocity joint and a cage for a constant velocity joint manufactured thereby, which significantly increases the surface hardness of the cage to prevent breakage and surface wear in high-angle and high-torque environments. The present invention provides a method for manufacturing a cage for a constant velocity joint and a cage for a constant velocity joint produced thereby, characterized by comprising the steps of: forming an exterior of a cage for a constant velocity joint including a plurality of windows and a web between the windows; and a heat treatment step of pre-forming the cage, upon completion of the exterior formation, through carburizing martempering, wherein the microstructure of the surface and the core comprises tempered martensite and lower bainite.

Description

Technology Field

[0001] The present invention relates to a method for manufacturing a cage for a constant velocity joint and a cage for a constant velocity joint manufactured thereby. More specifically, the invention relates to a method for manufacturing a cage for a constant velocity joint and a cage for a constant velocity joint manufactured thereby, which significantly increases the surface hardness of the cage to prevent breakage and surface wear in high-angle and high-torque environments. Background Technology

[0002] In Korean published patent 10-2007-0055423 and US registered patent US7758431, a quenching and tempering process is used to manufacture a cage. In this process, the surface is composed of high-carbon martensite, retained austenite, and a grain boundary oxide layer, while the core (depth of 1 mm or more) is composed of low-carbon martensite. Although the surface hardness is high, the ductility is poor, and furthermore, there is a problem that there is a significant difference in hardness between the surface and the core, and after rapid cooling, reheating for tempering is required.

[0003] In addition, there was also the problem that there was a possibility of cracks or deformation occurring due to quenching.

[0004] Meanwhile, Japanese Patent Publication No. 4731945 discloses a carburization process and a quenching-tempering process for cast low-carbon steel to achieve full hardening of the cage surface and core. However, this also had problems, such as the surface being composed of high-carbon martensite, retained austenite, and a grain boundary oxide layer, while the core (depth of 1 mm or more) being composed of low-carbon martensite, resulting in high surface hardness but poor ductility, and furthermore, a significant difference in hardness between the surface and the core. Additionally, reheating for tempering was required after rapid cooling. There was also a problem that there was a possibility of cracking or deformation occurring due to quenching.

[0005] Korean Patent No. 10-0737602 and Korean Patent No. 10-0727196 disclose a method for manufacturing a cage using austempering. However, in the case of Korean Patent No. 10-0737602 and Korean Patent No. 10-0727196, case hardening is used instead of full handening, and since low-carbon steel (carbon content 0.18~0.22 wt%) is used, carburization and diffusion processes are required, which resulted in a problem of prolonged processing time. Furthermore, the surface structure becomes lower bainite, and the core structure becomes tempered martensite, so the surface and core structures are non-uniform, resulting in a problem where there is a difference in strength or hardness between the core and the surface.

[0006] To solve these problems, the inventors invented a cage and a method for manufacturing the same, which are subjected to austempering treatment using medium carbon steel (carbon content 0.4~0.5%) without using a carburization process, and obtained patents for the same (Korean Patent Registration No. 10-2391538, published April 27, 2022, Method for manufacturing a cage), (Korean Patent Registration No. 10-2484456, published January 5, 2023, Cage for constant velocity joints)).

[0007] Through this, the surface and core regions of the cage both contained lower bainite, and compared to the cage that underwent carburization heat treatment using low-carbon steel, the hardness of the surface region was 590~600HV and the hardness of the core region was 530~550HV, confirming that the increase in hardness and the compressive strength, tensile strength, and elongation were significantly improved.

[0008] However, in the case of electric vehicles (EVs) and special purpose vehicles (PBVs), since they are driven on a front-wheel basis and heavy batteries are mounted or cabins forming the cargo area are positioned, the load on the constant velocity joints in the front wheels increases, making high-angle-of-turn specifications necessary.

[0009] However, in the case of a conventional austempered cage made of medium carbon steel (carbon content 0.4~0.5%), it can only be used in limited applications where contact stress is low (e.g., cross-groove joints), and it cannot withstand the high angles and loads required in electric vehicles (EVs) and special purpose vehicles (PBVs) (e.g., Burfield joints), resulting in surface cracking or breakage and frequent wear, thus raising the need for a cage with higher surface hardness. Prior art literature

[0010] Korean Registered Patent No. 10-0737602 (July 10, 2007) Korean Registered Patent No. 10-0727196 (June 13, 2007) Korean Published Patent No. 10-2007-0055423 (May 30, 2007) US Registered Patent US7758431 (July 20, 2010) Japanese Patent Publication JP 4731945 (April 28, 2011) Korean Registered Patent No. 10-2391538 (April 27, 2022) Korean Registered Patent No. 10-2484456 (January 5, 2023) The problem to be solved

[0011] The present invention is designed to satisfy this need, and specifically aims to provide a method for manufacturing a cage for a constant velocity joint that can prevent breakage and surface wear in high-angle and high-torque environments by significantly increasing the surface hardness of the cage, and a cage for a constant velocity joint manufactured thereby. means of solving the problem

[0012] The present invention, for achieving the above objective, provides a method for manufacturing a cage for a constant velocity joint, characterized by comprising: a step of forming an exterior of a cage for a constant velocity joint including a plurality of windows and a web between the windows; and a heat treatment step of pre-forming the cage, upon completion of exterior formation, through carburization martempering.

[0013] By undergoing the above heat treatment step, the surface hardness of the cage is Rockwell hardness HRC 58 or higher and HRC 61 or lower, or Vickers hardness 700 HV or higher and 750 HV, and the core hardness penetrating to a depth of 1 mm or more into the surface of the cage is Rockwell hardness HRC 53 or higher and HRC 55 or lower, or Vickers hardness 560 or higher and 600 HV.

[0014] The above heat treatment step comprises a step of raising the furnace temperature to 900~960℃;

[0015] When the heating step is completed, a preheating step of uniformly heating to a temperature of 900~960℃ for 10 to 20 minutes;

[0016] A step of performing carburization in an atmosphere with a carbon concentration of 1.0 to 1.2% by injecting a gas at a temperature of 900 to 960℃ for 120 to 180 minutes;

[0017] A step of allowing carbon to diffuse from the cage material in a gas atmosphere with a carbon concentration of 0.80–0.95% at a temperature of 900–960℃ for 60 to 100 minutes;

[0018] A step of cooling the temperature to an atmosphere of 820–880°C while maintaining a gas atmosphere with a carbon concentration of 0.80–0.95%;

[0019] A step of uniformly heating to a temperature atmosphere of 820–880°C while maintaining a gas atmosphere with a carbon concentration of 0.80–0.95% for 20 to 40 minutes;

[0020] A step of performing cooling in a salt bath treatment tank at a temperature range of 180 to 230°C for 20 to 40 minutes;

[0021] It is characterized by including a step of performing tempering at a temperature range of 160 to 220°C for 120 to 180 minutes.

[0022] It is characterized by increasing the carbon content ratio of the cage from 0.4~0.5 wt% to 0.8~0.9 wt% by heat treatment that hardens the cage through carburization martempering.

[0023] The microstructure of the surface and depth is characterized by being composed of tempered martensite and lower bainite.

[0024] In addition, the present invention relates to a cage for a constant velocity joint having a plurality of windows and a web separating the windows, wherein

[0025] The basic material of the cage is medium carbon steel with a carbon content of 0.4 to 0.5 weight percent relative to the total weight, and

[0026] The cage undergoes a heat treatment process in which it is fully hardened by carburization and martempering, and

[0027] The cage comprises a surface region defined from the outer surface to a depth of 1 mm inward, and a deep region defined from the outer surface to a depth of 1 mm or more, wherein

[0028] The present invention provides a cage for a constant velocity joint characterized by the fact that, through a total hardening heat treatment by carburizing and martempering, the hardness of the surface region is greater than the hardness of the core region, and the hardness of the surface region is Rockwell hardness HRC 58 or higher and HRC 61 or lower, or Vickers hardness 700HV or higher and 750HV.

[0029] The microstructure of the surface and the core is composed of tempered martensite and lower bainite, and is characterized by having a higher weight percentage of carbon content in the surface than in the core. Effects of the invention

[0030] According to the present invention, there is a technical advantage in that the surface hardness can be significantly increased compared to conventional methods by additionally infiltrating and diffusing carbon into medium-carbon alloy steel to greatly improve the carbon content of the cage.

[0031] In other words, the increase in compressive stress due to the increase in surface hardness can achieve an increase in fracture strength, and there are advantages such as reduced noise and extended cage lifespan resulting from improved wear resistance. Brief explanation of the drawing

[0032] FIG. 1(a) is a perspective view of a pipe used to manufacture the cage of the present invention. FIG. 1(b) is a micrograph of the microstructure of medium carbon steel used in the present invention. FIGS. 2(a) to 2(e) are drawings illustrating the process of processing the cage of the present invention. Figure 3 is a graph showing the time and temperature of the heat treatment process in the present invention. FIG. 4 is a side cross-sectional view of a finished cage product according to the present invention. FIG. 5(a) is a micrograph of the microstructure of the surface of a cage produced by the conventional technique (austempering). FIG. 5(a) is a micrograph of the microstructure of the core of a cage produced by the conventional technique (austempering). FIG. 6(a) is a micrograph of the microstructure of the surface of a cage according to the present invention (carburization martempering). FIG. 6(b) is a micrograph of the microstructure of the core of the cage according to the present invention (carburization martempering). Fig. 7 is a photograph of the cage. FIG. 8(a) is a photograph of the cut section of the web portion of a cage produced by the conventional technique (austempering) when cut along the line A-A' in FIG. 7. FIG. 8(b) is a photograph of the cut portion of the web section of the cage produced by the present invention (carburizing martempering) when cut along the line A-A' in FIG. 7. FIG. 9 is a comparison table of hardness according to the conventional austempering and the carburized martempering of the present invention. FIG. 10 is a hardness graph according to the conventional austempering and the carburized martempering of the present invention. Specific details for implementing the invention

[0033] The present invention is capable of various modifications and may have various embodiments, and specific embodiments are illustrated and described in the drawings.

[0034] However, this is not intended to limit the invention to specific embodiments, and it should be understood that it includes all modifications, equivalents, and substitutions that fall within the spirit and scope of the invention.

[0035] Terms including ordinal numbers, such as first, second, etc., may be used to describe various components, but said components are not limited by said terms.

[0036] The above terms are used solely for the purpose of distinguishing one component from another.

[0037] For example, without departing from the scope of the present invention, the second component may be named the first component, and similarly, the first component may be named the second component.

[0038] The term "and / or" includes a combination of multiple related listed items or any of the multiple related listed items.

[0039] When it is stated that one component is "connected" or "joined" to another component, it should be understood that while it may be directly connected or joined to that other component, there may also be other components in between.

[0040] On the other hand, when it is stated that one component is "directly connected" or "directly coupled" to another component, it should be understood that there are no other components in between.

[0041] The terms used in this application are used merely to describe specific embodiments and are not intended to limit the invention.

[0042] A singular expression includes a plural expression unless the context clearly indicates otherwise.

[0043] In this application, terms such as "comprising" or "having" are intended to specify the existence of the features, numbers, steps, actions, components, parts, or combinations thereof described in the specification, and should be understood as not precluding the existence or addition of one or more other features, numbers, steps, actions, components, parts, or combinations thereof.

[0044] Hereinafter, embodiments will be described in detail with reference to the attached drawings, provided that identical or corresponding components are given the same reference number regardless of the drawing symbols, and redundant descriptions thereof will be omitted.

[0045] FIG. 1(a) is a cylindrical pipe (1) that serves as the raw material for the cage according to the present invention. The pipe (1) is a seamless pipe and is manufactured through a drawing process.

[0046] In the case of the present invention, by utilizing medium carbon alloy steel such as 47MnB3, there is an advantage of being inexpensive and having a stable supply. In the case of the medium carbon alloy steel according to the present invention, the carbon content is 0.4 to 0.5 weight % with respect to the total weight, preferably 0.45 to 0.48 weight %, the boron (B) content is 0.00015 to 0.0004 weight %, and the manganese (Mn) content is 0.7 to 0.9 weight %.

[0047] Meanwhile, optionally, the above composition may contain 0.15–0.35 wt% silicon (Si), 0.001–0.03 wt% phosphorus (P), 0.01–0.03 wt% sulfur (S), 0.01–0.2 wt% chromium (Cr), 0.01–0.2 wt% nickel (Ni), 0.01–0.05 wt% molybdenum (Mo), 0.01–0.3 wt% copper (Cu), 0.015–0.05 wt% titanium (Ti), 0.015–0.045 wt% aluminum (Al), and the remainder being iron (Fe).

[0048] The microstructure of the medium carbon steel used in the present invention consists of a small amount of ferrite and a large amount of pearlite, as shown in FIG. 1(b).

[0049] In the inventors' prior patents, medium-carbon alloy steel was also used, but the carburization heat treatment process was excluded. This was because the carbon content of the medium-carbon steel alone was sufficient to cover the strength and hardness required for conventional vehicles, and thus, a separate carburization and diffusion process was unnecessary.

[0050] However, in the case of the present invention, since a cage for a constant velocity joint made of high-carbon alloy steel must be manufactured by adding more carbon to medium-carbon steel, the cage is manufactured using strong carburization technology rather than the conventional weak carburization.

[0051] As shown in FIG. 2(a), a cylindrical pipe is cut along its length to produce a structure (10) that forms the outer shape of the cage.

[0052] Then, as shown in FIG. 2(b), a forming step is performed to form a structure (20) having a curved surface on its outer surface, and as shown in FIG. 2(c), a turning process is performed to form a chamfer by removing a sharp edge portion from the edge surface of the structure (30), or to form a flat or curved surface.

[0053] When the turning process is completed, as shown in FIG. 2(d), a punching process is performed to form a window (41) that can accommodate a ball on the outer surface of the structure (40), and a broaching process is performed to process the edge of the window (51) on the structure (50) after the punching is completed.

[0054] Once the broaching process is completed in this manner, heat treatment is performed on the cage as shown in Fig. 3.

[0055] The specific detailed steps of the heat treatment according to the present invention are as follows.

[0056] 1) Heating step: A step of raising the furnace temperature to 900~960℃ (preferably 930℃).

[0057] 2) Crack preheating step: A step of preheating by uniformly heating to a temperature of 900 to 960°C (preferably 930°C) for 10 to 20 minutes (preferably 15 minutes).

[0058] 3) Carburization step: A step of performing carburization in an atmosphere at a temperature of 900 to 960°C (preferably 930°C) for 120 to 180 minutes (preferably 150 minutes) by injecting a gas with a carbon concentration of 1.0 to 1.2% (preferably 1.1%).

[0059] 4) Diffusion step: A step of allowing carbon to diffuse from the cage material in a gas atmosphere with a carbon concentration of 0.80–0.95% (preferably 0.85%) at a temperature atmosphere of 900–960°C (preferably 930°C) for 60 to 100 minutes (preferably 80 minutes).

[0060] 5) Cooling step: A step of cooling to a temperature atmosphere of 820 to 880°C while maintaining a gas atmosphere with a carbon concentration of 0.80 to 0.95% (preferably 0.85%).

[0061] 6) Cracking step: A step of uniformly heating to a temperature atmosphere of 820–880°C (preferably 850°C) while maintaining a gas atmosphere with a carbon concentration of 0.80–0.95% for 20 to 40 minutes.

[0062] 7) Salt bath cooling step: A step of cooling in a salt bath treatment tank at a temperature range of 180 to 230°C (preferably 210°C) for 20 to 40 minutes (preferably 30 minutes).

[0063] 8) Tempering step: A step of performing tempering at a temperature range of 160 to 220°C (preferably 210°C) for 120 to 180 minutes (preferably 150 minutes).

[0064] After the heat treatment process and cooling are completed, as shown in FIG. 4, grinding is performed on the window (61), inner diameter surface (64), and outer diameter surface (63) to remove fine small elements (e.g., burrs) that may interfere with the mating parts (ball and, inner ring, outer ring) in the constant velocity joint and to make the surface smooth.

[0065] FIG. 5(a) is a micrograph of the surface microstructure of a cage that has undergone austempering without a carburization process using medium carbon alloy steel in a prior patent by the applicant and inventors, and FIG. 5(b) is a micrograph of the microstructure of the core. Here, the core refers to an area that is indented by more than 1 mm from the surface.

[0066] As illustrated in FIGS. 5(a) and 5(b), the surface and core microstructure of the cage according to the prior patent consists of lower bainite.

[0067] However, in the case of the present invention, the carbon content is higher than that of the existing patent because the medium carbon alloy steel undergoes a so-called carburizing and diffusion process, and then undergoes cooling and tempering in a salt bath to achieve full hardening.

[0068] Therefore, as the carbon content ratio of the cage increases from 0.4 to 0.5 wt%, which is the carbon content ratio of medium carbon alloy steel, to more than 0.5 wt% to 0.9 wt%, the carbon content ratio increases significantly.

[0069] This is because it increases the carbon potential of carbon to enable strong carburization and diffusion, thereby allowing additional carbon atoms to be incorporated into the crystal lattice of γ-iron, which already contains carbon atoms at the level of medium carbon alloy steel.

[0070] General carburization refers to light carburization, which increases the carbon content of low-carbon alloy steel (carbon content 0.2~0.3 wt%) by carburizing it. However, the present invention is characterized by including a heavy carburization and diffusion step for performing additional carbon dissolution on medium-carbon steel that already has a sufficient carbon content, followed by salt bath cooling and tempering.

[0071] Accordingly, the surface and deep tissues also differ compared to conventional technology.

[0072] FIG. 6(a) is a micrograph of the surface microstructure of a cage manufactured by the method of the present invention, and FIG. 6(b) is a micrograph of the microstructure of the deep region. Here, the deep region refers to an area that is at least 1 mm deep from the surface.

[0073] Here, the surface and core consist of tempered martensite and lower bainite. Here, the proportion of tempered martensite constitutes at least 90% of the total weight.

[0074] However, there are slight differences in the carbon content between the surface and the core. That is, compared to FIGS. 5(a)(b), a dark area of ​​black color appears in FIGS. 6(a)(b), and this part is the high-carbon tempered martensite structure. Here, the carbon content in the high-carbon range refers to 0.5 wt% or more.

[0075] In other words, as the carbon content increases from the core to the surface, there is a tendency for the proportion of high-carbon tempered martensite to be relatively larger at the surface than at the core, and relatively smaller at the core than at the core. This is because carbon was additionally infiltrated and diffused mainly into the surface due to heavy carburization.

[0076] In other words, as described above, this is because additional carbon atoms are incorporated into the crystal lattice of γ-iron, which already contains carbon atoms at the level of medium carbon (carbon content 0.4~0.5 wt%) alloy steel, by increasing the carbon potential of carbon to induce strong carburization and diffusion. This is why a dark, black-colored structure is visible.

[0077] As illustrated in FIG. 7, if the middle of the web portion (62) between the windows (61) and windows (61) is cut along the A-A' line (red dotted line) and the cross-section is viewed, as in FIG. 8(a), in the case of a conventional patent (austempered cage without carburization and diffusion for medium carbon steel), no carburization pattern is formed on the outer surface, but as in FIG. 8(b), in the case of the present invention, a dark carburization pattern is formed on the surface (region outside the yellow boundary line).

[0078] In this way, by subjecting medium carbon alloy steel to a carburization (steel carburization) and martempering process, the carbon content of the cage is significantly increased, and the surface hardness is significantly increased.

[0079] As illustrated in FIGS. 9 and 10, in the case of the conventional technology (austempering), the hardness of the surface and the core was maintained within a certain range (Rockwell hardness HRC 53 or higher and HRC 55 or lower, or Vickers hardness 560 HV or higher and 600 HV). (Light green graph line in FIG. 10)

[0080] On the other hand, in the case of the present invention (carburizing martempering), it was confirmed that the surface hardness of the cage is Rockwell hardness HRC 58 or higher and HRC 61 or lower, or Vickers hardness 700 HV or higher and 750 HV, and the core hardness penetrating 1 mm or more from the surface of the cage is Rockwell hardness HRC 53 or higher and HRC 55 or lower, or Vickers hardness 560 or higher and 600 HV.

[0081] That is, regarding surface hardness, compared to conventional technology (austempering), the present invention improves the hardness of the surface in contact with the ball by 16 to 34% while maintaining the hardness of the core, thereby providing a cage suitable for front-wheel-based EVs (electric vehicles) or PBVs (special purpose vehicles) that are inevitably subjected to high torque, high angle of cut, high load, and high wear environments.

[0082] Although the present invention has been described above with reference to the embodiments illustrated in the drawings, this is merely for the purpose of explaining the invention, and those skilled in the art will understand that various modifications or equivalent embodiments are possible from the detailed description of the invention.

[0083] Therefore, the true scope of rights of the present invention must be determined by the technical concept of the patent claims. Explanation of the symbols

[0084] 1: Pipe 10;20;30;40;50: Intermediate structure for cage fabrication 60: Cage

Claims

Claim 1 A method for manufacturing a cage for a constant velocity joint comprising: a step of forming an exterior of a cage for a constant velocity joint including a plurality of windows and a web between the windows; and a heat treatment step of fully hardening the cage formed through carburizing martempering, wherein the cage comprises: a surface region defined from the outer surface to a depth of 1 mm inward, and a core region defined from the outer surface to a depth of 1 mm or more, wherein the hardness of the surface region is greater than the hardness of the core region due to the total hardening heat treatment by carburizing martempering, the hardness of the surface region is Rockwell hardness HRC 58 or higher and HRC 61 or lower, or Vickers hardness 700 HV or higher and 750 HV, and the microstructure of the surface and core comprises tempered martensite and lower bainite. Claim 2 A cage for a constant velocity joint having a plurality of windows and a web separating the windows, wherein the basic material of the cage is medium carbon steel having a carbon content of 0.4 to 0.5 weight percent relative to the total weight, the cage undergoes a heat treatment process in which it is fully hardened by carburizing martempering, and the cage comprises: a surface region defined from the outer surface to a depth of 1 mm inward, and a core region defined from the outer surface to a depth of 1 mm or more, wherein the hardness of the surface region is greater than the hardness of the core region due to the heat treatment in which it is fully hardened by carburizing martempering, the hardness of the surface region is Rockwell hardness HRC 58 or higher and HRC 61 or lower, or Vickers hardness 700 HV or higher and 750 HV, and the microstructure of the surface and core is composed of tempered martensite and lower bainite. Claim 3 A cage for a constant velocity joint according to paragraph 2, characterized in that the ratio of carbon content by weight% on the surface is higher than in the core.

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